Planar three-coordinate high-spin Fe(II) complexes with large orbital angular momentum: Mössbauer, electron paramagnetic resonance, and electronic structure studies

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

Spectroscopic and Theoretical Investigation of High-Spin Square-Planar and Trigonal Fe(II) Complexes Supported by Fluorinated Alkoxides

The electronic structures and spectroscopic behavior of three high-spin FeII complexes of fluorinated alkoxides were studied: square-planar {K(DME)2}2[Fe(pinF)2] (S) and quasi square-planar {K(C222)}2[Fe(pinF)2] (S') and trigonal-planar {K(18C6)}[Fe(OC4F9)3] (T) where pinF = perfluoropinacolate and OC4F9 = tris-perfluoro-t-butoxide. The zero-field splitting (ZFS) and hyperfine structure parameters of the S = 2 ground states were determined using field-dependent 57Fe Mössbauer and high-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopies. The spin Hamiltonian parameters were analyzed with crystal field theory and corroborated by density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations. Whereas the ZFS tensor of S has a small rhombicity, E/D = 0.082, and a positive D = 15.17 cm-1, T exhibits a negative D = -9.16 cm-1 and a large rhombicity, E/D = 0.246. Computational investigation of the structural factors suggests that the ground-state electronic configuration and geometry of T's Fe site are determined by the interaction of [Fe(OC4F9)3]- with {K(18C6)}+. In contrast, two distinct countercations of S/S' have a negligible influence on their [Fe(pinF)2]2- moieties. Instead, the distortions in S' are likely induced by the chelate ring conformation change from δλ, observed for S, to the δδ conformation, determined for S'.

Partial Deoxygenative CO Homocoupling by a Diiron Complex

One route to address climate change is converting carbon dioxide to synthetic carbon-neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo- and heterogeneous catalysis, deoxygenative coupling of CO to products with C-C bonds-as in liquid fuels-remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2 (CO)2 L (2), where L2- is a bis(β-diketiminate) cyclophane, gives [K(THF)5 ][Fe2 (CO)2 L] (3), which undergoes silylation to Fe2 (CO)(COSiMe3 )L (4). Subsequent C-OSiMe3 bond cleavage and C=C bond formation occurs upon reduction of 4, yielding Fe2 (μ-CCO)L. CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallest J value, with changes to local metal ion spin states and coupling schemes (ferro- vs. antiferromagnetism) based on DFT calculations, Mössbauer and EPR spectroscopy. Finally, reaction of 5 with KEt3 BH or methanol releases the C2 O2- ligand with retention of the diiron core.

Spin polarization assisted facile C-H activation by an S = 1 iron(iv)-bisimido complex: a comprehensive spectroscopic and theoretical investigation

High valent iron terminal imido species (Fe[double bond, length as m-dash]NR) have been shown to be key reactive intermediates in C-H functionalization. However, the detailed structure-reactivity relationship in Fe[double bond, length as m-dash]NR species derived from studies of structurally well-characterized high-valent Fe[double bond, length as m-dash]NR complexes are still scarce, and the impact of imido N-substituents (electron-donating vs. electron-withdrawing) on their electronic structures and reactivities has not been thoroughly explored. In this study, we report spectroscopic and computational studies on a rare S = 1 iron(iv)-bisimido complex featuring trifluoromethyl groups on the imido N-substituents, [(IPr)Fe(NC(CF3)2Ph)2] (2), and two closely related S = 0 congeners bearing alkyl and aryl substituents, [(IPr)Fe(NC(CMe3)2Ph)2] (3) and [(IPr)Fe(NDipp)2] (1), respectively. Compared with 1 and 3, 2 exhibits a decreased Fe[double bond, length as m-dash]NR bond covalency due to the electron-withdrawing and the steric effect of the N-substituents, which further leads to a pseudo doubly degenerate ground electronic structure and spin polarization induced β spin density on the imido nitrogens. This unique electronic structure, which differs from those of the well-studied Fe(iv)-oxido complexes and many previously reported Fe(iv)-imido complexes, provides both kinetic and thermodynamic advantages for facile C-H activation, compared to the S = 0 counterparts.

