A stable organic triradical with truncated π-conjugation as a model for single-component organic molecule-based ferrimagnetics

As a novel molecular design for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of ‘single-component ferrimagnetics.’ When a π-biradical with an S=1 ground state and a π-monoradical with S=1/2 are united by σ-bonds, the π-conjugation between the biradical and the monoradical moieties should be truncated in the resultant triradical. This gives magnetic degrees of freedom for both S=1 and S=1/2 in the single molecule, serving as a building block for organic molecular ferrimagnets. We have designed and synthesized a triradical, 2,2,6,6-tetramethyl-pipelidine-1-N-oxyl-4-carboxylic acid 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenyl ester (2) as a model compound for single-component ferrimagnetics. Solution-phase ESR spectra from 2 are explained by a perturbation treatment assuming that the exchange interaction within the biradical moiety is much larger than those between the biradical and the monoradical moieties, which is suitable for single-component ferrimagnetics. From susceptibility measurements for a cyclohexane-substituted biradical, cyclohexane carboxylic acid 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl) phenyl ester (4) as a biradical analogue of 2, it is shown that the intramolecular ferromagnetic interaction has been found to be unaffected by the chemical modification for anchoring the monoradical moiety.     

Benzyl-phenyl ether derivatives of nitronyl nitroxide triradicals as a model for single-component organic molecule-based ferrimagnetics

As a novel molecular design for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of “single-component organic molecule-based ferrimagnetics”. In this study, we have designed and synthesized a triradical p-trisNN (1) as a building block for single-component ferrimagnetics, in which a m-phenylene-based π-biradical and a π-monoradical are connected in a benzyl-phenyl ether framework. The triradical retains the magnetic degrees of freedom for both S = 1 and S = 1/2 spins in the single molecule. X-ray crystallographic analyses show that 1 has a head-to-tail packing with an approximately isosceles triangular geometry, which is favorable for a ferrimagnetic spin alignment. From the analysis of crystal and magnetic susceptibility measurements, the triradical is found to form exchange-coupled systems composed of six S = 1/2 spins in the repeating unit of the crystalline solid state.     

Magnetic properties and quantum phase transitions of purely organic molecule-based ferrimagnets based on Green's function theory

Magnetic properties of a Heisenberg diamondlike spin chain model for purely organic molecule-based ferrimagnets are investigated by means of the many-body Green's function method within random phase approximation. The molecule-based ferrimagnet is composed of S=1 biradical and S=1/2 monoradical molecules alternating with intermolecular antiferromagnetic (AF) interactions, and the S=1 site is composed of two S=1/2 spins by a finite intramolecular ferromagnetic (F) interaction. The numerical results reveal that occurrence of ferrimagnetic spin alignments along the chain is determined by the intermolecular AF interactions. Owing to the very small intermolecular AF interactions, the curves of the product of magnetic susceptibility and temperature (chiT) against temperature display as a round peak at low temperatures, and the ferrimagnetic phase transition could only be detected at ultralow temperatures in practical organic compounds. Temperature- and magnetic-field-induced magnetic phase transitions are discussed, which are consistent with the experimental findings. The lower spatial symmetry of intermolecular interactions makes it easy to form spin pairs with a singlet (S=0) ground state along the chain and to reduce Curie temperature. The formations of molecular dimers and trimers along the chain have contributions to bring about F alignments with effective S=1/2 magnetic supramolecules and to enhance Curie temperature. In addition, the experimental data of the magnetic susceptibility measurements for a molecule-based ferrimagnet are also fairly compared with our theoretical results.     

