Theoretical Study of Metallophilic Interactions and Excited States of

Organometallics 2010, 29, 3261–3270 DOI: 10.1021/om100080u

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Theoretical Study of Metallophilic Interactions and Excited States of Heterobimetallic d10-d8 Complexes with Bridging Ligands: The Tuning of Electronic Spectroscopy Qing-Jiang Pan,*,† Yuan-Ru Guo,‡ and Hong-Xing Zhang*,§ †

Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry and Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China, ‡College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, People’s Republic of China, and §State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received February 1, 2010

A series of heterobimetallic d10-d8 complexes, [AuIM0 (R1)2(R2)2] (M0 = PtII, AuIII; R1 = CN-, CtCH-; R2 = PH2CH2PH2, CH2PH2CH2-), were explored using ab initio methods and timedependent density functional theory (TD-DFT). In the ground states, d10-d8 distances were calculated at 2.88-3.00 A˚ with associated vibrational frequencies of 102-114 cm-1 by the MP2 method; energies of metallophilic interactions were estimated to be in the range 11.0-18.2 and 7.03-9.14 kcal/mol at the MP2 and CCSD levels, respectively. With the reference complex [AuIPtII(CN)2(PH2CH2PH2)2]þ (1), the variation of the d8 heterometal (M0 ), the coordination environment of the d8 metal (R1), and the bridging ligand (R2) changes the electronic structures and thus further tunes the electronic spectroscopy of these complexes. The known experimental absorption spectra were well reproduced by the present TD-DFT calculations. Upon excitation, UMP2 revealed the metal-metal interactions in their triplet excited states were strengthened. The corresponding phosphorescent emissions were attributed to the combination of metal-centered (MC) transition, metal-to-metal charge transfer (MM0 CT), and ligand-to-metal charge transfer (LMCT). The MC and MM0 CT transitions are responsible for the excited metal-metal contraction. The contraction in the AuI-AuIII complex is larger than that in corresponding AuI-PtII complex due to greater participation of MM0 CT transitions. 1. Introduction Interactions between closed-shell d10 metal centers, especially with gold(I), have attracted considerable experimental *To whom correspondence should be addressed. E-mail: panqj@ yahoo.com.cn. (1) Fackler, J. P. Inorg. Chem. 2002, 41, 6959–6972. (2) van Zyl, W. E.; Lopez-de-Luzuriaga, J. M.; Mohamed, A. A.; Staples, R. J.; Fackler, J. P. Inorg. Chem. 2002, 41, 4579–4589. (3) Schneider, J.; Lee, Y. A.; Perez, J.; Brennessel, W. W.; Flaschenriem, C.; Eisenberg, R. Inorg. Chem. 2008, 47, 957–968. (4) Wang, Q. M.; Lee, Y. A.; Crespo, O.; Deaton, J.; Tang, C.; Gysling, H. J.; Gimeno, M. C.; Larraz, C.; Villacampa, M. D.; Laguna, A.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 9488–9489. (5) Lee, Y. A.; Eisenberg, R. J. Am. Chem. Soc. 2003, 125, 7778–7779. (6) Lee, Y. A.; McGarrah, J. E.; Lachicotte, R. J.; Eisenberg, R. J. Am. Chem. Soc. 2002, 124, 10662–10663. (7) Tong, G. S. M.; Kui, S. C. F.; Chao, H. Y.; Zhu, N.; Che, C. M. Chem.;Eur. J. 2009, 15, 10777–10789. (8) Che, C. M.; Lai, S. W. Coord. Chem. Rev. 2005, 249, 1296–1309. (9) Phillips, D. L.; Che, C. M.; Leung, K. H.; Mao, Z.; Tse, M. C. Coord. Chem. Rev. 2005, 249, 1476–1490. (10) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931–1951. (11) Hutchings, G. J.; Brust, M.; Schmidbaur, H. Chem. Soc. Rev. 2008, 37, 1759–1765. (12) Schmidbaur, H.; Cronje, S.; Djordjevic, B.; Schuster, O. Chem. Phys. 2005, 311, 151–161. (13) Schmidbaur, H. Gold Bull. 2004, 37, 136–136. (14) Pyykko, P. Chem. Soc. Rev. 2008, 37, 1967–1997. (15) Pyykko, P. Inorg. Chim. Acta 2005, 358, 4113–4130. r 2010 American Chemical Society

and theoretical attention in recent years.1-16 The energy of the weak aurophilic interaction has been estimated to be at 7-20 kcal/mol, comparable to that of hydrogen bonds.17,18 Pyykk€ o et al. attributed the aurophilicity to correlation effects, strengthened by relativistic effects.19-22 Intra- or intermolecular interactions lead to gold(I) complexes with surprising structural arrangement such as the extended chain {[Au(CN)2]n23,24 and [Au2(SPR2S)2]n (R = Et, Ph)}.2,6 The aurophilicity was shown to play an important role in determining their photophysical properties.6 With various bridging ligands, binuclear AuI complexes display diversified luminescent properties of metal-centered (MC) transition, metal-to-ligand charge transfer (MLCT), and ligand-tometal charge transfer (LMCT). The MC transition is closely related to the aurophilic interaction. (16) Pyykko, P. Angew. Chem., Int. Ed. 2004, 43, 4412–4456. (17) Schmidbaur, H. Chem. Soc. Rev. 1995, 24, 391–400. (18) Schmidbaur, H. Gold Bull. 1990, 23, 11–21. (19) Pyykko, P. Chem. Rev. 1997, 97, 597–636. (20) Pyykko, P.; Mendizabal, F. Inorg. Chem. 1998, 37, 3018–3025. (21) Pyykko, P.; Mendizabal, F. Chem.;Eur. J. 1997, 3, 1458–1465. (22) Pyykko, P.; Runeberg, N.; Mendizabal, F. Chem.;Eur. J. 1997, 3, 1451–1457. (23) Pham, D. M.; Rios, D.; Olmstead, M. M.; Balch, A. L. Inorg. Chim. Acta 2005, 358, 4261–4269. (24) Stender, M.; Olmstead, M. M.; Balch, A. L.; Rios, D.; Attar, S. Dalton Trans. 2003, 4282–4287. Published on Web 07/14/2010

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Scheme 1. The Designed Heterobimetallic Complexes, [AuIM0 (R1)2(R2)2] (M0 = PtII, AuIII; R1 = CN-, CtCH-; R2 = PH2CH2PH2, CH2PH2CH2-)

