3PO

Kinetic Analysis and Sequencing of Si-H and C-H Bond Activation Reactions: Direct Silylation of Arenes Catalyzed by an Iridium- Polyhydride

Miguel A. Esteruelas,* Antonio Martínez, Montserrat Olivan, and Enrique Onate

ABSTRACT: The saturated trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1; xant(PiPr2)2 = 9,9- dimethyl-4,5-bis(diisopropylphosphino)xanthene) coordinates the Si-H bond of triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, and triphenylsilane to give the σ-complexes IrH3(η2-H- SiR3){κ2-cis-P,P-[xant(PiPr2)2]}, which evolve to the dihydride-silyl derivatives IrH2(SiR3){κ3- P,O,P-[xant(PiPr2)2]} (SiR3 = SiEt3 (2), SiMe(OSiMe3)2 (3), SiPh3 (4)) by means of the oxidative addition of the coordinated bond and the subsequent reductive elimination of H2. Complexes 2-4 activate a C-H bond of symmetrically and asymmetrically substituted arenes to form silylated arenes and to regenerate 1. This sequence of reactions defi nes a cycle for the catalytic direct C-H silylation of arenes. Stoichiometric isotopic experiments and the kinetic analysis of the transformations demonstrate that the C-H bond rupture is the rate-determining step of the catalysis. As a consequence, the selectivity of the silylation of substituted arenes is generally governed by ligand-substrate steric interactions.

■ INTRODUCTION
Transition metal-mediated cross-coupling reactions involving elemental steps of σ-bond activation in both substrates (Scheme 1) are challenging from a conceptual point of view.
The reason is the need of sequencing the splitting of the σ- bonds in the metal coordination sphere, which requires an adequate difference between the activation energies of the bond rupture reactions. Thus, the success of the cross-coupling demands the control of the sequencing process, for which it is necessary to know and to govern such parameters. The latter is achieved by having a deep knowledge of what is happening just before the σ-bond cleavage step. It is generally assumed that the fi rst step for a σ-bond activation reaction is the
The intermolecular C-H silylation of arenes without the use of directing groups is a particular type of this class of cross- coupling reactions of great interest. The silylated products are useful precursors to commercial polymers and copolymers and can be also used as intermediates for organic synthesis. The reactions are environmentally friendlier than the classical procedures of synthesis of arylsilanes, minimizing waste formation, and off er the possibility of reaching alternative regioselectivities.2 The catalysis involves the activation of the Si-H bond of the silane3 and a C-H bond of the arene,4 before the coupling of the substrates. Although transition metal-silyl derivatives are a prominent group of compounds, comparable in relevance to the aryl derivatives, there is significantly less information on silane Si-H bond splitting than on the arene C-H bond activation. Two features distinguish silicon from carbon and hydrogen: its higher electropositive character and the hypervalent ability. As a consequence, a greater variety of interactions M-HSi than M-eff ort has been centered on the stabilization and character- ization of these interactions, whereas the H-Si cleavage process has received less attention. Often, M-HSi compounds are treated as still pictures of a situation more than as transitory species on the way to the Si-H activation.6Catalysts for the arene C-H silylation include complexes of ruthenium,7 rhodium,8 iridium,9 nickel,10 platinum,11 and some rare-earth metals.12 Knowledge of the identity of species by which the catalysts functionalize the C-H bond is scarce. Recently, it has been proposed that the iridium systems generated from [Ir(μ-OMe)(η4-COD)]2 (COD = 1,5-cyclo- octadiene) and phenanthroline ligands work through only one cycle involving both dihydride-silyl and hydride-disilyl complexes (Scheme 2). In a sequential manner the former activates the Si-H bond of the silane, whereas the second adds a C-H bond of the arene to subsequently promote the silyl- aryl coupling.
The ligands used for stabilizing C-H silylation catalysts are usually monodentate and bidentate. Pincer ligands are having a tremendous impact in current catalysis because of their ability for stabilizing uncommon species, which open novel approaches.13 However, they have been scarcely used for supporting these catalysts. As far as we know, only one catalyst bearing a ligand of this class has been previously employed. In 2007, Tilley and co-workers reported that complex Ir(κ3- NSiN)H(OTf)(COE) (NSiN = bis(8-quinolyl)methylsilyl, OTf = trifl ate, COE = cyclooctene) is active for arylsilane redistribution and for the dehydrogenative silylation of arenes.14
9,9-Dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant- (PiPr2)2) is an ether-diphosphine that has demonstrated to have a higher capacity than other POP-diphosphines to act as pincer.15 However, its fl exibility along with the hemilabile character of the ether function allows it to adapt to the requirements of the participating intermediates of the catalytic cycles.16 As a proof-of-concept, species bearing the diphos- phine coordinated in κ3-mer,17 κ3-fac,18 κ2-cis,19 and κ2-trans20 fashions have been isolated, whereas notable catalysts of platinum group metals stabilized by this ligand have proved to be effi cient for a wide range of reactions. For the elements of the iron triad, it should be mentioned that the ruthenium complex RuH(η2-H2BH2){κ3-P,O,P-[xant(PiPr2)2]} is an efficient catalyst precursor for the hydrogen transfer from 2- propanol to ketones, the α-alkylations of phenylacetonitrile and acetophenone with alcohols, and the regio- and stereo-(PiPr2)2]} also catalyzes the latter22 and the hydroxo- osmium(IV) derivative OsH3(OH){κ3-P,O,P-[xant(PiPr2)2]} dehydrogenates formic acid to H2 and CO2.23 Reactions catalyzed by rhodium complexes include dehydropolymeriza- tion of H3B·NMeH2,24 dehydrogenation of ammonia borane25 and alkanes,26 monoalcoholysis of diphenylsilane,27 borylation of arenes,28 decyanative borylation of nitriles,29 borylation of alkynes,30 dehalogenation of chloroalkanes, and homocoupling of benzyl chloride.31 Recently, we have observed that the iridium-trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} catalyzes the direct borylation of arenes with the help of dihydride-boryl and hydride-diboryl derivatives. The three compounds are involved in two catalytic cycles, which have the dihydride-boryl complex as a common intermediate because of its ability to activate both B-H and C-H bonds (Scheme 3).
