Synthesis of trans bis-alkynyl complexes of Co(III) supported by a tetradentate macrocyclic amine: A spectroscopic, structural, and electrochemical analysis of π-interactions and electronic communication in the C[tbnd]C[sbnd]M[sbnd]C[tbnd]C structural unit

Abstract

Alkynyl complexes of the type trans-[Co(cyclam)(CCR)2]OTf have been prepared and characterized by UV–Vis spectroscopy, 1H NMR, vibrational spectroscopy (infrared and Raman), and cyclic voltammetry. Where appropriate the data is compared to the corresponding Cr(III) and Rh(III) complexes. Though the arylalkynyl ligands have been shown to act as π-donors for the corresponding Cr(III) complexes, vibrational spectroscopy suggests that the π-interactions between the arylalkynyl ligands and Co(III) are quite weak, and that the more electron withdrawing trifluoropropynyl ligand likely behaves as a weak π-acceptor toward Co(III). X-ray crystal structures for trans-[Co(cyclam)(CCCF3)2]OTf and trans-[Cr(cyclam)(CCCF3)2]OTf are also reported and analysis of the MC and CC bond lengths are consistent with this understanding of the trifluoropropynyl ligand. Cyclic voltammetry of the trans-[Co(cyclam)(CCR)2]OTf complexes demonstrates that when R=C6H5 or p-C6H4CH3, the CoIII/II reduction wave is chemically irreversible. However, when R=p-C6H4CF3, p-C6H4CN, or CF3, the CoIII/II reduction wave is chemically reversible. This suggests that the more electron withdrawing alkynyl ligands become π-acceptors toward the reduced form of cobalt. Finally, the ferrocenyl capped trans-[M(cyclam)(CCFc)2]OTf complexes (where M=Co(III) and Rh(III)) were prepared and studied. Cyclic voltammetry shows only a single 2e− wave for the ferrocenyl termini, indicating little to no electronic communication through the organometallic backbone.