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Paramagnetic ruthenium-biimidazole derivatives [(acac)2RuIII(LHn)]m,n/m = 2/+, 1/0, 0/-. Synthesis, structures, solution properties and anionreceptor features in solution state†‡

Tanaya Kundu, Shaikh M. Mobin and Goutam Kumar Lahiri*

Received 14th September 2009, Accepted 12th February 2010First published as an Advance Article on the web 25th March 2010DOI: 10.1039/b919036h

The paramagnetic ruthenium-biimidazole complexes [(acac)2RuIII(LH-)] (1 = red-brown),[(acac)2RuIII(LH2)](ClO4) (2 = pink) and Bu4N[(acac)2RuIII(L2-)] (3 = greenish yellow) comprising ofmonodeprotonated, neutral and bideprotonated states of the coordinated biimidazole ligand (LHn, n =1, 2, 0), respectively, have been isolated (acac- = acetylacetonate). Single-crystal X-ray diffraction of 1reveals that the asymmetric unit consists of three independent molecules: A–C, where molecule Acorresponds to complex 1 and the other two molecules B and C co-exist as a hydrogen bondeddimeric unit perhaps between the cationic 2+ and anionic 3-. The packing diagram further reveals thatthe molecule A in the crystal of 1 also forms a hydrogen bonded dimer with the neighbouringanother unit of molecule A. The formation of [(acac)2RuIII(LH2)](ClO4) (2) has also been authenticatedindependently by its single-crystal X-ray structure. The packing diagram of 2 shows multiple hydrogenbonds between the N–H protons of coordinated LH2 and the counter ClO4

-. Paramagnetic complexesshow 1H NMR spectra over a wide range of chemical shift, d (ppm), +10 to -35 in CDCl3. One-electronparamagnetic 1–3 (m/B.M. ~1.9) exhibit distinct rhombic-EPR spectra with relatively large ganisotropic factors: <g> 2.136–2.156 and Dg 0.65–0.77, typical for distorted octahedral ruthenium(III)complexes. The complexes 1–3 are inter-convertible as a function of pH. The pKa1 and pKa2 of 6.8 and11, respectively, for 2 are estimated by monitoring the pH dependent spectral changes. TheRu(III)–Ru(IV) couple near 1.25 V vs. SCE remains almost invariant in 1–3 whereas the correspondingRu(III)–Ru(II) couple varies appreciably in the range of -0.52 to -0.85 V vs. SCE based on theprotonated-deprotonated states of the coordinated biimidazole ligand. Compounds 1–3 exhibit oneweak ligand to metal charge transfer (LMCT) transition near 500 nm and intense intraligandtransitions in the higher energy UV region. The spectrophotometric titrations of 2 with the TBA(TBA = tetrabutylammonium) salts of a wide variety of anions, F-, Cl-, Br-, I-, HSO4

-, OAc-, H2PO4-

in CH3CN reveal that the possible hydrogen bonds between the N–H protons of LH2 in 2 and Cl- orBr- or I- or HSO4

- or H2PO4- anion are rather weak or negligible. However, in presence of excess

H2PO4- anion, the molar ratio of 2 to H2PO4

- being 1 : 4, simple liberation of one N–H proton of thecoordinated LH2 in 2 has been taken place which in effect yields 1 and H3PO4. On the contrary, thespectrophotometric titrations of 1 : 1 molar solution of 2 and OAc- or F- anion suggest the initialformation of hydrogen bonds between the N–H protons of LH2 in 2 and the anion with the calculatedlog K value of 5.92 or 4.7, respectively, which eventually leads to the transfer of one of the N–H protonsof LH2 in 2 to the anion, resulting in 1 and HOAc or HF. On addition of excess OAc- to the abovesolution of 1 (molar ratio of OAc- to 1, 4 : 1), further hydrogen bonding between the N–H proton ofLH- in 1 and OAc- occurs but without the abstraction of the N–H proton of LH-. However, excess F-

anion concentration (molar ratio of anion to 1, 5 : 1) facilitates the removal of the remaining N–Hproton of LH- in 1 which in turn yields 3 incorporating the bideprotonated form L2-.

Introduction

The potential of metal complexes incorporating the heterocyclicligand, 2,2¢-biimidazole (LH2) or its monodeprotonated form

Department of Chemistry, Indian Institute of Technology Bombay, Powai,Mumbai, 400076, India. E-mail: lahiri@chem.iitb.ac.in† Electronic supplementary information (ESI) available: Supplementarydata. CCDC reference numbers 747924 and 747925. For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/b919036h‡ Dedicated to Dr Sumit Bhaduri on the occasion of his 60th birthday.

(HL-) to function as receptors for anions as well as to participate inself-assembling processes through hydrogen bonding interactionsby virtue of the suitably positioned available N–H proton(s)associated with the coordinated LH2 have been well establishedin the solid state.1 The anion binding feature of a few selectivecomplexes of LH2 in the solution state has also been reportedrecently.2 The ruthenium chemistry of 2,2¢-biimidazole ligand,LH2, with two dissociable NH protons, in combination with s-donating NH3,3 H-,4 p-donating Cl-5 and p-acidic 2,2¢-bipyridine(bpy),6 2-phenylazopyridine (pap),7 triphenylphosphine (PPh3),4,8

4232 | Dalton Trans., 2010, 39, 4232–4242 This journal is © The Royal Society of Chemistry 2010

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Scheme 1

CO,4,8 cymene,9 NO5 co-ligands has drawn considerable attentionover the past years. The research activities are primarily centredaround the exploration of the coordination chemistry of thebiologically relevant imidazole based LH2 ligand including thehydrogen bonding aspects3–9 and anion binding features ofthe ruthenium coordinated LH2 moiety in the solid state.9,10 Thebinding of the neutral LH2 (pKa of free LH2, 11.53,11) to themetal ion through its imine nitrogen donors enhances the acidityof its N–H protons which in turn can facilitate the linkage ofthe coordinated biimidazole ligand with the second metal ionthrough the deprotonated N- donor centres. The dianionic L2-

bridged diruthenium complexes {(A)2Ru(m-L2-)Ru(A)2} (A =2,2¢-bipyridine (bpy) or 2-phenylazopyridine (pap)) have alsobeen investigated subsequently to analyse the effectiveness ofthe L2- bridge towards the intermetallic electronic couplingprocess in the mixed valent Ru(II)–Ru(III) state.6,7 More recently,attempts have however been made to explore the potential appli-cations of the mononuclear diamagnetic [(bpy)2RuII-LH2]2+12 and[RuIICl(cym)(LH2)]+ (cym = h6-para-isopropylmethylbenzene)9 tofunction as efficient receptors of a wide variety of anions viahydrogen bonding interactions particularly in the solution state.

