Hydrogenolysis of Tantalum Hydrocarbyl Complexes: Intermediates

Jul 7, 2015 - The formation of 3 via this hydride route is particularly attractive, as it ... In an effort to further develop the hydride route shown ...
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Hydrogenolysis of Tantalum Hydrocarbyl Complexes: Intermediates on the Road to a Dinuclear Tantalum Tetrahydride Derivative Kyle D. J. Parker, Dominik Nied, and Michael D. Fryzuk* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *

ABSTRACT: The synthesis, characterization, and reactivity with H2 of a series of tantalum hydrocarbyl complexes are reported. The reaction of [NPN*]TaMe3 (4, where NPN* = PhP(2-(Nmesityl)-5-Me-C6H3)2) with dihydrogen (H2, 4 atm) results in the formation of the dinuclear tetrahydride ([NPN*]Ta)2(μ-H)4 (5), without the observation of intermediates. The preparations of two alkyne benzyl complexes of the formula [NPN*]Ta(alkyne)(CH2Ph) (where alkyne = BTA = bis(trimethylsilyl)acetylene (6), 3-hexyne (7)) are reported starting from the respective chloroalkyne complexes [NPN*]Ta(alkyne)Cl, by addition of benzylpotassium. Hydrogenation of these two alkyne benzyl complexes ultimately results in the formation of the same dinuclear tetrahydride 5; however, using lower pressures of H2 and shorter reaction times results in the isolation of an intermediate in each case. Hydrogenation of 6 generates the alkene hydride complex [NPN*]Ta(trans-1,2-C2H2(SiMe3)2)H (8); addition of H2 to 7 gives [NPN*]Ta(1-hexene)H (9), in which the 3-hexyne moiety has been partially hydrogenated and isomerized to the 1-hexene regioisomer. Both of these alkene hydride complexes can be converted to the dinuclear tetrahydride complex 5 by addition of H2. A mechanism is proposed for the formation of the intermediates that involves hydrogenolysis of the alkyne moiety prior to the benzyl ligand; the formation of the trans-alkene units is suggested to be a result of a zwitterionic alkylidene intermediate that allows free rotation of a C−C single bond.

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INTRODUCTION

dinitrogen complex 3, with the N2 ligand coordinated in a bridging side-on end-on bonding mode. The formation of 3 via this hydride route is particularly attractive, as it proceeds without the use of external reducing agents, which can sometimes lead to the formation of unwanted side products or reactions.8,9 More importantly, 3 undergoes a number of stoichiometric transformations that result in the functionalization of the coordinated N2 unit10 by the formation of N−Si,11,12 N−B,13,14 N−Al,15 N−Zr,16 and N−C17 bonds. In addition to our own work, several other groups have investigated the potential applications of dinuclear group 5 polyhydrides as reductants for small molecules. For example, Wolczanski and co-workers have extensively studied the reactivity of Ta tetrahydride complexes prepared via the hydrogenolysis of low-valent Ta(III) siloxide precursors.18−21 Similarly, Kawaguchi and co-workers have synthesized a series of group 5 polyhydrides via the reduction of Ta and Nb aryloxide chloride complexes with borohydride reagents;22,23 several of these polyhydrides react spontaneously with N2 gas to afford complexes featuring strongly reduced dinitrogen ligands.24−26 In an effort to further develop the hydride route shown in Scheme 1 for the preparation of dinitrogen complexes, we have

Our interest in the synthesis of dinitrogen-containing metal complexes has evolved from the reduction of metal halide derivatives1−4 in the presence of N2 to the use of hydride complexes5−7 that activate N2 by displacement of dihydrogen. We have previously reported the reactivity of the ditantalum tetrahydride species 2, generated via hydrogenolysis of the mononuclear Ta(V) trimethyl complex 1 (Scheme 1);5,6 upon exposure to N2 gas, 2 generates the reactive ditantalum Scheme 1

Received: April 25, 2015 Published: July 7, 2015 © 2015 American Chemical Society

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Organometallics been examining new ligand systems that contain two amido units flanking a phosphine donor: i.e., NPN donor sets. In Scheme 2, the original NPN donor set with methylene− Scheme 2

Figure 1. ORTEP representation of the solid-state molecular structure of 4 (ellipsoids at 50% probability). All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta01−N01 2.055(4), Ta01−N02 2.137(4), Ta01−P01 2.6034(14), Ta01−C39 2.221(3), Ta01−C40 2.210(3), Ta01−C41 2.192(3); N01−Ta01−N02 121.59(10), P01−Ta01−C39 74.82(10), C39−Ta01−N02 88.17(12), P01−Ta01−N02 73.49(8), C40−Ta01−C41 79.12(14), C41−Ta01−N01 99.45(12), N01−Ta01−C40 99.54(12).

dimethylsilyl linkers is shown as A, and the modified NPN* with o-phenylene-bridged donors is shown as B (Mes = 2,4,6Me3C6H2). The NPN* ligand system has been shown to coordinate to Zr and be stable to reduction to generate a dinuclear dinitrogen complex that activates H2.27,28 In this work, we describe this same ligand set coordinated to tantalum alkyl and alkyne complexes and examine their reactivity with dihydrogen. While the resulting dinuclear tetrahydride of tantalum with NPN* does not react with dinitrogen under ambient conditions, we have studied the details of the hydrogenolysis process and report these results herein.

structure of [NPN*]TaMe3. In contrast to the Cs symmetry that is observed in the solution-state NMR spectra, complex 4 adopts a structure with C1 molecular symmetry in the solid state. The [NPN*] ligand coordinates facially to the Ta center, and the geometry at the metal is distorted trigonal prismatic, with one of the trigonal faces consisting of C39, N02, and P01 and the second trigonal face made up of C40, C41, and N01. The bond lengths between the [NPN*] ligand and the metal center (Ta01−N01, Ta01−N02, and Ta−P01) are consistent with those found in similar Ta complexes29 and are generally unremarkable. In addition, the three Ta−Me bond lengths (Ta01−C39, Ta01−C40, and Ta−C41) are essentially equal (within experimental error) and agree well with the Ta−Me bond lengths found in [P2N2]TaMe330 and [NPNSi]TaMe3 (1).6 As will be discussed below, the hydrogenolysis of 4 leads to the formation of the anticipated dinuclear ditantalum tetrahydride complex ([NPN*]Ta)2(μ-H)4 (5); however, no intermediates could be detected or isolated during the process, as one observes only the starting trimethyl species 4 and end product 5. In an effort to provide more information on the hydrogenolysis of Ta(V) trialkyl derivatives, we extended our study to include mixed benzyl alkyne complexes. The tantalum alkyne benzyl complexes [NPN*]Ta(BTA)(CH2Ph) (6, where BTA is bis(trimethylsilyl)acetylene) and [NPN*]Ta(3-hexyne)(CH2Ph) (7) were prepared via a salt metathesis reaction between the corresponding Ta alkyne chloride complex (C)29 and benzylpotassium, as depicted in Scheme 4. In C6D6, each benzyl derivative generates a singlet (6, δ 26.2; 7, δ 26.5) in their respective 31P{1H} NMR spectra. As with 4, the resonances attributable to the [NPN*] ligand in the 1H and 13 C{1 H} NMR spectra are indicative of C s-symmetric complexes. For 7, the benzyl methylene protons appear as a doublet (δ 2.75, 3JHP = 7.2 Hz) coupled to the phosphorus atom of [NPN*]; in contrast, the benzylic protons in 6 appear only as a broad singlet at δ 3.22. The methylene carbons of these benzyl moieties give rise to phosphorus-coupled doublets at δ 89.8 (6, 2JCP = 16 Hz) and δ 75.5 (7, 2JCP = 4 Hz) in their respective 13C{1H} NMR spectra.

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RESULTS AND DISCUSSION Synthesis of Tantalum Hydrocarbyl Complexes. A convenient precursor for incorporating the NPN* donor set is the dipotassium salt K2[NPN*](THF)x, as previously reported,29 which upon reaction with TaMe3Cl2 results in the formation of the six-coordinate trimethyl complex [NPN*]TaMe3 (4), as shown in Scheme 3. Scheme 3

In C6D6 solution, the 1H NMR spectrum of 4 displays [NPN*] ligand resonances indicative of Cs symmetry. However, the three Ta methyl groups give rise to a single broad resonance at δ 1.33, consistent with some fluxional process at room temperature. A resonance that corresponds to the TaMe3 groups could not be located in the 13C{1H}-APT spectrum, likely due to signal broadening caused by the rapid exchange of the Me groups on the NMR time scale; an 1H−13C HSQC experiment does correlate the proton resonance for the TaMe3 groups to a resonance at 72.5 ppm in the carbon spectrum, which is consistent with the NMR data from similar Ta trimethyl complexes synthesized by our group.6,30 The 31 1 P{ H} NMR spectrum features a sharp singlet at δ 42, typical of other Ta diamidophosphine complexes.5,6,29−31 Single crystals of 4 suitable for X-ray analysis were obtained from a saturated toluene solution cooled to −35 °C. Figure 1 shows an ORTEP representation of the solid-state molecular 3547

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Organometallics Scheme 4

Figure 2. ORTEP representation of the solid-state molecular structure of 6 (ellipsoids at 50% probability). All hydrogen atoms and the mesityl group at N01 (except for Cipso) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ta01−N01 2.038(2), Ta01−N02 2.106(2), Ta01−P01 2.6393(7), Ta01−C39 2.209(3), Ta01−C100 2.098(3), Ta01−C101 2.120(3), C100−C101 1.328(4); C100−C101−Si01 134.7(2), N01−Ta01−N02 122.39(9), P01− Ta01−C39 150.98(8), N01−Ta01−P01 73.49(7), N02−Ta01−P01 73.51(7), N01−Ta01−C39 99.25(10), N02−Ta01−C39 88.17(10), Ta01−C39−C40 133.5(2), C101−C100−Si02 136.7(2).

