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Mechanistic Studies on the Stereoisomerization between Two Stereoisomeric, Isolable Five-Coordinate Borylpalladium(II) Complexes Bearing a Phenylene-Bridged PSiP-Pincer Type Ligand Jun Takaya, Naohiro Kirai, and Nobuharu Iwasawa* Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan S Supporting Information *
ABSTRACT: The mechanism of the stereoisomerization of isolable five-coordinate borylpalladium(II) complexes bearing a phenylene-bridged PSiP-pincer ligand is investigated. The trigonal-bipyramidal borylpalladium(II) complexes undergo facile stereoisomerization under heating conditions from trans(B,Si) isomers having boryl and silyl ligands at apical positions to cis(B,Si) isomers in which boryl and silyl ligands are located in equatorial and apical positions, respectively. Kinetic studies and theoretical calculations clarified that the isomerization proceeds through a turnstile rotation mechanism via a five-coordinate transition state without dissociation of a monophosphine ligand. This is a quite rare example of stereoisomerization between two stereoisomeric five-coordinate group 10 metal complexes that are stable enough to be isolated. tereoisomerization of five-coordinate transition-metal complexes is a fundamental organometallic reaction. Berry pseudorotation and turnstile rotation are commonly proposed as representative mechanisms,1 although the latter has rarely been demonstrated unless conformationally restricted, multidentate ligands are employed.2 Such stereoisomerization is usually facile and accounts for the conformational flexibility of five-coordinate, d8 transition-metal complexes that interchange the position of ligands rapidly even at low temperature.3 Moreover, several metal-catalyzed synthetic reactions are proposed to involve the stereoisomerization reaction as a key step in the catalytic cycle.4 Thus, investigation and understanding of the mechanism of stereoisomerization has been an important issue in organometallic chemistry. There are numerous mechanistic investigations on the stereoisomerization of group 8 and 9 metal complexes. However, those on fivecoordinate group 10 metal complexes are rather limited, partially because these generally prefer a d8 square-planar geometry. Five-coordinate group 10 metal complexes are often proposed as transient intermediates in cis−trans isomerizations of 16-electron square-planar metal complexes via an associative, pseudorotation mechanism.5 Mechanistic studies on stereoisomerization based on the synthesis and isolation of both stereoisomers of interconvertible five-coordinate group 10 metal complexes have been a formidable challenge.6 Recently we have reported the selective synthesis of two stereoisomers of five-coordinate borylpalladium complexes bearing a phenylene-bridged PSiP-pincer ligand.7 The trans(B,Si) isomer 1 possesses boryl and silyl ligands in apical positions, whereas the cis(B,Si) isomer 2 bears boryl and silyl ligands in equatorial and apical positions, respectively (Figure 1). Although both complexes are conformationally stable enough to be isolated, the trans isomer 1 is easily converted
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© 2014 American Chemical Society
Figure 1. Two stereoisomeric five-coordinate borylpalladium(II) complexes bearing a phenylene-bridged PSiP-pincer type ligand.
to the cis isomer 2 through a HBpin-mediated reversible σ-bond metathesis reaction.7 On the other hand, we also found that thermal isomerization from the trans to the cis isomer proceeds as a minor pathway in this reaction. When a solution of trans(B,Si)-1 in benzene-d6 was allowed to stand at 303 K for 24 h, formation of a small amount of the cis isomer 2 was detected (14% from 1a, 5% from 1b; Scheme 1).8 These are quite rare examples of the synthesis and isolation of two stereoisomers of five-coordinate palladium(II) complexes that undergo thermally induced stereoisomerization, thus prompting us to investigate the mechanism in detail. Herein we report mechanistic investigations on the thermally induced stereoScheme 1. Preliminary Results on Thermal Stereoisomerization of trans(B,Si)-1 to cis(B,Si)-2
Received: October 13, 2013 Published: March 4, 2014 1499
dx.doi.org/10.1021/om401004c | Organometallics 2014, 33, 1499−1502
Organometallics
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isomerization of the five-coordinate borylpalladium(II) complexes bearing a phenylene-bridged PSiP-pincer ligand. Kinetic studies and theoretical calculations clarified that the isomerization proceeds through a rather rare turnstile rotation mechanism via a five-coordinate transition state without dissociation of a monophosphine ligand.