Spontaneous FeIII/FeII redox cycling in single-atom catalysts: Conjugation effect and electron delocalization

The mechanism of spontaneous Fe^III/Fe^II redox cycling in iron-centered single-atom catalysts (I-SACs) is often overlooked. Consequently, pathways for continuous SO₄ ^·-/HO⋅ generation during peroxymonosulfate (PMS) activation by I-SACs remain unclear. Herein, the evolution of the iron center and ligand in I-SACs was comprehensively investigated. I-SACs could be considered as a coordination complex created by iron and a heteroatom N-doped carbonaceous ligand. The ligand-field theory could well explain the electronic behavior of the complex, whereby electrons delocalized by the conjugation effect of the ligand were confirmed to be responsible for the Fe^III/Fe^II redox cycle. The possible pyridinic ligand in I-SACs was demonstrably weaker than the pyrrolic ligand in Fe^III reduction due to its shielding effect on delocalized π orbitals by local lone-pair electrons. The results of this study significantly advance our understanding of the mechanism of spontaneous Fe^III/Fe^II redox cycling and radical generation pathways in the I-SACs/PMS process.

Spin States, Bonding and Magnetism in Mixed-Valence Iron(0)-Iron(II) Complexes

"Xenophilic" complexes offer metal-metal bonds between disparate metal sites, but the nature of the metal-metal bonding is often unclear. Here, we describe two novel complexes with unsupported Fe-Fe bonds, Lx Fe-Fp (LX = β-aldiminate or β-diketiminate; Fp = Fe(CO)2 Cp), that offer insight into Fe-Fe bonding. Mössbauer, magnetism, and DFT analysis indicate that the most accurate electronic structure description is LFeII ←Fe0 (CO)2 Cp, in which the Fe(CO)2 Cp is low-spin iron(0) and acts as an X-type ligand toward the high-spin iron(II) of the LFe fragment. This largely electrostatic interaction has a bond order of only 0.5. The three-coordinate high-spin iron(II) site has large zero-field splitting, and in addition its Mössbauer parameters can be used to rank the Fp- "metalloligand" as a donor; it is nearly as strong a donor as phosphides and alkyls.

Slow magnetic relaxation in a trigonal-planar mononuclear Fe(II) complex

Based on a β-diketiminate ligand, an iron(III) tetrahedral high-spin complex, [LFeIII(Cl)2] (1), and an iron(II) high-spin triangular planar complex, [LFeIICl] (2), have been synthesized and structurally characterized. Also, complex 1...

Modulation of the architectures and magnetic dynamics in pseudotetrahedral cobalt(II) complexes

Two β-Diketiminate cobalt(II) compounds of formula [LCo(μ-Cl)]2∙2C6H14 (1) and [LCoClPy]∙0.5C7H8∙0.5C6H14 (2) (L = [PhC-(PhCN-Dip)2]−, Dip = 2,6-iPr2C6H3) have been synthesized and structurally characterized by single crystal X-ray diffraction. Compound 1...

High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (MesPDPPh)Fe(CPh2), where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Mössbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground MS = ±1 non-Kramers doublet that is well-separated from the MS = 0 sublevel due to very large axial zero-field splitting (D = -195 cm-1, E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis observed by Mössbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that (MesPDPPh)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (dxy)2{(dxz),(dz2)}3(dyz)1(dx2-y2)0 configuration featuring near-degenerate dxz and dz2 orbitals.

Trigonal Planar Iron(II) and Cobalt(II) Complexes Containing [RS(NtBu)3]n- (R = NtBu, n = 2; CH2PPh2, n = 1) as Acute Bite-Angle Chelating Ligands: Soft P Donor Proves Beneficial to Magnetic Co Species

