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Theoretical studies of (μ-CR) bridging complexes
Polyhedron Vol. 7, No. ‘O/l’, Printed in Great Britain
pp. 871479,
0277-5387/88 S3.00+.00 Pergamon Press plc
1988
THEORETICAL STUDIES OF (p-CR) BRIDGING COMPLEXES ELUVATHINGAL
D. JEMMISt
and BHARATAM
V. PRASAD
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad-500 134, India (Accepted
5 November
1987)
Abstract-Scrambling of substituents on the C3R3 bridging ligands in L2W(@Z3R3)(pCR)WL2 is studied using Extended Hiickel calculations by two pathways : deinsertion and cyclopropenium ring formation. The reaction of L,W@-CR),WL, with allenes to give L2W@-CHC(CR2)2)(@R)WL2 is also studied. The electronic structure of CpMeW&CMe),WMeCp is compared to that of L2W&-CR)2WL2.
Bimetallic systems bridged by small organic molecules are of current interest.1’ Carbyne bridged &-CR) complexes are of special interest, mainly because of their catalytic implications.’ Several reactions of bridging CH or CR groups have been reported recently.3 Bimetallic systems bridged by two such carbynes, 1, are especially interesting, because of the delocalization of the 4 72electrons of the M2C2 ring.4-89@)Some of these react selectively with small organic molecules.“’ We have studied the aromatic behaviour and reactivity of these complexes using molecular orbital theory.” 1 reacts specifically with those molecules having x bonds in orthogonal planes. Thus, alkynes, allenes, ketenes etc. but not alkenes react with 1.” ’ Reaction of alkynes with 1 results in the formation of a carboncarbon bond as shown in Scheme l.‘@‘)~(~) Allene reacts with 1 to give 4. The reaction with alkyne first yields an adduct, 2, which finally results in an q+3-didehydroallyl bridging complex, 3. Formation of 2 and its MO correlation to 3 have been discussed. ’ ’ The C3R3 bridge in 3 scrambles the CR units to give a product with a bulkier group on the central carbon atom. Here, we analyse two pathways for this process: One involves essentially a deinsertion step. Formation of a bridged cyclopropenyl ring and its rotation along a C3 axis and reformation of the product is another. We also dis-
t Author to whom correspondence should be addressed. $ Theoretical studies on single carbyne bridged bimetallic systems are in progress.
cuss the formation of 4. The electronic structure of the recently reported 1,3-dimetallacyclobutadiene, (CSMe,)MeW@-CMe)2WMe(C,Me,),8 5, is compared to that of 1 at the end. The Fragment Molecular Orbital (FMO) method within the Extended Hiickel approximation is used in obtaining the electronic structures.” All the calculations were carried out on models where the terminal ligands were CH3 and the substituents on bridging carbon fragments were hydrogens. 5 is modelled by CpCH3W(p-CH)2WCpCH3. The details of geometric and atomic parameters used in the Extended Hiickel calculations are given in the appendix.
Cp’=C,Me,
I
I
Me
R
b
6
THE C-C BOND FORMATION AND THE DEINSERTION PATHWAY FOR SCRAMBLING
One possible pathway for the conversion of 2 + 3, involves the formation of a transition state having a six membered ring, 6, formed by the in-plane movement of the acetylene and CH. The C3 unit then rotates around an axis perpendicular to the M-M axis to give 3. We studied the for871
872
E. D. JEMMIS and B. V. PRASAD
Scheme 1.
mation of 6 and its conversion to 3 separately. The formation of 6 indicates a barrier of 2.8 eV. An
optimized reaction coordinate should give a smaller barrier and the Walsh diagram is given in Fig. 1. The HOMO (a’) orbital initially goes up in energy due to the decrease in the bonding interaction between the acetylene n* and the nearest metal, and then comes down mainly due to the reformation of the metal-metal bond. The M-M bond is decreased from 2.915 to 2.544 A. HOMO- 1 shows an increase in energy due to the increasing antibonding interaction between the terminal carbons of the ally1
group and also an increasing 6* interaction between the metals. The LUMO + 1(6*) orbital comes down due to a decrease in the antibonding interaction between the n of the alkyne and 6* of the metal, and the increasing x combination within the allylic carbons. The MOs corresponding to the new C-C (Tand rcbonds formed are buried among lower lying ligand orbitals and mix well with other ligand based orbitals because of their lower symmetry. Therefore, it was not practical to point any out. Conversion of 6 to 3 involves twisting the allylic part of 6 by 90” and correspondingly adjusting the
Fig. 1. Walsh diagram for the conversion of 2 + 6. C, symmetry is maintained.
