Probing d8?d8 Interactions in Luminescent Mono- and Binuclear Cyclometalated Platinum(II) Complexes of 6-Phenyl-2,2‘-bipyridines

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Probing d

8

-d

8

_{Interactions in Luminescent Mono- and Binuclear Cyclometalated}

Platinum(II) Complexes of 6-Phenyl-2,2

′

-bipyridines

Siu-Wai Lai,† _{Michael Chi-Wang Chan,}†_{Tsz-Chun Cheung,}† _{Shie-Ming Peng,}‡ _and Chi-Ming Che*,†

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, and Department of Chemistry, National Taiwan University, Taipei, Taiwan

ReceiVed February 26, 1999

A series of luminescent mono- and binuclear cyclometalated platinum(II) complexes, namely [Pt(L1-6_)Cl]

(1a-6a; HL1-6 ) 4-(aryl)-6-phenyl-2,2′_{-bipyridine; aryl ) H (1), phenyl (2), 4-chlorophenyl (3), 4-tolyl (4),} 4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)), [Pt(L1_)E]+

(E ) py (7), PPh3(8)), [Pt2(L1-6_)2(µ-dppm)]2+ (1b-6b, dppm ) bis(diphenylphosphino)methane), [Pt2(L1_)2(µ-pz)]+_{(9, Hpz ) pyrazole), and [Pt2(L}1_)2(µ-dppC

n)]2+ (dppCn) bis(diphenylphosphino)propane (10, n ) 3) and -pentane (11, n ) 5)), were synthesized in order to examine fluid- and solid-state oligomeric d8-d8 _{and ligand-ligand interactions. The molecular structures of}

4b(ClO4)2and 9(PF6) reveal intramolecular Pt-Pt distances of 3.245(1) and 3.612(2) Å, respectively. While minimal metal-metal communication is expected for 9, weak π-π interactions are possible. All complexes described in this work are emissive in fluid solution at room temperature. Negligible changes in emission energy are detected by incorporating different aryl substituents into the 4-position of 6-phenyl-2,2′-bipyridine, and this indicates little electronic delocalization between them. Self-quenching of the3_{MLCT emission by the mononuclear} derivatives are observed in CH2Cl2at 298 K, and a red shift in the emission energy is exhibited by complex 7 in acetonitrile at 77 K. The fluid emissions of theµ-dppm species 1b-6b at λmax652-662 nm appear at substantially lower energies than their mononuclear counterparts and show dramatic solvatochromic effects. These emissions are ascribed to3_{[dσ*, π*] excited states. In contrast, the emission of 10 and 11, bearing long bridging diphosphine} ligands, are attributed to3_{MLCT states of non-interacting [Pt(L}1_{)] moieties. Significantly, the luminescence of} theµ-pyrazolate complex 9 displays transitional features which are reminiscent of both3_{[dσ*, π*] and}3_MLCT excited states. Hence a relationship is observed between emission energy, the nature of the lowest energy excited state, and metal-metal interactions. The excited-state redox potential [E(*Pt22+_/Pt2+_{)] of 1b has been estimated} by electrochemical studies (1.61 V vs NHE) and by quenching experiments with aromatic hydrocarbons (1.63 V vs NHE).

Introduction

Luminescent coordinatively unsaturated metal complexes are appealing from a photochemical perspective. While saturated congeners such as [Ru(bpy)3]2+ _{(bpy ) 2,2}′_{-bipyridine) are} restricted to outer-sphere interactions with substrates, these chromophores allow inner-sphere electron-transfer reactions, and applications for chemical sensing,1-6_{solar energy conversion,} and photocatalysis7-9_{have been developed. Investigations into} square planar d8_{platinum(II) compounds have been prominent}

since this class of molecules can mediate excited-state atom transfer reactions and bond activation. In particular, the prolific excited-state chemistry of the binuclear derivative [Pt2(µ-P2O5H2)4]4- _{has been demonstrated.}10 _{The triplet (dσ*, pσ)} excited state, which is a manifestation of the d8_-d8_interaction between the diplatinum centers, is capable of C-H and C-halogen bond cleavage and electron-transfer reactions.

The propensity for square planar d8_{complexes to engage in} metal-metal interactions and form extended linear-chain solid-state structures has been extensively studied.11_{Unusual colors} and strong emission, as well as highly anisotropic properties, are often the result of such stacking interactions. Platinum(II)

* Corresponding author. Fax: (852) 2857 1586. E-mail: cmche@hkucc. hku.hk.

†_{The University of Hong Kong.} ‡_{National Taiwan University.}

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R-diimine complexes display a variety of low-energy excited states,12-14 _{including R-diimine intraligand (IL,} _{π f π*)} transitions of both monomer and dimer and metal-to-ligand charge transfer (MLCT). In addition, when two platinum(II) R-diimine units are in close proximity so as to allow metal-metal and ligand-ligand (π-π) contacts, a low-energy photo-luminescence which is red-shifted from the 3_MLCT emis-sion of mononuclear species is typically observed. The elec-tronic excited-state associated with this emission is denoted as 3_{[dσ*, π*] (the metal-metal-to-ligand charge transfer (MMLCT)} notation is also used in the literature, see Figure 112a_). How-ever, it should be noted that the Pt(II)-Pt(II) interaction in the ground state is considerably weaker than a normal Pt-Pt single bond. Although these R-diimine compounds exhibit interesting photophysical properties, they are usually weak emitters or even non-emissive in fluid solution. The tuning of the excited-state properties of the related Pt(II) diimine dithio-late derivatives have been documented.15 _{In addition, d}8_-d8 and π-π interactions are invoked in oligomerization16-19 and excimer formation20-22 _{of mononuclear platinum(II)} spe-cies in solution, which leads to changes in their photophysical behavior.

Square planar d8_{complexes are proposed to be unstable with} respect to a D2d distortion, which is likely to result in non-radiative decay. Many researchers have therefore diverted their attention to the 2,2′:6′,2′′-terpyridine (tpy) ligand which shows strong preference for planar geometry.16-19,22,23_{Several binuclear} d8_-d8_{complexes of the type [Pt2(tpy)2(µ-L)]}n+_{(L ) bidentate} ligand) have been spectroscopically characterized to model

intermolecular interactions in Pt(II) polypyridine species.24-26 We became attracted to Pt(II) derivatives bearing cyclometalated 6-phenyl-2,2′-bipyridine (L1_{) and related ligands.}3,27-30_Earlier studies on bidentate analogues employing C-deprotonated 2-phenylpyridine and 2-(2′-thienyl)pyridine revealed low-lying metal-to-ligand charge transfer (MLCT) states with interesting photochemical properties.31_{We anticipated that the tridentate} ligand L1_{, which favors planar geometry upon cyclometalation,} would discourage a D2ddistortion, while the extendedπ system within L1_{and the strongly}_{σ-donating carbanion would increase} the energy difference between the ligand field (d-d) and the MLCT states.

Herein is described the preparation and spectroscopic proper-ties of a series of mono- and binuclear cyclometalated platinum-(II) complexes derived from 6-phenyl-2,2′-bipyridine. They are photoluminescent at room temperature in fluid solution. The binuclear derivatives are envisaged as models for investigating the photophysics and solid-state structures of cyclometalated Pt(II) oligomers, and our objective was to characterize the [dσ*, π*] excited state and d8_-d8_{interactions in detail. By} systemati-cally varying (1) the length of the bridging bidentate ligand and (2) the electronic nature of substituents on 6-phenyl-2,2′ -bipyridine, we set out to modify the degree of metal-metal andπ-π interactions and to monitor the effects upon the nature of the excited states.

Experimental Section

General Procedures. K2PtCl4 (Strem),

bis(diphenylphosphino)-methane, -propane, and -pentane (dppm, dppC3, and dppC5, respectively,

Aldrich), pyrazole (Hpz), and pyridine (py, Aldrich) were used as received. HL1-6_{(4-(aryl)-6-phenyl-2,2}′_{-bipyridine; aryl ) H (1), phenyl}

(2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)) were prepared by literature methods.32_Syntheses

of [Pt(L1_{)Cl] (1a), [Pt}

2(L1)2(µ-dppm)](ClO4)2(1b(ClO4)2), and [Pt(L1

)-PPh3]ClO4 (8(ClO4)) have been described previously.29 (Caution! Perchlorate salts are potentially explosiVe and should be handled with care and in small amounts.) Dichloromethane for photophysical studies

was washed with concentrated sulfuric acid, 10% sodium hydrogen carbonate, and water, dried by calcium chloride, and distilled over calcium hydride. Acetonitrile for photophysics was distilled over potassium permanganate and calcium hydride. The other solvents used were of analytical grade.

