Fulvene−Ruthenium and Cp−Ruthenium Complexes via [2 + 2 + 1]Cyclotrimerization of Phenylacetylene with [RuCl(Tp)(1,5-cod)]Afsar Ali, Frederick P. Malan, Eric Singleton,* and Reinout Meijboom*

Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park, 2006, Johannesburg, South Africa

*S Supporting Information

ABSTRACT: The complex [RuCl(Tp)(1,5-cod)], which bears the labile 1,5-codligand, was prepared from a high-yielding route involving the reaction of[RuCl2(1,5-cod)(CH3CN)2] with KTp (Tp = HB(pz)3). The reaction of[RuCl(Tp)(1,5-cod)] with phenylacetylene in either ethanol or methanol gaveanti-Markovnikov alkoxide-adduct complexes [Ru(Tp)(η6-C5H2Ph2-CH(Ph)R)](R = OMe, OEt). These adducts were formed by [2 + 2 + 1] cyclotrimerizationreactions of phenylacetylene mediated by the precursor complex, [RuCl(Tp)(1,5-cod)]. The ruthenium(II)−fulvene complex, [Ru(Tp)(η6-C5H2Ph2-CH(Ph))]

+,involved in these transformations was successfully isolated in the presence ofNH4PF6. These complexes were fully characterized by 1H NMR, 13C NMR, DEPT,HSQC, IR, and ESI-MS spectroscopy. The molecular structures of [Ru(Tp)(η6-C5H2Ph2-CH(Ph)R)] (R = OMe/OEt) and [Ru(Tp)(η6-C5H2Ph2-CH(Ph))]PF6have been determined by X-ray single-crystal diffraction. These complexes havepiano-stool structures around the ruthenium center where half of the coordination sites are occupied by the pyrazole ligand whilethe remaining sites are occupied by either the π-bonded cyclopentadiene (Cp) or fulvene ligand.

■ INTRODUCTION

The isolation of catalytically active transition-metal carbocycliccomplexes remains an actively investigated topic mainlybecause these organometallic species, which include transi-tion-metal vinylidene complexes, form the direct intermediatesin the industrially important oligomerization and polymer-ization of terminal alkynes.1,2 As part of the synthesis of theseorganometallic complexes, interest in the RuTp complexesfeaturing hydrido, dihydrogen, metal−carbon single and doublebonds is increasing.3 The involvement of carbene andvinylidene complexes as reactive intermediates in stoichiometricas well as catalytic transformations has also become importantmainly because the electrophilicity of the α-carbon can addamines, alcohols, phosphines, and fluoride.4

In ruthenium(II) systems, reactions involving terminalalkynes usually give either η6-benzene, cyclobutene, vinylidene,or alkylidenecyclobutadiene derivatives, with dien-ynes to givecyclohexenes, with diynes together with enynes and diazo-alkanes to give alkenylbicyclo[3.1.0]hexanes, 2,5-disubstitutedbiscarbene ruthenacycles, as well as other uncommon carbo-cycles.5−12 In addition, the mechanisms proposed for theseorganic transformations are derived from the successfulisolation of ruthenacyclopentatrienes, η4-cyclobutadienes, orη6-benzene complexes.5,13 The commonly employedruthenium(II) precursors (via the pseudo-14-electron speciesCpRuCl, and CpRu+) for which these transformations typicallyoccur: [Ru(Cp′)(1,5-cod)(L)] (Cp′ = C5H5, C5Me5; L = Cl,Br), [Ru(Cp′)(CH3CN)2(L)]

+ (L = CH3CN, PR3, AsR3, SbR3,CO), and [Ru(η5-C5H4CH2CH2-κ

1P-PPh2)(CH3CN)2]+.14,15

These reactive organometallic catalysts have been employed

in numerous organic transformations, for which a variety ofimportant organometallic intermediates have been reported todate.5,16 Tris(pyrazolyl)hydridoborato (Tp) and cyclopenta-dienyl (Cp) ligands have often been compared as these classesof 6-electron donor ligands exhibit the same charge, althoughthey differ in steric and electronic properties.17

