synthesis of silylene and silyl(silylene)metal complexes

291© 2002 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

Synthesis of Silylene andSilyl(silylene)metal Complexes

HIROSHI OGINOMiyagi Study Center, The University of the Air, 2-1-1 Katahira, Aobaku, Sendai 980-8577, Japan

Received 23 March 2002; Accepted 11 June 2002

ABSTRACT: In 1987, two research groups published the first-ever reports on the synthesis of sily-lene complexes and presented structural evidence. Since then, a range of synthetic methods have beendeveloped and a number of silylene complexes have been prepared. In 1988, we reported on the firstbase-stabilized bis(silylene) complexes that can be regarded as being masked silyl(silylene) complexes.These complexes occupy a unique position among silylene and silyl(silylene) complexes in that theyprovide a convenient tool for studying the reactivity of coordinated silylenes. They are stable enoughto be isolated, but the bond between the silylene silicon atom and the internal base can easily becleaved by thermal perturbation to generate real silyl(silylene) complexes. To date, a number of base-stabilized bis(silylene) complexes have been prepared in which the central metals range from group5 to group 9. Only two base-free silyl(silylene) complexes have been prepared. One is prepared byreacting a platinum complex with a stable silylene; the other is produced by the photolysis of a tung-sten complex in the presence of a hydrodisilane. © 2002 The Japan Chemical Journal Forum andWiley Periodicals, Inc. Chem Rec 2: 291–306, 2002: Published online in Wiley InterScience(www.interscience.wiley.com) DOI 10.1002/tcr.10034

Key words: silylene complex; silyl(silylene) complex; base-stabilized bis(silylene) complex; 1,3-migration; 1,2-migration

Introduction1

Silylene complex 1 is a silicon analog of carbene complexes.Silylene and silyl(silylene) complexes have attracted attentionas possible intermediates in the transition metal-catalyzeddehydrogenative coupling of hydrosilanes and the scramblingof substituents on silanes.2,3

As early as 1970, Kumada and colleagues reported thathydropentamethyldisilane in the presence of trans-PtCl2(PEt3)2

undergoes monomerization and oligomerization (Eq. 1),3a,4

which can be explained by the mechanism shown in Figure 1.The oxidative addition of the disilane to the coordinativelyunsaturated platinum complex PtCl2(PEt3), followed by thereductive elimination of the monosilane, produces a silylenecomplex. Then, the addition of disilane or oligosilanes to the silylene intermediate and the reductive elimination ofSinMe2n+1H regenerate the reactive platinum catalyst.

The Chemical Record, Vol. 2, 291–306 (2002)

T H EC H E M I C A L

R E C O R D

� Correspondence to: H. Ogino; e-mail: [email protected]

LnM SiR

R1

trans-PtCl2(PEt3)2

90 °C, 18 hSinMe2n+1H

n = 1 - 6

Si2Me5H

(1)

© 2002 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

T H E C H E M I C A L R E C O R D

292

Ojima and colleagues reported that in the presence ofWilkinson’s catalyst, dihydrosilane H2SiPhMe undergoes thescrambling of its substituents together with dehydrogenativesilane coupling (Eq. 2).3d The reaction can be explained by themechanism shown in Figure 2. The oxidative addition reactions

of two molecules of the dihydrosilane to the coordinativelyunsaturated complex [Rh] produce a silyl(silylene) complex thatis the key intermediate. A 1,3-R-shift in the complex, followedby the reductive elimination of a monosilane, causes a scram-bling of the substituents, whereas a 1,2-silyl shift in the complex,followed by further reductive elimination, produces the dehy-drogenative coupling product, dihydrodisilane. The formeddihydrodisilane can react with [Rh], which explains the forma-tion of the dihydrotrisilane. To confirm the roles of the silyleneand silyl(silylene) complexes, we would have to prepare and, ifpossible, isolate the silylene and silyl(silylene) complexes.

� Hiroshi Ogino was born in Matsue in 1938. He received his MSc degree from Tohoku University in 1962 and became a research associate there in the same year. He received his DScdegree in 1966, became associate professor in 1968, and professor in 1983. Dr. Ogino workedas Dean of the Graduate School of Science and Faculty of Science, Tohoku University, from1996 to 1999. He spent a year in 1968–1969 as a postdoctoral fellow at the University of Illinois with Professor John C. Bailar, Jr., one month in 1993 as an invited professor at Université Louis Pasteur in Strasbourg, France, and twice spent two weeks in 1990 and 1994as an invited professor of the Consortium of the University of the Philippines, Ateneo de ManillaUniversity, and De La Salle University. After his retirement from Tohoku University at the endof March 2001, he moved to the University of the Air, where he was appointed Director of theMiyagi Study Center. He is a member of the international advisory editorial boards of Journalof the Chemical Society, Dalton Transaction (Royal Society of Chemistry). He served as amember of the international advisory editorial boards on Organometallics (American Chemical Society). He also served as associate editor of the Bulletin of the Chemical Society ofJapan and as one of the editorial board members of Chemistry Letters. From 1994 to 1996 hewas one of the directors of the Chemical Society of Japan, from 1996 to 2000 he was presidentof the Society of Coordination Chemistry, Japan, and from 2000 to 2002 he was president ofThe Society of Silicon Chemistry, Japan. He received the Award of the Chemical Society of Japan for Distinguished Young Chemists in 1970 and the Award of the Chemical Society ofJapan in 2001. His early research interests focused on the kinetic and equilibrium studies of coordination compounds as well as the synthesis of Werner-type complexes. One of the resultswas the first synthesis of rotaxanes using the principle of spontaneous threading. His currentresearch interests include the synthesis, structure, and reactivity of organometallic andinorganometallic compounds. �

Fig. 1. Possible mechanism of monomerization and oligomerization of hydrodisilane by trans-PtCl2(PEt3).

