N-methylmonothiocarbamatopentamminecobalt(III): restricted C-N bond rotation and the acid-catalyzed O- to S-bonded rearrangement.
High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.
Jackson
W Gregory
WG
School of Physical, Environmental and Mathematical Sciences, Chemistry, University College, University of New South Wales, Canberra, ACT 2600, Australia. wgj@adfa.edu.au
McKeon
Josephine Anne
JA
Balahura
Robert J
RJ