Article pubs.acs.org/Organometallics
Reactions of (Cyclopentadienylidenehydrazono)triphenylphosphorane with Chlororuthenium(II) Complexes and Substituent Effect on the Thermodynamic Trend in the Migratory-Insertion Reactions of Chlororuthenium−Alkylidene Complexes Wei Bai, Ka-Ho Lee, Jiangxi Chen, 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: Reactions of (cyclopentadienylidenehydrazono)triphenylphosphorane with RuCl2(PPh3)3 or (η6-cymene)RuCl2(PPh3) in toluene produced the chlorocyclopentadienyl complex (η5-C5H4Cl)RuCl(PPh3)2, which was likely formed through the cyclopentadienylidene intermediate RuCl2{C(C4H4)}(PPh3)2. Treatment of (η6-cymene)RuCl2(PPh3) with (cyclopentadienylidenehydrazono)triphenylphosphorane in methanol gave [(η5-C5H4Cl)Ru(η6-cymene)]+. Computational studies reveal that the thermodynamic stability of the analogous chlororuthenium complexes RuCl2(CRR′)(PPh3)2 and [RuCl(CRR′)(PMe3)(η6-C6H6)]+, with respect to the formation of the corresponding η5-chloropentadienyl complexes via a migratory carbene-insertion reaction is in the order of cyclopentadienylidene < indenylidene < fluorenylidene < phenylcarbene.
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INTRODUCTION
used as catalysts or precursors for making catalysts for olefin metathesis. Related ruthenium carbocyclic carbenes11 such as cyclopropenylidene complex 512,13 and fluorenylidene complexes (e.g., 614,15) have also been isolated and wellcharacterized. Surprisingly, there have been no reports on the syntheses and/or properties of ruthenium−cyclopentadienylidene16 complexes (e.g., 7). Inspired by the remarkable catalytic activity of ruthenium− carbene and ruthenium−indenylidene complexes, we became interested in the chemistry of ruthenium−cyclopentadienylidene complexes. One of common approaches to prepare ruthenium−carbene complexes is to use the reactions of diazoalkanes with appropriate ruthenium complexes.17 With the intention to prepare the cyclopentadienylidene complex RuCl2{C(C4H4)}(PPh3)2, we have carried out the reaction of (cyclopentadienylidenehydrazono)triphenylphosphorane 1 8 {C5H4N2PPh3, 9 (see Scheme 1 for its structure), a source of diazocyclopentadiene, C5H4N2} with RuCl2(PPh3)3. Interestingly, the reaction led to the formation of the chlorocyclopentadienyl complex (η5-C5H4Cl)RuCl(PPh3)2 (11) instead of the expected cyclopentadienylidene complex RuCl2{C(C4H4)}(PPh3)2 (10) (Scheme 1). Subsequently, we have conducted computational (DFT) studies on the reactions of PhCHN 2 and diazocyclopentadiene (C 5 H 4 N 2 ) with
There has been much interest in the chemistry of group 8 transition-metal complexes with a metal−carbon double bond due to their relevance to organometallic synthesis and catalysis.1 In the area of ruthenium chemistry, a number of such complexes with various substituents at the carbene carbon have been reported, especially for those with the metal fragment RuCl(PR3). For example, ruthenium−carbene complexes of the structural formulas 12−5 and 26 and indenylidene complexes of the structural formulas 37,8 and 49,10 (Chart 1) are well-known organometallic compounds which have been Chart 1. Selected Examples of Ruthenium(II) Complexes with a Ru−Carbon Double Bond
Received: June 8, 2017 Published: August 29, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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Organometallics
or toluene showed a singlet peak at ca. 40 ppm. The species displaying the 31P signal at ca. 40 ppm can be isolated as a yellow solid after purification by column chromatography on silica gel and was identified to be the chlorocyclopendienyl complex (η5-C5H4Cl)RuCl(PPh3)2 (11) (Scheme 1). The structure of the complex (η5-C5H4Cl)RuCl(PPh3)2 (11) has been characterized both spectroscopically and crystallographically. The 1H NMR spectrum (in CDCl3) shows two broad singlets at 4.26 and 3.35 ppm for the protons of the cyclopentadienyl ligand. The 13C{1H} NMR spectrum (in CDCl3) shows a triplet at 106.1 ppm for CCl of the cyclopentadienyl ring as well as a triplet at 80.0 ppm and a singlet 76.5 ppm for the other carbon atoms of the ring. The structure has been confirmed by X-ray diffraction. As shown in Figure 1, the complex adopts a piano-stool structure with the
Scheme 1. Reactions of C5H4N2PPh3 with RuCl2(PPh3)3 and (η6-Cymene)RuCl2(PPh3)
RuCl2(PPh3)3 in order to understand why the carbene complex RuCl2(CHPh)(PPh3)2 can be isolated, while the chlorocyclopentadienyl complex 11, rather than the cyclopentadienylidene complex 10, was isolated in our experiments. The DFT studies reveal that the migratory-insertion reaction of the cyclopentadienylidene complex 10 to give the chlorocyclopentadienyl complex 11 is thermodynamically favorable, while a similar migratory-insertion reaction of the carbene complex RuCl2(CHPh)(PPh3)2 is thermodynamically unfavorable. These DFT results explain the experimental observations mentioned above. For a better understanding of the thermodynamic trend, we carried out studies on the migratory reactions of phenylcarbene, cyclopentadienylidene, indenylidene, and fluorenylidene complexes supported by the metal fragments RuCl2(PPh3)2 and [(η6-C6H6)RuCl(PMe3)]+. In this work, the interesting findings arising from these investigations are reported.
