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Published October 12, 2015 | Supplemental Material
Journal Article Open

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

Abstract

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 Ni^0 and Pd^0 via the phosphine donors and the π system of the heterocycle. Electrophilic reagents such as B(C_6F_5)_3, Me+, and BCy_2OTf bind the available pyridine nitrogen, generating the Ni complexes 2Ni-B(C_6F_5)_3, 2Ni-Me, and 2Ni-BCy_2OTf, respectively. Analogous compounds were prepared for Pd (2Pd, 2Pd-B(C_6F_5)_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 η^2 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(C_6F_5)_3-bound; 4Ni-H, 4Ni-Me, and 4Ni-B(C_6F_5)_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 [OMe_3][BF_4] and then [NO][BF_4]. 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.

Additional Information

© 2015 American Chemical Society. Received: June 29, 2015; Publication Date (Web): August 20, 2015. Special Issue: Gregory Hillhouse Issue. We thank Lawrence M. Henling and Mike Takase for crystallographic assistance. We are grateful to Caltech and the NSF (CHE-1151918) for funding. T.A. is grateful for Sloan, Cottrell, and Dreyfus fellowships. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF CRIF:MU award to Caltech (CHE0639094). The authors declare no competing financial interest.

Attached Files

Supplemental Material - om5b00562_si_001.pdf

Supplemental Material - om5b00562_si_002.cif

Supplemental Material - om5b00562_si_003.cif

Supplemental Material - om5b00562_si_004.cif

Supplemental Material - om5b00562_si_005.cif

Supplemental Material - om5b00562_si_006.cif

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Supplemental Material - om5b00562_si_008.cif

Supplemental Material - om5b00562_si_009.cif

Supplemental Material - om5b00562_si_010.cif

Supplemental Material - om5b00562_si_011.cif

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August 20, 2023
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