Molecular Flexibility in Solvated Crystals of the Dimer, Au2(µ-1,2-bis(diphenylphosphino)ethane)2I2, with Three-Coordinate Gold(I).

We report the ability to trap the dimer Au2(µ-dppe)2I2 (dppe is 1,2-bis(diphenylphosphino)ethane) with different separations between the three-coordinate gold ions in crystalline solvates. All of these solvates ((Au2(µ-dppe)2I2·4(CH2Cl2) (1), Au2(µ-dppe)2I2·2(CH2Cl2) (2), the polymorphs α-Au2(µ-dppe)2I2·2(HC(O)NMe2) (3) and ß-Au2(µ-dppe)2I2·2(HC(O)NMe2) (4), and Au2(µ-dppe)2I2·4(CHCl3) (5)) along with polymeric {Au(µ-dppe)I}n·n(CHCl3) (6)) originated from the same reaction, only the solvent system used for crystallization differed. In the different solvates of Au2(µ-dppe)2I2, the Au···Au separation varied from 3.192(1) to 3.7866(3) Å. Computational studies undertaken to understand the flexible nature of these dimers indicated that the structural differences were primarily a result of crystal packing effects with aurophillic interactions having a minimal effect.

Chiral Teropyrenes: Synthesis, Structure, and Spectroscopic Studies.

We present the inaugural synthesis of a chiral teropyrene achieved through a four-fold alkyne benzannulation catalyzed by InCl3, resulting in good yields. The product underwent thorough characterization using FT-Raman and FT-IR spectroscopies, demonstrating a close agreement with calculated spectra. X-ray crystallographic analysis unveiled a notable twist in the molecule's backbone, with an end-to-end twist angle of 51°, consistent with computational predictions. Experimentally determined enantiomeric inversion barriers revealed a significant energy barrier of 23 kcal/mol, facilitating the isolation of enantiomers for analysis by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopies. These findings mark significant strides in the synthesis and characterization of chiral teropyrenes, offering insights into their structural and spectroscopic properties.

Structure and Luminescence Studies of Salts of the Helical Dication, [Au2(µ-bis(diphenylphosphine)ethane)2]2+ and Comparison with Salts of [Au2(µ-bis(diphenylphosphine)propane)2]2.

Six salts ([Au2(µ-dppe)2](BF4)2·CHCl3, [Au2(µ-dppe)2](BF4)2·1,2-Cl2C2H4, [Au2(µ-dppe)2](PF6)2·CHCl3, [Au2(µ-dppe)2](PF6)2, [Au2(µ-dppe)2](SbF6)2, and [Au2(µ-dppe)2](OTf)2·2CHCl3), (dppe is bis(diphenylphosphine)ethane) containing the dication, [Au2(µ-dppe)2]2+, have been prepared and structurally characterized by single-crystal X-ray crystallography. Unlike the three-coordinate dppe-bridged dimers, Au2X2(µ-dppe)2 (X = Br, I), which show considerable variation in the distance between the gold(I) ions over the range 3.0995(10) to 3.8479(3) Å in various solvates, the structure of the helical dication, [Au2(µ-dppe)2], in the new salts is remarkably consistent with the Au···Au separation falling in the narrow range 2.8787(9) to 2.9593(5) Å. In the solid state, the six crystals display a green luminescence both at room temperature and at 77 K, which has been assigned as phosphorescence. However, solutions of the dication are not luminescent. Salts containing the analogous dication [Au2(µ-dppp)2](PF6)2 (dppp is bis(diphenylphosphine)propane) have been prepared to determine whether the longer bridging ligand might also twist into a helical shape. These salts include [Au2(µ-dppp)2](OTf)2 (OTf is triflate) and three crystalline forms of [Au2(µ-dppp)2](PF6)2: the solvate [Au2(µ-dppp)2](PF6)2·(CHCl3) and two polymorphs of the unsolvated salt. None of these crystals are luminescent, but all contain a similar dication, [Au2(µ-dppp)2]2+, that contains two nearly parallel, linear P-Au-P groups and a long separation between the gold ions that varies from 5.3409(4) to 5.6613(6)Å.

Tb2O@C2(13333)-C74: A Non-Isolated Pentagon Endohedral Fullerene Containing a Nearly Linear Tb-O-Tb Unit.

