Synthesis, spectroscopic, photophysical and electrochemical behaviour

of ruthenium(II) and copper(I) isocyano-bridged complexes with polypyridine

ligands: 2,2¢-bipyridine and 1,10-phenanthroline

Sudhir Ranjan*,** and Sheo K. DikshitDepartment of Chemistry, Indian Institute of Technology, Kanpur – 208016, UP, India

Received 13 September 2001; accepted 29 October 2001

Abstract

New binuclear complexes with [Cu(PPh3)3]þ and [Cu(PPh3)(NAN)]þ (NAN – 2,2¢-bipyridine, 1,10-phenanthroline)

moieties connected via the isocyanide group to [Ru(bpy)2(py)]þ and [Ru(phen)2(py)]

þ have been prepared andisolated as PF�

6 salts. In addition, new trinuclear complexes, [{(PPh3)3Cu(l-NC)}2Ru(bpy)2](PF6)2 and [{(NAN)-(PPh3)Cu(l-NC)}2Ru(bpy)2](PF6)2, have been synthesized using [Ru(bpy)2(CN)2]. The complexes have beencharacterized by elemental analyses, i.r., n.m.r., u.v.–vis., FAB mass spectra and by conductivity measurements. Thei.r. spectra reveal an increase in m(CN) in the isocyano-bridged complexes compared to the mononuclear parentcomplexes. The complexes are luminescent with emission wavelengths in the 458–550 and 600–636 nm ranges. Thehalf wave reduction potentials in MeCN are always more positive than those of the parent complexes. It is observedthat the isocyano-bridged complexes are more powerful excited state reductants than the cyano-bridged, Cu(I)(l-CN)Ru(II) complexes.

Introduction

The design of suitable chromophores to be used assensitizers in solar energy conversion is one of the aimsof coordination chemists [1]. Mono- and binuclearruthenium(II) polypyridine complexes show remark-able sensitizing properties [2, 3]. Such systems are alsoinvolved in electron and energy transfer processes [3]. Inrecent years, much work has been done on cyano-bridged homo- and heteropolynuclear complexes [4] toexplore various photophysical properties, includingsensitization, in which –RuðbpyÞ2þ2 – is present as oneof the chromophoric units. There are some reports onheteronuclear cyanide [5] bridged complexes containingruthenium(II) and other metal centres. The copper(I)centre, a d10 system, is very electron-rich and can bestabilized by ligands having p-acid character; viz. PPh3,bpy, phen, CO, CN� etc. We have been interested in theproperties of isocyano-bridged bimetallic complexes ofcopper(I) and ruthenium(II) [6]. Isocyanide-bridgedcomplexes containing copper(I) and other metal centreshave also been reported [7]. Since copper(I) complexesalso show photophysical and photochemical properties[8], it stimulated us to study various aspects of cop-per(I)–ruthenium(II) complexes. Our interests are four-fold: (a) to study the nature of the stretching frequencyof the –NC– group when it is bonded to two electron-

rich metal centres, copper(I) and ruthenium(II); (b) howthis –NC– bridging affects the electronic and photo-physical properties of the spectator ligands and subunit;(c) to study the photoinduced-intramolecular electronand energy transfer in the partially oxidized state; (d) tostudy the electrochemical properties. In this paper wereport photophysical, electrochemical and spectral stud-ies on variety of isocyano-bridged, Cu(I)(l-NC)Ru(II)complexes.

Experimental

Materials and instruments

1H- and 13C-n.m.r. experiments were carried out at 300and 75 MHz respectively on a Bruker DRX300 spec-trometer. I.r. spectra were recorded on a Shimadzu IR420 spectrometer using KBr pellets in the 4000–400 cm�1 range. U.v.–vis. spectra were recorded on aShimadzu double beam UV-160 spectrophotometer inMeCN. FAB mass spectra were recorded on a JEOL SX102/DA-6000 system using Xe (6 kV, 100 mA) as theFAB gas and m-nitrobenzylalcohol as a matrix. Meltingpoints were obtained on a Fisher–John melting pointapparatus and are uncorrected. Conductivity measure-ments were carried out in MeCN on a ToshniwalConductivity Bridge. Microanalyses were performed atthe Microanalytical Laboratory, IIT, Kanpur.Emission spectra were recorded on a Perkin-Elmer

Luminescence Spectrophotometer LS50B. Correction of

* Author for correspondence

** Present address: Department of Chemistry, National Tsing Hua

University, Hsinchu, Taiwan 30013, ROC.

