Biomimetic Model DOI: 10.1002/ange.201201825 Oxidative Carbon  Carbon Bond Cleavage of a a-Hydroxy Ketone by a Functional Model of 2,4’-Dihydroxyacetophenone Dioxygenase** Sayantan Paria, Partha Halder, and TapanKanti Paine* 2,4’-Dihydroxyacetophenone dioxygenase (DAD), an enzyme from aerobic soil bacterium Alcaligenes sp., is involved in the catabolism of 4-hydroxyacetophenone. [1–3] DAD catalyzes the cleavage of 2,4’-dihydroxyacetophenone into 4-hydroxy benzoate (HB) and formate in the presence of dioxygen. [3] In the C  C bond cleavage reaction, an oxygen atom from dioxygen is incorporated into each product (Scheme 1). [3] DAD was first purified and sequenced by Hopper and Kaderbhai revealing that the enzyme has little sequence homology to other dioxygenases. [3] This enzyme has been proposed to be a member of the “cupin” superfamily. [4] DAD, not yet structurally characterized, is a homotetramer and contains one atom of iron per molecule of enzyme. [3] However, little is known about the oxidation state of iron and its role in the C C bond cleavage mechanism. [3, 4] In biomim- etic studies, small-molecule transition-metal model com- plexes play an important role in understanding the mecha- nism of enzymatic reactions. [5–13] Thus to develop a mecha- nistic understanding of the reaction carried out by DAD, we have studied the dioxygen reactivity of biomimetic iron(II)– a-hydroxy ketone complexes. Here we report the synthesis, characterization, and dioxyen reactivity of an iron(II)–a- hydroxy ketone complex, [(Tp Ph2 )Fe II (HAP)] (1), where Tp Ph2 is hydrotris(3,5-diphenylpyrazolyl)borate and HAP-H is 2- hydroxyacetophenone. The model iron(II) complex (1) was prepared by reacting equivalent amounts of KTp Ph2 , iron(II) perchlorate, 2-hy- droxy acetophenone, and triethylamine in methanol. The complex was isolated as pink solid. Complex 1 exhibits weak charge transfer bands at 505 (460 m 1 cm 1 ) and 560 nm (325 m 1 cm 1 ) in benzene. Similar charge-transfer (CT) bands in the region between 500–600 nm have been reported for [(Tp Ph2 )Fe II (benzoylformate)], [8, 9] [(Tp Ph2 )Fe II - (pyruvate)], [9] [(Tp Me2 )Fe II (benzoylformate)], [14] and [(6- Me 3 -TPA)Fe II (benzoylformate)] [15] complexes (Tp Me2 = hydrotris(3,5-dimethylpyrazolyl)borate, 6-Me 3 -TPA = tris(6- methyl-2-pyridylmethyl)amine. The peaks have been attrib- uted to the charge-transfer transition from the filled d orbital of the iron(II) to the empty p* orbital of the keto group of a- keto acid. [9, 15] The CT bands of 1, by analogy to the reported ion(II)-benzoylformate complexes, may be attributed to the iron(II)-to-keto charge-transfer transitions. The optical spec- tral features suggest that the a-hydroxy ketone binds to the metal center in the keto form. The 1 H NMR spectrum of the complex in [D 6 ]benzene displays paramagnetically shifted proton resonances, typical for a high-spin iron(II) complex (Figure S1 in the Supporting Information). X-ray quality single-crystals of 1 were isolated from a solvent mixture of dichloromethane and methanol. The crystal structure of 1 shows a five-coordinate iron center ligated by three nitrogen donors from the facial tridentate ligand and two oxygen donors from a bidentate HAP anion (Figure 1). The average Fe  N bond distance of 2.13  is similar to that reported for other high-spin iron(II) com- plexes, [(Tp Ph2 )Fe II (benzoylformate)], [8, 9] [(Tp Ph2 )Fe II - Scheme 1. Reaction carried out by DAD. Figure 1. ORTEP plot of [(Tp Ph2 )Fe II (HAP)] (1) with 50 % thermal ellipsoids. All hydrogen atoms except those attached to C46 and B1 have been omitted for clarity. Selected bond lengths [] and angles [deg] for 1: Fe1–O1 1.915(6), Fe1–O2 2.230(5), Fe1–N2 2.096(7), Fe1– N4 2.116(6), Fe1–N6 2.199(6), C46–O1 1.386(10), C47–O2 1.236(10), C46–C47 1.500(12); O1-Fe1-N4 128.8(2), O1-Fe1-N6 106.6(2), O1-Fe1- N2 137.3(3), O2-Fe1-N2 90.1(2), O2-Fe1-N6 174.4(2), O2-Fe1-N4 89.6(2), O1-Fe1-O2 78.9(2), N2-Fe1-N4 91.8(2), N2-Fe1-N6 85.8(2), N4-Fe1-N6 86.8(2). [*] S. Paria, P. Halder, Dr. T.K. Paine Department of Inorganic Chemistry Indian Association for the Cultivation of Science 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032 (India) E-mail: ictkp@iacs.res.in [**] T.K.P. acknowledges the DST, Government of India (project number SR/S1/IC-51/2010) for financial support. S.P. and P.H. thank the CSIR, India, for fellowships. The crystal structure determination was performed at the DST-funded National Single Crystal Diffractometer Facility in the Department of Inorganic Chemistry, IACS. Prof. S. Bhattacharya, JU, is gratefully acknowledged for his help with the GC-MS measurements. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201201825. A ngewandte Chemi e 1 Angew. Chem. 2012, 124,1–6  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü