Synthesis, Structure, and Bonding Properties of Ruthenium

Article pubs.acs.org/Organometallics

Synthesis, Structure, and Bonding Properties of Ruthenium Complexes Possessing a Boron-Based PBP Pincer Ligand and Their Application for Catalytic Hydrogenation Takuma Miyada, Enrique Huang Kwan, and Makoto Yamashita* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, 112-8551 Tokyo, Japan S Supporting Information *

ABSTRACT: In addition to our previous works for PBP-pincer metal complexes, four PBP-pincer Ru complexes, [PBP]Ru(Cl)(CO) (2), [PBP]Ru(CO)(η2-BH4) (3), [PBP](μ-H)2Ru(OAc-κ2O) (4), and [PBP](μ-H)2Ru(η2-BH4) (5) were synthesized. All the obtained complexes were characterized by NMR and IR spectroscopy, X-ray crystallography, elemental analysis, and DFT calculation with AIM analysis. Through the structural analysis, two types of interaction between boron atoms and ruthenium atoms in 3−5 were revealed. One is the typical two-center−two-electron bond between the boron atom of the PBP ligand and the Ru atom, associated with bridging hydride ligand(s) on the Ru(IV) center. The other is an ionic interaction between the Ru fragment and the tetrahydroborate anion. On comparison of the structural features, vibrational analysis, NBO analysis, and AIM analysis of the obtained compounds with those of the previously reported complexes having “similar” interactions among B, H, and Ru atoms, the interactions in 4 and 5 were proven to be different from that previously reported. The obtained complexes 3−5 were applied as catalysts for the hydrogenation of aldehyde. Complex 5 showed the highest catalytic activity and widest range of substrate scope. Two mechanisms for the catalytic cycle were proposed with an initial dissociation of BH3 or anionic ligand, according to the control experiments.

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INTRODUCTION Since the discovery of cyclometalation reactions of phosphorustethered benzene derivatives with transition metals,1 the organometallic chemistry of cyclometalated phosphine-based pincer complexes has been widely explored together with the development of new structures of pincer ligands and advances in the catalytic transformation of organic compounds.2 Among the reported pincer complexes, the phosphorus-based anionic pincer system showed unique properties and catalytic activity. For example, iridium complexes possessing PCP, PNP, and PSiP pincer ligands have been applied to the catalytic dehydrogenation of alkanes3 and elementary X−H bond cleavage reactions (X = N, O).4 Ruthenium complexes having noninnocent PNP pincer systems were also effective for catalytic hydrogenation, dehydrogenation, and hydrogen transfer reactions.5 The anionic ligand in the central position of the pincer ligand played an important role in achieving the catalytic activity and unique bond cleavage reaction. Recently, we reported the synthesis of hydroboranes 1a−c as precursors for the boron-containing PBP pincer ligand (Chart 1) and their complexation with iridium to form the corresponding PBP-ligated iridium complexes.6 In the study on iridium complexes, the strong trans influence of the PBP ligand was proven by structural parameters. In the case of utilization of PBP ligand precursor 1a for complexation with rhodium metal, we could isolate a unique T-shaped, 14-electron [PBP]Rh complex as a crystalline solid.7 The complex reacted with the O−H bond of phenol and aliphatic alcohols in an oxidative addition fashion. Furthermore, the [PBP]Rh complex © 2014 American Chemical Society

Chart 1. PBP-Ligand Precursors 1a−c and Reported Examples of Their Complexation with Transition Metals

could also cleave C−C single bonds in 3,3-diphenylcyclobutanone and benzocyclobutene.8 In the latter reaction, the oxidative addition product, a five-membered rhodacycle, could be isolated. The PBP ligand system could also be introduced to platinum to form square-planar [PBP]PtX complexes (X = Cl, OTf, NTf2, H).9 Among them, the complex [PBP]PtCl was shown to be active as a catalyst for hydrosilylation of 1-decene. After our initial study of the synthesis of the hydroborane precursor, Hill reported the utilization of phenyl-substituted PBP ligand 1c to prepare the corresponding ruthenium and osmium complexes along with a study of their dynamic behavior and structures.10 We also reported the introduction of 1a to ruthenium, and the subsequent oxygenation reaction led to the formation of an oxidized [PBPO]Ru complex possessing a doubly bonded BO type ligand with an octahedral structure.11 Recently, Peters reported the introduction of 1a to cobalt and subsequent reduction under a nitrogen Received: June 1, 2014 Published: November 19, 2014 6760

dx.doi.org/10.1021/om500585j | Organometallics 2014, 33, 6760−6770

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atmosphere to form the [PBP]CoI(N2) complex.12a Their complex could reversibly react with dihydrogen to form a dihydroborate-type “[PBPH2]CoIII(H)2” complex. The resulting complex could be utilized as a catalyst for the dehydrogenation of an amine−borane complex and transfer hydrogenation of styrene with amine−borane. Subsequently, they also showed the PBP ligand could be utilized to prepare dinuclear Co and Ni complexes.12b Here, we report the synthesis of several new t Bu-substituted [PBP]Ru complexes, a systematic study of their properties, and their utilization as catalysts for the hydrogenation of aldehyde with TON values up to 31500.

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RESULTS Synthesis of Complexes 2−5. The synthesis of PBP-Ru complexes 2−5 is summarized in Scheme 1. Mixing the Scheme 1. Synthesis of PBP-Ru Complexesa Figure 1. ORTEP drawing of 2 (50% thermal ellipsoids, two cocrystallized THF molecules and hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): B1−Ru1 = 2.022(4), Ru1−Cl1 = 2.4354(10), Ru1−C1 = 1.785(4), C1−O1 = 1.172(5), P1−Ru1 = 2.4211(10), P2−Ru1 = 2.3899(10), B1−N1 = 1.416(5), B1−N2 = 1.436(5); P1−Ru1−P2 = 151.44(4), N1−B1−N2 = 106.8(3), B1−Ru1−C1 = 72.95(18), B1−Ru1−Cl1 = 157.80(13).