The diagonal relationship between the elements of rows 2 and 3 is well-known, being particularly marked for boron and silicon.33 With that in mind, we reasoned that complex IrH3{κ3-P,O,P-[xant(PiPr2)2]} should also be an effi cient catalyst for the silylation of arenes. Thus, in order to build a catalytic cycle for the arene C-H silylation, related to those shown in Scheme 3, we decided to study the activation of the Si-H bond of silanes promoted by this trihydride and the C- H bond activation of arenes promoted by the resulting silyl products, paying particular attention to the kinetics of the processes and to the reaction intermediates. This paper demonstrates that the iridium-promoted arene C-H silylation can also take place through trihydride and dihydride-silyl complexes without the help of hydride-disilyl species.

■ RESULTS AND DISCUSSION
Si-H Bond Activation. As expected for the diagonal relationship between boron and silicon, trihydride complex IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1) activates the Si-H bond of silanes such as triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisilox- ane, and triphenylsilane (Scheme 4). Treatment of toluene solutions of 1 with 1.0 equiv of the silanes, at room temperature, for 18 h leads to the corresponding dihydride- silyl derivatives IrH2(SiR3){κ3-P,O,P-[xant(PiPr2)2]} (SiR3 = SiEt3 (2), SiMe(OSiMe3)2 (3), SiPh3 (4)) and molecular hydrogen. Complexes 2-4 were isolated as white solids in 49- 55% yield.The monitoring of the activations by 1H and 31P{1H} NMR spectroscopy in toluene-d8 reveals that the reactions are quantitative and take place via the intermediates ASiR3 shown in Scheme 4. At room temperature, characteristic features of these transitory species are a broad hydride resonance centered between -11.2 and -12.2 ppm in the 1H NMR spectra and a broad signal centered between -1 and -5 ppm in the 31P{1H} NMR spectra. The half-life of these intermediates depends upon the substituents attached to the silicon atom, increasing in the sequence SiPh3 < SiMe(OSiMe3)2 < SiEt3. The half-life of intermediate ASiEt3 is long enough to allow its spectroscopic study at 183 K. Figure 2 collects the most informative NMR spectra at this temperature. The 31P{1H} NMR spectrum (a) shows two doublets (2JP-P = 18 Hz) at 8.7 (a1) and -5.9 (a2) ppm and a broad singlet at -12.8 (b1) ppm. The COSY 31P-31P spectrum (b) confi rms that the doublets correspond to the same molecule (A), which bears a κ2-P,P-cis- diphosphine with inequivalent PiPr2 groups, whereas the broad singlet represents other κ2-P,P-cis-diphosphine species with chemically equivalent PiPr2 moieties (B). The coordination polyhedron around the iridium center can be rationalized as a distorted octahedron with the ether-diphosphine coordinated in a mer fashion (P(1)-Ir(1)- P(2) = 156.83(15)° and 161.33(14)°, P(1)-Ir(1)-O(3) = 82.0(3)° and 81.1(3)°, and P(2)-Ir(1)-O(3) = 81.5(3)° and 81.4(3)°) and the silyl group located trans to the diphosphine oxygen atom (Si(1)-Ir(1)-O(3) = 170.4(3)° and 172.8(3)°). The Ir(1)-Si(1) bond lengths of 2.262(4) and 2.271(4) Å compare well with those reported for other iridium(III)-silyl complexes.34 The 1H, 31P{1H}, and 29Si{1H} NMR spectra of 2-4, in benzene-d6, at room temperature are consistent with the structure shown in Figure 1. In the 1H NMR spectra, the most noticeable feature is the resonance corresponding to the equivalent hydrides, which appears as a triplet (2JH-P = 17 Hz) between -5.2 and -6.2 ppm. The 31P{1H} NMR spectra show a singlet between 44 and 54 ppm, in agreement with equivalentbetween them is approximately 2:1. The HMBC 31P-1H spectrum (c) reveals hydride resonances for A at -9.7 (a1), -11.2 (a2), -12.6 (a3), and -14.4 (a4) ppm, whereas those of B appear at -9.7 (b1), -10.0 (b2), and -14.2 (b3+b4) ppm. According to an A:B molar ratio of 2:1, the 1H{31P} spectrum in the high-fi eld region (Figure S19) displays an a1+b1:b2:a2:a3:b3+b4:a4 intensity ratio of 3:1:2:2:2:2. Further- more, on the basis of this spectrum and the 1H one, a trans disposition of the hydrides corresponding to resonances a3 and a4 and the PiPr2 groups of A (2JH-P = 104 and 113 Hz, respectively), and between the hydrides assigned to signal b3+b4 and the PiPr2 moieties of B, can be deduced. The signal b3+b4 is the AA′ part of an AA′XX′ spin system with 2JA‑X = 2JA′‑X′ = 116 Hz. The decoupling of the 31P nucleus brings to light hidden 29Si satellites on the signal a2. At