Article pubs.acs.org/Organometallics
Abstraction of a Vinylic Hydrogen to Form Alkynes. Multinuclear and Multidimensional NMR Spectroscopy and Computational Studies Elucidating Structural Solution Behavior of Acetylene and Propyne Complexes of Titanium Marco G. Crestani, Anne K. Hickey, Balazs Pinter, Xinfeng Gao, and Daniel J. Mindiola*,† Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: The alkyne complexes [(PNP)Ti(η2-HCCH)(CH2tBu)] (2) and [(PNP)Ti(η2-HCCMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)TiCHtBu(OTf)] (1) with the Grignard reagents H2CCHMgCl and MeHCCHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2CC(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2MeCCMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H−31P HMBC, 1 H−31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol−1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)TiCHtBu(CHCHR)] followed by a concerted, metal-mediated β-hydrogen abstraction step that has been computed to have a barrier of 20−22 kcal mol−1. The geometry and rotational mechanism of the alkyne ligand in 2 are presented and compared with those of the ethylene derivative [(PNP)Ti(η2-H2CCH2)(CH2tBu)] (5), which does not display rotation of the bound ethylene under the same conditions.
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precursor such as [(PNP)TiCHtBu(OTf)] (1)5 (Scheme 1). If α-hydrogen migration was the chosen path, our approach would provide entry to rare examples of titanium vinylidenes, which are reactive intermediates in group transfer chemistry.6
INTRODUCTION We recently reported that the complex [(PNP)TiCHtBu(CH2tBu)] (PNP− N[2-PiPr2-4-methylphenyl]2) reacts with C2−C8 alkanes (in the case of C4 and above only linear alkanes were explored), cyclohexane, and methylcyclohexane to afford the corresponding olefin products [(PNP)Ti(η2-H2C CHR)(CH2tBu)] (R = H, Me, Et, nPr, nBu, nPentyl, nHexyl) by virtue of a 1,2-CH bond addition across the transient species [(PNP)TiC t Bu] (A) to form [(PNP)TiCH t Bu(CH2CH2R)], followed by a metal-mediated β-hydrogen abstraction step to form the bound terminal olefin.1,2 The second step is particularly important, since we do not observe any evidence for β-hydride elimination via a six-coordinate intermediate such as [(PNP)TiCHtBu(η2-H2CCHR)(H)]. Even though α-migration to form the tautomer [(PNP)TiCHCH2R(CH2tBu)] is feasible on the basis of isotopic labeling studies using D2CCD2,2 we have found that the latter process can be observed when no β-hydrogens are present, as in the case of [(PNP)TiCHtBu(CH2R′)] (R′ = SiMe3, C6H5, 3,5-Me2C6H3, CMe2SiOSiMe3, SiMe2CCSiMe3, C6F5).3 For this reason we turned our attention to vinylic groups, to test whether a strong C−H bond (111 kcal mol−1)4 would be amenable to activation. This would also allow us to establish if terminal titanium vinylidenes or alkynes could be prepared from the corresponding vinyl-delivering group using a © 2014 American Chemical Society
Scheme 1. Proposed Reaction To Understand Selectivity in α-Hydrogen Migration versus β-Hydrogen Abstraction in the Transient Alkylidene−Vinyl Species [(PNP)Ti CHtBu(HCCHR)]
Received: November 25, 2013 Published: February 19, 2014 1157
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Scheme 2. Syntheses of [(PNP)Ti(η2-HCCR)(CH2tBu)] (R = H, Me) Compounds 2 and 3
In contrast, if β-hydrogen abstraction ensued, this would also result in a rare ligand scaffold on titanium, a reactive η2alkyne.7−13 In this work we describe the transmetalation chemistry of 1 with various vinyl Grignards and report the synthesis of a rare example of an acetylene adduct of titanium, [(PNP)Ti(η2HCCH)(CH 2tBu)] (2), in addition to the propyne derivative [(PNP)Ti(η2-HCCMe)(CH2tBu)] (3). The structural and dynamic behaviors of these alkyne complexes of titanium were elucidated in solution using an array of state of the art multinuclear NMR spectroscopic experiments, including dqfCOSY, 1H−13C HSQC, 1H−13C and 1H−31P HMBC, 1H and 31P EXSY, and 1H−31P HOESY, in addition to variabletemperature (VT) 1H and 31P{1H} NMR. We established the barrier to rotation of the acetylene ligand via VT NMR spectroscopy and proposed a mechanism to rotation with the aid of theoretical studies. Likewise, with the support of theoretical studies, we have also probed the mechanism of formation of the alkyne and its dynamic behavior, as well as predicted the structural features of the [(PNP)Ti(η2-HC CH)(CH2tBu)] complex and compared them to those of the ethylene analogue [(PNP)Ti(η2-H2CCH2)(CH2tBu)].
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high-purity grade and used without further purification. Repeated attempts to obtain satisfactory microanalyses of the compounds failed because of their extreme sensitivity to air and moisture and quite possibly their incomplete combustion. We have therefore provided high-resolution 1D and 2D NMR spectra as proof of their purity and in lieu of combustion elemental analysis data.16 Samples for IR analysis were prepared by crushing and mixing the compounds with KBr (FTIR grade, ≥99% trace metals basis) and pressing the mixture into a pellet. Characterization. Unless otherwise stated, NMR spectroscopic characterization of all air- and moisture-sensitive compounds was done in solution using sealed J. Young NMR tubes, under argon. All 1D (1H, selective and fully decoupled 1H{31P}, 31P{1H}, 13C{1H}, DEPT-135) and 2D (dqfCOSY, multiplicity-edited 1H−13C gHSQC, 1H−13C HMBC, 1H−31P HMBC, 1H−31P HOESY, 31P-EXSY, 1H-EXSY) NMR spectra of isolated products were recorded from C6D6 solutions using Varian 500, 400, and 300 MHz NMR spectrometers operating at 25 °C. A mixing time of 0.8 s was used for both 1H and 31P EXSY experiments, as well as for 1H−31P HOESY. 1H−13C HMBC experiments were optimized for 8 Hz. 1H−31P HMBC experiments were optimized for 7 Hz. 1H and 1H{31P} chemical shifts are reported referenced to the internal residual solvent proton resonance (C6D5H; 7.160 ppm, s). 13C{1H} NMR spectra are referenced to the deuterated solvent (C6D6; 128.39 ppm, t). 31P{1H} NMR chemical shifts are reported relative to external H3PO4 (0.0 ppm). For 2D NMR, ⟨δ1, δ2⟩ coordinates extracted from the C−H decoupled multiplicity-edited gHSQC spectra (δ1, 1H; δ2, 13C{1H}) are reported for distinctive, key connectivities around titanium centers on the alkyne compounds. Also for these compounds, the one-bond coupling constant, 1JC−H, is indicated in parentheses, as measured from the respective C−H coupled multiplicity-edited gHSQC spectra. [(PNP)Ti(η2-HCCH)(CH2tBu)] (2). In a vial, a solution of [(PNP)TiCHtBu(OTf)] (0.0508 g, 0.073 mmol) in Et2O (∼10 mL) was cooled to −35 °C for 30 min and to it was added vinylmagnesium chloride as a 1.6 M solution in THF (H2C CHMgCl; 46 μL, 0.074 mmol; 1.0 equiv). The reaction mixture was stirred for 30 min and the solvent evaporated under vacuum. The residue was extracted into n-pentane and filtered through a frit charged with a pad of Celite and taken to dryness under vacuum, leaving a brown-red solid. Yield of 2, after workup: 88.1% (0.0372 g, 0.065 mmol). Spectroscopic assignments are given in the Supporting Information. [(PNP)Ti(η2-HCCMe)(CH2tBu)] (3). Method A: Reaction with 1-Propenylmagnesium Bromide. The synthesis was performed as described for 2, using [(PNP)TiCHtBu(OTf)] (0.0503 g, 0.072 mmol) and 1-propenylmagnesium bromide as a 0.5 M solution in THF ((H3C)HCCHMgBr; 200 μL, 0.1 mmol; 1.3 equiv). Yield, after workup: 94.4% of spectroscopically pure brown-red solid (0.040 g, 0.068 mmol).
