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Hemoproteins serve many diverse biological functions through the nearly identical heme prosthetic group. The elucidation of the electronic structures of synthetic hemes is quite important in understanding the function and catalytic processes of naturally occurring heme proteins. For example, an important step in cytochrome P450 catalysis has been the spin-state crossover upon substrate binding which thereby facilitates the reduction of the iron centre. Iron(III) porphyrins having intermediate-spin state have attained much of attention since Maltempo’s discovery of its crucial contributions in several bacterial heme proteins such as Cytochromes c′. We have analyzed here various factors responsible for stabilizing such intermediate-spin states of iron using the same iron(III)porphyrinato platform. We report the synthesis, X-ray structure and spectroscopic characterizations of saddle distorted six-coordinated iron(III) porphyrins having weak axial coordination such as THF and dioxane. Through combined solid as well as solution phase studies, it has been found that the complexes demonstrate iron(III) in pure intermediate-spin state. A brief comprehensive personal account for stabilizing intermediate-spin state of iron using the same iron(III) porphyrins has also been presented. Our work highlights the possible spin state switching of iron through external perturbations such as H-bonding interactions, weak axial coordination etc which are indeed important steps in the functioning of various heme proteins.

This study explored the thermochromic behavior of [(μ2-6-methyl-2-pyridinemethanolate)2{Cu(6-methyl-2-pyridinemethanol)}2](PF6)2. The complexes show no significant variation between 296 K and 100 K according to a single-crystal X-ray crystallographic investigation. The occupancy of the triplet at 300 K and 100 K is 55% and 16.9%, respectively based on magnetic susceptibility. Photographs of the complex at different temperatures and quantum calculation exhibit that the thermochromic behavior is induced by the alteration of spin state between triplet and singlet.

Manganese spin crossover (SCO) complexes form a small but ever expanding family of compounds with thermally accessible states of different electronic configuration and total spin. Accurate prediction of spin-state energetics is essential for the theoretical description of these systems. However, this represents a challenging problem that necessitates recourse to correlated wave function methods rather than the more approximate density functional theory (DFT). Here we present a detailed study of spin-state energetics for eight Mn(III) and Mn(II) SCO complexes using the domain-based local pair natural orbital approach to coupled cluster theory with singles, doubles, and perturbative triples, DLPNO-CCSD(T). The effects of reference determinants, basis set, triples excitations, and pair natural orbitals (PNO) thresholds are evaluated and analysed in detail, enabling us to propose a robust and efficient computational protocol based on a combined and balanced mix of extrapolation to the complete basis set and infinite PNO space limits. The results are subsequently used to evaluate multireference wavefunction-based (CASSCF/NEVPT2) and DFT approaches, highlighting their inability to provide a balanced description of spin-state energetics for these complexes. The DLPNO-CCSD(T) protocol proposed in this study can serve as a generally applicable reference-quality quantum chemical method for studying spin crossover systems.

Complex assembly preferences and resultant spin state choice and Jahn-Teller distortion mode in twelve Mn(III) complexes of varying nuclearity with Schiff base chelate ligands of different sizes are compared. Mononuclear complexes Mn(5-F-sal2323)]OTf (1), [Mn(5-F-sal2323)]ClO4 (2) and [Mn(5-F-sal2323)]NO3, (3) with the 8-carbon R-Sal2323 chelate type show rare axial compression at room temperature and complexes (2) and (3) also show reversible thermal spin state switching from spin quintet to triplet states on cooling. The prevalence of atypical axial compression is further probed by single crystal diffraction studies on high spin mononuclear complexes (4)-(7) with the longer 9-carbon R-Sal2333 chelate series using [Mn(3-OEt-sal2333)]BF4, (4) [Mn(3,5-diOMe-sal2333)]BF4·0.5PrOH, (5), [Mn(5-F-sal2333)]BPh4, (6) and Mn(5-NO2-sal2333)]BPh4, (7) as examples. Ring closure of both the 8-carbon R-Sal2323 and longer 9-carbon R-Sal2333 ligands was also observed yielding a ligand with a 6-membered ring in the mononuclear complex [Mn(3-OMe,5-NO2-sal23336R)·0.07MeOH·0.93MeCN (8) in contrast to a protonated 5-membered ring in the dimeric complex [Mn2(5-Cl-sal2323H5R)2](ClO4)2·1.85EtOH·0.33MeCN, (9) with the R-Sal2323 chelate type. Finally comparison of the assembly modes of Mn(III) complexes of the 8-carbon R-Sal2323 and 9-carbon R-Sal2333 series with those of the shorter 7-carbon R-Sal2322 ligand series reveal that the mononuclear assembly mode is not favoured with the smaller chelate. Instead three dimeric or cluster complexes with rearranged ligands were recovered in low yield. The structures of these complexes [Mn2(5-NO2-sal23225R)2]·2BuOH, (10), [Mn2(3,5-diBr-sal23225R)(3,5-diBr-sal)3]·0.7EtOH·2.61MeCN, (11) and [Mn2Na(3-NO2,5-OMe-sal23225R)]ClO4·0.5MeCN (12) are reported and compared with examples with the longer chain chelate ligands.

Based upon recent work on the Mn 3 monomer and the Mn(taa) there is an expectation that Mn III centers can exist in local spin states with S = 2 (3 d ↑ ↑ ↑ ↑ ) or S = 1 (3 d ↑ ↑ ↑ ↓ ). While the high-spin states are usually more stable based on Hunds first rule, the Mn 3 monomer, the Mn(taa) and [ Mn 6 III Mn III ] 3 + triplesalen provide examples where the presence of S = 1 and S = 0 low-spin states is energetically favorable. In this paper we examine the possibility that such S = 1 centers, referred to as Spin-Flip Excitons (SFE) can exist in the Mn 12 O 12 ( COOR ) 16 [ H 2 O ] 2 molecule ( Mn 12 -Ac). Our results show that up to eight of these excitons can exist as metastable electronic states. We examine the relative stability per exciton as a function of the number of these excitons and determine spin-crossover paths between the Mn 12 -Ac zero-exciton ground state (S = 10), a four-exciton state (S = 6) and an eight-exciton state (S = 2). The magnetic anisotropy of these states decreases monotonically and significantly with the number of excitons.

