Lecture 3. Applications of x-ray spectroscopy to inorganic chemistry

Transcription

1 Lecture 3. Applications of x-ray spectroscopy to inorganic chemistry 1. Bioinorganic chemistry/enzymology. Organometallic Chemistry 3. Battery materials MetE (cobalamin independent MetSyn) contains Zn Zn is tightly bound Zn is required for activity Is Zn involved in reaction, or does it play a structural role? 1

2 The Zn site in MetE has ZnS (O/N) ligation. Addition of homocysteine changes ligation to ZnS 3 (O/N). Fourier Transform Magnitude EXAFS k R + (Å) Native +Hcy k (Å -1 ) Fourier Transform Magnitude Changes in ligation are due to homocysteine binding to Zn 6 MetH (ZnS O) MetE (ZnS NO) Native 1 +Hcy +SeHcy R + (Å)

3 Combination of Zn + Se EXAFS consistent with only a small distortion from tetrahedral geometry in substrate-bound enzyme J. Am. Chem. Soc., 11 (1) 199 p p

4 EXAFS shows that CN does not remain bound Fourier Transform Magnitude CuCN Li + MeLi CuCN Li + MeLi CuCN Li Cu-CN Cu-C N-Cu Cu-Nearest Neighbor R+ (Å) Structures of cyanocuprates in THF Cu C N Cu N C Cu MeLi Me Cu C N MeLi Li C N Li Me Cu Me 4

5 Solution speciation of CuI+PhLi PhLi + CuI phenylcopper PhLi+ CuI diphenylcuprate Crystalline phenyl:copper species 1:1 Cu 4 Ph 4 (Me S) Cu 5 Mes 5 1.:1 [Cu 5 Ph 6 ] 1.5:1 [Cu 4 LiPh 6 ] [Cu 4 MgPh 6 ] :1 [CuPh ] [CuPh Li] [Cu 3 Li Ph 6 ] Titration of CuI+ n PhLi shows isosbestic behavior up to 1. equivalents 3 Normalized Absorbance n=. n=. n=.4 n=.6 n=.8 n=1. n=1.1 n= Energy ( ev ) 5

6 Titration of CuI+ n PhLi shows isosbestic behavior from 1.-. equivalents 3 Normalized Absorbance n=. n=1.8 n=1.7 n=1.5 n=1.4 n=1.3 n= Energy ( ev ) EXAFS data support XANES speciation Cu Composition 1 CuI - CuPh -.8 Cu 5 Ph PhLi / CuI Ratio [CuI ] [Cu 5 Ph 6 ] [CuPh ] 6

7 In-situ X-ray Absorption Spectroscopy Aniruddha Deb et al. J. Synchrotron Rad. (4). 11, In-situ X-ray Absorption Spectroscopy 7

8 Charge/Discharge at.1c Li 3 V 1.9 Mg.15 (PO 4 ) 3 - Cut off : 3. V-4.5V - Charge: 11 mah/g - Discharge: 14 mah/g Li 3 V 1.9 Mg.15 (PO 4 ) 3 - Cut off : 3. V-4.8V - Charge: 136 mah/g - Discharge: 1 mah/g Li 3 V 1.9 Mg.15 (PO 4 ) 3 8 Charge: V 8 Discharge: V Normalized Intensity Nor,malized Intensity Energy(eV) Energy(eV) Between V No change shown in the pre-edge region for a cut off of 4.5V Between 3-4.5V changes in the edge, observed, showing oxidation state change No significant e g -t g splitting observed in the pre-edge region Octahedral structure preserved 8

9 D Grap h 1 Li 3 V 1.9 Mg.15 (PO 4 ) 3 8 Charge:3.-4.8V 8 Discharge.-4.8V 6 Normalized Intensity Normalized Intensity Energy (ev) Energy (ev) Above 4.5V and below 3.V Above 4.5V change in edge consistent with formation of tetrahedral V 4+ or V 5+. XANES change is largely irreversible. Possible structural change below 3.V as the pre-edge intensity increases Li(Ni.4 Co.15 Al.5 Mn.4 )O 9

10 Li(Ni.4 Co.15 Al.5 Mn.4 )O Li(Ni.4 Co.15 Al.5 Mn.4 )O 1

11 Li(Ni.4 Co.15 Al.5 Mn.4 )O 11

12 Applications of EXAFS to Crystallographically Characterized Materials Cu Cu + Cu 1

13 Polarized XAS Fluorescence Detector x-ray beam Crystal or other oriented sample Sample positioner Polarized XAFS of Mo/Fe/S clusters Fourier Transform Magnitude S Fe S Fe S Fe S S Mo S S Mo Fe S S Fe Fe S S R+ (Å) Flank, Weininger, Mortenson, & Cramer, J. Am. Chem. Soc.,

14 Examples using XANES to determine electronic structure X-ray absorption spectroscopy XANES EXAFS Absorption 1s 3s 3p 3d 4s 4p 14

15 1s3d transitions Dipole forbidden (L = ) for centrosymmetric complexes Weak, but not absent, for all first row transition metals Possible mechanisms 3d+4p mixing (not possible for centrosymmetric complexes vibronic coupling direct quadrupole coupling Orientation dependence of 1s3d transition e 5 k 4 9 o Absorbance 3 1 Cu o Energy (ev) 15

16 Four-fold periodicity to 1s3d transition (degrees) Quadrupole transitions L= Theoret ical <s xy dx-y> = <s xy dxy > e Cu k Experiment al e Cu k 16

