Transcript Slide 1

Group
4
5
6
7
8
9
10
3d row
Ti
V
Cr
Mn
Fe
Co
Ni
4d row
5d row
Zr
Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Neutral stable compounds
0
I
II
III
IV
V
ML7
ML6
MXL6
MX2L6
MXL5
MX2L5
MX3L4 (16e)
MX4L4 (16e)
ML5
ML4
MXL3 (16e)
MX2L4
MX3L4
MX4L3 (16e)
MX2L2 (16e)
MX3L3
MX4L3
MX5L2 (16e)
Each X will increase the oxidation number of metal by +1.
Each L and X will supply 2 electrons to the electron count.
MX4L2
Group
4
5
6
7
8
9
10
3d row
Ti
V
Cr
Mn
Fe
Co
Ni
4d row
5d row
Zr
Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Stable monocationic compounds
0
I
II
III
IV
V
Now looking at compounds having a charge of +1 to obey 18 e rule.
Elec count: 4 (M) +2 (NO) +12 (L6) = 18
Group 4
5
6
7
8
9
10
3d row Ti
V
Cr
Mn
Fe
Co
Ni
4d row Zr
5d row Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Stable monocationic compounds
[M(NO)L6]+
0
[M(NO)L5]+
[ML6]+ (16e)
I
[MXL7]+
II
IV
[ML6]+
[MXL6]+
[MX2L5]+ (16e)
III
[M(NO)L4]+
[MX3L5,6]+
[ML4]+ (16e)
[MXL5]+
[MX2L5]+
[MX3L4]+ (16e)
MX2L2 (16e)
[MX2L4]+
[MX3L4]+
[MX4L3]+ (16e)
V
NO+ is isoelectronic to CO
X increases O N by 1
ML4
Elec Count: 4 (M) + 4 (L2) + 10 (L5)
MX4L2
Actors and spectators
Actor ligands are those that dissociate or undergo a chemical
transformation
(where the chemistry takes place!)
Spectator ligands remain unchanged during chemical
transformations
They provide solubility, stability, electronic and steric influence
(where ligand design is !)
Organometallic Chemistry
1.2 Fundamental Reactions
Fundamental reaction of organo-transition metal complexes
Reaction
D(FOS) D(CN) D(NVE)
Association-Dissociation of Lewis acids
0
±1
0
Association-Dissociation of Lewis bases
0
±1
±2
Oxidative addition-Reductive elimination
±2
±2
±2
0
0
0
Insertion-deinsertion
FOS: Formal Oxidation State;
CN: Coordination Number
NVE: Number of valence electrons
Association-Dissociation of Lewis acids
D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0
Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2
H
H
+ BF 3
W:
H
BF3
W
H
This shows that a metal complex may act as a Lewis base
The resulting bonds are weak and these complexes are called adducts
Association-Dissociation of Lewis bases
D(FOS) = 0; D(CN) = ± 1; D(NVE) = ±2
A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…)
in this case the metal is the Lewis acid
HCo(CO)4
HCo(CO)3 + CO
Crucial step in many ligand exchange reactions
For 18-e complexes, only dissociation is possible
For <18-e complexes both dissociation and association are possible
but the more unsaturated a complex is, the less it will tend to dissociate a ligand
Oxidative addition-reductive elimination
D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2
Mn+ + X-Y
M(n+2)+
X
Y
H
Ph3P
Cl
Ir
I
CO
PPh3
Vaska’s compound
+ H2
Ph3P
Ir
III
Cl
H
PPh3
CO
Very important in activation of hydrogen
Oxidative addition-reductive elimination
H becomes H-
Concerted reaction
H
Ph3P
CO
Ir I
Cl
+ H2
Cl
IrI
PPh3
H
H
M
PPh3
via
H
CO
Ir: Group 9
SN2 displacement
CO
Ir III
Cl
PPh3
Vaska’s
compound
Ph3P
Ph3P
+ CH3I
cis addition
CH3+ has become CH3+
CH3
Ph3P
IrIII
Cl
CO
PPh3
I-
CH3
Ph3P
IrIII
Cl
CO
PPh3
I
trans addition
Also radical mechanisms possible
Oxidative addition-reductive elimination
Mn+ + X-Y
M(n+2)+
X
Y
Not always reversible
