Thiocarbonyl, thiocarbyne and thiocarbene ligands in di- and polynuclear complexes

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Inorganica Chimica Acta 361 (2008) 3004–3011 www.elsevier.com/locate/ica

Review

Thiocarbonyl, thiocarbyne and thiocarbene ligands in di- and polynuclear complexes Luigi Busetto *, Valerio Zanotti Dipartimento di Chimica Fisica ed Inorganica, Universita` di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy Received 16 October 2007; received in revised form 21 December 2007; accepted 3 January 2008 Available online 9 January 2008 This work is a contribution for the special issue dedicated to Prof. Robert J. Angelici on the occasion of his 70th birthday.

Abstract Thiocarbonyl (CS), homologous to the ubiquitous carbonyl ligand, has interesting and unique properties as ligand. Nevertheless it did not reach the widespread use of CO in the formation of transition metal complexes. This short account, dedicated to professor R.J. Angelici, is focused on the multisite coordination of thiocarbonyl ligand in di- and poly-nuclear transition metal complexes, and to its transformation into thiocarbyne and thiocarbene ligands. These latter, in turn, can be transformed, providing access to a variety of new ligands and functionalities, which are here briefly reviewed. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Thiocarbonyl; Thiocarbyne; Thiocarbenes; Dinuclear complexes; Bridging ligands

Luigi Busetto Postdoctoral-fellow at the Iowa State (USA) from 1967 to 1968, full Professor of General ed Inorganic Chemistry, since 1980, at the University of Bologna, visiting professor at Bristol University (UK) in 1982. He is author of more than 120 papers on transition metal organometallic chemistry. His research interests includes: carbene and carbyne mono and dinuclear complexes, carbon sulfide activation, thiocarbonyl complexes, nucleophilic additions to carbonyl complexes, cyclopentadienyl compounds, C–C and C–H bond formation. From 1995 to 1997 Director of the Department of Chimica Fisica ed Inorganica and, from 1998 to 2000, Dean of the Faculty of Industrial Chemistry in Bologna, where he founded the inorganic chemistry research group. He has been President of the Inorganic Division of Italian Chemical Society (2001– 2003). From 2000 he is vice-director of the University of Bologna and he is italian representative member of the EuCheMS (Organometallic Division).

*

Corresponding author. Tel.: +39 0512093694; fax: +39 0512093690. E-mail address: [email protected] (L. Busetto).

0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.01.001

Valerio Zanotti graduated in Industrial Chemistry (cum Laude) at the University of Bologna in 1981. He joined the group of prof. L. Busetto, in the same university (1983), and in 1989–1990 he was postdoctoral-fellow in the group of Prof. R.J. Angelici at the Iowa State University. He received the ‘Flavio Bonati award’ by the Organometallic Group of the Italian Chemical Society in 1992, and was appointed professor of inorganic chemistry in 1998. He is currently full professor at the University of Bologna and his major scientific interests involve the activation of small molecules by dinuclear transition metal complexes.

L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011

3005

Contents 1. 2. 3. 4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CS coordinated to di- and polynuclear complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bridging thiocarbyne complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions of the bridging thiocarbyne [Fe2(l-CSMe)(CO)3(Cp)2]SO3CF3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Cleavage of the C–SR bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Conversion of bridging thiocarbynes into bridging thiocarbenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Coupling of the bridging thiocarbynes with olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S

