Redox Activity and Two-Step Valence Tautomerism in a Family of Dinuclear Cobalt Complexes with a Spiroconjugated Bis(dioxolene) Ligand

Opto-Spintronics: Photoisomerization-Induced Spin State Switching at 300 K in Photochrome Cobalt-Dioxolene Thin Films

Controllable quantum systems are under active investigation for quantum computing, secure information processing, and nonvolatile memory. The optical manipulation of spin quantum states provides an important strategy for quantum control with both temporal and spatial resolution. Challenges in increasing the lifetime of photoinduced magnetic states at T > 200 K have hindered progress toward utilizing photomagnetic materials in quantum device architectures. Here we demonstrate reversible light-induced magnetization switching in an organic thin film at device operating temperatures of 300-330 K. By utilizing photochromic ligands that undergo structural changes in the solid state, the changes in ligand field associated with photoisomerization modulate the ligand field and in turn the oxidation and spin state of a bound metal center. Green light irradiation (λ_exc = 550 nm) of a spirooxazine cobalt-dioxolene complex induces photoisomerization of the ligand that in turn triggers a reversible intramolecular charge-transfer coupled spin-transition process at the cobalt center. The generation of photomagnetic states through conversion between a low-spin Co(III)-semiquinone doublet and a high-spin Co(II)-bis-semiquinone sextet state has been demonstrated in both solution and the solid state and is described as a photoisomerization-induced spin-charge excited state (PISCES) process. The high transition temperature (325 K) and long-lived photoinduced state (τ = 10 s at 300 K) are dictated by the photochromic ligand. Theory provides effective modeling of the phenomenon and long-term strategies to further modulate the lifetimes of photomagnetic states for quantum information technologies at the single molecule level.

Cobalt complexes with redox-active anthraquinone-type ligands

Three anthraquinone-type multidentate ligands, HL1-3 (HL = 2-R-1H-anthra[1,2-d]imidazole-6,11-dione; HL1: R = (2-pyridyl), HL2; R = (4,6-dimethyl-2-pyridyl), HL3; R = (6-methoxy-2-pyridyl)), were prepared, and their complexation behaviour was investigated. Three bis-chelate cobalt complexes with the formula [CoII(L1-3)2]·n(solv.) (1, 2, and 3 for HL1, HL2, and HL3, respectively), in which the ligands adopted tridentate binding modes, were synthesized and structurally characterized by single-crystal X-ray analyses. Electrochemical studies of 1-3 in CH2Cl2 reveal three reversible redox waves, assigned to ligand and cobalt-centred processes. Additional complexes were obtained in which HL1 adopted a bidentate binding mode, stabilising the mono-chelate [CoII(HL1)(NO3)2(DMF)2] (4) species and tris-chelate [CoIII(L1)3] (5) complex in which the cobalt ion was in its 3+ state. The electrochemical properties of complex 5 were investigated in DMF, and the Co(ii)/Co(iii) redox couple was found to be negatively shifted compared to that of complex 1, while the ligand-based processes became irreversible. Tridentate chelation is found to stabilise the anthraquinone ligands and unlocks their redox multi-stability.

Hydrazone-based cobalt complexes toward multielectron redox and spin crossover

Hydrazone-based derivatives modified by substitution at different positions were utilized to prepare a series of bis-homoleptic cobalt complexes. One species, [Co^III(L1)₂]⁺ (1), which incorporated deprotonated ligands, adopted a Co(iii) diamagnetic ground state. However, the substituent of a hydrogen atom with a methyl group precluded the possibility of deprotonation upon metal coordination, which led to two species, [Co^II(L2^Me)₂]²⁺ (2) and [Co^II(L3^NO2)₂]²⁺, (3) which underwent a gradual spin crossover with an adjustable substituent effect and a mixed character of low-spin (doublet) and high-spin (quartet) populations in wide temperature ranges. Depending on the electronic effects of the substituents on the ligand, the multielectron redox behavior of the cobalt center was systematically modulated as well. This result demonstrates redox-switchable spin crossover in a new hydrazone-based Co(ii) system, in which the deprotonation of the coordination pocket and substituent groups in aromatic ligands can have a profound effect on the redox potential and spin state of the metal center.

Hydrazone-based derivatives modified by substitution at different positions were utilized to prepare a series of bis-homoleptic cobalt complexes toward multielectron redox and spin crossover.

Steric control in the metal–ligand electron transfer of iminopyridine–ytterbocene complexes

A systematic study of reactions between Cp*₂Yb(THF) and iminopyridine ligands featuring similar electron accepting properties but variable denticity and steric demand, has provided a new example of steric control on the redox chemistry of ytterbocenes.

