Crystal Structure of Orthorhombic {bis-[(pyridin-2-yl)methyl](3,5,5,5-tetrachloropentyl)amine-κ\u3csup\u3e3\u3c/sup\u3e\u3cem\u3eN,N\u27,N\u27\u27\u3c/em\u3e}chloridocopper(II) Perchlorate

In the title compound, [CuCl(C17H19Cl4N3)]ClO4, the CuII ion adopts a distorted square-planar geometry defined by one chloride ligand and the three nitro­gen atoms from the bis­[(pyridin-2-yl)meth­yl](3,5,5,5-tetra­chloro­pent­yl)amine ligand. The perchlorate counter-ion is disordered over three sets of sites with refined occupancies 0.0634 (17), 0.221 (16) and 0.145 (7). In addition, the hetero-scorpionate arm of the bis­[(pyridin-2-yl)meth­yl](3,5,5,5-tetra­chloro­pent­yl)amine ligand is disordered over two sets of sites with refined occupancies 0.839 (2) and 0.161 (2). In the crystal, weak Cu⋯Cl inter­actions between symmetry-related mol­ecules create a dimerization with a chloride occupying the apical position of the square-pyramidal geometry typical of many copper(II) chloride hetero-scorpionate complexes

Crystal structure of ammonium bis[(pyridin-2-yl)methyl]ammonium dichloride

In the title molecular salt, C12H14N3+·NH4+·2Cl-, the central, secondary-amine, N atom is protonated. The bis[(pyridin-2- yl)methyl]ammonium and ammonium cations both lie across a twofold rotation axis. The dihedral angles between the planes of the pyridine rings is 68.43 (8)°. In the crystal, N-H⋯N and N-H⋯Cl hydrogen bonds link the components of the structure, forming a two-dimensional network parallel to (010). In addition, weak C-H⋯Cl hydrogen bonds exist within the two-dimensional network

Determination of Rate Constants for the Activation Step in Atom Transfer Radical Polymerization Using the Stopped-Flow Technique

International audienceThe synthesis of macromolecules with well-defined compositions, architectures, and functionalities represents an ongoing effort in the field of polymer chemistry. Over the past few years, atom transfer radical polymerization (ATRP) has emerged as a very powerful and robust technique to meet these goals.1-3 The basic working mechanism of ATRP (Scheme 1) involves a reversible switching between two oxidation states of a transition metal complex.4,5 Typically, copper(I) halide is used in conjunction with a nitrogen-based complexing ligand 4,6-9

Kinetic and Mechanistic Aspects of Atom Transfer Radical Addition (ATRA) Catalyzed by Copper Complexes with Tris(2-pyridylmethyl)amine

Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris­(2-pyridylmethyl)­amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [Cu^I(TPMA)]⁺ cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (<i>K</i>_ATRA = <i>k</i>_a/<i>k</i>_d) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu^I(TPMA)­BPh₄ ((1.6 ± 0.2) × 10^–7) than Cu^I(TPMA)Cl ((2.8 ± 0.2) × 10^–8) complex. This difference in reactivity between Cu^I(TPMA)Cl and Cu^I(TPMA)­BPh₄ was reflected in the activation rate constants ((3.4 ± 0.4) × 10^–4 M^–1 s^–1 and (2.2 ± 0.2) × 10^–3 M^–1 s^–1, respectively). The fluxionality of Cu^I(TPMA)­X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (Δ<i><i>H</i></i>^⧧ = 29.7 kJ mol^–1, Δ<i><i>S</i></i>^<i>‡</i> = −60.0 J K^–1 mol^–1, Δ<i><i>G</i></i>^⧧₂₉₈ = 47.6 kJ mol^–1, and <i>k</i>_obs,298 = 2.9 × 10⁴ s^–1). Relatively strong coordination of halide anions in Cu^I(TPMA)­X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu^I(TPMA)­BPh₄ were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu^I(TPMA)­X was further confirmed by synthesizing tris­(2-(dimethylamino)­phenyl)­amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me₆TREN (tris­(2-dimethylaminoethyl)­amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu^I(TDAPA)­Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper­(I) center by replacing chloride anion with less coordinating counterions such as BF₄^– and BPh₄^–. The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu^I(TPMA)­X complexes catalyze the homolytic cleavage of carbon–halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm

Kinetic and Mechanistic Aspects of Atom Transfer Radical Addition (ATRA) Catalyzed by Copper Complexes with Tris(2-pyridylmethyl)amine

Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris­(2-pyridylmethyl)­amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [Cu^I(TPMA)]⁺ cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (<i>K</i>_ATRA = <i>k</i>_a/<i>k</i>_d) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu^I(TPMA)­BPh₄ ((1.6 ± 0.2) × 10^–7) than Cu^I(TPMA)Cl ((2.8 ± 0.2) × 10^–8) complex. This difference in reactivity between Cu^I(TPMA)Cl and Cu^I(TPMA)­BPh₄ was reflected in the activation rate constants ((3.4 ± 0.4) × 10^–4 M^–1 s^–1 and (2.2 ± 0.2) × 10^–3 M^–1 s^–1, respectively). The fluxionality of Cu^I(TPMA)­X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (Δ<i><i>H</i></i>^⧧ = 29.7 kJ mol^–1, Δ<i><i>S</i></i>^<i>‡</i> = −60.0 J K^–1 mol^–1, Δ<i><i>G</i></i>^⧧₂₉₈ = 47.6 kJ mol^–1, and <i>k</i>_obs,298 = 2.9 × 10⁴ s^–1). Relatively strong coordination of halide anions in Cu^I(TPMA)­X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu^I(TPMA)­BPh₄ were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu^I(TPMA)­X was further confirmed by synthesizing tris­(2-(dimethylamino)­phenyl)­amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me₆TREN (tris­(2-dimethylaminoethyl)­amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu^I(TDAPA)­Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper­(I) center by replacing chloride anion with less coordinating counterions such as BF₄^– and BPh₄^–. The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu^I(TPMA)­X complexes catalyze the homolytic cleavage of carbon–halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm

A DFT Study of R-X Bond Dissociation Enthalpies of Relevance to the Initiation Process of Atom Transfer Radical Polymerization

International audienceDFT calculations at the B3P86/6-31G** level have been carried out to derive the bond dissociation energies (BDE) and free energies for a number of R−X systems (X = Cl, Br, I, N3, and S2CNMe2) that have been or can potentially be used as initiators for atom transfer radical polymerization (ATRP). For selected systems, a conformational search was carried out for R−X and R by using semiempirical (PM3) and molecular mechanics (MM+ augmented with appropriately optimized parameters for the radical systems) methods. The MM+ technique is more suited to search for the most stable conformations. The computed energies are in good agreement with the experimentally available BDEs and reveal a small weakening effect caused by the substitution of an α-H atom with a CH3 group. The free energies are used to derive the relative equilibrium constant for the ATRP activation/deactivation process. These are compared with the equilibrium constants that have been determined from ATRP polymerization rates and from model studies of activation−deactivation−termination processes in the absence of monomer. These comparisons reveal the effectiveness of the DFT-computed BDEs for predicting polymerization rates for new monomers in ATRP processes

Reversible-Deactivation Radical Polymerization of Methyl Methacrylate and Styrene Mediated by Alkyl Dithiocarbamates and Copper Acetylacetonates

International audienceReversible-deactivation radical polymerization (RDRP) of methyl methacrylate (MMA) and styrene (St) was successfully mediated by copper(II) acetylacetonate, [Cu(acac)2], or copper(II) hexafluoroacetylacetonate, [Cu(hfa)2], in conjunction with 1-cyano-1-methylethyl diethyldithiocarbamate (MAN-DC) or 2-(N,N-diethyldithiocarbamyl)ethyl isobutyrate (EMA-DC) initiators/transfer agents in the absence of additional reducing agents. Linear first-order kinetic plots were obtained for the polymerization of MMA in the presence of [Cu(hfa)2] or [Cu(acac)2] and MAN-DC. [Cu(hfa)2] provided better control than [Cu(acac)2] for the polymerization of MMA, leading to PMMA with narrow molecular weight distribution, Mw/Mn ∼ 1.1. Polymerization of St was successfully carried out with either MAN-DC or EMA-DC in the presence of [Cu(hfa)2], also resulting in polymers with low Mw/Mn values. In the absence of alkyl dithiocarbamates or copper acetylacetonates, the polymerizations resulted in only trace amounts of polymers or polymers with high values of Mw/Mn. Thus, the combination of alkyl dithiocarbamates and copper(II) acetylacetonates provides a convenient way to prepare well-controlled PMMA and PSt. NMR analysis of low-MW polymers reveals the presence of DC groups as chain ends. DFT calculations show that DC group transfer from a H-MMA-DC model of the growing chain to the Cu(II) complexes is energetically accessible and more favorable than Br atom transfer, thus rationalizing the need for the Cu(II)/dithiocarbamate combination for successful control and suggesting that the process takes place by reversible DC group transfer involving a CuII/CuIII couple. Attempts to synthesize complexes [Cu(acac)2(DC)] and [Cu(hfa)2(DC)], in combination with DFT calculations, suggest that these complexes are thermodynamically unstable relative to the bis(diketonate)copper(II) and dithiuram disulfide, but this does not preclude the involvement of the Cu(III) species as a spin trap in RDRP controlled by DC group transfe