J. Am. Chem. SOC. 1994,116, 12103-12104
12103
Spontaneous Loss of Molecular Hydrogen from Tungsten(1V) Alkyl Complexes To Give Alkylidyne Complexes
Table 1. Kinetic data and oxidation potentials for methyl complexes"
Keng-Yu Shih, Karen Totland, Scott W. Seidel, and Richard R. S h o c k *
[(M~~S~NCHZCH~)~N]W(CH~) 351 30.6
Department of Chemistry 6-331 Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Received September 2, 1994
We have begun to explore the chemistry of Mo and W complexes that contain triamidoamine ligands, (R'NCHzCH2)3N, where R' is a silyl group (e.g., SiMe3)' or C,#S.~ (The only other reported W or Mo complex in this category is [(MeNCHzC H ~ ) ~ N ] M O)~We N . found ~ that paramagnetic [(R~S~NCHZCH&N]Mo(CH3) and [(R~S~NCHZCH~)~N]MO(CX!R") (R3Si = Me3Si, PhMezSi, or PhZMeSi; R" = Me, Ph, or SiMe3) complexes could be prepared readily from [(R3SiNCH2CH2)3N]MoC1.l We have now found that attempted syntheses of [(R'NCHZCHZ)~N] W(alky1) complexes usually yield alkylidyne complexes, even when p protons are present in the alkyl. The monochloride complexes [(R3SiNCH2CH2)3N]WCl (R3Si = Me&, MezPhSi, or MePhzSi) can be prepared in low yields (-20%) in THF from WCl4 1,2-dimethoxyethane) or WCL(THF)? and the trilithium salts of the ligands, (R3SiNLiCHzCH&N. (See supplementary material for details.) Addition of methyllithium to [ ( M ~ ~ S ~ N C H ~ C H Z ) ~inNether ]WC~ yields paramagnetic [(M~~S~NCHZCHZ)~N]W(CH~) in high yield. By the Evan's method the magnetic moment was found to be 2.5 p~ at 25 "C, slightly less than the expected spin-only value (2.83 p ~for ) two unpaired electrons. Two characteristic ligand methylene resonances can be observed in ca6 at -3 1.43 (width at half-height = WI/Z = 40 Hz) and -72.70 ppm (WIR= 23 Hz) at 25 "C, while the T M S methyl resonances are observed at 9.89 ppm. The CH3 resonance cannot be observed, although the CD3 resonance in [(Me3SiNCH2CH2)3N]W(CD3)is observed at 7.5 ppm (~112= 5 Hz) in the 2H NMR ~ p e c t r u m . ~[(Me3-~ SiNCH2CH&N] W(CH3) is converted virtually quantitatively into [(Me3SiNCHzCH2)3N]W=CH (6(Ca) = 272.6 ppm, JCW = 244 Hz, &Ha) = 7.08 ppm, JHW = 81 Hz) (eq 1) between 25 and 80 "C. The reaction is first order through several half-
lives and independent of concentration over a 5-fold range (Table 1) as determined by proton NMR at 500 MHz by following the decrease in the intensity of the TMS resonance. A resonance for molecular hydrogen can be observed, although dihydrogen's low solubility in toluene-dg does not allow the amount to be quantified by NMR. From the data presented in Table 1, values for @ = 19.2 kcal mol-' and A 9 = -16 eu (1) Shih, K.-Y.; Schrock, R. R.; Kempe, R. J. Am. Chem. SOC.1994, 116, 8804. (2) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem. SOC. 1994, 116, 4382. (3) Plass, W.; Verkade, J. G. J. Am. Chem. SOC. 1992, 114, 2275. (4) Often only ZHNMR resonances are observable in a paramagnetic species as a consequence of a significantly smaller line width than in 'H NMR ~pectra.~ The advantages of 2H NMR in transition metal chemistry have been demonstrated recently.6 (5) NMR of Paramagnetic Molecules; LaMar, G . N., Horrocks, W. D., Jr., Holm, R. H., Eds.; Academic: New York, 1973. (6) Johnson, A. R.; Wanandi, P. W.; Cummins, C. C.; Davis, W. M. Organometallics 1994, 13, 2907.
341 330 320
-0.81 (298 K) 12.2, 11.8 4.35 1.56, 1.49, 1.5OC 0.644 0.251 2.30 0.273 6.32 -0.63 (298 K)
310 300 [(MesSiNCHzCHz)3N]W(CD3) 34 1 320' [(MezPhSiNCHzCH2)3N]W(CH3) 341 320 0.764 [(M~P~zS~NCH~CH~)~N]W 341 ( C H ~1.89 ) 320 0.219
-0.49 (298 K)
"The decrease in the intensity of the downfield-shifted MeSi resonance was followed by NMR at 500 MHz in toluene-& in each case through at least two half-lives at a concentration of -0.020 M, except where noted. dichloromethane under dinitrogen using 0.1 M [NBw]+PFs- as the electrolyte and ferrocene as the intemal reference (0.47 V). Data were acquired at a 200 mV s-* scan speed using a Pt bead electrode. In all cases ip,&= ip,cand Ep,a- Ep.cvalues are in the range 90-120 mV. '0.0038 M.
