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A structural, photophysical and electrochemical investigation of complexes [LnII(Mentpa)2]2+ (Ln = Eu, Yb; n = 0, 2 and 3) reveals how the steric requirements of the ligands modulate the LnII ligand field and the resulting electronic properties.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3)(Br4Cat)]·X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br4Cat2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d24)(NO3)(Br4Cat)]·CH3CN-d3 (1-LnD; Ln = La, Tb, Dy) have been synthesised, with 1-TbD and the diamagnetic analogue 1-LaD measured by INS. The dynamic magnetic properties of 1-TbD and 1-DyD have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br4Cat)]·X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-TbD by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-TbD, while a previously reported anomalous phonon bottleneck effect in the 1-DyD analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices.

Magnetoelastic coupling has been investigated using inelastic neutron scattering. This has allowed the spin–phonon coupling of the candidate qubit Na9[Ho(W5O18)2]·35D2O to be determined in the absence of an applied magnetic field.

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Metal complexes that can exist in two different charge distributions often exhibit dramatic color changes when switched between them. The underlying spectral changes are fundamentally related to the switchable behavior. In valence tautomeric (VT) systems, the transition is a stimulated, reversible intramolecular electron transfer between a metal center and a ligand, while in charge-transfer-induced-spin-transition (CTIST), also known as electron-transfer-coupled-spin-transition (ETCST), systems, the electron is transferred between two metal centers. We discuss, herein, the relationships between the switchable behavior of these systems and two related optical phenomena: charge transfer and solvatochromism. The insights gained from analyzing these phenomena can illuminate important aspects of VT or CTIST behavior, for example, the energetic relationship between the electromeric forms, or the effects of molecular environment on a VT or CTIST thermal equilibrium. Such insights may assist efforts to employ these compounds as molecular scale components in data storage, sensor and display devices.

Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII}⇌{CoII-SQ-SQ-CoII} (cat2- = catecholate, SQ•- = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol (spiroH4) or 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII} states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {ZnII-cat-SQ-ZnII} analog have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.

Incorporating lanthanoid(III)-radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single-molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln-radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)-radical compounds of formula [Gd(18-c-6)X

Photophysical and magnetic properties arising from both ground and excited states of lanthanoid ions are relevant for numerous applications. These properties can be substantially affected, both adversely and beneficially, by ligand-to-metal charge-transfer (LMCT) states. However, probing LMCT states remains a significant challenge in f-block chemistry, particularly in the solid state. Intriguingly, the europium compounds [Eu

The combination of redox-active metals with redox-active ligands can lead to interesting charge transfer behaviours, including valence tautomerism and solvatochromism. With the aim of investigating a relatively underexplored redox-active metal/redox-active ligand combination, complexes [CoII(acac)2(X-BIAN)] (acac− = acetylacetonate; X-BIAN = bis(4-X-phenyl)iminoacenaphthene; 1: X = –CF3, 2: X = –Cl, 3: X = –H, 4: X = –Me) and [CoIII(acac)2(Me-BIAN)]+ (5+) have been synthesised and characterised. At all temperatures investigated, and in both the solid and solution state, complexes 1–4 exist in a CoII-BIAN0 charge distribution, while 5+ adopts a CoIII-BIAN0 charge distribution. In the case of 1–4, the potential CoIII-BIAN˙− valence tautomer is inaccesible; the energy ordering between the ground CoII-BIAN0 state and the excited CoIII-BIAN˙− state must be reversed in order for an entropically driven interconversion to be possible. The energy gap between the states can be monitored via metal-to-ligand charge transfer bands in the visible region. We demonstrate tuning of this energy gap by varying the electronic properties of the BIAN ligand, as well as by controlling the molecular environment through solvent choice. Solvatochromic analysis, in combination with crystallographic evidence, allows elucidation of the specific solvent–solute interactions that govern the molecular behaviour of 1–4, affording insights that can inform potential future applications in sensing and switching.

The reaction of yttrium or cerium nitrate and tetrachloro- or tetrabromocatecholate (X4Cat2- with X = Cl or Br) has afforded the compounds: (Et3NH)2[{Y(Cl4Cat)(H2O)2}2(-Cl4Cat)2]‧2MeOH‧2H2O (1-Cl)...

The ability to identify promising candidate switchable molecules computationally, prior to synthesis, represents a considerable advance in the development of switchable molecular materials. Even more useful would be the possibility of predicting the switching temperature. Cobalt-dioxolene complexes can exhibit thermally induced valence tautomeric switching between low-spin CoIII-catecholate and high-spin CoII-semiquinonate forms, where the half-temperature (T1/2) is the temperature at which there are equal amounts of the two tautomers. We report the first simple computational strategy for accurately predicting T1/2 values for valence tautomeric complexes. Dispersion-corrected density functional theory (DFT) methods have been applied to the [Co(dbdiox)(dbsq)(N2L)] (dbdiox/dbsq•- = 3,5-di-tert-butyldioxolene/semiquinonate; N2L = diimine) family of valence tautomeric complexes, including the newly reported [Co(dbdiox)(dbsq)(MeO-bpy)] (1) (MeO-bpy = 4,4'-dimethoxy-2,2'-bipyridine). The DFT strategy has been thoroughly benchmarked to experimental data, affording highly accurate spin-distributions and an excellent energy match between experimental and calculated spin-states. Detailed orbital analysis of the [Co(dbdiox)(dbsq)(N2L)] complexes has revealed that the diimine ligand tunes the T1/2 value primarily through π-acceptance. We have established an excellent correlation between experimental T1/2(toluene) values for [Co(dbdiox)(dbsq)(N2L)] complexes and the calculated lowest unoccupied molecular orbital energy of the corresponding diimine ligand. The model affords accurate T1/2(toluene) values for [Co(dbdiox)(dbsq)(N2L)] complexes, with an average error of only 3.7%. This quantitative and simple DFT strategy allows experimentalists to not only rapidly identify proposed VT complexes but also predict the transition temperature. This study lays the groundwork for future in silico screening of candidate switchable molecules prior to experimental investigation, with associated time, cost, and environmental benefits.