A number of d8 complexes, such as Au(III), Pt(II), Pd(II), Ni(II), and Rh(I), also exhibit weak metal-metal interactions and intensive luminescence.25-35 So far, the synthesis of (25) Dominique, F.; Gornitzka, H.; Sournia-Saquet, A.; Hemmert, C. Dalton Trans. 2009, 340–352. (26) Gimeno, M. C.; Laguna, A. Chem. Soc. Rev. 2008, 37, 1952– 1966. (27) Yam, V. W. W. Acc. Chem. Res. 2002, 35, 555–563. (28) Xia, B. H.; Che, C. M.; Phillips, D. L.; Leung, K. H.; Cheung, K. K. Inorg. Chem. 2002, 41, 3866–3875. (29) Xia, B. H.; Che, C. M.; Zhou, Z. Y. Chem.;Eur. J. 2003, 9, 3055–3064. (30) Yip, H. K.; Lai, T. F.; Che, C. M. J. Chem. Soc., Dalton Trans. 1991, 1639–1641. (31) Roundhill, D. M.; Gray, H. B.; Che, C. M. Acc. Chem. Res. 1989, 22, 55–61. (32) Striplin, D. R.; Crosby, G. A. J. Phys. Chem. 1995, 99, 7977– 7984. (33) Striplin, D. R.; Crosby, G. A. J. Phys. Chem. 1995, 99, 11041– 11045. (34) Stace, J. J.; Lambert, K. D.; Krause, J. A.; Connick, W. B. Inorg. Chem. 2006, 45, 9123–9131. (35) Cowie, M.; Dwight, S. K. Inorg. Chem. 1980, 19, 2500–2507. (36) Fernandez, E. J.; Laguna, A.; Lopez-De-Luzuriaga, J. M. Dalton Trans. 2007, 1969–1981. (37) Balch, A. L.; Catalano, V. J.; Olmstead, M. M. Inorg. Chem. 1990, 29, 585–586. (38) Balch, A. L.; Catalano, V. J. Inorg. Chem. 1991, 30, 1302–1308. (39) Esswein, A. J.; Dempsey, J. L.; Nocera, D. G. Inorg. Chem. 2007, 46, 2362–2364. (40) Dempsey, J. L.; Esswein, A. J.; Manke, D. R.; Rosenthal, J.; Soper, J. D.; Nocera, D. G. Inorg. Chem. 2005, 44, 6879–6892. (41) Cao, L. Y.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chem. 2007, 46, 1361–1368. (42) Raptis, R. G.; Porter, L. C.; Emrich, R. J.; Murray, H. H.; Fackler, J. P. Inorg. Chem. 1990, 29, 4408–4412. (43) Mazany, A. M.; Fackler, J. P. J. Am. Chem. Soc. 1984, 106, 801– 802. (44) Mendez, L. A.; Jimenez, J.; Cerrada, E.; Mohr, F.; Laguna, M. J. Am. Chem. Soc. 2005, 127, 852–853. (45) Xia, B. H.; Zhang, H. X.; Che, C. M.; Leung, K. H.; Phillips, D. L.; Zhu, N. Y.; Zhou, Z. Y. J. Am. Chem. Soc. 2003, 125, 10362– 10374. (46) Yip, H. K.; Lin, H. M.; Cheung, K. K.; Che, C. M.; Wang, Y. Inorg. Chem. 1994, 33, 1644–1651. (47) Yip, H. K.; Che, C. M.; Peng, S. M. J. Chem. Soc., Chem. Commun. 1991, 1626–1628.

complexes has been devoted to combining a d8 metal center with a d10 center through a heterometallic interaction.36-49 The heterobimetallic Au(I)-metal complexes can take advantage of the maximum relativistic effects of gold to increase the metallophilicity.36-40,45-49 Many reported complexes use bridging ligands to pull two metals into close proximity.36-49 These complexes not only have an undoubted theoretical interest but can comprise a new class of photoluminescent material as well.36,45,50-53 In this work we systematically investigated a series of d10-d8 complexes in which AuI is the d10 center and the d8 metal is either PtII or AuIII, as shown in Scheme 1. On one hand, this choice is because the AuI and AuIII complexes dominate the molecular chemistry of gold;1-16,26 on the other hand, a number of luminescent AuI-PtII complexes45-49 were reported with a heterobimetallic interaction. Two types of bidentate ligands, diphosphine (PH2CH2PH2) and phosphorus ylide (CH2PH2CH2-), were used to bridge the d10 and d8 metal centers to form an eight-membered-ring skeleton. The diphosphine ligand has been successfully used in luminescent materials of binuclear complexes such as [AuI2(PR2CH2PR2)2]2þ,8,54,55 [PtII2(CN)4(PR2CH2PR2)2],28 and [AuIPtII(CN)2(PR2CH2PR2)2]þ45-47 (R = Me, Cy, and Ph), whereas the phosphorus ylide can stabilize the high (48) Yip, H. K.; Lin, H. M.; Wang, Y.; Che, C. M. J. Chem. Soc., Dalton Trans. 1993, 2939–2944. (49) Yin, G. Q.; Wei, Q. H.; Zhang, L. Y.; Chen, Z. N. Organometallics 2006, 25, 580–587. (50) Pan, Q. J.; Zhou, X.; Guo, Y. R.; Fu, H. G.; Zhang, H. X. Inorg. Chem. 2009, 48, 2844–2854. (51) Pan, Q. J.; Zhou, X.; Fu, H. G.; Zhang, H. X. Organometallics 2008, 27, 2474–2482. (52) Pan, Q. J.; Zhou, X.; Zhang, H. X.; Fu, H. G. Chem. Phys. Lett. 2008, 453, 7–12. (53) Crespo, O.; Laguna, A.; Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Jones, P. G.; Teichert, M.; Monge, M.; Pyykko, P.; Runeberg, N.; Schutz, M.; Werner, H. J. Inorg. Chem. 2000, 39, 4786–4792. (54) Fu, W. F.; Chan, K. C.; Cheung, K. K.; Che, C. M. Chem.;Eur. J. 2001, 7, 4656–4664. (55) Fu, W. F.; Chan, K. C.; Miskowski, V. M.; Che, C. M. Angew. Chem., Int. Ed. 1999, 38, 2783–2785.

Article

oxidation state AuIII ion as reported in [AuIII2(X)4(CH2PPh2CH2)2] (X = Cl and Br)56,57 and [AuIAuIII(X)2(CH2PPh2CH2)2] (X = Cl, Br, CCPh, CCtBu, and CCSiMe3)42-44 complexes. The CN- and CCH- groups were adopted to modify the local chemical environment of the d8 metal center. A series of heterobimetallic AuI-PtII/AuIII complexes were explored using ab initio methods and density functional theory (DFT). It was predicted that the metallophilic d10-d8 interactions are present in the ground states. The variation of the d8 heterometal and its coordination environment and the choice of the bridging ligand were studied to determine their effects on the electronic spectra of the complexes in acetonitrile solution. The excited states and related emissive properties were discussed in detail.

2. Computational Details and Scope of the Current Study 2.1. Computational Details. In the calculations, we used hydrogen atoms to represent the R (R = Me, Et, and Ph) groups that bond to the real ligands (PR2CH2PR2, CH2PR2CH2, and CtCR). This kind of simplification has been successfully applied in previous works.20-22,50-53,58 Therefore, we began by exploring the reference complex [AuIM0 (R1)2(R2)2] (M0 = PtII, R1 = CN-, R2 = PH2CH2PH2; 1), which has been studied experimentally by structural and spectroscopic characterization.45-47 To study the effects of heterometal centers, the coordination environments of the d8 metal, and the bridging ligands on the metal-metal interaction, excited state, and electronic spectroscopy, we designed complexes with different heterometals (M0 = PtII, AuIII), bonding groups (R1 = CN-, CtCH-), and bridging ligands (R2 = PH2CH2PH2, CH2PH2CH2-) relative to those of 1. Scheme 1 includes [AuIPtIII(CN)2(PH2CH2PH2)2]2þ (1), [AuIAuIII(CN)2(PH 2CH2 PH 2)2]2þ (2), [AuIPtII(CCH)2 (PH2 CH 2PH 2)2]þ (3), [AuIPtII(CN)2(CH2PH2CH2)2]- (4), [AuIAuIII(CCH)2(PH2CH2PH2)2]2þ (5), [AuIPtII(CCH)2(CH2PH2CH2)2]- (6), [AuIAuIII(CN)2(CH2PH2CH2)2] (7), and [AuIAuIII(CCH)2(CH2PH2CH2)2] (8). The complexes, represented by theoretical models 1, 3, and 8, have been characterized experimentally.44-49 We employed the second-order Møller-Plesset perturbation (MP2)59 method to optimize the structures of 1-8 in the ground states. The subsequent frequency analysis confirms these structures as local minimum points on the potential energy surfaces. Optimizations of their triplet excited states were performed at the unrestricted MP2 (UMP2) level. The C2 symmetry was adopted to settle their conformations in the ground and excited states. On the basis of the optimized structures, the timedependent density functional theory (TD-DFT)60-62 and solvent-effect model were used to predict the electronic spectroscopy in the acetonitrile solution. The electronic spectra were simulated using the Gaussian function. In these simulated spectra, the calculated oscillator strength of the absorption spectrum is approximately proportional to the extinction coefficient of the experimentally observed peak and also to the amplitude of the Gaussian curve. Here we used the combination (56) Raptis, R. G.; Fackler, J. P.; Murray, H. H.; Porter, L. C. Inorg. Chem. 1989, 28, 4057–4059. (57) Dudis, D. S.; Fackler, J. P. Inorg. Chem. 1985, 24, 3758–3762. (58) Schwerdtfeger, P.; Bruce, A. E.; Bruce, M. R. M. J. Am. Chem. Soc. 1998, 120, 6587–6597. (59) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618–622. (60) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454–464. (61) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439–4449. (62) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218–8224. (63) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032–3041. (64) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255, 327–335.