Physical Measurements and Instrumentation. Fast atom

bombard-ment (FAB) mass spectra were obtained on a Finnigan Mat 95 mass spectrometer. Elemental analyses were performed by Butterworth Laboratory, Teddington, U.K.1_{H (in MHz, 300 or 500),}13_{C (126),}31_P

(202), and195_{Pt (107) NMR measurements were performed on a Bruker}

(12) (a) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446. (b) Houlding, V. H.; Miskowski, V. M. Coord. Chem. ReV. 1991, 111, 145. (c) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang, Y.

Inorg. Chem. 1993, 32, 2518.

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1989, 28, 3081.

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1998, 37, 3588. (b) Zheng, G. Y.; Rillema, D. P. Inorg. Chem. 1998, 37, 1392.

(15) Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Geiger, D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125.

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Chem. Soc. 1976, 98, 6159.

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1991, 1077.

(22) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591.

(23) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,

33, 722.

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Chem Commun. 1992, 1369.

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Chem. 1990, 29, 918.

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1993, 115, 11245.

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132, 87.

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Soc., Dalton Trans. 1996, 1645.

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Commun. 1998, 2295. (b) Lai, S. W.; Chan, M. C. W.; Peng, S. M.;

Che, C. M. Angew. Chem., Int. Ed. 1999, 38, 669.

(31) (a) Chassot, L.; Mu¨ller, E.; von Zelewsky, A. Inorg. Chem. 1984, 23, 4249. (b) Maestri, M.; Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet, P.; von Zelewsky, A. Chem. Phys. Lett. 1985, 122, 375. (c) Sandrini, D.; Maestri, M.; Balzani, V.; Chassot, L.; von Zelewsky, A. J. Am.

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(32) Kro¨hnke, F. Synthesis 1976, 1.

Figure 1. Schematic molecular orbital diagram illustrating d8_-d8_and

π-π interactions in binuclear platinum(II) polypyridine complexes.

d-d Interactions in Platinum(II) Complexes Inorganic Chemistry, Vol. 38, No. 18, 1999 4047

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DRX 300 or 500 FT-NMR spectrometer with TMS (1_{H and}13_{C), H} 3

-PO4(31P), and H2PtCl6(195Pt) as references. UV-vis absorption spectra

were obtained on a Perkin-Elmer Lambda 19 UV-visible spectropho-tometer.

Emission and Lifetime Measurements. Steady-state emission

spectra were recorded on a SPEX 1681 Fluorolog-2 series F111AI spectrophotometer. Low-temperature (77 K) emission spectra for glasses and solid-state samples were recorded in 5-mm diameter quartz tubes, which were placed in a liquid-nitrogen Dewar equipped with quartz windows. The emission spectra were corrected for monochromator and photomultiplier efficiency and for xenon lamp stability.

Sample and standard solutions were degassed with at least three freeze-pump-thaw cycles. The emission quantum yield was measured by the method of Demas and Crosby33 _{with [Ru(bpy)}

3](PF6)2 in

degassed acetonitrile as the standard (Φr) 0.062).

Emission lifetimes and flash-photolysis measurements were per-formed with a Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse output 355 nm, 8 ns). The emission signals were detected by a Hamamatsu R928 photomultiplier tube and recorded on a Tektronix model 2430 digital oscilloscope. Error limits are estimated: λ ((1 nm); τ ((10%); φ ((10%).

Syntheses. [Pt(L2-6_{)Cl] (2a-6a; HL}2-6_{) 4-(aryl)-6-phenyl-2,2}′ -bipyridine; aryl ) phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)). A modification

of Constable’s method was used.34_{A mixture of K}

2PtCl4(0.20 g, 0.48

mmol) and L2-6_{(except L}3_{, 0.48 mmol) in CH}

3CN/H2O (15/15 mL)

was refluxed for 18 h to give a deep red solution, which was evaporated to dryness. The product was extracted with dichloromethane, and the volume of extract was reduced to∼5 mL. Addition of diethyl ether yielded an orange solid which was recrystallized by vapor diffusion of diethyl ether into an acetonitrile solution to afford a reddish-orange crystalline solid.

2a. Yield: 0.22 g, 85%. MS (+ve FAB): m/z 538 (M+). Anal. Calcd for C22H15N2ClPt: C, 49.12; H, 2.81; N, 5.21. Found: C, 48.98; H, 2.78; N, 5.16.1_{H NMR (DMSO-d} 6, 300 MHz): δ 7.06-7.18 (m, 2H), 7.49-7.63 (m, 4H), 7.82 (d, 1H, J ) 6.2 Hz), 7.92 (t, 1H, J ) 6.4 Hz), 8.11-8.14 (m, 2H), 8.27 (s, 1H), 8.38 (t, 1H, J ) 8.5 Hz), 8.53 (s, 1H), 8.76 (d, 1H, J ) 8.1 Hz), 8.91 (d, 1H, J ) 4.4 Hz).13_C{1_H} NMR (DMSO-d6): δ 116.4, 117.0, 123.7, 124.1, 125.1, 127.5, 128.1, 129.0, 130.1, 130.2, 134.1, 136.2, 140.3, 142.3, 146.9, 148.0, 150.5, 154.5, 156.7, 165.3.

3a. A mixture of K2PtCl4(0.20 g, 0.48 mmol) and L3(0.17 g, 0.49

mmol) in H2O/CH3CN (10/40 mL) was refluxed for 30 h. An orange

suspension was formed, and the reaction mixture was evaporated to dryness. The solid residue was extracted using N,N-dimethylformamide (DMF) (25 mL× 2), and the volume of the extract was reduced to ∼5 mL. An orange precipitate appeared upon addition of diethyl ether and was filtered and washed with H2O and diethyl ether. Recrystallization

by vapor diffusion of diethyl ether into a DMF solution afforded reddish-orange crystals. Yield: 0.19 g, 69%. MS (+ve FAB): m/z 572 (M+). Anal. Calcd for C22H14N2Cl2Pt: C, 46.17; H, 2.47; N, 4.89.

Found: C, 46.10; H, 2.40; N, 4.78.1_{H NMR (DMSO-d} 6, 300 MHz): δ 7.10-7.16 (m, 2H), 7.51 (d, 1H, J ) 7.0 Hz), 7.69 (d, 2H, J ) 7.9 Hz), 7.83 (d, 1H, J ) 7.6 Hz), 7.93 (t, 1H, J ) 6.5 Hz), 8.18 (d, 2H, J ) 8.0 Hz), 8.30 (s, 1H), 8.39 (t, 1H, J ) 7.7 Hz), 8.54 (s, 1H), 8.76 (d, 1H, J ) 8.0 Hz), 8.92 (d, 1H, J ) 4.9 Hz).13_C{1_H}_NMR (DMSO-d6): δ 115.6, 116.4, 116.9, 123.8, 124.2, 125.3, 128.3, 129.1, 129.3, 130.3, 134.2, 140.4, 142.3, 146.2, 148.3, 149.2, 149.7, 154.5, 156.7, 165.8.

4a. Yield: 0.23 g, 86%. MS (+ve FAB): m/z 552 (M+). Anal. Calcd for C23H17N2PtCl: C, 50.05; H, 3.10; N, 5.08. Found: C, 49.78; H, 3.02; N, 4.96.1_{H NMR (DMSO-d} 6, 300 MHz): δ 2.41 (s, 3H, Me), 7.08-7.19 (m, 2H), 7.42 (d, 2H, J ) 7.9 Hz), 7.52 (d, 1H, J ) 7.2 Hz), 7.84 (d, 1H, J ) 6.6 Hz), 7.94 (t, 1H, J ) 6.4 Hz), 8.07 (d, 2H, J ) 8.0 Hz), 8.28 (s, 1H), 8.39 (t, 1H, J ) 7.7 Hz), 8.53 (s, 1H), 8.78 (d, 1H, J ) 7.9 Hz), 8.93 (d, 1H, J ) 5.2 Hz).13_C{1_H}_NMR (DMSO-d6): δ 20.8 (Me), 115.9, 116.5, 123.8, 124.1, 125.1, 127.3, 128.2, 129.7, 130.2, 133.2, 134.2, 140.3, 140. 4, 142.3, 147.0, 148.1, 150.4, 154.5, 156.8, 165.3.

5a. Yield: 0.21 g, 77%. MS (+ve FAB): m/z 568 (M+). Anal. Calcd for C23H17N2OClPt: C, 48.64; H, 3.02; N, 4.93. Found: C, 48.85; H,

2.90; N, 4.87.1_{H NMR (DMSO-d} 6, 300 MHz): δ 3.88 (s, 3H, OMe), 7.15 (d, 2H, J ) 6.6 Hz), 7.50-7.62 (m, 2H), 7.83 (d, 1H, J ) 7.6 Hz), 7.92 (t, 1H, J ) 6.4 Hz), 8.01 (d, 1H, J ) 8.7 Hz), 8.14 (d, 2H, J ) 8.7 Hz), 8.25 (s, 1H), 8.35-8.40 (m, 1H), 8.50 (s, 1H), 8.76 (d, 1H, J ) 8.0 Hz), 8.92 (d, 1H, J ) 5.0 Hz).13_C{1_H}_NMR (DMSO-d6): δ 56.2 (OMe), 115.4, 115.5, 118.9, 124.6, 128.0, 129.1, 129.4, 129.6, 130.0, 130.1, 141.1, 143.2, 148.0, 150.2, 150.9, 155.2, 157.7, 161.4, 162.1, 166.0.