Transition Metal Chemistry 27: 668–675, 2002. 668� 2002 Kluwer Academic Publishers. Printed in the Netherlands.

the luminescence intensity profile was performed byfluorescence data manager software. Luminescencequantum yields were evaluated by comparing areasunder the corrected luminescence on an energy scale andby using the following equation:

/s=/r ¼ FsAr=FrAs

where the Fs and Fr are the integrated fluorescenceintensities of the sample and [Ru(bpy)3]

2þ, As and Ar arethe absorbance of the sample and [Ru(bpy)3]

2þ atexcitation of the same wavelength. /r for [Ru(bpy)3]

2þ

was taken as 0.028 [9].For the life-time measurement a coherent synchro-

nously pumped cavity dumped dye laser(702-1) pumpedby a coherent CW mode-locked Nd:YAG laser (Antares76s) was used. The fundamental laser light at 600 nmwasfrequency doubled to produce exciting light at 300 nm.The emission was detected at a magic angle of 54.7polarization using a Hamamatsu MCP photomultipliertube (2809U). The response time of the setup is 50–60 ps.C.v. studies were carried out on EG/G PAR Model

273A polarographic analyzer interfaced to a computer.A three-electrode system consisting of a Pt workingelectrode, Pt mesh counter electrode and a commerciallyavailable saturated calomel electrode (SCE) as thereference electrode was used.RuCl3 Æ xH2O was obtained from Loba Chemie, India

and PPh3, 2,2¢-bipyridine and 1,10-phenanthroline fromE-Merck, Germany and used as received. All otherchemicals used were either AnalaR grade or chemicallypure. All manipulations were performed under an O2-free dry N2 atmosphere using standard Schlenk line andother techniques. The solvents were dried using standardprocedures before use.CuCN, CuCl, [Cu(PPh3)2CN], [Cu(PPh3)3Cl] and

their substituted analogues were prepared by modifyingthe literature procedures [6, 10]. [Ru(bpy)2(py)CN]PF6[11] and [Ru(bpy)2(CN)2] [12] were prepared accordingto literature methods. [Ru(phen)2(py)CN]PF6 was pre-pared according to reported literature [11] for the 2,2¢-

bipyridine analogue. The ligand numbering is shown inScheme 1.

Syntheses

Tris-(triphenylphosphine)copper(I)(l-isocyano)pyridine-bis-(2,2¢-bipyridine)ruthenium(II)hexaflurophosphate,[(PPh3)3Cu(NC)Ru(bpy)2(py)] (PF6)2 (1)A mixture of [Cu(PPh3)3Cl] (0.885 g, 1.0 mmol) and[Ru(bpy)2(py)CN]PF6 (0.663 g, 1.0 mmol) was suspend-ed in EtOH (30 cm3) and refluxed for 3 h. An equalvolume of H2O was then added and the solution wasrefluxed further for 6 h. The solution was then cooledand filtered. A highly concentrated solution of NH4PF6(3 cm3) was added, causing immediate precipitation ofcomplex. The precipitate was isolated by centrifugation,washed thoroughly with H2O followed by Et2O, andthen redissolved in EtOH and H2O. This procedure wasrepeated twice and the final orange-red product wasdried under vacuum. It was recrystallized from hotMeCN (12 cm3).MS (FAB): observed (calcd.) {relative abundance}

[assignment] m/z 1362 (1367) {10.3} [63CuANCA101-Ru]þ. 1H-n.m.r. (CD3CN): d 8.90 (d, 4H, J ¼ 5.8 Hz,H6;6

0), 8.51 (d, 4H, J ¼ 7.2 Hz, H3;30 ), 8.20–7.55 (m, 11H,

H4;40, H5;5

0and Ha;c;a0), 7.15–7.44 (m, 45H, AC6H5), 7.10

(t, 2H, Hb;b0). 13C-n.m.r. (CD3CN): d 158.01 (2¢), 157.43

(2), 154.57 (a), 154.19 (6¢), 153.06 (6), 138.83 (4¢), 138.72(4), 138.60 (c), 131.76 (C-2), 128.31 (C-3), 127.92 (C-4),127.80 (5¢), 127.61 (5), 127.0 (b), 125.50 (3), 125.12 (3¢).Found (calcd.) %: C 58.0 (57.9), H 4.0 (3.9), N 5.0 (5.0).M.p. (�C); 190(d). Colour: orange-red. Yield: 23%.Conductivity (X�1 cm2 mol�1): 244. I.r.: m(CN) 2080cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3 mol�1 cm�1)[assignment, entry], 436 (1.32) [dp(Ru) ! p(bpy) CT,A], 331 (1.34) [dp(Ru) ! p(bpy) CT, B], 285 (6.70)[p ! p(bpy) IL, C], 244 (sh) (4.94) [p ! p(bpy) IL, D],206 (7.41) [dp(Ru)! p(CN�) CT, E].

