Physical Methods in Organometallic Chemistry

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Physical Methods in Organometallic Chemistry

• ??? Hon Man Lee
• 94.2.?????? (?)
• Organometallic Chemistry (II)

Textbook The Organometallic Chemistry of the
Transition Metals, Ch.10, 4th Ed. by Robert H.
Crabtree
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Synthesize and Characterization

• For a new organometallic complex,
• Needs isolation first
• Air sensitive, especially d-block and f-block
  elements
• Schlenk glassware, glove-box
• Assign its stereochemistry
• Knows something about its properties
• Spectroscopic and crystallographic analysis
• Mainly NMR

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1H NMR spectroscopy

• Metal hydrides in empty region of spectrum (d 0
  to -40 ), shielded by d electrons.
• d -10.44 (d, 2J(P,H) 15.0 Hz, 1H, Ha)
• Trans coupling (90-160 Hz)/cis coupling (10-30
  Hz)
• Stereochemistry assignment

integration
splitting
J constant
assignment
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Figure 10.1
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Virtual coupling

• Phosphorus, I 1/2
• PMe3, PMe2Ph
• Two phosphorus atoms are cis ? a simple doublet
  for the Me group.
• When they are trans to each other ? a distorted
  triplet or virtual triplet.
• Large P-P coupling ? methyl group couples to its
  own P and the trans P equally.

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Figure 10.2
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Diastereotopy

• Diastereotopic groups
• No symmetry element of the molecule relates the
  groups.
• Resonate at different chemical shifts.
• Regardless of MP or CC bonds are freely
  rotating

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Chemical shifts

• Chemical shift in organometallic compounds are
  much variable.
• For example
• Free alkene (d 5 -7)
• Co-ordinated (d 2 5)
• Ir(III) hydride complex
• Trans to high field ligand e.g. H ? -10 ppm
• Trans to low field ligand e.g. H2O ? -40 ppm

• Neighboring group effect
• InIrHL2 vs CpIrHL2
• -0.27 ppm shift of the aromatic proton

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Paramagnetic complexes

• Large shifts in the NMR resonaces
• (h6-C6H6)2V at 290 ppm
• Broadened signals ? may be effectively
  unobserved.

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13C NMR spectroscopy

• 13C (I 1/2) only 1 of natural carbon
• Needs longer acquistion time
• 13C1H, proton-decoupled spectrum
• Off-resonace-decoupled spectrum
• Only 1-bond C,H couplings
• Reveals number of H attached to the C
• CH3, quartet
• CH2, triplet
• CH, doublet
• Coupling is transmitted through s bond, the
  higher the s character of a bond, the higher is
  the coupling. For example in CH
• sp3 125 Hz,
• sp2 160 Hz
• sp 250 Hz

10.5 and 10.6 inseparable
2 q, 2 t, 2 d, 2 s for each isomer
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13C NMR Spectrum

• Characteristic resonance positions
• Alkyls 40 to 20 ppm
• Alkenes, Cp, arenes 40 to 120 ppm
• Terminal C?O 150 to 220 ppm
• Bridging C?O 230 to 290 ppm
• Carbenes 200 to 400 ppm

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13C NMR Spectrum

• Relaxation problem e.g. C?O, relaxation reagent,
  Cr(acac)3
• Signals farther apart than in 1H NMR ?
  complicated molecules easier to assign.
• trans coupling 2J(C,P) 100 Hz gtgt cis coupling
  2J(C,P) 10 Hz

• Wide range of relaxation time, saturation problem
  ? Integration not reliable
• Coordination shift for polyene and polyenyl
  ligands (25 ppm to high field)
• Coupling to metal is also seen if th metal has I
  1/2

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31P NMR spectroscopy

• Very useful for phosphine complexes
• Usually 31P1H spectrum
• Phosphine vs. phosphites
• Coordination shift free and bound phosphines
• Chelation shift
• For example if the P atom is part of 4-, 5-, or
  6-membered ring, it will shift by -50, 35, -15
  ppm relative to a coordinated but noncoordinating
  ligand.
• Orgin of shift not well-understodd.

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Mechanistic Study of Wilkinson Hydrogenation
at 30 C

• 31P1H NMR
• Rh (I ½, 100 abundance)
• Fluxional process
• Non-rigid vs static structures

at -25 C
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Dynamic NMR

• Many organometallic species are nonrigid
  molecules giving fewer signals than their static
  structures.
• Rate of exchange (fluxional) process gtgt NMR
  timescale (10-1 to 10-6 s)
• For Fe(CO)5
• 13C NMR one signal at 25 C.
• IR spectrum two types of CO
• IR time scale 10-12 s

Berry pseudorotation Axial and equatorial COs
are exchanged.
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Rate of Fluxionality

• When rate of exchange is comparable to NMR time
  scale ? slow it down by cooling to get the static
  spectrum (low-temperature limit)
• Accelerate the exchange process by heating ?
  fully averaged spectrum (high- temperature limit)

decoalescence
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Rate of Fluxionality

• Rate at which A and B leave the site during the
  exchange process
• The exchange rate during coalescence
• ?v separation of the two resonance in static
  structure
• A single peak
• The rate is field dependent.

