Multiple Bonds Between Metal Atoms / 10-Osmium Compounds

Osmium Compounds

Ren 441

than the spin-only values (µ = [n(n+2)]1/2; n is the number of unpaired electrons) because of the large spin-orbit coupling intrinsic to Os. Hence, the high effective moment of Os2(ap)4Cl2 is peculiar.

Fig. 10.11. Temperature dependence of magnetic susceptibility (ρ, x 10-3 cgs) and effective magnetic moment (µ, B.M.) of Os2(DTolF)4Cl2 (taken from ref. 61).

The ground state configuration of Os2(hp)4Cl2 was initially assigned as diamagnetic μ2/4β2β*2 on the basis of a relatively short Os–Os bond (2.344(2) Å).1 Measurement of its magnetic susceptibility between 5 – 300 K revealed a room temperature magnetic moment of 1.7 B.M. and overall dependence described by the equation on the previous page, implying a ground state configuration of μ2/4β2/*2.5 Although the ρ−Ζ dependence is unavailable, both the Os–Os bond length and room temperature effective moment (1.76 B.M.) of Os2(PhCONH)4Cl2 are consistent with a μ2/4β2/*2 ground state.9 Small room-temperature

µeff values were reported for Os2(O2CCH3)2-(Ph2PC6H4)2Cl2 (0.29 B.M./Os) and Os2(O2CC2H5) 2(Ph2PC6H4)2Cl2 (0.07 B.M./Os),18 while the magnetism of the compound Os2Cl4(Ph2PC6H4)2

remains unknown.27 These orthometallated species have the shortest Os–Os bond lengths (2.23 – 2.27 Å) among all paddlewheel diosmium species, which supports μ2/4β2β*2 as the most probable ground state configuration. The weak paramagnetism in these compounds is certainly worthy of further investigation, although the presence of a paramagnetic impurity cannot be excluded.

The [Os2X8]2- anions are all diamagnetic with very short Os–Os bond lengths (2.20 – 2.22 Å), which is consistent with a closed-shell ground state and an Os–Os triple bond. A μ2/4β2β*2 ground state was derived from the SCF X_ calculation of Os2Cl82- in the eclipsed configuration, where the /* orbital (LUMO) was found to be 1.5 eV above the β* orbital (HOMO).37 On the basis of both the X_ results of the eclipsed Os2Cl82- and symmetry considerations, it was concluded that four valence electrons from the β-type orbitals on both Os centers are accommodated in a nonbonding e24 shell in the staggered Os2Cl82-.37 Hence, the staggered Os2X82- has a μ2/4 ground state configuration and an Os–Os triple bond.

The earliest studies of [Os2(O2CR)4Cl2]1- and [Os2(hp)4Cl2]1- found their room temperature magnetic moments (ca. 2.70 B.M.) much higher than those of the corresponding Os26+ parent compounds,45 indicating the S = 3/2 nature for Os2(II,III) molecules. An EPR study of the latter anion also revealed a pattern consistent with a MS = 1/2 ground state in thermal equilibrium with a MS = 3/2 state, both the consequence of zero-field splitting of an S = 3/2 configuration. Subsequently, magnetic properties of Os2(chp)4Cl and its derivatives were care-

Multiple Bonds Between Metal Atoms

442 Chapter 10

fully examined,10 which yielded both an effective moment of 2.90 B.M. and an EPR spectrum similar to that of [Os2(hp)4Cl2]1-. The ground state configuration for all Os25+ species appears to be μ2/4β2/*2β*1. This description fits the structural data as well: the Os–Os bond length in Os2(chp)4Cl (2.348(1) Å) is identical to that of Os2(hp)4Cl2 (2.344(2) and 2.357(1) Å) within the experimental errors, since the added β* electron in Os2(chp)4Cl is only weakly antibonding. Magnetic susceptibilities over a temperature range of 2 – 300 K were measured for both [Os2(chp)4(py)]BF4 and {[Os2(chp)4]2(µ-pyrazine)}(BF4)2.31 The temperature dependence of the former was modeled with a zero-field splitting of the S = 3/2 state, which corroborated the ground state configuration derived from previous EPR studies. Compared with [Os2(chp)4(py)]BF4, {[Os2(chp)4]2(µ-pyrazine)}(BF4)2 exhibited a much faster decay in the effective moment as temperature decreases, which is indicative of a significant antiferromagnetic coupling in the bridged compound.

Details about the Os–Os bonding in [Os(Porp)]2 remain elusive due to the absence of single crystal X-ray structures. Temperature-dependent magnetic properties are consistent with a ground state configuration of μ2/4β2β*2/*2 for [Os(Porp)]2 in analogy to that of Ru24+ compounds,53 and an Os–Os double bond.