Terbium has been added to the list of elements that form oxide clusters inside fullerene cages. Tb2O@C2(13333)-C74 has been isolated as a byproduct of the electric arc synthesis of the azafullerene Tb2@C79N. Cocrystallization of Tb2O@C2(13333)-C74 with Ni(OEP) (where OEP is the dianion of octaethylporphyrin) in toluene yielded black needles of Tb2O@C2(13333)-C74·NiII(OEP)·1.5C7H8 that have been examined by single-crystal X-ray diffraction. The resulting structure shows that a nearly linear Tb-O-Tb unit is contained in a C2(13333)-C74, which has two sites where pentagons share an edge to form pentalene units at opposite ends of the fullerene. Unlike the usual situations where metal atoms in fullerenes that do not obey the isolated pentagon rule are situated within the folds of the pentalene units, the Tb atoms in Tb2O@C2(13333)-C74 are positioned to the side of the pentalene units and near-neighboring hexagons. The magnetic properties of Tb2O@C2(13333)-C74 have been examined starting from the experimental geometry, using ab-initio multiconfigurational methods. The computations predict that Tb2O@C2(13333)-C74 will show strong axiality, which would make it a single-molecule magnet with a large magnetic anisotropy barrier.

Self-Assembled Encapsulation of CuX2- (X = Br, Cl) in a Gold Phosphine Box-like Cavity with Metallophilic Au-Cu Interactions.

Synthetic routes to the crystallization of two new box-like complexes, [Au6(Triphos)4(CuBr2)](OTf)5·(CH2Cl2)3·(CH3OH)3·(H2O)4 (1) and [Au6(Triphos)4 (CuCl2)](PF6)5·(CH2Cl2)4 (2) (triphos = bis(2-diphenylphosphinoethyl)phenylphosphine), have been developed. The two centrosymmetric cationic complexes have been structurally characterized through single-crystal X-ray diffraction and shown to contain a CuX2- (X = Br or Cl) unit suspended between two Au(I) centers without the involvement of bridging ligands. These colorless crystals display green luminescence (λem = 527 nm) for (1) and teal luminescence (λem = 464 nm) for (2). Computational results document the metallophilic interactions that are involved in positioning the Cu(I) center between the two Au(I) ions and in the luminescence.

Reactivity of Nitric Oxide and Nitrosonium Ion with Copper(II/I) Schiff Base Complexes: Mechanistic Aspects of Imine C═N Bond Cleavage and Oxidation of Pyridine-2-aldehyde to Pyridine-2-carboxylic Acid.

Four Schiff base ligands of the general formulas [6-(R)-2-pyridyl-N-(2'-methylthiophenyl)methylenimine] (RL1) and 6-p-chlorophenyl-2-pyridyl-N-(2'-phenylthiophenyl)methylenimine (RL2), where R = H, Me, p-ClPh, and their bis-ligand copper(II) and copper(I) complexes, 1-4 and 1'-4', respectively, were synthesized and characterized. The reactivities of 1-4 with nitric oxide (NO) gas and of 1'-4' with solid NOBF4 (NO+) were examined in dry acetonitrile in the presence and absence of water (H2O). The results revealed that, in the absence of H2O, complexes 1-4 (or 1'-4') reacts with NO (or NOBF4), leading to imine C═N bond cleavage of both (or one) Schiff base(s) that generates 2 (or 1) equiv of 2-(methyl/phenyl)thiobenzenediazonium perchlorates (5/6) and the corresponding picolaldehyde (RPial) via a copper nitrosyl of a {CuNO}10-type intermediate. In the presence of H2O, the in situ formed RPial get oxidized to the corresponding picolinic acid (RPicH) via an in situ formed LCuIOH intermediate (LCuI + HO-NO → LCuIOH + NO+; L = RL1/RL2/RPic- and νO-H of CuIOH = 3650 cm-1) and subsequently produces, with the aid of NO+ oxidant, the picolinate-ligated copper(II) complexes (i) [(HPic)2Cu] (7), [(MePic)4Cu3(NO3)2]n·H2O (8·H2O), or [(ClPhPic)2Cu] (9) when NO reacts with 1-4 or (ii) [(RPic)CuII(RL1/RL2)]+ when NO+ reacts with 1'-4'. The CuII to CuI reduction of [(RPic)CuII(RL1/RL2)]+ is essential for C═N cleavage of the remaining RL1/RL2 Schiff base; excess NO can do it. The X-ray structures (1, 1', 3', 5, 7, and 8) and spectroscopic results revealed the role of CuII/I, NO, NO+, and H2O, shedding light on the mechanism of C═N bond cleavage and the oxidation of pyridine-2-aldehyde to pyridine-2-carboxylic acid. The reaction of 1 with 15NO revealed that the terminal N of the N2+ group of 5 originates from 15NO [ν14N14N- = 2248 cm-1 and ν15N14N- = 2212 cm-1].

