Zn(4,4'-bipy)2SiF6.2H20 (4,4'-bipy = 4,4'-bipyridine) consists of two perpendicular and equivalent stacks of infinite, essentially square grid [Zn(H20)2(4,4'-bipy)2],,2n+ sheets, which interpenetrate so that any particular sheet has an infinite number of perpendicular ones enmeshed or concatenated with it.

The dioxo-Mo(VI) complexes LM0O2X [L = hydrotris(3,5-dimethylpyrazol-1 -yl)borate (La), hydrotris(3isopropylpyrazol-1 -yl)borate (Lb), hydrotris(3,5-dimethyl-1,2,4-triazol-1 -yl)borate (Lc); X = Cl, Br, NCS, OMe, OEt, OPh, SPr', SPh, SCH2Ph] have been synthesized and characterized by spectroscopic and structural methods.The infrared spectra of the complexes exhibit ( 2) bands at 940-920 and 910-890 cm-1, and the NMR spectra are indicative of molecular C, symmetry in solution. The X-ray crystal structures of three complexes are reported. LaMo02(SPh): monoclinic space group P2\lc, a = 18.265( 6) Á, b = 8.110(3) Á, c = 18.299(3) Á, ß = 117.06(2)°, V = 2414(1) Á* 123 with Z = 4. LbMo02(OMe): monoclinic space group Cllc, a = 30.365(4) Á, b = 8.373(1) Á, c = 19.646( 2) Á, ß = 113.28(1)°, V = 4588(1) Á3 5with Z = 8. LcMo02(SPh): orthorhombic space group P2,2,2,, a = 7.9302(13) k,b = 16.627(2) k,c= 17.543(2) k,V= 2313.1(9) Á3 with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043,0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries. The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMovC>2X]-. The requirements for reversible, one-electron electrochemical reduction of dioxo-Mo(VI) complexes appear to be (i) minimal conformational change, restricting substitution trans to the oxo groups, upon reduction and (ii) a steric or electrostatic barrier to the close approach and dinucleation of the reduced species. A number of oxo-hydroxo-Mo(V) complexes of the type LMovO-(OH)X were generated by protonation of the anions [LMov02X]~. Chemical reduction by Bun4NSH results in the sequential generation of [LMov02X]-and [LMovOSX]-anions (except for X = OPh, SPh, and SPr1, when only [LMov02X]is formed). The Mo(V) complexes have been characterized by EPR spectroscopy.

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 {Co III -cat-cat-Co III } ⇌ {Co III -cat-SQ-Co II }⇌{Co II -SQ-SQ-Co II } (cat 2− = 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 (spiroH 4 ) or 3,3,3′,3′-tetramethyl-1,1′-spirobi(indan)-4,4′,7,7′-tetrabromo-5,5′,6,6′-tetraol (Br 4 spiroH 4 ) with Me n tpa 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 {Co III -cat-cat-Co III } states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {Zn II -cat-SQ-Zn II } 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.

The family of complexes of general formula [Co(Me n tpa)(Xdiox)] + (tpa = tris(2-pyridylmethyl)amine, n = 0−3 corresponds to successive methylation of the 6-position of the pyridine rings; X = Br 4 , Cl 4 , H 4 , 3,5-Me 2 , 3,5-tBu 2 ; 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(Me 3 tpa)(Br 4 diox)] + (1 + ). One-electron oxidation of neutral precursor [Co II (Me 3 tpa)-(Br 4 cat)] (1) enabled isolation of target compounds 1(PF 6 ) and 1(BPh 4 ). 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(PF 6 ) and 1(BPh 4 ) reveal the onset of VT transitions above room temperature.