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of the B3LYP functional and the PCM (polarized continuum model),63-66 which presented results in agreement with experimental observations in previous studies.50-52 Relativistic effects were included by using Hay and Wadt67,68 effective core potentials (ECPs) for Au, Pt, and P. The LANL2DZ basis sets associated with the ECPs were employed. In order to better describe the molecular properties, Au (Rf = 0.20), Pt (Rf = 0.18), and P (Rd = 0.34) were implemented as an additional function.20-22,50-53,58 All the calculations were carried out using the Gaussian03 program package.69 2.2. Scope of the Current Study. In the study, the structures of 1-8 in the ground and triplet excited states were optimized. The intramolecular d10-d8 metallophilicity in the ground state was estimated by comparing the MP2 energies at the MP2 and HF optimized geometries (the computational details are described below). The metallophilic energy was also calculated with the higher level CCSD method.70-72 Absorption and emission spectra in the solution were obtained by the TD-DFT calculations associated with the solvent-effect PCM model. Heterobimetallic complexes 2-8 were newly optimized in this work. Heterobimetallic complex 1 was studied previously by comparing with its homobimetallic analogues [Au2I(PH2CH2PH2)2]2þ and [Pt2II(CN)4(PH2CH2PH2)2].52 A similar investigation was performed for [AuIRhI(CNH)2(PH2NHPH2)2Cl2] and its derivatives, but special attention was focused on the effect of bidentate PH2NHPH2 and PH2CH2PH2 ligands on redox properties of the Au-Rh complexes.50 The phosphorus ylide ligands CH2PR2CH2 (R = H, Me, Et, and Ph) readily stabilize diverse oxidation states of gold from I, to II, to III.42-44,51,56,57,73 Our previous study indicated that [AuIAuIIIX2(CH2PH2CH2)2] and [AuII2X2(CH2PH2CH2)2] (X = Cl and Br) feature a weak interaction and proximate single bonding between metal centers, respectively, although having the same 18 metal-centered 5d electrons.51 Our earlier theoretical studies were devoted to homobimetallic Au(I) and Pt(II) complexes with diphosphine ligands,74-81 which give us the confidence about the reliability of (65) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239–245. (66) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117–129. (67) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (68) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (69) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Wallingford, CT, 2004. (70) Purvis, G. D.; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910–1918. (71) Scuseria, G. E.; Janssen, C. L.; Schaefer, H. F. J. Chem. Phys. 1988, 89, 7382–7387. (72) Scuseria, G. E.; Schaefer, H. F. J. Chem. Phys. 1989, 90, 3700– 3703. (73) Carlson, T. F.; Fackler, J. P. J. Organomet. Chem. 2000, 596, 237–241. (74) Pan, Q. J.; Zhang, H. X.; Zhou, X.; Fu, H. G.; Yu, H. T. J. Phys. Chem. A 2007, 111, 287–294. (75) Pan, Q. J.; Zhang, H. X.; Fu, H. G.; Yu, H. T. Eur. J. Inorg. Chem. 2006, 1050–1059. (76) Pan, Q. J.; Fu, H. G.; Yu, H. T.; Zhang, H. X. Inorg. Chem. 2006, 45, 8729–8735. (77) Pan, Q. J.; Zhang, H. X. Inorg. Chem. 2004, 43, 593–601. (78) Pan, Q. J.; Zhang, H. X. J. Phys. Chem. A 2004, 108, 3650–3661.

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Figure 1. Optimized structures of [AuIM0 (R1)2(R2)2] (M0 =PtII, AuIII; R1 =CN-, CtCH-; R2 =PH2CH2PH2, CH2PH2CH2-). the MP2 and TD-DFT methods for calculations of transition metal complexes.

3. Results and Discussion 3.1. Ground-State Geometric and Electronic Structures. As shown in Scheme 1, we designed all the binuclear complexes with the reference of 1 via changing heterometal centers (M0 = PtII, AuIII), bonding groups (R1 = CN-, CtCH-), and bridging ligands (R2 = PH2CH2PH2, CH2PH2CH2-). The optimized geometry structures of 1-8 are presented in Figure 1. Selected geometry parameters obtained from our calculations are summarized and compared with the experimentally available values in Table 1.44-49 The calculations show that all the complexes consist of two bidentate units bridging a linearly coordinated AuI and a square-planar-coordinated PtII or AuIII atom, forming an eight-membered ring. The bonding groups CN- or CtCHcoordinate the d8 center in a trans arrangement. For 1-8, the bite distances of P 3 3 3 P and C 3 3 3 C in the bridging ligands were calculated in the range 3.07-3.11 and 3.04-3.05 A˚, respectively, comparable to experimental values of 3.14 and 2.97 A˚ available.45-49 The range of P 3 3 3 P separations is a little larger, which is a reflection of the fact that diphosphine is a softer ligand than phosphorus ylide. This is further proved by the observation that P-Au-M0 -P dihedral angles are larger than C-Au-M0 -C dihedral angles in the pairs 1/4, 2/7, and 3/6 but not for the pair 5/8. The calculated AuI-P/C and M0 -P/C bond lengths fall within the range of typical values,8,9,44-49 for example, AuI-P of ca. 2.37 A˚ and AuI-C of ca. 2.13 A˚. We obtained two types of C-M0 -AuI angles of ca. 88° in 1, 2, 3, and 5 and a little more than 90° in the others. Apparently, there is an interaction of the CtN/C triplet bond with the AuI center in the former series of complexes. Three electronic features were observed for these complexes. First, the HOMO, in most cases, is composed of metal centers except in 3 and 5, which have significant π(CtCH) character. This kind of higher occupied metal-based orbital has more dz2(PtII) participation than dz2-y2(AuI) in the AuI-PtII complex, but dz2-y2(AuI) more than dz2(AuIII) in the AuI-AuIII complex. The HOMO-2 of 3 and HOMO-4 (79) Pan, Q. J.; Zhang, H. X. Organometallics 2004, 23, 5198–5209. (80) Pan, Q. J.; Zhang, H. X. J. Chem. Phys. 2003, 119, 4346–4352. (81) Pan, Q. J.; Zhang, H. X. Eur. J. Inorg. Chem. 2003, 4202–4210.

