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 LM(OTf) [ M = Cu (4); Ni (5); Co (6)], along with our recently reported helicates LFe(OTf) (1), LZn(OTf) (2), and LFeZn(OTf) (3) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of LCu(OTf) (4) and LNi(OTf) (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 LFe(OTf) (1), LZn(OTf) (2), and LFeZn(OTf) (3). ESI-FTICR mass spectrometry and UV-vis spectroscopy studies show that helicates LFe(OTf) (1), LZn(OTf) (2), LFeZn(OTf) (3), and LCo(OTf) (6) can easily be transmetalated to helicate LCu(OTf) (4) in the presence of Cu(OTf). On the other hand, only a trace amount of heterometallic helicate LNiCu(OTf) forms even after several days, when Cu(OTf) is added to a the solution of homometallic helicate LNi(OTf) (5). Further, we have demonstrated the formation of a heterometallic helicate LNiCo(OTf) (7) from a 1:1:1 reaction mixture of L, Ni(OTf), and Co(OTf), which can also be prepared from homometallic helicate LCo(OTf) (6) by transmetalation with Ni(OTf).