Fundamental Properties of an Oxygen-Damaged Small
Molecule Model of Fe-only Hydrogenase: Studies in Intra- and
Intermolecular Ligand Exchange
Bin Li (Ben)
Group of Dr. Marcetta Y. Darensbourg
Department of Chemistry, TAMU
An Oxygen-Damaged Small Molecule Model of Fe-only Hydrogenase
OC
e-
H2
X
Cys
Fe
Fe
NC
(b)
Fe
C
O
CO
C
N
C
O
X= CH2, NH or O
(a)
Fe
CO
C
O
S S
[4Fe4S] S
H+
OC
C
O
O
C
SS
O
OC
OC
C
O
SS
Fe
O
C
Fe
CO
C
O
(c)
Active site of [FeFe] H2ase (a); Model Complex (b); Oxygen-Damaged Model Complex (c).
What is the influences of Sulfenato-oxygen upon the ligand exchange
into such oxygen damaged models?
Previous Kinetic Studies of Model of [FeFe]H2ase Active Sites
Marcetta Y. Darensbourg’s group
Thomas B. Rauchfuss’ and
Luca De Gioia‘s group
Rate = k2[FeFe][CN-]
Eric J, Lyon, Marcetta Y. Darensbourg et al, J.Am.Chem.Soc. 2001, 123, 3268-3278.
Aaron K. Justice, Luca De Gioia, Thomas B. Rauchfuss et al. Inorg. Chem. 2007,46,1655-1664.
Kinetic Studies of PMe3/CO Exchange into (μ-pdt)Fe2(CO)6
OC
OC
O
C
SS
Fe
C
O
Fe
OC
PMe3
Fe
OC
Me3P
CO
C
O
(μ-pdt)Fe2(CO)6 + xs PMe3
SS
Toluene
(μ-pdt)Fe2(CO)5PMe3+ xs PMe3
Rate = k2[(μ-pdt)Fe2(CO)6][PMe3]
Fe
O
C PMe3
CO
C
O
OC
Fe
OC
Me3P
(μ-pdt)Fe2(CO)5PMe3
Toluene
O
C
SS
Fe
PMe3
C
O
(1)
(μ-pdt)Fe2(CO)4(PMe3)2 (2)
Rate = k2’[(μ-pdt)Fe2(CO)5PMe3][PMe3]
Determination of Rate Constants
3
2.5
ln(A0/At)
2
ln(At/A0)= kobst
1.5
1
0.5
0
0
20
40
60
80
100
120
Time, (min)
Figure 4. Example of plot of ln(A0/At) vs time over five half-lives, Entry 4, Table 1.
-7.6
-7.8
ln(kobs )
-8
kobs=k2[PMe3]
-8.2
-8.4
y = 1.03x - 6.48
R2 = 0.997
-8.6
-8.8
-9
-2.4
-2.2
-2
-1.8
-1.6
-1.4
-1.2
-1
ln[PMe3]
Figure 5. Plot of ln(kobs) vs ln[PMe3] for the formation of (μ-pdt)Fe2(CO)5PMe3 from (μ-pdt)Fe2(CO)6 measured at 22°C.
Rate Constants of PMe3/CO Exchange into (μ-pdt)Fe2(CO)6
Table 3. Rate Data for the Reaction of PMe3 with (μ-pdt)Fe2(CO)6 (1) Measured at 22 °C and
with (μ-pdt)Fe2(CO)5PMe3 (2) Measured at 50 °C in Toluene Solution
entry
compd
[PMe3], M
104 kobs,
sec-1
1
1a
0.10
1.47
2
1
0.15
2.15
3
1
0.20
2.85
4
1
0.30
4.55
av k2 = 14.6 (± 0.6) × 10-4 M-1 sec-1
5
2a
0.10
1.28
6
2
0.15
1.97
7
2
0.20
2.62
8
2
0.30
4.00
av k2 = 13.1 (± 0.3) × 10-4 M-1 sec-1
a [Fe
2]
= 5.0 × 10-3 M with 20 — 60-fold excess PMe3.
