Novel spin transition between S = 5/2 and S = 3/2 in highly saddled iron(III) porphyrin complexes at extremely low temperatures

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

Dimerization of Paramagnetic Trinuclear Complexes by Coordination Geometry Changes Showing Mixed Valency and Significant Antiferromagnetic Coupling through -Pt···Pt- Bonds

Paramagnetic trinuclear complexes, trans-[Pt₂M(piam)₄(NH₃)₄](ClO₄)_x (t-M; piam = pivalamidate, M = Mn, Fe, Co, Ni, and Cu, x = 2 or 3), aligned as Pt-M-Pt were successfully synthesized and characterized. The dihedral angles between the Pt and M coordination planes in t-M are approximately parallel, showing straight metal-metal bonds with distances of approximately 2.6 Å. Except for t-Fe, the trinuclear complexes are dimerized with close contact (approximately 3.9 Å) between the end Pt atoms to form Pt-M-Pt···Pt-M-Pt alignments with high-spin M(+2) containing five (t-Mn), three (t-Co), two (t-Ni), and one (t-Cu) unpaired electrons localized on M atoms. Several physical measurements and calculations revealed that the dimerized structures were maintained in MeCN, where cyclic voltammograms for t-M exhibited two-step oxidation and reduction attributed to Pt-M(+2)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+3)-Pt via mixed-valent states. Magnetic susceptibility measurements for t-M showed antiferromagnetic interaction, t-Mn: J = -0.9 cm^-1, t-Co: J = -3.5 cm^-1, t-Ni: J = -7.3 cm^-1, and t-Cu: J = 0.0 cm^-1, between the two M centers with distances of 9.0 Å through Pt···Pt bonds.

Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies

High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the Fe^IV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of Fe^IV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k₂ = 3.6(1)-1.21(3) × 10⁴ M^-1 s^-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, Mn^IV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the M^III(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol^-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.

Isolation and characterization of a tetranuclear Pt-FeFe-Pt intermediate en route to the trinuclear Pt-Fe-Pt cluster

An intermediate compound, [{PtFe(piam)₂(NH₃)₂(OCH₃)}₂(μ-OCH₃)₂](ClO₄)₂ (1, piam = pivalamidate), in the synthetic process to form [Pt₂Fe(piam)₄(NH₃)₄](ClO₄)₃ (2) by mixing cis-[Pt(piam)₂(NH₃)₂]·2H₂O and iron sources was successfully isolated and characterized by single-crystal X-ray analysis. In 1, the platinum and iron atoms are bridged by two piam ligands to afford a dinuclear Pt-Fe structure and are further linked to each other by methoxide bridges at the equatorial positions of iron atoms to form a tetranuclear Pt-FeFe-Pt complex. The Pt-Fe distances in 1 are 3.0010(16) and 2.9883(17) Å, which are significantly longer than those in 2 (2.5566(15) and 2.5718(15) Å). X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and magnetic susceptibility measurements revealed that the oxidation states are Pt(+2)-Fe(+3)Fe(+3)-Pt(+2) (1) and Pt(+2)-Fe(+3)-Pt(+2) (2) with high-spin (S = 5/2) configurations in iron atoms. The magnetic susceptibility of 1 has a χT value of 5.83 cm³ mol^-1 K per Pt(+2)-Fe(+3)Fe(+3)-Pt(+2) unit at 300 K, which decreases down to 0.04 cm³ mol^-1 K at 7 K due to antiferromagnetic coupling (J = -28 cm^-1) of the two Fe(+3) centers. Compound 2 maintains its trinuclear structure in MeCN, exhibiting reversible one-electron reduction and oxidation, Pt(+2)-Fe(+2)-Pt(+2) ↔ Pt(+2)-Fe(+3)-Pt(+2), at E_1/2 = -0.19 V (vs. Fc/Fc⁺). However, in MeOH, compound 2 is decomposed into a dinuclear structure of Pt-Fe involving an equilibrium between 1 and 2.

An HASApf-redoxin complex causing asymmetric catalytic oxidation via the regenerative formation of a reactive oxygen species

A PP (pea)-HASApf-redoxin complex eluted from encapsulated PP gel with aeration displays asymmetric oxidation activity over 200 times greater than that of a similar protein expressed by E. coli cells. The intermediate spin, identified in the ESR spectrum, appears at g = 4.3 and g = 2.0, suggesting that an iron electron-transfer system for the asymmetric oxidation of secondary alcohols may be successfully created by the PP-HASApf-redoxin complex (39 kDa). FTIR experiments provided values νs(SO2) ≈ 950-1050 cm(-1) and νas(SO2) ≈ 1100-1200 cm(-1) for metal-bound sulfinate S-O and Fe-O vibrations. The sulfur and iron detected by physicochemical inspection (IC/ICP-AES) may facilitate the electron transport of a sulfate-iron complex (e.g., rubredoxin (6 kDa) or ferredoxin (9 kDa)) to the HASApf (21 kDa). The observations are consistently acceptable; i.e., the oxygen-driven PP-HASApf-redoxin complex functions regenerate via the successive asymmetric catalytic event - Fe(ii) + O2 → Fe(iii)-O-O(-) → Fe(iv) = O (oxidizing rac- or rac-) → Fe(ii) + H2O. Therefore, the use of a raw biomaterial as a PP-HASApf-redoxin complex-catalytic system for asymmetric oxidation is an important novelty, despite the apparent difficulties in working with pure dehydrogenase enzymatic/redox-cofactor systems for biotransformation.

Water oxidation catalysed by iron complex of N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. Spectroscopy of iron-oxo intermediates and density functional theory calculations

The macrocyclic [Fe^III(L1)Cl₂]⁺ (1, L1 = N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH₄]₂[Ce^IV(NO₃)₆] (CAN), NaIO₄, or Oxone as the oxidant. The mechanism of 1-catalysed water oxidation was examined by spectroscopic methods and by ¹⁸O-labelling experiments, revealing that Fe^IV 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O and/or Fe^V 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent Fe 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple (E_1/2 = +0.46 V vs. SCE at pH 1) and an irreversible anodic wave (E_pa = +1.18 V vs. SCE at pH 1) assigned to the Fe^III/Fe^II couple and the Fe^III to Fe^IV oxidation, respectively. DFT calculations showed that the E value of the half reaction involving [Fe^V(L1)(O)(OH)]²⁺/[Fe^IV(L1)(O)(OH₂)]²⁺ is +1.42 V vs. SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe^IV 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O reaction intermediate was suggested by ESI-MS and UV-vis absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on the concentrations of both 1 and CAN. Using NaIO₄ or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of 1, and a reactive Fe^V 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species with formula [Fe^V(L1)(O)₂]⁺ generated by oxidation with NaIO₄ or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe^V(L1)(O)₂]⁺ is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol^-1.

A mononuclear Fe(III) single molecule magnet with a 3/2↔5/2 spin crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.