Nitrile ligands activation in dinuclear aminocarbyne complexes

The diiron complexes [Fe(Cp)(CO){μ-η²:η²-C[N(Me)(R)]NC(C₆H₃R′)CCH(Tol)}Fe(Cp)(CO)] (R = Xyl, R′ = H, 3a; R = Xyl, R′ = Br, 3b; R = Xyl, R′ = OMe, 3c; R = Xyl, R′ = CO₂Me, 3d; R = Xyl, R′ = CF₃, 3e; R = Me, R′ = H, 3f; R = Me, R′ = CF₃, 3g) are obtained in good yields from the reaction of [Fe₂{μ-CN(Me)(R)}(μ-CO)(CO)(p-NCC₆H₄R′)(Cp)₂]⁺ (R = Xyl, R′ = H, 2a; R = Xyl, R′ = Br, 2b; R = Xyl, R′ = OMe, 2c; R = Xyl, R′ = CO₂Me, 2d; R = Xyl, R′ = CF₃, 2e; R = Me, R′ = H, 2f; R = Me, R′ = CF₃, 2g) with TolCCLi. The formation of 3 involves addition of the acetylide at the coordinated nitrile and C-N coupling with the bridging aminocarbyne together with orthometallation of the p-substituted aromatic ring and breaking of the Fe-Fe bond. Complexes 3a-e which contain the N(Me)(Xyl) group exist in solution as mixtures of the E-trans and Z-trans isomers, whereas the compounds 3f,g, which posses an exocyclic NMe₂ group, exist only in the Z-cis form. The crystal structures of Z-trans-3b, E-trans-3c, Z-trans-3e and Z-cis-3g have been determined by X-ray diffraction experiments.     

NMR J-based analysis of nitrogen-containing moieties and application to dysithiazolamide, a new polychlorinated dipeptide from Dysidea sp.

The methodology of J-based analysis applied to 1,3-methylcarboamido systems allowed us to deduce the relative configurations of the two leucine-like fragments of a new tetrachloro amino acid derivative dysithiazolamide, which was isolated from an unidentified sponge of the genus Dysidea. Furthermore, the absolute configuration was also proposed by comparison with analogous systems.     

Observation of an Unusual Molecular Switching Device. The Position of One 1,2-Dimethylimidazole Switched "On" or "Off" the Rotation of the Other 1,2-Dimethylimidazole in cis,cis,cis-Ru(II)Cl(2)(Me(2)SO)(2)(1,2-dimethylimidazole)(2)

Some cis,cis,cis-RuX(2)(Me(2)SO)(2)(1,2-Me(2)Im)L complexes [L = 1,2-Me(2)Im (1,2-dimethylimidazole) or Me(3)Bzm (1,5,6-trimethylbenzimidazole), X = Cl or Br, and Me(2)SO = S-bonded DMSO] have been synthesized and their rotamers studied in CDCl(3). From 2D NMR data, cis,cis,cis-RuCl(2)(Me(2)SO)(2)(1,2-Me(2)Im)(Me(3)Bzm) has 1,2-Me(2)Im in position "a" (cis to both Me(2)SO's and cis to "b") and Me(3)Bzm in position "b" (trans to one Me(2)SO and cis to the other). There are two stable atropisomers [head-to-tail (HT, 84%) and head-to-head (HH, 16%), defining the aromatic H of Ru-N-C-H as head for both ligands]. Me(3)Bzm has the same orientation in both atropisomers. In this orientation, the unfavorable interligand steric interactions of Me(3)Bzm with the Me(2)SO and 1,2-Me(2)Im ligands appear to be countered by favorable electrostatic attraction between the delta+ N(2)CH moiety of Me(3)Bzm and the delta- cis Cl ligands. The 1,2-Me(2)Im lacks a delta+ N(2)CH group, and its orientation is dominated by steric effects of the 2-Me group. The NMR spectrum of cis,cis,cis-RuCl(2)(Me(2)SO)(2)(1,2-Me(2)Im)(2) is consistent with four rotamers in restricted rotation about both Ru-N bonds: two HH and two HT. 2D NMR techniques (NOESY and ROESY) afforded complete proton signal assignments. The ligand disposition could be assessed from the large chemical shift dispersion of some 1,2-Me(2)Im ligand signals (Delta 0.86-1.52 ppm) arising from cis-1,2-Me(2)Im shielding modulated by deshielding influences of the cis halides. The relative stability of the four rotamers correlates best with steric interactions between the 2-Me groups and the Me(2)SO ligands. The most favorable conformer (46%) is the HH rotamer with both 2-Me groups pointing away from the Me(2)SO ligands. The least favorable conformer (14%) was also HH, but the methyl groups in this case point toward the Me(2)SO ligands. In the HT conformers of intermediate stability ( approximately 20%), one 2-Me group is toward and the other is away from the Me(2)SO ligands. The exchange cross-peaks in the 2D spectra are unusually informative about the dynamic processes in solution; the spectra provide evidence that the rotamers interchange in a definite pattern of succession. Thus, all conceivable exchange pathways are not available. 1,2-Me(2)Im "b" can rotate regardless of the orientation of 1,2-Me(2)Im "a". 1,2-Me(2)Im "a" can rotate only when "b" has the orientation with its 2-Me group directed away from "a". Thus, 1,2-Me(2)Im "b" can switch 1,2-Me(2)Im "a" rotation on or off.     

