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Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes
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stable [Cr(bpy)<sub>2</sub>(Ph)<sub>2</sub>](BPh<sub>4</sub>) complexes
undergo reductive elimination of biphenyl with visible-light photolysis
using household incandescent or compact fluorescent light bulbs. A
series of [Cr(R-bpy)<sub>2</sub>(Ar)<sub>2</sub>](X) complexes (R
= H or CMe<sub>3</sub>; Ar = Ph, C<sub>6</sub>H<sub>4</sub>-CMe<sub>3</sub>, or C<sub>6</sub>H<sub>4</sub>-OMe; X = I, BPh<sub>4</sub>, or PF<sub>6</sub>) were prepared, and the effect of varying the
bipyridine and aryl ligands on the UV–visible spectra and electrochemistry
of the chromium(III) complexes was investigated. Photolysis of a mixture
of two different bis(aryl) complexes gave only the homocoupled biaryl
products by <sup>1</sup>H NMR and gas chromatography/mass spectrometry
analysis. The initial product of photoinduced reductive elimination
of [Cr(bpy)<sub>2</sub>(Ar)<sub>2</sub>](PF<sub>6</sub>) was trapped
with bipyridine to generate [Cr(bpy)<sub>3</sub>](PF<sub>6</sub>)
and with benzoyl peroxide to form [Cr(bpy)<sub>2</sub>(O<sub>2</sub>CPh)<sub>2</sub>](PF<sub>6</sub>). The latter chromium(III) bis(benzoate)
complex was also synthesized by the addition of bipyridine and PhCO<sub>2</sub>H to Cp<sub>2</sub>Cr, followed by air oxidation. The neutral
Cr(bpy)(S<sub>2</sub>CNMe<sub>2</sub>)Ph<sub>2</sub> complex also
generated biphenyl upon visible-light photolysis. While the treatment
of Cr(<sup>t</sup>Bu-bpy)(dpm)Cl<sub>2</sub> [dpm = (OC<sup>t</sup>Bu)<sub>2</sub>CH] with AgO<sub>2</sub>CPh gave <i>trans</i>-Cr(<sup>t</sup>Bu-bpy)(dpm)(O<sub>2</sub>CPh)<sub>2</sub>, reaction
of the dichloro precursor with PhMgCl produced anionic [Cr(<sup>t</sup>Bu-bpy)Ph<sub>3</sub>]<sup>−</sup> with [Mg(dpm)(THF)<sub>4</sub>]<sup>+</sup> as the countercation, with both complexes characterized
by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph<sub>3</sub>(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph<sub>2</sub>, which generated biphenyl under visible-light photolysis,
and the initial product of reductive elimination was trapped by bipyridine
or benzoyl peroxide. A related Cr(bpy)(quin)<sub>2</sub> complex was
synthesized by protonolysis of Cr(bpy)[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> and characterized by single-crystal X-ray diffraction