2,2¢-Bipyridine(triphenylphosphine)copper(I)(l-isocyano)pyridinebis(2,2¢-bipyridine)ruthenium(II)hexafluorophos-phate, [(bpy)(PPh3)Cu(NC)Ru(bpy)2(py)](PF6)2 (2)A suspension of [Cu(bpy)(PPh3)Cl] (0.258 g, 0.5 mmol)and [Ru(bpy)2(py)CN]PF6 (0.331 g, 0.5 mmol) in a 1:1mixture of H2O and EtOH (50 cm3) was refluxed for10 h. The resulting solution was cooled and filtered, anda concentrated aqueous solution of NH4PF6 (1 cm

3) wasadded to the filtrate. A precipitate immediately ap-peared; it was centrifuged, washed several times withH2O and Et2O and redissolved in EtOH and H2O. Thisprocedure was repeated twice and the final brown-redproduct was dried under vacuum. It was recrystallizedfrom hot MeCN (5 cm3).MS (FAB): observed (calcd.) {relative abundance}

[assignment] m/z 1001 (999) {23.8} [63CuANCA101-Ru]þ. 1H-n.m.r. (CD3CN): d 8.93–7.05 (br, m, 44H).13C-n.m.r. (CD3CN): d 158.60 (2), 158.31 (2¢), 157.63(2), 154.57 (a), 154.14 (6¢), 153.46 (6), 153.10 (6), 139.25(4), 139.03 (4¢), 138.82 (4), 138.61 (c), 132.76 (C-2),Scheme 1. M @ Cu or Ru.

669

128.83 (C-3), 128.51 (C-4), 127.86 (5¢), 127.57 (5), 127.03(b), 126.76 (3), 125.54 (3), 125.42 (3¢), 125.07 (5). Found(calcd.)%: C 52.2 (52.2), H 3.4 (3.4), N 8.68 (8.68). M.p.(�C) 235(d). Colour: brown-red. Yield: 29%. Conduc-tivity (X�1 cm2 mol�1): 239. I.r.: m(CN) 2095 cm�1.U.v.–vis.: kmax/nm (e � 10�4/dm3 mol�1 cm�1) [assign-ment, entry], 438 (1.17) [dp(Ru) ! p(bpy) CT, A], 336(1.42) [dp(Ru) ! p(bpy) CT, B], 285 (7.24) [p ! p

(bpy) IL, C], 244 (3.60) [p ! p(bpy) IL, D], 209 (6.31)[dp(Ru) ! p(CN�) CT, E].

1,10-Phenanthroline(triphenylphosphine)copper(I)(l-isocyano)pyridinebis-(2,2¢-bipyridine)ruthenium(II)hexafluorophosphate, [(phen)(PPh3) Cu(NC)Ru(bpy)2(py)] (PF6)2 (3)The above procedure for the 2,2¢-bipyridine analoguewas followed, but in place of [Cu(bpy)(PPh3)Cl] its 1,10-phenanthroline analogue was used.MS (FAB): observed (calcd.) {relative abundance} [as-

signment] m/z 1025 (1023) {19.7} [63CuANCA101Ru]þ.1H-n.m.r. (CD3CN): d 8.9–7.25 (br, m, 42H), 6.93 (t, 2H,Hb;b0

). Found (calcd.)%: C 51.2 (51.2), H 3.4 (3.4), N 8.5(8.5). M.p. (�C): 182(d). Colour: orange-red. Yield: 23%.Conductivity (X�1 cm2 mol�1): 238. I.r.: m(CN) 2095cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3 mol�1 cm�1)[assignment, entry], 439 (1.06) [dp(Ru) ! p(bpy) CT,A], 334 (1.34) [dp(Ru) ! p(bpy) CT, B], 286 (6.97)[p ! p(bpy) IL, C], 244 (3.43) [p ! p(bpy) IL,D].

Tris-(triphenylphosphine)copper(I)(l-isocyano)pyridine-bis-(1,10-phenanthroline)ruthenium(II)hexafluorophos-phate, [(PPh3)3Cu(NC)Ru(phen)2(py)](PF6)2 (4)The method was the same as (1), except [Ru(phen)2-(py)CN]PF6 was used in place of [Ru(bpy)2(py)CN]PF6.MS (FAB): observed (calcd.) {relative abundance}

[assignment] m/z 1705 (1705) {2.3} [63CuANCA101Ru]-(PF6)2, 1408 (1415) {29.7} [

63CuANCA101Ru]þ. 1H-n.m.r. (CD3CN); d 9.38 (d, 4H, J ¼ 5.2 Hz, H2,9), 8.63(d, 4H, J ¼ 7.0 Hz, H4,7), 8.25–7.70 (m, 11H, H5,6, H3,8and Ha;c;a0), 7.18–7.54 (m, 45H, –C6H5), 7.15 (t, 2H,Hb;b0

