Article pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivities of Molybdenum and Tungsten PONOP Pincer Complexes Ruth Castro-Rodrigo,‡ Sumit Chakraborty,‡ Lloyd Munjanja,‡ William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: A new series of molybdenum and tungsten tricarbonyl pincer complexes, bearing pyridine-based PONOPtype pincer ligands, have been synthesized and fully characterized. Addition of HBF4·Et2O to these tricarbonyl complexes generated seven-coordinate molybdenum and tungsten hydride complexes, and these compounds have been isolated in good yields. These metal hydrides show fluxional behavior in solution. The hydride ligands on these metal complexes are acidic in nature and are readily deprotonated by bases. The molybdenum hydride complex is shown to catalyze isomerization of 1-hexene to internal isomers under mild conditions.
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(RPONOP = 2,6-(R2PO)2(C5H3N); R = iPr, tBu) could be synthesized analogously to Kirchner’s compounds and if the synthesis of the molybdenum and especially the tungsten tricarbonyl complexes could be synthesized directly from the M(CO)6 carbonyls. Herein, we report the synthesis and characterization of molybdenum and tungsten tricarbonyl and hydridotricarbonyl complexes supported by pyridine-based PONOP-type pincer ligands. Additionally, we have applied molybdenum hydridotricarbonyl complexes in the catalytic isomerization of terminal olefins to internal isomers.
INTRODUCTION Transition-metal complexes containing tridentate pincer-type ligands have attracted much attention recently.1 Part of the reason is that pincer ligands, in general, form robust metal complexes due to the chelating effect of the ligand. The steric, electronic, and stereoelectronic properties of the resulting metal complexes can be readily tuned by varying the substituents on the donor atoms or introducing functionalities in the pincer backbone. Among varieties of reported pincer ligands, PNpyPtype ligands that have a central pyridine nitrogen atom and disubstituted phosphorus-based donor sites linked with CH2 groups (i.e., −CH2PR2) have been widely used in organometallic chemistry including catalysis.1 Numerous examples of PNP pincer complexes are reported in the literature.2 Milstein and co-workers have demonstrated that the PNpyP ligand exhibits metal−ligand cooperativity through dearomatization/ aromatization of the pyridine ring.3 This unique behavior of the ligand when linked with a transition-metal center gives rise to interesting stoichiometric and catalytic reactivity. Of particular interest is the PNP pincer ligand-supported molybdenum and tungsten tricarbonyl and hydrido-tricarbonyl derivatives reported by Kirchner and co-workers.4 Although the molybdenum tricarbonyl complexes were synthesized directly from the corresponding PNP ligand and Mo(CO)3(MeCN)3 precursor, analogous tungsten tricarbonyl complexes were prepared in a multistep process by reduction of the bromocarbonyl tungsten species [(PNP)W(CO)3Br]Br with NaHg. The cationic bromocarbonyl compound was synthesized from the dinuclear tungsten precursor [W(CO)4(μ-Br)Br]2. Recently, they have also reported that the products can be obtained directly from the carbonyl precursors in a few hours using a solvothermal method in a sealed high-pressure microwave vial.5 We were curious if the related PONOP ligand © XXXX American Chemical Society
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RESULTS AND DISCUSSION Synthesis of the Molybdenum and Tungsten PONOP Pincer Complexes. Treatment of Mo(CO)3(CH3CN)36 and W(CO)3(CH3CN)37 precursors with the RPONOP ligands8 (1a: R = tBu, 1b: R = iPr) at room temperature resulted in the formation of the corresponding molybdenum and tungsten tricarbonyl complexes 2a−d (Scheme 1). These complexes were isolated as yellow/orange solids (74−86%) and have been completely characterized by a combination of 1H, 31P{1H}, and 13 C{1H} NMR spectroscopy, IR spectroscopy, and elemental analysis. Noticeable characteristic spectroscopic features of 2a− d include, in the 13C{1H} NMR spectrum, one low-field triplet (2JP−C = 6.1−10.1 Hz) and one low-field broad singlet resonance within the ranges δ 210−223 and 217−229 assignable to the carbonyl carbon atoms trans and cis to the pyridine nitrogen atom, respectively. In general, 13C resonances for the carbonyl carbon atoms present in the tungsten complexes appeared slightly upfield in comparison to the analogous molybdenum complexes. In the 31P{1H} NMR Received: June 8, 2016
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DOI: 10.1021/acs.organomet.6b00461 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