of 5 have such features. Second, the σ(pz) forms the LUMO of the AuI-PtII complex, while the dx2-y2(AuIII) plays an important role in the LUMO of the AuI-AuIII complex. The bridging ligand contribution is significant and cannot be neglected. Third, the diphosphine ligand has greater participation in the low-lying unoccupied orbitals than phosphorus ylide when comparing complexes with similar M0 and R1. The CtCH and CtN groups contribute more to high-lying occupied orbitals. The density of states (DOS) of 1-8 can be seen in S-Figures 1-8. 3.2. Metallophilic Interactions. Weak metal-metal interactions in binuclear complexes have attracted much attention because they not only determine the photophysical and photochemical properties of complexes but also play an important role in biological systems.1-13,82-85 Short AuI-PtII distances in 1 and 3 were calculated to be 2.912 and 2.881 A˚, respectively, corresponding to the reported experimental values of 2.953 and 2.910 A˚.45-49 Other d10-d8 distances were estimated to be in the range 2.96-3.00 A˚ (Table 1). These distances are comparable to those calculated in homobimetallic complexes of d10-d10 (3.03 A˚ for [AuI2(PH2CH2PH2)2]2þ75,78 and 2.99 A˚ for [AuI2(CH2PH2CH2)2]51) and d8-d8 (3.07 A˚ for [PtII2(CN)4(PH2CH2PH2)2]74 and 3.05 A˚ for [AuIII2X4(CH2PH2CH2)2] (X = Cl and Br))51 and close to those estimated in other d10-d8 complexes (2.95 A˚ for [AuIAuIIIX2(CH2PH2CH2)2] (X = Cl and Br)).51 The heterobimetallic complexes 1-8, with closed-shell electronic structures, share commonalities: the calculated AuI-M0 distances are slightly shorter than the van der Waals contacts;86,87 the bite distances of P/C 3 3 3 P/C are apparently longer than corresponding AuI-M0 separations; and the P/C-AuIM0 and P/C-M0 -AuI angles have been calculated to be more than right angles with the exception of 89.9° for the P-Pt-Au angle of 1 and 89.3 ° for C-Au-Pt of 6. These results suggest that a weak bonding interaction possibly occurs between the (82) Tu, S. P.; Sun, R. W. Y.; Lin, M. C. M.; Cui, J. T.; Zou, B.; Gu, Q.; Kung, H. F.; Che, C. M.; Wong, B. C. Y. Cancer 2009, 115, 4459– 4469. (83) Sun, R. W. Y.; Che, C. M. Coord. Chem. Rev. 2009, 253, 1682– 1691. (84) Blackburn, N. J.; Barr, M. E.; Woodruff, W. H.; Vanderooost, J.; Devries, S. Biochemistry 1994, 33, 10401–10407. (85) Hetmanska, B.; Tomasik, P.; Tuszynski, T. Water Air Soil Pollut. 1994, 74, 281–288. (86) Slater, J. C. J. Chem. Phys. 1964, 41, 3199–3204. (87) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.

2.783 2.171 2.170 2.019 1.255 2.915 179.3 173.2 89.7 93.4 177.4 91.3 8.4 2.969 2.085 2.121 2.003 1.207 2.971 175.8 178.4 87.7 90.8 177.5 91.2 13.8 2.975 2.130 2.157 2.016 1.257 3.044 179.6 177.3 90.2 91.4 179.6 90.2 6.8 2.613 2.140 2.416 2.049 1.177 3.010 178.0 163.9 91.0 98.0 168.6 84.3 6.4

8

2.962 2.130 2.166 2.026 1.220 3.038 179.1 177.3 90.4 91.3 179.7 90.2 6.2 2.577 2.127 2.465 2.032 1.247 3.003 179.8 161.6 89.9 99.2 179.3 89.6 8.9 2.977 2.131 2.136 2.002 1.267 3.054 178.6 175.0 89.3 92.5 179.2 90.4 7.0 2.586 2.389 2.584 2.031 1.258 3.098 177.6 160.0 91.2 100.0 170.1 85.0 5.5 2.989 2.372 2.373 2.017 1.254 3.101 178.2 176.8 90.9 91.6 177.5 88.7 5.7 2.686 2.126 2.465 2.052 1.192 2.979 179.3 168.5 90.4 95.7 179.4 90.3 7.3 2.976 2.131 2.141 2.010 1.224 3.050 179.3 175.8 89.7 92.0 179.3 90.3 6.3 177.0

174.0 175.7

2.603 2.347 2.411 2.027 1.254 3.076 178.9 159.9 90.5 100.0 169.3 84.6 -10.8 2.910 2.319 2.303 1.956 1.232

2.881 2.363 2.305 2.011 1.258 3.073 173.4 179.1 93.3 90.4 177.1 88.5 11.7 2.633 2.410 2.533 2.063 1.221 3.118 177.0 161.4 91.5 99.3 151.7 75.8 -5.9

3

The experimental values are from refs 44-49.

2.591 2.359 2.393 2.024 1.221 3.085 177.6 160.5 91.2 99.8 166.3 83.1 11.8

2.999 2.378 2.397 2.030 1.221 3.110 178.3 177.6 90.9 91.2 175.9 87.9 9.5 a

1

2.953 2.311 2.333 2.009 1.126 3.140 174.2 177.0 92.9 91.3 176.5 88.4 8.5 2.912 2.368 2.321 2.017 1.221 3.084 174.9 179.7 92.5 89.9 175.3 87.6 16.3 Au-M0 Au-P/C M0 -P/C M0 -C CtN/C P/C 3 3 3 P/C P/C-Au-P/C P/C-M0 -P/C P/C-Au-M0 P/C-M0 -Au C-M0 -C C-M0 -Au P/C-Au-M0 -P/C

1

A

expt

3

A

1

A

2

3

A

1

A

expt

3

A

1

A

4

3

A

1

A

5

3

A

1

A

6

3

A

1

A

7

3

A

1

A

expt

3

A

Organometallics, Vol. 29, No. 15, 2010 Table 1. Optimized Geometry Parameters of Complexes [AuIM0 (R1)2(R2)2] (M0 = PtII, AuIII; R1 = CN-, CtCH-; R2 = PH2CH2PH2, CH2PH2CH2-) Using the MP2 Method for the 1 A Ground States and the UMP2 Method for the 3A Excited States with Available Experimental Values (distances in angstroms and angles in degrees)a

