Laser Flash Photolysis of M(CO)6 (M = Cr, Mo, or W) in

(M = Cr, Mo, or W) in perfluoromethylcyclohexane have been identified as "naked" M(CO),. These species are extremely reactive, complexing with CO, M(C...
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3344

J. Phys. Chem. 1983, 87, 334-3349

Laser Flash Photolysis of M(CO)6 (M = Cr, Mo, or W) in Perfluoromethylcyclohexane. The Generation of Highly Reactive Coordinatively Unsaturated Species John M. Kelly,' Conor Long, Chemistry Department, Trinity Coiiege, Dublin 2, Ireland

and Roland Bonneau Laboratoire de Chime Physique, Universite de Bordeaux I, Talence, France (Received: October 25, 1982; In Nnai Form: December 29, 1982)

Transient species observed immediately after laser pulse ( T = 5 ns, A,, = 265 or 353 nm) excitation of M(CO)6 (M = Cr, Mo, or W) in perfluoromethylcyclohexane have been identified as "naked" M(CO),. These species are extremely reactive, complexing with CO, M(CO)6,cyclohexane, or N2 with rate constants exceeding lo9 dm3mol-' s-l. Visible absorption spectra of M2(CO)11and M(CO), (cyclohexane),and preliminary kinetic data for the reactions of Cr2(CO)11and Cr(CO)5(cyclohexane) with CO, are reported.

Introduction In many reactions of metal carbonyls and organometallic compounds, including those where these compounds act as catalysts, coordinatively unsaturated complexes are involved as reactive intermediates. Quantitative measurements of the reactivity of these species, however, are generally lacking. In cases where the coordinatively unsaturated complexes may be generated photochemically flash photolysis methods allow the recording of their electronic absorption spectra and the determination of kinetic data for their reactions. Perhaps the most studied system of this type is that of the pentacarbonyls of chromium, molybdenum, or tungsten.1-8 A study of the microsecond flash photolysis of Cr(CO), in cyclohexane revealed that the pentacarbonyl complex in this solvent is quite reactive, and indeed this high reactivity was responsible for earlier problems of irrepr~ducibility.'-~ The group-6 pentacarbonyls have also been extensively studied by matrix isolation methods*" and in low-temperature solvent g l a s s e ~ , ~ and ~ - ' ~these techniques have provided many useful data on the structure of these compounds. Of particular interest is the observation of a pronounced shift in the visible absorption band of M(CO), as the electron-donatingproperties of the matrix are varied (1)J. Nasielski, P.Kirsch, and L. Wilputte-Steinert, J. Organomet. Chem., 29, 269 (1971). (2)J. M. Kellv and A. Morris. Reu. Latinoam. Quim.. 2. 163 (1972). (3)J. M. Kell;, H. Hermann, k d E. Koerner v& G u s k f , J. Chem. SOC., Chem. Commun., 105 (1973). (4)J. M. Kellv. D. V. Bent. H. Hermann. D. Schulte-Frohlinde, and E. Koerner von Gustorf, J. Organomet. Chem., 69,259 (1974). (5)Y.M. Efremov, A. N. Samoilova, and L. V. Gurvich, Chem. Phys. Lett., 44, 108 (1976). (6)R. Bonneau and J. M. Kelly, J.Am. Chem. SOC.,102,1220(1980). (7)A. J. Lees and A. W. Adamson, Inorg. Chem., 20, 4381 (1981). (8)W.H. Breckenridge and N. Sinai, J. Phys. Chem., 85,3557(1981). (9)R. N. Perutz and J. J. Turner, J. Am. Chem. SOC.,97,4791(1975). (10)J. K. Burdett, M. A. Graham, R. N. Perutz, M. Poliakoff, A. J. Rest, J. J. Turner, and R. F. Turner, J. Am. Chem. SOC.,97,4805(1975). (11)J. J. Turner, J. K. Burdett, R. N. Perutz, and M. Poliakoff, Pure Appl. Chem., 49,271 (1977). (12)I. W.Stolz, G. R. Dobson, and R. K. Sheline, J. Am. Chem. SOC., 85, 1013 (1963). (13)M. J. Boylan, P. S. Braterman, and A. Fullarton, J.Organomet. Chem., 31,C29 (1971). (14)D. R. Tyler and D. P. Petrylak, J. Organomet. Chem., 212,389 (1981).

