Synthesis, Characterization, and Reactivity of the First Osmium Î²

Article pubs.acs.org/Organometallics

Synthesis, Characterization, and Reactivity of the First Osmium β‑Diketiminato Complexes and Application in Catalysis Dominique F. Schreiber,† Crystal O’Connor,†,‡ Christian Grave,† Helge Müller-Bunz,† Rosario Scopelliti,§ Paul J. Dyson,§ and Andrew D. Phillips*,† †

School of Chemistry & Chemical Biology, University College Dublin (UCD), Belfield, Dublin 4, Ireland SFI Strategic Research Cluster in Solar Energy Conversion, University College Dublin (UCD), Belfield, Dublin 4, Ireland § Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1014 Lausanne, Switzerland ‡

S Supporting Information *

ABSTRACT: The strongly chelating anionic β-diketiminate ligand has been employed to formulate complexes involving almost every metal of the periodic table; however, the heavier metals of the d block remain relatively unexplored. This paper describes the synthesis and characterization of the first two osmium β-diketiminato compounds, including a coordinatively unsaturated cationic complex. In parallel to the analogous Ru(II) complexes, the cationic (η6arene)osmium(II) complex demonstrates bifunctional behavior through [4 + 2] cycloaddition with ethylene, cleavage of dihydrogen under mild conditions, and protonation/chloride addition with [Et2OH]Cl. Metal-centered activity in both the Ru(II) and Os(II) β-diketiminates has until now remained elusive, as the cationic Os complex is shown to readily coordinate an aryl isonitrile. The applicability of Os(II) β-diketiminato complexes in catalytic olefin hydrogenation demonstrates significantly greater activity in terms of conversion and TOF for a range of substrates, including styrene, cyclohex-1-ene, and 1-methylcyclohex-1-ene. Moreover, selective hydrogenation of the exocyclic alkenyl group in limonene was observed, whereas the corresponding isostructural Ru(II) complexes are inactive. In contrast, the cationic (η6arene)ruthenium(II) β-diketiminato complex proved more active for the catalytic dehydrogenation of N,N-dimethylamine borane (Me2NBH3) than the equivalent Os(II) species. A detailed DFT study of the Ru(II) and Os(II) β-diketiminato species using charge decomposition analysis (CDA) demonstrates differences in metal−ligand interactions, which in turn considerably influences the extent of bifunctional reactivity.

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INTRODUCTION Within the group 8 transition metals, osmium has received comparatively less attention than iron and ruthenium, which is generally due in part to slower ligand exchange kinetics and hence lower catalytic activity. However, a handful of Os compounds have been shown to have relevant catalytic activity in their own right.1−3 In an extension of their work on Rumediated catalytic hydrogenation, Vaska et al. pioneered the first monometallic Os species active in homogeneous hydrogenation, [OsH(Cl)(CO)(PPh3)3].4,5 Later on, SanchezDelgado et al. published their findings on the rapid reduction of 1-hexene, 1,3-cyclohexadiene, and phenylacetylene by [OsH(Br)(CO)(PPh3)3], a brominated derivative of the complex developed by Vaska.6 The same authors investigated the reduction of L-carvone, featuring endo- and exocyclic CC bonds.7,8 More recent work on osmium complexes concentrated mainly on the reduction of carbonyl compounds.9−15 A particularly interesting example reported by Clapham and Morris described the ability of an amido hydrido Os complex, OsH(NHCMe2CMe2NH2)(PPh3)2, to reduce acetophenone.16 It was found that the corresponding Ru analogue showed conversion similar to that of the Os complex; however, the kinetic behavior indicated a continuously decreasing turnover frequency (TOF) as the reaction progressed. As part of the © 2013 American Chemical Society

recent increase in interest in Os chemistry, a number of reports have focused on the synthesis of a wide variety of novel Os complexes.17−23 Of particular relevance for the (η6-arene)osmium half-sandwich complexes discussed herein is the synthesis of the dinuclear complex [{(η6-p-cymene)Os}2(μη2:η2-L) (L = 2,5-bis[2-(methylthio)anilino]-1,4-benzoquinone) by Sommer et al.24 Previous work by some of us described the facile synthesis of the coordinatively unsaturated (η6-arene)ruthenium β-diketiminato complex [(η6-C6H6)Ru{(2,6-Me2-C6H3NCMe)2CH}]OTf (2-Ru), via anion metathesis of (η6-C6H6)RuCl{(2,6-Me2C6H3NCMe)2CH} (1-Ru) (Figure 1). This bifunctional complex demonstrated the ability to undergo [4 + 2] cycloadditions with both ethylene and acetylene and, importantly, heterolytically activate H2.25 Moreover, 2-Ru performs efficient homogeneous catalytic hydrogenation of styrene26 and dehydrogenation of amine−boranes.27 A key property of the coordinatively unsaturated complex 2-Ru is the thermal reversibility of the substrate addition, which is strongly influenced by the substitution pattern of the flanking aryl groups. Considering the propensity of Os compounds to Received: August 31, 2013 Published: November 27, 2013 7345

dx.doi.org/10.1021/om400875r | Organometallics 2013, 32, 7345−7356

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coordinatively saturated dichromatic purple-orange Os(II) complex 1-Os was obtained in 73% yield. The characteristic 1 H NMR signals in CD2Cl2 for the η6-C6H6 and β-CH protons were observed at δ (ppm) 5.07 and 5.66, respectively (Table 1). A comparison with the previously published solution NMR data of 1-Ru, with δ (ppm) 4.60 and 5.54, respectively, reflects a considerable downfield shift for the NMR signals in 1-Os. This trend is paralleled in the corresponding 13C{1H} NMR data (Table 1).25 Analogous to the case for 1-Ru, the protons of the o-CH3 substituents of the flanking aryl groups of 1-Os appear as a single 1H NMR signal, indicating apparent C2v symmetry as in the case for the cationic 2-Os. Importantly, this resonance considerably broadens on changing to a more polar NMR solvent, in this instance, THF-d8, where the signal for the omethyl groups is observed at 2.36 ppm. This suggests that in solution Cl is only weakly associated with the Os(II) metal center. In contrast, the solid-state structure of 1-Os reveals a Os−Cl distance of 2.496(1) Å (Figure 2), which is significantly shorter than the Ru−Cl bond of 2.521(1) Å in 1-Ru.25 Notably, the Os−Cl bond length in 1-Os is the longest (η6-C6H6)Os−Cl distance yet recorded in the Cambridge Crystallographic Database (CCDC), with a calculated median distance of 2.402 Å for the entire data set.33 The metal−halogen charge transfer (MXCT) resulting from the coordination of Os(II) to the electron-donating β-diketiminate ligand, together with the steric pressure exerted by the o-CH3 groups on the flanking aryls, are believed to be the major contributions to this unusually long Os−Cl bond. Moreover, the Os−Ccent (Ccent = Carene centroid) bond length is 1.682(2) Å, which is equivalent

Figure 1. β-Diketiminato metal complexes (η6-C6H6)MCl{(2,6-Me2C6H3NCMe)2CH} and [(η6-C6H6)M{(2,6-Me2-C6H3NCMe)2CH}]OTf (M = Ru/Os, OTf = CF3SO3−).

homogeneously catalyze the hydrogenation of various olefin and carbonyl substrates described above, we herein describe the synthesis, characterization, and catalytic application of the novel organoosmium β-diketiminato complexes (η6-C6H6)OsCl{(2,6Me2-C6H3NCMe)2CH} (1-Os) and [(η6-C6H6)Os{(2,6-Me2C6H3NCMe)2CH}]OTf (2-Os) (Figure 1).