The [2 + 2 + 2] cyclotrimerization of substituted alkynes bytransition-metal complexes has been established as a generalroute to give 1,3,5- or 1,2,4-trisubstituted benzene deriva-tives.20,21 However, apart from one reported22 account on theisolation of bidentate cyclopentadienyl-phosphine rutheniumcomplexes from the [2 + 2 + 1] cyclotrimerization of alkynes,no η5-cyclopentadienyl carbocycles with [Ru(Tp)]+-fragmentshave, to the best of our knowledge, been reported from in situreactions of ruthenium(II) complexes with terminal alkynes.Interestingly, mixed [M(Tp)Cp] compounds, where the twoligands directly compete for interactions with the metal, arerelatively scarce. These have typically been produced by thereaction of a suitable “CpM” reagent with KTp,18 such as[CpRu(CH3CN)3]

+ with KTp.19

Alkyne cyclotrimerization reactions in general have beenfound to occur mainly for the group 9 and 10 transition metalsCo, Rh, Ni, and Pd.13 The formation of five-memberedcarbocycles from phenylacetylene, in particular, have only beenreported for the late-transition-metal complexes of rhodi-um,23,24 iridium,21 and palladium25,26 through the formal [2 + 2+ 1] cyclotrimerization route. Specific mention could be made

Received: April 24, 2014Published: October 17, 2014

Article

pubs.acs.org/Organometallics

© 2014 American Chemical Society 5983 dx.doi.org/10.1021/om500432w | Organometallics 2014, 33, 5983−5989

of the reaction of [Rh(Cp*)(η6-2,6-(MeCH)2C6H3NH2)]-(OTf)2 with the terminal aryl alkynes HCCPh and HCCC6H4CH3 in alcohol (EtOH, n-BuOH), which gave the five-membered cycloadducts [Rh(Cp*)(η5-C5H2Ar2-CH(Ar)-OEt)]OTf (Ar = Ph, p-tolyl) via a [2 + 2 + 1]cyclotrimerization mechanism.23

Our research has focused on the organometallic chemistryinvolving the dimerization, cyclotrimerization, and cyclo-oligomerization of alkynes using ruthenium(II) systems. Duringthese studies, routes to novel ruthenium(II)−cyclopentatrienylcomplexes were defined. Here, we report the syntheticoutcomes of the reactions of [RuCl(Tp)(1,5-cod)] with phenylacetylene in alcohols (EtOH, MeOH). These reactionsproduced unusual five-membered cycloadducts as part of the[RuTp]+-fragment complexes, by the [2 + 2 + 1] cyclo-trimerization of those aryl alkynes. The details of the routes andsyntheses of these novel complexes, as well as the X-raystructures of the representative complexes, are described herein.

■ EXPERIMENTAL SECTIONGeneral Procedures. Prior to use, solvents were purified following

the standard procedures.23 Synthetic experiments were carried outunder an argon atmosphere using standard Schlenk techniques.27,28

Solvents were dried prior to use using standard techniques.29 Thecompounds [RuCl2(1,5-cod)]x

30 and [RuCl2(1,5-cod)(CH3CN)2]31

were prepared using literature methods. All other chemicals werepurchased from Sigma-Aldrich and used as received. 1H (400 MHz)and 13C{1H} (100 MHz) NMR spectra were recorded on a BrukerAvance III Ultrashield 400 MHz spectrometer fitted with a B-ACS 60autosampler using CDCl3 solutions. All measurements were performedat ambient temperature (∼296 K). Chemical shifts were referenced tothe internal residual protio impurities in the solvent (δH 7.24 forCDCl3) or carbon signals (δC 77.0 for CDCl3). Solid-state FT-IRexperiments were carried out on a Bruker Tensor 27 FT-IR, all aspressed KBr disks (4000−400 cm−1) in air. Melting points wereperformed in air on a Stuart SMP10 and are uncorrected. Themicroanalytical analyses (%CHNS) were performed at RhodesUniversity using an Elementar Vario Micro cube instrument with aTCD detector.Synthesis of [RuCl(Tp)(1,5-cod)] (Tp = HB(pz)3) (1). A