OO22-36S4/83/2087-3344$01 SOIO

(e.g., for Cr(CO), A- at 624 nm in Ne, 547 cm in CF4,533 nm in Ar,and 489 nm in CH4).9 This was interpreted as a weak bonding interaction of the matrix material with the available coordination site of the square-pyramidal (C4J Cr(C0)5. The absorption maximum of Cr(CO), in cyclohexane solution at room temperature is at about 503 nm.3 This might suggest that there is an interaction of the hydrocarbon with Cr(CO), similar to that found between Cr(CO),and methane at 12 K. However, the strength of this interaction is unknown and at the start of this investigation it was not clear whether this weak binding of the solvent would appreciably affect the reactivity of the complex. (An MO cal~ulation'~ has indicated an interaction energy of about 23 kJ mol-' for Mo(CO), and krypton). It may be deduced from the matrix studiesgthat the interaction with perfluoromethane is rather weak (comparable to that with argon), and it might be expected that the species generated by flash photolysis of Cr(CO), should also be more reactive in perfluorocarbon solution than in cyclohexane. We therefore carried out a flash photolysis study of Cr(CO)G in perfluoromethylcyclohexane using 5-ns, 353-nm laser pulse excitation of Cr(CO)6, and found that the pentacarbonyl species produced in this solvent was indeed more reactive than the pentacarbonyl species previously observed in cyclohexane.6 In this paper we present full details of these experiments and of similar ones with Mo(CO), and W(CO),. Also described are the results of preliminary experiments on the reaction of Cr2(CO)11or Cr(CO),(C,H,,) with CO. It was hoped that these experiments might provide some evidence for the strength of interaction of Cr(C0)5with Cr(CO)6or cyclohexane.

Experimental Section Materials. The group-6 hexacarbonyls (BDH or Strem Chemicals) were used without further purification. Perfluoromethylcyclohexane (Bristol Organics Ltd.) was spectroscopically pure, showing no absorption at wavelengths longer than 200 nm. For some experiments it was further purified by successive reflux with acidified KMn04 for 12 h, steam distillation, drying over Pz05,and passage down a column of dry silica. Cyclohexane was of spec~~~~

~

(15)A. Rossi, E.Kochanski, and A. Veillard, Chem. Phys. Lett., 66, 13 (1979).