EXPERIMENTAL SECTION
General Considerations. Unless otherwise stated, all manipulations involving air- or moisture sensitive compounds and their preparations were performed in double or single gloveboxes under purified nitrogen or argon atmospheres. Manipulations involving high vacuum and inert or reactive gases (N2O, propyne, ethylene) were performed using standard Schlenk techniques in single- and doublemanifold glass lines. The glassware used for all these manipulations was either flame- or oven-dried for at least 12 h at 145 °C and then cooled in antechambers under dynamic vacuum. Celite, alumina, and molecular sieves used for the purification of compounds and solvent drying and storage were first activated at 250 °C for 24 h under high dynamic vacuum, before use. Diethyl ether (Et2O) was dried by dual passage through activated alumina columns. Tetrahydrofuran (THF) was dried and distilled from deep purple sodium/benzophenone ketyl solutions.14 All bulk protio solvents were typically maintained over sodium and 4 Å molecular sieves. Benzene-d6 (C6D6,) and toluene-d8 were degassed by three consecutive freeze−pump−thaw cycles in a Schlenk line and then placed over sodium and molecular sieves for at least 12 h prior to use. [(PNP)TiCHtBu(OTf)] (1;15 PNP = N[2P(CHMe2)2-4-methylphenyl]2−, OTf = [OSO2CF3]−), [(PNP)Ti CHtBu(CH2tBu)],15 and [(PNP)Ti(CH2tBu)(η2-H2CCH2)]1 were prepared according to the procedures reported by our group. All other chemicals, chromatographic materials, and filter aids were purchased in 1158
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Method B: Reaction with Isopropenylmagnesium Bromide. The reaction and its workup were performed following the method described for 2, using [(PNP)TiCHtBu(OTf)] (0.0798 g, 0.115 mmol) and isopropenylmagnesium bromide as a 0.5 M solution in THF (H2CC(CH3)MgBr; 280 μL, 0.14 mmol; 1.2 equiv). Yield of 3, after workup: 91.3% of spectroscopically pure brown-red solid (0.0618 g, 0.105 mmol). Spectroscopic assignments are given in the Supporting Information.16 [(PNP)Ti(η2-MeCCMe)(CH2tBu)] (4). In a vial, a solution of [(PNP)TiCHtBu(OTf)] (0.0382 g, 0.055 mmol) in Et2O (∼10 mL) was cooled to −35 °C for 30 min and to it was added 1-methyl-1propenylmagnesium bromide as a 0.5 M solution in THF ((H3C)HCC(CH3)MgBr; 300 μL, 0.15 mmol; 2.7 equiv). The reaction mixture was stirred for 5 days, after which the solvent was evaporated under vacuum. The residue was extracted into hexanes and filtered through a frit charged with a pad of Celite and taken to dryness under vacuum, leaving a brown-red oil. Yield, after workup: 80.0% (26.3 mg, 0.044 mmol). Spectroscopic assignments are given in the Supporting Information.16
should be noted that 2 cannot be prepared from [(PNP)Ti CHtBu(CH2tBu)] and ethylene (low or high pressure), via dehydrogenation of the ethylene by transient A.2 Like complex 2, the propyne derivative [(PNP)Ti)(η 2 -HCCMe)(CH2tBu)] (3) was prepared by stoichiometric reaction with 1 equiv of 1-propenylmagnesium bromide (MeHC CHMgBr) in Et2O. The use of isopropenylmagnesium bromide (H2CC(Me)MgBr) was found to give an equal outcome (Scheme 2). Scheme 2 also shows Newman projections of the alkyne complexes 2 and 3. How these titanium alkyne products are obtained is important, since they seemingly originate from an intramolecular β-H abstraction of a vinylic ligand involving the common transient titanium vinyl intermediate [(PNP)Ti CHtBu(HCCHR)] (R = H, Me) (Scheme 3). The fact that complex 3 can be prepared from two different isomeric forms of the vinyl Grignard, isopropyl or 1-propenyl, implies that a vinylic hydrogen abstraction (as opposed to β-H elimination, αH abstraction, or allylic C−H abstraction) is the dominant pathway toward formation of the respective alkyne adduct. The identity of the alkyne derivatives 2 and 3 was conclusively established by a battery of multinuclear 1D and 2D NMR techniques, many of which are discussed in detail in the following sections. In the 1H NMR spectra, both compounds exhibit highly deshielded resonances at ∼10 ppm for the alkyne proton (Figures S14, S30, and S31, Supporting Information),16 which are coupled to the phosphorus atoms on the PNP backbone (vide infra). In the case of 2, the acetylene protons are highly deshielded with respect to the only other group 4 acetylene complex reported, namely the porphyrin species (TPP)Ti(η2-HCCH) (TPP2− = tetratolylporphyrinato) prepared by Woo and co-workers, which shows such a feature at 5.81 ppm (Table 1).7 Consistently, the resonances of the alkyne carbons also appear largely deshielded in the 13 C{1H} NMR spectrum, at ∼200 ppm (Figures S16 and S32, Supporting Information),16 deceivingly close to the range for titanium alkylidenes, but also in the range for reported titanium alkyne adducts (186−245 ppm; Table 1).6−13 Complexes 2 and 3 elicit spectroscopic evidence of a neopentyl fragment possessing diastereotopic methylenic protons in the aliphatic region,1 and thus, this feature rules out the possibility of an alkylidene−vinyl structure. Consistent with this fact, no vinyl protons were detected in the spectra of either of these compounds. Figure 1 clearly depicts selected expansions of double-quantum filtered COSY (dqfCOSY) spectra of 2 and 3, highlighting the correlations between diastereotopic neopentyl protons in both molecules. The latter correlations are highlighted in red and marked as a and b. The identity of the alkyne adducts 2 and 3 was further confirmed by 1H−13C heteronuclear multiple bond coherence (HMBC), which permitted us to systematically establish the connectivity in the core fragments, Ti(CH2tBu) and Ti(η2HCCR), by their two- and three-bond C−H correlations (Figure 2).17 As noted in Figure 2, both 2 and 3 display virtually the same three-bond distance correlations between the diastereotopic neopentyl protons and the methyl substituents on the tert-butyl groups, which is consistent with these complexes having a Ti(CH2tBu) moiety. In addition, compounds 2 and 3 exhibit the same type of two-bond distance correlations between the terminal alkyne protons and their coordinated carbons, which for compound 2, being symmetric, results in two overlapping contours with (1H, 13C) coordinates at (10.4, 198.25) ppm for the two HCCH or HCCH spin systems. For complex 3,
