Heptanuclear Mixed-Valence Co4IIICo3II Molecular Wheel─A Molecular Analogue of Layered Double Hydroxides with Single-Molecule Magnet Behavior and Electrocatalytic Activity for Hydrogen Evolution Reactions.

We present a bifunctional heptanuclear cobalt(II)/cobalt(III) molecular complex formulated as [Co7(µ3-OH)4(H2L1)2(HL2)2](NO3)6·6H2O (1) (where H5L1 is 2,2'-(((1E,1'E)-((2-hydroxy-5-methyl-1,3-phenylene)bis(methanylylidene))bis(azanylylidene))bis(propane-1,3-diol)) and H2L2 is 2-amino-1,3-propanediol). Compound 1 has been characterized by single-crystal X-ray diffraction analysis along with other spectral and magnetic measurements. Structural analysis indicates that 1 contains a mixed-valence Co7 cluster where a central Co(II) ion is connected to six different Co centers (four CoIII and two CoII ions) by four µ3-OH groups, giving rise to a planar heptanuclear cluster that resembles a molecular fragment of a layered double hydroxide (LDH). Two triply deprotonated (H2L1)3- ligands form the outer side of the cluster while two singly deprotonated (HL2)- ligands are located at the top and bottom of the central heptanuclear core. Variable temperature magnetic measurements indicate the presence of weak ferromagnetic CoII···CoII interactions (J = 3.53(6) cm-1) within the linear trinuclear CoII cluster. AC susceptibility measurements show that 1 is a field-induced single-molecule magnet (SMM) with τ0 = 8.2(7) × 10-7 s and Ueff = 11.3(4) K. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 in homogeneous phase shows an overpotential of 455 mV, with a Faradaic efficiency of 81% and a TOF of 8.97 × 104 µmol H2 h-1 mol-1.

Crossover from Antiferromagnetic to Ferromagnetic Exchange Coupling in a New Family of Bis-(µ-phenoxido)dicopper(II) Complexes: A Comprehensive Magneto-Structural Correlation by Experimental and Theoretical Study.

Five neutral bis(µ-phenoxido)dicopper(II) complexes, [Cu2(LMe,Me,Me)2] (1), [Cu2(LMe,Me,Et)2]·CH2Cl2 (2), [Cu2(L i-Pr,i-Pr,i-Pr)2]·2H2O (3), [Cu2(L t-Bu,Me,i-Pr)2] (4), and [Cu2(L t-Bu,t-Bu,i-Pr)2]·H2O (5) have been synthesized and characterized by single-crystal X-ray diffraction analyses, magnetic studies, and density functional theory (DFT) calculations, in which the ligands [H2LMe,Me,Me = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N',N'-dimethylethylene-1,2-diamine, H2LMe,Me,Et = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N',N'-dimethylethylene-1,2-diamine, H2L i-Pr,i-Pr,i-Pr = N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N',N'-diisopropylethylene-1,2-diamine, H2L t-Bu,Me,i-Pr = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N',N'-diisopropylethylene-1,2-diamine, and H2L t-Bu,t-Bu,i-Pr = N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N',N'-diisopropylethylene-1,2-diamine] contain the same [O,N,N,O]-donor atoms combination but differ in substituents at phenol rings and at an amino nitrogen atom. The effect of these remote substituents on the nature of exchange coupling interactions (ferromagnetic vs antiferromagnetic) between the copper(II) ions has been investigated. The average Cu-O-Cu angle, Cu-O-Cu-O torsion angle, and Cu···Cu separation in 1-5 are varied systematically by these remote ligand substituents in the range 98.6-83.3°, 26.0-46.5°, and 2.982-2.633 Å, respectively. As a result, the intramolecular spin-spin coupling in these complexes are changing gradually from a strong antiferromagnetic (J = -395 cm-1, where H = -JS 1 S 2) to a moderate ferromagnetic (J = +53.2 cm-1) regime. The crossover angle at which the magnetic interaction changes from antiferromagnetic to ferromagnetic (J = 0) is determined to be ca. 87° for this series of dicopper(II) complexes. DFT calculations support the experimentally determined crossover angle and disclose various magneto-structural correlations in the series 1-5.

