NMR Investigations of Dinuclear, Single-Anion Bridged Copper(II) Metallacycles: Structure and Antiferromagnetic Behavior in Solution

The nuclear magnetic resonance (NMR) spectra of single-anion bridged, dinuclear copper­(II) metallacycles [Cu₂(μ-X)­(μ-<b>L</b>)₂]­(A)₃ (<b>L</b>_{<i><b>m</b></i>} = <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene: X = F^–, A = BF₄^–; X = Cl^–, OH^–, A = ClO₄^–; <b>L</b>_{<i><b>m</b></i>}<b>*</b> = <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene: X = CN^–, F^–, Cl^–, OH^–, Br^–, A = ClO₄^–) have relatively sharp ¹H and ¹³C NMR resonances with small hyperfine shifts due to the strong antiferromagnetic superexchange interactions between the two <i>S</i> = ¹/₂ metal centers. The complete assignments of these spectra, except X = CN^–, have been made through a series of NMR experiments: ¹H–¹H COSY, ¹H–¹³C HSQC, ¹H–¹³C HMBC, <i>T</i>₁ measurements and variable-temperature ¹H NMR. The <i>T</i>₁ 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 (<i>S</i> = 1) and diamagnetic singlet (<i>S</i> = 0) states. This correlation allows the determination of antiferromagnetic exchange coupling constants, −<i>J</i> (<b>Ĥ</b> = −<i>J</i><b>Ŝ</b>₁<b>Ŝ</b>₂), in solution for the <b>L</b>_{<i><b>m</b></i>} compounds 338­(F^–), 460­(Cl^–), 542­(OH^–), for the <b>L</b>_{<i><b>m</b></i>}* 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 (−<i>J</i>_solid = 365, 536, 555, 160, 340, 720, 808, and 945 cm^–1, respectively). This method of using NMR to determine −<i>J</i> 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 −<i>J</i> values of these complexes confirms the information gained from the <i>T</i>₁ measurements: the structures are similar in the two states

Zinc(II) and Cadmium(II) Monohydroxide Bridged, Dinuclear Metallacycles: A Unique Case of Concerted Double Berry Pseudorotation

The reactions of M­(ClO₄)₂·6H₂O [M = Zn­(II), Cd­(II)] and the ligands <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene, <b>L</b>_{<i><b>m</b></i>}, or <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene, <b>L</b>_{<i><b>m</b></i>}*, in the presence of a base yield the hydroxide bridged dinuclear metallacycles [M₂(μ-OH)­(μ-<b>L</b>)₂]­(ClO₄)₃, <b>L</b> = <b>L</b>_{<i><b>m</b></i>}, M = Zn­(II) (<b>1</b>); <b>L</b> = <b>L</b>_{<i><b>m</b></i>}*, M = Zn­(II) (<b>2</b>), Cd­(II) (<b>3</b>). 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 ¹H NMR for all three complexes coupled with the determination of the hydrodynamic radius based on the diffusion coefficient of <b>1</b> 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 ¹¹³Cd NMR spectrum of [Cd₂(μ-OH)­(μ-<b>L</b><i>_<b>m</b></i>*)₂]­(ClO₄)₃ showing ^111/113Cd satellites (<i>J</i>¹¹¹_Cd‑¹¹³_Cd = 173 Hz). Complex <b>1</b> 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 Δ<i>G</i>_pz^⧧ = 15.2(±0.2) kcal/mol, Δ<i>H</i>_pz^⧧ = 6.6(±0.1) kcal/mol, and Δ<i>S</i>_pz^⧧ = −28.8(±0.4) cal/mol·K at 25 °C for this process. The same Δ<i>G</i>^⧧ 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 <i>both metal sites</i> 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 Δ<i>G</i>_pz^⧧; increasing the water concentration lowers Δ<i>G</i>_pz^⧧. Spin saturation transfer experiments demonstrated the exchange of the hydrogens between the water in the solvent and the bridging hydroxide group, with Δ<i>G</i>_OH^⧧ = 16.8(±0.2) kcal/mol at 25 °C, a value larger than the barrier of Δ<i>G</i>_pz^⧧ = 15.2(±0.2) kcal/mol for the “Columbia Twist and Flip”. Compounds <b>2</b> and <b>3</b> 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

