Synthesis, structure and reactivity of NO⁺, NO˙ and NO^- pincer PCN-Rh complexes

Synthesis of a pincer-type linear nitrosyl complex [Rh(P^tBuCN^Et)(NO)]⁺ (3⁺) is described. The product and all intermediates involved were fully characterized by FTIR, NMR, cyclic voltammetry and X-ray crystallography. Attempts at obtaining (3⁺) from its chlorinated precursor Rh(PCN)(NO)Cl (2) revealed that a relative stabilization of this complex ion is introduced by the BArF^- counteranion, as other counteranions-PF₆^-, BF₄^- and triflate-proved to coordinate to the metal center. Redox reactivity both of (3⁺) and of that of its five-coordinate derivatives (2) and [Rh(PCN)(NO)(CH₃CN)]⁺ (4⁺) was found to distinguish itself from analogous PCP complexes due to a relative stabilization of higher oxidation states. Oxidation of these three complexes was studied by FTIR spectroelectrochemistry. Reduction of complex (3⁺) to yield a short-lived {RhNO}⁹ species [Rh(PCN)(NO)]˙ (3˙) was also carried out. Complex (3˙) was proved able to activate carbon-halogen bonds in aryl halides, in much a similar way as that of its PCP analogue. Complex (3⁺) was also seen to establish a linear ↔ bent nitrosyl equilibrium upon addition of CO which could not be fully displaced with excess CO.