Reactivity Differences for the Oxidation of Fe^IIFe^II to Fe^III(µ‐O)Fe^III Complexes Caused by Pyridyl versus 6‐Methyl‐Pyridyl Ligands

Abstract

The dinucleating bis(tetradentate) ligand susan [= 4,7-dimethyl-1,

A Dinucleating Ligand System with Varying Terminal Donors to Mimic Diiron Active Sites

“…In our efforts to obtain doubly-bridged complexes with oxo or hydroxo donors, we also employed Fe II [72] indicating the hydrolysis of BF 4 À , which is a known reactivity. [73,74] Surprisingly, reacting susan Me with Fe (BF 4 ) 2 • 6H 2 O under the same aerobic conditions provided the (Figure 3b).…”

“…Under these harsher conditions, we could indeed achieve the synthesis of the μ-oxo-bridged complex [(susan Me ){FeF(μ-O) FeF}](ClO 4 ) 2 with terminal fluoro ligands ( Figure 3d). [72] However, analogous complexes with terminal chloro or acetato ligands have still not been accessible with susan Me even under these harsher oxidizing conditions.…”

“…[81] To obtain further insight we performed a systematic analysis of molecular structures exhibiting MÀ py units in the Cambridge Structural Database. [72] This provided, that the statement that MÀ N py bonds with 6-methyl pyridyl donors are always longer than the analogous complexes with unsubstituted pyridyl donors is only valid for six-coordinate complexes. [79,82,83] In five-or four-coordinate complexes, the difference in the MÀ N py bond lengths for 6-methyl pyridine and pyridine donors is mostly not significant.…”

“…We therefore concluded that a steric repulsion between the 6-methyl substituent and the ligand in cis position in octahedral coordination elongates MÀ N py bonds for 6-methyl substituted pyridine donors. [72] Indeed, this was manifested by comparing the molecular structures of the two μ-oxo-complexes [(susan){FeF(μ-O) FeF}] 2 + and [(susan Me ){FeF(μ-O)FeF}] 2 + with terminal fluoro donors ( Figure 4). It can be seen, that there is an overlap in [(susan Me ){FeF(μ-F)FeF}] 2 + of the 6-methyl group with the fluoro ligand coordinated cis indicating strong steric repulsion, which is almost absent in [(susan){FeF(μ-O)FeF}] 2 + .…”

“…It can be seen, that there is an overlap in [(susan Me ){FeF(μ-F)FeF}] 2 + of the 6-methyl group with the fluoro ligand coordinated cis indicating strong steric repulsion, which is almost absent in [(susan){FeF(μ-O)FeF}] 2 + . [72] This suggests, that the small F À ligand is the only ligand, which can be enforced to coordinate cis to the 6-methyl pyridine by favoring the Fe III Fe III state with H 2 O 2 addition. However, larger ligands as Cl À or OAc À cannot coordinate with a 90°angle to a 6-methyl pyridine group.…”