Theoretical studies of (p-CR) bridging complexes
873
Fig. 2. Walsh diagram for the conversion of 6 + 3. C2 symmetry is maintained.
distance between the C3R3 and the bimetallic fragment. The process has only CZ symmetry and the energy barrier for the process is around 1.68 eV. The Walsh diagram (Fig. 2) shows that the (THOMO (a,) gradually increases in energy due to an increase in the antibonding combination with the terminal carbons of the ally1 part. The non-bonding R* combination of the allylic group remains almost constant because it retains a 6 type of interaction with the 6* orbital of the M-M combination in the HOMO i 1 (Q). In this process, the M-M distance is kept constant. No other change is observed in the Walsh diagram. The above discussion indicates that the conversion of 2 to 3 through transition state 6 is an allowed process. Experimental studies on the insertion reaction have shown that the kinetically favoured product retains the substituents on the alkynes in the 1,2 positions, 9(b)but the thermodynamic product has the bulkier carbon substituent on the central carbon of the bridging C3 ligand. The thermodynamic preferences can be simply attributed to the steric factors, both in the ground state and the transition state. The transformation of the kinetic product to the thermodynamic one (the scrambling of the CXR3 group) takes place with ease. An alkyne deinsertion along the lines described for the C-C bond formation, together with alkyne rotation can account for the scrambling. If the processes from 2 to 6 and 6 to 3 are reversible, the new alkyne formed in 6 can include the original bridging because the two end carbons of the C3 units are equivalent. To make
way for the formation of the thermodynamically more favoured product from any possible set of starting materials, both a rotation of the acetylene in 2 and a dissociation-association step have to be involved. We find that rotation of the alkyne in 2 along an axis perpendicular to the C-C bond and passing through the metal, is a possible pathway, involving a barrier of only 1.5 eV. This is lower than that for 2 to 6 or 6 to 3. It should be emphasized here that these values are only to be taken as a crude estimate, as the method we use cannot give reliable energy differences. This scrambling mechanism, which involves deinsertion, demands that the rotation of the allylic group as in 6 + 3 should be observed. The detailed experiments of Chisholm et a1.9(b)do not indicate any such process, probably because the barrier for such a process may be higher than is possible thermally. This, in addition to the thermodynamics of the bonds involved does not allow us to support this mechanism conclusively, even though this is the simplest possible mechanism. SCRAMBLING PROCESS INVOLVING A CYCLOPROPENIUM RING FORMATION A general scheme which encompasses all the observed rearrangements was proposed by Chisholm et &9(b) This involves the formation of the L,W(p-rj’,$-cyclopropenyl)WL2 transition state, 7. It is envisaged that the transition state has the cyclopropenyl ring parallel to the L2W-WL2
874
E. D. JEMMIS
and B. V. PRASAD
R
-c’ I?
I
L\
‘p?
R
c2’
yJ_“-
,
WCs-
R
/
R R
I
3
Scheme 2. plane, as shown in Scheme 2. This involves the formation of a cyclopropenium ring by bringing the terminal carbon of C3R3 closer. Ring opening or closing in the normal cyclopropenium cation is not feasible thermally, but the presence of the transition metal changes the picture dramatically. Many experimental14 and theoretical studies13 are available on these systems. The cyclopropenium ion ring opening is complete in the case of d8-ML, complexes. Experimental results suggest that the ring opening is partial in several other systems. I4 A Walsh diagram for the conversion of 3 to 7 is plotted in Fig. 3. A least motion path is defined by pushing the terminal carbons of the allylic bridging ligand upwards and pulling the central carbon downwards. This is the only path which can have at least C, symmetry. The total energy increases continuously and the Walsh diagram shows HOME% LUMO crossing towards the end of the process. As
is clearly shown in Fig. 3, the 3a” (HOMO- l), 8 of 3 goes up in energy precipitously to become the LUMO, 9 of 7. Correspondingly, the 3a’ of 3 comes down slightly to become the HOMO of 7. The 34’ becomes destabilized due to the decrease in the 6 type of bonding interaction of the non-bonding ally1 rc* orbital with the 6* orbital of M-M in 3. There is an avoided crossing between 2a” and 3a” which exchanges their character. The HOMO of 3, the M-M 0 bond, now becomes the HOMO - 1. In 7, the HOMO-LUMO gap is small. The forbidden symmetry of the reaction may be an artifact of the reaction coordinate assumed and the restriction (C, symmetry) imposed on the studied reaction path. A minor change in 7 (as shown in 10) would
k IO
avoid the HOMO-LUMO crossing. coordinate which lacks any symmetry is also tried. The conversion should only on the basis of variation of energy
A reaction restrictions be analysed levels. Slight
-I
ev
L,
IO’
Fig. 3. Walsh diagram for the conversion of 3 + 7 (C, symmetry). Only important orbitals are shown.
