New Guanidine-Pyridine Copper Complexes and Their Application in ATRP

The guanidine hybrid ligands, (tetramethylguanidine)methylenepyridine (TMGpy) and (dimethylethyleneguanidine)methylenepyridine (DMEGpy), were proven to be able to stabilize copper complexes active in the solvent-free polymerization of styrene at 110 °C using 1-phenylethylbromide as the initiator. The polymerization proceeded after first-order kinetics, and polystyrenes with polydispersities around 1.2 could be obtained. Using the ligand, DMEGpy, three new copper guanidine-pyridine complexes could be synthesized and structurally characterized. Their structural characteristics are discussed.


Introduction
Atom transfer radical polymerization (ATRP) is one of the most important and most efficient controlled radical polymerization methods, which combines the advantages of radical polymerization (high tolerance towards functional groups and impurities, many possible monomers and mild conditions) with the controlled character of a living polymerization.The living character of the controlled-radical polymerization methods can be obtained through suppression of termination and side reactions.This is achieved by a fast dynamic equilibrium between a very small number of OPEN ACCESS growing free radicals (active species) and a large number of non-reactive so-called dormant species [1].Since the development of ATRP by K.Matyjaszewski in 1995, this field has run through a rapid progress in catalyst development, but also in the application to modern polymer technology [2,3].In ATRP, the dormant species is an alkyl halide, which gives the active species after activation by atom transfer to a transition metal complex.Numerous transition metal systems on the basis of Cu, Fe, Ru and other transition metals of Groups 6 to 11 can be used, but Cu complexes dominate the field, due to the fast and clean polymerization.Mostly, polyfunctionalized N donor ligands are used for the stabilization of suited activator complexes [4].Besides the classical ATRP, which starts with Cu(I), new ATRP methods have evolved that start with Cu(II) (e.g., Activators ReGenerated by Electron Transfer (ARGET)ATRP, Initiators for Continuous Activator Regeneration (ICAR)-ATRP [5,6] electrochemically mediated (e)ATRP [7,8]).Taking into account the idea of sustainability, intensive efforts have been undertaken to minimize the copper catalyst content.Here, there are still fundamental principles of the polymerization mechanism under discussion [9].New ligands can significantly contribute to fundamental mechanistical understanding.Tailored ligand design enables the ideal adjustment of ligand properties to requests.By the choice of donor function and bridging units, the denticity and ligand geometry can be adapted, which steers the metal coordination and the redox potential.As donor functions, mainly amines, imines and pyridines have been tested [10].
Guanidines represent a further class of N donor ligands with a highly basic and nucleophilic imine function.The modular synthetic protocol allows for the combination of different spacers, amine groups and guanidine groups for building up a ligand library [11].The donor properties can be tuned through the choice of guanidine substituents, amine and spacers.These ligands have already been intensely investigated in bioinorganic coordination chemistry [12][13][14][15][16][17][18], but also in the ATRP of styrene [19][20][21][22][23][24].In all of these studies, it appeared that the polyfunctional guanidines support the oxidation state change from Cu(I) over Cu(II) to Cu(III) and stabilize the corresponding complexes excellently.These properties make guanidines ideal ligands for catalysis.Hybrid guanidines combine one guanidine function with one different donor function, e.g., pyridine or quinoline [13,17,18].Here, we present three new copper guanidine-pyridine complexes and the first styrene ATRP studies with the hybrid guanidine ligands, (tetramethylguanidine)methylenepyridine (TMGpy) and (dimethylethyleneguanidine)methylenepyridine (DMEGpy).

