Next Generation of Guanidine Quinoline Copper Complexes for Highly Controlled ATRP: Influence of Backbone Substitution on Redox Chemistry and Solubility

Abstract

Ligands DMEG6etqu, TMG6etqu, DMEG6buqu, and TMG6buqu were developed on the basis of guanidine quinoline (GUAqu) ligands 1,3-dimethyl-N-(quinolin-8-yl)imidazolidin-2imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine (TMGqu). These ligands feature an alkyl substituent at the C6 of the quinoline backbone. The synthetic strategy developed here enables inexpensive syntheses of any kind of C6-substituted GUAqu ligands. On one hand, the alkylation increases the solubility of corresponding copper complexes in apolar atom transfer radical polymerization (ATRP) monomers like styrene. On the other hand, it has a significant electronic influence and thus an effect on the donor properties of the new ligands. Seven Cu I and Cu II complexes of DMEG6etqu and TMG6etqu [a]

Increasing the Activity of Copper Guanidine Quinoline Catalysts: Substitution at the Quinoline Backbone Leads to Highly Active Complexes for ATRP

“…So the first improvement was to modify the quinoline backbone with alkyl substituents (right of Scheme 1, R = Et, Bu) to increase the solubility. [10] Besides optimising the ATRP performance of this system by better solubility, we demonstrate here that it is also possible to vary and enhance the activity by electronic effects. Substituents at the C6-position of the quinoline were varied by electron withdrawing (right of Scheme 1, R = NO 2 , Br) and donating (R = OMe, NMe 2 ) groups with a large range of Hammett parameters.…”

“…K ATRP constants were measured for copper catalyst systems based on L2-L6 under the same conditions like previously reported measurements of guanidine quinoline complexes. [10] Resulting values are summarized in Table 3.…”

“…TMGqu [10] 8.6 � 0. [25] Green circles: GUA6Rqu values published by Herres-Pawlis et al [10] Red circles: New values for GUA6Rqu complexes. […”

“…TMGqu [10] 0.83 � 0.03 9.7 � 0. determination of k deact the K ATRP values obtained by the Matyjaszewski method were used. As expected the resulting rate constants for the deactivation reaction slightly decrease for more electron-rich catalysts but all complexes show sufficiently high values to perform standard ATRP.…”

“…Based on this tetrapodal ligand a K ATRP value of around 1 is possible. [9] Our group has its focus on guanidine ligands because they have excellent N-donor properties and several guanidine complexes show catalytical activity not just in ATRP [10][11] but also in ring-opening polymerization of lactide [12] or oxygen activation. [13] We showed already that copper catalysts of guanidine quinoline (GUAqu) ligands TMGqu and DMEGqu (right of Scheme 1, R = H) are highly active in ATRP reactions.…”