). 13C-n.m.r. (CD3CN): d 155.62 (a), 155.09 (2),154.86 (9), 154.55 (a,a¢), 141.04 (4), 140.78 (7), 138.65(c), 137.98(b,b0), 132.78 (C-2), 128.56 (C-3), 128.26 (C-4), 128.0 (b), 127.57 (5,6), 127.12 (3), 126.91 (8). Found(calcd.)%: C 60.0 (59.9), H 3.9 (3.9), N 5.0 (4.9). M.p.(�C): 180(d). Colour: mustard-brown. Yield: 24%. Con-ductivity (X�1 cm2 mol�1): 232. I.r.: m(CN) 2075 cm�1.U.v.–vis.: kmax/nm (e � 10�4/dm3 mol�1 cm�1) [assign-ment, entry], 415 (1.42) [dpðRuÞ ! pðphenÞ CT, F], 259(7.45) [p ! pðphenÞ IL, G], 223 (7.28) [p ! pðphenÞIL, H].

2,20-Bipyridine (triphenylphosphine)copper(I)(l-isocyano)pyridinebis(1,10-phenanthroline)hexafluorophosphate,[(bpy)(PPh3)Cu(NC)Ru(phen)2(py)](PF6)2 (5)The method was the same as (2), except [Ru(phen)2-(py)CN]PF6 was used in place of [Ru(bpy)2(py)CN]PF6.MS (FAB): observed (calcd.) {relative abundance}

[assignment] m/z 1050 (1047) {33.8} [63CuANCA101-

Ru]þ. 1H-n.m.r. (CD3CN): d 9.40–7.15 (br, m, 44H).13C-

n.m.r. (CD3CN): d 157.2 (2), 154.82 (a), 154.69 (2),154.56 (9), 154.43 (a,a0), 153.94 (6), 139.14 (4), 138.28 (7),137.85 (c), 137.58 (b,b0), 136.93 (4), 132.88 (C-2), 128.93(C-3), 128.50 (C-4), 128.23 (b), 128.07 (3), 127.37 (5,6),127.0 (3), 126.88 (8), 126.18 (5). Found (calcd.)%: C 50.0(50.0), H 3.3 (3.3), N 8.4 (8.4). M.p. (�C): 245(d). Colour:brown. Yield: 42%. Conductivity (X�1 cm2 mol�1): 229.I.r.: m(CN) 2085 cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3mol�1 cm�1) [assignment, entry], 425 (0.84) [dpðRuÞ !pðphenÞ CT, F], 264 (5.86) [p ! pðphenÞ IL, G], 224(5.94) [p ! pðphenÞ IL, H].

1,10-Phenanthroline (triphenylphosphine)copper(I)(l-iso-cyano)pyridinebis-(1,10-phenanthroline)ruthenium(II)hexafluorophosphate, [(phen)(PPh3)Cu(NC)Ru(phen)2(py)](PF6)2 (6)The preparation was the same as for (2), except[Ru(phen)2(py)CN]PF6 was used in place of [Ru(bpy)2-(py)CN]PF6.MS (FAB): observed (calcd.) {relative abundance} [as-

signment] m/z 1078 (1071) {42.0} [63CuANCA101Ru]þ.1H-n.m.r. (CD3CN): d 9.40–7.05 (br, m, 44H). M.p. (�C):211(d). Colour: brown. Yield: 26%. Conductivity(X�1 cm2 mol�1): 221. I.r.: m(CN) 2085 cm�1. U.v.–vis.:kmax/nm (e � 10�4/dm3 mol�1 cm�1) [assignment, entry],440 (0.83) [dpðRuÞ ! pðphenÞ CT, F], 266 (5.02)[p ! pðphenÞ IL,G], 223 (6.67) [p ! pðphenÞ IL,H].

Bis-{tris-(triphenylphosphine)copper(I)(l-isocyano)}bis-(2,20-bipyridine)ruthenium(II)hexafluorophosphate,[{(PPh3)3Cu(NC)}2Ru(bpy)2](PF6)2 (7)Same as (1), except [Ru(bpy)2(CN)2] was used in placeof [Ru(bpy)2(py)CN]PF6. The reaction was carried outin the dark. The brick-red superfine crystals of (7) wereobtained after dissolving the compound to 50 cm3 ofMeCN and leaving the solution to �20 �C for severaldays.1H-n.m.r. (DMSO-d6): d 9.30–7.10 (br, m, 63H).

