Transition metal boryl complex

In chemistry, a transition metal boryl complex is a molecular species with a formally anionic boron center coordinated to a transition metal.^[1] They have the formula L_nM-BR₂ or L_nM-(BR₂LB) (L = ligand, R = H, organic substituent, LB = Lewis base). One example is (C₅Me₅)Mn(CO)₂(BH₂PMe₃) (Me = methyl).^[2] Such compounds, especially those derived from catecholborane and the related pinacolborane, are intermediates in transition metal-catalyzed borylation reactions.

Structure of (PMe₃)₄RhB(pinacolate).^[3] (color code: pink=B, blue green=Rh, red=O, orange=P).

Structure of boryl ligands in metal complexes.

Synthesis

Oxidative addition is the main route to metal boryl complexes. Both B-H and B-B bonds add to low-valent metal complexes. For example, catecholborane oxidatively adds to Pt(0) to give the boryl hydride.^[4]

C₆H₄O₂BH + Pt(PR₃)₂ → C₆H₄O₂B Pt(PR₃)₂H

Addition of diboron tetrafluoride to Vaska's complex gives the triboryl iridium(III) derivative:

2 B₂F₄ + IrCl(CO)(PPh₃)₂ → Ir(BF₂)₃(CO)(PPh₃)₂ + ClBF₂