RESUMEN

A series of monochloride-bridged, dinuclear metallacycles of the general formula [M2(µ-Cl)(µ-L)2](ClO4)3 have been prepared using the third-generation, ditopic bis(pyrazolyl)methane ligands L = m-bis[bis(1-pyrazolyl)methyl]benzene (Lm), M = Cu(II), Zn(II), and L = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*), M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). These complexes were synthesized from the direct reactions of M(ClO4)2·6H2O, MCl2, and the ligand, Lm or Lm*, in the appropriate stoichiometric amounts. Three analogous complexes of the formula [M2(µ-Cl)(µ-L)2](BF4)3, L = Lm, M = Cu(II), and L = Lm*, M = Co(II), Cu(II), were prepared from the reaction of [M2(µ-F)(µ-L)2](BF4)3 and (CH3)3SiCl. The bromide-bridged complex [Cu2(µ-Br)(µ-Lm*)2](ClO4)3 was prepared by the first method. Three acyclic complexes, [Co2(µ-Lm)µ-Cl4], [Co2(µ-Lm*)Cl4], and [Co2(µ-Lm*)Br4], were also prepared. The structures of all [M2(µ-X)(µ-L)2]3+ (X = Cl-, Br-) complexes have two ditopic bis(pyrazolyl)methane ligands bridging two metals in a metallacyclic arrangement. The fifth coordination site of the distorted trigonal bipyramidal metal centers is filled by a bridging halide ligand that has an unusual linear or nearly linear M-X-M angle. The NMR spectra of [Zn2(µ-Cl)(µ-Lm*)2](ClO4)3 and especially [Cd2(µ-Cl)(µ-Lm*)2](ClO4)3 demonstrate that the metallacycle structure is maintained in solution. Solid state magnetic susceptibility data for the copper(II) compounds show very strong antiferromagnetic exchange interactions, with -J values of 536 cm-1 for [Cu2(µ-Cl)(µ-Lm)2](ClO4)3·xCH3CN, 720 cm-1 for [Cu2(µ-Cl)(µ-Lm*)2](ClO4)3, and 945 cm-1 for [Cu2(µ-Br)(µ-Lm*)2](ClO4)3·2CH3CN. Smaller but still substantial antiferromagnetic interactions are observed with other first row transition metals, with -J values of 98 cm-1 for [Ni2(µ-Cl)(µ-Lm*)2](ClO4)3, 55 cm-1 for [Co2(µ-Cl)(µ-Lm*)2](ClO4)3, and 34 cm-1 for [Fe2(µ-Cl)(µ-Lm*)2](ClO4)3. EPR spectra of [Cu2(µ-Cl)(µ-Lm*)2](BF4)3 confirm the dz2 ground state of copper(II). In addition, the sign of the zero-field splitting parameter D was determined to be positive for [Cu2(µ-F)(µ-Lm*)2](BF4)3. Electronic spectra of the copper(II) complexes as well as Mössbauer spectra of the iron(II) complexes were also studied in relation with the EPR spectra and magnetic properties, respectively. Density functional theory calculations were performed using ORCA, and exchange integral values were obtained that parallel but are slightly higher than the experimental values by about 30%.

RESUMEN

The reactions of Cu(ClO4)2 with NaCN and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*) yield [Cu2(µ-CN)(µ-Lm)2](ClO4)3 (1) and [Cu2(µ-CN)(µ-Lm*)2](ClO4)3 (3). In both, the cyanide ligand is linearly bridged (µ-1,2) leading to a separation of the two copper(II) ions of ca. 5 Å. The geometry around copper(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu2(µ-F)(µ-Lm)2](ClO4)3 and (CH3)3SiN3 yields [Cu2(µ-N3)(µ-Lm)2](ClO4)3 (2), where the azide adopts end-on (µ-1,1) coordination with a Cu-N-Cu angle of 138.0° and a distorted square pyramidal geometry about the copper(II) ions. Similar chemistry in the more sterically hindered Lm* system yielded only the coordination polymer [Cu2(µ-Lm*)(µ-N3)2(N3)2]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper(I) complex [Cu5(µ-I4)(µ-Lm*)2]I3. The complexes 1 and 3 show strong antiferromagnetic coupling, -J = 135 and 161 cm(-1), respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the dz(2) and the bridging ligand s and px orbitals. High field EPR studies confirmed the dz(2) ground state of the copper(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of D and E are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex 2 has -J = 223 cm(-1) and DFT calculations indicate a predominantly d(x(2)-y(2)) ground state.

