Diastereoisomers as probes for solvent reorganizational ...deanna/images/MyPublications... · meso diastereoisomer and parallel in the rac form. In addi-tion, ‘‘exterior clefts’’

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Chemical Physics 324 (2006) 8–25

Diastereoisomers as probes for solvent reorganizational effects onIVCT in dinuclear ruthenium complexes

Deanna M. D�Alessandro, F. Richard Keene *

Department of Chemistry, School of Pharmacy and Molecular Sciences, James Cook University, Townsville, Qld. 4811, Australia

Received 19 June 2005; accepted 1 September 2005Available online 3 October 2005

Abstract

IVCT solvatochromism studies on the meso and rac diastereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ (bpy = 2,2 0-bipyridine; bpm = 2,2 0-bipyrimidine) in a homologous series of nitrile solvents revealed that stereochemically directed specific solvent effects in the first solvationshell dominated the outer sphere contribution to the reorganizational energy for intramolecular electron transfer. Further, solvent pro-portion experiments in acetonitrile/propionitrile mixtures indicated that the magnitude and direction of the specific effect was dependenton the relative abilities of discrete solvent molecules to penetrate the clefts between the planes of the terminal polypyridyl ligands. Inparticular, the specific effects were dependent on the dimensionality of the clefts, and the number, size, orientation and location ofthe solvent dipoles within the interior and exterior clefts.

IVCT solvatochromism studies on the diastereoisomeric forms of [{Ru(bpy)2}2(l-dbneil)]5+ and [{Ru(pp)2}2(l-bpm)]5+

{dbneil = dibenzoeilatin; pp = substituted derivatives of 2,2 0-bipyridine and 1,10-phenanthroline} revealed that the subtle and systematicchanges in the nature of the clefts by the variation of the bridging ligand, and the judicious positioning of substituents on the terminalligands, profoundly influenced the magnitude of the reorganizational energy contribution to the electron transfer barrier.� 2005 Elsevier B.V. All rights reserved.

Keywords: Solvatochromism; Intervalence charge transfer; Dinuclear; Stereochemistry; Ruthenium; Solvent reorganization

1. Introduction

Dinuclear ligand-bridged mixed-valence complexes haveplayed a pivotal role in the assessment of activation barri-ers to intramolecular electron transfer since the disclosureof the Creutz–Taube ion, [{Ru(NH3)5}(l-pyz){Ru-(NH3)5}]5+ (pyz = pyrazine), in 1973 [1]. Systems of thisgenre have provided important experimental insights intothe roles of solvent dynamics [2–15], ion-pairing [16–21],encapsulation [22,23], temperature [24–28], and redoxasymmetry [29,30], and they have been used as model sys-tems to verify the salient predictions of several importanttheoretical models that describe the activation barrier toelectron transfer [31–35].

0301-0104/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemphys.2005.09.016

* Corresponding author. Tel.: +61 0 7 47814433; fax: +61 0 7 47816078.E-mail address: [email protected] (F.R. Keene).

The particular appeal of mixed-valence complexes of theform [{LnMII}(l-BL){MIIILn}]5+ (M = metal centers,L = terminal ligands and BL = bridging ligand) is theobservation of an absorption band in the near-infrared(NIR) region of the electronic spectrum which is identifiedas the optically induced intervalence charge transfer(IVCT) transition. IVCT measurements provide a sensitiveand powerful probe to elucidate aspects of intramolecularelectron transfer processes as the energy (mmax), intensity(e) and bandwidth (Dm1/2) of these transitions can be quan-titatively related to the factors which influence the activa-tion barrier to electron transfer [31,32].

For symmetrical, valence-localized mixed-valence sys-tems, Hush [31,32] proposed the relationship

mmax ¼ ki þ ko þ DE0; ð1Þwhere DE 0 represents the energy contributions due tospin–orbit coupling and/or ligand field asymmetry, and

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D.M. D�Alessandro, F.R. Keene / Chemical Physics 324 (2006) 8–25 9

ki and ko are the inner- and outer-sphere reorganizational(Franck–Condon) parameters, respectively: ki correspondsto the energy required for reorganization of the metal–li-gand and intra-ligand bond lengths and angles, and ko isthe energy required for reorganization of the surroundingsolvent medium. The solvent contribution is generallymodelled as a one-dimensional classical mode due to thelow frequencies of the coupled vibrations [31–34]. Thespherical cavity dielectric continuum model given by Eq.(2) provides a framework for the calculation of the sol-vent reorganizational contribution in which the electrondonor and acceptor are modelled as two non-interpene-trating spheres, embedded in the dielectric continuum[31,32].

ko ¼ e2 1

a� 1

d

� �1

Dop

� 1

Ds

� �. ð2Þ

The parameters a and d define the molecular radii and dis-tance between the donor and acceptor, e is the unit elec-tronic charge, and Ds and Dop are the macroscopic staticand optical dielectric constants of the solvent, respectively.In accordance with Eqs. (1) and (2), mmax should vary line-arly with the solvent dielectric function (1/Dop � 1/Ds),with slope e2(1/a � 1/d) and intercept ki + DE 0 at (1/Dop � 1/Ds) = 0, and when the length of the bridging li-gand is varied (at fixed a) mmax should vary linearly with1/d, with slope e2(1/Dop � 1/Ds) in a given solvent.

While the predictions of the dielectric continuum modelhave been consistent with the results from a number ofIVCT solvatochromism studies of mixed-valence dinuclearruthenium complexes [2,36,37], the model breaks downwhen the underlying assumptions of the classical modelare invalidated, or in the presence of specific solvent–soluteinteractions or dielectric saturation effects [2]. Theoreti-cally, the dielectric continuum model is formulated in termsof a one-dimensional classical mode for the solvent, but itis inadequate for systems which exhibit coupled high-fre-quency quantum modes which must be explicitly treatedthrough a quantum mechanical approach [15]. Eq. (2) alsoneglects the volume occupied by the donor and acceptor(the ‘‘excluded volume’’) and is valid only when the dis-tance between the redox centers exceeds the sum of their ra-dii (d� 2a). The corrections due to non-spherical fieldsaround the metal centers become increasingly importantas the distance between the metal centers is decreased.

Experimentally, the analysis of IVCT solvatochromismdata according to the spherical (and ellipsoidal[13,15,38,39]) dielectric continuum models has often beenseverely confounded by non-continuum effects. These is-sues have been addressed in an extensive review of mediumeffects on the IVCT properties of mixed-valence complexesby Chen and Meyer [2], and include specific solvent–soluteinteractions and dielectric saturation effects, in addition toion-pairing contributions from the counter and electrolyteions, the concentration of the chromophore and the chem-ical oxidant used for the generation of the mixed-valencecomplex.

The elucidation of the relative contributions of contin-uum and non-continuum effects is the subject of consider-able experimental interest in the attempt to develop moresophisticated theoretical models for solvent reorganiza-tional contributions to the electron transfer barrier [2,40].Dielectric continuum theory obscures the ‘‘molecularity’’of the solvent by neglecting the properties of individual sol-vent molecules, and this underpins the recent theoreticalinterest directed towards understanding the molecular basisof reorganizational effects [2,40].

The experimental strategy for extracting information atthe molecular level using IVCT solvatochromism studiesinvolves probing the first solvation shell separately fromthe bulk solution. Dinuclear ruthenium complexes incorpo-rating ammine and cyano ligands have been extensivelyinvestigated in this regard because of the existence ofstrong directional H-bonding and donor–acceptor interac-tions between the chromophore ligands and individual sol-vent molecules [6,11–15,41]. These specific solventinteractions coexist with, and often dominate dielectriccontinuum effects [2]. Correlations have been found be-tween the IVCT solvent shifts and empirical solvent param-eters such as the Gutmann donor and acceptor numbers[42]. In studies of dinuclear ruthenium mixed-valence com-plexes based on –Ru(NH3)5, –trans-Ru(NH3)4(py) and–Ru(bpy)(NH3)3 with pyz, 4-cyanopyridine and 4,4 0-bpybridging ligands [19,43,44], the IVCT energies correlate lin-early with the Gutmann solvent donor number (DN) dueto specific H-bonding interactions between the ammine li-gands and the solvent molecules. In each case, the magni-tude of the specific interaction increased with the donornumber of the solvent, and the number of NH3 ligands inthe chromophore.

IVCT solvatochromism studies in solvent mixtures havedemonstrated that the solvent reorganizational process oc-curs predominantly within the first solvation layer, andmay be profoundly influenced by the systematic replace-ment of individual solvent molecules in the immediatevicinity of the mixed-valence chromophore[2,5,7,13,14,19,45].

There is clearly a need for experimental studies of IVCTsolvatochromism which provide insights into the micro-scopic solvent reorganizational contributions to the intra-molecular electron transfer barrier.

1.1. Scope and objectives of the present study

The majority of experimental IVCT studies have beenconducted by varying global features of the complexes,such as the identities of the bridging and terminal ligands,and the constituent metal centers. In addition, the theoret-ical implications of the results have often been complicatedby ion-pairing, and ambiguities in the geometries of thecomplexes due to a lack of structural rigidity and/or stereo-isomeric purity [46,47].

In the present study, the investigation of the IVCT sol-vatochromism in the mixed-valence diastereoisomeric

10 D.M. D�Alessandro, F.R. Keene / Chemical Physics 324 (2006) 8–25

forms of [{Ru(pp)2}2(l-bpm)]5+ (pp = 2,2 0-bipyridine andits derivatives; bpm = 2,2 0-bipyrimidine) provides a newexperimental approach to probe the microscopic originsof solvent reorganizational effects on intramolecular elec-tron transfer processes. As an example, the dinuclear spe-cies [{Ru(bpy)2}2(l-bpm)]4+ (bpy = 2,2 0-bipyridine) existsin two diastereoisomeric forms – meso, KD (�DK) andracemic (rac) (illustrated in Fig. 1), the latter comprisingthe enantiomeric pair DD and KK. While the identity andcoordination environments of the metal centers are identi-cal in each diastereoisomeric form, a significant differencemay be discerned in the nature of the ‘‘clefts’’ formed be-tween the planes of the terminal bpy ligands [47]. ‘‘Interiorclefts’’ are formed between the bpy ligands immediately‘‘above’’ and ‘‘below’’ the plane of the bridging ligand:the terminal ligands are approximately orthogonal in themeso diastereoisomer and parallel in the rac form. In addi-tion, ‘‘exterior clefts’’ are evident between the planes of theterminal bpy ligands at either end of the complex and areidentical for two diastereoisomeric forms.