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RESULTS AND DISCUSSION Synthesis and Characterization of the Complexes. The synthesis of the β-diketiminate ligand with o-methyl substituents (2,6-Me2-C6H3NCMe)2CH2 was carried out as previously described via acid-catalyzed co-condensation of 2,6-dimethylaniline and 2,4-pentanedione.28−32 Analogous to the synthesis of the chloro-substitued Ru(II) complex (η6C6H6)RuCl{(2,6-Me2-C6H3NCMe)2CH} (1-Ru), nBuLi-mediated deprotonation of the β-diketiminate ligand to Li{(2,6-Me2C6H3NCMe)2CH} was necessary to achieve transmetalation with the dichloro(η6-C6H6)osmium(II) dimer.25 The resulting

Table 1. Selected Solution 1H and 13C{1H} NMR Chemical Shifts (ppm) for Compounds 1-Os−6-Os in CD2Cl2 compounda 1-Os

2-Os

3-Os

4-Os

5-Os

6-Os

1

H NMR

α-CH3 o-CH3 β-CH η6-arene CH aryl CH

1.79 1.49b 2.26 2.36b br 5.66 4.86b 5.07 4.60b 7.04−7.08 6.92−7.13b

2.23

2.14

1.98

2.05

1.58

2.13

2.31/2.34

2.06/2.23

1.96/2.23

2.22/2.26

7.00

4.31/4.81

3.14/4.44

4.54

5.13

5.79

5.05

4.67

4.67

4.85

7.18−7.22

7.10−7.14

7.05−7.08

7.10−7.13

7.06−7.34

23.2

24.8

23.8

22.7

22.4

19.1

18.7/19.0

18.0/18.2

18.0/18.9

18.5/19.7

77.1

79.9

79.0

80.1

86.2

107.9

49.2

54.9

101.1

103.5

128.3

128.3

127.7

128.1

129.8

158.6

153.0

153.2

152.8

154.7

165.7

183.0

177.8

180.0

164.2

13

α-CH3 o-CH3 η6-arene CH β-CH aryl p-CH i-C CN

a

23.3 23.2b 19.3 19.6b br 76.9 76.6b 103.4 101.9b 126.8 126.3b 156.8 156.5b 162.9 161.3b

C{1H} NMR

Spectra were recorded in CD2Cl2, except for those of 3-Os, which were measured in CDCl3. bSpectra recorded in THF-d8. 7346


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Figure 2. ORTEP diagrams of the Os(II)-β-diketiminato complexes 1-Os and 2-Os. Thermal ellipsoids are drawn at the 50% probability level. Solvates, anions, and internal disorder have been omitted for clarity. For 2-Os, the unit cell contains two crystallographically independent molecules, of which only one is shown. The provided bond lengths and angles for 2-Os represent averaged values. Relevant bond lengths (Å) and bond angles (deg): (1-Os)a) Os−Cl, 2.496(1); Os−N, 2.110(4); Os−Ccent, 1.682(2); N−Os−N, 87.3(1); Ccent−Os−Cl, 125.0(1); Ccent−Os−N,N′, 153.5(1); Os−N,N′−Cβ, 158.3(2); N−Cα−Cβ, 124.5(4). (2-Os) Os−N, 2.000(8), 2.016(8); Os−Ccent, 1.711(5); N−Os−N, 89.3(3); Ccent−Os−N,N′, 178.3(3); Os−N,N′−Cβ, 176.2(5); N−Cα−Cβ, 122.9(9). Selected torsion angle (deg): Ru−N−Cα−Cβ (1-Os) 11.0, −11.12, (2-Os) 0.15, 2.52. Closest contact between 2-Os and OTf counterion: η6-arene H22 to O2 2.416 Å. Ccent = Carene centroid, and N,N′ = Nligand midpoint.

Scheme 1. Reactivity of 2-Os with HCl, H2, C2H4, and CNXyl

procedure slightly modified from that used for 2-Ru.25 This is in contrast with the work presented by Carmona et al., where the Ag(I)-mediated chloride abstraction of the osmium halfsandwich complex [(η6-p-cymene)OsCl(L-α-aminocarboxylate)] led to trimerization and isolation of an Os cluster compound linked through OC groups.34 As is evident from the ORTEP representation, 2-Os features a vacant coordination site at the metal center (Figure 2). The shortest contact

to the distance of 1.687(3) Å in 1-Ru. The geometry at the metal center in 1-Os has an Os−N,N′−Cβ angle of 158.3(2)°, which is not as tripodal as in 1-Ru (N,N′ = Nligand midpoint) and thus reflects the stronger interaction between Os and the βdiketiminate ligand. The corresponding 16-valence-electron mononuclear cationic complex [(η 6 -C 6H 6 )Os{(2,6-Me 2C6H3NCMe)2CH}]OTf (2-Os) was prepared in 88% yield via a salt metathesis reaction of 1-Os and NaOTf, employing a 7347


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Figure 3. ORTEP diagrams of 3-Os and 4-Os. Thermal ellipsoids are drawn at the 50% probability level. Solvates and the triflate anion have been omitted for clarity. Relevant average bond lengths (Å) and bond angles (deg) are as follows. 3-Os: Os−Cl, 2.416(1); Os−N, 2.125(4); Os−Ccent, 1.693(3); N−Os−N, 84.7(2); Ccent−Os−Cl, 123.7(1); Ccent−Os−N,N′, 152.5(1); Os−N,N′−Cβ, 145.0(2); N−Cα−Cβ, 122.2(5). 4-Os: Os−H, 1.55(2); Os−N, 2.106(1); Os−Ccent, 1.715(1); N−Os−N, 85.2(1); Ccent−Os−H, 126.4(1); Ccent−Os−N,N′, 155.2(3); Os−N,N′−Cβ, 138.6(1); N−Cα−Cβ, 119.1(1). Ccent = Carene centroid, and N,N′ = Nligand midpoint.

between 2-Os and the counteranion OTf is 2.416 Ǻ , measured between O2 and H22 of the η6-coordinated benzene. Due to the rigid, well-defined steric environment provided by the ancillary β-diketiminate ligand, no dimerization in solution or in the solid-state occurs. In comparison to 2-Ru, the metal center in the cationic complex 2-Os is apparently more strongly bound to the supporting ligands (on the basis of bond lengths), mirroring a similar trend found between the chlorinesubstituted compounds 1-Ru and 1-Os.25 This is further evidenced by the shifted 1H NMR signals found for the η6C6H6 δ (ppm) 5.79, and β-CH δ (ppm) 7.00 groups within 1Ru (Table 1). A comparison of the Os−N and Os−Ccent bond lengths in 2-Os with the equivalent metal−ligand distances reported for 2-Ru features overall shortened Os(II)−ligand distances in comparison to 2-Ru.25 Further discussion regarding the solid-state structure of 2-Os is provided in the Supporting Information. Reactivity. The coordinatively unsaturated complex 2-Os is a Lewis acidic species and therefore should react to form stable adducts with a variety of neutral σ-donors such as phosphines, pyridines, and nitriles. However, no evidence for the reaction of 2-Os with PPh3, pyridine, or MeCN was observed by 1H NMR spectroscopy. Nevertheless, 2-Os undergoes various other types of reactions, as outlined in Scheme 1. Notably, 2-Os shows the ability to activate suitable substrates through simultaneous bifunctional metal and ligand interactions. Upon treatment of 2-Os with an excess of [Et2OH]Cl in Et2O, the saturated β-diimine complex [(η6-C6H6)OsCl{(2,6Me2-C6H3NCMe)2CH2}]OTf (3-Os) is rapidly and quantitatively formed as a bright orange solid. Presumably, protonation of the β-C site of the β-diketiminate ligand affords a highly electrophilic dicationic Os(II) species which rapidly combines with Cl−. In contrast to 1-Os, the dominant CN character of the β-diimine backbone in 3-Os is illustrated by a downfield shift observed for the corresponding 13C{1H} NMR signal with δ (ppm) 183.0 (Table 1). The neutral, weakly electron donating β-diimine ligand mediates a highly electron deficient Os(II) center, and consequently a shortened Os−Cl bond length of 2.416(1) Å is observed (Figure 3). The increase in Os−Cl bond strength is further underlined by the Cs-symmetric 1 H NMR spectrum, which shows two different sets of signals associated with the o-CH3 substituents on the flanking aryl

