The reduction of Nb2Cl6(tmeda)2 by R2NLi. Formation of a diamagnetic niobium(II) cluster with short Nb-Nb triple bond and amide C-N bond cleavage

Article abstract of DOI:10.1039/a703280c

Reduction of trivalent Nb₂Cl₆(tmeda)₂ with either lithium diisopropylamide or n-butyllithium yields a novel Nb^II cluster which performs oxidative addition into the amide C-N bond.

Full text of DOI:10.1039/a703280c

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The reduction of Nb2Cl6(tmeda)2 by R2NLi. Formation of a diamagnetic
niobium(ii) cluster with short Nb–Nb triple bond and amide C–N bond
cleavage
Maryam Tayebani, Aparna Kasani, Khalil Feghali, Sandro Gambarotta* and Corinne Bensimon
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Reduction of trivalent Nb2Cl6(tmeda)2 with either lithium
diisopropylamide or n-butyllithium yields a novel NbII
cluster which performs oxidative addition into the amide
C–N bond.
(M
=
Nb, Ta; tmeda
=
N,N,NA,NA-tetramethyl-
ethylenediamine)4 with R2NLi derivatives. Herein we describe
our preliminary findings.
A stoichiometric 1:3 ratio of Nb to amide, as well as an
extended reflux in thf, was always necessary to complete the
Halide complexes of divalent Nb and Ta are particularly rare
and limited to a few di- and poly-nuclear structures of halide
derivatives mainly bridged by tetrahydrothiophene (THT)
ligands.1 The M–M triple bonds in these species are surprisingly
long and thus to evaluate their properties it is important to
prepare a variety of multiply bonded systems with the greatest
possible diversity of ligand environments.
This work was prompted by the simple idea that low-valent
niobium and tantalum amides may be able to form polymetallic
structures with or without bridging ligands. The choice of these
anions as supporting ligands was advised by their well
established ability to stabilize highly reactive medium valent
early transition metals.2 With the dual purpose of bypassing the
difficulties arising from the treatment of MCl5 (M = Nb, Ta)
starting materials with anionic amides3 and possible complica-
tions arising from their subsequent reduction, and in an attempt
to gain some insight into the stability of trivalent Nb and Ta
amide, we have now reacted the dinuclear [MCl3(tmeda)]2
reactions
[RRANMA
of
[NbCl3(tmeda)]2
with
RRANMA
=
Ph2NNa, Pri2NLi, (C6H3Me2-3,5)(Ad)NLi
(Ad = adamantyl)] and to avoid the presence of unreacted
starting materials in the reaction mixtures. However, in the
particular case of LDA (lithium diisopropylamide) the reaction
was completed in a few hours at room temp. After suitable
work-up, well formed, dark-brown, and air-sensitive crystals of
a new diamagnetic product were isolated in poor yield. The IR
and the NMR of the crystalline product clearly indicated that the
complex contained two non-equivalent tmeda ligands. The
presence of lithium, suggested by qualitative flame tests, was
confirmed by 7Li NMR spectroscopy. Combustion analysis data
were in agreement with the formulation (tmeda)3Nb2Cl5Li 1.
An X-ray structure confirmed the formula and elucidated the
molecular connectivity (Fig. 1).†
The crystal structure of 1 provides a rare example of a
dinuclear niobium(ii) complex. The diamagnetism and the very
short Nb–Nb distance [Nb–Nb 2.4001(5) Å], suggest the
C(6)
Cl(2)
C(2)
N(2)
C(3)
Cl
M
Cl
M
N
N
Cl
Cl
N
C(5)
Cl(1)
Nb(1)
R2NLi
C(1)
C(4)
Nb(1)
N(1)
M = Nb
N
Cl
Cl
Cl(3)
Li(1)
Cl(4)
C(9)
N(3)
M = Ta LLi
M = Nb
N(4)
C(7)
LLi
C(10)
LLi
O
Ta
N
N
N
N
Nb
Nb
N
N
N
LLi = (C6H3Me2-3,5)(Ad)NLi
3
2
Fig. 1 Selected bond distances (Å) and angles (°) for complex 1: Nb(1)–Nb(1a) 2.4001(5), Nb(1)···Li(1) 3.217(8), Nb(1)–Cl(1) 2.5712(9), Nb(1)–Cl(2)
2.579(1), Nb(1)–Cl(3) 2.569(1), Nb(1)–Cl(4) 2.5583(9), Nb(1)–N(1) 2.390(2), Nb(1)–N(2) 2.366(3), Li(1)–Cl(4) 2.489(3), Li(1)···Cl(3) 2.840(8),
Li(1)···Cl(1) 3.031(9), Li(1)–N(3) 2.199(9), Li(1)–N(4) 2.162(9), Cl(1)–Nb(1)–Cl(2) 95.90(3), Cl(1)–Nb(1)–Cl(3) 108.13(3), Cl(1)–Nb(1)–Cl(4) 84.39(3),
Cl(1)–Nb(1)–N(1) 165.65(8), Cl(1)–Nb(1)–N(2) 85.83(7), Cl(2)–Nb(1)–Cl(3) 95.28(4), Cl(2)–Nb(1)–Cl(4) 84.16(3), Cl(2)–Nb(1)–N(1) 86.24(8), Cl(2)–
Nb(1)–N(2) 85.45(8), Cl(3)–Nb(1)–Cl(4) 84.16(3), Cl(3)–Nb(1)–N(1) 85.73(8), Cl(3)–Nb(1)–N(2) 165.82(7), Cl(4)–Nb(1)–N(1) 93.60(8), Cl(4)–Nb(1)–
N(2) 95.06(8), N(1)–Nb(1)–N(2) 80.2(1).
Chem. Commun., 1997
2001