Theoretical study on spin alignments in ferromagnetic heterospin chains with competing exchange interactions: a generalized ferrimagnetic system containing organic biradicals in the singlet ground state

Spin alignments in heterospin chains are examined from numerical calculations of model spin Hamiltonians. The Hamiltonians of the heterospin chains mimic an open-shell molecular assemblage composed of an organic biradical in a singlet (S = 0) ground state and a doublet (S = 1/2) monoradical, which are coupled by intermolecular ferromagnetic exchange interactions. It is found from numerical calculations of the spin Hamiltonians that the spin value S2 of the ground-state singlet biradical embedded in the exchange-coupled assemblage deviates from zero and contributes to the bulk magnetization. The alternating chain is found to have two kinds of ground spin states, a high- and a low-spin state. All the spins are parallel to each other in the high-spin state, which is characterized by the spin correlation function of (S(i).S(j)) = 0.25. On the other hand, the spin alignment in the low-spin state is found to be dependent on the topology of the intermolecular exchange interactions. The energy preference of the two states depends on the relative amplitude of the exchange interactions in the chain. The intermolecular ferromagnetic couplings are competing in the ground-state singlet biradical with the intramolecular antiferromagnetic interaction. The appearance of the two kinds of ground states is attributed to a quantum spin frustration effect inherent in the triangular motif of the competing interactions. Magnetic properties of a zigzag chain complex composed of a nitronyl nitroxide biradical with a singlet ground state and Cu(hfac)2 are examined on the basis of the theoretical calculations. The vanishing magnetic moments, or the product of susceptibility and temperature chiT, at low temperatures observed for the complex are consistent with those of the low-spin state predicted in the theoretical calculations.     

Peculiarity in the electronic structure of Cu(II) complex ferromagnetically coupled with bisimino nitroxides

By means of the electron spin resonance (ESR) technique, we have investigated the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu-bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1' H-imidazol-2'-yl)pyridine). It was clarified that Cu-bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn-bisimpy) and a copper(II) terpyridine complex (Cu-tpy) (tpy = 2,2';6',2'-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu-bisimpy, which has a small g anisotropy ( g x = 2.02, g y = 2.01, g z = 2.08) and small hyperfine coupling with Cu (|A x| = 42.0 MHz, |A y|相似文献   

Crystal structures and magnetic properties of metal complexes bearing four nitronyl nitroxide moieties in the same coordination sphere

Mononuclear transition metal complexes of the type [M(2,6-NITpy)2](ClO4)2 x solvent (2,6-NITpy = 2,6-bis-(3'-oxide-1'-oxyl-4',4',5',5'-tetramethylimidazolin-2'-yl)pyridine; M = Ni (1), Co (2), Zn (3), Mn (4), Cu (5)) have been synthesized and characterized by single-crystal X-ray diffraction studies. Crystal data: 1, monoclinic, P2(1)/c, Z = 4, a = 20.946(2) A, b = 12.0633(2) A, c = 21.173(2) A, beta = 113.55(1) degrees; 2, monoclinic, P2(1)/c, Z = 4, a = 20.902(2) A, b = 12.0981(8) A, c = 21.215(2) A, beta = 113.130(9) degrees; 3, triclinic, P1, Z = 2, a = 11.410(1) A, b = 12.932(1) A, c = 21.609(2) A, alpha = 96.040(2) degrees, beta = 102.24(1) degrees, gamma = 114.98(1); 4, monoclinic, P2(1)/n, Z = 4, a = 11.5473(8) A, b = 19.212(1) A, c = 25.236(2) A, beta = 98.772(9) degrees; 5, triclinic, P1, Z = 2, a = 12.1604(9) A, b = 12.6961(9) A, c = 18.103(2) A, alpha = 84.191(8) degrees, beta = 73.392(8) degrees, gamma = 66.072(8). The two 2,6-NITpy biradicals behave as terdentate ligands and bind almost perpendicular to each other in meridional positions. In compounds 1-4, the pyridine rings are axially ligated and four different nitronyl nitroxide radicals bind to the metal center through their O(nitroxyl) atoms, forming the equatorial plane of a distorted octahedron. On the contrary, in the copper(II) complex (5), the two N(pyridyl) atoms are found in equatorial positions. Only two nitroxide groups are then bound to the copper(II) ion in the equatorial plane, the other two being axially ligated. The two axially bound nitronyl nitroxide radicals couple ferromagnetically to the copper center (JCu-rad(ax) = + 10 K), whereas a strong antiferromagnetic coupling between this metal ion and the equatorial nitroxide groups (JCu-rad(eq) = -460 K) is observed. The other complexes exhibit strong antiferromagnetic metal-radical interactions: JNi-rad = -240 K, for 1; JMn-rad = -120 K, for 4. Interestingly, the study of the diamagnetic zinc(II) compound (3) reveals a moderate intramolecular antiferromagnetic interaction between radicals coordinated to the same metal center (Jrad-rad = -27.7 K). This interaction is transmitted through space and is also present in the other complexes: Jrad-rad = -14 K, for 1; Jrad-rad = -10 K, for 4; Jrad-rad = -20.5 K, for 5. Antiferromagnetic intermolecular interactions are also present in all the complexes herein studied.     

