Formations and electrochemical behavior of mononuclear and binuclear molybdenum dithiolene complexes with nitrosyl ligands: Evidence for the formation of a coordinatively unsaturated species [CpMo(NO)(dithiolene)]

The one-pot reaction of [Cp^∗Mo(NO)(CO)₂] with elemental sulfur and dimethyl acetylenedicarboxylate (C₂Z₂ (Z = COOMe)) gave the [2+2] cycloadduct of the mononuclear molybdenum dithiolene complex [Cp^∗Mo(NO)(S₂C₂Z₂)(C₂Z₂)] (1), and some binuclear complexes:[Cp^∗Mo(NO)(S₂C₂Z₂)]₂ (2), [Cp^∗₂Mo₂(NO)₂S₂(S₂C₂Z₂)] (3) and [Cp^∗Mo(NO)S₂]₂ (4).The reaction of [Cp^∗Mo(NO)(Cl)(μ-Cl)]₂ with OC{S₂C₂(COOMe)₂} in the presence of sodium methoxide also produced complex 2 and the paramagnetic Cp^∗Mo bisdithiolene complex [Cp^∗Mo(S₂C₂Z₂)₂] (5, Z = COOMe).The structures of complexes 1-5 were determined by X-ray crystal structure analysis.The nitrosyl ligands of complexes 1-4 showed a linear coordination to the molybdenum center (the Mo-N-O bond angles = 169-174°), and their N-O bond lengths were 1.17-1.20 Å.In the binuclear complexes 2-4, two nitrosyl ligands were placed at cis-position.Complexes 1 and 2 were characterized by cyclic voltammetry and spectroelectrochemistry (visible and IR). The electrochemical reduction of the dimeric complex 2 formed the monomeric dithiolene complex[Cp^∗Mo(NO)(S₂C₂Z₂)]⁻ (X⁻) whose lifetime was several minutes. When the anion X⁻ was electrochemically oxidized, the coordinatively unsaturated species X was generated, but it was immediately dimerized to afford the original dimeric complex 2. The reduction of the complex 1 included the elimination of the bridged DMAD moiety (C₂Z₂) to give the anion X⁻.     

Sulfur-rich CpCo(dithiolene) complexes: Isostructural or non-isostructural couples of CpCo(III) with CpNi(III) dithiolene complexes

Eight new sulfur-rich [CpCo(dithiolene)] complexes were synthesized from [Zn(dmit)₂]²⁻ as a starting material. The structures, electrochemical behavior and electronic absorption spectra of the sulfur-rich [CpCo(S₂C₂S₂Y)] complexes could be compared with the early data of analogous Ni complexes. [CpCo(pddt)] (Y = -(CH₂)₃-), [CpCo(dpdt)] (Y = -CH₂C(CH₂)CH₂-), [CpCo(bddt)] (Y = -(CH₂)₄-), [CpCo(dtdt)] (Y = -CH₂SCH₂-) and [CpCo(poddt)] (Y = -CH₂C(O)CH₂-) crystallized in all isostructural with the corresponding paramagnetic [CpNi(dithiolene)] complexes, but [CpCo(dmid)] (Y = CO), [CpCo(dddt)] (Y = -(CH₂)₂-) and [CpCo(F₂pddt)] (Y = -CH₂CF₂CH₂-) crystallized in non-isostructural with them. These molecules are associated with intermolecular short S?S contacts in the crystals. [CpCo(F₂pddt)] did not show any remarkable S?S contacts but indicated interesting fluorine segregation and Cp?Cp face-to-face interactions. Redox potentials of [CpCo(dithiolene)] complexes were obtained with the cyclic voltammetry measurements and dimerized by electrochemical oxidations. Electronic absorption spectra of [CpCo(dithiolene)] complexes showed visible absorption in the range of 585-701 nm as lowest energy wavelengths (? = 9800-11,800 M⁻¹ cm⁻¹) in solutions, and they were higher energy than those of [CpNi(dithiolene)] complexes (near-IR).     