1. Introduction Transition metal bonded thiocarbonyl (CS), homologous to the ubiquitous carbonyl ligand, has always represented a fascinating and challenging topic in organometallic chemistry. This short account, dedicated to professor R.J. Angelici, is focused on the chemistry of the thiocarbonyl ligand and of its congeners: the thiocarbyne and thiocarbene ligands, with particular regards to their coordination to di- and polynuclear metal complexes, and to our contribution in the field. Since the first thiocarbonyl complex, trans[RhCl(PPh3)(CS)], obtained by Wilkinson et al. [1], the number of CS containing complexes rapidly grew owing to the studies of several excellent research groups. Remarkable contributions from Angelici, Butler, Roper, Broadhurst, Werner, Petz have been reported, among others, in excellent reviews [2]. In spite of these efforts and of superior properties of CS as a ligand compared to CO, the thiocarbonyl complexes are still far from having the widespread use of metal carbonyls. The reason is mainly synthetic and mostly derives from the lack of a direct source of CS and of straight synthetic methods, which is still an open challenge for synthetic chemists. 2. CS coordinated to di- and polynuclear complexes The synthesis mononuclear thiocarbonyl compounds was soon followed by the development of bridging thiocarbonyl complexes, which exploited the excellent properties of CS to act as ligand capable of coordination to more than a single metal centre. As shown in Scheme 1, a variety of coordination modes have been found: bridging coordination (A), which is the most common; bridging side on (B) [3]; bridging end-to-end (C) [4] and triply bridging (D) [5]. Moreover, since the sulfur atom of these thiocarbonyl complexes maintains relevant coordination ability, an additional metal fragment can be coordinated generating the forms E [6] and F [5a,5b]. Type A complexes can be obtained by coupling of mononuclear thiocarbonyl complexes. As an example, the

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

3005 3005 3007 3007 3008 3008 3010 3010 3011 3011

S

C [M]

. . . . . . . . . .

C [M']

[M]

[M] C

B

A

S

C [M']

S

[M']

C

S

S [M]

[M]

[M'] [M']

C

C

[M]

[M] [M]

[M]

[M] [M]

D

E

F

Scheme 1.

reductive coupling of [Fe(CO)2(CS)(Cp)]+ (Cp = g5C5H5) affords a mixture of the mono- and di-thiocarbonyl complexes 1 and 2 (Scheme 2) [7]. Similarly, the homodinuclear l-thiocarbonyl complexes [Mn(CO)(NO)(CS)(Cp)]2 [8], [Ru2(CO)3(CS)(Cp)2] and [Ru(CO)(CS)(Cp)]2 [9], and the pentamethylcyclopentadienyl (Cp*) complexes analogue to 1 and 2 [10], have been synthesized by the same route. Studies on 1 and 2, as well as on their ruthenium analogues [Ru(CO)(CS)Cp]2 and [Ru2(CO)3(CS)Cp2], evidenced the preference of CS for bridging over a terminal position in these molecules. This was tentatively explained in terms of the weakness of the C„S p-interaction, which loses little p-bond stabilization in moving from a terminal (C„S) to a bridging (˜C@S) position. These arguments also suggested that CO should have a lower preference for a bridging position than CS [9].

S C

+

OC NaH Fe OC CS

CO

Fe

Fe C O 1

Scheme 2.

S C

OC

CO +

Fe

CO Fe

C S 2

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L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011 S C

OC +

M OC

CS

OC

M

Mn

Mn

Co Me3P

C O

CO THF

M = Rh, Ir

Co

S

C

CO

C

Mn S

Na[Mn(CO)5]

Scheme 5.

C O 4

Scheme 3.

Heterodinuclear l-thiocarbonyl complexes have been also obtained from the combination of mononuclear precursors. Examples include: [MnM(l-CO)(l-CS)(PR3) (CO)(Cp)2] [M = Co, Rh] (3) [11] and [FeMn(l-CO)(l-CS)(CO)5(Cp)] (4) [12] (Scheme 3). The obvious requirement is that at least one of the two complexes contains the CS ligand and that the two species exhibit some complementary reactivity (e.g. cation–anion or donor–acceptor properties), as usually necessary to form heterodinuclear species, satisfying the 18-electron rule. The dinuclear Pt–Mn, and Pt–Re complexes [(Cp)(CO)M(l-CS)(l-CO)Pt(PR3)2] (5) were the products of the reaction of [Pt(C2H4)(PR3)2] with [M(CO)2(CS)(Cp)] (M = Mn, Re) (PR3 = PMe2Ph, PMePh2) [13]. The syntheses follow the modular approach to the construction of clusters developed by Stones, and are based on the fruitful use of isolobel analogy concepts [14]. An alternative method for the preparation of bridging thiocarbonyl complexes consists of assembling mononuclear complexes, not containing CS, in combination with an appropriate source of CS. Indeed compound 1 was more efficiently formed by reacting Na[Fe(CO)2(Cp)] with SC(OPh)2 (Scheme 4) [15]. Complexes containing CS2 or CSSe as g2-coordinated ligand are also good sources of dinuclear thiocarbonyl complexes, since the combination with a second metal compound might promote the C–S or C–Se cleavage. In several cases, the CS ligand consequently generated is found terminally bonded to one metal atom instead of bridging [16,17]. As an example the complexes [LLM(l-E)Pt(CS)(PPh3)] (6) (M = Pd, Pt, E = S, Se; LL = 2 PPh3; Ph2PCH2CH2PPh2) are obtained by reacting [MLL(g2-CSE)] with [Pt(PPh3)2(C2H4)] or [Pt(PPh3)4] [17]. However, in the case of the diplatinum complex [(diphos)Pt(l-Se)Pt(CS)(PPh3)]

