A Prototype Hexacoordinate Carbonium Ion - American Chemical

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C7H122+: A Prototype Hexacoordinate Carbonium Ion Golam Rasul,* George A. Olah, and G. K. Surya Prakash Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, United States ABSTRACT: C7H122+ (1), the prototype hexacoordinate carbonium dication was found to be a viable minimum at the MP2/6-31G** and MP2/cc-pVTZ levels. Structure 1 is a propeller shaped molecule resembling a complex involving a C2+ with three ethylene molecules resulting in the formation of three two-electron, three-center (2e 3c) bonds. Isomeric structure 2 was found to be 21.8 kcal/mol more stable than structure 1. However, conversion of 1 into 2 through transition structure 3 has a barrier of 5.7 kcal/mol. Related structures 4, 5, and 8 were also located as minima for C7H122+. The isoelectronic boron analogue BC6H12+ (10) was also computed to be a minimum at the same level of calculations.

’ INTRODUCTION Kekule’s1 classical concept that carbon can form not more than four bonds (and that carbon carbon bonding results in chains or cyclic compounds) has become the backbone of organic chemistry. The classical tetravalence bond theory, however, is unable to explain bonding in higher coordinate carbocations that require the involvement of two-electron, three-center (2e 3c) bonds as suggested by Olah.2 CH5+ is considered the parent of nonclassical carbocations as it contains a fivecoordinate carbon atom (Scheme 1).3 Ab initio studies reconfirmed4 the preferred global minimum Cs symmetrical structure5 7 with a 2e 3c bond as suggested by Olah et al.8 The structure of the parent six-9 and seven-coordinate10 carbocations, CH62+ and CH73+, respectively, were also reported by Olah et al. (Scheme 1). Using gold(I) organometallic ligand (LAu+) as isolobal substitute for H+, Schmidbaur et al. have prepared cationic trigonalbipyramidal12 and dicationic octahedral13 gold complexes containing five- and six-coordinate carbons. The complexes were characterized by X-ray structural analysis (Scheme 2). They represent isolobal analogues of CH5+ and CH62+, respectively. The presence of significant metal metal bonding in these gold complexes renders them remarkably stable, even isolable, as crystalline salts. Many experimental and theoretical studies on hypercoordinate square-pyramidal carbocations were also reported.2 In 1970, Williams13 first suggested the hypercoordinate square-pyramidal structure for the (CH)5+ carbocation on the basis of the structure of isoelectronic isostructural pentaborane (Scheme 3). In 1972, Stohrer and Hoffmann concluded14 from a theoretical treatment using extended H€uckel theory that the energy minimum for the (CH)5+ cation does not correspond to a planar classical structure. The proposed structure was a three-dimensional one in the form of a square-pyramid with a four-electron, five-center (4e 5c) bond. r 2011 American Chemical Society

Scheme 1

In the same year, Masamune and co-workers presented15 experimental evidence for a dimethyl analogue (CH3)2C5H3+ in superacid solutions (Scheme 4) and concluded that the structure is indeed square-pyramidal, and thus a close structural and isoelectronic relationship with 1,2-(CH3)2B5H7 was established. The square-pyramidal (CH)5+ cation has also been the subject of many theoretical studies first by semiempirical and later by ab initio methods. Kollman et al.16 and Dewar and Haddon17 reported CNDO and MINDO/3 studies on (CH)5+, respectively. Ab initio calculations on capped annulene rings with six interstitial electrons were carried out by Schleyer et al.18,19 Results of this calculation show that the favorable pyramidal structure follows the H€uckel-like 4n + 2 interstitial electron rule. Although the parent cation, (CH)5+, has not yet been observed in superacids experimentally, a variety of related structures including the C7H9+ cation20 and the C8H9+ cation21 have been identified under stable ion conditions using 13C and 1H NMR spectroscopy. The pentagonal-pyramidal structure was also observed for the (CCH3)62+ dication containing a six-coordinate carbon.22 In continuation of our study of higher coordinate Received: October 24, 2011 Revised: November 28, 2011 Published: November 30, 2011 756

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Scheme 2

Scheme 5

Scheme 3

Table 1. Total Energies ( au), ZPE,a and Relative Energiesb

Scheme 4

MP2/6-31G**

ZPE

MP2/cc-pvtz

rel energy (kcal/mol)

