- Email: [email protected]
CO activation by dicyclopentadienyl derivatives of titanium, niobium and tantalum
Journal ofMo[ecufm- Ca~-[ysL% 13 (1981) 107 - 114 @ Else~ierSequoia S.A., Lausanne -- Printed in The Nether[ands
CO ACTIVATION BY TITA_NlCtYM, N I O B H J M
DICYCLOPENTADIENYL A~ND TANTALUM
DERIVATIVES
107
OF
J. H T E U B E N , E. J. M. D E B O E R S, A. H. K L A Z I I q G A t and E. IqLE[ Laboralorfum =]oor A n o r ~ a n ~ c h e Chernie. Rijl~su~uersite~ Gronrngen, N[]enborgh 16, 9 747 A G Groningen (The Netherlands)
Summary Exploratory stud/es on reactions of CO (room temperature, 1 atmosph~.re) w i t h C p z T ~ (1% = aryl, a l k y l , allyl, v i n y l ) a n d C p ~ 1 % (M = N b , T a ; 1% = a l k y l ) a r e r e p o r t e d . T h e t i t a n i u m c o m p o u n d s s h o w f o r m a t i o n o f C O c o m p l e x e s C p z T , ( C O ) R f o r 1% = CI, C s F s , a n d o f ~2-acyI c o m p l e x e s CpzTiCO1% f o r 1%= C~Hs, o - C H a C s H 4 , a n d C ( C H ~ ) = C [ H ) C H s . F o r 1% = C6H~, C H s , C H ~ C ( C H s ) s a n d ~ ~-CsHs m o r e c o m p 2cared r e a c t i o n s t a k e p l a c e a n d complete identification of the products was not posmble. The formation of k e t o n e s , 1%CO1%, a n d d i k e t o n e s , RCOCO1%, s h o w t h e f o r m a t i o n o f new- C - - C b o n d s in t h e s e s y s t e m s , p o s s i b l y t h r o u g h t h e r a d i c a l c h a r a c t e r i n d u c e d o n t h e h g a n d s b y b a c k d o n a t i o n o f t h e u n p a i r e d eleetro_a o f t i t a n i u m . 1%eduction of the acyl function and of ketones to the corresponding alcohols was o b s e r v e d . T h e n i o b i u m a n d t a n t a l u m c o m p o u n d s CpzM1% (1% = n-Call 7, n-C4Hg, n ~ [ - I n ) give s t a b l e c o m p l e x e s CpgM(CO)I% ,and f u r t h e r r e a c t m n o f these with CO was n o t observed.
Int, r o d u c t i o n C a r b o n m o n o x i d e activation b y early transition metal c o m p o u n d s has b e e n s h o w n to be a very promising field in C O chemistry. T h e first reports [I - 3] o n C O reacting with C p 2 T i R 2 * ~ n d Cp2TI(R}CI (1% = C H a, Cz£-I5, C H 2 C 6 H s ) s h o w e d , in addition to a facile insertion of the C O molecule into the Ti--C b o n d R n d reductive elimination of a ketone: CpzT~Rz
co
co
zTi~
' C p z T i ( C O ) z + 1%--C--1%
C-
O tprese-t address: Koninkli]ke/ShelILahoratorium, Badhuisweg 3, i031 ClV~Amsterdam. The Netherlands. *Cp = W S ~ s H s , Cp* = ~ S ~ s M e s.
108
new dihupfo
an interesting
bonding
mode of the acyl group, viz.,
This type of bonding has been found to be quite common for early tmnsrtion metals and underhnes the high oxygen affinity of these metals ]%51-
Quite recently elcwt studies by Rercaw on the reaction of Cpz*ZrH, with Cp2*Zr(CCj2 and other metal carbonyls provided 2 clear insight into the mechanism 01 the reduction of CO to C&OH and the formation of the MSH=CH+M complexes u& LM+D+%H mtermediates [S] . Group VB clicyclopentadienyl compounds seem to react differently. Flxiani reported the reaction of Cp2VR with CO ]5] _ Depending on R, CO can be inserted to give Cp,V(CO)COR (R = CHa, CH&Hs) or R is transferred to the cyclopentadienyl ring (R = C&IS):
q-$-; qR Cp,VR
=
Cp-V’
\
-
cp-v-0
(R = C&J.