the CS shifts from terminal to bridging coordination upon treatment of diphos, affording [(diphos)Pt(l-Se)(lCS)Pt(diphos)] [17]. Cleavage of the g2-CS2 ligand, through reactions with appropriate complexes, is also an efficient route to polynuclear thiocarbonyl complexes. In particular the synthesis of the first triply bridging thiocarbonyl complexes was accomplished by reaction of a cobalt-CS2 compound (Scheme 5) [5a,5b]. In spite of the presence of manganese, the reaction produces selectively 7, which consists of a cobalt triangle bridged by the CS ligand. Heteronuclear triply bridging thiocarbonyl complexes have been obtained by analogous methods, based on the assembling of mono and dinuclear species. Thus, the reaction of [Fe(PPh3)2(CO)2(g2-CS2)] with [Co(PPh3)2(Cp)] gave [{Co(Cp)}2{Fe(CO)2(PPh3)}(l3- S)(l3-CS)] [5d]. Complexes of type E (Scheme 1), containing a bridging CS ligand which is further coordinated to a third metal centre through the S atom, can be obtained by exploiting the electrophilic behavior of the CS. As an example, complex 1 reacts with a variety of Lewis acid (L) generating both cationic [Fe2(l-CSL)(l-CO)(CO)2(Cp)2]+ (L = HgMe, Fe(CO)2Cp) and neutral complexes [Fe2(l-CSL)(lCO)(CO)2(Cp)2] [L = HgCl2, HgBr2, Cr(CO)5, W(CO)5] (Scheme 6) [18]. Also triply bridging thiocarbonyl can coordinate, through the sulfur atom, a further metal fragment, generating four coordinated complexes (Scheme 1, type F). There-

S C

OC

CO

Fe

Fe

OC M(CO)5(THF)

C O 1

S C Fe

S C

OC SC(OPh)2

Fe CO

CO

M = Cr, Mo, W

Fe

Fe C O 1

Scheme 4.

+

Fe

Cr(CO)5 S Cp Co

CO Fe

C O

CO Fe

Scheme 6.

C

O C

OC

M(CO)5

C O 8

S

-

Cp

Mn(CO)4

Fe

OC CO CS

Co

7

S C

OC

Fe

OC

Co

Cp

3 +

Cp Co

C O

+

S

S

O C

Me3P

PR3

Cp

S

Co

C [Cr(CO)5THF]

Co

Cp Cp

Co S 9

7

Scheme 7.

Cp Co Co

Cp

L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011 Cp

Me

Co2(CO)8

Cp(CO)2Fe

C S

C O C

OC OC

Fe(CO)2Cp

+

Me S

S

Fe S

3007

Co

S Co

C

OC

CO CO

C Co CO O CO 10

Fe

CO Fe

Cp

C [Co(CO)4]-

OC

C O

Fe

OC Co

CO C O

11

Fe Cp 12

Scheme 10.

