Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis(alkyl) compound Sm{C(SiHMe2)3}2THF2 (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This synthesis is similar to that of previously reported Yb{C(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostructural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to 2400 cm−1 assigned to νBH in the IR spectra and highly paramagnetically shifted signals in the B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions. Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is important for accessing the catalytically active species. ■ INTRODUCTION Rare-earth-element−carbon bonds react via insertion, σ-bond metathesis, and protonolytic ligand substitution reactions that are important elementary steps in catalytic processes, including olefin polymerization, hydrosilylation, and dehydrocoupling. Trivalent rare-earth alkyls are typical catalysts for these processes, whereas fewer catalytic processes involve divalent rare-earth alkyl compounds. In the few catalytic chemistries involving divalent lanthanides, the metal center is typically a precatalyst that is activated by a one-electron oxidation: for example, in the initiation of acrylate polymerizations. Moreover, while homoleptic tris(alkyl) lanthanide compounds are useful and versatile starting materials for rare-earth organometallic chemistry, fewer homoleptic bis(alkyl) ytterbium(II), europium(II), and samarium(II) compounds are known. Note that, in this series of aqueous lanthanide ions, samarium(II) is the most reducing (−1.55 V vs NHE) followed by ytterbium(II) (−1.15 V) and europium(II) (−0.35 V). We recently synthesized homoleptic bis(alkyl) ytterbium(II) species Yb{C(SiHMe2)3}2L2 (L2 = THF2, TMEDA), investigated their nonclassical M↼H−Si-containing structures, and studied the reactivity of β-SiH groups with electrophiles. Because only a few divalent bis(alkyl) lanthanide compounds have been employed in catalytic processes, we decided to investigate Ln{C(SiHMe2)3}2THF2 (Ln = Yb, Sm) as catalysts for the dehydrocoupling of organosilanes and amines. In this reaction’s initiation, Ln{C(SiHMe2)3}2L2 could undergo protonolysis to give a rare-earth amide. During the cycle, amide transfer to an organosilane through a σ-bond metathesislike step would provide the silazane product and a rare-earth hydride. Protonolysis of this species would re-form the amide. These steps might be facile with divalent bis(alkyl) rare-earth compounds, or their zwitterionic monoalkyl derivatives, as precatalysts and not require oxidation to initiate the process. This dehydrocoupling pathway, initiated by Sm(II) alkyls, would be in contrast with the aforementioned polymerization initiation as well as the Cp*2SmTHF2-catalyzed hydroamination/cyclization of aminoalkenes, which is proposed to first form a samarium(III) amide. Catalytic cross-dehydrocoupling of silanes with amines is an excellent method for silazane preparation because hydrogen gas is the only byproduct, the degree of amine silylation or silane amination can, in principle, be controlled by the catalyst, and stoichiometric salt byproducts are not formed as is the case in halosilane amination. Silazanes are used extensively as bases, ligands, and silylating agents, including as protecting groups Received: February 18, 2016 Published: May 5, 2016 Article pubs.acs.org/Organometallics © 2016 American Chemical Society 1674 DOI: 10.1021/acs.organomet.6b00138 Organometallics 2016, 35, 1674−1683 in synthesis. F-element, yttrium, and group 2 complexes, along with Lewis acids and Lewis bases, catalyze SiH/NH cross-coupling. In SiH/NH dehydrocoupling reactions catalyzed by bis(disilazido)ytterbium(II) compounds, Cui and coworkers observed that an N-heterocyclic carbene (NHC) ligand is required for efficient conversion. The rate of crossdehydrocoupling of organosilanes and amines catalyzed by bis(disilazido) Yb{N(SiMe3)2}2L (L = 1,3-diisopropyl-4,5dimethylimidazol-2-ylidene (ImiC3H7), 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene (ImMes)) is dramatically increased in comparison to THF-coordinated analogues. A potential advantage of alkyl-based starting materials vs silazides is that the latter may participate in the N−Si bond forming reaction, giving 1−2 equiv of byproduct per catalyst molecule. Despite the reactivity of carbene-coordinated rare-earth complexes and the importance of NHC ligands in transition-metal-catalyzed processes, the catalytic chemistry of carbene-coordinated rare-earth alkyl compounds is underdeveloped. This limitation in the divalent series may be related to the few available starting organolanthanide(II) materials. In terms of homoleptic rare-earth alkyls, the series of trivalent compounds Ln(CH2SiMe3)3ImiC3H7 (Ln = Er, Lu) 26 and Ln(CH2SiMe3)3ImDipp (Ln = Y, Lu; ImDipp = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) have