Syntheses and Characterization of Tantalum Alkyl Imides and Amide

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Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α‑SiMe3 Abstraction by an Amide Ligand Seth C. Hunter,† Shu-Jian Chen,† Carlos A. Steren,† Michael G. Richmond,‡ and Zi-Ling Xue*,† †

Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996, United States Department of Chemistry, The University of North Texas, Denton, Texas 76203, United States

‡

S Supporting Information *

ABSTRACT: Reaction of TaCl2(NSiMe3)[N(SiMe3)2] (1) with alkylating reagents form the alkyl amide imide complexes TaR2(NSiMe3)[N(SiMe3)2] (R = Me (2), CH2Ph (3)) and mixed amide imide compounds Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)). The reaction of 2 and 0.5 equiv of O2 leads to preferential oxygen insertion into one Ta−Me bond, yielding the alkoxy-bridged alkyl dimer Ta2(μ-OMe)2Me2(NSiMe3)2[N(SiMe3)2]2 (6) as cis and trans isomers. Crystallization of the cis-6 and trans-6 mixture gave only crystals of trans-6. When the crystals of trans-6 were dissolved in benzene-d6, conversion of trans-6 to cis-6 occurred until the trans-6 ⇌ cis-6 equilibrium was reached with Keq = 0.79(0.02) at 25.0(0.1) °C. Kinetic studies of the exchange gave the rate constants k = 0.018(0.002) min−1 for the trans-6 → cis-6 conversion and k′ = 0.022(0.002) min−1 for the reverse cis-6 → trans-6 conversion at 25.0(0.1) °C. Complex 6 reacts with additional O2, forming the dialkoxy dimer Ta2(μ-OMe)2(OMe)2(NSiMe3)2[N(SiMe3)2]2 (7) as cis and trans isomers. Solid-state structures of 3 and trans-6 have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) have also been prepared by salt metathesis reactions employing TaCl3[N(SiMe3)2]2 (8). The pathway from 8 to 4 and 5 eliminates Me3Si−NR′2 (R′ = Me, Et), converting the amide N(SiMe3)2 ligand to the imide NSiMe3 ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and α-elimination involving the mixed amide species TaCl2(NMe2)[N(SiMe3)2]2 (9) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of 1−6 and 8 show that only the alkoxy-bridged cis-6 and trans-6 are dimers in benzene-d6 solution at 25 °C.

T

of Me3Si−NMe2 to form the imide dimer [Ta(NMe2)3(μ-NSiMe3)]2, as discussed below.9 Very little is known about the reactions of d0 transition-metal complexes with O2, which is in part due to their air sensitivity, requiring work under inert gases and vacuum.10 Our group has studied the reaction between the alkyl amide imide complex Ta(CH2But)2(NSiMe3)[N(SiMe3)2] and O2.10v Selective oxygen insertion into the two Ta−alkyl bonds was observed, yielding the alkoxide complex Ta(NSiMe3)[N(SiMe3)2](OCH2But)2. Ta(NSiMe3)[N(SiMe3)2](OCH2But)2 undergoes a ligand exchange to give Ta2(μ-NSiMe3)2(OCH2But)6 and Ta(NSiMe3)[N(SiMe3)2]2(OCH2But). We have prepared TaR2(NSiMe3)[N(SiMe3)2] (R = Me (2), CH2Ph (3)) and Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) from TaCl2(NSiMe3)[N(SiMe3)2] (1). The reaction between O2 and 2 has been studied. 4 and 5 have also been prepared from the reaction of TaCl3[N(SiMe3)2]2 (8) with LiNR′2 (R′ = Me, Et) through the abstraction of an α-SiMe3

he chemistry of early-transition-metal imide complexes is an active area of research, due to their potential uses as catalysts and as precursors in the preparation of microelectronic materials.1−3 Tantalum imide complexes in particular have been used as precursors for Ta2O53,4 and TaN5 thin films in chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. Imide ligands have been prepared by a variety of methods, mostly through intermolecular imidation.1,6,7 These intermolecular imidation reactions are achieved with primary amines, imines, nitriles, and other nitrogen-containing compounds.1,6 The rarer intramolecular imidation is generally conducted through 1,2-elimination of Me3SiCl or interligand transfer.6,7 Thermolysis of TaCl3[N(SiMe3)2]2 (8), for example, yields Me3SiCl and TaCl2(NSiMe3)[N(SiMe3)2] (1).6h A new type of intramolecular imidation by α-Si abstraction was reported by Herrmann and Baratta in the reaction of M(NMe2)5 (M = Nb, Ta) with [(η5-C5H4)−Si(CH3)2−(HNC6H5)].8 The Si−N bond is cleaved upon optical excitation by visible light to form an imide complex. Our group recently reported the second case of α-Si abstraction in the pentaamide Ta(NMe2)4[N(SiMe3)2] (10), which upon heating undergoes elimination © XXXX American Chemical Society

Received: June 26, 2015

A

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observed at 459.1499. These NMR features and the observation of [2 + H+] in the MS spectrum are consistent with a monomeric structure of 2. DFT calculations were performed on 2 (species A), and a fourcoordinate tantalum species possessing idealized tetrahedral geometry was confirmed as the ground-state minimum. The optimized structure of A is shown in Figure 1; a bond angle of

group by an amide ligand, an unusual intramolecular imidation reaction. The bonding in 2, 3, and trans-6 has been investigated by electronic structure calculations, and the imidation reaction originating from 8 has been computationally examined.

■

RESULTS AND DISCUSSION Preparation and Characterization of 2−5. Reaction of 2 equiv of MeLi with TaCl2(NSiMe3)[N(SiMe3)2] (1) gave TaMe2(NSiMe3)[N(SiMe3)2] (2) as a yellow liquid in 20.3% yield through distillation at 85−86 °C (0.05 mmHg vacuum; Scheme 1). A significant portion of the crude product Scheme 1. Preparation of 2−5

Figure 1. M06-optimized structure of A (2).

