DOI: 10.1002/chem.201402304
Communication
& Coordination Chemistry
Facile Synthesis of Dibenzopentalene Dianions and Their Application as New p-Extended Ligands Takuya Kuwabara,[a] Kazuya Ishimura,[b] Takahiro Sasamori,[c] Norihiro Tokitoh,[c] and Masaichi Saito*[a] Dedicated to Professor Renji Okazaki on the occasion of his 77th birthday
Abstract: Reduction of phenyl(silyl)ethynes with potassium followed by quenching with iodine gave dibenzopentalenes in moderate yields. The intermediates of the reactions, dipotassium dibenzopentalenides, were isolated. The first dibenzopentalene–transition-metal complex was successfully synthesized. The ruthenium atoms are located above the six-membered rings. However, X-ray diffraction analysis and theoretical calculations revealed that the aromatic nature of the five-membered rings was retained. The cyclic voltammetry of the Ru complex revealed two oxidation waves with relatively large separation.
Scheme 2. Previous methods for the synthesis of dibenzopentalene dianions.
lithium salt with a trans-h5 ;h5-mode reported by our group,[5] and the other is a barium salt with an h8-mode, which was recently reported by Xi and co-workers.[6] On the other hand, transition-metal complexes bearing dibenzopentalene ligands remain elusive. Because of its extended p-planar structure, a dibenzopentalene ligand would produce many kinds of coordination modes that cannot be accomplished by using a pentalene ligand. In spite of its unique p-extended system, the chemistry of dibenzopentalene has been much less explored due to its synthetic difficulties.[7] However, since 2009, effective and facile synthetic methods with transition-metal catalysts have successively been reported by several groups.[8] Moreover, the syntheses of dibenzopentalene derivatives without using transition-metal catalysts have also been achieved.[9] Xi’s group reported the surprising transformation of 1,4-dilithio-1,3-butadienes into barium salts of dibenzopentalene dianion (Scheme 2).[6] In contrast, our group has already reported the formation of a dilithium salt of dibenzopentalene dianion 1 a by the reduction of phenyl(triisopropylsilyl)ethyne 2 a with lithium.[5] However, the yield of 1 a was only 8 %, and the main product was 1,4-dilithio-1,3-butadiene. Herein, we report an improved method for the synthesis of dibenzopentalene dianion that has independently been found recently.[10] Furthermore, the synthesis, structure, and electronic properties of the first example of a dibenzopentalene–transition-metal complex are also reported. Reduction of phenyl(triisopropylsilyl)ethyne 2 a with potassium provided a purple solid, which reacted with iodine to give dibenzopentalene 3 a (Scheme 3).[11] Surprisingly, the yield of 3 a was drastically improved from 8 to 59 % by only changing
Coordination chemistry of pentalene dianions has attracted much attention for the last two decades.[1] Transition-metal complexes bearing pentalene ligands exhibit various coordination modes, such as h5 ;h5-coordination with cis or trans fashions,[2] double sandwich,[3] and h8-fashion (Scheme 1).[4] But to the best of our knowledge, metal complexes of dibenzo[a,e]pentalene, which is a p-extended molecule of pentalene, has been limited to only two examples (Scheme 2). One is a di-
Scheme 1. Various coordination modes in pentalene complexes.
[a] T. Kuwabara, Prof. Dr. M. Saito Department of Chemistry, Graduate School of Science and Engineering Saitama University, Shimo-okubo, Sakura-ku, Saitama, 338-8570 (Japan) Fax: (+ 81) 48-858-3700 E-mail: [email protected] [b] Dr. K. Ishimura Theoretical and Computational Chemistry Initiative Institute for Molecular Science Myodaiji, Okazaki, Aichi, 444-8585 (Japan) [c] Prof. Dr. T. Sasamori, Prof. Dr. N. Tokitoh Institute for Chemical Research, Kyoto University Gokasho, Uji, Kyoto 611-0011 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402304. Chem. Eur. J. 2014, 20, 1 – 6
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Communication bonds in the six-membered rings (ca. 1.38–1.46 ) slightly alter compared with those in the five-membered rings (1.44–1.46 ), which suggests that negative charges are delocalized over the five-membered rings. The reason for the drastic improvement of the yields can be attributed to two factors. One is the sizes of alkaline metals. When lithium is used as a reductant, s-cis 1,4-dilithio-1,3-butadiene is mainly formed, as was evidenced by X-ray diffraction analysis,[5, 13] because lithium atom is so small that each of the two lithium atoms can be sandwiched by the two terminal carbon atoms of the butadiene. However, when potassium is used, the formation of s-cis 1,4-dipotassio-1,3-butadiene would be difficult due to the steric repulsion between the two potassium atoms, and instead, s-trans 1,4-dipotassio-1,3-butadiene is mainly formed, which readily undergoes intramolecular cyclization with the evolution of H2 gas to generate 4 (Scheme 4).
