Hydrogenation of Alkenes Catalyzed by Rare-Earth Metal Phosphinophosphinidene Complexes: 1,2-Addition/Elimination Versus σ-Bond Metathesis Mechanism

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Hydrogenation of Alkenes Catalyzed by Rare-Earth Metal Phosphinophosphinidene Complexes: 1,2-Addition/Elimination Versus σ-Bond Metathesis Mechanism Bin Feng, Hao-Yu Zhang, Hongling Qin, Qian Peng, Xuebing Leng and Yaofeng Chen Bin Feng State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032 , Hao-Yu Zhang State Key Laboratory of Elemento-Organic Chemistry and Frontiers Science Center for New Organic Matter, College of Chemistry, Nankai University, Tianjin 300071 , Hongling Qin State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032 , Qian Peng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Elemento-Organic Chemistry and Frontiers Science Center for New Organic Matter, College of Chemistry, Nankai University, Tianjin 300071 , Xuebing Leng State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032 and Yaofeng Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200032 https://doi.org/10.31635/ccschem.021.202101468 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Rare-earth metal complexes have been used as catalysts for many types of reactions. However, the mechanistic studies showed that basically, these various reactions proceeded with σ-bond metathesis of RE–E bond (RE: rare-earth metals; E: main group elements) or RE ions acting as Lewis acid centers to activate the polar functional groups of the substrates. In this article, a lutetium phosphinophosphinidene complex, with Lu=P double-bond character, was synthesized and structurally characterized. This lutetium complex and our previously reported scandium phosphinophosphinidene complexes were able to catalyze the hydrogenation of terminal alkenes under mild conditions, and the lutetium complex showed higher catalytic activity than the scandium ones. More interestingly, isotopic labeling experiments indicated that the catalytic reaction proceeded through an addition/elimination mechanism rather than the traditional σ-bond metathesis mechanism. Density functional theory calculations provided insights into the 1,2-addition/elimination mechanism and regioselectivities of the H2 addition to the Lu=P double bond of the lutetium phosphinophosphinidene complex and the anti-Markovnikov alkene insertion into the Lu–H bond of the lutetium phosphinophosphido hydride intermediate. Download figure Download PowerPoint Introduction The past four decades have witnessed the development of rare-earth metal complexes as a new type of catalysts for various catalytic reactions, including alkene hydrofunctionalization (hydrogenation, hydroboration, hydroamination, hydrosilylation, hydrophosphinylation, etc.),1–4 alkene and polar monomer polymerization,5–8 C–H bond functionalization,7,9–15 as well as Aldol reaction, Ene reaction, Michael reaction, Hetero–Diels–Alder reaction, Friedel–Crafts acylation reaction, Biginelli reaction, Roskamp–Feng reaction, and so on.16–22 However, studies on the mechanisms revealed that basically, these catalytic reactions proceeded with σ-bond metathesis of RE–E bond (RE: rare-earth metals; E: main group elements)1–15 or RE ions acting as Lewis acid centers to activate the polar functional groups of the substrates.16–22 Due to energy mismatch between the frontier orbitals of the rare-earth metals and the ligand atoms, the complexes containing RE=E double bonds are extremely unstable and readily labile to aggregation and/or reaction with the ligand/environment, quenching the double-bond character.23–25 Only a few structurally characterized pincer-type mononuclear rare-earth metal alkylidene complexes possessing very low multiple bonding degree were reported before 2010.26,27 Since 2010, the landscape in the area changed dramatically, as a few rare-earth terminal imido,28–33 alkylidene,34,35 and oxo complexes were isolated and structurally characterized.36,37 Furthermore, we recently reported two scandium monometallic phosphinophosphine complexes and one scandium terminal boronylphosphinidene complex.38,39 Compared with the RE–E single bonds, the RE=E double