Model Complexes for the Ni_p Site of Acetyl Coenzyme A Synthase/Carbon Monoxide (CO) Dehydrogenase: Structure, Electrochemistry, and CO Reactivity

Abstract

Aliphatic thiolato-S-bridged tri-and binuclear nickel(II) complexes have been synthesized and characterized as models for the Ni p site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide (CO) dehydrogenase. Reaction of the in situ formed N 2 S thiol donor ligands withrespectively. The X-ray crystal structures of 1−4 revealed a central Ni II S 4 moiety in 1 and 2 and a Ni II P 2 S 2 moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Ni p site of ACS. The electrochemical reduction of both terminal Ni II ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni{(L Me(S) ) 2 Ni(NO)} 2 ](ClO 4 ) 2 ( 5) and [Ni{(L Br(S) ) 2 Ni(NO)} 2 ](ClO 4 ) 2 (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit ν NO at 1773 and 1789 cm −1 , respectively. In the presence of O 2 , both 5 and 6 transform to nitrite-bound monomers [(L Me(S−S) )Ni(NO 2 )]-(ClO 4 ) (7) and [(L Br(S−S) )Ni(NO 2 )](ClO 4 ) 2 (8). The nature of the ligand modification is evident from the X-ray crystal structure of 7. To understand the origin of multiple reductive responses of 1−4, complex [(L Me(SMe) ) 2 Ni](ClO 4 ) 2 ( 9) is considered. The central NiS 4 part of 1 is labile like the Ni p site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3 red -(CO) 2 species, as confirmed by Fourier transform infrared (ν CO = 1997 and 2068 cm −1 ) and electron paramagnetic resonance (g 1 = 2.18, g 2 = 2.13, g 3 = 1.95, and A P = 30−80 G) spectroscopy. The CO binding to Ni I of 3 red is relevant to the ACS activity.

Nickel(II)‐Mediated Reversible Thiolate/Disulfide Conversion as a Mimic for a Key Step of the Catalytic Cycle of Methyl‐Coenzyme M Reductase

“…One Ni is coordinated to two L SÀ which is bridged via its thiolato-S donors to another Ni that is ligated to four Ndonors of aL S-S ligand. Theavg.Ni ÀNand NiÀS thiol distances for the former Ni (Ni1 in Figure 1, middle) are 2.063(5) and 2.3992 (14) and those for the latter Ni (Ni2 in Figure 1, middle) are 2.100(4) and 2.4317 14,respectively.T he SÀSdistance of Ni-uncoordinated disulfide of L S-S is 2.064(2) which is 0.02 shorter than that of the h 1coordinated disulfide of L S-S present in 1 Cl .T he NiÀS thiol distances found in 2 are comparable to those of the other reported hexa-coordinated Ni II -thiolate complexes [22,27,28] and to that of the MCR ox1-silent structure; [25] Figure S15A) reveals as harp and intense stretch at 1734 cm À1 corresponding to the n CO of CH 3 CO 2 Et solvate in the structure.The n S-S stretch is absent, unlike in 1 Cl , 1 Solv and 2 where the n S-S stretch at % 475 cm À1 ( Figure S26) is observed. Theelectronic absorption spectrum of the greenish brown DMF solution of 3 ( Figure S15B) displays bands at l max /nm (e/m À1 cm À1 ) = 967 (975), 750 (1305), 580 (9350) and 304 (66 714).…”

“…Complex 1 Cl was synthesized either by reacting L S‐S with NiCl 2 ⋅6 H 2 O or via the oxidation of a CH 3 CN solution of 2 with the use of Cl 2 (g) as an oxidant ( 2 +Cl 2 (g)→ 1 Cl , see Figure S21A). The electronic absorption spectrum of 1 Cl in CH 3 CN features bands at λ max /nm ( ϵ / m −1 cm −1 )=912 (26), 550 sh (65), 323 (20 576), 294 (17 670) and 237 sh (32 750) comparable to those of reported octahedral Ni II complexes . The slow evaporation of a CH 3 CN/CH 2 Cl 2 solution of 1 Cl afforded crystals suitable for X‐ray diffraction.…”

“…Complex 2 was synthesized either by refluxing a CH 3 OH solution of a mixture of L S‐S and [Ni(H 2 O) 6 ](ClO 4 ) 2 (1:1, mol mol −1 ) or by reducing the CH 3 CN solution of 1 Cl with one equivalent of NaBH 4 (Figure S21B). The electronic spectrum of 2 in CH 3 CN (Figure S12B) displays bands at λ max /nm ( ϵ / m −1 cm −1 )=866 (176), 468 sh (7050), 316 (43 860), 270 sh (42 845) and 232 sh (67 620), comparable to those of bis‐thiolato‐S bridged hexa‐coordinated Ni II complexes . The intense shoulder at 470 nm is due to the thiolato‐S − to Ni II charge transfer transition that is absent in the spectrum of 1 Cl and 1 Solv .…”

“…Thee lectronic absorption spectrum of 1 Cl in CH 3 CN features bands at l max /nm (e/m À1 cm À1 ) = 912 (26), 550 sh (65), 323 (20 576), 294 (17 670) and 237 sh (32 750) comparable to those of reported octahedral Ni II complexes. [22][23][24] Thes low evaporation of aC H 3 CN/CH 2 Cl 2 solution of 1 Cl afforded crystals suitable for X-ray diffraction. TheX -ray structure of 1 Cl revealed it to be am onomer (Figure 1, top [23,24] but is % 0.07 longer than that of the MCR ox1-silent state of the enzyme where the Ni II has N 4 SO coordination and the Ni II -S CoM distance is 2.40 .…”

“…As a continuing interest in understanding the donor strength dependent stabilization and reactivity of various Ni oxidation states (0, +1, +2 and +3) in an NS donor environment, herein, we report first the Ni‐mediated reversible thiolate/disulfide transformation. This study offers Ni II complexes, [(L S‐S )Ni(Cl)](ClO 4 ) ( 1 Cl ), [(L S‐S )Ni (Solvent)](ClO 4 ) 2 ( 1 Solv ), [Ni(L S ) 2 Ni(L S‐S) )](ClO 4 ) 2 ⋅0.5 Et 2 O ⋅1.5 H 2 O ( 2 ) and [Ni(L S ) 2 Ni(L S )](ClO 4 )⋅CH 3 CO 2 Et ( 3 ), derived from a tridentate N 2 S thiol donor ligand, HL S and its disulfide analogue, L S‐S (Scheme a).…”