Small-Molecule Activation Mediated by a Uranium Bipyridyl

Article pubs.acs.org/Organometallics

Small-Molecule Activation Mediated by a Uranium Bipyridyl Metallocene Lei Zhang,† Congcong Zhang,† Guohua Hou,† Guofu Zi,*,† 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

‡

Organometallics 2017.36:1179-1187. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 10/09/18. For personal use only.

S Supporting Information *

ABSTRACT: Addition of potassium graphite (KC8) to a solution of (η5C5Me5)2UCl2 (1) and 2,2′-bipyridine (bipy) gives the uranium bipyridyl metallocene (η5-C5Me5)2U(bipy) (2) in good yield. In complex 2 a bipy radical anion is coordinated to a U(III) atom, and it is therefore an ideal starting material for small-molecule activation: e.g., it serves as a synthetic equivalent for the (η5C5Me5)2UII fragment on treatment with conjugated alkynes and a variety of heterounsaturated molecules such as imines, carbodiimide, organic azides, hydrazine, and azo derivatives. Alternatively, it may also react with aldehydes, ketones, nitriles, and α,β-unsaturated reagents such as pClPhCHO, (CH2)5CO, PhCN, and methyl methacrylate (MMA), forming the C−C bond coupling products (η5C5Me5)2U[(bipy)(p-ClPhCHO)] (10), (η5-C5Me5)2U[(bipy){(CH2)5CO}] (11), (η5-C5Me5)2U[(bipy)(PhCN)] (12), (η5C5Me5)2U[(bipy){CH2C(Me)CO(OMe)] (13a), and [(η5-C5Me5)2U{OC(OMe)C(Me)CH2−3-bipy}]2 (13b), respectively, in quantitative conversion. Furthermore, addition of CuI to complex 2 induces a single-electron-transfer process to form the uranium(III) iodide complex (η5-C5Me5)2U(I)(bipy) (14).

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INTRODUCTION Low-valent actinide compounds have sparked considerable attention because of their ability to activate small molecules and to accommodate multielectron-transfer processes.1 However, highly unfavorable redox potentials make divalent actinide complexes difficult synthetic targets, and only a limited number of thoroughly characterized divalent actinide compounds have therefore been prepared.2 One strategy to overcome these limitations is the in situ reduction of appropriate An(IV) precursors,3 but species formed under these conditions are highly reactive and their reactivity can be difficult to control.3 An alternative approach comprises well-defined organometallic complexes that can serve as synthons for the intrinsic reactivity of these difficult to access oxidation states. In this context, redox-active ligands such as 2,2′-bipyridine,4 1,4-diazabutadiene,5 pyridine diimine,6 and arenes7 are ideal, since they can “store” electrons and can provide them to the actinide atom when required. For example, Gambarotta demonstrated that naphthalene can be employed as a redox-active ligand to prepare well-characterized thorium arene complexes which act as sources for divalent thorium(II).7c,d In the course of our investigation, we have used diimine systems such as 2,2′bipyridine (bipy) for this purpose and reported the wellbehaved thorium metallocenes [η5-1,2,4-(Me3C)3C5H2]2Th(bipy), [η5-1,3-(Me3C)2C5H3]2Th(bipy), and (η5-C5Me5)2Th(bipy), in which a bipy dianion is coordinated to a Th(IV) atom. These compounds enabled us to explore the reactivity of the “Cp2ThII” fragment,8 and we are now extending these investigations to the related uranium compounds which have so far not been studied extensively.4j,o,q In this contribution, we © 2017 American Chemical Society

report on some observations concerning the chemistry of the uranium bipyridyl metallocene (η5-C5Me5)2U(bipy) (2) in small-molecule activation.

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RESULTS AND DISCUSSION Treatment of an excess of KC8 with a 1:1 mixture of 2,2′bipyridine and (η5-C5Me5)2UCl2 (1) in toluene solution gives the green bipyridyl metallocene (η5-C5Me5)2U(bipy) (2) in 90% yield (Scheme 1). This approach obviates the previous Scheme 1

need for sodium amalgam and a uranium(III) starting material.4i,q In contrast to a uranium(IV) complex containing a bipyridyl dianion, [K(2.2.2-crypt)][((Ad,tBuArO)3tacn)U(bipy)],4y complex 2 is best described as a U(III) complex with a bipyridyl radical anion. The electronic structure in 2 is supported by UV/vis and NIR spectroscopy and was independently reported by Bart4q and Ephritikhine.4i The identity of 2, which was isolated from our synthetic procedure, Received: January 26, 2017 Published: March 10, 2017 1179

DOI: 10.1021/acs.organomet.7b00064 Organometallics 2017, 36, 1179−1187

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Organometallics was confirmed by 1H NMR spectroscopy,4i,q,t and variabletemperature (20−100 °C) 1H NMR investigations confirmed that no bipy dissociation occurs when 2 is heated to 100 °C, consistent with a strong coordination of the bipyridyl ligand to the U(III) atom. Nevertheless, the bipyridyl ligand in 2 can be exchanged with conjugated alkynes. For example, treatment of 2 with 1 equiv of RCCCCR releases bipy and forms the metallacyclopentatrienes (η5-C5Me5)2U[η4-C4(R)2] (R = Ph (3),9a Me3Si (4)9b) in quantitative conversion (Scheme 2). The