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of thf. Accordingly, significant amounts of CH4 and CH3CH3
(27 and 31%, respectively based on Ta) were recovered from the
reaction mixture (Toepler pump and GC combined experi-
ments).
This work was supported by the Natural Sciences and
Engineering Council of Canada (NSERC).
Footnotes and References
HOMO
(-10.85 eV)
HOMO -1
(-11.24 eV)
HOMO -2
(-12.17 eV)
* E-mail: sgambaro@oreo.uottowa.ca
† Crystal data: C18H48LiCl5N6Nb2 1, M = 718.64, orthorhombic, space
group Cmc21, a = 15.2097(2), b = 16.8896(2), c = 12.0285(1) Å,
U = 3089.95(6) Å3, Z = 4, Dc = 1.545 g cm23, F(000) = 1471.49, m
= 11.9 cm21, T = 2145 °C, R = 0.026, Rw = 0.031, GOF = 1.01 for 196
parameters and 2972 reflections out of 3508 unique collected with a
Siemens CCD diffractometer. C40H58N2NbO 2, M = 675.81, monoclinic,
space group P21/c, a = 12.5969(2), b = 14.3835(1), c = 20.3103(3) Å,
Fig. 2 Pictorial view of the frontier orbitals of 1
presence of a Nb–Nb triple bond.5 However, the shortness of the
intermetallic distance contrasts with those of the few previously
reported divalent diniobium or ditantalum THF bridged com-
plexes,1 whose unusually long M–M distances were ascribed to
an intrinsic weakness of the metal–metal bonds.1b Theoretical
calculations carried out on the atomic crystallographic coordi-
nates of 1 show that the three highest occupied molecular
orbitals are mainly Nb–Nb centered with a metal atom
contribution of predominantly d-orbital character and a minor
but significant contribution from the p orbitals of the bridging
chlorine atoms. The HOMO–LUMO gap (0.53 eV) is rather
small yet sufficient to account for the observed diamagnetism in
solution. The three MOs are formed by the overlap of hybrid
atomic orbitals of the two niobium atoms. The shape of the
HOMO (Fig. 2) is reminiscent of a M–M d bond while the
LUMO (210.32 eV) is the corresponding out-of-phase combi-
nation. The next occupied molecular orbital, located at 211.24
eV is also M–M centered and has some p-bond character, while
the third next orbital (212.17 eV) is a regular s bond lying
symmetrically on the intermetallic vector. At this stage is not
clear which factor (electronic or steric) is responsible for the
short Nb–Nb distance. Certainly, we cannot rule out the
possibility that the lithium cation may ultimately determine the
intermetallic distance by bringing together the two vertices of
the two octahedra.
The reduction of the metal center to the divalent state by LDA
implies that the amide acted as a strong reductant.6 Thus, it was
possible to prepare 1 in higher yield by simply reacting
[NbCl3(tmeda)]2 with 2 equiv. of BunLi in ether and in the
presence of a small excess of tmeda. Even though the ability of
amides to act as reductants is documented in the literature,7 the
fact that the same reaction with Ph2NNa (instead of LDA) gave
the tetravalent derivative (Ph2N)4Nb (45%)8 suggests that the
reduction of the metal center during the formation of 1 might