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RESULTS AND DISCUSSION Syntheses of η2-Alkyne Complexes of Titanium. The compound [(PNP)Ti(η2-HCCH)(CH2tBu)] (2) was preScheme 3. Transmetalation of 1 with the Vinyl Grignard, Followed by β-Hydrogen Abstraction in the Vinyl−Titanium Intermediate To Form the Alkyne Derivatives 2 and 3
Table 1. 1H and 13C NMR Spectroscopic Features for Reported Alkyne Complexes of Titanium (a) 1
compound (TTP)Ti(η2-EtCCEt) (TTP)Ti(η2-MeCCMe) (TTP)Ti(η2-PhCCH)b (TTP)Ti(η2-HCCH) (OEP)Ti(η2-PhCCPh) Cp*2Ti(η2-PhCCPh) Cp*2Ti(η2-MeCCMe) Cp2Ti(CO)(η2-PhCCPh) Cp2Ti(η2-Me3SiCCSiMe3) Cp2Ti(η2-PhCCPh) [(PNP)Ti(η2-HCCH) (CH2tBu)] (2) [(PNP)Ti(η2-HCCMe) (CH2tBu)] (3) [(PNP)Ti(η2-MeCCMe) (CH2tBu)] (6)
H NMR CHalkyne
5.73 5.81
10.40 9.93
13
C{1H} NMR Calkyne (1JCH in Hz)
225.9 221.9 226.0 (PhC); 211.3 (CH) 216.0; 174.8 219.4 200.1 200.9 244.7 196.5 198.3 (176.9) 207.6 (MeC); 200.9 (HC, 123.1) 221.9 (MeC)
ref 7 7 7 7 8 9 9 10 11 11 this work this work this work
Abbreviations: Cp*, C5Me5−; TTP, tetratolylporphyrin(2−); OEP, octaethylporphyrin(2−). bNo 1JCH value was reported for this complex. a
pared cleanly by reaction of [(PNP)TiCHtBu(OTf)] (1)5 with 1.0 equiv of vinylmagnesium chloride (H2CCHMgCl) in chilled diethyl ether (Et2O), over a period of 30 min. It 1159
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Figure 1. Expansion of the dqfCOSY (in ppm) spectrum of complexes 2 (top) and 3 (bottom), highlighting the correlation between the diastereotopic protons in the neopentyl substituent.
(2JC−Pa = 17.6 Hz, 2JC−Pb = 7.1 Hz). The resonance for the Ti(η2-HCCMe) residue, visible in the DEPT-135 trace, was also assigned as a dd, centered at 201.70 ppm (2JC−Pb = 20.1 Hz, 2 JC−Pa = 9.6 Hz). For compound 2, the two alkyne carbon resonances, Ti(η2-HCCH), appear as a pseudotriplet (t) at δ 198.25 ppm, with 2JC−P = 14.8 Hz. It is possible that for compound 3, the side-on coordination of the alkyne is reflected on the differences in magnitude of the 2JC−P coupling constants to each of the two phosphorus atoms on the PNP ligand. However, for species 2 the multiplicity of the resonance and the apparent 2JC−P value are more in line with the fluxional character of this molecule being smaller and symmetric; features which will be discussed in detail later. It is worth
however, the two- and three-bond correlations stemming from the C−H couplings of the coordinated propyne give rise to four clearly resolved contours: (9.93, 23.55), (9.93, 207.58), (2.79, 200.90), and (2.79, 207.68) ppm, as a result of the (HCC− CH3), (HCC−CH3), (HCC−CH3) and (HCC−CH3) spin systems, respectively. Of these, the first and last systems elicit two-bond coupling between protons and a quaternary carbon and, therefore, no carbon resonance is observed in the DEPT-135 trace used for plotting the HMBC spectrum of this compound (Figure 2). Regardless, the resonance for the carbon in Ti(η2-HCCMe) in complex 3 was unequivocally assigned in the 13C{1H} NMR spectrum, as a doublet of doublets (dd) at 208.48 ppm due to coupling to the inequivalent phosphines 1160
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Figure 2. 1H−13C HMBC of 2 (top) and 3 (bottom) highlighting critical two- and three-bond C−H correlations in these compounds (reported in ppm). The asterisk denotes the chemical shift of the alkynyl carbon on Ti(η2-HCCMe) in 3, whose resonance cannot be observed in the DEPT135 trace.
noting that the analysis of the HMBC NMR spectra of 2 and 3 mentioned above permitted us to discriminate the possibility of α-H abstraction of the alkylidene−vinyl intermediate in Scheme 1, which could have led to a hypothetical terminal titanium vinylidene of the type [(PNP)Ti(CH2tBu)(CCH2)]. Closely related examples with early transition metals have been prepared from allyl intermediates.6 The fact that compound 3 can be indistinctively prepared from (Me)HCCHMgBr or H2CC(Me)MgBr further supports the critical role of β-H abstraction in the synthesis of these products. With this idea in mind, we evaluated the reactivity of 1 with 2-methyl-1-propenylmagnesium bromide (Me2CCHMgBr), a vinyl Grignard reagent lacking β protons, which we hypothesized would result in formation of the alkylidene−vinyl product [(PNP)TiCH tBu(HC CMe2)] or the tautomer [(PNP)TiCCMe2(CH2tBu)] shown in Scheme 4. Unfortunately, multiple attempts to
Scheme 4. Reaction of Complex 1 with 2-Methyl-1propenylmagnesium Bromide To Form 4 and with 1-Methyl1-propenylmagnesium Bromide To Form 6
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Figure 3. Expansion of the 31P{1H} NMR (162 MHz, 25 °C) spectrum of 2 showing two sharp doublets for the asymmetric phosphorus atoms on the PNP backbone. The phosphorus atoms have been arbitrarily assigned.
