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A dodecanuclear copper hydride L₄Cu₁₂H₁₂is shown to be a transient species after the cluster expansion from L₂Cu₄H₄to L₃Cu₆H₆and before the degradation to Cu(0), L, and H₂. 

Photoenols, formed through photoinduced intra-molecular H atom abstraction in o-alkyl-substituted arylketones,typically have limited utility as reactive intermediates owing to fastreversion to the starting material. Herein, we introduced an azidogroup on the o-alkyl substituent to render the photoreactionirreversible. Irradiation of 2-azidomethylbenzophenone (1) inmethanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchangedand that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet andsinglet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2