Experimental Section
General: Ligand syntheses were performed under argon by using standard Schlenk techniques; complexes were prepared in a glove box under nitrogen atmosphere.Solvents were purified according to literature procedures and kept under nitrogen [25].All chemicals were used as purchased, besides styrene, which was destabilized by eluting through a column of neutral Al 2 O 3 .The Vilsmeier salts, N,N′-dimethylethylenechloroformamidinium chloride (DMEG) and N,N,N′,N′-tetramethylchloroformamidinium chloride (TMG), were synthesized as described in the literature [11,26].The ligands DMEGpy and TMGpy were synthesized according to the protocol in the literature [27].
Crystal Structure Analyses: The crystal data for Compounds 1-3 are presented in Table 1.Data for these complexes were collected with an Xcalibur S diffractometer from Oxford Diffraction using Mo-K α radiation (λ = 0.71073 Å) and a graphite monochromator with the programs, CRYSALIS (Oxford Diffraction Ltd., Oxford, UK, 2008) and CRYSALIS RED (Oxford Diffraction Ltd., Oxford, UK, 2008).The structures were solved by direct methods (SHELXS90) [28] and conventional Fourier methods, and all non-hydrogen atoms refined anisotropically with full-matrix least-squares procedures based on F 2 (SHELXL97) [29].Hydrogen atoms were derived from difference Fourier maps and placed at idealized positions, riding on their parent carbon atoms, with isotropic displacement parameters U iso (H) = 1.2U eq (C) and U iso (H) = 1.5U eq (C methyl).All methyl groups were allowed to rotate, but not to tip.CCDC-987045 (for 1), CCDC-987046 (for 2) and CCDC-987047 (for 3) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (CCDC) [30].
Gel Permeation Chromatography: The average molecular weights and the weight distributions of the obtained polystyrene samples were determined by gel permeation chromatography (GPC) in THF as the mobile phase at a flow rate of 1 mL/min.The utilized GPCmax VE-2001 from Viscotek (Herrenberg, Germany) is a combination of an HPLC pump, an SDV column (PSS) with a porosity of 500 Å and a refractive index detector (VE-3580, Malvern, Herrenberg, Germany).The instrument was calibrated with standard polystyrene samples.Sample concentrations were 3 mg•mL −1 .

Results and Discussion
Ahead of polymerization experiments, we conducted complex synthesis and characterization experiments in order to structurally characterize our potential ATRP catalysts with the guanidine-pyridine ligands, TMGpy and DMEGpy, thoroughly.Hence, in Section 3.1, we firstly describe the complex syntheses of bis(chelate) and mono(chelate) copper guanidine-pyridine complexes together with their single crystal structure analyses and structural comparison to related copper complexes from the literature.With the ligand, TMGpy, we were not successful in preparation of single crystals.In Section 3.2, we describe then the ATRP experiments performed with the copper bromide catalyst species.(2) (Figure 1). 1 crystallizes in the triclinic space group P , and 2 in the monoclinic space group C2/c.In both complexes, the unit cell contains both isomers of the chiral cations.Selected geometrical data of these complexes are listed in Table 2.The pyridine donors reside in the axial positions of the trigonal-bipyramidal coordination polyhedra, whereas the guanidine donors and the halide form the equatorial plane.The structural parameter, τ 5 , indicates the characteristic of such a polyhedron in distortion toward the square-pyramid (one being indicative of the trigonal-bipyramidal and zero for square-pyramid) [31].The τ 5 values of 0.77 for 1 and 0.68 for 2 show that a distortion of the ideal trigonal-bipyramidal coordination occurs, which becomes clear in the increase in the equatorial angle, N gua -Cu-N gua ′, with 132.5(1) for 1 and 136.9(2) for 2. The N py -Cu-N py ′ angles do not deviate considerably from the ideal angle of 180° (178.6(1) for 1 and 177.6(2) for 2. The Cu-N gua bond lengths in 1 and 2 (2.041(2), 2.133(2) Å in 1; 2.029(4), 2.065(3) Å in 2) are longer than the Cu-N py bond lengths (1.988(2), 1.994(2) Å in 1; 1.991(4), 1.993(1) Å in 2).It is remarkable that the bonds to the axial ligands are shorter than those to the equatorial ligands [32,33].The Cu-N py bond lengths of both complexes are equal, whereas the Cu-N gua bond lengths deviate significantly between 1 and 2.

Complex Synthesis
The structural parameter ρ can be used to evaluate in guanidines and their complexes the degree of delocalization of the guanidine moiety.The delocalization is important for effective coordination behavior towards metals in different oxidation states [18].This parameter amounts in both complexes to 0.95, indicating a low charge delocalization within the CN 3 guanidine framework.The intra-guanidine torsion is rather small, as expected for DMEG units, with N amin,gua C 3 ,C gua N 3 plane angles of 14.7(av) (1) and 14.6(av) (2) [12].The reaction of one equivalent of DMEGpy with one equivalent of CuCl 2 gives the mono(chelate) complex, [Cu(DMEGpy)Cl 2 ] (3) (Figure 3).This complex crystallizes in the orthorhombic space group Pna2 1 .Selected geometrical data of this complex are listed in Table 2.The molecular structure of 3 is depicted in Figure 4. 3 is a four-coordinate complex with the coordination by one bidentate ligand and two chloride anions.Here, the τ 4 -value can give a measurement of the degree of distortion between tetrahedral and square-planar coordination (square-planar: zero; tetrahedral: one) [34].With a τ 4 -value of 0.49, the observed coordination geometry of 3 lies in the middle between both polyhedra.This is in accordance with the angle between the CuN 2 -and the CuCl 2 -planes of 48.3(1).The Cu-N gua bond is with 1.956(3) Å considerably shorter than the Cu-N py bond (2.016(3) Å).The ρ-value of 0.95 shows a small degree of charge delocalization within the guanidine unit.The intra-guanidine torsion is small (14.8°(av)), as expected for a DMEG unit [12].

