A lanthanide-gallium complex stabilized by the N-heterocyclic carbene group

Article abstract of DOI:10.1021/ja0710858

The complex [Nd(L′){Ga(NArCH)2}(N″)(THF)], which exhibits the first f-element-gallium bond, is formed from the reaction between the N-heterocyclic carbene-supported neodymium complex [Nd(L′)(N″)(I)] (L′ = Bu^tNCH₂CH₂{C(NCSiMe₃ CHNBu^t)}; N″ = N(SiMe₃)₂) and the anionic gallium(I) heterocycle [Ga(NArCH)₂][K(tmeda)] (Ar = 2,6-Prⁱ₂C₆H₃). The Nd-Ga bond energy is calculated to be 386 kJ mol^-1. Copyright

Full text of DOI:10.1021/ja0710858

Published on Web 04/06/2007
A Lanthanide-Gallium Complex Stabilized by the N-Heterocyclic Carbene
Group
Polly L. Arnold,*,† Stephen T. Liddle,† Jonathan McMaster,*,† Cameron Jones,*,‡,§ and
David P. Mills‡,§
School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham, NG7 2RD, U.K.,
Department of Chemistry, UniVersity of Wales, Cardiff, P.O. Box 912, Park Place, Cardiff, CF10 3TB, U.K., and
School of Chemistry, Monash UniVersity, P.O. Box 23, Victoria, 3800, Australia
Received February 14, 2007; E-mail: polly.arnold@nottingham.ac.uk; jonesca6@cardiff.ac.uk
Scheme 1. Synthesis of 3
The synthesis of molecular metal-metal bonds is fundamentally
important to the understanding of chemical bonding,1,2 catalysis,
the chemistry of metal surfaces, and of magnetic devices.3-5 Since
the report of quadruple bonds in salts of [Re2Cl8]2-,6 the study of
transition metal-metal bonds has generated significant new knowl-
edge, the most recent milestone exemplified by a Cr(I)-Cr(I) dimer
which formally features a quintuple bond.7 However, in contrast
to transition metals, lanthanide complexes featuring a metal-metal
bond are virtually unknown. The only reported lanthanide com-
plexes that contain unsupported metal-metal bonds are [(THF)-
(Cp)2Lu-Ru(Cp)(CO)2]8 and [(Cp*)2Ln-Al(Cp*)] (Ln ) Eu, Yb).9
In the former, the Lu-Ru bond may be regarded as a polarized
covalent bond, whereas in the latter pair the dative Ln-Al bonds
have been shown to be essentially ionic, with negligible charge
transfer or covalent contributions from the Ln and Al fragments.
Recently, we in the Arnold group reported the facile, one-pot,
high yield synthesis of [Nd(L′)(N′′)(µ-I)]2 (1)10 [L′ ) ButNCH2-
CH2{C(NCSiMe3CHNBut)}; N′′ ) N(SiMe3)2] from [Nd(L)(N′′)2]
and Me3SiI [L ) ButNCH2CH2{C(NCHCHNBut)}]. Compound 1
is an excellent synthon for exploring N-heterocyclic carbene (NHC)-
Ln-Ga bond from treatment of LnCl3 with 3 equiv of 2, or of
Ln(η-C5H5)2Cl with 2 (Ln ) Nd, Sm, Gd, or Er).
supported lanthanide chemistry by salt elimination routes, since the
NHC is tethered covalently to the lanthanide center and the iodide
ligand is easily substituted. The NHC in 1 is softer than in non-
silylated analogues.10,11 Thus, the NHC in 1 renders a potentially
reducible neodymium center very resistant to reduction.10 In parallel,
we in the Jones group have been exploring the coordination
chemistry of the gallium diyl, [Ga(NArCH)2][K(tmeda)] (2)12 [Ar
) 2,6-Pri2C6H3; tmeda ) Me2NCH2CH2NMe2]. 2 contains an
anionic gallium heterocycle which is valence iso-electronic to
NHCs, and strong parallels between the chemistry of 2 and NHCs
have been observed.13 The utility of 2 in salt elimination chemistry
has begun to be demonstrated.14 In some reactions the insertion of
2 into metal-halide bonds followed by reductive elimination to
afford gallium(II)-halide species may occur.15 Given the docu-
mented reluctance of 1 to engage in reduction chemistry,10 we
identified 1 and 2 as ideal precursors from which to prepare a
compound containing a lanthanide-gallium bond. Herein, we report
the synthesis and structure of the first f-element-gallium bond.
