Electronic Circular Dichroism of Transition Metal Complexes within TDDFT

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Electronic Circular Dichroism of Transition Metal
Complexes within TDDFT
Jing Fan University of Calgary
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Objectives

• To understand, experimental CD spectra, quantum
  mechanical calculations of electronic structure
  and CD based on TDDFT
• To elucidate the origin of CD in typical
  transition metal complexes, relationship between
  CD and molecular geometry
• To evaluate the reliability and accuracy of TDDFT
  for transition metal compounds

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Complexes Studied
Trigonal Dihedral
- ?-bonded complexes M(en)33 (M Co, Cr)
(d-to-d, L?MCT)
- both ?- and p-bonded complexes M(L-L)3n (M
Co, Cr L ox, acac, thiox, etc.) (d-to-d,
LMCT, MLCT, LC)
- complexes with conjugated ligands M(L-L)32
( M Fe, Ru, Os L bpy, phen) (LC exciton CD )
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Computational Details ADF package

• Basis sets (STO)
• ligand atoms frozen core triple-? polarized
  TZP -C, N, O (1S) S (2p)
• metal atoms
• -Co, Cr TZP (2p)
• -Fe, Ru, Os TZ2P (2p, 3d, 4f)
• Functionals VWN (LDA) BP86 (GGA)
• Relativistic effect for Fe group metals (scalar
  ZORA)
• Un-restricted calculations for Cr(III)
• The COnductor-like continuum Solvent MOdel
  (COSMO) of solvation

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?-bonded Complexes Co(en)33 and Cr(en)33
en

• Calculated ?E are systematically overestimated
  for the d-d region (by 5,500 cm-1)
  underestimated for the LMCT region (by 6,000
  cm-1)

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Assignment of Transitions
Lowest singlet excited states and their splitting
in D3 symmetry
?-Co(en)33
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Why Optically Active?
1A1g 1T1g d-d transitions magnetically
allowed 1A1g 1T1u LMCT transitions
electrically allowed
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Origin of Optical Activity

• Metal-ligand orbital interactions

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• Metal-ligand Orbital Interaction

• Metal-ligand Orbital Interaction

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MO diagram
Energy (eV)
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• Prediction of the Sign of Rotatory Strengths

and in terms of one-electron excitations 4e(d?)??
5e(d?)
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Both ?- and p-bonded Complexes
Metal and Ligand Frontier Orbitals
L?-orbitals
L?-orbitals
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• Metal-ligand Orbital Interaction

• Metal-ligand Orbital Interaction

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Symmetry Unique Metal-ligand Orbital Overlaps
Only p-orbitals on the N atoms are considered
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Case I Oh Case II D3
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CD spectra - acac
theor.
expt.

• d-to-d, LMCT as well as ML?CT and LC, etc.
• Global red-shift applied to the computed
  excitation energies
• Cr(III) 5.0 ? 103 cm1
• Co(III) 4.0 ? 103 cm1

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• - thiox

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Relationship between CD of the d-d transitions
and geometry in ?-M(L-L)3n
a Sign of rotatory strength of the E symmetry. b
Azimuthal distortion ?? 0? for ideal
octahedrons. c Trigonal splitting of the T1g
state. d Polar distortion s/h 1.22 for ideal
octahedrons.
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Rotatory strengths R ( ) and overlaps S(d?2,
) ( ) against ?
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Relationship between CD of the d-d transitions
and geometry in ?-M(L-L)3n
a Sign of rotatory strength of the E symmetry. b
Azimuthal distortion ?? 0? for ideal
octahedrons. c Trigonal splitting of the T1g
state. d Polar distortion s/h 1.22 for ideal
octahedrons.
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Complexes with Conjugated Ligands (Trigonal
Dihedral)
theor.
?? ? 5
expt.
?-Os(bpy)32
M Fe, Ru, Os N-N bpy, phen
For the ? configuration R(E) gt 0, R(A2) lt 0,
?(A2-E) gt 0
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ßp
Energy (eV)
ap
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(ap-gtßp)
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Energy Splitting of CD Bands

• d-to-d trigonal splitting of dp orbitals due to
  metal-ligand interactions

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• LC trigonal splitting of dp orbitals due to
  metal-ligand interactions and electron-electron
  repulsion energy involving different number of
  ligands

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Determination of Absolute Configuration by CD

• ?-bonded d-to-d, L?MCT ??

• ?/?-bonded d-to-d (might be safe), CT (not safe)

• ?/?-bonded (conjugated ligands) LC exciton
  excitations ?

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Complexes with Tripodal Tetradentate Ligands
(Trigonal bipyramidal)
MeTPA
MeBQPA
MeTQA