PhotosoIvation of [Co(CN),]3-, awns-[Cr(NH,),(N@S),1-, and &an$-[Cs(en),N@SFg' in aqueous mixed solvent systems

CARL FOOK CHOW WOPJG A N D ALEXAKDER DAVID KIRK Depurtt~lelli of Cl~e~~zistr:i., L~rtlicci:rii~~ of Vicforiu, Vicroriu. R.C., Cutludu V8 W 2Y2

Received June 28, 1976

CARL FOOK CHOW WONG and ALEXANDER DAVID KIRK. Can. J. Chem. 54, 3791 (1976). Photosolvation of [C~(cN)~] j - ( l ) , frur1s-[Cr(NHj)~(NCS)~]-(2) and tratls-[Cr(en),NCSF]+(3)

has been studied in various water;methanol, ethanol. acetonitrile. eth1,lene gllcol, glycerol, and acetone mixtures and in aqueous solutions of pol~\ii~ylp)rolidolle. The quantun~ yield for thiocyanate loss from 3 was found to be fairly independent of solveiit, while for thiocyanate loss from 2. large, and at higher concentrations of organic solvent, specific reductions of quantum yield were observed. The study of 1 using spectrophotometric analysis for [CO(CN)~H~O]'- yielded similar data to some in the literature. but the larger range of bysteins studied here did not support the reported reduction of quantum yield with bulk vixosity. Furthermore evidence from this and other work suggests that niany of the obserbed quantum jield red~~ctions are only apparent, based on a false assutnption that the photo~olvatioii is cjualitativeiy the same in all mixtures. The data are reinterpreted in terms of competitive sol\ation by both solvent com- ponents. It is argued that this study supports a dissociative model of reaction for 1, but an associative model for the chroniium con~plexes 2 and 3.

CARL FOOK CHOW WONG et ALEXANDER DAVID KIRK. Can. J . Chem. 54, 3791 (1976). On a 6tudiC la photosol\ atation du [CO(CN)~]'-(~), [Cr(NH3)z(NCS)4]-it.ii~~s (2) et [Cr(en)2NC-

SF]+ rrutl~ (3) dans divers inelallges d'eau methanol, Cthal~ol; acetonitrile, ethylene glycol, glyckrol et acetone et dans des solutions aqueuses de polyvinylpyrolidone. On a trouve que le rendenlent quantique pour la perte du thiocjanate B partir de 3 est pratiquement independante du solvant alors qu'il y a une grande dependance pour la perte du thiocyanate ?I partir de 2; de plus on a observe qu'a des conceatration:, plus Clevees cle solvanti organiques, des rCductions specifiques du rendelnent quantique peuvent Ctre observees. L'Ctude de 1> en faisant appel a l'analyse spectrophotometrique de [CO(CN)~H~O]~- , aconduit B des donnees semblablec celles que 1'011 peut retrouver dans la littirature; toutefois les etelldues plus grandes des systkmes CtudiCs dans le present travail n'apportent pas de support pour la reduction des rendetnents quantiques qui a CtC rapportees en fonction de l'augmentation de la viscositC globale. De plus des donnees rapportee dans ce travail et dans d'autres travaux suggkrent que pl~lsieurs reduc- tions dans les rendements quantiques ne sont qu'apparentes et basees uniquement sur I'hypo- thkse fausse que la photosolvatation est qualitativement la m&me dans tous les melanges. On rCinterprkte les donnees en terme de solvatation compktitive par les deux composants du solvant. On prCsente des arguments suggirant que notre etude supporte un modkle dissociatif pour la reaction de 1 mais un modkle a,sociatif pour les complexes 2 et 3 du chrome.

[Traduit par le journal]

Introduction Scandola et al. (7, 8) have reported large solvent The large majority of studies of ligand photo- effects on photolysis of some cobalt complexes.

substitution processes of coordination corn- These they correlated with solvent viscosity and pounds have been carried out in water as explained in terms of a photoinduced heterolytic solvent ( l , 2 ) . In recent years, however, a number bond fission to produce a five coordinate species of experiments have extended the range to non- aqueous, and mixed aqueous solvent systems. The influence of solvent on excited state proper- ties (3) and on photocheclical behaviour (4) is clearly not well understood a t present. Thus, while Langford and Tipping ( 5 ) and the authors (6) found evidence only for minor solvent effects for [Cr(RNH2)5C1]2- photolyses in DMSO water

and solvent separated ligand, with subsequent competition between solvent substitution and cage recombination.

