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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 metallaselones 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-tetrachalcogenametallolanes with phosphine reagents. Of these Tb(Ar)M=Y germanethione Tb(Tip)Ge=S was isolated as orange crystals, the molecular geometry of which was established by X-ray structural analysis.

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 substituents 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 compounds 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 substituted 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 using 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 introduction 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 diarylgermylene 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 wavelength 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 germylene-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 reactions of 6 with Cr(CO)5*THF. Of the two complexes 10 and 11, which are the first, base-free diarylgermylene-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 attributable 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 examples 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 highly 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.

(Scheme 4). Plumbylene 16, which was purple in ether, was found to be stable only in solution at low temperature (-40 °C) and underwent ready insertion reactions with methyl iodide, diphenyl dis- ulfide, and diphenyl diselenide to give the expected products 17,18, and 19 as shown in Scheme 5. Läppert et al. reported the sole example of a kinetically stabilized dialkyl substituted plumbylene, 17 [(Me3Si)2CH]2Pb:, ) which is known to undergo complexation with Mo(CO)6 to give the corre- 18 sponding plumbylene complex, [(Me3Si)2CH]2Pb=Mo(CO)5, in 4% yield. ) Among the heavier

Mel TiP\ /Me Pb 17 9% Tip I

(PhY) 2 TiP\/YPh m{Y=S) 33% /\ 12 (Y=Se) 30% Tip YPh

TiPs /S-S A Pb I + Tip2Pb' PbTip, + Tip2Pb' PbTip2 2 / \ ^S V \ / Tip S^ S S-S 20 20% 21 13% 22 14%

Scheme 5.

group 14 element carbene analogues it is most difficult to trap a plumbylene, since it easily poly- merizes to undergo disproportionation. The attempted reaction of [(Me3Si)2CH]2Pb: with methyl 18 iodide resulted in the formation of only Pbl2, not the expected insertion product. ) The successful isolation of the insertion product of 16 to C-l bond, /'. e. 17, is in sharp contrast with the case of the

58 Ν. Tokitoh, Υ. Matsuhashi, Κ. Shibata, Τ. Matsumoto, Main Group Metal Chemistry H. Suzuki, M. Saito, K. Manmaru and R. Okazaki Lappert's plumbylene. Furthermore, treatment of plumbylene 16 with elemental sulfur resulted in a formation of a novel Pb-containing cyclic polysulfide, 1,2,3,4,5-tetrathiaplumbolane 20, together with dithiaplumbetane 21 and trithiaplumbolane 22. Tetrathiaplumbolane 20 was isolated as orange-yellow crystals and the molecular structure was finally determined by X-ray crystallographic analysis, where the PbS4 ring exists in a half-chair conformation similarly to the cases of previously reported terathiametallolanes of other group 14 metals, Tb(Mes)MS4 [M = Si (1a), Ge (2a), and Sn (4a)] 13a,d)

2. Formation of Metallathiones and Metallaselones of Group 14 Metals via Kinetically Stabilized Divalent Group 14 Metal Compounds. With the stable germylene 6 and stannylene 12 in hand, we have examined their chalcogen- ation in hope of obtaining a stable group 14 metal-heavier chalcogen double bond compounds such as germanethione 23 and stannanethione 24. Although the reactions of 6 and 12 with an excess amount of elemental sulfur resulted in the exclusive formation of tetrathiametallolanes 2b and 4b as sulfurization products, the treatment of 6 and 12 generated in solution as mentioned above with styrene episulfide as S-| source followed by an addition of thiocumulenes such as carbon disul-

CS2 Tb /S 25 (M=Ge) 17% - Μ V C =c S (M=Ge) . / \ / 26 (M=Sn) 19% TiT p s (M=Sn) T PhNCS \ A 2Z (M=Ge) 31% Μ C=Ns 2S (M=Sn) 38% 7 N PhCH=CH2^ Tip S Vh

Tb S^ .Mes MesCNO Μ 2S (M=Sn) 40% _. / M\ TΝ Tip O"

23 (M=Ge) Tb 24 (M=Sn) V Η 3Q (M=Ge) 16% Tip s α Ph Ο Tbs /S 31 (M=Ge) 33% Μ rfwph / \ Δ 22 (M=Sn) 35% Tip O"^

Scheme 6.

fide and phenyl isothiocyanate afforded the corresponding [2+2]cycloadducts 25-28 of the intermediary metallathiones, 23 and 24, respectively (Scheme 6).19) Metallathiones here formed could also react with mesitonitrile oxide, 2,3-dimethyl-1,3-butadiene, and styrene oxide giving the corresponding adducts 29-32, respectively (Scheme 6).19) Furthermore, we have found that elemental

59 Vol. 17, Νos. 1-4, 1994 New Aspects in the Chemistry of Low-Coordinate Compounds of Group 14 Elements sulfur can also be used as S-| source for sulfurization of germylene 6 and stannylene 12 if the divalent species was treated with less than one equivalent amount of sulfur as shown in Scheme 7.