The present work thus originates from our interest to explorethe interaction of the biimidazole ligand (LH2) with the precursorruthenium fragment {RuII(acac)2} incorporating electron richs-donating acetylacetonate (acac-) ligand, which leads to theformation of paramagnetic complexes [(acac)2RuIII(LH-)] (1),[(acac)2RuIII(LH2)]ClO4 (2) and Bu4N[(acac)2RuIII(L2-)] (3) encom-passing all the three possible protonated-deprotonated states of thecoordinated biimidazole ligand. Herein, we report the synthesisand solution properties of paramagnetic 1–3, structural aspectsof 1 and 2 and the potentiality of the paramagnetic 2 to functionas an anion receptor of selective anions such as F-, Cl-, Br-, I-,HSO4

-, OAc-, H2PO4- in solution state.

Results and discussion

The monomeric complex [(acac)2RuIII(LH-)] (1) has been synthe-sized in moderate yield from 1 : 1 mixture of the precursor com-plex [RuII(acac)2(CH3CN)2] and 2,2¢-biimidazole ligand (LH2) inrefluxing ethanol under aerobic reaction condition. The complex

[(acac)2RuIII(LH2)]+ (2+) has been prepared via the protonation ofthe preformed [(acac)2RuIII(LH-)] (1) in methanol using HClO4

and subsequently isolated in pure form as its perchlorate salt,[(acac)2RuIII(LH2)]ClO4 (2). The gradual addition of base (NaOH)into the methanol–water (1 : 9) solution of [(acac)2RuIII(LH2)]+ (2+)(pink) quantitatively converts it initially to the monodeprotonated[(acac)2RuIII(LH-)] (1) (red-brown) followed by doubly deproto-nated [(acac)2RuIII(L2-)]- (3-) (greenish yellow) which has beenisolated as Bu4N[(acac)2RuIII(L2-)] (3). Similarly, the addition ofHClO4 to 3- generates 2+ via the intermediate 1 implying the pHdependent reversible processes, 2 ↔ 1 ↔ 3 (Scheme 1).

Complexes 1 and 2 are fairly stable both in the solid and solutionstates but complex 3 is found to be reactive. It changes to 1 incontact with any proton sources which indeed prevents to get theclean NMR spectrum of 3. The appreciably negative Ru(III)–Ru(II)potential (< -0.5 V vs. SCE, see later) in 1–3 due to the electronrich weak-field ligands facilitates the stabilisation of the Ru(III)state in atmospheric conditions as has also been reported earlierin case of s-donating NH3 co-ligand in [RuIII(NH3)4(LH2)]3+.3

However, in reported other ruthenium-biimidazole complexes[(A)nRu(LH2)]m with moderate to strong p-acidic co-ligands, A =bpy (2,2¢-bipyridine), pap (2-phenylazopyridine), PPh3, CO, NO+,the ruthenium ion is expectedly stabilised in the diamagnetic +2state.4–8

The electrically neutral and 1 : 1 complexes 1 and 2/3, re-spectively, give satisfactory microanalytical data. The ESI massspectral data match well with the calculated values (see Exper-imental). The n(NH) and n(ClO4

-)/n(NH) vibrations for 1 and2, respectively, appear at 3377 and 1088, 627/3336, 3373 cm-1,respectively.

The paramagnetic complexes 1–3 exhibit magnetic mo-ments m, 1.92, 1.98 and 1.88 B.M. at 298 K in the solidstate, respectively, correspond to one unpaired electron asso-ciated with the Ru(III) centres (low-spin t2g

5, S = 12). Con-

sequently 1/2/3 display rhombic EPR spectra in CH2Cl2–toluene (1 : 1) at 77 K with g1= 2.495/2.433/2.410, g2 =2.177/2.188/2.191, g3 = 1.725/1.780/1.754; Dg = 0.77/0.65/0.66,<g> = 2.156/2.150/2.136 (Dg = g1 - g3 and <g> = [1/3(g1

2 +g2

2 + g32)]1/2) (ESI, Fig. S1),† typical for distorted octahedral

Ru(III) complexes.13

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Table 1 Crystallographic data for 1 and 2

Compound 1 2

Empirical formula C32H38N8O8Ru2 C16H20ClN4O9RuMr 864.84 548.88Crystal symmetry Monoclinic MonoclinicSpace group C2/c C2/ca/A 21.951(2) 31.457(3)b/A 22.3352(12) 6.1523(2)c/A 15.1032(10) 26.643(2)a/◦ 90 90b/◦ 107.134(8) 125.435 (12)g /◦ 90 90V/A3 7076.0(9) 4201.2(5)Z 8 8T/K 150(2) 150(2)Dc/g cm-3 1.624 1.736m/mm-1 0.914 0.930F(000) 3504 2216q range/◦ 2.98 to 25.00 3.41 to 25.00Reflectionscollected/unique

26 995/6215[Rint = 0.1026]

14 545/3688[Rint = 0.0938]

Data/restraints/parameters 6215/0/468 3688/42/288Final R1, wR2 [I > 2s(I)] 0.0412, 0.0826 0.0936, 0.1903R1, wR2(all data) 0.0523, 0.0870 0.1268, 0.2043GOF 1.117 1.173

Compound 1 exhibits well resolved broad 1H NMR signalsover a wide range of chemical shift, d , +6 to -35 ppm in CDCl3

(ESI, Fig. S2a)† due to paramagnetic contact shifts.14 The twomagnetically inequivalent acac ligands in 1 with unsymmetricLH- exhibit two C–H (acac) signals at -9.1 and -26.4 ppm. FourCH3 groups of two acac ligands however appear as two widelyseparated signals at -5.3 and -18.4 ppm. The four C–H and oneN–H signals of LH- in 1 appear at 4.5 and -34.3 ppm, respectively.The symmetric paramagnetic complex 2 on the other hand exhibitsone C–H and two CH3 signals associated with the acac ligand at3.67 and -21.41/-15.0 ppm and two C–H protons and one N–Hproton signals of LH2 appear at 8.02 and -12.65 ppm, respectively(ESI, Fig. S2b).†

The single-crystal X-ray structure of 1 is shown in Fig. 1.The selected crystallographic parameters and bond distances andangles are given in Tables 1 and 2, respectively. Interestingly, theasymmetric unit consists of three independent molecules: onefull molecule (A) and two half molecules (B and C) (Fig. 1).The generation of N–H proton of the coordinated biimidazoleligand through Fourier map establishes that in molecule A thebiimidazole ligand exists in monodeprotonated LH- form leadingto the overall molecular configuration of [(acac)2RuIII(LH-)] (1).

On the other hand the generation of one N–H proton (N6-H) of the coordinated biimidazole ligand in one of the halfmolecules (B) through Fourier map (ESI, Fig. S3 and TableS3)† indicates the presence of symmetric neutral, LH2 anddianionic, L2- states in molecule B, [(acac)2RuIII(LH2)]+ (2+) andmolecule C, [(acac)2RuIII(L2-)]- (3-), respectively, which in turn co-exist as a hydrogen bonded dimeric unit (N6-H6N–N8#, d(D–A),2.870(4) and ∠(DHA), 165.59, ESI Table S1, Fig. 1, Table 2).†However, the quality of the data set is not good enough to identifyunambiguously the exact arrangements of molecules B and C inthe crystal of 1. Therefore, the above assignment of molecules Band C as 2+ and 3-, respectively, in the crystal of 1 is feasible butis not uniquely defined.