Magnetic properties of nitronylnitroxide and iminonitroxide triradicals as model compounds for generalized ferrimagnets

A novel iminonitroxide triradical, p-triIN (1), has been synthesized, in which a π-conjugated biradical with a singlet ground state and a doublet monoradical are united by σ-bonds. The intramolecular exchange interaction within the para-phenylene-based biradical moiety has been found to be antiferromagnetic (2J(π)/k_B = −30.8 K) from magnetic susceptibility measurements on a parent biradical, p-bisIN (2). The magnetic properties of 1 have been examined by magnetic susceptibility χ_m. Upon cooling, the χ_mT value for 1 has decreased and passed across 0.38 emu mol⁻¹ K, which is expected for one mol of S = 1/2 spin with g = 2.0. The magnetic behavior of 1 indicates that the ground state of the molecular assemblage of 1 in the crystal is diamagnetic, which is attributable to one of the exotic spin states as predicted in a theoretical model of generalized ferrimagnetism. The possible occurrence of the generalized ferrimagnetic spin alignment is concluded in view of the magnetic properties of 1.     

Synthesis, structure, and magnetism of a 1D compound engineered from a biradical [5,5'-Bis(3''-oxide-1''-oxyl-4'',4'',5'',5''-tetramethylimidazolin-2''-yl)-2,2'-bipyridine] and Mn(II)(hfac)2

The synthesis, crystal structure, and magnetic properties of a one-dimensional compound, {[Mn(hfac)2]2(biradical)}n (1), resulting from the coordination of bis(hexafluoroacethylacetonato)manganese(II) [Mn(hfac)2] with a biradical obtained by grafting two nitronyl nitroxide radicals in the 5 and 5' positions of a 2,2'-bipyridine ligand are described. Compound 1 crystallizes in the triclinic P space group with the following parameters: a = 11.905(2) A, b = 12.911(2) A, c = 20.163(3) A, alpha = 73.556(3) degrees , beta = 80.850(3) degrees , gamma = 82.126(3) degrees , Z = 2. The bipyridyl moiety acts as a chelate toward one [Mn(hfac)2] unit, while the pendent nitronyl nitroxide radicals are symmetrically bound in trans-configuration to additional [Mn(hfac)2] units. The result is infinite chains running along the c axis direction with the biradical bridging [Mn(hfac)2] units with pending bipyridine/Mn(hfac)2 cores. The magnetic behavior is characteristic of ferrimagnetic chains. Qualitatively we observe first the antiferromagnetic coupling (J2) of each manganese(II) center with two nitronyl nitroxide moieties, leading to a minimum in the chiT product of 6.63 emu K mol(-1) observed at 70 K and corresponding to a ground spin state S = 3/2 plus one extra spin S = 5/2 coming from the pending manganese(II) center. The increase of chiT at lower temperature is understood as a fictive ferromagnetic coupling related to the true antiferromagnetic coupling J1 of the pseudospin S = 3/2 with spin S = 5/2 of the pending manganese(II). Along this approach (H = -JSiSj) the best fit (300-8 K) of the experimental data leads to J1 = -0.622 +/- 0.022 cm(-1) and J2 = -203 +/- 3 cm(-1) with g(Rad) = 2.0017 +/- 0.0015 and g(Mn) = 2.0017 +/- 0.0015.     

