Model Complexes for the Nip Site of Acetyl Coenzyme A Synthase

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Model Complexes for the Nip Site of Acetyl Coenzyme A Synthase/ Carbon Monoxide (CO) Dehydrogenase: Structure, Electrochemistry, and CO Reactivity Anirban Bhandari,† Ram Chandra Maji,† Saikat Mishra,† Akhilesh Kumar,‡ Suman Kumar Barman,‡ Partha Pratim Das,‡ Kamran B. Ghiassi,§ Marilyn M. Olmstead,§ and Apurba K. Patra*,† †

Department of Chemistry, National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, India Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India § Department of Chemistry, University of California, Davis, California 95616, United States Inorg. Chem. Downloaded from pubs.acs.org by REGIS UNIV on 10/19/18. For personal use only.

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S Supporting Information *

ABSTRACT: Aliphatic thiolato-S-bridged tri- and binuclear nickel(II) complexes have been synthesized and characterized as models for the Nip site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide (CO) dehydrogenase. Reaction of the in situ formed N2Sthiol donor ligands with [Ni(H2O)6](ClO4)2 afforded the trinuclear complexes [Ni{(L Me(S) ) 2 Ni} 2 ](ClO 4 ) 2 ·CH 3 CN (1·CH 3 CN) and [Ni{(LBr(S))2Ni}2](ClO4)2·5H2O (2·5H2O) following self-assembly. Complexes 1 and 2 react with [Ni(dppe)Cl2] and dppe [dppe = 1,2-bis(diphenylphosphino)ethane] to afford the binuclear [Ni(dppe)Ni(LMe(S))2](ClO4)2·2H2O (3·2H2O) and [Ni(dppe)Ni(LBr(S))2](ClO4)2·0.75O(C2H5)2 [4·0.75O(C2H5)2], respectively. The X-ray crystal structures of 1−4 revealed a central NiIIS4 moiety in 1 and 2 and a NiIIP2S2 moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Nip site of ACS. The electrochemical reduction of both terminal NiII ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni{(LMe(S))2Ni(NO)}2](ClO4)2 (5) and [Ni{(LBr(S))2Ni(NO)}2](ClO4)2 (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit νNO at 1773 and 1789 cm−1, respectively. In the presence of O2, both 5 and 6 transform to nitrite-bound monomers [(LMe(S−S))Ni(NO2)](ClO4) (7) and [(LBr(S−S))Ni(NO2)](ClO4)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 [(LMe(SMe))2Ni](ClO4)2 (9) is considered. The central NiS4 part of 1 is labile like the Nip site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3red-(CO)2 species, as confirmed by Fourier transform infrared (νCO = 1997 and 2068 cm−1) and electron paramagnetic resonance (g1 = 2.18, g2 = 2.13, g3 = 1.95, and AP = 30−80 G) spectroscopy. The CO binding to NiI of 3red is relevant to the ACS activity.

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INTRODUCTION

The X-ray crystal structure determination of CODH/ACS, isolated from an acetogenic bacterium Moorella thermoacetica, revealed an α2β2 heterotetramer structure.3 The central β2 domain contains B, C, and D metalloclusters, where CO2 ↔ CO conversion occurs. Each end of the β2 complex is attached to an α subunit, containing the A cluster responsible for acetyl CoA formation. This and other reported X-ray crystal structures of this enzyme revealed an unprecedented architecture of the A cluster comprised of three metallic subunits; all are Cys-S-bridged to each other.4 These subunits are a Fe4S4 cubane, a four-coordinate central metal site in (Cys-S)3L (L is an unidentified nonprotein ligand) coordination environment known as the Mp site (proximal to Fe4S4

Carbon monoxide (CO) dehydrogenase/acetyl coenzyme A (CoA) synthase (CODH/ACS) is a bifunctional enzyme that catalyzes two key biological processes: one is the reversible reduction of carbon dioxide (CO2) to CO (CODH activity) that occurs at the C cluster of the enzyme, and the other is the synthesis of acetyl coenzyme A (ACS activity) using CODHgenerated CO, a methyl group, and the thiol CoA that occurs at the A cluster of the enzyme.1 Once acetyl CoA is formed, it is used either in respiration to produce energy or for the biomass synthesis in biology. This enzyme plays a key role in the global C cycle by its participation in the Wood−Ljungdahl pathway of autotrophic C fixation, through the action of a few anaerobic bacteria including acetogens, methanogens, and sulfate reducers.2 © XXXX American Chemical Society

Received: August 10, 2018

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DOI: 10.1021/acs.inorgchem.8b02276 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry cubane, where M = CuI, ZnII, or NiII), and a square-planar NiII bonded to two peptide N and two Cys-S donors known as the Nid site (distal to Fe4S4 cubane), as shown in Figure 1. The

occurrence of Cu or Zn at the Mp site is controversial. The structure reported by Doukov et al.3 revealed high occupancy of Cu at this site, while that reported by Darnault et al.4a showed Ni in one and Zn in the other molecule, although for both cases the enzyme isolated was from the same species, M. thermoacetica. The biochemical evidence indicates that only the Nip−Nid form is active and not the Cup−Nid or Znp−Nid form. This conclusion is further supported by spectroscopic and computational studies,5 including a crystal structure determination of the enzyme, isolated from a different species, Carboxydothermus hydrogenoformans.4b It is now well established that the Fe4S4 cubane and Nid fragments do not participate in any redox event during the enzyme’s catalytic action; the assembly of CO and CH3 to form a nickel acetyl intermediate and finally the transfer of this acetyl group to thiol CoA take place at the redox-active Nip site, which is reducible prior to CO binding. According to the proposed mechanism for the enzyme, CO binds to NipI but not to Nip0; the Ni0 state in ACS has never been observed or reported.1a However, the

Figure 1. ChemDraw depiction of the A cluster of the ACS subunit of the CODH/ACS enzyme. L is an unidentified nonprotein ligand, and M is NiII, CuI, or ZnII.

Scheme 1. ChemDraw Depiction of Ligands and Nickel Complexes and Their Syntheses and Reactivity

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Article

Inorganic Chemistry sequence of CO and +CH3 binding to NipI is debatable.6 From a pulse-chase study of acetyl CoA synthesis with CODH/ ACSMt and with ACS only, Seravalli and Ragsdale reported that either CO or the methyl group can bind first during catalysis of the enzyme.7 The Nip site of ACS/CODH is found to be labile. When ACS/CODH was titrated with a bidentate chelator such as 1,10-phenanthroline (phen), ∼30% removal of the Ni content and loss of the ACS activity of the enzyme were observed. The addition of NiCl2 to the phen-treated enzyme replenishes its activity as a confirmation that this “labile Ni” is essential for the ACS activity of the enzyme.5b,8 Presumably, because of the labile nature of Nip, the other metal ion, such as CuI or ZnII, is fortuitously captured by the Nid site and thus generates the Cup−Nid or Znp−Nid form. These forms of the enzyme have been X-ray structurally characterized.3,4 Mono- and binuclear complexes as models of the Nid, Nip− Nid, and Cup−Nid subunits of the A cluster of ACS/CODH are known.9,10 In addition to these, a few trinuclear9c,e−j,10c,d,g,h and heteromultinuclear9c,10d complexes, containing metal cores such as Ni3Cu2, Ni2Cu2, and Ni3Zn2, related to the A-cluster modeling of ACS are reported. In all of these models, either amidato-N-containing9 or tert-amine-N-containing10 thiol ligands are used to mimic the Nid part, which, in turn, utilizes its thiolato S to bridge another metal ion such as NiII, CuI, or ZnII, hence producing a Mp site. Employment of such N donors (tert-amine or amide) is reasonable because these can stabilize the NiII state of Nid to a great extent and prevent its reduction to NiI, ensuring that the modeled Nip is the sole reducible site. The Nip site is able to bind substrates such as CO following reduction and may proceed further to acetyl thioester formation such as acetyl CoA, produced by ACS/ CODH. Unfortunately, however, only a few models are reported to exhibit CO binding to reduced NiI of the Nip model site,9a,d−f,10a,b and even fewer are known to produce acetyl thioester.10a,b A literature survey surprisingly revealed no model complex with an imine-N-containing thiol ligand that may generate a S-rich coordination environment surrounding a NiII center that can mimic the properties of the Nip site like other models.9a,d−f,10a,b Also, a six-coordinate metallothiolate synthon that generates a Nip model is unknown.9,10 As a continuing interest to investigate the donor-strength-dependent stabilization of various Ni oxidation states (0, 1+, 2+, and 3+) in NS donor environments11 and to study their reactivity,12 herein, we first report the use of pyridine-/ imine-N-containing thiol ligands to synthesize thiolato-Sbridged bi- and trinuclear nickel complexes. These yield Nip site models by employing a six-coordinate NiN4S2 metallothiolate synthon. The central NiS4 unit of 1 and 2 is labile like the Nip site of the enzyme; complete replacement of this moiety is evident when a solution of 1 or 2 is treated with 1,10phenanthroline. The NiII ion of the NiS4 unit exhibits a more cathodic NiII/I reduction potential than the NiII/I and NiI/0 couples of the two terminal NiII ions; therefore, the CO reactivity with reduced NiS4 alone cannot be investigated. However, the NiII/I reduction potentials of the NiP2S2 fragments of 3 and 4 are less cathodic than those of their NiN4S2 counterparts, allowing us to investigate the CO reactivity of the one-electron-reduced form of 3 (i.e., 3red) like other reported models containing a NiP2S2 chromophore.9b,d,10f The NiI of 3red binds CO, as is evident from Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectra. To understand the nature (one- or twoelectron transfer) and origin (which Ni of the Ni3 unit) of

multiple reductive responses, displayed by 1 and 2, complexes 5−9 have been synthesized and characterized. The syntheses of ligands and nickel complexes and the reactivity studies carried out are shown in Scheme 1. The structural, spectroscopic, and electrochemical results reported herein offer a clue to the choice of peptide-N coordination to the Nid site of ACS/CODH, which, in turn, produces the Nip active site, where acetyl CoA synthesis occurs.

