Organometallic Chemistry

1
Organometallic Chemistry
JHU Course 030.442 Prof. Kenneth D. Karlin
Spring, 2009
Kenneth D. Karlin Department of Chemistry, Johns
Hopkins University
karlin_at_jhu.edu
http//www.jhu.edu/chem/karlin/
2
Organometallic Chemistry030.442 Prof.
Kenneth D. Karlin Spring, 2009Class
Meetings TTh, 1200 115 pm
p. 1

• Textbook The Organometallic Chemistry of the
  Transition Metals
• 4th Ed., R. H. Crabtree
• Course Construction Homeworks, Midterm Exams
  (1 or 2), Oral Presentations

Rough Syllabus Most or all of these topics
Introduction, History of the field Transition
Metals, d-electrons Bonding, 18 e Rule (EAN
Rule) Ligand Types / Complexes Types of
Compounds M-carbonyls, M-alkyls/hydrides
M-olefins/arenes M-carbenes (alkylidenes
alkylidynes) Other
Reaction Types Oxidative Addition
Reductive elimination Insertion Elimination
Nucleophilic/electrophilic Rxs. Catalysis
Processes Wacker oxidation Monsanto acetic
acid synthesis Hydroformylation
Polymerization- Olefin metathesis Water
gas-shift reaction Fischer-Tropsch reaction
3
p. 2
4
Reaction Examples
p. 3

• Oxidative Addition
• Reductive Elimination
• Carbonyl Migratory Insertion
• Reaction of Coordinated Ligands

Vaskas complex
5
Reaction Examples - continued
p. 4

• Wacker Oxidation
• C2H4 (ethylene) ½ O2 gt CH3CH(O)
  (acetaldehyde)
• Pd catalyst, Cu (co-catalyst)
• Monsanto Acetic Acid Synthesis
• CH3OH (methanol) CO gt CH3C(O)OH
  (acetic acid) (Rh catalyst)
• Ziegler-Natta catalysts Stereoregular
  polymerization of 1-alkenes (a-olefins)
• 1963 Nobel Prize
• Catalyst Ti compounds and organometalllic
  Al compound (e.g., (C2H5)3Al )
• Olefin metathesis variety of metal complexes
• 2005 Nobel Prize Yves Chauvin, Robert H.
  Grubbs, Richard R. Schrock

n CH2CHR gt CH2-CHRn
6
Organo-transition Metal Chemistry History-Timeline
p. 5

• Main-group Organometallics
• 1760 - Cacodyl tetramethyldiarsine
  ,
• from Co-mineral with arsenic
• 1899 gt 1912 Nobel Prize Grignard reagents
  (RMgX)
• 1827 Zeises salt - K (C2H4)PtCl3

n-Butyl-lithium
Synthesis PtCl4 PtCl2 in EtOH, reflux, add
KCl Bonding- Dewar-Chatt-Duncanson
model
7
Organo-transition Metal Chemistry
History-Timeline (cont.)
p. 6

• 1863 - 1st metal-carbonyl, PtCl2(CO)2
• 1890 L. Mond, (impure) Ni xs CO gt
  Ni(CO)4 (highly toxic)
• 1900 M catalysts organic hydrogenation (---gt
  food industry, margerine)
• 1930 Lithium cuprates, Gilman regent,
  formally R2CuLi
• 1951 Ferrocene discovered. 1952 --
  Sandwich structure proposed
• Ferrocene was first prepared unintentionally.
  Pauson and Kealy, cyclopentadieny-MgBr and FeCl3
  (goal was to prepare fulvalene) But, they
  obtained a light orange powder of "remarkable
  stability., later accorded to the aromatic
  character of Cp groups. The sandwich compound
  structure was described later this led to new
  metallocenes chemistry (1973 Nobel prize,
  Wilkinson Fischer). The Fe atom is assigned to
  the 2 oxidation state (Mössbauer spectroscopy).
• The bonding nature in (Cp)2Fe allows the Cp
  rings to freely rotate, as observed by NMR
  spectroscopy and Scanning Tunneling Microscopy.
  ----gt Fluxional behavior. (Note Fe-C bond
  distances are 2.04 Å).

