Research Article pubs.acs.org/acscatalysis
Synthesis of Triazolylidene Nickel Complexes and Their Catalytic Application in Selective Aldehyde Hydrosilylation Yingfei Wei,†,‡ Shi-Xia Liu,† Helge Mueller-Bunz,‡ and Martin Albrecht*,†,‡ †
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
‡
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S Supporting Information *
ABSTRACT: A set of nickel(II) Cp complexes with triazolylidenes containing various different wingtip groups (aryl, alkyl, pyridyl, CH2OH) have been synthesized by direct metalation of the corresponding triazolium salt with nickelocene. Distinct effects of the substituents on electronic and steric properties of the formed complexes are demonstrated by NMR spectroscopy and singlecrystal structural analysis, as well as by the catalytic activity of the complexes in the hydrosilylation of aldehydes. While all complexes display appreciable catalytic performance, the cyclometalated pyridyl-functionalized triazolylidene nickel complex 2e is highly active and reaches turnover frequencies of >20 000 h−1 with good catalyst stability (full conversion at 0.05 mol % catalyst, maximum turnovers of ∼6000). A variety of functional groups on the benzaldehyde are tolerated. Ketones are not converted, which identifies complex 2e as a highly selective precatalyst for the hydrosilylation of aldehydes in the presence of ketones. KEYWORDS: nickel, triazolylidene, hydrosilylation, N-heterocyclic carbene, aldehyde conversion
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ketones, alkynes, and alkenes.22 In an effort to exploit Earthabundant base metals,15c,23 a series of NHC complexes of copper(I),13c,24 manganese(I),25 and iron26 was recently developed for catalyzing the hydrosilylation of carbonyl compounds. Simultaneously, Royo and Ritleng reported the first well-defined nickel(II) complexes bearing an imidazolylidene ligand,5a,27 which efficiently catalyze the hydrosilylation of aldehydes and ketones at room temperature with a maximum turnover frequency (TOF) of 2300 h−1 at 0.5 mol % catalyst loading. Subsequent investigations included, in particular, carbene wingtip modifications and afforded catalysts for the hydrosilylation of nitroarenes,28 olefins,29 and imines.30 We have recently reported a series of nickel(II) complexes containing triazole- rather than imidazole-derived NHC ligands and their performance as catalyst precursors in Suzuki−Miyaura cross-coupling reactions.31 A drawback of these complexes was their low stability in the presence of boronic acids. We hypothesized that the milder conditions typically used in hydrosilylation will provide a more compatible environment for these complexes to be applied in catalysis. Moreover, chemical modification of the monodentate carbene triazolylidene ligand, as well as incorporation of different potentially chelating substituents such as pyridyl, CH2OH, and phenyl groups, provides a methodology for optimizing the stability and the electronic and steric properties of the complexes for catalytic
INTRODUCTION Reduction of carbonyl compounds, particularly of aldehydes and ketones, is a fundamental process in organic synthesis.1 This process can be achieved either by hydrogenation,1,2 transfer hydrogenation,1,3 or indirectly by hydrosilylation.1,4 While direct and transfer hydrogenation usually require harsh reaction conditions such as high pressure and temperature, and often a corrosive base as an additive, hydrosilylation is typically performed under mild conditions.5 In catalytic hydrosilylation, both the reduction of carbonyl functionality and protection of the resulting alcohol in the form of silylethers can be accomplished in a single step with high atom economy.1a,6 To date, most reported hydrosilylation reactions have been catalyzed by precious metals, such as iridium,7 ruthenium,5b,8 gold,9 platinum,10 or silver.9b,11As an alternative to these precious metals, Fe,12 Cu,13 and