Steric and Electronic Influences of Internal Alkynes on the Formation of

Article pubs.acs.org/Organometallics

Steric and Electronic Influences of Internal Alkynes on the Formation of Thorium Metallacycles: A Combined Experimental and Computational Study Bo Fang,† Guohua Hou,† Guofu Zi,*,† Wanjian Ding,*,† and Marc D. Walter*,‡ †

Department of Chemistry, Beijing Normal University, Beijing 100875, China Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

‡

S Supporting Information *

ABSTRACT: The formation of thorium metallacyclopentadiene and metallacyclopropene complexes is significantly influenced by the steric and electronic properties of the internal alkyne employed during their syntheses. The reduction of (η5-C5Me5)2ThCl2 (2) with potassium graphite (KC8) or lithium in the presence of internal phenyl(alkyl)acetylenes (PhCCR) selectively yields the corresponding Cssymmetric thorium metallacyclopentadienes (η5-C5Me5)2Th[η2-C(Ph)C(R)C(Ph)C(R)] (R = Me (4), iPr (5), C6H11 (6)), while the phenyl(silyl)acetylene PhCCSiHMe2 gives the C2v-symmetric metallacyclopentadiene (η5-C5Me5)2Th[η2C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] (7). However, the sterically more encumbered acetylene derivative PhCCSiMe3 affords the chloro metallacyclopropene complex [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)][Li(EDM)2] (8), whereas Me3SiC CSiMe3 forms the chloro alkenyl complex [(η5-C5Me5)2Th[C(SiMe3)CHSiMe3](Cl) (9), in which the chloro metallacyclopropene intermolecularly activates a C−H bond of the solvent (C7H8). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C−C bond formation.

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INTRODUCTION Metallacycles have rapidly emerged from laboratory curiosities to important intermediates in catalytic processes and in organic and material synthesis.1−3 In this context metallacyclopropenes and metallacyclopentadienes of group 4 metallocenes are readily accessible from the reaction of Cp′2M (Cp′ = (un)substituted η5-cyclopentadienyl) with alkynes or from the reduction of Cp′2MCl2 in the presence of a suitable alkyne and the resulting metallacycles exhibit unusual intrinsic reactivity.1,2 However, precise stoichiometric control of the added alkyne is usually a prerequisite to prevent the formation of metallacyclopentadienes1d and thereby to isolate the more strained metallacyclopropenes. However, in contrast to the plethora of group 4 metallacycles,1,2 the chemistry of the corresponding actinide derivatives is underdeveloped.4 Therefore, thorium, which adopts an intermediate position between group 4 and actinide metals, represents an interesting synthetic target for further investigations, since its electronic ground state of 7s26d2 relates thorium to the group 4 elements. For example, recent studies established that 5f orbitals contribute to the bonding in organometallic thorium complexes, which explains the distinctively different reactivity in comparison to that of group 4 complexes.5 We have recently prepared the thorium metallacyclopropene [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph2)6 and the thorium metallacyclopentadiene (η5-C5Me5)2Th(η2C4Ph4),7 and studied their intrinsic reactivity with various unsaturated substrates.6,7 The formation of the thorium metallacyclopentadiene (η5-C5Me5)2Th(η2-C4Ph4) most likely © XXXX American Chemical Society

proceeds via the thorium metallacyclopropene intermediate (η5-C5Me5)2Th(η2-C2Ph2),7 which is too reactive to be isolated. In contrast, the more sterically encumbered thorium metallacyclopropene [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph2) is stable in the presence of added alkynes.6 However, the tendency of the (η5-C5Me5)2Th fragment to form metallacyclopentadienes poses the question how this reactivity changes when sterically and electronically (un)symmetrically substituted internal alkynes R1CCR2 are employed; these studies are described in this article.

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RESULTS AND DISCUSSION Reaction of a 1:1 mixture of [η5-1,2,4-(Me3C)3C5H2]2ThCl2 (1) and internal alkynes PhCCR (R = Ph, iPr, C6H11) with an excess of KC8 in toluene solution gives the metallacyclopropenes [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph(R)) (R = Ph, iPr, C6H11) (Scheme 1).6,8 Metallacyclopropenes derived from phenyl(alkyl)acetylenes degrade to the cyclometalated alkenyl complexes via an intramolecular C−H bond activation of the 1,2,4-(Me3C)3C5H2 ligand (Scheme 1),8 whereas [η51,2,4-(Me3C)3C5H2]2Th(η2-C2Ph2) is stable.6 Under similar reaction conditions, [η5-1,2,4-(Me3C)3C5H2]2ThCl2 (1) reacts with dimethylacetylene (MeCCMe) and KC8 to give the thorium metallacyclopentadiene [η5-1,2,4-(Me3C)3C5H2]2ThSpecial Issue: Organometallics in Asia Received: November 11, 2015

