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 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 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). The triplet state of 10 respectively (G). The singlet state has an anomalously large exhibits a geometry that is similar to the other triplet state HOMO-LUMO gap (3.86 eV), driven by the formation of an derivatives. In 11 the distortion in the ethyl C-C bond is less exocyclic C=C bond coupled with extreme lengthening of the prevalent but still apparent, with the corresponding ethyl C-C terminal C-C bond as discussed above. Even if the singlet state bond distance being 1.60 Å. The C1-C6 bond is 1.44 Å, nearly were the ground state, the terminal bond in the ethyl substituent identical to the methyl-substituted derivatives. would be expected to be incredibly labile, given its extreme length.

Molecular orbital (MO) calculations for 6-11 reveal a  MO The singlet states of alkyl/CF3 substituted derivatives 8, 9 and framework similar to that exhibited by the borole (Figure 1), with 11 are predicted to be substantially more stable than the C6F5 the HOMO (or low energy SOMO for triplet states) primarily substituted compounds (6, 7) and 10. Compound 8, with four CF3 consisting of  interactions centred on C2-C3 and C4-C5. The groups and one CH3 group has a HOMO-LUMO gap of 2.53 eV, LUMO (or high energy SOMO for triplet states) has coefficients which is only marginally smaller than the experimentally known on atoms C1-C2-C5 together with a -interaction of opposite phase borole rings. However, the adiabatic S-T gap is much smaller centred on C3-C4. than the borole rings at 10.4 kJ/mol. Changing the CF3 groups at

Considering HOMO-LUMO and S-T gaps (Table 3), the the 4 and 5 positions of the ring to CH3 (9) unexpectedly results in experimentally known borole rings can be taken as a benchmark a derivative with a larger HOMO-LUMO gap of 2.56 eV and an with calculated HOMO-LUMO gaps of 2.62 eV for 4 and 2.69 eV adiabatic S-T gap of 22.2 kJ/mol. Additionally, replacing the CH3 for 5. The increased gap for 5 is consistent with the red-shift in group attached to the cationic carbon with an ethyl group (11) the absorption spectrum that is observed experimentally.[15] The gives a larger HOMO-LUMO gap of 2.64 eV and a adiabatic S-T adiabatic S-T gaps for the boroles are calculated to be 63.4 (4) gap of 27.8 kJ/mol, likely due to increased conjugative and 86.3 (5) kJ/mol. interactions associated with a C-C bond compared to a C-H bond. A comparison of the LUMOs of the two compounds (9, 11) illustrates these interactions, where in 11 the C-C or C-H  Table 3. B3LYP/def2-TZVP calculated HOMO-LUMO gaps (eV), interaction has negative overlap with the  system of the ring singlet-triplet adiabatic gaps (G, kJ/mol), and singlet-triplet vertical (Figure 2). This results in 11 being predicted to be the most gaps (E, kJ/mol) for compounds 1-14. electronically stable of the alkyl substituted derivatives. The vertical S-T gap (calculating the triplet energy at the singlet [a] Compound H-L S-Tadiabatic S-Tvertical geometry) indicates this derivative would have a kinetic stability of 1 2.04 -42.4 - 103 kJ/mol. This value is intermediate between that of the boroles 2 2.01 -33.7 - (ca. 130 kJ/mol) and compounds 8 and 9 (82-93 kJ/mol). 4 2.62 63.4 128.3 5 2.69 86.3 133.5 6 1.45 -1.1 - 7 1.36 -10.9 - 8 2.53 10.4 81.7 9 2.56 22.2 93.2 10 3.86 -0.8 - 11 2.64 27.8 102.8 12 2.49 28.7 115.3 13 2.52 39.7 117.9 14 2.41 39.4 115.9 Figure 2. LUMO of compound 11.

[a] Singlet state HOMO-LUMO gap.

