Exchange Interactions Through π–π Stacking in the Lamellar Compound [{Cu(bipy)(en)}{Cu(bipy)(H₂O)}{VO₃}₄]_n

Abstract

Structural, magnetic, and powder and single-crystal electron paramagnetic resonance (EPR) studies were performed on [{Cu(bipy)(en)}{Cu(bipy)(H(2)O)}{VO(3)}(4)](n) (bipy = 2,2'-bipyridine, en = ethylenediamine), which is a new copper-vanadium hybrid organic-inorganic compound containing Cu(II) and V(V) centers. The oxovanadium units provide an anionic scaffolding to the structure, where two types of Cu(II) coordination modes, octahedral (Cu1) and square pyramidal (Cu2), contribute to the magnetic properties. The crystal structure contains layers including Cu1 and Cu2 ions, separated by stacked arrangements of 2,2'-bipyridine molecules. Each type of Cu(II) ion in these layers forms parallel spin chains described by exchange coupling parameters J(1) and J(2) for Cu1 and Cu2, respectively (exchange couplings defined as H(ex)(i,j) = -J(ij)S(i)S(j)), which, for necessity, are assumed to be equal to J. These chains are coupled by much weaker Cu1-Cu2 exchange interactions J(3) connecting neighbor Cu1 and Cu2 ions within a layer, through paths acting as rungs of a ladder chain structure. The average coupling J, which is antiferromagnetic (J < 0), according to the susceptibility data, is estimated with similar results with a mean field approximation (J = -1.4 cm(-1)), and with a uniform chain model (J = -1.7 cm(-1)). The EPR spectra of powdered samples and oriented single crystals are shown to be independent of J(1) and J(2), but are dependent on the weak coupling J(3), and the data allow a lower limit to be established: |J(3)| > 0.04 cm(-1). The spectra are also strongly sensitive to extremely weak coupling interactions with average magnitude J(4) between copper atoms in neighboring layers, separated by ∼10 Å, using the stacked 2,2'-bipyridine molecules, which produce a 2D-to-3D quantum phase transition. This is observed in single-crystal samples when the energy levels are changed with the orientation of the magnetic field. From the characteristics of these transitions, we estimate a value of |J(4)| = 0.0034 ± 0.0004 cm(-1) between Cu(II) ions in neighboring layers. This work emphasizes the important possibilities of EPR to evaluate extremely small exchange couplings between metal ions in a solid material, even in the presence of other much larger couplings.

Magnetic-field-tuned phase transition of a molecular material from the isolated-spin to the coupled-spin regime

“…The work of Venegas et al assigned to a similar coupling of bipyridine molecules in lamellar [Cu(bipy)(en)Cu(bipy)(H 2 O)VO 34 ] n a coupling more than one order of magnitude smaller than in CuBMB. 87 Thus, we conclude that our results indicate that the largest exchange couplings in CuBMB are between Cu 1 and Cu 3 ions, related by inversion operations at distances of B6.5 Å. The DFT calculation confirms these experimental results with J c 3a,b 4 J c 2,4 .…”

Synthesis of a self-assembled copper(ii) metallo-rectangle with a guanosine-substituted terpyridine

“…It has been previously reported that π-π interactions can give rise to long-range ferromagnetic ordering between metal ions. 17,22,23 Therefore we propose that these inter-molecular interactions may serve as adequate superexchange mediators between the di-copper(II) centres and account for the observed ferromagnetic exchange.…”

“…A number of systems have been reported in which π-π stacking interactions play a major role in propagating the magnetic properties, eventually leading to long range magnetic ordering. [17][18][19][20] Some of the factors that play crucial roles in transmitting strong magnetic coupling are the distance between metal centres, the nature of the bridging ligands and the spin ground state. Recently, non-covalent interactions have been exploited in the design of polymetallic magnetic materials where supramolecular interactions bring enhanced magnetic ordering in the system.…”

Syntheses, structures and properties of two 2-D layered hybrid organic–inorganic materials based on different V4O12building units

“…The bond lengths of V-O range from 1.598(3) to 1.828(3) Å and the O-V-O angles range from 106.42(10)°to 112.07(15)°(Table S1a †), which are consistent with previously reported vanadates. [19][20][21] As shown in Fig. 3 chains in the [100] direction.…”