H2SiPhMeRhCl(PPh3)3

70 °C, 1 hSiPh3Me + HSiPh2Me

H SiPh

MeSiPh

MeH+ + H Si

Ph

MeSiPh

MeSiPh

MeH

Synthesis of a Silylene Complex by Schmid and Welz

When we began our work on silylene and silyl(silylene) com-plexes in 1985, there had only ever been one report on the syn-

(2)

S i l y l e n e a n d S i l y l ( s i l y l e n e ) m e t a l C o m p l e x e s

293

thesis of a silylene transition metal complex—that publishedby Schmid and Welz (Eq. 3).6 The irradiation of Fe(CO)5 inthe presence of HSiMe2(NEt2) produced a diethylamine-stabilized silylene complex (OC)4Fe(¨SiMe2◊NHEt2) 2.However, this compound is unstable at room temperature andhence cannot be structurally characterized by X-ray crystal-lography. The reaction may be explained by the mechanismshown in Equation 3. The photolysis of Fe(CO)5 generates the16-electron complex Fe(CO)4 that reacts with HSiMe2(NEt2)to give the oxidative addition product Int A. The hydrideligand in Int A migrates from the iron center to the diethy-lamino group to give the final product 2, as the diethyl–aminogroup is more basic than the iron atom.

Elusive to Conclusive Silylene Complexes

Until 1987, no X-ray structural evidence of a silylene complexhad been reported, largely because of the instability and highreactivity of these species. Ten years after Schmid and Welz,two groups reported on the synthesis of base-stabilized silylenecomplexes. Zybill and colleagues synthesized silyleneiron com-plexes by applying a salt elimination method (Eq. 4),8 whereasTilley and colleagues synthesized a silyleneruthenium complexby using a triflate abstraction method (Eq. 5).9 These productswere characterized by X-ray crystallography. As mentionedabove, complex 2 prepared by Schmid and Welz was unstable.Interestingly, Braunstein and colleagues succeeded in isolatingcomplex 3, closely related to 2, as a stable product using almostthe same method as that used by Schmid and Welz (Eq. 6)10

and determined its X-ray crystallographic structure.


Fig. 2. Proposed mechanism of reaction 2.

Fe(CO)5 + HSiMe2(NEt2)h �

-CO(OC)4Fe SiMe2

NHEt2

2

h �

-COFe(CO)4 (OC)4Fe

H

SiMe2

NEt2

Int A

The M¨Si double bond in the silylene complexes can bedescribed by the same kind of s-donation/p-back donationthat has been recognized for carbene complexes (Fig. 3).However, the back donation from the dp orbital of the metalcenter to the vacant p orbital of the silylene silicon atom isweak, compared with the carbene complexes, so that theM¨Si double bond is strongly polarized toward Md-

¨Sid+.This means that the silicon atom is highly electron-deficient.Consequently, the silylene ligand can be stabilized by coordi-nating a base. In this case, p-back donation occurs from thedp orbital to the s* orbital of the silicon-base bond. This viewof the M¨Si double bond has been supported by theoreticalinvestigations.7

Fig. 3. Schematic diagram of the M¨Si bonds of silylene complex (a) andbase-stabilized silylene complex (b). B denotes a base.

(OC)4Fe Si

OtBuOtBu

base

(tBuO)2SiCl2 + Na2Fe(CO)4+ base

base = HMPA, THF

- 2NaCl

(3)

(4)



294

In 1993, Tilley and his group reported on the synthesis ofthe first base-free silylene complexes 4 and 5.11 Since 1987,several synthetic routes to base-free and base-stabilized silylenecomplexes have been developed in addition to those shownabove.

Ligation of Free Silylene to Metal Complexes

The direct reaction of a coordinative unsaturated metal frag-ment [M] with a free silylene will provide the most straight-forward means of preparing silylene complexes (Eq. 7).Recently, the isolation of several divalent organosilicon specieshas been reported.12 Further, silylene complexes have been prepared by using these stable silylenes.12a,13 Compounds 6 and 7 are such examples.12a,13a This method can also beapplied to very unstable silylenes. Compounds 8 and 9are examples of silylene complexes that are prepared by thereaction of metal complexes with the free silylenes that are generated either thermally or photochemically (Eqs. 8, 9).14,15

Metal-Mediated Dihydrogen Elimination from Dihydrosilanes

Corriu and colleagues used this method to synthesize silylenecomplexes.16 Equation 10 shows an example.

Ru Si

Me3PMe3P

Ph

Ph

NCMeCp*(Me3P)2RuSiPh2(OTf) + NaBPh4

+ MeCN

- NaOTfBPh4

-

-CO(OC)4Fe Si

NMe2

NMe2

NHMe2

Fe(CO)5 + HSi(NMe2)3

h �

3

SiOsOCOC

CO

COSpTol

RuPt

H PCy3

Cy3P Si

SEt

SEt

BPh4

45

(6)

[M] + : SiR2 [M] SiR2

SiN

N

tBu

tBu

Ni(CO)2

RN

NR

Si Pt SiNR

RNCl

2 2 2

6 7

(7)

Pt(PCy3)2 + (Me3Si)2SiMes2

h �

-(Me3Si)2

Pt(Cy3P)2 SiMes2

9

Os(TPP)2 +Me2C CMe2

Me2Si D, THF

-Me2C=CMe2

(TPP)Os SiMe2

THF

8 (8)

(9)

Ph2SiH2 + Fe(CO)5

h �

+ HMPA-CO, -H2

(OC)4Fe SiPh2

HMPA

(10)

1,2-H Shift from Silyl Ligand to the Central Metal17

An example is shown in Equation 11.17a

PPt

P Me

Si

iPr

iPr H Mes

Mes+ B(C6F5)3

PPt

P H

Si

iPr

iPr Mes

Mes

iPr

iPriPr

iPr

MeB(C6F5)3

(11)

FeOC

OC

Si

pTolpTol

HFe

OCOC

Si

pTolpTol

HMPA

+ Ph3CPF6 PF6-

– Ph3CH

+ HMPA

(12)

Hydride Abstraction from a Hydrosilyl Complex

Hydride abstraction from a hydrosilyliron complex in the presence of HMPA afforded an excellent yield of a cationic silyleneiron complex that is stabilized by HMPA (Eq. 12).18

(5)

Coordinatively unsaturated disilanyl complexes mightgive rise to silyl(silylene) complexes through a 1,2-shift of the terminal silyl group to the central metal. We have exten-sively investigated these types of reactions and discuss thembelow.

