Cyclometalated PhosphinineâIridium(III) Complexes: Synthesis

Article pubs.acs.org/Organometallics

Cyclometalated Phosphinine−Iridium(III) Complexes: Synthesis, Reactivity, and Application as Phosphorus-Containing WaterOxidation Catalysts Leen E. E. Broeckx,† Alberto Bucci,‡ Cristiano Zuccaccia,‡ Martin Lutz,§ Alceo Macchioni,*,‡ and Christian Müller*,†,∥ †

Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands Department of Chemistry, Biology and Biotechnology and CIRCC, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy § Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands ∥ Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, D-14195 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: The novel phosphinine-based coordination compound [Cp*Ir(P∧C)(CH3CN)]CF3SO3 (P∧C = cyclometalated 2,4,6-triphenylphosphinine) could be synthesized by chloride abstraction from [Cp*Ir(P∧C)Cl] with AgOSO2CF3 and crystallographically characterized. It turned out that this species is the first phosphorus-containing Ir(III) complex which shows a remarkable activity in the cerium ammonium nitrate driven water oxidation reaction. In situ NMR spectroscopic investigations further reveal that water is added selectively to one of the PC double bonds with formation of four stereoisomers. Moreover, [Cp*Ir(P∧C)] species, possibly OH-functionalized but still having Cp* and P∧Cligands contemporary bound to iridium, are present in solution, even under catalytic conditions.

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INTRODUCTION

Pyridines and pyridine derivatives are one of the most important classes of ancillary ligands in organometallic chemistry.1 The replacement of a σ-donating pyridine by a more π-accepting λ3-phosphinine entity significantly modifies the properties of the ligand due to the electronic differences that exist between these heterocycles.2 More specifically, transition-metal complexes bearing phosphinines are often reactive toward nucleophiles. This occurs especially for coordination compounds with metal centers in higher oxidation states and low-substituted phosphinines.3 In these cases, traces of water or alcohols rapidly undergo 1,2-addition at the PC double bond of the coordinated phosphinine, destroying the aromaticity of the ligand with formation of a 1,2-dihydrophosphinine, as originally reported by Mathey and Venanzi et al. in 1991.4 The extreme sensitivity of these complexes toward nucleophilic attack has hampered their straightforward synthesis, handling, and characterization for a long time and made any potential application rather unattractive. We could recently achieve for the first time the facile synthesis and crystallographic characterization of hitherto unknown neutral phosphinine−M(II) (M = Pd, Pt) and cationic phosphinine−M(III) complexes (M = Rh, Ir) containing the chelating hybrid ligand 2-(2′-pyridyl)-4,6diphenylphosphinine (Figure 1, type I).5 Our strategy relied © XXXX American Chemical Society

Figure 1.

on a well-known procedure to stabilize PC moieties by increasing the steric bulk at the adjacent Cα atom to prevent nucleophile 1,2-addition at the double bond. Despite the fact that a tremendously increased kinetic stability toward nucleophiles was achieved in comparison to the few reported Received: April 2, 2015

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DOI: 10.1021/acs.organomet.5b00281 Organometallics XXXX, XXX, XXX−XXX

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Organometallics examples in the literature, we found that [Cp*Rh(P∧N)Cl]Cl and [Cp*Ir(P∧N)Cl]Cl (P∧N = 2-(2′-pyridyl)-4,6-diphenylphosphinine) do react with water at room temperature. Much to our surprise, however, the water addition proceeds both regio- and diastereoselectively at the external PC double bond, leading exclusively to the anti-addition products [Cp*MCl((P∧N)H·OH)]Cl (M = Rh, Ir), rather than to a product mixture (Figure 1, type II).6 Because 2-(2′-pyridyl)-4,6-diphenylphosphinine (P∧N) is isoelectronic with a formally C−H activated, anionic 2,4,6triphenylphosphinine ligand (P∧C), it was of interest to investigate the cyclometalation of 2,4,6-triphenylphosphinine as a phosphorus analogue of 2-phenylpyridine (Figure 1, types III and IV). Interestingly, we could show that the cyclometalation of 2,4,6-triphenylphosphinine with [Cp*IrCl2]2 and [Cp*RhCl2]2 as metal precursors in the presence of NaOAc as a base is indeed possible and even Ir(III) complexes containing three cyclometalated 2,4,6-triphenylphosphinines are now accessible.5c,7 This new reactivity pattern of 2-aryl-phosphinines toward the formation of novel and stable phosphinine-based metallacycles inspired us to investigate such systems in more detail and to explore new potential applications of these fascinating aromatic phosphorus heterocycles. As a matter of fact, cyclometalated coordination compounds often display unique chemical and photophysical properties, finding widespread application in several research fields, from homogeneous catalysis (e.g., Pd(II) in Suzuki coupling reaction, Hf(IV) in olefin polymerization) to optical devices (e.g., Ir(III)).8 Among catalytic applications, water oxidation surely represents a very interesting field, since the oxidation of water to molecular oxygen is broadly recognized as the slower step for the realization of an efficient artificial photosynthetic apparatus aimed at the production of alternative, renewable, and cost-attractive fuels.9−14 The development of efficient water oxidation catalysts (WOCs) is thus a fundamental priority, and several research groups are currently involved in this area.15 Recently, iridium d6-metal complexes emerged as highly active WOCs15−33 and a few of them have been successfully heterogenized on functional materials.34 In particular, complexes featuring cyclometalated 2-phenylpyridines (Figure 1, type IV) or similar derivatives have found to be highly active, in terms of both turnover frequency (TOF) and turnover number (TON). On the other hand, the nature of the ancillary ligands L in these complexes plays a critical role in modulating the catalytic performance35 and also in determining the nature of the active species, altering e.g. the tendency of the Cp* ligand to undergo oxidative transformations.20,36 Interestingly, Ir complexes featuring iridium−phosphorus bond(s) have not been previously investigated as potential WOCs, most likely due to the fact that P(III)-containing species are prone to oxidation reactions. However, considering the strict structural analogy of the cyclometalated-phosphinine−iridium complex [(Cp*Ir(P∧C)Cl] (Cp* = pentamethylcyclopentadienyl; Figure 1, type III) with respect to [Cp*Ir(2-phpy)Cl] reported by Crabtree,17 as well as to [Cp*Ir(bzpy)NO3] reported in our previous work,18 it was of interest to explore their catalytic behavior as a potential phosphorus-containing WOC. In this respect it should be pointed out that phosphinines are difficult to oxidize due to the low nucleophilicity of the formally sp2hybridized phosphorus atom in contrast to classical phosphorus(III) compounds. Thus, phosphinine oxides have so far been neither isolated nor properly characterized.37 Herein we show for the first time that an iridium catalyst,

coordinated to a phosphorus-containing ligand, is able to promote the oxidative splitting of water to molecular oxygen, paving the way to the development of a new class of catalysts.

