Publications (11)41.87 Total impact
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Article: Reactivity of coordinatively unsaturated iron complexes towards carbon monoxide: to bind or not to bind?
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ABSTRACT: An overview of the reactivity of coordinatively unsaturated iron complexes (in most cases Fe(II)) towards carbon monoxide is presented. Unsaturated iron complexes are known with coordination numbers (CN) of two to five adopting linear or slightly bent (CN = 2), trigonal (CN = 3), tetrahedral, square planar or trigonal pyramidal (CN = 4), and square-pyramidal or trigonal-bipyramidal geometries (CN = 5), respectively. The binding of CO depends strongly on the number and the nature of co-ligands (overall ligand field strength), the charge of the complex, the complex geometry, and the spin state of the unsaturated metal center. In many cases, CO addition to high-spin iron complexes takes place with concomitant spin state changes forming compounds in the lowest possible spin state, i.e., with S = 0. In several other cases, however, the addition of CO is reversible or is even totally rejected altogether for either thermodynamic or kinetic reasons. In the case of the latter such reactions are termed "spin-blocked" or "spin forbidden".Dalton Transactions 03/2011; 40(18):4778-92. · 3.84 Impact Factor -
Article: Kinetically Controlled Formation of Octahedral trans-Dicarbonyl Iron(II) PNP Pincer Complexes: The Decisive Role of Spin-State Changes§
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ABSTRACT: Treatment of either cis-[Fe(PNP)(X2)(CO)], trans-[Fe(PNP)(X2)(CO)], or [Fe(PNP)X2] (X = Cl, Br; PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine) with 1 equiv of AgBF4 in the presence of CO afforded selectively octahedral iron(II) complexes of the type trans-[Fe(PNP)(CO)2X]+. The same reaction carried out with trans-[Fe(PNP-iPr)(Cl)2(CO)] in the absence of CO affords also trans-[Fe(PNP-iPr)(CO)2Cl]+ together with unidentified paramagnetic species. This reaction involves an intermolecular CO transfer between coordinately unsaturated [Fe(PNP-iPr)(CO)(Cl)]+ intermediates. In all reactions studied, there was no evidence for the formation of cis dicarbonyl complexes. X-ray structures of representative complexes are presented. A detailed mechanism, based on DFT/B3LYP calculations, is presented, suggesting that upon irreversible removal of X− transient cationic intermediates [Fe(PNP)(CO)(X)]+ of two conformations, one with the CO in the apical and the halide in the basal position (A) and vice versa (B), are formed. These adopt a singlet ground state in the case of A and a triplet ground state in the case of B. The formation of trans-[Fe(PNP)(CO)2X]+ is kinetically controlled, with A in the singlet ground state being the key intermediate. Pathways originating from complexes with a triplet ground state are “spin-blocked” (spin forbidden) or thermodynamically disfavored.04/2010; -
Article: Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide
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ABSTRACT: Several new iron(II) complexes of the types [Fe(PNP)X2] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe−X···H−N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis- and trans-[Fe(PNP)(CO)(Cl)2], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis- or trans-[Fe(PNP)(CO)(X)2] leads to complete regeneration of analytically pure [Fe(PNP)(X)2]. Mössbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s−1) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s−1) or in solution (δ = 0.15(1) mm s−1). Magnetization studies led to a magnetic moment close to 4.9 μB, reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl2], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (≤3.4 kcal mol−1). The cis−trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol−1), after initial loss of chloride, and involves also spin-state changes12/2009; -
Article: Stereospecific and reversible CO binding at iron pincer complexes.
Angewandte Chemie International Edition 11/2008; 47(47):9142-5. · 13.45 Impact Factor -
Article: Modularly designed transition metal PNP and PCP pincer complexes based on aminophosphines: synthesis and catalytic applications.