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two metal centers. To characterize the interaction, we carried out frequency calculations on the complexes at the MP2 level. The frequencies at 102-114 cm-1 were attributed to Au-M0 stretching vibrations for 1-8 (Table 2). These are comparable to our previous results50-52,74,76,77,79-81,88 of d10-d10: [AuI2(PH2CH2PH2)2]2þ (91 cm-1), [AuI2(CH2PH2CH2)2] (74 cm-1), [AuI2(PH2CH2PH2)(S2CC(CN)2)] (110 cm-1), [AuI2(PH3)2(S2CC(CN)2)] (96 cm-1), [AuI2(PH2CH2PH2)(SCH2S)] (93 cm-1), trans-[AuI2(PH2CH2SH)2]2þ (96 cm-1), cis-[AuI2(PH2CH2SH)2]2þ (97 cm-1), [AuI2(SHCH2SH)2]2þ (101 cm-1); of d8-d8: [AuIII2X4(CH2PH2CH2)2] (X = Cl (101 cm-1) Br (74 cm-1)), trans-[PtII2X4(PH2CH2PH2)2] (X = CN (94 cm-1), CCH (76 cm-1), Cl (83 cm-1), Br (75 cm-1)), trans-[PdII2(CN)4(PH2CH2PH2)2] (106 cm-1), [PtII2(P2O5H2)4]4- (118 cm-1), [PtII2(P2O4CH4)4]4- (113 cm-1); and of d10-d8: [AuIAuIIIX2(CH2PH2CH2)2] (X = Cl (124 cm-1), Br (115 cm-1)), [AuIPtII(CN)2(PH2CH2PH2)2]þ (107 cm-1), and [AuIRhI(CNH)2(PH2CH2PH2)2]2þ (127 cm-1). Herein, the calculated frequencies of 1-8 correspond to 88, 85, and 93 cm-1 for [Au2(PCy2CH2PCy2)2] 3 (ClO4)2, [AuPt(CN)2(PCy2CH2PCy2)2] 3 (ClO4), and trans-[Pt2(CN)4(PCy2CH2PCy2)2] determined by resonance Raman spectra, respectively.28,45,89 So far, many bimetallic Au and Pt complexes have been characterized using infrared vibrational spectra and resonance Raman spectra. The featured metalmetal stretching vibrations, ν(M2), were reported; for example, Harvey’s group found ν(Au2) of [Au2I(PMe2CH2PMe2)2] 3 (PF6)2, [Au2I(PMe2CH2PMe2)3] 3 (PF6)2, and [Au2I(PMe2CH2PMe2)2] 3 Cl2 at ca. 70 cm-1 and ν(Pt2) of [Pt2(PPh2CH2PPh2)3] at 102 cm-1.90,91 In brief, the present calculated results, together with previous theoretical and experimental reports, predict the existence of weak metalmetal bonding interactions in these heterobimetallic d10-d8 complexes. To characterize the d10-d8 interaction quantitatively in 1-8, the intramolecular interaction energies between metal centers were calculated. The metallophilic interaction energy was estimated using the operational definition of Pyykk€ o:92,93 the difference between the MP2 energies of a system at Hartree-Fock and MP2 optimized geometries, corresponding to Model 1. The energy of the system is relaxed on going from the HF geometry to the MP2 geometry, giving an approximation for the intramolecular interaction energies. We list metallophilic energies and d10-d8 distances of 1-8 in Table 2. The metallophilic energies were estimated to be 11.01-18.24 kcal/mol by the MP2 calculations, indicative of the attractive d10-d8 interaction in nature. However, the MP2 results obviously overestimate the metal-metal interaction greatly when compared with reported metallophilic energies.17-19,29,45,92,93 This overestimation can be ascribed to the following: (i) the MP2 method naturally overestimates the interaction; (ii) there exists possibly “assisted metallophilic attraction” in bridging systems (1-8) with the bidentate ligands; (iii) it is not impossible that the bridging ligand atoms would play a role not only in pulling the metal atoms close together but also in participating in the transmission of the interaction with their own electron cloud; and finally (iv) the present calculations on intramolecular interactions do not involve the counterpoise correction for the basis set superposition error (BSSE), but calculations on the free dimer do.29,45,93 To describe the metallophilic interaction precisely, we used the higher level electronic correlation method CCSD, which is known to be more accurate and reliable for calculating the

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Table 2. Intramolecular d10-d8 Distances (A˚), Stretching Vibrational Frequencies (cm-1), and Metallophilic Interaction Energies (kcal/mol) of 1-8 HF geom

Δ(MP2-HF)a

MP2 geom

Δ(B3LYP-HF)a

Δ(SVWN-HF)a

isodesmicb

r(AuI-M) r(AuI-M0 ) ν(AuI-M0 ) Δr(AuI-M0 ) ΔE(MP2) ΔE(CCSD) Δr(AuI-M0 ) ΔE(MP2) Δr(AuI-M0 ) ΔE(MP2) ΔEcorrc ΔE(MP2) d 1 2 3 4 5 6 7 8

3.330 3.430 3.276 3.254 3.418 3.260 3.209 3.231

2.912 2.999 2.881 2.976 2.989 2.977 2.962 2.975

107 102 104 112 105 111 114 112

0.418 0.431 0.395 0.278 0.429 0.283 0.247 0.256

18.24 18.03 14.53 14.96 13.17 12.86 14.64 11.01

8.48 9.14 8.06 6.98 8.64 7.03 7.59 7.47

0.160 0.143 0.156 0.045 0.142 0.042 0.058 0.047

12.62 11.74 10.22 9.80 8.46 8.36 9.67 6.94

0.447 0.459 0.422 0.251 0.445 0.240 0.241 0.239

11.33 12.30 10.14 8.44 8.86 7.26 8.07 5.36

51.97 46.91 52.49 39.99 46.12 39.52 39.63 38.85

9.05 8.20 8.70 3.95 8.06 3.77 2.98 2.95

a The structures were optimized by HF, MP2, B3LYP, and SVWN methods; in the case of Δ(MP2-HF), for example, Δr(AuI-M0 ), ΔE(MP2), and ΔE(CCSD) correspond to the differences of d10-d8 distances, MP2 energies, and CCSD energies between the MP2 and HF optimized geometries, respectively. b The eight-membered ring system was formed by the isodesmic reaction as seen in S-Table 3 of the Supporting Information. c The change in the MP2 correlation energies in the isodesmic reaction. d The difference between MP2 energies of the isodesmic reaction at the HF and MP2 optimized geometries.

energy of a system.94 With Model 1, the CCSD single-point calculations presented the metal-metal interaction energies in the range 6.98-9.14 kcal/mol (Table 2), which are ca. 9 kcal/mol lower than those calculated by the MP2 method. The CCSD energies fall well within the range of reported values of weak metal-metal interactions.17-19,29,45,92,93 The calculated metallophilic energies are compared in Figure 2. According to Model 1, we compared the effect of X = CN- (7), CCH- (8), Cl- (9), and Br- (10) of [AuIAuIIIX2(CH2PH2CH2)2] on the weak metal-metal interaction. The present calculations estimated the metallophilic energies to be 14.64, 11.01, 6.08, and 6.46 kcal/mol at the MP2 level and 7.59, 7.47, 4.66, and 4.72 kcal/mol at the CCSD level for 7-10, respectively (see S-Table 1 of the Supporting Information). MP2 and CCSD present the same changing trend, but MP2 overestimates the energies relative to CCSD. It can be seen that the σ-donating Cl- and Br- do not facilitate the AuI-AuIII interaction, while the CN- and CCH- with combined σ and π properties show an energetic preference for the interaction. According to our previous studies on homobimetallic Au(I) complexes, the weak AuI-AuI interaction in the ground state comes from the weak bonding of electrons that disperse to Au 6s/6p orbitals due to relativistic effects of the heavy metal and the perturbation of ligands.75,77-81 However, in heterobimetallic AuI-AuIII complexes, the charge transfer of Au(I)fAu(III) also contributes to the metallophilic interaction. In 7 and 8, the π acceptors of CN- and CCH- decrease the electron clouds around Au(III) and induce greater Au(I)fAu(III) charge transfer. Therefore, the calculated metallophilic interactions of 7 and 8 are stronger that those of 9 and 10 with halide ligands. In addition, we used DFT optimized geometry to replace MP2 geometry in Model 1 and calculated the metallophilic interaction energy. The ground-state structures of 1-8 were (88) Pan, Q. J.; Zhou, X.; Zhang, H. X.; Fu, H. G. J. Photochem. Photobiol. A: Chem. 2007, 188, 287–292. (89) Leung, K. H.; Phillips, D. L.; Tse, M. C.; Che, C. M.; Miskowski, V. M. J. Am. Chem. Soc. 1999, 121, 4799–4803. (90) Perreault, D.; Drouin, M.; Michel, A.; Miskowski, V. M.; Schaefer, W. P.; Harvey, P. D. Inorg. Chem. 1992, 31, 695–702. (91) Harvey, P. D.; Dallinger, R. F.; Woodruff, W. H.; Gray, H. B. Inorg. Chem. 1989, 28, 3057–3059. (92) Pyykko, P.; Tamm, T. Organometallics 1998, 17, 4842–4852. (93) Mendizabal, F.; Pyykko, P. Phys. Chem. Chem. Phys. 2004, 6, 900–905. (94) O’Grady, E.; Kaltsoyannis, N. Phys. Chem. Chem. Phys. 2004, 6, 680–687.

