From NHC to Imidazolyl Ligand: Synthesis of Platinum
and Palladium Complexes d<sup>10</sup>-[M(NHC)<sub>2</sub>] (M = Pd,
Pt) of the NHC 1,3-Diisopropylimidazolin-2-ylidene
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Abstract
The widely held belief that N-heterocyclic
carbenes (NHCs) act
only as innocent spectator ligands is not always accurate, even in
the context of well-explored reactions. Ligand exchange in the conversion
of [Pt(PPh<sub>3</sub>)<sub>2</sub>(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)] (<b>3</b>) to [Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>2</sub>] (<b>2</b>) depends critically on the
particular reaction conditions employed, with slight changes leading
to vastly different outcomes. In addition to [Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>2</sub>] (<b>2</b>), complexes [Pt(<i>i</i>Pr<sub>2</sub>Im)(PPh<sub>3</sub>)(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)] (<b>5</b>) and <i>trans</i>-[Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>2</sub>(<i>i</i>Pr-Im*)(H)] (<b>6</b>) were isolated and in the case of <b>6</b> fully characterized. Complex <b>5</b> represents the
first mixed-olefin complex in transition metal chemistry containing
both an NHC and a phosphine ligand. Chemical degradation of the NHC
was shown to yield the new imidazole-2-yl <i>i</i>Pr-Im*
in <b>6</b>. Therefore, the synthesis of [Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>2</sub>] (<b>2</b>) via metallic reduction
of the ionic precursor [Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>3</sub>(Cl)]<sup>+</sup>Cl<sup>–</sup> (<b>9</b>) is
favorable, a procedure adaptable to analogous palladium compounds.
While [Pd(<i>i</i>Pr<sub>2</sub>Im)<sub>3</sub>(Cl)]<sup>+</sup>Cl<sup>–</sup> (<b>8</b>) is the only product
obtained from the reaction of <i>i</i>Pr<sub>2</sub>Im and
PdCl<sub>2</sub>, neutral [Pt(<i>i</i>Pr<sub>2</sub>Im)<sub>2</sub>(Cl)<sub>2</sub>] (<b>10</b>), formed as a mixture of
its two stereoisomers <i><b>cis</b></i><b>-10</b> and <i><b>trans</b></i><b>-10</b>, is available
through precise control of the stoichiometry in the reaction of PtCl<sub>2</sub> and exactly 2 equiv of <i>i</i>Pr<sub>2</sub>Im
