Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Formation of an α‑Diimine from Isocyanide Coupling Using Thorium(IV) and Uranium(IV) Phosphido−Methyl Complexes Pokpong Rungthanaphatsophon,† Iker del Rosal,‡ Robert J. Ward,† Sean P. Vilanova,† Steven P. Kelley,† Laurent Maron,*,‡ and Justin R. Walensky*,† †
Department of Chemistry, University of Missouri, 601 S. College Avenue, Columbia, Missouri 65211, United States Laboratoire de Physique et Chimie de Nano-objets, Universite de Toulouse, INSA-CNRS-UPS, 135 Avenue de Ranguiel, 31077 Toulouse, France
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‡
S Supporting Information *
ABSTRACT: To probe the reactivity of two potential sites at tetravalent thorium and uranium metal centers, we examined the monophosphido methyl complexes, (C5Me5)2An(CH3)[P(SiMe3)(Mes)], An = Th, U; Mes = 2,4,6-Me3C6H2. Reaction of these mixed ligand complexes with one, two, and three (or excess) equivalents of tBuNC was explored. When (C5Me5)2An(CH3)[P(SiMe3)(Mes)] is treated with one equivalent of tBuNC, the iminoacyl products, (C5Me5)2An[η2-tBuNCCH3][P(SiMe3)(Mes)], are formed. Using three equivalents (or excess) of tBuNC results in the formation of an α-diimine moiety, (C5Me5)2An[κ2-(N,N)−N(tBu)CCN(tBu)CN(tBu)CH2]. When two equivalents of tBuNC are added, only the mono-insertion or α-diimine was observed in a 3:1 ratio. Density functional theory calculations were carried out to determine the lowest energy pathway in the formation of the α-diimine product via the iminoacyl complex.
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INTRODUCTION Metal−phosphido bonds can be exploited for a variety of uses including hydrophosphination catalysts,1 small-molecule activation,2−5 and advancing our knowledge of their interactions with electropositive metals.6 A key observation in hydrophosphination is that the reactivity of metal−phosphido complexes involves the basicity of the phosphido ligand.7−14 For example, (η5-indenyl)Ru(PPh3)(Cl)(HPR2), R = Cy, iPr has the ability to catalyze the hydrophosphination of substituted alkenes, initially forming a reactive ruthenium phosphido complex upon deprotonation of the phosphine.15 Our interest in actinide−phosphido chemistry stems from the expectation that the heavier congener of nitrogen will give rise to unique physical properties and reactivity patterns. We anticipate that these differences between nitrogen and phosphorus will be due to the larger size of phosphorus, decrease in metal−phosphorus bond strength, PIII/PV redox capability, and the decreased basicity of phosphido ligands. This study seeks to build upon previous examination of reactions of phosphido (our work), phosphinidiide, and phosphinidene (Zi, Ding, & Walter) complexes with isocyanides, Scheme 1.4,5,16 Here, we have synthesized mixed phosphido−methyl complexes of the form, (C5Me5)2An(CH3)[P(SiMe3)(Mes)], Mes = 2,4,6-Me3C6H2, to examine their reactivity with isocyanides. Because isocyanides are known to undergo migratory insertion reactions into actinide−carbon bonds,17−20 as well as have the propensity to insert into metal−phosphido bonds21−27 and heavier main group-element bonds,16,28,29 the phosphido−methyl complexes setup a © XXXX American Chemical Society
competitive scenario where the isocyanide could react with the phosphido, methyl, or both. We demonstrate that the basicity of the phosphido ligand, combined with the reactivity of an actinide−carbon bond, produces a cascade of reactions in which three C−C bonds are formed, as well as one CC bond with extrusion of the phosphido ligand as the free phosphine, to produce an α-diimine.
