Reversible Transformation between Alkylidene, Alkylidyne, and

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Reversible Transformation between Alkylidene, Alkylidyne, and Vinylidene Ligands in High-Valent Bis(phenolate) Tungsten Complexes Haruka Nishiyama,† Keishi Yamamoto,† Andreas Sauer,‡ Hideaki Ikeda,† Thomas P. Spaniol,‡ Hayato Tsurugi,† Kazushi Mashima,*,† and Jun Okuda*,‡ †

Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany

‡

S Supporting Information *

ABSTRACT: A tungsten alkylidyne complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(CEt)X] (X = OtBu, 1a) with a bridged bis(phenolate) ligand was prepared from the alkoxy precursor (tBuO)3WCEt and the corresponding bis(phenol). The tertbutoxy ligand in 1a was substituted against chloride or trimethylsilylmethyl to give 1b (X = Cl) or 1c (X = CH2SiMe3). X-ray diffraction studies of 1a and 1c showed an octahedral cis α-configuration of the metal atom. Hydride abstraction from the βposition of the n-propylidyne ligand in 1a and 1c with [Ph3C][B(C6F5)4] gave high-valent cationic vinylidene complexes [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(CCHMe)X][B(C6F5)4] (2a: X = OtBu; 2c: X = CH2SiMe3), while protonation of 1a with [PhNMe2H][B(C6F5)4] gave the cationic n-propylidene complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(CHEt)(OtBu)][B(C6F5)4] (3a). The alkylidyne complex 1a was regenerated from the alkylidene 3a by deprotonation with Ph3PCH2, whereas 1c was regenerated from 2c by nucleophilic attack by a hydride at the β-carbon of the vinylidene moiety. The cationic tungsten alkylidyne complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(CEt)(PhNMe2)][B(C6F5)4] (4c) was obtained from [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(CHEt)(CH2SiMe3)][B(C6F5)4] (3c) in chlorobenzene at 60 °C.

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INTRODUCTION Transition-metal complexes with multiple metal−carbon bonds such as alkylidene, vinylidene, and alkylidyne ligands are used as catalysts in organic synthesis.1−3 “Schrock-type high-valent alkylidene and alkylidyne complexes” catalyze metathesis of alkenes and alkynes.1,2 Low-valent vinylidene complexes are regarded as key intermediates in transition-metal-mediated transformations of alkynes.3 The vinylidene moiety is prone to nucleophilic attack at the α-carbon, leading to vinyl metal complexes (Scheme 1b),3,4 except for [CpMo(CCHPh)Br{P(OMe)3}2], where the β-carbon is attacked.5 High-valent vinylidene complexes have rarely been isolated, and few reactivity studies exist.6 Electrophilic attack at the αcarbon and [2 + 2]-cycloaddition of the metal−carbon double bonds with alkenes/alkynes were reported.6c−g Here, we describe the isolation of a high-valent vinylidene complex supported by an (OSSO)-type ligand (OSSO = (OC6H2tBu24,6)2(SCH2CH2S)). Bis(phenolate)s of (OSSO)-type have extensively been used to stabilize oxophilic transition-metal centers. Nucleophilic addition at the β-carbon of the vinylidene © XXXX American Chemical Society

Scheme 1. Reactivity of Vinylidene Complexes with Nucleophiles

moiety produces the corresponding alkylidyne complex (Scheme 1a). We also present a new synthetic route to a high-valent cationic alkylidyne tungsten complex by abstracting a proton in the α-position of an alkylidene. Received: October 10, 2015

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DOI: 10.1021/acs.organomet.5b00855 Organometallics XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION We prepared tungsten n-propylidyne complexes with a chelating (OSSO)-type ligand (Scheme 2).7 Scheme 2. Synthesis of the Tungsten n-Propylidyne Complexes 1a−c