‘co
The aforementioned starting compounds and products are all even (16 or 18) electron systems. We decided to study the reactions of odd (15) electron systems, Cp,TrR. The unpaired electron can here be transferred by a-back-donation on to the ligand. grving radical character to certain parts of the molecule and possibly. in Consequence, mterestmg reactions. We found such reactions when Cp2T3R (R = aryl) was reacted with cyanides R’CN and isocyanides R’NC. With cyanides, coupling of the ligands W?Zthe Lyanide C-atom was observed with the Formation of [7] : R
R
\ T=P,
NY+ R’ R’
IsocYanides
insert into the Ti-C
bond
going
~2-immoacy~
complexes
[81
-
cp*Ti’ \-xII \
D’
tYe have f&ended our studies to the reaction of CpaTiR (R = a~$, alkyI, a&E, vinyl) with CO. We aLso investigated the reactions of Cp#R (M = N5, Ta; R = ah&) with CO to compare the results with those of CpsTiR and C&VR. Ln the ‘hy-pothetrczl’ compounds Cp&R (M = Nb, Ta), the size of the metals is expected Ltobe about the same as Tr; V is smaller. Steric aspects will be very simiEar for the compounds CpsMR (M = ‘Pi, Nb, Ta) but the eIectronic aspects are quite drfferent. The Nb and Ta complexes have two ekctrons available for back donation whereas the titanium compounds have one. By studying the compIexes we beheve we have a distinct possibihty of fmding an effect which depends on the number of electrons available for back-donation. En this communication we report the results of some exploratory studies. Parts of the study have been pubhshed ehewhere [9,101_ Experimenti Data on experimental conditions, equrpment, and analytical procedures can be found elsewhere ES, IO]. The compounds were prepared following published procedures: Cpe*TKI [IO], Cp,TiR (R = CsF5, C&s, o-CR3CsR4) [17], CpzT~cHs 1181, Cp2TiCH,C(CH3)3 [19], CpzTi(q3-C3H5) [ 20 1, Cp*_MR (M = Nb, Ta) [Sf _ C~,TLC(CH,)~(K)CH, was prepared from (Cp,TiC1), and LiC(C&)= C(H)C&. It was isoWed and fuHy characterized [21].
AJmost all 15electron compounds, Cp,TiR, react easily with CO (25 “C, 1 atm) both in solution and in the solid state. The products mainly depend on the nature of R; other variables such es solvent or temperature seem to be of secondary importance. No reaction takes place with complexes where the vacant site on the metal is inaccessibIe. This cao be for steric reasons (e.g., Cp,Tr-2,8-(CHs)s C&-I3 ElII ) or due to coordination with heteroaton donor Ligands (e-g., in dimeric (CpzTiCI), [X2), or IGIthe iutemahy coordinated compound below [X31.
(CH, );N-&Hz
110 For
skrong
ekctronegatwe
proceeds according Cp,TiR
R groups,
(R = CL*, C6F5),
Cp,Ti(CO)R.
+ CO -
For R = CsFs the adduct could be isolated and characterized Cp*,Ti(CO)C1 is thermally unstable and disproportionatis: 2Cp*2Ti(CO)C1
the reaction
co:
-
Cp*zTi(CO)2
completely;
f Cp*zTiC12.
The adducts are terminal CO complexes showmg v(C0) at 2060 (R = CsFs) and 2020 cm-l (R = Cl) [lo] _ They are comparable with the vanadium complex Cp,VKO)Cl (v(C0) 1950 cm-l) [5] taking into account that Cp,VCl (16 electron system) has two, and the titanium compolxds have only one election available for ;r-bonding with the CO ligand The formation of C~‘,TI(CO)~ from C~*,TI(CO)CI resembles in 2 way the formation of Cp,Ti(CO), from Cp2TiR2 (R = CsHsCH2) and CO. En the latter case zt is thought to result from carbonylation of titanocene formed by the reductive elimination of the ketone R-CO-R from the acyl alkyl compound Cp,Ti(COR)R [2]. In the case of Cp*,Tr(CO)Cl, at least an iAermolecd~ reaction is needed to explain the products_ The chemical reactivity of the CO molecule in these adducts is rather low. Despite the preference of early transition metals for insertion of CO rnto an M-C bond to give an q2-acyl, C-R M’
II ?O
this LSnot obsemed here Instead the CO ligand is easily losr; with formation of the origmal C22T~R (R = C,Fs) by warming a soiution of the adduct under reduced pressure. Less electionegative R groups (aryl, alkyl, allyl, vinyl) at once @ve insertion of CO into the Ti-R bond accompanied by side- and following reactions resulting in very complex mixtures. It is very probable that the Insertion step :s preceded by adduct formation, a process that has been observed in the closely related systems of Cp,TiR v&h cyanides R’CN and isocyamdes R’NC [7,8]. So far the only acy! compound sufficiently stable to isolate and study 1s Cp,TiCO+-CH,CsH,: CF,TI+-CH&,H,
f CO -
Cp,TiCOa-CH3C,H4.
Its stability 1s probably due to the steric mteraction of Ehe orfho methyl group on the phenyl rmg which shields the acyl carbon atom from further interaction with other titanium compounds. The acyl group is q2-bonded to the metal as can be concluded from the very low, characteristic v(C=O) at 1470 cm-l [3]. *In this case the permethyl complex Cp*2TKI It is moncrnerlc In solutior_
(Cp* = $C5hle5)
was studled because
AcyI formation wa.s aIso found for CpzVR (R = CH,, CH,C,H,) [5j. Here the insertion of CO into a V-C bond is accompanied by complexation of an extra CO molecule: cp*VR
f- 2CO -
Cp,V(CO)COR.