Nucleus independent chemical shift (NICS) calculations were

carried out to evaluate the antiaromatic nature of the rings being For compounds 6 and 7 bearing C F groups about the ring 6 5 considered (Table 4). While original formulations of NICS the singlet state HOMO-LUMO gaps of 1.36-1.45 eV are much considered isotropic NICS(0) and NICS(1), it has become evident smaller than that of the boroles. For 6 the small gap results in a that NICS(0)zz or NICS(1)zz yield a most effective triplet ground state, although the adiabatic S-T gap is a mere 1.1 25a index for planar  rings. Here we report NICS(1)zz values, with kJ/mol. While our DFT calculations do not allow us to conclusively NICS(0) included for direct comparison with previous reports exclude a singlet ground state, both the small HOMO-LUMO and (Table 4). S-T gaps suggest that 6 would not be a stable singlet cation at The NICS(0) values of ca. 0 ppm for the ground state triplet room temperature. The ground state for 7 is also a triplet, with a species 1, 2, 6 7 and 10 are consistent with previous reports,[25b] S-T gap of 10.9 kJ/mol. As such, it can be concluded that 6 and 7 as are the singlet state values of 13-17 ppm for the antiaromatic would likely not be thermodynamically stable in terms of attaining boroles 4 and 5 (24-32 ppm for NICS(1)zz), with the parent B-H a species that could be handled at room temperature and/or borole calculated to have a NICS(0) value of 20.3 ppm.[26] The stored. Compound 10 is predicted to be a ground state triplet by carbocationic ground state singlet derivatives (8, 9, 11) have less than 1 kJ/mol, although such a small S-T gap does not allow NICS(0) values of 24-28 ppm and for NICS(1)zz values of 52-65

3 DOI: 10.1002/chem.201xxxxxx

ppm indicating strong antiaromatic character, although the values the 3 and 4 positions would be the optimal derivative (13) to + are less than that calculated for planar cyclobutadiene at the stabilize [Cp] . Tetra-CF3 substituted 12 was also considered to [25b] same level of theory (33.1 ppm, NICS(0)). check for consistency with the trend for CF3/CH3 substitution, and For 1, 2, 6, and 7 with triplet ground states, the corresponding the 2,4,6-trimethylphenyl (mesityl) derivative of 13 (14) was also singlet state NICS(1)zz values are positive and large while the considered to check for consistency with our hypothesis and as a triplet states have negative NICS(1)zz values. This is indicative of preliminary examination of the effect of ring substitution. triplet aromaticity,[26] which is reflected in the equalization of ring The optimized geometries of 12-14 (Table 1 and Figure 3 for bond distances in the triplet ground states as discussed above. 12, 13) are consistent with the other singlet ground state compounds in that a localized geometry is predicted. For 13, the Table 4. B3LYP/def2-TZVP calculated NICS (ppm) for compounds exocyclic C-C bond is 1.403 Å and the bond distances in the 1-14. phenyl ring exhibit some distortion, although typical of that for an Compound Singlet Triplet aryl group bound to an active  system. The geometry of 14 is NICS(0) NICS(1)zz NICS(0) NICS(1)zz essentially identical to 13. For 12, 13 and 14 the S-T gaps are

[a] 28.7, 39.7 and 39.4 kJ/mol and the HOMO-LUMO gaps are 2.49, 1 85.5 196.5 -2.3 -25.6 2.52 and 2.41 eV, respectively. The difference between 12 and 2[a] 50.9 114.1 -1.3 -18.2 13 is consistent with the previous findings in that substitution of 4 13.4 24.2 3.7 -5.1 CH3 at the 3 and 4 positions provides greater stability. The 5 16.7 32.4 2.7 -8.0 vertical S-T gaps for 12-14 are larger than 11 at approximately 6[a] 23.3 45.7 3.9 -1.5 115 kJ/mol. 7[a] 41.3 92.8 2.2 -6.8

8 27.0 64.6 -4.8 -16.9

9 27.3 59.2 -2.7 -17.0 10[a] 2.4 9.7 -4.6 -15.1 11 24.1 51.8 -2.7 -16.5 12 12.2 32.8 0.6 1.7 13 16.3 36.4 0.8 -5.3 14 16.7 37.5 3.7 2.6

[a] Indicates a ground state triplet.

