Fluxional hopping of Fe(CO) 3 in some of its complexes with dienes

Fluxional hopping of Fe(CO)3 in some of its complexes with dienes

Bidisa Dasa,b, K.L. Sebastianb,*, A.G. Samuelsona,1

aDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Malleswaram, Bangalore 560 012, IndiabJawaharlal Nehru Center for Advanced Scientific Research, Bangalore 560 064, India

Received 8 March 2004; accepted 21 June 2004

Available online 26 November 2004

Abstract

We first present a theoretical study of the Cope rearrangement of syn-tricyclooctadiene molecule. We find that the barrier height for the

rearrangement is about 24 kcal/mol. We also investigate the possibility of fluxionality in hypothetical tricarbonyliron complexes of

hypostrophene, syn-tricyclooctadiene, semibullvalene and 1,5-hexadiene (boat) in which the double bonds of the ligands coordinate to the

Fe(CO)3 moiety. The ligands can undergo Cope rearrangement in which the double bonds are shifted and this can be accompanied by the

hopping of Fe(CO)3 unit so as to be coordinated by the newly formed double bonds. We find that the barrier height for this process, varies

from 22 to 37 kcal/mol.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Fluxionality; Diene; Hypostrophene; syn-Tricyclooctadiene; Semibullvalene; 1,5-Hexadiene

1. Introduction

In this paper, we consider organometallic compounds

in which a tricarbonyliron unit is coordinated to a diene.

The dienes chosen are fluxional and if they are still

fluxional in the coordinated state, then the fluxional

rearrangement with a concerted movement of Fe(CO)3,

leads to a new type of fluxional motion, that is quite

interesting on its own (see Fig. 2) and has not yet been

observed to the best of our knowledge. The dienes

considered here are hypostrophene, semibullvalene, syn-

tricyclooctadiene (syn-TOD) and 1,5-hexadiene (boat

form). All these molecules are capable of undergoing

degenerate Cope rearrangement and have a parallel or

nearly parallel pair of double bonds that can be

coordinated to Fe(CO)3 moiety to form p-complexes.

0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2004.08.010

* Corresponding author. Tel.: C91 80 2293 2385; fax: C91 80 2360

1552.

E-mail addresses: [email protected] (K.L. Sebastian), ashoka@

ipc.iisc.ernet.in (A.G. Samuelson).1 Fax: C91 80 2360 1552.

2. Diene iron complexes

A vast amount of literature exists on this type of

organometallic coordination complexes. The (1,3-diene)

tricarbonyliron complexes are very common, in which all

four carbon atoms of the 1,3-diene are coordinated to the

tricarbonyliron unit. The first compound of this type (h4-

butadiene) tricarbonyliron, was prepared in 1930 [1,2]. cis-

1,3-Diene fragments of open chain and cyclic ligands form

p-complexes with transition metals. A molecular orbital

description of the bonding in the (diene) tricarbonyliron

complexes has been given by Mason et al. [3–5]. The

structure of (h4-butadiene) tricarbonyliron has been inves-

tigated by single crystal X-ray diffraction techniques [1,2].

In all these molecules, bonding is via conjugated 1,3-diene

units donating electrons to iron. 1,4-Pentadienes and 1,4-

cyclohexadienes rearrange to the corresponding conjugated

dienes upon reaction with carbonyl iron reagents and

eventually form (1,3-diene) tricarbonyliron complexes.

However, norbornadiene and 1,5-cyclooctadiene form

stable tricarbonyliron derivatives in which the non-con-

jugated nature of the C–C double bonds are preserved [6].

(Diene)tricarbonyl complexes are often fluxional [7].

‘Ring whizzing’ and ‘carbonyl scrambling’ are the two

Journal of Molecular Structure: THEOCHEM 712 (2004) 21–32

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Fig. 1. The rotation of the CO bonds with respect to the diene fragment.

B. Das et al. / Journal of Molecular Structure: THEOCHEM 712 (2004) 21–3222

kinds of fluxional motion exhibited by this class of

organometallic molecules. (h4-Cyclooctatetraene) tricarbo-

nyliron and (h4-cyclooctatetraene)tricarbonylruthenium [8]

are the two most common examples which exhibit both the

types of fluxional motions. The process of ring whizzing and

carbonyl scrambling in these two systems have similar

activation parameters (approximately 8–9 kcal) and it is

impossible to say whether the two processes are truly

simultaneous (and therefore possibly independent) or not.

The ring whizzing is thought to be via 1,2-shifts in most of

the cases.