T\ 1/8Se T\ ΡΗνΔ T\ /S /M: -72 °C/THF * /M = S ^ Λ TPh Tip Tip Tip O"^

£ (M=Ge) 23 (M=Ge) 31 (M=Ge) 31% 12 (M=Sn) 24 (M=Sn) 32 (M=Sn) 30%

Scheme 7.

In the case of formation of stannanethione 24 by the reaction of stannylene 12 with styrene episulfide in hexane, monitoring of the reaction using UV-vis spectroscopy showed an appearance of a new absorption at 473 nm (shoulder) at the expence of the absorption of stannylene at 561 nm, as shown in Figure 1, the absorption of 473 nm being assignable to the η-π* transition of the tin-sulfur double bond of stannanethione 24. This is the first example of the spectroscopic obser- vation of stannanethione.19)

solvent; hexane

561 nm Tt>N

Sn: / \ \ ^^ 47V3 nm \\ / * \

T\ Sn=S x:—" Tip7

Hi 11 .... _ Η.... . ι .— 400 500 600 700

wavelength/nm

Fig. 1. UV-vis Spectral Change in the Reaction of Tb(Tip)Sn: (12) with Styrene Episulfide.

The successful transformation of stannylene 12 into stannanethione 24 described above prompted us to examine the selenation of 12 which leads to the formation of stannaneselone 33.

60 Ν. Tokitoh, Υ. Matsuhashi, Κ. Shibala, Τ. Matsumoto, Main Group Metal Chemistry Η. Suzuki, Μ. Saito, Κ. Kfanmaru and R. Okazaki When the stannylene 12 was treated with episelenide 34, which is a sole example of isolable episelenide, and then with styrene oxide, the expected adduct, 1,3,2-oxaselenastannolane 35, was isolated, suggesting the formation of intermediary stannaneselone 33 though in a very low yield

Tbx /Se~Se Sn -wPh + Sn I V -Se Tip' Tip' Se 35 5b 4% 46% 27%

TbSrfeYMeS Tip'SVN 3S 12%

B: elemental Se Scheme 8.

(Scheme 8). Furthermore, treatment of stannylene 12 with elemental selenium also resulted in the formation of stannaneselone 33, the formation of which was evidenced by its trapping experiments with styrene oxide and mesitonitriie oxide giving the corresponding adducts, 35 and 36, respectively (Scheme 8). Thus, we have succeeded in the formation of novel group 14 metal-chalcogen double bond compounds and revealed some of their reactivities. However, the methodology here we used seems to be an unsuitable synthetic route from a standpoint of isolation of such unstable double bond compounds because of the low, not quantitative, yields of the adducts of intermediary metallathiones 23 and 24 and metallaselone 33.

3. Synthesis of Metallathiones and Metallaselones of Group 14 Metals by Desulfurization of Tetra- chalcogenametallolanes. In order to isolate the double bond compounds between group 14 metals and heavier chalcogen atoms, we have examined desulfurization of the 1,2,3,4,5-tetrachalcogenametallolanes by phosphine reagents. When a mixture of tetrathiagermolane 2b and 3 molar equivalent of triphenylphosphine was refluxed in hexane, a quantitative amount of triphenylphosphine sulfide was precipitated. After fil- tration of the phosphine sulfide under argon the residual bright yellow solution was concentrated in a glovebox filled with argon to give pure germanethione 23 as orange crystals (mp. 163-165 °C) quantitatively (Scheme 9).20) Germanethione 23 thus obtained was found to be thermally quite stable under inert atmosphere and it showed a characteristic strong Raman line at 521 cm-1, attributable to the Ge-S stretching of the germathiocarbonyl unit, the value of which is in good agreement with the IR- -1 21 stretching (518 cm ) reported for matrix-isolated Me2Ge=S by Nefedov et al. ) In the electronic spectra, 23 showed an absorption maximum at 450 nm (ε 100) most likely due to the Ge=S η-π* transition and it was unchanged at all even when heated at 160 °C for 3 days in a sealed cell. The