Scheme 8.

fore, compound 7 reacts with [Cr(CO)5(THF)] affording the tetranuclear complex 9 (Scheme 7) [5a,5b]. An interesting example of tetranuclear thiocarbonyl complex is shown in Scheme 8. Again, cleavage of coordinated CS2, generates the thiocarbonyl ligand. In this case, the CS2 coordination is bridging two metal fragments, and the reaction with [Co(CO)8] afford 10, among other products. It should be remarked that in complex 10 the CS acts as a six electron donor, which represent the upper limit for the thiocarbonyl ligand [19]. 3. Bridging thiocarbyne complexes The stability and good properties of CS as metal ligand when bridging two metal centers is well evidenced by the fact that a variety of reactions can be carried out on thiocarbonyl complexes without influencing or producing the cleavage of the coordinated CS. Also nucleophilic addition at the CS, which is the most common feature in cationic mononuclear complexes [20] is not observed for l-CS ligands. Conversely, bridging thiocarbonyl exhibits electrophilic behavior. Beside the above mentioned addition of unsaturated metal fragments and Lewis acid, the l-CS ligands, as expected, generally react with strong alkylating reagents (MeSO3CF3, OMe3BF4, RI, R = alkyl ) leading to the formation of thiocarbyne ligands, which remain bridging. A number l-thiocarbyne complexes, mostly l-CSMe complexes, have been obtained following this procedure and examples include both diiron [15,21] (Scheme 9) diplatinum [22] and hetero dinuclear Mn–Pt [13] Mn–Co and Mn–Rh [11]. The X-ray crystal structural determination of [Fe2(lCSEt)(l-CO)(CO)2(Cp)2]BF4 [17], which is very similar to 11, shows that the l-C–S interaction results lengthened as ˚ versus a consequence of the alkylation (1.666(11) A ˚ 1.590(ave) A of cis-[Fe2(l-CS)(l-CO)(CO)2(Cp)2]). How-

Me

S C

OC Fe

S CO Fe

C

OC MeSO3CF3

C O

Fe

11

Scheme 9.

CO Fe

C O

1

+

ever, the CS bond remains substantially shorter than a single bond, indicating the presence of some p-interaction. This evidences the peculiar character of the thiocarbyne ligand: p donation from S to the carbyne carbon provides stabilization. On the other hand, due to this interaction, the ligand could not be accurately labelled as a true alkylidyne, which is expected give p-bond exclusively with the metal atoms. Triply bridging cationic thiocarbyne complexes have also been obtained by transforming the l3-CS into l3CSR, through direct alkylation of the S atom [5a,5b]. Alternative methods are also available: thus the reaction of [Fe(CO)2(PPh3)2(g2-CS2Me)][SO3CF3] with of [Co(Cp)(PPh3)2] results in the formation of [{Co(Cp)}2{Fe(CO)2(PPh3)}(l3-S)(l3-CSMe)][SO3CF3] [5d]. The mononuclear thiocarbyne complex [HB(pz)3](CO)2W(CSMe)] (pz = 1-pyrazolyl) is a source of triply bridging heterotrinuclear complexes, by assembling with appropriate metal fragments. In particular reaction with 2 equiv. of (L)Rh(CO)2 (L = g5-indenyl, C9H7) gave the complex [(L)2(l-CO)Rh2(l3-CSMe)W(CO)2{HB(pz)3}], whereas the reaction with [(Cp)Ni(CO)]2 afforded the trinuclear complexes [(Cp)2Ni2(l3-CSMe)W(CO)2{HB(pz)3}] [23]. The triply bridging thiocarbyne complex 12 was obtained by reacting [Fe2(l-CSMe)(CO)3(Cp)2]SO3CF3 (11) with [Co(CO)4] under photolytic conditions [24] (Scheme 10). It should be remarked that in 12 the CSMe group is bonded through the carbyne carbon to all three metals, and the S is coordinated to the Co atom. In a related reaction, the thiocarbyne 11 was converted into the trinuclear complex [Fe3(l3-CSMe)(CO)3Cp3] (13) upon treatment with sodium naphthalenide and [Fe2(CO)4(Cp)2], under UV irradiation [25]. 4. Reactions of the bridging thiocarbyne [Fe2(l-CSMe)(CO)3(Cp)2]SO3CF3 Among bridging thiocarbyne complexes, the diiron [Fe2(l-CSMe)(CO)3(Cp)2]SO3CF3 (11) [23] was, by far, the most investigated, and the following discussion will be essentially focused on its reactivity. Furthermore, the similarities between 11 and the l-methylidyne [Fe2(lCH)(CO)3(Cp)2]+ [26], which share the same dinuclear frame, make possible a direct and significant comparison between the methylidyne and the thiocarbyne ligands. As

