Dipyrromethene and β‑Diketiminate 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 DIPPDPM, was deprotonated with ZnEt2 to give (DIPPDPM)­ZnEt, which reacted with I2 to form (DIPPDPM)­ZnI. Reaction of the latter with
K­[N­(iPr)­HBH3] afforded a labile
Zn amidoborane complex which, after β-hydride elimination, formed
(DIPPDPM)­ZnH. Crystal structures of (DIPPDPM)­ZnX
(X = Et, I, H) revealed their monomeric nature. The Zn–N bond
distances are somewhat longer than those in the corresponding monomeric
β-diketiminate complexes (DIPPBDI)­ZnX (DIPPBDI = 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 (DIPPDPM)­ZnH with CO2 gave
(DIPPDPM)­Zn­(O2CH), which crystallized as a monomer
with a symmetrically bound η2-formate ligand. In
contrast, the β-diketiminate complex crystallizes as a dimer
[(DIPPBDI)­Zn­(O2CH)]2 with bridging
formate ligands. Reaction of (DIPPDPM)­Zn­(O2CH)
with various silanes regenerated the hydride complex (DIPPDPM)­ZnH. Catalytic CO2 hydrosilylation with (EtO)3SiH using (DIPPDPM)­ZnH as a catalyst gave full
reduction to [Si]–OMe species, whereas the catalyst (DIPPBDI)­ZnH only partially reduced CO2 to [Si]–OC­(O)­H.
The advantage of the DIPPDPM 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 DIPPBDI ligand, that in DIPPDPM is neutral. This
makes it less nucleophilic and Brønsted basic, as expected for
a true spectator ligand.