Dihydrogen Cleavage by a Zinc–Zinc Bond of a Heteroleptic Dizinc(I) Cation

Oxidative addition of dihydrogen across a metal–metal bond to form reactive metal hydrides in homogeneous catalysis is known for transition metals but not for zinc(I)–zinc(I) bond as found in Carmona's eponymous dizinconene [Zn2Cp*2] (Cp* = η5-C5Me5). Dihydrogen reacted with the heteroleptic zinc(I)–zinc(I) bonded cation [(L2)Zn–ZnCp*][BAr4F] (L2 = TMEDA, N,N,N′,N′-tetramethylethylenediamine, TEEDA, N,N,N′,N′-tetraethylethylenediamine; ArF = 3,5-(CF3)2C6H3) under 2 bar at 80 °C to give the zinc(II) hydride cation [(L2)ZnH(thf)][BAr4F] along with zinc metal and Cp*H derived from the intermediate [Cp*ZnH]. DFT calculations show that the cleavage of dihydrogen occurs through a highly unsymmetrical transition state. Mechanistic studies agree with a heterolytic cleavage of dihydrogen as a result of the cationic charge and unsymmetrical ligand coordination. To explore the existence of zinc(I) hydride, thermally unstable hydridotriphenylborate complexes of zinc(I) [(L2)Zn(HBPh3)–ZnCp*] (L2 = TMEDA, TEEDA; TMPDA, N,N,N′,N′-tetramethyl-1,3-propylenediamine) have been prepared by salt metathesis and were shown to undergo fast exchange with both BPh3 and [HBPh3]−.