A Kinetico-Mechanistic Study on CuII Deactivators ... - ACS Publications

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A Kinetico-Mechanistic Study on CuII Deactivators Employed in Atom Transfer Radical Polymerization Timothy J. Zerk,† Manuel Martinez,*,‡ and Paul V. Bernhardt*,† †

School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane 4072, Australia Departament de Química Inorgànica i Orgànica, Secció de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1-11, E-08028 Barcelona, Spain

‡

S Supporting Information *

ABSTRACT: Copper complexes of tertiary amine ligands have emerged as the catalysts of choice in the extensively employed atom transfer radical polymerization (ATRP) protocol. The halide ligand substitution reactions of fivecoordinate copper(II) complexes of tris[2-(dimethylamino)ethyl]amine (Me6tren), one of the most active ATRP catalysts, has been studied in a range of organic solvents using stopped-flow techniques. The kinetic and activation parameters indicate that substitution reactions on [CuII(Me6tren)X]+ (X− = Cl− and Br−) and [CuII(Me6tren)(Solv)]2+ (Solv = MeCN, DMF, DMSO, MeOH, EtOH) are dissociatively activated; this behavior is independent of the solvent used. Adjusting the effective concentration of the solvent by addition of an olefinic monomer to the solution does not affect the kinetics of the halide binding (kon) but can alter the outer-sphere association equilibrium constant (KOS) between reactants prior to the formal ligand substitution. Halide (X−/Y−) exchange reactions (X = Br and Y = Cl) involving the complex [Cu(Me6tren)X]+ and Y− reveal that the substitution is thermodynamically favored. The influence of solvent on the substitution reactions of [Cu(Me6tren)X]+ is complex; the more polar DMF confers a greater entropic driving force but larger enthalpic demands than MeCN. These substitution reactions are compared with those for copper(II) complexes bearing the tris[2-(diethylamino)ethyl]amine (Et6tren) and tris[2-(pyridyl)methyl]amine (tpa) ligands, which have also been used as catalysts for ATRP. Changing the ligand has a significant impact on the kinetics of X−/Y− exchange. These correlations are discussed in relation to the ability of five-coordinate [CuLX]+ complexes to deactivate radicals in ATRP.

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INTRODUCTION The advent of atom transfer radical polymerization1,2 (ATRP) as a technique for conducting controlled reversible deactivation polymerizations3 has rejuvenated interest in the study of transition-metal complexes bearing relatively simple polyamine ligands. ATRP relies on such complexes to selectively modulate the concentration of propagating radicals (R•), in what would otherwise be a free radical polymerization, by the reversible transfer of a halogen atom (X) from the dormant initiator R−X to the catalyst concomitant with electron transfer (Figure 1).4 Ideally, the activation (kact ∼ 102−10−4 M−1 s−1) and deactivation (kdeact ∼ 107−104 M−1 s−1) reactions are fast, with the condition kdeact ≫ kact, so that the ratio kact/kdeact = KATRP typically falls in the range 10−4−10−9.5 These small values of the equilibrium constant KATRP ensure that the concentration of radicals is kept low throughout, so that

undesirable bimolecular radical−radical reactions such as termination (kt ∼ 107 M−1 s−1) are minimized and reaction of the radical with monomer (propagation, kp) is the dominant process. In terms of catalyst, a range of metal/ligand combinations is available which can be tailored according to the properties the monomer to be polymerized. The majority of work performed to date has utilized copper complexes bearing bi-, tri-, or tetradentate polyamine ligands as catalysts. One of the most effective and versatile ATRP catalysts is the CuI complex [Cu(Me6tren)]+ (Me6tren = tris(2-dimethylaminoethyl)amine, Figure 2),6,7 for which kinetic studies with various initiators have revealed kact values in the range 102−104 M−1 s−1.8−11 Much attention has been directed toward understanding the sensitivity of the activation reaction to changes in the components of the system, such as catalyst, initiator, and solvent.8−19 The effect of solvent is particularly pronounced; one study found that the rate of activation can be tuned by up to 3 orders of magnitude simply by altering the polarity of the solvent.16 In ATRP, deactivation is an equally important step that is influenced by (1) the properties of the solvent in which the

Figure 1. Key reactions in ATRP.

