Article pubs.acs.org/Organometallics
Computations Offer an Unconventional Route to Metallaphosphabenzene from a Half-Phosphametallocene Chao Huang,† Yulei Hao,‡ Yufen Zhao,*,†,§ and Jun Zhu*,‡ †
Department of Chemistry, College of Chemistry and Chemical Engineering, and the Key Laboratory for Chemical Biology of Fujian Province, Xiamen University, Xiamen 361005, People’s Republic of China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China § Key Lab of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: Metallaaromatics have attracted continuing interest of both theoretical and experimental chemists since the first metallabenzene was predicted by Hoffmann and isolated by Roper. In sharp contrast to metallabenzenes, metallaphosphabenzene (MPB) is much less developed and has not been synthesized so far. Thus, developing synthetic approaches is urgent. Here we present thorough density functional theory (DFT) calculations on the thermodynamics and kinetics of the rearrangement between MPBs and the corresponding η5-phosphacyclopentadiene (η5-PCp) complexes. The effects of metal centers, ligands, and substituents on the metallacycles were examined systematically. Our results reveal that the third-row metal osmium has the highest possibility to form MPB in comparison with the first-row metal iron and second-row metal ruthenium. Substituents were found to have a significant effect on the thermodynamics and kinetics of the rearrangement reactions, leading to an interconversion between osmaphosphabenzenes (OsPBs) and the corresponding η5-PCp complexes by simply tuning the substituents on the metallacycles. Thus, all of these findings should invite experimentalists to test these unconventional methods to realize the first MPB.
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INTRODUCTION
One of the common reactions on the stabilities of metallabenzenes is their isomerization to cyclopentadiene (Cp) complexes.4a,13 Previous studies3 have shown that electron-donating groups (EDGs) on the metallacycle, such as the methoxy group, can improve the stability of metallabenzene in comparison with the Cp complex,14 whereas electron-withdrawing groups (EWDs), such as triphenylphosphonium, can also increase the stability of the metallabenzenes15 and isometallabenzenes,16 depending on the position of the substituents. Could these stabilizing factors be also applied to MPBs? If such a stabilizing factor is significant, could we realize MPBs from the isomerization of η5-PCp metal complexes (Scheme 1) which have been synthesized
In 1979 Hoffman and co-workers predicted three types of metallabenzenes.1 Three years later, the first example of metallabenzene, osmabenzene, was reported by Roper.2 Since then, varieties of stable metallabenzenes have been isolated and well-studied. Many important comprehensive reviews of metallabenzenes have been summarized to cover the advances in this field.3 In addition, many detailed theoretical studies on metallabenzenes have also been reported.4 On the other hand, the heterobenzenes, in which one main-group element is used to replace one of the CH groups of benzene, have also been reported. Particularly, heterobenzenes containing the group 13 element B,5 the group 14 elements6 Si, Ge, and Sn, and group 15 elements7 N, P, As, Sb, and Bi have been investigated and their aromatic character is still maintained.8 In sharp contrast, the main-group heteroatom-containing metallabenzenes are much less developed. The reported heteroatom-containing metallabenzenes have been limited to metallapyrylium,9 metallathiabenzene,9b,10 and metallapyridine.11 To the best of our knowledge, the phosphorus-containing metallabenzene, metallaphosphabenzene, has not been synthesized so far, although its stability was investigated theoretically by Solà and co-workers recently.12 © XXXX American Chemical Society
Scheme 1
Received: December 9, 2013
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dx.doi.org/10.1021/om401188v | Organometallics XXXX, XXX, XXX−XXX
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previously?17 Our ongoing interest in aromaticity has led us to investigate these phosphorus-containing metallaaromatics.16,18 Here we report a thorough theoretical study on the interconversion of MPBs and the η5-PCp metal complexes (Scheme 1).
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stability of 2-OsPB over 1-OsPB, which is also supported by the CCSD(T) calculations. Therefore, in the following study, the CO ligand is chosen at the cis position of the phosphorus atom on the ring. Isomerization of 2-OsPB to 2-η5-PCp. The kinetics of the conversion of the osmaphosphabenzene 2-OsPB to its η5 isomer is examined. As shown in Figure 2, the small reaction barrier (18.4 kcal/mol) and large exothermicity (19.4 kcal/ mol) demonstrate the great challenge of synthesizing osmaphosphabenzenes.