Theoretical studies of @-CR) bridging complexes
875
The electronic structure of 4 is studied using the FM0 method between fragments 13 and 14. Figure 4 gives the interaction diagram. The orbital pattern of fragment 13 can be easily obtained from the well known L2M-ML2 and carbyne fragments, and hence is not discussed in detail. It has three occupied metal orbitals with the following electronic configuration in the frontier range: la’ (A)*, 2~’ (a)*, la” (6)‘, 2a” (d*)‘, 3~’ (rc*)‘, 4~’ (o*)’ and 5~’ (0)‘. (Symmetry labels are given assuming C, symmetry rather than C2V, as 4 has only C, symmetry.) Fragment 14 is like trimethylmethylene except for the lack of one hydrogen. The framework of resembles that of trimethyleneCHC(CHJ2 methane and it can also be considered as having a carbyne (CH) attached to the central carbon of the allylic system. The four orbitals of the allylic part are clearly describable as in la’, 2a’, la” and 3~‘. The contribution from the CH group is through an sp hybrid as in 2a’ and 3a’. The CH group in REACTION OF L2W@-CR)2WLZ addition has ap orbital lying in the plane of the C4 WITH ALLENE unit, 2~“. The orbitals involved are shown clearly Allene satisfies the necessary conditions required in Fig. 4. Its interaction with fragment 13 gives 4. for its reaction with 1 and is experimentally found The HOMO in 4 is a slightly destabilized 2a’ (B) to give 4 ‘O,‘i (Scheme 3). A plausible mechanism, orbital of fragment 13. This indicates the presence via the formation of an intermediate (allene of a metal-metal (rr) single bond even though it has adduct), 11 and the transition state, 12, is such a long bond length (2.855 A). The la” orbital proposed. ” Here, we trace the reaction using the of fragment 14 becomes stabilized by its interaction molecular orbital methods and analyse the elec- with the LUMO (a*) orbital of fragment 13, which tronic structure of the final complex, 4. is pushed up to high energy. The HOMO (6) of Conversion of 1 and allene to 11 is a smooth fragment 13 stabilizes the 2a” of fragment 14. The process, where the HOMO becomes stabilized due metal-metal o* combination remains unaffected to the increasing bonding interaction between the [r and becomes the LUMO of 4. In this case also, orbital of 1 and the in-plane n* orbital of the allene. the second CH group is unreactive because of the No other orbital shows any considerable change absence of the metal-metal 6* LUMO which is used during the process. The electronic structure of 11 is to accept electrons from allene. similar to the corresponding alkyne adduct. The conversion of 11 to 12 is a symmetry allowed proELECTRONIC STRUCTURE OF cess but with a high energy barrier. The Walsh (C&Ie,)MeW(p-CMe),WMe(Me(C,Me,), 5 diagram is similar to Fig. 1. Conversion of 12 to 4 does not retain any symmetry and the barrier is Electronic structure studies on 1,3-dimetallasmall, so, formation of 4 along the lines discussed cyclobutadienes reported so far are with simple should be an allowed process. alkyl ligands. “s’~ In 5, an alkyl group on each metal
changes in the geometric parameters like increasing the M-M distance or changes in the C-C distances, make the transition state 7 more stable and the energy barrier small. Rotation of the cyclopropenyl ring, either in 7 or 10 along a pseudo C3 axis and the return to the ring opened structure 3 by breaking a different C-C bond results in complete scrambling. Rotation of the C3R3 ring along an axis passing through the ring centroid and the M-M mid point in 7 produced a tiny barrier (less than 0.2 eV), slightly preferring the structure which lacks any symmetry. This indicates that rotation is a facile process. At this stage, steric factors may play a role in preferentially opening a C-C bond which is opposite to the carbon containing bulky substituents, leading to the observed thermodynamically favoured products.
C
k
k
II
12
Scheme 3.
