Cooperative P–H Bond Activation with Ruthenium and Iridium

Article pubs.acs.org/Organometallics

Cooperative P−H Bond Activation with Ruthenium and Iridium Carbene Complexes Julia Weismann, Lennart T. Scharf,† and Viktoria H. Gessner*,† Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany † Inorganic Chemistry II, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany S Supporting Information *

ABSTRACT: The reactivity of nucleophilic ruthenium and iridium carbene complexes toward the P−H bond in secondary phosphines and phosphine oxides was studied. While the reactions of the free phosphines resulted in the formation of product mixtures, the oxidized congeners gave way to fast and selective P−H bond activation reactions by means of metal−ligand cooperation and net addition of the P− H bond across the metal−carbon double bond. The formed phosphoryl complexes were characterized in the solid state and in solution, indicating C−H···O hydrogen bonding as a structural motif. Computational studies to provide insights into the reaction mechanism revealed thatin contrast to other E−H bond activation reactions with carbene complexesno concerted 1,2-addition across the MC bond is operative. Instead, the P−H bond activation of the phosphine oxides preferably proceeds via coordination of the phosphinous acid tautomer to the metal, followed by hydrogen transfer to the carbenic carbon atom.

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INTRODUCTION Bond activation reactions via metal−ligand cooperation have attracted increasing attention over the past few years.1 Most recently, carbene complexes have also been established in this chemistry, being capable of the splitting of a variety of polar and nonpolar E−H bonds. For example, Piers and Iluc employed group 9 and 10 PCsp2P pincer complexes such as A in a number of activation reactions: e.g., of H−H, N−H, Si−H, or C−H bonds (Figure 1).2 Similarly, we were able to introduce complexes derived from dilithium methandiide precursors that showed analogous reactivities toward a series of E−H bonds.3,4 Thereby, ruthenium complex 1 turned out to be extremely active and allowed the isolation of various activation products.5 In contrast, the iridium congener 2 showed the tendency to undergo cyclometalation of the sulfur-bound phenyl group after

the activation processes, thus preventing the isolation and possible transfer reactions of the activated species.6 The activation of P−H bonds is an elementary step in many synthetic processes for the construction of phosphoruscontaining compounds, including hydrophosphination and dehydrocoupling/dehydropolymerization reactions.7 Thereby, different mechanisms including σ-bond metathesis reactions and the splitting of the P−H bond at a single metal center or by multimetallic systems have been realized.8,9 However, no P−H bond activation reactions by means of metal−ligand cooperation in carbene complexes orto the best of our knowledgein any other cooperating ligand system have been reported so far. Analogous to the activation of amines, which has been realized by different cooperating ligand systems, simple coordination of the phosphine to the metal center may be limiting factor for P−H bond activation.10 Hence, we focused on the reactivity of carbene complexes 1 and 2 toward secondary phosphines and phosphine oxides, R2P(O)H. In addition to the viability of cooperative P−H bond activation reactions with carbene complexes we were also interested in the selectivity of the reaction (e.g., formation of phosphido versus hydrido complexes) and the complex stability (e.g., competing cyclometalation). Here, we show that phosphines and phosphine oxides can be activated via metal−ligand cooperation and that coordination to the metal is a seminal step in these transformations.

Figure 1. Carbene complexes which are active in cooperative bond activation reactions. © XXXX American Chemical Society

Received: May 20, 2016

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DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION P−H Activation with Ruthenium Complex 1. To address the question of whether phosphines can be activated via metal−ligand cooperation in carbene complexes or whether coordination results in the deactivation of the metal center or in further side reactions, we first focused on the activation of secondary phosphines (see Scheme 1, left). To this end, the

via their tautomeric form, the corresponding phosphinous acid R2POH (eq 1). However, we assumed that due to the

preference of the phosphine oxide form this coordination should be less efficient than in the case of the free phosphines and thus hamper replacement of the cymene ligand. Addition of a selection of phosphine oxides Ar2P(O)H (Ar = 4-ClC6H4, 2-MeC6H4, 3,5-Cl2C6H4) to a solution of ruthenium carbene complex 1 in toluene at room temperature resulted in the typical color change from purple to orange. Fortunately, 31 1 P{ H} NMR spectroscopy confirmed the selective formation of a single new product and the successful P−H activation by the disappearance of the signal of the carbene complex (δP 66.6 ppm) at the expense of two new doublets. All activation products could be isolated in good (4b, 66%; 4c, 77%) to excellent (4a, 90%) yields and were characterized by multinuclear NMR spectroscopy, X-ray diffraction, and elemental analysis. Overall, the P−H bond activation by carbene complex 1 proceeds smoothly and isto the best of our knowledge the first example of a cooperative P−H bond activation in phosphine oxides. Transition-metal phosphoryl complexes reported so far have mostly been prepared via oxidation of the corresponding phosphido complexes or from phosphine oxides in the presence of an additional metal base.11,12 In addition, “classical” P−H bond activation reactions in phosphine oxides at a single metal center have only scarcely been reported, particularly without chelate assistance.13 In solution, the phosphoryl complex 4a exhibits two broadened signals (δP 60.3 and 86.9 ppm) in the 31P{1H} NMR spectrum, while 4b,c feature two doublets (at δP 57.3 and 71.0 ppm (4b) and δP 58.6 and 72.8 ppm (4c), respectively) with 3JPP coupling constants of about 20 Hz (for 4b,c). Additionally, the complexes feature characteristic doublets of doublets in the 1 H and 13C{1H} NMR spectra for the methanide hydrogen and carbon atom, respectively (δH ∼5−6 ppm and δC ∼26 ppm; see the Experimental Section). All other signals are consistent with the formulation of 4 as the corresponding phosphoryl complexes. No formation of any hydrido species was observed. Suitable crystals for X-ray diffraction analysis could be obtained by diffusion of n-pentane into saturated toluene solutions of 4a,b (Figure 2). Both compounds crystallize in the monoclinic space group C2/c. The molecular structures confirm the activation of the P−H bond via cis addition across the RuC double bond and selective protonation of the former carbene carbon atom. In both structures, the ruthenium center adopts a piano-stool geometry with acute angles between 75.1(1) and 82.8(1)° involving the three legs. Due to the change from a metal−carbon double bond to a single bond, the Ru−C bond elongates from 1.965(2) Å in 1 to 2.170(4) Å in 4a and 2.169(4) Å in 4b, respectively. The Ru−P distances of 2.390(1) and 2.353(1) Å, respectively, are comparable to Ru−P bonds reported for other ruthenium phosphoryl complexes.11 Interestingly, in both structures the oxygen atoms of the phosphoryl moieties point toward the hydrogen atom at the PCS bridge with C1−H1···O1 angles of 146(4) (4a) and 155(3)° (4b).14 This indicates the presence of hydrogen bonds, as has often been observed for phosphoryl ligands,15 particularly in an intramolecular fashion in bis(phosphoryl) complexes.16 However, the hydrogen bonds in 4b,c are rather

