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Bioinorganic chemistry of nickel and carbon dioxide: an Ni complex behaving as a model system for carbon monoxide dehydrogenase enzyme
ELSEVIER
Bioinorganic chemistry of nickel and carbon dioxide: an Ni complex behaving as a model system for carbon monoxide dehydrogenase enzyme ’
Eich step of the CODH complex putt~‘rnof rcuctivity hus hen shown to tukc plucc ut three diftcrcnt centers named, after Rugsdule. 14 I A, B und C und performing the uctivitics shown in Tublc 2. Csntcr A is proposedttobe un NiIFeeS cluster performing the reactions shown in Eqs. ( 2 1-f 5 1, Scheme I. The structure of ccntrr A huh not beenclcurly elucidated. but the informutinn okuincd from EXAMS unulysis. ullows the structure reported in Fig. I to be proposed.in which the nuturc of X und of the Ni-~oordinuting lipnnds 1.. bus IIOI ban ascatuin~ti I4h I. The eazymuGcmcchunism presents. to da~c. some t@~r points thut descrvuinvcstipution: I, the cxistcnec of different sites where the reduction of curbon dioxide und the formation of rhc C-C bond take place.rcspe32tivcly:
coDl4medkkcleyMmkelcohwnco2 (M)+CO2 + 2[H++e-1
--a
M-CO t H20
WMUgudlllt~M. (la 1)
C + H20 CO + H20
+ NibFefCO) --a Conin +
~~)NI-F~(CO) (CHs)Ni-k(W)
--a
C&C(O)-Ni-Fe
-OH (CH#4iiFe(CO)
(Eq. 2)
Ni--Fe-C(O)CHa
----, No-Fe(CO)(CHa) ---s Ni-Fe-C(O)CH&
NI--FeC(O)Ctl3
CO+H2 CO2 + Hz.
e
HolmkgstkndmMhanoltothe~.
cowmaBledaY~~~~~hno~ tzwrimg
+
+ HI ->
I
LnlM(CO)(CH3) 49
----> Cb(O)C-NI-Fe + C&s’ ---a NI-Fe + cOASC(O)C~
WC have slurId un e~t~ndd invcsGgution 1111 FC iId Ni CT&-lixalion Wdysts. WC report hcrcon SOIW Ni-mod:l sys~cm lhut lidly ndmic the cnzymuiic uclivity convurling CO9 inlo an t)l@lllic IliicWcr. via rcduclion lo CO and rcuction with Ihiols und un olelin.
con~plcxes us
2. Experlmcntul All opcrukms were curried OUI under dinirrogcn iIIuNK_ phcrc using vucuum-lint Icchniques. Solvc!nls wcrc clrictl ith descrihcd in the li~cruwrc and stored under dinitrogcn. CO, ( > 99.95%) was lioln SIO SpA. alI other used prducts were purchasedlkm Aldrich.
CH31+H@
LnM(C0) + CHgl G
M is Co, Fe. Rh. Ru
LnlM(CO)(CH3) Lnl(CO)MCJO)CH~
for
d m K KB Ircpantled, It im;rlyreci (PCy,)JWSPb. 7hc mother sohuion was added with pentanc and an orangeyellow sdi precipitated that was isolated and analyzed for
( PCy,)2Ni(CO)2.
was shownto he ( FCy, ):Ni( CO 1:. only slightly impure for
(PCy.dNi(CW.+ 200 mg of such a mixture were reacted in toluene under Nz atmosphere at 273 K with I -heptene( 44 pi, 0.3 I3 mmol 1 and PhSH (64 pl. 0.626 mol ) but no formation of the thio-
2.1. I. ~~r~~~~ c?l’tPC.Y,&WCo~~ m 1980 4s). I912 fs) cm-‘. 8(‘~C{‘H~):202.98~.S(?L’P(‘H}):39.Sl~. A&. Calc. forC,&NiPzO,: C. 67.56: H. 9.85; Ni. 8.69: P, 9.17. FoundsC. 67.54: H, 9.89; Ni, 8.66; P. 9.15%.
8 ( “PI’H} 1: 8.31 ppm. A&. Calc. for C&H,NiP&: P. f&I,
C. 68.8 I ; H. 9.11: Ni, 7.0 I : Foumt: C. (i11.79:H, 9.18: Ni. 7.00: P. 7.389.
ester was observed. ~b)O.~g(O.472m~)of~~y~)NitCO)~(IR~s at 2056 and 1973 cm-’ 191) were reacted with I-heptene (67 ~1. 0.470 mmoi ) and PhSH (97 pl.0.944 mmol 1 in toluene at 273 K. The thioester was not observedin the reaction mixtureafter 24 h.