0 1983 American Chemlcal Society

Laser Flash Photolysis of M(CO)6

troscopic grade (BDHLtd.). Argon, helium, carbon monoxide (all from L'Air liquide, 99.995%), and nitrogen (L'Air liquide 99.9%) were used without further purification. Apparatus. The laser flash photolysis apparatus, which has been described elsewhere,16was used in the crossedbeam arrangement with the sample in a 1X 1cm standard quartz fluorescence cuvette. This crossed-beam arrangement means that the concentration range of the experiments is limited by the optical density of the sample at the excitation wavelength. The most suitable optical density lies between 0.6 and 2.0; in this range the solution absorbs enough of the laser radiation to produce an adequate transient signal without the occcurrence of undesirable concentration gradients or shock waves. This limitation restricts the concentration range for the various hexacarbonyls which may be studied by using the third M for harmonic (353 nm) to about (0.3-1.0) X and Mo(CO)~and about (0.8-3.0) X M for W(CO)6. Although the higher extinction coefficients of the hexacarbonyl at the wavelength of the fourth harmonic (265 nm) should result in a greater yield of transient species for 265-nm as compared with 353-nm excitation at low concentrations of M(COI6,this advantage was counterbalanced by the much lower intensity of the 265-nm pulse. For this reason most studies were carried out with 353-nm pulses. Sample Preparation. Most samples were degassed by passing a stream of argon through the solution in a 1 X 1 cm quartz cuvette for 10 min. This technique was also used for other gas purges such as He, N2,and CO and has proved satisfactory for other nanosecond flash experiments. Some later samples were degassed by a freezepump-thaw procedure. This was conducted in a quartz cell fitted with a degassing bulb and greaseless taps. The vacuum used for degassing was better than torr, and the procedure was repeated 5 times for each sample studied. The results presented in this work would suggest that the simple gas purging technique is not entirely satisfactory for these systems. It is not clear whether such purging introduces impurities from the gas into the solution, or whether it is inefficient in the removal of volatile impurities. In all cases the concentration of the hexacarbonyl was verified after the degassing procedure by UV spectroscopy. Initially difficulty was experienced in obtaining reproducible transient absorptions (corresponding to MO(CO)~ or W(CO),) in argon-flushed perfluoromethylcyclohexane solutions of Mo(CO)~or W(CO)@This could be attributed to a fine brown precipitate, or colloidal suspension, produced upon each exposure to the laser radiation. The first flash of a newly prepared solution of the hexacarbonyl always resulted in an analyzable transient absorption. A similar fine precipitate was also observed in the chromium system after many (>40)exposures to the laser radiation. A typical trace is given in Figure la. A significant improvement in the shape and reproducibility of the transient absorptions could be obtained by rubbing the outside walls of the cuvette with a paper towel before each excitation pulse. Presumably this had the effect of removing any suspended material from the solution by electrostatic attraction to the walls of the cuvette. Figure l b shows a typical transient absorption after this treatment. The colloidal material appears to be formed by a slow thermal reaction of the transient species and not by a multiphoton process. This conclusion is supported by the fact that addition of carbon monoxide or of cyclohexane suppresses (16)R.Bonneau, J. Am. Chem. SOC.,102,3816 (1980).

The Journal of Physical Chemistry, Vol. 87, No. 17, 1983

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,\"

8

,\"

0

0,

L A 0

40

20

60

80

100

ns

Flgure 1. Comparison of the transient absorption changes caused by 353-nm laser excltatbn of argon-flushed C,F,, solution of W(CO)8 (1.6 X lo3 M) which had been previously subJected to laser excitation: (a) no treatment, (b) after rubbing cuvette with paper towel.

200

2 50

300

3 50

nm

Figure 2. Absorption spectrum of Cr(CO), (4.1 X solution.

M) in C7F,,

the formation of the precipitate. A full explanation as to why the presence of the colloidal suspension should influence the shape of the transients is not known. However, it is possible that the particles absorb the laser radiation, efficiently converting the light energy into heat, and hence causing a change in the refractive index of the solvent in the regions exposed to the laser radiation. Results The absorption spectra of the M(CO)6 compounds in perfluoro solvents are very similar to those of the compounds in alkane solvents (Figure 2). For these solutions it has been proposed1' that the bands at ca. 235 and ca. (17)(a) N. A. Beach and H. B. Gray, J. Am. Chem. SOC.,90, 5713 (1968);(b) W.C.Trogler, S. R. Desjardins, and E. I. Solomon, Inorg. Chem., 18, 2131 (1979).

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The Journal of Physical Chemistty, Vol. 87, No. 17, 1983

Kelly et ai.

TABLE I : Visible Absorption Band Maxima of M(CO), Complexes in Perfluoromethylcyclohexane and in Low-Temperature Matrices

Cr Mo W a

+ l o nm.

620 460 47 5

510 415 415

485 420 420

4 11 413

450 461

Reference 9.