The NMR data for the alkyne ligands in complexes 6 and 7 have several noteworthy features. While the quaternary alkyne carbons for either complex are not directly observable in the 13 C{1H} spectra, they can be detected indirectly by a 1H−13C HMBC experiment. Using this approach, both of the carbons of the BTA triple bond in 6 give rise to one resonance at δ 241.0, typical for alkyne ligands coordinated to early transition metals;32−34 the resonance attributed to the carbons of the 3hexyne carbon−carbon triple bond of 7 appears at δ 240.0. Additionally, there is only one 13C resonance for both of the methyl (δ 29.9) and methylene (δ 13.3) carbon atoms of the 3hexyne unit in complex 7. For 6, all six TMS methyl carbons give rise to a single resonance at δ 2.2. The 1H NMR data for both 6 and 7 are consistent with what is observed in the 13 C{1H} NMR spectra: the two TMS groups in 6 produce one singlet (integrating to 18 protons), and there is only one set of methyl and methylene protons for the 3-hexyne unit in 7 (a quartet and a triplet that integrate to four and six protons, respectively). Overall, these data suggest that in solution both 6 and 7 have alkyne units with equivalent halves and are thus bound to the Ta center perpendicular to the σv plane of symmetry in each molecule. ORTEP representations of the solid-state molecular structures of 6 and 7 are shown in Figures 2 and 3, respectively; the two benzyl complexes exhibit significant structural differences with regard to the relative orientation of the ligands around the Ta center. In the case of 6 the geometry about the metal is best described as distorted square pyramidal (assuming the alkyne unit occupies one site), with a τ − 5 value35 of 0.48; the alkyne ligand occupies the apical coordination site, and the equatorial plane is defined by N01, N02, P01, and the benzyl group. This ligand arrangement is similar to the structure of the parent tantalum alkyne chloride complex (C, R = TMS29), with the alkyne unit located cis to the phosphorus atom of the NPN ligand set. In contrast, the position of the benzyl group and the alkyne unit are reversed in 7, an orientation that results in a nearly idealized trigonal-bipyramidal geometry (τ − 535 = 0.87): the P1−Ta−C45 angle is almost perpendicular (81.2(3)°), and Ta01, P01, and the centroid of the C41− C42 bond form an essentially straight line (∼176°). In both benzyl complexes, the angle between the metal center and the carbons of the benzyl ligand (6, Ta01−C39−C40 = 133.5(2)°; 7, Ta01−C45−C46 = 114.8(10)°) is large enough so as to

Figure 3. ORTEP representation of the solid-state molecular structure of 7 (ellipsoids at 50% probability). All hydrogen atoms and the mesityl group at N02 (except for Cipso) have been omitted for clarity. The phenyl ring of the benzyl arm appears in the difference map as disordered over two discrete positions; although only one orientation is displayed, both orientations were located and modeled anisotropically. Selected bond lengths (Å) and angles (deg): Ta01−N01 2.117(10), Ta01−N02 2.043(9), Ta01−P01 2.625(3), Ta01−C45 2.238(10), Ta01−C41 2.065(13), Ta01−C42 2.096(11), C41−C42 1.306(18); C40−C41−C42 139.4(11), N01−Ta01−N02 118.9(4), P01−Ta01−C45 81.2(3), N01−Ta01−P01 74.9(2), N02−Ta01−P01 74.8(3), N01−Ta01−C45 104.9(4), N02−Ta01−C45 121.0(4), Ta01−C45−C46 114.8(10), C41−C42−C43 132.4(13).

unambiguously designate these groups as η1-coordinated ligands.36 Overall, the solid-state structural data are in accordance with what is observed in solution via NMR spectroscopy, specifically with regard to the equivalency of the halves of the alkyne units. In complex 6, the angle between the plane defined by C101− Ta01−C100 and the σv plane (P01−Ta01−C39) is ∼30°; in complex 7, the angle between the plane defined by C41− Ta01−C42 and the σv plane (P01−Ta01−C45) is ∼35°. While 3548

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Organometallics neither unit is oriented exactly perpendicular to the σv plane, it is probable that in solution the alkyne can “wag” diagonally back and forth and becomes trapped in one of these diagonal orientations upon crystallization. It appears that the difference in ligand orientation around the metal center has little effect on the strength of the various Ta− donor atom interactions, as these bond lengths are essentially the same in 6 and 7. However, these structural differences may explain the dissimilar rates of reaction with H2 observed for these two complexes (vide infra). Hydrogenolysis of Tantalum Hydrocarbyl Complexes. In analogy to the reaction sequence shown in Scheme 1 and the production of the dinuclear tetrahydride 2 from [NPNSi]TaMe3 (1), the hydrogenolysis of [NPN*]TaMe3 (4) was examined. Upon addition of H2 (4 atm) to 4, the dinuclear tetrahydride 5 is generated after 24 h as a dark red-brown powder (Scheme 5). As already mentioned, during this process no intermediates were detected.

Figure 4. ORTEP representation of the solid-state molecular structure of 5 (ellipsoids at 30% probability). All hydrogen atoms and the mesityl groups at N01 and N02 (except for Cipso) have been omitted for clarity. Half of the molecule is generated by the symmetry operation 2 − x, y, 1/2 − z. The bridging hydride moieties could not be located in the difference map. Selected bond lengths (Å) and angles (deg): Ta01−N01 2.081(9), Ta01−N02 2.108(10), Ta01−P01 2.574(3), Ta01−Ta01′ 2.6244(12); N01−Ta01−N02 117.0(3), N01−Ta01−P01 76.7(2), N02−Ta01−P01 75.3(3), P01−Ta01− Ta01′−P01′ (dihedral) 86.39.

Scheme 5

perpendicular (∼86°) relative to one another, likely to reduce interactions between the sterically encumbered mesityl amido donors. Unfortunately, the data obtained from the X-ray crystallographic study were of insufficient quality to allow for the unambiguous identification of the bridging hydrogen atoms. However, the assignment of 5 as a dinuclear Ta(IV)−Ta(IV) complex bridged by four hydrides is justified, on the basis of solution-state NMR data (vide infra) and by comparison to similar crystallographically characterized compounds. The two Ta atoms in 5 are separated by 2.6244(12) Å, which is slightly longer than the Ta−Ta distances found in similar dinuclear tetrahydride-bridged M(IV) complexes of Ta37,38 and Nb25 (2.51−2.57 Å) reported by other workers but is in good agreement with the distances in 2 (∼2.57 Å39) and another Ta tetrahydride previously synthesized in our laboratory, ([P2N2]Ta)2(μ-H)4 (2.6165(5) Å40); it is probable that the slight Ta− Ta bond elongation observed in 5 is due to steric interactions between the bulky mesityl amido groups of the two [NPN*] ligands. The NMR spectroscopic data also suggest that 5 adopts a diamagnetic, dinuclear structure in solution, with two [NPN*]TaIV units bridged by four hydrides; in C6D6, the 31P{1H} NMR spectrum contains only one singlet at δ 32.3. The roomtemperature (293 K, C6D6) 1H NMR spectrum of 5 features broad resonances which suggest that the two [NPN*]Ta units may rotate relative to one another along the axis of the Ta−Ta bond. However, in toluene-d8 at 263 K, the 1H NMR spectrum features sharp resonances that are indicative of a C2-symmetric structure similar to what is depicted in Figure 4, where the two [NPN*]Ta moieties are rotated 90° relative to one another. While the two [NPN*]Ta units that constitute 5 are equivalent to each other due to the C2 rotational axis of symmetry, the halves of each individual [NPN*]Ta unit are not equivalent, resulting in resonances for eight distinct aryl methyl groups and the concomitant aromatic protons of the ligand backbone. In contrast, the 1H NMR spectrum at 363 K is indicative of a more symmetric structure, where the two [NPN*]Ta units are

In addition, the reaction of H2 with the Ta alkyne benzyl complexes 6 and 7 also affords the ditantalum tetrahydride 5 (Scheme 6), albeit requiring much longer reaction times; with Scheme 6

