On the Stability of Perfluoroalkyl-Substituted Singlet Carbenes: A

Article pubs.acs.org/JPCA

On the Stability of Perfluoroalkyl-Substituted Singlet Carbenes: A Coupled-Cluster Quantum Chemical Study Alexander B. Rozhenko,*,†,‡,§ Wolfgang W. Schoeller,§ and Jerzy Leszczynski‡ †

Institute of Organic Chemistry of NAS of Ukraine, Murmans’ka str. 5, Kyiv 02094, Ukraine Department of Chemistry and Biochemistry, Jackson State University, Jackson, Mississippi 39217, United States § Department of Chemistry, Bielefeld University, P.O. Box 10 01 31, 33501 Bielefeld, Germany ‡

S Supporting Information *

ABSTRACT: A series of trifluoromethyl-substituted carbenes R−C(:)−CF3 (R = NMe2, OMe, F, PMe2, P(NMe2)2, P(N(Pr-i)2)2, SMe, Cl); (dimethylamino)(perfluoroalkyl)carbenes Me2N−C(:)−R (R = CF3, C2F5, n-C3F7, i-C3F7, and tC4F9) and symmetrically substituted carbenes R−C(:)−R (R = NMe2, OMe, F, PMe2, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD(T) approach also known in quantum chemistry as the “golden standard”, in combination with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (ΔEST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the ΔEST values. The 13C NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me2N− C(:)−CF3, Me2N−C(:)−C3F7-i) are proposed as good candidates for the experimental preparation.

■

INTRODUCTION The turn of the millennium marked broad development of carbene chemistry, presenting the world with a variety of unique chemical processes and amazing structures.1,2Singlet carbenes are being actively developed as new ligands for preparing new metal complexes as efficient and selective catalysts.3 To some extent, thermodynamic stability of carbenes correlates with a difference between the energies of the singlet and triplet ground state structures (singlet−triplet energy gap, ΔEST).4 Large ΔEST magnitudes are usually provided by electronegative π-donor substituents (NR2, OR, or F) attached to the methylidene carbon. However, it is known that the quite stable Bertrand-type push−pull carbenes possess rather small ΔEST magnitudes,2,5−7 which was previously explained by their existing in the phosphaacetylene mesomeric form.8 The number of species prepared based on this principle is constantly increasing.9 Among others, the relatively stable (phosphanyl)(trifluoromethyl)carbene was recently characterized spectroscopically and kept for days in solution at −30 °C.10 At the same time, the methoxytrifluoromethylcarbene (MeO−C(:)− CF3) turned out to be highly unstable.11,12 This was attributed to the predominance of the pull inductive effect of the CF3 group over the push resonance effect of the hard-donor methoxy group, but it can also be referred to the fact that the methoxy group is a poor kinetically stabilizing group. Later, matrix-isolated 2-benzothienyl(trifluoromethyl)carbene was generated by irradiation of the corresponding diazirine, and © 2014 American Chemical Society

characterized by IR and UV/vis spectroscopy, in situ trapping, and DFT modeling.13 Similarly, 2,2,2-trifluoroethylidene (CF3−C(:)−H) was generated and characterized by photolysis in argon matrix.14,15 Finally, the chloro- and bromotrifluoromethyl carbenes were also isolated in argon matrix at 10 K.16 The absence of signals in ESR spectra confirmed the singlet ground state for these species. Recently, the original direct synthesis of perfluoroalkyl- and trifluorovinyl-substituted Fischer carbene complexes of tungsten and chromium was reported by Yagupolskii et al.,17 elegantly omitting the preparation of the corresponding carbenes. Based on quantum-chemical analysis of the potential energy surface of the reaction of thioamides with phosphites, the perfluoroalkylsubstituted aminocarbenes have been considered to be an intermediate.18 We were prompted to search new prospective perfluoroalkyl-substituted carbenes, which are still less investigated than their alkyl- and aryl-homologues. A detailed review on computational methods in carbene chemistry has appeared recently.19 The pioneering report on Hartree−Fock or DFT calculations of simple model species should also be mentioned here.4 The first ab initio (MP4(sdtq)/6-311(2d,2p)//MP2(fc)/6-31G** and QCISD(t)/6311(2d,2p)//MP2-(FC)/6-3lG*) molecular orbital study of Received: September 2, 2013 Revised: January 27, 2014 Published: January 28, 2014 1479

dx.doi.org/10.1021/jp408778x | J. Phys. Chem. A 2014, 118, 1479−1488

The Journal of Physical Chemistry A

Article

RI-BP86) and smaller basis sets (TZVP and cc-pVDZ), have been employed. The role of the alkyl chain size we have studied for the series of perfluoroalkyl substituted dimethylaminocarbenes 1, 10−13 (R = CF3, C2F5, n-C3F7, iso-C3F7, t-C4F9). For the coupled cluster energy calculations, geometries were optimized using the superior and very efficient RI-CC2 approach implemented into the TURBOMOLE set of programs29 combined with the TZVP basis sets. The singlepoint energies were computed at the (RI)-CCSD(T)/cc-pVTZ (for larger structures, at the (RI)-CCSD(T)/cc-pVDZ) and RICCSD(T)/TZVP levels of theory, in all cases, using the frozen core approximation. For a comparison, the DFT RI-BP86 functional and RI-MP2 approaches were also used for geometry optimization and single-point energy calculations.

2,2,2-trifluoroethylidene was reported in the work of O’Gara and Dailey14 and later at the DFT (BPW91) level of theory.15 The triplet ground state was predicted for this species. Song and Sheridan studied the effect of trifluoromethyl substitution on singlet carbene stability using DFT (B3LYP/6-31+G(d,p)) level of theory.20 They found that substitution of a CF3 group for H in carbenes decreases the ΔEST by a small, but systematic, amount. The effective donor substituents generally used for stabilizing the singlet ground state in carbenes, such as −NR2, −OR, or −P(NR 2) 2 , were not tested theoretically in combination with the CF3 group. The effect of size of the perfluoroalkyl substituent on carbene stability was not analyzed. Recently,21 thermodynamic stabilities for a series of 92 carbenes, singlet and triplet state structures, have been evaluated on the basis of hydrogenation enthalpies calculated at the G3MP2 level of theory. However, the perfluoroalkylsubstituted carbenes were not included in the list. Some calculations were performed for carbenes at the coupled cluster level with single, double, and perturbative triple excitations [CCSD(T)], considered as the “gold standard” for quantumchemical modeling.22 Bacskay calculated excitation energies for CH3CF and CF3CF carbenes by a range of quantum chemical methods, including CCSD(T), equations of motion coupled cluster (EOM-CCSD), complete active space second-order perturbation (CASPT2), multireference configuration interaction (MRCI), and time-dependent density functional methods.23 Nemirowski and Schreiner presented recently a concept for the construction of electronically stabilized triplet ground state carbenes based on systematic CCSD(T)/cc-pVDZ studies. Several carbene structures calculated at the CCSD(T) level of theory were published by Schaefer III et al.24,25 Kassaee et al. studied a series of symmetrically substituted diheteroatom carbenes at the DFT and CCSD(T)/6-311G*//QCISD/631G* levels of theory.26 The absence of published results for the perfluoroalkyl-substituted singlet carbenes at the CCSD(T) level of approximation can be referred to the fact that such calculations are already time-consuming for medium-sized structures involving more than 20−30 atoms.27 However, the recent implementation of the RI approximation into the CCSD(T) calculation procedure within the TURBOMOLE set of programs28 significantly improved the efficiency of the coupled cluster energy calculations allowing the treatment of a large pool of carbenes at this advanced level of theory.