3008

L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011

mentioned above, the latter appears more stable and less reactive due to the C–S p interaction, which significantly reduces, the electrophilic character of the carbyne carbon. In spite of this stabilization effect, the l-CSMe displays a very rich chemistry and is involved in a large variety of reactions. Initial work evidenced that 11 undergoes one electron reduction generating the thiocarbyne radical [Fe2(lCSMe)(CO)3(Cp)2][27]. The relative stability of this latter species was exploited to promote substitution reactions of terminally coordinated CO with phosphanes, isocyanides and thiolates [27]. 4.1. Cleavage of the C–SR bond

R R P

Fe

2,4,6-R3C6H2PH(SiMe3)

C

OC Fe

CO Fe

C

OC

R

Me

S Me

R Fe

R

CO Fe

+

Fe

C O

C O 11

R H CN SR alkyl, Ph

CO

Fe

16 17 18 19

20 21

Scheme 12.

In spite of a reduced electrophilic character, the bridging thiocarbyne in 11 is susceptible to nucleophilic addition, affording a general route to the synthesis of bridging thiocarbene complexes. Thus, 11 undergoes addition at the bridging carbyne carbon by a variety of nucleophilic reagents including: NaBH4 [30], SR , cyanide, [31] and several organocopper reagents [32] (Scheme 12). In addition to the expected bridging thiocarbene complexes (16–19), in some cases the bridging ligand exhibit a S–Fe bond (20– 21). This latter coordination mode is promoted by loss of CO, easily achieved under photolytic conditions. The reactions described above for the thiocarbyne complex 11 should take place also in the related thiocarbyne complex [Fe2(l-CSMe)(l-CS)(CO)2(Cp)2]SO3CF3, originated from the bis-thiocarbonyl complex [Fe2(lCS)2(CO)2(Cp)2] (2). In theory, the presence of a second l-CS ligand should double all of the transformations observed for the CS ligand. Indeed, through a stepwise methylation of the S atom, followed by nucleophilic addition at the thiocarbyne ligand consequently generated, it was possible to transform one or both of the bridging CS into the corresponding thiocarbene ligands. This approach has been applied to the synthesis of a double bridging thiocarbene complex 22 (Scheme 13) [33]. Likewise, through a stepwise alkylation and NCO insertion the bis-isocyanide complex 23, analogous of 15 was obtained from 2 (Scheme 13) [34]. It should be noted that not all the nucleophilic reagents attack selectively the bridging carbon and that the reacO

C

C O

C

Me

O

Fe

SMe

C

C

OC Fe

CO Fe

C O

Cp

Fe

CO Fe

C NC

S

SMe

N

CN

S

N

C

OC Fe

Cp

CO Fe

C

Me

N MeS

22

C O

15

Scheme 11.

Fe C O

+

NCO-

S

C

OC

14

C

11

CO

C O

S

Fe

R

C

OC

OC

+

4.2. Conversion of bridging thiocarbynes into bridging thiocarbenes

The cleavage of the carbon–sulfur bond in bridging thiocarbonyl complexes is a rare occurrence and examples are limited to the conversion of bridging CS into l-CH2 by treatment of 1 and 2 with Ni-Raney [10]. The C–SR cleavage should be easier to accomplish due to lengthening of the C–S bond in the thiocarbyne ligand. Nevertheless, this reaction remains difficult to achieve and examples are limited to the two reactions reported in Scheme 11. Thus, it has been shown that the l-CSMe ligand in 11 can be transformed into a bridging isophosphaalkyne ligand (CPR) by the condensation with 2,4,6-R3C6H2PH(SiMe3) (R = Me, CHMe2, CMe3), affording the complex 14 [28]. The cleavage of the thiocarbyne ligand in 11 is also given by the insertion of NCO in the C–S bond, which yielded the isocyanide complexes 15 (Scheme 11) [29]. In both cases, the cleavage of the thiocarbyne provides synthetic routes to unusual ligands, otherwise unattainable, and is accompanied by the formation of a double bond between the bridging carbon and a group 15 heteroatom.

Me

Me S

Scheme 13.