The variation in the dimensions of the clefts between thediastereoisomeric forms of the same complex may have sig-nificant consequences for differential solvent and anionassociation at the molecular level. Indeed, the differentialassociation of eluent anions such as toluene-4-sulfonatefCH3ðC6H4ÞSO�3 g gives rise to the separation of the diaste-reoisomeric forms in the chromatographic cation-exchangeseparation process [46–49].

The diastereoisomers of symmetrical dinuclear polypyr-idyl complexes exhibit several attractive features over dinu-clear complexes which have been employed to date for theinvestigation of reorganizational contributions to theIVCT processes: the complexes are structurally rigid, andthe dimensions of the clefts may be varied in a subtle andsystematic way through stereochemical variation, bridgingligand modification, or the judicious positioning of substit-

Fig. 1. (a) Front view and (b) top view of the meso (KD) and rac (DD)diastereoisomers of [{Ru(bpy)2}2(l-bpm)]4+ illustrating the subtle varia-tion in the dimensions of the clefts above and below the plane of thebridging ligand. Hydrogen atoms are omitted for clarity.

uents on the terminal polypyridyl ligands – while maintain-ing the identity and coordination environments of thecomponent metal centers. For symmetrical complexes, ki

and DE 0 are expected to be identical for the diasterereoi-somers of the same complex, so IVCT solvatochromismstudies of the diastereoisomeric forms permit a direct probeof stereochemically induced ko variations on the intramo-lecular electron transfer barrier. This may provide newand intimate insights into solvent reorganizational effectsat the molecular level.

We report details of a three-pronged approach to thisproblem:

(a) IVCT solvatochromism studies. An investigation wasundertaken on the IVCT solvatochromism for thediastereoisomeric forms of [{Ru(bpy)2}2(l-bpm)]5+

in the homologous series of the nitrile solvents aceto-nitrile {CH3CN; AN}, propionitrile {CH3CH2CN;PN}, n-butyronitrile {CH3(CH2)2CN; BN}, iso-butyronitrile {(CH3)2HCCN; iBN} and benzonitrile{C6H5CN; BzN}. The macroscopic properties of thesolvents (as defined by the solvent parameter 1/Dop � 1/Ds) vary over the range 0.5127 (AN) to0.3897 (BzN), while the subtle variation in the molec-ular shape, size and symmetry through the series ofaliphatic and aromatic nitriles allows for a detailedanalysis of the microscopic origins of the solvent reor-ganizational energy due to stereochemically directedsolvent effects.

(b) Influence of the bridge on the selectivity of solvent asso-ciation. The IVCT solvatochromism properties of[{Ru(bpy)2}2(l-dbneil)]5+ (dbneil = dibenzoeilatin,Fig. 2) were examined to assess the effect of increasingthe dimensions of the interior clefts by comparisonwith the analogous diastereoisomers of [{Ru(b-py)2}2(l-bpm)]5+.

(c) Influence of the terminal ligands on the selectivity of

solvent association. This study involved the systematicmodification of the dimensions of the interior andexterior clefts by the judicious positioning of substit-uents on the terminal ligands in the series[{Ru(pp)2}2(l-bpm)]5+, where pp = 5,5 0-dimethyl-2,2 0-bipyridine (5,5 0-Me2bpy), 4,4 0,5,5 0-tetramethyl-2,2 0-bipyridine (Me4bpy), 2,9-dimethyl-1,10-phenan-throline (Me2phen) and 4,4 0-di-tert-butyl-2,2 0-bipyri-dine (tBu2bpy), shown in Fig. 3. In each case, theIVCT characteristics of the separated diastereoiso-mers were investigated in the homologous series ofnitrile solvents.

2. Experimental

2.1. Materials

Hydrated ruthenium trichloride (RuCl3 Æ 3H2O; Strem,99%), 2,2 0-bipyrimidine (bpm; Lancaster), 2,2 0-bipyridine

Fig. 2. The bpm and dbneil bridging ligands with the crystallographically determined Ru–Ru distances in their dinuclear complexes (meso-

[{Ru(Me2bpy)2}2(l-bpm)]4+ [48] and meso-[{Ru(bpy)2}2(l-dbneil)]4+ [68]).

Fig. 3. Terminal polypyridyl ligands used in the present study.


(bpy; Aldrich, 99+%), 4,4 0-dimethyl-2,2 0-bipyridine (Me2b-py; Aldrich), 5,5 0-dimethyl-2,2 0-bipyridine (5,5 0-Me2bpy;Aldrich), 2,9-dimethyl-1,10-phenanthroline (Me2phen;Monsanto), 4,4 0-di-tert-butyl-2,2 0-bipyridine (tBu2bpy;Aldrich, 98%), stannous chloride (SnCl2 Æ 2H2O; Ajax),lithium chloride (LiCl; Aldrich, 99+%), ammonium hexa-fluorophosphate (NH4PF6; Aldrich, 99.99%), potassiumhexafluorophosphate (KPF6; Aldrich, 98%), lithium tetrakis-(pentafluorophenyl)borate diethyletherate (Li{B(C6F5)4} ÆEt2O; Boulder Scientific), ethylene glycol (Ajax, 95%),sodium benzoate (Aldrich, 98%), sodium toluene-4-sulfo-nate (Natos; Aldrich, 98%), DOWEX� 1 · 8, 50–100 mesh(Aldrich) and Amberlite� IRA-400 (Aldrich) anion ex-change resins (Cl� form) and laboratory reagent solventswere used as received. Tetra-n-butylammonium hexaflu-orophosphate ([(n-C4H9)4N]PF6; Fluka, 99.9+%) was driedin vacuo at 60 �C prior to use and ferrocene (Fc; BDH) waspurified by sublimation prior to use. SP Sephadex C-25(Amersham Pharmacia Biotech), and silica gel (200–400mesh, 60 A, Aldrich) were employed for the chromato-graphic separation and purification of ruthenium com-plexes [47].

Acetonitrile (AN or CH3CN; Aldrich, 99.9+%), andpropionitrile (PN; Aldrich) were distilled over CaH2 beforeuse, while acetone (BDH, HPLC grade) was distilled overK2CO3 and dichloromethane over CaCl2 prior to use. n-Butyronitrile (BN; Aldrich, 99+%), iso-butyronitrile (iBN;Aldrich) and benzonitrile (BzN; Aldrich) were used as re-ceived. N,N-Dimethylformamide (DMF; Ajax) was dis-tilled under reduced pressure [50] (76 �C at 39 mm Hg)immediately prior to use.

4,4 0,5,5 0-Tetramethyl-2,2 0-bipyridine (Me4bpy) was sup-plied by Dr. Bradley Patterson (JCU) and was preparedaccording to the literature method [51,52].

2.2. Instrumentation and physical methods

1D and 2D 1H NMR spectra were performed on a Var-ian Mercury 300 MHz spectrometer. The 1H NMR chem-ical shifts for all complexes are reported relative to 99.9%d3-acetonitrile {CD3CN; Cambridge Isotope Laboratories(CIL)} at d = 1.93 ppm. 1H NMR assignments were per-formed with the assistance of COSY experiments to iden-tify each pyridine ring system.


Elemental microanalyses were performed at the Micro-analytical Unit in the Research School of Chemistry, Aus-tralian National University. For selected complexes, anallowance for hydration was necessary to account for anal-ysis figures within the acceptable limits (±0.4%).

Electrochemical measurements were performed underargon using a Bioanalytical Systems BAS 100A Electro-chemical Analyzer. Cyclic and differential pulse volta-mmograms were recorded under Ar in 0.02 M[(n-C4H9)4N]{B(C6F5)4}/CH3CN at +25 �C using a glassycarbon working electrode, a platinum wire auxiliary elec-trode and an Ag/AgCl (0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN) reference electrode. Ferrocene was added as aninternal standard on completion of each experiment {theferrocene/ferrocenium couple (Fc+/Fc0) occurred at+550 mV vs. Ag/AgCl}: all potentials quoted in millivoltsvs. Fc+/Fc0 [53]. Cyclic voltammetry was performed witha sweep rate of 100 mV s�1; differential pulse voltammetrywas conducted with a sweep rate of 4 mV s�1 and a pulseamplitude, width and period of 50 mV, 60 ms and 1 s,respectively. Potentials from DPV experiments arereported ±3 mV. In order to obtain reasonable electro-chemical responses, measurements in the 0.02 M[(n-C4H9)4N]{B(C6F5)4}/CH3CN electrolyte required aconcentration of complex which was approximatelydouble that in 0.1 M [(n-C4H9)4N]PF6/CH3CN. iR com-pensation was not employed for the electrochemicalmeasurements.

2.3. UV/Vis/NIR spectroelectrochemistry

Electronic spectra were recorded using a CARY 5EUV–visible–NIR spectrophotometer interfaced to VarianWinUV software. The absorption spectra of the electrogen-erated mixed-valence species were obtained in situ by theuse of an optically semi-transparent thin-layer electrosyn-thetic (OSTLE) cell (path length 0.685 mm) mounted inthe path of the spectrophotometer [54,55].

Solutions for the solvent proportion experiments wereprepared by serial dilution of solvent mixtures containingacetonitrile and propionitrile. To minimize artefacts inthe NIR spectral data due to ion-pairing and concentrationeffects which are known to influence the IVCT transitionsof dinuclear complexes [20,21,28,56,57], the spectra weremeasured using a constant concentration of complex(0.40 · 10�3 M) in 0.02 M [(n-C4H9)4N]{B(C6F5)4} at+25 �C. Spectroelectrochemical experiments were repeatedthree times at each concentration, and the results reportedas an average of the triplicate experiments.

