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Toward Thermodynamically Stable Triplet Carbenes Yumiao Ma* BSJ Institute, Haidian, Beijing, 100084 [email protected] Abstr

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Toward Thermodynamically Stable Triplet Carbenes Yumiao Ma* BSJ Institute, Haidian, Beijing, 100084 Ymma@Bsj-Institute.Top Abstr Toward Thermodynamically Stable Triplet Carbenes Yumiao Ma* BSJ Institute, Haidian, Beijing, 100084 [email protected] Abstract In sharp contrast to the widely studied and applied stable singlet carbenes, only several kinetically persistent triplet carbenes have been studied, and thermodynamically stable triplet carbenes are much less developed. With the Gibbs free energy of C-H bond insertion into methane as a probe, DFT calculations were employed to examine a variety of candidate molecules for stable triplet carbenes. Guided by these calculations, some molecules with significant stability against C-H insertion were designed by fine tuning of geometry and electronic structures. These compounds might be potential candidates for experimental development of stable triplet carbenes. Introduction Pioneered by Guy Bertrand, Armin Arduengo and others, stable singlet carbenes have been well developed since 1980s1-3, and have exhibited great importance as ligands, reagents, etc. On the other hand, stable, or even kinetically persistent triplet carbenes are still scarce. A persistent triplet carbene, 2, 2', 4, 4', 6, 6'-hexabromodiphenylcarbene, was reported by Hideo Tomioka et al in 19954, with a half-life at ~1 s. In 2001, persistent triplet carbene with half-life of 19 min at room temperature was reported5, which was a great breakthrough. Several other kinetically persistent triplet carbenes were also studied, and the half-life ranges from microseconds to a week6, 7. However, thermodynamically stable triplet carbenes are still unknown hitherto, to the best of our knowledge. In this work, a variety of potential candidates for stable triplet carbenes were designed and examined by quantum chemical calculations. Guided by calculations and rational design, some competitive candidates were obtained, which may be helpful for the development of stable triplet carbenes, and for the understanding of the chemical properties related to open shell organic compounds. Figure 1. Summary of this work Results and discussions Figure 2. (a)The probe for stability (G(ins)), the first generation structures and its G(ins) value (in kcal/mol). Values inside brackets were obtained at DLPNO-CCSD(T)-F12/cc-pVDZ-F12//M06- 2X/def2-SVP level, whereas others were obtained at M06-2X/def2-SVP level. Spin density isosurface of 31 with isovalue at 0.02 is shown in the inner picture. It is known that triplet carbenes are usually prior to undergo insertion into C-H (or other) bonds, or hydrogen abstraction from other compounds, or reaction with dioxygen. In this work, the thermodynamics of insertion into methane, G(ins), was used as a probe for the thermodynamical stability. A stable triplet carbene should exhibit a near-zero or positive insertion free energy. There are two methods to achieve this: to stabilize the carbene itself by enhancing electron delocalization, or to destabilize the insertion product. The design of first generation of candidates originates from the stable Blatter’s radical. A series of carbon analogues of Blatter’s radical were examined (1 – 5 in Figure 2) at M06-2X/def2-SVP level. High accuracy calculations at DLPNO-CCSD(T)-F12/cc-pVDZ-F12 were also performed for 1 and 2, showing that the DFT calculations have been perfectly accurate, despite the fact that the def2-SVP basis set seems moderate. Except 1, all compounds exhibited triplet ground state. It can be seen that 1 shows similar thermodynamics stability with a persistent carbene (di(2,4,6- trichlorophenyl)carbene), whereas further enlarging conjugation system provides extra stabilization. It was originally expected that conjugating group (-Ph) or polarizable group (-SMe) at C2 or C6 sites might provide stabilization toward the unpaired electron at C1 by weak interactions. Unfortunately, the effect of this substitution pattern is negligible. Figure 3. The G(ins) values for several “second-generation” candidates (in kcal/mol). The ground states for insertion products were shown in brackets, if not close-shell singlet (CSS). OSS means open-shell singlet. The DLPNO-CCSD(T)-F12/cc-pVDZ-F12//M06-2X/def2-SVP value for 6 was shown in bracket. In order to provide further stabilization toward the triplet carbenes, the second generation of structures was designed (Figure 3). The basic idea is to introduce an aromatic ring that would be lost upon insertion into C-H bond. Following this idea, the Gibbs free energy of insertion for compound 6 has been tuned to -35.3 kcal/mol. A fused quinone ring significantly stabilized the triplet carbene 7. It can be seen that 7 and its insertion product 7HMe adopt significantly different geometries (Figure 4). The ring A bearing two-coordinated carbon atom deviates from the plane