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Reactivity of methylidyne ligands on trinuclear clusters of group 8 metals
0277~5387/88 $3.W+ .OO 0 1988 Pergamon Press plc
Po,ykdron Vol. 7, No. lO/ll, Pp. 847-858, 1988 Printed in Great Britain
LIGANDS ON TRINUCLEAR CLUSTERS OF GROUP 8 METALS JEROME
B. KEISTER
Department of Chemistry, University at BufIalo, S.U.N.Y., Buffalo, NY 14214, U.S.A. (Accepted 5 November 1987)
Abstract-The chemistry of alkylidyne containing clusters HM &&X)(CO) i0 and H3M & ,-CX)(CO), is presented&-( 1) structures, bonding and spectroscopic characterizations, (2) syntheses involving modification of the methylidyne substituent, (3) alkyne-alkylidyne coupling, and (4) formation of H-CX bonds, Reactions of HRu, ~COMe)(CO),* or ~~Ru~(~X)(CO)~ fx = OMe, Ph or alkyl) with alkynes, produce HRu3(p3-q3-XCCRCR)(C0)g clusters containing 1,3-dimetalloallyl ligands. Hydrogenation of these products when X = OMe or SEt gives new alkylidynes H3Ru3(p3CCHRCH*R)(CO)~. For X = SEt, isome~zation of HRu~~~-~3-EtSCCRCR)(CO)~ to Ru3(fl-SEt)(~3-q3-CCRCHR)(C0)g, a presumed intermediate in the hydrogenation reaction, occurs under mild conditions. Under CO, reductive elimination of CH3X can be induced from H3Ru~(~~-CX~(CO)g (X = OMe, Ph, Cl or C02Me). Pyrolysis of H3Ru&CX)(CO),, X = COzMe and SEt, gives stabilized intermediates H,Ru,&-CHC02Me) (CO), and HRu3(p,-CH2SEt)(C0)9, respectively. Protonation of H3Ru3(fi3-CR)(C0)g (R = Et or CHPhCH~Ph) yields H3Ru~~3-HCR~CO~~+, in which the alkylidene hydrogen is agostic ; the protonated clusters undergo more rapid reductive elimination than the neutral precursors. The kinetics and mechanism of reductive elimination have been investigated. A mechanism is presented.
The comparison between the chemical reactivity of hydrocarbyl ligands bridging two or more metals and the reactivity of these ligands when bound to a single metal centre is clearly of fundamental interest. The chemistry of bridging hydrocarbyl ligands is also of interest because of the analogy to the chemistry of hydrocarbyl groups adsorbed on metal surfaces. The alkylidyne ligand has an extensive chemistry for monometal~c complexes and is implicated in acetylene metathesis. ’ Furthermore, alkylidynes have been identified in numerous studies of surface adsorbed hydrocarbons, although iittle is known of the reactivity of these surface bound species.’ Alkylidynes of metal clusters have also been known for many years. The best known class of clusters containing a bridging alkylidyne group has the genera1 formula Co3@c13-CR)(CO)g;the chemistry of these compounds has been mainly found to involve modification of the alkylidyne substituent or ligand substitution on the metal atoms.3 Comparisons of the reactivity of the alkylidyne ligand on single metal centres, on metal clusters and on metal surfaces are therefore of special interest.
Our research group has been concerned with the chemical reactivity of the cluster series having the general formula HOMER, M = Fe, Ru or OS and X = alkyl, aryl, alkoxy or other substituent (Fig. 1). These clusters are isoelectronic with, and close structural anaiogues of the Coax clusters. These series provide the opportunity to investigate the chemical reactivity of a variety of groups-hydride, carbonyl, M-C bond or C-X bond-as a function of the metal, methylidyne substituent and Lewis base ligands.
Fig. 1. Structure of (y-H),M&-CX)(CO), (M = Fe, Ru or OS; X = H, dkyl, aryl, halide, OMe, CO*Me or SEt).
847
848
J. B. KEISTER
This review will present the chemistry of the H~MJ(CX)(CO)~ series and the related class HM,(CX)(CO) 1,, : (1) structure, bonding and spectroscopic characterization, (2) syntheses involving modification of the X substituent, (3) insertion of alkynes into the M-CX bond, and (4) formation of H-CX bonds. Although this review will emphasize work done by our group on Ru clusters, results of other research groups will also be presented. STRUCTURES, BONDING AND SPECTROSCOPY
The structures of H3M3(CX)(C0)9 (Fig. 1: M = Fe, X = Me;4 M = Ru, X = CL5 Me,6 C6H44-Me7 and CH&Me,;’ M = OS, X = OBO(trimeric)g and CO,H”) all contain a triangle of metal atoms symmetrically capped by the methylidyne ligand and with each edge of the triangle bridged by a single hydride ligand. The coordination geometry around each metal is octahedral, with facially coordinating CO ligands and mutually cis hydrides and methylidyne ligands. For several Ru clusters the positions of the hydride ligands have been determined directly by X-ray methods. The Ru-H-Ru and angles are 97-108”, while the H-Ru-H C-Ru-H angles are 8587”. Much greater structural variation is found for clusters of the general formula HM,(CX)(CO)lo (Fig. 2: M = Fe, X = OMe,” NMe212 or O’-;‘3 M = Ru, X = 0Me,‘4~‘S NMe216 or O’-;17 M = OS, X = CCH2CHMe2,‘8 Ph,” H,20 NHCMe,2’ or C(Ph)CC(Ph)Re(CO)4PMe2Ph22). Although in all of these the hydride and methylidyne bridge a common edge, the bonding of the CX ligand varies from doubly bridging to almost symmetrically triply bridging. The dihedral angle 0 between the CM2 and M3 planes varies between 10470”, depending upon the metal and the methylidyne substituent, with the angle decreasing as X = O’- (Ru, 104”) > NMe2 (Ru, 100’) > C(Ph)CC(Ph)Re(CO),L (OS, 99”) > OMe (Ru, 95”)
Fig. 2. Structure of (~H)M&u-CX)(CO),~ (M = Fe, Ru or OS ; X = OMe, NR,, O-, H, or Ph).
> CH,CHMe2 (OS, 82”) > Ph (OS, 78”) > H (OS, 70”) and as M = OS > Ru > Fe ; a comparison between the structures of HRu,(COMe)(CO), o (0 = 94.90”) and HRu3(COMe)(CO)8(C6H8) (0 = 60.70”) shows that the other ligands on the cluster also have an influence. I4 For the doubly bridging alkylidynes, there is structural and spectroscopic evidence for rc-donation from the methylidyne substituent to the adjacent carbon. Thus, for HM&-CX)(CO) l o, X = OMe or NMe,, the M2(p-CX) unit is planar, the C-X bond lengths are substantially shorter than values expected for single bonds (for HRu,(p-COMe)(CO) lo, 1.305 A and for HRu3(p-CNMe2)(CO)lo, 1.280 A), and there is restricted rotation about the C-X bond (AG* = 56.5 kJ for HOs3(COMe)(CO)‘023~24 and > 80 kJ for HOs3(CNMeCH,Ph)(CO)‘023). The degree of C-X multiple bonding increases as the rc-donor ability of X increases: OR < NR2 < O’-. At this time there is no explanation for the dependence of the coordination geometry, assumed by the methylidyne in the ground state, upon the identity of the metal or other ligands. The explanation for the variation of the dihedral angle 6 with the nature of the methylidyne substituent may be found in the competition between X + C rc-donation, which is most favourable for a doubly bridging configuration, and M + C rc-donation, which is most favourable for a triply bridging one. I4 Fenske-Hall calculations and photoelectron spectroscopy have been used to investigate the bonding in H3M3(p3-CX)(C0)g.25 The “tilt” of the Ru(CO), units caused by the hydride ligands gives rise to some differences between the MO’s of the Ru clusters and those of the Co analogues. The Ru-H-Ru bonds are best described as threecentre, two-electron bonds with no direct Ru-Ru interaction. The methylidyne carbon atom may be described as sp-hybridized with delocalized bonding involving the 2~7~orbitals and little delocalization of the sp hybrid lone pair onto the cluster. The HOMO is mainly Ru-C bonding in character, unlike the Co analogues for which the HOMO is Co-Co bonding. The vibrational spectra of H3M3(CX)(C0)g (M=Ru,X=H,Me,Cl;M=Os,X=Me)have been analysed.26,27 The metal-hydride stretching modes occur near 1350 cm-‘. For H3Ru3(CH) (CO), the C-H stretching frequency is 2988 cm-’ and normal coordinate analysis found the Ru-C force constant to be 137 N rn- ‘, close to the value for the Co analogue. 13C NMR spectra have been reported for both HM,(CX)(CO),, and H,M,(CX)(CO),. The chemical shift of the methylidyne carbon varies between 370 and 118 ppm, depending upon the
849
Reactivity of methylidyne ligands Table 1. ’3C Chemical shift for the ~kyIidyne carbon Chemical shift Reference @pm)
Compound HRu,(COMe)(CO) 1o HRu~(~NMe~)(CO)io H,Ru,(CBt)(CO), B&u&Me)(CO), H,Ru,(CPh)(CO), H,Ru,(CSEt)(C0)9 H,Ru,(CC0,Me)(C0)9 HOs,(COMe)(CO)
,o HOs~~CCH~CHMe~~(CO), c.