Dinuclear adducts of di-o-iminoquinone ligands with CoII diketonates: computational insights into two-step valence tautomeric rearrangements

Density functional theory (DFT) computational modeling [B3LYP*/6-311++G(d,p)] of a series of potentially valence tautomeric (VT) dinuclear 2:1 adducts of Co^II bis-acetylacetonate, bis-trifluoroacetylacetonate and bis-hexafluoroacetylacetonate with redox-active tetradentate di-o-iminoquinone ligands has been performed. The significant energy preference of the low-spin electromeric forms of the complexes on the basis of Co^II bis-acetylacetonate points to a low probability of VT in these compounds. Electron-withdrawing CF₃-groups in the diketonate moiety of the mixed-ligand complexes promote narrowing of the energy gaps between the electronic states, which allows one-step VT rearrangements to be expected in the adducts of Co^II bis-trifluoromethylacetylacetonate. The most suitable energy parameters for the occurrence of two-step VT conversion (stability of the adduct with respect to dissociation into the components, small relative energies of electromeric forms and thermally achievable energy barriers to intramolecular electron transfers estimated as minimum energy crossing points) are found for the adduct of di-o-iminoquinone with Co^II bis-trifluoroacetylacetonates comprising a diphenylene linker.

Influence of Intermolecular Interactions on Valence Tautomeric Behaviors in Two Polymorphic Dinuclear Cobalt Complexes

Two polymorphic structures have been well determined in a valence tautomeric (VT) dinuclear cobalt complex. These polymorphs showed distinct thermal- and photomagnetic behavior, and are thus ideal for studying the "pure" intermolecular factors to VT transitions. In polymorph 1A, the VT cations are arranged head-to-waist with their neighbors and exhibit weak π⋅⋅⋅π interactions, resulting in a gradual and incomplete thermal VT transition. In contrast, the cations in polymorph 1B are arranged head-to-tail and exhibit relatively strong π⋅⋅⋅π interactions, leading to an abrupt and complete thermal VT transition with adjustable hysteresis loop at around room temperature. The VT process for both polymorphs can be induced by light, but the light-excited state of 1B⋅2H₂ O has a higher thermal relaxation temperature than that of 1A⋅3H₂ O.

The electronic and solvatochromic properties of [Co(L)(bipyridine)2]+ (L = o-catecholato, o-benzenedithiolato) species: a combined experimental and computational study

Complexes of Co(iii) containing mixed chelating diimine and o-quinone ligand sets are of fundamental interest on account of their fascinating magnetic and electronic properties. Whilst complexes of this type containing one diimine and two o-quinone ligands have been studied extensively, those with the reverse stoichiometry (two diimines and one o-quinone) are much rarer. Herein, we describe a ready route to the synthesis of the complex [Co^III(o-catecholate) (2,2'-bipyridine)₂]⁺ (1), and also report the synthesis of [Co^III(o-catecholate)(5,5'-dimethyl-2,2'-bipyridine)₂]⁺ (2) and [Co^III(o-benezenedithiolate)(5,5'-dimethyl-2,2'-bipyridine)₂]⁺ (3) for the first time. Spectroscopic studies show that complex 2 displays intriguing solvatochromic behaviour as a function of solvent hydrogen bond donation ability, a property of this type of complex which has hitherto not been reported. Time-dependent density function theory (TD-DFT) shows that this effect arises as a result of hydrogen bonding between the solvent and the oxygen atoms of the catecholate ligand. In contrast, the sulfur atoms in the benzenedithiolate analogue 3 are much weaker acceptors of hydrogen bonds from the solvent, meaning that complex 3 is only very weakly solvatochromic. Finally, we show that complex 2 has some potential as a molecular probe that can report on the composition of mixed solvent systems as a function of its absorbance spectrum.

Fluorescence behaviour of an anthracene-BODIPY system affected by spin states of a dioxolene-cobalt centre

Dioxolene cobalt complexes, [Co(L)(TPA)]PF6 () and [Co(L)(Me3TPA)]PF6 (), were synthesized using a catechol with anthracene and boron-dipyrromethene (BODIPY), H2L, and tris(pyridin-2-ylmethyl)amine (TPA) or tris[(6-methylpyridin-2-yl)methyl]amine (Me3TPA) and characterised by UV-vis absorption, IR and NMR spectroscopy. Complexes and are a low-spin cobalt(iii) catecholate form and a high-spin cobalt(ii) semiquinonate form, respectively, in CH3CN at room temperature. The effect of the spin states of the dioxolene-cobalt centre on fluorescence was investigated in CH3CN at room temperature. The fluorescence intensity of was clearly lower than that of , although both complexes showed weaker fluorescence compared with that of H2L. The difference in the intensity between and is mainly due to the enhancement of nonradiative decay processes based on the stronger metal-ligand interactions for the cobalt(iii) catecholate form of compared with the cobalt(ii) semiquinonate form of .

Redox-promoted associative assembly of metal-organic materials

We develop an associative synthesis of metal-organic materials that combines solid-state metal oxidation and coordination-driven self-assembly into a one-step, waste-free transformation. The methodology hinges on the unique reactivity of ortho-quinones, which we introduce as versatile oxidants for mechanochemical synthesis. Our strategy opens a previously unexplored route to paramagnetic metal-organic materials from elementary metals.

