Diverse coordination of two ligands in ferromagnetic [Cu(μ-HCO2)2(3-pyOH)]n and [Cu2(μ-HCO2)2(μ-3-pyOH)2(3-pyOH)2(HCO2)2]n

Two novel polynuclear complexes with methanoate anions and 3-hydroxypyridine ligands [Cu(μ-HCO2)2(3-pyOH)]n (1) and [Cu2(μ-HCO2)2(μ-3-pyOH)2(3-pyOH)2(HCO2)2]n (2), respectively, were synthesized and characterized. The central copper atom in 1 is surrounded by four methanoates and a 3-pyOH molecule, forming a square-pyramidal CuO3NO chromophore. All the methanoates are bidentate and serve as bridges between the adjacent copper ions via syn-anti and anti–anti coordination. The basal square coordination axes are formed by O(syn), N(3-pyOH) (1.974(2), 2.016(2) Å) and O(anti), O(anti) (1.945(2), 1.960(2) Å), while the third O(anti) (2.247(2) Å) is on the top of the pyramid. A ferromagnetic transition with an exchange constant 2J/kB = 9.2 cm−1 is found for 1 below 20 K. This interaction probably takes place through two syn-anti methanoates extended in a chain through the 2D structure. On the other hand, two monoatomic Cu–O–Cu intra-dinuclear asymmetric (1.986(2), 2.415(2) Å) bridges of two methanoates in [Cu2(HCO2)4(3-pyOH)4] (2) are present. An elongated distorted octahedral coordination sphere around each copper(II) atom is completed by an additional monodentate terminal methanoate (1.975(2) Å), two N-coordinated 3-pyOH (2.005(2), 2.002(2) Å) and the third weakly O-coordinated 3-pyOH (2.732(2) Å). Although a shorter Cu⋯Cu distance is noticed in 2 than in 1 (4.690(1) Å 1, 3.442(1) Å 2), much weaker ferromagnetism is found in 2.

The polynuclear complex [Cu(μ-HCO2)2(3-pyOH)]n (1) shows triatomic methanoate bridges of anti–anti and syn–anti modes, on the other hand [Cu2(μ-HCO2)2(μ-3-pyOH)2(3-pyOH)2(HCO2)2]n (2) reveals only monoatomic methanoate coordination inside the dinuclear moiety and 3-hydroxypyridine connecting these units by semi-coordination. A ferromagnetic interaction is found in both complexes, however it is significantly important only for 1 (2J = 9.2 cm−1). This interaction takes place via the syn–anti   methanoates, due to the magnetic Cu(II) dx2-y2dx2-y2 orbitals on both sides of the bridge. In all the other bridging cases in 1 and 2, the non-magnetic dz2dz2 orbital is always involved.Figure optionsDownload as PowerPoint slide