Synthetic and Computational Studies on the Thermal and

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Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe3 (NPN = PhP(CH2SiMe2NPh)2) and [MesNPN]TaMe3 (MesNPN = PhP(CH2SiMe2N(2,4,6-Me3C6H2))2) Aleksandra Zydor, Richard J. Burford, and Michael D. Fryzuk* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver BC, V6T 1Z1, Canada S Supporting Information *

ABSTRACT: The thermolysis of [PhP(CH2SiMe2NPh)2]TaMe3 leads to the elimination of methane and the formation of cyclometalated derivative [PhP(CH2SiMe2NPh)(CH2SiMe2N-oC6H4)]TaMe2, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an ortho-Nphenyl-C−H bond and a Ta-methyl unit. The photolysis of [PhP(CH2SiMe2NPh)2]TaMe3 takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP(CH2SiMe2NPh)2]TaCH2(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the N-phenyl substituent with N-Mesityl (Mesityl = 2,4,6-Me3C6H2) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H2 were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP(CH2SiMe2NPh)2]TaMe3 with H2 to generate dinuclear tetrahydride ([PhP(CH2SiMe2NPh)2]Ta)2(μ-H)4.

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INTRODUCTION We have previously reported the hydrogenolysis of the Ta(V) trimethyl complex [NPN]TaMe3 (1) (where NPN = PhP(CH2SiMe2NPh)2) to generate the dinuclear ditantalum tetrahydride complex ([NPN]Ta)2(μ-H)4 (2), which in turn reacts spontaneously with molecular nitrogen to produce the side-on end-on dinitrogen complex, ([NPN]Ta)2(μ-η2:η1N2)(μ-H)2 (3), as shown in Scheme 1.1,2 The ability to generate a highly activated dinitrogen complex in the absence of strong reducing agents such as KC8, Na/Hg, or Li, simply by reaction of N2 with a polyhydride complex has been exploited by a number of groups to advance the area of N 2 functionalization.3−7 In fact, dinitrogen complex 3 generated by this route has a rich functionalization chemistry that has been reviewed elsewhere.8 Hydrogenolysis of metal−alkyl complexes is often employed as a synthetic route to generate metal hydrides, for example, a mononuclear tantalum hydride9 and higher nuclearity metal hydride species10−19 can all be accessed by H2 addition to alkyl complexes. The conversion of 1 to 2 is a complex process that likely involves multiple hydrogenolysis steps to eliminate methane (CH4), Ta−Ta bond formation, and reduction from Ta(V) to two Ta(IV) centers, in some unknown sequence. We also noted that starting trimethyl 1 was somewhat thermally sensitive and quite photochemically unstable, both of which required protecting this complex from heat and light to ensure good yields of the tetrahydride 2.1 One of the curious aspects of © XXXX American Chemical Society

Scheme 1. Known Reactions of Tantalum-trimethyl 1 to Generate Tetrahydride 2 and Deuteride d12-2 and Formation of Dinitrogen Complex 3

the hydrogenolysis was the outcome when trimethyl 1 was allowed to react with excess D2; instead of the expected tetradeuteride ([NPN]Ta)2(μ-D)4 (d4-2), we isolated the d12 isotopologue, ([PhP(CH2SiMe2 N-o-C 6H3 D2) 2]Ta)2(μ-D) 4 Received: June 27, 2017

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DOI: 10.1021/acs.organomet.7b00487 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (d12-2), in which all eight of the ortho-phenyl protons in the NPh substituents have been deuterated.20 While one can imagine a stepwise hydrogenolysis process that involves converting Tamethyl groups to Ta-hydrides, with a dimerization step at some point, the involvement of the ortho-C−H bonds of the Nphenyl groups adds considerable complexity to any proposed reaction scheme. In this paper, we report computational and synthetic studies on the thermal reaction of trimethyl 1 in an effort to probe some of the early steps in the conversion of 1 to 2. Because of the involvement of the N-phenyl ortho-C−H bonds, we modified the NPN ligand by blocking those sites with methyl groups and examining the analogous reactions of the N-Mesityl version. We also include the results of photolysis of 1 and some preliminary reactions with H2 in an effort to provide information on the subsequent transformations of possible intermediates along the pathway to tetrahydride 2.

However, our initial attempts at calculating transition states did provide reasonable activation barriers (ΔH⧧) that were as low as 157.3 kJ/mol for one of three transition states found; we designate this lowest energy one as TS1, with the other two transition states, TS2 and TS3, having slightly higher barriers, at ΔH⧧ = 157.4 and 162.8 kJ/mol, respectively. Each of these three transition states (TS1, TS2, and TS3) connects to cyclometalated product 4, which is slightly endothermic with respect to 1, with enthalpy changes for the conversion calculated to be ΔH = 3.5, 6.3, and 9.9 kJ/mol, respectively; this is summarized in Figure 1a.

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RESULTS AND DISCUSSION We wondered about the possibility that the starting trimethyl complex 1 could undergo cyclometalation of one of the Nphenyl groups as an initial step.21−24 If this were an early stage process, then subsequent reaction with H2 or D2 could provide a possible pathway for the observed incorporation of deuterium at the ortho-position of this group. Starting with a computational analysis,25 the anticipated reaction pathway for C−H bond activation of one of the Nphenyl groups in 1 is shown in Scheme 2. One N-Ph group first Scheme 2. Themolysis of Trimethyl 1 to Cyclometalated Derivative 4 via Four-Centered Transition State Represented as TS

Figure 1. (a) Initial DFT results identifying possible transitions states and products. (b) Using data from the isolated X-ray crystal structure of 4, a new lower energy transition state TS4 was located and energies of the optimized product 4 recalculated to show its stability with respect to the reactants.