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RESULTS AND DISCUSSION Synthesis and Characterization. Synthesis of [(Cp*Ir(P ∧ C)Cl] (1). As recently demonstrated by us, 2,4,6triphenylphosphinine readily undergoes C−H activation with [Cp*IrCl2]2 as metal precursor and in the presence of NaOAc, which assists as an intramolecular base in the ortho-metalation reaction. The final product [(Cp*Ir(P∧C)Cl] can be obtained as yellow crystals in high yield after recrystallization from CH2Cl2/Et2O.7 Synthesis and Crystallographic Characterization of the Cationic Complex [Cp*Ir(P∧C)(NCCH3)]CF3SO3 (2). We were further interested whether 1 could be converted into the corresponding cationic complex of the type [Cp*Ir(P∧C)(L)]X (X = counterion) by chloride abstraction. A first attempt was made by using NaB(3,5-(CF3)2C6H3)4 (NaBArF), which is known to abstract Cl− from a variety of Ir(III) complexes. Addition of 1 equiv of NaBArF to a solution of 1 in a mixture of CH2Cl2/CH3CN resulted in the formation of a new species featuring a signal at δP 155.2 ppm in the 31P{1H} NMR spectrum. However, only 50% conversion of 1 could be observed after 4 days at room temperature and the product could not be isolated in pure form. A second attempt was made using silver nitrate (AgNO3), as silver salts are generally stronger abstraction agents in comparison to sodium salts. Unfortunately, upon addition of up to 2.8 equiv of AgNO3 to a solution of 1 in CD2Cl2/CH3CN at room temperature, several species could be detected by 31P{1H} NMR spectroscopy. Heating the reaction mixture to T = 40 °C did not result in the formation of only one product, as judged again from the 31 1 P{ H} NMR spectrum. Finally, silver triflate (AgOSO2CF3) was added to a solution of 1 in CH2Cl2/CH3CN and the mixture was heated to T = 40 °C for a few hours. This time a single species was observed by means of 31P{1H} NMR spectroscopy, which we attributed to the presence of the Scheme 1. Synthesis of Cationic Phosphinine-Based Complex 2

complex [Cp*Ir(P∧C)(NCCH3)][OTf] (2; Scheme 1). The 1 H NMR data of complex 2 in solution indicate that both a CH3CN and a phosphinine ligand are bound to the same Cp*Ir metal fragment. In particular a single resonance at δP 155.5 ppm is observed in the 31P{1H} NMR spectrum, while the presence of two characteristic doublets of doublets for the exocyclic protons H5 (δH 8.18 ppm, 1H, dd, 3JH,P = 23.3 Hz, 4JH,H = 1.5 Hz) and H3 (δH 8.48 ppm, 1H, dd, 3JH,P = 23.5 Hz, 4JH,H = 1.5 Hz) confirms the presence of an intact phosphinine core (Figure S1, Supporting Information). The resonances of both CH3CN (δH 2.60 ppm, 5JH,P = 1.2 Hz) and Cp* (δH 1.66 ppm, 4 JH,P = 3.3 Hz) appear as doublets due to scalar coupling with the phosphorus atom of the phosphinine ligand. B


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Organometallics

coplanar. The phosphorus−carbon and carbon−carbon bond lengths of the phosphinine core in 2 do not change considerably on passing from 1 to 2. Moreover, the metal center is not located in the ideal axes of the phosphorus lone pair and shifted toward the formally anionic carbon atom, as observed in related transition-metal complexes based on chelating phosphinines. This phenomenon can be attributed to a diffuse and less directional lone pair in phosphinines. The internal ∠C(1)−P(1)−C(5) angle of the phosphinine core in complex 2 is slightly larger in comparison to that in the neutral [Cp*Ir(P∧C)Cl] complex 1, suggesting a higher reactivity of this compound toward nucleophilic addition at the PC double bond. This phenomenon is due to a higher disruption of aromaticity and has been documented before.3g Synthesis and Crystallographic Characterization of the Cationic Dimeric Complex [[Cp*Ir(P∧C)(NCCH3)]CF3SO3· AgCF3SO3]2 (3). Addition of 2 equiv of silver triflate to 1 equiv of 1 resulted in the quantitative formation of a new species. This compound exhibits similar, albeit slightly different, chemical shift values with respect to 2, in both the 1H and 31 1 P{ H} NMR spectra. For example, the 31P{1H} resonance appears at δP 153.5 ppm, i.e. at 2 ppm lower frequency with respect to 2 (δP 155.5 ppm), while the methyl groups of the Cp* ligand were detected at the same chemical shift of δH 1.62 ppm. In analogy to 2, the resonance of the coordinated acetonitrile ligand is observed at δH 2.61 ppm in the 1H NMR spectrum, with a 5JH,P value of 1.2 Hz. By careful filtration of the reaction mixture over silica and Celite, using CH3CN as solvent, the silver chloride could be separated from the formed complex. Subsequent crystallization via slow diffusion of Et2O into a concentrated CH2Cl2 solution resulted in the formation of single crystals suitable for X-ray diffraction. The molecular structure of the new compound 3 is depicted in Figure 3, and selected bond lengths (Å) and angles (deg) are given in Table 1 and compared with those of 1 and 2.

Yellow single crystals of 2 suitable for X-ray diffraction were obtained by slow diffusion of Et2O into a saturated CH2Cl2 solution. Compound 2 crystallizes in the orthorhombic space group Pbca (No. 61), and the molecular structure in the crystal is depicted in Figure 2. Selected bond lengths (Å) and angles (deg) of 2 have been compared with those of the neutral starting complex [Cp*Ir(P∧C)Cl] (1) and are given in Table 1.

