Design of luminescent complexes with different Cu4I4 cores based on pyridyl phenoxarsines.

A series of luminescent Cu4I4 clusters with stair-step, cubane, and octahedral geometries supported by a novel type of cyclic As,N-ligand, pyridyl-containing 10-phenoxarsines, were synthesized and characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis. An unusual arrangement of As,N-bidentate and µ2-iodo ligands was found in the octahedral cluster. The structural diversity of the Cu(I) complexes is reflected in their photophysical properties: the phosphorescence spectra of the compounds display emission in a broad spectral range of 495-597 nm. The complex with the Cu4I4L2 stoichiometry bearing a stair-step Cu4I4 core demonstrates temperature-dependent dual emission. The luminescence properties of all complexes were rationalized by DFT calculations.

PNP Ligands in Cobalt-Mediated Activation and Functionalization of White Phosphorus.

Transition-metal mediated white phosphorus activation is of high interest as an ecological alternative to P4 chlorination pathway to the practically useful phosphorus products. Herein, we report a facile approach for P4 activation, transformation and subsequent functionalization using cobalt complexes bearing PNP ligands. The use of N,N-bis(diphenylphosphino)amine as a ligand allows one to transform P4 tetrahedron into a zig-zag chain with the formation of complex [Co(Ph2 PNHP(Ph2 )PPPPP(Ph2 )NHPPh2 )]BF4 (4). The presence of organic substituent at nitrogen atom in PNP ligand enables one to obtain complexes with η1 -coordinated P4 molecule, which indicates a crucial role of N-H bond in transformation of white phosphorus tetrahedron. Additionally, complex 4 can readily be functionalized by means of the reaction with Ph2 PCl leading to the formation of a new complex bearing unique P9 -ligand. The obtained results provide opportunities for facile construction of new polyphosphorus ligands in the coordination sphere of transition metal complexes.

Luminescent Cu4I4-cubane clusters based on N-methyl-5,10-dihydrophenarsazines.

Two luminescent Cu4I4-cubane tetramers with N-methyl-10-(p-halogenophenyl)-5,10-dihydrophenarsazine ligands were synthesized and characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis. The UV-Vis absorption and emission properties were studied and rationalized by DFT and time-dependent DFT calculations. The luminescence behavior was found to be rather different from that of recently reported tetranuclear copper iodide cubane clusters based on As,O-analogues - 10-(aryl)phenoxarsines. The crystalline powders of both complexes exhibit the temperature-dependent dual-band emission: the low-energy emission originates from the cluster-centered (3CC) triplet state, whereas the high-energy emission was attributed to the intraligand (3IL) triplet state.

Platinum(II) Complexes with 10-(Aryl)phenoxarsines: Synthesis, Cis/Trans Isomerization, and Luminescence.

Synthesis and structural and photophysical characterization of platinum dihalogenide complexes formulated as [PtHal2L2], where Hal = Cl and I, with different 10-(aryl)phenoxarsine ligands such as 10-(p-chlorophenyl)phenoxarsine, 10-(p-tolyl)phenoxarsine, and 10-(phenyl)phenoxarsine are reported. The structures of complexes were determined by NMR spectroscopy, mass spectrometry, and X-ray analysis. Cis/trans isomerism of the complexes in solution was studied by NMR spectroscopy. In the solid state, under UV irradiation, platinum diiodide trans complexes exhibit an intense orange-red emission, which was attributed to a metal halide-centered triplet state. The UV/vis absorption and emission properties were studied and rationalized by density functional theory (DFT) and time-dependent DFT calculations.

A new heterometallic pivalate {Fe8Cd} complex as an example of unusual "ferric wheel" molecular self-assembly.

The interaction of the pivalate complexes of iron(iii), [Fe3O(Piv)6(H2O)3]·HPiv, and cadmium(ii), [Cd(Piv)2], in Et2O resulted in one more type of "ferric wheel" family complex, namely [Fe8(Piv)16{Cd(Piv)2}(µ-OH)8]·Et2O (1). The complex is an octanuclear iron(iii) wheel with a {Cd(Piv)2} moiety asymmetrically incorporated into the ring.

Cu4I4-cubane clusters based on 10-(aryl)phenoxarsines and their luminescence.