Table 1. Effect of Additional Phosphines on the Thermal Stereoisomerization
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RESULTS AND DISCUSSION trans(B,Si)-1a and -1b were synthesized from the square-planar borylpalladium(II) complex 3 and PPh3 or PMe3 according to the previously reported procedure.7 X-ray analysis indicated that these complexes are five-coordinate with trigonalpyramidal coordination of the palladium. However, our previous studies also demonstrated that there exists a rapid equilibrium between the five-coordinate trans(B,Si)-1 and 3 in solution at room temperature through dissociation of the monophosphine ligand, especially in the case of R = Ph. The experimentally estimated energy differences at 298 K between the five-coordinate borylpalladium complexes 1 and 3 are only −0.8 kJ mol−1 for 1a and −27 kJ mol−1 for 1b.9 Therefore, we considered two possibilities for the stereoisomerization, one being direct stereoisomerization from the five-coordinate palladium(II) and the other a dissociative mechanism initiated by the dissociation of the monophosphine ligand (Scheme 2).10
entry
starting material
additive (amt/equiv)
kobs/10−4 s−1
1 2 3 4 5
1a (R = Ph) 1a (R = Ph) 1a (R = Ph) 1b (R = Me) 1b (R = Me)
none PPh3 (5) PPh3 (10) none PMe3 (5)
a 2.03 2.06 1.37 1.43
a The kobs value was not determined due to partial decomposition of the complexes.
reversible, and the reverse reactions are negligible for the determination of the rate constants. Scheme 3. Reverse Stereoisomerization of cis(B,Si)-2 to trans(B,Si)-1
Scheme 2. Possible Mechanisms for Stereoisomerization of trans(B,Si)-1 to cis(B,Si)-2
The entropies of activation were estimated to be 3 ± 16 J mol−1 K−1 for 1a and −13.8 ± 0.4 J mol−1 K−1 for 1b from rate constants at various temperatures (348−363 K) in the presence of an additional monophosphine ligand (Table 2 and Figure 2). Table 2. Thermal Stereoisomerization at Various Temperatures To gain insight into the mechanism of the isomerization, several kinetic experiments were carried out on the conversion of trans(B,Si)-1 to cis(B,Si)-2 under heating conditions. The PPh3-coordinated trans(B,Si)-1a underwent stereoisomerization to the cis(B,Si)-2a in the presence of 5 equiv of PPh3 in toluene-d8 at 353 K with a first-order kobs value of 2.03 × 10−4 s−1, as monitored by 1H and 31P NMR (entry 2, Table 1). The rate constant in the absence of an additional PPh3 could not be determined due to partial decomposition of the complex. Addition of 10 equiv of PPh3 did not retard the reaction at all (entry 3). Furthermore, the reaction of PMe3-coordinated trans(B,Si)-1b also exhibited almost the same first-order rate constants in the presence or absence of an additional PMe3 (entries 4 and 5), demonstrating that an additional monophosphine ligand does not affect the rate of the reaction. The reaction of PPh3-coordinated trans(B,Si)-1a was slightly faster than that of PMe3-coordinated 1b (entries 2 and 5). In all entries, the reactions were monitored until more than 90% of trans(B,Si)-1 was converted to cis(B,Si)-2 (about 3−4.5 h). It should be noted that cis(B,Si)-2 underwent reverse stereoisomerization slightly to trans(B,Si)-1 at 353 K, giving an equilibrium mixture of the cis(B,Si) and trans(B,Si) species in the ratios 97:3 (for R = Ph) and 96:4 (for R = Me) (Scheme 3). Therefore, the stereoisomerization is confirmed to be
entry
starting material
temp/K
kobs/10−4 s−1
1 2 3 4 5 6 7 8
1a (R = Ph) 1a (R = Ph) 1a (R = Ph) 1a (R = Ph) 1b (R = Me) 1b (R = Me) 1b (R = Me) 1b (R = Me)
348 353 358 363 348 353 358 363
1.08 2.03 3.15 5.90 0.83 1.43 2.43 4.05
The relatively small values of the entropy of activation and the independence of the rate constants on the additional monophosphine clearly indicate that the stereoisomerization reaction proceeds through an intramolecular rearrangement process via a five-coordinate palladium transition state without dissociation of a monophosphine ligand. We propose that a turnstile rotation mechanism is likely to be operative, since the 1500
dx.doi.org/10.1021/om401004c | Organometallics 2014, 33, 1499−1502
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Figure 4. Top view of the transition state TSAB. The red arrows exhibit main force vectors.
through a turnstile rotation mechanism between two isolable stereoisomeric five-coordinate group 10 metal complexes.11−13
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CONCLUSION We have demonstrated a quite rare example of stereoisomerization between two isolable stereoisomeric fivecoordinate palladium(II) complexes bearing a phenylenebridged PSiP-pincer ligand. Several kinetic studies and theoretical calculations clarify that the stereoisomerization proceeds through a turnstile rotation mechanism via a fivecoordinate palladium transition state without dissociation of a monophosphine ligand. Further investigations on the synthesis and reaction of other derivatives of five-coordinate borylpalladium(II) complexes are ongoing to elucidate the effect of ligand structure on the reaction rate and relative stability of the complexes.