6a. Yield: 0.21 g, 69%. MS (+ve FAB): m/z 628 (M+). Anal. Calcd for C25H21N2O3ClPt: C, 47.81; H, 3.37; N, 4.46. Found: C, 47.90; H, 3.25; N, 4.29.1_{H NMR (DMSO-d} 6, 300 MHz): δ 3.76 (s, 3H, p-OMe), 3.98 (s, 6H, m-OMe), 7.11-7.17 (m, 2H), 7.34 (s, 2H), 7.51 (d, 1H, J ) 6.7 Hz), 7.85 (d, 1H, J ) 7.2 Hz), 7.92-7.96 (m, 1H), 8.23 (s, 1H), 8.40-8.44 (m, 2H), 8.74 (d, 1H, J ) 7.6 Hz), 8.92-8.94 (m, 1H). 13_C{1_H}_{NMR (DMSO-d} 6): δ 55.4 (m-OMe), 59.1 (p-OMe), 115.5, 116.0, 116.9, 122.8, 123.2, 124.3, 127.3, 129.3, 130.8, 132.6, 133.2, 138.8, 139.4, 145.9, 147.1, 149.8, 152.4, 153.3, 155.8, 164.2. [Pt(L1_)py]ClO 4, 7(ClO4). A mixture of [Pt(L1)Cl] (0.25 g, 0.54

mmol) and excess pyridine (0.20 g, 2.70 mmol) in CH3CN/CH3OH

(20/20 mL) was stirred for 3 h at room temperature. Excess LiClO4

(0.2 g) was added to the resultant mixture, which was stirred for 5 h and then filtered and evaporated to∼5 mL. Addition of diethyl ether yielded a yellow-brown solid which was filtered and washed with diethyl ether. Recrystallization by vapor diffusion of diethyl ether into an acetonitrile solution yielded 0.28 g (86%) of yellow crystals. MS (+ve FAB): m/z 505 (M+). Anal. Calcd for C21H16N3O4ClPt: C, 41.70;

H, 2.67; N, 6.95. Found: C, 41.95; H, 2.55; N, 6.87.1_{H NMR} (DMSO-d6, 300 MHz): δ 6.28 (d, 1H, J ) 7.5 Hz), 7.03-7.15 (m, 2H), 7.70-7.85 (m, 4H), 8.07 (d, 2H, J ) 6.9 Hz), 8.15-8.43 (m, 4H), 8.58 (d, 1H, J ) 7.8 Hz), 9.09 (d, 2H, J ) 5.1 Hz).13_C{1_H}_NMR (DMSO-d6): δ 120.3, 124.2, 125.0, 125.6, 126.2, 128.1, 129.2, 131.4, 132.4, 140.2, 141.4, 141.7, 142.3, 147.5, 149.4, 153.4, 155.0, 156.6, 165.5.

[Pt2(L2-6)2(µ-dppm)](ClO4)2, 2b-6b(ClO4)2. A mixture of

[Pt-(L2-6_{)Cl] (except L}3_{, 0.32 mmol) and dppm (0.06 g, 0.16 mmol) in}

CH3CN/CH3OH (15/15 mL) was stirred for 12 h under a nitrogen

atmosphere. The resultant solution was filtered and evaporated to∼5 mL. Addition of excess aqueous LiClO4afforded a bright orange solid,

which was filtered and washed with water and diethyl ether. Recrys-tallization by vapor diffusion of diethyl ether into an acetonitrile solution yielded an orangish-red crystalline solid.

2b(ClO4)2. Yield: 0.20 g, 79%. MS (+ve FAB): m/z 1489 (M++

ClO4), 1389 (M+). Anal. Calcd for C69H52N4O8Pt2Cl2P2: C, 52.18; H,

3.30; N, 3.53. Found: C, 52.08; H, 3.27; N, 3.48.1_{H NMR} (DMSO-d6, 300 MHz): δ 5.26 (broad t, 2H,2J(PH) ) 12 Hz, PCH2P), 6.16 (s, 2H), 6.40-6.45 (m, 2H), 6.59-6.69 (m, 6H), 7.36-7.60 (m, 20H), 7.76-7.79 (m, 8H), 7.97-8.04 (m, 4H), 8.23 (s, 2H), 8.41-8.52 (m, 6H).13_C{1_H}_{NMR (DMSO-d} 6): δ 19.4 (t,1J(PC) ) 30.1 Hz, PCH2P), 116.7, 117.0, 124.4-134.9, 137.5, 140.0, 146.7, 150.7, 153.2, 153.4, 156.4, 162.3.31_P{1_H}_{NMR (CD} 3CN): δ 19.36 (1J(PtP) ) 4114 Hz, 3_{J(PtP) ) 87 Hz).}195_{Pt NMR (CD} 3CN): δ -4095 (d,1J(PPt) ) 4111 Hz).

3b(ClO4)2. Dppm (0.08 g, 0.20 mmol) in CH3CN (10 mL) was added

dropwise to a solution of [Pt(L3_{)Cl] (0.23 g, 0.40 mmol) in CH} 3CN/

CH2Cl2(10/10 mL) and stirred for 24 h under a nitrogen atmosphere

to afford a clear red-orange solution. After addition of methanolic LiClO4(0.2 g), the solution was further stirred for 2 h and then reduced

to∼5 mL. Addition of diethyl ether gave a red-orange solid which was filtered and washed with water and diethyl ether. Recrystallization by vapor diffusion of diethyl ether into an acetonitrile solution yielded an orangish-red crystalline solid. Yield: 0.23 g, 69%. MS (+ve FAB): m/z 1558 (M++ ClO4), 1458 (M+). Anal. Calcd for C69H50N4O8

-Pt2Cl4P2: C, 50.01; H, 3.04; N, 3.38. Found: C, 49.95; H, 2.95; N, 3.32.1_{H NMR (DMSO-d} 6, 300 MHz): δ 5.22 (broad t, 2H,2J(PH) ) 12 Hz, PCH2P), 6.23-6.39 (m, 4H), 6.54-6.73 (m, 6H), 7.40-7.61 (m, 18H), 7.78-7.87 (m, 8H), 7.99-8.04 (m, 4H), 8.28-8.37 (m, 6H), 8.51 (d, 2H, J ) 8.1 Hz).13_C{1_H}_{NMR (DMSO-d} 6): δ 20.3 (t,1J(PC)

(33) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (34) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.

Chem. Soc., Chem. Commun. 1990, 513.

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) 32.1 Hz, PCH2P), 116.3, 116.9, 124.4-139.9, 146.6, 150.9, 151.7, 153.3, 156.5, 162.4.31_P{1_H}_{NMR (CD} 3CN): δ 19.31 (1J(PtP) ) 4124 Hz,3_{J(PtP) ) 81 Hz).}195_{Pt NMR (CD} 3CN): δ -4087 (d,1J(PPt) ) 4129 Hz).

3.49; N, 3.47. Found: C, 52.70; H, 3.42; N, 3.45.1_{H NMR (CD} 3CN, 500 MHz): δ 2.42 (s, 6H, Me), 4.85 (broad t, 2H,2_{J(PH) ) 13 Hz,} PCH2P), 6.22 (s, 2H), 6.41 (t, 2H, J ) 6.9 Hz), 6.56 (t, 2H, J ) 6.1 Hz), 6.62 (t, 2H, J ) 7.3 Hz), 6.72-6.73 (m, 2H), 7.08 (d, 2H, J ) 7.1 Hz), 7.18 (d, 4H, J ) 7.8 Hz), 7.41-7.57 (m, 16H), 7.59 (s, 2H), 7.70-7.87 (m, 8H), 8.02 (d, 2H, J ) 7.8 Hz), 8.25-8.33 (broad s, 4H).13_C{1_H}_{NMR (CD} 3CN): δ 21.3 (t,1J(PC) ) 31.4 Hz, PCH2P), 21.5 (Me), 117.5, 117.7, 125.0-133.3, 139.1, 141.0, 142.9, 147.9, 152.4, 154.5, 155.0, 157.8, 163.8.31_P{1_H}_{NMR (CD} 3CN): δ 19.40 (1_{J(PtP) ) 4111 Hz,}3_{J(PtP) ) 83 Hz).}195_{Pt NMR (CD} 3CN): δ -4091 (d,1_{J(PPt) ) 4113 Hz).}