The ‘‘raw’’ absorption spectra (e vs. m) were scaled as�e(m)/mdm [31,58] and all data reported in both tabulatedand graphical format are presented as e/m vs. m.

Due to comproportionation of the mixed-valence spe-cies [59] a correction for the concentration of the speciespresent in solution was determined from Eq. 3. In all cases,the proportion, P of the complex in the mixed-valence form(at equilibrium) was >97.5%.

P ¼ K1=2c

2þ K1=2c

ð3Þ

Spectral deconvolution of the NIR transitions was per-formed using the curve-fitting subroutine implementedwithin the GRAMS 32 commercial software package.For the dinuclear complexes, convergence of the iterationprocedure was generally achieved for three gaussian-shaped bands under the IVCT manifold. Based on thereproducibility of the parameters obtained from the decon-volutions, the uncertainties in the energies (mmax), intensi-ties {(e/m)max} and bandwidths (Dm1/2) were estimated as±10 cm�1, ±0.0001 M�1 and ±10 cm�1, respectively. Inall cases, the correlation coefficient (R2) for the fits reportedwas >0.995.

2.4. Synthetic procedures

Microwave-assisted syntheses were conducted in around-bottom flask fitted with condenser, mounted withina modified microwave oven (Sharp, Model R-2V55; 600W,2450 MHz) on medium-high power [60–62]. A detailed ac-count of the column cation-exchange chromatographicprocedures for separation of the diastereoisomers of dinu-clear species has been reported previously [48,49].

[(n-C4H9)4N]{B(C6F5)4} was obtained by metathesisfrom tetra-n-butylammonium tetrakis(pentafluorophe-nyl)borate ([(n-C4H9)4N]{B(C6F5)4}) using an adaption ofthe literature procedure [63], which is described in Supple-mentary Material.

The mononuclear ruthenium complexes [Ru(DM-SO)4Cl2] [64], cis-[Ru(pp)2Cl2] Æ 2H2O {pp = bpy, Me2bpy}[65] and cis-[Ru(Me4bpy)2Cl2] [66] were obtained accordingto the literature procedures. [Ru(5,5 0-Me2bpy)2Cl2] wassynthesized using an adaption of the ‘‘ruthenium blue’’method reported by Togano et al. [65], and [Ru(Me2-phen)2Cl2] and [Ru(tBu2bpy)2Cl2] were prepared via amodification of a procedure reported by Sullivan et al.[67]: the details of syntheses and characterizations of thesethree [Ru(pp)2Cl2] species are provided in the Supplemen-tary Material.

2.4.1. [Ru(Me2phen)2(bpm)](PF6)2

[Ru(Me2phen)2Cl2] (500 mg, 0.850 mmol) and bpm(123 mg, 0.776 mmol) were heated at reflux in 3:1 EtOH/water (200 cm3) for 5.5 h during which time the solutionattained an orange coloration. Upon cooling, the mixturewas diluted with distilled water (50 cm3) and loaded ontoa column (15 cm · 3.5 cm) containing SP Sephadex C-25support. Separation of the mononuclear product fromthe crude mixture was achieved via a gradient elution pro-cedure using aqueous 0.1–0.4 M NaCl as the eluent. Theorange band of mononuclear material was precipitatedas the PF�6 salt from the eluate by addition of a saturatedsolution of aqueous KPF6. The solid was isolated by vac-uum filtration and washed with diethyl ether. Yield:433 mg (53%). Anal. Calculated for C36H30F12N8P2Ru:


C, 44.8; H, 3.11; N, 11.6%; Found: C, 44.8; H, 3.05; N,11.5%.

2.4.2. [Ru(tBu2bpy)2(bpm)](PF6)2

A suspension of bpm (120 mg, 0.760 mmol) in ethyleneglycol (3 cm3) was heated in a modified microwave ovenon medium-high power for 20 s to complete dissolution.[Ru(tBu2bpy)2Cl2] (700 mg, 0.988 mmol) was added andthe mixture heated at reflux for a further 5 min duringwhich time the solution attained an orange coloration.Upon cooling, the mixture was diluted with distilled water(50 cm3) and loaded onto a column (15 cm · 3.5 cm) con-taining SP Sephadex C-25 support. Separation of themononuclear product from the crude mixture was achievedvia a gradient elution procedure using aqueous 0.1–0.4 MNaCl as the eluent. The orange band of mononuclear mate-rial was precipitated as the PF�6 salt by addition to the elu-ate of a saturated solution of aqueous KPF6. The solid wasisolated by vacuum filtration and washed with diethylether. Yield: 609 mg (74%). Anal. Calculated forC44H54F12N8P2Ru: C, 48.7; H, 5.01; N, 10.3%; Found: C,48.6; H, 4.99; N, 10.3%.

[{Ru(Me2bpy)2}2(l-bpm)](PF6)4 was synthesized asdescribed previously [48,49], and [{Ru(bpy)2}2(l-dbneil)](PF6)4 was supplied by Prof. Moshe Kol [68]. De-tails of the separation of these complexes into their diaste-reoisomeric forms and the 1H NMR characterization of thediastereoisomers have been reported previously [48,49,69].

2.4.3. [{Ru(bpy)2}2(l-bpm)](PF6)4

[Ru(bpy)2Cl2] Æ 2H2O (300 mg, 0.5765 mmol) and bpm(41.4 mg, 0.262 mmol) were heated at reflux (�120 �C) in10% water/ethylene glycol (20 cm3) for 5 h. Upon cooling,the dark green solution was diluted with distilled water(50 cm3) and loaded onto a column (dimensions:15 cm · 3.5 cm) containing SP Sephadex C-25 support.Separation of the dinuclear product from the crude mixturewas achieved via a gradient elution procedure using aque-ous 0.1–0.5 M NaCl as the eluent. An orange band ofmononuclear material eluted first (0.2–0.3 M NaCl), fol-lowed by a dark green band of the dinuclear material(0.5 M NaCl), which was precipitated as the PF�6 salt byaddition of a saturated aqueous solution of KPF6, isolatedby vacuum filtration, washed with chilled water and diethylether, and dried in vacuo at 40 �C for 4 h. Yield: 385 mg(94%). The 1H NMR spectrum of the dinuclear productwas identical to that reported previously [70].

The diastereoisomeric mixture was converted to thechloride salt by stirring an aqueous suspension with DOW-EX� 1 · 8 anion exchange resin (50–100 mesh; Cl� form).The complex was sorbed onto a chromatography column(ca. 1 m in length · 1.6 cm in diameter) containing SPSephadex C-25 cation exchanger as the support, and sepa-ration of the diastereoisomers was achieved using aqueous0.25 M sodium toluene-4-sulfonate solution as the eluent[48]. The separation of the stereoisomers was not achievedin a single passage down the column, so the column was

sealed and the substrate recycled several times down itslength with the aid of a peristaltic pump. The ‘‘effectivecolumn length’’ (ECL) – which represents the length ofsupport travelled by the sample for visual band separation– was �180 cm. The two dark green bands were collectedand precipitated as the PF�6 salts by addition of a saturatedsolution of KPF6. The solid products from each band werepurified on silica gel: the complex was dissolved in a mini-mum volume of acetone and loaded onto a short column ofsilica gel (200–400 mesh equilibrated with AR acetone; ca.3 cm in length · 1.5 cm in diameter), washed alternatelywith copious amounts of water and acetone, and theneluted with AR acetone containing 5% NH4PF6. Followingthe addition of an equal volume of water and removal ofthe acetone under reduced pressure, the precipitate was iso-lated by vacuum filtration, washed with chilled water anddiethyl ether and dried in vacuo. The 1H NMR character-istics of the diastereoisomers are provided in the Supple-mentary Material.

2.4.4. [{Ru(5,5 0-Me2bpy)2}2(l-bpm)](PF6)4

[Ru(5,5 0-Me2bpy)2Cl2] (200 mg, 0.370 mmol) and bpm(26.6 mg, 0.168 mmol) were heated at reflux in ethylene gly-col (2 cm3) in a modified microwave oven according to themethod previously reported for [{Ru(Me2bpy)2}2(l-bpm)](PF6)4 [48,49] on medium-high power for 10 min.The separation of the orange mononuclear material fromthe desired dark green product was achieved via a gradientelution procedure as described previously for [{Ru(b-py)2}2(l-bpm)](PF6)4. Yield: 226 mg (85%). Anal. Calcu-lated for C56H56F24N12P4Ru2: C, 40.1; H, 3.37; N, 10.0%;Found: C, 40.0; H, 3.34; N, 10.0%.

The separation, isolation and purification of the diaste-reoisomeric forms was achieved as described above, butusing 0.25 M sodium benzoate solution as the eluent(ECL � 50 cm).

[{Ru(Me4bpy)2}2(l-bpm)](PF6)4 was synthesized, andthe diastereoisomers separated and isolated, in an analo-gous manner to that described above for [{Ru(5,5 0-Me2b-py)2}2(l-bpm)](PF6)4.

2.4.5. [{Ru(Me2phen)2}2(l-bpm)](PF6)4

[Ru(Me2phen)2bpm](PF6)2 (150 mg, 0.155 mmol) and[Ru(Me2phen)2Cl2] (160 mg, 0.250 mmol) were heated inethylene glycol (5 cm3) in a modified microwave oven onmedium-high power for 10 min. Upon cooling, the resul-tant dark green solution was diluted with distilled water(50 cm3) and loaded onto a column (15 cm · 3.5 cm) con-taining SP Sephadex C-25 support. Separation of the dinu-clear product from the crude mixture was achieved via agradient elution procedure using aqueous 0.1–0.5 M NaClas the eluent. An orange band of mononuclear materialeluted first, followed by the desired dark green product,which was precipitated as the PF�6 salt by addition of a sat-urated solution of aqueous KPF6. The solid was isolated byvacuum filtration and washed with diethyl ether. Yield:208 mg (76%). Anal. Calculated for C64H54F24N12P4Ru2:


C, 43.4; H, 3.05; N, 9.48%; Found: C, 43.3; H, 3.01; N,9.40%.

