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 amino substituents on the parent cage complexes using formaldehyde and formic acid. Further methylation with methyl iodide in dimethyl sulfoxide, or with dimethyl sulfate in N,N-dimethylformamide, converts the dimethylamino substituents to trimethylammonium substituents and essentially inverts the circular dichroism (CD) spectrum in comparison to that of the protonated parent cage complex, [Co{(NH3)(2)sar}](5+). A combination of NMR and electronic spectroscopic measurements indicates that 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 solid 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 = 14.604(3) Angstrom, c 16.998(6) Angstrom, beta = 100.28(3)degrees, Z= 4. The 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)2sar)](5+/4+) couple under the same conditions and is the slowest electron-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(en)(3)](3+/2+) and [Co(NH3)(6)](3+/2+) couples. This paper also provides hard evidence for the conformations of other Co(III) cage complexes in solution as well as the solid state.