suspension of [RuCl2(1,5-cod)(CH3CN)2] (400 mg, 1.19 mmol)and K[HB(pz)3] (330 mg, 1.31 mmol) in THF (20 mL) was heatedunder reflux for 6 h. The volume of the solution was reduced to about5 mL, after which a yellow residue precipitated by addition of ethanol.The yellow solid was washed with ether and dried under vacuum.Yield: (410 mg, 75%). All spectroscopic data were in agreement withthe literature.32 1H NMR (CDCl3, 400 MHz): δH 2.23 (d, 2H, CH2 ofcod), 2.40 (d, 2H, CH2 of cod), 2.67 (m, 2H, CH2 of cod), 2.93 (m,2H, CH2 of cod), 4.01 (s, 2H, CH of cod), 4.89 (s, 2H, CH of cod),6.19 (s, 2H, CH of pz), 6.31 (s, 1H, CH of pz), 7.55 (s, 2H, CH ofpz), 7.63 (s, 2H, CH of pz), 7.78 (s, 1H, CH of pz). 13C{1H} NMR(CDCl3, 100 MHz): δC 29.67, 30.34 (CH2 of cod), 86.99, 94.44 (CHof cod), 106.16, 106.08 (CHa of pz), 134.81, 137.50 (CHb of pz),141.66, 144.97 (CHc of pz).Synthesis of [Ru(Tp)(η6-C5H2Ph2-CH(Ph)OMe)] (2). A solution

of 1 (250 mg, 0.546 mmol) and phenylacetylene (300 μL, 2.730mmol) in methanol (15 mL) was heated under reflux for 6 h. Theresulting brown yellow solution was evaporated to dryness. Theresidue was extracted with pentane (2 × 5 mL) and diethyl ether (2 ×5 mL) to give a brown precipitate, which was collected on a glass fritand dried in vacuo. The brown residue was recrystallized frommethanol/ether to give brown crystals of the title compound. Yield:(227 mg, 65%). mp: >167 °C. Anal. Calcd for C33H30BN6ORu·CH3OH·2H2O: C, 58.42; H, 5.46; N, 11.68. Found: C, 57.79; H, 5.42;N, 11.89. FT-IR (KBr): (ν, cm−1), 3126, 3027 (CH), 2483, 1945(Ru-Cp), 1493, 1401 (−CC−), 1210, 1115, 1047 (PhCHOCH3),759, 697. 1H NMR (400 MHz, CDCl3): δH 3.42 (s, 3H, OCH3); 4.67

(d, 1H, 4JHH = 1 Hz, CH(6) of Cp); 4.83 (d, 1H, 4JHH = 1 Hz, CH(4)of Cp); 5.02 (s, 1H, CH(1) of Ph(OMe)CH); 5.90 (t, 3H, 3JHH = 2Hz, CH(b,b′,b″) of pz); 7.30 (d, 3H, 3JHH = 2 Hz, CH(a,a′,a″) of pz);7.43 (d, 3H, 3JHH = 2 Hz, CH(c,c′,c″) of pz); 6.95−7.09, 7.24−7.29,7.60−7.63 (m, 15H, CH of Ph). 13C{1H} NMR (100 MHz, CDCl3):δC 56.69 (OCH3), 73.12 (quaternary carbon C(2) of Cp), 73.77(quaternary carbon C(3) of Cp), 75.88 (CH(4) carbon of Cp), 80.70(CH(1) carbon of Ph(OMe)CH), 80.76 (CH(6) carbon of Cp), 87.96(quaternary carbon C(5) of Cp) 104.99 (CH(b,b′,b″) carbon of pz),133.99 (CH(a,a′,a″) carbon of pz), 143.16 (CH(c,c′,c″) carbon of pz),136.30, 137.5, 139.43 (quaternary carbon of Ph), 125.34, 125.69,126.91, 126.98, 127.28, 127.99, 128.39, 128.56, 129.65 (CH carbon ofPh). HRMS-ESI (m/z): [M] calcd for C34H31BN6ORu 652.1696,found 652.1730; [M − (CH3O)]

+ calcd for C33H28BN6Ru 621.1512,found 621.1533.

