Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl

Article pubs.acs.org/Organometallics

Efficient Lewis Acid Promoted Alkene Hydrogenations Using Dinitrosyl Rhenium(−I) Hydride Catalysts Yanfeng Jiang, Wenjing Huang, Helmut W. Schmalle, Olivier Blacque, Thomas Fox, and Heinz Berke* Anorganisch-Chemisches Institut, Universität Zürich, Winterthurerstraße 190, CH-8037, Zurich, Switzerland S Supporting Information *

ABSTRACT: Highly efficient alkene hydrogenations were developed using NOfunctionalized hydrido dinitrosyl rhenium catalysts of the type [ReH(PR3)2(NO)(NO(LA))][Z] (2, LA = B(C6F5)3; 3, LA = [Et]+, Z = [B(C6F5)4]−; 4, LA = [SiEt3]+, Z = [HB(C6F5)3]−; R = iPr a, Cy b). Lewis acid attachment to the NO ligand was found to facilitate bending at the NOLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order 4 > 3 > 2, which did not match with the order of increasing hydrogenation activities: 3 > 4 > 2. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the ONO−LA bonds. LA detachment from the ONO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “1/ hydrosilane/B(C6F5)3” (1:5:5) system exhibited the highest performance, with TOFs up to 1.2 × 105 h−1 (1-hexene, 1-octene, cyclooctene, cyclohexene). The cocatalyst [Et3O][B(C6F5)4] showed the smallest effect, presumably due to the strong Lewis acidic character of the reagent causing side-reactions before reacting with 1a,b. The catalytic reaction course crucially involves not only reversible bending at the NOLA atom but also loss of a PR3 ligand, forming 16e or 14e monohydride reactive intermediates, which drive an Osborn-type hydrogenation cycle with olefin before H2 addition.

1. INTRODUCTION In recent years tuning of middle transition metal complexes for homogeneous catalysis became a major research challenge, avoiding the use of platinum group metals.1 Middle transition metals in general tend to obey the 18e rule and are reluctant to form complexes with low coordination numbers; consequently they show little tendency for ligand exchanges as crucial properties for catalytic reaction courses. Tuning of the ancillary ligand sphere by ligand effects for self-labilization or labilization of other ligands turns out to be the only way to eventually establish appropriate conditions for a well-functioning catalytic cycle and to render high catalytic activities. Ligand properties used to stabilize 16- or 14-electron intermediates are the cis labilization effect,2 the trans influence and trans effect,3 noninnocent ligand properties,4 and adjustment of the πaccepting properties of ligands and of their steric properties via incorporation of large Tolman cone angle ligands and/or large bite angle chelate ligands.5 In line with some of the listed properties is the nitrosyl ligand, which possesses three prominent functions in transition metal based catalysis (Scheme 1).6 Two of these functions (1 and 2 of Scheme 1) became the main focus of our group in the exploration of efficient catalyses based on middle transition metal complexes. (1) First, the nitrosyl ligand can serve as an ancillary ligand exhibiting strong electron-withdrawing σ- and π-effects/ influences to activate the trans ligand.7 For σ-donors, such as the hydride ligand, and π-acceptors such as a H2 ligand trans to NO, NO may cause enhancement of the hydridic or acidic © 2013 American Chemical Society

Scheme 1. NO Function in Transition Metal Based Catalysis

property, respectively, prominent in secondary coordination sphere activation processes.8 For π-donors trans to NO, such as halide ligands, appropriate Lewis acidic cocatalysts often can support the elimination of these ligands and facilitate generation of highly electron-deficient metal centers with one vacant site. This was exemplified by the boron Lewis acid promoted catalytic activities of mononitrosyl rhenium bromo complexes in alkene hydrogenation reactions, where the trapping of the bromo ligand by external boron Lewis acid serves as the key initiating step.1a Received: July 25, 2013 Published: November 4, 2013 7043

dx.doi.org/10.1021/om400733u | Organometallics 2013, 32, 7043−7052

Organometallics

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(2) The nitrosyl ligand can be considered as a “noninnocent” ligand capable of supporting different oxidation states of metal centers via either linear (M−N−O of 160−180°) or bent (M− N−O of 110−140°) coordination modes.9 Bending triggers a NO+/NO− transformation, which is equivalent to a formal 2e oxidation of the metal center and often accompanied by a change in the coordination geometry with formation of a vacant site.10 This redox change induced by NO bending was also termed “stereochemical control of valence” by Enemark and Feltham.11 It is anticipated to have great impact on the activation of molecules in catalytic reaction courses, allowing them “silent” addition to metal centers without ligand exchange. Exploiting this property we recently reported a silylium cation triggered nitrosyl bending, boosting performance of mononitrosyl rhenium diiodo catalysts to high efficiencies in alkene hydrogenations, denoted as the “catalytic nitrosyl effect”.12 In situ NMR spectroscopy and DFT calculations verified the facilitation of NO bending upon attachment of the silylium cation to the ONO atom. This newly formed intermediate in turn promoted heterolytic H−H splitting and subsequent protonation of a phosphine ligand. (3) The nitrosyl ligand can serve as a “nitrosyl/nitro” redox couple, supporting secondary oxo- or oxene-transfer reactions,13 which however are not targets of this paper. In order to take advantage of properties 1 and 2 of the NO ligand in catalytic reaction courses (Scheme 1), the attachment of the Lewis acids (LA) to the NO ligand was intended to be probed, however not by starting from a cocatalytic mixture of the complex and LA, rather than by creating a singlecomponent catalyst attaching the Lewis acid to the ONO atom prior to catalysis. The halide-free hydrido dinitrosyl rhenium complexes [ReH(PR3)2(NO)2] (1, R = iPr a, Cy b), which are five-coordinate 18e, d8 Re(−I) complexes possessing a TBP coordination geometry,14 are eventually used as base components in homogeneous alkene hydrogenations. The ONO atoms were expected to be Lewis basic enough to enable attachment of Lewis acids.15 ONO functionalization of 1a,b was carried out with the following Lewis acids, the neutral B(C6F5)3, and the cationic [Et]+ and [SiEt3]+ moieties16 to induce the expected effects in catalytic hydrogenations.