Mn+ + R-X
Mn+ + R-H
M(n+2)+
X
R
M(n+2)+
H
R
Insertion-deinsertion
D(FOS) = 0; D(CN) = 0; D(NVE) = 0
M-X + L
(CO)5Mn-CH 3 + CO
M-L-X
O
(CO)5Mn-C-CH 3
Mn: Group 7
Very important in catalytic C-C bond forming reactions
(polymerization, hydroformylation)
Also known as migratory insertion for mechanistic reasons
Migratory Insertion
CH3
OC
CO
+ CO
CO
Mn
OC
OC
O
C
CH3
Mn
CO
OC
CO
CO
CO
k1
k2
O
OC
+ CO
C
CH3
Mn
OC
CO
CO
Also promoted by including bulky ligands in initial complex
Insertion-deinsertion
The special case of 1,2-addition/-H elimination
R2C
LnM
CR'2
H
LnM
R2
C  H
 C
R'2
A key step in catalytic isomerization & hydrogenation of alkenes
or in decomposition of metal-alkyls
Also an initiation step in polymerization
Attack on coordinated ligands
Nu- Favored for electron-poor complexes
(cationic, e-withdrawing ligands)
M
L
E+
Favored for electron-rich complexes
(anionic, low O.S., good donor
ligands)
Very important in catalytic applications and organic synthesis
Some examples of attack on coordinated ligands
Electrophilic addition
Nucleophilic addition
Cl
Pt
py
Cl
Et
Pt
Cl
py
py
O
O
+
N
Cl
Et3O+
+
Fe(CO)3
Fe(CO)3
Electrophilic abstraction
Nucleophilic abstraction
Cp
Cp
+
Ta
CH3
CH3
Cp
Me3PCH2
+ Me4P+
Ta
Cp
Cp
CH2
CH3
Fe
OC
OC
OH-
OH
Cp
Fe
OC
OC
+
OH2
-H2O
Cp
Fe
OC
OC
Brooklyn College
Chem 76/76.1/710G Advanced Inorganic Chemistry
(Spring 2009)
Unit 6
Organometallic Chemistry
Part 2. Some physical and chemical properties of important classes
of coordination and organometallic compounds
Suggested reading:
Miessler/Tarr Chapters 13 and 14
Metal Carbonyl Complexes
M-CO
CO as a ligand
s donor, π-acceptor
strong trans effect
small steric effect
CO is an inert molecule that becomes activated by complexation to metals
“C-like MO’s”
Frontier orbitals
Larger homo lobe on C
Mo(CO)6
anti bonding
“metal character”
non bonding
“18 electrons”
6CO ligands x 2s e each
12 s bonding e
“ligand character”
Metal carbonyls may be mononuclear or polynuclear
Synthesis of
metal carbonyls
Characterization of metal carbonyls
IR spectroscopy
M-C-O
(C-O bond stretching modes)
Effect of charge
Lower frequency, weaker CO bond
u(free CO)
2143 cm-1
Effect of other ligands
PF3 weakest donor (strongest acceptor)
PMe3 strongest donor (weaker acceptor)
The number of active bands
as determined by group theory
13C
13C
NMR spectroscopy
is a S = 1/2 nucleus of natural abundance 1.108%
1.6% as sensitive as 1H only
For metal carbonyl complexes d 170-290 ppm (diagnostic signals)
Very long T1
(use relaxation agents like Cr(acac)3 and/or enriched samples)
Typical reactions of metal carbonyls
Ligand substitution:
Cr(CO)6 + CH3CN
Cr(CO)5(CH3CN) + CO
Always dissociative for 18-e complexes, may be associative for <18-e complexes
Migratory insertion:
CH3
OC
Mn
OC
CO
CO
CO
H3C CO
H3C
C
CO
O
Mn
OC
CO
CO
CO
C
CO
O
Mn
OC
CO
CO
Metal complexes of phosphines
M-PR3
PR3 as a ligand
Generally strong s donors, may be π-acceptor
strong trans effect
Electronic and steric properties may be controlled
Huge number of phosphines available
Metal complexes of phosphines
M-PR3
Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3 > PCl3 > PF3
Can be measured by IR using trans-M(CO)(PR3) complexes
Steric properties:

M
R1
P
R2
R2P
M
PR2
R3
The cone angle
