The Kinetics of the Insertion Reaction of Tin(1I) Chloride with Hexacarbonylbis(tripheny1phosphite)dicobaIt

PETER FOWLER BARRETT Department of C l l e tn i~ t~ l , Trent Uni\,ersity, Peterborough, O~zturio K9J 7BS

Received November 26. 1973'

PETER FOWLER BARRETT. Can. J. Chem. 52,3773 (1974). The kinetics of the thermal insertion reaction of SnCl, with the metal-metal bonded complex

[P(OC,H,),CO(CO)~]~ have been studied by foilowing the change in the visible spectrum in T H F over the temperature range 35.0 to 55.0 'C. The activation enthalpy and entropy for the reaction are 24.7 i 0.4 kcal/mol and 2.4 i 1.3 cal m o l l deg-' respectively. The data are con- sistent with a two-stage mechanism identical to that proposed for the corresponding reaction with [ P ( M - C , H ~ ) ~ C O ( C O ) ~ ] ~ and from a comparison of the two reactions it is concluded that the cobalt-cobalt bond is slightly weakened when tributylphosphine is replaced by the better rc-electron acceptor triphenylphosphite. The insertion products [LCo(CO),],SnCl, are shown to undergo further reaction with [LCo(CO),], to form [LCO(CO),]~S~CI (L = CO, P(n- C4H9)3, P(OCsH5)z).

PETER FOWLER BARRETT. Can. J. Chem. 52,3773 (1974). Les cinetiques de la reaction d'insertion thermique de SnCI, avec le complexe a lien metal-

metal [P(OC,H5),Co(CO),], ont ete etudiees en suivant la variation du spectre visible dans le THF dans I'intervalle de temperature de 35.0 a 55.0 'C. L'enthalpie d'activation et l'entropie de la reaction sont respectivement de 24.7 0.4 kcaljniol et de 2.4 i: 1.3 cal niol-' deg-'. Les resultats sont conformes a un mecanisme en deux etapes, ident iq~~e a celui propose pour la reaction correspondante avec [P(n-C,H,),Co(CO),],, et de la comparaison des deux reactions on deduit que la liaison cobalt-cobalt est legerement affaiblie lorsque la tributylphosphinc est remplacee par la triphenylphosphite q11i est un meilleur accepteur d'electrons. Les produits d'insertion [LCo(C0)3],SnCIZ peuvent subir des changen~ents en reagissant a nouveau avec [LCo(CO),I, pour former [LCo(CO),],SnCl (L = CO, P(n-C,H9),, P(OC,H,),).

[Traduit par le journal]

Introduction The kinetics of the insertion of tin(I1) halides

into the metal-metal bond of a nuinber of transition metal carbonyl complexes have been reported recently (1-4). On the basis of the pro- posed mechanisms for these reactions, it has been possible in a number of cases to interpret the data in a way which provides a measure of the metal-metal bond strengths.

In an attempt to determine the effect of sub- stituents on metal-metal bond strengths, a kinetic investigation of the thermal insertion of SnCl, into the metal-metal bond of [P(OC,H,),- Co(CO),], is reported here and the results com- pared with those of the similar reaction with [P(iz-C,H,),Co(CO),], (4). Under the kinetic conditions, only the insertion reaction was ob- served to occur, although at much lower con- centrations of SnC1, a different product was obtained and is concluded to be the tris sub- stituted complex [P(OC,H,),Co(CQ),],SnCl.

'Revision received August 15, 1974.

The preparation by similar routes of the anal- ogous complexes [LCo(CO),],SnCl (L = CO, P(n-C,H,),) as well as the tetrakis complex [Co(CO),],Sn are reported, and the ease of formation of these products is used to cor- roborate the kinetic evidence regarding the rela- tive cobalt-cobalt bond strengths in [LCo- (C0),12 (L = P(n-C4H9)3, P(OC,H5)3).