Synthesis and magneto-structural studies on a new family of carbonato bridged 3d-4f complexes featuring a [CoLn(CO3)] (Ln = La, Gd, Tb, Dy and Ho) core: slow magnetic relaxation displayed by the cobalt(ii)-dysprosium(iii) analogue.

A new family of [3 + 3] hexanuclear 3d-4f complexes [(µ3-CO3){CoIILnIIIL(µ3-OH)(OH2)}3]-(ClO4)·mC2H5OH·nH2O (1-5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62-78%) following a self-assembly reaction between the ligand 6,6',6''-(nitrilotris(methylene))tris-(2-methoxy-4-methylphenol) (H3L), Co(OAc)2·4H2O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare η2:η2:η2-µ3 mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the GdIII analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the CoGd complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the CoII-GdIII superexchange, mediated via the bridging oxygen atoms. Only the CoII-DyIII compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/kB = 9.2 K and the relaxation time constant τ0 = 1.0 × 10-7 s.

Dinuclear Iron(III) and Cobalt(III) Complexes Featuring a Biradical Bridge: Their Molecular Structures and Magnetic, Spectroscopic, and Redox Properties.

Bis-bidentate ligand H4LB featuring two o-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(LR)MIII(•LB•)MIII(LR)](ClO4)2, M = Fe (1) and Co (2, 3), with HLR (R = CH3, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (LB)4- becomes oxidized, resulting in the formation of a rare example of a biradical (•LB•)2- bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by Mössbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species 1 contains two high-spin Fe(III) ions (S = 5/2) each coupled strongly antiferromagnetically (|J| > 150 cm-1; H = -2JS1S2) with a semiquinone π-radical (S = 1/2) form of the bridging (•LB•)2- ligand. The effective S = 2 spins of each [Fe(III)+R●] monomeric unit are then weakly ferromagnetically coupled with J = +0.22 cm-1. Species 2 and 3 reveal very similar electronic structures: the low-spin Co(III) ion is diamagnetic, which leaves the two-spin carriers at the bridging (•LB•)2- biradical to display an isotropic EPR signal at g = 1.995 for 2 (1.993 for 3) in solution at room temperature and in the frozen state with no hyperfine structure. The weak half-field signal at g = 3.988 for 2 (3.978 for 3) was also observed at 17 K for the spin-forbidden |ΔMS| = 2 transition due to ferromagnetically coupled S = 1/2 spins (J = +47 cm-1) of the bridging biradical. The compounds show rich electrochemistry, displaying two (1) or four (2, 3) one-electron reversible processes. Normal and differential pulse voltammetry as well as constant potential coulometry, combined with EPR experiments, confirmed that the observed electron transfers are all localized at the bridging noninnocent (•LB•)2- ligand.

Ligand-Induced Tuning of the Oxidase Activity of µ-Hydroxidodimanganese(III) Complexes Using 3,5-Di-tert-butylcatechol as the Substrate: Isolation and Characterization of Products Involving an Oxidized Dioxolene Moiety.

Oxidase activities of a µ-hydroxidodimanganese(III) system involving a series of tetradentate capping ligands H2LR1,R2 with a pair of phenolate arms have been investigated in the presence of 3,5-di-tert-butylcatechol (H2DBC) as a coligand cum-reductant. The reaction follows two distinctly different paths, decided by the substituent combinations (R1 and R2) present in the capping ligand. With the ligands H2Lt-Bu,t-Bu and H2Lt-Bu,OMe, the products obtained are semiquinonato compounds [MnIII(Lt-Bu,t-Bu)(DBSQ)]·2CH3OH (1) and [MnIII(Lt-Bu,OMe)(DBSQ)]·CH3OH (2), respectively. In the process, molecular oxygen is reduced by two electrons to generate H2O2 in the solution, as confirmed by iodometric detection. With the rest of the ligands, viz., H2LMe,Me, H2Lt-Bu,Me, H2LMe,t-Bu, and H2LCl,Cl, the products initially obtained are believed to be highly reactive quinonato compounds [MnIII(LR1,R2)(DBQ)]+, which undergo a domino reaction with the solvent methanol to generate products of composition [MnIII(LR1,R2)(BMOD)] (3-6) involving a nonplanar dioxolene moiety, viz., 3,5-di-tert-butyl-3-methoxy-6-oxocyclohexa-1,4-dienolate (BMOD-). This novel dioxolene derivative is formed by a Michael-type nucleophilic 1,4-addition reaction of the methoxy group to the coordinated quinone in [MnIII(LR1,R2)(DBQ)]+. During this reaction, molecular oxygen is reduced by four electrons to generate water. The products have been characterized by single-crystal X-ray diffraction analysis as well as by spectroscopic methods and magnetic measurements. Density functional theory calculations have been made to address the observed influence of the secondary coordination sphere in tuning the two-electron versus four-electron reduction of dioxygen. The semiquinone form of the dioxolene moiety is stabilized in compounds 1 and 2 because of extended electron delocalization via participation of the appropriate metal orbital(s).