Scheme 3

Scheme 2

of 2 with (Ph2CN)2 gives the U(IV) diiminato complex (η5C5Me5)2U(NCPh2)2 (7)9b,12 in quantitative conversion (Scheme 3), for which the proposed reaction mechanisms are assumed to be related to those outlined for the reaction with ((p-tolyl)NCH)2, and they are shown in the Supporting Information. The identity of 5−7 was established by a comparison of their 1H NMR spectroscopic data to those reported in the literature for these compounds.9b Moreover, the anionic bipyridyl ligand in 2 is also exchanged on addition of carbodiimides. For example, treatment of 2 with N,N′-dicyclohexylcarbodiimide (DCC) gives the four-membered uranium heterocycle (η5-C5Me5)2U[η2-N(C6H11)C( NC6H11)N(C6H11)] (8) (Scheme 4). Analogous to the reaction with PhCHNPh, DCC exchanges bipyridyl ligand to give a metallaaziridine,10 which converts to an imido complex by isonitrile C6H11NC dissociation. However, unlike

identity of 3 and 4 was confirmed by comparison of their 1H NMR spectra to the literature data.9 However, when complex 2 is treated with simple alkynes such as PhCCPh, MeC CMe, and PhCCMe, no reaction occurs even when the reaction mixture is heated at 100 °C for 1 week. Nevertheless, complex 2 may also react with heterounsaturated organic molecules via ligand exchange with concomitant two-electron transfer. For example, complex 2 reacts with 2 equiv of the imine PhCHNPh to yield the five-membered heterocyclic U(IV) complex (η5-C5Me5)2U[η2-N(Ph)CH(Ph)CH(Ph)N(Ph)] (5), resulting from a two-electron-transfer process inducing the bipyridyl replacement and C−C coupling (Scheme 2). For the formation of 5 the following reaction pathway may be proposed: the coordination of PhCHNPh to 2 induces a two-electron transfer, for which the U(III) atom and the bipyridyl radical anion provide two electrons. This leads to the reduction of PhCHNPh to [PhCH−NPh]2− and bipy dissociation to form a U(IV) metallaaziridine intermediate,10 which immediately couples with a second molecule of PhCHNPh to give 5 (Scheme 2). A two-electron-transfer process with concomitant bipy release is also observed in the reaction of 2 with the diazabutadiene ((p-tolyl)NCH)2, in which the five-membered U(IV)-containing heterocycle (η5C5Me5)2U[η2-N(p-tolyl)CHCHN(p-tolyl)] (6) is formed (Scheme 3). We propose a sequence similar to that outlined for the reaction with PhCHNPh; coordination of ((p-tolyl)N CH)2 forms a U(IV) metallaaziridine complex and releases bipy,10 which converts by a [1,3]-U migration to complex 6 (Scheme 3). Alternatively, similar to mechanisms proposed for the formation of the group 4 and ytterbium(III) diazabutadiene complexes,11 formation of 6 may also proceed via a transient diazabutadiene U(II) adduct,2a,b which undergoes instantaneous two-electron transfer to yield 6 (Scheme 3). Treatment

Scheme 4

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Organometallics the sterically more congested uranium imido complex [η5-1,2,4(Me3C)3C5H2]2UNMe,13 the Cp* derivative is less sterically hindered and can therefore immediately convert with a second molecule of DCC in a [2 + 2] cycloaddition reaction to 8 (Scheme 4). The molecular structure of 8 is shown in Figure 1. The U−N distances are 2.213(3) Å for N(1) and 2.265(3) Å for N(3), which are comparable to those found in (η5C 5 Me 5 ) 2 U[η 2 -N( i Pr)C(N i Pr)N( i Pr)] (2.258(4) and 2.218(4) Å).9b

of the pyridine-stabilized imido complex (η5-C5Me5)2U NPh(py) with PhN3,14 the less sterically encumbered Cp* ligand and the absence of a coordinating solvent renders the coordination sphere at the uranium atom open enough for a second molecule of (p-tolyl)N3 to coordinate, which forms 9 and N2 (Scheme 5). Complex 9 may also be accessed by the reaction of 2 with bis(p-tolyl)diazene (Scheme 5). Similar to the reaction with PhCHNPh, bis(p-tolyl)diazene exchanges bipyridyl ligand to form a three-membered U(IV) metallacycle, followed by two-electron transfer and N−N bond cleavage to give the U(VI) diimido complex 9 (Scheme 5). However, in contrast to the reaction with the imine PhCH NPh, the bipyridyl ligand in 2 cannot be exchanged by aldehydes and ketones. Instead, treatment of 2 with 1 equiv of p-ClPhCHO or cyclohexanone yields (η5-C5Me5)2U[(bipy)(pClPhCHO)] (10) and (η5-C5Me5)2U[(bipy){(CH2)5CO}] (11), respectively, in quantitative conversion (Scheme 6). As Scheme 6

Figure 1. Molecular structure of 8 (thermal ellipsoids drawn at the 35% probability level). Selected bond lengths (Å) and angles (deg): U−C(Cp) (av) 2.773(4), U−C(Cp) (range) 2.740(4)− 2.824(4), U− Cp(cent) (av) 2.499(4), U−N(1) 2.213(3), U−N(3) 2.265(3), U− C(27) 2.767(4); Cp(cent)−U−Cp(cent) 132.1(1), N(1)−U−N(3) 60.7(1).