first glance, the 1JH-Si value of 43 Hz suggests some degree of Si-H interaction and the formation of a “symmetric oxidative addition product”.35 The Si-H interaction in A and the value of the 1JH-Si are also supported by the HMQC 29Si-1H spectrum (d), which contains the cross-spot between the resonance a2 and the Ir-Si resonance, which is observed at 2.8 ppm in the 29Si{1H} NMR spectrum. This bidimensional spectrum also is consistent with the Si-H interaction in B, showing a cross-spot between Ir-Si and b1 resonances, although the value of 1JH-Si cannot be measured in this case. Optimization of this structure by DFT calculations (see Supporting Information) shows the silicon atom in position anti with regard to the oxygen of the diphosphine. Keeping this disposition there are several rotamers involving the alkyl substituents of the diphosphine and silane. The isopropyl substituents of the PiPr2 groups can be disposed in positions eclipsed or alternating, which gives rise to structures bearing equivalent or inequivalent PiPr2 groups, in agreement with the spectroscopic observations. The disposition of the isopropyl groups slightly modifies the position of the silicon atom, which changes the P-Ir-Si angles. The criterion of the value of 1JH-Si is ambiguous to establish the nature of the Si-H interaction. So, we decided to prepare a monodeuterated ASiEt3 species, by reaction of 1 with Et3SiD, after reasoning that the rupture of the Si-D bond should give rise to deuterium distribution between all hydride positions, since at room temperature the six signals observed at 183 K coalesce to give only one, while if the Si-D bond was maintained, only the intensities of resonances b1 and a2 should be modifi ed. The high-fi eld region of the 1H and 2H NMR spectra of the monodeuterated ASiEt3 intermediate at 183 K (Figure 3) clearly supports the second possibility; that is, ASiR3 are species on the way to the rupture of the Si-H bond. Having clarifi ed the nature of the ASiR3 intermediates, we subsequently analyzed the deuterium present in the final product from the reaction of 1 with DSiEt3, finding about 12% of deuterium in the hydride positions (Figure S20). It is a 75:25 mixture of isotopomers 2 and 2-d1 (Scheme 5). The presence of 2 in the mixture indicates that the creation of a coordination vacancy in 1 by reductive elimination of molecular hydrogen does not occur, in spite of its saturated character. Such reductive elimination takes place after the Si- Figure 3. High-fi eld region of the 1H NMR spectra at 183 K of ASiEt3 (500 MHz, toluene-d8) (a) and ASiEt3‑d (b) and of the 2H NMR spectrum (76.77 MHz, toluene) of ASiEt3‑d (c). The negative value of the activation entropy for the formation of ASiMe(OSiMe3)2 is consistent with the intermolec- ular character of the process and suggests an ordered transition state involving the participation of the silane. Because complex 1 is saturated and the coordination of the Si-H bond requires the κ3-to-κ2 change in the coordination of the diphosphine, by dissociation of the hemilabile oxygen, that entropy value points out in favor of a hypervalent hydride-silicon interaction previous to the oxygen dissociation. Such an interaction, which is broken in the coordination, should decrease the activation energy of the dissociation. The value of the activation entropy for the transformation from ASiMe(OSiMe3)2 to 3, close to zero, is in agreement with the fast nature of the elimination of molecular hydrogen and points out the Si-H rupture as the rate-determining step. Table 1 also contains values of the rate constants k1obs‑d and k2d obtained for reactions with DSiMe- (OSiMe3)2. The ratio k1obs/k1obs‑d of 1.16 ± 0.01 confirms that the silane does not have a direct participation in the rate- determining transition state for the transformation of 1 into ASiMe(OSiMe3)2, although it is present. In contrast, the ratio k2/ k2d gives a primary isotope eff ect of 2.40 ± 0.11, which corroborates the rupture of the Si-H bond as the rate- determining step for the formation of 3.38 Scheme 6 summarizes the Si-H bond activation of silanes promoted by the trihydride 1, on the basis of the previously mentioned results. The silane-assisted dissociation of the hemilabile ether of the diphosphine aff ords the unsaturated intermediates I, which subsequently coordinate the silane to give ASiR3. The oxidative addition of the coordinated Si-H bond, in the rate-determining step, leads to