Water affects the stereochemistry and dioxygen reactivity of carboxylate-rich diiron(II) models for the diiron centers in dioxygen-dependent non-heme enzymes

Carboxylate-bridged high-spin diiron(II) complexes with distinctive electronic transitions were prepared by using 4-cyanopyridine (4-NCC(5)H(4)N) ligands to shift the charge-transfer bands to the visible region of the absorption spectrum. This property facilitated quantitation of water-dependent equilibria in the carboxylate-rich diiron(II) complex, Fe(2)(mu-O(2)CAr(Tol))(4)(4-NCC(5)H(4)N)(2)] (1), where (-)O(2)CAr(Tol) is 2,6-di-(p-tolyl)benzoate. Addition of water to 1 reversibly shifts two of the bridging carboxylate ligands to chelating terminal coordination positions, converting the structure from a paddlewheel to a windmill geometry and generating Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)(H(2)O)(2)] (3). This process is temperature dependent in solution, rendering the system thermochromic. Quantitative treatment of the temperature-dependent spectroscopic changes over the temperature range from 188 to 298 K in CH(2)Cl(2) afforded thermodynamic parameters for the interconversion of 1 and 3. Stopped flow kinetic studies revealed that water reacts with the diiron(II) center ca. 1000 time faster than dioxygen and that the water-containing diiron(II) complex reacts with dioxygen ca. 10 times faster than anhydrous analogue 1. Addition of {H(OEt(2))(2)}{B}, where B(-) is tetrakis(3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to Fe(2)(mu-O(2)CAr(Tol))(3)(4-NCC(5)H(4)N)(2)](B) (5), which was also structurally characterized. Mossbauer spectroscopic investigations of solid samples of 1, 3, and 5, in conjunction with several literature values for high-spin iron(II) complexes in an oxygen-rich coordination environment, establish a correlation between isomer shift, coordination number, and N/O composition. The products of oxygenating 1 in CH(2)Cl(2) were identified crystallographically to be Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)].2(HO(2)CAr(Tol)) (6) and Fe(6)(mu-O)(2)(mu-OH)(4)(mu-O(2)CAr(Tol))(6)(4-NCC(5)H(4)N)(4)Cl(2)] (7).