Syntheses and reactivities of azido coordinated CpCo(dithiolene) complexes

Azido coordinated dithiolene complexes [CpCo(N₃){S₂C₂(CO₂Me)₂}(S-CHR¹R²)], where R¹, R² = H (4a); R¹ = H, R² = SiMe₃ (4b); R¹ = H, R² = CO₂Et (4c), were synthesized by the reactions of the corresponding Cl⁻ coordinated precursors [CpCo(Cl){S₂C₂(CO₂Me)₂}(S-CHR¹R²)] (3a-3c) with sodium azide. The Cl⁻ coordinated complex 3d (R¹, R² = CO₂Me) did not produce any N₃⁻ coordinated complexes but formed the CR¹R²-bridged alkylidene adduct [CpCo{S₂C₂(CO₂Me)₂}(CR¹R²)] (2d; R¹, R² = CO₂Me). The structure of 4a was determined by X-ray diffraction study. In the molecular structure of 4a, the coordinated N₃⁻ ligand and CHR¹R² group were located at the same side with respect to the dithiolene ring (syn form), although the corresponding Cl⁻ precursor (3a; R¹, R² = H) was anti form. A structural conversion of syn/anti was conceivable during the Cl⁻/N₃⁻ ligand exchange. Thermal (80 °C) and photochemical reactions (Hg lamp) of 4a-4c were performed. Among them, 4c was relatively well reacted compared with the others to form the CR¹R²-bridged alkylidene adduct (2c; R¹ = H, R² = CO₂Et), followed by a formal HN₃ elimination, and the reaction also produced non-adduct of the cobalt dithiolene complex [CpCo{S₂C₂(CO₂Me)₂}] (1). The electrochemical 1e⁻ reduction of 4c underwent a formal N₃⁻ ligand elimination, and successive second reduction caused the CHR¹R² group elimination or reformed the CR¹R²-bridged alkylidene adduct 2c.     

Cross-conjugated 3-ferrocenylmethylidene-2,4-dimethylpenta-1,4-diene in cationic cycloaddition and proton-catalyzed cyclodimerization reactions

Cationic cycloaddition of cross-conjugated 3-ferrocenylmethylidene-2,4-dimethylpenta-1,4-diene to 1-ferrocenyl-2-isopropenyl-3,3-dimethylallylium tetrafluoroborate and its proton-catalyzed cyclodimerization result in diferrocenylbicyclo[3.3.1]nonene derivatives. The spatial structure of one of the reaction products, viz., ferroceno[4,5]-{9-ferrocenylmethylidene-1,3,3,8,10-pentamethyltricyclo[6.3.1.0^2,6]dodec-2(6)-ene}, was established based on the data from X-ray diffraction analysis.     

Isostructural diamagnetic cobalt(III) and paramagnetic nickel(III) dithiolene complexes with an extended benzenedithiolate core [CpM(bdtodt)] (M = Co and Ni)

The isostructural diamagnetic [CpCo(bdtodt)] and paramagnetic [CpNi(bdtodt)] (Cp = η⁵-cyclopentadienyl, bdtodt:benzo[1,3]dithiol-2-one-5,6-dithiolato) complexes were prepared by starting from the corresponding bis(dithiocarbonate): benzo[1,2-d;4,5-d′]bis[1,3]dithiole-2,6-dione. Both Co and Ni complexes are isostructural and crystallize in the orthorhombic system, space group Pbca. The formally M^III (16-electron for Co^III and 17-electron Ni^III) complexes were investigated by X-ray structure analyses and exhibit the same two-legged piano-stool geometry. The CV of the radical [CpNi(bdtodt)] resulted in well-defined reversible reduction and oxidation waves. On the other hand, oxidation of [CpCo(bdtodt)] leads to dimerization in CH₂Cl₂ or reaction in the more coordinating CH₃CN solvent. The absorption maximum (λ_max) of [CpNi(bdtodt)] (741 nm) showed a more red shift compared with [CpCo(bdtodt)] (595 nm) in dichloromethane solution. The structural similarities, and electrochemical, spectroscopic and magnetic differences between various [CpCo(dithiolene)] and [CpNi(dithiolene)] complexes are further analyzed.     

Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.     

Formations and structures of cobalt dithiolene complexes with nitrogen group-substituted cyclopentadienyl ligands

The cobalt dithiolene complex with the sulfonylamide-substituted Cp ligand [(C₅H₄-NHTs)Co{S₂C₂(COOMe)₂}] (1, Ts = p-SO₂C₆H₄Me) reacted with TsOH · H₂O to give [(C₅H₄-NH₂)Co{S₂C₂(COOMe)(H)}] (2), [(C₅H₄-NHTs)Co(S₂C₂H₂)] (3) and [(C₅H₄-NHTs)Co{S₂C₂(COOMe)(H)}] (4). Complex 1 was dissolved in a basic aqueous solution, and the anion reacted with Me₂SO₄ to form the N-methylated product [{C₅H₄-N(Me)Ts}Co{S₂C₂(COOMe)₂}] (5); the carboxylic acid complex [{C₅H₄-N(Me)Ts}Co{S₂C₂(COOMe)(COOH)}] (6) formed by a base hydrolysis. The X-ray crystal structures of complexes 4-6 and the methylsulfonylamide-substituted Cp complex [(C₅H₄-NHMs)Co{S₂C₂(COOMe)₂}] (7, Ms = SO₂Me) were determined. In the crystal structures of complexes 4 and 7, intermolecular hydrogen bondings of NH?O (ca. 2.1 Å) and NH?S (ca. 3.1 Å) were observed. Complex 6 showed the OH?O intermolecular hydrogen bonding (ca. 1.6 Å) of COOH moiety between two molecules, and these two molecules were assembling each other. Complexes 5 and 6 showed an intramolecular π-π interaction between the aromatic cobaltadithiolene and benzene rings, and complex 5 also has intermolecular π-π interactions between two benzene rings, and between two cobaltadithiolene rings.     

