Symmetrical linear Co-3(6+) chains cocooned by two polypyridylamide ligands: How do they compare to open chains?
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abstract
Two extended metal atom chain (EMAC) compounds having a symmetrical Co 36+ metal chain encapsulated by two N,N- bis[(6-pyrid-2-yl)aminopyrid-2yl]-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 [Co 3(mpeptea)2]Cl2 two solvates have been crystallized. Anion exchange has also allowed isolation of [Co 3(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 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 T 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 Co 3(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. 2010 Elsevier B.V. All rights reserved.