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DOI: 10.1021/acs.organomet.5b00945 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1

Scheme 2

(η2-C4Me4) (3) regardless of the amount of MeCCMe employed (Scheme 1). We propose that in analogy to PhC CR (R = Ph, iPr, C6H11) the metallacyclopropene [η5-1,2,4(Me3C)3C5H2]2Th(η2-C2Me2) is formed initially, but it reacts with another 1 equiv of MeCCMe to yield the thermodynamically preferred metallacyclopentadiene 3. All these observations imply that the formation and stability of thorium metallacycles are significantly influenced by the substituents on the acetylene. In an attempt to further investigate this correlation, we decided to also vary the steric properties of the metallocene fragment by extending our investigations to the C5Me5 (Cp*) ligand. In contrast to the sterically more demanding 1,2,4(Me3C)3C5H2 ligand system, (η5-C5Me5)2ThCl2 (2) reacts with internal alkynes PhCCR (R = Ph, Me, iPr, C6H11) in the presence of KC8 to give the metallacyclopentadienes (η5C5Me5)2Th[η2-C(Ph)C(R)-C(Ph)C(R)] (R = Ph,7 Me (4), iPr (5), C6H11 (6)) in good yields (Scheme 2). Again, we propose an initial formation of the corresponding metallacyclopropenes (η5-C5Me5)2Th(η2-C2Ph(R)), but the steric protection is not sufficient to prevent the insertion of a second PhCCR to furnish the thermodynamically preferred metallacyclopentadienes 4−6. Nevertheless, the C−C bond formation occurs selectively, so that the phenyl end of the disubstituted acetylene is coupled to the alkyl-substituted end of the second acetylene, resulting in Cs-symmetric Th[η2C(Ph)C(R)C(Ph)C(R)] fragments. DFT computations regarding the formation of 5 and its possible insertion isomers reveal that the formation of the Cs-symmetric Th[η2-C(Ph) C(iPr)C(Ph)C(iPr)] fragment is indeed more thermodynamically favorable (ΔG(298 K) = −2.5 kcal/mol) than the C2v-symmetric species Th[η2-C(iPr)C(Ph)-C(Ph)C(iPr)] (P5a; ΔG(298 K) = 1.4 kcal/mol) and Th[η2-C(Ph)

C(iPr)C(iPr)C(Ph)] (P5b; ΔG(298 K) = −1.1 kcal/mol). Furthermore, this formation also proceeds with the lowest barrier (TS5; ΔG⧧(298 K) = 28.7 kcal/mol) (Figure 1), which is in total agreement with the experimental observations. In addition, the formation of 5 may proceed in two different ways: that is, via the transition state TS5 or TS5c (Figure 1C). However, the TS5 insertion (TS5; ΔG⧧(298 K) = 28.7 kcal/ mol) appears to be energetically more favorable than TS5c insertion (TS5c; ΔG⧧(298 K) = 30.3 kcal/mol), presumably due to the steric hindrance between the incoming alkyne and the (C5Me5)2Th fragment. The reaction of (η5-C5Me5)2ThCl2 (2) and PhCCSiHMe2 with KC8 also yields the metallacyclopentadiene complex (η5C5Me5)2Th[η2-C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] (7) in good yield, but with an interesting change in the selectivity for C−C bond formation (Scheme 2). In this case the phenylsubstituted end of PhCCSiHMe2 couples with the phenyl end of a second acetylene to give a C2v-symmetric Th[η2C(SiHMe 2 )C(Ph)C(Ph)C(SiHMe 2 )] moiety. This change in selectivity is also represented in our DFT computations, which show that the formation of the C2vsymmetric isomer (η5-C5Me5)2Th[η2-C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] is energetically more favorable (7; ΔG(298 K) = −7.0 kcal/mol) than the other species with C2v symmetry (P7a; ΔG(298 K) = 0.1 kcal/mol) or Cs symmetry (P7b; ΔG(298 K) = −4.3 kcal/mol) and it also proceeds with the lowest barrier ΔG⧧(298 K) = 22.7 kcal/mol (Figure 2), which agrees with our experimental findings. Nevertheless, in contrast to the formation of complex 5, the observed selectivity in the C−C bond formation event leading to complex 7 may also be rationalized by the Mulliken charges in the free alkyne PhCCSiHMe2, the thorium metallacyclopropene intermediate (η 5 -C 5 Me 5 ) 2 Th[η 2 -C 2 Ph(SiHMe2)], and the transition state TS7 (Figure 3). The more negatively charged end of the internal alkyne coordinates to the electropositive Th(IV) atom and therefore electronic effects dominate rather than steric effects. B


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Organometallics

Figure 1. Free energy profiles (kcal/mol) for the reaction of (η5C 5 Me 5 ) 2 Th[η 2 -C(Ph)C( i Pr)] + PhCC i Pr. [Th] = (η 5 C5Me5)2Th.