■

METHODOLOGICAL DETAILS IN CALCULATIONS All the structures were first optimized in their singlet and triplet states without any symmetry constrain using the TURBOMOLE set of programs (v 6.2 or 6.4).28,30,31 The Resolution of the Identity (RI) based BP86 approach32,33 was used in combination with the TZVP basis sets (the TZV basis sets of triple-ζ quality34 plus one p-function set for hydrogens or plus one d-function set for all other atoms). The vibrational frequencies were calculated analytically or numerically. If an imaginary frequency was found for the equilibrium geometry, the structure was distorted using the TURBOMOLE screwer script and the full procedure was repeated. Then, the structures corresponding to the singlet and triplet ground states were reoptimized2830 using RI-MP2(full)/TZVP35 and RI-CC2(full)/TZVP36 levels of theory. Appropriate symmetry were kept by geometry optimization. The vibrational frequencies were calculated numerically; no imaginary frequencies were found for the structures optimized within the used symmetry constraints. The CCSD(T) single-point total energies were calculated using the ORCA37 or TURBOMOLE (v 6.4) sets of programs in the both cases combined with cc-pVDZ and cc-pVTZ basis sets using the frozen-core approximation (fc). In the case of TURBOMOLE, the RI approximation and symmetry were used for the more efficient calculations. The ORCA and TURBOMOLE based energy values calculated for the same structures were almost identical (ΔE < 10−5 a.u.). Additionally, the single-point calculations at the RI-CCSD(T)(fc)/TZVP were carried out using the TURBOMOLE set of programs (v 6.4). The intermediate RI-MP2(fc)-SCS energy values were also analyzed. The singlet−triplet energy gaps (ΔEST) were calculated using total energy values including the corrections on vibrations at 0 K (zero-point energies or ZPE), where they were available. To calculate the corresponding free Gibbs energy values (ΔGST), the total energy values were corrected on the chemical potentials, where available. The latter were generated using the TURBOMOLE f reeh script under standard conditions (T = 298 K, p = 0.1 MPa). No energy corrections were used for ΔEST magnitudes derived from CCSD(T) and RI-MP2(fc)-SCS based total energies. The T1 diagnostic38 and largest T2 amplitude values were taken from the standard ORCA output resulted from the CCSD(T)/cc-pVTZ singlepoint energy calculations. The singlet stabilities39 were tested at the RI-BP86/TZVP level of theory using the standard escf procedure implemented in the TURBOMOLE program set. The CASSCF(8,8) geometry optimization (with the TZVP basis sets) and single-point total energy calculations (with cc-

Here we present the first detailed study of trifluoromethylcarbenes 1−9 with a variety of the second stabilizing group (R = NMe2, N(Bu-t)2, OMe, F, PMe2, P(NMe2)2, P(N(Pr-i)2)2, SMe, Cl). For comparison, the symmetrical species R−C(:)−R (R = NMe2, OMe, SMe, PMe2, F, Cl) were also studied using the same theoretical approaches. The most superior level of approximation used in the study was CCSD(T) combined with a correlation consistent polarized valence triple-ζ basis set, ccpVTZ. This level of approximation was, however, suitable only for the small size carbenes. For the larger structures, including the experimentally investigated carbenes, only the cheaper and less time-consuming methods (RI-MP2, RI-MP2-SCS, RI-CC2, 1480



Article

Figure 1. Calculated (RI-CC2/TZVP) singlet and triplet state structures of trifluoromethyl carbenes. 1481



Article

Table 1. Calculated Singlet-Triplet Energy Gaps (ΔEST = Etrip − Esingl) for Carbenes R−C(:)−X (1−15), T1 Diagnostics, and Largest T2 Amplitudes (calculated at the CCSD(T)(fc)/cc-pVTZ level of theory) (RI-)CCSD(T)(fc)

item

R

X

RIBP86/ TZVP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3CF2 CF3CF2CF2 (CF3)2CF (CF3)3C Me Ph

NMe2 N(Bu-t)2 OMe F PMe2 P(NMe2)2 P(N(Pr-i)2)2 SMe Cl NMe2 NMe2 NMe2 NMe2 NMe2 NMe2

29.2 17.2 22.3 10.7 14.2 12.0 12.4 16.1 2.0 28.2 28.1 32.0 23.3 32.6 24.5

RIMP2/ TZVP

RIMP2(SCS)/ TZVP

RICC2/ TZVP

34.3 19.6 26.5 12.0 17.9 22.1 22.6 18.9 0.3 33.7 33.4 36.6 28.2 34.9 54.2

37.8 23.5 30.0 14.2 17.4 23.0

34.7 20.3 27.1 12.4 19.3 22.6 24.1a 20.6 1.4 33.9 33.3 37.3 28.5 35.5 41.1

b

20.7 2.8 37.7 35.6 39.9 37.7 53.2

cc-pVDZ

TZVP

35.5 19.9 26.8 14.1 (11.6)c 14.1 16.0 b

cc-pVTZ

35.4

36.9

b

b

27.9 14.0 (9.0)d 13.3 16.4

28.1 15.4 (12.3)e 16.9 b

b

18.4 3.2 (1.2)c 35.4 35.6 37.2 28.8 34.8 31.6

b

18.6 2.6 (1.3)d 34.8 35.2 36.5

20.5 4.7 (3.2)e 35.2 35.1 37.3

b

b

35.2 31.8

37.2 32.5

T1 diagnostics

largest T2 amplitudes

0.0140 0.0124 0.0149 0.0155 0.0149 0.0140f 0.0171 0.0132 0.0143 -

0.0635 0.0591 0.0599 0.0853 0.1107 0.0910f 0.0871 0.0734 0.0362 -

a Uncorrected ΔE relative energy. bThe value was not calculated. cCalculated at the CASSCF(8,8)/cc-pVDZ//CASSCF(8,8)/TZVP approximation level. dCalculated at the CASSCF(8,8)/TZVP approximation level. eCalculated at the CASSCF(8,8)/cc-pVTZ//CASSCF(8,8)/TZVP approximation level. fCalculated at the CCSD(T)(fc)/cc-pVDZ approximation level.

