Coordination and Activation of EH Bonds (E = B, Al, Ga) at Transition Metal Centers

Ian M. Riddlestone; Joseph A.B. Abdalla; Simon Aldridge*    Inorganic Chemistry Laboratory, Oxford, United Kingdom
* Corresponding author: email address: simon.aldridge@chem.ox.ac.uk

1 Introduction

Over the last 20 years, significant research effort has been expended on probing the mode(s) of interaction of group 13 hydrides with transition metal centers. An initial driver for such work—notwithstanding issues relating to chemical reversibility—was potential applications of boron/nitrogen based materials containing a high weight% of hydrogen, as hydrogen storage materials; more recent work has focussed on such systems as building blocks in the construction of inorganic polymers.1 Applications of alanes in hydrogen storage media have also received some attention,2 but in general, the hydrides of the heavier group 13 elements have not been in the spotlight, primarily due to their lower percentages by weight of hydrogen and their greater thermal instability.3

Control of dihydrogen release or of polymer formation, for example, means that a significant part of this effort has been directed toward metal-mediated processes; the interaction of EH bonds with transition metal centers and their potential modes of activation are therefore of key importance.4,5 As a consequence, the factors underpinning the fundamental bonding interactions (σ-donation, π-acceptance) of BH, and to a lesser extent AlH and GaH bonds, with transition metals have been explored (Fig. 1). The resulting data offer comparison with much more widely established families of σ-complex, featuring coordinated HH, CH, or SiH bonds.6–8

Even leaving aside complexes containing anionic borohydride-type ligands,9 the coordination of BH bonds at transition metal centers is well precedented, and many BH σ-complexes featuring three- and four-coordinate boranes have been reported.5 Examples abound involving either κ1 or κ2 binding—that is featuring interactions with the metal center through one or two BH bonds, respectively. Within this sphere, there has been a wealth of investigation into the interaction of the BH bonds in amine- and aminoboranes with transition metals, with such σ-complexes having been postulated as intermediates in BN dehydrocoupling reactions.1,4 In-depth mechanistic details of such processes are not the focus of this chapter and will not be examined in depth beyond consideration of the modes of ligation of relevant species. Activation of one BH bond, generally involving oxidative addition at a metal center, is very well established, in part reflecting the implication of such a step in a number of processes leading to the borylation of both saturated and unsaturated hydrocarbons.10,11 However, activation of two BH bonds, leading ultimately to dehydrogenation and the formation of a subvalent borylene complex, is much less common.12,13

The related chemistry of the heaver group 13 elements aluminium and gallium is much less developed than that of boron.14,15 The interaction of AlH bonds with transition metal centers is limited to the formation of a relatively small number of σ-complexes featuring bridging MHAl motifs, with no reported propensity toward oxidative addition of AlH bonds or dehydrogenation at a transition metal center.16 σ-Alane complexes themselves are rare; the first example of an unsupported AlH σ-complex, (cdt)Ni[κ1-HAlMe2·quin], was reported by Pörschke and coworkers in 1990.14 There is an even greater paucity of examples of GaH coordination at transition metals, in part due to the much greater thermal fragility of GaH bonds.3 Ueno reported the first structurally characterized examples of GaH σ-complexes, involving coordination of the amine-stabilized gallane quin·GaH3 at [Cp′Mn(CO)2] and [M(CO)5] fragments (M = Cr, Mo, W).15 However, with the + 1 oxidation state more readily accessible for gallium (and the GaH bond being inherently weaker than its lighter congeners),3,17 the potential for novel and interesting chemistry initiated by the dehydrogenation of Ga(III) hydride complexes at transition metal centers is not only foreseeable but beginning to be realized in practice.15,18

This chapter will primarily focus on reviewing the coordination and activation of AlH and GaH bonds at transition metal centers, making reference to key examples of related BH σ-complexes in order to put fundamental issues of electronic structure and bonding into appropriate context. In the interests of space, and with a view to comparing the intrinsic electronic/geometric properties of the coordinated σ-bond, “tethered” systems in which the coordinated EH bond forms part of an existing metal-bound ligand are not as a rule included.

2 Borane (BH) σ-Complexes

The long-standing desire to use an organometallic system to selectively functionalize CH bonds,19 and the inherent challenges related to this process, have seen the development of a number of systems to model the CH bond activation process (involving, for example, silanes and boranes as alkane mimics).5,8 The isoelectronic relationship between CH4 and [BH4]− led to much interest in the formation of transition metal, lanthanide, and actinide borohydride complexes featuring κ1, κ2, and κ3 coordination modes (Fig. 2).9,20 Although isoelectronic with CH4, the inherent negative charge possessed by the [BH4]− anion provides a significant electrostatic contribution to bonding.9 Consequently, the coordination chemistry of neutral boranes is perceived as a better potential model for alkane coordination, and this area has been the subject of significant interest in the recent chemical literature. Two distinct classes of borane coordination complex have emerged, involving three- and four-coordinate boranes, respectively.

The formation of Lewis-base adducts of BH3 is well established, and being charge neutral, L·BH3 systems perhaps more closely resemble alkanes in their potential ligating properties, than does the borohydride anion. Of key importance in rationalizing the bonding in complexes containing L·BH3 ligands is the coordinative saturation of the boron center, and crucially the population of the vacant boron pz-orbital by the Lewis base (L). Consequently, the L·BH3 moiety has a limited role as a π-acceptor, as the BH σ*-orbital is typically too high in energy to interact significantly with metal-based d-orbitals.21 By contrast, three-coordinate boranes, archetypally represented by HBcat, possess a formally vacant pz-orbital at the...