The concurrent effects of single-ion anisotropy and exchange interactions on the electronic structure and magnetization dynamics have been analyzed for a cobalt(II)-semiquinonate complex. Analogs containing diamagnetic catecholate and tropolonate ligands were employed for comparison of the magnetic behavior and zinc congeners assisted with the spectroscopic characterization and assessment of intermolecular interactions in the cobalt(II) compounds. Low temperature X-band (ν ≈ 9.4 GHz) and W-Band (ν ≈ 94 GHz) electron paramagnetic resonance spectroscopy and static and dynamic magnetic measurements have been used to elucidate the electronic structure of the high spin cobalt(II) ion in [Co(Me3tpa)(Br4cat)] (1; Me3tpa = tris[(6-methyl-2-pyridyl)methyl]amine, Br4cat2− = tetrabromocatecholate) and [Co(Me3tpa)(trop)](PF6) (2(PF6); trop− = tropolonate), which show slow relaxation of the magnetization in applied field. The cobalt(II)-semiquinonate exchange interaction in [Co(Me3tpa)(dbsq)](PF6)·tol (3(PF6)·tol; dbsq− = 3,5-di-tert-butylsemiquinonate, tol = toluene) has been determined using an anisotropic exchange Hamiltonian in conjunction with multistate restricted active space self-consistent field ab initio modeling and wavefunction analysis, with comparison to magnetic and inelastic neutron scattering data. Our results demonstrate dominant ferromagnetic exchange for 3+ that is of similar magnitude to the anisotropy parameters of the cobalt(II) ion and contains a significant contribution from spin–orbit coupling. The nature of the exchange coupling between octahedral high spin cobalt(II) and semiquinonate ligands is a longstanding question; answering this question for the specific case of 3+ has confirmed the considerable sensitivity of the exchange to the molecular structure. The methodology employed will be generally applicable for elucidating exchange coupling between orbitally-degenerate metal ions and radical ligands and relevant to the development of bistable molecules and their integration into devices.

Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(μ-tetraoxolene)] (RE = Y and Dy; HBpz3- = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2-; 1-RE) or bromanilate (ba2-; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3- ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(μ-ba˙)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(iii) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes.

Electronically labile, or switchable, cobalt coordination polymers exhibit reversible spin crossover (SCO) or valence tautomeric (VT) transitions upon the application of an external stimulus, such as temperature variation. Spin crossover transitions at pseudo-octahedral cobalt(II) centers with an appropriate ligand field involve a heating-induced transition from the low-spin to high-spin electronic configurations. Valence tautomeric transitions are most commonly observed for cobalt-dioxolene systems, which undergo an intramolecular electron transfer and concomitant spin transition at the cobalt center, from low-spin-cobalt(III)-catecholate at low temperature to high-spin-cobalt(II)-semiquinonate upon heating. The VT transition can also be induced by light, while the cobalt(II) SCO transition cannot. Incorporation of these switchable moieties into coordination polymers (CPs) is generally achieved using polytopic nitrogen-donor linking ligands. All but one of the switchable cobalt CPs that have been structurally characterized are 1D chains, most of which exhibit VT transitions rather than SCO. There is a single example of a switchable 2D cobalt CP where VT cobalt-dioxolene moieties are linked by a tetratopic nitrogen-donor bridging ligand. Efforts to confer the VT properties to compounds suitable for applications as materials or in devices have mainly focused on the incorporation of the 1D chain VT CPs into nano- and microparticles, in which the VT transition is generally maintained. Valence tautomeric nanoparticles synthesized from 1D chain CPs have been grafted on to gold surfaces as self-assembled monolayers.

The cationic one-dimensional coordination polymers [{Co(pyterpy)2}{M(hfac)2}]2+ (M = Co(II) or Zn(II); pyterpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine; hfac− = hexafluoroacetyloacetonate) feature alternating {Co(pyterpy)2} and {M(hfac)2} units linked through heteroditopic pyterpy ligands. Cocrystallization with [M(hfac)3]− (M = Co(II) or Zn(II)) counteranions has afforded the compounds: [{Co(pyterpy)2}{Co(hfac)2}][Co(hfac)3]2 (1) and [{Co(pyterpy)2}{Co0.7Zn0.3(hfac)2}][Zn(hfac)3]2 (2). Variable temperature crystallographic and magnetic studies indicate low spin cobalt(II) ions in the {Co(pyterpy)2} components of the coordination polymers up to room temperature; magnetic studies suggest the onset of a thermally-induced spin crossover at these centers above room temperature.