Scheme 1. Activation of the P−H Bond in Phosphines and Phosphine Oxides with Ruthenium Complex 1

reactivity of ruthenium carbene complex 1 toward a variety of different aryl phosphines bearing chloro, methoxy, amino, and trifluoromethyl substituents was tested. Addition of 1 equiv of phosphine to a solution of ruthenium carbene complex 1 in toluene at room temperature resulted in an immediate color change from purple to orange. Analogous color changes had also been observed in previous activation reactions with complex 1, also suggesting successful P−H bond cleavage. Unfortunately, NMR spectroscopic studies of the crude reaction mixture revealed a rather unselective process and the formation of various products, which could not be fully identified. The same results were obtained when the reaction conditions were changed (temperature, −78 °C; solvent, CH2Cl2 or THF). The 1H and 31P{1H} NMR spectra of the different reaction mixtures showed similar sets of signals, one of which was also indicative for successful P−H bond activation across the MC bond and formation of desired phosphido complexes such as 3. For example, the product mixture of the reaction with (p-ClC6H4)2PH showed a doublet of doublets at δH 3.56 ppm (2JPH = 10.5 Hz, 3JPH = 4.18 Hz) in the 1H NMR spectrum indicative of the protonation of the PCS linkage and the formation of 3b. Consistently, the 31P{1H} NMR spectrum featured two doublets at δP 39.0 and 56.7 ppm. In addition to these signals of the activation products, additional sets of doublets and triplets were found in the 31P{1H} NMR spectra, suggesting that the P−H bond activation is also accompanied by coordination of the phosphines to the metal and replacement of the cymene ligand. This could be confirmed by the detection of uncoordinated p-cymene in the 1H NMR spectra of all reaction mixtures. In order to test this hypothesis and to hamper phosphine coordination, we turned our attention toward differently substituted phosphine oxides (see Scheme 1, right). In general, secondary phosphine oxides may also coordinate to the metal B

DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX

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basis of a second-order perturbation analysis, secondary orbital interactions between the oxygen and the C−H bond provide a clear stabilization of the complex (stabilization energy 10.7 kcal/mol). P−H Activation with Iridium Complex 2. In order to examine whether iridium carbene complex 3 reacts in a fashion similar to that of its ruthenium congener, analogous reactivity studies were performed using 3 and the corresponding phosphines and phosphine oxides. Overall, a comparable reactivity behavior was found. As such, the activation of phosphines gave no selective P−H activation. Instead, 31P{1H} NMR spectroscopy showed the formation of complex product mixtures, the composition of which additionally changed over the course of time and with the workup procedure. The reaction of iridium carbene complex 3 with phosphine oxides, however, gave complete conversion to the desired activation products 5a,b. In comparison to ruthenium complex 1, P−H activation with 2 in toluene at room temperature proceeded much more slowly (Scheme 2). Complete conversion as Scheme 2. P−H Bond Activation in Phosphine Oxides with Iridium Carbene Complex 2

indicated by a color change from deep red to yellow could only be observed within 3−5 days reaction time, respectively. Notably, the activation products were found to be stable in the solid state as well as in solution over a long period of time (several weeks). No cyclometalation via reliberation of the phosphine oxide was observed, as had been the case for the Si− H activation products.6 Both complexes could be isolated as yellow-orange crystalline solids in good yields (5a, 89%, 5b, 69%) and could be characterized by multinuclear NMR spectroscopy, X-ray diffraction, and elemental analysis. Single crystals of both complexes could be obtained by diffusion of n-pentane into saturated solutions of the crude product in benzene or toluene. The molecular structures (monoclinic space group P21/c) confirmed the expected constitution and revealed an elongation of the Ir−C bond length from 1.963(3) Å in 2 to 2.185(3) Å (5a) and 2.178(2) Å (5b), thus being in line with a change from an IrC double bond to an Ir−C single bond (Figure 3). In contrast to the ruthenium complexes 4a,b, the molecular structures of the iridium analogues 5a,b exhibit no O···H interaction between the phosphine oxide moiety and the PCHS hydrogen atom. Instead, the PO moiety points toward the bulky Cp* ligand, presumably to minimize steric repulsion with the phosphorus-bound aryl substituents (vide infra). In contrast to the solid-state structures, the solution structures of the iridium phosphoryl complexes were found to be more complex. While complex 5b with the sterically less encumbering p-chlorophenyl groups shows the expected two doublets in the 31P{1H} NMR spectrum (δP 53.6, 62.5; 3JPP = 16 Hz) and all NMR signals consistent with the solid-state structure, the o-tolyl-substituted compound 5a featured only broadened signals at room temperature. Here, the 31P{1H}