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EXPERIMENTAL SECTION

Materials and Reagents. The reagents 6-bromopyridine-2carboxaldehyde, 6-methylpyridine-2-carboxaldehyde, 2-aminoethanethiol, 2,2′-diaminodiethyl disulfide dihydrochloride, triethylamine (NEt3), 1,2-bis(diphenylphosphino)ethane (dppe), [Ni(dppe)Cl2], [Ni(H2O)6](ClO4)2, and sodium perchlorate (NaClO4) were purchased from Aldrich Chemical Co. and used without further purification. Acetonitrile (CH3CN), methanol (CH3OH), dimethylformamide (DMF), dichloromethane (CH2Cl2), trichloromethane (CHCl3), toluene (C7H8), and diethyl ether (Et2O) were used either for spectroscopic studies or for syntheses and were purified and dried following standard procedures prior to use. Synthesis safety note! Transition-metal perchlorates are hazardous and explosive upon heating and should be handled cautiously. No explosion occurred in the present study. Syntheses of Ligands and Complexes. The Schiff base ligands (HLMe(S) and HLBr(S)) were synthesized following a condensation reaction of substituted pyridine-2-carboxaldehyde and 2-aminoethanethiol (1:1, mol/mol) under nitrogen (N2) in dry CH3OH at refluxing conditions and reacted in situ with [Ni(H2O)6](ClO4)2. Attempts to isolate HLMe(S) or HLBr(S) following the usual workup in air resulted in formation of the corresponding 2-thiazolidine derivatives, capable of reacting with [Ni(H2O)6](ClO4)2 to form the same nickel complexes as those for the in situ formed HLMe(S) or HLBr(S) ligand. 2-(6-Methylpyridin-2-yl)thiazolidine Form of HLMe(S). To a stirred CH3OH solution (10 mL) of 6-methylpyridine-2-carboxaldehyde (0.1 g, 0.825 mmol) was added dropwise a 5 mL of a CH3OH solution of 2-aminoethanethiol (0.064 g, 0.825 mmol). The resulting solution was refluxed under N2 for 6 h and cooled to room temperature. CH3OH was then completely removed by rotary evaporation, and the resulting liquid residue was dissolved in 30 mL of chloroform and washed successively with distilled water, a brine solution, and finally with distilled water. The organic layer was dried with anhydrous sodium sulfate (Na2SO4) and filtered, and the chloroform was removed to yield a thick yellow liquid (yield: 0.133 g, 89%). Elem anal. Calcd for HLMe(S) (C9H12N2S): C, 59.96; H, 6.71; N, 15.54. Found: C, 59.86; H, 6.66; N, 15.48. Selected IR frequencies (KBr disk, cm−1): 3288 (νNH, m). 1H NMR (400 MHz, CDCl3): δTMS 7.5 (1H, t, pyridine proton), 7.05 (2H, m, pyridine proton), 5.57 (1H, s, CH), 3.96 (2H, t, methylene proton of the “−SCH2CH2−N” moiety attached to S), 3.80 (NH, Br), 3.11 (2H, m, methylene proton of the “−SCH2CH2N” moiety attached to NH), 1.24 (3H, m, methyl proton of the pyridine ring). ESI-MS: m/z 181.07 (M + 1H, 100%). 2-(6-Bromopyridin-2-yl)thiazolidine Form of HLBr(S). This ligand was synthesized following the same synthesis procedure as that of HLMe(S). The condensation of 0.1 g of 6-bromopyridine-2carboxaldehyde (0.537 mmol) with 0.042 g of 2-aminoethanethiol (0.537 mmol) in 15 mL of CH3OH yielded 0.121 g of HLBr(S) (yield: 74%). Elem anal. Calcd for HLBr(S) (C8H9N2SBr): C, 39.23; H, 3.71; N, 11.43. Found: C, 39.15; H, 3.65; N, 11.37. Selected IR frequencies (KBr disk, cm−1): 3293 (νNH, m). 1H NMR (400 MHz, CDCl3): δTMS 7.50 (1H, t, pyridine proton), 7.38 (1H, d, pyridine proton), 7.24 (1H, d, pyridine proton), 5.56 (1H, s, CH), 3.96 (2H, t, methylene proton of the “−SCH2CH2N” moiety attached to S), 3.76 (NH, m), 3.10 (2H, m, methylene proton of the “−SCH2CH2N” moiety attached to NH). ESI-MS: m/z 244.97 (M+, 100%). 2,2′-Disulfanediylbis[N-(6-methylpyridin-2-yl)methylene]ethanamine (LMe(S−S)). To a solution of 2,2′-diaminodiethyl disulfide dihydrochloride (0.464 g, 2.06 mmol) in 10 mL of dry CH3OH was C


Article

Inorganic Chemistry

Found: C, 40.23; H, 4.14; N, 11.12. IR frequencies (KBr disk, cm−1): 2250 (w, νCN (CH3CN)), 1598 (νCN, s), 1088 (νCl−O (ClO4−), vs), 624 (νCl−O (ClO4−), s). ESI-MS: m/z 446.02 (M2+/2, 50%), 416.06 ({M − 4Me}2+/2, 100%), 473.99 ({M + CH3CN + H2O}2+/2, 87%). Molar conductance, ΛM, in CH3CN: 258 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 233 (37590), 285 (34200), 390 (16480), 540 (sh, 1810), 838 (223). [Ni{(LBr(S))2Ni}2](ClO4)2·2.5H2O (2·2.5H2O). This compound was synthesized following exactly the same procedure as that of 1·CH3CN. The condensation of 0.1 g of 6-bromopyridine-2-carboxaldehyde (0.825 mmol) with 0.064 g of 2-aminoethanethiol (0.825 mmol), followed by the addition of 0.362 g of [Ni(H2O)6](ClO4)2 (0.99 mmol), yielded 0.577 g of dark-brown block-shaped crystals of 2· 2.5H2O (yield: 76% with respect to the Ni salt used). Elem anal. Calcd for 2·2.5H2O (C32H32Br4Cl2N8Ni3O10.5S4): C, 27.62; H, 2.32; N, 8.05. Found: C, 27.59; H, 2.24; N, 8.03. IR frequencies (KBr disk, cm−1): 3437 (νOH, br), 1587 (νCN, s), 1550 (m), 1082 (νCl−O (ClO4−), vs), 622 (νCl−O (ClO4−), s). ESI-MS: m/z 575.81 (M2+/2, 100%). Molar conductance, ΛM, in CH3CN: 265 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 235 (41360), 290 (28400), 395 (14490), 575 (1715), 840 (150). [Ni(dppe)Ni(LMe(S))2](ClO4)2·2H2O (3·2H2O). To a stirred reddishbrown degassed CH3CN (30 mL) solution of 1 (0.300 g, 0.262 mmol) was added the solid [Ni(dppe)Cl2] (0.138 g, 0.262 mmol) when the color change was observed from reddish brown to dark red. After 30 min of stirring of this solution at room temperature, the solid dppe (0.104 g, 0.262 mmol) was added under N2. The resulting reaction mixture was further stirred for 24 h, and then solid NaClO4 (0.962 g, 0.786 mmol) was added to the solution and stirred for 1 h. The solution was filtered to get a clean red solution. The volume of the filtrate was reduced to 5 mL using rotary evaporation, and dry Et2O (10 mL) was added to this solution and refrigerated at 4 °C. After 1 h, the dark-red solid precipitated out and was filtered, washed with ether, and vacuum-dried. To obatin X-ray-quality crystals, the red solid was redissolved in CH3CN, and slow ether diffusion resulted in the formation of block-shaped dark-red crystals of 3·2H2O (0.572; yield 95%). Elem anal. Calcd for 3·2H2O (C44H50N4O10Cl2P2S2Ni2): C, 47.64; H, 4.54; N, 5.05. Found: C, 47.45; H, 4.35; N, 4.91. IR frequencies (KBr disk, cm−1): 3602 (νOH, m), 3459 (νOH, m), 1598 (νCN, m), 1106 (νCl−O (ClO4−), vs), 623 (νCl−O (ClO4−), m). ESIMS: m/z 436.05 ({M + 2H}2+/2, 100%). Molar conductance, ΛM, in CH3CN: 260 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 230 sh (45725), 290 (34430), 340 (10275), 375 (8030), 480 (1280), 860 (105). [Ni(dppe)Ni(LBr(S))2](ClO4)2·0.75Et2O (4·0.75Et2O). This compound was synthesized following the same procedure as that for 3. For synthesis, 0.30 g (0.223 mmol) of 2, 0.117 g (0.223 mmol) of [Ni(dppe)Cl2], 0.088 g (0.223 mmol) of dppe, and 0.082 g (0.669 mmol) of NaClO4 were used. The dark-red microcrystalline precipitate was filtered, washed with Et2O, and dried under vacuum. The red microcrystals were redissolved in CH3CN layered with Et2O, following by cooling to 4 °C, yielding dark-red block-shaped crystals of 4·0.75Et2O (yield: 0.49 g, 89% with respect to the Ni salt used) suitable for X-ray diffraction. Elem anal. Calcd for 4·0.75Et2O (C45H47.50Br2Cl2N4Ni2O8.75P2S2): C, 42.94; H, 3.8; N, 4.45. Found: C, 42.91; H, 3.6; N, 4.42. IR frequencies (KBr disk, cm−1): 1589 (m, νCN), 1088 (νCl−O (ClO4−), vs), 623 (νCl−O (ClO4−), m). ESI-MS: m/z 501.95 (M2+/2, 100%). Molar conductance, ΛM, in CH3CN: 264 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 230 sh (43800), 290 (37985), 340 (10290), 375 (8760), 475 (1770), 870 (90). [Ni{(LMe(S))2Ni(NO)}2](ClO4)2 (5). Solid 1 (0.1 g, 0.089 mmol) was dissolved in 10 mL of dry CH3CN in a Schlenk flask under N2. Into the clear red solution, purified NO(g) was purged for 1 min. The reddish-brown color of the solution immediately changed to green. Degassed Et2O was then added and kept inside a refrigerator overnight at 4 °C. The dark-green crystals formed were washed with dry Et2O and dried under vacuum (yield: 0.094 g, 91%). Elem anal. Calcd for 5 (C36H44Cl2N10O10S4Ni3): C, 37.53; H, 3.85; N, 12.16.