were calculated (Figure 1). For the analogous [(Me3SiNCHzCH2)3N]W(CD3) complex kH/kD was found to be 5.6 at 47 "C and 5.3 at 68 "C (Table 1). These values are approximately the same as those found for a hydrogen abstraction reactions in do specie^.^-^ When [(Me3SiNCHzCH2)3N]W=CD was placed in C D 6 under 2 atm of dihydrogen, no [(Me3SiNCH2CH2)3N]WiCH was observed after a period of 14 days. Therefore the loss of hydrogen from [(Me3SiNCH2CH2)3N]W(CH3) appears to be essentially irreversible under ambient conditions. [ ( M ~ ~ S ~ N C H ~ C H Z ) ~ N ] W also ( CisHconverted ~) in high yield to [ ( M ~ ~ S ~ N C H ~ C H Z ) ~ N ] W in Sthe C Hsolid state (5 h at 90 "C under dynamic vacuum). Analogous decompositions of [(MezPhSiNCH2CH2)3N]W(CH3) and [(MePhSiNCHzCH2)3N]W(CH3) in solution proceed progressively more slowly (Table 1). [(R3SiNCH2CH2)3N]WCl complexes react with LiCH2R' reagents (R' = Me, Pr, SiMe3, CMe3) or KCHzPh in ether or THF at room temperature to give [(R~S~NCHZCH~)~N]W=CR' complexes in high yield. The alkyl complexes [(R3SiNCH2CH2)3N]W(CHzR') are not observed. Gas evolution is evident in all cases upon adding the alkylating agent. [(Me&NCH2CH2)3N]W=CCMe3 prepared in this manner is identical to a sample prepared from (dme)C13WWCMe3'O and (Me3SiNLiCHzCH2)3N,' while [ ( M ~ ~ S ~ N C H ~ C H ~ ) ~ Nprepared ] W G C Pin~this manner is identical to a sample prepared from [(Me3SiNCHzCH2)3N]WECCMe3 and PhCECH by a triple bond metathesis reaction. [(C&NCH2CH2)3N]WC12 also reacts with LiCH2R' reagents (R' = Pr or SiMe3) in toluene to give [(C&NCHZCHZ)~N]WSCR' complexes in moderate isolated yields. These results suggest that the phenomenon is not restricted to the silylated tren ligand system. It should be noted that molybdenum methyl complexes that contain silylated tren ligands, e.g., [(MesSiNCH2CHz)gN]Mo(CH3), are thermally stable.' The linear relationship (not shown) between the rates of decomposition of [(Me3SiNCH2CH2)3N]W(CH3), [ ( M ~ ~ P ~ S ~ N C HW(CH3), ~ C H Zand ) ~ [(MePhzSiNCHz~ (7) Wood, C. D.; McLain, S. J.; Schrock, R. R. J. Am. Chem. SOC.1979, 101, 3210.
( 8 ) Schrock, R. R. In Reactions of Coordinated Ligands; Braterman, P. R., Ed.; Plenum: New York, 1986. (9) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1. (IO)Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; RocMage, S. M.; Pedersen, S. F. Organometallics 1982, I, 1645. (11) Joel S. Freundlich, unpublished observations.
0002-7863/94/1516-12103$04.50/00 1994 American Chemical Society
12104 J. Am. Chem. SOC.,Vol. 116, No. 26, 1994
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Communications to the Editor
Another is a reaction in which a rhenium polyhydride complex is transformed in the presence of acid and an alkyne into a rhenium hydrido-alkylidyne complex.16 A third is the reaction -13 -12 R=0997 between MoH(y3-allyl)(dppe)2and a large excess of anhydrous HCl in THF to give (at high concentrations of HCI) MoH2C12...... ......... .................. .......... .......... (dppe)?, molecular hydrogen, and propyne.17 It is clear from 14 5 the results reported here that an alkylidyne complex can be ........ ................. :................... .................. sufficiently favorable thermodynamically that “a,a-dehydrogenation” of the metal-alkyl bond becomes possible. The findings reported here raise many interesting questions concerning the possibly multifaceted role that a substituted triamidoamine ligand1* might play in the chemistry of high 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 oxidation state species. It remains to be seen the extent to which Indehydrogenation of alkyl ligands is possible in other ligand Figure 1. Plot of ln(WT) versus 1/T for [(M~~S~NCHZCH~)~N]W(CH~) environments, or the extent to which “oxidation” of the metal employing data in Table 1. under typical reducing conditions in classical olefin and acetylene metathesis systemslg can be attributed to dehydrogeCH&N]W(CH3> and the potentials at which they each undergo nation of alkyl ligands. a reversible