168.3° for the Ta−Nimide−Si linkage and a Ta−Nimide distance of 1.783 Å were verified. The pairwise, equivalent Ta−CH3 bonds display a mean distance of 2.186 Å. The sum of the angles about the Namide center of the ancillary N(SiMe3)2 group is nearly 360°, confirming the existence of a planar nitrogen atom that donates its electrons through both σ and π manifolds. When the lone-pair electron contributions from the imide and amide ligands are included, A contains 14e and is highly unsaturated. Ta(CH2Ph)2(NSiMe3)[N(SiMe3)2] (3) was prepared from the reaction of 1 with 2 equiv of ClMgCH2Ph at −30 °C (Scheme 1). Crystals of 3 were obtained from saturated pentane solutions at −33 °C. Chemical shifts in the 1H, 13C{1H}, and 29 Si{1H} NMR spectra of 3 in benzene-d6 are given in Table 1. The 1H NMR spectrum of 3 in benzene-d6 (Figure S5 in the Supporting Information) shows that the CHaHb moiety on the benzyl ligand appears as a diastereotopic pair (Table 1), as demonstrated by a Newman projection of the molecule in Chart 1. An ORTEP view of the crystal structure of 3 is given in Figure 2 along with selected bond lengths and angles. The metal center adopts a tetrahedral geometry. The Ta−N(1) bond length of 1.768(8) Å of the imide NSiMe3 ligand and the Ta−N(2) length of 1.986(7) Å of the amide N(SiMe3)2 ligand are very similar to those (1.770(6) and 1.981(7) Å, respectively) in our previously reported Ta(CH2But)2(NSiMe3)[N(SiMe3)2].10v

decomposed during the distillation. The liquid contains unknown impurities, and 2 decomposes with storage. We were not able to obtain 2 as a liquid pure enough for elemental analysis. Complex 2 was characterized by 1H, 13C{1H}, and 1H−29Si HMBC (heteronuclear multiple bond correlation) NMR and MS spectroscopy. The identity of 2 was further confirmed through the characterization of Ta 2 (μ-OMe) 2 Me 2 (NSiMe 3 ) 2 [N(SiMe3)2]2 (6), the product of the reaction between 2 and 0.5 equiv of O2 discussed below. Chemical shifts in the 1H, 13C{1H}, and 29Si{1H} NMR spectra of TaMe2(NSiMe3)[N(SiMe3)2] (2) in benzene-d6 are given in Table 1. The 13C{1H} and 29Si{1H} shifts of the imide NSiMe3 ligand are shifted upfield from those of the amide N(SiMe3)2 ligand, as observed in other Ta compounds in the current studies (Table 1). HR-MS of [2 + H+] (Figure S4 in the Supporting Information), calculated at m/z 459.1510, was

Table 1. Selected NMR Chemical Shifts (ppm) for 2−5 and cis-6/trans-6 in Benzene-d6 at 25 °C Unless Noted 1

13

H NSiMe3 N(SiMe3)2

compd.

29

C

Si

NSiMe3 a

2 3

0.57 (Me) 2.34, 2.01 (CHaHb)

0.37 0.30

0.32 0.23

4 5

0.31 0.32

0.31 0.38

cis-6

3.13 (NMe2) 3.49−3.36 (CH2), 1.09 (CH2−CH3) 3.86a (OMe), 0.65a (Me)

0.33a

0.40a

52.94 (Me) 64.92a (CH2, 1JH−C = 129 Hz), 138.80a (Cipso) 47.20 (NMe2) 48.25 (CH2−CH3), 16.44 (CH2) 64.16a (OMe), 41.61a (Me)

trans-6

3.78a (OMe), 0.62a (Me)

0.31a

0.42a

63.09a (OMe), 42.21a (Me)

N(SiMe3)2

NSiMe3

N(SiMe3)2

2.70 2.97a

4.41 4.50a

−7.59 −7.52

−0.66 0.00

3.94 3.69

5.58 5.64

−9.67 −10.56

−0.66 −0.72

3.47a

4.39a (broad, overlapping) 4.39a (broad, overlapping)

a

3.55a

a

−7.20b (overlapping) −7.20b (overlapping)

−0.59b,c (overlapping) −0.59b,c (overlapping)

23 °C b40 °C cIn the 1H−29Si HMBC spectrum of a mixture of cis-6 and trans-6 at 25 °C, the N(SiMe3)2 ligands of both isomers give two broad peaks centered at ca. 4.1 and −4.6 ppm, respectively, in the 29Si projection, suggesting dynamic processes, most likely the slow rotation of the bulky ligands, in the complexes. At 40 °C, the 1H−29Si HMBC spectrum shows an overlapping coalesced peak at −0.59 ppm in the 29Si projection (Figure S21 in the Supporting Information). a