Scheme 2. Synthesis of Lewis Acid Functionalized Dinitrosyl Rhenium(−I) Hydrido Complexes

low NO bond order, which may be taken as an indication of enforced strong electron back-donation from the metal to the π* orbital of the NO ligand. Similarly, the reactions of 1a,b with 1 equiv of [Et3O][B(C6F5)4] in diethyl ether afforded at 23 °C within 1 h the cationic ethyl-derivatized dinitrosyl complexes [ReH(PR3)2(NO)(NOEt)][B(C6F5)4] (3, R = iPr a, Cy b) in 93% (3a) and 86% (3b) yield, respectively. In the 1H NMR spectra the hydride resonances were found shifted low-field to 5.50 (3a) and 5.41 (3b) ppm in comparison to those of 2a and 2b, indicating higher electron deficiencies at the rhenium centers, which seemed reasonable mainly as a consequence of the positive charge of 3a,b. The signal of the ethyl group was in either case observed as a quartet at 4.77 ppm (2JHH = 7 Hz) and a triplet at 1.44 ppm (2JHH = 7 Hz) for 3a and at 4.76 ppm (2JHH = 6 Hz) and 0.90 ppm (2JHH = 6 Hz) for 3b. In the 31 1 P{ H} NMR spectra singlet resonances appeared at 59.2 ppm for 3a and at 49.1 ppm for 3b, shifted low-field with respect to those of 1a and 1b.14a In the IR spectra the vibrations of the “ethyl-free” NO groups were observed at 1664 for 3a and at 1670 cm−1 for 3b. The vibrations of the ethyl-bonded NO groups were found at the very low wavenumbers of 1252 (3a) and 1249 cm−1 (3b), indicative of a strongly decreased N−O bond order. It has been reported by Piers and Oestreich that the reaction of B(C6F5)3 with hydrosilanes affords a reactive adduct described by the B··H−Si ↔ B−H··Si hyperconjugative resonance structures.18 These reagents constitute crypto silylium cation sources capable of derivatizing particularly oxygen functions due to the high oxophilicity of the silicon atom. When a mixture of 1.0 equiv of B(C6F5)3 and 1.5 equiv of Et3SiH in toluene was treated with 1a,b at 23 °C, ONO functionalization took place within 5 min with formation of the [ReH(PR3)2(NO)(NOSiEt3)][HB(C6F5)3] salts (4, R = iPr a, Cy b) in quantitative in situ yields and in 53% (4a) and 67% (4b) isolated yields. Silylium binding to the ONO atom was confirmed also via 29Si NMR spectroscopy, since singlets were observed for both 4a and 4b at 41.0 ppm, which appeared in the typical chemical shift range of [R3SiO]+ compounds.19 In the 1H/29Si correlation spectra the 29Si NMR singlets of both derivatives showed correlation with the 1H NMR resonances of the ethyl groups appearing at 0.56 (t) and 0.33 (q) ppm for 4a

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of NO-Functionalized Dinitrosyl Hydrido Rhenium(−I) Complexes. The reactions of 1a,b with various Lewis acids led indeed to formation of the isolable and stable ReNO-LA adducts 2a,b, 3a,b, and 4a,b, as depicted in Scheme 2. The preparations of compounds 2a and 3a were described in a previous short communication by our group.17 The [ReH(PR3)2(NO)(NOB(C6F5)3)] derivatives (2, R = iPr a, Cy b) were obtained in quantitative yields by treating 1a,b with 1 equiv of B(C6F5)3 in toluene at 23 °C within 30 min. The 1H NMR spectra revealed characteristic triplets for the hydride ligand, which were shifted slightly high-field to 4.07 (2a) and 4.17 (2b) ppm in comparison with those of 1a and 1b.14a The 31P{1H} NMR spectra exhibited a singlet at 53.5 for 2a and at 41.3 ppm for 2b, indicative of two chemically equivalent phosphorus nuclei in each coordination environment. In the IR spectra the ν(NO) vibration of the B(C6F5)3-bonded ligand appeared at the low wavenumbers of 1340 for 2a and 1339 cm−1 for 2b, shifted by over 300 cm−1 to lower wavenumbers in comparison with those of the “free” nitrosyl ligands at 1641 (2a) and 1644 cm−1 (2b). The lower wavenumbers of B(C6F5)3-attached ligands reflect a 7044


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Figure 1. Molecular structures of 2a, 2b, 3a, and 3b with 30% probability displacement ellipsoids. All hydrogen atoms were omitted for clarity except for the HRe atoms. The solvent molecules and the [B(C6F5)4]− counteranion in 3a and 3b were also omitted for clarity.

and at 0.83 (t) and 0.76 (q) ppm for 4b. The HRe signals were slightly shifted low-field to 4.38 for 4a and to 4.71 ppm for 4b in comparison with those of 1a and 1b. Similar downfield chemical shifts were observed in the 31P NMR spectra, revealing singlets at 57.7 for 4a and at 47.0 ppm for 4b, implying also electron deshielding of the rhenium centers. Moreover, evidence for the presence of the [HB(C6F5)3]− counterions was provided by the 11B NMR spectra, showing a doublet resonance at −24.5 for 4a and at −24.8 ppm for 4b. The structure of 4a was further substantiated by a NOE experiment, wherein dipolar H···H couplings were detected between the HRe atom and the SiEt3 group established through irradiation of the isopropyl CH signal of 4a at 1.98 ppm. ESIMS spectroscopy confirmed for 4a and 4b the presence of both the Re(−I) cations and the borate anions, respectively. In the IR spectra of 4a and 4b ν(NO) vibrations of the silyliumderivatized NO groups appeared at the low wavenumbers of 1340 (4a) and 1296 cm−1 (4b) apparently due to drainage of electron density from the rhenium to the NO ligand. It should also be noted at this point that the structurally related complexes bearing [BAr′4]− anions could previously be accessed from the reaction of the cationic 16e complexes [Re(PR3)2(NO)2][BAr′4] with Et3SiH through H−Si bond heterolytic cleavage.14b Further structural proof for these Lewis acid adducts was established by X-ray diffraction studies of single crystals of 2a, 2b, 3a, and 3b. Structural models are displayed in Figure 1. In comparison to 1a,b,14a the coordination of the Lewis acid to the ONO atom of the nitrosyl ligand only slightly changes the coordination geometries at the rhenium center. Distorted TBP geometries were invariably seen to be adopted in the four molecular structures bearing the two types of NO groups and one hydride in the trigonal plane and the two phosphine ligands occupying apical positions. The P−Re−P angles in the range ca. 135−143° deviate strongly from linearity (Table 1). These moieties show even stronger bending toward the HRe atom than in the structure of 1a (154°).14a This can be interpreted in terms of enhancement of π-back-donation from the rhenium center to the NO ligand.20 The bending of the phosphines polarizes the rhenium dxz orbital, providing better dxz−π*NO overlap. Steric repulsion of the bulky B(C6F5)3 groups with the phosphines in 2a,b might also contribute to the decrease in P−Re−P angles. Evidently the Lewis acid coordinates to the nitrosyl ligand in an attracto fashion, leaning over toward the other NO ligand. Noteworthy is the fact that the Lewis acid functionalized nitrosyl ligands remained essentially linear in all four cases of structures, showing Re−

Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 2a, 2b, 3a, and 3b

a

parameter

2a

2b

3a

3b

Re−N1 Re−N2 (LA) Re−P1 Re−P2 N1−O1 N2−O2(LA) P1−Re−P2 O1−N1−Re O2−N2−Re N1−Re−N2 τa

1.792(4) 1.794(3) 2.4274(10) 2.4252(10) 1.186(5) 1.261(4) 134.77(3) 167.9(4) 175.0(3) 126.09(16) 0.46

1.771(7) 1.763(7) 2.439(2) 2.430(2) 1.232(9) 1.294(9) 141.71(6) 168.3(6) 171.1(5) 126.6(3) 0.63

1.776(3) 1.755(3) 2.4169(10) 2.4249(9) 1.197(4) 1.319(4) 143.39(3) 164.5(3) 172.5(3) 126.11(16) 0.62

1.788(3) 1.775(3) 2.4295(9) 2.4185(10) 1.180(4) 1.317(4) 139.60(3) 167.4(3) 171.3(3) 129.39(16) 0.59

τ = (β − α)/60; see text.

N−O angles in the range ca. 171−175°. The Re−N bond distances are short, lying in the range ca. 1.76−1.79 Å, similar to those of linear nitrosyls.21 The “Lewis acid free” nitrosyls possess structures with NO double-bond character, showing distances of ca. 1.18−1.23 Å.21 In comparison, the Lewis acid coordinated N−O bonds are lengthened (range of ca. 1.26 to 1.32 Å), approaching N−O single-bond character. This is again an indication of strong π-back-donation from the Re dxz orbital to the π*NO orbital, which is also reflected in the shortened Re−N bonds of the Re-NOLA moieties in comparison with the “Lewis acid free” compounds 1a and 1b. This change in the Re−N and N−O bond distances upon Lewis acid attachment is more pronounced in the structures of 3a,b than in those of 2a,b, emphasizing the stronger electron-withdrawing effect of the [Et]+ cation than of the neutral B(C6F5)3 moiety. As the linear nitrosyl is formally viewed as a [NO]+ unit, an 18e configuration was assigned to the d8 Re(−I) centers. In order to put these structural results in more systematic context for pentacoordinate structures, we employed the structural index parameter τ, which is an angular parameter introduced by Addison et al.22 as τ = (β − α)/60 to differentiate between the TBP and SP geometries. As depicted in Scheme 3, the Lewis acid functionalized nitrosyls take the axial position, the largest basal angle (P−Re−P) is defined as β, and the second largest basal angle (H−Re−NO) is defined as α. The parameter τ is formulated as τ = (β − α)/60. In a perfect TBP with β = 180° and α = 120° τ amounts to 1. On the other hand, an ideal square pyramid has α = β = 180°, and its angular parameter τ is 0. Therefore, any structural intermediate between SP and TBP must have an angular 7045


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Scheme 3. Definition of the Parameter τ

energies of the ONO-LA moieties were found to possess the order [Et]+ > [SiMe3]+ > BF3. Coordination of BF3 to the ONO atom of [Re] is accompanied by an energy release of 11.3 kcal/ mol. In comparison, [Et]+ and [SiMe3]+ are more strongly bound to the ONO atom, showing an energy release of −83.3 and −64.9 kcal/mol, respectively. Subsequently the energy required for NO bending was studied. The optimized ground-state structures possess linear Re−N−OLA arrangements. Structures kept artificially at a Re− N−OLA bending angle of 125° at the N atom are shown in Figure 2. The “Lewis acid free” complex [Re] requires an energy of +7.7 kcal/mol to adopt a Re−N−O bending angle of 135°. Decreasing the Re−N−O angle further to 125° resulted in the higher energy of +12.9 kcal/mol. In comparison, the [Re-BF3] complex required a lower bending energy of +6.4 kcal/mol to reach an angle of 135°, and further bending to 125° indeed resulted in the relatively low-energy demand of +9.7 kcal/mol. This Lewis acid triggered nitrosyl bending became even more facilitated in the cationic [Re-Et]+ and [Re-SiMe3]+ model complexes, where the small energy demands of +4.3 or +3.3 kcal/mol were calculated upon Re−N−O bending to 135° for [Re-Et]+ or [Re-SiMe3]+, respectively. For the enhanced bending angle of 125° the relatively low energies of +5.4 kcal/ mol for [Re-Et]+ and +5.1 kcal/mol for [Re-SiMe3]+ were required. The more facile bending of the NO ligand with the [Et]+ and [SiMe3]+ cations attached is interpreted in terms of a stronger shift of electron densities from the rhenium to the NNO atom, forming a lone pair at this atom, facilitating the bending.

parameter τ in the range 0 ≤ τ ≤ 1. Thus, the angular parameter τ tells us how far or close a particular structure is from the ideal TBP or SP. Applying this definition, the parameter τ for 1a is calculated to be 0.53, which can be taken as a reference. In comparison, τ values of 0.46, 0.63, 0.62, and 0.59 were obtained for 2a, 2b, 3a, and 3b, respectively. These data emphasize the presence of geometries in between TBP and SP, but except for 2a the adopted geometries are slightly closer to TBPs on going from 1a,b to the Lewis acid derivatized nitrosyl ligands, which is in agreement with the observation that generally TBPs of five-coordinate d8 complexes are more stable than the corresponding SP geometries.23 2.2. DFT Evaluations of Lewis Acid Induced Nitrosyl Bending. In order to further evaluate the bonding between the ONO atom and the Lewis acid, DFT calculations were carried out at the B3LYP level with an SDD basis set for Re and 631+G(d) for the other atoms using a PMe3 model complex ReH(NO)2(PMe3)2 ([Re]) and BF3, [Et]+, and [SiMe3]+ attached to one ONO atom in an attracto fashion. The geometry optimizations led to local and global minima of [Re-BF3], [ReEt]+, and [Re-SiMe3]+ structures. In general, the association

Figure 2. Optimized ground states of [Re], [Re-BF3], [Re-Et]+, and [Re-SiMe3]+ upon bending of the Re−N−OLA angle to 125° with relative energies of the local minima (ΔE, kcal/mol). All the carbon and hydrogen atoms except for the C atoms of the Et+ and SiMe3+ groups and the hydride ligand are omitted for clarity. 7046


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Table 2. Relative Energies, Geometry Parameters of the Local Minima, and τ Values Obtained upon Bending of Nitrosyl Ligand [Re]

[Re-Et]+

[Re-BF3]

[Re-SiMe3]+

Re−N−O (deg)

172.9

135.0

125.0

177.4

135.0

125.0

177.3

135.0

125.0

177.2

135.0

125.0

ΔE (kcal/mol) H−Re−N1 (deg) P1−Re−P2 (deg) N1−Re−N2 (deg) Re−N1 (Å) Re−N2 (LA) (Å) N1−O1 (Å) N2−O2 (LA) (Å) ν(NO) ν(N-OLA) τ