Ni5c,14 have recently been applied in the reduction of the carbonyl functionality via hydrosilylation, mostly in the presence of a base or PPh3.9b,13d,15 The emergence of well-defined N-heterocyclic carbene (NHC)-metal complexes has boosted catalyst performance in many domains, in particular C−C cross coupling,16 transfer hydrogenation,17 oxidation,18 carbon−heteroatom bond formation,19 and polymerization reactions.20 The first example of hydrosilylation catalyzed by a NHC complex was reported in 1996 using a rhodium complex containing a naphthyl-derived carbene ligand.21 Since then, much effort has been devoted to the development of robust and efficient catalysts mainly based on noble-metal−NHC complexes for hydrosilylation of © 2016 American Chemical Society
Received: August 9, 2016 Revised: October 14, 2016 Published: October 20, 2016 8192
DOI: 10.1021/acscatal.6b02269 ACS Catal. 2016, 6, 8192−8200
Research Article
ACS Catalysis
system, as the potentially monodentate triazolylidene precursors 1a−1d yielded predominantly a bis-carbene complex, as well as demethylated triazole, when long reaction times were applied.31 All new complexes are stable in air both as solids and in solution. They were characterized by nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The Cp protons of the neutral monocarbene and cyclometalated complexes 2a−2d appeared as a singlet at ∼4.9 ppm, while this resonance is shifted downfield to 5.6 ppm in the cationic chelating triazolylidene complexes 2e and 2f, presumably because of the stronger electron-donor ability of iodide, compared to the pyridyl ligand. Moreover, the hydroxyl proton (CH2OH) in 2d was observed in the 1H NMR spectrum at 4.29 ppm, indicating that the triazolylidene ligand in complex 2d is bound in a monodentate and not in a C,O-bidentate chelating mode. The proton signals of the pyridyl group in complex 2e were slightly shifted, when compared to those of 2f (e.g., high field doublet at δH 8.02 and 8.45, respectively). These differences may be attributed to the different electronic impact of the triazolium nitrogen and carbon atoms to which the pyridyl substituents are attached. The Ni-bound carbenic carbon, Ctrz−Ni, appears in the 13C NMR spectrum at lower field when the carbene is chelating (δC 155 and 153 for 2e and 2f, respectively) than for the monodentate carbene complexes 2a−2d (δ C 148−149). These shift differences are a consequence of a combination of electronic and stereoelectronic factors, including the more electron-withdrawing properties of pyridyl ligands versus iodide, the different arrangement of the carbene heterocycle with respect to the Ni−Cp vector upon chelation, the altered Ni−Ctrz bond distance (see structural discussion below), and the increased yaw distortion34 upon chelation. Crystal Structure Analysis. Single crystals were obtained for the three complexes 2d, 2e, and 2f via the slow diffusion of pentane into a CH2Cl2 solution of the corresponding complex. In all complexes, the Ni center adopts a distorted trigonal planar coordination geometry with the sum of the bond angles adding to 360° (±0.3°) (see Figure 1 and Table 1). Two of the coordination sites are occupied with the η5-bound Cp ligand and the triazolylidene, respectively, and the trigonal geometry is completed by either an iodide ligand or a pyridyl nitrogen atom, consistent with NMR structural assignments in solution. The C−Ni−X angle is more acute in the chelating systems (∠C1−Ni−N4 = 84.66(6)° in 2e and 84.81(4)° in 2f) than when X = I in the monodentate carbene complexes (∠C1−Ni− I = 94.64(5)° in 2a31 and 98.03(7)° in 2d). Of note, the triazolylidene heterocycle is essentially coplanar with the
application. As a result of these investigations, here, we report on triazolylidene Ni(II) complexes with high catalytic activity in the selective hydrosilylation of aldehydes, achieving a maximum TOF up to 6000 h−1 at a low catalyst loading (0.05 mol %).