B

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Ta(1)−C(12) and Ta(1)−C(18) vectors. The magnitude of each value is comparable to those values reported for such groups, and these data reinforce the coordinative flexibility of the ancillary benzyl ligands in 3.13 The amide analogues Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) were prepared from the addition of a measured excess of LiNR′2 (R = Me, Et) to a stirred solution of 1 in Et2O at −30 °C. After filtration and removal of solvents, 4 and 5 were obtained in 81% and 90% yields, respectively, as liquids at room temperature. These same products were also obtained by the addition of solid LiNR′2 in either toluene or pentane as the reaction solvent at room temperature, provided the mixtures were stirred for several days. The liquid of 4 was found to be adequately pure for elemental analysis. Attempts to crystallize 4 at either −30 or −55 °C did not give its crystals. In the crude product of liquid 5, HN(SiMe3)2 was found to be an impurity, which is common in complexes using N(SiMe3)2 as ligands. The removal of HN(SiMe3)2 from the liquid of 5 through evacuation at several temperatures (0.05 mmHg) did not work. At 23 °C, HN(SiMe3)2 could not be completely removed. For example, at 70 °C extensive decomposition of 5 was observed, turning the liquid into an unknown solid. Thus, we were not able to obtain 5 as a liquid pure enough for elemental analysis. Complexes 4 and 5 were characterized by 1H, 13C{1H}, and 29 Si{1H} NMR and HR-MS (Figures S8−S15 in the Supporting Information). Chemical shifts in the 1H, 13C{1H}, and 29Si{1H} NMR spectra of Ta(NR′)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) in benzene-d6 are given in Table 1. HR-MS of [4 + H+], calculated at m/z 517.20407, was observed at 517.20465 (Figure S11). The 1H NMR spectrum of Ta(NEt2)2( NSiMe3)[N(SiMe3)2] (5) in benzene-d6 (Figure S12 in the Supporting Information) shows that the CH2 protons of the N(CHaHbCH3)2 ligands are diastereotopic and appear as a multiplet at 3.49−3.36 ppm. The Newman projection of 5 in Chart 1 shows the diastereotopic feature of the CHaHb moiety. Since the prochiral CHaHb moiety in 5 is farther away from the prochiral Ta center than that in 3, the Ha and Hb chemical shift difference is much smaller than that in 3. The peaks at 1.09 ppm are readily assigned to the CH3 groups associated with the NEt2 ligands. In the 13C{1H} NMR spectrum of 5 in benzene-d6 (Figure S13 in the Supporting Information), the CH2 and CH3 moieties of the NEt2 ligands appear at 16.44 and 48.25 ppm, respectively. HR-MS of [5 + H+] (Figure S15 in the Supporting Information), calculated at m/z 573.26667, was observed at 573.26569. Formation of Ta2(μ-OMe)2Me2(NSiMe3)2[N(SiMe3)2]2 (6) from the Reaction of 2 with 0.5 Equiv of O2, trans-6 ⇌ cis-6 Isomerization, and the X-ray Crystal Structure of trans-6. The reaction of O2 (0.5 equiv) with TaMe2( NSiMe3)[N(SiMe3)2] (2) led to the formation of the methoxy methyl dimer 6 as a cis-6 and trans-6 mixture (Scheme 2), which gave trans-6 as crystals suitable for X-ray diffraction analysis. In fact, 2 is very sensitive to trace O2 in nitrogen gas, forming cis-6 and trans-6 in solutions. Oxygen selectively inserts into one Ta−Me bond in 2. Such selective oxygen insertion is known.14 For example, Mo(N-2,6-Pri2C6H3)2Me2 reacts with O2 to form Mo2(N-2,6-Pri2C6H3)4Me2(μ-OMe)214a and (cyclo)Si(NBut)2TiMe2 reacts with O2 to furnish [(cyclo)Si(NBut)2Ti(μ-OMe)Me]2.14b A solution of trans-6 in benzene-d6, prepared from crystals of trans-6, slowly converted to its cis isomer cis-6, eventually reaching an exchange equilibrium, trans-6 ⇌ cis-6, with Keq = 0.79(0.02) at

Chart 1. Newman Projections of 3 and 5, Showing Diastereotopic H Atoms

Figure 2. (top) ORTEP view of Ta(CH2Ph)2(NSiMe3)[N(SiMe3)2] (3, left) and the M06-optimized structure B (right). (bottom) Two coordination modes of the benzyl ligands in 3. Selected X-ray diffraction bond distances (Å) and angles (deg): Ta(1)−N(2), 1.986(7); Ta(1)− N(1), 1.768(8); Ta(1)−C(10), 2.201(9); Ta(1)−C(11), 2.219(9); Si(2)−N(2)−Ta(1), 116.8(4); Si(1)−N(1)−Ta(1), 167.5(5); C(12)− C(10)−Ta(1), 86.7(5); C(18)−C(11)−Ta(1), 113.2(6). The bond distance (Å) displayed in B is for the Ta(1)−C(12) vector.

The Ta−C(10)−C(12) bond angle of 86.7° on one of the benzyl ligands suggests an η2 coordination to the metal center (Figure 2, bottom). This is reasonable, as the metal center is electron deficient. It is also in agreement with the observations of Parkin and co-workers on the binding modes of benzyl ligands.11 NMR features of 3 are consistent with the presence of the η2 coordination mode. In the η1 and η2 modes, C−H coupling constants for the −CH2− groups are ∼120 and ∼135 Hz, respectively, and chemical shifts for the Cipso atoms are around 150 and 140 ppm, respectively.11 In 3, 1JH−C = 129 Hz for the −CH2− groups and the 138.80 ppm chemical shift for the Cipso atoms (Table 1) support the η2-benzyl bonding mode. DFT calculations were performed on 3 in order to probe the coordinative preference of the ancillary benzyl ligands. The DFT-optimized structure of 3 (species B) is shown alongside the solid-state structure, and excellent agreement exists between the two structures. The presence of distinct η1 and η2 benzyl groups in B is confirmed on the basis of the angles of 88.6 and 108.0° for the C(12)−C(10)−Ta(1) and C(18)− C(11)−Ta(1) linkages, respectively.12 Using Andersen’s criteria for hapticity, we computed δipso values of 0.42 and 0.81 for the C

DOI: 10.1021/acs.organomet.5b00558 Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Synthesis of a cis-6/trans-6 Mixture, Its Crystallization To Give trans-6 Only, and the trans-6 ⇌ cis-6 Isomerization

25.0(0.1) °C (Scheme 2). It should be noted that, in cis-6, DFT calculations discussed below show that only the two methyl ligands are pointing in the same direction. Its imide and amide ligands remain mutually trans so the molecule maintains the overall C2 symmetry. 1H NMR experiments for the trans-6 ⇌ cis-6 exchange show that it follows first-order reversible kinetics (eqs 1 and 2),15 ⎡ I (e) − Icis‐6(t ) ⎤ ln⎢ cis‐6 ⎥ = −(k + k′)t ⎣ Icis‐6(e) − Icis‐6(0) ⎦ Keq =

k [cis‐6] = k′ [trans‐6]

(1)