Table 2. Calculated Singlet-Triplet Energy Gaps (ΔEST = Etripl − Esingl) for Carbenes 16−21 RI-CCSD(T)(fc) item

X

RI-BP86/ TZVP

16 17 18 19 20 21

NMe2 OMe F PMe2 SMe Cl

39.0 51.0 50.9 14.5 27.8 19.5

RI-MP2/ TZVP

RI-MP2(SCS)/ TZVP

RI-CC2/ TZVP

ccpVDZ

TZVP

ccpVTZ

T1 diagnostic

largest T2 amplitude

45.2 56.3 51.4 16.3 30.5 15.7

48.6 59.3 54.4 15.0 30.9 17.9

44.9 57.1 53.4 17.2 31.8 17.8

43.6 56.1 54.9 10.6 28.7 19.0

44.5 56.7 54.5 10.3 37.9 18.2

46.1 58.0 56.4 13.8 30.8 19.8

0.0134 0.0152 0.0174 0.0193 0.0188 0.0209

0.0315 0.0579 0.0912 0.0804 0.0530 0.0702

or below 0.02) and largest T2 amplitudes (below 0.1)38,44 affirmed that the single-determinant representation of the carbenes in the current work is robust. However, the stability tests for carbenes 4 and 9 at the RI-BP86 level of theory39 exhibited that the singlet ground state was unstable and possessed some admixture of triplet spin states (expectation values for S2 were 0.162 and 0.358, respectively). The removal of spin contamination and the following unrestricted RI-BP86/ TZVP reoptimization of the singlet state structures sunk their total energies by 0.2 and 1.0 kcal/mol, for 4 and 9, respectively. Therefore, the multiconfigurational self-consistent field approach (MCSCF) calculations using the complete active space SCF (CASSCF(8,8)) were additionally carried out for these cases (Table 1). The corresponding ΔEST values are presented in Table 1. The full set of data is collected in the Supporting Information. For comparison, the corresponding values calculated for dimethylaminomethyl- (14) and dimethylaminophenylcarbene (15) are also included into Table 1. They mimic the experimental dimethylaminoalkyl- and dimethylaminoarylcarbenes synthesized by Bertrand’s group. As indicated from the calculation data, the CF3 group’s behavior is similar to that of methyl substituent, i.e., it possesses rather weak stabilizing properties for the singlet ground state. However, in combination with such an effective π-donor as the dimethylamino moiety (carbene 1), the singlet ground state is

pVDZ and cc-pVTZ Dunning’s basis sets) were carried out for structures 4 and 9 using the GAUSSIAN 09 program packet.40 The reaction 1 energies (ΔG) were determined using the corresponding corrected RI-CC2(full)/TZVP energy values. For this purpose, the geometries for the corresponding hydrogenated species were optimized at the same level of theory. The isotropic nuclear magnetic shielding values (σiso) were computed using the GIAO procedures implemented in the TURBOMOLE set of programs.41,42 The RI-BP86 and RIMP2(full) levels of approximations were employed for the density matrix calculations. In the latter case, the intermediately derived Hartree−Fock magnitudes of σiso were also obtained. The TZVPP basis sets were derived from the above-mentioned TZV basis34 adding two sets of the p-type and one set of the dtype polarization functions for hydrogen and two sets of the dtype and one set of the f-type polarization functions for all heavy atoms. The equilibrium structures were plotted using the Jmol program.43

■

RESULTS AND DISCUSSION a. Singlet and Triplet State Structures of Trifluoromethyl Carbenes. Equilibrium (RI-CC2/TZVP level) singlet and triplet state structures of trifluoromethyl carbenes are shown in Figure 1. All the calculated T1 diagnostics (equal 1482



Article

predicted to be more favored and the calculated ΔEST values (35−37 kcal/mol) are considerably high. The C−N bond in 1 (1.307 Å) is much shorter than those in the symmetrical Alder’s carbene 16 (1.356 Å). The efficient n,p-donation from the single nitrogen atom in 1 provides the formation of perfluoroalkyl-dialkylaminocarbene as reactive intermediate by desulfurization of the thioamides.18,45 Large substituents at nitrogen, such as t-Bu groups in 2, destabilize the singlet state, decreasing the ΔEST. This is in accordance with the experimental observations for carbenes with bulky substituents additionally stabilizing the triplet ground state.46 In particular, the CCN bond angle in the singlet state structure of 2 (126.5°) is practically identical to the corresponding value in the triplet state geometry (126.2°), whereas for 1 the corresponding angles are significantly different (113.4° and 121.9°, respectively). In order to elucidate the effect of the CF3 group we compare the trifluoromethylcarbenes with the corresponding symmetrically substituted species (Table 2). The largest ΔEST values have been predicted not for the experimentally known and stable bis(dimethylamino)carbene (16),47 but for bis(dimethoxy)carbene (17). However, dimethylamino group interplays better with trifluoromethyl substituent in 1 providing the ca. 0.9 kcal/mol larger ΔEST magnitude than the corresponding methoxy-derivative 3. In the equilibrium structures corresponding to the triplet states of 1 and 3, the dimethylamino and methoxy groups are rotated out of the CCN (CCO) plane. The C−N bond in 1 is significantly longer in the triplet state structure than that in the singlet ground state: 1.359 vs 1.307 Å, respectively. The difference is much smaller for the C−O bond in 3 (1.320 vs 1.296 Å), probably due to the fact that oxygen possesses two electron lone pairs and in the triplet state structure one of the two lone pairs is capable of conjugating with the singly occupied p-orbital on the methylidene carbon atom. The symmetric difluorocarbene 18 also possesses extremely large ΔEST values (55−56 kcal/mol), which even exceed the magnitudes predicted for Alder’s carbene 16 (36−43 kcal/ mol). While 18 is widely used in organic syntheses, this carbene is much less stable than 16. Thus, the ΔEST magnitudes do not correlate directly with the stabilities of carbenes. This is probably due to a wide variety of pathways for possible carbene transformations. Another carbene, also frequently generated in situ, dichlorocarbene 21 (ΔEST 18−19 kcal/mol), seems to be significantly less favored than 16 and 18. Substitution of one of the two halogen atoms in 18 and 21 by CF3 group (structures 4 and 9) decreases the ΔEST values (14−15 and 3−5 kcal/mol, respectively). These species probably possess lower stability. The CASSCF(8,8) calculations provide similar ΔEST magnitudes (Table 1). It was demonstrated previously that the combination of the π-donating phosphino group and σ-withdrawing trifluoromethyl substituent brought enough stabilization for bis(dicyclohexylamino)phosphinotrifluoromethylcarbene (22) to be persistent in solution at low temperature for several days.10 The crystal structure of 22 is unknown. The model species (H2N)2P−C(:)−X (X = CF3, SiH3, PH3+) was studied theoretically by Schoeller at the DFT level of theory,7 and the rather low ΔEST value (11.8 kcal/mol) was predicted for the model compound (H2N)2P−C(:)−CF3 (23). Previously, the high contribution of the phosphaacetylene mesomeric form were assumed for the push−pull carbenes.5 However, the CCcarbP bond angle in the equilibrium structure of 23 (126.4°)