The separation, isolation and purification of the diaste-reoisomeric forms was achieved as described above, butusing 0.20 M sodium benzoate solution as the eluent.

[{Ru(tBu2bpy)2}2(l-bpm)](PF6)4 was synthesized, andthe diastereoisomers separated and isolated, in an analo-gous manner to that described above for [{Ru(Me2-

phen)2}2(l-bpm)](PF6)4.The detailed description of these syntheses and separa-

tion of the diastereoisomeric forms of all the new dinuclearspecies is provided in the Supplementary Material, togetherwith the details of the 1H NMR characterizations of thediastereoisomers.

3. Results and discussion

3.1. Diastereoisomer synthesis, separation and structural

characterization

The complexes [{Ru(bpy)2}2(l-bpm)]4+ and [{Ru-(Me2bpy)2}2(l-bpm)]4+ have been synthesized previously[30,48,49,70–77], either by a thermal method involvingthe reaction of the cis-[Ru(bpy)2Cl2] Æ 2H2O precursor withbpm in ethylene glycol under reflux, or a microwave-assisted synthetic procedure. In the present case, the lattermethod was used for the complexes [{Ru(pp)2}2(l-bpm)]4+

{pp = bpy, Me2bpy, 5,5 0-Me2bpy, Me4bpy}, which re-sulted in comparable yields to those obtained from thethermal methods but with a significant reduction in thereaction times (typically�10 min rather than 2 h for the ther-mal procedure).

For the complexes [{Ru(pp)2}2(l-bpm)]4+ {pp = tBu2b-py and Me2phen}, the one-step microwave-assisted methodproduced lower yields than a two-step procedure involvingthe initial synthesis of [Ru(pp)2(bpm)]2+, followed by its

Table 1Electrochemical data (in mV relative to the Fc+/Fc0 couple) for the dinucC4H9)4N]PF6/CH3CNa

Complex DEoxb Eox2

meso-[{Ru(bpy)2}2(l-bpm)]4+ 192 1384rac-[{Ru(bpy)2}2(l-bpm)]4+ 188 1380meso-[{Ru(5,50-Me2bpy)2}2(l-bpm)]4+ 188 1260rac-[{Ru(5,50-Me2bpy)2}2(l-bpm)]4+ 176 1248meso-[{Ru(Me4bpy)2}2(l-bpm)]4+ 200 1208rac-[{Ru(Me4bpy)2}2(l-bpm)]4+ 192 1200meso-[{Ru(tBu2bpy)2}2(l-bpm)]4+ 192 1280rac-[{Ru(tBu2bpy)2}2(l-bpm)]4+ 184 1280meso-[{Ru(Me2phen)2}2(l-bpm)]4+ 192 1270rac-[{Ru(Me2phen)2}2(l-bpm)]4+ 184 1266meso-[{Ru(bpy)2}2(l-dbneil)]4+ 180 1268rac-[{Ru(bpy)2}2(l-dbneil)]4+ 182 1270

All potentials are reported ±3 mV.a All potentials are associated with one-electron electrochemical processes ub DEox = Eox2 � Eox1.c Two-electron reduction process.d Process complicated by adsorption/desorption peaks.e Irreversible reduction process.

reaction with an excess of [Ru(pp)2Cl2]. This probably re-flects the unfavorable steric interactions between themethyl substituents in the tBu2bpy and Me2phen terminalligands in the formation of the dinuclear complex.

Separation of the diastereoisomeric forms of [{Ru-(pp)2}2(l-bpm)]4+ was achieved by cation-exchange chro-matography using SP Sephadex C-25 support with aqueoussodium toluene-4-sulfonate or sodium benzoate as the elu-ent [46–49]. The latter was found to provide a more effi-cient separation (i.e., shorter ECL) for the complexesincorporating alkylated terminal ligands (5,5 0-Me2bpy,Me4bpy, tBu2bpy and Me2phen).

The final products from the synthetic procedures werepurified chromatographically and investigated by NMRspectroscopy and electrochemistry for confirmation of theirstructural identity and purity [48,49].

3.2. 1H NMR studies

Structural characterizations of the meso and rac formsof [{Ru(pp)2}2(l-bpm)]4+ {pp = bpy, 5,5 0-Me2bpy, Me4b-py, Me2phen, tBu2bpy} were performed using one- andtwo-dimensional (1H COSY) NMR techniques, and are de-cribed in the Supplementary Material.

3.3. Electrochemistry and electronic spectroscopy

The electrochemical properties of the diastereoisomericforms of [{Ru(pp)2}2(l-bpm)]4+ {pp = bpy, 5,5 0-Me2bpy,Me4bpy, Me2phen and tBu2bpy} and [{Ru(bpy)2}2-(l-dbneil)]4+ were investigated by cyclic and differentialpulse voltammetry in acetonitrile containing 0.1 M[(n-C4H9)4N]PF6, and are reported in Table 1. The electro-chemical and spectroelectrochemical characteristics of[{Ru(bpy)2}2(l-bpm)]4+ (as a diastereoisomeric mixture)

lear complexes [{Ru(pp)2}2(l-BL)]4+ {BL = bpm, dbneil} in 0.1 M [(n-

Eox1 Ered1 Ered2 Ered3 Ered4

1192 �794 �1466 �1948c �2368c,e

1192 �792 �1480 �1928c �2360c,e

1072 �824 �1508 �2032c �22721072 �824 �1532 �2032c �22721008 �872 �1460 �2055c �23151008 �880 �1464 �2056c �23201088 �816 �1488 �2008c �22811088 �816 �1490 �2015c �22921078 �816 �1488 �2008d �22721082 �816 �1512 �2016d �2264d

1088 �544 �945 �1676 �1972e

1088 �540 �946 �1668 �1965

nless otherwise stated.

Table 2UV–visible–NIR spectral data of the reduced absorption spectra (e/m vs. m)for the [Ru(bpy)2(BL)]n+ and [{Ru(bpy)2}2(l-BL)]n+ {BL = bpm, dbneil}in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN at +25 �C

Complex n+ mmax ± 10/cm�1 {(e/m)max ± 0.0001/M�1}

bpm dbneil [69]

meso-[{Ru(bpy)2}2(l-BL)]n+ 4 16775 (0.6142) 14290 (1.975)sh 18200 (0.4173) sh 22270 (1.131)24270 (1.550) 23700 (1.591)

5 5055 (0.1781) 4650 (0.2112)13540 (0.2463) 14045 (1.007)16440 (0.2880) 23660 (1.005)24340 (0.6772)

rac-[{Ru(bpy)2}2(l-BL)]n+ 4 16730 (0.6143) 14290 (3.219)sh 18200 (0.3895) sh 22195 (1.813)24230 (1.551) 23780 (2.590)

5 5080 (0.1781) 4560 (0.2064)13540 (0.2504) 14070 (1.660)16395 (0.2952) 23070 (1.660)24300 (0.6801)

The NIR spectral data are indicated in bold type.sh, shoulder band.

Fig. 4. UV–visible–NIR spectroelectrochemical progression for the oxi-dation reaction rac-[{Ru(bpy)2}2(l-bpm)]4+ ! rac-[{Ru(bpy)2}2(l-bpm)]5+ in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN at +25 �C. The breakin the axis signifies that the spectra were obtained at different scan rates.Inset. Overlay of IVCT bands for meso (—) and rac (- - - -) diastereoisomersand the components obtained by Gaussian deconvolution for the meso

form.


have been detailed in a number of previous studies [30,71–74,78–83].

The electrochemical characteristics of the diastereoiso-mers of [{Ru(bpy)2}2(l-dbneil)]4+ have been discussed pre-viously [68,69,84].

The [{Ru(pp)2}2(l-bpm)]4+ {pp = bpy, 5,5 0-Me2bpy,Me4bpy, Me2phen and tBu2bpy} complexes are character-ized by two reversible one-electron redox processes corre-sponding to successive oxidation of the metal centers, inaddition to multiple reversible ligand-based reductions. Inthe cathodic region, the first two one-electron reductionprocesses are localized on the the bridging ligand (i.e.,bpm0/� and bpm�/2�), due to the stronger p-acceptor nat-ure of bpm relative to the terminal pp ligands [30,71–74,78–83]. The subsequent reduction processes are localized onthe terminal pp ligands.

The influence of the peripheral ligands on the electro-chemical characteristics of this series of substituted com-plexes [{Ru(pp)2}2(l-bpm)]4+ is consistent with thevariation in the relative p-accepting abilities of the ligands[85]. For the diastereoisomers of a given complex, Eox1 andEox2 shift cathodically as the peripheral ligands are variedthrough the series bpy > tBu2bpy �Me2phen > 5,5 0-Me2bpy > Me4bpy. Minor variations in DEox (DEox =Eox2 � Eox1) are also evident across the series of complexes,and between the diastereoisomeric forms of the same com-plex. The potentials of the ligand-based reduction processesshift cathodically through the series bpy > Me2phen �tBu2bpy > 5,5 0-Me2bpy > Me4bpy.

For a given complex, DEox is slightly greater for themeso compared with the rac diastereoisomeric form, how-ever this difference is significant for the Me2phen andtBu2bpy species only. The separation in the potentials be-tween the metal-based oxidation processes permitted thegeneration of the mixed-valence forms of the complexes.The relative magnitudes of DEox suggest that the stabilitiesof the mixed-valence species are comparable for the seriesof complexes [30,86], although some caution must be exer-cised in the interpretation of DEox values [87].

