Synthesis, magnetic properties, and X-ray structure of catena-µ3-nitrato-O,O′,O″-[µ3-hydroxo-1-nitrato-1,2;1,3;2,3-tris(µ-pyrazolato-N,N′)-2,3-bis(pyrazole-N2)tricopper(II) monohydrate]. An unusual chain of trinuclear copper clusters

Abstract

An unusual trimeric Cu^II species of formula [Cu₃(OH)(pz)₃(Hpz)₂(NO₃)₂]·H₂O (Hpz = pyrazole) has been prepared from air oxidation of [Cu(Hpz)₂(NO₃)] in the presence of moisture. The X-ray structure of the oxidation product has been determined by single-crystal diffractometer techniques, and has been compared with a few related structures, all containing the pyramidal trimeric unit Cu₃(OH). The compound crystallizes in the space group P2₁ with a= 7.756(2), b= 17.639(3), c= 8.883(1)Å, β= 91.18(1)°, and Z= 2. The intensities of 3 640 independent reflections were used to determine the structure. The structure was solved and refined using standard heavy-atom and least-squares techniques to a residual R value of 0.0241 for 3 128 reflections with I > 2σ(I). The structure consists of trimeric units Cu₃(OH)(pz)₃ containing an almost flat Cu₃N₆ ring and a pyramidal Cu₃O group with Cu–O distances of 1.994 Å and an (averaged) Cu–O–Cu angle of 114°. Each copper is co-ordinated by four ligands in an almost square planar manner with two nitrogens from the bridging pyrazolato-groups and one oxygen from the bridging OH group. The fourth equatorial ligand is either the N-atom of a neutral pyrazole (for two Cu atoms) or the oxygen of a unidentate co-ordinating nitrato-ligand (for the third Cu atom). The fifth axial ligand for each copper ion is another nitrato-group. This nitrato-group links trimers together in chains, with two oxygens bound to coppers of one trimer, and the third oxygen binding to the third copper of the neighbouring trimer, resulting in chains of –O₂NO–CuCu₂O₂NO–CuCu₂–O₂–. The dominant magnetic exchange in the structure occurs within each trimer, resulting in a compound having only one unpaired spin per three copper ions. An additional magnetic exchange interaction occurs between the trimers, using the nitrate-bridged chain. This interaction is only evident at very low temperatures. Due to the occurrence of rapid spin-lattice relaxation no e.s.r. spectra can be obtained at ambient temperatures, but instead n.m.r. spectra have been recorded.