occur through the disproportionation of an unstable niobium(iii)
intermediate. In addition, the reaction of [NbCl3(tmeda)]2 with
(C6H3Me2-3,5)(Ad)NLi yielded the tetravalent, dinuclear
and diamagnetic [C6H3Me2-3,5(Ad)N]Nb(3,5-Me2Ph)}2(m-
NAd)2·2Et2O 2 (Fig. 1).† However, this product is likely to be
originated by the further reaction of 1 with the lithium amide
(the reaction requires reflux in thf for several hours in this case)
to form a transient [(C6H3Me2-3,5)(Ad)N]2Nb species which
generates 2 via oxidative addition into the C–N bond of the
amide function.9 Accordingly, direct reaction of 1 with 2 equiv.
of (C6H3Me2-3,5)(Ad)NLi also gave complex 2 in crystalline
form.
b = 104.223(1)°, U = 3567.17(8) Å3, Z = 4, Dc = 1.258 g cm23
,
F(000) = 1436.34, m = 3.7 cm21, T = 2145 °C, R = 0.045, Rw = 0.057,
GOF = 1.05 for 397 parameters and 6960 reflections out of 9089 unique
collected with
a
Siemens CCD diffractometer.
C
a
64H95N3O2Ta 3,
M
=
1119.41, monoclinic, space group P21/n,
=
14.2622(2),
b = 25.3107(1), c = 16.1753(2) Å, b = 101.706(3), U = 5717.6(1) Å3,
Z
= 4, Dc = = 2355.36, m = ,
1.300 g cm23, F(000) 1.96 cm21
T = 2165 °C, R = 0.033, Rw = 0.041, GOF = 1.41 for 641 parameters and
11 040 reflections out of 14 513 unique collected with a Siemens CCD
diffractometer. CCDC/602.
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Engl., 1986, 25, 996; (d) D. M. Curtis and J. Real, Organometallics, 1985,
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3 Ad hoc synthetic procedures have been developed for niobium and
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1993, 12, 2899; Y. W. Chao, S. Polson and D. E. Wigley, Polyhedron,
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Inorg. Chem., 1983, 22, 2127.
6 Typically, strong reducing agents such as alkali metals, amalgams or
potassium graphite are necessary for obtaining divalent niobium. See for
example: M. A. Araya, F. A. Cotton, J. H. Matonic and C. A. Murillo,
Inorg. Chem., 1995, 34, 5424; E. Guggloz, M. L. Ziegler, H. Biersack and
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The reaction of [TaCl3(tmeda)]2 with amides took a different
pathway. While the majority of these reactions led to intractable
materials, in the case of the reaction with (C6H3Me2-
3,5)(Ad)NLi the pentavalent [(C6H3Me2-3,5)(Ad)N]3Ta(O) 3
was isolated and characterized.‡ The complex is likely to be
originated by the reaction of the initially formed [(C6H3Me2-
3,5)(Ad)N]3Ta species with thf since reactions carried out in
different solvent led to different results. This hypothetical
intermediate obviously possesses a greater stability with respect
to the niobium analogue and performs a two-electron reduction
8 Identified by comparison of the cell parameters and analytical data with
those of an analytically pure sample: S. G. Bott, D. M. Hoffman and
S. P. Rangarajan, Inorg. Chem., 1995, 34, 4305 and references therein.
Received in Bloomington, IN, USA; 12th May 1997; 7/03280C
2002
Chem. Commun., 1997