Stable formally zerovalent and diamagnetic monovalent niobium and tantalum complexes based on diazadiene ligands

Article abstract of DOI:10.1021/ja017303t

This paper describes the efficient synthesis and full characterization of rare formally zerovalent and diamagnetic monovalent pseudooctahedral niobium and tantalum complexes. The reaction of NbCl₄(thf)₂ with 4 equiv of Na and 3.5 equ

Full text of DOI:10.1021/ja017303t

Published on Web 03/20/2002
Stable Formally Zerovalent and Diamagnetic Monovalent Niobium and
Tantalum Complexes Based on Diazadiene Ligands
P. James Daff,§ Michel Etienne,*,§ Bruno Donnadieu,§ Sushilla Z. Knottenbelt,† and
John E. McGrady*,†
Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 Route de Narbonne,
31077 Toulouse Cedex 4, France, and Department of Chemistry, UniVersity of York,
Heslington, York YO10 5DD, United Kingdom
Received October 16, 2001
There has been a long-standing quest for readily available sources
of zerovalent niobium and tantalum complexes. The consistent
failure to generate paramagnetic 17-electron (17e) hexacarbonyl
complexes M(CO)6 (M ) Nb, Ta) on a synthetic scale contrasts
1
markedly with the availability of V(CO)6 and also of the more
highly reduced 18e complexes, [M(CO)6]-, [M(CO)5]3-, and
[M(CN-2,6-Me2C6H3)6]-.2 Relatively pure Ta(CO)4(dppe) [dppe )
1,2-bis(diphenylphosphino)ethane], a dimer in the solid state, can
be generated on a limited scale via hydride abstraction from TaH-
(CO)4(dppe).3 Metal vapor techniques have yielded M(arene)2 4 and
M(dmpe)3 [dmpe ) 1,2-bis(dimethylphosphino)ethane],5 but only
Nb(1,3,5-Me3C6H3)2 is available via solution chemistry.4b There
has been a brief report of M(2,2′-bipyridine)3 and M(1,10-phenan-
throline)3,6 but there is a clear need for more accessible routes to
stable zerovalent niobium and tantalum complexes. 1,4-Diazabuta-
1,3-diene (R-diimine) ligands have long been known to stabilize
transition metals in low-oxidation states,7 and there has been a recent
resurgence in interest in their chemistry with the heavier group 5
metals.8 We report herein the efficient synthesis and full charac-
terization of formally zerovalent 17e niobium and tantalum
complexes, M(iPr2-dad)3, containing the readily available 1,4-
diisopropyl-1,4-diazabuta-1,3-diene (iPr2-dad). These complexes are
also oxidized to diamagnetic 16e cations [M(iPr2-dad)3]+, and the
two oxidation states are investigated using density functional theory.
Addition of Na/naphthalene (5 mol equiv) in 1,2-dimethoxy-
ethane (dme) to a dme solution of TaCl5 at -60 °C generates an
orange-brown slurry to which iPr2-dad (3.5 mol equiv) was added.
Workup of the resulting dark red-brown slurry yielded Ta(iPr2-
dad)3 (1a) as red-brown crystals in 33% yield. Dark-green Nb-
(iPr2-dad)3 (1b) is more conveniently obtained from NbCl4(thf)2
and Na sand in 52% yield. Reactions have been conducted
repeatedly on multigram scales. 1a,b, although extremely air
sensitive, are indefinitely stable under an inert atmosphere up to
80 °C. Both compounds give satisfactory elemental analyses,9 and
molecular ions are observed in the mass spectra. These formally
zerovalent 17e complexes behave as paramagnets with one unpaired
electron (µeff ) 1.66 B. M. down to 11 K for 1a). Pentane or toluene
solutions (fluid or frozen) give a broad (∆Hpp ) 55 to 26 G) esr
signal centered at g ) 2.025 (1a) with no resolved coupling pattern,
indicating very little metal contribution to the SOMO. Dark-red
1a shows blue-shifted UV-vis absorption bands [512 (550 sh) nm
(ꢀ 2800 L‚mol-1‚cm-1), 374 (6100), 304 (8900)] as compared to
green 1b [636 (2700), 434 (5300), 336 (5200)]. 1a,b are isomor-
phous in the crystal (monoclinic, P2/c),10 and 1a adopts a distorted
Figure 1. Molecular structure of [Ta(iPr2-dad)3] (1a).
