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

Abstract

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, Cu(I)(TPMA*(1))Br, Cu(I)(TPMA*(2))Br, and Cu(I)(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 Cu(I)(TPMA*(2))Br and Cu(I)(TPMA*(3))Br complexes, whereas pyridine arm dissociation occurred in Cu(I)(TPMA*(1))Br. All copper(I) complexes with substituted TPMA ligands exhibited a high degree of fluxionality in solution. At low temperature, Cu(I)(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 Cu(I)(TPMA*(2))Br and Cu(I)(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 ([Cu(II)(TPMA*(1))Br][Br] (τ = 0.92) > [Cu(II)(TPMA*(3))Br][Br] (τ = 0.77) > [Cu(II)(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 [Cu(II)(TPMA)Br][Br] to [Cu(II)(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 β(I) = 13.4 ± 0.2, log β(II) = 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 Cu(I) species from the corresponding Cu(II).

Tris(2-pyridylmethyl)amine Based Ligands in Copper Catalyzed Atom Transfer Radical Addition (ATRA) and Polymerization (ATRP)

“…Very recently, crystal structures of six novel copper(I and II) complexes with TPMA *1 , TPMA *2 and TPMA *3 ligands were reported (98). Their structural features are briefly discussed in the following sections.…”

“…The motivation towards development of synthetic methodologies for modification was largely driven by research in the area of copper catalyzed oxygen activation (12, 18-21, 25, 97). Recently, copper complexes with TPMA based ligands containing 4-methoxy-3,5-dimethyl substituted pyridine arms (TPMA *1-*3 ) were also successfully utilized in ATRA and ATRP reactions (Scheme 4) (91,98).…”

Towards the development of highly active copper catalysts for atom transfer radical addition (ATRA) and polymerization (ATRP)‡

“…Generally, the equilibrium constant for atom transfer (K ATRP = k a /k d ) for a given alkyl halide can be directly correlated with E 1/2 Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 10/12/15 3:34 AM (Kaur et al, 2015b). Hydrogen atoms and bromide counterions have been omitted for clarity; structures are drawn from cif files using the Mercury 3.5.1 software.…”

“…(1). All values are reproduced from literature (Eckenhoff et al, 2008;Kaur et al, 2015b;Qiu et al, 2000).…”

“…This ratio may increase with more active catalysts, but it is not directly known whether this is the result of an increase in β II or a decrease in β I . However, another quantity, namely the β II /(β I ) 2 ratio, can be readily obtained from disproportionation studies as previously reported in literature (Bortolamei et al, 2010;Kaur et al, 2015b;Pintauer et al, 2003;Schröder et al, 2012b). The following section provides mathematical derivations used to calculate individual stability constants (β I and β II ) from experimental measurements.…”