of 2 would change the orientation of tBu groups from that of 6, with the result that the CO ligand leans closer to the boron atom (B1−Ru1−C1 = 72.95(18)°) in comparison with the C− Ru−CO angle (85.0(2)°) of 6. The 1H and 13C NMR spectrum of 2 showed a Cs-symmetrical pattern, indicating that CO and Cl ligands do not exchange each position. The 13C NMR signal of the carbonyl ligand in 2 resonated as a triplet at δC 204.5, and the carbonyl vibration was observed at 1907 cm−1 in the IR spectrum. The corresponding PCP derivative 6 showed a similar 13C NMR chemical shift of δC 209.1 and carbonyl vibration of 1909 cm−1, indicating a similar strength of π backdonation from Ru to the carbonyl ligand. Complex 3. The crystallographic analysis revealed the η2 coordination mode of the BH4 ligand to form a pseudooctahedral ruthenium(II) center in 3,5m,16 where the carbonyl ligand also leaned toward the boron center with a B1−Ru1−C1 angle of 72.8(3)° (Figure 2). All four hydrogen atoms of the BH4 anion were assigned to the residual peaks in the differential Fourier map. The Ru1−C1 (1.825(8) Å) and the B1−Ru1 (2.048(8) Å) bond lengths in 3 were slightly elongated in comparison with those of 2. These longer bonds in 3 are a result of the stronger trans influence of BH4 ligand in comparison to those of the chloride and the vacant site in 2. However, the boron atom in the BH4 ligand is apparently separated from the Ru center with an Ru1- - -B2 distance of 2.365(9) Å, which is the longest Ru- - -B distance among the previously reported Ru(BH4) complexes (2.08−2.254 Å),5m,16 probably due to the strong trans influence of the boryl ligand. This longer Ru1- - -B2 distance in comparison to the B1−Ru1 bond could also differentiate two types of interactions between boron and ruthenium atoms in 3. Additionally, the shape of the PBP plane and the diazaborole ring in 3 was similar to those of 2, indicating no contribution of σ-borane-type coordination between B1 and Ru1. The bent B1−Ru1−B2 angle of 150.9(3)° results from the two inequivalent hydrogen atoms in η2-coordinating BH4. The 1H and 13C NMR spectra of 3 were similar to those of 2 except for BH4 protons. Two distinct broad 1H NMR signals at δB −7.44 (full width at half-maximum

Legend: (a) RuHCl(CO)(PPh3)3, tol, 100 °C, 2 h, 65%; (b) NaBH4, THF, room temperature, 2 h, 88%; (c) RuH(OAc)(PPh3)2, tol, 100 °C, 15 h, 51%; (d) NaBH4, THF, room temperature, 1 h, 91%. a

hydroborane precursor 1a with RuHCl(CO)(PPh3)3 gave [PBP]Ru(Cl)(CO) (2) through loss of H2 and PPh3 ligands in 65% yield. Addition of NaBH4 to 2 induced ligand exchange from Cl to BH4 to form [PBP]Ru(CO)(η2-BH4) (3) in 88% yield. The reaction of 1a with Ru(H)(OAc)(PPh3)2 afforded the hydride-bridged complex [PBP](μ-H)2Ru(OAc-κ2O) (4) in 51% yield. Ligand exchange from OAc to BH4 could also be achieved by the reaction of 4 with NaBH4 to give the dihydridoruthenium tetrahydroborate complex [PBP](μH)2Ru(η2-BH4) (5) in 91% yield. Complexes 2−5 could all be characterized by NMR (1H, 11B, 13C, 31P) and IR spectroscopy, elemental analysis, and X-ray crystallographic analysis. Additionally, DFT calculations for complexes 3−5 with the B3LYP method, the LANL2DZ basis set for Ru, and the cc-pVDZ basis set for other atoms were performed for the characterization of these complexes. The resulting optimized structures opt-3−opt-5 were in good agreement with the crystallographically obtained structures of 3−5. In addition to the conventional DFT study, AIM analysis was also performed to clarify the bonding properties of these complexes. The following paragraphs describe the properties of each complex. Complex 2. A crystallographic analysis of 2 showed its distorted-trigonal-bipyramidal (“Y”-shaped)13 structure with two apical phosphine ligands (Figure 1). The P−Ru−P angle of 151.44(4)° in 2 was smaller than that (162.44(4)°) in the previously reported [PCP]Ru(CO)Cl (6),14 reflecting the smaller size of the boron-containing five-membered ring in comparison to the benzene ring in 6. This difference may lead to a Ru−B bond shorter (2.022(4) Å) than the Ru−C bond (2.076(4) Å) in 6, even though the boron atom has a larger atomic size than the carbon atom.15 These structural distortions 6761


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Figure 3. ORTEP drawing of 4 (50% thermal ellipsoids, one of the two independent molecules and hydrogen atoms except for the bridging hydrides omitted for clarity). Selected interatomic distances (Å) and angles (deg): B1−Ru1 = 2.009(8), Ru1−O1 = 2.240(5), Ru1−O2 = 2.199(5), Ru1- - -C1 = 2.565(7), C1−O1 = 1.272(9), C1− O2 = 1.256(9), P1−Ru1 = 2.3646(17), P2−Ru1 = 2.3709(17), B1− N1 = 1.427(10), B1−N2 = 1.443(9), P1−Ru1−P2 = 158.41(7), N1− B1−N2 = 105.6(6).

Figure 2. ORTEP drawing of 3 (50% thermal ellipsoids, cocrystallized THF molecule and hydrogen atoms except for the BH4 anion omitted for clarity). Selected interatomic distances (Å) and angles (deg): B1− Ru1 = 2.048(8), Ru1−C1 = 1.825(8), C1−O1 = 1.151(9), P1−Ru1 = 2.3957(18), P2−Ru1 = 2.3949(19), Ru1- - -B2 = 2.365(9), B1−N1 = 1.437(9), B1−N2 = 1.421(10), P1−Ru1−P2 = 153.05(6), N1−B1− N2 = 106.4(6), B1−Ru1−C1 = 72.8(3), B1−Ru1−B2 = 150.9(3).

(fwhm) = 80 Hz) and 2.70−3.30 ppm (fwhm = 276 Hz) could be assigned as two bridging hydrides and a terminal BH2 moiety, as were assigned in the similar PNN- and PNPRu(H)(BH4) complexes.5m,16d The reason why the bridging hydrides showed one broad signal could be attributed to a possible fluxional behavior consisting of a dissociation of one of the two bridging hydrides to form a η1-BH4 complex (see below in the discussion for the mechanism of catalytic hydrogenation; Scheme 3). At 100 °C in toluene, the former signal disappeared and the latter signal was broadened and shifted to lower field (see the Supporting Information),5m,16a,d indicating dissociation of the BH4 anion and exchange of bridging and terminal hydrides. The carbonyl ligand also showed a triplet signal at room temperature in the 13C NMR spectrum, similar to the case for 2. The IR spectrum (KBr) of 3 was useful to understand its bonding situation, in combination with assignments by using DFT calculation. Two characteristic vibrations at 1931 and 1937 cm−1 were observed as the sum of carbonyl vibrations and stretching of two B−H bonds showing the η2 coordination mode of the BH4 ligand (see the Supporting Information). A peak at 1283 cm−1 was proven to incorporate Ru1−B1 stretching as one of the components, suggesting a simple two-center−two-electron bond between the Ru1 and B1 atoms. Complex 4. The crystal structure of 4 showed the acetato ligand coordinated in a κ2O fashion to the ruthenium center through the two oxygen atoms O1 and O2 with similar Ru−O bond lengths (Figure 3). Two peaks in the difference Fourier map could be assigned to the bridging hydride ligands H1 and H2. The B1−Ru1 bond length of 2.009(8) Å in 4 was similar to that of 2, even in the presence of two bridging hydride ligands. The B1−Ru1 bond is shorter than those in all the previously reported η2-hydroborate complexes (2.047−2.141 Å) having two or more hydrides on the boron center,17 indicating that the interaction between the boron and ruthenium atoms is a boryltype 2c-2e bond. In accord with the coexistence of a short B1− Ru1 bond and two bridging hydride ligands, the P−Ru−P angle of 4 is larger than those of 2 and 3. The bridging hydrides between the PBP ligand and the ruthenium center in 4 resonated at δH −16.39 ppm (fwhm = 40 Hz) in its 1H NMR spectrum. Similar chemical shifts were observed for the