Figure 1. ORTEP drawing of complex (η5-C5H4Cl)RuCl(PPh3)2 (11). Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 40% probability level. Selected bond lengths (Å) and angles (deg): average Ru(1)−C(Cp), 2.207; Ru(1)−Cl(1), 2.4362(10); Ru(1)−P(1), 2.3078(10); Ru(1)−P(2), 2.3214(9); Cl(2)−C(1), 1.726(4); P(1)−Ru(1)−P(2), 100.77(3); P(1)−Ru(1)−Cl(1), 90.27(3); P(2)−Ru(1)−Cl(1), 90.77(3).
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chloride substituent on the cyclopentadienyl ring lying on the same side of the chloride ligand (on the ruthenium metal center). The structural feature is similar to that of the reported monosubstituted cyclopentadienyl complexes (η5-C5H4R)RuCl(PPh3)2.25 The formation of the chlorocyclopentadienyl complex 11 from the reaction of C5H4N2PPh3 with RuCl2(PPh3)3 is somewhat unexpected, especially in view of the fact that reactions of RuCl2(PPh3)3 with diazoalkanes RR′CN2 can give the carbene complex RuCl2(CRR′)(PPh3)2 and that the ruthenium−fluorenylidene complex 6 can be obtained from the reaction of 9-diazofluorene with RuCl2(PPh3)3 and tBu-PNP.14 It is likely that the reaction of RuCl2(PPh3)3 with C5H4N2PPh3 also produced the analogous cyclopentadienylidene complex 10, but the complex was unstable and rearranged to the chlorocyclopentadienyl complex 11 by migratory insertion of the carbene into a Ru−Cl bond. Migratory insertions of carbene into Ru−H26 and Ru-alkyl27 bonds are well-documented reactions. Although rare, there are precedents for migratory insertions of carbene into Ru−halide bonds. For example, the reactions of CO with the sixcoordinated difluorocarbene complexes RuHF(CF2)(CO)(L)2 (L = PiPr3, PtBu2Me) produced the six-coordinated alkyl complexes RuH(CF3)(CO)2(L)2.28 Migratory insertion of carbene into a Ru−Cl bond has been suggested to occur in
RESULTS AND DISCUSSION Reactions of C5H4N2PPh3 with RuCl2(PPh3)3 and (η6Cymene)RuCl2(PPh3) in Solvents of Low Polarity. The ruthenium−cyclopentadienylidene complex RuCl 2 {C(C4H4)}(PPh3)2 (10) is interesting especially in view of the remarkable catalytic activities of complexes 1 and 3 for olefin metathesis. Ruthenium complexes of the type RuCl2( CRR′)(PPh 3 ) 2 can be obtained by the reactions of RuCl2(PPh3)3 with diazoalkanes RR′CN2.2 Ruthenium−fluorenylidene complexes have been successfully prepared from the reactions of ruthenium complexes such as RuCl2(PPh3)314 and CpRuCl(PPh3)215 with 9-diazofluorene.19 We therefore anticipated that the cyclopentadienylidene complex 10 might be obtained from the reaction of RuCl2(PPh3)3 with diazocyclopentadiene (C5H4N2).20,21 Diazocyclopentadiene is a sensitive reagent that can readily decompose at room temperature in neat form and has to be stored in solution at low temperature.22,23 A possible substitute for diazocyclopentadiene is C5H4N2PPh3,18 which is thermally stable but can dissociate into PPh3 and free diazocyclopentadiene in organic solvents.24 Thus, we have used C5H4N2PPh3 instead of diazocyclopentadiene in our experiments. The in situ 31P{1H} NMR spectrum of the reaction of RuCl2(PPh3)3 (8) with C5H4N2PPh3 (9) in dichloromethane 3267
DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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Organometallics
Figure 2. Calculated energy profile for the reaction of the RuCl2(PPh3)3 with diazocyclopentadiene at 298 K. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.
the reactions of vinyldiazoacetates with (η6-arene)RuCl2 to produce η3-chloroallyl complexes29 and in the reactions of Cp*Ru(X)Cl2 (X = NO, η3-C3H5) with CH2N2 in the presence of copper powder to give Cp*Ru(X)(CH2Cl)Cl.30 As a closely related observation, the reactions of tetrahalodiazocyclopentadienes C5X4N2 (X = Cl, Br) with Cp*RuX2 (or [Cp*RuX]4) (X = Cl,21b Br21a) were reported to give Cp*Ru(η5-C5X5), although the reaction mechanism has not been described in detail. To gain experimental evidence that the ruthenium−cyclopentadienylidene 10 is involved in the formation of the chlorocyclopentadienyl complex 11, we have studied the reaction of C5H4N2PPh3 with (η6-cymene)RuCl2(PPh3) (12). The reaction of (η6-cymene)RuCl2 with N2CHSiMe3 in the presence of two equiv of PCy3 in dichloromethane is known to give RuCl2(CHSiMe3)(PCy3)2.31 Thus, it was expected that the reaction of C5H4N2PPh3 with (η6-cymene)RuCl2(PPh3) in a solvent of low polarity can provide an alternative route to generate the ruthenium−cyclopentadienylidene intermediate 10. In agreement with the expectation and the proposal that the chlorocyclopentadienyl complex 11 can be formed via the ruthenium−cyclopentadienylidene intermediate 10, the complex (η5-C5H4Cl)RuCl(PPh3)2 (11) was produced from the reaction of C5H4N2PPh3 with (η6-cymene)RuCl2(PPh3) in solvents of low polarity such as toluene and CHCl3 (Scheme 1). Computational Studies on the Reaction of RuCl2(PPh3)3 with C5H4N2. Further support to the proposal that the formation of the chlorocyclopentadienyl complex 11 from the reaction of RuCl2(PPh3)3 with C5H4N2PPh3 involves the ruthenium−cyclopentadienylidene intermediate 10 comes from computational studies.
