TWO PHOTON EXCITATION SPECTRUM OF A TWISTED QUADRUPLE BOND METAL-METAL COMPLEX

The overlap of two orbitals, each containing an electron, defines the most significant chemical interaction between atoms. The electron population of the resulting bonding and antibonding orbitals gives rise to four electronic states: a singlet ground state, one-electron triplet and singlet excited states, and a two-electron, singlet excited state. Although recognized at the inception of valence1 and molecular orbital2 bonding theories, these four states are difficult to spectroscopically characterize for an isolated twoelectron bond of a molecular species. The challenge comes in the identification of the highest energy singlet state, which amounts to the promotion of two electrons into an antibonding orbital. For a σ bond, this state is dissociative, and for a π bond, the state is energetically unstable with respect to rotation.3,4 Conversely, overlap of the dxy orbitals on the metal centers of a bimetallic core presents a stable two-electron δ bond, which is ideally adapted for spectroscopic investigation when incorporated within the sterically constrained coordination environment of quadruple metal-metal bonded species.5 In this case, the four states resulting from electron occupation of the δ HOMO and the δ* LUMO are 1δδ, 3δδ*, 1δδ*, and 1δ*δ*, each of which has been independently characterized. The dipole-allowedness of the 1δδ f 1δδ* transition led to the identification of the 1δδ ground and 1δδ* excited states early in the study of the electronic absorption spectroscopy of these compounds.5,6 The espial of the 3δδ* state has come in quadruple-bond complexes featuring bridging phosphines that induce rotation about the metal-metal bond. In these complexes, the δ bond is sufficiently weakened that the 3δδ* state is lowered in energy and can be populated at reasonable temperatures, thus allowing its energy to be determined from temperature-dependent measurements of the magnetic susceptibility7 and of ligand 31P{1H} NMR signals.8 Most recently, we have demonstrated that the 1δ*δ* excited state, which is forbidden by the selection rules of conventional optical spectroscopy, may be located by two-photon spectroscopy of quadruplebond metal-metal complexes possessing eclipsed ligand geometries.9 Although the methods for detection of the individual states that define the δ bond have been assembled, a complete spectroscopic description of the two-electron bond in a single complex has not yet been achieved owing to the spectroscopic incompatibility of the magnetic and electronic measurements. Significant population of the 3δδ* state requires weakening of the bond to such an extent that the electronic excitation and detection wavelengths required for the two-photon experiment occur in the spectrally troublesome near-infrared region. With advances in our spectroscopic instrumentation, we now report the successful detection of the 1δ*δ* excited state of the twisted â-Mo2Cl4(S,S-dppb)2 complex (S,Sdppb ) S,S-bis(diphenylphosphino)butane, o ) 24.7°). This result, together with Cotton’s previous measurements of the 3δδ* and 1δδ* excited states for this complex,8,10 provides a complete description of the four states that characterize a two-electron bond in a discrete molecular species. The 1δδ f 1δδ* transition of â-Mo2Cl4(S,S-dppb)2 in 3methylpentane appears at λabs,max ) 741 nm (Figure 1a).10 On the basis of this low-energy absorption, corresponding luminescence is expected to occur, if at all, in the near-infrared spectral region where the response function of conventional detectors is compromised. We addressed this issue by rebuilding the detection system of a previously described emission instrument so that the pertinent spectral region could be accessed.11 3-Methylpentane glasses of â-Mo2Cl4(S,S-dppb)2, excited with 740-nm light, produce the luminescence profile shown in Figure 1b. As is the case for the 1δδ* excited state of many quadruple bond metalmetal complexes, the emission spectrum (λem,max ) 921 nm) is nearly a mirror image of the absorption spectrum and a Stokes shift of ∼2500 cm-1 is observed. The detection of this near-infrared emission allows us to investigate the two-photon spectroscopy of the complex. In this experiment, the 1δ*δ* state is pumped by two near-infrared photons, followed by internal conversion to the emissive 1δδ* excited state. The 1δ*δ* excitation profile is constructed by monitoring the intensity of the 1δδ* luminescence as the infrared excitation wavelength (normalized in intensity) is scanned in to and out of resonance with the 1δ*δ* state. Consistent with this experimental scheme, excitation of the 3-methylpentane glass (60 K) of â-Mo2Cl4(S,S-dppb)2 over the spectral range 1100 to 1350 † Department of Chemistry, Michigan State University. (1) Heitler, W.; London, F. Z. Phys. 1927, 44, 455-72. (2) Coulson, C. A.; Fischer, I. Philos. Mag. 1949, 40, 386-93. (3) Merer, A. J.; Mulliken, R. S. Chem. ReV. 1969, 69, 639-56. (4) Transient absorption spectra consistent with a 1π*π* excited state of twisted olefins have recently been reported; see Piotrowiak, P.; Strati, G.; Smirnov, S. N.; Warman, J. M.; Schuddeboom, W. J. Am. Chem. Soc. 1996, 118, 8981-2. (5) Cotton F. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 2nd ed.; Clarendon Press: Oxford, 1993. (6) Trogler, W. C.; Gray, H. B. Acc. Chem. Res. 1978, 11, 232-9. (7) Hopkins, M. D.; Zietlow, T. C.; Miskowski, V. M.; Gray, H. B. J. Am. Chem. Soc. 1985, 107, 510-2. (8) (a) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. J. Am. Chem. Soc. 1992, 114, 4915-7. (b) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. Inorg. Chem. 1993, 32, 2104-6. (9) Engebretson, D. S.; Zaleski, J. M.; Leroi, G. E.; Nocera, D. G. Science 1994, 265, 759-62. (10) Agaskar, P. A.; Cotton, F. A.; Fraser, I. F.; Manojlovic-Muir, L.; Muir, K. A.; Peacock, R. D. Inorg. Chem. 1986, 25, 2511-9. (11) Emission spectra were measured on a high-resolution instrument modified with a liquid N2-cooled Ge detector from Applied Detector (403L spectrometer). The signal from the detector was phase-matched to a chopped excitation beam with a PARC EGG instrument control and digital data collection and manipulation were performed through a Labview Graphical User Interface. Figure 1. (a) One-photon absorption (295 K) and (b) emission (λexc ) 740 nm, 60 K) spectra of â-Mo2Cl4(S,S-dppb)2 in 3-methylpentane. (c) The two-photon excitation of fluorescence spectrum in the same solvent at 60 K. Spectrum (a) is described by the right ordinate axis and spectrum (c) by the left ordinate axis. The emission spectrum is normalized to that of the absorption profile and was not corrected for the detector response function. The sharp feature in trace (c) results from the 1184-nm overtone absorption band of 3-methylpentane (see text). 868 J. Am. Chem. Soc. 1999, 121, 868-869