Synthesis,structure and physical properties of nickel bis(dithiolene) complexes with different cations

A series of new complexes of multi-sulfur 1,2-dithiolene ligands, [Ru(bipy)₃][Ni(L)₂]₂ (bipy?=?2,2′-bipyridine; L?=?pddt (6,7-dihydro-5H-1,4-dithiepin-2,3-dithiolate), dddt (5,6-dihydro-1,4-dithiin-2,3-dithiolate)), have been synthesized and characterized. One typical complex, [Ru(bipy)₃][Ni(pddt)₂]₂·2H₂O (1), crystallized in an acentric space group of P2₁2₁2₁, with the cell dimensions of a?=?8.634(1), b?=?14.560(1), c?=?49.889(5)?Å, α?=?β?=?γ?=?90°, and Z?=?4. It consists of alternating columns of cations and anions along the a direction. The structure was refined by full matrix least squares methods to R ₁?=?0.0340, wR ₂?=?0.0670. Magnetic studies on [Ph₂Cr][Ni(dddt)₂] are also reported.     

Two near-UV and visible chromophores of CpCo(Dithiolene) complexes with pyridinium-dicyanomethylide group

Tetracyanoethylene oxide (TCNEO) reacted with [CpCo(dithiolene)] (Cp = η⁵-cyclopentadienyl) complexes having 4-pyridyl or 3-pyridyl group to undergo a dicyanomethylation to the nitrogen atom on the pyridyl group. The reaction of [CpCo(S₂C₂(⁴Py)₂)] (1) with TCNEO formed both the monodicyanomethylated [CpCo(S₂C₂(⁴Py)(⁴Py-C(CN)₂))] (1a) and bisdicyanomethylated [CpCo(S₂C₂(⁴Py-C(CN)₂)₂)] (1b). [CpCo(S₂C₂(²Py)(⁴Py))] (2) reacted with TCNEO to give [CpCo(S₂C₂(²Py)(⁴Py-C(CN)₂))] (2a) but no dicyanomethylation occurred on the 2-pyridyl group. 2 reacted with excess TCNEO to form the only dicyanomethylated acetylene derivative ²Py-CC-(⁴Py-C(CN)₂) (2c), followed by a dissociation of the CpCoS₂ fragment. The monodicyanomethylated [CpCo(S₂C₂(ⁿPy-C(CN)₂)(2-thienyl))] (n = 4 (4a) or 3 (5a)) complexes were also prepared from [CpCo(S₂C₂(ⁿPy)(2-thienyl))] (n = 4 (4) or 3 (5)) and TCNEO. 1b was structurally characterized by X-ray diffraction study. The all dicyanomethylated [CpCo(dithiolene)] complexes showed the dithiolene LMCT absorption in the range of 605-644 nm (ε = 7000-9200 M⁻¹ cm⁻¹) and very strong absorption due to their pyridinium-dicyanomethylide moieties in near-UV region (e.g. 1b: λ_max = 470 nm, ε = 43,400 M⁻¹ cm⁻¹). The CV of the all dicyanomethylated complexes exhibited two reduction waves. The first reduction is due to Co^III/Co^II and the second one is due to the reduction of the pyridinium-dicyanomethylide moiety. The reduced 1b⁻ is stable enough for several minutes according to the visible spectroelectrochemical measurement. The ESR spectrum of 1b⁻ indicated eight hyperfine splittings due only to the interaction with the nuclear spin of cobalt (I = 7/2).     

Oxidative-addition reactions of nitronium tetrafluoroborate with unsaturated transition metal complexes

The oxidative-addition reaction of nitronium tetrafluoroborate to unsaturated species such as Ru(O), Rh(I), Ir(I), Pt(O) has been used to prepare nitrosyl and nitro complexes of these transition metals. In the case of Ru(O), a nitro-nitrosyl complex is also found. The use of nitronium ion, NO₂⁺ to introduce the nitrosyl group into coordination complexes is novel. The reaction of NO₂⁺ with coordinatively unsaturated species appears to be analogous with that of NO⁺. The preparation and some chemical and physical properties of the complexes are discussed.     