Examination of the combined geometrical, aromatic and electronic parameters for compounds 6-11 sheds light on the most desirable combination of R groups about the ring for targeting a “stable” cyclopentadienyl cation. It is immediately evident that C6F5 as the electron-withdrawing group is not sufficient to drive the HOMO-LUMO gap far enough apart for a Figure 3. Optimized geometries of compound 12 and 13. molecule to likely display stability at ambient temperatures.

Substitution of CF3 at the 2 and 5 position and CH3 at the 3 and 4 position gives larger HOMO-LUMO and adiabatic S-T gaps than the tetra-CF3 substituted derivatives. This can be explained by the change in phase for the -orbitals for the 3 and 4 position versus the 1, 2 and 5 positions in the LUMO. Essentially, CF3 in the 3 and 4 positions results in a stabilization of the LUMO relative to having CH3 in these positions, driving the HOMO-LUMO gap smaller. Comparing compounds 6 and 7 it appears that a Ph group at the cationic carbon provides more energetic stabilization than a CH3 group, while significantly reducing the singlet antiaromatic character (from 93 to 46 ppm) as measured by

NICS(1)zz. The effect of conjugation in changing CH3 to Et gives minimal benefit and the calculations suggest the terminal C-C bond in the ethyl groups would be labile. Therefore, of the possibilities considered we conclude that Ph is the “best” R group to have on the cationic carbon. This would also provide opportunities to increase the steric shielding on the Figure 4. Charge distribution (NPA) in compound 13. cationic carbon for improved kinetic stability. For example, mesityl groups are able to render a 3-coordinate boron in a borepin ring system sufficiently unreactive with respect to water that it may be Calculated natural population analysis (NPA) charges (Table purified by column chromatography.[27] Our data suggests that a 2, Figure 4) indicate that for the aryl substituted derivatives some derivative with Ph at the 1 position, CF3 at the 2 and 5 and CH3 at delocalization of the formal positive charge onto the ring is present, with a partial charge at the “central carbon” somewhat

4 DOI: 10.1002/chem.201xxxxxx

reduced from the formal +1 charge (ranging from +0.26 to +0.30), isolatable trityl cation has recently been calculated to have a gas- - and a negative charge at the C6 carbon (ranging from -0.91 to - phase fluoride affinity of 599 kJ/mol, or 608 kJ/mol using [OCF3] 0.12). This compares to +0.46 to +0.49 for the central carbon in as a “softer” fluoride source.[28] More experimentally relevant the singlet alkyl substituted derivatives with corresponding partial solvent phase (CH2Cl2) calculations give values of 160 and 171 charges at C6 of -0.50 to -0.72. An examination of the LUMOs for kJ/mol, respectively. Another benchmark for isolatable cations 12 and 13 (Figure 5) shows that there are two negative could be the N-heterocyclic phosphenium cations (cationic P- interactions between the  system of the phenyl ring and that of centered analogues of N-heterocyclic carbenes). These have the central carbon, destabilizing the LUMO and giving the larger been calculated to have gas-phase fluoride affinities of 651 and HOMO-LUMO gap. A -bonding interaction is found in the 721 kJ/mol for the unsaturated and saturated derivatives, HOMO-3. For 14, the HOMO is based on the mesityl group, the respectively.[29] The corresponding solvent-corrected values were corresponding orbital for 12 and 13 is the HOMO-1. The calculated to be 277 and 344 kJ/mol. Finally, the Lewis super acid hyperconjugative effect of the methyl aryl substituents in 14 SbF5 was calculated to have a fluoride affinity of 439 kJ/mol in a + destabilizes this orbital. The HOMO-1 for 14 is the “Hückel” orbital, CH2Cl2 solvent forcefield. The unisolatable cation [Me2P] was and the gap for HOMO-1-LUMO is 2.52 eV, identical to the calculated to have a fluoride affinity of 959 and 535 kJ/mol in the corresponding gap in 13. gas phase and with a CH2Cl2 solvent, respectively. Compounds 8 - NICS(1)zz values for 12 (32.8 ppm), 13 (36.4 ppm) and 14 and 9 were calculated to have fluoride affinities (using [OCF3] in

