Fundamental Properties of an Oxygen-Damaged Small Molecule Model of Fe-only Hydrogenase: Studies in Intra- and Intermolecular Ligand Exchange

Transcription

1 Fundamental Properties of an xygen-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 hemistry, TAMU

2 An xygen-damaged Small Molecule Model of Fe-only Hydrogenase H + S S e - H 2 X ys [4Fe4S] S S S (b) N N X= H 2, NH or S S (a) (c) Active site of [FeFe] H2ase (a); Model omplex (b); xygen-damaged Model omplex (c). What is the influences of Sulfenato-oxygen upon the ligand exchange into such oxygen damaged models?

3 Previous Kinetic Studies of Model of [FeFe]H 2 ase Active Sites Marcetta Y. Darensbourg s group Thomas B. Rauchfuss and Luca De Gioia s group Rate = k 2 [FeFe][N - ] Eric J, Lyon, Marcetta Y. Darensbourg et al, J.Am.hem.Soc. 2001, 123, Aaron K. Justice, Luca De Gioia, Thomas B. Rauchfuss et al. Inorg. hem. 2007,46,

4 Kinetic Studies of / Exchange into (μ-pdt)fe 2 () 6 S S S S Fe Me 3 P Fe S S Me 3 P Toluene (μ-pdt)fe 2 () 6 + xs (μ-pdt)fe 2 () 5 (1) Toluene (μ-pdt)fe 2 () 5 + xs (μ-pdt)fe 2 () 4 ( ) 2 (2) Rate = k 2 [(μ-pdt)fe 2 () 6 ][ ] Rate = k 2 [(μ-pdt)fe 2 () 5 ][ ]

5 ln(kobs) ln(a0/at) Determination of Rate onstants ln(a t /A 0 )= k obs t Time, (min) Figure 4. Example of plot of ln(a 0 /A t ) vs time over five half-lives, Entry 4, Table k obs =k 2 [ ] y = 1.03x R 2 = ln[ ] Figure 5. Plot of ln(k obs ) vs ln[ ] for the formation of (μ-pdt)fe 2 () 5 from (μ-pdt)fe 2 () 6 measured at 22.

6 Rate onstants of / Exchange into (μ-pdt)fe 2 () 6 Table 3. Rate Data for the Reaction of with (μ-pdt)fe 2 () 6 (1) Measured at 22 and with (μ-pdt)fe 2 () 5 (2) Measured at 50 in Toluene Solution entry compd [ ], M 10 4 k obs, sec a av k 2 = 14.6 (± 0.6) 10-4 M -1 sec a av k 2 = 13.1 (± 0.3) 10-4 M -1 sec -1 a [Fe 2 ] = M with fold excess.

7 Kinetic Studies of / Exchange into (μ-pst)fe 2 () 6 S S S S Me 3 P Toluene (μ-pst)fe 2 () 6 + xs (μ-pst)fe 2 () 5 Rate = k 2 [(μ-pst)fe 2 () 6 ][ ]

8 ln(kobs) Determination of Rate onstants Table 4. Rate Data for the Reaction of with (μ-pst)fe 2 () 6 (1) Measured at 95 in Toluene Solution entry compd [ ], M 10 4 k obs, sec a a [Fe 2 ] = M with fold excess. y = 0.65x R 2 = Rate = k 2 [(μ-pst)fe 2 () 6 ][ ] k obs =k 2 [ ] The reaction does not follow a second-order rate expression well. It suggested a complicated, both S N 2 and S N 1 substituted process was involved at a high reaction temperature with dissociation of ligand of (μ-pst)fe 2 () 6. ln[ ] Figure 8. Plot of ln(k obs ) vs ln[ ] for the formation of (μ-pst)fe 2 () 5 from (μ-pst)fe 2 () 6 measured at 95.