[NPN*]Ta(BTA)(CH2Ph) (6), full conversion to 5 is achieved after approximately 10 days (under 4 atm of H2), while the 3hexyne congener (7) is even slower, requiring approximately 14 days. Subsequent investigations revealed that the hydrogenolysis of the Ta alkyne benzyl complexes initially leads to the formation of isolable alkene hydride intermediates (8 and 9, Scheme 6); these latter complexes will be addressed in detail in the next section. Figure 4 shows an ORTEP representation of the solid-state molecular structure of 5. The complex adopts a C2 -symmetric structure, with the two [NPN*] ligands rotated nearly 3549

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Organometallics rotating quickly with respect to each other on the NMR time scale, which results in a spectrum that displays resonances for only four aryl methyl groups and the associated aromatic protons of the [NPN*] backbone. The only non-[NPN*] ligand resonance found in the 1H NMR spectrum of 5 appears at δ 14.70 and integrates to 2 H per [NPN*]Ta unit. This signal is assigned to the four Ta−H− Ta units and gives rise to a well-resolved triplet coupled (2JHP = 3 Hz) to two equivalent phosphorus-31 nuclei (from two [NPN*] ligands) over the temperature range studied (263− 363 K). As cited above, the major motivation for these investigations into the hydrogenolysis chemistry of [NPN*]Ta complexes was the possibility of generating a dinuclear Ta(IV) tetrahydride complex capable of activating molecular nitrogen in a fashion analogous to that for ([NPNSi]Ta)2(μ-H)4 (2). Unfortunately, the synthesis of the analogue of ([NPNSi]Ta)2(η1:η2-μ-N2)(μH)2 (3) proved not to be possible. The dinuclear tetrahydride 5 shows no signs of reactivity with N2 under a variety of conditions (1 or 4 atm of N2, temperatures up to 100 °C). In this regard, complex 5 is similar to ([P2N2]Ta)2(μ-H)4, which also does not react with N2.40 While it is difficult to rationalize this result in the absence of theoretical calculations, it is noteworthy that 5 has amido substituents (N-mesityl vs Nphenyl) more sterically demanding than those of 2 and the phosphine unit is less basic in 5 than in 2. How important these differences are to the outcome of this N2 activation process is currently under investigation. Synthesis of Tantalum Alkene Hydrides via Hydrogenolysis. As mentioned above, the hydrogenolysis of the [NPN*]Ta alkyne benzyl complexes 6 and 7 ultimately affords the Ta tetrahydride 5 (Scheme 6). Although complete conversion to 5 requires exposure to 4 atm pressures of H2 for several days, 6 and 7 do react relatively rapidly to form Ta alkene hydride intermediates. For example, exposing a benzene solution of [NPN*]Ta(BTA)(CH2Ph) (6) to H2 (1 atm) over the course of approximately 36 h resulted in the quantitative formation of complex 8 (Scheme 6). Bright red crystals of 8 suitable for an X-ray diffraction study were obtained from a concentrated toluene/pentane solution cooled to −35 °C; an ORTEP representation of the solid-state molecular structure of 8 is shown in Figure 5. Complex 8 is generated by hydrogenolysis of both the alkyne and benzyl moieties of 7 and thus can be viewed as a Ta alkene hydride complex. Just as the structures of the Ta alkyne complexes previously reported29 by us (along with 7 and 8) are discussed as being intermediate between the Ta(V)−“alkenediyl” and Ta(III)−alkyne resonance forms, it is also possible to view this complex as a combination of the Ta(V)−“alkanediyl” and Ta(III)−alkene forms (8 and 8′ respectively, in Scheme 7). The solid-state structural details of this alkene hydride derivative suggest that the Ta(V) formalism, i.e., 8, is the better description. The C39−C43 distance is quite long (1.498(4) Å), placing it on the upper limit of the range of C−C bond lengths found in most other structurally characterized Ta and Nb olefin complexes (1.43−1.49 Å41−46). In addition, the Ta01−C39 (2.188(3) Å) and Ta01−C43 (2.154(3) Å) bonds are short in comaparison to the same literature examples cited above (2.18−2.30 Å41−46). Consequently, it is more appropriate to refer to the (Me3Si)CH−CH(SiMe3) moiety as a formally dianionic “alkanediyl” ligand, bound to a Ta(V) center by two Ta−C single bonds. However, just as 6 and 7 are referred to as Ta

Figure 5. ORTEP representation of the solid-state molecular structure of 8 (ellipsoids at 50% probability). All hydrogen atoms (except for H39, H43, and H100) and the mesityl group at N02 (except for Cipso) have been omitted for clarity; H39, H43, and H100 were located from the difference map and refined isotropically. Selected bond lengths (Å) and angles (deg): Ta01−N01 2.050(2), Ta01−N02 2.023(2), Ta01− P01 2.685(1), Ta01−H100 1.72(3), Ta01−C39 2.188(3), Ta01−C43 2.154(3), C39−C43 1.498(4); N01−Ta01−N02 124.29(10), P01− Ta01−H100 151.17(95), N01−Ta01−P01 74.75(7), N02−Ta01−P01 72.26(7), Si01−C43−H43 113(2), Si02−C39−H39 109.80(16), Si01−C43−C39−Si02 (dihedral) 111.54.

Scheme 7

alkyne benzyl complexes for the sake of simplicity, by extension 8 will be referred to as a Ta alkene hydride complex. The solution-state NMR spectroscopic data for 8 are in good agreement with the solid-state structure shown above. A C6D6 solution of 8 generates a 1H NMR spectrum that is indicative of C1 molecular symmetry, with resonances for the eight inequivalent aryl methyl groups and related aryl protons of the [NPN*] ligand backbone. The hydride ligand gives rise to a doublet at δ 24.3, strongly coupled to phosphorus-31 (2JHP = 44 Hz); both the extremely downfield chemical shift and the large 2 JHP coupling constant are indicative of a trans disposition of the hydride and phosphine donor, similar to the case for the Ta alkyne hydride complexes previously reported by our group.29 The “alkene” protons of the (Me3Si)CH−CH(SiMe3) fragment give rise to a pair of AB-coupled doublets (δ −1.06 and −1.11, 3 JHH = 22 Hz); this large coupling constant is consistent with the trans stereochemistry observed in the solid-state structure. A 1H−13C HSQC experiment correlates these protons with carbon atoms at δ 57.9 and 75.0, respectively; these 13C NMR resonances are slightly more downfield than the signals for most Ta and Nb olefin complexes but are still within the empirically observed range (δ ∼30−80).41−44,46−51 The trans stereochemistry of the bis(trimethylsilyl)ethylene ligand in complex 8 was surprising. Typically, the hydrogenation of a coordinated alkyne by a late-transition-metal complex such as Wilkinson’s catalyst, Rh(PPh3)3Cl,52 proceeds via migratory insertion into a metal hydride, followed by reductive elimination of the resulting vinyl moiety and another hydride ligand; such a mechanism necessarily leads to a cis3550

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Organometallics alkene.52−55 Although there are examples in the literature56−59 of trans-alkene moieties being generated via the insertion of an alkyne into a Ta−hydride bond, they are rare, and the mechanism for their formation is not well understood. In an effort to gain some insights into the mechanism for the conversion of 6 to 8, the progress of the reaction was monitored by 1H and 31P{1H} NMR spectroscopy. These data suggest that the reaction proceeds via the formation of several Ta hydride containing intermediates, evinced by the appearance of new resonances in the downfield region (δ >15) of the 1H NMR spectrum; attempts at isolating these intermediates proved unsuccessful, and consequently the structure and composition of these proposed Ta hydride complexes are presently unknown. One possible intermediate that we could dismiss is [NPN*]Ta(BTA)H (10), the putative result of initial hydrogenolysis of only the benzyl ligand. As we have previously reported,29 in the synthesis of this compound by hydride metathesis of the starting chloroalkyne complex [NPN*]Ta(BTA)Cl we were able to clearly establish that 10 is not detected during the hydrogenolysis of 6 to 8; in other words, none of the hydride resonances observed during the conversion of 6 to 8 matched that previously reported for 10. In fact, subsequent investigations revealed that 10 does react with H2 to generate 8, but the rate of this reaction (>10 days, 4 atm of H2) was significantly slower than the route starting from 6 (36 h, 1 atm of H2). This last observation rules out the possibility that complex 10 is a short-lived (and unobserved) intermediate in the latter reaction (Scheme 8).

Figure 6. Possible mechanism for the formation of Ta alkene hydride complexes (8 and 9′) from Ta alkyne benzyl species (6 and 7), which proceeds via a common Ta vinyl hydride intermediate ([Ta] = [NPN*]Ta).