The study of four different salts of a hybrid glycine–polyoxotungstate has revealed photocoloration that is dependent on the intramolecular hydrogen bonding between glycine ligand ammonium groups and the polyoxotungstate. Four compounds comprised of the hybrid polyoxometalate [As4{W3Y}W44Y4O159(Gly)8(H2O)12]9– (Gly = glycine) with glycinium, benzylammonium, 4-methylbenzylammonium, or benzyltriethylammonium countercations have been synthesized and structurally characterized. In the solid state, two of the compounds exhibit coloration associated with photoreduction, cycling from white to blue under UV irradiation, and bleaching back to white following reoxidation in air. The ability of the hybrid POM to undergo photoreduction correlates with the orientation of the zwitterionic ammonium groups of the glycine ligands and the intramolecular hydrogen bonding to POM oxo ligands. The kinetics of coloration of these POMs follows a pseudo-second-order rate law.

The combination of the divergent tetrakis-pyridyl linking ligand 1,2,4,5-tetra(4-pyridyl)benzene (tpb) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the complexes [Co(3,5-dbdiox)2(tpb)]∞ (1) and [Co(3,5-dbdiox)2(tpb)0.5]∞ (2). Both species are 1D coordination polymers that crystallize as the hydrated compounds 1·3H2O and 2·9H2O. While the tpb in complex 1 is a 2-connecting linker, affording a conventional 1D chain, the tpb in 2 is a 4-connecting linker, giving rise to a ribbon topology. Single crystal X-ray structural analysis indicates that 1·3H2O and 2·9H2O both possess the {CoIII(3,5-dbcat)(3,5-dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butyl-semiquinonate) charge distribution at 100 or 130 K, respectively. Variable temperature magnetic susceptibility studies of partially dehydrated 1·H2O indicate a thermally induced valence tautomeric (VT) transition above 300 K.

Compounds [Co(L-N4R2)(dbdiox)](BPh4) (L-N4R2 = N,N'-di-alkyl-2,11-diaza[3.3]-(2,6)pyridinophane, R = iPr (1a), Et (2a); dbdiox = 3,5-di-tert-butyldioxolene) and [M(L-N4iPr2)(dbdiox)](BPh4) (M = Mn (3a), Fe (4a)) have been synthesized and investigated with a view to possible valence tautomeric (VT) or spin crossover (SCO) interconversions. Single crystal X-ray diffraction data for all compounds at 100 or 130 K indicate trivalent metal cations and di-tert-butylcatecholate (dbcat2-) dioxolene ligands. Variable temperature magnetic susceptibility data for all species between 2 and 340 K are consistent with these redox states, with low spin configurations for the cobalt(iii) ions and high spin for the manganese(iii) and iron(iii) ions. Above 340 K, compound 1a exhibits an increase in magnetic susceptibility, suggesting the onset of a VT interconversion from low spin Co(iii)-dbcat to high spin Co(ii)-dbsq (dbsq- = di-tert-butylsemiquinonate) that is incomplete up to 400 K. In solution, variable temperature electronic absorption spectra and Evans NMR method magnetic susceptibility data indicate reversible VT interconversions for 1a in several solvents, with the transition temperature varying with solvent. Variable temperature electronic absorption spectra are temperature-invariant for 3a and 4a, while spectra for 2a in 1,2-dichloroethane suggest the onset of a VT transition at the highest temperatures measured. Density functional theory calculations on all four compounds and literature analogues provide key insights into the relative energies of the different electromeric forms and the possibilities for VT versus SCO interconversions in this family of compounds.

Three families of tetraoxolene-bridged dinuclear rare earth (RE) complexes have been synthesised and characterised, with general formula [((HB(pz)3)2RE)2(μ-tetraoxolene)] (HB(pz)3- = hydrotris(pyrazolyl)borate; tetraoxolene = chloranilate (1-RE), the dianionic form of 2,5-dihydroxy-1,4-benzoquinone (2-RE), or its 3,6-dimethyl analogue (3-RE)). In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and species with selected lanthanoid(iii) ions from Eu(iii) to Yb(iii) have been obtained, as well as the diamagnetic Y(iii) analogues. Use of the 3,6-dimethyl substituted tetraoxolene ligand (Me2-dhbq2-) has also afforded the two byproducts [((HB(pz)3)(MeOH)(B(OMe)4)Y)2(μ-Me2dhbq)] (4-Y) and [{((HB(pz)3)(MeOH)Y)2(μ-B(OMe)4)}2(μ-Me2dhbq)2]Cl2 (5-Y), with the B(OMe)4- ligands arising from partial decomposition of HB(pz)3-. Electrochemical studies on the soluble 1-RE and 3-RE families indicate multiple tetraoxolene-based redox processes. Magnetochemical and EPR studies of 3-Gd indicate the negligible magnetic coupling between the two Gd(iii) centres through the diamagnetic tetraoxolene bridge. Alternating current magnetic susceptibility studies of 1-Dy and 3-Dy reveal slow magnetic relaxation, with quantum tunnelling of the magnetisation (QTM) dominant in the absence of an applied dc field. The application of a dc field suppresses the QTM and relaxation data are consistent with an Orbach relaxation mechanism playing a major role in both cases, with effective energy barriers to magnetisation reversal determined as 47 and 24 K for 1-Dy and 3-Dy, respectively. The different dynamic magnetic behaviour evident for 1-Dy and 3-Dy arises from small differences in the local Dy(iii) coordination environments, highlighting the subtle structural effects responsible for the electronic structure and resulting magnetic behaviour.

Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid–polyoxometalate single-molecule magnet family: Na9[Ln(W5O18)2] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed to assign the INS trans...

