Chapter 21 Carbon—Sulfur B o n d Cleavage of Thiolates o n Electron-Deficient T r a n s i t i o n M e t a l s
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Kazuyuki Tatsumi and Hiroyuki Kawaguchi Department of Chemistry, Faculty of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Reactions of Cp*MCl4 (M=Ta, Mo, W) with aliphatic thiolates were examined, shedding light on C-S bond activation by electron-deficient early transition metals. From the reactions of Cp*TaCl4 with varying amounts of LiStBu, Cp*Ta(StBu) Cl (1), Cp*Ta(StBu)Cl (2), and Cp*Ta(S)(StBu)2 (3) were isolated, and C-S bond rupture was found to occur specifically in the reaction between 2 and LiStBu. Upon warming a toluene solution of Cp*Ta(SCH2CH S) at 100°C, we isolated Cp*Ta(SCH2CHS){SCHCHSCH(CH3)S} (9), while similar treatment of Cp*Ta{SC(CH)2CHS}2 gaveriseto Cp*Ta{SC(CH)2CH2S}{SSC(CH3)2CHS} (12). The reactions of Cp*MoCl4 with LiStBu, Li (SCH CH S), and Li2(SCH CH CH S) yielded the Mo(IV) complexes, Cp*Mo(StBu) (13), (PPh4)[Cp*Mo(SCH CH S) ] (14), and (PPh4)[{Cp*Mo(SCH2CHCH2S)2}Li Cl] (15), respectively. On the other hand, the analogous reactions of Cp*WCl4 with LiStBu and Li (SCH CH S) lead to W(VI) sulfide complexes, Cp*W(S)(StBu) (19)and Cp*W(S) (20). 2
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Aliphatic thiolate complexes of early transition metals occasionally undergo C-S bond cleavage reactions (1-3), resulting in formation of sulfide and sulfide/thiolate mixed ligand complexes. Christou et al. and Coucouvanis et al. have independently reported that the reaction of NbCls with NaSBu generates a variety of anionic complexes, [Nb(S)Cl4h [Nb(S) (StBu) ]-, and [Nb(S)(StBu)]- (4). On the other hand, the reaction between Zr(CHPh)4 and 4 equiv of HStBu was found to produce a trinuclear sulfide/tbiolate cluster, Zr (S)(StBu)io (5). In these intriguing reactions, C-S bond activation occurs concurrently with substitution of chloride or the benzyl group with tbutyl thiolate. For these d0 transition metal systems, the C-S bond cannot be activated via reductivefissionprocesses. Questions regarding mechanism revolve about which stage of these multi-step reactions the C-S bond is actually broken. Insight into the factors that influence the activation of C-S bonds is of fundamental importance in l
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0097-6156/96/0653-0336$15.00/0 © 1996 American Chemical Society In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
21. TATSUMI & KAWAGUCHI
Carbon-Sulfur Bond Cleavage ofThiolates
337
sulfur-based coordination chemistry and it is germane also to understanding the mechanism of the industrially important hydrodesulfurization processes (6). In this context, it is necessary to find systems with which one can trace individual steps of thiolate substitution and C-S bond cleavage. This paper summarizes our recent studies on C-S bond cleavage found in the reactions of Cp*MCU (Cp*^n -C5Mes; M=Ta, Mo, W) with aliphatic thiolates. 5
Reactions of C p * T a C l with LiS*Bu
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4
We have previously reported syntheses of pentamethylcyclopentadienyl tantalum dithiolate complexes , Cp*Ta(SCH CH S)2, Cp*Ta(SCH=CHS)2, and Cp*Ta(ndt) (ndt = norbornane-exo-2,3-dithiolate) from the reactions of Cp*TaCU with lithium salts of the corresponding dithiolales (7). Only bis-(dithiolate) complexes were isolated even from the reactions between Cp*TaCU and U (mthiolate) in 1:1 molar ratios, and formation of mono-dithiolate complexes, e.g., C p * T a C l ( S C H 2 C H 2 S ) , was not discernible (Scheme 1). 2
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Scheme 1
Scheme 2
El Mass Fragmentation
S
and
The EI mass spectrum of C p * T a ( S C H 2 C H S ) 2 shows signals derived from 2
