New Syntheses, Analytic Spin Hamiltonians, Structural and Computational Characterization for a Series of Tri-, Hexa- and Hepta-Nuclear Copper (II) Complexes with Prototypic Patterns

Abstract

We present a series of pyrazolato-bridged copper complexes with interesting structures that can be considered prototypic patterns for tri-, hexa- and hepta- nuclear systems. The trinuclear shows an almost regular triangle with a μ3-OH central group. The hexanuclear has identical monomer units, the Cu6 system forming a regular hexagon. The heptanuclear can be described as two trinuclear moieties sandwiching a central copper ion via carboxylate bridges. In the heptanuclear system, the pyrazolate bridges are consolidating the triangular faces, which are sketching an elongated trigonal antiprism. The magnetic properties of these systems, dominated by the strong antiferromagnetism along the pyrazolate bridges, were described transparently, outlining the energy levels formulas in terms of Heisenberg exchange parameters J, within the specific topologies. We succeeded in finding a simple Kambe-type resolution of the Heisenberg spin Hamiltonian for the rather complex case of the heptanuclear. In a similar manner, the weak intermolecular coupling of two trimer units (aside from the strong exchange inside triangles) was resolved by closed energy formulas. The hexanuclear can be legitimately proposed as a case of coordination-based aromaticity, since the phenomenology of the six-spins problem resembles the bonding in benzene. The Broken-Symmetry Density Functional Theory (BS-DFT) calculations are non-trivial results, being intrinsically difficult at high nuclearities.

Structural basis for Cu(ii) metallocycle hexamer formation

“…10–12 Particularly, hexanuclear copper-based exchange coupling has been compared to the bonding in benzene. 13 The DFT spin-density plot is consistent with magnetic ordering and shows that paramagnetic electron density is delocalized into the aromatic rings of the ligand and reflects the energy lowering of the metal-based orbitals as a result of the nephelauxetic effect. The coordination pattern Cu( ii )–CO amide –Cu( ii ) that drives metallocycle formation is dominated by 3d x 2 − y 2 character as shown in S3 (ESI†).…”

“…[10][11][12] Particularly, hexanuclear copper-based exchange coupling has been compared to the bonding in benzene. 13 The DFT spin-density plot is consistent with magnetic ordering and shows that paramagnetic electron density is delocalized into the aromatic rings of the ligand and reflects the energy lowering of the metal-based Fig. 1 Ligands were synthesized and complexed with Cu(II) to produce chelate monomers.…”

Spin Frustrated Pyrazolato Triangular CuII Complex: Structure and Magnetic Properties, an Overview

“…These triangular Cu II systems have been largely, studied since they formed the simplest spin triangle. This has allowed for the possibility of studying the magnetic properties of geometrically spin-frustrated systems in detail [21]. Among these systems, the ones with a hydroxy bridge (µ 3 −OH − ) are among the most reported in the literature [22,23].…”

Metal–organic framework structures of fused hexagonal motifs with cuprophilic interactions of a triangular Cu(i)3(pyrazolate-benzoate) metallo-linker

“…The more widespread, structurally close form of that cluster is {Cu 3 IJμ 3 -OH)IJpz) 3 } n-, based on CuIJI/II) or CuIJII) only (importantly, the pz = 3,5-H-pyrazolate ligand is very typical for the μ 3 -OHcentered clusters, unlike for the CuIJI)-only cluster). 37 As special feature of copperIJI), the {Cu I 3 (pz) 3 } clusters are very prone to associate via intermolecular cuprophilic interactions, which could affect the course of self-assembly of potential MOFs. 38,39 There is a fortunate opportunity for the synthesis of potential mixed-metal M II-IV /Cu I pyrazolatecarboxylate coordination polymers staring from Cu II as a precursor in N,N-dimethylformamide (DMF) and similar amide solvents.…”