added NEt3 (0.417 g, 4.128 mmol), and the solution was stirred for 30 min. Then the resulting solution was added to a solution of 6methylpyridine-2-carboxaldehyde (0.500 g, 4.13 mmol) in 10 mL of CH3OH. The resulting reaction mixture was refluxed for 6 h and cooled to room temperature. The CH3OH was completely removed by rotary evaporation, and the resulting residue was redissolved in 30 mL of chloroform and washed successively with distilled water, a brine solution, and finally with distilled water. The organic layer was dried with anhydrous Na2SO4, and the chloroform was removed, resulting a thick brown liquid (yield: 0.577 g, 78%). Elem anal. Calcd for LMe(S−S) (C18H22N4S2): C, 60.31; H, 6.18; N, 15.63. Found: C, 60.28; H, 6.11; N, 15.55. Selected IR frequencies (KBr disk, cm−1): 1647 (νCN, vs), 1589 (s). 1H NMR (500 MHz, CDCl3): δTMS 8.36 (2H, s, CHN), 7.75 (2H, d, pyridine proton), 7.60 (2H, t, pyridine proton), 7.15 (2H, d, pyridine proton), 3.95 (4H, t, methylene proton of the “−SCH2CH2N” moiety attached to S), 3.045 (4H, t, methylene proton of the “−SCH2CH2N” moiety attached to N), 2.56 (6H, s, methyl proton of the pyridine ring). ESI-MS: m/z 359 (M + 1H, 10%), 256 (M − {CH-6-MePy}, 40%), 179 (M/2, 30%), 122 (6-Me2-picolylamine, 70%), 108 (2-picolylamine, 100%). 2,2′-Disulfanediylbis[N-(6-bromopyridin-2-yl)methylene]ethanamine (LBr(S−S)). This ligand was synthesized following the same synthesis procedure as that of LMe(S−S). The treatment of 0.272 g of NEt3 (2.688 mmol) with 0.302 g of 2,2′-diaminodiethyl disulfide dihydrochloride (1.344 mmol), followed by condensation of the resulting solution with 0.5 g of 6-bromo-2-pyridinecarboxaldehyde (2.688 mmol) in 20 mL of CH3OH, yielded 0.511 g of LBr(S−S) (yield: 78%). Elem anal. Calcd for LBr(S−S) (C16H16Br2N4S2): C, 39.36; H, 3.3; N, 11.47. Found: C, 39.41; H, 3.2; N, 11.5. Selected IR frequencies (KBr disk, cm−1): 1644 (vs, νCN). 1H NMR (500 MHz, CDCl3): δTMS 8.33 (2H, s, CHN), 7.95 (2H, d, pyridine proton), 7.59 (2H, t, pyridine proton), 7.50 (2H, d, pyridine proton), 3.96 (4H, t, methylene proton of “−SCH2CH2N” moiety attached to S), 3.04 (4H, t, methylene proton of the “−SCH2CH2N” moiety attached to N). ESI-MS: m/z 488.92 {(M + 1H}+, 15%), 321.98 (M − {6BrPy-CH}, 100%), 244.95 (M/2, 42%), 108 (2-picolylamine, 90%). 2-Pyridyl-N-(2′-methylthioethyl)methyleneimine (LMe(SMe)). To a solution of 6-methylpyridine-2-carboxaldehyde (0.150 g, 1.23 mmol) in 10 mL of dry toluene was added 5 mL of a toluene solution of 2(methylthio)ethylamine (0.112 g, 1.23 mmol). The resulting solution was then refluxed for 6 h and cooled to room temperature. The toluene was completely removed by rotary evaporation, and the resulting yellow oil was redissolved in 30 mL of chloroform and washed successively with distilled water, a brine solution, and finally with distilled water. The organic layer was dried with anhydrous Na2SO4 and then removed completely to afford a thick yellow liquid (yield: 0.206 g, 86%). Elem anal. Calcd for LMe(SMe) (C10H14N2S): C, 61.82; H, 7.26; N, 14.42. Found: C, 61.77; H, 7.18; N, 14.37. Selected IR frequencies (KBr disk, cm−1): 1645 (vs, νCN). 1H NMR (400 MHz, CDCl3): δTMS 8.37 (1H, s, CHN), 7.77 (1H, d, pyridine proton), 7.62 (1H, t, pyridine proton), 7.17 (1H, d, pyridine proton), 3.87 (2H, t, methylene proton of the “−SCH2CH2N” moiety attached to S), 2.84 (2H, t, methylene proton of the “−SCH2CH2N” moiety attached to N), 2.58 (3H, s, methyl substituent of the pyridine ring), 2.14 (s, 3H, −SCH3). [Ni{(LMe(S))2Ni}2](ClO4)2·CH3CN (1·CH3CN). To a stirred degassed solution of 6-methylpyridine-2-carboxaldehyde (0.100 g, 0.825 mmol) in 5 mL of dry CH3OH, a CH3OH solution (2 mL) of 2aminoethanethiol (0.064 g, 0.825 mmol) was added dropwise. The mixture was refluxed under N2 for 90 min to generate a light-yellow color. To this solution was then added dropwise a CH3OH solution (2 mL) of [Ni(H2O)6](ClO4)2 (0.362 g, 0.990 mmol) under N2. Immediately, a dark-reddish-brown color appeared, and the solution was refluxed under N2 for 30 min and cooled to room temperature. The resulting dark-reddish-brown solid that precipitated out was collected by filtration, washed with ether, and dried under vacuum. Slow diffusion of Et2O into the CH3CN solution of this solid at 4 °C afforded dark-brown block-shaped crystals of 1·CH3CN (0.272 g, yield 71% with respect to Ni salt) after 3−4 days. Elem anal. Calcd for 1·CH3CN (C38H47Cl2N9Ni3O8S4): C, 40.28; H, 4.18; N, 11.13. D


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Inorganic Chemistry

CL220 conductivity meter, 1H NMR spectra were recorded on a JEOL JNM LA 500 or a JEOL JNM LA 400 spectrometer. Redox potentials were measured using a CHI 1120A potentiometer. For a constant potential electrolysis experiment, a platinum mesh working electrode was used, and a solute concentration of ∼(0.5−2) × 10−3 M was maintained. X-ray Crystallography. Crystals of 1−4 and 7, suitable for X-ray diffraction, were grown by the slow diffusion of dry Et2O to the CH3CN solution of the corresponding complexes kept at 4 °C. Singlecrystal intensity measurements for 1, 3, and 7 were collected at 100 (2) K, while those for 2 and 4 were collected at 90 (2) K with a Bruker Smart APEX II CCD area detector using either Mo Kα radiation (λ = 0.71073 Å; for 1−4) or a Bruker Photon 100 area detector and synchrotron radiation (λ = 0.7749 Å; for 7). The cell refinement, indexing, and scaling of the data sets were carried out using the SAINT and Apex2 or Apex3 programs.13 All structures were solved by dual-space (a mixture of direct and Patterson, i.e., reciprocal and direct space) methods with SHELXT14 and refined by full-matrix least squares based on F2 with SHELXL.15 The crystal structures of the trinuclear complexes, 1 and 2, display two molecules in the asymmetric unit. Each molecule is charge-balanced by two perchlorate anions, which confirms that all of the Ni atoms present are in the 2+ oxidation state. One perchlorate anion in 1 and two perchlorate anions in 2 are found to be severely disordered, which was modeled considering two positions of the O and Cl atoms of the perchlorate anions with partial fractional occupancy summing to 1. The perchlorate anions present in the structures 3, 4, and 7 do not show any disorder. The CH3CN molecule present as the solvent of crystallization in 1 occupies two positions with a half occupancy each. The positions of the C-bound H atoms were calculated assuming ideal geometry and refined using a riding model. Figures showing that displacement parameters were created using the program Mercury.16 Crystal data for complexes 1−4 and 7 are summarized in Table S1. Additional information on the structure determination is available in the deposited CIFs (1, CCDC 1861319; 2, CCDC 1861318; 3, CCDC 1861317; 4, CCDC 1861315; 7, CCDC 1861316).