The family of complexes of general formula [Co(Me ntpa)(Xdiox)]+ (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings; X = Br4, Cl4, H4, 3,5-Me2, 3,5- tBu2; diox = dioxolene) was investigated by density functional theory (DFT) calculations to predict the likelihood of valence tautomerism (VT). The OPBE functional with relativistic and solvent corrections allowed accurate reproduction of trends in spin-state energetics, affording the prediction of VT in complex [Co(Me3tpa)(Br4diox)]+ (1+). One-electron oxidation of neutral precursor [CoII(Me3tpa)(Br4cat)] (1) enabled isolation of target compounds 1(PF6) and 1(BPh4). Solution variable-temperature UV-vis absorption and Evans method magnetic susceptibility data confirm DFT predictions that 1+ exists in a temperature-dependent valence tautomeric equilibrium between low-spin Co(III)-catecholate and high-spin Co(II)-semiquinonate forms. The solution VT transition temperature of 1+ is solvent-tunable with critical temperatures in the range of 291-359 K for the solvents measured. Solid-state magnetic susceptibility measurements of 1(PF6) and 1(BPh4) reveal the onset of VT transitions above room temperature.

Remarkable electronic lability is evident from a preliminary study of a dinuclear cobalt complex with a bis(dioxolene) bridging ligand and tetraazamacrocylic ancillary ligands. Structural, magnetic, spectroscopic, and electrochemical data indicate coinciding metal- and ligand-based mixed-valence involving cobalt(II)/cobalt(III) and catecholate/semiquinonate couples. A thermally stimulated spin-crossover or valence-tautomeric transition is also observed above 200 K.

The combination of cobalt, 3,5-di-tert-butyldioxolene (3,5-dbdiox) and 1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane (tpch) yields two coordination polymers with different connectivities, i.e. a one-dimensional zigzag chain and a two-dimensional sheet. Poly[[bis(3,5-di-tert-butylbenzene-1,2-diolato)bis(1,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-yl-3-olato)[μ4-1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane]cobalt(III)]–ethanol–water 1/7/5], {[Co2(C14H20O2)4(C26H24N4O)]·7C2H5OH·5H2O} n or {[Co2(3,5-dbdiox)4(tpch)}·7EtOH·5H2O} n , is the second structurally characterized example of a two-dimensional coordination polymer based on linked {Co(3,5-dbdiox)2} units. Variable-temperature single-crystal X-ray diffraction studies suggest that catena-poly[[[(3,5-di-tert-butylbenzene-1,2-diolato)(1,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-yl-3-olato)cobalt(III)]-μ-1-hydroxy-1,2,4,5-tetrakis(pyridin-4-yl)cyclohexane]–ethanol–water (1/1/5)], {[Co(C14H20O2)2(C26H24N4O)]·C2H5OH·5H2O} n or {[Co(3,5-dbdiox)2(tpch)]·EtOH·5H2O} n , undergoes a temperature-induced valence tautomeric interconversion.

Longstanding and important applications make use of the chemical and physical properties of both rare earth metals and polyoxometalates of early transition metals. The catalytic, optical, and magnetic features of rare earth metal ions are well-known, as are the reversible multielectron redox and photoredox capabilities of polyoxomolybdates and polyoxotungstates. The combination of rare earth ions and polyoxometalates in discrete molecules and coordination polymers is of interest for the unique combination of chemical and physical properties that can arise. This Account surveys our efforts to synthesize and investigate compounds with rare earth ions and polyoxometalates (RE-POMs), sometimes with carboxylate-based organic coligands. Our general synthetic approach is "bottom-up", which affords well-defined nanoscale molecules, typically in crystalline form and amenable to single-crystal X-ray diffraction for structure determination. Our particular focus is on elucidation of the physical properties conferred by the different structural components with a view to ultimately being able to tune these properties chemically. For this purpose, we employ a variety of spectroscopic, magnetochemical, electrochemical, and scattering techniques in concert with theoretical modeling and computation. Studies of RE-POM single-molecule magnets (SMMs) have utilized magnetic susceptibility, inelastic neutron scattering, and ab initio calculations. These investigations have allowed characterization of the crystal field splitting of the rare earth(III) ions that is responsible for the SMM properties of slow magnetic relaxation and magnetization quantum tunneling. Such SMMs are promising for applications in quantum computing and molecular spintronics. Photophysical measurements of a family of hybrid RE-POMs with organic ligands have afforded insights into sensitization of Tb(III) and Eu(III) emission through both organic and polyoxometalate chromophores in the same molecule. Detailed variable-temperature studies have revealed the temperature dependence of the POM-based sensitization, which is relevant for potential applications in phosphor thermometry. Novel RE-POM coordination polymers demonstrate the promise of higher-dimensional materials for catalytic and sensing applications that can make use of either or both rare earth and polyoxometalate capabilities. Finally, structural, electrochemical, and density functional theory studies on a family of modular RE-POMs that incorporate molybdotungstates with amino acid coligands have revealed how closed Mo-oxo loops that are reduced preferentially can act as electron reservoirs in mixed-metal molybdotungstates. This has important implications for mixed-metal polyoxometalates in redox and photoredox catalysis. Notably, these hybrid RE-POMs are stable in solution and maintain the chirality induced by amino acid ligands. The RE-POMs surveyed in this Account provide a glimpse of possible structural features that are accessible with this family of compounds. The studies of the ensuing chemical and physical properties reveal the promise of RE-POMs for diverse and varied applications and lay an excellent foundation for the future development of this new class of functional materials.