The studies of (diene)Fe(CO)3 complexes in the late

70s [7,9,10] have shown that apart from ring whizzing

mentioned above, carbonyl scrambling is also taking

place in these molecules. (Cycloheptatriene)Fe(CO)3, (1,3-

hexadiene)Fe(CO)3, (butadiene)Fe(CO)3 and (cyclo-

octatetraene)Fe(CO)3, when studied revealed that in

these cases the metal carbonyl fragment rotates relative

to the polyene fragment (see Fig. 1). In the molecules

there should be at least two resonances in the ratio of 2:1

for the basal and the apical CO groups due to their

square-pyramidal geometry. But, 13C NMR spectra of

these molecules showed only one resonance for the

carbonyl groups. There is clearly an averaging of the

signals at high temperatures. At lower temperatures

the 2:1 signal was obtained. Also, the barrier height for

such kind of rotation has been estimated and varies from

7.4 to 12.0 kcal/mol [7].

Fig. 2. In Cope rearrangement the pair of double bonds in hypostrophene undergoe

the location of the new double bonds if the rearrangement takes place. (a) The ar

shows the progress of the rearrangement. (b) Alternatively one can think as if hy

3. The hopping motion of tricarbonyliron moiety when

bonded to fluxional ligands

The free molecules hypostrophene, semibullvalene,

syn-TOD and 1,5-hexadiene can undergo Cope rearrange-

ment. 1,5-Hexadiene exists in ‘chair’ and ‘boat’ confor-

mations, the chair form being more stable. Also TOD can

exist in syn- and anti-conformation with respect to the

position of the double bonds, of which the anti-confor-

mation is more stable. We consider only the case of the boat

molecule for 1,5-hexadiene and syn-isomer for TOD

because these isomers have a parallel pair of double bonds

which can be coordinated to Fe(CO)3 fragment. In Cope

rearrangement, which is a characteristic of 1,5-dienes, the

parallel pair of double bonds shift (see Fig. 2, for

hypostrophene). Now, we can consider these molecules

bonded to a tricarbonyliron fragment. The (diene)Fe(CO)3

molecule can undergo a fluxional motion where the diene

undergoes a Cope like rearrangement and Fe(CO)3 unit

moves so that the newly formed double bonds coordinate to

it. For hypostrophene and syn-TOD there would be five and

four identical positions, respectively, to which the Fe(CO)3

unit can move. For semibullvalene and 1,5-hexadiene (boat)

there are only two possible positions (see Sections 6.1–6.4).

The hopping motion of the tricarbonyliron unit as a

whole onto different positions on the ligand is similar to the

movement of Fe(CO)3 unit in the ring whizzer molecules,

already discussed in Section 2. One can denote it as a

change of the coordinating atoms of the ligand. Alterna-

tively, the rearrangement may be thought of as a rotational

motion of a pentagonal prism (for hypostrophene molecule),

whereas the Fe(CO)3 unit is static (as shown in Fig. 2(b)). Of

course in the actual rearrangement both move. In addition,

the rotation of Fe(CO)3 unit relative to the diene fragment,

i.e. CO scrambling is also present and we study the

energetics of that rotation as well.

s 1,2-shifts and hence if Fe(CO)3 is bonded to the diene part, it has to hop to

rows show the position of Fe(CO)3 unit in the molecule. The dashed arrow

postrophene molecule is rotating while Fe(CO)3 unit is static.

Table 1

The bond-lengths (in A) for free anti-TOD

1–2 2–3 4–5 3–6 1–H 2–H

1.52 1.56 1.34 1.58 1.09 1.09

Table 2

Bond-lengths (in A) for GS of free syn-TOD

1–1 0 3–3 0 1–2 2–3 3–4 1–H 3–H

1.59 3.02 1.56 1.51 1.34 1.09 1.09

B. Das et al. / Journal of Molecular Structure: THEOCHEM 712 (2004) 21–32 23

4. Methods of study

Density functional theory (DFT) is used in the calcu-

lations, as it seems to be the best technique presently

available for systems with metal atoms [11]. The hybrid

functional B3LYP [12,13] was used in all the calculations

and GAUSSIAN 98 [14] software was used for the study. In all

the studies, the basis-set used for Fe was 6-311G** and for

C, H and O we used 6-31G**. All the geometries were fully

optimized and normal mode analysis was carried out. The

transition state structures were confirmed by the single

imaginary frequency obtained from the normal mode

analysis and then visualizing the vibrational mode. The

geometry of the optimized ground state (GS) structures and

the transition state structures are all tabulated. Generally the

C–H distances for all the molecules do not vary much and

hence in all the tables we present only one average value of

the C–H bond-distance. The rate constants for the

rearrangements were estimated using the transition state

theoretical expression [15]

k Z cðkBT =hÞQ�=Q exp½KE0=kBTÞ� (1)

In this equation, Q and Q* are the partition functions for the

GS and TS, respectively, E0 is the barrier height and c is the

transmission coefficient. We take the transmission coeffi-

cient to be

c Z ðZu* =2kBTÞ=sinðZu�=2kBTÞ (2)

where u* is the frequency of the unstable mode [15].

5. Cope rearrangement of free fluxional molecules

Degenerate Cope rearrangement occurs for 1,5-dienes

and as a result an identical molecule with shifted double

bonds is formed (see Fig. 2 for hypostrophene molecule).