The relative permittivity of 1,2-dimethoxyethane and N,N-dimethylformamide mixtures from −10 to 40°C

Binary solutions of N,N-dimethylformamide and 1,2-dimethoxyethane have been investigated by means of dielectric measurements at temperatures ranging from –10 to +40°C, and for nine mixtures covering the whole miscibility field expressed by the mole fraction of one component (0X₁1). The experimental data were used to study the dependence of on T and X₁, of the type = (T), = (X₁), and = (T,X₁). Further, the excess mixing function _E has been evaluated in order to identify particular patterns of interaction between unlike molecules and any other factor that could modify such patterns. The minimum in the _E vs. composition plots suggests the formation of an adduct of stoichiometric ratio DMFDME=11 at all the investigated temperatures.     

Methylene‐Bridged Metallocenes with 2,2′‐Methylenebis[1H‐inden‐1‐yl] Ligands: Synthesis,Characterization, and Polymerization Catalysis of a Synthetically Simple Class of C2‐ and C2v‐Symmetric ansa‐Metallocenes

The acid‐catalyzed reaction between formaldehyde and 1H‐indene, 3‐alkyl‐ and 3‐aryl‐1H‐indenes, and six‐membered‐ring substituted 1H‐indenes, with the 1H‐indene/CH₂O ratio of 2 : 1, at temperatures above 60° in hydrocarbon solvents, yields 2,2′‐methylenebis[1H‐indenes] 1 – 8 in 50–100% yield. These 2,2′‐methylenebis[1H‐indenes] are easily deprotonated by 2 equiv. of BuLi or MeLi to yield the corresponding dilithium salts, which are efficiently converted into ansa‐metallocenes of Zr and Hf. The unsubstituted dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 1′ )]) is the least soluble in organic solvents. Substitution of the 1H‐indenyl moieties by hydrocarbyl substituents increases the hydrocarbon solubility of the complexes, and the presence of a substituent larger than a Me group at the 1,1′ positions of the ligand imparts a high diastereoselectivity to the metallation step, since only the racemic isomers are obtained. Methylene‐bridged ‘ansa‐zirconocenes’ show a noticeable open arrangement of the bis[1H‐inden‐1‐yl] moiety, as measured by the angle between the planes defined by the two π‐ligands (the ‘bite angle’). In particular, of the ‘zirconocenes’ structurally characterized so far, the dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[4,7‐dimethyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 5′ )] is the most open. The mixture [ZrCl₂( 1′ )]/methylalumoxane (MAO) is inactive in the polymerization of both ethylene and propylene, while the metallocenes with substituted indenyl ligands polymerize propylene to atactic polypropylene of a molecular mass that depends on the size of the alkyl or aryl groups at the 1,1′ positions of the ligand. Ethene is polymerized by rac‐dichloro{(1,1′,2,2′,3,3′,3a,3′a,7a,7′a‐η)‐2,2′‐methylenebis[1‐methyl‐1H‐inden‐1‐yl]}zirconium ([ZrCl₂( 2′ )])/MAO to polyethylene waxes (average degree of polymerization ca. 100), which are terminated almost exclusively by ethenyl end groups. Polyethylene with a high molecular mass could be obtained by increasing the size of the 1‐alkyl substituent.     