Received: July 15, 2016

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.6b01700 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Deactivation of an organic radical by [CuIILX]+, where X = Cl or Br and L = tris(2-dimethaminoethyl)amine (Me6tren), tris(2diethylaminoethyl)amine (Et6tren), or tris(2-pyridylmethyl)amine (tpa). CuII Me6tren Complexes. [Cu(Me6tren)(OH2)](ClO4)234 and [Cu(Me6tren)Br]Br35 were both isolated as crystalline solids that were pure, as determined by elemental microanalysis. [Cu(Me6tren)(Solv)]2+ (Solv = DMSO, MeCN, DMF, MeOH, or EtOH) complexes were generated in situ by dissolution of [Cu(Me6tren)(OH2)](ClO4)2 in neat solvent. [Cu(Me6tren)Cl]+ was also generated in situ by addition of 2 equiv of Bu4N·Cl to [Cu(Me6tren)(OH2)](ClO4)2 in the relevant solvent. In all cases, no UV−vis spectral changes occur after dissolution or halide addition, indicating that displacement of the aqua ligand with the solvent or halide occurs on the mixing time scale. This was likewise true for the Et6tren and tpa complexes below. CuII Et6tren Complexes. [Cu(Et6tren)(Solv)]2+ complexes were formed in situ by addition of 1.02 equiv of Et6tren to Cu(ClO4)2· 6H2O in the relevant solvent. Bromido and chlorido complexes were generated by the addition of 2.0 equiv of Bu4P·Br or Bu4N·Cl, respectively, to [Cu(Et6tren)(Solv)]2+. The UV−vis spectra observed for these complexes were very similar to those of the equivalent Me6tren complexes. CuII tpa Complexes. The complex [Cu(tpa)(OH2)](ClO4)2 was prepared as previously described.36,37 The crystalline solid was pure, as determined by elemental analysis, and redox and spectral behavior were consistent with previous reports. [Cu(tpa)Cl]+ was formed in situ by the addition of 2.0 equiv of Bu4N·Cl to [Cu(tpa)(OH2)](ClO4)2 in the relevant nonaqueous solvent. Kinetics. Standard kinetic measurements within the range 15−35 °C were performed using a stopped-flow mixing unit from Applied Photophysics. For experiments run at variable pressure, a previously described pressurized stopped-flow mixing unit setup was used.38,39 All setups were connected with fiber optics to a J&M TIDAS instrument, as described,39 allowing for the measurement of time-resolved spectra. Substitution experiments were carried out under pseudo-first-order kinetics ([X−]/[CuII] ≥ 10) in all solvents. The concentration of the copper(II) complex was kept constant at 2.0 × 10−4 M. In all cases, the full spectrum (250−850 nm) was collected and analysis was carried out using the programs SPECFIT40 or ReactLab KINETICS.41 The observed rate constants were derived from global analysis of the timedependent spectral data. The greatest changes were seen in the range 250−450 nm. The only case where the weaker d−d electronic transitions (ca. 600−850 nm) were used to analyze the data were for solvent−solvent substitution reactions. In these reactions, spectral changes were small and high copper complex concentrations were needed (3.8 × 10−3 M) to enable a significant change in visible absorption to be measured. In all cases the time-resolved spectral changes agree with the observation of a single first-order process; no secondary or parallel processes were detected. Typically, errors in the observed first-order rate constant (kobs) were less than 10%. The kobs values were plotted as a function of the concentration of the reagent in excess and modeled with a rate law appropriate to that mechanism; i.e.,

olefinic monomer is dissolved,5,8,20−23 (2) the identity of the halogen atom exchanged between the dormant initiator and the deactivator complex,8,24 and (3) the chelating ligand “L”. For example, when the ligand is changed from Me6tren to the related tris(2-diethylaminoethyl)amine (Et6tren), the deactivation rate is faster, but the overall polymerization reaction is less efficient due to a rise in undesirable radical−radical termination.25,26 When the terminal tertiary amines are replaced with pyridyl groups, as in tris(2-pyridylmethyl)amine (tpa), the deactivating properties are very different25−27 and can be further adjusted by incorporation of electron-donating groups on the pyridine rings.17,28 The CuII complexes of these ligands are each trigonal bipyramidal, as shown in Figure 2.29−31 Therefore, differences in deactivation (for a given halide) are related to the unique steric and electronic influences of the ligand on the CuII−X bond. In fact, each of the aforementioned variables is likely to affect this bond and ATRP reactivity. Herein we present a kinetico-mechanistic study using stopped-flow techniques to probe ligand substitution reactions at the CuIIL−X bond and correlate kinetic and/or mechanistic differences with the solvent, monomer, halide, and chelating ligand, our goal being to understand the fundamental origins of the observed differences in the deactivation capabilities of various [CuIILX]+ complexes in different ATRP systems. Such understanding is necessary for the progression of rational ATRP catalyst design and selective implementation.