Platinum(ii) complexes of mixed-valent radicals derived from cyclotricatechylene, a macrocyclic tris-dioxolene

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4'-bis(tert-butyl)-2,2'-bipyridyl { t Bu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6- catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or t Bu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.

Thermal and optical control of electronic states in a single layer of switchable paramagnetic molecules

The control of the charge distribution in a monolayer of switchable cobalt-dioxolene complexes undergoing Valence Tautomerism (VT) has been achieved by means of thermal and optical stimuli. Thiol-derivatised VT molecules have been grafted on polycrystalline gold surface as monolayers from solution. X-ray photoelectron spectroscopy and time-of-flight secondary ions mass spectrometry evidenced the formation of a covalent bond between intact VT molecules and the surface and excluded the presence of physisorbed molecules. X-ray absorption spectroscopy revealed that the temperature- and light-induced conversion profiles of the monolayer closely reproduce the ones found for the crystalline phase. This study demonstrates that a wet chemistry based approach allows to transfer switchable paramagnetic molecules at the nanoscale, widening the playground to develop new hybrid molecular based architectures for novel technologies.

Theoretical modeling of valence tautomeric dinuclear cobalt complexes. Adducts of CoII diketonates with cyclic redox-active tetraone ligands

B3LYP*/6-311++G(d,p) calculated two-step VT rearrangement of a Co(ii) diketonate and a pyrene-4,5,9,10-tetraone 2 : 1 adduct occurs with the migration of paramagnetic centers.

Valence tautomeric dinuclear adducts of Co(ii) diketonates with redox-active diquinones for the design of spin qubits: computational modeling

DFT calculations performed on dinuclear adducts of Co(ii) diketonates with redox-active diquinones showed that with the proper structural tuning of the ligands, these paramagnetic complexes possess the properties required in a 2-qubit molecular system.

Stable mixed-valent radicals from platinum(II) complexes of a bis(dioxolene) ligand

Three diplatinum(II) complexes [{PtL}₂(μ-thea)] (H₄thea=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene) have been prepared, with diphosphine or bipyridyl “L” co-ligands. One-electron oxidation of these complexes gave radical cations containing a mixed-valent [thea]³⁻ ligand with discrete catecholate and semiquinonate centers separated by quaternary methylene spacers. The electronic character of these radicals is near the Robin–Day class II/III border determined by UV/Vis/NIR and EPR spectroscopies. Crystal-structure determinations and a DFT calculation imply that oxidation of the thea⁴⁻ ligand may lead to an increased through-space interaction between the dioxolene π systems.

Dinuclear cobalt(ii) and cobalt(iii) complexes of bis-bidentate napthoquinone ligands

Seven dinuclear cobalt complexes with bridging bis-bidentate naphthoquinone ligands reveal the steric influence of the ancillary capping ligand on the redox potential of the cobalt centres, with metal-catalysed derivatisation of the bridging ligand also observed.

Redox properties of LDH microcrystals coated with a catechol-bearing phosphonate derived from dopamine

The surface of Zn–Al-chloride LDH microcrystals (LDH = Layered Double Hydroxides) was activated by grafting a redox active catechol bearing bis-phosphonate obtained by dopamine derivatization.

Magnetic and spectroscopic investigation of thermally and optically driven valence tautomerism in thioether-bridged dinuclear cobalt-dioxolene complexes

A series of dinuclear cobalt complexes of general formula [Co(Mentpa)(diox-S-diox)Co(Mentpa)](PF6)2·MeOH (n = 0, 2, 3) was prepared through the synthesis of the bis-bidentate ligand 6,6'-((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(3,5-di-tert-butyl-benzene-1,2-diol) (diox-S-diox). The ancillary ligands Mentpa are obtained by the tripodal tris(2-pyridylmethyl)amine (tpa) ligand through successive introduction of methyl groups into the 6 position of the pyridine moieties. As expected, the steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, determining the charge distribution of the metal-dioxolene adduct at room temperature. Magnetic measurements and X-ray photoelectron and X-ray absorption spectroscopies indicate that the charge distributions low-spin-Co(III)-catecholate and high-spin-Co(II)-semiquinonate characterize the complexes formed by the tpa and Me3tpa tetradentate ligands, respectively. The complex formed by the Me2tpa ligand undergoes a thermal- and light-induced interconversion of the two states, in agreement with the existence of a valence tautomeric equilibrium. All complexes were stable and behaved reproducibly under X-ray irradiation. This work points out a fast and simple chemical approach to structurally and electronically modify the catechol ring while leaving its coordination capabilities unaffected. These findings afford a robust chemical method to prepare sulfur-functionalized dioxolene ligands as new molecular bricks for chemical functionalization of noble metal surfaces with this class of molecular switches.