ChemComm View Article Online FEATURE ARTICLE View Journal | View Issue Magnesium, zinc, aluminium and gallium hydride complexes of the transition metals† Cite this: Chem. Commun., 2017, 53,1348 Michael J. Butler and Mark R. Crimmin* The preparation and applications of heterobimetallic complexes continue to occupy researchers in the fields of organometallic, main group, and coordination chemistry. This interest stems from the promise Received 12th July 2016, Accepted 23rd December 2016 these complexes hold as precursors to materials, reagents in synthesis and as new catalysis. Here we survey and organise the state-of-the-art understanding of the TM–H–M linkage (M = Mg, Zn, Al, Ga). DOI: 10.1039/c6cc05702k We discuss the structure and bonding in these complexes, their known reactivity, and their largely unrealised potential in catalysis. www.rsc.org/chemcomm studied type of this bonding mode.7–12 While the latter appear as Creative Commons Attribution 3.0 Unported Licence. 1. Introduction potential intermediates in alkene hydrosilylation via the Chalk– The catalytic practices of C–H bond functionalisation, dehydro- Harrod mechanism,13,14 the former bear significance for a range coupling (for hydrogen storage), hydroboration and hydrosilyla- of industrially relevant hydrogenation reactions.15,16 In C–H tion are all attractive prospects for the future chemical economy. borylation, stabilisation of catalytic intermediates by a TM–H–B The modern development in these methodologies continues to (TM = transition metal) interaction has been supported by be enhanced by the perception of borane and silane s-complexes significant experimental mechanistic studies.17–19 as intermediates in reaction mechanisms (Fig. 1). A 3-centre 2-electron interaction, the Z2-ligation of E–H to A s-complex can be described as an Z2-binding of the s-E–H TM can be viewed within the Dewar–Chatt–Duncanson model This article is licensed under a bond to a transition metal centre (E = C, Si, B, H).1,2 Along with and interpreted as a combination of: (i) donation of the dihydrogen complexes,3–6 s-silanes are the most comprehensively s-electrons in the E–H bond to a vacant orbital on the transi- tion metal and (ii) back-donation from the metal into the s*-orbital of the same E–H bond. The resulting s-E–H adduct Open Access Article. Published on 03 January 2017. Downloaded 10/7/2021 11:58:00 PM. Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, UK. E-mail: [email protected]; Tel: +44 (0)2075942846 represents an intermediate along the oxidative addition reaction † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc05702k coordinate (Fig. 1) – part of a continuum of bonding descriptions Michael J. Butler was born in MarkR.CrimminreceivedhisPhD Tottenham, London in 1989 and in main group chemistry and in 2012 graduated with an MSci catalysis from Imperial College from the University of Glasgow. London in 2008 supervised by These studies included a year at Prof. Mike Hill (now at Bath) and Bayer Crop Science AG (Frankfurt Prof. Tony Barrett. In the same am Main), working under Dr year, he moved to UC Berkeley to Klemens Minn. After some time study with Prof. Bob Bergman. and studying surface chemistry at Prof. Dean Toste. In 2011, he Sika AG in Zurich, he began an returned to London as a Royal organometallic chemistry PhD in Society University Research Fellow, the Crimmin group in 2013. initially at UCL and now back at Michael J. Butler Mark R. Crimmin Imperial. Work in the Crimmin group focuses on new methods to break strong carbon–fluorine, carbon–oxygen and carbon–hydrogen bonds in small molecules and the development of heterobimetallic complexes for catalysis. 1348 | Chem. Commun., 2017, 53, 1348--1365 This journal is © The Royal Society of Chemistry 2017 View Article Online Feature Article ChemComm The half-arrow notation and covalent bond classification advocated by Green, Green and Parkin are used to represent the TM–H–M 3-centre 2-electron interactions.11 This formalism is employed as an organisational principle not as an absolute interpretation of the bonding within TM–H–M units. The line Fig. 1 The continuum between s-complex and oxidative addition for the drawings are constructed from the perspective of the transition coordination of E–H bonds to a transition metal. metal coordination environment in order to represent charge neutral species rather than an accurate representation of the bonding within the TM–H–M group. The literature survey between free E–H and E–TM–H. It is clear that the nature of the is followed by discussion of the ‘‘continuum’’ of bonding bonding in TM–H–E containing complexes is a tuneable property, descriptions, reactivity and the potential these complexes hold being a