Table 2 Selected bond lengths (A) and angles (◦) for 1

Molecule A

Bond lengths Bond angles

Ru1–O4 2.000(2) O4–Ru1–O1 179.26(10)Ru1–O1 2.005(2) O4–Ru1–O3 92.96(10)Ru1–O3 2.009(2) O1–Ru1–O3 86.69(10)Ru1–O2 2.012(2) O4–Ru1–O2 87.98(10)Ru1–N3 2.024(3) O1–Ru1–O2 91.37(10)Ru1–N1 2.059(3) O3–Ru1–O2 90.77(10)N1–C13 1.335(5) O4–Ru1–N3 91.21(10)N1–C11 1.377(4) O1–Ru1–N3 89.46(11)N2–C13 1.337(4) O3–Ru1–N3 93.41(10)N2–C12 1.367(5) O2–Ru1–N3 175.78(10)N3–C14 1.355(4) O4–Ru1–N1 87.48(10)N3–C16 1.362(5) O1–Ru1–N1 92.95(10)N4–C14 1.328(5) O3–Ru1–N1 172.77(11)N4–C15 1.374(5) O2–Ru1–N1 96.46(11)C11–C12 1.354(5) N3–Ru1–N1 79.36(11)C15–C16 1.373(5)N2–H(2N) 0.81(4)

Molecule B

Bond lengths Bond angles

Ru2–O5 1.995(2) O5–Ru2–O5#1 178.48(13)Ru2–O6 2.011(2) O5–Ru2–O6#1 86.83(10)Ru2–N5 2.043(3) O5–Ru2–O6 92.13(9)N5–C24 1.345(4) O6#1–Ru2–O6 92.96(14)N5–C22 1.361(5) O5–Ru2–N5 91.09(10)N6–C24 1.330(5) O5#1–Ru2–N5 90.08(10)N6–C23 1.368(5) O6#1–Ru2–N5 172.79(10)C22–C23 1.365(5) O6–Ru2–N5 94.02(10)N6–H(6N) 0.76(7) N5–Ru2–N5#1 79.07(16)

Molecule C

Bond lengths Bond angles

Ru3–O8 1.997(2) O8#1–Ru3–O8 179.31(13)Ru3–O7 2.014(2) O8#1–Ru3–O7 87.35(9)Ru3–N7 2.047(3) O8–Ru3–O7 92.15(9)N7–C32 1.341(4) O8#1–Ru3–O7#1 92.14(10)N7–C30 1.369(4) O7–Ru3–O7#1 87.17(13)N8–C32 1.334(4) O8–Ru3–N7#1 86.77(10)N8–C31 1.381(4) O7–Ru3–N7#1 175.71(10)C30–C31 1.363(5) O8–Ru3–N7 93.76(10)

O7–Ru3–N7 96.92(10)N7#1–Ru3–N7 79.01(15)

#1: -x + 1, y, -z + 3/2.

The packing diagram reveals that the unsymmetric moleculeA in the crystal of 1 also forms intermolecular hydrogen bondeddimeric structure with the neighbouring another molecule A (N2–H2N–N4#, d(D–A), 2.836(5) A and ∠(DHA), 177.47◦, ESI TableS1, Fig. 2).†

The intra-ring bond distances associated with the biimidazoleligands in molecules A, B and C in the crystal of 1 are close to eachother, only slight variations exist, average C–N/C–C distancesare 1.354(4)/1.363(5), 1.351(5)/1.365(5) and 1.356(4)/1.363(5) A,respectively (Table 2). The Ru–N(biimidazole) distances in half-symmetric molecules B and C are 2.043(3) and 2.047(3) A,respectively, and the same in unsymmetric molecule A are 2.059(3)and 2.024(3) A, with an average value of 2.043 A. The Ru–O(acac)distances in molecules A, B and C in the crystal of 1 vary slightly

4234 | Dalton Trans., 2010, 39, 4232–4242 This journal is © The Royal Society of Chemistry 2010

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Fig. 1 Single-crystal X-ray structure of 1 showing three independent molecules, A, B and C in the asymmetric unit where the half symmetric moleculesB and C co-exist as a hydrogen bonded dimer. (Symmetry operation: -x + 1, y, -z + 3/2.)

Fig. 2 Packing diagram of molecule A in the crystal of 1 along the a-axis showing the hydrogen bonded dimeric structure between the twoneighbouring units of molecule A.

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within the range of 2.003-2.006 A (average Ru–O distance in eachmolecule) which match fairly well with the average RuIII–O(acac)distances of 1.990–2.025 A reported in analogous complexes.15

It should be noted that the closeness of the bond parameters inA, B and C in the crystal of 1 (Table 2) does not extend theunambiguous assignment of the protonated-deprotonated statesof the coordinated biimidazole ligands, however, the generationof N–H protons through Fourier map (ESI Table S3 and Fig.S3)† indicates their correspondence to 1, 2+ and 3-, respectively,as stated above.

The single crystals of the isolated [(acac)2RuIII(LH2)]ClO4 (2)could be grown independently (ESI Fig. S4, Tables 1 and 3).†The average bond distances associated with the biimidazole ligand(LH2) in the independently crystallised 2, Ru–N, 2.059(8); C–N,1.349(13); C–C, 1.354(15) A, (Table 3) are close to those of the halfsymmetric molecule B present in the asymmetric unit of the crystalof 1, 2.043(3), 1.351(5), 1.365(5), respectively, (Fig. 1, Table 2)particularly considering the fact of the relatively weakly diffractingsingle crystal of the former molecule (Table 1, Experimental).

The packing diagram of the independently crystallized 2 alongthe a-axis (Fig. 3, ESI Table S2)† reveals the presence of N–H–Ohydrogen bonds between the N–H protons (N2-H and N4-H) ofcoordinated LH2 and oxygen atoms of the counter perchlorateanions.

With the ruthenium ion being in the +3 oxidation state, com-plexes 1–3 exhibit one quasi-reversible Ru(III)–Ru(IV) oxidation

Table 3 Selected bond lengths (A) and angles (◦) for 2

Compound 2

Bond lengths Bond angles

Ru1–O4 1.998(7) O4–Ru1–O1 178.6(3)Ru1–O1 2.007(6) O4–Ru1–O2 89.0(3)Ru1–O2 2.030(7) O1–Ru1–O2 92.3(3)Ru1–O3 2.032(7) O4–Ru1–O3 92.8(3)Ru1–N3 2.053(8) O1–Ru1–O3 87.0(3)Ru1–N1 2.066(9) O2–Ru1–O3 88.0(3)N1–C13 1.323(13) O4–Ru1–N3 90.0(3)N1–C11 1.361(12) O1–Ru1–N3 88.7(3)N2–C13 1.345(14) O2–Ru1–N3 176.1(3)N2–C12 1.359(14) O3–Ru1–N3 95.9(3)N3–C14 1.339(12) O4–Ru1–N1 89.6(3)N3–C16 1.361(12) O1–Ru1–N1 90.4(3)N4–C14 1.329(12) O2–Ru1–N1 97.2(3)N4–C15 1.380(14) O3–Ru1–N1 174.3(3)C11–C12 1.358(15) N3–Ru1–N1 79.0(3)C15–C16 1.351(15)N2–H2 0.81(9)N4–H4 0.88

process near 1.25 V and one reversible Ru(III)–Ru(II) reductionprocess in the potential range of -0.5 to -0.8 V vs. SCE (Fig. 4,Table 4). The oxidation potential for 1–3 remains more or lessinvariant whereas the reduction potential varies appreciably basedon the protonated-deprotonated states of the biimidazole ligand

Fig. 3 Packing diagram of 2 along the a-axis showing the hydrogen bonds between the N–H protons of the coordinated LH2 and the ClO4- counter

anion.