(37.5 ppm) indicate that these compounds exhibit similar CH2Cl2 forcefield) of 416 and 299 kJ/mol, respectively. aromaticity properties to that of the isolatable pentaaryl boroles 4 Compound 11 gave a similar value as 9 at 301 kJ/mol. Finally and 5. compounds 12, 13 and 14 were calculated to have fluoride affinities of 336, 270 and 237 kJ/mol, respectively. In general the calculated fluoride affinities mirror trends in HOMO-LUMO gaps with compounds having larger gaps providing smaller predicted fluoride affinities. For compounds 13 and 14 in particular, these values are of the same magnitude as isolatable compounds such as N-heterocyclic phosphenium cations (which may be handled easily under inert atmosphere), suggesting that the derivatives we propose as the best synthetic targets would not be so Lewis acidic as to preclude their handling under typical air/moisture free organic solvent conditions. Finally, we modeled the outcomes of some obvious potential pathways to the decomposition of these complexes as a more rigorous evaluation of their potential stabilities. Non-aromatic cyclopentadiene is well known to dimerize via a Diels-Alder reaction, with the boroles also known to undergo Diels-Alder reactions.[30] A potential decomposition pathway for our proposed cyclopentadienyl cations is therefore dimerization via a Diels- Alder reaction. However, calculations of the Gibbs free energy change (G) associated with this transformation shows that it is unlikely. Diels-Alder reactions for 9, 11, 13 and 14 are Figure 5. Frontier molecular orbitals of compound 13. unfavourable by 195-212 kJ/mol (corrected to CH2Cl2 solvent field). This is likely due to a combination of the unfavourability of combining two positive charges, as well as for steric reasons; Cp*

for example is not prone to Diels-Alder reactions and our Table 5. Fluoride affinities (-∆xH298 K, kJ/mol) for compounds 8, 9, 11-14. proposed molecules have similar steric demands. To test this Calculated at B3LYP/aug-cc-pVTZ//B3LYP/def2-TZVP in both gas phase hypothesis in a simple manner, the Diels-Alder reaction of 9 with and solvent (CH2Cl2) corrected forms. acetylene was considered. Acetylene is a relatively poor [31] Gas phase Solvent (CH2Cl2) dienophile from a kinetic perspective, but for our purposes it is - - - - Compound F OCF3 F OCF3 neutral and not sterically encumbered. With compound 9, the

+ Diels-Alder reaction with acetylene is favourable by 43 kJ/mol, [CPh3] 599 608 160 171 8 904 912 406 417 demonstrating that these cyclopentadienyl cations are competent 9 772 780 289 299 dienes in Diels-Alder reactions. This lends support to the 11 766 774 290 301 hypothesis that dimerization via Diels-Alder is being shut down by 12 804 812 325 336 a combination of charge and steric factors. Another decomposition pathway is the elimination of a proton (9, 13, 14 13 715 723 260 270 from the C methyl) or methyl group (11). For compound 11 we 14 673 682 227 237 3 modeled methylation of imidazole in comparison with the same

reaction using MeI. The calculated G for methylating imidazole An examination of the predicted Lewis acidities was also using MeI is -32 kJ/mol, and for 11 it is -201 kJ/mol. This data carried out for compounds having a clear singlet ground state (8, indicates the 11 would be an exceptionally powerful methylating 9, 11-14) by calculating their fluoride ion affinities (Table 5). The agent, and is consistent with the previous supposition that the