C p * T a ( S 2 ) ( S C H 2 C H S ) + and Cp*Ta(S2)2 2
+
in addition to the parent ion isotopic
In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
338
TRANSITION METAL SULFUR CHEMISTRY
cluster. No other signals were observed in this mass region. Likewise, the EI fragmentation of Cp*Ta(ndt)2 consists of Cp*Ta(S2)(ndt)+ and Cp*Ta(S ) +. The stepwise liberation of the alkane portions in the spectra suggest that C-S bonds of these alkanedithiolates are broken cleanly in the EI mass condition (Scheme 2). In contrast to the synthesis of dithiolate complexes, the reaction of Cp*TaCU with LiStBu resulted in stepwise replacement of chloride in Cp*TaCU (8). When 1 equiv of LiStBu in THF was added at OKI to a THF solution of Cp*TaCU, the color of the solution turned immediately from orange to red-brown and a yellow powder gradually precipitated. The IR spectra of a red-brown powder, which was obtained by quick evaporation of the solvent, indicated the presence of Cp* and the t-butyl group. However, attempts to isolate the mono-thiolate complex Cp*Ta(S Bu)Cl3 were unsuccessful, because of instability of the red-brown powder, which decomposes in solution to give an insoluble yellow powder. On the other hand, we were able to isolate the bis-(thiolate) complex, cis-Cp*Ta(StBu)2Cl2 (1) as red needles in 81% yield from a similar treatment of 2 equiv of LiStBu with Cp*TaCU in THF followed by a standard workup and subsequent recrystallization from hexane. Analogously, the tris-(thiolate) complex Cp*Ta(StBu)3Cl (2) was synthesized as dark-red plates in 50% yield from the reaction between 3 equiv of LiStBu and CpTaCL*. When the amount of LiStBu was increased to 4 equiv, the expected tetrakis-(thiolate) complex was not obtained. Instead, a C-S bond rupture of the thiolate occurred to give rise to Cp*TaS(StBu)2 (3), which was isolated as yellow needles in 54% yield. These reactions are summarized in Scheme 3. The complexes 1-3 were fully characterized. The relatively low yield of 2 as crystals is partly due to its high solubility in hexane, and partly due to the formation of 3 as a side product (9% yield) in the (1:3) Cp*TaCU/LiStBu reaction system
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Scheme 3
Ta-S 2.376(2) A 2.390(3) CI l
S*Bu
BuSLI
l
3 'BuSLi
cry* S ; c i ci
cr/"
BuSU
Ta-S 2.425(3) A 2.381(2) S'Bu 2.422(2)
S^S'BU
CI
'BuSLI
4 OuSU Ta«S 2.166(2) A Ta-S 2.366(3)
Ta.. !
S^"S Bu S'BU
2.365(3) 3
By monitoring the UV-visible spectra, we found that the complex 1 was converted cleanly to 2 when 1 was treated with 1 equiv of LiStBu. Transformation of 2 into 3 occurred, again cleanly, upon addition of 1 equiv of LiStBu to 2. Thus, the C-S bond
In Transition Metal Sulfur Chemistry; Stiefel, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
21. TATSUMI & KAWAGUCHI
Carbon-Sulfur Bond Cleavage ofThiolates
was cleaved specifically in the process where the last chloride of C p T a C U was replaced by t-butylthiolate. Isobutane, tBuS*Bu, and tBuSS*Bu were detected by a GC-MS study of the byproducts of the latter reaction. The complex 3 can also be synthesized by treating 1 with L12S. On the other hand, the reaction of Cp*Ta(StBu) Cl (1) with 1 equiv of L i ( S C H C H S ) in T H F gave Cp*Ta(StBu) (SCH CH S) (4) in 80% yield. In the case of a similar reaction of 1 with 2 equiv of LiSPh, the major product was Cp*TaS(StBu)(SPh) (5), and a small amount of trans-Cp*Ta(StBu)2(SPh) (6) was also isolated (Scheme 4). The GC-MS analysis showed that the organic byproducts were tBuSPh and tBuSS^Bu. It seems likely that the tetrakis-thiolate complex 6 is the precursor, which, in turn, is converted into 5 and liberates tBuSPh. 2
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2
Scheme 4
1
us 2
LiSPh
.Ta,.
cry
!
S Bu
#
S^-s^u
CI
3
S'Bu
PhS^ 'BuS
!
S^-S Bu SPh
1 U2