differs from the other Bertrand’s carbenes, adopting much larger bond angle values (151.1° and 176.4° for X = SiH3 and PH3+, respectively).7 Therefore, in contrast to the silyl and phosphonium-substituted congeners, the species with the trifluoromethyl group represents the classical carbenes. One could also think that the bulky substituents will additionally increase the CCcarbP bond angle and decrease the ΔEST value (vide supra). However, we have found that within the series of carbenes R2P−C(:)−CF3 (R = Me (5), NMe2 (6), N(i-Pr)2 (7), N(cyc-Hex)2 (22)) calculated at the RI-CC2/TZVP level of theory, the CCcarbP bond angle does not change significantly (122.2, 123.0, 125.5, 121.3, respectively). Moreover, in all considered substituted structures 5−7, 22, the CCcarbP bond angles are even smaller than that previously predicted for the model carbene 23.7 A comparison of the ΔEST values derived at the CCSD(T)/cc-pVDZ level of theory demonstrates that the dimethylamino groups in 6 provide the increase of the ΔEST gap by only 2 kcal/mol compared to the dimethylphosphino congener 5. Unfortunately, increasing size of the structures by going to 7 and 22 prevents the coupled cluster calculations and computing the thermal corrections to Gibbs free energy at the RI-CC2 and RI-MP2 levels of theory. Obviously, the RI-MP2 and RI-CC2 approaches provide the ΔEST values, which are approximately 2−4 kcal/mol larger than the more superior CCSD(T) method, for 23 the expected ΔEST value is about 18−20 kcal/mol. For the rather stable trifluoromethylphosphinocarbenes, these magnitudes are surprisingly small when compared with 27−28 kcal/mol predicted for the unstable methoxytrifluoromethylcarbene 3.11,12 22 is probably to the greatest extent stabilized kinetically by the sterical protection against attacks at the carbene carbon. This supports the limited applicability of the singlet−triplet energy gap as the thermodynamic stability indicator. The small ΔEST values found for 19 support the previously noticed lower stabilizing ability of the phosphino substituents compared with the amino-group and are in agreement with the previously reported7 lower stability of bis-phosphino carbenes compared to the corresponding bis-amino-congeners (vide inf ra). However, the push−pull effect and sterically bulky substituents at phosphorus in 7 and 22 promote the planarity of the phosphine phosphorus. As a consequence, the Ccarb−P bonds in 5−7 and 22 (1.615−1.626 Å) correspond to the CP double bond length. Planarity violation is not important for the sulfur derivatives, where the both lone electron pairs on the sulfur atom can conjugate with the formally vacant p-orbital of the methylidene carbon. As a result, the symmetrical bis(methylthio)carbene 20 demonstrates increased ΔEST values (29−30 kcal/mol at the CCSD(T) level of theory, but this value is still much lower than the corresponding magnitudes derived for dimethoxy-substituted homologue 17 (56−58 kcal/mol). However, the dimethoxy carbene48 and [1,3]dithian-2-ylidenes49experimenexperimentally isolated in Ar matrix behaved similarly: they are very unstable and inclined to decomposition and/or di- and isomerization. The ΔEST magnitudes computed for trifluoromethyl-substituted congener 8 are even lower (ca. 20 kcal/ mol) and comparable to those predicted for 5 (14−17 kcal/ mol). b. Hydrogenation Reaction. An alternative approach for estimating the relative stabilities of carbenes is the hydrogenation reaction 1.20,21,49 The corresponding ΔG values (Table 3) are related to methyl(dimethylamino)carbene 14, 1483



Article

Table 3. Calculated Reaction 1 Energies (ΔG, kcal/mol) for Carbenes 1−21 in the Singlet (S) and Triplet (T) Ground States ΔG(BP86)

ΔG(MP2)

ΔG(CC2)

item

R

X

S

T

S

T

S

T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3CF2 CF3CF2CF2 (CF3)2CF (CF3)3C Me Ph NMe2 OMe F PMe2 SMe Cl

NMe2 N(Bu-t)2 OMe F PMe2 P(NMe2)2 P(N(Pr-i)2)2 SMe Cl NMe2 NMe2 NMe2 NMe2 NMe2 NMe2 -

−3.2 −4.4 −18.2 −40.9 −23.9 −22.2 −9.3 −21.6 −44.7 −2.2 −2.6 +3.6 −5.2 0.0 +2.3 +10.7 +14.5 −9.9 −14.3 0.0 −23.5

−0.2 −11.0 7.8 19.0 5.5 1.6 −10.9 5.0 14.1 −2.3 −1.1 −4.3 −4.1 0.0 −10.1 −4.4 3.8 28.1 −3.8 −4.9 10.4

−2.6 −6.3 −16.1 −38.4 −23.6 −21.1 −9.3 −22.7 −46.9 −1.1 −1.3 +2.2 −5.4 0.0 +1.8 +10.9 +17.3 −5.6 −17.6 −2.1 −26.1

1.9 −9.0 7.7 15.5 6.6 8.3 −3.0 6.7 12.3 −0.1 −0.2 −0.5 −1.4 0.0 16.9 0.4 4.1 22.2 −1.0 −2.3 6.9

−2.4 −5.3 −15.3 −38.4 −23.2 −19.7 −7.7 −20.6 −45.6 −0.9 −1.2 +3.0 −5.2 0.0 +2.2 +10.2 +18.0 −4.6 −16.1 +0.3 −24.3

1.7 −9.9 6.9 15.3 7.0 6.8 5.7 12.0 −0.6 −0.7 −1.1 −1.8 0.0 3.9 −0.8 3.6 22.5 −2.2 −4.0 6.7

Figure 2. Calculated (RI-CC2/TZVP) singlet and triplet state structures of dimethylamino-perfluoroalkylcarbenes.

