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-NCC5H4N) 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, [Fe2(μ-O2CArTol)4(4-NCC5H4N)2] (1), where -O2CArTol 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 [Fe2(μ-O2CArTol)2(O2CArTol)2(4-NCC5H4N)2(H2O)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 CH2Cl2 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(OEt2)2}{B

}, where
B

- is tetrakis(3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to [Fe2(μ-O2CArTol)3(4-NCC5H4N)2](B

) (5), which was also structurally characterized. Mössbauer 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 CH2Cl2 were identified crystallographically to be
[Fe2(μ-OH)2(μ-O2CArTol)2(O2CArTol)2(4-NCC5H4N)2]·2(HO2CArTol) (6) and [Fe6(μ-O)2(μ-OH)4(μ-O2CArTol)6(4-NCC5H4N)4Cl2] (7).