Functional Tetrametallic Linker Modules for Coordination Polymers and Metal−Organic Frameworks

The new biphenol-based tetranucleating ligand, 2,2‘,6,6‘-tetrakis(N,N-bis(2-pyridylmethyl)aminomethyl)-4,4‘-biphenolate,
dbpbp2-, comprises two linearly disposed phenolato-hinged dinucleating heptadentate units, each of which offer
one O and three N donors to a total of four metal ions. The ligand has been isolated as the zinc chloride complex
[Zn4(dbpbp)Cl4]2+, and the ZnII ions have been completely or partially substituted by CuII, FeIII, CoII, and CoIII in
metathesis reactions. Similarly, the chloride ligands of [Zn4(dbpbp)Cl4]2+ have been exchanged for solvent molecules
(acetonitrile and/or water) and bridging carboxylate ligands. The resulting complexes have been characterized by
single-crystal X-ray diffraction, ESI mass spectrometry (ESI-MS), cyclic voltammetry (CV), and EPR spectroscopy.
The structures containing [M4(dbpbp)Cl4]2+ with M = ZnII or CuII exhibit 2-D polymeric honeycomb sheets in which
intermolecular M···Cl interactions bridge between adjacent [M4(dbpbp)Cl4]2+ cations. Two mixed-metal tetrabenzoate
complexes [M4(dbpbp)(O2CC6H5)4]2+/3+ have also been prepared, namely a stoichiometric CuII2ZnII2 complex and a
nonstoichiometric FeIII/ZnII system. In the latter case, ESI-MS identifies FeZn3, Fe2Zn2, and Zn4 species, and X-ray
crystallography suggests an average composition of Fe0.8Zn3.2. Preparation of a CoII4 complex by metathesis was
considerably more difficult than preparation of [Cu4(dbpbp)Cl4]2+, requiring both a large excess of the cobalt source
and the presence of auxiliary benzoate. In the presence of 2 equiv of benzoate per starting [Zn4(dbpbp)Cl4]2+ unit
and excess CoII, dioxygen binds as peroxide at each end of the molecule to give the CoIII4 complex [Co4(dbpbp)(O2)2(O2CC6H5)2]4+. This latter complex, together with new tetra- and hexametallic benzenedicarboxylato- and
benzenetricarboxylato-bridged complexes of dinuclear [Co2(O2)(bpbp)]3+ units (bpbp- = 2,6-bis(N,N-bis-(2-pyridylmethyl)aminomethyl)-4-tert-butyl-phenolate), is a module for potential construction of 1-D and 2-D coordination
polymers/metal−organic frameworks (MOFs) capable of reversible O2 binding.