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RESULTS AND DISCUSSION We have previously reported the thorium complex, (C5Me5)2Th(CH3)[P(SiMe3)(Mes)], 1,30 and the uranium analogue was made similarly from the reaction of (C5Me5)2U(CH3)I with KP[(SiMe3)(Mes)], eq 1. Red crystals, suitable
for X-ray diffraction analysis, were obtained from a concentrated diethyl ether solution at −25 °C, Figure 1. The resulting complex, (C5Me5)2U(CH3)[P(SiMe3)(Mes)], 2, was obtained in nearly quantitative yield. While no resonance in the 31P NMR spectrum was found, the 1H NMR resonances were located at 9.45 ppm for the (C5Me5)1− ligands, 2.46, Received: January 23, 2019
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DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Reactivity of Thorium Phosphido, Phosphinidiide, and Phosphinidene Complexes with Isocyanides
Because 1 and 2 have two potentially reactive sites, reaction with one equivalent of tBuNC was attempted. The reaction with 1 yields a yellow-colored solution, eq 2. The 31P NMR
spectrum showed a resonance at −50.3 ppm, consistent for phosphido ligands with a silyl group coordinated to thorium.30 A resonance at 269.6 ppm in the 13C{1H} NMR spectrum was found as well. These two spectroscopic features indicated that the phosphido ligand was still bound to thorium, as well as the formation of an iminoacyl group. Indeed, the structure as obtained by X-ray crystallographic analysis showed (C5Me5)2Th[P(SiMe3)(Mes)][η2-N(tBu)C(CH3)], 3, Figure 2. We note in a similar competitive study with Ti(η5Ind)(NMe2)(Me) and tBuNC, insertion also occurred exclusively into the titanium−carbon (methyl) bond.35 In comparing 3 with (C5Me5)2Th(CH3)[P(Mes)(SiMe3)], 1, the thorium−phosphorus bond distance in 3 is 2.9536(13) Å, which is elongated from the 2.8713(12) Å in 1, because of the steric congestion of the iminoacyl ligand. The Th−C11 bond distance of 2.429(5) Å is similar to the Th−C bond distance of 2.430(6) Å in (C5Me5)2Th(CNtBu)[η2-N,C-(tBuNCP(2,4,6-iPr3C6H2))],16 while the Th−N bond distance in 3 is 2.467(4) Å, which is much longer than 2.346(5) Å in the phosphazaallene.16 Again, a similar product is obtained with uranium, (C5Me5)2U[P(SiMe3)(Mes)][η2-N(tBu)C(CH3)],
Figure 1. Thermal ellipsoid plot of 2 shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. A silyl group is shown in the wireframe for clarity.
−0.20, and −6.78 ppm for the Ph protons, −14.3 ppm for SiMe3, and −125.0 ppm for the methyl coordinated to uranium. These resonances bear similarity to (C5Me5)2U(CH3)(PPh2) which has been spectroscopically characterized with resonances at 11.08 ppm for the (C5Me5)1− ligands, −3.86, −4.08, and −41.04 ppm for the Ph protons, and −160.3 ppm for the coordinated methyl.31 Complex 2 is structurally identical to 1 with a uranium− phosphorus bond distance of 2.7933(10) Å, which is comparable to that of 2.789(4) Å in (C5Me5)2U(Cl)[P(SiMe3)2]32 and 2.883(2) Å in U(TrenTIPS)(PH2).33 The uranium−carbon (methyl) bond distance of 2.393(4) Å is slightly shorter than in (C5Me5)2U(CH3) 2 which has uranium−carbon (methyl) distances of 2.424(7) and 2.414(7) Å.34 The C21−U1−P1 bond angle is 88.58(10)°, slightly more acute than the 93.92(12)° in (C5Me5)2U(Cl)[P(SiMe3)2].32 B
DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 3. Thermal ellipsoid plot of 5 shown at the 50% probability level. Hydrogen atoms have been omitted for clarity. tert-Butyl groups are shown in the wireframe for clarity.
Figure 2. Thermal ellipsoid plot of 3 shown at the 50% probability level. The hydrogen atoms have been omitted for clarity. Silyl and tertbutyl groups are shown as the wireframe for clarity.
