Structural Modification of Functionalized Phosphine Sulfonate-Based

Article pubs.acs.org/Organometallics

Structural Modification of Functionalized Phosphine Sulfonate-Based Palladium(II) Olefin Polymerization Catalysts Timo M. J. Anselment,† Christian Wichmann,† Carly E. Anderson,† Eberhardt Herdtweck,‡ and Bernhard Rieger*,† †

WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany ‡ Lehrstuhl für Anorganische Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany S Supporting Information *

ABSTRACT: The influence of phosphine sulfonate ligands bearing a variety of functionalities on the conformation of their derived Pd(II) complexes and their catalytic behavior in olefin polymerization reactions was investigated. Analogous to the anisylderived 1a the methyl thioether-substituted 8 as well as methoxylated naphthalene-based compound 9 were successfully prepared. NOESY NMR spectroscopy has been applied for the interpretation of the complex configurations in solution and the correlation to the corresponding molecular structures of 1a and 9. Ethene homopolymerization reactions were used for the determination of reactivity trends and interpretation of effects originating from the altered ligand substitution. Detailed analysis of the PE microstructure shows that catalyst 9 acts as an efficient isomerization-type catalyst during the formation of low molecular weight PE. Additionally for compound 8 Pd−sulfur interactions with the introduced methyl thioether functionalities are proposed, based on NMR spectroscopic experiments. This catalyst promotes formation of minimal amounts of high molecular weight PE. Comparison of 1a, 1b, 8, and 9 indicates that the PE molecular weight is controlled by the protection of at least one axial position of the palladium center, but no clear trends concerning the catalyst activity could be observed.

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INTRODUCTION Investigation of late transition metal-based polymerization catalysts is of high interest due to the enhanced tolerance of these compounds toward polar-functionalized monomers. 1 Specifically the neutral Pd(II)-based phosphine sulfonate catalyst system 1 and its variations (e.g., 1a−f, Figure 1) show by far the highest potential for copolymerization reactions of ethene with polar-functionalized vinyl monomers. The rapid development of discrete single-component catalysts has been recently reviewed in the literature,2 and a large number of functionalized olefins as well as CO have also been reported to be suitable comonomers.3 Among these reports are detailed experimental and theoretical investigations that describe the characteristic features of polymerization reactions with phosphine sulfonate-based Pd(II) complexes, both for ethene homopolymerization and for copolymerization reactions with functionalized vinyl monomers.4 The importance of the nonsymmetric κ2-(P,O)-chelating monoanionic character of the phosphine sulfonate ligand has been highlighted.4f This results in a cis−trans isomerization of the substituents at the palladium center and a high energy barrier for β-hydride elimination. The combination of these effects is presumed to control the high linearity of the resulting polymers and a low © 2011 American Chemical Society

Figure 1. Overview of reported [κ2-(P,O)-phosphine sulfonate}PdMe(pyridine)] catalysts for olefin polymerization reactions. Received: August 5, 2011 Published: November 30, 2011 6602

dx.doi.org/10.1021/om200734x | Organometallics 2011, 30, 6602−6611

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degree of chain walking and isomerization. Ethene homopolymerization, which can be regarded as the parent system for copolymerization reactions with functionalized comonomers, is an ideal suitable test reaction for phosphine sulfonate-based catalyst development. Literature reports on base-stabilized [{κ2-(P,O)-phosphine sulfonate}PdMe(base)] (base: e.g., pyridine, lutidine, or dmso) complexes do not satisfactorily answer questions regarding the general catalyst structure−reactivity relationship in olefin polymerization. Some important influences such as the role of the stabilizing base have been thoroughly investigated 4a,c,5 and were successfully exploited for catalyst improvement by Mecking and co-workers.6 It was also found that the absence of a base or the presence of anionic functionalities leads to catalyst aggregation in solution.7 However, the role of the substituents at phosphorus is not yet completely understood. This can be seen from the three reported major variants of the neutral pyridine-stabilized catalyst systems; 1a, the derived methoxylated biphenyl system 1b,4c and the phenyl-substituted 1c, together with a series of polycyclic aromatic hydrocarbon modifications 1d−f4e (Figure 1). From this catalyst family only 1a,b can polymerize ethene with good activity and molecular weights higher than 20 × 103 g·mol−1.4c,6 Investigation of 1c−f with increasing ligand basicity and steric hindrance in ethene polymerization showed only the formation of low molecular weight PE combined with poor catalyst activities.4c Early reports on copolymerization reactions with the phosphine sulfonate ligand system 2 invariably mention one methoxy functionality aligned toward the metal center in the solid-state structure as observed from the molecular structures.3j,m,4a,f,8 However, the observed O−Pd distance of 3−4 Å does not allow clear statements on possible interactions with the metal center. Furthermore, DFT calculations show no distinct effect of the OMe group, although it is indicated that a certain degree of electrostatic influence is possible.4j Nevertheless, with regard to the cis−trans isomerization of substituents at the palladium center, even minor interactions could affect the energies of transition states required for polymerization reactions. An unusual activating effect of OMe functionalities has also been observed in the Pd(II)-based CO/ethene alternating copolymerization with 3. This catalyst is based on the κ2-(P,P)chelating bis-phosphine which bears 2-OMe-Ph substituents at phosphorus. Spatial proximity of the 2-OMe-Ph protons and the HAr6 proton (Figure 2) on the aryl substituents to the

center on the polymerization reaction is still not comprehensively answered. These reports inspired this investigation on the influence of functional groups present in the metal coordination sphere of phosphine sulfonate-based Pd(II) complexes on polymerization reactions, with focus on the determination of effects concerning the structure/reactivity relationship of these catalysts.