bridging hydride ligand(s) in the previously reported PBPligated [PBP](μ-H)RhCl complex 7 (δH −22.09 ppm at room temperature)7 and [PBP](μ-H)2Co(H)2 complex 8 (δH −4.09 ppm at −90 °C)12 (Chart 2). The 13C NMR spectrum also Chart 2. Reference Complexes 7 and 8 with the Same PBP Ligand and Bridging Hydride between the PBP Ligand and Transition-Metal Atom

reflected the C2v symmetry of the molecule. The IR spectrum of 4 showed two sets of characteristic signals (1541 and 1435 cm−1 (sym + antisym stretch of C1−O1 and C1−O2), 2066 cm−1 (sym + antisym stretch of Ru1−H1 and Ru1−H2)). The latter vibration could be assigned as a symmetrical stretching mode of two Ru1−H bonds, not B1−H bonds, by DFT calculations (see the Supporting Information for force constants). This result also indicated that the two bridging hydride ligands could be considered to be ruthenium hydrides interacting with the vacant p orbital of the boron atom, instead of adopting a η2-dihydroborate coordination mode. Therefore, 4 could be considered as a Ru(IV) complex having PBP, two hydrides, and acetate as anionic ligands. Complex 5. The crystal structure of 5 also showed a close contact of the tetrahydroborate anion with the ruthenium center (Figure 4). Similar to the case for 3 and 4, two and four peaks in the difference Fourier map were assigned as the bridging hydride ligands H1 and H2 and the hydrogen atoms of BH4 anion, respectively. The B1−Ru1 bond length (2.048(9) Å) was around the shortest edge of the reported two-center− two-electron B−Ru bond lengths (2.047−2.141 Å) of borylruthenium complexes.18 However, the Ru1- - -B2 distance (2.333(9) Å) was slightly shorter than that in 3 but still longer than those of reported Ru- - -BH4 distances (2.08−2.254 Å).5m,16 This Ru1- - -B2 distance in 5 is also close to the longest edge of the reported Ru- - -B distances (2.085−2.346 6762


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symmetric bending of B2−H5 and B2−H6), 2413 cm−1 (symmetric stretch of B2−H5 and B2−H6), and 2426 cm−1 (antisymmetric stretch of B2−H5 and B2−H6), as DFT calculations provided the assignment. Similar to the case for complex 4, complex 5 consisted of boryl−Ru(IV), two Ru−H ligands bridging to B1, and a η2-coordinating BH4 anion. Molecular Orbitals of opt-3−opt-5. The shape of molecular orbitals for opt-3−opt-5 also supported the twocenter−two-electron boryl-type interaction between the PBP ligand and the ruthenium center. HOMO-5 of opt-3 (Figure 5a) also showed a B−Ru σ-bond character, merged with π backdonation from the ruthenium to the CO ligand. This orbital overlap may be more effective in comparison with that between σ coordination of the BH4 ligand and the π* orbital of the CO ligand to be the reason for the small B1−Ru1−C1 angle of 3 (the same comparison of the B−Ru σ bond with the Ru−Cl σ bond in 2 would be expected, even we have no DFT calculations on 2). HOMO-4 of opt-4 (Figure 5b) and HOMO-4 of opt-5 (Figure 5c) have σ-bond character consisting of the sp2 orbital of the boron atom in the PBP ligand and the d orbital of ruthenium atom. The symmetrical shape of these two orbitals below and above the plane of the PBP ligand may reflect their C2v-symmetrical molecular structure. In the case of opt-4 and opt-5, a three-center interaction among the boron atom of the PBP ligand, the bridging hydrogen atoms, and the central ruthenium atom could also be confirmed in the molecular orbitals. Figure 5d,e shows two sets of three-center interactions, reflecting the phase of the p orbital of the boron center. Thus, a description of the molecular orbital could be as a separate two-center−two-electron boryl-type interaction and three-center interaction. NBO Analysis of opt-4 and opt-5. The NBO analysis of opt-4 and opt-5 using the NBO 3.0 package in Gaussian 09 provides a further understanding of the three-center interaction among B, 2H, and Ru atoms. In both cases, two sets of donor− acceptor interactions from the Ru−H bond (BD) as a donor to a vacant p orbital of the boron center (LP*) as an acceptor were found. This result indicates that the three-center

Figure 4. ORTEP drawing of 5 (50% thermal ellipsoids, minor part of disordered benzodiazaborole moieties and hydrogen atoms except for the bridging hydrides and BH4 anion omitted for clarity). Selected interatomic distances (Å) and angles (deg): B1−Ru1 = 2.048(9), P1− Ru1 = 2.376(2), P2−Ru1 = 2.383(2), Ru1- - -B2 = 2.333(9), B1−N1 = 1.444(10), B1−N2 = 1.439(11), P1−Ru1−P2 = 157.41(7), N1− B1−N2 = 105.9(7), B1−Ru1−B2 = 177.4(4).

Å) for the η2 coordination mode of an sp3 hydroborate anion having two or more hydride ligands.17 The linear B1−Ru1−B2 angle of 177.4(4) reflects a nearly symmetrical coordination mode of bridging μ2-hydrogen atoms and η2-coordinating BH4. The 1H NMR spectrum of 5 showed three distinct broad signals each having a 2H integral ratio. Two of them (δH −14.57 (H1, H2, fwhm = 42 Hz) and −5.78 ppm (H5, H6, fwhm =86 Hz)) are relatively sharp and one remaining signal is greatly broadened (δH 5.50−6.60 ppm (H3,H4, fwhm = 224 Hz)). In comparison with 3 having a BH4 ligand and 4 having bridging hydride ligands, the assignment of these three signals could also be supported. The pattern of the other signals in the 1 H NMR spectrum and the whole 13C NMR spectrum supported the C2v symmetry of the molecule. The IR spectrum of 5 showed several characteristic vibrations of 1364 and 1379 cm−1 (symmetric stretch of Ru1−H1 and Ru1−H2 +

Figure 5. Molecular orbitals involving the B(PBP ligand)−Ru bond ((a) HOMO-5 of opt-3, (b) HOMO-4 of opt-4, (c) HOMO-4 of opt-5) and the B(PBP ligand)−2H−Ru interaction ((d) HOMO-28 of opt-4, (e) HOMO-7 of opt-5). All hydrogen atoms except for the bridging 2H and BH4 moieties are omitted for clarity. 6763


