Optimization of the Cyclic Cross-Hyperconjugation in 1,4

Article pubs.acs.org/Organometallics

Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa-2,5-dienes Rikard Emanuelsson,†,§ Aleksandra V. Denisova,†,§ Judith Baumgartner,‡ and Henrik Ottosson*,† †

Department of Chemistry-BMC, Uppsala University, Box 576, 751 23 Uppsala, Sweden Institut für Chemie, Karl Franzens Universität Graz, Stremayrgasse 9, A-8010 Graz, Austria

‡

S Supporting Information *

ABSTRACT: Cyclic cross-hyperconjugation can exist to variable extents in 1,4ditetrelcyclohexa-2,5-dienes, i.e., all-carbon cyclohexa-1,4-dienes and 1,4-disila/ digerma/distanna/diplumbacyclohexa-2,5-dienes. In this study we first use density functional theory (DFT) computations to optimize the conjugation strength by seeking the optimal atom E and substituent group E′Me3 in the two saturated E(E′Me3)2 moieties (E and E′ as the same or different tetrel (group 14) elements). We reveal that the all-carbon cyclohexadienes with gradually heavier E′Me3 substituents at the two saturated carbon atoms display significant crosshyperconjugation. The first electronic excitations in these compounds, which formally have two isolated CC bonds, are calculated to reach wavelengths as long as 400 nm (excitation energies of 3.1 eV). These transitions are mostly forbidden, and the lowest allowed transitions are found at 387 nm (3.2 eV). The silicon analogues are also cross-hyperconjugated, while a decline is observed in the 1,4-digerma/ distanna/diplumbacyclohexa-2,5-diene. Experiments on two substituted 1,4-disilacyclohexa-2,5-dienes confirm the effect of the E′Me3 substituents, with regard to both electronic excitations and geometries as determined by UV absorption spectroscopy and X-ray crystallography, respectively. At the end, we reveal through computations how electron-donating and electron-withdrawing substituents at the CC double bonds influence the electronic properties of the all-carbon ring. We find that the first calculated excitation, which is forbidden, can be shifted to 440 nm (2.83 eV). This shows to what extent cyclic cross-hyperconjugation can affect the electronic and optical properties of a compound with two formally isolated CC double bonds.

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INTRODUCTION The conjugation concept, which describes delocalization of electrons across a molecule, is well established in chemistry, and it is a fundamental part of the organic chemistry curriculum. The conjugation between π-bonded molecular fragments is without doubt the most thoroughly explored and well-understood conjugation type. It is known that π-conjugation, in addition to the regular linear (or through) conjugation, also can be crossconjugated1,2 and in some rare cases also omniconjugated.3 A cross-conjugated compound consists of three unsaturated segments (A−C in Figure 1), between which there are two strong, linearly conjugated interactions involving interactions between

segments A and B and between B and C, respectively, while the third interaction between A and C is only weak. Omniconjugated compounds are branched, similar to cross-conjugated compounds, yet all parts are linearly conjugated. However, it should be noted that conjugation, or rather electron density delocalization, is not limited to π-bonded molecules but can also be found in pure σsystems,4,5 as well as between saturated (σ-bonded) groups with unsaturated π-systems. This last concept, named hyperconjugation, was pioneered by Mulliken in the 1930s and 1940s,6,7 and it is the interaction between a local π(ER2) or π(ER3) orbital of a saturated ER2 or ER3 segment and a π(CC) orbital of an unsaturated segment.8 The enhancement of hyperconjugation through organometallic substituents,9,10 as well as the conjugation between silicon and π-conjugated rings, has previously been reported.11,12 In addition, the hyperconjugative interaction across a saturated segment between two π-systems has been studied by Hoffmann and co-workers, who called this hyperconjugative interaction through-bond conjugation.13,14 We have recently shown that the interaction between the two π systems can be tuned by incorporation of an ER2 segment (E = C, Si, R = σ-electronwithdrawing group (σ-EWG), σ-electron-donating group (σ-EDG)) between these molecular segments. This is in line with what has

Figure 1. Examples of different conjugation topologies: (top) three different π-conjugation topologies; (bottom) conjugation across saturated segments. © 2014 American Chemical Society

Received: February 21, 2014 Published: June 10, 2014 2997

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been shown earlier for the hyperconjugative donor−acceptor behavior of C−X bonds15,16 and in cyclic systems with conjugated double bonds.17−19 However, we argue that when the interaction is strong the saturated segment behaves as a geminally connected C C double bond inserted between two π-systems; we call this interaction cross-hyperconjugation, as it is valence isolobal with regular cross-π-conjugation (Figure 2).20 It should be especially

Figure 3. Qualitative molecular orbital diagram for the highest few occupied orbitals, displaying the interaction between the fragment orbitals 2 × π(CC) and 2 × π(ER2) resulting from symmetryadapted contributions of the π(ER2) and the π(CC) local orbitals. The arrow indicates that the orbital energies, and therefore the interaction strengths between different orbitals, could be varied with different tetrel elements and substituents on the ER2 fragment. That is, electropositive E′Me3 groups raise the energy of the 2 × π(ER2) fragment orbitals (R = E′Me3), which mix with 2 × π(CC) so that the energy of the HOMO (2b1u) is raised. The energies of the 2 × π(ER2) fragment orbitals can be varied to such an extent that the 2 × π(ER2) fragment orbitals are higher in energy than the 2 × π(CC) fragment orbitals (vide infra).

Figure 2. Schematic displaying the valence isolobal analogy between a geminally connected CC double bond and an ER2 group. In the local π(ER2) orbital the two lobes at the R substituents approximate the pπ atomic orbital of the right C atom of the local π(CC) orbital.

strong when R = σ-EDG, as this polarizes the local π(ER2) orbital of the saturated segment toward the E atom of the ER2 group. The compound classes that we previously investigated are bis(phenylethynyl)silanes and -methanes as well as 1,4-disilacyclohexa-2,5-dienes substituted at the 1- and 4-positions.20−22 In the latter compound class, the cyclic aspect of crosshyperconjugation can be explored, since two ER2 segments are joined by two CC double bonds, and we earlier achieved the strongest cross-hyperconjugation with R = SiMe3. More specifically, 2,3,5,6-tetraethyl-1,1,4,4-tetrakis(trimethylsilyl)1,4-disilacyclohexa-2,5-diene had a first allowed absorption maximum at a wavelength of 273 nm (excitation energy 4.54 eV): i.e., exceptionally long (low) for a molecule with two formally isolated double bonds. The corresponding calculated transition was found at 276 nm (TD-PBE0/6-31+G(2d)//B3LYP/ 6-31G(d) level), while a symmetry-forbidden transition at 311 nm was also identified. Herein we now explore the possibilities for further tuning of the electronic properties, and we build on (i) the results from the study on the 1,4-disilacyclohexa-2,5-dienes and (ii) the study on bis(phenylethynyl)silanes and -methanes, for which also other group 14 element E′Me3 groups than silyl were investigated as substituents.20−22 Our earlier observations are in line with qualitative molecular orbital analysis, where more electropositive groups raise the energy of the π(ER2) group orbital to better match the local π(CC) orbitals (Figure 3). However, local orbital overlap also has an effect, and we have found that E as a carbon atom with electropositive substituents leads to a compound which is more similar to a cross-π-conjugated compound than a compound with E = Si having the same substituents.20 Indeed, studies in the early 1980s by Bock and Kaim, which primarily focused on radical cations, revealed that trialkylsilyl substituents at the saturated carbon atoms in cyclohexa-1,4-diene gave low first vertical ionization energies (7.0 eV), indicating strong hyperconjugative interaction also in the all-carbon ring.23 In this computational and experimental study we investigate all possible combinations of E and E′ as the tetrel elements C, Si, Ge, Sn, and Pb in order to probe the scope and limitations of the cross-hyperconjugation strength in 1,1,4,4-tetrasubstituted 1,4-ditetrelcyclohexa-2,5-dienes. The aim is to identify

new synthetic targets for the organic and molecular electronics fields. The compounds were named according to the nomenclature E′EMe3 so that, for example, the compound with E = C and E′Me3 = SiMe3 is labeled CSiMe3. To test our computed results against experiments, we also synthesized and investigated the spectroscopic properties of the tetrakis(trimethylsilyl)- and tetrakis(trimethylgermyl)-substituted 1,4-disilacyclohexa-2,5-dienes, both of which have ethyl groups at the CC double bonds. These two compounds are labeled SiSiMe3Et and SiGeMe3Et, respectively.

