Remarkably stable radical anions derived from clusters [HOs 3(CO) 9(L)], L= ortho-metallated α-diimine: a spectro-electrochemical study and theoretical rationalization

Abstract

Reduction of the cluster [Os 3(CO) 10( i Pr-PyCa)], i Pr–PyCa= σ- N, σ- N′-pyridine-2-carbaldehyde- N-isopropyl-imine, ( 1) follows an ECEC sequence. The initial one-electron reduction step produces the radical anion 1 − which was found stable on a subsecond time scale at low temperatures. At room temperature an open-structure radical anionic transient 1a − is formed which instantaneously reduces to 1a 2−. Subsequent attack of the latter species at yet nonreduced 1 yields an Os–Os bonded cluster dimer [ 1a– 1a] 2−. In contrast to the reactivity of 1 − , the radical anions of the clusters [HOs 3(CO) 9(C 5H 3N-2-C(H)N– i Pr)] ( 2), [HOs 3(CO) 9(6-CH 2–C 5H 3N-2-C(H)N– i Pr)] ( 3), containing the ortho-metallated ligands o- i Pr–PyCa{6-X} (X=H, Me}, are substantially more stable. At room temperature in THF, 2 − and 3 − slowly disproportionate to moderately stable dianionic species 2b and 3c. IR and 1H-NMR spectra of 2b have revealed the presence of bridging CO and terminal hydride ligands, and the dominant localization of the negative charge on the metallic core. The structure of 3c is probably similar. The latter dianions were also obtained by direct one-electron reduction of 2 − and 3 − , respectively. Both radical anions are inherently stable at T=253 K. According to extended Hückel molecular orbital calculations on models of the parent clusters 1– 3, the higher stability of 2 − and 3 − relative to that of 1 − has its origin in larger localization of the odd electron at the lowest π*-orbital of the ortho-metallated ligands compared to the σN, σN′-chelated i Pr–PyCa ligand, and in higher cluster-diimine binding energies of 2 and 3 which drop due the one-electron reduction less than calculated for the couple 1/ 1 − . IR and 1H-NMR spectra have revealed that 2,3-dipyrid-2′-ylbenzoquinoxaline (dpb) coordinates to the Os 3 core as an ortho-metallated ligand, forming [HOs 3(CO) 9(dpb-14-yl)] ( 4), in contrast to shorter 2,3-dipyrid-2′-ylpyrazine (dpp). The corresponding radical anion 4 − is inherently stable already at room temperature, mainly due to the large π-acceptor capacity of the ortho-metallated dpb ligand.