3.42; N, 3.40. Found: C, 51.80; H, 3.46; N, 3.48.1_{H NMR (CD} 3CN, 300 MHz): δ 3.88 (s, 6H, OMe), 4.84 (broad t, 2H,2_{J(PH) ) 13 Hz,} PCH2P), 6.15 (s, 2H), 6.20-6.41 (m, 2H), 6.55-6.60 (m, 4H), 6.71-6.80 (m, 2H), 6.88 (d, 4H, J ) 8.9 Hz), 7.07-7.14 (m, 2H), 7.42-7.59 (m, 18H), 7.70-8.04 (m, 10H), 8.25-8.47 (broad s, 4H).13_C{1_H} NMR (CD3CN): δ 21.6 (t,1J(PC) ) 31.3 Hz, PCH2P), 56.3 (OMe), 115.6, 116.5, 117.0, 125.0-135.4, 139.0, 140.9, 147.9, 152.3, 154.2, 154.4, 157.8, 163.2, 163.6.31_P{1_H}_{NMR (CD} 3CN): δ 19.52 (1J(PtP) ) 4108 Hz,3_{J(PtP) ) 85 Hz).}195_{Pt NMR (CD} 3CN): δ -4087 (d, 1_{J(PPt) ) 4110 Hz).}

3.65; N, 3.17. Found: C, 50.80; H, 3.52; N, 3.10.1_{H NMR (CD} 3CN, 300 MHz): δ 3.79 (s, 6H, p-OMe), 3.82 (s, 12H, m-OMe), 4.85 (t, 2H,2_{J(PH) ) 13.6 Hz, PCH} 2P), 6.25 (s, 2H), 6.36 (t, 2H, J ) 7.0 Hz), 6.50 (t, 2H, J ) 7.5 Hz), 6.69 (t, 2H, J ) 6.3 Hz), 6.79 (s, 6H), 7.32 (d, 2H, J ) 7.6 Hz), 7.44-7.94 (m, 22H), 8.15 (d, 2H, J ) 8.1 Hz), 8.28 (broad s, 4H).13_C{1_H}_{NMR (DMSO-d} 6): δ 19.9 (t,1J(PC) ) 30.5 Hz, PCH2P), 56.1 (m-OMe), 59.9 (p-OMe), 105.3, 116.3, 116.9, 124.3-140.1, 146.7, 150.9, 153.0, 153.1, 153.3, 156.7, 162.2.31_P{1_H} NMR (CD3CN): δ 19.65 (1J(PtP) ) 4134 Hz,3J(PtP) ) 78 Hz).195Pt NMR (CD3CN): δ -4083 (d,1J(PPt) ) 4125 Hz).

[Pt2(L1)2(µ-pz)]X, 9(X) (X ) ClO4and PF6). A mixture of

[Pt-(L1_{)Cl] (0.23 g, 0.50 mmol), pyrazole (Hpz) (0.02 g, 0.25 mmol), and}

potassium tert-butoxide (0.03 g, 0.25 mmol) in CH3CN/CH3OH (20/

10 mL) was heated at 60°C under a nitrogen atmosphere for 10 h. The orange suspension gradually became a clear red-orange solution and was allowed to cool to room temperature. A methanolic solution of LiClO4or NH4PF6(1.5 mmol in 10 mL) was added, and the resultant

mixture was stirred for 1 h, after which the solvent was evaporated to ∼5 mL. Addition of diethyl ether yielded an orange solid which was recrystallized by vapor diffusion of diethyl ether into an acetonitrile solution. Yield: 0.14 g, 55% for 9(ClO4); 0.14 g, 53% for 9(PF6). Anal.

Calcd for 9(ClO4), C35H25N6O4Pt2Cl: C, 41.24; H, 2.47; N, 8.25. Found:

C, 41.18; H, 2.63; N, 8.42. Calcd for 9(PF6), C35H25N6Pt2PF6: C, 39.48;

H, 2.37; N, 7.89. Found: C, 39.42; H, 2.52; N, 8.05.

Data for 9(ClO4) and 9(PF6). MS (+ve FAB): m/z 919 (M+).1H

NMR (DMSO-d6, 300 MHz): δ 6.78-6.80 (m, 4H), 6.88-7.12 (m, 5H), 7.48 (d, 2H, J ) 7.2 Hz), 7.83 (d, 2H, J ) 7.7 Hz), 7.93-8.09 (m, 8H), 8.25-8.36 (m, 4H).13_C{1_H} _{NMR (DMSO-d} 6): δ 107.4, 119.2, 119.3, 123.4, 124.3, 125.0, 127.1, 130.5, 133.7, 140.4, 140.8, 141.1, 141.7, 146.4, 151.0, 154.3, 155.4, 165.1.

[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2. The procedure for 1b(ClO4)2

was adopted using Pt(L1_{)Cl (0.18 g, 0.38 mmol) and}

bis(diphenylphos-phino)propane (0.08 g, 0.19 mmol) to yield 0.18 g (63%) of a yellow crystalline solid. MS (+ve FAB): m/z 1365 (M++ ClO4), 1265 (M+).

Anal. Calcd for C59H48N4O8Pt2Cl2P2: C, 48.40; H, 3.30; N, 3.83.

Found: C, 48.25; H, 3.26; N, 3.76.1_{H NMR (CD} 3CN, 500 MHz): δ 3.09-3.13 (m, 2H, PCH2CH2), 3.33-3.41 (m, 4H, PCH2), 6.25-6.33 (m, 4H), 6.61 (t, 2H, J ) 7.4 Hz), 6.74-6.75 (m, 2H), 6.86 (t, 2H, J ) 6.5 Hz), 7.04 (d, 2H, J ) 7.7 Hz), 7.35-7.38 (m, 8H), 7.43-7.47 (m, 4H), 7.58 (d, 2H, J ) 8.1 Hz), 7.72-7.76 (m, 8H), 7.84-7.97 (m, 6H), 8.08 (t, 2H, J ) 8.0 Hz).13_C{1_H}_{NMR (CD} 3CN): δ 26.1 (dd, 1_{J(PC) ) 37.2 Hz,}3_{J(PC) ) 15.6 Hz, PCH} 2), 26.9 (t,2J(PC) ) 7.2 Hz, PCH2CH2), 120.9, 121.2, 125.4-138.6, 141.9, 143.7, 148.3, 152.2, 154.4, 158.9, 164.1.31_P{1_H}_{NMR (CD} 3CN): δ 19.71 (1J(PtP) ) 3990 Hz).

[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2. The procedure for 1b(ClO4)2

was adopted using Pt(L1_{)Cl (0.18 g, 0.40 mmol) with}

bis(diphenylphos-phino)pentane (0.09 g, 0.20 mmol) to afford 0.19 g (64%) of a yellow crystalline solid. MS (+ve FAB): m/z 1393 (M++ ClO4), 1293 (M+).

Anal. Calcd for C61H52N4O8Pt2Cl2P2: C, 49.10; H, 3.51; N, 3.75.

Found: C, 48.96; H, 3.47; N, 3.70.1_{H NMR (DMSO-d}

6, 500 MHz):

δ 1.44; 1.78; 2.84 (broad multiplets, 10H, P(CH2)5P), 6.48 (broad s,

2H), 6.68 (broad s, 2H), 6.98-7.06 (m, 4H), 7.25-7.52 (m, 18H), 7.69-7.81 (m, 8H), 8.21-8.36 (m, 6H), 8.52-8.55 (m, 2H).13_C{1_H} NMR (DMSO-d6): δ 24.1 (d, 1J(PC) ) 37.8 Hz, PCH2), 25.0 (s, P(CH2)2CH2), 30.9-31.2 (m, PCH2CH2), 120.3, 124.7-136.8, 141.3, 142.9, 147.7, 150.9, 153.5, 158.0, 162.7.31_P{1_H}_{NMR (CD} 3CN): δ 19.53 (1_{J(PtP) ) 3972 Hz).}

X-ray Crystallography. Crystals of 4b(ClO4)2‚5H2O and 9(PF6)

were obtained by vapor diffusion of diethyl ether into acetonitrile solutions. Crystal data and details of data collection and refinement are summarized in Table 1. The following data are listed in the order

4b(ClO4)2‚5H2O/9(PF6). A total of 10 201/2841 unique reflections were

collected at 298 K on a Nonius diffractometer (λ(Mo KR) ) 0.7107 Å, 2θmax) 45/50°). The structure was solved by Patterson methods,

expanded using Fourier techniques, and refined by least-squares treatment on F2_{using the NRCVAX program: R ) 0.059/0.025, wR}

) 0.058/0.027, GoF ) 1.75/1.48, for 3760/2257 absorption-corrected (transmission 0.93-1.00/0.63-1.00) reflections with I > 2σ(I) and 847/ 229 parameters. The pairs of atoms N(1)/C(16) and N(3)/C(39) for

4b(ClO4)2‚5H2O and N(1)/C(12) for 9(PF6) were differentiated by their

temperature factors; interchanging the respective C and N atoms resulted in unreasonable temperature factors and/or higher R values. For 9(PF6),

the two halves of the cation are related by a 2-fold axis through C(18). Primed atoms are located at (-x, y,1_/

2-z).

