Experimental and Computational Studies of a Uranium

Article pubs.acs.org/Organometallics

Experimental and Computational Studies of a Uranium Metallacyclocumulene Lei Zhang,† 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 synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η5-C5Me5)2UCl2] (1) with potassium graphite (KC8) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me3SiCC−C CSiMe3) forms the uranium metallacyclocumulene [(η5C5Me5)2U{η4-C4(SiMe3)2}] (2) in good yield. Magnetic susceptibility data confirm that 2 behaves as a U(IV) complex, and density functional theory (DFT) studies indicate a substantial 5f orbital contribution to the bonding of the metallacyclopentatriene U(η4-CCCC) moiety, leading to more covalent bonds between the [(η5-C5Me5)2U]2+ and [η4-C4(SiMe3)2]2− fragments than those found in the related Th(IV) compound. Consequently, very different reactivity patterns emerge; e.g., 2 can act as a synthetic equivalent for the (η5-C5Me5)2U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η4-Me3SiCCCCSiMe3)]2− moiety in 2 may react as a nucleophile when exposed to a variety of simple heterounsaturated molecules such as aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, pyridines, and organic azides. DFT studies are included to complement the experimental observations.

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INTRODUCTION Metallacycles of d-transition metals play an important role in many catalytic and material science applications,1 and they have therefore been investigated in detail. In contrast, the related lanthanide and actinide derivatives have only sporadically been examined, and this research field has remained more or less dormant for more than 35 years following the initial reports by Marks and co-workers.2 During this time, however, metallacycles of group 4 elements have been studied extensively, and their reactivity is now well explored and understood, which has also led to interesting synthetic applications.1,3 An eminent question in organoactinide chemistry is the influence of the 5f orbitals on the bonding and reactivity of these compounds, because variations in the 5f orbital contribution can modulate reactivity, including the applications of these compounds in the activation and functionalization of small molecules.4−6 A few years ago, we started exploring the influence of 5f orbitals on the intrinsic reactivity of thorium organometallics to compare it to that of the related group 4 compounds.6 As part of our continued interest in this area, we also initiated a research program on the so far largely neglected actinide metallacycles.7−9 In this context, we recently reported the first isolable actinide metallacyclopentatriene [(η5-C5Me5)2Th(η4-C4Ph2)], in which the butadiyne is coordinated strongly and gives rise to interesting reactivity with various heterounsaturated molecules such as isothiocyanates, carbodiimides, aldehydes, ketones, nitriles, pyridines, and diazoalkane derivatives.10 We also established that the alkyne in the thorium metallacyclopropene [{η5-1,2,4-(Me3C)3C5H2}2Th(η2-C2Ph2)] reacts as a nucleo© XXXX American Chemical Society

phile toward heterounsaturated molecules or as a strong base inducing intermolecular C−H bond activations,8 whereas the related uranium metallacyclopropene [(η5-C5Me5)2U{η2C2(SiMe3)2}] serves as a synthetic equivalent for the (η5C5Me5)2U(II) fragment when reacted with unsaturated molecules.9 Motivated by this remarkable difference, we have extended our studies to the so far unexplored uranium metallacyclocumulenes. Some of our observations concerning the electronic structure and structure−reactivity relationship of the uranium metallacyclopentatriene [(η5 -C 5Me5 )2 U{η 4C4(SiMe3)2}] (2) are described herein along with the differences and similarities between the uranium and thorium metallacyclopentatrienes.

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RESULTS AND DISCUSSION Synthesis of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2). Reduction of a 1:1 mixture of (η5-C5Me5)2UCl2 (1) and 1,4bis(trimethylsilyl)butadiyne (Me3SiCC−CCSiMe3) with an excess of KC8 in a toluene solution yields the metallacyclopentatriene, [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2),11 as brown crystals in 88% yield (Scheme 1). The 29Si nuclear magnetic resonance (NMR) spectrum of 2 features a resonance at δ 237.9. This 29Si NMR chemical shift is much larger than the values between 0 and −150 ppm reported by Windorff and Evans for various U(IV) compounds.12 Nevertheless, in contrast to its group 4 analogues,1k no uranium metalReceived: December 16, 2016

A

DOI: 10.1021/acs.organomet.6b00936 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1

lacyclopropene [(η5-C5Me5)2U(η2-1,2-Me3SiC2CCSiMe3)] can be isolated, which may arise from the larger ionic radius of the U4+ cation.13 In addition, variable-temperature (20−100 °C) 1H NMR spectroscopic studies show no isomerization of 2 to a three-membered η2-metallacyclopropene P2, and the density functional theory (DFT) calculations show that P2 is less stable by more than 17.6 kcal/mol (see Figure 3). Solid State Magnetic Susceptibility Studies (SQUID). The electronic structure of 2 was further probed by solid state magnetic susceptibility studies, and the results are presented in Figure 1. Like the uranium metallacyclopropene [(η5-C5Me5)2-

Figure 2. Plots of the relevant MOs for 2 (the hydrogen atoms have been omitted for the sake of clarity).

molecular orbital (NLMO) analysis (Table 1) suggests that U− C σ-bonds, σ(U−C), are composed of a carbon hybrid orbital (76.5%; 30.5% s and 69.5% p) and a uranium hybrid orbital (20.2%; 30.7% 5f and 52.8% 6d). The other bonding interactions can be divided into two σ-bonds, σ1(CC) and σ2(CC), and three π-bonds, π1[U(CC)], π2[U(CC)], and π(CCCC). The σ1-bond is composed of a carbon sp3.7 hybrid orbital (46.2%) and a carbon sp1.3 hybrid orbital (51.0%), whereas the σ2-bond consists of pure carbon sp1.6 hybrid orbitals (97.3%). In the in-plane π-bond, π2[U(CC)], pure carbon p orbitals (86.8%) donate electron density to a uranium hybrid orbital (9.5%; 45.7% 5f and 53.1% 6d). The out-of-plane π-bond, π1[U(CC)], is composed of 86.5% carbon occupancy consisting of only p orbitals and a 7.1% contribution from a uranium hybrid orbital (59.5% 5f and 38.8% 6d), whereas the other out-of-plane delocalized π-bond, π(CCCC), is mainly formed by pure carbon p orbitals (90.8%). Furthermore, a spin density of 2.19 on uranium was computed (Table 1), which also supports the notion that electron density is transferred from the π-orbital of Me3SiC CCCSiMe3 to the electron deficient metal uranium atom. The uranium contribution to the U−C σ-bond (20.2%), and U−(CC) π1-bond (7.1%), and π2-bond (9.5%) is in the same range as those computed for the related zirconium [23.6% for the Zr−C σ-bond and 3.4 and 9.6% for two type Zr−(CC) π-bonds] metallacyclopentatrienes.15b Therefore, the bonding in 2 resembles that of the delocalized metallacyclopentatriene moiety reported for group 4 compounds.1k,15 However, in the thorium complex [(η5-C5Me5)2Th{η4-C4(SiMe3)2}] (2′), the metal contribution to the bonding of the Th[η4-C4(SiMe3)2] moiety decreases notably (16.2% Th for the Th−C σ-bond and 5.2 and 6.5% Th for the Th−(CC) π1- and π2-bonds, respectively) (Table 1). This is also reflected in an increased charge separation and therefore an increased level of electrostatic interaction between the individual [(η5-C5Me5)2An]2+ and [η4-C4(SiMe3)2]2− fragments, that is, 1.44 [for An = U (2′)] and 2.11 [for An = Th (2′)]. The Meyer bond order within the An−[C4(SiMe3)2] moiety increases from 0.69, 0.46, 0.46, and 0.69 (for 2) to 0.71, 0.49, 0.49, and 0.71 (for 2′) (Table 1). Both observations are consistent with a more polarized and more ionic bond between the metallocene (η5-