Bis(µ-thiolato)-dicopper Containing Fully Spin Delocalized Mixed Valence Copper-Sulfur Clusters and Their Electronic Structural Properties with Relevance to the CuA Site.

With aromatic and aliphatic thiol-S donor Schiff base ligands, the copper-sulfur clusters, [(L1)8CuI6CuII2](ClO4)2·DMF·0.5CH3OH (1) and [(L2)12CuI5CuII11(µ4-S)(µ4-O)6](ClO4)·4H2O, respectively, have been reported ( Chem. Commun. 2017, 53, 3334); HL1/HL2 are 2-(((3-methylthiophen-2-yl)methylene)amino)benzene/ethanethiol). Complex 1 comprises a wheel shaped Cu8S8 framework, made up of interlinked Cu2{µ-S(R)}2 units. To understand the properties with relevance to the CuA site and to check whether self-assembly generates similar type clusters to 1, three complexes, [(L3)8CuI6CuII2](ClO4)2·(C2H5)2O·2.5H2O (2), [(L3Cl)8CuI6CuII2](ClO4)2·1.25(C2H5)2O·1.25CH3OH·2H2O (3), and [(L3CF3)8CuI6CuII2](ClO4)2·2(C2H5)2O·H2O (4) have been synthesized with supporting ligands HL3X (HL3 = 2-((furan-2-ylmethylene)amino)benzenethiol when X = -H; X = -Cl or -CF3 para to thiol-S are HL3Cl and HL3CF3 ligands, respectively). The X-ray structures of 3 and 4 feature a similar Cu8S8 architecture to 1. The spectroscopic properties and the X-ray structures revealed that 2-4 are fully spin delocalized mixed valence (MV) of class-III type clusters. The structural parameters of the N2Cu2{µ-S(R)}2 units of 3 and 4 closely resemble those of the MV binuclear CuA site. With the aid of UV-vis-NIR, EPR, and spectroelectrochemical studies, the electronic properties of these complexes have been described in comparison with the MV model complexes and CuA site.

Role of Anions and Mixtures of Anions on the Thermochromism, Vapochromism, and Polymorph Formation of Luminescent Crystals of a Single Cation, [(C6H11NC)2Au].

Noncoordinating anions, which generally play a subordinate role in coordination chemistry, alter the structure, the luminescence, as well as the thermochromic and vapochromic behaviors of salts of the two-coordinate cation, [(C6H11NC)2Au]+. Thus whereas the yellow polymorphs of [(C6H11NC)2Au](PF6) and [(C6H11NC)2Au](AsF6) contain single chains of cations and are vapochromic, yellow [(C6H11NC)2Au](SbF6) does not form the same polymorph and is not vapochromic but contains two distinct chains of cations connected through aurophilic interactions. Mixed crystals such as [(C6H11NC)2Au](PF6)0.50(AsF6)0.50 have been prepared by adding diethyl ether to a dichloromethane solution containing equimolar amounts of [(C6H11NC)2Au](PF6) and [(C6H11NC)2Au](AsF6). The initial (kinetic) product for the three combinations of anions ((PF6)-/(AsF6)-, (PF6)-/(SbF6)-, and (AsF6)-/(SbF6)-) was a precipitate of fine yellow needles with a green emission, which were gradually transformed at rates that depended on the anions present into colorless crystals with a blue emission. Whereas neither polymorph of [(C6H11NC)2Au](PF6) nor [(C6H11NC)2Au](SbF6) is thermochromic, the colorless mixed crystal [(C6H11NC)2Au](PF6)0.50(SbF6)0.50 is thermochromic and converts from blue-emitting to green-emitting at 87-95 °C. The temperature required to transform a crystal of the type [(C6H11NC)2Au](PF6)n(AsF6)1-n from colorless (blue-emitting) to yellow (green-emitting) increases as the fraction of hexafluorophosphate ion in the crystal increases. The yellow crystals of [(C6H11NC)2Au](PF6)0.75(AsF6)0.25, [(C6H11NC)2Au](PF6)0.50(AsF6)0.50, and [(C6H11NC)2Au](PF6)0.25(AsF6)0.75 are vapochromic, whereas the yellow crystals of [(C6H11NC)2Au](PF6)0.50(SbF6)0.50 and [(C6H11NC)2Au](AsF6)0.50(SbF6)0.50 are not.

