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

Abstract

Complexes of MnII, CoII and ZnII with the hexadentate podand ligand tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate [L]– have been prepared and structurally characterised. In mononuclear [CoL][PF6]·CH2Cl2 all three bidentate arms of the ligand are co-ordinated to the CoII in a relatively strain-free manner to give a trigonal prismatic co-ordination geometry. In contrast in [Mn4L4][PF6]4·4MeCN·Et2O and [Zn4L4][PF6]3[OH]·12EtOH the [M4L4]4+ complex cations are tetrahedral clusters. Each ligand [L]– co-ordinates one bidentate arm to each of three metal ions in a κ2∶κ2∶κ2 co-ordination mode, such that each ligand caps one triangular face of the metal tetrahedron. This trinucleating co-ordination mode, and the 1∶1 correspondence of octahedral metal ions and hexadentate ligands, necessarily results in formation of the tetrahedral cluster in which all four metal tris(chelate) centres have the same chirality. Thus the mononucleating κ6 co-ordination mode results when the metal ion can tolerate a trigonal prismatic geometry, whereas the trinucleating κ2∶κ2∶κ2 mode occurs when the metal ions are octahedral. Spectroscopic evidence (1H 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 the decomposition product [Zn4L2(pypz)2(µ4-PO4)][PF6]3·2Me2CO·2Et2O [pypz = 3-(2-pyridyl)pyrazole], in which [L]– adopts the hitherto unseen binucleating (κ4∶κ2) co-ordination mode, the central bridging phosphate arising from hydrolysis of [PF6]– under ambient conditions.