HOs,(CPh)(CO),o BOs,(CH)(CO),o H,Os,(COMe)(CO), H~Os~(CMe)(CO)~ H,Os,(CH)(CO), HFe&NBtJ(CO)10 HFe~(COMe)(CO~ , o B$%(CH)(C%
366.5 312.6 232.6 219.3 213.0 204.4 183.5 or 180.2 352.2 320.5 (40°C) 312.0 (-SOOC) 314.2 (17°C) 304.2(-115°C) 219 (- 100°C) 205.2 154.7 118.4 317.3 356.5 232.0
15 28 29 27 30 31 32 24 18 19 23 27 20 33 11 34
bonding mode of the ligand, the metal and the methylidyne substituent (Table 1). In several examples the chemical shift of the methylidyne carbon has been found to be strongly temperature dependent. It has been proposed that in these instances there exists a fluxional equilibria between isomers having different coordination modes for the methylidyne ligand. 3s
HM,(CO);; Mel:HM,(COMe)(CO),, (M = Fe, Ru or OS)
HM ,(CX)(CO) I 0. (2) X=H-orPh)
(M=RuorOs;
The first member of the class H,M,(CX)(CO),, H~Ru~(CMe)(CO)g, was prepared by the reaction of H4Ru4(C0),2 ethylene ;3g subsequently the OS analogue was prepared by hydrogenation of H~Os3(CCH~)(CO)~, derived from ethylene and Fehlner and Wong more recently 0s3(c0)**.40 prepared H3Fe3(CR)(C0)g, R = H, Me or Et, by reduction of Fe(CO)5_4L These syntheses were not adaptable to other methylidyne derivatives. In 1976 we found that hydrogenation of HM3(COMe)(CO) i 0, M = Fe, Ru or OS, gave H,M,(COMe)(CO), [eq. (3)].23 With these new methylidyne clusters in hand we sought to prepare a complete series analogous to the Co,(CX)(CO), series. Seyferth had earlier developed a number of syntheses involving modification of the halome~ylidyne ligand. Therefore, our first goal was to convert H3M3(COMe)(CO)9 to H3M3(CX)(C0)9, X = halide. The approach in eq. (4) afforded the desired clusters in nearly quantitative yields. 42Then by application of procedures developed for the Co analogues, we were able to prepare a wide variety of substituted methylidyne species [eqs (5)-(8)].42*3’ HM,(COMe)(CO),
o+ Hz
--+H3M3(COMe)(CO)g+C0
H3M3(COMe)(C0)g-t
Relatively few methylidynes of the general formula HM,(CX)(CO) i0 have been synthesized. In 1975 Shriver ez al. prepared HFe~(COMe)(CO},~ by 0-methylation of HFe,(CO) f ;. ’ ‘,36The Ru and OS analogues were subsequently prepared by several groups using similar routes [eq. (I)]. 15*23*24 Substitution on HM,(CO)i; by CNR, followed by alkylation, gives the analogous HM3(CNR2) (CO),,,.23*33 Attack by nucleophiles, LiBHEt3 or LiPh on HOs3(COMe)(CO)r0, followed by treatment with Lewis acids, has been used to generate H%tCX)(CO) t 0, X = Hz0 or Ph;” HRu,(CX) (CO) 1o has been similarly prepared. 38 Treatment of H,Os,(CO) lo with 3,3-dimethyl~y~lopropene produces HOs3(CCH,CHMe2)(CO), o.” OH-
-
(3)
(M = Fe, Ru or OS)
SYNTHESES OF HM,(CX)(CO),, AND H3M3(CX)(CO)9
M3(CO)12-
HM 3(COMe)(CO) , o+ (1)X-(2)Me+
BX3 -
(4)
H,M,(CX)(CC)9+AlC13+C6H6 (M=RuorOs;
I-I3M&Ph)(C% X=BrorOMe)
(5)
H3M3(CX)(C0),+A1C1,+C0 -----+ H,M,(CCO)(CO)g (M=RuorOs; H,M,(CCO)(CO);
(6a)
X=BrorOMe)
f ROH -
H~Ms(CCGR)(CO)~
(6b)
H3Ru3(CBr)(C0)9+NEt3+HSR -
(1)
H 3M ,(CX)(C% X=ClorBr)
(M=RuorOs;
H3RuGSR)(CO)g
(7)
H3Ru3(CBr)(C0)g + HSnBu, -
H3Ru&H)(CO)9
(8)
J. B. KEISTER
850
HOs,(CHX)(CO), HOs~(CH)(CO),o+X(X = H- or pyridine)
0.
(9) Although the mechanisms of these reactions remain unknown, the reaction chemistry involving modification of the C-X unit may be considered to fall into three classes: (a) reactions which may be thought to proceed through the H3M3(C) (CO);+ ion, by the abstraction of halide or alkoxide from the neutral cluster [e.g. reactions (4), (5) and (611, (b) radical reactions proceeding through H,M,(C)(CO), [e.g. reactions (7) and (811,and (c) reactions which proceed by nucleophilic attack on the alkylidyne carbon [reaction (9)]. The Co,(CX)(CO), clusters display reactivities analogous to that of H,M,(CX)(C0)9. The apparent radical chemistry of the cluster in the presence of reducing agents has been proposed to follow the SRNl mechanism. Electron transfer to H,Ru,(CBr)(CO), followed by halide ion expulsion could form the H3R~3(C)(C0)9 radical, which may abstract hydrogen from donor substrates to form H,Ru3(CH)(CO),, or may add to substrates such as thiols. 3l The chemistry of the H3M ,(C)(CO)i+ ion is more thoroughly understood, although the cation may never exist in a free form. In the presence of large quantities of the reactive substrate the cation may be trapped without rearrangement. For example, in benzene the aryl product is obtained. However, in an unreactive medium, the cation picks up CO from solution to form the ketenylidene H3M3(CCO) (CO);+. The chemistry of this species is quite interesting. Attack by nucleophiles such as HX = ROH, R,NH or H,O generates the corresponding acyls H3M3(CCOX)(C0)9. ‘“,38,42Deprotonation forms HzM~(CCO)(CO)~ ; an X-ray crystallographic study of the OS cluster found an upright ~~-q’ bonding mode for the CC0 ligand.43 The neutral ketenylidene is also formed by pyrolysis of HOs,(CH)(CO) I o. Shapley has demonstrated nucleophilic addition and substitution on the alkylidyne carbon of HM3(CX)(CO),o (M = Ru or OS ; X = OMe or H). ‘9,20,38 In several instances the addition products can be isolated, but the reactions can be quite complex. At this time there are no definitive examples of nucleophilic attack on the alkylidyne carbon of clusters of the class H3M3(CX)(C0)9. This suggests that the chemical reactivity of the alkylidyne ligand may depend strongly upon its coordination mode.
since it is the central step in the alkyne metathesis reaction.44 Alkyne-alkylidyne coupling has also been noted on polymetallic systems,37 although less is known about the mechanism of the reaction. Stoichiometric hydrogenations of alkynes by H3R~3(CX)(C0)9 (X = OMe, Me, Ph or CH2 CH2CMe3) yield the corresponding cis-alkene ; in each case a second equivalent of alkyne combines with the cluster residue to produce clusters of the general formula HRu,(C(X)C(R)CR)(CO)~ (Fig. 3), in which alkyne-alkylidyne coupling has generated a ,u3-~3-1,3-dimetalloallyl ligand. For X = OMe these same clusters may be obtained in better yields by direct reaction of HM,(COMe) (CO),,, M = Ru or OS, with the alkyne.29 Clusters of this class have been previously synthesized through a variety of pathways. The clusters HRu~(~~-~~-RCCR’CR”)(CO)~ have been obtained from reactions of alkenes, dienes or alkynes with Ru~(CO)~~.~~Interestingly, a hydroxy derivative HOS~(~~-~~-HOCCHCH)(CO)~ is formed from Os3(CO), ,(NCMe)* and acetylene in wet THF ; in this reaction a CO ligand is the source of the “hydroxymethylidyne” fragment.46 The stability of these clusters and the numerous pathways for their formation suggests that the p3-q3-C3R3 fragment may also be a common intermediate during reactions of hydrocarbons adsorbed on metal surfaces. The reactions of H3M3(CX)(C0)9 or HM3 (COMe)(CO),, with alkynes provide an excellent opportunity to study alkyne-alkylidyne coupling in trimetallic cluster systems. Two regioisomers are obtained with unsymmetrical alkynes. The two regioisomers from the terminal alkynes are easily identified by virtue of their ‘H NMR spectra, in which the coupling constant between the hydride and the hydrogen on the central carbon is ca 3 Hz, but no coupling is observed between the hydride and the hydrogen on the terminal carbon. The ratio of the isomers [HRu,(C(X)C(R’)CR’)(CO),]/ [HRu3(C(X)C(R2)CR1)(CO),] depends upon the
T
i
X,c/-p+,R’ -M
INSERTION OF ALKYNES INTO THE M-CX BOND Alkyne-alkylidyne coupling is a reaction of some interest for monometallic alkylidyne complexes
‘I
ii7q -H-
I\
M-
Fig. 3. Structure of HM3(p3-q3-XCCRCR’)(C0)9
(M = Ru or OS ; X = OMe, SEt, Ph or alkyl ; R and R’ = H or alkyl).
851
Reactivity of methylidyne ligands
of the alkyne substituents: (R’, R*), ratio = (H, Ph), 3 : 1; (H, Bu), 2: 1; (H, CMe,), 20: 1; (H, CO,Me), 2: 1; (H, OEt), 1: 0; (Me, C7H15), 3 : 1; (C02Me, Ph), 2 : 1. The isomer distribution is kinetically determined, since the isomers do not interconvert even at temperatures far in excess of that required for coupling. Apparently both steric and electronic properties influence the orientation of coupling. Comparison of the isomer ratios from I-hexyne and from 3,3_dimethylbutyne indicates that bulkier substituents prefer the terminal carbon. However, since the sterically similar substituents OEt and Bu give rise to very different isomer ratios, electronic effects are also important. The steric and electronic requirements for alkyne-alkylidyne coupling are still poorly understood. Bis(trimethylsilyl)acetylene, trimethylsilylacetylene and dimethyl acetylenedicarboxylate give little or no coupled products with HRu,(COMe) (CO), ,,. Furthermore, coupled products could not be isolated from reactions of 2-butyne with HFe,(COMe)(CO) , o or with HRu,(CNMe,)(CO) 1,,. Obviously, the coupling is sensitive to the nature of the alkyne, the metal and the alkylidyne substituent. The carbon-carbon bond formation in these Ru, clusters is reminiscent of carbon chain growth on metal surfaces in the Fischer-Tropsch reaction,47 in which CO is reduced with hydrogen to give alkanes and water. The major products from CO hydrogenation are linear hydrocarbons, but minor products are branched hydrocarbons, primarily 2methyl branched. Since alkylidyne species have been identified on metal surfaces and may come together to form alkynes, we considered the possibility that the cluster reaction might act as a model for carbon chain growth on metal surfaces. If an adsorbed CH fragment were to couple with an adsorbed “alkyne” HC2R, then the products would be linear and 2-methyl branched hydrocarbon chains. Pertinent to this analogy, is the report of the formation of HOS&-~~-HOCCHCH)(CO)~ from water, CO, acetylene and OS~(CO),~ (NCMe)2.46 To investigate this analogy we treated HRu, (C(OMe)C(R)CR)(C0)9 with hydrogen (l-4 atm, 96110°C). In each case the major cluster products isolated were H4Ru4(C0),* and H3Ru3 (CCHRCH,R)(CQ), (15-90%). A minor product from the hydrogenation of HRu,(C(OMe) CHCII)(CO), was identified as H2Ru3(MeC20Me) (CO), (Fig. 4) ;48 this product shows that hydrogen can migrate from, as well as to, the hydrocarbon framework. The isolation of alkylidyne and alkyne products, indeed suggested the possibility of carbon chain growth by the coupling of C, and C2 fragments to give dimetalloallyl species. identities
‘I’ Fig. 4. Structure of (p-H)IRus@s-~3-MeCCOMe)(CO),.
