Synergy between Experimental and Computational Chemistry Reveals the Mechanism of Decomposition of Nickel–Ketene Complexes

Abstract

A series of (dppf)Ni(ketene) complexes were synthesized and fully characterized. In the solid state, the complexes possess η-(C,O) coordination of the ketene in an overall planar configuration. They display similar structure in solution, except in some cases, the η-(C,C) coordination mode is also detected. A combination of kinetic analysis and DFT calculations reveals the complexes undergo thermal decomposition by isomerization from η-(C,O) to η-(C,C) followed by scission of the C═C bond, which is usually rate limiting and results in an intermediate carbonyl carbene complex. Subsequent rearrangement of the carbene ligand is rate limiting for electron poor and sterically large ketenes, and results in a carbonyl alkene complex. The alkene readily dissociates, affording alkenes and (dppf)Ni(CO). Computational modeling of the decarbonylation pathway with partial phosphine dissociation reveals the barrier is reduced significantly, explaining the instability of ketene complexes with monodentate phosphines.

Electronic Effect of Ligands on the Stability of Nickel–Ketene Complexes

“…When ketene is added to an equimolar mixture of a substituted dppf ligand and Ni(COD) 2 , the formation of the Ni−ketene complex is observed as an appearance of two doublets in the 31 P NMR. X-ray crystal structures of the Ni−ketene complexes have shown that a square planar geometry exists around the nickel center, which, along with past Mayer bond analysis, 8 is indicative of a Ni(II) species. The two doublets seen in the 31 P NMR corresponds to the two inequivalent phosphine atoms on the complex.…”

“…Additionally, the solid-state structure of 9d was previously found to be disordered, with random orientation of the ketene with respect to the nickel center. 8 In light of these, no discernible correlation between the bond lengths of the metallacycle and their observed rate constant of decomposition can be made. However, the observed electronic trend can be explained using past computational modeling of the decomposition pathway of the Ni−ketene complex.…”

“…The procedure was adapted from a previous literature source. 8 In the N 2 -filled glovebox, the substituted (dppf)Ni(ketene) complex (0.01 mmol) and 1,3,5-trimethoxybenzene (0.008 mmol) standard were measured into a 4 mL scintillation vial with stir bar in the glovebox and dissolved in toluene-d 8 (0.75 mL) with vigorous stirring. The solution was then transferred through a pipet filter with glass wool to screw cap NMR tubes and analyzed by 1 H NMR (500 MHz).…”

“…Kinetic analysis on the thermal decomposition of (dppf)-Ni(ketene) complexes (4) has shown that ketenes appended with electron-donating moieties further stabilized the complexes by resisting the formation of the catalytically inert nickel carbonyl complex. 8 DFT modeling on the mechanism of decomposition revealed a buildup of negative charges on the β carbon of the ketene moiety during the rate-determining step (TS2, Figure 1). The presence of an electron-donating ketene can destabilize the buildup of this negative charge, which lends to the observed stability.…”

“…Past efforts from our research group have led to a synthesis of stable and isolable Ni–ketene complexes using various ketenes and chelating 1,1′-bis­(diphenylphosphino)­ferrocene (dppf) as the ligand (Scheme ). Key to the stability of these complexes versus previous complexes is the more robust ligation of a chelating ligand compared to that of the previously used monodentate phosphines.…”