Water Dissociation of a Dinuclear Bis(3,5‐dimethylpyrazolyl)methane Copper(II) Complex: X‐ray Diffraction Structure, Magnetic Properties, and Characteristic Absorption of the (CuN 2 Cl 2 ) 2 Core

Atomic-Scale Elucidation of Unusually Distorted Dimeric Complexes Confined in a Zr-Based Metal-Organic Framework

Nanoconfinement in metal-organic framework (MOF) pores can lead to the isolation of unusual or reactive metal complexes. However, MOFs that support the stabilization and precise structural elucidation of metal complexes and small metal clusters are rare. Here, we report a thermally and chemically stable zirconium-based MOF (University of Adelaide Material-1001, UAM-1001) with a high density of free bis-pyrazolyl units that can confine mono- and dinuclear metal complexes. The precursor MOF, UAM-1000, has a high degree of structural flexibility, but post synthetic modification with a bracing linker, biphenyl-4,4'-dicarboxylic acid, partially rigidifies the MOF (UAM-1001). This allows "matrix isolation" and detailed structural elucidation of postsynthetically added dimeric complexes bound within a tetradentate binding site formed by two linkers. Dimeric species [Co2Cl4], [Cu2Cl4], [Ni2Cl3(H2O)2]Cl, and [Rh2(CO)3Cl2] were successfully isolated in UAM-1001 and characterized by single-crystal X-ray diffraction. Comparison of the UAM-1001 isolated species with similar complexes in the solid state reveals that UAM-1001 can significantly distort the structures and enforce notably shorter metal-metal distances. For example, MOF tethering allows isolation of a [Cu2Cl4] complex that rapidly reacts with water in the solid state. The stability, porosity, and modulated flexibility of UAM-1001 provide an ideal platform material for the isolation and study of new dimeric complexes and their reactivity.

Magneto-structural maps and bridged-ligand effect for dichloro-bridged dinuclear copper(ii) complexes: a theoretical perspective

Theoretical understanding of magneto-structural correlations in dichloro-bridged dicopper(ii) complexes can guide the design of magnetic materials having broad-scale applications. However, previous reports suggest these correlations are complicated and unclear. To clarify possible correlations, magnetic coupling constants (J _calc) of variants of a representative {Cu-(μ-Cl)₂-Cu} complex A were calculated through BS-DFT. The variation of the Cu-(μ-Cl)-Cu angle (α), Cu⋯Cu distance (R ₀), and Cu-Cl-Cu-Cl dihedral angle (τ) followed by structural optimization and calculation of the magnetic coupling constant (J _calc) revealed several trends. J _calc increased linearly with R ₀ and τ, and initially increased and then decreased with α. Further, bridging ligand effects on J _calc for dicopper(ii) complexes were evaluated through BS-DFT; the results revealed that J _calc increased with increasing ligand field strength (I^- < Br^- < Cl^- < N₃ ^- < F^-). Furthermore, a linear relationship was found between the spin density of the bridging ligand and J _calc.

The effect of anions in the synthesis and structure of pyrazolylamidino copper(II) complexes

Six new pyrazolylamidino Cu(II) complexes are synthesized directly from the reactions of Cu(X)₂ salts (X = ClO₄^-, BF₄^-, or Cl^-) and pyrazole (pzH) in nitrile solution (RCN, R = Me or Et) at 298 K via the metal-mediated coupling of RCN with pzH: [Cu(HNC(R)pz)₂(X)₂] (X = ClO₄^- or BF₄^-, R = Me, 1 or 7 and Et, 2 or 8, respectively) and dichloro Cu(II) complexes [Cu₂Cl₂(μ-Cl)₂(HNC(Me)pz)₂] (3) and [CuCl₂(HNC(Et)pz)] (4). Four more new complexes, [Cu₂(μ-Cl)₂(HNC(Me)pz)₂(pzH)₂][X]₂ (X = ClO₄^-, 5 and BF₄^-, 9) and [Cu₂(μ-Cl)₂(HNC(Et)pz)₂(pzH)₂(X)₂] (X = ClO₄^-, 6 and BF₄^-, 10), are obtained indirectly from the anion substitution reaction with Cl^- ions in 1 and 7, and 2 and 8, respectively. All complexes are characterized by EA, FTIR, UV-vis and EPR spectroscopy and X-ray crystallographic analyses. HNC(Et)pz or pzH is unobserved in both the nitrile-exchange reaction of 2 to d₆-1 and the anion-substitution reaction of 2 to d₆-5 in the CD₃CN solution. The ¹H NMR results reveal that the pzH-RCN coupling is intramolecular and reversible on a Cu(II) center. The crystal structures of these complexes show diverse supramolecular assemblies through imino NH⋯anion hydrogen bonds and pyrazolylamidino pz-pz (π⋯π) and pz-Cu(II) (π⋯metal) interactions. EPR results suggest weak magnetic couplings between Cu(II) centers in the polynuclear Cu(II) complexes. The yield and rate of the formation of 1 are higher in the reaction of Cu(ClO₄)₂ with a 4-fold molar excess of pzH compared with a 2-fold excess, indicating that [Cu(pzH)₄]²⁺ is the more active species for pzH-RCN coupling. The highest rate for the formation of 1 is achieved when [Cu(pzH)₄(ClO₄)₂] is used in MeCN solution. Thus, a plausible synthetic path for synthesizing pyrazolylamidino Cu(II) complexes is established. An intermediate species, [Cu(HNC(Me)pz)₂(pzH)₂][ClO₄]₂ (1a), is proposed for the synthetic process based on spectroscopic studies and DFT calculations. The reaction of [Cu(pzH)₄X₂] (X = ClO₄^-, Cl^-, NO₃^-, or BF₄^-) in MeCN solution suggests that the lability of coordinated anions upon nitrile substitution affects the rate of the formation of bis-pyrazolylamidino Cu(II) complexes.

Crystal structures, and magnetic and thermal properties of basic copper formates with two-dimensional triangular-lattice magnetic networks

Basic copper formate, [Cu₃(OH)₄(HCOO)₂], was selectively prepared by hydrolysis of formate ions in concentrated aqueous solutions of copper formate. This material exhibits a two-dimensional distorted triangular-lattice magnetic network of Cu(ii) ions where S = 1/2. The dominance of antiferromagnetic interactions with J/k_B = 35.7(2) K and a magnetic anomaly at approximately 2.3 K that corresponds to a paramagnetic-to-antiferromagnetic ordering transition were revealed by magnetic measurements. The field dependence of the magnetisation at 2 K corresponds to the 1/3-magnetisation plateau that is commonly observed in a two-dimensional triangular-lattice system. Moreover, heat-capacity measurements found a λ-type anomaly at 2.15 K, which is the magnetic transition temperature. This material may be a good candidate for a geometrical frustration system with novel magnetic phenomena such as spin-liquid states.

Basic copper formate, [Cu₃(OH)₄(HCOO)₂], has a distorted two-dimensional triangular lattice magnetic network and showed antiferromagnetic ordering at 2.15 K.