octahedral geometry (Figure 1) with planar iPr2-dad ligands. The
1
Ta atom lies on a special position (0 y /4), and a C2 axis bisects
the C(1)-Ta-C(1)′ angle. Ta-N bond lengths are 2.088(6), 2.096-
(4), and 2.100(6) Å. C-N bond lengths of 1.365(7), 1.370(9), and
1.397(8) Å and C-C bond lengths of 1.348(12) and 1.369(8) Å
suggest some enediamido character. 1a,b remain electron-rich,
exhibiting two fully reversible single-electron processes in the CV
(1a, -1.93 and -1.03 V vs ferrocenium/ferrocene), as well as an
irreversible oxidation process (1a, 0.12 V). Whatever the electron
density distribution within these complexes (see below), 1a,b are
among the very few examples of fully characterized, readily
available, formally zerovalent Nb and Ta complexes. Both the
reductant and the nature of the ligand seem crucial to the successful
synthesis of 1b, since utilization of Li, tBu2-dad and NbCl5 (10:
5:2 ratio) yields a diamagnetic dimer that displays three different
coordination modes.8e
Addition of AgBPh4 to 1a,b generates the cation [M(iPr2-dad)3]+-
[BPh4]- [M ) Ta (2a), Nb (2b)] as dark-violet crystals.11 The
overall X-ray molecular structure12 of 2a (Figure 2) is conspicuously
similar to that of 1a although there is no special position occupied.
Ta-N bond lengths vary between 2.074(2) and 2.135(2) Å, C-N
bond lengths between 1.327(4) and 1.365(3) Å, and C-C bond
lengths between 1.347(4)) and 1.380(4) Å. Somewhat surprisingly,
the Ta-N bonds in 2a are essentially identical to those in 1a, but
there is a small but significant shortening of the C-N bonds.
Unusually for pseudo-octahedral 16e species, 2a,b are diamagnetic
* To whom correspondence should be addressed. E-mail: (M.E.) etienne@
lcc-toulouse.fr, (J.E.M.) jem15@york.ac.uk.
§ LCC, Toulouse.
† University of York.
9
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J. AM. CHEM. SOC. 2002, 124, 3818-3819
10.1021/ja017303t CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Structural Data for Ta(iPr2-dad)3 and [Ta(iPr2-dad)3]+ as
Determined by X-ray Diffraction (average) and by DFT
Calculations (Bond Lengths in Å, Energies in eV)
method
state
Ta−N
C−N
E
rel
2a
1a
X-ray
DFT
X-ray
DFT
DFT
-
2.107(2)
2.159
2.095(6)
2.156
1.347(4)
1.352
1.377(9)
1.371
-
-
-
1A1
-
2A2
2A1
0.0
2.182
1.351
+0.64
2A2 counterpart, indicating a clear thermodynamic preference for
a ligand-based redox process. This conclusion is reinforced by the
optimized structural parameters (Table 1), where the redox-induced
2
changes in bond length in the A2 state are far more consistent
2
with experiment than those in A1.
Acknowledgment. We thank the CNRS and the LCC (P.J.D.,
M.E.), the EPSRC (J.E.M.), and the ORS Award Scheme (S.Z.K.).
Supporting Information Available: Details of the synthesis and
characterization of 1a,b and 2a,b; X-ray diffraction data for 1a and
2a; computational details (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Molecular structure of the cation [Ta(iPr2-dad)3]+ (2a).
References
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(9) For 1a. Anal. Calcd for C24H48N6Ta: C, 47.91; H, 8.04; N, 13.97.
Found: C, 47.40; H, 8.03; N, 13.46. MS-CI (NH3): m/e 602 (M + H)+.
