STABILIZATION OF COBALT CAGE CONFORMERS IN THE SOLID-STATE AND SOLUTION

The cage complexes [Co{(NMe(2))(2)sar}](3+) and [Co{(NMe(2),Me)sar}](3 +) (sar = sarcophagine = 3,6,10,13,16,19 hexaazabicyclo[6.6.6]icosane; (NMe(2))(2)sar 1,8-bis(dimethylamino)sarcophagine; (NMe(2),Me)sar 1-( dimethylamino)-8-methylsarcophagine) are obtained by methylation of am ino substituents on the parent cage complexes using formaldehyde and f ormic acid. Further methylation with methyl iodide in dimethyl sulfoxi de, or with dimethyl sulfate in N,N-dimethylformamide, converts the di methylamino substituents to trimethylammonium substituents and essenti ally inverts the circular dichroism (CD) spectrum in comparison to tha t of the protonated parent cage complex, [Co{(NH3)(2)sar}](5+). A comb ination of NMR and electronic spectroscopic measurements indicates tha t the inversion of the CD spectra is due to conformational inversions in the 1,2-ethanediamine rings from mainly a lel conformation in [Co{( NH3)(3)sar}](5+) to an obs conformation in [Co{(NMe(3))(2)sar}](5+), ( without inversion at any of the seven stereogenic centers; Co plus six coordinated amines). This ob(3) conformation was identified in the so lid state by an X-ray crystallographic analysis of [Co{(NMe(3))(2)sar} ](NO3)(5).3H(2)O: monoclinic, P2(1)/n, a 14.382(4) Angstrom, b = 14.60 4(3) Angstrom, c 16.998(6) Angstrom, beta = 100.28(3)degrees, Z= 4. Th e electron self-exchange rate constant of the [Co((NMe(3))2sar)](5+/4) redox couple (+0.05 V vs NHE, 0.011(1) M(-1) s(-1) at 25 degrees C, I = 0.2 M (NaCl)) is a factor of 2 smaller than that of the [Co((NH3)2 sar)](5+/4+) couple under the same conditions and is the slowest elect ron-transfer rate so far measured for a hexaamine cage complex of this type. However, it is still much faster than those of the parent [Co(e n)(3)](3+/2+) and [Co(NH3)(6)](3+/2+) couples. This paper also provide s hard evidence for the conformations of other Co(III) cage complexes in solution as well as the solid state.