Figure 2. Molecular structures of the P−H activation products 4a,b in the solid state. Solvent molecules (4a, toluene; 4b, benzene) and all hydrogen atoms except for the hydrogen atoms at C1 are omitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): 4a, Ru1−C1 2.170(4), Ru−S1 2.455(1), Ru−P2 2.390(1), S1−P1 2.014(1), S2−O1 1.435(3), S2−O2 1.441(3), S2−C1 1.746(4), P1−C1 1.777(4), P2−O3 1.517(3), C1−Ru−P2 82.8(1), C1−Ru−S1 75.4(1), P2−Ru−S1 81.53(3), S2−C1−P1 125.6(2), S2−C1−Ru 121.3(2), P1−C1−Ru1 97.2(2); 4b, Ru−C1 2.169(4), Ru1−P2 2.353(1), Ru−S1 2.449(1), S1−P1 2.019(1), P1−C1 1.777(4), O1−S2 1.448(2), C1−S2 1.745(2), S2− O2 1.445(2), P2−O3 1.518(2), C1−Ru−P2 81.4(1), C1−Ru−S 1 75.3(1), P2−Ru−S1 81.8(1), S2−C1−P1 126.4(2), P1−C1−Ru 98.4(2), S2−C1−Ru 123.0(2).

weak. As such, no significant elongation of the C1−H bond length could be detected in 4a,b. However, the O3···C1 (2.649(5) Å (4a) and 2.855(4) Å (4b)) and O3···H1 distances (2.08(5) Å (4a) and 2.03 (4) Å (4b)) arealbeit longer than those reported in strong O···H−O hydrogen bonds15 significantly shorter than the sum of the van der Waals radii.17 Additionally, the 1H NMR shifts of the PC(H)S hydrogen atoms at about 5−6 ppm are shifted distinctly downfield in comparison to those of other activation products. For example, the methanide hydrogen of the H2 and Si−H bond activation products with complex 1 appeared at δH 3.77 and 3.74−4.06 ppm, respectively.5a,b Interestingly, natural bond orbital (NBO) analysis of the ruthenium complex (with Ph2POH) confirmed the weak hydrogen bonding. On the C

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Figure 4. Section of the VT 31P{1H} NMR studies of complex 5a in CD2Cl2 at low temperatures.

low-field shift suggests the presence of an O···H interaction, as has also been observed for the ruthenium complexes 4a,b (see above), which led us to the assumption that in solution both 5a and 5a′ exist. These two isomers are only observable for the o-tolylsubstituted phosphoryl ligand, presumably because of the hindered rotation around the Ir−P bond due to the steric demand of the Cp* ligand and the tolyl groups of the phosphoryl moiety. In the case of the less bulky p-chlorofunctionalized system 5b and the ruthenium complexes 4 rotation is still possible at room temperature, while elevated temperatures are required for 5a. As such, heating of the NMR sample of 5a/5a′ resulted in coalescence at approximately 60 °C and isomerization between 5a and 5a′ (see the Supporting Information for NMR spectra). Computational Studies. To get insights into the reaction mechanism and the different isomers of 5, density functional theory (DFT) calculations were performed (see Experimental Section and the Supporting Information for details). At first, three potential isomers of the product of the diphenylphosphine oxide, Ph2P(O)H, activation with iridium complex 2 were calculated. The studies revealed a clear energetic preference (ΔH = 32 and ΔG = 28 kJ/mol) of the cis isomer cis-5 observed in experiment (XRD) over the hypothetical trans isomer trans-5 with the hydrogen and phosphoryl moiety bound on opposite sides of the former MC double bond (Figure 5). Furthermore, isomer cis-5′ with an H bond between the methanide carbon and the phosphoryl group was revealed to be slightly disfavored (ΔΔH = 12 kJ/mol; ΔΔG = 9 kJ/

Figure 3. Molecular structures of the P−H activation products 5a,b in the solid state. Solvent molecules (5b, toluene) and all hydrogen atoms except the hydrogen atoms at C1 are omitted for clarity. Ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): 5a, Ir−C1 2.1850(3), Ir−P2 2.3217(8), Ir−S1 2.4365(8), S1−P1 2.0129(1), S2−O1 1.436(3), S2−O2 1.431(2), S2−C1 1.764(3), P1−C1 1.810(3); S2−C1−P1 122.63(2), S2−C1−Ir 124.94(2), P1−C1−Ir 95.63(1); 5b, Ir−C1 2.178(2), Ir−P2 2.2947(6), Ir−S1 2.4310(6), S1−P1 2.0015(9), S2− O1 1.4394(2), S2−O2 1.4413(2), S2−C1 1.759(2), P1−C1 1.802(2); S2−C1−P1 124.19(1), S2−C1−Ir 125.35(1), P1−C1−Ir 93.91(1).

NMR spectrum showed two pairs of signals at δP 53.6, 72.3 and 62.5, 64.1 ppm, indicating the presence of two isomeric forms of 5a. In order to further explore the solution behavior of 5a and the different isomeric forms, VT NMR spectroscopic studies in the temperature range from +90 to −90 °C were performed (Figure 4). While only broadened signals were detected from 25 to 0 °C, a sharpening of the first pair of signals occurred at −10 °C (δP 62.5 and 64.1 ppm; 3JPP = 20.6 Hz; black triangles in Figure 4). This was also accompanied by a sharpening of a characteristic doublet of doublets at δH 4.93 ppm (2JPH = 14.7 Hz, 3JPH = 3.23 Hz; PCHS) in the 1H NMR spectrum. Further cooling to −40 °C resulted in the sharpening of the second pair of doublets, which corresponded to a significantly downfield shifted signal for the hydrogen atom at the PCS bridge (δH 6.37 ppm) in the 1H NMR spectrum. This