The majority of Os2 compounds are deeply colored, reflecting the strong charge transfer nature of electronic absorption spectra as the result of Os-ligand orbital mixings. However, quantitative analysis of these spectroscopic signatures remains rare. A careful examination of both the solution and solid state (CsI pellet and single crystal) absorption spectra of Os2(O2CR)4X2, (R = Me, Prn and But, X = Cl and Br) provided detailed assignments of the observed transitions.26 Solution studies of Os2(O2CCMe3)4X2 with X = Cl, Br, and BF4– (Fig. 10.12) revealed that the intense peaks at 394 nm for Cl and 455 nm for Br are the ligand to metal charge transfer (LMCT) transitions from axial ligand X to the Os2 center. Analysis of single crystal polarized absorption spectra of Os2(O2CCMe3)4Cl2 (Fig. 10.13) yielded assignments of β β* band at 850 nm and /* β* band at 1200 nm, and vibronic progression of c. 220 cm-1 in both bands assigned as ι(Os–Os) in the excited states.

Fig. 10.12. Solution spectra of Os2(O2CCMe3)4Cl2 (solid line) and Os2(O2CCMe3)4Br2 (dashed line) at room temperature (taken from ref. 26).

It was noted that [Os2Cl8]2- undergoes two reversible one-electron oxidations at 235 K, which were assigned as

[Os2Cl8]2- - e- [Os2Cl8]- - e-[Os2Cl8]0

Osmium Compounds

Ren 443

Spectroelectrochemical characterization of [Os2Cl8]- was carried out at 233 K (Fig. 10.14), from which the β β* transition was unambiguously identified at a ιmax of 4600 cm-1.62 The observed ι(β β*) is substantially lower than those observed in [Re2Cl8]3- (6950 cm-1)63 and [Tc2Cl8]3- (6800 cm-1),64 reflecting a significant deviation from the eclipsed configuration in

[Os2Cl8]1-.65

Fig. 10.13. Electronic spectra of the (110) face of a single crystal of Os2(O2CCMe3)4Cl2 (parallel c polarization) at 20 (solid line) and 295 K(dashed line) (taken from ref. 26).

Fig. 10.14. Spectroelectrochemical data for [Os2Cl8]2-. (a) Spectral changes as oxidation progresses; (b) Spectrum of [Os2Cl8]1- showing three visible and near infrared transitions and their assignments (inset) (taken from ref. 62).

While the majority of infrared and Raman spectroscopic data were reported as part of rudimentary characterizations of Os2 compounds, resonance enhanced Raman data allow inferences as to Os–Os bond strengths. A resonance Raman study of Os2(O2CCH3)4Cl2 and Os2(O2CCD3)4Cl2 revealed an Os–Os stretching frequency at 229 cm-1,66 and a similar study of Os2(O2CCH2Cl)4Cl2, Os2(O2CC2H5)4Cl2, and Os2(O2CC3H7)4Cl2 yielded ι(Os–Os) ranging from 228 to 236 cm-1.67 These values are consistent with the triple bond nature of these Os2 species, considering that the ι(M-M) determined for other 5d paddlewheel species are 304 cm-1 for W2(O2CCH3)4 (quadruple bond),68 288 cm-1 for Re2(O2CCH3)4Cl2 (quadruple bond),69 and 158 cm-1 for [Pt2(P2O5H2)4Cl2]4- (single bond).70 A resonance Raman study of both [Os(Porp)]2 and its oxidized derivatives revealed that the Os–Os stretching frequency progressively increases with the increasing oxidation state: 233 cm-1 for [Os2], 254 cm-1 for [Os2]1+ and 266 cm-1 for

444

Multiple Bonds Between Metal Atoms

Chapter 10

[Os2]2+,71 which are consistent with the stepwise removal of /* electrons and consequently the increase of Os–Os bond order from 2 to 3 (Scheme 10.5).

Scheme 10.5. Change of the ground state configuration of [Os(porp)]2 upon oxidations.

10.5 Concluding Remarks

The chemistry of diosmium compounds is clearly dominated by compounds with an Os26+ core. Recent isolation of an Os27+ compound32 supported by hpp revealed diosmium compounds of higher oxidation state as a promising new focus area. Paddlewheel species having an Os26+ core display the propensity of axial halide ligation (Table 1) that is reminiscent of Ru25+ species, which should enable axial coordination of non-trivial ligands through metathesis. Theoretical understanding of diosmium species is limited to a few SCF-X_ calculations performed between late 80s and early 90s. Calculations capable of treating the relativistic effect accurately are much needed in providing better understanding of structures, magnetism and spectroscopy.

References

1.F. A. Cotton and J. L. Thompson, J. Am. Chem. Soc. 1980, 102, 6437.

2.D. S. Moore, A. S. Alves and G. Wilkinson, J. Chem. Soc., Chem. Comm. 1981, 1164.