Since the computational results suggested that this process could be viable, with a surmountable barrier, we examined the generation of 4 in a thermal process. Heating trimethyl 1 at 70 °C for 48 h results in the formation of a new species as evidenced by a singlet at δ 26.6 downfield-shifted from starting point of 1 at δ 13.5. Isolation of this material could be achieved in 38% yield upon recrystallization. The 1H NMR spectrum shows two broad separate Ta-CH3 resonances and four distinct sharp Si-CH3 resonances, indicative of a lack of symmetry in 4. Disappearance of a methyl group and a more complicated set of 1 H NMR resonances for the aryl region are consistent with a cyclometalation event; also, the lower symmetry generates four silyl methyl resonances and two inequivalent Ta-methyl resonances. Monitoring the thermolysis of 1 by 31P NMR spectroscopy shows that the reaction occurs over a few days at

rotates along the N−ipso-C axis and bends toward the Ta center, tightening the C(Ph)−Ta-C(Me) angle. In a fourmembered transition state (TS), the ortho carbon−hydrogen is then transferred from the N-Ph group to the Ta-methyl ligand, and methane is eliminated to generate [PhP(CH2SiMe2NPh)(CH2SiMe2N-o-C6H4)]TaMe2 (4). Using the known X-ray crystal structure coordinates for 1, we computed some possible pathways. Modeling a transition state for this process and identifying the one with the lowest energy is not a trivial task as many structures can be calculated. In particular, the potential energy surface of [NPN]TaMe3 (1) is very complex partly due to different possible configurations of the flexible diamidophosphine ligand as well as the fact that the Ta-Me’s are fluxional, any one of which could be eliminated. B


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Organometallics 50 °C and is quite clean, as just the consumption of 1 and no other side products are observed, while the singlet for 4 grows in heating to higher temperatures. Speeding up the process results in additional unidentified side products and lowers the yield. Confirmation of the proposed structure was obtained by single-crystal X-ray diffraction; the solid-state molecular structure is shown in Figure 2.

that is 26.5 kJ/mol more stable than the most stable one previously computed (ΔH⧧ = 130.8 kJ/mol, Figure 1b). Importantly, newly computed TS4 links via an Intrinsic reaction coordinate (IRC)26 the reactant and the product identified in the thermolysis reaction. The experimentally determined structures of trimethyl 1 and cyclometalated product 4 are very similar to the structures predicted with DFT models; a comparison of 4 using line drawings is shown in Figure 3.

Figure 2. ORTEP diagram of [PhP(CH2SiMe2NPh)(CH2SiMe2N-oC6H4)]TaMe2 (4) with thermal ellipsoids at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and bond angles (deg): Ta1−C1, 2.204(3); Ta1−C2, 2.209(3); Ta1−C3, 2.189(3); Ta1−N1, 2.098(3); Ta1−N2, 2.008(3); Ta1−P1, 2.6472(9); N1−Ta1−C1, 96.83(12); N1−Ta1−C2, 140.56(12); N1−Ta1−C3, 63.94(11); N1−Ta1−P1, 72.84(7); N2− Ta1−P1, 76.32(8); N2−Ta1−N1, 114.68(11); N2−Ta1−C1, 129.41(12); N2−Ta1−C2, 96.13(12); N2−Ta1−C3, 110.46(11); P1−Ta1−C3, 135.04(9).

Figure 3. Comparison of the X-ray structure (black) and the DFToptimized model (blue) of [PhP(CH2SiMe2NPh)(CH2SiMe2N-oC6H4)]TaMe2 (4).

The ball and stick calculated structure of TS4 is presented in Figure 4 along with similar representations of starting trimethyl 1 and cyclometalated product 4. Similar to the earlier computed transition states TS1 to TS3, the C−H bond activation of the N-Ph group proceeds via a four-membered transition state, whereby one of the N-Ph groups first rotates along the N−ipsoC axis and bends toward the Ta center, making a 70.7° angle for C(Ph)−Ta-C(Me). In TS4, hydrogen is being transferred from the ortho-Ph group (C(Ph)−H = 1.36 Å) to the methyl group (C(Me)−H = 1.48 Å), while the carbon of the Ph group repositions to coordinate to Ta, leading to the cyclometalated product shown in Figure 2. The four atoms involved in the reaction, Ta, C(Ph), C(Me), and H(Ph), are coplanar in the transition state. The C(Ph)−Ta−C(Me) angle is 70.7° and the C(Ph)−H−C(Me) angle is 166.5°. (See Table 1 and TS4 in Figure 4b for selected bonds length and angles of the calculated transition state.) During the cyclometalation (H-transfer) process,27 population analysis shows a very slight increase in positive charge on H in TS4 (Δq = 0.06 in Table 2), which suggests that the polarity of the C−H bond making and breaking process does not change. In addition, no spin-unpaired radicals are produced: Calculations show that the closed-shell singlets are the lowest energy structures present, as the electron pair is transferred from σ(C(Ph)−H) to σ(H−C(Me)). Furthermore, Table 2 presents the charge on the atoms that are involved in forming TS4 from trimethyl 1. The charge q on Ta decreases as the electron density on the metal increases upon going to TS4 (Δq = −0.34). As the reaction proceeds from TS4 to product 4 we again see that electron density on the metal decreases (q increases) due to formation of a new, single σ(Ta−C(Ph)) bond from dσ−pσ of the Ta−C(CH3) bond. The primary observation is that hydrogen is transferred as a H atom when