Asides Oxidation states US vs. UK 18-electrons ?
(Cp)2Fe Cp cyclopentadienyl anion)
(h5-C5H5)2Fe (pentahapto)
Solid-state structure
8
p. 7
Organo-transition Metal Chemistry
History-Timeline (cont.)
1955 - Cotton and Wilkinson (of the Text)
discover organometallic-complex fluxional
behavior (stereochemical non-rigidity) The
capability of a molecule to undergo fast and
reversible intramolecular isomerization, the
energy barrier to which is lower than that
allowing for the preparative isolation of the
individual isomers at room temperature. It is
conventional to assign to the stereochemically
non-rigid systems those compounds whose molecules
rearrange rapidly enough to influence NMR line
shapes at temperatures within the practical range
(from 100 C to 200 C ) of experimentation.
The energy barriers to thus defined
rearrangements fall into the range of 5-20
kcal/mol (21-85 kJ/mol).
Aside Oxidation State 18-electron Rule
9
p. 8
Fluxional behavior stereochemical non-rigidity
(cont.)
Butadiene iron-tricarbonyl

• Xray- 2 COs equiv, one diff., If retained in
  solution, expect, 21 for 13-C NMR. But, see
  only 1 peak at RT. Cooling causes a change to
  the 21 ratio expected.
• Two possible explanations
• Dissociation and re-association or (2) rotation
  of
• the Fe(CO)3 moiety so that COs become equiv.
• Former seems not right, because for example
  addition
• of PPh3 does NOT result in substitution to give
  (diene)M(CO)2PPh3.
• Note You can substitute PPh3 for CO, but that
  requires
• either high T or hv. So, the equivalency of the
  CO groups
• is due to rotation without bond rupture,
  pseudorotation.

13C-NMR spectra CO region, only
10
p. 9
Berry Pseudorotation
Pseudorotation Ligands 2 and 3 move from
axial to equatorial positions in the trigonal
bipyramid whilst ligands 4 and 5 move from
equatorial to axial positions. Ligand 1 does not
move and acts as a pivot. At the midway point
(transition state) ligands 2,3,4,5 are
equivalent, forming the base of a square pyramid.
The motion is equivalent to a 90 rotation about
the M-L1 axis. Molecular examples could be PF5
or Fe(CO)5.
11
p. 10
The Berry mechanism, or Berry pseudorotation
mechanism, is a type of vibration causing
molecules of certain geometries to isomerize by
exchanging the two axial ligands for two of the
equatorial ones. It is the most widely accepted
mechanism for pseudorotation. It most commonly
occurs in trigonal bipyramidal molecules, such as
PF5, though it can also occur in molecules with a
square pyramidal geometry. The process of
pseudorotation occurs when the two axial ligands
close like a pair of scissors pushing their way
in between two of the equatorial groups which
scissor out to accommodate them. This forms a
square based pyramid where the base is the four
interchanging ligands and the tip is the pivot
ligand, which has not moved. The two originally
equatorial ligands then open out until they are
180 degrees apart, becoming axial groups
perpendicular to where the axial groups were
before the pseudorotation.
12
Organo-transition Metal Chemistry
History-Timeline (cont.)
p. 11

• 1961 D. Hodgkin, X-ray structure Coenzyme
  Vitamin B12 (see other page)
• Oldest organometallic complex (because
  biological) (see other page)
• 1963 - Ziegler/Natta Nobel Prize, polymerization
  catalysts
• 1964 - Fischer, 1st Metal-carbene complex
• 1965 Cyclobutadieneiron tricarbonyl,
  (C4H4)Fe(CO)3
• theory before experiment
• (C4H4) is anti-aromatic (4 p-electrons)
• With -Fe(CO)3, C4H4 behaves as aromatic
• 1965 Wilkinson hydrogenation catalyst,
  Rh(PPh3)3Cl

Methylmalonyl-CoA gt Succinyl-CoA (CoA
coenzyme A)
Catalysis of 1,2-shifts (mutases) or Homocysteine
methylation
13
Vitamin B-12 is a water soluble vitamin, one
of the eight B vitamins. It is normally involved
in the metabolism of every cell of the body,
especially affecting DNA synthesis and
regulation, but also fatty acid synthesis and
energy production. Vitamin B-12 is the name
for a class of chemically-related compounds, all
of which have vitamin activity. It is
structurally the most complicated vitamin. A
common synthetic form of the vitamin,
cyanocobalamin (R CN), does not occur in
nature, but is used in many pharmaceuticals,
supplements and as food additive, due to its
stability and lower cost. In the body it is
converted to the physiological forms,
methylcobalamin (R CH3) and adenosylcobalamin,
leaving behind the cyanide.
Vitamin B-12 Co-enzyme
p.12
5-deoxyadenosyl group
14
Organo-transition Metal Chemistry
History-Timeline (cont.)
p. 13