Halide and Hydroxide Linearly Bridged Bimetallic Copper(II) Complexes: Trends in Strong Antiferromagnetic Superexchange Interactions

Centrosymmetric [Cu₂(μ-X)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (X = F^–, Cl^–, Br^–, OH^–, <b>L</b>_{<i><b>m</b></i>}* = <i>m</i>-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 −<i>J</i> = 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_<i>z</i>² orbital

Dinuclear Complexes Containing Linear M–F–M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] Bridges: Trends in Structures, Antiferromagnetic Superexchange Interactions, and Spectroscopic Properties

The reaction of M­(BF₄)₂·<i>x</i>H₂O, where M is Fe­(II), Co­(II), Ni­(II), Cu­(II), Zn­(II), and Cd­(II), with the new ditopic ligand <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L_<i>m</i>*</b>) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(BF₄)₃. The analogous manganese­(II) species, [Mn₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(ClO₄)₃, was isolated starting with Mn­(ClO₄)₂·6H₂O using NaBF₄ 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 ¹H, ¹³C, and ¹⁹F NMR spectra of the zinc­(II) and cadmium­(II) compounds and the ¹¹³Cd 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 <i>g</i>_<i>x</i> = 2.150, <i>g</i>_<i>y</i> = 2.329, <i>g</i>_<i>z</i> = 2.010, <i>D</i> = 0.173 cm^–1, and <i>E</i> = 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 <i>g</i>_<i>x</i> = 2.26, <i>g</i>_<i>y</i> = 2.29, <i>g</i>_<i>z</i> = 1.99, <i>J</i> = −16.0 cm^–1, <i>D</i>₁ = −9.89 cm^–1, and <i>D</i>₁₂ = −0.065 cm^–1 were obtained for the iron­(II) complex and <i>g</i>_<i>x</i> = <i>g</i>_<i>y</i> = <i>g</i>_<i>z</i> = 2.00, <i>D</i>₁ = −0.3254 cm^–1, <i>E</i>₁ = −0.0153, <i>J</i> = −6.7 cm^–1, and <i>D</i>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

Halide and Hydroxide Linearly Bridged Bimetallic Copper(II) Complexes: Trends in Strong Antiferromagnetic Superexchange Interactions

Centrosymmetric [Cu₂(μ-X)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (X = F^–, Cl^–, Br^–, OH^–, <b>L</b>_{<i><b>m</b></i>}* = <i>m</i>-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 −<i>J</i> = 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_<i>z</i>² orbital

Hydroxide-Bridged Cubane Complexes of Nickel(II) and Cadmium(II): Magnetic, EPR, and Unusual Dynamic Properties

The reactions of M­(ClO₄)₂·<i>x</i>H₂O (M = Ni­(II) or Cd­(II)) and <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene (<b>L</b>_<b>m</b>) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M₄(μ₃-OH)₄(μ-<b>L</b>_<b>m</b>)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 <b>L</b>_<b>m</b>, 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-<i>d</i>₃ has <i>C</i>₂ symmetry and undergoes an unusual dynamic process at higher temperatures (Δ<i>G</i>_Lm^‡ = 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 <b>L</b>_<b>m</b> ligand termed the “Columbia Twist and Flip:” twisting of the pyrazolyl rings along the C_pz–C_methine bond and 180° ring flip of the phenylene spacer along the C_Ph–C_methine 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-<i>d</i>₃. The nickel­(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an <i>S</i> = 4 ground state, and the nickel­(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni–O–Ni angles two types of interactions are observed: <i>J</i>₁ = 9.1 cm^–1 (97.9 to 99.5°) and <i>J</i>₂ = 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

Dinuclear Metallacycles with Single M–O(H)–M Bridges [M = Fe(II), Co(II), Ni(II), Cu(II)]: Effects of Large Bridging Angles on Structure and Antiferromagnetic Superexchange Interactions