Scheme 3. Reduction of phenyl(silyl)ethyne with potassium.
the reductant from lithium to potassium. Furthermore, the corresponding reactions using other phenyl(silyl)ethynes 2 b and c also gave dibenzopentalenes 3 b and c in moderate yields (58 and 40 %, respectively). In sharp contrast, the reduction of 2 b and c with lithium has never given dibenzopentalenes 3 b and c.[5] The generation of dibenzopentalenes 3 indicates that the purple intermediates are dipotassium dibenzopentalenides 4. In fact, compounds 4 a–c were successfully isolated in 35 to 89 % yields, and the molecular structure of 4 a was elucidated by X-ray diffraction analysis (Figure 1). In contrast to the bowshaped structure found in an anion radical of dibenzopentalene,[12] the dibenzopentalene skeleton in 4 a is planar. No C C bond alternation was found in the five-membered rings, suggesting their considerable aromatic nature. Interestingly, a coordination mode in 4 a is different from that found in dilithium salt 1 a. Each of the two potassium atoms in 4 a is situated above nearly the midpoint of C1 and C1# atoms, whereas each lithium atom is located above the Scheme 4. Plausible mechanism for the formation of 4. center of each five-membered ring with h5-coordination in 1 a. In both compounds 4 a and 1 a, the C C Such a mechanism has already been proposed to explain the conversion of 1,4-dilithio-1,3-butadiene to barium dibenzopentalenide.[6] The other factor is higher reactivity of C K bonds than C Li bonds. It is known that a carbanion bearing a potassium as a countercation has nucleophilicity stronger than those bearing a lithium.[14] Therefore, the intramolecular cyclization took place easily on 1,4-dipotassio-1,3-butadiene. This facile access to dibenzopentalene dianions from simple starting materials enables to study their coordination to transition metals. First, reaction of 4 a with [RuClCp*]4[15] (Cp* = h5C5Me5) was investigated, because its pentalene derivative, [trans-(RuCp*)2] pentalene complex, has already been synthesized.[16] In the 1H NMR spectrum of a crude product, signals derived from 4 a disappeared, and two sets of aryl protons were observed in a high field, (A (main): d = 4.40, 4.48, 6.15, Figure 1. ORTEP drawing of 4 a (left) and its top view (right; 50 % probabili6.52 ppm; B (minor): d = 4.27–4.45, 6.32, 6.60 ppm), which sugty). All hydrogen atoms were omitted for clarity. Selected bond lengths []: gests the formation of two different compounds, in which # C1 C1 1.433(3), C1 C2 1.463(2), C2 C3 1.455(2); C3 C4 1.420(2), C3 C8 each of the two six-membered rings in both products coordi1.463(2), C4 C5 1.382(2), C5 C6 1.401(2), C6 C7 1.381(2), C7 C8 1.411(2), nate to ruthenium atoms. However, no crystals were obtained, C8 C1# 1.442(2), K C1 2.9812(15), K C1# 2.9322(16). &
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Communication side of the Cp*. However, compound 5 appears to be not an h6-arene, but an h5-pentadienyl complex, because 5 has four shorter Ru C bonds of 2.17 to 2.23 (Ru1 C4/5/6/7 and Ru2 C12/13/14/15), relatively long Ru C bonds (Ru1 C8 and Ru2 C16; ca. 2.37 ), and very long Ru C bonds (Ru1 C3 and Ru2 C11; > 2.5 ).[22] Complex 5 could be also regarded as an h4arene complex. However, as one of the examples for h4-arene complexes, an h4-naphthalene complex has a bent naphthalene skeleton with the bent angle of approximately 408.[23] In contrast, the sum of the internal angles of the five-membered rings and the six-membered rings in compound 5 are 539.98 and 717.288 (average), suggesting that these rings are planar and slightly deviates from planarity, respectively. The small deviation of the dibenzopentalene skeleton in 5 excludes the possibility that compound 5 is an h4-arene complex. It is noted that no C C bond alternation was found even in the central pentalene skeletons. In contrast, dimetal dibenzopentalenides 1 a and 4 a have slightly altered C C bonds in the six-membered rings, and C C bond alternation was found in the fivemembered rings of a neutral dibenzopentalene.