(2)

where Icis‑6(0), Icis‑6(t), and Icis‑6(e) are the integrations of cis-6 at time t = 0, t = t, and equilibrium, respectively, and k and k′ are the rate constants for the forward and reverse reactions, respectively. One kinetic plot for the exchange is shown in Figure S16 in the Supporting Information. The kinetic studies gave k + k′ = 0.041(0.003) min−1 at 25.0(0.1) °C, leading to the computed values of k = 0.018(0.002) min−1 and k′ = 0.022(0.002) min−1 at this temperature. A similar trans-7 ⇌ cis-7 exchange was reported for Ta2(μ-OMe)2(OMe)2(NSiMe3)2[N(SiMe3)2]2 (7).6h 1 H, 13C{1H}, and 29Si{1H} NMR chemical shifts of cis-6 and trans-6 are given in Table 1. The 1H NMR spectra of trans-6 and a cis-/trans-6 mixture at equilibrium are given in Figures S17 and S18, respectively, in the Supporting Information. The assignments of the 13C{1H} NMR spectrum of the cis-6/trans-6 mixture in benzene-d6 (Figure S19 in the Supporting Information) were assisted by HSQC NMR (Figure S20 in the Supporting Information). For cis-6, the OMe peak was not directly observed in the 13C{1H} NMR spectra (Figure S19), but the HSQC spectrum (Figure S20) indicated that it is a broad peak at 64.16 ppm overlapping with the OMe peak in trans-6. The N(SiMe3)2 ligands for both isomers appear to undergo an exchange, as their 1H, 13C, and 29Si{1H} NMR peaks are broad. Their 29Si{1H} NMR peaks are very broad and overlapping, covering the −6.5 to +5.9 ppm range. The trans-6 ⇌ cis-6 exchange occurs at rates much slower than the NMR time scale. 1 H exchange spectroscopic (EXSY) studies of a cis-6/trans-6 mixture support our kinetic results, as EXSY revealed no exchange between the two isomers on the NMR time scale. The NMR peak broadening is likely due to restricted rotation of the bulky N(SiMe3)2 ligands in the dimeric cis-6/trans-6. HR-MS of [6 + H+] (Figure S22 in the Supporting Information) gave the monomer at m/z 475.14389 (calculated at m/z 475.14589). The ORTEP view of trans-6 is shown in Figure 3 along with selected bond angles and lengths. The structure of trans-6

Figure 3. ORTEP view of trans-6. Selected bond lengths (Å) and angles (deg): Ta(01)−N(2), 1.780(2); Ta(01)−N(1), 1.9967(19); Ta(01)− O(1), 2.1244(16); Ta(01)−O(1A), 2.1376(16); Ta(01)−C(1), 2.199(2); O(1)−Ta(01)−C(1), 86.24(8); O(1A)−Ta(01)−C(1), 149.89(8); Ta(01)−O(1)−Ta(0A), 112.06(7); O(1)−Ta(01)−O(1A), 67.95(7).

consists of two equivalent Ta centers, each adopting a distortedtrigonal-bipyramidal environment. One methoxy group occupies an axial site, and the other methoxy group exhibits an equatorial disposition in the edge-fused union of the two trigonal-bipyramidal units. The bridging methoxy groups symmetrically bind the two metals in trans-6. Bond lengths of the Ta−N imide and amide bonds in trans-6, 1.780 and 1.996 Å, respectively, are similar to those in the methoxy dimer analogue Ta 2(μ-OMe) 2(OMe)2 (NSiMe 3) 2[N(SiMe3 )2 ]2 (trans-7; 1.777, 2.023 Å).6h We computationally evaluated the different possible structures for 6 in an effort to establish the identity of the participant isomers in the solution equilibrium. The most stable isomer computed is species D1, whose structure shows an excellent correspondence to the solid-state structure of trans-6. The next most stable isomer is species D2, which lies 2.8 kcal/mol above D1. Species D2 exhibits idealized C2 symmetry where the amido and methyl groups are situated at basal sites at the squarepyramidal tantalum centers. The imido groups occupy axial sites in the MeO-bridged polyhedron. Given the cis disposition of the N(SiMe3)2 and Me groups, we designate this isomer as cis-6 for discussion purposes. Other stereoisomers based on 6 were examined by DFT, but they were found to be considerably less stable or collapsed to D1 or D2 during optimization. D

DOI: 10.1021/acs.organomet.5b00558 Organometallics XXXX, XXX, XXX−XXX

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Figure 4. DFT-optimized structures for TaMe(OMe)(NSiMe3)[N(SiMe3)2] (C) and the cis and trans isomers of 6 (D1 and D2). The quoted free energies in the monomer−dimer equilibria are in kcal/mol relative to 2 equiv of species C.

confirmed by 1H NMR and GC-MS. Typically, intermolecular imidation is more common for the formation of imide ligands.1,6,7 Intramolecular imidation typically occurs through 1,2-elimination of Me3SiCl or interligand transfer. This is, to our knowledge, the third case of α-SiMe3 abstraction by an amide ligand.8,9 For the intramolecular imidation in Ta(NMe2)4[N(SiMe3)2] (10), through α-SiMe3 abstraction by NMe2, to form [Ta(NMe2)3(μ-NSiMe3)]2, the half-life at room temperature is 7 days (Scheme 5).9 In comparison, the current reactions to form 4 and 5 in Scheme 4 are completed nearly instantaneously. We reported earlier that the reaction of TaCl(NMe2)3[N(SiMe3)2]

The bonding in the isomeric dimers D1 and D2 and their ground-state stability relative to that of the monomeric species TaMe(OMe)(NSiMe3)[N(SiMe3)2] have been investigated by DFT calculations. The DFT-optimized structures for the pertinent monomer (C) and the dimer (D1 and D2) equilibria are depicted in Figure 4. The computed features for dimer D1 closely mirror those bond distances and angles found in the crystallographic structure, and the pairwise equivalent ligands located on the adjacent metal centers are related to each by inversion through the midpoint of the molecule. The computed bond distances and angles in D2 are unexceptional with respect to D1. Monomer C exhibits idealized tetrahedral symmetry, and the subtended angles at tantalum range from 102.4° (C−Ta−Nimide) to 118.8° (Namide−Ta−O). Dimer D1 is energetically favored over monomer C by 17.5 kcal/mol. This particular reaction is not unlike that recently computed by us for the equilibrium between Ta(NMe2)3(NSiMe3) and [Ta(NMe2)3(NSiMe3)]2, where the free energy change lies in favor of the dimer (ΔG = −10.3 kcal/mol).9b The reaction of O2 with the cis-6/trans-6 mixture led to oxygen insertion into the remaining Ta−Me bonds in cis-6/trans-6 to give a mixture of cis-/trans-dimethoxy dimers Ta2(μ-OMe)2(OMe)2(NSiMe3)2[N(SiMe3)2]2 (cis-7/trans-7; Scheme 3)