Complex 1a was obtained in 95% yield by protonolysis of (tBuO)3WCEt8 with (OSSO)H2 in toluene at 60 °C. The tert-butoxy ligand of 1a was substituted against chloride by treating 1a with an excess of SiCl4 in toluene at 60 °C to a purple chloride complex 1b in 79% yield.9 Alkylation of 1b by LiCH2SiMe3 in diethyl ether afforded a pink complex 1c bearing a trimethylsilylmethyl ligand in 73% yield.10 The 1H NMR spectrum of 1a indicated C1-symmetry in solution. Signals for the ethyl group of the n-propylidyne ligand were detected as two doublets of quartets at δ 4.46 and 4.34 ppm (2JHH = 20.0 Hz, 3JHH = 7.6 Hz, WCCH2CH3) and a triplet at δ 1.03 ppm (3JHH = 7.6 Hz, WCCH2CH3). The signals for aryl-CH protons of the (OSSO)-type ligand appear at δ 7.55, 7.50, 7.30, and 7.14 ppm as four doublets (4JHH = 2.5 Hz). Four singlets were observed at 1.75, 1.67, 1.25, and 1.17 ppm for the tBu moieties bound to the (OSSO)-type ligand. Four AA′BB′ split resonances were displayed due to the −SCH2-CH2-S− unit centered at δ 2.77, 2.58, 1.98, and 1.86 ppm. The 13C{1H} NMR signal for the α-carbon of the npropylidyne ligand at 292.3 ppm is typical for alkylidyne tungsten complexes.11 The 1H NMR spectra of 1b and 1c reveal a similar pattern for the (OSSO)-type ligand and the npropylidyne moiety. Single crystals of 1a and 1c were grown from toluene or pentane solution, respectively. Figure 1 shows the molecular structures from X-ray diffraction. The structure of 1a is C1symmetric with helical cis α-coordination of the (OSSO)-type ligand12 to the tungsten atom in octahedral geometry. The structure of 1c is similar to that of 1a except for the trimethylsilylmethyl instead of the tert-butoxy ligand. The W1−C1 bond lengths of 1.775(6) Å for 1a and 1.762(4) Å for 1c are similar to those in other octahedral alkylidyne complexes with tridentate (ONO)- or (OCO)-type ligands (1.746(9)− 1.773(4) Å).13 On the other hand, the bond lengths of W1−O1 and O2 are slightly longer compared with those found for

Figure 1. Molecular structures of 1a (top) and 1c (bottom) with 50% displacement ellipsoids; all hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): 1a, W1−C1 1.775(6), W1−O1 1.984(4), W1−O2 2.001(4), W1−O3 1.890(4), W1−S1 2.7741(15), W1−S2 2.6024(13), O1−W1−O2 152.10(16), S1−W1− S2 80.29(4), C1−W1−O3 106.4(2); 1c, W1−C1 1.762(4), W1−O1 1.971(2), W1−O2 1.996(2), W1−S1 2.7413(10), W1−S2 2.6404(10), W1−C4 2.132(4), O1−W1−O2 149.36(10), S1−W1−S2 80.50(3), C1−W1−C4 99.94(16).

octahedral tungsten alkylidyne complexes (1.9354(19)− 1.953(2) Å).13a,14 The W1−S1 bond lengths are longer than W1−S2, indicating the large trans effect of the alkylidyne ligand in 1a and 1c. CCDC 1430276 (1a) and 1430277 (1c) contain crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Hydrogen atoms in the β-position of the WCCH2CH3 alkylidyne units in 1a and 1c were abstracted as hydrides by the trityl cation in [Ph3C][B(C6F5)4] in bromobenzene-d5 to give the cationic vinylidene complexes 2a and 2c, respectively (eq 1).

The formation of 2a was only observed in an NMR scale reaction. The low stability of 2a in common organic solvents B

DOI: 10.1021/acs.organomet.5b00855 Organometallics XXXX, XXX, XXX−XXX

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Scheme 3. Synthesis of the Cationic n-Propylidyne Tungsten Complex 4c