Figure 2. Eyring plot.
usually favorable pseudorotation is inhibited by the tridentate PSiP-pincer ligand.2a This mechanism was further supported by theoretical calculations using a model compound of 1b, in which the pinacolato moiety is replaced by an ethyleneglycolato group on boron. trans(B,Si)-A and cis(B,Si)-B are found to be connected by transition state TSAB, in which the geometry around the palladium is square pyramidal, bearing a silicon in the apical position (Figures 3 and 4). No intermediates are found
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EXPERIMENTAL SECTION
General Considerations. All operations were performed under an argon atmosphere. Toluene-d8 was purchased from Acros Chemicals and degassed by benzophenone ketyl. Borylpalladium complexes 1 and 2 were prepared according to previously reported procedures.7 General Procedure for Kinetic Experiments on the Stereoisomerization of 1 to 2 (Table 2, Entries 1−4). In a sealed NMR tube, borylpalladium complex 1a (5.3 mg, 0.005 mmol) and PPh3 (6.6 mg, 0.025 mmol) were dissolved in toluene-d8 (0.50 mL). The stereoisomerization was monitored by 1H NMR with a JEOL EX-500 spectrometer at the various temperatures indicated in Table 2. The formation of cis(B,Si)-2a was confirmed by the comparison of the spectrum with that of an authentic sample. The data were fit to firstorder kinetics. The activation parameters were calculated from the Eyring plot at four different temperatures. Other kinetic experiments were also carried out according to this procedure. All plots for the kinetic experiments are provided in the Supporting Information. Computational Details. All calculations were performed with the Gaussian 09 program package (revision B.01). Equilibrium and transition state structures were optimized by density functional theory (DFT) using the PW91PW91 hybrid functional with tight SCF convergence and ultrafine integration grids. The LANL2DZ basis set, including a double-ζ valence basis set with the Hay and Wadt effective core potential (ECP), was used for palladium, and the 6-31G(d,p) basis set was used for carbon, hydrogen, oxygen, boron, phosphorus, and silicon. Each of the stationary points was adequately characterized by normal coordinate analysis (no imaginary frequency for an equilibrium structure and one imaginary frequency for a transition state structure). Intrinsic reaction coordinates (IRC) were calculated to verify the relevance of transition state structures. The bulk effects of the THF solvent were taken into account by performing geometry optimizations with the polarizable continuum model (PCM). In all calculations, the temperature was set to 298.15 K. Structural
Figure 3. Energy profile of the stereoisomerization, calculated by DFT using PW91PW91 (6-31G(d,p)/LANL2DZ) in THF (PCM). The pinacolato moiety is replaced by ethyleneglycolato.
between the TSAB and products by IRC calculations. The activation energy is 116 kJ mol−1 from trans(B,Si)-A, which is in good agreement with the experimentally estimated value (ΔG⧧298 K = 112.2 ± 0.3 kJ mol−1) of the reaction of 1b. It should be noted that force vectors of the transition state TSAB mainly show the motion of a pair (PMe 3 and (ethyleneglycolato)B) as shown in Figure 4, supporting the turnstile rotation mechanism. Therefore, we conclude that this is a quite rare example of a stereoisomerization reaction 1501
dx.doi.org/10.1021/om401004c | Organometallics 2014, 33, 1499−1502
Organometallics
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(6) There is an example of stereoisimerization of five-coordinate platinum complexes, although isolation and structural analysis of both isomers before and after the isomerization were not achieved; see: (a) Albano, V. G.; Monari, M.; Orabona, I.; Ruffo, F.; Vitagliano, A. Inorg. Chim. Acta 1997, 265, 35. A solid-state phase transition of fivecoordinate nickel complexes via turnstile rotation has been proposed; see: (b) Rufińska, A.; Goddard, R.; Weidenthaler, C.; Bühl, M.; Pörschke, K.-R. Organometallics 2006, 25, 2308. (7) Kirai, N.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2013, 135, 2493. (8) The stereoisomerization of 1b was accelerated at 90 °C to give the isomer cis(B,Si)-2b in high yield. This procedure was utilized for the preparation of cis-2b. See ref 7 and its Supporting Information for more details. (9) See the Supporting Information of ref 7. (10) Anderson, G. K.; Cross, R. J. Chem. Soc. Rev. 1980, 9, 185. (11) We have reported a related isomerization reaction of a fivecoordinate platinum hydride complex bearing a PSiP-pincer ligand; see: Takaya, J.; Iwasawa, N. Dalton Trans. 2011, 40, 8814. (12) This is the 3-fold turnstile rotation which is also classified as the Muetterties mechanism 2. See refs 1b, 1c, and 2a. (13) A turnstile rotation mechanism has scarcely been demonstrated in the stereoisomerization of group 10 metal complexes; see ref 5c. Turnstile rotation is proposed as a preferred possibility in refs 5e and 6b.
parameters, energies, and Cartesian coordinates of A, B, and TSAB are provided in the Supporting Information.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Figures, tables, and an XYZ file giving plots for kinetic experiments, all computed molecule Cartesian coordinates for convenient visualization, and details of computational studies. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail for N.I.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research (A) (No. 24245019), a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” (No. 22105006), a Grant-inAid for Young Scientists (A) (No. 24685006), and a Grant-inAid for Scientific Research on Innovative Areas “Stimuliresponsive Chemical Species for the Creation of Functional Molecules” (No. 25109519) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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REFERENCES
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