groups (Table 1). As expected, the β-CH2 position in 3-Os shows distinctive sp3 hybridization, as evident by the 30.6° outof-plane folding with resspect to the planes defined by the “C N” bonds. In analogy with 2-Ru, 2-Os also mediates the heterolytic bifunctional activation of H2 under mild conditions. The synergic protonation of the nucleophilic β-carbon on the ligand and the addition of the hydride to the Os center results in the isolation of [(η6-C6H6)OsH{(2,6-Me2-C6H3NCMe)2CH2}]OTf (4-Os). Very few examples of this type of cooperative Os(II)−ligand activation of H2 have been reported in the literature. In particular, the Os complex OsH(NHCMe2CMe2NH2)(PPh3)2 of Clapham et al. showed heterolytic cleavage of H2 to form the dihydride complex trans-OsH2(NH2CMe2CMe2NH2)(PPh3)2, featuring a protonated nitrogen at the diamine ligand.16 In parallel with complex 3-Os, the bifunctional H2-addition product 4-Os also features an sp3-hybridized β-CH2 with strong diimine character (Figure 3). This observation is in stark contrast to the work of West et al. on Pt(II) β-diketiminato complexes, where the βdiketiminato β-CH group did not participate in the dehydrogenation reaction, remaining sp2 hybridized throughout.35 The Cs symmetry observed for 4-Os in solution is evident from the corresponding 1H and 13C{1H} solution NMR data (Table 1). Further evidence in favor of a strongly bound hydride in 4-Os is provided by a solution 1H NMR inversion recovery experiment. Complex 4-Os shows a diagnostic T1 value for the Os-bound hydride of 1.082 s, which is significantly shorter than the 1.392 s measured previously for the analogous Ru complex.25 A shorter T1 indicates more efficient relaxation due to a stronger Os−H bond. Upon prolonged storage in solution and the solid state, H2 is gradually lost, regenerating 2-Os. This process is accelerated when 4-Os is exposed to reduced pressure, whereas 3-Os is indefinitely stable even when exposed to an O2- and moisture-filled environment.25 As already observed in the case of 2-Ru,25 ethylene undergoes a rapid [4 + 2] cycloaddition with 2-Os via a synergic ligand−metal interaction, affording the light orange adduct 5-Os. As a result, the β-diketiminato ligand in 5-Os is transformed into a N,C,N-tridentate ligand with pronounced imino bonding character, evidenced by the 13C{1H} NMR C N shift of δ (ppm) 180.0 (Table 1). When it is coordinated to 7348


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Figure 4. ORTEP representations of complexes 5-Os and 6-Os. Thermal ellipsoids are drawn at the 50% probability level. Solvates, the triflate anion, and selected hydrogen atoms have been omitted for clarity. Relevant average bond lengths (Å) and bond angles (deg) are as follows. 5-Os: Os−C28, 2.164(2); Os−N, 2.108(2); Os−Ccent, 1.704(1); Cβ−C29, 1.569(3); C28−C29, 1.523(3); N−Os−N, 85.7(1); Ccent−Os−C28, 126.1(1); Ccent−Os− N,N′, 154.5(1); Os−N,N′−Cβ, 134.1(1); N−Cα−Cβ, 118.4(2); C28−C29−Cβ, 113.6(2). 6-Os: Os−C28, 1.964(3); Os−N, 2.102(3); Os−Ccent, 1.736(2); C28−N3, 1.164(5); N−Os−N, 86.8(1); Ccent−Os−C28, 127.8(1); Ccent−Os−N,N′, 150.5(1); Os−N,N′−Cβ, 153.8(2); N−Cα−Cβ, 123.9(3). Ccent = Carene centroid, and N,N′ = Nligand midpoint.

the ethylene group, the nucleophilic sp2-hybridized β-carbon of the complex becomes sp3-hybridized, as evidenced by a more shielded 1H NMR signal of δ (ppm) 4.54. The Os- and Cβbound ethylene carbons resonate in 13C{1H} NMR at δ (ppm) −7.5 and 16.9, respectively, and hence are more shielded than the corresponding values of δ (ppm) 3.7 and 18.6 for 2-Ru.25 In particular, the negative 13C{1H} chemical shift associated with the Os-bound ethylene carbon indicates strong metal− ethylene bonding. Even a less shielded resonance of δ (ppm) −23.0 is observed for the metal-bound methyl group in the Os(II) complex Os(CO)2(Me)(PMe3)2I.36 The solid-state structure of 5-Os is shown in Figure 4 and is isostructural with that of the Ru analogue, whereby the only difference is a greater bending of the flanking aryl groups. The 1.523(3) Å distance measured for the eclipsing C(H2)−C(H2) unit within 5-Os is indicative of a single bond, a consequence of more pronounced electron density at Os, as already suggested by the solution NMR analysis. The C(H2)−C(H2) distance in 5-Os is equivalent to that found in the structure of the dinuclear species Os2(CO)8(μ2-η1:η1-C2H2) reported by Norton et al.37 Due to the incorporation of the strongly electron releasing βdiketiminate ligand, the Os−C(H2) distance of 2.164(2) Å is longer than the mean distance of 2.030 Å compiled from the Cambridge Crystallographic Database (CCDC).33 Of note is the significantly long Cβ−C(H2) bond, 1.569(3) Å, as compared to the average value of 1.530 Å for general C(sp3)−C(sp3) bond connections. This suggests less than ideal sp3 character at the Cβ position, providing an indication that the ethylene addition to 2-Os is thermally reversible. Similar to the case for 5-Os and to the previously reported structurally related Ru(II) β-diketiminato acetylene complex,25 West et al. described a metal−ligand bifunctional cycloaddition of terminal alkynes to the Pt(IV) β-diketiminato complex SiPh3(H)2Pt−C2H2{(p-Cl-C6H4NCMe)2CH}.38 In contrast to the case for nitriles, strong π-accepting isonitriles, such as 2,6-dimethylphenyl isocyanide (CNXyl),

spontaneously reacted with 2-Os in anhydrous CH2Cl2 to afford the red-colored adduct 6-Os in 92% yield. Relevant solution NMR data are given in Table 1, and the solid-state structure is shown in Figure 4. The lengthened CN bond distance of 1.164(5) Å indicates considerable π back-donation from the metal to the isocyanide ligand. This is further supported by the increase in double-bond character, as evidenced by the IR ν(CN) stretch of 2144 cm−1, which is at lower energy than that of the unbound 2,6-dimethylphenyl isocyanide (i.e., 2160 cm−1).39 It is interesting to compare 6-Os to similar compounds in the literature, such as [(η6-pcymene)Os(CNCMe3)(Me)Cl],40 [(η6-mes)OsCl2(CNXyl)],41 and [(η6-mes)OsH2(CNXyl)],42 where ν(CN) is 2070, 2124, and 2055 cm−1, respectively. In comparison, 6-Os features the least amount of metal−ligand π back-bonding into the CN ligand. This result indicates that both the CNXyl and the β-diketiminate ligand demonstrate some degree of π-back-bonding with the metal center available through energetically low-lying π*-type MOs. The latter observation is well reflected in literature examples featuring a second π-accepting ligand, such as [(η6-p-cymene)Os(η2-1,3PhNNNPh)(CNtBu)]BPh4,43 [η6-p-cymene)(Rpz)Os(HRpz)(CNtBu)]BPh4,44 and [(η6-p-cymene)OsCl(η2-CH2CH2)(CNtBu)]BPh4,45 with ν(CN) values of 2181, 2170, and 2191 cm−1, respectively. Computational Analysis. The reactivities of 2-Ru25 and 2Os with small molecules are similar, and therefore two reaction pathways were examined by computational methods to allow for a differentiation between the Os and Ru complexes. Employing the Gaussian G09 package,46 high-level quantum calculations using density functional theory were performed to both probe all metal interactions and examine energy differences in reaction pathways of dihydrogen cleavage and ethylene addition. Where possible, previously established X-ray structures were used as a starting point for the gas-phase geometry optimizations, yielding model structures which are in 7349


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Figure 5. Gas-phase HOMO and LUMO electron densities associated with the cationic C2v complex 2-Os. The geometry was optimized on the basis of the X-ray structure, using the M06 density functional with a 6-31G(d,p) and SDDAll (Os) basis set, as implemented in Gaussian G09.46 For further details, see the Supporting Information.