Figure 5. Energetic differences between the different isomers of 5. D

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Figure 6. Possible pathways of the P−H activation in Ph2P(O)H with iridium complex 2 (free energies are given in kJ/mol).

mol), thus being in line with the observation of isomer cis-5 in the solid state. In contrast, in the case of the ruthenium system the H-bonded isomer was found to be favored by ΔΔG = 4 kJ/ mol, thus reflecting the preference of the different conformations for the ruthenium and iridium systems in the solid state. The energetic differences are due to the different steric bulk of the cymene in comparison to the Cp* ligand. In both complexes, the aromatic coligand disfavors the H-bonded isomer due to repulsion with the aryl groups at phosphorus. This repulsion is more pronounced for the Cp* than for the cymene ligand, thus leading to an overall preference for isomer cis-5. Overall, the energetic differences between the two isomers of the iridium and the ruthenium systems are small. This is also the case when the activation of the bulkier o-tolyl-substituted phosphine oxide to 5a is considered (ΔG = 14 kJ/mol; ΔH = 10 kJ/mol). Thus, both isomers are energetically easily accessible, which is also found in solution. We next addressed the question of possible reaction mechanisms of the P−H bond activation. So far, two different pathways have been reported for E−H bond activation reactions with cooperative carbene ligands: (i) a concerted 1,2-addition of the E−H bond across the MC double bond and (ii) addition of the E−H bond to the metal center followed by hydrogen transfer from the metal to the carbenic carbon atom.2,5 In the case of the P−H bond activation of secondary phosphine oxides, even a third mechanism might be operative due to the possible tautomeric equilibrium of the phosphine oxide with the corresponding phosphinous acid R2POH (eq 1). Generally, this equilibrium is completely shifted to the phosphine oxide structure. However, the phosphinous acid tautomer can be stabilized by employment of strongly electron withdrawing groups such as CF3 and C2F518 or by coordination to a metal center.12 The latter may be the case in the reaction of

phosphine oxides with 1 and 2. Hence, P−H bond activation may also proceed via coordination of the phosphinous acid to the metal and subsequent transfer of the hydrogen from the oxygen to the methanide carbon atom. Indeed, the calculations showed only a small energetic difference between Ph2P(O)H and Ph2P(OH) of 2 kJ/mol. Mechanistic studies were performed on a model system of iridium complex 2 with methyl groups at phosphorus and sulfur and a Cp instead of a Cp* ligand. The three possible pathways(1) 1,2-addition, (2) addition to the metal center, and (3) coordination of the phosphinous acidwere studied by means of the activation of diphenylphosphine oxide, Ph2(H)PO. The results are depicted in Figure 6, showing a clear preference of the pathway via the phosphinous acid tautomer over the oxidative addition of the P−H bond to iridium. No transition state for the concerted 1,2addition reaction was found. Optimizations always led to oxidative addition to the metal center. This oxidative addition shows an activation barrier of 108 kJ/mol (TS1), while coordination of Ph2P(OH) to iridium only requires 22 kJ/mol (TS1′). The formed intermediate Int1′ is energetically favored over the starting compounds and reacts virtually without a further need of energy (TS2′) to the final activation product. Overall, the calculations suggest that the P−H activation in secondary phosphine oxides with complexes 1 and 2 preferably proceeds via coordination of the phosphinous acid tautomer and subsequent PO−H activation. However, also the oxidative addition has a sufficiently low reaction barrier, so that this mechanism may compete particularly in the case of the sterically more demanding systems. Additionally, one also has to keep in mind that the tautomerization to the phosphinous acid form typically proceeds via a non-negligible activation barrier, which strongly depends on the substitution pattern and the reaction conditions (e.g. solvent, aggregation).19 The slow E