For Cp2Tio-CH&H, no further uptake of CO was observed. The avtiability of one electron may be insufficient to give the back donation needed tz~ bind an additional CO molecule, the bonding of the ~~“-acyl may be too strong, but steric reasons may also be responsible for this difference between vanadmm and ttaniurn compounds Cp,VR. Oxidation of CpzTiCo~-CH&H,) with the drsulfrde C&SSCsHs or iodine gives the diamagnetic, tetravalent compounds Cp2Ti(X)CO~-CH&H4 (X = I, C&S). The observed u(C=O) (1570 - 1580 cm-l) is now In a position comparable vnth the closely related Cp,Ti(I)COCHs (v(C=O) 1610 cm-l) 131. The reaction of CO with Cp,TiCsHs is similar but more complicated, and the products could not be identified in detail. The uptake of CO (in the case of Cp2Ti-c4!HsC,HE, exactly I CO/Ti) varies for different runs between 1.3 and 1.8 CO/T?. The i-r. spectrum shows an absorption at 1500 cm-r indiczztive of IJ2-acyl, but the iow solubiiity suggests ohgomeric or polymeric structures with bridging acyls:
B F”P +=-F= -
7”B 7-O
CP
CP
F” TiCp
The reaction of Cp,TiCOR with HCI/ether C6HC the acyl was oxidized quantitatively CpsTiCO-o-CH,C& For R = CsHs a compla CpaTiCOCsHs
+ HCI -
was studred. For R = o-CEI,
Cp2Ti(CI)CO+CH&H4
f&H,.
mixture resulted:
+ HCI -
CpaTiCla + CpaTi(C!)COCsHs
7 organic products.
The T&balance for the products does not add up to 106% and unidenbfied organotitanium compounds are still present. The organic part of the reaction product is very complicated. The presence of C&Is, C,H,-C&E,, C&H&OCOC.&s was demonstiated. The fact that coupling products such as biphenyl and &H&O-COCsHs are present indicate radical character. It is not clear whether this coupling takes place in the reaction with CO or in the HCl reaction. The presence of Cp,TiCl, and Cp,Ti(CI)COC& show - at least in retention of the Cp,Ti structure during the c.arbonyIation and Part -the work up. This is in contrast to the carbonylation of Cp2VCsHs, where the -+--,-I group IS tiansferred to the cyclopentadienyl ring. A very mteresting point is the presence of the diketone C&I&O-COC&, because it suggests a strong relation between the CO reactions in these systems and the Bercaw systems Cp*,Zr&/CO; the mechanisms of the reactions may show close similarity [6].
112
B
8. R-C--C-R
However, other mechanisms are conceivable. Carbonylanon of CpaTiCHs results in a complicated mixture of products. The main difference from the aryl compounds is the formation CP,TI(CO), (33%) and acetone, isopropyl alcohol and ethanol in minor quantities. Trobably, reaction of CpaTiCQCHa with unreacted Cp,TiCH, under CO produces Cp,Ti(CO), and acetone: CpsTiCOCHa
f CpsTiCHs
-
co
CpJi(CO),
of
f CHsCOCHs.
It resembles to some degree the formation of acetone from Cp2Ti(COCHs)Cl with CH$&CI under CO [Z] ; in our case, free CpaTiCH, xvould play the role of the Grignard reagent. The presence of ethanol and 2-propanol indicates a reduction of a carbonyl function during the reaction. Hydrogen Lor this process may be supplied by various sources, e.g , cyclopentadienyl ligands, methyl groups or solvent molecules [ 141 Attempts to control the carbonylation reaction and isolate the acyl CpaTiCOR were undertaken for R = CH&(CHs)s, but were unsuccessful. The results are basic&iy the same as those obtained for CpaTiCH,. Carbonylation of Cp,Ti(r13-C3Hs) (= CpeTi-z-allyl) is a rather simple reaction, CpaTi(CO), and (C,H&CO being the main products. It LS also observed for CpzTiR (R = dkyl) but there it is a minor reaction. The ~~coordination of the ally1 group apparently limits the number of possible reactions essenti,ally to one. The vinyl denvative Cp2Ti-C(CH3)= C(CH3 )H also gives a clear result but different from thar; of the ally1 compound: Cp2Ti-C(CH3)<(H)CH3
+ CO -
CpaTiG-C(CH+C(H)CHs. 0
cm-’
The acyl can be isolated in good yield and characterized ) as a normal pammagnetic q*-acyl compound,
(Y(CO)
1482
0 It can be oxidized by C&SSCsHs to the diamagnetic tetravalent CpaTi(SCsHs)(CO)-C(CH,)=C(H)CH, (u(C0) 1564 cm-‘) and shows close analogy with R = O-tolyl. The reason For the different course OF the carbonylation reactions for rl”+slIyl and viny1 compounds CpaTiR is not clear.