The reactions of M­(ClO₄)₂·<i>x</i>H₂O and the ditopic ligands <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}) or <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}*) in the presence of triethylamine lead to the formation of monohydroxide-bridged, dinuclear metallacycles of the formula [M₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (M = Fe­(II), Co­(II), Cu­(II)) or [M₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (M = Co­(II), Ni­(II), Cu­(II)). With the exception of the complexes where the ligand is <b>L</b>_{<i><b>m</b></i>} and the metal is copper­(II), all of these complexes have distorted trigonal bipyramidal geometry around the metal centers and unusual linear (<b>L</b>_{<i><b>m</b></i>}*) or nearly linear (<b>L</b>_{<i><b>m</b></i>}) M–O–M angles. For the two solvates of [Cu₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>})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, −<i>J</i> = 48–56 cm^–1, whereas the copper­(II) complexes show very strong antiferromagnetic coupling, −<i>J</i> = 555–808 cm^–1. The largest coupling is observed for [Cu₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃, the complex with a Cu–O–Cu angle of 180°, such that the exchange interaction is transmitted through the d_<i>z</i>² and the oxygen s and p_<i>x</i> 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_{<i>x</i>²–<i>y</i>²}. These intermediate geometries and magnetic interactions lead to spin Hamiltonian parameters for the copper­(II) complexes in the EPR spectra that have large <i>E</i>/<i>D</i> ratios and one <i>g</i> 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

Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates

The new ditopic organoiron ligand, [3,5-bis­(1-ferrocenyl-prop-3-enol-1-one)­(pyridine)] (H₂<b>L</b>^<b>3,5</b>), has been prepared and the reactions of its dianion (Na₂<b>L</b>^<b>3,5</b>) with M³⁺ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M₂(<b>L</b>^<b>3,5</b><b>)</b>₃, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variable-temperature ¹H NMR study of the Ga₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H₂<b>L</b>^<b>3,5</b>) and the M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5–6 Å range in both the free ligand and the helicates

Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates

The new ditopic organoiron ligand, [3,5-bis­(1-ferrocenyl-prop-3-enol-1-one)­(pyridine)] (H₂<b>L</b>^<b>3,5</b>), has been prepared and the reactions of its dianion (Na₂<b>L</b>^<b>3,5</b>) with M³⁺ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M₂(<b>L</b>^<b>3,5</b><b>)</b>₃, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variable-temperature ¹H NMR study of the Ga₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H₂<b>L</b>^<b>3,5</b>) and the M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5–6 Å range in both the free ligand and the helicates

Design, Synthesis, and Structural Characterization of a New Class of Ferrocene-Containing Heterometallic Triple-Stranded Helicates

The new ditopic organoiron ligand, [3,5-bis­(1-ferrocenyl-prop-3-enol-1-one)­(pyridine)] (H₂<b>L</b>^<b>3,5</b>), has been prepared and the reactions of its dianion (Na₂<b>L</b>^<b>3,5</b>) with M³⁺ ions (M = Ga or In) yield a new class of “3d-np block” heterometallic triple-stranded helicates, M₂(<b>L</b>^<b>3,5</b><b>)</b>₃, by the self-assembly process. The X-ray structural analysis of the new ligand shows that it is in the enolic form with each enolic carbon bonded to the pyridine ring and each carbonyl carbon connected to a ferrocene moiety; overall, the nonferrocenyl part of the molecule is nearly planar. The M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ (M = Ga or In) complexes are helicates with three ligand strands, each of which is twisted into an S-shape, coordinating to two metal ions, each of which is in a distorted octahedral geometry. The new helicates are observed as a racemic mixture in the solid state by single-crystal X-ray analysis, and in solution by NMR, with both the left-handed Λ,Λ- and the right-handed Δ,Δ-isomers present. Variable-temperature ¹H NMR study of the Ga₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicate indicates that the right-handed Δ,Δ-isomer and left-handed Λ,Λ-isomer equilibrate through a heterochiral Λ,Δ-intermediate by a concerted twist motion of one-half of the dinuclear complex through a trigonal prismatic transition state, according to the Bailar twist mechanism. Electrochemical properties of the ligand (H₂<b>L</b>^<b>3,5</b>) and the M₂(<b>L</b>^<b>3,5</b><b>)</b>₃ helicates were investigated through cyclic voltammetry, and the results indicate the lack of communication between the ferrocene units, because the separation between any two ferrocene units is greater than the 5–6 Å range in both the free ligand and the helicates