Fluorescent Bis(guanidine) Copper Complexes as Precursors for Hydroxylation Catalysis

Abstract

Bis(guanidine) copper complexes are known for their ability to activate dioxygen. Unfortunately, until now, no bis(guanidine) copper-dioxygen adduct has been able to transfer oxygen to substrates. Using an aromatic backbone, fluorescence properties can be added to the copper(I) complex which renders them useful for later reaction monitoring. The novel bis(guanidine) ligand DMEG2tol stabilizes copper(I) and copper(II) complexes (characterized by single crystal X-ray diffraction, IR spectroscopy, and mass spectrometry) and, after oxygen activation, bis(µ-oxido) dicopper(III) complexes which have been characterized by low-temperature UV/Vis and Raman spectroscopy. These bis(guanidine) stabilized bis(µ-oxido) complexes are able to mediate tyrosinase-like hydroxylation activity as first examples of bis(guanidine) stabilized complexes. The experimental study is accompanied by density functional theory calculations which highlight the special role of the different guanidine donors.

Room temperature stable multitalent: highly reactive and versatile copper guanidine complexes in oxygenation reactions

“…In 1990, Réglier and co-workers developed the first model system [Cu 2 (MeCN) 4 (BiPh(impy) 2 ](PF 6 ) 2 , catalyzing the oxygenation reaction of 2,4-di- tert -butyl phenol to 3,5-di- tert -butyl quinone [ 12 ]. Since then, further systems were reported by the working groups of Casella [ 13 ], Lumb and Ottenwaelder [ 14 – 16 ], Tuczek [ 17 – 24 ] and Herres-Pawlis [ 25 – 29 ], demonstrating that very different supporting ligand systems feature catalytic transformations of phenolic substrates (Fig. 1 ).…”

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Fig. 1 Ligand design in synthetic model systems capable of tyrosinase-like oxygenation reactions [ 5 , 10 – 29 ]

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The bridge towards a more stable and active side-on-peroxido (Cu2II(µ-η2:η2-O2)) complex as a tyrosinase model system

“…Mimicking the active site of tyrosinase by designing and synthesizing small structural analogues, provides the possibility to reveal the mechanism and transfer the enzyme's catalytic activity outside of its biological application, towards a synthetic catalytic system. Aer the rst catalytically active tyrosinase model system was reported by Regliér et al, 9 further systems were developed by the groups of, for example, Bulkowski et al, 10 Casella et al, [11][12][13] Stack et al, 14 Lumb and Ottenwaelder et al, [15][16][17][18][19] Tuczek et al [20][21][22][23][24][25][26][27][28] and Herres-Pawlis et al [29][30][31][32][33][34][35][36] (Fig. 1).…”