SPECTROSCOPIC STUDIES OF METAL CARBONYL
COMPLEXES FOR SMALL MOLECULE ACTIVATION





KEE JUN WEI





NATIONAL UNIVERSITY OF SINGAPORE
2013

SPECTROSCOPIC STUDIES OF METAL CARBONYL
COMPLEXES FOR SMALL MOLECULE ACTIVATION





KEE JUN WEI
(B.Sc. (HONS), NUS)



A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY
NATIONAL UNIVERSITY OF SINGAPORE
2013

Thesis Declaration
I hereby declare that this thesis is my original work, performed
independently under the supervision of A/P Fan Wai Yip, (in the IR and Laser
Research Laboratory), Chemistry Department, National University of
Singapore, between 8 August 2008 and 4 January 2013.
I have duly acknowledged all the sources of information which have
been used in the thesis. This thesis has not also been submitted for any degree
in any university previously.
The content of the thesis has been partly published in:
[1] Kee, J. W.; Fan, W. Y. “Infrared studies of halide binding with
CpMn(CO)2X complexes where X=ligands bearing the O-H or N-H group”
Journal of Organometallic Chemistry, 2013, 729, 14-19.
[2] Kee, J. W.; Chong, C. C.; Toh, C. K.; Chong, Y. Y.; Fan, W. Y.
“Stoichiometric H
2
Production from H
2
O upon Mn
2
(CO)
10
photolysis” Journal
of Organometallic Chemistry, 2013, 724, 1-6.
[3] Kee, J. W.; Tan, Y. Y.; Swennenhuis, B. H. G.; Bengali, A. A.; Fan, W. Y.
“Hydrogen Generation from Water upon CpMn(CO)3 Irradiation in a
Hexane/Water Biphasic System” Organometallics 2011, 30, 2154-2159.


Kee Jun Wei



20/07/2013

Name

Signature

Date

i

Acknowledgement
First and foremost, I would like to express my gratitude towards my
supervisor and mentor, Assoc. Prof. Fan Wai Yip, for his guidance and
patience which made the completion of my PhD possible. I am grateful for the
opportunities and advices that he has given me through the years.
My gratitude extends to the members of the group, whom I have had
the pleasure of working with, including Tan Sze Tat, Toh Chun Keong, Tan
Kheng Yee Desmond, Chong Yuan Yi, Fong Wai Kit, Chong Che Chang, Tan
Yong Yao, Tan Xiang Yeow, Alvin Then, Sum Yin Ngai, Soh Wei Quan
Daniel, Quek Linken, Lim Xiao Zhi, Chow Wai Yong, Goh Wei Bin, and
Yang Jiexiang. I would like to thank them for their help and support all these
years.
I also appreciate the support from Mdm Han Yanhui from the NMR
Laboratory and Mdm Patricia Tan from the Physical Chemistry Laboratory. I
would also like to extend my gratitude to the staff of the Chemistry

Department who helped me in various ways. I am also grateful to the National
University of Singapore for awarding me a research scholarship and giving me
the opportunity to pursue my degree.
Lastly, I would like to acknowledge the encouragement that my family
and wife, Zenn Ong, has given me throughout the years. Their support has
allowed me to persevere through.

ii

Table of Contents
Acknowledgement
i
Table of Contents
ii
Summary
viii
List of Tables
x
List of Figures
xii
List of Schemes
xvi
Abbreviations
xix
List of Symbols
xxi

CHAPTER 1 Introduction
1
1.1 Small Molecule Activation

2
1.2 Photochemical Water-splitting
5
1.3 Metal Carbonyl Compounds
6
1.3.1 Cyclopentadienyl Manganese Carbonyl
10
1.3.2 Dimanganese Decacarbonyl
14
1.4 Computational Organometallic Chemistry
18
1.5 Objectives of the study
21
1.6 References
24

iii

CHAPTER 2 O-H bond weakening in
CpMn(CO)
2
(CH
3
OH) : Generation of the
CpMn(CO)
2
(CH
3
O) radical upon H atom abstraction by
O