Since C5H4N2PPh3 is known to readily dissociate into PPh3 and free diazocyclopentadiene (C5H4N2), it is reasonable to assume that it is diazocyclopentadiene that reacted with RuCl2(PPh3)3 (8) to give the chlorocyclopentadienyl complex 11. Our calculations confirm that the dissociation of C5H4N2PPh3 into PPh3 and C5H4N2 is thermodynamically favorable with a calculated free-energy change of −12.5 kcal/ mol (and ΔE = 0.4 kcal/mol). To verify that the cyclopentadienylidene complex 10 can be the intermediate for the reaction, we first calculated the reaction profile for the reaction of RuCl2(PPh3)3 with diazocyclopentadiene to give the cyclopentadienylidene intermediate 10. Previous work has demonstrated that reactions of diazoalkanes with transition-metal complexes to give metallacarbene species most likely proceed through η1-C-coordinated diazoalkane complexes.32 In our system, the formation of the cyclopentadienylidene 10 could be initiated by the formation of either the six-coordinate η1-C-coordinated diazoalkane complex RuCl2{η1-(C)-C5H4N2}(PPh3)3 or the five-coordinate η1-Ccoordinated diazoalkane complex RuCl2{η1-(C)-C5H4N2}(PPh3)2. We have tried to define computationally the structure of a six-coordinate η1-C-coordinated diazoalkane complex RuCl2{η1-(C)-C5H4N2}(PPh3)3. When optimization starting from a structure with diazocyclopentadiene in close proximity of RuCl2(PPh3)3 was carried out, dissociation of diazocyclopentadiene from the ruthenium center occurred. The results may suggest that it is difficult for diazocyclopentadiene to coordinate to RuCl2(PPh3)3 to give a six-coordinated η1-Ccoordinated diazoalkane complex, probably because of steric effects. Alternatively, the structure of the five-coordinate η1-Ccoordinated diazoalkane complex 13 can be easily optimized 3268
DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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Organometallics
Figure 3. Calculated energy profile for the reaction of the RuCl2(PPh3)3 with PhCHN2 at 298 K. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.
11 can be accomplished by a migratory-insertion reaction to give an η2−C−Cl complex followed by ligand rearrangement. The migratory-insertion reaction of the cyclopentadienylidene complex 10 to give the η2−C−Cl complex 16 occurs with a free-energy change of 26.6 kcal/mol and a reaction barrier of 27.2 kcal/mol. Rearrangement of the η2−C−Cl complex 16 to the chlorocyclopentadienyl complex 11 has a free-energy change of −31.7 kcal/mol and a very small barrier of 3.5 kcal/mol. It is the formation of the chlorocyclopentadienyl complex 11 that drives the rearrangement reaction and makes the isolation of 10 difficult. We have also considered direct displacement of N2 by a chloride atom in the η1-C-coordinated diazoalkane complex RuCl2{η1-(C)-C5H4N2}(PPh3)2 (13) to give the chlorocyclopentadienyl complex 11. However, we have failed to locate such a transition state, indicating that the process is unlikely. Complex 11 could also be formed by initial dissociation of RuCl2(PPh3)3 to give [RuCl(PPh3)3]+ and a chloride anion Cl−. A nucleophilic attack of the chloride anion at the alkylidene carbon atom of the precursor C5H4N2PPh3 could give the chlorocyclopentadienyl anion C5H4Cl−, which reacts with the Ru(II)−chloro complex [RuCl(PPh3)3]+ to give 11. Our computational work suggests that this alternative mechanism is highly unlikely. The dissociation of the Cl− anion from the neutral Ru(II) complex RuCl2(PPh3)3 is thermodynamically highly unfavorable with a free-energy change of 111.3 kcal/mol, although the reaction of the Cl− anion with C5H4N2 to give chlorocyclopentadienyl anion C5H4Cl− is feasible (with a free-energy change of −34.2 kcal/ mol). The computational results described above indicate that the complex 11 can be formed by an initial formation of the fivecoordinate η1-C-coordinated diazoalkane complex RuCl2{η1(C)-C5H4N2}(PPh3)2 (13), which loses an N2 molecule to give the cyclopentadienylidene intermediate 10 followed by a carbene migratory-insertion reaction and ligand rearrangement.
