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AJELIAS L2‐S5
Infrared Spectroscopy‐ A spectro‐analytical tool in chemistry Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic
and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a
compound positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to
determine the chemical functional groups in the sample. Functional groups are identified based
on vibrational modes of the groups such a stretching, bending etc. Different vibrational modes
absorb characteristic frequencies of IR radiation. An infrared spectrophotometer is an
instrument that passes infrared light through a molecule and produces a spectrum that contains
a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared
radiation on the horizontal axis. Absorption of radiation lowers the percentage transmittance
value.
AJELIAS L2‐S6
Infrared Spectroscopy‐ Spectra of Metal Carbonyls OC
OC
OC
CO OC CO
Mn
CO
Mn
The range in which the band appears decides bridging or terminal .
CO
CO OC
terminal
The number of bands is only related to the symmetry of the molecule
OC
OC
OC
O
C
O
C
Fe
CO
Fe
C
O
CO
CO
terminal
bridging
AJELIAS L2‐S7
Terminal versus bridging carbonyls
O
M
O
C
C
C
M
M
M
M
O
M
terminal
ν
CO 2120-1850 cm-1
bridging μ 2
1850-1700 cm-1
bridging μ
3
1730-1620 cm-1
Cp
Fe
CO
CO
OC
OC
Cr
OC
Fe Cp
OC
CO
Fe
CO
CO
2000 cm‐1
Fe
Cp
2018, 1826 cm‐1
CO
1620 cm‐1
Cp
AJELIAS L2‐S8
Factors which affect νCO stretching frequencies
1.Charge on the metal
2. Effect of other ligands
Variation in νCO (cm–1) of the first row transition
metal carbonyls
free CO
2143
As the electron density on a metal centre increases, more π‐back‐bonding to the CO ligand(s) takes place. This weakens the C–O bond further as more electron density is pumped into the empty π* anti‐bonding carbonyl orbital. This increases the M–C bond order and reduces the C‐O bond order. That is, the resonance structure M=C=O becomes more dominant.
Ni(CO)4
2057
Co(CO)41890
Co2(CO)8
2044(av, ter)
[Fe(CO)4]21815
Fe(CO)5
2030
[Mn(CO)4]31600,1790
Mn(CO)6 +
2098
[Cr(CO)4]41462,1657
Cr(CO)6
2000
V(CO)6¯
1860
Mn2(CO)10
2013 (av)
V(CO)6
1976
Ti(CO)621747
M
C
ν
O
CO Higher
M
C
ν
O
CO Lower
More back bonding
AJELIAS L2‐S9
Other spectator ligands: Phosphines χ(cm–1)
PR3
νCO, (cm–1)
2056.1
0.0
PPh2(C6F5)
2074.8
18.7
PCy3
2056.4
0.3
P(OEt)3
2076.3
20.2
P(i-Pr)3
2059.2
3.1
P(p-C6H4-CF3)3
2076.6
20.5
PEt3
2061.7
5.6
P(OMe)3
2079.5
23.4
P(NMe2)3
2061.9
5.8
PH3
2083.2
27.1
PMe3
2064.1
8.0
P(OPh)3
2085.3
29.2
PBz3
2066.4
10.3
P(C6F5)3
2090.9
34.8
P(o-Tol)3
2066.6
10.5
PCl3
2097.0
40.9
PPh3
2068.9
12.8
PF3
2110.8
54.7
PPh2H
2073.3
17.2
P(CF3)3
2115.0
58.9
PR3
νCO, (cm–1)
P(t-Bu)3
Δ νCO wrt
P(t-Bu)3
χ(cm–1)
Δ νCO wrt
P(t-Bu)3
PR3
Lowest CO stretching frequency
Most donating phosphine
best σ donor
OC
Ni
Highest CO stretching frequency
Least donating phosphine
CO
best π acceptor
CO
AJELIAS L2‐S10
Effect of a ligands trans to CO
Effect of different co-ligands on νCO (cm-1) of
Mo(CO)3L3
CO
L
CO
Mo
L
CO
L
Complex
(fac isomers)
ν CO cm–1
Mo(CO)3(PF3)3
2090, 2055
Mo(CO)3(PCl3)3
2040, 1991
Mo(CO)3[P(OMe)3]3
1977, 1888
Mo(CO)3(PPh3)3
1934, 1835
Mo(CO)3(NCCH3)3
1915, 1783
Mo(CO)3(dien)*
1898, 1758
Mo(CO)3(Py)3
1888, 1746
More back bonding =
More lowering of the C=O bond order = More lower ν CO stretching frequency
With each negative charge added to the metal centre, the CO stretching frequency decreases by approximately 100 cm–1.
