Iron–molybdenum-oxo complexes as initiators for olefin autoxidation with O₂

Abstract

The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular [Mo-O-Fe-O-]2 core containing high-spin iron(ii) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(ii) and cobalt(ii) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)](2+). The cobalt analogue 4 reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O8(4-) structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*.

Reactivity Study of Pyridyl‐Substituted 1‐Metalla‐2,5‐diaza‐cyclopenta‐2,4‐dienes of Group 4 Metallocenes

“…This resulted in the thiadiazole 16 a,i nt he elimination of the Cp* ligand and in af ormal Ti species[ Cp*Ti] with only one Cp* ligand (Scheme 12). The eliminated Cp* ligand was identified as decamethylfulvalene (Cp* 2 ), [31] tetramethylfulvene [32] and pentamethylcyclopentadiene (Cp*H), [33] by comparison of the typical NMR signals to those published in the literature (below all three species are mentioned as [Cp*] for ab etter readability). In order to complete this conversion the mixture was warmed to 60 8Cf or 38 h. Afterwards, we identified [Cp*],t he formal [Cp*Ti] species, the thiadiazole 16 a and, unfortunately,s till 15 a.T hus, Cp* 2 TiCl 2 (13)s eems to be necessary for the conversion of 15 a to 16 a.T ot est this assumption,w ed issolved pure 15 a and warmed it to 85 8C.…”

A Cubic Fe4Mo4Oxo Framework and Its Reversible Four-Electron Redox Chemistry

“…While some purely inorganic polyoxometal aggregates − and coordination polymers ,− featuring Fe–O–Mo entities have been published, until 2013 only five structurally characterized molecular coordination compounds have been reported where molybdate units are bridging iron ions, ,− ligated, for instance, by the tris­(2-pyridylmethyl)­amine (TPA) ligand. Recently, we have extended this field to iron complexes being connected to organomolybdate units via oxido ligands, and these proved to be efficient initiators for autoxidation reactions . Also, in these investigations, the tetradentate TPA ligand had been chosen as a coligand at iron, leaving two coordination sites vacant at the iron center for interaction with the organomolybdate .…”

“…Recently, we have extended this field to iron complexes being connected to organomolybdate units via oxido ligands, and these proved to be efficient initiators for autoxidation reactions. 29 Also, in these investigations, the tetradentate TPA ligand had been chosen as a coligand at iron, leaving two coordination sites vacant at the iron center for interaction with the organomolybdate. 29 To increase the connectivity within the Fe−O−Mo framework, we then decided to replace TPA by tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 TACN) and to return to molybdate without organic ligands.…”

“…29 Also, in these investigations, the tetradentate TPA ligand had been chosen as a coligand at iron, leaving two coordination sites vacant at the iron center for interaction with the organomolybdate. 29 To increase the connectivity within the Fe−O−Mo framework, we then decided to replace TPA by tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 TACN) and to return to molybdate without organic ligands. This led to a cubic Fe II 4 Mo VI 4 compound, [[(Me 3 TACN)Fe][μ-(MoO 4 -κ 3 O,O′,O″)]] 4 (1), exhibiting a remarkable reversible redox chemistry.…”