We have studied the Cope rearrangement of the free

Fig. 3. (a) The anti-TOD and (b) The syn-TOD.

molecules and the barrier height and reaction rates are

presented in Table 11. There are studies available for Cope

rearrangement of free 1,5-hexadiene [9,10,16], semibullva-

lene [17–19], hypostrophene [20–22], but none for syn-

TOD. Our studies show that syn-TOD molecule behaves

like hypostrophene and possibly can undergo Cope

rearrangement with almost a similar barrier height. Below

we discuss the Cope rearrangements of syn-TOD, 1,5-

hexadiene (boat) and semibullvalene. We give the results

for the DFT (B3LYP) calculations for the free molecules

and the tricarbonyliron complexes. In the case of free

molecules, we find the DFT results to be closer to the

experimental values, wherever available. The Cope

rearrangement of hypostrophene has been reported earlier

[22] in detail.

5.1. Degenerate Cope rearrangement of free syn-TOD

Though no studies are available for the Cope rearrange-

ment of syn-TOD, Bally et al. [23] has investigated the

rearrangement products of the radical cation of syn-TOD.

The other isomer, anti-TOD is also very less studied and the

same authors have studied the rearrangement products of

this molecule [24]. We have studied anti-TOD and found it

to be more stable than the syn-isomer by 6.33 kcal/mol

(B3LYP/6-31G**). The structure of anti-TOD is shown in

Fig. 3 and geometry is given in Table 1. syn-TOD is a

rearrangement product of cubane molecule [25,26]. We

have studied the ground and transition states of syn-TOD for

Cope rearrangement and our results for various levels of

calculation are given in Table 2 (for GS) and Table 3

(for TS) and the rearrangement is schematically presented in

Fig. 4. The double bond-lengths in the GS for the free

molecule between 3–4 and 3 0–4 0 are 1.34 A. The non-

bonded inter-ring distances between 3,3 0 and 4,4 0 are

3.02 A. The inter-ring single bonds between 1–1 0 and 2–2 0

are strained (1.60 A). These bond-lengths are longer than

normal C–C single bonds, but similar bond-lengths are

already known for geometries calculated for free hypos-

trophene molecule by DFT methods [22,27]. In the TS, there

is delocalization involving three neighboring C atoms of

each ring 4–3, 3–2 and 4 0–3 0, 3 0–2 0 (see Fig. 4). There is

Table 3

Bond-lengths (in A) for TS of free syn-TOD

1–1 0 2–2 0 3–3 0 1–2 2–3 1–H 2–H 3–H

1.54 2.29 3.14 1.53 1.40 1.09 1.08 1.09

https://www.researchgate.net/publication/260903726_Theory_of_Elementary_Atomic_and_Molecular_Processes_in_Gases?el=1_x_8&enrichId=rgreq-0c71b70b018a2d79ae9088bcbe67ea0e-XXX&enrichSource=Y292ZXJQYWdlOzIxNTU0MTY3NztBUzoxMTE4ODY3Nzk1NTU4NDVAMTQwMzY4NzI5Njc1Mw==


Table 4

Bond-lengths (in A) for the GS geometry of free semibullvalene

3–3 0 4–4 0 5–5 0 1–2 2–3 3–4 4–5 5–1 1–H 2–H 3–H 4–H 5–H

1.61 3.10 2.36 1.55 1.50 1.47 1.34 1.53 1.09 1.09 1.08 1.08 1.08


only one real inter-ring single bond in the TS (1–1 0 with

bond-length of 1.54 A). The 2–2 0 and 4–4 0 distances

(2.28 A) are much larger than single bonds. The barrier

height for Cope rearrangement is found to be 24.25 kcal/mol

(B3LYP/6-31G**). The TS was found to have one

imaginary frequency of 479 cmK1 (B3LYP/6-31G**).

The rate constant of rearrangement can be estimated

using the transition state theoretical expressions (1) and

(2) given before and the rate constant was estimated to be

1.36!10K4 sK1 at 298.1 K.

It is to be noted that there can be another rearrangement

for syn-TOD molecule, where the pair of double bonds shift

to positions perpendicular to the earlier double bonds (from

4–3, 4 0–3 0 to 4–4 0, 3–3 0, respectively, as in Fig. 5),

eventually giving rise to the same molecule. This rearrange-

ment would involve cubane as an intermediate as shown in

Fig. 5 [28,29]. But, it has already been pointed out [30] that

rearrangement of cubane to syn-TOD is a symmetry

forbidden process and hence is likely to involve a very

high barrier height. Catalysts like Rh or Ag can bring down

this barrier [30].

Fig. 5. Rearrangement of syn-TOD via cubane as intermediate.

Fig. 4. Schematic energy profile for Cope rearrangement of syn-TOD.

GS I is converted to GS II via the TS.