[8b] To gain further insight into the electronic structure of 5, theoretical calculations were next performed by using the Gaussian 03 program.[24] The geometry of 5 was fully optimized with hybrid density functional theory at B3LYP[25] level of theory by using the LANL08(f) basis set for Ru[26] and the 6-31G(d) basis set for C, H, and Si.[27] The optimized geometry of 5 was in good agreement with the experimental one. To estimate aromaticity of each ring in 5, nucleus-independent chemical shifts (NICS)[28] were calculated at the B3LYP with Huzinaga’s (4333111/43111/4111) (TZ) basis set[29] for Ru and the 6-311G(d) for C, H, and Si.[30] Because the NICS(1) values for the six-membered rings (opposite sides to the ruthenium atoms) were calculated to be 12.6 and 12.7 ppm, their aromaticity were retained. Notably, the NICS(1) values of the five-membered rings are also negative ( 10.0 and 12.0 ppm for the same and the opposite sides toward the ruthenium atoms, respectively), revealing that the aromatic nature of the five-membered rings was retained, even though they do not participate in the coordination to the ruthenium atoms. To evaluate electronic characteristics of a dibenzopentalenide as a new ligand, cyclic voltammetry of 5 was performed by using a tailored glassware.[31] To stabilize the resulting cationic species formed after anticipated oxidation of 5, [Bu4N][TPFPB] (TPFPB = tetrakis(pentafluorophenyl)borate) was used as a supporting electrolyte.[32] The cyclic voltammogram of 5 recorded in THF at room temperature is shown in Figure S3 in the Supporting Information. Two reversible oxidation waves were observed at E1/2 = 0.46 and 0.13 V (vs. Fc/Fc + ). The first and the second oxidation waves are derived from the oxidations of RuII II to RuIII II and RuIII II to RuIII III, respectively. Such two-step oxidations were also found in a [trans-(RuCp*)2] pentalene complex by using a saturated calomel electrode (SCE) reference electrode.[16] Unfortunately, because an oxidation potential of ferrocene under the same conditions was not described in the paper, the comparison of the oxidation potential between the pentalene and dibenzopentalene complexes is not possible. The large separation of the first and the second oxidation
Scheme 5. Reaction of 1 a with [RuClCp*]4.
and the two compounds could not be separated. Instead, when dilithium salt 1 a, which is readily synthesized from the reduction of 3 a with lithium in 77 % yield,[17] was used as a starting material, the same signal patterns A and B were observed in the ratio of 2:1 (Scheme 5). Finally, dark brown crystals were obtained, and X-ray diffraction analysis revealed that one of the products, derived from A, is [cis-(RuCp*)2] dibenzopentalene (5; Figure 2).[18] It should be noted that such a cis-
Figure 2. ORTEP drawing of 5 (50 % probability). All hydrogen atoms and a diethyl ether molecule were omitted for clarity. Selected bond lengths []: C1 C2 1.451(4), C1 C9 1.422(4), C1 C16 1.441(4), C2 C3 1.404(4), C3 C4 1.441(4), C3 C8 1.480(4), C4 C5 1.414(5), C5 C6 1.416(5), C6 C7 1.416(5), C7 C8 1.418(4), C8 C9 1.440(4), C9 C10 1.455(4), C10 C11 1.404(4), C11 C12 1.426(4), C11 C16 1.482(4), C12 C13 1.417(4), C13 C14 1.408(5), C14 C15 1.419(5), C15 C16 1.426(4), Ru1 C3 2.527(3), Ru1 C4 2.234(3), Ru1 C5 2.176(3), Ru1 C6 2.174(3), Ru1 C7 2.200(3), Ru1 C8 2.370(3), Ru2 C11 2.499(3), Ru2 C12 2.221(3), Ru2 C13 2.176(3), Ru2 C14 2.183(3), Ru2 C15 2.210(3), Ru2 C16 2.364(3).
