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Antiferromagnetically Coupled Dimeric Dodecacopper Supramolecular Architectures of Macrocyclic Ligands with a Symmetrical μ6‑BO33– Central Moiety

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journal contribution
posted on 20.07.2015, 00:00 by Santokh S. Tandon, Scott D. Bunge, Sara A. Toth, Joaquin Sanchiz, Laurence K. Thompson, Jacob T. Shelley
Reactions between 2,6-diformyl-4-alkyl­(R)-phenol (R = CH3 or C­(CH3)3) and 1,3-diamino-2-hydroxypropane (1,3-DAP) in the presence of copper­(II) salts (Cu­(BF4)2·6H2O, Cu­(ClO4)2·6H2O/H3BO3/Ar) and triethylamine (TEA) in a single pot result in self-assembly of dimeric dodecacopper supramolecular architectures of 30-membered hexatopic macrocyclic ligands (H6L4 and H6L5) with unique and fascinating structures having the BO33– anion as the central species bonded to all six copper centers in a symmetrical fashion (μ6-BO33–). A number of closely related macrocyclic hexacopper complexes are reported: {[Cu6(L4)­(μ6-BO3)­(μ-H2O)­(C3H7NO)2(BF4)]­[BF4]2·3C3H7NO}2 (1) (DMF = C3H7NO), {[Cu6(L4)­(μ6-BO3)­(μ-C3H7NO)3]­[ClO4]3·3C3H7NO}2 (2), {[Cu6(L5)­(μ6-BO3)­(μ–OH)­(H2O)3(C3H7NO)]­[BF4]2·6C3H7NO·4C2H5OH·2H2O}2 (3), {[Cu6(L5)­(μ6-BO3)­(μ-CH3OH)­(CH3OH)2]­[ClO4]3·10H2O}2 (4), and {[Cu6(L5)­(μ6-BO3)­(μ-CH3CO2)­(μ-CH3O)­(CH3OH)]­[BF4]·13CH3OH·8H2O}2 (5). A polymeric side product {[Cu2(H2L2)­(CH3OH)­(BF4)]­[BF4]}n (6), involving a 2 + 2 macrocyclic ligand, was also isolated and structurally characterized. Complex 6 involves dinuclear copper­(II) units linked through BF4 anions to form a novel 1D single-chain polymeric coordination compound. This appears to be the first report in which a central BO33– species is linked to six copper­(II) ions held together by a single macrocyclic ligand through three μ1,1-O­(BO33–) and three μ1,3-O­(BO33–) bridges. In complexes 15 the BO33– is present in the center of the macrocyclic cavity and is bonded to all six metal centers arranged in a benzene-like hexagonal array. In the hexagonal array there are alternate double (μ-alkoxide and μ1,3-O­(BO33–)) and (μ-phenoxide and μ1,1-O­(BO33–)) bridges between the Cu­(II) centers. The symmetrical hexa-bridging nature of μ6-BO33– is unprecedented in transition metal complex chemistry, and along with alkoxide and phenoxide bridges in the equatorial plane provides effective pathways for an overall antiferromagnetic exchange interaction between six copper­(II) centers. In 1, 3, and 5 the BO33– moiety is produced in one step (synthetic) by an unusual copper­(II)-macrocycle complex catalyzed hydrolysis of BF4 ion in methanol. In 2 and 4 the central species (BO33–) comes from boric acid (H3BO3) which is added to reaction mixture of Cu­(ClO4)2/H6L4/H6L5 under inert conditions to confirm the identity of the central species.