CV (Bu4NBF4, THF, 100 mV‚s-1): E1/2 -1.93 [1a/1a-1], -1.03 [1a+/
1a], Ep -0.12 V vs ferrocenium/ferrocene. 1H NMR (dichloromethane-
d2, 300 MHz, 300 K): δ 0.29 (s, 3 H, w1/2 155 Hz, CH3), -2.34 (s, 3 H,
w1/2 90 Hz, CH3), other signals unobserved.
(10) Crystal data for 1a: C24H48N6Ta, monoclinic, P2/c, T ) 180(2) K, a )
9.955(2) Å, b ) 15.939(3) Å, c ) 9.385(2) Å, â ) 110.73(3)°, V )
1392.7(5) Å3, Z ) 2, R1 ) 0.0227, wR2 ) 0.0477 (2275 reflections, 147
parameters), GOF ) 1.011.
(11) For 2a: Anal. Calcd for C48H68BN6Ta: C, 62.60; H, 7.44; N, 9.13.
Found: C, 62.45; H, 7.42; N, 9.08. MS-CI (NH3): m/e 601 ([Ta(iPr2-
dad)3]+), MS-ES: m/e 319 (BPh4-). 1H NMR (dichloromethane-d2, 300
MHz, 300 K): δ 7.37 (br, 8 H, o-C6H5), 7.07 (t, 8 H, m-C6H5), 6.88 (t,
4 H, p-C6H5), 6.56 (s, 6 H, CHdN), 3.54 (septet, 6 H, CH(CH3)2), 1.29
(d, 18 H, CH3), 1.14 (d, 18 H, CH3).
Figure 3. Frontier molecular orbitals for [Ta(iPr2-dad)3]+.
and exhibit temperature-independent (300-193 K) 1H NMR
spectra.11
A number of the experimental features described above are
inconsistent with a simple pseudo-octahedral ligand field model,
prompting us to examine the detailed electronic structure of these
compounds using density functional theory (DFT).13 The frontier
molecular orbitals for [Ta(dad)3]+ are shown in Figure 3. The most
striking feature of the orbital array is the strong splitting within
the t2g subset of orbitals (19e and 11a1). In D3 symmetry, the
LUMOs (i.e., π*) of the three ligands transform as a2 + e, as a
result of which, only 19e is strongly stabilized by back-bonding,
leading to the unusual diamagnetic ground state. Moreover, the
relative energies of the metal- and ligand-based orbitals are such
that the nonbonding combination of ligand LUMOs, 10a2, falls in
the gap between 19e and 11a1 and forms the LUMO of complex
(12) Crystal data for 2a: C48H68BN6Ta, monoclinic, P21/n, T ) 298(2) K, a
) 12.873(5) Å, b ) 20.883(5) Å, c ) 17.307(5) Å, â ) 99.215(5)°, V )
4593(2) Å3, Z ) 4, R1 ) 0.0209, wR2 ) 0.0495 (7040 reflections, 517
parameters), GOF ) 1.022.
(13) DFT calculations (Amsterdam Density Functional package)14 were
performed using the gradient corrections of Becke15a and Perdew (BP86)15b
Double-ú + polarization and triple-ú basis sets were used for main group
and transition metals, respectively. Relativistic effects were included using
the zeroth order relativistic approximation (ZORA). In all cases, the
terminal CH3 groups were replaced with hydrogens.
2
2a. The A2 ground state of the neutral complex, 1a, arises from
addition of a single electron to 10a2, indicating that the neutral
complex is correctly formulated as [Ta]+ [(iPr2-dad)3]- (16e + 1e),
rather than [Ta]0 [(iPr2-dad)3]0 (17e). The unpaired electron has
negligible metal character and is instead delocalized over the ligand
array, rationalizing both the esr and magnetic behavior. The
alternative formulation, where the Ta center is zerovalent, is realized
in the 2A1 state, resulting from promotion of the unpaired electron
into the 11a1 orbital. This state is located some 0.64 eV above its
(14) (a) ADF 2000.02, Theoretical Chemistry; Vrije Universiteit: Amsterdam,
2000. (b) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 42-
51. (c) te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84-98.
(d) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322-328.
(15) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100. (b) Perdew, J. P.
Phys. ReV. B 1986, 33, 8822-8824.
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