orientation in [(MCp)(m-polyarene)(MCp)] (M = Group 8 metals) is still rare,[19] whereas a trans-orientation is commonly reported.[20] Based on the 1H NMR spectrum, the ruthenium atoms are located above not the five-membered rings, but the sixmembered rings. In general, RuCp* prefers a five-membered ring to a six-membered ring, because the 18-electron rule is satisfied by a coordination to a five-membered ring.[21] The reason why the six-membered rings coordinate to the ruthenium atoms is probably due to steric repulsion between Cp* and SiiPr3 groups. In fact, the silicon atoms deviate from the planar five-membered rings by 0.40 and 0.53 to the opposite Chem. Eur. J. 2014, 20, 1 – 6
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Communication waves of 0.59 V suggests that the monocation species 5 + is stabilized due to a delocalization of the cation charge over whole of the complex. On the other hand, the oxidation waves of decamethylruthenocene [RuCp*2] and pentamethylcyclopenta-dienyl nonamethylfluorenyl ruthenocene [RuCp*Flu*] were found at 0.08 and 0.32 V (vs Fc/Fc + ), respectively.[21] Although these data were recorded under different conditions ([Bu4N][ClO4] as a supporting electrolyte in CH2Cl2), it can be considered that dibenzopentalenide diruthenium complex 5 is oxidized more easily than ruthenocenes bearing electron-rich Cp* and Flu* ligands. One of the reasons for the easily oxidized character of 5 is that the dibenzopentalenide functions as more electron-rich ligand than Cp* and Flu*, because it has silyl groups and two negative charges, which causes its higher HOMO level. More importantly, the low oxidation potential indicates that the mixedvalent RuIII/II cationic complex 5 + can be stabilized effectively, because a positive charge can be delocalized over the dibenzopentalene skeleton. This hypothesis is supported by the separation of the two oxidation waves in 5 larger than that of the pentalene complex. Therefore, complex 5 is oxidized more easily than ruthenocenes bearing electron-donating Cp* and Flu* ligands. In summary, dibenzopentalenes 3 a–c were easily obtained in moderate yields (40–59 %) by the reduction of the corresponding phenyl(silyl)ethynes with potassium followed by quenching with iodine. The intermediate of the reactions, dipotassium dibenzopentalenides, were isolated and characterized by NMR and X-ray diffraction analysis. The first dibenzopentalene–transition-metal complex was successfully synthesized by the reaction of 1 a with [RuClCp*]4. Although two ruthenium atoms are coordinated not by the five-membered, but rather by the six-membered rings, aromaticity of the fivemembered rings was retained as was evidenced by X-ray diffraction analysis and theoretical calculations. The cyclic voltammetry measurements exhibited two oxidation waves with relatively large separation.
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Acknowledgements This work was partially supported by Grant-in-Aid for Scientific Research (B) (No. 22350015 for M.S.), Young Scientists (B) (No. 25730079 for K.I.), and Priority Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” [2408] (No. 25109510 and 24109013 for M.S. and N.T., respectively) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by the collaborative Research Program of Institute for Chemical Research, Kyoto University (No. 2013-69). M.S. acknowledges a research grant from the Mitsubishi Foundation. T.K. acknowledges the JSPS for a Research Fellowship for Young Scientists. We also thank Professor Mao Minoura, in Rikkyo University, Japan, for his refinement in X-ray analysis for compound 3 b. Keywords: coordination modes · cyclization · potassium · reduction · ruthenium &
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Communication
COMMUNICATION & Coordination Chemistry
Potassium is effective! Reduction of phenyl(silyl)ethynes with potassium gave dipotassium dibenzopentalenides in 35–89 % yield. The first dibenzopentalene–transition-metal complex was successfully synthesized and characterized. X-ray diffraction analysis revealed an unexpected coordination mode, in which the two ruthenium atoms are not coordinated by the five-membered rings, but rather by the six-membered rings (see scheme).