Oximato bridged {MnIII6} clusters are among the best known SMMs for transition metal clusters that exhibit a very large barrier height for magnetization reversal and decent blocking temperatures. Several attempts were made to improve the magnetic properties of this family, among others, employing dicyano-metallo linkers such as [Au/Ag(CN)2]- to link two {MnIII6} clusters gained attention as this paves the way forward to construct supramolecular architecture based on {MnIII6} units and also facilitate a way to control the intercluster exchange coupling – a parameter vital to Qubit application. In this work, we have employed DFT calculations to estimate various exchange coupling constants within the {MnIII6} clusters ([MnIII6(μ3-O)2(Et-sao)6(3-py)2(MeOH)2 (M(CN)2)2] M = Au (1), Ag (2) here sao2- is the dianion of salicylaldoxime or 2-hydroxybenzaldeyhyde oxime). We have identified four different Js within the cluster based on the orientation of the Jahn-Teller axes and the Mn N O M dihedral angles. The computed J values within the clusters are in the borderline of ferro and antiferromagnetic coupling, leading to a competing interaction within the cluster, and for this reason, several spin states are found to lie close to each other. The computed Js reproduce the experimental susceptibility data and yield very small/negligible {MnIII6-MI-MnIII6} intercluster interaction. The reason for such weak couplings was analysed, and a search for stronger exchange coupling was undertaken by studying in detail the J values in [{Mn(salen)(EtOH)}3{M(CN)6}] (here M = FeIII (3), CrIII (4); Salen = N,N′-bis(salicylidene)ethylenediamine) clusters. This predictive strategy suggests that incorporating a metal ion such as CrIII could be a game-changer as this promotes a very strong ferromagnetic coupling between two MnIII centers. If this prediction is attempted for oximato bridged {MnIII6} clusters, it could be very rewarding.

The iron mixed-valence complex ( n- C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ] (dto = C 2 O 2 S 2 ) exhibits a novel type of phase transition according electron transfer between Fe II and Fe III called charge transfer phase transition (CTPT). The existence of the CTPT highly depends on the 3d metal substitution with Zn, Mn, Cr, or Co for corresponding Fe site in ( n- C 3 H 7 ) 4 N[Fe II Fe III (dto) 3 ]. Here, we prepared a series of magnetically diluted complexes ( n- C 3 H 7 ) 4 N[Fe II Fe III 1− x V III x (dto) 3 ] ( x = 0–1), and carried out investigation of magnetic properties. Judging from the magnetic susceptibility measurement, there is no suppression of the CTPT unlike other 3d metal substituted cases. Moreover, 57 Fe Mossbauer spectroscopy revealed that about 60% of Fe II sites changed their spin states from high spin to low spin in case of substitution rate for x = 1.00. Since the difference of hardness and softness between metallic ions (Fe II , Fe III and V III ) and coordination sites of ligand (O or S in dto), this behavior is explained by the linkage isomerization of bridging ligand in formation process of the complex.

An interesting series of four complexes of acenaphthoquinone 3-(4-benzylpiperidyl) thiosemicarbazones (HL), with a general formula [Co II L 2 ]( 1 ), [Ni II L 2 ]( 2 ), [Cu II L 2 ]( 3 ), and [Zn II L 2 ]( 4 ), were prepared. The complexes were characterized by means of elemental analysis, IR, UV–Vis spectroscopy and ESI-MS methods. In all the complexes the ligand (HL) coordinate as a monobasic ligand in tridentate fashion. The crystal structure of the compounds 1 and 2 were determined by single crystal X-ray diffraction experiment. The geometry and bonding parameters of the compound 3 were determined from EPR spectra. The g values of 3 , calculated from EPR at 77 K, indicate a rhombic coordination geometry about copper(II). The room temperature magnetic moment of the cobalt(II) complex 1 , indicate that it is in high spin state, while the nickel(II) complex, 2 , is paramagnetic. The redox behavior of the ligand and its complexes were studied by cyclic voltammetry. The density functional theory (DFT) was used for geometry optimization, determination of HOMO and LUMO energies, HOMO–LUMO energy gap and dipole moment of the compounds. Antibacterial activity of the ligand and complexes 1 – 4 , have been screened against two gram negative bacteria Escherichia coli , Pseudomonas aeruginosa and two gram positive bacteria Staphylococcus aureus and Bacillus subtilis. The electron micrograph images display damage to microbial cell wall upon treatment with the compounds.

In this work, complexes of europium (III) based on β-naphthol and phenol were synthesized and investigated by the methods of Mossbauer spectroscopy, NMR spectroscopy, thermogravimetry, mass spectroscopy. It was found that the structure of the studied complexes determines the spin state of the central ion. The β-naphthol-based complex exhibits diamagnetic properties (S = 0), and the complex, where the ligand is phenol-based azomethine, is paramagnetic (S = 3). It was shown that the studied complexes exhibit a broad spectral response upon excitation in the ultraviolet part of the spectrum.

In this work, in order to determine the solvent and spin state effects on the molecular structure of deferiprone–ferric complex ([Fe(DFP)3]), density functional theory (DFT) calculations were performed at the CAM-B3LYP/6-31G(d) level of the theory. The results indicate that the stability and binding energy of [Fe(DFP)3] in the polar solvents are higher and lower, respectively, than non-polar ones. The analysis of the spin state energies of the complexes showed that the corresponding stability constants increase with the spin states increment. The binding energies in high-spin state is higher than intermediate- and low-spin states which showed that the complex formation in a high-spin state is more favorable. Furthermore, the evaluation of the IR spectrum in the various solvents reveals that the intermolecular hydrogen bond formation between the oxygen atom of the carbonyl group and the hydrogen atom of the solvent causes a spectral red shift. Because of the importance of the charge transfer in the complex formation, donor–acceptor interaction energies were evaluated. Based on this analysis, lone pair electrons of the oxygen atom interact with the antibonding orbitals of the iron, more than the other possibilities. Finally, some probable correlations between the quantum chemical reactivity indices of the complexes and solvent polarity were considered. According to the obtained results, an enhancement in the dielectric constant of the solvent decreases the electrophilicity index while the electronic chemical hardness increases. Recent study indicates a linear correlation between chemical hardness and binding energies of [Fe(DFP)3] complex.

Dinuclear cobalt(II) complexes with II–II oxidation state of 20-membered N4, binucleating macrocycle derived by a cyclic [2 + 2] condensation of isophthaldialdehyde with 1,3-diaminopropane have been synthesised and characterised by analytical, spectroscopic and X-ray diffraction methods. The solid and solution states investigations show that overall structures and geometries of the complexes are very similar. The molecular structure of [Co2L(OMe)(NCS)3], which has been determined by single crystal X-ray diffraction, indicate that the cobalt(II) ions have a distorted trigonal bipyramidal environment, with methoxy bridge in an equatorial positions and N-bound thiocyanate ion in axial positions to metals. Also, the structure of [Co2L(OMe)(NCS)3] complex was optimized using density functional theory for all of the possible spin states of the complex. The results indicated that septet spin state is the most stable form of the complex.