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EXPERIMENTAL SECTION

Synthesis. All general chemicals used were commercially available. Solvents, of at least HPLC grade, were used without further purification. Bu4P·Br and Bu4N·Cl were gently melted under high vacuum over the course of 4 h; cooling under vacuum afforded the dry salts. Safety Note. Caution! Perchlorate salts are potentially explosive. Although we have experienced no problems with the perchlorate salts used herein, they should never be heated in the solid state or scraped from sintered glass frits. Free Ligands. 2,2′,2″-Tris(dimethylamino)triethylamine (Me6tren) was prepared as previously described.32 2,2′,2″-Tris(diethylamino)triethylamine (Et6tren) was prepared as previously described from tris(aminoethyl)amine (tren); 1H and 13C NMR data were consistent with previous reports.33 Tris[(2-pyridyl)methyl]amine (tpa) was purchased from Sigma-Aldrich and used without further purification. B


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Inorganic Chemistry eq 1 from Scheme 1, which fits the data in Figure 4. KOS and kon are calculated from the fit.

and the reverse solvolysis reaction (koff) is negligible.42−46 From the temperature dependence of kon, the thermal activation parameters have also been determined for all systems using the Eyring equation. From the dependence of ln kon on pressure, the relevant volumes of activation have also been determined whenever possible.47 A summary of the kinetic and activation parameters determined for these reactions appears in Table 1. The entropies and volumes of activation are typically positive, while the activation enthalpies are relatively large. The data are thus consistent with a dissociative activation character for the anation reaction mechanism. The effect of styrene and methyl methacrylate (MMA) on the anation behavior was also examined given its possible influence under real polymerization conditions; both monomers have been successfully polymerized with [Cu(Me6tren)]+.12,48 The copper(II) species is not soluble in neat MMA or styrene, consequently the experiments were conducted by increasing the percent volume of these monomers in a mixture with DMSO. The data indicate that neither the identity nor the concentration of the monomer has a measurable effect on the rate of substitution (kon), but the value of KOS (Scheme 1) is dependent on the mixture and increases significantly with monomer concentration (Table 1, Figure 5). Such behavior is typical for kinetics with significant outer-sphere complex formation in which the polarity of the medium is tuned. The monomer has no direct effect on the copper(II) complex, but increasing monomer concentration lowers the dielectric constant of the medium and facilitates greater association between the 2+ cationic complex and the incoming anion. Halido Ligand Interchange. The halide (X− → Y−) ligand interchange reactions on [CuLX]+ were also studied using the same methodology. For the bromido → chlorido and chlorido → bromido substitution reactions, the values of kobs determined at different concentrations of entering ligand, both in MeCN and DMF, follow a linear trend with a nonzero intercept (Figures 6 and 7). This is distinct from the solvent → halide anation reactions (see previous section). This kinetic behavior is consistent across the series of CuII complexes of Me6tren, Et6tren, and tpa (Figures 6 and 7). The lack of significant curvature of the plots in Figures 6 and 7 (in contrast to Figure 4) indicates that there is no observable buildup of an outersphere complex (KOS is small) in the general reaction sequence of Scheme 1 and that the reaction is reversible. This decrease in

Scheme 1. Mechanism and Observed Rate Law for Anation of [Cu(Me6tren)(Solv)]2+

The desired halide solutions were prepared with Bu4P·Br or Et4N· Cl; the ionic strength was kept constant at 0.1 M for all experiments using LiClO4. Table S1 of the Supporting Information collects all the values of kobs as a function of the different concentration variables used in this study.

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RESULTS As indicated in the Experimental Section, all the substitution processes studied in this work occur during the time scale of manual mixing, so a stopped-flow instrument was necessary to obtain meaningful time-resolved spectral data. Figure 3 shows the typical spectral changes obtained for the solvato complex anation reaction (Figure 3A) and for the solvent interchange reactions (Figure 3B), which are complete in less than 0.5 s. The absorption maximum in the range 300−325 nm is a ligand to metal charge transfer transition, which is very sensitive to ligand substitution of solvent by incoming halide anion. The spectral changes for the solvent interchange reaction are much smaller because the reaction does not go to completion; i.e., the final spectrum in Figure 3B is a mixture of those of [Cu(Me6tren)(NCMe)]2+ and [Cu(Me6tren)(DMF)]2+. This is explained in greater detail when the solvent interchange kinetic results are presented later. Anation Kinetics. For the anation reaction of [Cu(Me6tren)(Solv)]2+ with chloride or bromide (X−) in different solvents, a typical [X−]-limiting substitution behavior (as shown in Figure 4A) is obtained for the values of kobs. This behavior agrees with the rate law and mechanism indicated in Scheme 1 and eq 1, where an outer-sphere complex precursor accumulates prior to the rate-limiting halide coordination (kon)

Figure 3. (Left, A) Time-resolved spectral changes for the reaction of [Cu(Me6tren)(NCMe)]2+ with Br− (0.0025 M in MeCN) at 25 °C and I = 0.1 M (LiClO4). (Right, B) Time-resolved spectral changes for the reaction of [Cu(Me6tren)(NCMe)]2+ with DMF (1.29 M in MeCN) at 25 °C and I = 0.1 M (LiClO4). Insets show the absorbance changes at the wavelengths indicated by the arrows. C


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Inorganic Chemistry

Figure 4. (Left, A) Plot of kobs versus [Br−] for the [Cu(Me6tren)(NCMe)]2+ + Br− reaction (in MeCN) at different temperatures and at I = 0.1 M (LiClO4). (Right, B) Corresponding temperature and pressure dependence plots of the limiting kon value.