In this work, we present the synthesis, structural and photophysical characterization, and theoretical study of tetranuclear copper(i) cubane-type Cu4I4 clusters 6-10 with different 10-(aryl)phenoxarsine ligands - 10-(p-fluorophenyl)phenoxarsine (1), 10-(p-ethoxyphenyl)phenoxarsine (2), 10-(phenyl)phenoxarsine (3), 10-(m-fluorophenyl)phenoxarsine (4), and 10-(o-methoxyphenyl)phenoxarsine (5), respectively. The structures of 1-5 were confirmed by NMR spectroscopy, mass spectrometry, elemental analysis and for complexes 6, 7, and 10 by single-crystal X-ray diffraction analysis. The UV/Vis absorption and emission properties were studied and rationalized by DFT and time-dependent DFT calculations. In the solid state, under UV irradiation, all complexes exhibit an intense green emission, which was attributed to a cluster-centered triplet state.

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism.

The reaction of (dpp-Bian)EuII(dme)2 (3) (dpp-Bian is dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane) with 2,2'-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium(II) complex (dpp-Bian)EuII(bipy)2 (9). In contrast the reaction of related ytterbium complex (dpp-Bian)YbII(dme)2 (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)YbIII(bipy)(bipy-̇) (10) - ytterbium(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex 3 reacts with N,N'-bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)EuII(mes-dad)(dme) (11) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-tert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-tert-butylphenoxy radical) with (dpp-Bian)EuII(dme)2 (3) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12). The molecular structures of complexes 9-12 have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2-310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium(II) complexes (11 and 12). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts.

Double exchange in a mixed-valent octanuclear iron cluster, [Fe8(µ4-O)4(µ-4-Cl-pz)12Cl4](-).

A combination of SQUID and pulsed high-field magnetometry is used to probe the nature of mixed valency in an Fe(II)Fe7(III) cluster. DFT-computed spin Hamiltonian parameters suggest that antiferromagnetic coupling dominates, and that electron transfer both between the four irons of the cubane core (t1) and between a cubane and three neighboring irons (t2) is significant. Simulations using the computed parameters are able to reproduce the key features of the measured effective magnetic moment, µeff(T), over the 2 < T < 300 K temperature range. In contrast, the field dependence of the molar magnetization, Mmol, measured at 0.4 K is inconsistent with substantial electron transfer: only values of t2∼ 0 place the separation between ground and first excited states in the region indicated by experiment. The apparent quenching of the cubane-outer electron transfer at very low temperatures indicates that vibronic coupling generates one or more shallow minima on the adiabatic potential energy surfaces that serve to trap the itinerant electron in the cubane core.

Detailed EPR study of spin crossover dendrimeric iron(III) complex.

The unusual magnetic behavior of the first dendritic Fe(3+) complex with general formula [Fe(L)2](+)Cl(-)·H2O based on a branched Schiff base has been investigated by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. EPR displays that complex consists of the three types of magnetically active iron centers: one S = 1/2 low-spin (LS) and two S = 5/2 high-spin (HS) centers with strong low-symmetry and weak distorted octahedral crystal fields. Analysis of the magnetic behavior reflected by I versus T (where I is the EPR lines integrated intensity of the spectrum) demonstrates that the dendritic Fe(3+) complex has sufficiently different behavior in three temperature intervals. The first (4.2-50 K) interval corresponds to the antiferromagnetic exchange interactions between LS-LS, LS-HS, and HS-HS centers. The appearance of a presumable magnetoelectric effect is registered in the second (50-200 K) temperature interval, whereas a spin transition process between LS and HS centers occurs in the third (200-330 K) one. The coexistence of the magnetic ordering, presumable magnetoelectric effect, and spin crossover in one and the same material has been detected for the first time. The Mössbauer spectroscopy data completely confirm the EPR results.

Crucial role of paramagnetic ligands for magnetostructural anomalies in "breathing crystals".

Breathing crystals based on polymer-chain complexes of Cu(hfac)(2) with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu(hfac)(2) complexes with tert-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets.

Theoretical analysis of spin crossover in iron(II) [2×2] molecular grids.

We use quantum-chemical density functional theory calculations to elucidate the origin of spin-crossover pathways in two iron(II) [2×2] molecular grids with carbohydrazide-based bridging ligands. The complexes are characterized energetically and structurally in five available spin states. Special attention is paid to analysis of the structural distortion induced on each iron center by spin transition on any of its neighbors. The evolution of coordination polyhedra is monitored using the Continuous Shape Measures. It is demonstrated that a succession of spin transitions on different centers depends on the character of the induced distortion, either approaching or getting them away from a more regular low-spin geometry. These effects, resulting from the elasticity of bridging ligands, can be modulated by weak perturbations such as a change of the positions of the hydrogen atoms.