Synthesis of [Ru(Tp)(η6-C5H2Ph2-CH(Ph)OEt)] (3). A solution of1 (250 mg, 0.546 mmol) and phenylacetylene (300 μL, 2.730 mmol)in ethanol (15 mL) was heated under reflux for 6 h. The resultingbrown yellow precipitate was filtered and washed with pentane anddiethyl ether. The brown microcrystals obtained were recrystallizedfrom ethanol/ether to give brown crystals of the title compound.Yield: (225 mg, 62%). mp: >183 °C. Anal. Calcd C35H32BN6ORu·H2O: C, 61.50; H, 5.16; N, 12.29. Found: C, 61.16; H, 5.18; N, 12.30.FT-IR (KBr): (ν, cm−1), 3125, 3028, 3059 (CH), 2928, 2975, 2891(CH2), 2485, 2094 (Ru-Cp), 1599, 1492, 1397 (−CC−), 1211,1115, 1046 (PhCHOCH2CH3), 760, 697, 620.

1H NMR (400 MHz,CDCl3): δH 1.38 (t, 3H, 3JHH = 7 Hz, CH3 of OCH2CH3); 3.56 (m,2H, CH2 of OCH2CH3); 4.68 (d, 1H, 3JHH = 2 Hz, CH(6) of Cp);4.82 (d, 1H, 4JHH = 1 Hz, CH(4) of Cp); 5.11 (s, 1H, CH(1) ofPh(OEt)CH); 5.90 (t, 3H, 3JHH = 2 Hz, CH(b,b′,b″) of pz); 7.30 (d,3H, 3JHH = 2 Hz, CH(a,a′,a″) of pz); 7.428 (d, 2H, 3JHH = 2 Hz,CH(c,c′,c″) of pz); 6.93−7.14, 7.27−7.28, 7.32 (m, 15H, CH of Ph).13C{1H} NMR (100 MHz, CDCl3): δC 15.70 (CH2 for OCH2CH3),64.57 (CH3 for OCH2CH3), 72.89 (quaternary carbon C(2) of Cp),74.14 (quaternary carbon C(3) of Cp), 75.94 (CH(4) carbon of Cp),79.33 (CH(1) carbon of Ph(OEt)CH), 80.15 (CH(6) carbon of Cp),80.91 (quaternary carbon C(5) of Cp), 104.98 (CH(b, b′, b″) carbonof pz), 125.26−129.73 (CH carbon of Ph), 133.38 (CH(a, a′, a″)carbon of pz), 136.39, 137.89, 140.15 (quaternary carbon of Ph),143.15 (CH(c, c′, c″) carbon of pz). HRMS-ESI (m/z): [M] calcd forC35H33BN6ORu 666.1852, found 666.1857; [M − (C2H5O)]

+ calcdfor C33H28BN6Ru 621.1512, found 621.1519.

Synthesis of [Ru(Tp)(η6-C5H2Ph2-CH(Ph))] (4). A solution of 1(250 mg, 0.546 mmol), phenylacetylene (300 μL, 2.730 mmol), andNH4PF6 (98 mg, 0.601 mmol) in ethanol (15 mL) was heated underreflux for 6 h. The resulting brown yellow solution was evaporated todryness. The resulting solids were washed with pentane and diethylether and then recrystallized from ethanol/ether to give brown crystalsof the title compound. Yield: (310 mg, 75%). mp: >251 °C. Anal.Calcd C32H28BN6PF6Ru·5H2O: C, 46.33; H, 4.48; N, 9.82. Found: C,45.38; H, 3.68; N, 9.61. FT-IR (KBr): (ν, cm−1), 3143 (CH), 2853(CH2), 2520 (Ru-η4Cp), 1504, 1474, 1456, 1399 (−CC−), 837(PF6).

