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*)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 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 (ν5) and 1350 cm-1
(ν3), 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-1
(ν2) 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 (ν1),
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.