II. The latter rapidly eliminates molecular hydrogen to give III. Finally, the recoordination of the diphosphine ether group yields 2-4. C-H Bond Activation. Dihydride-silyl complexes 2-4 react in benzene solution to give R3Si-Ph and the trihydride derivative 1 (Scheme 7). ■ CONCLUDING REMARKS This study reveals that the Si-H bond activation of silanes promoted by the trihydride IrH3{κ3-P,O,P-[xant(PiPr2)2]} and the C-H bond activation of arenes mediated by dihydride-silyl derivatives of formulas IrH2(SiR3){κ3-P,O,P-[xant(PiPr2)2]} can be sequenced in order to build catalytic reactions involving direct silylation of arenes mediated by the trihydride IrH3{κ3- P,O,P-[xant(PiPr2)2]}. Stoichiometric isotopic labeling experiments as well as the results of a detailed kinetic study have demonstrated that the Si-H bond activation takes place through the σ-complexes IrH3(η2-H-SiR3){κ2-cis-P,P-[xant(PiPr2)2]} and that the oxida- tive addition of the coordinated bond to the metal center is the determining step of the activation. Isotopic labeling experiments and kinetic results of the C-H bond activation indicate that it occurs through a classical mechanism where the C-H bond cleavage is the rate- determining step. Its activation energy is higher than that of the Si-H bond activation. Thus, the C-H bond rupture is the rate-determining step of the catalysis, and, as a consequence, the selectivity of the silylation of monosubstituted and 1,3- disubstituted arenes is generally governed by ligand-substrate steric interactions. In summary, the catalytic cycle for the direct silylation of arenes catalyzed by a saturated polyhydride bearing a pincer ligand has been built on the basis of stoichiometric isotopic labeling experiments, the kinetic analysis of the involved σ- bond activation reactions, and the full characterization of the key σ-intermediate for the Si-H bond activation. ■ EXPERIMENTAL SECTION General Information. All reactions were carried out with exclusion of air using Schlenk-tube techniques or in a drybox. Instrumental methods and X-ray details are given in the Supporting Information. In the NMR spectra the chemical shifts (in ppm) are referenced to residual solvent peaks (1H, 13C{1H}) or external 85% H3PO4 (31P{1H}), SiMe4 (29Si), or CFCl3 (19F). Coupling constants J and N (N = JP-H + JP′-H for 1H and N = JP-C + JP′-C for 13C{1H}) are given in hertz. Preparation of IrH2(SiEt3){κ3-P,O,P-[xant(PiPr2)2]} (2). A solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16 mmol) in toluene (3 mL) was treated with HSiEt3 (25 μL, 0.16 mmol), and the resulting mixture was stirred at room temperature for 18 h. After this time, the yellowish solution was evaporated to dryness to aff ord a yellow residue. Pentane was added to afford a white solid, which was washed with pentane (2 × 1 mL) and dried in vacuo. Yield: 65 mg (55%). Anal. Calcd for C33H57IrOP2Si: C, 52.70; H, 7.64. Found: C, 52.81; H, 7.59. HRMS (electrospray, m/z): calcd for C33H56SiIrOP2 [M - H]+ 751.3200; found 751.3203. IR (cm-1): ν(Ir-H) 1757 (w), ν(C-O-C) 1095 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.00 (m, 2H, CH-arom POP), 6.72 (d, 3JH-H = 7.5, 2H, CH-arom POP), 6.61 (t, 3JH-H = 7.5, 2H, CH-arom POP), 2.35 (m, 4H, PCH(CH3)2), 1.17 (dvt, 3JH-H = 7.2, N = 18.0, 24H, PCH(CH3)2), 1.04 (m, 6H, Si(CH2CH3)3), 0.95 (m, 9H, Si(CH2CH3)3, 0.90 (s, 6H, CH3), -5.91 (t, 2JH-P = 17.4, 2H, Ir-H). 13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 156.8 (vt, N = 10.3, C-arom), 132.5 (vt, N = 4.9, C-arom), (s, CH-arom), 125.9 (s, CH-arom), 125.0 (vt, N = 31.4, C- arom), 124.3 (vt, N = 5.2, CH-arom), 34.4 (s, C(CH3)2), 28.9 (s, C(CH3)2), 26.0 (vt, N = 29.1, PCH(CH3)2), 19.5 (vt, N = 4.9, PCH(CH3)2), 18.4 (s, PCH(CH3)2), 14.9 (t, 3JC-P = 2.4, Si(CH2CH3)3), 10.6 (s, Si(CH2CH3)3). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 47.6 (s). 29Si{1H} NMR (59.63 MHz, C6D6, 298 K): δ -9.3 (t, 2JSi-P = 8.8). Preparation of IrH2[SiMe(OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]} (3). A solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16 mmol) in toluene (3 mL) was treated with 1,1,1,3,5,5,5-heptamethyl- trisiloxane (45 μL, 0.16 mmol), and the resulting mixture was stirred at room temperature for 18 h. After this time, the yellowish solution was evaporated to dryness to aff ord a yellow residue. Pentane was added to aff ord a white solid, which was washed with cold pentane (2 × 1 mL) and dried in vacuo. Yield: 70 mg (49%). Anal. Calcd for C34H63IrO3P2Si3: C, 47.58; H, 7.40. Found: C, 47.58; H, 7.41. HRMS (electrospray, m/z): calcd. for C34H62Si3IrO3P2 [M - H]+ 857.3104; found 857.3135. IR (cm-1): ν(Ir-H) 1758 (w), δs(Si-CH3) 1246 (m), ν(C-O-C) 1033 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 7.24 (m, 2H, CH-arom), 6.93 (d, 3JH-H = 7.5, 2H, CH-arom), 6.84 (t, 3JH-H = 7.5, 2H, CH-arom), 2.67 (m, 4H, PCH(CH3)2), 1.44 (dvt, 3JH-H = 7.5, N = 16.2, 12H, PCH(CH3)2), 1.21 (dvt, 3JH-H = 6.9, N = 13.8, 12H, PCH(CH3)2), 1.12 (s, 6H, CH3), 0.79 (s, 3H, SiMe(OSiMe3)2), 0.43 (s, 18H, SiMe(OSiMe3)2), -6.11 (t, 2JH-P = 17.1, 2H, Ir-H). 