876
E. D. JEMMIS
and B. V. PRASAD
x
3 0” 6~7’
50’ 40’ eV
-I(
13
Fig. 4. Interaction diagram for the construction of the orbitals of L,W(p-CRC(CR,)&-CR)WLz from L*W(p-CR)WL* and RCC(CR,),. Only the orbitals of the later fragment are shown.
is replaced by a Cp*. The electronic structure of Cp*MeW(p-CMe),WMeCp* is analysed here. Assuming that CR is a 3- ligand, the oxidation state of the metal is 5+ (d’). There are 16 electrons around W in 5 as opposed to the 12 electron count found in 1. The electronic structqe of 1 (R = SiMe,, L = CH2SiMe3) has a metal-metal (d,2 - dz2) bond as the HOMO and a (d,, - &.,,)d*as the LUMO. The HOMO- 1 shows the delocalization that removes the antiaromatic character from the four-membered ring. Extended Hiickel calculations on 5 show that it has the same orbital pattern as that of 1 (R = SiMe,, L = CHzSiMe3). The HOMO corresponds to a M-M IJ bond and the LUMO is a 6* (d,, - &,) orbital. The HOMOLUMO gap is around 0.82 eV), which is of the same order as that in 1. Here also, the aromatic nature of the molecule can be traced to the HOMO- 1 orbital. Neither a change due to Cp* nor to out-ofplane bending in 5 over 1 seems to cause any change in the electronic structure. (The corresponding planar structure is less stable by 4 kcal mol- ‘.) We expect this molecule to have a similar reactivity to that of 1, with small organic molecules, which have two orthogonal A orbitals, like alkynes, allene, ketenes, etc. but not with alkenes. A word ofcaution is in order here-the bulky terminal ligands may cause some hindrance to the incoming reagent,
also, 5 is already strained and puckered in order to release it. So, the formation of an intermediate adduct with an alkyne, which is shown to be crucial, may be difficult, because it involves the out-of-plane bending of terminal ligands. With Cp ligands instead of Cp* the steric problems should be reduced. Scrambling of substituents on the C3R3 ligands in L2W(@Z3R3)(p-CR)WL, is studied in detail. The deinsertion path demands a rotation of the C3 unit between 3 and 6. The cyclopropenium ring formation path would demand the transition state to be more like 10 than 7. Reaction of 1 with allenes is shown to be a symmetry allowed process, but with a high energy barrier. The electronic structure of 5 is very similar to that of 1. thank the University Grants Commission for financial assistance and the University of Hyderabad’s computer centre for computations. Acknowledgements-We
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879
Theoretical studies of (p-CR) bridging complexes APPENDIX The geometric parameters (internal coordinates) for structures are given in Table 1. In structures 7 and 10, d is taken as the mid point of the two metals and d’ is the centroid of the cyclopropenium ring. Process 2 + 6 is described by the in-plane movement of the alkyne in 2 towards C’. W-W and C’-C* distances decreased and the C*-C’ distance increased. The ter-
minal ligands on W* are brought back to the L2W’W2 plane. The process maintains C, symmetry. Process 6 + 3 is designed to maintain C2 symmetry. Rotation of the ally1 ligand along the M-M mid point and C* axis is performed in six equal intervals. C, symmetry is maintained in process 3 + 7. Cz is brought down and C’ and C3 are pushed upwards as shown in Scheme 2. This process is also studied in six steps.
Table 1.
Parameter WI-W2 WI-C W2-C W’-C’ W2-C W2-C W-C(L) C’-C2 C2-C’ h-d d’--c* C’-C2-C3 w’-C’---w2 w’-C’_w2
W’-WZ-C(L) w2-WI-C(L) C(L)-W-C(L) W’-W’-C(@‘)
1
2
3
2.550 1.910 1.910 1.910
2.915 1.730 2.158 1.730 2.000 2.000 2.100
2.550 1.910 1.910 2.220 2.450 2.220 2.100 1.410 1.410
2.100
1.300
79.9 79.9 125.0 125.0 110.0 48.1
86.7 86.9 125.0 115.0 103.0 45.2
106.0 79.0 69.9 125.0 125.0 110.0 60.3
Structure 4 5 2.858 1.972 1.972 1.928
2.606 1.960 1.960 1.960
2.100 1.470 1.440
2.199
120.0 86.2 125.0 125.0 110.0
c&d’-C*
Distances are in angstrom units and angles in degrees.