We prepared four new complexes, 4a,b and 5a,b, from polyimido sulfur-centered ligands with Fe^II and Co^II amides. Their molecular structures were elucidated by single-crystal X-ray diffraction. Cobalt magnetic investigations and multiconfigurational calculations provided insight into magneto-structural correlations between the acute N,N' chelating bite angle and P-side arm donation. The deviation from an ideal trigonal planar geometry and the magnetic performance correlated in an unprecedented manor. Mononuclear cobalt species 4b and 5b showed slow magnetic relaxation under a small applied dc field with energy barriers of up to 33.0 and 21.9 cm^-1, respectively. Although they possess some of the largest zero-field splitting parameters among three-coordinate cobalt single-ion magnets, both theory and experiment suggest that the high rhombicity (E/D) hampers large effective energy barriers to spin reversal at zero field from being obtained.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(μ-H)3]3+ Clusters

CO₂ insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO₂ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe₃H₃L², 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe₃H₃L¹ (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO₂ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD₃ requires Fe-O bond formation in the product to offset an endergonic CO₂ insertion. μ₃-hydride coordination is noted to lower the activation barrier for the first CO₂ insertion event in DFT calculations.

Hepta-coordinated Ni(II) assemblies - structure and magnetic studies

Two mononuclear complexes [Ni(dapsc)(H2O)2]Cl(NO3)·H2O (1) and [Ni(dapsc)(NCS)2] (2), and a bimetallic CN-bridged trinuclear molecule [NiII(dapsc)(H2O)]2[WIV(CN)8]·11H2O (3) (dapsc = 2,6-diacetylpyridine-bis(semicarbazone)) were synthesised and characterised in terms of structure and magnetic properties. All three compounds contain Ni(ii) ions in a pentagonal bipyramid coordination geometry afforded by the equatorial pentadentate ligand (dapsc) and two O- or N-donating axial ligands. The compounds differ in the relative arrangement of the complexes, intermolecular interactions and distortion from the ideal coordination geometry. The high-field EPR and magnetometric studies show large anisotropy of the Ni(ii) centres with the D parameters in the range of -10.5 to -21.2 cm-1 and negligible antiferromagnetic interactions. The easy-axis magnetic anisotropies of 1-3 were reproduced by ab initio CASSCF/NEVPT2 calculations. The ground states consist mainly of the |MS = |±1 states, which is consistent with the fact that no out-of-phase signal can be detected in the AC magnetic susceptibility measurements.

Catalytic Carbodiimide Guanylation by a Nucleophilic, High Spin Iron(II) Imido Complex

Reduction of the three-coordinate iron(III) imido [Ph₂B(^tBuIm)₂Fe═NDipp] (1) affords [Ph₂B(^tBuIm)₂Fe═NDipp][K(18-C-6)THF₂] (2), a rare example of a high-spin (S = 2) iron(II) imido complex. Unusually for a late metal imido complex, the imido ligand in 2 has nucleophilic character, as demonstrated by the reaction with DippNH₂, which establishes an equilibrium with the bis(anilido) complex [Ph₂B(^tBuIm)₂Fe(NHDipp)₂][K(18-C-6)THF₂] (3). In an unusual transformation, formal insertion of ⁱPrN═C═NⁱPr into the Fe═N(imido) bond yields the guanidinate [Ph₂B(^tBuIm)₂Fe(ⁱPrN)₂CNDipp][K(18-C-6)THF₂] (4). Reaction of 4 with excess DippNH₂ provides 3, along with the guanidine (ⁱPrNH)₂C═NDipp. As suggested by these stoichiometric reactions, 2 is an efficient catalyst for the guanylation of carbodiimides, converting a wide range of aniline substrates under mild conditions.

Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO₂-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe^II(TPP)] complex (1; TPP^2- = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate ³A_2g state with a (d_xy)²(d_z²)²(d_xz,yz)² configuration or a degenerate ³E_g^θ state with a (d_xy)²(d_xz,yz)³(d_z²)¹/(d_z²)²(d_xy)¹(d_xz,yz)³ configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1. Highly correlated wave-function-based computations predicted that ³A_2g and ³E_g^θ are well-isolated from other triplet states by ca. 4000 cm^-1, whereas their splitting Δ_A-E is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm^-1). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the ³A_2g and ³E_g^θ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field ⁵⁷Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ_A-E, an angle θ that describes the mixing of (d_xy)²(d_xz,yz)³(d_z²)¹ and (d_z²)²(d_xy)¹(d_xz,yz)³ configurations, and the ⟨r_d^-3⟩ expectation value of the iron d orbitals that is necessary to estimate the ⁵⁷Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both ³A_2g (<88%) and ³E_g (>12%), owing to their accidental near-triple degeneracy with Δ_A-E = +950 cm^-1. As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g_⊥ ≈ 3 and g_∥ ≈ 1.7) and a large and positive internal field at the ⁵⁷Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g_⊥ ≫ g_∥ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of 1 for CO₂ reduction are discussed.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII , CoII , and NiII Single-Ion Magnets