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RESULTS AND DISCUSSION Synthesis of New Ligands and Complexes. Ligand modification, complex synthesis, and investigation of the corresponding structure in the solid state as well as in solution are performed. This is carried out for the determination of general influences of functionalized ligands on the behavior of phosphine sulfonate-based Pd(II) polymerization catalysts. Here the ligand synthesis aims for (i) the exchange of the OMe group in the phosphine sulfonate ligand 2 (Drent ligand), which alters the size and Lewis basicity of the functional group, as well as (ii) an alteration of the OMe functionality position on the ligand framework, with respect to the metal center. Synthesis of the novel phosphine sulfonate ligands 5 and 7, which address this concept, was achieved by modification of literature-known procedures (Scheme 1).10 Scheme 1. Synthesis of the Phosphine Sulfonate Ligands 5 and 7

In situ formation of the Li[2-(dichlorophosphino)benzene sulfonate] intermediate 4 and subsequent reaction with 2-lithiothioanisole, obtained from lithiation of 2-bromo-thioanisole with n-BuLi, leads to formation of the methyl thioetherfunctionalized phosphine sulfonic acid 5. This novel phosphine sulfonate ligand was characterized by multinuclear NMR spectroscopy, supported by high-resolution ESI-MS spectrometry. 5 was obtained as the free sulfonic acid and not in the zwitterionic form (in contrast to 2), as shown from the singlet resonance in the proton-coupled 31P NMR spectrum. This is attributed to a decreased basicity of the phosphine by exchange of the 2-OMe-Ph with the 2-SMe-Ph substituent. Crystallization was not achieved, and thus 5 was used as received for subsequent complexation reactions. Likewise the regioselective lithiation of 1-methoxynaphthalene could be successfully exploited for the synthesis of the methoxylated naphthalene-based phosphine sulfonate ligand 7. Following a literature procedure 8-lithio-1-methoxynaphthalene 6 was obtained from the reaction of 1-methoxynaphthalene with tert-BuLi.10c Isolation and characterization of 6 shows minor impurities resulting from the presence of the 2-lithiated

Figure 2. Structures of the phosphine sulfonate ligand 2 introduced by Drent and the 2-OMe-Ph-substituted dppp-based catalyst 3 with labeling scheme for aryl substituents.

Pd-Me protons (HPd‑Me) has been observed in solution by ROESY NMR spectroscopy and the molecular structure in the solid state.9 Despite these findings, the question concerning the exact influence of these ligand-substituent interactions with the metal coordination 6603


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byproduct, which is attributed to a change of the solvent from cyclohexane to pentane. Subsequent reaction with the in situ prepared reactive dichlorophosphine species 4 provides the zwitterionic phosphine sulfonate ligand 7, which could be isolated from the crude product mixture by crystallization from chloroform. The proposed structure of 7 was confirmed by multinuclear and multidimensional NMR spectroscopy, supported by high-resolution ESI-MS spectrometry. In solution, a nonequivalent ligand conformation could be observed by 1H and 13C NMR spectroscopy due to the different alignment of the two naphthalene substituents with respect to the protonated phosphorus. Therefore the corresponding naphthalene signals in the 1H NMR spectrum are observed as being inequivalent, which is most pronounced for the methoxy functionalities (δOMe(1) = s, 3.89 and δOMe(2) = s, 3.22 ppm).

Figure 3. Structure of the bimetallic κ1-(N)-tmeda-bridged phosphine sulfonate Pd(II) intermediate 10.

the reported molecular structure.3m The obtained data confirm that the structures of 1a in solution and in the solid state are very similar, as reported for a complex closely related to 1a.4a In Figure 6 the spatial proton interactions that are relevant for the interpretation of the complex configurations of 1a, as well as 8 and 9, in solution are displayed (vide infra). Integration of the NOESY NMR cross-peaks allows approximation of the corresponding proton distances with the pyridine H py2,3 interaction as a reference value (Figure 6). 11 Furthermore, the obtained results for 1a and the novel synthesized methoxylated naphthalene-based complex 9 are directly compared to the H−H distances from the molecular structures in the solid state (details can be found in the Supporting Information). Data for 1a show a very reasonable agreement of this correlation, if an increased divergence for groups with dynamic flexibility in solution is taken into account. Good examples for such a deviation are the 2-OMe functionalities of 1a due to an observed and previously reported fast chelate ring inversion and relatively high rotational flexibility of these functional groups as well as of the corresponding aryl substituents.4a Interactions of the HPd‑Me protons with (i) those (HOMe) from the 2-OMe functionality (1aC) and (ii) the HAr6 proton of the aryl substituents (1aD) at phosphorus are assigned. These data confirm the two different alignments of the OMefunctionalized aryl substituents in solution, which is also observed for 1a in the solid state. Due to the equivalence of the methoxy signal in the 1H NMR spectra, this indicates a fast exchange due to chelate ring inversion as reported previously. 4a Most notable is the close contact of the HAr6 proton to the palladium center (interaction most likely to be of anagostic nature due to the high Pd−H−C angle of ca. 122°12) observed in the molecular structure, an observation that is also found for other phosphine-based palladium complexes and polymerization catalysts.9,11b Evaluation of the NOESY NMR spectra confirms the proposed structure of 1a in solution and indicates a steric protection of one axial position on the palladium center.3m This type of axial position blocking has been previously reported to be critical for polymerization reactions with late transition metal-based catalysts in terms of obtained molecular weight and activity. This concept could be validated for the phosphine sulfonate system with the complex 1b (Figure 1) and related Ni-based complexes, but it appears that only one axial position has to be blocked in these catalyst systems.1b,13 Analysis of complex 8 by 1H and 31P NMR spectroscopy shows broadened signals, a situation that is tentatively ascribed

Scheme 2. Synthesis of the Neutral Pyridine-Stabilized Phosphine Sulfonate-Based Pd(II) Complexes 8 and 9

The corresponding pyridine-stabilized neutral [{κ2-(P,O)phosphine sulfonate}PdMe(pyridine)] complexes 8 and 9 were prepared by a slightly modified literature procedure.10a 8 and 9 could be easily obtained by reaction of ligands 5 and 7 with (tmeda)PdMe 2 (tmeda: N,N,N′,N′-tetramethylethylenediamine) and subsequent addition of pyridine. Crystallization of the complexes was achieved only for 9 from CHCl3/pentane (1:2), which also gave crystals suitable for determination of the molecular structure by X-ray diffraction (Figure 4). Both compounds were investigated by 1D and 2D NMR spectroscopy for evaluation of their solution structures. Furthermore the κ1-(N)-tmeda-bridged dimeric phosphine sulfonate complex 10 could be isolated as an intermediate product in the reaction from 7 to 9. This compound displays limited solubility in CHCl3 and could also be crystallized from CHCl3/pentane (1:2) for determination of the molecular structure (Figure 5). For comparative reasons the reference catalyst 1a was prepared according to the literature.4a Investigation of the Complex Structures. As a model for comparison, the conformation of 1a was analyzed by NOESY NMR spectroscopy (CDCl3, 25 °C) and compared to 6604


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Figure 4. Ortep-style plot of compound 9 in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å] and bond angles [deg]: Pd1−P1 2.2434(5), Pd1−O1 2.1650(13), Pd1−N1 2.1043(15), Pd1−C1 2.022(2); P1−Pd1−O1 81.49(4), P1−Pd1−N1 174.29(4), P1−Pd1−C1 96.26(6), O1−Pd1−N1 93.16(5), O1−Pd1−C1 177.72(7), N1−Pd1−C1 89.10(7).