4236 | Dalton Trans., 2010, 39, 4232–4242 This journal is © The Royal Society of Chemistry 2010

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Table 4 Electrochemical data in acetonitrilea

CompoundE◦

298/V (DEp/mV)RuIV–RuIII couple

E◦298/V (DEp/mV)

RuIII–RuII couple DEb K cc

1 1.25(120) -0.76(60) 2.01 1.2 ¥1034

2 1.23(82) -0.52(88) 1.75 4.6 ¥1029

3 1.26(110) -0.85(86) 2.11 5.8 ¥1035

a In CH3CN/0.1 mol dm-3 Et4NClO4/versus SCE/scan rate 100 mV s-1.b DE = Separation in potential between the two couples. c RT lnK c =nF(DE) for T = 298 K.

Fig. 4 Cyclic (——) and differential pulse (---) voltammograms of[(acac)2RuIII(H2L)](ClO4) (2) vs. SCE in CH3CN, 0.1 mol dm-3 Et4NClO4.

Scan rate 100 mV s-1.

in the complexes. The reversible Ru(III)–Ru(IV) oxidation of theparent RuIII(acac)3 takes place at 0.98 V vs. SCE.16 Therefore, thereplacement of one of the “acac” ligands from the tris-RuIII(acac)3

by the biimidazole ligand either in neutral (LH2) or deprotonated(LH-/L2-) form increases the relative stability of the ruthenium(III)state or in other words destabilises the corresponding Ru(IV)state. The Ru(III)-Ru(II) reduction potential on the other handdecreases with the increasing electron density on the coordinatedbiimidazole ligand via the successive deprotonations of the N–Hprotons of LH2 in 1–3 (Table 4) and the potential of -0.76 V for1 having the monodeprotonated NH- is closest to the Ru(III)–Ru(II) potential of Ru(acac)3, -0.79 V.16 The appreciably negativeRu(III)–Ru(II) reduction potential (< -0.5 V) indeed facilitatesthe stabilisation of the Ru(III) state in 1–3 under atmosphericconditions. The separation in potential between the Ru(III)–Ru(IV)and Ru(III)–Ru(II) couples in Ru(acac)3 of DE = 1.77 V leads to thecomproportionation constant (K c) (calculated using the equationRT lnK c = nF(DE)13,17) value of 1030 16 which is comparable tothe K c values of 1029–1035 calculated for the same Ru(III)–Ru(IV)and Ru(III)–Ru(II) couples of 1–3 (Table 4) implying the inherentstability of the intermediate Ru(III) state in such complexes.

The reversible reduction of Ru(III) to Ru(II) for the analogousamine derivative, [RuIII(NH3)4(LH2)]3+ takes place near 0.0 V vs.SCE in the acidic pH region (pH < 5.5) and at higher pH (> 6),the reduction is reported to proceed with the loss of one proton,

Table 5 UV-vis spectral data for 1–3 in CH3CN

Compound lmax/nm (e/dm3 mol -1cm-1)

1 636 (790), 500 (1580), 348 (8220), 276 (27 780)2 516 (2240), 334 (8750), 278 (22 140)3 748 (670), 478 (1380), 360 (7550), 276 (29 670)

[RuIII(NH3)4(LH2)]3+ + e- � [RuII(NH3)4(LH-)]+ + H+.3 The 500mV negative shift of the Ru(III)–Ru(II) couple of 2 relative to theamine derivative implies better stability of the Ru(III) state in 2.

The Ru(III)–Ru(II) couples of the bpy (2,2¢-bipyridine)and pap (2-phenylazopyridine) derivatives, [RuII(bpy)2(LH2)]2+6

and [RuII(pap)2(LH2)]2+/[RuII(pap)2(LH-)]+/[RuII(pap)2 (L2-)]7 inCH3CN are reported to appear, respectively, at 1.04 and1.70/0.96/0.38 V (irreversible) vs. SCE. The large positive shiftof the Ru(III)–Ru(II) couple (1.3–2.3 V) on replacing “acac” in 1–3by “bpy/pap” co-ligands due to the difference in their electronicproperties, s/p-donating vs. p-acidic indeed stabilises the Ru(II)sate in the bpy and pap derivatives irrespective of the protonatedor deprotonated state of the biimidazole ligand.

In acetronitrile solution the ruthenium(III) complexes 1, 2 and3 exhibit one weak ligand to metal charge transfer transition(LMCT) in the visible region around 500 nm (Fig. 5a and Table 5)as expected from the Ru(III) complexes with electron rich s/p-donor ligands.3,18 The LMCT band energy gradually increasesfrom 516 (2) to 500 (1) to 478 (3) nm with the increase inelectron density on the coordinated biimidazole ligand via thesuccessive deprotonation processes which in turn increases therelative stability of the ligand based HOMO in the order of 3 >

1 > 2. The complexes 1 and 3 with the monoanionic LH- anddianionic L2-, respectively, show an additional weak and broadabsorption at the lower energy part above 600 nm. However, ligandbased intense transitions in the UV region are detected for all thethree complexes, 1–3.

The pKa values of coordinated biimidazole ligand in 2 have beendetermined by monitoring the pH dependent spectral changes inMeOH–H2O (1 : 9). All the three protonated-deprotonated formsof the coordinated biimidazole ligand: LH2(2), LH-(1), L2-(3)are accessible and interconvertible depending on the pH of thesolution. Therefore, stepwise slow increase in pH from 2 to 12results in the distinct transformation of 2 (pink) → 3 (greenishyellow) via the intermediacy of 1 (red brown). The pH depen-dent spectral changes proceed through distinct isosbestic points(Fig. 5b) implying the exclusive involvements of the concernedspecies, 1, 2+ and 3- throughout the experimental pH range of 2–12. The computed pKa1 and pKa2 values of 6.8 and 11, respectively,are closer to the corresponding diamagnetic [(bpy)2RuII(LH2)]2+

(bpy = 2,2¢-bipyridine) derivative, 7.2 and 12.16b but appreciablyhigher than those of the corresponding [(pap)2RuII(LH2)]2+ (pap =2-phenylazopyridine) complex, 4.2 and 8.0,7a respectively. Themuch higher acidity of the NH proton of coordinated biimidazoleligand (LH2) in the pap complex relative to the bpy derivativecan be attributed to the stronger p-accepting ability of pap thanbpy. The direct comparison of the pKa values of the paramagnetic[(acac)2RuIII(LH2)]+ (2+) with the analogous but diamagnetic bpyor pap derivative is not straightforward or rather meaningful dueto their in-built difference in formal charges, ruthenium(III) vs.ruthenium(II). On the other hand the pKa1 of the other known

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Fig. 5 (a) Electronic spectra of [(acac)2RuIII(HL-)] (1), [(acac)2RuIII-(H2L)](ClO4) (2) and (Bu)4N[(acac)2RuIII(L-)] (3) in CH3CN. (b) pHdependent spectral changes of 2 in MeOH–H2O (1 : 9, 10-4 mol dm-3).

