Organometallics 1986, 5 , 99-104
99
Ligand Substitution Kinetics of the Triruthenium Hydride Ion HRu,(CO) 11Douglas J. Taube and Peter C. Ford" Department of Chemistry, Universirv of California, Santa Barbara, California 93 106 Received April 26, 1985
A kinetics investigation of the substitution of CO on HRU,(CO)~> k-lk-,[CO] and kz[PPh3] >> [PPh31 k-l[co], and thus eq 5 simplifies to kobsd = kl. Figure 4. Plots of koM vs. [PPh,] for the reaction of HRu~(CO)~< The rate constant k l can also be obtained under less plus PPh, in THF under varied Pco at 25 "C (curves drawn for than limiting conditions (e.g., 10:90 CO/N2 or 25:75 illustrative purposes only). CO/N2) by the use of the following analysis. The rate expression in eq 6 can be broken into forward and reverse >> [HRu3(CO),,-]. The reaction was monitored by the contributiondl (eq 8). decrease in absorbance at 430 nm. Plots of In ( A - A,) vs. time were linear for more than 3 half-lives, indicating k-1k-z[CO] hkz[PPh31 + that the reaction is first order with respect to cluster (8) kobsd = k_1[CO] + kz[PPh3] k-l[CO] kz[PPh3] concentration. Experiments were carried out varying [PPh3] over the range 0.0022-0.1 M and [CO] over the For simplicity the term on the left will be defined as "k? range 0.00164.016 M (0.10-1.0 atm).1° Plots of kobsd vs. and the right as "k,". The value of kf can be calculated [PPh3] were nonlinear and leveled to limiting values a t from the following relationship.'l O
'
O J
Y
0:02
0.6 4
0. 66
0.b E
0.7
+
(10) "Solubilities of Inorganic and Metal Organic Compounds"; 4th
ed.;Seidell and Linke, Eds.; D. Nostrand Co.: Princeton NJ, 1958. [Note added in proof Carbon monoxide concentrations were calculated by
using the assumption that the solubility of CO in THF is the same as in diethyl ether (0.016 mol/L atm at 25 "C). We have been unable to find a literature value for this parameter, but further consideration has led us to conclude that this is too large a value for CO solubility in THF. The use of solution theory ("Encyclopedia of Chemical Technology", 3rd ed.; Wiley: New York, 1978; Vol21, p 377) gives an estimated CO solubility of 0.0084 mol/L atm. Use of this latter estimate instead would change several of the parameters calculated from the experimental data. The key changes would be for the equilibrium constant K (eq 2) which would assume the values 0.11 f 0.1 (9.0 "C), 0.12 f 0.01 (25.0 "C), and 0.14 f 0.01 (37.3 "C). The other changes would be the rate constant ratio k - l / k z (eq 4 and 5), which would assume the values 29 f 5 for L = PPh3 and 27 i 4 for L = PBq. These new values lead to no substantive changes in the conclusions drawn in this paper.]
If the mechanism shown in Scheme I applies, then a double reciprocal plot of (kf)-l vs. [PPh3]-l should be linear according to eq 10
with an intercept of (k1)-l and a slope of k-l[COl/klkz. Figure 5 represents such plots for three different values (11)Doeff, M. M.; Sweigart, D. A. Inorg. Chem. 1981, 20, 1683.
102 Organometallics, Vol. 5, No. 1, 1986
Taube and Ford
Table I. Rate Data for the Forward Reaction of HRu8(CO),,-Plus PPh3-As Calculated According to the Kinetic Analysis Described for Scheme I solv
temp, OC
atm
k , , s?