0.0 116.8 157.8 126.4 1.815 1.815 1.205 1.205

+7.7 143.7 159.4 108.4 1.806 1.852 1.207 1.213

+12.9 146.7 161.8 105.6 1.809 1.879 1.206 1.220

0.68

0.26

0.25

0.0 108.8 160.6 126.9 1.807 1.790 1.195 1.257 1744 1511 0.86

+6.4 138.4 160.9 109.1 1.799 1.806 1.200 1.270 1716 1371 0.38

+9.7 145.2 162.5 105.2 1.806 1.825 1.198 1.280 1718 1312 0.29

0.0 103.6 147.5 127.0 1.804 1.789 1.186 1.302 1784 1313 0.73

+4.3 137.5 150.3 107.3 1.797 1.796 1.189 1.325 1765 1171 0.21

+5.4 146.0 152.2 103.8 1.808 1.808 1.188 1.337 1765 1098 0.10

0.0 105.8 146.6 126.2 1.808 1.783 1.188 1.302 1774 1374 0.68

+3.3 140.4 149.2 107.2 1.803 1.793 1.190 1.321 1757 1215 0.15

+5.1 146.5 151.2 104.0 1.811 1.806 1.189 1.334 1758 1138 0.08

Table 3. Hydrogenation of Alkenes Catalyzed by Rhenium Dinitrosyl Complexes with or without Cocatalystsa

entry

alkene (mmol)

[Re]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) 1-hexene (20) cyclooctene (16) cyclooctene (16) cyclooctadiene (16) 1,7-octadiene (14) 1-hexene (24) 1-hexene (24) 1-hexene (24) 1-hexene (24) 1-octene (16) 1-octene (16) cyclooctene (16) cyclooctene (16) cyclohexene (25) cyclohexene (25)

1a 1b 2a 2a 2b 2b 3a 3a 3b 3b 4a 4a 4b 4b 3a 3b 3b 3b 1a 1b 1a 1b 1a 1b 1a 1b 1a 1b

cocatalyst (5 equiv)

B(C6F5)3 B(C6F5)3 [Et3O][B(C6F5)4] [Et3O][B(C6F5)4] Et3SiH/B(C6F5)3 Et3SiH/B(C6F5)3

Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Et3SiH/B(C6F5)3 Et3SiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3 Me2PhSiH/B(C6F5)3

time (h)

conversion (%)

15.0 39.0 4.0 1.0 4.5 24/60 2.0 2.0 1.7 1.5 3.0 45/60 2.5 18/60 1.5 1.0 7.0 16.0 8/60 8/60 20/60 22/60 5/60 4/60 30/60 20/60 40/60 45/60

24 94 83 98 100 100 100 92 100 99 76 100 80 100 100 100 66b 100 76 91 97 99 100 100 85 71 82 86

TON 2405 9400 8260 9778 1.0 × 1.0 × 1.0 × 9240 1.0 × 9930 7600 1.0 × 8022 1.0 × 8000 8000 5280 7000 9176 1.1 × 1.2 × 1.2 × 8000 8000 6792 5714 1.0 × 1.1 ×

104 104 104 104

104 104

104 104 104

104 104

TOF (h−1) 160 241 2065 9778 2222 2.5 × 5000 4600 5882 6620 2533 1.3 × 3209 3.3 × 5333 8000 754 437 6.9 × 8.2 × 3.5 × 3.2 × 9.6 × 1.2 × 1.4 × 1.7 × 1.5 × 1.4 ×

104

104 104

104 104 104 104 104 105 104 104 104 104

Reactions were performed in a steel autoclave using 0.002 mmol of rhenium complex under 40 bar of H2 at 100 °C. In the case of application of cocatalysts, these were added in a 5:1 molar ratio with respect to the Re catalyst. The reaction progress was monitored by the decrease in H2 pressure. bWith a distribution of 61% of cyclooctene and 39% of cyclooctene. a

parameter τ of each state. When BF3 is coordinated, the τ value sharply decreases from 0.86 in the ground state to 0.29 in the bent form (125°), approaching indeed a distorted SP geometry. In the case of [Re-Et]+ and [Re-SiMe3]+, the bent form at 125° is much closer to an ideal SP coordination, exhibiting a τ value of 0.10 and 0.08, respectively. The distortion toward the SP geometry triggered by the bending of the apical NOLA ligand

The geometric parameters of each structural state are listed in Table 2. Importantly, along the bending course the H−Re− NO angles open up drastically with concomitant decrease of the N−Re−N angles, indicating a geometric transformation from TBP (ground state) toward SP at the NO bending angle of 125°. In comparison, the P−Re−P angle displayed an only slight increase during the bending course. The geometry change became still more evident when comparing the structural index 7047


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Scheme 4. Proposed Catalyst Activation Pathways

creates a vacant site opposite of this ligand, expected for complexes with a d6 configuration. 2.3. NO-Functionalized Hydrido Complexes as Catalysts in Hydrogenations of Alkenes. The generation of a vacant site via Re−N−O or Re−N−OLA bending was anticipated to be beneficial or even crucial for the catalytic performance of rhenium compounds, which normally prefer 18e coordination spheres and show low tendency for ligand exchange reactions. Assuming the capability of facile Re−N− OLA bending of 1a,b−4a,b, the catalytic potential in hydrogenation of alkenes was still to be manifested.24 For the different series of catalysis experiments conditions were chosen to be as much as possible comparable with each other to allow comparison. Typically solvent-free conditions were applied using 2.5 mL of 1-hexene (20 mmol) with 0.01 mol % of the rhenium catalyst under 40 bar of H2 at 100 °C. The results for 1-hexene, cyclic alkenes (standing for disubstituted alkenes), and dienes are listed in Table 3. The non-LA-functionalized complexes 1a,b showed low hydrogenation activities, despite their relative moderate energy demand for NO bending to offer vacant sites. Applying 1a as catalyst in 1-hexene hydrogenation, a conversion of 24% was accomplished within 15 h, corresponding to a TOF of 160 h−1 (entry 1). 1b afforded a TON of 9400, but only after the long reaction time of 39 h (entry 2). In contrast to this, the NOfunctionalized complexes 2a,b−4a,b demonstrated significant catalytic accelerations. For instance, the hydrogenation with 2b proceeded quickly, affording a TON of 1.0 × 104 and a TOF of 2222 h−1 within 4.5 h (entry 5). 2a turned out to perform slightly less well than 2b and furnished a TON of 8260 at 83% conversion after 4 h, corresponding to a TOF of 2065 h−1

