Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on January 21, 2010. Copyright Margret J. Geselbracht 2010. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
Titanium-Catalyzed Ethylene Trimerization
1. While trolling the literature, I found a really cool organometallic paper on titanium-catalyzed
ethylene trimerization, discussing the important role of hemilabile ligands in this catalysis. A
hemilabile ligand combines both a tightly bound donor group with a substitutionally labile
group. In this case, the titanium catalyst contains a tightly bound cyclopentadienyl ligand
modified with a pendant labile arene group, which can coordinate to the titanium in a number
of different ways.
(a) The active catalyst, [(Cp-arene)TiMe2]+, is formed in two steps according to Scheme 2. A
large series of similar materials was prepared in order to explore the effects of minor
variations in structure on catalytic activity. X-ray crystal structures of 3b, 3c, and 3f
confirm the so-called ansa-Cp-arene coordination where both the Cp and the arene ring
coordinate to the titanium. For each of the 3 classes of complexes below (1, 2, and 3),
determine the titanium oxidation state and count the number of electrons in the complex.
Show your counting methods in detail. Note: It is actually quite common to not achieve
18 e- in titanium complexes…remember, it’s only a guideline! (0.5 pts for each complex,
1.5 pts. total)
I favor the Donor Pair method of electron counting, where chlorides and methyl ligands
carry a negative charge and each donate 2 e- to a metal center. The cyclopentadiene
ligand carries a negative charge and donates 6 π e- to a metal center.
Complex 1: Oxidation state of Ti is +4 to balance the 3 Cl– and the anionic Cp ligand,
resulting in a neutral complex. Ti(IV) is d0, so this is a 6 + 3(2) = 12 e- complex.
Complex 2: Oxidation state of Ti is +4 to balance the 3 CH3– and the anionic Cp ligand,
resulting in a neutral complex. Ti(IV) is d0, so this is a 6 + 3(2) = 12 e- complex.
Complex 3: Oxidation state of Ti is +4 to balance the 2 CH3– and the anionic Cp ligand,
leaving the complex with an overall +1 charge. Counting electrons on this can be tricky,
depending on how you interpret the dashed line to the arene pendant. The Ti(IV) is still
d0, and the Cp and methyls contribute 6 + 2(2) = 10 e-. A reasonable assumption based
on the drawing is that the coordinated arene donates all 6 π e- to the Ti(IV) resulting in a
16 e- complex.
(b) The proposed catalytic cycle for ethylene trimerization based on computational studies is
shown below in Scheme 1. For each complex in the cycle, determine the titanium
oxidation state. Determine the number of electrons in the complex, again showing the
method in your madness. (0.5 pts. for each complex, 2.5 pts. total)
First of all, I should start by saying that two organometallic chemists from VIPEr have
called some of the proposed intermediates in this catalytic cycle “wacky” and unusual.
The exact meaning of the dashed line to one vertex of the arene ring in the three
intermediates on the top and right sides of Scheme 1 is unclear. It could indicate an
agostic interaction with a C–H bond of the arene, where the C–H bonding electrons are
weakly donated to the Ti center. Or another interpretation is that there is a weak
electrostatic cation–π interaction between the cationic Ti center and the π-cloud of the
arene. For the purposes of accounting, I will assume that there are 2 e- donated to the Ti
center in this “slipped” coordination mode of the pendant arene.
Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on January 21, 2010. Copyright Margret J. Geselbracht 2010. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
The coordinated arene is assumed to be neutral in all cases as are the coordinated alkenes.
Coordinated alkyl ligands are assigned a negative charge, except for an agostic C–H
interaction, which is neutral.
