Christian M. Frech

Dichloro-bis(aminophosphine) complexes of palladium: highly convenient, reliable and extremely active suzuki-miyaura catalysts with excellent functional group tolerance.

Dichloro-bis(aminophosphine) complexes are stable depot forms of palladium nanoparticles and have proved to be excellent Suzuki-Miyaura catalysts. Simple modifications of the ligand (and/or the addition of water to the reaction mixture) have allowed their formation to be controlled. Dichlorobis[1-(dicyclohexylphosphanyl)piperidine]palladium (3), the most active catalyst of the investigated systems, is a highly convenient, reliable, and extremely active Suzuki catalyst with excellent functional group tolerance that enables the quantitative coupling of a wide variety of activated, nonactivated, and deactivated and/or sterically hindered functionalized and heterocyclic aryl and benzyl bromides with only a slight excess (1.1-1.2 equiv) of arylboronic acid at 80 degrees C in the presence of 0.2 mol % of the catalyst in technical grade toluene in flasks open to the air. Conversions of >95 % were generally achieved within only a few minutes. The reaction protocol presented herein is universally applicable. Side-products have only rarely been detected. The catalytic activities of the aminophosphine-based systems were found to be dramatically improved compared with their phosphine analogue as a result of significantly faster palladium nanoparticle formation. The decomposition products of the catalysts are dicyclohexylphosphinate, cyclohexylphosphonate, and phosphate, which can easily be separated from the coupling products, a great advantage when compared with non-water-soluble phosphine-based systems.

Suzuki Cross‐Coupling Reactions Catalyzed by an Aliphatic Phosphine‐Based Pincer Complex of Palladium: Evidence for a Molecular Mechanism

A new phosphine‐based pincer complex with an adamantyl core, [{C10H13‐1,3‐(CH2PCy2)2}Pd(Cl)] (1), has proved to be an excellent Suzuki catalyst. Catalyst 1 enables the quantitative coupling of a wide variety of electronically activated, deactivated, and/or sterically hindered and highly functionalized aryl bromides with phenylboronic acid in pure water with NaOH as base within very short reaction times and low catalyst loadings and without the need for exclusion of air. Hydrophobic substrates, which lead to inefficient conversions in aqueous solution, are efficiently and quantitatively coupled by 1 in toluene with K3PO4 as base. Mechanistic investigations indicate that palladium nanoparticles are probably not the catalytically active form of 1. Experimental results strongly indicate that the phenyl pincer complex [{C10H13‐1,3‐(CH2PCy2)2}Pd(Ph)] (3) is a key intermediate in the catalytic cycle of the Suzuki reaction in both toluene and aqueous solution. Treatment of 1 with phenylboronic acid exclusively yields 3 under catalytic reaction conditions. Moreover, stoichiometric reactions of 3 with aryl bromides lead to the exclusive formation of [{C10H13‐1,3‐(CH2PCy2)2}Pd(Br)] (2) and the corresponding biaryl, thus indicating that biaryl formation occurs either by oxidative addition of aryl bromides to 3, to form neutral hexacoordinated pincer‐type PdIV intermediates with the general formula of [{C10H13‐1,3‐(CH2PCy2)2}Pd(Ar)(Br)(Ph)], followed by reductive elimination of the coupling products or by direct biaryl formation on the PdII center of 3, via a four‐centered transition state.

Alkyne Hydrothiolation Catalyzed by a Dichlorobis(aminophosphine) Complex of Palladium: Selective Formation of cis-Configured Vinyl Thioethers

Cis all round: Dichlorobis[1-(dicyclohexylphosphanyl)piperidine]palladium, [(P{(NC(5)H(10))(C(6)H(11))(2)})(2)Pd(Cl)(2)], is a highly efficient alkyne hydrothiolation catalyst and the first generally applicable system that selectively generates cis-configured anti-Markovnikov adducts in excellent yields within only a few minutes at 120 °C in the presence of only 0.05 mol % of the catalyst (see scheme).

[Pd(Cl)2{P(NC5H10)(C6H11)2}2]--a highly effective and extremely versatile palladium-based Negishi catalyst that efficiently and reliably operates at low catalyst loadings.

[Pd(Cl)(2){P(NC(5)H(10))(C(6)H(11))(2)}(2)] (1) has been prepared in quantitative yield by reacting commercially available [Pd(cod)(Cl)(2)] (cod=cyclooctadiene) with readily prepared 1-(dicyclohexylphosphanyl)piperidine in toluene under N(2) within a few minutes at room temperature. Complex 1 has proved to be an excellent Negishi catalyst, capable of quantitatively coupling a wide variety of electronically activated, non-activated, deactivated, sterically hindered, heterocyclic, and functionalized aryl bromides with various (also heterocyclic) arylzinc reagents, typically within a few minutes at 100 °C in the presence of just 0.01 mol% of catalyst. Aryl bromides containing nitro, nitrile, ether, ester, hydroxy, carbonyl, and carboxyl groups, as well as acetals, lactones, amides, anilines, alkenes, carboxylic acids, acetic acids, and pyridines and pyrimidines, have been successfully used as coupling partners. Furthermore, electronic and steric variations are tolerated in both reaction partners. Experimental observations strongly indicate that a molecular mechanism is operative.

