Electronic Structures of Nickel(II)-Bis(indanyloxazoline)-dihalide Catalysts: Understanding Ligand Field Contributions That Promote C(sp <sup>2</sup> )–C(sp <sup>3</sup> ) Cross-Coupling
Brendon J. McNicholas, Zhengjia Tong, Daniel Bím, Raymond F. Turro, Nathanael P. Kazmierczak, Jakub Chalupský, Sarah E. Reisman, Ryan G. Hadt
Abstract
Ni II (IB) dihalide [ IB = (3a R,3a′ R,8a S,8a′ S )-2,2′-(cyclopropane-1,1-diyl)bis(3a,8a-dihydro-8 H -indeno[1,2- d ]-oxazole)] complexes are representative of a growing class of first-row transition-metal catalysts for the enantioselective reductive cross-coupling of C(sp 2 ) and C(sp 3 ) electrophiles. Recent mechanistic studies highlight the complexity of these ground-state cross-couplings but also illuminate new reactivity pathways stemming from one-electron redox and their significant sensitivities to reaction conditions. For the first time, a diverse array of spectroscopic methods coupled to electrochemistry have been applied to Ni II -based precatalysts to evaluate specific ligand field effects governing key Ni-based redox potentials. We also experimentally demonstrate DMA solvent coordination to catalytically relevant Ni complexes. Coordination is shown to favorably influence key redox-based reaction steps and prevent other deleterious Ni-based equilibria. Combined with electronic structure calculations, we further provide a direct correlation between reaction intermediate frontier molecular orbital energies and cross-coupling yields. Considerations developed herein demonstrate the use of synergic spectroscopic and electrochemical methods to provide concepts for catalyst ligand design and rationalization of reaction condition optimization.