Experimental and Results Materials

Hexacarbonylbis(tripheny1phosphite)dicobat was pre- pared by the method of Sacco (5) and dichlorobis(tetra- carbonylcobalt) tin(1V) by the method of Graham and co-worker (6). Tin(I1) chloride and bromide Mere dehy- drated using acetic anhydride (7), the entire operation being carried out in an atmosphere of nitrogen in a glove bag. Tetrahydrofuran was purified by refluxing over sodium hire, and was freshly distilled and deoxygenated by bubbling nitrogen through it for about 20 min before being used in the kinetic runs,

Kinetics The thermal insertion of tin(I1) chloride into the

metal-metal bond of [P(OC6H,),Co(C0)3]2 was studied by following the disappearance of the starting material at

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3774 C A N . J . CHEM

570 nm using a Unicain SPlSOO spectrophotometer. Infrared spectra of the reaction products were recorded on a Perkin-Elmer 621 spectrophotometer and showed that at concentrations of SnCl, greater than 0.1 PI, only the insertion product was formed, hut that at lower con- centrations increasing quantities of another product were being formed. This byproduct proved to be the tris com- plex [P(OC6H,),Co(COj3]3SnC1, and in order to keep its formation to a minimum in the kinetic runs, it was neces- sary to avold using colutions of low SnCl, concentration. Reaction solutions approximately 5 x M in [P(OC6H5),Co(CO),J2 were prepared as reported earlier (1 j.

Pseudo first order kinetics a-ere observed in all cases with the tin(1I) chloride in excess. Plots of log (A, - A , ) against time, where A, and A, refer to the absorbances at 570 nm at times t and x, respectively, were linear for between 50 to 75% of the reaction. A, was measured experimentally for the faster reactions and estimated in other cases.

The pseudo first order rate constants, k,,,, are piotted in Fig. 1 as a function of tin(I1) chloride concentration. The lines drawn through the experimental points are theoretical ones calc~~lated on the basis of rate param- eters derived by a least-squares treatment (8).

Reactions of T11X2 and 'LCo(C0) 3.72SnX2 wirh [LCojCO) 3,72

Dichlorobis(tvicarbonyltriphenylphosphifecoba1t) - rin(IV), .~PIOC6H,)3Co(CO)312SnC12

To a refluxing solution of SnCI, (1.3 g, 6.9 rnmol) in 20 ml THF under N, was added gradually [P(OC6H5),- Co(CO),], (0.40 g, 0.43 mniolj. After 45 min the THF was removed under reduced pressure, the residue ex- tracted with CH,CI,, filtered, and the residue obtained on evaporation recrystallized from ethanol yielding the desired product (9).

Chlorotris (tricarbonyltriphenylplzosphitecobalt) tin (IV), LrP(OC6H5)3Co(CO)3~'3SnCI

Method A : A solution of [P(OC6H5)3Co(CO)3IZ (0.40 g, 0.43 minol) and SnC1, (0.05 g, 0.26 mmol) in 25 ml THF was refluxed under N2 for 45 min. The T H F was then removed under reduced pressure and the residue extracted with ethanol, filtered, and recrystallized from ethanol yielding 0.15 g of orange platelet crystals, m.p. 119-122 "C.

Anal. Calcd. for [P(OC6H5)3Co(CO)3]3SnC1: C, 49.98; H, 3.00; C1, 2.34. Found: C, 49.26: H, 3.55; Cl, 2.62.

Method B: A solution of [P(OC6H,)~Co(CO)3]2 (0.064 g, 0.068 mmol) and [P(OC6H5),Co(CO3],SnCl, (0.064 g, 0.058 mmol) in 20 ml THF was refluxed under N2 for 30 min. The THF was removed under reduced pressure and 0.03 g product isolated as in Method A.

Bron~otris(~ricarbonylrripheny~hosphitecobat) tin (IV), /P(OC6H5) 3C~(CO)3- -3SnB~

This compound was prepared in an identical manner to that employed in Method A for the chloro analog using 0.40 g (0.43 mmol) [P(OC6H,),Co(CO)3]2 and 0.06 g (0.22 mmol) SnBr, in 25 ml THF yielding 0.14 g of orange needle crystals, m.p. 128-1 31 "C.