The better the σ donating capability of the other ligands on the metal, more
electron density given to the metal, more back bonding (electrons in the
antibonding orbital of CO) and lower the CO stretching frequency.
AJELIAS L2‐S11
Synthesis of Metal Carbonyls
Direct carbonylation
Reductive carbonylation
AJELIAS L2‐S12
Reactions of Metal Carbonyls
Reduction : Carbonyl anions
V(CO)6
+ Na
Mn2(CO)10
Co2(CO)8
Na[V(CO)6]
2 Na[Mn(CO)5]
+ 2Na
2 Na[Co(CO)4]
+ 2Na
Fe(CO)5 + Na/Hg
Na 2Fe(CO)4
Oxidation : Iodocarbonyls
Mn2(CO)10
Fe(CO)5
+ I2
2 Mn(CO)5I
+ I2
Fe(CO)4I2
Photochemical substitution
W(CO)6 + PPh3
Fe(CO)5 +
hν
W(CO)5(PPh3) + CO
hν
Fe(CO)3 + 2CO
In the presence of UV radiation a monodentate
ligand displaces only one CO unit
AJELIAS L2‐S13
Reactions of Metal Carbonyls
Nucleophilic addition to CO
R
CO
OC
OCH 3
C
CO
W
OC
R
OLi
OC
RLi
CO
ether
C
CO
W
OC
OC
OC
[Me 3O]BF 4
CO
W
OC
CO
CO
OC
OC
Fischer Carbene
Carbenes are catalysts
for olefin metathesis
Migratory insertion of CO
Me
Me
OC
C
CO
Mn
OC
CO
OC
O
CO
high pressure
OC
CO
Mn
OC
CO
OC
Problem solving ‐ synthesis
Give a scheme for the synthesis of Mn(CO)4(PPh3)[C(O)CH3] starting from Manganese acetate, Mn(OAc)2.
2 Mn(OAc)2 + 4 Na + 10 CO
high temp
high pressure
Mn2(CO)10 + 4 NaOAc
Mn2(CO)10 + 2 Na
2 NaMn(CO)5
NaMn(CO)5 + CH3I
CH3Mn(CO)5
CH3Mn(CO)5 + CO
CH3C(O)Mn(CO)5 + PPh3
CH3C(O)Mn(CO)5 ( migratory insertion)
CH3C(O)Mn(CO)4PPh3
hv
Or at step 3 direct reaction with acyl chloride instead of MeI. Step 1 other
reducing agents e.g. AlEt3 can also be used.
AJELIAS L2‐S14
Metal‐ Sandwich compounds
Hapticity of sandwich compounds varies from 1‐8
AJELIAS L2‐S15
Why metal – sandwich compounds are important? 1. Transition metal/ metal ion embedded inside an organic matrix: Makes a metal ion soluble
even in hydrocarbon solvents. E.g. Ferrocene is soluble in hexane while Fe2+ as such is not.
