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

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(mu-O2CArTol)(2)(O2CArTol)(2)(THF)(2)] (1), where (ArCO2-)-C-Tol 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(O2CArTol)(2)(Me-2-napy)] (2), whereas dinuclear [Fe-2(mu-DMP)(mu-O2CArTol)(2)-(O2CArTol)(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(O2CArTol)(2)(vpy)(2)] (4) upon addition of 2-vinylpyricline (vpy). The diiron analogue of 4, [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(vpy)(2)] (5), was prepared directly from 1. When the sterically more demanding ligand 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2-) was used, mononuclear [Fe(O2CAr4-tBuPh)(2)(THF)(2)] (6) and [Fe(O2CAr4-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))(mu-O2CArTol)(3)](OTf) (15) and [Cu-2(Et(2)BCQEB(Et))(mu-l)(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.