5.2. Degenerate Cope rearrangement of free semibullvalene

The free semibullvalene molecule is less symmetric (Cs,

see Table 4 for the structure) than it is in the TS (C2v, see

Table 5 for the structure) for Cope rearrangement and the

barrier height is very small. In the free molecule the double

bonds between 4–5 and 4 0–5 0 (first GS, GS I) are shifted to

4–3 and 4 0–3 0 (second GS, GS II) through a TS shown in

Fig. 6. In the GS the double bond-distance between the pairs

5–4 and 5 0–4 0 is 1.32 A which is converted to a delocalized

(half) double bond in the TS with the bond-distances

between 5–4, 5 0–4 0, 4–3 and 4 0–3 0 equal to 1.38 A.

The barrier height is rather small (experimental: 4.8–5.2

kcal/mol [18,17], calculated: 5.5 kcal/mol B3LYP/

6-31G**) and hence the rate constant calculated using

Eqs. (1) and (2) is 3.7!109 sK1 at 298.1 K which is an

extremely rapid conversion rate.

5.3. 1,5-Hexadiene [C6H10]

1,5-Hexadiene has been studied extensively in connec-

tion with pericyclic reactions [31]. 1,5-Hexadiene exists in

two possible structures differing in the position of the

double bonds in the molecule: boat and chair. They are

discussed separately below. The detailed geometries of the

boat and chair conformation of 1,5-hexadiene in GS as well

as the transition structure (TS) for Cope rearrangement are

presented in Tables 6–10. The values of barrier heights are

in agreement with previous theoretical studies [32–34].

5.3.1. The boat form

In the boat form of 1,5-hexadiene (Cs) the double bonds

are on the same side of the molecule and it is higher in

energy than the chair form. But we find that the symmetric

boat (Cs) becomes unstable due to the gauche interaction of

Table 5

Bond-lengths (in A) for the TS geometry of free semibullvalene

3–3 0 4–4 0 1–2 2–3 3–4 1–H 3–H 4–H

2.10 3.18 1.57 1.50 1.39 1.08 1.08 1.08

Table 6

The bond-lengths (in A) for Cs geometry for the Cope rearrangement of

1,5-hexadiene (boat) as shown in Fig. 7

1–2 2–3 3–4 5–2 1–6 C–H

1.33 1.50 1.57 2.89 4.55 1.09

The hydrogens bonded to 3 and 4 interact strongly with each other, hence,

this form is not a minimum in the potential energy surface.

Fig. 6. Schematic energy profile for Cope rearrangement of semibullvalene.

GS I is converted to GS II via the TS.

Table 11

The barrier height for degenerate Cope rearrangement of all the free

fluxional molecules

Molecule Cope rearrang.

E0 (kcal/mol)

Unstable vib.

mode (cmK1)

Rate const.

(sK1)

Hypostrophene 25.31 510 1.8!10K5

Semibullvalene 5.51 226 3.7!109

syn-TOD 24.25 479 1.36!10K4

1,5-Hexadiene

(boat)

42.53 520 6.01!10K19

1,5-Hexadiene

(chair)

33.47 568 3.91!10K13

Rate constants are calculated at 298.1 K.

Table 10

The bond-lengths (in A) for TS geometry of Cope rearrangement of 1,5-

hexadiene (chair) as shown in Fig. 8

1–2 1–3 3–4 C–H

1.40 2.43 1.98 1.09

3–4 is the bond in formation and 1–3 is a non-bonded distance.


the two adjacent C (4 and 3) atoms (see Table 6 for the

structure of the symmetric GS). The study of symmetric

boat form yields an imaginary frequency (156 cmK1)

corresponding to the movements of H atoms, which

encounter the ‘gauche interaction’. So the symmetric boat

distorts to minimize the interaction between the H atoms

resulting in a totally asymmetric geometry (C1, see Table 7

for the structure). The asymmetric boat is more stable than

the symmetric one by 5.1 kcal/mol. We find that

the energies of the chair and the distorted boat form are

Table 7

The bond-lengths (in A) for GS geometry of 1,5-hexadiene (boat) isomer as

shown in Fig. 7

1–2 2–3 3–4 4–5 5–6 5–2 1–6 C–H

1.33 1.50 1.55 1.50 1.33 3.31 4.94 1.09

This form is totally asymmetric, 5–2 and 1–6 are the non-bonded distances.

Table 8

The bond-lengths (in A) for TS geometry of the Cope rearrangement of

1,5-hexadiene (boat) as shown in Fig. 7

1–2 3–4 1–3 C–H

1.39 2.22 2.43 1.09

3–4 is the bond in formation and 1–3 is a non-bonded distance.