The molecular structure shows some differences as compared to that of starting trimethyl complex 1, most of which can be traced to the extra strain imposed by the cyclometalated-phenyl unit. However, the majority of the bond lengths in 1 and 4 are very similar, for example, the Ta−P bonds differ by roughly 0.13 Å (shorter in 4) while the Ta−C bonds are slightly contracted for the remaining methyl groups (2.224(5), 2.228(5), and 2.204(5) Å in 1 to 2.209(3) and 2.189(3) in 4). The change from C-sp3 to C-sp2 ligation at tantalum in 4 shows no deviation from the shortest Ta−C bond in 1 (2.204(3) vs 2.204(5) Å, respectively). Interestingly, the noncyclometalated Ta−N bonds of 1 differ slightly (2.025(4) and 2.078(4) Å) and remain unchanged in 4 (2.0008(3) Å), yet the cyclometalated arm results in the longest Ta−N contact at 2.098(3) Å. The 4-membered Ta1−N1−C8−C3 ring is perfectly planar with a tight N1−C8−C3 angle of 107.2(3) and a 63.95(11)° N1−Ta−C3 angle. A slight expansion in the N−Ta−N angle in 4 versus that of 1 (116.62(11) and 113.1(2)°, respectively) is accompanied by an expansion in tightest angle between two Ta−C bonds (80.15(14) and 78.2(2)°, respectively). At this point, the results from the thermolysis experiment can serve to validate the model and provide feedback on the computational analysis. DFT optimization of the experimental structure of 4 leads to a cyclometalated product that is 34.0 kJ/ mol more stable than in the best initial iteration (Figure 1a). In turn, this result helped to locate a new transition state, TS4, C


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Organometallics

Figure 4. (a) Ball and stick representations of computed structures of trimethyl 1, TS4, and cyclometalated product 4 along the reaction coordinate; some CH3 and Ph groups, and H atoms were omitted for clarity. White = H, gray = C and Si, blue = Ta, dark blue = N, and orange = P. (b) Bond distances calculated in TS4.

attempts. Assuming a distorted trigonal bipyramidal structure, the methylene unit can be cis or trans to the phosphine; since we observe no coupling to the carbon and only a small 3JP = 1.2 Hz for the methylene protons from the 31P nucleus, we favor the cis site as indicated in eq 1.

Table 1. Selected Bond Distances and Angles Parameters of the DFT Optimized Structures of [NPN]TaMe3 (1), Optimized Transition State (TS4) and Cyclometalated Product 4 1 Ta−C(Ph) Ta−C(CH3) Ta···H CH3−H H−C(Ph) C(Ph)−Ta−C(CH3)

Distance (Å) 3.79 2.23 3.82 3.05 1.09 Angle [°] 64.4

TS4

4

2.34 2.53 1.98 1.48 1.36

2.19

1.09

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70.7

LIGAND MODIFICATION: N-PHENYL TO N-MESITYL The ortho-H’s of the N-Ph group of the tridentate NPN ligand are clearly involved in the chemistry of trimethyl 1, both in the thermolysis process described above as well as in the deuterium incorporation into the ortho positions of the dinuclear tetradeuteride, d12-2. We reasoned that blocking these positions with methyl groups would eliminate this complexity; therefore we examined the synthesis of the N-Mesityl derivative (Mesityl = 2,4,6-trimethylphenyl). The synthesis of the dilithio NPN ligand precursor is shown in Scheme 3 and follows from the reported NPN ligand synthesis;1 reaction of the monolithiomesitylamide with chloromethyldimethyl-chlorosilane (ClCH2SiMe2Cl) followed by in situ reaction with dilithium phenylphosphide in the presence of an additional 2 equiv of BuLi leads to the formation of the dilithio phosphinediamide, Mes [NPN]Li2(THF)4, which is isolated with four THF molecules. Subsequent reaction of the dilithio derivative Mes[NPN]Li2(THF)4 with TaMe3Cl2 results in the formation of the corresponding tantalum trimethyl complex Mes[NPN]TaMe3 (6) as confirmed by X-ray crystallographic analysis (Figure 5). Even though 6 and 1 differ by 9 methyl substituents on the flanking rings, the added steric demand of the ligand results in only minor changes to the metrical parameters. For example,

Table 2. Charge q of the Involved Atoms Ta, ortho-C(N-Ph), C(Ta-CH3), and H from NBO Population Analysis q[Ta] q[ortho-C(N-Ph)] q[C(Ta-CH3)] q[H]

1

TS4

4

1.321 −0.256 −1.082 0.259

0.985 −0.359 −1.26 0.314

1.244 −0.359 −0.962 0.250

σ(C(N-Ph)−H) transforms into σ(H−C(Ta-CH3)), and the change from σ(Ta−C(Ta-CH3)) to σ(Ta−C(Ph)) happens via the metal center with the involvement of the d orbital. Given that the thermolysis provided clean formation of cyclometalated 4, for completeness, we examined the photolysis of 1 at 350 nm, which resulted in a different outcome.28 Monitoring this process by 31P NMR spectroscopy shows that 1 disappears completely after 4−5 h concomitant with the formation of single new species that displays a singlet at δ 30.8. Purification by crystallization results in the isolation of 5 in 38% yield. The 1H NMR spectrum displays a sharp doublet at δ 9.16 (3JPH = 1.4 Hz), indicative of a TaCH2 moiety.29 This is also evident from the 13C NMR spectrum, which shows a singlet at δ 239.6 for the methylidene carbon TaCH2.30,31 Attempts to grow single crystals for X-ray analysis failed, despite many D


