Water was found to be an ideal solvent for the remarkably high-yield, fast preparation of easily hydrolyzable 2-pyrrolecarbaldimines. In the presence of Cu 2 þ , the reaction afforded, in one step, the corresponding Cu(II) chelates which were demonstrated to meet the initial set of current microelectronics criteria for Cu metal deposition via a CVD/ALD process. Electronic effects of the substituent on the imino nitrogen of the Cu complexes were shown to strongly influence the coordination geometry at the Cu center.Keywords: chelating agents; chemical vapor deposition; copper; imines; Schiff bases Since the original 1929 report [1] of the first imino derivatives of pyrrole-2-carboxaldehyde and its Co and Cu chelates, numerous 2-pyrrolecarbaldiminato-metal complexes have been synthesized and have found a number of important applications. Recent examples of exciting applications of 2-pyrrolecarbaldiminato complexes include efficient post-metallocene catalysis of ethylene polymerization with titanium [2] and hafnium [3] complexes, and facile C À H activation with platinum derivatives under mild conditions. [4] Most of the reported ligands of the type 2-pyrrolyl-CH ¼ NR are more stable derivatives of aromatic amines (R ¼ Ar), whereas much more easily hydrolyzable alkyl counterparts are scarce. [2,3,5 -8] The Schiff reaction [Eq.(1)] [9] is used for the preparation of pyrrolecarbaldimines from the aldehyde and a primary amine.
ð1ÞThe condensation of pyrrole-2-carboxaldehyde is commonly run in anhydrous alcohols such as EtOH [3,5,8] or dry ether [6,7] to minimize the reverse process, hydrolysis. Azeotropic distillation of the H 2 O byproduct has also been employed to maximize the yield. [10,11] For aromatic amines, catalysis with acids is often used, which, while speeding up the reaction, can also cause product decomposition and coloring. Regardless of the solvent and conditions employed, the yields for isolated, pure 2-pyrrolecarbaldimines rarely exceed 70% and often are not specified. Close to quantitative isolated yields for pure, colorless pyrrolecarbaldimines are seldom reported.While the above considerations of the equilibrium in Eq. (1) suggested that H 2 O should be avoided as thoroughly as possible, we found that water is a superior solvent for the reaction of pyrrole-2-carboxaldehyde with RNH 2 . Running the synthesis in water afforded the desired imines in pure form in ! 90% isolated yield [Eq. (2)]. For alkylamines, the reaction is fast, taking only a few minutes to furnish the products as colorless or slightly yellow crystalline materials or oils. The less nucleophilic aniline reacted with pyrrole-2-carboxaldehyde in water more slowly to give the imine in 94% isolated yield after 3 days. The observed ca. 100% selectivity at full conversion eliminated the need for additional purification of the products. Although the alkylimines precipitated out of the aqueous medium, their isolation was performed via extraction with hexanes, followed by filtration through a short Na 2 SO 4 column, [12] evaporatio...