It is these latter observations, the apparent differences in observed bellaviour. and the important implications for theoretical models of transition metal photochemistry that prompted this investigation of solvent effects for two

and acetone water respectively, in contrast chromium complexes, similar but of opposite

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WONG AND KIRK

Ethylene glycol

TABLE 1. I'hotochetnical yields as a function of mediun-i

Wt. 4 . GoC

v Water: (( X H ~ O (cP) la 2 b 3C

Glycerol 10 0.98 i . 3 0.93 0.93 -

20 0.95 1.7 - - 0.93 24 0 .91 2 .0 0.79 0 .83 -

10 0.88 3.7 0.62 0 .63 0 .90 48 0.85 5 .6 0.51 0 .53 -

56 0.80 8 . 3 0.31 0 .36 -

60 0.76 10.7 - 0.85 61 0.74 13.6 0.32 0 .39 -

16 0.95 1.5 0 .86 0 .83 -

32 0 .88 2 .3 0 .72 0 .65 -

48 0.79 3 .5 0.59 0 .53 -

60 0.70 5 .0 0 .46 0.46 -

70 0.60 6 .8 0.36 0.40 -

80 0.46 9 . 5 0.30 0.37 -

Ethallol 7 . 5 0 .97 1 .4 0.92 - -

15.3 0.9? 1 . 9 0.86 0.78 0.95 27.5 0.87 2 .6 0.82 - -

40.5 0 .99 2 .8 0.79 0.59 0 .93 54.2 0.68 2 .8 0 .78 - -

68.9 0 .54 2.1 0.77 0 .55 -

hlethanol 30 0.81 1.8 - - 0.93 60 0.54 1 .9 - - 0.95 75 0.37 1 . 2 0.81 - -

Acetone 20 0.93 1 .5 - 0.73 -

30 0.88 1.6 - - 0.99 10 0.83 1 . 6 - 0.63 -

60 0.68 1 . 2 - 0.61 1 .O

Acetonitrile 20 0.90 1.1 0.56 0.85 0 .97 40 0.77 0 .95 0 .45 0 .80 1 . 0 60 0 .60 0.73 0.43 0.79 -

Polyvinylpyrolidone 0 . 5 1 . 6 0.92 - -

1 .O 2 . 3 0 .90 - -

2 .0 4 .3 0.86 - 0.99 3 .0 7 .2 0.84 - -

1 .0 10.6 0.82 - 0.96

aCalculated from absorbance at 380 nm o n assuniption that sole product was [Co(CN)jH2OI2-; see text. Measured a t 20 "C, h, ,,,, i = 31 3 nm.

"Quantum yield for thiocyanate release. Measured a t 20 "C, X,,,,, = 546 nm. CQ = quantum yield in solvent, 40 quantum yield 111 pure water, see text.

charges, and a re-investigation of solvent efTects for [Co(CN)6]3- photolysis.

Results and Discussion

Table 1 sho\vs the quantum yields of photoly- sis of the three complexes [CO(CN)~]" (I); trur~.s-[Cr(NH3)2(NCS)4]-(2), and r~a~zs-[Cr(en)2- NGSFI- (3), relative to the quantum yields in pure uater a t 20 "C, namely 0.313? 0.262, and 0.266 respectively, a t irradiation wivelengths of 313 nm for 1 and of 546 nm for 2 and 3. The

quantum yields for 1 were determined spectro- photometrically a t 380 nm follouing Scandola et 01. (7) (that is, assuming that [Co(CN)SH20]'- is the sole photo product), ~vhile for 2 and 3 the yields are based on direct measurement of reieased thiocyanate. For 3 a small yield of ethylenediamine aquation, 4 = 0.09, also occurs, but can be neglected for the purposes of this u ork.

In brief, the results reveal large solve~lt effects for the two negative ions (but see later discussion for [Co(CN)6j3-) b u t the efTects on the positive

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3796 CAN. J. CHEM. VOL. 54, 1976

ion are much smaller. in most cases on the order of the cumulative experimental uncertainty.