Table 9

The bond-lengths (in A) for the GS geometry of 1,5-hexadiene (chair) as

shown in Fig. 8

1–2 2–3 3–4 5–2 1–6 C–H

1.33 1.50 1.55 3.05 4.48 1.09

5–2 and 1–6 are the non-bonded distances.

almost comparable. The TS for Cope rearrangement of the

boat form is of C2v symmetry (see Table 8 for the structure

of the TS). Also, the Cope rearrangement in the boat form

is less facile than in the chair form. The barrier height for

the Cope rearrangement of the distorted boat form

(42.53 kcal/mol B3LYP/6-31G**) is high in comparison

to the barrier height of the chair form (33.47 kcal/mol

B3LYP/6-31G**). Experimental value of the barrier height

for the Cope rearrangement of 1,5-hexadiene (boat) is

44.7G2.0 kcal/mol [35] (Fig. 7).

Fig. 7. The GS and the TS of the Cope rearrangement of 1,5-hexadiene

(boat form).

Fig. 8. The GS and the TS for the Cope rearrangement of 1,5-hexadiene

(chair form).

Fig. 11. Schematic presentation of the energy profile diagram of hopping of

Fe(CO)3 unit. GS (Fig. 17(a)) and TS (Fig. 17(b)) geometries are shown,

along with the GS and the TS geometries of the free syn-TOD molecule.Fig. 9. Schematic presentation of the energy profile diagram of rotation of


along with the GS and the TS geometries of the free hypostrophene

molecule.


Fe(CO)3 unit. GS (Fig. 16(a)) and TS (Fig. 16(c)) geometries are shown,

along with the GS and the TS geometries of the free semibullvalene

molecule.


5.3.2. The chair form

In the chair form of 1,5-hexadiene the double bonds are

on the opposite side. The chair form (C2) is known to be the

more stable and symmetric one (see Table 9 for the structure

of the GS). In the case of chair isomer the TS for Cope

rearrangement has C2h symmetry (see Table 10 for the

structure of the TS). There are many studies of the chair



along with the GS and the TS geometries of the free 1,5-hexadiene

molecule.

Fig. 13. The variation of energy when the Fe(CO)3 fragment is rotated relative to the diene fragment of semibullvalene molecule. The horizontal axis is the

average of the three dihedral angles (18–17–8–7, 19–17–8–7 and 2–17–8–7). The potential is periodic with a periodicity of 1208. The rotation of the Fe(CO)3

fragment is visible in the ‘top view’ of the molecule. The geometries marked in the plot are given in top view and ‘side view’.


form of 1,5-hexadiene and the Cope rearrangement of this

molecule [32–34]. The barrier height for the Cope

rearrangement of the chair form is 33.47 kcal/mol

(B3LYP/6-31G**). Experimental value of the barrier height

for the Cope rearrangement of 1,5-hexadiene (chair) is

33.3G0.5 kcal/mol [36] (Fig. 8).

6. Study of (diene)Fe(CO)3 complexs

We now investigate the possibility of the hopping

motion of Fe(CO)3 moiety onto different faces of the

fluxional molecules studied earlier (see Fig. 2). The

hopping motion of the tricarbonyliron unit is a result of

a degenerate ‘Cope like’ rearrangement of the ligands.

Schematic presentations of the energy profile diagrams for

the hopping of Fe(CO)3 moiety on the molecules are shown

in Figs. 9–12. The TS for such a motion has only two short

C–Fe bonds which are shorter than the C–Fe bonds in the

GS. For all the molecules there are different isomers

formed by the rotation of Fe(CO)3 unit relative to the diene

molecule. The variations in energies due to this rotation for

(semibullvalene)Fe(CO)3 and (1,5-hexadiene)Fe(CO)3 are

shown in Figs. 13 and 14. Here we have plotted the change

in potential energy w.r.t to the change in the average of the

dihedral angles that three Fe–CO bonds make with the

diene fragment (for the details of the dihedral angles

see Figs. 13 and 14). The plots clearly show that when

Fig. 14. The variation of energy when the Fe(CO)3 fragment is rotated relative to the diene fragment of 1,5-hexadiene molecule. The horizontal axis is the

average of the three dihedral angles (3–XX–17–19, 3–XX–17–20 and 3–XX–17–21, where XX is a dummy atom at the center of the four sided polygon formed

by 1–4). The potential is periodic with a periodicity of 1208. The rotation of the Fe(CO)3 fragment is visible in the top view of the molecule. The goemetries

marked in the plot are given in top view and side view.

Table 12

The bond-lengths (in A) for GS as shown in Fig. 15(a) of (hypostrophene)Fe(CO)3

1–2 5–6 9–10 9–7 7–3 3–2 1–21 2–21 3–21 4–21 21–24 21–22 C–O C–H

2.88 1.57 1.55 1.56 1.53 1.41 2.14 2.14 2.17 2.17 1.78 1.77 1.15 1.09

The C–Fe distances are slightly different.

