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Study of the structure and reactivity of the complexes of cyclopentadienyl titanium derivatives with alkylaluminium halides
43
Joumal of !ifolecu&r Catulys& 10 (1981) 43 - 56 0 Ekevim
Sequoia
S4.
Lausame
-Prided
in the Nethedands
V.E.LVOVSKY ‘Phstpolymer’, E. A. FUSIr&ifute
Okktz Research ad
RotiucLi~r- A.sociatio.x, Leniagmd (~~S.S.R.)
a3d F. S. DYACEEROVSKY
of Chemicd
Pkysics. U.S.S.R. Academy of Sciences. Moscow
(U2LS.R.)
(Received Jaauary 10,198O)
Alternative structures of several compIexe5 L2TiRCl-AlC13 (L = Cp, Cl; R = Cl, CH3, CBH5, CsH7) have been considered in the framework of the semiempirical CNDO method. Retention of the bent structure of the Cp2Ti fragment has been found to be a characteristic feature of biscyclopentsdienyl derivatives of titanium. Replacement of the Cp-ligand by a chlorine atom increases the probability of formation of a complex with two bridgkg chlorine atoms and especially a bipyramidal structure. Retention of the Cp,Ti ment structrrre can also be considered in the five-coordinate transition state of ligand exchange betvzen the derivatives of titanium and aluminium. The high catiytic activity of the complex in olefm polymerization correlates witi the ease with which it forms a ‘quasi~ctahedral’ structure which ensures maximum reactivity of tie Ti-C o-bond. Activation of a complex with a bent Cp*Ti &agment in the presence of more “acidic” compounds of aluminium may be connected with its increasing polarity.
At present it is known that the catalytic activity of the ZiegkrNatta type systems in ole6n polymerization is caused by the o-bond between the transition metal azd carbon. This condition is, however, not always sufkient, as there exist isolated o*rganometallic transition meti derivatives v&h either low or no polymerization activity. Some authors 111 b&eve that the activity of these compounds with alkylaluminium halides can be attributed to the formation of “qzlasi-0ctahedml” compkxes with an enhanced &and reactivity in the transition metal coordination sphere. However, quantumchemical studies of such systems [2 - 51, did not cotider tie skudural features of the catalytic complex and their interrelation between the struc-
44 tural features and reactivity without any significant simplifications. This relates primarily to homogeneous systems based on Ti(IV) cyclopentadienyl derivatives and alkylaluminium halides. The structural features of tie complexes formed in this system, as well as their role in the formation of the Ti-C bond and subsequent polymerization, are dealt with here.
Calculational
methods
All valence electron restricted MO-SCF calculations were carried out by the CNDO semiempirical method with different electron rep-&ion parameters for functions of s, p and d type. The parametrization method and choice of atomic orbital radial parts for alkyltitanium and alkylaluminium halides are discussed in ref. 6_ -4ccording +a this work, the energies of the tEUlsition (S, + S,) to lower singlet electronicsxcited states and their oscillator strengths calculated by the configurational interaction methd permit a satisfactiry prediction of the longwave absorption boundary for the simplest alkyltitanium (IV) halides. For example, for TiCl,, this method yields lower transition energies of 4.65 and 5.40 eV, which agree with the experimental long-wave bands with maxima at 4.43 and 5.38 eV [7]. Figure i presents the calculated and experiienti [S] specti for Cp,TiC12* representing the simplest compound containing a cyclopentadienyl ligand. It can be seen here that the method is equally suitable for a semiquantitative characterization of the longwave absorption boun&_r. It should, however, be noted that satisfactory agreement between the calculated spectrum and experimental absorpticn curve can be expected only if the calculations are carried out with the experimental equilibrium *The experimental the czlculations.
equilibrium
Cp2TiC12
--I\ 2-oo D
-I-
I.00
025
geometry
according
to ref. 9 was used in
f
o-01
1
11!L-
O-JO1
L
300
Fig. 1. Comparison mental ahsorption
6co
iI
of the calculated curve for C&lWl2_
emzitation
energies
(vertical
Lines) with
the experi-
45
geometryofthecomplex.I~is~e~Foreadvisable~use~e s/stem’stoti energy obtained by the same method to fiad the equilibrium geomeky of the compounds. For this purpose, by analogy with refs. IO and 11, for fixed electronic parameters, the core interaction potential was calibrated from the geome&y of the simplest represenktives of the compounds under study. The shape of the potential was drawn from ref. 12_ The only variable parameter @AS for each pair of &xn.s was so chosen that the optimal* (with respect to the toti energy) geometry of the compounds used in the e&bration was consistent tith the experimental equilibrium geometry. The prediction ability of the method (for equilibrium geometry) with such a parametrization depends largely on its power to reproduce not so much the bonding energies as their relationship. Table 1 lists the calculated and experimentat atomization energies for the simpkst compounds containing the majority of the bonds of inkrest. As is seen kom the data presented,tberelatiouship between'~ea~mizationenergiesis~maiEntained forhydromrbons_Thechecking~c~tionsindicate thatinthiscasethe bond lengths and angles are reproducibie to within 0.03 A and 2 - 3”, respectively- Agreement between the calculated and ~perimenkl geometries worseas with the increasin g error in the relationships between the atomization energies in the case of alumintim and titanium derivatives. In this case the error in the predicted geometry may be an order of magnitude greater (Table 2). The use of 2 semiempirical method [S, 171, however, demonstrates that the variation in the equilibrium internuclear distances (within 0.1 A) in *Tte
optimization
in coUabor&ion
progmmm e intended with G. E. Ehalimsky.
to fmd such a geome’ky
has been de=zlop=l
TABLE1 Atomizationenergies EA (eV) Compound
E"
EiLJEXt
lcxp~. [I3 H2 CH4 C2H4 c2I-k <=12 c(=l4
HCl
dH3 ma3 75(=I4 cp2Tia2 WCH3
13
ourcdc.
OurCdC.
- 151
4.4s 18.1s 24.36 30.82 2.51 13.35 4.43 8.99 13.26 17B4 108.48 45.82
CNDO/%
CNDOWithd
vwithout d 11.61 49.14 66.15 8&3‘s 6.68 36.25 10.56 19.57 30.01 36.61 297.44 136.31
2.59 2.70 2.72 2.74 2.66 2.72 2.38 2.Z8 2.26 2.05 2.74 2.91
lWithout~ngintoaccountthed-AOonthe(=1eto~.