[c~(civ)fj]~- The results for this compound, 1, while agree-

ing with those of Scandola ei d., encompass a larger range of solvents and of composition, and it becomes apparent that the correlation with viscosity observed by those workers was more apparent than real. Thus small, but signifi- cant, differences are observed between the ethylene giycol, bvater system and glycerol ~ a t e r systems. The most dramatic departures, how- ever, are for polyvinylpyrolidone, uhich in- creases bulk viscosity markedly at low mole fraction; similar observations by Natarajan (9) have been discounted by Scandola cr ul. (7, who interpret them to indicate only that the relationship between bulk viscosity and structure of the solvent cage may be different for viscous polymer solutions and for mixed solvent sys- tems; and most in~portantly for acetonitrile which gives a marked lo\$ering of measured quantum yield in solntions of redirced viscosity. We conclude that the viscosity correlation observed (7) was fortuitous aiid is not supported by our larger data set.

Since the experimental phase of this work was completed, Nakamaru et trl. (10) have pub- lished a study of the photochemistry of % in water, acetonitrile: dimethyi formamide, meth- anol, ethanol, and some other solvents, which shows rhat a [Co(CN)5SI2- solvent n~onosubsti- t~ l ted product is formed in each case, with quantum yields in the range 0.28 + 0.32, that is, roughly the sanie as that in water. They also find that for water ethanol mixtures the propor- tions of ethanol and water subst~tution var) in parailcl n ~ t h composition as might bc cxpcctcd.

Thus the spectropholometric calculations used in d e n v ~ n g the Table 1 data for 1 are almost certa~nly in crror, since the p r o d ~ ~ c t is likely to be an u n k n o ~ n mixture of [CO(CN)~I-I~O]~- and [Co(CN)5SI2- rather than purely the former a s presumed.

The data for the acetonitrile water systein support such an interpretation 15ell. From a plot of q u m t u n ~ yield u,. mole fraction of water a lower limit quantum yield of about 0.125 appears to be reached at high acetonitrile concentrations. If this is due to product [ C O ( C N ) ~ C H ~ C N ] ~ - being produced with C$ = 0.28 (10) one can

calculate its molar absorptivity at 380 nn! (the analysis wavelength) to be 135 I mol-I cm-I. This is in good agleement with the value calcia- !ated by us f r o n the publ~shed spectrum of this compound (I I), namely I30 1 mol-I cm-I For the single methanol polnt of ocr work, assunling the lob lln:lt apparent quantum ~ ~ e l d has been reached (this secms reasonable as X H ~ ~ has been reduced to 0 3. ~ h i l e for aceton~tr~ie uatcr and ethanol uater mixtures the quantum jield approaches the linlitlng value at XI,,,, as hlgh as 0.6) one ca!culates t3so -- 230 1 molP1 cm-I again 111 agreement with the lalue u e calculate from Wakarnaru's spectrum, 220 I ~ n o l - ~ cm-I. For ethanol there is a d~screpancy: €380 [Co(CN)5- C2tI50Hl2- 230 1 mo!-I cm-I (our uo ik ) us. 170 I niol-I c r ~ i - ~ (Nakan~aru data. our calcula- tion). Hoi$ever, note that Unkamaru"s niolar absorptivlties for the ethanol monosubstituied compound are quite 101% compclred to the methanol analogue so the former might be sub- ject to some error In order to explaln the apparent quantum q~elcl decredse for the elycerol ndter dnd ethqleile glqcol hate1 qys- u

tems In the same uaq requires the hypotheticai monosubstituted products to have molar absorp- tivltles at 380 nrn of about 60 and 75 1 inol-I cm-' respectivelj These are rather low values and suggest that for these molecules some or all of the apparent decrease 111 quantum yleld ulth increased concentration of oiganic component maq be real