Photochemical Reactions of Disilanyl(carbonyl)Complexes

Pannel’s group19 and our group20 reported that the irradiationof a disilanyliron complex 10 gave rise to a monosilyliron


295

complex 11 (Fig. 4). Extensive mechanistic investigations19–24

revealed that the reaction is based on the mechanism shown inFigure 4. Irradiation causes the elimination of a carbonylligand, after which a 1,2-silyl shift of the terminal silyl groupto the iron atom occurs to produce the silyl(silylene) complex.However, this is unstable and the silylene ligand is replaced bycarbon monoxide to produce the monosilyl complex.

To determine which silicon atom of the disilanyl ligand isleft on the metal center, two substituents (R) were introducedon the terminal silicon atom in 10. We were surprised to findthat the photolysis of 12 gave a mixture of three monosilyl-

iron complexes 13a–13c. Mechanistic studies revealed that thenet reaction has four steps and that silyl(silylene) complexesare key intermediates (Fig. 5). (i) Photochemical extrusion ofa carbonyl ligand; (ii) 1,2-migration of the terminal silyl groupfrom the silicon atom to the iron atom; (iii) 1,3-alkyl groupshift from the silyl group to the silylene group to give threedifferent silyl(silylene) complexes; and (iv) ligand substitutionof the silylene ligands by carbon monoxide to give the scram-bling products of substituents 13a–13c. The 1,3-alkyl shift[step (iii)] occurs quickly enough to attain an equilibrium, thatis, step (iii) is faster than the step (iv): the product ratio in thephotolysis of CpFe(CO)2SiMe2SiMe(CD3)2 was found to be13a :13b :13c = 57 :33 :10. The observed ratio is very close tothe statistically predicted product ratio (60 :30 :10) if weassume that step (iv) occurs after attaining equilibrium in theinterconversion between the three intermediate silyl(silylene)complexes [step (iii)].22

According to the mechanism shown in Figure 4,dimethylsilylene must be released into solution upon the irradiation of disilanyliron complex 10. To confirm this, thephotolysis of 10 was performed in the presence of various sily-lene-trapping agents, namely, 2,3-dimethyl-1,3-butadiene,diethylmethylsilane, and hexamethylcyclotrisiloxane.20,21

However, no silylene-trapped product was detected. Pannelland colleagues found that the irradiation of 10 in the presenceof triethylsilane (silylene-trapping agent) produces a significant


Fig. 4. Proposed mechanism of the photochemical formation of 11 from 10.

Fe SiMe2SiMeR2OC

FeSiMe2R

OC SiMeR

– COFe SiMe2SiMeR2OC

OC

+CO

Fe

SiMe2

SiMeR2

Fe SiMe2ROC

OC

+CO

FeSiMe3

OC SiR2

Fe SiMeR2OCOC

+CO

Fe SiMe3OCOC

hν

12 : R = CD3, Et

– [:SiMeR]– [:SiMe2]– [:SiR2]

13a 13b 13c

(i)

(ii)

(iii)( iii)

(iv) (iv) (iv)

CO

Fig. 5. Possible mechanism for scrambling of substituents upon irradiation of CpFe(CO)2SiMe2SiMeR2.



296

amount of oligosiloxane (Me2SiO)n and a very small amount(approximately 1%) of dimethylsilylene-trapped productEt3SiSiMe2H.24 This suggests that dimethylsilylene is indeedgenerated upon irradiation, but that it is effectively trapped bycarbon monoxide to produce the oligosiloxane.

The 1,3-R shift of the substituents has been observed in anumber of mononuclear and dinuclear silyl(silylene) com-plexes.19–23,25 Two examples of this are discussed below. Berryand colleagues found that interconversion occurs between themethyl and X groups of silicon atoms in disilyltungstenocene(Eq. 13).25f This reaction can be easily understood if we considerthe formation of a cationic silyl(silylene)tungsten complex as anintermediate and the 1,3-methyl shift within it (Eq. 14).

Figure 6. The reaction of the starting complex with two mol-ecules of diarylsilane produces a silyl(silylene) complex that isaccompanied by the elimination of H2. The 1,3-migration ofan aryl group in the silyl group to the silylene ligand, togetherwith the reductive elimination of triarylsilane, produces thefinal product.

Fe FeP

SiOCOC

OC COPh Ph

HR

Fe FeP

COCOC

OC COPh Ph

O

2R2SiH2 R3SiH

R = Ph, pTolr.t. ~ 80 °C

– CO, – H2+ +

(15)

Fe Fe

P

COC

OC

OC COPh Ph

O

Fe Fe

P

OCOC

OC COPh Ph

H

SiHR2

COFe Fe

POC

Si

OC COPh Ph

HSi

COH

R RHR

RH

Fe Fe

POC

Si

OC COPh Ph

H

Si

CO

RR

RRH

Fe Fe

POC

Si

OC COPh Ph

HSi

CO

RR RHR

Fe Fe

P

Si

OCOC

OC CO

Ph Ph

HR1,3-Rshift

H2

R2SiH2 R2SiH2

– CO

R3SiH

Fig. 6. Possible mechanism of reaction 15.