Table 1. Kinetic and Activation Parameters for the Anation Reaction [Cu(Me6tren)(Solv)]2+ + X− (X = Br or Cl) (I = 0.1 M LiClO4, T = 298 K) in Different Solvents Solv MeCN DMF MeOH EtOH 100% DMSO 25% MMAc 50% MMAc 25% styrenec 50% styrenec

X− −

Br Cl− Br− Br− Cl− Br− Cl− Br− Br− Br− Br− Br−

298

KOS (M‑1)

1.3 1.4 2.7 75 61 64 69 1.1 3.3 5.3 2.0 3.5

× 10 × 102 × 102 b 2

× × × × ×

102 102 102 102 102

298

kon (s‑1)

53 54 1.7 36 48 47 66 1.1 1.1 1.1 1.0 1.1

ΔH⧧ (kJ mol‑1)

66 ± 1 7.0 ± 3 67 ± 5 11 ± 16 − − 69 ± 1 16 ± 4 72 ± 2 28 ± 7 72 ± 3 25 ± 9 72 ± 4 31 ± 13 too fast at higher temperatures and solutions solidify upon cooling

× 102 b

× × × × ×

ΔS⧧ (J K‑1mol‑1)

102 102 102 102 102

ΔV⧧ (cm3 mol‑1) 6 ± 1(289 K)a − − − − − − − − − − −

Determined at [Br−] = 0.045 M, where kobs ≈ kon; see Figure 4a. bMeasured at 288 K. cSolutions containing an olefinic monomer (styrene or methyl methacrylate) are quoted as a percentage volume in DMSO.

a

tpa ligand. Where measurement of these thermodynamic parameters is possible, the values indicate a general dissociative activation for the formal halide interchange reaction with large values of ΔH⧧ and generally positive values of ΔS⧧. Furthermore, the ΔS⧧ values are solvent-dependent, indicating an involvement of the solvent in the reaction (vide infra). Solvent Ligand Interchange. Substitution of MeCN by DMF or MeCN by DMSO could not be studied by directly mixing an MeCN solution of [Cu(Me6tren)(NCMe)]2+ with neat DMF or DMSO, as the reaction was too fast. Careful concentration screening of DMF/MeCN or DMSO/MeCN solvent mixtures reacting with [Cu(Me6tren)(NCMe)]2+ in MeCN solution led to measurable time-resolved spectral changes (see Figure 3B). For the MeCN → DMF interchange, a final DMF concentration of 1.29 M in MeCN was necessary for the reaction to be observable. Higher DMF concentrations led to reactions that were too fast, while lower concentrations resulted in spectral changes that were too small and unreliable due to the equilibrium favoring the reactant [Cu(Me6tren)(NCMe)]2+. Similarly, for the MeCN → DMSO interchange, only a DMSO concentration of 7.05 M produced reliable results for the process. In all cases, the reverse solvent exchange reactions were too fast to measure under the conditions of the study. The kinetic data collected in Table 2 correspond to values acquired under these specific conditions, indicating a relatively slow first-order reaction. Furthermore, spectral changes were very small, which necessitated higher concentrations of copper complex (3.75 × 10−3 M) and monitoring

Figure 5. Plot of kobs versus [Br−] for the [Cu(Me6tren)(DMSO)]2+ + Br− reaction in DMSO/MMA. Black squares, 50% (vol) MMA; red circles, 25% (vol) MMA; blue triangles, 0% MMA. Fitted functions (according to Scheme 1/eq 1) are extrapolated to high [Br−], where kobs → kon.

KOS is consistent with the lower reactant charges involved, i.e., 1± for halide exchange versus 2± for halide anation. Table 2 collects the corresponding kinetic and thermal activation parameters determined for the halide exchange reactions whenever possible. The rapid kinetics of some of these reactions, even at low temperature, prohibits the determination of thermal activation parameters, especially in the case of the D


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Inorganic Chemistry

Figure 6. Concentration and temperature dependence of kobs for the halido ligand exchange reaction ([Cu(Me6tren)X]+ with Y−) in MeCN, I = 0.1 M (LiClO4): (left) X = Br, Y = Cl and (right) X = Cl, Y = Br.