Figure 1. Plots of 1/χ and μeff vs T for 2.

U{η2-C2(SiMe3)2}], complex 2 exhibits temperature-independent paramagnetism at low temperatures. This behavior is typical for U(IV) compounds for which a nonmagnetic ground state with a thermally accessible first excited state can be realized.14 Furthermore, the effective magnetic moment [μeff(4 K) = 0.73(1) μB] (see the Supporting Information for details) is in the typical range found for U(IV) compounds in general (∼0.5−0.8 μB at approximately 4 K).14 These magnetic results are consistent with the description of the uranium(IV) metallacyclopentatriene that was inferred from the structural data of 2.9a Bonding Studies. The interaction between the uranium atom and the Me3SiCCCCSiMe3 moiety in 2 was further evaluated by density functional theory (DFT) computations at the B3PW91 level of theory and compared to that in the related thorium analogue [(η5-C5Me5)2Th{η4C4(SiMe3)2}] (2′). In both cases, the computed structures are in excellent agreement with the experimental data (for details, see the Supporting Information) and show that the Me3SiC CCCSiMe3 fragment is coordinated to the (η5-C5Me5)2An moiety by two An−C σ-bonds and one in-plane π-bond in an η4-σ,σ,π2 fashion, as illustrated in Figure 2. A natural localized B


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Organometallics

Reactivity Studies. In analogy to the thorium metallacyclopentatrienes,10 variable-temperature (20−100 °C) 1H NMR investigations provide no indication for alkyne dissociation upon heating of 2 to 100 °C. This is consistent with a strong coordination of the bis(trimethylsilyl)butadiyne moiety to the uranium atom, which is responsible for the thermal stability of 2 at high temperatures. Nevertheless, contrary to thorium metallacyclopentatrienes,10,16 the coordinated bis(trimethylsilyl)butadiyne in 2 can be readily replaced with conjugated alkynes or 2,2′-bipyridine (bipy). For example, addition of diphenylbutadiyne (PhCCCCPh) to 2 at 70 °C furnishes uranium metallacyclopentatriene [(η5-C5Me5)2U(η4-C4Ph2)] (3)9b (Scheme 2). Nevertheless, simple alkynes

Table 1. Natural Localized Molecular Orbital (NLMO) Analysis of An−[C4(SiMe3)2] Bonds,a Spin Densities on Metals, Bond Order, and the Natural Charge of the [η4C4(SiMe3)2] Unit and Metal for [Cp*2An{η4-C4(SiMe3)2}] Complexes σ An−C

π1 An−(CC)

π2 An−(CC)

π C4 σ1 CC

σ2 CC

NBO charge (An) NBO charge [C4(SiMe3)2] Mülliken spin density (An) Mayer bond order {An−[C4(SiMe3)2]}

% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %

An s p d f C s p An p d f C2 p An s p d f C2 p C4 p C s p C s p C2 s p

2 (U)

2′ (Th)

20.2 12.3 4.2 52.8 30.7 76.5 30.5 69.5 7.1 1.6 38.8 59.5 86.5 100 9.5 0.3 0.9 53.1 45.7 86.8 100 90.8 100 46.2 21.4 78.6 51.0 44.1 55.9 97.3 38.5 61.5 0.96 −0.48 2.19 0.69 0.46 0.46 0.69

16.2 15.6 3.8 66.3 14.2 80.5 28.6 71.4 5.2 3.7 51.7 44.6 88.1 100 6.5 0.7 2.7 74.7 21.9 91.1 100 92.9 100 46.1 26.1 73.9 50.5 45.3 54.7 96.8 38.9 61.1 1.37 −0.74 0.00 0.71 0.49 0.49 0.71

Scheme 2

such as RCCR (R = Ph, Me, or Me3Si) do not replace the coordinated bis(trimethylsilyl)butadiyne ligand in complex 2 even when it is heated at 100 °C for 1 week. In contrast, the bis(trimethylsilyl)butadiyne moiety in 2 can be exchanged for diazabutadiene (p-tolylNCH)2, in which the five-membered metallaheterocycle [(η5-C5Me5)2U{η2-N(p-tolyl)CHCHN(p-tolyl)}] (4)9a is formed (Scheme 2). In a similar manner, the uranium bipy complex [(η5-C5Me5)2U(bipy)] (5)9a,17 can also be formed after addition of 2,2′-bipyridine (bipy) to complex 2 (Scheme 2). Nevertheless, in analogy to the reactivity of the thorium metallacyclopentatriene [(η5-C5Me5)2Th(η4-C4Ph2)] toward isothiocyanates,10 insertion of 1 equiv of PhNCS into the uranium metallacyclopentatriene moiety of 2 yields the sevenmembered heterocyclic complex [(η5-C5Me5)2U{SC(NPh){C4(SiMe3)2}}] (6) in quantitative conversion (Scheme 3). DFT computations suggest that the metallacyclopropene adduct COM6 forms when PhNCS initially coordinates to 2 (Figure 3). In the next step, PhNCS inserts into the U− [C4(SiMe3)2] moiety of COM6 via transition state TS6 to yield the thermodynamically favorable product 6 [ΔG(298 K) = −37.2 kcal/mol]. The molecular structure of 6 is shown in Figure 4, and selected bond distances and angles are listed in Table 2. The U−C bond distances [U−C(29), 2.645(6) Å; U− C(30), 2.589(6) Å; and U−C(31), 2.581(6) Å] are stretched relative to those found in 2 [2.515(5), 2.434(5), 2.435(5), and 2.487(5) Å],9a whereas the U−S bond distance of 2.762(2) Å is

a

The contributions by the atom and orbital are averaged over all the ligands of the same character (complexes of U and Th) and over α and β orbital contributions (complex of U).

