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Electronic Circular Dichroism of Transition Metal Complexes within TDDFT

Electronic Circular Dichroism of Transition Metal Complexes within TDDFT. Jing Fan University of Calgary. Objectives. To understand, experimental CD spectra, quantum mechanical calculations of electronic structure and CD based on TDDFT

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Electronic Circular Dichroism of Transition Metal Complexes within TDDFT

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

  2. 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

  3. Complexes Studied: Trigonal Dihedral: − -bonded complexes: [M(en)3]3+ (M = Co, Cr) (d-to-d, LMCT) − both - and π-bonded complexes: [M(L-L)3]n+ (M = Co, Cr; L =ox, acac, thiox, etc.) (d-to-d, LMCT, MLCT, LC) − complexes with conjugated ligands: [M(L-L)3]2+ ( M = Fe, Ru, Os; L = bpy, phen) (LC exciton CD ) Trigonal bipyramidal: − complexes with conjugated ligands [M(L)X]+ (M = Cu, Zn; L = MeTPA, MeBQPA, MeTQA; X = Cl-, NCS-) (LC exciton CD)

  4. 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

  5. -bonded Complexes : [Co(en)3]3+ and [Cr(en)3]3+ 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) d-d LMCT

  6. Assignment of Transitions Lowest singlet excited states andtheir splitting in D3 symmetry Λ-[Co(en)3]3+ (2A2) (1E) (2E) (1A2) (3E)

  7. Why Optically Active? 1A1g1T1g d-d transitions: magnetically allowed 1A1g1T1u LMCT transitions: electrically allowed Rotatory Strengths: electric transition dipole moment magnetic transition dipole moment

  8. Metal d-orbitals: Origin of Optical Activity Symmetry Metal and Ligand Frontier Orbitals • Metal-ligand orbital interactions L-orbitals: 8 • Metal-ligand Orbital Interaction • Metal-ligand Orbital Interaction

  9. Overlaps Expression , , , , s s s s s s = - + = + ˆ ˆ 1 e ( 3 / 2 ) 1 e ( 1 / 2 ) 2 e , 2 e ( 1 / 2 ) 1 e ( 3 / 2 ) 2 e Semi-quantitative Metal-ligand Orbital Overlaps In general Overlaps Case I Case II Case III Case I: Oh,  = 60 Case II: D3,  = −6.3 Case III: D3,  = +6.3 , 1.225 S 1.220 S 1.193 S -[Co(en)3]3+ , 0 -0.001 S 0.013 S 0 0.047 S 0.287 S , 0 0.096 S -0.102 S , 0 -0.021 S -0.160 S

  10. MO diagram Energy (eV) MOs as linear combinations of symmetry ligand and metal d-orbitals Main components from DFT calculations bonding anti-bonding anti-bonding 10

  11. positive negative s s ˆ 2 e ˆ 1 e y x • Prediction of the Sign of Rotatory Strengths Band 2: and in terms of one-electron excitations 4e(d) 5e(d)

  12. Metal d-orbitals: Both - and π-bonded Complexes Metal and Ligand Frontier Orbitals L-orbitals: L-orbitals: 12 • Metal-ligand Orbital Interaction • Metal-ligand Orbital Interaction

  13. overlap overlap* Symmetry Unique Metal-ligand Orbital Overlaps ,, . * Only p-orbitals on the N atoms are considered 13

  14. Overlaps Overlaps Case I(Oh) Case I(Oh) Case II (D3) Case II (D3) -type -type 0.612 S 0.612 S 0.610 S 0.610 S 1.061 S 1.061 S 1.058 S 1.058 S 0 0 -0.034 S -0.034 S 0 0 -0.064 S -0.064 S 0 0 0.059 S 0.059 S -type -type 0 0 -0.138 S -0.138 S 0 0 0.110 S 0.110 S 1.632 S 1.632 S 1.590 S 1.590 S -0.816 S -0.816 S -0.892 S -0.892 S -2.002 S -2.002 S -1.976 S -1.976 S Case I: Oh Case II: D3

  15. 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 cm–1 • Co(III): –4.0  103 cm–1 15

  16. - thiox

  17. Early rule proposed for Λ-configuration: Azimuthal contraction ( < 0)  positive R(E) Polar compression (s/h > 1.22)  (E) < (A2) Relationship between CD of the d-d transitions and geometry in Λ-[M(L-L)3]n+ σ-bonded a Sign of rotatory strength of the E symmetry. b Azimuthal distortion; Δ = 0for ideal octahedrons. c Trigonal splitting of the T1g state. d Polar distortion; s/h = 1.22 for ideal octahedrons. 17

  18. Rotatory strengths R ( ) and overlaps S(d2, ) ( ) against  (1E) (2E) R /10-40 cgs S / S S R(2E1) (1A2) R(1E1) / degree

  19. Relationship between CD of the d-d transitions and geometry in Λ-[M(L-L)3]n+ a Sign of rotatory strength of the E symmetry. b Azimuthal distortion; Δ = 0for ideal octahedrons. c Trigonal splitting of the T1g state. d Polar distortion; s/h = 1.22 for ideal octahedrons. 19 19

  20. Complexes with Conjugated Ligands (Trigonal Dihedral) Exciton CD (LC π-π* transitions) theor.  5 expt. E [Λ-Os(bpy)3]2+ M: Fe, Ru, Os N-N: bpy, phen For the Λ configuration: R(E) > 0, R(A2) < 0, υ(A2−E) > 0 A2

  21. βπ Energy (eV) απ

  22. (απ−>βπ) R(A2) < 0 and R(E) > 0 for 0 < < 90

  23. Energy Splitting of CD Bands • d-to-d: trigonal splitting of dπ orbitals due to metal-ligand interactions • d-Lσ E polar compression (s/h > 1.22) (E) < (A2) [Co(en)3]3+ A2 E [Cr(en)3]3+ A2 • d-Lπ/σ E polar elongation (s/h < 1.22) (E) > (A2) Co(acac)3 and Cr(acac)3 A2

  24. LC: trigonal splitting of dπ orbitals due to metal-ligand interactions and electron-electron repulsion energy involving different number of ligands

  25. 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 excitonexcitations ✔ 25

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

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