Cobalt Complexes with “Click”-Derived Functional Tripodal Ligands: Spin Crossover and Coordination Ambivalence

Fluorinated Click-Derived Tripodal Ligands Drive Spin Crossover in both Iron(II) and Cobalt(II) Complexes

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently,...

Spin-state control of cobalt(II) and iron(II) complexes with click-derived tripodal ligands through non-covalent and fluorine-specific interactions

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF₅TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF₅TA)₂(BF₄)₂·2EtOH displays spin crossover (SCO).

Molecular Valence Tautomeric Metal Complexes for Chemosensing

Switchable valence tautomeric metal complexes have been long suggested for applications as chemosensors. However, no such molecular sensors have been yet reported. Here, we present a concept for sensing and the first prototype molecular sensor based on valence tautomeric cobalt-dioxolenes. A valence tautomeric cobalt-dioxolene complex [ls-Co^III(SQ^•)(Cat)(stypy)₂] ⇄ [hs-Co^II(SQ^•)₂(stypy)₂] 1 (ls = low spin, hs = high spin, Cat = 3,5-di-tert-butylcatecholate(2-), SQ = one-electron oxidized, benzosemiquinone(1-) form of Cat, stypy = trans-4-styrylpyridine) has been used as a molecular sensor. The lability of axial stypy ligands of 1 in solution allows us to exchange stypy ligands by dimethyl sulfoxide and simple pyridine analytes in a controllable way, which triggers colorimetric and magnetic responses.

Copper(ii) and zinc(ii) complexes of mono- and bis-1,2,3-triazole-substituted heterocyclic ligands

Chelating 1,4-disubstituted mono- (8a-8d) and bis-1,2,3-triazole-based (9a-11a) ligands were prepared by regioselective copper(i)-catalysed 1,3-dipolar cycloaddition of terminal alkynes with aromatic azides, together with bioconjugate 13a synthesized by amide coupling of l-phenylalanine methyl ester to 11a. Cu(ii) and Zn(ii) complexes were prepared and single crystal structures were determined for complexes 8aCu, 8dCu, 9cCu and 10cCu, as well as the free ligands 10a and 10c. The in situ prepared Zn(ii) complexes were studied by NMR spectroscopy, while the stoichiometry of the Cu(ii) complexes in solution was determined by UV-Vis titrations and confirmed by the electronic structure DFT calculations at the (SMD)/M05-2X/6-31+G(d)/LanL2DZ+ECP level of theory.

Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes

A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R¹)-4-(R²)-pyrrolide anions (dpp^R1,R2)^-, varying in the nature of the R¹ and R² substituents [R¹, R² = CN, Ph; CO₂Et, CO₂Et; CO₂Me, 4-Py; CO₂Me, Me; Me, Me], has been used to generate the analogous family of neutral [Co^II(dpp^R1,R2)₂] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [Co^II(dpp^Me,Me)₂], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl₃ solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn-Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible Co^III/II redox potentials are predictably tuned by choice of substituents R¹ and R², from -0.95 (Me,Me) to -0.45 (CN,Ph) V vs Fc⁺/Fc, with a linear correlation observed between E_1/2(Co^III/II) and the Swain-Lupton parameters of the pyrrolide substituents.

Structural snapshots in the copper(ii) induced azide–nitrile cycloaddition: effects of peripheral ligand substituents on the formation of unsupported μ1,1-azido vs. μ1,4-tetrazolato bridged complexes

Peripheral substituents on click-derived tripodal ligands dictate the reactivity of their copper(ii) azido complexes.

Probing bistability in FeIIand CoIIcomplexes with an unsymmetrically substituted quinonoid ligand

A quinone ligand with a [O,O,O,N] donor set is presented. Its Fe^IIcomplex displays bistability as a function of temperature, pressure and photoexcitation and its Co^IIcomplex displays a redox-induced spin-state change.

Exploring potential cooperative effects in dicopper(i)-di-mesoionic carbene complexes: applications in click catalysis

Dicopper(i) dimesoionic carbene complexes are active catalysts for the azide–alkyne cycloaddition reaction and are more active than their mononuclear counterparts.

Distortion Pathways of Transition Metal Coordination Polyhedra Induced by Chelating Topology

A continuous shape measures analysis of the coordination polyhedra of a host of transition metal complexes with bi- and multidentate ligands discloses the distortion pathway associated with each particular topology of the chelate rings formed. The basic parameter that controls the degree of distortion is the metal-donor atom bond distance that induces nonideal bond angles due to the rigidity of the ligands. Thus, the degree of distortion within each family of complexes depends on the atomic size, on which the high- or low-spin state has a large effect. Special attention is therefore paid to several spin-crossover systems and to the enhanced distortions that go along with the transition from low- to high-spin state affected by temperature, light, or pressure. Several families of complexes show deviations from the expected distortion pathways in the high-spin state that can be associated to the onset of intermolecular interactions such as secondary coordination of counterions or solvent molecules. Also, significant displacement of counterions in an extended solid may result from the changes in metal-ligand bond distances when ligands are involved in intermolecular hydrogen bonding.

