Chemical Society Reviews Low-Valent Group 14 Hydride Chemistry: Towards Catalysis Journal: Chemical Society Reviews Manuscript ID CS-SYN-09-2017-000649.R2 Article Type: Review Article Date Submitted by the Author: 06-Apr-2018 Complete List of Authors: Jones, Cameron; Monash University, School of Chemistry Hadlington, Terrance; Metallorganics and Inorganic Materials, Institute of Chemistry Driess, Matthias; Metallorganics and Inorganic Materials, Institute of Chemistry Page 1 of 53 Chemical Society Reviews 1 Low-Valent Group 14 Element Hydride Chemistry: Towards Catalysis 2 Terrance J. Hadlington,*,† Matthias Driess,† and Cameron Jones*,‡ 3 †Department of Chemistry, Metalorganics and Inorganic Materials, Techniche Universitat Berlin, Strasse 4 des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany 5 ‡School of Chemistry, Monash University, P.O. Box 23, Melbourne, VIC 3800, Australia 6 7 Abstract 8 The chemistry of group 14 element(II) hydride complexes has rapidly expanded since the first stable 9 example of such a compound was reported in 2000. Since that time it has become apparent that these 10 systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal 11 (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic 12 substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 13 element(II) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how 14 the chemistry of these hydride compounds has advanced since their early development. Throughout, 15 there is a focus on the importance of ligand effects in these systems, and how ligand design can 16 greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency. 1 Chemical Society Reviews Page 2 of 53 1 Low-Valent Group 14 Element Hydride Chemistry: Towards Catalysis 2 Terrance J. Hadlington,*,† Matthias Driess,† and Cameron Jones*,‡ 3 †Department of Chemistry, Metalorganics and Inorganic Materials, Techniche Universitat Berlin, Strasse 4 des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany ‡ 5 School of Chemistry, Monash University, P.O. Box 23, Melbourne, VIC 3800, Australia 6 7 Author Biographies: 8 9 Terrance Hadlington completed his MChem(Hons) at the University of Bath, UK, in 2011. He then 10 attained his PhD from Monash University, Australia, in 2015, working under Professor Cameron 11 Jones, with a short research stay at Oxford University (UK) working with Professor Simon Aldridge. 12 This work led to developments in the stabilisation of low-valent group 14 element complexes, and 13 their reactivity directed towards transition-metal like catalytic transformations. He has been a 14 postdoctoral fellow of the Cluster of Excellence UniCat, TU Berlin (Germany), since May 2016, 15 working with Professor Matthias Driess. His research interests currently lie in the synergistic interplay 16 between transition-metal fragments and low-valent group 14 ligands, low-valent main-group 17 compounds as ligands in catalysis, and main-group element centered catalysis. 2 Page 3 of 53 Chemical Society Reviews 1 2 Matthias Driess obtained his Ph.D. degree in 1988 and completed his habilitation at the University of 3 Heidelberg (Germany) in 1993. Since 2004, he is full professor of metalorganics and inorganic 4 materials at the Department of Chemistry of the Technische Universität Berlin. He serves as 5 spokesperson of the Cluster of Excellence UniCat, co-director of the UniCatBASF joint laboratory 6 (BasCat) and director of Chemical Invention Factory (CIF) in the Berlin area. He has received several 7 awards, including the Alfred Stock Memorial Award of the German Chemical Society in 2010 and the 8 Wacker Silicone Award in 2011. He is a member of the German National Academy of Sciences 9 (Leoplodina) and of the Berlin-Brandenburg Academy of Sciences and Humanities. His current 10 research interests include coordination chemistry of main-group elements and transition-metals in 11 unusual coordination and oxidation states and synthesis of functional inorganic materials, for 12 example, heterobimetallic oxide nanoparticles, employing molecular architecture for catalysis. 13 14 3 Chemical Society Reviews Page 4 of 53 1 2 Cameron Jones obtained his BSc(Hons) degree at the University of Western Australia (1984). His 3 PhD degree (1992) was gained from Griffith University, Brisbane, under the supervision of Professor 4 Colin Raston. He then moved to a postdoctoral fellowship (1992–1994) at Sussex University under 5 the supervision of Professor John Nixon FRS. From 1994 he held a lectureship at The University of 6 Wales, Swansea before moving to a Readership in Inorganic Chemistry at Cardiff University (1998). 7 There, he was promoted to a Personal Chair in 2002. In 2007 he moved to Monash University, 8 Melbourne, where he is currently Professor of Chemistry, in addition to being the Director of the 9 Monash Centre for Catalysis (MonCat). His research interests include the stabilisation of low 10 oxidation state and hydrido s- and p-block metal complexes, and their application to synthesis, small 11 molecule activations and catalysis. 