DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes

Density Functional Theory (DFT) has been used to investigate the alkyne-to-vinylidene isomerisation reaction mediated by [Rh(PXNXP)]⁺ complexes (X = CH₂: 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP) and X = O: 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP)) for terminal alkynes HC[triple bond, length as m-dash]CR, where R = Bu and Ar' (3,5- Bu₂C₆H₃). Calculations suggest the reaction mechanism proceeds via the slippage of π-bound alkyne at the Rh centre into a Rh-alkyne σ_C-H complex followed by an indirect 1,2-H shift to give the Rh-vinylidene species. NBO (Natural Bond Orbital) analysis of the transition states corresponding to the latter indirect 1,2-H shift step indicates that the migrating hydrogen atom exhibits protic character and hence, the basicity of the H-accepting centre (C_β) is controlled by the substituents at that same atom and can tune the 1,2-H shift transition state. QTAIM (Quantum Theory of Atoms in Molecule) and NBO analyses of the Rh-vinylidene complexes indicate that these species exhibit a Rh ← C dative bond as well as π-back bonding from the Rh centre into the empty p orbital of the carbene centre (C_α), showing the Rh-vinylidene complexes are Fischer type carbenes. Analysis of the alkyne and vinylidene complex HOMOs show that the equilibrium between the isomers can be tuned by the P-Rh-P bite angle of the [Rh(pincer)]⁺ fragment. Dictated by the nature of the pincer backbone, wider bite angles shift the equilibrium toward the formation of the Rh-vinylidene isomer (e.g., X = CH₂ and R = Ar'), while tighter bite angles shift the equilibrium more to the formation of the Rh-alkyne isomer (e.g., X = O and R = Ar').