The mechanism of the hydrogenation reaction was investigated. Hydrogenation of DRu3(C (OMe)C(H)CCMe3)(C0)9 produced exclusively H3Ru3(CCH,CHDCMe3)(CO),, indicating transfer of the hydride to the 3-carbon, prior to the addition of molecular hydrogen. Comparison of the rates of reaction of HRu,(C(OMe)C(H)CR)(C0)9, R = Bu and CMe3, showed that bulkier substituents on the 3-carbon facilitate the hydrogenation, and consistent with this, hydrogenation of HRu3 (MeOCC(H)COEt)(C0)9 produces primarily H3Ru3(CCH2CH20Et)(C0)+ These observations suggest that the hydride ligand is transferred to the 3-carbon early in the reaction. Low yields precluded further kinetic studies. No alkylidyne products are obtained by hydrogenation of HRu,(C(X)C(R)CR)(CO), when X = H, alkyl or NMe2. However, hydrogenation of HRu3(C(SEt)C(Me)CMe)(CO)9 does form H3Ru3 (CCHMeCH,Me)(C0)9. Thus, a relatively weak C-X bond is required to allow rearrangement under conditions mild enough to allow isolation of the product. More information concerning intermediates in the hydrogenation was obtained by careful analysis of the products from the coupling of alkynes RC2R, R = Me or Ph, with H3Ru3(CSEt)(C0)9. Although the major product of the reaction is the expected HRu,(C(SEt)C(R)CR)(CO),, a minor product was found to be Ru,(CCRCHR)(SEt)(CO),; the structure of this product (Fig. 5), determined by X-ray crystallography for R = Ph, is closely related to the possible intermediate in the hydrogenation process. 4g The hydride ligand has been transferred
Ph
Fig. 5. Structure of Ru3@3-q3-CCPhCHPh)(pSEt)(CO)9.
J. B. KEISTER
852
to the 3-carbon and the CSEt bond has been cleaved, as is required in the hydrogenation. Although the alkylidyne carbon is triply bridging, there are only two Ru-Ru bonds. Presumably, the strong Ru-S bonds are responsible for retention of the SEt moiety in the cluster, whereas pyrolysis of HRu,(MeOCCRCR)(C0)9 does not form an analogous, stable product. Pyrolysis of HRu3 (EtSCCRCR)(CO), at 70°C forms Ru,(CCRCHR) (SEt)(CO), in nearly quantitative yield ; preliminary kinetic measurements have found a first order rate law with a rate constant of 7.6 x 10e6 s- ’ at 70°C. Thus, Ru,(CCRCHR)(SEt)(CO), seems to be a possible intermediate in the hydrogenation of HRu3 (EtSCCRCR)(C0)9 to H,Ru,(CCHMeCH?Me) (CO),. From the results obtained so far we can propose the mechanism shown in Fig. 6 for alkyne-alkylidyne coupling and subsequent hydrogenation. This work has shown that : (1) alkyne-alkylidyne coupling occurs on trimetallic clusters as well as for monometallic species, but pJ-q3-bridging makes the cluster-bound metallocycles much more stable, and (2) hydrogenation of 1,3-dimetalloallyls can form alkylidynes. These reactions provide a pathway for hydrocarbon chain growth on polymetallic units, which may be related to hydrocarbon chain growth on metal surfaces. Although the pu-q3-C3R3 fragment has not yet been identified on a metal surface,
the stability of these clusters and the large variety of paths which have been shown to lead to clusters of this class suggest that the fragment is likely to be an important product of hydrocarbon adsorption in heterogeneous catalysis. Further studies of the mechanism of the isomerization and hydrogenation reactions are in progress. REDUCTIVE ELIMINATION OF H-CX BONDS Reductive eliminations of C-H bonds from metals are the product-releasing steps in many catalytic processes. The mechanism is well understood for monometallic complexes,50 but not for metal clusters or surfaces. Reactions of H3M3(CX)(C0)9 with CO (l-35 atm, 6&l 1O’C) produce CH3X and Ru~(CO),~/RU(CO)~ when X = Ph, C02Me, Cl or H ; ” when X = OMe, dimethyl ether can be identified after decomposition under a CO/H2 mixture. Elimination of CH3X from H3M3(CX)(CO), may model desorption of alkanes from metal surfaces. With this in mind, we undertook an investigation of the kinetics and mechanism of this reaction. Calvert and Shapley have previously demonstrated the reverse process, “adsorption” of a methyl group by sequential C-H oxidative additions to give methylene and methylidyne fragments (Fig. 7). 52 Importantly, a facile equilibrium
X
>R”/c\--/““< ’ ‘\
+ 2 C2R2 >
.H
- C2H2R2
/”
H,
/y\
I
R H&H .H$R 1 /Ru
F\
+ 3 H2
/
-H --4”-\ I I RuAH H -
< -HX
‘I’
R
H
Fig. 6. Reaction pathway for alkyne-alkylidyne coupling and hydrogeneration of a 1,3-dimetalloallyl ligand. (X = OMe or SEt.)
853
Reactivity of methylidyne ligands
.2L /
OS~(CO),~L~
+ CH4 (COj3
Fig. 7. Interconversion
of alkyl, alkylidene and alkylidyne ligands on a OS, cluster.
between hydridomethylene and a methyl ligand with an agostic hydrogen was demonstrated. Furthermore, HOs3(CH3)(CO), o reacted under mild conditions with Lewis bases to form methane. These OS clusters provide valuable models for intermediates in the reductive elimination, but the difficulty and expense of obtaining substantial quantities make kinetic studies of this system impractical. Based upon our results and those of others we can propose the mechanism shown in Fig. 8 for
the reductive elimination from H3M3(CX)(C0)9, M = Ru or OS. The basis for this proposition is given below. That C-H elimination from H,Ru,(CX)(CO), occurs in a sequential manner is suggested by the isolation of stabilized alkylidene and alkyl intermediates. Pyrolysis of H3Ru3(CC02Me)(C0)9 in the absence of CO forms HZRu3(CHC0,Me)(C0)9 in 56% yield (Fig. 9). 32 This product is formed by reductive elimination of a single C-H bond followed by coordination of the acyl to the unsatu-
X
X, c -ii\ toa3y
/-
M3(C0),2
(CO13
M(CO)3
-
H.M\
/
Hi
(co)3
+ CH-,X
t
+co
X
\
,H
F- (OC13M \
y(co)3
\ HtM/H (CO)3
CHZX
I
t°CgM\
j-.(co)4 H-M
+co/ 7 X
(C 01,
lC/H
(OC)3M/-- \
-co
H .M\
y(co), .H
Fig. 8. Mechanism for reductive elimination of CHpX from HSM3(p3-CX)(C0)9 (X = H, alkyl, Cl, Ph or C02Me ; M = Ru or OS).
854
J. B. KEISTER OMe
(10). The near-zero entropy of activation for the limiting rate constant, a, is consistent with an intramolecular rate-determining step. Kinetic isotope effects were unexpectedly less than or equal to one, being 0.64 for X = Ph and 1.00 for X = CO,Me.
rate = ac[Col
b+c[CO]
Fig. 9. Structure of (p-H),Ru,(p,-CC0,Me)(C0)9.
rated metal site thus created. Pyrolysis of H3Ru3 (CSEt)(CO), yields HRu3(CH2SEt)(C0)9 (Fig. 10); in this case the sulphur atom stabilizes the two vacant sites formed by reductive elimination.3’ Under CO, complete elimination of CH3X ultimately yields RUDE 2, which is in equilibrium with RUG under the reaction conditions. A number of lines of evidence indicate that the rate of C-H reductive elimination increases for successive steps. First, no intermediate alkylidene is observed during pyrolysis of H3Ru3(CSEt)(C0)9, suggesting that formation of H,Ru3(CHSEt) (CO), is slower than its conversion to HRu, (CH,SEt)(CO),. Second, direct comparison of the reactivity of the HzOs3(CHJ(CO)10/HOs3(CHJ (CO),, equilibrium mixture found that the rate of conversion of H20s3(CH2)(CO)lo to HOs3(CH3)(CO)io is slower than the reaction of CO with the latter, forming methane and 0s3(CO), 2. Third, structural evidence suggests that the Ru-C bond strength decreases upon each successive C-H elimination ; the Ru-C bond lengths decrease in the order: H3R~3(CR)(C0)9 < H2Ru3 (CHC02Me)(CO), < HRu3(CH2SEt)(C0)9. 31 A study of the mechanism of the reductive elimination process was undertaken. A crossover experiment revealed that the acetates formed from a mixture of H3Ru3(C02Et)(C0)9 and D3Ru3 (C02Me)(CO), were labelled in a manner consistent with intramolecular formation of at least two of the three C-H bonds. The rate-determining step involves formation of only the first C-H bond. The rate law for toluene elimination from H3Ru3 (CPh)(CO), was determined to be as shown in eq.
[H,Ru,(CPh)(CO),].
(10)
Although several pre-equilibria may be postulated to account for the kinetics, the most attractive, shown in Fig. 11, involves reversible migration of a hydrogen atom from a position bridging a Ru-Ru vector, to a position bridging a Ru-C vector. The agostic C-H-M bond53 has recently been established in a number of monometallic complexes, as well as in polymetallic ones. In particular there are two excellent models for this intermediate. First, Fehlner and coworkers have shown that deprotonation of H3Fe3(CH)(C0)9 forms HFe, (HCH)(CO)i-, shown by ‘H and i3C NMR spectroscopy to contain a Fe--H-C bridge. Further work by this group has shown that H,Fe,(CH) (CO), exists in solution in equilibrium with H2Fe3 (HCH)(C0)9 and HFe3(H2CH)(C0)9 (Fig. 12).34 Second, although the equilibrium constant for the tautomerization is not so favourable for the agostic Ru analogues, we have found that dissolution of H3M3(CR)(C0)9, where M = Ru and R = Et or CHPhCH2Ph, or M = OS and R = Me, in fluorosulphonic or trifluoromethanesulphonic acid forms (Fig. 13), by protonation H,M,(HCR)(CO);+ of the M-C bond. These cations have been characterized by NMR spectroscopy.30 For H3Ru3 (HCEt)(CO)i+, the chemical shift of the agostic hydrogen is -9.45 ppm while a singlet resonance at - 18.37 ppm is found for the three hydride ligands. The propylidene c1 carbon resonance occurs at 143.3 ppm (d) with a coupling constant to the agostic hydrogen of 58 Hz. These values of the spectral parameters are very similar to corresponding values for H,Fe,(HCH)(CO)i-. Protonation of the Ru-C bond is expected since the HOMO is Ru-C bonding in character.25 The proposed structure should give rise to two signals for the hydride ligands. To account for the obser-
X I
‘I’
’ \’ Fig. 10. Structure of (pH)Ru,(p,-CH,SEt)(CO),.
/I’
Fig. 11. Equilibrium between ground state (p-H)JM3(p3CX)(CO), and the higher energy tautomer (p-H)2M3(p3HCX)(CO), with an agostic hydrogen.
Reactivity of methylidyne ligands
PPN
855
Cl
>
Fig. 12. Interconversions between various tautomers of H4Fe3(C)(C0)9 and deprotonation H)SFe&-CH)(C0)9 to give @-H)Fe&-HCH)(CO);.
of only a single hydride resonance, we must postulate that the hydrides are averaged by some dynamic process ; however, there is no evidence for line broadening in the NMR spectrum even at -80°C. Fluxional migration of the agostic hydrogen to each of the three M-C edges would explain the observed spectrum. The exchange process must be intramolecular, because “C-hydride coupling is retained. No exchange occurs between the agostic hydrogen and the hydride ligands on the NMR timescale. Isotopic labelling has shown that exchange between the agostic hydrogen and the metal hydrides does not take place over a period of several hours at 25°C. The protonated clusters undergo much faster reductive elimination than the neutral precursors. Shapley has found that dissolution of H30s3 (CH)(CO), in sulphuric acid gives methane and HzOs3(03SO)(CO),.54 Dissolution of H,Ru,(CR)
vation
Fig.