complexes, the PNP pincer ligand adopts a meridional geometry. The solid-state structures of 2a−d were determined by single-crystal X-ray crystallography. X-ray structures are shown in Figure 1, with selected bond distances and angles in Table S2. Molybdenum and tungsten centers adopt a distorted octahedral geometry with P1−M−P2 and trans-CCO−M−CCO bond angles deviating significantly from linearity. For example, for the molybdenum complexes 2a and 2b, these angles were found to be 150.023(16)° and 152.93(4)°, respectively, and for the tungsten complexes 2c and 2d, 150.113(14)° and 152.62(2)° were observed. Irrespective of the metal centers, the trans-C−M−C bond angle seems to vary more (∼4°) than the P1−M−P2 bond angle (∼2°) on changing the substituents on the phosphorus atoms. Molybdenum and tungsten metals reside in the same square plane defined by the P1−N−P2− C(CON‑trans) atoms. The M−CON‑cis bond distances were found to be slightly longer than the M−CON‑trans distances possibly due to the greater trans influence of the CO ligand in comparison to the pyridine nitrogen atom. Protonation Reactions of the (PONOP)Mo and -W Tricarbonyl Complexes. As part of our interest in generating the seven-coordinate molybdenum and tungsten hydride complexes, we began to investigate the reactivity of 2a−d with a strong acid, HBF4·Et2O (pKa ≈ −0.4 in H2O9). Addition of 1 equiv of HBF4·Et2O to a CH2Cl2 solution of [(RPONOP)Mo(CO)3] (2a and 2b) or [(RPONOP)W(CO)3] (2c and 2d)
Scheme 1. Synthesis of the (PONOP)Mo and -W tricarbonyl complexes
spectra, singlet resonances were observed between δ 210 and 246 for 2a−d, which is indicative of C2-symmetric structures. Tungsten complexes 2c and 2d each exhibit a single resonance at δ 221−210 with 1JW−P coupling constants (183W: 14% abundance, I = 1/2) of 326 Hz. Solid-state IR spectra of these complexes show three absorption bands in the range 1800− 2000 cm−1, consistent with a C2v symmetry of the molecule and a meridional CO arrangement. These bands are assignable to one strong asymmetric and one weaker plus one stronger symmetric νCO stretching mode. In all of these Mo and W
Figure 1. X-ray crystal structures of 2a, 2b, 2c, and 2d. Ellipsoids are at the 50% probability level. B
DOI: 10.1021/acs.organomet.6b00461 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
the reported molybdenum hydrides are acidic, we carried out stoichiometric reactions with organic and inorganic bases. Treatment of 3b with 1 equiv of triethylamine generated 2b quantitatively and the corresponding triethylammonium salt (Scheme 2). Interestingly, the hydride ligand on 3b is
resulted in an instant color change from bright yellow to pale yellow. This observation is consistent with the protonation of the Mo(0) and W(0) metal centers in 2a−d to generate Mo(II) and W(II) hydride species [(RPONOP)Mo(CO)3H]BF4 (3a: R = tBu; 3b: R = iPr) and [(RPONOP)W(CO)3H]BF4 (3c: R = t Bu; 3d: R = iPr), respectively (eq 1). Similar experimental
Scheme 2. Deprotonation of 3b with Organic and Inorganic Bases
observation was reported for the analogous PNP-based molybdenum and tungsten complexes by Kirchner et al.4 Seven-coordinate metal complexes often show fluxional behavior in solution,10 and such dynamic behavior is also observed for the molybdenum hydride 3b in the variabletemperature 1H NMR spectroscopy (Figure 2). For example, in
sufficiently acidic to be deprotonated by tetrahydrofuran (as solvent) to afford the corresponding oxonium salt in nearly 40% yield. Similarly, a stoichiometric reaction of 3b with a basic transition-metal hydroxide, [Tp′Ni(μ-OH)]2 (Tp′: tris(3,5dimethylpyrazolyl)borate), instantaneously formed 2b and presumably the corresponding paramagnetic nickel-aqua species. These reactions suggest that the hydride moieties present in these molybdenum complexes are indeed acidic in nature. Catalytic Isomerization of 1-Hexene with 3b. Catalytic isomerization of terminal alkenes to internal isomers is an atom-economical organic reaction that has several applications in industry.15 Although this is a thermodynamically favorable process, catalytic reactions often produce a mixture of internal alkenes with E- and Z-isomers.16 Currently, there are a variety of homogeneous catalysts reported that can carry out this process with E-selectivity16c,e,f,17 and a few with Z-selectivity.18 Since the hydride moiety present in 3b is quite acidic, we hypothesized that the molybdenum hydride species might catalyze olefin isomerization reactions. We observed that indeed 3b (10 mol %) catalyzed the isomerization of 1-hexene to 2-hexenes (both E- and Z-isomers) and 3-hexene (only Eisomer) within 9 h at 65 °C in CD2Cl2 (eq 2).19 Isomer Z-3-
Figure 2. Variable-temperature (°C) 1H NMR spectra of 3b.