consideration of the symmetry and energy of the frontier for catalysis. orbitals of the transition metal fragment along with the sub- stituents on, and nature of, E. ’ A fundamental question that arises when considering this 2. r-Complexes (TM H–M) model, is: what happens when C, B and Si are replaced by We have reported 1, a Zn congener of structurally related metallic main group elements such as Mg, Zn, Al, or Ga? The s-alane complexes (Fig. 3).27 The binding of the zinc hydride increased ionic contribution within the TM–H–M interaction to the CuI centre is weak and reversible. In toluene or benzene will necessarily give another dimension to the bonding descrip- solution, an equilibrium exists between the heterobimetallic tion. As with silicon, the ability of these elements to expand complex and the Z2-arene complex of CuI. The electronic their coordination sphere leads to the possibility of forming structure of the three-centre linkage has been investigated by additional bonding interactions with existing ligands on the DFT calculations. The analysis suggests that the formal L donation TM fragment. Furthermore, for the heavier group 13 elements I Creative Commons Attribution 3.0 Unported Licence. of the M–H s-bond to the 4s orbital on Cu is accompanied I the formation of low-valent M ligands through manifestation by weak Cu-M back-donation into the M–H s*-orbital of the inert-pair effect becomes an important consideration. (see Discussion section). In combination, the data allow 1 to The TM–H–M motif is one way of adjoining two metal be described as a weakly bound s-complex. centres bearing at least one reactive hydride ligand (Fig. 2a). Coordination of Al–H bonds to CuI has also been examined; This motif can also be obtained by coordinating a transition complexes possessing four-coordinate and five-coordinate metal hydride to a neutral main group metal fragment (Fig. 2b), aluminium centres have been isolated (Fig. 4, 2 and 3). The and multiply bridged species formed by a combination of the bindingofCu–H–Alisagainweakandreversiblebasedon two aforementioned donor–acceptor interactions (Fig. 2c). solution NMR studies and crossover experiments. In DFT This article is licensed under a Herein we survey the known heterobimetallic complexes of calculations, the coordination of exogenous ligands to the transition metal and main group hydrides (M = Mg, Zn, Al, Ga). CuI fragment was found be increasingly exergonic across the The coordination chemistry of heterobimetallic transition series C F H–B H–Si C H H–Zn H–Al – a trend 20–22 6 6 o o o 6 6 o o metal hydrides, and of Al, Ga, In and Zn-based ligands at 27 Open Access Article. Published on 03 January 2017. Downloaded 10/7/2021 11:58:00 PM. that is manifest in the experimental data. 23–26 transition metal centre have been summarised previously. In 2 the CuÁÁÁAl vector lies outside of the H–Al–H wedge – To focus the discourse, a loose definition ‘heterobimetallic the only example of this structural feature we are aware of in hydride complexes’ is employed. The complexes mostly fit these main group metal s-complexes.27 The ‘lee side’ coordination of criteria: (a) crystallographically characterised; (b) 1 : 1 ratio of the Al–H bond and very long intermetallic distance in 2 are transition : main group metal; (c) the absence of TMÁÁÁTM notable. For comparison, s-complexes of HBpin (pinacolborane) interactions; (d) a hydride ligand in a bridging role. The survey or HBcat (catecholborane) show different coordination geometries is arranged: TM’H–M, TM–H-M, TM–Hn–M (n 4 1). 28,29 to those of four-coordinate boranes BH3ÁEMe3 (E = N, P). The discrepancy has been rationalised by disruption of the TMÁÁÁB back-donation due to the absence of a vacant orbital of suitable Fig. 2 (a) The continuum for addition of M–H to a transition metal. (b) Lewis adducts between a TM hydride and M, (c) combination of interactions. Fig. 3 A s-zincane complex of CuI. This journal is © The Royal Society of Chemistry 2017 Chem. Commun., 2017, 53, 1348--1365 | 1349 View Article Online ChemComm Feature Article Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 03 January 2017. Downloaded 10/7/2021 11:58:00 PM. Fig. 4 s-Alane, s-gallane and s-zincane complexes of late TM. 31–35 energy in BH3ÁEMe3, be it the boron p-orbital or the s*-orbital of groups 6 and 7 transition metal carbonyls. Like 2 and 3, the the B–H bond.28 While steric factors are undoubtedly important, a structures of 4–7 contain s-Al–H adducts and derive from similar effect may explain the solid state structures of 2 and 3. ligand exchange reactions, in this case from the parent metal DFT calculations are consistent with reduced back-donation from carbonyl under either thermal or photochemical conditions.