Supramolecular self-assembly of cytidine monophosphate-di-copper building blocks

We report the synthesis and characterization of three di-copper compounds, [Cu2phen2CMP(µ-OH)(µ-X)]·3H2O (with X = Cl− (1) and Br− (2)) and [Cu2phen2CMP(µ-OH)(µ-NO3)]·[Cu2phen2CMP(µ-OH)(NO3)]·7H2O (3). 1 and 3 present a dinuclear core assembled by an unusual bis-monodentate phosphate and µ2-hydroxo double bridge, further supported by bridging anions. The spontaneous self-assembly of an extended coordination structure of higher dimensionality, although possible in principle, has been hindered by the use of strongly coordinating counterions (Cl−, Br−) and, unexpectedly, even by the less coordinating nitrate ions. The neutral complexes feature unsaturated coordination sites, having both the N(3) and O(2) of the cytosine moieties of the CMP ligands still available for coordination. π···π Stacking and hydrogen bonding interactions established between the discrete chiral entities mainly drive the self-assembling of a 3D supramolecular network. Small hydrophilic voids are defined within the 3D net, which are filled by solvent water molecules. These small nuclearity complexes with coordinatively unsaturated CMP ligands are likely to act as tectons. They could be used as complex-as-ligands for further binding additional metal ions in a predictable manner, favoring a more rational synthetic approach aimed to a stricter structural and chemical control of the resulting material.</p

Coligand Effects on the Field-Induced Double Slow Magnetic Relaxation in Six-Coordinate Cobalt(II) Single-Ion Magnets (SIMs) with Positive Magnetic Anisotropy

Two mononuclear cobalt­(II) compounds of formula [Co­(dmphen)2(OOCPh)]­ClO4·1/2H2O·1/2CH3OH (1) and [Co­(dmbipy)2(OOCPh)]­ClO4 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6′-dimethyl-2,2′-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt­(II) ion is six-coordinate with a distorted octahedral CoN4O2 environment. The complex cations are interlinked leading to supramolecular chains (1) and pairs (2) that grow along the crystallographic c-axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm–1 ≤ D < 60.7 cm–1 (1) and 63.8 cm–1 ≤ D –1 (2). HFEPR spectra of polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and provide an estimate of the E/D quotient [0.147/0.187 (1) and 0.052 (2)]. Detailed ac and dc magnetic studies reveal that 1 and 2 are new examples of field-induced single-ion magnets (SIMs) with small transversal anisotropy. CASSCF/NEVPT2 calculations support these results. Two Orbach processes or one Orbach plus a direct relaxation mechanism provide similar agreements with the nonlinear experimental Arrhenius plots at Hdc = 500 and 2500 G for 1. Two independent relaxation processes occur in 2, but in contrast to 1, an observed linear dependence of ln­(τ) vs 1/T substantiates Orbach processes against the most widely proposed Raman and direct mechanisms. The analysis of each relaxation process in 2 provided values for Ea and τ0 that are very close to those found for 1, validating the predominant role of the Orbach relaxations in both compounds and, probably, also in other cobalt­(II) SIMs. A mechanism based on a spin-phonon coupling is proposed to account for the SIM behavior in 1 and 2 with any Raman or direct processes being discarded