Figure 6. Gas-phase energy diagrams for H2 (a) and C2H4 (b) adduct formation with 2-Os (black) and 2-Ru (gray). The relative energies are given in kcal/mol, with the total energies for 2-Os/Ru and free substrate set to 0. The extent of the imaginary bond vibration associated with the transition states of 4-Os (−961.51) and 5-Os (−332.53) is depicted graphically in the top part of the diagram. The calculated structural parameters for the transition states are given in the Supporting Information.

bonding observed for the β-diketiminate ligand in the case of 2Os. Moreover, the high electron density associated with the metal in the metal β-diketiminate fragment leads to considerable charge transfer to the η6-arene, which in turn increases π back-donation, particularly in the case of 2-Os. From the corresponding HOMO and LUMO orbitals of 2Os depicted in Figure 5, it is apparent that the strong πdonating character of the β-diketiminate ligand significantly reduces the potential for the binding by a third ligand. However, the observed bifunctional reactivity of these complexes with molecules such as H2 (4-Os/Ru) and C2H4 (5-Os/Ru) is the result of simultaneous weakening of the πdonor ability of the β-diketiminate and σ-type bonding to the metal center. Such interactions are dependent on symmetrymatched interactions between the HOMO and LUMO of the molecules involved. To estimate the energy differences involved in the formation of the H2 (4-Os/Ru) and C2H4 (5-Os/Ru) adducts, the coordinatively unsaturated 2-Os/Ru complexes were considered as the respective starting points of the reactions (Figure 6). In both sets of transition states, 4-Os/ Ru shows a simultaneous binding of C2H4 to metal and Cβ center, while for 5-Os/Ru direct heterolytic cleavage of H2 by the metal and Cβ site is calculated. A more detailed description

good agreement with the experimentally measured structural parameters (see the Supporting Information). The coordinatively unsaturated cationic complexes 2-Os and 2-Ru show ΔEHOMO−LUMO gaps of 6.5 and 5.9 eV, respectively. On this basis, the Os complex can be categorized as less reactive in comparison to its Ru analogue. The shapes of the probability distributions associated with the HOMO and LUMO are very similar for the two complexes 2-Os and 2Ru (Figure 5).25 From a qualitative perspective, the HOMO is β-diketiminate metallacycle centered, whereas the LUMO spreads predominantly over the metal−arene fragment. With respect to the differences in reactivity between 2-Os and 2-Ru, it is informative to determine the degree by which the two supporting ligands, η6-arene and β-diketiminate, engage in synergic σ donation and π back-donation with the metal center. Using the postanalytic electron partitioning package AOMix,47,48 the metal−ligand bonding was investigated through a charge decomposition analysis (CDA), as developed by Frenking et al.49 CDA describes quantitative charge transfer between the η6-arene/metal and β-diketiminate/metal fragments. The results indicate that the more electron-rich Os ion tends to accept less electron density from donating ligands in comparison to Ru. This is supported by the slightly weaker σ 7350


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Figure 7. Gas-phase electron densities associated with the dominant σ-donating and π-back-donating MOs, HOMO and HOMO-9, respectively, of the isonitrile complex 6-Os. The geometry was optimized on the basis of the X-ray structure, using the M06 density functional with a 6-31G(d,p) and SDDAll (Os) basis set, as implemented in Gaussian G09.46 For further details, see the Supporting Information.

modeling indicate that the β-diketiminate folds along the N−N axis and, to a lesser degree, along the M−Cβ axis. An inspection of the MOs shows that distortion does not significantly disrupt the π delocalization across the core atoms of the β-diketiminate ligand but reduces the metal to ligand σ and π interactions. Accordingly, the CDA indicates a decrease in charge transfer to the metal center of 0.899 versus 1.046 in 2-Os. Catalytic Studies. 1-Os and 2-Os were screened for the ability to mediate the catalytic hydrogenation of styrene under conditions similar to those employed for the corresponding Ru(II) analogues (1-Ru and 2-Ru) and related chelating bisphosphine Os complexes.25,26,51 All hydrogenation reactions were performed with the careful exclusion of O2 to avoid deactivation of the catalyst. In a separate experiment, a THF solution containing 2-Os was opened to the atmosphere for several hours. During this time, the solution darkened and a dark brown insoluble powder was formed. Moreover, solution 1 H NMR of 2-Os in nondegassed CD2Cl2 proved inconclusive with respect to reaction with O2; however, no uncoordinated βdiketiminate ligand was observed. Both 1-Os (TOF = 1842 h−1) and 2-Os (TOF = 1246 h−1) showed significantly higher activity in the hydrogenation of styrene in comparison to the Ru analogues (Table 2). With respect to the relative reactivity of Ru and Os complexes, Bertoli et al. have observed a similarly higher activity for the isostructural Os complexes over the corresponding Ru analogues in the base-mediated transfer hydrogenation of acetophenone.52 However, Baratta et al. reported equal reactivity in the hydrogen transfer to ketones catalyzed by the pincer complexes MCl(CNN)(Ph 2 P(CH2)4PPh2) (M = Ru, Os).13 Given that the OTf complexes 2-Os and 2-Ru feature a vacant coordination site, the lower activity in comparison to the chloro analogues 1-Os and 1-Ru was unexpected. It is important to note that, for complexes 1-Os and 1-Ru, no base cocatalyst was added to facilitate halogen abstraction, in contrast with literature reports.14,15,53 However, on the basis of the above solution NMR analysis of 1-Os (Table 1), the chloride anion is only weakly interacting with the metal center, leading to a cationic species present in solution. This weak Cl− Os interaction therefore does not impede the catalytic activity. In contrast, the weak interaction with the chloride anion could

of the transition states is provided in the Supporting Information. The slightly higher reaction energies associated with the adduct formation of H2 and C2H4 reflect the generally lower reactivity associated with Os, in comparison to Ru. Examining the structures of 4-Os and 4-Ru, interesting insights with respect to the metal−hydride bond are possible. In general, the strength of a metal−hydride bond is rather challenging to assess. Although solution NMR and IR spectroscopy provide useful diagnostics, a more complete understanding can be gained in combination with DFT calculations. The Mayer bond indices provide a normalized integration of the electron population associated with each bond, therefore enabling a direct comparison between different bond types.50 The corresponding Mayer bond indices for 4-Os-H and 4-Ru-H are 0.8288 and 0.8172, confirming the experimental solution NMR analysis, attributing the stronger metal−hydride bond to 4-Os-H (see the Supporting Information for details). A CDAbased comparison of the resulting neutral β-diimine in 4-Os and the anionic β-diketiminate in 2-Os shows a reduction in the charge transfer from ligand to metal with 0.711 versus 0.932. Interestingly, the difference is more dramatic in the case of 4Ru and 2-Ru, with 0.723 versus 1.009. In both complexes, minor back-donation from the metal HOMO dz2 orbital to the π* NC bonds of the β-diimine ligand occurs, as indicated by the CDA values of 0.056 for Ru and 0.038 for Os. Aside from the bifunctional reactivity of 2-Os/Ru, coordination of a third ligand to the metal center is possible only when moderate to strong π-accepting molecules are introduced. Here, the prototypical example is an aryl isonitrile bound to a cationic complex (6-Os). The CDA of the interaction between 2-Os and CNXyl shows only a moderate charge transfer of 0.492 from the C lone pair of the isonitrile to the Os dxy orbital, formerly the LUMO of 2-Os (Figure 7). The corresponding back-donation from the tilted Os dz2 orbital to the π* CN orbital amounts to a charge transfer of 0.129, which justifies the observed moderate decrease in the CN stretching frequency to 2144 cm−1, in comparison to unbound CNXyl (2160 cm−1).39 It is possible to probe the electronic changes associated with the β-diketiminate ligand upon the addition of a third ligand. Both solid-state structures and computational 7351