with LiN(SiMe3)2 gave (Me2N)3TaN(SiMe3)SiMe2CH2 with a four-membered metallaheterocyclic ring through γ-H abstraction in Ta(NMe2)3[N(SiMe3)2]2.16 Thus, clearly the current reaction of TaCl3[N(SiMe3)2]2 (8) with 3 equiv of LiNMe2 (Scheme 4), forming the imide Ta(NMe2)2(NSiMe3)[N(SiMe3)2] (4), does not involve Ta(NMe2)3[N(SiMe3)2]2 as an intermediate. We tested if an intermediate could be observed in the reaction of 8 with 1 equiv of amide LiNR′2(R′ = Me, Et). The reactions gave a mixture of 4 or 5 along with unreacted 8 and TaCl2( NSiMe3)[N(SiMe3)2] (1). Neither of the monosubstituted species TaCl2(NR′2)[N(SiMe3)2]2 (R′ = Me (9), Et) were observed in those reactions monitored by 1H NMR spectroscopy. The possibility of a direct attack of LiNMe2 on a silyl group associated with one of the N(SiMe3)2 moieties in 8 to furnish Me3SiNMe2 and dimer 1 (0.5 equiv), the latter of which could then yield 4 through successive LiNMe2 metatheses of the existing chlorides, was explored by DFT calculations. The reaction profiles computed for different step-scan calculations revealed highly unfavorable energies (>48 kcal/mol). Parallel calculations using NMe2− as the attacking nucleophile failed to give suitable transition-state structures, leading us to abandon these pathways as potential manifolds to compounds 4 and 5. Taken collectively, these data suggest that the mixed amides TaCl2(NR′2)[N(SiMe3)2]2 (R′ = Me, Et) are not stable and an α-SiMe3 group is abstracted by the newly incorporated amide to form 1. Details related to the stability and reactivity of the putative intermediates TaCl2(NMe2)[N(SiMe3)2]2 (9) were addressed computationally for the species derived from LiNMe2. The relative stabilities of the ground-state isomers of TaCl2(NMe2)[N(SiMe3)2]2 (E1−E3; Figure 5) were evaluated, and three structures with a trigonal-bipyramidal geometry were found to be stable. Structures based on a square-pyramidal geometry were unstable and collapsed to either species E1 or E2 during optimization. Predictably, species E1, whose three amide ligands occupy equatorial sites, is thermodynamically preferred. The isomeric species E2 and E3 contain axially disposed NMe2 and

Scheme 3. Reaction of the cis-6/trans-6 Mixture with O2 Leading to Oxygen Insertion into Ta−Me Bonds To Give a cis-7/trans-7 Mixture

reported by Bradley and co-workers earlier.6h They have prepared 7 from the reaction of LiOMe with TaCl2( NSiMe3)[N(SiMe3)2] (1). Substitution of the Cl ligands in 1 by the OMe− anion gave 7. In contrast, our preparation of 7 involves sequential oxidation of the methyl ligands in TaMe2( NSiMe3)[N(SiMe3)2] (2) by O2, first yielding methyl methoxy 6 and then dimethoxy 7. Studies of the Intramolecular Imidation To Form 4 and 5. Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) were initially prepared from TaCl2(NSiMe3)[N(SiMe3)2] (1) and LiNR′2 (R = Me, Et; Scheme 1), as discussed earlier. These two compounds have also been prepared from the reactions of TaCl3[N(SiMe3)2]2 (8) with 3 equiv of LiNR′2 (R = Me, Et). The solutions were filtered, and upon removal of solvent in vacuo, liquid 4 and 5 were obtained (Scheme 4) along with silyl amines Me3Si−NR′2, which were E

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Organometallics Scheme 4. (Top) Preparation of 4 and 5a and (Bottom) Proposed Pathway in the Formation of 4/5 and 1

a

In the reaction with 1 equiv of the amide, TaCl2(NSiMe3)[N(SiMe3)2] (1), along with 4/5 and unreacted 8, was observed.

Scheme 5. α-SiMe3 Abstraction by an NMe2 Ligand in Ta(NMe2)4[N(SiMe3)2] (10) To Form [Ta(NMe2)3(μ-NSiMe3)]29

Figure 5. DFT-optimized structures and ground-state energy ordering for the TaCl2(NMe2)[N(SiMe3)2]2 (9) stereoisomers (E1−E3). The quoted free energies are in kcal/mol relative to species E1.

Figure 6. DFT-optimized structures and free energy surface for the α-elimination in E1 and E2 to give Me3SiNMe2 (F) and TaCl2( NSiMe3)[N(SiMe3)2] (G). The structure for F is not shown.

N(SiMe3)2 groups and lie 3.7 and 13.4 kcal/mol, respectively, above E1. The transformation of TaCl2(NMe2)[N(SiMe3)2]2 (9) to Me3Si-NMe2 (F) and TaCl2(NSiMe3)[N(SiMe3)2] (G) was next examined, and both E1 and E2 give the expected α-elimination products F and G.17 Step-scan calculations on species E3 failed to furnish a viable transition structure for the α-elimination process, and its examination was abandoned. Figure 6 shows the reaction surface and the optimized structures for TSE1FG, TSE2FG, and the products F and G. While the two reactants E1 and E2 both afford the products F and G, transition state TSE1FG lies 4.7 kcal/mol above TSE2FG. The overall reaction is exergonic by 6.5 kcal/mol.18

As shown in Figure 6, the monomer G is preferred and this is consistent with the observations by DOSY. The nature of the α-SiMe3 abstraction in E2 parallels the process computed earlier by us for the all-nitrogen derivative Ta(NMe2)4[N(SiMe3)2] (10; Scheme 5).9b The barrier height computed for TSE2FG is clearly overestimated by several kcal/mol, given that the reaction of 8 and LiNMe2 proceeds at room temperature overnight, a feature not uncommon in the computation of transition-state energies.19 Rapid metathetical replacement of the chloride ligands in G with LiNMe2 furnishes 4 (not shown in Figure 6), in concert with the documented product distribution from the reaction of 8 with LiNMe2 (1:1 mole ratio). F

