Journal of Molecular Structure, 93 (1983) 3 3 3 - - 3 3 9 THEOCHEM Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THEORETICAL INVESTIGATION ON HYDROFORMYLATIONREACTIONS. I I I .
V. BELLAGAMBA, R. ERCOLI and A. GAMBA
I s t i t u t o di Chimica Fisica, Universita di Sassari, Via Vienna 2, 1-07100 Sassari (Italy)
ABSTRACT The formation of possible intermediates from a hydrocarbonyliron and propylene, the simplest model system of an o l e f i n with an asymmetric double bond, was in vestigated. Extended H~ckel calculations predict four stable isomers of the ~ complex HpFe(CO)R-propylene generated by reciprocal rotation of the component mo i e t i e s . T~eir optimized geometries were calculated, and t h e i r s t a b i l i z a t i o n curT ves turned out to have no b a r r i e r s . The minimum energies of the conformers are s t r i c t l y comparable, and t h e i r interconversion path was determined. Two of these isomers seem apter to turn into a linear a l k y l and the other ones are more favou rable to a branched a l k y l .
INTRODUCTION The mechanism of Reppe synthesis ( r e f . 1 ) catalyzed by hydrocarbonylirons was recently investigated ( r e f . 2 ) by theoretical computations at Extended H~ckel Theory (EHT) level of approximation ( r e f . 3 ) . In our previous paper ( r e f . 2 ) H2Fe(CO)4 and HFe(CO)4, which were indicated by experimentalists as active catal y t i c species in the reaction ( r e f . 4 ) , were allowed to interact with ethylene, the simplest model system with a double CC bond. No stable complexes were predi~ ted, thus we turned our attention to the possible c a t a l y t i c a c t i v i t y of CO d e f i cient hydrocarbonylirons deriving from H2Fe(CO)4 and HFe(CO)~._ H2Fe(CO)3 and HFe(CO)3 turned out to be able to coordinate ethylene giving stable ~-complexes. Their geometries were determined through an optimization process of the most relevant geometrical parameters, in p a r t i c u l a r the ~-complex formed by ethylene and H2Fe(CO)3 resulted to be more stable than the corresponding one with HFe(CO~ In the present work we study the interaction between H2Fe(CO)3 and propylene,
*For parts I and I I see refs.5 and 2. 0166-1280/83/$03.00
© 1983 Elsevier Science Publishers B.V.
the simplest olefin with an asymmetrically substituted double bond. This enables us to compare the behaviour of the same hydrocarbonyliron toward different olefins and to investigate, at the ~-complex level, the isomerization that occurs in the final reaction products.
METHOD OF CALCULATION EHT was employed in its standard formulation (ref.3). Both the double C parametrization and the energy minimization technique adopted are reported in ref. 2. This computational method is particularly convenient when a large number of geometrical parameters has to be included in the geometry optimization process. Moreover, i t proved to give a satisfactory agreement with the experimental data on the geometries of small transition metal hydrocarbonyls(refs.2 and 5) available in the literature.
RESULTS AND DISCUSSION Since H2Fe(CO)3 gave the more stable complex with ethylene (ref.2), i t seemed reasonable to choose i t as active species to coordinate propylene. The optimized geometrical parameters of this hydrocarbonyliron (I in Fig.l) and of the olefin (2 in Fig.l) are reported in Table I.
Fig.1. Geometries of isolated H2Fe(CO)3 (|)and propylene (2). The formation of a complex obtained from the two fragments described was studied letting propylene approach I on its less sterica]ly hindered side. The energy was optimized at several values of the distance between Fe and the midpoint A of the CC double bond. Four ways of approach were found, depending on the reciprocal orientation of I and 2. The two fragments, while getting closer to eachothe~ tend to place themselves in such a way that the CC double bond nearly eclipses
one of the equatorial carbonyls and the opposite hydrogen atom in I. .~ X
Fe~--.--- ....- H
~ / C
C~~ H 1
H H Fig. 2. Geometries of two stable isomers of the H2Fe(CO)3-propylene complex.
Stable intermediates are formed, due to a x-interaction between the two frag ments. The complexes 3 and 4 shown in Fig.2 are representative of this result. In 4 the olefin is rotated around the FeA axis of about 180° with respect to 3. The tota] energy curves of 3 and 4 VSo the FeA distance are depicted, in Fig.3 and show that the formation of the complex takes place without any barrier. The
Fig. 3. Trend of total energy vs. the distance r between iron and the midpoint of the propylene double bond for complexes 3 and 4 (see Fig.2).
stabilization energies with respect to propylene and H2Fe(CO)3 isolated are 7.5 kcal mol-I for 3 and 6.9 kcal mol-I for 4, the minima corresponding to FeA=2.20 and 2.25 A. While the fragments get closer, the hydrocarbonyl rearranges slightly, since one of the equatorial carbonyls and one hydrogen atom are repelled by the eclipsing CIC2 bond. As for propylene, the double bond lengthens from 1.24 A to 1.35 and 1.32 A in 3 and 4 respectively, and the hydrogen atoms of CH2 and CH groups are no longer coplanar with CIC2C3 because of the significant change in the hybridization of the olefinic carbons. The optimized geometrical parameters in the minima and the corresponding energies are collected in Table I.
TABLE 1 Optimized geometrical parameters a f o r complexes 3 and 4 and for 2 at i n f i n i t e distance.
fragments ! and
C FeC eq eq
bDistance in A, angles in degrees. Experimental values f o r propylene (in parentheses) are taken from r e f . 6 , from which the fixed values of C2C3= 1.501 and CH= 1.09 were deducted. ~See r e f . 2 . OE (1) + E (2).
