CH Bond Activation of Unsaturated Hydrocarbons by a Niobium

Article pubs.acs.org/Organometallics

CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction Pascal Oulié,†,‡ Chiara Dinoi,†,‡,∥ Chen Li,†,§ Alix Sournia-Saquet,†,‡ Kane Jacob,†,‡ Laure Vendier,†,‡ and Michel Etienne*,†,‡ †

CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, BP44099, F-31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse Cedex 4, France § Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse Cedex 4, France ‡

S Supporting Information *

ABSTRACT: The transient intermediate η2-cyclopropene/bicyclobutane niobium complex [TpMe2Nb(η2-c-C3H4)(MeCCMe)] A, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [TpMe2NbMe(cC3H5)(MeCCMe)] (1), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [TpMe2NbR(c-C3H5)(MeCCMe)] (R = 2-furyl, 2-thienyl, 1-alkynyl, 1cyclopentenyl, 1-ferrocenyl (Fc), pentafluorophenyl). The activation of the C−H bond occurs stereospecifically via a 1,3-CH addition across the Nb(η2-cyclopropene) bond of A. Full characterization of several of these complexes includes multinuclear NMR spectroscopy, X-ray diffraction, UV/vis spectroscopy, and electrochemical data. A charge transfer between the ferrocenyl moiety and the niobium center is responsible for the characteristic purple color of the bimetallic complex [TpMe2NbFc(cC3H5)(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the pKa of the CH bond that is cleaved. The pentafluorophenyl complex [TpMe2Nb(C6F5)(c-C3H5)(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling to give two isomeric η4-butadienyl complexes, with [TpMe2Nb(C6F5)(η4-CMeCMeCHCHMe)] as the major isomer.

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INTRODUCTION Early-transition-metal centers can activate the CH bonds of simple hydrocarbons by oxidative addition1 and 1,2-addition at imido ligands.2 Carbon-based ligands are also capable of achieving this challenging reaction. σ-Bond metathesis3 is most probably the most prominent mechanism, followed by 1,2-CH addition at unsaturated alkylidene4 or alkylidyne5 complexes. In addition, a number of transient complexes with aryne/alkyne,6 allene or diene,7 and also, more rarely, alkene8 ligands are able to cleave strong and inert CH bonds in what can be coined a net 1,3-CH bond addition. Due to its unique interactions with an early-transition-metal fragment, the cyclopropene ligand is involved in this transformation. As sketched in Scheme 1, the transient η2-cyclopropene/bicyclobutane niobium complex A generated by intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [TpMe2NbMe(c-C3H5)(MeCCMe)] (1) is able to cleave a CH bond of benzene (to give the phenyl cyclopropyl complex [TpMe2NbPh(c-C3H5)(MeCCMe)] (2)),9 alkyl aromatics,10 and even methane11 under mild conditions. [Cp2Zr(η2-c-C3H4)], a group 4 analogue of A generated from Cp2Zr(c-C3H5)2, was found to activate the more reactive CH bonds of furan or thiophene by the same mechanism.12 Clearly, the problem of selectivity (substrate, regio- and stereoselectivity) in these CH bond activation reactions is a critical issue. In this article, we summarize our © XXXX American Chemical Society

Scheme 1. Generation of Intermediate A and CH Bond Activation of Benzene

findings concerning the CH bond activation reactions of A with a variety of unsaturated hydrocarbons including 1-alkene, 1alkyne, heteroaromatics, pentafluorobenzene and ferrocene.

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RESULTS AND DISCUSSION The CH bond activation chemistry we report is summarized in Scheme 2. Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: June 20, 2016

A

DOI: 10.1021/acs.organomet.6b00506 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 2. CH Bond Activation with Complex 1

Figure 1. X-ray molecular structure of 3a. Relevant bond lengths (Å) and angles (deg): Nb1−C8 2.214(4), Nb1−C1 2.179(4), C1−C2 1.513(5), C1−C3 1.528(5), C2−C3 1.467(6), C8−C9 1.343(5), C10−C11 1.319(7), C9−C10 1.438(7), O1−C8 (1.417(4), O1−C11 1.355(5); C1−Nb1−C8 110.47(14), Nb1−C8−O1 118.0(2), Nb1− C1−C3 113.9(3).

than O1−C8 (1.417(4) Å). There appears to be no other structurally characterized group 5 furyl complex. For early transition metals, an ansa-molybdenocene dihydride activates furan nonselectively at positions 2 and 3 by loss of H2 followed by oxidative addition. The X-ray crystal structure of [Me2Si(C5Me5)2MoH(2-C4H3O)] has been reported.13 [(C5Me5)2YH]2 activates furan, most probably by σ-bond metathesis, to give ultimately the X-ray-characterized [(C5Me5)2Y(2-C4H3O)(thf)], which exhibits very similar bonding parameters within the 2-furyl ligand in comparison to 3a.14 Similar parameters are also found in half-sandwich 18e iron complexes formed by CH activation.15 It is worth comparing here the behavior of a few different heteroaromatics. Whereas 1 readily activates a CH bond of furan or thiophene to yield the corresponding heteroaryl complexes and methane, pyridine is not activated in the first place. 1 has been shown previously to eliminate methane to yield intermediate A, which was trapped by pyridine acting as a Lewis base to yield the fully characterized η2-cyclopropene pyridine complex [TpMe2Nb(η2-c-C3H4)(MeCCMe)(NC5H5)] (A-py).9a Pyridine is a stronger base and, hence, a better ligand than either furan or thiophene. Activation of 1-Alkyne and Alkene Derivatives. Complex 1 readily activates phenylacetylene at the terminal position to give the alkynyl derivative [TpMe2Nb(c-C3H5)(CCPh)(MeCCMe)] (4) in 40% yield. Key spectroscopic data include, in the 1H NMR spectrum, three aryl signals in a 2:2:1 ratio and, in the 13C spectrum, NbCαCPh and NbCCβPh resonances at δ 149.2 and 125.9, respectively. No sign of aryl activation is seen, as expected on thermodynamic and kinetic grounds (see below). When PhCCD is reacted with A-py, release of pyridine and fast ( C4H4O > C4H4S, C6F5H > C6H6 > C5H10 > CH4 (Scheme 5).20 In addition, this D


