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.