Article pubs.acs.org/Organometallics
Alkyne Metathesis Reactions of Rhenium(V) Carbyne Complexes Wei Bai, Ka-Ho Lee, Herman H. Y. Sung, Ian D. Williams,* Zhenyang Lin,* and Guochen Jia* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *
ABSTRACT: Although alkyne metathesis reactions mediated by highvalent d0 carbyne complexes are well established, similar reactions mediated by well-defined low-valent (non-d0) carbyne complexes are rare. This work demonstrates that d2 Re(V) carbyne complexes Re(CR)Cl2(PMe2Ph)3 (R = CH2(o-C6H4Br), Ph, CO2Et) can undergo stoichiometric alkyne metathesis reactions. For example, reactions of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 with TMSCCR (R = CO2Et, CH2Ph) and PhCCPh produced carbyne complexes Re(CR)Cl2(PMe2Ph)3 and Re(CPh)Cl2(PMe2Ph)3, respectively. Theoretical studies suggest that the metathesis reactions most likely proceed through six-coordinate alkyne-carbyne intermediates Re(CR)Cl2(η2-alkyne)(PMe2Ph)2 that undergo reversible cycloaddition reactions.
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(non-d0) carbyne complexes that can readily mediate alkyne metathesis reactions. In this work, we report stoichiometric alkyne metathesis reactions mediated by d2 rhenium(V) carbyne complexes Re(CR)Cl2(PMe2Ph)3, the second example of well-defined non-d0 carbyne complexes that can undergo alkyne metathesis reactions.
INTRODUCTION There has been much interest in the synthesis and properties of transition metal carbyne complexes.1 Previous studies have led to the isolation of a variety of carbyne complexes, including those with a non-d0 metal center (a carbyne ligand is viewed as a trianionic ligand, and such complexes are referred to as lowvalent carbyne complexes in this work)2 and those with a d0 metal center (such complexes are referred to as high-valent carbyne complexes in this work).3 One of the most interesting chemical properties of carbyne complexes is that they can undergo metathesis reactions with alkynes. In fact, alkyne metathesis reactions mediated by welldefined high-valent d0 carbyne complexes,4 both stoichiometrically5,6 and catalytically,7 are now well documented especially for Mo(VI), W(VI), and Re(VII) carbyne complexes. However, similar reactions involving well-defined low-valent carbyne complexes are scarce, despite the fact that many low-valent carbyne complexes are known and that reactions of such complexes with alkynes have been actively investigated.8−11 To the best of our knowledge, no catalytic alkyne metathesis reactions mediated by well-defined non-d0 carbyne complexes have been reported. Even stoichiometric metathesis reactions involving non-d0 carbyne complexes are very rare. The only reported examples we are aware of are the reactions of W( CMe)Cl(PMe3)4 (1) with alkynes RCCR′ (R = R′ = Ph, Et; R = H, R′ = CMe3), which give carbyne-alkyne complexes 2 in one day at room temperature and then the metathesis products 3 in 2 weeks (eq 1).12
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RESULTS AND DISCUSSION Alkyne Metathesis Reactions of d2 Rhenium(V) Carbyne Complexes Re(CR)Cl2(PMe2Ph)3. It is well accepted that alkyne metathesis proceeds through reversible cycloaddition reactions of alkyne-carbyne complexes to give metallacyclobutadienes. In agreement with the mechanism, many metallacyclobutadiene complexes have been isolated from the reactions of alkynes with high-valent carbyne complexes.14 It is probably not surprising that there are still no efficient alkyne metathesis catalysts based on typical well-defined non-d0 carbyne complexes, since even cycloaddition reactions of welldefined non-d0 carbyne complexes with unactivated alkynes are virtually unknown, although such intermediates have been proposed occasionally in the reactions of a few carbonylcontaining non-d0 carbyne complexes with alkynes,9 and cycloadded products 5 have recently been isolated from the reactions of “activated” alkynes HCCR (R = CO2H, OEt) with the osmapentalyne 4, which contains a strained OsC bond (eq 2).11
One possible reason for the rarity of metathesis or cycloaddition reactions of non-d0 carbyne complexes with In view of the great success of low-valent d4 Ru carbenes (a carbene ligand is viewed as a dianionic ligand) in olefin metathesis,13 it is highly desirable to search for other low-valent © XXXX American Chemical Society