Comparative Structural Discussion
In this section, we compare the presented complexes, 1 and 2, with five-coordinate copper(II) complexes with the symmetric ligand, 2,2′-bipyridine (bpy): [Cu(bpy) 2 Cl]Cl•6H 2 O (4) [35] and [Cu(bpy) 2 Br]Br (5) [36] (Figure 5 and Table 3).Table 3. Key geometric parameters of 4 [35] and 5 [36].The bonds to the axial ligands are shorter than the bonds to the equatorial ligands in the guanidine-pyridine complexes, 1 and 2, as well as in the comparative bipyridine complexes, 4 and 5. Hence, the metal bonding influence is as strong as the donor difference.The angles of the coordination polyhedra of 4 and 5 are very similar to those of 1 and 2. Interestingly, the Cu-halide distances in 1 and 2 are longer than those in 4 and 5, which might be indicative of the larger donor strength of the guanidine functions.

Atom Transfer Radical Polymerization of Styrene
The ligands, TMGpy and DMEGpy, together with CuBr as the copper source, have been investigated towards their activity in styrene ATRP with the initiator 1-phenylethylbromide (PEBr).The ratio of styrene/ligand/CuBr/PEBr was 100/2/1/1.The reaction temperature was 110 °C, and samples were drawn in equidistant time intervals and plotted semilogarithmically (Figure 6).It has to be noted that the polymerization occurs in a homogeneous solution of the in situ formed complexes in the styrene bulk.The styrene-ATRP with the catalysts 2, TMGpy/CuBr and 2 DMEGpy/CuBr, follows a first-order kinetics, which indicates a constant radical concentration and, thus, the living character of the polymerization.After a polymerization time of 35 min, the conversion reaches a value of 57% with 2 TMGpy/CuBr and of 63% with 2 DMEGpy/CuBr.The apparent rate constant (k app ) amounts to 4.20 × 10 −4 s −1 (2 TMGpy/CuBr) and 4.69 × 10 −4 s −1 (2 DMEGpy/CuBr).Hence, we can detect only a small amount of activity difference between the two guanidine complexes.The polymerization speed is high compared to related systems with pyridine-based copper catalysts, such as CuBr/2bipy or CuBr/2dNBipy, which mediate a significantly slower polymerization [37].Moreover, the bipy system was reported to proceed under heterogeneous conditions [37].The progress of the polymerization as marked by the development of the molecular weight and the polydispersities is depicted in Figure 7. Selected polymerization data are given in Table 4.For both polymerizations, the average molecular weights increase linearly, but deviate significantly from the theoretical molecular weights at conversion >40%.The initiator efficiency can be calculated as the slope of the linear function of M n,th vs. M n,GPC [38].Here, it points towards a small deactivation rate (f TMGpy = 0.72; f DMEGpy = 0.76).The polydispersities decrease during polymerization with 2 TMGpy/CuBr to values under 1.2 and increase again to 1.24, indicating a loss of control by the small deactivation rate.Using 2 DMEGpy/CuBr, the polydispersity only decreases to a value of 1.24.In summary, the catalysts, 2 TMGpy/CuBr and 2 DMEGpy/CuBr, show a high activity.Due to small deviations of the averaged and theoretical molecular weights and the small polydispersities, the polymerization control can be rated as medium.We relate the increase in polymerization speed to the changed donor situation of the copper complexes with one guanidine and one pyridine donor combined within the ligands.

Conclusions
Herein, we report three new guanidine-pyridine copper(II) complexes.The bis(chelate) complexes, 1 and 2, exhibit trigonal-bipyramidal coordination geometries with axial pyridine donors and equatorial guanidine and halide ligands.The bonds to the pyridine donors are slightly shorter than to the guanidine donors.Overall, the guanidine donor seems to be the stronger donor, as was shown in the mono(chelate) complex, 3, with considerably shorter Cu-N gua bonds.The corresponding guanidine-pyridine ligands were shown to mediate with their copper complexes controlled by styrene ATRP.Remarkably and in contrast to the bipy systems, the polymerization mixture stayed homogeneous.The polymerization proceeds considerably faster than with dNbipy/2CuBr, but with a smaller degree of control.

Table 2 .
Key geometric parameters of Complexes 1

Table 4 .
Conversion, M n,GPC , M n,th and M w /M n for the kinetics of styrene ATRP with 2 TMGpy/CuBr and 2 DMEGpy/CuBr and PEBr at 110 °C after 10 and 35 min.