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 Lambda and Delta 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).