Structure and dynamic NMR behavior of rhodium complexes supported by Lewis acidic group 13 metallatranes

Monovalent Rh was installed into the group 13 metallatranes, M[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃] (where M = Al and Ga, abbreviated as ML) to generate Rh → M bonds in the parent complexes, Cl-RhAlL (1-Cl) and Cl-RhGaL (2-Cl). The electron-withdrawing nature of the group 13 metalloids was probed by cyclic voltammetry, and Rh-Ga was found to be more electron-deficient than Rh-Al (E_pc = -2.07 and -1.95 V vs. Fc⁺/Fc for 1-Cl and 2-Cl, respectively). Both 1-Cl and 2-Cl were further functionalized through metathesis reactions using MeLi to generate 1-CH₃ and 2-CH₃, respectively, or using LiHBEt₃ to form 1-H and 2-H, respectively. The solid-state structures of all Rh-M bimetallics feature Rh-M bond lengths that are less than the sum of the covalent radii of Rh and M (Rh-M: 2.50-2.54 Å for 1-X and 2.49-2.46 Å for 2-X, where X = Cl, CH₃, and H). In the Rh-M structures, the Rh center is distorted from square pyramidal geometry due to steric interactions between X and the isopropyl substituents of L. Finally, all the Rh-M bimetallics exhibit fluxionality that involves phosphine exchange. Of note, the two phosphines cis to the X ligand become inequivalent at low temperature. The activation barrier to exchange these two phosphine donors is: 14.9, 14.2, 10.9, and 11.5 kcal mol^-1 for 1-Cl, 2-Cl, 1-H, and 2-H, respectively. The activation barriers for 1-CH₃ and 2-CH₃ are both >15.2 kcal mol^-1. At high temperature, 2-Cl was also found to exchange all three phosphine donors. Mechanisms for the different types of phosphine exchange are proposed.