The preference for incorporation of molybdenum over tungsten into specific sites of a family of yttrium-arsenotungstates with amino acid ligands prompted exploration of the incorporation of other metals, affording three new vanadium-containing (V/W and V/Mo/W) analogues: K2(GlyH)10[As4(V2W2)W44Y4O160(Gly)8(H2O)12]·11Gly (1), (MBAH)9(L-NleH)3[As4(V2W2)W44Y4O160(L-Nle)8(H2O)12] (2), and (MBAH)9(L-NleH)3[As4(V2W2)Mo2W42Y4O160(L-Nle)8(H2O)12] (3) (Gly=glycine and L-Nle=l-norleucine, MBAH=4-methylbenzylammonium). These hybrid polyoxometalates all possess a tetrametallic oxo-bridged {VIV2WVI2} central core surrounded by an amino acid-ligated cyclic metal-oxo framework. X-Ray photoelectron, UV-visible reflectance, and electron paramagnetic resonance spectroscopy, together with metal analysis, confirm the incorporation of vanadium into the polyoxometalates, while single crystal X-ray diffraction analysis supports the location of the vanadium atoms in the central core.

The reaction of ambidentate sulfonium ylide (Me)2SCHC(O)C6H4-p-OCH3(Y) with HgX2 [X = Cl (1), Br (2) and I (3)] in equimolar ratios using methanol as the solvent leads to two types of products. Single-crystal X-ray diffraction analysis reveals (i) a two-dimensional polymer, [{HgCl2(Y)}4]n, in which the mercury(II) atom is five-coordinated and ligand Y displays C-coordination as well as an additional interaction, including O-coordination to different mercury centers and (ii) an asymmetric halide-bridged binuclear structure of the type [HgBr2(Y)]2, in which the mercury(II) center is four-coordinate and ligand Y displays C-coordination to the metal. The characterization of these compounds by IR, 1H- and 13C NMR spectroscopy confirms the coordination of the ylide to the metal through the carbon atom. Analytical data indicate a 1:1 stoichiometry between the ylide and the Hg(II) halide in all complexes.

The first members of a promising new family of hybrid amino acid-polyoxometalates have emerged from a search for modular functional molecules. Incorporation of glycine (Gly) or norleucine (Nle) ligands into an yttrium-tungstoarsenate structural backbone, followed by crystallization with p-methylbenzylammonium (p-MeBzNH3(+)) cations, affords (p-MeBzNH3)6K2(GlyH)[As(III)4(Y(III)W(VI)3)W(VI)44Y(III)4O159(Gly)8(H2O)14]⋅47 H2O (1) and enantiomorphs (p-MeBzNH3)15(NleH)3[As(III)4(Mo(V)2Mo(VI)2)W(VI)44Y(III)4O160(Nle)9(H2O)11][As(III)4(Mo(VI)2W(VI)2)W(VI)44Y(III)4O160(Nle)9(H2O)11] (generically designated 2: L-Nle, 2 a; D-Nle, 2 b). An intensive structural, spectroscopic, electrochemical, magnetochemical and theoretical investigation has allowed the elucidation of site-selective metal substitution and photoreduction of the tetranuclear core of the hybrid polyanions. In the solid state, markedly different crystal packing is evident for the compounds, which indicates the role of noncovalent interactions involving the amino acid ligands. In solution, mass spectrometric and small-angle X-ray scattering studies confirm maintenance of the structure of the polyanions of 2, while circular dichroism demonstrates that the chirality is also maintained. The combination of all of these features in a single modular family emphasizes the potential of such hybrid polyoxometalates to provide nanoscale molecular materials with tunable properties.

The defining characteristic of a valence tautomeric (VT) transition in a metal complex with a redox-active organic ligand is a stimulated intramolecular electron transfer between metal and ligand. Reversible VT behavior is well established for octahedral cobalt complexes with o -dioxolene ligands, with other combinations of metal centers and organic ligands also capable of exhibiting the phenomenon. Although the thermodynamic basis of VT transitions in cobalt-dioxolene systems has been understood for some time, it has not necessarily been elucidated for the less common systems. In general a number of factors additional to the redox-active metal–ligand unit influence the manifestation of the VT transition. These include the ancillary ligands, counterions, solvent molecules and intermolecular interactions, with cooperativity between VT molecules important for hysteresis and bistability. Recent advances in the field include the development of new metal–ligand combinations that can display VT transitions and the recognition that stimuli in addition to heat, light and pressure can induce the transitions. Considerable effort is also currently being directed toward assembling multiple VT moieties into polynuclear and polymeric complexes and combining VT behavior with other physical and chemical properties to generate bifunctional molecule-based materials.

The combination of bridging bis-bidentate redox-active ligands derived from 3,3-bis-2-hydroxy-1,4-naphthoquinone (bhnqH2), ancillary ligands based on tris(2-pyridylmethyl)amine (tpa) and cobalt salts has afforded a new family of dinuclear cobalt complexes. Compounds of the complexes [Co2(bhnq)(tpa)2](2+) (1), [Co2(bhnq)(Metpa)2](2+) (2), [Co2(bhnq)(Me2tpa)2](2+) (3) [Co2(bhnq)(Me3tpa)2](2+) (4), [Co2(bhnq)(tpa)2](4+) (5), [Co2(bhMenaph)(tpa)2](2+) (6) and [Co2(bhPronaph)(tpa)2](2+) (7) (Mentpa involves n = 0, 1, 2 and 3 methyl groups at the 6-position of the tpa pyridine rings; bhMenaphH4 = bis-3,4-dihydroxy-4-methoxynaphthalene-1-one; bhPronaphH4 = bis-3,4-dihydroxy-4-(2-oxopropyl)naphthalen-1(4H)-one) have been characterised by single crystal X-ray diffraction. While complexes 1-4 possess divalent cobalt centres, trivalent cobalt is evident in complexes 5-7. The bis-bidentate redox-active bridging ligand remains in the diamagnetic quinone bhnq(2-) redox state in complexes 1-5. Metal-catalysed reaction with methoxide or acetone enolate ions gives rise to the derivatised bridging ligands present in 6 and 7. The electronic properties of compounds of 1-7 have been explored in the solid state by infrared spectroscopy and variable temperature magnetic measurements and in solution by electronic absorption spectroscopy and cyclic voltammetry.