k-,lCOl/k1k7"
THF 25.0 100% CO 2.1 f O.lb 0.108 THF 25.0 25% CO 2.3 f 0.2b 0.0321 THF 25.0 10% CO 2.0 f 0.2b 0.0106 THF 9.4 N, 0.36 f 0.01' THF 25.0 N, 2.1 f 0.1c THF 37.6 N, 5.1 f 0.lc AH* = 16.0 f 1.7 kcal/mol; A S * = -1.9 f 3.0 cal/(deg mol)
8.1 N2 0.26 f 0.02c DMTHF 25.0 N2 1.6 f O . l e DMTHF 38.1 N2 5.8 f 0.1' DMTHF AH*= 17.9 f 0.3 kcal/mol; A S = 2.7 f 0.8 cal/(deg mol) Slopes of plots in Figure 5. *Calculated from the inverse of the intercepts of plots as in Figure 5. CLimitingkobadvalues measured a t high [PPh,].
4v 2
C
C.2
0.4
0.6
0.8
1.0
1.2
1.4
CPPh31 / [COI
of Pco with intercepts independent of Pco and slopes linearly dependent on Pco as predicted. From these plots an average value of k1 = 2.1 f 0.1 is derived, consistent with the limiting koM measured under dinitrogen (Table I). Given that [CO] = 0.016 M in 25 "C THF'O under Pco = 1.0 atm, the ratio k-,/k2 = 15 f 4 can be calculated from the slopes of these plots. This represents the discrimination of the intermediate (presumably HRu,(CO),{) toward reaction with CO relative to reaction with PPh,. Kinetics of the forward reaction were run under N2 for the temperatures 9.4 and 37.6 "C as well. The k , values obtained from kobsd(limiting)at these temperatures (0.36 and 5.1 s-l, respectively) were plotted according to the Eyring equation to give the activation parameters for kl in THF: AH* = 16.0 f 1.7 kcal/mol and AS* = -1.9 f 3.0 cal/(deg mol). One possible explanation for the reactively low value of AH*would be that this reflects an associative reaction of a solvent molecule with HRU,(CO)~; to labilize a carbonyl. This possibility was explored by using a similar, but more sterically hindered and less coordinating solvent12 2,5-dimethyltetrahydrofuran (DMTHF). The reaction conditions for studying the forward reaction were otherwise identical to those in THF. The kinetics, followed spectrophotometrically a t 430 nm, gave linear In ( A - A , ) vs. time plots. The kl(DMTHF) values derived from the intercepts of plots of (hob&' vs. [PPh,]-l a t 8.0, 25.0, and 38.1 "C were found to equal 0.26 f 0.02 s-l, 1.6 f 0.1 s-l, and 5.8 f 0.1 s-l, respectively. These values gave activation parameters of AH* = 17.9 f 0.3 kcal/mol and AS* = 2.7 f 0.8 cal/(deg mol). It is notable that the k, values are only marginally different in DMTHF than in T H F and that the activation parameters in DMTHF are within experimental uncertainity to those obtained for kl in THF. Rates of the reverse reaction were obtained for the reaction of [PPN][HRU~(CO)~,,PP~,] with CO in THF. The substituted cluster [PPN][ H R U ~ ( C O ) ~ , , Pwas P ~ prepared ~] in situ by the addition of a 10-40-fold excess of PPh, to solutions of [PPN][HRu,(CO),,] under N2. The solutions were degassed by four freeze-pumpthaw cycles to remove the resultant CO. The reactions were then monitored by observing the decrease in absorbance at 430 nm upon mixing with solvent containing CO. Plots of In ( A - A , ) vs. time obtained over the [CO] range of 0.0008-0.008 M were linear for more than 3 half-lives showing the reaction to be first order in [ H R U ~ ( C O ) ~ ~ P PPlots ~ , - ] . of kobsd vs. [co]at constant [PPh,] were nonlinear, but limiting kobsd (12) Gutmann, V. Coord. Chem. Reu. 1976, 18, 225.