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Organometallics Scheme 5. CH BDE and pKa Values for Hydrocarbons Used in This Work

establishes that the BDE of the ferrocene CH bond is approximately in the range 439−477 kJ mol−1. A comparison based on pKa values is also meaningful, since the Nb−C bonds have some ionic character and the transition state for the formation/cleavage of the hydrocarbon CH bond involving intermediate A can be described as an intramolecular proton transfer (σ-bond metathesis) between two negatively charged carbons. The most acidic C2−H bond of furan or thiophene is activated. The pKa of FcH has been determined,22 and its value fits with the trend for the hydrocarbons used in this work.21,27 Both scales are summarized in Scheme 5 and found to fit with the behavior of hydrocarbyl complexes 1−7. Electrochemistry. Electrochemical studies (Table 1) were conducted to probe not only the influence of the hydrocarbyl

Figure 6. Cyclic voltammogram of 6 with ferrocene as internal standard. (thf, [n-Bu4N][PF6], Pt electrode, E/V vs FcH+/FcH, scan rate 0.2 V s−1).

better stabilization of the LUMO. The Fc substituent fits in between sp2- and sp-hybridized carbons, displaying better πaccepting properties than the cyclopentenyl ligand. The niobium fragment [TpMe2Nb(c-C3H5)(MeCCMe)] is strongly electron donating, as 6 exhibits a reversible one-electron oxidation process 0.25 V less positive than that of ferrocene. This is similar to that of [(C5H5)(C5Me5)Fe] (0.04 V less positive).29 UV/Vis Spectroscopy. Whereas dihydrocarbyl complexes of niobium in the series [TpMe2NbRR′(MeCCMe)] are yellow to yellow-orange, the ferrocenyl complex 6 is a brick red solid giving burgundy red solutions, different also from yelloworange ferrocene. The data for the electronic absorption spectra in the near UV/vis range of 1, 4, 5, and 6 in thf (same solvent as in the electrochemical study) are summarized in Table 1. The spectra of 1, 4, and 5 are characterized by strong bands in the UV region (likely TpMe2-based π−π* and MLCT) with tails in the high-energy visible region. The low-energy shoulders are responsible for the color of the complexes. The two similar lower energy shoulders for the three complexes are blue-shifted in the order 1 > 5 > 4. This likely results from the more strongly σ donating, more weakly π accepting properties of the hydrocarbyl substituent along the series C(sp3) > C(sp2) > C(sp), in accord with the electrochemical data. Most probably these transitions occur between singlet states originating from a bonding Nb−π-alkyne HOMO and the dπ LUMO on Nb.28 The ferrocenyl complex 6 (Figure 7) exhibits a different spectrum with two well-resolved low-energy bands at 394 and 536 nm, which are responsible for its unique color. In thf, ferrocene displays bands at 327 (ε = 64 M−1 cm−1) and 443 (ε = 128 M−1 cm−1) nm; relatively weak absorptions in this range characterize d−d transitions in ferrocenyl derivatives whose substituents are devoid of significant acceptor or donor character.30 Thus, the two bands in the visible region with significantly increased intensities observed for 6 are due to the presence of both the iron- and niobium-based moieties. They are reminiscent of MLCT bands originating from oxidizable ferrocenyl moieties as observed in organic-,31 inorganic-,32 or organometallic-substituted33 ferrocene derivatives. We note that the difference in redox potentials for 6 (ΔE1/2 = 2.4 V, 19440 cm−1) matches the energy of these CT bands (25380, 18660 cm−1), suggesting they involve the ferrocenyl moiety acting as a donor and the niobium center acting as an acceptor. Hence the CT band can be defined as metal−metal charge transfer (MMCT) mediated by the cyclopentadienyl ligand. As

Table 1. Electrochemical and Electronic Absorption Data complex

E1/2/Va,b

ΔEp/mVc

1

−2.91

83

4

−2.43

92

5

−2.77

103

6

−2.65 −0.25

98 93

λmax/nmd 296 407 264 330 440 308 415 325

(sh) (sh) (sh) (sh) (sh) (sh) (sh) (sh) 394 536

ε/M−1 cm−1 5075 590 33000 10500 700 9130 600 2400 980 630

a Cyclic voltammetry at a Pt electrode in thf, with 0.1 M [nBu4N][PF6] (scan rate 0.2 V s−1). bE1/2 vs FcH+/FcH. c|Epa − Epc|. d thf solution.

ligand R (Me, c-C5H7, CCPh, Fc) on the redox properties of the corresponding complexes [Tp M e 2 NbR(c-C 3 H 5 )(MeCCMe)] (1, 4, 5, and 6, respectively) but also the influence of the fragment [TpMe2Nb(c-C3H5)(MeCCMe)] on the redox properties of Fc. A cyclic voltammogram (CV) of 6 is shown in Figure 6, which also includes FcH as a reference (0.56 V vs SCE). At a scan rate of 0.2 V s−1, a quasi-reversible (ΔEp = |Epox − Epred| ≤ 0.10 V; Ipox/Ipred ≈ 1) one-electron wave at negative potentials is observed in all of the CVs of 1, 4, 5, and 6. We assign these processes as reversible niobium-centered reductions. These complexes are formally pseudo-octahedral 16e species in which the LUMO is essentially a dπ orbital that extends approximately in the plane containing two pyrazolyl nitrogens and the NbCα.28 The complexes are more easily reducible in the order 1 < 5 < 4, which follows the hybridization of the niobium-bound carbon: sp3 (Me) < sp2 (c-C5H7) < sp (CCPh). This can be ascribed to an increase in the π-accepting properties of the hydrocarbyl group and a E


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Organometallics

Figure 8. X-ray molecular structure of 8a,b. Relevant bond lengths (Å) and angles (deg): Nb1−C1 2.002(5), Nb1−C2 2.348(4), Nb1−C3 2.347(4), Nb1−C4 2.226(4), Nb1−C11 2.344(4), C1−C2 1.412(6), C2−C3 1.421(6), C3−C4 1.428(7), C1−C5 1.472(6), C4−C6 1.501(7); C1−C2−C3 121.0(4), C2−C3−C4 125.8(4), C1−C2− C7a 121.1(4), Nb1 C1−C2 85.0(3), Nb1−C4−C3 67.2(2).