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Organometallics

spectrum, but these products have resisted isolation because of their low yield. Given our success with the thermolysis of the N-phenyl trimethyl complex 1 from both a computational perspective and synthetically, we subjected the bulkier trimethyl 6 to DFT analysis to examine a possible reaction outcome. Given the proximity of the ortho-methyl substituents, we computed a transition state of methane elimination from one of the Ta-Me’s and a benzylic C−H bond of one of the ortho-methyl groups. We managed to find a transition state, TS5, that was similar in energy (ΔH⧧ = +120 kJ/mol) to that found for the optimized N-phenyl derivative and the anticipated product in an overall exothermic process (ΔH = −65.5 kJ/mol). The reaction coordinate diagram is shown in Figure 6.

Scheme 3. Synthetic Method for the Preparation of N-Mes Substituted Complex Mes[NPN]TaMe3 6

Figure 6. Preliminary DFT results for thermolysis of the N-Mesityl derivative showing the presumed transition state in the loss of CH4 to generate an ortho-benzyl unit.

Figure 5. ORTEP diagram of [PhP(CH2SiMe2NMes)2]TaMe3 (6) with thermal ellipsoids at the 50% probability level. Hydrogen atoms and pentane in the crystal lattice have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ta1−C1, 2.208(5); Ta1−C2, 2.231(5); Ta1−C3, 2.240(5); Ta1−N1, 2.086(4); Ta1−N2, 2.035(4); Ta1−P1, 2.6852(12); N1−Ta1−C1, 82.57(18); N1−Ta1− C2, 135.02(19); N1−Ta1−C3, 91.24(18); N1−Ta1−P1, 75.91(11); N2−Ta1−P1, 75.78(11); N2−Ta1−N1, 123.97(14); N2−Ta1−C1, 95.87(19); N2−Ta1−C2, 97.42(18); N2−Ta1−C3, 126.83(18); P1− Ta1−C3, 76.59(15); C1−Ta1−C2, 75.8(2); C1−Ta1−C3, 130.2(2); C2−Ta1−C3, 74.5(2).

Surprisingly, attempted thermolysis of trimethyl 6 at 50 °C did not result in any change in the 31P NMR spectrum over a period of 2 days. Continued thermolysis for 2 weeks still showed the presence of starting material and a number of new products that have resisted isolation (Scheme 4). There are Scheme 4. When 6 is Thermolyzed a Mixture of Products Is Obtained That Has Resisted Purification

the Ta−C and Ta−N bond lengths are effectively unchanged, yet an expansion in the N−Ta−N angle in 6 versus that in 1 (123.97(14) and 113.1(2)°, respectively) is accompanied by a contraction in tightest angle between two Ta−C bonds (74.5(2) and 78.2(2)°, respectively). The largest change in bond length appears in the anchoring Ta−P bond which is approximately 0.1 Å shorter in 6 than that in 1 (2.6852(12) and 2.7713(13) Å, respectively). This derivative is very thermally sensitive and is best stored at −30 °C in the dark. In an effort to examine the effect of introducing substituents at the ortho position of the NPN ligand, we examined the thermal stability of 6 along with its reactivity with H2. The reaction of 6 with H2 was first examined. Surprisingly, there is no reaction with excess H2 at room temperature. In the presence of H2, 6 slowly decomposes over a period of weeks. There appear to be small Ta−H or Ta(μ-H)Ta type resonances in the downfield region (greater than δ 9) of the 1H NMR

many peaks in the 1H and 31P NMR spectra, but none are identifiable, which makes any conclusions about the viability of the anticipated cyclometalation process in Figure 6 debatable. In this case, while DFT did allow identification of a possible reaction outcome, there are other unanticipated pathways that occur. It is interesting to note that ortho-tert-butyl groups in E


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Scheme 5. Hydrogenolysis Outcomes of Trimethyl 1, Cyclometalated 4, and Methylidene 5 to Generate Tetrahydride 2 Presumably via Formation of Putative Dimethylhydride [NPN]TaMe2H

Figure 7. Time-dependent changes in concentration of 1, 4, and 5 upon hydrogenolysis (4 atm H2), as well as the concomitant increase in concentrations of 2 from each of the starting tantalum hydrocarbyl complexes as determined by 31P{1H} NMR spectroscopy.