SiiPr2X

SiMe3

SiiPr2Me

SiMe2XW

X = Cl (160 °C), OSO2CF3 (25 °C)

W

(13)

SiiPr2

SiMe3W

1,3-Me shift SiiPr2Me

SiMe2WX- X-

(14)

CpFe(CO)2 2Si Me5 + Et2MeSiHh �

CpFe(CO)2SiMeEt2

14- HSi2Me5

CpFe(CO)2SiMe2Rh �

R = SiMe3, Me

CpFe(CO)SiMe2R + CO

(16)

(17)

As mentioned above, the photolysis of 10 in the presenceof diethylmethylsilane does not produce a silylene-trappedproduct but instead produces a significant amount of silylgroup exchanged species CpFe(CO)2SiMeEt2 14 (Eq. 16).20,21

The same product was also obtained upon the photolysis ofCpFe(CO)2SiMe3 in the presence of diethylmethylsilane. Theformation of 14 can be interpreted as evidence that the reac-tions involve the successive oxidative addition and reductiveelimination of hydrosilane (Eqs. 17–20).

The reaction of an unsymmetrical phosphido-bridged dinuclear complex with a diarylsilane produces amonoarylsilylene-bridged complex and a tertiary silane (Eq.15).25e The reaction can be explained by the formation of asilyl(silylene) complex, the mechanism of which is proposed in

CpFe(CO)SiMe2R + Et2MeSiH CpFe(CO)H(SiMe2R)(SiMeEt2) (18)

CpFe(CO)H(SiMe2R)(SiMeEt2) CpFe(CO)SiMeEt2 + RMe2SiH (19)


297

Synthesis of Silyl(silylene) Complexes

Fischer and Maasböl reported the first stable carbene complex 17with a methoxy group on the carbene carbon atom.32 Thecomplex is stabilized by several resonance forms. We believed thatwe would be able to achieve a similar stabilization for the silylenecomplexes: if the CpFe(CO)2SiMe(OMe)SiMe3 starting mater-ial 18 was to be photolyzed, silyl(silylene) complex 19 might beformed (Eq. 25) because the methoxy group is located on the sily-lene silicon atom. If the methoxy substituent on the silylenesilicon atom sufficiently stabilizes the silylene ligand, the photol-ysis of CpFe(CO)2SiMe2SiMe2(OMe) 20 could be expected toproduce the same product 19 as that described in Equation 25through 1,3-migration in the silyl(silylene) complex (Eq. 26).


CpFe(CO)SiMeEt2 + CO CpFe(CO)2SiMeEt2h �

Fe

OCOC

SiMe3

O

Fe FeC

Si

H

ROC CO–2HSiMe3

RSiH32h �

+

R = tBu, (CMe2)2H

15

O CO

Fe FeC

Si

X

tBu

OC

O

Fe FeC

Si

X

tBu

OC CO

+

O

Fe FeC

Si

H

tBuOC CO

r.t.

CCl4CHBr3,or CH2I2

cis-16 trans-16

(20)

(21)

(22)

The silyl group exchange reaction mentioned above pro-vides a convenient method of preparing silylene-bridged diironcomplexes.26 The photolysis of CpFe(CO)2SiMe3 in the pres-ence of RSiH3 (R = tBu, (CMe2)2H) gives a silylene-bridgeddiiron complex 15 (Eq. 21). The Si¶H bond on the bridg-ing silylene ligand in 15 was found to be highly activated. Thesilylene ligand undergoes halogenation by halomethane sol-vents under mild conditions (Eq. 22).26d,27,28 The Si¶X bondin product 16 is also reactive toward substitution reactions: theiodine atom in the iodosilylene-bridged complex is eliminatedas an iodide ion by the addition of a strong Lewis base, N-methylimidazole (NMI) or 4-(dimethylamino)pyridine(DMAP), to produce the first base-stabilized silylyne bridgeddinuclear complex (Eq. 23).28 Germylene-bridged diiron com-plexes were also prepared by the photochemical reaction ofCp¢Fe(CO)2SiMe3 (Cp¢ = Cp, Cp*) with tBuGeH3, pTolGeH3,and pTol2GeH2.29 The first base-stabilized germylyne-bridgeddinuclear complexes were also prepared by using a slightlymodified version of the method shown in Equation 23.30 Thephotolysis of CpFe(CO)2Me in the presence of a dihydrosilanewith bulky aryl groups produced a diiron complex that is triplybridged by a silylene ligand and two carbonyl ligands (Eq. 24).This is the only known example of a silylene-bridged complexhaving a triplet ground state.31

Si

Fe Fe

CO

tBu I

COOC

CH3CNFe

CO

OCFe

COSi

tBu Base

I+

+

Base

Base = NMI, DMAP

Et

Et

iPr

iPriPr

Me

Me

Me

Fe

OCOC

Me

Si

Fe Fe

CC

Ar Ar

Ar2SiH2

O O

Ar = , ,

+h �

2–2CO–2CH4

(24)

(23)

(OC)5Cr COMe

Me

CpFe(CO)2SiMe(OMe)SiMe3h �

-COFe

Si

Me

Si

OC

MeOMe

MeMe18

19

17

CpFe(CO)2SiMe2SiMe2(OMe)h �

-CO

20

1,3-Me shiftFe

SiOMe

Si

OC

Me

Fe

Si

OMe

Si

OC

MeMe

MeMe

Me

Me

Me

(25)

(26)

Contrary to our expectations, this was not the case. Thephotolysis of 18 as well as 20 produced the same product 21,which was an unprecedented complex containing a four-membered chelate ring (Eq. 27).33 The two silicon atoms areequivalent, such that the 29Si NMR spectrum exhibits only onesignal. The chemical shift of the signal appears in a remarkably

