William J. Marshall

IMIDAZOLYLIDENES, IMIDAZOLINYLIDENES AND IMIDAZOLIDINES

Abstract Starting from glyoxal, 1,3-diarylimidazolinium chlorides 3 were obtained in a three-step sequence via the diimines (1) and ethylene diamine dihydrochlorides (2). Reduction of 1,3-diarylimidazolinium chlorides (3) with lithium alumnium hydride furnished the 1,3-diarylimidazolidines (4) while their deprotonation with potassium hydride in thf gave access to stable carbenes (1,3-diarylimidazolin-2-ylidenes, 5). Similarly substituted imidazol-2-ylidenes are described for comparison.

Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl3+2Mn4+2O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.

New, efficient electroluminescent materials based onorganometallic Ir complexes

Reaction of IrCl3 with fluorinated 2-arylpyridines in the presence of AgO2CCF3 affords fac-tris-cyclometalated arylpyridine Ir complexes exhibiting excellent processing and electroluminescent properties which can be fine-tuned via systematic control of the nature and position of the substituents on the aromatic rings.

C−H Insertion Reactions of Nucleophilic Carbenes

Syntheses and characterizations are described for C−H insertion products derived from 1,3-dimesityldihydroimidazol-2-ylidene (1) with acetylene, acetonitrile, methyl phenyl sulfone, and chloroform. In the reaction with acetylene, both acetylenic H-atoms are reactive so that 1 : 1 and 2 : 1 adducts can be obtained. The acetylene and methyl-phenyl-sulfone adducts are structurally characterized by means of single-crystal X-ray structure determinations. The reactions of 1,3,4,5-tetramethylimidazolidin-2-ylidene (8) with chloroform or chlorodifluoromethane are shown to yield 2-(dihaloalkyl)imidazolium salts that arise from a failure of the intermediate 2-protioimidazolium salt to capture the initially formed halocarbanion.

Mechanisms of Catalyst Poisoning in Palladium-Catalyzed Cyanation of Haloarenes. Remarkably Facile C−N Bond Activation in the [(Ph3P)4Pd]/[Bu4N]+ CN- System

Reaction paths leading to palladium catalyst deactivation during cyanation of haloarenes (eq 1) have been identified and studied. Each key step of the catalytic loop (Scheme 1) can be disrupted by excess cyanide, including ArX oxidative addition, X/CN exchange, and ArCN reductive elimination. The catalytic reaction is terminated via the facile formation of inactive [(CN)4Pd]2-, [(CN)3PdH]2-, and [(CN)3PdAr]2-. Moisture is particularly harmful to the catalysis because of facile CN- hydrolysis to HCN that is highly reactive toward Pd(0). Depending on conditions, the reaction of [(Ph3P)4Pd] with HCN in the presence of extra CN- can give rise to [(CN)4Pd]2- and/or the remarkably stable new hydride [(CN)3PdH]2- (NMR, X-ray). The X/CN exchange and reductive elimination steps are vulnerable to excess CN- because of facile phosphine displacement leading to stable [(CN)3PdAr]2- that can undergo ArCN reductive elimination only in the absence of extra CN-. When a quaternary ammonium cation such as [Bu4N]+ is used as a phase-transfer agent for the cyanation reaction, C-N bond cleavage in the cation can occur via two different processes. In the presence of trace water, CN- hydrolysis yields HCN that reacts with Pd(0) to give [(CN)3PdH]2-. This also releases highly active OH- that causes Hofmann elimination of [Bu4N]+ to give Bu3N, 1-butene, and water. This decomposition mode is therefore catalytic in H2O. Under anhydrous conditions, the formation of a new species, [(CN)3PdBu]2-, is observed, and experimental studies suggest that electron-rich mixed cyano phosphine Pd(0) species are responsible for this unusual reaction. A combination of experimental (kinetics, labeling) and computational studies demonstrate that in this case C-N activation occurs via an S(N)2-type displacement of amine and rule out alternative 3-center C-N oxidative addition or Hofmann elimination processes.

1,3,4,5-Tetraphenylimidazol-2-ylidene: The Realization of Wanzlick's Dream

Questions remain as to why the title compound 1 was never isolated before; contrary to long-held opinion, this compound is not unstable. With a modified version of the original procedure by H.-W. Wanzlick et al. for the synthesis of the imidazolium salt starting material, 1 has now been isolated as a stable crystalline solid at room temperature.

trans-Difluoro complexes of palladium(II).

Complexes trans-[(Py)(2)Pd(Ph)(F)] (1; Py = pyridine) and trans-[(t-BuPy)(2)Pd(Ph)(F)] (2; t-BuPy = 4-tert-butylpyridine) have been prepared from the corresponding iodides and AgF. Thermal decomposition of 1 and 2 in anhydrous benzene at 80 degrees C did not result in C-F bond formation, but Pd black and Ph(2) were produced instead, along with novel difluorides trans-[(Py)(2)Pd(F)(2)] (3) and trans-[(t-BuPy)(2)Pd(F)(2)] (4). Both 3 and 4, the first trans-difluoro d(8) square complexes, were independently synthesized from the corresponding diiodides and AgF and fully characterized. Contrary to filled/filled d(pi)-p(pi) repulsion considerations, the Pd-F bond distances in 3 and 4 are unprecedentedly short, being only 1.947(4)-1.958(4) A and shorter than those in 1 and 2 by 0.12-0.13 A. The mechanism of formation of 3 and 4 and bonding in these complexes are discussed.

