Peter H. M. Budzelaar

Efficient palladium catalysts for the carbonylation of alkynes

Abstract A class of highly efficient homogeneous palladium catalysts has been developed for the carbonylation of alkynes. One application of interest is the selective production of methyl methacrylate by methoxycarbonylation of propyne. The essential feature of the new catalyst systems is that they are formed by the combination of a ligand containing a 2-pyridylphosphine moiety with a palladium(II) species and a proton source containing weakly coordinating anions. High turn-over numbers of more than 40000 mol (mol Pd) −1 h −1 and selectivities towards methyl methacrylate of up to 99.95% can be obtained under mild conditions. It is suggested that the 2-pyridylphosphine ligand plays an essential role both as a chelating PN ligand in the selectivity-determining step and as a monocoordinated ligand in the rate-determining step of the catalytic cycle.

β‐Diiminato Complexes of VIII and TiIII – Formation and Structure of Stable Paramagnetic Dialkylmetal Compounds

(Mono-β-diiminato)titanium(III) and -vanadium(III) dichlorides LMCl2 [L = ArNC(R)CHC(R)NAr–] are easily accessible from the metal trichlorides and LLi in THF. The crystal structures of LVCl2 (Ar = 2,6-iPr2C6H3, R = Me) and LTiCl2 (Ar = 2,4,6-Me3C6H2, R = Me and Ar = 2,4,6-Me3C6H2, R = tBu) reveal tetrahedral metal environments. Treatment of LVCl2 with alkyllithium reagents affords surprisingly stable dialkylvanadium(III) compounds; the structure of LV(nBu)2 (Ar = 2,6-iPr2C6H3, R = Me) is similar to that of the dichloride. The corresponding dialkyltitanium(III) compounds are less stable; only the dimethyl derivatives could be obtained in pure form (from LTiCl2 and MeMgI), and only for ligands bearing 2,6-disubstituted aryl groups. The structure of LTiMe2 (Ar = 2,4,6-Me3C6H2, R = Me) is also similar to that of the dichloride. Reaction of LTiMe2 with B(C6F5)3 produces a catalyst for α-olefin polymerization, but the corresponding VIII derivatives are inactive.

Homogeneous catalysis by cationic palladium complexes. Precision catalysis in the carbonylation of alkynes

Abstract A class of highly efficient homogeneous palladium cationic catalysts has been developed for the carbonylation of alkynes. An interesting application is

Redox-active ligands and organic radical chemistry.

Knowledge about bonding in diiminepyridine (L) halide, alkyl, and dinitrogen complexes of the metals iron, cobalt, and nickel is summarized, and two new examples are added to the set: L(1)Ni(Me) and L(1)Ni(N(2)). Reactivity of these types of complexes is discussed in terms of organic radical chemistry. New C-C couplings with L(2)CoAr complexes are described and proposed to involve halide abstraction and radical coupling. Calculations support the high tendency of the diiminepyridine ligand to accept an electron coming from a metal-carbon bond and so facilitate loss of a radical.

Multiple Pathways for Dinitrogen Activation during the Reduction of an Fe Bis(iminepyridine) Complex