113
The niobiuiu and tantalum compounds Cp.&R (R = alkyi, aryl) are not Emown. Their complexes CpzMRL can, however, be easiry formed by reaction of endo-olefim-hydride complexes Cp&I(CH,=CHR’)H with dono?acceptor Ligands, L, such as CO or isocyanides [9] : Cp&(CH,aR’)H
- co
Cp2M(CO)R (M = Nb, Ta; R = n-C3H7, n-C,H9,
n-C&Ill).
The u(C0) absorptrons are found at 1880 - 1925 cm-l, indrcattig consrderable back donation horn the metal into the r*;-orbit&s of the CO moEecuIe. 7n consequence, the CO molecule is firmly bound to the metal and the Ligand shows little reactivity. Formation of the acyl compleges Cp&ICOR, commonly found for titanium and vana&um [5, IO], was not observed_ By contrast, the CO complexes CpzM(CO)R are thermally rather robust. Deccmposition tith loss of CO is observed for M = Nb fIom about 130 “C; for M = Ta, decomposition sets Ln at about 150 “C. In this respect they behave like CpzTi(CO)CsFS. The low reactrvity of the Nb and Ta alkyd carbonyl complexes with respect to V IS surprising, since on gomg from 3d to 4d or 5d metal compounds the latter are often more reactive thar, their 3d an&yes (cf. CO reactIons of Cp2MR2, M = Ti, Zr, Hf 12, 3,153 ). On the other hand our results seem to fit in with observations by other workers; e.g., Cp2Nb(CO)COC&I, could be produced from Cp,Nb(C,H,)C,H, by high temperature uld CO pressure, and Cp,Nb(CO)CH, does not react with CO under reflex in benzene in a CO atmosphere [16].
AcknowIedgemenb The authors thank Prof. Dr H. J_ de Liefde Meijer and Prof. Dr F. Jellinek for their stimulating interest. Mrs A. Jonkman-Beuker 1s thanked for her able experimental assistance. This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the ddvancement of Pure Research (ZWO).
References 1 I-L Masai, K. Somgashirn end N. Eiz&arr+ B&L Chem. Sot. JAR. &I (1968) 750 . 2 G. Fechinetti and C. FEotii, J_ Orgammef_ Cfrem , 71 (1974) C5. Tram, (1977) 2297. 3 G. Fachinetti, G Fachi and C. Floriani, J. Cfrem. Sot.. Ddfon 4 G. Fachmetti, C. E’lorizmi. F. Marchatti and S. Merlino_ J. Cfwm Sot.. Chem. Commun_, (1976) 522. 5 G. Fachinetti, S. Del Nero and C. Floristi, J. Cfwm. Sot , Dalfon ‘T’rmzs.. (1976) 203. 121. 6 P. T. Wol czzmski and J. E. Bercaw, Act CFrem. Res.. I3 (l9SC) 7 E. J. M. de Boer and J. H. Teubea, L Urganomef. Chem., I40 (1977) 41.
114 8 F_ vz- &olhuls, E J. _%I.de Boer and J. H. Teaben, 6. OrganomeL Chem.. Z 70 (X979) 599. 9 A. H Klazinga and J. H Teuben, J. r)rgcnomec CFrem.. I84 (1960) 309. 10 E J. M de Boer, L C. ten Gate, A. G. J. Staring and J. H. Teuben. J UrganomeL Chem , Z8Z (1979) 61 11 G J Olthof znd F. vzn Eoihuis, J. Orgunomet Cfiem., I22 (2976) 47. 12 R. Jung& D Sekutowskr, J. Davis, M. Luly and G Stucky. Zrrorg. Cfrem., 16 (1977) 1645. 13 H F. J. vzn der Wal and H R. van der Wd, J. OrganomeL Chem , Z53 (1978) 335. 14 C P Boekel, J_ H. Teuben and H UT.de Liefde Meijer, J Urgancmet. Chem.. BZ (1974) 371. 15 G. Fachinetti, G Fochi and C. Fioriani, J Chem. Sac.. D&OR Trans.. (1977) 1946. 16 R R Schrock .znd G. W. Pan&all, Cizem. Rev., 76 (1976) 261. 17 J. H. Teuben, J. Orgunomet Chem.. 69 (1974) 241 18 E_ Keel end J. H Teuben, J_ Orgmomet. Chem. 188 (1980) 97. 19 F. Vi. van der WVaij,H_ Scholtens and J. H Teubea, J Orgunomet. Chem.. 127 (1977) 299 20 H. A_ Martin 2nd F. Jellinek, J 0:ganomet. Chem., 8 (1967) 115. 21 E. Klei, to be published