400

500

5

10

5

10

600

nm

Flgure 3. Absorption spectrum of transient absorption recorded immediately after excitation of C,F,, solutions of M(CO)o (M = Cr, Mo, or W).

-

280 nm correspond to M 7r* CO charge-transfer transitions whereas the absorption between 300 and 360 nm is predominantly a d-d (LF) band. The enhanced absorption of W(CO)6in the region 340 360 nm has been attributed to the lowest (lAIg 3T1g)singlet-triplet transition. Most of the experiments reported here were carried out by using 353-nm radiation although for more dilute solutions the 265-nm pulse was used. From the above assignments excitation at 353 nm causes initial population of LF states whereas 265-nm radiation leads in the first instance to a M CO CT state. Formation of M(CO)5and Its Reaction with M(C0)6, CO, and N2.Excitation (353 nm) of perfluoromethylcyclohexane solutions of M(CO)6 M), previously flushed with high-purity argon (>99.995%), leads to the formation of species having the characteristic absorption spectra shown in Figure 3. These transients, which are also produced by 265-nm laser pulses, are already completely formed in the shortest time measurable (5 ns). If the wavelengths of maximum absorption of these species are compared with those reported for the photoproducts of M(CO16 in the gas phases or low-temperature mat rice^,^ it seems reasonable to assign these absorptions to the "naked" M(CO)5species (Table I). The chemical reaction being observed is that of eq 1. No evidence was

-

cone x

io3

M

Figure 4. plots of the first-ordw rate constants for the decay of M(CO), in C7F,, against the concentratbn of M(CO),. a: M = Cr; (0)results results obtained with pwffied C,F,,. obtained with CF,, as received; (0) M = W. b: (0)M = Mo, (0)

+

+

-

M(CO)6 M(C0)5 + co (1) found for a precursor to these species, so that it may be stated that the lifetime of any excited states (of M(CO)6 or possibly of M(CO),) must be shorter than 5 ns. The lifetime of M(CO)5is sensitive to the concentration of M(CO),, of CO, or of other compounds such as nitrogen

400

500

6 00

nm Flgure 5. Absorption spectrum of transient absorption recorded 200 ns after excitation of argon-flushed C7F,, soiutbns of M(CO), (M = Cr, Mo, W).

or cyclohexane. In Figure 4 the first-order rate constants for the disappearance of M(CO)5are plotted against the concentration of the corresponding M(CO)& Concurrent with the disappearance of M(CO)5a new species is formed,

Laser Flash Photolysis of M(CO),

The Journal of Physical Chemistry, Vol. 81, No. 17, 1983

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TABLE 11: Rate Constants for Reaction of M(CO), with M(CO), or with Cyclohexane in Perfluoromethylcyclohexane Solution kM(CO),3a

Cr Mo W

kC6H12~a

dm3 mo1-ls-l

dm3 mol-'^-^

4 x 109 4 x 109 4 x 109

1.8 x 109 1.8 x 109

L

-

1.4 x 109

I-,

,

,

,

,

,

,

2

4

6

8 10 12 14 16

.

,

.

,

%20%.

its rate constant for formation being similar to that for the disappearance of M(C0)6. The spectra of these species, which we ascribe to M2(CO)11formed by reaction 2, are M(C0)5 + M(C0)6

-

M2(CO)11

(2)

