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Molecular Structures and FT-Raman Spectroscopy of Luminescent Niobium and Tantalum Arylimido Compounds

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posted on 19.11.1997, 00:00 by Kurt S. Heinselman, Vincent M. Miskowski, Steven J. Geib, Louis C. Wang, Michael D. Hopkins
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*)Cl3(dme) (1), Ta(NAr*)Cl3(tmeda) (4), Ta(NAr*)Br3(tmeda) (6), and Nb(NAr*)Cl3(dme) (7) were studied by single-crystal X-ray diffraction. The M⋮N bond lengths (M = Ta, 1.771(6)−1.785(6) Å; M = Nb, 1.746(4) Å) and nearly linear M−N−C linkages (>174°) are characteristic of a formal metal−nitrogen triple bond. With the exception of the M⋮N bond distance, which is 0.025 Å 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*)Cl3(py)2 (2), Ta(NAr*)Cl3(thf)2 (3), and Ta(NAr*)Br3(dme) (5) were additionally studied by FT-Raman spectroscopy. The low-frequency regions (≤600 cm-1) of the FT-Raman spectra of 17 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, 160−180 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 (ν5) and 1350 cm-13), exhibit strong shifts as a function of the metal and are assigned to symmetric and antisymmetric combinations of the nominal ν(M⋮N) and ν(N−C) oscillators, respectively; the ν(N−C) mode correlates to mode 13 of benzene and is intrinsically strongly mixed with Ar* internal modes. Two bands at ca. 1300 (ν4) and 1430 cm-12) also shift upon metal substitution, indicating that they, too, arise from modes that contain some ν(M⋮N) character; they correlate to modes 9a and 19a of benzene, respectively. A fifth band, at ca. 1585 cm-11), is assigned to Ar*-localized mode 8a. Bands ν1−ν4 exhibit preresonance enhancement from a [π(M⋮NAr*) → π*(M⋮NAr*)] electronic transition. The implications of these results for the excited-state properties of the compounds are discussed.