Figure 5. Expansions of the overlaid selective and fully decoupled 1 H{31P} NMR spectra (400 MHz, 25 °C) of 2, showing the changes in multiplicity of (A) the alkyne resonance and (B) the more shielded methylene resonance as a result of decoupling from P1 or P2.
prepare the vinyl or vinylidene species resulted in complicated mixtures, even after brief mixing of the Grignard with 1. We then attempted the synthesis of a 2-butyne analogue, [(PNP)Ti(η2-MeCCMe)(CH2tBu)] (4), by reaction of 1 with 1-methyl-1-propenylmagnesium bromide ((Me)HC C(Me)MgBr), which we envisioned would also demonstrate the importance of hydrogen in the β position (Scheme 4). The reaction is, however, exceedingly slow in forming a new product (>96 h, room temperature), and as with Me2CCHMgBr, we also observe some decomposition, albeit in minor amounts. Regardless, the 2-butyne complex 6 can be characterized unambiguously using multiple NMR spectroscopic techniques (Table 1 and Figures S56−S58 (Supporting Information)).
This compound shows spectroscopic features similar to those of 2, except now displaying a methyl resonance (1H NMR:, 2.37 ppm; 13C NMR, 221.9 ppm) instead of the terminal HC unit. In the 13C{1H} NMR spectrum, the alkyne carbon resolves into a pseudotriplet with 2JCP = 11.8 Hz, a value slightly lower than that determined for 2 (14.8 Hz). Unfortunately, we have been unable to observe the vinyl intermediate leading to 6 (Scheme 4). 1D and 2D Multinuclear NMR Spectroscopic Studies of Alkyne Complexes and Fluxional Behavior. [(PNP)Ti(CH2tBu)(η2-HCCH)] (2). Similarly to the previously addressed olefin-bound titanium compounds [(PNP)Ti(η2-
Figure 4. 1H NMR spectrum (500 MHz, 25 °C) of 2, showing as insets the expansions of the peaks due to the core fragments Ti−CH2tBu and Ti(η2-HCCH). 1162
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Figure 6. 1H−31P HMBC NMR spectrum (400 MHz, 25 °C) of 2, indicating the correlation between resonances of the core fragments Ti−CH2tBu and Ti(η2-HCCH). Only the diastereotopic hydrogen labeled “a” correlates to one phosphorus atom of the PNP ligand.
Figure 7. Stacked expansions of the 1H VT-NMR (400 MHz) spectra of 2 spanning from −63 to +90 °C, showing decoalescence of the Ti(η2-HCCH) resonances at the low-temperature limit.
Figure 9. Expansions of overlaid selective and fully decoupled 1H{31P} NMR spectra (500 MHz, toluene-d8) of 2 at −85 °C, showing the change in multiplicity of one of the two alkyne resonances as a result of the selective decoupling from P1 or P2.
Figure 8. (a) Simulated and (b) experimental 1H VT-NMR traces of 2 (400 MHz, +25 to −63 °C). Exchange constants are shown compared versus the respective temperatures for each of the actual spectra. 1163
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Figure 10. Multiplicity-edited C−H decoupled gHSQC NMR (500 MHz, 25 °C) spectrum of 2, displaying in the insets the contours of the Ti(η2HCCH) (red denoting positive) and the Ti−CH2tBu (blue denoting negative) fragments. The protons of the latter moiety are diastereotopic and give individual contours aligned with the negative peak at 115.06 ppm in the DEPT trace.
Figure 11. Multiplicity-edited C−H coupled gHSQC NMR (500 MHz, 25 °C) spectrum of 2, displaying in the insets the contours of the Ti(η2HCCH) and Ti−CH2tBu fragments as doublets. The one-bond C−H coupling constants (1JC−H) are indicated in maroon.
H2CCH2)(CH2tBu)],1,2 the 31P{1H} NMR spectrum of 2 (Figure 3) exhibits two doublets at 23.3 and 22.3 ppm, with 2JPP = 29.5 Hz: very close in both chemical shift range and coupling constant magnitude to those of the former (for the olefinbound compound the doublets appear at 28.1 and 26.05 ppm, with 2JPP = 22.2 Hz). The 1H NMR spectrum of 2 (Figure 4) shows two distinct resonances integrating to 1H each, at 1.62 (d, 2JHH = 11 Hz)
and 0.42 ppm (dd, 2JHH = 11.3 Hz, 3JHP = 2.7 Hz), consistent with diastereotopic protons on the Ti−CH2tBu fragment, a feature this compound additionally shares with the olefin adduct [(PNP)Ti(η2-H2CCH2)(CH2tBu)]. For the latter ethylene complex, there is simultaneous geminal and phosphorus coupling, indicative of being locked in a single orientation with no rotation of the ethylene ligand. As mentioned before, the alkyne protons on the Ti(η2-HC 1164
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Figure 12. (left) 1H−31P HOESY NMR spectrum of 2 at 400 MHz, showing equal-probability correlation for each proton in the compound to either phosphorus in the PNP backbone. (right) Expected 1H−31P HOESY correlations for complex 2 with indicated red arrows.
CH) core fragment of 2 yield a pseudotriplet at 10.39 ppm (3JHP = 2.5 Hz) that integrates to 2H, therefore showing equivalence (Figure 4, top left). Each of the protons in the acetylene moiety of 2 are coupled to only one of the phosphorus atoms in the PNP ligand, and such a feature was confirmed by selectively decoupled 1H{31P} NMR (Figure 5), and 1H−31P HMBC spectra (Figure 6). Alike the more shielded Ti−CH2tBu resonance, which depicts simultaneous coupling to these atoms, the pseudotriplet resonance for the alkyne moiety is in fact an envelope of two individual doublets (one for each proton) with coincidental chemical shifts. The latter feature was unequivocally confirmed by variable-temperature NMR spectroscopy (VT-NMR), which upon cooling to −63 °C permitted us to resolve the resonance into two distinct broad resonances, each of which integrates to 1H (Figure 7). Coalescence was observed at −40 °C, and such a result points to fast rotation of the acetylene ligand around the Ti−alkyne σ bond of 2 at room temperature. This feature contrasts with our prior observations for the seemingly similar ethene analogue [(PNP)Ti(η2-H2CCH2)(CH2tBu)] (5), which exhibits a nonfluxional and more rigid conformation.2,16 In light of these results, we set to estimate the energy barrier for the rotation of the bound acetylene in 2. Gradual decoalescence of the overlapping alkyne resonances was modeled using SpinWorks,18,19 and the calculated trace was generated using decreasing values for exchange rate constants (k) against actual spectra obtained at different temperatures (Figure 8). The energy of activation (Ea) was determined from a linear plot of ln k vs T−1, with the enthalpy (ΔrH⧧) and entropy (ΔrS⧧) of activation extracted from the corresponding plot of ln(k/T) vs T−1.16,18 Figure 8 depicts how our simulated 1 H VT NMR spectroscopic trace is in full agreement with our experimental data. On the basis of our VT spectroscopic data we can extract a relatively low energy of activation (Ea) of 10.2 kcal mol−1 for this process, with ΔrH⧧= 9.7 kcal/mol and ΔrS⧧ = −5.4 eu. A Gibbs free energy of activation (ΔrG⧧) of 11.0 kcal mol−1 at the coalescence temperature of −40 °C was determined for the process, which only slightly increased to 11.3 kcal mol−1 at 25 °C. These results are consistent with the proposed rapid rotational exchange between the two protons of the bound acetylene at room temperature (nondissociative), which is slowed down at low temperatures but not fully frozen. The broadness of the two resonances due to the alkyne protons, which were not resolved even at −63 °C, is consistent with this
Figure 13. 31P EXSY NMR spectrum of 2 at 162 MHz, showing dynamic exchange between both phosphorus atoms in the compound, demonstrated by the cross peaks observed between each 31P nucleus.