Pentanuclear 3d-4f Heterometal Complexes of M(II)3Ln(III)2 (M = Ni, Cu, Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism, and Photoluminescence Properties.

A new family of pentanuclear 3d-4f heterometal complexes of general composition [Ln(III)2(M(II)L)3(µ3-O)3H](ClO4)·xH2O (1-5) [Ln = Nd, M = Zn, 1; Nd, Ni, 2; Nd, Cu, 3; Gd, Cu, 4; Tb, Cu, 5] have been synthesized in moderate yields (50-60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., N,N-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-N('),N(')-diethylethylenediamine (H2L). Single-crystal X-ray diffraction analyses have been used to characterize these complexes. The compounds are all isostructural, having a 3-fold axis of symmetry that passes through the 4f metal centers. The [M(II)L] units in these complexes are acting as bis-bidentate metalloligands and, together with µ3-oxido bridging ligands, complete the slightly distorted monocapped square antiprismatic nine-coordination environment around the 4f metal centers. The cationic complexes also contain a H(+) ion that occupies the central position at the 3-fold axis. Magnetic properties of the copper(II) complexes (3-5) show a changeover from antiferromagnetic in 3 to ferromagnetic 3d-4f interactions in 4 and 5. For the isotropic Cu(II)-Gd(III) compound 4, the simulation of magnetic data provides very weak Cu-Gd (J1 = 0.57 cm(-1)) and Gd-Gd exchange constants (J2 = 0.14 cm(-1)). Compound 4 is the only member of this triad, showing a tail of an out-of-phase signal in the ac susceptibility measurement. A large-spin ground state (S = 17/2) and a negative value of D (-0.12 cm(-1)) result in a very small barrier (8 cm(-1)) for this compound. Among the three Nd(III)2M(II)3 (M = Zn(II), Ni(II), and Cu(II)) complexes, only the Zn(II) analogue (1) displays an NIR luminescence due to the (4)F(3/2) → (4)I(11/2) transition in Nd(III) when excited at 290 nm. The rest of the compounds do not show such Nd(III)/Tb(III)-based emission. The paramagnetic Cu(II) and Ni(II) ions quench the fluorescence in 2-5 and thereby lower the population of the triplet state.

Heterobimetallic µ-oxido complexes containing discrete V(V)-O-M(III) (M = Mn, Fe) cores: targeted synthesis, structural characterization, and redox studies.