(entry 3). Further improvement of the activities was accomplished with [Et3Si]+ functionalization of 4a,b, which afforded TONs of 7600 for 4a and 8022 for 4b within 3.0 and 2.5 h, corresponding to TOFs of 2533 h−1 (4a) and 3209 h−1 (4b), respectively. The [Et]+-functionalized complexes 3a,b proved to be the most active ones. A TON of 1.0 × 104 and a TOF of 5000 h−1 were achieved with 3a within 2.0 h at full conversion (entry 7). 3b showed slightly better performance than 3a, affording 100% conversion within 1.7 h, corresponding to a TOF of 5882 h−1 (entry 9). Other alkenes, such as cyclooctene, cyclooctadiene, and 1,7-octadiene, were also tested as substrates in the hydrogenations with 3a,b as catalysts, performing best in the given series of experiments. Surprisingly the reactions of the disubstituted olefin cyclooctene turned out to be very efficient. Within 1.5 h for 3a and 1.0 h for 3b not only were full conversions accomplished but also a maximum TOF of 8000 h−1 (entry 16) was reached. In contrast, the related hydrogenations of dienes required a relatively long reaction time to reach reasonable TONs (entries 17 and 18). Generally, no alkene isomerized or polymerized product was observed in the case of full conversion, and only trace amounts ( 4a,b > 2a,b, correlating with the calculated order of the ONO−LA bond strength of the [Re-Et]+, [Re-SiMe3]+, and [Re-BF3] species. For 2a,b, showing the weakest Lewis acid attachment, we anticipated a too easy loss of the coordinated Lewis acid approaching the poor catalytic level of 1a,b. Therefore, we reckoned that addition of a stoichiometric excess of B(C6F5)3 (5 equiv) as a cocatalyst to 2a,b could 7048


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alternative pathways are followed depending on the type of Lewis acid functionalization. 1. Path 1. The generated vacant site of I-1 gets occupied by an alkene ligand, affording intermediate I-2, possessing an 18e pseudo-octahedral structure. The alkene ligand is presumably orientated in the H−Re−NO plane, prone to facile alkene insertion into Re−H when entering the Osborn-type hydrogenation cycle,26 which includes elementary steps of alkene insertion into the M−H bond, coordination of H2, and reductive elimination. DFT geometry optimizations on the [ReH(PMe3)2(NO)(NOSiMe3)(η2-C2H4)]+ model complex of I-1 however showed no energetic minimum for the ethylene coordination, and furthermore the full energy hypersurface to simultaneously add the olefin and transfer the hydride onto it could not be calculated. Despite this, this pathway looks reasonable from a more empirical point of view and cannot be excluded, particularly in the case of 1, where low catalytic activities were recognized. 2. Path 2. The excellent performance exhibited by type 2 or “2/B(C6F5)3” catalysts in comparison with 1 strongly suggested different activation pathways involving reversible phosphine dissociation from the rhenium center. In this way the “double Lewis acidic” 14e species I-3 bearing two vacant sites is assumed to be generated. Taking into account Negishi’s notion of catalytic enhancement by formation of “double Lewis acidic” “superelectrophiles”,27 the Lewis acid promoted NO bending together with the generation of another vacant site is essential for excellent catalytic performance, since the “double Lewis acid” will “melt” the two Lewis centers temporarily together to form a Negishi-type “superelectrophile”. The generated super vacant site of I-3 is then occupied by an alkene ligand, resulting in the 16e Re(I) species I-4, which can enter the Osborn-type hydrogenation cycle. The highly electron-deficient nature of I-4 is expected to lower the barrier for the insertion reaction of the olefin into the Re−H bond, contributing to the overall acceleration in the hydrogenation catalyses. However, it can be argued that the 14e species I-3 is too reactive to exist. This view could be compromised by arguing that this species is expected to be very short-lived, and its “resting state” is the linear NO-LA form I-5 with a 16e Re(-I) configuration and a TBP geometry and just one vacant site. A related intramolecular redox process with NO bending was also proposed for ruthenium complexes by Eisenstein and Caulton.28 Coordination of the alkene would lead to formation of I-6 ready for entering the Osborn-type catalytic cycle.26 The 18e Re(−I) species I-6 is supposed to equilibrate with the 16e Re(I) species I-4. The proposed unsupported reversible phosphine dissociations look reasonable for the cases of initiation courses of 2a,b or the B(C6F5)3 attached cases 2a,b, since the formation of phosphonium cations or R3P-B(C6F5)3 adducts could not be observed. 3. Path 3. The formation of the phosphonium cation as a byproduct in the hydrogenations catalyzed by the cationic type 3 and 4 complexes or those obtained in situ with the “hydrosilane/B(C6F5)3” reagents as cocatalysts suggests the existence of the third activation pathway, involving irreversible phosphine dissociation from the rhenium center. Ethylene and H2 have similar binding affinities to transition metal centers.29 Therefore instead of ethylene, the small H2 molecule could enter the coordination sphere of I-1 to occupy the vacant site, affording the 18e Re(I) species I-7.30 Heterolytic cleavage of the H−H bond could occur by deprotonation with a neighboring phosphine to form [HPR3]+ phosphonium salts