Figure 14. 31P EXSY NMR spectrum of 5 at 162 MHz, showing a very small extent of dynamic exchange between the two phosphorus atoms in PNP (compare with the much more intense relative intensities of the correlation peaks in Figure 13).
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Figure 15. 1H NMR spectrum (500 MHz, 25 °C) of 3, showing as insets the expansions of the peaks due to the core fragments Ti−CH2tBu and Ti(η2-HCCMe).
Figure 16. 1H{31P} NMR spectrum (400 MHz, 25 °C) of 3, showing as insets the expansions of the peaks from the core fragments Ti−CH2tBu and Ti(η2-HCCMe), with f ull 31P decoupling.
relatively unchanged upon selective decoupling to either of the two phosphorus atoms. The doublet exhibits 3JH−P = 7.7 Hz due to coupling to the less shielded of the two phosphorus atoms on the PNP backbone (δ 23.31 ppm), a feature that was confirmed by selective decoupling. The magnitude of the 3JH−P value measured for this doublet at −85 °C is larger than that measured at room temperature of 2.5 Hz (vide supra), suggesting that the latter apparent coupling constant is greatly affected by the rapid rotational exchange of the coordinated alkyne. The fact that only one of the two resonances for the bound alkyne exhibits measurable coupling to phosphorus at the low temperatures is consistent with an asymmetric chemical
conclusion. We also propose that wobbling of the HCCH fragment might be causing broadening of these resonances, and modeling by computational studies are consistent with such dynamic behavior (vide infra). In an effort to demonstrate the nonequivalence of the acetylene protons in 2, we set to acquire the selective and fully decoupled 1H{31P} NMR spectra at −85 °C. The experiment was performed in toluene-d8 solution at 500 MHz, aiming to improve peak separation and resolution (Figure 9). A clear evidence of a doublet at the less shielded acetylenic resonance (δ 10.64 ppm) was obtained in this way, with the more shielded resonance remaining a broad singlet. The latter resonance was 1166
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on the DEPT trace indicative of an methylenic carbon. The phases of both spin systems and their (1H, 13C) correlations were confirmed by C−H decoupled, multiplicity-edited gradient heteronuclear single quantum correlation (gHSQC) spectroscopy (Figure 10), and the magnitude of the 1JC−H coupling constants of these fragments was established by the respective C−H coupled gHSQC NMR spectroscopic experiment (Figure 11). In the former, the red and blue contours indicate positive and negative phases, respectively. Expansions of each of the contours of interest in both spectra appear as insets, additionally indicating for the C−H coupled spectrum the magnitude of the 1JC−H constants as extracted from each of the doublets: 176.9 Hz for the acetylene in Ti(η2-HCCH) and 97.6 and 103.7 Hz for the diastereotopic methylenic protons on Ti−CH2tBu. The 1JC−H value for the acetylene ligand in 2 is quite similar to that reported by Woo and coworkers7 for the only other reported acetylene adduct of titanium (175 Hz; Table 1). All of the 1JC−H constants in the alkyl or acetylene fragments in 2 are much smaller than the corresponding constants in the respective free alkane (H−CtBu; 114.2 Hz) or alkyne (H−C CH; 249 Hz)20 as a result of their binding to the electrophilic titanium(IV) center, a feature also noted in [(PNP)Ti(η2H2CCH2)(CH2tBu)] and other olefin adducts of the type [(PNP)Ti(η2-H2CCHR)(CH2tBu)], already reported by our group.2 In the case of [(PNP)Ti(η2-H2CCH2)(CH2tBu)], however, the 1JC−H coupling constants from Ti−CH2tBu and Ti(η2-H2CCH2) are only slightly smaller than the reported values for free alkane and ethylene, respectively.21 The greater decrease in magnitude observed for the bound alkyne in 2 with respect to free acetylene suggests a greater degree of rehybridization of the sp orbitals on the bound carbons, maximizing the s orbital to accommodate a large negative charge around them, a feature well established for organolithium and organomagnesium compounds.22 We turned our attention to heteronuclear Overhauser effect spectroscopy (HOESY), since this technique is specifically
Figure 17. Expansions of the overlaid selective and fully decoupled 1 H{31P} NMR spectra (400 MHz, 25 °C) of 3, showing the changes in multiplicity of (A) the alkyne resonance and (B) the more shielded methylene resonance as a result of decoupling.
environment and supports the premise of a broken-symmetry Ti(η2-HCCH) core having nonisometric Ti−C distances. The 13C{1H} NMR (Figure S18, Supporting Information)16 and DEPT-135 (Figure S19, Supporting Information)16 spectra of 2 were acquired and studied as well, aiming at a further understanding of the structure of this compound. The acetylene carbons on the Ti(η2-HCCH) moiety give a pseudotriplet at 198.3 ppm (2JC−P = 14.8 Hz), which mirrors the spectroscopic signature of the corresponding protons exhibiting coincidental chemical shifts by 1H NMR spectroscopy (vide supra). In addition, the sign of this resonance is positive in the DEPT trace, which is consistent with sp-hybridized functionalities. Conversely, the resonance for the methylene carbon on the Ti−CH2tBu moiety (δ 115.06 ppm) yields a negative resonance
Figure 18. 1H−31P HMBC NMR spectrum (400 MHz, 25 °C) of 3, with colored traces highlighting the proton−phosphorus correlations of the core fragments Ti−CH2tBu and Ti(η2-HCCMe). 1167
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Figure 19. C−H decoupled 1H−13C gHSQC NMR (500 MHz, 25 °C) of 3, including in the insets the expansions of select contours. Red and blue contours represent positive and negative peaks, respectively. The 1H NMR and DEPT-135 NMR spectra are used as traces for the 2D spectrum.