• 1973 Commercial synthesis of L-Dopa
  (Parkinsons drug)
• asymmetric catalytic hydrogenation
• 2001 Nobel Prize catalytic asymmetric
  synthesis, W. S. Knowles (Monsanto Co.)
• R. Noyori,, (Nagoya, Japan), K. B. Sharpless
  (Scripps, USA)
• 1982, 1983 Saturated hydrocarbon oxidative
  addition, including methane
• 1983 Agostic interactions (structures)

15
p. 14
AGOSTIC INTERACTIONS Agostic derived
from Greek word for "to hold on to oneself C-H
bond on a ligand that undergoes an interaction
with the metal complex resembles the transition
state of an oxidative addition or reductive
elimination reaction. Detected by NMR
spectroscopy, X-ray diffraction Compound above
MoH 2.1 angstroms, IR bands were observed at
2704 and 2664 cm1 and the agostic proton was
observed at 3.8 ppm. The two hydrogens on the
agostic methylene are rapidly switching between
terminal and agostic on the NMR time scale.
16
p. 15

• Organometallic Chemistry
• Definition Definition of an organometallic
  compound
• Anything with MR bond R C, H (hydride)
• Metal (of course) Periodic Table down left
• electropositive element (easily
  loses electrons)
• NOT
• Complex which binds ligands via, N, O, S, other
• M-carboxylates, ethylenediamine, water
• MX where complex has organometallic behavior,
  reactivity patterns
• e.g., low-valent
• Oxidation State
• Charge left on central metal as the ligands are
  removed in their usual closed shell
  configuration (examples to follow).
• d n for compounds of transition elements
• N d lt (N1) s or (N1) p in compounds
• e.g., 3 d lt 4 s or 4 p

17

• d n computation very important in transition
  metal chemistry
• d n zero oxidation state of M in M-complex has
  a configuration d n where n is the group .
• Examples Mo(CO)6 Mo(0) d n d 6 (CO,
  neutral)
• HCo(CO)4 H is hydride, H, --gt --gt Co(I), d n
  d 8
• Group 5 Group 6 Group 7
• V(CO)6 Cr(CO)6 Mn(CO)6
• V(1) Cr(0) Mn(1)
• d 6 d 6 d 6
• Isoelectronic and isostructural compounds
  (importance of d n)
• Effective Atomic Rule 18-Electron Rule (Noble
  gas formalism)
• of electrons in next inert gas
• Metal valence electrons s (sigma) electrons
  from ligands
• Rule For diamagnetic (spin-paired) mononuclear
  complexes in organotransition metal compounds,
  one never exceeds the E.A.N.

p. 16
18
p. 17

• Cr(CO)6 Cr ---gt d6 6 electrons
• (CO)6 e - pairs from 6 ligands 12
  electrons
• gt to Ar configuration 18
  electrons
• (will see more in M.O. diagram)
• Consequence of EAN Rule
• leads to prediction of maximum in coordination
  
• Max coordination (18 n) / 2 n is from
  d n .
• d n 10 8 6 4 2 0
• Max Coord 4 5 6 7 8 9
• Change in 2-electrons results in change of only
  one in Coord.
• Any Coord. less than Max ---gt
  coordinatively unsaturated
• Fe(CO)42 Fe(CO)5
• 18 e 18 e
• Fe(2) Fe(0)
• d 10 d 8