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ABSTRACT: Transition metal complexes are indispensable tools for any synthetic chemist. Ideally, any metal-mediated process should be fast, clean, efficient, and selective and take place in a catalytic manner. These criteria are especially important considering that many of the transition metals employed in catalysis are rare and expensive. One of the ways of modifying and controlling the properties of transition metal complexes is the use of appropriate ligand systems, such as pincer ligands. Usually consisting of a central aromatic backbone tethered to two two-electron donor groups by different spacers, this class of tridentate ligands have found numerous applications in various areas of chemistry, including catalysis, due to their combination of stability, activity, and variability. As we focused on pincer ligands featuring phosphines as donor groups, the lack of a general method for the preparation of both neutral (PNP) and anionic (PCP) pincer ligands using similar precursor compounds as well as the difficulty of introducing chirality into the structure of pincer ligands prompted us to investigate the use of amines as spacers between the aromatic ring and the phosphines. By introduction of aminophosphine and phosphoramidite moieties into their structure, the synthesis of both PNP and PCP ligands can be achieved via condensation reactions between aromatic diamines and electrophilic chlorophosphines (or chlorophosphites). Moreover, chiral pincer complexes can be easily obtained by using building blocks obtained from the chiral pool. Thus, we have developed a modular synthetic strategy with which the steric, electronic, and stereochemical properties of the ligands can be varied systematically. With the ligands in hand, we studied their reactivity towards different transition metal precursors, such as molybdenum, ruthenium, iron, nickel, palladium, and platinum. This has resulted in the preparation of a range of new pincer complexes, including various iron complexes, as well as the first heptacoordinated molybdenum pincer complexes and several pentacoordinated nickel complexes by using a controlled ligand decomposition pathway. In addition, we have investigated the use of some of the complexes as catalysts in different C-C coupling reactions: for example, the palladium PNP and PCP pincer complexes can be employed as catalysts in the well known Suzuki-Miyaura coupling, while the iron PNP complexes catalyze the coupling of aromatic aldehydes with ethyl diazoacetate under very mild reaction conditions to give selectively 3-hydroxyacrylates, which are otherwise difficult to prepare. While this Account presents an overview of current research on the chemistry of P-N bond containing pincer ligands and complexes, we believe that further investigations will give deeper insights into the reactivity and applicability of aminophosphine-based pincer complexes.Accounts of Chemical Research 03/2008; 41(2):201-13. · 21.64 Impact Factor -
Article: Iron(II) Complexes Bearing Tridentate PNP Pincer-Type Ligands as Catalysts for the Selective Formation of 3-Hydroxyacrylates from Aromatic Aldehydes and Ethyldiazoacetate
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ABSTRACT: Several new iron(II) complexes of the types [Fe(PNP)(CO)(CH3CN)2]2+, [Fe(PNP)(CO)2Br]+, and Fe(PNP)Cl2 containing tridentate PNP pincer-type ligands have been prepared and characterized. Some of them turned out to be efficient catalysts for the coupling of aromatic aldehydes with ethyldiazoacetate (EDA) to give 3-hydroxyacrylates. These reactions are highly chemoselective, proceed under mild conditions, exhibit good scope, and circumvent the need of slow addition of EDA at low temperature.11/2006; -
Article: Selective Phosphoramidite Cleavage as a Route to Novel Chiral and Achiral Pentacoordinated Nickel(II) PNP Pincer Complexes
Berichte der deutschen chemischen Gesellschaft 09/2006; 2006(21):4374 - 4379. · 2.94 Impact Factor -
Article: A Modular Approach to Achiral and Chiral Nickel(II), Palladium(II), and Platinum(II) PCP Pincer Complexes Based on Diaminobenzenes