Figure 7. UV/vis electronic absorption spectrum of 6 in thf.

allyl group is particularly noteworthy, since the allyl and cyclopropyl groups are isomers. The major isomer 8a is characterized by key NMR features that are summarized in Scheme 7, akin to those found in the

far as we are aware, none of the X-ray-characterized 1ferrocenyl(MLn) compounds have been the subject of similar studies, although [Cp2Zr(L)Fc][MeB(C6F5)3] (L = thf, PMe3) exhibit deep blue to purple colors, likely due to related CT from the electron-rich ferrocenyl moiety to the reducible Zr(IV) center.23b Thermal Rearrangement and Intramolecular Coupling in 7. Upon gentle heating in benzene at 318 K, and in contrast to the chemistry of the phenyl derivative 2,9 7 undergoes a slow intramolecular cyclopropyl ring opening and 2-butyne coupling to give an inseparable 88:12 mixture of the two regioisomers 8a,b, respectively (Scheme 6). This ratio was

Scheme 7. 1H (Blue) and 13C NMR (Black Italics) Data of 8a,ba

Scheme 6. Thermal Rearrangement of 7

consistently found when the reaction was repeated. The interpretation of spectroscopic data and the identity of 8a,b have been ascertained by an X-ray diffraction analysis on a single crystal. Remarkably, a 88:12 statistical disorder of two methyl groups over the two central carbons of a five-membered niobacycle pentafluophenyl complex was observed in the crystal of 8a,b (see Figure 8). The ratio observed in the solid state matches that observed in solution. The existence of these positional isomers has implications for the mechanism by which they are formed. We first describe the characterization of 8a,b before addressing mechanistic issues. 8a,b belong to a large class of η4-butadienyl (η4-C4R5) complexes. This bonding motif, initially highlighted by Curtis,34 has been observed in several group 5 complexes,17b,35 including in the TpMe2Nb family. We have observed these structures in reactions coupling a coordinated alkyne with acyl,36 iminoacyl,37 and rearranged ethyl28a or allyl38 groups. The case of the

a

Additional couplings to 19F are not pictured.

previous examples. Together with the X-ray data, they translate to the limiting structures shown in Scheme 8. A deshielded NbCα with alkylidene character at δ 260.2 in the 13C NMR spectrum is seen together with the three other carbons along Scheme 8. Limiting Structures for 8a,b

F


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Organometallics

Å41), thereby suggesting that limiting structures with Nb(V) would be dominant even if steric crowding might play a role as well. The Nb−C6F5 bond is significantly longer (0.09 Å) than the Nb−C6H5 bond in 2. This can tentatively be assigned to steric interactions in the different coordination spheres. A remarkable feature of the X-ray structure of 8a,b is the disorder that involves specifically the Cβ and Cγ carbons, and the hydrogen and methyl group directly attached to them. No other atoms in the structure, most strikingly none of the other atoms of the butadienyl ligand, are involved in the disorder. This can be judged from the well-defined, small thermal ellipsoids of C1, C4, C5, and C6, for example. Consequently Nb−C1 (NbCα) and Nb−C4 (NbCδ) exhibit well-defined double- and single-bond character, respectively. Thus, the disorder cannot result from a statistical distribution of H4 between C1 and C4. A mechanism occurring through the coupling of the 2-butyne ligand with a rearranged cyclopropyl group in 7 leading to 8a would result in (C5, C1, C2, C7a) and (C3, C4, C6) originating from the alkyne and cyclopropyl ligands, respectively. Looking at 8b, one realizes that a similar route would necessitate C−C bond cleavage in the alkyne ligand, a potentially disfavored event. Alternatively, (C6, C4, C3, C7b) and (C5, C1, C2) might originate from the alkyne and cyclopropyl ligands, respectively, a much more reasonable assumption. However, in that case, the mechanism would imply the migration of at least one more hydrogen, namely H4. Recall that in solution, the same 88:12 ratio between 8a and 8b is observed and that there is no evidence that 8a and 8b interconvert. We have not attempted to address this problem experimentally any further, and DFT computations led to intricate unrealistic mechanisms and/or energy barriers. Finally, a possible reason the rearrangement occurs within the pentafluorophenyl complex 7 but not from the phenyl complex 2 might be due to an enhanced electrophilic character facilitating the C−C coupling between the cyclopropyl groupor rearranged allyl group38and the 2-butyne.