1.410
83.3 83.3 115.6 115.6
6
7
10
2.550 1.910 1.910 1.926
2.550 1.910 1.910 2.136 1.948 1.948 2.100 1.410
106.0 79.9
2.550 1.910 1.910 1.922 2.111 2.111 2.100 1.410 1.410 1.790 0.866 60.0 79.9
125.0 125.0 110.0
125.0 125.0 110.0
125.0 125.0 110.0
90.0
118.0
1.926 2.100 1.410 1.410
1.790 0.866 60.0 79.9
48.4
pp. 871479,
0277-5387/88 S3.00+.00 Pergamon Press plc
1988
THEORETICAL STUDIES OF (p-CR) BRIDGING COMPLEXES ELUVATHINGAL
D. JEMMISt
and BHARATAM
V. PRASAD
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad-500 134, India (Accepted
5 November
1987)
Abstract-Scrambling of substituents on the C3R3 bridging ligands in L2W(@Z3R3)(pCR)WL2 is studied using Extended Hiickel calculations by two pathways : deinsertion and cyclopropenium ring formation. The reaction of L,W@-CR),WL, with allenes to give L2W@-CHC(CR2)2)(@R)WL2 is also studied. The electronic structure of CpMeW&CMe),WMeCp is compared to that of L2W&-CR)2WL2.
Bimetallic systems bridged by small organic molecules are of current interest.1’ Carbyne bridged &-CR) complexes are of special interest, mainly because of their catalytic implications.’ Several reactions of bridging CH or CR groups have been reported recently.3 Bimetallic systems bridged by two such carbynes, 1, are especially interesting, because of the delocalization of the 4 72electrons of the M2C2 ring.4-89@)Some of these react selectively with small organic molecules.“’ We have studied the aromatic behaviour and reactivity of these complexes using molecular orbital theory.” 1 reacts specifically with those molecules having x bonds in orthogonal planes. Thus, alkynes, allenes, ketenes etc. but not alkenes react with 1.” ’ Reaction of alkynes with 1 results in the formation of a carboncarbon bond as shown in Scheme l.‘@‘)~(~) Allene reacts with 1 to give 4. The reaction with alkyne first yields an adduct, 2, which finally results in an q+3-didehydroallyl bridging complex, 3. Formation of 2 and its MO correlation to 3 have been discussed. ’ ’ The C3R3 bridge in 3 scrambles the CR units to give a product with a bulkier group on the central carbon atom. Here, we analyse two pathways for this process: One involves essentially a deinsertion step. Formation of a bridged cyclopropenyl ring and its rotation along a C3 axis and reformation of the product is another. We also dis-
t Author to whom correspondence should be addressed. $ Theoretical studies on single carbyne bridged bimetallic systems are in progress.
cuss the formation of 4. The electronic structure of the recently reported 1,3-dimetallacyclobutadiene, (CSMe,)MeW@-CMe)2WMe(C,Me,),8 5, is compared to that of 1 at the end. The Fragment Molecular Orbital (FMO) method within the Extended Hiickel approximation is used in obtaining the electronic structures.” All the calculations were carried out on models where the terminal ligands were CH3 and the substituents on bridging carbon fragments were hydrogens. 5 is modelled by CpCH3W(p-CH)2WCpCH3. The details of geometric and atomic parameters used in the Extended Hiickel calculations are given in the appendix.
Cp’=C,Me,
I
I
Me
R
b
6
THE C-C BOND FORMATION AND THE DEINSERTION PATHWAY FOR SCRAMBLING
One possible pathway for the conversion of 2 + 3, involves the formation of a transition state having a six membered ring, 6, formed by the in-plane movement of the acetylene and CH. The C3 unit then rotates around an axis perpendicular to the M-M axis to give 3. We studied the for871
872
E. D. JEMMIS and B. V. PRASAD
Scheme 1.
mation of 6 and its conversion to 3 separately. The formation of 6 indicates a barrier of 2.8 eV. An
optimized reaction coordinate should give a smaller barrier and the Walsh diagram is given in Fig. 1. The HOMO (a’) orbital initially goes up in energy due to the decrease in the bonding interaction between the acetylene n* and the nearest metal, and then comes down mainly due to the reformation of the metal-metal bond. The M-M bond is decreased from 2.915 to 2.544 A. HOMO- 1 shows an increase in energy due to the increasing antibonding interaction between the terminal carbons of the ally1
group and also an increasing 6* interaction between the metals. The LUMO + 1(6*) orbital comes down due to a decrease in the antibonding interaction between the n of the alkyne and 6* of the metal, and the increasing x combination within the allylic carbons. The MOs corresponding to the new C-C (Tand rcbonds formed are buried among lower lying ligand orbitals and mix well with other ligand based orbitals because of their lower symmetry. Therefore, it was not practical to point any out. Conversion of 6 to 3 involves twisting the allylic part of 6 by 90” and correspondingly adjusting the
Fig. 1. Walsh diagram for the conversion of 2 + 6. C, symmetry is maintained.