CO ACTIVATION BY TITA_NlCtYM, N I O B H J M
DICYCLOPENTADIENYL A~ND TANTALUM
DERIVATIVES
107
OF
J. H T E U B E N , E. J. M. D E B O E R S, A. H. K L A Z I I q G A t and E. IqLE[ Laboralorfum =]oor A n o r ~ a n ~ c h e Chernie. Rijl~su~uersite~ Gronrngen, N[]enborgh 16, 9 747 A G Groningen (The Netherlands)
Summary Exploratory stud/es on reactions of CO (room temperature, 1 atmosph~.re) w i t h C p z T ~ (1% = aryl, a l k y l , allyl, v i n y l ) a n d C p ~ 1 % (M = N b , T a ; 1% = a l k y l ) a r e r e p o r t e d . T h e t i t a n i u m c o m p o u n d s s h o w f o r m a t i o n o f C O c o m p l e x e s C p z T , ( C O ) R f o r 1% = CI, C s F s , a n d o f ~2-acyI c o m p l e x e s CpzTiCO1% f o r 1%= C~Hs, o - C H a C s H 4 , a n d C ( C H ~ ) = C [ H ) C H s . F o r 1% = C6H~, C H s , C H ~ C ( C H s ) s a n d ~ ~-CsHs m o r e c o m p 2cared r e a c t i o n s t a k e p l a c e a n d complete identification of the products was not posmble. The formation of k e t o n e s , 1%CO1%, a n d d i k e t o n e s , RCOCO1%, s h o w t h e f o r m a t i o n o f new- C - - C b o n d s in t h e s e s y s t e m s , p o s s i b l y t h r o u g h t h e r a d i c a l c h a r a c t e r i n d u c e d o n t h e h g a n d s b y b a c k d o n a t i o n o f t h e u n p a i r e d eleetro_a o f t i t a n i u m . 1%eduction of the acyl function and of ketones to the corresponding alcohols was o b s e r v e d . T h e n i o b i u m a n d t a n t a l u m c o m p o u n d s CpzM1% (1% = n-Call 7, n-C4Hg, n ~ [ - I n ) give s t a b l e c o m p l e x e s CpgM(CO)I% ,and f u r t h e r r e a c t m n o f these with CO was n o t observed.
Int, r o d u c t i o n C a r b o n m o n o x i d e activation b y early transition metal c o m p o u n d s has b e e n s h o w n to be a very promising field in C O chemistry. T h e first reports [I - 3] o n C O reacting with C p 2 T i R 2 * ~ n d Cp2TI(R}CI (1% = C H a, Cz£-I5, C H 2 C 6 H s ) s h o w e d , in addition to a facile insertion of the C O molecule into the Ti--C b o n d R n d reductive elimination of a ketone: CpzT~Rz
co
co
zTi~
' C p z T i ( C O ) z + 1%--C--1%
C-
O tprese-t address: Koninkli]ke/ShelILahoratorium, Badhuisweg 3, i031 ClV~Amsterdam. The Netherlands. *Cp = W S ~ s H s , Cp* = ~ S ~ s M e s.
108
new dihupfo
an interesting
bonding
mode of the acyl group, viz.,
This type of bonding has been found to be quite common for early tmnsrtion metals and underhnes the high oxygen affinity of these metals ]%51-
Quite recently elcwt studies by Rercaw on the reaction of Cpz*ZrH, with Cp2*Zr(CCj2 and other metal carbonyls provided 2 clear insight into the mechanism 01 the reduction of CO to C&OH and the formation of the MSH=CH+M complexes u& LM+D+%H mtermediates [S] . Group VB clicyclopentadienyl compounds seem to react differently. Flxiani reported the reaction of Cp2VR with CO ]5] _ Depending on R, CO can be inserted to give Cp,V(CO)COR (R = CHa, CH&Hs) or R is transferred to the cyclopentadienyl ring (R = C&IS):
q-$-; qR Cp,VR
=
Cp-V’
\
-
cp-v-0
(R = C&J.
