D. Christopher Roe

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.

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.

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)].

Fluoroacetate-Mediated Toxicity of Fluorinated Ethanes

A series of 1-(di)halo-2-fluoroethanes reported in the literature to be nontoxic or of low toxicity were found to be highly toxic by the inhalation route. Experiments were performed that showed the compounds, 1,2-difluoroethane, 1-chloro-2-fluoroethane, 1-chloro-1,2-difluoroethane, and 1-bromo-2-fluoroethane to be highly toxic to rats upon inhalation for 4 hr. All four compounds had 4-hr approximate lethal concentrations of < or = 100 ppm in rats. In contrast, 1,1-difluoroethane (commonly referred to as HFC-152a) has very low acute toxicity with a 4-hr LC50 of > 400,000 ppm in rats. Rats exposed to the selected toxic fluoroethanes showed clinical signs of fluoroacetate toxicity (lethargy, hunched posture, convulsions). 1,2-Difluoroethane, 1-chloro-2-fluoroethane, 1-chloro-1,2-difluoroethane, and 1-bromo-2-fluoroethane were shown to increase concentrations of citrate in serum and heart tissue, a hallmark of fluoroacetate intoxication. 19F NMR analysis confirmed that fluoroacetate was present in the urine of rats exposed to each toxic compound. Fluorocitrate, a condensation product of fluoroacetate and oxaloacetate, was identified in the kidney of rats exposed to 1,2-difluoroethane. There was a concentration-related elevation of serum and heart citrate in rats exposed to 0-1000 ppm 1,2-fluoroethane. Serum citrate was increased up to 5-fold and heart citrate was increased up to 11-fold over control citrate levels. Metabolism of 1,2-difluoroethane by cytochrome P450 (most likely CYP2E1) is suspected because pretreatment of rats or mice with SKF-525F, disulfiram, or dimethyl sulfoxide prevented or delayed the toxicity observed in rats not pretreated. Experimental evidence indicates that the metabolism of the toxic fluoroethanes is initiated at the carbon-hydrogen bond, with metabolism to fluoroacetate via an aldehyde or an acyl fluoride. The results of these studies show that 1-(di)halo-2-fluoroethanes are highly toxic to rats and should be considered a hazard to humans unless demonstrated otherwise.

High resolution NMR techniques in catalysis

High resolution NMR techniques are applicable to a variety of aspects of catalysis. Methods for studying homogeneously-catalyzed systems under high gas pressure are described along with approaches for obtaining mechanistic and dynamic information. Many of the same techniques may be applied to heterogeneous catalysis by following the reaction chemistry by gas phase NMR.

An unusual rotational distortion in an [(η-indenyl)RhL2] complex: molecular structures of [(η-1,2,3-Me3C9H4)Rh(η-C2H4)2] and [(η-C9H7)Rh(CO)2]

The molecular structures of [(η-1,2,3-Me3C9H4)Rh(η-C2H4)2] and [(η-C9H7)Rh(CO)2], barriers to hindered ethylene rotation in a series of indenyl rhodium bis(ethylene) complexes bearing different degrees of indenyl-ring methylation, and the barrier to hindered indenyl-ring rotation in [(η-1-MeC9H6)Rh(CO)2] are reported.

Studies on Organoaluminum Precursors OP Aluminum Nitride Fibers

A melt-spinnable precursor of aluminum nitride fibers derived from triethylaluminum and ammonia contains AlNH, AlNH 2 groups, and a small number of AlN units characteristic of aluminum nitride. The molecular weight of a spinnable composition is 070, corresponding to an average molecular weight of 13 organoaluminum groups. Ammonia, a curing agent for the fibers, accelerates elimination of ethane from the material, and decreases its solubility in toluene.

Synthesis of the new metalloligands [η6-(PPh2)xC6H6-x]Cr(CO)2(L) [x = 1,2; L = CO and PR3] and some rhodium(I) complexes

Abstract Transmetallation of {η 6 -1,4-(Bu n 3 Sn) 2 C 6 H 4 }Cr(CO) 2 (PBu n 3 ) with n BuLi followed by treatment with diphenylchlorophosphine afforded as the major product {η 6 -1,4-(PPh 2 ) 2 C 6 H 4 }Cr(CO) 2 (PBu n 3 ) ( 4 ) and a minor amount of {η 6 -1-(SnBu n 3 )-4-(PPh 2 )C 6 H 4 }Cr(CO) 2 (PBu n 3 ) ( 5 ). Treatment of {η 6 -1,4-(PPh 2 ) 2 C 6 H 4 }Cr(CO) 2 (L) ( 9 , L = CO; 4 , L = PBu n 3 ) with [(CO) 2 RhCl] 2 liberated carbon monoxide and gave the heterobimetallic coordination polymers {[η 6 -1,4-(PPh) 2 C 6 H 4 ]Cr(CO) 2 L]Rh(CO)Cl} n ( 10a , L = CO; 10b , L = PBu 6 3 ). Coordination of both PPh 2 substituents to rhodium led to hindered rotation about the Cr-(η 6 -arene) axis (ΔG ‡ = 11.7 ± 0.1 kcal mol −1 for 10b , estimated from the 31 P DNMR spectra). The transmetallation chemistry was also used to prepare {η 6 -(PPh 2 )C 6 H 5 }Cr(CO) 2 (L) ( 8a , L = CO; 8b , L = PPh 3 ] in excellent yields.

Homogeneous Models for Mechanisms of Surface Reactions: Propene Ammoxidation

The proposed active sites on the surface of the catalyst for propene ammoxidation have been successfully modeled by structurally characterized pinacolato W(VI) tert-butyl imido complexes. These compounds exist as an equilibrating mixture of amine-bis(imido) and imido-bis(amido) complexes, the position of this equilibrium being dependent on the electronic nature of the glycolate ligand. Both of the C-N bond-forming reactions proposed in recent studies by Kartisek and Grasselli [J. Catal. 81, 489 (1983)] have been reproduced using discrete Group VI d° organoimido complexes under mild conditions suitable for detailed mechanistic studies. These reactions are 1) oxidative trapping of radicals at molybdenum imido sites and 2) migration of the allyl group from oxygen to an imido nitrogen atom.