Postsynthetic modifications of [2,2,2-(H)(PPh3)2-closo-2,1-RhSB8H8] with Lewis bases: cluster modular tuning.

It has been demonstrated that the reaction of [2,2,2-(H)(PPh3)2-closo-2,1-RhSB8H8] () with PPh3 affords the boron substituted rhodathiaborane-PPh3 adduct, [6,6-(PPh3)2-9-(PPh3)-arachno-6,5-RhSB8H9] (). Building upon this reaction, we report herein that the 10-vertex hydridorhodathiaborane reacts with the Lewis bases, PCy3, py, 2-Mepy, 2-Etpy, 3-Mepy and 4-Mepy to form the rhodathiaborane-ligand adducts, [6,6-(PPh3)2-9-(L)-arachno-6,5-RhSB8H9], where L = PCy3 (), 2-Mepy (), 2-Etpy (), py (), 3-Mepy () or 4-Mepy (), and [8,9-μ-(H)-9-(PPh3)2-8-(L)-arachno-9,6-RhSB8H8], where L = py (), 3-Mepy () or 4-Mepy (). The selectivity of the reactions depended on the nature of the entering Lewis bases, affording the 6,5-isomers, , , and as single products for PPh3, PCy3, 2-Mepy and 2-Etpy; and mixtures of the 6,5-/9,6-regioisomers, /, / and / for py, 3-Mepy and 4-Mepy, respectively. The molecular structures of both regioisomers were characterized by X-ray diffraction analysis for the 6,5-isomers, and , and for the 9,6-isomers, and . Variable temperature NMR studies of the reaction between and PPh3 or 2-Mepy demonstrated that at low temperatures there is formation of the 9,6-species that subsequently isomerizes to the 6,5-regioisomer, indicating that for the more sterically hindered Lewis bases, PPh3, 2-Mepy and PCy3, the latter isomer is more stable and accessible through an intramolecular {Rh(PPh3)2} vertex flip. The formation of both isomers with py, 3-Mepy and 4-Mepy indicates that the kinetic and thermodynamic energies of the 6,5 and 9,6 rhodathiaborane-ligand adducts are similar for these Lewis bases. Lewis base bonding to exo-polyhedral boron vertices results in a change of the metal coordination from pseudo-octahedral Rh(iii) in to pseudo-square planar Rh(i) in the adducts. The chemistry described here highlights the remarkable structural flexibility of these polyhedral boron-containing compounds, their modular architecture and their easy postsynthetic modification.