13C{1H} NMR (75.48 MHz, C6D6, 298 K): δ 156.6 (vt, N = 10.7, C-arom), 133.2 (vt, N = 4.8, C-arom), 130.2 (s, CH- arom), 126.4 (s, CH-arom), 125.6 (vt, N = 30.9, C-arom), 124.4 (vt, N = 5.1, CH-arom), 34.4 (s, C(CH3)2), 29.8 (s, C(CH3)2), 25.8 (vt, N = 30.6, PCH(CH3)2), 19.4 (vt, N = 5.3, PCH(CH3)2), 18.3 (s, PCH(CH3)2), 15.5 (s, SiMe(OSiMe3)2), 3.2 (s, SiMe(OSiMe3)2). 31P{1H} NMR (161.99 MHz, C6D6, 298 K): δ 53.0 (s, triplet under off -resonance decoupling conditions). 29Si{1H} NMR (59.63 MHz, C6D6, 298 K): δ -7.9 (s, SiMe(OSiMe3)2), -58.2 (t, 2JSi-P = 10.9, SiMe(OSiMe3)2). Preparation of IrH2(SiPh3){κ3-P,O,P-[xant(PiPr2)2]} (4). A solution of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (100 mg, 0.16 mmol) in toluene (3 mL) was treated with HSiPh3 (41 mg, 0.16 mmol), and the resulting mixture was stirred at room temperature for 18 h. After this time, the yellowish solution was evaporated to dryness to aff ord a yellow residue. Pentane was added to aff ord a white solid, which was washed with pentane (2 × 1 mL) and dried in vacuo. Yield: 72 mg (51%). Anal. Calcd for C45H57IrOP2Si: C, 60.31; H, 6.41. Found: C, 60.08; H, 6.56. HRMS (electrospray, m/z): calcd for C45H56SiIrOP2 [M - H]+ 895.3201; found 895.3177. IR (cm-1): ν(Ir-H) 1772 (w), ν(C-O-C) 1087 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 8.39 (d, 3JH-H = 7.2, 6H, SiPh3), 7.29 (t, 3JH-H = 7.2, 6H, SiPh3), 7.18 (m, 3H, SiPh3), 7.01 (m, 2H, CH-arom POP), 6.94 (d, 3JH-H = 7.5, 2H, CH-arom), 6.81 (t, 3JH-H = 7.5, 2H, CH-arom POP), 1.59 (m, 4H, PCH(CH3)2), 1.16 (s, 6H, CH3), 1.02 (dvt, 3JH-H = 7.2, N = 13.8, 24H, PCH(CH3)2), -5.28 (t, 2JH-P = 16.8, 2H, Ir-H). 13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 156.8 (vt, N = 10.6, C-arom), 144.4 (s, C SiPh3), 138.5 (s, CH SiPh3), 132.4 (vt, N = 4.8, C-arom), (s, CH-arom), 127.1 (s, CH SiPh3), 125.9 (s, CH SiPh3), 125.7 (s, CH-arom), 125.0 (vt, N = 31.0, C-arom), 124.2 (vt, N = 5.6, CH- arom), 34.2 (s, C(CH3)2), 28.6 (br s, C(CH3)2), 23.6 (vt, N = 30.1, PCH(CH3)2), 18.7 (vt, N = 5.1, PCH(CH3)2), 17.3 (s, PCH(CH3)2). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 44.9 (s). 29Si{1H} NMR (59.63 MHz, C6D6, 298 K): δ -25.5 (t, 2JSi-P = 10.4). Spectroscopic Characterization of IrH3(η2-H-SiEt3){κ2-cis- P,P-[xant(PiPr2)2]} (ASiEt3). In the glovebox, an NMR tube was charged with a solution of 1 (10 mg, 0.016 mmol) and HSiEt3 (3 μL, 0.02 mmol) in toluene-d8 (0.42 mL), and the NMR spectra of the resulting solution were recorded immediately. 1H NMR (400.13MHz, C7D8, 298 K): δ 7.01 (m, 4H, CH-arom POP), 6.91 (t, JH-H = 7.6, 2H, CH-arom POP), 2.38 (m, 4H, PCH(CH3)2), 1.39 (s, 6H, CH3), 1.28 (dd, 3JH-H = 7.6, 3JH-P = 15.2, 12H, PCH(CH3)2), 1.13 (dd, 3JH-H = 7.2, 3JH-P = 12.8, 12H, PCH(CH3)2), 0.96 (m, 6H, Si(CH2CH3)3), 0.79 (m, 9H, Si(CH2CH3)3, -12.09 (br, 4H, Ir-H). 1H NMR (500.13 MHz, C7D8, 183 K, high-fi eld region, relative intensities): δ -9.73 (m, 3, Ir-H), -9.99 (m, 1, Ir-H), -11.19 (t, 2JH-P = 15.5, 2, Ir-H), -12.49 (dd, 2JH-P = 15.5, 2JH-P = 104, 2, Ir- H), -14.04 (m, 2, Ir-H), -14.33 (dd, 2JH-P = 21, 2JH-P = 111, 2, Ir- H). 1H{31P} NMR (500.13 MHz, C7D8, 183 K, high-fi eld region, relative intensities): δ -9.73 (br s, 3, Ir-H), -9.99 (br s, 1, Ir-H), -11.19 (br s, 2, Ir-H), -12.60 (br s, 2, Ir-H), -14.12 (br s, 2, Ir- H), -14.40 (br s, 2, Ir-H). 31P{1H} NMR (202.46 MHz, C7D8, 298 K): δ -1.2 (br s). 31P{1H} NMR (202.46 MHz, C7D8, 183 K): δ 8.7 (d, 2JP-P = 18.1), -5.9 (d, 2JP-P = 18.1), -12.8 (br s). 