A series of dihalide M(II) mononuclear complexes in a cis configuration, Mn(HL1)2Cl2·CH3OH·H2O (1), Co(HL1)2Cl2·CH3OH·H2O (2), Ni(HL1)2Cl2·2H2O (3) and CoL2Cl2·0.5dmf (4) (HL1 – Schiff base ligand 4-N-(2′-pyridylimine)benzoic acid; L2 – pyridin-2-yl(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-1-yl) methanone, both formed in situ from 2-pyridinecarbaldehyde), have been obtained as single crystals and characterized by elemental analysis, IR and EPR spectroscopy, and single-crystal X-ray diffraction techniques. The neutral M(L1)2Cl2 and CoL2Cl2 molecules in 1–3 and 4, respectively, show no crystallographically imposed symmetry with the metal atoms in a MN4Cl2 distorted octahedral (1–3) and CoN3Cl2 (4) elongated trigonal–bipyramidal geometries. In the crystal lattice molecules of 1–3 are arranged in columns propagating along the c axis; the structure of 4 can be described as layers of CoL2Cl2 and dmf molecules alternating along the a axis. The minimum M⋯M separations in a column (1–3) and inside a layer (4) are above 8 A. The X-band solid-state EPR spectra of 1 at room temperature and 77 K exhibit broad fine structure signals at 1600 and 3000 G (geff ∼ 4.14 and 2.22, respectively) resulting from ZFS of the spin states for the high-spin d5 metal ion. The solid-state EPR spectra of 2 are temperature dependent showing a wide unresolved line at geff ∼ 2.09 at room temperature and a very broad signal (ΔHpp = 2500 G) at geff ∼ 4.21 at 77 K. The Ni analogue 3 is EPR-silent. The EPR spectra of 4 at both temperatures show a broad asymmetrical signal centred at g ∼ 2 suggesting the low-spin configuration; the line width is nearly independent of temperature.

A new trinuclear complex [(CuLα-Me)2Co(bnz)2] (1) has been synthesized by using a metalloligand [CuLα-Me] (H2Lα-Me = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine, bnz = benzoate) with trans-coordinated syn–syn bridging benzoate group (1κO:2κO′). The syn–syn coordinative selectivity of carboxylate towards this trinuclear unit leads exclusively to the formation of linear coordination cluster. Such coordinative adaptability is exploited for supramolecular assembly using a dicarboxylate linker, terephthalate (tph, 1,4-benzenedicarboxylate) which yielded a tailored one-dimensional quasi-linear coordination polymer [(CuLα-Me)2Co(tph)]n·2nH2O (2) having the linear trinuclear node. Isothermal magnetization measurement at 2 K suggests that both 1 and 2 posses S = 1/2 ground spin state indicating the presence of antiferromagnetic coupling at low temperature. The variable-temperature magnetic susceptibility measurements also reveal that both compounds are antiferromagnetically coupled with exchange coupling constants (J) of −17.3 and −9.2 cm−1 for 1 and 2, respectively. The nature and magnitude of exchange interactions are further corroborated by density functional calculations.

The carboxylate substitution reaction between the reported all-benzoate complex [Ce3Mn8O8(PhCO2)18(PhCO2H)2] (1) and 2-Cl-benzoic acid has led to isolation of [Ce3Mn8O8(2-Cl-PhCO2)15.3(PhCO2)2.7(H2O)2] (2). The structures of 1 and 2 are very similar, both containing a tetragonally-elongated MnIII8 cuboid with a CeIV at its center and two CeIII attached to opposite faces, but 2 shows extensive disorder in the ligands and the Cl atom positions and occupancies. Variable-temperature (T) solid-state dc and ac magnetic susceptibility data for 2 are significantly different from those for 1 and are assigned as due to small ligand-induced structural perturbation of the core that has brought an A-type antiferromagnetic (AF) excited state much closer to the C-type AF ground state. Fits of the dc χMT vs T data support this interpretation but due to the ligand disorder in 2, a more definitive description of the lowest energy spin states of 2 is not possible. Nevertheless, these results provide important support for the feasibility of ground state spin switching by ligand-induced core perturbation (‘chemical pressure’) in this family, and they encourage further studies.

Bis(borole)nickel derivatives (η5-C4H4BR)2Ni, isoelectronic with the corresponding ferrocenes (η5-C5H4R)2Fe, have been synthesized and characterized structurally by X-ray crystallography as sandwich compounds with parallel borole rings. The geometries, energetics, and preferred spin states of the complete series of bis(methylborole)metal sandwich compounds (C4H4BCH3)2M of the first row transition metals (M = Ti to Ni) have now been investigated by density functional theory. The late transition metal derivatives (η5-C4H4BCH3)2M energetically prefer metallocene-like sandwich structures having parallel pentahapto borole rings with singlet, doublet, triplet, quartet, and quintet spin states for M = Ni, Co, Fe, Mn, and Cr, respectively. However, the early transition metal (η5-C4H4BCH3)2M derivatives energetically prefer bent sandwich structures having ∼30° dihedral angles between the pentahapto borole rings with singlet, doublet, and triplet spin states for M = Ti, V, and Cr, respectively. For (η5-C4H4BCH3)2Cr the closeness in energy (within ∼1 kcal/mol) of the parallel quintet and bent triplet structures suggests interesting magnetic properties. The titanium and vanadium derivatives are predicted to form marginally stable [(C4H4BCH3)2M]2 dimers (M = Ti, V) with the two halves linked by a metal–metal bond as well as two bridging borole carbon atoms through agostic hydrogen atoms.

Cyclooctatetraene has been found experimentally to form the perpendicular binuclear sandwich compounds (η3,η3-C8H8)2Ni2 and [η5,5-(Me3Si)2C8H6]2Ti2. The entire series of such cyclooctatetraene sandwich compounds (C8H8)2M2 has now been examined by density functional theory. Bis(pentahapto) folded η5,η5-C8H8 rings are found in the lowest energy (C8H8)2M2 derivatives of the early transition metals Ti, V, Cr, and Mn. The lowest energy Ti derivative is the singlet spin state structure (η5,η5-C8H8)2Ti2 containing two such rings with geometry close to the experimental structure for the known silylated derivative [η5,5-(Me3Si)2C8H6]2Ti2. The lowest energy (C8H8)2M2 structures for V, Cr, and Mn are (η5,η5-C8H8)M2(η3,η3-C8H8) having metal–metal multiple bonds of various orders. For (η5,η5-C8H8)V2(η3,η3-C8H8) singlet and triplet spin state structures are approximately equal in energy. For (η5,η5-C8H8)M2(η3,η3-C8H8) (M = Cr, Mn) the triplet state structures are the lowest energy structures. The lowest energy (C8H8)2M2 structures for the later transition metals Fe, Co, Ni are triplet spin state structures for Fe and Co and a singlet spin state structure for Ni. All three such (η3,η3-C8H8)2M2 (M = Fe, Co, Ni) structures have two bis(trihapto) η3,η3-C8H8 rings containing uncomplexed C C double bonds.

