Two Cu coordination polymers [CuCl(pyz)](BF) 1 and [CuBr(pyz)](BF) 2 (pyz = pyrazine) were synthesized in the family of quasi two-dimensional (2D) [Cu(pyz)] magnetic networks. The layer connectivity by monatomic halide ligands results in significantly shorter interlayer distances. Structures were determined by single-crystal X-ray diffraction. Temperature-dependent X-ray diffraction of 1 revealed rigid [Cu(pyz)] layers that do not expand between 5 K and room temperature, whereas the expansion along the c-axis amounts to 2%. The magnetic susceptibility of 1 and 2 shows a broad maximum at ∼8 K, indicating antiferromagnetic interactions within the [Cu(pyz)] layers. 2D Heisenberg model fits result in J = 9.4(1) K for 1 and 8.9(1) K for 2. The interlayer coupling is much weaker with | J| = 0.31(6) K for 1 and 0.52(9) K for 2. The electron density, experimentally determined and calculated by density functional theory, confirms the location of the singly occupied orbital (the magnetic orbital) in the tetragonal plane. The analysis of the spin density reveals a mainly σ-type exchange through pyrazine. Kinks in the magnetic susceptibility indicate the onset of long-range three-dimensional magnetic order below 4 K. The magnetic structures were determined by neutron diffraction. Magnetic Bragg peaks occur below T = 3.9(1) K for 1 and 3.8(1) K for 2. The magnetic unit cell is doubled along the c-axis ( k = 0, 0, 0.5). The ordered magnetic moments are located in the tetragonal plane and amount to 0.76(8) μ/Cu for 1 and 0.6(1) μ/Cu for 2 at 1.5 K. The moments are coupled antiferromagnetically both in the ab plane and along the c-axis. The Cu g-tensor was determined from electron spin resonance spectra as g = 2.060(1), g = 2.275(1) for 1 and g = 2.057(1), g = 2.272(1) for 2 at room temperature.

INTRODUCTIONShortly after the element iodine was discovered two centuries ago, polyiodides were first observed. The first crystal structure determination had to wait until 1935, for (NH 4 )I 3 . 1 Meanwhile, the structural chemistry of polyiodides has developed considerably, and hundreds of structures have been determined. 2,3 Many of these polyiodides have been discovered by fortunate coincidence when the syntheses of other compounds were designed. These are mostly triiodides; [Cs(b18c6) 2 ]-(I 3 ) and [Cs 2 (b18c6) 3 ](I 3 ) 2 are recent examples with sandwich and tripledecker cations. 4 Polyiodides may be constructed (formally and literally) by adding iodine, I 2 , to iodide, I À . A number of series with different charges in accord with the general formula I 2m+n nÀ (with m and n integers larger than zero, n = 1, 4) 3 have been shown to exist. As much as crystal growth is often straightforward and a large area may be covered in a reasonable time, the design of specific polyiodide anions or anionic networks is difficult to achieve. Only very slowly patterns emerge. To date there is no definitive prescription of how a desired polyiodide anion and/or a special anionic architecture could be constructed. Larger cations, however, seem to have a templating effect, attested for example by the largest polyiodide anion known, I 29 3À , with three large ferrocenium cations. 5 Crown ethers with encapsulated (metal) cations offer a wide range of shape and charge around which polyiodide anions of different sizes and geometries as well as charges can be arranged. [Lu(H 2 O) 3 (db18c6)(Thf) 6 ] 4 (I 3 ) 2 (I 5 ) 6 (I 8 )(I 12 ) is an especially prolific example when the giant cations and the four different polyiodide anions forming a three-dimensional network are regarded. 6 In a more general approach, we have investigated the influence of different cations (H 3 O + , H 5 O 2 + , mono-, di-, and trivalent metal cations), encapsulated in crown-ethers of different sizes, on the formation of polyiodide networks. Apart from the choice of crown-ethers and cations and the concentrations of iodine and iodide, the solvents also play an important role. The present picture is also dictated by the solubility product as single crystals are needed for structure determination, which is the only reliable method to determine the composition of the salt that crystallizes from a specific solution. As part of a broad study, we here report on a number of new metal(II)Àcrown-etherÀ polyiodides with the general formulas [M(crown) 2 ] 2+ ([I 2m+1 ] À ) 2 (m = 1, 2, 3) and [M(crown) 2 ] 2+ [I 2m+2 ] 2À (m = 5, 7, 8) with M = Fe, Mn, Co, Ni employing relatively small crown-ethers, 12crown-4 (12c4), benzo-12-crown-4 (b12c4), and benzo-15crown-5 (b15c5).' EXPERIMENTAL SECTION Syntheses. Generally, the polyiodide salts [M(crown) 2 ] 2+ ([I 2m+1 ] À ) 2 (m = 1, 2, 3) and [M(crown) 2 ] 2+ [I 2m+2 ] 2À (m = 5, 7, 8) were synthesized by dissolving metal diiodide, MI 2 (M = Mn, Fe, Co, Ni),

Pressure-driven orbital reordering in the quantum magnet [CuF2(H2O)2(pyz)], (pyz = pyrazine), dramatically affects its magnetic exchange interactions. The crystal chemistry of this system is enriched with a new phase above 3 GPa, surprisingly concomitant with other polymorphs. Moreover, we discovered an unprecedented compound with a different stoichiometry, [(CuF2(H2O)2)2(pyz)], featuring magnetic bi-layers.

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF_{2}(H_{2}O)_{2}]_{2}pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.

Abstract:The reaction of bismuth triiodide with iodine and benzo-15-crown-5 in ethanol/dichloromethane yielded red single crystals of (b15c5)BiI 3 (I 2 ) (monoclinic, P2 1 /c (no. 14), a = 1376.9(1), b = 1172.7(1), c = 1700.2(2) pm, β = 115.197(6), V = 2484.1(4)·10 6 pm 3 , Z = 4). Neutral pseudo-octahedral complexes (b15c5)BiI 3 are connected by secondary bonding interactions via iodine molecules to chains. Electronic structure calculations of the neutral complex (b15c5)BiI 3 reveal that the compound can indeed be described as b15c5 interacting with a molecular BiI 3 unit. However, bonding has to be mainly electrostatic as the interactions of the bismuth 6s lone pair with the 2p orbitals of the oxygen atoms of the crown ether are clearly antibonding.