bonds have one more π interaction between the rare-earth metals and the ligand atoms. This additional π bonding is highly active, thus, providing a new substrate activation function. The regeneration of the RE=E bonds achieved under specific conditions leads to a new mechanism of rare-earth metal-catalyzed reactions: The 1,2-addition/elimination mechanism (Scheme 1a) is distinct from the traditional σ-bond metathesis mechanism (Scheme 1b). Alkene hydrogenation is one of the most important catalytic reactions. We found that rare-earth metal phosphinophosphine complexes were able to catalyze the hydrogenation of terminal alkenes under mild conditions. Furthermore, our isotopic labeling experiments and density functional theory (DFT) calculations revealed that the catalytic reaction proceeded in a 1,2-addition/elimination mechanism. Herein, we report these results. Scheme 1 | Rare-earth metal-catalyzed addition reactions of A–B with an unsaturated substrate (C=D) via 1,2-addition/elimination mechanism (a) or σ-bond metathesis mechanism (b). Download figure Download PowerPoint Experimental and Computational Methods General considerations Experiments were carried out under an argon atmosphere using Schlenk techniques or in a nitrogen-filled glovebox. All solvents and reagents were rigorously dried and deoxygenated before use. H2 (>99.9%) and D2 (>99%) were dried by passing through an activated 4 Å molecular sieves before use. Hydrogen deuteride (HD; 96%) was used as received. Synthesis of [L′Lu{η2-PP[N(DIPP)CH2CH2N(DIPP)]}(THF)] (3) KCH2(C6H5) (13 mg, 0.10 mmol) was added to a tetrahydrofuran (THF) solution (2 mL) of H2PP{N(DIPP)CH2CH2N(DIPP)} (DIPP = 2,6-(i-Pr)2C6H3)) (44 mg, 0.10 mmol) at −35 °C. After standing at −35 °C overnight, a THF solution (2 mL) of [L′Lu(Me)Cl] (L′ = [MeC(NDIPP)CHC(Me)(NCH2CH2N(Me)2)]−) (55 mg, 0.10 mmol) was added. The reaction yielded an orange solution with precipitates at room temperature in 12 h, which was filtered. The filtrate was concentrated to about 1 mL, then 5 mL of toluene was added. The resultant solution was concentrated to about 1 mL and stored at −35 °C overnight. The orange solid formed was isolated, washed with hexane (0.5 mL), and dried under vacuum to give complex 3 (61 mg, 60% yield). General procedure for hydrogenation of 1-hexene with an initial H2 pressure of one atmosphere In a glovebox, a C6D6 solution (0.4 mL) containing rare-earth metal phosphinophosphinidene complex (0.010 mmol), 1-hexene (0.10 mmol), and internal standard mesitylene (0.033 mmol) were loaded into a special 6 mL NMR tube sealed with a rubber septum, then 6 mL H2 was injected into the NMR tube at room temperature. The reaction was monitored by 1H NMR spectroscopy. DFT calculations All DFT calculations were carried out without any symmetry constraints using the Gaussian 16 program at the B3LYP-D3(BJ) level of theory. For the basis set, the Stuttgart 4f-in-core effective core potential (ECP) MWB60 (including 60 electrons in core with quasi-relativistic) and its most recent basis set (including f and g polarization) was used for Lu, while for other atoms (H, C, N, O, P), 6-31G(d) was used. The single point energies were further estimated using PBE0-D3(BJ) under a larger basis set ma-def2-TZVP for all atoms, including Stuttgart 4f-in-valence ECP MWB28 for Lu under solvation model based on density (SMD). The natural population analysis (NPA) was carried out based on an optimized structure using Gaussian 16 program. The optimized structures were displayed using CYLview visualization software ( http://www.cylview.org/). Further experimental and computational details, as well as characterization data, are provided in the Supporting Information. Results and Discussion Scandium phosphinophosphinidene complexes [LSc{η2-PP[N(DIPP)CH2CH2N(DIPP)]}(THF)] (L = [MeC(NDIPP)CHC(NDIPP)Me]− ( 1) and [L′Sc{η2-PP[N(DIPP)CH2CH2N(DIPP)]}(THF)] ( 2) (Figure 1) were synthesized as we previously reported.38 Attempts to synthesize lutetium phosphinophosphinidene complex [LLu{η2-PP[N(DIPP)CH2CH2N(DIPP)]}(THF)] from the reaction of [LLu(Me)Cl] with an in situ-generated phosphinophosphido potassium salt K[HPP{N(DIPP)CH2CH2N(DIPP)}] failed. On the other