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 = CH(2): 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 = (t)Bu and Ar′ (3,5-(t)Bu(2)C(6)H(3)). 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(z) 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 = CH(2) 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′).