Complex 2 also reacts with organic azides, e.g., it forms the U(VI) bis-imido complex (η5-C5Me5)2U[N(p-tolyl)]2 (9) with (p-tolyl)N3 (Scheme 5). The 1H NMR spectroscopic data Scheme 5

suggested by Bart and co-workers,4q initial coordination of the carbonyl-containing substrate to 2 via the carbonyl oxygen atom induces a single-electron transfer (SET) from the uranium(III) atom to the π* orbital of the CO group, which oxidizes the U(III) in 2 to U(IV), to form the radical anion [R1R2CO]•−. Finally, both radicals [R1R2CO]•− and [bipy]•− participate in a C−C coupling reaction to yield 10 and 11, respectively (Scheme 6). In a similar manner, organic nitriles such as PhCN can also interact with the bipyridyl moiety to yield (η5-C5Me5)2U[(bipy)(PhCN)] (12) (Scheme 6). Moreover, the reaction of 2 with the α,β-unsaturated reagent methyl methacrylate (MMA) produces a mixture of the isomers (η 5 -C 5 Me 5 ) 2 U[(bipy){CH 2 C(Me)CO(OMe)] (13a) and [(η 5 -C 5 Me 5 ) 2 U{OC(OMe)C(Me)CH 2 −3bipy}]2 (13b) in quantitative conversion (Schemes 7 and 8).

of 9 agree with those reported in the literature.9b In analogy to the reactions of the more sterically encumbered actinide complexes [η5-1,2,4-(Me3C)3C5H2]2An(bipy) (An = Th, U) with (p-tolyl)N3,4j,8a it is reasonable to assume that (p-tolyl)N3 exchanges the coordinated bipyridyl ligand in 2 to form initially the uranium(IV) imido species (η5-C5Me5)2UN(p-tolyl) accompanied by N2 loss. However, in analogy to the reaction 1181


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(Scheme 7). Alternatively, when the CO bond is reduced, an enolyl radical (B) ensues, which immediately couples with bipyridyl moiety to give a U(IV) complex. In a next step, this complex then converts to a diradical species, which dimerizes to the final product 13b (Scheme 8). Presumably, the selectivitydetermining step for the formation of 13a,b is the radical coupling event: i.e., for 13a the formation of a five-membered ring occurs with a slightly reduced barrier in comparison to the formation of 13b, which yields a less favorable eight-membered ring. The molecular structure of 13b is shown in Figure 2. The

Scheme 7

Scheme 8

Figure 2. Molecular structure of 13b (thermal ellipsoids drawn at the 35% probability level). Selected bond lengths (Å) and angles (deg): U−C(Cp) (av) 2.878(5), U−C(Cp) (range) 2.763(5)−2.947(5), U− Cp(cent) (av) 2.614(5), U−N(1) 2.611(4), U−N(2) 2.451(4), U−O 2.200(3); Cp(cent)−U−Cp(cent) 128.6(1), N(1)−U−N(2) 65.9(1).

relatively long U−N(1) distance of 2.611(4) Å is consistent with a datively coordinated nitrogen atom. While the U−N(2) distance of 2.451(4) Å is shorter than the distance of U−N(1) (2.611(4) Å), it is longer than those found in uranium(IV) amide complexes, e.g., (η5-C5H5)3UNPh2 (2.29(1) Å)15 and (η5-C5Me5)2U[NH(2,6-Me2C6H3)]2 (2.267(6) Å),16 presumably because of the steric hindrance of the more bulky chelate amido ligand. In addition, the U−O distance of 2.200(3) Å is longer than those found in (η5-C5Me5)2U(O-2,6-iPr2-C6H3)I (2.114(6) Å)17 and (η 5 -C 5Me5 ) 2 U(O-2,6- iPr2 -C6 H 3)CH 3 (2.126(4) Å).18 These structural parameters are comparable to those found in (η5-C5Me5)2U[(bipy)(p-MePhCHO)].4q Complex 2 can also be oxidized in a SET process by metal halides such as CuI. Addition of 1 equiv of CuI to 2 affords the uranium(III) iodide complex (η5-C5Me5)2U(I)(bipy) (14)19 in quantitative conversion (Scheme 6), in which bipy serves as a neutral ligand.4i The molecular structure of 14 is shown in Figure 3. The U−N distances are 2.721(4) Å for N(1) and 2.592(4) Å for N(2), whereas the U−I distance is 3.176(5) Å. The relevant C(25)−C(26) distance of 1.467(7) Å is clearly indicative of a neutral bipy ligand being coordinated to the U(III) atom. Overall, these parameters are consistent with those previously reported by Ephritikhine and co-workers.4i