RESUMEN

The reactions of M(ClO4)2·xH2O (M = Ni(II) or Cd(II)) and m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M4(µ3-OH)4(µ-Lm)2(solvent)4](ClO4)4, where solvent = dimethylformamide, water, acetone. In the solid state the metal centers are in an octahedral coordination environment, two sites are occupied by pyrazolyl nitrogens from Lm, three sites are occupied by bridging hydroxides, and one site contains a weakly coordinated solvent molecule. A series of multinuclear, two-dimensional and variable-temperature NMR experiments showed that the cadmium(II) compound in acetonitrile-d3 has C2 symmetry and undergoes an unusual dynamic process at higher temperatures (ΔGLm‡ = 15.8 ± 0.8 kcal/mol at 25 °C) that equilibrates the pyrazolyl rings, the hydroxide hydrogens, and cadmium(II) centers. The proposed mechanism for this process combines two motions in the semirigid Lm ligand termed the "Columbia Twist and Flip:" twisting of the pyrazolyl rings along the Cpz­Cmethine bond and 180° ring flip of the phenylene spacer along the CPh­Cmethine bond. This dynamic process was also followed using the spin saturation method, as was the exchange of the hydroxide hydrogens with the trace water present in acetonitrile-d3. The nickel(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an S = 4 ground state, and the nickel(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni­O­Ni angles two types of interactions are observed: J1 = 9.1 cm(­1) (97.9 to 99.5°) and J2 = 2.1 cm(­1) (from 100.3 to 101.5°). "Broken symmetry" density functional theory calculations performed on a model of the nickel(II) compound support these observations.

Asunto(s)

RESUMEN

The reactions of M(ClO4)2·xH2O and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (Lm) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*) in the presence of triethylamine lead to the formation of monohydroxide-bridged, dinuclear metallacycles of the formula [M2(µ-OH)(µ-Lm)2](ClO4)3 (M = Fe(II), Co(II), Cu(II)) or [M2(µ-OH)(µ-Lm*)2](ClO4)3 (M = Co(II), Ni(II), Cu(II)). With the exception of the complexes where the ligand is Lm and the metal is copper(II), all of these complexes have distorted trigonal bipyramidal geometry around the metal centers and unusual linear (Lm*) or nearly linear (Lm) M-O-M angles. For the two solvates of [Cu2(µ-OH)(µ-Lm)2](ClO4)3, the Cu-O-Cu angles are significantly bent and the geometry about the metal is distorted square pyramidal. All of the copper(II) complexes have structural distortions expected for the pseudo-Jahn-Teller effect. The two cobalt(II) complexes show moderate antiferromagnetic coupling, -J = 48-56 cm(-1), whereas the copper(II) complexes show very strong antiferromagnetic coupling, -J = 555-808 cm(-1). The largest coupling is observed for [Cu2(µ-OH)(µ-Lm*)2](ClO4)3, the complex with a Cu-O-Cu angle of 180°, such that the exchange interaction is transmitted through the dz(2) and the oxygen s and px orbitals. The interaction decreases, but it is still significant, as the Cu-O-Cu angle decreases and the character of the metal orbital becomes increasingly d(x(2)-y(2)). These intermediate geometries and magnetic interactions lead to spin Hamiltonian parameters for the copper(II) complexes in the EPR spectra that have large E/D ratios and one g matrix component very close to 2. Density functional theory calculations were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with the experiments.