23

L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011 Me +

S O

C

C

CO

Fe

Fe C O

RMgCl

LiC CR

Me O C

Me

11

S O C

C Fe

Fe

S

H

C O 24

Fe

O C Fe

Fe

O RC C C

H C

C C CR

C O

R

Me S

O

C

OC

CO R

C

OC

Fe

Me

Fe

C O

S Fe

C O

25

26

Scheme 14.

tions shown in Scheme 12 lack general character. Indeed, other organometallic reagents (e.g. Grignard reagents or lithium acetylides) react with the ancillary ligands (Cp or CO), rather than giving addition at the carbyne ligand of 11 [32]. However, in these cases, the initial attack at the Cp or CO is followed by intramolecular rearrangements, which involve the thiocarbyne and ultimately afford stable bridging thiocarbene complexes. As an example, the addition of Grignard reagents (RMgCl; R = allyl, Ph, CH2Ph, Pri) to the Cp ligands of 11 lead to the formation of the cyclopentadiene compounds [Fe2(l-CSMe)(l-CO)(CO)2(C5H5R)(Cp)] (24) which, in turn, undergo hydride migration from cyclopentadiene to the l-carbyne carbon, affording the l-thiocarbene 25. (Scheme 14) [32,35]. Likewise, nucleophilic attacks at the terminally coordinated CO are followed by acyl migration to generate the l-thiocarbene complexes [Fe2{l-C(SMe)COR}(l-CO)(CO)(Cp)2] (26) (Scheme 14) [35]. Among the broad variety of bridging thiocarbene complexes (e.g. 16–22, 25–26), generated by addition of nucleophiles to the bridging thiocarbyne ligand the complexes [Fe2{l-C(R)SMe}(l-CO)(CO)2(Cp)2] (R = H, 16; CN, 17) resulted of particular interest since they could be transformed into their corresponding sulphonium complexes

27 and 28, respectively, by methylation of the S atom [36] (Scheme 15). The sulphonium carbenes complexes 27–28 provided access to a broader variety of bridging alkylidene complexes via the facile displacement of SMe2 occurring with: amines [37], phosphanes [38], alcohols [39], carbon nucleophiles and hydrides [36,40] (Scheme 16 ). It should be remarked that the overall sequence of S-methylation and nucleophilic addition at the carbyne carbon lead to the cleavage of the l-C–S bond. In particular, the reactions above described demonstrate that the conversion of the bridging CS into a methylidene ligand (l-CH2), which in the case of 1 was obtained upon treatment with Ni-Raney [10], can be accomplished through a step-by-step procedure where each of the intermediate species have been isolated and characterized (Scheme 17). Since the CS can model the behavior of CO on metal surfaces, and in consideration of the fact that the conversion of CO to methylene (CH2) is believed a fundamental step in the hydrocarbon chain growth on metal surfaces (Fischer–Tropsch process) [41], the overall sequence described in Scheme 16 might be considered among the numerous examples of organometallic complexes which

HNR2 -H+ -SMe2

Me

R C

OC

OC Fe

Fe

Fe

R = H, 27; CN, 28

Scheme 15.

Fe C O

PR2

R C

OC

CO HPR2

Fe C O

Fe

CO Fe

C O

-H+ -SMe2

27-28 LiR' -SMe2

R'

R C

OC Fe

CO Fe

C O

Scheme 16.

Me S + S Me H C C OC C O NaBH O C CO 4 Fe Fe Fe Fe C Cp Cp C Cp Cp O O 11 16

S C

OC

Me+

CO Fe

C O 1

Cp

Me

+ S Me

H

CO

C O

CO

Fe

S Me

S Me C

C

+

Me

Cp

R

NR2

R OC

Fe

+

3009

+

Me

C

OC Fe Cp

CO Fe

C O 27

Cp

H NaBH4 -SMe2

Scheme 17.

H C

OC Fe Cp

CO Fe

C O

Cp

3010

L. Busetto, V. Zanotti / Inorganica Chimica Acta 361 (2008) 3004–3011 +

Me

Me

S

H

C

OC

CO Fe

Fe C O 11

1.