1

272.218 38

104.6

272.475 41

21.8

2 3 TS

272.252 76 272.210 30

103.6 103.6

272.508 61 272.464 80

0.0 27.5

4

272.273 01

102.3

272.528 08

13.5

5

272.274 78

102.4

272.529 93

14.6

6 TS

272.238 54

102.5

272.492 84

8.8

7 TS

272.252 49

102.2

272.508 19

1.1

8

272.280 30

101.0

272.535 57

19.5

9 TS

272.263 45

101.2

272.519 31

9.1

10

259.520 72

103.7

259.771 52

a

Zero point vibrational energies (ZPE) at MP2/6-31G**//MP2/631G** scaled by a factor of 0.95. b Relative energy (kcal/mol) at MP2/cc-pVTZ //MP2/cc-pVTZ + ZPE level.

carbocations, we have now extended our investigation to the sixcoordinate C7H122+ carbodication. The C7H122+ is another example of the simplest carbocation containing a six-coordinate carbon (involving seven carbons). The structure will be compared with Hogeveen’s intriguing pentagonal-pyramidal (Scheme 5) structure (CCH3)62+.22

’ CALCULATIONS Geometry optimizations, frequency calculations, and natural bond orbital charge calculations were carried out with the Gaussian 09 program.23 The geometry optimizations were performed at the MP2/6-31G** level. Vibrational frequencies at the MP2/6-31G**//MP2/6-31G** level were used to characterize stationary points as minima (number of imaginary frequency (NIMAG) = 0) or transition state (NIMAG) = 1) and to evaluate zero point vibrational energies (ZPE), which were scaled by a factor of 0.95.24 The transition states were also checked by IRC (intrinsic reaction coordinate) calculations.23 The MP2/6-31G** geometries were further optimized at the higher MP2/cc-pVTZ level. Calculated energies are given in Table 1. Atomic charges at the MP2/cc-pVTZ//MP2/cc-pVTZ level were obtained using the natural bond orbital analysis (NBO) method.25 NMR chemical shifts were calculated by the GIAO (gauge invariant atomic orbitals) method26 using MP2/cc-pVTZ geometries. GIAO-CCSD(T), GIAO-MP2, and GIAO-SCF calculations using tzp/dz basis set27,28 have been

performed with the ACES II program.29 The 13C NMR chemical shifts were computed using tetramethylsilane (TMS; calculated absolute shift, i.e., σ(C); tzp/dz = 193.9 (GIAO-SCF), 199.6 (GIAO-MP2), and 197.9 (GIAO CCSD(T)) as a reference. The 11B NMR chemical shifts were first computed using B2H6 (calculated absolute shift i.e σ(B) = 102.4 (GIAO-SCF/tzp), 98.1 (GIAO-MP2/tzp), 97.7 (GIAO-CCSD(T)/tzp) as a reference. The 11B NMR chemical shifts were finally referenced to BF3:OEt2 (δ(B2H6) 16.6 vs. BF3:OEt2).

’ RESULTS AND DISCUSSION At the MP2/6-31G** level, the C3 symmetric form 1 (Figure 1) was found to be a viable minimum for C7H122+. This is confirmed by frequency calculations at the same the MP2/631G** level. For the MP2/6-31G** optimized structures, further geometry optimizations were carried out at the MP2/cc-pVTZ level. Dication 1 is a propeller-shaped molecule. The structure is a hexacoordinate carbonium ion as it contains a six-coordinate carbon. It resembles a complex between C2+ and three ethylene molecules resulting in the formation of three 2e 3c bonds. The Cc-Cp (Cc is for central carbon and Cp is for peripheral carbon) bond distance of the 2e 3c interaction is 1.703 Å. The Cp Cp bond distance of the 2e 3c interaction is 1.411 Å. This is 0.079 Å longer than that found in the neutral ethylene C C 757

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Scheme 6

Figure 2. Part of the potential energy surface of C7H122+ calculated at the MP2/cc-pVTZ //MP2/cc-pVTZ + ZPE level.

