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 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][PF₆]·CH₂Cl₂ 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 [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.