Figure 7. Concentration dependence of kobs for the reaction of (left) [Cu(Et6tren)Cl]+ with Br− at different temperatures or (right) [Cu(tpa)Cl]+ with Br− at 288 K in MeCN, I = 0.1 M (LiClO4) in both cases. The reaction of [Cu(tpa)Cl]+ was too fast to measure above 288 K.

Table 2. Summary of the Rate Constants and Activation Parameters for Halide and Solvent Interchange on [CuII(L)X]+ and [CuIIL(Solv)]2+ (X = Br or Cl, Solv = DMF, MeCN, DMSO)

a

solvent

reaction

MeCN DMF MeCN MeCN DMF MeCN MeCN DMF DMSO

+

288 −

[Cu(Me6tren)Br] + Cl [Cu(Me6tren)Br]+ + Cl− [Cu(Et6tren)Br]+ + Cl− [Cu(Me6tren)Cl]+ + Br− [Cu(Me6tren)Cl]+ + Br− [Cu(Et6tren)Cl]+ + Br− [Cu(tpa)Cl]+ + Br− [Cu(Me6tren)(NCMe)]2+ + DMF [Cu(Me6tren)(NCMe)]2+ + DMSO

kX−Y (M−1 s‑1) 97 1.7 × 4.3 × 13 8.7 1.1 × 3.5 × 36a 42b

288

kY−X (M‑1 s‑1) 75 6.0 2.3 1.0 7.5 1.9 3.8

103 105

103 105

× × × × × ×

103 104 102 102 104 105

ΔHon⧧ (kJ mol‑1)

ΔSon⧧ (J K−1 mol‑1 )

58 ± 2 −6 ± 7 65 ± 3 40 ± 8 too fast at higher temperatures 71 ± 1 15 ± 2 92 ± 6 92 ± 20 74 ± 3 73 ± 2 too fast at higher temperatures conditions specified in the text

In s−1, at 288 K. bIn s−1 at 298 K.

time-resolved changes of the d−d electronic transitions to resolve spectral differences.

ground state has a nondegenerate (dz2) ground state, so no Jahn−Teller distortion is operative and no coordinate bonds are weakened.55,56 Crystal structures of complexes bearing these tripodal ligands systematically reveal a five-coordinate, trigonal bipyramidal geometry with the three terminal amine donors of the ligand occupying the equatorial coordination sites and the tertiary amine coordinated via a slightly shorter bond at one of the axial positions (Figure 8). The remaining axial site is occupied by a monodentate ligand,57 e.g., MeCN,58 H2O,35,58,59 HCO2−,60 Br−,34 Cl−,61 or CN−,62 and the bond length at this position is shorter than the trans-Cu−Naxial bond for C-, N- or O-donors such as MeCN, CN−, CF3SO3−, and H2O.35,58 The axial coordinate bond lengthens for the Cl− and Br− ligands due to their increased covalent radii.61 While no crystal structures are currently available for complexes bearing the Et6tren ligand, EPR measurements reveal that trigonal bipyramidal geometry is conserved in solution.30,31 Interestingly, while [CuII(tren)X]n+ complexes react via an Ia mechanism,63,64 increasing the steric

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DISCUSSION Mechanism of Ligand Substitution. Solvent-exchange reactions on CuII complexes lacking any coligands, [Cu(Solv)6]2+, are extremely fast, and DMF,49 H2O,49 and MeOH 50 exchange have been accepted to undergo a dissociative interchange (Id) substitution (presumably) at the axial coordination sites due to the inherent axial elongation of the Cu−Solv bonds as a consequence of the Jahn−Teller effect. 51,52 While some ambiguity surrounds the exact coordination number and geometry of solvated CuII complexes,49,53,54 the addition of a tripodal tetradentate ligand such as tren, or its hexa-methylated analogue Me6tren, simplifies the situation. These ligands favor a trigonal bipyramidal geometry for CuII, with an additional monodentate coligand, which removes the orbital degeneracy of the d9 ground state present in an octahedral ligand field. The trigonal bipyramidal d9 E