Ru(III)-biimidazole complex, [(NH3)4RuIII(LH2)]3+ is reported tobe 5.65,3 suggesting greater acidic character of the N–H proton ofLH2 in comparison to that of the acac derivative in 2 as can alsobe revealed from the difference in their Ru(III)–Ru(II) potentials(see above). However, the pKa1 of 2 is much lower than that ofthe free biimidazole ligand (LH2), 11.5, which in turn enhancesthe potentiality of 2 to function as a suitable receptor for selectiveanions particularly in the solution state through possible hydrogenbonding.

The designing of metal complex derived sensors capable ofrecognising selective anions through weak or strong hydrogenbonding is considered to be an important aspect as anions areknown to play important roles in various chemical, biochemicaland environmental processes.19 In this regard the potential of thecoordinated biimidazole ligand (LH2) to participate in hydrogenbonding through its acidic free amino (N–H) protons in syn-conformation with a wide variety of anions has been well

recognised particularly in the solid state.1 However, the anionbinding features of coordinated LH2 in the solution state have beenreported recently in selective systems.2,9,12 Thus, the second sphereinteractions of 2, having two NH protons associated with thecoordinated neutral biimidazole ligand (LH2), with the selectiveanions, F-, Cl-, Br-, I-, HSO4

-, H2PO4- and OAc- have been

investigated spectrophotometrically in acetronitrile in order toexplore the anion receptor features of 2 in the solution state.

The addition of acetonitrile solution of the TBA (tetrabuty-lammonium) salt of Cl- or Br- or I- or HSO4

- to the acetonitrilesolution of 2 in a molar ratio of 1 : 1 to 5 : 1 does not alter theLMCT band of 2 at 516 nm (ESI, Fig. S5 and S6),† implyingvery weak or negligible hydrogen bonding interaction of 2 withthe aforesaid anions. Similar weak hydrogen bonding interactionsbetween the anions such as Cl-, Br-, I-, HSO4

- and the analogousdiamagnetic bpy complex [(bpy)2RuII(LH2)]2+ in the solution statehave recently been reported.12 The addition of H2PO4

- anion tothe acetonitrile solution of 2 in 1 : 1 molar ratio also fails to alterthe LMCT band of 2 (ESI, Fig. S5).† However, unlike Cl-, Br-, I-,HSO4

- the addition of excess H2PO4- into the acetonitrile solution

of 2 (molar ratio of H2PO4- and 2 = 4 : 1) causes the blue-shift of

the LMCT band of 2 from 516 nm to 500 nm, corresponding tothe LMCT band of 1 (Fig. 6) with the distinct change in colourfrom pink (2) to red-brown (1) implying simple proton abstractionprocess at higher concentration of the anion. However, unlike 2the analogous bipyridine derivative [(bpy)2RuII(LH2)]2+ interactsmoderately strongly with the H2PO4

- anion in 1 : 1 molar ratiowith the binding constant of log K = 3.32.12

Fig. 6 UV-vis spectral changes of 2 (5 ¥10-5 mol dm-3) in CH3CN onsequential additions of 0–4 equiv. of [TBA][H2PO4] in acetonitrile.

On the other hand, the successive additions of TBA salt ofOAc- to the acetonitrile solution of 2 up to a molar ratio of1 : 1 gradually shift the LMCT band of 2 from 516 nm (pinksolution) to the higher energy region at 500 nm (red brownsolution) (Fig. 7a), corresponding to the LMCT band of 1incorporating monodeprotonated LH-. This indicates the initialhydrogen bonding between the OAc- anion and the N–H protonsof LH2 in 2 as shown in the equilibrium process in Scheme 2.

The binding constant, K of 2 and OAc- has been calculatedbased on eqn (1).20

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Scheme 2

D D D DA=

+ + ± + +e e e([S] [L] 1/K) ([S] [L] 1/K) - 4 [S][L]2 2 2÷2

(1)

In eqn (1), DA represents the change in the initial absorbanceof 2 at 516 nm on each addition of the anion OAc- and [S] and[L] are the concentrations of 2 and OAc-, respectively, duringspectrophotometric titrations. The binding constant K and thechange in molar extinction coefficient (De) were estimated from thenon-linear curve fitting procedure using DA at each concentrationof OAc-, which yields the binding constant, log K = 5.92 (ESI, Fig.S7a).† The observed stronger hydrogen bonding between 2 andOAc- in relation to HSO4

- and H2PO4- can be tentatively attributed

to the difference in pKa1 values of 4.75, 1.89, 1.43, for HOAc,H2SO4 and H3PO4, respectively.21 The relatively stronger basicityof OAc- facilitates its interaction with the acidic N–H protons ofLH2 in 2. The binding constant of the paramagnetic 2 and OAc-

in terms of log K = 5.92 is even higher than that reported forthe diamagnetic [(bpy)2RuII(LH2)]2+, log K = 5.44.12 The slightlylower pKa1 value of 2, 6.8 compared to 7.2 for [(bpy)2RuII(LH2)]2+

may be the controlling factor for the stronger hydrogen bondinginteraction in the present case.

On further stepwise additions of OAc- to the earlier in situgenerated 1 (up to 4 : 1 molar ratio) the LMCT band of 1 at500 nm gradually blue shifted to 485 nm. However, the LMCTband remains unaltered on subsequent increase in the ratio ofOAc-:1 (Fig. 7b and ESI Fig. S8)† implying the formation ofhydrogen bonding between 1 and OAc- in presence of excess anionconcentration but without any proton transfer process (Scheme 3).

Interaction of 2 with the TBA salt of F- in a molar ratio of1 : 1 similarly blue shifts the LMCT band of the pink 2 at 516 nmto 500 nm (Fig. 8a) corresponding to the LMCT band of the redbrown 1. The quantitative conversion of 2 to 1 in presence of oneequivalent amount of F- suggests the initial hydrogen bondingbetween the N–H proton of LH2 in 2 and F-, which subsequentlyfacilitates the transfer of N–H proton to F- leading to 1 and HFas observed in case of OAc- (Scheme 2). The value of log K for theequilibrium process of the interaction of 2 with F- (eqn (2)) hasbeen estimated to be 4.7 (ESI, Fig. S7b)† based on eqn (1) whichis less than that reported as 5.29 for

[Ru(acac)2LH2]+ (2) + F-� [[Ru(acac)2LH2].F]→[Ru(acac)2LH](1) + HF (2)

K = −

[ . ]

[ ][ ]