Rigid structures create chiral complexes
apex angle of a cone that encompasses
the van der Waals radii of the outermost
atoms of the ligand
Tolman’s electronic and steric parameters of phosphines
Typical reactions of metal-phosphine complexes
Ligand substitution:
HCo(CO)4 + PBu 3
HRh(CO)(PPh3)3 + C2H4
HCo(CO)3(PBu3) + CO
HRh(CO)(PPh3)2(C2H4) + PPh3
presence of bulky ligands (large cone angles)
can lead to more rapid ligand dissociation
Very important in catalysis
Mechanism depends on electron count
Metal hydride and metal-dihydrogen complexes
M
M
H
H
Terminal hydride (X ligand)
M
Bridging hydride (m-H ligand, 2e-3c)
H
Coordinated dihydrogen (h2-H2 ligand)
M
H
Hydride ligand is a strong s donor and the smallest ligand available
H2 as ligand involves s-donation and π-back donation
Synthesis of metal hydride complexes
IrCl(CO)(PPh3)2 + H2
RuCl2(PPh 3) 3 + H2
Ir(H) 2Cl(CO)(PPh3)2
Et 3N
RuHCl(PPh 3) 3 + Et 3N.HCl
Co2(CO) 8 + H2
2 HCo(CO)4
[Fe(CO) 4] 2- + H+
[HFe(CO)4]-
Cp2ZrCl2 + NaBH 4
Cp2ZrHCl
Characterization of metal hydride complexes
1H
NMR spectroscopy
High field chemical shifts (d 0 to -25 ppm usual, up to -70 ppm possible)
Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz
Coupling between inequivalent hydrides J(H-H) = 1-10 Hz
Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans
IR spectroscopy
n(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging
n(M-H)/n(M-D) = √2
Weak bands, not very reliable
Some typical reactions of metal hydride complexes
Transfer of HCp2Zr(H)2 + 2CH2O
Cp2Zr(OCH3)2
Transfer of H+
HCo(CO)4
H+ + [Co(CO)4]-
A strong acid !!
Insertion
IrH(CO)(PPh3)3 + (C2H4)
Ir(CH2CH3)(CO)(PPh3)3
A key step in catalytic hydrogenation and related reactions
Bridging metal hydrides
Anti-bonding
Non-bonding
2-e ligand
4-e ligand
bonding
Metal dihydrogen complexes
M
Characterized by NMR (T1 measurements)
H
H
i
H
M
H
OC
OC
P Pr3
CO H
W
H
Very polarized
d+, d-
PiPr3
back-donation to s* orbitals of H2
the result is a weakening and lengthening of
the H-H bond in comparison with free H2
If back-donation is strong, then the H-H bond is broken (oxidative addition)
Metal-olefin complexes
2 extreme structures
sp3
sp2
Zeise’s salt
metallacyclopropane
π-bonded only
Net effect weakens and lengthens
the C-C bond in the C2H4 ligand (IR, X-ray)
Effects of coordination on the C=C bond
Compound
C-C (Å)
M-C (Å)
C2H4
1.337(2)
C2(CN)4
1.34(2)
C2F4
1.31(2)
K[PtCl3(C2H4)]
1.354(2)
2.139(10)
Pt(PPh3)2(C2H4)
1.43(1)
2.11(1)
Pt(PPh3)2(C2(CN)4)
1.49(5)
2.11(3)
Pt(PPh3)2(C2Cl4)
1.62(3)
2.04(3)
Fe(CO)4(C2H4)
1.46(6)
CpRh(PMe3)(C2H4)
1.408(16)
2.093(10)
C=C bond is weakened (activated) by coordination
Characterization of metal-olefin complexes
IR
n(C=C) ~ 1500 cm-1 (w)
NMR
1H
and 13C, d < free ligand
X-rays
C=C and M-C bond lengths
indicate strength of bond
Synthesis of metal-olefin complexes
[PtCl4]2- + C2H4  [PtCl3(C2H4)]- + Cl-
RhCl3.3H2O + C2H4 + EtOH  [(C2H4)2Rh(m-Cl)2]2
Reactions of metal-olefin complexes
Metal alkyl, carbene and carbyne complexes
Metal-alkyl complexes
Main group metal-alkyls known since old times
(Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903))
Transition-metal alkyls mainly from the 1960’s onward
W(CH3)6
Ti(CH3)6
Cp(CO)2Fe(CH2CH3)6
PtH(CCH)L2
[Cr(H2O)5(CH2CH3)6]2+
Why were they so elusive?