oxidation (Table 1) suggests that any methyl compound whose E1/2(ox) is greater than -0.43 V might not Acknowledgment. R.R.S. thanks the National Science lose hydrogen. In fact, the E1/2(0x) value for [(Me3SiNCH2Foundation (CHE 91 22827) for research support, and K.T. CH2)3N]Mo(CH3) is -0.36 V.’ thanks the Natural Sciences and Engineering Research Council All data in hand suggest that the conversion of [(R3SiNCH2of Canada for a postdoctoral fellowship. CH2)3N]W(CH3) to [ ( R ~ S ~ N C H ~ C H Z ) ~ N ] W is unimolecuWH lar. Although we cannot discount the possibility that the ligand Supplementary Material Available: Experimental details is directly involved in removing one or both a protons, a for the syntheses of [(R3SiNCH2CH2)3N]WCI (R3Si = MesSi, plausible explanation would consist of “a elimination” to give MezPhSi, or MePhzSi), [(R3SiNCH2CH2)3N]W(CH3), [(Me3a W(V1) methylene hydride complex, perhaps with concomitant SiNCH2CH2)3N]W(CD3), [(R3SiNCH2CH2)3N]WICH,[(Me3dissociation of the apical nitrogen donor (eq 2), followed by SiNCH2CH2)3N]WsCD, [(Me3SiNCH2CH2)3N]W=CCH3, “a abstraction” of the alkylidene Ha by the hydride to give [ ( P ~ M ~ z S ~ N C H ~ C H ~ ) W W C[(Me3SiNCH2CH2)mWWH~, molecular hydrogen. When no j3 protons are present in the alkyl n-Pr, [(PhMe2SiNCH2CH2)3N]WIC-n-Pr, [(Me3SiNCH2CH2)mW=C-t-Bu, [ ( P ~ M ~ ~ S ~ N C H ~ C H Z ) N W W [(Mes-~-BU, SiNCH2CH2)3N]W~SiMe3,[(PhMe2SiNCHzCH2)3N]WWSiMe3, [(Me3SiNCH2CH2)3N]W=CPh, [(C&‘5NCH2CH2)3N]WECSiMe3, and [(C&NCH2CH2)3N]W=CPr (8 pages). This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. (alkyl = CH2Ph, CH2SiMe3, or CH2CMe3), we propose that (12) Interestingly, preliminary investigations 2H, and ‘H NMR) of the mechanism is most likely to be analogous to that for the the butylidyne products of the reactions between LiCDzCH2CHzCH3 and methyl complex, even though the alkyl intermediates have not [(MesSiNCHzCH2)3N]WCl or [(C&NCH2CH2)3N]WCl suggest that the yet been observed. However, when ,8 protons are present (as deuterons are not present in the butylidyne ligand in [(MejSiNCHzin the butyl complex), we cannot exclude the possibility that j3 CH2)3N]WqCH2CH2CH3,but they are scrambled throughout the burylidyne chain in the C6ps wen derivative. Additional studies are under way. hydride elimination and other, more unusual proton migration (13) Reactions in which a methylidyne is formed from a methyl complex reactions are parf of a series of rapid steps that lead to the stable are known in multimetallic chemistry; see, for example, the low-yield alkylidyne complexes.12 Almost certainly, the steric protection conversion of [Ruz(CH,)(CO)(dppm)Cp2]+into [Ru~(CH)(C0)(dppm)Cp~]+ reported by Davies et al.: Davies, D. L.; Gracey, B. P.; Guerchais, V.; against intermolecular reactions provided by the substituted Knox, S. A. R.; Orpen, A. G. J. Chem. Soc., Chem. Commun., 1984,481. triamidoamine ligand combined with the low probability of (14)Sharp. P. R.; Holmes, S. J.; Schrock, R. R.; Churchill, M. R.; dissociation of the triamidoamine ligand are important features Wasserman, H. J. J. Am. Chem. SOC. 1981, 103, 965. (15) (a) Holmes, S. J.; Clark, D. N.; Turner, H. W.; Schrock, R. R. J. of the chemistry observed here. Am. Chem. SOC.1982,104,6322. (b) Atagi, L. M.;Critchlow, S. C.;Mayer, Several reactions in the literature that involve monomeric J. M. J. Am. Chem. SOC. 1992, 114, 1483. species13may be related, at least distantly, to the conversions (16)Leeaphon, M.; Fanwick, P. E.; Walton, R. A. J. Am. Chem. SOC. 1992, 114, 1890. observed here. One concerns a reaction between W(PMe3)4(17) Oglieve, K. E.; Henderson, R. A. J. Chem. Soc., Chem. Commun. Cl2 and 2 equiv of AlMe3 to give trans-C1(PMe3)4WzCH in 1994,474. -65% yield and 0.9-1.2 equiv of a gas that consists of 70(18) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483. (19) Ivin, K. J. Olefin Metathesis; Academic: New York, 1983. 85% methane and 30-15% h y d r ~ g e n ’ ~(see * ’ ~also ~ ref 15b). -1 1 L , ’ ,
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