Control of molecular topology by stereochemical preferences of metal ions: double helical versus side-by-side structures in tetranuclear copper(ii) and nickel(ii) complexes

Abstract

The novel polydentate ligand N,N′-bis(5-(2-pyridyl)-4H-pyrazole-3-carbonyl)-1,2-diaminoethane (L) was prepared in one-pot synthesis, employing Claisen condensation of 2-acetylpyridine and diethyl ethylene-1,2-bis(oxamate) followed by treatment with hydrazine hydrate. L possesses four bidentate chelating units and is expected to be able to bind metal ions in bis(binucleating) or tetradentate modes. Reaction of L with 1 equiv. of copper(II) or nickel(II) hydroxides in the presence of 2 equiv. KOH in DMSO affords mononuclear anionic complexes K₂[(Cu(L − 4H)]·5H₂O (1) and K₂[(Ni(L − 4H)]·6H₂O (2) containing metal ions in square-planar environment of two pairs of deprotonated amide and pyrazolato nitrogens. Reaction of 1 with 2 equiv. of Cu(NO₃)₂ leads to complete rearrangement of the coordination sphere, yielding a tetranuclear product which was crystallised in the presence of picric acid in the form [Cu₄(L − 2H)₂(DMSO)₂(EtOH)₂](picrate)₄ (3a). Complex [Cu₄(L − 2H)₂(H₂O)₄](NO₃)₄·4H₂O (3b) was obtained as a result of spontaneous self-assembly from L and copper(II) nitrate in aqueous acetic acid. Reaction of 2 with 2 equiv. of Ni(NO₃)₂ yields a tetranuclear cationic complex [Ni₄(L − 2H)₂(DMSO)₆(NO₃)(H₂O)](NO₃)₃·H₂O·3DMSO·(CH₃)₂CO (4) which contains four octahederal Ni ions coordinated in bis(binucleating) mode to two ligand threads disposed in side-by-side fashion and cis with respect to the Ni(1)–Ni(2) axis. In both 3a and 3b the ligand is coordinated in the same bis(binucleating) mode as in Ni complex 4, however, the Cu ions are square-pyramidal and the two ligand threads are disposed trans with respect to the Cu(1)–Cu(2) axis, thus giving rise to the double helical structures. 3a and 3b are the first examples of helicates comprising of bimetallic units. Formation of different topologies in 3a,b and 4 is elucidated in terms of different stereochemical preferences of these ions in mixed N,O-donor environments. All compounds were characterised by elemental analysis, ESI and MALDI mass spectrometry and UV–VIS spectroscopy, and complexes 3a, 3b and 4 by single crystal X-ray analysis. Magnetic susceptibility measurements (1.7–300 K) of powdered samples of tetranuclear complexes revealed strong (3b) or moderate (4) antiferromagnetic coupling within the dimeric units incorporating double pyrazolato bridges between paramagnetic centres.