Exploration of the reactivity of the versatile polyoxometalate precursor [As2W19O67(H2O)]14− with yttrium salts and organic proligands derived from 2-picolinic acid: 2,5-pyridinedicarboxylic acid (pdcH2) and 5-(methoxycarbonyl)-2-pyridinecarboxylic acid (mcpcH), has afforded the three new hybrid complexes [Y{AsW8O30}2(AsO)2{WO2(pdc)}2]13− (1) and [{Y(H2O)3}2{As2W19O68}{WO2(L)}2]8− (L− = pdcH− (2) or mcpc− (3)). The structurally-related and organic ligand free species [{Y(H2O)3}4{Y(H2O)6}2{As2W19O68}2{WO3(H2O)}4(AsO)2]20− (4) was also obtained. Structural characterization of the mixed salts of the four complexes, K8H5[1]·9H2O, K2.5H5.5[2]·37H2O, K2.5H5.5[3]·34H2O and K6Na2H4[4]·30H2O, has revealed that five membered N,O-chelated {WO2(L)} units play a key structural role in complexes 1–3. With no picolinate-based ligand present in 4, additional arsenite fragments bind to the analogous tungsten coordination sites.

Protonated sulfonium ylide chlorocadmate compounds of type [ ( Me ) 2 SCH 2 C ( O ) C 6 H 4 R ] + [ CdCl 3 ] - (R = H( 1 ); p -Br ( 2 ); p -Me ( 3 ); p -OMe ( 4 ); p -NO 2 ( 5 )) were prepared by the reaction of α-keto stabilized sulfonium ylides (Me) 2 SCHC(O)C 6 H 4 R with CdCl 2 ⋅H 2 O in 1:2 M ratios using methanol as solvent. Characterization of the compounds by IR, 1 H- and 13 C NMR spectroscopy indicated the absence of coordination of the protonated sulfonium ylide to the metal through either the carbon or oxygen atoms. The crystal structures of compounds 1 and 2 were both found to consist of organic cations, [ ( Me ) 2 SCH 2 C ( O ) C 6 H 4 R ] + (R = H( 1 ) and p -Br ( 2 )) and infinite one-dimensional chains of formula [ CdCl 3 ] n - In both structures each Cd atom is five-coordinate with square pyramidal coordination geometry.

Reaction of α-keto stabilized sulfonium ylides (Me)2SCHC(O)C6H4R (R = p-NO2 (Y) and p-Br(Y′)) with HgX2 (X = Cl, Br and I) in equimolar ratios using methanol as solvent leads to two types of products. Single crystal X-ray diffraction analysis reveals (i) binuclear complex of [HgI2(Y)]2 (3) with an asymmetric halide-bridged structure and (ii) one-dimensional polymer of [HgI2(Y′)]n (6) that the monomeric –Hg–I–Hg– bridging leads to a zig-zag polymeric chain in which mercury assumes a distorted tetrahedral geometry. Characterization of the compounds by IR, 1H and 13C NMR spectroscopy confirmed coordination of the ylide to the metal through the carbon atom. Analytical data indicate a 1:1 stoichiometry between the ylide and Hg(II) halide in all of products. The Hg(II) complexes with different ligands evaluated for their antibacterial activity using disc diffusion method. The results show that all complexes represent antibacterial activity against bacteria tested especially on Gram positive ones.

Reaction of α-keto stabilized sulfonium ylides (Me)2SCHC(O)C6H4R (R = p-Me (a); p-Cl (b)) with HgX2 (X = Cl, Br and I) in equimolar ratios using methanol as solvent leads to binuclear products of the type [HgX2(ylide)]2 (X = Cl (1), Br (2) and I (3)). Single crystal X-ray diffraction analysis reveals the presence of unexpected asymmetric halide-bridged dinuclear structures for 1a and 2b. Characterization of the compounds by IR, 1H- and 13C NMR spectroscopy confirmed coordination of the ylide to the metal through the carbon atom. In addition, the antibacterial effects of DMSO-solutions of the complexes were investigated by the disc diffusion method against three Gram positive and three negative bacteria. All complexes represent antibacterial activity against these bacteria with high levels of inhibitory potency exhibited against the Gram-positive species.

Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal-oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal-oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled MoV centers in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal-oxo cores.