T. Kuwabara, K. Ishimura, T. Sasamori, N. Tokitoh, M. Saito* && – && Facile Synthesis of Dibenzopentalene Dianions and Their Application as New p-Extended Ligands
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Communication
& Coordination Chemistry
Facile Synthesis of Dibenzopentalene Dianions and Their Application as New p-Extended Ligands Takuya Kuwabara,[a] Kazuya Ishimura,[b] Takahiro Sasamori,[c] Norihiro Tokitoh,[c] and Masaichi Saito*[a] Dedicated to Professor Renji Okazaki on the occasion of his 77th birthday
Abstract: Reduction of phenyl(silyl)ethynes with potassium followed by quenching with iodine gave dibenzopentalenes in moderate yields. The intermediates of the reactions, dipotassium dibenzopentalenides, were isolated. The first dibenzopentalene–transition-metal complex was successfully synthesized. The ruthenium atoms are located above the six-membered rings. However, X-ray diffraction analysis and theoretical calculations revealed that the aromatic nature of the five-membered rings was retained. The cyclic voltammetry of the Ru complex revealed two oxidation waves with relatively large separation.
Scheme 2. Previous methods for the synthesis of dibenzopentalene dianions.
lithium salt with a trans-h5 ;h5-mode reported by our group,[5] and the other is a barium salt with an h8-mode, which was recently reported by Xi and co-workers.[6] On the other hand, transition-metal complexes bearing dibenzopentalene ligands remain elusive. Because of its extended p-planar structure, a dibenzopentalene ligand would produce many kinds of coordination modes that cannot be accomplished by using a pentalene ligand. In spite of its unique p-extended system, the chemistry of dibenzopentalene has been much less explored due to its synthetic difficulties.[7] However, since 2009, effective and facile synthetic methods with transition-metal catalysts have successively been reported by several groups.[8] Moreover, the syntheses of dibenzopentalene derivatives without using transition-metal catalysts have also been achieved.[9] Xi’s group reported the surprising transformation of 1,4-dilithio-1,3-butadienes into barium salts of dibenzopentalene dianion (Scheme 2).[6] In contrast, our group has already reported the formation of a dilithium salt of dibenzopentalene dianion 1 a by the reduction of phenyl(triisopropylsilyl)ethyne 2 a with lithium.[5] However, the yield of 1 a was only 8 %, and the main product was 1,4-dilithio-1,3-butadiene. Herein, we report an improved method for the synthesis of dibenzopentalene dianion that has independently been found recently.[10] Furthermore, the synthesis, structure, and electronic properties of the first example of a dibenzopentalene–transition-metal complex are also reported. Reduction of phenyl(triisopropylsilyl)ethyne 2 a with potassium provided a purple solid, which reacted with iodine to give dibenzopentalene 3 a (Scheme 3).[11] Surprisingly, the yield of 3 a was drastically improved from 8 to 59 % by only changing
Coordination chemistry of pentalene dianions has attracted much attention for the last two decades.[1] Transition-metal complexes bearing pentalene ligands exhibit various coordination modes, such as h5 ;h5-coordination with cis or trans fashions,[2] double sandwich,[3] and h8-fashion (Scheme 1).[4] But to the best of our knowledge, metal complexes of dibenzo[a,e]pentalene, which is a p-extended molecule of pentalene, has been limited to only two examples (Scheme 2). One is a di-
Scheme 1. Various coordination modes in pentalene complexes.
[a] T. Kuwabara, Prof. Dr. M. Saito Department of Chemistry, Graduate School of Science and Engineering Saitama University, Shimo-okubo, Sakura-ku, Saitama, 338-8570 (Japan) Fax: (+ 81) 48-858-3700 E-mail: [email protected] [b] Dr. K. Ishimura Theoretical and Computational Chemistry Initiative Institute for Molecular Science Myodaiji, Okazaki, Aichi, 444-8585 (Japan) [c] Prof. Dr. T. Sasamori, Prof. Dr. N. Tokitoh Institute for Chemical Research, Kyoto University Gokasho, Uji, Kyoto 611-0011 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402304. Chem. Eur. J. 2014, 20, 1 – 6
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Communication bonds in the six-membered rings (ca. 1.38–1.46 ) slightly alter compared with those in the five-membered rings (1.44–1.46 ), which suggests that negative charges are delocalized over the five-membered rings. The reason for the drastic improvement of the yields can be attributed to two factors. One is the sizes of alkaline metals. When lithium is used as a reductant, s-cis 1,4-dilithio-1,3-butadiene is mainly formed, as was evidenced by X-ray diffraction analysis,[5, 13] because lithium atom is so small that each of the two lithium atoms can be sandwiched by the two terminal carbon atoms of the butadiene. However, when potassium is used, the formation of s-cis 1,4-dipotassio-1,3-butadiene would be difficult due to the steric repulsion between the two potassium atoms, and instead, s-trans 1,4-dipotassio-1,3-butadiene is mainly formed, which readily undergoes intramolecular cyclization with the evolution of H2 gas to generate 4 (Scheme 4).
