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

摘要

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 structurally characterised. In mononuclear [CoL][PF_6] centre dot CH_2Cl_2 all three bidentate arms of the ligand are co-ordinated to the Co~(II) in a relatively strain-free manner to give a trigonal prismatic co-ordination geometry. In contrast in [Mn_4L_4][PF_6]_4 centre dot 4MeCN centre dot Et_2O and [Zn_4L_4][PF_6]_3[OH] centre dot 12EtOH the [M_4L_4]~(4+) complex cations are tetrahedral clusters. Each ligand [L]~- co-ordinates 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 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 #kappa#~6 co-ordination mode results when the metal ion can tolerate a trigonal prismatic geometry, whereas the trinucleating #kappa#~2: #kappa#~2: #kappa#~2 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 [Zn_4L_4][PF_6]_4 from acetone-diethyl ether resulted in formation of a few crystals of the decomposition product [Zn_4L_4](pypz)_2(#mu#_4-PO_4)[PF_6]_3 centre dot 2Me_2CO centre dot 2Et_2O [pypz = 3-(2-pyridyl)pyrazole], in which [L]~- adopts the hitherto unseen binucleating (#kappa#~4: #kappa#~2) co-ordination mode, the central bridging phosphate arising from hydrolysis of [PF_6]~- under ambient conditions.