Figure 2. Calculated metallophilic interaction energies of 1-8. MP2[Δ(MP2-HF)] means the differences between MP2 energies at the MP2 and HF optimized geometries and similar cases for MP2[Δ(B3LYP-HF)] and MP2[Δ(SVWN-HF)]; MP2[Δ(Isodesmic)] denotes the difference between MP2 energies of the isodesmic reaction at the HF and MP2 optimized geometries.

optimized using the B3LYP and SVWN density functionals. The selected geometry parameters are listed in S-Table 2. As far as the d10-d8 distances are concerned, B3LYP overestimates them compared with those calculated by MP2, but SVWN presents close results. According to Model 1, the MP2 interaction energies of 6.94-12.12/5.36-12.30 kcal/ mol were obtained on the basis of HF and B3LYP/SVWN optimized geometries. These results are located between the MP2 and CCSD values (Figure 2). Another model (Model 2) was suggested to calculate the metallophilic energy for the ring system.92,93 This kind of system was formed by a hypothetical chemical reaction from a “monomeric fragment” containing only one metal atom and thus no metal-metal interaction. The isodesmic reaction conserves the number and character of the chemical bonds involved. To obtain a measure of the metallophilic interaction energy, the change in the MP2 correlation energies in such isodesmic reactions were calculated, i.e., ΔEcorr in Table 2. The energies of fragments calculated are listed in S-Table 1 and corresponding isodesmic reactions in S-Table 3. Just like previous studies,92,93 the ΔEcorr energies are much higher than the values of ΔE(MP2) and ΔE(CCSD)

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Table 3. Calculated Absorptions of 1-8 in Acetonitrile Solution at the TD-B3LYP/PCM Level, Associated with the Absorptions Observed in the Experiment state 1

2

3

1

Ad A 1 B 1 B 1 d B 1

1

A 1 B 1 d A 1 d B 1

B 1 d A 1 d

B

4

1

Ad B

1

1 d

B

conf

|CI coeff| > 0.2

E (eV)a

λ (nm)a

fb

exptc

27af28a 27af29a 26bf28a 26bf29a 25bf29a 20bf28a 27af28a 25bf28a 27af29a 20bf28a 21bf28a 26bf28a 26af28a 26bf27b 24bf28a 25bf29a 28af29a 28af31b 25bf29a 25bf30a 24bf29a

0.668 0.675 0.680 0.672 0.595 0.262 0.692 0.661 0.653 -0.423 0.422 0.680 0.620 -0.266 0.637 0.268 0.683 0.630 -0.256 0.525 -0.374

4.03 4.49 4.66 5.14 6.21

308 276 266 241 199

0.236 0.013 0.053 0.010 0.097

316 (23200) 279 (4320) 259 (7200) 245 (6370)

3.87 4.26 4.97 6.00

320 291 250 207

0.024 0.089 0.118 0.330

3.25 3.95

381 314

0.059 0.131

393 (10348) 329 (15066)

5.20

238

0.029

246 (38641)

4.08 5.34

304 232

0.246 0.018

5.71

217

0.034

state 5

1

B B B 1 d A 1 1

1 d

B

6 7

1

d

A A 1 d B 1 d A 1

1

Ad

1 d

B

8

1

B

1

Ad B

1 d

conf

|CI coeff| > 0.2

E (eV)a

λ (nm)a

fb

26bf28a 25bf28a 26bf29a 26af29a 25af28a 22bf28a 20bf28a 28af29a 28af30a 25bf30a 27af29a 28af30a 27af29a 28af30a 27bf31a 28af30b 26bf29a 27bf29a 28af30a 25bf31a 22bf30a

0.687 0.685 0.675 0.546 0.403 0.509 0.378 0.691 0.660 0.593 0.497 0.465 0.493 -0.472 0.504 -0.266 0.544 -0.334 0.649 0.456 0.236

3.07 3.47 4.08 4.50

404 357 304 276

0.007 0.036 0.058 0.070

6.05

205

0.135

3.71 4.17 5.22 5.05

334 297 238 245

0.164 0.075 0.011 0.081

5.11

243

0.111

6.39

194

0.282

4.93

252

0.123

5.13 6.43

241 193

0.193 0.270

a Calculated absorption spectra in nm and eV. b Oscillator strength. c Experimental absorptions from refs 45-49 and molar absorption coefficient (dm3 mol-1 cm-1) listed in parentheses. d Band I with the 1A electronic excited state and band II with the 1B state.

with Model 1. In fact, the ΔEcorr (ΔEcorr = ΔEcorr(Au-M0 ) þΔEcorr(other bonds)) includes much more correlation energies with the exception of the metal-metal weak bonding, so it inevitably results in the overestimation of the metallophilic interaction. To eliminate these errors, we combined Models 1 and 2 and calculated the difference between the MP2 energies of the isodesmic reaction at the HF and MP2 optimized geometries (Model 3). All the ΔE(MP2) energies with Model 3, corresponding to the term ΔEcorr(Au-M0 ) in the above equation, were estimated to be below 9.1 kcal/mol, as shown in Table 2 and Figure 2. In summary, ΔE(CCSD) with Model 1 presents a reasonable estimate for the metallophilic interaction energies of 1-8, as compared to the reported values in experiment and theory.17-19,29,45,92,93 In contrast, the energies are overestimated in the MP2 calculations. Model 2, with the isodesmic reaction, overestimates the interaction energy greatly, in agreement with previous studies.92,93 Model 3, combining the two first models, displays interaction energies close to those of CCSD for diphosphine complexes. From the viewpoint of structural features, the d10-d8 interaction is determined by combined effects of heterometal centers (M0 = PtII, AuIII), bonding groups (R1 = CN-, CtCH-), and bridging ligands (R2 = PH2CH2PH2, CH2PH2CH2-). First, from HF optimized geometry to MP2 (Table 2 and S-Table 2), the AuI-M0 distances lengthen by 0.40-0.43 and 0.25-0.28 A˚ for diphosphine and phosphorus ylide complexes, respectively, so diphosphine complexes 1, 2, 3, and 5 have larger interaction energies than phosphorus ylide ones, 4, 7, 6, and 8, respectively. (Figure 2) This is also evidence that the diphosphine ligand is softer. Here, every pair of complexes that we compare differ in only one of M0 , R1, and R2, with the other two unchanged. Second, because the CN- accepts electrons more easily than CCH-, both ΔE(MP2) and ΔE(CCSD) energies (Model 1) of 1, 2, and 7 are higher than those of 3, 5, and 8, respectively. Finally, the difference of PtII and AuIII results in energies following 1 > 2, 3 > 5, 4 > 7 and 6 > 8 for ΔE(MP2) but taking the converse sequence for ΔE(CCSD).