13C-n.m.r. (DMSO-d6): 157.90 (2), 157.63 (2

0), 154.35 (6),153.81 (60), 139.0 (4), 138.53 (40), 131.50 (C-2), 128.21(C-3), 127.93 (C-4), 127.60 (5), 127.0 (50), 125.21 (3),125.0 (30). Found (calcd.)%: C 63.4 (63.5), H 4.3 (4.5), N3.4 (3.4). M.p. (�C): 282(d). Colour: brick-red. Yield:40%. Conductivity (X�1 cm2 mol�1): 224. I.r.: m(CN)2083, 2092 cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3mol�1 cm�1) [assignment, entry] 477 (0.75) [dpðRuÞ! pðbpyÞ CT, I], 331 (0.82) [dpðRuÞ ! pðbpyÞ CT, J],285 (5.27) [p ! pðbpyÞ IL, K], 243 (sh) (3.52) [p ! p

ðbpyÞ IL, L], 209 (7.45) [dpðRuÞ ! pðCN�Þ CT, M].

Bis-{2,20-bipyridine(triphenylphosphine)copper(I)(l-iso-cyano)}bis-(2,20-bipyridine)ruthenium(II)hexafluoropho-sphate, [{(bpy)(PPh3)Cu(NC)}2 Ru(bpy)2](PF6)2 (8)Same as (2), except [Ru(bpy)2(CN)2] was used in placeof [Ru(bpy)2(py)CN]PF6. The reaction was carried out inthe dark. The brick-red superfine crystals of (8) wereobtained after dissolving the compound to 60 cm3 ofMe-CN and leaving the solution to �20 �C for several days.

670

MS (FAB): observed (calcd.) {relative abundance}[assignment] m/z 1433 (1432) {11.0} [65CuANCA104-Ru]þ. 1H-n.m.r. (DMSO-d6): d 9.34–6.95 (br, m, 41H).13C-n.m.r. (DMSO-d6): 158.31 (2), 157.77 (2), 157.54(20), 155.50 (6), 154.05 (6), 153.81 (60), 139.25 (4), 139.0(4), 138.73 (40), 131.0 (C-2), 128.21 (C-3), 128.03 (C-4),127.55 (5), 127.0 (50), 126.47 (3), 125.13 (3), 124.92 (30),124.70 (5). Found (calcd.)%: C 55.0 (54.5), H 3.6 (3.6),N 8.2 (8.1). M.p. (�C): 230(d). Colour: brick-red. Yield56%. Conductivity (X�1 cm2 mol�1): 215. I.r.: m(CN)2083, 2095 cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3mol�1 cm�1) [assignment, entry], 467 (0.72) [dpðRuÞ !pðbpyÞ CT, I], 333 (0.75) [dpðRuÞ ! pðbpyÞ CT, J],291 (4.86) [p ! pðbpyÞ IL, K], 244 (sh) (3.18) [p !pðbpyÞ IL, L], 207 (7.48) [dpðRuÞ ! pðCN�Þ CT, M].

Bis-{1,10-phenanthroline(triphenylphosphine)copper(I)-(l-isocyano)}bis-2,20-bipyridine)ruthenium(II)hexafluo-rophosphate, [f (phen)(PPh3)Cu(NC)g 2Ru(bpy)2](PF6)2 (9)Same as (3), except [Ru(bpy)2(CN)2] was used in place of[Ru(bpy)2(py)CN]PF6. The reaction was carried out inthe dark. The brick-red superfine crystals of (9) wereobtained after dissolving the compound to 60 cm3 ofMe-CN and leaving the solution to �20 �C for several days.MS (FAB): observed (calcd.) {relative abundance}

[assignment] m/z 1480 (1480) {11.0} [65CuANCA104-Ru]þ. 1H-n.m.r. (DMSO-d6): d 9.30–7.0 (br, m, 41H).Found (calcd.)%: C 56.0 (55.7), H 3.5 (3.5), N 8.0 (7.9).M.p. (�C): 245(d). Colour: brick-red. Yield: 88%. Con-ductivity (X�1 cm2 mol�1): 217. I.r.: m(CN) 2083, 2093cm�1. U.v.–vis.: kmax/nm (e � 10�4/dm3 mol�1 cm�1)[assignment, entry] 469 (0.52) [dpðRuÞ ! pðbpyÞ CT, I],329 (0.63) [dpðRuÞ ! pðbpyÞ CT, J], 290 (4.04)[p ! pðbpyÞ IL, K], 246 (sh) (2.64) [p ! pðbpyÞ IL,L], 206 (7.01) [dpðRuÞ ! pðCN�Þ CT, M].

Results and discussion

The analytical data for the complexes are consistent withthe stoichiometries proposed. All of the complexes are1:2 electrolytes [13], and diamagnetic at room tempera-ture. The FAB mass spectrum molecular ion peakusually is of low intensity, however all the complexesshow an M–PF6 peak. Other fragments of lower abun-dance such as [Ru(L)2L

0]þ and [Cu(PPh3)(L)]þ, L – bpy,

phen and L0 – py, can be attributed to fragmentation ofthe bridged complexes. All of these fragments exhibitisotopic peaks and appear as bands. The variation in them/z values is mainly due to presence of the heavier atomisotopes of ruthenium (96–104), Cu (63, 65) and Cl (35,37) [6]. Free ligands and their fragments are also presentat lower m/z values in higher abundance.