61 Vol. 17, Nos. 1-4, 1994 New Aspects in the Chemistry of Low-Coordinate Compounds of Group 14 Elements molecular structure of 23 was determined by X-ray analysis, which also confirmed that 23 exists as a monomer even in the solid state, showing a remarkable shortening of the Ge-S bond length [2.049(3) Ä] and the completely trigonal planar geometry (the sum of bond angles around Ge is 359.4').20)

T S 3Ph3P, hexane, Δ 160 °C GVe I -/h Ge Ge -S -3Ph3P=S 3 days / ^ sealed tube Tip S Tip V Y quant. 2b

Exposure of 23 to the open air resulted in an instantaneous and quantitative formation of the hydroxymercaptogermane 37. As previously mentioned, germanethione 23 here obtained also underwent ready cycloaddition reactions with 2,3-dimethyl-1,3-butadiene, mesitonitrile oxide, and phenyl isothiocyanate to give the corresponding cycloadducts, 30, 38, and 27, in very good yields, respectively (Scheme 10).

Scheme 10.

Similarly, tetrathiastannolane 4b was readily desulfurized by triphenylphosphine (3 equiv.) in toluene at room temperature to give hydroxymercaptostannane 39 without any formation of 1,3,2,4- dithiadistannetane derivatives suggesting that the intermediary stannanethione 24 did not undergo a dimerization. Although the isolation of 24 as solid materials has not been successful yet, the stannenethione 24 generated in solution reacted with 2,3-dimethyl-1,3-butadiene, phenyl isothiocyanate, and styrene oxide to give the expected cycloadducts 40, 28, and 32 in good yields as

62 Ν. Tokitoh, Υ. Matsuhashi, Κ. Shibata, Τ. Matsumoto, Main Group Metal Chemistry Η. Suzuki, Μ. Saito, Κ. Manmaru and R. Okazaki shown in Scheme 11.22)

40 56%

3 Ph3P=S

PhNCS τ\ /\ Sn C=N 28 93% Tip'' V Ph

Ph Ο VA Sn ^Ph 32 33% Tip/ o-^

32 52% Scheme 11.

On the other hand, in the case of deselenation of tetraselenastannolane 5b with triphenylphosphine under similar reaction conditions to those for desulfurization of 4b, both cis- and trans-isomers

Tbv Se Tb Se Tb, Se .Tb \ / Se ", / \ Sn | Sn Sn ''· / \ / Sn Sn Tiτ- p / Se Tip^ NSe \ip Tip/ Se Tip 41 13% 42 65%

3 Ph3P

-78 °C - r.t. 3Ph3P=S^

Ph Ο V-Λ Sn ^wPh -78 °C ~ r.t. Tip/ W

35 53%

Scheme 12.

63 Vol. 17, Νos. 1-4, 1994 New Aspects in the Chemistry of Low-Coordinate Compounds of Group 14 Elements of 1,3,2,4-diselenadistannetanes 41 and 42, the dimers of intermediary stannaneselone 33, were obtained, while at low temperature the initially formed stannaneselone 33 could be trapped by styrene oxide in a moderate yield. These results suggest that the steric hindrance around the tin- selenium bond of 33 is not large enough to prevent the dimerization of longer tin-chalcogen double bond of 33 than that of stannanethione 24 .

Conclusion

Novel, kinetically stabilized metallathiones and metallaselones of group 14 metals were synthesized by taking advantage of a new steric protection group Tb. Especially in the case of germanethione Tb(Tip)Ge=S (23), we have succeeded in its isolation as orange crystals and characterization by X-ray crystallographic analysis. Metallathiones and metallaselones here obtained underwent ready cycloadditions with various reagents, which are of great importance from the viewpoint of elucidating the intrinsic nature of group 14 metal-chalcogen double bonds. Particular- ly striking are their [2+4]cycloaddition reactions with 2,3-dimethyl-1,3-butadiene, which demon- strate that these metal-chalcogen double bonds have a considerable extent of ene-character like their carbon analogues such as thioketones and selenoketones.

Acknowledgment

This work was partly supported by Grant-in-Aid for Scientific Research from the Ministry of Educatin, Science and Culture, Japan. We are grateful to Dr. Midori Goto, Institute for Material and Chemical Research, and Dr. Yukio Furukawa, the University of Tokyo, for the crystallographic analysis of new dichalcogenastannetanes and the measurement of the FT-Raman spectrum of germanethione, respectively. We also thank ASAI Germanium Research Institute, Shin-etsu Chemical Co., Ltd., and Tosoh Akzo Co., Ltd. for the generous gift of tetrachlorogermane, chlorosilanes, and alkyllithiums, respectively.

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Received: September 29, 1993