2

2

F

F

the interaction of F- with the analogous but diamagnetic[Ru(bpy)2(LH2)]2+.12 Unlike OAc- (Fig. 7b and ESI Fig. S8),† thegradual additions of excess F- anion (maximum six equivalents)to the solution of 1, generated initially via the reaction of 2 andF- in 1 : 1 molar ratio, shift the LMCT band of 1 at 500 nm to478 nm corresponding to the doubly deprotonated 3 (yellowishgreen) (Fig. 8b). Further deprotonation of LH- in 1 leading to3 in presence of excess F- is believed to be accompanied by theconcomitant formation of the stable dimeric unit, HF + F- �HF2

-.22

Scheme 3

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Fig. 7 (a) UV-vis spectral changes of 2 (5 ¥10-5 mol dm-3) in CH3CNon gradual additions of acetonitrile solution of [TBA][OAc] up to oneequivalent. Inset shows the changes in absorbances at 516 and 636 nm as afunction of equivalents of the OAc- anion. (b) UV-vis spectral changes ofpre-generated 1, through 1 : 1 reaction of 2 (5 ¥ 10-5 mol dm-3) and OAc-

anion in CH3CN (see ‘a’), on further additions of [TBA][OAc] up to sevenequivalents.

Conclusions

The present work demonstrates the following unique fea-tures: (i) Isolation of the paramagnetic [(acac)2RuIII(LH-)] (1),[(acac)2RuIII(LH2)](ClO4) (2) and NBu4[(acac)2RuIII(L2-)] (3) en-compassing the possible protonated-deprotonated states of the2,2¢-biimidazole ligand (LH2). (ii) The crystal structure of 1 revealsthe co-existence of three independent molecules, A, B and C inthe asymmetric unit having protonated-deprotonated forms of thebiimidazole ligand where molecules B and C exist as a hydrogenbonded dimer. (iii) The crystal structure of the independentlysynthesized 2 establishes the presence of multiple hydrogenbonding interactions between the N–H protons of coordinatedLH2 and the counter anion, ClO4

-. (iv) The moderately acidic N–H protons of the coordinated LH2 in 2 with pKa1 and pKa2 of6.8 and 11, respectively, interact selectively with the anions, OAc-

Fig. 8 (a) UV-vis spectral changes of 2 (5 ¥10-5 mol dm-3) in CH3CN ongradual additions of [TBA][F] up to one equivalent. (b) UV-vis spectralchanges of 2 (5 ¥ 10-5 mol dm-3) in CH3CN on gradual additions of 0–6equivalents of [TBA][F]. Inset shows the changes in absorbances at 360and 636 nm as a function of equivalents of the F- anion.

and F- in acetonitrile solution with log K values of 5.92 and 4.7,respectively.

Experimental

Materials

The precursor complex [Ru(acac)2(CH3CN)2] was prepared ac-cording to the reported procedure.23 The ligand 2,2¢-biimidazolewas synthesized according to the literature report.24 The chemicalsterabutylammonium salts of F-, Cl-, Br-, I-, HSO4

-, OAc-, H2PO4-

were purchased from Aldrich or Alfa Aesar. Other chemicals andsolvents were of reagent grade and used as received.

Physical measurements

UV-vis spectra were recorded on a Perkin-Elmer 950 lamdaspectrophotometer. FT-IR spectra were taken on a Nicoletspectrophotometer with samples prepared as KBr pellets. Solutionelectrical conductivity was checked using a Systronic 304 conduc-tivity bridge. The EPR measurements were made with a Varianmodel 109C E-line X-band spectrometer fitted with a quartz dewarfor 77 K. 1H NMR spectra were obtained with a 300 MHz VarianFT spectrometer. Magnetic susceptibilities were measured using

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a Faraday magnetic balance. Cyclic voltammetric, differentialpulse voltammetric and coulometric measurements were carriedout using a PAR model 273A electrochemistry system. Platinumwire working and auxiliary electrodes and an aqueous saturatedcalomel reference electrode (SCE) were used in a three-electrodeconfiguration. The supporting electrolyte was 0.1 M [NEt4]ClO4

and the solute concentration was ~10-3 mol dm-3. The half-wave potential E◦

298 was set equal to 0.5(Epa + Epc), where Epa

and Epc are the anodic and cathodic cyclic voltammetric peakpotentials, respectively. A platinum wire-gauze working electrodewas used in the Coulometric experiments. The elemental analyseswere carried out with a Perkin-Elmer 240C elemental analyzer.Electrospray mass spectra were recorded on a Micromass Q-ToFmass spectrometer.

In spectrophotometric titrations, 2 mL aliquots of the TBA-saltof the respective anions (5 ¥ 10-3 mole dm-3) in acetonitrile wereadded by a micro- syringe in each step in 2 cm3 of 5 ¥ 10-5 mol dm-3

acetonitrile solution of 2 using quartz cuvettes of 1 cm path lengthand the volume of 2 cm3.

Caution! Perchlorate salts of metal complexes with organicligands are potentially explosive. Heating of dried samples mustbe avoided, handling of small amounts has to proceed with greatcaution using protection.

Preparation of complexes 1–3

[(acac)2RuIII(HL-)] (1). The starting complex [Ru(acac)2-(CH3CN)2] (100 mg, 0.26 mmol) and 2,2¢-biimidazole (H2L)(35 mg, 0.26 mmol) were taken in 40 cm3 ethanol and the mixturewas heated at reflux for 14 h. The initial orange colour of thesolution gradually changed to red-brown. The solvent was thenremoved under reduced pressure. The solid residue thus obtainedwas purified by using a silica gel (60–120 mesh) column. A red-brown solution corresponding to the desired product 1 was elutedwith a mixture of CH3CN–CH3OH (4 : 1). 1: Yield, 48 mg (43%).Anal. calcd For C16H19N4O4Ru: C 44.34, H 4.42, N 12.93. Found:C 44.29, H 4.37, N 12.99. Electrospray mass spectral data: m/z =433.93 corresponding to [1]+ (calcd molecular weight: 433.04). 1HNMR in CDCl3 (d/ppm): -34.27 (1 H, NH of LH-), -26.42 (1 H,CH of acac-), -18.42 (6 H, CH3 groups of acac-), -9.1 (1 H, CH ofacac-), -5.29 (6 H, CH3 groups of acac-), 4.53 (4 H, CH of LH-).IR (KBr disk): n(NH), 3377 cm-1.

Transformation of [(acac)2RuIII(HL-)] (1) to [(acac)2RuIII-(H2L)](ClO4) (2). The compound [(acac)2RuIII(HL-)] (1)(100 mg, 0.23 mmol) was dissolved in 25 cm3 methanol andperchloric acid (30 mL, 0.35 mmol) was added to it and themixture was stirred for 0.5 h. The colour of the solution waschanged from red-brown to pink. The solvent was then removedunder reduced pressure. The resulted solid mass was redissolved inminimum volume of acetonitrile followed by addition of saturatedaqueous solution of NaClO4 yielded dark pink precipitation. Theprecipitate thus obtained was filtered and washed thoroughly bywater. It was then dried under vacuum over P4O10.