Kinetic Studies of PMe3/CO Exchange into (μ-pst)Fe2(CO)6
O
OC
OC
SS
Fe
C
O
O
C
Fe
PMe3
CO
C
O
(μ-pst)Fe2(CO)6 + xs PMe3
O
OC
SS
Fe
OC
Me3P
Toluene
O
C
Fe
CO
C
O
(μ-pst)Fe2(CO)5PMe3
Rate = k2[(μ-pst)Fe2(CO)6][PMe3]
Determination of Rate Constants
Table 4. Rate Data for the Reaction of PMe3 with (μ-pst)Fe2(CO)6 (1) Measured at 95 °C in Toluene Solution
a [Fe
2]
entry
compd
[PMe3], M
104 kobs,
sec-1
1
3a
0.10
0.43
2
3
0.20
0.62
3
3
0.30
0.85
4
3
0.40
1.07
= 5.0 × 10-3 M with 20 — 80-fold excess PMe3.
-9
Rate = k2[(μ-pst)Fe2(CO)6][PMe3]
-9.2
ln(kobs )
-9.4
kobs=k2[PMe3]
y = 0.65x - 8.59
R2 = 0.986
The reaction does not follow a second-order rate
expression well.
It suggested a complicated, both SN2 and SN1
substituted process was involved at a high
reaction temperature with dissociation of CO
ligand of (μ-pst)Fe2(CO)6.
-9.6
-9.8
-10
-10.2
-2.7
-2.2
-1.7
-1.2
-0.7
ln[PMe3]
Figure 8. Plot of ln(kobs) vs ln[PMe3] for the formation of (μ-pst)Fe2(CO)5PMe3 from (μ-pst)Fe2(CO)6 measured at 95°C.
Kinetic Studies of CN-/CO Exchange into (μ-pst)Fe2(CO)6
1OC
OC
SS
Fe
C
O
O
OC
O
C Et4N+CN-
Fe
CO
OC
C
O
(μ-pst)Fe2(CO)6 + xs Et4NCN
Fe
O
C
Et4N+CN-
Fe
CO
C
N
C
O
CH3CN
(μ-pst)Fe2(CO)5(CN)- + xs Et4NCN
Rate = k2[(μ-pst)Fe2(CO)6][CN-]
O
SS
CH3CN
2OC
OC
SS
Fe
C
N
O
O
C
Fe
CN
C
O
(μ-pst)Fe2(CO)5(CN)(μ-pst)Fe2(CO)4(CN)22-
Rate = k2’[(μ-pst)Fe2(CO)5(CN)-][CN-]
Determination of Rate Constants
1.4
-8.2
1.2
-8.4
-8.6
ln(kobs)
ln(A0/At)
1
0.8
-8.8
-9
0.6
y = 1.00x - 7.14
R2 = 0.982
-9.2
0.4
-9.4
0.2
-9.6
-2.4
0
0
10
20
30
40
50
60
70
-2.2
80
-2
-1.8
-1.6
-1.4
-1.2
ln[CN ]
Time, (min)
Figure 11. Plot of ln(kobs) vs ln[CN-] for the formation of (μ-pst)Fe2(CO)4(CN)22from (μ-pst)Fe2(CO)5(CN)- measured at 40°C.
Figure 10. Example of plot of ln(A0/At) vs time over three half-lives.
Table 5. Rate Data for the Reaction of Et4N+CN- with (μ-pst)Fe2(CO)6 (3) Measured at 0 °C and with [Et4N+ ][(μ-pst)Fe2(CO)5(CN)-] (4)
Measured at 40 °C in CH3CN Solution
a [Fe
b [Fe
entry
compd
[CN-], M
104 kobs,
sec-1
1
3a
0.10
22.43
2
4b
0.10
0.83
3
4
0.15
1.08
4
4
0.20
1.62
5
4
0.30
2.43
= 5.0 × 10-3 M with 20-fold excess CN-.
-3
2] = 5.0 × 10 M with 20 — 60-fold excess CN .
2]
-1
-
av k2 = 7.94 (± 0.8) × 10-4 M-1 sec-1
Determination of Activation Parameters
Eyring plot
Arrhenius plot
-4
-10
-10.5
-5
-11.5
ln k2
ln(k2/T)
-11
-12
-12.5
-6
-7
-13
-13.5
-8
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
3
1/T (×10 )
Figure 12. Eyring plot for the formation of (μ-pdt)Fe2(CO)5PMe3 from (μpdt)Fe2(CO)6 (□), formation of (μ-pdt)Fe2(CO)4(PMe3)2 from (μpdt)Fe2(CO)5PMe3 (△) and formation of (μ-pst)Fe2(CO)4(CN)22- from (μpst)Fe2(CO)5(CN)- (○).