Figure 6. P l o t of l / k , vs. [PPh3]/[CO] according t o eq 11 for t h e reaction of H R U ~ ( C O ) ~ ~ plus P P ~CO ~ - in THF a t 25 OC.
values were not obtained even at the highest Pco employed. The k , term defined above (eq 8) can be written as kobsd
(11)
A plot of (kJ1 vs. [PPh,]/[CO] is linear with a nonzero intercept, ( k 2 ) - l (Figure 6). The value of k-2 obtained at 25 "C is 0.46 f 0.02 s-l. The reverse reaction was run at 10.5, 25.0, and 37.6 "C, and the rate constants k-2 determined for these tempertures were 0.11 f 0.02 s-l, 0.46 f 0.02 s-l, and 2.0 f 0.2 s-l, respectively. The activation parameters for the loss of PPh, from [PPN][HRu3(CO)loPPh3]in T H F are AH* = 18.2 f 1.2 kcal/mol and AS* = 1.0 f 4.0 cal/(deg mol). The dissociative mechanism proposed in Scheme I predicts the limiting kl values to be independent of the nature of the incoming ligand L. This prediction was tested by examining the kinetics for the reaction of HRu,(CO),,- with excess PBu, (0.007-0.10 M) in T H F under a 10% CO (90% N,) atmosphere. First-order rate behavior was observed by following the disappearance of HRU,(CO)~,-a t 376 nm (25 "C). The double reciprocal plot, kOM-l vs. [PBuJl, was found to be linear as predicted by Scheme I with slope and intercept virtually identical with those observed for reaction with PPh, under the same conditions. The k , value, 2.1 f 0.2 s-l (intercept)-', so calculated is identical with the value found for PPh,, as would be expected for the proposed dissociative mechanism. Notably, the k - , / k 2 value for PBu,, calculated as above, is 14 f 2, within experimental uncertainty the same as found for PPh,. Thus, it appears that while the suggested intermediate HRu3(CO)lo-is roughly an order of magnitude more reactive with CO than with either phosphine, it differentiates little between PPh3 and PBu,. Discussion The substitution of a carbonyl on HRu3(CO),< by PPh3 in T H F solution is a particularly facile process occurring at rates conveniently measured with a stopped-flow spectrophotometer and displaying a relatively low activation enthalpy. The reverse reaction has similar characteristics. These properties contrast strongly with the analogous substitution reactions of the neutral clusters Ru3(60)1sand Ir4(CO)12which are orders of magnitude less labile toward substitution.13J4 Another key difference is
Kinetics of the Triruthenium Hydride Ion
Organometallics, Vol. 5, No. 1, 1986 103
and if the analogous intermediate formed by isomerization the reluctance of the hydride triruthenium core to undergo of HRu3(C0),$Ph< were quenched by reacting with PPh3 multiple substitutions in the presence of PPh3. However, or proceeded along the back reaction coordinate by respectral changes indicating the formation of disubstitution acting with CO to give the “ H R U ~ ( C O ) ~ , P Pinterme~~-” products were observed in the presence of excess PBu3, and diate. Although such a mechanism involving at least five trisubstitution products were observed in the presence of different intermediates can be constructed without vioexcess P(OCH3)3.1 lating the principle of microscopic reversiblity, there apIn eq 4 and 5 it was suggested that the reaction kinetics, pears to be no evidence a t present supporting so detailed especially the inhibition of CO substitution by excess CO, a hypothesis. Furthermore, the pathways for which cluster could be interpreted in terms of a mechanism involving intermediates of the type depicted in B or C have been CO dissociation to give the unsaturated intermediate proposed5J3 are concerned with cluster fragmentation or HRu3(CO),,- which would undergo competitive addition displacement of the hydride, and there is no indication of with CO or PPh3. The rate law is fully consistent with this either of these processes occurring in