Since the last decade, the focus in the area of single-molecule magnets (SMMs) has been shifting constructively towards the development of single-ion magnets (SIMs) based on transition metals and lanthanides. Although ground-breaking results have been witnessed for Dy^III -based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on Co^II ion are very prominent as they often exhibit high magnetic anisotropy or zero-field splitting parameters and offer a large barrier height for magnetisation reversal. Although Co^II possibly holds the record for having the largest number of zero-field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various Co^II SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the Co^II centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the Co^II ion. Additionally, some important Fe^II /Fe^III and Ni^II complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ-Base Resistance

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η¹-allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the alkene substrate is bound, the catalyst has high immunity to typical σ-base poisons due to the antibonding interactions of the high spin state.

Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes

The combined experimental and theoretical investigation of the magnetic properties of the cobalt(ii) NHC complexes (NHC = N-heterocyclic carbene); [Co(CH2SiMe3)2(IPr)] (1), [CoCl2(IMes)2] (2) and [Co(CH3)2(IMes)2] (3) revealed a large easy plane anisotropy for 1 (D = +73.7 cm-1) and a moderate easy axis anisotropy for 2 (D = -7.7 cm-1) due to significant out-of-state spin-orbit coupling. Dynamic magnetic measurements revealed slow relaxation of the magnetization for 1 (Ueff = 22.5 K, τ0 = 3 × 10-7 s, 1000 Oe) and for 2 (Ueff = 20.2 K, τ0 = 1.73 × 10-8 s, 1500 Oe). The molecular origin of the slow relaxation phenomena was further supported by the retention of AC signal in 10% solutions in 2-MeTHF which reveals a second zero field AC signal in 1 at higher frequencies. Compound 3 was found to be an S = 1/2 system.

Self-assembly of an organometallic Fe9O6 cluster from aerobic oxidation of (tmeda)Fe(CH2tBu)2

Aerobic oxidation of (tmeda)Fe(CH₂^tBu)₂ in toluene or THF solution leads to the self-assembly of a magic-sized all-ferrous oxide cluster containing the Fe₉O₆ subunit and bearing organometallic and diamine ligands. Mössbauer studies of the cluster are consistent with an all-ferrous assignment and magnetometry reveals complex intracluster and intercluster magnetic interactions.

Syntheses and characterizations of iron complexes of bulky o-phenylenediamide ligand

We report the reactivity of the iron complexes of a bulky phenylenediamide ligand.

The influences of carbon donor ligands on biomimetic multi-iron complexes for N2 reduction

The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N₂ to NH₄⁺, implicating the central carbon and its interaction with Fe as important in the mechanism of N₂ reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers. Here, we present functional and structural models that test the impacts of carbon donors and sulfide donors in simpler iron compounds. We report the first example of a diiron complex that is bridged by an alkylidene and a sulfide, which serves as a high-fidelity structural and spectroscopic model of a two-iron portion of the active-site cluster (FeMoco) in the resting state of Mo-nitrogenase. The model complexes have antiferromagnetically coupled pairs of high-spin iron centers, and sulfur K-edge X-ray absorption spectroscopy shows comparable covalency of the sulfide for C and S bridged species. The sulfur-bridged compound does not interact with N₂ even upon reduction, but upon removal of the sulfide it becomes capable of reducing N₂ to NH₄⁺ with the addition of protons and electrons. This provides synthetic support for sulfide extrusion in the activation of nitrogenase cofactors.

High-spin diiron alkylidenes give insight into the electronic structure and functional relevance of carbon in the FeMoco active site of nitrogenase.