Anal. Calcd. for [P(OC6H5)3Co(CO)3],SnBr : C, 48.56; H, 2.91; Br, 5.13. Found: C, 47.98; H, 3.21; Br, 4.63.

. VOL. 5 2 , 1974

FIG. 1. Plot of the observed pseudo first order rate constant cs. tin(1Ij chloride concentration for the thermal reaction [P(OC6H,)3Co(CO)3]2 t SnCI, + [P(OC,H5f3- Co(CO),],SnCI, in THF. The curves drawn through the experimental points ar: theoretical lines calculated from the rate constants in Table 2.

Dichlorobis(rvicarbon~~ltri-n-b~ityiphosphinecobalt) - tin(ZV), [P(n-C4H9)3Co(CO)3.-2SnC12

A solution of [P(n-C4H,)3Co(CO)3]2 (0.40 g, 0.5X mmol) and SnCl, (0.40 g, 2.1 mmol) in 20 ml THF was refluxed under N, for 1 h. The THF was removed under reduced pressure and the residue extracted with CH,CI,. Thc remainder of the isolation procedure was identical to that reported by Bonati et a/. (9) yielding 0.25 g of product.

Chlorotris(tricarbon.yltri-n-butylphosphit~ecobalt) - tin(IV), [P(n-C,H9) 3Co(CO)3.73SnC1

A solution of [ P ( I I - C , H , ) ~ C ~ ( C O ) ~ ] ~ S ~ C ~ ~ (0.20 g, 0.23 ~ n n ~ o l ) and [P(n-C,H,),Co(CO),], (0.16 g, 0.23 mmol) in 20 ml THF was refluxed under N, for 5 h. The THF was removed under reduced pressure and the residue extracted with boiling methanol, filtered, and recrystallized from methanol-water yielding 0.11 g orange crystals, n1.p. 151-155 "C.

Anal. Calcd. for [P(n-C,H,),Co(CO,],SnCI: C, 45.42; H, 6.86; C1, 2.98. Found: C, 46.62; H, 7.34; CI, 2.28.

Chlorotri~~.(tetracarbo~iylcobalt) rin(IV), ~Co(CO),],SnCl and Tetrakis(tetracarbonj~1- >obalt)tin(lV), [CojCO),j4Sn

A solution of [Co(CO),], (0.20 g, 0.59 mmol) and [CO(CO)~],S~CI, (0.20 g, 0.38 mmol) in 20 ml THF was stirred under N, for 10 min at room temperature. The THF was removed under reduced pressure and the residue extracted with n-pentane, filtered, and crystallized

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BARRETT: REACTION OF SnC12 WITH [ P ( O C ~ H ~ ) ~ C O ( C O ) I ] ~

TABLE i . Infrared spectra

Compound v(CO) in CH,CI, (cm-')

Cornpound

*From ref. 16.

from n-pentane yielding a red product which the infrared indicated was a mixture of unreacted [Co(CO),], and [ C O ( C O ) ~ ] ~ S ~ C I (10). An infrared spectrum of the red material not soluble in the 11-pentane extractions indi- cated it to be the tetrakis product [Co(CO),],Sn (11).

Table 1 gives the infrared spectra in the carbonyl region for those cornplexes first reported here and also the tin-halogen frequencies for these and a number of other related con~plexes.

Discussion Figure 1 shows the variation of the pseudo

first order rate constants Ic,,, with the concen- tration of tin(I1) chloride for the thermal insertion reaction of [P(OC,H,),Co(CO),], with excess SnCl, in THF a t the temperatures indicated. Although a t a given temperature the observed rate constants d o not vary consider- ably, there does appear to be a tendency for them to increase with increasing SnCI, concen- tration. This behavior could be explained by postulating that both a unimole~ular and - bimolecular mechanism were operating simul- taneously. Under these circumstances a straight line could be drawn through the experimental points, and from the intercept and slope the rate constants for these two mechanisms determined. Inspection of the experimental points indicates that because of the low values of the slopes that would be obtained, the bulk of the reaction would have to proceed via the unimolecular mechanism.