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similar complexes in the literature. Stradiotto and Sabo-Etienne independently reported a detailed AIM analysis on the four DFT-optimized ruthenium complexes [Ru(H2BMes)]+[B(C6F5)4]− (opt-9; Ru = Cp*Ru(PiPr3)), [Ru(H3BMes)] (opt-10), (PMe 3 ) 2 RuHCl(H 2 BNMe 2 ) (opt-11), and (PMe3)2RuH2(H2BNMe2) (opt-12), having similar two bridging hydrogen atoms between boron and ruthenium atoms.17f,h,18d,h The interatomic distances and bond paths obtained from an AIM study for DFT-optimized structures of these examples and those of our complexes opt-3−opt-5 are summarized in Table 1. On the basis of the interatomic distance between B and Ru atoms, all B−H−Ru interactions could be classified into the three groups A−C as indicated by colors in the structural formula. The H2BMes ligand in opt-9 and H2BNMe2 ligand in opt-11 and opt-12 coordinated to Ru center in an η2 fashion with a short Ru−B distance (1.996 Å for opt-9, 1.969 Å for opt11, 1.924 Å for opt-12), due to strong π back-donation from the filled d orbital of the Ru atom to the vacant p orbital of the boron atom, to be classified as group A (red). The group A complexes can also be considered as Ru(IV) species, if the π back-donation is described as a coordination bond with a positive charge on Ru and a negative charge on B. Interaction between the Ru center and hydroborate with a relatively long Ru- - -B distance (2.283 Å for opt-10, 2.384 Å for opt-3, 2.326 Å for opt-5) could be considered as η2 coordination of two B− H bonds without π back-donation from the Ru center. This type of interaction was classified as group B (purple). In opt-4 and opt-5, slightly longer B−Ru distances (2.032 Å for opt-4, 2.069 Å for opt-5) in comparison with those of group A were found to be classified as group C. The reason for longer B−Ru distances in group C in comparison to those in group A could be attributed to the Coulombic interaction between Ru and B. A comparison of Ru−H and B−H distances also gives a similar understanding. The interaction of group B has the longest Ru−H distances (1.789 Å for opt-10, 1.876 and 1.931 Å for opt-3, 1.843 and 1.860 Å for opt-5), reflecting the weak η2 coordination of hydroborate. In the case of group A with the shortest Ru−B distance, Ru−H distances are slightly shorter (1.773 and 1.780 Å for opt-9, 1.814 and 1.816 Å for opt-11, 1.618 and 2.000 for opt-12). In contrast, the group C interaction has apparently short Ru−H distances (1.595,

interaction consists of the Ru(IV) dihydride complex and the sp2 boron atom of the boryl ligand, rather than the Ru(II) complex and the dihydroborate ligand (Chart 3), providing an η2 interaction of dihydroborate with the Ru(II) center. Chart 3. Two Possible Descriptions for Complexes 4 and 5

AIM Analysis of opt-3−opt-5. Furthermore, AIM analysis19 for opt-3−opt-5 using the AIMALL software package20 also revealed the difference in interaction between the two boron atoms and the ruthenium center (Figure 6). In opt-3, a bond path was found between B1 and Ru1 atoms to indicate that B1 is covalently bonded to the ruthenium center as a boryl ligand. No bond path was found between Ru1 and B2 in opt-3, indicating the η2 coordination mode of the BH4 ligand. Therefore, opt-3 could be described as a (boryl)RuII tetrahydroborate complex. Although bond paths were found between Ru1 and the bridging hydride ligands in opt-4, no bond path was found between the boron atom of the PBP ligand and bridging hydride ligands, indicating we should consider opt-4 as a (boryl)RuIV dihydride acetate complex. This result is consistent with the conclusion from detailed vibrational analysis by using IR spectroscopy and DFT calculations. The structure of opt-5 could be described by a combination of opt3 having a BH4 ligand and opt-4 having two bridging Ru−H ligands to the boron atom. Since a bond path was also found between B1 and H1 in opt-5, we would expect that H1 and H2 rapidly exchange to bond to B1, as the 1H NMR spectrum of 5 exhibited a C2v-symmetrical pattern with all four tBu groups being magnetically equivalent. Thus, opt-5 could be considered as a (boryl)RuIV η2-tetrahydroborate complex. Comparison of B−H−Ru Interactions in opt-3−opt-5 with the Previously Reported Examples. The interaction among the boron atom of the PBP ligand, two bridging hydrogen atoms, and the central ruthenium atom in opt-4 and opt-5 may be different from that in the previously reported

Figure 6. Molecular graphs of opt-3 (a), opt-4 (b), and opt-5 (c) with bond critical points (red spheres) and bond paths (solid lines: electron density at BCP ≥ 0.025 au) calculated by AIM analysis. Atom labeling is based on the numbering in crystallographic analysis as illustrated in Figures 2−4 6764


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Table 1. Comparison of Ru- - -B Interatomic Distances and Results of AIM Analysis among opt-9−opt-12 and opt-3−opt-5a

a Solid bonds indicate bond paths found by AIM analysis (for opt-9−opt-12, bond paths are illustrated as reported in the literature). Ru = Cp*Ru(PiPr3), and Ru′ = (PMe3)2Ru. Distances are based on the results of the previously reported or present DFT-optimized structures. Distances with colored atoms correspond to colored atoms in the structural formula. The interaction between Ru and colored boron atoms are classified by color as follows: red (group A), η2 coordination of two B−H bonds in dihydroborane with π back-donation from the d orbital of Ru to a vacant p orbital of the boron atom; purple (group B), η2 coordination of two B−H bonds in sp3 borate with no π back-donation; blue (group C), donor/ acceptor interaction from Ru−H donor to a vacant p orbital of the B(PBP ligand) atom in the borylruthenium(IV) dihydride complex.