Figure 2. Molecular structure of 2 in the crystal. Displacement ellipsoids are shown at the 50% probability level.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Coordination Compounds 1−3 17 Ir(1)−Cp*(cent) Ir(1)−N(1) Ir(1)−P(1) Ir(1)−C(11) Ag(1)−C(9−10) Ag(1)−C(22) Ag(1)−O(1)a Ag(1)−O(4) P(1)−C(1) P(1)−C(5) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(11)−Ir(1)−P(1) C(1)−P(1)−C(5) N(1)−C(34)−C(35) P(1)−C(5)−C(6)−C(11) a

1.861(2) 2.2396(12) 2.076(5)

1.724(5) 1.724(4) 1.411(6) 1.389(6) 1.406(6) 1.391(6) 1.476(6) 78.72(14) 105.7(2) 12.2(5)

2 1.8582(11) 2.0320(19) 2.2370(6) 2.098(2)

1.712(2) 1.717(2) 1.399(3) 1.396(3) 1.398(3) 1.391(3) 1.474(3) 78.30(6) 106.80(11) 179.0(3) 4.2(3)

3 1.855(2) 2.047(4) 2.2461(11) 2.079(4) 2.365(4) 2.530(5) 2.335(14)/ 2.563(12) 2.299(4) 1.715(4) 1.717(4) 1.402(6) 1.401(6) 1.405(6) 1.391(6) 1.469(6) 78.31(12) 106.4(2) 178.4(6) 3.7(5)

Disordered triflate ligand.

As shown in Figure 2, the molecular structure of the cationic cyclometalated phosphinine−Ir(III) complex 2 displays a characteristic three-legged “‘piano-stool”’ arrangement around the metal center similar to that of its neutral counterpart [Cp*Ir(P∧C)Cl] (1). The overall bond lengths (Å) and angles (deg) are rather similar (see Table 1), although the torsion angle between the cyclometalated aryl group and the phosphinine heterocycles is at 4.2(3)° smaller than those observed for the reference complex 1 (12.2(5)°). Nevertheless, both six-membered rings are in general still considered to be

Figure 3. Molecular structure of 3 in the crystal. Displacement ellipsoids are shown at the 50% probability level. Only the major conformation of the disordered triflate ligands is shown. Hydrogen atoms and severely disordered solvent molecules (dichloromethane and pentane) have been omitted for clarity. Symmetry code i: 1 − x, 1 − y, 1 − z. C


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Organometallics Scheme 2. Synthesis of Ag-Bridged Dimeric Species 3

Interestingly, it turned out that compound 3 is a centrosymmetric dinuclear species, in which two cationic complexes 2 are bridged by the remaining two Ag(I) cations (Scheme 2). The Ag ion shows a η2 coordination to C9 and C10 of the phenyl ring with Ag−C distances of 2.448(5) and 2.486(5) Å. The triflate anions in 3 are severely disordered in the crystal. Nevertheless, the silver−oxygen coordination bond distances in 3 are in the correct range for a silver−oxygen single bond (2.33−2.41 Å). It should be pointed out, however, that it is not clear whether these complexes are different in solution or only in the solid state, due to the striking similarity of complexes 2 and 3 on the basis of NMR spectroscopy as well as single-crystal X-ray structure analysis. Catalytic Activity of 2 in the Water Oxidation Reaction. As mentioned in the Introduction, [Cp*Ir(2phpy)Cl] is an excellent catalysts in the CAN (CAN = cerium ammonium nitrate) driven water oxidation reaction (eq 1).17 4Ce 4 + + 2H 2O → 4Ce3 + + 4H+ + O2

Figure 4. TON versus t trends for catalytic experiments with different amounts (%V) of acetonitrile ([2] = 5 μM and [CAN] = 10 mM).

formation of any precipitate. All other catalytic tests (using different detection methods) were then carried out using these optimized conditions (Table 3).

(1)

The striking analogy of complexes 1 and 2 with this system encouraged us to study their catalytic behavior. Unfortunately, the introduction of three aromatic rings to sterically protect the PC double bond also renders insoluble both the neutral complex 1 and the cationic complex 2 in water. Since addition of a small amount of acetonitrile resulted in an enhanced solubility of 2, we decided to focus on the catalytic tests with compound 2. Considering that acetonitrile is a rather coordinating solvent and usually decreases the catalytic performance, we initially set out a series of experiments in order to determine the minimum amount of acetonitrile necessary to maintain the catalytic system completely homogeneous without slowing down the catalytic reaction too much. The results of this preliminary screening are reported in Table 2 and graphically presented in Figure 4. As expected, the catalytic activity of 2 is indeed substantially reduced on increasing the amount of acetonitrile. We finally found out that using 4% acetonitrile in volume is the optimum compromise to retain a good catalytic activity while avoiding

Table 3. Summary of Results for Catalytic Water Oxidation Driven by CAN TOF (min−1)a entry

technique

[2] (μM)

[CAN] (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

manometry manometry manometry manometry oximetry oximetry oximetry oximetry oximetry UV−vis UV−vis UV−vis UV−vis UV−vis UV−vis UV−vis UV−vis UV−vis UV−vis

5 5 2.5 2.5 5 5 2.5 2.5 1 5 5 4 3 3 2 2 1 2.5 5

5 10 5 10 10 10 10 10 10 1 1 1 1 1 1 1 1 10 10

Table 2. Manometric Results Obtained on Changing the Percentage of Acetonitrile [2] (μM)

[CAN] (mM)

MeCN (%)

TON (cycles)

TOF (min‑1)

5 5 5 5

10 10 10 10

1 2 4 8

250 250 250 250

20.4 18.6 5.2 2.8

TOFI

TOFLT

TON (cycles)

3.6 4.8 5.6 4.3

250 500 500 1000

5.4 4.1 5.3 4.5 5.9 4.1 5.7 5.4 3 6

50 50 62.5 83 83 125 125 250 1000 500

12.6 12.1 16.5 15.8 17.5

18.5 14.6

a

Since a discontinuity is usually observed in kinetic trends, initial and long-term turnover frequencies, TOFI and TOFLT, respectively, have been separately evaluated. D


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Organometallics

[Cp*Ir(bzpy)NO3] shows average TOF values ranging from 13 to 8 min−1 in the absence of acetonitrile. Reactivity of Coordination Compounds 1 and 2 with H2O. To get a deeper insight into the behavior of 2 during catalysis, we performed a series of in situ NMR studies on the reaction of 2 with water and CAN. For comparison reasons we also investigated the reactivity of coordination compound 1. [Cp*Ir(P∧C)Cl] (1). The reactivity of the cyclometalated Ir(III) complex 1 toward H2O was investigated first by in situ 31 1 P{ H} NMR spectroscopy. The addition of a small amount of water to a solution of 1 in CD2Cl2 at T = 298 K resulted in the gradual consumption of 1 (δP 170.8 ppm) and formation of a new species (1A), which displays a 31P chemical shift at δP 98.7 ppm (Figure 6). The conversion of 1 reaches 50% after 15 h