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Inorganic Chemistry

pyridine leads to a rather complicated kobs−pyridine concentration dependence and produces a final UV−vis spectrum typical of a tetragonally elongated octahedral CuII complex. In our case, the characteristic visible−NIR spectral feature of a trigonal bipyramidal [CuII(Me6tren)(ligand]n+ complex is always preserved; thus, we have no evidence for partial dissociation of the Me6tren ligand at any stage. Solvent Composition. A recent study showed that the concentration of monomer significantly altered the value of KATRP (kact/kdeact, Figure 1) for polymerizations catalyzed by copper complexes of Me6tren and tpa.27 As the polymerization reaction mixture was altered from pure DMSO to bulk monomer (methyl acrylate) the value of KATRP decreased by 4 orders of magnitude. Our data indicate that the monomer concentration is more likely to affect activation rather than deactivation provided that the absolute concentration of deactivator remains the same as the concentration of monomer is increased (a reasonable assumption for the DMSO/methyl acrylate system). This proposal is consistent with the wellestablished ability of olefin monomers to coordinate to CuIL complexes rather than the CuII complexes.74−78 In this respect, an earlier study also found that the deactivation rate was significantly less affected than the activation rate (in a mixed solvent/monomer system) by changing the identity of the monomer.79 Halide Substitution. Especially relevant for the atom transfer deactivation reaction (Figure 2) are the data collected in Table 2, referring to halide and solvent interchange on the [CuIIL(X or Solv)]n+ complex. Again, positive entropies of activation suggest that substitution of the initially coordinated halide occurs via an Id mechanism. The two-step sequence indicated in Scheme 2 may account for the solvent-dependent

Figure 8. Comparison of the CuII−N coordinate bonds (Å) across the homologous series [Cu(Me6tren)(NCMe)]2+ (BPh4− salt),59 [Cu(Me3tren)(NCMe)]2+ (ClO4− salt),70 and [Cu(tpa)(NCMe)]2+ (ClO4− salt).71 Structures obtained from the Cambridge Structural Database and rendered with Mercury (version 3.5.1). The N-donors are shown in blue.

bulk by methylation of the three terminal amines (Me6tren) produces a definite shift toward an Id mechanism, as shown for DMF and diethylformamide interchange65,66 or azide− and thiocyanate−water substitution.67 The intermediate bulk of the Me3tren complexes (with three terminal secondary amines, Figure 8) appears to still generate substitutions via an associatively activated Ia mechanism,68 thus illustrating a subtle mechanistic tuning as a function of steric hindrance in the vicinity of the exchanging coordination site. Likewise, the less hindered [Cu(tpa)(Solv)]2+ complex also undergoes much faster solvent exchange than [Cu(Me6tren)(Solv)]2+ and via an Ia activated mechanism.69 Nevertheless, these results relate to aqueous solution processes, which might be relevant for the differences observed. There are many published crystal structures of both tpa and Me6tren as their CuII complexes. Complexes of Me3tren are less common, but all three have been structurally characterized with MeCN in the axial coordination site. The comparison in Figure 8 shows that increasing steric bulk on the terminal N-donors lengthens the equatorial Cu−N bonds but has no effect on the axial coordinate bond lengths. Thus, the observed dissociative character of [Cu(Me6tren)(Solv)]2+ substitution reactions is not due to axial ligand destabilization but rather steric encumbrance, which hinders any incoming ligand from close approach to the five-coordinate CuII ion. With this background in mind, as well as the data collected in this work, it is evident that the anation reactions of [Cu(Me6tren)(Solv)]2+ studied show the expected dissociatively activated mechanism with large enthalpies of activation ΔH⧧, positive volumes of activation ΔV⧧, and small but positive activation entropies ΔS⧧. The two latter parameters have to be considered along with the fact that the entering halide anions are already poorly solvated in the outer-sphere encounter complex due to charge compensation (Scheme 1). From the data in Table 1 it is also clear that the fastest reactions occur with [CuII(Me6tren)(DMF)]2+, which bears the most sterically bulky solvent ligand of the series, which is again consistent with dissociative activation. Similarly no significant differences are obtained for the anation rate constants kon with chloride or bromide, despite the stronger CuII−Cl bond being formed.61 An alternative substitution mechanism has been reported that involves partial dissociation of the Me6tren ligand from [CuI(Me6tren)]+ 61,72,73 and [CuII(Me6tren)(OH2)]2+ 68 when very high concentrations of incoming ligands are present, but this can be discarded here. Under the conditions of these reported studies, the reaction of [Cu(Me6tren)(OH2)]2+ with

Scheme 2. Possible Solvent-Involved Mechanism for X-to-Y Interchange on the [CuII(Me6tren)(ligand)]n+ Moiety

kinetics of this dissociatively activated process.45,80 The mechanism indicated, with [Cu(Me6tren)(Solv)]2+ under steady state (s.s.) or pre-equilibrium (p.eq.) conditions,42,47 produces the alternative approximate rate laws shown (eqs 2a and 2b, respectively, of Scheme 2) if outer-sphere complexes do not build up during the anation reactions, as indicated in the Results section (i.e., k−1 = k′−1KOS(X) and k2 = k′2KOS(Y), where k′−1 and k′2 are equivalent to kon in Scheme 1). Obviously, from the experimentally observed linear increase in kobs as a function of entering halide concentration (Figures 6 and 7), the steady-state approach (eq 2a, Scheme 2) does not apply, as kobs should tend to k1 at high concentrations of the entering halide Y−. Furthermore, the pre-equilibrium approximation (valid under the assumption that k1 and k−2 are very F


