Modeling the Syn Disposition of Nitrogen Donors at the Active Sites of Carboxylate-Bridged Diiron Enzymes. Enforcing Dinuclearity and Kinetic Stability with a 1,2-Diethynylbenzene-Based Ligand

Abstract

The syn coordination of histidine residues at the active sites of several carboxylate-rich non-heme diiron enzymes has been difficult to reproduce with small molecule model compounds. In this study, ligands derived from 1,8-naphthyridine, phthalazine, and 1,2-diethynylbenzene were employed to mimic this geometric feature. The preassembled diiron(II) complex [Fe(2)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(THF)(2)] (1), where Ar(Tol)CO(2)(-) is the sterically hindered carboxylate 2,6-di(p-tolyl)benzoate, served as a convenient starting material for the preparation of iron(II) complexes, all of which were crystallographically characterized. Use of the ligand 2,7-dimethyl-1,8-naphthyridine (Me(2)-napy) afforded the mononuclear complex [Fe(O(2)CAr(Tol))(2)(Me(2)-napy)] (2), whereas dinuclear [Fe(2)(micro-DMP)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(THF)] (3) resulted when 1,4-dimethylphthalazine (DMP) was employed. The dinuclear core of compound 3 is kinetically labile, as evidenced by the formation of [Fe(O(2)CAr(Tol))(2)(vpy)(2)] (4) upon addition of 2-vinylpyridine (vpy). The diiron analogue of 4, [Fe(2)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(vpy)(2)] (5), was prepared directly from 1. When the sterically more demanding ligand 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO(2)(-)) was used, mononuclear [Fe(O(2)CAr(4)(-)(tBuPh))(2)(THF)(2)] (6) and [Fe(O(2)CAr(4)(-)(tBuPh))(2)(DMP)(2)] (7) formed. The difficulty in stabilizing a dinuclear core with these simple (N)(2)-donor ligands was circumvented by preparing a family of 1,2-diethynylbenzene-based ligands, from which were readily assembled the complexes [Fe(2)(Et(2)BCQEB(Et))(micro-O(2)CAr(Tol))(3)](OTf) (15) and [Cu(2)(Et(2)BCQEB(Et))(micro-I)(2)] (16), where Et(2)BCQEB(Et) is 1,2-bis(3-ethynyl-8-carboxylatequinoline)benzene ethyl ester. The Et(2)BCQEB(Et) framework provides both structural flexibility and the desired syn nitrogen donor geometry, thus serving as a good first-generation ligand in this class.