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RESULTS AND DISCUSSION First, we present a computational study on the scope and limitations of cross-hyperconjugation, where variations in geometries and orbital energies as well as excitation energies are discussed and analyzed in relation to the choice of E and E′Me3. Thereafter, the experimental results from UV absorption and X-ray structural investigations of two compounds are analyzed in relation to our computed results. Finally, we investigate how the cross-hyperconjugation can be further optimized through the choice of substituents at the two CC double bonds, when based on the cycle with the E(E′Me3)2 segments that lead to the strongest crosshyperconjugation. Quantum Chemical Calculations. The geometric aspect of conjugation, i.e., the elongation of double bonds and shortening of single bonds, can be described through different resonance structures, and this is especially instructive for 2998

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π-bonded compounds. However, hyperconjugation can also be described in this way by the use of bond/no-bond resonance structures, and in the 1,4-ditetrelcyclohexa-2,5-diene case this results in several different resonance structures, as shown in Figure 4. Here, structure I is the standard Lewis structure, while

Figure 5. CC double bond lengths of the 1,4-ditetrelcyclohexa-2,5dienes with different E′Me3 substituents. Results are from PBE0/ LANL2DZdp calculations.

Figure 4. Examples of the possible resonance structures for the 1,1,4,4tetrasubstituted-1,4-ditetrelcyclohexa-2,5-dienes displaying the bond (I) and no-bond (II and III) resonance structures. There are in total one structure of type I, four structures of type II, and eight structures of type III. The corresponding structures (I′−III′) for a cross-πconjugated p-xylylene, which has two exocyclic CCX2 moieties instead of two E(E′R3)2 groups, are also shown.

we expect structures II and III to influence the compounds when the element E′ is more electropositive than E, leading to an E−E′ bond polarization. Structure III should, together with structures I and II, contribute in compounds that are strongly cross-hyperconjugated. On the other hand, if one has the opposite relationship between the electronegativities of elements E and E′, then positive charge will, to various extents, be delocalized at the ring atoms, leading to a corresponding set of resonance structures with charge distributions which are reversed to those of structures II and III. Compounds influenced by strong cross-hyperconjugation would, according to the resonance structures of Figure 4, exhibit longer endocyclic CC double bonds and shorter endocyclic C−E single bonds than those with weaker or no such conjugation. Our earlier investigations have indicated that more electropositive E′Me3 groups enhance the conjugation, and therefore, we expect the bond lengths to vary accordingly with longer CC double bonds and shorter E−C single bonds when going from tBu to PbMe3 substituents within a compound class with constant E. The results presented below come from PBE0/LANL2DZdp calculations; however, the geometric trends are reproduced with the M06 and ωB97XD methods (see Computational Methods and the Supporting Information). From Figures 5 and 6 this trend is clearly seen for E = C, Si, and the effect is largest for the all-carbon rings. Both the CC and E−C bond lengths are influenced, with the effect being the largest for the latter bond. On the other hand, the CC and E−C bond length variations for the compounds with E = Ge are minute, regardless of substituent. Interestingly, the effect for the cycles with E = Sn, Pb are reversed in comparison to those with E = C, Si, with slightly shorter CC and longer C−E bonds when going from E′Me3 = tBu to PbMe3. Using natural resonance theory (NRT) as implemented in the NBO6 program, we also calculated the NRT bond orders. These bond orders are derived from the contributing resonance

Figure 6. E−C bond length differences of the 1,4-ditetrelcyclohexa2,5-dienes with different E′Me3 substituents in comparison to the tBusubstituted analogue for each compound class (E constant). Results are from PBE0/LANL2DZdp calculations.

structures and weighted according to their importance. The corresponding NRT analysis of cross-π-conjugated p-xylylene gives bond orders of 1.85 for the internal and 1.86 for the external CC double bonds and 1.06 for the C−C single bonds (Figure 7): i.e., a tendency for equalization of the bond