Results and Discussion

Synthesis and Characterization. The cyclometalating ligands

HL1-6 _{(Figure 2) are readily prepared by Kro¨hnke syntheses} using the appropriate enone and 2-pyridacylpyridinium iodide in the presence of excess ammonium acetate.32_Complexes

1a-6a are synthesized by modification of the procedure reported

by Constable and co-workers.34_{The coordinated chloride ligand} in this series allows facile derivatization of the [Pt(L1-6_)] moieties. The cationic derivative [Pt(L1_)py]+_{(7) is afforded by} reaction of 1a with pyridine at room temperature.

Table 1. Crystal Data

4b(ClO4)2‚5H2O 9(PF6)

formula C71H66N4Cl2O13P2Pt2 C35H25N6PF6Pt2

fw 1706.36 1064.77

cryst system monoclinic monoclinic

space group C2/c C2/c

color orange orange

cryst size, mm 0.35× 0.35 × 0.35 0.20× 0.30 × 0.40 a, Å 31.338(8) 15.339(2) b, Å 21.767(5) 13.309(1) c, Å 27.515(6) 16.264(5) β, deg 123.37(2) 103.03(2) V, Å3 _15675(6) _3235(1) Z 8 4 Dc, g cm-3 1.447 2.186 µ, cm-1 _37.68 _88.50 F(000) 6752 2008 R,a_R wb 0.059, 0.058 0.025, 0.027 GoFc _1.75 _1.48 residual F, e Å-3 -0.57, +0.87 -0.75, +1.50 a_{R )}Σ||F o| - |Fc||/Σ|Fo|.bRw) [Σw(|Fo| - |Fc|)2/Σw|Fo|2]1/2.cGoF ) [Σw(|Fo| - |Fc|)2/(n - p)]1/2.

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The binuclear complexes 1b-6b and 9-11 are conveniently obtained in moderate to high yields by treatment of the corresponding mononuclear precursor with the diphosphine or pyrazole (for 9). The 31_P{1_H}_{NMR spectra of the dicationic} derivatives 1b-6b contain one signal with multiple 195_Pt satellites due to one- and three-bond couplings. These spectral patterns indicate the presence of a Pt2(µ-dppm) unit with chemically equivalent phosphorus atoms.35 _{In the} 1_{H NMR} spectra, the dppm methylene group appears as a distinctive triplet resonance. The positive FAB mass spectra for 1b-6b contain signals corresponding to the (M++ ClO4) fragment as well as the molecular cation M+.

Crystal Structures and Pt-Pt Distances. The crystal

structures of complex 4b bearing the tolyl group and the pyrazolate derivative 9 were determined by X-ray crystal-lography (Figures 3 and 4 respectively, Table 2). Since the structural parameters for 1b29_{and 3b}3a _{have previously been} elucidated, we can compare the effects of different substituents (H: 1b; 4-chlorophenyl: 3b; 4-tolyl: 4b) on 6-phenyl-2,2′ -bipyridine upon the solid-state and in particularπ-π stacking interactions in these complexes. As in 1b and 3b, the platinum centers in 4b reside in distorted square planar environments and the molecular structure consists of two [Pt(L4_{)] units linked by} a dppm bridge. The adjacent [Pt(6-phenyl-2,2′-bipyridine)] fragments are virtually parallel with a dihedral angle of 3.4° (cf. 4.6° in 3b and 6.1° in 1b). The intramolecular Pt-Pt contacts for 1b, 3b and 4b (3.270(1), 3.150(1), and 3.245(2) Å, respectively) are slightly longer than those in the related derivatives [Pt2(tpy)2(µ-L)]3+ _{(L ) anion of guanidine}25 _and canavanine:26_{mean 3.081 and 2.988 Å, respectively), but they} nevertheless fall within the range of intermetal distances (3.09-3.50 Å) observed in monomeric Pt(II) extended linear-chain structures.11a

The distinct difference between these structures is the torsion angle about the Pt-Pt axis, which is 44.6, 27.2, and 20.7°for

1b, 3b, and 4b (defined by the angle between the Pt(1)-(35) Langrick, C. R.; McEwan, D. M.; Pringle, P. G.; Shaw, B. L. J. Chem.

Soc., Dalton Trans. 1983, 2487. Figure 2.

Figure 3. Perspective view of [Pt2(L4)2(µ-dppm)]2+(4b, 30%

prob-ability ellipsoids).

Figure 4. Perspective view of [Pt2(L1)2(µ-pz)]+(9, 40% probability

ellipsoids). The two halves of the cation are related by a 2-fold axis through C(18).

Table 2. Selected Bond Lengths (Å) and Angles (deg)

Complex 4b(ClO4)2‚5H2O Pt(1)-Pt(2) 3.245(2) Pt(2)-P(2) 2.241(8) Pt(1)-P(1) 2.251(8) Pt(2)-N(3) 2.13(2) Pt(1)-N(1) 2.17(2) Pt(2)-N(4) 2.01(2) Pt(1)-N(2) 1.99(2) Pt(2)-C(39) 1.94(3) Pt(1)-C(16) 2.04(3) P(2)-C(71) 1.79(3) Pt(2)-Pt(1)-P(1) 89.6(2) Pt(1)-P(1)-C(71) 113.3(9) Pt(2)-Pt(1)-N(1) 94.1(5) P(1)-Pt(1)-N(1) 104.0(5) Pt(2)-Pt(1)-N(2) 94.8(5) P(1)-Pt(1)-N(2) 175.2(6) Pt(2)-Pt(1)-C(16) 88.4(7) P(1)-Pt(1)-C(16) 101.2(7) Pt(1)-Pt(2)-P(2) 89.3(2) Pt(2)-P(2)-C(71) 111.5(9) Pt(1)-Pt(2)-N(3) 93.4(6) P(2)-Pt(2)-N(3) 109.8(6) Pt(1)-Pt(2)-N(4) 89.4(5) P(2)-Pt(2)-N(4) 176.0(6) Pt(1)-Pt(2)-C(39) 90.2(8) P(2)-Pt(2)-C(39) 96.9(8) Complex 9(PF6) Pt-N(1) 2.113(5) N(3)-N(3’) 1.368(8) Pt-N(2) 1.959(5) N(3)-C(17) 1.340(8) Pt-N(3) 2.009(4) C(17)-C(18) 1.384(9) Pt-C(12) 1.999(6) Pt-Pt’ 3.612(2) N(1)-Pt-N(2) 79.3(2) N(3)-Pt-C(12) 96.2(2) N(1)-Pt-N(3) 102.4(2) Pt-N(3)-N(3’) 123.6(3) N(1)-Pt-C(12) 161.4(2) N(3)-N(3’)-C(17) 108.3(5)

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Pt(2)-N(2) and Pt(1)-Pt(2)-N(4) planes), respectively. The orientation in 1b optimizes theπ-stacking interactions of the aromatic ligand L1_{, since face-to-face overlap of}_{π orbitals in} an eclipsed manner (i.e. torsion angle ca. 0°) introduces repulsive forces.36_{The reduced torsion angles in 3b and 4b are presumably} manifestations of furtherπ-π interactions between the respec-tive 4-chlorophenyl and tolyl substituents in achieving the most stable conformation. In addition, these partially staggered geometry essentially eliminate steric repulsion between the chloro and methyl groups in 3b and 4b, respectively. We therefore attribute the varying torsion angles in these structures to the different geometrical demands forπ-stacking interactions by the cyclometalating ligands L1_{, L}3_{and L}4_.

The platinum centers in the structure of complex 9 are in distorted square planar geometry, but unlike in the dppm derivatives, the [Pt(L1_{)] moieties are not parallel due to the rigid} coordination mode of the pyrazolate linker (Figure 4). The comparable bond lengths within the pz ring (range 1.340(8)-1.384(9) Å) is consistent with the expectedπ-delocalization. The resultant Pt-Pt distance of 3.612(2) Å is significantly longer than that in [Pt2(tpy)2(µ-pz)](ClO4)3(3.432(1) Å)24_{and implies} negligible metal-metal communication, although weak π-π interactions are possible. The effect of this and the general consequences of longer metal-metal andπ-π separations upon the excited-state properties of these binuclear complexes will be discussed later. Close intermolecular contacts are not observed in the crystal lattice of 4b and 9.

Absorption and Emission Spectroscopy. Mononuclear Complexes. The UV-visible spectral data of the monomeric

complexes [Pt(L1-6_{)Cl] (1a-6a), [Pt(L}1_)py]+_{(7), and [Pt(L}1 )-PPh3]+(8) are listed in Table 3. Their absorption spectra obey Beer’s law in the concentration range 10-6-10-4M. The optical spectrum of 1a has been described previously.29_{Similarly, the} high-energy region (λ < 370 nm) in the absorption spectra of

2a-6a, with 4-aryl substituents on the 6-phenyl-2,2′-bipyridine ligand, is dominated by1_{IL (π f π*) transitions, while the} moderately intense low-energy bands withλmax in the range 434-438 nm are assigned to1_{MLCT (5d)Pt f}_{π*(L) transitions} (see Figure 5 for 6a). The weak absorption tails at 519-525 nm ( < 600 dm3 _mol-1 _cm-1_{) are attributed to} 3_MLCT transition.