Pincer‐Type Heck Catalysts and Mechanisms Based on PdIV Intermediates: A Computational Study

Pincer-type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd(II) centers, controversial Pd(II)/Pd(IV) cycles have been often proposed as potential catalytic mechanisms. However, pincer-type Pd(IV) intermediates have never been experimentally observed, and computational studies to support the proposed Pd(II)/Pd(IV) mechanisms with pincer-type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd(IV) intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts with styrene and phenyl bromide as substrates and (E)-stilbene as coupling product. The potential-energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd(II)/Pd(IV) mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)] [X=NH, R=piperidinyl (1 a); X=O, R=piperidinyl (1 b); X=O, R=iPr (1 c); X=CH(2), R=iPr (1 d)] to yield cationic, three-coordinate, T-shaped 14e(-) palladium intermediates of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd](+) (2). An alternative reaction path to generate complexes of type 2 (relevant for electron-poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)(CH(2)=CHPh)] (4) and consecutive dissociation of the chloride ligand to yield cationic square-planar styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CH(2)=CHPh)](+) (6) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd(IV) complexes of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))](+) (11), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))(CH(2)=CHPh)](+) (12). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(CHPhCH(2)Ph)](+) (13). Subsequent beta-hydride elimination induces direct HBr liberation to yield cationic, square-planar (E)-stilbene complexes with general formula [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CHPh=CHPh)](+) (14). Subsequent liberation of (E)-stilbene closes the catalytic cycle.

Cyanation of Aryl Bromides with K4[Fe(CN)6] Catalyzed by Dichloro[bis{1‐(dicyclohexylphosphanyl)piperidine}]palladium, a Molecular Source of Nanoparticles, and the Reactions Involved in the Catalyst‐Deactivation Processes

Dichloro[bis{1-(dicyclohexylphosphanyl)piperidine}]palladium [(P{(NC(5)H(10))(C(6)H(11))(2)})(2)PdCl(2)] (1) is a highly active and generally applicable C-C cross-coupling catalyst. Apart from its high catalytic activity in Suzuki, Heck, and Negishi reactions, compound 1 also efficiently converted various electronically activated, nonactivated, and deactivated aryl bromides, which may contain fluoride atoms, trifluoromethane groups, nitriles, acetals, ketones, aldehydes, ethers, esters, amides, as well as heterocyclic aryl bromides, such as pyridines and their derivatives, or thiophenes into their respective aromatic nitriles with K(4)[Fe(CN)(6)] as a cyanating agent within 24 h in NMP at 140 °C in the presence of only 0.05 mol % catalyst. Catalyst-deactivation processes showed that excess cyanide efficiently affected the molecular mechanisms as well as inhibited the catalysis when nanoparticles were involved, owing to the formation of inactive cyanide complexes, such as [Pd(CN)(4)](2-), [(CN)(3)Pd(H)](2-), and [(CN)(3)Pd(Ar)](2-). Thus, the choice of cyanating agent is crucial for the success of the reaction because there is a sharp balance between the rate of cyanide production, efficient product formation, and catalyst poisoning. For example, whereas no product formation was obtained when cyanation reactions were examined with Zn(CN)(2) as the cyanating agent, aromatic nitriles were smoothly formed when hexacyanoferrate(II) was used instead. The reason for this striking difference in reactivity was due to the higher stability of hexacyanoferrate(II), which led to a lower rate of cyanide production, and hence, prevented catalyst-deactivation processes. This pathway was confirmed by the colorimetric detection of cyanides: whereas the conversion of β-solvato-α-cyanocobyrinic acid heptamethyl ester into dicyanocobyrinic acid heptamethyl ester indicated that the cyanide production of Zn(CN)(2) proceeded at 25 °C in NMP, reaction temperatures of >100 °C were required for cyanide production with K(4)[Fe(CN)(6)]. Mechanistic investigations demonstrate that palladium nanoparticles were the catalytically active form of compound 1.

Mizoroki–Heck reactions catalyzed by palladium dichloro-bis(aminophosphine) complexes under mild reaction conditions. The importance of ligand composition on the catalytic activity