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activity in comparison to the case for styrene. For the reduction of cyclohexene, the Os complexes are more active than the corresponding Ru analogues. In contrast to the reactions involving styrene, the cationic complexes 2-Os/Ru show higher activity for the conversion of cyclohexene. The propensity of a homogeneous Os complex to activate cyclohexene type substrates was investigated by Harman et al., who reported an X-ray structure of [Os(NH3)5(η2-3-methoxycyclohexene)]2+.60 Only very few Os complexes have been tested in the hydrogenation of cyclohexene. Examples include OsHCl(CO)(PPh3)3 (TOF = 191 h−1),61 [OsH(CO)(NCMe)2(PPh3)2][BF4] (TOF = 167 h−1),61 and [{Pd(cod)}2(μ-biim)2Os(OPPh3)2](NO3)3.62 The heterotrinuclear Pd2−Os complex only showed marginal activity, and when the {Pd(cod)} moiety was omitted, no activity was observed. In comparison to the examples cited, 2-Os is clearly the most active homogeneous Os hydrogenation catalyst for the reduction of cyclohexene to cyclohexane. However, as evidenced by a literature comparison, Os is not the metal of choice for this type of transformation, as for example Crabtree’s catalyst [Ir(cod)(PCy3)(py)]PF6 significantly exceeds the activity of 2-Os.63,64 The reduction of 1-methylcyclohex-1-ene to methylcyclohexane proceeds significantly more slowly than for cyclohexene, as a consequence of the increased steric bulk of the substrate. Only 2-Ru shows acceptable activity with a TOF of 406 h−1 (Table 3). While the [RuH(CO)(MeCN)(tppms) 3 ]BF 4 complex reported by Baricelli et al. is inactive in the reduction of 1-methylcyclohex-1-ene,65 [(η4-Ph4C4CO)(CO)2Ru]2 is by 1 order of magnitude more active than 2-Ru.66 2-Ru has similar reactivity to that of [(cod)Ir(PCy3)(py)]PF667 but shows higher TOFs than [Ir(PPh3)2(H)2(ClCH2CH2Cl)]BArF68 and (MeCNC)Fe(N2)2 (MeCNC = (2,6-Me2-C6H3-imidazol-2-ylidene)2-C5H3N).69 However, the most active catalyst for this type of transformation involves a Rh species.70 Limonene is a prominent member of the terpene class, featuring endo- and exocyclic alkenyl bond. The naturally occurring (+)-limonene is used in the flavor and fragrance industry. Importantly, due to its high availability, it can also serve as a cheap building block in asymmetric synthesis. Moreover, work by Grzybek et al. and Shephard et al. showed the preference for the exocyclic reduction of limonene by Rh and Ru/Cu transition-metal complexes.71,72 Complexes 1-Os/ Ru and 2-Os/Ru mediated exclusive exocyclic hydrogenation of limonene, with the osmium complexes being considerably more active than the Ru counterparts (Table 4). In the case of 1-Os, quantitative conversion to p-menthene was achieved with a TOF of 647 h−1. To the best of our knowledge, this is the first report of an Os-catalyzed hydrogenation of limonene. However, considering the similarity of L-carvone, the work by SanchezDelgado et al. needs to be mentioned; they found selective reduction of the exocyclic CC bond in L-carvone by the complex [OsH(Br)(CO)(PPh3)3] in the presence of H2 (1 atm).6 Moreover, the study also revealed that, under an increased H2 pressure of 5 atm, both CC bonds of L-carvone were hydrogenated. The TOF for the hydrogenation of limonene catalyzed by 1-Os is one of the highest reported in the literature for a monometallic homogeneous complex. Even in comparison to certain heterogenized systems, such as Pd/ Al2O 3,73 Pd-AEAPSi/SiO2,74 NH2 -SBA-15-[RuHCl(CO)(PPh3)2],75 and ionic liquid suspended [Ru(cod)(cot)] derived nanoparticles,76 1-Os shows higher reactivity. In their initial report on the olefin hydrogenation of limonene to p-menthene

Table 2. Hydrogenation of Styrene to Ethylbenzene Catalyzed by Complexes 1-Os/Ru and 2-Os/Rua

complex

time (h)

conversion (%)b

TOF (h‑1)c

1-Os 2-Os 1-Ru 2-Rud

0.5 0.5 0.5 1.0

92 62 66 84

1842 1246 1326 838

a

Conditions: 1 g (9.6 mmol) of styrene and 0.1 mol % of catalyst were dissolved in 12 mL of anhydrous and N2-saturated THF under inert conditions and transferred to an individually stirred multicell autoclave. The sealed reactor was purged and pressurized to 40 bar of H2 and heated to 80 °C. bAfter the appropriate time the conversion was measured by gas chromatography. cTurnover frequency (TOF) values were calculated on the basis of the listed conversion and reaction time. dResults taken from earlier reports.26

facilitate the adoption of a tripodal geometry at the metal center, leading to increased activity of 1-Os over 2-Os. Only very few Os complexes have been reported to show catalytic activity in the hydrogenation of styrene and closely related substrates.54−58 More recently, Albertin et al. showed that the p-cymene Os triazenide complex [Os(η2-1,3-ArNNNAr)(η6-pcymene)L]BPh4 (Ar = Ph; L = P(OEt)3) hydrogenates styrene to ethylbenzene with a TOF of approximately 22 h−1 at 100 °C.43 In comparison to the previously reported mono- and tetranuclear osmium complexes [OsHCl(CO)(PiPr3)2],59 OsH2Cl2(PiPr3),56 and H3Os4(CO)12(I),54 1-Os shows considerably higher activity. The complexes were further tested in the ability to mediate the catalytic hydrogenation of cyclohexene and 1-methylcyclohex-1-ene (Table 3). The increase in steric bulk of these substrates is clearly reflected in a decrease of the catalytic Table 3. Catalytic Hydrogenation of Cyclohexene (R = H) and 1-Methylcyclohex-1-ene (R = Me) by Complexes 1-Os/ Ru and 2-Os/Rua

complex

R

time (h)

conversion (%)b

TOF (h‑1)c

1-Os 1-Ru 2-Os 2-Ru 1-Os 1-Ru 2-Os 2-Ru

H H H H Me Me Me Me

0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0

55 42 75 64 7 7 6 41

1090 840 1492 1276 66 69 60 406

a

Conditions: 1 g of substrate (cyclohexene, 12.2 mmol; 1methylcyclohex-1-ene, 10.4 mmol) and 0.1 mol % of catalyst were dissolved in 12 mL of anhydrous and N2-saturated THF under inert conditions and transferred to an individually stirred multicell autoclave. The sealed reactor was pressurized with H2 (cyclohexene, 40 bar; 1-methylcyclohex-1-ene, 50 bar) and heated to 80 °C. bAfter the appropriate time, the conversion was measured by gas chromatography. cTurnover frequency (TOF) values were calculated on the basis of the listed conversions and reaction times. 7352


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Table 4. Hydrogenation of Limonene to p-Menthene Catalyzed by Complexes 1-Os/Ru and 2-Os/Rua

complex

time (h)

conversion (%)b

TOF (h‑1)c

1-Os 1-Ru 2-Os 2-Ru

1.5 1.0 1.5 1.0

97 8 50 20

647 84 334 195

bonding impedes the rapid release of H2, as evidenced through the above T1 NMR analysis of 4-Os.

