Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)₂ Dimer

Abstract

Square-planar high-spin Fe(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO4 core. The trianionic pincer-type ligand [CF3-ONO]H3 (1) supports the high-spin square-planar Fe(II) complex {[CF3-ONO]FeCl}{Li(Sv)2}2 (2). In the solid state, 2 forms the dimer complex {[CF3-ONO]Fe}2{(μ-Cl)2(μ-LiTHF)4} (3) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent (57)Fe Mössbauer investigation of 3 revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative D parameter. In solution, 2 is in equilibrium with the solvento complex {[CF3-ONO]FeCl(THF)}{Li2(Sv)4} (2·Sv) and the dimer 3. A combination of frozen solution (57)Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of 2·Sv was investigated. Complex 2 reacts readily with the one-electron oxidizing agent CuCl2 to give the Fe(III) complex {[CF3-ONO]FeCl2}{Li(THF)2}2 (4). Also, 2·Sv reacts with 2 equiv of TlPF6 to form the Fe(III) complex [CF3-ONO]Fe(THF)3 (5).

Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand

“…In this particular ground state configuration, the asymmetry in charge distribution caused by the additional electron in the d z 2 orbital is compensated by ligand field contributions from the four ligands in the xy -plane resulting in a near radially symmetric electric field gradient at the iron nucleus. Veige and co-workers have suggested that small values of |Δ E Q | can be considered a fingerprint of square-planar high-spin Fe II . , This proposal was further supported by data from Limberg and co-workers and is consistent with earlier studies of the electronic structure of the iron sites in gillespite. , …”

“…Veige and co-workers have suggested that small values of |ΔE Q | can be considered a fingerprint of square-planar high-spin Fe II . 27,28 This proposal was further supported by data from Limberg and co-workers 26 and is consistent with earlier studies of the electronic structure of the iron sites in gillespite. 51,52 Inspired by the unusual electronic structure of ( Mes PDP Ph )-Fe(OEt 2 ), the synthesis of ( Mes PDP Ph )FeL complexes with other L-type ligands was investigated.…”

“…24−26 Additionally, Veige and coworkers showed that the common FeO 4 motif can be expanded to an FeO 2 NCl environment by replacing two alkoxide ligands with amide and chloride donors that retain the dianionic nature of the complex. 27,28 Breaking with requirement (b), the groups of Doerrer 29 and Tamm 30 Often overlooked early examples for compounds with square-planar high-spin Fe II centers are bis(imino)pyridine iron halide complexes reported by Chirik and co-workers in 2005. 31 However, structural confirmation for a distorted square-planar coordination environment in these compounds was only provided in 2013.…”

“… On the basis of their report, Holland proposed several conditions that favor the high-spin state in a square-planar ligand field through electronic rather than steric effects: (a) anionic ligands imposing an overall negative charge on the complex that prevents further ligand binding via electrostatic effects, (b) alkali metal binding to stabilize the negative charge, and (c) weak-field ligands that reduce the splitting between the d x 2 –y 2 orbital and the remaining d orbitals via weak σ-donation (reducing the energy of d x 2 –y 2 ) and strong π-donation (raising the energy of d xz , d yz , and d xy ) . Following these design principles, several complexes carrying chelating alkoxide or siloxide ligands have been reported by the groups of Doerrer and Limberg, respectively. − Additionally, Veige and co-workers showed that the common FeO 4 motif can be expanded to an FeO 2 ­NCl environment by replacing two alkoxide ligands with amide and chloride donors that retain the dianionic nature of the complex. , Breaking with requirement (b), the groups of Doerrer and Tamm reported alkali metal-free examples.…”

Identification of α-Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations

“…More recently, however, a number of molecular compounds containing high-spin square planar FeL 4 arrangements in a non-strained environment were presented, suggesting that specific ligand environments may inherently electronically stabilize a square planar over a tetrahedral structure. [27][28][29][30][31] In ref. 27 the preference for a square planar structure was rationalized with the help of DFT calculations, as a Jahn-Teller (JT) flattening of the usual FeL 4 tetrahedron, caused by four highly charged, π-basic ligands L. In the present work this explanation will be further explored in the specific context of α-Fe in a zeolite environment, making use of a combined DFT/CASPT2 approach.…”