21

18 or 20h �

-COFe

Si

OMe

Si

OC

MeMe

MeMe

(27)



298

low field (123.7ppm), which is characteristic of species con-taining sp2 silicon atoms.34

The first X-ray structurally characterized complex of this type was Cp*Fe(CO){SiMe2◊◊◊O(Me)◊◊◊SiMe(OMe)} 22(Fig. 7), and the results were reported in 1988.33a At that time,we used the nomenclature for the complex “methoxy-stabilizedbis(silylene)iron complex” because the complex can be expressed as structure A shown in Chart 1. In this structure, amethoxide ion is coordinated with the two silylene ligands of the cationic bis(silylene) complex. Another extreme of theexpression for 22 is structure C in Chart 1: all four bonds in the four-membered ring comprise covalent bonds, and the ring contains an onium ion. However, the charge separationseems to be very slight because 22 and its related complexes arevery soluble in typical nonpolar organic solvents. Therefore,complex 22 can be best expressed as structure B in Chart 1,which lies in the midst of the two canonical structures A and C.The Fe¶Si bond lengths of 22 [2.207(3) and 2.222(3) Å] arevery short, the former being the shortest ever observed at thetime. On the other hand, the bond lengths between each silicon

atom and the trivalent oxygen atom O(2) (1.793(9) and1.799(8) Å) are much longer than the normal Si¶O singlebonds, e.g., Si(2)¶O(3) [1.632(9) Å]. These bond lengthsstrongly support the structure of 22 with its partial double-bondcharacter for the Fe¶Si bonds and partial dative–bond charac-ter for the Si¶O(2) bonds. The sums of the bond angles at thesilicon atoms (C¶Si(1)¶C and two Fe¶Si(1)¶C’s andC¶Si(2)¶O(3), Fe¶Si(2)¶C, and Fe¶Si(2)¶O(3)) are352.5° at Si(1) and 352.6° at Si(2). These values are between thesum of the three valence angles around an ideal sp2-hybridizedatom (360°) and sp3-hybridized atom (328.5°).

Several synthetic methods have been developed for syn-thesizing base- or donor-stabilized bis(silylene) complexes.

Photolysis of LnM(CO)SiMe2SiMe2Do (Do = donor)(method A)

The reaction shown in Equation 27 exemplifies this method.A plausible mechanism for explaining the reaction is shown inEquation 28. First, photolysis causes the elimination of a COligand to give a 16-electron complex. Then, a 1,2-silyl shift ofthe terminal silyl group from silicon to iron takes place to giverise to the silyl(silylene) complex. Finally, internal coordina-tion of the donor to the electron-deficient silylene ligandoccurs to produce the internally donor-stabilized bis(silylene)complex. OMe,33,35 OtBu,33 CH3COO,36 NEt2,37 and SpTol38

groups have been employed as donors. Germylene analogsbase-stabilized bis(germylene)- and germylene(silylene)ironcomplexes were also synthesized.39

Fig. 7. ORTEP view of Cp*Fe(CO){SiMe2◊◊◊O(Me)◊◊◊SiMe(OMe)} 22.

Fe

SiMe

Me

Si OMe

Me

O MeOC

Fe

SiMe

Me

Si OMe

Me

O MeOC

Fe

SiMe

Me

Si OMe

Me

O MeOC

A B C

Chart 1

LnFe SiMe2SiMe2DoCO

h �LnFe SiMe2SiMe2Do

18 electron complex 16 electron complex

1,2-silyl shiftLnFe

SiMe2Do

SiMe2

cyclization

18 electron complex

LnFe

Me2Si

SiMe2

Do

- CO

(28)


299© 2002 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

It is interesting that the photolysis of Cp¢W(CO)3Me (Cp¢ = Cp*, C5Me4Et) in the presence of excess HSiMe2SiMe3

afforded a self-stabilized silyl(silylene)tungsten complex with adimeric structure, where carbonyl ligands act as the base (Eq.33).42 The result demonstrates the strong Lewis acidity of thesilylene ligand, which may be stabilized by the coordination ofa carbonyl ligand.

FeOC

Si

Si

Me Me

Me Me

O

OC MeFe

OCSiMe2SiMe2

OC

O CO

Meh �

-CO

R5 R5

R = H, Me

2423

(29)

FeOC

SiMe2SiMe2

OC

O

R = Me

N h �

-CO

FeOC

Si

Si

Me Me

Me Me

O

R5

CO

Me

Fe

OC

Si

Si

Me Me

Me Me

ON

2526

24' 26'

OC

Si

Si

Me Me

Me Me

N

O

Fe

The irradiation of acetoxydisilanyl complex 23 and 2-pyridyloxydisilanyl complex 25 produced base-stabilizedbis(silylene) complexes having a six-membered chelate ring, 24 and 26, respectively (Eqs. 29, 30).36 Complex 26has an asym-metrical six-membered chelate ring in which the 2-pyridyloxygroup bridges two silylene ligands. The formation of complexeswith a four-membered chelatering, 24¢ and 26¢, was notdetected. This result apparently arises from the fact that the six-membered ring structures of 24 and 26 are less strained than the four-membered structures.