For acctonitrilc. methanol, dnd probdbl) ethanol, ho~xever. fie beliebe that the quantum yield of reaction remains approximatel) con- stant, and uith incrcascd concentration of organic solvent substitution by the organic coin- poneni successf~~lly competes with water substi- tution. The measured absorptivities at 380 nm, together with the published spectra of the I-!lono- substituted products, enables one to calculate tllc proportion of aquo and solvento-rnono- substit~ited products as a function of solvent composition. The data are not sufficiently ample or reliable to investigate the dependence of the proportion on composition (mole fraction) but it reveals the interesting result that acetonitrile preferentially photosubstitutes over water by a considerable factor, about 40. Methanol appears favored b\ a factor of about t h o , while ethanol appears to be on an approximately equal footing with water. Whether these results reflect prefer-

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WONG AND KIRK 3797

ential solvation effects or rates of substitution is uncertain. The favoring of the \+eak nucleophile acetonitrile, hobvever, suggests that preferential solvation (or greater mobility due to lack of hydrogen bonding) rnust provide the explana- tion. Preferential solvation by acetonitrile would be favored by the similar nature and polariz- ability of acetonitrile and I. Yakamaru et ( [ I . (10) noted spectral evidence for strong, polarizability effects in [Co(CN)6I3- acetonitrile solutions; some charge transfer interaction is even possible.

In summarv, this analvsis of the data indicates that many quantum yield changes observed for 1 (7) were only apparent based on an incorrect a s sum~t ion as to the nature of the ~ r o d u c t . The data for polyvinylpyrolidone solutions and pos- sibly for glycerol and ethylene glycol d o suggest some residual composition dependence but not well correlated with bulk viscosit~. The case for cage recombination is not established by the data since other explanations for such solvent effects exist: such as c h a n e s in the extent of solvent structure (1 1) and preferential or mixed solvation with consequent alteration of effective water activity in the solvation shell of the ion.

tr~ns-[Cr(NH~)~(NCS)41- cmd Ircms- [Cr(en)2NCSF]+

Table 1 shows that definite changes of +xcs- occur as a function of solvent composition for 2. Since these yields were directly determined there is no doubt about their magn i t~~de and the reality of the variations observed, although we did not, due to the experimental difficulty involved, attempt to determine the nature of the solvent substit~i&xl proclucts.

The changes in quantum yield observed for 2 d o not correlate with solvent viscosity, and again the data for acetonitrile are a good ili~lstration of this. A plot of relative quantum yield for thiocyanate from 2 us. mole fraction water, X H ~ ~ (Fig. 1) shows that for each solvent ex- amined a different limiting quantum yield is approached at high mole fraction organic sol- vent. For very much lower mole fractions of water a further change in quantum yield might occur, as water was progressiveljr excluded from the .system. Such a regime might not be acces- sible due to solubility problems. However, a t high rnoie fraction water ( 2 0 . 9 ) the relative quantum yield is close to the sanle linear f l~nc- tion of X g z ~ for all soivents except acetonitrile.

FIG. 1. Relatibe quantum yields for photosolvation of thiocyanate ion from I ~ I I I ~ . S - [ C ~ ( N H ~ ) ~ ( N C S ) ~ ~ - as a function of aqiieous niixed \ol\ent conipo\ition. + =

ql~antum yield for thiocya~nate in inixed solvent. 40 =

qLiantum lield for thioc).anate in water. x,,, = mole fraction water in iolution. Legend. 0, acetonitrile; [7. acetone: 3, ethanol: A, ethllene glycol; @, glycerol.

This suggests a similar lowering of water activity a t low conceritratior~s of o-ganic solvent, with the appearance of specific solvent effects a t hieher concentrations. u

In contrast the data for the unipositive ion, 3, show only small effects of solvent com- position. This parallels earlier data (5, 6) for [Cr(RNH2)5CI]"+ photolyses and suggests a charge eirect; that perhaps generally, positively charged chromiurn complexes show little sensi- tivitp of quantunl yield-to solvent composition in aqueous non-aqueous media.

These observations can be rationalized in terms of a model which considers the variation of the composition of the primary solvation sphere, the effectiveness of the components as nucleophiles, the steric constraints on substitu- tion and the influence of solvent structure includ- ing hydrogen bonding. From the point of view of predictive po\+er a model with so many variable parameters is bound to leave something t o be desired, but it dces seem from the data obtained here, and from studies of solvation and solvent structure (1 1 and references therein),

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3798 CAN. 3. CHEM. VOL. 54, 1976

that all of the above factors might be expected to play a role.

