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

Claire McMullin, Nasir Rajabi

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Abstract

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-CR, where R = tBu and Ar′ (3,5- tBu 2C 6H 3). Calculations suggest the reaction mechanism proceedsviathe slippage of π-bound alkyne at the Rh centre into a Rh-alkyne σ C-Hcomplex 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 zorbital 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 2and 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′).

Original languageEnglish
Pages (from-to)11793-11803
Number of pages11
JournalRSC Advances
Volume11
Issue number20
DOIs
Publication statusPublished - 23 Mar 2021

Bibliographical note

Funding Information:
The authors acknowledge EPSRC (UK) for nancial support under award EP/R020752/1 (CLM and NAR). This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath. The authors would like to thank Adrian Chaplin (University of Warwick) for discussions on his group's experimental research that initiated this study.

Funding Information:
The authors acknowledge EPSRC (UK) for financial support under award EP/R020752/1 (CLM and NAR). This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath. The authors would like to thank Adrian Chaplin (University of Warwick) for discussions on his group's experimental research that initiated this study.

Publisher Copyright:
© The Royal Society of Chemistry 2021.

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