Stepwise acetylide ligand substitution for the assembly of ethynylbenzene-linked Co(III) complexes

• Published in: Inorganica Chimica Acta 2012-01, Vol.380, p.174-180
• Main Authors: Hoffert, Wesley A., Kabir, Md. Khayrul, Hill, Ethan A., Mueller, Sara M., Shores, Matthew P.
• Format: Article
• Language: English
• Subjects: Cobalt complexes; Ethynylbenzene ligands; Ligand substitution; Metallodendrimer; Synthesis
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2011.08.051

A family of mono-, di- and trinuclear chloro-ethynylbenzene complexes containing [(cyclam)Co]3+ units are prepared via dehydrohalogenation routes and structurally characterized; stepwise acetylide addition is demonstrated in the synthesis of [(cyclam)Co(C2Ph)](BPh4), potentially offering control of complex nuclearity. [Display omitted] ► A family of chloro-ethynylbenzene complexes containing [(cyclam)Co]3+ units are prepared via dehydrohalogenation routes. ► Co:ligand reactant ratios and concentration of exogenous base affect product nuclearity. ► Stepwise ligand substitution is demonstrated, offering control of complex nuclearity. ► Electrochemical characterization shows solution instability of reduced Co-ethynylbenzene complexes. We describe the preparation and structural characterization of a family of chloro-ethynylbenzene complexes containing [(cyclam)Co]3+ units: [(cyclam)CoCl(CCPh)]X (X=Cl (1a), BPh4 (1b)), [(cyclam)2Co2Cl2(p-DEB)]X2 (X=Cl (2a), BPh4 (2b)), [(cyclam)3Co3Cl3(1,3,5-TEB)]X3 (X=Cl (3a), BPh4 (3b)), [(cyclam)2Co2Cl2(1,3,5-TEBH)]X2 (X=Cl (4a), BPh4 (4b)), where the bridging ligands p-DEB and TEB are the bis- and tris-deprotonated anions of 1,4-diethynylbenzene and 1,3,5-triethynylbenzene, respectively. Cyclic voltammetry of [(cyclam)3Co3Cl3(1,3,5-TEB)]Cl3 (3a) suggests that electrochemical reduction of the Co(III) centers to Co(II) is accompanied by complex dissociation resulting from the increased lability of the Co(II) ions. Inclusion of the tetraphenylborate anions in 1b–4b allows for the possibility of stepwise acetylide substitution by imparting complex cation solubility in tetrahydrofuran: trans-[(cyclam)Co(CCPh)2](BPh4) (5) is prepared by mixing LiCCPh with 1b. Isolation of 5 demonstrates the feasibility of metallodendrimer synthesis using octahedral first-row transition metal nodes, which awaits demonstration.