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

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journal contribution
posted on 12.10.2015 by Kyle T. Horak, David G. VanderVelde, Theodor Agapie
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.