(Figure 2). The reaction of RuCl2(PPh3)3 with diazocyclopentadiene to give the five-coordinate η1-C-coordinated diazoalkane complex 13 was found to be thermodynamically unfavorable by 22.6 kcal/mol. Further calculations show that the η1-C-coordinated diazoalkane complex 13 can lose a N2 molecule in an exoergic process (by 58.1 kcal/mol, 13 → 10) almost without any barrier to give the cyclopentadienylidene complex 10 (Figure 2). Overall, the reaction of RuCl2(PPh3)3 with diazocyclopentadiene to give the cyclopentadienylidene complex 10 is thermodynamically favored by 35.5 kcal/mol. Reactions of diazoalkanes with mononuclear transition-metal complexes can give stable diazoalkane complexes33 with an η1N-diazoalkane ligand coordinated at the terminal nitrogen atom34 or an η2-NN-diazoalkane ligand bound through the N− N multiple bond.35 We have therefore also carried out computational studies to see if such species could be involved in the formation of the cyclopentadienylidene complex 10. It was found that the η1-N-diazoalkane complex RuCl2{η1-(N)C5H4N2}(PPh3)2 (14) and the η2-NN-diazoalkane complex RuCl2{η2-(NN)-C5H4N2}(PPh3)2 (15) are actually thermodynamically more stable than the η1-C-coordinated complex RuCl2{η1-(C)-C5H4N2}(PPh3)2 (13) (Figure 2). However, evolution of the N-coordinated diazoalkane complexes to the cyclopentadienylidene complex 10, which can proceed through the η2-CN four-membered ring transition state TS15−10, has a barrier significantly higher than that (TS13−10) from the η1-Cdiazoalkane complex 13. The TS15−10 structure derived from the N-coordinated diazoalkane complexes is lying 17.4 kcal/ mol higher in energy than the TS13−10 structure derived from the η1-C-diazoalkane complex 13. The results imply that the Ncoordinated diazoalkane compounds are not responsible for the formation of the cyclopentadienylidene complex 10. We next investigated the conversion of the cyclopentadienylidene complex 10 to the chlorocyclopentadienyl complex 11. Consistent with the experimental observation, the complex 11 was found to be thermodynamically more stable than the complex 10 by 5.1 kcal/mol. The conversion of complex 10 to 3269
DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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Organometallics Computational Studies on the Reaction of RuCl2(PPh3)3 with PhHCN2. It is interesting to note that the carbene complexes RuCl2(CRR′)(PPh3)2 can be isolated from the reactions of RR′CN2 with RuCl2(PPh3)32, while the chlorocyclopentadienyl complex 11, instead of the cyclopentadienylidene complex RuCl2{C(C4H4)}(PPh3)2 (10), was isolated from the reaction of C 5 H 4 N 2 PPh 3 with RuCl2(PPh3)3. To understand the difference, it is necessary to know the energy profiles for the reactions of diazoalkane RR′CN2 with RuCl2(PPh3)3. However, such information is still absent in the literature. For this reason, we have calculated the energy profiles for the reaction of PhCHN2 with RuCl2(PPh3)3 to give the phenylcarbene complex RuCl2(CHPh)(PPh3)2 and the migratory-insertion reaction of the carbene complex RuCl2(CHPh)(PPh3)2. The results are shown in Figure 3. In consideration of the computational results obtained for the reaction of RuCl2(PPh3)3 with diazocyclopentadiene, it is reasonable to assume that the reaction RuCl2(PPh3)3 with PhCHN2 to give the carbene complex RuCl2(CHPh)(PPh3)2 proceeds through an initial formation of the five-coordinate η1C-coordinated diazoalkane complex RuCl2{η1-(C)-PhCHN2}(PPh3)2 (17). The reaction of PhCHN2 with RuCl2(PPh3)3 to give the η1-N-diazoalkane complex RuCl2{η1-(C)-PhCHN2}(PPh3)2 (17) has a free-energy change of 9.6 kcal/mol, which is 13.0 kcal/mol less than that calculated for the reaction of diazocyclopentadiene with RuCl2(PPh3)3 to give the η1-Ndiazoalkane complex RuCl2{η1-(C)-C5H4N2}(PPh3)2 (13). Like the case of the η1-N-diazocyclopentadiene complex 13, loss of an N2 molecule from the η1-C-coordinated diazoalkane complex 17 occurs in an exoergic process (by 55.0 kcal/mol) almost without any barrier to give the carbene complex RuCl2(CHPh)(PPh3)2 (18) (Figure 3). Overall, the reaction of RuCl2(PPh3)3 with PhCHN2 to give the carbene complex RuCl2(CHPh)(PPh3)2 is thermodynamically favored by 45.4 kcal/mol, which is 9.9 kcal/mol more than that (35.5 kcal/mol) calculated for the reaction of RuCl2(PPh3)3 with diazocyclopentadiene to give the cyclopentadienylidene complex 10. It is interesting to note that the carbene ligands of complexes 10 and 18 adopt different orientations. In the optimized structure of 10, the carbocycle is almost perpendicular to the plane formed among the metal center and the two chloride ligands. In the optimized structure of 18, the H and Ph groups are coplanar with the plane. We next calculated the reaction profiles for the migratoryinsertion reaction of the ruthenium−carbene complex RuCl2(CHPh)(PPh3)2 (18). The migratory-insertion reaction of the complex 18 to give the 16e− η2−C−Cl complex 19 has a free-energy change of 29.7 kcal/mol and a reaction barrier of 30.0 kcal/mol, which are similar to or slightly larger than those (being 26.6 and 27.2 kcal/mol, respectively) calculated for the reaction of the cyclopentadienylidene complex 10 to give 16. Unlike the rearrangement of the η2−C−Cl complex 16 to give the 18e− η5-chlorocyclopentadienyl complex 11, the rearrangement of the η2−C−Cl complex 19 to the 18e− η5cyclohexadienyl complex 20 is thermodynamically unfavorable (by 10.6 kcal/mol). The computational results clearly indicate that the migratory-insertion reaction of the phenylcarbene complex RuCl2(CHPh)(PPh3)2 to give an η2−C−Cl complex and conversion of the η2−C−Cl complex to an η5cyclohexadienyl complex are both thermodynamically unfavorable, consistent with the experimental observation that the phenylcarbene complex RuCl2(CHPh)(PPh3)2 can be isolated from the reaction of PhCHN2 with the ruthenium