Scheme 3. Reduction of phenyl(silyl)ethyne with potassium.
the reductant from lithium to potassium. Furthermore, the corresponding reactions using other phenyl(silyl)ethynes 2 b and c also gave dibenzopentalenes 3 b and c in moderate yields (58 and 40 %, respectively). In sharp contrast, the reduction of 2 b and c with lithium has never given dibenzopentalenes 3 b and c.[5] The generation of dibenzopentalenes 3 indicates that the purple intermediates are dipotassium dibenzopentalenides 4. In fact, compounds 4 a–c were successfully isolated in 35 to 89 % yields, and the molecular structure of 4 a was elucidated by X-ray diffraction analysis (Figure 1). In contrast to the bowshaped structure found in an anion radical of dibenzopentalene,[12] the dibenzopentalene skeleton in 4 a is planar. No C C bond alternation was found in the five-membered rings, suggesting their considerable aromatic nature. Interestingly, a coordination mode in 4 a is different from that found in dilithium salt 1 a. Each of the two potassium atoms in 4 a is situated above nearly the midpoint of C1 and C1# atoms, whereas each lithium atom is located above the Scheme 4. Plausible mechanism for the formation of 4. center of each five-membered ring with h5-coordination in 1 a. In both compounds 4 a and 1 a, the C C Such a mechanism has already been proposed to explain the conversion of 1,4-dilithio-1,3-butadiene to barium dibenzopentalenide.[6] The other factor is higher reactivity of C K bonds than C Li bonds. It is known that a carbanion bearing a potassium as a countercation has nucleophilicity stronger than those bearing a lithium.[14] Therefore, the intramolecular cyclization took place easily on 1,4-dipotassio-1,3-butadiene. This facile access to dibenzopentalene dianions from simple starting materials enables to study their coordination to transition metals. First, reaction of 4 a with [RuClCp*]4[15] (Cp* = h5C5Me5) was investigated, because its pentalene derivative, [trans-(RuCp*)2] pentalene complex, has already been synthesized.[16] In the 1H NMR spectrum of a crude product, signals derived from 4 a disappeared, and two sets of aryl protons were observed in a high field, (A (main): d = 4.40, 4.48, 6.15, Figure 1. ORTEP drawing of 4 a (left) and its top view (right; 50 % probabili6.52 ppm; B (minor): d = 4.27–4.45, 6.32, 6.60 ppm), which sugty). All hydrogen atoms were omitted for clarity. Selected bond lengths []: gests the formation of two different compounds, in which # C1 C1 1.433(3), C1 C2 1.463(2), C2 C3 1.455(2); C3 C4 1.420(2), C3 C8 each of the two six-membered rings in both products coordi1.463(2), C4 C5 1.382(2), C5 C6 1.401(2), C6 C7 1.381(2), C7 C8 1.411(2), nate to ruthenium atoms. However, no crystals were obtained, C8 C1# 1.442(2), K C1 2.9812(15), K C1# 2.9322(16). &
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Communication side of the Cp*. However, compound 5 appears to be not an h6-arene, but an h5-pentadienyl complex, because 5 has four shorter Ru C bonds of 2.17 to 2.23 (Ru1 C4/5/6/7 and Ru2 C12/13/14/15), relatively long Ru C bonds (Ru1 C8 and Ru2 C16; ca. 2.37 ), and very long Ru C bonds (Ru1 C3 and Ru2 C11; > 2.5 ).[22] Complex 5 could be also regarded as an h4arene complex. However, as one of the examples for h4-arene complexes, an h4-naphthalene complex has a bent naphthalene skeleton with the bent angle of approximately 408.[23] In contrast, the sum of the internal angles of the five-membered rings and the six-membered rings in compound 5 are 539.98 and 717.288 (average), suggesting that these rings are planar and slightly deviates from planarity, respectively. The small deviation of the dibenzopentalene skeleton in 5 excludes the possibility that compound 5 is an h4-arene complex. It is noted that no C C bond alternation was found even in the central pentalene skeletons. In contrast, dimetal dibenzopentalenides 1 a and 4 a have slightly altered C C bonds in the six-membered rings, and C C bond alternation was found in the fivemembered rings of a neutral dibenzopentalene.