Theoretical studies of (p-CR) bridging complexes
873
Fig. 2. Walsh diagram for the conversion of 6 + 3. C2 symmetry is maintained.
distance between the C3R3 and the bimetallic fragment. The process has only CZ symmetry and the energy barrier for the process is around 1.68 eV. The Walsh diagram (Fig. 2) shows that the (THOMO (a,) gradually increases in energy due to an increase in the antibonding combination with the terminal carbons of the ally1 part. The non-bonding R* combination of the allylic group remains almost constant because it retains a 6 type of interaction with the 6* orbital of the M-M combination in the HOMO i 1 (Q). In this process, the M-M distance is kept constant. No other change is observed in the Walsh diagram. The above discussion indicates that the conversion of 2 to 3 through transition state 6 is an allowed process. Experimental studies on the insertion reaction have shown that the kinetically favoured product retains the substituents on the alkynes in the 1,2 positions, 9(b)but the thermodynamic product has the bulkier carbon substituent on the central carbon of the bridging C3 ligand. The thermodynamic preferences can be simply attributed to the steric factors, both in the ground state and the transition state. The transformation of the kinetic product to the thermodynamic one (the scrambling of the CXR3 group) takes place with ease. An alkyne deinsertion along the lines described for the C-C bond formation, together with alkyne rotation can account for the scrambling. If the processes from 2 to 6 and 6 to 3 are reversible, the new alkyne formed in 6 can include the original bridging because the two end carbons of the C3 units are equivalent. To make
way for the formation of the thermodynamically more favoured product from any possible set of starting materials, both a rotation of the acetylene in 2 and a dissociation-association step have to be involved. We find that rotation of the alkyne in 2 along an axis perpendicular to the C-C bond and passing through the metal, is a possible pathway, involving a barrier of only 1.5 eV. This is lower than that for 2 to 6 or 6 to 3. It should be emphasized here that these values are only to be taken as a crude estimate, as the method we use cannot give reliable energy differences. This scrambling mechanism, which involves deinsertion, demands that the rotation of the allylic group as in 6 + 3 should be observed. The detailed experiments of Chisholm et a1.9(b)do not indicate any such process, probably because the barrier for such a process may be higher than is possible thermally. This, in addition to the thermodynamics of the bonds involved does not allow us to support this mechanism conclusively, even though this is the simplest possible mechanism. SCRAMBLING PROCESS INVOLVING A CYCLOPROPENIUM RING FORMATION A general scheme which encompasses all the observed rearrangements was proposed by Chisholm et &9(b) This involves the formation of the L,W(p-rj’,$-cyclopropenyl)WL2 transition state, 7. It is envisaged that the transition state has the cyclopropenyl ring parallel to the L2W-WL2
874
E. D. JEMMIS
and B. V. PRASAD
R
-c’ I?
I
L\
‘p?
R
c2’
yJ_“-
,
WCs-
R
/
R R
I
3
Scheme 2. plane, as shown in Scheme 2. This involves the formation of a cyclopropenium ring by bringing the terminal carbon of C3R3 closer. Ring opening or closing in the normal cyclopropenium cation is not feasible thermally, but the presence of the transition metal changes the picture dramatically. Many experimental14 and theoretical studies13 are available on these systems. The cyclopropenium ion ring opening is complete in the case of d8-ML, complexes. Experimental results suggest that the ring opening is partial in several other systems. I4 A Walsh diagram for the conversion of 3 to 7 is plotted in Fig. 3. A least motion path is defined by pushing the terminal carbons of the allylic bridging ligand upwards and pulling the central carbon downwards. This is the only path which can have at least C, symmetry. The total energy increases continuously and the Walsh diagram shows HOME% LUMO crossing towards the end of the process. As
is clearly shown in Fig. 3, the 3a” (HOMO- l), 8 of 3 goes up in energy precipitously to become the LUMO, 9 of 7. Correspondingly, the 3a’ of 3 comes down slightly to become the HOMO of 7. The 34’ becomes destabilized due to the decrease in the 6 type of bonding interaction of the non-bonding ally1 rc* orbital with the 6* orbital of M-M in 3. There is an avoided crossing between 2a” and 3a” which exchanges their character. The HOMO of 3, the M-M 0 bond, now becomes the HOMO - 1. In 7, the HOMO-LUMO gap is small. The forbidden symmetry of the reaction may be an artifact of the reaction coordinate assumed and the restriction (C, symmetry) imposed on the studied reaction path. A minor change in 7 (as shown in 10) would
k IO
avoid the HOMO-LUMO crossing. coordinate which lacks any symmetry is also tried. The conversion should only on the basis of variation of energy
A reaction restrictions be analysed levels. Slight
-I
ev
L,
IO’
Fig. 3. Walsh diagram for the conversion of 3 + 7 (C, symmetry). Only important orbitals are shown.