Found: C, 37.35; H, 3.81; N, 12.11. Selected IR frequencies (KBr disk, cm−1): 1773 (νNO, s), 1598 (νCN, s), 1088 and 623 (νCl−O (ClO4−), vs). ESI-MS: m/z 503.95 (M − {(NO)Ni(LMe(S))2, 8%), 475.99 (M2+/2, 10%), 461.46 ([(NO2)Ni(LMe(S−S))]+, 100%). Molar conductance, ΛM, in CH3CN: 244 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 233 (40400), 290 (34020), 385 (4240), 590 (740), 890 (70). [Ni{(LBr(S))2Ni(NO)}2](ClO4)2 (6). This complex was synthesized following a synthesis procedure similar to that of 5, but instead of taking 1, here 2 (0.1 g, 0.071 mmol) is considered (yield: 0.097 g, 93%). Elem anal. Calcd for 6 (C32H32Br4Cl2N10Ni3O10S4): C, 27.23; H, 2.28; N, 9.92. Found: C, 27.16; H, 2.24; N, 9.86. Selected IR frequencies (KBr disk, cm−1): 1789 (νNO, s), 1589 (νCN, s), 1090 and 623 (νCl−O (ClO4−), vs). ESI-MS: m/z 635.76 (M − {(NO)Ni(LBr(S))2, 32%), 605.67 (M2+/2, 8%), 590.85 ([(NO2)Ni(L Br(S−S) )] + , 8%), 575.86 ({M − 2NO} 2+ /2, 82%). Molar conductance, ΛM, in CH3CN: 238 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 233 (47670), 295 (30650), 400 (7850), 600 (650), 905 (120). [(LMe(S−S))Ni(NO2)](ClO4) (7). To the solid sample of 5 (0.05 g, 0.042 mmol), taken in a Schlenk flask, was added 5 mL of degassed CH3CN. A clear green solution was generated. The color of the solution faded to light-green when exposed to O2. This solution was further stirred for 1 h. Et2O diffusion to this solution at 4 °C afforded X-ray-quality crystals of 7 in high yield after 3−4 days. The mother liquor was decanted, and the crystals were filtered, washed with Et2O, and vacuum-dried (yield: 0.04 g, 83%). Elem anal. Calcd for 7 (C18H22ClN5NiO6S2): C, 38.42; H, 3.94; N, 12.45. Found: C, 38.35; H, 3.87; N, 12.42. Selected IR frequencies (KBr disk, cm−1): 1599 (νCN, s), 1386 (νNO2, m), 1208 (νNO2, s), 1090 and 623 (νCl−O (ClO4−), vs), 476 (νS−S, w). ESI-MS: m/z 461.02 (M+, 100%). Molar conductance, ΛM, in CH3CN: 140 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 234 (14860), 295 (15400), 585 (10), 975 (6). [(LBr(S−S))Ni(NO2)](ClO4)2 (8). This compound was synthesized following the same procedure as that of 7 but dissolving 6 (0.05 g, 0.034 mmol) instead of 5 to afford 0.041 g of 8 (yield: 85%) as a microcrystalline solid. Elem anal. Calcd for 8 (C16H16Br2N5O6S2ClNi): C, 27.75; H, 2.32; N, 10.11. Found: C, 27.69; H, 2.29; N, 10.05. Selected IR frequencies (KBr disk, cm−1): 1591 (νCN, s), 1390 (νNO2, s), 1175 (νNO2, m), 1090 and 622 (νCl−O (ClO4−), vs), 472 (νS−S, w). ESI-MS: m/z 590.85 ({M + H}+, 50%). Molar conductance, ΛM, in CH3CN: 135 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 235 (16450), 300 (12910), 600 (40), 1010 (40). [(LMe(SMe))2Ni](ClO4)2 (9). To the stirred yellow solution of ligand LMe(SMe) (0.05 g, 0.257 mmol) in 10 mL of dry CH3OH was added dropwise to a 5 mL CH3OH solution of [Ni(H2O)6](ClO4)2 (0.047 g, 0.128 mmol). The resulting greenish-yellow solution was stirred for 2 h at room temperature and then kept inside the refrigerator at 4 °C overnight. The greenish-yellow microcrystals formed were filtered, washed with Et2O, and vacuum-dried (yield: 0.076 g, 92%). Elem anal. Calcd for 9 (C20H28Cl2N4NiO8S2): C, 37.17; H, 4.37; N, 8.67. Found: C, 37.11; H, 4.30; N, 8.61. IR frequencies (KBr disk, cm−1): 1602 (νCN, s), 1085 and 622 (νCl−O (ClO4−), vs). ESI-MS: m/z 223.05 (M2+/2, 100%). Molar conductance, ΛM, in CH3CN: 245 Ω−1 cm2 mol−1. Electronic absorption spectrum [λmax, nm (ε, M−1 cm−1), in CH3CN]: 233 (14370), 290 (17420), 580 (35), 890 (85). Physical Measurements. The FTIR spectra of the ligands and their complexes were recorded on a Thermo Nicolet iS10 spectrometer using KBr pellets in the range 4000−400 cm−1. The electronic spectra were recorded on an Agilent 8453 diode-array spectrophotometer. Elemental analyses were carried out on a PerkinElmer 2400 series II CHNS or Thermo Fisher Flash 2000 CHNS analyzer. EPR spectra were obtained using a MiniScope MS 5000 Magenttech EPR spectrometer at 298 and 77 K. Mass spectrometry (MS) spectra were recorded on a Waters HAB213 spectrometer or a Thermo Finnigan LCQ Deca mass spectrometer (for 9). Solution conductivity was measured using a CHEMILINE

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RESULTS AND DISCUSSION Synthesis and Characterization. The Schiff base ligands HLMe(S) and HLBr(S) were synthesized in situ following the condensation of substituted pyridine-2-carboxaldehyde and 2aminoethanethiol in dry CH3OH under N2 at refluxing conditions and next reacted with [Ni(H2O)6](ClO4)2 to form the trinuclear nickel complexes, 1 and 2. Attempts to isolate the in situ formed HLMe(S) or HLBr(S) afford 2thiazolidine derivatives, characterized by 1H NMR (δNH at 3.80 and 3.76 ppm, respectively), FTIR (νN−H at 3286 and 3293 cm−1, respectively), and MS spectra (Figures S1−S6). Following a similar condensation of the ortho-substituted pyridine-2-carboxaldehyde with 2,2′-diaminodiethyl disulfide or 2-(methylthio)ethylamine, the ligands LMe(S−S), LBr(S−S), or LMe(SMe) were prepared and characterized by 1H NMR, MS, and FTIR. Their νCN occur at 1647, 1644, and 1645 cm−1, respectively (Figures S7−S14). The absence of νN−H and the comparable νCN of 1−9 in the range ∼1590−1600 cm−1 confirm the imine-N ligation to NiII ions. Except for complexes 5 and 6, the ESI-MS spectra displayed intense peaks that correspond to their molecular-ion peak (100% for 2−4, 7, and 9; 50% for 1 and 8; ∼10% for 5 and 6; Figures S15−S23) and thus support their formulation as proposed in Scheme 1. For 5, the most intense peak observed at m/z 461.46 is the same as the molecular-ion peak of 7. This is due to the conversion of 5 → 7 that may occur in the presence of air/oxygen, as is evident from the UV−vis spectral changes (vide infra, Figure 6B). In fact, the reaction of 5 with O2 in CH3CN yields complex 7, as confirmed by X-ray crystal structure determination. Similarly, E


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Inorganic Chemistry

of 7 (Figure S25) are most probably due to the antisymmetric and symmetric stretching frequencies of the NiII-coordinated nitrite anion (NO2−).18 The corresponding stretching frequencies for analogous 8 are 1390 and 1175 cm−1 (Figure S26). The weak stretching frequencies at 476 and 472 cm−1 of 7 and 8 are most likely due to νS−S of the disulfide functional group of the modified ligand LMe(S−S) or LBr(S−S), formed in situ during the transformation of 5 or 6 → 7 or 8, respectively. The EPR spectrum of reduced 3 (3red) is axial, whereas that of CO-treated 3red is rhombic, supporting the CO ligation to NiI of 3red. This change of the spectral profile, from axial to rhombic, is due to a change in the coordination geometry from square-planar to square-pyramidal or octahedral around NiI of 3red, which generates three different g tensors (rhombic signal) for CO-treated 3red. The addition of ether to the reaction mixture of 3red plus CO(g) afforded a red solid that displays two stretching frequencies at 1997 and 2068 cm−1 in its FTIR spectrum (Figure S27), supporting the presence of two terminally coordinated CO molecules in this species. The solution conductivity measurement in CH3CN revealed that 1−6 and 9 behave as 2:1 electrolytes (Λ ranges from 238 to 265 Ω mol−1 cm−1), whereas 7 and 8 behave as 1:1 electrolytes (Λ ranges from 135 to 140 Ω mol−1 cm−1).19 The MS spectral results, microanalytical data, and other information on 1−9 collectively support their formulations, as stated in Scheme 1. Structures of Complexes 1−4 and 7. Perspective full molecule views of the cationic parts of 1, 3, and 7 are shown in Figures 2−4 while those of compounds 2 and 4 are shown in