> sp3.61 Accordingly, the greater degree of s-orbital character in the Ta−Calkyne bond in complex 6 means that it would better stabilize the attendant four-membered σ-bond metathesis transition state and react more readily than the Ta−Cbenzyl bond. Thus, the first step in our proposed mechanism (Figure 6) posits the hydrogenolysis of the coordinated alkyne ligand into a vinyl moiety; a zwitterionic alkylidene resonance structure can also be envisioned for this latter complex. The α insertion of the Ta−alkylidene moiety into the Ta−H bond (and not the Ta−CH2Ph bond, which would be significantly slower62) results in a zwitterionic alkyl species (shown in brackets in Figure 6) that undergoes a ring-closing step to afford the Ta alkene alkyl complex; prior to ring closure, free rotation about the C−C bond (labeled in red) allows the least sterically crowded conformer to result, which results in a sterically favorable trans geometry of the alkene unit. The remaining benzyl ligand of this Ta alkene complex must then undergo a σbond metathesis with H2 faster than that with the coordinated alkene to generate the observed complex 8. While the mechanism in Figure 6 can rationalize the trans stereochemistry of the isolated alkene hydride complexes 8 and 9′ (vide infra), we do make the following assumptions: (i) no reductive elimination processes are operative, and (ii) the separately synthesized alkyne hydride complex 10 is not a productive intermediate in the these hydrogenolysis processes (see Scheme 8). These two assumptions are actually related, as any reductive elimination process under hydrogenolysis conditions (i.e., excess H2) would likely result in oxidative addition of H2 to form a vinyl dihydride intermediate, which as shown in Scheme 9 would also result from hydrogenolysis of the alkyne hydride complexes (e.g., 10) previously reported by us. As this latter process has already been discussed as being too slow and therefore unlikely to be productive as a means of forming the alkene hydride complexes, the two assumptions above are therefore defensible. Synthesis of [NPN*]Ta(1-hexene)H via Hydrogenolysis and Alkene Isomerization. In addition to the [NPN*]Ta(BTA) benzyl complex 6, the 3-hexyne analogue [NPN*]Ta(3-hexyne)(CH2Ph) (7) reacts with H2 to afford [NPN*]Ta(1-hexene)H (9), a Ta alkene hydride complex that is analogous to 8, but with the added complexity of isomerization

Scheme 8

A possible mechanism for the synthesis of 8 from 6 that is consistent with these observations is shown in Figure 6. This postulate requires that hydrogenolysis first occurs at one of the Ta−Calkyne bonds, rather than at the Ta−Cbenzyl bond via a σbond metathesis reaction. The Ta−Cbenzyl bond features a carbon atom that is sp3 hybridized, in comparison to the Ta− Calkyne bond, where the carbon atom is (formally) sp 2 hybridized. There is compelling experimental evidence60 which suggests that a key factor in determining the rate of reactivity of a particular substrate for σ-bond metathesis is the degree of s character in the orbitals that constitute the wellknown four-membered transition state. Investigations with a series of d0 scandium alkyl complexes revealed that the rate of reaction with various hydrocarbons followed the trend sp > sp2 3551

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Organometallics

The proposed mechanism in Figure 6 does not rationalize the formation of the 1-hexene isomer; rather, it provides a route to the trans-3-hexene isomer 9′. One of the most interesting aspects of this transformation is that, of the hydrogenolysis reactions discussed thus far, the conversion of 7 into 9 is the fastest and is complete in 6 h using 1 atm of H2. Just as a reminder, the conversion of the other alkyne benzyl complex 6 to the corresponding alkene hydride 8 takes 36 h at 1 atm of H2. A key difference glossed over in Figure 6 and in Scheme 9 is that the geometry of the 3-hexyne benzyl starting material 7 is different from that observed in 6, both in solution and in the solid state (see Scheme 4 and Figure 3). While the alkyne unit is trans to the phosphine in 7, as opposed to cis in 6, it is not obvious how important the stereochemistry at the tantalum center is to the overall hydrogenolysis processes. For the sake of completeness, it is also noteworthy that the stereochemistry of the coordinated alkyne in the previously reported alkyne hydride derivatives 10 and 11 is distinct from that of both of the alkyne benzyl complexes 6 and 7 (i.e., the alkyne unit is oriented perpendicular to the N−Ta−N plane), and these complexes undergo exceedingly slow hydrogenolysis reactions of the alkyne units. Regardless of these differences in ligand arrangement, the formation of 9 from 7 is believed to proceed via a mechanism similar to that depicted in Figure 6; the progression of the reaction was monitored periodically by 1H and 31P{1H} NMR spectroscopy and, similar to the formation of 8, the presence of several new resonances downfield of δ 15 indicate that the reaction proceeds via the intermediacy of several unidentified Ta hydride containing complexes. As mentioned above, the formation of the previously reported29 Ta alkyne hydride 11 (Scheme 10) is an unlikely intermediate in the formation of 9.

Scheme 9

of the 3-hexene ligand to the 1-hexene isomer. Although the solid-state molecular structure of 9 has not yet been determined, solution-state NMR spectroscopy provides compelling evidence for the 1-hexene isomer. In C6D6, the 1H NMR spectrum of 9 features [NPN*] ligand resonances indicative of a C1-symmetric complex, as well as an extremely downfield doublet resonance characteristic of a hydride ligand strongly coupled to a phosphorus-31 atom (δ 23.7, 2JHP = 36 Hz). On the basis of the similarly large 2JHP coupling constants, this last piece of data suggests that the Ta−H is disposed trans to the phosphine donor of the NPN* ligand, analogous to that found for 8. The 1H and 13C NMR resonances attributable to the 1hexene ligand of 9 are given in Table 1. These spectral

Scheme 10

Table 1. 1H and 13C NMR Spectral Assignments for the 1Hexene Ligand of Complex 9

proton(s) 1

C Ha/b C2H C3H2 C4H2 C5H2 C6H3

chem shift (δ) 0.43, 2.07 1.17 1.21 1.84 1.52 0.77

location (13C NMR chem shift (δ)) C1 C2 C3 C4 C5 C6

(43.9) (74.7) (23.4) (36.8) (37.5) (14.5)

assignments were made on the basis of the results of the 1 H−1H COSY, 1H−13C HSQC, 1H−13C HMBC, and 13C-APT NMR experiments; further details regarding these NMR experiments can be found in the Supporting Information. The protons of the methyl group (C6H3) generate a wellresolved triplet resonance at δ 0.77 (2JHH = 7 Hz), and the methylene protons (C5H2, C4H2, C3H2) give rise to three broad multiplets in the region between δ ∼1.2 and 1.8; the resonance for the C2H proton is nearly coincident with that of the two C3H2 protons. The two diastereotopic C1H2 protons give rise to multiplets at δ 2.07 and 0.43, with well-resolved coupling to phosphorus-31 (3JHP = 4 Hz), vicinal coupling to H2 (3JHH = 17 Hz), and geminal coupling (2JHH = 13 Hz).

Not only is there no NMR spectroscopic evidence for its formation but also exposure of 11 to H2 (1 atm) for a period of 24 h leads to no observable reaction. While the hydrogenolysis of 11 does ultimately lead to the low-yield formation of 9 over a period of several days (under 4 atm of H2), the details of this transformation are complicated and are not discussed here. As indicated above, the 1H and 13C NMR data for 9 show that the 3-hexyne moiety in 7 is ultimately converted to coordinated 1-hexene, rather than coordinated 3-hexene. This result has precedent: the isomerization of alkene ligands is a well-known process for early-transition-metal hydride com3552

DOI: 10.1021/acs.organomet.5b00344 Organometallics 2015, 34, 3546−3558

Article

Organometallics plexes, a classic example being the hydrozirconation reaction between an internal alkene and Schwartz’s reagent, Cp2Zr(H)Cl. Insertion of the alkene into the Zr−H bond ultimately affords the terminal Zr alkyl moiety;63−65 this isomerization process is hypothesized to occur via a “chain walking” mechanism66 consisting of successive olefin insertion and βhydride elimination cycles, with the terminal isomer being preferred as a means of reducing steric interactions at the metal center.67 Thus, presuming that the hydrogenolysis of 7 initially results in the formation of a [NPN*]Ta(3-hexene)H intermediate (9′ in Figure 6), the alkene ligand must undergo rearrangement to generate the terminal 1-hexene isomer. Figure 7 depicts one

The results of several deuterium-labeling experiments with 9 support a mechanism that involves successive hexene isomerization steps, similar to the olefin insertion/β-hydride elimination process depicted in Figure 7. Treating 7 with D2 gas resulted in the deuterated analogue 9-dn, where the hydride ligand and protons of the 1-hexene moiety are replaced by deuterium atoms. Although the mass spectroscopic data (lowresolution electron ionization) for 9-dn indicates a significant level of deuterium incorporation, evidence for the fully deuterated isotopologue 9-d13 was inconclusive. Nevertheless, the 1H NMR spectrum of 9-dn lacks all of the resonances attributed to the protons of the 1-hexene ligand in 9 along with the Ta hydride resonance at δ 23.7. However, the resonances for these deuterons are readily located at the expected chemical shifts in the 2H NMR spectrum of 9-dn (with the exception of C1Da/b, whose 2H NMR resonances either are very weak in intensity or are too broad to be observed). In addition, in the 13 C{1H} NMR spectrum of 9-dn, the resonances attributed to the carbons of the 1-hexene chain are not visible. This is a common phenomenon for carbon atoms attached to deuterons, due to both a decrease in signal intensity as a result of longer spin relaxation times (T1) and signal broadening due to 13 C−2H J coupling.73 Full details of these 1H and 2H NMR experiments can be found in the Supporting Information. In separate experiments, complexes 9 and 9-dn both readily undergo H/D exchange with D2 or H2 gas, respectively, to generate the corresponding isotopologues (Scheme 12); the Scheme 12