3. Resultsanddi.xussh
3. I. CO, wdttctiottto CO
I ( 0.030 g, 0.0448 mmol ) was dissolved in 4 ml of anhydrous tohrcneunder Nz atmo@em and the solution was cooledto 2!!3 K. 7 )ri ( O.&W6mmol 1 of I -hepme and 4.6 pi ( 0.0448 mmoi1ofPhSHwere addedto theorangesolution with vigomusstirring.AtIer additionof PhSH the reaction mixture immediatelytumed red-brown. The solution was stirredfor 2 h at the tempemtureof 253 K, then hydrolyzed with rurhydrous #Cl andthe~lvent~v~~ in vucuo.The organicfmctignextra&d in Rt$J fromtherusiduurl solidwus treatedw~th~H~OH/Rp~t I:! vol.~und,~~erRP~hydmiysis with water. sp&cw VI-W (Scheme3) were ~~nt~~i~ by tX=MS unuiyntw, -Thererarlhtlrproductswere fbntitid by compuringthe Mtentiontimesand massfragmentswith thaw of uutheak sumpkl undby eJ~nWRtul unulysis, VI: Al * = #de 344 A#ul, CMC. for C,,H,,OS,: e, 69.73; H, 7.Q2.Found:e, 699’10:H, 7,050, WI: M” =m/c 168. A&. Cuk, f6r C,,H,O,S: C, 57.12: H. 4,79. pouml:C. 57.m H, 4.81%. VM M + -de St, Atutl, Cult. furC,,H,,OS: C. 71.75: H, 7.74, pound:6,71,79: H, 7.@%. tx: iU ’ =wd8 -Xl& Amt. 6uk. Cm6, \H9rS:6, 74,clw:H, W3. Fou& CT,74,vQ:H, 9.65%.
l#e Wng deep-tedsolutionimmediatelyturnedy40w. tR spc&um of this solutionrev~kd tlrp prctsmcw of t~WBPndsut 198Qund i9130m’tduetothe tPQ&NitCO)~ speciesand of two very weak additiunai ~~~~ l973em”‘~to(PCy~)Ni(CO)~pffsent in #ar’r?s. The y&W solid isolatedafteraddition of pentam: The
For a long time. Ni has been considered to be, in the enzymaticsystem.the metalcenterbindingCO and probably catalyzingthe CO2 reduction(Eq. ( I 1) [ 101.The involved couple of oxidation states of the metal (N&N?‘. Nin-Ni’n. Ni”-Nil) hasbeenu matttxof discussionandinveatigutionin view of the factthatE!JRstudieshaveshownthatthe inactive form of the enzymeis ESR silent (Nil’ ‘?I. while the active form showsthe presenceof unpainzdelectrons( Ni’?) I I I 1. Where the electron is located, if nt the Ni or at the C atom, hasulsobeena mattetof discussion.Whether Ni or Fe is the cvnter whem COz is rtzduced to CO is still unclear.but it has been proposedthut CO is bound to R prior to the ucetyi moietybeinggencmted( 4 1. Chemkui modelsystemsindkutc thutboth Ni 18 1 and Fe 1 I2 1 complexesarc ubieto bind curbondioxide.Concerning the CO, reductionto CO. ut the presenttime only cvidcnc~ thut the Ni complex (PCy,)jNi(COz) (I) (Cy=oyclo= hexyl 1 cutalyzerthe CO2 reductionreuctionin the ptusencu! of protonund ek!tron donorshusbeencollrsttd I I3 1. The fluxionai behuviorof the curbondioxide moietycoordinated to the metal hasbeenunumbigu~ly establishedin the case of I und the tatutionalfree energybarrier( AG* = 39.6 kcul mol ‘) determined1141. I in toluene,u~rdinit~gen atmosphere, reactswith sevcml Broenstedacids,under&c&on tmnsferconditions.and showsa different behavioraczvrdingto the speciesand the reactionWnditions. PhSH is a quiteinterestingreagent:in factwhenit isudded to a solutionOFI in tohtencthe IR spectrumof the solution showsthe immediatedisappcumnrc of thebandat I745 cm ._* (due to coordinatedCO2) while two new bandsat 1980 and 1912 cm 1 ’ indiiute the formation of the carbonyl-bound species(PCy&Ni(CO)2 (II). 11 and (PCy,)tNi(SPh)z ( 111) wax is&ted from the reactionmixtureus thermodynamk productsand characterizedby elementalanalysis,IR und ‘.‘C. ‘tP NMR qtwmcopy. Water is alsoan end productof the reaction.while PhSSPh.which isakineticproductwith’(PCy,),Ni(CO)’ (IV) (Scheme 2). was detwted in the reactionmedium but not