I.r. spectra

The i.r. spectra in the CN stretching region (Figure 1)give support to the structure proposed for complexes

(1)–(9). In each case the m(CN) value is shifted tohigher energy by ca. 5–20 cm�1 compared to the parentcomplexes on bridging. This is indicative of coordina-tion by a Lewis acid. Rather than pulling electrondensity away from the group 8 metal through thecyanide bridge, which would decrease m(CN), a Lewisacid allows for cyanide nitrogen lone pair coordination(ACBN:!) [7]. In addition, copper(I) is an electron-richcentre, which is also bonded to triphenylphosphine, astrong r-donor and poor p-acceptor; and this probablyprevents large blue shifts of m(CN). The characteristicbands due to other ligands, namely, 2,20-bipyridine and1,10-phenanthroline, are found in their appropriatespectral regions.

N.m.r. spectra

The n.m.r. spectra were recorded in CD3CN (1)–(6)and in DMSO-d6 (7)–(9). The

1H-n.m.r. spectra ofcomplexes (1)–(9) show a similar pattern of resonancesfrom 6.80 to 9.40 p.p.m. (d) due to phenyl, bpy and phenprotons [6, 14]. The spectra become more complicatedwhen two molecules of PPh3 attached to copper arereplaced by bpy ligand or its analogue (Figure 2).Therefore, 13C-n.m.r. spectra were used to characterizethese complexes [15, 16]. The 13C-n.m.r. signal of thebridging cyanide group was not observed.

Fig. 1. The expanded FT-IR spectra of [Ru(bpy)2(CN)2] (—), (7)

(- - - - -) (8) (-�-�-�) and (9) (� � �) in the CN stretching frequency region.

671

U.v.–vis. spectra

All the electronic spectra were recorded in MeCNsolution at room temperature. The isocyano-bridgedcomplexes show five (complexes (1)–(3) and (7)–(9)) orthree (complexes (4)–(6)) bands. These bands aredenoted as A–E, F–H and I–M in order of increasingenergy in the experimental section.They are characteristicof [Ru-(bpy)2(py)CN]

þ [11], [Ru(phen)2(py)CN]þ [17]

and [Ru-(bpy)2(CN)2] [12] chromophores in MeCNsolution. A band in the 203–210 nm range is due to the[dp(Ru)! p(CN�)]MLCT transitions. Small blue shiftsare observed in bands A, F and I due to strong electron-withdrawing effect of [Cu(PPh3)(N–N)]

þ units on theparent complexes, [Ru(bpy)2(py)(CN)]

þ {kmax ¼ 461nm}, [Ru(phen)2(py)CN]

þ {kmax ¼ 456} nm, and [Ru-(bpy)2(CN)2] {kmax ¼ 494 nm}, and d ! p nature ofthe relevant transitions. This effect is entirely consistent

with observations in a number of closely related systemsin which other transition metal ions and metal com-plexes act as a Lewis acid [4, 18, 19]. In this context thedp(Ru) ! p(bpy) or dp(Ru) ! p(phen) transitionsact as spectator transitions for the ACBN: ! Cuinteractions. For [Ru(bpy)2(py)2]

2þ complexes, no sep-arate dp(Ru)! p(py) band was detected [11], althougha study of similar complexes assigned high energyshoulders on the MLCT bands as dp(Ru) ! p(py)transitions [20] or the absorption band at ca. 346 nm hascontributions from dp(Ru) ! p(bpy) and dp(Ru)! p(py) MLCT transitions [4]. The observation ofhigh energy bands as broad A and F, or appearance ofbands B as a shoulder on C in the complexes (1)–(3)may then be tentatively associated with such a dp (Ru)! p(py) transitions. Bands below 300 nm are assignedas p ! p intraligand transitions. The characteristicbands of [Cu(PPh3)3Cl], [Cu(bpy)(PPh3)Cl] and[Cu(phen)(PPh3)Cl], which appear at 260.5, 279.0 and267.0 nm respectively, are either merger or reside as ashoulder on bands C and D, band G and band K.