Figure 2. Free energy profiles (kcal/mol) for the reaction of (η5C5Me5)2Th[η2-C(Ph)C(SiHMe2)] + PhCCSiHMe2. [Th] = (η5C5Me5)2Th.

Interestingly, treatment of (η5-C5Me5)2ThCl2 (2) and PhCCSiMe3 with lithium sand in toluene/THF gives, after recrystallization from a solvent mixture of benzene and ethylene glycol dimethyl ether (EDM), the chloro metallacyclopropene complex [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)][Li(EDM)2] (8) in good yield (Scheme 3). In contrast, when the sterically more encumbered acetylene Me3SiCCSiMe3 is used as the substrate, the chloro alkenyl complex [(η5-C5Me5)2Th[C(SiMe3)CHSiMe3](Cl) (9) is isolated (Scheme 3). Similar to the reaction with PhCCSiMe3, a plausible mechanism includes the initial formation of a chloro metallacyclopropene, followed by intermolecular C−H bond activation of the solvent (C7H8) to give the chloro alkenyl complex 9. This may again be attributed to electronic effects. As discussed above (Figure 3), an alkylsilyl group exerts a −I effect, which increases the negative charge on the alkylsilyl end of the dianionic [η2alkenediyl]2− ligand and therefore protonation should occur preferentially at the more basic alkylsilyl end. In addition, the thermal stability of the PhCCSiMe3 derived anion [(η5C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)]− in 8 may also reflect the reduced basicity of the phenyl-substituted [η2-alkenediyl]2− ligand; therefore, the anion [(η5-C5Me5)2Th(η2-C2(SiMe3)2)(Cl)]− derived from Me3SiCCSiMe3 is thermally converted

Figure 3. Mulliken charges of the free alkynes, the respective thorium metallacyclopropenes, and transition state complexes. [Th] denotes the Th(IV) atom.

to the alkenyl complex 9 by deprotonation of the solvent (toluene). Complexes 3−9 are stable under a dry nitrogen atmosphere, but they are moisture sensitive. They were characterized by various spectroscopic techniques, elemental analyses, and single-crystal X-ray diffraction analyses. The solid-state C


Article

Organometallics Scheme 3

Figure 6. Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).

molecular structures of 3−7 are shown in Figures 4−8, while selected bond distances and angles are given in Table 1. In each




distorted-tetrahedral geometry with average Th−C(Cp) distances of 2.801(6)−2.885(4) Å, Cp(cent)−Th−Cp(cent) angles of 133.6(4)−146.2(2)°, and C−Th−C angles of 75.4(2)−82.2(4)°. The Th−C distances of 2.399(5)− 2.499(11) Å are comparable to reported Th−C(sp2) σ bonds (2.420(3)−2.654(14) Å).9 The C−C distances within the butadiene are 1.360(7), 1.536(7), and 1.385(8) Å for 3, 1.341(8), 1.515(8), and 1.343(7) Å for 4, 1.342(19),


molecule, the Th4+ ion is η5-bound to two Cp rings and σcoordinated to two carbon atoms of the dianion [η2-C4Me4]2− (for 3), [η2-C(Ph)C(R)C(Ph)C(R)]2− (for 4−6), or [η2C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)]2− (for 7) in a D


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Organometallics Table 1. Selected Distances (Å) and Angles (deg) for Compounds 3−9a compound

C(Cp)−Thb

C(Cp)−Thc

Cp(cent)−Thb

3 4 5 6 7 8 9

2.885(4) 2.801(6) 2.829(14) 2.831(11) 2.810(8) 2.858(5) 2.806(10)

2.803(4)−2.980(4) 2.752(6)−2.831(5) 2.775(14)−2.866(14) 2.802(11)−2.864(11) 2.792(7)−2.821(7) 2.821(5)−2.886(5) 2.787(10)−2.834(10)