Experimental evidence for the triplet-like spin state appearing in ground-state singlet biradicals as a key feature for generalized ferrimagnetic spin alignment

The authors have previously proposed a theoretical model for exotic spin alignment in organic molecular assemblages: The alternating chain of organic biradicals in a singlet (Sb=0) ground state and monoradicals with S=1/2 has a ferrimagnetic ground state for the whole chain, which has been termed generalized ferrimagnetism. An important feature of the generalized ferrimagnetic spin alignment has been found in the deviation of the expectation value Sb2 of the biradical spin from zero. Even a triplet-like spin state Sb2=2 (Sb=1) has been predicted in the theoretical calculations. In this study, we have found experimental evidence for the pseudo-triplet state appearing in the ground-state singlet biradical of a real open-shell compound. At first, we have demonstrated from theoretical calculations that the singlet biradical has Sb2=2 (Sb=1) in a molecular pair with an S=1 metal ion as well as with the S=1/2 monoradical. The pseudo-triplet state of the biradical affords a singlet state of the whole system of the biradical-metal ion pair, which is readily detectable in experiments for verifying the theoretical prediction. As a model compound for the biradical-metal ion pair, a transition metal complex, [(bnn)(Ni(hfac)2)1.5(H2O)] (1), has been synthesized from a nitronyl nitroxide-based ground-state singlet biradical bnn and Ni(hfac)2. From X-ray crystallographic analyses, the compound contains a molecular pair of bnn and Ni(hfac)2, which serves as a model system under the above theoretical studies. It has been found from the analysis of the temperature dependence of magnetic susceptibility that the bnn-Ni(hfac)2 pair has the singlet (S=0) ground state. The singlet ground state of the pair results from an antiparallel coupling of the pseudo-triplet of the biradical and the S=1 spin on the Ni ion. The pseudo-triplet state in the ground-state singlet biradical has thus been verified experimentally, which is crucially important to realize the generalized ferrimagnetic spin alignment.     

Synthesis,Crystal Structure and Magnetic Properties of a Trinuclear Copper(II) Complex Based on P-Cresol-Substituted Bis(α-Nitronyl Nitroxide) Biradical

Trinuclear copper(II) complex [Cu^II₃(NIT₂PhO)₂Cl₄] was synthesized with p-cresol-substituted bis(α-nitronyl nitroxide) biradical: 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (NIT₂PhOH). The crystal structure of this heterospin complex was determined using single-crystal X-ray diffraction analysis and exhibits four unusual seven-membered metallocycles formed from the coordination of oxygen atoms of the N-O groups and of bridging phenoxo (µ-PhO⁻) moieties with copper(II) ions. The crystal structure analysis reveals an incipient agostic interaction between a square planar copper center and a hydrogen-carbon bond from one methyl group carried on the coordinated nitronyl-nitroxide radical. The intramolecular Cu∙∙∙H-C interaction involves a six-membered metallocycle and may stabilize the copper center in square planar coordination mode. From the magnetic susceptibility measurements, the complex, which totals seven S = 1/2 spin carriers, has almost a ground state spin S = 1/2 at room temperature ascribed to strong antiferromagnetic interaction between the nitronyl nitroxide moieties and the copper(II) centers and in between the copper(II) centers through the bridging phenoxo oxygen atom.     