presented in Figure 5. Although we have no definite evidence for this, it is probable that the M(CO)6fragment is bound to M(CO), by a carbonyl oxygen to metal bond (i.e., (CO),MOCM(CO)& A species assigned to Cr2(CO)11 has previously been identified in argon matrices containing high concentrations of cr(co)6.10 Derived rate constants for reaction 2 are given in Table 11. I t will be noted from the plots of Figure 4 that extrapolation of the rate constant for decay of M(CO), to zero concentration of M(CO)6reveals that, as well as reaction 2, a further process is responsible for the decay of M(CO), under the experimental conditions used. Rigorous purification of the solvent prior to outgassing did not affect the rate of disappearance of Cr(CO), as is demonstrated by the data in Figure 4a. To investigate the nature of the nonzero intercept further, a series of experiments was carried out under conditions where reaction 2 was suppressed (i.e., at low concentrations of Cr(CO)6; 2.2 x M in each case). As discussed in the Experimental Section these experiments were carried out by using 265-nm excitation pulses because of the very low absorbance of Cr(CO), at 353 nm at this concentration. Although the much lower intensity of the 265-nm line allowed only semiquantitative results to be obtained, it was found that in argon-flushed solutions the Cr(CO), decayed with a lifetime of ca. 100 ns (as predicted from Figure 4a) with the resultant production of a species having maximum absorption at 480 f 20 nm. The same result was obtained with helium-degassed samples. However, in samples degassed by using the vacuum technique (see Experimental Section) found to be most effective for microsecond flash photolysis studies of cr(co)6: it was found that the lifetime of Cr(CO), was 350 ns and the product formed had maximum absorption at 485 f 10 nm. These results suggest that the nonzero intercept of Figure 4 is caused by some impurity in the solution which is not removed by argon or helium flushing. The lifetime of M(CO), in C7Fl, solution saturated by flushing with carbon monoxide or nitrogen is much shorter than that in argon-flushed solution. The first-order rate constants for M(CO), under these conditions are given in

I

,

.

.

.

.

0 4 08 12 16 2 0 2 4 2 6 32 time (us) Figure 6. (a) Decay of Cr,(CO),, in CO-flushed C7Fi4 solution ([Cr(co),] = 7.4 X M) monitored at 480 nm. (b) Decay of Cr(C0)&HI2) in CO-flushed c7F14 solutlon ([Cr(CO),] = 5.0 X M; [C,H,,] = 2.5 X lo-' M) monitored at 500 nm. I

500

400

603

nm

Figure 7. Absorption spectra of M(CO),(C,H,,) recorded 200 ns after excitation of C7F14 solutions of M(CO), containing C,H,,.

Table 111, which also gives the rate constants calculated for reactions 3 and 4. The absorption spectrum of Cr(CM(CO),

-

+ CO

M(C0)5 + N2

M(CO)6

(3)

M(CO),(NJ

(4)

O),(NJ was found to peak at 370 f 10 nm, in agreement with the literature.l8

TABLE 111: Rate Constants for Reaction of M(CO), with N, or CO rate constant, s-l Ar co Cr Mo W

1.8 X 2.1 x 107" 2.4 x 107"

5.3 x 1 0 7 ~

7.7 x 107" 5.2 x 107"

kcO,C

dm3

mol-' s-'

3 x 109 5 x 109 2 x 109

rate constant, s-l Ar N, 3.1 x 1.8 x

4.7 x 107b 3.0 X

'-;a.,"

dm3 s-l

1.3 x 109 1 x 109

a [Cr(CO),l = 2.6 x lO-'M, [Mo(CO),l = 5.8 x M, [W(CO),] = 1.8 x M. [Cr(CO),] = 3.7 x M, [Mo(CO),] = 4.1 X M. These are calculated by subtracting the rate constant for decay of M(CO), in Ar-flushed solution from that in CO- or N,-flushed solution and dividing by the [CO] or [ N , ] at equilibrium under 1 atm ( 1 . 2 x lo-, M).19 Derived values for k c o and k,, therefore *30%.

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Kelly et al.