2.620(4) 2.529(6) 2.557(14) 2.559(11) 2.541(8) 2.590(5) 2.537(10)

a

Th−X C(36) C(22) C(21) C(21) C(21) C(27) C(22)

2.404(4), C(39) 2.399(5) 2.443(5), C(25) 2.453(6) 2.465(13), C(24) 2.467(14) 2.499(11), C(24) 2.456(11) 2.476(7), C(24) 2.463(6) 2.415(5), C(28) 2.432(5), Cl 2.747(1) 2.486(8), Cl 2.663(2)

Cp(cent)−Th− Cp(cent)

X−Th−X/Y

146.2(2) 143.8(3) 136.9(4) 133.6(4) 138.0(2) 134.2(2) 136.9(2)

81.4(2) 75.4(2) 78.0(4) 82.2(4) 76.7(2) 32.6(2)d 93.7(2)

Cp = cyclopentadienyl ring. bAverage value. cRange. dThe angle C(27)−Th(1)−C(28).

1.553(14), and 1.380(19) Å for 5, 1.382(15), 1.521(15), and 1.351(15) Å for 6, and 1.364(9), 1.528(9), and 1.359(9) Å for 7, which are in a range similar to those observed in other actinide metallacyclopentadiene complexes (η5-C5Me5)2U(η2C4Ph4) (1.365(3), 1.509(4), and 1.365(3) Å)4e and (η5C5Me5)2Th(η2-C4Ph4) (1.362(3), 1.516(4), and 1.362(3) Å).7 The molecular structure of 8 consists of well-separated, alternating layers of the discrete octahedral [Li(EDM)2]+ cations and [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)]− anions. Furthermore, one molecule of benzene cocrystallized. In the anion [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)]−, the Th4+ ion is η5-bound to two Cp rings and σ-coordinated to two carbon atoms of the dianion [η2-C2Ph(SiMe3)]2− and one chlorine atom (Figure 9) with an average Th−C(Cp) distance of


The Th−Cl distance (2.663(2) Å) is comparable to those found in [η5-1,2,4-(Me3C)3C 5H2]2ThCl2 (2.619(1) and 2.624(1) Å)10 and [η5-1,2,4-(Me3C)3C5H2]2Th(Cl)[N(ptolyl)SiH2Ph] (2.613(3) Å)11 but shorter than that in 8 (2.747(1) Å).

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CONCLUSIONS In conclusion, the steric and electronic influences of alkyne substituents on the formation of the thorium metallacycles were investigated. The Th(η2-PhCCR) moiety in the thorium metallacyclopropenes [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph(R)) (R = Ph, iPr, C6H11) exhibits no reactivity toward additional alkynes. However, whereas the thorium metallacyclopropene [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph2) is thermally stable,6,8 the [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C2Ph(R)) (R = iPr, C6H11) derivatives are too reactive to be isolated and thermally convert to the cyclometalated complexes via an intramolecular C−H bond activation of the 1,2,4-(Me3C)3C5H2 ligand.8 When the cyclopentadienyl ligands on the thorium metallacyclopropene are changed from 1,2,4-(Me3C)3C5H2 to the less sterically demanding C5Me5, a distinct change in reactivity is noted, which is illustrated by their reactivity toward a second internal alkyne. The metallacyclopropenes (η5C5Me5)2Th(η2-C2Ph(R)) (R = Ph, Me, iPr, C6H11) cannot be isolated but convert to metallacyclopentadienes via the C−C coupling with a second alkyne PhCCR regardless of the amount of alkyne added during the synthesis. Furthermore, the coupling of the internal alkynes PhCCR (Me, iPr, C6H11) proceeds selectively to give the Cs-symmetric thorium metallacyclopentadienes (η5-C5Me5)2Th[η2-C(Ph)C(R)C(Ph) C(R)] (4−6), whereas only the C2v-symmetric isomer (η5-

Figure 9. Molecular structure of the anion [(η5-C5Me5)2Th(η2C2Ph(SiMe3))(Cl)]− in 8 (thermal ellipsoids drawn at the 35% probability level).

