Square-Planar Rhodium(I) Complexes Partnered with [arachno-6-SB₉H₁₂]^-:  A Route toward the Synthesis of New Rhodathiaboranes and Organometallic/Thiaborane Salts

Treatment of [RhCl(η⁴-diene)]₂ (diene = nbd, cod) with the N-heterocyclic ligands 2,2‘-bipyridine (bpy), 4,4‘-dimethyl-2,2‘-bipyridine (Me₂bpy), 1,10-phenanthroline (phen), and pyridine (py) followed by addition of Cs[arachno-6-SB₉H₁₂] affords the corresponding salts, [Rh(η⁴-diene)(L₂)][SB₉H₁₂] [diene = cod, L₂ = bpy (1), Me₂bpy (3), phen (5), (py)₂ (7); diene = nbd, L₂ = bpy (2), Me₂bpy (4), phen (6), (py)₂ (8)]. These compounds are characterized by NMR spectroscopy and mass spectrometry, and in addition, the cod−Rh species 1 and 3 are studied by X-ray diffraction analysis. These saltlike reagents are stable in the solid state, but in solution the rhodium(I) cations, [Rh(η⁴-diene)(L₂)]⁺, react with the polyhedral anion [SB₉H₁₂]^- leading to a chemistry that is controlled by the d⁸ transition element chelates. The nbd−Rh(I) complexes react faster than the cod−Rh(I) counterparts, leading, depending on the conditions, to the synthesis of new rhodathiaboranes of general formulas [8,8-(L₂)-nido-8,7-RhSB₉H₁₀] [L₂ = bpy (9), Me₂bpy (10), phen (11), (py)₂ (12)] and [8,8-(L₂)-8-(L‘)-nido-8,7-RhSB₉H₁₀] [L‘ = PPh₃, L₂ = bpy (13), Me₂bpy (14), phen (15); L‘ = NCCH₃, L₂ = bpy (16), Me₂bpy (17), phen (18)]. Compound 13 is characterized by X-ray diffraction analysis confirming the 11-vertex nido-structure of the rhodathiaborane analogues 14−18. In dichloromethane, 1 and 3 yield mixtures that contain the 11-vertex rhodathiaboranes 9 and 10 together with new species. In contrast, the cod−Rh(I) reagent 5 affords a single compound, which is proposed to be an organometallic rhodium complex bound exo-polyhedrally to the thiaborane cage. In the presence of H₂(g) and stoichiometric amounts of PPh₃, the cod−Rh(I) reagents, 1, 3, and 5, afford the salts [Rh(H)₂(L₂)(PPh₃)₂][SB₉H₁₂] [L₂ = bpy (19), Me₂bpy (20), phen (21)]. Similarly, in an atmosphere of CO(g) and in the presence of PPh₃, compounds 1−6 afford [Rh(L₂)(PPh₃)₂(CO)][SB₉H₁₂] (L₂ = bpy (22), Me₂bpy (23), phen (24)]. The structures of 19 and 24 are studied by X-ray diffraction analysis. The five-coordinate complexes [Rh(L₂)(PPh₃)₂(CO)]⁺ undergo PPh₃ exchange in a process that is characterized as dissociative. The observed differences in the reactivity of the nbd−Rh(I) salts versus the cod−Rh(I) analogues are rationalized on the basis of the higher kinetic lability of the nbd ligand and its faster hydrogenation relative to the cod diene.