Transition-Metal Complexes Featuring Dianionic Heavy Group 14

Article Cite This: Acc. Chem. Res. 2018, 51, 160−169

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Transition-Metal Complexes Featuring Dianionic Heavy Group 14 Element Aromatic Ligands Masaichi Saito* Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Shimo-okubo, Sakura-ku, Saitama-city, Saitama 338-8570, Japan CONSPECTUS: The synthesis of dilithio-stannoles and -plumboles, dianionic aromatic compounds containing tin and lead atoms in their π-skeletons, opened a new field of transition-metal chemistry. Since the discovery of ferrocene (Cp2Fe), which is composed of anionic aromatic ligands (Cp: cyclopentadienyl) and Fe(II), ferrocene-type sandwich complexes have long played important roles in many fields of chemistry. During the last few decades, the electronic and structural properties of the Cp ligand have been modified by introducing electron-donating, electron-withdrawing, and sterically encumbered substituents on the skeletal carbon atoms to obtain desirable properties of the resulting sandwich complexes. In terms of modifying the Cp ligand, we focused our attention on introducing a heavy group 14 atom into the π-skeleton. This idea was originally inspired by a question of whether or not aromaticity was retained after the replacement of a skeletal carbon atom by a heavy group 14 atom. After we succeeded in the synthesis of aromatic dilithio-stannoles and -plumboles, revealing that the concept of conventional aromaticity was expanded to lead-containing π-systems, we undertook the present project on applying these dianionic aromatic heavy Cp analogues as ligands for transition-metal complexes. The combination of a stannole and Cp*Ru units accomplished the creation of a neutral triple-decker complex and an anionic ruthenocene, which was not be accessible using Cp and its related ligands that are composed of only carbon atoms. The anionic ruthenocene reacted with electrophiles to afford ruthenocene-type sandwich complexes, and the structures of the stannole skeletons were highly dependent on the substituents on the tin atoms, in sharp contrast to the planar Cp ligand. The dianionic plumbole ligand was also found to function as an η5-coordinating ligand in an anionic ruthenocene, which is noteworthy in terms of incorporating the heaviest group 14 atom into a π-ligand to produce a ferrocene-type sandwich complex. The anionic ruthenocene bearing the plumbole ligand reacted with electrophiles to afford ruthenocene-type plumbole complexes, which have oxidation potentials lower than those of the corresponding tin analogues, demonstrating the effect of introduction of a lead atom heavier than a tin atom. In the reactions of dilithiostannoles with group 4 metals, the resulting complexes were found to have exotic electronic structures that cannot be constructed by the Cp ligand. The transition-metal complexes derived from dilithio-stannoles and -plumbole therefore exhibit remarkable differences as well as similarities to the traditional Cp-based transition-metal complexes. These results spotlight the introduction of heavy group 14 atoms into carbon-based π-skeletons, which can perturb the electronic properties of conventional transition-metal complexes and open a new chemistry of transition-metal complexes.

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aromatic by quantum-chemical calculations,13 whereas experimental investigations revealed that aromaticity of monoanion equivalents of siloles14,15 is dependent on the substituent on the silicon atom, and germoles15,16 do not exhibit aromatic character. Using the metallole monoanion equivalents afforded hafnocene-,17 ferrocene-,18 and ruthenocene-type19,20 sandwich complexes (Figure 1). The heavy ruthenocene exhibits an oxidation potential lower than that of ferrocene because of the strong electron-donating nature of the heavy germole ligand. In contrast, decamethylruthenocene, bearing electron-donating methyl groups displays an oxidation potential higher than that of ferrocene.21 More recently, heavy ferrocene and ruthenocene, derived from an aromatic Cp ligand containing three heavy group 14 atoms,22,23 were reported to possess low oxidation potentials, resulting from the strong electron-

