Summary of chapter 2.5
2.6. Investigation of the crossover step in co-oligomerization reactions via ESI- ESI-TOF MS
The investigation of the polymer-species formed in the ROMP process after quenching was performed with MALDI-TOF mass spectrometry in the previous chapter. To detect and evaluate the catalyst species involved in this process, an analytical method was required that allowed the investigation directly from solution. The monitoring of the catalytic species via NMR spectroscopy is limited to the detection of the initiating and propagating carbene and the determination of the kp/ki ratio, with no detailed information on the number of monomer units incorporated. Thus, the catalytic species present in solution were investigated via ESI-TOF MS, which allowed the transfer of the living species from solution to the gas phase by a gentle ionization. For the investigations the selection of catalysts G1 and G3 was extended to the catalysts Umicore M1 (U1) and Umicore M3 (U3) which differ by their indenylidene-ligand from the Grubbs type catalysts. The set of monomers (1-4) remains unchanged (Scheme 2.16).
[Ru]
R
O O
O O
[Ru]
O O
O O
R O
O O O
[Ru]
R
O O
O O
O
O O
O
N O
N O
O N O
(CH2)3 C4F9
ESI-TOF MS
O
1 2 3 4
G1, G3, U1, U3 (1)
monomers 2, 3, 4
Monomers
Scheme 2.16. Co-oligomerization reactions monitored via ESI-TOF MS, using the monomers (1-4) and catalysts (G1, G3, U1 and U3), Mes: mesitylene, Ph: phenyl, sum formulas: 1 (C11H14O4), 2 (C15H12F9N1O3), 3 (C10H10), 4 (C27H42N2O6), G1 (C43H72Cl2P2Ru1), G3 (C38H40Br2Cl2N4Ru1), U1 (C51H76Cl2P2Ru1), U3 (C41H41Cl2N3Ru1).
67 The chain length of the polymeric species during crossover reaction was significantly reduced in comparison to the samples measured by MALDI MS from 15 units of monomer 1 to one unit since ESI-TOF is limited in its detection of the mass range to about 6000 Da. Furthermore, the reduction in chain length allows to study the very first step of the initiation and propagation, thus visualizing the reacted and unreacted catalysts species as a result of the kp/ki-ratio.
The preparation of the samples for the measurement was done by diluting the reaction solution (DCM) by a factor of 100. The dilute solution is then mixed with a solution of LiCl (0.1 mg in 10 mL) in MeOH/acetonitrile (v/v = 100/1). Subsequently, the ESI MS measurements in ion positive mode were done by direct injection of the premixed analyte solution into the ESI mass spectrometer. To get an insight into the crossover reaction before and after the point of crossover, two sets of experiments were performed: as first experiments, the addition of 1 equiv. of monomer 1 to the catalysts G1, G3 and U1, U3 was conducted. Secondly the addition of 1 equiv. of monomer 1 and the subsequent addition of one equivalent of the monomers 2, 3 or 4 to the selected catalysts (Scheme 2.16) was performed to study the crossover-reactions.
2.6.1. Overview of detected ions
Before starting the discussion of the mass spectra, a nomenclature for the different types of ions, detected via ESI-TOF MS, is introduced. An overview of this is given in Scheme 2.17, describing only the ions a letter, omitting neutral species, as they are not detected by ESI-TOF MS. As an example, the molecule ion by loss of chloride [M – Cl]+ for Grubbs catalyst 1st-generation (G1) will be denominated as G1a. For oligomer species with inserted monomer the same nomenclature will be applied, thus designating an ion of Grubbs catalyst 1st-generation, generated by loss of one chlorine and one phosphine with two inserted units of 1 [M + two units of 1 – Cl – PCy3]+ as G1b-(1)2. The co-oligomer species will be named analogously, e.g. catalyst G1 after insertion of one unit of monomer 1 followed by insertion of one unit of monomer 3 and ionized by loss of chloride and phosphine [M + one unit of 1 + one unit of 3 – Cl – PCy3]+ will be designated as G1b-(3)1(1)1. The order G1-(3)x(1)y is a consequence of the olefin metathesis mechanism where the monomer is always inserted into the metal carbene double bond, thus the second monomer is attached to the catalyst. The structures labeled with * and # in Scheme 2.17 depict the neutral catalysts and therefore the starting structures for catalyst G3 and the catalysts G1, U1, U3 respectively.
