Elsevier

Journal of Organometallic Chemistry

Volume 751, 1 February 2014, Pages 573-578
Journal of Organometallic Chemistry

Group-13 carbenoid ligands at tungsten: Coordination and C–H activation

Dedicated to Professor Wolfgang A. Herrmann on the occasion of his 65th birthday.
https://doi.org/10.1016/j.jorganchem.2013.08.039Get rights and content

Highlights

  • •

    Structural characterization of the shortest W–Al bond in a molecular compound.

  • •

    C–H activation of ethylene.

  • •

    Formation of a novel organoaluminium cage structure acting as a ligand for tungsten.

Abstract

Reactions of zero valent tungsten complexes with AlCp∗ and GaCp∗ (Cp∗ = pentamethylcyclopentadienyl) are presented. The treatment of [W(C2H4)2(PMe3)4] with 2 equiv of GaCp∗ or AlCp∗ leads to the formation of cis-[W(GaCp∗)2(PMe3)4] (1) or trans,cis,cis-[W(AlCp∗)2(C2H4)2(PMe3)2] (2) under cleavage of ethylene or PMe3 ligands, respectively. Treatment of either [W(C2H4)2(PMe3)4] or 2 with 4 or 6 equiv of AlCp∗ leads to the C–H activated species of over-all composition [W(AlCp∗)6(C2H4)2] (3), which features terminal W–H as well as bridging W–H–Al hydride ligands and C–H activated C2H4 resulting in Al–C bonds. All new compounds are characterized by solution NMR, IR, elemental analyses (EA), liquid injection field desorption ionization mass spectrometry (LIFDI-MS) as well as single crystal x-ray crystallography.

Graphical abstract

Zero valent tungsten complex [W(C2H4)2(PMe3)4] reacts with AlCp* and GaCp* (Cp* = pentamethylcyclopentadienyl) in the formation of cis-[W(GaCp*)2(PMe3)4] (1), trans,cis,cis-[W(AlCp*)2(C2H4)2(PMe3)2] (2), or a C–H activated species of over-all composition [W(AlCp*)6(C2H4)2] (3).

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Introduction

Carbenoid group 13 metal organyls EIR (E = Al, Ga, In; R = steric demanding organic residue) and their coordination chemistry have been in the focus of research for two decades [1], [2], [3], [4], [5], [6]. The first synthetic routes to transition metal complexes of these exotic, very strong 2e donor ligands have been the CO/EIR substitution reactions of transition metal carbonyls like [Ni(CO)4] or [Co2(CO)8]. For example, a certain amount of GaCp∗ could be introduced in the coordination sphere of the transition metal leading to the selective formation of defined substitution products such as [Ni4(GaCp∗)4(CO)6] or [Co2(GaCp∗)2(CO)6] under evolution of CO. However, coordination of GaCp∗ leads to an increase of the M–CO bond strength in the resulting products, i.e. the number of CO ligands which can be substituted by ECp∗ is intrinsically limited and homoleptic complexes [M(ECp∗)n] (n ≥ 4) cannot be obtained from the respective homoleptic carbonyls [7], [8]. This intrinsic limitation is not given when homoleptic olefin complexes are used as starting materials [7], [9], [10], [11], and various mono- and multinuclear complexes [Ma(ECp∗)b] (M = Ni, Pd, Pt; E = Ga, Al) could be obtained [9], [10], [12], [13], [14]. Compounds EIR as well as their interactions with transition metal centres were thoroughly investigated on the density functional level of theory (DFT), revealing the M–ECp dissociation energies to follow the order B > Al > Ga > In > Tl with a major contribution of the electrostatic attraction Mδ−–Eδ+ to the total bond dissociation energies [15], [16]. Different from the heavier homologues Ga and In, the enhanced Lewis acidic character of the aluminium site in AlCp∗ complexes of electron rich metal centres has been shown to enable C–H and Si–H activation reactions, e.g. [M(AlCp∗)5] (M = Fe, Ru) reveal C–H activated CH3-groups and M–H–Al and Al–C bonds. [9], [17]

We have been particularly interested to expand the series of [M(ECp∗)n] compounds with n > 5 and were aiming to achieve [M(ECp∗)6] compounds as analogues to the classic hexacarbonyl complexes [M(CO)6] (M = Cr, Mo, W). Recently, we reported on the successful synthesis of the homoleptic MoGa6 complex [Mo(GaCp∗)6] prepared by hydrogenolysis of [Mo(η4-butadiene)3] in the presence of GaCp∗ [18]. However, the AlCp∗ analogous complex of MoAl6 composition is not accessible using this hydrogenolytic route, since AlCp∗ readily reacts with H2 under very mild conditions. Also, the homologous WGa6 complex [W(GaCp∗)6] has been inaccessible due to the low reactivity of [W(η4-butadiene)3] towards H2 [19]. Despite numerous complexes featuring W–AlIIIR3 interaction (with or without bridging hydrides), the number of tungsten complexes with EIR ligands is quite small and only a few examples with unsupported W–EIR bonds are known [8], [20], [21], [22], [23], [24]. Herein, we would like to give an account on some new results on our way to [M(ECp∗)n] (n > 5) and in particular we will discuss the reactivity of the tungsten complex trans-[W(C2H4)2(PMe3)4] towards AlCp∗ and GaCp∗ and discuss characterization and the structural properties of the products isolated from these reactions.

Section snippets

Synthesis

According to Scheme 1, cis-[W(GaCp∗)2(PMe3)4] (1) and trans,cis,cis-[W(AlCp∗)2(PMe3)2(C2H4)2] (2) can be obtained in good yields by ligand exchange reactions from trans-[W(C2H4)2(PMe3)4] and 2 equiv of ECp∗ (E = Al, Ga). Complex 1 represents the tungsten analogue of the previously reported complex [Mo(GaCp∗)2(PMe3)4], obtained by substitution of N2 and 1 equiv of PMe3 by GaCp∗ from [Mo(N2)(PMe3)5]. In both cases the use of an excess of GaCp∗ does not lead to further substitution of phosphine

Conclusion

The reactivity of ECp∗ (E = Al, Ga) towards the zero-valent tungsten complex [W(C2H4)2(PMe3)4] was investigated. Reaction with GaCp∗ exclusively yields cis-[W(GaCp∗)2(PMe3)4] (1), while reaction with AlCp∗ leads to two different products depending on stoichiometry. trans,cis,cis-[W(AlCp∗)2(PMe3)2(C2H4)2] (2) is formed on reaction of [W(C2H4)2(PMe3)4] if 2 equiv of AlCp∗ are used. Complex 2 exhibits the to date shortest, first reported covalent W–Al bond (2.424(3)–2.432(3) (a.v. 2.43) Å). The

General remarks

All manipulations were carried out in an atmosphere of purified argon using standard Schlenk and glove box techniques. Hexane and toluene were dried using an MBraun Solvent Purification System. The final H2O content in all solvents was checked by Karl Fischer titration and did not exceed 5 ppm. trans-[W(C2H4)2(PMe3)4] [30], trans,trans,trans-[W(C2H4)2(CO)2(PMe3)2] [30], [AlCp∗] [31], [32], and [GaCp∗] [33] were prepared according to known methods from the literature. Elemental analyses were

Acknowledgements

The dissertation project of MM is supported by German Chemical Industry Fund (fellowship) and the Ruhr University Research School (http://www.research-school.rub.de). Support by the German Research Foundation grant No Fi 502/23-1 is gratefully acknowledged.

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