29Si{1H} NMR (99.36 MHz, C7D8, 183 K): δ 2.8 (br). Reaction of IrH3{κ3-P,O,P-[xant(PiPr2)2]} (1) with DSiEt3. Two Wilmad screw-cap NMR tubes were charged with 1 (10 mg, 0.016 mmol). To the fi rst NMR tube was added 0.42 mL of toluene and to the second was added 0.42 mL of toluene-d8. DSiEt3 (3 μL, 0.02 mmol) was added to both samples, and they were periodically checked by NMR spectroscopy. After 28 h, the 1H and 2H NMR spectra showed the presence of 2 and 2-d1. The 1H NMR (300.13 MHz, C7D8, 298 K) data were identical to those reported for 2 with the exception of the decrease of the intensity of the triplet at -5.76 ppm (2JH-P = 17.4 Hz) corresponding to IrH2 and the appearance of a new triplet at -5.61 ppm (2JH-P = 17.4 Hz) corresponding to the IrHD isotopomer, with the deuterium incorporation at the hydride positions being 25%. 2H NMR (46.07 MHz, toluene, 298 K): δ -5.63 (s, IrD). NMR Spectroscopic Study of the Transformation of IrH3{κ3- P,O,P-xant(PiPr2)2} (1) into ASiMe(OSiMe3)2 and IrH2[SiMe- (OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]} (3). The experimental proce- dure is described for a particular case, but the same method was used in all experiments, which were run in duplicate. In the glovebox, an NMR tube was charged with a solution of 1 (10 mg, 0.016 mmol) and 1,1,1,3,5,5,5-heptamethyltrisiloxane (129 μL, 0.47 mmol) in toluene (0.42 mL), and a capillary tube fi lled with a solution of the internal standard (PPh3) in benzene-d6 was placed in the NMR tube. The tube was immediately introduced into an NMR probe at the desired temperature, and the reaction was monitored by 31P{1H} NMR at diff erent intervals of time. Determination of the Reaction Order for 1,1,1,3,5,5,5- Heptamethyltrisiloxane in the Transformation of 1 into ASiMe(OSiMe3)2. The experimental procedure is analogous to that described for the transformation of 1 into ASiMe(OSiMe3)2 and 3, starting form 1 (10 mg, 0.016 mmol, 0.0373 M) and variable concentrations of 1,1,1,3,5,5,5-heptamethyltrisiloxane (from 0.747 to 1.495 M) in toluene (0.42 mL). The experiments were carried out at 288 K. NMR Spectroscopic Study of the Transformation of IrH2[SiMe(OSiMe3)2]{κ3-P,O,P-[xant(PiPr2)2]} (3) into IrH3{κ3- P,O,P-xant(PiPr2)2} (1). The experimental procedure is described for a particular case, but the same method was used in all experiments, which were run in duplicate. In the glovebox, an NMR tube was charged with a solution of 3 (14 mg, 0.016 mmol) in benzene (0.42 mL), and a capillary tube fi lled with a solution of the internal standard (PPh3) in benzene-d6 was placed in the NMR tube. The tube was immediately introduced into an NMR probe preheated at the desired temperature, and the reaction was monitored by 31P{1H} NMR at diff erent intervals of time. NMR Spectroscopic Study of the Catalysis. In the glovebox, an NMR tube was charged with a solution of 1 (15 mg, 0.023 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (64 μL, 0.23 mmol), and cyclo- hexene (24 μL, 0.23 mmol) in benzene (0.42 mL). The tube was introduced into an NMR probe preheated at 85 °C, and the reaction was monitored by 31P{1H} NMR at diff erent intervals of time. Determination of the Activation Parameters of the Catalytic Silylation of Benzene. The experimental procedure is described for a particular case, but the same method was used in all experiments, which were run in duplicate. In the glovebox, an NMR tube was charged with a solution of 1 (7.7 mg, 0.012 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (33 μL, 0.12 mmol), and cyclo- hexene (12 μL, 0.12 mmol) in benzene (0.42 mL), and a capillary tube fi lled with a solution of 1,4-dioxane (used as internal standard) in benzene-d6 was placed in the NMR tube. The tube was immediately introduced into an NMR probe preheated at the desired temperature, and the reaction was monitored by 1H NMR at diff erent intervals of time (a d1 = 10 s was used in order to ensure accurate integration of the signals). General Procedure for the Silylation Reactions. In an argon- fi lled glovebox an Ace pressure tube was charged with 1 (12.7 mg, 0.02 mmol), HSiMe(OSiMe3)2 (100 μL, 0.36 mmol), cyclohexene (33 μL, 0.36 mmol), pentadecane (10 μL, 0.036 mmol), as internal standard, and 1.5 mL of the arene. The resulting mixture was stirred at 110 °C for 18 h. After this time the yield of the silylation reaction was determined by GC on an Agilent Technologies 6890N gas chromatograph with a flame ionization detector, using an HP- Innowax column (30 m × 0.25 mm; film thickness 0.25 μm). The injector temperature was 250 °C, and the FID temperature was 300 °C. The oven temperature began at 60 °C for 5 min, then 15 °C per minute to 200 °C, and fi nally 200 °C for 13 min. Then the arene was evaporated under reduced pressure to aff ord a crude reaction mixture. The identity of the silylation product was