found to be 21.8 kcal/mol more stable than structure 1. We have located a transition structure, 3 (Figure 1), for the conversion of 1 into 2. Structure 3 lies 5.7 kcal/mol higher in energy than structure 1. The conversion between 1 into 2 through transition state 3, therefore, has a considerable energy barrier. Dication 2 is a pentacoordinate carbonium ion as it contains a five-coordinate carbon with two 2e 3c bonds and a 2e 2c bond. The structure can also be considered as a cyclopropylcarbinyl cation. Secondary carbocationic center of structure 2 is stabilized by two cyclopropyl groups. Structures 4, 5, and 8 were also located as minima on the potential energy surface of C7H122+ (Figure 1). Cyclopropylcarbinyl structures 4 (Cs symmetrical) and 5 (C2h symmetrical) are rotational isomers to each other with 5 being about 1 kcal/mol more stable than 4. Structure 8 (trimethylenemethane dication) is the global minimum, being 4.9 kcal/mol more stable than 5 (Table 1). Related (hexaphenyltrimethylene)methane dication has been prepared and found stable in solution under superacidic stable ion conditions as shown by Head et al.30 The spectroscopic data and AM1 theoretical modeling indicate that although the entire π-system is twisted, phenyl groups stabilize the positive charges to a degree similar to those in the trityl cation. In all of the cases studied no evidence for “Y-aromatic” stabilization was found. Conversion of 2 into 4 has an energy barrier of 8.8 kcal/mol through transition structure 6. On the other hand, the rotational barrier of conversion of 4 into 5 through transition state 7 was computed to be 12.4 kcal/mol. Subsequent conversion of 5 into 8 was also found to have a barrier of 5.5 kcal/mol through transition structure 9. The potential energy surface of C7H122+

Figure 1. MP2/cc-pVTZ calculated structures of 1 10 and NBO charges (given in parentheses) of 1 and 10.

bond (1.332 Å) calculated at the same MP2/cc-pVTZ level. The plane of each of the 2e 3c units is 14.2° rotated around its axis from the central axis of the molecule. The NBO charge calculations (Figure 1) show that the central carbon atom of 1 bears 0.22 of charge. The peripheral three-ethylene molecules as a whole bear +2.22 of charge. This is due to the transfer of positive charge from the formal dicationic carbon center to the more electropositive hydrogens. Although both are hexacoordinate dication, structure 1 is significantly different from the structure of (CCH3)62+ dication.22 Structure 1 contains a six-coordinate carbon involving three symmetrical 2e 3c bonds. On the other hand, structure (CCH3)62+ (Scheme 5) contains a six-coordinate carbon involving a fourelectron, six-center (4e 6c) bond and a two-electron, two-center (2e 2c) bond. Attempts to find a stable minimum for structure I (Scheme 6) corresponding to formula C7H122+ failed because of spontaneous rearrangement to the thermodynamically more stable dication structure 1 at both the MP2/6-31G** and MP2/cc-pVTZ levels. Isomeric structure 2 (Figure 1) was also found to be a minimum for C7H122+ at the MP2/cc-pVTZ level. At the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level structure 2 was 758

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Scheme 7

Table 2. Calculateda 13C NMR Chemical Shifts no

atom

GIAO-SCF

GIAO-MP2

GIAO-CCSD(T)

1

Cc

58.7

41.6

39.7

2

Cp C (C)

85.7 19.3

91.3 37.0

90.5 35.9

Cav (CH2)

4

5

87.0

97.0

95.9

C (CH)

298.4

291.6

290.2

C (CH3)

35.3

44.3

44.6

C (C)

78.9

95.3

93.4

C (CH2)

81.4

95.1

93.6

Cav (CH)

285.1

277.7

275.9

Cav (CH3) C (C)

35.3 82.8

45.0 98.5

45.5 96.4

C (CH2)

83.5

97.8

96.1

C (CH)

289.3

282.8

281.0

C (CH3) 8

10

moderately deshielded at δ13C 90.5, indicating the relatively more nonclassical nature of the ion in accord with its relatively elongated Cc Cp bonds. Interestingly, the GIAO-CCSD(T) calculated 13C NMR data for the ions 2, 4, and 5 indicates that they involve substantial nonclassical charge delocalizations into the cyclopropyl rings. Similar calculations for isomer 10 show that the central boron atom is also highly shielded at δ11B 84.4 and the peripheral carbon (C p ) is moderately deshielded at δ 13 C +69.8, also indicating a relatively more nonclassical nature of the ion in accord with its relatively elongated B Cp bonds.