Fig. 15. The optimized geometries of GS and TS structures of

(hypostrophene)Fe(CO)3; (a) is the GS (b) is the corresponding TS for

the motion of Fe(CO)3 unit as a whole.


the dihedral angle changes by 1208 one set of Fe–CO is

exchanged with another with the rest of the structure

remaining unaffected. So the periodicities of the curves are

1208. For (hypostrophene)Fe(CO)3 and (syn-TOD)Fe(CO)3

the energies required for the rotation of Fe(CO)3 fragment

are rather small (only 0.11 and 0.43 kcal/mol, respect-

ively), which means the potential energy curve is relatively

flat and is therefore not plotted. Here, one is reminded of

the rotation of the tail of bacteriophage w.r.t the

icosahedral head membrane, where the head has icosahe-

dral symmetry and the portion that joins the tail to the head

has six-fold axis and hence there is a symmetry mismatch,

which makes the potential curve almost flat [37]. In these

Table 13

The bond-lengths (in A) for TS, as shown in Fig. 15(b) of (hypostrophene)Fe(CO)3

3–4 9–10 7–8 9–7 7–3 9–6 3–21 21–22 21–24 C–O C–H

2.51 1.56 1.57 1.57 1.53 1.55 2.06 1.76 1.82 1.15 1.09


two cases [(hypostrophene)Fe(CO)3 and (syn-

TOD)Fe(CO)3] there are symmetry mismatches, because

hypostrophene or syn-TOD has C2-axis whereas Fe(CO)3

fragment has C3-axis. Possibly, due to this symmetry

mismatch the potential energy curve is fairly flat. In these

two cases the periodicity for the rotation of Fe(CO)3

relative to the diene unit is 608. Due to the small

differences in energies between different isomers arising

from the rotation of Fe(CO)3 fragment the rotation would

easily occur at room temperature. For the TS also one can

think of different isomers obtained by the rotation of

Fe(CO)3 unit. But, we found that only one of them has the

particular imaginary vibrational mode we are looking for

(except (semibullvalene)Fe(CO)3). We have looked for and

identified the structure that has the appropriate normal

mode with imaginary frequency. Rotation of Fe(CO)3,

keeping the other parts fixed, would change the energy of

the state by roughly the same magnitude as in the GS. We

find the C–Fe distances in the GS structures to be at least

0.1 A longer than the C–Fe distances in the transition

states.

Fig. 16. The optimized geometries of the GS and TS structures of

(semibullvalene)Fe(CO)3; (a) is the GS (b) A TS structure and (c) The TS

structure corresponding to (a) for the motion of Fe(CO)3 unit as a whole.

6.1. Hypostrophene(tricarbonyliron) [(C10H10)Fe(CO)3]

This is a hypothetical molecule, which is obtained by

imagining the coordination of hypostrophene molecule to

Fe(CO)3. When Fe(CO)3 is bonded to hypostrophene and

the rearrangement takes place then Fe(CO)3 unit can hop to

five equivalent positions. All these have the same energies.

Though the molecule has not been synthesized, similar

type of molecules with cyclobutadiene and cyclooctate-

traene are known [7]. In the equilibrium structure the

Fe(CO)3 unit would have interactions with four C-atoms.

There is Cs symmetry for the GS and the TS structure of

(hypostrophene)Fe(CO)3. In the TS the Fe(CO)3 fragment

is attached to only two C-atoms. The small C–Fe distance

(2.06 A) in the TS suggests this bond to be a s-bond. The

molecule (hypostrophene)Fe(CO)3 can have isomers in

which Fe(CO)3 unit is rotated relative to the diene

fragment.

The different geometries have similar energies (differ

maximum by w0.11 kcal/mol). We give the detailed

geometry for only the lowest energy GS structure in

Table 12. The TS for the hopping motion of Fe(CO)3 unit

is shown in Fig. 15(b) and the detailed structure is given in

Table 13. For (hypostrophene)Fe(CO)3 the barrier height for

the rearrangement is 33.87 kcal/mol (B3LYP/6-31G** for

C, H, O and 6-311G** for Fe). The barrier is not low

and the reaction rate is very small and the reaction will not

take place at ordinary temperatures.

It is to be noted that though the Fe atom is at a bonding

distance from all the four C-atoms at the base of the

molecule, the bonds still retain some of its double bond

character. This is reflected from its comparatively shorter

single bond-length (w1.40 A). The detailed structure shows

that the GS is an example of a p-complex of tricarbonyliron

with all four C–Fe distances being nearly equal, and a pair

of short single bonds between 3–2 and 1–4. The small

difference between the four C–Fe distances is quite well

known [6].

6.2. Semibullvalene(tricarbonyliron) [(C8H8)Fe(CO)3]

Free semibullvalene molecule has Cs symmetry, but if one

attaches a Fe(CO)3 unit to this molecule it can loose all its

symmetry. Semibullvalene coexists in two structures and

there are rapid interconversions between the two, due to

degenerate Cope rearrangement. Fe(CO)3 can be coordinated

equally well to both the structures. Degenerate Cope

rearrangement together with Fe(CO)3 hopping would lead

to interconversion between the two coordinated structures.