9 Kinetic Studies of N - / Exchange into (μ-pst)fe 2 () 6 S S S S Fe N Fe (μ-pst)fe 2 () 6 + xs Et 4 NN H 3 N (μ-pst)fe 2 () 5 (N) - H (μ-pst)fe 2 () 5 (N) - 3 N + xs Et 4 NN (μ-pst)fe 2 () 4 (N) Et 4 N + N - Et 4 N + N - S S N N 2- Rate = k 2 [(μ-pst)fe 2 () 6 ][N - ] Rate = k 2 [(μ-pst)fe 2 () 5 (N) - ][N - ]

10 ln(a0/at) ln(kobs) Determination of Rate onstants y = 1.00x R 2 = Time, (min) Figure 10. Example of plot of ln(a 0 /A t ) vs time over three half-lives ln[n - ] Figure 11. Plot of ln(k obs ) vs ln[n - ] for the formation of (μ-pst)fe 2 () 4 (N) 2 2- from (μ-pst)fe 2 () 5 (N) - measured at 40. Table 5. Rate Data for the Reaction of Et 4 N + N - with (μ-pst)fe 2 () 6 (3) Measured at 0 and with [Et 4 N + ][(μ-pst)fe 2 () 5 (N) - ] (4) Measured at 40 in H 3 N Solution entry compd [N - ], M 10 4 k obs, sec a a [Fe 2 ] = M with 20-fold excess N -. b [Fe 2 ] = M with fold excess N b av k 2 = 7.94 (± 0.8) 10-4 M -1 sec -1

11 ln(k2/t) Determination of Activation Parameters Eyring plot -4-5 Arrhenius plot ln k /T ( 10 3 ) Figure 12. Eyring plot for the formation of (μ-pdt)fe 2 () 5 from (μpdt)fe 2 () 6 ( ), formation of (μ-pdt)fe 2 () 4 ( ) 2 from (μpdt)fe 2 () 5 ( ) and formation of (μ-pst)fe 2 () 4 (N) 2-2 from (μpst)fe 2 () 5 (N) - ( ). ln(k 2 /T) = (-ΔH /R)T -1 + ln(k B /h) + (ΔS /R) ln(k 2 /T) vs T -1 Slope= -ΔH /R, Intercept = ln(k B /h) + (ΔS /R) /T ( 10 3 ) Figure 13. Arrhenius plot for the formation of (μ-pdt)fe 2 () 5 from (μ-pdt)fe 2 () 6 ( ), formation of (μ-pdt)fe 2 () 4 ( ) 2 from (μ-pdt)fe 2 () 5 ( ) and formation of (μ-pst)fe 2 () 4 (N) 2 2- from (μ-pst)fe 2 () 5 (N) - ( ). k 2 = A exp(-ea / RT) lnk 2 vs T -1 Slope= -Ea/R

12 Determination of Activation Parameters Table 6. Temperature Dependence of Reaction of with (μ-pdt)fe 2 () 6, (μ-pdt)fe 2 () 5 and Reaction of Et 4 N + N - with [Et 4 N + ][(μ-pst)fe 2 () 5 (N) - ] compound T, (10 4 ) k 2, M -1 s -1 activation parameters (μ-pdt)fe 2 () 6 a (μ-pdt)fe 2 () 5 a E a = 48 kj/mol ΔH = 45 kj/mol ΔS = J/mol K E a = 53 kj/mol ΔH = 50 kj/mol ΔS = J/mol K Ea = 61 kj/mol ΔH = 58 kj/mol ΔS = - 99 J/mol K a [Fe2] = M; [PMe3] = 0.1 M. b [Fe2] = M; [N-] = 0.1 M.

13 Proposed mechanism for N - / exchange reaction in (μ-pst)fe 2 () 6 - N - S S N - S S N Ea' N S S N - - Ea S S N S S N N S S N - - We propose that the transition states play an critical role in the intermolecular / and N - / 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 N - / attack.

14 onclusions The / exchange process into (μ-pdt)fe 2 () 6 follows a strict second-order rate law which is first order in both the substrate and. The evolved results suggest an associative or Ia mechanism. The larger activation energy barrier of / exchange process into (μpst)fe 2 () 6 responds to the high reaction temperature, which finds both SN2 and SN1 substituted mechanism occurred. N - exchanges with into (μ-pst)fe 2 () 6 by associative processes produce both stable monocyano anionic and dicyano dianionic products. The second step N - / exchange reaction was found to be rate-limiting step, which is converse to the N - / exchange process into (μ-pdt)fe 2 () 6.