Received: August 14, 2016
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DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics alkynes is that it is difficult for non-d0 metal centers to form alkyne-carbyne complexes in which the alkyne ligand is parallel to the carbyne ligand, which is required for the cycloaddition reactions to proceed. For a typical octahedral non-d0 carbynealkyne complex, the isomer 6A, in which the alkyne ligand is perpendicular to the carbyne ligand, is electronically preferred to isomer 6B, in which the alkyne ligand is parallel to the carbyne ligand for the following reason. For a d2 M(CR)L4 fragment with a structure 6′, the “t2g” set d orbitals are not degenerate. The dxy and dyz orbitals are higher in energy than the dxy orbital (which is filled with two electrons) due to their interactions with the π-type orbitals of the carbyne ligand. For 6A, the filled dxy orbital can interact with the empty π*2 orbital of the alkyne ligand, while the empty dyz orbital can interact with the filled π1 orbital of the alkyne ligand, both of which are stabilizing. On the other hand, for 6B, there will be a repulsive interaction between the filled dxy orbital with the filled π1 orbital of the alkyne ligand. The energetic preference of isomer 6A over isomer 6B may be one of the factors causing the cycloaddition reactions of non-d0 carbyne complexes (and metathesis reactions) to have high barriers.6
metathesis reaction of the carbyne complex 9 with the alkyne TMSCCCO2Et (Scheme 1). In agreement with the occurrence of the metathesis reaction, the alkyne TMSC CCH2(o-C6H4Br) could also be isolated from the reaction mixture. Scheme 1. Metathesis Reaction between the Complex 9 and TMSCCCO2Et
Further studies show that the complex 9 could also undergo metathesis reactions with other alkynes. The complex 9 reacted with the diarylacetylene PhCCPh to give the arylcarbyne complex Re(CPh)Cl2(PMe2Ph)3 (12) (Scheme 2). MetaScheme 2. Metathesis Reaction between the Complex 9 and Internal Alkynes
In order to develop alkyne metathesis reactions mediated by non-d0 carbyne complexes, it is necessary to find strategies to lower the barrier for the cycloaddition reactions. One might expect that the barrier for cycloaddition reaction could be lowered if the energy of isomer 6B relative to 6A is lowered. One possible way to make isomer 6B energetically more favorable is to use steric effects by placing a small ligand trans to the carbyne ligand and bulky ligands cis to the alkyne ligand as shown in 8. We have recently synthesized d2 Re(V) carbyne complexes15 of the type mer-Re(CR)Cl2(PMe2Ph)3. We reasoned that such carbyne complexes could undergo substitution reactions with alkynes to give alkyne-carbyne complexes with a structural feature similar to that of 8, and therefore they may undergo cycloaddition or metathesis reactions with alkynes. To test the hypothesis, we first studied the reactivity of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with ethyl 3(trimethylsilyl)propiolate (TMSCCCO2Et). As monitored by the in situ 31P{1H} NMR, after a mixture of the complex Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) and 8 equiv of TMSCCCO 2 Et in toluene was heated under an N 2 atmosphere at 110 °C for 12 h, the 31P{1H} signals of 9 (a doublet at −17.7 ppm and a triplet at −24.5 ppm) disappeared completely, and a doublet at −19.5 ppm and a triplet at −31.8 ppm appeared. The species associated with the new signals can be isolated as a blue solid from the reaction mixture and was identified to be the new carbyne complex Re(CCO2Et)Cl2(PMe2Ph)3 (10), which was presumably formed through
