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]­[(IP^2–)₂Al] (<b>1c</b>) with disulfides RSSR (where X = C­(S)­NMe₂, <b>4</b>; SMe, <b>5</b>). 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 [(^RIP^2–)₂Al]⁻ (R = H, <b>1c</b>; Me, <b>1d</b>) with ZnX₂ affords (^RIP^–)₂AlX (where IP = iminopyridine, R = H, and X = Cl, <b>2</b>; CCPh, <b>6</b>; N₃, <b>7</b>; SPh, <b>8</b>; or R = Me and X = NHPh, <b>9</b>). 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 <b>4</b> is six-coordinate and is best described as distorted octahedral. Variable temperature magnetic susceptibility measurements indicate that each of the complexes <b>3–9</b> has a biradical electronic structure similar to previously reported <b>2</b>. Magnetic exchange coupling constants in the range <i>J</i> = −94 to −212 cm^–1 were fit to the data for <b>2–9</b> to describe the energy of antiferromagnetic interaction between ligand radicals assuming a spin Hamiltonian of the form <i>Ĥ</i> = −2<i>JŜ</i>_L(1)·<i>Ŝ</i>_L(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 <b>3–9</b> displayed a reversible two-electron reduction and two quasi-reversible one-electron oxidation processes. The energy of the ligand based redox processes for <b>2–9</b> 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 <i>K</i>_c = 10^5.7 to 10^7.9 for <b>3–9