Synthesis and x-ray characterization of the [Ir6(CO)15COEt]-Anionic Cluster

The reaction of Ir₆(CO)₁₆ with a mixture of CO, H₂, and ethylene yields the [Ir₆(CO)₁₅COEt]^- anion, which has been shown by X-ray diffraction to contain an octahedral iridium cluster bearing a —bonded acyl group; the arrangement of the 11 terminal and 4 edge-bridging carbonyl groups is different from that found in both the analogous rhodium complex and the parent Ir₆(CO)₁₆ carbonyl.     

Axial Imidazole Distortion Effects on the Catalytic and Binding Properties of Chelated Deuterohemin Complexes

The effect of strain in the axial coordination of imidazole to the heme has been studied in the chelate complexes deuterohemin-histidine (DH-His) and deuterohemin-alanylhistidine (DH-AlaHis). Molecular mechanics calculations indicate that three types of distortion of the axial ligand occur in DH-His, due to the relatively short length of the arm carrying the donor group: tilting off-axis, tipping, and inclination of the imidazole plane with respect to the axial Fe-N bond. The effects of tilting (Deltagamma approximately 10 degrees ) and inclination of the imidazole ring (Deltadelta approximately 17 degrees ) are dominant, while tipping is small and is probably of little importance here. By contrast, the axial imidazole coordination is normal in DH-AlaHis and other computed deuterohemin-dipeptide or -tripeptide complexes where histidine is the terminal residue, the only exception being DH-ProHis, where the rigidity of the proline ring reduces the flexibility of the chelating arm. The distortion in the axial iron-imidazole bond in DH-His has profound and negative influence on the binding and catalytic properties of this complex compared to DH-AlaHis. The former complex binds more weakly carbon monoxide, in its reduced form, and imidazole, in its oxidized form, than the latter. The catalytic efficiency in peroxidative oxidations is also reduced in DH-His with respect to DH-AlaHis. The activity of the latter complex is similar to that of microperoxidase-11, the peptide fragment incorporating the heme that results from hydrolytic cleavage of cytochrome c.     

Photochemistry of Dihydrido(hydrotris(3,5-dimethylpyrazolyl)borato)(Z-cyclooctene)iridium. Synthetic Intermediates and Mechanism of the Photochemical Formation of Hydridophenyl(hydrotris(3,5-dimethylpyrazolyl)borato)(trimethyl phosphite)iridium

The photolysis of a benzene solution of [Tp(Me2)IrH(2)(COE)], 1 (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate, COE = Z-cyclooctene), in the presence of P(OMe)(3), gives the stable novel complex [Tp(Me2)IrH(C(6)H(5))(P(OMe)(3))], 3a. The photochemical syntheses of [Tp(Me2)IrH(2)(P(OMe)(3))], from 1 and P(OMe)(3) in diethyl ether, and [Tp(Me2)IrH(2)(CH(2)=CHCOO(t)Bu)], from 1 in tert-butyl acrylate, are also reported. The above reactions and several experiments using C(6)D(6) and P(OCD(3))(3) show that, in all cases, the primary photoproduct is the 16-electron, five-coordinate iridium(III) intermediate {Tp(Me2)IrH(2)}, 6a, produced by loss of COE from 1. The above experiments also allow the postulation of a mechanistic pathway for the formation of 3a which involves the oxidative addition of an aromatic C-H bond by 6a. Furthemore, the photochemical reaction of 1 in the presence of P(OCD(3))(3) shows that, under the reaction conditions used, oxidative addition of C-H bonds of P(OMe)(3) and of coordinated Tp(Me2)-ligands, presumably, to the intermediates 6a and {Tp(Me2)IrH(C(6)H(5))}, also occurs. Thus, coordinatively unsaturated iridium(III) species readily activate C-H bonds.     

