Catalysis by low-valent diamagnetic transition-metal complexes of the cis—trans isomerization of azobenzenes

Journal of Molecular Catalysis, l(1975176) 417 - 429 @ Eisevier Sequoia S-A_, Lausanne - Printed in the Netherlands

CATALYSIS COMPLEXES

AKIRA

BY LOW-VALENT DIAMAGNETIC TRANSITION-METAL OF THE CIS-TRANS ISOMERIZATION OF AZOBENZENES

NAKAMURA,

Department Engineerirzg (Received

KENJI

DOI,

KAZUYUKI

TATSUMI

and SE1 OTSUKA

of Chemistry. and Department of Chemical Engineering, Science, Osaka University_ Toyonuka, Osaka (Japan)

January

Summary

415

26,1976;

accepted

March

Faculty

of

15,1976)

_

Diamagnetic hydrido complexes, MH~(Q-C~H~)~ (M = MO and W) and zerovalent dl” complexes, ML2-4 (M = Pt, Pd and Ni) are catalytically active in the cis-tram isomerization of azohenzenes and their p-substituted derivatives_ The importance of a a interaction between the catalyst and azobenzene in inducing the isomerization is shown by (1) kinetic studies, (2) steric and electronic effects, and (3) a semi-empirical INDO calculation. By considering the 1~acidity of an N=N linkage, a r-basic metal species is suggested as playing a vital role in the isomerization-

Introduction The interactions between transition-metal complexes and azo compounds (di-imides) deserve investigation because of their important bearing on nitrogen fixation_ Azo compounds can behave either as q~r or as 7’ ligands depending on the identity of the metal species as evidenced by the existence of ~l-azobenzene(dichloro)palladium(II) ([PdCl,(PhN=NPh)] =) [l] or q2azobenzenenickel(0) complexes (NiL=(PhN=NPh); L = Bu’NC, PPh,) [2 - 73 _ The a-acidic character of azobenzene exhibited in its n2 coordination [2] suggests the possibility of a strong interaction with zerovalent platinum or palladium even though the isolation of q2-azobenzene complexes has not yet been reported_ We thought that this interaction may be revealed in the cis-bans isomerization of azobenzenes catalyzed by low-valent metals. We have examined a number of transition-metal complexes as catalysts and have found some that are very effective. These may be divided into two distinct classes; one comprises low-valent diamagnetic complexes, e.g. ML, (M = Ni, Pd, Pt; L = PArs), and the other paramagnetic coo&natively unsaturated labile complexes, e-g. MXzLz (M = Fe, Co, Ni; L = PPhs, etc.), Although there have been a number of detailed experimental and theoretical studies on the thermal as well as the photochemical cis-trans isomerization of azobenzenes [S] , catalytic isomerization has received little

418

attention. A few papers [8b, 91 report only a slight catalytic activity for some protic (e-g- acetic or suEuric acid) or basic reagents (e.g. pyridine, etc.). Very recently, Ittel and Ibers have briefly mentioned [lo] catalysis by nickel (0) complexes_ We have already published a preliminary report 1111 of this catalysis and we now report fulIy the first example of catalysis by various low-valent diamagnetic metal complexes, which act as u or s donors towards some acceptors_ Two distinct modes of interaction of these donor moIecules with an azo Iinkage are discernible: (a) (Tdonation to the azo n* orbital; and (b) s donation to the a* orbital. Similar modes of interaction have aIready been proposed for systems of n-acidic olefins and metal donor moIecuIes_ It is a purpose of this paper to discuss which interaction is more important in the catalytic isomer&&ion of c&-azobenzene-

Results Group

VI _metaZ diamagnetic compzexes as catalysts Some d6 complexes of predominant 0 basicity,

(CsHs)zMHz (M = MO, W), are weakly active (e.g., about 1 h is required for completion at 20 “C with a catalyst concentration of 2.5 mmoll-l) allowing us to obtain reliable kinetic data around room temperature from the visibIe spectrum (cfi Table 1) The observed rate has a first-order dependence both on the concentration of azobenzene and of the catalyst (M = MO)_ The rate in benzene is ca. 2 - 3 times faster than in THF (cf- Table 1) The absence of a polar transition state is implied. The influence on the rate of the par-a substituent in Ph-N= N-CsH&p-X) is reiatively small [the relative rates at 20 “C with the catalyst CpzMoHs (2.5 mm01 1-l in benzene) being X = CHsO, 1.46; CHs, 1.9; Cl, l-9; H, l-0; cfi Table 21) catalyst Ar\ nr'

