Janus-Faced Reactivity of Heterobimetallic Al–Zn Complexes

p-Block complexes containing unsupported metal–metal bonds can activate small molecules via mechanisms that complement and contrast with those of d-block metal complexes. Within this compound class, Al–Zn heterobimetallics have shown a remarkable ability to cooperatively activate heteroallenes by migratory insertion into the Al–Zn bond. Here we report the synthesis of a new family of heterobimetallic Al–Zn complexes [(ArNacnac)Zn–Al(Cp*)(X)] (Ar = Dep, X = Cl (2a), Br (2b), I (2c); Ar = Dipp, X = Cl (3a), Br (3b); ArNacnac = {HC[C(Me)N(Ar)]2}; Ar = Dep, C6H3Et2-2,6; Dipp, C6H3iPr-2,6), by insertion of in situ-generated “AlCp*” (Cp* = C5Me5) into the Zn–X bonds of parent dimeric zinc ArNacnac halide complexes [Zn(ArNacnac)(μ-X)]2. These complexes were characterized by NMR and ATR-IR spectroscopy, elemental analysis, single crystal X-ray diffraction, and density functional theory calculations. We find that the reactions of the newly formed Al–Zn complexes with N,Nʹ-dicyclohexylcarbodiimide (DCC) proceed by migratory insertion into the Al–Zn bond to give [Zn(ArNacnac){μ2-C(NCy)2-κ1-C,κ2-N,Nʹ}Al(Cp*)(X)] (Ar = Dep, X = Cl (4a), Br (4b), I (4c); Ar = Dipp, X = Cl (5a), Br (6a)) as expected, but surprisingly for the DippNacnac derivatives a second equivalent of DCC inserts into the Al–Cp* moiety to afford [Zn(DippNacnac){μ2-C(NCy)2-κ1-C,κ2-N,Nʹ}Al(Cl){κ2-(NCy)2C-η1-Cp*-κ2-N,Nʹ}] (5b) or [Zn(DippNacnac)(Br){μ2-C(NCy)2-κ1-C,κ2-N,Nʹ}Al{κ2-(NCy)2C-η1-Cp*-κ2-N,Nʹ}] (6b); for 6b concomitant halide migration gives a Zn–Br bond. These surprising results show that non-innocent reactivity of Al–Cp*, widely assumed to be a spectator ligand in heterobimetallic complexes, can be switched on by subtly tuning ligand steric bulk about Al–Zn bonds.