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C{1H} NMR (125.8 MHz, CD2Cl2): δ 16.7 (CH3,Cymene), 20.9 (CH(CH3)2), 21.9 (d, 3JPC = 3.16 Hz; CH3), 22.5 (d, 3JPC = 2.89 Hz; CH3), 24.5 (CH(CH3)2), 26.3 (dd, 1JPC = 19.8 Hz, 2JPC = 9.78 Hz; PCHS), 30.9 (CH(CH3)2), 80.7 (CHCymene), 86.9 (d, 2JPC = 9.33 Hz; CHCymene), 90.8 (CHCymene), 94.6 (d, 2JPC = 2.91 Hz; CHCymene), 95.7 (CCH 3 ), 115.3 (CCH(CH 3 ) 2 ), 124.0 (d, 3 J PC = 13.3 Hz; CHP(S)Ph,meta), 124.5 (br d, 3JPC = 9.69 Hz; CHP(O)Ph,meta), 127.05 (CHSPh,ortho),127.1 (d, 1JPC = 75.4 Hz; CP(O)Ph,ipso), 128.36 (CHSPh,para), 128.4 (d, 4JPC = 2.18 Hz; CHP(O)Ph,para), 128.5 (d, 3JPC = 11.0 Hz; CHP(S)Ph,meta), 129.0 (d, 4JPC = 2.17 Hz; CHP(O)Ph,para), 129.1 (CHSPh,meta), 131.1 (d, 3JPC = 11.3 Hz; CHP(S)Ph,ortho), 131.7 (d, 3JPC = 7.15 Hz; CHP(O)Ph,meta), 132.0 (d, 4JPC = 3.09 Hz; CHP(S)Ph,para), 132.2 (d, 2JPC = 2.89 Hz; CHP(O)Ph,ortho), 133.4 (d, 4JPC = 3.04 Hz; CHP(S)Ph,para), 134.7 (d, 3JPC = 10.5 Hz; CHP(S)Ph,ortho), 134.8 (d, 2JPC = 21.5 Hz; CHP(O)Ph,ortho), 136.5 (d, 1JPC = 54.4 Hz; CP(S)Ph,ipso), 138.4 (CCH3), 142.8 (CSPh,ipso), 143.3 (d, 1JPC = 47.4 Hz; CP(S)Ph,ipso), 144.6 (CCH3), 146.7 (d, 1JPC = 38.9 Hz; CP(O)Ph,ipso). 31P{1H} NMR (202.5 MHz, CD2Cl2): δ 60.3 (br; PO), 86.9 (br, PS). Anal. Calcd for C47H49O3P2S2Ru: C, 63.49; H, 5.56; S, 7.21. Found: C, 63.88; H, 5.65; S, 7.17. Synthesis and Characterization of Complex 4b. In a J. Young NMR tube, 30.0 mg (49.5 μmol) of carbene complex 1 and 13.4 mg (49.5 μmol) of bis(p-chlorophenyl)phosphine oxide were dissolved in 0.6 mL of C6D6. After 10 min, a color change from purple to yellow was observed and 31P{1H} NMR spectroscopy revealed complete conversion to product 4b. The complex was isolated by diffusion of npentane into the reaction mixture, affording complex 4b as an orange crystalline solid (27.0 mg, 32.8 μmol, 66%). 1H NMR (500.1 MHz, CD2Cl2): δ 0.99 (d, 3JHH = 6.98 Hz, 3 H; CH(CH3)2), 1.11 (d, 3JHH = 6.82 Hz, 3 H; CH(CH3)2), 1.65 (s, 3 H; CH3), 2.34−2.42 (sept., 3JHH = 6.80 Hz, 1 H; CH(CH3)2), 4.92 (d, 3JHH = 6.02 Hz, 1 H; CHCymene), 5.43 (dd, 2JPH = 14.0, 3JPH = 5.01 Hz, 1 H; PCHS), 5.01−5.03 (br, 1 H; CHCymene), 5.31 (d, 3JHH = 5.80 Hz, 1 H; CHCymene), 5.89 (d, 3JHH = 6.12 Hz, 1 H; CHCymene), 6.79−6.83 (m, 2 H; CHP(S)Ph,meta), 7.03− 7.06 (td, 3JHH = 7.80 Hz, 4JHH = 2.78, 2 H; CHP(S)Ph,meta), 7.21−7.35 (m, 8 H; CHSPh,ortho,para + CHP(S)Ph,para + CHP(O)Ph,ortho), 7.61 (t, 3JHH = 8.48 Hz, 2 H; CHSPh,meta), 7.66−7.72 (m, 4 H; CHP(O)Ph,meta), 7.75− 7.81 (m, 3 H; CHP(S)Ph,ortho+ CHP(S)Ph,para), 8.33−8.38 (m, 2 H; CHP(S)Ph,ortho). 13C{1H} NMR(125.8 MHz, CD2Cl2): δ 17.5 (CH3), 20.6 (CH(CH3)2), 24.4 (CH(CH3)2), 26.7 (dd, 1JPC = 17.7 Hz, 2JPC = 10.4 Hz; PCHS), 30.9 (CH(CH3)2), 80.9 (CHCymene), 88.0 (d, 2JPC = 10.0 Hz; CHCymene), 91.6 (CHCymene), 94.3 (d, 2JPC = 2.89 Hz; CHCymene), 96.6 (CCH3), 115.6 (CCH(CH3)2), 126.9 (CHSPh,ortho), 127.1 (CCl), 127.7 (CCl), 127.77 (CHSPh,para), 127.8 (d, 2JPC = 9.89 Hz; CHP(O)Ph,ortho), 128.6 (d, 3JPC = 11.7 Hz; CHP(S)Ph,meta), 128.7 (d, 3 JPC = 12.8 Hz; CHP(O)Ph,meta), 129.1 (CHSPh,meta), 130.6 (d, 3JPC = 11.3 Hz; CHP(S)Ph,meta), 132.2 (d, 4JPC = 2.89 Hz; CHP(S)Ph,para), 133.2(d, 2 JPC = 10.8 Hz, CHP(S)Ph,ortho), 133.5 (d, 4JPC = 2.88 Hz; CHP(S)Ph,para), 134.0 (d, 2JPC = 10.7 Hz; CHP(S)Ph,ortho), 134.7 (d, 1JPC = 48.3 Hz; CP(S)Ph,ipso), 136.5(d, 1JPC = 52.5 Hz; CP(S)Ph,ipso), 143.9 (CSPh,ipso), 144.9 (d, 1JPC = 44.6 Hz; CP(O)Ph,ipso), 146.8 (d, 1JPC = 41.1 Hz; CP(O)Ph,ipso). 31P{1H} NMR (202.5 MHz, CD2Cl2): δ 57.3 (d, 3JPP = 20.5 Hz; PO), 71.0 (d, 3JPP = 20.6 Hz; PS). Anal. Calcd for C41H38O3P2S2RuCl2: C, 56.16; H, 4.37; S, 7.31. Found: C, 54.31; H, 4.25; S, 6.90. HRMS: [M − H]•+ calcd 877.0231, found 877.0219. Synthesis and Characterization of Complex 4c. A 40.0 mg portion (66.0 μmol) of carbene complex 1 was dissolved in 1 mL of toluene, and 22.4 mg (65.9 μmol) of bis(3,5-dichlorophenyl)phosphine oxide was added at room temperature. When it was stirred overnight, the reaction mixture turned from purple to orange, indicating complete conversion. The solvent was removed in vacuo, and the residue was washed with n-pentane (3 × 4 mL), affording product 4c as an orange solid (48.0 mg, 50.8 μmol, 77%). 1H NMR (500.1 MHz, CD2Cl2): δ 1.06 (d, 3JHH = 6.99 Hz, 3 H; CH(CH3)2), 1.17 (d, 3JHH = 6.77 Hz, 3 H; CH(CH3)2), 1.83 (s, 3 H; CH3), 2.43− 2.52 (sept., 3JHH = 6.89 Hz, 1 H; CH(CH3)2), 4.83 (dd, 2JPH = 13.2, 3 JPH = 5.52 Hz, 1 H; PCHS), 4.90 (d, 3JHH = 5.97 Hz, 1 H; CHCymene), 5.03 (t, 3JHH = 8.45, 4.22 Hz, 1 H; CHCymene), 5.34 (d, 3JHH = 5.47 Hz, 1 H; CHCymene), 6.02 (d, 3JHH = 6.12 Hz, 1 H; CHCymene), 6.67 (dd, J = 13

tautomerization is presumably also responsible for the slow reaction process.