DOI: 10.1021/acs.organomet.5b00558 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Nuclearity of 1−6 and 8 in Solution at 25 °C Studied by Diffusion-Ordered Spectroscopy (DOSY) Experiments. The nuclearity of organometallic compounds in solution is an important component of understanding reaction pathways and intermediates formed in reactions. Along with mass spectrometry, diffusion-based NMR experiments such as DOSY may shed light on the nuclearity of compounds in solution.20 DOSY is a 2D NMR technique that gives the diffusion coefficient in one dimension and the chemical shift in the other.20 The diffusion coefficient is related to the size of the particle, and for compounds of similar density and shape, it may be related to their molecular weights. We have used DOSY to identify the nuclearity of 1−6 and 8 in deuterated benzene. Shown in Figure 7 is a plot of the diffusion

The new mixed amide imides Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (R′ = Me (4), Et (5)) have been prepared from two different reactions. The preparation of Ta(NR′2)2(NSiMe3)[N(SiMe3)2] (4 and 5) from TaCl3[N(SiMe3)2]2 (8) and LiNR′2 involves rare intramolecular imidation through α-SiMe 3 abstraction by NR′2, converting the amide N(SiMe3)2 to the imide NSiMe3 ligand through Me3Si−NR′2 elimination (R′ = Me, Et). DFT calculations support the proposed α-elimination sequence, although the computed free energy of activation is slightly higher than the experimental reaction. Future benchmark studies are planned in order to evaluate the best density functional for the general analysis of the tantalum compounds under investigation by our groups.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under a dry nitrogen atmosphere with the use of either a drybox or standard Schlenk techniques. All solvents such as diethyl ether, THF, hexanes, toluene, and pentane were dried over potassium/benzophenone, distilled, and stored under nitrogen. TaCl5 (Strem), LiN(SiMe3)2 (Aldrich), LiNMe2 (Aldrich), and LiMe (Acros) were used as received. TaCl2(NSiMe3)[N(SiMe3)2] (1), TaCl3[N(SiMe3)2]2 (8), and ClMgCH2Ph were prepared by the literature methods.6h,21,22 LiNEt2 was obtained by addition of BunLi to a hexanes solution of diethylamine and then removal of volatiles. NMR spectra were recorded on a Varian VNMRS 500 MHz, Varian Mercury Vx 300 MHz, Bruker AMX-400, or Bruker AC250/Tecmag spectrometer and were referenced to an appropriate solvent resonance. 1H-gated-decoupled 13C NMR spectra were used to obtain the 1JH−C coupling constants. 29Si{1H} NMR chemical shifts were obtained from 1H−29Si HMBC spectroscopy. Mass spectra were recorded on a JEOL AccuTOF DART mass spectrometer. Elemental analyses were conducted via Complete Analysis Laboratories, Inc., Parsippany, NJ. Preparation of TaMe2(NSiMe3)[N(SiMe3)2] (2). MeLi (1.40 mL, 2.2 mmol) in Et2O was added dropwise to a stirred solution of 1 (542 mg, 1.09 mmol) in Et2O (30 mL) with stirring at −50 °C. The solution was stirred for 18 h as the temperature was gradually increased to 23 °C. The solvent was then removed in vacuo, and pentane was added. The solution was then filtered and solvent removed in vacuo to give 2 (414 mg, 0.903 mmol, 83.1% yield) as a crude brown liquid. Distillation of the crude liquid at 85−86 °C under 0.05 mmHg vacuum gave a yellow liquid of 2 (101 mg, 0.220 mmol, 20.3% yield) that was collected on a cold finger at −15 °C. 1H NMR (benzene-d6, 499.7 MHz, 25 °C): δ 0.57 (s, 6H, Me), 0.37 (s, 9H, NSiMe3), 0.32 [s, 18H, N(SiMe3)2]. 13C NMR (benzene-d6, 75.5 MHz, 23 °C): δ 52.94 (Me), 4.41 [N(SiMe 3 ) 2 ], 2.70 (NSiMe3 ). 29Si NMR (benzene-d6 , 99.28 MHz, 25 °C): δ −7.59 (NSiMe3), −0.66 [N(SiMe3)2]. MS for [2 + H+]: m/z calculated 459.1510, found 459.1499. Preparation of Ta(CH 2 Ph) 2 (NSiMe 3 )[N(SiMe 3 ) 2 ] (3). ClMgCH2Ph (309 mg, 2.05 mmol) in Et2O was added dropwise to a stirred solution of 1 (512 mg, 1.02 mmol) in Et2O (30 mL) at −30 °C. The solution was stirred for 18 h and then filtered. The volatiles were then removed in vacuo, giving 3 (520 mg, 0.85 mmol, 83% yield) as a brown solid. Crystals were obtained from a saturated solution of 3 in pentane at −33 °C. 1H NMR (benzene-d6, 499.7 MHz, 25 °C, J in Hz): δ 7.11−7.04 (m, 3JH(meta)−H(ortho) = 6.9 Hz, Hortho and Hmeta on Ph), 6.96 (t, 3JH(meta)−H(para) = 7.1 Hz, Hpara on Ph), 2.34 (d, 2JH−H = 11.6, 2H, CHaHbPh), 2.01 (d, 2H, CHaHbPh), 0.30 (s, 9H, NSiMe3), 0.23 [s, 18H, N(SiMe3)2]. 13C NMR (benzene-d6, 62.9 MHz, 23 °C): δ 138.80 (Cipso, CH2Ph), 131.13 (Cortho, CH2Ph), 129.07 (Cmeta, CH2Ph), 125.11 (Cpara, CH2Ph), 64.92 (CH2Ph, 1JH−C = 129 Hz), 4.50 ([N(SiMe3)2], 1 JH−C = 118 Hz), 2.97 [(NSiMe3) 1JH−C = 114 Hz]. 29Si NMR (benzene-d6, 99.28 MHz, 25 °C): δ −7.52 (NSiMe3), 0.00 [N(SiMe3)2]. Anal. Calcd for C23H41N2Si3Ta: C, 45.23; H, 6.77. Found: C, 45.07; H, 6.79. At the time of the elemental analysis, the N analysis was not requested. Preparation of Ta(NMe2)2(NSiMe3)[N(SiMe3)2] (4). Preparation from TaCl2(NSiMe3)[N(SiMe3)2] (1). LiNMe2 (261 mg,