INTRODUCTION Since the first synthesis1 and characterization2,3 of ferrocene (Cp2Fe), which is composed of anionic aromatic ligands (Cp: cyclopentadienyl) and Fe(II), ferrocene-type sandwich complexes have long played important roles in many fields of chemistry such as structural,4,5 catalytic,6−8 polymer,9 supramolecular,10 and medicinal11,12 chemistry. During the last few decades, the electronic and structural properties of the Cp ligand have been modified by introducing electron-donating, electron-withdrawing, and sterically encumbered substituents on the skeletal carbon atoms to obtain desirable properties of the resulting sandwich complexes. In terms of modifying the Cp ligand, another strategy is the replacement of skeletal carbon atoms in the Cp ligand by heavy group 14 atoms, that is, the utilization of group 14 metalloles as ligands. This idea was originally inspired by a question of whether or not aromaticity was retained after the replacement of a skeletal carbon atom by a heavy group 14 atom. Lithiosilole was predicted to be © 2017 American Chemical Society

Received: July 25, 2017 Published: December 20, 2017 160

DOI: 10.1021/acs.accounts.7b00367 Acc. Chem. Res. 2018, 51, 160−169

Article

Accounts of Chemical Research Scheme 1. Formation of Germole Dianion Complex

Figure 1. Sandwich transition-metal complexes bearing group 14 metallole ligands.

analogues with transition-metal reagents were never reported, although such reactions were thought to be the most straightforward for the investigation on dianionic metallole transition-metal complexes. In this Account, we highlight our recent study on the reactions of dilithio-stannoles and -plumboles with transition-metal reagents, which afforded expected and unpredictable complexes with unique electronic features. All these complexes exhibit both similarities and differences to the Cp-derived complexes. The effects of the introduction of a tin and a lead atom on electronic properties will also be demonstrated.

donating nature of the ligand (Figure 1).24−26 In parallel with the investigations on the synthesis of heavy Cp monoanion equivalents and their application as ligands for sandwich complexes, dianion equivalents of group 14 metalloles received considerable attention in terms of their potential aromaticity predicted by quantum-chemical calculations.27,28 After the generation of dianion equivalents of siloles29−31 and germoles32 had been reported, the X-ray characterization and quantumchemical calculations elucidated that dimetalla-siloles15,33,34 and -germoles15,35,36 possess considerable aromatic nature (Figure 2). Inspired by the previous works, a question was raised: do

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TRIPLE-DECKER RUTHENOCENES AND THEIR RELATED COMPOUNDS

Preparation of Triple-Decker Ruthenocenes and an Anionic Ruthenocene50

The triple-decker sandwich-type transition-metal complexes composed of Cp and its related ligands are generally cationic,51−53 as Cp and its related ligands function as monoanionic ligands. On the other hand, neutral triple-decker complexes would exhibit electronic properties different from those of the Cp-based cationic triple-decker complexes. To prepare neutral triple-decker complexes, dianionic ligands are necessary. As for the five-membered ring systems, dianionic ligands containing more than two boron atoms have already been utilized to prepare neutral triple-decker complexes.54−57 Inspired by the previous reports, we envisioned that our dilithiostannoles would function as dianionic ligands to form neutral triple-decker complexes. Reaction of dilithiostannole 1a, which has trimethylsilyl groups on the α-positions, with [Cp*RuCl]4 (0.5 equiv) proceeded cleanly to afford the expected neutral triple-decker ruthenocene-type complex 2a (Scheme 2).50 When using dilithiostannole 1b, bearing a bulky t-butyldimethylsilyl group on the α-position, the stepwise introduction of a Cp*Ru moiety was possible and anionic ruthenocene 3b was obtained. Compound 3b reacted with [Cp*RuCl]4 to afford another triple-decker ruthenocene 2b. The molecular structures of compounds 2 and 3b were determined by X-ray diffraction analysis. The stannole rings in 2 and 3b are nearly planar with a sum of the internal bond angles of 540° and negligible C−C bond alternation, suggesting that the stannole moieties should retain aromaticity. The Sn− Ru distances of approximately 2.73 Å in 2 and 3b lie in the range of those previously reported (2.543−3.141 Å).58,59 The distances between the ruthenium atom and the carbon atoms of the stannole ring (for example, avg 2.26 Å in 2a) are similar to those between the ruthenium atom and the carbon atoms of the Cp* ring (for example, av. 2.22 Å in 2a). These structural