isolated. In fact the disultide reacts with the Ni( 0) exisring in solution according to Eq. ( I I ): ‘(PCy,)2Ni’+PhS-SPh-,(PCy3)2Ni(SPh)2
(II)
The overall stoichiometry of the reaction is reported in Eq. ( 12): 3(PCy~)1Ni(CO~)+lPhSH-,(PCyz)~Ni(CO),
(I21
+2(PCy,)2Ni(SPh),+C0,+2H20 The yield of the dicarbonyl species based on Ni decreases with the temperature and with the time elapsed between the dissolution of the complex and the PhSH addition. A 100% yield with respect to Ni. Uccording to E&l. ( I2 1, was ohluincd carrying OUI the reaction under CO, almosphcre ( PC.C,2= 0. IO I MPa 1 that prevents the carbon dioxidcdecoordinurion from Ni. These fuels demonstrate that c&on dioxide reduction is noI a ‘gus phase’ reaction: CO, has to be coordinated to the metal in order to he reduced to CO. The mechanism shown in Scheme 2 huh been proved for the reduction of COP with chiols. The reaction of ( PCy,j2Ni( CO,) wide H?S or HCI WUh also studied. The lower yield (~72 of the SkuIing CO, W;LS reduced to CO) obtidned using HCI (a stronger Ucid than PhSH ) infers (hill the prolon WUnsfer may not be the tirst step. supporting Ihe mcchimism reported in Schc~nc I. III Ull cUses. the end clcclroii-lrclnsfer implies rhn~ Ni is oxidized lo Nit II I. iIs found in he enzyme.
WW?~lee?)+@(0
kq 14)
FQa)2W02)
( 20 )
-sPh+CO+H,O
3.2. Crurplirtg of CO with dkyl ~rolrpz Quite recently Rugsdale and co-worker!; 14 ] suggested that ;t bimetallic mechanism takes place at ccnrcr A where Fe and Ni huve been proposed to play ;I dll’fercnt role: Fe hiIs been reported Usthe aton1 where CO produced at ~:‘~tc i C’is bound. while Ni is proposed to be involved in the t~ir~til a CH, group from a Co-corrinoid protein 10 the iron bound CO. wilh C-c’ bond l’orm;rGon ( I$. ( 2 ) and ( 3 1, Scheu~ I 1.The SOI’ormed Ucetyl group is Ul~imUtely rclouscd us CH ,Cf 0 )with Ni probilbly involved SCOA hy reilction with GASH. iii the IiOill tronslkr skp ( Eq. ( 5 ) 1. The ellZymi\~i~ pu~hwuy IISC~ 10 build up the UcetyI t’rirgnlcnl ha!, closr similarity 10 inducwial hynthctic procwcs, nmonp which is the Monsun~o process I Scheme I ( B 1; kq, ( IO) I. AIIWII~ melul ccnterh thut pmmolc the rertclions reponcd in Scheme I ( 8). FIJ complcxc~ arc well known lo cUtUlyX thy synthrsih of the Ucetyl moiety from Ihe methyl group Und CO 1 IS I. vi;1 Un intramolcculer monometallic
of
lllWtlilllis~ll. (Eq. 15)
PhS’ + Ii+
PhSW +
7PhSH+COt==P’
III oUr opinion. titkinp into i\ccouIIt111~’ chemkll
~‘;IL’Is.Ihe
ucclyl formirtion in CODH could be explilined in krms PhS
$
1/2PhS.SPh
ofNi-
@a 16)
mediated rrunsfer of iI methyl group from u corrinoid en/.ymc
(EQ 17)
irou-hound CO ( Fig. 2 ). would producc the acclyl moiety.