The UV–visible–NIR spectral data for the un-oxidized(+4) and mixed-valence (+5) forms of the meso and rac

diastereoisomers of [{Ru(bpy)2}2(l-bpm)]n+ and [{Ru(b-py)2}2(l-dbneil)]n+ [87] over the range 3050–30000 cm�1

are reported in Table 2, and the spectral progressionsaccompanying the formation of the mixed-valence formsof the rac diastereoisomers are shown in Figs. 4 and 5,respectively. The spectral features for the diastereoisomericforms of the complexes are consistent with previous litera-ture reports for [{Ru(bpy)2}2(l-bpm)]4+ [30,71–74,88] (as adiastereoisomeric mixture) and [{Ru(bpy)2}2(l-dbneil)]4+

[68,69].The UV–visible spectra over the region 10000–

30000 cm�1 for the un-oxidized +4 forms of [{Ru-(bpy)2}2(l-BL)]4+ {BL = bpm, dbneil} are characterizedby a combination of overlapping dp(RuII)! p*(BL) anddp(RuII)! p*(bpy) singlet metal-to-ligand (1MLCT) tran-sitions. The lowest energy transitions at 16775 and

16730 cm�1 in the meso and rac diastereoisomers of[{Ru(bpy)2}2(l-bpm)]4+, respectively, and at 14300 and14290 cm�1 in the meso and rac diastereoisomers of[{Ru(bpy)2}2(l-dbneil)]4+, respectively, are assigned asdp(RuII)! p*(BL) transitions. These assignments are sup-ported by comparisons with the mononuclear complexes[Ru(bpy)2(bpm)]2+ [72,73,80] and [Ru(bpy)2(dbneil)]2+

[68] and the well-documented transitions in [Ru(bpy)3]2+

[89,90].

Fig. 5. UV–visible–NIR spectroelectrochemical progression for the oxi-dation reaction rac-[{Ru(bpy)2}2(l-dbneil)]4+! rac-[{Ru(bpy)2}2(l-dbneil)]5+ in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN at +25 �C. Inset.Overlay of IVCT bands for meso (—) and rac (- - - -) diastereoisomers andthe components obtained by Gaussian deconvolution for the meso form.


Spectroelectrochemical generation of the mixed-valenceforms of meso- and rac-[{Ru(bpy)2}2(l-BL)]5+ (BL = bpmand dbneil) revealed stable isosbestic points in the spectralprogressions, as shown in Figs. 4 and 5, respectively. TheMLCT absorption bands decreased in intensity, and expe-rienced a slight red-shift following one-electron oxidationto the mixed-valence species, with the appearance of anew band in the NIR region 3000–9000 cm�1 (Figs. 4and 5; Table 2). The NIR bands are absent in the spectrumof the un-oxidized +4 species, and decrease in intensity asthe potential is held at a value beyond that required forgeneration of the +6 species. On this basis the bands at4900 and 4890 cm�1 in meso- and rac-[{Ru(bpy)2}2-(l-bpm)]5+ and at 4650 and 4560 cm�1 in meso- andrac-[{Ru(bpy)2}2(l-dbneil)]5+ are assigned as IVCTtransitions.

The characterization of the +6 states of the complexeswas not possible as the complete generation of the fully oxi-dized forms could not be achieved reversibly. The energiesof the RuIII-based LMCT transitions in the dinuclear spe-cies could not be established, however a comparison withthe mononuclear species [Ru(bpy)2(bpm)]3+ [72,73,80]and [Ru(bpy)2(dbneil)]3+ [68] suggests that such transi-tions, if present, should occur in the visible region between10000 and 15 000 cm�1. This provides further support forthe assignment of the NIR bands as IVCT, rather thanLMCT, transitions.

In previous work on [{Ru(bpy)2}2(l-bpm)]5+ (as a dia-stereoisomeric mixture) [30,78,91], an analysis of theIVCT characteristics was precluded as the mixed-valenceform was not sufficiently stable for a meaningful band-width to be obtained. In the present case, a fast scanningtechnique was employed over the wavelength range 3200–9200 cm�1 (at a rate of 8000 cm�1/min), and permittedthe generation of the mixed-valence species for a sufficienttime to provide characterization of the IVCT bands be-

fore appreciable net reduction of the 5+ to the 4+ ion oc-curred. Chemical decomposition was eliminated as asource of the instability of the mixed-valence species asthe regeneration of the un-oxidized +4 species wasachieved with >98% reversibility, and the decrease inthe IVCT intensity (following the attainment of the max-imum intensity) was consistent with the disproportion-ation of the mixed-valence species according to expectedsecond-order kinetics [36,92,93].

3.4. Intervalence charge transfer

IVCT measurements for the complexes [{Ru(bpy)2}2(l-bpm)]5+ and [{Ru(bpy)2}2(l-dbneil)]5+ were performed in0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN containing a uni-form low concentration of the given diastereoisomer(0.40 · 10�3 M) to eliminate ion-pairing artefacts[20,21,57,94]. The {B(C6F5)4}� anion is known to associateweakly in comparison with PF�6 , traditionally used for elec-trochemical measurements [63] . The NIR spectra of thedinuclear systems were scaled as �e(m)/mdm [31,58] anddeconvoluted by use of the software package GRAMS32.The results of the band maxima, mmax, molar extinctioncoefficients, (e/m)max and bandwidths, Dm1/2, are summa-rized in Table 3.

The NIR band manifolds appear asymmetrical andslightly narrower on the lower energy side with bandwidthsat half-height (Dm1/2) of 2800 and 2620 cm�1 for meso- andrac-[{Ru(bpy)2}2(l-bpm)]5+, respectively, and 1970 and2050 cm�1 for meso- and rac-[{Ru(bpy)2}2(l-dbneil)]5+,respectively. On the basis of a classical two-state modelthe theoretical band-width at half-height, Dm0

1=2, is givenby [16RT ln (2)mmax]1/2 for a weakly coupled (Class II [95])system, where 16RT ln (2) = 2310 cm�1 at 298 K [96]. Therelatively narrow observed bandwidths suggest that thelocalized Class II description may be inappropriate forthese systems [97].

The parameter C provides a criterion for the degree ofelectronic coupling in the system [35]:

C ¼ 1� ðDm1=2Þ=½16RT lnð2Þmmax�1=2

¼ 1� ðDm1=2Þ=ðDm01=2Þ; ð4Þ

where 0 < C < 0.5 for weak to moderate coupling (ClassII), C � 0.5 at the Class II–III transition, and C > 0.5 forstrongly coupled (Class III) systems within the Robin andDay classification scheme [95]. For [{Ru(bpy)2}2(l-bpm)]5+, C = 0.175 (meso) and 0.229 (rac) and for [{Ru-(bpy)2}2(l-dbneil)]5+, C = 0.40 (meso) and 0.37 (rac),which suggests that all systems lie between the fully local-ized (Class II) and delocalized (Class III) regimes, in theClass II–III transition region.

Within the framework of the classical model, the asym-metric appearance of the bands is ascribed to the bandcut-off which occurs at hm = 2Hab [35,95,97], where Hab isthe electronic coupling parameter. When 0 < C < 0.5, Hab

is given by

Table 3Characteristics of the IVCT bands for the absorption spectra scaled as (e/m vs. m) for the dinuclear mixed-valence complexes in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/CH3CN at +25 �C (parameters for overall envelope are shown in bold type: details of deconvoluted bands are in normal type)a,b

Complex mmax ± 10/cm�1 (e/m)max ± 0.0001/M�1 Dm1/2 ± 10/cm�1 Dm01=2/cm�1 C Hab/cm�1

meso-[{Ru(bpy)2}2(l-bpm)]5+ 5055 0.1781 2800 3395 0.175 420

3615 0.0664 995 2870 0.347 1105055 0.1779 2145 3395 0.368 3657185 0.0193 1505 4050 0.372 145

rac-[{Ru(bpy)2}2(l-bpm)]5+ 5080 0.1781 2620 3400 0.229 405

3920 0.0479 710 2990 0.237 2805090 0.1775 2145 3405 0.370 3707160 0.0187 1520 4040 0.376 140

meso-[{Ru(bpy)2}2(l-dbneil)]5+ [69] 4650 0.2112 1970 3270 0.398 250

4570 0.1549 1595 3240 0.508 1905530 0.07953 2390 3265 0.268 200

rac-[{Ru(bpy)2}2(l-dbneil)]5+ [69] 4560 0.2064 2050 3232 0.366 220

4495 0.1493 1465 3215 0.544 1555515 0.08983 2290 3560 0.357 185

a Dm01=2=[2310(mmax)]1/2 at 298 K.

b Lower limits for Hab using rab = 7.9 A for dbneil [68] and 5.56 A for bpm [48]. These may differ for the two diastereoisomers.


H ab ¼ 2:06� 10�2ðmmaxemaxDm1=2Þ1=2=rab; ð5Þ

where rab is the effective electron transfer distance. Whilerab is often equated with the through-space geometrical dis-tance between the metal centers [98], the effective chargetransfer distance is decreased relative to the geometric dis-tance as electronic coupling across the bridge increases, andthis equation provides a lower limit only for Hab [98]. Withthis caveat noted, Hab values for the meso and rac diastere-oisomers are shown in Table 3, with rab equated with theapproximate geometric metal–metal separation of 7.9 Afor the dbneil-bridged species [68] and 5.56 A for thebpm-bridged analogue [48].