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Mechanism of Fluxionality

• Fluxionality relates to coordination number.
• very common in 5-coordinate TBP complexes
• also in 7-, 8,-, 9- coordinate complexes
• 4-, 6- coordinate complexes are usually rigid.
• CN unrelated fluxionality

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Mechanistic study

• Different degree of initial broadening ?
  1,2-shift vs. 1,3-shift
• 1,2-shift
• one of Hc still in Hc site
• all Hb sites are different.
• Exchange rate of Hb 2 x that of Hc
• Hc less initial broadening
• 1,3-shift Hb less initial broadening

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Mechanism of Fluxionality

• Bridge-terminal exchange in CO complexes
• one signal for Cp in 1H NMR at R.T.
• Separate signals for cis and trans isomer at -50
  C
• Adams-Cotton mechanism
• via open form
• The trans isomer gives faster exchange between
  terminal and bridging COs. (greater initial
  broadening of signal in 13C NMR)

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Spin Saturation Transfer

• A low temperature limiting spectrum at all
  accessible temperatures, exchange is slow on NMR
  timescales.
• Too slow to affect the NMR line shapes.
• Spin saturation transfer
• To know which proton is exchanging to which.
• Irradiate MeA (saturating the signal), MeB signal
  diminished.
• Exchange rate k

Need to know T1(B)
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Spin-lattice relaxation time, T1

• T1 spin lattice relaxation
• Inversion recovery method
• 180 pulse
• Sample spins after 0.1s, 0.2 s by applying 90
  pulse to bring the net magnetization back to xy
  plane
• A first order process
• Rate constant 1/T1

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T1 and Dihydrogen Metal Complexes

• Application of T1 measurement
• Distinguish between M(H2) and MH2 complexes
• When 2 protons are close to each other, they can
  relax via dipole-dipole mechanism.
• Relate to tumbling of molecules in solution ? no
  dipole-dipole splitting seen.
• Rate is very sensitive to r

MH2 gt 1.6 Å ? 0.5 s M(H2) 0.85 Å ? 10 ms
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Nuclear Overhauser Effect

• A valuable technique for conformational study in
  solution
• Ha and Hb relax via dipole-dipole mechanism
• The two nuclei need to be lt3 Å apart.
• 50 increase in intensity
• Usually 5 10

NOE effect observed
No NOE effect observed
Origin Irradiate Ha ? equalize Has a and b
states Dipole-dipole relaxation transfer some of
the increased upper b state of Ha to the lower a
state of Hb ? increase in intensity NOE factor
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Isotopic Perturbation of Resonance

• IPR technique
• For fluxional system in the fast exchange limit
  at all accessible temperatures
• 10.15 and 10.16 exchange rapidly even at -100 ?C

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IR Spectroscopy

• Vibrational modes of molecule
• u depends on strength of the bond
• Carbonyl complexes at 1700-2100 cm-1
• Intensity is large ? dipole moment change, dm/dr,
  is large.
• COs vibrate in concert ? depends on the symmetry
  of the M(CO)n fragment.

characteristic for a trans CO complex
facial (fac) or meridional (mer)
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IR Spectroscopy

• Shift in frequency
• Other ligands
• M-H ? bond low polarity ? weak intensity, 2000
  cm-1
• Complexes of CO2, SO2, NO ? intense bands
• Oxo ligand ? 500-1000 cm-1
• Agostic C-H ? 2800 cm-1
• M-X ? 200-400 cm-1 (not practical)

• Isotopic subsitution
• A band at 2000 cm-1 may due to M-H or M-CO
• M-D shift to lower frequency according to
• At about 2000/v2 1414 cm-1

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Raman Spectroscopy

• Useful for detecting nonpolar bonds (absorbs
  weakly in IR)
• Rarely applied to organometallic species

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Crystallography

• Extremely important
• X-ray and neutron diffraction
• Unit cell
• Braggs law
• Diffractions give a pattern of spots.
• The intensity of spots carry information about
  the locations of the atoms in the unit cell
• The relative positions of the spots carry
  information about the arrangement of the unit
  cells in space (space group)

• Limitations
• X-rays diffracted by electron-clouds around each
  atom
• Hydrogen atoms difficult to locate (MH2, M(H2),
  bond angle at metal-ethylene complex)
• Neutron diffraction ? large crystal needed. ? few
  labs in the world has this facilities.
• Single crystal ? bulk material
• Solution structure ? solid state structure
• May be the least soluble tautomer
• IR technique (solid state solution )
• Solid-state NMR

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Other Methods

• Microanalysis, Elemental analysis
• C, H, N. acceptable if 0.03
• Solvent may be present
• Conductivity measurements
• UV-Vis
• Electron diffraction
• Much less information
• In vacuum, solvation and crystal packing are
  absent

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Paramagnetic Organometallic Complexes

• Magnetic moment ? Evans method
• Measure the solvent resonance and a solution of
  paramagnetic complex
• NMR shifted broadening
• EPR
• LNiII oxidation
• ? Ni(II)(L?) g 2 or Ni(III)L g ? 2
• Electrochemical methods
• Cyclic voltammetry

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Volatile Species

• Mass spectrometry
• Hard and soft ionization
• Photoelectron spectroscopy (PES)
• Molecular energy level
• X-ray can ionize core electrons

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• Photoelectron Spectroscopy Where Orbital
  Energies Come From
• Radiation can dislodge an electron from a
  molecule
• UV radiation removes outer e-
• X-Ray radiation can remove inner e-
• The KE of the expelled e- tells us the energy of
  the orbital it came from
• Ionization energy is equivalent to the orbital
  energy
• IE hn KE
• N2 spectrum

• Lower E at top (outer orbital)
• Fine structure is due to vibrational
• Energy levels within electronic levels
• i. Many levels bonding orbital
• ii. Few levels less bonding

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• O2 Spectrum

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Computational Methods

• Molecular orbital (MO theory) based on quantum
  mechanical methods
• Hückel method by Roald Hoffmann (Cornell)
• Fenske-Hall methods
• Ab initio (fewer assumptions, based on physics of
  the system)
• Density Functional Theory (DFT) (the present
  standard method)
• Molecular mechanics (MM) (useful for organic
  chemistry)