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Cobaltocene reduction of LMoVIO2(SPh) complexes yielded extremely dioxygen-sensitive cobaltocenium salts of the cis-dioxo-Mo(V) radical anions cis-[LMoVO2(SPh)]-, while sodium acenaphthalenide reduction of LaMoVIO2(SPh) yielded a complex sodium salt containing [LaMoVO2(SPh)]- [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (La), hydrotris(3-isopropylpyrazol-1-yl)borate (Lb), or hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Lc)]. Crystals of [CoCp2][LcMoVO2(SPh)]·toluene were orthorhombic, space group Pbca, with a = 17.695(3) Å, b = 19.490(3) Å, c = 21.925(4) Å, V = 7561(2) Å3 for Z = 8. The structure, refined using a full-matrix least-squares procedure and 2269 data, converged with R = 0.067 (R w = 0.067). In the distorted octahedral anion, the metrical parameters of the cis-MoVO2 fragment [Mo−O = 1.742(9) Å, O−Mo−O = 112.1(4)°] are significantly larger than those of LcMoVIO2(SPh) [average Mo−O = 1.700(6) Å, O−Mo−O = 103.9(2)°]. 18O-Substitution of [CoCp2][La,bMoVO2(SPh)] permitted the assignment of bands at ca. 870 and 770 cm-1 to the νs and νas modes, respectively, of the cis-MoVO2 fragment. Freshly prepared solutions of [CoCp2][LMoVO2(SPh)] exhibited a broad EPR signal (g, 1.92; W 1/2, 20 G) characteristic of a cis-[MoVO2]+ complex. The signal was stable when L = Lc but when L = La or Lb it was replaced by a proton-coupled signal characteristic of a cis-[MoVO(OH)]2+ center. The complex LaMoVO(OH)(SPh) was isolated as a coprecipitate with its conjugate base salt [CoCp2][LaMoVO2(SPh)]. It was characterized by infrared bands at 915 and 535 cm-1, assigned to ν(MoO) and ν(MoOH) modes, respectively. The anions were readily converted to air-stable LMoVO(OSiMe3)(SPh) upon reaction with Me3SiCl and reacted with dioxygen to regenerate LMoVIO2(SPh). The paper reports the first isolation in substance of compounds containing novel cis-[MoVO2]+ and cis-[MoVO(OH)]2+ centers and the first crystallographic characterization of a cis-dioxo-Mo(V) species.

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: Na[Ln(WO)] (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 transitions allowed by selection rules and interpret the observed relative intensities of the INS peaks. Ultimately, we have been able to establish the relationship between the wave function composition of the CF split Ln ground multiplets and the experimentally measured magnetic and spectroscopic properties for the various analogs of the Na[Ln(WO)] family.

Five members of a new family of polyoxometalate (POM)-ligated tetranuclear rare earth metal complexes have been synthesized and characterized. These compounds have the general formula (HDABCO)(8)H(5)Li(8)[Ln(4)As(5)W(40)O(144)(H(2)O)(10)(gly)(2)]·25H(2)O [Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Y = (5), HDABCO = monoprotonated 1,4-diazabicyclooctane, gly = glycine] and were synthesized from the preformed POM precursor [As(2)W(19)O(67)(H(2)O)](14-). The structure is comprised of two {As(2)W(19)O(68)} building blocks linked by a unit containing four rare earth ions and two additional tungsten centers, with the two glycine ligands playing a key bridging role. Two crystallographically distinct rare earth ions are present in each complex, both of which possess axially compressed, approximate square antiprismatic coordination geometry. The variable-temperature magnetic susceptibility profiles for 2-4 are dominated by population/depopulation of the M(J) sublevels of the relevant ground terms, and fitting of the data has afforded the ligand field parameters in each case, from which the energies of the M(J) sublevels can be calculated. Alternating current magnetic susceptibility data have revealed the onset of slow magnetic relaxation for 3, with the energy barrier to magnetization reversal determined to be 3.9(1) K. As for other lanthanoid complexes that display slow magnetic relaxation, this energy barrier is due to the splitting of the M(J) sublevels of the Dy(3+) ions such that the ground sublevel has a relatively large |M(J)| value, thereby affording Ising-type magnetic anisotropy. This complex is thus the first POM-supported polynuclear lanthanoid-based SMM. Simulation of the W-band EPR spectrum of 1 has afforded the spin Hamiltonian parameters for this species, while the X-band EPR spectrum of 3 indicates the presence of a non-negligible fourth-order transverse component of the anisotropy, which is responsible for the small effective energy barrier observed for 3 and the absence of slow magnetic relaxation for 4.