28 and 35%, respectively, represent the crystalline product but only one product is observed in the 1H NMR spectrum. As expected, the resonances in the 1H NMR spectrum of 6 are paramagnetically shifted with the (C5Me5)1− signal located at 8.47 ppm, and the tert-butyl resonances at 6.41, −12.5, and −24.7 ppm and the methylene resonance at 31.6 ppm. In the infrared spectrum, both 5 and 6 contain a strong absorption at 1670 and 1688 cm−1, respectively, for the CN bond stretch. The structure of 5, Figures 3, and 6, Figure S21, resemble thorium36,37 and uranium38 α-diimine complexes, respectively. Complexes 5 and 6 are the result of an insertion of tBuNC into the actinide−carbon bond, coupling of three equivalents of t BuNC as well as the deprotonation of the methyl group formerly coordinated to the metal center. While obvious because of the use of an alkane solvent, the methylene was verified to have originated from the methyl using 13C labeling experiments (see Supporting Information). The coupling of isocyanides is well-known in organometallic chemistry;39−45 however, the synthesis from intramolecular deprotonation of a methyl group has not been reported, to our knowledge. Similar products have been observed from isocyanide insertion and coupling, however these usually involve the cyclobutene group already being formed prior to insertion.46−50 In 5, the Th−N bond distances of 2.316(2) and 2.364(2) Å and the Th−C1 and Th−C2 bond distances are 2.781(2) and 2.784(2) Å. In comparison, the Th−N bond distances are close to the 2.294(3)−2.316(2) Å range, while the thorium−carbon bond distances are slightly shorter than the 2.811(3)−2.850(3) Å observed in (MesDABMe)2Th(THF).36 This shortening of the metal−carbon bonds in the backbone is reflected in the elongation of the C−C bond length of 1.404(4) Å in 5 and 1.406(5) Å in 6. The origin for this interaction in uranium αdiimine complexes involves population of the π* orbitals in the C−C backbone and has been previously studied.51 These distances are consistent with other thorium and uranium interactions with C−C π bonds.52 With respect to the
4, from the reaction of 2 with one equivalent of tBuNC, Figure S20. In the case of both 3 and 4, the methyl group, C12 in 3 shown in Figure 2, which is to be deprotonated is pointed away from the phosphido ligand, hence the iminoacyl must rotate in order for 3 to be produced. This rotational barrier is most likely the reason why 3 and 4 can be isolated (see the Computational Details section). The reaction of 1 and 2 with two equivalents of tBuNC produced two products. One could be identified as the monoinsertion product (3 and 4), but the other was a new product. When three equivalents (or excess) of tBuNC were used, then only the unknown product was observed in the 1H NMR spectrum. The reaction of 1 with an excess of tBuNC, produces an instant color change from dark red to orange, eq 3. Only the
parent phosphine, HP[(SiMe3)(Mes)], was observed in the 31 P NMR spectrum. The 1H NMR spectrum of the reaction mixture showed one (C5Me5)1− resonance at 2.06 ppm, three tert-butyl resonances at 1.81, 1.50, and 1.36 ppm. Another resonance at 3.69 ppm integrated to two protons, as referenced to the (C5Me5)1− signal, and corresponded to a CH2 group located in the 13C DEPT spectrum at 47.1 ppm. Upon crystallization from a saturated toluene solution at −25 °C, the structure was identified as (C5Me5)2Th[κ2-(N,N)−N(tBu)C CN(tBu)CN(tBu)CH2], 5. The analogous product is obtained when 2 is treated with an excess of tBuNC, and (C5Me5)2U[κ2-(N,N)−N(tBu)CCN(tBu)CN(tBu)CH2], 6, eq 3, was isolated. In the case of 5 and 6, the reported yields,
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DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 4. Computed enthalpy profile for the reaction of 1 with an excess of tBuNC.