Magnetic Properties of Conjugated Phenoxy Polyradicals

Formation and magnetic properties of π-conjugated phenoxy radicals bearing a porphyrin macrocycle or a polyacetylenic backbone are described. Tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphinato-metals (7-M) [M = Zn(II), VO(II), and Cu(II)] were oxidized to give the corresponding stable monoradical and biradical. Magnetic properties of the radical species depend on the magnetic orbital of the central metal ion. ESR hfs structure reveals that the biradical for 7-Zn and—VO are in the triplet (S = 1) and quartet state (S = 3/2), respectively. Poly(3,5-di-t-butyl-4-hydroxyphenylacetylene) (8) and poly(p-ethynylphenylhydrogalvinoxyl) (9) with molecular weights of ～ /10⁴ are characterized. ESR spectra with hyperfine structure for 8 indicate that the phenoxy radicals are conjugated with the polyacetylenic main chain over ～7 monomer units as a “neutral soliton.” Formed polyradicals for 8 and 9 are surprisingly stable, even in the solid state, due to resonance stabilization and/or steric effect of the conjugated main chain. An antiferromagnetic interaction is observed for oxidized 8 with a spin concentration above 10 mol%.     

S = (3)/(2) <= => S = (1)/(2) spin crossover behavior in five-coordinate halido- and pseudohalido-bis(o-iminobenzosemiquinonato)iron(III) complexes

Five-coordinate halido- and pseudohalido-bis(o-iminobenzosemiquinonato)iron(III) complexes [Fe(III)X(L(ISQ))(2)] (X = Cl(-) (1), Br(-) (2a, 2b), I(-) (3), N(3)(-) (4), and NCS(-) (5)) have been synthesized where (L(ISQ))(1)(*)(-) represents the pi radical anion N-phenyl-o-imino(4,6-di-tert-butyl)benzosemiquinonate(1-). The molecular structures of the two polymorphs 2a and 2b have been determined at 100, 220, and 295 K, respectively, by single crystal X-ray crystallography. Variable temperature magnetic susceptibility data reveal the following electronic ground states, S(t): For 1, it is (3)/(2). Polymorph 2a contains a 1:1 mixture of (3)/(2) and (1)/(2) forms in the range 4.2 to approximately 150 K; above 150 K the latter form undergoes a spin crossover (1)/(2) --> (3)/(2). Polymorph 2b contains only the S(t) = (3)/(2) form (4-300 K). Complex 3 contains the S(t) = (1)/(2) form in the range 4-130 K, but above 130 K, a spin crossover to the (3)/(2) form is observed which is confirmed by three crystal structure determinations at 100, 220, and 295 K. Complex 4 possesses an S(t) = (1)/(2) ground state at 80 K and undergoes a spin crossover at higher temperatures. Complex 5 has a temperature-independent S(t) = (3)/(2) ground state. All crystal structures of 1, 2a, 2b, 3, 4, and 5, regardless at which temperature the data sets have been measured, show that two o-iminobenzosemiquinonate(1-) pi radical anions are N,O-coordinated in all of these neutral iron complexes. The Fe-N and Fe-O bond distances are longer in the S(t) = (3)/(2) and shorter in the S(t) = (1)/(2) forms. The S(t) = (3)/(2) ground state is attained via intramolecular antiferromagnetic coupling between a high spin ferric ion (S(Fe) = (5)/(2)) and two ligand pi radicals whereas the S(t) = (1)/(2) form is generated from exchange coupling between an intermediate spin ferric ion (S(Fe) = (3)/(2)) and two ligand radicals.     

Magnetic phase transition in a heteromolecular hydrogen-bonded complex of nitronylnitroxide radicals

An organic molecular acid-base complex has been synthesized from pyridine-substituted biradical 2 in a triplet (S = 1) ground state and a benzoic acid derivative of monoradical 3 with S = 1/2. The two constituent molecules are bound by an OH-N hydrogen bond in a crystalline solid state. The complex has been found to exhibit an antiferromagnetic phase transition at 5 K. The complex is the first example of a hydrogen-bonded heterospin, heteromolecular complex exhibiting a magnetic phase transition in purely organic molecule-based materials.     