The Journal of Physical Chemistry, Vol. 87, No. 17, 1983

Cr(CO)5-Cr(CO)6Complex. Preliminary experiments have also been carried out on the reaction of Cr2(CO)11 with CO to reform Cr(CO)G (reaction 5). Figure 6a shows the cr*(Co)ll + co 2Cr(CO)6 (5)

-

transient absorption change at 480 nm for a 7.4 X lov3M solution of Cr(CO), flushed with CO. It may be seen that the decay consists of two portions: a strong transient of lifetime 450 ns and a weaker long-lived species (T > 1 ps). The long-lived transient absorption is probably caused by the impurity complex and the shorter-lived decay may be attributed to reaction of Cr2(CO)11with CO. Two mechanisms may be considered for this reaction: either a direct bimolecular reaction, in which case the rate of decay of Cr2(CO)11will be k5[Cr2(CO),,][CO], or dissociation of Cr2(CO)11(reaction 6) and combination of Cr(C0)5with Cr2(CO)11 CI-(CO)~ + Cr(CO)6 (6)

-

CO, in which case the rate of decay will be as shown in eq 7. These mechanisms are therefore distinguishable by a -d [Crz(CO) /dt = k6k3[Cr2(CO)ll][CO]/(k2[Cr(C0)6] + ks[Co]) (7) study of the variation of the rate of decay of Cr2(CO)11as a function of Cr(CO)6 concentration. Values for the pseudo-first-order rate constant for decay of Cr2(CO)11are 4.5 X lo6s-l for [Cr(CO),] = 3.5 X M, 2.2 X lo6s-l for M. These M, and 2.2 X 10, s-l for 1.1 X 7.4 X preliminary results might then best be analyzed in terms of a combination of both dissociative and bimolecular reaction mechanisms. To ascertain this definitely, experiments will have to be carried out with samples which have been degassed by vacuum pumping and then saturated with CO, so that the concentration of the impurity complex is kept very low. It should, however, be mentioned that, even when these precautions are taken, the range of Cr(CO),concentrations which may be studied by using the present experimental setup of crossed excitation and monitoring beams is limited (as discussed in the Experimental Section). M(CO)5-Cyclohexane Complex. M(CO)5 also reacts with cyclohexane in perfluoromethylcyclohexane solution. The species formed have absorption maxima (Table I and Figure 7) at wavelengths similar to those reported for the transient species observed on flash photolysis of M(COI6 in pure cyclohexane solution (503 nm for M = Cr; 415 nm for M = W20). The first-order rate constant for decay of M(CO)5depends on the concentration of cyclohexane (e.g., Figure 8). These observations are consistent with the occurrence of reaction 8 and allow the determination of M(C0)5 + C&iz M(C0)5(C6Hi2) (8) the rate constants for the process (Table 111). Preliminary kinetic studies have also been performed on the reaction of Cr(C0)5(cyclohexane) in CO-saturated perfluoromethylcyclohexane solution containing cyclohexane. These were carried out by monitoring absoption changes at 500 nm following 353-nm excitation of Cr(CO)6 (5 X M) in the presence of cyclohexane. A typical decay trace is shown in Figure 6b. A detailed study of the decay kinetics of the Cr(C0)5(C6H,2)is made difficult by the fact that Cr2(CO)ll and the impurity complex, both of which absorb at 500 nm, are also present (the latter +

(18)(a) M.Wyart, ThBse, Universit4 Libre, Bruxelles, 1976;(b) J. K. Burdett, A. J. Downs,G. P. Gaskill, M. A. Graham, J. J. Turner, and R. F. Turner, Znorg. Chem., 17, 523 (1978). (19) J. C. Gjaldbaek and J. H. Hildebrand,J.Am. Chem. Soc., 71,3147 (1949);J. C.Gjaldbaek, Acta Chem. Scand., 6,623 (1952). (20)L. Flamigni, Radiat. Phys. Chem., 13, 133 (1979).