N=N\Ar

N=N\Ar

a-not

but some acceleration was fomid for an electron-attracting p-chloro substituent (relative rates: PhN=NPh, 1; p-CICsH4N=NPh, 1.9; p-CICsH4N=NCsH4(P-cI), 2-7) Group

VIII

metaZ diamagnetic

complexes

as catazysts

Some labile Group VIII metal complexes with a formal oxidation state of 0 - 2 are compared. Thus, a trace amount of the zerovalent dl” complexes Ni(PPh&, Pd(PPh& or Pt(PPhs)4 caused an instant isomerization of cisazobenzene to the trans isomer in toluene even at -20 “CL The activity of Pt(PPh,), was measurable even when present in small amount (10 min to completion at catalyst concentration 0.03 mmol l-l; kobs: 1.1 X 10-l min-l). The incremental addition of PPhs lowered the isomerization rate in a linear manner (cfi experimental section)_

419 TABLE

1

Comparison Solvent

of kinetic

parameters

in benzene

CataIyst (conc_Zmmoll-x)

k ob$,x lo* Imin?

GH,

cP2MoHd2.5)

THF w%

cpzM0H~(2.5) cP2wH2(3-5)

3.3 1.6 0.15

%rbstrater

TABLE

czk-azobenzene

(50 mmol

and in THF

9.9 14 -

at 20 “C”

7.8X105 7.2x IO8 -

40 -26 -

l-l)_

2

Substituent effect on the first-order azoarenes, p-X-Ar-N=N-Ar-(p-Y) Y

X

rate constants

kz(-t-)

X 102/min-1

kl (therm-)_l X 104/min

of cis-trans isomerization

kl (cat-1

of

Relative value

kl(therm.) (a) CpaMoHa

as catalyst

CZ

H

6.3a

1.03u

610

1.02

Cl

Cl

s.9a

1.4u

640

1.03

H

H

3.3a

0.55u

600

1.0

CH3 CHaO

H

6.3a

1_45b

430

_ 0.6

H

4.fJa

2.24=

210

0.3

CH3

CH3

4,6a

o-70=

660

1.1

1.5

(b) Pt(PPh3)4/PPh3system

as catalyst

cl

H

6.22d

1.03=

603

H

H

2.27d

o-55=

615

CH3

H

l-16*

1.45e

80.0

“[azobenzene] = 50.0 mmol 1-l; [Cp2MoH2] = 2.5 mmol l-l, in benzene bTaken from literature (Ref. [8b]). =Measured by us at 20 “C d[azobenzene] = 3.00 mmol 1-l; [Pt] = O-0264 mm01 1-l; [PPha (added)] nun01 1-1, in toluene at 20 %_ =Taken from literature (Ref_ [8b])_ The value obtained in benzene.

1 0.2 at 20 “C.

= O-163

Theobservedinitialrateofisomerization with Pt(PPh3), showsafirstorderdependenceontheconcentrationofcis_azobenzene[equation(1)]_ Rate= kobs [cis-azobenzene]

(1)

- Thefirs&orderrateconstant,atermrepresentedask,,,,incorporatesthe concentrationofthe aCtiVeCatdyStsincek,b, wasroughly proportionalto theamountofcatalystadded.

420

kohs

=

kz [active catalyst]

The concentration of the active catalytic species may be determined the following equilibria [equations (3) and (4)] 1121. PtL, PtL,

Kl K,_

l

PtLa+L PtL2 + L

(2) from (3) (4)