Dichloro-bis(aminophosphine) complexes of palladium with the general formula [(P{(NC5H10)3−n(C6H11)n})2Pd(Cl)2] (where n = 0–2) are easily accessible, cheap and air stable, highly active and universally applicable C–C cross-coupling catalysts, which exhibit an excellent functional group tolerance. The ligand composition of amine-substituted phosphines (controlled by the number of P–N bonds) was found to effectively determine their catalytic activity in the Heck reaction, for which nanoparticles were demonstrated to be their catalytically active form. While dichloro{bis[1,1′,1′′-(phosphinetriyl)tripiperidine]}palladium (1), the least stable complex (towards protons) within the series of [(P{(NC5H10)3−n(C6H11)n})2Pd(Cl)2] (where n = 0–3), is a highly active Heck catalyst at 100 °C and, hence, a rare example of an effective and versatile Heck catalyst that efficiently operates under mild reaction conditions (100 °C or below), a significant successive drop in activity was noticed for dichloro-bis(1,1′-(cyclohexylphosphinediyl)dipiperidine)palladium (2, with n = 1), dichloro-bis(1-(dicyclohexylphosphinyl)piperidine)palladium (3, with n = 2) and dichloro-bis(tricyclohexylphosphine)palladium (4, with n = 3), of which the latter is essentially inactive (at least under the reaction conditions applied). This trend was explained by the successively increasing complex stability and its ensuing retarding effect on the (water-induced) generation of palladium nanoparticles thereof. This interpretation was experimentally confirmed (initial reductions of 1–4 into palladium(0) complexes of the type [Pd(P{(NC5H10)3−n(C6H11)n})2] (where n = 0–3) were excluded to be the reason for the activity difference observed as well as molecular (Pd0/PdII) mechanisms were excluded to be operative) and thus demonstrates that the catalytic activity of dichloro-bis(aminophosphine) complexes of palladium can – in reactions where nanoparticles are involved – effectively be controlled by the number of P–N bonds in the ligand system.

Dinitrosyl rhenium complexes for ring-opening metathesis polymerization (ROMP)

The treatment of benzene solutions of the cations [Re(NO)2(PR3)2][BArF4] (R = Cy and R = iPr; [BArF4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) with phenyldiazomethane afforded the moderately stable cationic rhenium(I) benzylidene dinitrosyl bis(trialkyl) phosphine complexes as [BArF4]- salts in good yields. The cationic rhenium dinitrosyl bisphosphine complexes catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights, and Z configurations of the double bonds in the polymer chain backbones of over 80 %. The benzylidene derivatives are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave first hints for the initial formation of carbene complexes when [Re(NO)2(PR3)2][BArF4] was treated with norbornene. The carbene formation is initiated by an unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylid carbene complex. The [2+2] addition of a norbornene molecule to the Re=C bond leads to the rhenacyclobutane complex, which is expected to be converted into an iminate complex by attack of the ylid function onto one of the NNO atoms followed by Wittig-type phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. This species is thought to drive the ROMP metathesis with alternating rhenacyclobutane formations and cycloreversions. The proposed mechanism is supported by density functional theory (DFT) calculations.

Reactions within Molecular Single Crystals of Inorganic and Organometallic Compounds: Recent Advances and Implications for Catalysis

Chemical transformations within single crystals have received great general interest. Aside from ecological points of view (green chemistry), such interest is mainly because most device applications, such as molecular machines, motors, and functional materials (chemical sensors for toxic gaseous or volatile organic compounds, for example), require a maximum degree of order and regularity. 2] In addition, chemical reactions might be highly selective inside single crystals, whereas the same reactions in solution are not. However, single crystal-tosingle crystal transformations are in general very rare, because the rearrangement of atoms in the solid state mostly leads to the loss of single crystallinity and the formation of microcrystalline powders as the solid-state reaction proceeds. Single crystal-to-single crystal transformations may be divided into two main categories, of which transformations of porous crystalline compounds such as coordination polymers or metal-organic frameworks comprise the major category. Apart from a few redox reactions, most such transformations involve reversible liberation or exchange of solvent molecules in the coordination sphere or networks. The other category involves reactions within crystals of nonporous molecular compounds. Whereas reactions within organic crystals typically involve photochemically induced coupling reactions of alkenes or alkynes, most reactions within inorganic or organometallic systems involve the reversible loss of a ligand or a metal-coordinated solvent molecule. For example, the trinuclear, acetate-bridged iron complex [Fe3(h -O)(h-CH3COO)6(C5H5NO)2(H2O)]ClO4·3 H2O, [6a] which consist of two iron centers ligated with pyridin-2(1H)-one and one with water, loses the coordinated water molecule under exposure to MeOH vapor at 25 8C. The solvent exchange reaction is reversible and selective towards methanol and water (no solvent exchange took place with other alcohols). However, although probably no real application may be found for simple solvent exchange reactions, the reversible absorption of a gaseous (or volatile) organic compounds may be relevant for gas storage devices and optoelectronic (gas-triggered) switches, as was assumed to be the case for an amine-derived pincer complex of platinum, which reversibly adsorbs SO2, thereby changing color from colorless to orange. However, few examples have been described in which ligands are transferred within molecular single crystals. The first example of a chemical transformation of a metal-coordinated ligand into a new one was reported in 2006 for a cationic rhenium dinitrosyl bisphosphine complex; exposure of single crystals to air was found to result in a stepwise and irreversible oxidation of one of the nitrosyl ligands into a nitrato ligand within two weeks at 25 8C (Scheme 1). X-ray diffraction

Bis[2,6‐bis(dipiperidin‐1‐ylphosphanyloxy)phenyl]bromidopalladium(II)

The title compound, [PdBr(C26H43N4O2P2)], a so-called palladium pincer complex, is a very efficient catalyst for the Suzuki cross-coupling reaction. The Pd atom exhibits a distorted square-planar coordination, typical for PdII complexes.