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CONCLUSIONS Employing standard synthetic methodology for the transmetalation of aryl-substituted β-diketiminate ligands, the preparation and characterization of the first β-diketiminato osmium complexes, 1-Os and 2-Os, has been achieved. The Ru and Os arene β-diketiminate complexes demonstrate a strong propensity to engage in bifunctional activity involving the metal and β-carbon site. This work demonstrates, however, that the metal-based coordination and activation of a third ligand is only possible if the latter has π-accepting capabilities. Both spectroscopic evidence and DFT calculations indicate that the η6-arene Os(II) β-diketiminate differ from the isostructural Ru(II) counterparts in σ- and π-based interactions between the metal and supporting ligands. This differentiation is further evidenced in the hydrogenation of various types of olefins, where Os(II) proved to be more active. In contrast, Ru seems to be the superior catalyst in dehydrogenation reactions, as evidenced by the results obtained with N,N-dimethylamine borane. Interestingly, using 2-Os, it is possible to hydrogenate the exo-olefinic bond of limonene, which is normally limited to heterogeneous-based catalysts.

a

Conditions: 1 g (9.6 mmol) of limonene and 0.1 mol % of catalyst were dissolved in 12 mL of anhydrous and N2-saturated THF under inert conditions and transferred to an individually stirred multicell autoclave. The sealed reactor was pressurized to 50 bar of H2 and heated to 80 °C. bAfter the appropriate time, the conversion was measured by gas chromatography. cTurnover frequency (TOF) values were calculated on the basis of the listed conversion and reaction time.

by ( iPrPDI)Fe(N2) 2 ( iPrPDI = {(2,6-CHMe 2) 2C 6H 3N CMe}2C5H3N), Bart et al. stated a TOF of 164 h−1 in toluene77 and a TOF of 1085 h−1 in n-pentane.78 Very recently, the most active systems for the selective hydrogenation of limonene have been reported, both based on Rh nanoparticles, suspended either in ethanol 79 or in the ionic liquid [bimim]PF6.80 In view of the ongoing interest in hydrogen storage related catalytic dehydrogenation of amine borane substrates,81−85 2Os was tested for its capability to catalytically dehydrocouple N,N-dimethylamine borane (DMAB). As depicted in Figure 8,

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EXPERIMENTAL SECTION

The complexes were synthesized using standard Schlenk techniques, whereas subsequent syntheses and manipulations of all products and reagents were performed in a glovebox with a N2 atmosphere containing less than 1 ppm of O2 and H2O. All glassware was dried at 130 °C, and the flasks underwent three N2 purge/refill cycles prior to the introduction of solvents or reagents. All solvents were dried according to literature procedures involving distillation over the appropriate drying agents and storage in Schlenk flasks equipped with a Teflon stopcock.86 Extra-dry THF was purchased from Acros Organics (Fisher Scientific). Benzene-d6, n-pentane, and toluene were dried and distilled over sodium. THF-d8 was purchased dry from Aldrich and used as received. CH2Cl2 and CD2Cl2 were distilled over CaH2 and stored under inert conditions. As a filtration aid, Celite was kept in an oven at 130 °C and flushed with N2 prior to use. The precursor dichloro(η6-C6H6)osmium(II) dimer was prepared according to literature procedures.25,87 Other reagents were purchased from commercial sources and used as received if not specified otherwise. NMR spectra were recorded using Varian VNMRS 300 and 400 and a Varian INOVA 500 instruments. Chemical shifts for 1H and 13C{1H} NMR spectra were referenced to the relevant solvent peaks, observed as residual signals. 19F NMR spectra were referenced to the relevant residual solvent peak and CCl3F. Infrared spectra were recorded on a Varian 3100FT-IR Excalibur spectrometer. Samples were prepared as Nujol mulls on KBr disks. Elemental microanalyses were obtained using an Exeter Analytical EA-1110 elemental analyzer. Mass spectra were recorded on a Waters alliance HT Micromass Quattro LCT (MeOH/H2O, 60/40) TOF instrument with a cone voltage of 35 V and a capillary voltage of 2800 V (+) and 2500 V (−). Gas chromatography measurements were performed on a Varian CP-3380 Gas Chromatographic Analyzer. Synthesis of (η6-C6H6)OsCl{(2,6-Me2-C6H3NCMe)2CH} (1-Os). Dichloro(η6-C6H6)osmium(II) dimer (300 mg, 0.44 mmol) was weighed into a 50 mL Schlenk tube under N2. In a second 50 mL Schlenk tube, Li{(2,6-Me2-C6H3NCMe)2CH} (304 mg, 0.97 mmol) was dissolved in dried and N2-saturated CH2Cl2 (15 mL) and added to the dichloro(η6-C6H6)osmium(II) dimer by cannula. The reaction mixture was stirred for 24 h under N2. The mixture was filtered over Celite under N2, and the solvent was reduced to 1 mL. Dried and N2saturated n-pentane was added to precipitate the crude product. After filtration under N2 and three washings with n-pentane, 1-Os (392 mg, 73%) was obtained as a light brown solid. Anal. Found (calcd): C,

Figure 8. H2 gas evolution (DMAB equivalents) as a function of time (h) for the dehydrogenation of DMAB (3.2 M in THF) at 42 °C, catalyzed by 0.5 mol % of 2-Os (□) and 2-Ru (○), respectively. The background reaction without catalyst added is shown as a solid line (). Further details are provided in the Experimental Section.

0.5 mol % of 2-Os affords nearly quantitative dehydrogenation of DMAB after 30 h at 42 °C. The observed activity is significantly lower than that previously reported for 2-Ru, which showed quantitative dehydrogenation of DMAB after only 0.5 h.27 The modest reactivity of 2-Os for amine borane dehydrogenation is in strong contrast with the superior activity of Os observed in the above olefin hydrogenation reactions. A possible explanation might be that the strong Os−hydride 7353