-
1.20 2.09 2.44 2.35 1.93 1.90 0.83 0.82 OS6 0.35= -
5.33 3.15 1.03 1.28 2.47 0.39 -
2.08
2.40
46
TABLE2 Equilibriumgeome&yofAl2(CH3)6 Parameter R(d=-H) E(Ch-H) R(A_M+)
Experiment[l6] 1 )
1.11.7= 0.002A 2.140 = O_OOCj$ l-957 f 0-003x 2.619 f 0.005A 75.5 5 0.1" 117.3 5 1.5'
R(_%l-CL’) R(.Al-Al:
Al-c-AI C-A-C
CdlCUhtiOU
1.102.h 1.115 A 2.220A 1.965A 2.710A 75.7" 117.9'
molecular models does not affect the qualitative conclusions and is therefore acceptable. As can be seen from Table 1, the parametrization used leads to overrated atomization energies. It does, however, permit satisfactory reproduction of the experimental relationship between these quantities, far better than the standard CNDO/B method [18] .We cm thus use, exercising, of course, the caution usual for semiempirical methods, the cahxlated energies of different configurations of complexes as a relative measur-e for the energy benefit. The structure of complexes
L*TiRCl-
AICI,
First, we shall deal with cyclopentadienyl titanium derivatives (L = Cppentahaptocyclopentadienyl). In the literature there exist two different views on the structure of complexes based on CplTiC12. According to one viewpoint [ 191, complexing takes place via scheme (1) and is not accompanied by any substantial variation in the angles between the normals to planes of the Cp r.gs.
According to the other approach [l] , complexing is accompanied by substantial distortion of the geometry of the Cp,Ti fragment, the resultant complex having a bridge structure with parallel Cp rings: G CL
T/-
CL
‘R
+
ALRCL,
1
c;/),CL
R-Ti
‘CC
A &
xR ‘CL
(2)
47
Taking these hypotheses into account, we calculated the optimal geometry of three alternative struc~s (configurations I and 11 in (I) and ifI in (2)) for each complex of the series Cp,TiRCl. AhIs under study, where R = Cl, CHa, C&IS, C&5,. To save computer time, we discarded the optimization of internal cyclopenkdierryl hgand variables. The geometrical characteristics of this ligand were taken from the experimental structure Cp2TiC12 [9] _ In our cakulations F5e also dkegarded the variations in the internal variables of alkyl ligands and assigned standard values to them [IS] _ The remaining variables, including the Ti-Cp bond length and the angle Cp-T&I@, were varied. The initial angle for configurations I and U psas taken to be 128” and for configuration III, 180”. Figure 2 illustra~s some calculated
Fig. 2. Models
of optimize
d configurations
I, II, Ei of the
complex CpaTi(312~AlCI3.
geometrical characteristics for all three configurations of the complex Cp2TiCl,~NC13. Figure 3 (solid lines) shows the variation in the relative energies of these configurations iu the series under study. The energy of the complex of coufrguration I is taken as the origiu. As w-ill be _en in Fig. 3, the bipyramidal complexes have the highest energy, tie difference in energy for the semiempirical method being fairly large at about 2 eV. Hence, their predominance in sohrtion is hardly possible*. This re_wlt can be extended to compounds of more general formul Cp2TiRCl~A1Rs_,vClNr because even for the strongest complexiug agent, Ah&, the geometical distortion that converts tie complex to a bipyramidal form witi parallel Cp rings, require substantial energy expenditure. The relatively small difference between the energies of configurations I and iI, obtained in the above method, does not permit an unequivocal selection of the most advantageous con@uration of the complex, and does not exclude the simultaneous presence of the two configurations in solution in commensurate concentitions. For small differences in energies of configurations I and PI the decisive factor may be the entropy contribution to the equilibrium constant I =LI, associated with the grea%r rigidity of complex I. In addition, the position of
*At least ior the conventionalLy
used nonpolar
solvents.
48
--
c--___ ------*------
Fig. 3. Relative energies of alternative configurations I, II, III of the complexes Cp~TiRClAlCl~ (solid lines) and Cl~1FRCI-AKl~ (broken lines). The energies of the complexes of in conflgurstion I are taken as the origin_ Tlze subsiituents are arranged on #he absciie an arbi-7 order.
this equilibrium in solution can be substantially influenced by the different degrees of s&&ion of complexes I and II, As the toti energy calculahzd by this method is only a semiquantitative characteristic, we also cekulated, for each co_n!igmation* of the complexes under study, the energies of transition to the lowest singlet electronic excited states and theu correlated them with the absorption spectra. Such a co&tion is illustrated in Fig. 4(a) - (c) by the experiienti 1201 and caktited spectra of the complex Cp2TiC12- Ah&. Analysis of the a~tral data ieads to a conclusion identical with that obtained by comparkn of energies. In fact, for bipymmidal compounds 111the cakulated spectrum w-i’& longwave transitions in the region 800 - 900 nm (Fig. 4(a)) corresponds to a shift of this band, as compared wi+h its position in Cp,TiC12, by 280 - 380 nm, which is far in excess of the otied value of 65 - 70 nm. Within the calculational error the other two configurations have indkcernible specka that agree fairly well with the experjmenti shift in Fig. 4(b), (c)_ The resu% obtained suggest that the complexing of cyclopentadienyl titanium derivatives witi alkylal uminium halides does not involve any substantial distortion of the Cp*Ti kgment and formation of bipyramidal III iype structures*+. *An optimized geometry was used in the spectral calculation for each conflation of the CoDplex. **For the 1:l complexes, w&u a complex is formed with tie AI mol4~ disfzortion of the geometry, as shovn by pmLmiwry calcrrlatious. is more probable.
43
;
Qat
(a) D
zoo \ 1.80
02.5
\\\ ;1\ .-
\
\
f P.01
\ \
0.001
.l_
(b)
600
100
l(nm)
Fig.4.Comprisun ofthe calculated ercitafion energies(verticallines)ofthRe configmatio~~ofthecompI~ Cp~TiCi2-AlCl3(() t ;wQfQudionm;@).couGguratioPEI; (c),co~tion I)ariththe espe&ne~talahsarptioncu~~forthiscompk (solidLine). The broken Line &s@des the eaprimellti &mrption cl.Lmefor cp2Ticl*.
50
Analogous configurations I’, II’ and LII’ were examined for the complexes C&TiRCl-AlCla, where R = Cl, CHs, C2H5, C3HT. Fig%re 5 lisk some calculated geometric parameters of these configurations for the complex C&TiCHaCl- AlCl,. A com_parison of energies of different forms of these complexes in the series (Fig. 3, broken lines) shows that the complexing feature considered above is specific to Ti compounds containing cyclopentadienyl ligands. Replacement of these ligands by chlorine atoms with appreciably :veaker donor properties [ 171 results in a higher acceptor capacity of the ment L2Ti and favours the formation cf complexes with two bridging chlorine atoms. As can be seen in Fig. 3 (curve II’), a linearly’ structured complex with a bridging chlo&e atom is least suitable in energy within the series under study. The relationship between energies of fivecoordination compounds (configurations I and III) varies too (Fig. 3, curve III’), which, as compared with cyclopentadienyl derivatives, results in a substantially higher probability for the presence of bipyramidal complexes of con&u.ration III in solution, especially of size of the alkyl ligand.
Fig. 5. Mod& of optimized configurations I’, II’, III’ of the complex Cl~TiCH3Cl-Al<=13.