Synthesis and magnetic characterization of a dinuclear complex of low-coordinate iron(II)

Low-coordinate ions possess exciting magnetic, optical, and reactive properties that may afford novel material physics. Hence, it is important to test both synthetic methods for realizing extended solids of such ions as well as the properties of smaller molecular fragments of envisioned future materials. Herein, we report the synthesis and characterization of a new dinuclear Fe species, [{(Me₃Si₂)₂N}Fe{μ-p-{HN(SiMe₃)}(C₆Me₄){N(SiMe₃)}}₂Fe{N(SiMe₃)₂}] (1), formed through a transamination reaction between [Fe{N(SiMe₃)₂}₂]₂ and the bulky diamine p-{HN(SiMe₃)}₂(C₆Me₄) (L). The Fe centers of this dimer assume a pseudo-trigonal-planar, three-coordinate conformation in 1, bridged by two aromatic diamines. Single-crystal X-ray diffraction, IR spectroscopy, and Mössbauer spectroscopy enable the assignment of both Fe centers as the 2+ oxidation state. Magnetic studies show that 1 displays a weak antiferromagnetic exchange interaction (J = -2.33 cm^-1) and moderate zero-field splitting (D = 7.51 cm^-1). Importantly, these studies demonstrate the viability of using transamination to bridge high-spin low-coordinate metal ions and hence the technique may, in the future, produce new extended structures.

Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene

We report the first Fe─CPh₃ complex, and show that the long Fe─C bond can be disrupted by neutral π-acceptor ligands (benzophenone and phenylacetylene) to release the triphenylmethyl radical. The products are formally iron(I) complexes, but X-ray absorption spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description of all the complexes is iron(II). In the formally iron(I) complexes, this does not imply that the π-acceptor ligand has radical character, because the iron(II) description arises from doubly-occupied frontier molecular orbitals that are shared equitably by the iron and the π-acceptor ligand, and the unpaired electrons lie on the metal. Despite the lack of substantial radical character on the ligands, alkyne and ketone fragments can couple to form a high-spin iron(III) complex with a cyclized metalladihydrofuran core.

Magnetic anisotropy in trigonal planar Fe(ii) bis(trimethylsilyl)amido complexes of the type [Fe{N(SiMe3)2}2L]-experiment and theory

Systematic ac (alternating current) magnetic investigations on four new trigonal planar high-spin Fe2+ complexes [Fe{N(SiMe3)2}2L] reveal that complexes which comprise a phosphine or arsine type ligand (L = PPh3, PMe3 and AsPh3) display slow magnetic relaxation at temperatures below 8 K under applied dc (direct current) fields, whereas a complex with a phosphine oxide ligand (L = OPPh3) does not. Accordingly, the parameters characteristic for magnetic anisotropy, derived both from dc magnetic measurements and quantum chemical calculations, reveal distinct differences for these two types of complexes. Extensive ab initio calculations of multi-reference wave function type were performed on the four new complexes listed above and the related reported ones with L = py, thf and PCy3 in order to get a reasonable description of the local electronic states involved in the magnetic relaxation. These calculations confirm that strong spin-orbit effects generate the magnetic anisotropy of complexes with L = PPh3, PMe3, AsPh3 and PCy3. On the other hand, the complexes with L = OPPh3, py and THF exhibit only small spin-orbit splittings, consistent with the fast relaxation found experimentally.

A Synthetic Model of Enzymatic [Fe4S4]-Alkyl Intermediates

Although alkyl complexes of [Fe₄S₄] clusters have been invoked as intermediates in a number of enzymatic reactions, obtaining a detailed understanding of their reactivity patterns and electronic structures has been difficult owing to their transient nature. To address this challenge, we herein report the synthesis and characterization of a 3:1 site-differentiated [Fe₄S₄]²⁺–alkyl cluster. Whereas [Fe₄S₄]²⁺ clusters typically exhibit pairwise delocalized electronic structures in which each Fe has a formal valence of 2.5+, Mössbauer spectroscopic and computational studies suggest that the highly electron-releasing alkyl group partially localizes the charge distribution within the cubane, an effect that has not been previously observed in tetrahedrally coordinated [Fe₄S₄] clusters.