‘co
The aforementioned starting compounds and products are all even (16 or 18) electron systems. We decided to study the reactions of odd (15) electron systems, Cp,TrR. The unpaired electron can here be transferred by a-back-donation on to the ligand. grving radical character to certain parts of the molecule and possibly. in Consequence, mterestmg reactions. We found such reactions when Cp2T3R (R = aryl) was reacted with cyanides R’CN and isocyanides R’NC. With cyanides, coupling of the ligands W?Zthe Lyanide C-atom was observed with the Formation of [7] : R
R
\ T=P,
NY+ R’ R’
IsocYanides
insert into the Ti-C
bond
going
~2-immoacy~
complexes
[81
-
cp*Ti’ \-xII \
D’
tYe have f&ended our studies to the reaction of CpaTiR (R = a~$, alkyI, a&E, vinyl) with CO. We aLso investigated the reactions of Cp#R (M = N5, Ta; R = ah&) with CO to compare the results with those of CpsTiR and C&VR. Ln the ‘hy-pothetrczl’ compounds Cp&R (M = Nb, Ta), the size of the metals is expected Ltobe about the same as Tr; V is smaller. Steric aspects will be very simiEar for the compounds CpsMR (M = ‘Pi, Nb, Ta) but the eIectronic aspects are quite drfferent. The Nb and Ta complexes have two ekctrons available for back donation whereas the titanium compounds have one. By studying the compIexes we beheve we have a distinct possibihty of fmding an effect which depends on the number of electrons available for back-donation. En this communication we report the results of some exploratory studies. Parts of the study have been pubhshed ehewhere [9,101_ Experimenti Data on experimental conditions, equrpment, and analytical procedures can be found elsewhere ES, IO]. The compounds were prepared following published procedures: Cpe*TKI [IO], Cp,TiR (R = CsF5, C&s, o-CR3CsR4) [17], CpzT~cHs 1181, Cp2TiCH,C(CH3)3 [19], CpzTi(q3-C3H5) [ 20 1, Cp*_MR (M = Nb, Ta) [Sf _ C~,TLC(CH,)~(K)CH, was prepared from (Cp,TiC1), and LiC(C&)= C(H)C&. It was isoWed and fuHy characterized [21].
AJmost all 15electron compounds, Cp,TiR, react easily with CO (25 “C, 1 atm) both in solution and in the solid state. The products mainly depend on the nature of R; other variables such es solvent or temperature seem to be of secondary importance. No reaction takes place with complexes where the vacant site on the metal is inaccessibIe. This cao be for steric reasons (e.g., Cp,Tr-2,8-(CHs)s C&-I3 ElII ) or due to coordination with heteroaton donor Ligands (e-g., in dimeric (CpzTiCI), [X2), or IGIthe iutemahy coordinated compound below [X31.
(CH, );N-&Hz
110 For
skrong
ekctronegatwe
proceeds according Cp,TiR
R groups,
(R = CL*, C6F5),
Cp,Ti(CO)R.
+ CO -
For R = CsFs the adduct could be isolated and characterized Cp*,Ti(CO)C1 is thermally unstable and disproportionatis: 2Cp*2Ti(CO)C1
the reaction
co:
-
Cp*zTi(CO)2
completely;
f Cp*zTiC12.
The adducts are terminal CO complexes showmg v(C0) at 2060 (R = CsFs) and 2020 cm-l (R = Cl) [lo] _ They are comparable with the vanadium complex Cp,VKO)Cl (v(C0) 1950 cm-l) [5] taking into account that Cp,VCl (16 electron system) has two, and the titanium compolxds have only one election available for ;r-bonding with the CO ligand The formation of C~‘,TI(CO)~ from C~*,TI(CO)CI resembles in 2 way the formation of Cp,Ti(CO), from Cp2TiR2 (R = CsHsCH2) and CO. En the latter case zt is thought to result from carbonylation of titanocene formed by the reductive elimination of the ketone R-CO-R from the acyl alkyl compound Cp,Ti(COR)R [2]. In the case of Cp*,Tr(CO)Cl, at least an iAermolecd~ reaction is needed to explain the products_ The chemical reactivity of the CO molecule in these adducts is rather low. Despite the preference of early transition metals for insertion of CO rnto an M-C bond to give an q2-acyl, C-R M’
II ?O
this LSnot obsemed here Instead the CO ligand is easily losr; with formation of the origmal C22T~R (R = C,Fs) by warming a soiution of the adduct under reduced pressure. Less electionegative R groups (aryl, alkyl, allyl, vinyl) at once @ve insertion of CO into the Ti-R bond accompanied by side- and following reactions resulting in very complex mixtures. It is very probable that the Insertion step :s preceded by adduct formation, a process that has been observed in the closely related systems of Cp,TiR v&h cyanides R’CN and isocyamdes R’NC [7,8]. So far the only acy! compound sufficiently stable to isolate and study 1s Cp,TiCO+-CH,CsH,: CF,TI+-CH&,H,
f CO -
Cp,TiCOa-CH3C,H4.
Its stability 1s probably due to the steric mteraction of Ehe orfho methyl group on the phenyl rmg which shields the acyl carbon atom from further interaction with other titanium compounds. The acyl group is q2-bonded to the metal as can be concluded from the very low, characteristic v(C=O) at 1470 cm-l [3]. *In this case the permethyl complex Cp*2TKI It is moncrnerlc In solutior_
(Cp* = $C5hle5)
was studled because
AcyI formation wa.s aIso found for CpzVR (R = CH,, CH,C,H,) [5j. Here the insertion of CO into a V-C bond is accompanied by complexation of an extra CO molecule: cp*VR
f- 2CO -
Cp,V(CO)COR.