R

S Cα

C C

H H 2. NaH 3. Me3NO

OC

Me

H R

Cβ Fe

Fe

S

Cγ

H Cβ

OC

Fe

Fe

C O

C O

29

29

Me S

Cγ

Cα H

Me

COOMe

OC

CH2Cl2

Fe

Fe

COOMe Cγ H

C O

R = CN, CO2Me

NaBH4

Scheme 18. H

Cβ Cα

OC

Fe

Fe

30 -SMe2

H

act as molecular models for the conversion of syn-gas (CO/ H2) into hydrocarbons [42].

Cβ Cα

MeCF3SO3

H

+

H

COOMe Cγ H

C O 31

4.3. Coupling of the bridging thiocarbynes with olefins Scheme 19.

The coupling of bridging ligands, like carbynes and carbenes, with small molecules (mainly olefins and acetylenes) open new perspective in the field of metal assisted C–C bond formation and attract a considerable interest due to the implications with the organic synthesis and the hydrocarbyl ligand transformations [43]. Unfortunately, bridging thiocarbyne ligand appears reluctant to react, at least in comparison with the l-methylidyne complex [Fe2(l-CH)(l-CO)(CO)2(Cp)2]+. The latter is readily attacked by olefins in a reaction named ‘hydrocarbation’, because of the analogies with the hydroboration reaction [26b,44]. However, very recently, we found that the thiocarbyne ligand in 11 undergoes a coupling reaction with olefins activated by electron withdrawing substituents (Scheme 18) [45]. The reaction, affording the l-allylidene complexes 29, represents a rare example [46] of olefin incorporation into a bridging ligand producing a C1 to C3 chain growth. Moreover, the reaction is regio and stereoselective: indeed, the addition of asymmetric alkenes might produce two regio-isomers depending on which of the two nonequivalent alkene carbons forms the C–C bond with the thiocarbyne ligand. The observed coupling occurs selectively between the CH2 termination and the carbyne carbon. Likewise, each of the two geminal hydrogen in the olefin might undergo deprotonation, affording the corresponding E or Z isomers, (Cb–H and Cc–H hydrogen, in the allylidene ligand, on the same or on the opposite side of the Cb–Cc double bond). In the observed products 29 the two hydrogen atoms (Cb–H and Cc–H) are exclusively found trans (E isomer). A major difference with the nucleophilic addition at the thiocarbyne ligand described in the previous paragraph, is that the coupling reaction require the presence of Me3NO, to promote CO loss and generate a vacant coordination site. This latter is presumably required to coordinate the olefin in the initial step of the reaction sequence. Therefore, the reaction is the result of an intramolecular coupling rather than the consequence of a direct olefin or vinyl anion attack at the carbyne carbon.

Compound 29, like the thiocarbene complexes described above, can be methylated at the S atom, leading to the formation of the sulphonium species 30. Again, the methylation promote the desulfurization of the bridging ligand (Scheme 19) although in complex 30 the displacement of SMe2 by hydride or other nucleophiles is less easily achieved, compared to the carbene sulphonium complexes 27 and 28. The overall result, considering the thiocarbonyl complex 1 as starting material, consists of a multi-step transformation of a bridging CS into a bridging allylidene unit. The C–S cleavage permits the formation of C–C and C–H bonds and the regio and stereo-controlled formation of the bridging allylidene complex 31. 5. Conclusions Bridging thiocarbonyl complexes exhibit specific reaction profiles, distinct from those of the corresponding mononuclear complexes. In particular, bridging CS in neutral dinuclear complexes are readily alkylated (methylated) to form bridging thiocarbyne ligands which, in turn, can be transformed into bridging thiocarbene ligands. Both thiocarbyne (l-CSMe) and thiocarbene (l-CS(Me)R) are more stable and less reactive compared to the corresponding alkylidyne and alkylidene ligands which do not contain heteroatoms. Nevertheless, taking advantage of the activation due to the coordination to cationic diiron complexes, the bridging thiocarbyne undergoes a variety of reactions, including the addition of nucleophilic reagents, intramolecular rearrangements, CS cleavage, coupling and insertion reactions. On the other hand, bridging thiocarbene complexes can be transformed into more reactive sulphonium alkylidene complexes, upon methylation of the S atom. The displacement of SMe2 from the bridging ligand allows the formation of new C–C and C-heteroatom bonds, with consequent transformations of the bridging hydrocarbyl

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