when 6 is dissolved in CH3CN and kept in air or under oxygen, complex 8 is formed, as is evident from the UV−vis spectral changes observed (Figure S24) similar to that of 5. The isotopic mass distribution of peaks found at m/z 475.99 (10%) for 5 and at 605.67 (8) for 6 matches well with the mass of [Ni{(LMe(S) or LBr(S))2Ni(NO)}2]2+/2, the cation of 5 or 6, respectively, and thus supports the dinitrosyl formulation of 5 and 6 (Figure S20). In the case of 6, the most intense peak observed at m/z 575.86 (82%) corresponds to the molecularion peak of 2, except the two NO molecules of 6. In the MS spectrum of both 5 and 6, the masses corresponding to the [Ni{(LBr(S) or LMe(S))2Ni(NO)}]+ fragments (m/z 503.95 (10%) for 5 and 635.75 (32%) for 6) indicate that possibly the central NiII is not ligated to NO. The FTIR spectrum of 6 displayed a strong νNO stretch at 1789 cm−1, as was observed for 5, which displayed this νNO at 1773 cm−1. These νNO values of 5 and 6 are in the range of νNO observed for the {NiNO}10 type nickel nitrosyls, where NO is assigned as either •NO or + NO but not as NO−, for which much less value of νNO (≤1600 cm−1) is reported.17 Considering the attached nitrosyl group as either •NO or +NO, the two possibilities to assign the nitrosyl complexes 5 and 6 are (a) NiI−NO+ ↔ NiII−NO•, which generates {NiNO}9 type species, and (b) NiI−NO• ↔ NiII−NO−, which generates {NiNO}10 type species. According to the latter possibility, a neutral complex is expected (as both terminal Ni atoms are ligated to NO); however, the conductivity measurement of the CH3CN solutions of 5 and 6 revealed that the biunivalent conductance behavior, which is further supported by the appearance of a strong band at ∼1090 cm−1 in their FTIR spectra, corresponds to the presence of ClO4− as the counteranion. A comparison of the FTIR spectra (spectrum of 5 with 1 or that of 6 with 2) revealed that, except for the extra νNO band, all other stretching frequencies of 5 and 6 are similar in profile and position to those of their precursors 1 and 2, respectively (Figures S25 and S26), clearly indicate symmetrical NO ligation to both terminal Ni ions. No shift of the imine stretch indicates that, like 1 or 2, in the case of 5 and 6, the terminal Ni ions are in the 2+ oxidation state. Furthermore, the lower-energy UV−vis bands at λmax = 890 and 590 nm for 5 and 905 and 585 nm for 6 indicate the presence of an octahedral NiII ion in these complexes. Finally, the susceptibility of Ni-coordinated NO to attack on an electrophile such as O2, which generates the corresponding nitrite species 7 and 8, strongly supports that the nitrosyl groups present in these complexes are either NO• or NO− but definitely not +NO. On the basis of the elemental analysis, FTIR, ESI-MS, and electronic spectral results, conductivity measurements, and O2 reactivity studies, complexes 5 and 6 can be best described as {NiNO}9 type species of the formulation NiII−NO• but not as NiI−NO+. These nitrosyls are unstable like other reported {NiNO}9 type species.17a It is noteworthy that νNO of any {NiNO}9 type complex is unknown; thus, the present NO stretch displayed by 5 and 6 in their FTIR spectra could not be compared. Attempts to obtain the crystal structures of 5 and 6 are underway that may shed more light on the formulation of these {NiNO}9 type nitrosyls. The notion of the simultaneous reduction of both terminal Ni atoms is supported by the mole percentages of 7 and 8 formed, which are almost double the amounts of their precursors, 5 and 6, respectively. A symmetric κ2-O,O binding of NO2− to NiII is evident [the Ni−O(NO2) distances are 2.093(3) and 2.106(3) Å] from the X-ray structure of 7. The stretches observed at 1386 and 1208 cm−1 in the IR spectrum

Figure 2. Thermal ellipsoid (probability level 30%) plot of 1·CH3CN with a partial atom-labeling scheme. Only one of the two molecules present in the asymmetric unit cell is shown. H atoms, perchlorate anions, and solvent molecules are omitted for clarity.

Figures S28 and S29. The crystal structures of 1 and 2 contain two trinuclear molecules in the asymmetric unit. One molecule of 1 is shown in Figure 2. The trinuclear unit consists of two terminal NiII ions (designated as NiT: Ni1 and Ni3 in the case of one molecule of 1 shown in Figure 2) that are sixcoordinate, giving rise to a distorted octahedral coordination geometry. Two ligands are bonded to NiT in a meridional fashion. The imine-N and thiolato-S donors of the two ligands occupy trans and cis positions, respectively, resulting in an N4S2 coordination zone surrounding each NiT ion. The average NiT−Npy, NiT−Nimine, and NiT−S distances of Ni1 are 2.185(5), 2.010(6), and 2.391(2) Å, respectively, comparable to the corresponding values of Ni3, which are 2.215(6), 2.005(6), and 2.378(2) Å, respectively. The shorter NiT−Nimine than NiT−Npy distances indicate stronger coordination of the imine N than the pyridine N to the NiT ions. The F


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Inorganic Chemistry

Table 1. Average Ni−S Distances (Å) of Trinuclear Ni3 Complexes (in the XNiY Term, Where X = 4C or 6C Means Four- or Six-Coordinate NiII and Y = T or C Means the Terminal or Central Position of NiII in the Trinuclear Architecture; the Asterisk Is Five-Coordinate NiII)

complexes.9c,e−j,10c,d,g,h,20 For 2, a trinuclear structure similar to 1 is observed, and the bond distances and angles of the NiII coordination zones for both are comparable to each other (Table S2). Out of the two molecules present in the unit cell of 2, in one molecule, a pyridine ring of a ligand and its attached methylene C is found to be disordered and occupy two positions. All four perchlorate anions exhibit some disorder. One of these was split into two orientations, each with 0.5 occupancy. A limited number of thiolato-S-bridged trinuclear nickel complexes, related to the A-cluster modeling of ACS/CODH, are reported in the literature.9c,e−j,10c,d,g,h These and other similar complexes (not related to the ACS/CODH modeling)20 are shown in Table 1. The table indicates that, except

metallothiolato-S donors of the two terminal six-coordinate NiN4S2 units are linked to a central NiII ion (designated as NiC: Ni2 in structure 1; Figure 2), thereby generating a nearly square-planar S4 coordination geometry surrounding NiC. An average NiC−S distance of 2.221(2) Å observed in 1 is ∼0.16 Å shorter than the average NiT−S distance. This appreciable shortening of the Ni−S distance can be traced to the lower coordination number of NiC than NiT ions. The three Ni atoms are collinear and situated on a plane comprised of Ni1, N1, and N3 atoms; however, all of the S atoms are not coplanar; the two Ni2S2 squares formed with the NiC ion as a shared corner makes a twist angle of 13.11°. The Ni1−Ni2 and Ni2−Ni3 distances of 3.348 and 3.339 Å are comparable to those of other reported thiolato-S-bridged trinuclear nickel G


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Inorganic Chemistry

near the pyridine-N donor of the HLBr(S) ligand, bonded to the six-coordinate NiII ion in 4, decreases the pyridine-N donor strength,11 as observed by the lengthening of the average Ni− Npy length in 4 compared to 3, where the electron-donating Me substituent is ortho to pyridine N in the ligand frame of HLMe(S). This reduced donor strength of pyridine N, observed in 4, is compensated for by the stronger thiolato-S coordination of LBr(S) (compared to LMe(S) in 3) to NiII resulting in ∼0.05 Å shortening of its Ni−S distance compared to 3. The second Ni present in the structures of 3 and 4 has a square-planar coordination geometry like that of other reported models.9d,10f This four-coordinate NiII ion is displaced by only 0.086 and 0.015 Å from the P2S2 plane of 3 and 4, respectively. The average Ni−P [2.1637 (6) Å] and Ni−S [2.2486 (6) Å]) distances for 3 compared to 2.174 (2) and 2.2632 (19) Å, respectively, for 4 are similar to those of other four-coordinate complexes having a NiIIP2S2 chromophore, as reported by Mascharak et al.9d and Schröder et al.10f The Ni−Ni distances of 3.3279(5) and 3.323(1) Å, observed for 3 and 4, respectively, are ∼0.3−0.5 Å higher than those of the reported complexes,9d,10f which may be due to the thiolato-S link of sixcoordinate NiN4S2 to the {(dppe)Ni} fragment instead of a four-coordinate NiN2S2 to the {(dppe)Ni} fragment, as is evident in the X-ray structure of the reported models.9d,10f It is notable that the X-ray structure of the Ni−Ni−[4Fe−4S] cluster of the A site of ACS revealed Nip−S distances in the range 2.2−2.3 Å and a Ni−Ni distance of 3.0 Å. The two water molecules present in 3 are hydrogen-bonded to each other (Ow1−Ow2 = 2.814 Å) and to one O atom of each of the two perchlorate anions (Ow1−Op1 = 2.871 Å and Ow2−Op2 = 2.909 Å, where w = water, p = ClO4− group, and 1 or 2 are the number of two molecules), thus forming a chain of two water molecules and two perchlorate anions. The thermal ellipsoid diagram of the cationic part of 7 is shown in Figure 4. It shows a single NiII ion in a distorted

for this work, in none of the cases is (i) a six-coordinate NiTthiolato synthon used to construct the NiC coordination zone as models of the Nip site and (ii) the ligand containing a pyridine- or an imine-N-donor atom used to create the NiT coordination zone. Therefore, the structural data of the present work may enrich the database of thiolato-S-bridged nickel complexes, reported as models of the A cluster of ACS/ CODH. The data tabulated in Table 1 emphasize the dependence of the Ni−S bond length on the coordination number of NiII. In the structure containing L3, two DMF ligands are coordinated to the central Ni, thus lengthening the Ni−S bond.9e A similar observation is reported by Darensbourg and co-workers20d and others.9j The central NiS4 moiety of other species (Table1, including 1 and 2) are fourcoordinate; thus, comparable Ni−S distances for all are expected and observed, although different N-donor types such as imine-/pyridine-/tert-amine-/amidato N are involved in bonding. The binuclear Ni−Ni complexes, 3 and 4, were synthesized by the reaction of a CH3CN solution of the respective trinuclear species (1 or 2) with an equivalent amount of dppe and [Ni(dppe)Cl2], assuming the successive steps of reactions such as dppe coordination to NiC of 1 or 2 after the detachment of a NiTN4S2 unit followed by an association of this detached unit via its thiolato S to [Ni(dppe)Cl2] replacing Cl− ions. In this way, the formation of two molecules of the same binuclear Ni−Ni complex from one molecule of a trinuclear complex was expected. The X-ray structures of 3 and 4 revealed that these complexes are binuclear thiolato-Sbridged nickel(II) complexes, as expected. The thermal ellipsoid drawing of the cationic part of 3 is shown in Figure 3, and that of 4 is shown in Figure S29. One NiII ion in 3 or 4

Figure 3. Thermal ellipsoid (probability level 80%) plot of the cationic part of 3·2H2O with a partial atom-labeling scheme. H atoms, perchlorate anions, and solvent molecules are omitted for the sake of clarity.