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CONCLUSIONS In conclusion, the reactivity of the uranium bipyridyl metallocene (η5-C5Me5)2U(bipy) (2) toward small molecules was investigated. These studies established that, in analogy to the reactivity of the related thorium derivative (η5-C5Me5)2Th(bipy),8d the uranium complex 2 may serve as a synthetic equivalent for the (η5-C5Me5)2UII fragment, as illustrated by its reaction with unsaturated moleclules such as conjugated alkynes, imines, carbodiimide, organic azides, hydrazine, and

The presence of the two isomers 13a,b in C6D6 solution in a ratio of 3:1 can be verified by 1H NMR spectroscopy, but unfortunately, this mixture cannot be converted to a single isomer. In analogy to the formation of complexes 10 and 11, complex 13a arises from the reduction of the CO bond followed by radical (A) coupling with the bipyridyl moiety 1182


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EXPERIMENTAL SECTION

General Procedures. 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. 2,2′Bipyridine was purified by sublimation prior to use. KC8,20 pCH3C6H4N3,21 and (η5-C5Me5)2UCl2 (1)9b,22 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 obtained from KBr pellets on an Avatar 360 Fourier transform spectrometer. 1H and 13 C{1H} NMR spectra were at 25 °C recorded on a Bruker AV 400 spectrometer at 400 and 100 MHz, respectively. All chemical shifts are reported in δ units and referenced to the residual protons of the deuterated solvents, which are 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. The bulk purity of new compounds was established by NMR spectroscopy and elemental analyses, unless stated otherwise. Preparation of (η5-C5Me5)2U(bipy) (2). KC8 (1.42 g, 10.5 mmol) was added to a toluene (20 mL) solution of (η5-C5Me5)2UCl2 (1; 2.00 g, 3.5 mmol) and 2,2′-bipyridine (bipy; 0.55 g, 3.5 mmol) with stirring at room temperature. After this solution was stirred at 70 °C for 2 days, the suspension was filtered. The residue was washed with toluene (5 mL × 3) and filtered. The combined filtrate was evaporated to dryness, and the green powder was washed with n-hexane (10 mL × 2) and dried under vacuum at 50 °C overnight. Yield: 2.09 g (90%). 1H NMR (C6D6): δ 0.14 (s, 30H), −19.94 (d, J = 9.1 Hz, 2H), −41.38 (s, 2H), −81.28 (s, 2H) −93.81 (s, 2H) ppm. These spectroscopic data were in agreement with those reported in the literature.4i,q,t,9b Preparation of (η5-C5Me5)2U(η4-C4Ph2) (3). Method A. A toluene (5 mL) solution of PhCCCCPh (81 mg, 0.4 mmol) was added to a toluene (10 mL) solution of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) with stirring at room temperature. After the solution was stirred at 50 °C overnight, the solvent was removed. The residue was extracted with benzene (10 mL × 3) and filtered. The volume of the filtrate was reduced to 5 mL; brown microcrystals of 3 were isolated when this solution was kept at room temperature for 1 week. Yield: 213 mg (75%). 1H NMR (C6D6): δ 6.69 (t, J = 7.0 Hz, 2H), 5.18 (t, J = 6.8 Hz, 4H), 2.09 (m, 4H), −0.95 (s, 30H) ppm. These spectroscopic data agreed with those reported in the literature.9a,23 Method B. NMR Scale. A C6D6 (0.3 mL) solution of PhCCC CPh (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 3 and those of 2,2′-bipyridine (1H NMR (C6D6): δ 8.72 (d, J = 8.0 Hz, 2H), 8.53 (d, J = 4.0 Hz, 2H), 7.22 (t, J = 1.8 Hz, 2H), 6.68 (m, 2H) ppm) were observed by 1H NMR spectroscopy (100% conversion) when this solution was heated at 50 °C overnight. Preparation of (η5-C5Me5)2U[η4-C4(SiMe3)2] (4). Method A. This compound was prepared as brown microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and Me3SiCCCCSiMe3 (78 mg, 0.40 mmol) in toluene (15 mL) at 70 °C and recrystallization from an n-hexane solution by a procedure similar to that in the synthesis of 3. Yield: 225 mg (80%). 1H NMR (C6D6): δ 2.17 (s, 18H), −2.45 (s, 30H) ppm. These spectroscopic data were consistent with those reported in the literature.9b,24 Method B. NMR Scale. A C6D6 (0.3 mL) solution of Me3SiC CCCSiMe3 (3.9 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 4 and those of 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion) when this solution was heated at 70 °C overnight. However, when this resulting solution was heated at 130 °C for 3 days, the resonances of 2 and those of Me3SiCCCCSiMe3 were observed by 1H NMR

