Reactions of [(dmpe)2MnH(C2H4)]: synthesis and characterization of manganese(i) borohydride and hydride complexes

Abstract

Reactions of trans-[(dmpe)₂MnH(C₂H₄)] (1) with BH₃(NMe₃), 9-BBN, and HBMes₂ yielded the manganese(I) borohydride complexes [(dmpe)₂Mn(μ-H)₂BR₂] (3: R = H, 4: R₂ = C₈H₁₄, 5: R = Mes). The reaction of 1 with BH₃(NMe₃) proceeds via ethylene substitution. By contrast, a detuerium labelling study indicates that the reaction of 1 with HBMes₂ involves initial isomerization of 1 to an unobserved 5-coordinate ethyl intermediate, [(dmpe)₂MnEt], which reacts with the hydroborane to afford EtBR₂ and [(dmpe)₂MnH], followed by reaction with a second equivalent of hydroborane to generate 5 (an analogous pathway is likely followed for other base-free hydroboranes such as 9-BBN). Identification of 3–5 as κ²-borohydride complexes, as opposed to boryl dihydride or hydroborane hydride isomers, is supported by ¹¹B NMR spectroscopy, X-ray diffraction, and Atoms in Molecules calculations. Two byproducts were observed in the syntheses of 3–5: [{(dmpe)₂MnH}₂(μ-dmpe)] (6) and [(dmpe)₂MnH(κ¹-dmpe)] (7). These complexes were independently prepared by exposure of 1 to free dmpe under an atmosphere of Ar or H₂, and the generality of this synthetic route was demonstrated by the reaction of 1 with PMe₃ (under H₂) to form [(dmpe)₂MnH(PMe₃)] (8). Complexes 6–8 can exist as isomers with either a trans or a cis relationship between the hydride and κ¹-coordinated phosphine ligands on manganese. trans to cis isomerization of 6–8 is photochemically induced, whereas the reverse reaction occurs under thermal conditions. X-ray crystal structures were obtained for 3–5, trans,trans-6, cis,cis-6, trans-7, and trans-8.