Electrochemical properties

The electrochemical data of complexes (3), (6) and (9)are reported in Table 1. The cyclic voltammograms(versus SCE in MeCN) of complexes (3) and (9) wererecorded at a scan rate of 100 and 200 mV sec�1. Thesecompounds show two reversible couples at ca. 0.40 and1.30 V, which are attributed to the CuI! CuII and RuII

! RuIII couples respectively. E1/2 (RuIII/RuII) appears

at 1.30 and 1.32 V for (3) and (9) respectively, asexpected for its higher charge with respect to parentcomplexes ([Ru(bpy)2(py)CN])PF6 [21], E1/2: 1.04,�1.48 and �1.72 V; Ru(bpy)2(CN)2 [22], E1/2: 0.89,�1.60 V), but E1/2: (CuII/CuI) appears at lower valuesthan parent complex ([Cu(phen)(PPh3)2]

þ [23], E1/2:þ0.68 V, no cathodic wave was observed). However,during subsequent scan (200 mV s�1) the anodic wavesharpened further. An additional reversible couple(RuII–RuIII) appears at 1.03 V in complex (3) and1.06 V in complex (9). The most likely explanation ofthese results is that ligand dissociation occurs, especiallyafter oxidation to copper(II), where the assumption is

Fig. 2. The structure of [(bpy)(PPh3)Cu(NC)Ru(bpy)2(py)]2þ.

Table 1. Luminescence and electrochemical properties at 298 K

No. Luminescence data Electrochemical data Ligand

based (V)

Excited state properties

kem (nm) s (ns) /s E1/2(RuII–RuIII)

E1/2(CuI–CuII)

Ered(CuI–Cu0)

E00 *E1/2(RuII–RuIII)

*E1/2(CuI–CuII)

*Ered(CuI–Cu0)

(3) 504.0a 1.485 1.30 0.40 �1.02 �0.41600.2b 0.118 7.7 · 10�4 1.03 �1.33 2.06 �1.03 �1.66 1.04

�1.54(6) 505.0a 1.636 1.06 0.35 �1.00 �0.40

628.0b 0.139 3.4 · 10�3 �1.28 1.97 �0.91 �1.62 0.97

(9) 550.0a 2.515 1.32 0.39 �1.03 �0.60636.0b 0.183 2.4 · 10�3 1.06 �1.56 1.95 �0.91 �1.56 0.92

aDenotes metal-centred or metal-to-ligand charge-transfer excited state arising from Cu sub-units;bDenotes Ru ! L MLCT excited states.

672

that either a solvent molecule or PF�6 =ClO

�4 fill the

coordination sites. McMillin and co-workers observed asimilar effect in their studies involving the 2,9-dineopen-tyl-1,10-phenanthroline ligand with mononuclear cop-per(I) complexes [24]. Moreover these complexesundergo a limited amount of metathesis, presumablydue to CuANCA bridge cleavage by the solventproducing non-separable products, probably, [Ru-(bpy)2(PPh3)CN]

þ or [Ru(bpy)2(MeCN)]2þ as a pre-

dominant product in addition to the mononuclearcopper complex. This can be seen by changes in theabsorption spectra in the presence of supporting elec-trolyte, tetra-n-butylammonium hexafluorophosphate(TBAH) or tetra-n-butylammonium perchlorate (TBAP)(recorded at regular time intervals (120 s) for 50 min.[25] (Figure 3). An additional reversible couple (RuII–RuIII) observed at 1.03 V for complex (3) and 1.06 Vfor complex (9), is consistent with the reported litera-ture value of similar complexes [20]. Complex (6)exhibits similar behaviour. The difference in potentialsof RuII–RuIII couples of complexes (3) and (6) can beassigned to better stabilization of phenanthroline com-pared to bipyridine, a consequence of its stronger p-acidcharacter [24]. Such a mechanism depends on the abilityof the cyanide ligand to act as a conducting bridge andmoreover, it involves in intramolecular electron transfer.There is one irreversible reductive wave in the range�1.00 – �1.45 V, attributable to the process CuI ! Cu0

[6, 23]. Comparison of the electrochemical data withprevious investigations carried out on these types ofcomplexes show that Cuþ is oxidized at a potential lesspositive than Ru2þ and copper(I) interacts with ruthen-ium(II) through the isocyanide bridge.

Luminescence properties

Room temperature emission band maxima, life timesand luminescence quantum yields are reported inTable 1. Representative luminescence spectra are shownin Figures 4–6. Two emission maxima are observed inthe 458–550 and 600–636 nm ranges. The assignment of

these electronic states poses an interesting problem. Theabsorption spectra of isocyano-bridged complexes are

Fig. 3. Electronic spectra of complex (9) in presence of supporting

electrolyte, TBAP, in MeCN at regular time interval of 120 s for

50 min.

Fig. 4. Luminescence spectra of complexes (1)–(3).

Fig. 5. Luminescence spectra of complexes (4)–(6).