with K1 = 1 and Kz = 1 X 10d6 mole’_ It is important to note that the addition of PPha hardly changes the equilibrium concentration of PtL, since Kl is large and the catalyst concentration is low (10B5 mol l-l)_ Hence, the PtLz moiety is most likely the main active species. Owing to the uncertainty in the Kz value, reliable secondorder rate constants (k, values) could not be calculated_ It is not possible to exclude the possibility that PtL, may also be weakly active. Further, the available data do not exclude the existence of an extremely small concentration of PtL which might be very effective for the isomer&&ion (see discussion)_ The apparent energy of activation (E,) was 14.7 kcal mol-l (10 - 30 OC, cptw 0 = 2.65 X low5 mol l-l, [L] = 1.6 X 10m4 mol l-l in toluene) and the entropy of activation was 17 cal K-l mol-l_ Pt(CzH4)(PPhs)z was very active at a concentration of 10m5 mol l-i, but the activity was greatly reduced when ethylene was added in excess to suppress the dissociation which generates the highly active Pt(PPhs), species_ The suppressing effect of ethylene may also be due to hindrance in the exchange reaction of the v2-ethylene Iigand in Pt(C2H4)(PPha), with cis-azobenzene which takes place without detectable dissociation of the q2-ethylene ligand. More stable (undissociable) olefin complexes, e.g. Pt(maleic anhydride) (PPhz)z, were inactive_ Formally divalent diamagnetic square complexes, e-g_ MXz(PPhs), (M = Pd, Pt; X = I), MX2(ButNC), (M = Ni, Pd; X = I), PdCl,(PhCN)2, [+ZaH,PdCl] 2, were also inactive_ In an attempt to isolate the expected catalytic intermediate, reactions of &ens-azobenzene with zerovalent platinum phosphine complexes, e_g Pt(PPhs)4, Pt(C,H,)(PPhs)z, were attempted_ However, no stable crystalline azobenzene complex could be isolated although the corresponding very labile nickel phosphine or nickel isocyanide complexes, Ni(PhN = NPh)L2 (L = PhsP, ButNC), are known c2,31For confirmation of a coordinatively unsaturated catalytically active species, the 2-coordinate phosphine-palladium(O) complex, Pd[PBui] z was examined as a catalyst_ An efficient activity was actually_ found at a catalyst concentration of lOA mol l-1 at 20 “C_ A platinum(O) complex 1131 containing a slightly less bulky phosphine ligand, Pt[PB&Ph] 2, was also active but the activity deteriorated rapidly afta isomerization of ce_ 30 mol of cisazobenzene per mole of the platinum catalyst_ The isomerization with this Pt catalyst is not inhibited when the reaction is carried out in air- An excess of the free phosphine or tmns-azobenzene does not affect the life of the

422

frans and the cis complex are 0.9367 and 0.9293, respectively. The bond indices for the Ni-N bonds are 0.8547 and 0.8475 for the fmns ad cis isomers, respecftively.

Discussion lsomerization mechanism The experimentally

determined kinetic rate equations for the platinum and molybdenum catalysts clearly indicate that the rate-determining step involves a bimolecular reaction_ The molecularity suggests the presence of an intermediate consisting of the catalyst and azobenzene- The rate equation aIso suggests that cis-frans isomerization within the metal-azobenzene intermediate compIex is a fast step_ Three possibIe modes of interaction between a transition-metal compIex and azobenzene may be considered as illustrated below: (1) o coordination with a lone pair on the azobenzene bonding to a vacant coordination site on the metal; (2) c donation from a filled metal orbital to the vacant antibonding v* orbital of the N=N bond of azobenzene; and (3) a-complex formation mainly through v back-bonding. (1)

(2)

&--

6+/N--

N \

(Tea-ordination ($-azobenzene

Udonation

to the metal complex)

of the metal electron to the fl

orbital

of xobenzene

(3)

7Fcomplex

formation

(7’)Tazobenzene

complex)

Examples of interaction (1) have been found in, for example, [Pdn(transPhN=NPh)C12] a [Z J and of interaction (3) in Ni(frans-PhN=NPh)(ButNC)2

PI -

Of these three interactions, the remarkably high catalytic activity of zerovalent dr” complexes, e-g. M(PPhs),, as compared to divalent d8 complexes, e-g PdCIa(PhCN),, indicates the inadequacy of interaction (1) for inducing the isomerization. The observed steric effect of o-methyl substifuents (cf, Fig- 1) on the azobenzene ring atso prechrdes interaction (l), since e coordination is not sterically blocked. Interaction (2) implies a polar transition state which is favored in polar solvents or with asymmetrically substituted azobenzenes, Our resuhs with the strong u-donor complexes -

423

Fig- I_ Steric hindrance

in the (T or T coordination

of o, o’-azotoluene

to a metal.