1.603 Å for opt-4, 1.615, 1.629 Å for opt-5), indicating the strongest Ru−H interaction as a Ru−H bond. Similarly, bridging B−H distances (1.619, 1.557 Å for opt-4, 1.474, 1.585 Å for opt-5) of group C are longer than those of groups A and B (group A, 1.311, 1.309 Å for opt-9, 1.320 Å for opt-11, 1.546, 1.272 Å for opt-12; group B, 1.324, 1.328 Å for opt-10, 1.284, 1.327 Å for opt-3, 1.311, 1.316 Å for opt-5). Thus, Ru− H and B−H distances are also consistent with the Ru(IV) dihydride character of opt-4 and opt-5. The AIM analysis of these complexes also afforded the same conclusion (detailed parameters for opt-3−opt-5 are summarized in Table S1 in the Supporting Information). In the case of opt-9, opt-11, and opt-12, a bond path between Ru and B atoms exists, but no bond path for opt-9 and opt-11 and only one bond path for opt-12 was found between Ru and H atoms. In contrast, opt-4 and opt-5 have two bond paths between Ru and H atoms. Thus, the interaction between the PBP ligand and Ru in opt-4 and opt-5 could be differentiated from the Ru−B interaction in opt-9, opt-11, and opt-12. One possible reason for this difference may be the orientation of a boroncentered plane defined by three atoms around the boron center and the d orbital of the Ru center. For example, the boroncentered planes in opt-11 and opt-12 were perpendicular to the P−Ru−P axis of two trans-ligating phosphine ligands, and in contrast, the boron-centered planes of PBP ligand in opt-4 and opt-5 were coplanar with the P−Ru−P axis. On the other hand, the neutral borate complex opt-10 had a longer Ru- - -B distance of 2.283 Å in an η2 fashion with bond paths between Ru and H atoms. The AIM analysis of our Ru tetrahydroborate complexes opt-3 and opt-5 similarly showed bond paths between Ru and two bridging hydrides of the BH4 anion. Thus, the bonding situation of the B−2H−Ru moiety in these complexes could be classified into three types on the basis of the results of an AIM study. Catalytic Hydrogenation Using Complexes 3−5. The [PBP]Ru complexes 3−5 were subjected to a simple catalytic hydrogenation of 1-hexanal (Table 2), because the hydro-

Table 2. Catalytic Hydrogenation of 1-Hexanal using PBPRu Complexes 3−5

run cat. 1 2 3 4 5

3 4 5 5 5

x (mol %)

temp (°C)

time (h)

conversna (%)

yielda (%)

TON

0.01 0.01 0.01 0.002 0.002

140 140 140 160 180

23 23 23 20 22

98 53 100 85 94

87 31 99 43 63

8700 3100 9900 21500 31500

a

Estimated by GC with internal standard of dodecane and authentic samples.

genation of linear aliphatic aldehydes is one of the most important industrial processes for the production of linear alcohols. Heating [PBP]Ru(CO)(η2-BH4) (3; 0.01 mol %) with 1-hexanal at 140 °C for 23 h under 6.0 MPa of H2 induced nearly complete consumption of the substrate and 1-hexanol was formed in 87% yield, as judged by gas chromatography (run 1). Replacement of the catalyst with [PBP](μ-H)2Ru(OAc-κ2O) (4) lowered the conversion and yield (run 2). Use of [PBP](μ-H)2Ru(η2-BH4) (5) improved the conversion and yield to close to quantitative with a TON value of 9900 (run 3). Decreasing the catalyst loading to 0.002 mol % and elevating the temperature to 160 °C led to an increase of TON up to 21500 (run 4). The higher temperature of 180 °C with the same catalyst loading gave a better TON of 31500 (run 5). The obtained catalytic activity of 5 is higher than those of 3d metal (Co, Fe) based homogeneous hydrogenation catalysts for 1octanal21 but is lower than that of an Os-based catalyst for 1hexanal.22 The catalyst system 5 was applied to the hydrogenation of a variety of aldehyde substrates (Table 3). Benzaldehyde could be transformed to benzyl alcohol in 95% yield (run 1). 6765


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Table 3. Substrate Scope for Catalytic Hydrogenation of Aldehyde using PBP-Ru Complex 5a

Scheme 2. Control Experiments for Complex 3

THF-d8 liberated free NaBH4 within 1 h (Scheme 2, right, and Figure S11 (Supporting Information)). Considering the BH4 anion could dissociate rapidly in THF-d8 upon addition of NaBD4, we could imagine a fluxional process containing [PBP]Ru(CO)(η1-BH4) (3′) in C6D6, as illustrated in Scheme 3. This process is consistent with an unsymmetrical η2Scheme 3. Possible Fluxional Behavior of BH4 Ligand in 3

coordination mode of the BH4 ligand in the solid state and the magnetic equivalency of two coordinating hydrogen atoms in the BH4 ligand in C6D6 (see above for structural details of 3). In the case of 5, leaving a solution of 5 in THF-d8 led to no reaction (Scheme 4, left). However, the treatment of 5 with

Conditions: aldehyde (10 mmol), 5 (10 μmol), THF (2.0 mL). Yields were estimated by the 1H NMR spectrum with 1,3,5trimethoxybenzene as an internal standard. a b

Introduction of an electron-withdrawing group on the aromatic ring of benzaldehyde did not alter the chemical yield of benzylic alcohols (run 2). The reaction of p-anisaldehyde gave the corresponding alcohol in moderate yield probably due to the electron richness of the carbonyl group (run 3). Changing the position of a methoxy group to the meta position led to a higher yield of the reduced product (run 4). This result would be consistent with the Hammett σ value of the methoxy group. Placing a methoxy group in the ortho position also gave a similar yield (run 5), indicating that steric hindrance or the directing ability of the methoxy group increases the efficiency of the catalyst even though electronic effects were proven to decrease the efficiency in the case of run 3. As a heteroaromatic compound, furfural could also be reduced to 2-furylmethanol with a yield similar to that for m- or o-anisaldehyde (run 6). An ester functionality could survive the hydrogenation conditions to give a 75% yield of the benzylic alcohol (run 7). A sterically bulky aldehyde, pivalaldehyde, was also converted to neopentyl alcohol (run 8). Cyclic and acyclic secondary alkyl-substituted aldehydes afforded the corresponding alcohol in around 80% yield (runs 9 and 10). Thus, the present catalytic hydrogenation could accept a wide range of substrates. Mechanistic Study. To gain insights into the catalytic cycle, several control experiments were performed. Leaving a THF-d8 solution of 3 at room temperature for 20 h led to an appearances of a triplet signal at hydride region (1H, −9.56 ppm, 2JPH = 16 Hz) and a set of PBP ligand skeleton in the aliphatic and aromatic regions in the 1H NMR spectrum (Scheme 2, left, and Figure S9 (Supporting Information)). These signals may be tentatively assigned as [PBP]Ru(CO)H complex 11. No further change after a prolonged time suggested that the present reaction was in equilibrium. Additionally, exposure of 3 to atmospheric H2 in THF-d8 afforded no hydridic signal other than 3 and 11, indicating that 3 and 11 did not react with H2 (Figure S10 (Supporting Information)) However, the treatment of 3 with NaBD4 in

Scheme 4. Control Experiments for Complex 5

NaBD4 induced a formation of free NaBH4 and 5-d4, as judged by the 1H NMR spectrum (Scheme 4, right, and Figure S12 (Supporting Information)) with a slower exchange rate in comparison with that of 3. This result may reflect the lower electron density on the Ru(IV) center of 5 in comparison to that on the Ru(II) center of 3. Complexes 3 and 5 were also subjected to the reaction with p-F-C6H4CHO in THF-d8 (Scheme 5). In the reaction of 3, all of the broadened BH4 signals immediately disappeared with no hydride signal of 11. Additionally, many signals in the range 3.5−5 ppm, assignable to CH2 groups of the ligand or of the resulting alkoxide Scheme 5. Reactions of Complexes 3 and 5 with Aldehyde