Figure 7. Plot of diffusion coefficients at 25 °C versus molecular weights on the logarithmic scale. A total of seven runs on SiMe4 gave random uncertainty in the diffusion coefficient δDran/D = 2%. The total uncertainty δD/D of 5.4% was calculated from the estimated systematic uncertainty of δDsys/D = 5% and δDran/D by (δD/D)2 = (δDran/D)2 + (δDsys/D)2.

coefficients of our tantalum compounds along with a few known organic compounds for calibration. All compounds are monomers in solution except for the cis/trans isomers of 6. In other words, among the Ta compounds in the current DOSY work, only trans-6 is a dimer in both the solid state (Figure 3) and solution, using the OMe ligands to bridge two Ta centers. Although the dichloride compound 1 is a dimer in the solid state,6h it is a monomer in solution on the basis of the DOSY experiment. Another interesting finding is that the cis and trans isomers of 6 (e.g., Figure S23 in the Supporting Information) have slightly different diffusion coefficients, although the differences are within the errors. In the current work, syntheses and characterization of tantalum alkyl imides (2 and 3) and amide imides (4 and 5) are reported. The crystal structure of Ta(CH2Ph)2(NSiMe3)[N(SiMe3)2] (3) reveals η2 coordination to the Ta center by a phenyl ring on one of the benzyl ligands. Oxygen initially inserts into only one Ta−Me bond of the two methyl ligands in 2 to form the methoxy dimer Ta2(μ-OMe)2Me2(NSiMe3)2[N(SiMe3)2]2 (cis-6 and trans-6). A mixture of cis-6 and trans-6 reacts with additional O2 to give Ta2(μ-OMe)2(OMe)2(NSiMe3)2[N(SiMe3)2]2 (cis-7 and trans-7). The reversible isomerization of trans-6 and cis-6 has been investigated by 1H NMR spectroscopy, and the first-order rate constants (k1 and k−1) at room temperature are reported. G

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Organometallics

At least two separate studies were performed to give errors in both Keq and k + k′ from the slope in the kinetic plot. The solution of the trans-6 ⇌ cis-6 mixture gradually decomposed at room temperature, as Icis‑6 decreased significantly a few hours after equilibrium was reached. DOSY Studies. Samples were prepared in 5 mm Young tubes under an inert nitrogen atmosphere. DOSY studies of all samples were individually conducted in dry benzene-d6 with SiMe4 as the internal standard. NMR measurements were performed on a Varian VNMRS 500 MHz spectrometer equipped with an OneNMR probe, with a z-axis gradient coil. The maximum field gradient strength was 60 G cm−1. DOSY experiments were carried out with a double-stimulated echo pulse sequence with bipolar gradient pulses and convection compensation (Dbppste_cc). The diffusion delay (Δ) was set to 0.05 s, the gradient pulse length (δ) to 0.002 s, and the delay between scans to 5 s. The number of transients was 16. A set of 20 spectra were acquired, with the pulsed field gradient strength (G) varied between 2 and 28 G cm−1, incremented in equal steps of gradient squared. The temperatures of the samples were regulated at 25 °C. Processing of the spectra was done with MestReNova (Mestrelab Research S.L., Santiago de Compostela, Spain). The spectral phase and baseline were corrected before integration of the area of each NMR line of interest. The diffusion coefficient for each individual NMR line is obtained from the slope of the linear regression fit of the natural logarithm of the integration areas as a function of δ2γ2G2(Δ − δ/3). γ is the 1H gyromagnetic ratio. Determination of X-ray Crystal Structures of 3 at 173(2) K and trans-6 at 100(2) K. X-ray diffraction data were collected on a Bruker AXS Smart 1000 X-ray diffractometer equipped with a CCD area detector and a graphite-monochromated Mo source (Kα radiation, 0.71073 Å). Suitable crystals were coated with Paratone-N oil (Exxon Chemical Americas, Houston, TX) and mounted on a cryoloop and measured under an open-flow nitrogen cryostream. Global refinements for the unit cell and data reduction were performed with the SAINT program.23 Empirical absorption correction was performed with SADABS.24 The space group for structure solution and refinement was determined from systematic absences using XPREP.23 Computational Methodology. All DFT calculations were carried out with the Gaussian 09 package of programs25 using M06 as the DFT functional.26 The choice of this functional was dictated by our recent computational study involving the α-SiMe3 abstraction in Ta(NMe2)4[N(SiMe3)2] (9), where this functional accurately reproduced the experimentally determined barrier height.9b The tantalum atom was described with the Stuttgart−Dresden effective core potential and SDD basis set,27 and the 6-31G(d′) basis set28 was employed for all remaining atoms. All reported geometries were fully optimized, and analytical second derivatives were evaluated at each stationary point to verify whether the geometry was an energy minimum (positive eigenvalues) or a transition structure (one negative eigenvalue). Unscaled vibrational frequencies were used to make zero-point and thermal corrections to the electronic energies. The resulting free energies are reported in kcal/mol relative to the specified standard. Standard state corrections were applied to all species to convert concentrations from 1 atm to 1 M according to the treatise of Cramer.29 Intrinsic reaction coordinate (IRC) calculations were performed on TSE1FG and TSE2FG in order to establish the reactant and product species associated with these transition-state structures. The geometry-optimized structures have been drawn with the JIMP2 molecular visualization and manipulation program.30