CpsMHs, which give a slower rate in a polar solvent with less marked substituent effects, indicate that interaction (2) does not cause the isomerization. Furthermore, the relative basicity trend in CpZMHP, i-e_ W > MO, (p& values in aqueous dioxan, 8.6 and 12.5) is contrary to the observed trend in the catalytic activity, i.e. MO > W. Interaction (3), ie_ a ?;-complex mechanism, is thus favored where ‘ITback-donation from the filled metal orbital (x basicity) virtually determines the catalytic activity. Thep-substituent effect on the rate also supports the n-interaction mechanism for Pt” catalysis. The kinetic result only provides information about the rate-determining step and the steps preceding it in some cases- The important intramolecular isomerization step in the metal-azobenzene complex still remains unknown. In view of the high catalytic activity of coordinatively unsaturated platinum (als6 nickel or palladium) species, the preparation of a cis-azobenzene complex, e.g. MLs(cis-PhN=NPh), seems to us to be impossible even at very low temperatures [lo] _ With the MH&p, (M = MO, W) catalyst systems, no evidence other than isomer&&ion catalysis was obtained from IR and visible spectra for complex formation with tins-azobenzene in solution. Since experimental methods for investigating the fast intramolecular isomerization step are presently unavailable, the problem was attacked by theoretical calculation on a closely related hypothetical q2-di-imide-nickel(O) complex, Ni(n2-NH=NH)(CNH),. A modified INDO calculation of the relative stabilities (total energies), covalent bond strength of Ni-N bonds ( Wni-_N)* and x-bond orders in the cis- and frans-di-imide-nickel complexes revealed that the frans-di-imide complex is more stable (cf. bond indices WNi_n) and that the N-N bond order is considerably reduced horn that of the free diimide. The a-orbital population (qr) decreased to l-8 with a concomitant increase of the x*-orbital population (qs*) to 0.93. The corresponding values for the free di-imide are 2.0 and 0.0, respectively_ Hence the N-N rbond order in the complex is 0.5. Although approximations using pseudo-ligands, e_g NH=NH and CNH, are unavoidable for simplification of the calculation, the result implies that the intramolecular isomerization of the cis-azobenzene complex to the fmnsazobenzene complex is a sterically and energetically favored process. A similar trend in stability (trans > cis) has been observed by Tolman 1153 in the study of equilibrium constants for the reaction of Ni[P(Os-tol)s]s with cis- and &ens-hex-Zene. A nickel(O)-dimethyl maleate complex,

424

Ni(CNBu)z(CHaOzCCH=CH-CO&H,). readily isomerizes to the dimethyl fumarate complex on heatiig in toluene solution above 80 OC 1163 _ In contrast, the cis isomer generally interacts more strongly with some electrophilic reagents (cationic center), e.g. Ag* or carbonium ions [17]. The stabilization of the frans-di-imide-nickeI(0) complex is ascribable to enhanced back-donation relative to the cis-di-imide complex, because the Z* level of the trans-di-imide molecule is Iower than that of the cis isomer. The values of the n-orbital populations (qs,( II) and qirg (II)) indicate that the backdonation in the trans complex is greater than that of the cis complex (qsrz (II); a measure of x back-donation; trans complex 0.9367, cis complex O-9293)_ Considering the similarity of the metaIT2-azo systems (M-N-N) to the organic diaziridine rings (C-N-N j where stereoisomers are interconverted by pyramidal inversion attheatoms, the cis- and frans-azobenzene complexes may easily Le interconverted into each other favoring the trans compltx probably via pyramidal inversion at one of the coordinated nitrogen atoms. The accurate, minimum energy pathways are, however, still obscure and extensive calculation of the systems as a whole is requireciThe combined results now suggest metal v basicity to be the most important factor influencing the catalysis_ When the 5~acidity of azobenzene as measured by the magnitude of u(EN) of Ni(Bu’NC),(PhN=NPh) is considered, n2 coordination to a x-basic metal atom is probable for the catalytic intermediate where the N=N double bond may be weakened to a level approaching that of an N-N single bond- Then cis-fmns isomerization may occur in the x complex by a process of pyramidal inversion at the coordinated nitrogen atom. Weakening of the double bond by coordination thus lowers the activation energy (23 kcal mol-l) of thermal isomerization to 10 - 14 kcal mol-’ for the catalyzed reaction (cf- Table 1). pyramidal 4

En=-_-_----gN\ Ph’

N=N\Ph

F

Ph


Ph H<__/___N LNOi; \ Ph

\ Ph

Scheme

I_

Latent TThsicity of Cp2MH2 molecules and catalytk isomerizafion The symmetry of the highest-filled molecular orbital (HOMO)

valent metal Some recent hedral form planar form [18,19]_

in lowcomplexes is critically dependent on the stereochemistry [18]_ studies on CpzMH2 moIecuIes suggest that the pseudo-tetra(wedged metallocene structure) should be (Tbasic and the pseudo(parallel metallocene structure) should be v basic (see below)

