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.