2.858(5) Å and a Cp(cent)−Th−Cp(cent) angle of 134.2(2)°. The Th−C distance is 2.415(5) Å for C(27) and 2.432(5) Å for C(28), and the C(27)−Th−C(28) angle is 32.6(2)°. These parameters are close to those found for the thorium metallacyclopropene complex [η5-1,2,4-(Me3C)3C5H2]2Th(η2C2Ph2) with a Th−C distance of 2.395(2) Å and a C−Th−C angle of 32.6(1)°.6 However, the Th−Cl distance of 2.747(1) Å is slightly elongated in comparison to those in [η5-1,2,4(Me3C)3C5H2]2ThCl2 (2.619(1) and 2.624(1) Å).10 The molecular structure of 9 is shown in Figure 10. The Th4+ ion is η5-bound to two Cp rings and σ-coordinated to a carbon atom of the anion [C(SiMe3)CHSiMe3]− and a chlorine atom with an average Th−C(Cp) distance of 2.806(10) Å and a Cp(cent)−Th−Cp(cent) angle of 136.9(2)°. The Th−C(22) distance (2.486(8) Å) is in the range of those previously reported for Th−C(sp2) σ bonds (2.420(3)−2.654(14) Å).9 E


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Organometallics C5Me5)2Th[η2-C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] (7) is isolated when PhCCSiHMe2 is used as the substrate. The change in selectivity is also reflected in our DFT computations, which showed that the formation of the Cs-symmetric Th[η2C(Ph)C(iPr)C(Ph)C(iPr)] fragment is energetically more favorable for 5, while the formation of the C2v-symmetric Th[η2-C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] is preferred for 7. The observed selectivity for the formation of complex 7 can be traced to the electronic influence of the substituents polarizing the CC triple bond, and the more negative end then coordinates to the electropositive Th(IV) atom, while steric hindrance dominates in the formation of complex 5. However, when the steric bulk of the substituents on the thorium metallacyclopropene is increased further from phenyl or alkyl (Me, iPr, C6H11) to Me3Si the reactivity changes substantially. The thorium metallacyclopropene derived from PhCCSiMe3 is isolated as a LiCl adduct [(η5-C5Me5)2Th(η2C2Ph(SiMe3))(Cl)][Li(EDM)2] (8), but the corresponding thorium metallacyclopropene derived from Me3SiCCSiMe3 is too reactive and converts to a chloro alkenyl complex by deprotonation of the solvent (toluene). Further development of novel actinide metallacycles and the exploration of their intrinsic reactivity are ongoing projects in our laboratories.