2

32
2.1 Introduction
33
2.2 Experimental Section
35
2.2.1 Materials and methods
35
2.2.2 Synthesis of CpMn(CO)
2
(RNH
2
)
36
2.2.3 Synthesis of CpMn(CO)
2
(C
12
H
25
SH)
36
2.2.4 Synthesis of CpMn(CO)
2
(ROH) complexes and
investigation of their reactions with air
37
2.2.5 In situ NMR spectroscopy of CpMn(CO)
3

photolysis in
CD
3
OD
37
2.2.6 H atom abstraction reactions with dpph and H
2
O
2

38
2.2.7 PPh
3
substitution reactions of CpMn(CO)
2
(CH
3
O) radical
38
2.3 Results and Discussion
39
2.3.1 Evidence for CpMn(CO)
2
(RO) radical complex formation
39
2.3.2 Computational studies of bond weakening
48
2.3.3 Electron Delocalization and NBO Spin Analyses
52
2.3.4 Evaluation of OH bond activation for other complexes

54
2.4 Conclusion
62
iv

2.5 References
63

CHAPTER 3 Hydrogen Generation from Water upon
CpMn(CO)3 Irradiation in a Hexane/Water Biphasic
System
68
3.1 Introduction
69
3.2 Experimental Section
71
3.2.1 Materials and methods
71
3.2.2 Photolysis of CpMn(CO)
3
in hexane/water mixture
71
3.2.3 NMR quantification of cyclopentadiene
72
3.2.4 Mass spectrometric determination of hydrogen
72
3.2.5 Deuteration studies
73
3.2.6 Analysis of hydrogen peroxide production
73

3.2.7 Photolysis of CpMn(CO)
3
suspended in water
75
3.2.8 Photolysis of CpMn(CO)
3
in cyclopentadiene
76
3.2.9 Time-Resolved Infrared Spectroscopy
76
3.2.10 Reaction of CpMn(CO)
2
(THF) with water
77
3.3 Results and Discussion
77
3.3.1 H
2
and H
2
O
2
production
77
3.3.2 NMR and IR spectroscopy
81
v

3.3.3 Mechanism and DFT Studies
85

3.3.4 Attempts to improve water activation
92
3.4 Conclusion
93
3.5 References
94

CHAPTER 4 Stoichiometric H
2
Production from H
2
O
upon Mn
2
(CO)
10
photolysis
97
4.1 Introduction
98
4.2 Experimental Section
100
4.2.1 Materials and measurements
100
4.2.2 Mass spectrometric determination of H
2
and CO
2
100
4.2.3 Photolysis of Mn

2
(CO)
10
in cyclohexane/water mixture
101
4.2.4 Photolysis of Mn
2
(CO)
10
under a variety of conditions
102
4.2.5 Deuteration studies
103
4.2.6 Time Profile Monitoring
103
4.2.7 Photolysis of MnH(CO)
5
in cyclohexane/water
104
4.2.8 Photolysis of MnH(CO)
5
in dried cyclohexane
104
4.2.9 H-D exchange studies of MnD(CO)
5

105
4.2.10 Attempted thermal activation of H
2
O using Mn

2
(CO)
10

or MnH(CO)
5

105
4.2.11 Chemical analysis of solid residue
106
vi

4.2.12 Photolysis of Mn
2
(CO)
10
in acetic acid
106
4.3 Results and Discussion
107
4.3.1 IR and NMR Studies
107
4.2.2 Generation of H
2
and identification of H source
110
4.2.3 Proposed Mechanism and Computational Studies
114
4.4 Conclusion
123

4.5 References
123

CHAPTER 5 Infrared studies of halide binding with
CpMn(CO)
2
X complexes where X = ligands bearing the
O-H or N-H group
127
5.1 Introduction
128
5.2 Experimental Section
130
5.2.1 Materials and methods
130
5.2.2 Syntheses of CpMn(CO)
2
L complexes
130
5.2.3 Addition of halides to CpMn(CO)
2
L complexes
131
5.2.4 Incremental addition of fluoride to CpMn(CO)
2
(3-
hydroxyl-pyridine) complex
131
5.2.5 Displacement studies of ligands by PPh
3


