Secondary Deuterium Kinetic Isotope Effect for Aquation, Solvolysis, and Isomerization Reactions of trans-[Co(en)₂(OSMe₂)N₃]²⁺, and the Resolution of a Mechanistic Anomaly

Abstract

The two closely spaced NH signals in the (1)H NMR spectrum of trans-[Co(en)(2)(OSMe(2))(N(3))](2+) have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co-N(3)) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND(3)(l) has been reinterpreted as "normal", with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me(2)SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me(2)SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me(2)SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.

26 Mechanisms of reactions in solution

“…Careful examination of the 1 H NMR spectrum and application of 2D NMR methods has allowed a re-interpretation of the structure of trans-[Co(en) 2 -(OSMe 2 )(N 3 )] 2+ , such that the previously accepted unusual syn-/anti-rearrangement during base catalysed rearrangement has now been recognised as being in fact a ''normal'' rearrangement. 127 Also, an earlier secondary deuterium isotope effect study is extended to include solvolysis reactions of this complex in DMSO and acetonitrile. 128 [Ni(NCO)(L)] 2 is the product of reaction of CO and [Ni(N 3 )(L)] 2 (L = bi(2mercaptophenyl)sulfide) and has been shown by DFT to be formed in three key steps.…”