[NPN]TaMe2H. As shown in Scheme 5, this could also form by selective hydrogenolysis of the cyclometalated unit of 4, and by addition of H2 across the Ta−C double bond of the Ta CH2 unit of 5. While we could not intercept or detect the putative monohydride, [NPN]TaMe2H, we could monitor the relative disappearance of the indicated starting tantalum hydrocarbyl complexes, 1, 4, and 5, in the presence of excess H2 and observe the appearance of the tetrahydride 2. This is shown graphically in Figure 7. The disappearance rates of these three species are remarkably different. Under identical concentrations and H2 pressure, trimethyl complex 1 has τ1/2 ≈ 2.5 h, cyclometalated product 4

tantalum aryloxide derivatives do cyclometalate, albeit at much higher temperatures.28

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HYDROGENOLYSIS OF TANTALUM HYDROCARBYL COMPLEXES To determine the importance of cyclometalated product 4 in the hydrogenolysis process, we devised experiments to compare relative rates of hydrogenation under similar conditions. As already mentioned, the hydrogenolysis of trimethyl 1 is a complex process involving many steps. However, one can imagine that the first product formed by hydrogenolysis of one of the Ta-Me groups would be the dimethyl-hydride, F


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Organometallics has τ1/2 < 0.5 h, and methylene 5 has τ1/2 ≈ 15 h (Figure 7). While it is tempting to invoke the first steps as involving the cyclometalated complex 4 in the hydrogenolysis of trimethyl 1, the rate of cyclometalation to form 4 in the absence of H2 likely precludes this possibility. Nevertheless, it is interesting that the rates of hydrogenolysis of these three species are different with the cyclometalated complex being the fastest likely in part due to relief of strain by hydrogenolysis of the cyclometalated unit.

referenced to an external sample of P(OMe)3 (δ 141.0 with respect to 85% H3PO4 at δ 0.0). Computational Details. The molecules were modeled as isolated molecules in vacuum. The ground state electronic wave function of each molecule was calculated self-consistently within Kohn−Sham density functional theory (DFT) using the Gaussian 09 suite of quantum chemical programs.33 All species were closed shell. Unconstrained optimization of the molecular geometry was carried out on the DFT potential energy hypersurface. Vibrational analysis was performed for each considered structure in order to characterize the nature of the stationary point. A stationary point found by a geometry optimization is a minimum (local or global following its stability) when all the vibrational frequencies are real. In contrast, it is a transition state (TS) linking two minima when there is one imaginary frequency. Gibbs free energies were obtained at T = 298.15 K. IRC calculations were performed to confirm the connections of the optimized transition states. The electronic density (at the DFT level) has been analyzed using the natural bond orbital (NBO) technique.34 A good trade-off between accuracy and computational cost was obtained by using the hybrid functional B3PW91. The small core Stuttgart−Dresden relativistic effective core potential in combination with its adapted basis set was used on Ta atoms.35,36 C, N, and H atoms were described with a 6-31G(d,p) double-ζ basis set.37 Si and P atoms were modeled with the Stuttgart−Dresden ECP in combination with its adapted basis set and additional d polarization functions.38,39 Synthesis of [PhP(CH2SiMe2NPh)(CH2SiMe2N-o-C6H4)]TaMe2 (4). To a glass reactor fitted with a Teflon needle valve containing [PhP(CH2SiMe2NPh)2]TaMe3 (1) (0.130 g, 0.20 mmol) was added 20 mL of toluene to afford a yellow solution. The vessel was sealed and heated to 70 °C for 48 h. The volatiles were removed from the resulting dark yellow solution in vacuo to give a yellow residue. The residue was recrystallized in 0.5 mL of pentane at −35 °C to generate crystalline orange 4 in 38% yield (0.048 g, 0.75 mmol). 1H NMR (400 MHz, C6D6): δ 8.27 (dd, 3JHH = 7.0, 4JHP = 1.2 Hz, 1H, CH, ometalated Ar), 7.66 (ddd, 3JHH = 9.6, 3JHH = 8.1, 4JHH = 1.5 Hz, 2H, CH, Ar), 7.36 (td, 3JHH = 7.6, 4JHH = 1.5 Hz, 1H, CH, o-metalated Ar), 7.24 (m, 8H, CH, Ar), 7.19 (m, 4H, CH, Ar), 7.14 (s, 1H, CH, Ar), 7.07 (m, 4H, CH, Ar), 7.03 (m, 2H, CH, Ar), 6.65 (dd, 3JHH = 7.6, 4 JHH = 0.9 Hz, 1H, CH, o-metalated Ar), 1.56 (br, 3H, Ta-CH3), 1.28 (m, 4H, P−CH2-Si), 0.90 (br, 3H, Ta-CH3), 0.51 (s, 3H, Si(CH3)2), 0.42 (s, 3H, Si(CH3)2), 0.16 (s, 3H, Si(CH3)2), −0.10 (s, 3H, Si(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 181.7 (d, 2JCP = 16.0 Hz, C−Ta, o-metalated Ar), 162.9 (d, 3JCP = 16.2 Hz, C−N, ometalated Ar), 145.6 (d, 3JCP = 9.5 Hz, C−N, Ar), 136.3 (d, 1JCP = 20.7 Hz, C−P, Ar), 132.1 (d, 4JCP = 11.2 Hz, CH, Ar), 131.3 (s, CH, ometalated Ar), 131.0 (s, CH, o-metalated Ar), 130.3 (s, CH, Ar), 129.0 (s, CH, Ar), 128.8 (d, 4JCP = 8.8 Hz, CH, Ar), 128.5 (s, CH, Ar), 124.9 (s, CH, Ar), 121.1 (s, CH, o-metalated Ar), 113.0 (d, 4JCP = 3.7 Hz, CH, o-metalated Ar), 15.9 (d, 1JCP = 4.6 Hz, P−CH2−Si), 13.9 (d, J = 4.8 Hz, Ta-CH3), 2.4 (s, Si(CH3)2), 1.6 (d, 3JCP = 3.0 Hz, Si(CH3)2), 1.5 (d, 3JCP = 4.0 Hz, Si(CH3)2), 1.3 (d, 3JCP = 5.5 Hz, Si(CH3)2). 31 P{1H} NMR (162 MHz, C6D6): δ 26.6. Anal. Calcd For C26H36N2PSi2Ta: C, 48.44; H, 5.63; N, 4.35. Found: C, 47.10; H, 5.59; N, 4.02. C31H49N2PSi2Ta (complex plus pentane): C, 51.87; H, 6.88; N, 3.90. Found: C, 51.53; H, 5.69; N, 4.21. Other attempts also failed: Found: C, 42.58; H, 5.51; N, 3.51; Found: C, 42.84; H, 5.67; N, 3.40; Found: C, 43.95; H, 5.64; N, 3.14. Synthesis of [PhP(CH2SiMe2NPh)2]TaCH2(Me) (5). To a glass reactor fitted with a Teflon needle valve with [PhP(CH2SiMe2NPh)2]TaMe3 (1) (0.318 g, 0.48 mmol) was added 20 mL of toluene to afford a yellow solution. The vessel was sealed and irradiated at 350 nm for 4 h. The volatiles were removed from the resulting brown solution in vacuo to give a pale brown residue. The dark brown impurities were washed away with hexanes, while the desired beige product was only sparingly soluble and was isolated by filtration in 38% yield (0.118 g, 0.18 mmol). 1H NMR (400 MHz, C6D6): δ 9.16 (d, 3JHP = 1.4 Hz, 2H, TaCH2), 8.28 (ddd, 3JHH = 10.8 Hz, 5JHP = 8.2 Hz, 4JHH = 1.4 Hz, 2H, CH, Ar), 7.27 (td, 3JHH = 7.9, 7.5 Hz, 4JHH = 1.7 Hz, 2H, CH, Ar), 7.22 (m, 4H, CH, Ar), 7.15 (m, 5H, CH, Ar), 6.93 (m, 2H, CH, Ar), 1.23 (m, 4H, P−CH2−Si), 0.58 (d, 3JHP = 1.0