C5 Me5 )2 Th2+ and the butatriene-diyl fragment [η 4-C 4(SiMe3)2]2−. Consequently, the π-donation from the π-MO of the butatriene-diyl fragment to the metal atom decreases, which is attributed to an increase in the 5f orbital energy of the thorium atom relative to those of the uranium atom.4g,h The 5f orbital contribution to the bonding in 2 [30.7% for the U−C σbond and 59.5 and 45.7% for the U−(CC) π1- and π2-bonds, respectively] is substantially larger than that of the 5f orbitals in [(η5-C5Me5)2Th{η4-C4(SiMe3)2}] (2′) [14.2% for the Th−C σ-bond and 44.6 and 21.9% for the Th−(CC) π1- and π2bonds, respectively], which is in line with previously reported systems4d,f,7,9a and also leads to a different reactivity of the uranium (2) and thorium metallacyclopentatrienes.10,16 C


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Figure 4. Molecular structure of 6 (thermal ellipsoids drawn at the 35% probability level).

Moreover, exposure of 2 to 4-(dimethylamino)pyridine (DMAP) rearranges the compound to [(η5-C5Me5)(η6:η5-{2(4-Me 2 NC 4 H 3 N)C(SiMe 3 )CCHC(CHSiMe 3 )CH 2 }C5Me4)U] (8) (Scheme 4). In analogy to the reaction of the thorium metallacyclopentatriene [(η5-C5Me5)2Th(η4-C4Ph2)] with DMAP,10 DMAP inserts into the uranium metallacyclopentatriene moiety of 2 to form a seven-membered metallaheterocycle, which converts by a [1,5]-H migration and C−H activation to final product 8 (Scheme 4). The molecular structure of 8 is shown in Figure 6, and selected bond distances and angles are listed in Table 2. The U−C bond distances [U− C(21), 2.636(10) Å; U−C(22), 2.611(10) Å; U−C(23), 2.561(10) Å; and U−C(24), 2.755(10) Å] are in the same range as those found in 6 (Table 2), whereas the U−N bond distance of 2.452(8) Å is markedly longer than that found in 7 [2.247(7) Å]. However, under similar reaction conditions, insertion of 1 equiv of quinoline (C9H7N) into the uranium metallacyclopentatriene moiety of 2 furnishes [(η5-C5Me5)(η 4 :η 5 -{2-(C 9 H 6 N)C(CHSiMe 3 )CHC(SiMe 3 )CH 2 }C5Me4)U] (9) (Scheme 5), which is presumably a consequence of the sterically more demanding quinoline. We propose that in a reaction similar to that found for DCC, quinoline initially coordinates to 2 to form a η2-metallacyclopropene adduct, followed by a nucleophilic attack to afford a five-membered metallaheterocycle that further converts by an [1,3]-H migration and C−H bond activation to final product 9 (Scheme 5). The molecular structure of 9 is presented in Figure 7, whereas selected bond distances and angles can be found in Table 2. The U−C bond distances [U−C(23), 2.653(13) Å; U−C(24), 2.463(12) Å; and U−C(31), 2.746(15) Å] are comparable to those found in 6 and 8 (Table 2), whereas the U−N bond distance of 2.344(10) Å is longer than that observed in 7 [2.247(7) Å] but shorter than that found in 8 [2.452(8) Å]. When complex 2 is exposed to p-ClPhCHO, a ninemembered metallaheterocycle [(η5-C5Me5)2U{OCH(p-ClPh){C4(SiMe3)2}CH(p-ClPh)O}] (10) (Scheme 5) is the only isolated product. Presumably as in the case of PhNCS, complex

Figure 3. Free energy profile for the reaction of 2 with PhNCS. R = Me3Si. [U] = (η5-C5Me5)2U.

longer than that found in [(η5-C5Me5)2U(SMe)2] [2.639(3) Å].18 The IR spectrum of 6 features a characteristic CC CC absorption at 1867 cm−1, which is significantly reduced relative to that (2068 cm−1) found in 29a but comparable to those (1865−1910 cm−1) observed in related group 4 metallacyclopentatrienes.19 Under similar reaction conditions, treatment of 2 with N,N′-dicyclohexylcarbodiimide (DCC) gives the five-membered metallaheterocycle [(η5-C5Me5)2U{N(C 6 H 11 )C(NC 6 H 11 )C(CCSiMe 3 )C(SiMe 3 )}] (7) (Scheme 3). We propose that DCC initially coordinates to 2 to form a η2-metallacyclopropene adduct, followed by a nucleophilic attack to form 7 (Scheme 3). Figure 5 depicts the molecular structure of 7, while selected bond distances and angles are listed in Table 2. The U−C bond distance of 2.499(8) Å is contracted compared to those observed in 6 (Table 2), but the U−N bond distance of 2.247(7) Å is in a range similar to the range of those found in other uranium(IV) amido compounds, e.g., [(η5-C5H5)3UNPh2] [2.29(1) Å]20 and [(η5-C5Me5)2U{NH(2,6-Me2C6H3)}2] [2.267(6) Å].21 D


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Organometallics Table 2. Selected Distances (angstroms) and Angles (degrees) for Compounds 6−16a compound

a

C(Cp)−Ub

C(Cp)−Uc

Cp(cent)−Ub

U−X

Cp(cent)−U−Cp(cent)

X−U−X/Y

C29, 2.645(6); C30, 2.589(6) C31, 2.581(6); S1, 2.762(2) C21, 2.499(8); N1, 2.247(7) C21, 2.636(10); C22, 2.611(10) C23, 2.561(10); C24, 2.755(10) N1, 2.452(8) C23, 2.653(13); C24, 2.463(12) C31, 2.746(15); N1, 2.344(10) O1, 2.125(4); O2, 2.111(5) O1, 2.125(4); O2, 2.109(4) O1, 2.131(10); O2, 2.108(9) O1, 2.106(6); O2, 2.111(6) O1, 2.107(4); O2, 2.111(4) N1, 2.030(4); N2, 2.046(4) C22, 2.291(9); N1, 2.260(7)

137.8(2)

122.3(2)d

131.9(2) 127.3(3)

72.7(3) 118.8(3)e

131.1(4)

78.3(4)f

131.7(2) 126.1(2) 127.9(3) 125.6(2) 128.8(2) 142.2(2) 135.3(3)

109.8(2) 105.9(2) 103.5(3) 100.0(2) 107.4(2) 132.6(2) 64.1(3)

6

2.729(6)

2.705(6) to 2.752(6)

2.451(6)

7 8

2.797(10) 2.765(11)

2.746(10) to 2.860(9) 2.688(10) to 2.830(11)

2.514(9) 2.495(10)

9

2.805 (14)

2.742(14) to 2.892(14)

2.523(12)

10 11 12 13 14 15 16

2.769(7) 2.793(7) 2.808(15) 2.820(10) 2.795(6) 2.739(5) 2.749(9)

2.706(7) to 2.834(7) 2.738(6) to 2.843(7) 2.749(15) to 2.867(15) 2.766(9) to 2.868(10) 2.724(6) to 2.837(6) 2.717(5) to 2.752(5) 2.709(9) to 2.789(8)

2.494(9) 2.521(7) 2.532(15) 2.547(9) 2.521(6) 2.460(5) 2.469(8)

Cp = cyclopentadienyl ring. bAverage value. cRange. dThe C31−U−S1 angle. eThe C21−U−N1 angle. fThe C24−U−N1 angle.