MSi

OC

Me Me

Si SiOMe

MeMeMe

Me

M

OCOC

SiMe2SiMe2SiMe2OMe

M = Fe, Ru

h �

-CO

27

h �

-[SiMe2]M

Si

OMe

Si

OC

MeMe

MeMe

28 (31)

As mentioned above, the silyl(silylene)iron complexformed by the photolysis of disilanyliron complex 10 cannot

FeSiMe3

Me2Si

OCHMPAFe

OCSiMe2SiMe3

OC

+ HMPA– CO

h �

29

W

SiMe3

SiMe2

OCW

Me2SiCO

SiMe3R

ROC

CO

R

WOCOC CO

Me + 2 HSiMe2SiMe3h �

-2CO, -2CH4

2

R = Me, Et

(32)

(33)

Reactions of Thermally Generated UnsaturatedComplexes with HSiMe2SiMe2OR (R = Me, tBu) (method B)

Base-stabilized bis(silylene) complexes can be prepared notonly by photochemical methods but also by thermal methods.As shown in Equation 28, the key step for the formation ofthe silyl(silylene) intermediate is the 1,2-shift of the terminalsilyl group to a coordinatively unsaturated metal center. We found that a coordinative unsaturated (disilanyl)metalintermediate can be generated by the thermal reaction ofCpRu(PPh3)2Me with HSiMe2SiMe2OMe at 90 °C, directlygiving methoxy-stabilized bis(silylene)ruthenium complex 30via 1,2-silyl shift (Eq. 34).43 Similarly, base-stabilized bis(sily-lene)ruthenium complexes containing the pentamethylcy-clopentadienyl ligand Cp*Ru(PMe3){SiMe2◊◊◊O(R)◊◊◊SiMe2} (R = Me, tBu) were also prepared.44

The photolysis of a 3-methoxytrisilanyl complex pro-duced methoxy-stabilized disilanyl(silylene) complex 27.40a

Prolonged irradiation of the product gave rise to the methoxy-stabilized bis(silylene) complex 28 (Eq. 31). However, complex28 can revert to 27 upon the insertion of dimethylsilylene,which is photochemically generated from (SiMe2)6.40b

be isolated (Fig. 4). However, the photolysis of 10 in the presence of HMPA was found to produce HMPA-stabilizedsilyl(silylene) complex 29 (Eq. 32).25i This implies that if astrong Lewis base is used, the silyl(silylene)iron intermediatemay be stabilized without the aid of the chelate effect.Silyl(silylene)tungsten complexes stabilized by external basesCp(OC)2W(SiMe3)(¨SiMe2◊base) have also been synthesizedby the photolysis of CpW(CO)3Si2Me5 in the presence of bases(THF, HMPA).41 The HMPA-stabilized bis(silylene)tungstencomplex was isolated and its molecular structure was deter-mined by X-ray crystallography.

(30)



300

The following three reactions may also belong to this method: the reaction of Ir{h2-Me2Si(CH2)2PPh2}(PMe3)3

with HSiMe2SiMe2OMe gives a methoxy-stabilized bis(silylene)iridium complex (Eq. 35).45 The thermal reactionof Ru(xantsil)(CO)(h6-C6H5CH3) with HSiMe2SiMe2OtBugives a tert-butoxy-stabilized bis(silylene) complex (Eq. 36).46

The third reaction is shown in Equation 37. The reaction of Cp2Ta(H)(h2-C3H6) 31 with HSiMe2SiMe2OMe 32produces hidridobis(disilanyl)tantalocene 33 and methoxy-stabilized bis(silylene)tantalocene 34.47 These products can be separated by fractional crystallization. The structures ofthese products were determined by X-ray crystallography. Theproposed formation mechanisms of 33 and 34 are shown inFigure 8.

+ HSiMe2SiMe2Ot Bu90 min, r.t.

- toluene

O

Me2SiMe2Si

Ru

CO

RuSi

O

MeMe

Si

CO

HSi

Si

Ot BuMe

Me

Me

Me

Me

Me

MeMe

H

HH

(36)

TaH

HSiMe2SiMe2OMe+

3eq.

TaSiMe2SiMe2OMe

SiMe2SiMe2OMe

H Ta

Me2Si

SiMe2

OMe+

31

32

33 34 (37)

Ru

PPh3

Ph3PMe Ru

Ph3PMe-PPh3

+ HSiMe2SiMe2OMe

- CH4

RuPh3P

SiMe2SiMe2OMe Ru

Si

OMe

Si

Ph3P

MeMe

MeMe

30

Ph2P IrPMe3

PMe3

SiMe2

PMe3

+ HMe2SiSiMe2OMer.t., toluene

- 2PMe3Ir

Si

PPh2

Me2Si

PMe3

H

SiMe2

Me

Me

OMe

(35)

Fig. 8. Proposed formation mechanism of complexes 33 and 34.

Hybrid Method of (A) and (B)

The combination of methods A and B, namely, the pho-tolysis of Ln(OC)M-Me in the presence of HSiMe2SiMe2Doshould produce a donor-stabilized bis(silylene) complex by thereaction sequence given in Equation 38. Indeed, donor-stabilized bis(silylene)chromium, molybdenum, and tungstencomplexes Cp(OC)2M{SiMe2◊◊◊Do◊◊◊SiMe2} (Do¨OMe,NEt2) have been prepared by this method.48

LnM MeCO

h �

-COLnM Me

HSiMe2SiMe2DoLnM SiMe2SiMe2Do

18-e complex

LnMSiMe2

SiMe2DoLnM

Me2Si

SiMe2

Do

16-e complex- CH4

(38)

(34)


301

1,2-Elimination of Amines fromHydridobis(aminosilanyl) Complexes

Malisch and colleagues reported that the treatment of hydri-dobis(chlorodimethylsilyl)iron complex with dialkylamineproduces an amino-stabilized bis(silylene)iron complex (Eq. 39).49 Roper and colleagues prepared a similar type of ruthenium complex by reacting a five-coordinate ruthenium(II) complex with a dimethylaminohydrosilane (Eq. 40).50 It is most likely that the key step in both reactions is the 1,2-elimination of amines from the hydridobis(aminosilyl) complex Int B that is formed duringthe reaction (Eq. 41).

Using these methods, we have obtained a number of base-stabilized bis(silylene)metal complexes in which the centralmetals are Ta, Cr, Mo, W, Mn, Fe, Ru, and Ir.