Optimization of literature synthetic procedures has afforded, in moderate yield, homogeneous and crystalline samples of the five analogues Na11[{RE(OH2)}3CO3(PW9O34)2] (1-RE; RE = Y, Tb, Dy, Ho, and Er). Phase-transfer methods have allowed isolation of the mixed salts (Et4N)9Na2[{RE(OH2)}3CO3(PW9O34)2] (2-RE; RE = Y and Er). The isostructural polyanions in these compounds are comprised of a triangular arrangement of trivalent rare-earth ions bridged by a μ3-carbonate ligand and sandwiched between two trilacunary Keggin {PW9O34} polyoxometalate ligands. Alternating-current (ac) magnetic susceptibility studies of 1-Dy, 1-Er, and 2-Er reveal the onset of frequency dependence for the out-of-phase susceptibility in the presence of an applied magnetic field at the lowest measured temperatures. Inelastic neutron scattering (INS) spectra of 1-Ho and 1-Er exhibit transitions between the lowest-lying crystal-field (CF) split states of the respective J = 8 and (15)/2 ground-state spin-orbit multiplets of the Ho(III) and Er(III) ions. Complementary ab initio calculations performed for these two analogues allow excellent reproduction of the experimental magnetic susceptibility and low-temperature magnetization data and are in reasonable agreement with the experimental INS data. The ab initio calculations reveal that the slight difference in coordination environments of the three Ln(III) ions in each complex gives rise to differences in the CF splitting that are not insignificant. This theoretical result is consistent with the observation of multiple relaxation processes by ac magnetic susceptibility and the broadness of the measured INS peaks. The ab initio calculations also indicate substantial mixing of the MJ contributions to the CF split energy levels of each Ln(III) ion. Calculations indicate that the CF ground states of the Ho(III) centers in 1-Ho are predominantly comprised of contributions from small MJ, while those of the Er(III) centers in 1-Er are predominantly comprised of contributions from large MJ, giving rise to slow magnetic relaxation. Although no direct evidence for intramolecular RE···RE magnetic coupling is observed in either magnetic or INS studies, on the basis of the ab initio calculations, we find noncollinear magnetic axes in 1-Er that are coplanar with the erbium triangle and radially arranged with respect to the triangle's centroid; thus, we argue that the absence of magnetic coupling in this system arises from dipolar and antiferromagnetic superexchange interactions that cancel each other out.

The combination of the divergent bis-pyridyl linking ligands 1,2-bis(4-pyridyl)ethane (1,2-bpe), 4,4'-trans-azopyridine (azpy), and 1,3-bis(4-pyridyl)propane (1,3-bpp) with cobalt and 3,5-di-tert-butyldioxolene (3,5-dbdiox) ligands has afforded the complexes [Co(3,5-dbdiox)2(1,2-bpe)]∞ (1), [Co(3,5-dbdiox)2(azpy)]∞ (2), [trans-Co(3,5-dbdiox)2(1,3-bpp)]∞ (3a), and [cis-Co(3,5-dbdiox)2(1,3-bpp)]∞ (3b). All species are 1D coordination polymers that crystallize as solvated forms; the geometric isomers 3a,b cocrystallize. Complexes 1, 2, and 3a exhibit around the Co centers a trans disposition of the N-donor atoms from the pyridyl linkers, while an unusual cis disposition is evident in 3b. Single-crystal X-ray structural analysis at 100 or 130 K of solvated forms of these complexes indicates that all complexes possess the {Co(III)(3,5-dbcat)(3,5-dbsq)} (3,5-dbcat = 3,5-di-tert-butylcatecholate; 3,5-dbsq = 3,5-di-tert-butylsemiquinonate) charge distribution at the temperature of data collection. Variable-temperature magnetic susceptibility studies reveal that 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O (3 = 3a·3b) all exhibit thermally induced valence tautomeric (VT) transitions above 200 K. Multiple heating and cooling cycles indicate that in some cases the behavior is strongly dependent on desolvation processes. Most notably, further desolvation of 1·1.5MeCN·2H2O above 340 K affords χmT values that suggest unusual ferromagnetic coupling in the {hs-Co(II)(3,5-dbsq)2} valence tautomer. Compound 3·MeCN·H2O exhibits a two-step VT transition that may be ascribed to the presence of the cis and trans geometric isomers. Compounds 1, 1·1.5MeCN·2H2O, 2·2EtOH, and 3·MeCN·H2O all also exhibit a single photoinduced VT transition, comparable to those generally observed for nonpolymeric cobalt-dioxolene complexes.

The new coordination polymer [e-PMo12O37(OH) 3{La(H2O)3}4(L)]·28H 2O [H4L = tetrakis(4-carboxyphenyl)silane] can be synthesized by combining equal quantities of two types of 4-connecting building blocks: organic L4- ligands and inorganic polyoxometalate-derived [e-PMo12O37(OH)3{La(H2O) 4}4]4+ polyoxocations. The resulting coordination polymer exhibits the diamondoid topology, where both building blocks act as pseudotetrahedral nodes. The structural and spectroscopic characteristics of the parent polyoxocation are retained in the coordination polymer. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reaction of the pseudotetrahedral tetracarboxylic acid proligand tetrakis(4-carboxyphenyl)silane (H4L) with salts of various lanthanoid metals has afforded three new lanthanoid coordination polymers: {[H3O]2[Ce2(L)2(H2O)2]·2DMF·2H2O}∞ (1), {[Eu3L2(NO3)(DMF)4(H2O)]·0.5DMF·6H2O}∞ (2), and {Eu(HL)(DMF)2(H2O)]·DMF·2.5H2O}∞ (3) (DMF = N,N′-dimethylformamide), which have been structurally characterized by single-crystal X-ray diffraction. Compounds 1 and 2 are both noninterpenetrated three-dimensional networks that display the rare fluorite (CaF2) topology. In compound 1, the fully deprotonated L4– ligands act as pseudotetrahedral 4-connecting nodes and dinuclear cerium-carboxylate building blocks act as 8-connecting nodes, while in compound 2 the L4– ligands and trinuclear europium-carboxylate units are 4- and 8-connecting, respectively. In contrast, compound 3 exhibits a two-dimensional layered structure with triply deprotonated HL3– ligands acting as 3-connecting units, linking single europium centers within...