the present case. mechanism as is the observation that the identical limiting kl value was measured for the reaction of H R u ~ ( C O ) ~ ~ - Thus, we believe that a dissociative mechanism as indicated in eq 4 and 5 is the most likely explanation of the with PBu3. However, the kinetic data would be equally observed substitution rate behavior. However, the high consistent with an alternative mechanism proceeding via lability and low activation enthalpy of the reactions of both an unimolecular step giving an intermediate subject to HRU~(CO),~and HRu3(CO),$Ph< need to be addressed competitive trapping by these two ligands. An example in the context of rate studies other triruthenium clusters. of such an alternative would be the associative reaction For example, investigation of the substitution reactions with solvent to displace the leaving group and form a of R u ~ ( C Oin ) ~hydrocarbon ~ solutions have concluded that solvent0 intermediate complex, e.g., eq 12, followed by two mechanisms are operative, a dissociation mechanism H R u ~ ( C O )+ ~ ~S- HRu3(CO)loS- + CO (12) analogous to eq 4 and 5 and a second-order mechanism. The k, value of the former pathway (1.1X s-l at 40 nucleophilic attack by either CO or PPh, to give HRu3“C) is more than 5 orders of magnitude less than that for (CO),,- or H R U ~ ( C O ) ~ , P Prespectively. ~~-, The limiting “unimolecular” rate constant kl would be for eq 12, since HRu3(CO),,- and has activation enthalpy 14 kcal/mol larger.13 Even more significant is the report17 that mesolvent would not appear in the rate law. However, the thylation of the bridging CO of H R u ~ ( C O ) ~to < give values and activation parameters for kl are nearly identical H R U ~ ( ~ - C O C H ~ ) (leads C O ) ~to~reduced lability toward for reactions run in THF and DMTHF (Table I), a strong first-order CO labilization by roughly 5 orders of magniindication that the solvent is not as intimately involved tude (kobsd= 3.2 X s-l in heptane at 24 OC for 0.010 as suggested by eq 12. M PPh3 under N2 atmosphere) with a corresponding inAnother alternative for the kl step would be a unimocrease in AH* (27 kcal/mol).17 Notably even the presence lecular isomerization to give a more reactive species of the same chemical composition, e.g., eq 13. Such an isomer of alkali-metal cations (rather than PPN+) as the counterion to HRU~(CO),~has been shown18 to slow the exH R u ~ ( C O ) ~ ~‘‘HRU~(CO)~~-” (13) change between coordinated and solution CO in THF, a could be one where a metal-metal bond has been broken result which can be attributed to formation of tight ion simultaneous with motion of a ligand to or from a bridging pairs at the bridging CO, the most negative site on the c ~ n f i g u r a t i o n ~ (e.g., ~ * J ~B) or one where migration of the cluster. l9 hydride to a carbonyl has occurred to give a formyl comSeveral rationalization can be offered for the high lability plex5J6 (e.g., C), in each case opening a coordination site of HRu3(CO),< toward CO dissociation. For example, the structure of this anion displays elongated Ru-CO bonds for those carbonyls trans to bridging carbonyl2 suggesting some ground state destabilizing contribution to the CO dissociation. However, similar elongations are seen in other HRu,(p-CX)(CO),, derivatives (X = OCH3 or N(CH3)2),20 yet there is no correlation between the extent of such bond lengthening and the observed substitution rates. (C) The low AH*values for H R u ~ ( C O )are ~ ~ likely to reflect (9) a lowering of the barrier for ligand dissociation owing to to reaction with a two-electron donor. Rate behavior of some special stabilization of the intermediate “HRu3the type described here would be duplicated if the inter(CO),,”. A likely structure for such an intermediate is D. mediate “ H R u ~ ( C O ) ~were ~ - ” quenched by reacting irreHowever, it is not clear why an analogous species formed versibly with CO or proceeded along the forward reaction via CO dissociation from other HRu3(p-CX)(CO),, derivcoordinate by reacting with PPh3, first to give a new inatives would not be equally stabilized, unless the reason termediate, ” H R U ~ ( C O ) ~ ~ P Pand ~ , - ”then , the product, lies in the tendency of anionic complexes to favor species with bridging carbonyls.*l (13)(a) Poe, A.;Twigg, M. V. J. Chem. SOC., Dalton. Trans. 1974,