consisted of ring B and C in 7, whereas in 7HMe (with closed shell singlet (CSS) as ground state) the coplanarity between A and B is largely kept, although serious distortion is introduced into both ring B and C. Thus it is natural to propose that an extra rigid fused ring may destabilized the inserted structure. Indeed, the Gibbs free energy change for insertion was further tuned into -20.9 kcal/mol for 8. Since the spin density of 7 shows that unpaired electron mostly distributes at the O1 site (Figure 4), it is safe to introduce further modification to O2. Due to the fact that the ring A and B tends to be more coplanar in the inserted structure, the O2 atom was replaced with two methyl group to achieve more geometry constraint (Figure 3d). The resulted structure 11 exhibits an insertion Gibbs free energy of only -15.3 kcal/mol. On the basis of the observations above, some modification was added to 11 in order to obtain more sterically rigid and constraining and thus less negative G(ins). Unfortunately, the bottleneck was soon met when the ground state of insertion product changed from close shell singlet (CSS) to open shell singlet (OSS) upon large distortion. Obviously a new backbone is in need to overcome the last 10 kcal/mol gap between current G(ins) and ideal 0 kcal/mol. Figure 4. The geometries for 7 and its insertion product 7HMe are shown in (a) and (b) respectively. (c) The spin density isosurface of 7 with isovalue at 0.02. (d) A schematic representation for the design of 11 and related structures. Figure 5. (a) G(ins) for the “third generation” candidates (in kcal/mol). The ground states for insertion products were shown in brackets, if not CSS. (b) The geometry and spin density isosurface (at 0.02) of 18. (c) The geometry for 18HMe (CSS state). The third generation of backbone directly comes from the second generation molecules. There are two core designs. The first, the introduction of N-N bond will provide an extra aromatic pyridine ring for triplet carbenes, which is expected to be lost upon insertion (into CSS product). The second, a distant quinone motif promotes intramolecular charge transfer, which is proposed to be beneficial for radical delocalization. The so-designed structure 18 exhibits insertion free energy of only -13.0 kcal/mol, much more positive than its second generation analog 7, proving this strategy effective. The spin density of 19 is very located at the quinone ring C. In contrast to the case of 7, in which spin density strongly tends to distribute at O1 but not O2, both two oxygen atoms bear similar spin density, indicating that the electronic structure of 19 should be better considered as a single electron reduced quinone ring and close-shelled aromatic ring A and D. The aromaticity of ring A and D can be clearly seen in bond length, and is significantly lost after insertion. With 19 as a starting point, several structures were tested in combination with all design strategies mentioned above. Although introduction of electron-withdrawing groups might lead to more favorable electron transfer at the first glance, it actually leads to an open shell singlet state insertion product, and a more negative insertion G(ins). This can be understood considering the fact that too much intramolecular electron transfer will lead to small spatial overlap of the two single electrons, and thus stabilize the triplet state (and the related OSS state) of insertion product. Also too large R2 and R3 lead to insertion products with triplet state as its ground state, in consistent with the fact that ring A and D comes perpendicular for 20 and 21, for which the insertion free energies are much more negative than that expected for an CSS state product. Then came a contradictory. While spin delocalization stabilizes unpaired electron and large R2 and R3 destabilize the CSS state of insertion products, they also stabilize its OSS and triplet state. Another bottleneck appears now. Figure 6. G(ins) for the “modified third generation” candidates (in kcal/mol). Huge efforts were paid to overcome this bottleneck (Figure 6). A seven-member ring was introduced to fix the dihedral angle between A and D (compound 25). Although it could be seen that the conformation of this seven-member ring changed after insertion (see Supporting Information), further conformational constraints failed to improve the G(ins) (27-30). It is proposed that the conformation has very complex and significant influence, although our efforts to understand it was in vain. Electron-withdrawing cyano groups at Ring C again failed to improve the G(ins). Modifications at Ring B also failed, which was proposed to originate from the fact that any substitution at Ring B will change the relative conformation between Ring B and D. The Hirshfeld atom charge8 showed that for the triplet and OSS insertion products, charges on most atoms remains unchanged in comparison with the carbene, except the C2 and C6 (inner page of Figure 6), of which charges get more positive upon insertion.
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