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center. The other one is an ionic interaction between the Ru fragment and tetrahydroborate anion. From a comparison of structural features, vibrational analysis, NBO analysis, and AIM analysis of the obtained compounds with those of the previously reported complexes having “similar” interactions among B, H, and Ru atoms, interactions in 4 and 5 were proven to be different from those previously reported. The obtained complexes 3−5 were applied as a catalysts for the hydrogenation of aldehyde. Complex 5 showed the highest catalytic activity and a wide range of substrate scope. Two mechanisms for the catalytic cycle were proposed with an initial dissociation of BH3 or anionic ligand, according to the control experiments.

intermediate, inhibited us from characterizing the product. In the case of the reaction of 5, no change was observed. According to the results of control experiments, two possible reaction mechanisms for hydrogenation with complexes 3−5 were proposed (Scheme 6). The mechanism with 3 involved an Scheme 6. Proposed Mechanism for Catalytic Hydrogenation using Complexes 3−5

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out in an argon-filled glovebox (Miwa MFG) unless otherwise noted. The 1H, 11 B, 11B{1H}, 13C{1H}, and 31P{1H} NMR spectra were recorded on 500, 400, and 300 MHz spectrometers with residual protiated solvent for 1H and 1H{11B}, deuterated solvent for 13C{1H}, external BF3 for 11 B and 11B{1H}, and external 85% H3PO4 for 31P{1H} used as reference. Elemental analyses were performed by A Rabbit Science Co., Ltd. using a Yanaco CHN corder MT-5 instrument. X-ray crystallographic analysis was performed on a Rigaku VariMax Saturn CCD diffractometer. Melting points were measured on a MPA100 Optimelt Automated Melting Point System and are uncorrected. Benzene, hexane, pentane, toluene, and THF were purified by passing through a solvent purification system (Grass Contour). All other common chemicals were purchased from Kanto Chemical Co., Inc., Tokyo Chemical Industry Co., Ltd., and Aldrich, and they were used after degassing with freeze−pump−thaw cycles. Argon and dihydrogen were purchased from Suzuki Shokan Co., Ltd. The hydroborane precursor 1,6a RuHCl(CO)(PPh3)3,23 and RuH(OAc)(PPh3)324 were synthesized according to the literature procedures. Synthesis of 2. A mixture of RuHCl(CO)(PPh3)3 (206 mg, 0.216 mmol) and 1 (100 mg, 0.229 mmol) in toluene (4.0 mL) was stirred at 100 °C for 2 h. The resulting suspension was evaporated under reduced pressure. The crude product was precipitated by an addition of the benzene solution to hexane. The resulting suspension was filtered, and recrystallization of the residue from benzene/hexane at room temperature gave single crystals of 2 (89.6 mg, 0.150 mmol, 69%): 1H NMR (C6D6, 500 MHz) δ 1.25 (vt, 3JPH = 7 Hz, 18H), 1.26 (vt, 3JPH = 7 Hz, 18H), 3.67 (d, J = 13 Hz, 2H), 3.87 (d vt, J = 13, 3 Hz, 2H), 6.96 (dd, J = 6, 2 Hz, 2H), 7.13 (dd, J = 6, 2 Hz, 2H); 11 1 B{ H} NMR (C6D6, 160 MHz) δ 44.8 (br s); 13C{1H} NMR (C6D6, 126 MHz) δ 30.39 (vt, 2JPC = 3 Hz, CH3), 30.41 (vt, 2JPC = 2 Hz, CH3), 37.7 (vt, 1JPC = 5 Hz, 4°), 37.8 (vt, 1JPC = 5 Hz, 4°), 40.5 (vt, 1 JPC = 15 Hz, CH2), 108.5 (CH), 118.8 (CH), 140.5 (vt, 3JPC = 6 Hz, 4°), 204.5 (t, 2JPC = 12 Hz, CO); 31P{1H} NMR (C6D6, 202 MHz) δ 82.9 (s); mp 210.5−216.6 °C dec; IR (KBr; see the Supporting Information for whole spectrum) νCO 1907 cm−1. Anal. Calcd for C25H45BClN2P2Ru: C, 50.13; H, 7.57; N, 4.68. Found: C, 49.80; H, 7.37; N, 4.76. Synthesis of 3. A solution of 2 (200 mg, 0.334 mmol) in THF (3.0 mL) was added to a solution of NaBH4 (63.0 mg, 1.67 mmol) in THF (3.0 mL), and the resulting solution was stirred at room temperature for 3 h. The resulting suspension was filtered, and the filtrate was evaporated under reduced pressure. Recrystallization of the crude product from THF/pentane at −35 °C gave single crystals of 3 (170 mg, 0.294 mmol, 88%): 1H{11B} NMR (C6D6, 500 MHz) δ −7.44 (br, 2H, BH4, disappeared in tol-d8 at 105 °C, see the Supporting Information), 1.21 (vt, 3JPH = 6 Hz, 18H), 1.26 (vt, 3JPH = 6 Hz, 18H), 2.70−3.30 (br, 2H, BH4, slightly shifted to higher field in tol-d8 at 105 °C, see the Supporting Information), 3.52 (d, J = 12 Hz, 2H), 3.89 (d vt, J = 12, 3 Hz, 2H), 6.95 (dd, J = 6, 3 Hz, 2H), 7.11 (dd, J = 6, 3 Hz, 2H); 11B{1H} NMR (C6D6, 160 MHz) δ 3.9 (br s), 46.8 (br s); 13C{1H} NMR (C6D6, 126 MHz) δ 30.19 (vt, 2JPC = 2 Hz, CH3), 30.22 (vt, 2JPC = 3 Hz, CH3), 37.35 (vt, 1JPC = 5 Hz, 4°), 37.42 (vt, 1JPC = 7 Hz, 4°), 41.9 (vt, 1JPC = 16 Hz, CH2), 108.8 (CH), 118.8

initial dissociation of BH3 to produce the coordinatively unsaturated monohydrido complex 11 in polar media such as THF or neat aldehyde, as experimentally observed. Coordination and insertion of aldehyde to 11 would form the corresponding alkoxo complex 12. The invisibility of hydride or BH4 anion in the reaction of 3 with aldehyde under an Ar atmosphere also supported this pathway. Oxidative addition of dihydrogen to 12 and subsequent reductive elimination of alcohol from the resulting Ru(IV) dihydride complex 13 would be expected to regenerate 11. Instead, a direct hydrogenolysis from 12 to 11 through σ-bond metathesis is also probable. In the case of catalytic hydrogenation using Ru(IV) complexes 4 and 5, dissociation of anionic acetate or BH4 from the Ru center generates the cationic complex 14. Coordination and insertion of aldehyde to 14 would give the alkoxo hydrido complex 15. This pathway may be slow at room temperature, because we did not observe the direct reaction of 5 with aldehyde even the dissociation equilibrium between 5 and 14 could be observed. The subsequent O−H bond-forming reductive elimination would occur to form product alcohol and cationic Ru(II) complex 16. Addition of dihydrogen to 16 regenerates the dihydride complex 14. The higher catalytic activity of 5 than that of 3 may be attributed to the cationic charge on the Ru(IV) center to accelerate coordination− insertion sequence of aldehyde or the subsequent reductive elimination during catalytic cycle. Considering the high reaction temperature, it may be difficult to rule out the heterogeneous mechanism with dissociation of tridentate pincer ligand forming nanoparticles etc.