1H NMR: (400 MHz, (CH3)2CO): δH 5.94 (t, 3H, 3JHH = 2 Hz,CH(b) of pz), 6.07 (t, 3H, 3JHH = 2 Hz, CH(b′) of pz), 6.25 (t, 3H,3JHH = 2 Hz, CH(b″) of pz), 6.32 (d, 1H, 4JHH = 2 Hz, CH(6) ofη4Cp), 6.82 (d, 1H, 4JHH = 1 Hz, CH(4) of η4Cp), 7.02 (d, 1H, 4JHH =1 Hz, CH(1) of CH(Ph)), 7.39 (d, 3H, 3JHH = 2 Hz, CH(a) of pz),7.73 (d, 3H, 3JHH = 2 Hz, CH(a′) of pz), 7.75 (d, 3H, 3JHH = 2 Hz,CH(a″) of pz), 7.79 (dd(br), 3H, CH(c′,c″) of pz), 8.04 (d, 3H, 3JHH= 2 Hz, CH(c″) of pz), 7.27 (t, 3H, CH of Ph), 7.37 (d, 2H, CH ofPh), 7.41 (d, 1H, CH of Ph), 7.45 (dt, 1H, CH of Ph), 7.49−7.53 (m,2H, CH of Ph), 7.58 (t, 2H, CH of Ph), 7.69 (dt, 1H, CH of Ph), 7.75(d, 1H, CH of Ph), 7.86 (t, 2H, CH of Ph). 13C{1H} NMR (100 MHz,(CH3)2CO): δC 87.85 (CH(1) carbon of CH(Ph), 90.78 (CHcarbon of Ph), 95.42 (quaternary carbon of Ph), 107.27, 107.42,107.54 (CH(a, a′, a″) carbon of pz), 108.19 (quaternary carbonCH(2) of η4Cp), 116.78 (CH carbon of Ph), 128.77 (quaternarycarbon CH(3) of η4Cp), 129.44 (CH(b′, b″) carbon of pz), 129.70(quaternary carbon of Ph), 129.97, 130.07, 130.17 (CH carbon of Ph),130.82 (CH(c, c′) carbon of pz), 131.35, 131.72, 131.85 (CH carbon

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of Ph), 133.68 (quaternary carbon of Ph), 136.79, 138.30 (CH carbon

of Ph), 137.52 (CH(c″) carbon of pz), 142.89 (CH carbon CH(6) of

η4Cp), 144.54 (CH(b) carbon of pz), 145.39 (CH carbon CH(4) of

η4Cp). HRMS-ESI (m/z): [M − PF6] calcd for C33H28BN6Ru

621.1512, found 621.1523.X-ray Crystallography. X-ray structural determinations were

performed on a Rigaku MiniFlex 600 diffractometer equipped with a

Mo X-ray tube. Intensity data were corrected for using multiscan data.

All the relevant calculations were executed using SHELXTL programs,

and the refinement thereof by direct methods using SHELXL

programs. All non-hydrogen atoms were refined anisotropically. All

hydrogen atoms were refined to exhibit ideal positions, riding on their

parent atoms. The crystallographic data collection and structure

refinement parameters are summarized in Table S1 (contained in the

Supporting Information). The CIF file for the subject compounds are

deposited with the Cambridge Crystallographic Data Centre (CCDC

numbers, 997419, 997420, and 997421).

■ RESULTS AND DISCUSSION

The air-stable polymeric complex [RuCl2(1,5-cod)]x wasemployed as a convenient and reactive starting material,which was synthesized from RuCl3·xH2O and 1,5-cyclo-octadiene in boiling EtOH as reported previously.27 Theneutral complex [RuCl2(1,5-cod)(CH3CN)2] was subsequentlysynthesized from the polymeric [Ru(COD)Cl2]x to be used asthe high-yielding synthetic precursor to the known [RuCl-(Tp)(1,5-cod)] complex. In this reaction, a THF solution of[RuCl2(1,5-cod)(CH3CN)2] was reacted with KTp (Tp =HB(pz)3) and heated under reflux for 6 h to give [RuCl(Tp)-(1,5-cod)] 1 (Scheme 1), as was confirmed by 1H and 13CNMR.The reaction of 1 with phenylacetylene in MeOH at 70 °C

gave the unusual substituted cyclopentadienyl carbocycliccomplex of [Ru(Tp)(η6-C5H2Ph2-CH(Ph)OMe)], 2, as aproduct of a [2 + 2 + 1] cyclotrimerization reaction ofphenylacetylene (Scheme 2). A mechanistic explanation for the

