Uncommon coordination behaviour of P(S) and P(Se) units when bonded to carboranyl clusters: experimental and computational studies on the oxidation of carboranyl phosphine ligands.

Oxidation of closo-carboranyl diphosphines 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr) and closo-carboranyl monophosphines 1-PR(2)-2-R'-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr, Cy; R'=Me, Ph) with hydrogen peroxide, sulfur and elemental black selenium evidences the unique capacity of the closo-carborane cluster to produce uncommon or unprecedented P/P(E) (E=S, Se) and P=O/P=S chelating ligands. When H(2)O(2) reacts with 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr), they are oxidized to 1,2-(OPR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr). However, when S and Se are used, different reactivity is found for 1,2-(PPh(2))(2)-1,2-closo-C(2)B(10)H(10) and 1,2-(PiPr(2))(2)-1,2-closo-C(2)B(10)H(10). The reaction with sulfur produces mono- and dioxidation products for R=Ph, whereas Se produces the mono-oxidation product only. For R=iPr, only monooxidation takes place with S, and the second C(c)-PiPr(2) bond breaks to yield 1-SPiPr(2)-1,2-closo-C(2)B(10)H(11). When Se is used, only 1-SePiPr(2)-1,2-closo-C(2)B(10)H(11) is formed. The potential of the mono-chalcogenide carboranyl diphosphines 1-EPPh(2)-2-PPh(2)-1,2-closo-C(2)B(10)H(10) (E=S, 9; Se, 15) to behave as unsymmetric chelating bidentate ligands was studied for different metal complexes, different solvents and in the solid state. Dechalcogenation takes place in each case. Computational studies provided information on the P=E (E=S, Se) bonds. Steric effects block the bonding ability of the P=E group due to interactions between the chalcogen and the neighbouring hydrogen atoms (three from the phenyl rings and one from the carborane cluster). The electronic effects originate from the strongly electron-withdrawing character of the closo carborane cluster, which polarizes the P=E (E=S, Se) bond towards the phosphorus atom. As a consequence, the E atom is the electron-poor site and the P atom the electron-rich site in the P=E bond.