Figure 5. Ortep-style plot of compound 10 (molecule A) in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å] and bond angles [deg]: Pd1−P1 2.2379(6)/2.2373(8), Pd1−O1 2.175(2)/2.184(2), Pd1−N1 2.174(2)/2.169(3), Pd1−C1 2.026(3)/2.033(3); P1−Pd1−O1 79.42(5)/80.28(6), P1−Pd1−N1 173.89(6)/171.10(8), P1−Pd1−C1 95.52(8)/ 96.36(9), O1−Pd1−N1 94.86(8)/92.09(10), O1−Pd1−C1 174.92(9)/176.36(11), N1−Pd1−C1 90.22(10)/91.37(12). Symmetry operation to equivalent atom positions a: −x, 2−y, −z. In italics the values for a second, crystallographically independent, molecule, B. Both molecules A and B are located on a center of symmetry.

corresponding harder, oxygen donor in complex 1a. Variabletemperature 1H and 31P NMR spectroscopic experiments from 20 to 120 °C in C2D2Cl4 were employed to evaluate the dynamic behavior and temperature stability of 8 (in the absence of ethene). In these experiments a peak broadening of the phosphorus NMR resonance was observed during heating from 20 to 80 °C, followed by a decomposition of the complex. 8 was examined by NOESY NMR spectroscopy, and observed

to Pd−S interactions, as observed for similar complexes with phosphines bearing 2-SMe-Ph substituents at the palladium center.14 Furthermore, the κ2-(P,S)-chelate binding potential of these P{Ph(2-SMe-Ph)2} phosphines is established for a wide range of transition metal complexes.14,15 An increased tendency of the SMe functionality to interact with the metal center is expected in the present case due to the increased size and the softer donor nature of the sulfur in 8 compared to the 6605


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the corresponding naphthalene substituent (naphthalene fold angle 2.1° and 4.5°, respectively; for details see Supporting Information). Due to the constrained conformation, this substituent shows a hindered dynamic behavior in solution, which is attributed to the steric hindrance from the large naphthalene substituents or a weak Pd−O interaction, which forces the ligand substituents in this position. The complex conformation of 9 is apparently relatively rigid, as free rotation of the functionalized naphthalene substituents seems to be hindered for 9 due to the bulky ligand and the stable κ2-(P,O) chelate, as observed from the molecular structure in the solid state and due to the absence of any corresponding NOE interactions (no interactions of the HNp8-protons with the Pd-Me group; corresponding solid-state distances >4 Å). Nevertheless the slightly broadened 1H NMR (CDCl3, 25 °C) resonance for the OMe(1) functionality indicates a certain degree of dynamic freedom at this position. With regard to the general structure of 9 it is surprising, that despite the steric bulk of the phosphine sulfonate ligand near the palladium center, the axial positions of the metal center remain accessible as the complex structure shows an opened coordination environment (Figure 7).

Figure 6. Labeling scheme for the description of the proton relations observed by NOESY NMR spectroscopy with complexes 1a, 8, and 9.

cross-peaks from intramolecular spatial proton relations (Figure 6) were employed for the determination of the corresponding H−H distances (Table 1). However, despite an apparent Table 1. Correlation of the Determined Distances for Spatial Proton Relations in Solution by NOESY NMR Spectroscopy for 1a, 8, and 9 to the Corresponding Solid-State Proton Distances from the Molecular Structure of 1a and 9 H−H relation xa

dx (Å)b solution

dx (Å)d molecular struct

ref (Hpy2,3) 1aA 1aB 1aC 1aD 8A 8B 9A 9B1 9B2 9C1 9C2

2.3c 2.7 2.0 3.1 2.7 2.4 2.3 2.8 2.4 2.4 3.4 2.7

2.3 2.6 2.2 3.7 2.8 n.d. n.d. 2.4 2.3 2.3 3.0 2.5

a

NOE proton relation defined in Figure 6. bDetermined average distance (from f1 and f2). cReference value from the molecular structure. dShortest distance observed in the solid state, n.d. = not determined; see Supporting Information 1 for further details.

interaction of the SMe group with the metal center, no spatial interactions of the SMe functionality and the Pd-Me fragment can be observed in the NOESY NMR spectrum. 1 H and 13C NMR spectroscopy of complex 9 shows a nonequivalent orientation of the methoxylated naphthalene substituents at phosphorus in solution (see Supporting Information for NMR spectra). Analogous to the corresponding ligand 7 the observed NMR signals for both OMe groups are separated, which is attributed to a different conformation with respect to the palladium center. Likewise the molecular structure of 9 confirms the nonequivalent alignment of these methoxy functionalities, with respect to the palladium center (Figure 4). Furthermore the detailed analysis of 9 by NOESY NMR spectroscopy has been employed for the determination of the present spatial proton interactions, which are shown in Figure 6. The derived distances (Table 1) are compared to the molecular structure for evaluation of the conformation in solution, which shows the close resemblance of these structures. The molecular structure reflects the difference between the methoxy groups by a longer Pd1−O5 distance (3.318(2) Å, OMe(1)) in comparison to the Pd1−O4 distance (3.134(1) Å, OMe(2)) in the solid state. Also it can be seen that the close contact of the OMe(2) group to the methyl group on the palladium center leads to a significantly increased distortion of

Figure 7. Model of complex 9 with removed pyridine and methyl substituents that shows the open coordination sphere at the palladium center as well as the accessible axial positions.

