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

摘要

In this study benzyl and ethyl groups were appended to pyridine and aniline ancillary ligands in diiron(II) complexes of the type [Fe2(μ-O2CAr^R)2(O2CAr^R)2(L)2], where ^-O2CAr^R 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 analogs of the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). Use of 2-benzylpyridine (2-Bnpy) afforded doubly-bridged [Fe2(μ-O2CAr^Tol)2(O2CAr^Tol)2(2-Bnpy)2] (>1) and [Fe2(μ-O2CAr^4-FPh)2(O₂CAr^4-FPh)₂(2-Bnpy)₂] (>4), whereas tetra-bridged [Fe₂(μ-O₂CAr^Tol)₄(4-Bnpy)₂] (>3) resulted when 4-benzylpyridine (4-Bnpy) was employed. Similarly, 2-(4-chloro-benzyl)pyridine (2-(4-ClBn)py) and 2-benzylaniline (2-Bnan) were employed as N-donor ligands to prepare [Fe₂(μ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(2-(4-ClBn)py)₂] (>2) and [Fe₂(μ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(2-Bnan)₂] (>5). 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-O₂CAr^Tol)₂(O₂CAr^Tol)₂(2-Etpy)] (>6), [Fe₂(μ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(3-Etpy)] (>7), [Fe₂(μ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(4-Etpy)] (>8), and [Fe₂(μ-O₂CAr^4-FPh)₂(O₂CAr^4-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.