-
-
1860. (b) Malik, S.K., Poe, A. Znorg. Chem. 1978,17,1484and references therein. (14)(a) Karel, K. J.; Norton, J. R. J. Am. Chem. SOC. 1974,96,6812. (b) Sonnenberger, D. C.; Atwood, J. D. Inorg. Chem. 1981,20,3243.(c) 1982,104,2113. Sonnenberger, D. C.; Atwood, J. D. J. Am. Chem. SOC. (d) Darensbourg, D. J.; Baldwin-Zuschke, B. J. J.Am. Chem. SOC. 1982, 104,3906. (15)(a) A similar mechanism has been suggested for several photochemical reactions of polynuclear metal compounds including photofragmentation reactions of R U ~ ( C O ) (b) ~ ~Desrosiers, ’~ M. F.; Ford, P. C. Organometallics 1982,1,1715. (c) Malito, J.; Markiewicz, S.; Poe, A. Inorg. Chem. 1982,21,4335. (16)Pearson, R. G.; Walker, H. W.; Mauermann, H.; Ford, P. C. Inorg. Chem. 1981,20,2741.
(17)Dalton, D. M.; Barnett, D. J.; Duggan, T. P.; Keister, J. B.; Malik, P. T.; Modi, S.; Shaffer, M. R.; Smeski, S. A.; submitted for publication. (18)Shore, S. G.; Bricker, J. C.; Bhattacharyya, A. A.;Nagel, C. C., reported a t the 1984 International Chemical Congress of Pacific Basin Societies, Symposium on Metal Cluster chemistry, Honolulu, HI, Dec 1984;Paper 07N07. (19)Keister, J. B. J . Organomet. Chem. 1980,190, C36. (20)(a) Johnson, B. F. G.; Lewis, J.; Guy Orpen, A.; Raithby, P. R.; Suss, G.J . Organomet; Chem. 1979, 173, 187. (b) Churchill, M. R.; Beanan, L. R.; Wasserman, H. J.; Bueno, C.; Abdul Rahman, Z.; Keister, J. B. Organometallics 1983,2,1179.(c) Churchill, M. R.; DeBoer, B. G.; Rotella, F. J. Inorg. Chem. 1976,15, 1843. (21) Avanzino, S. C.; Jolly, W. L. J. Am. Chem. SOC. 1976,98, 6505.
104
Organometallics 1986,5, 104-109
(E)
forming adducts with coordinated CO are particularly effective catalysts for the ligand substitution reactions of metal cluster^*^^^^ suggest an alternative possible role of the F-CO group, namely, nucleophilic attack on an axial CO of the unique ruthenium rather than at the metal center itself.25 The resulting intermediate F would then be expected to undergo rapid ligand dissociation given the lability of such nucleophilic adducts.23 In summary, it is clear that the hydride anion HRu3(C0)11is dramatically more reactive toward carbonyl substitution than the neutral carbonyl clusters R u ~ ( C Oor )~~ HRU~&-COCH~)(CO The ) ~ ~substitution kinetics suggest a dissociative mechanism, and the low AH* value may indicate an unusually stabilized transition state and/or intermediate for CO dissociation. However, the present information is not sufficient to characterize the precise nature of such an intermediate.
(r
Another attractive proposal was the suggestion by KeisteP that the ligand substitution in the HRu3(pCX)(CO)loseries may reflect an internal nucleophilic attack of the X group on the unique ruthenium center, displacing a CO to give an intermediate such as E. The higher reactivity of H R u ~ ( C O )could ~ ~ - then be attributed to the more nucleophilic character of the oxygen of the bridging CO relative to, for example, the oxygen of the p-COCH, group of H R U ~ ( ~ L - C O C H ~ ) (In CO support ) ~ ~ of such a proposal, there is ample precedent for the interaction of a bridging ligand with all three metals of a triangular cluster to form stable q2-p3complexes.22 However, the recent demonstrations that nucleophiles capable of
SOC.1984, 106, 3696.