improve the performance of the olefin hydrogenations by pushing the dissociation equilibrium of the Lewis acid to the side of 2a,b. Much to our pleasure, the reaction catalyzed with 0.01 mol % of the “2b/B(C6F5)3” system (1:5) afforded full conversion within 24 min, corresponding to a TON of 1.0 × 104 and a TOF of 2.5 × 104 h−1 (entry 6), which suggested an 11 times acceleration with respect to the case of no extra B(C6F5)3 (entry 5). Similarly, the “2a/B(C6F5)3” system (1:5) showed a 5 times increase in activity for the hydrogenation of 1-hexene, affording a TOF of 9778 h−1 within 1.0 h (entry 4). It should be mentioned that formation of the PR3/B(C6F5)3 addition products [(R3P)(C6F4)BF(C6F5)2] (R = iPr, Cy) reported by Stephan et al.25 was not observed after catalyses, excluding the ability of B(C6F5)3 to abstract a phosphine ligand from the rhenium center. These results strongly suggested that the lower activities of the 2a,b catalysts with no extra B(C6F5)3 added were mainly due to ONO−B(C6F5)3 bond cleavages and subsequent B(C6F5)3 scavenging in the catalytic course by various nucleophiles evolved in the catalytic reactions. The same trick to add extra Lewis acid was applied using 4a,b as catalysts. The addition of 5 equiv of “Et3SiH/B(C6F5)3” (1:1) to 4b revealed a ca. 10 times acceleration in the hydrogenation of 1-hexene, showing a TON of 1.0 × 104 and a TOF of 3.3 × 104 h−1 within 18 min (entry 14). The promotion effect of the added cocatalytic system was less pronounced with 4a. A ca. 5 times acceleration was observed upon addition of 5 equiv of “Et3SiH/B(C6F5)3” as cocatalyst to 4a (entry 12). Unfortunately, such methodology to improve activities could not be applied to catalyses with 3a,b. The additional presence of [Et3O][B(C6F5)4] did not significantly improve the hydrogenation performance in comparison with the catalyses using solely 3a,b. Among other effects this was attributed not only to the poor solubility of [Et3O][B(C6F5)4] in 1-hexene but also to the highly reactive character of this species, reacting not only with 1a,b in the reaction mixture. When the cationic systems of 3b, 4b, “4b/Et3SiH/B(C6F5)3”, and “1b/Et3SiH/B(C6F5)3” were applied as catalysts, the phosphonium cation [HPCy3]+ combined with the [B(C6F5)4]− or [HB(C6F5)3]− anions was invariably observed as the major phosphorus-containing species after catalysis. [HPCy3]+ was previously identified in the hydrogenation system of “Re(I) diiodo/hydrosilane/B(C6F5)3”.12 In comparison, this phosphonium cation was not observed in hydrogenations catalyzed by 1b, 2b, or the “2b/B(C6F5)3” system. These observations not only suggested that the higher catalytic activities can be achieved by removal of at least one phosphine ligand from the rhenium center of the catalyst but also implied that different activation pathways are operative for functionalization with the neutral Lewis acid B(C6F5)3 and the cationic Lewis acids [Et]+ and [SiEt3]+. It should be noted that it is the same as the previous case12 that the reaction of free PCy3 and B(C6F5)3 with 1-hexene and H2 under the reaction conditions did not reveal the phosphonium salt [HPCy3][HB(C6F5)3]. Therefore a frustrated Lewis pair (FLP)-type hydrogenation as a competitive reaction step seemed less plausible. 2.4. The “Catalytic Nitrosyl Effect”: Initiation of Catalysis via NO Bending. The mechanism for catalyst activation was proposed in Scheme 4. The crucial step corresponds to the NO bending assisted by the coordinated Lewis acid, denoted as the “catalytic nitrosyl effect”. The 18e TBP Re(−I) monohydride complexes of types 2−4 form first a distorted square pyramidal Re(I) 16e intermediate I-1 possessing a vacant site trans to the bent NO moiety. Three 7049


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and a dihydride of type I-8,31 which are neutral species when the attached LAs are cations. Subsequent coordination of the alkene to the vacant site possibly oriented in plane with either the H−Re−NO(linear) or H−Re−NO(bent) arrangement would allow this species to enter the Osborn-type hydrogenation cycle.26 Path 3, involving cationic Lewis acids, seems to also get support from the positive charge enhancing the H− H heterolytic cleavage by PR3 so that catalysis could also get more efficient than those following path 2. This is in accord with experimental observations that 3, 4, and the “4 or 1/ hydrosilane/B(C6F5)3” systems are more active than 2 and the “2/B(C6F5)3” systems. The monohydride alkene complexes I-2, I-4, I-6, and I-9 could join into the Osborn-type hydrogenation cycle, undergoing the elementary steps of alkene insertion into the M−H bond, subsequent coordination and oxidative addition of H2, and reductive elimination of the alkane product.26 Generally, the considerable enhancement of the hydrogenations upon addition of a Lewis acidic cocatalyst, such as B(C6F5)3 or hydrosilane/B(C6F5)3, was interpreted in terms of a shift of the dissociation equilibrium of the coordinated Lewis acid toward higher concentrations of the adduct. The central adduct species I-1 of the initiation course was attempted to be traced by various NMR techniques, which however proved to be unsuccessful.