Scheme 4


acetophenone, cyclohexanone, and 1-indanone exclusively also result in double-insertion products [(η5-C5Me5)2U{OCPh2{C4(SiMe3)2}CPh2O}] (11), [(η5-C5Me5)2U{OCMePh{C4(SiMe 3 ) 2 }CMePhO}] (12), [(η 5 -C 5 Me 5 ) 2 U{OC(CH 2 ) 5 {C4(SiMe3)2}C(CH2)5O}] (13), and [(η5-C5Me5)2U{OC{2C6H4(CH2)2}{C4(SiMe3)2}C{2-C6H4(CH2)2}O}] (14), respectively (Scheme 5). The molecular structures of 10 and 11 are shown in Figures 8 and 9, whereas the structures of 12− 14 are provided in the Supporting Information. The U−O bond distances are 2.125(4) and 2.111(5) Å for 10, which are virtually identical within statistical uncertainty to those found in 11−14 [2.106(6)−2.131(10) Å] (Table 2). Furthermore, double insertion also occurs when 2 is exposed to PhCN to give the nine-membered heterocyclic complex [(η5-C5Me5)2U{NC(Ph){C4(SiMe3)2}C(Ph)N}] (15) (Scheme 5),

2 converts initially with p-ClPhCHO to a seven-membered metallaheterocycle, followed by the rapid insertion of a second equivalent of p-ClPhCHO to yield final product 10. Related reactions of 2 with ketones such as benzophenone, E


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which again proceeds in a manner independent of the amount of PhCN added to the reaction mixture. Figure 10 depicts the molecular structure of 15, and selected bond distances and angles are compiled in Table 2. The U−N distances are 2.030(4) and 2.046(4) Å, which are significantly shorter than those found in [(η5-C5Me5)2U(NCPh2)2] [2.169(6)−2.185(5) Å].9a,22 Interestingly and in contrast to the reaction of the uranium metallacyclopropene [(η5-C5Me5)2U{η2-C2(SiMe3)2}] with organic azides,9a no bis(imido) products are obtained from the reaction of 2 with Me3SiN3, but a four-membered metallaheterocycle [(η5-C5Me5)2U{N(SiMe3)C(SiMe3)C(C2SiMe3)}] (16) is formed (Scheme 6). A plausible reaction pathway to account for the formation of 16 includes a metallacyclopropene adduct, followed by a nucleophilic attack of the coordinated Me3SiN3 to form an eight-membered


metallaheterocycle, which converts to 16 by N2 loss and [1,5]U migration (Scheme 6). The results of our DFT computations F


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mol, and kinetically, this reaction is driven by a low activation barrier [ΔG⧧(298 K) = 0.6 kcal/mol]. Intermediate INT16b then converts to final product 16 [ΔG(298 K) = −61.6 kcal/ mol] via intermediate INT16c and two transition states (TS16c and TS16d) with a low activation barrier [ΔG⧧(298 K)] of 1.2 kcal/mol. Consistent with these computational results, no intermediates can be isolated experimentally or detected by NMR spectroscopy. The molecular structure of 16 is shown in Figure 12, and selected bond distances and angles are listed in Table 2. The U−C bond distance of 2.291(9) Å is significantly shorter than those found in 6−9 (Table 2), whereas the U−N bond distance of 2.260(7) Å is comparable to that found in 7 [2.247(7) Å] but shorter than those found in 8 [2.452(8) Å] and 9 [2.344(10) Å]. A characteristic CC absorption at 2081 cm−1 is detected in the IR spectrum of 16, which is close to that (2113 cm−1) found in 7.

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CONCLUSIONS The intrinsic reactivity of the uranium(IV) metallacyclopentatriene, [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2), was evaluated and compared to that of related uranium and thorium metallacycles. In analogy to the uranium metallacyclopropene [(η5-C5Me5)2U{η2-C2(SiMe3)2}],9a DFT computations indicate substantial contributions of the 5f orbitals to the σ- and π-bonds of the U(η4-CCCC) fragment, which leads to more covalence in the bonds between the [(η5-C5Me5)2U]2+ and [η4C4(SiMe3)2]2− fragments than that found in the related thorium metallacyclopentatriene. While the butatriene-diyl fragment in the thorium metallacyclopentatrienes is inert to alkyne substitution, it reacts as a nucleophile toward heterounsaturated molecules.10,16 However, the reactivity


agree with this mechanistic proposal. Intermediate COM16 is formed when Me3SiN3 initially coordinates to 2 (Figure 11). In the next step, Me3SiN3 inserts into the U−[C4(SiMe3)2] moiety of COM16 via transition state TS16a to yield the thermodynamically favorable eight-membered metallaheterocycle INT16a [ΔG(298 K) = −22.6 kcal/mol]. However, loss of N2 from INT16a to form INT16b and N2 is more thermodynamically favorable by a ΔG(298 K) of −40.1 kcal/ G


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Organometallics

Figure 11. Free energy profile for the reaction of 2 with Me3SiN3. R = Me3Si. [U] = (η5-C5Me5)2U.

such as carbodiimides, nitriles, and organic azides,9 whereas these substrates insert into the U[η4-C4(SiMe3)2] moiety of uranium metallacyclopentatriene 2. However, thorium and uranium metallacyclopentatrienes can also exhibit similar reactivity patterns, e.g., when exposed to aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, and pyridines, for which single or double insertion into the actinide metallacyclopentatriene moieties yields five-, seven-, or nine-membered heterometallacycles, or rearranged products. 10,16 Further studies of the unique reactivity of actinide metallacyclopentatrienes are in progress and will be detailed in due course.