Figure 3. Molecular structure of 14 (thermal ellipsoids drawn at the 35% probability level). Selected bond lengths (Å) and angles (deg): U−C(Cp) (av) 2.813(5), U−C(Cp) (range) 2.758(5)−2.856(5), U− Cp(cent) (av) 2.541(5), U−N(1) 2.721(4), U−N(2) 2.592(4), U−I 3.176(5), C(25)−C(26) 1.467(7); Cp(cent)−U−Cp(cent) 135.1(1), N(1)−U−N(2) 61.1(1).

azo derivatives, in which the coordinated bipyridyl ligand was readily exchanged during the reaction. Nevertheless, the electronic ground states in (η5-C5Me5)2Th(bipy)8d and (η5C5Me5)2U(bipy) are different. For Th, a bipy dianion coordinates to a Th(IV) atom, whereas in the case of U a bipyridyl radical anion is bound to a U(III) center. Hence, in a two-electron-transfer process these electrons are provided exclusively by the dianionic ligand in the case of Th, while the two electrons for the related uranium compound are provided by the U(III) atom and the bipyridyl radical anion. Alternatively, similar to the reactivity of the thorium bipyridyl complex (η5-C5Me5)2Th(bipy),8d a C−C σ bond is formed on reaction of 2 with the CO, CN, and CC−CO functionalities of aldehydes, ketones, nitriles, and α,βunsaturated reagents. Moreover, complex 2 can also be oxidized by metal halides such as CuI in a single-electron-transfer (SET) process. In addition, the reactivity patterns observed in these transformations are significantly influenced by the steric demand of the cyclopentadienyl ligand. For example, mixing of the more sterically hindered [η5-1,2,4-(Me3C)3C5H2]2U(bipy) with the organic azide (p-tolyl)N3 yields the imido complexes [η5-1,2,4-(Me3C)3C5H2]2UN(p-tolyl),4j while exposure of the less bulky 2 to (p-tolyl)N3 affords the U(VI) diimido complex (η5-C5Me5)2U[N(p-tolyl)]2 (9). Furthermore, we also observed a difference between the related uranium and thorium compounds: while the reaction of (η5C5Me5)2Th(bipy) with imine PhCHNPh furnishes the insertion product (η5-C5Me5)2Th[(bipy)(PhCHNPh)],8d the bipyridyl ligand in the related uranium derivative 2 is cleanly replaced by [PhCH−NPh]2−. The attenuated reactivity can be attributed to the more covalent bonds formed between uranium and the coordinated ligands.9b Further exploration of actinide complexes with redox-noninnocent ligands such as 2,2′bipyridine is ongoing and the results will be reported in due course. 1183


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Organometallics spectroscopy (100% conversion),24 indicating that an equilibrium between 2 + TMS2C4 and 4+bipy exists in the C6D6 solution. Preparation of (η5-C5Me5)2U[η2-N(Ph)CH(Ph)CH(Ph)N(Ph)] (5). Method A. This compound was prepared as purple microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and PhCHNPh (145 mg, 0.80 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that in the synthesis of 3. Yield: 279 mg (80%). 1 H NMR (C6D6): δ 33.61 (s, 2H), 10.93 (s, 30H), 8.25 (br s, 4H), 6.81 (s, 4H), 6.33 (s, 2H), 1.23 (s, 4H), 0.88 (s, 4H), −10.15 (s, 2H) ppm. These spectroscopic data were consistent with those reported in the literature.9b Method B. NMR Scale. A C6D6 (0.3 mL) solution of PhCHNPh (7.2 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 5 and those of 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight. Reaction of (η5-C5Me5)2U(bipy) (2) with PhCHNPh. NMR Scale. A C6D6 (0.2 mL) solution of PhCHNPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of 5 along with those of unreacted 2 and 2,2′-bipyridine were observed by 1H NMR spectroscopy (50% conversion based on 2) when this solution was kept at room temperature overnight. Preparation of (η5-C5Me5)2U[η2-N(p-tolyl)CHCHN(p-tolyl)] (6). Method A. This compound was prepared as brown microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and ((p-tolyl)NCH)2 (94 mg, 0.40 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that in the synthesis of 3. Yield: 268 mg (90%). 1 H NMR (C6D6): δ 5.09 (s, 30H), 1.39 (s, 2H), 0.17 (s, 6H), −0.88 (d, J = 7.6 Hz, 4H), −30.67 (s, 2H), −33.85 (br s, 2H). These spectroscopic data were in agreement with those reported in the literature.9b Method B. NMR Scale. A C6D6 (0.3 mL) solution of ((p-tolyl)N CH)2 (4.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 6 and those of 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion). Preparation of (η5-C5Me5)2U(NCPh2)2 (7). Method A. This compound was prepared as brown microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and (Ph2CN)2 (144 mg, 0.40 mmol) in toluene (15 mL) at 100 °C, and recrystallization from a benzene solution by a procedure similar to that in the synthesis of 3. Yield: 320 mg (92%). 1H NMR (C6D6): δ 7.69 (d, J = 6.8 Hz, 4H), 7.35 (d, J = 7.4 Hz, 4H), 7.10 (m, 6H), 7.01 (m, 6H), −1.83 (s, 30H) ppm. These spectroscopic data were in agreement with those reported in the literatures.9b,12 Method B. NMR Scale. A C6D6 (0.3 mL) solution of (Ph2CN)2 (7.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 7 and those of 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion) when this solution was heated at 100 °C overnight. Preparation of (η 5 -C 5 Me 5 ) 2 U[η 2 -N(C 6 H 11 )C(NC 6 H 11 )N(C6H11)] (8). Method A. This compound was prepared as brown crystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and DCC (165 mg, 0.80 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a procedure similar to that in the synthesis of 3. Yield: 266 mg (82%). Mp: 208−210 °C dec. 1H NMR (C6D6): δ 26.88 (s, 1H), 15.72 (s, 1H), 5.78 (s, 2H), 5.40 (s, 1H), 5.22 (m, 3H), 5.00 (s, 30H), 4.16 (s, 2H), 2.76 (m, 1H), 2.38 (s, 1H), 2.09 (s, 2H), 1.21 (s, 2H), 0.86 (s, 1H), −5.61 (s, 1H), −7.13 (s, 2H), −7.45 (m, 3H), −9.69 (s, 1H), −10.37 (s, 4H), −12.52 (s, 1H), −29.53 (s, 1H), −34.85 (s, 1H), −35.82 (s, 1H), −47.44 (s, 1H) ppm. 13C{1H} NMR (C6D6): δ 327.1, 58.8, 50.9, 31.9, 28.2, 28.1, 23.0, 15.4, 14.3, 10.4, 4.9, −31.2 ppm. IR (KBr, cm−1): 2924 (s), 2848 (s), 1639 (s, CN), 1570 (s), 1444 (s), 1402 (s), 1259 (s), 1097 (s), 1018 (s), 800 (s). Anal. Calcd for