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dominated by Ru-polypyridine units and the contribu-tion from [Cu(PPh3)(NAN)]þ is negligible. Complexeshaving general formula [Cu(PPh3)(NAN)]þ are knownto exhibit emission properties both at low and roomtemperature. McMillin and co-workers [23, 26] observedemission band maxima at 608 and 575 nm in [Cu-(PPh3)2(phen)]

þ, and 620 nm in [Cu(PPh3)2(bpy)]þ upon

excitation at ca. 360 nm wavelength, and these wereassigned as metal-to-ligand charge-transfer transitions.However, a metal-centered d ! s orbital transition wasnot ruled out. Similar emission behaviour was alsoobserved in polynuclear copper(I) complexes. In thetrinuclear copper(I) complex [Cu3(dpmp)(MeCN)2-(l-Cl)2]

þ, where dpmp is bis(diphenylphosphinometh-yl)methyl phosphine, an emission at 560 nm wasassigned as a metal-centered 3d94s1 ! 3d10 transition[27]. In the binuclear copper(I) complex [PPh3Cu2Cl2-(py)] a similar type of assignment was made by Zink andco-workers [28]. We have seen in the cyano-bridgedcomplexes of copper(I) and ruthenium(II) [6] that inaddition to ruthenium to bpy MLCT emission bands,copper(I) also exhibits emission in the 525–545 nmrange, tentatively assigned as MLCT/MC transitions. Itis interesting that emission energies of compounds (1)–(9) are similar in comparison to earlier reportedCuACNARu systems [6], despite the difference in modeof bridging (–NC–) in complexes. Free triphenylphos-phine, bpy and phen exhibit very weak emissions around350–450 nm MeCN at room temperature. The 458–550nm emissions in (1)–(9) cannot come from ligand-

centred excited states because of the large red shift inenergy. We therefore assign this emission to a coppermetal-centred 3d9s1 ! d10s0 transition and metal-to-ligand charge-transfer transition.The emission spectra of compounds (1)–(9) have

maxima in the 600–636 nm range, assigned as Ru(II)!L (L – bpy, phen) MLCT transitions [29]. These bandsare blue shifted with respect to the parent complexes,confirming the electron-accepting ability of the Lewisacid, [Cu(PPh3)(NAN)]þ, as also indicated by otherspectroscopic techniques. It is apparent that C-bondedcyanide acts as a typical strong-field p-acid ligand. Incontrast, the N-bonded cyanide behaves like a mediumfield, mainly r-donor ligand. These complexes undergobi-exponential decay showing life times at 0.118 ns(A1 ¼ 0:95) and 1.485 ns (A2 ¼ 0:05) for complex (3),0.139 ns (A1 ¼ 0:95) and 1.636 ns (A2 ¼ 0:05) for com-plex (6) and 0.183 ns (A1 ¼ 0:67) and 2.515 ns(A2 ¼ 0:33) for complex (9). Figure 7 shows the bi-exponential decay profile of a representative complex(9). This confirms the presence of the two species in theexcited state, namely the copper and ruthenium sub-units. A comparison of the absorption and emissionspectra of these complexes shows that bridging betweenruthenium moieties alters the relative energy ordering inthe original chromophores to a sufficient extent that anew high energy state is present in the system, i.e. MCexcited state. Thus the lowest d ! p excited state ineach complex is not localized on ruthenium, but alsoinvolves the copper centre and appears as a smallshoulder on emission band maxima of MC excited state.It follows that energy transfer occurs from ruthenium tocopper sub-unit due to ligand dissociation and thesolvent (MeCN) takes the place of the copper coordi-nation site. On account of the increase in the formaloxidation state, either of the metal centres will be astronger Lewis acid in the CT excited state. The excitedstate species can be better oxidants or reductants thancorresponding ground state, and the ground and excitedstate potentials are related to each other via equations;

EðMþ=MÞ ¼ EðMþ=MÞ þ E00ðM–MÞEðM=M�Þ ¼ EðM�=MÞ � E00ðM–MÞ

Fig. 6. Luminescence spectra of complexes (7)–(9).

Fig. 7. Fluorescence decay curve of complex (9) at emission wave-

length 636.0 nm.

674

where E00(M–M*) is the one electron potential corre-sponding to the zero–zero spectroscopic energy of theemitting state, E(Mþ/M*) and E(M�/M) are excitedstate oxidation and reduction potentials respectively,and E(Mþ/M*) and E(M�/M) refer to ground stateoxidation and reduction potentials respectively. InTable 1, zero–zero energies have been used to calculatethe excited state redox potentials, and the *E1/2 (M

þ/M*) values indicate that compound (3), (6) and (9)should behave as very powerful excited state reductants[23]. The CuANCARu system is a more powerfulexcited state reductant than the CuACNARu system [6].

Acknowledgements

The authors are grateful to Prof T.K. Chandrashekar,Department ofChemistry, Indian Institute ofTechnology(Kanpur, India) for the use of a luminescence spectro-photometer and a cyclic voltammetric instrument.

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