In comparison to the closely related naphthalene-based phosphine sulfonate 1d4e the introduction of functional groups apparently assists the fixation of the complex structure and also facilitates crystallization. Ethene Polymerization as Test Reaction. Test reactions with catalysts 8 and 9 in ethene homopolymerization focus on the investigation of the general reactivity trends. Table 2 shows the obtained data of these polymerizations for comparison with literature-reported data based on catalysts 1a−f, as well as reference experiments with 1a for specific reaction conditions (Table 2, entries 12, 13). The microstructure of these synthesized polymers is investigated in detail to determine possible catalyst substitution effects. 6606


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Table 2. Preliminary Ethene Homopolymerization Results for Comparison of the Catalysts 8, 9, 10, and 1a entrya 1 2 3 4 5 6 7 8 9 10 11 12 13

cat. 9 9 9 9 9 9 9 9 10b 8 8 1a 1ab

T (°C) 50 80 100 100 80 80 80 80 80 50 80 50 100

p (bar) 5 5 5 10 10 20 5 5 5 20 20 20 5

t (h) 2 2 2 2 2 2 4 16 2 20 20 2 2

mPol. (g) 0.28 0.64 0.65 0.88 2.33 2.49 4.09 4.48 0.78 0.07 0.10 0.70 1.64

act.c 2.8 6.4 6.5 4.4 12 6.2 21 5.6 7.8 0.02 0.03 1.8 33

Mwd 1.8 0.8 0.6e 0.7e 0.9 0.9 0.9 0.8 1.7 500f 450f 33 13

PDId 2.2 1.6 1.5 1.7 2.2 2.1 1.8 1.8 3.6 6.3f 3.9f 1.7 1.9

Tm (°C)g h

117 105h 111h 115h 109h 110h 107h 105h 106h 133 135 131 127

Mni

%int.j

1.3 0.8 1.0 1.0 1.0 1.0 1.0 1.0 1.0 n.d.k n.d. n.d. n.d.

100 100 94.3 85.5 100 100 100 90.4 100 n.d. n.d. n.d. n.d.

a

200 mL stainless steel reactor, 30 mL of toluene, 10 μmol of catalyst. b5 μmol of catalyst. cActivity kgPol·gPd−1·barethene−1·h−1. dDetermined by GPC, RI detection 103 g·mol−1. eBimodal distribution with minor amount of a higher molecular weight fraction; entries 3 and 4: 56 × 103 and 79 × 103 g · mol−1 respectively. fDetermined by GPC, multidetection. gDetermined by DSC, second heating cycle. hBroad melting area before sharp melting point. i103 g·mol−1; determined by 1H NMR spectroscopy at 120 °C in C2D2Cl4. jInternal olefin content, no branching observed by 1H NMR spectroscopy; determined by 1H NMR spectroscopy at 120 °C in C2D2Cl4. kn.d. = not determined.

Figure 8. Olefinic region (13C{1H} NMR spectrum, C2D2Cl4, 120 °C) of the low molecular weight PE sample from Table 2, entry 2.

Data from the polymerization reactions with the methoxylated naphthalene-based catalyst 9 (Table 2, entries 1−8) show a relatively low polymerization activity in comparison to 1a and literature reports on related catalysts.4a,f This is combined with an unexpected reduction of molecular weights leading to a Mw of approximately 1000 g·mol−1. As this is at the lower detection limit of GPC analysis, the molecular weights were also confirmed by high-temperature NMR spectroscopy, and additionally these experiments were employed for the analysis of the PE microstructure. 1H NMR spectroscopy shows olefinic species that are attributed to chain termination via β-hydride elimination and subsequent olefin exchange. For polymerizations performed up to 80 °C no terminal 1-olefins are observed, and the presence of a high degree of 3-olefins and higher can be shown by 13C NMR spectroscopy (Figures 8, 9), which is surprising for phosphine sulfonate-based catalysts. Although olefin isomerization could be previously confirmed

for 1-octene and high degrees of internalization of the terminal olefin functionality (more than 70% internal olefins) have been previously reported with catalysts 1a and, very recently, with the PCy2-based analogue, only a low tendency for olefin internalization far into the polymer backbone is observed. 3c,4a,16 Double-bond isomerization from the presence of Pd(0) residues, a possible origin for olefin isomerization, is considered to be unlikely due to the high temperature stability of the phosphine sulfonate catalysts, the low polymerization temperature for the experiment of entry 1, Table 2, and the consistency of this observation for the presented polymerization runs with 9 presented in Table 2. Furthermore, as no internal branches are observed by 1H NMR spectroscopy, a completely linear PE backbone is present as expected for this catalyst type. Also, the ethene pressure has apparently no significant influence on the observed molecular weight and the degree of olefin internalization (Table 2, entries 2, 5, 6) at the applied conditions. 6607


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Figure 9. Proposed reaction mechanism for the PE formation with 9 starting from a model alkyl complex: (a) ethene coordination followed by (b) insertion, which can lead to chain growth; (c, c′) 1-olefin formation by β-hydride elimination, (d) generation of a secondary alkyl group by 2,1insertion; (e, f) ethene coordination cannot lead to subsequent insertion of the secondary alkyl group and branch formation; (g) β-hydride elimination and reinsertion forms a 2-olefin; either (h) chain termination by olefin replacement or (i) further isomerization continues; ligands and cis−trans isomerization steps are omitted for clarity.