thesis reaction of 9 with the silyl/alkyl alkyne TMSC CCH2Ph also occurred to give the carbyne complex Re( CCH2Ph)Cl2(PMe2Ph)3 (11).15a We have also carried out the reaction of 9 with unsymmetrically substituted aryl/alkyl alkyne PhCC(CH2)5CH3, which was found to give the expected carbyne complexes 12 and 1315a in a molar ratio of 2.2:1. Other Re(V) carbyne complexes analogous to 9 can also undergo alkyne metathesis reactions. For example, the complex 10 reacted with PhCCPh to give the carbyne complex 12, which in turn can react with TMSCCCO2Et to go back to the carbyne complex 10 (Scheme 3). The structure of the complex 10 has been confirmed by Xray diffraction. As shown in Figure 1, the complex contains three meridionally bound phosphine ligands, two cis-diposed chloride ligands, and a carbyne ligand with a CO2Et group attached to the carbyne carbon. The ReC bond distance is 1.760(4) Å, which is typical for rhenium carbyne complexes. Scheme 3. Metathesis Reactions of Complexes 10 and 12
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DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
reaction of CpRe{C(OMe)Ph}(CO)2 with BCl3,20a [Re(OTf){C(2,4,6-C6H2Me3)}(CO)4]OTf was prepared by the reaction of Re{C(O)(2,4,6-C6H2Me3)}(CO)5 with (CF3SO2)O,22a and ReH(CPh)(PNP) (PNP = (iPr2PCH2SiMe2)2N) was obtained by the reaction of PhCH3 with (PNP)ReH4 or (PNP)ReH2(η2-cyclooctyne).23 Theoretical Studies. We assume that the metathesis reaction proceeds by initial formation of a six-coordinate alkyne-carbyne complex, Re(CR)Cl2(η2-alkyne)(PMe2Ph)2, which undergoes reversible cycloaddition reactions. To verify whether the proposed mechanism is reasonable or not, we have calculated the energy profile for the metathesis reaction of the complex 14 (a model for 9) with MeCCMe involving the six-coordinate intermediate Re(CMe)Cl2(η2-MeCCMe)(PMe3)2 (16). As shown in Figure 2, the cycloaddition reaction of 14 with MeCCMe to give the metallacyclobutadiene 17 involving the
Figure 1. Molecular structure of Re(CCO2Et)Cl2(PMe2Ph)3 (10). The hydrogen atoms of the PMe2Ph ligands are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Re(1)−Cl(1) 2.5198(9), Re(1)−Cl(2) 2.4692(9), Re(1)−P(1) 2.4443(10), Re(1)−P(2) 2.3998(10), Re(1)−P(3) 2.4371(9), Re(1)−C(1) 1.760(4), C(1)− C(2) 1.464(5), Cl(1)−Re(1)−Cl(2) 101.12(13), Cl(1)−Re(1)−C(1) 175.58(13), Re(1)−C(1)−C(2) 177.4(3).
Consistent with the solid-state structure, the complex showed a characteristic quartet at 3.64 ppm and a triplet at 0.81 ppm in the 1H NMR spectrum in C6D6 for the CH2CH3 group of the ester substituent. The 13C{1H} spectrum showed the ReC carbyne signal at 248.5 ppm. The identity of the alkyne TMSCCCH2(o-C6H4Br) can be readily confirmed by the NMR data (see Experimental Section for details). The structure of the new carbyne complex 12 can be readily assigned on the basis of its NMR data. For example, its 31P{1H} NMR spectrum of Re(CPh)Cl2(PMe2Ph)3 (12) showed a doublet at −20.1 ppm and a triplet at −24.1 ppm, the chemical shifts of which are similar to those of 10. The 13C{1H} spectrum of 12 showed the ReC carbyne signal at 260.2 ppm. The metathesis reactions are interesting, as reported examples of well-defined non-d0 carbyne complexes that undergo alkyne metathesis reactions are extremely rare and are limited to W(CMe)Cl(PMe3)412 as mentioned in the Introduction. Furthermore, the transformation provides a way to make Re(V) carbyne complexes that cannot be easily achieved by other routes. The complex 10 is the first rhenium carbyne complex with a carbonyl group attached to the carbyne carbon. In fact, only a few carbyne complexes with a carbonyl group attached to the carbyne carbon are known. They include Tp′(CO)2WC− C(O)Ph (obtained from the reaction of Tp′(CO)2WC−Li with PhCOCl, Tp′ = hydridotris(3,5-dimethylpyrazolylborate)),16 W(CCOR)(OCMe3)3 (R = Me or OMe, obtained from reactions of W2(OCMe3)6 with EtCCOR),17 OsCl3( CC(O)Ph)(PCy3)2 (obtained from the reaction of Os( C)(PCy3)2Cl2 with PhCOCl),18 and Mn(dmpe)2(CCH)( CCO2) (obtained from the reaction of [Mn(dmpe)2(C CSiR3)2]+ with Bu4NF and 2,2,6,6-tetramethyl-1-piperidinyloxy).19 The complex 12 is the first example of rhenium arylcarbyne complexes obtained by alkyne metathesis. Previously reported rhenium arylcarbyne complexes are usually obtained by electrophilic abstraction of aryl alkoxycarbene20 or acyl21,22 complexes and dehydrogenation of methyl groups of toluene derivatives with electron-rich rhenium complexes.23,24 For example, [CpRe(CPh)(CO)2]BCl4 was synthesized by the
Figure 2. Calculated energy profile for the cycloaddition reaction of the carbyne complex 14 with MeCCMe. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.