The ligand field of the azido ligand: insights into bonding parameters and magnetic anisotropy in a Co(II)-azido complex

The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)-azido complex (1), which has been characterized experimentally (magnetically and by electronic d-d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = -10.7 cm(-1), E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d-d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)-azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3(-) ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co-N bond distances) of the axial ligand on the N3(-) site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.

Hybrid Pyrazolyl-1,2,3-Triazolyl Tripodal Tetraamine Ligands: Click Synthesis and Cobalt(III) Complexes

A family of tripodal tetraamine ligands incorporating two pyrazolyl and one 1,2,3-triazolyl donor arm have been synthesized in modest-to-excellent yields (42–90 %) using the copper(i)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Mono-, bis-, and tris-tripodal ligand scaffolds were readily generated using this method. The coordination chemistry of the ligands with cobalt(iii) ions has been studied, and cobalt(iii) carbonato complexes of the ligands have been isolated and characterized spectroscopically and crystallographically. X-ray crystallography and NMR spectroscopy of the mono-metallic complexes showed that racemic mixtures of the cis-isomer are formed selectively. The di- and tri-metallic systems could not be crystallized, but NMR spectroscopy indicates that these compounds were isolated as mixtures of stereoisomers.

Photoswitchable stable charge-distributed states in a new cobalt complex exhibiting photo-induced valence tautomerism

We report the synthesis and magnetic and photomagnetic behaviour of a novel valence tautomeric cobalt complex, [Co(3,5-dbbq)₂(μ-bpym)] (1) (3,5-dbbq = 3,5-di-tert-butyl-1,2-benzoquinone and μ-bpym = 2,2′-bipyrimidine). The synthesis is performed by reacting Co₂(CO)₈ and μ-bpym in the presence of the ligand 3,5-dbbq in a mixed solvent under inert atmosphere. The magnetic behavior clearly shows the presence of electron transfer from the catecholate ligand to the cobalt center, producing valence tautomers of [Co^II(SQ)₂] with a transition temperature (T_1/2) of 215 K. Photomagnetic studies, performed via both SQUID magnetometry and X-band electron paramagnetic resonance, show the clear presence of photoinduced valence tautomerism, at temperatures considerably higher than previous systems. A metastable charge distribution is observed, strengthening previous investigations on the character of mixed valence ligands. Entropy-driven valence tautomeric interconversion is observed, and drives the transition to the most stable charge distribution. The complex has the ability to coordinate and can be used as a photoswitchable building block, with the photomagnetic characterisation evidencing a metastable state lifetime of the photo-induced valence tautomeric process of ca. 2.9 × 10⁴ s below 20 K. The observed yields are higher than ones in similar systems, showing that tiny changes in the molecular structures may have a huge impact.

Cyclometalated mono- and dinuclear Ir(III) complexes with "click"-derived triazoles and mesoionic carbenes

Orthometalation at Ir(III) centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C-H activation sites, we present here mono- and dinuclear Ir(III) complexes with "click"-derived 1,2,3-triazole and 1,2,3-triazol-5-ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir(III) centers. Structural characterization of the complexes reveal a piano stool-type of coordination around the metal centers with the "click"-derived ligands bound either with C^N or C^C donor sets to the Ir(III) centers. Furthermore, whereas bond localization is observed within the 1,2,3-triazole ligands, a more delocalized situation is found in their 1,2,3-triazol-5-ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer-pure, dinuclear cyclometalated Ir(III) complexes with poly-mesoionic-carbene ligands.

Ruthenium complexes of tripodal ligands with pyridine and triazole arms: subtle tuning of thermal, electrochemical, and photochemical reactivity

Electrochemical and photochemical bond-activation steps are important for a variety of chemical transformations. We present here four new complexes, [Ru(L(n) )(dmso)(Cl)]PF6 (1-4), where L(n) is a tripodal amine ligand with 4-n pyridylmethyl arms and n-1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L(2) , two isomers (with respect to the position of the triazole arm, equatorial or axial), trans-2sym and trans-2un , could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru(II) /Ru(III) oxidation potentials, but it increases the stability of the RuSdmso bond. Hence, the oxidation waves become more reversible from trans-1 to trans-4, and whereas the dmso ligand readily dissociates from trans-1 upon heating or irradiation with UV light, the RuS bond of trans-4 remains perfectly stable under the same conditions. The strength of the RuS bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans-2sym and trans-2un . A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.