the metallacycle. Their chemical shifts and 1JCH values, when appropriate, are characteristic of their C(sp2) nature. In the 1H NMR spectrum, JHH couplings testify to the position of the hydrogens and the methyl groups. Hγ and Hδ (characteristic shielding) are in a trans disposition. Additional through-space couplings with one fluorine from the C6F5 ring that sits on top of the metallacycle (Scheme 6 and Figure 8) are seen for the βMe group (JHF = 5, JCF = 15 Hz) as well as for the γ-CH (JHF = 4, JCF = 6 Hz). The 1H−19F HOESY spectrum showed correlations of these protons with the o-fluorine at δ −115.7 (dt) and δ −109.5 (br d). The latter sharpens to a doublet of triplets upon 1H decoupling. m-F (complex multiplets at δ −164.1 and −163.0) and p-F (δ − 164.1 (t)) are also observed. Apart from the 19F signals that are unique to 8a, the data match perfectly those of the allyl-rearranged product [TpMe2NbCl( CPhCMeCHCHMe)].38 Definitive assignment of the position of the hydrogens and methyl groups along the metallacycle in 8b (Scheme 7) follows from a comparison of the NMR data with those for 8a. It is clear that Hβ and Hδthe latter with a characteristic shieldingdo not couple with each other and are thus isolated. The former couples to the α-Me protons, while the latter couples with the δ-Me protons as for 8a. Additional throughspace 1H−19F couplings to C6F5 are observed for Hβ and γ-Me protons. Neither a change in the 88:12 ratio nor line broadening was observed in the 1H NMR spectra when a 8a,b mixture was heated in the temperature range 298−343 K in cyclohexane-d12. No cross-peak was observed in EXSY spectra. This establishes the absence of easy interconversion between the two isomers. This potentially rules out a reversible hydrogen shift between Cα and Cδ positions as being responsible for the presence of the two isomers, since such a mechanism would likely have a low energy barrier. This suggests that the 88:12 ratio of 8a,b results from competitive processes during the rearrangement of 7 (see below). The X-ray crystal structure of 8a,b is depicted in Figure 8. The noteworthy features are (i) the disordered η4-trimethylbutadienyl unit and (ii) the presence of the pentafluophenyl ring. The latter is perpendicular (88°) to the nearly planar (C1− C2−C3−C4, 10°) four-membered niobacycle and sits above the Cβ (C2) and Cγ (C3) carbons and their hydrogen and methyl substituents, justifying the through-space couplings observed with the o-fluorine atoms in the NMR spectra. The observed disorder was successfully modeled in a 88 (8a):12 (8b) ratio of two occupancies of methyl groups C7a on C2 (H3 on C3) and C7b on C3 (H2 on C2). The five-membered metallacycle is folded around the C1−C4 axis (113°) so that all four carbons are at bonding distance of the niobium. There is a double bond between Nb1 and C1 (Nb1−C1 = 2.002(5) Å) and a single bond between Nb1 and C4 (Nb1−C4 = 2.226(4) Å). C2 and C3 interact symmetrically with the niobium center (Nb1−C2, 2.348(4) Å; Nb1−C3, 2.347(4) Å). All C−C bonds in the four-carbon chain are identical, pointing to a fully delocalized picture. These data match the solution NMR data. They are fully reminiscent of those for [TpMe2NbCl( CPhCMeCHCHMe)] in particular38 and are consistent with the limiting structures in Scheme 8. The Nb−C6F5 bond length (Nb1−C11, 2.344(4) Å) can be compared to those found in various organometallic complexes where the niobium presents different oxidation states ([C5H5SiMe3)2Nb(CO)(C6 F 5 )], 2.294(7) Å;39 [CpNb(C 6F 5 ){(μ-H)(η 5 -C 5 H4 B(C6F5)2)}], 2.228(2) Å;40 [NBu4][NbO(C6F5)5], 2.356(3)

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CONCLUSION Methane and the Nb(η 2 -cyclopropene) intermediate [TpMe2Nb(η2-c-C3H4)(MeCCMe)] (A) are formed readily by an intramolecular abstraction of a β-H from the cyclopropyl group of [TpMe2NbMe(c-C3H5)(MeCCMe)]. By the microscopic reverse, i.e. 1,3-CH addition, A activates a CH bond of a variety of unsaturated hydrocarbons, including alkene, alkyne, selected heteroaromatics, and ferrocene. This establishes this reaction sequence as an alternative to the more common reversible α-H abstraction/1,2-CH addition events. The selectivity for the activation of these CH bonds follows qualitatively those previously observed, and ferrocene is included on this scale for the first time.

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EXPERIMENTAL SECTION

All experiments were carried out under a dry argon atmosphere using either Schlenk tube or glovebox (JACOMEX GP Concept) techniques. Diethyl ether was obtained after refluxing purple solutions of Na/benzophenone under argon. Benzene, toluene, cyclohexane, pentane, and dichloromethane were dried by refluxing over CaH2 under argon. Alternatively solvents were passed over activated alumina columns under argon and degassed by freeze−pump−thaw cycles. Deuterated NMR solvents were dried over activated molecular sieves, degassed by freeze−pump−thaw cycles, and stored under argon. Other reagents were used as received from the vendors except unless stated otherwise. 1H, 13C, and 19F NMR spectra were obtained on Bruker Avance 300, Avance 400, or Avance 500 spectrometers at T = 298 K G