3.5. IVCT solvatochromism – the diastereoisomers of

[{Ru(bpy)2}2(l-bpm)]5+ as probes for solvent

reorganizational effects in IVCT

The NIR spectra for the diastereoisomers of [{Ru-(bpy)2}2(l-bpm)]5+ were measured in the range of solventsAN, PN, BN, iBN and BzN. The energies of the IVCTband maxima (mmax) as a function of 1/Dop � 1/Ds are pre-

Table 4IVCT solvatochromism data of the reduced absorption spectra (e/m vs.C4H9)4N]{B(C6F5)4}/solvent at +25 �C; MLCT energies for the +4 states are

Solvent 1/Dop � 1/Ds meso

mmax ±10/cm�1

Dm1/2 ±10/cm�1

mMLCT(1) ±10/cm�1

AN 0.5127 5055 2800 16775PN 0.5011 5435 2710 16900iBN 0.4795 5420 3200 16850BN 0.4762 5395 2686 16800BzN 0.3897 5065 2570 16695

a IVCT characteristics are reported as an average of triplicate experiments.b The absolute intensities of the IVCT bands, (e/m)max, are not tabulated as al

sented in Table 4. The plot in Fig. 6 reveals the predictedlinear trend for both diastereoiosmers in all solvents exceptAN. From the data (excluding AN), the following valueswere obtained for the slope and intercept: (meso)slope = 3545 ± 390 cm�1 A�1 and intercept = 3390 ±245 cm�1 (R2 = 0.98); (rac) slope = 3690 ± 180 cm�1 A�1

and intercept = 3780 ± 115 cm�1 (R2 = 0.99). The slopesof the plots are identical (within experimental error) forboth diastereoisomers while the intercept for the rac formis marginally greater than the meso form. Minor differencesin the energies of the IVCT bands (mmax (meso � rac) inTable 4) are also apparent between the diastereoisomersacross the series of solvents.

The origin of the energy disparity in AN is attributed toa specific solvent effect which overwhelms the dielectriccontinuum description. The clefts between the terminalbpy rings in the diastereoisomeric forms of [{Ru-(bpy)2}2(l-bpm)]5+ permit the AN molecules to approachthe metal centers more closely than is permitted by the the-oretical model: the clefts between the ligands at the twoRu(bpy)2 terminii (the ‘‘exterior clefts’’) are identical inboth diastereoisomeric forms while the dimensions of the

m) for the diastereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ in 0.02 M [(n-also tabulateda,b

rac

mmax ±10/cm�1

Dm1/2 ±10/cm�1


Dmmax (meso � rac) ±10/cm�1

5080 2620 16775 �255470 2770 16900 �355425 2952 16780 �55375 2860 16750 205095 2674 16660 �30

l spectra were normalized at the maximum intensity of the IVCT manifold.

Fig. 6. mmax as a function of the solvent parameter 1/Dop � 1/Ds for thefor meso (—) and rac (- - - -) diastereoisomeric forms of [{Ru(bpy)2}2(l-bpm)]5+ in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/solvent at +25 �C.

Fig. 7. mmax as a function of solvent composition for the meso (—) and rac

(- - - -) diastereoisomeric forms of [{Ru(bpy)2}2(l-bpm)]5+ in AN and PNmixtures containing 0.02 M [(n-C4H9)4N]{B(C6F5)4}/solvent at +25 �C.The inset shows mmax as a function of the absolute number of ANmolecules (relative to one mole of complex).


clefts above and below the plane of the bridging ligand (the‘‘interior clefts’’) are distinguishable in the two forms, asshown in Fig. 1. The minor differences between the energiesof the IVCT bands for the diastereoismers suggest that spe-cific ‘‘stereochemically directed’’ interactions may be pres-ent in all solvents due to solvent penetration within theinterior clefts. The red-shifts in mmax of 380 ± 10 and390 ± 10 cm�1 between PN and AN for the meso and rac

diastereoisomers, respectively, are not expected in view ofthe similar bulk dielectric properties of AN and PN, andthe similarity of the structures which differ only in the pres-ence of an additional methyl group for PN. Despite thesubtle structural differences between the two solvents, theresults suggest that the relatively small AN moleculesmay penetrate the clefts and approach the metal centersmore closely than is permitted for PN due to steric hin-drance of that additional methyl group.

Two issues arise in the attempt to rationalize theseobservations. Firstly, the red-shift of the IVCT energy inAN relative to PN is surprising since the general expecta-tion is that increased specific solvation induces an addi-tional contribution to the reorganizational energy, andhence a blue-shift. Previous solvatochromism experimentsfor complexes based on –Ru(NH3)5 [2] have revealed corre-lations between the energies of the IVCT bands and empir-ical solvent basicity parameters such as DN due to specificsolvent–ammine H-bonding. In the present case, solvent–solute interactions are likely to involve electrostatic interac-tions between the solvent molecules and pockets of electrondensity on the bpy and bpm ligands. Secondly, specific sol-vation effects will be present in addition to dielectric contin-uum solvation. In previous studies of complexes based on–Ru(NH3)5, –trans-Ru(NH3)4(py) and –Ru(bpy)(NH3)3

with pyz, 4-cyanopyridine and 4,4 0-bpy bridging ligands[19], mmax was fitted to a dual-parameter equation includingboth 1/Dop � 1/Ds and DN. In the present case, the appar-ent success of dielectric continuum theory in explaining the

linear solvent dependence of mmax on 1/Dop � 1/Ds in PN,BN, iBN and BzN is surprising if all the solvents engagein some form of specific solvation. In polar solvents,1/Dop� 1/Ds, and the value of Dop in the first solvationshell may be similar to that in bulk solution. This would ex-plain the apparent success of the continuum theory, even inthe presence of specific solvation effects for these solvents.The anomalous result in AN suggests that these moleculesengage in a different form of specific interaction with thediastereoisomers compared with PN, BN, iBN and BzN.

To address the issue of continuum vs. specific solvation,the IVCT characteristics of the diastereoisomeric forms of[{Ru(bpy)2}2(l-bpm)]5+ were investigated as a function ofsolvent composition in a series of solvent mixtures contain-ing varying mole fractions of AN (nAN) and PN(nPN = 1 � nAN) (Fig. 7 and Supplementary Table S1).

Fig. 7 reveals striking differences in the dependence ofmmax on solvent composition for the two diastereoisomericforms. Three regions are discernable. For 0 6 nAN 6 0.02,the rac diastereoisomer exhibited a 150 ± 10 cm�1 red-shiftwith the addition of two mole equivalents of AN while themeso form exhibited a 30 ± 10 cm�1 red-shift over the samerange. The ratio of the number of moles of AN to [{Ru(b-py)2}2(l-bpm)]5+ is 2:1 at nAN = 0.02. The results suggestthat the specific solvent effect in dilute AN occurs withinthe interior clefts, since the exterior clefts are identical inboth diastereoisomeric forms. The specific effect for therac diastereoisomer may well correspond to the associationof two AN molecules, one within each identical cleft eitherside of the bpm plane. The closer distance of approach ofthe AN molecules in the rac form gives rise to the largermagnitude of the specific effect. For 0.02 6 nAN 6 0.8(meso) and 0.2 6 nAN 6 0.6 (rac), mmax is relatively invari-


ant to solvent composition. In the rac form, the AN mole-cules located in the interior clefts block access to associa-tion by additional solvent molecules, while thecomposition of molecules within the exterior clefts remainsessentially constant. The specific association of solventmolecules within the first solvation shell increases the effec-tive radius and decreases the sensitivity of mmax to variationin the solvent composition. The composition in the exteriorclefts remains constant over the same range for the meso

form, and the composition at the interior clefts also re-mains relatively constant due to the larger orthogonal-shaped cleft compared with the rac form. In the latter theAN molecules associated within the relatively smaller inte-rior clefts are not accessible to interaction with solventmolecules in the bulk solvent. Given the more open natureof the interior clefts in the meso diastereoisomer, the sol-vent molecules associated within these clefts are moreaccessible to solvent molecules in the outer solvation shellsand the bulk solution. The increased solvent–solvent inter-actions may give rise to the consistently higher mmax for themeso compared with the rac form. For 0.8 6 nAN 6 1.0(meso) and 0.6 6 nAN 6 1.0 (rac), mmax decreases for bothdiastereoisomers. The dependence of mmax on solvent com-position is greater for the meso diastereoiosmer, as quanti-fied by the slope of �2020 ± 130 cm�1 per unit nAN (meso)vs. �843 ± 60 cm�1 per unit nAN (rac) over the range0.8 6 nAN 6 1.0. At nAN = 0.2 (and nPN = 0.8), the ratiosof AN and PN molecules to the mixed-valence complexare 36:1 and 8.8:1, respectively. The sharp red-shift in mmax

with increasing concentration of AN occurs at higher nAN,and more rapidly for the meso diastereoisomer. This formexhibits a greater sensitivity to solvent structure effectscompared with the rac diastereoisomer, and these effectsare more pronounced for solvent mixtures containing highconcentrations of AN [99]. The parameter mmax decreasesmore gradually for the rac diastereoisomer as the solventmolecules associated within the interior clefts are relativelymore restricted towards interactions with the bulk solvent,and hence to solvent structure effects.

The results indicate that the specific solvent effect of dis-crete solvent molecules in the immediate vicinity of thecomplex dominate the solvent reorganizational energy.The diastereoisomers offer a detailed insight into the sol-vent reorganizational contribution from specific solventmolecules on ko: the results support the general hypothesisthat the distance of approach and orientation of the discretesolvent molecules dictate the solvent shifts.

Due to the specific nature of the solvent interactions, aquantitative model for such effects must treat the solventon the molecular level as discrete entities. The specific sol-vent effect (DEs), due to the presence of an oriented solventmolecule on a charge transfer transition, may be inferredfrom London dipole solvation theory [100,101] (Eq. (6)),where ls is the ground state dipole moment of the solventmolecule, Dl is the dipole change due to charge transfer, r

is the cavity radius and f(�) is a function of the solventdielectric properties.