six-coordinate alkyne-carbyne intermediate 16 has an overall barrier of 29.7 kcal/mol. The dissociation of PMe3 from 14 to give the five-coordinate intermediate 15 is slightly endothermic, with a barrier of 20.8 kcal/mol. The complex 15 binds a molecule of MeCCMe to give the six-coordinate alkynecarbyne intermediate 16, which is thermodynamically unfavored by 13.4 kcal/mol. Interestingly, the cycloaddition reaction of 16 to give the metallacyclobutadiene 17 via TS16−17 has a barrier of only 15.0 kcal/mol, and the cycloreversion of 17 has a moderate barrier of 21.5 kcal/mol. From 16 to the transition state TS16−17, the dihedral angle between the ReC and CC bonds changes from −101.42° (in 16) to −7.88° (in TS16−17). It is noted that species 17 belongs to the point group C2v. Thus, the structure shown in Figure 2 is one of its resonance structures. The calculated results indicate that the major factors contributing to the reaction barrier of the cycloaddition reaction include the substitution reaction of the complex 14 with MeCCMe to give the alkyne-carbyne complex 16 and the alignment of the alkyne ligand with the carbyne ligand. In agreement with the calculated results, we found that addition of CuI, a phosphine scavenger, can speed up the metathesis reaction. In the presence of 30 mol % of CuI, the metathesis reaction of the complex 9 with 6 equiv of PhCCPh was essentially completed in 20 h after heating at 110 °C in toluene. In the absence of CuI, the reaction was completed in about 96 h under similar conditions. C
DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
We noted that reactions of W(CMe)Cl(PMe3)4 (1) with alkynes RCCR′ (R = R′ = Ph, Et; R = H, R′ = CMe3) give five-coordinated carbyne-alkyne complexes 3. Isolation of fivecoordinate carbyne-alkyne complexes suggests that the metathesis reactions most likely proceed through five-coordinated species. Indeed, our computational work confirms that the sixcoordinate complex W(CMe)(MeCCMe)Cl(PMe3)3 is 30.3 kcal/mol higher in energy than the five-coordinate complex W(CMe)(MeCCMe)Cl(PMe3)2. The metathesis reactions of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with TMSCCCO2Et and TMSC CCH2Ph produced carbyne complexes 10 and 11, respectively. In principle, the reactions could also give the carbyne complex Re(CTMS)Cl2(PMe2Ph)3 (18) and the alkyne EtO2CC CCH2(o-C6H4Br) (Figure 4). However, we have no evidence
In the discussion above, we mentioned that an alkyne metathesis reaction may proceed through a structure like isomer 6B, in which the alkyne ligand is parallel to the carbyne ligand. Our calculations show that the alkyne-carbyne complex in which the alkyne ligand is almost parallel to the carbyne ligand (similar to isomer 6B mentioned earlier) corresponds to the rotational transition state TS16−16, which is 12.3 kcal/mol in energy above the alkyne-carbyne complex 16 and only 2.7 kcal/ mol below TS16−17. As shown in Figure 3, TS16−17 and TS16−16
Figure 3. Selected bond distances (in Å) and dihedral angles of the calculated transition-state structures TS16−16 and TS16−17.