Fluxionality of [(Ph3P)3Rh(X)]: The extreme case of X = CF3

[(Ph(3)P)(3)Rh(F)] reacts with CF(3)SiMe(3) to produce trans-[(Ph(3)P)(2)Rh(CF(2))(F)] (1; X-ray), which is equilibrated with a number of species in solution. Addition of excess Ph(3)P shifts all of the equilibria to [(Ph(3)P)(3)Rh(CF(3))] (2; X-ray) as the only NMR-observable and isolable (84%) species. Complex 2 is uniquely highly fluxional in solution, maintaining ligand exchange even at -100 degrees C (12.1 s(-1)). Activation parameters have been determined (variable-temperature (31)P NMR) for the similar but slower exchange in the Me analogue of 2, [(Ph(3)P)(3)Rh(CH(3))]: E(a) = 16.5 +/- 0.6 kcal mol(-1), DeltaG(double dagger) = 12.9 kcal mol(-1) (calculated at -30 degrees C), DeltaH(double dagger) = 16.0 +/- 0.6 kcal mol(-1), and DeltaS(double dagger) = 12.8 +/- 2.3 e.u. Intramolecular exchange in [(R(3)P)(3)Rh(X)] occurs (DFT, MP2//BP86) via a distorted trigonal transition state (TS) with X in an axial position trans to a vacant site. The rearrangement is governed by a combination of steric and electronic factors and is facilitated by bulkier ligands on Rh as well as by strongly donating X that stabilize the TS. The Rh atom in [(H(3)P)(3)Rh(X)] has been shown to be more negatively charged (NPA) for X = CF(3) than for X = CH(3), despite the strongly oppositely charged carbon atoms of the CF(3) (+0.79e) and CH(3) (-0.96e) ligands. Clarification of stereochemical rigidity (X = halide, CN, OR, NR(2)) versus fluxionality (X = H, Alk, Ar, CF(3)) is provided, along with a resolution of the long-standing contradiction between the electron-withdrawing effect of CF(3) in organic compounds and its strong trans influence (electron donation) in metal complexes.

Carbene-Based Lewis Pairs for Hydrogen Activation

A series of Lewis acid–base pairs containing sterically demanding carbenes were investigated for hydrogen activation that could potentially be reversible for use in hydrogen storage applications. When electron-rich boranes are employed as electrophiles, the imidazolium cation is reduced to a 2H-imidazoline (aminal). The aminals were synthesized independently by reduction of imidazolium cations with strong reducing agents. Carbocations were also found to act as electrophiles for hydrogen activation. Preliminary results revealed that it is possible to reduce an alcohol to an alkane using hydrogen gas as a reducing agent in these systems. Finally, it was demonstrated that a transition metal can be used as an electrophile to activate hydrogen through heterolytic cleavage.

Fluxionality of [(Ph3P)3M(X)] (M = Rh, Ir). the red and orange forms of [(Ph3P)3Ir(Cl)]. Which phosphine dissociates faster from wilkinson's catalyst?

NMR studies of intramolecular exchange in [(Ph(3)P)(3)Rh(X)] (X = CF(3), CH(3), H, Ph, Cl) have produced full sets of activation parameters for X = CH(3) (E(a) = 16.4 +/- 0.6 kcal mol(-1), DeltaH(double dagger) = 16.0 +/- 0.6 kcal mol(-1), and DeltaS(double dagger) = 12.7 +/- 2.5 eu), H (E(a) = 10.7 +/- 0.2 kcal mol(-1), DeltaH(double dagger) = 10.3 +/- 0.2 kcal mol(-1), and DeltaS(double dagger) = -7.2 +/- 0.8 eu), and Cl (E(a) = 16.3 +/- 0.2 kcal mol(-1), DeltaH(double dagger) = 15.7 +/- 0.2 kcal mol(-1), and DeltaS(double dagger) = -0.8 +/- 0.8 eu). Computational studies have shown that for strong trans influence ligands (X = H, Me, Ph, CF(3)), the rearrangement occurs via a near-trigonal transition state that is made more accessible by bulkier ligands and strongly donating X. The exceedingly fast exchange in novel [(Ph(3)P)(3)Rh(CF(3))] (12.1 s(-1) at -100 degrees C) is due to strong electron donation from the CF(3) ligand to Rh, as demonstrated by computed charge distributions. For weaker donors X, this transition state is insufficiently stabilized, and hence intramolecular exchange in [(Ph(3)P)(3)Rh(Cl)] proceeds via a different, spin-crossover mechanism involving triplet, distorted-tetrahedral [(Ph(3)P)(3)Rh(Cl)] as an intermediate. Simultaneous intermolecular exchange of [(Ph(3)P)(3)Rh(Cl)] with free PPh(3) (THF) via a dissociative mechanism occurs exclusively from the sites cis to Cl (E(a) = 19.0 +/- 0.3 kcal mol(-1), DeltaH(double dagger) = 18.5 +/- 0.3 kcal mol(-1), and DeltaS(double dagger) = 4.4 +/- 0.9 eu). Similar exchange processes are much slower for [(Ph(3)P)(3)Ir(Cl)] which has been found to exist in orange and red crystallographic forms isostructural with those of [(Ph(3)P)(3)Rh(Cl)].