34.0

43.2

43.8

C (C)

139.9

149.9

148.0

C (CH)

265.0

259.3

256.8

C (CH3)

33.0

42.7

43.5

B Cp

79.7 65.8

77.3 66.8

84.4 69.8

a 13

C and 11B NMR chemical shifts were referenced to TMS and Et2O: BF3; Cc is for central carbon, and Cp is for peripheral carbon.

’ CONCLUSION The present study shows that the prototype hexacoordinate carbonium ion C7H122+ (1) is a viable minimum on its potential energy surface. Conversion of 1 into more stable 2 through transition structure 3 was found to have a barrier of 5.7 kcal/mol. The C3 symmetric structure 1 is a propeller shaped molecule resembling a complex between C2+ with three ethylene molecules resulting in formation of three 2e 3c bonds. Isomeric structure 2 was found to be 21.8 kcal/mol more stable than structure 1. The isoelectronic boron analogue 10 was also found to be as a viable minimum for BC6H12+. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. calculated at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level is depicted in Figure 2. Structure 8 can be considered as resonance stabilized allylic cations as shown in Scheme 7. The BC6H12+ monocation is isoelectronic with the C7H122+ dication. The MP2/cc-pVTZ optimization shows that the propeller-shaped C3 symmetrical structure 10 (Figure 1) is also a viable minimum on the potential energy surface of BC6H12+. Structure 10 resembles a complex between B+ and three ethylene molecules resulting in formation of three 2e 3c bonds. The plane of each of the 2e 3c units is slightly rotated (5.0°) around its axis from the central axis of the molecule. The B Cp bond distance of the 2e 3c interaction is 1.747 Å. The Cp Cp bond distance of the 2e 3c interaction is 1.403 Å. This is 0.071 Å longer than that found in the neutral ethylene C C bond (1.332 Å) calculated at the same MP2/cc-pVTZ level. The NBO charge calculations (Figure 1) show that the central boron atom of 1 bears +0.08 of charge. The peripheral three ethylene molecules as a whole bear +1.92 of charge. The 13C NMR chemical shifts for the calculated structures were computed using GIAO-SCF, GIAO-MP2, and GIAOCCSD(T) methods (Table 2). The more reliable GIAO-CCSD(T) calculations for isomer 1 show that the central carbon (Cc) is highly shielded at δ13C 39.7 and the peripheral carbon (Cp) is

’ ACKNOWLEDGMENT Support of our work by the Loker Hydrocarbon Research Instititue is gratefully acknowledged. ’ REFERENCES (1) Kekule, F. A. Ann. Chem. Pharm. 1858, 106, 129. Kekule, F. A. Ann. Chem. Pharm. 1857, 104, 129. (2) Olah, G. A. In Chemical Reactivity and Reaction Paths; Klopman, G., Ed.; Wiley: London, 1974. (3) Olah, G. A.; Prakash, G. K. S.; Wade, K.; Molnar, A.; Williams, R. E. Hypercarbon Chemistry; 2nd ed., Wiley, New Jersy, 2011. Schwarz, H. Angew. Chem., Int. Ed. Engl. 1981, 20, 991. (4) Marx, D.; Parrinello, M. Nature 1995, 375, 216. (5) Schreiner, P. R.; Kim, S.-J.; Schaefer, H. F.; Schleyer, P. v. R. J. Chem. Phys. 1993, 99, 3716. (6) Scuseria, G. E. Nature 1993, 366, 512. (7) White, E. T.; Tang, J.; Oka, T. Science 1999, 284, 135. (8) Olah., G. A.; Klopman, G.; Schlosberg, R. H. J. Am. Chem. Soc. 1969, 91, 3261. Olah., G. A.; Schlosberg, R. H. J. Am. Chem. Soc. 1968, 90, 2726. (9) (a) Lammertsma, K.; Olah., G. A.; Barzaghi, M.; Simonetta, M. J. Am. Chem. Soc. 1982, 104, 6851. (b) Lammertsma, K.; Barzaghi, M.; 759

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