Figure 4. Thermal ellipsoid (probability level of 80%) plot of 7 with a partial atom-labeling scheme. H atoms and perchlorate anions are omitted for the sake of clarity.

is six-coordinate, comprised of two N2S donor ligands sitting in meridional positions, as observed for the NiT ions of 1 and 2, whereas the other NiII ion is four-coordinate, consisting of two P donors of a “dppe” ligand and two thiolato-S donors of the six-coordinate NiN4S2 unit. The average Ni−Npy, Ni−Nimine, and Ni−S distances of 2.1647(18), 2.0299(18), and 2.4527(6) Å observed for the six-coordinate NiII ions of 3 are comparable to those of 4, which are 2.174(6), 2.043(5), and 2.4081(20) Å, respectively. The electron-withdrawing ortho-Br substituent

octahedral coordination geometry comprised of four N donors of a modified disulfide ligand LMe(S−S) (originating from two LMe(S) ligands of the precursor nitrosyl species 5) and two O atoms of a nitrite anion (NO2−), generated from the oxidation of a nitrosyl (NO) group attached to NiT of 5. The average Ni−Npy and Ni−Nimine distances of 2.165 (3) and 2.041(3) Å are observed. The similar Ni1−O1 [2.106 (3) Å] and Ni1−O2 H


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Inorganic Chemistry

to the intraligand n → π* and π → π* transitions, respectively. To test whether the NiC ion in 1 and 2 is labile like the Nip site of ACS/CODH, we titrated a CH3CN solution of 1 or 2 with a CH3CN solution of 1,10-phenanthroline and the reaction progress was monitored using UV−vis spectroscopy. The spectral changes occurring for 1 with the progress of titration are shown in Figure 6A, which features an abrupt decrease of the 390 nm band with steady growth of two new bands at 265 and 225 nm, corresponding to those of authentic [Ni(phen)3]2+ (Figure S30). This observation of the complete disappearance of the 390 nm band indicates a loss of the NiS4 chromophore, i.e., removal of the central NiII from the trinuclear structure. A mononuclear square-planar nickel(II) complex of the ligand N,N′-dimethyl-N,N′-bis(βmercaptoethyl)ethylenediamine was transformed to a thiolato-S-bridged trinuclear nickel(II) species following the reaction with NiCl2, and the development of an intense band at ∼400 nm was observed and reported by Reedjik and coworkers.10c This band at ∼400 nm was tentatively assigned to the 1A1g → 1A2g transition of the resulting NiS4 chromophore. Therefore, the absence of the 390 nm band in the final spectrum of the titration process (Figure 5A, black trace) supports Reedjik’s report that the square-planar NiS 4 chromophore is the origin of the ∼400 nm band. The detached central NiII of 1 forms a stable complex [Ni(phen)3]2+, following the reaction with externally added 1,10phenanthroline. Interestingly, complexes 1, 2, 5, and 6 display an absorption band in the range of 385−400 nm that strongly supports the presence of the NiS4 unit in their structure, as proposed in Scheme 1. However, in the case of 3 and 4, the appearance of an extra band at 340 nm in addition to the ∼380−400 nm band may be due to the presence of mixed S and P donors surrounding the Ni II ion of the square-planar NiP 2 S 2 chromophore. The trinuclear bis-nitrosyl species 5 or 6 in CH3CN slowly changed to the corresponding mononuclear nitrite-bound species 7 or 8, respectively, in the presence of O2. An attempt was made to follow the 5 → 7 and 6 → 8 transformations by using UV−vis spectroscopy. The spectral changes occurring within 1 h during 5 → 7 conversion are shown in Figure 6B. The 590 nm band steadily decreases and develops a weak shoulder at 530 nm and a new peak at 870 nm. All other higher-energy bands remain in the same positions (at 385, 290, and 233 nm) and have intensities

[2.093 (3) Å] distances indicate a symmetric κ2-O,O binding of the nitrite anion. The two imine-N donors (N2 and N4) of LMe(S−S) and the three atoms of the nitrite group, namely, O1, O2, and N5, are situated on a plane, and the Ni atom is 0.027 Å out of this plane toward the pyridine N atom, N1. Electronic Spectra. The electronic spectra of 1−9 are measured in CH3CN, and the spectral data are tabulated in Table 2. The spectra of the set of complexes derived from Table 2. Electronic Spectral Data of Nickel(II) Complexes 1−9 in CH3CN at 298 K compound

λmax, nm (ε, M−1 cm−1)

1

233 (37590), 285 (34200), 390 (16480), 540 sh (1810), 838 (223) 235 (41360), 290 (28400), 395 (14490), 575 sh (1715), 840 (150) 230 sh (45725), 290 (34430), 340 (10275), 375 (8030), 480 (1280), 860 (105) 230 (43800), 290 (37985), 340 (10290), 375 (8760), 475 (1770), 870 (90) 233 (40400), 290 (34020), 385 (4240), 590 (740), 890 (70) 233 (47670), 295 (30650), 400 (7850), 600 (650), 905 (120) 234 (14860), 295 (15400), 585 (10), 975 (6) 235 (16450), 300 (12910), 600 (40), 1010 (40) 233 (14370), 290 (17420), 580 (35), 890 (85)

2 3 4 5 6 7 8 9

ligand LMe(S) (1, 3, 5, and 7) are shown in Figure 5A,B. All complexes exhibit a weak and broad lower-energy band in the range 840−1010 nm, followed by a band in the range 475−600 nm, attributable to 3A2g(F) → 3T2g(F) and 3A2g(F) → 3T1g(F) transitions designated as ν1 and ν2, respectively. These d−d transitions are characteristic of a nickel(II) complex of octahedral parentage.21 Low extinction coefficients (i.e., ε in M−1 cm−1) for both ν1 and ν2 are expected for all of the complexes; however, for 1−6, ν2 has much higher ε value than expected for a d−d transition. This higher ε of ν2 is possibly due to the presence of a nearby intense band in the range 380−400 nm, attributable to a thiolato S− to NiII chargetransfer transition. For the mononuclear complexes 7−9, where thiolato-S coordination to NiII and hence the band at ∼380−400 nm is lacking, low ε values for both ν1 and ν2 have been observed, indicating a spin-allowed but Laporteforbidden d−d transition as their origin. The other lowerenergy bands observed for 1−9 at ∼300 and ∼230 nm are due

Figure 5. Electronic absorption spectra of (A) 1 (red) and 3 (blue) and (B) 5 (red) and 7 (blue) in CH3CN (for the right-hand side y axis, x = 2 and 3 are for 7 and 5, respectively). I


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Inorganic Chemistry

Figure 6. (A) Electronic absorption spectral changes of 1 (1 × 10−5 M, orange trace) during titration with 1,10-phenanthroline in CH3CN. A total of 15 equiv of 1,10-phenanthroline is dissolved in 200 μL of CH3CN, and then a 10 μL aliquot was added each time and spectrum taken. (B) Electronic absorption spectral changes of 5 (black) in the presence of O2 in CH3CN showing the formation of intermediate 5# (pink) prior to the formation of 7.

Scheme 2. Conversion of Trinuclear Bis-nitrosyls (5 and 6) to Mononuclear Nitrite Complexes (7 and 8) in the Presence of O2

Ared-CO is its IR spectrum, which displays a strong stretch at 1996 cm−1 corresponding to the νCO band of a NipI-bound terminal CO group. To test whether a nickel(II) model complex can functionally mimic the NipII site of ACS/CODH, the first requirement is to investigate whether the NiII ion is reducible to NiI and whether the empty binding site around NiI is available. The next step is if its CO binding affinity may resemble that of the NipI site and its CO-bound form, Ared-CO. To explore the susceptibility of NiII ions toward reduction, cyclic voltammetry (CV) of the complexes was recorded in CH3CN using the same cell setup. The CV trace of 1 is shown in Figure 7 that exhibits three reduction waves within the potential range of 0.0 to −1.8 V. The first two quasi-reversible reductive responses, one at E1/2 = −1.06 V and the other at E1/2 = −1.29 V, are due to the reduction of two terminal NiII ions (designated as NiT) corresponding to 2NiTII + 2e → 2NiTI and 2NiTI + 2e → 2NiT0 electron-transfer processes whereas the third reversible reductive response of E1/2 = −1.60 V is due to reduction of the central NiII ion (designated as NiC) corresponding to NiCII + e → NiCI conversion. The NiC ion of 1 is bonded to four negatively charged metallothiolato-S donors, whereas each of the two NiT ions is coordinated to two thiolato-S donors; therefore, a more cathodic response for the former is expected and observed. The CV trace of 1 (Figure 7) featured a doubled current height of the first two more anodic reductive responses compared to that of the third one, indicating that each of the former responses is responsible for a two-electron-reduction