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EXPERIMENTAL SECTION

General Procedures. All reactions and product manipulations were performed 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. KC823 and [(η5-C5Me5)2UCl2] (1)2c,9a were prepared following literature methods. All other chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. and used as received unless otherwise noted. The bulk purity of all new compounds was established by NMR spectroscopy and elemental analyses. Infrared spectra were recorded in KBr pellets on an Avatar 360 Fourier transform spectrometer. 1H and 13 C{1H} NMR spectra were recorded at 25 °C on a Bruker AV 400 spectrometer at 400 and 100 MHz, respectively. 29Si NMR spectra were recorded on a JEOL 600 spectrometer at 119.2 MHz. All chemical shifts are reported in δ units and referenced to the residual protons of the deuterated solvents, which served as internal standards, for proton and carbon chemical shifts, and to external Me4Si for silicon chemical shifts. The magnetic susceptibility data were recorded on a


patterns of uranium complex 2 change notably, so that 2 serves as a synthon for the (η5-C5Me5)2U(II) fragment when it is exposed to conjugated organic molecules such as butadiyne, bipy, and diazabutadiene derivatives. In addition, independent of the metal atom, the steric strain within the metallacycles also modulates their reactivity; e.g., the coordinated alkyne in the uranium metallacyclopropene [(η5-C5Me5)2U{η2-C2(SiMe3)2}] can be replaced upon addition of heterounsaturated molecules H


Article

Organometallics

Anal. Calcd for C37H53NSSi2U: C, 53.03; H, 6.37; N, 1.67. Found: C, 53.01; H, 6.35; N, 1.70. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of PhNCS (2.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 6 were observed by 1H NMR spectroscopy (100% conversion within 20 min). Preparation of [(η5-C5Me5) 2U{N(C6H11)C(NC6H11)C(C CSiMe3)C(SiMe3)}] (7). Method A. This compound was prepared as orange crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and DCC (52 mg, 0.25 mmol) in toluene (15 mL) at room temperature for 2 days and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 186 mg (82%). Mp: 129−131 °C. 1H NMR (C6D6): δ 27.75 (s, 1H, CH), 8.99 (s, 1H, CH2), 8.72 (s, 30H, CpCH3), 7.73 (s, 2H, CH2), 7.50 (s, 2H, CH2), 5.24 (s, 2H, CH2), 3.78 (m, 1H, CH), 3.18 (s, 1H, CH2), 3.12 (m, 2H, CH2), 2.15 (s, 2H, CH2), 1.19 (s, 2H, CH2), 0.83 (s, 2H, CH2), −1.45 (s, 9H, SiCH3), −2.55 (s, 2H, CH2), −7.60 (m, 1H, CH2), −11.16 (m, 1H, CH2), −23.67 (s, 9H, SiCH3). 13C{1H} NMR (C6D6): δ 323.6 (UC), 108.1 (ring C), 57.8 (NC), 55.7 (CCSi), 55.2 (CCSi), 49.3 (SiCC), 35.3 (CH2), 30.3 (CH2), 29.1 (CH2), 28.5 (NCH), 25.8 (CH2), 24.8 (CH2), 10.0 (NCH), 9.7 (CH2), −7.8 (SiCH3), −29.0 (SiCH3), −63.9 (CpCH3). 29Si{1H} NMR (C6D6): δ −4.6 (Me3Si), −144.2 (Me3Si). IR (KBr, cm−1): 2926 (s), 2848 (s), 2113 (w, CC), 1581 (s), 1446 (s), 1406 (s), 1244 (s), 1122 (s), 1099 (s), 1041 (s), 1020 (s), 837 (s). Anal. Calcd for C43H70N2Si2U: C, 56.80; H, 7.76; N, 3.08. Found: C, 56.81; H, 7.73; N, 3.10. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of DCC (4.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 7 were observed by 1H NMR spectroscopy (100% conversion) after this solution was kept at room temperature for 2 days. Preparation of [(η 5 -C 5 Me 5 )(η 6 :η 5 -{2-(4-Me 2 NC 4 H 3 N)C(SiMe3)CCHC(CHSiMe3)CH2}C5Me4)U] (8). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and DMAP (31 mg, 0.25 mmol) in toluene (15 mL) at 70 °C for 2 days and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 167 mg (81%). Mp: 156−158 °C dec. 1H NMR (C6D6): δ 98.21 (s, 1H, py), 59.70 (s, 1H, py), 50.43 (s, 3H, CH3), 20.64 (s, 1H, py), 8.66 (s, 15H, CpCH3), −3.11 (s, 3H, CH3), −4.85 [s, 6H, N(CH3)2], −6.01 (s, 9H, SiCH3), −14.57 (s, 9H, SiCH3), −16.13 (s, 1H, CH2), −19.90 (s, 1H, CH2), −30.51 (s, 3H, CH3), −36.38 (s, 3H, CH3), −58.37 (s, 1H, CCH), −172.21 (s, 1H, SiCH). 13C{1H} NMR (C6D6): δ 361.5 (UC), 271.1 (UC), 258.2 (UC), 256.7 (UC), 197.2 (ring C), 153.4 (ring C), 137.8 (py C), 129.3 (py C), 125.6 (py C), 123.5 (py C), 111.7 (py C), 30.1 (CpCH3), 13.4 (CpCH3), 8.2 (SiCH3), 1.3 (SiCH3), −20.3 (CpCH3), −67.5 (CpCH2). 29Si{1H} NMR (C6D6): δ 84.8 (Me3Si), −194.6 (Me3Si). IR (KBr, cm−1): 2945 (s), 2897 (s), 1597 (s), 1436 (s), 1363 (s), 1276 (s), 1259 (s), 1161 (s), 844 (s). Anal. Calcd for C37H58N2Si2U: C, 53.86; H, 7.09; N, 3.40. Found: C, 53.83; H, 7.11; N, 3.42. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of DMAP (2.5 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 8 were observed by 1H NMR spectroscopy (100% conversion) after the sample was heated at 70 °C for 2 days. Preparation of [(η5-C5Me5)(η4:η5-{2-(C9H6N)C(CHSiMe3)CHC(SiMe3)CH2}C5Me4)U] (9). Method A. This compound was prepared as orange crystals from the reaction of [(η5-C5Me5)2U{η4C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and quinoline (33 mg, 0.25 mmol) in toluene (15 mL) at 70 °C for 2 days and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 175 mg (84%). Mp: 162−164 °C dec. 1H NMR (C6D6): δ 65.15 (d, J = 13.4 Hz, 1H, aryl), 49.45 (s, 1H, aryl), 45.38 (d, J = 13.1 Hz, 1H, aryl), 42.05 (d, J = 9.0 Hz, 1H, aryl), 11.07 (s, 15H, CpCH3), 8.36 (s, 9H, SiCH3), 3.00 (s, 1H, SiCCH), 2.21 (d, J

