Redox Routes to Substitution of Aluminum(III): Synthesis and Characterization of (IP–)2AlX (IP = α‑iminopyridine, X = Cl, Me, SMe, S2CNMe2, CCPh, N3, SPh, NHPh)

Redox active ligands are shown to facilitate a variety of group transfer reactions at redox inert aluminum­(III). Disulfides can be used as a two-electron group transfer reagent, and we show that (IP–)2AlSR can be formed by reaction of [(THF)6Na]­[(IP2–)2Al] (1c) with disulfides RSSR (where X = C­(S)­NMe2, 4; SMe, 5). In a more general redox route to substitution of aluminum bis­(iminopyridine) complexes, we report zinc­(II) salts as a group transfer reagent. Reaction of [(RIP2–)2Al]− (R = H, 1c; Me, 1d) with ZnX2 affords (RIP–)2AlX (where IP = iminopyridine, R = H, and X = Cl, 2; CCPh, 6; N3, 7; SPh, 8; or R = Me and X = NHPh, 9). Single crystal X-ray diffraction analysis of the complexes reveal that each of the five coordinate complexes reported here has a trigonal bipyramidal geometry with τ = 0.668 – 0.858. We observed a correlation between the greatest deviations from ideal trigonal bipyramidal symmetry (lowest τ values), the bond lengths consistent with smallest degree of ligand reduction, and the least polarizable X ligand in (IP–)2AlX. Complex 4 is six-coordinate and is best described as distorted octahedral. Variable temperature magnetic susceptibility measurements indicate that each of the complexes 3–9 has a biradical electronic structure similar to previously reported 2. Magnetic exchange coupling constants in the range J = −94 to −212 cm–1 were fit to the data for 2–9 to describe the energy of antiferromagnetic interaction between ligand radicals assuming a spin Hamiltonian of the form Ĥ = −2JŜL(1)·ŜL(2). The strongest coupling occurs when the angle between the ligand planes is smallest, presumably to afford good overlap with the Al–X σ* orbital. Electrochemical properties of the complexes were probed using cyclic voltammetry and each of 3–9 displayed a reversible two-electron reduction and two quasi-reversible one-electron oxidation processes. The energy of the ligand based redox processes for 2–9 differ by about 150 mV over all complexes and show a correlation with the degree of IP– reduction observed crystallographically; more reduced IP– ligands require higher potentials for further reduction. Comproportionation constants that describe the equilibrium for the reaction (IP–)2AlX + (IP)2AlX ↔ (IP–)­(IP)­AlX fall in the range of Kc = 105.7 to 107.9 for 3–9.