Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes.

Known stable [Cr(bpy)(Ph)](BPh) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)(Ar)](X) complexes (R = H or CMe; Ar = Ph, CH-CMe, or CH-OMe; X = I, BPh, or PF) 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 H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)(Ar)](PF) was trapped with bipyridine to generate [Cr(bpy)](PF) and with benzoyl peroxide to form [Cr(bpy)(OCPh)](PF). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCOH to CpCr, followed by air oxidation. The neutral Cr(bpy)(SCNMe)Ph complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(Bu-bpy)(dpm)Cl [dpm = (OCBu)CH] with AgOCPh gave trans-Cr(Bu-bpy)(dpm)(OCPh), reaction of the dichloro precursor with PhMgCl produced anionic [Cr(Bu-bpy)Ph] with [Mg(dpm)(THF)] as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph, 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) complex was synthesized by protonolysis of Cr(bpy)[N(SiMe)] and characterized by single-crystal X-ray diffraction.