The results of the manometric experiments (selected trends of oxygen evolution vs t are depicted in Figure S2 of the Supporting Information) showed a long-term activity (TOFLT) of about 4−5 min−1 (Table 3, entries 1−4). CAN was completely consumed in all experiments, thus indicating that catalyst 2 is capable of completing at least 1000 cycles (Table 3, entry 4). However, it is known that the initial rate of oxygen evolution cannot be accurately measured by means of manometry, due to a rather slow instrument response. For this reason, the initial TOF (TOFI) was evaluated by means of direct measurement of dissolved oxygen by means of a Clark electrode. On the other hand, Clark electrode measurements are known to afford reliable results on initial formation of oxygen, whereas subsequent oxygen evolution may be not quantitatively detected due to the formation of microbubbles. In some sense the two detection methods are complementary. The TOFI values obtained using a Clark electrode are given in Table 3. Most strikingly, these values range from ∼12 to ∼18 min−1, which is rather remarkable and comparable with that of other iridium catalysts reported so far in the literature.15−33 Finally, the catalytic performance of 2 was tested following the CAN depletion by means of UV−vis spectroscopy. The results are collected in Table 3, and selected trends of the absorbance vs t are shown in Figure 5. Kinetics do not show a

Figure 6. Time-dependent 31P{1H} NMR spectra for the addition of water to complex 1 at room temperature with formation of 1A. Conditions: CH2Cl2, 0.6 mL; 1, 48 mM; H2O, 923 mM.

and is practically complete after 70 h. However, we noticed that small quantities of other, still unidentified, species were formed during the course of the reaction, which feature signals in the 31 1 P{ H} NMR in the range 70−85 ppm. The 31P NMR chemical shift difference between the starting complex 1 and the new phosphorus-containing species 1A formed upon water addition is on the order of ΔδP ≈ −100 ppm, which is very similar to the value observed for the quantitative conversion of [Cp*Ir(P∧N)Cl]Cl to compound II (Figure 1).6 Moreover, the two characteristic resonances in the 1 H NMR spectrum for protons H6 and H5 of 1A can be observed at δH 4.91 ppm (dd, 2JH,P = 21.2 Hz, 3JH,H = 2.8 Hz) and δH 6.20 ppm (ddd, 3JH,P = 21.6 Hz, 3JH,H = 3.2 Hz, 4JH,H = 1.6 Hz), respectively. This strongly suggests the main formation of a single H2O-addition product, which indicates at the same time that the addition of H2O to the PC double proceeds selectively to the external PC double bond, in strict analogy with the formation of II (Figure 1).6 Unfortunately, all attempts to obtain single crystals suitable for X-ray diffraction failed up to this point and the exact regio- and stereochemistry of 1A thus remains to be clarified (Scheme 3). Interestingly, the reaction of 1 with H2O under the same experimental conditions (CD2Cl2, room temperature) is much slower in comparison to the reaction of the corresponding cationic complex [Cp*Ir(P∧N)Cl]Cl with H2O, which features the neutral P,N-hybrid ligand.6 [Cp*Ir(P∧C)(NCCH3)]CF3SO3 (2). In order to better mimic the catalytic conditions, the reaction of 2 with water was initially investigated under acidic conditions. The addition of 0.4 mL of acidic D2O (pH 1 by DNO3) to a dilute solution of 2

Figure 5. Kinetics of CAN depletion followed by means of UV−vis spectroscopy.

linear trend for the whole catalytic run: while at the beginning the catalytic activity is high (TOFI; Table 3, entries 18 and 19), a substantial drop in the performance of 2 is observed after a few minutes, resulting in TOFLT values of about 4−5 min−1 independently of the catalyst concentration (Table 3, entries 10−17), thus suggesting a first-order dependence of the longterm rate constant on catalyst concentration (Figure S3, Supporting Information). These values are nicely consistent with those determined by Clark electrode and manometry, respectively. Such catalytic behavior, i.e. rather different TOFI and TOFLT values, has been noticed before for other catalytic systems. For instance, the benchmark system [Cp*Ir(2phpy)Cl], reported by Crabtree et al., is able to promote the water-splitting reaction with TOFI = 10 min−1 without any induction period. However, the activity drops to 6 min−1 during the course of the reaction.17 The catalytic activity of 2 is slightly better than that of [Cp*Ir(2-phpy)Cl] in terms of TOFI and slightly lower in term of TOFLT. However, it is important to emphasize that the activity of 2 has been evaluated on adding some acetonitrile for solubility reasons that has a detrimental effect. Despite this, compound 2 exhibits an activity comparable to that of [Cp*Ir(bzpy)NO3].18 As a matter of fact, E


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Organometallics Scheme 3. Addition of Water to Coordination Compound 1 under Formation of 1A