12 13 1. Introduction 14 Our fundamental understanding of the reactive capacity of the main-group (MG) elements has been 15 challenged and significantly developed over recent decades. Only in 1976 was the ‘Double-Bond 16 Rule’ disproved,1 with the publication of the first heavier MG multiple bond described by M. F. 2 II I 17 Lappert, in [{(Me3Si)2CH}2Sn]2. This was soon followed by related low-valent Si and P t 18 compounds (viz. [{(Mes)2Si}2] and [{(Mes*)2P}2]; Mes = C6H2-Me3-2,4,6; Mes* = C6H2-Bu 3-2,4,6) 19 in 1981, both of which are heavier element π-bonded systems.3 Since that time, synthetic inorganic 4 Page 5 of 53 Chemical Society Reviews 1 chemists have achieved numerous breakthroughs in heavier group 14 element chemistry. For 2 example, heavier alkene and alkyne analogues have been described for SiPb,4 in addition to 5 3 monomeric heavier tetrylenes, [R2E:] (E = SiPb). Their reactivity has been shown to be extremely 4 far removed from that of their EIV analogues. For example, the facile activation of synthetically 5 important small molecules such as dihydrogen,6 ethylene,7 and both CII and CIV oxides8 has been 6 effected by heavier group 14 element compounds in the +1 and +2 oxidation states, bringing this 7 fascinating area of research into the spotlight since the turn of the millennium. This remarkable 8 reactivity has led to direct comparisons between the chemistry of the transition-metals (TMs) and the 9 group 14 elements.9 This, of course, goes hand-in-hand with the underlying concepts of catalysis: 10 TMs are extremely important for otherwise challenging organic transformations, and rely heavily on 11 oxidative-addition/reductive elimination cycles to achieve this.10 These key reactions have now been 12 observed at low-valent group 14 centres.11,12 This comes at a time where combating man-made 13 environmental issues is at the core of research efforts across the globe, with the potential toxicity and 14 high cost of precious d-block elements fuelling the search for benign yet efficient catalytic systems. 15 Frustrated Lewis Pairs (FLPs) have proven powerful towards this end, since the first demonstration of 16 their ability to cleave typically unreactive small-molecules.13 Broadly studied on a more fundamental 17 basis, low-valent E-H complexes (E = group 14 element) have shown great promise, with their 18 hydridic reactivity far more pronounced than classical EIV congeners, allowing for direct 19 hydroelementation of even unactivated alkenes, in some cases reversibly.14 In 2015, our (the Jones) 20 group extended such systems to catalytic hydroboration reactions,15 marking the beginning of what we 21 believe will be a fruitful area of MG catalysis into the future. This review aims to discuss progress 22 towards the application of low-valent group 14 element hydride species in hydrofunctionalisation 23 catalysis. This will firstly address the fundamental electronic features of tetrylenes which can lead to 24 extremely reactive hydride complexes and related catalytically significant transformations. This 25 should act as a guide for the synthetic chemist in the design of novel low-valent group 14 complexes 26 which extend the catalytic potential of such species. A discussion of the advances in the literature 27 towards the synthesis and reactivity of hydrido tetrylenes will highlight key points in accessing 28 isolable, yet highly reactive systems of this type, leading to how these factors have allowed for 5 Chemical Society Reviews Page 6 of 53 1 efficient catalysis to be achieved at a low-valent heavier group 14 centre. Throughout, there will be a 2 focus on the importance of ligand effects in these systems, and how ligand design can greatly modify 3 a coordinated complex's electronic structure, reactivity, and dynamic solution processes. 4 6 Page 7 of 53 Chemical Society Reviews 1 2. Frontier Orbitals for Small-Molecule Activation 2 The most stable electronic ground-state of the heavier parent tetrylenes (i.e. [H2E]) is the singlet state, 3 whereby E holds a lone-pair of electrons (the HOMO) and a vacant p-orbital (the LUMO), as 16 4 calculated by Trinquier for the parent tetrylenes. The same is typically observed for [L2E] species, 5 where L is a monoanionic ligand.5 The stability of the monomeric singlet state increases as group 14 6 is descended due to the inert pair effect, which in turn reduces the degree of sp-mixing at E with 7 increasing quantum number. The important interactions between a singlet tetrylene and small 8 molecules (e.g. H2, C2H4; Figure 1) can be compared with those for a TM fragment, the mechanisms 9 for the latter being typically very wellestablished (e.g.