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C6H6), 6.95 (m, 4H, phenyl), 2.05 (s, 3H, CH3), 1.99 (s, 3H, CH3), 1.94 (s, 30H, ring CH3) ppm. 13C{1H} NMR (C6D6): δ 221.2 (ThCPh), 220.6 (ThCCH3), 148.0 (ThCCPh), 147.4 (ThC CCH3), 144.9 (phenyl C), 141.3 (phenyl C), 129.0 (phenyl C), 128.5 (phenyl C), 128.4 (phenyl C), 128.0 (C6H6), 127.1 (phenyl C), 125.6 (phenyl C), 124.2 (ring C), 122.8 (phenyl C), 23.7 (CH3), 21.8 (CH3), 11.4 (ring CH3) ppm. IR (KBr, cm−1): ν 2904 (s), 2856 (s), 1589 (s), 1436 (s), 1379 (s), 1199 (s), 1070 (s), 1024 (s), 956 (s), 846 (s), 756 (s). Anal. Calcd for C41H49Th: C, 63.63; H, 6.38. Found: C, 63.72; H, 6.43. Preparation of (η5-C5Me5)2Th[η2-C(Ph)C(CHMe2)C(Ph)C(CHMe2)] (5). This compound was prepared as yellow crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and PhC CCHMe2 (0.61 g, 4.2 mmol) in the presence of KC8 (0.85 g, 6.3 mmol) in toluene (20 mL) at 70 °C and recrystallization from an nhexane solution by a procedure similar to that in the synthesis of 3. Yield: 1.38 g (83%). Mp: 150−152 °C. 1H NMR (C6D6): δ 7.41 (d, J = 7.1 Hz, 2H, phenyl), 7.33 (t, J = 7.7 Hz, 2H, phenyl), 7.27 (t, J = 7.6 Hz, 2H, phenyl), 7.10 (m, 3H, phenyl), 6.99 (t, J = 7.3 Hz, 1H, phenyl), 3.25 (m, 1H, CH(CH3)2), 3.05 (m, 1H, CH(CH3)2), 2.03 (s, 30H, ring CH3), 1.08 (d, J = 6.7 Hz, 6H, CH(CH3)2), 0.93 (d, J = 7.2 Hz, 6H, CH(CH3)2) ppm. 13C{1H} NMR (C6D6): δ 233.2 (ThCPh), 219.1 (ThC(iPr)), 151.2 (phenyl C), 147.9 (phenyl C), 147.6 (phenyl C), 144.9 (phenyl C), 130.5 (phenyl C), 128.5 (phenyl C), 127.9 (phenyl C), 127.4 (phenyl C), 125.8 (ThCCPh), 125.4 (ring C), 123.0 (ThCC(i-Pr)), 35.7 (CH(CH3)2), 33.1 (CH(CH3)2), 24.6 (CH(CH3)2), 24.3 (CH(CH3)2), 12.3 (ring CH3) ppm. IR (KBr, cm−1): ν 2959 (s), 2926 (s), 1598 (m), 1463 (s), 1382 (s), 1073 (s), 1027 (s), 915 (s), 755 (s). Anal. Calcd for C42H54Th: C, 63.78; H, 6.88. Found: C, 63.72; H, 6.93. Preparation of (η5-C5Me5)2Th[η2-C(Ph)C(C6H11)C(Ph)C(C6H11)] (6). This compound was prepared as pale yellow crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and PhCC(C6H11) (0.77 g, 4.2 mmol) in the presence of KC8 (0.85 g, 6.3 mmol) in toluene (20 mL) at 70 °C and recrystallization from an n-hexane solution by a procedure similar to that in the synthesis of 3. Yield: 1.61 g (88%). Mp: 186−188 °C. 1H NMR (C6D6): δ 7.41 (d, J = 7.4 Hz, 2H, phenyl), 7.33 (t, J = 7.5 Hz, 2H, phenyl), 7.27 (t, J = 7.5 Hz, 2H, phenyl), 7.12 (m, 3H, phenyl), 7.00 (t, J = 7.1 Hz, 1H, phenyl), 2.81 (t, J = 11.8 Hz, 1H, CH), 2.69 (t, J = 11.4 Hz, 1H, CH), 2.06 (s, 30H, ring CH3), 1.83 (m, 2H, CH2), 1.74 (m, 4H, CH2), 1.45 (m, 6H, CH2), 1.16 (m, 6H, CH2), 0.92 (m, 2H, CH2) ppm. 13C{1H} NMR (C6D6): δ 231.1 (ThCPh), 219.0 (ThCCy), 151.2 (phenyl C), 148.7 (phenyl C), 147.6 (phenyl C), 145.2 (phenyl C), 130.2 (phenyl C), 128.5 (phenyl C), 127.6 (phenyl C), 127.4 (phenyl C), 125.8 (CPh), 125.4 (ring C), 123.0 (CCy), 48.4 (CH), 45.3 (CH), 34.7 (CH2), 33.4 (CH2), 27.8 (CH2), 26.9 (CH2), 26.6 (CH2), 26.5 (CH2), 12.4 (ring CH3) ppm. IR (KBr, cm−1): ν 2923 (s), 2850 (s), 1585 (m), 1445 (s), 1383 (s), 1072 (s), 1027 (s), 971 (s), 758 (s). Anal. Calcd for C48H62Th: C, 66.19; H, 7.17. Found: C, 66.25; H, 7.15. Preparation of (η5-C5Me5)2Th[η2-C(SiHMe2)C(Ph)C(Ph)C(SiHMe2)] (7). This compound was prepared as orange crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and PhC CSiHMe2 (0.67 g, 4.2 mmol) in the presence of KC8 (0.85 g, 6.3 mmol) in toluene (20 mL) at 70 °C and recrystallization from an nhexane solution by a procedure similar to that in the synthesis of 3. Yield: 1.24 g (72%). Mp: 204−206 °C. 1H NMR (C6D6): δ 6.93 (m, 8H, phenyl), 6.76 (m, 2H, phenyl), 4.20 (m, 2H, SiH), 2.14 (s, 30H, CH3), 0.02 (d, J = 3.5 Hz, 12H, Si(CH3)2) ppm. 13C{1H} NMR (C6D6): δ 232.5 (ThC), 163.0 (CPh), 147.9 (phenyl C), 129.2 (phenyl C), 127.3 (phenyl C), 125.2 (phenyl C), 125.0 (ring C), 11.8 (ring CH3), 0.1 (SiCH3) ppm. IR (KBr, cm−1): ν 2956 (s), 2904 (s), 2139 (m), 1552 (m), 1438 (s), 1381 (s), 1244 (s), 1072 (s), 1022 (s), 879 (s). Anal. Calcd for C40H54Si2Th: C, 58.37; H, 6.61. Found: C, 58.45; H, 6.69. Preparation of [(η5-C5Me5)2Th(η2-C2Ph(SiMe3))(Cl)][Li(EDM)2]· C6H6 (8·C6H6). This compound was prepared as orange crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and PhC CSiMe3 (0.37 g, 2.1 mmol) in the presence of lithium sand (44 mg, 6.3 mmol) in a toluene and THF mixture (20 mL; 1/1 v/v) at 70 °C and