13. Proposed structure for @H),M&-HCR) (CO): (M = Ru or OS; R = Et or CHPhCH,Ph).
(CO),, R = H, Ph, Et, CHPhCH,Ph, in trifluoromethanesulphonic acid results in the formation of the corresponding alkane RH and a variety of Ru species including HRu,(CO) i :, HRu(C0) :’ and what may be H2Ru3(03SCF3)(CO)i+. The alkane product was isolated in the cases of R = CHPhCH2Ph and Ph. Although the alkyl derivatives are moderately stable (tllz N 24 h for CHPhCH2Ph), dissolution of benzylidyne or methylidyne clusters causes immediate decomposition. The pre-equilibrium between H3R~3(CX)(C0)9 and its agostic tautomer accounts nicely for the inverse isotope effect for the reductive elimination of CH3X, since the vibrational modes of an agostic hydrogen should be considerably higher in energy than those of a bridging hydride (e.g. stretching frequencies of ca 2600 cm- ’ for C-H-M vs 1350 cm- ’ for M-H-M). If only the stretching modes are considered, the equilibrium isotope effect upon the equilibrium constant for the equilibrium shown in Fig. 11 can be calculated to be 0.54 (cf. 0.64 for reductive elimination from H,Ru,(CPh)(CO),). The rate-determining step following the pre-equilibrium tautomerization to the agostic alkylidene intermediate, is cleavage of the C-H-Ru bond to form an unsaturated Ru centre. Addition of CO to the unsaturated cluster produces a saturated alkylidene of the formula H2M 3(p-CHX)(CO)1 o. Stable analogues of this alkylidene intermediate are HzRu3(CHCOzMe)(C0)9andH,0s3(CH,)(CO),,.55 Methylidyne substituents which can act as Lewis bases, such as CO*Me, C(O)NR? or SEt, greatly
856
J. B. KEISTER
accelerate the reductive elimination reaction. In these cases, the fact that the relative rates increase in the same order as the basicity of the substituent (C(O)OMe < C(0)NR2 < SEt), and the fact that stable products can be isolated in which the substituents are coordinated to a metal centre, suggest that the acceleration is due to direct attack of the substituent upon a metal atom as the agostic bond is cleaved. The next step in the reductive elimination of the alkane is reversible conversion of H2M3 (J&HX)(CO), o to HM,@-HCHX)(CO), ,,, a tautomer containing an agostic bond. Indeed, Shapley has shown that in solution, H20s3(CH2)(C0)i0 is in equilibrium with HOs,(HCH,)(CO),,, a slightly less stable tautomer (& = 0.28).52 These workers have established the same tautomeric equilibrium for
H,Os,(CHMe)(CO), o/HWHCHMe)(W
1o
K,
0.14) ; in this system /?-elimination from the ethyl group is thermodynamically favoured, but aelimination is cu 100 times faster at WCs6 The next step in the proposed mechanism is cleavage of the agostic bond of HM,(HCHX) (CO),,, forming an unsaturated metal centre, followed by coordination of CO to give the hydridoalkyl HM,(CH,X)(CO) ,,,. Stable analogues of these intermediates are HOs3(CH2C02R) (CO),, (Fig. 14),57HOs3(RCHOMe)(CO)Io,5* HOs, HOs,(NCO)(succinoyl) (MeCHSPh)(CO) , o,5g (CO),o6o and HRu,(CH,SEt)(CO),. Finally, reductive elimination of the third C-H bond and coordination of CO would produce and CH3X. If fact, both HOs, M,(W12 (HCH,)(CO) I o and HOs3(CH2C02Et)(CO) 1o react with Lewis bases under very mild conditions (room temperature and below) in this fashion. In the latter case an intermediate of the probable formula HOs3(CH2C02Et)(CO)io(PMe,Ph) was observed by NMR spectroscopy during the reaction with PMe*Ph. 6’ An alternative path for “desorption” of the hydrocarbon from the cluster alkyl is /&elimination, producing an alkene and a metal hydride. In fact, decomposition of H3Ru3(CCHPhCHzPh) under CO produces exclusively alkenes (CO)9 =
Fig. 14. Proposed
structure
of HOs,(CH,CO,Et)(CO),e.
-CH2---_CPhCH2Ph and cis- and truns-MeC (Ph)=CHPh,62 while fumarate esters are the only products of the reaction between stabilized alkyls HOS,(C(CO~R)CH~CO~R)(CO)~~ and Lewis bases. 6’ Thus, j&elimination, when possible, is the prefered path for decomposition of hydridoalkyl clusters. At this point, isolatable analogues are available for all the probable intermediates in the “desorption” of an alkane, adsorbed as alkylidyne, from a small molecular cluster. Determinations of the energetics and mechanism are only beginning. These studies should continue to provide analogies applicable to metal surface chemistry. CONCLUSION
Since the structures and compositions of cluster compounds vary so widely, it is not surprising that generalizations concerning chemical reactivity are difficult to come by. However, cluster chemistry is unique from the chemistry of monometallic complexes in that : (1) Bonding of hydrocarbyl ligands to several metal centres is the rule in cluster chemistry. (2) One may expect facile hydrogen migration from metal to carbon as well as from metal to metal, and the various tautomers may differ only slightly in energy. (3) The wide variety of bonding modes available for hydrocarbyls on clusters makes the number of potential reaction pathways much greater than for monometallic complexes. (4) Metal clusters have already provided useful models for the bonding of hydrocarbyl ligands to metal surfaces and will continue to provide useful models for reaction mechanisms.
REFERENCES 1 (a) M. H. Chisholm, K. Folting, D. M. Hoffman and J. C. Huffman, J. Am. Chem. Sot. 1984, 106, 6794 ; (b) R. R. Schrock, M. L. Listemann and L. G. Sturgeoff, J. Am. Chem. Sot. 1982, 104, 4291; (c) J. H. Wengrovius, J. Sancho and R. R. Schrock, J. Am. Chem. Sot. 1981, 103,3932 ; (d) G. J. Leigh, M. T. Rahman and D. R. M. Walton, J. Chem. Sot., Chem. Commun. 1982,541. 2. (a) L. L. Kesmodel, L. H. Dubois and G. A. Somorjai, J. Chem. Phys. 1979,70,2180; (b) T. P. Beebe, Jr, M. R. Albert and J. T. Yates, Jr, J. Catal. 1985, 96, 1; (c) M. M. Hills, J. E. Parmeter and W. H. Weinberg, J. Am. Chem. Sot. 1987, 109, 4224; (d) Ts. S. Marinova and K. L. Kostov, Surf. Sci. 1987, 181, 573 ; (e) M. M. Hills, J. E. Parmeter and W. H. Weinberg, J. Am. Chem. Sot. 1986, 108, 7215 ; (f) K. M. Ogle and J. M. White, Surf. Sci. 1986, 165, 234; (g) B. E. Koel, B. E. Bent and G. A. Somorjai,
Reactivity of methylidyne ligands
3.
4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14.
15.
16. 17. 18. 19. 20.
21. 22.
23. 24. 25. 26. 27. 28.
Surf. Sci. 1984, 146,211; (h) J. E. Parmeter, M. M. Hills and W. H. Weinberg, J. Am. Chem. Sot. 1987, 109,72. (a) G. Palyi, F. Piacenti and L. Marko, Znorg. Chim. Actu Rev. 1970,4, 109; (b) D. Seyferth, Adv. Organomet. Chem. 1976, 24, 97; (c) B. R. Penfold and B. H. Robinson, Act. Chem. Res. 1973,6, 73. K. S. Wong, K. J. Haller, T. K. Dutta, D. M. Chipman and T. P. Fehlner, Znorg. Chem. 1982,21, 3197. N. J. Zhu, C. Lecomte, P. C. Coppens and J. B. Keister, Acta Cryst. 1982, B38, 1286. G. M. Sheldrick and J. P. Yesinowski, J. Chem. Sot., Dalton Trans. 1975, 873. M. R. Churchill, T. P. Duggan, J. B. Keister and J. W. Ziller, Acta Cryst. 1987, C43,203. M. Castiglioni, G. Gervasio and E. Sappa, Znorg. Chim. Acta 1982,49,217. S. G. Shore, D.-Y. Jan, W.-L. Hsu, L.-Y. Hsu, S. Kennedy, J. C. Huffman, T.-C. L. Wang and A. G. Marshall, J. Chem. Sot., Chem. Commun. 1984,392. J. Krause, D.-Y. Jan and S. G. Shore, J. Am. Chem. Sot. 1987,109,4416. D. F. Shriver, D. Lehman and D. Strope, J. Am. Chem. Sot. 1975,97, 1594. F. H. Herbstein, Acta Crust. 1981, B37, 339. E. Iiskola, T. A. Pakkanen, T. T. Pakkanen and T. Venalainen, Acta Chem. Stand. 1983, A37, 125. M. R. Churchill, L. R. Beanan, H. J. Wasserman, C. Bueno, Z. Abdul Rahman and J. B. Keister, Organometallics 1983,2, 1179. B. F. G. Johnson, J. Lewis, A. G. Orpen, P. R. Raithby and G. Suss, J. Organomet. Chem. 1979, 173, 187. M. R. Churchill, B. G. DeBoer and F. J. Rotella, Znorg. Chem. 1976, 15, 1843. B. F. G. Johnson, J. Lewis, P. R. Raithby and G. Suss, J. Chem. Sot., Dalton Trans. 1979, 1356. M. Green, A. G. Orpen and C. J. Schaverein, J. Chem. Sot., Chem. Commun. 1984,37. W.-Y. Yeh, J. R. Shapley, Y. J. Li and M. R. Churchill, Organometallics 1985,4, 767. J. R. Shapley, M. E. Cree-Uchiyama, G. M. St. George, M. R. Churchill and C. Bueno, J. Am. Chem. Sot. 1983,105, 140. R. D. Adams and N. M. Golembeski, Znorg. Chem. 1979,18,2255. A. A. Kordize, 0. A. Kizas, N. E. Kolobova, A. I. Yanovsky and Yu. T. Struchkov, J. Organomet. Chem. 1986,302,413. J. B. Keister, M. W. Payne and M. J. Muscatella, Organometallics 1983, 2, 219. P. D. Gavens and M. J. Mays, J. Organomet. Chem. 1978,162,389. D. E. Sherwood, Jr and M. B. Hall, Organometallics 1982, 1, 1519. I. A. Oxton, Spectrochimica Acta 1982,38A, 181. J. Evans and G. S. McNulty, J. Chem. Sot., Dalton Trans. 1984,79. D. M. Dalton, D. J. Bamett, T. P. Duggan, J. B. Keister, P. T. Malik, S. P. Modi, M. R. Shaffer and S. A. Smesko, Organometallics 1985,4, 1854.