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(ppm) −6.65 (s, 1H, Os-H), 1.98 (s, 6H, o-CH3), 2.06 (s, 6H, oCH3′), 2.23 (s, 6H, α-CH3), 3.14 (d, 2JHH = 14,8 Hz, 1H, β-CH), 4.44 (d, 2JHH = 14,8 Hz, 1H, β-CH), 4.67 (s, 6H, η6-C6H6), 7.05−7.08 (m, 2H, Ar p-CH), 7.12−7.14 (m, 2H, Ar m-CH), 7.21−7.22 (m, 2H, Ar m-CH′). 13C{1H} NMR (100 MHz, 25 °C, CD2Cl2): δ (ppm) 18.0 (s, o-CH3), 18.2 (s, o-CH3′), 23.8 (s, α-CH3), 54.9 (s, β-CH), 79.0 (s, η6C6H6), 121.2 (q, 1JCF = 321.5 Hz, CF3SO3−), 127.7 (s, Ar p-CH), 127.9 (s, Ar o-C), 129.0 (s, Ar m-CH), 129.2 (s, Ar m-CH′), 129.5 (s, Ar oC′), 153.2 (s, Ar i-C), 177.8 (s, CN). Note: T1 of Os−H is 1.082 s (as measured by the inversion recovery method). FT-IR (25 °C, ATR): ν (cm−1) 3067 (vw), 2976 (w), 2922 (w), 2867 (w), 1981 (w, Os−H), 1633 (vw), 1589 (vw), 1562 (w), 1511 (w), 1469 (w), 1431 (w), 1383 (w), 1469 (w), 1431 (w), 1383 (w), 1346 (w), 1305 (w), 1259 (vs), 1223 (m), 1149 (s), 1095 (w), 1067 (vw), 1030 (s), 910 (w), 859 (w), 825 (m), 771 (m), 753 (w), 715 (w), 637 (s). Synthesis of [(η6-C6H6)Os(C2H4){(2,6-Me2-C6H3NCMe)2CH}]OTf (5-Os). The title compound was isolated as a light orange solid (43 mg, 41%), following a procedure identical with that reported for the corresponding Ru(II) complex.25 Anal. Found (calcd): C, 46.95 (47.99); H, 4.50 (4.70); N, 3.75 (3.73). Elemental analysis was affected by the partial loss of ethylene. ESI MS+ (25 °C, CH2Cl2) (m/ z): 577.6035 [parent (M − C2H4 + H+)+, 100%, 577.2163]. 1H NMR (25 °C, 400 MHz, CD2Cl2): δ (ppm): 1.35 (m, 2H, Os−CH2), 1.96 (s, 6H, o-CH3), 2.05 (s, 6H, α-CH3), 2.23 (s, 6H, o-CH3′), 2.35 (m, 2H, Os−CβH2), 4.54 (s, 1H, β-H), 4.67 (s, 6H, η6-C6H6), 7.10−7.13 (m, 2H, Ar p-CH), 7.16−7.18 (m, 2H, Ar m-CH), 7.22−7.24 (m, mCH′). 13C{1H} NMR (25 °C, 100 MHz, CD2Cl2): δ (ppm) −7.5 (s, Os-CαH2), 16.9 (s, Os-CβH2), 18.0 (s, o-CH3), 18.9 (s, o-CH3′), 22.7 (s, α-CH3), 61.4 (s, β-CH), 80.1 (s, η6-C6H6), 121.6 (q, 1JCF = 321.5 Hz, CF3SO3−), 128.1 (s, Ar p-CH), 128.9 (s, Ar o-C), 129.4 (s, Ar mCH), 129.6 (s, Ar o-C′), 129.7 (s, Ar m-CH′), 152.8 (s, Ar i-C), 180.0 (s, CN). FT-IR (25 °C, ATR): ν (cm−1) 2955 (vw), 2917 (vw), 1626 (vw), 1612 (w), 1590 (vw), 1471 (w), 1420 (w), 1398 (w), 1381 (w), 1333 (vw), 1260 (vs), 1223 (m), 1183 (w), 1150 (s), 1132 (m), 1098 (w), 1028 (vs), 998 (vw), 883 (vw), 832 (m), 804 (w), 791 (w), 782 (w), 774 (m), 766 (m), 753 (w), 718 (w), 711 (vw). Synthesis of [(η6-C6H6)-Os(CNXyl){(2,6-Me2-C6H3NCMe)2CH}]OTf (6-Os). 2-Os (100 mg, 0.138 mmol) and 2,6-dimethylphenyl isocyanide (19 mg, 0.145 mmol) were weighed into a 50 mL Schlenk tube under inert conditions and dissolved in dried and N2-saturated CH2Cl2 (10 mL). The reaction mixture was stirred overnight under N2 and the solvent removed under vacuum to afford 6-Os as a red solid (108 mg, 92%). Crystals suitable for X-ray analysis were grown from a concentrated CH2Cl2 solution by slow vapor diffusion of n-pentane at −10 °C. Anal. Found (calcd): C, 51.43 (51.92); H, 5.13 (4.71); N, 4.47 (4.91). ESI MS+ (25 °C, CH2Cl2) (m/z): 707.6621 [parent (M + H+)+, 100%, 707.3000]. 1H NMR (400 MHz, 25 °C, CD2Cl2): δ (ppm) 1.58 (s, 6H, α-CH3), 2.22 (s, 6H, Ar o-CH3), 2.26 (s, 6H, Ar o′CH3), 2.56 (s, 6H, Xyl Ar o-CH3), 4.85 (s, 6H, η6-C6H6), 5.13 (s, 1H, β-CH), 7.06−7.34 (m, 9H, Ar CH). 13C{1H} NMR (100 MHz, 25 °C, CD2Cl2): δ (ppm) 18.5 (s, Ar o-CH3), 19.6 (s, Xyl o-CH3), 19.7 (s, Ar o′-CH3), 22.4 (s, α-CH3), 86.2 (s, η6-C6H6), 103.5 (s, β-CH), 121.6 (q, 1 JCF = 321.5 Hz, CF3SO3−), 127.4 (s, Xyl p-CH), 129.1 (s, Xyl Ar mCH), 129.7 (s, Ar m-CH), 129.8 (s, Ar p-CH), 130.4 (s, Xyl Ar o-CH), 131.6 (s, Ar o′-CH), 132.8 (s, Ar o-C), 136.6 (s, Xyl Ar i-CH), 154.7 (s, Ar i-C), 164.2 (s, NCCH3). FT-IR (25 °C, ATR): ν (cm−1) 3062 (w), 2953 (w), 2920 (w), 2144 (s, CN), 1552 (w), 1523 (w), 1453 (m), 1435 (m), 1387 (s), 1263 (vs), 1225 (m), 1187 (m), 1168 (w), 1148 (m), 1134 (m), 1101 (vw), 1027 (vs), 975 (vw), 858 (vw), 839 (m), 808 (vw), 770 (s), 754 (w), 713 (w), 669 (w). Hydrogenation Catalysis. The alkene substrates were distilled from CaH2 and degassed with three freeze−thaw cycles prior to use. The experiments were conducted in a custom-built multicell (10 mL each) steel autoclave reactor. A THF stock solution (2 mL) containing a magnetic Teflon stirrer and 0.1 mol % of 1-Os/Ru or 2-Os/Ru along with the appropriate alkene (1.0 g) and dried (CaH2) and degassed noctane (100 mg) as a GC standard were added to the autoclave under inert conditions. After the autoclave was sealed, the cells were purged with high-purity H2 gas (99.8%, 5 bar). The autoclave was heated to the required temperature (80 °C) under H2 pressure (10 bar). After

