Protonation of dppbz- and binap-ligated rhodathiaboranes yielding proton, hydride and hydrogen exchange

Abstract

The reaction of the eleven-vertex rhodathiaborane [8,8,8-(H)(PPh3)2-3-(NC5H5)-nido-7,8-RhSB9H10] (1) with 1,2-bis(dipheylphosphine)benzene (dppbz) and with (S)-(–)-2,2′-bis(diphenylphosphino)-1,1′-binaphyl (binap) affords [1,1-(2-dppbz)-3-(NC5H5)-closo-1,2-RhSB9H8] (2) and [1,1-(2-binap)-3-(NC5H5)-closo-1,2-RhSB9H8] (3), respectively. These eleven-vertex closo-rhodathiaborane chelates result from PPh3 ligand substitution at the rhodium centre and nido-to-closo structural cluster transformation driven by H2 loss. Treatment of compounds 2 and 3 with triflic acid (HTfO) leads to the formation of cationic clusters [(dppbz)(NC5H5)RhSB9H9]+ (4) and [(binap)(NC5H5)RhSB9H9]+ (5). In these clusters, the protons bind to the polyhedral clusters, acquiring hydride character and providing chemically non-rigidity that manifests through metal vertex-to-thiaborane pseudo-rotations and concomitant proton tautomerisms. The resulting cations react with hydrogen to form mixtures of protons, hydrogen and hydridorhodathiaboranes in equilibrium. These boron-based metal-hydride mixtures can be formulated as [(dppbz)(NC5H5)RhSB9H11]+ (6) and [(binap)(NC5H5)RhSB9H11]+ (8), resulting from the heterolytic cleavage of the H–H bond with the clusters’ full participation and the addition of hydrogen atoms to the cages. From the reaction mixture of 6 under H2, we have characterized the crystal structure of triflate-substituted [1,1-(dppbz)-3-(NC5H5)-closo-1,2-RhSB9H8] (7). In the reactions, there is conversion of electron and protons into hydrogen and of hydrogen into hydride ligands, demonstrating that these boron-based metal compounds act as electrons reservoirs, capable of promoting multielectron processes.