Figure 7. One iteration of the proposed “chain walking” mechanism for the formation of the 1-hexene ligand in complex 9 from the putative 3-hexene isomer 9′.

such iteration, in which the putative 3-hexene derivative 9′ is converted to the 2-hexene ligand; a second iteration can be envisioned that would convert the 2-hexene ligand to the 1hexene isomer, consistent with the NMR spectroscopic data for complex 9. Drawing on the analogy of Schwartz’s reagent, this process is envisioned as proceeding via a series of olefin insertion and β-hydride elimination steps involving the Ta(III) alkene resonance form of 9 (cf. Scheme 7 for 8). A key intermediate is the Ta(III) 3-hexyl species γ-I, shown in Figure 7; it should be noted that this isomerization process occurs in the presence of excess H2 (or D2 in subsequent experiments), which could oxidatively add to form a Ta(V) 3-hexyl dihydride species. We suggest without proof that the equilibrium lies far to the side of the unsaturated Ta(III) alkyl species (Scheme 11) perhaps due to α- or β-agostic interactions.68−72

synthesis of tetrahydride 5 from 9 is exceedingly slow under these conditions, and thus the formation of 5 (or 5-d4) is not observed alongside these H/D exchange processes. However, additional experiments confirm that tetrahydride 5 and tetradeuteride 5-d4 also undergo H/D exchange with D2 or H2, respectively, to generate the corresponding isotopologue, as is also shown in Scheme 12. Furthermore, the 3-hexyne hydride complex [NPN*]Ta(3hexyne)H (11) undergoes fast H/D exchange with D2 (1 atm, ∼16 h) to generate the monodeuteride complex [NPN*]Ta(3hexyne)D (11-d1), as evidenced by the disappearance of the Ta hydride 1H NMR resonance at δ 21; this exchange process is slow but is significantly faster than the conversion of 11 to 9 as already mentioned, and thus no deuteration of the alkyne unit is observed. Scheme 13 depicts a proposed mechanism for the incorporation of deuterium into the hexene ligand of 9, on the basis of the data presented above; similar to the case in Figure 7, the Ta(III) alkene resonance form of 9 is invoked for

Scheme 11

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DOI: 10.1021/acs.organomet.5b00344 Organometallics 2015, 34, 3546−3558

Article

Organometallics Scheme 13

of experiments showed that the rate of hydrogenation of these hydrocarbyl groups follows the order alkyne > benzyl. Also of interest is that the alkene hydride complexes undergo deuterium incorporation and scrambling, which suggests that the overall release of alkane to generate the dinuclear tetrahydride final product is slow and is preceded by a series of equilibria that involve reversible alkene and hydride migratory insertion and β-elimination processes. Further work is in progress to generate new ligand designs related to NPN* and NPNSi that can generate ditantalum tetrahydride complexes which are reactive to molecular nitrogen.

the sake of simplicity and consistent terminology. In the presence of excess D2 gas, the hydride ligand of 9 would exchange for a deuteride via a σ bond metathesis type interaction. This deuterium atom could then be incorporated into the hexene chain via an olefin insertion/β-hydride elimination cycle to afford a new hydride ligand; Scheme 13 depicts two possible olefin insertion products (α-I and β-I), which can lead to two possible β-hydride elimination results (d1-1-hexene and d1-2-hexene). While the steric bulk of the [NPN*] ligand ensures that 1hexene is ultimately the favored regioisomer, work done by Bercaw and co-workers on the reactivity of alkenes with Nb(III) metallocene hydride complexes42,44 suggests that the kinetic barrier between the possible olefin insertion/β-hydride elimination pathways could be low enough such that in solution a number of different regioisomers (e.g., α-I and β-I) exist in equilibrium with one another. Thus, as the hydride ligand in d11-hexene or d1-2-hexene can also exchange with D2 to generate a deuteride, subsequent olefin insertion/β-hydride elimination cycles can be envisioned that will eventually lead to all the hydrogen atoms along the hexene chain being replaced by deuterium. As discussed above, it is assumed that the Ta(III) alkyl intermediates α-I and β-I in Scheme 13 do not undergo oxidation addition with the excess D2 present; rather, the deuterium incorporation occurs via exchange of the tantalum hydride/deuteride in the 1-hexene, d1-1-hexene, and d1-2hexene regioisomeric complexes, which because of the equilibria shown result in further deuterium incorporation.

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EXPERIMENTAL SECTION

Unless otherwise noted, all experiments were conducted by means of standard Schlenk line techniques or in a glovebox (Innovative Technology) equipped with a freezer (−35 °C), under an atmosphere of dry oxygen-free dinitrogen, using oven-dried (200 °C) glassware cooled under dynamic vacuum. Anhydrous benzene, toluene, and diethyl ether were purchased from Aldrich, sparged with dinitrogen, and dried further by passage through towers containing activated alumina and molecular sieves. Pentane was refluxed over sodium benzophenone ketal, distilled under positive argon pressure, and degassed via several freeze−pump−thaw cycles. Deuterated benzene (C6D6) was stirred over sodium benzophenone ketal, vacuum transferred, and freeze−pump−thaw degassed; toluene-d8 was stirred over activated molecular sieves and freeze−pump−thaw degassed. Benzylpotassium,74 TaMe3Cl2,75 [NPN*]K2(THF)0.5, [NPN*]Ta(3hexyne)Cl, and [NPN*]Ta(BTA)Cl29 were prepared according to literature methods. Hydrogen (Praxair) and deuterium (Cambridge Isotope Laboratories, HD 0.4%) gases were passed through a column containing activated molecular sieves prior to use. NMR spectra were recorded on a Bruker AV-400 MHz or AV-300 MHz spectrometer. Except where noted, all spectra were recorded at room temperature. 1 H NMR spectra were referenced to residual proton signals in C6D6 (δ 7.16) or toluene-d8 (δ 2.09). 31P{1H} NMR spectra were referenced to an external sample of P(OMe)3 (δ 141.0 with respect to 85% H3PO4 at δ 0.0). 13C{1H} NMR spectra were referenced to the solvent resonances of C6D6 (δ 128.06) or toluene-d8 (δ 20.9). Elemental analyses (EA) were performed using a FISONS Elemental Analyzer 1108 at the Department of Chemistry, University of British Columbia. Electron ionization−mass spectrometry (EI-MS) analyses were performed using a Kratos MS-50 spectrometer (70 eV source). [NPN*]TaMe3 (4). A solution of TaMe3Cl2 (0.68 g, 2.29 mmol) in Et2O (30 mL) was added dropwise via cannula to a −78 °C solution of [NPN*]K2(THF)0.5 (1.53 g, 2.29 mmol) in 200 mL of Et2O over the

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CONCLUSIONS The impetus for this study was originally focused on developing a synthesis of the ditantalum tetrahydride complex 5 to investigate its ability to activate dinitrogen, in analogy to that found for tetrahydride 2 (Scheme 1). While 5 does not activate N2, we did discover a number of different tantalum hydrocarbyl complexes that served as precursors to generate this dinuclear tetrahydride. Of particular note were a series of mononuclear tantalum alkyne benzyl complexes that allowed us to intercept intermediates in which the benzyl ligand has been replaced by a hydride and the alkyne unit has been partially hydrogenated to generate a trans-alkene, in the case of bis(trimethylsily)acetylene, and 1-hexene, in the case of the 3-hexyne. A number 3554