Figure 20. C−H coupled 1H−13C gHSQC NMR (500 MHz, 25 °C) of 3, including in the insets the expansions of select doublets. The one-bond C− H coupling constants (1JC−H) are indicated in maroon. The 1H NMR and DEPT-135 NMR spectra are used as traces for the 2D spectrum.
each proton in the compound to either phosphorus atom in the PNP ligand. This unexpected result prompted us to measure the EXSY (exchange spectroscopy) spectrum of 2. Strong cross peaks in the 31P EXSY of 2 (Figure 13) and 1H EXSY (Figures S28 and S29, Supporting Information) spectra of 2 are observed, leading to the conclusion that, in solution, the phenylene backbone in PNP must be exchanging on the NMR time scale. This is opposite to the 1H−31P HOESY spectrum of complex [(PNP)Ti(η2-H2CCH2)(CH2tBu)], which shows single correlations between the protons and fixed phosphorus atoms of the PNP ligand, indicative of a more rigid structure (Figure S52, Supporting Information).1,16 The cross peaks exhibited in
utilized to establish spatial correlations between two NMRactive nuclei.23 HOESY has been used to elucidate the stereochemistry of organometallic molecules,24−26 as well as to observe hydrogen bonding in small organic molecules.27 Due to the configuration of the neopentyl group relative to the PNP backbone for 2, we expected two particular cross peaks to be exhibited in the HOESY spectrum: the spatial correlation of the tert-butyl group to one P atom of the PNP backbone and a correlation of a diastereotopic neopentyl proton to the other P atom. These anticipated interactions are shown by the red arrows in the Newman projection in Figure 12 (right). Contrary to our hypothesis, the 1H−31P HOESY of 2 at 400 MHz shows equal-probability, through-space correlation of 1168
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(dd, 2JH−H = 11.0 Hz, 3JH−P = 4.0 Hz). As observed in [(PNP)Ti(η2-H2CCH2)(CH2tBu)] and other olefin congeners,1,2 the more shielded resonance displays strong coupling to one of the phosphorus atoms, resolving to a doublet on acquisition of the fully decoupled 1H{31P} NMR spectrum (Figure 16). Figure 17 (A component) shows clearly how selective decoupling of one or two P atoms in PNP affects the multiplicity in the HCCMe resonance, while the right set of stacked spectra shows the effect of selective decoupling of P from the diastereotopic methylene protons in −CH2tBu. From the B component in Figure 17 it is clear that one of the diastereotopic methylene protons in the latter moiety is weakly coupled to at least one of the two phosphorus atoms in the PNP backbone, barely noticeable in the 1H{31P} NMR spectra but more evidently observed in the 1H−31P HMBC of this compound (Figure 18). Figures 17 and 18 illustrate the differences in the spectrum of the aliphatic region as a result of selective and full decoupling to phosphorus. Also, as noted in Figure 18, the methyl substituent on the alkyne surprisingly exhibits approximately equal, although weak, long-range couplings to both phosphorus atoms on the PNP ligand. The alkyne proton also couples to both of these phosphorus atoms on the basis of this spectrum, but it does so considerably more strongly to one of them, pointing to a Ti(η2-HCCMe) core, which we again propose not to possess isometric Ti−C distances. Since the diastereotopic methylene protons exhibit a similar behavior as per judgment of their 1H−31P correlations in the HMBC NMR spectrum, it follows that the neopentyl group is probably also subject to some degree of distortion. Coupling of the propyne carbons to phosphorus has been established on the basis of an analysis of the 13C{1H} NMR spectrum of compound 3 (Figure S36, Supporting Information)16 wherein two doublets of doublets (dd) centered at 208.48 (dd, 2JC−Pa = 17.6 Hz, 2JC−Pb = 7.1 Hz) and 201.70 ppm (dd, 2JC−Pb = 20.1 Hz, 2JC−Pa = 9.6 Hz) correspond to the Ti(η2-HCCMe) and Ti(η2-HCCMe) carbons, respectively. These assignments have been corroborated by comparison with the DEPT-135 NMR spectra (Figure S37, Supporting Information).16 As mentioned above, the degree of covalency on the Ti−C bonds can be indirectly addressed by accurate determination of the one-bond C−H coupling constants (1JC−H) of the Ti(η2HCCMe) and Ti-CH2tBu core fragments in 3, which can be extracted from the C−H coupled 1H−13C gHSQC NMR spectrum. The latter is important, since titanium, being highly electropositive, affects the magnitude of these constants, which typically deviate to smaller values the more strongly the carbon atoms in these f ragments are bound. Figures 19 and 20 illustrate the C−H decoupled and C−H coupled 1H−13C gHSQC NMR spectra of 3, respectively. The magnitude of the 1JC−H constant of the alkyne C−H bond is 123.1 Hz (Table 1), akin to that of compound 2 (176.9 Hz; vide supra, Table 1). The 1JC−H constants for the diastereotopic protons on the Ti−CH2tBu moiety from 3 are 106.4 and 100.4 Hz, respectively. The magnitudes of these constants suggest a high degree of covalency due to binding to a highly electropositive titanium center, analogous to the case for 2. The 1H−31P HOESY NMR spectrum of complex 3 at 400 MHz (Figure 21) shows single correlations between the protons and fixed phosphorus atoms, indicative of discriminated through-space correlations, in contrast to the ones exhibited by complex 2 (see Figure 12). For 3, the diastereotopic neopentyl proton at δ ∼ 0.5 only shows spatial
Figure 21. 1H−31P HOESY NMR spectrum of 3 at 400 MHz, showing well-discriminated correlations for each proton in the compound to a single phosphorus in the PNP ligand.
Figure 22. 31P-EXSY NMR of 3 at 162 MHz showing some degree of conformational exchange between the two phosphorus atoms in PNP (compare with Figure 13).
the ethylene complex 5 are consistent with different atoms of the neopentyl group interacting with different phosphorus atoms through space. Consistent with exhibiting discriminated through-space correlations, complex 5 does not demonstrate a high degree of conformational exchange in solution, as evidenced by the 31P EXSY (Figure 14) and 1H EXSY NMR (Figures S54 and S55, Supporting Information) spectra. [(PNP)Ti(η2-HCCMe)(CH2tBu)] (3). Akin to compound 2, the 31P{1H} NMR spectrum of 3 displays two phosphorus resonances for the PNP backbone with a 2JP−P coupling constant of ∼30 Hz (Figure S31, Supporting Information).16 Likewise, the alkyne proton resonance appears highly deshielded in the 1H NMR spectrum (Figure 15), centered at ∼9.93 ppm, although much less resolved as a result of asymmetric coupling to both phosphorus atoms in the PNP backbone. The latter resonance was unequivocally confirmed by acquisition of the fully decoupled 1H{31P} NMR spectrum, in which the multiplicity of the resonance collapses to a singlet (Figure 16). No long-range coupling to the methyl substituent (which gives a sharp singlet at 2.79 ppm in both spectra) is observed, confirming that the broadness of the alkyne resonance is entirely due to phosphorus coupling. As noted earlier, the Ti(CH2tBu) fragment in 3 exhibits diastereotopic protons, which is a distinctive feature shared by olefin1,2 and alkyne adducts having a (PNP)Ti scaffold. For 3, the latter peaks were assigned at 1.69 (d, 2JH−H = 10.5 Hz) and 0.34 ppm 1169
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Figure 23. Proposed mechanism for the formation of complexes 2 and 3 from the alkylidene−vinyl intermediates [(PNP)Ti(CHtBu)(HCCHR)] (R = H (A), Me (B)) as well as rotation and wobbling of the alkyne ligand. The tolyl methyl of the PNP has been omitted for clarity.