C39H63N3U: C, 57.69; H, 7.82; N, 5.18. Found: C, 57.73; H, 7.81; N, 5.15. Method B. NMR Scale. A C6D6 (0.3 mL) solution of DCC (8.2 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 8 and those of 2,2′-bipyridine and C6H11NC (1H NMR (C6D6): δ 3.13 (m, 1H, NCH), 1.85 (m, 2H, CH2), 1.58 (m, 2H, CH2), 1.34 (m, 6H, CH2) ppm) were observed by 1H NMR spectroscopy (100% conversion). Reaction of (η5-C5Me5)2U(bipy) (2) with DCC. NMR Scale. A C6D6 (0.2 mL) solution of DCC (4.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of 8 along with those of unreacted 2 and 2,2′-bipyridine and C6H11NC were observed by 1H NMR spectroscopy (50% conversion based on 2). Preparation of (η5-C5Me5)2U[N(p-tolyl)]2 (9). Method A. This compound was prepared as brown microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and (p-tolyl)N3 (107 mg, 0.80 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that in the synthesis of 3. Yield: 224 mg (78%). 1H NMR (C6D6): δ 9.14 (d, J = 8.1 Hz, 4H), 7.59 (s, 6H), 4.10 (s, 30H), 2.70 (d, J = 8.1 Hz, 4H) ppm. These spectroscopic data were in agreement with those reported in the literatures.9b Method B. NMR Scale. A C6D6 (0.3 mL) solution of (p-tolyl)N3 (5.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 9 and those of 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion). Reaction of (η5-C5Me5)2U(bipy) (2) with (p-tolyl)N3. NMR Scale. A C6D6 (0.2 mL) solution of (p-tolyl)N3 (2.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of 9 along with those of unreacted 2 and 2,2′-bipyridine were observed by 1H NMR spectroscopy (50% conversion based on 2). Reaction of (η5-C5Me5)2U(bipy) (2) with Bis(p-tolyl)diazene. NMR Scale. A C6D6 (0.2 mL) solution of bis(p-tolyl)diazene (4.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of 9 and 2,2′-bipyridine were observed by 1H NMR spectroscopy (100% conversion). Preparation of (η5-C5Me5)2U[(bipy)(p-ClPhCHO)] (10). Method A. This compound was isolated as pink-red microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and pClPhCHO (57 mg, 0.40 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that described in the synthesis of 3. Yield: 274 mg (85%). Mp: 229−231 °C dec. 1H NMR (C6D6): δ 71.67 (s, 1H), 54.06 (s, 1H), 24.28 (d, J = 8.7 Hz, 2H), 16.57 (s, 1H), 14.45 (d, J = 8.2 Hz, 1H), 13.27 (d, J = 9.1 Hz, 2H), 12.91 (d, J = 11.5 Hz, 1H), 7.35 (d, J = 9.7 Hz, 1H), 1.82 (s, 15H), 0.80 (s, 15H), −3.34 (s, 1H), −6.51 (s, 1H), −6.88 (s, 1H) ppm. 13C{1H} NMR (C6D6): δ 162.0, 153.3, 147.0, 146.6, 140.2, 137.8, 135.0, 128.5, 127.9, 117.4, 111.0, 95.6, 93.1, 58.9, 36.5, −39.8, −41.1 ppm. IR (KBr, cm−1): ν 2964 (m), 1600 (m), 1471 (m), 1402 (s), 1261 (s), 1082 (s), 1024 (s), 1010 (s), 796 (s). Anal. Calcd for C37H43N2ClOU: C, 55.19; H, 5.38; N, 3.48. Found: C, 55.15; H, 5.41, N, 3.46. Method B. NMR Scale. A C6D6 (0.3 mL) solution of p-ClPhCHO (2.8 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances due to 10 were observed by 1H NMR spectroscopy (100% conversion). Preparation of (η5-C5Me5)2U[(bipy){(CH2)5CO)] (11). Method A. This compound was isolated as purple crystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and (CH2)5CO (40 mg, 0.40 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a procedure similar to that described in the synthesis of 3. Yield: 244 mg (80%). Mp: 218− 220 °C dec. 1H NMR (C6D6): δ 101.73 (s, 1H), 35.84 (m, 1H), 35.27 1184