lo iron ( Eq. ( 3b) ). A subse+~ent nucleophilic iW\ck to the (W~s,2Nl(C02~
(PQ~)~NI(COOH)+
+ H+
e’ + H+
+
‘(PCW2NI(CO)”
2=(PCy3)2NI(CO)’ + PhS.SPh
+
(PQ3)2NI(CO)2+
+ YO
(Ea 12)
(M (m3)2NI(Sm)2
(Eq ‘2)
This xhemc may bc preferred to the direct methyl transfer I’rom Ni- 10 Fe-bound CO ( Eq. ( 3 ) I. Pathway ( 3h 1 could hcttcr cxpl;lin the fact thul removal ol’one clcctron from iron pronwks IIW Ibr~l~utio~~ of the WCQ~moiety 1 I h I. WC hilvc extended oUr studies on Ni complexes ’ in order 10 ;\sccrlUin if Ni itself is Ublc to cUtUlyX the form;rtion ol’iI thioestcr. reproducing alone thr full cnzymalic cycle W U%!h CO? for TIC form;ltion of the lhiocstcr group.
how closely chemicalmodels can interpret enzymatic rex(ions, and to pmposca role for the metal centers. Acknowledgements Financial suppon by MURST (National Proprnmmes. 40% and of MY%) is gratefully acknowledged. References
By reacting( PCy, ~,Ni(CQ 1 with PhSH in the prcscncc of I =hcptcnr.under dinitrogrn atmosphere.the f’~rmi~tionol the following productsWRS ohscrvcd(Scheme 3 1. &oducts Vi und VII1 demonstratethat the formation of it thioester from CC& and thiols canbepromotedby a single Ni center. The coordinativcly unsukmatedspecies ‘t PCy \ I :Nit CO,’ ( IV 1, that is the kinetic productin the Ni-promoted CO2 reduction. must play u key role in this process. In fact. when I& or Ji-cnrbonyl species. namely ( PCy ,)Nil(CO) t t X) or ( PCyl )?Nit 031~ ( II 1 wkctvu.scxiin the samere;\ction conditions.the I’ormutionofthioesters WBSnot observed. Thih tinding helps to explainwhy the reactionaffording the thiocstcr gives u quite Vtiriitble yield with the I\?Wion conditions. The ‘monocerhonyl specks i IV I can cithcr generatethe tliourbonyl ( that is not renctiw to\vMd
Bioinorganic chemistry of nickel and carbon dioxide: an Ni complex behaving as a model system for carbon monoxide dehydrogenase enzyme ’
Eich step of the CODH complex putt~‘rnof rcuctivity hus hen shown to tukc plucc ut three diftcrcnt centers named, after Rugsdule. 14 I A, B und C und performing the uctivitics shown in Tublc 2. Csntcr A is proposedttobe un NiIFeeS cluster performing the reactions shown in Eqs. ( 2 1-f 5 1, Scheme I. The structure of ccntrr A huh not beenclcurly elucidated. but the informutinn okuincd from EXAMS unulysis. ullows the structure reported in Fig. I to be proposed.in which the nuturc of X und of the Ni-~oordinuting lipnnds 1.. bus IIOI ban ascatuin~ti I4h I. The eazymuGcmcchunism presents. to da~c. some t@~r points thut descrvuinvcstipution: I, the cxistcnec of different sites where the reduction of curbon dioxide und the formation of rhc C-C bond take place.rcspe32tivcly:
coDl4medkkcleyMmkelcohwnco2 (M)+CO2 + 2[H++e-1
--a
M-CO t H20
WMUgudlllt~M. (la 1)
C + H20 CO + H20
+ NibFefCO) --a Conin +
~~)NI-F~(CO) (CHs)Ni-k(W)
--a
C&C(O)-Ni-Fe
-OH (CH#4iiFe(CO)
(Eq. 2)
Ni--Fe-C(O)CHa
----, No-Fe(CO)(CHa) ---s Ni-Fe-C(O)CH&
NI--FeC(O)Ctl3
CO+H2 CO2 + Hz.
e
HolmkgstkndmMhanoltothe~.
cowmaBledaY~~~~~hno~ tzwrimg
+
+ HI ->
I
LnlM(CO)(CH3) 49
----> Cb(O)C-NI-Fe + C&s’ ---a NI-Fe + cOASC(O)C~
WC have slurId un e~t~ndd invcsGgution 1111 FC iId Ni CT&-lixalion Wdysts. WC report hcrcon SOIW Ni-mod:l sys~cm lhut lidly ndmic the cnzymuiic uclivity convurling CO9 inlo an t)l@lllic IliicWcr. via rcduclion lo CO and rcuction with Ihiols und un olelin.
con~plcxes us
2. Experlmcntul All opcrukms were curried OUI under dinirrogcn iIIuNK_ phcrc using vucuum-lint Icchniques. Solvc!nls wcrc clrictl ith descrihcd in the li~cruwrc and stored under dinitrogcn. CO, ( > 99.95%) was lioln SIO SpA. alI other used prducts were purchasedlkm Aldrich.
