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

Abstract

The reaction of Re₂(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)₃Re⁰–Re⁰ (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^I(L)(CO)₃Cl 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(L¹)(CO)₃Cl molecules which are non-superimposable mirror images; d and l enantiomers, exist in a 1:1 ratio. The complexes 2 display irreversible Re^I→Re^II 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 dπ(Re)→π^∗L metal-to-ligand charge-transfer transitions near 500 nm and intraligand n–π and π–π^∗ transitions near 400 and 300 nm, respectively. The complexes 2 are susceptible to a spontaneous chloride exchange reaction.