hand, when [L′Lu(Me)Cl] was used, a lutetium phosphinophosphinidene complex [L′Lu{η2-PP[N(DIPP)CH2CH2N(DIPP)]}(THF)] ( 3) was obtained in 60% isolated yield (Scheme 2). The formation of the phosphinophosphinidene complex at room temperature indicated that the 1,2-methane elimination of the reaction intermediate, lutetium phosphinophosphido methyl complex, was facile. Complex 3 was characterized by NMR spectroscopy (1H, 13C{1H}, and 31P{1H}). In the 31P{1H} NMR spectrum of 3 in THF-d8, the Pα(phosphinidenene) signal appeared at δ = 176.7 ppm, much downshifted in comparison with the Pα(phosphido) signal of the potassium salt K[HP{PN(DIPP)CH2CH2N(DIPP)}] (−119.0 ppm), but significantly upshifted in comparison with the Pα(phosphinidenene) signals of the scandium complexes 1 and 2 (402.3 and 312.2 ppm). Figure 1 | Scandium phosphinophosphinidene complexes 1 and 2. Download figure Download PowerPoint Complex 3 was further characterized by single-crystal X-ray diffraction (Figure 2). In 3, the phosphinophosphinidene dianion [PP{N(DIPP)CH2CH2N(DIPP)}]2− coordinated to the lutetium center through Pα and Pβ atoms in an η2-fashion, similar to that observed in 1 and 2. The Lu–Pα bond length (2.608(1) Å) was almost the same as the Sc–Pα bond length in 2 (2.484(2) Å) when the difference in metal ion radii was counted (Lu3+ (0.861 Å) and Sc3+ (0.745 Å) for coordination number of 6).40 The Lu–Pβ bond length (2.838(1) Å) was much longer than the Lu–Pα bond length (2.608(1) Å) and longer than the Lu–P single-bond length in a previously reported lutetium phosphido complex [{2-(i-Pr2P)-4-Me-C6H3}(Mes)N]2Lu(PHMes) (2.735(1) Å).41 The Pα–Pβ bond length (2.091(2)) was close to those in 1 and 2 (2.105(1) and 2.095(1) Å), consistent with the similar 1JP–P values observed in the 31P{1H} NMR spectra of 1, 2, and 3 (501, 519, and 501 Hz, respectively). Figure 2 | Molecular structure of 3 with ellipsoids at 30% probability level. DIPP isopropyl groups and hydrogen atoms are omitted for clarity. Download figure Download PowerPoint Complexes 1– 3 were tested for H2 activation by dissolving in C6D6, followed by exposure to H2 (∼1.0 atm) at room temperature. We observed no new compounds in the 1H and 31P NMR spectra of the reaction solutions. But we noted a situation that needs to be considered: the H2 activation of 1– 3 was reversible; the formed rare-earth metal phosphido hydrides quickly eliminated H2 to regenerate the phosphinophosphinidene complexes; however, the concentrations of the phosphido hydrides were too low to be detectable in the NMR spectra. Therefore, the reactions of complexes 1– 3 with an H2/D2 mixture (1∶1) were studied, and the formation of HD was clearly observed (Scheme 3, Figures 3a and 3b, and Supporting Information Figures S2–S5). The isotope exchange of H2 and D2 revealed H2 activation by the phosphinophosphinidene complexes and σ-bond metathesis of the phosphido hydrides ( A) or phosphido deuterides ( A-d2) with D2 or H2, as shown in Scheme 4. (DFT calculations showed 1,2-H2 addition to RE=Pα bond was more favorable than 1,3-H2 addition to RE–Pβ bond, see DFT calculation paragraph below and Supporting Information Figures S25 and S26 for details.) Scheme 2 | Synthesis of lutetium phosphinophosphinidene 3. Download figure Download PowerPoint Scheme 3 | Isotope exchange of H2 and D2 catalyzed by phosphinophosphinidene complexes 1–3. Download figure Download PowerPoint As complexes 1– 3 were able to activate H2, their application for alkene hydrogenation was investigated. The hydrogenation of 1-hexene was tested initially, and the complexes 1– 3 catalyzed the 1-hexene hydrogenation at room temperature with an initial H2 pressure of 1 atm (Table 1, entries 1–3). With 10 mol % of lutetium complex 3, the conversion of 1-hexene was >95% in 48 h, which was higher than those achieved by scandium complexes 1 and 2 (55% and 40%, respectively). Also, we observed that the catalytic efficiency of the complexes could be improved by increasing the H2 pressure (Table 1, entries 4–6). Under 6 atm of H2, using 1 or 2 as the catalyst, the conversion of 1-hexene was >95% in 16 h; with 3 as the catalyst, the conversion of 1-hexene reached >95% in 4 h. Table 1 | Hydrogenation of 1-Hexene Catalyzed by 1–3a Entry P (atm) Catalyst Time (h) Conv. (%)b 1 1 1 48 55 2 1 2 48 40 3 1 3 48 >95 4 6 1 16 >95 5 6 2 16 >95 6 6 3 4 >95 a0.010 mmol of rare-earth metal complex, 0.10 mmol of 1-hexene, 0.4 mL of C6D6. bDetermined by quantitative 1H NMR using mesitylene as an internal standard. Scheme 4 | Reaction pathways for isotope exchange of H2 and D2 catalyzed by 1–3. Download figure Download PowerPoint Subsequently, complex 3 was applied for the catalytic hydrogenation of a variety of alkenes. For 4-methylhex-1-ene and 5-butoxy-pent-1-ene, using 10 mol % of 3, high conversions of the alkenes (>95%) were achieved in 4 h at room temperature under 6 atm of H2 (Table 2, entries 1 and 2). For 3-methylhex-1-ene, the conversion was slightly lower (85%), which could be ascribed to the steric effect of the substrate (Table 2, entry 3). When the reaction time was extended to 5 h, the conversion of 3-methylhex-1-ene reached >95%. For 6-chlorohex-1-ene, no hydrogenation product was observed (Table 2, entry 4). This might be due to the halogen abstraction by the rare-earth metal center, as such observation was made previously in the reaction of 2,6-difluoropyridine with a scandium phosphinoalkylidene complex42 since the formation of lutetium chloride deactivated the catalyst. With complex 3 as the catalyst, styrene and its derivatives bearing –Me, –NMe2, –SMe, or –OMe substituent also underwent the hydrogenation reactions, but the reactivities were lower than those of the aliphatic alkenes (Table 2, entries 5–11). The conversions of styrene, 1-ethenyl-3-methyl-benzene, and 1-ethenyl-4-methyl-benzene in 8 h were 78%, 75%, and 72%, respectively; the conversion of 1-ethenyl-2-methyl-benzene was slightly lower (60%) due to the steric effect from the methyl group at the ortho position of the phenyl ring. The conversions of N,N-dimethy-4-vinylaniline, 4-methythiostyrene, and 4-methoxystyrene in 8 h were 75%, 65%, and 48%, respectively; the lower conversion of 4-methoxystyrene could be ascribed to the interaction of the OMe group with the oxophilic rare-earth metal ion. When the reaction time was extended to 16 h, the high conversions of these substrates (92 ∼ >95%) were observed (Table 2, entries 5–11). The hydrogenation of Cl- or NO2-substituted styrene was unsuccessful (Table 2, entries 12 and 13), possibly due to the complex 3 undergoing halogen abstraction with the Cl-substituted styrene, thereby quenching the hydrogenation reaction. For the NO2-substituted styrene, the DFT studies indicated the competitive coordination of the NO2-group showed significantly strong binding affinity to the Lu3+ ion, which formed a dynamic well inhibiting the catalytic reactivity by occupying the coordination site (see Supporting Information Figure S32, for details). We also found that hydrogenation did not occur when the internal alkenes (2-hexene and 1-propenylbenezene) were used as the substrates (Table 2, entries 14 and 15), attributable to the weak coordination of internal alkenes to the hard Lewis acidic rare-earth metal ion. Table 2 | Hydrogenation of Alkenes Catalyzed by 3a Entry Substrate Product Conv. (%)b 1 >95 (4 h) 2 >95 (4 h) 3 85 (4 h) >95 (5 h) 4 0 (4 h) 5 78 (8 h) >95 (16 h) 6 75 (8 h) >95 (16 h) 7 72 (8 h) >95 (16 h) 8 60 (8 h) >95 (16 h) 9 75 (8 h) >95 (16 h) 10 65 (8 h) >95 (16 h) 11 48 (8 h) 92 (16 h) 12 0 (16 h) 13 0 (16 h) 14 0 (16 h) 15 0 (16 h) a0.040 mmol of complex 3, 0.40 mmol of alkene, 1.6 mL of C6D6, 6.0 atm of H2. bDetermined by quantitative 1H NMR using mesitylene as an internal standard. Figure 3 | 1H NMR spectroscopy, showing the isotope exchange of H2 and D2 catalyzed by 3 (400 MHz, C6D6, 25 °C). (a) 30 min, (b) 24 h. H2 decreases, and HD (δ 4.43 ppm, 1∶1∶1 t, 1JHD = 42.8 Hz) increases with time. Download figure Download PowerPoint Plausible pathways for alkene hydrogenation catalyzed by complexes 1– 3 are shown in Scheme 5. The 1,2- or 1,3-addition of H–H to the RE=Pα or RE–Pβ bond of the phosphinidene complex gave phosphido hydrides ( A or A′), and alkene insertions into the RE−H bond of A to generate the phosphido alkyl intermediates under linear or branched-chains' selectivities. (The 1,2-addition product A