Also, for (semibullvalene)Fe(CO)3, there are different

possible orientations (see Fig. 13) of the Fe(CO)3 w.r.t the

double bonds, which gives rise to different isomers (rota-

tional), with different energies (maximum energy difference

is 7.5 kcal/mol) as well as different symmetries. In this

case we have plotted the variations in energy with the average

of the dihedral angles between the atoms 18–17–8–7,

19–17–8–7 and 20–17–8–7 along the horizontal axis. The

isomer in Fig. 16(a) is found to be most stable form.

There are two probable TSs for the movement of the

tricarbonyl on semibullvalene molecule and both have Cs

symmetry. An analysis of normal modes of the structures

shown in Fig. 16(b) and (c) showed both to be the TSs for

Table 15

The bond-lengths (in A) for the TS, the structure shown in Fig. 16(c) of (semibullvalene)Fe(CO)3

5–6 1–2 7–8 6–7 2–6 2–17 17–18 17–20 C–O C–H

1.53 2.53 1.51 1.52 1.53 2.05 1.75 1.82 1.15 1.09

Table 14

The bond-lengths (in A) for GS, the structure shown in Fig. 16(a) of (semibullvalene)Fe(CO)3

1–2 3–4 5–6 7–8 8–3 3–2 2–6 2–17 3–17 4–17 1–17 17–18 17–19 C–O C–H

2.83 2.24 1.52 1.54 1.52 1.41 1.51 2.13 2.16 2.16 2.13 1.78 1.78 1.15 1.09

Table 16

The bond-lengths (in A) for the geometry shown in Fig. 16(b) of (semibullvalene)Fe(CO)3

5–6 1–2 7–8 6–7 2–6 2–17 17–18 17–20 C–O C–H

1.51 2.54 1.51 1.52 1.53 2.07 1.76 1.82 1.15 1.09


the motion of Fe(CO)3 unit as a whole, with the later one

being more stable by 6.9 kcal/mol. The structure in Fig. 16(c)

is the TS for the structure shown in Fig. 16(a) with an

imaginary mode of 99 cmK1 corresponding to the motion we

are looking for. The geometry shown in Fig. 16(b) is also a TS

for the same kind of motion of tricarbonyliron moiety. The

difference between the structures shown in Fig. 16(b) and (c)

is just the changed orientation of the CO bonds in space. The

structure in Fig. 13(b) has an imaginary mode of 431 cmK1

which corresponds to the same hopping motion. When the

GS is the structure as in Fig. 16(a) and the TS is as in

Fig. 16(c) then the barrier height is 37.25 kcal/mol (B3LYP/

6-31G** for C, H, O and 6-311G** for Fe). The two

transition state structures have nearly identical geometries

only differing in the orientation of Fe(CO)3 unit w.r.t the rest

of the molecule. In case of semibullvalene we note that for the

free molecule the Cope rearrangement barrier is much

lower in comparison to (semibullvalene)Fe(CO)3. The free

molecule (GS) is very strained and hence there is

energetically less difference with the TS. But, bonding

the free molecule to a Fe(CO)3 unit already releases some

strain in the GS in comparison to the TS and hence the

barrier height increases.

In the GS for the fluxional motion of Fe(CO)3 unit, a

p-complex is formed, and the four C–Fe distances differ

slightly. The pair of double bonds in the parent semibullva-

lene molecule retains some of its identity, which is reflected

in a shortened single bond of 1.41 A. In the TS there are

only two C–Fe bonds which are much shorter than the C–Fe

bonds we find in the GS structure. The detailed bond-

distance for GS structure and the two TS structures are given

in Tables 14–16.

Fig. 17. The optimized geometries of GS and TS structures of (syn-

TOD)Fe(CO)3; (a) is the GS (b) is the corresponding TS for the motion of

Fe(CO)3 unit as a whole.

6.3. syn-TOD (tricarbonyliron) [(C8H8)Fe(CO)3]

We have already presented the study of Cope rearrange-

ment of free syn-TOD before. Now we present our study for

(syn-TOD)Fe(CO)3. For (syn-TOD)Fe(CO)3 degenerate

Cope rearrangement along with Fe(CO)3 hopping can shift

the Fe(CO)3 unit into four possible equivalent positions, all

having same energies. It is a surprising fact that syn-TOD

molecule retains its tricyclic structure when coordinated to

Fe(CO)3, as it is already known that for cyclobutene

molecule, bonding to a Fe(CO)3 unit results in ring opening

[38]. For (syn-TOD)Fe(CO)3 the GS is not symmetric, but

the TS has Cs symmetry. For (syn-TOD)Fe(CO)3 molecule

there can be various possible geometries for the GS which

differ only in the orientation of the C–O bonds and these

isomers have nearly the same energy (maximum difference

is 0.43 kcal/mol). In Fig. 17(a) there is a small asymmetry in

the four C–Fe bond-lengths. The detailed geometries of

different isomers of (syn-TOD)Fe(CO)3 are given in

Tables 17 and 18. The structure shown in Fig. 17(b) is the

transition state structure corresponding to the motion of

Fe(CO)3 unit and has been confirmed by vibrational analysis

which shows an unstable mode corresponding to the motion

of Fe(CO)3 unit. It has two short C–Fe bonds. The barrier

height for the fluxional motion of the tricarbonyl fragment is

32 kcal/mol (B3LYP/6-31G** for C, H, O and 6-311G**

for Fe).