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Organometallics

1.37 (m, 1 H, c-C3H5 β′), 1.29, 0.71 (m, 1 H each, c-C3H5 β). 13C NMR (100.6 MHz, benzene-d6): δ 239.3, 233.0 (MeCC), 153.2, 151.4, 150.6, 144.0, 143.4, 143.3 (TpMe2CCH3), 149.2 (br s, NbC CPh), 130.9 (m-C6H5), 128.1 (o-C6H5), 127.1 (ipso-C6H5), 126.2 (pC6H5), 125.9 (br s, CCPh), 107.5, 107.1, 106.9 (TpMe2CH), 77.9 (br d, JCH = 146 Hz, Nb-c-C3H5 α), 23.2, 8.4 (both t, JCH = 159, 163 Hz, c-C3H5 β, β′), 21.8, 21.1 (CH3C), 16.2, 15.4, 14.7, 12.7, 12.6, 12.3 (TpMe2CH3). Generation of [TpMe2Nb(c-C3H4D)(CCPh)(MeCCMe)] (4-D). In a J. Young valve NMR tube in the glovebox, A-py (35 mg, 0.062 mmol) was dissolved in benzene-d6 (0.5 mL) to give a dark green solution. PhCCD (6.8 μL, 6.4 mg, 0.62 mmol) was added via syringe at room temperature. The solution turned yellow-orange instantly. 1H and 13C{1H} NMR spectra recorded after ca. 30 min at room temperature showed formation of a single diastereomer of 4-D together with free pyridine (see Figures 3 and 4). 1H NMR (400 MHz, benzene-d6): as for 4 except for different multiplet shapes for c-C3H4D and the absence of the signal at δ 2.25. 13C{1H} NMR (100.6 MHz, benzene-d6): as for 4 except δ 8.4 (t, ratio 1:1:1, 1JCD = 24 Hz, c-C3H5 β′). [TpMe2Nb(c-C3H5)(c-1-C5H7)(MeCCMe)] (5). 1 (0.30 g, 0.60 mmol) was dissolved in pure cyclopentene (2.5 mL) to give a yellow solution. After 22 h at 308 K when the solution had turned to yellowbrown, the solvent was evaporated to leave an oily yellow-brown residue. The residue was dissolved in 20 mL of pentane, and the solution was filtered through a pad of Celite that was subsequently rinsed with pentane (3 × 5 mL). The solution was concentrated until microcrystals started to form, filtered and stored at 233 K overnight to yield a yellow microcrystalline powder of 5 (0.220 g, 0.40 mmol, 66%). X-ray quality crystals were obtained from a pentane solution stored at 273 K. Anal. Calcd for C27H40BN6Nb: C, 58.71; H, 7.30; N, 15.21. Found: C, 58.63; H, 7.66; N, 15.02. 1H NMR (400 MHz, benzene-d6): δ 6.29 (br s, 1H, CCH), 5.80, 5.70, 5.61 (all s, 1 H each, TpMe2CH), 2.94, 2.29 (both s, 3 H each, CH3C), 2.47, 2.26, 2.19, 2.14, 2.13, 1.70 (all s, 3 H each, TpMe2CH3), 2.36 (m, 2 H, CCH−CH2), 2.09 (m, 1 H, CH2CCH), 2.03 (m, 1 H, c-C3H5 β′), 1.89 (m, 1 H, CH2CCH), 1.76 (m, 1 H, c-C3H5 α), 1.65, 1.56 (both m, 1 H each, CH2CH2CCH), 1.49 (m, 1 H, c-C3H5 β′), 1.35, 0.89 (m, 1 H each, c-C3H5 β). 13C NMR (100.6 MHz, benzene-d6): δ 240.2, 239.0 (MeC), 201.6 (br s, NbCCH), 152.3, 151.1, 149.8, 143.6, 143.2, 143.1 (TpMe2CCH3), 136.4 (d, 1JCH = 160 Hz, NbCCH), 107.2, 106.9, 106.7 (TpMe2CH), 66.5 (br d, 1JCH = 136 Hz, Nb c-C3H5 α), 43.7 (CH2CCH), 34.6 (CCHCH2), 25.4 (CCHCH2CH2), 22.1, 19.1 (CH3C), 21.1, 12.3 (both t, 1JCH = 159, 161 Hz, c-C3H5 β, β′), 15.5, 15.2, 14.1, 12.8, 12.6, 12.5 (TpMe2CH3). [TpMe2NbFc(c-C3H5)(MeCCMe)] (6). 1 (0.250 g, 0.500 mmol) and ferrocene (FcH; 0.186 g, 1 mmol) were heated in cyclohexane (10 mL) at 313 K overnight. The solution changed from bright orange to purple. The solvent was evaporated to dryness. The residue was dissolved in 100 mL of pentane and filtered. The resulting solution was cooled at 273 K overnight. Purple crystals of 6 were collected by filtration, rinsed with a small amount of pentane, and dried under vacuum (0.140 g, 0.21 mmol, 42%). X-ray-quality crystals were obtained from a cyclohexane/pentane solution stored at 273 K. Anal. Calcd for C32H42BFeN6Nb: C, 57.34; H, 6.32; N, 12.54. Found: C, 57.35; H, 6.77; N, 11.65. The low nitrogen content could be due to a small amount of remaining FcH. 1H NMR (500.3 MHz, benzene-d6, 287 K): δ 5.79, 5.66, 5.57 (all s, 1 H each, TpMe2CH), 4.32, 3.74 (d, 2 JHH = 1.2 Hz, 1 H each, FcCH), 4.09 (s, 5 H, FcCH), 4.05 (t, 2JHH = 1.6 Hz, 2 H, FcCH), 3.21, 2.24 (all s, 3 H each, CH3C), 2.54, 2.37, 2.14, 2.10, 2.07, 2.00 (all s, 3 H each, TpMe2CH3), 2.21 (partly obscd, 1 H, c-C3H5β′), 1.70 (tt, 1 H, c-C3H5 α), 1.41 (m, 1 H, c-C3H5β′), 1.27, 0.96 (m, 1 H each, c-C3H5β). 13C NMR (125.8 MHz, benzene-d6, 283 K): δ 246.1, 237.4 (CH3C), 153.3, 151.1, 151.0, 144.0, 143.9, 143.3 (TpMe2CMe), 107.6, 107.1, 106.9 (TpMe2CH), 80.0, 78.3 (both d, 1JCH = 172 MHz, NbC5H4, Cβ), 69.4 (d, 1JCH 174 MHz, C5H5), 68.6, 68.0 (both d, 1JCH = 171 MHz, NbC5H4, Cγ), 66.6 (br d, 1JCH = 139 MHz, Nb c-C3H5 α), 22.8, 20.4 (CH3C), 21.5, 13.7 (both t, 1JCH = 158, 161 MHz, Nb c-C3H5 β, β′), 17.0, 16.2, 15.7, 13.1, 12.9, 12.8 (TpMe2CH3). The niobium-bound NbC5H4 was not observed.

