Properties and ATRP Activity of Copper Complexes with Substituted Tris(2-pyridylmethyl)amine-Based Ligands

Synthesis, characterization, electrochemical studies, and ATRP activity of a series of novel copper(I and II) complexes with TPMA-based ligands containing 4-methoxy-3,5-dimethyl-substituted pyridine arms were reported. In the solid state, CuI(TPMA*1)Br, CuI(TPMA*2)Br, and CuI(TPMA*3)Br complexes were found to be distorted tetrahedral in geometry and contained coordinated bromide anions. Pseudo-coordination of the aliphatic nitrogen atom to the copper(I) center was observed in CuI(TPMA*2)Br and CuI(TPMA*3)Br complexes, whereas pyridine arm dissociation occurred in CuI(TPMA*1)Br. All copper(I) complexes with substituted TPMA ligands exhibited a high degree of fluxionality in solution. At low temperature, CuI(TPMA*1)Br was found to be symmetrical and monomeric, while dissociation of either unsubstituted pyridine and/or 4-methoxy-3,5-dimethyl-substituted pyridine arms was observed in CuI(TPMA*2)Br and CuI(TPMA*3)Br. On the other hand, the geometry of the copper(II) complexes in the solid state deviated from ideal trigonal bipyramidal, as confirmed by a decrease in τ values ([CuII(TPMA*1)Br][Br] (τ = 0.92) > [CuII(TPMA*3)Br][Br] (τ = 0.77) > [CuII(TPMA*2)Br][Br] (τ = 0.72)). Furthermore, cyclic voltammetry studies indicated a nearly stepwise decrease (ΔE ≈ 60 mV) of E1/2 values relative to SCE (TPMA (−240 mV) > TPMA*1 (−310 mV) > TPMA*2 (−360 mV) > TPMA*3 (−420 mV)) on going from [CuII(TPMA)Br][Br] to [CuII(TPMA*3)Br][Br], confirming that the presence of electron-donating groups in the 4 (−OMe) and 3,5 (−Me) positions of the pyridine rings in TPMA increases the reducing ability of the corresponding copper(I) complexes. This increase was mostly the result of a stronger influence of substituted TPMA ligands toward stabilization of the copper(II) oxidation state (log βI = 13.4 ± 0.2, log βII = 19.3 (TPMA*1), 20.5 (TPMA*2), and 21.5 (TPMA*3)). Lastly, ARGET ATRP kinetic studies show that with more reducing catalysts an induction period is observed. This was attributed to slow regeneration of CuI species from the corresponding CuII.