The monomeric cyclometalated platinum(II) complexes stud-ied in this work are emissive in solution and solid states (Tables

4 and 5, respectively). With reference to earlier work, the structureless emissions of complexes 2a-6a in CH2Cl2at room temperature are assigned as3_{MLCT in nature (see Figure 5 for}

6a). Variation of the emission maxima is small (range

562-568 nm), and no trends are apparent for the different para-substituents. Hence there appears to be limited electronic communication between the 4-aryl group and the planar 6-phenyl-2,2′-bipyridine moiety. Minor solvatochromic effects are observed for 2a-6a: for example, the room-temperature emission of 2a shifts from 560 nm in acetonitrile to 567 nm in dichloromethane. The blue-shifted emission maxima for the cationic complexes 7 and 8 correlates with the greater charge on the Pt(II) center, which is expected to increase the energy of the3_{MLCT transition.}

The orange to red colors of 1a-6a in the solid state are noteworthy since these compounds absorb weakly at

wave-(36) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.

Table 3. UV-Visible Spectral Data for Complexes 1a-6a, 7, and 8a

complex λmax/nm (/dm3mol-1cm-1)

[Pt(L1_{)Cl], 1a} _{278 (19 600), 325 (9500), 360 (5000),} 430 (1550), 510 (180) [Pt(L2_{)Cl], 2a} _{285 (35 000), 334 (16 100), 369 (7600),} 435 (3100), 519 (150) [Pt(L3_{)Cl], 3a} _{288 (20 000), 332 (7800), 369 (4000),} 434 (1950), 523 (260) [Pt(L4_{)Cl], 4a} _{292 (34 000), 334 (17 800), 368 (7800),} 438 (3500), 522 (200) [Pt(L5_{)Cl], 5a} _{278 (21 800), 314 (18 700) 340 (17 100),} 437 (3300), 522 (540) [Pt(L6_{)Cl], 6a} _{278 (29 700), 336 (19 200), 418 (5000),} 437 (5200), 525 (570) [Pt(L1_)py]ClO 4, 7(ClO4)b 259 (37 500), 335 (16 000) 350 (15 700), 422 (590), 498 (90) [Pt(L1_)PPh 3]ClO4, 8(ClO4)b 266 (24 000), 336 (11 200), 350 (10 700), 425 (500)

a_{Measured at room temperature in CH}

2Cl2unless otherwise stated.

b_{In acetonitrile.} Figure 5. UV-vis absorption and emission (inset: λex350 nm) spectra of 6a in CH2Cl2at 298 K.

Table 4. Solution Emission Data for Complexes 1a-6a, 7, and 8a

complex 298 K data: λmax/nm; τo/µs; φo; kq/M-1s-1 77 K data: λmax/nm [Pt(L1_{)Cl], 1a} _{565; 0.51; 0.025; 3.1}_{× 10}9 _{535, 567 (max),} 610, 600 [Pt(L2_{)Cl], 2a} _{564; 0.60; 0.052; 4.1}× 109 _{555 (max), 592} [Pt(L3_{)Cl], 3a} _{568; 0.52; 0.054; 2.4}_{× 10}9 _{572 (max), 595} [Pt(L4_{)Cl], 4a} _{562; 0.62; 0.064; 2.2}_{× 10}9 _{575 (max), 623} [Pt(L5_{)Cl], 5a} _{563; 0.72; 0.068; 2.9}_{× 10}9 _{578 (max), 613} [Pt(L6_{)Cl], 6a} _{566; 0.63; 0.057; 1.6}_{× 10}9 _{552 (max), 589} [Pt(L1_)py]ClO 4, 7(ClO4) 540; 1.47; 0.066; 1.5× 109 _{522 (max), 554}b [Pt(L1_)PPh 3]ClO4, 8(ClO4) 535; 0.89; 0.062; 3.2× 108 _{527 (max), 560}b a_{Complex concentration 1}× 10-5_{M, in CH} 2Cl2unless otherwise

stated; 1/τ ) kobs) kq[complex] + ko.bIn acetonitrile.

Table 5. Solid-State Emission Data for Complexes 1a-6a, 7, and 8

complex 298 K data: λmax/nm (τo/µs) 77 K data: λmax/nm [Pt(L1_{)Cl], 1a} _{665 (0.40)} ₇₀₀ [Pt(L2_{)Cl], 2a} _{603 (max), 646 (0.66)} _{597 (max), 643, 702} [Pt(L3_{)Cl], 3a} _{600 (max), 640 (0.27)} _{594 (max), 643, 700} [Pt(L4_{)Cl], 4a} _{602 (max), 646 (0.51)} _{591 (max), 637, 700} [Pt(L5_{)Cl], 5a} _{676 (0.34)} ₇₁₆ [Pt(L6_{)Cl], 6a} _{687 (0.25)} ₇₂₂ [Pt(L1_)py]ClO 4, 7(ClO4) 578 (max), 615 (1.80) 572 (max), 611 [Pt(L1_)PPh 3]ClO4, 8(ClO4) 600 (2.41) 550 (max), 570

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lengths longer than 450 nm in solution. Their intense colors are attributed to the propensity of these square planar Pt(II) species, like established diimine relatives, to undergo solid-state intermolecular metal-metal and ligand-ligand interactions which yield low-energy [dσ* f π*] transitions (Figure 1).12 Structurally determined cyclometalated analogues exhibit Pt-Pt distances in the range 3.28-3.37 Å (for [Pt-Pt(L1 )(CH3CN)]-PF634_{and [Pt(dpphen)(CH3CN)]ClO4}27_{(Hdpphen )} 2,9-diphenyl-1,10-phenanthroline), respectively) and π-π separations of around 3.35 Å (for [Pt(L1_)PPh3]ClO429_{). At room temperature,} microcrystalline samples of complexes 1a, 5a, and 6a emit with λmax at 665, 676, and 687 nm, respectively (Table 5). Upon cooling of the samples to 77 K, the bandwidths of the emissions reduce and the emission maxima shift to 700, 716, and 722 nm, respectively. These data are comparable to those for the binuclear analogues 1b, 5b, and 6b which display metal-metal and/or ligand-ligand interactions (see later) and suggest that the solid-state emissions of 1a, 5a, and 6a are3_{[dσ*, π*] in} nature. The red-shift for these solid-state emissions upon cooling can be rationalized by shortening of intermolecular Pt-Pt and π-π separations in the crystal lattice, which re-sults in 3_{[dσ*, π*] emissions of lower energy. In contrast,} complexes 2a-4a and 7 show a structured band at 298 K with emission maxima at significantly higher energies (λmax 578-603 nm) than the3_{[dσ*, π*] emissions, while a blue shift is} detected at 77 K;3_{MLCT excited states are therefore tentatively} assigned.

Self-Quenching and 77 K Frozen Emission. Like planar

aromatic systems, a small number of square planar platinum-(II) complexes are known to display low-energy excimeric emission in concentrated fluid solutions20-22_{(eq 1 , NkN )}

4,7-diphenyl-1,10-phenanthroline, 4,4′-di-tert-butyl-2,2′ -bipy-ridine).

Self-quenching of the 3_{MLCT emission has been detected} for complexes 1a-6a, 7, and 8 at 298 K in CH2Cl2(Table 4), but no excimeric emission with complex concentration up to 10-3 M has been observed. In each case, a linear plot of 1/τ against complex concentration was produced. The intrinsic luminescence lifetimesτorange from 0.51 to 1.47µs, and self-quenching rate constants kqof 3.2× 108_{(for PPh3}_{complex 8)} to 4.1× 109_dm3_mol-1_s-1_{are observed. The emission quantum} yieldsφoof the substituted complexes 2a-6a range from 0.052 to 0.068 and are higher than that for 1a (φo 0.025). This is reminiscent of studies on phenyl-substituted tpy complexes of ruthenium, where the increased quantum yield was ascribed to extended conjugation which stabilizes the emissive MLCT excited state relative to a radiationless d-d transition.37

The emissions of 1a-6a, 7, and 8 at 77 K in frozen dichloromethane or acetonitrile have been studied (Table 4). Except for 7, their emissions are insensitive to the complex concentration in the range 10-6-10-3 M. They are highly structured, with the most intense vibronic component beingν′ ) 0 to ν′′ ) 0, which is characteristic of3_{MLCT emissions.} The concentration-dependent emissive behavior of 7 in acetonitrile at 77 K has been investigated (Figure 6). At low concentrations (∼10-6M), vibronic yellow emission with peak maxima at 522 and 554 nm (vibronic progression 1110 cm-1) is observed. Increasing the complex concentration yielded a new red emission centered at 670 nm at the expense of the yellow band. Indeed, at complex concentration > 10-3 M, the high-energy emission is completely replaced by the low-high-energy band at 670 nm. We note that the UV-vis absorption spectrum of 7 in acetonitrile at 298 K follows Beer’s law for concentration e 10-3 M; hence, no ground-state oligomerization is evident at 298 K. Emission from solid-state phases in frozen acetonitrile is discounted because this occurs atλmax572 and 611 nm at 77 K (Table 5). We suggest that, in frozen acetonitrile solution, the complex cations of 7 form weakly interacting dimeric pairs, and the 670 nm band can therefore be ascribed to a3_{[dσ*, π*]} excited state.