unless stated otherwise. In the 1H NMR spectra, the BH signal is not observed due to quadrupolar broadening with the 10B and 11B nuclei. Only pertinent 1JCH values are quoted in the 13C spectra. Electronic absorption spectra were recorded on a PerkinElmer LAMBDA 950 UV/vis/NIR spectrophotometer; the samples (thf solutions) were prepared and introduced in airtight quartz cuvettes of 1 cm width in the glovebox. Elemental analyses were obtained from the Analytical Service of our laboratory. [TpMe2NbMe(c-C3H5)(MeCCMe)] (1) and [TpMe2Nb(c-C3H4)(NC5H5)(MeCCMe)] (A-py) were prepared according to published procedures.9 [TpMe2Nb(c-C3H5)(2-C4H3X)(MeCCMe)] (3a, X = O; 3b, X = S). A procedure identical with that described here for 3a was followed for both complexes. Complex 1 (0.660 g, 1.32 mmol) was dissolved in pentane (50 mL), and excess furan (0.44 mL, 0.420 g, 6.17 mmol) was added. After 2 days at room temperature, the solvent was evaporated to dryness. Pentane (10 mL) was added and stripped off. The yellow powder was extracted with pentane (50 mL) and filtered through a pad of Celite. The solvent was stripped off to leave 3a as a yellow powder (0.450 g, 0.82 mmol, 62%). Crystallization from a benzene/ pentane mixture yielded X-ray-quality crystals. Data for 3a are as follows. Anal. Calcd for C26H36BN6NbO: C, 56.54; H, 6.57; N, 15.22. Found: C, 55.86; H, 6.33; N, 15.43. 1H NMR (400 MHz, benzene-d6): δ 7.35 (d, 1 H, J = 1.5 Hz, 5-H-furyl), 6.49 (d, 1 H, J = 3.0 Hz, 3-Hfuryl), 6.21 (dd, 1 H, J = 1.5, 3.0 Hz, 4-H-furyl), 5.79, 5.77, 5.52 (all s, 1 H each, TpMe2CH), 3.16, 2.27 (both s, 3 H, CH3C), 2.26, 2.20, 2.19, 2.10, 2.07, 1.34 (all s, 3 H each, TpMe2CH3), 2.27 (partly obscd m, 1 H, c-C3H5 β′), 2.03 (m, 1 H, c-C3H5 α), 1.51 (m, 1 H, c-C3H5 β′), 1.36, 0.76 (m, 1 H each, c-C3H5 β). 13C NMR (100.6 MHz, benzened6): δ 242.3, 238.7 (MeC), 210.4 (v br, 2-C-furyl), 153.3, 151.2, 150.9, 143.8, 143.3, 143.2 (TpMe2CCH3), 142.3 (ddd, JCH = 195, 11, 7 Hz, 5-C-furyl), 120.5 (dt, JCH = 168, 6 Hz, 4-C-furyl), 108.2 (ddd, JCH = 169, 14, 6 Hz, 3-C-furyl), 107.3, 107.0, 106.6 (TpMe2CH), 72.2 (br d, JCH = 142 Hz, c-C3H5 Cα), 22.0, 20.3 (CH3C), 22.3, 10.3 (both t, JCH = 160, 162 Hz, c-C3H5 Cβ, Cβ′), 15.2, 14.9, 13.5, 12.7, 12.6, 12.4 (TpMe2CH3). Following a procedure identical with that for 3a, 1 (0.580 g, 1.16 mmol) and thiophene (0.46 mL, 0.500 g, 5.94 mmol) gave 3b as a yellow powder (0.350 g, 0.62 mmol, 53%). Anal. Calcd for C26H36BN6NbS: C, 54.94; H, 6.38; N, 14.79; S, 5.64. Found: C, 55.26; H, 6.52; N, 15.13; S, 5.42. 1H NMR (300.1 MHz, benzene-d6): δ 7.57 (dd, J = 4.5, 0.6 Hz, 5-H-thienyl), 7.01 (dd, 1 H, J = 4.5, 3.3 Hz, 4-H-thienyl), 6.85 (br s, 1 H, 3-H-thienyl), 5.85, 5.84, 5.63 (all s, 1 H each, TpMe2CH), 3.32, 2.35 (both s, 3 H, CH3C), 2.38, 2.28, 2.25, 2.23, 2.11, 1.40 (all s, 3 H each, TpMe2CH3), 2.52 (m, 1 H, c-C3H5β′), 2.25 (obscd m, 1 H, c-C3H5 α), 1.73 (m, 1 H, c-C3H5β′), 1.50, 0.96 (m, 1 H each, c-C3H5β). 13C NMR (75.5 MHz, benzene-d6): δ 242.1, 240.6 (MeC), 153.6, 151.6, 151.1, 143.8, 143.3 (TpMe2CCH3), 136.0 (ddd, JCH = 164, 10, 6 Hz, 3-C-thienyl), 129.0 (ddd, JCH = 169, 9, 8 Hz, 4-C-thienyl), 126.0 (ddd, JCH = 163, 8, 5 Hz 5-C-thienyl), 107.6, 107.1, 106.8 (TpMe2CH), 74.6 (br d, JCH = 139 Hz, Nb-c-C3H5 α), 22.4, 12.3 (both t, JCH = 159, 161 Hz, c-C3H5 β, β′), 22.1, 20.8 (CH3C), 15.7, 15.2, 14.2, 12.8, 12.7, 12.5 (TpMe2CH3); Nb-2-Cthienyl unobserved. [TpMe2Nb(c-C3H5)(CCPh)(MeCCMe)] (4). 1 (0.30 g, 0.60 mmol) was dissolved in cyclohexane (4 mL), and phenylacetylene (1.32 mL, 30 mmol) was added. The solution was heated at 35 °C for 22 h. The yellow solution turned orange, and orange blocks formed (suggesting the formation of poly(phenylacetylene)). After solvent evaporation, the orange blocks were scratched and the orange solid was extracted with pentane (2 × 20 mL). The combined solutions were filtered and concentrated until some crystals started to form. The solution was stored at 233 K overnight, and orange crystals of 4 were collected by filtration and dried under vacuum (0.139 g, 0.237 mmol, 40%). X-rayquality crystals were obtained from a pentane solution stored at −40 °C. Anal. Calcd for C30H38BN6Nb: C, 61.45; H, 6.53; N, 14.33. Found: C, 61.79; H, 6.64; N, 14.01. 1H NMR (400 MHz, benzene-d6): δ 7.47 (d, 2 H, o-C6H5), 7.02 (t, 2 H, m-C6H5), 6.94 (t, 1 H, p-C6H5), 5.81, 5.76, 5.43 (all s, 1 H each, TpMe2CH), 3.25, 2.30 (both s, 3 H each, CH3C), 2.93, 2.22, 2.21, 2.19, 2.03, 2.02 (all s, 3 H each, TpMe2CH3), 2.25 (m, 1 H, c-C3H5 β′), 2.18 (obscd m, 1 H, c-C3H5 α), H


