Broken-symmetry Density Functional Theory Calculations Research Articles

Magnetic and spectroscopic properties of chloride-bridged guanidinate dilanthanide complexes

The guanidinate ligand scaffold offers a unique combination of hard anionic charges, steric tunability, and electronic modulation, rendering it an ideal candidate for the molecular design of lanthanide complexes. Here, we present two dilanthanide chloride complexes [{(Me3Si)2NC(NiPr)2}2Ln(μ–Cl)]2 (Ln = Gd (1), and Tb (2)) where each metal ion is ligated to two guanidinate anions and bridged to the other metal ion by two chloride ligands. The isostructural complexes 1 and 2 were synthesized through salt metathesis reactions and characterized through single–crystal X–ray diffraction analysis, IR and UV–Vis spectroscopy, and SQUID magnetometry. The half-filled f-electron valence shell of GdIII ion allows an accurate study of magnetic coupling through fitting of the dc magnetic susceptibility data to a spin–only Hamiltonian, affording values of J = -0.0242 cm−1 and g = 2.0192. The operative superexchange coupling mechanism was confirmed through broken–symmetry density functional theory calculations, which additionally provide insight into the spectral analysis of 1.

Shape polarization in the tin isotopes near N = 60 from precision g-factor measurements on short-lived 11/2− isomers

The g factors of 11/2− isomers in semimagic 109Sn and 111Sn (isomeric lifetimes τ=2.9(3) ns and τ=14.4(7) ns, respectively) were measured by an extension of the Time Differential Perturbed Angular Distribution technique, which uses LaBr3 detectors and the hyperfine fields of a gadolinium host to achieve precise measurements in a new regime of short-lived isomers. The results, g(11/2−;109Sn)=−0.186(8) and g(11/2−;111Sn)=−0.214(4), are significantly lower in magnitude than those of the 11/2− isomers in the heavier isotopes and depart from the value expected for a near pure neutron h11/2 configuration. Broken-symmetry density functional theory calculations applied to the sequence of 11/2− states reproduce the magnitude and location of this deviation. The g(11/2−) values are affected by shape core polarization; the odd 0h11/2 neutron couples to Jπ=2+,4+,6+... configurations in the weakly-deformed effective core, causing a decrease in the g-factor magnitudes.

A Remarkably Unsymmetric Hexairon Core Embraced by Two High-Symmetry Tripodal Oligo-α-pyridylamido Ligands.

Oligo-α-pyridylamides offer an appealing route to polyiron complexes with short Fe-Fe separations and large room-temperature magnetic moments. A derivative of tris(2-aminoethyl)amine (H6tren) containing three oligo-α-pyridylamine branches and 13 nitrogen donors (H6L) reacts with [Fe2(Mes)4] to yield an organic nanocage built up by two tripodal ligands with interdigitated branches (HMes = mesitylene). The nanocage has crystallographic D3 symmetry but hosts a remarkably unsymmetric hexairon-oxo core, with a central Fe5(μ5-O) square pyramid, two oxygen donors bridging basal sites, and an additional Fe center residing in one of the two tren-like pockets. Bond valence sum (BVS) analysis, density functional theory (DFT) calculations, and electrochemical data were then used to establish the protonation state of oxygen atoms and the formal oxidation states of the metals. For this purpose, a specialized set of BVS parameters was devised for Fe2+-N3- bonds with nitrogen donors of oligo-α-pyridylamides. This allowed us to formulate the compound as [Fe6O2(OH)(H3L)L], with nominally four FeII ions and two FeIII ions. Mössbauer spectra indicate that the compound contains two unique FeII sites, identified as a pair of closely spaced hydroxo-bridged metal ions in the central Fe5(μ5-O) pyramid, and a substantially valence-delocalized FeII2FeIII2 unit. Broken-symmetry DFT calculations predict strong ferromagnetic coupling between the two iron(II) ions, leading to a local S = 4 state that persists to room temperature and explaining the large magnetic moment measured at 300 K. The compound behaves as a single-molecule magnet, with magnetization dynamics detectable in zero static field and dominated by an Orbach-like mechanism with activation parameters Ueff/kB = 49(2) K and τ0 = 4(2) × 10-10 s.