been prepared by substituting THF with NHC. Only a handful of organoytterbium(II) and organosamarium(II) alkyl carbene compounds have been crystallographically characterized. These include the divalent metallocene adduct Cp*2Sm(ImMe4)2 (ImMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene), in which two THF groups from Cp*2SmTHF2 are replaced with ImMe4 (the monocarbene adduct Cp*2Sm(ImMe4) is formed but was not crystallographically characterized). To the best of our knowledge, N-heterocyclic carbene coordinated divalent bis(alkyl)lanthanide complexes were previously unknown. This fact, as well as the increase in catalytic activity affected by an NHC ligand for Yb(II)-catalyzed amine−silane dehydrocoupling, motivated our preparation of “carbon-only coordination” divalent rare-earth bis(alkyl) compounds. Here, we report the synthesis of a divalent samarium bis(alkyl) compound, which is compared to Yb{C(SiHMe2)3}2THF2 in reactions of a Lewis acid and coordination of carbene ligands. The NHC-coordinated compounds are efficient precatalysts for cross-dehydrocoupling of organosilanes with primary and secondary amines, and kinetic studies provide a rationale for the enhanced activity of NHC-coordinated divalent catalysts. ■ RESULTS AND DISCUSSION Synthesis of Sm{C(SiHMe2)3}2THF2. The homoleptic bis(alkyl)samarium compound Sm{C(SiHMe2)3}2THF2 (1b) is synthesized by the reaction of SmI2THF2 and 2 equiv of KC(SiHMe2)3, which proceeds in THF over 12 h at room temperature. The solution changes color from blue-green to dark green upon mixing and then to black after 1 h. The diamagnetic isostructural analogues Yb{C(SiHMe2)3}2THF2 (1a) and Ca{C(SiHMe2)3}2THF2 (1c) were previously synthesized under similar conditions, but the synthesis of those diamagnetic compounds does not show the dramatic color changes observed for samarium. Sm{C(SiHMe2)3}2THF2 is isolated as black blocks by crystallization from pentane at −40 °C. The H NMR spectrum of 1b (all NMR spectra reported were acquired in benzene-d6 at room temperature unless otherwise specified) contained signals assigned on the basis of their relative integrated ratio, chemical shifts, and line widths. The last two properties are influenced by the nuclei’s interaction with the paramagnetic center. The highly upfield, broad resonance at −66.5 ppm (6 H, 342 Hz at half-height) and the sharp singlet at −1.12 ppm (36 H, 32 Hz at half-height) were assigned to the C(SiHMe2)3 ligand. The remaining signals at 11.9 and 2.78 ppm were assigned to THF. The H NMR spectrum of 1b was unchanged after its benzene-d6 solution was heated at 80 °C for 80 h. Si or C spectra acquired either through direct or indirect experiments did not contain signals, likely the result of the paramagnetic Sm(II) center. The infrared spectrum of 1b contained signals at 2107, 2062, and 1867 cm−1 (KBr) attributed to silicon−hydrogen stretching modes. The first was assigned to a two-center−two electron (2c-2e) bonded SiH, while lower energy signals were assigned to groups engaged in three-center−two-electron (3c-2e) interactions with the samarium(II) center. Three comparable stretching modes in ytterbium and calcium analogues (1a, 2101, 2065, and 1890 cm−1; 1c, 2107, 2066, and 1905 cm−1) suggest that the paramagnetic samarium compound contains structural features similar to those of the ytterbium and calcium derivatives. A single-crystal X-ray diffraction study confirms this idea, showing that the Sm center in 1b adopts a distortedpseudotetrahedral geometry based on C1/C1# and O1/O1# coordination (Figure 1). The ∠O1−Sm1−O1# angle of 89.24(7)° is acute, while the ∠C1−Sm1−C1# angle is larger at 133.96(7)°. The Sm1−C1 interatomic distance of 2.733(2) Å is ca. 0.14 Å longer than the corresponding distance in 1a, as expected on the basis of the larger ionic radii of 7-coordinate Sm (1.22 Å) than 7-coordinate Yb (1.08 Å). The Sm1− C1 interatomic distance is significantly longer than those in related Sm(II) alkyl compounds, including Sm{C(SiMe3)3}2 4d (2.58 Å) and (C5Me5)SmCH(SiMe3)2(μ-C5Me5)KTHF2 4c ( 2 . 6 4 ( 1 ) Å ) , b u t s h o r t e r t h a n i n Sm { C (SiMe3)2(SiMe2OMe)}2THF (2.787(5) and 2.845(5) Å). 