Insertion Reactions of copper (I) Aryloxides with heterocumulenes

Abstract

Studies on alkoxide and aryloxide complexes of copper(I) are significant due to their relevance to biological systems and their potential in small molecule activation. In the present investigation, the insertion reaction of copper(I) aryloxide complexes with heterocumulenes (electronic analogs of CO, like CS and RNCS—primarily MeNCS) has been studied. This reaction results in the in situ formation of a substituted methanethiolate ligand complexed to copper. Oligomeric complexes having copper(I) ion bonded to soft donor ligands like sulfur and nitrogen are formed, and these are quite stable. The steric and electronic factors that govern the insertion reaction have been studied.

Steric effects were found to dictate the structure and nuclearity of the resulting oligomeric complexes. Earlier studies with PhNCS have shown that the presence of sterically demanding substituents on the aryloxide led to the formation of species with lower nuclearity. In the present work, it has been shown that this steric congestion could be relieved by employing less bulky groups on the heterocumulene—thereby resulting in complexes of higher nuclearity even with bulky phenols. It has also been shown how a gradual reduction in size leads to more symmetry in the structure of the resulting oligomeric complex. Thus, by introducing suitable steric variations in the phenol and the heterocumulene, the structure and nuclearity of the oligomeric complexes could be controlled.

Electronic effects were also found to play an important role in the insertion reaction. While steric factors alter the structure of the product, electronic effects modify the reactivity patterns. The insertion reaction was found to be favorable when the carbon atom of the heterocumulene is electrophilic and the aryloxide group is highly nucleophilic. The insertion of the heterocumulene across the Cu(I)-OAr bond becomes reversible when the electronic factors are unfavorable. Electronic variations were introduced by employing heterocumulenes and aryloxides with varying electronic features. The insertion reaction of MeNCS, a less electrophilic heterocumulene, is reversible in the presence of PPh. Based on the reactivity patterns, a possible mechanistic pathway is postulated. Similarly, the reactions with phenols having electron-withdrawing groups were also unfavorable.

The reversible nature of the insertion allows exchange of either the heterocumulene or the aryloxide group in the oligomeric complex in the presence of catalytic amounts of PPh. A less electrophilic heterocumulene could be substituted by a more electrophilic one, and an electron-deficient aryloxide could be replaced by a more nucleophilic one. The reversible behavior is apparently contrary to HSAB principles since a Cu-S bond is replaced with a Cu-O bond, showing the limitations of HSAB theory.

Treatment of these oligomeric complexes with PPh results in the formation of bis-triphenylphosphine Cu(I)L species. The structures of these complexes were compared with those of copper(I) phosphine complexes of heterocyclic thiones, as these two ligands are similar in structure. The comparison showed that the electronic properties of these ligands are different and, as a result, their bonding patterns are also different. Examination of the solid-state structures of these complexes revealed an interesting supramolecular motif. The phenyl rings of the PPh are involved in a concerted supramolecular interaction, which has been called the t-H step. The C-H step is made up of a series of alternating C—H··· and ··· interactions among four phosphines.