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CONCLUSION We have shown the reactivity of ruthenium and iridium carbene complexes toward differently substituted phosphines and phosphine oxides. Overall, both complexes exhibited a similar reactivity behavior. While the free phosphines resulted in the formation of complex product mixtures, the phosphine oxides gave way to selective P−H activation via metal−ligand cooperation and selective net addition of the P−H bond across the MC bond. The phosphoryl complexes were stable and showed weak C−H···O hydrogen bonding as a structural motif. Computational studies provided insights into the reaction mechanism, revealing thatin contrast to other E−H bond activation reactions with carbene complexesno concerted 1,2-addition across the MC bond is operative. Instead the formal P−H bond activation in phosphine oxides preferably proceeds via the phosphinous acid tautomer, which at first coordinates to the metal, followed by hydrogen transfer from oxygen to the carbenic carbon atom. The presented net P−H bond activation underlines the applicability of the carbene complexes 1 and 2 in cooperative bond activation processes and their mechanistic flexibility to enable E−H bond activation reactions.

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EXPERIMENTAL SECTION

General Considerations. All experiments were carried out under a dry, oxygen-free argon atmosphere using standard Schlenk techniques. The solvents used were dried over sodium or potassium (or over P4O10, CH2Cl2) and distilled prior to use. H2O is distilled water. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Avance-500, Avance-400, and Avance-300 spectrometers at 22 °C if not stated otherwise. All values of the chemical shifts are in ppm with regard to the δ scale. All spin−spin coupling constants (J) are given in hertz (Hz). To display multiplicities and signal forms correctly, the following abbreviations were used: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad signal. The signal assignment was supported by DEPT and HMQC experiments. Elemental analyses were performed on an Elementar vario MICRO-cube elemental analyzer. All reagents were purchased from Sigma-Aldrich, ABCR, or Acros Organics and used without further purification. Carbene complexes 1 and 2 and the corresponding phosphines/phosphine oxides were prepared according to literature procedures.3a,20 A general procedure for the P−H activation of phosphines with complex 1 as well as the corresponding NMR spectra are given in the Supporting Information. Synthesis and Characterization of Complex 4a. A solution of 40.0 mg (66.1 μmol) of ruthenium carbene complex 1 in 1 mL of toluene was treated with a solution of 15.2 mg (66.1 μmol) of bis(omethylphenyl)phosphine oxide in 1 mL of dry toluene at room temperature. After 1 h reaction time the mixture slowly turned from purple to red. Reaction monitoring by 31P{1H} NMR spectroscopy showed complete conversion to 4a after 3 h reaction time. Isolation of the activation product was achieved by diffusion of n-pentane into the saturated reaction mixture, giving 4a as an orange crystalline solid (49.8 mg, 59.6 μmol, 90%). 1H NMR (500.1 MHz,CD2Cl2): δ = 1.03 (d, 3JHH = 6.96 Hz, 3 H; CH(CH3)2), 1.30 (d, 3JHH = 6.82 Hz, 3 H; CH(CH3)2), 1.53 (s, 3 H; CH3,Cymene), 2.37 (s, 3 H; CH3), 2.51−2.59 (sept., 3JHH = 6.83 Hz, 1 H; CH(CH3)2), 2.70 (br s, 3 H; CH3), 5.01 (br, 1 H; CHCymene), 5.17 (br, 1 H; CHCymene), 5.49 (d, 3JHH = 6.36 Hz, 1 H; CHCymene), 5.98 (d, 3JHH = 5.92 Hz, 1 H; CHCymene), 6.40 (br, 1 H; PCHS), 7.03−7.26 (m, 14 H; CHP(O)Ph,meta + CHP(O)Ph,para + CHP(S)Ph,ortho + CHP(S)Ph,meta+ CHP(S)Ph,para + CHSPh,para), 7.29−7.32 (m, 1 H; CHP(O)Ph,ortho), 7.65−7.69 (m, 2 H; CHSPh,meta), 7.72(d, 3JHH = 7.22 Hz, 2 H; CHSPh,ortho), 7.75−7.79 (m, 1 H;CHP(S)Ph,para), 7.82− 7.86 (m, 1 H; CHP(O)Ph,ortho), 8.22−8.26 (m, 2 H; CHP(S)Ph,ortho). F

DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

75.3 Hz; CP(S)Ph,ipso), 127.0 (CHSPh,ortho), 127.6 (d, 3JPC = 10.7 Hz; CHP(S)Ph,meta), 127.8 (d, 3JPC = 9.46 Hz; CHP(S)Ph,meta), 128.1 (d, 2JPC = 24.5 Hz; CHP(O)Ph,ortho), 128.3 (d, 2JPC = 24.3 Hz; CHP(O)Ph,ortho), 128.6 (d, 3JPC = 12.9 Hz; CHP(O)Ph,meta), 128.8 (d, 3JPC = 11.5 Hz; CHP(O)Ph,meta), 129.2 (CHSPh,meta), 130.2 (d, 2JPC = 11.2 Hz; CHP(S)Ph,ortho), 132.4 (d, 4JPC = 2.99 Hz; CHP(S)Ph,para), 132.7 (CHSPh,meta), 133.8 (d, 4JPC = 3.08 Hz; CHP(S)Ph,para), 134.2 (d, 2JPC = 11.2 Hz; CHP(S)Ph,ortho), 134.8 (d, 4JPC = 2.48 Hz; CCl), 135.3 (d, 4 JPC = 2.76 Hz; CCl), 139.9 (d, 1JPC = 49.7 Hz; CP(S)Ph,ipso), 143.0 (CSPh,ipso), 143.2 (d, 1JPC = 42.8 Hz; CP(O)Ph,ipso), 143.6 (d, 1JPC = 39.3 Hz; CP(O)Ph,ipso). 31P{1H} NMR (202.5 MHz, CD2Cl2): δ 37.3 (d, 3JPP = 16.5 Hz; PO), 66.9 (d, 3JPP = 16.2 Hz; PS). Anal. Calcd for C50H48O3P2S2Cl2Ir: C, 55.29; H, 4.45; S, 5.90. Found: C, 55.55; H, 4.49; S, 5.53. X-ray Crystallography. The crystals of all compounds were mounted in an inert oil (perfluoro polyalkyl ether). Data collection was conducted with a Bruker APEX II CCD diffractometer (D8 threecircle goniometer) at 100 K. The structures were solved using the intrinsic phasing method (ShelXT), refined with the ShelXLprogram,21 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions, except for those bound to C1 in 4a,b, which were also refined. Crystallographic data (including structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1450795− 1450798. For structural details, see also the Supporting Information. Computational Details. All calculations were performed without symmetry restrictions. Starting coordinates were obtained with GaussView 5.0.9 or directly from the crystal structure analyses. All calculations were done with the Gaussian09 (Revision D.01) program package.22 Geometry optimizations were performed using density functional theory (DFT) with the dispersion-corrected M062X functional23 together with 6-311+G(d) (for all nonmetal atoms) and LANL2TZ(f)+ECP (for Ru and Ir). The metrical parameters of the energy-optimized geometries compare well with those determined by X-ray diffraction. Harmonic vibrational frequency analyses were performed on the same levels of theory for the hydrogen-substituted model systems. For the real systems, thermal corrections to SCF energies were taken from frequency calculations using 6-31+G(d) and the LANL2DZ basis set. The vibrational frequency analyses showed no imaginary frequencies for the ground states and one imaginary frequency for the transition states, corresponding to the expected translational motion of the transition states. Intrinsic reaction coordinate (IRC) calculations24 were carried out to ensure the connectivity of the reported minima and transition states. Tables S10 and S18 in the Supporting Information give the energies of all calculated compounds, and Tables S11−S17 and S19−S29 in the Supporting Information give the Cartesian coordinates of all optimized compounds.