Figure 7. Calculated NRT bond orders of p-xylylene and (Z)-hexa1,3,5-triene at the PBE0/LANL2DZdp level.

orders which implies conjugation. In comparison, (Z)-hexa1,3,5-triene has NRT bond orders of 1.84 and 1.93 for the internal and external CC double bonds and 1.07 for the C−C single bonds, respectively. The bond orders for the various 1,4-ditetrelcyclohexa-2,5-dienes are given in Table 1. For the all-carbon rings these show in the gradual decrease (increase) in the bond orders of the CC double (C−C single) bonds when progressively heavier E′Me3 groups are used as substituents at the two sp3-hybridized carbon atoms. Notably, in the ring of CPbMe3 the bond orders are clearly approaching those of p-xylylene. The same trend, albeit weaker, is evident in the 1,4-disilacyclohexa-2,5-dienes. The weaker effect for the 1,4-disilacyclohexa-2,5-dienes in comparison to that for the all-carbon cyclohexadienes should be an effect of the poorer π-type overlap between local π(CC) and π(SiR2) 2999

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Table 1. Calculated NRT Bond Orders for the CC and C−E Bonds of the Ringsa

a

One of each bond type is displayed in red. Results are from from NRT calculations at the PBE0/LANL2DZdp level.

orbitals than between local π(CC) and π(CR2) orbitals as a result of better size match in the latter case, similar to what is manifested in silenes in comparison to alkenes.24 Descending group 14 further, the effects are nonexistent or reversed for the 1,4-digerma/distanna/diplumbacyclohexa-2,5-dienes, revealing very limited cross-hyperconjugation in these compound classes. The highest occupied molecular orbitals (HOMOs) are of the same character (2b1u, Figure 3) for all of the compounds, regardless of atom E and substituent E′Me3. They are also analogous to the HOMO of p-xylylene (Figure 8). The energies of these orbitals (εHOMO) show the effect of gradually heavier E′Me3 groups at the E atoms, as such replacement raises εHOMO (Figure 9). The most pronounced shift to higher energies of HOMO is found for E = C (ΔεHOMO = 2.03 eV), indicating gradually stronger hyperconjugative overlap between the local π(CR2) and π(CC) orbitals when the R = E′Me3 become heavier. However, the shift ΔεHOMO is only half of this when E = Si and is much smaller for the heavier compounds. More precisely, when E is either Si, Ge, Sn, or Pb, then the change of the E′Me3 group from SiMe3 to PbMe3 gives only modest variations in εHOMO from −5.90 to −5.65 eV (Figure 9). In contrast, CSiMe3 is calculated to have a somewhat higher εHOMO (−5.40 eV) than each of its analogues with E = Si−Pb, and εHOMO is raised even further for CGeMe3, CSnMe3, and CPbMe3. Although the absolute values and differences should be treated with caution, the trend is clear with large substituent effects on εHOMO for the all-carbon cyclohexadiene and much smaller effects for the heavier 1,4-ditetrelcyclohexa-2,5-dienes. For a comparison of the eight π-orbitals of p-xylylene, CSnMe3, and the parent molecule cyclohexa-1,4-diene see Figure S3 in the Supporting Information. The orbital next below HOMO (the HOMO-1 orbital) is of π-character, except for PbCMe3, PbSiMe3, and PbGeMe3, for which one instead finds one or two σ-orbitals. The HOMO-1 orbitals of the carbon compounds are of 1b3g character with electron density on the CC double bonds, while for E = Si, Ge, Sn, Pb, the orbital is of 1b2g character with density across the saturated E(E′Me3)2 segment. Tracking the 1b3g-symmetric orbital (the CC double-bond orbital) through the different compounds reveals that its energy changes minutely with substituent, and its designation as HOMO-n goes to larger n when descending group 14 because heavier atoms E provide highenergy σ-orbitals. The 1b2g orbital, on the other hand, shows the effect of the gradually more electropositive substituent. For the carbon compounds the 1b2g orbital varies by 1.0 eV, from CtBu to CPbMe3, while there are smaller shifts for the heavier species (Si, 0.6 eV; Ge, 0.5 eV; Sn, 0.3 eV; Pb, 0.2 eV). This again shows the larger substituent effect which heavy E′Me3 groups have in the all-carbon cyclohexadienes in comparison to