From Ylides to Doubly Yldiide-Bridged Iron(II) High Spin Dimers via Self-Protolysis

A synthetic strategy for the preparation of novel doubly yldiide bridged iron(II) high spin dimers ([(μ2-C)FeL]2, L = N(SiMe3)2, Mesityl) has been developed. This includes the synthesis of ylide-iron(II) monomers [(Ylide)FeL2] via adduct formation. Subsequent self-protolysis at elevated temperatures by in situ deprotonation of the ylide ligands results in a dimerization reaction forming the desired bridging μ2-C yldiide ligands in [(μ2-C)FeL]2. The comprehensive structural and electronic analysis of dimers [(μ2-C)FeL]2, including NMR, Mössbauer, and X-ray spectroscopy, as well as X-ray crystallography, SQUID, and DFT calculations, confirm their high-spin FeII configurations. Interestingly, the Fe2C2 cores display very acute Fe-C-Fe angles (averaged: 78.6(2)°) resulting in short Fe···Fe distances (averaged: 2.588(2) Å). A remarkably strong antiferromagnetic coupling between the Fe centers has been identified. Strongly polarized Fe-C bonds are observed where the negative charge is mostly centered at the μ2-C yldiide ligands.

Diiron oxo reactivity in a weak-field environment

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform tBudmxH2 with Fe2(Mes)4 results in formation of a diiron complex ( tBudmx)Fe2(Mes)2. Treatment of ( tBudmx)Fe2(Mes)2 with one equivalent of water yields the diiron μ-oxo complex ( tBudmx)Fe2(μ-O) and free mesitylene. A two-electron oxidation of ( tBudmx)Fe2(μ-O) gives rise to the diferric complex ( tBudmx)Fe2(μ-O)Cl2, and one-electron reduction from this FeIIIFeIII state allows for isolation of a mixed-valent species [Cp2Co][( tBudmx)Fe2(μ-O)Cl2]. Both ( tBudmx)Fe2(μ-O) and [Cp2Co][( tBudmx)Fe2(μ-O)Cl2] exhibit basic character at the bridging oxygen atom and can be protonated using weak acids to form bridging diferrous hydroxide species. The basicity of the diferrous oxo ( tBudmx)Fe2(μ-O) is quantified through studies of the pK a of its conjugate acid, [( tBudmx)Fe2(μ-OH)]+, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( tBudmx)Fe2(μ-O)(thf)2, the basicity is increased as observed through an increase in the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)(thf)2]+ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( tBudmx)Fe2(μ-OH)(thf)2]+ resulted in isolation of ( tBudmx)Fe2(μ-O)Cl2 with concomitant loss of a proton, consistent with the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)Cl2]+ determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron μ-oxo units.

Electronic and Structural Factors Controlling the Spin Orientations of Magnetic Ions

Magnetic ions M in discrete molecules and extended solids form ML_n complexes with their first-coordinate ligand atoms L. The spin moment of M in a complex ML_n prefers a certain direction in coordinate space because of spin-orbit coupling (SOC). In this minireview, we examine the structural and electronic factors governing the preferred spin orientations. Elaborate experimental measurements and/or sophisticated computational efforts are required to find the preferred spin orientations of magnetic ions, largely because the energy scale of SOC is very small. The latter is also the very reason why one can readily predict the preferred spin orientation of M by analyzing the SOC-induced highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) interactions of the ML_n complexes in terms of qualitative perturbation theory. The strength of this HOMO-LUMO interaction depends on the spin orientation, which is governed by the selection rules based on the minimum |ΔL_z| value (i.e., the minimum difference in the magnetic quantum numbers) between the HOMO and LUMO. With the local z axis of ML_n chosen as its n-fold rotational axis, the preferred spin orientation is parallel to the z axis (∥z) when |ΔL_z| = 0 but perpendicular to the z axis (⊥z) when |ΔL_z| = 1.