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CONCLUSION We first synthesized trimethyl 1 more than 20 years ago and have used this complex countless times during this period as an entry point to generate ditantalum tetrahydride 2 and subsequent conversion to the side-on end-on dinitrogen complex 3.1,8 In this study, we have focused on the thermal and photochemical stability of 1 and have discovered that it cleanly cyclometalates to generate ortho-phenyl derivative 4 via methane elimination. DFT studies played a prominent role in urging us to examine this thermal process. In fact, the DFT optimized structure of the product closely matches that found by single crystal X-ray diffraction. We also investigated the photolysis process and found that it accesses a different manifold of reaction to generate the methyl−methylidene derivative. The effects of changing the ligand substituents on the above reactions proved profound. When the N-phenyl substituents of the NPN ligand are replaced by the bulkier N-Mesityl groups, the thermolysis of trimethyl 6 does not result in any clean product. In this case, DFT analysis did suggest a viable outcome for cyclometalation of one of the ortho-benzyl groups, but this was not observed. Probably the most significant finding in this work is the rate of hydrogenolysis of the three hydrocarbyl complexes that have N-phenyl amido units in the ligand backbone. It is noteworthy that cyclometalated complex 4 undergoes hydrogenolysis an order of magnitude faster than parent trimethyl complex 1. While it is unlikely that cyclometalation to generate 4 is an important first step in the overall hydrogenation process to produce the dinuclear complex 2, it does suggest that if cyclometalation occurs at any point in the conversion of 1 to 2 then cleavage of the cyclometalated unit will be facile. Given the deuterium labeling results, it is clear that cyclometalation events are occurring.20,32 While we cannot detect any intermediates in the conversion of 1 to 2, we can conclude that these cyclometalation processes do not impede the formation of tetrahydride 2.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise indicated, all reagents were stored in an IT glovebox housing an N2 atmosphere and all reactions were performed on a dual-manifold Schlenk line under N2. Solvents including anhydrous hexanes, toluene, diethyl ether, and tetrahydrofuran were purchased from Aldrich, sparged with N2, and dried further by passage through towers containing activated alumina and molecular sieves. Pentane was refluxed over sodium benzophenone ketyl, distilled under positive N2 pressure, degassed via several freeze−pump−thaw cycles and stored over activated sieves. THF-d8 and C6D6 were stirred over sodium benzophenone ketyl, vacuum transferred, and freeze−pump−thaw degassed. [NPN]TaMe3 (1) was prepared according to the literature reported method.1,2 NMR spectra were collected on a Bruker AV-400 MHz Spectrometer at room temperature. 1H NMR spectra were referenced to the residual solvent signal at δ 7.16 (C6D6). 13C{1H} NMR spectra were referenced to the residual solvent signal at δ 128.06 (C6D6). 31P{1H} NMR spectra were G