Figure 2. Aromatic group 14 dilithiometalloles.

the tin and lead congeners possess aromaticity and function as ligands for transition-metal complexes? We first prepared dilithiostannoles37−40 by the reactions of the corresponding diphenylstannoles with lithium. Based on their molecular structures found by the X-ray diffraction analysis and their magnetic properties elucidated by the 7Li NMR41,42 and nucleus independent chemical shift (NICS) calculations,43,44 which are now widely used as effective probes for diatropic ring currents, we concluded that dilithiostannoles are aromatic. We next focused our attention on the introduction of a lead atom, the heaviest group 14 atom, into a carbon π-framework. We prepared dilithioplumboles by two different methods: the reaction of a diphenylplumbole with lithium45 and the twoelectron reduction of a plumbacyclopentadienylidene46 with lithium.47 The X-ray characterization, 7Li NMR analysis, and NICS calculations revealed that the dilithioplumboles also possess considerable aromatic character. The concept of conventional aromaticity can now be expanded to leadcontaining π-systems.48 The next step was to apply these aromatic, dianionic heavy Cp analogues as ligands for transition-metal complexes. However, there was only one example of a sandwich-type complex, where a heavy Cp functions as a dianionic ligand (Scheme 1).49 The complex was prepared from a monoanionic lithiogermole, and an unexpected reaction occurred to afford the germole dianion complex. However, surprisingly, reactions of dianionic heavy Cp 161


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Accounts of Chemical Research

hand, the stannole ring of chloro derivative 4d considerably deviates from planarity with a bent angle of 41.2°, and the tin atom remarkably pyramidalized with a sum of the angles being 306.3°. Therefore, compound 4d can be regarded as a complex composed of a cationic Cp*Ru fragment coordinated by a butadiene and anionic tin moiety. The structure of ethyl derivative 4c is in between them. The diverse structures can be explained by both electronic and steric reasons due to the substituent introduced onto the tin atom. In the trimethylsilyl derivative 4a, as a trimethylsilyl group is the least electronegative among the four substituents, the lone pair on the tin atom has large p character, contributing to the HOMO of 4a, which can efficiently interact with the ruthenium moiety to afford η5-coordination (Figure 5). In contrast, in the chloro derivative 4d, the negative chlorine atom enhances the s character of the lone pair on the tin atom, as shown in HOMO1 of 4d, which cannot participate in η5-coordination. Furthermore, the bulkiness of the trimethylsilyl group on the tin atom can cause flattening of the stannole ring. Calculations of model compounds suggested that a SiH3-substituted derivative should have a more bent stannole ring (18.3°) than those of the t-butyl (12.3°) and SiMe3 derivatives (9.7°) (Figure 4). The functionalization on the tin atom of anionic ruthenocene 3b enables preparation of the sandwich-type complexes with various types of substituents on the atoms, whereas the sandwich-type complexes previously reported have only silyl groups on the group 14 atoms.17,19,20,49

Scheme 2. Preparation of Triple-Decker Ruthenocenes 2 and Anionic Ruthenocene 3b

features suggest that the stannole rings coordinate the ruthenium atoms in an η5-fashion. Electronic Properties of Triple-Decker Ruthenocene 2a

The cyclic voltammogram of triple-decker ruthenocene 2a measured in CH2Cl2 by the use of [nBu4N][TPFPB] (TPFPB = tetrakis(pentafluorophenyl)borate) as a supporting electrolyte60 in CH2Cl2 revealed a quasi-reversible oxidation wave (Epa = −0.36, Epc = −0.50, E1/2 = −0.43 V vs Fc/Fc+), indicating that triple-decker ruthenocene 2a is oxidized more easily than decamethylruthenocene (0.08 V under similar conditions) and even ferrocene, and the oxidation potential of 2a is comparable to those of ruthenocenes bearing a η5-Si3C226 and a η5-Si2GeC225 ligands (Figure 3). It can be concluded that the introduction of only one tin atom into the π-ligand effectively increases the HOMO energy level.

