Molecular structures and FT-Raman spectroscopy of luminescent niobium and tantalum arylimido compounds

Metal-imido compounds of the type mer-cis-M(NAr*)X3L2 (M = Nb, Ta; Ar* = 2,6-diisopropylphenyl; X = Cl, Br; L2 = MeOCH2CH2OMe (dme), Me2NCH2CH2NMe2 (tmeda), (C4H8O)(2) ((thf)(2)), (C5H5N)(2) ((py)(2))) have been synthesized and characterized. The compounds Ta(NAr*)Cl-3(dme) (1), Ta(NAr*)Cl-3(tmeda) (4), Ta(NAr*)Br-3(tmeda) (6), and Nb(NAr*)Cl-3(dme) (7) were studied by single-crystal X-ray diffraction. The M drop N bond lengths (M = Ta, 1.771(6)-1.785(6) Angstrom; M = Nb, 1.746(4) Angstrom) and nearly linear M-N-C linkages (>174 degrees) are characteristic of a formal metal-nitrogen triple bond. With the exception of the M drop N bond distance, which is 0.025 Angstrom shorter for 7 than for 1, the structure of the MNAr* fragment is insensitive to the natures of M, X, and L. These four compounds and the derivatives Ta(NAr*)Cl-3(py)(2) (2), Ta(NAr*)Cl-3(thf)(2) (3), and Ta(NAr*)Br-3(dme) (5) were additionally studied by FT-Raman spectroscopy. The low-frequency regions (less than or equal to 600 cm(-1)) of the FT-Raman spectra of 1-7 and of difference spectra of pairs of chloride/bromide and niobium/tantalum compounds exhibit bands attributable to M-X stretching modes (X = Cl, 250-360 cm(-1); X = Br, 180-220 cm(-1)) and bending modes (X = Cl, 160-150 cm(-1); X = Br, 120 cm(-1)); analogous M-L modes could not be identified. In the region 900-1600 cm(-1), five bands associated with the MNAr* fragment are observed. Two bands, at ca. 995 (nu 5) and 1350 cm(-1) (nu 3), exhibit strong shifts as a function of the metal and are assigned to symmetric and antisymmetric combinations of the nominal nu(M drop N) and nu(N-C) oscillators, respectively; the nu(N-C) mode correlates to mode 13 of benzene and is intrinsically strongly mixed with Ar* intemalmodes. Two bands at ca. 1300 (nu 4) and 1430 cm(-1) (nu 2) also shift upon metal substitution, indicating that they, too, arise from modes that contain some nu(M drop N) character; they correlate to modes 9a and 19a of benzene, respectively. A fifth band, at cn. 1585 cm(-1) (nu 1), is assigned to Ar*-localized mode 8a. Bands nu 1-nu 4 exhibit preresonance enhancement from a [pi(M drop NAr*) --> pi*(M drop NAr*)] electronic transition. The implications of these results for the excited-state properties of the compounds are discussed.