[8b] To gain further insight into the electronic structure of 5, theoretical calculations were next performed by using the Gaussian 03 program.[24] The geometry of 5 was fully optimized with hybrid density functional theory at B3LYP[25] level of theory by using the LANL08(f) basis set for Ru[26] and the 6-31G(d) basis set for C, H, and Si.[27] The optimized geometry of 5 was in good agreement with the experimental one. To estimate aromaticity of each ring in 5, nucleus-independent chemical shifts (NICS)[28] were calculated at the B3LYP with Huzinaga’s (4333111/43111/4111) (TZ) basis set[29] for Ru and the 6-311G(d) for C, H, and Si.[30] Because the NICS(1) values for the six-membered rings (opposite sides to the ruthenium atoms) were calculated to be 12.6 and 12.7 ppm, their aromaticity were retained. Notably, the NICS(1) values of the five-membered rings are also negative ( 10.0 and 12.0 ppm for the same and the opposite sides toward the ruthenium atoms, respectively), revealing that the aromatic nature of the five-membered rings was retained, even though they do not participate in the coordination to the ruthenium atoms. To evaluate electronic characteristics of a dibenzopentalenide as a new ligand, cyclic voltammetry of 5 was performed by using a tailored glassware.[31] To stabilize the resulting cationic species formed after anticipated oxidation of 5, [Bu4N][TPFPB] (TPFPB = tetrakis(pentafluorophenyl)borate) was used as a supporting electrolyte.[32] The cyclic voltammogram of 5 recorded in THF at room temperature is shown in Figure S3 in the Supporting Information. Two reversible oxidation waves were observed at E1/2 = 0.46 and 0.13 V (vs. Fc/Fc + ). The first and the second oxidation waves are derived from the oxidations of RuII II to RuIII II and RuIII II to RuIII III, respectively. Such two-step oxidations were also found in a [trans-(RuCp*)2] pentalene complex by using a saturated calomel electrode (SCE) reference electrode.[16] Unfortunately, because an oxidation potential of ferrocene under the same conditions was not described in the paper, the comparison of the oxidation potential between the pentalene and dibenzopentalene complexes is not possible. The large separation of the first and the second oxidation
Scheme 5. Reaction of 1 a with [RuClCp*]4.
and the two compounds could not be separated. Instead, when dilithium salt 1 a, which is readily synthesized from the reduction of 3 a with lithium in 77 % yield,[17] was used as a starting material, the same signal patterns A and B were observed in the ratio of 2:1 (Scheme 5). Finally, dark brown crystals were obtained, and X-ray diffraction analysis revealed that one of the products, derived from A, is [cis-(RuCp*)2] dibenzopentalene (5; Figure 2).[18] It should be noted that such a cis-
Figure 2. ORTEP drawing of 5 (50 % probability). All hydrogen atoms and a diethyl ether molecule were omitted for clarity. Selected bond lengths []: C1 C2 1.451(4), C1 C9 1.422(4), C1 C16 1.441(4), C2 C3 1.404(4), C3 C4 1.441(4), C3 C8 1.480(4), C4 C5 1.414(5), C5 C6 1.416(5), C6 C7 1.416(5), C7 C8 1.418(4), C8 C9 1.440(4), C9 C10 1.455(4), C10 C11 1.404(4), C11 C12 1.426(4), C11 C16 1.482(4), C12 C13 1.417(4), C13 C14 1.408(5), C14 C15 1.419(5), C15 C16 1.426(4), Ru1 C3 2.527(3), Ru1 C4 2.234(3), Ru1 C5 2.176(3), Ru1 C6 2.174(3), Ru1 C7 2.200(3), Ru1 C8 2.370(3), Ru2 C11 2.499(3), Ru2 C12 2.221(3), Ru2 C13 2.176(3), Ru2 C14 2.183(3), Ru2 C15 2.210(3), Ru2 C16 2.364(3).