(22) (a) Kaesz, H. D.; Knobler, C. B.; Andrews, M. A.; van Busbird, G. Pure Appl. Chem. 1982, 54, 131. (b) Schink, K. P.; Jones, N. L.; Sekula, P.; Boag, N. M.; Labinger, J. A.; Kaesz, H. D. Inorg. Chem. 1984, 23,2204. (c) Barner-Thorsen, C.; Rosenberg, E.; Saatjian, G.; Aime, S.; Milone, L.; Osella, D. Inorg. Chem. 1981, 20, 1592.
(24) (a) Morris, D. E.; Basolo, F. J. Am. Chem. SOC. 1968,90,2536. (b) Brown, T. L.; Bellus, P. A. Inorg. Chem. 1978,17,3726. (c) Darensbourg, D. L.; Gray, R. L.; Pala, M. Organometallics 1984,3,1928. (d) Lavigne, G.; Kaesz, H. D. J. Am. Chem. SOC. 1984, 106,4647. (25) In this context it is notable that according to the atomic coordinates given in ref 2, the p C 0 oxygen of A is -0.9 A closer to the carbon of the axial CO than to the metal atom of the unique Ru(CO), group.
Acknowledgment. This work was supported by a US. Department of Energy (office of Basic Energy Sciences) contract to P.C.F.. We thank Johnson-Matthey Inc. for a loan of ruthenium and Dr. J. Keister (SUNY-Buffalo) for communicating a copy of ref 17 prior to publication. D.J.T. thanks Dr. M. M. Doeff for many helpful discussions. Registry No. HRU,(CO)~~-, 60496-59-5;PPh3,603-35-0;PBu,, 998-40-3. (23) Anstock, M.; Taube, D.; Gross, D. C.; Ford, P. C. J . Am. Chem.
x
Diamagnetic Anisotropy of Organometallic Moieties: Values for M(CO)3 (M = Cr, Mo, W) and for Ferrocene Michael J. McGlinchey,*t Robert C. Burns,t Roger Hofer,+ Siden Top,$ and GQrard Jaouent Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M 1, Canada, and Ecole Nationale Sup6rieure de Chimie, 75231 Paris Cedex 05, France Received May 30, 1985
Proton NMR chemical shifts are profoundly influenced by their proximity to metal carbonyl moieties. It is shown that the McConnell relationship can be used to evaluate the diamagnetic anisotropy of carbonyl ligands in M(CO), fragments, where M = Cr, Mo, and W. The 500-MHz 'H NMR spectra of the a-and &Cr(CO), complexes of an estradiol derivative reveal that protons proximate to the tripod are deshielded relative to their resonance positions when they are distal to the metal carbonyl moiety. The chemical shift differences together with the geometric terms yield a x value of -490 X lo-%m3/moleculefor a chromium carbonyl ligand. Analogous calculations are presented for molybdenum and tungsten carbonyls and also for the ferrocene molecule. Introduction The effed of the anisotropy in of certain organic functional groups upon the NMR chemical shifts of neighboring nuclei in the molecule is a well-established phenomenon. perhaps the clearest examples are provided by the alkyne and benzene systems. +
McMaster University.
* Ecole Nationale Superieure de Chimie. 0276-7333/86/2305-OlO4$01.50/0
In the first case, one can readily envisage that when the applied magnetic field Bo is along the molecular axis, there is no hindrance to circulation of the electrons in the CGC bond and so the temperature-independent paramagnetic term will be zero. In contrast, when BO is perpendicular to the axis, the nuclei hinder the circulation and the paramagnetic effect is strong.' (1) Harris, R. K. "Nuclear Magnetic Resonance Spectroscopy"; Pitman: London, England, 1983; p 193.
0 1986 American Chemical Society