bind the phosphido and methyl ligands, respectively) of the thorocene. Because of the relatively low-bonding character of the 2a1 orbital, the coordination is weak (−0.8 kcal/mol) and is mainly a balance between electrostatic attraction by the thorium and repulsion by the two anionic ligands, Int1. Therefore, the system easily undergoes a migratory insertion onto the Th−CH3 bond (barrier of 11.8 kcal/mol) to yield Int2 (equivalent to complex 3 experimentally) that is thermodynamically favorable (−26.9 kcal/mol). Int2 can isomerize to a less stable intermediate Int3 through a low-lying transition state (11.4 kcal/mol), in which the carbon and nitrogen have exchanged position in the iminoacyl ligand. This intermediate allows a proton exchange between the methyl group of the iminoacyl and the phosphido ligand. The associated barrier is 22.2 kcal/mol, consistent with a thermally possible reaction. The transition evolves to the formation of a phosphine−iminoacyl complex (Int4), which is not stable with respect to either Int3 (+10.5 kcal/mol) or Int2 (+16.8 kcal/mol). However, in the presence of additional isocyanide, the phosphine to isocyanide ligand exchange thermodynamically drives the reaction to the formation of intermediate Int5, which is the methylene equivalent of the reaction product with a bis(phosphido) thorium complex (Scheme 1). This nicely explains why no complex could be
cyclobutene fragment in 5 comprised of C1, C2, C3, and C4, the C1−C2, 1.404(4) Å, C2−C3, 1.484(3) Å, C3−C4, 1.544(4) Å, and C1−C4, 1.520(3) Å. The cyclobutene is essentially planar with a torsion angle of 0.11°. The same is true in 6 at 0.16°. The reaction of 3 with excess tBuNC yields 5, eq 4. Finally, when two equivalents of tBuNC are reacted with 1, then a mixture of 3 and 5 is produced in a 3:1 ratio by 1H NMR spectroscopy, eq 5. Therefore, a reactive intermediate is formed in the interim that we could not isolate or observe by 1 H or 31P NMR spectroscopy. In investigating the rotation of the iminoacyl as well as the identity of the reactive intermediate formed with two equivalents of tBuNC, density functional theory (DFT) calculations were carried out to determine a plausible energy profile. Several reaction pathways were considered (Figure S22), but only the lowest energy one is discussed, Figure 4. Even though the final product exhibits a CH2 fragment, the reaction does not begin by the formation of a ThCH2 carbene complex through loss of the phosphine. This is computed to be less favorable than the mechanism reported here (see Supporting Information). The reaction begins by the coordination of tBuNC in between the two equatorial ligands involving the 2a1 orbital (the 1a1 and 1b2 orbitals being used to D
DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics isolated with two equivalents of tBuNC. Rather than undergoing a nucleophilic attack, another migratory insertion is observed at low energy (barrier of 5.1 kcal/mol), yielding Int6, which is marginally stabilized with respect to Int5 (−1.2 kcal/mol). After isomerization into Int7 (mainly to change the position of the nitrogen of the lately inserted isocyanide molecule), the system binds another isocyanide molecule. As the thorium center is not accessible, the coordination takes place at the most available acidic atom, the carbon of the last inserted isocyanide (charge C = +1.1). The associated barrier is also 22.2 kcal/mol. This coordination yields a strongly stabilized intermediate Int9 (−56.5 kcal/mol, −23.4 kcal/ mol with respect to Int6). Despite this strong stabilization, this intermediate readily undergoes a nucleophilic attack with the carbene fragment (barrier of 9.1 kcal/mol). Following the intrinsic reaction coordinates, it yields the very stable complex 5, whose formation is favored by 27.4 kcal/mol with respect to Int9 (83.9 kcal/mol with respect to the entrance channel). Cooperative chemistry has been an area of increased attention but typically done with multimetallic systems.53 Here, we have demonstrated that manipulating the coordination environment of heteroleptic metallocene thorium and uranium with reactive actinide-carbon and actinide-phosphido bonds produces unusual reactivity at a single metal center. Further investigations are underway.
