Rotational isomerism and fluxional behavior of bis(carbon dioxide) adducts of molybdenum

Article abstract of DOI:10.1021/ja00024a030

The exchange reactions involving the bis(carbon dioxide) complex trans-Mo(CO₂)₂(PMe₃)₄ (1) and chelating phosphines P-P (Me₂PCH₂PMe₂, dmpm; Me₂PCH₂PMe₂, dmpe; Et₂PCH₂CH₂PEt₂, depe; and Ph₂PCH₂CH₂PPh₂, dppe) and isocyanides, CNR, provide efficient routes to the new (CO₂)₂ adducts trans-Mo(CO₂)₂(P-P)(PMe₃)₂ (P-P = dmpe (3a), depe (3b), dmpm (3c), dppe (3d)), trans-Mo(CO₂)₂(P-P)₂ (P-P = dmpe (4a), depe (4b)), and trans-Mo(CO₂)₂(depe)(PMe₃)(CNR) (R = Bu^t (5a), Cy (5b)). Due to the asymmetric nature of the coordinated CO₂ groups and to the existence of different ligands (PMe₃, P-P, CNR) in the plane perpendicular to the O₂C-Mo-CO₂ bond axis, several isomers arising from the different conformations of the CO₂ ligands are possible for each of the above type of complex. Their existence has been demonstrated by variable-temperature NMR spectroscopy, and they have furthermore been shown to interconvert in solution. Four nondissociative mechanisms have been considered for this exchange process, of which one that involves exchange of the coordinated and free oxygen atoms of one of the CO₂ ligands has been shown to be inoperative. Two rotational mechanisms, namely an independent rotation and a disrotatory motion, have also been excluded. A concerted rotation of the CO₂ groups in which both molecules rotate in the same direction is in accord with all the experimental results.