132
5.2.6 Displacement of halides
132
5.3 Results and Discussion
133
vii

5.3.1 Spectroscopic characterization of CpMn(CO)
2
L
complexes
133
5.3.2 Spectroscopic studies of halide interactions
135
5.3.3 PPh
3
displacement reaction of halide-bound
CpMn(CO)
2
(3-OHpy)

139
5.3.4 Computational modeling of halide interactions
143
5.4 Conclusion
148
5.5 References
149
Appendix

151


viii

Summary
In this thesis, the reactivity of manganese carbonyl complexes
with various small molecules was studied using spectroscopic
techniques. In Chapter 1, an introduction to the use of transition metals
complexes in small molecule activation was presented. The ability of
the complexes to adopt various oxidation states and coordination
modes, as well as their photochemistry and spectroscopic
characterization, is key to the study of the Mn complexes, namely
CpMn(CO)
3
and Mn
2
(CO)
10
. Computational chemistry is used to
support the experimental results obtained throughout this dissertation.
The reactivity of CpMn(CO)
3
in the activation of OH bonds in
alcohols was explored in Chapter 2. Photogenerated
CpMn(CO)
2
(CH
3
OH) was found to react with oxygen, 1,1-diphenyl-2-

picrylhydrazyl radical or H
2
O
2
to give the radical complex of
CpMn(CO)
2
(CH
3
O), as supported by magnetic susceptibility studies,
NMR and IR spectroscopic studies. DFT computational studies were
employed to understand the weakening of the O-H bond. NBO spin
analysis suggested that the bond weakening was due to the transfer of
the single electron from the O atom to the Mn atom.
In Chapter 3, the stoichiometric generation of hydrogen
peroxide and hydrogen was observed upon photolysis of CpMn(CO)
3

in a hexane/water biphasic system, as supported by various chemical
assays and mass spectrometric studies. The main decomposition
ix

pathway of CpMn(CO)
3
is the protonation of the Cp ring to CpH as
evident by the formation of CpH complexes.
The photoreactivity of Mn
2
(CO)
10

with water was evaluated in
Chapter 4. Stoichiometric production of H
2
was observed along with
HMn(CO)
5
production. HMn(CO)
5
is proposed as the key intermediate
in this reaction as the production of H
2
correlated well with the
concentration of HMn(CO)
5
observed.
Finally, in Chapter 5, halide binding is tested on CpMn(CO)
2
L
complexes, where L = 4-hydroxypyridine, 3-hydroxypyridine, 3,5-
dimethylpyrazole or imidazole. These ligands possess OH and NH
groups for anion binding. ESI-mass spectrometric and IR
spectroscopic studies are used to study the halide binding. DFT
computational studies were used to model the proposed halide binding
on the ligand, showing agreement with the IR spectroscopic data
obtained for the complexes.

x

List of Tables
Table 2.1

ν
CO
IR stretching frequencies of selected
CpMn(CO)
2
(alcohol) complexes and the radical product
after exposure to air
41
Table 2.2
Experimental νCO

frequencies of selected CpMn(CO)
2
L
complexes
42
Table 2.3
Calculated enthalpies and νCO

frequencies for
CpMn(CO)
2
L complexes.
50
Table 2.4
Calculated enthalpies of H abstraction by oxygen from the
CpMn(CO)
2
L


to form the respective radical complexes and
HO
2
.
51
Table 2.5
Calculated bond dissociation enthalpies (BDE) of X-H
bonds.
52
Table 2.6
Calculated NBO spin densities for Mn atom and X atoms
of CpMn(CO)
2
(RX) radical complexes and the free radical
ligands.
55
Table 2.7
Calculated O-H bond dissociation and NBO spin analyses
for selected transition metal carbonyl complexes.
58
Table 2.8
Calculated O-H bond dissociation and NBO spin analyses
for selected CpM(CO)
x
complexes.
59
Table 2.9
Calculated O-H bond dissociation and NBO spin analyses
for CpRe(CO)
3

and CpMn(CO)
3
complexes.
59
Table 2.10
Calculated O-H bond dissociation and NBO spin analyses
for Cp
R
Mn(CO)
3
complexes.
60
xi