19
p. 18
ReH92 e.g., as Ba2 salt Re(VII), (Mn,Tc, Re
triad) d 0, 9 hydride ligands CN
9 Geometry Face capped trigonal prism
A compound not obeying an rules Fe5(CO)15C Iron-c
arbonyl carbide
20
p. 19
Eighteen-Electron Rule - Examples
Co(NH3)63
Cr(CO)6 Obey 18-electron rule for different
reasons Carbonyl Compounds in Metal-Metal
Bonded Complexes less straightforward Fe2(CO)9
p-Cp)Cr(CO)32
Co2(CO)8 (2 isomers)
21
p. 20
d6 Octahedral maximum of 6 coordinate
22
p. 21
Picture of Octahedral Complex Various
representations (ignore s orbital
23
p. 22
The five d-orbitals form a set of two bonding
molecular orbitals (eg set with the dz2 and the
dx2-y2), and a set of three non-bonding orbitals
(t2g set with the dxy, dxz, and the dyz
orbitals).
eg orbitals point at ligands (antibonding) appropr
iate symmetry for s-bonds to ligands s-bonds will
be six d2sp3 hybrids ndz2, ndx2-y2, (n1)s,
(n1)px,py,pz t2g orbital set left as
non-bonding
24
p. 23
25
p. 24
Standard MO diagram for Octahedral ML6
complexes with s-donor ligands e.g.,
Co(NH3)63 (18 e) e.g., W(Me)6 (12 e)
Case I Electron-configuration unrelated to
18-Rule 1st Row-Complexes with weak ligands Do
small or relatively small, eg only weakly
antibonding No restriction on of d-electrons
12 to 22 electrons
26
p. 25
27
Case II Compounds which follow rule insofar as
they never exceed the 18-e rule Metal in
high oxidation state Do is large(r) (for a
given ligand) radius is small -gt ligands
approach closely gt stronger bonding 2nd
or 3rd Row Metal - 4d, 5d Do is large(er) (for
a given ligand) d-orbitals larger, more
diffuse. Complex d n Total e Complex d n Total
e ZrF62 0 12 OsCl62 4 16 ZrF73 0 14 W(CN)
83 1 17 Zr(C2O4)44 0 16 W(CN)64 2 18 WCl6 0
12 PtF6 4 16 WCl6 1 13 PtF6 5 17 WCl62 2
14 PtF62 6 18 TcF62 3 15 PtCl42 8 16
Less than 18 e, but
rarely exceed 18 e
p. 26
28
p. 27
Similar Result if ligands are high in
Spectrochemical Series e.g., CN Do is
larger V(CN)63 d2 Cr(CN)63 d3 Mn(CN)63 d4
Less than or equal to 6
d-electrons Fe(CN)63 d5 eg not
occupied Fe(CN)63 d6 Co(CN)63 d6
however Co(II) d7 gt Co(CN)53
Ni(II) d8 gt Ni(CN)42 and
Ni(CN)53 Can have less than maximum of
non-bonding (t2g) electrons, because they are
nonbonding. Addition or removal of e has little
effect on complex stability
29
p. 28
Do can get (or is) very small with p-donor ligands

• F example (could be Cl, H2O, OH, etc.)
• Filled p-orbitals are the only orbitals capable
  of p-interactions
• 1 lone pair used in s-bonding
• Other lone pairs p-bond
• The filled p-orbitals are lower in energy than
  the metal t2g set
• Bonding Interaction
• 3 new bonding MOs filled by Fluorine electrons
• 3 new antibonding MOs form t2g set contain
  d-electrons
• Do is decreased (weak field)
• Ligand to metal (L? M) p-bonding
• Weak field, p-donors F, Cl, H2O
• Favors high spin complexes