Unsymmetrical Coordination of Bipyridine in Three-Coordinate Gold(I) Complexes.

The unsymmetrical coordination of gold(I) by 2,2'-bipyridine (bipy) in some planar, three-coordinate cations has been examined by crystallographic and computational studies. The salts [(Ph3P)Au(bipy)]XF6 (X = P, As, Sb) form an isomorphic series in which the differences in Au-N distances range from 0.241(2) to 0.146(2) Å. A second polymorph of [(Ph3P)Au(bipy)]AsF6 has also been found. Both polymorphs exhibit similar structures. The salts [(Et3P)Au(bipy)]XF6 (X = P, As, Sb) form a second isostructural series. In this series the unsymmetrical coordination of the bipy ligand is maintained, but the gold ions are disordered over two unequally populated positions that produce very similar overall structures for the cations. Although many planar, three-coordinate gold(I) complexes are strongly luminescent, the salts [(R3P)Au(bipy)]XF6 (R = Ph or Et; X = P, As, Sb) are not luminescent as solids or in solution. Computational studies revealed that a fully symmetrical structure for [(Et3P)Au(bipy)]+ is 7 kJ/mol higher in energy than the observed unsymmetrical structure and is best described as a transition state between the two limiting unsymmetrical geometries. The Au-N bonding has been examined by natural resonance theory (NRT) calculations using the "12 electron rule". The dominant Lewis structure is one with five lone pairs on Au and one bond to the P atom, which results in a saturated (12 electron) gold center and thereby inhibits the formation of any classical, 2 e- bonds between the gold and either of the bipy nitrogen atoms. The nitrogen atoms may instead donate a lone pair into an empty Au-P antibonding orbital, resulting in a three-center, four-electron (3c/4e) P-Au-N bond. The binuclear complex, [µ2-bipy(AuPPh3)2](PF6)2, has also been prepared and shown to have an aurophillic interaction between the two gold ions, which are separated by 3.0747(3) Å. Despite the aurophillic interaction, this binuclear complex is not luminescent.

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

According to the well-accepted mechanism, methyl-coenzyme M reductase (MCR) involves Ni-mediated thiolate-to-disulfide conversion that sustains its catalytic cycle of methane formation in the energy saving pathways of methanotrophic microbes. Model complexes that illustrate Ni-ion mediated reversible thiolate/disulfide transformation are unknown. In this paper we report the synthesis, crystal structure, spectroscopic properties and redox interconversions of a set of NiII complexes comprising a tridentate N2 S donor thiol and its analogous N4 S2 donor disulfide ligands. These complexes demonstrate reversible NiII -thiolate/NiII -disulfide (both bound and unbound disulfide-S to NiII ) transformations via thiyl and disulfide monoradical anions that resemble a primary step of MCR's catalytic cycle.

The Preparation of Luminescent, Mechanochromic Molecular Containers from Non-Emissive Components: The Box Cations, [Au6 (Triphos)4 Br]5+ and [Au6 (Triphos)4 Br2 ]4.

Two new molecular boxes, the mono-bromo box [Au6 (Triphos)4 Br](SbF6 )5 ⋅6(CH2 Cl2 ), 4 mB, and the dibromo box, [Au6 (Triphos)4 Br2 ⋅H2 O](SbF6 )4 ⋅4(CH2 Cl2 ), 5 dB, have been prepared in crystalline form. Although constructed from non-luminescent components, both are strongly luminescent. Like its chloro counterpart, the mono-bromo box [Au6 (Triphos)4 Br](SbF6 )5 ⋅6(CH2 Cl2 ), 4 mB, is mechanochromic. Under grinding, it loses its luminescence. The bromo-bridged helicate, [(µ-Br){Au3 (Triphos)2 }2 ](CF3 SO3 )5 ⋅2(CH2 Cl2 ), 3µ-H, with a cation that is isomeric with the box [Au6 (Triphos)4 Br]5+ , has also been prepared and crystallographically characterized. Unlike its chloro analogue, [(µ-Br){Au3 (Triphos)2 }2 ](CF3 SO3 )5 ⋅2(CH2 Cl2 ) is not luminescent. Thus, the cation produced upon grinding may be the cation present in the bromo-bridged helicate, [(µ-Br){Au3 (Triphos)2 }2 ](CF3 SO3 )5 ⋅2(CH2 Cl2 ), 3µ-H. The dibromo box, [Au6 (Triphos)4 Br2 ⋅H2 O](SbF6 )4 ⋅4(CH2 Cl2 ), 5 dB, is not significantly mechanochromic.