Structure and reactivity As a first step, we shti turn our attention to ligand exchange reactions between cyclopentadieuyl titanium derivatives and alky!alumini-um halides. Assuming that they occur via a monomolecukar mechanism [ 211, we *alI correlate our data with the results of kinetic studies. As reported in r&s. 8, 21, these reactions are preceded by the formation of the 1:l complex which then may participate in two parallel processes, namely intramolecular exchange due to the rearrangement in the complex and intermolecular exchange presupposlbg its intern&ion with the monomeric AIEtB_,&lN present in the system. According SKIthe estimates reported in ref. 21, both routes have similar activation parameters: AE’ - 10 - 12 kcal/mol, and AS’ - -30 e. u. Since solvation plays h&y any part in the monomolecular reaction [21], the high negative value of AS’ in this case may be attributed mainly to the formation of 2 cyclic km&ion state with a bridging alkyl group more rigid
than the initial state. The closeness of KS’ then excludes the substantid distortion of #e CpzTi &agment, as suggested in ref. 8, in the transition s&ate of the bimolecular reaction, because the formation of two bridges would othee result in this case in a .substeMiahy greater entropy loss. On the other hand, small differences ic AE* do not admit such distortion in the transition state of the intramoleculer reaction, since it requires substantially greater energies in the I:1 complex. The relatively high absolute values of &SE make it possible to relate the activation bader in both cases w-itb the bond rupture preceding the migration of the ahmhium component or its removal from the complex. Consequently, we can rep*‘esent the infxamolecrrlar alkylation (dealkylation) mechanism as:
(3)
Accordingly,
Q
--
‘R’
6
for the bimolecular
‘ct
+
Ate&t
:
process:
+
AtEtCr, (4)
Both the ligand exchange routes are reversible but the bimolecular mechanism is preferable in the back reaction. Intxamolecular dealkylation requires, as a first stage, the migration of the ahuninium component, accompanied by the replacement of the preferred chlorine bridge by the alkyl bridge, which should lead to a higher energy barrier. The bimolecular route does not require such a migration, so the alkyl group may exist in the bridge without any substantial energy expenditures. Thus, the exchange reactions of alkyl groups demonstrate that it is quite possible for an additional coordination site to be present in *he complex with the bent fragment Cp,Ti:. Let us now turn to the polymerization reaction and examine how the structure of the complex is related to its catalytic activity. Such an activity would obviously be conditioned by the maximum reactiviQ of the metalcarbon o-bond coupled with the existence of a coordination site. We shall evaluate the reactivities of this bond in various forms of the complex, using AE% _ =. as a measure, the latter being the energy Gnsition to the lowest triplet electronsxcited state poszssing antibonding properties with respect to this bond. Such a transition corresponds, to the best extent, to the transfer of an electron tiom participating in the bond forming A0 of
52
titanium to its d-AO. The physical meaning of this parameter becomes clear on comparing the homolytic metal-carbon bond cleavage in Ti and AI alErJr1 halides. The unrestricted Hartree-Fock (UHF) method may be useful for describing the _~ecific features of this proces. It is known 122,231 that the restrict& Hartree-Rock (RHF) me*%od may be u.m&hIe during separation of atoms. the appearance of triplet in&ability of the singlet RHF solution being rela’ted to the beginuGng of orbital splitting of singlet pairs of electins forming the bond. As compared with a compound of a metal of the main group, the instability of this type begins in derivatives of titanium (IV) at a much shorter stretching of the meti-carbon bond [S}. It points to the appearance of a ‘spin density wave’ (SDW) type UHF solution with a lower energy, and is primarily induced by the beginning of localisation of an xipaired electron on the d-A0 of titanium in the &agment being formed*. According to ref. 25, the transition of such un electron to the d-A0 of Ti (III) in the liberated f?agment is capable of promoting homolytic cleavage of the o-bond Ti-C by rxlucing the energy of its dissociation D,-,=. Some data [26,27] compel us to doubt that such a reduction is significant.‘But, for i;nner-sphere reactions proceeding via cleavage, what is more important in our opinion is the formation of a biradical state associated with the re arrangement of the electronic structure of the transition metal, even at a low bond stretching. In other words, we believe that szlch 2 rearran gement takes place partially even in the transition state of these reactions and causes a kinetically perceptible reduction of its energy, i.e. it may be o3e of the main reasons responsible for the higher reactivity of the Ti-C bond as compared with the bond of the basic group metal, ir particular, in polymerization. The nonlocal nature of the bond between the transition metaI and the alkyl group may be an additional factor contibuting to the effect of metal electronic structure rearrangement. As has been shown for the ethyl l&and [6,17], in addition to the Ti-C, interaction, which is generally taken into consideraticn, a perceptible contribution to the total bonding is also made by the interaction of the metal with a saturated fragment CDs. In this case the “triplet instability” and rearran gement of the electronic structure of the transition metal appear only during stretching of the Ti-C, bond. In other words, for a polyatomic ligand there exists a predominant direction for its rupture, wherein the bonding of the metal with saturated fragments is retained to the maximum possible degree, and consequently requires less energy to attain the same degree of rearrangement of the metal sticture.
‘The clarrical picture corresponding ti different spatial localization of electrons with antiparallel spins is m&able i% interpret the motion of eleckous wi& a strong correlation. According to ref. 24, the increase in the spin density on the d-A0 of tihnium during shefxhing of the Ti-C bond shovs that the correlation increases. merefore the phrases “begiunhg oi lhtion of an unpaired electron” or “partial rearrangement of the eleckoric structure of 2 metal” should be more rig-~&y interpreted a5 the apmce of spin density on the d-A0 cf titaaium in the SDW solution
53
For such a iigand, the dative confribution of this rearrangement to the decrease in the energy of the transition state may be expected to be higher than that to the decrease in the dissociation energy. On the basis of the Iacts sMed above, the energy dEs, _. =, of i;he tramition, CalcuMed in the vicini@r of the equU.ibri~ geometry, cm s3me as a relative measure of the effect of the reamangement of the metal sfzucture on the k-an&ion state energy of similar reactions. of the complex. CalcuMLion of bEs0 - T in different co~tioos Cp2TfCJS3Cl- AK& (Table 31 indicAzs that the most favourable structure. in this sense, is configuration III which ensures not ody maximum weakening of the metalabon bond [28], i.e. decreasing D,*, but also its higher reactivity than noncomplexed titanium compounds*. Complexes with such a structure, which appear with a high probability, may be responsible for the high cak&tic activity of the system C&‘I’iR~AlRC12 [ 291. For cyclopentadienyl derivatives, as shorn above, this confition of the complex is practically noctsistent. We shall now discuss the possibility of its appearance when a monomer (ethylene) is included in the coordination environment of titanium, i.e. the formation of a ‘qua.six&.ahedral active cenke suggesti in ref. 1. Calculation of two alternative *ctures (IV and V) of the complex CpBTiCH,CI_4.1C13. C&i, reveals that in this case, too,.the distortion of the Cp,Ti fragment is
*Configuration due to the abserc2
TABLE
I, &o characterized by a re’ratirely of sty coordination site.
cannot
be active
3
Characteristics
of cydopenkdienyl
G3mpor;nci
‘E(N)
@Es cp~TiCH3cl~Alci3
I
II JII Cp~TiCH$ cp,‘IFCH3(=1 mean value cp*EcH,Cl-AlCl~.