the 1H NMR spectra of 3b, the hydride ligand appeared as a triplet (A2X spin system, where A = P and X = H) at δ −5.17 at room temperature. Upon cooling the NMR solution to −57 °C, the triplet Mo−H resonance collapsed to a doublet of doublets (AMX spin system) with one small and one large coupling constant of 22 and 60 Hz, respectively.11 Similarly, in the 31 1 P{ H} NMR spectrum of 3b, there are two broad singlets observed at room temperature. However, upon lowering the temperature to −80 °C, a sharp pair of doublet resonances is observed at δ 221.12 and 194.55 with 2JP−P = 84 Hz. From the variable-temperature 1H NMR data, the barrier for the fluxional process can be estimated as ΔG⧧ = 14.0(1.2) kcal mol−1 at 14 °C.12 Complexes 3a−d have been isolated in good yields as pale yellow powders and characterized by 1H, 13C{1H}, 31P{1H} NMR spectroscopy, IR spectroscopy, and elemental analysis (see Supporting Information). Despite several attempts to grow single crystals of complexes 3a−d, we were unable to obtain crystalline material suitable for X-ray crystallographic studies. Deprotonation of the Mo−H in 3b with Organic/ Inorganic Bases. It is well precedented in the literature that group VI metal hydrides are acidic in nature,13 and this often gives rise to unique reactivity.14 In order to determine whether
hexene was not detected in the reaction mixture by 1H NMR spectroscopy. The relative ratio of E-3-hexene and 2-hexenes was found to be approximately 1:11 after 9 h with 98% consumption of 1-hexene. Monitoring the reaction over time (Figure 3) showed comparable formation of E-2-hexene and Z-2-hexene within the C
DOI: 10.1021/acs.organomet.6b00461 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
inantly formed from the isomerization of Z-2-hexene. In addition, the data show that the thermodynamic ratio of Z/E-2hexene of 2:7 (0.28) is easily accessible under the catalytic conditions, thus indicating that steric bulk of the iPrPONOP ligand on 3b does not hinder the E/Z-2-hexene isomerization equilibrium. No isomerization activity was observed when 10 mol % of 3b in CD2Cl2 was reacted with E-3-hexene for 9 h (eq 5). The thermodynamic ratio of E/Z-3-hexene isomers is never reached since catalyst 3b starts degrading beyond 24 h. 1 H NMR spectra recorded at different time intervals during catalysis reveal that 3b is the resting state of the catalyst. Catalysis could also be performed with 0.7 mol % of 3b, affording a 98% conversion of 1-hexene in 9 h. Remarkably, the molybdenum hydride complex remains intact after the reaction and catalysis resumes as soon as a new batch of substrate is introduced, as demonstrated three times with 0.7 mol % of 3b. Next we compared the reactivity of the iPr-substituted molybdenum hydride (3b) with the bulkier tBu-substituted version 3a in the isomerization of 1-hexene to understand the effect of steric crowding on this reaction. Compound 3a catalyzed the isomerization of 1-hexene sluggishly under the same reaction conditions since only 68% of 1-hexene was converted in 9 h to afford Z-2-hexene and E-2-hexene with a relative ratio of 15:53 (0.28) (eq 6). Unlike the isomerization
Figure 3. Monitoring the isomerization of 1-hexene over time. Conditions: [3b] = 32 mM, [1-hexene] = 328 mM, [internal standard, 1,4-dibromobenzene] = 328 mM, 0.5 mL of CD2Cl2. The lines are a simulated fit for equilibrating isomers.
first hour, with E-3-hexene being the minor isomer. After the initial hour the formation of Z-2-hexene remains almost constant, while E-2-hexene and E-3-hexene increase. These results indicate that 3b isomerizes a terminal alkene at a much faster rate than an internal olefin, and therefore internal 2hexene isomers build up in higher concentrations during the initial stage of catalysis. In addition, the data denote that all four isomers are in a reversible equilibrium. Beyond 8 h of reaction, the amounts of Z-2-hexene and E-2-hexene decrease slightly at the same rate, giving the ratio of 2:7 (0.28) at 16 h, consistent with their thermodynamic ratio.20 The E/Z-2-hexene isomerization equilibrium is achieved within 3 h of reaction, whereas E/Z-3-hexene isomerization equilibrium was not achieved under the catalytic conditions. Catalyst 3b is selective for the 2-alkenes over 3-alkenes in the early reaction times. During the course of the reaction the concentration of E-3-hexene increases with no formation of Z-3-hexene, consistent with an inaccessible transition state for Z-3-hexene. The reverse reaction for the formation of 1-hexene is less favorable. To understand further the isomerization process, a catalytic reaction of Z-2-hexene with 3b was performed in a separate experiment. Z-2-Hexene isomerized to yield 1-hexene, E-2hexene, and E-3-hexene in the ratio of 1:59:25 after 9 h with 85% conversion of Z-2-hexene (eq 3). On the other hand, the
with 3b, no formation of 3-hexene isomers is observed in this reaction, suggesting 3a is presumably an inefficient catalyst for the isomerization of internal alkenes in 9 h, although E/Z equilibration occurs without double-bond migration. To test this hypothesis, a catalytic reaction at 65 °C in CD2Cl2 was performed with 10 mol % of 3a in the presence of Z-2-hexene (eq 7). After 9 h the ratio of Z-2-hexene and E-2-hexene is 15:53 (0.28). In addition minimal amounts of 1-hexene (