Isolation and characterization of a high-spin mixed-valent iron dinitrogen complex

We report a rare example of a mixed-valence iron compound with an FeNNFe core, which gives insight into the structural, spectroscopic, and magnetic influences of single-electron reductions and oxidations. In the new compound, the odd electron is localized as judged from Mössbauer spectra at 80 K and infrared spectra at room temperature, and the backbonding into the N2 unit is intermediate between diiron(i) and diiron(0) congeners. Magnetic susceptibility and relaxation studies on the series of FeNNFe compounds show significant magnetic anisotropy, but through-barrier pathways enable fairly rapid magnetic relaxation.

Effects of N2 Binding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex

Distinguishing the reactivity differences between N2 complexes having different binding modes is crucial for the design of effective N2-functionalizing reactions. Here, we compare the reactions of a K-bridged, dinuclear FeNNFe complex with a monomeric Fe(N2) complex where the bimetallic core is broken up by the addition of chelating agents. The new anionic iron(0) dinitrogen complex has enhanced electron density at the distal N atoms of coordinated N2, and though the N2 is not as weakened in this monomeric compound, it is much more reactive toward silylation by (CH3)3SiI (TMSI). Double silylation of N2 gives a three-coordinate iron(III) hydrazido(2-) complex, which is finely balanced between coexisting S = 1/2 and S = 3/2 states that are characterized by crystallography, spectroscopy, and computations. These results give insight into the interdependence between binding modes, alkali dependence, reactivity, and magnetic properties within an iron system that functionalizes N2.

High-Frequency Fe-H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT

High-spin iron species with bridging hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe-H bonds in high-spin multinuclear iron systems. An ⁵⁷ Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(μ-H)₂ Fe model complex reveals Fe-H stretching vibrations for bridging hydrides at frequencies greater than 1200 cm^-1 . These isotope-sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying hydrides in this high-spin iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal iron hydride core.

Synthetic, Structural, and Spectroscopic Characterization of a Novel Family of High-Spin Iron(II) [(β-Diketiminate)(phosphanylphosphido)] Complexes

This work describes a series of iron(II) phosphanylphosphido complexes. These compounds were obtained by reacting lithiated diphosphanes R₂PP(SiMe₃)Li (R = t-Bu, i-Pr) with an iron(II) β-diketiminate complex, [LFe(μ₂-Cl)₂Li(DME)₂] (1), where DME = 1,2-dimethoxyethane and L = Dippnacnac (β-diketiminate). While the reaction of 1 with t-Bu₂PP(SiMe₃)Li yields [LFe(η¹-Me₃SiPP-t-Bu₂)] (2), that of 1 with equimolar amounts of i-Pr₂PP(SiMe₃)Li, in DME, leads to [LFe(η²-i-Pr₂PPSiMe₃)] (3). In contrast, the reaction of 1 with (i-Pr₂N)₂PP(SiMe₃)Li provides not an iron-containing complex but 1-[(diisopropylamino)phosphine]-2,4-bis(diisopropylamino)-3-(trimethylsilyl)tetraphosphetane (4). The structures of 2-4 were determined using diffractometry. Thus, 2 exhibits a three-coordinate iron site and 3 a four-coordinate iron site. The increase in the coordination number is induced by the change from an anticlinal to a synclinal conformation of the phoshpanylphosphido ligands. The electronic structures of 2 and 3 were assessed through a combined field-dependent ⁵⁷Fe Mössbauer and high-frequency and -field electron paramagnetic resonance spectroscopic investigation in conjunction with analysis of their magnetic susceptibility and magnetization data. These studies revealed two high-spin iron(II) sites with S = 2 ground states that have different properties. While 2 exhibits a zero-field splitting described by a positive D parameter (D = +17.4 cm^-1; E/D = 0.11) for 3, this parameter is negative [D = -25(5) cm^-1; E/D = 0.15(5)]. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations provide insights into the origin of these differences and allow us to rationalize the fine and hyperfine structure parameters of 2 and 3. Thus, for 2, the spin-orbit coupling mixes a z²-type ground state with two low-lying {xz/yz} orbital states. These interactions lead to an easy plane of magnetization, which is essentially parallel to the plane defined by the N-Fe-N atoms. For 3, we find a yz-type ground state that is strongly mixed with a low-lying z²-type orbital state. In this case, the spin-orbit interaction leads to a partial unquenching of the orbital momentum along the x axis, that is, to an easy axis of magnetization oriented roughly along the Fe-P bond of the phosphido moiety.