An experiment for the κ1-(N)-tmeda-bridged bimetallic complex 10 shows a similar behavior, although the slightly increased molecular weight is attributed to the different coordination behavior of the present tmeda base. The proposed mechanism leading to the obtained PE microstructure is displayed in Figure 9. Due to the strong Lewis basic character of the employed pyridine (py) in complex 9, py-coordinated intermediates are assumed.4d Starting from an alkyl-coordinated species either chain growth by (a) ethene coordination and (b) migratory insertion or (c, c′) β-hydride elimination to a 1-olefin can occur. A possible (d) 2,1reinsertion of the latter gives a secondary alkyl group. Due to the linear character of the obtained PE, it is indicated that the catalyst system does not permit migratory insertion of the secondary alkyl moiety (e, f) after ethene coordination, which is in accordance with the literature.4a,f Instead of such a branch formation, the (h) reorientation and β-hydride elimination can generate 2-olefins. In this case either (h) chain termination by olefin exchange or (i) continued isomerization to further internalized olefin species (higher than 3-olefins) can take place. However the inability of 9 to promote branch formation traps the active species in a series of catalyst resting states until the completely reversible isomerization reaction reaches a chain end or olefin exchange occurs, a combination that results in the linear low molecular weight PE. By comparison to literature polymerization data with 1a these observations show that 9 apparently is a very effective isomerization-type catalyst within the phosphine sulfonate catalyst system. High degrees of internal olefins (∼70%) have been previously reported for 1a, but only the presence of 1- to 3-olefins could be observed.4a

By contrast the methyl thioether-functionalized catalyst 8 promotes only minimal PE formation at long reaction times with high ethene pressure. However, this is combined with a significant increase of the PE molecular weight (Table 2, entries 10, 11) in comparison to a reference experiment with the catalyst 1a (Table 2, entry 12). It was also observed that an increase of the reaction temperature leads to decreased molecular weights, as expected due to higher rates of chain termination under these conditions.

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CONCLUSIONS Comparison of the presented neutral phosphine sulfonatebased Pd(II) catalysts 8 and 9 to the literature-known systems 1a and 1b shows that the most striking difference of both catalysts in ethene polymerization is the remarkably high and low PE molecular weight, respectively. A detailed investigation of the catalyst structure of 1a and 9 shows a constrained confirmation of the latter compound with relatively unprotected axial positions on the metal center. Accessibility of these positions is expected, as well-known from other late transition metal-based olefin polymerization catalysts, to facilitate chain transfer by associative olefin exchange. Therefore, this lack of steric protection is suggested to be the origin of the obtained low molecular weight PE with 9. In this context for 1a the steric protection of one axial position by the anagostic interaction of the HAr6 proton with the palladium center observed in the molecular structure is apparently sufficient to significantly enhance the obtained PE molecular weight. Analogously it is suggested that the proposed S−Pd interaction of the 2-SMe functionalities in 8 leads to an enhanced protection of the Pd center and therefore to higher molecular weight PE. 6608


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With respect to the obtained PE microstructure the main difference of the low molecular weight PE obtained with 9, compared to related catalysts in the literature, is the drastically enhanced olefin isomerization and internalization. In this respect the catalyst system 9 behaves in a more similar manner to other late transition metal-based catalysts (vide infra) than the reference catalyst 1a. Either steric, electronic, or a combination of those effects could be responsible for this behavior, and a distinct factor cannot be identified. However, this facilitated isomerization pathway can be regarded as a competing reaction for chain propagation, as branch formation, even at low ethene pressures, is clearly hindered for phosphine sulfonate-based Pd(II) catalysts and continuation of the polymerization requires the metal to be situated on a terminal position of the growing polymer chain. Therefore, it is unclear if the higher catalyst activities for 1a are due to a lower tendency for olefin isomerization or due to an intrinsically higher polymerization activity compared to 9. High levels of PE branching have been observed in κ2-(N,N)- or κ2-(P,N)-derived catalyst systems (e.g., based on α-diimine,1b,17 amino-bis(imino)phosphorane,18 or phosphinosulfonamide19 ligands) with Ni(II) as the catalytically active metal. Specifically the latter complex type was reported to produce branched PE oligomers with a high degree of internalized olefins from isomerization.19 These investigations highlight the significance of functional groups on the employed ligand framework for (i) the stabilization of the complex conformation and (ii) the polymerization reactions. Furthermore, they give enhanced insights into the structure/reactivity relationship of the ethene homopolymerization reaction with phosphine sulfonate-based Pd(II) catalysts, especially concerning the molecular weight of the obtained PE. Investigations with respect to the nature of the influence of functional groups on the polymerization steps, the employed catalytically active metal, and the implications of the detailed effects on copolymerization reactions of ethene with polarfunctionalized olefins are in progress.

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Figure 10. NMR signal assignment for complex 9 and the corresponding ligand 7. laboratory of the Department of Inorganic Chemistry at Technische Universität München. Compound analyses indicate the presence of residual solvent molecules, which could not be removed. Ligand characterization is assisted by high-resolution ESI-MS spectra, obtained on a Bruker micrOTOF-Q, calibrated against sodium formiate 5 mmol/L with 0.2% (v/v) formic acid in a water/2propanol 1:1 mixture. Synthesis of 2-(Bis-2-methylthiophenylphosphine)benzenesulfonic Acid, 5. For safety during synthesis of this compound the reaction and workup procedure was carried out with extreme precaution to avoid contact with possible hazardous and volatile byproducts. Benzenesulfonic acid (2.05 g, 13.0 mmol) was dissolved in tetrahydrofuran (50 mL, flask A), n-BuLi (13.0 mL, 26.0 mmol, 2.0 M in hexane) was added at −78 °C, and the reaction was stirred for 3 h at room temperature. PCl 3 (1.14 mL, 1.79 g 13.0 mmol) was added to a separate flask charged with tetrahydrofuran (80 mL, flask B) at −78 °C, and the contents of flask A were slowly added via a dropping funnel with vigorous agitation. The obtained yellow solution was stirred for 1 h at −78 °C. In parallel a third flask charged with 2bromothioanisole (5.30 g, 26.1 mmol) and tetrahydrofuran (100 mL, flask C) was cooled to −78 °C, and n-BuLi (13.0 mL, 26.0 mmol, 2.0 M in hexanes) was added. The obtained orange solution was transferred by cannula to flask B and stirred 30 min at −78 °C followed by warming to room temperature. After stirring for 1 h termination of the reaction was achieved by addition of H 2O (5 mL) followed by removal of all volatiles in vacuo. After washing with pentane (20 mL) and drying under vacuum the solid was dissolved in a mixture of methylene chloride (180 mL) and water (100 mL), acidified with an aqueous HCl solution (5 mL, 37%), for extraction. The aqueous phase was removed and extracted with methylene chloride (50 mL), and the combined organic phases were dried over MgSO4. Subsequent removal of volatiles in vacuo provides an orange crude product mixture, which was washed with pentane (2 × 10 mL), tetrahydrofuran (3 × 10 mL), and Et2O (2 × 10 mL). 5 is obtained as a yellow powder after drying at reduced pressure (3.49 g, 8.03 mmol, 62%) and is used as received despite a low content of residual impurities, as purification by crystallization could not be achieved. 1 H NMR (300 MHz, CD2Cl2): δ 8.34−8.17 (m, 1H), 7.85−7.59 (m, 4H), 7.46 (t, J = 7.1 Hz, 1H), 7.42−7.22 (m, 3H), 7.18−6.86 (m, 3H), 2.48 (s, 6H). 31P NMR (121 MHz, CD2Cl2): δ −8.4 (s, br). HRESI-MS (neg., MeCN): m/z = 433.0164 (M − ). Calcd for C20H18PS3O3: m/z = 433.0156 (M − H+). Synthesis of 2-(Bis-8-methoxynaphthalene-1-phosphine)benzenesulfonic Acid, 7. Benzenesulfonic acid (2.06 g, 13.0 mmol) dissolved in tetrahydrofuran (50 mL, flask A) and n-BuLi (10.4 mL, 2.5 M in hexane, 16.0 mmol) was added at −78 °C, and the reaction was stirred for 3 h after slow warming to room temperature. PCl3 (1.12 mL, 1.79 g, 13.0 mmol) was added to a separate flask charged with tetrahydrofuran (80 mL, flask B) at −78 °C followed by slow addition of the contents of flask A via a dropping funnel at vigorous agitation, and the obtained yellow solution was stirred for 1 h at −78 °C. In parallel, 6 (4.70 g, 28.7 mmol) was suspended in pentane (100 mL, flask C) at −78 °C, and the contents were transferred to flask B via a V-connection tube. The obtained brown suspension cleared slowly and was stirred for 16 h after warming to room temperature. Termination of the reaction was achieved by