the corresponding singlet states for bis-substituted carbenes 18 and 21 are also significantly less stable (ΔG = −4.6 and −24.3 kcal/mol, respectively), whereas the triplets are more stable than 14 (ΔG = +22.5 and +6.7 kcal/mol). As expected, the high positive reaction energy values have been predicted for the singlet states of diamino and dimethoxy derivatives 16 and 17. In contrast, the Bertrand’s carbenes are less preferable in reaction 1: the corresponding ΔG values for the singlet states of 4 and 5 lay in the range of −20 kcal/mol and hence are similar to the sulfur congener 6. Therefore, sterical shielding of the carbene center by the bulky substituents is really important for the experimental species 22. A surprisingly high stability is predicted for the bis(methylthio)congener 20 (ΔG = +0.3 kcal/mol). They are significantly more stable than the diphosphino-homologue 19 (ΔG = −16.1 kcal/mol), probably due to the absence of the planarization obstacle for sulfur. We expect that in combination with sterically bulky groups at the sulfur atoms, the synthetic preparation of stable congeners of type 20 might be an achievable challenge for experimentalists. c. Larger Fluorinated Alkyl Groups vs Trifluoromethyl Substituent. Increasing the size of perfluoroalkyl group does not significantly affect the thermodynamic stability of the corresponding carbenes (Figure 2, Tables 1 and 3, and

modeling experimentally known [di(iso-propyl)amino]tertbutylcarbene.50 The corresponding comparison was made both for the singlet and triplet ground states of the carbenes. Positive ΔG values mean higher thermodynamic stability of the species compared to 14. X1C(: )X2 + H3CCH 2NMe2 → X1CH 2X2 + MeC(: )NMe2 + ΔG

(1)

It can be seen from the data listed in Table 3 that (dimethylamino)(methyl)carbene 14 possesses a slightly higher thermodynamic stability than its trifluoromethyl-substituted congener 1. For 2, both the singlet and triplet ground states are slightly destabilized compared to 14. In turn, all the trifluoromethyl-substituted carbenes are less stable than the symmetrically disubstituted species. In spite of relatively large predicted ΔEST for 3 and 6 (R = OMe and SMe, respectively), they are much less stable than 14 (ΔG = −15.3 and −20.6 kcal/mol, here and further, the ΔG values derived at the CC2/ TZVP level of theory are analyzed). Fluoro- and chlorotrifluoromethyl derivatives 4 and 9 exhibit the lowest thermodynamic stability in the singlet ground state; in contrast, the corresponding triplet states are strongly stabilized relative to 14 (+15.3 and +12.0, respectively). It is however noteworthy that 1484



Article

Table 4. Isotropic 13C Shielding Constants (σiso, ppm), 13C NMR Chemical Shift Values (δC, ppm), Calculated for Carbenes 1− 21 at Different Approximation Levels, and Experimental 13C Chemical Shifts for Carbenes (δCexp, ppm) RHF/TZVPPa

RI-BP86/TZVPP

RI-MP2/TZVPP

item

R

X

σiso(C)

δC

σisoBP(C)

δCBP

σisoMP2(C)

δCMP2

δCexp59

1 2 3 4 5 6 7 8 9 14 15 16 17 18 19 20 21 22

CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 CF3 Me Ph NMe2 OMe F PMe2 SMe Cl CF3

NMe2 N(Bu-t)2 OMe F PMe2 P(NMe2)2 P(N(Pr-i)2)2 SMe Cl NMe2 NMe2 P(N(cyc-Hex)2)2

−170.3 −219.7 −263.8 −439.1 −119.3 47.5 −400.4 −737.4 −196.9 −186.4 −120.4 −144.4 −136.6 −149.1 −326.0 −565.2 -

358.4 407.8 451.9 627.2 307.4 140.6 588.5 925.5 385.0 374.5 308.5 332.5 324.7 337.2 514.1 753.3 -

−138.1 −142.0 −227.8 −381.7 −82.7 53.6 19.1 −285.5 −546.9 −147.9 −152.1 −70.2 −124.4 −123.6 −152.2 −192.0 −373.9 22.3

326.2 330.1 415.9 569.8 270.8 134.5 169.0 473.6 735.0 336.0 340.2 258.3 312.5 311.7 340.3 380.1 562.0 165.8

−100.2 −112.1 −193.6 −354.6 −26.3 98.2 −222.4 −519.3 −124.8 −115.1 −66.8 −100.4 −98.8 −23.3 −160.8 −356.9 -

288.3 300.2 381.7 542.7 214.4 89.9 410.5 707.4 312.9 303.2 254.9 288.5 286.9 211.4 348.9 545.0 -

− 135b 135b 326.3c 314.2d 259.7e 135

The RHF-based σiso calculations were carried out using the structures optimized at the RI-MP2/TZVP level of theory. bF3C−C(:)−P(N(cycHex)2)2.10 c(i-Pr)2N−C(:)−Bu-t.50 dMe2N−C(:)−Mes*, Mes* = 2,4,6-tri(tert-butyl)phenyl.60 eIn toluene-d8.61

a

implemented in several popular quantum chemistry program sets. At the MP2 level of theory, the experimental NMR chemical shifts can be well reproduced for both 13C57 and heavy nuclei.58 However, this computationally demanding approach is not suitable even for medium-sized structures and hence the less superior approaches should be used instead. The BP86 level of theory was also shown to be useful for the 13C nuclear shielding calculations of thiocarbonyl compounds, due to the cancellation of errors of a different nature.57 We report here the first systematic computational study aimed at deriving 13C NMR chemical shift values for carbenes at the DFT and MP2 levels of theory. The calculations were carried out both for the stable carbenes with the known experimental carbon NMR chemical shift values59 and for the short-lived species (Table 4). First of all, we compared the calculated and experimental magnetic shielding values for the known carbenes. For bis(dimethylamino)carbene 16, excellent agreement has been found for the data calculated at the RIBP86 and RI-MP2 levels of theory. In the other cases (6, 7, 14, and 15) the corresponding experimental δC values for the exact structures were not available and the calculated data have been compared with the most similar experimental species. For the model carbene 6, which mimics the experimentally known compound 22, the value of δC at the MP2 level of theory seems to be too low. However, the difference between the chemical shift values, calculated at the RI-BP86 level of theory for 6 and 7 (approximately 34 ppm) or for 6 and 22 (approximately 31 ppm) can be referred to the sterical effect of the substituents at nitrogens. Adding this difference to the MP2-derived value of δC for 6, a quite good agreement with the experiment has been achieved. Taking into account the significant difference between the substituents in 14 and 15 and the experimental structures prepared by Bertrand et al., the achieved agreement with the experiment at the RI-MP2 level of theory within 11 and 15 ppm, respectively, is acceptable. In general, the RI-BP86 calculated δC values seem to be overestimated by 20−50 ppm compared to the more superior