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Inorganic Chemistry

considerations, the solvent dependence of the kinetic and activation parameters obtained for the Cl → Br and Br → Cl interchange should be related to a more effective desolvation of the ions (complex and halide) in the more polar DMF on approach to the transition state. It is important here to clarify that the kinetics of deactivation in ATRP is reported to be faster in more polar solvents,8 while the ef f iciency of deactivation is decreased. The latter observation has logically been attributed to substitution of the halido ligand by the polar solvent to yield a copper(II) complex that cannot deactivate the radical by halogen atom transfer (i.e., [CuIIL(Solv)]2+).23 We propose then that the differences in the kinetics are due to desolvation differences on approach to the transition state during deactivation. Ligand Dependence of Exchange. The rate of the reaction [CuIILCl]+ + Br− when L is Et6tren is ∼3 orders of magnitude faster than the corresponding rate when L is Me6tren. The reverse reaction between [CuII(Et6tren)Br]+ and Cl− was too fast to be measured. Overall these observations are consistent with increased steric crowding around the axial coordination site and the dissociative nature of the exchange. It is rather interesting to note the significant difference in the entropy for activation between the two reactions with Et6tren vs Me6tren. Apparently, the driving force for these faster exchange reactions with Et6tren is not enthalpic in origin but rather related to the entropic rearrangement of the first and second coordination spheres on approach to the transition state. While two new methods have recently been developed for the direct measurement of fast deactivation rate constants,83,84 very few experimentally determined rates exist in the literature; this is especially true for complexes that activate initiators and halide-capped polymers rapidly. However, it is possible to measure kact (or estimate it8,85,86) and, using known values for KATRP for a given solvent, initiator, or temperature, determine kdeact from the simple relation KATRP = kact/kdeact.26 This last reference provides a direct comparison of kdeact for the complexes [CuII(Me6tren)Br]+ and [CuII(Et6tren)Br]+ (kdeact = 1.5 × 106 M−1 s−1 vs 4.7 × 107 M−1 s−1, respectively). These rates are consistent with our observations that dissociatively activated halide exchange is faster with the bulkier Et6tren ligand. Again, it is important to distinguish this observation from the synthetic observation that deactivation is less ef f icient with Et6tren, which relates to the kact/kdeact ratio.25 When the aliphatic amine donors of Me6tren or Et6tren are replaced with aromatic N-donors, as in tpa, the kinetics for halide exchange on [CuIILBr]+ is faster again. Previous work has demonstrated that exchange reactions on [CuII(tpa)(OH2)]2+ most likely proceed via an Ia mechanism.69 This change in the mechanism would account for the unexpected lability of exchange we observe despite the reduction of steric crowding on going from either Me6tren or Et6tren to tpa. It seems that increasing the steric interactions between Br− and L served to increase the rate of exchange once the electronics of the system has established the mechanism of the substitution process. The value of kdeact quoted for [CuII(tpa)Br]+ is 3.3 × 106 M−1 s−1,26 which is intermediate between those for the Me6tren and Et6tren complexes. Our kinetics data would suggest that deactivation should be faster for this ligand, but it must be remembered that the redox potential of the CuII/I complex also influences kdeact.85,87 The redox potential is more positive for tpa complexes of copper(II) than Me6tren, which explains the decreased deactivation rate.

low, as observed in the Experimental Section) is not expected to produce a definitive intercept in kobs versus [Y−] plots at the very low X− concentration present in the reaction medium ([X−] = 4 × 10−4 M = 2 × [CuII]). Furthermore, the value for the K−2 = k−2/k2 equilibrium constant is expected to be low (halide complexes are formed on the stoichiometric addition of the corresponding anion to the solvato species; see Experimental Section and previous work10), which again diminishes any intercept. Thus, despite the large solvent concentration and the dissociatively activated nature of the processes,45,80 the direct X → Y substitution reaction is the only mechanism agreeing with the data (Scheme 3), with kobs = Scheme 3. Halide Exchange Reaction Mechanism and Rate Law for Reaction of Y− with [Cu(Me6tren)X]+