857
29. L. R. Beanan and J. B. Keister, OrganometaZZics 1985,4, 1713. 30. D. K. Bower and J. B. Keister, J. Organomet. Chem. 1986,312, C33. 31. M. R. Churchill, J. W. Ziller, D. M. Dalton and J. B. Keister, Organometallics 1987,6, 806. 32. M. R. Churchill, T. S. Janik, T. P. Duggan and J. B. Keister, Organometallics 1987,6, 799. 33. J. A. S. Howell and P. Mathur, J. Chem. Sot., Dalton Trans. 1982,43. 34. T. K. Dutta, J. C. Vites, G. B. Jacobsen and T. P. Fehlner, Organometallics 1987,6,842. 35. (a) L. J. Farrugia, J. Organomet. Chem. 1986, 310, 67 ; (b) A. A. Aitchison and L. J. Farrugia, Orgunometallics 1986, 5, 1103. 36. H. A. Hodali and D. F. Shriver, Znorg. Chem. 1979, 18, 1236. 37. (a) J. Ros, G. Commenges, R. Mathieu, X. Solans and M. Font-Altaba, J. Chem. Sot., Dalton Trans. 1985, 1087; (b) M. H. Chisholm, J. A. Heppert and J. C. Hulfman, J. Am. Chem. Sot. 1984,106, 1151. 38. J. S. Holmgren and J. R. Shapley, Organometallics 1984, 3, 1322. 39. A. J. Canty, B. F. G. Johnson, J. Lewis and J. R. Norton, J. Chem. Sot., Chem. Commun. 1972, 1331. 40. A. J. Deeming and M. Underhill, J. Chem. Sot., Chem. Commun. 1973,277. 41. K. S. Wong and T. P. Fehlner, J. Am. Chem. Sot. 1981,103,906. 42. J. B. Keister and T. L. Horling, Znorg. Chem. 1980, 19, 2304. 43. J. R. Shapley, D. S. Strickland, G. M. St. George, M. R. Churchill and C. Bueno, Organometallics 1983, 2, 185. 44. (a) M. R. Churchill, J. W. Ziller, J. H. Freudenberger and R. R. Schrock, Organometallics 1984, 3, 1554 ; (b) J. H. Freudenberger, R. R. Schrock, M. R. Churchill, A. L. Rheingold and J. W. Ziller, Organometallics 1984,3, 1563. 45. (a) 0. Gambino, M. Valle, S. Aime and G. A. Vaglio, Znorg. Chim. Actu 1974,8, 71 ; (b) M. Castiglioni, L. Milone, D. Osella, G. A. Vaglio and M. Valle, Znorg. Chem. 1976, 15, 394 ; (c) M. I. Bruce, M. A. Cairns and M. Green, J. Chem. Sot., Dalton Trans. 1972, 1293 ; (d) S. Aime, G. Jannon, D. Osella, A. J. Arce and A. J. Deeming, J. Chem. Sot., Dalton Trans. 1984, 1987. 46. B. E. Hanson, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Sot., Dalton Trans. 1980, 1852. 47. (a) C. K. Rofer-DePoorter, Chem. Rev. 1981, 81, 447 ; (b) E. L. Muetterties and J. Stein, Chem. Rev. 1979, 79,479. 48. M. R. Churchill, J. C. Fettinger, J. B. Keister, R. F. See and J. W. Ziller, Organometallics 1985,4, 2112. 49. D. K. Bower, J. W. Ziller, M. R. Churchill and J. B. Keister, unpublished results. 50. For leading references see : (a) R. A. Periana and R. G. Bergman, J. Am. Chem. Sot. 1986,108,7332; (b) W. D. Jones and F. J. Feher, J. Am. Chem. Sot. 1986,108,4814; (c)W. D. Jones and F. J. Feher, J. Am. Chem. Sot. 1985,107,620; (d) R. H. Crabtree,
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E. M. Holt, M. Lavin and S. M. Morehouse, Znorg. Chem. 198524, 1986; (e) A. H. Janowicz and R. G. Bergman, J. Am. Chem. Sot. 1983,105,3929. 51. T. P. Duggan, D. J. Barnett, M. J. Muscatella and J. B. Keister, J. Am. Chem. Sot. 1986, 108,6076. 52. R. B. Calvert and J. R. Shapley, J. Am. Chem. Sot. 1977,99,5225 ; 1978,100,6544. 53. M. Brookhart and M. L. H. Green, J. Organomet. Chem. 1983,250,395. 54. R. L. Keiter, D. S. Strickland, S. R. Wilson and J. R. Shapley, J. Am. Chem. Sot. 1986, lO& 3846. 55. R. B. Calvert, J. R. Shapley, A. J. Schultz, J. M. Williams, S. L. Suib and G. D. Stucky, J. Am. Chem. Sot. 1978,100,6240.
56. M. Cree-Uchiyama, J. R. Shapley and G. M. St. George, J. Am. Chem. Sot. 1986,108, 1316. 57. J. B. Keister and J. R. Shapley, J. Am. Chem. Sot.
1976,98, 1056. 58. E. Boyar, A. J. Deeming, A. J. Arce and Y. De Sanctis, J. Organomet. Chem. 1984,276, C45. 59. E. Boyar, A. J. Deeming, K. Henrick, M. McPartlin and A. Scott, J. Chem. Sot., Dalton Trans. 1986, 1431. 60. J. L. Zuffa and W. L. Gladfelter, J. Am. Chem. Sot.
1986,108,4669. 61. J. B. Keister, Ph.D. thesis, University (1978). 62. T. P. Duggan, Ph.D. thesis, University (1987).
of Illinois at Buffalo
Po,ykdron Vol. 7, No. lO/ll, Pp. 847-858, 1988 Printed in Great Britain
LIGANDS ON TRINUCLEAR CLUSTERS OF GROUP 8 METALS JEROME
B. KEISTER
Department of Chemistry, University at BufIalo, S.U.N.Y., Buffalo, NY 14214, U.S.A. (Accepted 5 November 1987)
Abstract-The chemistry of alkylidyne containing clusters HM &&X)(CO) i0 and H3M & ,-CX)(CO), is presented&-( 1) structures, bonding and spectroscopic characterizations, (2) syntheses involving modification of the methylidyne substituent, (3) alkyne-alkylidyne coupling, and (4) formation of H-CX bonds, Reactions of HRu, ~COMe)(CO),* or ~~Ru~(~X)(CO)~ fx = OMe, Ph or alkyl) with alkynes, produce HRu3(p3-q3-XCCRCR)(C0)g clusters containing 1,3-dimetalloallyl ligands. Hydrogenation of these products when X = OMe or SEt gives new alkylidynes H3Ru3(p3CCHRCH*R)(CO)~. For X = SEt, isome~zation of HRu~~~-~3-EtSCCRCR)(CO)~ to Ru3(fl-SEt)(~3-q3-CCRCHR)(C0)g, a presumed intermediate in the hydrogenation reaction, occurs under mild conditions. Under CO, reductive elimination of CH3X can be induced from H3Ru~(~~-CX~(CO)g (X = OMe, Ph, Cl or C02Me). Pyrolysis of H3Ru&CX)(CO),, X = COzMe and SEt, gives stabilized intermediates H,Ru,&-CHC02Me) (CO), and HRu3(p,-CH2SEt)(C0)9, respectively. Protonation of H3Ru3(fi3-CR)(C0)g (R = Et or CHPhCH~Ph) yields H3Ru~~3-HCR~CO~~+, in which the alkylidene hydrogen is agostic ; the protonated clusters undergo more rapid reductive elimination than the neutral precursors. The kinetics and mechanism of reductive elimination have been investigated. A mechanism is presented.
The comparison between the chemical reactivity of hydrocarbyl ligands bridging two or more metals and the reactivity of these ligands when bound to a single metal centre is clearly of fundamental interest. The chemistry of bridging hydrocarbyl ligands is also of interest because of the analogy to the chemistry of hydrocarbyl groups adsorbed on metal surfaces. The alkylidyne ligand has an extensive chemistry for monometal~c complexes and is implicated in acetylene metathesis. ’ Furthermore, alkylidynes have been identified in numerous studies of surface adsorbed hydrocarbons, although iittle is known of the reactivity of these surface bound species.’ Alkylidynes of metal clusters have also been known for many years. The best known class of clusters containing a bridging alkylidyne group has the genera1 formula Co3@c13-CR)(CO)g;the chemistry of these compounds has been mainly found to involve modification of the alkylidyne substituent or ligand substitution on the metal atoms.3 Comparisons of the reactivity of the alkylidyne ligand on single metal centres, on metal clusters and on metal surfaces are therefore of special interest.
Our research group has been concerned with the chemical reactivity of the cluster series having the general formula HOMER, M = Fe, Ru or OS and X = alkyl, aryl, alkoxy or other substituent (Fig. 1). These clusters are isoelectronic with, and close structural anaiogues of the Coax clusters. These series provide the opportunity to investigate the chemical reactivity of a variety of groups-hydride, carbonyl, M-C bond or C-X bond-as a function of the metal, methylidyne substituent and Lewis base ligands.
Fig. 1. Structure of (y-H),M&-CX)(CO), (M = Fe, Ru or OS; X = H, dkyl, aryl, halide, OMe, CO*Me or SEt).
847
848
J. B. KEISTER
This review will present the chemistry of the H~MJ(CX)(CO)~ series and the related class HM,(CX)(CO) 1,, : (1) structure, bonding and spectroscopic characterization, (2) syntheses involving modification of the X substituent, (3) insertion of alkynes into the M-CX bond, and (4) formation of H-CX bonds. Although this review will emphasize work done by our group on Ru clusters, results of other research groups will also be presented. STRUCTURES, BONDING AND SPECTROSCOPY
The structures of H3M3(CX)(C0)9 (Fig. 1: M = Fe, X = Me;4 M = Ru, X = CL5 Me,6 C6H44-Me7 and CH&Me,;’ M = OS, X = OBO(trimeric)g and CO,H”) all contain a triangle of metal atoms symmetrically capped by the methylidyne ligand and with each edge of the triangle bridged by a single hydride ligand. The coordination geometry around each metal is octahedral, with facially coordinating CO ligands and mutually cis hydrides and methylidyne ligands. For several Ru clusters the positions of the hydride ligands have been determined directly by X-ray methods. The Ru-H-Ru and angles are 97-108”, while the H-Ru-H C-Ru-H angles are 8587”. Much greater structural variation is found for clusters of the general formula HM,(CX)(CO)lo (Fig. 2: M = Fe, X = OMe,” NMe212 or O’-;‘3 M = Ru, X = 0Me,‘4~‘S NMe216 or O’-;17 M = OS, X = CCH2CHMe2,‘8 Ph,” H,20 NHCMe,2’ or C(Ph)CC(Ph)Re(CO)4PMe2Ph22). Although in all of these the hydride and methylidyne bridge a common edge, the bonding of the CX ligand varies from doubly bridging to almost symmetrically triply bridging. The dihedral angle 0 between the CM2 and M3 planes varies between 10470”, depending upon the metal and the methylidyne substituent, with the angle decreasing as X = O’- (Ru, 104”) > NMe2 (Ru, 100’) > C(Ph)CC(Ph)Re(CO),L (OS, 99”) > OMe (Ru, 95”)
Fig. 2. Structure of (~H)M&u-CX)(CO),~ (M = Fe, Ru or OS ; X = OMe, NR,, O-, H, or Ph).
