Rh(I) Complex with a Tridentate Pyridine–Amino–Olefin Actor Ligand–Metal–Ligand Cooperative Activation of CO₂ and Phenylisocyanate under C–C and Rh–E (E = O, N) Bond Formation

We synthesized and characterized the novel olefinic multidentate actor ligand dbap-py. The ligand consists of a 2-methyl pyridine unit and an azepine component fused by an N-alkylation. The coordination chemistry to Rh­(I) centers was explored and dbap-py revealed interesting tridentate chelation (pyridine–amine–olefin), which can suit square planar as well as distorted trigonal bipyramidal coordination spheres in Rh­(I) complexes. Most notably, dbap-py in [Rh­(dbap-py)­(PPh3)­Cl] (3) entails an acidic benzylic methylene moiety, which is readily deprotonated allowing for the formation of the neutral square planar complex [Rh­(dbap-py*)­(PPh3)] (4, the asterisk indicates the deprotonated ligand). As a result, the deprotonation disrupts the aromatic π-system of the pyridine unit in 4 and a C-nucleophilic methine moiety is formed. Complex 4 reacts in tetrahydrofuran solution at ambient temperature rapidly with CO bonds present in CO2 or the NC bond of phenyl isocyanate (Ph-NCO) to give the penta-coordinated rhodium κ1-O carboxylate [Rh­(dbap-COO)­(PPh3)] (5) and the rhodium κ1-N amidate [Rh­(dbap-NCO)­(PPh3)] (6), respectively. Both reactions are characterized by a C–C and Rh–E (E = O (5); N (6)) bond formation under concomitant redistribution of the ligand’s π-system of the pyridine unit (“rearomatization”). To the best of our knowledge, compound 6 gives precedence to Rh­(I) complexes with a κ1-N amidate ligand. Remarkably, metal–ligand cooperation is key to the uptake of Ph-NCO and allows for a convenient access to an amidate ligand motif