Frustrated Al/M Heterobimetallic Complexes (M = Cr, Mo, W) That Exhibit Both Lewis and Radical Pair Behavior

Exploration of new heterobinuclear Al/M combinations
is relevant
to contemporary strategies for cooperative bond activation. Here,
we report the synthesis and characterization of six new Al/M heterobimetallic
complexes (M = Cr, Mo, W) that exhibit end-on “isocarbonyl”-type
AlOCM bridges with metalloketene character
rather than featuring AlMCO motifs with metal–metal
bonding. The new compounds were characterized experimentally by nuclear
magnetic resonance and infrared spectroscopies and theoretically using
density functional theory, natural bond orbital, and quantum theory
of atoms in molecules calculations. Factors influencing AlOCM
vs AlMCO isomerism were probed both experimentally
and computationally. Crossover experiments between different group
VI Al/M derivatives and regioselective epoxide ring opening indicate
that the Al/M complexes act as masked frustrated Lewis pairs in solution
under certain conditions. However, crossover experiments between group
VI Al/M complexes and a previously studied Al–Fe complex, as
well as computational modeling, imply that the same complexes can
also reasonably act as masked frustrated radical pairs (FRPs). FRP
reactivity with the group VI Al/M complexes was achieved under photochemical
conditions, producing unsaturated metal–carbonyl dimers [(CpCr)2(CO)3]2– and [Mn2(CO)8]2–, which would otherwise be unstable under
standard conditions but that are isolable here due to Al(III) coordination.
The metal–metal bonding in these unsaturated metal–carbonyl
dimers was also analyzed theoretically.