Scheme 1

Scheme 2

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anti-Markovnikov addition of the alcohols (MeOH, EtOH) inthe [2 + 2 + 1] cyclotrimerization reaction has been proposedpreviously by Han and Lee20 for the formation of the analogousrhodium complex [Rh(Cp*)(η5-C5H2Ar2-CH(Ar)OEt)]OTf(Ar = Ph, p-tolyl). They proposed that the precursor complex,[Rh(Cp*)(η5-2,6-(MeCH)2C6H3NH2)](OTf)2, reacts with 1equiv of phenylacetylene to form a rhodium−vinylidene speciesthrough proton transfer and coordination of the OEt-anion tothe rhodium center, followed by the insertion of two furthermolecules of phenylacetylene to give a metallacyclohexadieneintermediate complex. This intermediate then undergoesreductive elimination to give a fulvene-type species, which isfollowed by the nucleophilic attack of the coordinated OEt-anion on the exocyclic carbon of the fulvene-intermediatecomplex to give the final product. Only two mechanisms on the[2 + 2 + 1] cyclotrimerization of terminal alkynes are known,one which involves a metallacyclopentadiene intermediate, andthe other an uncommon metallacyclohexadiene route, of whichthe latter is more applicable to this particular reaction. Theformation of complex 2 is, therefore, proposed to follow theroute described above analogous to that reported for theformation of the rhodium complex [Rh(Cp*)(η5-C5H2Ar2-CH(Ar)OEt)]OTf.20 To the best of our knowledge, this is thefirst account of the formation of five-membered cyclic adductswith the [RuTp]+-fragment through [2 + 2 + 1] cyclo-trimerization.The structures of the formed fulvene intermediate complexes

in general may exist as either of two resonance structures, inwhich one is the cyclopentadienyl-fragments, with the exo-carbon exhibiting a slight positive charge, and, therefore,making it susceptible to nucleophilic attack.17 It is important tonote that, in these reactions, the alcohol employed acts both asa proton donor and as a nucleophile to the labile 1,5-cyclooctadiene ligand. Upon changing the solvent to EtOH, the−OEt anti-Markovnikov adduct, [Ru(Tp)(η6-C5H2Ph2-CH-(Ph)OEt)] 3, analogous to 2, is obtained (Scheme 2). Thisfinding shows that the slightly less polar, although bulkier,ethoxy nucleophile continues to react in the same way as themethoxide nucleophile. The FT-IR spectra of both 2 and 3show characteristic O−C stretching and bending frequencies atν 1047 cm−1 (for 2) and 1046 cm−1 (for 3) for the alkoxidegroups. The 1H NMR spectrum of complex 2 exhibits a singletat δ 3.42 ppm for the OCH3 functionality, while resonances forthe OCH2CH3 functionality in complex 3 appeared as a tripletat δ 1.38 (OCH2CH3) and a multiplet at δ 3.56 (OCH2CH3)ppm. It is interesting to see a multiplet instead of quartet forCH2 protons of the OCH2CH3 group in complex 3. Thisindicates that the CH2 protons of the OEt group are prochiraldue to the presence of the adjacent chiral center C*(Cp)(H)-(Ph)(OEt). Hence, these are diastereotopic protons and arenonequivalent and appeared as a multiplet. We could notobserve the B-H proton of the ligand due to coupling to the10/11B atom. We have also studied variable low temperature 1HNMR of complex 3 at various temperatures: 0, −10, −20, −30,−40, and −50 °C. The CH2 protons of OCH2CH3 in complex3 appeared as a multiplet even at −30 °C, but these appeared astwo sets of quintets at 3.46 and 3.56 ppm at −55 °C (FigureS10, Supporting Information). The crystal structure of 3 (seelater) showed a weak C−H···π interaction (2.5 Å) between oneof the CH2 protons and π electrons of the phenyl groupattached to the exocarbon atom (Figure S11, SupportingInformation). This leads to restricted rotation of the CH2CH3

group along the C−O bond, and this might be an additionalreason for nonequivalency of CH2 protons.The exo-carbon proton of 2 resonates at δ 5.02 ppm, whereas