was analyzed as an example.) Regeneration of A could be achieved by two possible pathways: (a) a direct σ-bond metathesis of the phosphido alkyl intermediates with H2 gave a hydrogenation product and regenerated A; (b) an intramolecular 1,2-alkane elimination of the phosphido alkyl internediates released the hydrogenation product and yielded a rare-earth metal phosphinidene complex, which subsequently reacted with H2 to regenerate A. Scheme 5 | Plausible pathways for alkene hydrogenation catalyzed by 1–3. Download figure Download PowerPoint To establish whether the catalytic reaction proceeded via a σ-bond metathesis (path a) or 1,2-elimination mechanism (path b), the reactions were conducted with an initial HD of 1 atm pressure. To avoid the error of measuring the amount of hexane (we use hexane very often in the glove box, and the atmosphere in the glove box contains hexane), 1-pentene was used as the substrate. For the reaction with complex 1 as the catalyst, the ratio of d1-products:(d0-product + d2-product) was 89:11 (Table 3, entry 1); this indicated that the catalytic reaction most likely proceeded via the path b. The proportion of d1-products accounted for the majority but failed to approach 100% might for the following reasons: (1) the purity of HD used was 96%, and there were small amounts of H2 and D2; (2) the intermediate ( A-d1) underwent a hydrogen isotope exchange with HD to form A-d0 or A-d2, then the d0- or d2-product could be formed following the reaction performed in the 1,2-elimination mechanism; (3) small amounts of d0- and d2-products were formed through the σ-bond metathesis mechanism. The reaction catalyzed by complex 3 was also performed; similarly, the d1-products dominated the d0- and d2-products (82:18). The higher proportion of d0- and d2-products with complex 3 as the catalyst might be due to the faster σ-bond metathesis between the lutetium phosphido hydride or phosphido deuteride ( A-d1) with HD, which was observed in the complex 3-catalyzed hydrogen isotope exchange of H2 with D2. The reactions under an H2/D2 mixture (1:1) were also studied (Table 4). As expected, the reactions yielded the major d0- and d2-products, which further indicated the catalytic reaction likely proceeded in path b. Table 3 | Hydrogenation of 1-Pentene with HD Catalyzed by 1 or 3a Entry Catalyst Time (h) Conv. (%)b d1-products:d0 and d2-productsc 1 1 5 30 89∶11 2 3 4 30 82∶18 a0.010 mmol of rare-earth metal complex, 0.10 mmol of 1-pentene, 0.4 mL of C6D6, initial 1.0 atm of HD (96%). bDetermined by quantitative 1H NMR using mesitylene as an internal standard. cDetermined by gas chromatography–mass spectrometry. Table 4 | Hydrogenation of 1-Pentene with an H2/D2 Mixture (1:1) Catalyzed by 1 or 3a Entry Catalyst Time (h) Conv. (%)b d1-products:d0 and d2-productsc 1 1 5.5 30 11∶89 2 3 4.5 30 22∶78 a0.010 mmol of rare-earth metal complex, 0.10 mmol of 1-pentene, 0.4 mL of C6D6, initial 1.0 atm of H2/D2 (1:1). bDetermined by quantitative 1H NMR using mesitylene as an internal standard. cDetermined by gas chromatography–mass spectrometry (GC-MS). DFT studies by the Gaussian 16 program at the SMD-PBE0-D3(BJ)/ma-def2-TZVP//B3LYP/MWB60(Lu)/6-31G(d) level of theory43–48 were carried out to understand the isotopic labeling results and probe the mechanistic details, especially for the different regioselectivities shown in Scheme 5. These possible pathways would lead to indistinguishable products, and thus, were difficult to determine by the experimental studies. The calculated free energy profiles based on lutetium complex 3 are shown in Figure 4. The reaction initiated by the THF dissociation of 3 to vacate a coordination site on lutetium formed the catalytic species 4. This step required an estimated 11.8 kcal/mol kinetic barrier and is a 5.8 kcal/mol thermodynamical endothermic process (see Supporting Information Figures S29 and S30, for details).a The calculated structure of 4 revealed comparable bond lengths with the X-ray structure of complex 3, although the calculated Lu=Pα (2.57 Å) and Lu–Pβ (2.74 Å) bonds were slightly shorter than those observed in the crystal structure Å) due to the of THF The following mechanism would of H2 alkene