title: 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).
abstract: 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.
authors.affiliation: Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand.
authors.forename: Nicola E.
authors.initials: NE
authors.lastname: Brasch
authors.forename: David A.
authors.initials: DA
authors.lastname: Buckingham
authors.forename: Charles R.
authors.initials: CR
authors.lastname: Clark
authors.forename: Andrew J.
authors.initials: AJ
authors.lastname: Rogers