Although the use of ball milling to induce reactions between solids (mechanochemical synthesis) can
provide lower-waste routes to chemical products by avoiding solvent during the reaction, there are further potential advantages in using one-pot multistep syntheses to avoid the use of bulk solvents for the purification of intermediates. We report here two-step syntheses involving formation of salen-type ligands from diamines and hydroxyaldehydes followed directly by reactions with metal salts to provide the corresponding metal complexes. Five salen-type ligands 2,2’-[1,2-ethanediylbis[(E)-nitrilomethylidyne]]bis-phenol,‘salenH2’,1; 2,2’-[(±)-1,2-cyclohexanediylbis-[(E)-nitrilomethylidyne]]bis-phenol,2 ;2,2’-[1,2-phenylene-bis(nitrilomethylidyne)]-bis-phenol, ‘salphenH2’ 3; 2-[[(2-aminophenyl)imino]methyl]-phenol,
4; 2,2’-[(±)-1,2-cyclohexanediylbis[(E)-nitrilomethylidyne]]-bis[4,6-bis(1,1-dimethylethyl)]-phenol, ‘Jacobsen ligand’,5) were found to form readily in a shaker-type ball mill at 0.5 to 3 g scale from their corresponding
diamine and aldehyde precursors. Although in some cases both starting materials were liquids, ball milling
was still necessary to drive those reactions to completion because precipitation of the product and or
intermediates rapidly gave in thick pastes which could not be stirred conventionally. The only ligand
which required the addition of solvent was the Jacobsen ligand 5 which required 1.75 mol equivalents of
methanol to go to completion. Ligands
1–5 were thus obtained directly in 30–60 minutes in their hydrated forms, due to the presence of water by-product, as free-flowing yellow powders which could be dried by heating to give analytically pure products. The one-armed salphen ligand 4 could also be obtained selectively by changing the reaction stoichiometry to 1 : 1. SalenH2 1 was explored for the one-pot two-step synthesis of metal complexes. In particular, after in situ formation of the ligand by ball milling, metal salts (ZnO, Ni(OAc)2·4H2O or Cu(OAc)2·H2O) were added directly to the jar and milling continued for a further 30 minutes. Small amounts of methanol (0.4–1.1 mol equivalents) were needed for
these reactions to run to completion. The corresponding metal complexes [M(salen)] (M = Zn,6; Ni,7;or Cu,8) were thus obtained quantitatively after 30 minutes in hydrated form, and could be heated briefly to give analytically pure dehydrated products. The all-at-once ‘tandem’ synthesis of [Zn(salen)] 6 was also explored by milling ZnO, ethylene diamine and salicylaldehyde together in the appropriate mole ratio for 60 minutes. This approach also gave the target complex selectively with no solvent needing to be added.
Overall, these syntheses were found to be highly efficient in terms of time and the in avoidance of bulk solvent both during the reaction and for the isolation of intermediates. The work demonstrates the applicability of mechanochemical synthesis to one-pot multi-step strategies.