COMPUTATIONAL METHODS
The B3LYP level19 of density functional theory was applied to optimize all of the structures studied in this work. Frequency calculations at the same level of theory have also been performed to identify all stationary points as minima (zero imaginary frequency). Calculations of intrinsic reaction coordinates (IRC)20 were also carried out on transition states to ensure that such structures are indeed connecting two minima. On the basis of the frequency calculations, Gibbs free energies were evaluated at 298 K. The LanL2DZ basis set was employed to describe Os, Ru, Fe, P, and Cl, whereas the standard 6-31+G(d)21 basis set was used for all other atoms. Polarization functionals were added for Os (ζ(f) = 0.886), Ru (ζ(f) = 1.235), Fe (ζ(f) = 2.462), P (ζ(d) = 0.340), and Cl (ζ(d) = 0.514).22 The PCM model23 with benzene (ε = 2.2706) as the solvent and the UAKS cavity were used to optimize all of the structures. To check the reliability of the DFT calculations, we performed single-point CCSD(T)24 calculations on 1-OsPB and 2-OsPB (Figure 1) using the same basis set. The latter is computed to be more stable by 2.8 kcal/mol than the former, indicating the reliability of our DFT calculations. All of the calculations were carried out using the Gaussian 03 package.25
Figure 2. Energy profiles for the conversion of 2-OsPB to 2-η5-PCp. The free energies and relative energies (in parentheses) are given in kcal/mol.
Effect of Substituents on 2-OsPB. To tune the stability of OsPB and its η1 and η5 isomers, we investigated the substituent effect of both an EWG (PH3+) and an EDG (OMe) on the metallacycle (Scheme 2). Some general observations can be
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RESULTS AND DISCUSSION General Considerations. In this work, we focus on ometallaphosphabenzene (Scheme 1), as a phosphorus atom at the ortho position on metallaphosphabenzene was reported by Solà and co-workers to be more stable than those at the meta and para positions.12 As shown in Figure 1, for the previously
Scheme 2
made, despite some deviations. Specifically, as shown in Table 1, the methoxy group has a small effect on the stability of η1PCp and OsPB. In contrast, a phosphonium group can stabilize them dramatically. This is understandable, as our previous studies have shown that the phosphonium group can stabilize metallabenzenes15 and isometallabenzenes.16 Very interestingly, the η1-PCp complexes become even more stable than the corresponding η5-PCp complexes. It should be noted that all of
Figure 1. Relative thermodynamic stability of two isomers. The free energies and relative energies (in parentheses) are given in kcal/mol.
reported complex Cl(CO)Os(PH3)2(PC4H4) (1-OsPB), the phosphorus atom was trans to the CO ligand. Apparently, there is another stereoisomer, 2-OsPB, in which the CO ligand is on the cis position of the phosphorus atom on the ring plane. As a strong π acceptor, the CO ligand should enhance the interaction between the metal center and the CO ligand due to the significant back-donation, leading to a stronger metal− carbon bond in comparison with that on the ring. Thus, 2OsPB is expected to be more stable than 1-OsPB due to the trans influence.26 Indeed, the calculated Wiberg bond index (1.24) of the metal−phosphorus bond in 2-OsPB is larger than that (0.97) in 1-OsPB (Figure S1, Supporting Information). In addition, Os−P in 2-OsPB also has a stronger trans influence than Os−carbene in 1-OsPB ,as evidenced by the Os−Cl bond (2.572 Å) in 2-OsPB being longer than that (2.560 Å) in 1OsPB. It should be noted that due to the strong trans influence of the CO ligand, the strength of the metal−carbon bond on the ring is also decreased. However, the bond order of the Os− C bond on the ring is reduced by 24.3%, whereas the Os−P bond on the ring is increased by 27.8%, leading to a higher
Table 1. Effect of Substituents on the Stability of η1-PCp and OsPB Complexes Relative to the Corresponding η5-PCp Complexesa complex
R
2 3 4 5 6 7 8 9 10
H 1-OMe 2-OMe 3-OMe 4-OMe 1-PH3+ 2-PH3+ 3-PH3+ 4-PH3+
η1-PCp 6.4 6.4 3.1 4.2 8.9 −8.1 −10.2 −1.1 −5.5
(−3.7) (−4.2) (−8.2) (−6.0) (−1.4) (−17.0) (−19.2) (−10.2) (−16.3)
OsPB 19.4 21.1 10.7 19.4 15.0 11.3 16.4 18.8 9.8
(7.7) (9.4) (−2.0) (8.2) (3.1) (−0.4) (5.1) (7.5) (−2.4)
a
The free energies and relative energies (in parentheses) are given in kcal/mol.
