ELSEVIER Inorganica Chimica Acta 257 ( 1997) 241-245 Sensitized photolysis of dihydrobis ( 1 -pyrazolyl) borate copper( II) ; general reaction pattern G.E. Buono-Core *, AH Klahn, C. Bahamondes, F. Aros, M. Tejos, V. Astorga htitlcro & Quiniica. Oniversidod Car&co dc Valparako, CaFiIla 4059. Vaipamiko, Chile Received 12 February 19%; revised 9 .Septemkr 1996 Abstract Several aspects of the sensitized photoreduction of dihydrobis( I-pyrazolyl)borate Cu( II) have been studied. It was fotmdthatthescnsitizcd reduction was much more efficient than direct photolysis in hydrogen-donating solvents such as a products were identified as Cu( 0). the protonated ligand and oxidation products derived from the were quantitatively evahtated by determining the quantum yields, !@, in different solvents and in results showed that the sensitized photoreduction was more efficient in solvents with better hydrop hydrogen abstraction is a key step in the reaction. A lack of correlation exists between the triplet energy le quantum yields of the photoreduction. The rate constant (k,) for quenching of the benxophenone triplet state by the Cu(lI) determined by Stem-Volmer type experiments. An overall mechanism for the sensitized photoreduction has been proposed Keywords: Sensitized photolysis; Copper complexes 1. Introductioa Transition metal ions and complexes of Sdiketonates are known to be efficient quenchers of triplet statesof a number of organic sensitizers [l-4] and several quenching mecha- nisms such as paramagnetic quenching [ 11, energy transfer [ 31 and electron transfer [ 4 ] have been proposed. A careful evaluation of these studies seems to ‘rdicate that no single mechanism is capable of explaining the whole range of quenching reactions, and that the overall mechanism isindeed a complex one. lvloreover, it has been recently demonstrated that for Cu( acac)* and Ni( acac)2, the mechanism is strongly influenced by solvents and ligands [S 1. Although the sensi- tized photochemistry of transition metal 1,3diketonates has been extensively studied [ 4,5], no results can be found in the literature for metal complexes with polypyrazolylborate ligands. We have shown in 3 preliminary report [6] that the dihy- drobis( 1-pyrazolyl)borate Cu(II) complex, Cu(Bp),, can undergo benzophenone sensitized photoreduction in hydro- gen donating solvents such as alcohols and tetrahydrofuran, In order to gain more insight into the mechanism of photo reduction, we have now undertaken a more detaiIed study on * Corresponding author. 0020-1693/97/517.00 Q 1997 Elsevier Science S.A. All rights reserved PlISOO20-1693(96)05487-4 the sensitized photochemistry of Cu(Bp), and the results are reported here. 2 Experilueut3l Malting points were determined on a Mel-Tamp melting point apparatus. IR spectra were recorded on a Perkin-Elmer model 1605 FI’/IR spectrophotometer.NMR spectra were recorded on 3 Bruker AMX 300 spectmmeter. W-Vis spectra were determined on 3 Hewlett Packard HP- 8452A diode array specuopbotometer. 2.1. Materials Literature methods were used to prepare the salt K[H2B(pz)J and the copper complex CUE [7]. Ben- zophenone (Merck) was recrystallized from ethanol/W. All other sensitizers were purcM &om Aldrich and used 3s received. Solvents were of analyticaI andusedwitlk out hutherpurihcation. ~etrahydrofuran (THF) was&tilled fromLiAIH,andst~oversodiumwire.Oxygtn-freeni~ gen was used as the inert gas in all the experiments.