are structurally similar to each other. The small differences between these two transition states include (i) the change in the dihedral angle C1−Re−C2−C3 from 16 (−101.42°) to TS16−16 (−5.50°) is slightly greater than that to TS16−17 (−7.88°), and (ii) the alkyne in TS16−17 is positioned closer to the metal−carbyne bond than that in TS16−16. These results suggest that once the alkyne is oriented close and parallel to the carbyne ligand, the species will readily evolve to the metallacyclobutadiene 17. It is well accepted that metathesis involving d0 group 6 metal carbyne complexes X3MCR (M = Mo, W) proceed through five-coordinate complexes X3MCR(η2-alkyne).4 Olefin metathesis mediated by d0 Re(VII) alkylidyne/alkylidene complexes of the type (X)(Y)(RC)Re(CHR′) also involve fivecoordinate intermediates (X)(Y)(RC)Re(CHR′)(η2-olefin).25 We have therefore also calculated the energy profile for the metathesis reaction of the complex 14 (a model for 9) with MeCCMe involving the five-coordinate intermediate Re( CMe)Cl2(η2-MeCCMe)(PMe3) (16′). As shown in Figure 2, the five-coordinate intermediate 16′ is slightly more stable (by 3.6 kcal/mol) than the six-coordinate intermediate Re(CMe)Cl2(η2-MeCCMe)(PMe3)2 (16, see Figure 2 for its structure). However, the metallacyclobutadiene intermediate 17′ (see Figure 2 for its structure) is significantly less stable (by 17.0 kcal/mol) than the metallacyclobutadiene intermediate 17 (see Figure 2 for its structure). The barrier for the cycloaddition reaction of 16′ to give metallacyclobutadiene 17′ (25.8 kcal/mol) is also significantly higher than that for the cycloaddition reaction of 16 to give metallacyclobutadiene 17 (15.0 kcal/mol). The results suggest that the metathesis reactions most likely proceed through the path involving six-coordinate intermediates.
Figure 4. Calculated thermodynamics for the alkyne metathesis reactions of 9. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.
for these products. Our computational studies suggest that the reaction outcome is thermodynamically controlled. The reaction of the carbyne complex 9 with TMSCCCO2Et leading to the carbyne complex Re(CCO2Et)Cl2(PMe2Ph)3 (10) is thermodynamically favored by 5.1 kcal/mol, while the reaction leading to the carbyne complex Re(CTMS)Cl2(PMe2Ph)3 (18) is thermodynamically unfavored by 14.4 kcal/ mol. The reaction of the carbyne complex 9 with TMSC CCH2Ph leading to the carbyne complex Re(CCH2Ph)Cl2(PMe2Ph)3 (11) is almost thermodynamically neutral (ΔG = 0.3 kcal/mol), while the reaction leading to the carbyne complex Re(CTMS)Cl2(PMe2Ph)3 (18) is thermodynamically unfavored by 11.1 kcal/mol. The metathesis reaction of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with PhCC(CH2)5CH3 produced carbyne complexes Re(CPh)Cl2(PMe2Ph)3 (12) and Re{C(CH2)5CH3}Cl2(PMe2Ph)3 (13) in a molar ratio of 2.2:1 (Scheme 2). Consistent with the experimental observation, our computational work indicates that formation of the carbyne complexes 12 and 13 from reaction of the carbyne complex 9 with PhCC(CH2)5CH3 have similar free energy changes, with the formation of 12 being slightly more favorable (by 1.1 kcal/mol). D
DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