spectrometer. Microanalyses were carried out at the AnorganischChemisches Institut of the University of Zurich. Complexes 1 were prepared according to reported procedures.14a 1-Hexene, 1-octene, cyclooctene, and 1,5-cyclooctadiene were purified by distillation. General Procedure for Hydrogenation of Alkenes Catalyzed by the Rhenium Dinitrosyl Complexes. In a 30 mL steel autoclave equipped with a stirring bar, a certain amount of substrate alkene (e.g., 1-hexene: 2.5 mL, 20 mmol; 1-octene: 2.5 mL, 16 mmol; cyclooctene: 1 mL, 8 mmol; 1,5-cyclooctadiene: 1 mL, 8 mmol; cyclohexene: 2.5 mL, 25 mmol; 1,7-octadiene: 1 mL, 7 mmol), 0.002 mmol of rhenium catalyst 1−8 with or without 0.01 mmol of B(C6F5)3 (5.2 mg), and 0.01 mmol of hydrosilane (Et3SiH: 1.6 μL; Me2PhSiH: 1.6 μL) were mixed. The system was charged with 40 bar of H2 and kept stirring at 100 °C. The progress of the reactions was monitored by the decreased pressure of the autoclave. The hydrogenation products were characterized by 1H NMR spectroscopy in CDCl3. [ReH(PR3)2(NO)(NOB(C6F5)3)] (2, R = iPr a, Cy b). In a 20 mL vial in a glovebox, 1a (56.8 mg, 0.10 mmol) and B(C6F5)3 (51.2 mg, 0.10 mmol) were mixed in 3 mL of toluene. A yellow solution immediately formed, and after stirring at room temperature for 30 min, the solution was filtered off and the filtrate was dried in vacuo. The residue was washed with pentane (2 × 1 mL) and dried, giving a yellow solid, 2a. Yield: 98 mg, 91%. IR (cm−1, ATR): ν (NO) 1641, ν (N-OB) 1340. 1H NMR (300.08 MHz, toluene-d8, ppm): δ 4.07 (t, 1H, 2JHP = 41 Hz, Re-H), 1.79 (m, 6H, P(CHMe2)3), 0.77 (m, 36H, P(CHMe2)3). 13C{1H} NMR (75.5 Hz, toluene-d8, ppm): δ 148.8 (dm, 1JCF = 242 Hz, o-C6F5), 137.7 (dm, 1JCF = 244 Hz, m-C6F5), 140.4 (dm, 1JCF = 235 Hz, p-C6F5), 120 (m, ipso-C6F5), 28.4 (t, 1JCP = 14 Hz), 19.4 (s), 18.9 (s). 31P{1H} NMR (121.5 MHz, toluene-d8, ppm): δ 53.5 (s). 19F NMR (282.3 MHz, toluene-d8, ppm): δ −131.3 (d, 6F, 3JFF = 21 Hz, o-C6F5), −159.6 (t, 3F, 3JFF = 20 Hz, p-C6F5), −165.8 (m, 6F, m-C6F5). Anal. Calcd for C36H43BF15N2O2P2Re (1079.68): C, 40.05; H, 4.01; N, 2.59. Found: C, 40.17; H, 3.85; N, 2.63. In a similar procedure, the orange-brown-colored 2b was prepared from 1b (40.6 mg, 0.05 mmol) and B(C6F5)3 (25.9 mg, 0.05 mmol) in 2 mL of toluene within 1 h. Yield: 58 mg, 88%. IR (cm−1, ATR): ν (NO) 1644, ν (N-OB) 1339. 1H NMR (300.08 MHz, toluene-d8, ppm): δ 4.17 (t, 1H, 2JHP = 42 Hz, Re-H), 1.04−1.95 (m, 66H, PCy3). 13C{1H} NMR (75.5 Hz, toluene-d8, ppm): δ 38.4 (t, 1JCP = 14 Hz), 30.2 (s), 30.0 (s), 27.4 (m), 26.3 (s). 31P{1H} NMR (121.5 MHz, toluene-d8, ppm): δ 41.3 (s). 19F NMR (282.3 MHz, toluene-d8, ppm): δ −131.7 (d, 6F, 3JFF = 20 Hz, o-C6F5), −159.7 (t, 3F, 3JFF = 20 Hz, p-C 6 F 5 ), −166.1 (m, 6F, m-C 6 F 5 ). Anal. Calcd for C54H67BF15N2O2P2Re (1320.41): C, 49.13; H, 5.12; N, 2.12. Found: C, 49.01; H, 5.32; N, 2.26. [ReH(PR3)2(NO)(NOEt)][B(C6F5)4] (3, R = iPr a, Cy b). In a 20 mL vial in a glovebox, 1a (56.8 mg, 0.10 mmol) and [Et3O][B(C6F5)4] (79.0 mg, 0.10 mmol) were mixed in 5 mL of ether. After stirring for 1 h the formed yellow solution was filtered off and dried in vacuo. The residue was washed with ether and pentane (1:10) (3 × 3 mL) and dried, giving a yellow powder, 3a. Yield: 118.2 mg, 93%. IR (cm−1, ATR): ν (NO) 1664, ν (N-OEt) 1252. 1H NMR (300.1 MHz, THFd8, ppm): δ 5.50 (t, 1H, 2JHP = 38 Hz, Re-H), 4.77 (q, 2H, 2JHH = 7 Hz, NOCH2), 2.71 (m, 6H, P(CHMe2)3), 1.44 (t, 3H, 2JHH = 7 Hz, NOCH2CH3), 1.36 (m, 36H, P(CHMe2)3). 13C{1H} NMR (75.5 MHz, THF-d8, ppm): δ 81.0 (s, NOCH2CH3), 28.2 (t, 1JCP = 15 Hz), 19.7 (s), 18.5 (s), 13.0 (s, NOCH2CH3). 31P{1H} NMR (121.5 MHz, THF-d8, ppm): δ 59.2 (s). 19F NMR (282.3 MHz, THF-d8, ppm): δ −135.1 (d, 8F, 3JFF = 20 Hz, o-C6F5), −167.8 (t, 4F, 3JFF = 21 Hz, pC6F5), −171.3 (m, 8F, m-C6F5). Anal. Calcd for C44H48BF20N2O2P2Re (1275.80): C, 41.42; H, 3.79; N, 2.20. Found: C, 41.75; H, 3.43; N, 2.28. In a similar procedure, the orange-colored 3b was prepared from 1b (81.0 mg, 0.10 mmol) and [Et3O][B(C6F5)4] (79.0 mg, 0.10 mmol) in 5 mL of ether within 1 h. Yield: 131 mg, 86%. IR (cm−1, ATR): ν (NO) 1670, ν (N-OEt) 1249. 1H NMR (300.08 MHz, THFd8, ppm): δ 5.41 (t, 1H, 2JHP = 39 Hz, Re−H), 4.76 (q, 3JHH = 6 Hz, NOCH2CH3), 1.35−2.40 (m, 66H, P(C6H11)3), 0.90 (t, 3JHH = 6 Hz, NOCH2CH3). 13C{1H} NMR (75.5 MHz, THF-d8, ppm): δ 80.0 (s, NOCH2CH3), 37.2 (t, 1JCP = 14 Hz), 30.2 (s), 28.9 (s), 26.9 (m), 25.9 (s), 13.4 (s, NOCH2CH3). 31P{1H} NMR (121.47 MHz, THF-d8,

3. CONCLUSIONS In summary, we have presented the Lewis acid functionalized dinitrosyl rhenium hydride complexes as highly efficient catalysts for alkene hydrogenations. NO bending and opening up of a vacant site for reversible substrate binding are the main effects to promote catalysis summarized under the term “catalytic nitrosyl effect”. NO bending was found to be facilitated by Lewis acid coordination to the ONO atom. Different activation pathways were proposed for functionalization with the neutral or cationic Lewis acid. This work also demonstrated ways of how to tune “uncatalytic” middle transition metal centers for high catalytic performance in olefin hydrogenations using the structural flexibility of NO ligands. Detailed DFT evaluations on the catalyst activation pathway and further exploitation of the “catalytic nitrosyl effect” of various other nitrosyl complexes are currently under way in our group.

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EXPERIMENTAL SECTION

General Experimental Procedures. All manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or in a glovebox (M. Braun 150B-G-II) filled with dry nitrogen. Solvents were freshly distilled under N2 by employing standard procedures and were degassed by freeze−thaw cycles prior to use. The deuterated solvents were dried with sodium/benzophenone (toluene-d8, benzene-d6, THF-d8) or P2O5 (CD2Cl2, C6D5Cl) and vacuum transferred for storage in Schlenk flasks fitted with Teflon valves. 1H NMR, 13C{1H} NMR, 31P{1H} NMR, and 11B NMR data were recorded on a Varian Gemini-300, Varian Mercury 200, or Bruker DRX 500 spectrometer using 5 mm diameter NMR tubes equipped with Teflon valves, which allow degassing and further introduction of gases into the probe. Chemical shifts are expressed in parts per million (ppm). 1H and 13C{1H} NMR spectra were referenced to the residual proton or 13C resonances of the deuterated solvent. All chemical shifts for the 31P{1H} NMR data are reported downfield in ppm relative to external 85% H3PO4 at 0.0 ppm. Signal patterns are reported as follows: s, singlet; d, doublet; t, triplet; m, multiplet. IR spectra were obtained by using a KBr pellet with a Bio-Rad FTS-45 FTIR 7050


