2‑(Methylamido)pyridine–Borane: A Tripod κ³‑N,H,H Ligand in Trigonal Bipyramidal Rhodium(I) and Iridium(I) Complexes with an Asymmetric Coordination of Its BH₃ Group

The complexes [M­(κ³-N,H,H-mapyBH₃)­(cod)] (M = Rh, Ir; HmapyBH₃ = 2-(methylamino)­pyridine–borane; cod = 1,5-cyclooctadiene), which contain a novel anionic tripod ligand coordinated to the metal atom through the amido N atom and through two H atoms of the BH₃ group, were prepared by treating the corresponding [M₂(μ-Cl)₂(cod)₂] (M = Rh, Ir) precursor with K­[mapyBH₃]. X-ray diffraction studies and a theoretical Quantum Theory of Atoms in Molecules analysis of their electron density confirmed that the metal atoms of both complexes are in a very distorted trigonal bipyramidal coordination environment, in which two equatorial sites are asymmetrically spanned by the H–B–H fragment. While both 3c–2e BH–M interactions are more κ¹-H (terminal σ coordination of the B–H bond) than κ²-H,B (agostic-type coordination of the B–H bond), one BH–M interaction is more agostic than the other, and this difference is more marked in the iridium complex than in the rhodium one. This asymmetry is not evident in solution, where the cod ligand and the BH₃ group of these molecules participate in two concurrent dynamic processes of low activation energies (variable-temperature NMR and density functional theory studies), namely, a rotation of the cod ligand that interchanges its two alkene fragments (through a square pyramidal transition state) and a rotation of the BH₃ group about the B–N bond that equilibrates the three B–H bonds (through a square planar transition state). While the cod rotation has similar activation energy in <b>2</b> and <b>3</b>, the barrier to the BH₃ group rotation is higher in the iridium complex than in the rhodium one