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Download Article (PDF) NEW ASPECTS IN THE CHEMISTRY OF LOW-COORDINATE COMPOUNDS OF GROUP 14 ELEMENTS Norihiro Tokitoh, Yasusuke Matsuhashi, Kazusato Shibata, Tsuyoshi Matsumoto, Hiroyuki Suzuki, Masaichi Saito, Kyoko Manmaru and Renji Okazaki* Department of Chemistry, Graduate School of Science,The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Abstract: Introduction of a new steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tb in this article) onto heavier group 14 elements together with other bulky aryl groups such as mesityl (Mes) or 2,4,6-triisopropylphenyl (Tip) enabled us to synthesize unprecedented, stable diaryl substituted tetrachalcogenametallolanes, Tb(Ar)MY4 (M = Si, Ge, and Sn; Y = S and Se), as well as the heavier group 14 element carbene analogues, Tb(Ar)M: (M = Ge and Sn), stable in solution. Furthermore, the first examples of kinetically stabilized metallathiones and met- allaselones of group 14 metals Tb(Ar)M=Y were obtained by taking advantage of the following two different approaches, i) the reactions of the overcrowded divalent metal species with some epichal- cogenides or elemental chalcogen, and ii) the dechalcogenation of the 1,2,3,4,5-tetrachalcogena- metallolanes with phosphine reagents. Of these Tb(Ar)M=Y germanethione Tb(Tip)Ge=S was iso- lated as orange crystals, the molecular geometry of which was established by X-ray structural anal- ysis. Introduction Since the isolation of the first stable double bond compounds containing heavier group 14 and 15 elements such as Si=C,1) Si=Si,2) and P=P,3) a remarkable progress has been made in the chemistry of unsaturated compounds of heavier typical elements, especially in the field of group 14 metals.4) In the chemistry of low-coordinate compounds of group 14 elements (Si, Ge, Sn, and Pb), the double bond compounds between such metals and chalcogen atoms are among the most fascinating and challenging target molecules because of their interesting bonding character and potential synthetic utility. Although there have been reported some examples of silanethione,5) sil- aneselone,5) and germathiourea6) stabilized by the intramolecular coordination of nitrogen substitu- ents to the double bond (thermodynamic stabilization), very little is known for stable metallathiones and metallaselones of heaview group 14 metals RR'M=Y (M = Si, Ge, Sn, and Pb; Y = S and Se). In this paper we present our recent results on the kinetic stabilization of the low-coordinate com- pounds of group 14 elements by taking advantage of our new steric protection group, 2,4,6-tris[bis (trimethylsilyl)methyl]phenyl,7) leading to the synthesis and reactions of stable matallathiones and metallaselones. Results and Discussion 1. Kinetic Stabilization of Heavier Group 14 Element Carbene Analogues. The chemistry of germylene and stannylene, well-known as highly reactive, unstable low- coordinate group 14 metal species, has been an active area of stimulative and fascinating research for both organometallic and inorganic chemists.8) Although there have been a number of reports concerning the synthesis and characterization of gemylenes and stannylenes, they are restricted to dialkyl- and heteroatom-substituted examples. With regards to dialkylgermylenes, Läppert et al. 9 have described the first stable compounds [(Me3Si)2CH]2Ge: in 1976, ) and very recently a'genu- inely monomeric dialkylgermylene [(Me3Si)3C](Me3Si)2CH]Ge: has been synthesized and crystallo- graphically analyzed by Jutzi et al.10) However, as for the diaryl substituted counterpart there have been no examples of stable germylenes at ambient temperature. In the case of stannylene, Kira 55 Vol. 17, Νos. 1-4, 1994 New Aspects in the Chemistry of Low-Coordinate Compounds of Group 14 Elements and Sakurai et al. recently succeeded in the isolation of stable, monomeric dialkylstannylene, 2.2,5,5-tetrakis(trimethylsilyl)-1-stannacyclopentene-1,1-diyl,11) while as for the stable diaryl substi- tuted stannylene only the bis[2,4,6-tris(trifluoromethyl)phenyl]stannylene, strongly perturbed by the intramolecular coordination of the fluorine atoms in the o-substituents, has been reported by Grützmacheret al.12) On the other hand, we have already reported the synthesis and structural analysis of novel group 14 metal-containing cyclic polychalcogenides, 1,2,3,4,5-tetrachalcogenametallolanes 1-5 us- ing Tb group together with mesityl or 2,4,6-triisopropylphenyl group as steric protection groups.13) Recently, we have found that Tb group is effective to stabilize also the divalent species of group 14 metals such as germylene and stannylene. Tb χ / Y Me3Si.H Η SiMe, Μ I Me Si Ar / \ -Y Tb = 3 1; M=Si, Y=S 2; M=Ge, Y=S 3; M=Ge, Y=Se 4: M=Sn, Y=S a; Ar=mesityl (Mes) 5; M=Sn, Y=Se b; Ar=2,4,6-triisopropylphenyl (Tip) Thus, a new type of overcrowded diarylgermylene Tb(Tip)Ge: (6) was readily obtained by the sequential reaction of diiodogermane with TbLi and TipLi in THF in the presence of hexamethyl- phosphoramide (HMPA). Germylene 6 showed a dark red color in THF and brownish green color (Xmax = 580 nm) after solvent exchange into hexane. Studies on the electronic spectra of a variety of germylenes generated in glass matrices at cryogenic temperatures have revealed that introduc- tion of bulky substituents on a germanium atom resulted in a dramatic red shift of the n-p transition 14 of germylenes [for example: Ph2Ge: (466 nm), Mes2Ge: (550 nm), and Tip2Ge: (558 nm)]. ) The remarkable red shift observed for the λΓΤ13χ value of 6 is consistent with these results and it is the longest λΓη3χ value reported so far for C-substituted germylenes. Tb Me Gel 2 Mel V 7 26% 1. TbLi Tip/ Ν THF/HMPA 2. TipLi Tb Η νγ 8 41% Tiipp ' νΛ Ph Ph Tbx O^Ph Μο ο Ge 9 19% N Tip'' 0.3- .'p h T\ /s-s Ge I 2b 25% Tip Scheme 1. Under inert atmosphere germylene 6 was found to be stable in solution in contrast to bis- 56 Ν. Tokitoh, Υ. Matsuhashi, Κ. Shibata, Τ. Matsumoto, Main Group Metal Chemistry Η. Suzuki, Μ. Saito, Κ. Manmaru and R. Okazaki (2,4,6-tri-f-butylphenyl)germylene prepared by du Mont et al., the sterically most crowded diarylger- mylene reported so far, which reportedly survived only below -10 °C and underwent intramolecular cyclization at higher temperatures.15) It was found that in the electronic spectra of 6 the wave- length and absorption coefficient of the absorption maximum at 580 nm were almost unchanged in hexane within a temperature range from -73 °C to 60 °C, indicating the absence of an equilibrium between monomeric 6 and the corresponding dimer, digermene. In spite of the extreme congestion around the germanium atom, 6 underwent ready insertion and cycloaddition reactions with methyl iodide, 2,3-dimethyl-1,3-butadiene, and benzil giving the expected products 7-9 as shown in Scheme 1. Germylene 6 also reacted with excess amount of elemental sulfur to give the corresponding tetrathiagermolane 2b (Ar = Tip). The remarkable stabil- ity of the germylene 6 prompted us to examine its complexation with transition metal carbonyl com- Tb \ M(CO) 5.THF Ge: Scheme 2. THF/r. t. Tip/ 6 10 (M=W) II (M=Cr) plexes. When a THF solution of 6 was treated with W(CO)5*THF, the expected mononuclear ger- mylene-tungsten complex 10 was obtained as reddish orange crystals (Scheme 2). Analogous pentacarbonylchromium(O) complex 11 was also obtained as bright orange crystals by the reac- tions of 6 with Cr(CO)5*THF. Of the two complexes 10 and 11, which are the first, base-free diaryl- germylene-transition metal complexes, the molecular structure of 10 was determined by X-ray crystallographic analysis. The combination of Tb and Tip groups was also useful to stabilize the heavier metal analogue of 6, stannylene 12 (Scheme 3). Stannylene 12 was also found to be strikingly stable at ambient temperature, showing a deep purple color ^max = 561 nm) in hexane and only one signal attribut- able to a divalent organotin species at 2208 ppm in the 119Sn NMR. The possibility that stanny- Tb Me Sn' 13 11% 1. TbLi V Ni 2. TipLi /S 14 37% TiP OC Tb /°-rPh 1515 22% T>Tip so-^PI h Tb, /S-S Sn I 4b 50% Tip" V® Scheme 3. 57 Vol. 17, Νos. 1-4, 1994 New Aspects in the Chemistry of Low-Coordinate Compounds of Group 14 Elements lene 13 exists as an equilibrated mixture with the corresponding dimer, distannene, can be ruled out, since no spectral change was observed in the UV-vis and 119Sn NMR spectra of 12 within a temperature range from -30 °C to 60 °C. Stannylene 12 also reacted with a variety of reagents to give the expected insertion and cycloaddition products 13-15 and 4b as shown in Scheme 3. The formation of germylene 6 and stannylene 12 is worthy of note not only as the first exam- ples of stable heavier group 14 element carbene analogues with aryl attachment but also as the successful application of a new steric protection group, Tb group, to the kinetic stabilization of high- ly reactive low coordinate group 14 metal species. Although an attempt at introducing Tb group onto a lead atom by the reactions of TbLi with lead dichlroride or iodide has failed due to inevitable electron-transfer reactions, ligand exchange of bis[bis(trimethylsily!)amino]plumbylene16) by nucleophilic substitution with TipLi, a less hindered aryllithium than TbLi, resulted in the formation of a new kinetically stabilized diarylplumbylene 16 2(Me 3Si) 2NLi 2TipLi PbCI 2 — [(Me 3Si)2N]2Pb: ^ Tip2Pb: -40 °C/Et 20 -40 °C 1£ Scheme 4.
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