¹⁷O NMR Study of Solvent Exchange in Some Aqueous [Co(tren)(X)(OH₂/OH)]ⁿ⁺, [Co(cyclen)(X)(OH₂/OH)]ⁿ⁺, and [Co(N-Mecyclen)(X)(OH₂/OH)]ⁿ⁺ Systems (X = NH₃, OH₂/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.

Reactions of mer(exo)‐ and mer(endo)‐[Co(dien)(dapo)X]2+ Isomers (X=OH, Cl, ONO2, N3)

“…These values are in agreement with our previous suggestion [5] [6] that, in the sixcoordinate system, the concerted transfer of an endo-NH proton provides a low-energy pathway for the loss of OH À (as H 2 O). Several other cis-NH protons appear to be geometrically available for such a transfer, but the concordance with the base hydrolysis result locates this proton precisely.…”

“…O-NMR experiment (such experiments require both care and significant amounts of complex [5]) with 17 O-labelled mer(exo)-[Co(dien)(dapo)OH 2 ](ClO 4 ) 3 in 0.50m NaClO 4 , pH 4.0 (258) gave less than ideal data (due to a large residual solvent signal), but the increase in the solvent signal with time accorded with t 1/2 % 30 min for exchange. This corresponds to k ex for the hydroxo complex of 0.013 s À1 based on the reported pK a of $ 5.5 [2b].…”

First turnover analysis of water-oxidation catalyzed by Co-oxide nanoparticles

“…While water exchange of terminal ligands on Co II complexes occurs generally with characteristic time constants in the order of 1 μs, [78][79][80] significantly slower exchange is expected for terminal ligands on Co III , which is the dominant oxidation state in the resting state of Co-oxide catalysts. 78 Characteristic times published for molecular complexes vary with ligand type and span a range of 10 days to 100 ms. 81 To the best of our knowledge, no values are published so far for terminal water or hydroxo ligands on Co-oxides or Co(oxo)hydroxides. 78 Characteristic times published for molecular complexes vary with ligand type and span a range of 10 days to 100 ms. 81 To the best of our knowledge, no values are published so far for terminal water or hydroxo ligands on Co-oxides or Co(oxo)hydroxides.…”

“…30,35,52,54 The retarded ligand exchange of Co III is due to the fact that it is a low spin diamagnetic t 2g system. 78 Characteristic times published for molecular complexes vary with ligand type and span a range of 10 days to 100 ms. 81 To the best of our knowledge, no values are published so far for terminal water or hydroxo ligands on Co-oxides or Co(oxo)hydroxides.…”

First aminoacetone chelate: [Co(tren){NH2CH2C(O)CH3}]3+—a substrate binding and activation model for zinc(ii)-dependent 5-aminolaevulinic acid dehydratase

“…A relevant question is whether the cyclization is achieved by substitution primarily at the Co centre or at the aminoacetone C-2 atom. An 17 O NMR study of solvent exchange in labelled p-[Co(tren)(OH 2 )(NH 3 )] 3+ and p-[Co(tren)(OH)(NH 3 )] 2+ ions revealed slow O exchange at the p-site consistent with half lives of at least 1 h. 12 In view of the close structural resemblance with the present system, it would be expected to behave in a similar manner and thereby obviate substitution at the cobalt(III) centre as a relevant step in the reaction 3 → 4. This leaves substitution at the carbonyl C atom as the likely cyclization path which could occur concertedly with or subsequent to hydrolysis of the acetal in 3.…”