Reduction of the bis(iminopyridine) FeCl(2) complex {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}FeCl(2) using NaH has led to the formation of a surprising variety of structures depending on the amount of reductant. Some of the species reported in this work were isolated from the same reaction mixture, and their structures suggest the presence of multiple pathways for dinitrogen activation. The reaction with 3 equiv of NaH afforded {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(20PhN-C=CH(2)](C(5)H(3)N)}Fe(micro,eta(2)-N(2))Na (THF) (1) containing one N(2) unit terminally bound to Fe and side-on attached to the Na atom. In the process, one of the two imine methyl groups has been deprotonated, transforming the neutral ligand into the corresponding monoanionic version. When 4 equiv were employed, two other dinitrogen complexes {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe(micro-N2)Na(Et(2)O)(3) (2) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(micro-N(2))Na[Na(THF)(2)] (3) were obtained from the same reaction mixture. Complex 2 is chemically equivalent to 1, the different degree of solvation of the alkali cation being the factor apparently responsible for the sigma-bonding mode of ligation of the N(2) unit to Na, versus the pi-bonding mode featured in 1. In complex 3, the ligand remains neutral but a larger extent of reduction has been obtained, as indicated by the presence of two Na atoms in the structure. A further increase in the amount of reductant (12 equiv) afforded a mixture of {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe-N(2) (4) and [{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe-N(2)](2)(micro-Na) [Na(THF)(2)](2) (5) which were isolated by fractional crystallization. Complex 4, also containing a terminally bonded N(2) unit and a deprotonated anionic ligand bearing no Na cations, appears to be the precursor of 1. The apparent contradiction that excess NaH is required for its successful isolation (4 is the least reduced complex of this series) is most likely explained by the formation of the partner product 5, which may tentatively be regarded as the result of aggregation between 1 and 3 (with the ligand system in its neutral form). Finally, reduction carried out in the presence of additional free ligand afforded {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(eta(1)-N(2)){2,6-[2,6-(iPr)(2)PhN=C(CH(3))](20(NC(5)H(2))}[Na(THF)(2)] (6) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(NC(5)H(2))}Na(THF)(2)) (7). In both species, the Fe metal is bonded to the pyridine ring para position of an additional (L)Na unit. Complex 6 chemically differs from 7 (the major component) only for the presence of an end-on coordinated N(2).

The oxo-synthesis catalyzed by cationic palladium complexes, selectivity control by neutral ligand and anion

Abstract Catalyst systems consisting of a palladium(II) diphosphine complex with weakly or non-coordinating counterions are efficient catalysts for the hydrocarbonylation of both aliphatic and functionalized olefins. Moreover, variations of ligand, anion and/or solvent can be used to steer the reaction towards alcohols, aldehydes, ketones or oligoketones. Non-coordinating anions and arylphosphine ligands produce primarily (oligo)ketones; increasing ligand basicity or anion coordination strength shifts selectivity towards aldehydes and alcohols. For the mechanisms of the aldehyde-producing step, we propose heterolytic dihydrogen cleavage, assisted by the anion. At high electrophilicity of the palladium center, selective ketone formation is observed. The reactions described here constitute the first examples of selective formation of ketones by hydrocarbonylation of higher olefins.

Rhodium and Iridium β‐Diiminate Complexes – Olefin Hydrogenation Step by Step

The bulky β-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2)]– (X = Me, LMe; X = Cl, LCl) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14- complex LMeRh(COE) (COE = cyclooctene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LMeRh(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3-dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of LMe and LCl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β-diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.

Geometry optimization using generalized, chemically meaningful constraints

An external geometry optimizer (BOptimize) is described that can be used together with a number of existing quantum‐chemical codes (Gaussian, Gamess‐UK, Turbomole, ADF, Orca, Priroda, Spartan‐PM3, Mopac) and allows flexible and general constrained optimizations. Some details of the implementation are discussed, and examples are provided of constrained optimizations that would be difficult or impossible to perform with existing optimizers.

Structure-activity relationship in olefin polymerization catalysis: is entropy the key?

Activation parameters for propene polymerization mediated by a bis(phenoxyamine)Zr-dibenzyl catalyst in combination with MAO have been measured experimentally and calculated by DFT; experiment and calculation consistently indicate that the entropic term is the most important reason for the low chain propagation rate with this system. Based on this finding and a review of literature data on a variety of olefin polymerization catalysts, we propose a strong correlation between the propagation rate and how catalysts deal with the entropy loss of monomer capture.

Mimicking Ziegler-Natta Catalysts in Homogeneous Phase, 1. C2-Symmetric Octahedral Zr(IV) Complexes with Tetradentate [ONNO]-Type Ligands

Results of propene polymerization in the presence of two known octahedral C2-symmetric Zr complexes bearing tetradentate [ONNO]-type ligands are reported for the first time. Depending on the steric hindrance at the active metal, isotactic site-controlled or weakly syndiotactic chain-end-controlled polymers were obtained, in both cases via highly regioselective 1,2 (primary) monomer insertion. In this respect, the complexes mimic the behavior of the active Ti species on the surface of the heterogeneous Ziegler-Natta catalysts of which they might represent good structural models.