Terminal-ligand dissociation

Upon recrystallization, [Ni(tpzlmtacn)]2+ affords [Ni(L)(MeCN)]2+ (L = l,4-bis(pyrazol-l-ylmethyl)-l,4,7-triazacyclononane) via a N-dealkylation reaction and loss of a pendent arm.1420 More rational routes to Ni complexes of tacn ligands with only one or two pendent arms have been developed.1431,1432 In [Ni(L)(X) ]x (e.g., L= l-(3-aminopropyl)-l,4,7-triazacyclononane (n = 2) or l-(l-methylimidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 2) or l,4-bis(l-methyl-imidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 1), the coordination sphere is completed by additional ligands that bind either terminal (X = C1 , H20) or bridge two metal ions (X = N3 , OH, oxalate). The Ni11 complex of l,4-bis(2-pyridylmethyl)-l,4,7-triazacyclononane has been shown to be extremely inert to ligand dissociation in aqueous solution.1433 In (562), the tacn ligand provides a single bidentate arm.1434... 

The tetrahedral nickel isonitrile cluster Ni4(CNR)7 (R = cyclohexyl or f-Bu) contains three bridging isonitrile ligands and a terminal one at each nickel (52) the bridging ligands are mobile on the cluster surface and a ligand dissociation step occurs (33, 390, 391). 

In another protocol using a ruthenium(pyridinebisoxazoline)(pyridinedicarboxylic acid) catalyst 26 and iodosylbenzene as the terminal oxidant, Seller and co-workers <03TL7479> found that the addition of water and protic solvents resulted in a 100-fold acceleration of the epoxidation, presumably due to a ligand dissociation effect that promotes the oxidation of the ruthenium catalyst. Thus, the conversion of rrans-stilbene 27 to the corresponding 5,S-epoxide 28 required 96 h in anhydrous toluene, but only 1 h in the presence of r-butanol and water. The enantioselectivity of the reaction was not significantly affected (63% and 57% ee, respectively). 

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