A Trinuclear High-Spin Iron(III) Complex with a Geometrically Frustrated Spin Ground State Featuring Negligible Magnetic Anisotropy and Antisymmetric Exchange.

The trinuclear high-spin iron(III) complex [Fe3Cl3(saltagBr)(py)6]ClO4 {H5saltagBr = 1,2,3-tris[(5-bromo-salicylidene)amino]guanidine} was synthesized and characterized by several experimental and theoretical methods. The iron(III) complex exhibits molecular 3-fold symmetry imposed by the rigid ligand backbone and crystallizes in trigonal space group P3̅ with the complex cation lying on a crystallographic C3 axis. The high-spin states (S = 5/2) of the individual iron(III) ions were determined by Mößbauer spectroscopy and confirmed by CASSCF/CASPT2 ab initio calculations. Magnetic measurements show an antiferromagnetic exchange between the iron(III) ions leading to a geometrically spin-frustrated ground state. This was complemented by high-field magnetization experiments up to 60 T, which confirm the isotropic nature of the magnetic exchange and negligible single-ion anisotropy for the iron(III) ions. Muon-spin relaxation experiments were performed and further prove the isotropic nature of the coupled spin ground state and the presence of isolated paramagnetic molecular systems with negligible intermolecular interactions down to 20 mK. Broken-symmetry density functional theory calculations are consistent with the antiferromagnetic exchange between the iron(III) ions within the presented trinuclear high-spin iron(III) complex. Ab initio calculations further support the absence of appreciable magnetic anisotropy (D = 0.086, and E = 0.010 cm-1) and the absence of significant contributions from antisymmetric exchange, as the two Kramers doublets are virtually degenerate (ΔE = 0.005 cm-1). Therefore, this trinuclear high-spin iron(III) complex should be an ideal candidate for further investigations of spin-electric effects arising exclusively from the spin chirality of a geometrically frustrated S = 1/2 spin ground state of the molecular system.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A MnIII(μ-O)(μ-OH)FeIII species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the MnIII(μ-O)(μ-OH)FeIII state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded MnIII(μ-OH)FeIII species. This final intermediate precedes formation of the MnIII/FeIII resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

Strong magnetic exchange coupling in a radical-bridged trinuclear nickel complex.

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene hexahydrochloride (HATP·6HCl) and (TpPhNi)Cl (TpPh = tris(3,5-diphenyl-1-pyrazolyl)borate) produces the radical-bridged trinickel complex [(TpPhNi)3(HITP)] (HITP3-˙ = 2,3,6,7,10,11-hexaiminotriphenylene). Magnetic measurements and broken-symmetry density functional theory calculations reveal strong exchange coupling persisting at room temperature between HITP3-˙ and two of the three Ni2+ centers, a rare example of strong radical-mediated magnetic coupling in multimetallic complexes. These results demonstrate the potential of radical-bearing tritopic HITP ligands as building blocks for extended molecule-based magnetic materials.

Analysis of the Puzzling Exchange-Coupling Constants in a Series of Heterobimetallic Complexes.

