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

Metal−imido compounds of the type mer-cis -M(NAr*)X 3 L 2 (M = Nb, Ta; Ar* = 2,6-diisopropylphenyl; X = Cl, Br; L 2 = MeOCH 2 CH 2 OMe (dme), Me 2 NCH 2 CH 2 NMe 2 (tmeda), (C 4 H 8 O) 2 ((thf) 2 ), (C 5 H 5 N) 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⋮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*)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 (≤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, 160−180 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 (ν 5 ) and 1350 cm - 1 (ν 3 ), 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 (ν 4 ) and 1430 cm - 1 (ν 2 ) 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 - 1 (ν 1 ), is assigned to Ar*-localized mode 8a. Bands ν 1 −ν 4 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.