> CH,CHMe2 (OS, 82”) > Ph (OS, 78”) > H (OS, 70”) and as M = OS > Ru > Fe ; a comparison between the structures of HRu,(COMe)(CO), o (0 = 94.90”) and HRu3(COMe)(CO)8(C6H8) (0 = 60.70”) shows that the other ligands on the cluster also have an influence. I4 For the doubly bridging alkylidynes, there is structural and spectroscopic evidence for rc-donation from the methylidyne substituent to the adjacent carbon. Thus, for HM&-CX)(CO) l o, X = OMe or NMe,, the M2(p-CX) unit is planar, the C-X bond lengths are substantially shorter than values expected for single bonds (for HRu,(p-COMe)(CO) lo, 1.305 A and for HRu3(p-CNMe2)(CO)lo, 1.280 A), and there is restricted rotation about the C-X bond (AG* = 56.5 kJ for HOs3(COMe)(CO)‘023~24 and > 80 kJ for HOs3(CNMeCH,Ph)(CO)‘023). The degree of C-X multiple bonding increases as the rc-donor ability of X increases: OR < NR2 < O’-. At this time there is no explanation for the dependence of the coordination geometry, assumed by the methylidyne in the ground state, upon the identity of the metal or other ligands. The explanation for the variation of the dihedral angle 6 with the nature of the methylidyne substituent may be found in the competition between X + C rc-donation, which is most favourable for a doubly bridging configuration, and M + C rc-donation, which is most favourable for a triply bridging one. I4 Fenske-Hall calculations and photoelectron spectroscopy have been used to investigate the bonding in H3M3(p3-CX)(C0)g.25 The “tilt” of the Ru(CO), units caused by the hydride ligands gives rise to some differences between the MO’s of the Ru clusters and those of the Co analogues. The Ru-H-Ru bonds are best described as threecentre, two-electron bonds with no direct Ru-Ru interaction. The methylidyne carbon atom may be described as sp-hybridized with delocalized bonding involving the 2~7~orbitals and little delocalization of the sp hybrid lone pair onto the cluster. The HOMO is mainly Ru-C bonding in character, unlike the Co analogues for which the HOMO is Co-Co bonding. The vibrational spectra of H3M3(CX)(C0)g (M=Ru,X=H,Me,Cl;M=Os,X=Me)have been analysed.26,27 The metal-hydride stretching modes occur near 1350 cm-‘. For H3Ru3(CH) (CO), the C-H stretching frequency is 2988 cm-’ and normal coordinate analysis found the Ru-C force constant to be 137 N rn- ‘, close to the value for the Co analogue. 13C NMR spectra have been reported for both HM,(CX)(CO),, and H,M,(CX)(CO),. The chemical shift of the methylidyne carbon varies between 370 and 118 ppm, depending upon the
849
Reactivity of methylidyne ligands Table 1. ’3C Chemical shift for the ~kyIidyne carbon Chemical shift Reference @pm)
Compound HRu,(COMe)(CO) 1o HRu~(~NMe~)(CO)io H,Ru,(CBt)(CO), B&u&Me)(CO), H,Ru,(CPh)(CO), H,Ru,(CSEt)(C0)9 H,Ru,(CC0,Me)(C0)9 HOs,(COMe)(CO)
,o HOs~~CCH~CHMe~~(CO), c.
HOs,(CPh)(CO),o BOs,(CH)(CO),o H,Os,(COMe)(CO), H~Os~(CMe)(CO)~ H,Os,(CH)(CO), HFe&NBtJ(CO)10 HFe~(COMe)(CO~ , o B$%(CH)(C%
366.5 312.6 232.6 219.3 213.0 204.4 183.5 or 180.2 352.2 320.5 (40°C) 312.0 (-SOOC) 314.2 (17°C) 304.2(-115°C) 219 (- 100°C) 205.2 154.7 118.4 317.3 356.5 232.0
15 28 29 27 30 31 32 24 18 19 23 27 20 33 11 34
bonding mode of the ligand, the metal and the methylidyne substituent (Table 1). In several examples the chemical shift of the methylidyne carbon has been found to be strongly temperature dependent. It has been proposed that in these instances there exists a fluxional equilibria between isomers having different coordination modes for the methylidyne ligand. 3s
HM,(CO);; Mel:HM,(COMe)(CO),, (M = Fe, Ru or OS)
HM ,(CX)(CO) I 0. (2) X=H-orPh)
(M=RuorOs;
The first member of the class H,M,(CX)(CO),, H~Ru~(CMe)(CO)g, was prepared by the reaction of H4Ru4(C0),2 ethylene ;3g subsequently the OS analogue was prepared by hydrogenation of H~Os3(CCH~)(CO)~, derived from ethylene and Fehlner and Wong more recently 0s3(c0)**.40 prepared H3Fe3(CR)(C0)g, R = H, Me or Et, by reduction of Fe(CO)5_4L These syntheses were not adaptable to other methylidyne derivatives. In 1976 we found that hydrogenation of HM3(COMe)(CO) i 0, M = Fe, Ru or OS, gave H,M,(COMe)(CO), [eq. (3)].23 With these new methylidyne clusters in hand we sought to prepare a complete series analogous to the Co,(CX)(CO), series. Seyferth had earlier developed a number of syntheses involving modification of the halome~ylidyne ligand. Therefore, our first goal was to convert H3M3(COMe)(CO)9 to H3M3(CX)(C0)9, X = halide. The approach in eq. (4) afforded the desired clusters in nearly quantitative yields. 42Then by application of procedures developed for the Co analogues, we were able to prepare a wide variety of substituted methylidyne species [eqs (5)-(8)].42*3’ HM,(COMe)(CO),
o+ Hz
--+H3M3(COMe)(CO)g+C0
H3M3(COMe)(C0)g-t
Relatively few methylidynes of the general formula HM,(CX)(CO) i0 have been synthesized. In 1975 Shriver ez al. prepared HFe~(COMe)(CO},~ by 0-methylation of HFe,(CO) f ;. ’ ‘,36The Ru and OS analogues were subsequently prepared by several groups using similar routes [eq. (I)]. 15*23*24 Substitution on HM,(CO)i; by CNR, followed by alkylation, gives the analogous HM3(CNR2) (CO),,,.23*33 Attack by nucleophiles, LiBHEt3 or LiPh on HOs3(COMe)(CO)r0, followed by treatment with Lewis acids, has been used to generate H%tCX)(CO) t 0, X = Hz0 or Ph;” HRu,(CX) (CO) 1o has been similarly prepared. 38 Treatment of H,Os,(CO) lo with 3,3-dimethyl~y~lopropene produces HOs3(CCH,CHMe2)(CO), o.” OH-
-
(3)
(M = Fe, Ru or OS)
SYNTHESES OF HM,(CX)(CO),, AND H3M3(CX)(CO)9
M3(CO)12-
HM 3(COMe)(CO) , o+ (1)X-(2)Me+
BX3 -
(4)
H,M,(CX)(CC)9+AlC13+C6H6 (M=RuorOs;
I-I3M&Ph)(C% X=BrorOMe)
(5)
H3M3(CX)(C0),+A1C1,+C0 -----+ H,M,(CCO)(CO)g (M=RuorOs; H,M,(CCO)(CO);
(6a)
X=BrorOMe)
f ROH -
H~Ms(CCGR)(CO)~
(6b)
H3Ru3(CBr)(C0)9+NEt3+HSR -
(1)
H 3M ,(CX)(C% X=ClorBr)
(M=RuorOs;
H3RuGSR)(CO)g
(7)
H3Ru3(CBr)(C0)g + HSnBu, -
H3Ru&H)(CO)9
(8)
J. B. KEISTER
850
HOs,(CHX)(CO), HOs~(CH)(CO),o+X(X = H- or pyridine)
0.
(9) Although the mechanisms of these reactions remain unknown, the reaction chemistry involving modification of the C-X unit may be considered to fall into three classes: (a) reactions which may be thought to proceed through the H3M3(C) (CO);+ ion, by the abstraction of halide or alkoxide from the neutral cluster [e.g. reactions (4), (5) and (611, (b) radical reactions proceeding through H,M,(C)(CO), [e.g. reactions (7) and (811,and (c) reactions which proceed by nucleophilic attack on the alkylidyne carbon [reaction (9)]. The Co,(CX)(CO), clusters display reactivities analogous to that of H,M,(CX)(C0)9. The apparent radical chemistry of the cluster in the presence of reducing agents has been proposed to follow the SRNl mechanism. Electron transfer to H,Ru,(CBr)(CO), followed by halide ion expulsion could form the H3R~3(C)(C0)9 radical, which may abstract hydrogen from donor substrates to form H,Ru3(CH)(CO),, or may add to substrates such as thiols. 3l The chemistry of the H3M ,(C)(CO)i+ ion is more thoroughly understood, although the cation may never exist in a free form. In the presence of large quantities of the reactive substrate the cation may be trapped without rearrangement. For example, in benzene the aryl product is obtained. However, in an unreactive medium, the cation picks up CO from solution to form the ketenylidene H3M3(CCO) (CO);+. The chemistry of this species is quite interesting. Attack by nucleophiles such as HX = ROH, R,NH or H,O generates the corresponding acyls H3M3(CCOX)(C0)9. ‘“,38,42Deprotonation forms HzM~(CCO)(CO)~ ; an X-ray crystallographic study of the OS cluster found an upright ~~-q’ bonding mode for the CC0 ligand.43 The neutral ketenylidene is also formed by pyrolysis of HOs,(CH)(CO) I o. Shapley has demonstrated nucleophilic addition and substitution on the alkylidyne carbon of HM3(CX)(CO),o (M = Ru or OS ; X = OMe or H). ‘9,20,38 In several instances the addition products can be isolated, but the reactions can be quite complex. At this time there are no definitive examples of nucleophilic attack on the alkylidyne carbon of clusters of the class H3M3(CX)(C0)9. This suggests that the chemical reactivity of the alkylidyne ligand may depend strongly upon its coordination mode.