3.T. Behling, G. Wilkinson, T. A. Stephenson, D. A. Tocher and M. D. Walkinshaw, J. Chem. Soc., Dalton Trans. 1983, 2109.

4.T. A. Stephenson, D. A. Tocher and M. D. Walkinshaw, J. Organomet. Chem. 1982, 232, c51.

5.F. A. Cotton, T. Ren and M. J. Wagner, Inorg. Chem. 1993, 32, 965.

6.F. A. Cotton, A. R. Chakravarty, D. A. Tocher and T. A. Stephenson, Inorg. Chim. Acta 1984, 87, 115.

7.T. W. Johnson, S. M. Tetrick and R. A. Walton, Inorg. Chim. Acta 1990, 167, 133.

8.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, Inorg. Chim. Acta 1984, 89, L15.

9.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, Inorg. Chem. 1985, 24, 1334.

10.F. A. Cotton, K. R. Dunbar and M. Matusz, Inorg. Chem. 1986, 25, 1585.

11.F. A. Cotton, K. R. Dunbar and M. Matusz, Inorg. Chem. 1986, 25, 1589.

12.F. A. Cotton, T. Ren and J. L. Eglin, Inorg. Chem. 1991, 30, 2559.

13.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, Inorg. Chem. 1984, 23, 4693.

14.T. Ren, D. A. Parrish, J. R. Deschamps, J. L. Eglin, G.-L. Xu, W.-Z. Chen, M. H. Moore, T. L. Schull, S. K. Pollack, R. Shashidhar and A. P. Sattelberger, Inorg. Chim. Acta 2004, 357, 1313.

15.Y.-H. Shi, W.-Z. Chen, G.-L. Xu, J. L. Eglin, A. P. Sattelberger, C. Hare and T. Ren, manuscript in preparation.

16.R. Clérac, F. A. Cotton, L. M. Daniels, J. P. Donahue, C. A. Murillo and D. J. Timmons, Inorg. Chem. 2000, 39, 2581.

17.F. A. Cotton, C. A. Murillo, X. Wang and C. C. Wilkinson, Inorg. Chim. Acta 2003, 351, 191.

18.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, J. Chem. Soc., Chem. Commun. 1984, 501.

19.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, Inorg. Chem. 1984, 23, 4697.

20.S. M. Tetrick and R. A. Walton, Inorg. Chem. 1985, 24, 3363.

21.R. P. Tooze, P. Stavropoulos, M. Motevalli, M. B. Hursthouse and G. Wilkinson, J. Chem. Soc., Chem. Commun. 1985, 1139.

Osmium Compounds

Ren 445

22.P. Stavropoulos, P. D. Savage, R. P. Tooze, G. Wilkinson, B. Hussain, M. Motevalli and M. B. Hursthouse, J. Chem. Soc., Dalton Trans. 1987, 557.

23.F. A. Cotton, M. P. Diebold and M. Matusz, Polyhedron 1987, 6, 1131.

24.R. P. Tooze, M. Motevalli, M. B. Hursthouse and G. Wilkinson, J. Chem. Soc., Chem. Commun. 1984, 799.

25.R. P. Tooze, G. Wilkinson, M. Motevalli and M. B. Hursthouse, J. Chem. Soc., Dalton Trans. 1986, 2711.

26.V. M. Miskowski and H. B. Gray, Topics Cur. Chem. 1997, 191, 41.

27.F. A. Cotton and K. R. Dunbar, J. Am. Chem. Soc. 1987, 109, 2199.

28.F. A. Cotton and M. Matusz, Polyhedron 1987, 6, 1439.

29.F. A. Cotton, K. R. Dunbar and M. Matusz, Polyhedron 1986, 5, 903.

30.F. A. Cotton and M. Matusz, Inorg. Chim. Acta 1988, 143, 45.

31.F. A. Cotton, Y. M. Kim and D. L. Shulz, Inorg. Chim. Acta 1995, 236, 43.

32.F. A. Cotton, N. S. Dalal, P. Huang, C. A. Murillo, A. C. Stowe and X. Wang, Inorg. Chem. 2003, 42, 670.

33.P. E. Fanwick, M. K. King, S. M. Tetrick and R. A. Walton, J. Am. Chem. Soc. 1985, 107, 5009.

34.P. E. Fanwick, S. M. Tetrick and R. A. Walton, Inorg. Chem. 1986, 25, 4546.

35.F. A. Cotton and K. Vidyasagar, Inorg. Chem. 1990, 29, 3197.

36.C. L. Gross, S. R. Wilson and G. S. Girolami, Inorg. Chem. 1995, 34, 2582.

37.P. A. Agaskar, F. A. Cotton, K. R. Dunbar, L. R. Falvello, S. M. Tetrick and R. A. Walton, J. Am. Chem. Soc. 1986, 108, 4850 .