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Article

Organometallics

with cold pentane (2 × 10 mL) to afford 0.86 g (0.95 mmol, 68%) of the product. 1 H NMR (C6D6, 400 MHz): δ 7.54 (ddd, 3JHP = 11 Hz, 3JHH = 8 Hz, 4JHH = 2 Hz, 2H, ArH), 7.46 (dd, 3JHP = 8 Hz, 4JHH = 2 Hz, 2H, ArH), 7.05 (m, 5H, ArH), 6.93 (d, 3JHH = 8 Hz, 2H, ArH), 6.86 (d, 3 JHH = 8 Hz, 4H, ArH), 6.79 (m, 3H, ArH), 6.07 (dd, 4JHP = 5.6 Hz, 3 JHH = 8.4 Hz, 2H, ArH), 2.75 (d, 3JHP = 7.2 Hz, 2H, CH2Ph), 2.22 (q, 3 JHH = 7.2 Hz, 4H, hexyne CH2), 2.28 (s, 6H, ArCH3), 2.21 (s, 6H, ArCH3), 2.16 (s, 6H, ArCH3), 1.93 (s, 6H, ArCH3) 0.65 (t, 3JHH = 7.2 Hz, 6H, hexyne CH3). 31P{1H} NMR (C6D6, 160 MHz): δ 26.5 (s). 13 C{1H} NMR (C6D6, 100 MHz): δ 163.6 (d, 2JCP = 33 Hz, ArC), 151.4 (d, 3JCP = 7 Hz, ArC), 151.3 (d, 3JCP = 4 Hz, ArC), 135.4 (d, 4JCP = 2 Hz, ArC), 134.8 (d, 4JCP = 2 Hz, ArC), 134.1, 134.0 (d, 1JCP = 21 Hz, ArC) 133.2 (d, 2JCP = 11 Hz, ArC), 133.0 (ArC), 132.6 (ArC), 130.2 (d, 4JCP = 2 Hz, ArC), 129.9 (d, 3JCP = 4 Hz, ArC), 129.5 (d, 3JCP = 5 Hz, ArC), 129.2 (d, 2JCP = 9 Hz, ArC), 128.4 (ArC), 128.1 (ArC), 127.6 (ArC), 122.5 (ArC), 117.8 (d, 1JCP = 38 Hz, ArC), 115.7 (d, 3JCP = 12 Hz, ArC), 75.5 (d, 2JCP = 4 Hz, CH2Ph), 29.9 (hexyne CH2), 20.8 (ArCH3), 20.1 (ArCH3), 19.7 (ArCH3), 19.5 (ArCH3), 13.3 (hexyne CH3). Multiple attempts to obtain acceptable elemental analyses failed; a representative set is shown. Anal. Calcd for C51H56N2PTa: C, 67.39; H, 6.21; N, 3.08. Found: C, 59.67; H, 5.67; N, 3.67. ([NPN*]Ta)2(μ-H)4 (5). Caution! These reactions result in a pressure of ∼4 atm within a sealed vessel upon warming to room temperature and were performed with great care and were always manipulated behind a blast shield. Method A (from 4). A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a yellow solution of 4 (0.50 g, 0.64 mmol) in toluene (20 mL), and the headspace gas was removed via one freeze−pump−thaw cycle. The vessel was then cooled in liquid nitrogen, filled with H2 gas, and sealed under approximately 1 atm of pressure. The reaction mixture was warmed to room temperature, while the pressure inside the vessel slowly rose to approximately 4 atm. After the mixture was stirred at room temperature for 1 week, a dark brick red solution of ([NPN*]Ta)2(μ-H)4 was obtained. The excess H2 gas was carefully vented from the reaction vessel, and the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue. This residue was then extracted with Et2O (20 mL) and cooled in a glovebox freezer (−35 °C), whereupon a dark-colored solid precipitated out of solution. The precipitate was collected on a sintered-glass frit, washed with a minimum amount of cold pentane, and dried in vacuo to afford 0.32 g (0.21 mmol, 69%) of a dark redbrown powder. The low yield is due to the appreciable solubility of 5 in pentane. Method B (from 6 or 7). A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a benzene solution (4 mL) of 6 (0.11 g, 0.11 mmol) or 7 (0.10 g, 0.11 mmol), and the headspace gas was removed via one freeze−pump−thaw cycle. The vessel was then cooled in liquid nitrogen, filled with H2 gas, and sealed under atmospheric pressure. The reaction mixture was warmed to room temperature, while the pressure inside the vessel slowly rose to approximately 4 atm. After the mixture was stirred at room temperature for 10 days (6) or 14 days (7), a dark brick red solution of ([NPN*]Ta)2(μ-H)4 was obtained. The excess H2 gas was carefully vented from the reaction vessel, and the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue. This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (−35 °C), whereupon a dark-colored solid precipitated out of solution. The precipitate was collected on a sintered-glass frit, washed with a minimum amount of cold pentane, and dried in vacuo to afford 64 mg (43 μmol, 39%, from 6) or 55 mg (37 μmol, 34%, from 7). Low yields were obtained due to the appreciable solubility of 5 in pentane. 1 H NMR (C6D6, 300 MHz, 298 K): δ 14.70 (t, 2JHP = 3.2 Hz, 4H, Ta(μ-H)4Ta), 7.57 (m, 4H, ArH), 7.20 (bs, 4H, ArH), 7.13 (m, 6H, ArH), 6.99 (s, 4H, ArH), 6.93 (s, 2H, ArH), 6.60 (2bd, JHH = 8.7 Hz, 4H, ArH), 6.33 (s, 2H, ArH), 5.95 (m, 4H, ArH), 2.45 (bs, 6H, ArCH3), 2.22 (bs, 6H, ArCH3), 2.17 (bs, 6H, ArCH3), 2.10 (bs, 6H, ArCH3), 1.87 (bs, 12H, ArCH3), 1.82 (bs, 12H, ArCH3)..31P{1H}

course of approximately 10 min. The resulting dark brown solution was stirred for 30 min at −78 °C before it was warmed to 0 °C and stirred for an additional 30 min, during which the formation of a lightcolored precipitate was observed. The reaction mixture was then evaporated to dryness in vacuo to afford a dark brown residue, which was extracted with 50 mL of toluene and filtered through a pad of silica on a sintered-glass frit. The filtrate was again evaporated to dryness, and the resulting brown residue was triturated with 20 mL of pentane and cooled in a glovebox freezer (−35 °C), whereupon a yelloworange precipitate formed. This precipitate was collected on a frit, washed with a minimum amount of cold pentane, and dried in vacuo to afford 1.55 g (1.99 mmol, 87%) of a yellow-orange powder. 1 H NMR (C6D6, 400 MHz): δ 7.60 (dd, 3JHP = 11 Hz, 2JHH = 8 Hz, 2H, ArH), 7.32 (d, 2JHH = 8 Hz, 2H, ArH), 7.06 (m, 3H, ArH), 6.86 (s, 4H, ArH), 6.80 (d, 2JHH = 8 Hz, 2H, ArH), 6.06 (dd, 4JHP = 6 Hz, 2JHH = 8 Hz, 2H, ArH), 2.34 (s, 6H, ArCH3), 2.14 (s, 6H, ArCH3), 2.12 (s, 6H, ArCH3), 1.95 (s, 6H, ArCH3), 1.33 (bs, 9H, Ta(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ 41.9 (s). 13C{1H} NMR (C6D6, 100 MHz): δ 165 (d, 2JCP = 32 Hz, ArC), 144 (d, 3JCP = 5 Hz, ArC), 138.0 (ArC), 137.1 (ArC), 135.9 (ArC), 135.0 (d, 4JCP = 2 Hz, ArC), 134.2 (d, 2JCP = 2 Hz, ArC), 133.3 (d, 2JCP = 9.9 Hz, ArC), 132.2 (d, 1JCP = 39.3 Hz, ArC), 130.8 (ArC), 130.7 (ArC), 130.4 (ArC), 129.7 (d, 3JCP = 5.3 Hz, ArC), 129.0 (d, 3JCP = 9.4 Hz, ArC), 120.9 (d, 1JCP = 41.1 Hz, ArC), 117.3 (d, 3JCP = 11.2 Hz, ArC) 21.2 (ArCH3), 20.5 (ArCH 3 ), 20.31 (ArCH 3 ), 19.3 (ArCH 3 ). Anal. Calcd for C41H48N2PTa: C, 63.07; H, 6.20; N, 3.59. Found: C, 62.82; H, 6.34; N, 3.50. [NPN*]Ta(BTA)CH2Ph (6). A slurry of benzylpotassium (0.20 g, 1.53 mmol) in toluene (10 mL) was cooled to −35 °C and stirred vigorously. To this was added a solution of [NPN*]Ta(BTA)Cl (1.44 g, 1.53 mmol) in toluene (10 mL) via cannula over approximately 10 min. The resulting dark red solution was warmed to room temperature and stirred for 48 h, during which the formation of a light-colored precipitate was observed. This suspension was filtered through a pad of silica on a sintered-glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark red residue. This residue was triturated with 20 mL of pentane and cooled to −35 °C, whereupon a red precipitate formed. A maroon solid was collected on a frit and washed with cold pentane (2 × 10 mL) to afford 0.92 g (0.92 mmol, 60%) of the product. 1 H NMR (C6D6, 300 MHz): δ 7.62 (m, 4H, ArH), 7.05 (m, 3H, ArH), 6.90 (m, 3H, ArH), 6.81 (bs, 1H, ArH), 6.79 (bs, 1H, ArH), 6.67 (s, 2H, ArH), 6.55 (d, 2JHH = 7 Hz, 2H, ArH), 6.49 (s, 2H, ArH), 5.97 (dd, 3JHP = 5.4 Hz, 2JHH = 8.4 Hz, 2H, ArH), 3.22, (s, 2H, CH2Ph) 2.33 (s, 6H, ArCH3), 2.01 (s, 6H, ArCH3), 1.97 (s, 6H, ArCH3), 1.80 (s, 6H, ArCH3), 0.10 (s, 18H, Si(CH3)3). 31P{1H} NMR (C6D6, 160 MHz): δ 26.2 (s). 13C{1H} NMR (C6D6, 100 MHz): δ 163.6 (d, 2JCP = 32 Hz, ArC), 148.5 (ArC), 142.1 (d, 3JCP = 4 Hz, ArC), 136.8 (ArC), 136.4 (d, 2JCP = 32 Hz, ArC), 135.1 (ArC), 135.0 (d, 2JCP = 38 Hz, ArC), 134.3 (ArC), 134.2 (ArC), 133.7 (ArC), 130.4 (d, 1JCP = 34 Hz, ArC), 129.9 (ArC), 129.1 (d, 3JCP = 5 Hz, ArC), 128.6 (d, 3JCP = 9 Hz, ArC), 128.4 (ArC), 128.1 (ArC), 126.5 (ArC), 122.7 (ArC), 120.6 (d, 1JCP = 38 Hz, ArC), 115.6 (d, 3JCP = 11 Hz, ArC), 89.8 (d, 2JCP = 16 Hz, CH2Ph), 20.9 (ArCH3), 20.4 (ArCH3), 20.2 (ArCH3), 19.3 (ArCH3), 2.2 (SiCH3). Multiple attempts to obtain acceptable elemental analyses failed; a representative set is shown. Anal. Calcd for C53H64N2PSi2Ta: C, 63.84; H, 6.47; N, 2.81. Found: C, 65.81; H, 6.56; N, 2.63. [NPN*]Ta(3-hexyne)CH2Ph (7). A stirred slurry of benzylpotassium (0.18 g, 1.38 mmol) in toluene (10 mL) was cooled to −35 °C. A solution of [NPN*]Ta(3-hexyne)Cl (1.19 g, 1.39 mmol) in toluene (10 mL) was added via cannula over approximately 10 min. The resulting dark brown solution was warmed to room temperature and stirred for 48 h, during which the formation of a light-colored precipitate was observed. This suspension was filtered through a pad of silica on a sintered-glass frit, and the filtrate was evaporated to dryness in vacuo to afford a dark brown residue. This residue was triturated with 20 mL of pentane and cooled to −35 °C, whereupon a yellow precipitate formed. A yellow solid was collected on a frit and washed 3555