Light up the POMs: A luminescent lanthanoid complex with polyoxometalate (POM) and organic ligands has been structurally characterized (see picture). Comparison of this octanuclear TbIII complex of 2-picolinate and tungstoarsenate ligands with a dinuclear relative reveals the role of the organic ligands as chromophores, identifies the luminescent Tb centers, and determines the relationship between POM coordination mode and luminescence quenching.

Two one-dimensional chain complexes with building blocks comprised of polyoxotungstate-ligated lanthanoid centers have been synthesized from the adaptable precursor [As2W19O67(H2O)]14− through combined disassembly and reassembly processes. Careful control of the reaction and crystallization conditions affords compounds formulated as H3K7Na6[Nd3As4W41O141OH(H2O)10]·60H2O (1) and KNa[Dy4As2W22O76(H2O)19(C2H5NO2)2]·20.5H2O (2).

A detailed investigation of a valence tautomeric (VT) transition for the new complex [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)₂]/[Co(II)(3,5-DBSQ)₂(py)₂] (1) is reported, where 3,5-DBCatH₂ is 3,5-di-tert-butyl-catechol, 3,5-DBSQH is 3,5-di-tert-butyl-semiquinone and py is pyridine. Complex 1 exists as a mixture of the two valence tautomers, with the relative proportion of each depending on the external conditions. Three differently solvated forms of the complex have been synthesized and variable temperature structural and magnetic investigations of one of these, 1·0.5py, reveals that this compound undergoes a thermally-induced VT transition from the [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)₂] tautomer at temperatures below 150 K to a 1 : 1 mixture of the two tautomers at temperatures above 300 K. The VT transition may also be photo-induced at 9 K, affording a similar mixture of the two tautomers. In both cases the incomplete transition is attributed to the presence of π-π stacking interactions between the pyridine molecules of solvation and one of the two crystallographically independent complex molecules, which inhibits the expansion of this molecule that would accompany a VT transition. Studies on alternatively solvated forms 1·2MeCN and 1·1.67hexane also suggest a significant dependence of the VT transition on solvation-induced packing effects and/or intermolecular interactions.

Seven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe(3)(OAc)(3)(L)(3)] (1), where LH(2) = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe(3)(OAc)(Cat)(L)(3)] (2), [Fe(6)(OAc)(2)(Cat)(4)(L)(4)] (3), [Fe(4)(3,5-DBCat)(2)(L)(4)] (4), [Bu(4)N][Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)] (5a, 5(-) is the complex monoanion [Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)](-)), [Fe(4)(OAc)(3,5-DBCat)(3)(3,5-DBSQ)(L)(2)] (6), [Fe(2)(Cl(4)Cat)(2)(L)(LH(2))(H(2)O)] (7), and [Et(3)NH](2)[Fe(2)(Cl(4)Cat)(2)(L)(2)] (8a, 8(2-) is the complex dianion [Fe(2)(Cl(4)Cat)(2)(L)(2)](2-)), where CatH(2) = catechol; 3,5-DBCatH(2) = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl(4)CatH(2) = tetrachlorocatechol. While compounds 2-4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV-visible spectroscopy, voltammetry, UV-visible spectroelectrochemistry, and magnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5(-), and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5(-) is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ(-) ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibility measurements for compounds 3, 4, 5a, 6, 7, and 8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5(-), 7 and 8(2-), and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxo-bridged ferric complexes.

5 pages; International audience; The reaction of Cu(OAc)2 and Ln(OAc)3 (Ln = Gd, Tb and Dy) with 2-amino-2-methyl-1,3- propanediol (ampdH2) under solvothermal conditions has afforded a new family of isostructural octanuclear Cu4Ln4 complexes with the formula [Cu4Ln4(OAc)12(ampdH)8(OH2)2] (Ln = Gd (1), Tb (2) and Dy(3)) in good yield. Variable temperature magnetic susceptibility measurements reveal weak intramolecular exchange interactions for 1 and 2. Ferromagnetic coupling is observed for 1 and attributed to Cu · · ·Gd interactions. In contrast, the magnetic susceptibility behaviour of 2 arises from a combination of intramolecular exchange interactions and the crystal field splitting of the 7F6 ground state of the TbIII ions.

The defining characteristic of a valence tautomeric (VT) transition in a metal complex with a redox-active organic ligand is a stimulated intramolecular electron transfer between metal and ligand. Reversible VT behavior is well established for octahedral cobalt complexes with o -dioxolene ligands, with other combinations of metal centers and organic ligands also capable of exhibiting the phenomenon. Although the thermodynamic basis of VT transitions in cobalt-dioxolene systems has been understood for some time, it has not necessarily been elucidated for the less common systems. In general a number of factors additional to the redox-active metal–ligand unit influence the manifestation of the VT transition. These include the ancillary ligands, counterions, solvent molecules and intermolecular interactions, with cooperativity between VT molecules important for hysteresis and bistability. Recent advances in the field include the development of new metal–ligand combinations that can display VT transitions and the recognition that stimuli in addition to heat, light and pressure can induce the transitions. Considerable effort is also currently being directed toward assembling multiple VT moieties into polynuclear and polymeric complexes and combining VT behavior with other physical and chemical properties to generate bifunctional molecule-based materials.