A series of bis(triazole)-Fe(II) coordination polymers, {[Fe(NCS)2(btm)2]·H2O}n (1), {[Fe(bte)2(NCS)2][Fe(bte)(H2O)2(NCS)2]}n (2), [Fe(NCS)2(btp)2]n (3), {[Fe(NCS)2(btb)2]a[Fe(NCS)2(btb)2]b}n (4) and [Fe(NCS)2(bth)2]n (5) (btm = bis-(1,2,4-triazol-1-yl)methane, bte = 1,2-bis-(1,2,4-triazol-1-yl)ethane, btp = 1,3-bis-(1,2,4-triazol-1-yl)propane, btb = 1,4-bis(1,2,4-triazol-1-yl)butane, bth = 1,6-bis(1,2,4-triazol-1-yl)hexane), have been synthesized and structural characterized by X-ray single-crystal diffraction. Compounds 1–3 are 1D chains, complex 4 is a mixture of 2D networks and 1D chains, while complex 5 has 2-fold interpenetrated frameworks of [Fe(NCS)2(bth)2]n 2D planes. The iron(II) ions in complexes 1–5 all remain in the high spin state down to 2 K and the experimental data were fitted by using the single ion approximation for Fe(II) ion with zero-field splitting of ground-state quintet.

Synthesis, structural, magnetic, Mossbauer and thermal studies of isothiocyanate iron (II) complexes with substituted pyrazines (iodo-, bromo- and amino-derivatives) are discussed here. Complexes with iodo- and bromo-derivatives were found to have the composition [Fe(Ipz)2(SCN)2(H2O)2]·2Ipz (1) and [Fe(Brpz)2(SCN)2(H2O)2]·2Brpz (2), whereas in the case of amino-functionalized pyrazine the formation of [Fe(NH2pz)4(SCN)2] (3) was observed. 3D organization of the molecular complexes is stabilized within different hydrogen, halogen and lone pair–π interactions. Spin state of iron (II) ions in 1–3 was determined as high spin by Mossbauer and magnetic measurements. DFT calculations for the obtained iron (II) complexes were carried out to analyze the electronic structure and to support experimentally measured Mossbauer parameters with theoretical predictions.

Reported herein are the magnetic and structural properties as well as solution equilibrium investigation for a pair of iron (II) pyridyl-imidazoline complexes: [Fe(pizMe)Cl2]2 (1) and [Fe(pizMe)3](FeCl4) (2), where pizMe = 2-(2′-pyridinyl)-4,5-dihydro-1-methylimidazole. Dinuclear complex 1 contains high-spin iron (II) centers and a “flat” pizMe ligand (planar at the imidazoline N atom), while the pizMe-containing complex in 2 exhibits characteristics of a low-spin iron (II) center and a pyramidalized pizMe ligand. In solution, the two complexes are present in equilibrium as evinced through NMR and electronic absorption spectral studies. Spin-state switching between 1 and 2 in methanolic solution is a result of ligand lability.

2,6-Di(1-methyl-pyrazol-3-yl)pyridine (L), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (L), 2,6-di(1-benzyl-pyrazol- 3-yl)pyridine (L) and di(1-isopropyl-pyrazol-3-yl)pyridine (L ) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(L)] complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(L)]X (X = BF , ClO and PF ), [Fe(L)][BF ], [Fe(L)][BF] and [Fe(L)][PF ] have been determined at 150 K. All of these contain high-spin iron centres except [Fe(L)][BF ]·xHO, which is predominantly low-spin at that temperature. All the complexes are high-spin between 5 and 300 K as solvent-free bulk powders, and are also high-spin in (CD) CO solution between 193 and 293 K. This was unexpected, since the parent complex [Fe(3-bpp)] undergoes spin-crossover in the same solvent with T = 247 K [40]. The high-spin nature of the [Fe(L )] complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand 'R' substituents.

A novel heterotetranuclear chromium(III)–copper(II) complex of formula {[CrIII(bpy)(ox)2]2CuII2(bpy)2(ox)}·6H2O (1) has been synthesized by the ligand exchange reaction between Ph4P[CrIII(bpy)(ox)2]·H2O and [CuII(bpy)2(NO3)]NO3·MeOH in methanol (bpy = 2,2′-bipyridine; ox2− = oxalate dianion). The X-ray crystal structure of 1 consists of neutral oxalato-bridged CrIII2Cu2II zigzag entities which are formed by the monodentate coordination of two [CrIII(bpy)(ox)2]− mononuclear anionic units through one of its two oxalato groups toward a [CuII2(bpy)2(ox)]2+ dinuclear cationic moiety featuring relatively long axial bonds at the square pyramidal CuII ions. Variable temperature (2.0–300 K) magnetic susceptibility and variable-field (0–5.0 T) magnetization measurements for 1 have been interpreted on the basis of the linear tetranuclear topology of the CrIII2CuII2 entities [H = –J(S1 · S2 + S3 · S4) – J′ S2 · S3 with S1 = S4 = SCr = 3/2 and S2 = S3 = SCu = 1/2]. A weak antiferromagnetic coupling occurs between the outer CrIII and inner CuII ions through the peripheral bidentate/monodentate(outer) oxalates (J = –8.7 cm−1), while a strong antiferromagnetic coupling is operative between the inner CuII ions across the central bis-bidentate oxalate (J′ = –472.2 cm−1), leading thus to a S = (SCr – SCu) – (SCr – SCu) = 0 ground spin state for the CrIII2CuII2 entity of 1. A simple orbital analysis of the electron exchange interaction in the oxalato-bridged CuII2 and CrIIICuII fragments identify the σ-type pathways involving the d x 2 - y 2 (Cu)/ d x 2 - y 2 (Cu) and dxy(Cr)/ d x 2 - y 2 (Cu) pairs of magnetic orbitals as the two main contributions responsible for the different strength of the intramolecular magnetic coupling parameters for 1. A magneto-structural correlation between the nature and magnitude of the magnetic coupling and the bending angle at the axial carbonyl-oxygen to copper bond has been established for 1 and related oxalato-bridged chromium(III)–copper(II) dinuclear complexes.

