Canted Ferromagnetism and Other Magnetic Phenomena in Square-Planar, Neutral Manganese(II) and Iron(II) Octaethyltetraazaporphyrins

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 (^MesPDP^Ph)Fe(CPh₂), where [^MesPDP^Ph]^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 (^MesPDP^Ph)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 M_S = ±1 non-Kramers doublet that is well-separated from the M_S = 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 (^MesPDP^Ph)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 Fe^II center with a (d_xy)²{(d_xz),(d_z²)}³(d_yz)¹(d_x²-y²)⁰ configuration featuring near-degenerate d_xz and d_z² orbitals.

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.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

The relationship between ligand design and spin state in base metal compounds is surveyed. Implications and applications of these principles for light-harvesting dyes, catalysis and materials chemistry are summarised.

A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene)

Using the structural and electronic tunability of molecules to control magnetism is a central challenge of inorganic chemistry. Herein, a ten-member family of the high-ordering temperature (Tc) molecule-based magnetic coordination networks of the form V[x-ClnPTCE]2·yCH2Cl2 (PTCE = phenyltricyanoethylene, y < 0.5) were synthesized and characterized, where x is (are) the position(s) and n is the number of chlorine substitutions on the phenyl ring. These chlorophenyltricyanoethelenes are tunable analogs of the more commonly investigated tetracyanoethylene (TCNE). Varying the number and position of chlorine substitution around the phenyl ring engendered a family of network solids with significantly different magnetic ordering temperatures ranging from 146 to 285 K. The Tcs of these ferrimagnets were rationalized with the aid of cyclic voltammetry and Density Functional Theory (DFT) calculations.

New Insight into the Ground State of FePc: A Diffusion Monte Carlo Study

We have applied DMC to evaluate relative stability of the possible electronic configurations of an isolated FePc under D _4h symmetry, considering some fixed nodes generated from different methods. They predict A _2g ground state consistently, supporting preceding DFT studies, with confidence overcoming the ambiguity about exchange-correlation (XC) functionals. By comparing DMC with several XC, we clarified the importance of the short-range exchange to describe the relative stability. We examined why the predicted A _2g is excluded from possible ground states in the recent ligand field based model. Simplified assumptions made in the superposition model are identified to give unreasonably less energy gain for A _2g when compared with the reality. The state is found to have possible reasons for the stabilization, reducing the occupations from an unstable anti-bonding orbital, avoiding double occupation of a spatially localized orbital, and gaining exchange energy by putting a triplet spin pair in degenerate orbitals.

Gd3N@C84(OH)x: a new egg-shaped metallofullerene magnetic resonance imaging contrast agent

Water-soluble derivatives of gadolinium-containing metallofullerenes have been considered to be excellent candidates for new magnetic resonance imaging (MRI) contrast agents because of their high relaxivity and characteristic encapsulation of the lanthanide ions (Gd(3+)), preventing their release into the bioenvironment. The trimetallic nitride template endohedral metallofullerenes (TNT EMFs) have further advantages of high stability, high relative yield, and encapsulation of three Gd(3+) ions per molecule as illustrated by the previously reported nearly spherical, Gd3N@I(h)-C80. In this study, we report the preparation and functionalization of a lower-symmetry EMF, Gd3N@C(s)-C84, with a pentalene (fused pentagons) motif and an egg-shaped structure. The Gd3N@C84 derivative exhibits a higher (1)H MR relaxivity compared to that of the Gd3N@C80 derivative synthesized the same way, at low (0.47 T), medium (1.4 T), and high (9.4 T) magnetic fields. The Gd3N@C(s)-C84 derivative exhibits a higher hydroxyl content and aggregate size, as confirmed by X-ray photoelectron spectroscopy (XPS) and dynamic light scattering (DLS) experiments, which could be the main reasons for the higher relaxivity.

Synthesis, coordination chemistry, and physico-chemical properties of the 2-chloroethoxy-iron(III)(ethylthio) porphyrazine

Reaction of octakis(ethylthio)porphyrazine ( H 2 OESPz ) with FeBr 2 in ClCH 2 CH 2 OH at 135 °C affords the 2-chloroethoxy-iron(III)-(ethylthio)porphyrazine, ( ClCH 2 CH 2 O ) Fe III OESPz , ( LFe III OESPz ) in good yield. The spectroscopic, redox, and coordination properties of the complex and its μ-oxo dimer derivative, [ Fe III OESPz ]2 O , are investigated and compared to those of the iron(III)porphyrin analogs.

