Dichlorobis(pyrimidine-N)cobalt(II) and its bromo derivative

Low-Dimensional Metal-Organic Magnets as a Route toward the S = 2 Haldane Phase

Metal-organic magnets (MOMs), modular magnetic materials where metal atoms are connected by organic linkers, are promising candidates for next-generation quantum technologies. MOMs readily form low-dimensional structures and so are ideal systems to realize physical examples of key quantum models, including the Haldane phase, where a topological excitation gap occurs in integer-spin antiferromagnetic (AFM) chains. Thus, far the Haldane phase has only been identified for S = 1, with S ≥ 2 still unrealized because the larger spin imposes more stringent requirements on the magnetic interactions. Here, we report the structure and magnetic properties of CrCl₂(pym) (pym = pyrimidine), a new quasi-1D S = 2 AFM MOM. We show, using X-ray and neutron diffraction, bulk property measurements, density-functional theory calculations, and inelastic neutron spectroscopy (INS), that CrCl₂(pym) consists of AFM CrCl₂ spin chains (J₁ = -1.13(4) meV) which are weakly ferromagnetically coupled through bridging pym (J₂ = 0.10(2) meV), with easy-axis anisotropy (D = -0.15(3) meV). We find that, although small compared to J₁, these additional interactions are sufficient to prevent observation of the Haldane phase in this material. Nevertheless, the proximity to the Haldane phase together with the modularity of MOMs suggests that layered Cr(II) MOMs are a promising family to search for the elusive S = 2 Haldane phase.

1,4,8,11-Tetra-azoniacyclo-tetra-decane tetra-chloridocobaltate(II) dichloride

The asymmetric unit of the title compound, (C₁₀H₂₈N₄)[CoCl₄]Cl₂, contains two half-mol­ecules of the macrocycle, which are both completed by crystallographic inversion symmetry. In the dianion, the Co²⁺ cation is tetra­hedrally coordinated by four Cl atoms; the Co—Cl bond lengths correlate with the number of hydrogen bonds that the chloride ions accept. The crystal cohesion is supported by electrostatic inter­actions which, together with numerous N—H⋯Cl, N—H⋯(Cl,Cl) and C—H⋯Cl hydrogen bonds, lead to a three-dimensional network.

Structural diversity in two-dimensional coordination polymers constructed from simple building-blocks; a rare example of coordination polymer polymorphs structurally characterised from multiple crystals

A family of two-dimensional coordination polymers formed from the reaction of Cd(NO(3))(2) with pyrazine or pyrimidine is reported, including rare examples of polymorphic coordination polymers which crystallise as multiple crystals. Six coordination polymers have been structurally characterised, four for pyrazine and two for pyrimidine-based systems, all of which form two-dimensional arrays utilising pyrazine/pyrimidine bridging, in some instances in combination with nitrate bridging. The compounds form either 4(4) grids (1,3,4,5), or in one instance, a 6(3) herringbone sheet structure (2). In the case of 3, two polymorphs have been identified, 3a and 3b, in which the three-dimensional arrangements of the coordination polymers differ only in the relative ordering of adjacent two-dimensional sheets. It was found that these two polymorphs crystallise in a simultaneous fashion such that each crystal studied was found to contain regions of both polymorphs and was believed to be a multiple crystal. Assessment of the phase purity of the product from the reaction of Cd(NO(3))(2) with either pyrazine or pyrimidine indicates that compounds 1and 5 are not formed when the products are formed by rapid precipitation but only when using slow-diffusion methods. It is also apparent that in almost all instances more than one product is formed from a given reaction thereby illustrating the complexity of coordination polymer formation even when using simple building-blocks. For the crystal engineer this complexity is perhaps best illustrated by the simultaneous formation of 3a and 3b where no chemical interactions differentiate the two polymorphs, presenting a seemingly insurmountable complexity in the engineering of these systems.