The exchange-coupling constants (J) in a series of bimetallic complexes with an M2+(μ-OH)Fe3+ core (M = Mn, Fe, Ni, and Cu; series 1), which were reported in a recent study ( Sano et al. Inorg. Chem. 2017 , 56 , 14118 - 14128 ), have been analyzed with the help of density functional theory (DFT) calculations. The experimental J values of series 1 showed the remarkable property that they were virtually independent of metal M. This behavior contrasts with that observed for a related series of complexes with M2+Fe3+ cores reported by Chaudhuri and co-workers ( Biswas et al. Inorg. Chem. 2010 , 49 , 626 - 641 ) (series 2) in which J increases toward the upper end of the series. Broken symmetry DFT calculations for J, which yielded values in good agreement for the MnFe and NiFe complexes of series 1, gave for the CuFe complex a J value that was persistently much larger than that obtained from the experiment. Attempts to bridge the discrepancy by invoking various basis sets and corrections for hydrogen-bonding effects on J were not successful. The J values for series 1 were subsequently analyzed in the context of an exchange pathway model. From this analysis, it emerged that, in addition to the regular 2e-pathways, which contribute antiferromagnetic terms to J, there are also 3e-pathways that contribute ferromagnetic terms and have the propensity to keep J constant along series 1. It is shown that, while DFT evaluates the 2e-pathway terms reliably, this method seriously underestimates the 3e-pathway contributions, resulting in a too high J value for the CuFe complex of series 1. The pathway analysis of series 2 reveals that the 3e-pathway contributions to J are considerably smaller than those in series 1, resulting in J values that increase toward the upper end of the series, in accordance with the experiment.

Low-dimensional compounds containing cyanido groups. Part XXXV. Structure, spectral, thermal and magnetic properties of a binuclear CuII-biquinoline complex with bridging and terminal dicyanamide ligands.

From the system CuCl2-biq-NaN(CN)2 (biq is 2,2'-biquinoline), the binuclear molecular complex bis(μ-dicyanamido-κ2N1:N5)bis[(2,2'-biquinoline-κ2N,N')(dicyanamido-κN1)copper(II)], [Cu2(C2N3)4(C18H12N2)2] or [Cu2(biq)2(dca)2(μ1,5-dca)2] (1) [dca is dicyanamide, N(CN)2-] was isolated and characterized by crystal structure analysis, and spectral, thermal and magnetic measurements. IR spectroscopy confirmed the presence of the biq and dca ligands in 1. Its solid-state structure consists of discrete centrosymmetric binuclear copper(II) units with double end-to-end dca bridges. Each CuII atom is in a distorted square-pyramidal environment with the equatorial plane formed by two nitrile N atoms from bridging dca groups, one of the two N atoms of the chelate biq molecule and one nitrile N atom from a terminal dca ligand, whereas the second biq N atom occupies the axial position. Thermal decomposition of 1 in an air atmosphere proceeds gradually, with copper(I) cyanide being the final decomposition product. Magnetic measurements revealed the formation of alternating spin chains and a relatively strong exchange interaction within the binuclear units was also confirmed by Broken Symmetry DFT (density functional theory) calculations.

Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study.

The three- and four-membered redox series [Cr(MePDP)2]z (z = 1-, 2-, 3-) and [Mo(MePDP)2]z (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand MePDP2- (H2MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closed-shell MePDP2- ligands and +III oxidation states (d3, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(MePDP)2]1- (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(MePDP)2 which was characterized by X-ray crystallography, UV/vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a MoIV complex (d2, S = 1) with two MePDP2- ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of MePDP•3- radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M(MePDP)2]2- (S = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d3) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively.

Structural and Magnetic Investigations of a Heterospin System Composed of High-spin Mn(II) and Pyridyl-containing Triarylmethyl Radicals

A novel molecular heterospin system composed of high-spin MnII (S = 5/2) and (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM; S = 1/2) was developed. The MnII ion formed distorted octahedral coordination geometry with two PyBTM radicals coordinated in the trans configuration. Magnetic studies disclosed an intramolecular antiferromagnetic exchange interaction between the spins on PyBTM and MnII with JR–Mn/kB of −9.7 K. The antiferromagnetic interaction was confirmed by broken-symmetry density functional theory calculations. This study reveals the definite intramolecular exchange interaction in the MnII–triarylmethyl radical system for the first time.

