Cadmium(II) mediated addition of methanol to nitrile-functionalized 1,4,7-triazacyclononanes

• Cd(II) mediated methanolysis of nitrile pendant arm in tacn ligands is reported.
• The X-ray crystal structures of three Cd(II)-imino ether complexes are described.
• The methanolysis process is controlled by the length of pendant arm and anions.

The nucleophilic addition of methanol to two nitrile-functionalized triazamacrocyclic derivatives 1,4,7-tris(cyanomethyl)-1,4,7-triazacyclononane (L1) and 1,4,7-tris(cyanoethyl)-1,4,7-triazacyclononane (L2) were systematically studied under the mediation of various Cd2+ salts with different counter anions. When L1 was reacted with Cd(OAc)2·2H2O in refluxing methanol for 4 h at a ratio of 1:1, complete solvolysis of three pendant CH2CN groups of L1 was achieved and then a trigonal prismatic complex [Cd(L3)]2+ with the resulting imino ether ligand L3 was in situ produced and crystallized upon the addition of stoichiometric NaClO4. After raising the molar ratio of Cd(OAc)2·2H2O/L1 to 1.5:1 or higher and shortening the reaction time to 2 h, L1 was methanolyzed selectively into L5, incorporating two CH2C(OMe)NH and one CH2CN pendant arms because an acetate can still bind to Cd2+ and limits the conversion of the remaining nitrile arm. By adding NaClO4, a mono-capped trigonal prismatic complex [Cd(L5)(OAc)](ClO4) was easily precipitated from the methanolic solution. In contrast to L1, perchlorate, nitrate, and acetate salts of Cd2+ were found to promote the transformation of L2 bearing longer CH2CH2CN pendant arms into the entire solvolysis product L4 after refluxing for 4 h, which utilizes its three CH2CH2C(OMe)NH groups and tacn (tacn = 1,4,7-triazacyclononane) backbone to capture Cd2+ yielding an octahedral complex [Cd(L4)]2+. Additionally, L1 and L2 did not undergo methanolysis when treated with CdCl2·4H2O and two neutral compounds [Cd(L1)Cl2] and [Cd(L2)Cl2]·0.5H2O containing the original nitrile ligands were obtained. The latter was recrystallized from a DMF/CH3CN solution to afford an octahedral solvent-coordinated complex [Cd(L2)(DMF)Cl2]·CH3CN.

Selective or complete methanolysis of nitrile-functionalized triazamacrocyclic derivatives has been accomplished by employing various Cd(II) salts as Lewis acid promoters. Length of pendant arm and counter-anions of different sizes and coordination abilities have been proved to play important effects in manipulating the extent of solvolysis of pendant nitrile groups.Figure optionsDownload as PowerPoint slide