Silicagel support mediated nonphotolytic cleavage of the rhenium-rhenium bond of [{NC5H4N=NC6H4(R)}(CO)3Re0]2.: Synthesis of the monomeric species ReI{NC5H4N=NC6H4(R)}(CO)3Cl, crystal structure, spectroscopic and electron-transfer properties

The reaction of Re-2(CO)(10) with the 2-arylazopyridine ligand, L [L=NC5H4-N=N-C6H4(R), R-H, o-Me/Cl, m-Me/Cl] in dry THF under a dinitrogen atmosphere afforded a metal-metal bonded product of the type (L)(CO)(3)Re-0-Re-0 (CO)(3)(L) 1. On a silica gel column and in the presence of chlorinated solvents (CHCl3 or CH2Cl2) the complex 1 transformed into a mononuclear complex of composition Re'(L)(CO)(3)Cl 2, where the cleavage of Re-Re bond of 1 and the oxidative addition of chlorine to the rhenium center have taken place consecutively. The molecular structure of the complex 2a (R=H) has been determined by single crystal X-ray diffraction. The crystal lattice consists of two crystallographically independent Re(L-1)(CO)(3)Cl molecules which are non-superimposable mirror images; d and 1 enantiomers, exist in a 1:1 ratio. The complexes 2 display irreversible Re-1--->Re-11 oxidation processes near 1.5 V versus SCE and two quasi-reversible azo (N=N) reductions in the ranges -0.28 to -0.48 V and -0.83 to -1.06 V versus SCE. The complexes of the type 1 exhibit only four successive azo reductions in the range 0 to -2 V versus SCE. Both the complexes (1 and 2) display d pi(Re) --> pi*L metal-to-ligand charge-transfer transitions near 500 nm and intraligand n-pi* and pi-pi* transitions near 300 and 300 nM, respectively. The complexes 2 are susceptible to a spontaneous chloride exchange reaction. (C) 1999 Elsevier Science S.A. All rights reserved.