Article

Organometallics Hz, 3H, Ta-CH3), 0.29 (s, 6H, Si(CH3)2), −0.34 (s, 6H, Si(CH3)2). 13 C{1H} NMR (101 MHz, C6D6): δ 239.6 (s, TaCH2), 146.3 (d, 4 JCP = 10.6 Hz, CH, Ar), 136.0 (d, 1JCP = 23.8 Hz, C−P, Ar), 134.0 (d, 2 JCP = 14.9 Hz, CH, Ar), 130.8 (s, C−N, Ar), 129.6 (s, CH, Ar), 129.2(s, CH, Ar), 128.9 (d, 4JCP = 9.6 Hz, CH, Ar), 124.3 (CH, Ar), 51.1 (d, 2JCP = 29.7 Hz, Ta-CH3), 17.3 (d, 1JCP = 3.3 Hz, P−CH2−Si), 4.0 (d, 3JCP = 2.2 Hz, Si(CH3)2), 0.2 (d, 3JCP = 5.0 Hz, Si(CH3)2). 31 P{1H} NMR (162 MHz, C 6D6): δ 30.8. Anal. Calcd for C26H36N2PSi2Ta: C, 48.44; H, 5.63; N, 4.35. Found: C, 44.57; H, 5.53; N, 3.41. Despite several attempts (Found: C, 44.30; H, 5.07; N, 4.08; Found: C, 44.08; H, 5.15; N, 3.61), results for elemental analysis did not improve. Similar combustion analysis problems with transition metal carbene complexes have been reported previously.40,41 Synthesis of 1-(Chloromethyl)-N-Mesityl-1,1-dimethylsilanamine. To a flask charged with freshly distilled mesitylamine (6.10 mL, 43 mmol) were added 20 mL of diethyl ether and 10 mL of THF. Upon cooling to 0 °C, a 1.6 M solution of n-butyllithium (27.13 mL, 43 mmol) was added dropwise and stirred for 1 h. The reaction mixture was cannulated to a separate flask containing a THF solution (10 mL) of chloro(chloromethyldimethyl)silane (5.72 mL, 43 mmol) cooled to −78 °C. Upon warming to room temperature, white precipitate formed in the reaction mixture. After stirring for an additional 3 h at room temperature, the volatiles were removed in vacuo, the product was extracted into hexanes and filtered through Celite. Solvent removal in vacuo resulted in a pale brown liquid which was purified by distillation (2×) at 85 °C and 2 × 10−1 Torr. The colorless liquid product was isolated in 84% yield (8.82 g, 36 mmol). 1 H NMR (400 MHz, C6D6): δ 6.77 (s, 2H, Mes, CH, Ar), 2.56 (s, 2H, Si(CH3)2CH2Cl), 2.15 (s, 3H, Mes, p-CH3), 2.09 (s, 6H, Mes, o-CH3), 2.00 (br, 1H, NH), 0.06 (s, 6H, Si(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 139.2 (s, C−N, Ar, Mes), 132.9 (s, o-C−CH3, Ar, Mes), 131.9 (s, p-C−CH3, Ar, Mes), 129.2 (s, CH, Ar, Mes), 31.5 (s, Si(CH2)Cl), 20.7 (s, p-CH3, Mes), 19.5 (s, o-CH3, Mes), −2.9 (s, Si(CH3)2). HRMS (EI) calculated for C12H20NSiCl (M+) 241.10536, found 241.10506. Synthesis of [PhP(CH2SiMe2NMes)2]Li2THF4. To a flask charged with 1-(chloromethyl)-N-Mesityl-1,1-dimethylsilanamine (5.21 g, 0.022 mol) and phenylphosphine (1.19 g, 0.011 mmol) was added 40 mL of Et2O. The flask was cooled under a positive pressure of argon to 0 °C, and n-butyllithium solution (1.6 M, 27.0 mL, 0.043 mol) was added dropwise over 0.5 h. The reaction mixture was stirred at 0 °C for 3 h, and the volatiles were removed in vacuo. The resulting yellow residue was extracted with toluene and filtered through Celite, and the solvent was removed in vacuo. To crystallize the resulting residue, 10 mL of hexanes was added, followed by 3.5 mL (4 equiv) of THF and the mixture being cooled to −35 °C overnight. The resulting crystals were isolated by filtration and washed with cold hexanes to yield a colorless crystalline solid in 80% yield (5.94 g, 0.009 mol). 1H NMR (400 MHz, C6D6): δ 7.80 (t, J = 7.8, 7.8 Hz, 2H, CH, P−Ph, meta), 7.35 (t, J = 7.4, 7.4 Hz, 2H, CH, P−Ph, ortho), 7.26 (d, J = 6.3 Hz, 1H, CH, P−Ph, para), 6.94 (s, 4H, N-Mes, CH, Ar, meta), 3.51 (m, 16H, O−CH2, THF), 2.35 (s, 12H, N-Mes, ortho-CH3), 2.33 (s, 6H, N-Mes, para CH3), 1.44 (m, 16H, CH2, THF), 0.43 (s, 6H, Si(CH3)2), 0.33 (s, 6H, Si(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 153.6 (s, C−N, Ar, N-Mes), 144.7 (d, J = 10.2 Hz, d, C−P, P−Ph), 131.4 (d, J = 17.4 Hz, s, CH, P−Ph, meta), 130.8 (s, N-Mes, C−CH3, ortho), 129.5 (s, N-Mes, CH, Ar, meta), 128.5 (d, J = 7.2 Hz, CH, P− Ph, ortho), 128.2 (s, CH, P−Ph, para), 124.9 (s, N-Mes, C−CH3, para), 67.8 (s, O−CH2, THF), 25.5 (s, CH2, THF), 20.6 (s, N-Mes, CH3 para), 20.5 (s, N-Mes, CH3, ortho), 5.2 (d, J = 2.5 Hz, Si(CH3)2), 3.2 (s, Si(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ −35.6. Anal. Calcd for C46H75N2O4PSi2Li2: C, 67.29; H, 9.21; N, 3.41. Found: C, 67.04; H, 9.69; N, 3.23. Synthesis of [PhP(CH2SiMe2NMes)2]TaMe3 (6). To a flask appended to a swivel frit was added [PhP(CH2SiMe2NMes)2]Li2THF4 (0.494 g, 0.60 mmol). The neck of the swivel frit was charged with TaMe3Cl2 (0.178 g, 0.60 mmol), and the vessel was placed under vacuum. After cooling the flask to −78 °C, 20 mL of diethyl ether was vacuum transferred into the flask. The reagent solution was stirred for