Article

Organometallics (m, 1H), 34.64 (m, 1H), 30.78 (s, 1H), 27.61 (s, 1H), 27.31 (m, 1H), 26.25 (s, 1H), 18.76 (m, 1H), 18.43 (s, 1H), 17.46 (m, 1H), 16.80 (m, 1H), 16.24 (m, 1H), 16.10 (m, 1H), 13.51 (m, 1H), 0.14 (d, J = 5.72 Hz, 1H), −0.24 (s, 15H), −0.53 (s, 15H), −4.08 (s, 1H), −6.37 (s, 1H) ppm. 13C{1H} NMR (C6D6): δ 157.5, 147.8, 128.9, 112.6, 111.8, 111.5, 111.3, 109.6, 99.4, 84.0, 73.8, 60.3, 52.5, 51.9, 51.4, 49.8, 45.1, −44.7, −50.7 ppm. IR (KBr, cm−1): ν 2924 (s), 2852 (s), 1600 (s), 1471 (s), 1446 (s), 1369 (s), 1276 (s), 1141 (s), 1033 (s), 964 (s), 748 (s). Anal. Calcd for C36H48N2OU: C, 56.68; H, 6.34; N, 3.67. Found: C, 56.65; H, 6.36; N, 3.65. This complex was also characterized by Xray diffraction analysis, and its molecular structure is shown in the Supporting Information: The quality of the data was rather poor because of crystal twinning but was sufficient to establish the overall connectivity. Method B. NMR Scale. A C6D6 (0.3 mL) solution of (CH2)5CO (2.0 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances due to 11 were observed by 1H NMR spectroscopy (100% conversion). Preparation of (η5-C5Me5)2U[(bipy)(PhCN)] (12). Method A. This compound was isolated as brown microcrystals from the reaction of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and PhCN (41 mg, 0.40 mmol) in benzene (15 mL) at room temperature and recrystallization from a benzene solution by a procedure similar to that described in the synthesis of 3. Yield: 261 mg (85%). Mp: 215− 217 °C. 1H NMR (C6D6): δ 46.32 (s, 1H), 13.51 (s, 1H), 13.11 (s, 1H), 10.15 (s, 1H), 9.80 (s, 1H), 9.35 (s, 2H), 5.60 (s, 2H), 4.74 (s, 1H), 2.10 (s, 1H), 1.83 (s, 15H), 1.38 (s, 15H), 0.09 (s, 1H), −5.24 (s, 1H) ppm. 13C{1H} NMR (C6D6): δ 187.0, 169.9, 156.3, 156.0, 141.8, 129.2, 128.8, 128.5, 115.6, 101.8, 95.6, 89.3, 85.3, 61.0, −37.8, −40.3 ppm; other carbons overlapped. IR (KBr, cm−1): ν 2964 (s), 2899 (s), 1604 (s), 1465 (s), 1446 (s), 1261 (s), 1095 (s), 1022 (s), 796 (s). Anal. Calcd for C37H43N3U: C, 57.88; H, 5.64; N, 5.47. Found: C, 57.92; H, 5.66, N, 5.45. Method B. NMR Scale. A C6D6 (0.3 mL) solution of PhCN (2.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). The resonances due to 12 were observed by 1H NMR spectroscopy (100% conversion). Preparation of (η5-C5Me5)2U[(bipy){CH2C(Me)CO(OMe)] (13a) and [(η5-C5Me5)2U{OC(OMe)C(Me)CH2-3-bipy}]2 (13b). Method A. A toluene (5 mL) solution of MMA (40 mg, 0.4 mmol) was added to a toluene (10 mL) solution of (η5-C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) with stirring at room temperature. After the solution was stirred at room temperature overnight, the solvent was removed. The residue was extracted with n-hexane (10 mL × 3) and filtered. The volume of the filtrate was reduced to 5 mL. After this solution was kept at room temperature for 1 week, brown-red microcrystals were isolated. Yield: 229 mg (75%). The NMR spectrum recorded in C6D6 showed the presence of two isomers in a 3:1 ratio. Data for 13a are as follows. 1H NMR (C6D6): δ 54.46 (s, 1H), 25.50 (s, 1H), 16.14 (s, 1H), 14.06 (d, J = 10.6 Hz, 1H), 13.69 (s, 3H), 10.30 (s, 3H), 6.11 (d, J = 7.0 Hz, 1H), 3.88 (s, 15H), 2.19 (s, 1H), 1.50 (s, 15H), 1.09 (s, 1H), −0.94 (s, 1H), −3.69 (s, 1H), −20.00 (d, J = 10.6 Hz, 1H) ppm. 13C{1H} NMR (C6D6): δ 198.6, 198.4, 156.1, 118.1, 116.0, 101.6, 101.5, 98.5, 98.4, 78.5, 78.2, 77.0, 65.6, 32.2, 29.4, −33.3, −44.1. Data for 13b are as follows. 1H NMR (C6D6): δ 115.01 (s, 1H), 32.95 (s, 3H), 31.34 (s, 1H), 22.07 (d, J = 10.3 Hz, 1H), 20.09 (s, 3H), 6.56 (s, 1H), 2.20 (d, J = 10.3 Hz, 1H), −0.06 (s, 15H), −0.86 (s, 15H), −3.44 (s, 1H), −3.65 (s, 1H), −4.87 (s, 1H), −8.93 (s, 1H), −24.00 (s, 1H) ppm. 13C{1H} NMR (C6D6): δ 233.5, 163.2, 151.5, 134.7, 112.7, 109.7, 109.4, 107.5, 76.83, 76.76, 76.5, 70.3, 61.9, 43.4, 23.0, 20.3, 19.9, −46.0, −50.0. The unambiguous assignment of the NMR resonances corresponding to 13a,b is not a trivial task. However, isomers 13a,b are obtained in different ratios (ca. 3:1) and therefore their NMR resonances feature different intensities, and we used this information for the assignment of the respective NMR resonances. In addition, complexes 13a,b were not isolated as pure materials on a synthetic scale because of their very similar solubilities. However, a few orange crystals of 13b·