Outcome: a hydrocarbon soluble additive/catalyst
2. Coordination to an electropositive metal often changes the reactivity and electronic properties of the π system bound to it (benzene vs ferrocene)
3. A stericially protected metal site where a wide range of catalytic applications are possible on the. e.g alkene polymerization
4. Metal sandwich compounds are excellent substrates to make planar chiral compounds. Applications as chiral catalysts in asymmetric catalysis X
X
Y
Y
Fe
Planr chirality:
Non‐ super‐imposable mirror images
Fe
AJELIAS L2‐S16
Cyclopentadienyl (Cp−)
• Cyclopentadienyl (Cp−) the most important of all the polyenyl ligands
• It gets firmly bound to the metal • generally inert to nucleophilic reagents. • used as a stabilising ligand for many complexes. M
η1
M
η3
Least
com m on
M
η5
m ost
com m on
(η5‐Cp)(η3‐Cp)W(CO)2
•Neutral cyclopentadiene (C5H6) is a weak acid with a pKa of around 15 •Deprotonated with strong base or alkali metals to generate the anionic Cp−
AJELIAS L2‐S17
Synthesis of Cp (C5H5‐) based sandwich compounds
Cp2Fe + 2 [Et2NH2]Cl
FeCl2 + 2 C5H6 + 2 Et2NH
Cp2Ru + C5H8 + 3/2 Zn2+
RuCl3(H2O)n + 3C5H6 + 3/2 Zn
H 2O
2 C5H6 + 2 KOH + Tl2SO4
2 CpTl + K2SO4 + H2O
(poisonous)
CpTl + Mn(CO)5Cl
CpMn(CO)3 + TlCl + 2 CO
CpTl based chemistry is not practiced nowadays due to toxicity
H
H
Na
180°C
2 NaCp + H2
cracking
H H
H
H
dicyclopentadiene
MCl2 +
2 NaCp
Cp2M [ M = V, Cr, Mn, Fe, Co]
Solvent: THF, DME, Liquid NH3 etc
AJELIAS L2‐S18
Ferrocene: synthesis
Fe
Lab Synthesis
Fe + 2 (R 3NH)Cl
FeCl2 + 2 R 3N + H 2
FeCl2 + 2 C 5H 6 + 2 R 3 N
Cp 2Fe + 2(R 3NH)Cl
FeCl2 + 2 NaCp
Cp2Fe
AJELIAS L2‐S19
Reactions of Ferrocene
Ferrocene undergoes electrophilic substitution reactions. Many of its reactions are faster than similar reactions of benzene
Necessary requirement: The electrophile should not be oxidizing in nature
I2
Fe
FeCp2 + HBF4.OEt2
Fe
p- benzoquinone
Et2O
FeCl3
FeCp2 + NH4PF6
H2O/Acetone
I3
[FeCp2][BF4]
[FeCp2][PF6]
The oxidized Cp2Fe+, ferrocenium cation, will repel the electrophile away. Therefore direct nitration, halogenation and similar reactions cannot be carried out on ferrocene. Acetylation
H3C(O)C
Ac2O/ H3PO4
Fe
60 min, 50 °C
Fe
CH3C(O)Cl
AlCl3(1:2:2)
C(O)CH3
C(O)CH3
C(O)CH3
Fe
Fe
C(O)CH3
90 %
90 %
traces
3.3 x 106 times faster than benzene
AJELIAS L2‐S20
Chloromercuration (hazardous)
Hg(OAc)
Hg(OAc)2
Fe
HgCl
LiCl
Fe
Fe
Br2/I2
Br, I derivatives
109 times faster than benzene
Mannich reaction
H2
C
NR2
HCHO/R2NH
Fe
Fe
H3PO4
Does not happen with benzene; only with phenols/anilines
Lithiation reaction
Li
Fe
Li
t-BuLi
Fe
n-BuLi
N
Fe
TMEDA
Li
N
(3:2 adduct)
Does not happen with benzene; only with bromobenzene
AJELIAS L2‐S21
Lithiation and 1,1’‐di‐lithiation – access to range of new derivatives
Li
CO2/H+
HOOC
SiCl4
Fe
Cl3Si
Fe
Fe
(
) 3B
O
Bu
1/8 S
8
H+
(HO)2B
SLi
I2
Fe
Fe
I
Fe
NaCN
CN
Fe
dppf
[1]ferrocenophane
AJELIAS L2‐S22
Polymers with ferrocene in the backbone
Me Me
Si
Me
Fe
130 °C
Si
Fe
Me
n
M. Wt: 3.4 X 105
Bisbenzene chromium: Prepared by Fischer and Hafner
3 CrCl3 + 2 Al + AlCl3 + 6 C6H6
3
Cr
AlCl4
Na2S2O4
KOH
Cr
Problem solving ‐ synthesis
Starting fro m ferrocene show minimum number of steps for preparing 1,1’‐ ferrocene dicarboxylic acid