EXPERIMENTAL SECTION

General Methods. All reactions and product manipulations were carried out under an atmosphere of dry dinitrogen with rigid exclusion of air and moisture using standard Schlenk or cannula techniques or in a glovebox. All organic solvents were freshly distilled from sodium benzophenone ketyl immediately prior to use. KC8,12 [η5-1,2,4(Me3C)3C5H2]2ThCl2 (1),10,13 and (η5-C5Me5)2ThCl2 (2)7 were prepared according to literature methods. All other chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. and used as received unless otherwise noted. Infrared spectra were recorded in KBr pellets on an Avatar 360 Fourier transform spectrometer. 1H and 13C{1H} NMR spectra were recorded on a Bruker AV 400 spectrometer at 400 and 100 MHz, respectively. All chemical shifts are reported in δ units with reference to the residual protons of the deuterated solvents, which served as internal standards, for proton and carbon chemical shifts. Melting points were measured on an X-6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer. Preparation of [η5-1,2,4-(Me3C)3C5H2]2Th(η2-C4Me4) (3). KC8 (0.81 g, 6.0 mmol) was added to a toluene (20 mL) solution of [η51,2,4-(Me3C)3C5H2]2ThCl2 (1; 1.54 g, 2.0 mmol) and MeCCMe (0.22 g, 4.0 mmol) with stirring at room temperature. After this solution was stirred for 1 day at 35 °C, the solvent was removed. The residue was extracted with benzene (10 mL × 3) and filtered. The volume of the filtrate was reduced to 4 mL, and colorless crystals of 3 were isolated when this solution was kept at room temperature for 2 days. Yield: 1.32 g (82%). Mp: 203−205 °C. 1H NMR (C6D6): δ 6.29 (s, 4H, ring CH), 2.21 (s, 6H, CH3), 2.02 (s, 6H, CH3), 1.43 (s, 36H, C(CH3)3), 1.34 (s, 18H, C(CH3)3) ppm. 13C{1H} NMR (C6D6): δ 215.3 (ThCMe), 143.1 (ring C), 143.0 (ring C), 142.4 (ring C), 112.8 (CMe) 34.5 (C(CH3)3), 34.2 (C(CH3)3), 33.2 (C(CH3)3), 33.0 (C(CH3)3), 25.3 (CH3), 17.9 (CH3) ppm. IR (KBr, cm−1): ν 2961 (m), 1455 (m), 1385 (s), 1358 (s), 1261 (s), 1237 (s), 1020 (s), 818 (s). Anal. Calcd for C42H70Th: C, 62.51; H, 8.74. Found: C, 62.56; H, 8.68. Preparation of (η5-C5Me5)2Th[η2-C(Ph)C(Me)-C(Ph)C(Me)]·0.5C6H6 (4·0.5C6H6). This compound was prepared as pale yellow crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and PhCCCH3 (0.49 g, 4.2 mmol) in the presence of KC8 (0.85 g, 6.3 mmol) in toluene (20 mL) at 70 °C and recrystallization from an n-hexane solution by a procedure similar to that in the synthesis of 3. Yield: 1.30 g (80%). Mp: 138−140 °C. 1H NMR (C6D6): δ 7.34 (m, 4H, phenyl), 7.22 (m, 2H, phenyl), 7.15 (s, 3H, F