Binuclear Complexes. Our earlier report on the parent

6-phenyl-2,2′-bipyridine complex [Pt2(L1_)2(µ-dppm)]2+ _(1b) assigned the lowest energy absorption band at 420 nm in acetonitrile to a singlet [dσ* f π*] transition, while the emission at 652 nm was ascribed to a triplet [dσ*, π*] excited state (the MMLCT notation was previously used).29_{We now proceed to} examine the effects upon the [dσ*, π*] excited state of (A) different aryl substituents at the 4-position of 6-phenyl-2,2′ -bipyridine and of (B) bidentate ligands with various bridging lengths and geometry.

(A) Complexes with the µ-dppm Ligand: Effects of the

4-Aryl Substituent. The UV-vis absorption spectra of 2b-(37) Hecker, C. R.; Gushurst, A. K. I.; McMillin, D. R. Inorg. Chem. 1991,

30, 538.

Table 6. UV-Visible Spectral Data for Complexes 1b-6b and 9-11 in Acetonitrile

complex λmax/nm (/dm3mol-1cm-1)

[Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2 340 (16 500), 420 (3900), 478 (sh, 2550), 504 (sh, 1900) [Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2 286 (52 400), 316 (39 600), 421 (4000), 481 (sh, 2700), 511 (sh, 1800) [Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2 293 (47 900), 320 (41 200), 420 (3900), 488 (sh, 2600), 515 (sh, 1850) [Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2 292 (45 400), 325 (45 300), 420 (4500), 477 (sh, 2650), 510 (sh, 1700) [Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2 345 (46 200), 482 (2600), 510 (sh, 1750) [Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2 346 (38 700), 485 (2600), 522 (sh, 1450) [Pt2(L1)2(µ-pz)](ClO4), 9(ClO4) 266 (34 800), 323 (13 300), 356 (9700), 409 (2500), 510 (sh, 250) [Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2 336 (17 000), 350 (15 200), 426 (900), 494 (150) [Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2 334 (18 800), 350 (17 500), 422 (850), 494 (120)

Figure 6. Normalized emission spectra of 7 at different concentrations

in acetonitrile at 77 K (λex350 nm).

[Pt(NkN)(CN)₂]* + [Pt(NkN)(CN)₂] f

[Pt(NkN)(CN)₂]₂* (1)

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6b are generally similar to that of 1b (Table 6, see Figure 7 for 2b). All binuclear complexes in this work are photoluminescent

in solution (Table 7). Complexes 2b-6b exhibit structureless emissions with peak maxima at 654-662 nm in acetonitrile at room temperature, while at 77 K blue-shifted emissions are observed at 633-644 nm. These are comparable to the emissive behavior of 1b. Trends arising from the different nature of aryl substituents are not apparent. The time-resolved emission profiles for 2b-6b (Figure 8 for 5b) each show a mono-exponential decay, thus implying a single emitting species is present. The 77 K excitation spectrum of 5b in acetonitrile (monitoring emission at 640 nm; see Supporting Information)

corresponds with the absorption spectral bands, particularly in the 480-510 nm region (Table 6). In conjunction with the observed intramolecular Pt-Pt contacts in 3b and 4b, the emissions of 2b-6b are ascribed to3_{[dσ*, π*] excited states} such as for 1b. Self-quenching of the emissions in acetonitrile is difficult to quantify accurately due to the relatively short lifetimes. In dichloromethane, self-quenching rate constants of 1.2 × 108 _{and 2.0} _{× 10}8 _dm3 _mol-1 _s-1 _{for 2b and 5b} respectively were estimated, which are comparable with that for the mononuclear phosphine derivative 8 (3.2 × 108 _dm3 mol-1s-1).

The solvatochromic behavior of the3_{[dσ*, π*] emissions of}

1b(ClO4)2 and 5b(ClO4)2 has been studied (Table 8). The

respective emission energy, lifetime, and quantum yield are highly sensitive to the solvent polarity but insensitive to the complex concentration. For 5b(ClO4)2, the emission maximum shifts by 689 cm-1from dichloromethane to dimethylformamide (DMF) while the luminescence lifetimes (τo) and quantum yields (φo) change from 2.45 to 0.30µs and 0.20 to less than 0.01, respectively. This is ascribed to greater rates of non-radiative decay in highly polar solvents such as DMF. Significantly, these cyclometalated derivatives exhibit improved photophysical properties compared to the tpy congeners, which show very weak emission (φo in order of 10-4) in solution at room temperature.24,25

The solid-state emissions of 1b-6b (Table 9) are red-shifted in energy as the temperature is lowered to 77 K. This may be attributed to the shortening of Pt-Pt distances due to lattice contraction. The emissions of 1b, 5b, and 6b are similar to that for the mononuclear counterparts (see above), and we propose that they have the same electronic origin, namely 3_{[dσ*, π*].}

(B) Complexes with the 6-Phenyl-2,2′-bipyridine (L1₎

Ligand: Effects of the Bridging Ligand. To influence the

distance and interaction between [Pt(L1_{)] moieties in a binuclear} molecule, the pyrazolate derivative 9 and the C3 and C5 diphosphine complexes 10 and 11, respectively, were prepared. Consequences upon the excited-state properties can be probed by comparisons with the dppm-bridged species 1b.

Because of longer carbon chains between the phosphorus atoms, the intramolecular separations between the two [Pt(L1_)] units in 10 and 11 are expected to be greater than in 1b. This is reflected in their absorption spectra, which closely resembles that of the mononuclear phosphine congener [Pt(L1_)PPh3]+

(8). No moderately intense absorption is evident above 400 nm like the 1_{[dσ* f π*] absorption band in 1b; hence, the [Pt(L}1_)] fragments in 10 and 11 are proposed to behave as discrete non-interacting moieties from a spectroscopic viewpoint. In contrast, the absorption spectrum of complex 9 in acetonitrile at room temperature displays a moderately intense band at 409 nm ( ) 2500 dm3_mol-1 _cm-1_{). This low-energy band is absent in} complex 10 and the mononuclear pyridine derivative [Pt(L1 )-py]+(7), but the intensity of its tail (>500 nm) is noticeably

Figure 7. UV-vis absorption and emission (inset: λex350 nm, /

denotes instrumental artifact) spectra of 2b in acetonitrile at 298 K.

Table 7. Emission Data for Complexes 1b-6b and 9-11 in Acetonitrile (Complex Concentration 5× 10-5M)

complex 298 K data: λmax/nm;τo/µs; φo 77 K data: λmax/nm

[Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2 652; 0.14; 0.015 638 [Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2 659; 0.23; 0.016 641 [Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2 661; 0.19; 0.009 644 [Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2 654; 0.29; 0.024 633 [Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2 655; 0.40; 0.025 640 [Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2 662; 0.60; 0.018 644

[Pt2(L1)2(µ-pz)](ClO4), 9(ClO4) 548; 0.2; 0.003 555 (max), 590, 651

[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2 547; 1.9; 0.048 530 (max), 561

[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2 544; 0.8; 0.020 532 (max), 565 Figure 8. Time-resolved emission profile for 5b (0.01 mM in

acetonitrile) at 298 K (from top, 120, 200, 300, 400, 500, 600, 700, 800, 900, 1000 ns;λex350 nm).

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smaller than that for complex 1b in the same spectral region (Figure 9). For assignment purposes, this absorption band exhibits characteristics which are transitional between1_MLCT and1_{[dσ* f π*]. This is in accordance with the relatively long} Pt-Pt distance in 9 (3.612(2) Å), which implies virtually no metal-metal interaction, although the presence of intramolecular π-π contacts is feasible.