17 Selection rules for quadrupole transitions z x y xy yz xz 6 5 sin cos sin 5 5 sincso cos cos sin cos sin cos cos cos sin yz cos sin sin cos cos cos xz 5 sin z 5 sin sin sin cos cos cos xy x y Brouder, C. J. Phys. Condens. Matter, (199) s3d transitions for Fe porphyrins Normalized Absorption d d xy x -y Energy(eV) 17

18 .. Isolated 1s3d for Fe(OEP)(-MeIm) 5 d d z xy dx -y d xz+yz. ev dx-y Normalized Absorption ev.5 ev. ev dz dxz,yz dxy Energy(eV) XANES for Fe(III) complexes 4 coordinate 5 5 coordinate 6 coordinate Energy ( ev) Percent Fe 4p character Roe et al., J. Am. Chem. Soc., 16,

19 1s3d intensity Weak for square planar complexes Strong for tetrahedral complexes Correlates with coordination number Electronic information is (often) enhanced by studying L-edge rather than K-edge 19

20 Ti Ti Ti 4 3 Multiplet effects (1 s ) ( s ) ( p ) (3 s ) (3 p ) S (1 s ) ( s ) ( p) (3s ) (3 p) (3 d ) D (1 s ) ( s ) ( p ) (3 s ) (3 p ) (3 d ) F P PD G D S G, 5 4,3, L edge of Ti 4+,1, 3,, ( 1s) (s) ( p) (3s) (3p) (1s ) (s) ( p) (3s) (3p) (3d 1 S P, D, F, P, ( 3d ) ( p) (3d P P, 3 3 D, 3 1 ) 1 D, 3 D, 3 3 F, F, Allowed transitions: J = ±1 S P 1, P 1, D1 F 4 5,4,3 ) 1 1 S Simulations of L edges DeGroot, Coord. Chem. Rev. 5, 49, 31 63

21 But life is not really this simple need to consider ligand field: d terms vs ligand field Ti 4+ in the presence of a ligand field 1Dq=1.8 ev 1

22 Determination of Oxidation State L Cu +1 L 3 Cu +1 L 4 Cu +1 Cu + Kau et al., J. Am. Chem. Soc., 1987, 19, 6433

23 By defining generic Cu(I) and Cu(II) XANES can determine change in Cu oxidation state Sensitivity of XANES to geometry is consistent with expected density of unoccupied Cu 4p orbitals CuS CuS 3 3

24 Theoretical calculations of XANES Rehr and Ankudinov, Coord. Chem. Rev. 5, Mean-free path increases dramatically for energies near the edge accounts for some of the difficult with XANES caculations Penn, Phys. Rev. B 35,

25 The difficulty of simulating XANES is aalso due (in part) to multiple scattering Zn N NH Scattering probability as a function of scattering angle Ni Barton and Shirley, Phys. Rev. B 1985, 3, 189 5

26 Importance of linear multiple scattering J. Am. Chem. Soc. 1996, 118, MXAN can simulate XANES spectra (at least for some compounds under some conditions) Fe(CN) 6 3 Benfatto et al, J. Synchr.Rad. 3. 1, 51-57; J. Am. Chem. Soc. 4, 16,

27 XANES studies are not limited to metals George and Gorbaty, J. Am. Chem. Soc , 318 Ligand edges show pronounced dependence on metal identity Zn 4 Cu 4 Hedman, et al., J. Am. Chem. Soc. 199, 11,

28 Pre-edge transition due to M 3d +L 3p mixing Cu 4 Changes in pre-edge intensity correlate with changes in covalency 8

29 XANES (and potentially EXAFS) as a probe of solution structure XANES spectra show only very small changes with added thiolate 35 3 Absorption (cm /g) Zn(SR) 4 Zn(SR) mm (TMA)RS Energy (ev) 9

30 Conventional normalization misses changes in XANES MBACK reveals subtle changes when thiolate is added Equilibria for Zn(SPh) 4 - in DMSO Zn(SR) TMA Zn(SR) TMA 4 4 K IP = 13 4 M -1 Zn(SR) TMA 4 Zn(DMSO)(SR) 3 SR K D,IP =.1.9 M TMA Zn(SR) 4 Zn(DMSO)(SR) SR 3 K D =.13.1 M For 5 mm Zn, >75% dissociation Wilker and Lippard, Inorg. Chem. (1997) 36,

31 Flow system can be used for time resolved measurements Requirements (for reasonable sample volumes): Rapid scanning Small sample (i.e., small beam) 31

32 Flow direction [Reactants] x-ray t= t=t capillary 1 L/sec=1 mm/sec 1 m beam 1 ms resolution 3

33 Oxidation of Mn(III)hmpab by oxone is slow at º C 33

34 Oxidation is significantly faster at room temperature Reaction appears to be first order in Mn and oxone 34

35 Initial rate it temperature dependent G =3 kj/mol However, product that is formed is Mn(IV)=O not Mn(V)=O 35

36 Mn(III) shows significant radiation damage Room temperature, 3 minute scans Increasing time Low temperature (4K) reduces but does not eliminate radiation damage Low temperature can t be used if thermochromic 36

37 Flowing fluid samples can prevent radiation damage Mn 3+ (salpn)(acac) in Acetone + 15% H O Flow rate = L/s Residence time in beam = s - 16 ms