17O NMR Study of Solvent Exchange in Some Aqueous [Co(tren)(X)(OH(2)/OH)](n)()(+), [Co(cyclen)(X)(OH(2)/OH)](n)()(+), and [Co(N-Mecyclen)(X)(OH(2)/OH)](n)()(+) Systems (X = NH(3), OH(2)/OH; n = 3, 2, 1)(1).
A (17)O NMR study (I = 1.0 M, NaClO(4) or NaOSO(2)CF(3), 25.0 degrees C) of solvent exchange in labeled p- and t-[Co(tren)(NH(3))OH(2)](3+), [Co(tren)(OH(2))(2)](3+), [Co(cyclen)(OH(2))(2)](3+), and [Co(N-Mecyclen)(OH(2))(2)](3+) ions (ca. 30% (17)O) in aqueous solution has shown that loss of coordinated OH(2) is slow for all of the complexes (k(ex)/s(-)(1) = 1.1 x 10(-)(5), 1.2 x 10(-)(5), 3.7 x 10(-)(5) (p-site)/8.7 x 10(-)(6) (t-site), 2 x 10(-)(4), and 2 x 10(-)(4), respectively). Values of k(ex) for solvent exchange in [Co(tren)(OH)(2)](+) have been determined as 9.7 x 10(-)(5) s(-)(1) (p-site) and 2.2 x 10(-)(7) s(-)(1) (t-site) Coordinated OH(-) in both p- and t-[Co(tren)(NH(3))OH](2+) also exchanges only slowly with solvent (k(ex)/s(-)(1)= 1.7 x 10(-)(4), and <1 x 10(-)(6), respectively), whereas exchange of coordinated solvent in the aqua-hydroxo complexes [Co(tren)(t-OH(2))(p-OH)](2+), [Co(cyclen)(OH(2))OH](2+), and [Co(N-Mecyclen)(OH(2))OH](2+) is much more rapid (k(ex)/s(-)(1) = 0.03 (p-site)/0.01 (t-site), 12 and 15, respectively). Ligand-OH(-) exchange in these latter systems is interpreted as occurring via (indirect) S(N)1(CB)-type processes on the corresponding aqua complexes: [Co(amine)OH](2+) right harpoon over left harpoon [Co(amine-H)OH(2)](2+) --> exchange. This type of pathway is seen to be more efficient when leaving-group departure is synchronous with proton transfer, and this appears to be more important for exchange in the cyclen and N-Mecyclen complexes where a reasonably acidic, adjacent syn NH proton is involved.
Brasch
Nicola E.
NE
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand.
Buckingham
David A.
DA
Clark
Charles R.
CR
Rogers
Andrew J.
AJ