Preparation, Crystal Structure, and Unusually Facile Redox Chemistry of a Macrocyclic Nitrosylrhodium Complex

The reaction between NO and L2(H2O)Rh2+ (L2 = meso-Me6-1,4,8,11-tetraazacyclotetradecane) generates a sky-blue L2(H2O)RhNO2+, a {RhNO}8 complex. The crystal structure of the perchlorate salt features a bent Rh−N−O moiety (122.1(11)o), short axial Rh−NO bond (1.998(12) Å) and a strongly elongated Rh−OH2 (2.366(6) Å) trans to NO. Acidic aqueous solutions of L2(H2O)RhNO2+ are stable for weeks, and are inert toward oxygen. The complex is oxidized rapidly and reversibly with Ru(bpy)33+, kf = (1.9 ± 0.1) × 105 M−1 s−1, to an intermediate believed to be L2(H2O)RhNO3+. This unprecedented {RhNO}7 species has a lifetime of about 90 s at room temperature at pH 0. The reverse reaction between L2(H2O)RhNO3+ and Ru(bpy)32+ has kr = (1.5 ± 0.4) × 106 M−1 s−1. The kinetic data define the equilibrium constant for the L2(H2O)RhNO2+/Ru(bpy)33+ reaction, K = kf/kr = 0.13, and yield a reduction potential for the L2(H2O)RhNO3+/2+ couple of 1.31 V. Both the redox thermodynamics of L2(H2O)RhNO3+/2+ and the kinetics of the reactions with Ru(bpy)33+/2+ are quite similar to those of uncoordinated NO+/NO.