PI-BONDED INTERMEDIATES IN LINKAGE ISOMERIZATION-REACTIONS OF A TETRAZOLE COORDINATED TO PENTAAMMINECOBALT(III) - AN N-15 NMR-STUDY

Using regiospecifically N-15-labeled complexes, the N-1 (adjacent-Me) to N-2 (remote-Me) linkage isomerization reaction of (5-methyltetrazol ato)pentaamminecobalt(III) has been shown to be intramolecular in Me2S O and H2O as solvents. The results exclude a ligand dissociation and r eentry mechanism or rearrangement via a symmetrical eta5 intermediate; an eta2 pi-bonded tetrazolato intermediate is suggested. A degenerate remote-Me to remote-Me rearrangement (N-2 to N-3) has also been ident ified through the labeling experiments and shown to be intramolecular also. It is slower (25-fold) than the N-1 to N-2 rearrangement, as mig ht be anticipated because the Me interaction with the bound NH3 groups is diminished in the remote configuration. The N-1 isomer of the unch arged tetrazole ligand is some 100-fold faster to rearrange than its d eprotonated form, but N-15 experiments show that this rearrangement is also intramolecular and does not proceed via an eta5 pi-bonded interm ediate or through dissociation and reentry of the ligand. A degenerate (and 10-fold slower) N-2 to N-3 rearrangement has also been identifie d for the uncharged ligand complex. Degenerate N-1 to N-4 rearrangemen ts were not competitive with the N-1 to N-2 processes for either the a nionic or uncharged ligand complexes. The large upfield shifts for the N-15 signals and in particular the strong negative NOEs observed in t he proton-decoupled spectra establish N4 as the site of protonation fo r the N-1 and N-2 isomers; the tautomers involved have not been establ ished previously. Faster rearrangement for the more weakly bound neutr al ligand complexes is consistent with an essentially dissociative, al though still intramolecular, rearrangement process.