Figure 8. Frontier orbitals and frontier orbital energies for p-xylylene, CSiMe3, and SnSiMe3 calculated at the PBE0/LANL2DZdp level. Hydrogen atoms are omitted for clarity. For the full list of all πsymmetric orbitals of p-xylylene and CSnMe3 see Figure S4 in the Supporting Information.

the 1,4-digerma/distanna/diplumbacyclohexadienes. Here it should be mentioned that a comparison between all eight π-orbitals of p-xylylene and CSnMe3, and with those of cyclohexa1,4-diene, is found in Figure S4 of the Supporting Information. 3000

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consist mainly of the HOMO → LUMO excitation. The second excitation is allowed and is composed mainly of the HOMO → LUMO+1 excitation. This second excitation is also calculated to be stronger when E = Si, Ge than when E = C. Here, the same trend as in the geometry and NRT calculations is found, with the all-carbon compounds being more tunable than the compounds with E = Si, Ge. Notably, CSnMe3 and CPbMe3 reach forbidden calculated first excitations with energies as low as 3.1 eV (400 nm), while the second lowest excitations are allowed and found at 3.55 and 3.23 eV (349 and 380 nm), respectively. Somewhat higher forbidden first excitations are found for the analogous disilacyclohexadienes (at ∼3.7 eV (335 nm) for both SiSnMe3 and SiPbMe3) with the first allowed transitions at 4.3 and 3.9 eV (286 and 317 nm), respectively. The 1,4-digermacyclohexa-2,5-dienes have the first forbidden excitation 0.3 eV higher in energy and the first allowed excitation 0.1 eV above that of the analogous compounds with E = Si. Third, the configurations and order of the electronic excitations for the distanna- and diplumbacyclohexadienes (E = Sn, Pb) are different from those of the other compounds, and direct comparisons are therefore more difficult. However, while the trend for these compounds is to have excitation energies higher than those calculated for the analogous digermacyclohexadienes, there are exceptions when E′Me3 = SnMe3, PbMe3. In these, the calculated allowed excitation can reach energies down to 3.9 eV (320 nm). Thus, with a proper choice of substituents the 1,4-ditetrelcyclohexa-2,5-dienes can reach low calculated excitation energies. When E = C, Si, Ge, the amount of cross-hyperconjugation associated with each structure is a good indicator of the optical

Figure 9. Energies of the highest occupied molecular orbitals (εHOMO) for the 1,1,4,4-tetrasubstituted 1,4-ditetrelcyclohexa-2,5-dienes with different E atoms and E′Me3 substituents. Results are from PBE0/ LANL2DZdp calculations.

Next, we investigated the optical properties of these compounds at the TD-PBE0/LANL2DZdp//PBE0/LANL2DZdp level. In this context it should be noted that p-xylylene has its first calculated excitation at 4.16 eV (298 nm), which is an allowed transition (f = 0.770). The excitation is mainly described as the HOMO → LUMO excitation and is in good agreement with experiments on p-xylylene derivatives (290−294 nm).25 The TD-DFT calculations on the 1,4-ditetrelcyclohexa-2,5dienes reveal some general trends. First, for all compounds with E′Me3 = tBu, a gradually heavier E atom lowers the calculated first excitation energies (5.7 eV in CtBu to 4.6 eV in PbtBu; Table 2). Second, in compounds with E = C, Si, Ge and with E′Me3 ≠ tBu, the electronic transitions are of similar character, as we calculated earlier for SiSiMe3Et:22 i.e., with the first excitation symmetry forbidden (or nearly so) and calculated to

Table 2. Calculated First and Second Electronic Excitations of the EE′Me3 Compoundsa

a

Excitation energies (in eV), oscillator strengths ( f), and the main configurations of the excitations are reported. Results are from TD-PBE0/ LANL2DZdp//PBE0/LANL2DZdp calculations. The main configurations that describe the excitation are given (when possible) with H → L corresponding to HOMO → LUMO, H → L+1 corresponding to HOMO → LUMO+1, etc. 3001

