Emslie Group Research: M---B
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Late Transition Metal Ambiphilic Ligand Complexes

Why make Borane-Containing Transition Metal Complexes ?

Boranes [e.g. triphenylborane or tris(pentafluorophenyl)borane] are commonly used as Lewis acids in organic or transition metal chemistry, and late transition metals hold a prominent position in the field of catalysis. However, there are few examples where these two fields have been combined (beyond electrophilic abstraction reactivity). Our focus is on the use of ambiphilic ligands (ligands containing one or more traditional donor group in addition to a Lewis acidic borane) for the study/development of: (1) unique or unusual metal-borane bonding and metal-ligand-borane bridging interactions, and (2) cooperative reactivity which relies on both the tethered borane and a metal centre within an ambiphilic ligand complex. Examples of metal-borane and metal-ligand-borane interactions of particular interest are shown below:

Background - Borane-Containing Transition Metal Complexes

Due to the high reactivity of Lewis acidic boranes, reactivity is typically limited to:

- abstraction of an alkyl group (this is the key to their role as co-catalysts for insertion polymerization).x
- abstraction of a phosphine ligand.x
- adduct formation (e.g. with oxo, nitrido, alkylidene, alkylidyne, halide, cyano, carbonyl, acyl, enolate or nitrile ligands).

The transition metal chemistry of group 13 Lewis acid containing ambiphilic ligands has seen a surge of activity over the past decade, much of it directed towards the isolation and study of unusual metal–Lewis acid bonds where the Lewis acid is considered a zero-electron donor Z-type ligand. However, very few examples of cooperative reactivity involving ambiphilic ligands have been reported, and only a handful of complexes containing a free, tethered electrophilic borane have been prepared.

Strategy to Prepare and Control the Reactivity of Borane-Containing Transition Metal Complexes

Our research has focused on a uniquely rigid phosphine-thioether-triarylborane ambiphilic ligand (TXPB) as well as the highly electrophilic vinylborane, PhH=CH–B(C6F5)2 (VBPh).

The TXPB ligand is unique in that it contains Lewis acidic borane within a particularly rigid ligand framework.
Highlights from our research with TXPB include:

- Unprecedentated metal-borane complexes in which the borane is h3-coordinated to the metal (Rh, Pd or Ni) via boron as well as the ipso- and ortho-carbon atoms of one B-phenyl ring.x
- A unique metal-borane complex in which the borane is h2-coordinated to the metal (Rh) via boron and the ipso-carbon atom of one B-phenyl ring.x
- A complex in which an enone (dba) is coordinated between Pd and B, and is structurally activated towards the h3-boratoxyallyl extreme.x
- Rare (X = Cl) or unprecedented (X = Br or I) examples of complexes containing R3B-X-M linkages.
- Zwitterionic [Rh(CO)(TXPB-F)] in which fluoride has been abstracted from rhodium by the borane.

The VBPh vinylborane provided access to the first 1-borataalkene-like complex, and the first complex displaying an alkyl/borataalkene-like coordination mode. In both cases the vinylborane is h3-coordinated to the metal via boron and the two sp2-hybrdized carbon atoms.

Selected X-Ray crystal structures, NMR spectra and DFT calculations from our TXPB and VBPh research:

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