in CD3CN (0.3 mL) at room temperature led to the disappearance of 2 in about 30 min with concomitant formation of several new species (Figures S4 and S5, Supporting Information). All of them display 31P{1H} NMR resonances in the region between δp 100 and 60 ppm as well as doublets for the Me groups of the Cp* ring in the region between δH 2.2 and 1.6 ppm in the 1H NMR spectrum. Subsequently, the initial complex reaction mixture slowly transformed and only four main species (2A−D) were observed after 46 h (Figures S4 and S5, Supporting Information). We observed 31P{1H} NMR signals for 2D, 2B, 2A, and 2C at δP 90.3, 86.7, 80.8, and 75.2 ppm, respectively, with relative abundances of 27% (2A), 10% (2B), 59% (2C), and 4% (2D). The reaction of 2 with a small excess of water in CD3CN was attempted in order to better follow the formation of 2A−D and obtain some structural information. Unfortunately, the reaction was very slow, most likely due to the coordination of CD3CN to the metal center. For this reason we decided to investigate the reaction of 2 with water in CD3COCD3 (see the Experimental Section for the assignment of relevant NMR signals). Addition of 125 equiv of H2O to a solution of 2 in CD3COCD3 resulted in the formation of several species already after 10 min, while the starting material 2 was fully consumed during the course of several hours. All of the new species present in solution after 18 h display 31P{1H} NMR resonances in the region between δP 110 and 60 ppm, showing long-range correlations with 1HNMR doublets (below 2 ppm) in the 1H−31P HMBC NMR spectrum. Although a number of species are present, four of them show 31P{1H} NMR chemical shifts very similar to those of 2A−D observed in CD3CN and account for about 75% of the total.38 The 1H−31P HMBC NMR spectrum confirms that Cp*, CH3CN, and a modified phosphorus ligand are coordinated to the metal center in complexes 2A−C (Figure 7). In particular, the modified phosphorus ligand features two typical 1H NMR signals in the regions between δH 4−5 ppm and 6−7 ppm that show scalar coupling with the corresponding 31P NMR resonances (Figure 7). In analogy with the results discussed above for 1, which undergoes regio- and stereoselective water addition exclusively at one PC double bond of the phosphinine as shown in Scheme 3, the two resonances mentioned above are assigned to protons H6 and H5, respectively. Consequently, we propose that species 2A−C are three of the four possible isomers that can form upon the addition of H2O to the PC6 double bond of 2, which can occur in a syn or anti fashion and through the Re or the Si face of the PC6 double bond (Scheme 4). The NMR parameters of 2D in CD3COCD3 are, however, different from those of 2A−C. In particular, (i) the 31P{1H} NMR signal (δP 87 ppm) is rather broad at room temperature, (ii) the 1H−31P HMBC NMR spectrum confirms that both a Cp* (δH 1.61 ppm, 4JPH = 2.7 Hz) and a modified phosphorus

Figure 7. Two sections of the 1H−31P HMBC NMR spectrum obtained after 18 h of the addition of 125 equiv of water to a solution of 2 in CD3COCD3.

Scheme 4. Reaction of 2 with Water

ligand are coordinated to the metal center, whereas no indication of acetonitrile coordination at the metal center is obtained, and (iii) two doublets appear at δH 6.21 ppm (nJPH = 65 Hz) and δH 5.22 ppm (nJPH = 2.2 Hz) in the 1H NMR spectrum. It is difficult to rationalize these data and propose a structure for 2D. However, it is interesting to note that the values of JPH of the two doublets at δH 6.21 and 5.22 ppm are significantly different from those of 2A−C. Particularly, JPH = 65 Hz for the doublet at δH 6.21 is too large for nJPH with n ≥ 2 and may suggest the formation of a P−H moiety. However, the 1 JPH values are usually much larger. Possible explanations of such a reduced value of the 1JPH might derive from the presence of a water molecule coordinated at iridium undergoing hydrogen bonding with P−H or an agostic P−H/Ir interaction. These hypotheses are also consistent with the lack of CH3CN coordination in 2D and broad resonances of both Cp* and phosphorus. Clearly, further studies are necessary to clarify the exact structure of 2D. Reaction of 2A−D with CAN. To further explore the behavior of 2 under conditions as close as possible to those for catalysis, the reaction of 2 with CAN was investigated by in situ NMR spectroscopy. Addition of solid CAN (about 16 equiv) to a solution containing 2A−D in CD3CN/D2O (pH 1 by DNO3) resulted in their complete disappearance, formation of acetic acid (8% with respect to starting complex 2), and the appearance of three new organometallic species featuring 31 1 P{ H} NMR resonances at δP 151.6, 150.0, and 135.6 ppm. These high chemical shift values are very similar to those of the initial aromatic phosphinine ligand, suggesting that a rearomatization of the phosphinine ligand occurred. It appears that water reversibly adds to the PC double bond and, consequently, ligand/metal cooperation might play a role in F


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Organometallics

water elimination. It might be supposed that the reversible addition of water at the PC double bond might play a role in the water oxidation process, thus indicating ligand/metal cooperation, but at the present, this is just speculation. Since we could detect scalar correlation between the phosphorus and the Cp* signals by 1H−31P HMBC NMR spectroscopy, we conclude that [Cp*Ir(P∧C)] species, possibly OH-functionalized but still having Cp* and P∧C-ligand comtemporary bound to iridium, are present in solution, even under catalytic conditions. Our studies reported here describe the first phosphorus-containing Ir(III) complex, which is remarkably active in the CAN-driven water oxidation reaction. These findings significantly enlarge the scope of phosphinine-based coordination compounds in more applied research fields.

the oxidative splitting of water. In any case, the most important aspect for our purposes is to demonstrate that these new species still contain the phosphinine ligand and the Cp* moiety on the same metal center. Since we could detect scalar correlation between the phosphorus signals and the Cp* signals at δH ∼2.2 ppm in the 1H−31P HMBC NMR spectrum (Figure 8), we conclude that a relevant amount of species bearing an intact [Cp*Ir(P∧C)] moiety is present in solution even under catalytic conditions.

