DOI: 10.1002/chem.201xxxxxx █ Stabilized cyclopentadienyl cation A computational study on a strategy for isolating a stable cyclopentadienyl cation Kalon J. Iversen, David J. D. Wilson*[a] and Jason L. Dutton*[a] Abstract: A computational study has been carried out to derivative that may approach the stability of isolobal (and examine the feasibility of generating a simple monocyclic isolatable) borole rings, as evaluated by HOMO-LUMO and cyclopentadienyl cation that may be sufficiently stable to singlet-triplet gaps. These Cp+ derivatives may therefore be an isolate and handle at ambient temperatures. Using judicious attractive target for synthetic isolation. placement of electron withdrawing groups (CF3) about the ring we have identified a Introduction The isolobal neutral boron centred analogues (boroles) have recently experienced a resurgence of interest.[12] The + + [13] The antiaromatic cyclopentadienyl cation ([C5H5] ; [Cp] ; 1) has pentaphenyl borole ring (4) was reported some time ago, but long fascinated synthetic and theoretical chemists alike. The the determination of the solid state structure by Braunschweig in parent cation 1, has a triplet ground state and has been observed 2008 led to a substantial increase in activity.[14] Piers reported the at low temperatures via EPR studies, as have several substituted perfluorinated analogue (5) shortly thereafter.[15] These [1-3] analogues. However, as yet no isolatable monocyclic species compounds are indefinitely stable when stored under N2 or Ar and has been reported. The instability of the cation can be both species have demonstrated unusual and interesting [16-17] [18-22] rationalized from simple Hückel theory, where a 4- electron reactivity, such as H2 activation, reduction chemistry, triplet diradical is predicted (Figure 1). The diradical nature has and CO ligation.[23] The boroles owe their increased stability been observed from ESR studies of a variety of different (although they are very strong Lewis acids) to a loss in substituted derivatives of 1, all of which are unstable at ambient degeneracy in the highest occupied molecular orbital (HOMO) temperatures. For some bulkier derivatives a singlet ground state from [Cp]+, which is caused by the smaller electronegativity of is predicted due to Jahn-Teller distortions, but the very small boron in comparison with carbon in [Cp]+ (Figure 1).[12] The orbital singlet-triplet (S-T) gaps lead to substantial diradical nature and containing the contribution from boron (symmetric MO) rises subsequently these derivatives are not isolatable.[4-6] There was sufficiently in energy to yield a ground state singlet for boroles, an erroneous report in 2002 of the isolation of with localization of bonding (distinct C-C single and double bonds pentamethylcyclopentadienyl cation 2 as a surprisingly stable about the ring), and with the symmetric orbital centred on boron solid.[7] However, very soon thereafter the isolated material was becoming unoccupied. The HOMO-LUMO gap is small, resulting identified as 3.[8-11] in highly reactive deeply blue (4) or purple (5) compounds. Figure 1. -Molecular orbital framework (not to scale) and electron configuration for the parent cyclopentadienyl cation and parent borole. Adapted [a] Dr. D. J. D. Wilson, Dr. J. L. Dutton from Ref. 12. Department of Chemistry, La Trobe Institute for Molecular Science La Trobe University Melbourne, Victoria 3086 (Australia) We have investigated the stability of a number of derivative E-mail: [email protected] + [email protected] cyclopentadienyl cations [Cp] using a similar tact – if the boron centred borole cation is stable, can this stability be transferred to an all-carbon cycle? Our hypothesis was that ring stability could Supporting information for this article is available on the WWW be accomplished by making one carbon in the ring act as though under http://www.chemeurj.org/ or from the author. 