complex RuCl2(PPh3)3. To the best of our knowledge, there have been no reported examples of migratory insertion of carbene into a Ru−Cl bond for ruthenium−carbene complexes of the type RuCl2 (CRR′)(PR″ 3)(L), although other decomposition pathways for such complexes are known.36 It has been reported that LiNPh2 reacts with RuHCl(PPh3)3 to give the piano-stool η5-cyclohexadienylimine complex RuH[(η5-C6H5)NPh](PPh3)2 via the σ-arylamide intermediate RuH[(σ-C6H5)NPh](PPh3)3.37 Similarly, reactions of KOAr (Ar = 4-tBuC6H4, Ph) with RuCl2(PPh3)3 and RuHCl(PPh3)3 in nonalcohol solvents affords π-aryloxide derivatives Ru(η5ArO)(o-C6H4PPh2)(PPh3) and RuH(η5-ArO)(PPh3)2, respectively (via the intermediates Ru(σ-OAr)2(PPh3)3 and RuH(σOAr)(PPh3)3, respectively).38 The rearrangement of the η2− C−Cl complex 19 to the 18e− η5-cyclohexadienyl complex 20 is reminiscent of the σ-to-π rearrangement reactions of the abovementioned Ru(OAr) and Ru(NAr2) complexes. It is interesting to note that the conversion of the complex 19 to the complex 20 is thermodynamically unfavorable, while the σ-to-π rearrangement of the Ru(OAr) and Ru(NAr2) complexes is thermodynamically favorable.37,38 Thermodynamic Trend for the Migratory-Insertion Reactions of Carbene, Cyclopentadienylidene, Indenylidene, and Fluorenylidene Complexes. The computational results described above indicate that the thermodynamic stabilities of the cyclopentadienylidene complex 10 and the phenylcarbene complex 18 with respect to the formation of the corresponding η2−C−Cl complexes via migration of a chloride ligand to the carbene carbon are similar (the carbene complexes are more stable than the corresponding η2−C−Cl complexes by ca. 28 kcal/mol). However, the thermodynamic stabilities with respect to the formation of the corresponding η5-complexes are significantly different. It is thermodynamically favorable for the cyclopentadienylidene complex RuCl2{C(C4H4)}(PPh3)2 (10) to rearrange to the η5-chlorocyclopentadienyl complex 11, but it is thermodynamically unfavorable for the phenylcarbene complex RuCl2(CHPh)(PPh3)2 (18) to undergo a similar rearrangement reaction. To better appreciate the difference, the relative energies of the carbene complexes and their corresponding η5 isomers resulting from rearrangement reactions are calculated and shown in Figure 4. The calculated results described above might imply that analogous indenylidene complexes may, like the cyclopentadienylidene RuCl2{C(C4H4)}(PPh3)2 (10), also be thermodynamically unstable and would rearrange to η5-indenyl complexes. This inference seems to be in conflict with the fact that indenylidene complexes 3 can be isolated.7 To determine whether rearrangement reactions of indenylidene complexes RuCl2(indenylidene)(PPh3)2 to give η5indenyl complexes are thermodynamically feasible or not, we have calculated the thermodynamics for the rearrangement reaction of the model ruthenium−indenylidene complex 21 (Figure 4). The reaction of the indenylidene complex 21 to give the η5-chloroindenyl complex 22, unlike the case of the cyclopentadienylidene complex 10, is thermodynamically unfavorable (by 15.9 kcal/mol), consistent with experimental observations that indenylidene complexes 3 can be isolated. The dramatic difference in the reactivity of the indenylidene complex 21 and the cycloppentadienylidene complex 10 promoted us to study the rearrangement reaction of the analogous fluorenylidene complex 23. It was found that the fluorenylidene complex 23 is significantly more stable. The rearrangement of the fluorenylidene complex 23 to give the η53270
DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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4 are catalytically active for alkene metathesis. We have therefore calculated the thermodynamics for the migratoryinsertion reactions of the model Ru(CRR′) complexes 25, 27, 29, and 31 supported by the [RuCl(PMe3)(η6-C6H6)]+ fragment. The calculation results are given in Figure 5.
Figure 4. Relative stabilities of carbene and η5-pentadienyl complexes containing the metal fragment RuCl(PPh3)2. The relative free energies and electronic energies (in parentheses) are given in kcal/mol. Figure 5. Energy changes for the rearrangement reactions of [Ru( CRR′)Cl(PMe3)(η6-C6H6)]+. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.
chlorofluorenyl complex 24 was found to be thermodynamically unfavorable by 33.3 kcal/mol. It is interesting to note that the thermodynamic stabilities of the complexes 10, 18, 21, and 23 with respect to the formation of the corresponding η5 complexes are in the order of 18 > 23 > 21 > 10. The trend in the relative stability of the cyclopentadienylidene, indenylidene, and fluorenylidene complexes with respect to the formation of the corresponding η5chlorocyclopentdienyl complexes can be related to the strength of the Ru−η5-pentadienyl interactions in the η5-pentadienyl complexes. The average Ru−C (ring) distances of the η5pentadienyl complexes are in the order of cyclopentadienyl complex 11 (2.291 Å) < indenyl complex 22 (2.369 Å) < fluorenyl complex 24 (2.485 Å), suggesting that the Ru−(η5chlorocyclopentadienyl) bond is the strongest and the Ru−(η5chlorofluorenyl) bond is the weakest. Thus, the formation of the chlorocyclopentadienyl complex 11 is most favored, and the formation of the chlorofluorenyl complex 24 is least favored. The formation of the η5-cyclohexadienyl complex 20 from phenylcarbene complex 18 is unfavorable probably because the delocalization of electrons in the aromatic ring of 18 is disrupted in the formation of the η5-cyclohexadienyl complex 20. The interesting substituent effect on the thermodynamics for the rearrangement reactions of the carbene complexes 10, 18, 21, and 23 supported by RuCl 2 (PPh 3 ) 2 to give the corresponding η5-dienyl complexes promoted us to study the thermodynamics for the conversion of analogous ruthenium complexes supported by other ligands to the corresponding η5dienyl complexes. Complexes supported by [(η6-arene)RuCl(PR3)]+ are interesting in view of the fact that complexes 2 and