qf R
ns
1.6 2.6 1.5 2.2 2.7
0’=j
derivatives Fmgnzent
Trandtion
energy
%e
mmll dEsO-+-,,
(eV)
charge
Bond
length& (A)
QcP:=
QCH,
%X--C
%X-Cl
+0.64 +0.63 +0.67 +1.00 +0.47
-0.12 -0.19 -0.07 -0.13 -0.23
2.26 2.18 2.25 2.14 2.15
2-65 2.41 2.74 2.30
is given for wnfigurations I and IEI
of tke wnpler
stii an u.nfavourabIe process kom an energy point of view. The energy difference between these con@urations of the complex. is about 1 eV. Sime the entropy contribution to the eq-uilibrium cons-tad is Large, m-k displacement of ‘this equilibrium towards configuration IV and its predominant existence in solution can he assumed in this case, too. Figures 6 and 7 show the scale ‘tkeedimensional models of con@urations V and IV of the complex Cp,TiCH,Cl~AlCls~C&i, constructed on the basis of their optimized geometry. It can be seen that the relative arrange ment of alkyl and ethylene in the coordination titanium sphere in V is largely controlled by their stetic interactio~n v&h the hydrogen atoms of the Cp rings. The latter is likely to impair the reaction of monomer insertion into the active Ti-C -bond, requiring their simultaneous displacement in the coordination metal sphere ] 3]_ This fact excludes enhanced activity of such a complex, which would compensate for its low concentration in solution_ The geometrical distortion of ‘the CpsTi ment of configuration IV (Fig. 7) decreases the hindrances created by the hydrogen atoms of the rings; the alkyd and monomer are positioned here closer to each other, so that the insertion naction does not require them to be significantiy displaced. However, such a complex shows little or no difference in reactivity of the Ti-C bond as compared with noncomplexed titanium (IV) derivatives displaying no activity (Table 3). The s~~ctural behaviour discussed explains the absence of catiytic activity for the complex Cp,TiEtCl-AlEtCl, reported in refs. 21 and 30. Probably, one reason for its appearance in the presence of traces of water, leading to the formation of more ‘acidic’ aluminium compounds (e.g. alum-
(4
(b)
Fig. 6. Side ‘dxee4imensional model ((a), top view; (b), front view) of optimized COPfiguration V of the complex Cp2TiCH3(=I~AlCl3-C2H4. +, hydrogen of the Cp ring; &, hydrogen oE ethylene; HE, hydrogen of methyl group.
55
t (b)
(al
Fig. 7. Me three-dimensional model ((a). top view; (b), front view) of the opW configuration IV OF the complex Cp~TiCH3Cl-AICl3 - C2H4.
oxanes) 131,321, isthe a.c~tion of acomplex of conSguration Iv by these compounds. Enhanc& acceptor properties of the dumfnium component in the complex should result in an increased length of the bridging Ti-Cl bond and reduced steric hindrances for monomef coordination. Moreover, an increase in the polzrity of the complex CpzTiR- Cl.XlL, not only promotes titanium-monomer interaction, but also results in higher reactivity of the ti’anium-arbon bond (Table 3). Reference5 1 G. Hemici+live and S. Olive. iLdu. Polym. SC. 6 (1969) 421. Chem. 165 (1979) 303. 2 P. Canour. F. Cramier and I. F. Lahure, d. Oganomet. E. Blaisten-Barojzs. E. Clementi, G. Guinebi and M_ Ruiz-Vizcaya. d_ 3 0. Novzo, Chem BhyL. 68 (1978) 2 337. 4 D. R. Armstrong, R. Fortune aid P. G. Perkins, J Ck~L. 42 (1976) 435. 5 V. I_ Avdeev, I. I. Zakharov, V. A. Zakhzrov, G. D. Bukatov and Yu. I. Ermakov, Zh. Strukt_ Khia.
I8 (1977)
525.
Kood. Khim. 4 (1978) 1662. 6 V. E. Lvovsky, and I. Trebjeg, 7 C. A. L. Becker, C. J. Ballbawn
Theor. Chim rice
(Bed).
13
(1969) 355. 8 W. P. Long and D. S. Breslow, J. Am. Chem Sot. 82 (1960) 1953. 9 I_ A_ Ronom end N. V. Alekseyev, Zh. Shkt. Skim. 18 (1977) 212.
10 V. M_ ‘Iktyak, Khim.
0. V. Sizova end G. V. Kozkevnikow.
V. I. Barano&q,
19 (1978)
594.
_
_
_.
.
Zh
Strukc
56 11 A. V. Band=, (1976)
N. P. Novosyolov
and R. A. -mredov, m
Tear.
Eksper.
Khim.,
12
598.
12 R. J. Boyd and M. A. Whit&end, J. Chen. Sot.. Dalton Trans.. (1972) 73. 13 0. P. Charkin. Outer Orbits mu.?Enofchemica! Bon4 Deposit. in the VINFIT, S.S.S.R_ Acad. SC%.,No. 4418-72. 14 M. B. Smith, J. Oeanomet. Chem.. 70 (1974) 13. 15 V. I. Telnyi and I. E. Rabinovich; Usp. Khim., 8 (1977) 1337. 16 A. Almenningen, S. Halvorsen and A. Haaland, Acta Chem Scud.. 25 (1971) 1937. 17 V. E. Lvcvsky e-rd G. B. Ertmlimsky, Koord. Khim., 2 (1976) 122. 113 J. A. Pople and 9. L. Beveridge, Apprurimute Molecukw Orbitd Theory, McGraw Hill, New York, 1970. D. W. Clack, N. S. Hush and J. S. Yandle, 6. Chem. Phys.. 57 (1972) 3 503. ‘1729. 19 J. W. Lauher and R. Hoffrnamz, b. -4m. Chen. Sot.. 98 (1976) 20 W. P. Long, J_ Am Chem. Sot.. 81 <1959) 5 312. 21 L. N. Sctsnowkaya, E. A_ Fmshman, L. F_ Bmisova and A. N. Shupik, in the press. 22 H. Fclcutome, Frog_ Theoret. Phys. 50 (1973) 1433. 23 K. Yamaguchi, Chem Phys. 29 (1978) 1.17. 24 K. Ymnaguchi and T. Fueno, Chem. Phys.. 19 (1977) 35. 25 P. S. Brakrman and R. J. Cros, J. Chem. Sot., D&on Tnura, (1972) 657. 26 P. J. Davidson, M. F. Lappert and R. Pearce, Chem. Rev.. 76 (1976) 219. 27 R. J. H. Clark, S. Moorhouse and J. A. Stockwell, J. Oqanomet. Chem. Libr., 3 .(1977) 223. 28 0. S. Rctshchupkina, N. E. Khrushch, F. S. Dyachkowky and Yu G. Borodko. Zh. Fiz. Khim.. 45 (1971) 1329. 29 H. Bestien 2nd K. Claus, Angew. Chem.. 75 (1963) 1066. 30 K. R. Meyer and K. H. Reichert, Angew. MakromoL Chem, 12 (1970) 175. 31, G. Fink, Ft. Roftler, D. Schnell and W_ Zoller, J. Appl. PoL Sci. 20 (1976) 2 779. 32 L. F. Borimva, L. N. Scsnovskaya, E. A. Fushmzn and A. N. Shupik, Summ eryoia paper given at the 1st All-Union Conference on OrganomebJlic Chemistry, Moscow, 1979.