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ppm): δ 49.1 (s). 19F NMR (282.3 MHz, THF-d8, ppm): δ −133.9 (m, 8F, ortho-C6F5), −166.3 (t, 3JFF = 20 Hz, 4F, para-C6F5), −168.4 (m, 8F, meta-C6F5). Anal. Calcd for C62H72BF20N2O2P2Re (1516.44): C, 49.11; H, 4.79; N, 1.85. Found: C, 49.41; H, 4.98; N, 1.67. [ReH(PR3)2(NO)(NOSiEt3)][HB(C6F5)3] (4, R = iPr a, Cy b). In a 3 mL Young NMR tube, B(C6F5)3 (18.7 mg, 0.033 mmol) and Et3SiH (6.5 μL, 0.045 mmol) were mixed in 0.5 mL of toluene-d8. Then 1a (17.0 mg, 0.03 mmol) or 1b (24.4 mg, 0.03 mmol) was added, and the tube was vigorously shaken for 5 min. A brown oil precipitate was formed with a light yellow supernatant solution. When the same reaction was carried out in chlorobenzene-d5, the solution remained homogeneous. NMR spectroscopy indicated 100% in situ formation of the silylium-coordinated nitrosyl complex salt. After removal of the solvent in vacuo, the residue was washed with pentane (4 × 2 mL) through vigorous shaking and dried, affording 4a as a light yellow oil or 4b as light brown powders. 4a: 19.0 mg. Yield: 53%. IR (cm−1, ATR): ν (NO) 1641, ν (N-OSi) 1340. 1H NMR (300.08 MHz, C6D5Cl, ppm): δ 4.38 (t, 1H, 2JHP = 41 Hz, Re-H), 1.98 (m, 6H, PCH(CH3)2), 0.83 (m, 36H, PCH(CH3)2), 0.56 (t, 3JHH = 8 Hz, SiCH2CH3), 0.33 (q, 3JHH = 8 Hz, SiCH2CH3). 13C{1H} NMR (75.47 MHz, C6D5Cl, ppm): δ 28.6 (t, J(PC) = 15 Hz, P-CH), 19.8 (s, PCH(CH3)2), 19.3 (s, PCH(CH3)2), 6.3 (s, SiCH2CH3), 4.0 (s, SiCH2CH3). 31P{1H} NMR (121.47 MHz, C6D5Cl, ppm): δ 57.7 (s). 11B NMR (96.28 MHz, C6D5Cl, ppm): δ −24.47 (d, 1JHB = 91 Hz). 19F NMR (188.13 MHz, toluene-d8, ppm): δ −133.96 (m, 6F, ortho-C6F5), −165.59 (t, 1JCF = 19 Hz, 3F, para-C6F5), −168.32 (m, 6F, meta-C6F5). 29Si NMR (99.4 MHz, toluene-d 8 , ppm): 41.00 (s). ESI-MS: m/z 683.1 M + {[ReH(PiPr3)2(NO)(NOSiEt3)]+}, 567.1 (M − SiEt3)+·, 512.8 [HB(C 6 F 5 ) 3 ] − , 542.8 [CH 3 OB(C 6 F 5 ) 3 ] − . Anal. Calcd for C42H59BF15N2O2P2ReSi (1195.97): C, 42.18; H, 4.97; N, 2.34. Found: C, 41.86; H, 4.79; N, 2.29. 4b: 29.1 mg. Yield: 67%. IR (cm−1, ATR): ν (NO) 1657, ν (N-OSi) 1296. 1H NMR (300.08 MHz, C6D5Cl, ppm): δ 4.71 (t, 1H, 2JHP = 42 Hz, Re-H), 1.02−2.00 (m, 66H, P(C6H11)3), 0.83 (t, 3JHH = 8 Hz, SiCH2CH3), 0.76 (q, 3JHH = 8 Hz, SiCH2CH3). 13C{1H} NMR (75.47 MHz, C6D5Cl, ppm): δ 38.4 (t, J(PC) = 14 Hz, P-C), 30.7, 30.2, 27.5, 26.3, 6.5 (s, SiCH2CH3), 4.2 (s, SiCH2CH3). 31P{1H} NMR (121.47 MHz, C6D5Cl, ppm): δ 47.0 (s). 11B NMR (96.28 MHz, C6D5Cl, ppm): δ −24.82 (d, 1JHB = 89 Hz). 19F NMR (188.13 MHz, C6D5Cl, ppm): δ −134.00 (m, 6F, orthoC6F5), −165.79 (t, 1JCF = 21 Hz, 3F, para-C6F5), −168.43 (m, 6F, meta-C6F5). 29Si NMR (99.4 MHz, toluene-d8, ppm): 40.95 (s). ESIMS: m/z 922.3 M+ {[ReH(PCy3)2(NO)(NOSiEt3)]+}, 807.2 (M − SiEt3)+·, 512.8 [HB(C6F5)3]−. Anal. Calcd for C60H83BF15N2O2P2ReSi (1436.36): C, 50.17; H, 5.82; N, 1.95. Found: C, 49.91; H, 5.61; N, 1.87. Computational Methods. All calculations were performed with the Gaussian03 program package32 using the popular functional B3LYP33 in association with the Stuttgart/Dresden effective core potentials (SDD) basis set34 and the standard 6-31+G(d)35 for the remaining atoms. Sum of electronic and zero-point energies are taken as relative energies. Crystallographic Studies of Compounds 2a, 2b, 3a, and 3b. Single-crystal X-ray diffraction data were collected at 183(2) K on an image plate detector system (STOE IPDS diffractometer)36 using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The selected suitable single crystals were mounted using polybutene oil on top of a glass fiber fixed on a goniometer head and immediately transferred to the diffractometer. The unit cell constants and orientation matrixes for data collections were obtained from a total of 8000 reflections selected from the whole limiting sphere. The intensities were corrected for Lorentz and polarization effects, and numerical absorption corrections37 were applied using a video camera installed within the IPDS diffractometer. The structures were solved by direct methods and the Patterson method using SHELXS97.38 All refinements were performed by full-matrix least-squares on F2 with SHELXL97.38 PLATON39 was used to check the result of the X-ray analyses, and publCIF40 was used to finalize the crystallographic information files (CIF). For more details about data collections and refinement parameters, see the crystallographic information files. CCDC-919258 (for 2a), CCDC-919259 (for 3a), CCDC-919260 (for

2b), and CCDC-919261 (for 3b) contain the supplementary crystallographic data (excluding structure factors) for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.

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ASSOCIATED CONTENT

S Supporting Information *

The NMR spectra of 4a, comparison of catalytic activities of 2− 4 with or without cocatalyst, IR spectra of Lewis acid functionalized rhenium dinitrosyl complexes, crystallographic details and CIF files, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swiss National Science Foundation, Lanxess AG, Leverkusen, Germany, the Funds of the University of Zurich, the DFG, and SNF within the project “Forschergruppe 1175 - Unconventional Approaches to the Activation of Dihydrogen” are gratefully acknowledged.

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REFERENCES

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