similar to those of the parent complex 5. These spectral features (Figure 6B, final pink trace) are mismatched to those of 7 (Figure 5B), where no band at 390 nm and a much less intense band at 290 nm compared to 5 are observed, suggesting the formation of an intermediate species 5# rather than the final product 7 during this 1 h period of time of 5 → 7 conversion. The existence of the 390 nm band in 5# strongly supports that the NiS4 unit of the trinuclear architecture is intact yet in 5#, as presented in Scheme 2. Similar observation during the transformation of 6 → 8 via the intermediate 6# is evident from the UV−vis spectroscopy (Figure S24). Therefore, the final products such as 7 or 8 required more time to form from their intermediate congeners, 5 # and 6 # , respectively, following detachment of the central NiII ion, most possibly as a solvated species, [Ni(solv)6]2+. Redox Chemistry and CO Reactivity. Trinuclear Nickel Complexes. The A cluster of ACS/CODH is known to exist in two different oxidation states, diamagnetic Aox and the oneelectron-reduced form of Aox with bound CO, i.e., Ared-CO. This Ared-CO adduct displays an intense rhombic EPR signal with three g values at 2.03, 2.07, and 2.08. The nature of this characteristic EPR signal has been thoroughly investigated. It has been seen that, with isotopic substitution with 61Ni, 57Fe, and 13CO, dipolar broadening in the EPR spectrum occurs that firmly indicates its origin; thus, it is referred to as the “NiFeC signal”.22 This typical EPR signal strongly corroborates the binding of CO to NipI but not to Nip0, which is diamagnetic and EPR-inactive. Another characteristic spectral evidence of J


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Inorganic Chemistry

chemical reducing agent such as NO(g), the simultaneous reduction of both NiT ions is evident as follows. The reaction of a CH3CN solution of 1 or 2 with NO(g) leads to the formation of bis-nitrosylated trinuclear nickel complexes 5 and 6. The isolation of 2 equiv of the nitrite-bound monomer 7 or 8 from 1 equiv of their precursor trinuclear species 5 or 6, respectively, validates that reductive nitrosylation of both NiT occurs simultaneously, as proposed in Scheme 2. The electronic absorption spectral changes occurring during electrolysis are recorded, and the 220−700 nm range of the spectra are shown in Figure 8. The isosbestic points observed

Figure 7. Cyclic voltammograms (scan rate 50 mV s−1) of 1 (the inset is the CV trace of 9, orange trace) in a CH3CN solution containing 0.1 M [(n-Bu)4N]ClO4 as the supporting electrolyte at 298 K at a platinum working electrode at a scan rate of 50 mV s−1 using a saturated calomel electrode as the reference electrode (E1/2 of Fc/Fc+ = +0.42 V).

process, whereas the third is due to a single electron-transfer process. Because both NiT have similar N4S2 donor set surroundings and are decoupled from each other via the central NiII ion, the reduction of both NiT simultaneously at the same potential is more likely than the two-electron reduction of a single NiT center. Therefore, the assignments for the first and second reductive responses as 2NiTII + 2e → 2NiTI and 2NiTI + 2e → 2NiT0, respectively, are more likely than the other possibilities of 2NiTII + 2e → NiTIINiT0 and NiTIINiT0 + 2e → 2NiT0 processes where two-electron reduction of the same NiII ion is required. To get a clue on whether the NiT ions are responsible for the first two reductive responses, we synthesized the six-coordinate bis-ligand nickel(II) complex 9 having a N4S2 donor set similar to that of the NiT ions of 1 and the CV scan was performed. Like 1, complex 9 exhibits two quasi-reversible reduction waves (Figure 7, inset), supporting that NiT ions of 1 are responsible for the E1/2 = −1.06 and −1.29 V responses. The corresponding potentials of 9, observed at E1/2 = −0.64 and −0.92 V, are more anodic than the potentials of 1 (E1/2 = −1.06 and −1.29 V), which is due to the presence of neutral thioether-S donors surrounding the NiII ion of 9 instead of negatively charged thiolato-S donors that are structurally evident for the NiT sites of 1. These reduction potentials of 9, assigned to NiII/NiI and NiI/ Ni0 couples. respectively (Figure 7, inset), are comparable to those of other reported monomeric bis-ligand nickel complexes of similar N2S-type donor ligands, comprising N4S2 donor sets surrounding NiII ions.11 The bulk electrolysis experiments of a CH3CN solution of 1 and 9, carried out at a fixed potential of −1.24 and −0.81 V, respectively, revealed an electron count of 1.96 for the former complex and 0.97 for the latter complex, confirming that the first reductive response of 1 is due to a two-electron-transfer process, 2NiTII + 2e → 2NiTI. The EPRsilent behavior of the reduced CH3CN solution of 1 further supports this notion of one-electron reduction of both NiT ions simultaneously, which produced two NiTI ions; the unpaired electrons of which are antiferromagnetically coupled through bridging thiolato-S atoms and, hence, form a diamagnetic species. Apart from this bulk electrolysis experiment, using a

Figure 8. Electronic absorption spectral changes of a CH3CN solution of 1 (black trace) occurring during electrolysis at −1.24 V showing the formation of a two-electron-reduced product (pink trace) of 1.

at 613, 435, 282, and 260 nm indicate clean transformation of 1 to its two-electron-reduced form. The appreciable changes occurring in the spectrum are the 390 nm band decreased almost to half of its original intensity, a new band at 450 nm is generated, and the peak at 285 nm is shifted to 275 nm. The new band generated at 450 nm is most possibly due to the NiTI-to-ligand charge-transfer transition. The appearance of the 390 nm band (although the intensity decreased compared to its precursor 1) supports that the NiS4 moiety is intact yet following reduction. The four-coordinate NiC of 1 and 2, comprising a NiS4 unit that resembles the S-rich Nip site of ACS/CODH, has two empty coordination sites, where it can accommodate other ligands such as CO after its reduction (NiCII → NiCI), mimicking the CO reactivity of the Nip site of ACS/CODH. However, the CV trace of 1 revealed that the reduction potential of NiC is more cathodic than that of the two NiT sites. Therefore, prior to the reduction of NiCII → NiCI, the reductions of NiTII → NiTI and NiTI → NiT0 occur (Figure 7), diminishing the opportunity to explore the CO reactivity of NiCI alone because CO may also bind to NiTI/0 ions. Unlike 1, complex 2 exhibits a CV trace with three completely irreversible reduction waves at Epc = −1.00, −1.26, and −1.46 V. The present electrochemical results imitate that, possibly to ensure the reduction of Nip rather than the Nid site, the enzyme ACS/CODH chooses strong σ-donor peptido N atoms instead of other N-donor types (amine or imine) to construct the Nid part that acts as a synthon for the Nip site. So, to mimic the functional activity of ACS, the amine- or K


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Inorganic Chemistry

NiIP2S2 chromophore, corresponding to the 4CNiII + e → 4CNiI electron-transfer process. Because the potential of −0.88 V is the most anodic one among the three responses, the CO reactivity at this Ni site following its reduction is feasible without reducing the other NiII (i.e., 6CNi) present in 3. Passage of CO(g) through the electrochemically reduced solution of 3 and measurement of the EPR spectrum of the resulting solution revealed a rhombic EPR spectrum featuring g1 = 2.18, g2 = 2.13, and g3 = 1.96, as shown in Figure 10B.

imine-N-containing thiol ligands are not a good choice to synthesize a Nip model site. Binuclear Ni−Ni Complexes. The CV trace of the binuclear nickel complex 3 exhibited three quasi-reversible reductive responses at E1/2 = −0.88, −1.10, and −1.39 V, as shown in Figure 9. The current heights of all three responses are

Figure 9. Cyclic voltammograms (scan rate 50 mV/s) of 3 in a CH3CN solution containing 0.1 M [(n-Bu)4N]ClO4 as the supporting electrolyte at 298 K at a platinum working electrode at a scan rate of 50 mV s−1 using a saturated calomel electrode as the reference electrode (E1/2 of Fc/Fc+ = +0.42 V).

Figure 10. X-band EPR spectra measured at 77 K in a CH3CN solution of 3red (red trace) generated via electrolysis and its simulated spectrum (blue trace) and of 3red-(CO)2 (green trace). Selected g values are indicated in the spectra. Spectrometer settings: microwave frequency = 9.416 GHz, power = 10 mW, modulation frequency = 100 kHz, and modulation amplitude = 5 G.

comparable to each other, unlike that observed for the case of 1, where the first two reductive responses have current heights almost double to that of the third one (Figure 7). The comparable current heights indicate that all three responses are responsible for the same number of electron-transfer processes. In 3, the geometries surrounding two NiII sites are different: one is six-coordinate (6C), and the other is four-coordinate (4C) comprised of N4S2 and P2S2 donor sets, designated as 6C Ni and 4CNi, respectively. The first reduction observed at E1/2 = −0.88 V is due to the one-electron reduction of 4CNiII → 4CNiI, whereas the other two more cathodic responses of E1/2 = −1.10 and −1.39 V are due to the one-electron reductions of 6CNiII → 6CNiI and 6CNiI → 6CNi0, respectively. The latter two potentials of 3 are comparable to the reduction potentials of NiTII/NiTI and NiTI/NiT0 couples of 1, which are observed at E1/2 = −1.06 and −1.29 V, respectively. This is quite reasonable because similar N4S2-type donor sets surrounding 6CNi of 3 and the six-coordinate NiT of 1 exist. To confirm that the first reductive response of E1/2 = −0.88 V is due to the 4CNiII + e → 4CNiI process, the bulk electrolysis experiment of a CH3CN solution of 3 was performed at a fixed potential of −1.05 V. The electronic absorption spectral changes monitored during electrolysis (Figure S31) revealed a steady decrease of the 290 nm band and the formation of a new band at 385 nm, abolishing two shoulders at 340 and 375 nm. The clear isosbestic points observed at 230, 385, 460, and 570 nm demonstrate a clean transformation of 3 to its oneelectron-reduced form, 3red. This reduced solution at 77 K exhibits an axial EPR signal with g values at 2.24 and 2.16. Each of these lines is split into a triplet due to hyperfine coupling of the NiI unpaired spin with the nuclear spin of two 31P nuclei (AisoNi = 64 G, AisoP1 = 27 G, and AisoP2 = 44 G from simulation) of the “dppe” ligand frame, confirming that the E1/2 = −0.88 V response is due to the reduction of NiIIP2S2 →