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CONCLUSION In conclusion, we demonstrated the synthesis of four PBPpincer Ru complexes, [PBP]Ru(Cl)(CO) (2), [PBP]Ru(CO)(η2-BH4) (3), [PBP](μ-H)2Ru(OAc-κ2O) (4), and [PBP](μH)2Ru(η2-BH4) (5). All of the obtained complexes were characterized by NMR and IR spectroscopy, X-ray crystallography, elemental analysis, and DFT calculations with AIM analysis. Through the structural analysis, two types of interactions between boron atoms and ruthenium atoms in 3−5 were revealed. One is the typical two-center−two-electron bond between the boron atom of the PBP ligand and Ru atom, associated with bridging hydride ligand(s) on the Ru(IV) 6767


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Table 4. Crystal Data and Structure Refinement Detailsfor 2−5 formula fw T (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g/cm3) μ (mm−1) F(000) cryst size (mm) 2θ range (deg) no. of rflns collected no. of indep rflns/Rint params GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

2

3

4

5

C33H60BClN2O3P2Ru 742.10 93(2) monoclinic Cc 14.817(2) 15.856(2) 17.075(3) 90 115.1418(18) 90 3631.6(9) 4 1.357 0.628 1568 0.25 × 0.14 × 0.14 3.037 to 27.467 14678 7288/0.0325 400 1.019 0.0290, 0.0618 0.0329, 0.0634

C29H56B2N2O2P2Ru 649.39 113(2) monoclinic Cc 19.318(4) 16.123(3) 11.440(3) 90 109.855(2) 90 3351.4(12) 4 1.287 0.590 1376 0.55 × 0.40 × 0.20 1.69 to 25.00 10436 4588/0.0440 418 1.073 0.0352, 0.0780 0.0381, 0.0816

C26H49BN2O2P2Ru 595.49 113(2) triclinic P1̅ 11.396(2) 15.906(3) 17.642(3) 75.798(5) 71.795(6) 89.331(7) 2937.6(9) 4 1.346 0.667 1256 0.25 × 0.15 × 0.10 1.89 to 25.00 18981 10027/0.0708 731 1.048 0.0636, 0.1386 0.0972, 0.1740

C24H50B2N2P2Ru 551.29 93(2) orthorhombic Pbca 12.164(3) 14.605(4) 32.030(8) 90 90 90 5690(3) 8 1.287 0.678 2336 0.15 × 0.15 × 0.10 2.10 to 25.50 44637 5296/0.0495 517 1.225 0.0467, 0.1109 0.0500, 0.1200

160−224 °C dec; IR (KBr, assignment based on DFT, see the Supporting Information for whole spectrum) ν[H2B-(μ-H)2-Ru] 2428, 2415 cm−1, ν(terminal BH2) 1379, 1362 cm−1. Anal. Calcd for C24H50B2N2P2Ru + H2O: C, 50.63; H, 9.21; N, 4.92. Found: C, 51.01; H, 9.13; N, 4.89. Details of X-ray Crystallographic Study. Details of the crystal data and a summary of the intensity data collection parameters for 2− 5 are given in Table 4. A suitable crystal was mounted with mineral oil on the glass fiber and transferred to the goniometer of a Rigaku Mercury CCD or Rigaku VariMax Saturn CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71070 Å). In all of the following procedures for analysis, Yadokari-XG 200925 was used as a graphical interface. The structure was solved by direct methods with SIR-200426 and refined by full-matrix least-squares techniques against F2 (SHELXL-2014).27 The intensities were corrected for Lorentz and polarization effects. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed using AFIX instructions and refined without any constraint. The resulting CIF files have been deposited as Supporting Information. Computational Details. Geometry optimizations (B3LYP/ LanL2DZ for Ru/cc-pVDZ for others) were performed with the Gaussian 09 (rev. C.01) package.28 The crystal structures were used as an initial structure for the optimization of 3−5. The optimized structures of 3−5 were confirmed to have no imaginary frequency and are deposited in the Supporting Information in xyz file format, readable with Mercury software. General Procedure for the Hydrogenation of 1-Hexanal using 3−5 in Table 1. In a 50 mL stainless autoclave with a magnetic stirring bar were placed a mixture of decane (0.975 mL, 5.00 mmol) and [PBP]Ru complex (3; 5.7 mg, 5.0 μmol (entry 1); 4, 3.0 mg, 5.0 μmol (entry 2); 5, 2.9 mg, 5.0 μmol (entry 3)). The substrate 1hexanal (6.10 mL, 50.0 mmol) was added, and then the autoclave was pressurized with an appropriate pressure of H2. After completion of the reaction under conditions given in the table, the autoclave was cooled to room temperature with a water bath. The gas pressure was released, and then the resulting solution was directly analyzed by GC with authentic samples to estimate the yield. Procedure for the Hydrogenation of 1-Hexanal using 0.0005 mol % of 5 in Table 1 (Entry 4). In a 300 mL stainless