The complex [ FeL 2 1 ](ClO4)2·H2O (1a) (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine) was synthesized by the reaction of Fe(ClO4)2·6H2O with new NNN-tridentate pyrazolylpyrimidine ligand L1 in MeOH/EtOH mixture. It is the first example of iron(II) spin crossover complexes with 2,4(6)-di(1H-pyrazol-1-yl)pyrimidines and manifests gradual spin transition centered around 150 K on the χMT versus T curve without hysteresis. A broad heat capacity anomaly with the maximum at 150 K was observed on the Cp(T) dependence, ΔtrsH = 5.50 kJ/mol and ΔtrsS = 42.0 J/(mol K). Surprisingly, the syntheses of iron(II) perchlorate compound with L1 performed in EtOH and iPrOH afford another modification of the complex, [ FeL 2 1 ](ClO4)2·H2O (1b), being high spin in the 78–300 K temperature range. The complexes with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L2), [ FeL 2 2 ](ClO4)2·EtOH (2), [ FeL 2 2 ](CF3SO3)2 (3), [FeL2Cl2] 0.5H2O (4) and [FeL2(NCS)2] (5) were synthesized in EtOH solutions. They are in the high spin state within 78–300 K temperature range.

[(CoLCl)CoCl3], [(CoMeLCl)CoCl3], [(CoLBr)CoBr3], [CoLBr]+ and [CoMeLBr2] complexes, L = hexakis(2-pyridyloxy)cyclotriphosphazene, MeL = hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene, in the most stable high spin states are investigated at DFT level of theory using hybrid B3LYP functional. The exchange coupling parameter evaluated using a broken symmetry treatment increases with the ligands mass. Electron density is evaluated in terms of QTAIM (Quantum Theory of Atoms-in-Molecule) topological analysis of electron density. The bonds of central Co atoms with phosphazene nitrogens are shorter, stronger and more polar than with the aromatic pyridine nitrogens and their higher ellipticities may be explained by the π contribution from the phosphazene ring. The atomic charges of phosphazene nitrogens are ca. twice more negative than at the pyridine ones.

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy) 4 (X) 2 ] (stpy = styrylpyridine, X = NCS, NCBH 3 ) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe( trans -stpy) 4 (NCS) 2 ], while [Fe( cis -stpy) 4 (NCS) 2 ] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH 3 , due to the stronger ligand field of NCBH 3 , the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe( trans -stpy) 4 (NCBH 3 ) 2 ]. In the case of [Fe( cis -stpy) 4 (NCBH 3 ) 2 ], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65–1.09 GPa where [Fe(stpy) 4 (NCBH 3 ) 2 ] undergoes LD-LISC at room temperature.

A novel two-dimensional network bimetallic Fe Au spin crossover coordination polymer based on 3-phenylpyridine-coordinated iron centers and linear gold cyanide bridges {Fe(3-phenylpyridine)2[Au(CN)2]2}n (1), has been synthesized. The compound is characterized by elemental analysis, IR, single-crystal X-ray analysis at 300 and 90 K and magnetic measurements. The FeII ions in 1 have octahedral FeIIN6 coordination geometries, which are linked by [Au(CN)2]− units at the equatorial plane to form a polymeric 2D sheet architecture. The two pyridine rings coordinate in axial position. Variable-temperature (2–300 K) magnetic susceptibility measurements of 1 were performed to determine the spin transition behavior. SQUID data show that high and low spin states exist in a 1:1 ratio at 90 K. However, only one kind of FeII atom is apparent crystallographically at 90 K, indicating that the high and low spin sites are disordered in the polymeric 2D framework.

High-frequency Electron Paramagnetic Resonance (EPR) and Quantum Tunneling of Magnetization (QTM) measurements have been carried out on single-crystal samples of a mixed-valent Mn 2 II Mn 2 III single-molecule magnet (SMM). Frequency, temperature and field-oriented studies reveal an S = 9 spin ground state with low-lying excited spin states. Numerical simulations were performed via exact diagonalization of a multi-spin Hamiltonian, in which the anisotropy of the molecule is based on the anisotropy of the individual ions and their couplings. We found that the EPR and QTM spectra may adequately be explained with two s = 5/2 MnII ions weakly coupled to a rigid s = 4 spin, formed by the two ferromagnetically coupled MnIII ions.

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II)4O4 model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn4O4(OAc)6 and Mn3CaO4(OAc)6 using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV)4O4(OAc)6 although this model cluster consists of superexchange type of units, Mn(IV)2O2 that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O–O bond formation in the OEC.

The bzimpy iron(II) complexes, 1–3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T1/2↑ = 236 K to T1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.

Three new complexes [Ni(β-dkt)2(NH2-quin)] {β-dkt = 2,2,6,6-tetramethylheptane-3,3-dionate (tmhd) 1, hexafluoroacetylacetonate (hfac) 2, 1,3-diphenylpropanedionate (dbm) 3} have been prepared by reacting [Ni(β-dkt)2(H2O)2] with 8-aminoquinoline (NH2-quin). [Ni(tmhd)2(NH2-quin)] is found to be solvatochromic exhibiting a square planar geometry in CH2Cl2, acetone, and acetonitrile with the tmhd ligand acting unusually as a counteranion while in THF and DMSO the complex is octahedral. In contrast, 2 and 3 are octahedral in all solvents. Single crystal X-ray diffraction studies reveal octahedral nickel centres with a cis arrangement of the β-diketonates. The molecular packing consists of hydrogen bonded dimers which in the case of 2 and 3 are connected to one another via π···π and C–H⋯π interactions, respectively. Cyclic voltammetry shows 1 and 3 oxidise irreversibly at 0.46 and 1.17 V, respectively.

The structure of a series of [M(bdt)2]q complexes, M = Cu, Ni or Co, bdt = benzene-1,2-dithiolate, q = −1, −2 or −3 with up to two unpaired electrons, has been optimized at B3LYP/6-311G* level of theory as the 6-31G* basis incorrectly produces non-planar metallocycles. Square-planar 1[Cu(bdt)2]−, 2[Ni(bdt)2]− and 3[Co(bdt)2]− systems are the most stable ones in agreement with experimental data. The formal oxidation state of the central atom M defined using the total complex charge q differs from the real M oxidation state based on its d-electron population which is always between M(I) and M(II). The greatest deal of the electron structure changes during the reduction/oxidation is related to the bdt ligands, the strength of their ‘non-innocent’ character depends on M and on the spin state of the complex. These changes are not restricted to sulphur atoms only, including spin density distribution.