Spin crossover in tetranuclear cyanide-bridged iron(II) square complexes: a theoretical study.

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.

Experimental and theoretical Mössbauer study of an extended family of [Fe8(µ4-O)4(µ-4-R-px)12X4] clusters.

Six [Fe(8)(µ(4)-O)(4)(µ-4-R-pyrazolato)(12)X(4)] complexes containing an identical Fe(8)(µ(4)-O)(4) core have been structurally characterized and studied by Mössbauer spectroscopy. In each case, an inner µ(4)-O bridged Fe(III) cubane core is surrounded by four trigonal bipyramidal iron centers, the two distinct sites occurring in a 1:1 ratio. The Mössbauer spectrum of each of the clusters consists of two quadrupole doublets, which, with one exception (X = NCS, R = H), overlap to give three absorption lines. The systematic variation of X and R causes significant changes in the Mössbauer spectra. A comparison with values for the same clusters computed using density functional theory allows us to establish an unequivocal assignment of these peaks in terms of a nested model for the overlapping doublets. The changes in Mössbauer parameters (both experimental and computed) for the 1-electron reduced species [Fe(8)(µ(4)-O)(4)(µ-4-Cl-pyrazolato)(12)Cl(4)](-) are consistent with a redox event that is localized within the cubane core.

Redox potential of the Rieske iron-sulfur protein quantum-chemical and electrostatic study.

Quantum-chemical study of structures, energies, and effective partial charge distribution for several models of the Rieske protein redox center is performed in terms of the B3LYP density functional method in combination with the broken symmetry approach using three different atomic basis sets. The structure of the redox complex optimized in vacuum differs markedly from that inside the protein. This means that the protein matrix imposes some stress on the active site resulting in distortion of its structure. The redox potentials calculated for the real active site structure are in a substantially better agreement with the experiment than those calculated for the idealized structure. This shows an important role of the active site distortion in tuning its redox potential. The reference absolute electrode potential of the standard hydrogen electrode is used that accounts for the correction caused by the water surface potential. Electrostatic calculations are performed in the framework of the polarizable solute model. Two dielectric permittivities of the protein are employed: the optical permittivity for calculation of the intraprotein electric field, and the static permittivity for calculation of the dielectric response energy. Only this approach results in a reasonable agreement of the calculated and experimental redox potentials.

Electronic structure and magnetic properties of a trigonal prismatic Cu(II)6 cluster.

A combination of detailed magnetisation studies and electronic-structure analysis using broken-symmetry DFT is used to explore the electronic structure of a trigonal prismatic Cu(II)(6) cluster. The presence of six paramagnetic metal centres with S = 1/2 gives rise to a maximum multiplicity of S = 3 and a total of 31 broken-symmetry states with M(S) < 3. Computed differences in energy between the high-spin and broken-symmetry states are expressed in terms of the 15 distinct Heisenberg exchange coupling parameters, J(ij), and the equations are solved by a least-squares fitting procedure. By inspection of the errors introduced by progressive symmetrisation of the Hamiltonian to reduce the number of independent J(ij), we arrive at a minimal model containing only four distinct J(ij) (three intra- and one inter-triangular). The computed values then guide the fitting of the magnetisation data. The computed trends in J(ij) can only be reproduced when antisymmetric exchange is included in the model Hamiltonian. The use of this Hamiltonian provides a reasonable description of the magnetic behaviour at all temperatures and fields. If a simpler isotropic model Hamiltonian is used instead, the best fit values of J(ij) are compromised by the need to fit the low-temperature region where antisymmetric exchange dominates the shape of the curve.

Exchange coupling of paramagnetic ions in a polyoxometalate matrix: density functional study of diiron(III) substituted gamma-silicotungstates.

The quantum chemical (density functional) analysis of the antiferromagnetic interactions between the two iron centers incorporated into the gamma-silicotungstate is performed. The influence of the polyoxometalate framework on the exchange coupling within the diiron core unit is studied. The dependence of the strength of antiferromagnetic exchange on the protonation of the core bridges is considered. It is shown that the magnetic coupling is very sensitive to the distortions in core geometry. Variations in the core structure induced by the environment (such as the polyoxometalate framework) change the significance of superexchange pathways and give rise to new superexchange mechanisms. This effect is especially pronounced in the case of the hydroxo-bridged diiron core.