Cleavage of Carbon Disulfide by n-Propyldiphenylphosphine and Nickel(II) Bromide.

Carbon disulfide is cleaved by n-propyldiphenylphosphine and nickel(II) bromide in a one-step process, to form two unprecedented complexes: orange, [Ni(S2 C2 (Pn PrPh2 )2 )Br(Pn PrPh2 )]Br⋅CS2 (1) and purple [Ni{η2 -SC(Pn PrPh2 )2 }Br(Pn PrPh2 )]Br⋅0.5CS2 (2). Orange (1) contains a dithiolene-related ligand that results from carbon-carbon bond formation, while purple (2) contains a remarkable ligand in which two n-propyldiphenylphosphine molecules have added to a carbon atom of a CS unit that is coordinated to nickel.

Vapoluminescent Behavior and the Single-Crystal-to-Single-Crystal Transformations of Chloroform Solvates of [Au2 (µ-1,2-bis(diphenylarsino)ethane)2 ](AsF6 )2.

The mono- and di-chloroform solvates of [Au2 (µ-1,2-bis(diphenylarsino)ethane)2 ](AsF6 )2 undergo single-crystal-to-single-crystal transformations that result in gain (over 12 hours) or slow loss (over five years) of only one chloroform molecule. The change in solvation results in changes in the structure and luminescence of the digold cation. The cation consists of a pair of slightly bent As-Au-As units that are connected through the two bridging dpae ligands and by aurophilic interactions with Au⋅⋅⋅Au contacts of 3.05152(15) Šin the disolvate or 2.9570(5) Šin the monosolvate.

Isolation and Crystallographic Characterization of Two, Nonisolated Pentagon Endohedral Fullerenes: Ho3 N@C2 (22010)-C78 and Tb3 N@C2 (22010)-C78.

Purified samples of Ho3 N@C2 (22010)-C78 and Tb3 N@C2 (22010)-C78 have been isolated by two distinct processes from the rich array of fullerenes and endohedral fullerenes present in carbon soot from graphite rods doped with Ho2 O3 or Tb4 O7 . Crystallographic analysis of the endohedral fullerenes as cocrystals with Ni(OEP) (in which OEP is the dianion of octaethylporphyrin) shows that both molecules contain the chiral C2 (22010)-C78 cage. This cage does not obey the isolated pentagon rule (IPR) but has two sites where two pentagons share a common C-C bond. These pentalene units bind two of the metal ions, whereas the third metal resides near a hexagon of the cage. Inside the cages, the Ho3 N or Tb3 N unit is planar. Ho3 N@C2 (22010)-C78 and Tb3 N@C2 (22010)-C78 use the same cage previously found for Gd3 N@C2 (22010)-C78 rather than the IPR-obeying cage found in Sc3 N@D3h -C78 .

Acyclic Stereocontrol in the Additions of Nucleophilic Alkenes to α-Chiral N-Sulfonyl Imines.

Diastereoselective Lewis acid-mediated additions of nucleophilic alkenes to N-sulfonyl imines are reported. The canonical polar Felkin-Anh model describing additions to carbonyls does not adequately describe analogous additions to N-sulfonyl imines. Herein, we describe the development of conditions to produce both syn and anti products with high diastereoselectivity and good yields. A stereoelectronic model consistent with experimental outcomes is also proposed.

Utilization of a Nonemissive Triphosphine Ligand to Construct a Luminescent Gold(I)-Box That Undergoes Mechanochromic Collapse into a Helical Complex.