Dramatic 5'-residue effect on conformer distribution of short oligonucleotide retro models of the cisplatin-DNA cross-link: implications for the Lippard and cross-link distorted base pair steps present in cisplatin-DNA duplex adducts

The cisplatin anticancer drug preferentially attacks the GG sequence of DNA duplexes. Virtually all DNAs containing the key G*G* lesion (G* = N7 platinated G) have large distortions in the cross-link (G*G*) base pair (bp) step and also in the adjacent Lippard (XG*) bp step, making the adducts very different from B-form DNA in the XG*G* region. The XG*G* strand in duplexes also differs in several ways from single-strand (ss) models with G*G* and XG*G* sequences. In the duplex, the X residue has an N sugar, the 5'-G* and 3'-G* bases have slight "R" canting (3'-G* H8 atom toward the 5'-G* base), and there is no or weak H-bonding by the NH3 ligands. In most XG*G* ss models, X has an S sugar, the 5'-G* base normally cants strongly toward the 3'-G* base (L canting), and the NH3 forms an H-bond. Well-defined ss models exist in the solid state, but dynamic motion obscures the properties of the ss models in solution. In this work, we employ retro models (better defined, less dynamic ss models) to understand the differences between duplex and ss models. The retro models in this study lack carrier ligand NH's, thus eliminating H-bonding. To correlate previous ss solid-state models with our solution work, we constructed hybrid molecules by overlaying parts of known structures. The combined model and experimental information indicates that the X N-pucker is not favorable in L-canted ss models, that X residue steric effects (not H-bonding) favor L canting in ss models, that X N-pucker is needed for favorable WC hydrogen bonding and stacking interactions in duplexes, and that X N-pucker minimizes X base clashes with bases in the complementary strand in duplexes. The R canting minimizing clashes between the X and G* residues of the Lippard bp step (independent of X pucker) and the repositioning of the X residue base caused by the change from S-pucker to N-pucker together lead to the unusual features of the Lippard bp step in the duplex.     

Synthesis and Characterization of Rhenium(V) Oxo Complexes with a New Thiol-Amide-Thiourea Ligand System. X-ray Crystal Structure of [1-Phenyl-3-[2-((2-thioacetyl)amino)ethyl]thioureato]oxorhenium(V)

General methods for preparing Re(V)O complexes with a novel series of thiol-amide-thiourea (TATU) ligands, a new class of N(2)S(2) chelates, were developed. The TATU ligands, the first multidentate systems designed with a bidentate thiourea moiety, have been used to prepare the first high-valent transition metal complexes with bidentate thiourea coordination. Direct reaction of N-(2-aminoethyl)-2-((triphenylmethyl)thio)acetamide (1) with phenyl, 4-methoxyphenyl, 4-chlorophenyl, and methyl isothiocyanate afforded ready access to the corresponding S-protected TATU ligands in one step. A two-step preparation of the N,N-dimethylthiourea TATU ligand derived from 1 was also developed. Deprotection of thiols in trifluoroacetic acid with triethylsilane followed by a ligand exchange reaction with Re(V)O precursors yielded neutral ReO(TATU) complexes. The structure of [1-phenyl-3-[2-((2-thioacetyl)amino)ethyl]thioureato]oxorhenium(V) (6a) was determined by X-ray diffraction methods. Crystal data for 6a: C(11)H(12)N(3)O(2)ReS(2), fw 468.6, orthorhombic, Pca2(1); a = 22.605(5) ?, b = 13.029(3) ?, c= 9.698(2) ?; V = 2856.3(11) ?(3); Z = 8. The coordination environment of 6a was pseudo-square-pyramidal with a deprotonated thiol S, deprotonated amide N, deprotonated thiourea N, and thiocarbonyl S coordinated in the basal plane and the oxo ligand in the apical position. The thiourea function forms a four-membered chelate ring in the multidentate TATU ligands. The two N-C and the S-C bond distances within the monodeprotonated thiourea moiety were typical of bonds with multiple-bond character. Solution (1)H NMR data for all five complexes were consistent with the solid-state structure of 6a. A broad singlet attributable to the uncoordinated NH group of thiourea was observed for the monosubstituted thiourea complexes but was not present for the N,N-dimethylthiourea derivative. Instead, two singlets of equal intensity were observed for the two methyl groups, indicating that there is restricted rotation around the C-N(CH(3))(2) bond and an extended pi system in the thiourea moiety. The four-membered ring might cause difficulty because the M-S distance would be relatively long in an undistorted ligand. This may be the reason such chelate ligands have not been previously investigated. However, the N-C-S angle narrows to approximately 105 degrees, permitting a Re-S bond with a typical bond length to be formed. We conclude that such a ring represents a versatile new building block to create multidentate ligands.