EXPERIMENTAL SECTION

General Considerations. All reactions were routinely carried out under an inert atmosphere using standard Schlenk techniques or a glovebox unless otherwise stated. Chemicals were obtained from Sigma Aldrich, Acros Organics, or ABCR and used as received without further purification unless stated otherwise. 2-Bromothioanisole was purified by column chromatography, and benzenesulfonic acid was dried by azeotropic distillation with toluene before use. Anhydrous solvents were obtained from an MBraun MB-SPS-800 solvent purification system, and all other solvents were degassed prior to use. All palladium compounds were stored under exclusion of light at −20 °C in a glovebox refrigerator. The compounds (tmeda)PdMe2,20 2,10a and 1a4a and the lithium salt 610c were prepared according to slightly modified literature procedures. Solution NMR spectra were collected at room temperature using a Bruker ARX300 or AV500 spectrometer. High-temperature solution NMR spectra were recorded at 120 °C in C2D2Cl4. 1H and 13C NMR spectra are referenced to the residual solvent peak of SiMe 4 or in the case of 31P NMR spectra to 85% phosphorus acid as external standard. Polymer samples were filtered at 160 °C before GPC on a Polymer Laboratories PL-GPC 220 high-temperature chromatograph, equipped with two Olexis 300·7.5 mm columns, and triple detection was performed by a differential refractive index detector, a PL-BV 400 HT viscometer, and a Precision Detectors model 2040 light scattering detector (15°, 90°). The solvent was 1,2,4-trichlorobenzene (BHT stabilized) at 160 °C, with a PE standard. DSC was measured on a TA Instruments DSC Q2000 calorimeter at a heat/cooling rate of 10 K/min. Elemental analyses were performed by the micro analytical 6609