Electron donor-bridge-acceptor molecules with bridging nitronyl nitroxide radicals: influence of a third spin on charge- and spin-transfer dynamics

Appending a stable radical to the bridge molecule in a donor-bridge-acceptor system (D-B-A) is potentially an important way to control charge- and spin-transfer dynamics through D-B-A. We have attached a nitronyl nitroxide (NN*) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANI-Ph(NN*)-NI, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, and NI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, NN*, and NI are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NN* influences the spin dynamics of the photogenerated triradical states (2,4)(MeOAn(+)*-6ANI-Ph(NN*)-NI(-)*), resulting in slower charge recombination within the triradical compared to the corresponding biradical lacking NN*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn(+)(*) and NI(-)(*) is not altered by the presence of NN*, which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of (2,4)(MeOAn-6ANI-Ph(NN*)-(3)NI), in which NN* is strongly spin-polarized. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, net spin polarization appears on NN* with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NN* and the local triplet state (3)NI, which is populated following charge recombination. This requires an effective switch in the spin basis set between the triradical and the three-spin charge recombination product having both NN* and (3)NI present.     

Peculiarities of magnetic and spin effects in a biradical/stable radical complex (three-spin system). Theory and comparison with experiment

The field dependencies of biradical recombination probability in the presence of paramagnetic species with spins S(3) = 1 and S(3) = (1)/(2) have been calculated in the framework of the density matrix formalism. To describe the effect of the "third" spin on the spin evolution in biradical, we have also considered the spin exchange interaction between the added spin and one of the paramagnetic biradical centers. A characteristic feature of the calculated field dependencies is the existence of several extrema with positions and magnitudes depending on the signs and values of the exchange integrals in the system. The method proposed can be used to describe the effect of spin catalysis. It is shown that for the system with the third spin S(3) = 1 spin catalysis manifests itself stronger than in the case of spin S(3) = (1)/(2). The dependence of spin catalysis efficiency on the exchange interaction with the third spin has an extremum with position independent of the value of the spin added.     

Influence of the Coordinated Transition Metal Ion on Magnetic Relaxation of Lanthanide Based Complexes with Imino Nitroxide Biradical Ligands

In spite of achievement of a lot of Ln-radical SMMs, how to improve magnetic behavior of Ln-radical system remains challenging. Here, two series of Ln-radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln₂(hfac)₆(ImPhPyobis)₂] (Ln^III=Gd 1 , Tb 2 , Dy 3 ) and [Ln₂Cu₂(hfac)₁₀(ImPhPyobis)₂] (Ln^III=Gd 4 , Dy 5 ; hfac=hexafluoroacetylacetonate and ImPhPyobis=5-(4-oxypyridinium-1-yl)-1,3-bis(1’-oxyl-4’,4’,5’,5’-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). For these biradical-metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4-oxypyridinium units to produce a binuclear {Ln₂O₂} unit. Those imino nitroxide groups are free for complexes 1 – 3 , however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5 . The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(U_eff) of 74.0 K while complex 5 exhibits dual relaxation processes with U_eff values for the fast- and slow-relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation.     

Experimental and theoretical studies on ferromagnetically coupled metal complexes with imino nitroxides

Copper(II), zinc(II), and nickel(II) complexes with tridentate imino nitroxyl diradicals, [CuCl(bisimpy)(MeOH)](PF(6)) (1), [ZnCl(2)(bisimpy)] (2), and [NiCl(bisimpy)(H(2)O)(2)]Cl x 2H(2)O (3) (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazol-2'-yl)pyridine), were prepared, and their magnetic properties were studied. In 1, the Cu(II) ion has a square pyramidal coordination geometry, of which the equatorial coordination sites are occupied by three nitrogen atoms from the bisimpy and a chloride ion. The coordination geometry of the Zn(II) ion in 2 can be described as a trigonal bipyramid, with two chloride ions and a bisimpy. In 3, the Ni(II) ion has a distorted octahedral coordination geometry, of which four coordination sites are coordinated by the bisimpy and chloride ion, and two water molecules occupy the remaining cis positions. Magnetic susceptibility and EPR measurements revealed that in 1 and 3 the Cu(II) and Ni(II) ions with imino nitroxyl diradicals were ferromagnetically coupled, with the coupling constants J (H = -2J(ij) summation operator S(i)S(j)) of +165(1) and 109(2) cm(-1), respectively, and the intraligand ferromagnetic interactions in 1-3 were very weak. DFT molecular orbital calculations were performed on the diradical ligand, 1, and 2 to study the spin density distribution before and after coordination to the metal ions.     

