Nuclear magnetic resonance studies of dynamic cobalt and rhodium clusters in solution and in the solid state

Abstract

The intramolecular carbonyl exchange which occurs in solution for the dinuclear and tetranuclear cobalt complexes containing the bisphosphines DPM, bis(diphenylphosphino)methane; DMPM, (dimethyl-diphenylphosphino)methane; and DMM, bis(dimethylphosphino)methane is very fast at temperatures down to -80°C. For the tetranuclear clusters Co₄(CO)₈(DPM)₂, Co₄(CO)₈(DMPM)₂, Co₄(CO)₈(DMM)₂, and Rh₄(CO)₈(DPM)₂, this exchange is slow at -80°C on the NMR time scale. The postulated mechanism for carbonyl exchange is based on a previously proposed mechanism, which is the expansion of the ligand icosahedron into a cubooctohedron. Because of the constraints imposed by the bisphosphine ligands, only one ligand icosahedron can be formed that is consistent with the known structure. Racemization of enantiomers by rotation of the ligands on the apical metal atom can occur, and thus enables complete carbonyl exchange to take place.

The series of binary metal carbonyls: Co₄(CO)₈, Fe₃(CO)₁₂, Co₄(CO)₁₂, Co₃Rh(CO)₁₂, Co₂Rh₂(CO)₁₂ and Rh₄(CO)₁₂, has been studied by variable temperature MAS (magic angle spinning) ¹³C NMR spectroscopy. All of these molecules except for Rh₄(CO)₁₂ show dynamic behavior as evidenced by their solid state ¹³C NMR spectra. Since carbonyl ligands cannot move within the crystalline lattice to an extent sufficient to render bridging and terminal carbonyls equivalent, then the dynamic behavior observed for the binary metal carbonyls must be described as metal atom movement within the carbonyl cage. The tetranuclear clusters which contain rhodium show a higher coalescence temperature in their NMR spectra than Co₄(CO)₁₂. As the rhodium content increases the activation energy for carbonyl exchange for exchange increases. cluster Rh₄(CO)₁₂ does not exhibit dynamic behavior in the solid state. lt is concluded that the rhodium tetrahedron is too large to move within the carbonyl cage.

The cobalt “A-Frame" complexes Co₂(CO)₃,(DPM)₂I₂, Co₂(CO)₃(DMM)(DPM)I₂, and Co₂(CO)₃(DPM)₂S were synthesized but show no dynamic behavior in solution. The crystal structure of Co₂(CO)₃(DMM)(DPM)l₂, shows that this "A-Frame" complex is coordinatively saturated around the cobalt atoms. Thus, these molecules are relatively inert, and show no evidence of carbonyl scrambling.