Yield, 104 mg (85%). Anal. calcd for C16H20N4O8ClRu: C 36.02,H 3.78, N 10.51. Found: C 35.98, H 3.70, N 10.47. ESI MS(in CH3CN): m/z = 434.11 corresponding to [2 - ClO4]+ (calcdmolecular weight: 434.05). 1H NMR in CDCl3 (d/ppm): -21.41 (3H, CH3 group of acac-), -15.00 (3 H, CH3 group of acac-), -12.65(1 H, NH of LH2), 3.67 (1 H, CH of acac-), 8.02 (2 H, CH of LH2).

IR (KBr disk): n(ClO4-), 1088, 627 cm-1, n(NH), 3336, 3373 cm-1.

Molar conductivity [KM/X-1cm2 mol-1] in acetonitrile: 120.

Transformation of [(acac)2RuIII(HL-)] (1) to Bu4N[(acac)2-RuIII(L2-)] (3). The solid NaOH (7 mg, 0.17 mmol) was addedto the dry methanolic solution (25 cm3) of 1 (50 mg, 0.115 mmol)and the solution was stirred for 0.5 h. The colour of the solutionwas changed from red-brown to greenish yellow. The volume ofthe solution was reduced to 5 cm3 under reduced pressure and thenexcess tetrabutyl ammonium iodide (63 mg, 0.17 mmol) was addedto it which resulted in the solid product. The solid mass was furtherdissolved in dry acetonitrile and dry diethylether was added to itwhich resulted in the precipitation of excess reagents. The greenishyellow solution of 3 was then filtered and the solvent mixture wasremoved under reduced pressure. The solid mass thus obtainedwas dried in vacuo. The compound 3 was found to be unstableand transforms to 1 in presence of any proton sources, therefore,appropriate care needs to be taken during the synthetic work upand freshly prepared sample needs to be used in each time. TheUV-vis. spectrum of the freshly prepared 3 matches well with the insitu generated 3 by adding NaOH base into 1. Yield, 62 mg (80%).Anal. calcd for C32H54N5O4Ru: C 56.95, H 8.07, N 10.38. Found:C 56.89, H 8.01, N 10.42. ESI MS (in CH3CN): m/z = 431.5corresponding to [3 - Bu4N]- (calcd molecular weight: 432.04).Molar conductivity [KM/X-1cm2 mol-1] in acetonitrile: 142.

Crystal structure determination

Single crystals of 1 and 2 were grown by slow evaporation oftheir methanol and methanol–toluene (1 : 1) solutions, respec-tively. Despite several attempts we failed to get fully satisfactorysingle crystals of 2, however, the structure of 2 could be solvedwith a weakly diffracting crystal. X-Ray diffraction data werecollected using an OXFORD XCALIBUR-S CCD single-crystalX-ray diffractometer. The structures were solved and refined byfull-matrix least-squares techniques on F 2 using the SHELX-97 program.25 The absorption corrections were done by themulti-scan technique. All data were corrected for Lorentz andpolarization effects, and the non-hydrogen atoms were refinedanisotropically. The N–H hydrogen atoms in 1 and 2 weregenerated by Fourier map and all other hydrogen atoms in 1 and2 were included in the refinement process as per the riding model.The complex 2 crystallised with one water molecule in the unitcell, however, the hydrogen atoms of the water of crystallisationcould not be located.

Acknowledgements

Financial support received from the Department of Science andTechnology, New Delhi, India is gratefully acknowledged. X-Raystructural studies were carried out at the National Single crystalDiffractometer Facility, Indian Institute of Technology, Bombay.Special acknowledgement is made to the Sophisticated AnalyticalInstrument Facility (SAIF), Indian Institute of Technology,Bombay, for providing the NMR and EPR facilities.

References

1 (a) R. Atencio, M. Chacon, T. Gonzalez, A. Briceno, G. Agrifoglioand A. Sierraalta, Dalton Trans., 2004, 505; (b) S. Fortin, P.-L. Fabre,

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M. Dartiguenave and A. L. Beauchamp, J. Chem. Soc., Dalton Trans.,2001, 3520; (c) R. Atencio, K. Ramırez, J. A. Reyes, T. Gonzalez and P.Silva, Inorg. Chim. Acta, 2005, 358, 520; (d) L. M. Gruia, F. D. Rochonand A. L. Beauchamp, Inorg. Chim. Acta, 2007, 360, 1825; (e) Y. Liand P. Yang, J. Chem. Crystallogr., 2008, 38, 529; (f) M. Tadokoro andK. Nakasuji, Coord. Chem. Rev., 2000, 198, 205; (g) M. Tadokoro, H.Kanno, T. Kitajima, H. Shimada-Umemoto, N. Nakanishi, K. Isobeand K. Nakasuji, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4950; (h) M.Tadokoro, T. Inoue, S. Tamaki, K. Fujii, K. Isogai, H. Nakazawa, S.Takeda, K. Isobe, N. Koga, A. Ichimura and K. Nakasuji, Angew.Chem., Int. Ed., 2007, 46, 5938; (i) A. K. Ghosh, D. Ghoshal, E.Zangrando and N. Ray Chaudhuri, Polyhedron, 2007, 26, 4195; (j) Y.-R.Zhong, M.-L. Cao, H.-J. Mo and B.-H. Ye, Cryst. Growth Des., 2008,8, 2282.

2 (a) L. Ion, D. Morales, S. Nieto, J. Perez, L. Riera, V. Riera, D. Miguel,R. A. Kowenicki and M. McPartlin, Inorg. Chem., 2007, 46, 2846; (b) S.Fortin and A. L. Beauchamp, Inorg. Chem., 2001, 40, 105.

3 R. Sahai, W. R. Murphy, Jr. and J. D. Petersen, Inorg. Chim. Acta, 1986,114, 137.

4 M. P. Garcia, A. M. Lbpez, M. A. Esteruelas, F. J. Lahoz and L. A.Oro, J. Chem. Soc., Dalton Trans., 1990, 3465.

5 T. W. Stringfield, K. V. Somayajula, D. C. Muddiman, J. W. Flora andR. E. Shepherd, Inorg. Chim. Acta, 2003, 343, 317.

6 (a) E. V. Dose and L. J. Wilson, Inorg. Chem., 1978, 17, 2660; (b) M-A.Haga, Inorg. Chim. Acta, 1983, 75, 29; (c) M.-A. Haga and A.Tsunemitsu, Inorg. Chim. Acta, 1989, 164, 137.

7 (a) P. Majumdar, S.-M. Peng and S. Goswami, J. Chem. Soc., DaltonTrans., 1998, 1569; (b) P. Majumdara, S. Goswami and S.-M. Peng,Polyhedron, 1999, 18, 2543; (c) P. Majumdar, K. K. Kamar, A.Castineiras and S. Goswami, Chem. Commun., 2001, 1292.