Table 2.11
Calculated O-H bond dissociation and NBO spin analyses
for CpMn(CO)
2
L complexes.
61

Table 5.1

CO
frequencies of CpMn(CO)
2
L complexes upon addition
of halide salt.
136
Table 5.2

Effect of solvents on the 
CO
frequency shifts of
CpMn(CO)
2
(3-OHpy) upon 10 equivalents of F
-

interaction.
138
Table 5.3
Enthalpies of selected CpMn(CO)
2
L complexes and
ligands with the 
CO
frequencies (if any) and Mn-L bond
enthalpies.
144

xii

List of Figures
Figure 1.1
Half-sandwich structure of CpMn(CO)
3
.

11
Figure 1.2

Structure of Mn
2
(CO)
10
comprising of two Mn(CO)
5
in a
staggered conformation.
15

Figure 2.1
(a) IR spectra of the product mixture upon photolysis of
CpMn(CO)
3
in neat methanol.
(b) Subtraction IR spectra for coordination product and
subsequent radical product as a result of oxidation by air.
40
Figure 2.2
(a) IR spectrum of CpMn(CO)
2
(CH
3
NH
2
) in THF.
(b) IR spectrum showing the CpMn(CO)
2
(PhNH
2

) in THF before
and after exposure to air, indicating that the formation of
CpMn(CO)
2
(PhNH) from CpMn(CO)
2
(PhNH
2
).
43
Figure 2.3
1
H-NMR spectrum of CpMn(CO)
2
(CH
3
OH) after 1 mole
equivalent of DPPH was added, indicating the production of
DPPH-H.
45
Figure 2.4
ESR spectra of (a) the initial radical complex (attributed to
CpMn(CO)
2
(CH
3
O) radical) formed upon the introduction of air
to CpMn(CO)
2
(CH

3
OH) and (b) its secondary decomposition
radical product upon longer exposures of air.
47
Figure 2.5
SOMO Diagram calculated for CpMn(CO)
2
(CH
3
O) radical
complex.
53
Figure 2.6
Optimised structures of selected CpMn(CO)
2
radical complexes.
56

xiii

Figure 3.1
(a). Mass spectra of the headspace content upon
CpMn(CO)
3
photolysis in a dodecane/H
2
O or dodecane/D
2
O
mixture. (b) Gas-phase mass spectra of the headspace

obtained from the hexane photolysis of CpMn(CO)
3
with
(i)H
2
O and (ii)D
2
O.
78
Figure 3.2
UV-Vis spectrum showing absorption maximum at 420nm
of 9.29 x 10
-4
M K
3
Fe(CN)
6
, (a) before addition, (b) after
addition of H
2
O
2
produced from 20 mins of photolysis,
corresponding to 0.95 x 10
-4
M H
2
O
2
and (c) after addition

of H
2
O
2
from 360 mins of photolysis corresponding to 2.95
x 10
-4
M H
2
O
2
.
80
Figure 3.3
(a) Photolysis of CpMn(CO)
3
in wet hexane, producing 2
sets of product peaks at CpMn(CO)
2
(η
2
-C
5
H
6
) and
CpMn(CO)
2
(-
2

,
2
-CpH)CpMn(CO)
2
(b) Photolysis of
CpMn(CO)
3
in neat cyclopentadiene. (c) Photolysis of
CpMn(CO)
3
in hexane solution of cyclopentadiene
(CpMn(CO)
3
:CpH = 2:1).
82
Figure 3.4
Difference FTIR spectrum obtained upon photolysis of
CpMn(CO)
3
in a water-saturated hexane solution.
84
Figure 3.5
Relative Enthalpies (in kJ per mole of CpMn(CO)
3
) of the
intermediates proposed in Scheme 3.3.
89

Figure 4.1
FTIR spectra recorded after a 2-hour broadband irradiation

of Mn
2
(CO)
10
in biphasic cyclohexane/water. (a) The
108
xiv

production of MnH(CO)
5
in the cyclohexane layer (b) The
production of CO, CO
2
and MnH(CO)
5
in the headspace
above the reaction mixture.
Figure 4.2
FTIR spectrum of MnH(CO)
5
and MnD(CO)
5
recorded
after a 2-hour broadband irradiation of Mn
2
(CO)
10
in a
biphasic cyclohexane/H
2