30
p. 29
31
p. 30
Have discussed s-donor and p-donor now
p-acceptor
32
p. 31
CASE III L high in spectrochemical series CO,
NO, CN, PR3, CNR p-acid ligands
p-acceptors Can form strong p-bonds 18 e rule
followed rigorously Orbitals on M used in such
p-bonding are just those which are
non-bonding Result Increase in Do Imperative
to not Have electrons in eg orbitals Want to
maximize occupation of t2g because they are
stabilizing
33
p. 32
34
p. 33
Implications of 18e Rule for Complexes with
p-accepting ligands In octahedral geometry
almost always have 6 d-electrons 12 electrons
from ligands Other cases d-electrons and
coordination complementary Coordination
exactly determined by electron-configuration and
vice-versa (see previous notes) BrMn(CO)5 (d ?)
I2Fe(CO)4 (d ?) Fe(CO)5 (d
?) Ni(PF3)4 (d ?) All
18-electron When M has odd electron gt
metal-metal bond (often bridging COs) Mn2(CO)10
Co2(CO)8 Some 17 electron species
known V(CO)6 d 5 Mo(CO)2(diphos)2 d
5 See MO diagram Want to fill stable MOs
there is a large gap to LUMO
35
p. 34
Major Exception d 8 square-planar complexes As
one goes across periodic table, d and p orbital
energy Level splitting gets larger hard to use
p orbitals for s-bonding Common to have
4-coordinate SP complexes dsp2 hybridization
Which d-orbitals?
Common for Rh(I), Ir(I) Pd(II), Pt(II)
Rationalize d-orbital splittings look at
d-orbital pictures/axes
36
p. 35
37
p. 36
Again, examples of complexes dn C.N. Coord.
Geom. Example(s) d10 4 Td Ni(CO)4,
Cu(py)41 d10 3 Trig.planar
Pt(PPh3)3 d10 2 Linear (PPh3)AuX,
Cu(py)2 d8 5 TBP
Fe(PF3)5 d8 4 (square) planar
Rh(PPh3)2(CO)Cl (trans) d4 7 capped
octahedral Mo(CO)5X2 d2 8
sq. antiprism ReH5(PMePh2)3,
Mo(CN)84 d0 9 D3h symmetry ReH92
tricapped trig. prism
38
p. 37
LIGANDS in Organometallic Chemistry
Ligands, charge, coordination (i.e.,
denticity) X SnCl3 H (hydride) CH3 (alkyl,
perfluoroalkyl) Ar RC(O) (acyl) R3E (E P, As,
Sb, N) R2P CO RNC (isonitrile, isocyanide) R2C
(cabenoid, carbene) R2N N2 C2H4 (olefin,
alkene) R2C2 (acetylene) C4H4 (cyclobutadiene)
C3H5 (p-allyl) CHCH-CH2
(s-allyl) benzene (arenes) p-C5H5 (p-Cp) p-C7
H7 (tropylium) p-C3H3 (cyclopropenium, ) O
(O-atom oxide) NO (nitrosyl) ArN2
(diazonium)
39
p. 38
Carbon Monoxide exceedingly important ligand
CO-derivatives known for all transition
metals Structurally interesting, important
industrially, catalytic Rxs Source of pure metal
Ni (Mond) Fe contaminated with Cu,
purify via Fe(CO)5 Fe Ni only metals that
directly react with CO Source of oxygen in
organics RC(O)H, RC(O)OH, esters Processes
hydroformylation, MeOH gt acetic acid double
insertion into olefins, hydroquinone synthesis
(acetylene CO Ru catalyst), acrylic acid
synthesis (acetylene, CO, Ni catalyst) Fischer
Tropsch Rx CO H2 gt gt CnH2n2
H2O Most of these involve CO insertion
40
p. 39
Metal-Carbonyl Synthesis Reduction of available
(in our O2-environment) metal salts, e.g., MX2,
MX3, other (e.g., carbonates) M-carbonyls
generally in low-valent oxidation states gt
Reductive Carbonylation Reductants CO
itself ( gt CO2), H2, Na-dithionite Some
Reactions WMe6 xs CO gt W(CO)6
3 Me2CO NiO H2 (400 C) CO gt
Ni(CO)4 Re2O7 xs CO gt
(OC)5ReRe(CO)5 7 CO2 RhCl3 CO
pressure (Cu, Ag, Cd, Zn) gt Rh4(CO)12
or Rh6(CO)16 Structures Possible X-ray
diffraction, Infrared spectroscopy Ni(CO)4 Fe(CO
)5 M(CO)6 Td D3h Oh 2058
cm1 2013, 2034 cm1 2000 cm1
Cl acceptor/reductant
41
p. 40
H3BCO 2164 cm1 no backbonding possible
13C NMR spectroscopy of M-CO fragments 180
250 ppm Useful to use 13C enriched carbon
monoxide Can be useful to observed coupling to
other spin active nuclei, e.g., 103Rh or 13P
42
p. 41
Metal-Carbonyl Structures (cont.)
Polynuclear Metal-Carbonyls
43
p. 42
44
p. 43
45
p. 44
46
p. 45
The backbonding between the metal and the CO
ligand, where the metal donates electron density
to the CO ligand forms a dynamic synergism
between the metal and ligand, which gives unusual
stability to these compounds.
Valence Bond formalism
47
p. 46
CO stretching frequencies, n(C-O) Put more
electron density on metal by charge by
ligands which cannot p-accept Remaining COs
have to take up the charge (e-density) on the
metal See effects on n(C-O). Ni(CO)4 Co(CO)4
Fe(CO)42 2057 cm1 1886 cm1 1786 cm1
gt gt more ve
charge Mn(dien)(CO)3 2020, 1900
cm1 Cr(dien)(CO)3 1900, 1760 cm1 (dien not
p-acceptor)