Quantum Design MPMS XL5 SQUID magnetometer. The sample for magnetic susceptibility measurements was sealed in quartz tubes according to literature procedures.24 Magnetic susceptibility data were corrected for diamagnetism using Pascal’s constants25 for all the constituent atoms. 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-C5Me5) 2U{η 4-C4(SiMe 3)2}] (2) via a Modified Method.9a KC8 (1.19 g, 8.8 mmol) was added to a toluene (20 mL) solution of [(η5-C5Me5)2UCl2] (1; 2.00 g, 3.5 mmol) and bis(trimethylsilyl)butadiyne (0.68 g, 3.5 mmol) while being stirred at room temperature. After this solution had been stirred for 2 days at room temperature, the solvent was removed. The residue was extracted with n-hexane (3 × 10 mL) and filtered. The volume of the filtrate was reduced to 10 mL, and brown crystals of 2 were isolated when this solution was kept at −20 °C for 2 days. Yield: 2.16 g (88%). Mp: 168−170 °C dec. 1H NMR (C6D6): δ 2.17 (s, 18H, SiCH3), −2.45 (s, 30H, CpCH3). 29Si{1H} NMR (C6D6): δ 237.9 (Me3Si). Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with Diphenylbutadiyne on the NMR Scale. A C6D6 (0.2 mL) solution of PhC C−CCPh (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of [(η5-C5Me5)2U(η4C4Ph2)] (3)9b [1H NMR (C6D6): δ 6.69 (t, 2H, J = 7.0 Hz, phenyl), 5.18 (t, 4H, J = 6.8 Hz, phenyl), 2.09 (m, 4H, phenyl), −0.95 (s, 30H, CpCH3)] and Me3SiCCCCSiMe3 [1H NMR (C6D6): δ 0.01 (s, 18H, SiCH3)] were observed by 1H NMR spectroscopy (60% conversion) after this solution was kept at 70 °C for 1 day. However, prolonged heating resulted in the formation of unidentified degradation products. Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with (p-TolylNCH)2 on the NMR Scale. A C6D6 (0.2 mL) solution of (ptolylNCH)2 (4.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of [(η5-C5Me5)2U{η2N(p-tolyl)CHCHN(p-tolyl)}] (4)9a [1H NMR (C6D6): δ 5.16 (s, 30H, CpCH3), 1.41 (s, 2H, CH), 0.14 (s, 6H, tolylCH3), −1.06 (s, 4H, phenyl), −31.56 (s, 2H, phenyl), −34.73 (br s, 2H, phenyl)] and Me3SiCCCCSiMe3 were observed by 1H NMR spectroscopy (100% conversion) after this solution was kept at 100 °C for 2 days. Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with 2,2′Bipyridine on the NMR Scale. A C6D6 (0.2 mL) solution of bipy (3.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of [(η5-C5Me5)2U(bipy)] (5)9a [1H NMR (C6D6): δ 0.14 (s, 30H, CpCH3), −20.27 (d, J = 9.0 Hz, 2H, bipy), −41.28 (s, 2H, bipy), −81.37 (s, 2H, bipy), −95.28 (s, 2H, bipy)] and Me3SiCCCCSiMe3 were observed by 1H NMR spectroscopy (100% conversion) after this solution was kept at 130 °C for 3 days. Preparation of [(η5-C5Me5)2U{SC(NPh){C4(SiMe3)2}}] (6). Method A. A toluene solution (5 mL) of PhNCS (34 mg, 0.25 mmol) was added to a toluene (10 mL) solution of [(η5C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) while being stirred at room temperature. After the solution had been stirred at room temperature overnight, the solvent was removed. The residue was extracted with n-hexane (2 × 10 mL) and filtered. The volume of the filtrate was reduced to 3 mL, and brown crystals of 6 were isolated when this solution was kept at room temperature for 1 week. Yield: 161 mg (77%). Mp: 163−165 °C dec. 1H NMR (C6D6): δ 10.07 (s, 30H, CpCH3), 3.32 (s, 1H, phenyl), 1.46 (s, 2H, phenyl), −1.61 (s, 2H, phenyl), −9.04 (s, 9H, SiCH3), −15.65 (s, 9H, SiCH3). 13C{1H} NMR (C6D6): δ 441.6 (UCSi), 167.8 (UC), 129.3 (phenyl C), 128.5 (phenyl C), 128.1 (phenyl C), 127.9 (phenyl C), 125.6 (NC), 123.4 (ring C), 111.1 (SiCC), 89.5 (SiCC), −0.5 (SiCH3), −0.8 (SiCH3), −18.9 (CpCH3). 29Si{1H} NMR (C6D6): δ 18.3 (Me3Si), −191.0 (Me3Si). IR (KBr, cm−1): 2958 (s), 2900 (s), 1867 (w, C CCC), 1546 (s), 1402 (s), 1246 (s), 1066 (s), 1020 (s), 839 (s). I


