Variable Temperature IR Spectroelectrochemical Investigation of the Stability of the Metal-Metal-Bonded Radical Anions [(CO)5MnRe(CO)3(L)]•- (L = 2,2'-Bipyridine (BPY), 2,2'-Bipyrimidine (BPYM), 2,3-Bis(2-pyridyl)pyrazine (DPP)) and [(CO)5MnRe(CO)3(L)Re(Br)(CO)3]- (L = BPYM, DPP) Controlled by the Lowest π* (α-Diimine) Orbital Energy

Abstract

Cyclic voltammetry in combination with IR thin-layer spectroelectrochemistry was employed for detection and characterization of the radical anionic complexes [(CO)5MnRe(CO)3(L)]•- (L = 2,2'-bipyridine (BPY), 2,2'-bipyrimidine (BPYM), and 2,3-bis-(2-pyridyl)pyrazine (DPP)) and [(CO)5MnRe(CO)3(μ-L)Re(Br)(CO)3]•-(μ-L = μ-BPYM, μ-DPP) as the primary products formed upon one-electron reduction of the neutral parent compounds. The stability of the radical anions is determined by the degree of polarization of the Mn-Re bond which increases in the order Mn-Re(μ-BPYM)Re < Mn-Re(μ-DPP)Re < Mn-Re(BPYM) ~ Mn-Re(DPP) < Mn-Re(BPY), i.e., with increasing energy of the π* LUMO of the α-diimine ligand in the parent complexes and with increasing α- and π-donor character of the α-diimine anion in the reduced species. In the same order, the unpaired electron in the metal-metal-bonded radical anions becomes more delocalized over the (Mn)Re-L chelate bond. This delocalization in turn also considerably destabilizes the dz2 orbital of Re(L) involved in the Mn-Re bonding, making its occupation no more energetically convenient. Consequently, the Mn-Re bond will split heterolytically to give [Mn(CO)5]- and [Re(CO)3(L)]•-. The trinuclear radical anion with μ-L = μ-BPYM was found to be stable already at room temperature and could be characterized also by UV-vis and ESR spectroscopy. The related radical anion with μ-DPP could only be stabilized at properly low temperatures, just as for the compounds with nonbridging BPYM and DPP. In the case of the BPY compound, the metal-metal-bonded radical anion could not be detected at all. Descriptions of the secondary reaction pathways of the radicals [Re(CO)3(L)]•, formed in the course of the irreversible reduction of the above Mn-Re-bonded complexes, are given as well. © 1995, American Chemical Society. All rights reserved.