Article

Organometallics [TpMe2Nb(C6F5)(c-C3H5)(MeCCMe)] (7). 1 (0.40 g, 0.80 mmol) was dissolved in pentane (6 mL), and C6F5H (3.2 g, 19.04 mmol) was added. After 72 h of stirring at room temperature, the solution changed from yellow-brown to dark brown, with the formation of a yellow precipitate. The solvent was partially evaporated, and the resulting suspension was cooled to 233 K overnight. The resulting yellow microcrystalline solid was washed with cold pentane (2 × 5 mL, 273 K) and dried under vacuum, affording 7 (0.48 mmol, 60%). Anal. Calcd for C28H33BF5N6Nb: C, 51.56; H, 5.10; N, 12.88. Found: C, 51.86; H, 5.78; N, 12.12. 1H NMR (500 MHz, dichloromethane-d2, 233 K): δ 5.85, 5.82, 5.72 (all s, 1 H each, TpMe2CH), 2.88 (d, 3 H, JHF 5.6 Hz, CH3C), 2.13 (s, CH3C), 2.47, 2.42, 2.43, 1.87, 1.75, 1.22 (all s, 3 H each, TpMe2CH3), 2.72 (tt, 1 H, c-C3H5 α), 2.47 (partly obscd, 1 H, c-C3H5β′), 1.65 (m, 1 H, c-C3H5β′), 1.53, 0.24 (m, 1 H each, c-C3H5β). 13C{1H} NMR (125.8 MHz, dichloromethane-d2, 233 K): δ 246.0 (MeC) 238.7 (d, JCF = 7.6 Hz, MeC), 151.0, 150.7, 149.4, 144.6, 143.6, 143.5 (TpMe2CCH3), 148.9 (dd, JCF = 28, 235 Hz, o-C6F5), 146.9 (dd, JCF = 21, 228 Hz, o-C6F5), 138.7 (d, JCF = 245 Hz, m-C6F5), 135.9 (m, 1JCF = 252 Hz, p-C6F5), 106.8, 106.7, 106.5 (TpMe2CH), 86.6 (s, c-C3H5 α), 23.2 (d, JCF = 11 Hz, CH3C), 22.4 (s, CH3C), 24.9 (s, c-C3H5 β) 2.1 (d, JCF = 9 Hz, c-C3H5 β′), 15.4, 15.1, 14.0, 13.1, 13.0, 12.7 (all s, TpMe2CH3). 19F NMR (282.4 MHz, benzene-d6, 298 K): δ −100.2, −117.5 (both m, 1 F each, o-C6F5), −157.6 (m, 1 F, p-C6F5), −162.5, −163.2 (both m, 1 F each, m-C6F5). Thermolysis of 7 To Give a Mixture of [TpMe2Nb(C6F5)(CMeCMeCHCHMe)] (8a) and [TpMe2Nb(C6F5)(CMeCHCMeCHMe)] (8b). 7 (0.120 g, 0.18 mmol) was heated in benzene (2 mL) for 24 h at 318 K. The solution changed from yellow to orange. The solvent was evaporated to dryness, and the resulting solid was washed with pentane (2 × 2 mL, 273 K) and dried under vacuum to leave an inseparable mixture of 8a and 8b (88:12 ratio, respectively, by 1H NMR) as an orange solid (0.096 g, 0.147 mmol, 80%). X-ray-quality crystals were obtained from a toluene/pentane solution stored at 233 K. Anal. Calcd for C28H33BF5N6Nb: C, 51.56; H, 5.10; N, 12.88. Found: C, 51.96; H, 5.62; N, 13.12. Data for 8a are as follows. 1H NMR (400 MHz, benzene-d6): δ 6.09 (dd, 1 H, JHF = 4, JHH = 12 Hz, (CMeCMeCHCHMe), 5.83, 5.61, 5.37 (all s, 1 H each, TpMe2CH), 2.65 (d, 3 H, JHF = 5 Hz, CMeCMeCHCHMe), 2.40 (d, 3 H, JHH = 6 Hz, CMeCMeCHCHMe), 2.13 (s, 3 H, CMeCMeCHCHMe), 2.30, 2.25, 2.09, 1.90, 1.22, 0.93 (all s, 3 H each, TpMe2CH3), 1.05 (m, 1 H, CMeCMeCHCHMe). 13C{19F,1H} NMR (125.8 MHz, benzene-d6): δ 260.2 (CMeCMeCHCHMe), 151.8, 148.7, 148.3, 145.8, 144.8, 143.8 (TpMe2CCH3), 149.6, 148.5 (o-C6F5), 138.9 (m-C6F5), 136.7 (p-C6F5), 135.9 (br s, ipso-C6F5), 120.0 (CMeCMeCHCHMe), 107.5, 107.4, 106.7 (TpMe2CH), 101.6 (CMeCMeCHCHMe), 87.3 (CMeCMeCHCHMe), 24.4 (CMeCMeCHCHMe), 22.9 (CMeCMeCHCHMe), 21.1 (CMeCMeCHCHMe), 16.0, 15.7, 15.1, 13.0, 12.9, 12.7 (TpMe2CH3). 19F NMR (376.5 MHz, benzene-d6): δ −109.5 (dt, 1 F, JFF 19, 9 Hz, oC6F5), −115.7 (dt, 1 F, JFF 30, 10 Hz, o-C6F5), −158.3 (m, 1 F, JFF 20, p-C6F5), −163.0, −164.1 (both m, 1 F each, m-C6F5). Data for 8b are as follows. 1H NMR (400 MHz, benzene-d6): δ 7.60 (br s, 1 H, CMeCHCMeCHMe), 5.72, 5.70, 5.36 (all s, 1 H each, TpMe2CH), 2.49 (m, 3 H, CMeCHCMeCHMe), 2.42 (d, 3 H, JHH = 2 Hz, CMeCHCMeCHMe), 2.15 (d, 3 H, JHH = 6 Hz, CMeCHCMeCHMe), 2.32, 2.22, 2.07, 1.97, 1.27, 1.01 (all s, 3 H each, TpMe2CH3), 0.23 (br q, 1 H, JHH = 6 Hz, CMeCHCMeCHMe). 13 C{19F,1H} NMR (125.8 MHz, benzene-d6): δ 254.9 (CMeCHCMeCHMe), 151.8, 148.7, 148.3, 145.7, 144.6, 143.8 (TpMe2CCH3), 149.6, 148.5 (o-C6F5), 138.9 (m-C6F5), 136.7 (p-C6F5), 135.9 (br s, ipso-C 6 F5 ), 108.9 (CMeCHCMeCHMe), 107.3, 107.2, 106.4 (Tp Me2 CH), 105.0 (CMeCHCMeCHMe), 94.3 (CMeCHCMeCHMe), 26.7 (CMeCHCMeCHMe), 21.9 (CMeCHCMeCHMe), 17.0 (CMeCHCMeCHMe), 15.8, 15.6, 15.2, 13.0, 12.9, 12.6 (TpMe2CH3). 19F NMR (376.5 MHz, benzene-d6): −104.7 (dt, 1 F, JFF = 19, 9 Hz, o-C6F5), −115.7 (dt, 1 F, JFF = 19, 10 Hz, o-C6F5), −158.4 (t, 1 F, JFF = 20, p-C6F5), −163.4, −164.3 (both m, 1 F each, m-C6F5). Electrochemistry. Cyclic voltammetric measurements were carried out with a Autolab PGSTAT100 potentiostat controlled by