CH31+H@
LnM(C0) + CHgl G
M is Co, Fe. Rh. Ru
LnlM(CO)(CH3) Lnl(CO)MCJO)CH~
for
d m K KB Ircpantled, It im;rlyreci (PCy,)JWSPb. 7hc mother sohuion was added with pentanc and an orangeyellow sdi precipitated that was isolated and analyzed for
( PCy,)2Ni(CO)2.
was shownto he ( FCy, ):Ni( CO 1:. only slightly impure for
(PCy.dNi(CW.+ 200 mg of such a mixture were reacted in toluene under Nz atmosphere at 273 K with I -heptene( 44 pi, 0.3 I3 mmol 1 and PhSH (64 pl. 0.626 mol ) but no formation of the thio-
2.1. I. ~~r~~~~ c?l’tPC.Y,&WCo~~ m 1980 4s). I912 fs) cm-‘. 8(‘~C{‘H~):202.98~.S(?L’P(‘H}):39.Sl~. A&. Calc. forC,&NiPzO,: C. 67.56: H. 9.85; Ni. 8.69: P, 9.17. FoundsC. 67.54: H, 9.89; Ni, 8.66; P. 9.15%.
8 ( “PI’H} 1: 8.31 ppm. A&. Calc. for C&H,NiP&: P. f&I,
C. 68.8 I ; H. 9.11: Ni, 7.0 I : Foumt: C. (i11.79:H, 9.18: Ni. 7.00: P. 7.389.
ester was observed. ~b)O.~g(O.472m~)of~~y~)NitCO)~(IR~s at 2056 and 1973 cm-’ 191) were reacted with I-heptene (67 ~1. 0.470 mmoi ) and PhSH (97 pl.0.944 mmol 1 in toluene at 273 K. The thioester was not observedin the reaction mixtureafter 24 h.
3. Resultsanddi.xussh
3. I. CO, wdttctiottto CO
I ( 0.030 g, 0.0448 mmol ) was dissolved in 4 ml of anhydrous tohrcneunder Nz atmo@em and the solution was cooledto 2!!3 K. 7 )ri ( O.&W6mmol 1 of I -hepme and 4.6 pi ( 0.0448 mmoi1ofPhSHwere addedto theorangesolution with vigomusstirring.AtIer additionof PhSH the reaction mixture immediatelytumed red-brown. The solution was stirredfor 2 h at the tempemtureof 253 K, then hydrolyzed with rurhydrous #Cl andthe~lvent~v~~ in vucuo.The organicfmctignextra&d in Rt$J fromtherusiduurl solidwus treatedw~th~H~OH/Rp~t I:! vol.~und,~~erRP~hydmiysis with water. sp&cw VI-W (Scheme3) were ~~nt~~i~ by tX=MS unuiyntw, -Thererarlhtlrproductswere fbntitid by compuringthe Mtentiontimesand massfragmentswith thaw of uutheak sumpkl undby eJ~nWRtul unulysis, VI: Al * = #de 344 A#ul, CMC. for C,,H,,OS,: e, 69.73; H, 7.Q2.Found:e, 699’10:H, 7,050, WI: M” =m/c 168. A&. Cuk, f6r C,,H,O,S: C, 57.12: H. 4,79. pouml:C. 57.m H, 4.81%. VM M + -de St, Atutl, Cult. furC,,H,,OS: C. 71.75: H, 7.74, pound:6,71,79: H, 7.@%. tx: iU ’ =wd8 -Xl& Amt. 6uk. Cm6, \H9rS:6, 74,clw:H, W3. Fou& CT,74,vQ:H, 9.65%.