structures 1, 10−13). Only the perfluoroisopropyl group provides a slight stabilization in comparison to 1, as indicated by the increased singlet−triplet energy gap (ca. by 1 kcal/mol) and hydrogenation reaction free energy values: ca. by 5 kcal/ mol for the singlet ground state, with the slightly destabilized triplet state compared to 14. This can be referred to the especially favorable hydrogen bonding between the fluorine atom in the C2CF moiety of the i-C3F7 substituent and two hydrogen atoms of the methyl group (H···F 2.421 Å). As a consequence, in the perfluoroisopropyl congener 12 the calculated N−Ccarb bond length is slightly shortened (1.299 Å) compared to the corresponding distances in 1, 10, 11, and 13 (1.306−1.307 Å). This makes the N(p)−C(p) conjugation more effective, stabilizing the singlet ground state better. On the other hand, the CCcarbN bond angle in 1, 10, and 11 (113.4−113.6°) is smaller than the values predicted for 12 and 13 (120.1° and 121.2°). The corresponding equilibrium structure for 12 adopts Cs symmetry. The additional stabilization resulting from the overlap of the electron lone pairs on the fluorine atoms with the formally vacant carbenic porbital can also not be excluded.51 d. Theoretical 13C NMR Chemical Shifts for Carbenes 1−9, 14−22. Carbenes are the chemical compounds that possess rather small energy differences between the highest occupied and lowest vacant molecular orbitals. Consequently, a large paramagnetic contribution into the magnetic nuclear shielding (low-field chemical shifts) is expected.52 While the experimental δC values for the short-lived carbenes are unknown, the corresponding data can be derived theoretically. In spite of a number of papers devoted to the theoretical studies of carbenes, reports on calculations of chemical shifts for these species are rather rare.53,54 This is not the case, because the acceptable agreement with the experimental data can only be achieved by using the post-Hartree−Fock approaches with proper treatment of electron correlation effects.55−57 Up to the present date, only the MP2-based routines for the magnetic nuclear shielding calculations41 are 1485



Article

Figure 3. JMol plots of calculated (RI-CC2/TZVP) C2 (left) and C1 (right) symmetrical structures of bis(dimethylphosphino)carbene 19.

modeling, have been used for the estimation of relative stability of trifluoromethyl-, perfluoroalkyl-substituted, and symmetrically substituted carbenes. The dimethylamino-methyl- and dimethylamino-phenylcarbenes have also been calculated for comparison. For all investigated structures, the singlet ground state seems to be more favored. The calculations predict the largest singlet−triplet energy gap (ΔEST), the most positive hydrogenation energy (the energy of tearing off hydrogen molecule by the carbene from H3CCH2NMe2), and the highest magnetic shielding for the methylidene carbon for the experimentally known and relatively stable carbenes. These are, for example, Alder’s carbene Me2NC(:)NMe2, dimethylaminoalkyl-, or dimethylaminoarylcarbenes. The perfluoroalkylsubstituted aminocarbenes demonstrate thermodynamic stability similar to (dimethylamino)(methyl)carbene, or (in the case of perfluoroisopropyl-substituted congener) even exceed it and hence could probably be prepared experimentally. In contrast, the rather small ΔEST magnitudes and strongly negative hydrogenation energies have been found for carbenes Me2P−C(:)−CF3 and (R2N)2P−C(:)−CF3, which can be referred to the significant energy necessary for creating planar environment at phosphorus as the donor atom. This is also the reason why the equilibrium structure of bis(dimethylphosphino)carbene (19), optimized at the MP2 or CC2 level of theory, does not adopt the C2-symmetrical conformation, as in Alder’s bis(dimethylamino)carbene, but includes only one planar phosphorus atom. 19 also reveals the small ΔEST value and highly negative hydrogenation energy, but the surprisingly high magnetic shielding for the methylidene carbon nuclei. The optimized structures do not agree with the phosphaacetylenic Lewis structure for carbenes (R2N)2P− C(:)−CF3 and 19. The sulfur homologue MeS−C(:)−CF3 seems to possess poor stability and a strongly low-field 13C NMR resonance, but the corresponding symmetrically bissubstituted species MeS−C(:)−SMe is a promising candidate for synthesis under the condition of using sterically bulky substituents instead of the methyl group for the additional kinetic stabilization. The least stable are halogenocarbenes Hal−C(:)−CF3 (Hal = F, Cl), which also demonstrate the extremely high deshielding of the methylidene carbon nuclei.

RI-MP2 level of theory. The only exceptions are structures 16 (both values almost exact) and 19, for which geometry optimization at the DFT and RI-MP2 approaches yields different equilibrium structures and, hence, different magnetic shielding values (vide inf ra). The substitution of the dimethylamino group in Alder’s carbene 16 by trifluoromethyl substituent obviously reduces the stability of carbene. This is reflected in the calculated δC values, but the downfield shift predicted for the methylidene carbon (approximately 34 ppm) is not large. The MP2 calculated chemical shift value is only 38 ppm lower than the magnitude derived at the BP86 level of theory. The RI-BP86 method yields for 19 the C2-symmetrical structure with two equivalent C−P bonds (1.684 Å). At the RIMP2 or RI-CC2 levels of theory, the optimized structures are nonsymmetrical (Figure 3, right) with one P−C bond significantly shorter (1.634 Å) than another one (1.757 Å, the values are related to the RI-CC2 approach), whereas the C2symmetrical structure (Figure 3, left) corresponds to the transition state with one imaginary frequency (−45.3 cm−1). The difference in the calculated energies for the conformations is small (ΔGRI‑CC2 = 2.2 kcal/mol; see the Supporting Information for more detail). The structural changes mentioned above for 19 cause a significant difference in the calculated shielding values (+340.3 ppm for the C 2 -symmetrical structure at the RIBP86/TZVPP//RI-BP86/TZVP level of theory, +211.4 ppm for the C1-symmetrical structure at the RI-MP2 level of approximation). Interestingly, the latter structure in some extent mimics the trifluoromethyl substituted congener 5 with the short C−P bond (1.626 Å) and 5 reveals almost the same calculated δC value (214.4 ppm) as 19. Thus, by the type of interactions and predicted chemical shift values, 5 is related to the Bertrand’s type carbenes 6, 7, and 22. In some disagreement with the high stability of 20 predicted above, the calculated (RI-MP2 level of theory) chemical shift for this carbene is +348.9 ppm. The lowest energy conformation is possessed by the S-formed structure (see Figure S1 in the Supporting Information). The trifluoromethyl substituted species 8 shows the resonance in the lower field (+410.5 ppm). Within the series 14−21 the most deshielded is the divalent carbon in dichlorocarbene 21 (+545.0 ppm). This reflects the high electrophilicity of the unstable carbene 21. The theoretically predicted carbon chemical shift for the corresponding trifluoromethyl-substituted species 9 is even larger (+707.4) and agrees with the extremely low thermodynamic stability of this carbene concluded above from the ΔEST value and hydrogenation reaction 1.

■

ASSOCIATED CONTENT

S Supporting Information *

Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org

■

AUTHOR INFORMATION

Corresponding Author

■

*Tel +380 44 499 4610, Fax: +380 44 576 2643, e-mail: a_ [email protected].