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron β-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

Pentagonal Bipyramid FeII Complexes: Robust Ising-Spin Units towards Heteropolynuclear Nanomagnets

Pentagonal bipyramid Fe^II complexes have been investigated to evaluate their potential as Ising-spin building units for the preparation of heteropolynuclear complexes that are likely to behave as single-molecule magnets (SMMs). The considered monometallic complexes were prepared from the association of a divalent metal ion with pentadentate ligands that have a 2,6-diacetylpyridine bis(hydrazone) core (H₂ L^N3O2R ). Their magnetic anisotropy was established by magnetometry to reveal their zero-field splitting (ZFS) parameter D, which ranged between -4 and -13 cm^-1 and was found to be modulated by the apical ligands (ROH versus Cl). The alteration of the D value by N-bound axial CN ligands, upon association with cyanometallates, was also assessed for heptacoordinated Fe^II as well as for related Ni^II and Co^II derivatives. In all cases, N-coordinated cyanide ligands led to large magnetic anisotropy (i.e., -8 to -18 cm^-1 for Fe and Ni, +33 cm^-1 for Co). Ab initio calculations were performed on three Fe^II complexes, which enabled one to rationalize the role of the ligand on the nature and magnitude of the magnetic anisotropy. Starting from the pre-existing heptacoordinated complexes, a series of pentanuclear compounds were obtained by reactions with paramagnetic [W(CN)₈ ]^3- . Magnetic studies revealed the occurrence of ferromagnetic interactions between the spin carriers in all the heterometallic systems. Field-induced slow magnetic relaxation was observed for mononuclear Fe^II complexes (U_eff /k_B up to 53 K (37 cm^-1 ), τ₀ =5×10^-9  s), and SMM behavior was evidenced for a heteronuclear [Fe₃ W₂ ] derivative (U_eff /k_B =35 K and τ₀ =4.6 10^-10  s), which confirmed that the parent complexes were robust Ising-type building units. High-field EPR spectroscopic investigation of the ZFS parameters for a Ni derivative is also reported.

Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)

The addition of (trimethylsilyl)diazomethane and its conjugate base to iron β-diketiminate precursors gives novel dinuclear complexes in which the bridges are either diazomethane derivatives or an alkylidene. One product is an unusual bridging alkylidene complex containing two three-coordinate iron(II) centers. On the other hand, syntheses using the deprotonated diazomethane give two bridging diazomethyl species with binding modes that have not been observed in iron complexes previously. In the presence of a coordinating tetrahydrofuran solvent, a diiron(II) compound with μ-N bridges rearranges to a more stable isomer with μ-N,C bridges, a process that is accompanied by a 1,3-shift of a silyl group.

A trigonal prismatic anionic iron(iii) complex of a radical o-iminobenzosemiquinonate derivative: structural and spectral analyses

The complex [Fe^III(L²⁻˙)₂]⁻is a rare example of a trigonal prismatic Fe(iii) complex with twoS= 1/2 radical isoquinone ligands showing strong antiferromagnetic coupling.

Three and four coordinate Fe carbodiphosphorane complexes

Carbodiphosphoranes (CDPs) are a family of divalent carbon ligands that are known for their exceptional electron donor properties. Herein, the preparation and reactivity of a family of three and four co-ordinate Fe carbodiphosphorane complexes is described. Hexaphenylcarbodiphosphorane (HCDP) [1] is shown to react with FeCl₂(PPh₃)₂ to form the three coordinate adduct Fe(HCDP)Cl₂ [2], which is equilibrium with its four coordinate dimer. Reaction of [2] with two equivalents of benzyl Grignard yields the corresponding dialkyl complex (HCDP)FeBn₂ [3]. Combination of [2] with LiHMDS results in salt metathesis and the formation of the monosilylated derivative Fe(HCDP)Cl(N(SiMe₃)₂) [4]. Subsequent anion exchange leads to the three coordinate Fe(HCDP)(OTf)(N(SiMe₃)₂) [5] which was characterized crystallographically and in solution.