confi rmed by 1H, 13C{1H}, and 29Si{1H} NMR spectroscopies, as well as by GC-MS analyses. The isolated yields were calculated after purifi cation of the crude reaction mixture by flash chromatography over silica gel using diethyl ether as eluent and by evaporation to dryness. ■ ASSOCIATED CONTENT sı* Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.0c07578. Experimental details, NMR data of the silylation products, crystallographic data, computational details and energies of calculated complexes, and NMR spectra (PDF) Cartesian coordinates of the optimized structures (XYZ) X-ray crystal data (CIF) Accession Codes CCDC 2014866 contains the crystallographic data for this paper. These data can be obtained free of charge via www.ccdc. cam.ac.uk/data_request/cif, or by e-mailing data_request@ ccdc.cam.ac.uk, or by contacting the Cambridge Crystallo- graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033. ■ AUTHOR INFORMATION Corresponding Author Miguel A. Esteruelas - Departamento de Quımica Inorganica, Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH), Centro de Innovacion en Quımica Avanzada (ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain; orcid.org/0000-0002-4829-7590; Email: [email protected] Authors Antonio Martínez - Departamento de Quımica Inorganica, Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH), Centro de Innovacion en Quımica Avanzada (ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain; orcid.org/0000-0002-7292-8591 Montserrat Olivan - Departamento de Quımica Inorganica, Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH), Centro de Innovacion en Quımica Avanzada ■ ACKNOWLEDGMENTS Financial support from the MINECO of Spain (Projects CTQ2017-82935-P and RED2018-102387-T (AEI/FEDER, UE)), Gobierno de Aragon (Group E06_20R, project LMP148_18, and predoctoral contract to AM.), FEDER, and the European Social Fund is acknowledged. ■ REFERENCES (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure, Theory and Reactivity; Kluwer: New York, 2001. (b) Kubas, G. J. Metal-dihydrogen and σ-bond coordination: the consummate extension of the Dewar-Chatt-Duncanson model for metal-olefin π bonding. J. Organomet. Chem. 2001, 635, 37-68. (c) Esteruelas, M. A.; Lopez, A. M.; Olivan, M. Polyhydrides of Platinum Group Metals: Nonclassical Interactions and σ-Bond Activation Reactions. Chem. Rev. 2016, 116, 8770-8847. (d) Esteruelas, M. A.; Olivan, M.; Onate, E. Sigma-bond activation reactions induced by unsaturated Os(IV)- hydride complexes. Adv. Organomet. Chem. 2020, 74, 53-104. (a) Cheng, C.; Hartwig, J. F. Catalytic Silylation of Unactivated C-H Bonds. Chem. Rev. 2015, 115, 8946-8975. (b) Yang, Y.; Wang, C. Direct silylation reactions of inert C-H bonds via transition metal catalysis. Sci. China: Chem. 2015, 58, 1266-1279. (c) Hartwig, J. F.; Romero, E. A. Iridium-catalyzed silylation of unactivated C-H bonds. Tetrahedron 2019, 75, 4059-4070. (d) Richter, S. C.; Oestreich, M. Emerging Strategies for C-H Silylation. Trends Chem. 2020, 2, 13-27. (a) Corey, J. Y. Reactions of Hydrosilanes with Transition Metal Complexes and Characterization of the Products. Chem. Rev. 2011, 111, 863-1071. (b) Corey, J. Y. Reactions of Hydrosilanes with Transition Metal Complexes. Chem. Rev. 2016, 116, 11291-11435. (a) Jones, W. D.; Feher, F. J. Comparative Reactivities of Hydrocarbon C-H Bonds with a Transition-Metal Complex. Acc. Chem. Res. 1989, 22, 91-100. (b) Shilov, A. E.; Shul’pin, G. B. Activation of C-H Bonds by Metal Complexes. Chem. Rev. 1997, 97, 2879-2932. (c) Balcells, D.; Clot, E.; Eisenstein, O. C-H Bond Activation in Transition Metal Species from a Computational Perspective. Chem. Rev. 2010, 110, 749-823. (d) Eisenstein, O.; Milani, J.; Perutz, R. N. Selectivity of C-H Activation and Competition between C-H and C-F Bond Activation at Fluorocar- bons. Chem. Rev. 2017, 117, 8710-8753. (e) Gunsalus, N. J.; Koppaka, A.; Park, S. H.; Bischof, S. M.; Hashiguchi, B. G.; Periana, R. A. Homogeneous Functionalization of Methane. Chem. Rev. 2017, (a) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. J. Am. Chem. Soc. 2004, 126, 13794-13807. (b) Taw, F. L.; Bergman, R. G.; Brookhart, M. Silicon-Hydrogen Bond Activation and Formation of Silane Complexes Using a Cationic Rhodium(III) Complex. Organometallics 2004, 23, 886-890. (c) Matthews, S. L.; Pons, V.; Heinekey, D. M. Silane Complexes of Electrophilic Metal Centers. Inorg. Chem. 2006, 45, 6453-6459. (d) Vyboishchikov, S. F.; Nikonov, G. I. Rhodium