Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates

A hybrid tris-bidentate neutral ligand (L) composed of a central 2,2′-bipyridine and two terminal triazolyl-pyridine chelating units connected by methylene spacers is employed to synthesize trinuclear triple-stranded side-by-side helicates of first-row transition-metal­(II) ions. Three such new homometallic helicates L₃M₃(OTf)₆ [ M = Cu²⁺ (4); Ni²⁺ (5); Co²⁺ (6)], along with our recently reported helicates L₃Fe₃(OTf)₆ (1), L₃Zn₃(OTf)₆ (2), and L₃Fe₂Zn­(OTf)₆ (3) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L₃Cu₃(OTf)₆ (4) and L₃Ni₃(OTf)₆ (5) show the formation of trinuclear triple-stranded side-by-side helicates with alternating Λ and Δ chiralities at the metal ions as earlier observed in cases of L₃Fe₃(OTf)₆ (1), L₃Zn₃(OTf)₆ (2), and L₃Fe₂Zn­(OTf)₆ (3). ESI-FTICR mass spectrometry and UV–vis spectroscopy studies show that helicates L₃Fe₃(OTf)₆ (1), L₃Zn₃(OTf)₆ (2), L₃Fe₂Zn­(OTf)₆ (3), and L₃Co₃(OTf)₆ (6) can easily be transmetalated to helicate L₃Cu₃(OTf)₆ (4) in the presence of Cu­(OTf)₂. On the other hand, only a trace amount of heterometallic helicate L₃Ni₂Cu­(OTf)₆ forms even after several days, when Cu­(OTf)₂ is added to a the solution of homometallic helicate L₃Ni₃(OTf)₆ (5). Further, we have demonstrated the formation of a heterometallic helicate L₃Ni₂Co­(OTf)₆ (7) from a 1:1:1 reaction mixture of L, Ni­(OTf)₂, and Co­(OTf)₂, which can also be prepared from homometallic helicate L₃Co₃(OTf)₆ (6) by transmetalation with Ni­(OTf)₂.