Luminescent gold(I) complexes ([Au6(Triphos)4Cl](PF6)5·2(CH3C6H5), [Au6(Triphos)4Cl](AsF6)5·8(CH3C6H5), and [Au6(Triphos)4Cl](SbF6)5·7(CH3C6H5) where Triphos = bis(2-diphenylphosphinoethyl)phenylphosphine) with a boxlike architecture have been prepared and crystallographically characterized. A chloride ion resides at the center of the box with two of the six gold(I) ions nearby. Mechanical grinding of blue luminescent crystals containing the cation, [Au6(Triphos)4Cl]5+, results in their conversion into amorphous solids with green emission that contain the bridged helicate cation, [µ-Cl{Au3(Triphos)2}2]5+. A mechanism of the mechanochromic transformation is proposed. The structures of the blue-emitting helicate, [Au3(Triphos)2](CF3SO3)3·4(CH3C6H5)·H2O, and the green-emitting bridged-helicate, [µ-Cl{Au3(Triphos)2}2](PF6)5·3CH3OH have been determined by single crystal X-ray diffraction.

Identifying a Needle in a Haystack: Isolation and Structural Characterization of Er2 C94 as the Carbide Er2 C2 @D3 (85)-C92.

A method has been developed for isolating a pure sample of Er2 C94 from the myriad of fullerenes and endohedral fullerenes that are formed in the electric arc process. Crystallographic analysis of Er2 C94 in a cocrystal formed with Ni(OEP) reveals that the molecule is the carbide Er2 C2 @D3 (85)-C92 . Crystals of Er2 C2 @D3 (85)-C92 ⋅Ni(octaethylporphyrin)⋅2 C7 H8 are isostructural with those of Sm2 @D3 (85)-C92 ⋅Ni(octaethylporphyrin)⋅2 (chlorobenzene). Comparisons are made between the four crystallographically characterized endohedrals (Er2 C2 @D3 (85)-C92 , Gd2 C2 @D3 (85)-C92 , La2 C2 @D3 (85)-C92 , and Sm2 @D3 (85)-C92 ) that utilize the chiral D3 (85)-C92 cage.

A Copper(II) Nitrite That Exhibits Change of Nitrite Binding Mode and Formation of Copper(II) Nitrosyl Prior to Nitric Oxide Evolution.

The proton-coupled reduction of CuII-bound nitrite (NO2-) to nitric oxide (NO2- + 2H+ + e- → NO(g) + H2O), such as occurs in the enzyme copper nitrite reductase, is investigated in this work. Our studies focus on the copper(II/I) model complexes [(L2)Cu(H2O)Cl] (1), [(L2)Cu(ONO)] (2), [(L2)Cu(CH3CO2)] (3), and [Co(Cp)2][(L2)Cu(NO2)(CH3CN] (4), where HL2 = N-[2-(methylthio)ethyl]-2'-pyridinecarboxamide. Complex 1 readily reacts with a NO2- anion to form the nitrito-O-bound copper(II) complex 2. Electrochemical reduction of CuII → CuI indicates coordination isomerization from asymmetric nitrito-κ2-O,O to nitro-κ1-N. Isolation and spectroscopic characterization of 4 support this notion of nitrite coordination isomerization (νCu-N ∼ 460 cm-1). A reduction of 2, followed by reaction with acetic acid, causes evolution of stoichiometric NO via the transient copper(II) nitrosyl species and subsequent formation of the acetate-bound complex 3. The probable copper nitrosyl intermediate [(L2)Cu(NO)(CH3CN)]+ of the {CuNO}10 type is evident from low-temperature UV-vis absorption (λmax = 722 nm) and electron paramagnetic resonance spectroscopy. A density functional theory (DFT)-optimized model of [(L2)Cu(NO)(CH3CN)]+ shows end-on NO binding to Cu with Cu-N(NO) and N-O distances of 1.989 and 1.140 Å, respectively, and a Cu-N-O angle of 119.25°, consistent with the formulation of CuII-NO•. A spin-state change that triggers NO release is observed. Considering singlet- and triplet-state electronic configurations of this model, DFT-calculated νNO values of 1802 and 1904 cm-1, respectively, are obtained. We present here important mechanistic aspects of the copper-mediated nitrite reduction pathway with the use of model complexes employing the ligand HL2 and an analogous phenyl-based ligand, N-[2-(methylthio)phenyl]-2'-pyridinecarboxamide (HL1).

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

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{(LMe(S))2Ni}2](ClO4)2·CH3CN (1·CH3CN) 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.