Four coordination polymers, {[Co2(4-DPSS)4(DMF)4](ClO4)4}n (1), {[Mn2(4-DPSS)4(DMF)4](ClO4)4}n (2), {[Co(4-DPSS)2(H2O)(NO3)]NO3·3H2O}n (3) and [Co(4-DPSS)2(NCS)2]n (4) have been synthesized to study the reactivity of cobalt(II) and manganese(II) salts (perchlorate, thiocyanate and nitrate) with the flexible ligand di(4-pyridyl)disulfide (4-DPSS). These compounds have repeated rhomboidal structures in which the di(4-pyridyl)disulfide ligands coordinate the metal centers in a double bridged fashion. Each double chain and the metallocycle cavities are chiral and contain either the M- or the P-enantiomeric forms of 4-DPSS and are independent on recrystallization solvents or the metallic precursors. The crystal packing of these coordination polymers reveals that, in all cases, weak interactions such as classical and non-classical hydrogen bonds, as well as S⋯S interactions, connect the polymeric chains to increase the dimensionality of the network. Low temperature electron paramagnetic resonance (EPR) measurements on powdered samples show that the transition-metal ions, Co (II) and Mn (II) ions, are in the high spin states S = 3/2 and 5/2, respectively, and adopt octahedral geometry in case of compounds 1, 2 and 3, and rhombic environment in case of 4.

The reaction of salicylaldoxime (H2salox) with Mn(ClO4)2 · 6H2O, NaN(CN)2 and NEt3 in MeOH affords a MnIII6 hexanuclear complex of [Mn6O2(salox)6(MeOH)6(NCNCONH2)2] (1), while reaction of H2salox with MnCl2 · 4H2O and NEt4OH in EtOH affords a MnIII6 hexanuclear complex of [Mn6O2(salox)6(EtOH)4(H2O)2Cl2] (2). Both complexes 1 and 2 contain a [MnIII6(μ3-O)2]14+ core, which is a known structural type in the family of Mn6 complexes. Variable temperature magnetic susceptibilities and magnetization measurement of complexes 1 and 2 have been carried out. Exchange interactions of metal centers for complexes 1 and 2 are fitted by a full diagonalization matrix method. The fitting results indicate that both complexes 1 and 2 have the ground-state spin value of S = 4, and the ground state of complex 1 has the much closer energy to low-lying spin states than that of complex 2. Magnetization measurements at 2.0–4.0 K and 10–70 kG confirm that the ground state is S = 4, with significant magnetoanisotropy as gauged by the D value of −0.82 cm−1 and −1.18 cm−1, for 1 and 2, respectively. The frequency dependence of the out-of-phase component in alternating current magnetic susceptibilities for both complexes 1 and 2 indicates the slow magnetic relaxation of superparamagnetic behaviour with a Ueff of 27.0(1) K and τ0 = 3.8(2) × 10−9 s for complex 1, and Ueff of 25.1(6) K and τ0 = 4.6(1) × 10−8 s for complex 2.

We performed quantum spin dynamics in anisotropic electron-spin systems to clarify the mechanism in single molecular magnets (SMM) theoretically. Usually, SMMs have large negative anisotropic D value and the large total spin angular moment (Stotal). The splitting ground spin states in one SMM molecule are usually caused by the anisotropic terms (D and E), which become more important as well as isotropic term (J). In order to examine the effects of such anisotropic magnetic terms, model systems with large spin multiplicity of the ground state (Stotal = 5 and 2) were treated. First, energy diagram in external magnetic field is shown, since negative D terms are responsible to the zero-field magnetic moment. Next, the quantum Liouvile approach is used for our spin dynamics simulation involving the relaxation processes of the system. Our simulations in Stotal = 5 system successfully reproduced magnetic quantum tunneling (MQT) behavior in DC-external magnetic field. Moreover, in Stotal = 2 system under AC-external magnetic field, frequency dependence of real (χ′) and imaginary (χ″) parts of χ can be shown with Fourier transformation. Our theoretically drawn half circles in Cole-Cole plot (χ′ versus χ″) will give information to many experimental studies.

A photo-induced phase transition in the spin crossover complex Fe(pyrazine)[Pt(CN) 4 ] has been triggered by a 4 ns green pulsed laser excitation within its thermal hysteresis loop. Complete spin conversion from the low spin to the high spin state has been repeatedly observed by irradiating a polycrystalline powder sample. In addition, the non-linear behaviour of the photoconversion efficiency and, in particular, the occurrence at ca. 5 mJ/cm 2 of an energy density threshold, below which no light-induced phase transition appears, is observed. This threshold is shown to be temperature dependent, which is confirmed by a simple theoretical description.

The reaction of [Mn(O2CMe)2] · 4H2O with pdH2 (1,3-propanediol) or mpdH2 (2-methyl-1,3-propanediol) in the presence of NaN3 in MeCN/py (py = pyridine) results in the formation of two new one-dimensional coordination polymers composed of a [MnIII 6MnII 11(μ4-O)8(μ3-N3)4]25+ octahedral unit. The peripheral ligation is completed by pd2- (or mpd2-), acetate, pyridine and μ-1,3-N3 - ligands. The latter bridges each Mn17 unit to its neighboring one, resulting in the formation of the two 1-D coordination polymers. Variable-temperature dc magnetic susceptibility studies indicate the existence of predominantly ferromagnetic interactions and a resulting giant ground state spin within the Mn17 units and intermolecular antiferromagnetic exchange interactions between the neighboring Mn17 units that result in diamagnetic ground spin states for both polymeric compounds. © 2008 Elsevier Ltd. All rights reserved. 28 9-10 1814 1817

In the case of iron mixed-valence complexes whose spin states are situated in the spin-crossover region, conjugated phenomena coupled with spin and charge are expected. In general, the Fe site coordinated by six S atoms is in the low-spin state, while the Fe site coordinated by six O atoms is in the high-spin state. From this viewpoint, we have synthesized and investigated physical properties for an monothiooxalato-bridged (mto = C 2 O 3 S) iron mixed-valence complex, ( n -C 4 H 9 ) 4 N[Fe II Fe III (mto) 3 ], consisting of Fe III O 3 S 3 and Fe II O 6 octahedra, which behaves as a ferrimagnet with its magnetic transition temperature of T N = 38 K and Weiss temperature of θ = −93 K. From the analysis of 57 Fe Mossbauer spectra of 57 Fe enriched complexes, ( n -C 4 H 9 ) 4 N[ 57 Fe II Fe III (mto) 3 ] and ( n -C 4 H 9 ) 4 N[Fe II57 Fe III (mto) 3 ], the charge transfer between Fe II and Fe III exists in the paramagnetic phase. Considering the time window of 57 Fe Mossbauer spectroscopy, the time scale of the valence fluctuation is at least slower than 10 −7 s. In order to confirm the valence fluctuation between Fe II and Fe III , we investigated the dielectric constant and found an anomalous enhancement attributed to the Fe valence fluctuation between 170 and 250 K.