Synthesis, Structures, and Properties of Plumbole-Bearing Ruthenocene-Type Complexes62

The next step was to utilize a dilithioplumbole as a ligand for transition-metal complexes to evaluate the effects of the introduction of a heavier lead atom on structures and properties of the resulting transition-metal complexes. The starting dilithioplumbole 5 was prepared by the two-electron reduction of THF-coordinated plumbacyclopentadienylidene 6 46 (Scheme 4).47 Reaction of 5 with [Cp*RuCl]4 (0.25 equiv) afforded anionic ruthenocene 7 (Scheme 5). The ruthenocene structure of 7 was confirmed by X-ray diffraction analysis and quantum-chemical calculations revealing a considerable bonding interaction between the Pb and Ru atoms (Figure 6). The plumbole ring in 7 is almost planar with a sum of the internal bond angles of 539.4° and has negligible C−C bond alternation. The Wiberg bond index (WBI) of the Pb−Ru bond was calculated to be 0.42, which is reasonably large to show the bonding interaction between the lead and ruthenium atoms and is slightly larger than those of the Ru−C bonds in the Cp* ligand (0.34−0.35). It is noted that the plumbole, which has the heaviest group 14 atom in its skeleton, also

Synthesis of Ruthenocene-Type Complexes Derived from Anionic Ruthenocene 3b and Their Structures61

Anionic ruthenocene 3b reacted with various electrophiles such as chlorotrimethylsilane, iodomethane, bromoethane, and carbon tetrachloride to afford the corresponding adducts 4 that appeared to have ruthenocene-type structures (Scheme 3). However, the structures of the stannole moieties are highly dependent on the substituents on the tin atoms (Figure 4). The stannole ring of trimethylsilyl adduct 4a is slightly bent (bent angle of 12.0°) with a sum of the internal angles in the stannole ring of 538.3°, as was observed in Tilley’s complexes,19 and the pyramidalization at the tin atom is negligible. On the other

Figure 3. Oxidation potentials (V) based on Cp*2Ru as a standard. Each of the data is the difference in oxidation potentials between the complex and Cp*2Ru measured under the same conditions. 162


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Accounts of Chemical Research Scheme 3. Reactions of Anionic Ruthenocene 3b with Electrophiles

Figure 4. X-ray determined and calculated bent angles of the stannole rings and the sum of the angles around the tin atoms of complexes 4.

Figure 6. Molecular structure of plumbole-bearing anionic ruthenocene 7 with the thermal ellipsoid plots (50% probability). All the substituents on the carbon atoms and the cationic moiety are omitted for clarity. Selected bond lengths (Å) (above) and their calculated WBIs (below) are also shown. Figure 5. HOMO of 4a (left) and HOMO-1 of 4d (right) (isovalue = 0.05).

functions as an η5-coordinating ligand. In terms of sandwichtype transition-metal complexes bearing ligands containing the sixth row elements, a bismaferrocene has previously been reported.63 Functionalization on the lead atom was also achieved by the treatment of anionic ruthenocene 7 with electrophiles. Interestingly, the reactivity of anionic ruthenocene 7 is slightly different from that of the tin analogue 3b. Reaction of 7 with iodomethane did not afford the expected methyl derivative but afforded iodo derivative 8d, though for unclear reasons. The plumbole rings in isopropyl derivative 8b and halogenated derivatives 8c and 8d highly deviate from planarity, as clarified by X-ray diffraction analysis, although the degree of the deviation was dependent on the substituent on the lead atom (Figure 7). The ruthenium atoms are η4-

Scheme 4. Preparation of Dilithioplumbole 5

Scheme 5. Preparation of Plumbole-Bearing Anionic Ruthenocene 7 and Its Reactivity

Figure 7. Bent angles of the plumbole rings and the sum of the angles around the lead atoms of complexes 8.

