Predicting valence tautomerism in diverse cobalt-dioxolene complexes: elucidation of the role of ligands and solvent.

The ability of molecular switches to reversibly interconvert between different forms promises potential applications at the scale of single molecules up to bulk materials. One type of molecular switch comprises cobalt-dioxolene compounds that exhibit thermally-induced valence tautomerism (VT) interconversions between low spin Co(iii)-catecholate (LS-CoIII-cat) and high spin Co(ii)-semiquinonate (HS-CoII-sq) forms. Two families of these compounds have been investigated for decades but have generally been considered separately: neutral [Co(diox)(sq)(N2L)] and cationic [Co(diox)(N4L)]+ complexes (diox = generic dioxolene, N2L/N4L = bidentate/tetradentate N-donor ancillary ligand). Computational identification of promising new candidate compounds prior to experimental exploration is beneficial for environmental and cost considerations but requires a thorough understanding of the underlying thermochemical parameters that influence the switching. Herein, we report a robust approach for the analysis of both cobalt-dioxolene families, which involved a quantitative density functional theory-based study benchmarked with reliable quasi-experimental references. The best-performing M06L-D4/def2-TZVPP level of theory has subsequently been verified by the synthesis and experimental investigation of three new complexes, two of which exhibit thermally-induced VT, while the third remains in the LS-CoIII-cat form across all temperatures, in agreement with prediction. Valence tautomerism in solution is markedly solvent-dependent, but the origin of this has not been definitively established. We have extended our computational approach to elucidate the correlation of VT transition temperature with solvent stabilisation energy and change in dipole moment. This new understanding may inform the development of VT compounds for applications in soft materials including films, gels, and polymers.

Modulating the electronic properties of divalent lanthanoid complexes with subtle ligand tuning.

Five new compounds of formula [LnII(Mentpa)2](BPh4)2 (Ln = Eu, n = 0 (1-Eu), n = 2 (2-Eu) and n = 3 (3-Eu); Ln = Yb, n = 0 (1-Yb) and n = 2 (2-Yb); tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings of Mentpa) have been synthesized and their structural, photophysical and electrochemical properties investigated. The LnII ions in the five complexes possess cubic coordination geometry and exhibit only small structural differences, due to the lengthening of the Ln-N bonds to accommodate the additional steric bulk associated with increasing methylation of the Mentpa ligands. Photophysical studies indicate moderate shifts in absorbance, emission and excitation bands associated with the 4f7 ↔ 4f65d1 (EuII) and 4f14 ↔ 4f135d1 (YbII) transitions, while electrochemistry reveals modulation of the redox potential of the LnII to LnIII oxidation. There is a strong correlation between Ln-N bond lengths and both the photophysical transition energies and metal redox-potentials, revealing how subtle ligand changes and ligand field effects can be used to modulate the electronic properties of complexes of divalent lanthanoid ions. Utilization of these insights may ultimately afford design and property tuning strategies for future functional molecular complexes based on divalent lanthanoid metals.

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism.

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-BIAN0 (L0), monoanionic radical Ar-BIAN•- (L•-), and dianionic Ar-BIAN2- (L2-) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN)3]n+ complexes (Ar = Ph, n = 2 (12+), 1 (1+), 0 (1); Ar = 3,5-CF3-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (32+), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-CoII-L0 containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-CoIII-(L•-)3 ⇌ HS-CoII-(L•-)2(L0) VT interconversion. Experimentally, structural and magnetic data reveal a HS-CoII-L0 containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-CoII-L0-containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-CoII-(L•-)2(L0) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-CoIII-(L•-)3 ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.

Rich redox-activity and solvatochromism in a family of heteroleptic cobalt complexes.

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.

Lanthanoid Complexes as Molecular Materials: The Redox Approach.

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.

A large axial magnetic anisotropy in trigonal bipyramidal Fe(ii).

The first trigonal bipyramidal Fe(ii) complex to display slow relaxation of magnetisation has been isolated, with this behaviour found to arise through a combination of a large magnetic anisotropy (D = -27.5 cm-1) and a pseudo-D3h symmetry at the Fe(ii) centre, as investigated through ab initio and magnetic studies.

Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes.

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.

Trigonal to Pentagonal Bipyramidal Coordination Switching in a Co(II) Single-Ion Magnet.

In molecular magnetism and single-ion magnets in particular, the observation of slow relaxation of the magnetization is intimately linked to the coordination environment of the metal center. Such systems typically have blocking temperatures well below that of liquid nitrogen, and therefore detailed magnetic characterization is usually carried out at very low temperatures. Despite this, there has been little advantage taken of ultralow temperature single-crystal X-ray diffraction techniques that could provide a full understanding of the crystal structure in the same temperature regime where slow magnetic relaxation occurs. Here, we present a systematic variable temperature single crystal X-ray diffraction study of [CoII(NO3)3(H2O)(HDABCO)] (1) {DABCO = 1,4-diazabicyclo[2.2.2]octane} conducted between 295 to 4 K. A reversible and robust disorder-to-order, single-crystal to single-crystal phase transition was identified, which accompanied a switching of the coordination geometry around the central Co(II) from 5- to 7-coordinate below 140 K. The magnetic properties were investigated, revealing slow relaxation of the magnetization arising from a large easy-plane magnetic anisotropy (+D) in the Co(II) pentagonal bipyramidal environment observed at low temperatures. This study highlights the importance of conducting thorough low temperature crystallographic studies, particularly where magnetic characterization is carried out at such low temperatures.