Each of these lines splits into a triplet showing the hyperfine splitting parameter AP ranges from 30 to 80 G. This rhombic signal is due to the change of the NiI coordination geometry from square planar to either square pyramidal or octahedral that occurred due to the CO binding to NiI of 3red. A similar EPR spectral profile (g1 = 2.20, g2 = 2.12, g3 = 1.98, and AP = 30−60 G) of a CO-ligated square-pyramidal NiI species, [NiI(dppe)(CO)NiII(PhPepS)]−, obtained after CO(g) purging of its precursor four-coordinate NiI complex having a P2S2 donor set, is reported by Mascharak and co-workers.9d To confirm whether CO binds to NiI, we attempted to isolate this {NiI-(CO)x} intermediate and carry out FTIR spectroscopic studies. Anticipating that the {NiI/0-(CO)x} intermediates are thermally unstable, as reported,9a,d,e,10a,b the following experiment at −40 °C is performed to isolate this species, if formed. To a concentrated DMF solution of 3 at −40 °C was added a stoichiometric amount of a chemical reducing agent such as NaBH4, and next CO(g) was purged. The solution color changed from brown to red. The addition of dry Et2O (precooled at −40 °C) anaerobically to this reaction mixture precipitated out a red compound, the FTIR spectrum of which displayed two stretches at 1997 and 2068 cm−1, corresponding to the νCO of two CO molecules, terminally coordinated to 3red, thereby supporting its formulation as 3red-(CO)2. Isolated 3red-(CO)2 is unstable at 298 K and loses its CO with time, which is evident from its FTIR spectra, collected with a KBr pellet at different time intervals (Figure S32). The rhombic EPR spectrum similar to that of trace B of Figure 10 is obtained when 3red-(CO)2 is synthesized following the above procedure, i.e., by using NaBH4 as a reductant (that can L


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Inorganic Chemistry quickly generate 3red) and maintaining −40 °C temperature of the reaction mixture where the formed 3red-(CO)2 is more stable (Figure S33) than it is at 25 °C. The νCO values of 1997 and 2068 cm−1, observed in the FTIR spectrum of 3red-(CO)2, are comparable to that of Ared-CO of ACS (which is 1996 cm−1) and to that of its model complexes reported.9a,d,e,10a,b For example, Mascharak and co-workers have reported the νCO values in the range 1960−2040 cm−1 for a CO-bound NiIS4 or NiIP2S2 moiety.9e One other five-coordinate CO-bound NiIS4 model with an aliphatic thiolato thioether donor set, reported by Yamamura et al.,23 displays a strong νCO at 1940 cm−1 with a less intense peak at ∼2050 cm−1 like 3red-(CO)2 displays. However, in their case, the ∼2050 cm−1 peak intensity is much less compared to the 1940 cm−1 peak unlike 3red-(CO)2, where both the peaks at 1997 and 2068 cm−1 are equally intense, indicating an octahedral bis-CO-ligated species. It is noteworthy that this νCO is sensitive to the Ni oxidation state to which it is attached. For example, Rauchfuss and co-workers have reported a complex, (Et4N)2[{(CO)2Ni}{NiS2N’2}], that is the first structurally characterized Ni−Ni model of the CObound form of the enzyme ACS. This model exhibits two νCO bands at 1948 and 1866 cm−1 corresponding to two CO molecules, terminally bonded to Ni0. Another Ni0 complex [Ni(bpy)(CO)2] (bpy = 2,2′-bipyridine), reported by Holm and co-workers, displays the νCO bands at 1872 and 1973 cm−1. For a metal carbonyl complex, the lower metal oxidation state promotes more π-back-donation to the π* orbital of CO that decreases the C−O bond order and hence the νCO values. Therefore, the higher νCO values for 3red-(CO)2 than those recorded for the Ni0 carbonyl complexes as mentioned above clearly indicate that in 3red-(CO)2 the CO molecules are bonded to a NiI ion not to Ni0. With the same cell setup, we have measured the CV of 4 in CH3CN, which displayed three completely irreversible reductive responses at Epc = −0.88, −0.99, and −1.29 V corresponding respectively to 4CNiII → 4C I 6C II Ni , Ni → 6CNiI, and 6CNiI → 6CNi0 electron-transfer processes (Figure S34). Because the first two more anodic potentials of 4 (Epc = −0.88 V, −0.99 V) are not well separated from each other compared to 3 (ΔEpc = 110 mV for 4 and ΔEpc = 240 mV for 3), the reduction of 4CNiII → 4CNiI will always contain some 6CNiI as a mixture, and both NiI species will exhibit CO binding affinity in the case of 4, unlike 3, where CO reaction with only the 4CNiI center is possible as discussed above. From the reduced product of 4, we could not isolate any CO-ligated species fruitfully, following reaction with CO(g).

generate one-electron-reduced 3red, which binds CO and resembles a step involved in the mechanism of acetyl CoA formation by ACS/CODH. The binuclear complex 4 exhibits one-electron-reduction potentials of the two NiII ions very close to each other (ΔE = 110 mV for 4 and 240 mV for 3); thus, selective reduction of the sole NiP2S2 part without reducing the other NiII is not possible. Attempts to isolate any fruitful CO-bound complex of reduced 4 failed. Easier reduction of an imine-N-bound NiII center than the Nip model part supports the choice of strong σ-donor peptide coordination to Nid of ACS/CODH, which shuts off Nid-site reduction, hence ensuring reduction of only the Nip site during catalysis. The electrochemical results indicate that, irrespective of the ligand’s N-donor types (amine, imine, pyridine, or amide) used, modeling the Nip site and mimicking its properties are possible provided the reduction potential of the NiII/NiI couple of the Nip model site is the most anodic and well separated from that of the other NiII ions present.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02276. FTIR, ESI-MS, and 1H NMR spectra of the thiazolidine form of ligands HLMe(S), HLBr(S), LMe(S−S), LBr(S−S), and LMe(S−Me) (Figures S1−S14), ESI-MS spectra of 1−9 (Figures S15−S23), UV−vis spectral changes during 6 + O2 → 8 conversion (Figure S24), FTIR spectra of 5 and 6 (Figures S25 and S26), FTIR spectrum of 3red-(CO)2 species (Figure S27), ORTEP diagrams of 2 and 4 (Figures S28 and S29), UV−vis spectra of authentic [Ni(Phen)3]2+ (Figure S30), UV−vis spectral changes occurring during electrolysis of 3 at a fixed potential of −1.05 V (Figure S31), FTIR spectra of KBr pellets of 3red-(CO)2 at different time intervals (Figure S32), EPR spectrum of a mixture of 3 + NaBH4 + CO(g) in DMF (Figure S33), CV of 4 in CH3CN (Figure S34), and tables of bond lengths (Tables S1−S6) (PDF) Accession Codes

CCDC 1861315−1861319 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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CONCLUSIONS In summary, the thiolato-S-bridged tri- and binuclear nickel(II) complexes have been synthesized and characterized. To the best of our knowledge, here we report the first synthesis of imine-N-containing thiol ligation of nickel(II) complexes where either NiS4 or NiP2S2 moieties exist as models of the Nip site. Like the labile Nip site of ACS/CODH, the central NiS4 unit of trinuclear complexes (1 and 2) is replaced by a chelator, 1,10-phenanthroline. The NiII ion of the NiS4 unit is harder to reduce than the two terminal NiII ions, thereby the CO binding affinity of the sole NiIS4 unit cannot be explored. Incorporating another ligand “dppe” in addition to the N2Sthiol donor ligands, the binuclear complexes (3 and 4) have been synthesized to afford a NiP2S2 unit. A 720 mV anodic shift of the reduction potential of the NiP2S2 (E1/2 = −0.88 V) unit of 3 compared to the NiS4 unit (E1/2 = −1.60 V) of 1 allows us to

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; apurba.patra@ch. nitdgp.ac.in. ORCID

Kamran B. Ghiassi: 0000-0002-3557-2813 Marilyn M. Olmstead: 0000-0002-6160-1622 Apurba K. Patra: 0000-0001-7232-1274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Science Engineering and Research Board (SERB), Government of India, for financial support (Grant M


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EMR/2014/001059), Department of Science and Technology, Fund for Improvement of S&T Infrastructure, Government of India, for providing basic infrastructural facilities (Grant SR/ FST/CSI-267/2015), and Alexander von Humboldt, Berlin, Germany, for an equipment donation grant of a spectroelectrochemical analyzer.

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DEDICATION Dedicated to Prof. Karl Wieghardt, MPI Müllheim, Germany, on the occasion of his 77th birthday. REFERENCES

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Model Complexes for the Nip Site of Acetyl Coenzyme A Synthase

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