In addition, we depicted the diagram of the orbital-energy levels for heterobimetallic 1 and its homobimetallic [Au2I(PH2CH2PH2)2]2þ and [Pt2II(CN)4(PH2CH2PH2)2] in S-Figure 9.52 This diagram presents an intuitive understanding of the Au-Au, Au-Pt, and Pt-Pt interactions. 3.3. Electronic Absorption. In Table 3, the transition energies (nm/eV) and oscillator strengths of featured absorptions of 1-8 are listed, together with experimental absorption spectra.45-49 Simulated spectra using the Gaussian function are described in Figures 3-5. We provide detailed information on the MOs involved in absorption transitions in S-Tables 4-11. The general spectral pattern of [AuPt(CN)2(PCy2CH2PCy2)2] 3 (ClO4) observed in the experiment45 was well reproduced by the theoretical spectrum of 2 (Figure 3). An intensive peak at 308 nm (band I) is comparable to the experimental one at 316, which was attributed to the transitions of σ*[dz2-y2(Au)-dz2(Pt)]fσ[pz(Au-Pt)]/ligand. The MC transition is closely related to metallophilic interaction. Another strong peak labeled band II occurs at ca. 190 nm, with characteristics of π*[dxz(Au-Pt)]/ligand/CN-fσ[pz(Au-Pt)]/ ligand/dx2-y2(Pt) charge transfers. Besides these, the absorptions of 276, 266, and 241 nm correspond to experimental 279, 259, and 245 nm, respectively.45-47 With the exchange of the PtII metal center with AuIII, the electronic structures of 2 are modified, especially for the unoccupied orbitals. For instance, the LUMO and LUMOþ1 in complex 1 have properties of σ[pz(Au-Pt)]þligand and dx2-y2(Pt)þσ*(Pt-P), respectively, whereas they are reversed in 2. As a result of the rising of the orbital with σ[pz(Au-Pt)]þligand in 2, band I with the MC/MLCT transition property was found at 250 nm, shifting to higher energy relative to that at 308 nm for 1. Second, we obtained a low-lying absorption at 320 nm (3.87 eV) for 2, contributed by the HOMOfLUMO configuration. It was assigned to the σ*[dz2-y2(AuI)-dz2(AuIII)]fdx2-y2(AuIII)þσ*(AuIII-P) transition. Finally, the calculated 207 nm absorption (band II) of 2 has an energy and properties similar to the 190 nm band of 1. Therefore, the change of the d8 heterometal from PtII to AuIII

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Figure 3. Simulated absorption spectra in acetonitrile for 1-4 using TD-B3LYP/PCM calculations.

Figure 4. Simulated absorption spectra in acetonitrile for 1 and 5-7 using TD-B3LYP/PCM calculations.

Figure 5. Simulated absorption spectra in acetonitrile for 1 and 8 from TD-B3LYP/PCM calculations.

increases the energy of the featured band I significantly, which is evident in the calculated longer d10-d8 distance of 2 (2.999 A˚) than that of 1 (2.912 A˚).

Pan et al.

As seen in Figure 3, the calculated electronic absorptions of 1 and 3 are very similar for the featured bands I and II except for one additional band at 381 nm for 3. This absorption was attributed to dyz(Pt)þπ(CtC)fσ[pz(Au-Pt)]þligand, comparable to the 393 nm band of [AuPt(CtCPh)2(PPh2CH2PPh2)] 3 (SbF6) observed in solution.49 The presence of the Au-Pt interaction in binuclear 3 makes the band far longer than the absorption (302 nm) with dyz(Pt)þπ(CtC)fpz(Pt)þpz(P) character in mononuclear trans-[PtII(CtCH)2(PH3)2].95 A slightly longer wavelength of band I was predicted for 3 (314 nm) than 1 (308 nm), but the different electron-donating abilities of CtCH- and CtN- cause a large difference in their band II. Since the bridging ligand is greatly involved in the transitions of bands I and II, its properties should have an important effect on the featured absorptions by varying PH2CH2PH2 of 1 to CH2PH2CH2- of 4. It was found in S-Tables 4 and 7 that the bridging ligand contribution in the low-lying unoccupied orbitals decreases on going from 1 to 4. There are more metal participations in the transitions of bands I and II of 4. However, the absorption wavelength of bands I and II changes a little, especially for band I (304 nm) in 4. As concluded above for band I, the change of d8 heterometal from PtII to AuIII results in a ca. 7500 cm-1 blue shift, whereas the different participation of bridging ligands and different donor abilities of CtN- and CtCH perturb the energies to a smaller extent, corresponding to a ca. 400 cm-1 blue shift and 600 cm-1 red shift, respectively. Therefore, we hope that two or more variations in 5-8 relative to 1 can roughly follow the above trend, thus allowing for the tuning of the electronic spectra of these complexes. As shown in Figures 4 and 5, we found in 5-8 the bands I and II with transition properties similar to those of 1. As the introduction of AuIII into 1 causes the largest decrease of absorption wavelength among all three variations, we found a shorter absorption wavelength of band I in 5, 7, and 8, at 276, 245, and 241 nm, respectively. In contrast, band I of 6 was calculated at 334 nm. In brief, we can roughly estimate the absorption energies of band I when changing the chemical environment of the complex relative to that of 1. 3.4. Excited-State Properties. Geometry optimizations at the UMP2 level of theory were used to obtain the structures of the 3A triplet excited states for 1-8. One can note the following differences in comparison with their respective 1A ground state (Table 1). First, metal-metal bond distance contractions were found in the excited states of heterobimetallic AuI-PtII/AuIII complexes. For example, the Au-Pt distances of 1 and 3 shrink by ca. 0.32 and 0.28 A˚, respectively, as compared to 0.36 A˚ for [Au2(PH2CH2PH2)2]2þ75 and 0.28 A˚ for [Pt2(CN)4(PH2CH2PH2)2].74 Experimental resonance Raman spectra estimated the metal-metal contractions as 0.16, 0.10, and 0.11 A˚ for [Au2(PCy2CH2PCy2)2] 3 (ClO4)2,89 [AuPt(CN)2(PCy2CH2PCy2)2] 3 (ClO4),45 and trans-[Pt2(CN)4(PCy2CH2PCy2)2],28 respectively. Earlier investigations on [Pt2(pop)4]4- (pop = P2O5H22-) in the lowest energy triplet excited state revealed a 0.21-0.29 A˚ Pt-Pt distance reduction.76,96-101 Second, the bite distances of (95) Pan, Q. J.; Fu, H. G.; Yu, H. T.; Zhang, H. X. Inorg. Chim. Acta 2006, 359, 3306–3314. (96) Novozhilova, I. V.; Volkov, A. V.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 1079–1087. (97) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. J. Phys. Chem. B 2004, 108, 12175–12180. (98) Kim, C. D.; Pillet, S.; Wu, G.; Fullagar, W. K.; Coppens, P. Acta Crystallogr., Sect. A 2002, 58, 133–137.