Synthesis and characterization of sterically encumbered β-ketoiminate complexes of iron(II) and zinc(II)

The synthesis, structure, and spectroscopic signatures of a series of four-coordinate iron(II) complexes of β-ketoiminates and their zinc(II) analogues are presented. An unusual five-coordinate iron(II) triflate with three oxygen bound protonated β-ketoimines is also synthesized and structurally characterized. Single-crystal X-ray crystallographic analysis reveals that the deprotonated bis(chelate)metal complexes are four-coordinate with various degrees of distortion depending on the degree of steric bulk and the electronics of the metal center. Each of the high-spin iron(II) centers exhibits multiple electronic transitions including ligand π to π*, metal-to-ligand charge transfer, and spin-forbidden d-d bands. The (1)H NMR spectra of the paramagnetic high-spin iron(II) centers are assigned on the basis of chemical shifts, longitudinal relaxation times (T(1)), relative integrations, and substitution of the ligands. The electrochemical studies support variations in the ligand strength. Parallel mode EPR measurements for the isopropyl substituted ligand complex of iron(II) show low-field resonances (g > 9.5) indicative of complex aggregation or crystallite formation. No suitable solvent system or glassing mixture was found to remedy this phenomenon. However, the bulkier diisopropylphenyl substituted ligand exhibits an integer spin signal consistent with an isolated iron(ii) center [S = 2; D = -7.1 ± 0.8 cm(-1); E/D = 0.1]. A tentative molecular orbital diagram is assembled.

Square Planar vs Tetrahedral Geometry in Four Coordinate Iron(II) Complexes

The geometric preferences of a family of four coordinate, iron(II) d6 complexes of the general form L2FeX2 have been systematically evaluated. Treatment of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with monodentate phosphine and phosphite ligands furnished square planar trans-P2Fe(Mes)2 derivatives. Identification of the geometry has been accomplished by a combination of solution and solid-state magnetometry and, in two cases (P = PMe3, PEt2Ph), X-ray diffraction. In contrast, both tetrahedral and square planar coordination has been observed upon complexation of chelating phosphine ligands. A combination of crystallographic and magnetic susceptibility data for (depe)Fe(Mes)2 (depe = 1,2-bis(diethylphosphino)ethane) established a tetrahedral molecular geometry whereas SQUID magnetometry and Mössbauer spectroscopy on samples of (dppe)Fe(Mes)2 (dppe = 1,2-bis(diphenylphosphino)ethane) indicated a planar molecule. When dissolved in chlorinated solvents, the latter compound promotes chlorine atom abstraction, forming tetrahedral (dppe)Fe(Mes)Cl and (dppe)FeCl2. Ligand substitution reactions have been studied for both structural types and are rapid on the NMR time scale at ambient temperature.

A Family of Decamethylmetallocene Charge-Transfer Salt Magnets Using Methyl Tricyanoethylenecarboxylate (MTCE) as the Electron Acceptor

Methyl tricyanoethylenecarboxylate, MTCE, has been used as a one-electron acceptor building block for the synthesis of isomorphous decamethylmetallocene charge-transfer salt magnets of the formula [MCp*2][MTCE], M = Cr, Mn, and Fe. Functionally and electrochemically, MTCE is a hybrid between tetracyanoethylene (TCNE) and dimethyl dicyanofumarate (DMeDCF), two acceptors that have previously been found to support ferromagnetism. The X-ray crystal structure of the chromium analogue, [CrCp*2][MTCE], shows it to exist in the expected mixed stack structure in the orthorhombic space group Pnma with a = 14.739(3) A, b = 10.7869(19) A, and c = 15.771(3) A and Z = 4. As anticipated, all three family members exhibit dominant ferromagnetic coupling, which is presumed to reflect intrastack interactions. However, the bulk magnetic properties mostly differ from simple interpolations or extrapolations of the properties of their TCNE and DMeDCF analogues. Density functional theory calculations have been used to shed some light on this observation.