l#e Wng deep-tedsolutionimmediatelyturnedy40w. tR spc&um of this solutionrev~kd tlrp prctsmcw of t~WBPndsut 198Qund i9130m’tduetothe tPQ&NitCO)~ speciesand of two very weak additiunai ~~~~ l973em”‘~to(PCy~)Ni(CO)~pffsent in #ar’r?s. The y&W solid isolatedafteraddition of pentam: The
For a long time. Ni has been considered to be, in the enzymaticsystem.the metalcenterbindingCO and probably catalyzingthe CO2 reduction(Eq. ( I 1) [ 101.The involved couple of oxidation states of the metal (N&N?‘. Nin-Ni’n. Ni”-Nil) hasbeenu matttxof discussionandinveatigutionin view of the factthatE!JRstudieshaveshownthatthe inactive form of the enzymeis ESR silent (Nil’ ‘?I. while the active form showsthe presenceof unpainzdelectrons( Ni’?) I I I 1. Where the electron is located, if nt the Ni or at the C atom, hasulsobeena mattetof discussion.Whether Ni or Fe is the cvnter whem COz is rtzduced to CO is still unclear.but it has been proposedthut CO is bound to R prior to the ucetyi moietybeinggencmted( 4 1. Chemkui modelsystemsindkutc thutboth Ni 18 1 and Fe 1 I2 1 complexesarc ubieto bind curbondioxide.Concerning the CO, reductionto CO. ut the presenttime only cvidcnc~ thut the Ni complex (PCy,)jNi(COz) (I) (Cy=oyclo= hexyl 1 cutalyzerthe CO2 reductionreuctionin the ptusencu! of protonund ek!tron donorshusbeencollrsttd I I3 1. The fluxionai behuviorof the curbondioxide moietycoordinated to the metal hasbeenunumbigu~ly establishedin the case of I und the tatutionalfree energybarrier( AG* = 39.6 kcul mol ‘) determined1141. I in toluene,u~rdinit~gen atmosphere, reactswith sevcml Broenstedacids,under&c&on tmnsferconditions.and showsa different behavioraczvrdingto the speciesand the reactionWnditions. PhSH is a quiteinterestingreagent:in factwhenit isudded to a solutionOFI in tohtencthe IR spectrumof the solution showsthe immediatedisappcumnrc of thebandat I745 cm ._* (due to coordinatedCO2) while two new bandsat 1980 and 1912 cm 1 ’ indiiute the formation of the carbonyl-bound species(PCy&Ni(CO)2 (II). 11 and (PCy,)tNi(SPh)z ( 111) wax is&ted from the reactionmixtureus thermodynamk productsand characterizedby elementalanalysis,IR und ‘.‘C. ‘tP NMR qtwmcopy. Water is alsoan end productof the reaction.while PhSSPh.which isakineticproductwith’(PCy,),Ni(CO)’ (IV) (Scheme 2). was detwted in the reactionmedium but not
isolated. In fact the disultide reacts with the Ni( 0) exisring in solution according to Eq. ( I I ): ‘(PCy,)2Ni’+PhS-SPh-,(PCy3)2Ni(SPh)2
(II)
The overall stoichiometry of the reaction is reported in Eq. ( 12): 3(PCy~)1Ni(CO~)+lPhSH-,(PCyz)~Ni(CO),
(I21
+2(PCy,)2Ni(SPh),+C0,+2H20 The yield of the dicarbonyl species based on Ni decreases with the temperature and with the time elapsed between the dissolution of the complex and the PhSH addition. A 100% yield with respect to Ni. Uccording to E&l. ( I2 1, was ohluincd carrying OUI the reaction under CO, almosphcre ( PC.C,2= 0. IO I MPa 1 that prevents the carbon dioxidcdecoordinurion from Ni. These fuels demonstrate that c&on dioxide reduction is noI a ‘gus phase’ reaction: CO, has to be coordinated to the metal in order to he reduced to CO. The mechanism shown in Scheme 2 huh been proved for the reduction of COP with chiols. The reaction of ( PCy,j2Ni( CO,) wide H?S or HCI WUh also studied. The lower yield (~72 of the SkuIing CO, W;LS reduced to CO) obtidned using HCI (a stronger Ucid than PhSH ) infers (hill the prolon WUnsfer may not be the tirst step. supporting Ihe mcchimism reported in Schc~nc I. III Ull cUses. the end clcclroii-lrclnsfer implies rhn~ Ni is oxidized lo Nit II I. iIs found in he enzyme.