kX−Y[Y−] + kY−X[X−], resulting in a slope kX→Y and intercept kX−Y[X−]. The low donor strength of the solvent, relative to the halide, and an early transition state with little dissociation of the exiting ligand (generally not very positive activation entropies) can explain this observation. It is important to indicate that the values of kY−X must be taken with extreme caution given the large inherent uncertainty involved in the determination (errors as large as 70−80% of the reported value; see Figures 6 and 7). Despite the complexity in interpretation of the mechanism for halide exchange of these complexes, a number of key observations can be made from the data in Table 2. The enthalpies of activation for the halide exchange reactions of [CuII(Me6tren)X]+ suggest that breaking the Cu−X bond is more energetically demanding for [CuIILCl]+ than for [CuIILBr]+. This may explain the experimentally observed sluggishness of atom transfer of chloride to R• in the deactivation reaction (Figure 2) compared with bromide in polymer synthesis.8,81 As a whole, our data indicate that the ATRP-relevant deactivation reaction to produce R−X and [CuIL]+ species will be more favored for X = Br than X = Cl, both from a thermodynamic and a kinetic perspective. In view of the fact that eq 2b (Scheme 2) is not applicable, the observed solvent-dependence of the activation entropies has to be necessarily related to the effect of the solvent in the outersphere precursor complexation occurring in eq 3 (Scheme 3). Solvent Exchange. Despite the obvious correlations between the halide dependence of the Cu−X bond lability and the measured deactivation rates, the role that solvent plays in tuning kdeact represents a crucial point. The solvolytic influence is not solely related to the existence of solventcoordinated species in solution (as in Scheme 2), but it is also associated with stabilization of the transition state along the reaction coordinate. Halide substitution shows a more favorable entropic driving force in more polar solvents (ENT = 0.7775 for DMF, 0.460 for MeCN),82 which is consistent with the implicit inclusion of KOS in the kX−Y term (see above). However, higher enthalpies obtained in these more polar solvents seem to be dominant in determining the observed sluggishness of deactivation in that media. Therefore, given the above G


Inorganic Chemistry

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CONCLUSIONS The substitution reactions on [Cu(Me6tren)X)]+ and [Cu(Me6tren)(Solv))]2+ have been studied across a range of polar organic solvents and halides and were found to undergo substitution via a dissociatively activated process. This behavior is consistent with observations for similar ligand substitutions in aqueous conditions. When the solvent is initially coordinated to the copper center, the size of this outgoing ligand determines the rate of substitution; this rate is not sensitive to the nature of the incoming nucleophile. When halide is originally bound to the copper center, the rate of substitution is faster when the outgoing ligand is the larger, and weaker, Br− nucleophile. The more compact, more effective Cl− nucleophile produces slower reaction rates. The full behavior is consistent with slower rates of deactivation in ATRP when using CuII−Cl deactivators instead of the CuII−Br analogues. Interestingly, although the substitution reaction is dissociatively activated, the rate is sensitive to the nature of the solvent, which is attributed to the influence of solvent on the desolvation process, which occurs during outer-sphere association between the reacting species. In this respect, for the halide interchange reactions studied, where no buildup of such an encounter-complex is detected, the more polar the solvent is, the greater the enthalpic demands but the more favorable the entropic driving force. While solvents that have the capacity to bind copper(II) influence the rates of these atom transfer reactions, introduction of an olefinic monomer, such as styrene or methyl methacrylate, does not change the kinetics of atom transfer. The only effect on increasing the volume percent of monomer is to alter the outer-sphere association equilibrium constant KOS. This is consistent with reports that the monomer does not alter kdeact and the observed changes in KATRP relating to monomer variation have their origins in the term kact, which relates to the reactivity of copper(I). Finally, for a dissociatively activated atom transfer reaction, such as is found for complexes of Me6tren and Et6tren, increasing steric bulk around the site of exchange increases the kinetics of exchange. The increased driving force has its origins in entropic rearrangement in the first and second coordination spheres during the reaction. Again, the increased kinetics for halide exchange with Et6tren can explain the faster deactivation kinetics reported for this ligand over its relative Me6tren. The dependence of the rates of anation and halide exchange are therefore sensitive to the solvent, halide, and ligand L, and these sensitivities logically predict the observed variations of kdeact as each variable is modified. We would thus predict that deactivation kinetics is fastest for copper(II)−bromido complexes bearing ligands that crowd the coordination site of the CuIIL−X bond if the deactivation is dissociatively activated. The effect of the solvent is complex and not predictable. Further work on a range of complexes that activate via an associative mechanism will permit a more complete understanding of the effect of the ligand.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministerio de Economiá y Competitividad (CTQ2015-65707-C2-1/ FEDER to M.M.) and the Australian Research Council (DP150104672 to P.V.B.). T.J.Z. acknowledges receipt of a M.G. & R.A. Plowman Scholarship in Inorganic Chemistry awarded by the School of Chemistry and Molecular Biosciences (University of Queensland). The help from Dr. Carlos ́ Rodriguez in the initial stages of this work is gratefully acknowledged.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01700. Table of observed rate constants as a function of reactant concentrations, temperature, and pressure (PDF) H


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A Kinetico-Mechanistic Study on CuII Deactivators ... - ACS Publications

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