CONCLUSIONS Three different theoretical approaches, including CCSD(T) considered as the “gold standard” for quantum-chemical

Notes

The authors declare no competing financial interest. 1486



■

Article

(19) Gerbig, D.; Ley, D. Computational Methods for Contemporary Carbene Chemistry. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2013, 3, 242−272. (20) Song, M.-G.; Sheridan, R. S. Effects of CF3 Groups and Charged Substituents on Singlet Carbene Stabilities−a Density Functional Theory Study. J. Phys. Org. Chem. 2011, 24, 889−893. (21) Gronert, S.; Keeffe, J. R.; More O’Ferrall, R. A. Stabilities of Carbenes: Independent Measures for Singlets and Triplets. J. Am. Chem. Soc. 2011, 133, 3381−3389. (22) Rahalkar, A. P.; Mishra, B. K.; Ramanathan, V.; Gadre, S. R. “Gold Standard” Coupled-Cluster Study of Acetylene Pentamers and Hexamers via Molecular Tailoring Approach. Theor. Chem. Acc. 2011, 130, 491−500. (23) Bacskay, G. Quantum Chemical Characterization of the Ground and Lowest Excited Singlet and Triplet States of CH3CF and CF3CF and Their Photochemical Isomerization and Dissociation Pathways. Mol. Phys. 2003, 101, 1955−1965. (24) Xie, Y.; Schaefer, H. Singlet-Triplet Splitting of Carbohydroxycarbene. Mol. Phys. 1996, 87, 389−397. (25) Seidl, E. T.; Schaefer, H. F. Equilibrium Geometry of the HCCN Triplet Ground State: Carbene or Allene? An Open-Shell Coupled Cluster Study Including Connected Triple Excitations. J. Chem. Phys. 1992, 96, 4449−4452. (26) Kassaee, M. Z.; Ghambarian, M.; Shakib, F. A.; Momeni, M. R. From Acyclic Dialkylcarbene to the Unsaturated Cyclic Heteroatom Substituted Ones: a Survey of Stability. J. Phys. Org. Chem. 2011, 24, 351−359. (27) Pitoňaḱ , M.; Neogrády, P.; Č erný, J.; Grimme, S.; Hobza, P. Scaled MP3 Non-Covalent Interaction Energies Agree Closely with Accurate CCSD(T) Benchmark Data. ChemPhysChem 2009, 10, 282− 289. (28) TURBOMOLE v 6.4 2012, a Development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007, TURBOMOLE GmbH, since 2007; http://www.turbomole.com. (29) Hättig, C. Optimization of Auxiliary Basis Sets for RI-MP2 and RI-CC2 Calculations: Core-Valence and Quintuple-ζ Basis Sets for H to Ar and QZVPP Basis Sets for Li to Kr. Phys. Chem. Chem. Phys. 2005, 7, 59−66. (30) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation Computers: The Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165−169. (31) Furche, F.; Ahlrichs, R.; Hättig, C.; Klopper, W.; Sierka, M.; Weigend, F. Turbomole. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2013, DOI: 10.1002/wcms.1162. (32) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (33) Perdew, J. Density-Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas. Phys. Rev. B 1986, 33, 8822−8824. (34) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (35) Weigend, F.; Häser, M.; Patzelt, H.; Ahlrichs, R. RI-MP2: Optimized Auxiliary Basis Sets and Demonstration of Efficiency. Chem. Phys. Lett. 1998, 294, 143−152. (36) Christiansen, O.; Koch, H.; Jørgensen, P. The Second-Order Approximate Coupled Cluster Singles and Doubles Model CC2. Chem. Phys. Lett. 1995, 243, 409−418. (37) Neese, F. The ORCA Program System. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73−78. (38) Lee, T. J.; Rice, J. E.; Scuseria, G. E.; Schaefer, H. F. Theoretical Investigations of Molecules Composed Only of Fluorine, Oxygen and Nitrogen: Determination of the Equilibrium Structures of FOOF, (NO)2 and FNNF and the Transition State Structure for FNNF CisTrans Isomerization. Theor. Chim. Acta 1989, 75, 81−98. (39) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. A Spin Correction Procedure for Unrestricted Hartree-Fock and Møller-

ACKNOWLEDGMENTS The authors thank Prof. Uwe Manthe and Dr. Thorsten Tönsing, University of Bielefeld (Germany), for access to the computer cluster and the TURBOMOLE set of programs (v 6.2) and for technical support for the calculations. We are also grateful to the team of the SKIT Supercomputer at the Institute of Cybernetic, NAS, of Ukraine providing access to the computer cluster for A.B.R. The generous financial support from the Alexander von Humboldt Foundation (Germany) for buying computers and the license for the TURBOMOLE set of programs is gratefully acknowledged. The authors thank support of the NSF CREST Interdisciplinary Nanotoxicity Center NSF-CREST - Grant # HRD-0833178.

■

REFERENCES

(1) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 2000, 100, 39−91. (2) Vignolle, J.; Cattoën, X.; Bourissou, D. Stable Noncyclic Singlet Carbenes. Chem. Rev. 2009, 109, 3333−3384. (3) Slaughter, L. M. Acyclic Aminocarbenes in Catalysis. ACS Catal. 2012, 2, 1802−1816. (4) Räsänen, M.; Raaska, T.; Kunttu, H.; Murto, J. Ab Initio Studies on Carbenes; Singlet and Triplet Conformers and Vibrational Spectra of Hydroxy-, Dihydroxy- and Methylhydroxy-Carbene. J. Mol. Struct. THEOCHEM 1992, 208, 79−90. (5) Bertrand, G.; Reed, R. λ3-Phosphinocarbenes λ5-Phosphaacetylenes. Coord. Chem. Rev. 1994, 137, 323−355. (6) Schoeller, W. W.; Eisner, D.; Grigoleit, S.; Rozhenko, A. B.; Alijah, A. On the Transition Metal Complexation (Fischer-Type) of Phosphanylcarbenes. J. Am. Chem. Soc. 2000, 122, 10115−10120. (7) Schoeller, W. W. On the Electronic Properties of Substituted Phosphanylcarbenes. Eur. J. Inorg. Chem. 2000, 2000, 369−374. (8) Nyulászi, L.; Szieberth, D.; Réffy, J.; Veszprémi, T. H2PCH: a Phosphinocarbene or a Phosphaacetylene? a Revisited Problem. J. Mol. Struct. THEOCHEM 1998, 453, 91−95. (9) Denis, P. A.; Iribarne, F. Theoretical Investigation of CarbonSulfur Triple Bonds. Chem.Eur. J. 2011, 17, 1979−1987. (10) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Stable Versions of Transient Push-Pull Carbenes: Extending Lifetimes from Nanoseconds to Weeks. Science 2000, 288, 834−836. (11) Moss, R. A.; Zdrojewski, T.; Ho, G.-J. Push-Pull Carbenes: Methoxytrifluoromethylcarbene. J. Chem. Soc.: Chem. Commun. 1991, 946−947. (12) Brahms, D. L. S.; Dailey, W. P. Fluorinated Carbenes. Chem. Rev. 1996, 96, 1585−1632. (13) Wang, J.; Sheridan, R. S. A Singlet Aryl-CF3 Carbene: 2Benzothienyl(trifluoromethyl)carbene and Interconversion with a Strained Cyclic Allene. Org. Lett. 2007, 9, 3177−3180. (14) O’Gara, J. E.; Dailey, W. P. Matrix-Isolation and Ab Initio Molecular Orbital Study of 2,2,2-Trifluoroethylidene. J. Am. Chem. Soc. 1994, 116, 12016−12021. (15) Cramer, C. J.; Hillmyer, M. A. Perfluorocarbenes Produced by Thermal Cracking. Barriers to Generation and Rearrangement. J. Org. Chem. 1999, 64, 4850−4859. (16) Seburg, R. A.; McMahon, R. J. Ground-State Multiplicity of Halo(trifluoromethyl)carbenes. J. Org. Chem. 1993, 58, 979−980. (17) Tyrra, W.; Yagupolskii, Y. L.; Kirij, N. V.; Rusanov, E. B.; Kremer, S.; Naumann, D. A General Route to Perfluoroalkyl- and Trifluorovinyl-Substituted Fischer Carbene Complexes of Tungsten and Chromium − Syntheses, Characterisation and Structures. Eur. J. Inorg. Chem. 2011, 2011, 1961−1966. (18) Mykhailichenko, S. S.; Pikun, N. V.; Rozhenko, A. B.; Shermolovich, Y. G. In Proceedings of the 25th International Symposium on the Organic Chemistry of Sulfur; Czes̨ tohowa, Poland, June 24−29, 2012; p 142. 1487