orientation in [(MCp)(m-polyarene)(MCp)] (M = Group 8 metals) is still rare,[19] whereas a trans-orientation is commonly reported.[20] Based on the 1H NMR spectrum, the ruthenium atoms are located above not the five-membered rings, but the sixmembered rings. In general, RuCp* prefers a five-membered ring to a six-membered ring, because the 18-electron rule is satisfied by a coordination to a five-membered ring.[21] The reason why the six-membered rings coordinate to the ruthenium atoms is probably due to steric repulsion between Cp* and SiiPr3 groups. In fact, the silicon atoms deviate from the planar five-membered rings by 0.40 and 0.53 to the opposite Chem. Eur. J. 2014, 20, 1 – 6
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Communication waves of 0.59 V suggests that the monocation species 5 + is stabilized due to a delocalization of the cation charge over whole of the complex. On the other hand, the oxidation waves of decamethylruthenocene [RuCp*2] and pentamethylcyclopenta-dienyl nonamethylfluorenyl ruthenocene [RuCp*Flu*] were found at 0.08 and 0.32 V (vs Fc/Fc + ), respectively.[21] Although these data were recorded under different conditions ([Bu4N][ClO4] as a supporting electrolyte in CH2Cl2), it can be considered that dibenzopentalenide diruthenium complex 5 is oxidized more easily than ruthenocenes bearing electron-rich Cp* and Flu* ligands. One of the reasons for the easily oxidized character of 5 is that the dibenzopentalenide functions as more electron-rich ligand than Cp* and Flu*, because it has silyl groups and two negative charges, which causes its higher HOMO level. More importantly, the low oxidation potential indicates that the mixedvalent RuIII/II cationic complex 5 + can be stabilized effectively, because a positive charge can be delocalized over the dibenzopentalene skeleton. This hypothesis is supported by the separation of the two oxidation waves in 5 larger than that of the pentalene complex. Therefore, complex 5 is oxidized more easily than ruthenocenes bearing electron-donating Cp* and Flu* ligands. In summary, dibenzopentalenes 3 a–c were easily obtained in moderate yields (40–59 %) by the reduction of the corresponding phenyl(silyl)ethynes with potassium followed by quenching with iodine. The intermediate of the reactions, dipotassium dibenzopentalenides, were isolated and characterized by NMR and X-ray diffraction analysis. The first dibenzopentalene–transition-metal complex was successfully synthesized by the reaction of 1 a with [RuClCp*]4. Although two ruthenium atoms are coordinated not by the five-membered, but rather by the six-membered rings, aromaticity of the fivemembered rings was retained as was evidenced by X-ray diffraction analysis and theoretical calculations. The cyclic voltammetry measurements exhibited two oxidation waves with relatively large separation.
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Acknowledgements This work was partially supported by Grant-in-Aid for Scientific Research (B) (No. 22350015 for M.S.), Young Scientists (B) (No. 25730079 for K.I.), and Priority Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” [2408] (No. 25109510 and 24109013 for M.S. and N.T., respectively) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and by the collaborative Research Program of Institute for Chemical Research, Kyoto University (No. 2013-69). M.S. acknowledges a research grant from the Mitsubishi Foundation. T.K. acknowledges the JSPS for a Research Fellowship for Young Scientists. We also thank Professor Mao Minoura, in Rikkyo University, Japan, for his refinement in X-ray analysis for compound 3 b. Keywords: coordination modes · cyclization · potassium · reduction · ruthenium &
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Received: February 22, 2014 Published online on && &&, 0000
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
COMMUNICATION & Coordination Chemistry
Potassium is effective! Reduction of phenyl(silyl)ethynes with potassium gave dipotassium dibenzopentalenides in 35–89 % yield. The first dibenzopentalene–transition-metal complex was successfully synthesized and characterized. X-ray diffraction analysis revealed an unexpected coordination mode, in which the two ruthenium atoms are not coordinated by the five-membered rings, but rather by the six-membered rings (see scheme).
T. Kuwabara, K. Ishimura, T. Sasamori, N. Tokitoh, M. Saito* && – && Facile Synthesis of Dibenzopentalene Dianions and Their Application as New p-Extended Ligands
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Chem. Eur. J. 2014, 20, 1 – 6
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