GPES 4.09 software. Experiments were performed at room temperature in a homemade airtight three-electrode cell connected to a vacuum/argon line. The reference electrode consisted of a saturated calomel electrode (SCE) separated from the solution by a glass frit. The counter electrode was a platinum wire of ca. 1 cm2 apparent surface. The working electrode was a Pt microdisk (0.5 mm diameter). The supporting electrolyte, [n-Bu4N][PF6] (99% electrochemical grade), was dried at 120 °C and degassed under Ar. The thf solutions of 1 and 4−6 used for the electrochemical studies were typically 10−3 M in complex and 0.1 M in supporting electrolyte. All solutions were prepared in the glovebox. Before each measurement, the solutions were degassed by bubbling Ar and the working electrode was polished with a polishing machine. Under the experimental conditions employed in this work, the half-wave potential (E1/2) of the FcH+/ FcH couple in thf occurred at E1/2 = 0.56 V vs SCE. Potentials are referenced to the FcH+/FcH couple. X-ray Crystallography. A summary of crystallographic data and refinement parameters for all structures can be found in Table S1 in the Supporting Information. Data for 3a and 5 were collected at 180 K on a Gemini Agilent diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and equipped with an Oxford Instrument Cooler Device. Data for 4, 6 and 8a,b were collected at 180 K on a Bruker Kappa Apex II diffractometer using graphitemonochromated Mo Kα radiation and equipped with an Oxford Cryosystems Cryostream Cooler Device. The structures have been solved using either SHELXS-9742 or SIR9243 and refined by means of least-squares procedures on F2 with the aid of the program SHELXL97 included in the software package WinGX version 1.63.44 All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were refined by using a riding model. Absorption corrections were introduced by using the MULTISCAN program.45 Drawings of molecules were performed with the program ORTEP46 with 30% probability displacement ellipsoids for non-H atoms. Complex 5 presents a discrete positional disorder: the atom C10 occupies two sites. We used the PART instruction of the program SHELXL-97. Two different positions (C10A and C10B) were formulated. Their relative occupancies were refined (respectively 73% and 27%), and their thermal motions were constrained to be the same. Complex 8a,b presents a statistical disorder. The atom C7 partially occupies two different sites, C7A and C7B, in proportions of 88% and 12%. Thus, their parent atoms (C2 and C3, respectively) are carrying an H atom in the complementary proportions 12% and 88% (H2 and H3, respectively). This disorder has been treated using the PART instruction of SHELXL-97. The two positions C7A and C7B were formulated and handled via PART 1 and PART 2. The relative occupancies were refined with the help of a free variable. The free variable is equivalent to the occupancy of the atoms in component one (e.g. 0.88 for 88%). The thermal motions of C7A and C7B were constrained to be the same. For compounds 6 and 8, it was not possible to resolve diffuse electron-density residuals (enclosed solvent molecules). Treatment with the SQUEEZE facility from PLATON,47 with a localized void of about 190 Å3 and 75 recovered electrons for complex 6 and with a localized void of about 449 Å3 and 125 recovered electrons for compound 8, resulted in smooth refinements. For both compounds, since a few low-order reflections are missing from the data set, the electron count is underestimated. Thus, the values given for D(calc), F(000), and the molecular weight are only valid for the ordered part of the structure.

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(pdf), (cif). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.organomet.6b00506. I


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Organometallics

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Key NMR spectra, cyclic voltammograms, and UV/vis spectra for all new complexes and crystallographic details of 3a, 4−6, and 8a,b (PDF) Crystallographic data for 3a, 4−6, and 8a,b (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for M.E.: [email protected]. Present Address

∥ LPCNO INSA, 135 Avenue de Rangueil, F-31077 Toulouse Cedex 4, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Université de Toulouse and Région MidiPyrénées for a PhD grant to C.L. and the ANR for support (ANR-07-BLAN-0149). We thank M. Vedrenne and P. Lavedan for 19F VT NMR spectroscopy. Drs. A. Locati and F. Maseras (ICIQ, Tarragona, Spain) are thanked for computational efforts and fruitful discussions.

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REFERENCES

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K


CH Bond Activation of Unsaturated Hydrocarbons by a Niobium

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