12.8, 7.89 Hz, 2 H; CHP(S)Ph,meta), 7.01−7.06 (m, 2 H; CHP(O)Ph,ortho), 7.26−7.28 (m, 3 H; CHarom), 7.34−7.37 (m, 2 H; CHarom), 7.58 (dd, J = 8.48, 1.70 Hz, 2 H; CHP(O)Ph,ortho.), 7.65−7.70 (m, 4 H; CHarom), 7.75−7.78 (m, 4 H; CHSPh + CHP(S)Ph,meta), 8.28−8.33 (m, 2 H; CHP(S)Ph,ortho). 13C{1H} NMR(125.8 MHz, CD2Cl2): δ 17.7 (CH3), 20.4 (CH(CH3)2), 24.5 (CH(CH3)2), 26.2 (dd, 1JPC = 18.0 Hz, 2JPC = 10.3 Hz; PCHS), 31.2 (CH(CH3)2), 81.6 (CHCymene), 88.3 (d, 2JPC = 10.6 Hz; CHCymene), 93.4 (d, 2JPC = 10.6 Hz; CHCymene), 94.9 (d, 2JPC = 2.24 Hz; CHCymene), 97.0 (CCH3), 116.4 (CCH(CH3)2), 125.6 (CHSPh), 127.0 (CHSPh), 127.4 (d, 1JPC = 78.2 Hz; CP(S)Ph,ipso), 128.7 (d, 2JPC = 11.7 Hz; CHP(O)Ph,ortho), 128.8 (CHP(O)Ph,para), 128.9 (d, 2JPC = 12.9 Hz; CHP(O)Ph,ortho), 129.2 (CHP(O)Ph,para), 129.3 (d, 2JPC = 10.1 Hz, CHP(S)Ph,ortho), 130.0 (d, 3JPC = 10.7 Hz; CHP(S)Ph,meta), 130.1 (d, 3 JPC = 11.3 Hz; CHP(S)Ph,meta), 132.4 (d, 4JPC = 2.84 Hz; CHP(S)Ph,para), 132.8 (CHSPh), 133.8 (d, 2JPC = 10.7 Hz; CHP(S)Ph,ortho), 134.8 (3JPC = 4.83 Hz; CCl), 134.9 (3JPC = 4.00 Hz; CCl), 136.0 (d, 1JPC = 53.3 Hz; CP(S)Ph,ipso), 143.7 (CSPh,ipso), 149.9 (d, 1JPC = 34.7 Hz; CP(O)Ph,ipso), 151.3 (d, 1JPC = 31.3 Hz; CP(O)Ph,ipso). 31P{1H}-NMR (202.5 MHz, CD2Cl2): δ 58.6 (d, 3JPP = 20.7 Hz; PO), 72.9 (d, 3JPP = 20.7 Hz; PS). Anal. Calcd for C41H36O3P2S2RuCl4: C, 52.07; H, 3.84; S, 6.78. Found: C, 51.91; H, 4.20; S, 5.64. HRMS: [M − H]•+ calcd 877.0231, found 877.0219. Synthesis and Characterization of Complex 5a. A 21.1 mg portion (30.2 μmol) of iridium carbene complex 2 was dissolved in 4 mL of toluene, and the dark red solution was treated with 7.00 mg (30.2 μmol) of bis(o-methylphenyl)phosphine oxide. After the mixture was stirred for 2 days at room temperature, a color change from deep red to yellow-orange was observed, which changed to yellow within 5 days. Isolation of complex 5a was achieved by diffusion of n-pentane into the yellow reaction mixture, giving way to the activation product 5a as an orange crystalline solid (25 mg, 26.9 μmol, 89%). Data for 5a are as follows. 1H NMR (300.2 MHz, CD2Cl2, −90 °C): δ 1.48 (s, 15 H; Cp(CH3)5), 2.17 (s, 3 H; CH3), 2.98 (s, 3 H; CH3), 4.93 (dd, 2JPH = 14.7 Hz, 3JPH = 3.23 Hz, 1 H; PCHS), 5.43 (dd, 3JPH = 12.2 Hz, 3JHH = 7.95 Hz, 2 H; CHarom), 6.93 (dt, 3JHH = 7.47, 1.83 Hz, 2 H; CHarom), 6.82−6.89 (m, 1 H; CHarom), 6.94−7.03 (m, 4 H; CHarom), 7.13−7.23 (m, 7 H; CHarom), 7.44 (d, J = 7.62 Hz; CHarom), 8.26−8.33 (m, 2 H; CHarom). 31P{1H} NMR (121.5 MHz, CD2Cl2, − 90 °C): δ 62.5 (d, 3 JPP = 20.6 Hz; PO), 64.1 (d, 3JPP = 20.6 Hz; PS). Data for 5a′ are as follows. 1H NMR (300.2 MHz, CD2Cl2, − 90 °C): δ 1.28 (s, 15 H; Cp(CH3)5), 2.34 (s, 3 H; CH3), 2.89 (s, 3 H; CH3), 6.37 (dd, 2JPH = 10.7, 3JPH = 8.29 Hz, 1 H; PCHS), 7.22−7.26 (m, 8 H; CHarom), 7.59− 7.80 (m, 8 H; CHarom), 7.99 (dd, J = 8.12, 13.4 Hz; 2 H, CHarom), 8.31−8.37 (m, 2 H; CHarom). 31P{1H} NMR (121.5 MHz, CD2Cl2, − 90 °C): δ 53.6 (d, 3JPP = 12.3 Hz; PO), 72.3 (d, 3JPP = 12.3 Hz; PS). 13 C{1H} NMR (125.5 MHz, CD2Cl2, −90 °C) of 5a and 5a′: δ 8.82 (Cp*-CH3), 9.01 (Cp*-CH3), 13.9 (CH3), 20.3 (CH3), 21.4 (br d, JPC = 4.07 Hz; PCHS), 22.2 (br d, JPC = 2.76 Hz; PCHS), 22.4 (CH3), 23.3 (CH3), 92.08, 92.10 (CCp,ipso), 122.3−144.8 (Carom). Anal. Calcd for C43H45O3P2S2Ir: C, 55.65; H, 4.88; S 6.91. Found: C, 55.38; H, 5.02; S, 6.80. Synthesis and Characterization of Complex 5b. A 30.0 mg portion (43.0 μmol) of iridium carbene complex 3 was dissolved in 0.5 mL of dry C6D6 and treated with 11.7 mg (43.0 μmol) of bis(pchlorophenyl)phosphine oxide. After 3 days of vigorous stirring at room temperature a color change from dark red to orange could be observed. Monitoring the reaction mixture by 31P{1H} NMR spectroscopy indicated complete conversion to the P−H activation product 5b. The compound was isolated by diffusion of n-pentane into a saturated benzene solution, affording 5b as a yellow crystalline solid (32.2 mg, 29.6 μmol, 69%). 1H NMR (500.1 MHz, CD2Cl2): δ 1.49 (d, 4JPH = 2.27 Hz, 15 H; CH3), 5.58 (dd, 2JPH = 14.6, 3JPH = 3.82 Hz, 1 H; PCHS), 6.14 (m, 2 H; CHP(O)Ph,ortho), 6.93 (dt, 3JHH = 7.90, 2.77 Hz, 2 H; CHP(S)Ph,meta), 7.17 (t, 3JHH = 7.87 Hz, 2 H; CHP(S)Ph,meta), 7.24−7.27 (m, 1 H; CHP(S)Ph,para), 7.31−7.33 (m, 5 H; CHP(S)Ph,ortho + CHSPh,meta+ CHP(O)Ph,ortho), 7.59 (d, 2 H; CHSPh,ortho), 7.62−7.69 (m, 4 H; CHP(O)Ph,meta), 7.73−7.77 (m, 1 H; CHP(S)Ph,para), 7.98 (t, 3JHH = 8.80 Hz, 2 H; CHSPh,meta), 8.28−8.32 (m, 2 H; CHP(S)Ph,ortho). 13C{1H} NMR (125.5 MHz, CD2Cl2): δ 9.62 (CH3), 10.6 (dd, 1JPC = 19.1, 2JPC = 4.35 Hz; PCHS), 95.7 (d, 2JPC = 2.57 Hz; CCp,ipso), 126.2 (d, 1JPC =

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00408. Details of the P−H activation of free phosphines, NMR spectra of all isolated compounds, and computational and crystallographic details (PDF) Crystallographic data (CIF) Cartesian coordinates for the calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.H.G.: [email protected]. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (Emmy Noether grant to V.H.G.; DA1402/1) and the Fonds der Chemischen Industrie for financial support (PhD fellowship to L.T.S.). We are also grateful to Sebastian Molitor for the synthesis of the phosphines and phosphine oxides and to Christoph Mahler for HR MS measurements. We also thank Marie-Luise Schäfer and Dr. Rüdiger Bertermann for VT NMR studies.

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DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00408 Organometallics XXXX, XXX, XXX−XXX