the same proton in 3 resonates slightly more downfield at 5.11ppm; this might be due to the presence of the bulky ethylsubstituent on the exo-carbon of 3. Near identical resonancesfor the CH protons of the substituted cyclopentadienyl ligandare observed for both 2 and 3 at δ 4.67 (d, 4JHH = 1 Hz) and4.81 (d, 4JHH = 1 Hz), and 4.67 (d, 4JHH = 1 Hz) and 4.82 (d,4JHH = 1 Hz), showing that negligible change in the electronicenvironment within the carbocycles between 2 and 3 isobserved. Typical resonances for the pyrazolyl ligands in 2 and3 are observed between δ 5.89−5.90, 7.29−7.30, and 7.42−7.43ppm. The 13C NMR spectra of both 2 and 3 confirm the abovewith the exo-carbon of 2 resonating at δ 80.7 ppm, as comparedto the same carbon of 3 resonating at 79.3 ppm. The presenceof OCH3 and OCH2CH3 groups was confirmed in the samespectra with singlets appearing at δ 56.7 (OCH3), 15.7(OCH2CH3), and 64.6 (OCH2CH3) ppm.The molecular structures of 2 and 3 are shown in Figures 1

and 2, respectively. Complexes 2 and 3 are isostructural, with

both structures having slightly distorted piano-stool structures ifthe substituted cyclopentadienyl ligands are considered as thebasal planes, and the three pyrazolyl ligands of the Tp ligandform the three legs of the stool structures. The cyclo-pentadienyl ligands in both structures are comparable witheach other, where both ligands exhibit a nearly flat carbocyclicplane, and a Ru−Cp (π-bond) length of 2.154 Å (in 2) and2.155 Å (in 3). The distinct differences between the twocomplexes is, apart from the OMe and OEt functionalities,revealed in the bond lengths of the Cp−exo-carbon atom, as

Figure 1. Molecular structure of complex 2 with thermal ellipsoidsrepresenting 50% probability level. Most of the hydrogen atoms areomitted for clarity.

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well as the exo-carbon−O atom. The Cp−exo-carbon atombond length is 1.500(4) Å in 2, and 1.505(4) Å in 3. The exo-carbon−O atom bond length is 1.435(4) Å in 2, and 1.431(3)Å in 3. Furthermore, typical Ru−N bond lengths from the Tpligands are observed with an average Ru−N bond length of 2.14Å (2) and 2.08 Å (3), respectively (Table 1).In the proposed mechanism of formation for complexes 2

and 3, an important ruthenium(II)-fulvene complex is formed.The synthetic isolation of these highly reactive reactionintermediates is uncommon in the literature. Lim et al.20

have proposed the analogous fulvene intermediate, [Rh(Cp*)-(η5-C5H2Ar2-CH(Ar))]OTf in the synthesis of [Rh(Cp*)(η5-C5H2Ar2-CH(Ar)OEt)]OTf, but they were not able to isolateand characterize the intermediate fulvene-Rh complex. Thereaction of 1, [RuCl(Tp)(1,5-cod)], with phenylacetylene inCH3OH with 1.1 molar equivalents of NH4PF6 has resulted inthe fulvene complex 4, [Ru(Tp)(η5-C5H2Ph2-CH(Ph))]PF6, ingood yield (75%). It is interesting to note that the presence ofNH4PF6 inhibits the anti-Markovnikov addition of CH3OHover the formed fulvene complex during reaction. The complex2 on reaction with NH4PF6 in CH3OH produces fulvene

complex 4, [Ru(Tp)(η5-C5H2Ph2-CH(Ph))]PF6 (Scheme 3).Then, we decided to reflux complex 4 in MeOH, but thisreaction did not result in the formation of complex 2. Thisindicates that complex 4 is more stable than complex 2, and thismight be due to the stabilizing effect of the PF6

− counteranion.We believe that the in situ formation of NH3 and HPF6 fromNH4PF6 and, subsequently, the generation of H+ inhibit thereduction of the fulvene CC bond to form the −OMe and−OEt anti-Markovnikov products. This is also confirmed byHRMS-ESI mass spectroscopy. The high-resolution massspectra of complex 2, 3, and 4 showed the molecular ionpeak corresponding to [M]+ at m/z = 652.17, 666.18, and621.15, respectively. In the case of complexes 2 and 3, the basepeak appeared at m/z 621.15, which is attributed to [M −(OR)]+, and this corresponds to the molecular peak of fulvenecomplex 4.The resulting complex 4 has the tridentate Tp ligand