Lappert and colleagues synthesized the first donor-freesilyl(silylene) complex 7 by reacting a platinum complexwith a stable silylene.13 We recently reported the only other

examples of donor-free silyl(silylene) complexes: the photo-lysis of Cp¢W(CO)3Me 36 (Cp¢ = Cp*, C5Me4Et) in the presence of HSiMe2SiMeMes2 gave donor-free silyl(silylene)complex 37 (Fig. 9).42 The 29Si NMR signal of 37 appears at380ppm, which is the lowest reported chemical shift amongthe silylene complexes. X-ray crystallography revealed that thetungsten-silylene silicon bond (2.3850(12) Å) is the shortestamong the W¶Si bond lengths reported to date, and that the geometry around the silylene silicon atom is planar (sumof the three bond angles around silylene silicon atom is359.9(3)°). The proposed mechanism (Fig. 9) consists of thefollowing processes: (i) The photochemical elimination of the carbonyl ligand from 36, (ii) oxidative addition ofHSiMe2SiMeMes2 and the reductive elimination of methaneto give a 16-electron disilanyltungsten intermediate, and (iii) 1,2-silyl shift of the terminal group to form a silyl(silylene)tungsten com-plex Int D. However, this is not thefinal product, because the silylene ligand in the final producthas two mesityl substituents, whereas that in the Int D has twomethyl substituents. Migration of 1,3-Me must take place togive complex 37. This process apparently reduces the stericcongestion in Int D.

Reactivity of Silylene and Silyl(silylene)Complexes

More examples of silylene and silyl(silylene) complexes areappearing. To date, several reactivity studies have been reportedusing isolated complexes.

For the internally base-stabilized bis(silylene) complex, thebonding model B in Chart 1 can be expressed as a combina-tion of the two resonance forms D and E (Eq. 44). This meansthat the base-stabilized bis(silylene) complex can be regardedas being a masked silyl(silylene) complex. The base-stabilizedbis(silylene) complexes can thus act as convenient synthons for


Fe

SiMe2Cl

SiMe2Cl

OCFe

Si

NR2

Si

OC

MeMe

MeMe

HHNR2

(R = Me, Et)

(39)

ClRu

CO

SiEt3

PPh3

PPh3

- HSiEt3, -HNMe2 Ph3PRu

Ph3PMe2Si

SiMe2

Cl

CO

NMe2

2 HSiMe2(NMe2)

(40)

LnM

SiMe2NR2

H

SiMe2NR2

1,2-elimination

Int B

LnM

SiMe2NR2

SiMe2

Int C

LnM

Me2Si

SiMe2

NR2

HNR2- HNR2

(41)

Recently, we found that the reaction of base-stabilizedbis(silylene)iron complex 26 with 2-hydroxypyridine giveshydridobis(2-pyridyloxysilyl)iron complex 35 quantitatively(Eq. 42). However, the treatment of 35 with Lewis acid AlEt3 (two equiv.) gave 26 (Eq. 43). The latter reaction corresponds to the 1,2-elimination of 2-hydroxypyridine from 35 to give the base-stabilized bis(silylene) complex 26.51

FeOC

Si

Si

Me Me

Me Me

N

O+

NHO Fe

OC

Si

Si

H

O

O

N

N

Me Me

Me Me

r.t.

26 35

FeOC

Si

Si

H

O

O

N

N

Me Me

Me Me

35

AlEt3

Et2AlON

-EtH

-

FeOC

Si

Si

Me Me

Me Me

N

O

26

(42)

(43)

Fe

SiMe

Me

Si OMe

Me

O MeOC

Fe

SiMe

Me

Si OMe

Me

O MeOC

D E (44)



302

studying the reactivity of silylene and silyl(silylene) complexes.They are stable enough for isolation, but the bond between thesilylene silicon atom and the internal base can easily be cleavedby thermal perturbation to generate a real silyl(silylene)complex.

The base-stabilized silylene complex [Cp*(Me3P)2Ru(¨SiPh2◊NCMe)]BPh4 reacts with alcohol to produce analkoxysilane (Eq. 45). As part of the reaction, the formation ofthe alcohol-addition product [Cp*(Me3P)2Ru(H)SiPh2OR]BPh4 or [Cp*(Me3P)2Ru(h2-HSiPh2OR)]BPh4 has been pos-tulated.52 The reaction of Cp¢Fe(CO){SiMe2◊◊◊O(Me)◊◊◊SiMe2}38 (Cp¢ = Cp, Cp*) with MeOH gave the methanol-addition product Cp¢Fe(CO)H(SiMe2OMe)2 quantitatively.53

The reactions of methoxy-stabilized bis(silylene)rutheniumcomplex 30 with MeOH and H2O are summarized in Figure 10.54 The complex reacts vigorously with MeOH to produce the methanol-addition product 39 in a mannersimilar to the reaction of 38 with MeOH. In this case, theRu¶Si bond is retained. The reaction is believed to be initiated by the nucleophilic attack of MeOH on the electron-deficient silylene ligand. Complex 39 subsequently reacts very slowly with the excess MeOH to give complex 40and Me2Si(OMe)2. The reaction of 30 with H2O gave a metallacycle 41. The reaction is also believed to be initiated

by the nucleophilic attack of H2O at the silylene ligand to give Int E (Eq. 46). The subsequent intramolecular con-densation with the elimination of MeOH takes place to give 41 with siloxane bonds. Complex 41 further reacts with H2O to afford dimeric complex 42, which isbelieved to be formed through H2O-addition productCp(Ph3P)Ru(H)2(SiMe2OSiMe2OH).