DEs ¼ls.Dlr3f ð�Þ ð6Þ

The 1/r3 dependence of DEs is such that solvent moleculeslocated in closer proximity to the metal centers will domi-nate the specific solvent effect. The distance dependencemay account (in part) for the larger energy shifts in AN:due to their relatively small size, the discrete AN moleculescan approach the metal centers more closely than the othersolvents of the series. The greater magnitude of the specificeffect for the rac vs. the meso diastereoisomer at low ANconcentration is believed to reflect the ability of the ANmolecules to associate more strongly within the clefts be-

tween the metal centers.The dependence of DEs on the vector dot product of ls

and Dl gives rise to the orientation dependence of the sol-vent molecules located within the clefts of the chromo-phore. All else being constant, |DEs| is smallest when thedipole moment of the solvent molecule is oriented perpen-dicularly to the plane of the bridging ligand and greatestwhen oriented parallel to this plane. Assuming that thered-shift of 150 ± 10 cm�1 in mmax corresponds to the for-mation of a 2:1 complex between AN and the dinuclearcation for the rac diastereoisomer, the results suggest thateach AN dipole is oriented with the nitrogen of AC„Ndirected inwards towards the clefts at an angle0 6 h < 90� to the Ru–Ru charge transfer axis. The smallsizes of the AN molecules are such that they may associ-ate sufficiently close to solvate both metal centers simulta-neously. By comparison, the additional methyl group inPN may restrict the close approach of the solvent dipolesto the metal centers. Each PN dipole is thus oriented to-wards the metal center with the higher partial positivecharge (d+), giving rise to a greater positive reorganiza-tional contribution to mmax. For PN and the other mem-bers of the nitrile series, the magnitudes of the specificeffects are relatively smaller compared with AN, such thatwhen they are superimposed on their respective contin-uum contributions, apparent conformity with the theoret-ical prediction is obtained.

Clearly, a quantitative test of the applicability of Eq. (6)depends on a realistic estimation of the dimensions of theclefts (which is ambiguous due to their non-spherical nat-ure). However, the qualitative predictions for the orienta-tion and distance dependence from Eq. (6) arecompatible with the experimental results. DEs is thus super-imposed on the 1/Dop � 1/Ds continuum theory predictionand can be regarded as an additional energy contributionto ko.

3.6. Varying the selectivity of solvent association – influenceof the bridging ligand

The IVCT solvatochromism properties in the diastereoi-osomeric forms of [{Ru(bpy)2}2(l-dbneil)]5+ (dbneil =dibenzoeilatin, Fig. 2) were examined to assess the effectof increasing the dimensions of the interior clefts by

Table 5IVCT solvatochromism data of the reduced absorption spectra (e/m vs. m) for the diastereoisomers of [{Ru(bpy)2}2(l-dbneil)]5+ in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/solvent at +25 �C

Solvent meso rac Dmmax (meso � rac)± 10/cm�1

mmax ±10/cm�1

(e/m)max ±0.0001/M�1

Dm1/2 ±10/cm�1


mmax ±10/cm�1

(e/m)max ±0.0001/M�1

Dm1/2 ±10/cm�1


AN 4650 0.2112 1970 14290 4560 0.2064 2050 14290 90

4570 0.1549 1595 4495 0.1493 14655530 0.07953 2390 5515 0.08983 2290

PN 4610 0.2081 1929 14335 4670 0.1515 1764 14335 �60

BN 4590 0.1890 1884 14340 4585 0.0855 1948 14340 5

iBN 4610 0.1953 1871 14350 4600 0.1929 1917 14350 10

BzN 4600 0.1800 1880 14170 4590 0.1500 1880 14170 10

MLCT energies for the +4 states are also tabulated. Parameters for overall envelope are shown in bold type: details of deconvoluted bands (in AN only)are in normal type.

Fig. 8. mmax as a function of the solvent parameter 1/Dop � 1/Ds for the meso (—) and rac (- - - -) diastereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ and[{Ru(bpy)2}2(l-dbneil)]5+ in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/solvent at +25 �C.

Fig. 9. Chem 3D representations of the diastereoisomeric forms of[{Ru(bpy)2}2(l-dbneil)]n+ illustrating the subtle variation in the dimen-sions of the interior clefts above and below the plane of the bridgingligand. Hydrogen atoms are omitted for clarity.


comparison with the analogous diastereoisomers of[{Ru(bpy)2}2(l-bpm)]5+. The variation of the energies ofthe IVCT band maxima (mmax) as a function of 1/Dop

� 1/Ds for [{Ru(bpy)2}2(l-dbneil)]5+ are presented inTable 5 and Fig. 8. The complete NIR–UV–visible spectraldata for the un-oxidized (+4) and mixed-valence (+5)forms of [{Ru(bpy)2}2(l-dbneil)]n+ have been publishedpreviously [69].

The energies of the IVCT bands of [{Ru(bpy)2}2(l-dbneil)] are essentially solvent-independent for both diaste-reoisomers, as shown in the plot of mmax as a function of1/Dop � 1/Ds (Fig. 8). The decreased slope of the solvato-chromism plots compared with bpm-bridged analoguescontradicts the prediction from Eq. (2) that ko increasesas the intra-metal distance is increased from 5.56 A (mea-sured for meso-[{Ru(Me2 bpy)2}2(l-bpm)]5+ [48]) for thebpm-bridged systems to 7.9 A [68] for the dbneil-bridgedsystems (at fixed a). The interior clefts are of similar dimen-

sion for the dbneil-bridged diastereoisomers (Fig. 9), butlarger than for a given diastereoisomer compared with theirbpm-bridged analogues. Since the exterior clefts are identi-

Fig. 10. mmax as a function of the solvent parameter 1/Dop � 1/Ds for themeso and rac diastereoisomers of [{Ru(pp)2}2(l-bpm)]5+ {pp = bpy (—),5,5 0-Me2bpy (– – –), Me4bpy (- - - -), Me2phen (–Æ–Æ–) and tBu2bpy (. . .. . .)}in 0.02 M [(n-C4H9)4N]{B(C6F5)4}/solvent at +25 �C. Error bars areomitted for clarity.


cal for the diastereoisomers of both complexes, the resultssuggest that solvation in the interior clefts is more impor-tant than solvation about the entire dimer. In particular,the solvent reorganizational contribution to the IVCT en-ergy is greater for the bpm-bridged diastereoisomers whichcontain the relatively smaller interior clefts.

Alternatively, the dbneil-bridged diastereoisomers mayexhibit relatively greater delocalization than their bpm-bridged analogues due to the extensive aromatic frame-work of the bridging ligand in the former case. The slopeof the solvatochromism plot is given by e2(1/a � 1/d)according to Eq. (2), however the equation is frequently ex-pressed in terms of the effective amount of charge trans-ferred, and the slope is given instead as (De)2(1/a � 1/d).Since the effective amount of charge transferred is reducedfrom unit charge transfer by delocalization, this explana-tion may provide a qualitative rationale for the lesser slopefor the dbneil-bridged systems. The data for the IVCTparameters in Table 3 do indeed suggest that the latterare more delocalized as the bands are narrower and moreintense.

The solvent dependence of mmax for the dbneil diastereo-isomers (Fig. 8) arises from a superposition of the solventreorganizational contributions due to the continuum andspecific solvation effects. A comparison of the differencein mmax between the diastereoisomers {Dmmax (meso � rac)}for [{Ru(bpy)2}2(l-dbneil)]5+ and [{Ru(bpy)2}2(l-bpm)]5+

reveals that the differences in the specific solvent interac-tions between the diastereoisomeric forms are present forboth complexes in each solvent. In particular, the differencein the specific solvent interaction with the diastereoisomersof the same complex in AN and PN is greater for the dbneilcomplexes. This reflects the larger cleft available for solventpenetration relative to the bpm-bridged species.

3.7. Varying the selectivity of solvent association – influence

of the terminal ligands

The final investigation involved the systematic modifica-tion of the dimensions of the interior and exterior clefts bythe judicious positioning of substituents on the terminalligands in the series [{Ru(pp)2}2(l-bpm)]5+, wherepp = 5,5 0-dimethyl-2,2 0-bipyridine (5,5 0-Me2bpy), 4,4 0,5,5 0-tetramethyl-2,2 0-bipyridine (Me4bpy), 2,9-dimethyl-1,10-phenanthroline (Me2phen) and 4,4 0-di-tert-butyl-2,2 0-bipyridine (tBu2bpy), shown in Fig. 3.

The results for the energies of the IVCT band maxima asa function of 1/Dop � 1/Ds for the diastereoisomeric formsof the series [{Ru(pp)2}2(l-bpm)]5+ {pp = 5,5 0-Me2bpy,Me4bpy, Me2phen and tBu2bpy} in a homologous seriesof nitrile solvents are reported Supplementary Table S2and shown in Fig. 10.

Specific solvation effects clearly dominate the magnitudeof ko, and the consideration of the subtle and systematicstructural variation between the different complexes, andbetween the diastereoisomeric forms of the same complex,provide insights into the detailed nature of the reorganiza-

tional contributions of specific solvent molecules to the to-tal energy of the IVCT transition. The energies of the IVCTbands for the substituted derivatives exhibit a lesser butscattered dependence on 1/Dop � 1/Ds (Fig. 10) relativeto the linear dependence of mmax on 1/Dop � 1/Ds for thediastereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ in PN, BN,iBN and BzN. Negligible slopes are obtained for all com-plexes (neglecting the results in AN). The striking disconti-nuity between PN and AN in the plot of mmax vs.1/Dop � 1/Ds for the [{Ru(bpy)2}2(l- bpm)]5+ diastereoiso-mers is also evident for the substituted derivatives, with thenotable exceptions being the diastereoisomers of [{Ru-(tBu2bpy)2}2(l-bpm)]5+. The observation may be rational-ized by the steric restriction of the bulky tert-butylsubstituents to access of solvent molecules to both the inte-rior and exterior clefts. However, minor differences in theIVCT energies are evident between the two diastereoiso-meric forms in all solvents, as quantified by Dmmax (me-

so � rac) – provided in Supplementary Table S2 andSupplementary Figure S1 – which represents differences

Fig. 11. Chem 3D representations of [{Ru(pp)2}2(l-bpm)]5+ {pp = 5,5 0-Me2bpy, Me4bpy, Me2phen, tBu2bpy} illustrating the subtle variation in thenature of the clefts above and below the plane of the bridging ligand. Hydrogen atoms are omitted for clarity.


in the specific solvent interactions at the interior clefts. Therac diastereoisomer exhibits a negligible solvent shift(13 ± 10 cm�1) between AN and PN due to the smallerand less solvent-accessible interior cleft compared withthe meso form, where the shift is 90 ± 10 cm�1. If the con-tribution of specific effects are negligible for rac-[{Ru-(tBu2bpy)2}2(l-bpm)]5+, then Dmmax (meso � rac) providesa quantitative measure of the specific solvent effect at theinterior cleft, and hence provides a clear partition betweenthe magnitudes of the continuum and specific solvation ef-fects to ko.