B
dx.doi.org/10.1021/om401188v | Organometallics XXXX, XXX, XXX−XXX
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these η1-PCp osmium complexes are thermodynamically more stable than the corresponding OsPBs, leading to an obstacle to realize MPB through the isomerizaton of an η5-PCp complex. Thus, another ligand environment should be considered to overcome the obstacle caused by the higher stability of the η1PCp complexes over the corresponding OsPBs. Although this situation is not suitable for realizing MPBs, it is a good model to obtain η1-PCp complexes, especially for 8-η1PCp, which is thermodynamically more stable than the corresponding η5-PCp complex by 10.2 kcal/mol. The kinetic study on the isomerization of 8-OsPB to 8-η1-PCp has been carried out. As shown in Figure 3, the reaction barrier from 8-
remarkably to 32.9 kcal/mol in comparison with the corresponding 11-η5-PCp. When the π-acceptor ligand CO is replaced by the PH3 ligand, a slight stabilization is found in 12OsPB. When the π-acceptor ligand CO is replaced by chloride, the stabilization increases strikingly to 4.9 kcal/mol in 13OsPB. Such a stability change can be rationalized by the strength of the Os−P and Os−C bonds in the metallacycle. For instance, 13-OsPB has the largest sum of the bond orders of Os−P and Os−C bonds, whereas 11-OsPB has the smallest sum (Figure S1, Supporting Information). The small energy difference between 13-OsPB and 13-η5-PCp encouraged us to perform a detailed study on 13-OsPB. Effect of the Metal Center. As shown in Figure 5, the stability of group 8 MPBs, Cl2(PH3)2M(PC4H4) (M = Fe, Ru,
Figure 3. Energy profiles for the formation of 8-η1-PCp from 8-OsPB. The free energies and relative energies (in parentheses) are given in kcal/mol. Figure 5. Effect of metal centers on the stability of MPBs relative to η5-PCp complexes. The free energies and relative energies (in parentheses) are given in kcal/mol.
1
OsPB to 8-η -PCp is 14.7 kcal/mol, which is lower by 3.7 kcal/ mol than that in Figure 2, possibly due to the stabilization of the phosphonium group on the transition state. Effect of Ligands. The effect of ligands has also been investigated to tune the stability of OsPBs. Our results indicate that the ligands indeed have a significant effect on the relative stability of OsPBs in comparison with the related η5-PCp complexes (Figure 4). Specifically, when CO is introduced to replace the chloride, the stability of 11-OsPB decreases
Os), has been investigated in comparison with the η5-PCp complexes. Apparently, down the group, the stability of MPBs increases dramatically due to the more diffuse d orbitals, which is in line with previous results that 5d metallacycles are more stable than their 4d analogues.3,12 Isomerization of 13-η5-PCp to 13-OsPB. As shown in Figure 6, the reaction barrier from 13-η5-PCp to 13-OsPB is
Figure 6. Energy profiles for the formation of 13-OsPB from 13-η5PCp. The free energies and relative energies (in parentheses) are given in kcal/mol.
computed to be 26.5 kcal/mol. The reverse barrier from 13OsPB to 13-η5-PCp is 21.6 kcal/mol, which is higher than that (18.4 kcal/mol) in Figure 2. The slightly higher reaction barrier can be attributed to the stronger Os−C and Os−P bonds (1.963 and 2.279 Å, respectively) on the metallacycle in 13OsPB in comparison with those (2.098 and 2.300 Å, respectively) in 2-OsPB (Figure S1, Supporting Information). Effect of Substituents on 13-OsPB. Similarly, the substituent effect has also been examined with both an EWG (PH3+) and EDG (OMe) at different positions (Scheme 3). As shown in Table 2, a π-donor substituent at the para or ortho position on the metallacycle will increase the stability of the
Figure 4. Effect of ligands on the stability of OsPBs relative to η5-PCp complexes. The free energies and relative energies (in parentheses) are given in kcal/mol. The selected Os−P and Os−C bond lengths (Å) and Wiberg bond indices (in italics) in OsPBs are given on the right. C
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Surprisingly, when an EWG is introduced at the ortho position, an unexpected stabilizing effect is observed, which can be rationalized by longer average Os−C bonds (2.214 Å) in 21-η5PCp in comparison with that (2.180 Å) in 19-η5-PCp (Figure S1). When these two stabilizing effects caused by EDG and EWG are taken into account simultaneously, in other words, methoxy is introduced at the para position and PH3+ at the ortho position, 22-OsPB becomes thermodynamically more stable by 7.9 kcal/mol than 22-η5-PCp. The reaction barrier from 22-η5PCp to 22-OsPB is computed to be 20.4 kcal/mol (Figure 8),
Scheme 3
Table 2. Effect of Substituents on the Stability of OsPBs Relative to the Corresponding η5-PCp Complexesa complex
R
13 14 15 16 17 18 19 20 21 22
H 1-OMe 2-OMe 3-OMe 4-OMe 1-PH3+ 2-PH3+ 3-PH3+ 4-PH3+ 2-OMe, 4-PH3+
OsPB 4.9 6.2 −1.0 4.8 −1.2 7.2 17.6 9.2 −2.9 −7.9
(−7.2) (−5.1) (−12.1) (−7.0) (−14.4) (−3.7) (7.4) (−1.2) (−14.8) (−19.9)
Figure 8. Energy profiles for the formation of 22-OsPB from 22-η5PCp. The free energies and relative energies (in parentheses) are given in kcal/mol.
a
The free energies and relative energies (in parentheses) are given in kcal/mol.
