Vladimir V. Grushin

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.

The trans influence of F, Cl, Br and I ligands in a series of square-planar Pd(II) complexes. Relative affinities of halide anions for the metal centre in trans-[(Ph3P)2Pd(Ph)X]

Abstract Single crystal X-ray diffraction studies of trans-[(Ph3P)2Pd(Ph)X] (X = F (1), Cl (2), Br (3), and I (4) were carried out. The four structures split in two isostructural and isomorphous groups, namely orthorhombic for 1 and 2 (space group Pbca, Z = 8) and triclinic for 3 and 4 (space group P-1, Z = 2). According to the Pdue5f8C bond length, the trans influence of X within these pairs follows the trend Cl>F and 1>Br. However, the trans influence of Cl is slightly stronger than that of Br. Both structural and 13C NMR studies revealed that electron-donating effects of (Ph3P)2PdX increase along the series X=I Cl− > Br− > I− is characteristic of halide preference for the Pd complexes. Dissolving 1 and PPN Cl in dry CH2Cl2 resulted in the release of ‘naked’ F− which fluorinated the solvent smoothly to give a mixture of CH2ClF and CH2F2 in high yield. When chloroform was used instead of CH2Cl2, dichlorocarbene was generated slowly, forming the corresponding cyclopropane in the presence of styrene. All observations were rationalized successfully in terms of the filled/filled effect and push/pull interactions.

PALLADIUM(II) COMPLEXES OF HYBRID PHOSPHINE-PHOSPHINE OXIDE LIGANDS

Abstract Complexes of the type [L2PdCl2] , where L = Ph2PCH2P (O) Ph2 (dppmO) , Ph2P (CH2) 2P (O) Ph2 (dppeO) , Ph2P (CH2) 3P (O) Ph2 (dpppO) , Ph2P (CH2) 4P (O) Ph2 (dppbO) , and Ph2PC5H4FeC5H4P (O) Ph2 (dppfcO) , were prepared in high yield from Na2 [PdCl4] and 2 equiv. of the corresponding L in dichloromethane-methanol. All complexes, except for [ (dppfcO) 2PdCl2] , can exist as the cis and trans isomers, or mixtures of both, with the cis⧹trans ratio decreasing in media of low polarity. The ferrocene containing complex, [ (dppfcO) 2PdCl2] , appeared to be exclusively trans in both the solid state and solution, as established by single crystal X-ray diffraction of its 1 : 3 benzene solvate. The ion exchange extraction of dichloromethane solutions of [ (dppmO) 2PdCl2] and [ (dppeO) 2PdCl2] with aqueous NaBF4 furnished the corresponding cationic P,O-chelates, cis- [ (dppmO) 2Pd] 2 (BF−4)2 and cis- [ (dppeO) 2Pd] 2 (BF−4)2. Alternatively, these cations can be generated by the reaction between the chloro complexes, [ (dppmO) 2PdCl2] and [ (dppeO) 2PdCl2] , with Ag+. Single crystal X-ray diffraction of cis- [ (dppeO) 2Pd] 2 (BF−4)2 revealed distinct conformations (a puckered chair and a distorted sofa) for the two chelate rings situated around each Pd atom. A polymeric, poorly soluble material, { [ (dpppO) 2Pd] 2 (BF−4)2}n, was isolated from the reaction between [ (dppO) 2PdCl2] and AgNO3⧹ NaBF4, with both coordinated and free phosphoryl groups being present in the product (IR) . In dilute solutions of the { [ (dpppO) 2Pd] 2 (BF−4)2}n, depolymerization occurred, giving rise to the monomeric cationic chelate, cis- [ (dpppO) 2Pd] 2(BF−4)2.

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)].

Generation of “Naked” Fluoride Ions in Unprecedentedly High Concentrations from a Fluoropalladium Complex

An F- /Cl- ligand exchange in the stable organopalladium fluoro complex 1 generates naked fluoride ions in unusually high concentrations. The released F- readily fluorinates dichloromethane under exceedingly mild conditions and deprotonates chloroform to produce dichlorocarbene.

Nucleophile-catalyzed, facile, and highly selective C-H activation of fluoroform with Pd(II).

Exceedingly facile (23 °C) and chemoselective H-CF3 activation with [(dppp)Pd(Ph)(OH)] in the presence of a Lewis base promoter such as n-Bu3P leads to Pd-CF3 bond formation in nearly quantitative yield. A combined experimental and computational study points to a new mechanism that involves H-bonding Pd-O(H)···H-CF3 and nucleophilic attack of the promoter on the metal, followed by a push-pull-type collapse of the resultant five-coordinate Pd(II) intermediate via a polar transition state.

Exceedingly Facile Ph-X Activation (X = Cl, Br, I) with Ruthenium(II): Arresting Kinetics, Autocatalysis, and Mechanisms.

Abstract [(Ph3P)3Ru(L)(H)2] (where L=H2 (1) in the presence of styrene, Ph3P (3), and N2 (4)) cleave the Ph—X bond (X=Cl, Br, I) at RT to give [(Ph3P)3RuH(X)] (2) and PhH. A combined experimental and DFT study points to [(Ph3P)3Ru(H)2] as the reactive species generated upon spontaneous loss of L from 3 and 4. The reaction of 3 with excess PhI displays striking kinetics which initially appears zeroth order in Ru. However mechanistic studies reveal that this is due to autocatalysis comprising two factors: 1)u2005complex 2, originating from the initial PhI activation with 3, is roughly as reactive toward PhI as 3 itself; and 2)u2005the Ph—I bond cleavage with the just‐produced 2 gives rise to [(Ph3P)2RuI2], which quickly comproportionates with the still‐present 3 to recover 2. Both the initial and onward activation reactions involve PPh3 dissociation, PhI coordination to Ru through I, rearrangement to a η2‐PhI intermediate, and Ph—I oxidative addition.

The first perfluoroacetylacetonate metal complexes: as unexpectedly robust as tricky to make

First metal complexes containing the perfluoroacetylacetonato ligand have been prepared by the reaction of anhydrous Ln(OAc)3 (Ln = Eu, Tb, Tm) with heptafluoroacetylacetone, optionally in the presence of other ligands Ph3PO, bpy, and PyO.