Joumal of !ifolecu&r Catulys& 10 (1981) 43 - 56 0 Ekevim
Sequoia
S4.
Lausame
-Prided
in the Nethedands
V.E.LVOVSKY ‘Phstpolymer’, E. A. FUSIr&ifute
Okktz Research ad
RotiucLi~r- A.sociatio.x, Leniagmd (~~S.S.R.)
a3d F. S. DYACEEROVSKY
of Chemicd
Pkysics. U.S.S.R. Academy of Sciences. Moscow
(U2LS.R.)
(Received Jaauary 10,198O)
Alternative structures of several compIexe5 L2TiRCl-AlC13 (L = Cp, Cl; R = Cl, CH3, CBH5, CsH7) have been considered in the framework of the semiempirical CNDO method. Retention of the bent structure of the Cp2Ti fragment has been found to be a characteristic feature of biscyclopentsdienyl derivatives of titanium. Replacement of the Cp-ligand by a chlorine atom increases the probability of formation of a complex with two bridgkg chlorine atoms and especially a bipyramidal structure. Retention of the Cp,Ti ment structrrre can also be considered in the five-coordinate transition state of ligand exchange betvzen the derivatives of titanium and aluminium. The high catiytic activity of the complex in olefm polymerization correlates witi the ease with which it forms a ‘quasi~ctahedral’ structure which ensures maximum reactivity of tie Ti-C o-bond. Activation of a complex with a bent Cp*Ti &agment in the presence of more “acidic” compounds of aluminium may be connected with its increasing polarity.
At present it is known that the catalytic activity of the ZiegkrNatta type systems in ole6n polymerization is caused by the o-bond between the transition metal azd carbon. This condition is, however, not always sufkient, as there exist isolated o*rganometallic transition meti derivatives v&h either low or no polymerization activity. Some authors 111 b&eve that the activity of these compounds with alkylaluminium halides can be attributed to the formation of “qzlasi-0ctahedml” compkxes with an enhanced &and reactivity in the transition metal coordination sphere. However, quantumchemical studies of such systems [2 - 51, did not cotider tie skudural features of the catalytic complex and their interrelation between the struc-
44 tural features and reactivity without any significant simplifications. This relates primarily to homogeneous systems based on Ti(IV) cyclopentadienyl derivatives and alkylaluminium halides. The structural features of tie complexes formed in this system, as well as their role in the formation of the Ti-C bond and subsequent polymerization, are dealt with here.
Calculational
methods
All valence electron restricted MO-SCF calculations were carried out by the CNDO semiempirical method with different electron rep-&ion parameters for functions of s, p and d type. The parametrization method and choice of atomic orbital radial parts for alkyltitanium and alkylaluminium halides are discussed in ref. 6_ -4ccording +a this work, the energies of the tEUlsition (S, + S,) to lower singlet electronicsxcited states and their oscillator strengths calculated by the configurational interaction methd permit a satisfactiry prediction of the longwave absorption boundary for the simplest alkyltitanium (IV) halides. For example, for TiCl,, this method yields lower transition energies of 4.65 and 5.40 eV, which agree with the experimental long-wave bands with maxima at 4.43 and 5.38 eV [7]. Figure i presents the calculated and experiienti [S] specti for Cp,TiC12* representing the simplest compound containing a cyclopentadienyl ligand. It can be seen here that the method is equally suitable for a semiquantitative characterization of the longwave absorption boun&_r. It should, however, be noted that satisfactory agreement between the calculated spectrum and experimental absorpticn curve can be expected only if the calculations are carried out with the experimental equilibrium *The experimental the czlculations.
equilibrium
Cp2TiC12
--I\ 2-oo D
-I-
I.00
025
geometry
according
to ref. 9 was used in
f
o-01
1
11!L-
O-JO1
L
300
Fig. 1. Comparison mental ahsorption
6co
iI
of the calculated curve for C&lWl2_
emzitation
energies
(vertical
Lines) with
the experi-
45
geometryofthecomplex.I~is~e~Foreadvisable~use~e s/stem’stoti energy obtained by the same method to fiad the equilibrium geomeky of the compounds. For this purpose, by analogy with refs. IO and 11, for fixed electronic parameters, the core interaction potential was calibrated from the geome&y of the simplest represenktives of the compounds under study. The shape of the potential was drawn from ref. 12_ The only variable parameter @AS for each pair of &xn.s was so chosen that the optimal* (with respect to the toti energy) geometry of the compounds used in the e&bration was consistent tith the experimental equilibrium geometry. The prediction ability of the method (for equilibrium geometry) with such a parametrization depends largely on its power to reproduce not so much the bonding energies as their relationship. Table 1 lists the calculated and experimentat atomization energies for the simpkst compounds containing the majority of the bonds of inkrest. As is seen kom the data presented,tberelatiouship between'~ea~mizationenergiesis~maiEntained forhydromrbons_Thechecking~c~tionsindicate thatinthiscasethe bond lengths and angles are reproducibie to within 0.03 A and 2 - 3”, respectively- Agreement between the calculated and ~perimenkl geometries worseas with the increasin g error in the relationships between the atomization energies in the case of alumintim and titanium derivatives. In this case the error in the predicted geometry may be an order of magnitude greater (Table 2). The use of 2 semiempirical method [S, 171, however, demonstrates that the variation in the equilibrium internuclear distances (within 0.1 A) in *Tte
optimization
in coUabor&ion
progmmm e intended with G. E. Ehalimsky.
to fmd such a geome’ky
has been de=zlop=l
TABLE1 Atomizationenergies EA (eV) Compound
E"
EiLJEXt
lcxp~. [I3 H2 CH4 C2H4 c2I-k <=12 c(=l4
HCl
dH3 ma3 75(=I4 cp2Tia2 WCH3
13
ourcdc.
OurCdC.
- 151
4.4s 18.1s 24.36 30.82 2.51 13.35 4.43 8.99 13.26 17B4 108.48 45.82
CNDO/%
CNDOWithd
vwithout d 11.61 49.14 66.15 8&3‘s 6.68 36.25 10.56 19.57 30.01 36.61 297.44 136.31
2.59 2.70 2.72 2.74 2.66 2.72 2.38 2.Z8 2.26 2.05 2.74 2.91
lWithout~ngintoaccountthed-AOonthe(=1eto~.
-
1.20 2.09 2.44 2.35 1.93 1.90 0.83 0.82 OS6 0.35= -
5.33 3.15 1.03 1.28 2.47 0.39 -
2.08
2.40
46
TABLE2 Equilibriumgeome&yofAl2(CH3)6 Parameter R(d=-H) E(Ch-H) R(A_M+)
Experiment[l6] 1 )
1.11.7= 0.002A 2.140 = O_OOCj$ l-957 f 0-003x 2.619 f 0.005A 75.5 5 0.1" 117.3 5 1.5'
R(_%l-CL’) R(.Al-Al:
Al-c-AI C-A-C
CdlCUhtiOU
1.102.h 1.115 A 2.220A 1.965A 2.710A 75.7" 117.9'
molecular models does not affect the qualitative conclusions and is therefore acceptable. As can be seen from Table 1, the parametrization used leads to overrated atomization energies. It does, however, permit satisfactory reproduction of the experimental relationship between these quantities, far better than the standard CNDO/B method [18] .We cm thus use, exercising, of course, the caution usual for semiempirical methods, the cahxlated energies of different configurations of complexes as a relative measur-e for the energy benefit. The structure of complexes
L*TiRCl-
AICI,
First, we shall deal with cyclopentadienyl titanium derivatives (L = Cppentahaptocyclopentadienyl). In the literature there exist two different views on the structure of complexes based on CplTiC12. According to one viewpoint [ 191, complexing takes place via scheme (1) and is not accompanied by any substantial variation in the angles between the normals to planes of the Cp r.gs.