Heterobimetallic compounds [L'OV(V)(µ-O)M(III)L]n (n = 1, M = Mn, 1-5; n = 2, M = Fe, 6 and 7) containing a discrete unsupported V(V)-O-M(III) bridge have been synthesized through a targeted synthesis route. In the V-O-Mn-type complexes, the vanadium(V) centers have a square-pyramidal geometry, completed by a dithiocarbazate-based tridentate Schiff-base ligand (H2L'), while the manganese(III) centers have either a square-pyramidal (1 and 3) or an octahedral (2 and 5) geometry, made up of a Salen-type tetradentate ligand (H2L) as established by X-ray diffraction analysis. The V-O-Mn bridge angle in these compounds varies systematically from 155.3° to 128.1° in going from 1 to 5 while the corresponding dihedral angle between the basal planes around the metal centers changes from 86.82° to 20.92°, respectively. The V-O-Fe-type complexes (6 and 7) are tetranuclear, in which the two dinuclear V(µ-O)Fe units are connected together by apical iron(III)-aryl oxide interactions, forming a dimeric structure with a pair of Fe-O-Fe bridges. The X-ray data also confirm the V═O → M canonical form to contribute predominantly on the overall V-O-M bridge structure. The molecules in solution also retain their heterobinuclear composition, as established by electrospray ionization mass spectrometry and (51)V NMR spectroscopy. Electrochemically, these complexes are quite interesting; the manganese(III) complexes (1-5) display three successive reductions (processes I-III), each with a monoelectron stoichiometry. Process I is due to a Mn(III)/Mn(II) reduction (E1/2 ranges between -0.32 and -0.05 V), process II is a ligand-based reduction, and process III (E1/2 = ∼1.80 V) owes its origin to a V(V)O/V(IV)O reduction; all potentials are versus Ag/AgCl. The iron(III) compounds (6 and 7), on the other hand, show at least four irreversible processes, appearing at Epc = -0.20, -1.0, -1.58, and -1.68 V in compound 6 (processes IV-VII), together with a reversible process (process VIII) at E1/2 = -1.80 V (ΔEp = 80 mV). While the first two of these are due to Fe(III)/Fe(II) reductions at the two iron(III) centers of these tetranuclear cores, the reversible reduction at a more negative potential (ca. -1.80 V) is due to a V(V)O/V(IV)O-based electron transfer.

Tetranuclear hetero-metal [Co(II)2Ln(III)2] (Ln = Gd, Tb, Dy, Ho, La) complexes involving carboxylato bridges in a rare µ4-η(2):η(2) mode: synthesis, crystal structures, and magnetic properties.

A new family of 3d-4f heterometal 2 × 2 complexes [Co(II)2(L)2(PhCOO)2Ln(III)2(hfac)4] (1-5) (Ln = Gd (compound 1), Tb (compound 2), Dy (compound 3), Ho (compound 4), and La (compound 5)) have been synthesized in moderate yields (48-63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co(II)(H2L)(PhCOO)2] (H2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine) as a metalloligand and [Ln(III)(hfac)3(H2O)2] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn-Dy analog [Zn(II)2(L)2(PhCOO)2Dy(III)2(hfac)4] 6 to help us in understanding the magnetic properties of these compounds. The compounds 1-6 are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare µ4-η(2):η(2) bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co(II)2Dy(III)2] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads U/kB = 8.8 K (τ0 = 2.0 × 10(-7) s) at BDC = 0, and U/kB = 7.8 K (τ0 = 3.9 × 10(-7) s) at BDC = 0.1 T. The [Zn(II)2Dy(III)2] compound also behaves as a single-molecule magnet with U/kB = 47.9 K and τ0 = 2.75 × 10(-7) s.

Homo- and heterometal complexes of oxido-metal ions with a triangular [V(V)O-MO-V(V)O] [M = V(IV) and Re(V)] core: reporting mixed-oxidation oxido-vanadium(V/IV/V) compounds with valence trapped structures.

A new family of trinuclear homo- and heterometal complexes with a triangular [V(V)O-MO-V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido-metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, N,N'-bis(3-hydroxysalicylidene)-diiminoalkanes/arene (H4L(1)-H4L(3)). The upper compartment of these ligands with N2O2 donor combination (Salen-type) contains either a V(IV) or a Re(V) center, while the lower compartment with O4 donor set accommodates two V(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in 1-3. Compound 1 crystallizes in a monoclinic space group P2(1)/c, while both 2 and 3 have more symmetric structures with orthorhombic space group Pnma that renders the vanadium(V) centers in these compounds exactly identical. In DMF solution, compound 1 displays an 8-line EPR at room temperature with and values of 1.972 and 86.61 × 10(-4) cm(-1), respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p(3/2) and 2p(1/2) electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds 1 and 2. Electrochemically, compound 1 undergoes two one-electron oxidations at E(1/2) = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H4L(3) is spacious enough to accommodate Re(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (3) as established by X-ray crystallography.