A comparison of the coordination behaviour of R2PCH2BMe2 (R = Me vs. Ph) ambiphilic ligands with late transition metals

Abstract

A new synthesis that avoids the use of Me₂PH is reported for (Me₂PCH₂BMe₂)₂, and this method was extended to the synthesis of (Ph₂PCH₂BMe₂)₂. The ligand precursor (Me₂PCH₂BMe₂)₂ did not react with [{M(μ-Cl)(cod)}₂] (cod = 1,5-cyclooctadiene; M = Ir and Rh) or [PtCl₂(cod)] at room temperature. However, after 12–48 hours at 65–70 °C, these reactions afforded (a) [Ir(cod)(μ-Cl)(Me₂PCH₂BMe₂)] (1), (b) an equilibrium mixture of (Me₂PCH₂BMe₂)₂, [{Rh(μ-Cl)(cod)}₂] and [Rh(cod)(μ-Cl)(Me₂PCH₂BMe₂)] (2), and (c) cis-[Pt(μ-Cl)₂(Me₂PCH₂BMe₂)₂] (3), respectively. By contrast, reactions between the phenyl-substituted analogue, (Ph₂PCH₂BMe₂)₂, and [{M(μ-Cl)(cod)}₂] (cod = 1,5-cyclooctadiene; M = Ir and Rh) proceeded over the course of 1 hour at 20 °C to generate [M(cod)(μ-Cl)(Ph₂PCH₂BMe₂)] (M = Ir (4) and Rh (5)), indicative of room temperature (Ph₂PCH₂BMe₂)₂ dissociation. Room temperature reactions of (Ph₂PCH₂BMe₂)₂ with [{Rh(μ-Cl)(coe)₂}₂] (coe = cyclooctene) using a 1 : 1 or 3 : 1 stoichiometry also afforded [{Rh(coe)(μ-Cl)(Ph₂PCH₂BMe₂)}₂] (6) or [RhCl(Ph₂PCH₂BMe₂)₃] (7), respectively, where the latter is a borane-appended analogue of Wilkinson's catalyst, and reactions of (Ph₂PCH₂BMe₂)₂ with [PtX₂(cod)] (X = Cl or Me) yielded cis-[Pt(μ-Cl)₂(Ph₂PCH₂BMe₂)₂] (8) and cis-[PtMe₂(Ph₂PCH₂BMe₂)₂] (9). Compounds 1–9, (Me₂PCH₂BMe₂)₂ and (Ph₂PCH₂BMe₂)₂ were crystallographically characterized. In compounds 1–5 and 8, each chloride co-ligand is coordinated by the borane of an R₂PCH₂BMe₂ ligand. Additionally, in the solid state structure of 6, each bridging chloride ligand interacts weakly with a pendent borane, and in 7, the chloride ligand is tightly coordinated to the borane of one Ph₂PCH₂BMe₂ ligand and weakly coordinated to the borane of a second Ph₂PCH₂BMe₂ ligand. By contrast, both boranes in 9 (and one of the three boranes in 7) are non-coordinated.