5 DOI: 10.1002/chem.201xxxxxx

[43-45] terminal C-C bond is very labile. For proton elimination, the method in a dichloromethane (CH2Cl2) solvent. Affinities were analysed - - reactions were modeled against the protonation of benzene. The using both F and OCF3 as anion sources; the latter Christe suggests better replicate experimental conditions.[46] + [C6H7] cation is a highly reactive superacid, but may be isolated (and is stable up to 150ºC) when paired with the carborane anions pioneered by Reed.[32] For compound 9, elimination of a Acknowledgements proton from the C1 methyl group to protonate benzene is favourable by 6 kJ/mol (CH2Cl2 solvent corrected), indicating that We thank The La Trobe Institute for Molecular Science for their 9 would be approximately as acidic as the [C H ]+ cation. generous funding of this work. This project is partially supported 6 7 by an ARC DECRA to JLD (DE130100186). VPAC, NCI-NF and Compounds 13 and 14 have the potential to lose a proton from La Trobe University are acknowledged for computing resources. the C3/C4 position methyl groups. Modeling this reaction for 13 We also thank an anonymous referee for several suggestions gives the protonation of benzene to be unfavourable by 9 kJ/mol which strengthened the manuscript. (unfavourable by 51 kJ/mol in the gas phase), making this + compound a slightly weaker acid than [C6H7] , but in the same Keywords: • theoretical chemistry • carbocations • regime. Compound 12 with four CF3 groups, despite being slightly less electronically stable is not susceptible to these [1] M. Saunders, R. Berger, A. Jaffe, J. M. McBride, J. O'Neill, R. Breslow, J. decomposition pathways. M. J. Hoffman, C. Perchonock, E. Wasserman, R. S. Hutton, V. J. Kuck, J. Am. Chem. Soc. 1973, 95, 3017. Conclusion [2] W. J. Hehre, P. v. R. Schleyer, J. Am. Chem. Soc. 1973, 95, 5837. [3] R. Breslow, J. M. J. Hoffman, J. Am. Chem. Soc. 1972, 94, 2110.

We have identified simple cyclopentadienyl cations using [4] R. Breslow, H. W. Chang, W. A. Yager, J. Am. Chem. Soc. 1963, 85, relatively simple functional groups that should display 2033. substantially greater stability over such compounds previously [5] R. Breslow, R. Hill, E. Wasserman, J. Am. Chem. Soc. 1964, 86, 5349. targeted. For 12-14, the electronic and magnetic parameters point to increased stabilities over 6-11. Compound 11 is only marginally [6] H. Sitzmann, R. Bock, T. Dezember, Z. Havias, W. Kaim, M. Moscherosch, L. Zanathy, J. Am. Chem. Soc. 1993, 115, 12003. less electronically stable, but very susceptible to losing a CH3 group. The vertical S-T gaps of 12-14 are sufficiently large that [7] J. B. Lambert, L. Lin, V. Rassolov, Angew. Chem. 2002, 114, 1487; even if the compounds are not long-lived in solution at room Angew. Chem. Int. Ed. 2002, 41, 1429. temperature, they may be long-lived at modestly reduced [8] M. Otto, D. Scheschkewitz, T. Kato, M. M. Midland, J. B. Lambert, G. temperature or amenable to storage in the solid state. The Bertrand, Angew. Chem. 2002, 114, 2379; Angew. Chem. Int. Ed. 2002, prediction that compounds with lower fluorine loading such as 13 41, 2275. and 14 give greater electronic stabilities may also be convenient [9] J. B. Lambert, Angew. Chem. 2002, 114, 2382; Angew. Chem. Int. Ed. for future potential synthetic work. Coupled with outstanding 2002, 41, 2278. achievements in weakly coordinating and inert anion technology [10] T. Muller, Angew. Chem. 2002, 114, 2379; Angew. Chem. Int. Ed. 2002, over recent years, this may allow for these anti-aromatic 41, 2276. [33] carbocations to be “bottled”, which we put forward as a [11] J. N. Jones, A. H. Cowley, C. L. B. Macdonald, Chem. Commun. 2002, pathway worth investigating for the synthetic chemistry 1520. community. [12] a) H. Braunschweig, T. Kupfer, Chem. Commun. 2011, 47, 10903. b) H. Braunschweig, I. Krummenacher, J. Wahler, Ad. Organomet. Chem. 2013, 61, 1.