According to the other approach [l] , complexing is accompanied by substantial distortion of the geometry of the Cp,Ti fragment, the resultant complex having a bridge structure with parallel Cp rings: G CL
T/-
CL
‘R
+
ALRCL,
1
c;/),CL
R-Ti
‘CC
A &
xR ‘CL
(2)
47
Taking these hypotheses into account, we calculated the optimal geometry of three alternative struc~s (configurations I and 11 in (I) and ifI in (2)) for each complex of the series Cp,TiRCl. AhIs under study, where R = Cl, CHa, C&IS, C&5,. To save computer time, we discarded the optimization of internal cyclopenkdierryl hgand variables. The geometrical characteristics of this ligand were taken from the experimental structure Cp2TiC12 [9] _ In our cakulations F5e also dkegarded the variations in the internal variables of alkyl ligands and assigned standard values to them [IS] _ The remaining variables, including the Ti-Cp bond length and the angle Cp-T&I@, were varied. The initial angle for configurations I and U psas taken to be 128” and for configuration III, 180”. Figure 2 illustra~s some calculated
Fig. 2. Models
of optimize
d configurations
I, II, Ei of the
complex CpaTi(312~AlCI3.
geometrical characteristics for all three configurations of the complex Cp2TiCl,~NC13. Figure 3 (solid lines) shows the variation in the relative energies of these configurations iu the series under study. The energy of the complex of coufrguration I is taken as the origiu. As w-ill be _en in Fig. 3, the bipyramidal complexes have the highest energy, tie difference in energy for the semiempirical method being fairly large at about 2 eV. Hence, their predominance in sohrtion is hardly possible*. This re_wlt can be extended to compounds of more general formul Cp2TiRCl~A1Rs_,vClNr because even for the strongest complexiug agent, Ah&, the geometical distortion that converts tie complex to a bipyramidal form witi parallel Cp rings, require substantial energy expenditure. The relatively small difference between the energies of configurations I and iI, obtained in the above method, does not permit an unequivocal selection of the most advantageous con@uration of the complex, and does not exclude the simultaneous presence of the two configurations in solution in commensurate concentitions. For small differences in energies of configurations I and PI the decisive factor may be the entropy contribution to the equilibrium constant I =LI, associated with the grea%r rigidity of complex I. In addition, the position of
*At least ior the conventionalLy
used nonpolar
solvents.
48
--
c--___ ------*------
Fig. 3. Relative energies of alternative configurations I, II, III of the complexes Cp~TiRClAlCl~ (solid lines) and Cl~1FRCI-AKl~ (broken lines). The energies of the complexes of in conflgurstion I are taken as the origin_ Tlze subsiituents are arranged on #he absciie an arbi-7 order.
this equilibrium in solution can be substantially influenced by the different degrees of s&&ion of complexes I and II, As the toti energy calculahzd by this method is only a semiquantitative characteristic, we also cekulated, for each co_n!igmation* of the complexes under study, the energies of transition to the lowest singlet electronic excited states and theu correlated them with the absorption spectra. Such a co&tion is illustrated in Fig. 4(a) - (c) by the experiienti 1201 and caktited spectra of the complex Cp2TiC12- Ah&. Analysis of the a~tral data ieads to a conclusion identical with that obtained by comparkn of energies. In fact, for bipymmidal compounds 111the cakulated spectrum w-i’& longwave transitions in the region 800 - 900 nm (Fig. 4(a)) corresponds to a shift of this band, as compared wi+h its position in Cp,TiC12, by 280 - 380 nm, which is far in excess of the otied value of 65 - 70 nm. Within the calculational error the other two configurations have indkcernible specka that agree fairly well with the experjmenti shift in Fig. 4(b), (c)_ The resu% obtained suggest that the complexing of cyclopentadienyl titanium derivatives witi alkylal uminium halides does not involve any substantial distortion of the Cp*Ti kgment and formation of bipyramidal III iype structures*+. *An optimized geometry was used in the spectral calculation for each conflation of the CoDplex. **For the 1:l complexes, w&u a complex is formed with tie AI mol4~ disfzortion of the geometry, as shovn by pmLmiwry calcrrlatious. is more probable.
43
;
Qat
(a) D
zoo \ 1.80
02.5
\\\ ;1\ .-
\
\
f P.01
\ \
0.001
.l_
(b)
600
100
l(nm)
Fig.4.Comprisun ofthe calculated ercitafion energies(verticallines)ofthRe configmatio~~ofthecompI~ Cp~TiCi2-AlCl3(() t ;wQfQudionm;@).couGguratioPEI; (c),co~tion I)ariththe espe&ne~talahsarptioncu~~forthiscompk (solidLine). The broken Line &s@des the eaprimellti &mrption cl.Lmefor cp2Ticl*.
50
Analogous configurations I’, II’ and LII’ were examined for the complexes C&TiRCl-AlCla, where R = Cl, CHs, C2H5, C3HT. Fig%re 5 lisk some calculated geometric parameters of these configurations for the complex C&TiCHaCl- AlCl,. A com_parison of energies of different forms of these complexes in the series (Fig. 3, broken lines) shows that the complexing feature considered above is specific to Ti compounds containing cyclopentadienyl ligands. Replacement of these ligands by chlorine atoms with appreciably :veaker donor properties [ 171 results in a higher acceptor capacity of the ment L2Ti and favours the formation cf complexes with two bridging chlorine atoms. As can be seen in Fig. 3 (curve II’), a linearly’ structured complex with a bridging chlo&e atom is least suitable in energy within the series under study. The relationship between energies of fivecoordination compounds (configurations I and III) varies too (Fig. 3, curve III’), which, as compared with cyclopentadienyl derivatives, results in a substantially higher probability for the presence of bipyramidal complexes of con&u.ration III in solution, especially of size of the alkyl ligand.
Fig. 5. Mod& of optimized configurations I’, II’, III’ of the complex Cl~TiCH3Cl-Al<=13.