referenced external to 85% H3PO4 and SiMe4, respectively. If coupling is not specified, then the origin is not definitively known. Infrared spectra were recorded as KBr pellets on a PerkinElmer Spectrum One FT-IR spectrometer. Caution! Thorium-232 and depleted uranium (primarily U-238) are alpha-emitting radiometals with half-lives of 1.4 × 1010 years and 4.47 × 109 years, respectively. All work was carried out in a radiological laboratory with appropriate personal protective and counting equipment. Synthesis of (C5Me5)2U(CH3)[P(Mes)(SiMe3)], 2. Complex 2 was prepared using(C5Me5)2U(CH3)I (252 mg, 0.39 mmol) in toluene (5 mL) and KP(Mes)(SiMe3) (102 mg, 0.39 mmol) in toluene (5 mL). The reaction was allowed to stir overnight, during which time the color changed from dark red to black. A white precipitate was separated by filtration and the solvent removed under vacuum. Analytically pure sample and X-ray quality crystals of 2 were obtained from a concentrated diethyl ether solution at −25 °C (285 mg, 99%). 1 H NMR (C6D6, 500 MHz, 298 K): δ 9.45 (s, 30H, C5(CH3)5), 2.46 (s, 2H, m-Mes), −0.20 (s, 3H, CH3-p-Mes), −6.78 (s, 6H, CH3-oMes), −14.3 (s, 9H, Si(CH3)3), −125.0 (s, 3H, U−CH3). IR (KBr, cm−1): 2949 (m), 2908 (s), 2867 (m), 1438 (m), 1376 (m), 1237 (m), 1102 (w), 1044 (w), 1020 (w), 936 (w), 880 (w), 836 (vs), 749 (w). Anal. Calcd for C33H53SiPU: C, 53.07; H, 7.15. Found: C, 52.91; H, 7.04. Synthesis of (C5Me5)2Th[(η2-(N(tBu)CCH3))][P(C6H2Me3-2,4,6)(SiMe3)], 3. Complex 3 was prepared using 1 (128 mg, 0.173 mmol), tert-butyl isocyanide (14 mg, 0.17 mmol), and pentane (5 mL) in a 20 mL scintillation vial. An analytically pure sample as well as X-ray quality crystals of 3 were obtained after recrystallization in diethyl ether at −25 °C (63 mg, 45%). 1H NMR (C6D6, 600 MHz, 298 K): δ 7.14 (s, 2H, m-Mes), 2.98 (s, 6H, CH3-o-Mes), 2.46 (s, 3H, CH3−CN), 2.28 (s, 3H, CH3-p-Mes), 2.02 (s, 30H, C5(CH3)5), 1.23 (s, 9H, (H3C)3CNC), 0.50 (d, 9H, 3JH−P = 4.2 Hz, SiMe3). 13 C{1H} NMR (C6D6, 150 MHz, 298 K): 269.6 (CN), 146.5 (d, 2 JC−P = 4.5 Hz, Ph), 141.1 (d, 1JC−P = 15 Hz, Ph), 134.5 (Ph), 128.3 (d, 3JC−P = 4.0 Hz, Ph), 124.0 (C5Me5), 62.2 (CMe3), 31.4 (s, 4JC−P = 2.0 Hz, Me), 28.2 (d, 3JC−P = 9.2 Hz, Me), 23.1 (Me), 21.2 (C5Me5), 12.2 (d, 2JC−P = 1.5 Hz), 5.4 (d, 2JC−P = 10.5 Hz, SiMe3). 31P{1H} NMR (C6D6, 120 MHz): δ −50.3. 29Si INEPT NMR (C6D6, 60 MHz): δ 4.94 (d, 1JSi−P = 11 Hz). IR (KBr, cm−1): 2966 (s), 2945 (s), 2904 (br-vs), 2862 (s), 1438 (br-m), 1391 (w), 1376 (m), 1365 (m), 1259 (w), 1235 (m), 1190 (m), 1096 (br-m), 1026 (br-m), 944 (w), 924 (w), 893 (w), 833 (vs), 758 (w), 742 (w), 672 (w) 627 (m). Anal. Calcd for C38H62NPSiTh: C, 55.39; H, 7.58; N, 1.70. Found: C, 55.21; H, 7.41; N, 1.56. Synthesis of (C5Me5)2U[(η2-(N(tBu)CCH3))][P(C6H2Me3-2,4,6)(SiMe3)], 4. Complex 4 was prepared in a similar manner to 3 using 2 (98 mg, 0.131 mmol), tert-butyl isocyanide (11 mg, 0.131 mmol), and pentane (15 mL) in a 20 mL scintillation vial. X-ray quality crystals of 4 were grown from a concentrated pentane solution at −25 °C (56 mg, 67%). 