Theoretical studies of @-CR) bridging complexes
875
The electronic structure of 4 is studied using the FM0 method between fragments 13 and 14. Figure 4 gives the interaction diagram. The orbital pattern of fragment 13 can be easily obtained from the well known L2M-ML2 and carbyne fragments, and hence is not discussed in detail. It has three occupied metal orbitals with the following electronic configuration in the frontier range: la’ (A)*, 2~’ (a)*, la” (6)‘, 2a” (d*)‘, 3~’ (rc*)‘, 4~’ (o*)’ and 5~’ (0)‘. (Symmetry labels are given assuming C, symmetry rather than C2V, as 4 has only C, symmetry.) Fragment 14 is like trimethylmethylene except for the lack of one hydrogen. The framework of resembles that of trimethyleneCHC(CHJ2 methane and it can also be considered as having a carbyne (CH) attached to the central carbon of the allylic system. The four orbitals of the allylic part are clearly describable as in la’, 2a’, la” and 3~‘. The contribution from the CH group is through an sp hybrid as in 2a’ and 3a’. The CH group in REACTION OF L2W@-CR)2WLZ addition has ap orbital lying in the plane of the C4 WITH ALLENE unit, 2~“. The orbitals involved are shown clearly Allene satisfies the necessary conditions required in Fig. 4. Its interaction with fragment 13 gives 4. for its reaction with 1 and is experimentally found The HOMO in 4 is a slightly destabilized 2a’ (B) to give 4 ‘O,‘i (Scheme 3). A plausible mechanism, orbital of fragment 13. This indicates the presence via the formation of an intermediate (allene of a metal-metal (rr) single bond even though it has adduct), 11 and the transition state, 12, is such a long bond length (2.855 A). The la” orbital proposed. ” Here, we trace the reaction using the of fragment 14 becomes stabilized by its interaction molecular orbital methods and analyse the elec- with the LUMO (a*) orbital of fragment 13, which tronic structure of the final complex, 4. is pushed up to high energy. The HOMO (6) of Conversion of 1 and allene to 11 is a smooth fragment 13 stabilizes the 2a” of fragment 14. The process, where the HOMO becomes stabilized due metal-metal o* combination remains unaffected to the increasing bonding interaction between the [r and becomes the LUMO of 4. In this case also, orbital of 1 and the in-plane n* orbital of the allene. the second CH group is unreactive because of the No other orbital shows any considerable change absence of the metal-metal 6* LUMO which is used during the process. The electronic structure of 11 is to accept electrons from allene. similar to the corresponding alkyne adduct. The conversion of 11 to 12 is a symmetry allowed proELECTRONIC STRUCTURE OF cess but with a high energy barrier. The Walsh (C&Ie,)MeW(p-CMe),WMe(Me(C,Me,), 5 diagram is similar to Fig. 1. Conversion of 12 to 4 does not retain any symmetry and the barrier is Electronic structure studies on 1,3-dimetallasmall, so, formation of 4 along the lines discussed cyclobutadienes reported so far are with simple should be an allowed process. alkyl ligands. “s’~ In 5, an alkyl group on each metal
changes in the geometric parameters like increasing the M-M distance or changes in the C-C distances, make the transition state 7 more stable and the energy barrier small. Rotation of the cyclopropenyl ring, either in 7 or 10 along a pseudo C3 axis and the return to the ring opened structure 3 by breaking a different C-C bond results in complete scrambling. Rotation of the C3R3 ring along an axis passing through the ring centroid and the M-M mid point in 7 produced a tiny barrier (less than 0.2 eV), slightly preferring the structure which lacks any symmetry. This indicates that rotation is a facile process. At this stage, steric factors may play a role in preferentially opening a C-C bond which is opposite to the carbon containing bulky substituents, leading to the observed thermodynamically favoured products.
C
k
k
II
12
Scheme 3.