coordinated, as evidenced by the small WBIs of the Pb−Ru interactions being 0.27 and 0.21 for 8a and 8c, respectively. The effects of the introduction of a lead atom into the π-ligand were obviously demonstrated by the electrochemical properties: the plumbole derivatives 8b and 8c revealed oxidation potentials lower than those of tin analogues 4c and 4d, respectively (Table 1). These results spotlight the importance 163


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while that of 10 has a butterfly structure with a dihedral angle of 113° (Figure 8). More interestingly, the Ru−Ru distance in 10 is 2.3428(6) Å, which is remarkably shorter than those of silylene-bridged diruthenium complexes that are considered to possess Ru−Ru triple bonds (2.4686(5) and 2.4492(9) Å).66 Quantum-chemical calculations revealed that the Ru−Ru bond in 10 was mainly composed of one banana bond derived from d orbitals of the ruthenium atoms and two three-center-twoelectron bonds delocalized over the three-membered rings in the Sn2Ru2 skeleton (Figure 9). These MOs over the Ru−Ru moiety result in a very short Ru−Ru bond.

of the introduction of heavy atoms to perturb the electronic properties of materials and catalysts. Table 1. Oxidation Potentials of Stannole and Plumbole Complexes 4 and 8 EOX (E/V vs Fc/Fc+)

8b

8c

4c

4d

−0.01

0.23

0.04

0.48

Dinuclear Ruthenium Complexes Bearing Butterfly and Inverse-Sandwich Structures64

Using another dilithiostannole 4c bearing an ethyl group on each of the carbon atoms39 and the same ruthenium reagent, a different result was produced. Treatment of 4c with [Cp*RuCl]4 (0.2 equiv) afforded dilithium complex 9 bearing a rare inverse-sandwich structure composed of an inorganic ring containing transition metals (Scheme 6).65 On the other hand, Scheme 6. Formation of Dinuclear Ruthenium Complexes 9 and 10 Figure 9. MOs around Ru−Ru bond in complex 10. Banana bond (left) and one of the two orbitals delocalized over the three-membered rings (right) (isovalue = 0.05).

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GROUP 4 METAL COMPLEXES DERIVED FROM DILITHIOSTANNOLE

Reactions of Dilithiostannole with Hafnocene Dichloride

Inspired by the report on the synthesis of stannylene-group 4 metal complexes,67 the reaction of a dilithiostannole with hafnocene dichloride was next investigated. When monitoring the reaction of dilithiostannole 4b with an equivalent amount of hafnocene dichloride in THF at −30 °C by 119Sn NMR spectroscopy, a broad signal was observed at 984 ppm, which is in a region similar to the 119Sn resonances of stannylene-group 4 metal complexes,67 indicating the formation of fulvene-type stannylene-hafnium complex 11 likely coordinated by THF (Scheme 7). However, after warming to room temperature, the 119 Sn NMR spectrum of the crude product exhibited a main signal at −1020 ppm, which indicated that a compound having an electronic structure completely different from that of

using 0.5 equiv of [Cp*RuCl]4 provided butterfly type diruthenium complex 10 instead. Remarkably, complex 10 was obtained by the oxidation of dilithium complex 9 with [Cp*RuCl]4 or oxygen gas, and reduction of 10 with lithium provided 9. The Sn2Ru2 skeleton in compound 9 is planar,

Figure 8. ORTEP drawings of Ru complexes 9 and 10 with the thermal ellipsoid plots (50% probability). All hydrogen atoms and solvating diethyl ether molecules are omitted for clarity. 164


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Accounts of Chemical Research Scheme 7. Reaction of Dilithiostannole 4b with 1 equiv of Hafnocene Dichloride

Figure 11. HOMO (left) and HOMO-10 (right) of complex 12a (isovalue = 0.04).