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EXPERIMENTAL SECTION

All experiments were performed under an inert argon atmosphere, using standard Schlenk techniques or in an MBraun drybox, unless stated otherwise. All glassware was dried prior to use by heating under vacuum to remove traces of water. [Cp*Ir(P∧C)Cl]7 was prepared according to the literature procedure. The solvents were dried and deoxygenated using custom-made solvent purification columns filled with Al2O3. The 1H, 13C{1H}, 19F{1H}, and 31P{1H} NMR spectra were recorded on a Varian Mercury 400 MHz spectrometer, and chemical shifts are reported relative to the residual proton resonance of the deuterated solvents. Elemental analyses were performed by H. Kolbe, Mikroanalytisches Laboratorium, Mülheim a.d. Ruhr, Germany. 1D- and 2D-NMR spectra were measured on a Bruker Avance III HD 400 spectrometer equipped with a smartprobe, using standard Bruker pulse sequences. Referencing is relative to external TMS (1H and 13C), CCl3F (19F), and 85% H3PO4 (31P). Chemical shifts (δ) are quoted in ppm and scalar coupling constants (nJXY) in Hz. Synthesis and Characterization of New Complexes. racAcetonitrile(η5-pentamethylcyclopentadienyl)((2-phenylene)-κC24,6-diphenylphosphinine-κP)iridiumIII (2). A mixture of [Cp*Ir(P∧C)Cl] (1; 90.2 mg, 0.131 mmol, 1.0 equiv) and AgOTf (35.2 mg, 0.137 mmol, 1.0 equiv) was suspended in CH2Cl2 (1.5 mL) and transferred to a Schlenk tube in a drybox. Subsequently, 1.5 mL of CH3CN was added and the orange reaction mixture was heated to T = 40 °C for 3 h, after which complete conversion was apparent from the 31 1 P{ H} NMR spectrum. The silver salt was removed by filtration of the yellow-orange reaction mixture over silica and Celite and elution with CH3CN, which was subsequently removed in vacuo. Yellow crystals were obtained by slow diffusion of diethyl ether into a mixture of the compound in CH2Cl2. Yield: 108.8 mg, 98.4%. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 1.66 (15H, d, 4JH,P = 3.2 Hz, C5Me5), 2.65 (3H, d, 5JH,P = 1.2 Hz, CH3CN), 7.17−7.29 (2H, m, Harom), 7.45−7.51 (1H, m, Harom), 7.52−7.75 (10H, m, Harom), 7.86−7.91 (1H, m, Harom), 8.18 (1H, dd, 3JH,P = 23.3 Hz, 4JH,H = 1.5 Hz, H5), 8.48 (1H, dd, 3JH,P = 23.5 Hz, 4JH,H = 1.5 Hz, H3). 13C NMR (100 MHz, CD2Cl2): δ 4.7 (CH3CN), 9.3 (C5Me5), 98.4 (d, 2JC,P = 2.3 Hz, C5Me5), 119.9 (br d, 2JC,P = 1.1 Hz, CH3CN), 121.6 (d, 2JC,P = 19.5 Hz, CH), 125.3 (CH), 128.1 (d, 4JC,P = 3.3 Hz, CH), 128.3 (d, 3JC,P = 11.2 Hz, CH), 128.8 (CH), 129.3 (d, 3JC,P = 10.5 Hz, CH), 129.6 (CH), 129.8 (d, 4JC,P = 1.9 Hz, CH), 130.3 (CH), 130.8 (d, 4JC,P = 2.0 Hz, CH), 136.1 (d, 3JC,P = 11.5 Hz, CH), 139.2 (d, 3JC,P = 10.7 Hz, Cquat), 139.7 (CH), 141.9 (d, 3JC,P = 5.8 Hz, C4), 142.6 (d, 2JC,P = 27.1 Hz, 2/6-Ar(1′-C)), 144.6 (d, 2JC,P = 29.9 Hz, 2/6-Ar(1′-C)), 148.1 (d, JC,P = 1.9 Hz, Cquat), 155.4 (d, 1JC,P = 30.9 Hz, C2/6), 167.6 (d, 1J(C,P) = 43.6 Hz, C2/6). 19F NMR (376 MHz, CD2Cl2, 298 K): δ −78.9. 31P NMR (162 MHz, CD2Cl2, 298 K): δ 155.5. Relevant NMR data of 2 in other solvents: 1H NMR (400 MHz, CD3COCD3, 298 K): δ 1.72 (15H, d, 4JP,H = 3.3, C5Me5); 2.79 (3H, d, 5JP,H = 1.3 Hz, CH3CN); 8.29 (1H, dd, 3JP,H = 23.2 Hz, 4JH,H = 1.6 Hz, H5); 8.72 (1H, dd, 3JP,H = 23.5 Hz, 4JHH = 1.6 Hz, H3). 31P{1H} NMR (400 MHz, CD3COCD3, 298 K): δ 155.4 (s). 1H NMR (400 MHz, CD3CN, 298 K): δ 1.61 (15H, d, 4JP,H = 3.3, C5Me5); 8.24 (1H, dd, 3JP,H = 23.3 Hz, 4JH,H = 1.6 Hz, H5); 8.59 (1H, dd, 3JP,H = 23.6 Hz, 4JHH = 1.6 Hz,

Figure 8. Section of the 1H−31P HMBC NMR spectrum recorded about 60 min after the addition of solid CAN (about 16 equiv) to the solution of 2A−D in CD3CN/acidic D2O.

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CONCLUSIONS In summary, we have synthesized and crystallographically characterized the novel, cationic phosphinine-based Ir(III) complex [Cp*Ir(P∧C)(CH3CN)]CF3SO3 by reaction of [Cp*Ir(P∧C)Cl] with AgCF3SO3 in acetonitrile. With excess silver triflate used to generate the cationic species, the formation of a silver-bridged, dimeric coordination compound can be observed. The striking analogy of these Ir(III) complexes with previously reported phosphorus-free systems inspired us to study their catalytic behavior in the cerium ammonium nitrate (CAN) driven water oxidation reaction. Surprisingly, it turned out that remarkable TOFI values of 12− 18 min−1 were obtained, which are comparable with those of the iridium catalysts reported so far in the literature. In order to get a deeper insight into the behavior of the phosphinine-based complexes during catalysis, in situ NMR studies on their reaction with water and CAN were performed. When used in a small amount in CD2Cl2, water selectively adds to the external PC double bond of 1, leading to the formation of a single species bearing a P(OH)−CH moiety. Under conditions similar to those used in catalysis, the formation of four main species can be observed, which were attributed to isomers generated by the syn or anti addition of water to one of the two PC double bonds of the phosphinines ring (2A−C, likely bearing the P(OH)−CH moiety; 2D, reasonably derived from water addition with the opposite regiochemistry, leading to the PH−C(OH) moiety). Interestingly, addition of CAN seems to cause rearomatization of the phosphinine ring by means of G