Article

P/C 3 3 3 P/C remain constant (less than 0.01 A˚) in 1-3, 5, and 6, while they are reduced ca. 0.03-0.13 A˚ in others. So combined with the variation of P/C 3 3 3 P/C, the separation between two metal atoms decreases ca. 0.22-0.40 A˚ for 1-7 but only ca. 0.06 A˚ for 8 upon excitation from the ground state. Third, the bond lengths between metal and ligand increase in most cases. Finally, the C-M0 -Au angles are reduced, indicating that π electrons of CtN or CtCH ligands bonding to the d8 metal atoms could overlap more with orbitals of the AuI atom in the excited states. The σ*(dz2)fσ(spz) electronic excitations can provide an explanation for the first three structural variations, which has been established in previous homobimetallic complexes.74-81,95-97 However, the introduction of new metal centers in the heterobimetallic AuI-PtII/AuIII complexes can modify the relative energy of the frontier orbitals of gold and create new orbitals, as a consequence of the new goldmetal interaction. This can lead to new emissive states as the possibilities for electronic transitions are multiplied. To rationalize the electronic transitions in heterobimetallic complexes and further reveal their differences from homobimetallic ones, the TD-B3LYP calculations were carried out in the gas phase and acetonitrile solution on the basis of the UMP2 optimized geometries. The calculated phosphorescent emissive properties (Table 4) will be helpful for their potential applications in luminescent materials and devices. In the calculations, TD-DFT predicts the phosphorescent emission energies of 1 at 2.30 eV (539 nm) in the gas phase and 2.28 eV (544 nm) in acetonitrile. The solution emission falls within the range of experimental values of 450-570 nm, which depends on the R group in [AuPt(CN)2(PR2CH2PR2)2]þ (R = Cy and Ph).45-47 The analyses on the wave functions of these excited states indicate that the phosphorescent emission comes from three electronic transitions: σ[pz(Au-Pt)]f σ*[d z2 -y 2(Au)-dz2 (Pt)] (MC transition), [dx2 -y2 (Pt)]f[s, dz2-y2(Au)] (metal-to-metal charge transfer, MM0 CT), and ligandfσ*[dz2-y2(Au)-dz2(Pt)] (LMCT). The assignment obeys the direction of electronic transition in the emissive process, which is opposite the electronic absorption. The introduction of solvent molecules does not change the emissive nature, but leads to a small shift of emission relative to that in the gas phase. A similar case has been found in other complexes. For instance, the emission energy of 3 at 2.26 eV (548 nm) in solution was slightly underestimated with respect to experimental values of 2.68-2.43 eV (462-510 nm) phosphorescence of [AuPt(CCPh)2(PPh2CH2PPh2)2]2þ.48,49 The difference between experimental and calculated values could be caused by the replacement of the phenyl group with a hydrogen atom, the use of the B3LYP density functional, and even the exclusion of counterions. As indicated in the previous studies, the lowest energy phosphorescent emissions of homobimetallic complexes were assigned as the combined MC and LMCT transitions. Apparently, the MM0 CT is present due to the heterometal orbitals of heterobimetallic complexes. As a matter of fact, the excited states of heterobimetallic complexes differ in many aspects. For example, the AuI-PtII distance of 1 shortens by ca. 0.32 A˚ upon excitation, whereas that of (99) Leung, K. H.; Phillips, D. L.; Che, C. M.; Miskowski, V. M. J. Raman Spectrosc. 1999, 30, 987–993. (100) Thiel, D. J.; Livins, P.; Stern, E. A.; Lewis, A. Nature 1993, 362, 40–43. (101) Rice, S. F.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 4571– 4575.


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Table 4. Calculated Phosphorescent Emissions of 1-8 in the Gas Phase and Acetonitrile Solution at the TD-B3LYP/PCM Level, Associated with the Experimental Values medium states 1 2 3 4 5 6 7 8

gas sol. gas sol. gas sol. gas sol. gas sol. gas sol. gas sol. gas sol.

3

A A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3 A 3

|CI coeff| > 0.2 E (eV)a λ (nm)a

conf 27af28a 27af28a 27af28a 27af28a 26af28a 26af28a 28af29a 28af29a 26af28a 26af28a 28af29a 28af29a 28af29a 28af29a 28af30a 28af30a

0.740 0.742 0.745 0.746 0.724 0.725 0.728 0.727 0.733 0.733 0.727 0.718 0.737 0.736 0.704 0.704

2.30 2.28 1.81 1.80 2.27 2.26 1.83 1.88 1.66 1.67 1.73 1.78 1.89 1.91 3.58 3.61

539 544 686 688 547 548 676 658 747 741 716 697 655 651 346 344

exptb 450 450, 550, 570 484, 521 462, 510

a Calculated absorption spectra in nm and eV. b Experimental spectra from refs 45-49.

AuI-AuIII in 2 decreases ca. 0.37 A˚. The larger metal-metal contraction was found in the AuI-AuIII complex with keeping other ligands the same for comparison. We relate it to the MM0 CT transition that contributes more in the AuI-AuIII complexes. One can prove it by checking the HOMOs of complexes where dz2(PtII) contribution (40.1% for 1) in the AuI-PtII complex is far more than dz2(AuIII) (6.7% for 2) in the AuI-AuIII complex. The more dz2 participation in the HOMO inevitably decreases the MM0 CT contribution to the whole electronic transition of the complex.

4. Conclusions In this work, the metallophilicity and ground- and excitedstate properties of 1-8 were explored by the ab initio method and TD-DFT. MP2 calculations predicted the AuI-PtII/ AuIII distances at 2.88-3.00 A˚, shorter than the van der Waals contacts. The calculated stretching vibrational frequencies and interaction energies of metal-metal confirmed the metallophilic attraction; the energies of metallophilic interactions were calculated in the range 7.03-9.14 kcal/ mol by the CCSD method, while they were overestimated significantly by the MP2 method. 1-8 feature two intensive absorption bands (labeled I and II) in acetonitrile solution at the TD-B3LYP/PCM level, in agreement with available experimental values.45-49 With respect to 1, the variation of the d8 heterometal (M0 ), the coordination environment of the d8 metal (R1), and the bridging ligand (R2) can tune the electronic spectroscopy of 2-8 by modifying their electronic structures. The change of M0 from PtII to AuIII results in a large blue shift of wavenumber for band I, whereas variations of R1 and R2 affect the absorption slightly. Upon electronic excitation from the ground states, UMP2 revealed that the metal-metal interactions were enhanced in their triplet excited states. The lowest energy phosphorescent emissions of 1-8 were assigned as the combination of MC, MM0 CT, and LMCT transitions. In these transitions, the MM0 CT was found to be responsible for larger metal-metal contraction in the AuI-AuIII complex than that in the corresponding AuI-PtII complex.

Acknowledgment. Q.-J.P. thanks Professor Fred Drew and Dr. Samuel Odoh (University of Manitoba, Canada)

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for helping with the manuscript. This work was supported by the Natural Science Foundation of China (No. 20703015, 30901136), the New Teacher Foundation of Education Ministry of China (200802251002), the Youth Science Foundation of Heilongjiang Province of China (QC08C15), the Program for New Century Excellent Talents of Common Universities of Heilongjiang Province of China (1154-NCET-010), and the Supporting Plan for Excellent Youth of Common Universities of Heilongjiang Province of China (1153G028).

Pan et al. Supporting Information Available: Tables of MP2, SVWN, B3LYP, and HF optimized geometry parameters of 1-8; total energies of 1-8 and their fragments; isodesmic reactions of 1-8; and partial molecular orbital contributions of 1-8 in acetonitrile using TD-B3LYP/PCM calculations; diagrams of orbitalenergy levels of Au-Au, Au-Pt, and Pt-Pt complexes; figures of density of states (DOS) of 1-8 in the ground states; and figure of simulated absorption spectra in acetonitrile for 1-8 from TDB3LYP/PCM calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

Theoretical Study of Metallophilic Interactions and Excited States of

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