Anion-induced exchange interactions in binuclear complexes of Cu(II) with flexible hexadentate bispicolylamidrazone ligands

Two recently synthesized copper(II) complexes with spacer-armed bispicolylamidrazone ligands have been theoretically studied at the density functional theory (DFT) level accounting for empirical dispersion correction and intrinsic anionic environment by perchlorate ions. The exchange parameter between the open-shell singlet and triplet states of the studied complexes has been estimated by broken symmetry DFT calculations. The mechanism of spin-spin exchange interaction between the unpaired electrons via the σ-bond aliphatic chain (Gusev et al., 2015) is confirmed. Instead, a anion-induced mechanism is proposed which means that the anionic grid participates in the exchange interaction between the unpaired electrons.

Mechanism of the Intramolecular Hexadehydro-Diels-Alder Reaction.

Theoretical analysis of the mechanism of the intramolecular hexadehydro-Diels-Alder (HDDA) reaction, validated against prior and newly measured kinetic data for a number of different tethered yne-diynes, indicates that the reaction proceeds in a highly asynchronous fashion. The rate-determining step is bond formation at the alkyne termini nearest the tether, which involves a transition-state structure exhibiting substantial diradical character. Whether the reaction then continues to close the remaining bond in a concerted fashion or in a stepwise fashion (i.e., with an intervening intermediate) depends on the substituents at the remaining terminal alkyne positions. Computational modeling of the HDDA reaction is complicated by the significant diradical character that arises along the reaction coordinate, which leads to instabilities in both restricted singlet Kohn-Sham density functional theory (DFT) and coupled cluster theory based on a Hartree-Fock reference wave function. A consistent picture emerges, however, from comparison of broken-symmetry DFT calculations and second-order perturbation theory based on complete-active-space self-consistent-field (CASPT2) calculations.

Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study.

Whereas reaction of [(η(5)-Cp*)Ti(IV)Cl3](0) (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η(5)-Cp*)Ti(III)(bpy(•))2](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η(5)-Cp*)Ti(μ-Cl)(bpy(•))}2](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η(5)-Cp*)Ti(μ-Cl)(phen(•))}2](0) (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy-Cpy distances of 1.420(3) and 1.431(4) Å in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(•))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy(•))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 μB, which is indicative of an S = (1)/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η(5)-Cp* ligand and two (bpy(•))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the Ti(III) center and on the two (bpy(•))(1-) ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N'-coordinated (bpy(•))(1-)/(phen(•))(1-) ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}(2-)/{(phen)2}(2-) bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each Ti(III) center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.

Spin decontamination of broken-symmetry density functional theory calculations: deeper insight and new formulations.

This work re-examines the problem of the broken-symmetry Density-Functional Theory (DFT) solutions in diradical systems, in particular for the calculation of magnetic couplings. The Ms = 0 solution is not an eigenfunction of the S(2) spin operator and the evaluation of the singlet state energy requires a spin-decontamination. A popular approximation is provided by the so-called Yamaguchi formula, which operates using the expectation values of S(2) relative to both Ms = 1 and Ms =0 solutions. Referring to a previous decomposition of the magnetic coupling in terms of direct exchange, kinetic exchange and core polarization, it is shown that this expression will lead to unreliable values of the singlet-triplet energy gap when the spin polarization of the core orbitals becomes large. The here-proposed method of spin-decontamination is based on the Effective Hamiltonian Theory and uses the overlap between the two degenerate Ms = 0 solutions. An approximate and convenient formula, which uses the expectation values of S(2) of the Ms = 0 solutions before and after core polarization is proposed, which is free from the Yamaguchi's formula artefact, as illustrated on an organic diradical presenting a very high value of 〈S(2)〉 for the Ms = 0 solution, the antiferromagnetic coupling being due to the spin polarization mechanism.

On ammonia binding to the oxygen-evolving complex of photosystem II: a quantum chemical study.