CpCo(dithiolene) complexes of highly flexible oxddt ligand with two different Z-shaped and U-shaped structures

[CpCo(oxddt)] complex (2, oxddt = o-xylenediyldithioethylene-1,2-dithiolate, Cp = η⁵-cyclopentadienyl) was obtained from o-xylenediyldithioethylene-1,3-dithiol-2-one (OC(oxddt)) (1). 2 further reacted with diazoalkanes (N₂CHR) to form some alkylidene-bridged adducts [CpCo(CHR)(oxddt)] (R = H (3a), SiMe₃ (3b)). Adduct 3a further reacted with protic acids (HX) to give some S-methylated adducts [CpCo(X)(oxddt)(S-Me)] (X = Cl (4a), OCOCF₃ (4c)), followed by the Co-C bond cleavage in the three-membered cobaltathiirane ring. Two different Z-shaped and U-shaped molecular structures were observed by X-ray diffraction studies. In the former structure (Z), the dithiolene and o-xylylene planes are located at almost parallel position each other, and in the latter structure (U), both planes are not parallel but the o-xylylene moiety is located closer to the dithiolene plane than the Z-shaped one. The Z-shaped structure involves 1 and 2. The U-shaped structure involves 3a, 3b, 4a and 4c. Complex 1 showed a one-dimensional chain through intermolecular π-π interaction in the crystal. Complex 2 had a dimeric interaction between dithioethylenedithiolate moieties (S₂C₂S₂) in the oxddt. The SiMe₃ group in 3b was placed at an exo-position with respect to the cobaltadithiolene ring due to a steric hindrance from the U-shaped oxddt ligand. In 4a, the X and Me groups are located at the opposite side of the dithiolene plane (anti-form) but in 4c, both groups are presented at the same side of the dithiolene plane (syn-form). The NMR analysis of 4a in solution indicated existence of both anti- and syn-isomers (7:1).     

BODIPY-bridged push-pull chromophores: optical and electrochemical properties

A series of BODIPY bridged push-pull chromophores were prepared by a sequence of reactions involving: i) cross-coupling reactions promoted by palladium complexes; ii) Knoevenagel condensation leading to dicyano derivatives; iii) [2+2] cycloaddition. The last reaction is regioselective, providing mono-derivatives with anisole substituents. With dimethylaminophenyl donor groups, double addition is feasible, providing highly colored dyes displaying remarkable electrochemical properties.     

Pd-catalyzed boronations and cross-coupling reactions of aromatic cobaltadithiolene complexes with aryl halides

The Pd-catalyzed reaction of [CpCo(S2C2(Ph)(Bpin))] (1, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaboronate) with 1-iodonaphthalene or 2-bromothiophene gave the cross-coupling product [CpCo(S2C2(Ph)(Ar))] (Ar = 1-Np (4) or 2-Th (5)), although an early paper described the reaction of 1 with 3-bromopyridine or 9-bromoanthracene (Ar = 3-Py (2) or 9-Anth (3)). The boronation of the brominated precursor [CpCo(S2C2(p-C6H4Br)(H))] (7) with Bpin-H in the presence of Pd catalyst gave the expected boronated product [CpCo(S2C2(p-C6H4Bpin)(H))] (8) but also underwent an unexpected direct boronation on the dithiolene carbon to form [CpCo(S2C2(p-C6H4Br)(Bpin))] (9). The brominated complex 7 or [CpCo(S2C2(Ph)(p-C6H4Br))] (10) was synthesized by thermal reaction and the microwave-enhanced reaction relatively gave better yield with shorter reaction time than that of the conventional heating reaction. The cross-coupling reactions of the boronated or [CpCo(S2C2(Ph)(p-C6H4Bpin))] (11) with aryl halides successfully produced the corresponding cross-coupling products such as [CpCo(S2C2(p-C6H4Py)(H))] (12) or [CpCo(S2C2(p-C6H4Anth)(H))] (13) from 8 and [CpCo(S2C2(Ph)(p-C6H4Py))] (14) from 11. The structures of 7, 9, 11, 12, 13 and 14 were determined by X-ray diffraction studies. Electronic absorption maxima (λ_max) due to dithiolene LMCT in dichloromethane solution can be modified in the range of 574-602 nm by a substituent effect on the dithiolene ring. Redox potentials obtained from CV measurement were also reported.     