425

0\ G

c

.H

-

/LH

pseudo-tetrahedral form

pseudo-planar form

Cp,MoH:, and Cp2WH, assume the pseudo-tetrahedral form in the ground state [19] _ On thermal excitation, the molecules will convert to the pseudo-planar form because of polytopal rearrangement [ZO] . A thermal equilibrium between these forms is anticipated_ The term “latent x basicity” has been proposed 1211 for designating the s-basic property of a molecule which is exhibited upon thermal excitation wi’th accompanying structural change. Thus, CpzMHz molecules are expected to show such latent Z- basicity, Since metal 7r basicity is important for catalytic isomerization, the ease in assuming the pseudo-planar structure should be related to the catalytic activity. Then, the catalytic activity is a measure of the effective latent x basicity under the experimental conditions. The higher activity of CpsMoHa relative tc that of CpaWHz suggests a greater effective x basicity for CpzMoH, towa-*ls azobenzene. The intrinsic metal basicity, however, follows the sequence W > MO as is evident from the pK, values for Cp,MHa in aqueous dioxan. Since the effective z basicity is a consequence of a structural change which may be classified as a polytopal rearrangement [21] ) the reduced activity of CpsWHs may be related to the higher potential barrier for the polytopal rearrangement_ The established trend for the barrier among the transition-metal polyhydride complexes HaML, (M = Fe, Ru, OS) or H4ML4 (M = MO, W) is, in general, first row metal < second row metal < third row metal [22], a sequence reverse to the relative catalytic activity. The catalysis of the isomerization by CpzMoH, also provides support for the intermediacy of a thermally excited x-basic CpsMoHz molecule in oiefin or acetylene ck insertion, as may be inferred from the kinetics and also from steric and electronic substituent effects [21] _ There are three different pathways for the collapse of the intermediate CpzMHz-PhN=NPh: (a) dissociation of azobenzene (rate constants, kc* or k,*); (5) pyramidal inversion at the nitrogen atom (kpc or k,,); and (c) insertion of the N=N bond into the M-H bond (hi, or kir) (cf. Scheme 1). Experimentally, catalysis of the isomerization is observed at lower temperatures (0 - 30 “C) than those at which insertion occurs (above 40 “C) in the same solvent. This result indicates that hi, is smaller than the rate constants involved in the coordination equilibria and in the pyramidal inversion. The suggested s-coordination trend, frans-PhN=NPh > cis-PhN=NPh, indicates the relationships kt/kt* > kc/k,- and k,, > k,,. A quantitative determination of these rate constants requires a knowledge of the concentration of thermally excited species and therefore remains as a problem for the future.

426

* Ph

/N=H\

+

CP2M

CPp+2

\ N-NH

Ph Ph’ k

%

k

Pt

\Ph

PC

Ph N=N

:

kt

/ l

CP2f342

kt’

Ph’

Scheme

Ph

tPP3TH

e

I Ph

2.

37Basicity of low-vale& transition-metal As described above, the role of s

complexes

hasicity in the isomerization catalysis of azobenzenes is important. To confirm this roIe, we have examined some platinum(O) complexes containingp-substituted triarylphosphine Zigands. The results obtained cIea.rlyindicate the decreased activity on p-chloro substitution_ However, this cannot be taken as reflecting the trend in zrcoordination of c&-azobenzene to the metaI or in the subsequent pyramidal inversion_ The main cataiyticahy active species is not P~(PAQ)~ but the elusive Pt(PAr& which is generated as depicted in equations (3) and (4). Subsequently the activity may be governed by the dissociation of Pt(PAra)a to Pt(PAr& and also by the intrinsic R basicity of the Pt(PAr& species_ The aromatic substituent effect is thus a composite result of these two factors. The high catalytic activity of authentic ML2 complexes Cl33 (M = Pd and pt; L = bulky phosphine) in the isomerixation of axobenxene seems to support the inference that the ML2 species generated by ligand dissociation of the MLs species are the active ones. However, the excesstipebulkiness of the ligand, e-g PBu$ , may be so large as to prevent the q2 co_ordination of cis-azobenxene. A study of the &and exchange of Pd[PB$Ph Ja with PBuiPh and its reaction with some oIefins 1131 suggests the presence of the associative intermediates [ML, J* or [ML,L’] *%where distortion of the interligand angles and a lengthening of the metal-ligand bond are expected_ A similar distorted intermediate is also suspected in the catalyzed isomerixation of azobenxene with these Z-coordinate compIexes.