68
Ru L
Cl L'
R Cl
Ru L
L' R Cl
Ru L
R Cl
Ru L
R -H
Ru L
Cl Ru
L
-3H -R, -2H
+ +
+
+ + +CH3CN
Ru L
L' R Cl
+
H3CCN
Ru L
R Cl
+
NCCH3
Li+
Ru L
Cl L'
R Cl
-LiCl
+
Ru L
Cl R Cl
+
Li Li
-LiCl
- HCl -2H +L'
-L'
-CH3CN
+L' -L'
+L' -L'
a b c
d e
f g h i
Ru L
R -H +
L' k
Ru L
Cl L'
R Cl
-L' +L' L'
-HCl
+CH3CN
-CH3CN
- R
- Cl- - HCl
+L' -LiCl
L =
PCy3(G1,U1) NHC (G3,U3)
L' =
PCy3(G1,U1) 3-bromopyridine (G3) pyridine (U3)
R =
benzylidene (G1,G3) indenylidene (U1,U3)
#
*
neutral catalyst
*(G3)
# (G1,U1,U3)
Scheme 2.17. Overview on types of detected ions and proposed fragmentation pathway, NHC: N-heterocyclic carbene (C21H26N2), benzylidene (C7H6), indenylidene (C15H10).
In solution, the neutral catalysts cleave off and rebind the neutral ligands L’, thus e.g. an equilibrium exist between mono and bisphosphine species for catalyst G1. The types of detected ions in the mass spectrum show that there are multiple ways for the ruthenium complexes G1, G3, U1 and U3, with or without oligomer attached, to be ionized. Structures a and b, ionized by loss of chloride, appear together with their acetonitrile adducts d and e. The loss of the second chlorine as hydrogen chloride leads to structure c. Further fragment ions (catalyst species h and i), generated by loss of the carbene ligand are detected in the range of 400-500 m/z. Structures f and g are formed by the addition of alkali metal ions like lithium to the neutral catalyst species, as described by Wang and Metzger.79 Cleavage and rebinding of neutral ligands like L’ or acetonitrile are likely to happen during the ESI-process.80 Other fragmentation steps like loss of hydrogen chloride or the carbene ligand are assumed to be irreversible.
69
2.6.2. Reaction of catalysts G1, U1 with monomer 1
For the first experiments, 1 equiv. of monomer 1 was reacted with the catalysts G1, G3, U1 and U3. For the reaction catalyst G1 with 1 equiv. of monomer 1 the most prominent peaks could be assigned to unreacted catalyst species G1a (787.4 m/z), G1b (507.2 m/z), G1c (471.2 m/z) and G1d (828.4 m/z).
Species with inserted monomer 1 (up to 5 units) could be detected with much lower intensities e.g. at G1e-(1)1 at 758.3 m/z and G1e-(1)2 at 968.4 m/z. compared to the unreacted catalyst species (Figure 2.17, appendix, Table 5.1). A similar picture is seen for the reaction of 1 equiv. of monomer 1 with catalyst U1 (Appendix, Table 5.3). The main peaks can be assigned to the unreacted catalysts species U1a (887.4 m/z), U1b (607.2 m/z), U1d (928.4 m/z) and U1e (648.2 m/z). Species with inserted monomer 1 were detected with significant lower intensities e.g. U1b-(1)1 (817.3 m/z), U1c-(1)1 (781.3 m/z) and U1e-(1)1 (858.3 m/z).
Figure 2.17. ESI-TOF MS spectra in the range of 400 to 1100 m/z, a) reaction of catalyst G1 with 1 equiv.
of monomer 1, b) reaction of catalyst G1 with 1 equiv. of monomer 1 with 5 equiv. of hydrochloric acid (solution in diethyl ether).
70 Thus in both the cases, unreacted catalyst species dominate the mass spectrum. Under the chosen measurement conditions the ruthenium complexes are mainly ionized by loss of chlorine. Acetonitrile as neutral ligand can coordinate to the ruthenium complexes, e.g. G1d (828.4 m/z) or U1e-(1)1 (858.3 m/z).
The species with incorporated monomer are mostly observed as monophosphine complexes, while the unreacted catalyst species are observed as bisphosphine complexes e.g. G1a, G1d.
To investigate the effect of an additive, the reaction of catalyst G1 with 1 equiv. of 1 was repeated in the presence of 5 equiv. of hydrochloric acid, since the addition of acid is known to accelerate the ROMP -process by trapping cleaved off phosphine as phosphonium salt thus converting the inactive catalyst in its active form (Scheme 2.18).30,31
O O O
O O
O O O
Ph Ru
Cl Ph Cl
PCy3 PCy3
Ru Cl Ph
Cl PCy3 -PCy3
+PCy3
Ru Cl Ph
Cl
PCy3 PCy3
Ru Cl Ph
Cl PCy3 +HCl
H PCy3 Cl
Ru PCy3 Cl
Cl
+PCy3 -PCy3
O O
O O Ph Ru
PCy3 Cl Cl
PCy3
Polymerization
inactive active
active
inactive
ESI-process -Cl -Cl + CH3CN
G1b-(1)1-3 G1e-(1)1-3 (G1)
(G1) (1)
Scheme 2.18. Effect of the HCl-addition on the reaction of catalyst G1 with monomer 1, and possible