Alternatively, the following two-step mech- anism that has been postulated in a number of similar reactions (1, 3, 4) could account for the observed behavior :

kl k , Az 2 A2* -i ABA

k-1 B

2058w, 20311~1, 1989vs, -i960m, sh 2057w, 2030m, I986vs, 1960m, sli 2 0 2 9 ~ : 2001m, 1954vs, 1927111, sh

v(Sn-X) in Csl disc (cni-')

where A, represents the metal-metal bonded complex [P(OC,H,),Co(CO),],, A,* is a reac- tive intermediate, B is the tin(I1) chloride, and ABA the insertion product. On the basis of this mechanism, the observed rate constants should level off a t a constant value of k , at high con- centrations of SnCl,. All the experimental values can be interpreted as having nearly approached the limiting value of k , even a t concentrations of SnCi, as low as 0.1 M. Unfortunately, due to the production of the byproduct [P(OC,H,),Co- (CO),],SnCl a t lower concentrations of SnCl,, it was not possible to determine whether the rate constants dropped dramatically under these conditions. In a limited kinetic study using SnBr2 there was little change in the observed rate constants with changing SnBr, concentra- tion and the magnitude of these rate constants was very near the limiting value of the rate con- stants determined at high SnCI, concentrations at the same temperature. In other reactions in which a bimolecular mechanism has been pro- posed for the attack of stannous halides on metal-metal bonds (2, 4), SnBr, has reacted much more readily than SnC1,. Since no such difference has been found here, it is reasonable to conclude that no bimolecular mechanism is operating in this reaction, and the two-step mechanism is therefore proposed to account for our results.

As discussed previously ( I ) , the rate law for this mechanism can be expressed

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CAN. J . CHEM. VOL. 5 2 . 1974

TABLE 2. Kinetic parameters for the SnClz thermal reaction*

Temperature lo4kl 102k- , / k 2 ("'4 ( s - l ) (mol I - ' )

35.0 0 . 5 3 i 0 . 0 2 4 . 4 k 1 . 0 40.0 1 .30k0.03 9 . 0 1 0 . 8 45.0 2 .42 i0 .07 6 . 0 1 0 . 3 50.0 4 .1910.09 0 . 8 i 0 . 3 55.0 7 . 9 5 i 0 . 2 5 0 . 6 k 0 . 7

*The uncertainties are standard deviations; AH1* = 24.7 ? 0.4 kcal m o i l ; AS,* = 2.4 1.3 cal mol-l deg-1.

FIG. 2. Plot showing linear relationship between values of l/k,,, and l;[SnCl,] taken from Fig. I . The straight lines are least-squares plots and lead to the rate constants given in Table 2.

and by rearrangement of eq. 1 the following expression can be obtained

A plot of l/k,,, against 1/[B] should yield a straight line with slope k-,/k,k, and intercept Ilk,. Figure 2 shows plots of this type for the reaction of [P(OC,H,),CO(CO),]~ with SnCI,. Table 2 gives the kinetic parameters obtained from a least squares treatment assuming equal uncertainties for all values of k,,,. The activation enthalpy and entropy were obtained from a weighted least-squares treatment of the kinetic parameters (8).

The corresponding activation enthalpy and entropy for the analogous reaction of SnC1, with [P(n-C,H,),Co(CO),], have been reported to be 26.4 1 0.7 kcal/mol and 5 1 2 cal mol-I deg-I (4). The AH* for the reaction of the triphenyl- phosphite complex is marginally smaller (1.7 kcal/mol) than for the tributylphosphine com- plex while the AS* is essentially the same within experimental error for the two complexes. Since the two reactions are so similar in all respects, it would therefore seem reasonable to assume that they also react by the same mechanism.