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of SiSiMe3Et22 confirms this. The CC double bonds in SiGeMe3Et are slightly longer than in SiSiMe3Et (1.350(2) vs 1.3456(18) Å, Figure 10), and the Si−C single bonds are shorter (1.8762(15) Å (C2 symmetry) vs 1.8815(13)/1.8845(13) Å (Ci symmetry)). This elongation/shortening in SiGeMe3Et takes place despite reduced steric crowding resulting from the GeMe3 groups in comparison to that from the SiMe3 groups in SiSiMe3Et due to the Si−Ge bonds being longer than Si−Si bonds (2.3990(6) vs 2.3685(6) Å). For further crystal structure data see the Supporting Information. UV Absorption Spectroscopy. In comparison to SiSiMe3 the TD-DFT calculations indicate that SiGeMe3 has slightly (∼0.1 eV, 5 nm) red shifted absorptions. Here it is important to note that SiSiMe3 and SiGeMe3, which were investigated computationally, have no substituents at the double bonds, in contrast to SiSiMe3Et and SiGeMe3Et investigated experimentally. Previously we observed from computations that alkyl groups at the CC bonds produce small red shifts (∼0.2 eV, 10 nm), and both SiSiMe3Et and SiGeMe3Et display such red shifts in comparison to the calculated values for SiSiMe3 and SiGeMe3. In the experimental UV spectra shown in Figure 11

properties, with a more conjugated system having lower calculated excitation energies and vice versa. However, for the heavier distanna- and diplumbacyclohexadienes this relationship is less straightforward, with allowed transitions to orbitals associated with the heavy E(E′R3)2 segments. Synthesis. To provide support for our computed findings, we examined two of the compounds experimentally. In addition to our previously reported 2,3,5,6-tetraethyl-1,1,4,4-tetrakis(trimethylsilyl)-1,4-disilacyclohexa-2,5-diene (here labeled SiSiMe3Et),22 we synthesized the 2,3,5,6-tetraethyl- and tetrakis(trimethylgermyl)-substituted analogue SiGeMe3Et (Scheme 1). Scheme 1. Synthetic Route to the 1,4-Disilacyclohexa-2,5dienes SiSiMe3Et and SiGeMe3Et

Using the same synthetic methodology as for SiSiMe3Et, SiGeMe3Et was synthesized by mixing chlorotrimethylgermane, 1,1,4,4-tetrachlorodisilacyclohexa-2,5-diene, and lithium at −78 °C. SiGeMe3Et could be isolated after workup and recrystallization from chloroform. Unfortunately, no stannyl-substituted product could be detected when using this methodology with trimethylstannyl chloride. Several attempts using different conditions, including higher temperature, other stannyl sources (Bu3SnCl or tricyclohexylstannyl chloride), or a change in addition order (first formation of R3SnLi, with subsequent addition to the chlorosilane at −78 °C), were also unsuccessful. Crystal Structure Comparison. The geometric features that would indicate conjugation are, as noted above, longer C C double bonds and shorter Si−C single bonds in SiGeMe3Et than in SiSiMe3Et (Figure 10), a trend which is found in the

Figure 11. Experimentally recorded UV absorption spectra of SiSiMe3Et and SiGeMe3Et in cyclohexane.

the computed lower excitation energy for the first visible transition of the compound with E′Me3 = GeMe3 is confirmed (SiSiMe3Et, 273 nm (4.54 eV); SiGeMe3Et, 277 nm (4.48 eV)). A comparison with our calculated results shows agreement, as a first calculated transition at ∼300 nm (4.1 eV) is forbidden while the wavelength for the second excitation, which is allowed (SiSiMe3, 261 nm (4.74 eV); SiGeMe3, 273 nm (4.54 eV)), is in accord with the experimental findings. This is despite the obvious problems of comparing an idealized vertical excitation calculated in the gas phase with an experiment performed in solution.26 Thus, both the geometric and optical trends from the calculations have experimental support. Effects of Substitution at the CC Bonds. We also computed the 2,3,5,6-tetrafluoro- and 2,3,5,6-tetrastannylsubstituted cyclohexadienes (CSnMe3F and CSnMe3SnH3, respectively) in order to examine if substitution at the CC double bonds further enhances the cross-hyperconjugation strength and causes red shifts of the electronic transitions. The σ-electron-withdrawing fluoro and the σ-electron-donating stannyl groups represent limiting points of electronic modulation without significantly extending the π-system, and investigations of the two compounds should enable a guidance for future synthetic efforts. The strongly cross-hyperconjugated CSnMe3 was chosen as a template for these studies, as the