(CH), 141.0 (vt, 2JPC = 7 Hz, 4°); 31P{1H} NMR (C6D6, 202 MHz) δ 90.7 (s); mp 180−227 °C dec; IR (KBr, see the Supporting Information for whole spectrum) νCO 1929 cm−1. Anal. Calcd for C25H49B2N2OP2Ru: C, 51.92; H, 8.54; N, 4.84. Found: C, 52.07; H, 8.17; N, 4.92. Synthesis of 4. A mixture of RuH(OAc)(PPh3)3 (869 mg, 0.917 mmol) and 1 (400 mg, 0.917 mmol) in toluene (10.0 mL) was stirred at 100 °C for 12 h. The resulting suspension was filtered, and the filtrate was evaporated under reduced pressure. The crude product was reprecipitated from benzene/hexane. The resulting suspension was filtered, and recrystallization of the residue from benzene/hexane at room temperature gave single crystals of 4 (285 mg, 0.467 mmol, 51%): 1H NMR (C6D6, 500 MHz) δ−16.39 (br, 2H, sharpened as vt in 1H{11B} NMR spectrum with 2JPH = 15 Hz), 1.24 (vt, 3JPH = 7 Hz, 36H), δ 1.96 (s, 3H), 3.41 (s, 4H), 6.81 (dd, J = 6, 3 Hz, 2H), 7.06 (dd, J = 6, 3 Hz, 2H); 11B{1H} NMR (C6D6, 160 MHz) δ 44.2 (br s); 13 C{1H} NMR (C6D6, 126 MHz) δ 29.8 (vt, 2JPC = 3 Hz, CH3), 36.1 (vt, 1JPC = 5 Hz, 4°), 41.0 (vt, 1JPC = 15 Hz, CH2), 107.3 (CH), 118.0 (CH), 142.5 (vt, 3JPC = 6 Hz, 4°), 181.8 (br, OAc); 31P{1H} NMR (C6D6, 202 MHz) δ 95.8 (s); mp 160−210 °C dec; IR (KBr, assignment based on DFT, see the Supporting Information for whole spectrum) ν[B-(μ-H)2-Ru] 2068 cm−1, ν(OAc) 1541 cm−1. Anal. Calcd for C26H49BN2O2P2Ru + H2O (water molecule may get into the sample during weighing process, not originally included in the sample): C, 50.90; H, 8.38; N, 4.57. Found: C, 50.60; H, 8.10; N, 4.49. Synthesis of 5. A solution of 4 (50.0 mg, 0.0820 mmol) in THF (1.0 mL) was added to a solution of NaBH4 (15.5 mg, 0.420 mmol) in THF (1.0 mL), and the resulting solution was stirred at room temperature for 1 h. The resulting suspension was filtered, and the filtrate was evaporated under reduced pressure. Recrystallization of the residue from benzene/hexane at room temperature gave single crystals of 5 (40.0 mg, 0.0726 mmol, 89%): 1H{11B} NMR (C6D6, 500 MHz) δ −14.57 (br s, 2H, BH4), −5.78 (br s, 2H, N2B-(μ-H)2-Ru), 1.18 (vt, 3 JPH = 7 Hz, 36H), 3.52 (vt, 1JPH = 2 Hz, 4H), 5.50−6.60 (br, 2H, BH4), 6.82 (dd, J = 6, 3 Hz, 2H), 7.07 (dd, J = 6, 3 Hz, 2H); 11B{1H} NMR (C6D6, 160 MHz) δ 11.95 (br s), 40.84 (br s); 13C{1H} NMR (C6D6, 126 MHz) δ 29.8 (vt, 2JPC = 3 Hz, CH3), 35.6 (vt, 1JPC = 5 Hz, 4°), 41.1 (vt, 1JPC = 15 Hz, CH2), 107.8 (CH), 118.4 (CH), 142.4 (vt, 3 JPC = 6 Hz, 4°); 31P{1H} NMR (C6D6, 202 MHz) δ 101.8 (s); mp 6768


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autoclave with magnetic stirring bar was charged a mixture of decane (9.75 mL, 50.0 mmol) and 5 (5.5 mg, 10 μmol). The substrate 1hexanal (61.0 mL, 500 mmol) was added, and then the autoclave was pressurized with 6.0 MPa of H2. After completion of the reaction under conditions given in the table, the autoclave was cooled to room temperature with a water bath. The gas pressure was released, and then the resulting solution was directly analyzed by GC with authentic samples to estimate the yield. General Procedure for the Hydrogenation of Aldehyde using 5 as a Catalyst in Table 2. In a 50 mL stainless autoclave with magnetic stirring bar was charged a mixture of dodecane (1.06 g, 1.00 mmol) and 5 (5.5 mg, 100 μmol) with 2 mL of solvent. The substrate aldehyde (10.0 mmol) was added, and then the autoclave was pressurized with the appropriate pressure of H2. After completion of the reaction under conditions given in the table, the autoclave was cooled to room temperature with a water bath. The gas pressure was released, and the resulting solution was analyzed by GC. Control Experiments for Mechanistic Study. Confirmation of the Stability of 3 and 5 in THF-d8. In a screw-capped NMR tube with 0.500 mL of THF-d8 was placed the [PBP]Ru complex (3, 4.9 mg, 8.4 μmol; 5, 4.6 mg, 8.4 μmol). The reaction was monitored with 1H and 31 P NMR spectroscopy. The resulting NMR spectra for 3 are illustrated as Figure S9 in the Supporting Information. The observed triplet signal at −9.56 ppm and a set of signals of the PBP ligand were tentatively assigned to [PBP]Ru(CO)H complex 11. In the case of leaving 5 in THF-d8, there was no apparent change. Confirmation of the Reaction of 3 and 5 with H2 in THF-d8. In a J. Young NMR tube with 0.500 mL of THF-d8 was placed the [PBP]Ru complex (3, 4.9 mg, 8.4 μmol; 5, 4.6 mg, 8.4 μmol). The NMR tube was soaked in liquid nitrogen very slowly. The argon gas inside the NMR tube was removed in vacuo and replaced three times with dihydrogen gas. The mixture was then warmed to room temperature. The reaction was monitored with 1H and 31P NMR spectroscopy. The resulting NMR spectra for 3 are illustrated as Figure S10 in the Supporting Information. The observable signals except for those of 3 after mixing could be assigned as 11, indicating that 3 did not react with H2. In the case of 5, no change was observed in the 1H NMR spectra even after 16 h, also indicating that 5 did not react with H2. Confirmation of the Reaction of 3 and 5 with NaBD4. In a screwcapped NMR tube with 0.500 mL Tof HF-d8 and NaBD4 (3.2 mg, 84 mmol), was placed the [PBP]Ru complex (3, 4.9 mg, 8.4 μmol; 5, 4.6 mg, 8.4 μmol). The reactions were monitored with 1H NMR spectroscopy. In the case of 3, free BH4 anion immediately appeared after mixing even just after 1 h, as summarized in Figure S11 in the Supporting Information. In the case of 5, free BH4 anion appeared with a slower rate in comparison with the case of 3, as illustrated in Figure S12 in the Supporting Information. Confirmation of the Reaction of 3 and 5 with p-F-C6H4CHO in THF-d8. In a screw-capped NMR tube with 0.500 mL of THF-d8 and [PBP]Ru complex (3, 4.9 mg, 8.4 μmol; 5, 4.6 mg, 8.4 μmol), was placed p-F-C6H4CHO (9.0 μL, 84 μmol). The reaction was monitored with 1H and 31P NMR spectroscopy. In the case of 3, all three broadened signals of the BH4 anion immediately disappeared after mixing even just after 1 h, as summarized in Figure S13 in the Supporting Information. In the case of 5, no apparent change was confirmed in the 1H NMR spectra even after 4 h at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.Y.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas (“Stimulus-responsible Chemical Species for Creation of Functional Molecules” (24109012)) from MEXT and research grants from the TEPCO Memorial Foundation, Toray Science Foundation, the Mitsubishi Foundation, and the Asahi Glass Foundation. The computations were performed using the Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures, a table, and CIF and xyz files giving all IR spectroscopic data, VT NMR experiments, results for vibrational analysis using DFT with assignment of characteristic vibrations, crystallographic data of 2−5, and Cartesian coordinates of all optimized structures 3-opt−5-opt by DFT. This material is available free of charge via the Internet at http://pubs.acs.org. 6769


Organometallics

Article

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