Synthesis, Characterization, and Preliminary Oxygenation Studies of Benzyl- and Ethyl-Substituted Pyridine Ligands of Carboxylate-Rich Diiron(II) Complexes

In this study benzyl and ethyl groups were appended to pyridine and aniline ancillary ligands in diiron(II) complexes
of the type [Fe2(μ-O2CArR)2(O2CArR)2(L)2], where -O2CArR is a sterically hindered terphenyl carboxylate, 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh, respectively). These crystallographically characterized
compounds were prepared as analogues of the diiron(II) center in the hydroxylase component of soluble methane
monooxygenase (MMOH). The use of 2-benzylpyridine (2-Bnpy) yielded doubly bridged [Fe2(μ-O2CArTol)2(O2CArTol)2(2-Bnpy)2] (1) and [Fe2(μ-O2CAr4-FPh)2(O2CAr4-FPh)2(2-Bnpy)2] (4), whereas tetra-bridged [Fe2(μ-O2CArTol)4(4-Bnpy)2]
(3) resulted when 4-benzylpyridine (4-Bnpy) was employed. Similarly, 2-(4-chlorobenzyl)pyridine (2-(4-ClBn)py) and
2-benzylaniline (2-Bnan) were employed as N-donor ligands to prepare [Fe2(μ-O2CArTol)2(O2CArTol)2(2-(4-ClBn)py)2] (2) and [Fe2(μ-O2CArTol)2(O2CArTol)2(2-Bnan)2] (5). The placement of the substituent on the pyridine ring had
no effect on the geometry of the diiron(II) compounds isolated when 2-, 3-, or 4-ethylpyridine (2-, 3-, or 4-Etpy) was
introduced as the ancillary nitrogen ligand. The isolated [Fe2(μ-O2CArTol)2(O2CArTol)2(2-Etpy)] (6), [Fe2(μ-O2CArTol)2(O2CArTol)2(3-Etpy)] (7), [Fe2(μ-O2CArTol)2(O2CArTol)2(4-Etpy)] (8), and [Fe2(μ-O2CAr4-FPh)2(O2CAr4-FPh)2(2-Etpy)2] (9)
complexes all contain doubly bridged metal centers. The oxygenation of compounds 1−9 was studied by GC-MS
and NMR analysis of the organic fragments following decomposition of the product complexes. Hydrocarbon fragment
oxidation occurred for compounds in which the substrate moiety was in close proximity to the diiron center. The
extent of oxidation depended upon the exact makeup of the ligand set.