O and cyclohexane/D
2
O
respectively.
109
Figure 4.3
XRD analysis of the white precipitate MnCO
3
.
110
Figure 4.4
Mass spectra taken of the headspace content upon a 5-hour
photolysis of Mn
2
(CO)
10
in a hexane/H
2
O mixture,
representing the signal at m/e = 2. The signals obtained for
a 50 Torr H
2
standard and the headspace content of a
similar mixture prior to photolysis are included for
comparison.
111
Figure 4.5
Time profile showing the absorbance of Mn
2
(CO)

10
and
MnH(CO)
5

112
Figure 4.6
Time profile showing the percentage yield of H
2
per
Mn
2
(CO)
10
used at different time intervals throughout the
photolysis period.
112
Figure 4.7
Relative Enthalpies (in kJ per mole of Mn
2
(CO)
10
) of the
intermediates proposed in Scheme 4.3 (a) and (b).
118
Figure 4.8
Relative Enthalpies (in kJ per mole of Mn
2
(CO)
10

) of the
intermediates proposed in Scheme 4.3 (c).
120

xv

Figure 5.1
FTIR spectra of the 
CO
bands of CpMn(CO)
2
(3-
hydroxypyridine) (a) with 10 equivalents of F
-
and (b)
without F
-
in THF.
134
Figure 5.2
Negative-ion ESI mass spectra for CpMn(CO)
2
(3-
hydroxypyridine) (a) before and (b) after addition of Cl
-
.
137
Figure 5.3

The redshift of the two ν

CO
peaks of CpMn(CO)
2
(3-OHpy)
upon incremental addition of 1M THF solution of F
-
.
139
Figure 5.4

IR Spectra of the reaction mixture of CpMn(CO)
2
(3-OHpy)
and PPh
3
in chloroform in the absence of fluoride. (a)
Before the addition of PPh
3
(b) 30 minutes after the
addition of PPh
3
.
140
Figure 5.5
Time profile showing the change in the IR absorbances of
(a) CpMn(CO)
2
(3-OHpy) and (b) CpMn(CO)
2
PPh

3
with
and without F
-
binding upon addition of excess PPh
3
into a
chloroform

solution containing CpMn(CO)
2
(3-OHpy) at
50˚C.

141
Figure 5.6
Optimized structures of CpMn(CO)2(3-OHpy) and the
fluoride-bound CpMn(CO)2(3-OHpy), using b3lyp/lanl2dz.
145


xvi

List of Schemes
Scheme 1.1
Catalytic cycle for the hydroformylation of ethene,
catalyzed by HCo(CO)
4
.
3

Scheme 1.2
An example of manganese-oxo complexes that can act as
water oxidation catalysts.
4
Scheme 1.3
Mechanism of water splitting in a Z-scheme
photocatalytic system consisting of Ru/SrTiO
3
:Ph and
PRGO/BiVO
4
under visible-light irradiation.
6
Scheme 1.4
Reactions of metal carbonyl complexes.
7
Scheme 1.5
Cyclopentadienyl iron reactions characterized by νCO
stretching peaks.
8
Scheme 1.6
Different electronic transitions in metal carbonyl
complexes.
9
Scheme 1.7
Synthetic route to CpMn(CO)
3

10
Scheme 1.8

Reactions of the Cp ring on CpMn(CO)
3
.
12
Scheme 1.9
CpMn(CO)
3
derivatives recognized by recognized by
LAT1.
13
Scheme 1.10
Synthesis of CpMn(CO)
2
(THF) and subsequent reaction
to give CpMn(CO)
2
(N
2
) and CpMn(CO)
2
(PhN=NH).
14
Scheme 1.11
Reactions of Mn
2
(CO)
10
.
15
Scheme 1.12

Hydrosilylation of alkene with tertiary silanes by Mn-
2
(CO)
10
17
Scheme 1.13
Isomerization and hydrogenation of α-alkene by catalyzed
HMn(CO)
4
(PPh
3
) under photochemical conditions.
18
xvii

