Vol. 18 No. 5 CMNESE JOURNAL OF CHEMXWRY

A new class of rhodium complexes containing free donor atoms and their intramolecular substitution reaction

Used as catalysts for methanol carbonylation to acetic acid and acetic anhydride in hanogenews phase catalytic reaction,' square pIanar %(I) Complexes are typical metal ccanplexes due to their high catalytic activi- ty. However, they am usually unstable. nKy were cur- rently anionic cornplexee which have high catalytic activ- ity but low stability in &c reactioa.2 A new dess of c ic -d ihnyl %(I) carionic complexes exhibiting spe- cial p e e s is reported he=. They not imly possess high catalytic activity but also o u m stability at- tributed to their special stnlcm.

?he ligands were prepared Bccofding to refer- 3-1 ealws.

added dropwise to a tetmhydrofwan solution containing excessive ligand under stining to give a miTture solu- tion. Ihe hadim complex was obtained after dropping a tetrahyddumn solution of excessive NaB- in the above mhue solution. The product was Gltered off, washed with diethyl ether and dried to constant weight under VBcUum at n o d atmDspheric temperaaue.

The XPS nwmmmmt ofthe complexa (solid s m - ple immediately) was canied out an a Krato ES 3OO X- ray p-- spectroscogy using Mg Klll.2 x-*- tion at 1245 eV with the vacuum of 266.67 x la' Pa. nKir IR spectra were recorded on a VR-10 IR spec- t m p h t e r .

Resultsanddiscussion

Fonnation of rhodium canplex is shown in scheme 1. Ihe l i d contains two Menmt donors, N and 0,

nate bonds. The strong N+Rh bond makes the complex stable while the weak 0-Rh bond makes it active in catalytic reaction. with the f d m of the two coordi- nate bonds, the electmic charge transfemce happens. As a d t the binding energies of N1, , 01, and change following the charge transference. As is shown in Tablel, Thebindingene1gyofN1,irrcreesesfmn398.3 eV of the ligand to 399.8 eV of the complex, which in-

which coardinate with lhodium atrm to form two coordi-

152 Rhodium complae.9 JIANG et al. I

dicates that the nitmgen donor donatea electmnic charge to M u m atom. The binding energy of 01, increases h 5 3 2 . 1 eV dthe ligand to 533.0 eV ofthe cun-

plar, which indicates that the oxygen donor donates dec& charge to rhodium. Hawever, the binding en- ergy of &m decreases fxum 310. 0 eV of [ Rh-

(CO)2Cl]2 to 308.8 eV of the complex, which indicates that the rhodium atrm accepts electmnic charge from ni-

trogen and oxygen atans. The complex's IR spec- disp~yingtwoequalintenaitypeaicsreflectedthecharac- teristic absorption of terminal &yl in 1,990 em'' and 2,070 ad.

la 398.3 532.1 l c 399.8 533.0 308.8

caapared to the current &odium canplexe repolt- ed before, the above complex has been h g e l y improved onits stability. But it is still easy to decompose at the temperature in reaction condition due to no fnx donors. On the con-, the copolymer medium complex WMRh we f d has very excellent stability in reaction system wfiile let done in air.'PvMRh was obtained ac-

CDzding to above method by using Zvinyl-piridind methyl a c r y h copolymer as ligand. After analyzing its structue, it is f d that the s t r u m unit ofwMRh is similar to that of coIlrm3Dn o q p i c rhadium can$-. Bdh have quam planar cis-dicarfmnyl rhodium strucbure

unit.However, alargeamnrntoffmeN,Odonolsin copolymer chain, which am ixnp0-t for the high stabil- ity of the complex, differentiate copolymer rhodium con- plexes fmn cornmoll organic rhodium caplexes. In or- der to study the mechamm . ofitshighstability, wesyn- thesized some small lnolecular rhodium complexes con- taining fnx donors as shown in Scheme 2 and Table 2."

n CH=N N=CH d. o h

k

HSCH~COCH~CH~CHZOCCH 2SH 11 II 0 0

2d

2c'

HSCH2COCH2CH2CH20CCH 2SH II 0

co 2d'

Vol. 18 No. 5 2000 Chin- Journal of C h e m i s q 753

Table 2 Bbdkig mrgy & (ev) d bande (ad) d ligands end ~~ c o m p k d

Binding enag~ Eb(eV) uco (ad)

N1 I q, s, R1sdsLl semple

2s 399.5 532.0 ( imp. ) rn 1980 399.5 (&) 532.0 (imp.)

2a'

2b 398.8 532.7 2ooo 2100 398.8 (free) 532.7 2b'

k 399.3 532.1

400.8 (d.) 533.8 (d.) 310.4

399.8 (ad.) 533.4 (coord.) W.8

k' 399.8 (d.) 532.1 310.1

2d 532.1 162.1

2# 534.6 (d.) 163.3 (d.) 3*o.1 533.0 162.1 (flee)

d.: csonhml ; imp.: implrity.