RESUMEN

The nuclear magnetic resonance (NMR) spectra of single-anion bridged, dinuclear copper(II) metallacycles [Cu2(µ-X)(µ-L)2](A)3 (L(m) = m-bis[bis(1-pyrazolyl)methyl]benzene: X = F(-), A = BF4(-); X = Cl(-), OH(-), A = ClO4(-); L(m)* = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene: X = CN(-), F(-), Cl(-), OH(-), Br(-), A = ClO4(-)) have relatively sharp (1)H and (13)C NMR resonances with small hyperfine shifts due to the strong antiferromagnetic superexchange interactions between the two S = 1/2 metal centers. The complete assignments of these spectra, except X = CN(-), have been made through a series of NMR experiments: (1)H-(1)H COSY, (1)H-(13)C HSQC, (1)H-(13)C HMBC, T1 measurements and variable-temperature (1)H NMR. The T1 measurements accurately determine the Cu···H distances in these molecules. In solution, the temperature dependence of the chemical shifts correlate with the population of the paramagnetic triplet (S = 1) and diamagnetic singlet (S = 0) states. This correlation allows the determination of antiferromagnetic exchange coupling constants, -J (H = -JS1S2), in solution for the L(m) compounds 338(F(-)), 460(Cl(-)), 542(OH(-)), for the L(m)* compounds 128(CN(-)), 329(F(-)), 717(Cl(-)), 823(OH(-)), and 944(Br(-)) cm(-1), respectively. These values are of similar magnitudes to those previously measured in the solid state (-Jsolid = 365, 536, 555, 160, 340, 720, 808, and 945 cm(-1), respectively). This method of using NMR to determine -J values in solution is an accurate and convenient method for complexes with strong antiferromagnetic superexchange interactions. In addition, the similarity between the solution and solid-state -J values of these complexes confirms the information gained from the T1 measurements: the structures are similar in the two states.

Asunto(s)

RESUMEN

The reactions of M(ClO4)2·6H2O [M = Zn(II), Cd(II)] and the ligands m-bis[bis(1-pyrazolyl)methyl]benzene, L(m), or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene, L(m)*, in the presence of a base yield the hydroxide bridged dinuclear metallacycles [M2(µ-OH)(µ-L)2](ClO4)3, L = L(m), M = Zn(II) (1); L = L(m)*, M = Zn(II) (2), Cd(II) (3). In the solid state, the coordination environment of the metals is distorted trigonal bipyramidal with the bridging hydroxide in an equatorial position and M-O-M angles greater than 161°. The observation of two equal intensity resonances for each type of pyrazolyl-ring hydrogen in the (1)H NMR for all three complexes coupled with the determination of the hydrodynamic radius based on the diffusion coefficient of 1 that matches that observed in the crystal structure, demonstrate this structure is retained in solution. Additional proof of the dinuclear structures in solution is given by the (113)Cd NMR spectrum of [Cd2(µ-OH)(µ-L(m)*)2](ClO4)3 showing (111/113)Cd satellites (J(111)(Cd-)(113)(Cd) = 173 Hz). Complex 1 is dynamic in solution, with the resonances for each type of pyrazolyl-ring hydrogen broadening and averaging at higher temperatures. Detailed variable temperature studies show that ΔG(pz)(‡) = 15.2(±0.2) kcal/mol, ΔH(pz)(‡) = 6.6(±0.1) kcal/mol, and ΔS(pz)(‡) = -28.8(±0.4) cal/mol·K at 25 °C for this process. The same ΔG(‡) value for the dynamic process was also determined by saturation transfer experiments. The most plausible mechanism for this dynamic process, which exchanges the axial and equatorial positions of the pyrazolyl rings in the trigonal bipyramidal arrangement, involves Berry pseudorotation at both metal sites using the bridging oxygen atom as the pivot ligand, coupled with the ring flip of the ligand's phenylene spacer by 180°, a rearrangement process we termed the "Columbia Twist and Flip". This process was shown to be influenced by trace amounts of water in the solvent, with a linear relationship between the water concentration and ΔG(pz)(‡); increasing the water concentration lowers ΔG(pz)(‡). Spin saturation transfer experiments demonstrated the exchange of the hydrogens between the water in the solvent and the bridging hydroxide group, with ΔG(OH)(‡) = 16.8(±0.2) kcal/mol at 25 °C, a value larger than the barrier of ΔG(pz)(‡) = 15.2(±0.2) kcal/mol for the "Columbia Twist and Flip". Compounds 2 and 3 do not show dynamic behavior involving the pyrazolyl-rings in solution because of steric crowding caused by the methyl group substitution, but do show the exchange between the water in the solvent and the bridging hydroxide group.