15.8 Hz, ReC), 169.0 (s, CO), 143.7 (d, 1J = 46.9 Hz), 141.6 (t, 1J = 19.9 Hz), 131.6 (t, 1J = 4.8 Hz), 130.6 (d, 1J = 7.8 Hz), 129.5 (s), 128.3−129.0 (m), 128.1 (d, 1J = 7.5 Hz), 61.2 (s, CH2CH3), 20.6 (d, 1J = 36.4 Hz), 20.4 (t, 1J = 18.1 Hz), 14.3 (s, CH2CH3), 11.8 (t, 1J = 16.6 Hz). Anal. Calcd for C28H38Cl2O2P3Re: C, 44.45; H, 5.06. Found: C, 44.19; H, 5.11. The characterization data of TMSCCCH2(oC6H4Br) are given in the Supporting Information. Reaction of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with TMSCCCH 2 Ph. A mixture of Re{CCH 2 (o-C 6 H 4 Br)}Cl2(PMe2Ph)3 (9) (30 mg, 0.035 mmol) and TMSCCCH2Ph (66 mg, 0.35 mmol) in toluene (0.8 mL) in an NMR tube was heated under N2 at 110 °C for 120 h. After cooling, the volatile was removed and the dark residue was extracted with diethyl ether (0.5 mL × 2). A yellow solid was obtained after addition of hexane (1 mL) to the ether extract. The solid was washed with hexane (0.5 mL × 2) and dried under vacuum to give the complex Re(CCH2Ph)Cl2(PMe2Ph)3 (11). Yield: 8 mg, 29%. The 1H and 31P{1H} NMR data are consistent with the ones reported.15a Reaction of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with PhCCPh. A mixture of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) (100 mg, 0.12 mmol) and PhCCPh (125 mg, 0.70 mmol) in toluene (5 mL) was stirred at 110 °C for 96 h. After cooling, the volatile was removed and the dark residue was washed with methanol (5 mL × 2) and diethyl ether (5 mL × 2) to give a light purple solid of Re(CPh)Cl2(PMe2Ph)3 (12), which was dried under vacuum. Yield: 55 mg, 62%. 31P{1H} NMR (162.0 MHz, CD2Cl2): δ −20.1 (d, 2J(PP) = 12.5 Hz, 2P), −24.1 (t, 2J(PP) = 12.3 Hz, 1P). 1H NMR (400.1 MHz, CD2Cl2): δ 1.48 (d, 2J(PH) = 9.6 Hz, 6 H, PMe2Ph), 1.83 (t, 2 J(PH) = 4.0 Hz, 6 H, PMe2Ph), 1.89 (t, 2J(PH) = 4.0 Hz, 6 H, PMe2Ph), 6.84−7.05 (m, 6 H, Ph), 7.17−7.33 (m, 9 H, Ph), 7.38−7.42 (m, 5 H, Ph). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 260.2 (q, 2 J(PC) = 15.9 Hz, ReC), 150.1 (s), 144.5 (d, 1J = 46.4 Hz), 142.0 (t, 1 J = 20.1 Hz), 130.9 (t, 1J = 4.5 Hz), 130.1 (d, 1J = 8.2 Hz), 129.4 (s), 128.5−129.1 (m), 21.4 (d, 1J = 35.2 Hz), 17.6 (t, 1J = 17.6 Hz), 12.6 (t, 1J = 15.6 Hz). Anal. Calcd for C31H38Cl2P3Re: C, 48.95; H, 5.04. Found: C, 48.72; H, 5.09. Reaction of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with PhCC(CH 2 ) 5 CH 3 . A mixture of Re{CCH 2 (o-C 6 H 4 Br)}Cl2(PMe2Ph)3 (9) (20 mg, 0.023 mmol) and PhCC(CH2)5CH3 (40 mg, 0.21 mmol) in toluene (0.5 mL) was heated at 110 °C for 80 h. The in situ 31P{1H} NMR spectrum showed that the complex 9 has almost been completely consumed, and the carbyne complexes Re( CPh)Cl2(PMe2Ph)3 (12) and Re{C(CH2)5CH3}Cl2(PMe2Ph)3 (13)15a were present in a molar ratio of 2.2:1. Reaction of Re(CPh)Cl2(PMe2Ph)3 (12) with TMSC CCO2Et. A mixture of Re(CPh)Cl2(PMe2Ph)3 (12) (16 mg, 0.021 mmol) and TMSCCCO2Et (0.040 mL, 0.21 mmol) in toluene (0.5 mL) was heated at 110 °C for 15 h. After cooling, the volatile was removed and the dark residue was washed with methanol (0.5 mL × 2) to give a blue solid of Re(CCO2Et)Cl2(PMe2Ph)3 (10), which was dried under vacuum. Yield: 6 mg, 38%. Reaction of Re(CCO2Et)Cl2(PMe2Ph)3 (10) with PhCCPh. A mixture of Re(CCO2Et)Cl2(PMe2Ph)3 (10) (23 mg, 0.030 mmol) and PhCCPh (54 mg, 0.30 mmol) in toluene (0.8 mL) was heated at 110 °C for 96 h. After cooling, the volatile was removed and the dark residue was washed with methanol (0.5 mL × 3) and diethyl ether (0.5 mL × 2) to afford a light purple solid of Re( CPh)Cl2(PMe2Ph)3 (12), which was dried under vacuum. Yield: 16 mg, 69%. X-ray Crystallography. A single crystal of 10 suitable for X-ray diffraction was grown from a toluene solution layered with hexane. Intensity data of 10 were collected on a Rigaku-Oxford Diffraction SuperNova diffractometer at 100 K. Diffraction data were processed using the CrysAlisPro software (version 1.171.35.19). Empirical absorption corrections were performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm in the CrysAlisPro software suite. Structure solution and refinement for all compounds were performed using the Olex2 software package (which embedded SHELXL).27,28 All the structures were solved by direct methods, expanded by difference Fourier syntheses, and refined by full