A recent EPR study (M. Perrez Navarro et al., Proc. Natl. Acad. Sci. 2013, 110, 15561) provided evidence that ammonia binding to the oxygen-evolving complex (OEC) of photosystem II in its S2 state takes place at a terminal-water binding position (W1) on the "dangler" manganese center MnA. This contradicted earlier interpretations of (14)N electron-spin-echo envelope modulation (ESEEM) and extended X-ray absorption fine-structure (EXAFS) data, which were taken to indicate replacement of a bridging oxo ligand by an NH2 unit. Here we have used systematic broken-symmetry density functional theory calculations on large (ca. 200 atom) model clusters of an extensive variety of substitution patterns and core geometries to examine these contradictory pieces of evidence. Computed relative energies clearly favor the terminal substitution pattern over bridging-ligand arrangements (by about 20-30 kcal mol(-1)) and support W1 as the preferred binding site. Computed (14)N EPR nuclear-quadrupole coupling tensors confirm previous assumptions that the appreciable asymmetry may be accounted for by strong, asymmetric hydrogen bonding to the bound terminal NH3 ligand (mainly by Asp61). Indeed, bridging NH2 substitution would lead to exaggerated asymmetries. Although our computed structures confirm that the reported elongation of an Mn-Mn distance by about 0.15 Å inferred from EXAFS experiments may only be reproduced by bridging NH2 substitution, it seems possible that the underlying EXAFS data were skewed by problems due to radiation damage. Overall, the present data clearly support the suggested terminal NH3 coordination at the W1 site. The finding is significant for the proposed mechanistic scenarios of OEC catalysis, as this is not a water substrate site, and effects of this ammonia binding on catalysis thus must be due to more indirect influences on the likely substrate binding site at the O5 bridging-oxygen position.

Ab initio study of exchange coupling for the consistent understanding of the magnetic ordering at room temperature in V[TCNE] x

We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]− anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.

Tuning Spin–Spin Coupling in Quinonoid-Bridged Dicopper(II) Complexes through Rational Bridge Variation

Bridged metal complexes [{Cu(tmpa)}2(μ-L(1)-2H)](ClO4)2 (1), [{Cu(tmpa)}2(μ-L(2)-2H)](ClO4)2 (2), [{Cu(tmpa)}2(μ-L(3)-2H)](BPh4)2 (3), and [{Cu(tmpa)}2(μ-L(4)-2H)](ClO4)2 (4) (tmpa = tris(2-pyridylmethyl)amine, L(1) = chloranilic acid, L(2) = 2,5-dihydroxy-1,4-benzoquinone, L(3) = (2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, L(4) = azophenine) were synthesized from copper(II) salts, tmpa, and the bridging quinonoid ligands in the presence of a base. X-ray structural characterization of the complexes showed a distorted octahedral environment around the copper(II) centers for the complexes 1-3, the donors being the nitrogen atoms of tmpa, and the nitrogen or oxygen donors of the bridging quinones. In contrast, the copper(II) centers in 4 display a distorted square-pyramidal coordination, where one of the pyridine arms of each tmpa remains uncoordinated. Bond-length analyses within the bridging ligand exhibit localization of the double bonds inside the bridge for 1-3. In contrast, complete delocalization of double bonds within the bridging ligand is observed for 4. Temperature-dependent magnetic susceptibility measurements on the complexes reveal an antiferromagnetic coupling between the copper(II) ions. The strength of antiferromagnetic coupling was observed to depend on the energy of the HOMO of the bridging quinone ligands, with exchange coupling constants J in the range between -23.2 and -0.6 cm(-1) and the strength of antiferromagnetic coupling of 4 > 3 > 2 > 1. Broken-symmetry density functional theory calculations (DFT) revealed that the orientation of magnetic orbitals in 1 and 2 is different than that in 3 and 4, and this results in two different exchange pathways. These results demonstrate how bridge-mediated spin-spin coupling in quinone-bridged metal complexes can be strongly tuned by a rational design of the bridging ligand employing the [O] for [NR] isoelectronic analogy.