The synthesis, structure, spectroscopic and electro-spectrochemical properties of sterically constrained Schiff-base ligands (LnH) (n = 1, 2, and 3) (L = N-[m-(methylmercapto)aniline]-3,5-di-t-butylsalicylaldimine, m = 4, 3, and 2 positions, respectively) and their copper(II) complexes [Cu(Ln)2] are described. Three new dissymmetric bidentate salicylaldimine ligands containing a donor set of ONNO were prepared by reaction of different primary amine with 3,5-di-t-butyl-2-hydroxybenzaldehyde (3,5-DTB). The copper(II) metal complexes of these ligands were synthesized by treating an methanolic solution of the appropriate ligand with an equimolar amount of Cu(Ac)2 · H2O. The ligands and their copper complexes were characterized by FT-IR, UV–Vis, 1H and 13C NMR and elemental analysis methods in addition to magnetic susceptibility, molar conductivity, and spectroelectrochemical techniques. Analytical data reveal that copper(II) metal complexes possess 1:2 metal–ligand ratios. On the basis of molar conductance, the copper(II) metal complexes could be formulated as [Cu(Ln)2] due to their non-electrolytic nature in dimethylforamide (DMF). The room temperature magnetic moments of [Cu(Ln)2] complexes are in the range of 1.82–1.90 B.M which are typical for mononuclear of Cu(II) compounds with a S = 1/2 spin state. The complexes did not indicate antiferromagnetic coupling of spin at this temperature. Electrochemical and thin-layer spectroelectrochemical studies of the ligands and complexes were comparatively studied in the same experimental conditions. The results revealed that all ligands displayed irreversible reduction processes and the cathodic peak potential values of (L3H) are shifted towards negative potential values compared to those of (L1H) and (L2H). It is attributed to the weak-electron-donating methyl sulfanyl group substituted on the ortho (m = 2) position of benzene ring. Additionally, all copper complexes showed one quasi-reversible one-electron reduction process in the scan rates of 0.025–0.50 V s−1, which are assigned to simple metal-based one-electron processes; [Cu(2+)(Ln)2] + e− → [Cu(1+)(Ln)2]−. The spectral changes corresponding to the ligands and complexes during the applied potential in a thin-layer cell confirmed the ligand and metal-based reduction processes, respectively.

The new ligand, 2,7-bis(α-pyrimidylamino)-1,8-naphthyridine (H2bpmany), was prepared by the reaction of 2,7-dichloro-1,8-naphthyridine with 2-aminopyrimidine in the presence of sodium tert-butoxide under palladium-catalyzed conditions. The linear hexanickel Ni 6 11 + complexes [Ni6(μ6-bpmany)4X2]Cl (X = Cl (1); X = NCS (2)) and their singly oxidized products [Ni6(μ6-bpmany)4X2](BF4)2 (X = Cl (3); X = NCS (4)) have been synthesized, and compounds 1, 2 and 4 have been crystallographically characterized. The crystal structures of the Ni 6 11 + complexes show remarkably short Ni–Ni distances (ca. 2.22 A), clearly indicative of partial metal–metal bonding in the mixed-valence Ni 2 3 + unit. This is also verified by the axial X-band EPR spectra of the complexes in solution. Magnetic susceptibility measurements reveal that the Ni 6 11 + complexes exhibit antiferromagnetic interactions (J = −47 cm−1) between the terminal Ni2+ ion and the central Ni 2 3 + unit, and the Ni 6 12 + complexes exhibit weak antiferromagnetic interactions (J = −5 cm−1) between the two terminal Ni2+ ions. The cyclic voltammograms display three reversible redox waves at E(1)1/2 = +0.87, E(2)1/2 = −0.02 and E(3)1/2 = −0.46 V for 1, and E(1)1/2 = +0.96, E(2)1/2 = −0.01, and E(3)1/2 = −0.41 V for 2. The relatively low potentials of E(2)1/2 suggest that the Ni 6 11 + complexes can be easily converted to their Ni 6 12 + forms.

A survey of mononuclear iron(II) complexes with heterocyclic N-donor ligation is presented. A brief introduction to spin-crossover chemistry and low-temperature spin-trapping is provided, since many of these compounds undergo thermal spin-transitions upon cooling or heating. These are highlighted, and the structural changes underlying spin-crossover are discussed where this is known. Materials showing spin-trapping behaviour following thermal quenching or irradiation at very low temperatures are also described.

In this work, 4-pyridyl-phenylanthracene-iminonitroxide radical 2 was synthesized, which can make the coordination to metal ions. It was confirmed by the time-resolved ESR experiments that 2 has a photo-excited quartet (S = 3/2) high-spin state. Cu(II)(hfac)2(2)2 and Mn(II)(hfac)2(2)2 were synthesized by using 2 as a ligand. Their magnetic properties on the ground states were analyzed by three-spincluster model S1 − SM − S2 (S1 = S2 = SM = 1/2 for Cu(II)(hfac)2(2)2 and S1 = S2 = 1/2, SM = 5/2 for Mn(II)(hfac)2(2)2). The exchange interactions (J/kB) between 2 and the metal ions were very weak (J/kBs were ferromagnetic for Cu(II)(hfac)2(2)2 and antiferromagnetic for Mn(II)(hfac)2(2)2). The molecular orbital calculations of 2 have suggested the strong interaction between the paramagnetic center of the metal ions and the photo-excited quartet high-spin state.

Two π-radicals, 3-pyridinyl-phenylanthracene(iminonitroxide) (3) and 3-pyridinyl-phenylanthracene-(nitronylnitroxide) (4) were designed as candidates of the ligand for the metal complexes to clarify the exchange interactions between the paramagnetic centers of the metal ions and the photo-excited high-spin states of the purely organic π-radical. Compounds 3 and 4 were synthesized and their magnetic properties were examined, showing weak antiferromagnetic interactions, θ = −1.5 K for 3 and −0.7 K for 4. The photo-excited states of 3 and 4 were investigated by time-resolved ESR and clarified that both π-radicals have the quartet (S = 3/2) high-spin states as their lowest photo-excited states. Two metal complexes [Fe(III)(L)(4)] · (BPh4) (Low spin) (LH2 = N,N′-bis(1-hydroxy-2-benzyliden)-1,7-diamino-4-azaheptane) and [Cu(II)(hfac)2(4)2] using 4 were prepared. Their magnetic behaviors are well analyzed with the Bleaney–Bowers model with J/kB = − 0.86 K and three S = 1/2 spin cluster model with J/kB = −1.0 K, respectively, showing weak antiferromagnetic interactions between the paramagnetic centers of the metal ions and the π-radical in the ground state.