6.4. 1, 5-Hexadiene(tricarbonyliron) [C6H10Fe(CO)3]

We have chosen 1,5-hexadiene in the boat confor-

mation. In this form the double bonds are on the same side,

Table 20

The bond-lengths (in A) for TS, the structure as given in Fig. 18(b) of (1,5-hexadiene)Fe(CO)3

1–2 3–4 2–6 2–17 17–18 17–20 C–O C–H

2.46 1.55 1.53 2.07 1.76 1.82 1.15 1.09

Table 19

The bond-lengths (in A) for GS, the structure in Fig. 18(a) of (1,5-hexadiene)Fe(CO)3

1–2 1–3 3–4 1–5 5–6 2–6 2–4 3–17 4–17 1–17 2–17 17–18 17–19 17–20 C–O C–H

2.80 1.38 3.11 1.51 1.53 1.52 1.42 2.25 2.09 2.28 2.13 1.78 1.77 1.77 1.15 1.09

Table 18

The bond-lengths (in A) for TS, the structure shown in Fig. 17(b) of (syn-TOD) Fe(CO)3

14–15 5–6 1–2 5–15 1–5 1–7 7–8 7–10 C–O C–H

1.55 1.57 2.48 1.57 1.56 2.04 1.76 1.82 1.15 1.09

Table 17

The bond-lengths (in A) for GS, the structure given in Fig. 17(a) of (syn-TOD)Fe(CO)3

1–2 3–4 5–6 1–3 1–5 5–15 1–7 2–7 3–7 4–7 7–8 7–10 C–O C–H

2.92 2.83 1.56 1.41 1.54 1.57 2.14 2.13 2.17 2.16 1.78 1.79 1.15 1.09


and the pair of double bonds can accommodate the

Fe(CO)3 unit. Here the situation is very similar to (semi-

bullvalene)Fe(CO)3 and Fe(CO)3 unit has two alternative

position to coordinate. The equilibrium geometries are

asymmetric here.

When Fe(CO)3 is bonded to 1,5-hexadiene there may be

many possible isomers arising from the relative position of

the C–O bonds with respect to the diene fragment of the

molecule. The energy changes for the rotation of the

Fe(CO)3 unit is shown in Fig. 14. The geometries for the GS

and the TS are given in Tables 19 and 20. The GS is shown

in Fig. 18(a) for which there are four C–Fe bonds similar in

length. The TS for the hopping motion is shown in

Fig. 18(b) and this structure has a Cs symmetry. For (1,5-

hexadiene)Fe(CO)3 we find the lowest energy barrier

(21.92 kcal/mol, B3LYP/6-31G** for C, H, O and

6-311G** for Fe) for the hopping motion. All the barrier

heights and rates are tabulated in Table 21.

Fig. 18. The optimized geometries of the GS and TS structures of (1,5-

hexadiene)-Fe(CO)3; (a) is the GS and (b) is the TS for the motion of

Fe(CO)3 unit as a whole.

7. Conclusions

We predict that syn-TOD molecule, like hypostrophene

molecule can undergo Cope rearrangement and the barrier

height (24.25 kcal/mol) is also close to the barrier for Cope

rearrangement of hypostrophene molecule. We have also

presented DFT studies on four hypothetical tricarbonyliron

molecules derived form hypostrophene, semibullvalene,

syn-TOD and 1,5-hexadiene. We have studied the geome-

tries and the energetics of the hopping of the tricarbonyl unit

from one face of the ligand to an adjacent face. The barrier

heights are in the range of 21.92–37.25 kcal/mol for

different molecules. (1,5-Hexadiene)Fe(CO)3 molecule has

the lowest barrier height. Though the barrier heights that we

have obtained are not that small, it is an interesting kind of

fluxional motion that we find. Perhaps a bigger metal atom

may reduce the barrier heights. Also it should be possible to

tune the barrier heights by substituting the parent fluxional

molecule as well.

Table 21

The barrier heights for the hopping of Fe(CO)3 unit onto the faces of the

molecules

Molecule E0 (kcal/

mol)

Unstable vib.

mode (cmK1)

Rate const.

(sK1)

(Hypostrophene)Fe(CO)3 33.87 203 2.16!10K12

(Semibullvalene)Fe(CO)3 37.25 99 1.06!10K14

(syn-TOD)Fe(CO)3 32.00 259 4.88!10K11

(1,5-Hexadiene)Fe(CO)3 21.92 215 5.80!10K4

Rate constants axe calculated at 298.1 K.


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Documents

Fluxional hopping of Fe(CO) 3 in some of its complexes with dienes