Ti
Me
Me
Ti
Ti
Ti(IV): 2 anionic methyls
+ anionic Cp and overall
cationic charge
6 e- (Cp) + 6 e- (arene) +
(2)2 e-(methyl) = 16 e-
Ti
H
H
Ti(II): anionic Cp and
overall cationic charge
6 e- (Cp) + 6 e- (arene) +
2 e- (ethylene) + 2 e- (Ti)
= 16 eTi(II): anionic Cp and
overall cationic charge
6 e- (Cp) + 2 e- (arene) +
4 e- (2x ethylene) + 2e(Ti) = 14 e-
Ti
H
H
Ti
Ti(IV): 2 anionic alkyls
+ anionic Cp and
overall cationic charge
6 e- (Cp) + 2 e- (arene)
+ (2)2 e- (alkyl) + 2 e(agostic C–H) = 14 eTi(IV): 2 anionic alkyls
+ anionic Cp and
overall cationic charge
6 e- (Cp) + 2 e- (arene)
+ (2)2 e- (alkyl) + 2 e(agostic C–H) = 14 eTi(II): anionic Cp and
overall cationic charge
6 e- (Cp) + 6 e- (arene)
+ 2 e- (alkene) + 2 e(Ti) = 16 e-
(c) Variable temperature NMR spectroscopy provided evidence for hemilabile coordination
of the Cp-arene ligand. The authors provided the following potential mechanism for
arene exchange to explain their NMR data. Briefly describe what you would see for the
arene methyl resonances in the 1H NMR as a function of temperature if this arene
exchange is occurring. Why did these studies need to be carried out on 3d–3h, the
trimethylsilyl derivatives, rather than 3a–3c? (2 pts. for correct answers, partial credit
given)
If the proposed arene exchange mechanism (dissociation of the arene from the Ti
followed by rotation of the pendant arene group and then coordination again to the Ti
center) is occurring rapidly on the NMR timescale, then the two arene methyl groups,
labeled with a red A and a green B in Scheme 3, see an average environment and would
result in one resonance in the 1H NMR spectrum. However, as the temperature is
lowered and this intramolecular motion is frozen out, methyls A and B will each see a
different environment and two distinct resonances should be observed. The trimethylsilyl
derivatives are necessary to study this behavior in order to introduce asymmetry into the
molecule. If the Cp ring is unsubstituted as in 3a–3c, a plane of symmetry renders the
arene methyl groups A and B equivalent, leading to one resonance in the NMR regardless
of the temperature.
(d) The Cp-arene titanium dicarbonyl cations were also prepared. Scheme 4 shows two
different “resonance contributors” (the authors’ language, not mine) for the structure of
the dicarbonyl and the authors state that the X-ray crystal structure of 4b provides
evidence for the importance of resonance contributor B, a Ti(IV)/dienedialkyl complex of
Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on January 21, 2010. Copyright Margret J. Geselbracht 2010. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
the cyclohexadiene dianion. What sort of structural evidence would you look for to back
up this claim? Be specific. (1 pt., must have at least 2 predictions)
If resonance contributor B is important, the aromaticity of the pendant arene is broken
and we would expect a deviation from planarity in the six-membered ring. Examining
the C–C bond angles should reveal non-planarity in the arene ring. Furthermore, if the πelectrons of the arene have localized in a dienedialkyldianion fashion, we should expect
variation in the C–C bond distances within the arene ring. Specifically, the C–C bonds of
the localized double bonds should be shorter than the C–C single bonds. In fact, this is
exactly what is observed, 1.387(6) and 1.396(6) Å for the double bonds and an average of
1.419 Å for the C–C single bonds.
(e) Finally (actually the paper keeps going, but we will stop here), the IR data for the
dicarbonyl complex and related complexes are shown in Table 6. The authors argue that
the IR data also supports the importance of resonance contributor B above. Explain why
this is the case, in particular where you would expect to see for the CO frequencies if
contributor A was important. Rationalize the trends in CO frequencies. It is not
sufficient to say, “because it would then look like the frequencies in these other
compounds.” (1 pt.)
One way to formally distinguish resonance contributors A and B is the formal Ti
oxidation state: resonance contributor A with an 6- neutral arene is formally Ti(II),
whereas resonance contributor B with a dialkyldienedianion arene is formally Ti(IV).
Ti(II) is d2 compared to d0 Ti(IV), and one would expect more π-back bonding in
carbonyl complexes of Ti(II) compared to Ti(IV). In fact, Ti(IV) carbonyls are quite rare
since the lack of d electrons eliminates the stabilizing effects of π-back bonding for the
carbonyl ligand. More π-back bonding in Ti(II) carbonyls puts more electron density in
the π* orbitals of CO, weakening the coordinated C–O bond. This would be observed in
lower CO stretching frequencies in the IR spectrum as is seen in Table 6 for the Ti(II)
carbonyl Cp2Ti(CO)2 as compared to the Ti(IV) carbonyl [Cp2Ti(CO)2]2+. The CO
stretching frequency for compound 4b is intermediate between these two extremes
although closer to the Ti(IV) complex than the Ti(II) complex. This supports the
importance of contributor B in the real description although like true resonance
structures, contributor A cannot be completely disregarded.