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
1/T (×103)
Figure 13. Arrhenius plot for the formation of (μ-pdt)Fe2(CO)5PMe3
from (μ-pdt)Fe2(CO)6 (□), formation of (μ-pdt)Fe2(CO)4(PMe3)2 from
(μ-pdt)Fe2(CO)5PMe3 (△) and formation of (μ-pst)Fe2(CO)4(CN)22from (μ-pst)Fe2(CO)5(CN)- (○).
ln(k2/T) = (-ΔH‡/R)T-1 + ln(kB/h) + (ΔS‡/R)
k2 = A exp(-Ea / RT)
ln(k2/T) vs T-1
lnk2 vs T-1
Slope= -ΔH‡/R, Intercept = ln(kB/h) + (ΔS‡/R)
Slope= -Ea/R
Determination of Activation Parameters
Table 6. Temperature Dependence of Reaction of PMe3 with (μ-pdt)Fe2(CO)6, (μ-pdt)Fe2(CO)5PMe3 and Reaction
of Et4N+CN- with [Et4N+ ][(μ-pst)Fe2(CO)5(CN)-]
compound
T,°C
(104) k2,
M-1s-1
activation parameters
22
14.67
Ea = 48 kJ/mol
30
25.50
ΔH‡ = 45 kJ/mol
40
45.00
ΔS‡ = - 126 J/mol K
50
80.83
50
12.83
Ea = 53 kJ/mol
60
26.67
ΔH‡ = 50 kJ/mol
70
44.83
ΔS‡ = - 127 J/mol K
80
68.67
40
8.33
Ea = 61 kJ/mol
50
16.33
ΔH‡ = 58 kJ/mol
60
31.17
ΔS‡ = - 99 J/mol K
70
65.50
(μ-pdt)Fe2(CO)6a
(μ-pdt)Fe2(CO)5PMe3a
[Et4N+ ][(μ-pst)Fe2(CO)5(CN)-]b
a [Fe2] = 5.0 × 10-3 M; [PMe3] = 0.1 M.
b [Fe2] = 5.0 × 10-3 M; [CN-] = 0.1 M.
Proposed mechanism for CN-/CO exchange reaction in (μ-pst)Fe2(CO)6
OC
O
SS
Fe
OC
CN-
O
C
Ea
Fe
SS
Fe
OC
CO
C
O
OC
N
C
Fe
C
O
C
O
C
O
O
CO
C
O
CO
CN-
OC
O
SS
Fe
OC
-
N
C
OC
Fe
Fe
OC
CO
C
O
SS
O
Fe
CO
C
O
C
O
N
C
C
O
Ea'
-NC
OC
SS
Fe
C
O
O
N
C
Fe
C
O
CO
C
O
--
CO
NC
OC
SS
Fe
C
O
O
N
C
Fe
CO
C
O
We propose that the transition states
play an critical role in the intermolecular
PMe3/CO and CN-/CO exchange
processes.
The sulfenato-oxygen influence the
coordination sphere intramolecular
rearrangements due to steric and
electronic effects.
The increase of rotation barriers due to
the presence of sulfenato-oxygen
interaction thus increases the activation
energy for the CN-/PMe3 attack.
Conclusions
•
The PMe3/CO exchange process into (μ-pdt)Fe2(CO)6 follows a strict second-order
rate law which is first order in both the substrate and PMe3. The evolved results
suggest an associative or Ia mechanism.
•
The larger activation energy barrier of PMe3/CO exchange process into (μpst)Fe2(CO)6 responds to the high reaction temperature, which finds both SN2 and
SN1 substituted mechanism occurred.
•
CN- exchanges with CO into (μ-pst)Fe2(CO)6 by associative processes produce both
stable monocyano anionic and dicyano dianionic products. The second step CN-/CO
exchange reaction was found to be rate-limiting step, which is converse to the CN-/CO
exchange process into (μ-pdt)Fe2(CO)6.