Surface Functionalization DOI: 10.1002/anie.201305994 Enhanced Reactivity of a Biomimetic Iron(II) a-Keto Acid Complex through Immobilization on Functionalized Gold Nanoparticles** Debobrata Sheet, Partha Halder, and Tapan Kanti Paine* Immobilization of transition-metal complexes by surface functionalization of gold nanoparticles (AuNPs) has recently attracted the attention for several applications. [1] Thiol- protected AuNPs [2] are stable and soluble in organic solvents. Therefore, immobilization of metal complexes on AuNPs permits reactions in common organic solvents and also induces properties of the metal complex to the NP. [3] AuNPs functionalized by thiol-appended transition metal complexes are expected to find applications as immobilized catalysts to bridge between homogeneous and heterogeneous catalysis. The high surface area of a nanocatalyst increases the contact between the reactant and catalyst dramatically. These cata- lysts are easy to synthesize through desired surface modifi- cation and can be characterized by different analytical and spectroscopic techniques. Moreover, the catalyst can easily be separated from the reaction mixture. Several reports are now available where immobilization of metal catalysts on AuNPs has been shown to increase the catalytic reactivity. [4] Improved catalytic activities of transition-metal catalysts through surface functionalization of AuNPs have inspired us to initiate a project on immobilization of biomimetic iron complexes. In this direction, we have investigated the dioxygen reactivity of nonheme iron complexes on function- alized AuNPs. Dioxygen-activating a-ketoglutarate (a-KG)- dependent oxygenases that carry out versatile biological reactions are well-studied in biology and biomimetic chemis- try. [5] The iron(II) centers of the enzymes activate dioxygen, upon binding of a bidentate a-KG and the substrate, to form an iron–oxygen intermediate. [6] The heterolytic OÀO bond cleavage of the intermediate results in formation of an iron(IV)–oxo oxidant to affect substrate oxidations. Iron(IV)–oxo intermediates have been trapped and charac- terized in studies with several a-ketoglutarate-dependent enzymes. [7] In biomimetic chemistry, a number of iron(II)–a- keto acid complexes have been reported as functional models of a-ketoglutarate-dependent oxygenases. [8] The model com- plexes exhibit versatile reactivity towards dioxygen and, in some cases, iron(IV)–oxo intermediates have been inter- cepted in the decarboxylation of a-keto acids. Model iron(II)– a-keto acid complexes of N 4 donor ligands have been reported to react with O 2 for days to undergo stoichiometric decarboxylation of the coordinated a-keto acids. [8a] More- over, they often form m-oxo diiron(III) complex at the end of the decarboxylation reaction, leading to the deactivation of the complex. [8a] Such deactivation reaction may be avoided with a properly immobilized complex on the surface of AuNPs and is expected to exhibit improved/catalytic reac- tivity upon immobilization. As an outcome of our investiga- tion, we report herein an enhanced reactivity of a biomimetic iron(II)–benzoylformate complex of a thiol-appended N 4 ligand (TPASH, Scheme 1) towards dioxygen upon immobi- lization on functionalized AuNPs. The reactivity of the immobilized complex is compared with that of a related iron(II)–benzoylformate complex supported by a tetradentate nitrogen-donor ligand, TPA-O-allyl (Scheme 1). The tetradentate ligand TPA-O-allyl was synthesized from [6-(hydroxymethyl)-2-pyridylmethyl]bis(2-pyridylme- thyl)amine (TPAOH) [9] according to Scheme 1. The iron(II)–a-keto acid complex [(TPA-O-allyl) 2 Fe 2 (BF) 2 ]- (ClO 4 ) 2 (1) (where BF = monoanionic benzoylformate) was isolated from the reaction of TPA-O-allyl, Fe(ClO 4 ) 2 ·x H 2 O, and NaBF in methanol (see the Experimental Section in the Supporting Information). The complex displays a paramag- netically shifted 1 H NMR spectrum in CDCl 3 indicating the high-spin nature of the iron(II) center (Supporting Informa- tion, Figure S1). The ESI-MS (in positive-ion mode, CH 3 CN) of the complex shows an ion peak at m/z = 565.2 with the isotope distribution pattern calculated for the [(TPA-O- Allyl)Fe(BF)] + ion (Figure 1). The solid-state structure of the complex cation, however, displays a symmetrical dinu- clear complex in which each iron(II) center is coordinated by a ligand and two carboxylate oxygen donors from two Scheme 1. Ligands used in this study. AIBN = azobis(isobutyronitrile). [*] D. Sheet, Dr. 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, Govt. of India (Project SR/S1/IC-51/ 2010) for financial support. D.S. thanks CSIR, India, for a fellowship. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201305994. . Angewandte Communications 13314 # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2013, 52, 13314 –13318