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