Experimental Section [13] J. J. Eisch, J. E. Galle, S. Kozima, J. Am. Chem. Soc. 1986, 108, 379.

Angew. Chem. All geometries were optimized using the M06-2X density functional[34] with the [14] H. Braunschweig, I. Fernandez, G. Frenking, T. Kupfer, 120 Angew. Chem. Int. Ed. 47 def2-TZVP basis set[35] within Gaussian 09,[36] unless specifically noted 2008, , 1977; 2008, , 1951. otherwise in the text. Singlet states employed a restricted M06-2X approach, [15] C. Fan, W. E. Piers, M. Parvez, Angew. Chem. 2009, 121, 2999; Angew. while triplet states were optimized with an unrestricted M06-2X approach. All Chem. Int. Ed. 2009, 48, 2955. structures are minima with no imaginary frequencies. Thermodynamic corrections were taken from these calculations (standard state of T = 298.15 K [16] C. Fan, L. G. Mercier, W. E. Piers, H. M. Tuononen, M. Parvez, J. Am. and p = 1 atm). Other density functionals (B3LYP)[37] and basis sets (6-31G(d), Chem. Soc. 2010, 132, 9604. def2-SVP)[33-40] resulted in comparable geometries. Unless noted, all energetic J. and magnetic parameters were taken from single-point B3LYP/def2-TZVP [17] A. Y. Houghton, V. A. Karttunen, C. Fan, W. E. Piers, H. M. Tuononen, Am. Chem. Soc. 135 calculations, including HOMO-LUMO gaps, atomic partial charges, NICS, and 2013, , 941. singlet-triplet gaps since the B3LYP results produced trends in closer [18] H. Braunschweig, C. Chiu, K. Radacki, T. Kupfer, Angew. Chem. 2010, agreement with experiment and previous calculated results. Natural population 122, 2085; Angew. Chem. Int. Ed. 2010, 49, 2041. analysis (NPA) partial charges were calculated with NBO 5.9.[41] NICS[42] values for NICS(0) were calculated at the centroid of the ring, with NICS(1) calculated [19] H. Braunschweig, C. Chiu, K. Radacki, P. Brenner, Chem. Commun. 1.0 Å above and below the centroid of the ring (perpendicular to the plane of the 2010, 46, 916. ring). For vertical singlet-triplet energy gaps, triplet energies were determined at [20] H. Braunschweig, F. Breher, C. Chiu, D. Gamon, D. Nied, K. Radacki, the optimized singlet geometry of the compounds. Reported G energies are Angew. Chem. 122 Angew. Chem. Int. Ed. 49 defined from the combination of the B3LYP/def2-TZVP electronic energy and 2010, , 9159; 2010, , 8975. the M06-2X/def2-TZVP thermochemical correction. For comparison with a [21] H. Braunschweig, V. Dyakonov, O. C. Jimenez-Halla, K. Kraft, I. [28] previous study, the fluoride affinity calculations were performed at the Krummenacher, K. Radacki, A. Sperlich, J. Wahler, Angew. Chem. 2012, B3LYP/aug-cc-pVTZ level of theory incorporating the B3LYP/aug-cc-pVTZ 124, 3031; Angew. Chem. Int. Ed. 2012, 51, 2977. electronic energies with B3LYP/def2-TZVP geometries and thermal corrections. Solvent effects were included for electronic energies using the IEFPCM SCRF