8 B. K. Panda, S. Sengupta and A. Chakravorty, Eur. J. Inorg. Chem.,2004, 178.

9 L. Ion, D. Morales, J. Perez, L. Riera, V. Riera, R. A. Kowenickib andM. McPartlinc, Chem. Commun., 2006, 91.

10 S. Derossi, H. Adams and M. D. Ward, Dalton Trans., 2007, 33.11 F. Holmes, K. M. Jones and E. G. Torrible, J. Chem. Soc., 1961, 4790.12 Y. Cui, H.-J. Mo, J.-C. Chen, Y.-L. Niu, Y.-R. Zhong, K.-C. Zheng and

B.-H. Ye, Inorg. Chem., 2007, 46, 6427.13 (a) S. Maji, B. Sarkar, S. Patra, J. Fiedler, S. M. Mobin, V. G. Puranik, W.

Kaim and G. K. Lahiri, Inorg. Chem., 2006, 45, 1316; (b) S. Ghumaan,S. Kar, S. M. Mobin, B. Harish, V. G. Puranik and G. K. Lahiri, Inorg.Chem., 2006, 45, 2413; (c) S. Maji, B. Sarkar, S. M. Mobin, J. Fiedler,W. Kaim and G. K. Lahiri, Dalton Trans., 2007, 2411; (d) S. Kar, N.Chanda, S. M. Mobin, F. A. Urbanos, M. Niemeyer, V. G. Puranik, R.Jimenez-Aparicio and G. K. Lahiri, Inorg. Chem., 2005, 44, 1571; (e) D.Kumbhakar, B. Sarkar, S. Maji, S. M. Mobin, J. Fiedler, F. A. Urbanos,R. Jimenez-Aparicio, W. Kaim and G. K. Lahiri, J. Am. Chem. Soc.,2008, 130, 17575.

14 (a) S. Ghumaan, B. Sarkar, S. Maji, V. G. Puranik, J. Fiedler, F. A.Urbanos, R. Jimenez-Aparicio, W. Kaim and G. K. Lahiri, Chem.–Eur. J., 2008, 14, 10816; (b) S. Maji, B. Sarkar, S. M. Mobin, J. Fiedler,F. A. Urbanos, R. Jimenez-Aparicio, W. Kaim and G. K. Lahiri,

Inorg. Chem., 2008, 47, 5204; (c) T. Koiwa, Y. Masuda, J. Shono, Y.Kawamoto, Y. Hoshino, T. Hashimoto, K. Natarajan and K. Shimizu,Inorg. Chem., 2004, 43, 6215; (d) D. R. Eaton, J. Am. Chem. Soc., 1965,87, 3097; (e) R. A. Palmer, R. C. Fay and T. S. Piper, Inorg. Chem.,1964, 3, 875; (f) R. H. Holm and F. A. Cotton, J. Am. Chem. Soc.,1958, 80, 5658; (g) R. C. Fay and T. S. Piper, J. Am. Chem. Soc., 1963,85, 500; (h) J.- L. Chen, X.-U. Zhang, L.-Y. Zhang, L.-X. Shi and Z.-N.Chen, Inorg. Chem., 2005, 44, 1037.

15 (a) G. K. J. Chao, R. L. Sime and R. J. Sime, Acta Crystallogr., Sect.B: Struct. Crystallogr. Cryst. Chem., 1973, 29, 2845; (b) S. Patra, T. A.Millar, B. Sarkar, M. Niemeyer, M. D. Ward and G. K. Lahiri, Inorg.Chem., 2003, 42, 4707; (c) S. Patra, B. Sarkar, S. M. Mobin, W. Kaimand G. K. Lahiri, Inorg. Chem., 2003, 42, 6469; (d) S. Kar, B. Sarkar,S. Ghumaan, D. Janardanan, J. van Slageren, J. Fiedler, V. G. Puranik,R. B. Sunoj, W. Kaim and G. K. Lahiri, Chem.–Eur. J., 2005, 11, 4901.

16 (a) E. A. Seddon and K. R. Seddon, The Chemistry of Ruthenium,Elsevier, Oxford, 1984, 203; (b) A. Endo, M. Watanabe, S. Hayashi, K.Shimizu and G. P. Sato, Bull. Chem. Soc. Jpn., 1978, 51, 800.

17 C. Creutz, Prog. Inorg. Chem., 1983, 30, 1.18 S. Kar, N. Chanda, S. M. Mobin, A. Datta, F. A. Urbanos, V. G.

Puranik, R. Jimenez-Aparicio and G. K. Lahiri, Inorg. Chem., 2004,43, 4911.

19 (a) J. L. Sessler and J. M. Davis, Acc. Chem. Res., 2001, 34, 989;(b) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40,486; (c) 35 Years of Synthetic Anion Receptor Chemistry 1968–2003,ed. P. A. Gale, Coord. Chem. Rev., 2003, 240, 1; (d) R. Martinez-Manez and F. Sancenon, Chem. Rev., 2003, 103, 4419; (e) AnionSensing, ed. I. Stibor, Topics in Current Chemistry 255, Springer-Verlag, Berlin, 2005; (f) K. Bowman-James, Acc. Chem. Res., 2005, 38,671; (g) Anion Receptor Chemistry, ed. J. L. Sessler, P. A. Gale, W. S. Choand J. F. Stoddart, Monographs in Supramolecular Chemistry, RoyalSociety of Chemistry, Cambridge, U.K., 2006; (h) Anion CoordinationChemistry II, ed. P. A. Gale, Coord. Chem. Rev., 2006, 250, 2917; (i) V.Amendola, E. D. Gomez, L. Fabbrizzi and M. Licchelli, Acc. Chem.Res., 2006, 39, 343.

20 (a) Y. Liu, B.-H. Han and Y.-T. Chen, J. Phys. Chem. B, 2002, 106,4678; (b) Y. Liu, B.-H. Han, S.-X. Sun, T. Wada and Y. Inoue, J. Org.Chem., 1999, 64, 1487; (c) C. Basu, S. Biswas, A. P. Chattopadhyay,H. Stoeckli-Evans and S. Mukherjee, Eur. J. Inorg. Chem., 2008, 4927;(d) L. Wang, X.-J. Zhu, W.-Y. Wong, J.-P. Guo, W.-K. Wong and Z.-Y.Li, Dalton Trans., 2005, 3235; (e) B. Valeur, J. Pouget, J. Bouson, M.Kaschke and N. P. Ernsting, J. Phys. Chem., 1992, 96, 6545; (f) Y. Liu,B. Li, C.-C. You, T. Wada and Y. Inoue, J. Org. Chem., 2001, 66, 225.

21 Lange’s Handbook of Chemistry, ed. J. A. Dean, Science Press andMcGraw-Hill Education, Beijing, 1999, 15th edn.

22 Y. Cui, Y.-L. Niu, M.-L. Cao, K. Wang, H.-J. Mo, Y.-R. Zhong andB.-H. Ye, Inorg. Chem., 2008, 47, 5616.

23 T. Kobayashi, Y. Nishina, K. G. Shimizu and G. P. Sato, Chem. Lett.,1988, 1137.

24 J.-C. Xiao and J. M. Shreeve, J. Org. Chem., 2005, 70, 3072.25 G. M. Sheldrick, SHELXS-97, Program for Crystal Structure Solution

and Refinement, University of Gottingen, Germany, 1997.

4242 | Dalton Trans., 2010, 39, 4232–4242 This journal is © The Royal Society of Chemistry 2010

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