Article

Plesset Wavefunctions for Singlet Diradicals and Polyradicals. Chem. Phys. Lett. 1988, 149, 537−542. (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, rev A.01; Gaussian, Wallingford, CT, 2009. (41) Kollwitz, M.; Gauss, J. A Direct Implementation of the GIAOMBPT(2) Method for Calculating NMR Chemical Shifts. Application to the Naphthalenium and Anthracenium Ions. Chem. Phys. Lett. 1996, 260, 639−646. (42) Schreckenbach, G.; Ziegler, T. Calculation of NMR Shielding Tensors Using Gauge-Including Atomic Orbitals and Modern Density Functional Theory. J. Phys. Chem. 1995, 99, 606−611. (43) Jmol: An Open-Source Java Viewer for Chemical Structures in 3D. See also: http://www.jmol.org/. (44) Crawford, T. D.; Schaefer, H. F. Rotational Constants for the C̃ 2 A2 State of NO2. J. Chem. Phys. 1993, 99, 7926−7928. (45) Rozhenko, A. B.; Mykhaylychenko, S. S.; Pikun, N. V.; Shermolovich, Y. G.; Leszczynski, J. Intermediate Carbene Formation in the Reaction of Thioamides with Phosphorus (III) Derivatives: Quantum Chemical Investigation. Int. J. Quantum Chem. 2014, 114, 241−248. (46) Hirai, K.; Itoh, T.; Tomioka, H. Persistent Triplet Carbenes. Chem. Rev. 2009, 109, 3275−3332. (47) Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F.; Schütz, J. When and How Do Diaminocarbenes Dimerize? Angew. Chem., Int. Ed. 2004, 43, 5896−5911. (48) Reisenauer, H. P.; Romanski, J.; Mloston, G.; Schreiner, P. R. Dimethoxycarbene: Conformational Analysis of a Reactive Intermediate. Eur. J. Org. Chem. 2006, 2006, 4813−4818. (49) Schreiner, P. R.; Reisenauer, H. P.; Romanski, J.; Mloston, G. [1,3]Dithian-2-ylidene. Angew. Chem., Int. Ed. 2006, 45, 3989−3992. (50) Lavallo, V.; Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Synthesis, Reactivity, and Ligand Properties of a Stable Alkyl Carbene. J. Am. Chem. Soc. 2004, 126, 8670−8671. (51) Despagnet-Ayoub, E.; Solé, S.; Gornitzka, H.; Rozhenko, A. B.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. (Phosphino)(aryl)carbenes: Effect of Aryl Substituents on Their Stabilization Mode. J. Am. Chem. Soc. 2003, 125, 124−130. (52) Calculation of NMR and EPR Parameters: Theory and Applications; Kaupp, M., Bühl, M., Malkin, V. G., Eds.; Wiley VCH: Weinheim. (53) Alder, R. W.; Blake, M. E.; Oliva, J. M. Diaminocarbenes; Calculation of Barriers to Rotation About Ccarbene−N Bonds, Barriers to Dimerization, Proton Affinities, and 13C NMR Shifts. J. Phys. Chem. A 1999, 103, 11200−11211. (54) Wrackmeyer, B. Calculated NMR Parameters (Chemical Shifts and Coupling Constants) of Cyclic C4H2 and C4H4 Molecules Containing Carbene Centers, and of Some of Their Boron Analoga, Using Density Functional Theory (DFT). Z. Naturforsch., B 2004, 59, 37−43. (55) Bühl, M.; van Mourik, T. NMR Spectroscopy: QuantumChemical Calculations. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2011, 1, 634−647. (56) Helgaker, T.; Jaszuński, M.; Ruud, K. Ab Initio Methods for the Calculation of NMR Shielding and Indirect Spin−Spin Coupling Constants. Chem. Rev. 1999, 99, 293−352. (57) Rozhenko, A. B.; Trachevsky, V. V. Specificity of 13C NMR Shielding Calculations in Thiocarbonyl Compounds. Phosphorus Sulfur Silicon Relat. Elem. 2009, 184, 1386−1405. (58) Rozhenko, A. B.; Povolotskii, M. I.; Schoeller, W. W. Conjugation in Phosphabutadienes: Ab Initio Investigation and NMR Spectral Manifestation: II. Magnetic Shielding in Iminophosphines and Heterobutadienes Derived from Them. Russ. J. Gen. Chem. 2004, 74, 500−514. (59) Tapu, D.; Dixon, D. A.; Roe, C. 13C NMR Spectroscopy of “Arduengo-Type” Carbenes and Their Derivatives. Chem. Rev. 2009, 109, 3385−3407.

(60) Solé, S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. (Amino)(aryl)carbenes: Stable Singlet Carbenes Featuring a Spectator Substituent. Science 2001, 292, 1901−1903. (61) Otto, M.; Conejero, S.; Canac, Y.; Romanenko, V. D.; Rudzevitch, V.; Bertrand, G. Mono- and Diaminocarbenes from Chloroiminium and -amidinium Salts: Synthesis of Metal-Free Bis(dimethylamino)carbene. J. Am. Chem. Soc. 2004, 126, 1016−1017.

1488


On the Stability of Perfluoroalkyl-Substituted Singlet Carbenes: A

Recommend Documents