Homonuclear and heteronuclear complexes of a four-armed octadentate ligand: synthetic control based on matching ligand denticity with metal ion coordination preferences

The octadentate ligand 1,2,4,5-tetrakis-[3-(2-pyridyl)pyrazol-1-yl]benzene (L), with four bidentate arms radiating from a central phenyl ring, combines with 6-coordinate and 4-coordinate metal ions (as their tetrafluoroborate salts) in different ways according to the coordination number preferences of the metal ions. The four bidentate arms are not ideally matched to the requirement of octahedral metal ions, such that complexes with Cd(II), such as tetranuclear [Cd4L2(mu-F)(2)(solv)(4)](BF4)(6) (solv = MeOH or MeCN), and Co(II)/Ni(II), such as hexanuclear [Co6L4(mu-F)(2)][BF4](10), require monodentate ancillary ligands (solvent molecules or fluoride ions) to provide coordinative saturation. In contrast, a mixture of octahedral [M = Co(II) or Ni(II)] and tetrahedral [Ag(I)] metal ions reacts with L to afford the simpler trinuclear heterometallic complexes [Co2AgL2](BF4)(5) in which the requirements of the metal ions (for 3 + 3 + 2 bidentate arms) are matched by two four-armed ligands. The maximum site occupancy principle can accordingly be used to direct self-assembly of heterometallic complexes.