Article

Organometallics H3). 31P{1H} NMR (400 MHz, in CD3CN, 298 K): δ 157.2 (s). Anal. Calcd for C36H34F3IrNO3PS (840.91 g/mol): C, 51.42; H, 4.08; N, 1.67. Found: C, 51.41; H, 4.11; N, 1.65. [rac-Acetonitrile(η5-pentamethylcyclopentadienyl)((2-phenylene)-κC2-4,6-diphenylphosphinine-κP)iridium(III) silver(I) (bistriflate)] Dimer (3). A mixture of [Cp*Ir(P∧C)Cl] (1; 30 mg, 0.044 mmol, 1.0 equiv) and AgOTf (30.8 mg, 0.120 mmol, 2.7 equiv) was suspended in CH2Cl2 (0.6 mL) and transferred to a Young NMR tube in a drybox. Subsequently, 0.6 mL of CH3CN was added and complete conversion was apparent from the 31P{1H} NMR spectrum after 3 h at room temperature. The silver salt was removed by filtration of the yellow-orange reaction mixture over silica and Celite and elution with CH3CN, which was subsequently removed in vacuo. Yellow crystals were obtained by slow diffusion of diethyl ether into a mixture of the compound in CH2Cl2. Yield: 35.5 mg, 74.0%. 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 1.62 (30H, d, 4JH,P = 3.2 Hz, C5Me5), 2.61 (6H, d, 5 JH,P = 1.2 Hz, CH3CN), 7.29 (2H, dddd, 3JH,H = 7.2 Hz, 3JH,H = 6.9 Hz, 4JH,H = 1.6 Hz, 4JH,H = 1.6 Hz, Harom), 7.34 (2H, dddd, 3JH,H = 7.2 Hz, 3JH,H = 6.9 Hz, 4JH,H = 1.6 Hz, 4JH,H = 1.6 Hz, Harom), 7.46−7.52 (2H, m, Harom), 7.53−7.67 (10H, m, Harom), 7.71−7.75 (8H, m, Harom), 7.81 (2H, dd, 3JH,H = 7.2 Hz, 4JH,H = 1.6 Hz, Harom), 7.98 (2H, ddd, 3JH,H = 7.6 Hz, 4JH,H = 1.6 Hz, 4JH,H = 1.6 Hz, Harom), 8.17 (2H, dd, 3JH,P = 23.6 Hz, 4JH,H = 1.7 Hz, Hβ), 8.49 (2H, dd, 3JH,P = 23.4 Hz, 4 JH,H = 1.7 Hz, Hβ) ppm. 13C NMR (100 MHz, CD2Cl2): δ 4.8 (CH3CN), 9.3 (C5Me5), 98.4 (d, 2JC,P = 2.2 Hz, C5Me5), 120.4 (CH3CN), 120.8 (q, 1J(C,F) = 318.5 Hz, CF3), 121.6 (d, 2JC,P = 19.2 Hz, CH), 124.2 (CH), 127.7 (br s, CH), 128.1 (d, 4JC,P = 3.2 Hz, CH), 128.4 (d, 3JC,P = 11.1 Hz, CH), 128.7 (CH), 129.4 (CH), 129.8 (d, 4 JC,P = 1.3 Hz, CH), 129.9 (d, 3JC,P = 10.6 Hz, CH), 130.1 (CH), 136.5 (d, 3JC,P = 11.5 Hz, CH), 137.8 (CH), 139.2 (d, 3JC,P = 10.5 Hz, Cquat), 141.9 (d, 3JC,P = 5.6 Hz, C4), 142.8 (d, 2JC,P = 27.3 Hz, 2/6-Ar(1′-C)), 145.6 (d, 2JC,P = 30.1 Hz, 2/6-Ar(1′-C)), 148.7 (d, JC,P = 1.8 Hz, Cquat), 155.2 (d, 1JC,P = 31.6 Hz, C2/6), 166.9 (d, 1JC,P = 44.3 Hz, C2/ 6) ppm. 19F NMR (376 MHz, CD2Cl2) δ −77.9 ppm. 31P NMR (162 MHz, CD2Cl2): δ 153.5 ppm. Anal. Calcd for C74H68Ag2F12Ir2N2O12P2S4 (2195.70 g/mol): C, 40.48; H, 3.12; N, 1.2. Found: C, 40.46; H, 3.11; N, 1.29. Generation of 2A−D. To a dilute (∼10 mM) solution of 2 in CD3COCD3 at 298 K was added 125 equiv of water. The reaction smoothly proceeded with formation of several intermediate species, and full consumption of 2 was observed after 18 h. Although a number of species were present in solution, four of them (2A−D) accounted for about 75% of the total initial amount of 2. Relevant NMR data of 2A−C are reported below. For NMR features of 2D see the main text. Data for 2A are as follows. 1H NMR (400 MHz, in CD3COCD3, 298 K): δ 1.92 (d, η5-C5(CH3)5,4JPH = 2.4); 2.04 (d, CH3CN, 5JPH = 0.7); 4.55 (ddt, H6, 3JPH = 16.6, 3JHH = 7.3, 4JHH = 1.2); 7.07 (ddd, H5, 3 JPH = 18.0, 3JHH = 7.3, 4JHH = 1.5); 7.13 (dd, H3,3JPH = 24.5, 4JHH = 1.5). 31P{1H} NMR (400 MHz, in CD3COCD3, 298 K): δ 79.4 (s). Data for 2B are as follows. 1H NMR (400 MHz, in CD3COCD3, 298 K): δ 1.99 (d, η5-C5(CH3)5,4JPH = 2.5); 2.04 (d, CH3CN, 5JPH = 0.9); 4.84 (dd, H6, 3JPH = 18.2, 3JHH = 6.9); 6.67 (ddd, H5, 3JPH = 14.0, 3 JHH = 6.9, 4JHH = 1.2); 7.47 (dd, H3, 3JPH = 25.0, 4JHH = 1.3). 31P{1H} NMR (400 MHz, in CD3COCD3, 298 K): δ 84.8 (s). Data for 2C are as follows. 1H NMR (400 MHz, in CD3COCD3, 298 K): δ 1.65 (d, η5-C5(CH3)5,4JPH = 2.3); 2.80 (d, CH3CN, 5JPH = 0.8); 4.14 (dd, H6, 3JPH = 7.0, 3JHH = 3.0); 6.60 (ddd, H5, 3JPH = 8.9, 3 JHH = 3.0, 4JHH = 1.3); 7.3 (d, H3,3JPH = 23.9). 31P{1H} NMR (400 MHz, in CD3COCD3, 298 K): δ 74.4 (s). Catalytic Water Oxidation. Compound 2 was tested as a water oxidation catalyst using CAN as sacrificial oxidant (eq 1). Catalytic activity was monitored by means of a multitechnique approach based on both the detection of the production of oxygen and the depletion of Ce4+. In particular, differential manometry and oximetry were employed to follow the kinetics of formation of molecular oxygen in the gas phase and in solution, respectively. Instead, the depletion of CAN was followed by means of UV−vis at 340 and 410 nm, depending on its concentration. Typically, stock solutions of catalyst were prepared by dissolving 1−2 mg (1.2−2.4 μmol) of 2 in 50 mL of

