Mesoionic Complexes of
Platinum(II) Derived from “Rollover” Cyclometalation:
A Delicate Balance between Pt–C(sp<sup>3</sup>) and Pt–C(sp<sup>2</sup>) Bond Cleavage as a Result of Different Reaction Conditions
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Abstract
“Rollover” cyclometalation is a particular
case of metal-mediated C–H bond activation, and the resulting
complexes constitute an emerging class of cyclometalated compounds.
In the case of 2,2′-bipyridine “rollover cyclometalation”
has been used to synthesize the complexes [Pt(bipy-H)(Me)(L)] (L =
PPh<sub>3</sub>, PCy<sub>3</sub>, P(OPh)<sub>3</sub>, P(<i>p</i>-tolyl)<sub>3</sub>), whose protonation produces a series of stable
corresponding pyridylenes [Pt(bipy*)(Me)(L)]<sup>+</sup>. The unusual
bipy* ligand may be described as an abnormal-remote heterocyclic chelated
carbene or simply as a mesoionic cyclometalated ligand. These cationic
species spontaneously convert in solution, through a retro-rollover
reaction, to the corresponding isomers [Pt(bipy)(Me)(L)]<sup>+</sup>, where the 2,2′-bipyridine is coordinated in the classical
N,N bidentate mode. Isomerization is achieved at different rates (ranging
over three orders of magnitude), depending on the nature of the phosphane
ligand, the most basic (PCy<sub>3</sub>) providing the fastest reaction.
The mesoionic species [Pt(bipy*)(Me)(L)]<sup>+</sup> contain two Pt–C
bonds: the balance between the Pt–C(sp<sup>2</sup>) and Pt–C(sp<sup>3</sup>) bond rupture is subtle, and competition is observed according
to the reaction conditions. In the presence of an external neutral
ligand L′ methane is released to give the cationic derivatives
[Pt(bipy-H)(L)(L′)]<sup>+</sup>, whereas reaction of the neutral
[Pt(bipy-H)(Me)(L)] with HCl may follow different routes depending
on the nature of the neutral ligand L. Assuming all reactions take
place through the formation of a hydride intermediate, quantum chemical
calculations show that computed energy barriers are qualitatively
consistent with observed reaction rates