Cp¢

WOC Me

OC CO

Cp¢ = Cp*, C5Me4Et

+ HSiMe2SiMeMes2

hnCp¢

WMes2Si SiMe3

OC CO

hn -CO

Cp¢

WOC Me

OC HSiMe2SiMeMes2

Cp¢

WOC

OCSiMe2SiMeMes2

Cp¢

WMeMes2Si SiMe2

OC CO

1,2-silyl shift

1,3-Me shift

Int D

36 37

-CH4

-CO, -CH4

Fig. 9. Photochemical reaction of complex 36 with HSiMe2SiMeMes2 and a plausible reaction mechanism.

[Cp*(Me3P)2Ru(=SiPh2 NCMe)]BPh4 + ROH

[Cp*(Me3P)2Ru(NCMe)]BPh4 + HSiPh2OR

(R = Me, Et, tBu)

(45)

RuSi

SiOMe

Ph3P

MeMe

Me Me

Ru Si

SiMe2OH

Ph3P

OMe

H

RuSiMe2

SiO

Ph3PH

OH

H

Me2

Me2

30 Int E 41

- MeOH

(46)

When a toluene solution of 30 was heated to 130 °C, a C¶H bond of a phenyl group in PPh3 was activated by the Ru¨Si double bond and the complex CpRuSiMe2


303

The dynamic behavior of the 1,3-Me migration from thesilyl ligand to the silylene ligand in Cp*(OC)Fe(SiMe3)(¨SiMe2◊HMPA) 29 was demonstrated by variable tempera-ture 1H NMR spectroscopy.25i As the temperature is raisedfrom 250 K, the two singlet signals for the diastereotopic Megroups in the SiMe2 ligand broaden, coalesce, and become asharp singlet signal at 280 K. Two doublet signals for the freeand coordinated HMPA also become a sharp doublet signalabove 280 K. These spectral changes suggest that the exchangeof the Me groups in the SiMe2 ligand occurs concurrently withthe exchange of the coordinated HMPA molecule with freeHMPA in solution. As the temperature is raised from 280 K,the two singlet signals for the SiMe2 and SiMe3 ligands gradually broaden, then coalesce at 318 K, producing a sharp singlet signal at 360 K. This change indicates that the1,3-Me migration from SiMe3 to SiMe2 ligands occurs on the NMR time scale. A similar fluxional behavior was alsoobserved for donor-stabilized silyl(silylene)tungsten complexCp(OC)2W(SiMe3)(¨SiMe2◊THF).41


Fig. 10. Reaction of 30 with ROH (R = Me, H).

IrSi

PPh2

H

PMe3

Me

Me

MePMe3

IrSi

PPh2

H

PMe3

PMe3

HMe

Me+ H3SiR

(R = n-butyl, pentyl, hexyl)

45 °C

44 45

+ H2MeSiR

(48)

(o-C6H4PPh2)(H)(SiMe2OMe) 43 was formed.55 A possiblemechanism for this reaction is given by Equation 47. TheSi¶O bond in the four-membered chelate ring of 30 iscleaved to generate Int F, and the electron-deficient silylenesilicon atom electrophilically attacks the ortho carbon of thephenyl group in the PPh3 ligand. The proton in the ortho posi-tion is abstracted by the electronegative ruthenium center toproduce 43. Roper and colleagues reported that the reactionof OsHCl(PPh3)3 with Hg(SiMe3)2 produces OsSiMe2(o-C6H4PPh2)(o-C6H4PPh2)(CO)(PPh3) as one of the products.56

They believed that the formation of this compound mayinvolve an electrophilic silylene complex as an intermediate.

Fig. 11. Possible mechanism of reaction 48.

Recently, we found that the reaction of hydri-do(methyl)iridium complex 44 with trihydrosilane (SiRH3; R= pentyl, hexyl, n-butyl) resulted in silicon-carbon bond for-mation to give H2MeSiR and the dihydridoiridium complex45 (Eq. 48).57 Mechanistic studies revealed that the reactionproduces a methyl(silylene) complex (Int G) after which 1,2-methyl migration occurs from the metal center to the silyleneligand (Fig. 11). The mechanism was supported by theoreticalcalculations using the B3LYP methods.58 Recently, anotherexample of a similar 1,2-shift was reported by Ozawa and colleagues.59

RuSiMe2

Me2Si

OMePh3P

Ru SiMe2

SiMe2OMe

Ph2P

Ru SiMe2

SiMe2OMe

Ph2P H

RuSiMe2

SiMe2OMe

Ph2P

H

d

30

d

Int F

43 (47)



304

Conclusion and Future Work

Reactivity studies have shown that the M¨Si double bond inthe silylene and silyl(silylene) complexes is strongly polarizedtowards Md-

¨Sid+, even though the experiments have beenlimited to the central transition metals. In other words, we cansay that these complexes easily undergo nucleophilic attack atthe silylene silicon atom. This situation is similar to thatobserved for the Fischer carbene complexes. However, in theSchrock carbene complexes that are found with the early transition metals, the carbene carbon atom undergoes an electrophilic, as opposed to a nucleophilic, attack. Therefore,the reactivities of the silylene complexes of the early transitionmetals should also be very interesting.

The 1,3-substituent migration of silyl(silylene) complexesis well documented. The 1,2-methyl shift from a metal centerto the silylene ligand that we recently observed in our labora-tory may be applied to the transition metal-mediated func-tionalization of alkanes.

The photolysis of transition metal-carbonyl complexescontaining disilanyl ligands Ln(OC)MSiMe2SiMe3 producedsilyl(silylene) complexes. We are currently investigating theeffect of irradiation on Ln(OC)MSiMe2ERm (E denotes typicalelements: e.g., N, P, S, Se, Te).60

This work was conducted by the Department of Chemistry,Graduate School of Science, Tohoku University. I sincerelythank Professors H. Tobita (Tohoku University), K. Ueno(Gunma University), and J. R. Koe (International Christian University); Drs. H. Hashimoto and M. Okazaki; and all those persons who contributed to our work reviewed here.

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synthesis of silylene and silyl(silylene)metal complexes

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