Kinetically unstable (although thermodynamically stable)
Reactions of transition-metal alkyls
R
LnM
LnM + R-X
X
LnM
R
+ H+
LnM + + R-H
Blocking kinetically favorable pathways allows isolation of stable alkyls
Metal-carbene complexes
pz
pz
sp2
sp2
:
.
R
C
ds
M
L ligand
Late metals
Low oxidation states
Electrophilic
:
C
triplet carbene
C
R
M
.
ds
.
.
.
C
R
Fischer carbene
M
R
R
d
:
C
R
singlet carbene
d
.
R
Schrock carbene
OR
R
M
-
M
+
C
OR
R
R
C
R
R
X2 ligand
Early metals
High oxidation states
Nucleophilic
Fischer-carbenes
Schrock-carbenes
Synthesis
t-Bu
Cl
Np 3Ta
t-Bu
2LiNp
-NpH
Np 3Ta
Np3 Ta
H
Cl
t-Bu
Typical reactions
O
X
t-Bu
Np 3Ta
Np3Ta
+
X
Y
O
H
+
Y
+ olefin metathesis (we will speak more about that)
t-Bu
Grubbs carbenes
Excellent catalysts for olefin metathesis
Metal cyclopentadienyl complexes
M
Metallocenes
(“sandwich compounds”)
M
Bent metallocenes
“2- or 3-legged
piano stools”
M
M
L
L
L
L
L
Homogeneous catalysis:
an important application of organometallic compounds
M
H
M
CO
M
H
Very important fundamentally
M
PR3
Many synthetic and industrial applications
M
Cp
Catalysis in a homogeneous liquid phase
M
Comparison of heterogeneous and homogeneous
catalysts
• Usually distinct solid phase
• Readily separated
• Readily regenerated and
recycled
• Rates not usually as fast as
homogeneous
• May be difussion limited
• Quite selective to poisons
• Lower selectivity
• Long service life
• Often high-energy process
• Poor mechanistic understnding
•
•
•
•
•
•
•
•
•
Same phase as reaction medium
Often difficult to separate
Expensive/difficult to recycle
Often very high rates
Not diffusion controlled
Usually robust to poisons
High selectivity
Short service life
Often takes place under mild
conditions
• Often mechanism well
understood
Difficulties in separation and catalyst regeneration have
prevented a wider use of homogeneous catalysts in industry
Fundamental reaction of organo-transition metal complexes
Reaction
D(FOS) D(CN) D(NVE)
Association-Dissociation of Lewis acids
0
±1
0
Association-Dissociation of Lewis bases
0
±1
±2
Oxidative addition-Reductive elimination
±2
±2
±2
0
0
0
Insertion-deinsertion
Combining elementary reactions
H
H
MLn
H
H
-L
+
(oxidative addition)
MLn
MLn + H 2
H
H
H
MLx
H
MLx
H
C
MLn
C
(ligand exchange)
H
(insertion)
Completing catalytic cycles
Olefin isomerization
H
H
H
MLx
H
C
C
H
-H elimination
no net reaction
MLn
H
ML x
H
H
H3C
H
C
ML n
CH3
CH3
C
H3C
H
ML x
H
CH3
-H elimination resulting in C=C bond migration
Completing catalytic cycles
Olefin isomerization
H
H
ML x
H
H3C
H2
H
H
ML x
H
ML x
ML x
CH3
H
H3 C
H
C
ML n
CH3
C
H
CH3
Completing catalytic cycles
Olefin hydrogenation
H
H
MLx
H
C
MLn
C
H
C
C
H
(insertion)
MLn
H
MLn +
H
H
C
C
(reductive elimination)
Completing catalytic cycles
Olefin hydrogenation
H
H
H2
H2 C CH2
ML x
H
H
ML x
ML x
H3C CH3
H
H
H
C
ML n
H
C H
H
Wilkinson’s hydrogenation catalyst
RhCl(PPh3)3
Very active at 25ºC and 1 atm H2
Very selective for C=C bonds
in presence of other unsaturations
AcO
H2 RhCl(Ph3)3
Widely used in organic synthesis
AcO
Prof. G. Wilkinson won the Nobel Prize in 1973
H
H
The mechanism of olefin hydrogenation by Wilkinson’s catalyst
Other hydrogenation catalysts
[Rh(H)2(PR3)2(solv)2]+
With a large variety of phosphines
including chiral ones for enantioselective hydrogenation
RuII/(chiral diphosphine)/diamine
Extremely efficient catalysts for the enantioselective hydrogenation
of C=C and C=O bonds
Profs. Noyori, Sharpless and Knowles won the Nobel Prize in 2001
Olefin hydroformylation
R
+ H 2 + CO
cat
R
R
+
H
O
O
n-isomer
Cat:
i-isomer
HCo(CO)4; HCo(CO)3(PnBu3)
HRh(CO)(PPh3)3; HRh(CO)(TPPTS)3
6 million Ton /year of products worldwide
Aldehydes are important intermediates towards plastifiers, detergents
Olefin hydrogenation
H
H
H
MLx
H
C
C
C
C
H
(insertion)
MLn
H
MLn
MLn +
H
H
C
C
(reductive elimination)
What else could happen if CO is present?