The thermodynamic trend in the migratory-insertion reaction of [(η6-cymene)(PMe3)Ru(CRR′)Cl]+ was found to be the same as that of Ru(CRR′)Cl2(PPh3)2. The trend could also be related to the strengths of the Ru-(η5pentadienyl) bonds (Figure 5). As shown in Figure 5, only the reaction of the cyclopentadienylidene complex 25 to give the chlorocyclopentadienyl complex 26 was found to be thermodynamically favorable. Compared with those of Ru(CRR′)Cl2(PPh3)2, the migratory insertion reactions of [(η6-cymene)(PMe3)Ru( CRR′)Cl]+ to give the corresponding η5-pentadienyl complexes are in general thermodynamically more favorable. To confirm the prediction that the cyclopentadienylidene complex 25 is unstable with respect to the formation of chlorocyclopentadienyl complex 26, we have carried out the reaction of C5H4N2PPh3 with (p-cymene)RuCl2(PPh3) (12) in methanol. (p-Cymene)RuCl2(PPh3) is known to readily dissociate a chloride ion in a polar solvent.39 We therefore anticipated that (p-cymene)RuCl2(PPh3) may dissociate to [RuCl(η6-cymene)(PPh3)]+ which can react with C5H4N2PPh3 to give the carbene intermediate [RuCl(η6-cymene){C(C4H4)}(PPh3)]+ (33) that may rearrange to the cationic chlorocyclopentadienyl complex [(η5-C5H4Cl)Ru(η6-cymene)]+ (35Cl) via intermediate 34 (Scheme 2). The expectation was confirmed experimentally, and the cationic chlorocyclopentadienyl complex was isolated as the BPh4 salt 35BPh4. 3271
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H) to give Cp*Ru(η5-C5X4Cl) are thermodynamically favored by 31.9 kcal/mol for X = H and by 22.0 kcal/mol for X = Cl (see the Supporting Information). Summary. We have demonstrated experimentally that reactions of C5H4N2PPh3 (a source of diazocyclopentdiene) with RuCl2(PPh3)3 or (η6-cymene)RuCl2(PPh3) produced the chloro−cyclopentadienyl complexes rather than cyclopentadienylidenes. Computational studies reveal that the reactions of RuCl2(PPh3)3 with PhCHN2 and C5H2N2 to give haloruthenium complexes of the type RuCl2(CRR′)(PPh3)2 proceed through similar mechanisms, with the reaction involving PhCHN2 being both thermodynamically and kinetically more favorable. The thermodynamic stability of analogous haloruthenium complexes RuCl2(CRR′)(PPh3)2 and [RuCl( CRR′)(PMe3)(η6-C6H6)]+ with respect to the formation of the corresponding η5-chloropentadienyl complexes via migratory carbene insertion reaction is in the order of cyclopentadienylidene < indenylidene < fluorenylidene < phenylcarbene.
Scheme 2. Reaction between C5H4N2PPh3 and (pCymene)RuCl2(PPh3) in Methanol
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EXPERIMENTAL SECTION
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 and ether), sodium (toluene), or calcium hydride (CH2Cl2). Other reagents were used as purchased from Aldrich Chemical Co., U.S.A. Microanalyses were performed by M−H−W Laboratories (Phoenix, AZ). The complexes RuCl2(PPh3)3,41 (η6-cymene)RuCl2(PPh3),42 and C5H4N2PPh318 were prepared according to literature methods. 1H, 13 C{1H}, and 31P{1H} spectra were collected on a Bruker ARX-400 spectrometer (400 MHz) or a Bruker ARX-300 spectrometer (300 MHz). 1H and 13C NMR shifts are relative to TMS, and 31P chemical shifts are relative to 85% H3PO4. Synthesis of (η5-C5H4Cl)RuCl(PPh3)2 (11). Method A. A mixture of (p-cymene)RuCl2(PPh3) (197 mg, 0.347 mmol) and C5H4N2PPh3 (128 mg, 0.360 mmol) in 15 mL of toluene was heated at 110 °C for 1 h. The solvent was then removed, and the residue was redissolved in 1:1 hexanes/DCM (v/v) and loaded onto a column and purified by column chromatography on silica gel using 1:1 hexanes/DCM and 8:1 hexanes/acetone as eluents. The yellow band was collected and dried under vacuum. Yield: 142 mg, 53.9%. Method B. A mixture of RuCl2(PPh3)3 (200 mg, 0.209 mmol), C5H4N2PPh3 (74 mg, 0.209 mmol) in 6 mL of toluene was heated at 80 °C for 5 h. The solvent was then removed, and the residue was redissolved in 1:1 hexanes/ DCM (v/v) and loaded onto a column and purified by column chromatography on silica gel using 1:1 hexanes/DCM and 8:1 hexanes/acetone as eluents. The yellow band was collected and dried under vacuum. Yield: 52 mg, 32.8%. 1H NMR (400.1 MHz, CDCl3): 7.38−7.34 (m, 12H, PPh3), 7.23−7.20 (m, 6H, PPh3), 7.13−7.09 (m, 12H, PPh3), 4.26 (br s, 2H, C5H4), 3.35 (br s, 2H, C5H4). 13C{1H} NMR (100.6 MHz, CDCl3): 138.5−137.5 (m, PPh3), 134.1 (t, 1J = 4.9 Hz, PPh3), 129.0 (s, PPh3), 127.7 (t, 1J = 4.6 Hz, PPh3), 106.1 (t, 2JPC = 3.4 Hz, CCl on Cp), 80.0 (t, 2JPC = 4.1 Hz, CH on Cp), 76.5 (s, CH on Cp). 31P{1H} NMR (162.0 MHz, CDCl3): 38.6 (s). Anal. Calcd for C41H34Cl2P2Ru: C, 64.74; H, 4.51. Found: C, 64.59; H, 4.68. Synthesis of the Sandwich Complex (η5-C5H4Cl)Ru(pcymene)(BPh4) (35BPh4). A mixture of (p-cymene)RuCl2(PPh3) (100 mg, 0.175 mmol), C5H4N2PPh3 (68 mg, 0.192 mmol) in 6 mL of methanol was heated at 60 °C for 4 h and then cooled down to room temperature. The reddish solution was filtered, and NaBPh4 (120 mg, 0.351 mmol) in 3 mL of methanol was added while stirring. The pale brown solids precipitating out from the reddish solution were collected by filtration and washed with methanol (2 mL, ×2). The product was dried under vacuum. Yield: 90 mg, 78.5%. 1H NMR (400.1 MHz, DMSO-d6): 7.19 (br s, 8H, BPh4), 6.93 (t, J = 7.2 Hz, 8H, BPh4), 6.80 (t, J = 7.2 Hz, 4H, BPh4), 6.26−6.21 (m, 4H), 5.86 (t, J = 1.8 Hz, 2H, C5H4), 5.37 (t, J = 1.8 Hz, 2H, C5H4), 2.72−2.62 (m, 1H, CH(CH3)2), 2.21 (s, 3H, CH3), 1.20 (d, J = 7.2 Hz, 6H, CH(CH3)2).