For Cp2Tio-CH&H, no further uptake of CO was observed. The avtiability of one electron may be insufficient to give the back donation needed tz~ bind an additional CO molecule, the bonding of the ~~“-acyl may be too strong, but steric reasons may also be responsible for this difference between vanadmm and ttaniurn compounds Cp,VR. Oxidation of CpzTiCo~-CH&H,) with the drsulfrde C&SSCsHs or iodine gives the diamagnetic, tetravalent compounds Cp2Ti(X)CO~-CH&H4 (X = I, C&S). The observed u(C=O) (1570 - 1580 cm-l) is now In a position comparable vnth the closely related Cp,Ti(I)COCHs (v(C=O) 1610 cm-l) 131. The reaction of CO with Cp,TiCsHs is similar but more complicated, and the products could not be identified in detail. The uptake of CO (in the case of Cp2Ti-c4!HsC,HE, exactly I CO/Ti) varies for different runs between 1.3 and 1.8 CO/T?. The i-r. spectrum shows an absorption at 1500 cm-r indiczztive of IJ2-acyl, but the iow solubiiity suggests ohgomeric or polymeric structures with bridging acyls:
B F”P +=-F= -
7”B 7-O
CP
CP
F” TiCp
The reaction of Cp,TiCOR with HCI/ether C6HC the acyl was oxidized quantitatively CpsTiCO-o-CH,C& For R = CsHs a compla CpaTiCOCsHs
+ HCI -
was studred. For R = o-CEI,
Cp2Ti(CI)CO+CH&H4
f&H,.
mixture resulted:
+ HCI -
CpaTiCla + CpaTi(C!)COCsHs
7 organic products.
The T&balance for the products does not add up to 106% and unidenbfied organotitanium compounds are still present. The organic part of the reaction product is very complicated. The presence of C&Is, C,H,-C&E,, C&H&OCOC.&s was demonstiated. The fact that coupling products such as biphenyl and &H&O-COCsHs are present indicate radical character. It is not clear whether this coupling takes place in the reaction with CO or in the HCl reaction. The presence of Cp,TiCl, and Cp,Ti(CI)COC& show - at least in retention of the Cp,Ti structure during the c.arbonyIation and Part -the work up. This is in contrast to the carbonylation of Cp2VCsHs, where the -+--,-I group IS tiansferred to the cyclopentadienyl ring. A very mteresting point is the presence of the diketone C&I&O-COC&, because it suggests a strong relation between the CO reactions in these systems and the Bercaw systems Cp*,Zr&/CO; the mechanisms of the reactions may show close similarity [6].
112
B
8. R-C--C-R
However, other mechanisms are conceivable. Carbonylanon of CpaTiCHs results in a complicated mixture of products. The main difference from the aryl compounds is the formation CP,TI(CO), (33%) and acetone, isopropyl alcohol and ethanol in minor quantities. Trobably, reaction of CpaTiCQCHa with unreacted Cp,TiCH, under CO produces Cp,Ti(CO), and acetone: CpsTiCOCHa
f CpsTiCHs
-
co
CpJi(CO),
of
f CHsCOCHs.
It resembles to some degree the formation of acetone from Cp2Ti(COCHs)Cl with CH$&CI under CO [Z] ; in our case, free CpaTiCH, xvould play the role of the Grignard reagent. The presence of ethanol and 2-propanol indicates a reduction of a carbonyl function during the reaction. Hydrogen Lor this process may be supplied by various sources, e.g , cyclopentadienyl ligands, methyl groups or solvent molecules [ 141 Attempts to control the carbonylation reaction and isolate the acyl CpaTiCOR were undertaken for R = CH&(CHs)s, but were unsuccessful. The results are basic&iy the same as those obtained for CpaTiCH,. Carbonylation of Cp,Ti(r13-C3Hs) (= CpeTi-z-allyl) is a rather simple reaction, CpaTi(CO), and (C,H&CO being the main products. It LS also observed for CpzTiR (R = dkyl) but there it is a minor reaction. The ~~coordination of the ally1 group apparently limits the number of possible reactions essenti,ally to one. The vinyl denvative Cp2Ti-C(CH3)= C(CH3 )H also gives a clear result but different from thar; of the ally1 compound: Cp2Ti-C(CH3)<(H)CH3
+ CO -
CpaTiG-C(CH+C(H)CHs. 0
cm-’
The acyl can be isolated in good yield and characterized ) as a normal pammagnetic q*-acyl compound,
(Y(CO)
1482
0 It can be oxidized by C&SSCsHs to the diamagnetic tetravalent CpaTi(SCsHs)(CO)-C(CH,)=C(H)CH, (u(C0) 1564 cm-‘) and shows close analogy with R = O-tolyl. The reason For the different course OF the carbonylation reactions for rl”+slIyl and viny1 compounds CpaTiR is not clear.
