Three novel three-dimensional framework materials in the manganese(II)–phosphate–oxalate system have been prepared hydrothermally using organic amines as structure-directing agents. [H₃NC₆H₁₀NH₃][Mn₂(HPO₄)₂(C₂O₄)(H₂O)₂] 1 and [H₃N(CH₂)₂NH₃][Mn₂(HPO₄)₂(C₂O₄)(H₂O)₂] 2 have related, but not isomorphous, structures based on layers of vertex-sharing MnO₆ and PO₄ polyhedra linked by bridging oxalate groups. [H₃N(CH₂)₂NH₃][Mn₄(HPO₄)₂(C₂O₄)₃(H₂O)₂]·2H₂O 3 has a very different structure composed of edge-sharing dimeric MnO₆ units linked into chains by PO₄ groups, and into a 3-D framework by oxalate units. 3 can be topotactically dehydrated, with loss of both extra-framework and co-ordinated water, to produce a new phase 4. The magnetic susceptibilities of phases 1, 3 and 4 all show Curie–Weiss behaviour above about 50 K, and a maximum in the susceptibility curve in the vicinity of 7–12 K, indicating significant antiferromagnetic exchange. Structural relationships to other phosphate–oxalate frameworks are discussed.

Two new three-dimensional manganese phosphate oxalate frameworks have been synthesized and their structures determined by single crystal X-ray diffraction. The compound [Mn₄(PO₄)₂(C₂O₄)(H₂O)₂] 1 is monoclinic, space group P2₁/c, a = 10.263(3), b = 6.526(2), c = 10.082(3) Å and β = 116.84(2)°. MnO₅ and MnO₆ polyhedra share edges to form tetramers, which are further linked by PO₄ tetrahedra to form layers in the bc plane. The layer, which contains an extended array of direct Mn–O–Mn linkages, is in turn linked to neighbouring layers by pillaring bisbidentate oxalate groups. Magnetisation measurements indicate antiferromagnetic interactions, and suggest that the material may be a two dimensional antiferromagnet. Thermogravimetric analysis shows the structure to be stable up to 240 °C. The compound [H₃N(CH₂)₃NH₃][Mn₂(HPO₄)₂(C₂O₄)(H₂O)₂] 2 is monoclinic, space group P2₁/n, a = 5.474(2), b = 15.737(2), c = 9.056(2) Å, β = 93.47(3)°, and includes a disordered 1,3-diaminopropane dication in channels parallel to the a direction. In contrast to 1, no direct Mn–O–Mn linkages occur.

We report the crystal structures and magnetic properties of two pseudopolymorphs of the $S=1/2 {\mathrm{Ti}}^{3+}$ coordination framework, $\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$. Single-crystal x-ray and powder neutron diffraction measurements on $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of ${\mathrm{Ti}}^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1\approx 7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of ${\mathrm{Ti}}^{3+}$ in $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ is suppressed from its spin-only value to $0.62\left(3\right){\mu }_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$, on the other hand, realizes a three-dimensional diamondlike magnetic network of ${\mathrm{Ti}}^{3+}$ moments within a hexagonal $P{6}_{2}22$ structure. An antiferromagnetic exchange coupling of $J\approx 54$ K—an order of magnitude larger than in $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$—is extracted from magnetometry and specific heat data. $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ undergoes Néel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79{\mu }_B$ per ${\mathrm{Ti}}^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\alpha$ and $\beta$ pseudopolymorphs. Given their observed magnetic behaviors, we propose $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ and $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.