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

A hybrid tris-bidentate neutral ligand (<b>L</b>) 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⁺ (<b>4</b>); Ni²⁺ (<b>5</b>); Co²⁺ (<b>6</b>)], along with our recently reported helicates L₃Fe₃(OTf)₆ (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), and L₃Fe₂Zn­(OTf)₆ (<b>3</b>) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L₃Cu₃(OTf)₆ (<b>4</b>) and L₃Ni₃(OTf)₆ (<b>5</b>) 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 (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), and L₃Fe₂Zn­(OTf)₆ (<b>3</b>). ESI-FTICR mass spectrometry and UV–vis spectroscopy studies show that helicates L₃Fe₃(OTf)₆ (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), L₃Fe₂Zn­(OTf)₆ (<b>3</b>), and L₃Co₃(OTf)₆ (<b>6</b>) can easily be transmetalated to helicate L₃Cu₃(OTf)₆ (<b>4</b>) 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 (<b>5</b>). Further, we have demonstrated the formation of a heterometallic helicate L₃Ni₂Co­(OTf)₆ (<b>7</b>) from a 1:1:1 reaction mixture of <b>L</b>, Ni­(OTf)₂, and Co­(OTf)₂, which can also be prepared from homometallic helicate L₃Co₃(OTf)₆ (<b>6</b>) by transmetalation with Ni­(OTf)<sub>2