The reaction between 1 and 2 in cold THF proceeds smoothly,
with concomitant elimination of KI, to give a dark red solution.
Workup and recrystallization from toluene affords 3 as large red
blocks in moderate yield (Scheme 1).16 The spectroscopic and
elemental analysis data for 3 are consistent with the proposed
formulation. We were unable to isolate a product containing a
Notably, 3 is stable in solution, in contrast to [(Cp*)2Ln-Al-
(Cp*)] (Ln ) Eu, Yb),9 which immediately dissociate in solution
to [(Cp*)2Ln] and [Al(Cp*)]. A toluene solution of 3 does not
decompose until heated to 100 °C for 16 h. This results in deposition
of elemental gallium and an unidentified compound, which we
suggest contains a Nd-bound diazabutadienide anion.
A single-crystal X-ray diffraction experiment was performed;17
the molecular structure of 3 is depicted in Figure 1. The neodymium
center adopts a distorted trigonal bipyramidal geometry, such that
the two neutral donors O(1) and C(2) are axially disposed, and the
anionic donors N(2), N(4), and Ga(1) reside in the equatorial plane.
The Nd-Ga bond length of 3.2199(3) Å is without precedent and
so cannot be compared with other data, but it is slightly longer
than the sum of covalent radii (Nd-Ga ) 2.89 Å).18 The Nd-Ga
bond length is about 0.15 Å shorter than the Eu-Al bond length
of 3.3652(10) Å reported for [(Cp*)2Eu-Al(Cp*)].9 Divalent Eu
is slightly larger than trivalent Nd, and the covalent radii of Al(I)
and Ga(I) are very similar (1.30 vs 1.25 Å, respectively),19 so the
two bonds seem fairly similar in length, considering the differences
in their components. The Nd(1)-C(2) bond length of 2.669(2) Å
is toward the higher end of the range of Nd-NHC bonds20 and
reflects the presence of the nucleophilic, anionic gallium hetero-
cycle.
DFT calculations were carried out on the model geometry 3a
(Scheme 1) to probe the nature of the Nd-Ga bond in 3. The DFT
geometry optimization of 3a reproduces successfully the principal
† University of Nottingham.
‡ University of Wales.
§ Monash University.
9
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J. AM. CHEM. SOC. 2007, 129, 5360-5361
10.1021/ja0710858 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
The calculated Nd-Ga bond energy, corrected for thermal and
zero point energies, the preparation energies for the [Ga(MeNCH)2]-
and {Nd}+ fragments, and for basis set superposition errors, is 386
kJ mol-1. This energy is similar to that for the reaction [M(OMe2)5-
{Ga(MeNCH) 2}]+ + [Ga(MeNCH)2]- f [M(OMe2)5{Ga(MeNCH)
2}2] + Me2O (M ) Ca, ∆E ) 348.9 kJ mol-1 and M ) Sr, ∆E )
358.4 kJ mol-1 21
) and an order of magnidude greater than the bond
energy of the Ln-Al bonds in [(Cp*)2LnII-AlI(Cp*)] (Ln ) Eu,
Yb, ca. 30 kJ mol-1).9 Thus, 3 is considerably more stable than
[(Cp*)2Ln-Al(Cp*)] in solution because of the stronger Ln-M
bond in 3. The relatively strong Nd-Ga bond involves charge
transfer from a more polarizable Ga(I) center to a more polarizing
Nd(III) center leading to a bond with Nd-Ga covalent character.