The complex 35BPh4 has been characterized by NMR spectroscopy and elemental analysis. The signals of protons of the cyclopentadienyl ring were found at 5.37 and 5.86 ppm. In the 13C{1H} NMR spectrum, the signal of the carbon connected to the chloride atom was observed at 112.4 ppm. The structure of 35BPh4 has been confirmed by X-ray diffraction (Figure 6). It has an average Ru−C(Cp) bond length (2.179 Å) slightly shorter than that of 11 (2.208 Å) and comparable to that of [(η5-C5H5)Ru(η6-cymene)]BPh4 (2.184 Å).40
Figure 6. ORTEP drawing of [(η5-C5H4Cl)Ru(η6-cymene)]BPh4 (35BPh4). The counteranion BPh4− and hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 40% probability level. Selected bond lengths (Å) and angles (deg): average Ru(1)−C(Cp), 2.179; average Ru(1)−C(arene), 2.216; Cl(1)−C(1), 1.726(5).
The computational results described above suggest that halo cyclopentadienylidene complexes have a high tendency to undergo rearrangement reactions by migration of the halide ligand to the carbene carbon to give halocyclopentadienyl complexes. Reactions of C5X4N2 (X = Cl, Br) with Cp*RuX2 or [Cp*RuX]4 (X = Cl, Br) are known to produce η5halocyclopentadienyl complexes Cp*Ru(η5-C5X5).21a,b The products may also be formed by carbene insertion reactions of the intermediates Cp*RuCl{C(CX)4}. Our computational study confirms that reactions of Cp*RuCl{C(CX)4} (X = Cl, 3272
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13
C{1H} NMR (100.6 MHz, DMSO-d6): 163.90−162.44 (m, BC), 135.35 (s, BPh4), 125.12 (s, BPh4), 121.35 (s, BPh4), 112.42 (s, CCl on Cp), 101.76 (s), 99.02 (s), 87.66 (s), 85.38 (s), 79.99 (s), 78.95 (s), 30.76 (s), 22.70 (s), 18.17 (s). Anal. Calcd for C39H38BClRu: C, 71.62; H, 5.86. Found: C, 71.49; H, 6.00. X-ray Crystallography: Crystals of 11 and 35BPh4. Crystals suitable for X-ray diffraction were grown from CH2Cl2 solutions layered with hexane. Intensity data of 11 and 35BPh4 were collected on a Rigaku-Oxford Diffraction SuperNova diffractometer at 100 K (for 35BPh4) or at 173 K (for 11). 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).43,44 All of the structures were solved by direct methods, expanded by difference Fourier synthesis and refined by full matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically with a riding model for the hydrogen atoms except noted separately. Further crystallographic details are summarized in Table S1. Computational Details. All structures were optimized without any constraint at the B3LYP level of density functional theory (DFT).45 The standard 6-31G* basis set was used for C and H atoms,46 where the effective core potentials (ECPs) of Lanl2dz were used to describe Ru, Cl, and P atoms,47 with polarization functions for Ru (ζ(f) = 1.235), Cl (ζ(d) = 0.640) and P (ζ(d) = 0.387) being added.48 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 in the gas-phase results, corrections of −2.6 kcal/mol in free energies were made for 2:1 transformations.49 This free-energy correction was applied in a number of earlier computational studies.50 All the calculations were performed with the Gaussian 03 software package.51
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ACKNOWLEDGMENTS This work was supported by the Hong Kong Research Grants Council (Project Nos. 601812, 602113, CUHK7/CRF/12G-2, 16321516).
