Tuning of Metal Complex Electronics and Reactivity by Remote Lewis Acid Binding to π‑Coordinated Pyridine Diphosphine Ligands

Metal
complexes supported by ligands with chemically modifiable
pendant groups are of interest for controlling reactivity. We report
on the coordination chemistry and reactivity of a multidentate phosphine
ligand framework that contains a Lewis acid binding site. 3,5-Bis­(2-phosphinophenyl)­pyridine
coordinates low-oxidation-state metal centers such as Ni0 and Pd0 via the phosphine donors and the π system
of the heterocycle. Electrophilic reagents such as B­(C6F5)3, Me+, and BCy2OTf
bind the available pyridine nitrogen, generating the Ni complexes 2Ni-B­(C6F5)3, 2Ni-Me, and 2Ni-BCy2OTf, respectively. Analogous compounds were prepared for Pd (2Pd, 2Pd-B­(C6F5)3, and 2Pd-H). The effect of Lewis acid binding was evaluated by
single-crystal X-ray diffraction studies and spectroscopy. Lewis acid
binding to 2Pd leads to a stronger η1 interaction between the metal and the heterocycle π system.
Ni binds in an η2 fashion, but the Lewis acid free
species is not monomeric. Ni coordination results in disruption of
pyridine aromaticity, as indicated by localization of double- and
single-bond character in the solid state. CO adducts were prepared
for Lewis acid free (4Ni) and Lewis acid bound species
(H+-, Me+-, and B­(C6F5)3-bound; 4Ni-H, 4Ni-Me, and 4Ni-B­(C6F5)3) that show a significant
shift of the CO stretching frequency from 1930 to 1966–1976
cm–1, respectively, indicating communication of
ligand electronics to the metal center. An NO adduct (5Ni) with negligible metal–pyridine interactions was obtained
upon sequential reaction of 2Ni with [OMe3]­[BF4] and then [NO]­[BF4]. Treatment of 2Ni with silanes and boranes results in pyridine dearomatization
involving heteroatom–H bond activation, with the heteroatom
binding to the pyridine nitrogen and the hydrides delivered to the
ortho position of pyridine. This reactivity demonstrates that the
pendant pyridine is drastically affected by metal binding, enabling
unusual ligand-based substrate activation. The described chemistry
highlights a strategy for tuning the properties of metal centers by
ligand postsynthetic modifications.