6 DOI: 10.1002/chem.201xxxxxx

[22] H. Braunschweig, C. Chiu, J. Wahler, K. Radacki, T. Kupfer, Chem. Eur. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. J. 2010, 16, 12229. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. [23] A. Fukazawa, J. L. Dutton, C. Fan, L. G. Mercier, A. Y. Houghton, Q. Wu, Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, W. E. Piers, M. Parvez, Chem. Sci. 2012, 3, 1814. O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. [24] R. Hoffmann, P. v R. Schleyer, H. F. Schaefer III, Angew. Chem. Int. Ed., Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. 2008, 47, 7164. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09 Revision D.01, Gaussian, [25] a) H. Fallah-Bagher-Shaidaei, C. S. Wannere, C. Corminboeuf, R. Inc., Wallingford CT, 2009. Puchta, P. v R. Schleyer, Org. Lett. 2006, 8, 863. b)V. Gogonea, P. v R. Schleyer, P. R. Schreiner, Angew. Chem. Int. Ed., 1998, 37, 1945. [37] A. D. Becke, Physical Review A 1988, 38, 3098.

[26] P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. van Eikema [38] W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56, 2257. Hommes, J. Am. Chem. Soc. 1996, 118, 6317. [39] P. C. Hariharan, J. A. Pople, Chimica acta 1973, 28, 213.

[27] L. G. Mercier, W. E. Piers, M. Parvez, Angew. Chem. 2009, 121, 6224; [40] A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571. Angew. Chem. Int. Ed. 2009, 48, 6108. [41] E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. [28] S. A. Couchman, D. J. D. Wilson, J. L. Dutton, Eur. J. Org. Chem. 2014, Bohmann, C. M. Morales, and F. Weinhold NBO 5.9. (Theoretical 18, 3902. Chemistry Institute, University of Wisconsin, Madison, WI, 2011); [29] J. M. Slattery, S. Hussein, Dalton Trans. 2012, 41, 1808-1815. http://www.chem.wisc.edu/~nbo5

[30] C. Fan, W. E. Piers, M. Parvez, R. McDonald, Organometallics, 2010, 29, [42] P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. v. E. 5132. Hommes, J. Am. Chem. Soc. 1996, 118, 6317.

[31] E. H. Fort, M. S. Jeffreys, L. T. Scott, Chem. Commun, 2012, 48, 8102. [43] C. Pomelli, J. Tomasi, V. Barone, Theor. Chim. Acta 2001, 105, 446.

[32] a) C. A. Reed, N. L. P. Fackler, K. Kim, D. Stasko, D. R. Evans, J. Am. [44] E. Cances, B. Mennucci, J. Tomasi, J. Chem. Phys. 1997, 107, 3032. Chem. Soc. 1999, 121, 6314. b) C. A. Reed, K. Kim, E. S. Stoyanov, D. [45] J. Tomasi, B. Mennucci, E. Cances, J. Mol. Struct. (THEOMCHEM) 1999, Stasko, F. S. Tham, L. J. Mueller, P. D. W. Boyd, J. Am. Chem. Soc. 464, 211. 2003, 125, 1796. [46] K. O. Christe, D. A. Dixon, D. McLemore, W. W. Wilson, J. A. Sheehy, J. [33] For example, use of the carborane family of anions allowed for the tert- A. Boatz, J. Fluorine Chem. 2000, 101, 151. butyl cation to be isolated as a solid at room temperature. "Classic" anions only allowed for the tert-butyl cation to be handled at low temperatures or under super acidic conditions. See: T. Kato, C. Reed, Angew. Chem. Int. Ed., 2004, 43, 2908 and references therein.

[34] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215. Received: ((will be filled in by the editorial staff)) [35] A. Schäfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829. Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) [36] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.

7 DOI: 10.1002/chem.201xxxxxx

Entry for the Table of Contents

Layout 2:

FULL PAPER

█ Stabilized cyclopentadienyl cation

Kalon J. Iversen, David J. D. Wilson* and Jason L. Dutton*

Computational alchemy: A theoretical evaluation of a variety of simple, monocyclic ■■ – ■■ cyclopentadienyl cations has revealed a potential candidate for the isolation of the first Cp+ cation at ambient conditions. The presence of strong electron withdrawing A computational study on a groups at the 2 and 5 position of the ring is key to providing a sufficiently large strategy for isolating a stable HOMO-LUMO gap for a singlet ground state. cyclopentadienyl cation

8