Investigation of the magnetic anisotropy in a series of trigonal bipyramidal Mn(ii) complexes.

Understanding how the magnetic anisotropy in simple coordination complexes can be manipulated is instrumental to the development of single-molecule magnets (SMMs). Clear strategies can then be designed to control both the axial and transverse contributions to the magnetic anisotropy in such compounds, and allow them to reach their full potential. Here we show a strategy for boosting the magnetic anisotropy in a series of trigonal bipyramidal Mn(ii) complexes - [MnCl3(HDABCO)(DABCO)] (1), [MnCl3(MDABCO)2]·[ClO4] (2), and [MnCl3(H2O)(MDABCO)] (3). These have been successfully synthesised using the monodentate [DABCO] and [MDABCO]+ ligands. Through static (DC) magnetic measurements and detailed theoretical investigation using ab initio methods, the magnetic anisotropy of each system has been studied. The calculations reveal that the rhombic zero-field splitting (ZFS) term (E) can be tuned as the symmetry around the Mn(ii) ion is changed. Furthermore, an in silico investigation reveals a strategy to increase the axial ZFS parameter (D) of trigonal bipyramidal Mn(ii) by an order of magnitude.

In-depth investigation of large axial magnetic anisotropy in monometallic 3d complexes using frequency domain magnetic resonance and ab initio methods: a study of trigonal bipyramidal Co(ii).

The magnetic properties of 3d monometallic complexes can be tuned through geometric control, owing to their synthetic accessibility and relative structural simplicity. Monodentate ligands offer great potential for fine-tuning the coordination environment to engineer both the axial and rhombic zero-field splitting (ZFS) parameters. In [CoCl3(DABCO)(HDABCO)] (1), the trigonal bipyramidal Co(ii) centre has two bulky axial ligands and three equatorial chloride ligands. An in-depth experimental and theoretical study of 1 reveals a large easy-plane magnetic anisotropy (+ve D) with a negligible rhombic zero-field splitting (E) due to the strict axial symmetry imposed by the C 3 symmetric ligand and trigonal space group. The large easy-plane magnetic anisotropy (D = +44.5 cm-1) is directly deduced using high-field EPR and frequency-domain magnetic resonance (FDMR) studies. Ab initio calculations reveal a large positive contribution to the D term arising from ground state/excited state mixing of the 4E'' states at ∼4085 cm-1 and a minor contribution from the spin-flip transition as well. The nature of the slow relaxation in 1 is elucidated through analysis of the rates of relaxation of magnetisation, taking into account Raman and direct spin-lattice relaxation processes and Quantum Tunnelling of the Magnetisation (QTM). The terms relating to the direct process and QTM were found based on the fit of the field-dependence of τ at 2 K. Subsequently, these were used as fixed parameters in the fit of the temperature-dependence of τ to obtain the Raman terms. This experimental-theoretical investigation provides further insight into the power of FDMR and ab initio methods for the thorough investigation of magnetic anisotropy. Thus, these results contribute to design criteria for high magnetic anisotropy systems.

Probing the origin of the giant magnetic anisotropy in trigonal bipyramidal Ni(ii) under high pressure.

Understanding and controlling magnetic anisotropy at the level of a single metal ion is vital if the miniaturisation of data storage is to continue to evolve into transformative technologies. Magnetic anisotropy is essential for a molecule-based magnetic memory as it pins the magnetic moment of a metal ion along the easy axis. Devices will require deposition of magnetic molecules on surfaces, where changes in molecular structure can significantly alter magnetic properties. Furthermore, if we are to use coordination complexes with high magnetic anisotropy as building blocks for larger systems we need to know how magnetic anisotropy is affected by structural distortions. Here we study a trigonal bipyramidal nickel(ii) complex where a giant magnetic anisotropy of several hundred wavenumbers can be engineered. By using high pressure, we show how the magnetic anisotropy is strongly influenced by small structural distortions. Using a combination of high pressure X-ray diffraction, ab initio methods and high pressure magnetic measurements, we find that hydrostatic pressure lowers both the trigonal symmetry and axial anisotropy, while increasing the rhombic anisotropy. The ligand-metal-ligand angles in the equatorial plane are found to play a crucial role in tuning the energy separation between the d x2-y2 and d xy orbitals, which is the determining factor that controls the magnitude of the axial anisotropy. These results demonstrate that the combination of high pressure techniques with ab initio studies is a powerful tool that gives a unique insight into the design of systems that show giant magnetic anisotropy.

Ligand-directed synthesis of {Mn} twisted bow-ties.

Two isostructural polymetallic complexes [Mn(µ3-O)2(CH3COO)4(L1)4]- and [Mn(µ3-O)2(CH3COO)4(L2)4]- have been synthesised by using two Schiff base ligands derived from 3,5-diamino-1,2,4-triazole, following two different preparative routes, either using the pre-formed ligand (for L1) or via a metal-mediated template synthesis (for L2). The {Mn} structure is unusual, being based on two corner-sharing perpendicular {Mn3} triangles forming a twisted bow-tie. The magnetic studies reveal antiferromagnetic coupling between Mn(iii) ions while electrochemical experiments are consistent with a quasi-reversible Mn(iii)↔Mn(iv) redox process at the central manganese ion.