Figure 10. Crystal structure of SiGeMe3Et. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si(3)−C(7) 1.8769(15), Si(3)−C(8) 1.8762(15), C(7)−C(8′) 1.350(2), Ge(1)− Si(3) 2.3990(6); C(8)−Si(3)−C(7) 112.48(7). Corresponding bond lengths (Å) and angles (deg) in SiSiMe3Et:16 Si(3)−C(7) 1.8815(13), Si(3)−C(8) 1.8845(13), C(7)−C(8′) 1.3456(18), Siexocyclic−Si(3) 2.3685(6); C(8)−Si(3)−C(7) 111.66(6).

computed results. Indeed, a comparison between the crystal structure of SiGeMe3Et and our previously reported structure 3002

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Si(E′Me3)2 fragments might serve as substitutes to access optically interesting compounds. Endocyclic tetrel elements (E) display less cross-hyperconjugation with CC π-bonds, presumably due to less orbital overlap, as indicated by geometry and NRT calculations. Moreover, our findings are likely not limited to 1,4ditetrelcyclohexa-2,5-dienes but should be applicable to a range of π-conjugated linear and cyclic structures and could be used to design new structures of interest for organic light-emitting diodes (OLEDs) and other organic electronics applications. For example, silafluorene-based polymers have recently been investigated for such applications, as they have a narrow blue/violet emission in the solid state,27−33 and our findings reported herein could potentially be useful for the design of similar types of polymers.

all-carbon cycle displayed a very large effect with regard to the E′Me3 substituents while still being synthetically realistic.

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EXPERIMENTAL SECTION

Computational Methods. All computations were performed with the Gaussian 09 program package, revision C.01 or D.01.34 The structures discussed were optimized at the PBE0/LANL2DZdp hybrid density functional theory level,35−37 and frequency calculations were performed at the same level to verify that stationary points correspond to minima. We chose to use this basis set for all compounds so as to be able to compare the relative differences within and between compounds with different tetrel elements. In order to demonstrate that the results are not functional dependent, the CE′Me3 and SnE′Me3 series were also investigated with two other functionals: M0638,39 as well as ωB97XD,40−43 which reproduced the PBE0 trends (see Figures S1−S3 in the Supporting Information for the full comparison). Time-dependent density functional theory (TD-DFT) calculations were performed as implemented in Gaussian 09, using TD-PBE0/LANL2DZdp on the optimized PBE0 geometries.44−46 The five lowest excitations were calculated. Furthermore, natural resonance theory (NRT) analyses,47−49 as implemented in the NBO6 program,50 were carried out at the PBE0/LANL2DZdp level. Here, we analyze the NRT bond orders, which are weighted according to the importance of the contributing resonance structures and therefore give summaries of all contributing structures. General Remarks and Experiments. All reactions involving airsensitive compounds were carried out under an atmosphere of dry argon using standard Schlenk techniques. NMR spectra were recorded on a Varian Unity 500 (1H at 499.97 MHz) or an Agilent MR-400 spectrometer (1H at 399.97 MHz and 13C at 100.57 MHz) at 25 °C. Chemical shifts are reported in ppm referenced to tetramethylsilane via the residual solvent (CHCl3, 1H at 7.26 ppm and 13C at 77.0 ppm). UV absorption spectra were recorded in cyclohexane solution with a quartz cell of 1.0 cm length. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC 985055. Synthesis of 2,3,5,6-Tetraethyl-1,1,4,4-tetrakis(trimethylgermyl)1,4-disilacyclohexa-2,5-diene (SiGeMe3Et). To a mixture of 1,1,4,4tetrachloro-2,3,5,6-tetraethyl-1,4-disilacyclohexa-2,5-diene (0.50 g, 1.38 mmol) and trimethylgermyl chloride (1.85 mL, 10.86 mmol) in THF (15 mL) was added Li (0.075 g, 11.04 mmol) at −78 °C. The reaction mixture was warmed to room temperature overnight upon continued stirring. After 22 h, more trimethylgermyl chloride (0.50 mL) was added and the reaction mixture was stirred for an additional 1 h. Thereafter, all volatile substances were removed under reduced pressure. The remaining solid material was dissolved in DCM (15 mL) and washed with water (3 × 10 mL). The organic phase was dried over MgSO4. After filtration, the solvent was removed in vacuo. The residue was dissolved in CHCl3 and filtered through Celite, which removed more insoluble material. After removal of solvent a white solid was obtained (0.30 g, 0.47 mmol, 31%); subsequent crystallization from CHCl3 (slow evaporation) yielded analytically pure SiGeMe3Et as white crystals (mp 240 °C dec). 1H NMR (CDCl3, 499.97 MHz) δ 2.12 (q, 3J = 7.5 Hz, 8H), 1.01 (t, 3J = 7.5 Hz, 12H), 0.29 (s, 36H). 13C NMR (CDCl3, 100.57 MHz) δ 152.0, 25.9, 15.3, 0.4. UV/vis (cyclohexane): λmax/nm (ε/104 dm3 mol−1 cm−1) 277 (2.0), 237 (1.8). Anal. Calcd for C24H56Ge4Si2 (691.43): C, 41.69; H, 8.16; Ge, 42.02; Si, 8.12. Found: C, 41.55; H, 8.25.

