自旋翻转-spin crossover

Metal-Ligand Distances
The antibonding orbitals (eg*) are populated only in the HS state LS, S = 0
eg t2g
HS, S = 2
eg
t2g
Useful relationship:
10Dq ~ 1/rn (n = 5 – 9)
1A 1A 1A 3 1 T1 3 1 T2
1 1 T1
(10)
LS
HS
5T
2
5E
10DqHS = 11 800 cm–1
Hauser, A. J. Chem. Phys. 1991, 94, 2741.
10DqLS = 19 410 cm–1
Pairing Energy and Ligand-Field Strength
Spin Crossover in
Transition Metal Complexes
Michael Shatruk
Department of Chemistry and Biochemistry Florida State University, Tallahassee, FL
ICMM-2012 October 7-11, 2012
Halftime Update
1:1
Messi (31') C. Ronaldo (23')
Outline
1. The origins of spin crossover (SCO)
2. Characterization methods
3. Examples of SCO in transition metal complexes 4. The role of cooperativity in SCO 5. Pressure- and light-driven SCO 6. Charge-transfer induced spin transitions
Mössbauer Spectroscopy
Nucleus 1 Excited state E ES Nucleus 2
EES
Ground state E GS -source -receiver
EGS
Mössbauer Periodic Table
Experimental Implementation
Tanabe-Sugano Diagrams
d2 Ion d3 Ion
Tanabe-Sugano Diagrams
d5 Ion d6 Ion
The History of SCO Phenomenon
Observed for d4, d5, d6, d7 metal ions These ions exhibit two possible
Absorptions Spectra of HS and LS Complexes
10Dq, cm–1
[Fe(H2O)6]2+
10,000
[Fe(CN)6]3–
33,000
Absorption Spectra of SCO Complexes
[Fe(ptz)6](BF4)2
1 A 1 T 1 2
LS
57Fe
Mössbauer Spectroscopy Fe(II) LS
4.2 K 123 K 160 K
Other spectroscopic techniques
Absorption
LS Fe(II) HS
-4 -2 0
EQ
2 4
298K
HS
Velocity (mm/s)
Magnetometry
Example: High-Spin Fe2+ and Fe3+ Ions
HS Fe2+ S=2
E0ВН
If 10DqHS < 10,000 cm–1 or
10DqLS > 23,000 cm–1, the SCO is impossible
Conditions for observation of
SCO in Fe(II) complexes: 10DqHS 11,000 – 12,500 cm–1 10DqLS 19,000 – 22,000 cm–1
V, Å3 d(Fe–N)av, Å
(N-Fe-N), deg
12
i 90
i 1
1
i
2
Phan, H.; Chakraborty, P.; Chen, M.; Calm, Y. M.; Kovnir, K.; Keniley, L. K.; Hoyt, J. M.; Knowles, E. S.; Besnard, C.; Meisel, M. W.; Hauser, A.; Achim, C.; Shatruk, M. Chem. Eur. J. In press.
Orgel was the first to suggest the
Interesting sequence in 1950s: - Ligand field theory established - Mössbauer effect discovered - Spin crossover explained
r(Fe–L):
1.95-2.00 Å
2.15-2.20 Å
Using the average r(Fe–L) for
SCO complexes, one can estimate that 10DqLS/10DqHS ~ 1.75
Important: the Fe-N bond
lengths and orbital overlap change upon SCO, and therefore 10Dq is different for the LS and HS states of the same complex
eg t2g
HS, S = 2
eg t2g
10Dq >
10Dq <
10Dq – ligand-field splitting
– electron pairing energy
Complexes: HS SCO LS
Hauser, A. Top. Curr. Chem. 2004, 233, 49-58.
Splitting of d-Orbitals by the Ligand Field
Octahedral symmetry
eg 10Dq t2g
Mn+,
d
Spectrochemical series of ligand-field strength:
I– < Br– < Cl– < SCN– < NO3– < F– < OH– < H2O < NCS– < py < NH3 < en < NO2– < PPh3 < CN– < CO
LS, S = 1/2
HS, S = 5/2
1931 – Cambi and Szego observed
SCO in Fe(III) dithiocarbomates
Pauling initially explained their
results by the change in the bond type from covalent to ionic correct explanation based on the change in the spin state of the ion
Absorption Spectroscopy
Optical absorption spectrum of [Ti(H2O)6]3+
Wavelength, Å
eg
10Dq
t2g
Ti3+, 3d1
Molar absorptivity
10Dq = o 20,000 cm–1
Frequency, cm–1 10–3
10DqВС 10DqНС
E0HL = E0HS – E0LS
Characterization Methods
Crystallography
LS
d(FeII-N) (Å) 1.95-2.00
Mangetometry
HS
2.15-2.20
HS emu·K/mol
T = 3.3-3.8
T 0
mI
3 2 3 2 1 2
The quadrupole splitting arises from asymmetric distribution (gradient) of electron density around the absorbing nucleus
I


E0EA1 21 2Spin Crossover – Texas Style (Highway 6)
Spin Crossover (SCO) in Fe(II) complexes
About 90% of reported cases of SCO have been observed in Fe(II) complexes LS, S = 0
electron configurations in the octahedral ligand field
Example – d5 ion
eg t2g eg t2g
Switching between the states is
achieved by changing temperature, pressure, or by photoexcitation