113
The niobiuiu and tantalum compounds Cp.&R (R = alkyi, aryl) are not Emown. Their complexes CpzMRL can, however, be easiry formed by reaction of endo-olefim-hydride complexes Cp&I(CH,=CHR’)H with dono?acceptor Ligands, L, such as CO or isocyanides [9] : Cp&(CH,aR’)H
- co
Cp2M(CO)R (M = Nb, Ta; R = n-C3H7, n-C,H9,
n-C&Ill).
The u(C0) absorptrons are found at 1880 - 1925 cm-l, indrcattig consrderable back donation horn the metal into the r*;-orbit&s of the CO moEecuIe. 7n consequence, the CO molecule is firmly bound to the metal and the Ligand shows little reactivity. Formation of the acyl compleges Cp&ICOR, commonly found for titanium and vana&um [5, IO], was not observed_ By contrast, the CO complexes CpzM(CO)R are thermally rather robust. Deccmposition tith loss of CO is observed for M = Nb fIom about 130 “C; for M = Ta, decomposition sets Ln at about 150 “C. In this respect they behave like CpzTi(CO)CsFS. The low reactrvity of the Nb and Ta alkyd carbonyl complexes with respect to V IS surprising, since on gomg from 3d to 4d or 5d metal compounds the latter are often more reactive thar, their 3d an&yes (cf. CO reactIons of Cp2MR2, M = Ti, Zr, Hf 12, 3,153 ). On the other hand our results seem to fit in with observations by other workers; e.g., Cp2Nb(CO)COC&I, could be produced from Cp,Nb(C,H,)C,H, by high temperature uld CO pressure, and Cp,Nb(CO)CH, does not react with CO under reflex in benzene in a CO atmosphere [16].
AcknowIedgemenb The authors thank Prof. Dr H. J_ de Liefde Meijer and Prof. Dr F. Jellinek for their stimulating interest. Mrs A. Jonkman-Beuker 1s thanked for her able experimental assistance. This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the ddvancement of Pure Research (ZWO).
References 1 I-L Masai, K. Somgashirn end N. Eiz&arr+ B&L Chem. Sot. JAR. &I (1968) 750 . 2 G. Fechinetti and C. FEotii, J_ Orgammef_ Cfrem , 71 (1974) C5. Tram, (1977) 2297. 3 G. Fachinetti, G Fachi and C. Floriani, J. Cfrem. Sot.. Ddfon 4 G. Fachmetti, C. E’lorizmi. F. Marchatti and S. Merlino_ J. Cfwm Sot.. Chem. Commun_, (1976) 522. 5 G. Fachinetti, S. Del Nero and C. Floristi, J. Cfwm. Sot , Dalfon ‘T’rmzs.. (1976) 203. 121. 6 P. T. Wol czzmski and J. E. Bercaw, Act CFrem. Res.. I3 (l9SC) 7 E. J. M. de Boer and J. H. Teubea, L Urganomef. Chem., I40 (1977) 41.
114 8 F_ vz- &olhuls, E J. _%I.de Boer and J. H. Teaben, 6. OrganomeL Chem.. Z 70 (X979) 599. 9 A. H Klazinga and J. H Teuben, J. r)rgcnomec CFrem.. I84 (1960) 309. 10 E J. M de Boer, L C. ten Gate, A. G. J. Staring and J. H. Teuben. J UrganomeL Chem , Z8Z (1979) 61 11 G J Olthof znd F. vzn Eoihuis, J. Orgunomet Cfiem., I22 (2976) 47. 12 R. Jung& D Sekutowskr, J. Davis, M. Luly and G Stucky. Zrrorg. Cfrem., 16 (1977) 1645. 13 H F. J. vzn der Wal and H R. van der Wd, J. OrganomeL Chem , Z53 (1978) 335. 14 C P Boekel, J_ H. Teuben and H UT.de Liefde Meijer, J Urgancmet. Chem.. BZ (1974) 371. 15 G. Fachinetti, G Fochi and C. Fioriani, J Chem. Sac.. D&OR Trans.. (1977) 1946. 16 R R Schrock .znd G. W. Pan&all, Cizem. Rev., 76 (1976) 261. 17 J. H. Teuben, J. Orgunomet Chem.. 69 (1974) 241 18 E_ Keel end J. H Teuben, J_ Orgmomet. Chem. 188 (1980) 97. 19 F. Vi. van der WVaij,H_ Scholtens and J. H Teubea, J Orgunomet. Chem.. 127 (1977) 299 20 H. A_ Martin 2nd F. Jellinek, J 0:ganomet. Chem., 8 (1967) 115. 21 E. Klei, to be published