Molecular-Programmed Self-Assembly of Homo- and Heterometallic Tetranuclear Coordination Compounds: Synthesis, Crystal Structures, and Magnetic Properties of Rack-Type Cu2IIM2II Complexes (M = Cu and Ni) with Tetranucleating Phenylenedioxamato Bridging Ligands

New homo- and heterobimetallic tetranuclear complexes of formula [Cu-4(mpba)(Me(4)en)(4)(H2O)(4)](ClO4)(4)center dot 3H(2)O (1), [Cu-4(mpba)(Me(4)en)(4)(H2O)(4)](PF6)(4)center dot 2H(2)O (2), [Cu-4(ppba)(Me(4)en)(4)(H2O)(4)](ClO4)(4)center dot 2H(2)O (3), [Cu-4(mpba)(dipn)(4)](ClO4)(4)center dot 3H(2)O (4), [Cu-4(ppba)(dipn)(4)](ClO4)(4)center dot 2H(2)O (5), and [Cu2Ni2(ppba)(dipn)(4)(H2O)(2)](PF6)(4) (6) [mpba= N,N'-1,3-phenylenebis(oxamate), ppba = N,N'-1,4-phenylenebis(oxamate), Me(4)en = N,N,N',N'-tetramethylethylenediamine, and dipn = dipropylenetriamine] have been synthesized and structurally and magnetically characterized. Complexes 1-6 have been prepared following a molecular-programmed self-assembly method, where a heteropolytopic tetranucleating phenylenedioxamato bridging ligand (L = mpba or ppba) is bound to four metal ions of identical or different natures (M = Cu-II and/or Ni-II) with partially blocked coordination sites by bi- or tridentate polyamine terminal ligands (L' = Me(4)en or dipn). The structures of 1-6 consist of cationic tetranuclear (Cu2M2II)-M-II entities with an overall R-4 rack-type architecture, which is made up of two oxamato-bridged homo-(1-5) or heterodinuclear (6) (CuMII)-M-II units (M = Cu and Ni) connected through either a meta- (1, 2, and 4) or a para-substituted (3, 5, and 6) phenylene spacer between the Cu-II ions. The magnetic properties of 1-6 have been interpreted according to their "dimer-of-dimers" structure [H = -J(S-1 center dot S-2 + S-3 center dot S-4) - J'(S-1 center dot S-3) with S-1 = S-3 = S-Cu = 1/2 and S-2 = S-4 = S-M =1/2 (M = Cu) or 1 (M = Ni)]. The homometallic Cu-4(II) complexes exhibit either strong (-J = 330-350 cm(-1)) or weak-to-moderate (-J = 4.8-87.1 cm(-1)) antiferromagnetic intradimer couplings through the oxamato bridge, depending on the bi- (1-3) or tridentate (4 and 5) nature of the terminal ligand, respectively. The heterometallic (Cu2Ni2II)-Ni-II complex with a tridentate terminal ligand (6) shows instead a moderate antiferromagnetic intradimer coupling (-J= 50 cm(-1)). Otherwise, the nature and magnitude of the interdimer coupling cannot be unambiguously determined except for the pair of homo- and heterometallic (Cu2M2II)-M-II complexes [M = Cu (5) and Ni (6)] with the p-phenylenedioxamato bridging ligand and a tridentate terminal ligand, which show a weak antiferromagnetic interdimer coupling (-J' =14 and 23 cm(-1)) across the para-substituted phenylene spacer.