Structure and reactivity As a first step, we shti turn our attention to ligand exchange reactions between cyclopentadieuyl titanium derivatives and alky!alumini-um halides. Assuming that they occur via a monomolecukar mechanism [ 211, we *alI correlate our data with the results of kinetic studies. As reported in r&s. 8, 21, these reactions are preceded by the formation of the 1:l complex which then may participate in two parallel processes, namely intramolecular exchange due to the rearrangement in the complex and intermolecular exchange presupposlbg its intern&ion with the monomeric AIEtB_,&lN present in the system. According SKIthe estimates reported in ref. 21, both routes have similar activation parameters: AE’ - 10 - 12 kcal/mol, and AS’ - -30 e. u. Since solvation plays h&y any part in the monomolecular reaction [21], the high negative value of AS’ in this case may be attributed mainly to the formation of 2 cyclic km&ion state with a bridging alkyl group more rigid
than the initial state. The closeness of KS’ then excludes the substantid distortion of #e CpzTi &agment, as suggested in ref. 8, in the transition s&ate of the bimolecular reaction, because the formation of two bridges would othee result in this case in a .substeMiahy greater entropy loss. On the other hand, small differences ic AE* do not admit such distortion in the transition state of the intramoleculer reaction, since it requires substantially greater energies in the I:1 complex. The relatively high absolute values of &SE make it possible to relate the activation bader in both cases w-itb the bond rupture preceding the migration of the ahmhium component or its removal from the complex. Consequently, we can rep*‘esent the infxamolecrrlar alkylation (dealkylation) mechanism as:
(3)
Accordingly,
Q
--
‘R’
6
for the bimolecular
‘ct
+
Ate&t
:
process:
+
AtEtCr, (4)
Both the ligand exchange routes are reversible but the bimolecular mechanism is preferable in the back reaction. Intxamolecular dealkylation requires, as a first stage, the migration of the ahuninium component, accompanied by the replacement of the preferred chlorine bridge by the alkyl bridge, which should lead to a higher energy barrier. The bimolecular route does not require such a migration, so the alkyl group may exist in the bridge without any substantial energy expenditures. Thus, the exchange reactions of alkyl groups demonstrate that it is quite possible for an additional coordination site to be present in *he complex with the bent fragment Cp,Ti:. Let us now turn to the polymerization reaction and examine how the structure of the complex is related to its catalytic activity. Such an activity would obviously be conditioned by the maximum reactiviQ of the metalcarbon o-bond coupled with the existence of a coordination site. We shall evaluate the reactivities of this bond in various forms of the complex, using AE% _ =. as a measure, the latter being the energy Gnsition to the lowest triplet electronsxcited state poszssing antibonding properties with respect to this bond. Such a transition corresponds, to the best extent, to the transfer of an electron tiom participating in the bond forming A0 of
52
titanium to its d-AO. The physical meaning of this parameter becomes clear on comparing the homolytic metal-carbon bond cleavage in Ti and AI alErJr1 halides. The unrestricted Hartree-Fock (UHF) method may be useful for describing the _~ecific features of this proces. It is known 122,231 that the restrict& Hartree-Rock (RHF) me*%od may be u.m&hIe during separation of atoms. the appearance of triplet in&ability of the singlet RHF solution being rela’ted to the beginuGng of orbital splitting of singlet pairs of electins forming the bond. As compared with a compound of a metal of the main group, the instability of this type begins in derivatives of titanium (IV) at a much shorter stretching of the meti-carbon bond [S}. It points to the appearance of a ‘spin density wave’ (SDW) type UHF solution with a lower energy, and is primarily induced by the beginning of localisation of an xipaired electron on the d-A0 of titanium in the &agment being formed*. According to ref. 25, the transition of such un electron to the d-A0 of Ti (III) in the liberated f?agment is capable of promoting homolytic cleavage of the o-bond Ti-C by rxlucing the energy of its dissociation D,-,=. Some data [26,27] compel us to doubt that such a reduction is significant.‘But, for i;nner-sphere reactions proceeding via cleavage, what is more important in our opinion is the formation of a biradical state associated with the re arrangement of the electronic structure of the transition metal, even at a low bond stretching. In other words, we believe that szlch 2 rearran gement takes place partially even in the transition state of these reactions and causes a kinetically perceptible reduction of its energy, i.e. it may be o3e of the main reasons responsible for the higher reactivity of the Ti-C bond as compared with the bond of the basic group metal, ir particular, in polymerization. The nonlocal nature of the bond between the transition metaI and the alkyl group may be an additional factor contibuting to the effect of metal electronic structure rearrangement. As has been shown for the ethyl l&and [6,17], in addition to the Ti-C, interaction, which is generally taken into consideraticn, a perceptible contribution to the total bonding is also made by the interaction of the metal with a saturated fragment CDs. In this case the “triplet instability” and rearran gement of the electronic structure of the transition metal appear only during stretching of the Ti-C, bond. In other words, for a polyatomic ligand there exists a predominant direction for its rupture, wherein the bonding of the metal with saturated fragments is retained to the maximum possible degree, and consequently requires less energy to attain the same degree of rearrangement of the metal sticture.
‘The clarrical picture corresponding ti different spatial localization of electrons with antiparallel spins is m&able i% interpret the motion of eleckous wi& a strong correlation. According to ref. 24, the increase in the spin density on the d-A0 of tihnium during shefxhing of the Ti-C bond shovs that the correlation increases. merefore the phrases “begiunhg oi lhtion of an unpaired electron” or “partial rearrangement of the eleckoric structure of 2 metal” should be more rig-~&y interpreted a5 the apmce of spin density on the d-A0 cf titaaium in the SDW solution
53
For such a iigand, the dative confribution of this rearrangement to the decrease in the energy of the transition state may be expected to be higher than that to the decrease in the dissociation energy. On the basis of the Iacts sMed above, the energy dEs, _. =, of i;he tramition, CalcuMed in the vicini@r of the equU.ibri~ geometry, cm s3me as a relative measure of the effect of the reamangement of the metal sfzucture on the k-an&ion state energy of similar reactions. of the complex. CalcuMLion of bEs0 - T in different co~tioos Cp2TfCJS3Cl- AK& (Table 31 indicAzs that the most favourable structure. in this sense, is configuration III which ensures not ody maximum weakening of the metalabon bond [28], i.e. decreasing D,*, but also its higher reactivity than noncomplexed titanium compounds*. Complexes with such a structure, which appear with a high probability, may be responsible for the high cak&tic activity of the system C&‘I’iR~AlRC12 [ 291. For cyclopentadienyl derivatives, as shorn above, this confition of the complex is practically noctsistent. We shall now discuss the possibility of its appearance when a monomer (ethylene) is included in the coordination environment of titanium, i.e. the formation of a ‘qua.six&.ahedral active cenke suggesti in ref. 1. Calculation of two alternative *ctures (IV and V) of the complex CpBTiCH,CI_4.1C13. C&i, reveals that in this case, too,.the distortion of the Cp,Ti fragment is
*Configuration due to the abserc2
TABLE
I, &o characterized by a re’ratirely of sty coordination site.
cannot
be active
3
Characteristics
of cydopenkdienyl
G3mpor;nci
‘E(N)
@Es cp~TiCH3cl~Alci3
I
II JII Cp~TiCH$ cp,‘IFCH3(=1 mean value cp*EcH,Cl-AlCl~.