Article

Organometallics = 8.7 Hz, 1H, aryl), −4.12 (s, 3H, CH3), −5.95 (s, 1H, CH2), −9.80 (s, 1H, CH2), −11.41 (s, 9H, SiCH3), −19.26 (s, 3H, CH3), −19.58 (s, 3H, CH3), −20.37 (s, 3H, CH3), −48.13 (s, 1H, aryl), −126.96 (s, 1H, UCH). 13C{1H} NMR (C6D6): δ 392.5 (UC), 222.5 (UC), 220.6 (UC), 206.6 (ring C), 187.4 (ring C), 164.7 (ring C), 130.8 (aryl C), 128.5 (aryl C), 128.1 (aryl C), 127.9 (aryl C), 126.1 (aryl C), 117.9 (aryl C), 117.4 (aryl C), 105.5 (CHCSi), 82.2 (SiCCH), 27.3 (CpCH3), 27.1 (CpCH3), 10.1 (SiCH3), −8.1 (SiCH3), −52.3 (CpCH3), −65.3 (CpCH2). 29Si{1H} NMR (C6D6): δ −0.1 (Me3Si), −107.6 (Me3Si). IR (KBr, cm−1): 2957 (s), 1596 (s), 1417 (s), 1383 (s), 1258 (s), 1246 (s), 1085 (s), 1018 (s), 832 (s). Anal. Calcd for C39H55NSi2U: C, 56.30; H, 6.66; N, 1.68. Found: C, 56.32; H, 6.65; N, 1.67. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of quinoline (2.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 9 were observed by 1H NMR spectroscopy (100% conversion) after the sample was heated at 70 °C for 2 days. Preparation of [(η5-C5Me5)2U{OCH(p-ClPh){C4(SiMe3)2}CH(pClPh)O}] (10). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and p-ClPhCHO (70 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from a toluene solution by a procedure similar to that used for the synthesis of 6. Yield: 192 mg (78%). Mp: 141−143 °C dec. 1H NMR (C6D6): δ 35.94 (s, 2H, CH), 12.20 (s, 4H, phenyl), 9.33 (s, 4H, phenyl), −0.34 (s, 30H, CpCH3), −1.17 (s, 18H, SiCH3). 13C{1H} NMR (C6D6): δ 179.2 (ring C), 141.6 (CO), 139.2 (phenyl C), 137.6 (phenyl C), 131.8 (phenyl C), 128.5 (phenyl C), 122.1 (CCSi), 114.2 (SiCC), −1.9 (SiCH3), −44.6 (CpCH3). 29Si{1H} NMR (C6D6): δ −0.4 (Me3Si). IR (KBr, cm−1): 2960 (s), 1905 (w, CCCC), 1402 (s), 1246 (s), 1083 (s), 1055 (s), 1014 (s), 839 (s). Anal. Calcd for C44H58Cl2O2Si2U: C, 53.70; H, 5.94. Found: C, 53.65; H, 6.01. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of pClPhCHO (5.6 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 10 were observed by 1 H NMR spectroscopy (100% conversion within 10 min). Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with p-ClPhCHO on the NMR Scale. A C6D6 (0.3 mL) solution of p-ClPhCHO (2.8 mg, 0.02 mmol) was added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 10 along with those of unreacted 2 were observed by 1H NMR spectroscopy (50% conversion based on 2 within 10 min). Preparation of [(η5-C5Me5)2U{OCPh2{C4(SiMe3)2}CPh2O}]· 0.5C6H6 (11·0.5C6H6). Method A. This compound was prepared as orange crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and Ph2CO (91 mg, 0.50 mmol) in toluene (15 mL) at room temperature overnight and recrystallization from a benzene solution by a procedure similar to that used for the synthesis of 6. Yield: 238 mg (86%). Mp: 135−137 °C dec. 1H NMR (C6D6): δ 17.19 (br s, 4H, phenyl), 10.40 (s, 4H, phenyl), 9.46 (s, 2H, phenyl), 7.15 (s, 3H, C6H6), 6.38 (s, 6H, phenyl), 5.62 (br s, 4H, phenyl), 1.42 (s, 9H, SiCH3), 1.15 (s, 9H, SiCH3), −0.35 (s, 30H, CpCH3). 13C{1H} NMR (C6D6): δ 179.9 (ring C), 168.8 (ring C), 152.5 (phenyl C), 137.8 (phenyl C), 131.4 (phenyl C), 129.3 (phenyl C), 128.0 (C6H6), 126.8 (CO), 112.6 (CCSi), 107.0 (SiCC), 0.37 (SiCH3), −35.8 (CpCH3), −38.9 (CpCH3). 29Si{1H} NMR (C6D6): δ 1.8 (Me3Si). IR (KBr, cm−1): 2962 (m), 1564 (m), 1403 (s), 1260 (s), 1084 (s), 1046 (s), 1020 (s), 824 (s). Anal. Calcd for C59H71O2Si2U: C, 64.05; H, 6.47. Found: C, 64.03; H, 6.48. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of Ph2CO (7.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 11 were observed by 1H NMR spectroscopy (100% conversion) after this solution was kept at room temperature overnight.

Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with Ph2CO on the NMR Scale. A C6D6 (0.3 mL) solution of Ph2CO (3.7 mg, 0.02 mmol) was added to a J. Young NMR tube charged with [(η5C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 11 and unreacted 2 were observed by 1H NMR spectroscopy (50% conversion based on 2) after this solution was kept at room temperature overnight. Preparation of [(η5-C5Me5)2U{OCMePh{C4(SiMe3)2}CMePhO}] (12). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and acetophenone (60 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 193 mg (82%). Mp: 124−126 °C dec. 1H NMR (C6D6): δ 14.27 (s, 6H, CH3), 7.60 (s, 4H, phenyl), 6.75 (d, J = 11.8 Hz, 4H, phenyl), 6.47 (s, 2H, phenyl), 0.57 (s, 15H, CpCH3), 0.06 (s, 18H, SiCH3), 0.01 (s, 15H, CpCH3). 13C{1H} NMR (C6D6): δ 171.9 (ring C), 153.9 (ring C), 134.6 (phenyl C), 129.2 (phenyl C), 128.5 (phenyl C), 127.6 (phenyl C), 121.8 (CO), 105.8 (CCSi), 105.3 (SiCC), 59.7 (OCCH3), 0.2 (SiCH3), −38.9 (CpCH3), −40.9 (CpCH3). 29Si{1H} NMR (C6D6): δ −0.5 (Me3Si). IR (KBr, cm−1): 2963 (m), 1570 (m), 1444 (s), 1404 (s), 1260 (s), 1247 (s), 1118 (s), 1096 (s), 1029 (s), 941 (s), 839 (s). Anal. Calcd for C46H64O2Si2U: C, 58.58; H, 6.84. Found: C, 58.52; H, 6.85. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of acetophenone (4.8 mg; 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 12 were observed by 1H NMR spectroscopy (100% conversion within 10 min). Preparation of [(η 5 -C 5 Me 5 ) 2 U{OC(CH 2 ) 5 {C 4 (SiMe 3 ) 2 }C(CH2)5O}]·0.5C6H14 (13·0.5C6H14). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and cyclohexanone (49 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 179 mg (76%). Mp: 108−110 °C dec. 1H NMR (C6D6): δ 9.46 (m, 4H, CH2), 9.06 (m, 4H, CH2), 7.05 (m, 4H, CH2), 4.94 (m, 4H, CH2), 4.31 (m, 2H, CH2), 4.07 (m, 2H, CH2), 0.88 (m, 7H, C6H14), 0.40 (s, 18H, SiCH3), 0.07 (s, 30H, CpCH3). 13C{1H} NMR (C6D6): δ 164.0 (ring C), 145.3 (CCSi), 124.2 (SiCC), 99.5 (CO), 68.1 (CH2), 31.9 (C6H14), 29.6 (CH2), 29.5 (CH2), 23.0 (C6H14), 14.3 (C6H14), 2.2 (SiCH3), −39.8 (CpCH3). 29Si{1H} NMR (C6D6): δ −0.7 (Me3Si). IR (KBr, cm−1): 2958 (s), 2927 (s), 2129 (w, CCCC), 1564 (s), 1442 (s), 1404 (s), 1083 (s), 1049 (s), 1020 (s), 802 (s). Anal. Calcd for C45H75O2Si2U: C, 57.36; H, 8.02. Found: C, 57.32; H, 8.05. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of cyclohexanone (4.0 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 13 were observed by 1H NMR spectroscopy (100% conversion within 10 min). Preparation of [(η5-C5Me5)2U{OC{2-C6H4(CH2)2}{C4(SiMe3)2}C{2-C6H4(CH2)2}O}] (14). Method A. This compound was prepared as orange crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and 1-indanone (66 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from an nhexane solution by a procedure similar to that used for the synthesis of 6. Yield: 208 mg (86%). Mp: 119−121 °C dec. 1H NMR (C6D6): δ 26.09 (t, 2H, J = 7.8 Hz, phenyl), 18.60 (m, 2H, CH2), 15.00 (m, 2H, phenyl), 11.10 (m, 2H, CH2), 10.57 (d, J = 9.3 Hz, 2H, phenyl), 7.45 (s, 2H, CH2), 5.11 (s, 2H, CH2), 0.03 (s, 18H, SiCH3), −0.04 (s, 2H, phenyl), −0.48 (s, 15H, CpCH3), −0.82 (s, 15H, CpCH3). 13C{1H} NMR (C6D6): δ 173.6 (ring C), 154.7 (ring C), 151.7 (phenyl C), 143.1 (phenyl C), 133.2 (phenyl C), 130.9 (phenyl C), 126.7 (phenyl C), 125.4 (phenyl C), 124.6 (CO), 94.9 (CCSi), 89.4 (SiCC), 44.1 (CH2), 43.9 (CH2), 33.0 (CH2), 30.9 (CH2), −0.7 (SiCH3), −41.5 (CpCH3), −41.8 (CpCH3). 29Si{1H} NMR (C6D6): δ 1.3 (Me3Si). IR (KBr, cm−1): 2962 (s), 1404 (s), 1260 (s), 1068 (s), 1019 (s), 836 (s). Anal. Calcd for C48H64O2Si2U: C, 59.61; H, 6.67. Found: C, 59.62; H, 6.65. J


