No abstract

No abstract

No abstract

No abstract

Fischer's landmark discovery of transition-metal alkylidyne complexes two decades ago' initiated an extensive research effort that has resulted in the development of a large, diverse class of molecules that exhibit fascinating reaction chemistry.2 Much of the descriptive chemistry of these species hinges on the attribution of a formal bond order of 3 to the linkage between the alkylidyne ligand (CR) and the metal. This bonding description was initially based on the required valency of the ligated carbon atom1 and on X-ray crystallographic studies, the latter of which yielded the expected findings of a short M-C bond and a nearly linear M-C-R linkage.3 Direct evidence for the high multiplicity of the M-C bond order was ultimately provided by the sizable M-C force constant (k(W=C) = 7.00-7.40, k ( C e C ) = 5.18 mdyn A-l), which was determined, together with the M=C stretching frequency (v(M=C) = 1300-1400 cm-I), from an elegant series of vibrational spectroscopic experiments undertaken by Dao, Fischer, and co-workers on thearchetypal M(=CR)(C0)4X (M = Cr, Mo, W; R = Me, Ph; X = anionic ligand) complexes.4 Although v(M=CR) bands with similar frequencies have subsequently been observed for other alkylidyne compounds,2e~5 an acknowledged complication with the determination of v(M=C) and k(M=C) for M(CR)(CO)dX complexes arises from the vibrational complexity of their R groups; the M=C stretching coordinate is strongly coupled with other symmetry coordinates, especially the MC-C stretch and deformations of the R group, f University of Pittsburgh. t Wabash College. (1) Fischer, E. 0.; Kreis,G.; Kreiter, C. G.; Muller, J.; Huttner,G.; Lorenz, H. Dao,N. Q.; Fevrier, H.; Jouan, M.; Huy, N. H. T.; Fischer, E. 0.; Neugebauer, D. J. Organomet. Chem. 1982,241, C53-C56. (f) Dao, N. Q.; Fevrier, H.; Jouan, M.; Fischer, E. 0. Noun J. Chim. 1983,7,719-724. (9) Fischer, E. 0.; Friedrich, P.; Lindner, T. L.; Neugebauer, D.; Kreissl, F. R.; Uedelhoven, W.; Dao, N. Q.; Huttner, G. J . Organomet. Chem. 1983,247,239-246. (h) Dao, N. Q.; Fevrier, H.; Jouan, M.; Fischer, E. 0.; Rbll, W. J. Orgunomer. Chem. 1984,275,191-207. (i) Dao, N. Q.; Jouan, M.; Fonseca, G. P.; Huy, N. H. T.; Fischer, E. O.;J. Organomet. Chem. 1985,287,215-219. 6 ) Dao, N. Q.; Foulet-Fonseca, G. P.; Jouan, M.; Fischer, E. 0.; Fischer, H.; Schmid, J. C. R. Acad. Sci. Ser. 2 1988, 307, 245-250. (k) Foulet-Fonseca, G. P.; Jouan, M.; Dao, N. Q.; Fischer, H.; Schmid, J.; Fischer, E. 0. Spectrochim. Acta 1990,46A, 339-354. (1) Foulet-Fonseca, G. P.; Jouan, M.; Dao, N. Q.; Huy, N. H. T.; Fischer, E. 0.

The electrochemistry and electronic structures of over 30 tungsten-alkylidyne compounds of the form W(CR)L(n)L'(4-n)X (R = H, Bu(t), Ph, p-C6H4CCH, p-C6H4CCSiPr(i)3; X = F, Cl, Br, I, OTf, Bu(n), CN, OSiMe3, OPh; L/L' = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)3, CO, CNBu(t), py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal-ligand interactions. The first oxidation potentials are found to span a range of ∼2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal d(xy) orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the d(xy) orbital, being much more sensitive to the nature of the equatorial L ligands (π symmetry, with respect to d(xy), ΔE1/2 ≅ 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to d(xy), ΔE(1/2) < 0.3 V/L). The oxidation potential is linearly correlated with the calculated d(xy) orbital energy (slope ≅ 1, R(2) = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.

Complexes that luminesce in fluid solution from ligand-tometal charge-transfer (LMCT) excited states are scarce, in marked contrast to the ubiquitous metal-to-ligand charge-transfer chromophores whose emission properties have been the dominant subjects of study in inorganic photochemistry over the past two decades.' LMCT states are formally characterized by a hole in the electron shell at the ligand; complexes for which such states are long-lived in solution are therefore of interest as potential photochemical oxidants. Unfortunately, the systematic development of emissive LMCT chromophores is hindered by the fact that the few such complexes reported to luminesce in fluid solution-d5-configured ReCp*2 (Cp* = C5-Me5)2 and [Re(PMe2CH2CH2PMe2)3I2+ and do-configured ScCp*2X (X = C1, NHPh): [T~(NR'){OZP(OR>~}~I~ (R = CMe3, SiMe3; R' = CMe3, CMe2Et),5 and TaCp*& and TaCp*Cl3X' (X = C1, Br; X' = 02CR, 03SMe)6-constitute too electronically disparate a class of compounds for general design principles to have been deduced from them. Herein we report the spectroscopic and photophysical properties of group V do-configured aryl-imido compounds of the type cis,mer-M(~NAr*)X3L2,7.8 which luminesce in fluid solution at room temperature, and comment on the potential generality of the emissive LMCT state they possess.Ta CI dme 1 Ta CI (tho, 3 Ta CI tmeda 4 Ta Br h e 5 Ta Br uneda 6 Nb C1 dme 7 Ta CI ( P Y~ 2The electronic-absorption spectra of niobium and tantalum M(NAr*)X3L2 compounds in toluene solution at room temperature exhibit a weak band (6 lo2 M-' cm-' ) with a maximum in the range 19 000-22 500 cm-' (band I; Table 1) and a strong band ( E c 104 M-' cm-I) in the range 31 000-34 000 cm-l (band 11; Table 1) as the two lowest-energy features. Figure 1 shows the absorption spectrum of 1, which is representative of those of the M(NAr*)X3L2 class. Emission from M(NAr*)-X3L2 compounds upon excitation into band I or I1 is observed (1) (a) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum hess: New York, 1994. (b) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: San Diego, 1992. (2) Bandy, J. A.; Cloke, F. G. N.; Cooper, G.; Day, J. P.; Girling, R. B.; Graham, R. G.; Green, J. C.; Grinter, R.; Perutz, R. N.(8) Complexes 4-7 were prepared by procedures analogous to those reported previously for 1-3 (ref 7). Elemental analyses (C, H, N) and 'H-NMR spectra consistent with the formulations of 4-7 were obtained; these data are available as supporting information. Ligand abbreviations: Ar* = 2,6-diisopropylphenyl; dme = 1.2-dimethoxyethane; py = pyridine; thf = tetrahydrofuran; tmeda = N,N,K,K-tetramethylethylenediamine. 114, 6905-6906. 3860-3868. 0002-7863/95/15 17-12340$09.00/0Table 1. Electronic Spectroscopic and Photophysical Data for M(NAr*)X3L2 Compounds at Room Temperature" emission absorption V,,, cm-' (6, M-' cm-I) kn k m

No abstract