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Coordination and activation of aminoboranes

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The activation of BH bonds by late transition metal centres plays a central role in a number of key synthetic methodologies, including metal-catalysed hydroboration and alkane/arene C-H functionalization/borylation protocols. Within this sphere, aminoboranes, H2BNRR ’, are the subject of significant additional interest not only as the first-formed products in the dehydrogenation of a class of BN-containing hydrogen storage materials, but also as the monomeric building blocks from which a number of novel well-defined inorganic polymers can be constructed. The fundamental mode(s) of interaction of monomeric aminoboranes with catalytically relevant late transition metal systems are therefore of significant interest. The intrinsic two-electron donor capabilities of these compounds have been probed through coordination at 16-electron [CpRu(PR3)2]+ fragments. In contrast to the side-on binding of isoelectronic alkene donors, an alternative mono(σ-BH) mode of aminoborane ligation is established for H2BN Cy2 (e.g. I), with binding energies only ~8 kcal mol-1 greater than those for analogous dinitrogen complexes.[1] Variations in ground state structure and exchange dynamics as a function of the phosphine ancillary ligand set are consistent with chemically significant back-bonding into an orbital of B H σ* character.

I    II    III

The use of cationic 14-electron ruthenium, rhodium or iridium metal systems allows an alternative κ2 mode of coordination to be accessed, featuring significantly tighter ligand binding (e.g. II).[2] Moreover, by systematic variation in the electronic properties of the metal centre, varying degrees of B-H bond activation can be induced, leading to the formation of primary boryl species LnM{B(H)NR2} (by oxidative addition of a single B-H bond), or even of borylene dihydride complexes via additional B-to-M α-hydride transfer (see III and Scheme).[3]

[1] Vidovic, D.; Addy, D.A.; Krämer, T.; McGrady, J.; Aldridge, S. J. Am. Chem. Soc. 2011, 133, 8494. [2] (a) Tang, C.Y.; Thompson, A.L.; Aldridge, S. Angew. Chem., Int. Ed. 2010, 49, 921-925; (b) Tang, C.Y.; Thompson, A.L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578-10591. [3] O’Neill, M.; Addy, D.A.; Riddlestone, I.; Kelly, M.; Phillips, N.; Aldridge, S. J. Am. Chem. Soc. 2011, 133, 11500.

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