In CSnMe3F the optical properties were altered markedly in comparison to CSnMe3 because the transition energy of the first excitation was blue-shifted from 3.12 to 3.82 eV. Notably, this transition is still the analogous HOMO → LUMO excitation as in unsubstituted CSnMe3. Moreover, the tetrafluoro substitution in CSnMe3F resulted in shortening of the CC bonds to the same length as found in CtBu (1.344 Å). The tetrastannyl-substituted CSnMe3SnH3, on the other hand, gave the opposite effect with a first forbidden calculated transition at 2.83 eV (440 nm) (analogous to that of CSnMe3) and longer formal CC double bonds (1.366 vs 1.352 Å in CSnMe3). Here it should be noted that the allowed excitation in CSnMe3SnH3 shifts to 3.45 eV (359 nm): i.e., 0.1 eV lower than in CSnMe3. Interestingly, tetrafluoro as well as tetrastannyl substitution lowers εHOMO from −4.70 eV in CSnMe3 to −5.1 eV in both CSnMe3F and CSnMe3SnH3. However, the tuning of the excitation energies is caused by a lowering of the εLUMO value in the case of tetrastannyl substitution (−1.25 vs −0.43 eV), while with tetrafluoro substitution εLUMO remains at a constant level in comparison to CSnMe3 (−0.42 vs −0.43 eV). Clearly, there are possibilities for measurable tuning of the properties of cyclohexadienes. However, the compounds are synthetically challenging. In this regard, it is noteworthy that benzannulation at the double bonds of CSnMe3, which produces a tentatively more realistic synthetic target, results in first and second excitations at 3.47 and 3.52 eV, respectively. It seems that benzannulation could be an alternative to reach optically interesting molecules.

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CONCLUSIONS AND OUTLOOK In this study we have shown the potential for small cyclic compounds with two formally isolated π-bonds and two saturated carbon or silicon fragments substituted with electropositive groups to display extensive cyclic cross-hyperconjugation. The calculated geometric parameters show conjugation which could be as strong as in the purely π-conjugated p-xylylene. The calculated optical properties for these compounds indicate that they have low-lying first transitions when E and E′Me3 are properly selected. However, the most promising C(E′Me3)2 fragment remains a synthetic challenge, and 3003

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Organometallics

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Article

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and xyz and CIF files giving a comparison among PBE0, M06, and ωB97XD results, a full comparison of the energies and shapes of the eight π orbitals in p-xylylene, cyclohexa-1,4-diene, and CSnMe3, crystallographic data, NMR spectra, absolute electronic energies, and all computed molecule Cartesian coordinates in a format for convenient visualization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.O.: [email protected]. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Uppsala University for supporting the U3MEC initiative on molecular electronics and the Swedish Research Council (Vetenskapsrådet) for financial support. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for generous allotment of computer time at the National Supercomputer Center (NSC).

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