(t-Bd3P

(t-Bu)$

-

Pd -

P(t-Bt+

__ ‘-.

____f Ph \N=y

/

.-

.P(t-Bd3

.Pd;’ i *. N-=N

Ph Ph

/

\ Ph

427

The equilibrium constants of reactions (5) and (6) have been reported [22]. The values increase by cu. twofold on the introduction of ap-methyl substituent. This result indica& that ?r coordination of C&H4 is more favored for the P@-t~lyl)~ complex than for the PPhs complexRhCl(PAr& qpAr,13

+ CzH4 e + G?H,

-

RhCl(CzH&(PArs)s E’WGdXd(PAr3

+ PArs

13 + PAr,

(5) (6)

Although the azobenzene isomerization catalysis and CzH, x coordination a&seemingly different, these two are closely related since an elementary step in the catalysis involves s coordination to a coordinatively UTsaturated metal species. These two phenomena are thus based on the x basicity which is exhibited after ligand dissociation and may be called “effective ‘IZbasicity”. Then, electron-donating aromatic substituents in such complexes increase the “effective ‘ITbasicity” as revealed by the catalysis ZisM(PA%&or4 of azobenzene. The catalytic activity in the isomerization of azohenzene seems to be proportional to the product of the ligand dissociation constants (h,) of PtL3 and intrinsic x basicity (B) of the PtL2 species. When the value of the product (h,-B) is so large as to prevent the dissociation of the intermediate q2azobenzene complex into trans-azobenzene, no catalytic activity will be found, however_ Thus the isomerization catalysis of a metal complex may be linked with its effective K interaction through a discrete range of h,.B values.

Experimental The following metal complexes were prepared by methods cited in the literature; M(PPh,),, M12(PPh3), (M = Pd, Pt), Pt(C,H,)(PPh,), [23, 241, dihydrido-bis(q-cyclopentadienyl)-molybdenum and -tungsten [ 251. A nitrogen atmosphere was maintained during the manipulation of air-sensitive metal complexes. Substituted cis-azobenzenes were prepared by photo-isomerization of the corresponding frans-azobenzenes [26] which were, in turn, obtained by standard procedures 1271. The instruments used were the same as described previously [5]. Visible spectra were measured on a Hitachi EPS3T instrument. Procedure for the kinetic Method A

studies

of the isomerization

A typical kinetic run with Group VI diamagnetic metal hydrides (Cp,MH,, M = MO and W) was as follows. A benzene or toluene solution of cisazobenzene under nitrogen was weII mixed with a catalyst solution in a thermostatted bath at a given temperature (20.1 “C) (cf. Table 1). An aliquot was removed intermittently and the reaction quenched by adding the aliquot to a solution of dilute acetic acid in benzene (20 mmol cmw3) in air. Visible absorption spectra at 440 nm of the quenched and diluted solutions were

428

measured just after quenching_ Comparison with the calibration hue indicated that the reaction was complete in 1 - 6 h. No correction of the rate for acidcataIyzed or thermal isomer&&ion after quenching was made because the metal-catalyzed rate is IOO-times faster than those expected for these other processes_ Method B A typical kinetic run with Group VIII metal complex catalysts proceeded as follows_ A toluene solution containing c&azobenzene (3.0 X 10e3 mol l-l) and the catalyst was prepared with rigorous exclusion of air at -30 “C. A portion of this solution was transferred to a cell (2 mm thick) for spectral measurement and the absorbance at 440 nm was continuously monitored at the given temperature for I- 2 h_ The rates of isomerization were calculated from the decrease in absorption with time (observed first-order rate constant k, = l-7 X 10-l min-l ; catalyst cont. 1 mol% at 30 “C). Thermal rates are negligible (k, = 2-7 X 10e4 mind1 at 30 “C) at temperatures below 35 “C_ The light intensity used in our spectral measurement was so low that virtually no isomerization was induced during the kinetic run_ _ The dependence of the observed rate on the temperature (10 - 30 “C) of the isomer&ration gave an apparent activation energy E, = 14.7 kcal mol-r, and an entropy of activation As = -17.0 cal K-l mol-‘_ For convenience in making the rate measurement, free triphenylphosphine (0.16 mmoi 1-l) was added to the catalyst (0.030 mmol l-I)_ The inverse of the observed rates (l/k,,) in a catalytic system involving free tiphenyl phosphine may then be plot&l against the concentration of free tziphenylphosphine (triphenylphosphine liberated from the catalyst Pt(PPha), by ligand dissociation via equations (3) and (4) was also taken into account)_ A straight-line relationship was found enabling the product of the second-order rate constant (kz) and equilibrium constant (Ka) to be estimated from the slope (1.9 X lo2 mm mmol-l)_

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