9 These examples are the only three crystallographically characterized dialkyl samarium(II) compounds found in the Cambridge Structural Database. The second notable feature of 1b is the conformation of C(SiHMe2)3 ligands, which each contains one nonclassical Sm↼H−Si interaction, giving short Sm1−H 1s and Sm1−Si1 interatomic distances of 2.64(2) and 3.303(5) Å, and Sm1−C1−Si1 and Sm1−H 1s-Si1 angles of 90.94(7) and 104(1)°. Note that the two C(SiHMe2)3 ligands are related by a crystallographic C2 axis that bisects the C1− Sm1−C1# angle. Reactions with B(C6F5)3. The samarium compound 1b and B(C6F5)3 react to transfer hydride from the C(SiHMe2)3 ligand to the Lewis acid, giving zwitterionic hydridoborate compounds. These reactions follow the pathway observed for interactions of the ytterbium complex 1a and B(C6F5)3, despite the more reducing nature of Sm vs Yb. Thus, 1b and 1 or 2 equiv of B(C6F5)3 give SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively (Scheme 1). The Organometallics Article DOI: 10.1021/acs.organomet.6b00138 Organometallics 2016, 35, 1674−1683 1675 byproduct of each hydride transfer is 0.5 equiv of the 1,3disilacyclobutane {Me2Si-C(SiHMe2)2}2, which is the head-totail dimer of the silene Me2SiC(SiHMe2)2. X-ray-quality crystals are not yet available for zwitterionic compounds 2b and 3b. The infrared spectra (KBr) of analytically pure 2b, easily isolated after pentane washes, showed bands at 2389, 2306, and 2110 cm−1. In contrast, the IR spectrum of YbC(SiHMe2)3HB(C6F5)3THF2 (2a, KBr) contained bands at 2310 (νBH), 2074 (νSiH), and 1921 cm −1 (νSiH). The presence of only one SiH band at high energy in 2b suggests that the C(SiHMe2)3 group lacks secondary interactions present in 1b and 2a. While the change in IR spectrum is surprising, elemental analysis shows that one C(SiHMe2)3 is present in 2b, and the IR band at 2110 cm −1 is not from the hydrocarbon-soluble {Me2Si-C(SiHMe2)2}2 that is removed during workup. In addition, the IR spectrum of 3b contained bands at 2388 and 2318 cm−1 assigned to B−H stretching modes, while no bands were detected in the region associated with Si−H stretching modes (2100−1800 cm−1). Direct comparisons of samarium and ytterbium structures by NMR are complicated by paramagnetic effects. Nonetheless, signals in the B and F NMR spectra of 2b and 3b indicate that tris(perfluorophenyl)hydridoborate groups are closely associated with the samarium center. The B NMR spectra acquired in bromobenzene-d5 for 2b and 3b contained paramagnetically shifted broad signals at −88 and −100 ppm, respectively, whereas B NMR signals for diamagnetic ytterbium analogues appeared at −20.8 (2a) and −26.4 ppm (3a). Only two signals were observed in the F NMR spectrum in the ratio of 1:2, assigned to the para and meta fluorines on C6F5 groups for compounds 2b and 3b, in comparison to the spectra for the Yb analogues that revealed three fluorine signals. Reaction with 1,3-Di-tert-butylimidazol-2-ylidene (ImtBu). Compounds 1a,b and 1 equiv of 1,3-di-tertbutylimidazol-2-ylidene (ImtBu) react almost instantaneously at room temperature in benzene or pentane to displace the THF ligands, affording Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)) quantitatively during in situ reactions (Scheme 2). Recrystallization from pentane at −40 °C affords red crystals of Yb{C(SiHMe2)3}2ImtBu (4a) or dark red crystals of Sm{C(SiHMe2)3}2ImtBu (4b) in moderate isolated yield. The H NMR spectrum of diamagnetic 4a contained a doublet at 0.45 ppm (JHH = 3.6 Hz, 36 H) and a septet at 4.86 ppm with silicon satellites (JSiH = 144 Hz, 6 H) assigned to the SiHMe2 moiety, as well as singlets at 1.43 and 6.33 ppm assigned to the coordinated ImtBu ligand. These signals remained sharp even in spectra acquired at low temperature (∼200 K) in toluene-d8. In the C{H} NMR spectrum, a signal at 196.9 ppm for the carbene carbon appeared with a chemical shift similar to that for other lanthanide tris(alkyl) carbene adducts. Neither N NMR signals of ImtBu nor the Si NMR of the SiHMe2 moieties are changed significantly in 4a in comparison with starting materials. The SiH regions of the infrared spectra for 4a and 4b were distinct from each other as well as from those of 1a,b. In 4a, a broad signal at 2058 cm−1 was poorly resolved from bands at 2083 and 2114 cm−1, with one low-energy band at 1871 cm−1. In contrast, the spectrum of the samarium analogue contained sharper signals at 2108, 2076, 2064, and 2044 cm−1 as well as two lower energy bands at 1910 and 1796 cm−1. Note that the IR spectra of 1a,b (described above) only contained three νSiH bands, suggesting that compounds 1 and 4 have inequivalent conformations. This idea is supported by single-crystal X-ray diffraction analysis. Figure 1. Rendered thermal ellipsoid plot of Sm{C(SiHMe2)3}2THF2 (1b). Ellipsoids are plotted at 50% probability. Hydrogen atoms bonded to silicon were located objectively in the Fourier difference map and were refined isotropically. All other H atoms were placed in idealized positions, were refined with relative isotropic displacement coefficients of neighboring atoms, and were not plotted for clarity. Significant interatomic distances (Å): Sm1−C1, 2.733(2); Sm1−Si1, 3.303(5); Sm1−Si2, 3.6443(6); Sm1−Si3, 3.5990(7); Sm1−H1s, 2.64(2); Sm1−H2s, 3.57(3); Sm1−H3s, 3.69(2). Significant interatomic angles (deg): C1−Sm1−C1#, 133.96(7); O1−Sm1−C1, 108.57(5); O1−Sm1−O1, 89.24(7); Sm1−C1−Si1, 90.94(7); Sm1− C1−Si2, 104.48(7); Sm1−C1−Si3, 102.49(7); Sm1−H1s−Si1,