Organometallics

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recrystallization from a benzene and ethylene glycol dimethyl ether (EDM) mixture (2 mL; 10/1 v/v) by a procedure similar to that in the synthesis of 3. Yield: 1.70 g (76%). Mp: 156−158 °C. 1H NMR (C6D6): δ 7.15 (s, 6H, C6H6), 7.01 (m, 2H, phenyl), 6.89 (d, J = 7.4 Hz, 2H, phenyl), 6.71 (t, J = 7.1 Hz, 1H, phenyl), 3.30 (t, J = 4.6 Hz, 8H, OCH2CH2OCH3), 3.14 (t, J = 4.6 Hz, 8H, OCH2CH2OCH3), 3.05 (s, 12H, OCH3), 2.30 (s, 30H, CH3), 0.44 (s, 9H, Si(CH3)3) ppm. 13C{1H} NMR (C6D6): δ 231.9 (ThCSi), 195.4 (ThCPh), 147.2 (phenyl C), 132.8 (phenyl C), 129.3 (phenyl C), 125.6 (phenyl C), 121.3 (ring C), 71.9 (OCH2CH2OCH3), 70.3 (OCH2CH2OCH3), 58.7 (OCH2CH2OCH3), 12.4 (ring CH3), 4.7 (SiCH3) ppm. IR (KBr, cm−1): ν 2922 (s), 2854 (s), 1583 (m), 1452 (s), 1379 (s), 1246 (s), 1083 (s), 1018 (s), 837 (s). Anal. Calcd for C49H78ClLiO6SiTh: C, 55.23; H, 7.38. Found: C, 55.31; H, 7.43. When KC8 was used as reducing reagent, unreacted (η5C5Me5)2ThCl2 (2) and PhCCSiMe3 were always detected by 1H NMR spectroscopy: i.e., the conversion was not complete. Preparation of [(η5-C5Me5)2Th[C(SiMe3)CHSiMe3](Cl) (9). This compound was prepared as colorless crystals from the reaction of (η5-C5Me5)2ThCl2 (2; 1.20 g, 2.1 mmol) and Me3SiCCSiMe3 (0.36 g, 2.1 mmol) in the presence of KC8 (0.85 g, 6.3 mmol) in toluene (20 mL) at 70 °C and recrystallization from a benzene solution by a procedure similar to that in the synthesis of 3. Yield: 0.98 g (66%). Mp: 188−190 °C. 1H NMR (C6D6): δ 6.63 (s, 1H, CCH), 1.99 (s, 30H, CH3), 0.43 (s, 9H, Si(CH3)3), 0.32 (s, 9H, Si(CH3)3) ppm. 13 C{1H} NMR (C6D6): δ 250.4 (ThC), 145.3 (ThCCH), 125.2 (ring C), 12.1 (ring CH3), 2.7 (SiCH3), 1.6 (SiCH3) ppm. IR (KBr, cm−1): ν 2961 (s), 2900 (s), 1438 (s), 1382 (s), 1260 (s), 1089 (s), 1018 (s), 839 (s), 800 (s). Anal. Calcd for C28H49ClSi2Th: C, 47.41; H, 6.96. Found: C, 47.45; H, 7.01. When perdeuterated toluene (C7D8) was used as solvent, the resonance at δ 6.63 ppm corresponding to the Me3SiCCHSiMe3 fragment in 9 disappeared, which was also confirmed by 2H NMR spectroscopy, supporting the notion that a hydrogen atom was transferred from toluene to the alkenyl group. However, benzylpotassium (PhCH2K) was not isolated as a bright red solid, since the graphite that formed masked its color. X-ray Crystallography. Single-crystal X-ray diffraction measurements were carried out on a Bruker Smart APEX II CCD diffractometer at 100(2) K using graphite-monochromated Mο Kα radiation (λ = 0.71073 Å). An empirical absorption correction was applied using the SADABS program.14 All structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL-97 program package.15 All other hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 3−9 are summarized in Tables S1 and S2 in the Supporting Information. Selected bond lengths and angles are given in Table 1. Computational Methods. All calculations were carried out with the Gaussian 09 program (G09),16 employing the B3PW91 functional plus a polarizable continuum model (PCM) (denoted as B3PW91PCM) with standard 6-31G(d) basis set for C, H and Si elements and Stuttgart RLC ECP from the EMSL basis set exchange (https://bse. pnl.gov/bse/portal) for Th,17 to fully optimize the structures of reactants, complexes, transition state, intermediates, and products and also to mimic the experimental toluene solvent conditions (dielectric constant ε = 2.379). All stationary points were subsequently characterized by vibrational analyses, from which their respective zero-point (vibrational) energies (ZPEs) were extracted and used in the relative energy determinations; in addition, frequency calculations were also performed to ensure that the reactant, complex, intermediate, product and transition state structures resided at minima and first-order saddle points, respectively, on their potential energy hypersurfaces.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00945. Crystal parameters for compounds 3−9, computational details, and 1H NMR spectra of complex 9 (PDF) Cartesian coordinates of all stationary points optimized at the B3PW91-PCM level (XYZ) X-ray crystallographic data for compounds 3−9 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for G.Z.: [email protected]. *E-mail for W.D.: [email protected]. *E-mail for M.D.W.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21472013, 21172022, 21272026, 21573021), and the Deutsche Forschungsgemeinschaft (DFG) through the Emmy-Noether and Heisenberg program (WA 2513/2 and WA 2513/6, respectively).

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