Since. for chromium complexes, the lifetime of the excited quartet state leading to reaction (12-15) is iikely to be only a few picoseconds (16), the quantum yield of substit~ltion can be expected to depend on the rapid availability of a suitably oriented nucleophile. For a positive ion the primary solvation sphere, which is almost certain to contain a t least one water molecule even in a fairly dry organic solvent, has the water n~olecules oriented with the oxygen lone pair suitably placed for nucleophilic substi- tution. Thus one may understand that substitu- tion is facile and little change in quantum yield occurs on addition of organic solvent to the bulk solvation sphere, that is, until the obvious limit of extremely low mole fractions of water is reached. (Such a limit might, in practice, be beyond reach due to insolubility.)

In coiltrast, for a ncgativc ion, the water in the solvation sphere is inappropriately oriented for nucleophilic substitution and must rotate (or pseudo rotatc) in order to substitute. Since molecular rotational relaxation times are of the order of 5 X 10-12 s for water and longer for organic solvents, this can bc expected to be a non-trivial part of the substitution mechanism. It is known that as organic solvents are added to water initially an incrcasc in structuring and hydrogen bonclinz of the water occurs relatively independent of the nature of the organic com- ponent. If water molecules in thc s01vcnt sphere are participating in such stri~ctural changes this couid be expected to decrease the rate of water substitiition. Furthermore a t significant mole fractions of organic solvent it will ccmpete for sites in the solvation sphere, as shown by the results for [CO(CW)~]~-, and this could also lead to a decrease in the rate of substitution.

Consideration of the data on this basis sug- gests that the early linear portion of Fig., I corresponds to an increase in hydrogen-bondlng of solvation sphere molecules, with decrease in quantum yield due to their decreased mobility enabling radiationless decay to compete better with nucleophilic substitution involving the excited state. At higher concentrations of organic solvent conlpetition for sites ir, the solvation sphere may begin to be important, Thc limiting quantum yields achieved, Fig. 1, may then be determined by the ability of the organic solvent

to substitute on chromium in place of water. Such an explanation suggests the-order of substi- tution efficiency to be acetonitrile > acetone > ethanol > ethylene glycol > glycerol, certainly not an order of nucleophiiicity but quite reason- able as a sequence of mobility. iif this illode1 is correct, and we hope in future to be able to undertake analytical work to investigate this, it will dramaticallv illustrate the imwrtance of specific solvation and structural effects in chromium photochem~stry in mixed solvents, a t least for negative ions.

This analysis of the data suggests a real ditfer- ence between the behaviour of the cobalt corn- plex, 1 , and the chron~ium complex, 2. For I, the data suggest the quantum q ie ldrema~ns constdnt Independent of the substituting iigand (w~th the posslbllitj of small viscoslt) mob~llty effects) Ironicallq this supports the o r ~ g ~ n a l proposed d~ssociatrve mechanism involv~ng interruedldte [Co(CN)51Z-. followed by complete scavenging.

In contrast the neeatlve chrornlu~n(lEl) Ion sho~vs discrin~ination between entering nucleoph~les. This might be argued also to support a dissocla- tike mode of reaction with incomplete scaveng- ing. However there is no evidence for such a cage mechanism and it would not explain the contrast in behaviour of the negative and positive ions. In fact one ~vould expect recombinations to competc Il7oi.e favorably in the case of the positive ion, 3, where the fragments are of opposite charge. The observations better support an associative mode of reaction in which nuclco- philic substitutions must compete uith non- reactive degradative processes of the reactive excitcd state.

Experimental

.&fuirr.inls K I C O ( C F ~ ) ~ \\a, hindl? donated by A. Ludi. University

of' Bern, Switrerlasld. The spectroscopic data for the first ligand field band \+ere h ,n,, - 310 nm. t,,, = 192 1 11101-1 ~ 1 1 1 - 1 (lit. ( 1 7) A,,, = 3 13 I I ~ , t,,, = 170 1 mol-1 cm-I). K[ t r i~ /~ . \ - [Cr( idH~)~~NCCi)~] \\as prepared from arnmoliiun? reinecltate (18) (DDH).