WW?~lee?)+@(0
kq 14)
FQa)2W02)
( 20 )
-sPh+CO+H,O
3.2. Crurplirtg of CO with dkyl ~rolrpz Quite recently Rugsdale and co-worker!; 14 ] suggested that ;t bimetallic mechanism takes place at ccnrcr A where Fe and Ni huve been proposed to play ;I dll’fercnt role: Fe hiIs been reported Usthe aton1 where CO produced at ~:‘~tc i C’is bound. while Ni is proposed to be involved in the t~ir~til a CH, group from a Co-corrinoid protein 10 the iron bound CO. wilh C-c’ bond l’orm;rGon ( I$. ( 2 ) and ( 3 1, Scheu~ I 1.The SOI’ormed Ucetyl group is Ul~imUtely rclouscd us CH ,Cf 0 )with Ni probilbly involved SCOA hy reilction with GASH. iii the IiOill tronslkr skp ( Eq. ( 5 ) 1. The ellZymi\~i~ pu~hwuy IISC~ 10 build up the UcetyI t’rirgnlcnl ha!, closr similarity 10 inducwial hynthctic procwcs, nmonp which is the Monsun~o process I Scheme I ( B 1; kq, ( IO) I. AIIWII~ melul ccnterh thut pmmolc the rertclions reponcd in Scheme I ( 8). FIJ complcxc~ arc well known lo cUtUlyX thy synthrsih of the Ucetyl moiety from Ihe methyl group Und CO 1 IS I. vi;1 Un intramolcculer monometallic
of
lllWtlilllis~ll. (Eq. 15)
PhS’ + Ii+
PhSW +
7PhSH+COt==P’
III oUr opinion. titkinp into i\ccouIIt111~’ chemkll
~‘;IL’Is.Ihe
ucclyl formirtion in CODH could be explilined in krms PhS
$
1/2PhS.SPh
ofNi-
@a 16)
mediated rrunsfer of iI methyl group from u corrinoid en/.ymc
(EQ 17)
irou-hound CO ( Fig. 2 ). would producc the acclyl moiety.
lo iron ( Eq. ( 3b) ). A subse+~ent nucleophilic iW\ck to the (W~s,2Nl(C02~
(PQ~)~NI(COOH)+
+ H+
e’ + H+
+
‘(PCW2NI(CO)”
2=(PCy3)2NI(CO)’ + PhS.SPh
+
(PQ3)2NI(CO)2+
+ YO
(Ea 12)
(M (m3)2NI(Sm)2
(Eq ‘2)
This xhemc may bc preferred to the direct methyl transfer I’rom Ni- 10 Fe-bound CO ( Eq. ( 3 ) I. Pathway ( 3h 1 could hcttcr cxpl;lin the fact thul removal ol’one clcctron from iron pronwks IIW Ibr~l~utio~~ of the WCQ~moiety 1 I h I. WC hilvc extended oUr studies on Ni complexes ’ in order 10 ;\sccrlUin if Ni itself is Ublc to cUtUlyX the form;rtion ol’iI thioestcr. reproducing alone thr full cnzymalic cycle W U%!h CO? for TIC form;ltion of the lhiocstcr group.
how closely chemicalmodels can interpret enzymatic rex(ions, and to pmposca role for the metal centers. Acknowledgements Financial suppon by MURST (National Proprnmmes. 40% and of MY%) is gratefully acknowledged. References
By reacting( PCy, ~,Ni(CQ 1 with PhSH in the prcscncc of I =hcptcnr.under dinitrogrn atmosphere.the f’~rmi~tionol the following productsWRS ohscrvcd(Scheme 3 1. &oducts Vi und VII1 demonstratethat the formation of it thioester from CC& and thiols canbepromotedby a single Ni center. The coordinativcly unsukmatedspecies ‘t PCy \ I :Nit CO,’ ( IV 1, that is the kinetic productin the Ni-promoted CO2 reduction. must play u key role in this process. In fact. when I& or Ji-cnrbonyl species. namely ( PCy ,)Nil(CO) t t X) or ( PCyl )?Nit 031~ ( II 1 wkctvu.scxiin the samere;\ction conditions.the I’ormutionofthioesters WBSnot observed. Thih tinding helps to explainwhy the reactionaffording the thiocstcr gives u quite Vtiriitble yield with the I\?Wion conditions. The ‘monocerhonyl specks i IV I can cithcr generatethe tliourbonyl ( that is not renctiw to\vMd