两个氮氧双自由基桥连的稀土一维链的晶体结构、磁性和荧光性能

选用1,2-二苯氧基乙烷取代的氮氧双自由基(BNPhOEt)与稀土金属反应,得到了2例氮氧双自由基-稀土配合物[Ln(hfac)3(BNPhOEt)]·C6H14(Ln=Tb(1)、Ho(2);hfac=六氟乙酰丙酮),其均为2p-4f一维链状结构.磁性研究表明,在配合物1和2中分别存在铁磁和反铁磁耦合.此外,对2个配...     

A structural and Mössbauer study of complexes with Fe(2)(micro-O(H))(2) cores: stepwise oxidation from Fe(II)(micro-OH)(2)Fe(II) through Fe(II)(micro-OH)(2)Fe(III) to Fe(III)(micro-O)(micro-OH)Fe(III)

Dinuclear non-heme iron clusters containing oxo, hydroxo, or carboxylato bridges are found in a number of enzymes involved in O(2) metabolism such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases. Efforts to model structural and/or functional features of the protein-bound clusters have prompted the preparation and study of complexes that contain Fe(micro-O(H))(2)Fe cores. Here we report the structures and spectroscopic properties of a family of diiron complexes with the same tetradentate N4 ligand in one ligand topology, namely [(alpha-BPMCN)(2)Fe(II)(2)(micro-OH)(2)](CF(3)SO(3))(2) (1), [(alpha-BPMCN)(2)Fe(II)Fe(III)(micro-OH)(2)](CF(3)SO(3))(3) (2), and [(alpha-BPMCN)(2)Fe(III)(2)(micro-O)(micro-OH)](CF(3)SO(3))(3) (3) (BPMCN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane). Stepwise one-electron oxidations of 1 to 2 and then to 3 demonstrate the versatility of the Fe(micro-O(H))(2)Fe diamond core to support a number of oxidation states with little structural rearrangement. Insight into the electronic structure of 1, 2', and 3 has been obtained from a detailed M?ssbauer investigation (2' differs from 2 in having a different complement of counterions). Mixed-valence complex 2' is ferromagnetically coupled, with J = -15 +/- 5 cm(-)(1) (H = JS(1).S(2)). For the S = (9)/(2) ground multiplet we have determined the zero-field splitting parameter, D(9/2) = -1.5 +/- 0.1 cm(-)(1), and the hyperfine parameters of the ferric and ferrous sites. For T < 12 K, the S = (9)/(2) multiplet has uncommon relaxation behavior. Thus, M(S) = -(9)/(2) <--> M(S) = +(9)/(2) ground state transition is slow while deltaM(S) = +/-1 transitions between equally signed M(S) levels are fast on the time scale of M?ssbauer spectroscopy. Below 100 K, complex 2' is trapped in the Fe(1)(III)Fe(2)(II) ground state; above this temperature, it exhibits thermally assisted electron hopping into the state Fe(1)(II)Fe(2)(III). The temperature dependence of the isomer shifts was corrected for second-order Doppler shift, obtained from the study of diferrous 1. The resultant true shifts were analyzed in a two-state hopping model. The diferric complex 3 is antiferromagnetically coupled with J = 90 +/- 15 cm(-)(1), estimated from a variable-temperature M?ssbauer analysis.     

Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)?J(OVZ).