an acidic water (pH 1 by HNO3)/acetonitrile mixture (4/1 in volume). Differential Manometry. Experiments were performed by dissolving 13.7−27.4 mg of CAN (25−50 μmol) in 4−4.5 mL of acidic water in a working cell. The same amount of solvent was transferred into a second identical reference cell. Both cells were equipped with a side arm for the connection to the manometer and with a septum to seal the system, which was kept at a constant temperature (T = 25 °C) and allowed to equilibrate with stirring for at least 20 min. When a steady baseline was achieved, the solvent (0.5− 1.0 mL) was added into the reference cell, and 0.5 mL (0.25 μmol) to 1.0 mL (0.5 μmol) of a solution of 2 was added into the working cell. Final concentrations of CAN were 5 and 10 mM, whereas the concentration of 2 was changed between 2.5 and 5 μM. TOF values were calculated from the linear trend of TON vs time, assuming that molecular oxygen is the only gas produced. Oximetry. The Clark electrode experiments were performed by transferring 39−40.6 mL of an aqueous solution of CAN (410 μmol) into a reactor, which was subsequently sealed. The system was thermostated at 25 °C with stirring for at least 20 min. As soon as a stable baseline was obtained, 0.4 mL (0.041 μmol) to 2 mL (0.205 μmol) of a solution of 2 was injected into the reactor to reach a final volume of 41 mL. The concentration of CAN was kept constant at 10 mM, and the catalyst concentration was changed between 1 and 5 μM. TOF values for the reaction were obtained from the linear part of a TON vs time trend. UV−Vis. The general procedure to perform UV−vis experiments was rather different from those of the other methods. A stock solution of catalyst was diluted to obtain five different solutions with catalyst concentrations in the 7.5−1.5 μM range. A 2 mL portion of each solution was transferred in a cuvette that was thermostated at T = 25 °C with continuous stirring. After background correction, 1 mL of a solution of CAN (3 and 30 mM) was injected into the cuvette monitoring the depletion of Ce4+ at λ 340 nm (3 mM) and λ 410 nm (30 mM). The concentrations of CAN after dilution were 1 and 10 mM, whereas the catalyst concentrations ranged within 1−5 μM. From the trends observed by UV−vis spectroscopy (Figure 8) it was possible to evaluate both TOFI and TOFLT (Table 2, entries 10−19), because a clear discontinuity was visible in the kinetics trends. Both TOF values were determined on the basis of a zero-order kinetic treatment, in particular, by using the following integrated equation: A(t)/A0 = 1− 4kosst/C0. X-ray Crystallography. Compound 2: [C35H34IrNP](CF3O3S), fw = 840.87, yellow plate, 0.26 × 0.22 × 0.04 mm3, orthorhombic, Pbca (No. 61), a = 14.6265(6) Å, b = 16.8203(6) Å, c = 27.3682(10) Å, V = 6733.2(4) Å3, Z = 8, Dx = 1.659 g/cm3, μ = 4.13 mm−1. A total of 111352 reflections were measured on a Bruker Kappa ApexII diffractometer with sealed tube and Triumph monochromator (λ = 0.71073 Å) at a temperature of 150(2) K up to a resolution of (sin θ/ λ)max = 0.65 Å−1. X-ray intensities were integrated with Eval15 software.39 Analytical absorption correction and scaling was performed with SADABS40 (correction range 0.47−0.81). A total of 7728 reflections were unique (Rint = 0.036), 6498 of which were observed (I > 2σ(I)). The structure was solved with direct methods using SHELXS-97.41 Least-squares refinement was performed with SHELXL-201442 against F2 of all reflections. Non-hydrogen atoms were refined freely with anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier maps and refined with a riding model. A total of 421 parameters were refined with no restraints. R1/wR2 (I > 2σ(I)): 0.0184/0.0404. R1/wR2 (all reflections): 0.0265/0.0426. S = 1.032. The residual electron density was between −0.62 and 0.96 e/Å3. Geometry calculations and checking for higher symmetry were performed with the PLATON program.43 Compound 3: C74H68Ag2F12Ir2N2O12P2S4 + disordered solvent, fw = 2195.62*, yellow-brown needle, 0.35 × 0.17 × 0.06 mm3, monoclinic, P21/c (No. 14), a = 17.3777(4) Å, b = 14.0362(3) Å, c = 17.9409(4) Å, β = 95.4251(12)°, V = 4356.49(17) Å3, Z = 2, Dx = 1.674 g/cm3*, μ = 3.70 mm−1* (the asterisk denotes that the derived value does not contain the contribution of the disordered solvent). A H


Article

Organometallics total of 57991 reflections were measured on a Bruker Kappa ApexII diffractometer with sealed tube and Triumph monochromator (λ = 0.71073 Å) at a temperature of 150(2) K up to a resolution of (sin θ/ λ)max = 0.65 Å−1. X-ray intensities were integrated with Saint software.44 Multiscan absorption correction and scaling was performed with SADABS40 (correction range 0.28−0.43). A total of 9904 reflections were unique (Rint = 0.031), 8555 of which were observed (I > 2σ(I)). The structure was solved with direct methods using SHELXS-97.41 Least-squares refinement was performed with SHELXL-201442 against F2 values of all reflections. The crystal structure contains large voids (591 Å3/unit cell) filled with disordered solvent molecules. Their contribution to the structure factors was secured by back-Fourier transformation using the Squeeze routine,45 resulting in 193 electrons/unit cell. Both of the coordinated triflate ligands were refined with disorder models. Non-hydrogen atoms were refined freely with anisotropic displacement parameters. All hydrogen atoms were included in calculated positions and refined with a riding model. A total of 601 parameters were refined with 441 restraints (distances, angles, and displacement parameters of the disordered triflates). R1/wR2 (I > 2σ(I)): 0.0339/0.0824. R1/wR2 (all reflections): 0.0422/0.0860. S = 1.069. The residual electron density was between −1.12 and 1.85 e/Å3. Geometry calculations and checking for higher symmetry were performed with the PLATON program.43

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AUTHOR INFORMATION

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S Supporting Information *

Additional figures giving NMR characterization and catalytic experiments and a CIF file giving crystallographic data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00281. Corresponding Authors

*A.M.: e-mail, [email protected]. *C.M.: e-mail, [email protected]; web, http://www.bcp. fu-berlin.de/ak-mueller. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank (i) The Netherlands Organiziation for Scientific Research (NWO-Vidi) for financial support and for financing the X-ray diffractometer and (ii) SABIC, COST Action CM1205 (CARISMA), and Regione Umbria (POR FSE Projects) for financial support.

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REFERENCES

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