PPh3.72 However, very recently, the silicon73 and germanium74 analogues of 12 were synthesized and described as Si(II) and Ge(II) species, respectively (Figure 12), and quantum-chemical intermediate 11 was formed. The X-ray diffraction analysis provided the answer: pyramid-like hafnium complex 12a bearing Cp and Cl groups on the hafnium atom was the final product, which can be formed by the isomerization of the intermediate.68 As a byproduct, another pyramid-like hafnium complex 12b bearing two Cp groups on the hafnium atom was also characterized by NMR and X-ray diffraction analyses. The next step was to understand the electronic structures of complexes 12. The possible electronic structures of 12 are shown in Figure 10. As the Sn−Cα and Sn−Cβ distances in 12

Figure 12. Si(II) and Ge(II) analogues (left) similar to compound 12 and model compounds for the calculations (right).

calculations of model compounds were performed using the M06-2X density functional method in combination with a triple-ζ basis set and relativistic pseudo potentials for Sn, Pb, and Hf (M06-2X/def2-tzvp). A calculated model compound of our complexes 12 was also suggested to have Sn(II) nature using the quantum-chemical calculations that were different from our methods. On the other hand, using the same dilithiostannole and the Hf reagent in a different solvent provided a different compound. Treatment of dilithiostannole 4b with two equivalents of hafnocene dichloride in toluene afforded hafnium-stannole complex 13 bearing two η1-coordinating Cp2HfCl moieties (Scheme 8).75 Using an equivalent of

Figure 10. Possible electronic structures of 12.

are 2.300(5)−2.341(5) and 2.419(6)−2.451(4) Å, respectively, which are 5−12% longer than those of the starting dilithiostannole 4b (approximately 2.19 Å) 40 and are comparable to the Sn−C distances found in [(Ar′Sn)2(μ2η2:η3-cot)] (cot = cyclooctatetraene) (2.364−2.590 Å),69 stannapyramidane Sn[η4-C4(SiMe3)4] (2.399−2.343 Å),70 and (η3-allyl)Sn(II) complexes (2.380−2.418 Å),71 the Sn−C interactions in 12 are considered to be of π-type rather than σ-type. The geometry of 12a was fully optimized with hybrid density functional theory at the B3PW91 level using the Lanl2dz basis set for Hf and augmented by d polarization functions (d exponent 0.186) for Sn along with the 6-31G(d) basis set for the other atoms. The π-type Sn−Cα and Sn−Cβ interactions were also supported by their small WBIs of 0.376 and 0.174, respectively, in the quantum-chemical calculations of 12a. Furthermore, two types of lone pairs on the tin atom were found (Figure 11). The HOMO-10 is dominated by an s-type lone pair on the tin atom. More importantly, the HOMO is composed of a p-type lone pair of the tin atom, which interacts with both Cα and π orbital of the Cβ−Cβ bond. The p-type lone pair suggests that complex 12a could be regarded as an Sn(0) species stabilized by a butadiene as a 4π donor. The zerovalent character of the tin atom in 12a can also be supported by the calculated first and second proton affinities of 217 and 158 kcal mol−1, respectively, of which the second one is comparable to those of Sn(0) species stabilized by N-heterocyclic carbene and

Scheme 8. Reaction of Dilithiostannole 4b with 2 equiv of Hafnocene Dichloride

hafnocene dichloride provided complex 13 and unreacted 4b, indicating that the introduction of the second Cp2HfCl is much faster than the first step, and that the formation of 11 was suppressed likely due to the lack of THF coordinating to the tin atoms to stabilize 11. The X-ray diffraction analysis of 13 revealed that the stannole ring loses its original aromaticity with C−C bond alternation (1.362(4) to 1.500(5) Å), and functions as a μ−η1;η1-coordinating ligand. Reactions of Dilithiostannole with Titanocene Dichloride76

Reaction of dilithiostannole 4c with titanocene dichloride gave complex 14 with a TiSn2 three-membered ring that was considered to adopt a new type of an electronic structure (Scheme 9). The Sn−Ti distances of 2.6867(16) and 165


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Accounts of Chemical Research Scheme 9. Reaction of Dilithiostannole 4c with Titanocene Dichloride