Organometallic dithiolene complexes of the group 8-10 metals: reactivities, structures and electrochemical behavior

Research progress in the organometallic dithiolene complexes such as [Cp(or Cp*)M(dithiolene)] (M = Co, Rh, Ir, Ni), [(C(6)R(6))Ru(dithiolene)] and [(C(4)R(4))Pt(dithiolene)] complexes during the past decade is described and the reactivities, structures and electrochemical behavior are summarized in this paper. The five-membered metalladithiolene ring (MS(2)C(2)) undergoes addition reactions to the M[double bond, length as m-dash]S bond to form 18-electron adducts by an imido, alkylidene, alkene or norbornene group and also undergoes dimerizations on the basis of the unsaturation in the ring. The aromaticity of the ring causes substitution reactions on the dithiolene carbon by a C-centered radical, S-centered radical or succinimide group when the ring has a C-H bond. Furthermore a dithiolene-dithiolene homo-coupling reaction by an acid or dithiolene-aryl cross-coupling occurs based on the aromaticity in the ring. Dissociations of the 18-electron adducts are observed by those thermolyses, photolyses, electrochemical redox reactions and other chemical reactions with tertiary phosphorus compounds. One representative example of them is the imido adduct dissociation with PR(3) under heating toward the intramolecular imido migration to a Cp ligand. Since all products are rearomatized by those adduct dissociations, it is concluded that the 'coexistence of aromaticity and unsaturation' in the metallacycle mediates the diverse chemical reactions.     

A computational study of cycloaddition reactions of d8 metal tetroxide (iron, ruthenium, osmium) complexes with C60

The potential energy surfaces of the cycloaddition reactions MO(4)(NC(5)H(5))(2) + C(60)→ MO(4)(NC(5)H(5))(2)(C(60)) (M = Fe, Ru, and Os) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that there should be two competing pathways in these reactions, which can be classified as a [6,5]-attack (path A) and a [6,6]-attack (path B). Our theoretical calculations indicate that, given the same reaction conditions, the cycloaddition reaction of C(60)via [6,6]-attack is more favorable than that via [6,5]-attack both kinetically and thermodynamically. This is in good agreement with the available experimental observations. A qualitative model, which is based on the theory of Pross and Shaik, has been used to develop an explanation for the barrier heights. As a result, our theoretical findings suggest that the singlet-triplet splitting ΔE(st) (= E(triplet)- E(singlet)) of the d(8) MO(4)(NC(5)H(5))(2) and C(60) species can be a guide to predict their reactivity towards cycloaddition. Our model results demonstrate that the reactivity of d(8) metal tetroxide cycloaddition to C(60) decreases in the order FeO(4)(NC(5)H(5))(2) > RuO(4)(NC(5)H(5))(2) > OsO(4)(NC(5)H(5))(2). In consequence, we show that both electronic and geometric effects play a decisive role in determining the energy barriers as well as the reaction enthalpy.     

Synthesis and cycloaddition reactions of sulfur-substituted quinolizidine dienes

Synthesis of sulfur-substituted quinolizidine dienes via ring-closing enyne metathesis (RCEYM) of the corresponding enynes has been achieved. The cycloaddition reactions of these dienes with electron-deficient dienophiles give tricyclic and tetracyclic nitrogen-containing products.     

1,4-Zwitterionic intermediates formed by cleavage of a cyclobutane ring and their cycloaddition reactions

Cycloaddition reaction is an efficient organic reaction because all of the substrate components are introduced into a product skeleton. Recently, more than six-membered rings have been synthesized by Lewis acid-promoted ring cleavage of a cyclobutane ring followed by addition to an unsaturated bond. Typical examples in the literature of these types of reactions using 1-donor–2-acceptor cyclobutanes and 3-donor cyclobutanones with Lewis acids are presented in this Letter.     

Cyclometalated platinum(II) complexes bearing o-phenylenediamine derivatives: Synthesis and electrochemical behavior

New cyclometalated Pt(II) complexes bearing 1,2-phenylenediamine (pda) derivatives were synthesized and their chemical and electrochemical properties were investigated. Two Pt complexes, [Pt(bzqn) (pda)](PF₆) (bzqn = benzo[h]quinoline, [1](PF₆): pda = 1,2-phenylenediamine, [2](PF₆): pda = 4,5-dimethyl-1,2-phenylenediamine), were synthesized by the reaction of (Bu₄N)[PtCl₂(bzqn)] with corresponding pda derivatives. The Pt-pda complexes were converted to Pt-bda (bda = 1,2-benzenediamide) complexes by treatment of 2 mol equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and they showed two-step reversible redox couples in cyclic voltammetry. The cyclometalated ligand gave a strong electronic effect to the bda ligand to take place the negative shift of the bda-based redox potentials.