5 min at which point the frit neck was cooled to vacuum transfer diethyl ether onto the TaMe3Cl2 reagent and rinse the material into the reaction mixture. The now yellow reaction mixture was stirred at −78 °C for 5 h, and the volatiles were removed in vacuo while being warmed to room temperature. Hexanes was added via vacuum transfer to the crude material to extract the product, which was filtered through the frit followed by solvent removal in vacuo to yield an orange residue. The resulting residue was washed with cold pentane to yield a beige solid in 72% yield (0.324 g, 0.43 mmol). 1H NMR (400 MHz, C6D6): δ 7.87 (m, 2H, CH, Ar, ortho-P−Ph), 7.25 (m, 3H, CH, Ar, PPh), 6.98 (d, J = 2.2 Hz, 2H, N-Mes, CH, Ar, meta), 6.92 (d, J = 2.2 Hz, 2H, NMes, CH, Ar, meta), 2.51 (s, 6H, N-Mes, CH3, Ar), 2.41 (s, 6H, NMes, CH3, Ar), 2.26 (s, 6H, N-Mes, CH3, Ar), 1.51 (m, 4H, Me2Si− CH2−PPh), 1.49 (m, 6H, Ta-CH3), 1.11 (s, 3H, Ta-CH3), 0.26 (s, 6H, Si(CH3)2), −0.02 (s, 6H, Si(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 148.8 (d, J = 8.5 Hz, C−N, Ar, N-Mes), 137.9 (d, J = 20.0 Hz, C−P, Ar, PPh), 134.9 (s, N-Mes, C−CH3, Ar), 133.5 (s, N-Mes, C−CH3, Ar), 132.7 (s, N-Mes, C−CH3, Ar), 132.2 (d, J = 2.5 Hz, CH, Ar, PPh), 132.0 (d, J = 10.2 Hz, CH, Ar, PPh), 129.9 (s, N-Mes, CH, Ar, meta), 129.7 (s, N-Mes, CH, Ar, meta), 128.6 (d, J = 8.2 Hz, CH, Ar, PPh), 80.2 (d, J = 14.9 Hz, Ta-CH3), 78.8 (s, br, Ta-CH3), 67.3 (s, br, Ta-CH3), 21.6 (s, N-Mes, CH3, Ar), 21.4 (s, N-Mes, CH3, Ar), 20.6 (s, N-Mes, CH3, Ar), 15.2 (d, J = 4.1 Hz, Me2Si-CH2−PPh), 4.9 (d, J = 4.6 Hz, Si(CH3)2), 1.6 (d, J = 3.0 Hz, Si(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 22.5. Anal. Calcd for C33H52N2PSi2Ta: C, 53.21; H, 7.04; N, 3.76. Found: C, 53.70; H, 7.41; N, 3.91. Relative Hydrogenolysis Experiments to Synthesize 2 from 1, 4, and 5. To three J. Young NMR tubes were added 1 (0.015 g, 0.023 mmol), 4 (0.015 g, 0.023 mmol), and 5 (0.015 g, 0.023 mmol) respectively. To each tube were added a flame-sealed capillary tube containing 25 μL of phosphoric acid, 0.60 mL of C6D6, and 1 drop of diethyl ether. All samples were either yellow or yellow/orange initially. 31 1 P{ H} and 1H NMR spectra of each tube prior to further perturbation was recorded. Each sample was degassed at 5 × 10−5 Torr with three iterations of freeze/pump/thaw. Upon charging each tube with 4 atm of ultrahigh purity hydrogen gas (1 atm of H2 added to the tube cooled to −197 °C, ideal gas law estimates roughly 19 equiv based on the headspace of the sealed tube at room temperature) and mixing by constant vertical rotation, 31P{1H} and 1H NMR spectra were recorded at 0.5, 1, 2.5, 4, 18, and 24 h. After 18 h, all three samples were deep purple in color. All integrals are relative to the internal−external reference. Stacked spectra plots are shown in Figures S1−S3.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00487. Full experimental procedures, representative NMR spectra, and crystallographic data (PDF) Crystallographic data (MOL) Accession Codes

CCDC 1560032−1560033 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 604 822-2847. Tel.: +1 604 822-2897. ORCID

Michael D. Fryzuk: 0000-0002-3844-098X H


Article

Organometallics Notes

(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (34) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926. (35) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408−3420. (36) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta. 1990, 77, 123−141. (37) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (38) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuß, H. Mol. Phys. 1993, 80, 1431−1441. (39) Höllwarth, A.; Böhme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 237−240. (40) Voges, M. H.; Rømming, C.; Tilset, M. Organometallics 1999, 18, 529−533. (41) Gründemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree, R. H. Organometallics 2001, 20, 5485−5488.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.D.F. thanks the NSERC of Canada for a Discovery Grant. REFERENCES

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