Synthesis and characterization of a ditriflate-bridged, diiron(II) complex with syn-N-donor ligands: [Fe2(μ-OTf)2(PIC2DET)2](BARF)2

Abstract

The synthesis and characterization of the diiron(II) complex [Fe 2 (μ-OTf) 2 -(PIC 2 DET) 2 ](BARF) 2 (2), where PIC 2 DET is a 2,3-diethynyltriptycene-linked dipicolinic methyl ester ligand, are described. The dication in 2, contains, [Fe 2 (μ-OTf) 2 (PIC 2 DET) 2 ] 2+ two symmetry-equivalent iron atoms with octahedral coordination geometries. Each metal ion has a N 2 O 4 atom donor set that includes four atoms from two picolinic ester N,O chelate rings, as well as two oxygen atoms from the bridging trifluoromethanesulfonate groups. The Fe 2 (μ-OTf) 2 core of 2 is stabilized by two PIC 2 DET ligands that bind the two metal ions in a head-to-head fashion, leading to an Fe···Fe distance of 5.173(1)Å. Molar conductivity data for 2 are consistent with Fe 2 (μ-OTf) 2 (PIC 2 DET) 2 ] 2+ retaining its identity in acetone solutions, where it behaves as a 2:1 electrolyte. 1 H NMR spectroscopic, solution (d 6 -acetone) and solid-state magnetic susceptibility data all indicate that the iron atoms of 2 are highspin (S = 2). A fit of the magnetic data (2 -300K) to a spin-only isotropic exchange Hamiltonian H = −2JS 1 ·S 2 are consistent with weak antiferromagnetic coupling between the two iron atoms with J ~ −0.99(2) cm −1 and g = 2.10(1).

Toward Functional Carboxylate-Bridged Diiron Protein Mimics: Achieving Structural Stability and Conformational Flexibility Using a Macrocylic Ligand Framework

“…Other syn N -donor variants, such as H 2 L and H 2 BIPS (Chart 1), primarily afforded bis(ligand)diiron complexes formed through interdigitation of two dinucleating ligands when complexed with iron(II). 39,40 …”

“…Our initial studies using covalently linked N -donor ligands were promising, 37,38 but the diiron(II) complexes prepared were either too kinetically labile or contain two interdigitated syn N -donor units rather than one. 39,40 We overcame the previously encountered difficulties by designing a new macrocylic ligand, H 2 PIM, containing phenoxylimine metal binding groups (Chart 1). We used H 2 PIM and sterically-hindered carboxylates to synthesize the most accurate structural models of the carboxylate-bridged diiron active sites reported to date.…”

Current challenges of modeling diiron enzyme active sites for dioxygen activation by biomimetic synthetic complexes

“…The structures of several such diiron complexes were recently determined by X-ray crystallography, and interesting structural features were noted upon inspection of dimetallic compounds with this ligand scaffold. The complexes revealed M–M distances that range from 3.11 to 5.17 Å (Chart 1), suggesting that this seemingly rigid linker is flexible enough to accommodate changes in the Fe–Fe distance upon reaction with dioxygen 67,70,71. Additionally, the diethynylbenzene backbone provides a pocket in which a bridging oxo-group can be accommodated, as may occur in intermediate Q of MMOH.…”

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

“…The compound bis(picolinic methyl ester)diethynyltriptycene (PIC 2 DET) containing pyridine methyl ester groups afforded a heterometallic [Fe II Na(PIC 2 DET)(μ-TrpCO 2 ) 3 ] ( 31 ) complex that could exchange sodium for iron; the resulting diiron(II) compound could not be structurally characterized, however [107]. Two syn N -donor ligands can also bridge two iron(II) centers, giving [Fe II 2 ( syn N -donor) 2 ] species [108]. Several examples are shown in Chart 2 [S. Friedle, L.H.…”