Silyl Hydrides in Oxidation State + 5: Classical or Nonclassical? Organometallics 2007, 26, 4160-4169. (e) McGrady, G. S.; Sirsch, P.; Chatterton, N. P.; Ostermann, A.; Gatti, C.; Altmannshofer, S.; Herz, V.; Eickerling, G.; Scherer, W. Nature of the Bonding in Metal-Silane σ-Complexes. Inorg. Chem. 2009, 48, 1588-1598. (f) Gutsulyak, D. V.; Vyboishchikov, S. F.; Nikonov, G. I. Cationic Silane σ-Complexes of Ruthenium with Relevance to Catalysis. J. Am. Chem. Soc. 2010, 132, 5950-5951. (g) Scherer, W.; Meixner, P.; Barquera-Lozada, J. E.; Hauf, C.; Obenhuber, A.; Bruck, A.; Wolstenholme, D. J.; Ruhland, K.; Leusser, D.; Stalke, D. A Unifying Bonding Concept for Metal Hydrosilane Complexes. Angew. Chem., Int. Ed. 2013, 52, 6092-6096. (h) Komuro, T.; Arai, T.; Kikuchi, K.; Tobita, H. Synthesis of Ruthenium Complexes with a Nonspectator Si,O,P Chelate Ligand: Interconversion between a Hydrido(η2-silane) Complex and a Silyl Complex Leading to Catalytic Alkene Hydrogenation. Organometallics 2015, 34, 1211-1217. (i) Mai, V. H.; Korobkov, I.; Nikonov, G. I. Half-Sandwich Silane σ-Complexes of Ruthenium Supported by NHC Carbene. Organometallics 2016, 35, 936-942. (j) Price, J. S.; Emslie, D. J. H.; Berno, B. Manganese Silyl Dihydride Complexes: A Spectroscopic, Crystallographic, and Computational Study of Nonclassical Silicate and Hydrosilane Hydride Isomers. Organometallics 2019, 38, 2347-2362. Klare, H. F. T.; Oestreich, M.; Ito, J.; Nishiyama, H.; Ohki, Y.; Tatsumi, K. Cooperative Catalytic Activation of Si-H Bonds by a Polar Ru-S Bond: Regioselective Low-Temperature C-H Silylation of Indoles under Neutral Conditions by a Friedel-Crafts Mechanism. J. Am. Chem. Soc. 2011, 133, 3312-3315. (a) Sakakura, T.; Tokunaga, Y.; Sodeyama, T.; Tanaka, M. Catalytic C-H Activation. Silylation of Arenes with Hydrosilane or Disilane by RhCl(CO)(PMe3)2 under Irradiation. Chem. Lett. 1987, 16, 2375-2378. (b) Ezbiansky, K.; Djurovich, P. I.; LaForest, M.; Sinning, D. J.; Zayes, R.; Berry, D. H. Catalytic C-H Bond Functionalization: Synthesis of Arylsilanes by Dehydrogenative Transfer Coupling of Arenes and Triethylsilane. Organometallics 1998, 17, 1455-1457. (c) Cheng, C.; Hartwig, J. F. Rhodium- Catalyzed Intermolecular C-H Silylation of Arenes with High Steric Regiocontrol. Science 2014, 343, 853-857. (d) Cheng, C.; Hartwig, J. F. Mechanism of the Rhodium-Catalyzed Silylation of Arene C-H Bonds. J. Am. Chem. Soc. 2014, 136, 12064-12072. (e) Lee, K.; Katsoulis, D.; Choi, J. Intermolecular C-H Silylation of Arenes and Heteroarenes with HSiEt3 under Operationally Diverse Conditions: Neat/Stoichiometric and Acceptor/Acceptorless. ACS Catal. 2016, 6, 1493-1496. (a) Gustavson, W. A.; Epstein, P. S.; Curtis, M. D. Homogeneous Activation of the C-H Bond Formation of Phenyl- siloxanes from Benzene and Silicon Hydrides. Organometallics 1982, 1, 884-885. (b) Ishiyama, T.; Sato, K.; Nishio, Y.; Miyaura, N. Direct 117, 8521-8573. (f) Esteruelas, M. A.; Olivan, M. C-H Activation Coupling Reactions. In Applied Homogeneous Catalysis with Organo- metallic Compounds: A Comprehensive Handbook in Four Volumes, 3rd ed.; Cornils, B.; Herrmann, W. A.; Beller, M.; Paciello, R., Eds.; Wiley, 2017; Chapter 23, pp 1307-1332. (g) Zhao, Q.; Meng, G.; Nolan, S. P.; Szostak, M. N-Heterocyclic Carbene Complexes in C-H Activation Reactions. Chem. Rev. 2020, 120, 1981-2048. Nikonov, G. I. Recent Advances in Nonclassical Interligand Si··· H Interactions. Adv. Organomet. Chem. 2005, 53, 217-309. (b) Lachaize, S.; Sabo-Etienne, S. σ-Silane Ruthenium Complexes: The Crucial Role of Secondary Interactions. Eur. J. Inorg. Chem. 2006, 2006, 2115-2127. Synthesis of Aryl Halosilanes through Iridium(I)-Catalyzed Aromatic C-H Silylation by Disilanes. Angew. Chem., Int. Ed. 2003, 42, 5346- 5348. (c) Ishiyama, T.; Sato, K.; Nishio, Y.; Saiki, T.; Miyaura, N. Regioselective aromatic C-H silylation of five-membered heteroarenes with fluorodisilanes catalyzed by iridium(I) complexes. Chem. Commun. 2005, 5065-5067. (d) Saiki, T.; Nishio, Y.; Ishiyama, T.; Miyaura, N. Improvements of Efficiency and Regioselectivity in the Iridium(I)-Catalyzed Aromatic C-H Silylation of Arenes with Fluorodisilanes. Organometallics 2006, 25, 6068-6073. (e) Lu, B.; Falck, J. R. Efficient Iridium-Catalyzed C-H Functionalization/ Silylation of Heteroarenes. Angew. Chem., Int. Ed. 2008, 47, 7508- 7510. (f) Ishiyama, T.; Saiki, T.; Kishida, E.; Sasaki, I.; Ito, H.;3PO