A possible reactive intermediate, A,", pro-

posed in the reaction of the tributylphosphine complex was a radical pair trapped in a solvent cage, in which the Co-Co bond had been homolytically broken and the AH* could be regarded as a measure of the energy required to break this bond. Alternatively, a singly carbonyl bridge species

was postulated in which the metal-metal bond had also been broken, and in this case the AN* could be regarded as the lower limit of the energy required to break the Co-Co bond because the degree of bond making involved in forming the carbonyl bridge could offset the energy to break the Co-Co bond. Carbonyl bridged interme- diates have been proposed for a number of -reactions (12, 13) and might well be anticipated in the reactions of the cobalt complexes because all complexes of the type [LCo(CO),12 (L = CO, or Group Va ligand) have been postulated to exist as bridged as well as non-bridged isomers in solution (14, 15).

Regardless of what the actual mechanism is, as long as it can be assumed that the tributyl- phosphine and triphenylphosphite complexes react via the same mechanism, it has been shown that replacement of tributylphosphine by tri- phenylphosphite in the cobalt carbonyl complex results in a slight reduction (1.7 kcal,/mcl) in the A H * . Since the greatest contribution to this activation energy results from the breaking of the Co-Co bond according to any of the pro- posed mechanisms, we can conclude that this bond is weaker in the triphenylphospite complex than in the tributylphosphine complex.

It has been shown that the complexes [LCo(CO),], react not only with SnCI, to form [LCo(CO),],SnCI,, but also with [LCo(CO),],-

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BARRETT: REACTION OF SI

SnCl, to form [LCo(CO),],SnCl, (L = CO, P(OC,H,),, or P(r2-C,H,),). Patmore and Graham 11 1) have reported the similar reaction of SnX, ( X = F, acetate) with [Co(CO),], to form [Co(CO\,],SnX. The ease of these reactions depends very much on L, and the rates decrease with changing l, in the order CO > P(OC,- H,), > P(rz-G,H,), which parallels the order of decreasing x-accepting ability of L and hence increasing electron density on the cobalt atoms. Adams et a!. (16) have suggested that this should result in a weakening of the Sn-C1 bond and this is borne out in the dramatic decrease in the Sn--C1 stretching frequencies along both the [LCo(CO),],SnC1, and [LCo(CO),],SnCl series.

In the following proposed reaction to produce the tris-substituted complexes,

[ L C O ( C O ) ~ ] ~ + [LCo(CO)3]2SnC!2 -t [ L C O ( C O ) ~ ] ~ S ~ C I + LCo(CO),CI(?)

4 decomposes

both Co-Co bonds and Sn-Cl bonds must be broken. Since the ease of the reaction decreases with L in the order CO > P(OC,H,), > P(n-C,H,),, a.nd since this is the order of de- creasing Sn-C1 bond strengths, it would seem logical that it must be the order of increasing Co-Co bond strengths. Although such an argument does not carry much weight alone in the absence of any information regarding the mechanism of the reaction, it does help to sub- stantiate the conclusions drawn from the kinetic study of the insertion reaction.

As has been discussed in a previous paper ( I ) , the substitut~on of tributylphosphine by the stronger x-electron acceptor ligand triphenyl- phosphite might be expected to affect the metal- metal bond in two ways, one of which could strengthen the bond, the other weaken it. De- creased electron density on the metals could strengthen the bond by reducing the repulsion between electrons in filled nonbonding orbitals

on the metals, while at the same time shrinking of the o-bonding orbitals would occur which would tend to weaken the bond. While the former effect has been proposed to predominate in reactions of some compounds containing Fe-Fe and Mo-Mo bonds (I), the latter is evidently more important in reactions of the Co-Co complexes. Hence while it may be possible to predict the effect of substituents on metal-metal bond strengths for a given metal system, it would clearly be a dangerous assump- tion to make genera!izations to other systems.

The author is indebted to the National Research Council of Canada for financial support, and to Mr. L. L. Annett for performing the kinetic experiments.

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