The luminescence properties of 9-11 in acetonitrile solution have been examined. Their respective emission energy is independent of complex concentrations from 10-6to 10-3M. The emission spectra of 10 and 11 in acetonitrile at room temperature show a structureless band centered at ca. 545 nm. At 77 K, the emissions blue-shift in energy and become vibronically resolved with spacing of ca. 1100 cm-1. This emissive behavior is similar to that for 8 and is evidently different from the3_{[dσ*, π*] excited state of 1b, which emits} at lower energies without vibronic character. The solution emissions of 10 and 11 are assigned as3_{MLCT in nature.}

The pyrazolate-bridged complex 9 emits in acetonitrile at room temperature with a peak maximum at 548 nm. This structureless emission is similar to the3_{MLCT emissions of 10} and the mononuclear species 7, but the 77 K frozen acetonitrile emission is strikingly different (Figure 10). A vibronically structured band is evident with the most intense peak at 555 nm and shoulders at 590 and 651 nm. The room-temperature emission of a microcrystalline sample of 9 at room temperature is diffuse, but at 77 K, a well-resolved vibronic structure (peaks at 580, 624, and 685 nm) is observed. While 9 displays negligible Pt-Pt contacts at room temperature (as indicated by the crystal structure), the intramolecular separation between [Pt-(L1_{)] units may decrease at 77 K to afford weak} _π-π interactions. These would give rise to the low-energy shoulders in the solution- and solid-state emissions.

It is interesting to compare the solid-state emission spectra of [Pt2(tpy)2(µ-L′)]3+ _(L′_{) anionic N-N bridging ligand) at} 298 K.24_{For example, the emission maxima for L}′) azaindolate (Pt-Pt ) 3.13(2) Å) and pyrazolate (Pt-Pt ) 3.432(1) Å) appear at 690 and 630 nm, respectively; hence, a blue shift is evident with greater Pt-Pt separations. In our study on

[Pt2-(L1_)2(µ-L)]n+_{, the 298 K solid-state emission maxima for L )} dppm (Pt-Pt ) 3.270(1) Å) and pyrazolate (Pt-Pt ) 3.612(2) Å) appear at 630 and 596 nm, respectively. It is apparent that as the (L1_)Pt-Pt(L1_{) separation is increased by lengthening L,} the origin of the excited state changes from 3_{[dσ*, π*] to} 3_{MLCT and the solid-state emission is displayed at higher} energies (<600 nm). From its transitional luminescent behavior, the µ-pyrazolate derivative 9 apparently approaches the limit for metal-metal interaction.

Photoredox Properties of [Pt2(L1)2(µ-dppm)](ClO4)2(1b). The electro- and photochemical properties of 1b have been examined. The cyclic voltammogram of 1b reveals two quasi-reversible one-electron reduction waves [(ipc/ipa)≈ 1] at -0.54 and -0.77 V vs NHE with 0.1 Mn_Bu4NPF6_{in acetonitrile (eqs} 2 and 3, respectively). The potentials are independent of scan

rates from 20 to 100 mV/s. The excited-state potential can be estimated by eq 4 (Pt2) [Pt2(L1_{)2(µ-dppm)]).}

Table 8. Solvent Dependence of the3_[d_{σ*, π*] Emission of} 1b(ClO4)2and 5b(ClO4)2

λmax/nm;τo/µs; φo solvent [Pt2(L1)2(µ-dppm)]2+, 1b [Pt2(L5)2(µ-dppm)]2+, 5b dichloromethane 640; 2.60; 0.18 645; 2.45; 0.20 chloroform 643; 2.45; 0.17 647; 2.10; 0.19 acetone 657; 0.49; 0.026 657; 0.85; 0.051 acetonitrile 652; 0.14; 0.015 655; 0.40; 0.025 methanol 654; 0.2;φo< 0.01 653; 0.30;φo< 0.01 dimethylformamide non-emissive 675; 0.30;φo< 0.01 Table 9. Solid-State Emission Data for Complexes 1b-6b and 9-11 complex 298 K data: λmax/nm (τo/µs) 77 K data: λmax/nm [Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2 630 (1.6) 640 [Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2 647 (1.6) 656 [Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2 643 (1.2) 655 [Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2 665 (1.5) 668 [Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2 643 (1.4) 650 [Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2 670 (1.7) 674

[Pt2(L1)2(µ-pz)](ClO4), 9(ClO4) 596 (max),

620 (1.0)

580 (max), 624, 685 [Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2 577 547, 573

[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2 579 541, 572

Figure 9. Lowest energy UV-vis absorption bands of 1b, 9, and 10

in acetonitrile at 298 K.

Figure 10. Normalized emission spectra of 1b, 9, and 10 in acetonitrile

at 77 K (λex350 nm). Pt₂2++ e-f_Pt 2 + E(Pt₂2+/Pt₂+) ) -0.54 V vs NHE (2) Pt₂++ e-f_Pt 2 E(Pt2 + /Pt₂) ) -0.77 V vs NHE (3) E(*Pt₂2+/Pt₂+) ) E(Pt₂2+/Pt₂+) + E_0-0 (4)

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The 0-0 transition energy of Pt22+_{, E0-0, can be estimated} from the overlap of the emission and excitation spectra, which occurs at 2.15 eV. Thus *Pt22+_{is a strong oxidant with E(*Pt2}2+_/ Pt2+) at 1.61 V vs NHE. The excited-state redox potential has also been estimated through quenching studies with a series of aromatic hydrocarbons in dichloromethane. The chosen aromatic hydrocarbons have similar size and electronic structure but different reduction potentials E(D+/D°). For each quencher, the Stern-Volmer plot is linear over the range of quencher concentrations. Analysis of the quenching rate data was performed by the method of Meyer and co-workers.38_Quenching rate constants and the reduction potentials of the quenchers are summarized in Table 10. From the plot of ln kq′vs E(D+/D°) (D ) quencher; see Supporting Information), the excited-state potential E(*Pt22+_/Pt2+

) is estimated to be 1.63 V vs NHE, which is in close agreement with the value of 1.61 V calculated from the spectroscopic and electrochemical data.

Concluding Remarks

The mono- and binuclear complexes reported in this study exhibit luminescence with relatively large quantum yields in solution at room temperature. We have shown that cyclometa-lating tridentate ligands based on 6-phenyl-2,2′-bipyridine offer favorable photophysical characteristics compared to 2,2′:6′,2′′ -terpyridine in square planar platinum(II) systems. Because of the ease in modifying the bridging ligands, these complexes represent a new class of luminophores with potentially tunable

excited-state properties. The planar geometry of the mononuclear derivatives facilitates face-to-face interactions, and self-quench-ing of the emissions have been observed at room temperature in dichloromethane.

On the basis of the structural parameters and spectroscopic assignments described in this work, it is proposed that metal-metal interactions in this system will yield3_{[dσ*, π*] excited} states which emit above 600 nm. As the Pt-Pt separation lengthens, the energy of the [dσ*, π*] excited state increases and the nature of the excited state accordingly switches to MLCT with emissions below 600 nm. The transitional behavior of [Pt2(L1_)2(µ-pz)]+

, from structural and photophysical perspec-tives, is noteworthy and provides a “reference point” for future investigations into d8_-d8 _{interactions. Hence a correlation} between (1) the energy of the emission band, (2) the nature of excited state, and (3) the degree of metal-metal interaction is established.

Acknowledgments. We are grateful for financial support

from The University of Hong Kong. The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China [HKU 7298/99P].

Supporting Information Available: Listings of crystal data, atomic

coordinates, calculated coordinates, anisotropic displacement param-eters, and bond lengths and angles for 4b(ClO4)2•5H2O and 9(PF6),

excitation spectrum of 5b in acetonitrile at 77 K, and a plot of ln kq′vs E(D+/D°). This material is available free of charge via the Internet at http://pubs.acs.org.

IC990238S (38) For details of data treatment, see: Bock, C. R.; Connor, J. A.;

Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815, and references therein.

Table 10. Rate Constants for Reductive Quenching of the3_[d_{σ*, π*] Emission of 1b (Pt}

22+) by Organic Donors in Dichloromethane at 20 (

0.2°C

organic quencher E(D+/D°)a_/V _k

q/dm3mol-1s-1 kq′/dm3mol-1s-1 ln kq′b N,N,N′,N′-tetramethyl-p-phenylenediamine 0.35 1.08× 1010 _2.35_{× 10}10 _23.88 N,N,N′,N′-tetramethylbenzidine 0.67 1.16× 1010 _2.76× 1010 _24.04 o-phenylenediamine 0.75 4.80× 109 _6.32× 109 _22.57 benzidine 0.79 6.21× 109 _9.01_{× 10}9 _22.92 phenothiazine 0.86 6.65× 109 _9.96_{× 10}9 _23.02 diethylaniline 0.94 4.61× 109 _5.99_{× 10}9 _22.51 diphenylamine 1.07 1.24× 109 _1.31_{× 10}9 _21.00 p-chloroaniline 1.31 6.18× 108 _6.38_{× 10}8 _20.27 2,4-dichloroaniline 1.46 6.02× 106 _6.02× 106 _15.61 1,4-dimethoxybenzene 1.58 1.20× 106 _1.20× 106 _14.00 1,2,3-trimethoxybenzene 1.66 2.06× 105 _2.06× 105 _12.24

a_{Reduction potential versus NHE.}b_1/k

q′) 1/kq- 1/kd, where kdis taken as 2× 1010dm3mol-1s-1.