1 2 conc of cyclohexane x

3

10'M

Flgure 8. Plot of first-order rate constants for the decay of t ~ l ( C 0 ) ~ M = W. against [C8H,2]: (0)M = Cr; (0)

presumably accounts for the long-lived absorption noted). The initial portion of the decay follows the first-order kinetics, the rate of decay decreasing with increasing cyclohexane concentration (e.g., 5.8 X lo5 s-l with [C6H12] = 0.14 M and 3.7 X lo5 s-l with 0.25 M). Although this trend is that expected for a mechanism based on the dissociation of the complex (reaction 9) and subsequent Cr(C0)5(C~Hi2) Cr(CO)5 C6Hi2 (9) reaction of Cr(CO), with CO or cyclohexane,the inhibition of the rate of decay of Cr(C0)5(C6H12) is much less than that expected fom this scheme alone. This phenomenon might best be explained in terms of a combination of a dissociative mechanism and an association process (reaction 10). To test this experiments will have to be perCT(C0)5(C6H12)+ co Cr(CO),j + CsHi2 (10) formed using solutions dilute in Cr(CO)6and with samples degassed by freeze-pump-thaw procedure and then saturated with CO.

-

-

Discussion It may be concluded from the above experimental observations that the first species observable (7 > 5 ns) on flash photolysis of M(C0)6 in argon-flushed perfluoromethylcyclohexane is an M(CO)5species. A comparison of the ,A, for these species with those recorded for M(CO), in the gas phase or in rare gas or fluoromethane matrices reveals that they are the square pyramidal (C4J isomers and that their interaction with the perfluoro solvent is very weak. The absorption maximum of the Cr(CO),.species in C7Fl4is within the error of that of Cr(C0)5in neon matrices where the interaction is expected to be very weak indeed. I t is not clear however whether the Cr(C0)6is quite uncoordinated by the solvent as it is not known where the naked Cr(C0)5would absorb and this must await the results of gas-phase flash photolysis on a submicrosecond time scale.s No evidence has been found for a precursor to the ClV isomer and it may be presumed that any excited state or intermediate (e.g., the D% isomer) is very short-lived with lifetimes in the picosecond range or less.*l

J. Phys. Chem. 1903, 87, 3349-3354

The essentially naked character of the M(CO)5species in C7F14 is reflected in their high reactivity toward CO, Nz, cyclohexane, or M(CO)6. In each case the rate constant is within 1 order of magnitude of the rate constant expected for diffusion-controlled processes in C7F14. It is noteworthy that the rate constant of reaction of M(CO)5 with CO is about 3 orders of magnitude faster in C7F14 than those found in cyclohexane. This is expected from the observed rapid coordination of M(CO)5to cyclohexane and the fact that the spectrum indicates that equilibrium 11 M ( C ~ ) & C ~ H ~ M(C0)5 Z) -k C6Hiz (11) lies to the left at room temperature. The strength of the interaction between M(CO)5 and cyclohexane could be determined if values of the equilibrium constant for equilibrium 11could be obtained at various temperatures. Such a determination has not been possible in the present study. However, our preliminary results do show that Cr(CO)&C6HlZ) is very labile (Itdise1 106 8-9 and there also appears to be evidence for an associative reaction of CO with C ~ ( C O ) ~ ( C ~ H ~ Z ) . (21)In a recent paper on the picosecond flash of Cr(CO)8 in cyclohexane, benzene, and methanol it was shown that the Cr(CO)5 (solvent) complex was formed within 25 ps of excitation. (22)J. A. Welch, K. S.Peters, and V. Vaida, J. Phys. Chem., 86,1941 (1982).