In summary, we have described the synthesis and structure of
the first f-element-gallium bond in a complex which is stable in
solution as well as in the solid state. We are currently investigating
the reactivity of 3 since it contains a lanthanide center bonded to
both an NHC and an isoelectronic anionic gallium-NHC analogue,
both of which have potentially ambiphilic character.
Figure 1. Molecular structure of 3 (thermal ellipsoids at 40% probability
levels, H atoms and Me groups omitted). Selected bond lengths (Å) and
angles (deg): Nd(1)-Ga(1), 3.2199(3); Nd(1)-C(2), 2.669(2); Nd(1)-N(2),
2.223(2); Nd(1)-N(4), 2.357(2); Nd(1)-O(1), 2.5578(19); Ga(1)-N(5),
1.932(2); Ga(1)-N(6), 1.932(2); C(2)-N(1), 1.365(3); C(2)-N(3), 1.367-
(3); N(2)-Nd(1)-N(4), 120.37(8); N(2)-Nd(1)-O(1), 89.64(7); N(4)-
Nd(1)-O(1), 91.43(7); N(2)-Nd(1)-C(2), 82.45(7); N(4)-Nd(1)-C(2),
100.53(8); O(1)-Nd(1)-C(2), 167.84(7); N(2)-Nd(1)-Ga(1), 104.76(6);
N(4)-Nd(1)-Ga(1), 134.69(5); O(1)-Nd(1)-Ga(1), 84.59(4); C(2)-Nd-
(1)-Ga(1), 88.49(5); N(5)-Ga(1)-N(6), 83.89(9); N(1)-C(2)-N(3),
102.8(2).
Acknowledgment. We gratefully acknowledge financial support
from the U.K. Leverhulme Trust, the U.K. EPSRC, and the
Australian Research Council, and we thank Dr L. Maron for
discussions concerning the calculation of 3a.
Supporting Information Available: Experimental, X-ray, and
computational data for 3/3a. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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(16) See Supporting Information for details.
(17) Crystallographic data for 3 at 150 K with Mo KR radiation (λ ) 0.71073
Å): orthorhombic, space group P212121, a ) 14.0875(5), b ) 19.1557-
(6), c ) 22.0078(7) Å, V ) 5938.9(3) Å3, Z ) 4, R1 ) 0.0259 for 12575
(I > 2σ(I)) data, wR2 (all data) ) 0.0632.
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Figure 2. The Nd-Ga bond NBO in 3a.
features of the structure of 3 as determined by X-ray crystal-
lography; the Nd(1)-Gd(1) and Nd(1)-C(2) distances are 3.227
and 2.682 Å, respectively, ca. 0.01 Å longer than in the X-ray
crystal structure of 3. A notable difference between the experimental
geometry of 3 and the calculated geometry 3a is the relative
orientation of the [Ga(MeNCH)2]- and {Nd}+ fragments. In 3a
the angle between the planes defined by Nd(1)N(2)N(4) and
Ga(1)N(5)N(6) is 24.7° whereas in 3 this angle is 60.2°. The
calculated structure converged at this geometry irrespective of the
relative orientations of the [Ga(MeNCH)2]- and {Nd}+ fragments
used in the input geometry. Thus, it appears that the steric bulk of
the pendant substituents in 3 controls the relative orientation of
the [Ga(NArCH)2]- and {Nd}+ fragments in 3. However, given
the spherical symmetry and composition of the Nd-Ga bond in
3a (see below) it is likely that the electronic structure and Nd-Ga
bond energy of 3a provide good models for those of 3.
A natural bond orbital (NBO) analysis of 3a reveals natural
charges for Nd(1) and Ga(1) of +2.40 and +0.38, respectively,
consistent with charge transfer from a formal Ga(I) center to a
Nd(III) center to form a Nd-Ga bond with a Wiberg bond order
of 0.827. A NBO analysis of the Nd-Ga bond shows that this bond
is 87% Ga and 13% Nd in character (Figure 2) and involves Nd
6s6p0.015d0.36 and Ga 4s4p1.67 hybrid orbitals, the latter correspond-
ing to the expected sp2 hybridization of the formally Ga(I) center.
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