1H NMR (C6D6, 600 MHz, 298 K): δ 15.4 (s, 1H, Ph), 4.6 (s, 9H, tBu), 0.13 (s, 6H, Me), −0.75 (s, 30H, C5Me5), −5.4 (s, 9H, SiMe3), −6.2 (s, 1H, Ph). IR (KBr, cm−1): 2961 (s), 2917 (s), 2863 (b), 2359 (s), 2342 (s), 1635 (m), 1456 (m), 1437 (m), 1375 (m), 1261 (s), 1086 (sb), 1044 (s), 1022 (s), 835 (s). No satisfactory elemental analysis could be obtained. Synthesis of (C5Me5)2Th[κ2-(N,N)−N(tBu)CCN(tBu)CN(tBu)CH2], 5. A solution of 1 (147 mg, 0.198 mmol) in pentane (5 mL) was placed in a −25 °C freezer for 30 min prior to the next step. To this solution, tert-butyl isocyanide (50 mg, 0.6 mmol) was added dropwise. The mixture was then let to stir at room temperature overnight. Removal of volatiles in vacuo left an orange solid. An analytically pure sample and X-ray quality crystals of 5 were obtained after recrystallization in pentane at −25 °C (43 mg, 28%). 1H NMR (C6D6, 500 MHz, 298 K): δ 3.69 (s, 2H, CH2), 2.06 (s, 30H, C5(CH3)5), 1.81 (s, 9H, (H3C)3CNC), 1.50 (s, 9H, (H3C)3CNC), 1.36 (s, 9H, (H3C)3CNC). 13C{1H} NMR (C6D6, 175 Hz, 298 K): 157.5 (CNtBu), 132.3 (CC), 125.0 (C5Me5), 112.3 (CC), 58.2 (CMe3), 57.9 (CMe3), 56.0 (CMe3), 47.1 (CH2), 34.6 (tBu), 33.9 (tBu), 32.4 (tBu), 13.0 (Me). IR (KBr, cm−1): 2960 (vs), 2907
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CONCLUSIONS The reactivity of thorium(IV) and uranium(IV) phosphido− methyl complexes with an excess of tBuNC led to a series of cascade reactions involving the coupling of three isocyanides through C−C bond formation, deprotonation of the methyl group, and phosphine elimination. With one equivalent, the iminoacyl is formed from insertion into the actinide−carbon bond. Two equivalents only led to a mixture of iminoacyl to αdiimine in a 3:1 ratio. DFT calculations showed that the reason for isolation of the iminoacyl is that additional equivalents of t BuNC displace the phosphine to drive the reaction to the ultimate α-diimine. This demonstrates the basicity of the phosphido ligand, which parallels the hydrophosphination reactivity in which a phosphido deprotonates a neighboring phosphine,15 in concert with the reactivity of actinide-carbon bonds yielding unpredictable results.
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EXPERIMENTAL SECTION
General Considerations. All syntheses were carried out under an inert atmosphere of dinitrogen using standard Schlenk and glovebox techniques. Solvents were purified in the MBRAUN solvent purification system prior to use. tert-Butyl isocyanide (Aldrich) and KN(SiMe3)2 (Aldrich) were used as received. (C5Me5)2ThCl2,54 (C5Me5)2UCl2,54 (C5Me5)2U(CH3)I,55 Mg(13CH3)2,56 (C5Me5)2Th(CH3)[P(Mes)(SiMe3)], 1,30 were prepared according to literature procedures. (C5Me5)2Th(13CH3)2 was made from (C5Me5)2ThCl2 and Mg(13CH3)2. (C5Me5)2Th(13CH3)(Cl) was synthesized in a similar fashion to the unlabeled methyl, that is, (C5Me5)2Th(13CH3)2 and (C5Me5)2ThCl2. All yields are reported as crystalline products. Elemental analyses were performed at the University of California, Berkeley Microanalytical Facility using a PerkinElmer Series II 2400 CHNS analyzer. C6D6 (Cambridge) was dried over molecular sieves and degassed with three freeze−pump−thaw cycles. 1H and 13C NMR experiments were performed on either a Bruker AVANCE III 500 or 600 MHz spectrometer. 1H and 13C NMR spectra are reported in ppm referenced to internally to residual proton resonances.57 31P and 29 Si NMR experiments were performed on a Bruker AVII+ 300 MHz spectrometer. 31P and 29Si NMR resonances are reported in ppm E
DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics (vs), 2864 (s), 2825 (w), 1670 (s, CN stretch), 1595 (w), 1492 (s), 1452 (w), 1424 (w), 1386 (w), 1355 (m), 1315 (s), 1258 (w), 1229 (w), 1203 (vs), 1118 (w), 1086 (w), 1024 (w), 982 (w), 966 (w), 919 (w), 846 (w), 799 (m), 768 (m). Anal. Calcd for C36H59N3Th: C, 56.45; H, 7.76; N, 5.49. Found: C, 56.13; H, 7.77; N, 5.19. Synthesis of 5 from 3. To a J. Young NMR tube charged with 15 mg of 3, an excess amount of tert-butyl isocyanide was added at room temperature. After 5 min, the 1H NMR spectrum showed the complete conversion of 3 to 5. Synthesis of (C5Me5)2U[κ2-(N,N)−N(tBu)CCN(tBu)CN(tBu)CH2], 6. Complex 6 was prepared in a manner similar to 5 using 2 (241 mg, 0.323 mmol), tert-butyl isocyanide (80 mg, 0.96 mmol), and pentane (10 mL) in a 20 mL scintillation vial. The reaction mixture turned from black to brown. An analytically pure sample of 6 was obtained after recrystallization in diethyl ether (87 mg, 35%). X-ray quality crystals of 6 were grown from a concentrated pentane solution −25 °C. 1H NMR (C6D6, 600 MHz, 298 K): δ 31.6 (s, 2H, CH2), 8.47 (s, 30H, C5(CH3)5), 6.41 (s, 9H, (H3C)3CNC), −12.5 (s, 9H, (H3C)3CNC), −24.7 (s, 9H, (H3C)3CNC). IR (KBr, cm−1): 2962 (vs), 2908 (br-s), 1668 (s, CN stretch), 1598 (w), 1560 (w), 1492 (m), 1454 (m), 1385 (w), 1357 (m), 1322 (m), 1261 (m), 1200 (vs), 1088 (br-m), 1023 (m), 957 (w), 915 (w), 845 (m), 800 (m), 767 (w). Anal. Calcd for C36H59N3U: C, 56.02; H, 7.70; N, 5.44. Found: C, 55.78; H, 7.56; N, 5.14. Computational Details. All DFT calculations were carried out with the Gaussian 09 suite of programs.58 Geometries were fully optimized in the gas phase without symmetry constraints, employing the B3PW91 functional.59,60 The nature of the extrema was verified by analytical frequency calculations. The calculation of electronic energies and enthalpies of the extrema of the potential energy surface (minima and transition states) were performed at the same level of theory as the geometry optimizations. IRC calculations were performed to confirm the connections of the optimized transition states. Thorium atoms were treated with a small-core effective core potential (60 MWB), associated with its adapted basis set.61−63 Stuttgart effective core potentials64 and their associated basis set were used for silicon augmented by a set of polarization functions (ζd = 0.284). For the other elements (H, C, N, and P), Pople’s double-ζ basis set 6-31G(d,p) was used.65−69 The electronic charges (at the DFT level) were computed using the natural population analysis (NPA) technique.70
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Laurent Maron: 0000-0003-2653-8557 Justin R. Walensky: 0000-0003-0221-2675 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.R.W. gratefully acknowledges support for this work from the U.S. Department of Energy, Office of Science, Early Career Research Program under Award DE-SC-0014174. L.M. is a member of the Institut Universitaire de France, and acknowledges the HPCs CALcul en Midi-Pyrnes (CALMIP-EOS grant 1415).
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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.9b00043. Further experimental details, crystal data, and spectral information (PDF) xyz coordinates for computed structures (XYZ) Accession Codes
CCDC 1827001−1827002, 1827167, 1845451, and 1892451 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 data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
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Steven P. Kelley: 0000-0001-6755-4495 F
DOI: 10.1021/acs.organomet.9b00043 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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