qf R
ns
1.6 2.6 1.5 2.2 2.7
0’=j
derivatives Fmgnzent
Trandtion
energy
%e
mmll dEsO-+-,,
(eV)
charge
Bond
length& (A)
QcP:=
QCH,
%X--C
%X-Cl
+0.64 +0.63 +0.67 +1.00 +0.47
-0.12 -0.19 -0.07 -0.13 -0.23
2.26 2.18 2.25 2.14 2.15
2-65 2.41 2.74 2.30
is given for wnfigurations I and IEI
of tke wnpler
stii an u.nfavourabIe process kom an energy point of view. The energy difference between these con@urations of the complex. is about 1 eV. Sime the entropy contribution to the eq-uilibrium cons-tad is Large, m-k displacement of ‘this equilibrium towards configuration IV and its predominant existence in solution can he assumed in this case, too. Figures 6 and 7 show the scale ‘tkeedimensional models of con@urations V and IV of the complex Cp,TiCH,Cl~AlCls~C&i, constructed on the basis of their optimized geometry. It can be seen that the relative arrange ment of alkyl and ethylene in the coordination titanium sphere in V is largely controlled by their stetic interactio~n v&h the hydrogen atoms of the Cp rings. The latter is likely to impair the reaction of monomer insertion into the active Ti-C -bond, requiring their simultaneous displacement in the coordination metal sphere ] 3]_ This fact excludes enhanced activity of such a complex, which would compensate for its low concentration in solution_ The geometrical distortion of ‘the CpsTi ment of configuration IV (Fig. 7) decreases the hindrances created by the hydrogen atoms of the rings; the alkyd and monomer are positioned here closer to each other, so that the insertion naction does not require them to be significantiy displaced. However, such a complex shows little or no difference in reactivity of the Ti-C bond as compared with noncomplexed titanium (IV) derivatives displaying no activity (Table 3). The s~~ctural behaviour discussed explains the absence of catiytic activity for the complex Cp,TiEtCl-AlEtCl, reported in refs. 21 and 30. Probably, one reason for its appearance in the presence of traces of water, leading to the formation of more ‘acidic’ aluminium compounds (e.g. alum-
(4
(b)
Fig. 6. Side ‘dxee4imensional model ((a), top view; (b), front view) of optimized COPfiguration V of the complex Cp2TiCH3(=I~AlCl3-C2H4. +, hydrogen of the Cp ring; &, hydrogen oE ethylene; HE, hydrogen of methyl group.
55
t (b)
(al
Fig. 7. Me three-dimensional model ((a). top view; (b), front view) of the opW configuration IV OF the complex Cp~TiCH3Cl-AICl3 - C2H4.
oxanes) 131,321, isthe a.c~tion of acomplex of conSguration Iv by these compounds. Enhanc& acceptor properties of the dumfnium component in the complex should result in an increased length of the bridging Ti-Cl bond and reduced steric hindrances for monomef coordination. Moreover, an increase in the polzrity of the complex CpzTiR- Cl.XlL, not only promotes titanium-monomer interaction, but also results in higher reactivity of the ti’anium-arbon bond (Table 3). Reference5 1 G. Hemici+live and S. Olive. iLdu. Polym. SC. 6 (1969) 421. Chem. 165 (1979) 303. 2 P. Canour. F. Cramier and I. F. Lahure, d. Oganomet. E. Blaisten-Barojzs. E. Clementi, G. Guinebi and M_ Ruiz-Vizcaya. d_ 3 0. Novzo, Chem BhyL. 68 (1978) 2 337. 4 D. R. Armstrong, R. Fortune aid P. G. Perkins, J Ck~L. 42 (1976) 435. 5 V. I_ Avdeev, I. I. Zakharov, V. A. Zakhzrov, G. D. Bukatov and Yu. I. Ermakov, Zh. Strukt_ Khia.
I8 (1977)
525.
Kood. Khim. 4 (1978) 1662. 6 V. E. Lvovsky, and I. Trebjeg, 7 C. A. L. Becker, C. J. Ballbawn
Theor. Chim rice
(Bed).
13
(1969) 355. 8 W. P. Long and D. S. Breslow, J. Am. Chem Sot. 82 (1960) 1953. 9 I_ A_ Ronom end N. V. Alekseyev, Zh. Shkt. Skim. 18 (1977) 212.
10 V. M_ ‘Iktyak, Khim.
0. V. Sizova end G. V. Kozkevnikow.
V. I. Barano&q,
19 (1978)
594.
_
_
_.
.
Zh
Strukc
56 11 A. V. Band=, (1976)
N. P. Novosyolov
and R. A. -mredov, m
Tear.
Eksper.
Khim.,
12
598.
12 R. J. Boyd and M. A. Whit&end, J. Chen. Sot.. Dalton Trans.. (1972) 73. 13 0. P. Charkin. Outer Orbits mu.?Enofchemica! Bon4 Deposit. in the VINFIT, S.S.S.R_ Acad. SC%.,No. 4418-72. 14 M. B. Smith, J. Oeanomet. Chem.. 70 (1974) 13. 15 V. I. Telnyi and I. E. Rabinovich; Usp. Khim., 8 (1977) 1337. 16 A. Almenningen, S. Halvorsen and A. Haaland, Acta Chem Scud.. 25 (1971) 1937. 17 V. E. Lvcvsky e-rd G. B. Ertmlimsky, Koord. Khim., 2 (1976) 122. 113 J. A. Pople and 9. L. Beveridge, Apprurimute Molecukw Orbitd Theory, McGraw Hill, New York, 1970. D. W. Clack, N. S. Hush and J. S. Yandle, 6. Chem. Phys.. 57 (1972) 3 503. ‘1729. 19 J. W. Lauher and R. Hoffrnamz, b. -4m. Chen. Sot.. 98 (1976) 20 W. P. Long, J_ Am Chem. Sot.. 81 <1959) 5 312. 21 L. N. Sctsnowkaya, E. A_ Fmshman, L. F_ Bmisova and A. N. Shupik, in the press. 22 H. Fclcutome, Frog_ Theoret. Phys. 50 (1973) 1433. 23 K. Yamaguchi, Chem Phys. 29 (1978) 1.17. 24 K. Ymnaguchi and T. Fueno, Chem. Phys.. 19 (1977) 35. 25 P. S. Brakrman and R. J. Cros, J. Chem. Sot., D&on Tnura, (1972) 657. 26 P. J. Davidson, M. F. Lappert and R. Pearce, Chem. Rev.. 76 (1976) 219. 27 R. J. H. Clark, S. Moorhouse and J. A. Stockwell, J. Oqanomet. Chem. Libr., 3 .(1977) 223. 28 0. S. Rctshchupkina, N. E. Khrushch, F. S. Dyachkowky and Yu G. Borodko. Zh. Fiz. Khim.. 45 (1971) 1329. 29 H. Bestien 2nd K. Claus, Angew. Chem.. 75 (1963) 1066. 30 K. R. Meyer and K. H. Reichert, Angew. MakromoL Chem, 12 (1970) 175. 31, G. Fink, Ft. Roftler, D. Schnell and W_ Zoller, J. Appl. PoL Sci. 20 (1976) 2 779. 32 L. F. Borimva, L. N. Scsnovskaya, E. A. Fushmzn and A. N. Shupik, Summ eryoia paper given at the 1st All-Union Conference on OrganomebJlic Chemistry, Moscow, 1979.