the formation of the acetylene ligand, the computed structure of A-TS adopts a quasi trigonal-bipyramidal (tbp) geometry (Figure 24) in which the migrating hydrogen is located midway between the donor carbon atom and the acceptor carbon atom with C···H distances of 1.49 Å. The relatively short Ti···H distance of 1.71 Å, a Wiberg bond order of 0.25, and an electron density of 0.86 e at the transferred hydrogen in A-TS suggest this step to be best categorized as metal-mediated hydrogen atom transfer. In our static computations, the titanium acetylene product, complex 2, exhibits two almost isoenergetic (ΔG = 1.3 kcal mol−1) forms, labeled 2a and 2b. Both isomers adopt tbp geometries and, as indicated in Figure 24, the only difference between the two structures is the orientation of the acetylene fragment with respect to the P−Ti−P axis. In the case of the parallel arrangement, 2-TS∥, the P−Ti−P axis and the C−C bond of acetylene align perfectly parallel, whereby the acetylene fragment rotates slightly clockwise in 2a and slightly counterclockwise in 2b. When the alkyne unit is oriented perpendicular to the P−Ti−P axis we denote it eq⊥. Notably, the parallel arrangement in 2-TS∥ represents a very low-lying transition state, which allows for rapid wobbling of the acetylene fragment between the two extremes, 2a and 2b. Since this motion appears to be faster than the NMR time scale, compound 2 can be best represented as an average structure of 2a and 2b, where the envisioned fast wobbling of acetylene most likely explains the broad resonances observed in the 1H NMR spectra at low temperatures (vide supra). Wobbling of the acetylene ligand might also explain why there is a chemical drift of the 31P{1H} NMR spectroscopic resonances when the temperature is lowered (Figure 7, vide supra). Rotation of the acetylene
correlation with one of the two phosphorus atoms. In comparison to the acetylene complex 2, the propylene complex 3 shows diminished conformational exchange in solution, as evidenced by the weaker cross peaks observed in the 31P EXSY (Figure 22) and 1H EXSY NMR spectra (Figures S46 and S47, Supporting Information). This could be a result of steric interaction between the methyl substituent of the propylene ligand with the neopentyl moiety or steric interaction of the same methyl substituent with the isopropyl groups of the PNP ligand. The overall effect is a decrease in fluxional behavior, i.e. conformational exchange, in solution. As a result, compound 2 shows f luxional behavior, while complex 3 is more rigid and hence exhibits single through-space correlations. Theoretical Studies Detailing the Formation and Dynamic Process of the Alkyne Ligand in Complexes 2 and 3. Using DFT methods, we scrutinized the mechanism to formation of titanium alkyne complexes 2 and 3, as well as the rotation process of the alkyne ligand. Figure 23 shows the most plausible reaction pathway leading to the alkyne complexes. Starting from the alkylidene−vinyl type intermediates [(PNP)Ti(CH t Bu)(HCCH 2 )], [(PNP)Ti(CH t Bu)(HCCH(CH3))], and [(PNP)Ti(CHtBu)(CH3CCH2)], a metalmediated and concerted β-hydrogen abstraction step results in the direct formation of the alkyne unit traversing the transition states A-TS, B-TS, and C-TS, respectively. This type of step is associated with a computed barrier of 20−22 kcal mol−1, slightly dependent on the substituents on the vinyl group ligand.28 As expected, having the methyl proximal to the metal center destabilizes such an intermediate, with C being 2.9 kcal mol−1 higher than A or B. Formation of the alkyne from the vinyl intermediate is exergonic by about 3 kcal mol−1. During 1170
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Figure 24. Calculated structures for reactant A, the transition state leading to the alkyne complex A-TS, the isomeric forms of complex 2 (2a and 2b), and transition states involving the rotation and wobbling of the acetylene ligand (2-TS∥ and 2-TS⊥). Values are given in Å.
Figure 25. Simplified MO diagram for a π-acceptor acetylene ligand coordinated to the titanium(II) framework “(PNP)Ti(CH2tBu)” in either a parallel (eq∥) or perpendicular (eq⊥) fashion. Interactions in black represent back-donation from the metal to the π1* orbital of acetylene, whereas combinations in red represent donation from the π2 orbital to the metal. Note that the δ-type d−π2* interaction (blue) has no contribution to the metal−acetylene interaction because neither the bonding nor the antibonding combination is filled. The σ donation from the π1 orbital of acetylene to the d(a1) orbital is not shown for simplicity.