and intramolecular σ-bond The 1,2-H2 addition via to form phosphido hydride intermediate A was kcal/mol and with a low energy barrier 10 consistent with the experimental observation that the phosphido hydride readily the 1,2-H2 elimination to form the phosphinidene The of 4 via displayed a favorable kcal/mol energy with the 1,3-H2 addition via that the reaction likely on the Lu=Pα bond rather than the Lu–Pβ bond (see the energy of 1,3-H2 for details, Supporting Information Figure of 4 indicated a Pα of to activate H2, acting as the which the hydrogen to form the lutetium hydride intermediate A. In the Pβ in 4 with lutetium for the H2 activation (see Supporting Information Table for comparable results in and The Lu–H intermediate A would the formation of a linear alkyl phosphido intermediate under kcal/mol energy barrier via a This anti-Markovnikov alkene insertion is an while the other for the intermediate is not as more than kcal/mol activation energy (see Supporting Information Figure could be by the steric between the formed alkyl group and the ligand and of the lutetium complex, in a longer in the than the linear This of steric might the reactivity of internal alkene, which is also by the experimental results that hydrogenation of internal alkene did not occur (Table 2, entries 14 and To the catalytic intramolecular 1,2-alkane elimination of intermediate to regenerate 4 is kcal/mol more favorable than the σ-bond metathesis path via This could be to the steric of the ligand and the coordination of The ligand in intermediate would the and the bond in Å for and respectively). the weak interaction between Lu and Pβ to in with Å with Å in these calculations indicated that the 1,2-elimination of the phosphido alkyl intermediate is more likely to be the terminal step of this reaction, rather than the σ-bond which with the isotopic labeling results in Table 3 with elimination our calculations further small amounts of d0- and d2-products, likely formed through the σ-bond metathesis of the intermediate ( A-d1) with HD, followed by the 1,2-alkane elimination of the intermediate or is the steric phosphido hydride intermediate A would be more to the σ-bond metathesis with HD, with the phosphido alkyl intermediate might also be extended to different metal complexes for the slightly higher proportion of d0- and d2-products with complex 3, that the steric effect in Lu complex would interaction and the σ-bond metathesis of intermediate A rather than the one in Table 3. Figure 4 | energy profiles of hydrogenation of alkene catalyzed by lutetium phosphinophosphinidene complex, the bond lengths are shown in In the all C–H and are omitted for clarity. and Download figure Download PowerPoint The of rare-earth monometallic complexes was by the addition of lutetium complex 3. This complex was achieved using a ligand as the In the complex, the phosphinophosphinidene ligand coordinated to the lutetium ion in a η2-fashion, and the Lu–Pα bond is very (2.608(1) The hydrogen isotope exchange of H2 and D2 indicated that this lutetium complex and the previously reported scandium phosphinophosphinidene complexes ( 1 and 2) were able to activate H2, and the reactions were studies showed that complexes 1– 3 catalyzed the hydrogenation of terminal alkenes under mild conditions. The studies indicated that the hydrogenation reaction proceeded in a 1,2-addition/elimination mechanism rather than the traditional σ-bond metathesis mechanism. The DFT calculations also revealed that the 1,2-H2 addition to the Lu=Pα bond and anti-Markovnikov alkene insertion into the Lu–H bond were favorable in the regioselectivities of this catalytic a The mechanism of the to of phosphinophosphinidene ligand of the THF dissociation is not see Supporting Information Figure for Supporting Information Supporting Information is and the experimental and computational details, X-ray data, and NMR of is no of to Information This was by the Science of and the Science of Tianjin and the Tianjin for the Frontiers Science Center for New Organic University the and the Shanghai of Science and This is to the of Nankai A. in Organic 2. 3. A. A. of in the

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