ti.i1iis-[Cr(en)~(NCS)F]C1C~~: a 10g sample of ti.ii/ls-

[Cr(en)2H?OF] (Clod):. pr-epared as described by Vaughn. ei i i l . , (19) was vigoro~~sly 5tirreci with 5 g of dried NH4NCS ( 1 h at 100 (3) in 50 mi of acidified (0.2 ml of 3 M HCIO,) methanol in the dark at 40-45 C for 24 h. The solid residue \\as collected bq filtration. mashed with niethanol. and air-dried. The crude complex was re- crystallized from water by drop-\vise addition of 60'2 HCIB4 to the cooled solutioi~. The orange crystalline prociuct was filtered off, washed with methanol, and dried

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WONG AND KIRK 3799

it1 cclccro. Yield 5SC1. Allni. calcd for Cr(enj2(NCS)F- (C104):: Cr 14.92. C 17.20, N 20.10. H 4.60: found: C r 15.1, C 17.3, N 20.0, H 4.7.

The identity of the product is suggested by its rnethod of preparation and was confirmed by elemental analysis and the LIV-viiible spectrum. The preparation of rrr111.r- [ C ~ ( ~ I I ) ~ X F ] - (X = Cl, Br) by aiiation of trails-[Cr- ( ~ I I ) ~ H ~ O F ] ' - in methanol is weli-known (20), and generally, only the stereoretentive product is easil) isolable. The elemental ana lp is is in good agreement n i th the nioiecular formula and the LIV-visible spectrum confirms that it is the ti.iit~s isomer. The spectral charac- teristics, particularly the molar ahsorptivities (t), are significantly different fro111 those for the (,is isomer (namely A,,, = 375 nm ( t = 33.9) and 509 n m ( t =

41.2) for the above conipound but A,,, 375 nni (t = 52) and 500 nn1 ( t = 85) (19) for the c , i~ isomer.

Pl~orob,.ti~ A/,i,iiriir~i.t r~iril P~.oc.c.tlcrre A PEK air-cooled 203-1004 200-W high pressure

rnercury lamp with a Bausch and Lamb grating niono- chromator (set for 20 nn1 spectral band width) yielded light at 3 13 and 546 nm, with effective line half-widths of about 5 and 9 n11n respectively. In each case appropriate hloclting filterb (Corning) were ~ ~ s e c i to renioLe higher energy scattered light. Light ll~lxes were measured either by ferrioxalate (21) (313 nm) or reineckate (18) actino- metry. I11 addition a coiistaiit fraction of the light beam was continuously lnonitored by a photo-tube d~lring phorolys~s runs so that intensity fluctuation5 or drifts were eaiily observecl and correctctl.

Sample\ were photolyied in thermostatted 1 cni path length rectangular g l a s spectrophotonietcr cells, n i th continuous magnetic stirring.

RIIII Piocet1~rre.r ritlcl Ai~ci/~..sis A solution of the desired complex, 6 X :M for

K3Co(CN)6 ancl 1.0 X 10-2 34 for rruil.s-K[Cr(NH3):(N- CS)4] and r i c i i l . , - [CT(~~)~NCSF]CIQ~ . was niacie LIP under dim red light in the appropriate rtocl< solution. Two 3-nil aliquots were pipetted into I-cm path spectrophotometer cells and after thermal eili~ilibration (5 min) one saniplc was photolyzed to give about j', ( K 3 C o ( C N ) ~ ) or 2.5( , conversion.

For K,Co(CN), the extent of reaction was determined by measuring the absorbancei of the dark aiid photolyzed soli~tions a t 380 nm ( E = 298 I 11101-~ ctn-I) (22). The extent of reaction for K-reinecitatc and t~.ut~,t-[Cr(ei~)~PdC- SF]CI04 was determined by riieasurii~g he releasetl thiocyanate as tlescribcd by Wegner and Adanison ( 18). Corrections for thernial reaction were appliecl by means of the data on the dark iolutions.

Acknowledgements

The authors wish to thank the National Research Council of Canada and the University

of Victoria for financial support. We also thank A. Ludi, University of Bern for kindly donating a sample of K3co(CT\J)6. C.F.C.W. is grateful for a University of Victoria Scholarship and A.D.K. is grateful to the University of Sussex for use of their facilities during 1975-1976.

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