Synthetic Advances for P2N2 Molecular Electrocatalysts with Pendant Bases in the Second Coordination Sphere

Efficient electrocatalytic reduction of small molecules such as CO2, N2, and even protons relies upon the precise delivery of electrons and protons to an active site. The coordination of these reactions can involve sophisticated ligands controlling the coordination spheres around a reactive center. The incorporation of a pendant base for proton shuttling is one way to achieve such control. In order to explore the reactivities of a class of complexes, Ni(P2N2), versatile syntheses are required. Seven 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands with alkyl- and carbonyl- substituted phosphines have been synthesized via two versatile methods that allows for improved control of the phosphine substituent. The methyl-, ethyl-, benzyl-, 3-propanoic acid, 3-propanamide, and ethyl 3-propanoate-phosphino- substituted phosphine P2N2 ligands (PMe2NPh2, PEt2NPh2, PBn2NPh2, PPA2NPh2, PAmide2NPh2, PEtE2NPh2, PEtE2NPhOMe2) were synthesized and characterized by 31P{1H} NMR, 1H NMR, and elemental analysis, and their corresponding [Ni(PR2NPh2)2](BF4)2 complexes were synthesized and characterized by 31P{1H} NMR, 1H NMR, and electrochemistry, and their corresponding Ni(PR2NR’2)X2 (X= Cl-, BF4-, CF3SO3-, PF6-) complexes have been synthesized and characterized by 31P{1H} NMR, 1H NMR, X-ray crystallography, and elemental analysisThe nickel complexes of P2N2 ligands synthesized and characterized by these new methods were investigated for hydrogen evolution and formate oxidation electrocatalysis. The newly synthesized complexes with alkyl- and carbonyl- phosphine substituents rapidly evolve hydrogen and oxidize formate more slowly than previously synthesized cyclohexyl- and phenyl- substituted phosphine ligands, due to a more negative reduction potential leading to a decrease in the driving force of formate oxidation and increase for hydrogen evolution.