As previously mentioned, the t r a j e c t o r i e s found f o r the approaching of | and 2 are four, and two of them have just been described. The two remaining ways f o r the formation of a H2Fe(CO)3-propylene intermediate lead to complexes 5 and 6 which are very s i m i l a r to 3 and 4 respectively. In p a r t i c u l a r , 3 and 5 have
the carbon atom CI eclipsing an equatorial carbonyl and C2 eclipsing a hydride, and they differ for the methyl group orientation. Vice versa, in 4 and 6, C2 lies near a carbonyl, while again CH3 is differently oriented in the two isomers. Letting propylene rotate around the FeA axis, the confermers 3, 5, 4 and 6 are formed in sequence. The trend of the total energy vs. the dihedra] angle =CIAFeCeq is shown in Fig. 4, where the four energy minima are separated by E(eV)
o s ~ , s ~
Fig. 4. Trend of the total energy of the complex H2Fe(CO)3-propylene vs. ~= dihedral angle CIAFeCe_, with minima corresponding to structures 3, 5, 4 and 6. The projections of the s~able isomers and of the transition states on an equatorial plane are sketched as wet1. The energyofthe 21st occupied MOin the extremum points is shown at the bottom.
338 the t r a n s i t i o n states l , 8, 9 and |0, and the energy barriers range between 2.1 and 3.5 kcal mol - I . In Fig. 4 the projections of the complex on the equatorial plane xy in the minima and in the saddle points are sketched showing that s t e r i c a l hindrance be tween I and 2 is a d r i v i n g f a c t o r f o r the shape of the energy curve. The optim~ zed values of the most s i g n i f i c a n t geometrical parameters in these points are reported in Table 2 together with t h e i r energies.
TABLE 2 The most s i g n i f i c a n t optimized variables f o r isomers and t r a n s i t i o n states the
~-complex (see Fig. 4).
In order to get f u r t h e r i n s i g h t in the nature of the m e t a l - o l e f i n bond
the complex, the occupied MO's were analyzed. Among the o r b i t a l s that mainly con t r i b u t e to the bonding there are the 27th MO (a mixture of the d
AO of Fe and
the Pz AO of C2), and the 26th MO (an overlap of dz2 and the ~ o r b i t a l of propylene) whose energy contour map is c l o s e l y related to that of the 23rd MO of the complex H2Fe(CO)3 with ethylene (see Fig. 6 in r e f . 2). Another important MO is the low l y i n g 21st, an in-phase overlap of the d 2 AO of Fe and the a MO of z the propylene double bond. The same 21st MO is also determining f o r the energy barriers due to the reciprocal r o t a t i o n about the FeA axis in the complex. I t s energy levels in the minima and in the saddle points are shown in Fig. 4, and a s t r i k i n g s i m i l a r i t y with the trend of the t o t a l energy [email protected]
can be noticed.
339 A comparison between the complex formed by H2Fe(CO)3 with propylene and with ethylene ( r e f . 2) points out several analogies, such as
the p o s i t i o n of the CC
double bond e c l i p s i n g an equatorial carbonyl and a hydride, the o r b i t a l s respons i b l e f o r the binding, the shape of the energy curve vs. FeA. On the other hand, the s t a b i l i z a t i o n energy is 12 kcal mo1-1 f o r ethylene, lower than in the case of propylene, and t h i s can be ascribed to a larger deloca]ization of the ~-syste~ in the l a t t e r o ] e f i n . In the hydroformylation reaction scheme ( r e f .
I) the formation of a c a t a l y t i c
p r e c u r s o r - o l e f i n intermediate is followed by the migration of an H atom from the t r a n s i t i o n metal carbonyl to the substrate, while the metal is bound to one of the o l e f i n i c carbons. Considering the complex H2Fe(CO)3-propylene, the stable i somers 3 and 5 are s t e r i c a l l y apter to promote a migration of the hydrogen towards C2, while 4 and 6 are more favourable to a migration towards CI . Since the f i n a l reaction product is a mixture of l i n e a r and branched aldehydes, generally with a normal/iso r a t i o ~ I ( r e f s .
I and 4), 3 and 5 might be re-
garded as the precursors of the l i n e a r and 4 and 6 of the branched products. The data reported in Table 2 show that the minimum energies of the four isomers are c l o s e l y comparable, 3 and 5 being the most stable ones. The energy d i f ferences are very small, and we feel that no computational method is r e l i a b l e , at present, in the p r e d i c t i o n of such small ~E values f o r these large systems. However i t can be presumed t h a t the presence of a bulky substituent in the olef i n or in the c a t a l y t i c precursor can deeply modify the energy p r o f i l e shown in Fig. 4, in the sense t h a t one or more r o t a t i o n a l isomers might be s i g n i f i c a n t l y favoured with respect to the remaining ones. The influence of a large ligand on the energy curve is an obvious development of t h i s i n v e s t i g a t i o n .
P. Pino, F. Piacenti and M. Bianchi, in I . Wender and P. Pino (Eds.), Organic Syntheses via Metal Carbonyls, Vol. I I , Wiley, New York, 1977, 199 pp. 2 V. Bellagamba, R. Ercoli and A. Gamba, J. Organometal. Chem., 235 (1982) 201. 3 R. Hoffmann, J. Chem. Phys., 39 (1963) 1397. 4 H.C. Kang, C.H. Mauldin, T. Cole, W. Slegeir, K. Cann and R. P e t t i t , J. Am. Chem. Soc., 99 (1977) 8323. 5 V. Bellagamba, R. E r c o l i , A. Gamba and G.B. S u f f r i t t i , J. Organometal. Chem., 190 (1980) 381. 6 D.R. Lide Jr. and D. Christensen, J. Chem. Phys., 35 (1961) 1374.