prevented its isolation, but 2c was isolated as a yellow solid in 62% yield if the reaction was carried out in chlorobenzene on a preparative scale. Formation of 2c was further supported by MS analysis. The high-resolution ESI mass spectrum of 2c showed a prominent peak at m/z 811.3230 assigned to the cation [(OSSO)W(CCHMe)(CH 2 SiMe 3 )] + (calcd m/z 811.3230).15 The 1H NMR spectrum of 2c in bromobenzene-d5 exhibits two sets of resonances in a 3:2 ratio due to methyl groups located syn and anti to a trimethylsilylmethyl ligand.16 Two broad singlets for two diastereomers of WC CHMe were observed at δ 11.8 and 11.4 ppm. On the other hand, the 1H NMR spectrum of 2a shows only one set of signals, indicating the selective formation of one isomer. In the 13 C{1H} NMR spectrum of 2c, signals for the vinylidene αcarbon atoms appear at 335.2 and 327.2 ppm, comparable with those in previously reported low-valent vinylidene tungsten complexes.17 A signal for the vinylidene β-carbon atom was not observed, even at low temperature. The high-valent vinylidene complex 2c reacted with the hydride source NaHBEt3 under regeneration of the npropylidyne complex 1c (eq 2). Nucleophilic addition occurred at the β-carbon of the vinylidene ligand of 2c, although nucleophiles tend to react at the α-carbon in low-valent vinylidene complexes. This reactivity of 2c was supported by DFT calculation of the frontier molecular orbitals (B3LYP level with the basis sets 6-311G for H, C, O, S, and Si and LANL2DZ for W).18 The LUMO is consistent with the result of nucleophilic addition of hydride to the β-carbon of the vinylidene ligand in 2c, and its main contributions come from the dxz orbital of tungsten and the px orbital of the vinylidene βcarbon.

propylidene ligand in 3c is revealed in the 1H NMR spectrum by a triplet for the γ-protons at δ 1.14 ppm (3JHH = 7.2 Hz); the n-propylidyne ligand in 4c is indicated by a multiplet centered at δ 5.50 ppm for the β-protons. If the mixture in chlorobenzene was kept at 60 °C, 3c finally converted into 4c along with the formation of tetramethylsilane. In the spectrum of 4c, the methyl groups of the N,N-dimethylaniline appear magnetically inequivalent as two singlets because they are coordinated to tungsten. The 13C{1H} NMR spectrum displays a resonance for the alkylidyne α-carbon at δ 325.6 ppm, consistent with previously reported alkylidyne tungsten complexes.11

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CONCLUSION We have described here the synthesis of a high-valent vinylidene tungsten complex 2c stabilized by an (OSSO)-type bis(phenolate) ligand by hydride abstraction from the β-carbon of a propylidyne moiety. Complex 1c could be regenerated by nucleophilic addition of hydride to the β-carbon of the vinylidene group in 2c. The selectivity was distinct from that observed for low-valent vinylidene complexes, where nucleophiles normally react with the α-carbon to form vinyl metal species. The (OSSO)-type ligand also made the cationic propylidyne complex 4c accessible by protonation of neutral propylidyne complex 1c with [PhNMe2H][B(C6F5)4] and subsequent α-H elimination at high temperature.

Complex 1a was selectively protonated at the α-carbon of the alkylidyne with [PhMe2NH][B(C6F5)4] to afford a cationic alkylidene complex 3a (eq 3). The alkylidene group of 3a is revealed in the 1H NMR spectrum by a triplet at 9.39 ppm (3JHH = 7.2 Hz) for the α-proton as well as by two doublets of quartets at δ 5.01 and 4.79 ppm (2JHH = 15.0 Hz, 3JHH = 7.6 Hz) for the β-protons. The 13C{1H} NMR spectrum displays a resonance at 282.5 ppm for the alkylidene α-carbon atom, which agrees with other previously reported alkylidene tungsten complexes. 19 Treating 3a with methylene(triphenyl)phosphorane (Ph3PCH2) regenerated complex 1a (eq 3).

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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.5b00855. Experimental details; selected 1H, 13C{1H}, 2D 1H−1H COSY, and 1H−13C HMBC NMR spectra of complexes 1−4c; ESI-MS spectra of 2c; crystallographic data and refinement details for 1a and 1c; and summary of the DFT calculation (PDF)

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Treatment of 1c with [PhMe2NH][B(C6F5)4] gave two complexes, a cationic alkylidene complex 3c and a cationic alkylidyne complex 4c (Scheme 3). Formation of the n-

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.). C

DOI: 10.1021/acs.organomet.5b00855 Organometallics XXXX, XXX, XXX−XXX

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft for financial support through the International Research Training Group “Selectivity in Chemo and Biocatalysis” (GRK 1628). H.N. and K.Y. express their thanks for financial support provided by the Japan Society for the Promotion of Science through JSPS Research Fellowships for Young Scientists.

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DEDICATION This work is dedicated to the memory of Peter Hofmann. REFERENCES

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DOI: 10.1021/acs.organomet.5b00855 Organometallics XXXX, XXX, XXX−XXX