Two types of organic open-shell polymer containing phenothiazine cation radicals as spin resources were synthesized and the magnetic behaviors were investigated. The one is the 1,3-phenylene-linked polymer, and the other is the arylamine nitrogen-linked polymer. Precursor polymers were prepared by the Suzuki-Miyaura coupling and the Pd-catalyzed amination. The formation of complexes between the phenothiazine moieties in precursor polymers and antimony(V) chloride gave stable open-shell polymers. The magnetic behavior of 1,3-phenylene-linked polymer was intramolecularly ferromagnetic, but intermolecularly antiferromagnetic. Whereas, the arylamine nitrogen-linked polymer was in the S = 1 spin state in spite of low spin concentration of 0.16 spins per repeating unit.

The electronic and spin structures of a [2Fe–2S] model complex: Fe 2 S 2 ( SH ) 4 3 - (1), were investigated by hybrid density functional theory (HDFT) calculations. Generalized spin orbital (GSO) approach has been examined to investigate the delocalization nature at the reduced form. The calculated results showed that the delocalization is largely dependent on the geometrical [2Fe–2S] core structure. At the symmetrical geometry, the GSO solution was a mixed-valence state: Fe2.5+Fe2.5+, with a noncollinear spin state. The energy difference between the GSO solution and an unrestricted state was extremely small (4 cm−1). On the other hand at the asymmetrical geometry, the GSO solution converged into a trapped valence state: Fe3+Fe2+ with a collinear spin state, which was almost the same as an unrestricted result. Natural orbital analysis was performed to elucidate the magnetic orbital interactions at the GSO and unrestricted solutions. These results indicated that the delocalization of the odd electron is largely suppressed for the strong antiferromagnetic interactions between two irons. In conclusion, the GSO descriptions were not drastically improved the ground state energies of the [2Fe–2S] cluster, however, the GSO approach has opened to investigate a variety of electronic structures in the mixed valence states, which could not be investigated by a broken symmetry (BS) approach.

Stable 1,3-phenylene linked polyradicals bearing nitroxides in the π-conjugated main and side chains were synthesized and the intramolecular spin interaction was investigated. The S values numerically calculated based on the degree of polymerization and the spin concentration of polyradicals was in good agreement with the experimental values. This showed that the spin defect intercepts the intramolecular spin interaction in the polyradicals bearing nitroxides only in the π-conjugated main chain, whereas, in the polyradical bearing another nitroxide in the side chain, there is the intramolecular spin interaction across the spin defect.

The present work deals with a class I mixed valence trinuclear complex [ Co II ( μ -OAc ) 2 ( μ -L ) 2 Co 2 III ( NCS ) 2 ] ( 1 ) formed by partial aerial oxidation (H 2 L = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene). The terminal Co III N 3 O 3 centres exhibit a uniform facial arrangement of both N 3 and O 3 donor sets and the Co II centre is coordinated to six oxygen atoms. Variable temperature magnetic susceptibility measurement reveals that the intermediate Co(II) paramagnetic centre undergoes a very weak spin exchange ( θ = −6.39 K, antiferromagnetic) interaction with the two terminal Co(III) diamagnetic centres over the range 300–2 K. A three line EPR spectra at 4 K was characterized with highly anisotropic g tensors and the one line broad spectra at 300 K indicates the strong influence of temperature on the metal spin states, consistent with a high spin Co(II) complex. Two anodic responses appeared at +0.85 (Co 2+ → Co 3+ , reversible) and +1.7 V (Co 3+ → Co 4+ , irreversible) corresponding to one and two electron transfer oxidation respectively. Another reversible signal attributed to Co 3+ → Co 2+ reduction occurred at −0.5 V, associated with a two electron stoichiometry. Compound 1 reacted with hydrazine to form a square planar orange-red [Co II (L)] species, 2 . Inter conversion between 1 and 2 involves electron exchange coupled with proton transfer.

In this paper, we report the design, synthesis and electronic state of a π-conjugated stable iminonitroxide radical, 1 , the electron donor property of which is improved by an attachment of dimethylamino group. The photo-excited quartet high-spin state was observed by a time-resolved ESR experiment. CV measurement has clarified that the π-HOMO of the ground state is located higher ca. 0.35 eV in energy than 2 reported previously. These results show that the electron donor property is improved and the nature of the photo-excited spin alignment is conserved. Two CT complexes ( 1 -TCNQ and 1 -BQF 4 ) were synthesized using 1 as an electron donor. Their complexes have shown strong CT bands. The time-resolved ESR signal without the fine-structure splitting was observed for 1 -TCNQ CT complex. The signal may arise from the photo-induced electron transfer from the quartet excited state of 1 to the TCNQ acceptor.

The magnetic property of a one-dimensional magnetic chain, 5,10,15,20-tetrakis(4-bromophenyl)porphyrinatomanganese(III)tetracyanoethenide ([MnTBrPP][TCNE]), is investigated by using a model complex and theoretical calculations. In the model, MnTBrPP is reduced to a manganese porphyrin dimer. Hybrid UDFT (UB3LYP, UB2LYP) and UHF methods are used for the calculation of the model complex and effective exchange integrals ( J ) values are deduced from the calculated results. In order to estimate the spin coupling of two electronic states such as ferromagnetic and antiferromagnetic state, we calculated both high and low spin states. The calculated results indicate that the exchange interaction between Mn–TCNE is antiferromagnetic and exhibits ferrimagnetism. Natural orbital analyses are also performed to elucidate the antiferromagnetic interaction.

Quantum tunneling of the magnetization in a single-molecule magnet has been studied in experiments that combine microwave spectroscopy (10–50 GHz) with low temperature high sensitivity micro-Hall effect magnetometry (T = 0.4 K). This method enables the monitoring of spin-state populations in the presence of microwave radiation and a direct measure of the energy splitting between low lying high-spin states. We present results that show the level repulsion between such states as a function of magnetic field in the SMM Ni4 (S = 4), which clearly indicates the formation of high-spin superposition states. The absorption linewidths provide a lower bound on the transverse relaxation time (s2) or decoherence time of these superposition states of � 0.5 ns. Studies as a function of microwave power and magnetic field sweep rate suggest that the energy relaxation rate decreases with increasing longitudinal field and energy splitting between states. � 2005 Elsevier Ltd. All rights reserved.