1 DOI: 10.1002/chem.201xxxxxx it was less electronegative than the others, effectively “disguising” groups bound to C3 and C4. Compounds 9 and 11 exhibit it as a boron atom, via selective substitution of electron- perfectly planar rings. withdrawing groups around the rest of the ring. Results and Discussion Table 1. Selected M06-2X/def2-TZVP optimized bond distances (Å) for ground states of compounds 1-14 (singlet state unless noted). In this study we examined, in silico, the geometries and electronic Compound E1-C6 E1-C2 C2-C3 C3-C4 structure of cyclopentadienyl cations 6-13, containing selected 1[a] - 1.417 1.417 1.417 patterns with CF /CH and C F /C H substitution. While it is 3 3 6 5 6 5 2[a] 1.478 1.423 1.425 1.425 tempting to opt for strong electron-withdrawing groups such as 4 (E = B) 1.545 1.583 1.349 1.526 NO or CN, our calculations indicate that the electron-withdrawing 2 5 (E = B) 1.550 1.580 1.342 1.519 + CF3 and C6F5 groups are optimal for stabilizing [Cp] , which is 6[a] (singlet) 1.407 1.469 1.351 1.520 additionally appealing due to their potential synthetic feasibility 6[a] (triplet) 1.437 1.433 1.428 1.408 and similarity to the related isolatable boroles (4-5). 7[a] (singlet) 1.457 1.402 1.391 1.530 7[a] (triplet) 1.478 1.423 1.426 1.425 8 1.433 1.462 1.338 1.545 9 1.450 1.449 1.347 1.566 10[a] (singlet) 1.396 1.490 1.334 1.499 10[a] (triplet) 1.468 1.434 1.423 1.400 11 1.442 1.455 1.346 1.560 12 1.385 1.485 1.340 1.502 13 1.403 1.472 1.347 1.525 14 1.401 1.471 1.347 1.513 [a] Ground state triplet. We have defined the term “stable” (or “viable” using the suggested terminology of Hoffmann, Schleyer and Schaefer)[24] as having a sufficient HOMO-LUMO and S-T gap to allow for a clear ground state singlet at ambient temperatures. Geometry optimizations were carried out at the M06-2X/def2- TZVP level of theory, with subsequent properties investigated with B3LYP/def2-TZVP. Pertinent geometrical and electronic parameters for the derivatives considered as well as comparative data for compounds 1, 2, 4 and 5 are collected in Tables 1 and 2, respectively. For comparison, the dispersion-corrected B3LYP- Table 2. NPA partial charges (e) calculated at B3LYP/def2-TZVP for compounds 1-14. D3BJ produced equivalent structures, with variation in bond distances of less than 0.04 Å (mean absolute deviation of 0.006 Compound E1 C2 C3 C4 C5 C6 Å) from the M06-2X results detailed in Table 1. 1[a] -0.059 -0.059 -0.059 -0.059 -0.059 - The optimized geometries of the parent ground state triplet 2[a] 0.109 0.106 0.107 0.107 0.106 -0.659 cyclopentadienyl cations 1 and 2 reveal a delocalized geometry 4 0.852 -0.344 0.055 0.055 -0.344 -0.415 with ring C-C bond distances of 1.41-1.42 Å, consistent with 5 0.867 -0.325 0.074 0.074 -0.325 -0.483 [2] previous studies. Conversely, the singlet borole rings 4 and 5 6[a] 0.068 0.077 0.058 0.058 0.077 -0.068 display localized bonding with distinct C2-C3/C4-C5 double bonds 7[a] 0.142 0.053 0.077 0.078 0.048 -0.653 and a C3-C4 single bond. Compound 6, with a triplet ground state, 8 0.490 -0.237 0.146 0.085 -0.205 -0.717 also displays a localized geometry with a distinct C2-C3 double 9 0.462 -0.288 0.247 0.247 -0.288 -0.706 bond and C3-C4 single bond, while C1-C2 is intermediate between 10[a] 0.182 0.046 0.020 0.020 0.043 -0.463 a single and double bond at 1.41 Å (equivalent to delocalized C-C 11 0.470 -0.288 0.235 0.235 -0.288 -0.497 bonds in 1-2). The fluorinated aryl derivatives 6 and 7 have triplet 12 0.256 -0.166 0.036 0.036 -0.165 -0.091 ground states, with 7 featuring a delocalized geometry while 6 13 0.298 -0.245 0.186 0.185 -0.245 -0.123 exhibits localized bonding. 14 0.280 -0.246 0.168 0.169 -0.246 -0.106 Compounds with varying CH3/CF3 substitution patterns 8-11 exhibit singlet ground states with localized bonding, with 10 being [a] Ground state triplet. an exception. Methyl and ethyl substituents (electron donors) were considered on the formally cationic carbon centre, and the In the singlet state of 10 there is a further deviation involving C3 and C4 positions of the ring were considered with CF3 or CH3 extreme lengthening of the C-C bond in the ethyl substituent substituents. In all cases the C2 and C5 positions are occupied by (1.70 Å), with a shortening of C6-E1 (1.40 Å), which may be a CF3. In compounds 8 and 10 the rings are slightly distorted from strong conjugative interaction. Dispersion-corrected B3LYP- planarity, which is driven by steric clashes between the CF3 D3BJ/def2-TZVP yields equivalent results of 1.64 and 1.42 Å, 2 DOI: 10.1002/chem.201xxxxxx respectively, indicating that the bond distances are not overly a conclusive prediction about the ground state multiplicity. Indeed, influenced by dispersion. However, the E1-C6-C bond angle (ring- the S-T gap is sensitive to computational method; B3LYP-D3BJ ethyl) is sensitive to method, ranging from 73.3o (M06-2X/def2- and M06-L predict a singlet ground state by 18.1 and 4.2 kJ/mol, TZVP) to 99.1o (B3LYP-D3BJ/def2-TZVP).