O
H
OC
C
MLn
C
H
H
CO
C
O
C
MLn
CO insertion
C
C
H
MLn +
H
reductive elimination
C
C
H
Olefin hydroformylation
H
H
H2C
H2
CH2
ML x
H
ML x
ML x
H3C
H2
C
H
CHO
O
H
CH3
C
C
H
H
H
H
C
H
H
C
H
H
ML n
ML n
CO
Catalysts for polyolefin synthesis
Polyolefins are the most important products of organometallic catalysis
(> 60 million Tons per year)
•Polyethylene (low, medium, high, ultrahigh density) used in packaging,
containers, toys, house ware items, wire insulators, bags, pipes.
•Polypropylene (food and beverage containers, medical tubing, bumpers,
foot ware, thermal insulation, mats)
Catalytic synthesis of polyolefin
H2 C
CH2
isotactic
H2 C
C
H
syndiotactic
CH3
atactic
Monomers
Polymerization
catalysts
Polymers
Catalytic synthesis of polyolefin
H2C
CH2
High density polyethylene (HDPE) is linear, d  0.96
“Ziegler catalysts”: TiCl3,4 + AlR3
Vacant site
Ti
Cl + R 3Al
+
Ti
R
Electrophilic metal center
Insoluble (heterogeneous) catalyst
Coordinated alkyl
Catalytic synthesis of polyolefin
H2C
C
H
CH3
Isotactic polypropylene is crystalline
“Natta catalysts”: TiCl3 + AlR3
Vacant site
Ti
Cl + R 3Al
+
Ti
R
Coordinated alkyl
Electrophilic metal center
Insoluble (heterogeneous) catalyst, crystal structure determines tacticity
Catalytic synthesis of polyolefin
H2C
C
H
CH3
“Kaminsky catalysts”
Vacant site
+
Zr
X
+
Zr
+ MAO
R
X
R
Coordinated alkyl
Electrophilic metal center
Soluble (homogeneous) catalyst, structural rigidity determines tacticity
Polymerization mechanism
M
X + "R -Al"
initiation
M
R
R'
+
M
M
R
R
R'
M
propagation
M
-H
+H2
M
H +
P
M
H +
P
M
X +
P
+HX
termination
The catalytic synthesis of acetaldehyde
(Wacker process, oxidation of ethylene)
C2H4 + PdCl2
Pd(0) + 2CuCl2
2CuCl + 2HCl + 1/2O2
C2H4 + 1/2O2
CH3CHO + Pd(0) + 2 HCl
PdCl2 + 2CuCl
2CuCl2 + H2 O
CH3CHO
The catalytic synthesis of acetaldehyde
(Wacker process, oxidation of ethylene)
C2H4 + PdCl2
CH3CHO + Pd(0) + 2 HCl
H+/O2
H2O
2Cu+
2Cu2 +
OH 2
PdII
H+
Pd(0)
CH 3CHO
II
HO
H Pd
H
H
H
OH
PdII
H2 C
CH2
OH
PdII
Nucleophilic attack
Olefin metathesis
The Nobel Prize 2005 (Chauvin, Schrock, Grubbs)
2RCH=CHR'
RCH=CHR + R'CH=CHR'
N
N
H
R
R
N
Cl
Ru
Mo
Cl
PCy3
Grubbs catalyst
CMe 2Ph
Ph
H3 C(F2C)2CO
O-C(CF3)2 CH3
Schrock catalyst
ring-closing (RCM)
(CH2)n
(CH2)n
+
ring-opening (ROM)
ADMET
n
ROMP
n
The metathesis mechanism (Chauvin, 1971)