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addition of H2O (5 mL), removal of volatiles in vacuo, and subsequent dissolution of the residue in a mixture of methylene chloride (180 mL) and water (100 mL), acidified with 5 mL of aqueous HCl solution (37%), for extraction. The aqueous phase was removed and extracted with methylene chloride (100 mL), and the combined organic phases were dried over MgSO4 followed by removal of the volatiles in vacuo. Afterward the obtained residue was washed with tetrahydrofuran (6 × 20 mL), Et2O (2 × 20 mL), and pentane (2 × 20 mL), which provides the crude product mixture after drying in vacuo (4.59 g, 9.13 mmol, 70% yield, 81% content of 5 determined by 31P NMR spectroscopy in CDCl3). Crystallization of this crude product mixture from hot CHCl 3 gave colorless needles of the pure ligand 7 (682 mg, 28%). 1 H NMR (500 MHz, CDCl3): δ 10.62 (d, 1J(H−P) = 674.3 Hz, 1H, H-P), 8.50 (dd, 3J(H−H) = 7.7 Hz, 4J(H−P) = 4.8 Hz, 1H, H3), 8.12−8.05 (m, 2H, H14, 24), 7.73 (m, 1H, H4), 7.60−7.56 (m, 3H, H11, 12, 22), 7.47 (m, 2H, H21, 25), 7.36−7.28 (m, 2H, H5, 15), 7.26 (d, 3J(H−H) = 7.3 Hz, 1H, H26), 7.12 (dd, 3J(H−P) = 18.0 Hz, 3 J(H−H) = 7.3 Hz, 1H, H16), 7.00 (dd, J(H−H) = 6.7, 2.0 Hz, 1H, H10), 6.93 (dd, 3J(H−P) = 14.4 Hz, 3J(H−H) = 7.7 Hz, 1H, H6), 6.80 (d, 3J(H−H) = 7.7 Hz, 1H, H20), 3.89 (s, 3H, OMe1), 3.22 (s, 3H, OMe2). 13C{1H} NMR (75 MHz, CDCl3): δ 154.9 (s, C9), 154.2 (s, C19), 151.3 (d, 2J(C−P) = 8.2 Hz, C2), 136.8 (d, 2J(C−P) = 9.2 Hz, C26), 135.7 (d, 2J(C−P) = 9.1 Hz, C16), 135.5 (s, C23), 135.4 (s, C13), 134.5 (d, 4J(C−P) = 3.2 Hz, C24), 134.3 (s, C14), 133.9 (d, 4 J(C−P) = 3.3 Hz, C4), 133.9 (d, 2J(C−P) = 4.1 Hz, C6), 130.4 (d, 3J(C−P) = 8.7 Hz, C3), 129.8 (d, 3J(C−P) = 12.2 Hz, C5), 128.3 (s, C11), 127.7 (s, C21), 126.3 (d, 3J(C−P) = 15.0 Hz, C25), 125.8 (d, 3 J(C−P) = 14.7 Hz, C15), 125.4 (d, 2J(C−P) = 5.1 Hz, C8), 124.6 (d, 2 J(C−P) = 5.3 Hz, C18), 122.5 (s, C12), 121.8 (s, C22), 118.1 (d, 1J(C−P) = 94.7 Hz, C17), 117.0 (d, 1J(C−P) = 89.7 Hz, C7), 115.9 (d, 1J(C−P) = 92.8 Hz, C1), 107.9 (s, C10), 107.8 (s, C20), 56.3 (s, OMe1), 55.1 (s, OMe2). 31P NMR (121 MHz, CDCl3): δ +16.5 (ddd, 1J(H−P) = 674.3, J(H−P) 32.2, 15.5 Hz). HR-ESI-MS (neg., MeCN): m/z = 501.0936 (M−). Calcd for C28H22PSO5: m/z = 501.0926 (M − H+). Synthesis of [{κ2-(P,O)-2-(Bis-2-methylthiophenylphosphine)benzenesulfonate}PdMe (pyridine)], 8. The phosphine sulfonate 5 (348 mg, 0.80 mmol, 1 equiv) was dissolved in methylene chloride (10 mL), and (tmeda)PdMe2 (202 mg, 0.80 mmol, 1 equiv) was added. Evolution of gas was observed during formation of a yellow solution, which was stirred for 30 min. Pyridine was added (0.5 mL, excess), and the solution became orange. After stirring for 1 h at room temperature the solution was reduced to 5 mL, and the complex precipitated by addition of Et2O (20 mL). The liquid was removed by filtration, and the obtained solid was reprecipitated from methylene chloride (10 mL)/Et2O (20 mL). After filtration and removal of volatiles 8 was obtained as an off-white solid (145 mg, 0.23 mmol, 29%). Crystallization of this compound could not be achieved, and residual solvent has been observed by 1H NMR spectroscopy; hence, a low EA was obtained. 1 H NMR (500 MHz, CDCl3): δ 8.88 (s, 1H), 8.40 (s, 1H), 7.85 (s, 1H), 7.67−7.08 (m, 10H), 2.42 (s, 4H), 0.69 (s, 2H). 31P NMR (121 MHz, CD2Cl2): δ +34.5. Anal. Calcd for C26H26PNS3O3Pd: C 49.25, H 4.13, N 2.21, S 15.17. Found: C 47.26, H 4.03, N 1.86, S 14.76. Synthesis of [{κ 2-(P,O)-2-(bis-8-methoxynaphthalene-1phosphine)benzenesulfonate}PdMe (pyridine)], 9. 7 (200 mg, 0.40 mmol, 1 equiv) was dissolved in methylene chloride (10 mL), and (tmeda)PdMe2 (100 mg, 0.40 mmol, 1 equiv) was added. Evolution of gas was observed during formation of a pale yellow solution, which was stirred for 20 min followed by pyridine addition (0.25 mL, excess). After stirring for 1 h at room temperature the solution was reduced to 2 mL and precipitated by addition of pentane (8 mL). The solvent was filtered off, and the obtained solid was recrystallized from CHCl 3 (7 mL)/pentane (20 mL). A 150 mg amount of complex 9 (0.21 mmol, 53%) was obtained as yellow crystals. 1 H NMR (500 MHz, CDCl3): δ 8.75 (d, 3J(H−H) = 5.3 Hz, 2H, Hpy1), 8.39−8.28 (m, 1H, H3), 7.94 (d, 3J(H−H) = 8.1 Hz, 1H, H14), 7.86 (d, 3J(H−H) = 8.1 Hz, 1H, H24), 7.67 (t, 3J(H−H) = 7.6 Hz, 1H, Hpy3), 7.55−7.39 (m, 5H, H4, 11, 12, 21, 22), 7.39−7.32 (m, 1H, H15), 7.32−7.23 (m, 4H, Hpy2, 16, 25), 7.16 (t, 3J(H−H) = 7.6 Hz,