The results in Figure 4 show that the silylcarbyne complex 18 is considerably unstable in comparison to alkylcarbyne complexes such as 11 and 13. To understand the observation, we have estimated the free energy changes for the reactions of 9 with alkynes PhCH2CCSiH3 and EtO2CCCR (R = SiH3, OMe, CMe3, Me). The calculated results (see Figure S2 in the Supporting Information for details) show that both steric and electronic effects are important in determining the relative thermodynamic stability of the metal carbyne products. The carbyne complexes with a bulkier substituent are less stable (−Me vs −CMe3, −SiH3 vs −TMS). A π-donating substituent (e.g., −OMe) has a stabilizing effect on the metal carbyne complex, while a π-accepting substituent (e.g., −SiH3 and −TMS) has a destabilizing effect. This is because the carbyne complexes are Fischer-type, and it is well-known that the carbyne carbon of a Fischer-type carbyne complex is electrondeficient. Therefore, a π-donating substituent is expected to stabilize the electron-deficient carbyne carbon center and a πaccepting substituent would do the opposite. The substituent −CO2Et, despite its π-accepting property, stabilizes the metal carbyne carbon center, because it can strengthen the C−C πinteraction through conjugation. Summary. We have demonstrated that air-stable d2 Re(V) carbyne complexes Re(CR)Cl2(PMe2Ph)3 can undergo alkyne metathesis reactions. Theoretical studies suggest that the reactions most likely proceed through six-coordinate alkyne-carbyne intermediates Re(CR)Cl 2 (η 2 -alkyne)(PMe2Ph)2, which undergo reversible cycloaddition reactions. The reactions described here represent rare examples of alkyne metathesis reactions mediated by well-defined non-d0 carbyne complexes.
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EXPERIMENTAL SECTION
General Considerations. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques unless otherwise stated. Solvents were distilled under nitrogen from sodium benzophenone (hexane, ether) or sodium (toluene). The starting materials Re{CCH 2 (o-C 6 H 4 Br)}Cl 2 (PMe 2 Ph) 3 (9) 15c and TMSCCCH 2 Ph 26 were prepared following the procedures described in the literature. All other reagents were used as purchased from Aldrich Chemical Co. Microanalyses were performed by M-H-W Laboratories (Phoenix, AZ, USA). 1H, 13C{1H}, and 31P{1H} NMR spectra were collected on a Bruker-400 spectrometer (400 MHz). 1H and 13C NMR shifts are relative to tetramethylsilane, and 31P chemical shifts are relative to 85% H3PO4. Reaction of Re{CCH2(o-C6H4Br)}Cl2(PMe2Ph)3 (9) with TMSCCCO 2 Et. A mixture of Re{CCH 2 (o-C 6 H 4 Br)}Cl2(PMe2Ph)3 (9) (200 mg, 0.234 mmol) and TMSCCCO2Et (0.36 mL, 1.9 mmol) in toluene (10 mL) was stirred at 110 °C for 12 h. After cooling, the volatile was removed under vacuum, and the dark residue was washed with hexane (5 mL) and methanol (5 mL × 2) to give a blue solid of Re(CCO2Et)Cl2(PMe2Ph)3 (10), which was dried under vacuum. Yield: 69 mg, 39%. The filtrate was collected, and the volatile was removed under vacuum again. The residue was redissolved in hexane, and the mixture was subjected to column chromatography with a silica gel column using hexane as the eluent to afford the alkyne TMSCCCH2(o-C6H4Br) as a pale yellow oil. Yield: 28 mg, 45%. Characterization data of 10 are as follows. 31P{1H} NMR (162.0 MHz, C6D6): δ −19.5 (d, 2J(PP) = 13.3 Hz, 2P), −31.8 (t, 2J(PP) = 13.4 Hz, 1P). 1H NMR (400.1 MHz, C6D6): δ 0.81 (t, 3J(HH) = 7.0 Hz, 3 H, CH2CH3), 1.32 (d, 2J(PH) = 9.6 Hz, 6 H, PMe2Ph), 2.03 (t, 2 J(PH) = 4.2 Hz, 6 H, PMe2Ph), 2.22 (t, 2J(PH) = 4.2 Hz, 6 H, PMe2Ph), 3.64 (q, 3J(HH) = 7.2 Hz, 2 H, CH2CH3), 6.85−6.87 (m, 3 H, PMe2Ph), 7.01−7.11 (m, 8 H, PMe2Ph), 7.67−7.71 (m, 4 H, PMe2Ph). 13C{1H} NMR (100.6 MHz, C6D6): δ 248.5 (q, 2J(PC) = E
DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
Organometallics
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matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically with a riding model for the hydrogen atoms except where noted separately. Further crystallographic details are summarized in Table S1. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1446039. Copies of these data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Details. All structures were optimized without any constraint at the B3LYP level of density functional theory (DFT).29 The standard 6-31G* basis set was used for O, C, and H atoms,30 where the effective core potentials (ECPs) of Lanl2dz were used to describe Re, Br, Cl, Si, and P atoms,31 with polarization functions for Re (ζ(f) = 0.869), Br (ζ(d) = 0.428), Cl (ζ(d) = 0.640), Si (ζ(d) = 0.284), and P (ζ(d) = 0.387) being added.32 Frequency calculations were also performed at the same level of theory to identify all the stationary points as minima (zero imaginary frequency) or transition states (one imaginary frequency), as well as to provide free energies at 298.15 K. To reduce the overestimation of the entropy contribution often seen in the gas-phase results, corrections of −2.6 (or +2.6) kcal/mol to the gas-phase free energies were made for 2-to-1 (or 1-to-2) transformations, which have been used in many earlier work.33,34 All the calculations were performed with the Gaussian 03 software package.35 To validate the results calculated with the 6-31G* basis set and to reveal the solvent effect on the calculated results, we carried out singlepoint energy calculations on optimized structures 14, TS14−15, 15, 16, and TS16−17 with a better basis set (6-311+G** for O, C atoms and SDD with polarization function described above for Re, Cl, and P atoms)32 in both gas phase and solvent phase. In calculating the singlepoint energies in solution, E(Sol)sp, the SMD continuum solvation model and the solvent toluene were employed.36 The solvationcorrected free energy, G(Sol)sp, can be calculated by the following equation:
ACKNOWLEDGMENTS This work was supported by the Hong Kong Research Grants Council (Project Nos. 602113, 601812, CUHK7/CRF/12G-2, 16321516).
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These additional calculations were performed with the Gaussian 09 (rev. D.01) package.37 The free energies, corrected by the single-point energy calculations with the better basis set and solvent effect included, of the selected structures with respect to the energy reference point are roughly the same as those obtained with the lower level gas-phase calculations (see Table S2 in the Supporting Information for details). The small differences in the relative free energies indicate that the basis set and solvent have a small and limited influence on the calculated results.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available ACS Publications website at DOI: met.6b00653. NMR spectra and crystallographic Cartesian coordinates of all the (XYZ) Crystallographic data of 10 (CIF)
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G(Sol)sp = G(Gas)opt − E(Gas)opt + E(Sol)sp
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Article
free of charge on the 10.1021/acs.organodata (PDF) calculated structures
AUTHOR INFORMATION
Corresponding Authors
*E-mail (I. D. Williams):
[email protected]. *E-mail (Z. Lin):
[email protected]. *E-mail (G. Jia):
[email protected]. ORCID
Ka-Ho Lee: 0000-0002-8435-4986 Zhenyang Lin: 0000-0003-4104-8767 Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.6b00653 Organometallics XXXX, XXX, XXX−XXX