38.F. A. Cotton and K. Vidyasagar, Inorg. Chim. Acta 1989, 166, 109.

39.P. E. Fanwick, I. F. Fraser, S. M. Tetrick and R. A. Walton, Inorg. Chem. 1987, 26, 3786.

40.D. C. Ware, M. M. Olmstead, R. Wang and H. Taube, Inorg. Chem. 1996, 35, 2576.

41.S. S. Lau, W. G. Wu, P. E. Fanwick and R. A. Walton, Polyhedron 1997, 16, 3649.

42.F. A. Cotton, S. A. Duraj, C. C. Hinckley, M. Matusz and W. J. Roth, Inorg. Chem. 1984, 23, 3080.

43.G. A. Heath, D. G. Humphrey and K. S. Murray, J. Chem. Soc., Dalton Trans. 1998, 2417.

44.S. F. Gheller, G. A. Heath, D. C. R. Hockless, D. G. Humphrey and J. E. McGrady, Inorg. Chem. 1994, 33, 3986.

45.S. M. Tetrick, V. T. Coombe, G. A. Heath, T. A. Stephenson and R. A. Walton, Inorg. Chem. 1984, 23, 4567.

46.A. R. Chakravarty, F. A. Cotton and D. A. Tocher, Inorg. Chem. 1985, 24, 172.

47.A. R. Chakravarty, F. A. Cotton and W. Schwotzer, Polyhedron 1986, 5, 1821.

48.J. P. Collman, C. E. Barnes and L. K. Woo, Proc. Natl. Acad. Sci., USA 1983, 80, 7684.

49.J. P. Collman and H. J. Arnold, Acc. Chem. Res. 1993, 26, 586.

50.J. P. Collman, H. J. Arnold, K. J. Weissman and J. M. Burton, J. Am. Chem. Soc. 1994, 116, 9761.

51.J. P. Collman, S. T. Harford, S. Franzen, A. P. Shreve and W. H. Woodruff, Inorg. Chem. 1999, 38, 2093.

52.J. P. Collman and J. M. Garner, J. Am. Chem. Soc. 1990, 112, 166.

53.H. A. Godwin, J. P. Collman, J.-C. Marchon, P. Maldivi, G. T. Yee and B. J. Conklin, Inorg. Chem. 1997, 36, 3499.

54.J. P. Collman, J. T. McDevitt, C. R. Leidner, G. T. Yee, J. B. Torrance and W. A. Little, J. Am. Chem. Soc. 1987, 109, 4606.

55.J. P. Collman, J. T. McDevitt, G. T. Yee, C. R. Leidner, L. G. McCullough, W. A. Little and J. B. Torrance, Proc. Natl. Acad. Sci., USA 1986, 83, 4581.

56.J. P. Collman, D. S. Bohle and A. K. Powell, Inorg. Chem. 1993, 32, 4004.

57.M. Hanack and P. Vermehren, Inorg. Chem. 1990, 29, 134.

58.R. Caminiti, M. P. Donzello, C. Ercolani and C. Sadun, Inorg. Chem. 1998, 37, 4210.

59.F. A. Cotton, V. M. Miskowski and B. Zhong, J. Am. Chem. Soc. 1989, 111, 6177.

60.F. A. Cotton, T. Ren and J. L. Eglin, J. Am. Chem. Soc. 1990, 112, 3439.

446

Multiple Bonds Between Metal Atoms

Chapter 10

61.T. Ren Ph.D. Dissertation, Texas A&M University, 1990.

62.S. F. Gheller, G. A. Heath and R. G. Raptis, J. Am. Chem. Soc. 1992, 114, 7924.

63.G. A. Heath and R. G. Raptis, Inorg. Chem. 1991, 30, 4106.

64.F. A. Cotton, P. E. Fanwick, L. D. Gage, B. Kalbacher and D. S. Martin, J. Am. Chem. Soc. 1977, 99, 5642.

65.F. A. Cotton and D. G. Nocera, Acc. Chem. Res. 2000, 33, 483.

66.R. J. H. Clark, A. J. Hempleman and D. A. Tocher, J. Am. Chem. Soc. 1988, 110, 5968.

67.R. J. H. Clark and A. J. Hempleman, J. Chem. Soc., Dalton Trans. 1988, 2601.

68.D. J. Santure, J. C. Huffman and A. P. Sattelberger, Inorg. Chem. 1985, 24, 371.

69.C. Oldham, J. E. D. Davies and A. P. Ketteringham, J. Chem. Soc., Chem. Comm. 1971, 572.

70.M. Kurmoo and R. J. H. Clark, Inorg. Chem. 1985, 24, 4420.

71.C. D. Tait, J. M. Garner, J. P. Collman, A. P. Sattelberger and W. H. Woodruff, J. Am. Chem. Soc. 1989, 111, 7806.