Article

Organometallics Method B on the NMR Scale. A C6D6 (0.3 mL) solution of 1indanone (5.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 14 were observed by 1 H NMR spectroscopy (100% conversion within 10 min). Preparation of [(η5-C5Me5)2U{NC(Ph){C4(SiMe3)2}C(Ph) N}] (15). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and PhCN (52 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that used for the synthesis of 6. Yield: 191 mg (84%). Mp: 152−154 °C dec. 1H NMR (C6D6): δ 9.24 (d, J = 6.6 Hz, 4H, phenyl), 7.59 (m, 6H, phenyl), 4.03 (s, 30H, CpCH3), 0.83 (s, 18H, SiCH3). 13C{1H} NMR (C6D6): δ 202.7 (NC), 170.0 (ring C), 131.1 (phenyl C), 130.6 (phenyl C), 129.1 (phenyl C), 126.0 (phenyl C), 117.7 (CCSi), 79.3 (SiCC), 15.6 (CpCH3), 5.1 (SiCH3). 29Si{1H} NMR (C6D6): δ −60.8 (Me3Si). IR (KBr, cm−1): 2960 (m), 2038 (w, CCCC), 1591 (m), 1402 (s), 1259 (s), 1089 (s), 1018 (s), 833 (s). Anal. Calcd for C44H58N2Si2U: C, 58.13; H, 6.43; N, 3.08. Found: C, 57.98; H, 6.50; N, 3.12. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of PhCN (4.2 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 15 were observed by 1H NMR spectroscopy (100% conversion within 10 min). Reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) with PhCN on the NMR Scale. A C6D6 (0.3 mL) solution of PhCN (2.1 mg, 0.02 mmol) was added to a J. Young NMR tube charged with [(η5C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 15 and unreacted 2 were observed by 1H NMR spectroscopy (50% conversion based on 2 in 10 min). Preparation of [(η 5 -C 5 Me 5 ) 2 U{N(SiMe 3 )C(SiMe 3 )C(C2SiMe3)}] (16). Method A. This compound was prepared as brown crystals from the reaction of [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 176 mg, 0.25 mmol) and Me3SiN3 (29 mg, 0.25 mmol) in toluene (15 mL) at 60 °C overnight and recrystallization from an n-hexane solution by a procedure similar to that used for the synthesis of 6. Yield: 162 mg (82%). Mp: 134−136 °C dec. 1H NMR (C6D6): δ 4.40 (s, 12H, SiCH3), 0.01 (s, 36H, CpCH3 and SiCH3), −4.22 (s, 9H, SiCH3). 13C{1H} NMR (C6D6): δ 216.5 (UC), 201.3 (ring C), 138.8 (UCC), 133.4 (CCSi), 129.3 (CCSi), 9.3 (SiCH3), 4.2 (SiCH3), 1.3 (SiCH3), −47.5 (CpCH3). 29Si{1H} NMR (C6D6): δ 163.0 (Me3Si), −72.9 (Me3Si), −134.4 (Me3Si). IR (KBr, cm−1): 2960 (s), 2081 (w, CC), 1402 (s), 1259 (s), 1246 (s), 1083 (s), 1018 (s), 839 (s). Anal. Calcd for C33H57NSi3U: C, 50.17; H, 7.27; N, 1.77. Found: C, 50.21; H, 7.23; N, 1.75. Method B on the NMR Scale. A C6D6 (0.3 mL) solution of Me3SiN3 (2.3 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2; 14 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances of 16 were observed by 1 H NMR spectroscopy (100% conversion) after this solution was kept at 60 °C overnight. X-ray Crystallography. Single-crystal X-ray diffraction measurements were taken 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 SADABS.26 All structures were determined by direct methods and refined by full-matrix least squares on F2 using the SHELXL program package.27 All the hydrogen atoms were geometrically fixed using the riding model. The crystallographic details for 6−17 are summarized in the Supporting Information. Selected bond lengths and angles are listed in Table 2. Computational Methods. All calculations were performed with the Gaussian 09 program (G09),28 employing the B3PW91 functional, with a polarizable continuum model (PCM) (denoted B3PW91PCM), with the standard 6-31G(d) basis set for C, H, N, S, and Si elements and a quasi-relativistic 5f-in-valence effective-core potential (ECP60MWB) treatment with 60 electrons in the core region for U and the corresponding optimized segmented ((14s13p10d8f6g)/ [10s9p5d4f3g]) basis set for the valence shells of U,29 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. Frequency calculations were performed to ensure that the reactant, complex, intermediate, product, and transition state structures resided at minima and first-order saddle points on their potential energy hypersurfaces. To consider the dispersion effect for the reactions of 2 with PhNCS and 2 with Me3SiN3, single-point B3PW91-PCM-D330 calculations, based on B3PW91-PCM geometries, were performed.

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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.6b00936. Additional experiments, crystal parameters for compounds 6−17, figures, magnetic susceptibility, and computational studies (PDF) X-ray crystallographic data for compounds 6−17 (CIF) Cartesian coordinates of all stationary points optimized at the B3PW91-PCM level (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guohua Hou: 0000-0002-3571-456X Marc D. Walter: 0000-0002-4682-8749 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 (Grants 21472013, 21573021, and 21672024) and the Deutsche Forschungsgemeinschaft (DFG) through the Heisenberg program (WA 2513/6).

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REFERENCES

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