In rhodium camplexes, coo- donors' bind- bond in the square planar metal complex has been stud- ing energy was ditTenmt tmn that of fme donors. The ied and c~ntrmed.'~ However, all the intramoleclllar binding enew of coordinated donor increased because substitution reactions reported before occuxred in solu- the donor atom tranafered electronic charge to rhodium tion. Here, we studied the substitution process in solid atom, however, the binding enew &free donor was e- state.

pualtothatofligand.'Ihesemodiumcomplexesare Here, we take the complex 2d in scheme 2, quite stable for possessing special pmpedes. If being named PMRh, for example. After PMRh being heated in placed in air for severalday~ or heated,- their peaks of air at 509: for U) min, the double peaks reflecting the terminal &nyl in IR spectnun would gradually change characteristic absorption of the terminal carbony1 groups from double peaks to single peak. Furthermore, after changed into single peak, which indicates that one ofthe b e i i heated at further temperature for a period of time, two texminal cabnyls had been dropped off."-' As a

ffected the characteristic absorption of terminal &yl, II. Even the singlepeak disappeared when the complex were totally lost up to now. However, these complexes of s t ~ ~ c t u e II was heated at 1009: for 1.5 h and struc-

do not demmpose for the loss of the te& cdonyl , ture III was obtained. To our surprise, the double peaks which is the main reason for the deconposition of the were recovered in its IR spectnun if PMRh with structure

medium complexes r e p o d before. W h y these complex- III was heated at 1009: for 2 h under an atmosphere of es were 90 stable? We consider that it is because of the CO. According to the above phenamem, we pm+ a

existing of the free donors. They pmtect the &(I) ac- reversible intnmolecular substitution reaction of such tive unit fkm dexqming by intramolecular &tutim complexes containing two fnx donor atoms as shown in reaction. The substitution p~ocess of the COoTdinated Scheme 3.

they lost the single peak remained. Peaks, which re- result the complex changed tirnn structure I to structure

do co struclure I

CH* /

A co

structure 111 structure 11

754 Rhodium complexes W G et d.

One of the two terminal carbonyls was drupped off if PMRh was heated without efficient pmtection of the CO, at the s ~ m e time, neighboring free oxygen donor immedi- ately replaced it to form a O+a coordinate bond. After that, the uncoo-ed sulfur donor replaced the re- maining terminal carbony1 to fom quadridentate complex given further heating. And it should be noted that this substitution ~"X'RSS is reversible. Under an atmosphere of CO, it will retum to its on@ structure I . 'Ihat is the reason why the IR spectrum changes.

As is shown in Table 3, further evidence of the change of bindmg energy conforms that the above in- tramolecular substitution reaction is reasonable. Eb of sulfur atom which coordinated with rhodium in stn~cture I increased from 162. 1 eV of ligand to 163.3 eV, but that o f h e sulfur atom remained constant. Ihe same was the case with oxygen. In structure II, the free carfmnyl oxygen atom of complex replaces the terminal &nyl to form tridentate complex. But its XPS spectrum does not change because the coordinate status does not change. With the second terminal &yl being dropped off, the h e sulfur donor immediately coordinates with rhodi- um atom to form a stable quadridentate complex. As a mdt the E b of Sp d increases to 163.4 ev due to no free sulfur donors remained.

163.3" 534.5" 309.5 162.1b 533.1*

r[ 2060

534.5" ~ . 5 m 163.4" 533. l b

Table 3 shows an downwad tendency in the bindmg energyof Rh3dn. campared with 311.2 eV in [a- (CO>,a]2 (according to the measurement of refer- ence"), it decreases to 310.1 eV in the s t r u c h ~ e I. %s case is due to the fact that the negative charge transfer is from sulfur and oxygen donors to medium in the pmxss of formation of the complex. With the change of the structure I to structure II, the b d m g energy de-

crease to 309.5 eV can be attributed to m~ charge ac- cepted by rhodium. For the same reason, it further de- creases to308.5 eV after the structure changes liwn II to rn.

Reversible inh.amolecular substitution reaction largely hrpmes stability of complex in air. In catalytic reaction system, experiments showed that it was the key factor of high stability for this class of rhodium complex- es. Fig. 1 shows the catalyst PMRh for the carbonylation from methanol to acetic acid. After it reacted 150 min,

we gave out CO fmm reactor and aerated air (stirring at n o d temperatwe, 10 kg/cm-' pressure) into it to test its stability. After 30 min, we let out air and aerated CO to reactor. ?he reaction went on without any diaiculty. This indicates that the catalyst remains its catalytic activ- ity. Compared to the catalyst used More, 15 its stability is greatly impmved when lack of protection of CO.

Time (mm)

Fnp. 1 carbonylation of methanol. Reaction oonditions: lac, Pm 4-5 ma. (&OH 120 mL, CHJ 30 mL, Rh6xlO'mol. reactor25OmL. e) cnnver-

aimofmahand; b)yidddmthylscecate; c)

yieldofeceticacid; d) preglurersduoeofC0~- crrmulativeabsolption.

Ihe new class of square planar ck-di&nyl Rh- ( I ) complexes, on one hand, remains high active in catalytic reaction. On the other hand, its stability is largely impmved. So it is a pmpective catalyst for methanol carbonylation.

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