OsPB complex in comparison with the η5-PCp complex, whereas when it is located at the meta position, such a stabilizing effect disappears. This general observation is understandable because it is known that a π-donor substitutent at the carbene carbon of a given metal carbene complex plays a stabilizing role.27 OsPB, as a special type of metal carbene complex, is expected to follow this rule. An ortho substituent plays a role similar to that of a carbene substituent. A para substituent is normally expected to have an effect similar to that of an ortho substituent. Thus, both o- and p-methoxy substituents stabilize the OsPB complex in comparison with the η5-PCp complex. When an EWG is introduced at the ortho or para position, a destabilizing effect is expected. Indeed, PH3+ at the para position on the metallacycle gives a significant destabilization. Particularly, 19-OsPB becomes thermodynamically less stable by 17.6 kcal/mol than 19-η5-PCp, which produces a great opportunity to realize η5-PCp from the related metallaphosphabenzene (Figure 7). The reaction barrier from
which is lower by 6.1 kcal/mol than that from 13-η5-PCp to 13-OsPB in Figure 6. This is rationalized by the change in the Os−P bond. For instance, it is shortened by 0.098 Å from 22η5-PCp to 22-TS, whereas the change is 0.221 Å from 13-η5PCp to 13-TS (Figure S1, Supporting Information). Thus, less activation energy is required. In a word, a phosphonium group at the ortho position and a methoxy group at the para position not only stabilize osmaphosphabenzene but also lower the reaction barrier. These results indicate that an interconversion between OsPBs and the corresponding η5-PCp complexes can be achieved by simply tuning the substituents on the metallacycles.
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CONCLUSION The rearrangement between a series of MPBs and the η5-PCp complexes have been studied thoroughly by DFT methods. The effects of metal centers, ligands, and substituents on the metallacycles were examined systematically. Our results reveal a more stable stereoisomer (2-OsPB) of the previously reported complex Cl(CO)Os(PH3)2(PC4H4) (1-OsPB). Substituents were found to have a significant effect on the thermodynamics and kinetics of the rearrangement reactions. A phosphonium group at the para position on the metallacycle stabilizes the η1PCp complex drastically, which becomes more stable than the OsPB and η5-PCp complexes. Meanwhile, for the MPB [Cl2(PH3)2M(PC4H4)]− (M = Fe, Ru, Os), the instability of MPBs decreases in comparison with the corresponding η5-PCp complexes on going down the group. For the OsPB [Cl2(PH3)2Os(PC4H4)]−, a π-donor substituent at the para or ortho position on the metallacycle will increase its stability in comparison with the η5-PCp complex, whereas such a stabilizing effect disappears on substitution at the meta position. An EWG at the para position on the metallacycle gives a significant destabilization, whereas at the ortho position it gives a stabilizing effect. Finally, an interconversion between OsPBs and the corresponding η5-PCp complexes can be achieved by simply tuning the substituents on the metallacycles. Thus, our results could be valuable for experimentalists to realize the first metallaphosphabenzene and η1-PCp metal complex.
Figure 7. Energy profiles for the formation of 19-η5-PCp from 19OsPB. The free energies and relative energies (in parentheses) are given in kcal/mol.
19-OsPB to 19-η5-PCp is 15.4 kcal/mol, which is 6.2 kcal/mol lower than that of the unsubstituted case in Figure 6. It is understandable that the Os−C elongation (0.189 Å) from 19OsPB to 19-TS is smaller than that (0.209 Å) from 13-OsPB to 13-TS (Figure S1, Supporting Information). Thus, less activation energy is demanding. In a word, realizing η5-PCp complexes from MPBs becomes feasible under such conditions. D
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ASSOCIATED CONTENT
S Supporting Information *
Text, a figure, and tables giving the complete ref 25, additional computational results, and Cartesian coordinates and electronic energies. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.Z.);
[email protected] (Y.Z.). Home page: http://junzhu.chem8.org (J.Z.); http:// chem.xmu.edu.cn/group/yfzhao/zhao-home.html (Y.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the Chinese National Natural Science Foundation (21103142, 21133007, and 21232005), the National Basic Research Program of China (2011CB808504 and 2012CB821600), the Program for New Century Excellent Talents in University (NCET-13-0511), and the Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities (2012121021).
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