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REFERENCES
(1) Examples of recent work in osmium chemistry: (a) Buil, M. L.; Cardo, J. J. F.; Esteruelas, M. A.; Oñate, E. J. Am. Chem. Soc. 2016, 138, 9720−9728. (b) Casanova, N.; Esteruelas, M. A.; Gulías, M.; Larramona, C.; Mascareñas, J. L.; Oñate, E. Organometallics 2016, 35, 91−99. Ruthenium chemistry (c) Cesar, V.; Zhang, Y.; Kosnik, W.; Zielinski, A.; Rajkiewicz, A. A.; Ruamps, M.; Bastin, S.; Lugan, N.; Lavigne, G.; Grela, K. Chem. - Eur. J. 2017, 23, 1950−1955. (d) Teator, A. J.; Shao, H.; Lu, G.; Liu, P.; Bielawski, C. W. Organometallics 2017, 36, 490−497. (e) Gawin, R.; Kozakiewicz, A.; Gunka, P. A.; Dabrowski, P.; Skowerski, K. Angew. Chem., Int. Ed. 2017, 56, 981− 986. (f) Mikus, M. S.; Torker, S.; Xu, C.; Li, B.; Hoveyda, A. H. Organometallics 2016, 35, 3878−3892. (g) Smit, W.; Koudriavtsev, V.; Occhipinti, G.; Tornroos, K. W.; Jensen, V. R. Organometallics 2016, 35, 1825−1837. Iron chemistry (h) de Brito Sá, É.; RodríguezSantiago, L.; Sodupe, M.; Solans-Monfort, X. Organometallics 2016, 35, 3914−3923. (i) Lindley, B. M.; Jacobs, B. P.; MacMillan, S. N.; Wolczanski, P. T. Chem. Commun. 2016, 52, 3891−3894. (j) Lindley, B. M.; Swidan, A.; Lobkovsky, E. B.; Wolczanski, P. T.; Adelhardt, M.; Sutter, J.; Meyer, K. Chem. Sci. 2015, 6, 4730−4736. (2) Examples of ruthenium−carbene complexes RuCl2(CRR′) (PPh3)2 prepared by the reactions of diazoalkanes with RuCl2(PPh3)3: (a) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J., Jr.; Hoveyda, A. H. J. Am. Chem. Soc. 1999, 121, 791−799. (b) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100−110. (c) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2039−2041. (3) Examples of ruthenium−carbene complexes RuCl2(CRR′) (PPh3)2 prepared by other routes: (a) Amoroso, D.; Snelgrove, J. L.; Conrad, J. C.; Drouin, S. D.; Yap, G. P. A.; Fogg, D. E. Adv. Synth. Catal. 2002, 344, 757−763. (b) Volland, M. A. O.; Rominger, F.; Eisenträger, F.; Hofmann, P. J. Organomet. Chem. 2002, 641, 220−226. (c) Wu, Z.; Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1995, 117, 5503−5511. (d) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1992, 114, 3974−3975. (4) Examples of other ruthenium−carbene complexes RuCl2( CRR′)(PR″3)2: (a) van Lierop, B. J.; Fogg, D. E. Organometallics 2013, 32, 7245−7248. (b) Collado, A.; Esteruelas, M. A.; López, F.; Mascareñas, J.; Oñate, E.; Trillo, B. Organometallics 2010, 29, 4966− 4974. (c) Poverenov, E.; Milstein, D. Chem. Commun. 2007, 3189− 3191. (d) Roberts, A. N.; Cochran, A. C.; Rankin, D. A.; Lowe, A. B.; Schanz, H. Organometallics 2007, 26, 6515−6518. (e) Esteruelas, M. A.; Fernandez-Alvarez, F. J.; Onate, E. J. Am. Chem. Soc. 2006, 128, 13044−13045. (f) Gandelman, M.; Naing, K. M.; Rybtchinski, B.; Poverenov, E.; Ben-David, Y.; Ashkenazi, N.; Gauvin, R. M.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 15265−15272. (g) Ferrando, G.; Coalter, J. N., III; Gérard, H.; Huang, D.; Eisenstein, O.; Caulton, K. G. New J. Chem. 2003, 27, 1451−1462. (h) Oliván, M.; Caulton, K. G. Inorg. Chem. 1999, 38, 566−570. (i) Cucullu, M. E.; Li, C.; Nolan, S. P.; Nguyen, S. B. T.; Grubbs, R. H. Organometallics 1998, 17, 5565− 5568. (j) Belderrain, T. R.; Grubbs, R. H. Organometallics 1997, 16, 4001−4003. (k) Wilhelm, T. E.; Belderrain, T. R.; Brown, S. N.; Grubbs, R. H. Organometallics 1997, 16, 3867−3869. (l) Oliván, M.; Caulton, K. G. Chem. Commun. 1997, 1733−1734. (m) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993, 115, 9858−9859. (5) Examples of ruthenium−carbene complexes of the type RuCl2( CRR′)(PR″3)(L): (a) Engl, P. S.; Fedorov, A.; Coperet, C.; Togni, A. Organometallics 2016, 35, 887−893. (b) Lummiss, J. A. M.; Perras, F. A.; McDonald, R.; Bryce, D. L.; Fogg, D. E. Organometallics 2016, 35, 691−698. (c) Salem, H.; Schmitt, M.; Herrlich, U. B.; Kühnel, E.; Brill, M.; Nägele, P.; Bogado, A. L.; Rominger, F.; Hofmann, P. Organometallics 2013, 32, 29−46. (d) Lummiss, J. A. M.; Beach, N. J.; Smith, J. C.; Fogg, D. E. Catal. Sci. Technol. 2012, 2, 1630−1632.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00427. Crystallographic data of complexes 11 and 35BPh4; NMR spectra (PDF) Cartesian coordinates for all calculated structures (XYZ) Accession Codes
CCDC 1556062 and 1556065 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*G.J.: E-mail:
[email protected] *Z.L.: E-mail:
[email protected] ORCID
Ka-Ho Lee: 0000-0002-8435-4986 Jiangxi Chen: 0000-0001-6534-2552 Zhenyang Lin: 0000-0003-4104-8767 Guochen Jia: 0000-0002-4285-8756 Notes
The authors declare no competing financial interest. 3273
DOI: 10.1021/acs.organomet.7b00427 Organometallics 2017, 36, 3266−3275
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