Magnetic property studies of manganese-phosphate complexes

Phosphoric acid forms two distinct coordination compounds with manganese salts in aqueous media, a two-dimensional layered structure, [Mn(HPO4).(H2O)3], 1, under ambient conditions, and a three-dimensional synthetic mineral, [Mn5(mu-OH2)2(HPO4)2(PO4)2(H2O)2], 2, under hydrothermal procedures, at 120 degrees C. In compound 1, the oxygen atom of the doubly deprotonated phosphoric acid interconnects the metal centers to form a layered structure. The neutral hydrophilic layers of 1 are separated by 5.5 A and may potentially intercalate hydrophilic organic guest molecules. The metal centers in 2 are octahedral and bridged by PO4(3-), HPO4(2-), and OH2 groups to form a complex three-dimensional network. XPS analysis on 1 and 2 confirms that manganese exists in the +2 oxidation state. Compound 2 is a poor ion exchanger, but some improvement is observed on partial dehydration. The magnetic properties of both 1 and 2 were studied in detail to examine the amplitude of the magnetic interactions through phosphate ligand bridges. While 1 reveals dominant antiferromagnetic interactions between the spin carriers, a complete investigation of the magnetic properties of 2 revealed its true nature to be a glassy magnet.

A Novel One-Dimensional Cyano-Bridged Ni3Fe2 Ferromagnet Constructed from Bimetallic Molecular Squares

Reaction of the complex [Ni(rac-CTH)](2+) (rac-CTH = rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) with [Fe(CN)(6)](3-) leads to a novel cyano-bridged Ni(3)Fe(2) complex, [[Ni(rac-CTH)](3)[Fe(CN)(6)](2)](4). The structure consists of an alternating arrangement of [Fe(CN)(6)Ni(rac-CTH)](2) squares and trans-planar [Ni(rac-CTH)](2+) units bridged by cyanide groups to give a neutral 1D chain running along the a axis. Magnetic measurements reveal the occurrence of ferromagnetic coupling between Fe(III) and Ni(II) ions and 3D magnetic ordering at 3 K due to interchain interactions. Canting of the moments is inferred from the low value of the magnetization of the saturation below T(c).

Observation of a very large internal hyperfine field (62.4 T) in the ferromagnetically ordered state of the S = 1 alpha-iron(II) octaethyltetraazaporphyrin

The origins of the extraordinarily large internal hyperfine field (62.4 T) for the three-dimensional (weak) ferromagnetically ordered ground state of alpha-Fe(OETAP) are discussed semiquantitatively in terms of existing physical theory. In particular, the classical Fermi-contact contribution to the internal field is found to be highly enhanced by a very large orbital contribution and a significant dipolar term of the same sign. A rationale for the unexpected ordering of this S = 1 non-Kramers system is also presented.

Synthesis, structure, and magnetic characterization of a hysteretic charge-transfer salt metamagnet, decamethylferrocenium 2,3-dicyano-1,4-naphthoquinonide, [Fe(Cp*)2][DCNQ]

The synthesis and single-crystal structure of a new one-to-one charge-transfer salt, derived from decamethylferrocene and 2,3-dicyano-1,4-naphthoquinone, are described. [Fe(Cp*)2][DCNQ] crystallizes in the orthorhombic space group Pbca, with a = 17.3149(5) A, b = 14.6862(4) A, c = 21.0507(6) A, and Z = 8. Magnetization vs temperature data obtained in 100 G suggest that the compound exhibits dominant one-dimensional ferromagnetic coupling and that it subsequently undergoes an antiferromagnetic phase transition below TN approximately 4 K. Results of magnetization vs applied field experiments show that the compound is a metamagnet with a critical field of approximately 3 kG at 1.8 K. In the nominally antiferromagnetic state, apparent canting of the moments gives rise to a small amount of hysteresis. This picture is supported by ac susceptibility data. The 57Fe Mössbauer spectrum exhibits the expected decamethylferrocenium unresolved quadrupole doublet (delta = 0.53 mm/s) at 77 K and magnetic hyperfine splitting, Hint = 37.9 T, corresponding to long-range magnetic order at 1.63 K.