3349

The observation that M(CO)5reacts with M(CO), to give M2(CO)11could also be relevant to studies in alkane solvents as it had been a concern in earlier studies that Mz(CO)ll might be formed and possibly be the reason for the observed “impurity complexes”. Our data show that Crz(CO)ll is labile (kdimL lo6 s-l) and therefore it is extremely unlikely that the formation of these species plays an important role in the reactions of Cr(C0)5 in dilute alkane solutions of Cr(CO)6. In this connection it is also worth noting that we have found no definite evidence for the isocarbonyl complex M(C0)50C. (From matrix studies it is expected that Cr(CO)50Cabsorbs at about 460 nm.9) In conclusion the results presented here demonstrate the high reactivity of 16-electron coordinatively unsaturated species and show how their reactivity is modified by the formation of solvent complexes even in hydrocarbon solution. Acknowledgment. We thank the National Board for Science and Technology (Ireland) and the C.N.R.S. (France) for travel and subsistence expenses. C.L. acknowledges postgraduate awards from the Department of Education and Trinity College, Dublin. Registry No. Cr(C0)6, 13007-92-6; MO(CO)~, 13939-06-5; W(CO)6,14040-11-0;Cr(CO)5,26319-33-5;Mo(CO),, 32312-17-7; w(co)s, 30395-19-8; co, 630-08-0; Nz, 7727-37-9; C&12,110-82-7.

Cycloamylose Complexation of Inorganic Anions Robert I. Geib, Lowell M. Schwartz,’ Mlchael Radeos, and Daniel A. Laufer Deperlment of Ctmmlsby, Unverslty of Massachusetts, Boston, Massachusetts 02125 (Received: June 1, 1982; I n Final Form: January 28, 1983)

Formation constants for inclusion complexes of cyclohexaamylose (a-cyclodextrin) and cycloheptaamylose (/3-cyclodextrin)with inorganic anions ClO;, SCN-, I-, Br-, NO3-, and 10,- in aqueous solution are determined at various temperatures by a novel pH potentiometric method. Complexes with chloride ion even at 1 M C1could not be detected. AH” and AS” values for cyclohexaamylose complexation of C104-, SCN-, and I- are found to conform with a previously reported correlation based on corresponding complexation with a variety of substrate species including aliphatic and aromatic carboxylic acids and carboxylate anions and substituted phenols and phenolate anions. This correlation serves as a basis for theorizing a common binding mechanism for the complexes, Le., polar interaction between cyclohexaamylose and substrate. Thus, the same binding mechanism is indicated for small inorganic ion substrates.

Complexes of cycloamyloses (also cyclodextrins, to be denoted by Cy) and their derivatives have been reported with substrates of widely varying structures. These complexes have been detected by several physical and chemical techniques but pH potentiometry has proven to be particularly successful in terms of accuracy and precision. We have reported pH potentiometric determinations of complexation constants of several aqueous organic acids, phenols, anions and small solvent molecules.14 Others5s6 (1)Gelb, R. I.; Schwartz, L. M.; Johnson, R. F.; Laufer, D. A. J. Am. Chem. SOC.1979,101, 1869-74. (2)Gelb, R. I.; Schwartz, L. M.; Laufer, D. A. Bioorg. Chem. 1980,9, 450-61. (3)Gelb, R. I.; Schwartz, L. M.; Cardelino, B.; Furhman, H. S.;Johnson, R. F.; Laufer, D. A. J. Am. Chem. SOC.1981,103,1750-7. (4)Gelb, R. I.; Schwartz, L. M.; Radeos, M.; Edmonds, R. B.; Laufer, D.A. J. Am. Chem. SOC.1982,104,6283-8. 0022-3654/83/2087-3349~0~ .50/0

have also employed pH potentiometry similarly. The method involves measuring the pH of an aqueous acidbase conjugate which will also be called the buffer components and denoted by HB and B-. If cycloamylose is added to such a solution and forms a complex with either or both buffer components, the buffer/H+ equilibrium will be shifted and a pH perturbation will be observed except in the unlikely case that complexes of exactly equal strength are formed with both buffer components. The extent of pH perturbation, indeed, depends on the difference in such complex strength. Experimentally one measures the pH of a series of solutions containing variable amounts of buffer and cycloamylose and then fits the data (5)Miyaji, T.;Kurono, Y.; Uekama, K. Chem. Pharm. Bull. 1976,24, 1155-9. (6)Connors, K. A.; Lipari, J. M. J . Pharm. Sei. 1976,65, 379-83.

0 1983 American Chemical Society