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RESULTS AND DISCUSSION Synthesis of the Complexes. Copper-catalyzed “click” reactions of the appropriate azide and terminal alkyne,32 and subsequent methylation using MeOTf or MeI afforded the triazolium salts 1a−1f (Scheme 1). Because of the high Scheme 1. Synthesis of Triazolylidene Nickel Complexes 2a−2f
versatility of the [2 + 3] cycloaddition, the substituents on the triazole heterocycle can be flexibly varied, in terms of their donor properties and their chelating coordination motifs. Here, we have probed aryl vs alkyl substitution (1a−1c), as well as the introduction of potentially chelating donor groups attached to either C4 (R2 = CH2OH, 2-pyridyl in 1d, 1e) or at N1 (R1 = 2-pyridyl, 1f). All these triazolium salts were metalated with nickelocene (Scheme 1) following the procedure pioneered for imidazolium salt metalation,33 and previously applied for the synthesis of 2a and 2b.31 Variation of the reaction parameters revealed that an excess of nickelocene, elevated reaction temperatures, and extended reaction times improved the yield of complexes 2e and 2f by some 20%, when compared to the standard conditions used for complexes 2a−2d. These optimized reaction parameters are unique to the chelating
Figure 1. ORTEP representations of the complexes 2d, 2e, and 2f (50% probability ellipsoids; hydrogen atoms and noncoordinating OTf− anions are omitted for the sake of clarity). 8193
DOI: 10.1021/acscatal.6b02269 ACS Catal. 2016, 6, 8192−8200
Research Article
ACS Catalysis Table 1. Selected Bond Lengths and Angles for Complexes 2a,a 2d, 2e, and 2f 2a (X = I)a
a
Ni−C1 Ni−X Ni−Cpcentroid
1.879(3) 2.5133(4) 1.756(1)
∠C1−Ni−X ∠C1−Ni−Cpcentroid ∠X−Ni−Cpcentroid ∠N1−C1−Ni−Cpcentroid
98.03(7) 127.47(8) 134.34(2) 82.1(3)
2d (X = I) Bond Lengths (Å) 1.8747(15) 2.5089(2) 1.753(1) Bond Angles (°) 94.64(5) 130.68(5) 134.62(1) 78.19(15)
2e (X = N4)
2f (X = N4)
1.8667(15) 1.9287(12) 1.730(1)
1.8648(10) 1.9124(8) 1.726(1)
84.66(6) 139.86(5) 135.17(4) 6.26(3)
84.81(4) 135.73(3) 139.45(3) 2.61(5)
From ref 31.
Table 2. Hydrosilylation of Benzaldehyde Catalyzed by 2ea
trigonal metal coordination plane in the chelate complexes (dihedral angle ∠CpCentroid−Ni−C1−N1 = 6.26° in 2e and 2.61° in 2f), whereas, in the monodentate triazolylidene structure 2d, a pronounced out-of-plane arrangement is noted with a dihedral angle of 78.19°. The distance of the centroid of the Cp ligand to the Ni center is slightly longer for the monodentate carbene complexes 2a and 2d (1.756(1) and 1.753(1) Å, respectively) than in chelates 2e and 2f (1.730(1) Å and 1.726(1) Å, respectively). This difference probably originates from the specific orientation of the triazolylidene heterocycle in 2d, which positions the phenyl substituent ortho to the carbene carbon in the direction of the Cp ligand, causing steric congestion (cf dihedral angels above, Figure 1). The Ni−Ctrz bond length is unaffected by this orientational differences and is essentially identical in all four complexes, despite the metallacyclic structure in two of the four complexes (Ni−C = 1.870(6) Å). All nickel−carbene distances are within the range of reported values for analogous complexes containing imidazol-2-ylidene ligands.33 The Ni−Npyr bond distances are essentially equal in 2e and 2f (1.9287(12) and 1.9124(8) Å, respectively), and both bond lengths compare well with the distances of related nickel complexes with pyridine-containing ligands.35 The C,N-bidentate coordination mode is reported here, for the first time, for triazolylidene nickel complexes, although several examples with IrIII, CoIII, RuII, and OsII metal centers have appeared in the literature previously and the ligand geometries are very similar.36 Catalytic Activity of 2a−2f in Hydrosilylation. Complexes 2a−2f were evaluated as catalyst precursors for the hydrosilylation of carbonyl groups, a reaction that has been predominantly performed with catalysts based on noble metals,5b,9b,37 but recently also with Arduengo-type NHC nickel complexes.5a An initial set of experiments was directed toward evaluating suitable reaction conditions using benzaldehyde as a model substrate in the presence of 2 mol % of complex 2e (Table 2). Hydrosilylation with phenylsilane in 1,2dichloroethane gave moderate to good conversion of benzaldehyde to the corresponding benzyloxysilane when the reaction was performed at room temperature (74% in 22 h, entry 1 in Table 2). For analytical purposes, the hydrosilylation product was subsequently converted into the corresponding benzyl alcohol by treatment with a methanolic NaOH solution (1 M) for 16 h at room temperature, which provided yields of the corresponding alcohol products for each reaction. Catalytic activity was substantially improved when the reaction was performed at slightly elevated temperatures (40 °C), affording the product essentially quantitatively within