fragment through the perpendicularly oriented transition state, 2-TS⊥, is also represented in Figures 23 and 24. Such a concerted transformation possesses a calculated barrier of 11 kcal mol−1, which is in perfect agreement with our experimental barrier using variable-temperature 31P{1H} NMR spectroscopic data (Figures 7 and 8). The situation for the propylene complex 3 is slightly more complex. Due to the asymmetry created by the PNP backbone, there exist two nearly isoenergetic isomers (ΔG = 0.4 kcal mol−1), and further discrepancy arises on whether the methyl substituent of the propylene points toward the side of P1 (labeled 3s) or P2 (labeled 3r) (Figure 23). Analogous to the case for 2, the slightly rotated extreme forms (3a and 3b) exist for both of these conformers, but only on the potential energy surface (PES). When one includes thermal, entropic, and solvent contributions, the structure with propylene oriented strictly along the P−Ti−P vector (along the z axis, or eq∥) turns out to be the most stable arrangement, thus implying a very shallow free energy surface in this region. Finally, the calculated energy for rotation of the propyne ligand in 3 is associated with a barrier of 9.5 kcal mol−1 through 3-TS⊥. The ease with which the alkyne ligand rotates in 2 and 3 is puzzling in the light of the static ethylene fragment in the analogous complex [(PNP)Ti(CH2tBu)(η2-H2CCH2)] (5) (ΔGrot⧧ = 23.7 kcal mol−1). We recently rationalized the structure of the latter within the MO framework.2 The tbp geometry with the C−C bond parallel to the P−Ti−P vector originates from the orbital interaction between the lowest lying d orbital in the titanium(II) framework, (PNP)Ti(CH2tBu) (b1, filled), with the π* of the ethylene or, as in the case of 2, the
acetylene fragment (Figure 25). In the quasi octahedral structure of 5, eq⊥, where the CH2CH2 fragment is perpendicular to P−Ti−P, the orbital interaction between π* and a high energy d orbital (b2, unoccupied) is actually stronger due to the better overlap and smaller energy mismatch between them.29 However, only a fraction of the latter interaction is manifested directly in the stabilization of the perpendicular structure, eq⊥, because two electrons are promoted from b1 to b2. Congruently, the perpendicular structure is less stable than the parallel structure by 23.7 kcal mol−1 and corresponds to the rotational transition state of the ethylene ligand in [(PNP)Ti(CH2tBu)(η2-H2CCH2)] (5). The same rationalization also applies for the acetylene complex 2, and the b1−π1* interaction accounts for the preferred tbp structure in which the acetylene is lying parallel to the P−Ti−P vector. The major difference between alkene and alkyne as ligands, however, is the second, orthogonal set of occupied and vacant π orbitals that are available for the latter, which allow π donation from the alkyne to the metal, and they facilitate δ back-donation from the metal to the ligand.30 However, these extra interactions are typically of minor importance.31−33 Moreover, δ back-donation does not take 1171
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place at all in 2, because the MO representing this interaction (blue in Figure 25) remains unoccupied in the complex. Nevertheless, in the acetylene complex 2, π donation is more stabilizing when the alkyne fragment is perpendicular (eq⊥) to the P−Ti−P axis, as a result of the better energy match of the b1 d orbital with π2 as opposed to b2 and π2 in the parallel arrangement (Figure 25). For the acetylene derivative, this effect results in the relative stabilization of the rotational transition state with eq⊥ arrangement, 2-TS⊥, with respect to 2a and 2b (eq∥), thereby allowing facile rotation of the acetylene fragment even at low temperatures.34 As is often invoked,31−33,35,36 the metal−ligand interaction is stronger in the alkyne complexes 2 and 3, investigated in this work, than in the alkene analogues addressed beforehand, which is clearly manifested in the bond distances around the titanium core: the Ti−C distances are significantly shorter in the computed structure of the acetylene complex (∼2.03 Å) than they are in the ethylene derivative (∼2.14 Å). The C1−C2 bond of acetylene is also notably elongated to 1.31 Å, which resembles a carbon−carbon double bond more so than a triple bond. Additionally, the acetylene fragment in 2a is strongly bent, exhibiting a H−C1−C2 angle of 135.6°. These structural motifs are more characteristic of a Ti(IV)−cyclopropene moiety rather than a Ti(II)−acetylene π adduct. In contrast, the ethylene complex [(PNP)Ti(η2-H2CCH2)(CH2tBu)] features approximately equal contributions from a Ti(IV)− cyclopropane and a Ti(II)−ethene π adduct.2 As a result, 2 can be formally described as exhibiting some character of a Ti(IV)− cyclopropene structure, in which the bound acetylene formally acts as a 4e− donor ligand [C2H2]2−,37 consistent with the observed decrease in magnitudes of the 1JC−H coupling constants when going from free acetylene to 2. Likewise, complexes such as the olefin-bound [(PNP)Ti(η2-H2C CHR)(CH2tBu)] react rapidly with N2O to release the olefin, whereas compound 2 does not form an NMR-active, (PNP)Ti complex from oxidation7 or insertion38 by N2O. Only decomposition is observed by 31P{1H} NMR spectroscopy, without any free acetylene detected in the 1H NMR spectrum after 24 h from charging with N2O (Figures S8 and S9, Supporting Information). Notably, there is no exchange between 2 or 3 and ethylene, and treatment with oxidants such as N3Ad (Ad = 1-adamantyl) did not result in any evidence of oxidation to form [(PNP)TiNAd(CH2tBu)], even though a new set of unidentifiable (PNP)Ti complexes were formed (Figures S3 and S4, Supporting Information).16 In contrast, the complex [(PNP)Ti(η2-H2CCH2)(CH2tBu)(CH2tBu)] reacts rapidly with propyne, releasing ethylene and neopentane to form a new (PNP)Ti complex that thus far has eluded full characterization (Figures S10−S12, Supporting Information).
fast wobbling and rotation of the formally 4e− donor acetylene ligand. Similarly to the ethylene analogue, the CC functionality prefers an almost parallel arrangement to the P−Ti−P vector in the trigonal-bipyramidal equilibrium structure of [(PNP)Ti(CH2tBu)(η2-HCCH)]. An intuitive rationalization for this orientation can be extracted from the first-order MO diagram (Figure 25): the π back-donation from the TiII center to the π1* orbital of the acetylene stabilizes more the tbp structure, 2a and 2b, than the quasi-octahedral one with a perpendicular arrangement (eq⊥), 2-TS⊥. Notably, it is not the better orbital overlap or energy match that makes the back-donation more effective but the symmetry that allows a direct combination of π1* with the filled d orbital (b1) in the tbp structure. This contrasts with the quasi octahedral structure, in which π1* combines with a formally vacant d orbital and electron promotion of high cost must take place in order to initialize back-donation and, thus, a structural stabilization. The ease with which the alkyne fragment rotates in the [(PNP)Ti(η2-HCCH)(CH2tBu)] complex originates from the interaction between the metal d orbitals and the orthogonal π2 orbital available for acetylene. The donation from π2 to the appropriate d orbital is de facto weak but nevertheless more stabilized in the octahedral structure with perpendicular CC and P−Ti−P vectors (i.e., the transition state corresponding to the rotation 2-TS⊥) than in the tbp ground state (2a and 2b). This difference in the donation from π2 reduces the rotational barrier to ∼10 kcal mol for alkynes, therefore allowing facile rotation of the alkyne ligands in these titanium complexes. This feature contrasts with the seemingly analogous olefin complexes, which do not exhibit any rotational behavior at room temperature.
CONCLUSIONS We have described here the synthesis and spectroscopic characterization of titanium alkyne complexes (acetylene and propyne) that form via a β-hydrogen abstraction pathway from the alkylidene−vinyl intermediate [(PNP)TiCHtBu(HC CHR)] (R = H, Me). Our study presents state of the art multinuclear and multidimensional NMR spectroscopic studies to elucidate what the structural behavior of these titanium alkyne species is in solution and in addition establishes how the bound alkynes interact with the titanium and phosphorus centers. Also in this study we discussed the fluxional behavior of the titanium alkyne complexes and provided clear evidence for
ACKNOWLEDGMENTS Financial support of this research was provided by the National Science Foundation. M.G.C. acknowledges CONACYT for a postdoctoral fellowship.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Text and figures giving experimental details and NMR characterization data for complexes 2−4 and [(PNP)Ti(CH2tBu)(η2-H2CCH2)], as well as reactivity studies and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail for D.J.M.:
[email protected]. Present Address †
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States. Notes
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The authors declare no competing financial interest.
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REFERENCES
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NOTE ADDED AFTER ASAP PUBLICATION Due to a conversion error in production, the spectra in the version of SI published on February 19, 2014 were not represented accurately. Revised SI was reposted on February 24, 2014.
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