DOI: 10.1021/acs.organomet.5b00344 Organometallics 2015, 34, 3546−3558

Article

Organometallics NMR (C6D6, 120 MHz): δ 32.3 (s). 13C{1H} NMR (C6D6, 75 MHz): δ 162.6 (ArC), 161.0 (ArC), 152.9 (ArC), 151.4 (ArC), 136.5 (d, 1JCP = 41 Hz, ArC), 135.8 (ArC), 134.4 (ArC), 134.1 (ArC), 133.4 (d, 2JCP = 11 Hz, ArC), 133.3 (ArC), 132.9 (ArC), 132.7 (ArC), 130.8 (ArC), 130.2 (ArC), 129.5 (ArC), 129.1 (ArC), 128.4 (ArC), 123.7 (d, 1JCP = 41 Hz, ArC), 116.9 (ArC), 115.8 (ArC), 21.3 (ArCH3), 21.25 (ArCH3), 20.7 (ArCH3), 20.2 (ArCH3), 19.9 (ArCH3), 19.5 (ArCH3). 1 H NMR (toluene-d8, 400 MHz, 298 K): δ14.67 (t, 2JHP = 3.2 Hz, 4H, Ta(μ-H)4Ta), 7.56 (m, 4H, ArH), 7.15 (s, 10H, ArH), 6.97 (bs, 4H, ArH), 6.88 (bs, 2H, ArH), 6.53 (bd, 2JHH = 8.8 Hz, 4H, ArH), 6.26 (bs, 2H, ArH), 5.89, (bs, 2H, ArH) 5.79 (bs, 2H, ArH), 2.45 (bs, 6H, ArCH3), 2.19 (bs, 6H, ArCH3), 2.02 (bs, 6H, ArCH3), 1.88 (bs, 12H, ArCH3), 1.82 (bs, 12H, ArCH3) 1.72 (bs, 6H, ArCH3). 1H NMR (toluene-d8, 400 MHz, 253 K): δ14.54 (t, 2JHP = 3.2 Hz, 4H, Ta(μH) 4 Ta), 7.56 (m, 4H, ArH), 7.2−6.9 (overlapping signals, approximately 16 aromatic protons and residual toluene-d8 protons), 6.53 (bd, 2JHH = 8.8 Hz, 4H, ArH), 6.28 (bs, 2H, ArH), 5.97, (dd, 3JHP = 6 Hz, 2JHH = 8 Hz, 2H, ArH), 5.86 (dd, 3JHP = 6 Hz, 2JHH = 8 Hz, 2H, ArH), 2.47 (s, 6H, ArCH3), 2.23, (s, 6H, ArCH3), 2.18 (s, 6H, ArCH3), 2.06 (s, 6H, ArCH3), 1.88 (s, 6H, ArCH3), 1.85 (s, 6H, ArCH3), 1.80 (s, 6H, ArCH3) 1.77 (s, 6H, ArCH3). 1H NMR (toluene-d8, 400 MHz, 363 K): δ 14.81 (t, 2JHP = 4 Hz, 4H, Ta(μH)4Ta), 7.56 (m, 4H, ArH), 7.20 (bs, 6H, ArH), 7.12 (s, 2H, ArH), 6.95 (s, 4H, ArH), 6.86 (s, 4H, ArH), 6.53 (d, 2JHH = 8 Hz, 6H, ArH), 5.74 (dd, 3JHP = 6 Hz, 2JHH = 8 Hz, 4H, ArH), 2.28 (s, 12H, ArCH3), 1.90 (bs, 12H, ArCH3), 1.87 (s, 12H, ArCH3), 1.81 (bs, 12H, ArCH3). Multiple attempts to obtain acceptable elemental analyses failed; a representative set is shown. Anal. Calcd for C76H82N4P2Ta2: C, 61.87; H, 5.60; N, 3.80. Found: C, 59.61; H, 6.03; N, 3.46. [NPN*]Ta(trans-1,2-bis(trimethylsilyl)ethene)H (8). A thickwalled 200 mL glass vessel equipped with a Teflon valve was charged with a bright red benzene solution (4 mL) of 6 (011 g, 11 μmol), and the headspace gas was removed via one freeze−pump−thaw cycle. The vessel was back-filled with H2 gas and sealed under atmospheric pressure at room temperature. After the mixture was stirred at room temperature for 36 h, a dark brown-red solution of 8 was obtained; the reaction mixture was evaporated to dryness in vacuo to yield a dark red-brown residue. This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (−35 °C), whereupon a dark red solid precipitated out of solution. The precipitate was collected on a sintered -glass frit, washed with a minimum amount of cold pentane, and dried in vacuo to afford 67 mg (74 μmol, 68%) of the product. 1 H NMR (C6D6, 400 MHz): δ 24.41 (d, 2JHP = 44 Hz, 1H, TaH), 7.68 (s, 1H, ArH), 7.58 (d, 2JHH = 7 Hz, 1H, ArH), 7.52 (s, 1H, ArH), 7.11 (m, 4H, ArH), 6.84 (s, 1H, ArH), 6.77 (d, 2JHH = 10 Hz, 2H, ArH), 6.76 (s, 1H, ArH), 6.70 (d, 2JHH = 10 Hz, 2H, ArH), 6.20 (dd, 3 JHP = 5 Hz, 2JHH = 8 Hz, 1H, ArH), 5.78 (dd, 2JHH = 8 Hz, 3JHP = 5 Hz, 1H, ArH), 2.80 (s, 3H, ArCH3), 2.24 (s, 3H, ArCH3) 2.12 (s, 3H, ArCH3), 2.10 (s, 3H, ArCH3), 2.07 (s, 3H, ArCH3), 2.04 (s, 3H, ArCH3), 1.90 (s, 3H, ArCH3), 1.82 (s, 3H, ArCH3), 0.04 (s, 9H, Si(CH3)3), −0.12 (s, 9H, Si(CH3)3), −1.06 (d, 3JHH = 22 Hz, 1H), −1.11 (d, 3JHH = 22 Hz) (two protons for TMS(H)CC(H)TMS). 31 1 P{ H} NMR (C6D6, 160 MHz): δ 16.0 (s). 13C{1H} NMR (C6D6, 100 MHz): δ 160.4 (d, 1JCP = 32 Hz, ArC), 158.7 (d, 1JCP = 28 Hz, ArC), 142.9 (ArC), 139.0 (d, 3JCP = 4 Hz, ArC), 138.2 (d, 3JCP = 4 Hz, ArC), 137.8 (ArC), 136.3 (ArC), 135.9 (ArC), 134.8 (ArC), 134.7 (ArC), 134.5 (ArC), 134.4 (ArC), 134.0 (ArC), 133.9 (ArC), 133.8 (ArC), 131.0 (d, 3JCP = 5 Hz, ArC), 130.6 (ArC), 130.5 (ArC), 130.2 (d, 4JCP = 5 Hz, ArC), 129.9 (ArC), 129.8 (d, 4JCP = 2 Hz, ArC), 129.6 (ArC), 128.8 (ArC), 128.7 (ArC), 124.6 (ArC), 124.5 (ArC), 123.5 (d, 1JCP = 36 Hz, ArC), 122.0 (d, 1JCP = 38 Hz, ArC), 116.0 (d, 2JCP = 9 Hz, ArC), 115.4 (d, 2JCP = 10 Hz, ArC) 75.0, 57.9 (two carbons for TMS(H)C=C(H)TMS), 22.4 (ArCH3), 21.1 (ArCH3), 20.8 (ArCH3), 20.3 (ArCH3), 20.2 (ArCH3), 19.9 (ArCH3), 19.7 (ArCH3), 19.1 (ArCH 3 ), 2.9 (SiMe 3 ), 0.6 (SiMe 3 ). Anal. Calcd for C46H60N2Si2P1Ta1: C, 60.78; H, 6.65; N, 3.08. Found: C, 61.06; H, 6.60; N, 3.31. [NPN*]Ta(1-hexene)H (9). A thick-walled 200 mL glass vessel equipped with a Teflon valve was charged with a dark yellow benzene solution (4 mL) of 7 (0.10 mg, 0.11 mmol), and the headspace gas was