RESUMEN

The reaction of M(BF(4))(2)·xH(2)O, where M is Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II), with the new ditopic ligand m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (L(m)*) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M(2)(µ-F)(µ-L(m)*)(2)](BF(4))(3). The analogous manganese(II) species, [Mn(2)(µ-F)(µ-L(m)*)(2)](ClO(4))(3), was isolated starting with Mn(ClO(4))(2)·6H(2)O using NaBF(4) as the source of the bridging fluoride. In all of these complexes, the geometry around the metal centers is trigonal bipyramidal, and the fluoride bridges are linear. The (1)H, (13)C, and (19)F NMR spectra of the zinc(II) and cadmium(II) compounds and the (113)Cd NMR of the cadmium(II) compound indicate that the metallacycles retain their structure in acetonitrile and acetone solution. The compounds with M = Mn(II), Fe(II), Co(II), Ni(II), and Cu(II) are antiferromagnetically coupled, although the magnitude of the coupling increases dramatically with the metal as one moves to the right across the periodic table: Mn(II) (-6.7 cm(-1)) < Fe(II) (-16.3 cm(-1)) < Co(II) (-24.1 cm(-1)) < Ni(II) (-39.0 cm(-1)) ≪ Cu(II) (-322 cm(-1)). High-field EPR spectra of the copper(II) complexes were interpreted using the coupled-spin Hamiltonian with g(x) = 2.150, g(y) = 2.329, g(z) = 2.010, D = 0.173 cm(-1), and E = 0.089 cm(-1). Interpretation of the EPR spectra of the iron(II) and manganese(II) complexes required the spin Hamiltonian using the noncoupled spin operators of two metal ions. The values g(x) = 2.26, g(y) = 2.29, g(z) = 1.99, J = -16.0 cm(-1), D(1) = -9.89 cm(-1), and D(12) = -0.065 cm(-1) were obtained for the iron(II) complex and g(x) = g(y) = g(z) = 2.00, D(1) = -0.3254 cm(-1), E(1) = -0.0153, J = -6.7 cm(-1), and D(12) = 0.0302 cm(-1) were found for the manganese(II) complex. Density functional theory (DFT) calculations of the exchange integrals and the zero-field splitting on manganese(II) and iron(II) ions were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with experiment.

RESUMEN

Centrosymmetric [Cu(2)(µ-X)(µ-L(m)*)(2)](ClO(4))(3) (X = F(-), Cl(-), Br(-), OH(-), L(m)* = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene)], the first example of a series of bimetallic copper(II) complexes linked by a linearly bridging mononuclear anion, have been prepared and structurally characterized. Very strong antiferromagnetic exchange coupling between the copper(II) ions increases along the series F(-) < Cl(-) < OH(-) < Br(-), where -J = 340, 720, 808, and 945 cm(-1). DFT calculations explain this trend by an increase in the energy along this series of the antibonding antisymmetric combination of the p orbital of the bridging anion interacting with the copper(II) d(z(2)) orbital.