since it is the central step in the alkyne metathesis reaction.44 Alkyne-alkylidyne coupling has also been noted on polymetallic systems,37 although less is known about the mechanism of the reaction. Stoichiometric hydrogenations of alkynes by H3R~3(CX)(C0)9 (X = OMe, Me, Ph or CH2 CH2CMe3) yield the corresponding cis-alkene ; in each case a second equivalent of alkyne combines with the cluster residue to produce clusters of the general formula HRu,(C(X)C(R)CR)(CO)~ (Fig. 3), in which alkyne-alkylidyne coupling has generated a ,u3-~3-1,3-dimetalloallyl ligand. For X = OMe these same clusters may be obtained in better yields by direct reaction of HM,(COMe) (CO),,, M = Ru or OS, with the alkyne.29 Clusters of this class have been previously synthesized through a variety of pathways. The clusters HRu~(~~-~~-RCCR’CR”)(CO)~ have been obtained from reactions of alkenes, dienes or alkynes with Ru~(CO)~~.~~Interestingly, a hydroxy derivative HOS~(~~-~~-HOCCHCH)(CO)~ is formed from Os3(CO), ,(NCMe)* and acetylene in wet THF ; in this reaction a CO ligand is the source of the “hydroxymethylidyne” fragment.46 The stability of these clusters and the numerous pathways for their formation suggests that the p3-q3-C3R3 fragment may also be a common intermediate during reactions of hydrocarbons adsorbed on metal surfaces. The reactions of H3M3(CX)(C0)9 or HM3 (COMe)(CO),, with alkynes provide an excellent opportunity to study alkyne-alkylidyne coupling in trimetallic cluster systems. Two regioisomers are obtained with unsymmetrical alkynes. The two regioisomers from the terminal alkynes are easily identified by virtue of their ‘H NMR spectra, in which the coupling constant between the hydride and the hydrogen on the central carbon is ca 3 Hz, but no coupling is observed between the hydride and the hydrogen on the terminal carbon. The ratio of the isomers [HRu,(C(X)C(R’)CR’)(CO),]/ [HRu3(C(X)C(R2)CR1)(CO),] depends upon the
T
i
X,c/-p+,R’ -M
INSERTION OF ALKYNES INTO THE M-CX BOND Alkyne-alkylidyne coupling is a reaction of some interest for monometallic alkylidyne complexes
‘I
ii7q -H-
I\
M-
Fig. 3. Structure of HM3(p3-q3-XCCRCR’)(C0)9
(M = Ru or OS ; X = OMe, SEt, Ph or alkyl ; R and R’ = H or alkyl).
851
Reactivity of methylidyne ligands
of the alkyne substituents: (R’, R*), ratio = (H, Ph), 3 : 1; (H, Bu), 2: 1; (H, CMe,), 20: 1; (H, CO,Me), 2: 1; (H, OEt), 1: 0; (Me, C7H15), 3 : 1; (C02Me, Ph), 2 : 1. The isomer distribution is kinetically determined, since the isomers do not interconvert even at temperatures far in excess of that required for coupling. Apparently both steric and electronic properties influence the orientation of coupling. Comparison of the isomer ratios from I-hexyne and from 3,3_dimethylbutyne indicates that bulkier substituents prefer the terminal carbon. However, since the sterically similar substituents OEt and Bu give rise to very different isomer ratios, electronic effects are also important. The steric and electronic requirements for alkyne-alkylidyne coupling are still poorly understood. Bis(trimethylsilyl)acetylene, trimethylsilylacetylene and dimethyl acetylenedicarboxylate give little or no coupled products with HRu,(COMe) (CO), ,,. Furthermore, coupled products could not be isolated from reactions of 2-butyne with HFe,(COMe)(CO) , o or with HRu,(CNMe,)(CO) 1,,. Obviously, the coupling is sensitive to the nature of the alkyne, the metal and the alkylidyne substituent. The carbon-carbon bond formation in these Ru, clusters is reminiscent of carbon chain growth on metal surfaces in the Fischer-Tropsch reaction,47 in which CO is reduced with hydrogen to give alkanes and water. The major products from CO hydrogenation are linear hydrocarbons, but minor products are branched hydrocarbons, primarily 2methyl branched. Since alkylidyne species have been identified on metal surfaces and may come together to form alkynes, we considered the possibility that the cluster reaction might act as a model for carbon chain growth on metal surfaces. If an adsorbed CH fragment were to couple with an adsorbed “alkyne” HC2R, then the products would be linear and 2-methyl branched hydrocarbon chains. Pertinent to this analogy, is the report of the formation of HOS&-~~-HOCCHCH)(CO)~ from water, CO, acetylene and OS~(CO),~ (NCMe)2.46 To investigate this analogy we treated HRu, (C(OMe)C(R)CR)(C0)9 with hydrogen (l-4 atm, 96110°C). In each case the major cluster products isolated were H4Ru4(C0),* and H3Ru3 (CCHRCH,R)(CQ), (15-90%). A minor product from the hydrogenation of HRu,(C(OMe) CHCII)(CO), was identified as H2Ru3(MeC20Me) (CO), (Fig. 4) ;48 this product shows that hydrogen can migrate from, as well as to, the hydrocarbon framework. The isolation of alkylidyne and alkyne products, indeed suggested the possibility of carbon chain growth by the coupling of C, and C2 fragments to give dimetalloallyl species. identities
‘I’ Fig. 4. Structure of (p-H)IRus@s-~3-MeCCOMe)(CO),.
The mechanism of the hydrogenation reaction was investigated. Hydrogenation of DRu3(C (OMe)C(H)CCMe3)(C0)9 produced exclusively H3Ru3(CCH,CHDCMe3)(CO),, indicating transfer of the hydride to the 3-carbon, prior to the addition of molecular hydrogen. Comparison of the rates of reaction of HRu,(C(OMe)C(H)CR)(C0)9, R = Bu and CMe3, showed that bulkier substituents on the 3-carbon facilitate the hydrogenation, and consistent with this, hydrogenation of HRu3 (MeOCC(H)COEt)(C0)9 produces primarily H3Ru3(CCH2CH20Et)(C0)+ These observations suggest that the hydride ligand is transferred to the 3-carbon early in the reaction. Low yields precluded further kinetic studies. No alkylidyne products are obtained by hydrogenation of HRu,(C(X)C(R)CR)(CO), when X = H, alkyl or NMe2. However, hydrogenation of HRu3(C(SEt)C(Me)CMe)(CO)9 does form H3Ru3 (CCHMeCH,Me)(C0)9. Thus, a relatively weak C-X bond is required to allow rearrangement under conditions mild enough to allow isolation of the product. More information concerning intermediates in the hydrogenation was obtained by careful analysis of the products from the coupling of alkynes RC2R, R = Me or Ph, with H3Ru3(CSEt)(C0)9. Although the major product of the reaction is the expected HRu,(C(SEt)C(R)CR)(CO),, a minor product was found to be Ru,(CCRCHR)(SEt)(CO),; the structure of this product (Fig. 5), determined by X-ray crystallography for R = Ph, is closely related to the possible intermediate in the hydrogenation process. 4g The hydride ligand has been transferred
Ph
Fig. 5. Structure of Ru3@3-q3-CCPhCHPh)(pSEt)(CO)9.
J. B. KEISTER
852
to the 3-carbon and the CSEt bond has been cleaved, as is required in the hydrogenation. Although the alkylidyne carbon is triply bridging, there are only two Ru-Ru bonds. Presumably, the strong Ru-S bonds are responsible for retention of the SEt moiety in the cluster, whereas pyrolysis of HRu,(MeOCCRCR)(C0)9 does not form an analogous, stable product. Pyrolysis of HRu3 (EtSCCRCR)(CO), at 70°C forms Ru,(CCRCHR) (SEt)(CO), in nearly quantitative yield ; preliminary kinetic measurements have found a first order rate law with a rate constant of 7.6 x 10e6 s- ’ at 70°C. Thus, Ru,(CCRCHR)(SEt)(CO), seems to be a possible intermediate in the hydrogenation of HRu3 (EtSCCRCR)(C0)9 to H,Ru,(CCHMeCH?Me) (CO),. From the results obtained so far we can propose the mechanism shown in Fig. 6 for alkyne-alkylidyne coupling and subsequent hydrogenation. This work has shown that : (1) alkyne-alkylidyne coupling occurs on trimetallic clusters as well as for monometallic species, but pJ-q3-bridging makes the cluster-bound metallocycles much more stable, and (2) hydrogenation of 1,3-dimetalloallyls can form alkylidynes. These reactions provide a pathway for hydrocarbon chain growth on polymetallic units, which may be related to hydrocarbon chain growth on metal surfaces. Although the pu-q3-C3R3 fragment has not yet been identified on a metal surface,
the stability of these clusters and the large variety of paths which have been shown to lead to clusters of this class suggest that the fragment is likely to be an important product of hydrocarbon adsorption in heterogeneous catalysis. Further studies of the mechanism of the isomerization and hydrogenation reactions are in progress. REDUCTIVE ELIMINATION OF H-CX BONDS Reductive eliminations of C-H bonds from metals are the product-releasing steps in many catalytic processes. The mechanism is well understood for monometallic complexes,50 but not for metal clusters or surfaces. Reactions of H3M3(CX)(C0)9 with CO (l-35 atm, 6&l 1O’C) produce CH3X and Ru~(CO),~/RU(CO)~ when X = Ph, C02Me, Cl or H ; ” when X = OMe, dimethyl ether can be identified after decomposition under a CO/H2 mixture. Elimination of CH3X from H3M3(CX)(CO), may model desorption of alkanes from metal surfaces. With this in mind, we undertook an investigation of the kinetics and mechanism of this reaction. Calvert and Shapley have previously demonstrated the reverse process, “adsorption” of a methyl group by sequential C-H oxidative additions to give methylene and methylidyne fragments (Fig. 7). 52 Importantly, a facile equilibrium
X
>R”/c\--/““< ’ ‘\
+ 2 C2R2 >
.H
- C2H2R2
/”
H,
/y\
I
R H&H .H$R 1 /Ru
F\
+ 3 H2
/
-H --4”-\ I I RuAH H -
< -HX
‘I’
R
H
Fig. 6. Reaction pathway for alkyne-alkylidyne coupling and hydrogeneration of a 1,3-dimetalloallyl ligand. (X = OMe or SEt.)
853
Reactivity of methylidyne ligands
.2L /
OS~(CO),~L~
+ CH4 (COj3
Fig. 7. Interconversion
of alkyl, alkylidene and alkylidyne ligands on a OS, cluster.
between hydridomethylene and a methyl ligand with an agostic hydrogen was demonstrated. Furthermore, HOs3(CH3)(CO), o reacted under mild conditions with Lewis bases to form methane. These OS clusters provide valuable models for intermediates in the reductive elimination, but the difficulty and expense of obtaining substantial quantities make kinetic studies of this system impractical. Based upon our results and those of others we can propose the mechanism shown in Fig. 8 for
the reductive elimination from H3M3(CX)(C0)9, M = Ru or OS. The basis for this proposition is given below. That C-H elimination from H,Ru,(CX)(CO), occurs in a sequential manner is suggested by the isolation of stabilized alkylidene and alkyl intermediates. Pyrolysis of H3Ru3(CC02Me)(C0)9 in the absence of CO forms HZRu3(CHC0,Me)(C0)9 in 56% yield (Fig. 9). 32 This product is formed by reductive elimination of a single C-H bond followed by coordination of the acyl to the unsatu-
X
X, c -ii\ toa3y
/-
M3(C0),2
(CO13
M(CO)3
-
H.M\
/
Hi
(co)3
+ CH-,X
t
+co
X
\
,H
F- (OC13M \
y(co)3
\ HtM/H (CO)3
CHZX
I
t°CgM\
j-.(co)4 H-M
+co/ 7 X
(C 01,
lC/H
(OC)3M/-- \
-co
H .M\
y(co), .H
Fig. 8. Mechanism for reductive elimination of CHpX from HSM3(p3-CX)(C0)9 (X = H, alkyl, Cl, Ph or C02Me ; M = Ru or OS).