The subtle structural differences between the various[{Ru(pp)2}2(l-bpm)]5+ complexes provide further opportu-nities to quantitatively assess stereochemically directed spe-cific solvent effects. The diastereoisomers of the respectivecomplexes are shown in Fig. 11.

3.7.1. [{Ru(5,5 0-Me2bpy)2}2(l-bpm)]5+ and[{Ru(Me4bpy)2}2(l-bpm)]5+

The dimensions of the interior and exterior clefts be-tween the same diastereoiomeric forms of these two com-plexes differ only with respect to the additional methylsubstituents at the 4,4 0-positions in [{Ru(Me4bpy)2}2(l-bpm)]5+, which may hinder the access of solvent molecules

from directly above the interior clefts. For both complexes,the methyl substituents at the 5,5 0-positions restrict solventaccess at the convergence of the terminal ligands in themeso form, while the dimensions of the clefts in the rac

form are comparable to those of the unsubstituted bpycomplex. Despite the presence of the methyl substituents,the magnitude of the specific solvent effect due to AN ismaintained for both diastereoisomers of both complexes.The difference between the diastereoisomers {Dmmax

(meso � rac)} is more pronounced in AN for the Me4bpyderivatives compared with all the substituted complexes,which suggests that substitution at the 4,4 0-positions ofthe terminal bpy-type ligands results in the most pro-nounced difference in the specific interaction of the ANmolecules at the interior clefts. For a given diastereoiso-mer, the energies of the IVCT transitions in PN, BN,iBN and BzN are greater for the 5,5 0-Me2bpy derivative,which may reflect the smaller a (and hence larger contin-uum contribution, according to Eq. (2)) for the diastereoi-somers of [{Ru(5,5 0-Me2bpy)2}2(l-bpm)]5+ compared withthe corresponding forms of [{Ru(Me4bpy)2}2(l-bpm)]5+.For both diastereoiosomers of [{Ru(Me4bpy)2}2(l-bpm)]5+, mmax remains essentially constant over the seriesPN, BN, iBN and BzN compared with the relatively


greater solvent effects for the diastereoisomers of [{Ru-(5,5 0-Me2bpy)2}2(l-bpm)]5+. The results indicate that thespecific solvent interactions for the diastereoisomeric formsof the 5,5 0-Me2bpy derivative may occur via solvent pene-tration from directly above or below the plane of the bridg-ing ligand, since the only difference between the samediastereoisomer for the two complexes is substitution atthe 4,4 0-positions of the Me2bpy rings. When tert-butylsubstituents occupy these positions, solvent access is se-verely restricted, even for the relatively small ANmolecules.

3.7.2. [{Ru(Me2phen)2}2(l-bpm)]5+

The methyl substituents at the 2,9-positions of 1,10-phe-nanthroline (phen) induce a steric crowding close to themetal centers which should restrict the solvent access inthe immediate vicinity of the metal centers to a greater ex-tent than substitution at the 4,4 0- and/or 5,5 0-positions ofthe bpy-based ligands. In the meso diastereoisomer, themethyl substituents induce a crowding at the convergenceof the terminal rings on either side of the bridging ligandplane, and the size of the orthogonal-shaped interiorcavities are comparable to those in meso-[{Ru(pp)2}2(l-bpm)]5+ {pp = bpy, 5,5 0-Me2bpy and Me4bpy}. The simi-lar nature of the interior clefts for the complexes wouldaccount for the similar nature of the specific solvent effectobserved in AN. The magnitude of the specific solventeffect between AN and PN for rac-[{Ru(Me2phen)2}2(l-bpm)]5+ is comparable to that for rac-[{Ru(bpy)2}2(l-bpm)]5+, which suggests that the AN and PN dipolesmay assume similar orientations and distances of approachin both complexes.

Substitution at the 2,9-positions gives rise to a dramaticdifference in mmax between the meso and rac diastereoiso-mers in iBN, where Dmmax (meso � rac) is 180 ± 10 cm�1.The difference is striking in view of the subtle structural dif-ference between iBN, PN and BN. The additional stericbulk provided by the branched position of the methylgroup in iBN compared with PN and BN may be sufficientto inhibit penetration of these solvent molecules within theinterior clefts of the rac form, compared with the relativelymore open clefts in the meso diastereoisomer. The magni-tude of the difference between the diastereoisomers is sig-nificantly greater than that observed for the 5,5 0-Me2bpyand Me4bpy derivatives. This suggests that the presenceof methyl substituents at the 2,9-positions in Me2phengives rise to the greatest stereochemically induced differen-tiation between the diastereoisomeric forms.

3.7.3. [{Ru(tBu2bpy)2}2(l-bpm)]5+

The tert-butyl substituents at the 4,4 0-positions of theterminal tBu2bpy ligands in [{Ru(tBu2bpy)2}2(l-bpm)]5+

provide significantly greater steric hindrance to solventpenetration into the internal (and to a lesser extent, exter-nal) clefts relative to the diastereoisomers of[{Ru(pp)2}2(l-bpm)]5+ {pp = bpy, 5,5 0-Me2bpy, Me4bpy,Me2phen}. Most notably, the specific solvent effect de-

scribed previously for the diastereoisomers of the lattergroup of complexes in AN is absent as the tert-butyl sub-stituents block access to the interior clefts for the smallestsolvent of the nitrile series. Small differences in mmax areevident between the diastereoisomeric forms of [{Ru-(tBu2bpy)2}2(l-bpm)]5+ as the solvent molecules mayapproach the interior clefts more readily in the meso diaste-reoisomer compared with the rac form.

The solvent dependence of the IVCT energies for thediastereoisomers of the series [{Ru(pp)2}2(l-bpm)]5+

{pp = 5,5 0-Me2bpy, Me4bpy, Me2phen and tBu2bpy} areconsistent with the superposition of continuum and specificsolvation effects, in which the magnitude of the latter maydominate the ko contribution to mmax. The subtle variationsin the dimensions of the interior clefts between the diaste-reoisomeric forms of the same complex, and between boththe interior and exterior clefts for the same diastereoisomerover the series of complexes, give rise to a marked distanceand orientation dependence of the specific effects on ko.The solvent dependence of the IVCT energies for the dia-stereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ exhibits the theo-retically predicted linear dependence on the dielectricparameter 1/Dop � 1/Ds for all solvents except AN. Thestriking discontinuity for AN arises from penetration ofthe small AN molecules within the clefts between the termi-nal bpy rings. This specific interaction is also evident forthe substituted variants [{Ru(pp)2}2(l-bpm)]5+

{pp = 5,5 0-Me2bpy, Me4bpy, Me2phen}, however the effectis absent when solvent penetration to the clefts is restrictedby bulky tert-butyl substituents in [{Ru(tBu2bpy)2}2(l-bpm)]5+. The comparable magnitudes of the specific effectsfor both the unsubstituted bpy, and methyl-substituted{pp = 5,5 0-Me2bpy, Me4bpy, Me2phen} complexes, sug-gest that the AN dipoles must assume a similar orientationto the bpm plane in each case. The red-shift of mmax be-tween PN and AN indicates that the AN molecules areassociated within the interior clefts so as to solvate both

metal centers simultaneously, as discussed previously forthe [{Ru(bpy)2}2(l-bpm)]5+ diastereoisomers. By compari-son, the slightly larger PN molecules are oriented towardsthe metal center with the higher partial positive charge(d+), thus giving rise to a greater positive reorganizationalcontribution to mmax. The nature of the specific solventinteractions is markedly dependent on the position of themethyl substituents on the terminal ligands, and the sizeof the solvent molecules. The difference in the IVCT ener-gies between the meso and rac forms is largest for[{Ru(Me2phen)2}2(l-bpm)]5+ and provides the most pro-nounced example of differential stereochemically directedsolvent effects on ko.

4. Conclusions

IVCT solvatochromism studies on the meso and rac dia-stereoisomers of [{Ru(bpy)2}2(l-bpm)]5+ in a homologousseries of nitrile solvents reveal that stereochemically direc-ted specific solvent effects in the first solvation shell domi-


nate the outer sphere contribution to the reorganizationalenergy for intramolecular electron transfer. Solvent pro-portion experiments in AN/PN solvent mixtures demon-strate that the magnitude and direction of the specificeffect is dependent on the relative abilities of discrete sol-vent molecules to penetrate the clefts between the planesof the terminal polypyridyl ligands. In particular, the spe-cific effects are dependent on the dimensionality of theclefts, and the number, size, orientation and location ofthe solvent dipoles within the interior and exterior clefts.

IVCT solvatochromism studies on the diastereoisomericforms of [{Ru(bpy)2}2(l-dbneil)]5+ and [{Ru(pp)2}2(l-bpm)]5+ {pp = 5,5 0-Me2bpy, Me4bpy, Me2phen, tBu2bpy}reveal that the subtle and systematic changes in the natureof the clefts by the variation of the bridging ligand, and thejudicious positioning of substituents on the terminalligands profoundly influence the magnitude of the reorga-nizational energy contribution to the electron transferbarrier.

The present study provides an experimental platformwhich addresses the paucity of experimental data availableto guide and test developing solvation models which linkthe dynamics of individual solvent molecules and the Mar-cus–Hush theory of electron transfer [2,8,40,101,102].

Acknowledgments

We thank Professor Noel Hush and Dr. Jeff Reimers forhelpful discussions on this work, and gratefully acknowl-edge the financial support of the Australian ResearchCouncil.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.chemphys.2005.09.016.

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