Dipyrromethene and β-Diketinninate Zinc Hydride Complexes: Resemblances and Differences

A new dipyrromethene (DPM) ligand with bulky DIPP-substituents is introduced (DIPP = 2,6-diisopropylphenyl). The ligand, abbreviated as (DPM)-D-DIPP, was deprotonated with ZnEt2 to give ((DPM)-D-DIPP)ZnEt, which reacted with I-2 to form ((DPM)-D-DIPP)ZnI. Reaction of the latter with K[N(Pr-i)HBH3] afforded a labile Zn amidoborane complex which, after beta-hydride elimination, formed ((DPM)-D-DIPP)ZnH. Crystal structures of ((DPM)-D-DIPP)ZnX (X = Et, I, H) revealed their monomeric nature. The Zn-N bond distances are somewhat longer than those in the corresponding, monomeric beta-diketiminate complexes ((BDI)-B-DIPP)ZnX ((BDI)-B-DIPP = CH[C(Me)N-DIPP](2)). This is in agreement with calculated NPA charges, which are lower on the N atoms of DPM compared to those on BDI. Reaction of ((DPM)-D-DIPP)ZnH with CO2 gave ((DPM)-D-DIPP)Zn(O2CH), which crystallized as a monomer with a symmetrically bound eta(2)-formate ligand. In contrast, the beta-diketiminate complex crystallizes as a dimer [((BDI)-B-DIPP)Zn(O2CH)](2) with bridging formate ligands. Reaction of ((DPM)-D-DIPP)Zn(O2CH) with various silanes regenerated the hydride complex ((DPM)-D-DIPP)ZnH. Catalytic CO2 hydrosilylation with (EtO)(3)SiH using ((DPM)-D-DIPP)ZnH as a catalyst gave full reduction to [Si]-OMe species, whereas the catalyst ((BDI)-B-DIPP)ZnH only partially reduced CO2 to [Si]-OC(O)H. The advantage of the (DPM)-D-DIPP ligand is the arrangement of the DIPP-substituents, which form a pocket around the Zn-X unit, preventing dimerization and influencing its reactivity. In addition, in contrast to the negatively charged central backbone carbon in the (BDI)-B-DIPP ligand, that in (DPM)-D-DIPP is neutral. This makes it less nucleophilic and Bronsted basic, as expected for a true spectator ligand.