Effects of metal co-ordination geometry on self-assembly: a monomeric complex with trigonal prismatic metal co-ordination vs. tetrameric complexes with octahedral metal co-ordination

Complexes of Mn-II, Co-II and Zn-II with the hexadentate podand ligand tris [3-(2-pyridyl)pyrazol-1-yl]hydroborate[L](-) have been prepared and structu rally characterised. In mononuclear [CoL][PF6]. CH2Cl2 all three bidentate arms of the ligand are co-ordinated to the Co-II in a relatively strain-fre e manner to give a trigonal prismatic co-ordination geometry. In contrast i n [Mn4L4][PF6](4). 4MeCN . Et2O and [Zn4L4][PF6](3)[OH]. 12EtOH the [M4L4]( 4+) complex cations are tetrahedral clusters. Each ligand [L](-) co-ordinat es one bidentate arm to each of three metal ions in a kappa(2) : kappa(2) : kappa(2) co-ordination mode, such that each ligand caps one triangular fac e of the metal tetrahedron. This trinucleating co-ordination mode, and the 1 : 1 correspondence of octahedral metal ions and hexadentate ligands, nece ssarily results in formation of the tetrahedral cluster in which all four m etal tris(chelate) centres have the same chirality. Thus the mononucleating kappa(6) co-ordination mode results when the metal ion can tolerate a trig onal prismatic geometry, whereas the trinucleating kappa(2) : kappa(2) : ka ppa(2) mode occurs when the metal ions are octahedral. Spectroscopic eviden ce (H-1 NMR and UV/VIS spectroscopy and electrospray mass spectrometry, as appropriate) suggests that the monomeric and tetrameric forms are retained in solution and do not interconvert. Attempts to recrystallise [Zn4L4][PF6] (4) from acetone-diethyl ether resulted in formation of a few crystals of t he decomposition product [Zn4L2(pypz)(2)(mu(4)-PO4)][PF6](3). 2Me(2)CO . 2E t(2)O [pypz = 3-(2-pyridyl)pyrazole], in which [L](-) adopts the hitherto u nseen binucleating (kappa(4) : kappa(2)) co-ordination mode, the central br idging phosphate arising from hydrolysis of [PF6](-) under ambient conditio ns.