π‑Rich σ<sup>2</sup>P‑Heterocycles: Bent η<sup>1</sup>‑P- and μ<sup>2</sup>‑P-Coordinated 1,3-Benzazaphosphole Copper(I) Halide Complexes
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Abstract
The reaction of 1-neopentyl-1,3-benzazaphosphole <b>1</b> with CuCl, CuBr, or Cu(SMe<sub>2</sub>)Br in THF at room
temperature
provides sparingly soluble [Cu<sub>7</sub>(μ<sup>2</sup>-L<sub>6</sub>)(μ<sup>2</sup>-X<sub>7</sub>)]<sup>+</sup>[CuX<sub>2</sub>]<sup>−</sup> cluster complexes <b>2a</b>,<b>b</b> (L indicates coordinated <b>1</b>, <b>a</b> X
= Cl, <b>b</b> X = Br), with loosely bound THF, in high yields.
The conversions proceed via transient THF-soluble labile [(L<sub>2</sub>CuX)<sub>2</sub>] complexes. Separation before complete conversion,
combined with suitable conditions for crystallization, allowed these
intermediates to be trapped. Depending on the reactant ratios, crystals
of the clusters or of dimeric L<sub>2</sub>CuX complexes were formed.
The crystal structure analyses of <b>2a</b>·4THF and the
dimers <b>3b</b> [{Cu(η<sup>1</sup>-L)<sub>2</sub>(μ<sup>2</sup>-Br)}<sub>2</sub>], <b>4b</b> [{Cu(μ<sup>2</sup>-L)(η<sup>1</sup>-L)(κBr)}<sub>2</sub>], <b>5a</b>·2MeOH, and <b>5b</b>·2MeOH [{Cu(μ<sup>2</sup>-L)(η<sup>1</sup>-L)(κX···HOMe)}<sub>2</sub>] generally display μ<sup>2</sup>-P- and/or tilted η<sup>1</sup>-P-coordination, contrasting with the preference for the η<sup>1</sup>-P in-plane coordination mode of phosphinine ligands in their
copper(I) halide complexes. DFT studies of geometry-optimized monomers
LCuBr, L(CuBr)<sub>2</sub>, L<sub>2</sub>CuBr, and the dimers <b>3b</b> and <b>4b</b>, calculated at the ωB97xD/cc-PVDZ
level, suggest that weak competing interactions with the solvent THF
and the entropy factor of the dimerization result in lability and
a subtle balance between the different complexes in solution, whereas
the particular coordination observed in the crystals is attributable
to conservation of the delocalized π-system in the ligand. The
HOMO of <b>4b</b> is composed of Cu d orbitals and the π-type
HOMO of the bridging ligand. Interestingly, despite the rather short
Cu···Cu interatomic separation (2.726 Ǻ),
no bond critical point could be located in <b>4b</b>, indicating
the absence of weak cuprophilic interactions in this compound