854
J. B. KEISTER OMe
(10). The near-zero entropy of activation for the limiting rate constant, a, is consistent with an intramolecular rate-determining step. Kinetic isotope effects were unexpectedly less than or equal to one, being 0.64 for X = Ph and 1.00 for X = CO,Me.
rate = ac[Col
b+c[CO]
Fig. 9. Structure of (p-H),Ru,(p,-CC0,Me)(C0)9.
rated metal site thus created. Pyrolysis of H3Ru3 (CSEt)(CO), yields HRu3(CH2SEt)(C0)9 (Fig. 10); in this case the sulphur atom stabilizes the two vacant sites formed by reductive elimination.3’ Under CO, complete elimination of CH3X ultimately yields RUDE 2, which is in equilibrium with RUG under the reaction conditions. A number of lines of evidence indicate that the rate of C-H reductive elimination increases for successive steps. First, no intermediate alkylidene is observed during pyrolysis of H3Ru3(CSEt)(C0)9, suggesting that formation of H,Ru3(CHSEt) (CO), is slower than its conversion to HRu, (CH,SEt)(CO),. Second, direct comparison of the reactivity of the HzOs3(CHJ(CO)10/HOs3(CHJ (CO),, equilibrium mixture found that the rate of conversion of H20s3(CH2)(CO)lo to HOs3(CH3)(CO)io is slower than the reaction of CO with the latter, forming methane and 0s3(CO), 2. Third, structural evidence suggests that the Ru-C bond strength decreases upon each successive C-H elimination ; the Ru-C bond lengths decrease in the order: H3R~3(CR)(C0)9 < H2Ru3 (CHC02Me)(CO), < HRu3(CH2SEt)(C0)9. 31 A study of the mechanism of the reductive elimination process was undertaken. A crossover experiment revealed that the acetates formed from a mixture of H3Ru3(C02Et)(C0)9 and D3Ru3 (C02Me)(CO), were labelled in a manner consistent with intramolecular formation of at least two of the three C-H bonds. The rate-determining step involves formation of only the first C-H bond. The rate law for toluene elimination from H3Ru3 (CPh)(CO), was determined to be as shown in eq.
[H,Ru,(CPh)(CO),].
(10)
Although several pre-equilibria may be postulated to account for the kinetics, the most attractive, shown in Fig. 11, involves reversible migration of a hydrogen atom from a position bridging a Ru-Ru vector, to a position bridging a Ru-C vector. The agostic C-H-M bond53 has recently been established in a number of monometallic complexes, as well as in polymetallic ones. In particular there are two excellent models for this intermediate. First, Fehlner and coworkers have shown that deprotonation of H3Fe3(CH)(C0)9 forms HFe, (HCH)(CO)i-, shown by ‘H and i3C NMR spectroscopy to contain a Fe--H-C bridge. Further work by this group has shown that H,Fe,(CH) (CO), exists in solution in equilibrium with H2Fe3 (HCH)(C0)9 and HFe3(H2CH)(C0)9 (Fig. 12).34 Second, although the equilibrium constant for the tautomerization is not so favourable for the agostic Ru analogues, we have found that dissolution of H3M3(CR)(C0)9, where M = Ru and R = Et or CHPhCH2Ph, or M = OS and R = Me, in fluorosulphonic or trifluoromethanesulphonic acid forms (Fig. 13), by protonation H,M,(HCR)(CO);+ of the M-C bond. These cations have been characterized by NMR spectroscopy.30 For H3Ru3 (HCEt)(CO)i+, the chemical shift of the agostic hydrogen is -9.45 ppm while a singlet resonance at - 18.37 ppm is found for the three hydride ligands. The propylidene c1 carbon resonance occurs at 143.3 ppm (d) with a coupling constant to the agostic hydrogen of 58 Hz. These values of the spectral parameters are very similar to corresponding values for H,Fe,(HCH)(CO)i-. Protonation of the Ru-C bond is expected since the HOMO is Ru-C bonding in character.25 The proposed structure should give rise to two signals for the hydride ligands. To account for the obser-
X I
‘I’
’ \’ Fig. 10. Structure of (pH)Ru,(p,-CH,SEt)(CO),.
/I’
Fig. 11. Equilibrium between ground state (p-H)JM3(p3CX)(CO), and the higher energy tautomer (p-H)2M3(p3HCX)(CO), with an agostic hydrogen.
Reactivity of methylidyne ligands
PPN
855
Cl
>
Fig. 12. Interconversions between various tautomers of H4Fe3(C)(C0)9 and deprotonation H)SFe&-CH)(C0)9 to give @-H)Fe&-HCH)(CO);.
of only a single hydride resonance, we must postulate that the hydrides are averaged by some dynamic process ; however, there is no evidence for line broadening in the NMR spectrum even at -80°C. Fluxional migration of the agostic hydrogen to each of the three M-C edges would explain the observed spectrum. The exchange process must be intramolecular, because “C-hydride coupling is retained. No exchange occurs between the agostic hydrogen and the hydride ligands on the NMR timescale. Isotopic labelling has shown that exchange between the agostic hydrogen and the metal hydrides does not take place over a period of several hours at 25°C. The protonated clusters undergo much faster reductive elimination than the neutral precursors. Shapley has found that dissolution of H30s3 (CH)(CO), in sulphuric acid gives methane and HzOs3(03SO)(CO),.54 Dissolution of H,Ru,(CR)
vation
Fig.
13. Proposed structure for @H),M&-HCR) (CO): (M = Ru or OS; R = Et or CHPhCH,Ph).
(CO),, R = H, Ph, Et, CHPhCH,Ph, in trifluoromethanesulphonic acid results in the formation of the corresponding alkane RH and a variety of Ru species including HRu,(CO) i :, HRu(C0) :’ and what may be H2Ru3(03SCF3)(CO)i+. The alkane product was isolated in the cases of R = CHPhCH2Ph and Ph. Although the alkyl derivatives are moderately stable (tllz N 24 h for CHPhCH2Ph), dissolution of benzylidyne or methylidyne clusters causes immediate decomposition. The pre-equilibrium between H3R~3(CX)(C0)9 and its agostic tautomer accounts nicely for the inverse isotope effect for the reductive elimination of CH3X, since the vibrational modes of an agostic hydrogen should be considerably higher in energy than those of a bridging hydride (e.g. stretching frequencies of ca 2600 cm- ’ for C-H-M vs 1350 cm- ’ for M-H-M). If only the stretching modes are considered, the equilibrium isotope effect upon the equilibrium constant for the equilibrium shown in Fig. 11 can be calculated to be 0.54 (cf. 0.64 for reductive elimination from H,Ru,(CPh)(CO),). The rate-determining step following the pre-equilibrium tautomerization to the agostic alkylidene intermediate, is cleavage of the C-H-Ru bond to form an unsaturated Ru centre. Addition of CO to the unsaturated cluster produces a saturated alkylidene of the formula H2M 3(p-CHX)(CO)1 o. Stable analogues of this alkylidene intermediate are HzRu3(CHCOzMe)(C0)9andH,0s3(CH,)(CO),,.55 Methylidyne substituents which can act as Lewis bases, such as CO*Me, C(O)NR? or SEt, greatly
856
J. B. KEISTER
accelerate the reductive elimination reaction. In these cases, the fact that the relative rates increase in the same order as the basicity of the substituent (C(O)OMe < C(0)NR2 < SEt), and the fact that stable products can be isolated in which the substituents are coordinated to a metal centre, suggest that the acceleration is due to direct attack of the substituent upon a metal atom as the agostic bond is cleaved. The next step in the reductive elimination of the alkane is reversible conversion of H2M3 (J&HX)(CO), o to HM,@-HCHX)(CO), ,,, a tautomer containing an agostic bond. Indeed, Shapley has shown that in solution, H20s3(CH2)(C0)i0 is in equilibrium with HOs,(HCH,)(CO),,, a slightly less stable tautomer (& = 0.28).52 These workers have established the same tautomeric equilibrium for
H,Os,(CHMe)(CO), o/HWHCHMe)(W
1o
K,
0.14) ; in this system /?-elimination from the ethyl group is thermodynamically favoured, but aelimination is cu 100 times faster at WCs6 The next step in the proposed mechanism is cleavage of the agostic bond of HM,(HCHX) (CO),,, forming an unsaturated metal centre, followed by coordination of CO to give the hydridoalkyl HM,(CH,X)(CO) ,,,. Stable analogues of these intermediates are HOs3(CH2C02R) (CO),, (Fig. 14),57HOs3(RCHOMe)(CO)Io,5* HOs, HOs,(NCO)(succinoyl) (MeCHSPh)(CO) , o,5g (CO),o6o and HRu,(CH,SEt)(CO),. Finally, reductive elimination of the third C-H bond and coordination of CO would produce and CH3X. If fact, both HOs, M,(W12 (HCH,)(CO) I o and HOs3(CH2C02Et)(CO) 1o react with Lewis bases under very mild conditions (room temperature and below) in this fashion. In the latter case an intermediate of the probable formula HOs3(CH2C02Et)(CO)io(PMe,Ph) was observed by NMR spectroscopy during the reaction with PMe*Ph. 6’ An alternative path for “desorption” of the hydrocarbon from the cluster alkyl is /&elimination, producing an alkene and a metal hydride. In fact, decomposition of H3Ru3(CCHPhCHzPh) under CO produces exclusively alkenes (CO)9 =
Fig. 14. Proposed
structure
of HOs,(CH,CO,Et)(CO),e.
-CH2---_CPhCH2Ph and cis- and truns-MeC (Ph)=CHPh,62 while fumarate esters are the only products of the reaction between stabilized alkyls HOS,(C(CO~R)CH~CO~R)(CO)~~ and Lewis bases. 6’ Thus, j&elimination, when possible, is the prefered path for decomposition of hydridoalkyl clusters. At this point, isolatable analogues are available for all the probable intermediates in the “desorption” of an alkane, adsorbed as alkylidyne, from a small molecular cluster. Determinations of the energetics and mechanism are only beginning. These studies should continue to provide analogies applicable to metal surface chemistry. CONCLUSION
Since the structures and compositions of cluster compounds vary so widely, it is not surprising that generalizations concerning chemical reactivity are difficult to come by. However, cluster chemistry is unique from the chemistry of monometallic complexes in that : (1) Bonding of hydrocarbyl ligands to several metal centres is the rule in cluster chemistry. (2) One may expect facile hydrogen migration from metal to carbon as well as from metal to metal, and the various tautomers may differ only slightly in energy. (3) The wide variety of bonding modes available for hydrocarbyls on clusters makes the number of potential reaction pathways much greater than for monometallic complexes. (4) Metal clusters have already provided useful models for the bonding of hydrocarbyl ligands to metal surfaces and will continue to provide useful models for reaction mechanisms.
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