2C6H6 suitable for X-ray diffraction analysis were selected from those microcrystals that recrystallized from a benzene solution at room temperature. Method B. NMR Scale. A C6D6 (0.3 mL) solution of MMA (2.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 13a,b were observed by 1H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight. The 13a/13b ratio is ca. 3/1. The sample was monitored periodically by 1H NMR spectroscopy, but no changes in the 1H NMR spectrum were observed on heating at 100 °C for 1 week, indicating that the mixture of 13a and 13b could not be converted to a single isomer. Preparation of (η5-C5Me5)2U(I)(bipy) (14).19 Method A. This compound was isolated as black crystals from the reaction of (η5C5Me5)2U(bipy) (2; 266 mg, 0.40 mmol) and CuI (77 mg, 0.40 mmol) in benzene (15 mL) at 80 °C and recrystallization from a benzene solution by a procedure similar to that described in the synthesis of 3. Yield: 261 mg (80%). Mp: 238−240 °C dec. 1H NMR (C6D6): δ 31.34 (s, 1H), 16.41 (s, 1H), 5.88 (s, 30H), 2.13 (s, 1H), 0.48 (s, 1H), −0.66 (s, 1H), −5.25 (s, 1H), −36.36 (s, 36), −39.60 (s, 1H) ppm. IR (KBr, cm−1): ν 2904 (m), 1595 (s), 1435 (s), 1383 (s), 1155 (m), 1060 (s), 1014 (s), 879 (s). Anal. Calcd for C30H38IN2U: C, 45.52; H, 4.84; N, 3.54. Found: C, 45.54; H, 4.86; N, 3.47. Method B. NMR Scale. CuI (3.8 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.5 mL). The resonances due to 14 were observed by 1H NMR spectroscopy (100% conversion) when this solution was heated at 80 °C for 2 h. Reaction of (η5-C5Me5)2U(bipy) (2) with PhCCPh, MeC CMe, or PhCCMe. NMR Scale. An excess amount of PhCCPh, MeCCMe, or PhCCMe was added to a J. Young NMR tube charged with (η5-C5Me5)2U(bipy) (2; 13 mg, 0.02 mmol) and C6D6 (0.5 mL). In each case, the sample was monitored periodically by 1H NMR spectroscopy. No changes in the 1H NMR spectrum were observed on heating at 100 °C for 1 week. X-ray Crystallography. Single-crystal X-ray diffraction measurements were carried out on a Rigaku Saturn CCD diffractometer at 100(2) K using graphite-monochromated Mο Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54184 Å). An empirical absorption correction was applied using the SADABS program.25 All structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL program package.26 All hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 8, 13b, and 14 are summarized in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site at DOI: 10.1021/xxxxx. (CIF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00064. Synthesis of 7, molecular structure of 11, and crystal parameters for compounds 8, 13b, and 14 (PDF) X-ray crystallographic data for compounds 8, 13b, and 14 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

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

Guohua Hou: 0000-0002-3571-456X Guofu Zi: 0000-0002-7455-460X Marc D. Walter: 0000-0002-4682-8749 1185


Article

Organometallics Notes

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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, 21672024), and the Deutsche Forschungsgemeinschaft (DFG) through the Heisenberg program (WA 2513/6).

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