1H, H5), 7.13−7.07 (s, 1H, H26), 7.07−7.01 (m, 1H, H6), 6.85 (d, 3 J(H−H) = 7.6 Hz, 1H, H20), 6.70 (d, 3J(H−H) = 7.6 Hz, 1H. H10), 3.71 (s, br, 3H, OMe1), 3.05 (s, 3H, OMe2), 0.49 (s, 3H, PdMe). 13 C{1H} NMR (75 MHz, CDCl3): δ 156.1 (s, C19), 155.8 (s, C9), 150.9 (s, py1), 137.7 (s, py3, C16), 136.2 (d, J = 7.5 Hz), 135.5 (d, J = 5.2 Hz, C26), 135.4 (d, J = 7.6 Hz), 133.4 (d, 2J(C−P) = 1.6 Hz, C6), 132.0 (s), 131.8 (s, C24), 131.3 (s, C14), 130.3 (s, C4), 129.9 (s, br, C3, 5), 127.1 (s), 126.9 (s, C11), 126.4 (s, C21), 126.0 (d, 3J(C−P) = 8.5 Hz, C25), 125.5 (d, 3J(C−P) = 10.5 Hz, C15), 125.1−124.7 (m, py2, C16), 122.0 (s, C22), 120.9 (s, C12), 107.1 (s, C20), 105.3 (s, C10), 56.5 (s, br, OMe2), 53.7 (s, OMe1), −0.17 (s, Pd-Me). 31P{1H} NMR (121 MHz, CDCl3): δ +47.0. Due to overlapping peaks, most quaternary carbon atoms could not be assigned and detected. Anal. Calcd for C35H31NPPdSO5: C 58.79, H 4.37, N 1.96, S 4.48. Found: C 57.15, H 4.32, N 2.11, S 4.42. As observed from X-ray diffraction the compound cocrystallizes with CHCl3 and the residual solvent could not be removed. Synthesis of [κ1-(N)-tmeda][{κ2-(P,O)-2-(bis-8-methoxynaphthalene-1-phosphine)benzene sulfonate}PdMe]2, 10. This intermediary product for the reaction of 7 to 9 could be obtained as a pale yellow solid from a synthesis of 9 without addition of pyridine. Precipitation of 10 by addition of Et2O (yield 67.2%) was followed by recrystallization from CHCl3/pentane. The low solubility of 10 in CHCl3 precludes characterization by 13C NMR spectroscopy. 1H NMR (300 MHz, CDCl3): δ 8.24 (dd, J = 7.4, 3.7 Hz, 1H), 7.90 (d, J = 8.1 Hz, 1H), 7.79 (d, J = 8.2 Hz, 1H), 7.65 (dd, J = 14.0, 7.3 Hz, 1H), 7.53−7.36 (m, 5H), 7.34−7.19 (m, 4H), 7.09 (t, J = 7.5 Hz, 1H), 7.03−6.86 (m, 3H), 6.66 (d, J = 6.6 Hz, 1H), 3.68 (s, 3H), 2.98 (s, 3H), 2.17 (s, 3H), 1.55 (s, 3H), 0.16 (s, 3H). 31P{1H} NMR (121 MHz, CDCl3): δ +47.6. Anal. Calcd for C64H66N2P2Pd2S2O10: C 56.43, H 4.88, N 2.06, S 4.71. Found: C 53.18, H 4.77, N 2.01, S 4.17. As observed from X-ray diffraction the compound cocrystallizes with CHCl3 and the residual solvent could not be removed. Single-Crystal X-ray Structure Determinations. 9: yellow prism, C34H30NO5PPdS, Mr = 702.03; monoclinic, space group C2/c (No. 15), a = 18.3019(3) Å, b = 15.5260(3) Å, c = 24.6690(5) Å, β = 93.5722(9)°, V = 6996.2(2) Å3, Z = 8, λ(Mo Kα) = 0.71073 Å, μ = 0.674 mm−1, ρcalcd = 1.333 g cm−3, T = 173(1) K, F(000) = 2864, θmax 25.47°, R1 = 0.0225 (5970 observed data), wR2 = 0.0579 (all 6482 data), GOF = 1.060, 391 parameters, Δρmax/min = 0.32/−0.34 e·Å−3. Unresolved solvent molecules had to be removed with the SQUEEZE procedure. 10: yellow fragment, C64H66N2O10P2Pd2S2·CHCl3, Mr = 1481.44; monoclinic, space group P21/c (No. 14), a = 23.1506(9) Å, b = 14.7312(6) Å, c = 24.5348(9) Å, β = 110.6878(17)°, V = 7827.7(5) Å3, Z = 4, λ(Mo Kα) = 0.71073 Å, μ = 0.705 mm−1, ρcalcd = 1.257 g·cm−3, T = 123(1) K, F(000) = 3024, θmax 25.46°, R1 = 0.0344 (13 066 observed data), wR2 = 0.0935 (all 14451 data), GOF = 1.056, 785 parameters, Δρmax/min = 1.65/−1.01 e·Å−3. Besides the solvent molecule, well located in the difference Fourier maps, unresolved solvent molecules remained and had to be removed with the SQUEEZE procedure. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-853890 (9) and CCDC-853891 (10). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; e-mail: [email protected]). For more and detailed information see the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Additional NMR spectroscopic data including detailed peak assignments and tables for interpretation of NOESY spectra. Representative examples for polymer characterization and crystallographic data for complexes 9 and 10. This material is available free of charge via the Internet at http://pubs.acs.org. 6610


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(10) (a) Allen N. T., Godall B. L., McIntosh, L. H.; III. U.S. Pat. Appl. 0049712 a1, 2007. (b) Laitinen, R. H.; Heikkinen, V.; Haukka, M.; Koskinen, A. M. P; Pursiainen, J. J. Organomet. Chem. 2000, 598, 235. (c) Betz, J.; Bauer, W. J. Am. Chem. Soc. 2002, 124, 8699. (11) (a) Dani, P.; van Klink, G. P. M.; van Koten, G.; Europ., J. Inorg. Chem. 2000, 1465. (b) Mooibroek, T. J.; Lutz, M.; Spek, A. L.; Bouwman, E. Dalton Trans. 2010, 39, 11027. (12) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. 2007, 104, 6908. (13) (a) Meinhard, D.; Wegner, M.; Kipiani, G.; Hearley, A.; Reuter, P.; Fischer, S.; Marti, O.; Rieger, B. J. Am. Chem. Soc. 2007, 129, 9182. (b) Perrotin, P.; McCahill, J. S. J.; Wu, G.; Scott, S. L. Chem. Commun. 2011, 47 (24), 6948. (14) Abel, E. W.; Dormer, J. C.; Ellis, D.; Orrell, K. G.; Sik, V.; Hursthouse, M. B.; Mazid, M. A. J. Chem. Soc., Dalton Trans. 1992, 1073. (15) (a) Dyer, G.; Meek, D. W. Inorg. Chem. 1965, 4, 1398. (b) Hirsivaara, L.; Haukka, M.; Jaaskelainen, S.; Laitinen, R. H.; Niskanen, E.; Pakkanen, T. A.; Pursiainen, J. J. Organomet. Chem. 1999, 579, 45. (c) Meek, D. W.; Ibers, J. A. Inorg. Chem. 1969, 8, 1915. (16) Kanazawa M., Ito S., Nozaki K. Organometallics 2011, 30, 6049. (17) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (b) Guan, Z. B.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059. (18) (a) Keim, W.; Appel, R.; Storeck, A.; Kruger, C.; Goddard, R. Angew. Chem., Int. Ed. 1981, 20, 116. (b) Mohring, V. M.; Fink, G. Angew. Chem., Int. Ed. 1985, 24, 1001. (19) Rachita, M. J.; Huff, R. L.; Bennett, J. L.; Brookhart, M. J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 4627. (20) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1989, 8, 2907.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ACKNOWLEDGMENTS We thank S. Kaviani-Nejad and Prof. Dr. P. Köhler of the Deutsche Forschungsanstalt für Lebensmittelchemie for measurements of high-resolution ESI-MS spectra. Assistance of PD Dr. W. Eisenreich with NMR spectroscopy as well as helpful discussions with Dr. S. I. Vagin are highly appreciated.

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Structural Modification of Functionalized Phosphine Sulfonate-Based

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