removed via one freeze−pump−thaw cycle. The vessel was back-filled with H2 gas and sealed under atmospheric pressure at room temperature. After the mixture was stirred at room temperature for 6 h, a dark brown solution of 9 was obtained; the reaction mixture was evaporated to dryness in vacuo to yield a dark brown residue. This residue was then extracted with Et2O (10 mL) and cooled in a glovebox freezer (−35 °C), whereupon a dark brown solid precipitated out of solution. The precipitate was collected on a sintered-glass frit, washed with a minimum amount of cold pentane, and dried in vacuo to afford 40 mg (49 μmol, 44%) of the product. Low yields were obtained due to the appreciable solubility of 9 in pentane. 1 H NMR (C6D6, 400 MHz): δ 23.70 (d, 2JHP = 36 Hz, 1H, TaH), 7.77 (dd, 2JHH = 20 Hz, 3JHP = 12 Hz, 2H, ArH), 7.63 (dd, 2JHH = 11 Hz, 3JHP = 7 Hz, 2H, ArH), 7.11 (m, 2H, ArH), 6.91 (s, 1H, ArH), 6.86 (s, 2H, ArH), 6.83 (dd, 2JHH = 11 Hz, 3JHP = 8, 2H, ArH), 6.59 (s, 2H, ArH), 6.00 (dd, 2JHH = 12 Hz, 3JHP = 8 Hz, 1H, ArH), 5.83 (dd, 2 JHH = 12 Hz, 3JHP = 8 Hz, 1H, ArH), 2.87 (s, 3H, ArCH3), 2.64 (s, 3H, ArCH3), 2.08 (s, 3H, ArCH3), 2.07 (m, 1H, H1a/b), 2.03 (s, 3H, ArCH3), 2.01 (s, 6H, ArCH3), 1.84 (m, 1H, H4), 1.72 (s, 3H, ArCH3), 1.60 (s, 3H, ArCH3), 1.52 (m, 1H, H5), 1.21 (m, 1H, H3), 1.17 (m, 1H, H2), 0.77 (t, 2JHH = 7 Hz, 1H, H6), 0.43 (ddd, 3JHH = 17 Hz, 2JHH = 13 Hz, 3JHP = 4 Hz, 1H, H1a/b). 31P{1H} NMR (C6D6, 160 MHz): δ 30.0 (s). 13C{1H} NMR (C6D6, 100 MHz): δ 162.1 (d, 1JCP = 34 Hz, ArC), 161.4 (d, 1JCP = 34 Hz, ArC), 141.4 (ArC), 140.9 (ArC), 136.9 (ArC), 136.8 (ArC), 136.75 (d, 2JCP = 25 Hz, ArC), 135.5 (ArC), 135.0 (ArC), 134.9 (d, 4JCP = 2 Hz, ArC), 134.8 (d, 4JCP = 2 Hz, ArC), 134.5−134.4 (four ArC carbons), 134.3 (ArC), 132.1 (d, 2JCP = 12 Hz, ArC), 131.4 (d, 3JCP = 6 Hz, ArC), 130.5 (d, 3JCP = 4 Hz, ArC), 130.4 (ArC), 130.2 (ArC), 129.9 (ArC), 129.7 (ArC), 129.6 (ArC), 129.1 (d, 3JCP = 9 Hz, ArC), 125.5 (d, 1JCP = 36 Hz, ArC), 124.5 (d, 1JCP = 36 Hz, ArC), 116.3 (d, 2JCP = 11 Hz, ArC), 115.5 (d, 2JCP = 10 Hz, ArC) 74.7 (C2), 43.9 (C1), 37.5, 36.8, 23.3 (three carbons for C3, C4, C5), 21.7 (ArCH3), 21.2 (ArCH3), 21.1 (ArCH3), 20.9 (ArCH3), 20.4 (ArCH3), 20.3 (ArCH3), 18.6 (ArCH3), 17.7 (ArCH3), 14.5 (C6). Multiple attempts to obtain acceptable elemental analyses failed; a representative set is shown. Anal. Calcd for C46H60N2Si2P1Ta1: C, 64.38; H, 6.39; N, 3.41. Found: C, 62.30; H, 5.91; N, 5.04. EI-MS (m/ z): 816 (15, [M − 4 H]+), 734 (80, [Ta{NPN*}]+), 556 (30, [NPN*]+), 541 (100, [{NPN*} − Me]+). [NPN*]Ta(1-hexene-dn)D (9-dn). A sample of 9-dn was prepared using 7 and D2, in a manner identical with that for 9. 1 H NMR (C6D6, 400 MHz): δ 7.77 (dd, 2JHH = 20 Hz, 3JHP = 12 Hz, 2H, ArH), 7.63 (dd, 2JHH = 11 Hz, 3JHP = 7 Hz, 2H, ArH), 7.11 (m, 2H, ArH), 6.91 (s, 1H, ArH), 6.86 (s, 2H, ArH), 6.83 (dd, 2JHH = 11 Hz, 3JHP = 8, 2H, ArH), 6.59 (s, 2H, ArH), 6.00 (dd, 2JHH = 12 Hz, 3 JHP = 8 Hz, 1H, ArH), 5.83 (dd, 2JHH = 12 Hz, 3JHP = 8 Hz, 1H, ArH), 2.87 (s, 3H, ArCH3), 2.64 (s, 3H, ArCH3), 2.08 (s, 3H, ArCH3), 2.03 (s, 3H, ArCH3), 2.01 (s, 6H, ArCH3), 1.72 (s, 3H, ArCH3), 1.60 (s, 3H, ArCH3). 31P{1H} NMR (C6D6, 160 MHz): δ 30.0 (bs). 13C{1H} NMR (C6D6, 100 MHz): δ 162.1 (d, 1JCP = 34 Hz, ArC), 161.4 (d, 1 JCP = 34 Hz, ArC), 141.4 (ArC), 140.9 (ArC), 136.9 (ArC), 136.8 (ArC), 136.75 (d, 1JCP = 25 Hz, ArC), 135.5 (ArC), 135.0 (ArC), 134.9 (d, 1JCP = 2 Hz, ArC), 134.8 (d, 1JCP = 2 Hz, ArC), 134.5−134.4 (four ArC carbons), 134.3 (ArC), 132.1 (d, 2JCP = 12 Hz, ArC), 131.4 (d, 3JCP = 6 Hz, ArC), 130.5 (d, 3JCP = 4 Hz, ArC), 130.4 (ArC), 130.2 (ArC), 129.9 (ArC), 129.7 (ArC), 129.6 (ArC), 129.1 (d, 3JCP = 9 Hz, ArC), 125.5 (d, 1JCP = 36 Hz, ArC), 124.5 (d, 1JCP = 36 Hz, ArC), 116.3 (d, 2JCP = 11 Hz, ArC), 115.5 (d, 2JCP = 10 Hz, ArC), 21.7 (ArCH3), 21.2 (ArCH3), 21.1 (ArCH3), 20.9 (ArCH3), 20.4 (ArCH3), 20.3 (ArCH3), 18.6(ArCH3), 17.7 (ArCH3). EI-MS (m/z): 825 ((5, [9-d5]+), 816 (5, [9 − 4 H]+), 738 (80), 556 (30, [NPN*]+), 541 (100, [{NPN*} − Me]+).

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving full experimental procedures, representative NMR spectra, and crystallographic data. The Supporting Information is available free of charge on 3556

DOI: 10.1021/acs.organomet.5b00344 Organometallics 2015, 34, 3546−3558

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Organometallics

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the ACS Publications website at DOI: 10.1021/acs.organomet.5b00344.

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AUTHOR INFORMATION

Corresponding Author

*M.D.F.: e-mail, [email protected]; fax, +1 604 822-8710, tel, +1 604 822-2471. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.D.F. thanks the NSERC of Canada for a Discovery Grant, and K.D.J.P. thanks the NSERC for Postgraduate Scholarships; D.N. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Fellowship. Dedicated to Professor F. Ekkehardt Hahn on the occasion of his 60th birthday.

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