5.12 mmol) in Et2O (30 mL) was added to 1 (1.20 g, 2.40 mmol) in Et2O (30 mL) with stirring at −30 °C. The mixture was stirred for 18 h. Et2O was then removed in vacuo, and the residue was dissolved in pentane. The solution was then filtered, and the volatiles were removed in vacuo to give 4 as a yellow liquid (1.00 g, 1.94 mmol, 81% yield). Preparation from TaCl3[N(SiMe3)2]2 (8). Solid LiNMe2 (128 mg, 2.51 mmol) was added slowly to 8 (475 mg, 0.78 mmol) in toluene (60 mL) with stirring at −30 °C. The mixture was stirred overnight. Toluene was then removed in vacuo, and the residue was dissolved in pentane. The solution was then filtered, and the volatiles were removed in vacuo to give 4 as a yellow liquid (284 mg, 0.55 mmol, 71%). 1H NMR (benzene-d6, 499.7 MHz, 25 °C): δ 3.13 (s, 12H, NMe2), 0.31 [s, 18H, N(SiMe3)2], 0.31 (s, 9H, NSiMe3). 13C NMR (benzene-d6, 125.7 MHz, 25 °C): δ 47.20 (NMe2), 5.58 [N(SiMe3)2], 3.94 (NSiMe3). 29 Si NMR (benzene-d6, 99.28 MHz, 25 °C): δ −9.67 (NSiMe3), −0.66 [N(SiMe3)2]. Anal. Calcd for C13H39N4Si3Ta: C, 30.22; H, 7.61; N, 10.84. Found: C, 30.13; H, 7.51; N, 10.55. MS for [4 + H+]: m/z calculated 517.20407, found 517.20465. Preparation of Ta(NEt2)2(NSiMe3)[N(SiMe3)2] (5). Preparation from TaCl2 (NSiMe 3 )[N(SiMe 3 )2 ] (1). LiNEt 2 (380 mg, 4.81 mmol) in Et2O (30 mL) was added dropwise to 1 (1.16 g, 2.34 mmol) in Et2O (30 mL) with stirring at −30 °C. The mixture was stirred for 18 h. Et2O was then removed, and the residue was dissolved in pentane. The solution was then filtered, and the volatiles were removed in vacuo to give 5 as a brown liquid (1.20 g, 2.10 mmol, 90% yield). Preparation from TaCl3[N(SiMe3)2]2 (8). Solid LiNEt2 (212 mg, 2.68 mmol) was added slowly to 8 (510 mg, 0.84 mmol) in Et2O (30 mL) with stirring at −30 °C. The mixture was stirred overnight. Et2O was then removed, and the residue was dissolved in pentane. The solution was then filtered, and the volatiles were removed in vacuo to give 5 as a brown liquid (229 mg, 0.40 mmol, 69% yield). 1H NMR (benzene-d6, 499.7 MHz, 25 °C, J in Hz): δ 3.49−3.36 (m, 8H, N−CH2Me), 1.09 (t, 3JH−H = 7, 12H, N−CH2Me), 0.38 [s, 18H, N(SiMe3)2], 0.32 (s, 9H, NSiMe3). 13C NMR (benzene-d6, 125.7 MHz, 25 °C): δ 48.25 (N−CH2Me), 16.44 (N−CH2Me), 5.64 [N(SiMe3)2], 3.69 (NSiMe3). 29Si NMR (benzene-d6, 99.28 MHz, 25 °C): δ −10.56 (NSiMe3), −0.72 [N(SiMe3)2]. MS for [5 + H+]: m/z calculated 573.26667, found 573.26569. Synthesis of Ta2(μ-OMe)2Me2(NSiMe3)2[N(SiMe3)2]2 (cis-6 and trans-6). 2 (242 mg, 0.528 mmol) in pentane (20 mL) in a Schlenk flask was frozen in a liquid nitrogen bath, and the Schlenk flask was evacuated. A 1/2 equiv amount of O2 (0.264 mmol) was introduced through a gas manifold. The flask was gradually warmed to room temperature, and the solution was stirred for 6 h. The volatiles were removed in vacuo, giving a crude mixture of cis-6 and trans-6. The crude product was dissolved in pentane at 23 °C. Within a few minutes, white crystals of trans-6 began to appear. Cooling the mixture at −30 °C yielded more crystals of trans-6 (147 mg, 0.150 mmol, 58.8% yield). 1 H NMR (benzene-d6, 399.9 MHz, 23 °C): cis-6, δ 3.86 (s, 6H, μ-OMe), 0.65 (s, 6H, Me), 0.40 [s, 36H, N(SiMe3)2], 0.33 (s, 18H, NSiMe3); trans-6, δ 3.78 (s, 6H, μ-OMe), 0.62 (s, 6H, Me), 0.42 [s, 36H, N(SiMe3)2], 0.31 (s, 18H, NSiMe3). 13C NMR (benzene-d6, 75.5 MHz, 23 °C): cis-6, δ ∼64.16 (broad, μ-OMe), 41.61 (Me), 4.39 [overlapping peak, N(SiMe3)2], 3.47 (NSiMe3); trans-6, δ 63.09 (μ-OMe), 42.20 (Me), 4.39 [overlapping peak, N(SiMe3)2], 3.55 (NSiMe3). The 13C NMR assignments were made with the help of HSQC. 29Si NMR (benzene-d6, 99.28 MHz, 40 °C): δ −7.20 (overlapping for both isomers, NSiMe3), −0.59 [overlapping peak, N(SiMe3)2]. Anal. Calcd for C22H66N4O2Si6Ta2 (crystals of trans-6): C, 27.84; H, 7.01; N, 5.90. Found: C, 27.69; H, 6.83; N, 5.71. MS for [6 + H+]: m/z calculated 475.14589, found 475.14389. Kinetic and Thermodynamic Studies of the trans-6 ⇌ cis-6 Exchange. To a Young NMR tube was added 2.2−5.0 mg of crystals of trans-6 and 4.0 mg of 4,4′-dimethylbibenzyl as an internal standard. Benzene-d6 was then added. The formation of cis-6 was followed by 1 H NMR spectroscopy, with a spectrum recorded every 8−10 min at 25.0(0.1) °C until no change in Icis‑6 was observed. The Ta−Me peak in cis-6 was used and compared to the internal standard to follow the change in concentration vs time in the kinetic studies. At equilibrium, the Ta−Me peak in trans-6 was also used to calculate Keq.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00558. 1 H and 13C NMR and HR-MS spectra, crystal data and structure refinement details, and atomic coordinates of all optimized ground-state stationary points and transition states (PDF) Crystallographic data (CIF) H

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.-L.X.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (CHE-1362548 to Z.-L.X.) and the Robert A. Welch Foundation (Grant B-1093-MGR). Computational resources through the High Performance Computing Services and CASCaM at the University of North Texas are acknowledged, and we thank Prof. Michael B. Hall (Texas A&M University) for providing us a copy of his JIMP2 program, which was used to prepare the geometry-optimized structures reported here. We also thank Tabitha M. Cook for help with HRMS studies.

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DOI: 10.1021/acs.organomet.5b00558 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.5b00558 Organometallics XXXX, XXX, XXX−XXX