Symmetrical linear Co36+ chains cocooned by two polypyridylamide ligands: How do they compare to open chains?

Two extended metal atom chain (EMAC) compounds having a symmetrical Co36+ metal chain encapsulated by two N,N′-bis[(6′-pyrid-2″-yl)aminopyrid-2′yl]-bismethyl-2,6-diaminopyridinate (mpeptea) ligands have been prepared in good yield, and they have been structurally characterized by X-ray crystallography, magnetic and electrochemical measurements and spectroscopic techniques. For the EMAC [Co3(mpeptea)2]Cl2 two solvates have been crystallized. Anion exchange has also allowed isolation of [Co3(mpeptea)2](BPh4)2. In these three species the Co36+ units are cocooned within two polypyridine ligands having nine nitrogen atoms although only seven of these coordinate to the metal centers. The cations [Co3(mpeptea)2]2+ are similar and have Co ··· Co separations of ca. 2.3 Å at 213 K. These distances are consistent with partial bond formation between Co atoms. Electrochemical measurements show a unique one-electron oxidation process that differs from that in open chain species which show two reversible oxidation processes. At 300 K, the χT value is 7.32 emu K mol−1 but this value drops to 1.47 emu K mol−1 as the temperature is lowered to 2 K. The X-band EPR spectra for Co3(mpeptea)22+ show g values of 2.3 at room temperature. The magnetic behavior is quite different from that in compounds with open Co36+ units. A discussion is provided.

Three species with extended metal atom chains (EMACs) having symmetrical Co36+ cores and partial Co ··· Co bonds cocooned by two polypyridyl ligands have been synthesized and characterized by X-ray crystallography, magnetic and electrochemical measurements and spectroscopic techniques. Variable temperature magnetic susceptibility measurements suggest that at 300 K there may be up to seven unpaired electrons but electron coupling at low temperatures lowers the magnetism. This magnetic behavior differs from that of the open chain compound Co3(dipyridylamide)4Cl2 that has a maximum of five unpaired electrons at room temperature.Figure optionsDownload as PowerPoint slide