47.23 (46.84); H, 4.54 (4.74), N, 3.85 (3.77). 1H NMR (400 MHz, 25 °C, CD2Cl2): δ (ppm) 1.79 (s, 6H, α-CH3), 2.26 (s, 12H, o-CH3), 5.07 (s, 6H, η6-C6H6), 5.66 (s, 1H, β-CH), 7.04−7.08 (m, 2H, Ar pCH), 7.21−7.23 (m, 4H, m-CH). 13C{1H} NMR (100 MHz, 25 °C, CD2Cl2): δ (ppm) 19.3 (s, o-CH3), 23.3 (s, α-CH3), 76.9 (s, η6-C6H6), 103.4 (s, β-CH), 126.8 (s, Ar p-CH), 128.9 (s, Ar m-CH), 132.4 (s, Ar o-C), 156.8 (s, Ar i-C), 162.9 (s, CN). ESI MS+ (25 °C, CH2Cl2) (m/z): 575.240 [parent M − Cl−, 100%, calcd 575.201]. FT-IR (25 °C, ATR): ν (cm−1) 3391 (m, br), 3054 (w), 2956 (w), 2919 (m), 1641 (w), 1622 (w), 1590 (w), 1553 (w), 1528 (s), 1465 (w), 1450 (m), 1428 (m), 1392 (vs), 1334 (w), 1264 (w), 1242 (w), 1186 (s), 1172 (w), 1098 (w), 1090 (w), 1024 (w), 978 (w), 907 (w), 852 (s), 843 (m), 817 (w), 798 (w), 773 (m), 767 (vs), 760 (s), 756 (s), 711 (w). Synthesis of [(η6-C6H6)Os{(2,6-Me2-C6H3NCMe)2CH}]OTf (2Os). 1-Os (300 mg, 0.49 mmol) and NaOTf (93 mg, 0.54 mmol) were added to a 50 mL Schlenk tube under a N2 atmosphere. The solids were dissolved in dried and N2-saturated CH2Cl2 (10 mL) and stirred overnight. The solution was filtered over Celite under inert conditions to remove NaCl. The solvent was evaporated, and the crude product was washed with dried and N2-saturated n-pentane, decanted, and dried under vacuum to afford 2-Os (312 mg, 88%) as a dark brown solid. Anal. Found (calcd): C, 46.40 (46.35); H, 4.31 (4.36); N, 3.87 (3.83). ESI MS+ (25 °C, CH2Cl2) (m/z): 575.267 [parent M+, 100%, 575.201]. 1H NMR (400 MHz, 25 °C CD2Cl2): δ (ppm) 2.13 (s, 12 H, o-CH3), 2.23 (s, 6H, α-CH3), 5.79 (s, 6H, η6-C6H6), 7.00 (s, 1H, βCH), 7.18−7.22 (m, 2H, Ar p-CH), 7.33−7.35 (m, 4H, m-CH). 13 C{1H} NMR (101 MHz, 25 °C, CD2Cl2): δ (ppm) 19.1 (s, Ar oCH3), 23.2 (s, α-CH3), 77.1 (s, η6-C6H6), 107.9 (s, β-CH), 121.6 (q, 1 JCF = 320.9 Hz, CF3SO3−), 128.3 (s, Ar p-CH), 129.2 (s, Ar m-CH), 130.0 (s, Ar o-C), 158.6 (s, Ar i-C), 165.7 (s, CN). 19F NMR (376 MHz, 25 °C, CD2Cl2): δ (ppm) −78.89 (s, CF3SO3−). FT-IR (25 °C, ATR): ν (cm−1) 3068 (vw), 2919 (vw), 1592 (w), 1560 (w), 1512 (w), 1470 (w), 1430 (w), 1382 (w), 1345 (w), 1271 (vs, S−O), 1221 (m), 1180 (w), 1152 (s, C−F), 1098 (w, C−F), 1088 (w), 1029 (s), 979 (w), 876 (w), 863 (m), 778 (m), 752 (w), 716 (w). Synthesis of [(η6-C6H6)OsCl{(2,6-Me2-C6H3NCMe)2CH2}]OTf (3-Os). 2-Os (100 mg, 0.138 mmol) was suspended in dried and N2-saturated Et2O (10 mL) in a 50 mL Schlenk tube under inert conditions and cooled to −20 °C in a N2(l)/EtOH slurry. A 1 N solution of Et2O/HCl (0.15 mL, 0.15 mmol) was added dropwise by syringe with stirring. The reaction was warmed to room temperature and stirred for 2 h under N2. The crude product was filtered under inert conditions and washed with Et2O to afford 3-Os (97.6 mg, 93%) as a light green solid. Crystals suitable for X-ray analysis were grown by diffusion of n-pentane into a saturated solution of the title compound in CH2Cl2. Anal. Found (calcd): C, 44.72 (44.29); H, 4.64 (4.25); N, 3.26 (3.69). ESI MS+ (25 °C, MeCN) (m/z): 575.2101 [parent (M − Cl)+, 100%, 575.2102]. 1H NMR (400 MHz, 25 °C, CDCl3): δ (ppm) 2.14 (s, 6H, α-CH3), 2.31 (s, 6H, o-CH3), 2.34 (s, 6H, o-CH3′), 4.31 (d, 1H, 2JHH = 20.2 Hz, β-CH), 4.81 (d, 2JHH = 20.2 Hz, 1H, β-CH′), 5.05 (s, 6H, η6-C6H6), 7.10−7.14 (m, 2H, Ar p-CH), 7.19−7.22 (m, 4H, Ar m-CH). 13C{1H} NMR (101 MHz, 25 °C, CDCl3): δ (ppm) 18.7 (s, o-CH3), 19.0 (s, o-CH3′), 24.8 (s, α-CH3), 49.2 (s, β-CH2), 79.9 (s, η6-C6H6), 128.3 (s, Ar p-CH), 129.2(6) (s, Ar m-CH), 129.2(9) (s, Ar m-CH′), 129.8 (s, Ar o-CH), 130.2 (Ar o-CH′), 153.0 (Ar i-C), 183.0 (CN). 19F NMR (376 MHz, 25 °C, CDCl3): δ (ppm) −78.25 (s, CF3SO3−). FT-IR (25 °C, Nujol mull, KBr disks), ν (cm−1) 2953 (s), 2898 (s), 2861 (s), 1641 (w), 1463 (m), 1375 (m), 1267 (vs), 1229 (w), 1155 (s), 1099 (vw), 1030 (m), 910 (vw), 853 (w), 776 (w), 720 (vw), 637 (m). Synthesis of [(η6-C6H6)OsH{(2,6-Me2-C6H3NCMe)2CH2}]OTf (4Os). 2-Os (120 mg, 0.17 mmol) was weighed into a narrow 20 mL Schlenk tube under inert conditions and dissolved in dried and N2saturated THF (10 mL). The solution was degassed under slight vacuum, and H2(g) was run over the solution for 2 min and allowed to diffuse into the THF solution at −10 °C for 48 h to afford orange crystals suitable for X-ray analysis. Elemental analysis was affected by partial loss of hydrogen. Anal. Found (calcd): C, 45.98 (46.40); H, 4.33 (4.59); N, 3.78 (3.87). 1H NMR (400 MHz, 25 °C, CD2Cl2): δ 7354


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the desired temperature was reached, the reactor was fully pressurized to 40 or 50 bar with stirring. The reaction temperature (80 °C) was maintained for the duration of the experiment. Afterward, all catalytic reactions were quenched by depressurizing and cooling the reactor to ambient conditions. The autoclave was opened under air, and small aliquots were taken and diluted with toluene. The conversion and selectivity of the individual hydrogenation reactions were analyzed by GC employing the following conditions. A Varian CP-3380 Gas Chromatographic Analyzer was used with a CP-Sil 5CB normal phase (low polarity) 25 m × 0.25 mm column. The reported TOF values are averages from three separate runs. Dehydrogenation Catalysis. A typical procedure involved charging a N2-flushed 50 mL Parr 100 bar stainless steel pressure reactor with N,N-dimethylamine borane (DMAB; 954 mg, 16.2 mmol) and 2-Os or 2-Ru (0.5 mol %) under a stream of N2. The reactor was preheated to 42 °C using an oil bath, anhydrous N2-saturated THF (5 mL) was added through the sampling valve via syringe, and the reactor was sealed. The reaction mixture was left without stirring at 42 °C for the duration of the experiment. The reaction temperature was measured by an internal thermocouple. The pressure increase was monitored at regular intervals using an automated pressure gauge (Impress Sensors and Systems). The recorded pressure was converted to H2 equivalents using the ideal gas law with a reactor volume of 84 mL (including head space). To avoid contamination of the reactor interior by traces of catalyst, a base/acid cleaned glass liner was used for each experiment. The stirrer and thermocouple probe were sanded prior to each run. Unless otherwise stated, the reactions were carried out without stirring.

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ASSOCIATED CONTENT

* Supporting Information S

Text, tables, figures, and CIF files giving crystallographic details for all relevant complexes, computational methodology, Mayer bond indices, and a detailed comparison of the charge decomposition analysis for selected model complexes. 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 for A.D.P.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The initial part of this research was funded through the European Commission Marie Curie Action EIF (ADP, MEIFCT-2005-025287) and the Swiss National Science Foundation. This research was further supported through a commercialization grant from Enterprise Ireland, project no. CF20111039, and by Science Foundation Ireland (SFI) through a grant supporting the Solar Energy Conversion Cluster (07/SRC/ B1160). Finally, A.D.P. thanks Science Foundation Ireland (SFI) for a Stokes Lectureship at UCD.

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REFERENCES

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Organometallics

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