Scheme 1.14
Proposed mechanism for the displacement of arene from
CpMn(CO)
2
(arene) by pyridine.
20
Scheme 1.15
Optimized structures for stationary points on the reaction
coordinate for migratory CO insertion in
[Ir(CO)I
3
(COMe)]
-
, and [Ir(CO)
2

I
2
Me], and the respective
activation energies (ΔE
‡
) and enthalpies for migratory
insertion (ΔE
mig
).
21

Scheme 2.1
Formation of radicals from CpMn(CO)
2
complexes.
34

Scheme 3.1
Consecutive thermal H
2
and Light-induced O
2
evolution
from water promoted by Ru complex.
69
Scheme 3.2
Modes of addition of water to a transition metal complex.
70
Scheme 3.3
Proposed mechanism for the generation of H

2
and H
2
O
2

from CpMn(CO)
3
photolysis in hexane/water biphasic
system.
85
Scheme 3.4
Proposed mechanism, involving oxidative addition, for
the generation of H
2
and H
2
O
2
proceeding after the
formation of CpMn(CO)
2
(H
2
O) in hexane/water biphasic
system.
91
Scheme 3.5
Substitution of an additional CO by H
2

O, for the
generation of H
2
and H
2
O
2
proceeding after the formation
of CpMn(CO)
2
(H
2
O) in hexane/water biphasic system.
91
Scheme 3.6
Anchoring of Cp ring upon the substitution of a CO
93
xviii

ligand by the N atom of the 8-quinolyl arm.

Scheme 4.1
Proposed Mechanism for the photochemical reaction
between Mn
2
(CO)
10
and HCl.
98
Scheme 4.2

Photodissociation pathways of Mn
2
(CO)
10
.
114
Scheme 4.3
(a) Mechanism showing Mn-Mn bond homolytic fission,
leading to an eventual production of MnH(CO)
5
and
Mn(OH)(CO)
5
(adopted from [14]). (b) Mechanism
showing the H
2
production from MnH(CO)
5
and H
2
O.
(c) Mechanism showing the H
2
production from
Mn(OH)(CO)
5
and H
2
O.
115


Scheme 5.1
Proposed mode of anion binding observed for 1,2-
diaminoanthraquinone.
128
Scheme 5.2
The various CpMn(CO)
2
L complexes prepared from UV
photolysis of CpMn(CO)
3
and L.
129
Scheme 5.3
Proposed mechanism of displacement of 3-OHpy from
CpMn(CO)
2
(3-OHpy) by PPh
3
.
146


xix

Abbreviations
1
H
Proton
3-OHpy

3-hydroxypyridine
4-OHpy
4-hydroxypyridine
BDE
Bond Dissociation Energy
CO
carbonyl
CDCl
3

Deuterated Chloroform
CpMn(CO)
3

Cyclopentadienyl Manganese Tricarbonyl
CpH
Cyclopentadiene
Cp’Mn(CO)
3
Methylcyclopentadienyl Manganese Tricarbonyl
CyH
Cyclohexane
D
2
O
Deuterated Water
DFT
Density Functional Theory
DPPH
1,1-diphenyl-2-picryl hydrazyl radical

DPPH-H
1,1-diphenyl-2-picryl hydrazine
ESI
Electrospray Ionization
ESR
Electron Spin Resonance
xx

FTIR
Fourier-Transform Infrared
LAT1
L-type amino acid transporter 1
NMR
Nuclear Magnetic Resonance
MeOH
Methanol
Mn
2
(CO)
10
Dimanganese decacarbonyl
MS
Mass Spectroscopy
NBO
Natural Bond Orbitals
Ph
Phenyl
PPh
3


Triphenylphosphine
ROH
Alcohol
RSH
Thiol
THF
Tetrahydrofuran
TMS
Tetramethylsilane
UV
Ultraviolet


xxi

List of Symbols
%
percent
Å
Ångström
ºC
degrees Celsius
cm
centimetre
Hz
Hertz
m/e
mass-to-charge ratio
mg
milligrams

mJ
millijoules
mm
millimetre
mM
millimoles per cubic decimetre
mmol
millimole
nm
nanometre
uL
microlitre



1

CHAPTER 1
Introduction