2.7254(17) Å found by the X-ray diffraction analysis are remarkably shorter than those of the previously reported Sn−Ti single bonds (2.842−2.984 Å)77 but are comparable to those of the bis(stannylene) titanium complex (2.7122(13) and 2.7154(14) Å).67 The Sn−Sn distance is 3.0576(11) Å, which falls into the range of a single bond, as was observed in the Sn− Sn distances of bistannoles (2.7682(2),78 2.7822(7),79 and 2.9059(5) Å).80 The 119Sn NMR signal was observed at 1332.5 ppm, in an area typical to the 119Sn resonances of stannylenegroup 4 metal complexes,67,81−83 suggesting that the two tin atoms should have considerable stannylene character. Moreover, the large 1J(119Sn−117Sn) coupling constant of 3433 Hz, which lies in the range of normal 1J(119Sn−117Sn) coupling constants (2500−4500 Hz),84 suggests a bonding interaction between the two tin atoms. The quantum-chemical calculations reveal three important MOs localized over the three-membered ring to evaluate the electronic structure of complex 14 (Figure 13). The HOMO represents a back-donation from the Ti moiety to the vacant p orbitals of the stannylene moieties, resulting in a p−p interaction between the two tin atoms with a WBI of 0.52. The HOMO-3 originates from the donation of the stannylene moieties to the Ti moiety. More interestingly, the HOMO-6, which has a σ symmetry, is dominated by a donation from the Sn2 moiety to the vacant orbitals on the Ti moiety, delocalized over the three-membered ring. The NBO and second-order perturbation theory analyses reveal electrondonation from the Sn−Sn bond to the Ti moiety (Figure 14). The delocalized σ symmetric orbital produces a certain degree of aromaticity, which was evidenced by the NICS(0) and NICS(1) values of −36.7 and −18.6 ppm, respectively. Based on these MO descriptions, we can conclude that both bis(stannylene) and the σ aromatic structures are important to understand the electronic structure of complex 14 (Figure 15).

Figure 14. NLMO (natural localized molecular orbital) of dative bond from the Sn2 moiety to the Ti in complex 14 (isovalue = 0.02).

Figure 15. Important resonance structures in complex 14.

ligands produced exotic electronic structures caused by the introduction of heavy atoms with some similarities to the Cpbearing transition-metal complexes. These novel electronic structures provide us with more fundamental insight into the understanding of chemical bonds. These results spotlight the importance of the introduction of heavy atoms into carbonbased compounds and could be recognized as new strategies to stimulate future materials and catalytic chemistry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masaichi Saito: 0000-0001-6176-3034 Notes

The author declares no competing financial interest. Biography

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Masaichi Saito was born in Tokyo, Japan in 1967. He obtained his B.S. degree (1991) and Ph.D. (1996) at The University of Tokyo under the supervision of Professor Renji Okazaki. He started his academic carrier as an assistant professor at Saitama University in 1996. He became a full professor in 2009. His interest is focused on the fundamental main group chemistry, which includes the synthesis of

CONCLUSION AND OUTLOOK In summary, we have prepared transition-metal complexes derived from dilithio-stannoles and -plumbole, aromatic dianionic ligands containing tin and lead atoms, which are inaccessible utilizing the common Cp ligand. Using such unique

Figure 13. HOMO (left), HOMO-3 (middle), and HOMO-6 (right) of complex 14 (isovalue = 0.05). 166


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compounds with exotic electronic structures to gain deep insight into chemical bonds.

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ACKNOWLEDGMENTS The author acknowledges the graduate students and collaborators, in particular, Dr. Kuwabara, Ms. Nakada, and Mr. Hamada for their experiments, Professor Nagase, Dr. Ishimura, Dr. Guo, and Professor Hada for performing theoretical calculations, Professor Tokitoh and Professor Sasamori for helping us to measure a cyclic voltammetry, and Professor Minoura for his help on the X-ray diffraction analysis. This work was partially supported by Grants-in-Aid for Scientific Research (B) (Nos. 22350015 and 15H03774) and for Scientific Research on Innovative Area “π-System Figuration” (No. 26102006) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The author acknowledges research grants from the Mitsubishi Foundation.

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DEDICATION The author would like to dedicate this Account to Professor Michikazu Yoshioka on the occasion of his 77th birthday.

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Transition-Metal Complexes Featuring Dianionic Heavy Group 14

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