coordinated to the central ruthenium atom, along with two η2-bonds (diene-like) of the cyclopentadienyl ligand, as well as anη2-bond of the fulvene moiety to the ruthenium atom. Thepresence of the fulvene functionality has been confirmed fromthe 1H NMR spectrum with the fulvene CH proton resonatingas a singlet at δ 5.92 ppm, as well as from the 13C NMRspectrum of the endo-fulvene carbon resonating as a singlet at δ115.2 ppm. The cyclopentadienyl ligand in 4 differs from thoseof 2 and 3 in that the extent of conjugation between the sp2-carbon atoms is limited, and, therefore, bonds to the rutheniumatom in a more formal “diene”-fashion. Typical CH protons ofthe carbocyclic ligand appear as singlets in the 1H NMRspectrum at δ 6.26 and 6.40 ppm, which corresponds well totypical η2-bound diene resonances, and are shifted significantlymore downfield compared to the similar CH protons observedfor complexes 2 and 3.From the X-ray structure determined for 4 (Figure 3), the

coordination geometry of the ruthenium atom differs from thepiano-stool like structures of 2 and 3, as the side-oncoordinated fulvene bond distorts the traditional piano-stoolstructure. In complex 4, the Cp−C(1) exo-bond and the planeof the Cp make an angle of 118.4(11)° with each other,whereas this angle is less distorted than that in other reported[MCp(fulvene)]+ complexes.33,34 This indicates that thepresence of the exo-Ph substituent group stabilizes the positivecharge of the exo-C(1) atom. Although no formal π-bondbetween the cyclopentadiene ligand and the ruthenium atomexists in 4, a cyclopentadiene−ruthenium π-bond distance ismentioned for direct comparison with the cyclopentadienylcounterparts of 2 and 3, respectively. The Cp′−Ru π-bonddistance in 4 is 2.207 Å (average), which is noticeably longerthan the Cp′−Ru π-bonds found for 2 (average 2.154 Å) and 3

Figure 2. Molecular structure of complex 3 with thermal ellipsoidsrepresenting 50% probability level. Most of the hydrogen atoms areomitted for clarity.

Table 1. Selected Bond Lengths [Å] and Bond Angles [deg] for Complexes 2−4

2 3 4

Ru−N1 2.151(3) 2.090(10) 2.141(2)Ru−N3 2.131(2) 2.082(10) 2.157(2)Ru−N5 2.124(3) 2.077(8) 2.126(2)average Ru−C(C2−C6 π-bond) 2.154 2.155average Ru−C(C3−C6 π-bond) 2.207average Ru−C(C1−C2 π-bond) 2.370N(1)−Ru−N(3) 84.65(10) 81.7(4) 84.87(9)N(1)−Ru−N(5) 82.90(10) 87.5(4) 83.84(9)N(3)−Ru−N(5) 83.60(10) 83.6(4) 82.54(8)

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(average 2.155 Å), respectively. The tridentate Tp ligandcoordinates to the central ruthenium atom as usual, with anaverage Ru−N bond length of 2.14 Å, which is comparable tothe average Ru−N bond length of 2.08 Å found for complex 3(Table 1).

■ CONCLUSIONS

In summary, a high-yielding route to anti-Markovnikovalkoxide-adduct products in the [2 + 2 + 1] cyclotrimerizationof phenylacetylene has been devised. These transformationsoccur with high chemoselectivity and regioselectivity. Fur-thermore, the isolation of the catalytically active fulveneintermediate cationic complex allows for the critical evaluationand insight into the mechanistic pathway followed in the overall[2 + 2 + 1] cycloaddition reaction. This work suggests that theancillary ligands employed and the presence of external anionsare crucial in the overall chemodirected outcome of the [2 + 2+ 1] cyclotrimerization reaction.

■ ASSOCIATED CONTENT

*S Supporting InformationNMR and ESI-MS spectral data for 2, 3, and 4. This material isavailable free of charge via the Internet at http://pubs.acs.org.CCDC 997419, 997420, and 997421 contain the supplemen-tary crystallographic data for this article.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: rmeijboom@uj.ac.za (R.M.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is based on the research supported, in part, by theNational Research Foundation of South Africa (Grant specificunique reference number (UID) 85386. The New Generation

Scheme 3

Figure 3. Molecular structure of complex 4 with thermal ellipsoids representing 50% probability level. Most of the hydrogen atoms are omitted forclarity.

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Scholarship (NGS) of the University of Johannesburg (UJ) isgratefully acknowledged for a stipend for F.P.M.

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