Oxidative addition of carbon-carbon bonds with a redox-active bis(imino)pyridine iron complex.

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(μ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = Me, (i)Pr), resulted in oxidative addition of a C-C bond at ambient temperature to yield the corresponding iron biphenyl compounds, ((R)PDI)Fe(biphenyl). The molecular structures of the resulting bis(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.

Broken symmetry DFT calculations of exchange coupling constants for manganese–porphyrin quasi-one-dimensional molecular magnets

We report the results of broken symmetry density functional theory (BS-DFT) calculations providing the exchange coupling constants for two quasi-one-dimensional manganese–porphyrin compounds, [MnOEP][HCBD] and [MnTtBuPP][HCBD] (OEP = octaethylporphyrinato, TtBuPP = meso-tetrakis-(4′-tert-butylphenyl)porphinato, HCBD = hexa-cyanobutadiene). The different magnetic behaviour of these materials is due mainly to the distinct binding modes of the cyanocarbon unit. We compare and contrast the results of BS-DFT calculations for magnetic dinuclears, which properly model the actual molecular magnets, to determine the geometry dependence of the exchange interaction. The exchange coupling constants resulting from BS-DFT calculations vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the constants determined experimentally. Structure-properties correlations reveal the determinant role of the Mn–(N≡C)TCNE bond angle on the ferrimagnetic coupling between the S 1 = 2 spin located on the MnIII–porphyrin donor and the S 2 = 1/2 spin positioned on the cyanocarbon acceptor. Based on a phenomenological model providing the geometry dependence of the exchange coupling constants, we fitted the results of the DFT calculations and obtained parameters describing the ferromagnetic and the antiferromagnetic parts of the superexchange interaction. The large differences between the magnetic properties of the two Mn–HCBD systems are explained based on the correlation between the exchange coupling constant and the overlap between the Mn(III) d and the HCBD π* orbitals and on the torsion angle of the butadiene backbone, which affects dramatically the nature of the π* orbital in the case of the nonplanar HCBD.

DFT study of structure–properties correlations in [MnTPP][TCNE] quasi-one-dimensional molecular magnets

We report the first band structure calculations of the quasi-one-dimensional [MnTPP][TCNE] compounds (TPP = meso-tetraphenylporphyrinato, TCNE = tetracyanoethylene), based on Density Functional Theory (DFT) methods, in order to interpret the magnetic ordering in these prototypic systems. We compare and contrast the results of broken-symmetry DFT calculations for extended systems, with periodic boundary conditions, and for finite systems, magnetic dimers modeling the actual molecular magnets. By varying systematically the main angles, we are able to determine the geometry dependence of the exchange interaction. Structure–properties correlations in these charge-transfer salts reveal the determinant role of the Mn-(N≡C)TCNE bond angle on the strength of the ferrimagnetic coupling between the S1 = 2 spin located on the MnIII-porphyrin donor and the S2 = 1/2 spin positioned on the cyanocarbon acceptor. When the Mn-(N≡C)TCNE angle is decreased, the intrachain magnetic coupling strengthens, correlated with the increase in the \( d_{{z^{2} }} - \pi * \) orbital overlap. The exchange coupling constants resulting from DFT calculations of extended systems, with periodic boundary conditions, were found to be consistent with those obtained for the dimers, but systematically smaller. The exchange constants vary strongly with the functional used, hybrid functionals such as B3LYP leading to results that better correlate with the experimental mean-field critical temperatures. The coupling constant varies significantly with the type of broken-symmetry approach, depending on the overlap between magnetic orbitals, but weakly on the basis set once polarization effects are included. The electronic structure calculations for the extended systems provide a density of states consistent with the energy spectrum of the corresponding dimer, allowing for an intuitive explanation of the intrachain ferrimagnetic ordering.