876
E. D. JEMMIS
and B. V. PRASAD
x
3 0” 6~7’
50’ 40’ eV
-I(
13
Fig. 4. Interaction diagram for the construction of the orbitals of L,W(p-CRC(CR,)&-CR)WLz from L*W(p-CR)WL* and RCC(CR,),. Only the orbitals of the later fragment are shown.
is replaced by a Cp*. The electronic structure of Cp*MeW(p-CMe),WMeCp* is analysed here. Assuming that CR is a 3- ligand, the oxidation state of the metal is 5+ (d’). There are 16 electrons around W in 5 as opposed to the 12 electron count found in 1. The electronic structqe of 1 (R = SiMe,, L = CH2SiMe3) has a metal-metal (d,2 - dz2) bond as the HOMO and a (d,, - &.,,)d*as the LUMO. The HOMO- 1 shows the delocalization that removes the antiaromatic character from the four-membered ring. Extended Hiickel calculations on 5 show that it has the same orbital pattern as that of 1 (R = SiMe,, L = CHzSiMe3). The HOMO corresponds to a M-M IJ bond and the LUMO is a 6* (d,, - &,) orbital. The HOMOLUMO gap is around 0.82 eV), which is of the same order as that in 1. Here also, the aromatic nature of the molecule can be traced to the HOMO- 1 orbital. Neither a change due to Cp* nor to out-ofplane bending in 5 over 1 seems to cause any change in the electronic structure. (The corresponding planar structure is less stable by 4 kcal mol- ‘.) We expect this molecule to have a similar reactivity to that of 1, with small organic molecules, which have two orthogonal A orbitals, like alkynes, allene, ketenes, etc. but not with alkenes. A word ofcaution is in order here-the bulky terminal ligands may cause some hindrance to the incoming reagent,
also, 5 is already strained and puckered in order to release it. So, the formation of an intermediate adduct with an alkyne, which is shown to be crucial, may be difficult, because it involves the out-of-plane bending of terminal ligands. With Cp ligands instead of Cp* the steric problems should be reduced. Scrambling of substituents on the C3R3 ligands in L2W(@Z3R3)(p-CR)WL, is studied in detail. The deinsertion path demands a rotation of the C3 unit between 3 and 6. The cyclopropenium ring formation path would demand the transition state to be more like 10 than 7. Reaction of 1 with allenes is shown to be a symmetry allowed process, but with a high energy barrier. The electronic structure of 5 is very similar to that of 1. thank the University Grants Commission for financial assistance and the University of Hyderabad’s computer centre for computations. Acknowledgements-We
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879
Theoretical studies of (p-CR) bridging complexes APPENDIX The geometric parameters (internal coordinates) for structures are given in Table 1. In structures 7 and 10, d is taken as the mid point of the two metals and d’ is the centroid of the cyclopropenium ring. Process 2 + 6 is described by the in-plane movement of the alkyne in 2 towards C’. W-W and C’-C* distances decreased and the C*-C’ distance increased. The ter-
minal ligands on W* are brought back to the L2W’W2 plane. The process maintains C, symmetry. Process 6 + 3 is designed to maintain C2 symmetry. Rotation of the ally1 ligand along the M-M mid point and C* axis is performed in six equal intervals. C, symmetry is maintained in process 3 + 7. Cz is brought down and C’ and C3 are pushed upwards as shown in Scheme 2. This process is also studied in six steps.
Table 1.
Parameter WI-W2 WI-C W2-C W’-C’ W2-C W2-C W-C(L) C’-C2 C2-C’ h-d d’--c* C’-C2-C3 w’-C’---w2 w’-C’_w2
W’-WZ-C(L) w2-WI-C(L) C(L)-W-C(L) W’-W’-C(@‘)
1
2
3
2.550 1.910 1.910 1.910
2.915 1.730 2.158 1.730 2.000 2.000 2.100
2.550 1.910 1.910 2.220 2.450 2.220 2.100 1.410 1.410
2.100
1.300
79.9 79.9 125.0 125.0 110.0 48.1
86.7 86.9 125.0 115.0 103.0 45.2
106.0 79.0 69.9 125.0 125.0 110.0 60.3
Structure 4 5 2.858 1.972 1.972 1.928
2.606 1.960 1.960 1.960
2.100 1.470 1.440
2.199
120.0 86.2 125.0 125.0 110.0
c&d’-C*
Distances are in angstrom units and angles in degrees.
1.410
83.3 83.3 115.6 115.6
6
7
10
2.550 1.910 1.910 1.926
2.550 1.910 1.910 2.136 1.948 1.948 2.100 1.410
106.0 79.9
2.550 1.910 1.910 1.922 2.111 2.111 2.100 1.410 1.410 1.790 0.866 60.0 79.9
125.0 125.0 110.0
125.0 125.0 110.0
125.0 125.0 110.0
90.0
118.0
1.926 2.100 1.410 1.410
1.790 0.866 60.0 79.9
48.4