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ABSTRACT: The synthesis and characterization of a series of nickel, palladium, and platinum complexes containing new achiral and chiral PCP pincer ligands based on 1,3-diaminobenzene, 5-trifluoromethyl-1,3-diaminobenzene, and 3,5-diamino-4-chloroisobutylbenzoate are reported. The new PCP ligands are prepared conveniently in high yield by treatment of the respective diaminobenzene with 2 equiv of a variety of achiral and chiral R2PCl compounds in the presence of base. PCP complexes of Ni(II), Ni(PCP)Cl, were synthesized by the reaction of NiCl2·6H2O with 1 equiv of a PCP ligand. In similar fashion, treatment of M(COD)X2 (M = Pd, Pt; X = Cl, Br) with 1 equiv of a PCP ligand yields the square-planar complexes M(PCP)X. Palladium PCP complexes featuring a coordinated TFA ligand (TFA = CF3COO-) are obtained by the reaction of Pd(TFA)2 with 1 equiv of a PCP ligand. Alternatively, palladium PCP complexes can also be generated via an oxidative addition route. Addition of 2 equiv of PCP ligands based on 3,5-diamino-4-chloroisobutylbenzoate to Pd2(dba)3 affords the respective Pd(PCP)Cl pincer complexes in high yields. X-ray structures of representative Ni, Pd, and Pt PCP complexes have been determined. Finally, the use of the palladium complexes as catalysts for the Suzuki−Miyaura coupling of some aryl bromides and phenyl boronic acid has been examined.06/2006; -
Article: Achiral and Chiral Transition Metal Complexes with Modularly Designed Tridentate PNP Pincer-Type Ligands Based on N-Heterocyclic Diamines
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ABSTRACT: The synthesis and characterization of a series of molybdenum, iron, ruthenium, nickel, palladium, and platinum complexes containing new achiral and chiral PNP pincer-type ligands based on the N-heterocyclic diamines 2,6-diaminopyridine, N,N‘-di-10-undecenyl-2,6-diaminopyridine, N,N‘-dihexyl-2,6-diaminopyridine, and 2,6-diamino-4-phenyl-1,3,5-triazine are reported. The new PNP ligands are prepared conveniently in high yield by treatment of the respective N-heterocyclic diamines with 2 equiv of a variety of achiral and chiral R2PCl compounds in the presence of base. Molybdenum PNP complexes of the type [Mo(PNP)(CO)3PNP] are obtained by treatment of [Mo(CO)3(CH3CN)3] with 1 equiv of the respective PNP ligand. They were found to react with I2 to give novel seven-coordinate pincer complexes of the types [Mo(PNP)(CO)3I]+ and [Mo(PNP)(CO)2(CH3CN)I]+ depending of whether the reaction is carried out in CH2Cl2 or CH3CN. With [Fe(H2O)6](BF4)2 and 1 equiv of PNP ligand in acetonitrile dicationic complexes of the type [Fe(PNP)(CH3CN)3](BF4)2 are obtained. The cis and trans dichloride complexes [Ru(PNP)(PPh3)Cl2] are prepared by a ligand exchange reaction of [RuCl2(PPh3)3] with a stoichiometric amount of the respective PNP ligand. Cationic PNP complexes of Ni(II), [Ni(PNP)Br]Br, were synthesized by the reaction of [NiBr2(DME)] with 1 equiv of PNP ligand. In similar fashion, treatment of [M(COD)X2] (M = Pd, Pt; X = Cl, Br) with 1 equiv of PNP ligand yields the cationic square-planar complexes [M(PNP)X]X. If the reaction is carried out in the presence of the halide scavenger KCF3SO3, complexes of the type [M(PNP)X]CF3SO3 are obtained, which are better soluble in nonpolar solvents than the analogous halide compounds. X-ray structures of representative Mo, Fe, Ru, Ni, and Pd PNP complexes have been determined. Finally, the use of the palladium complexes as catalysts for the Suzuki−Miyaura coupling of some aryl bromides and phenyl boronic acid has been examined.03/2006; -
Article: Synthesis and Reactivity of Palladium and Nickel β-Diimine Complexes: Application as Catalysts for Heck, Suzuki, and Hiyama Coupling Reactions†
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ABSTRACT: The synthesis of a range of sterically hindered β-diimine ligands and their complexes with palladium(II) and nickel(II) of the formula Pd(κ2N,N-RNCHC(CH2)n−CHNR)(Cl2) (R = 2,6-i-Pr2Ph, 2,6-Me2Ph, Cy, t-Bu, n = 4, 5) and Ni(κ2N,N-RNCHC(CH2)4−CHNR)(Br2) (R = 2,6-i-Pr2Ph, 2,6-Me2Ph) has been investigated. Representative X-ray structures of both ligands and complexes have been determined. The use of the palladium complexes as catalysts for Suzuki coupling of aryl halides and arylboronic acids has been examined. In addition, it has been shown that the palladium complexes are also active in the Heck reaction of aryl bromides and methyl acrylate as well as in the Hiyama coupling of aryl halides and phenyltrimethoxysilane.05/2005; -
Article: Synthesis and Characterization of Ni(II) and Pd(II) Complexes Bearing Achiral and Chiral Bidentate Aminophosphine Ligands
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ABSTRACT: The synthesis of a range of achiral and chiral bidentate aminophosphine ligands and their complexes with nickel(II) and palladium(II) has been investigated. The ligands and the complexes have been characterized by NMR spectroscopy, and X-ray structures of representative compounds have been determined. In addition, DFT calculations have been performed to investigate different geometries of the nickel(II) complexes in the solid state and in solution.
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Institutions
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2006–2011
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Vienna University of Technology
- Institute of Applied Synthetic Chemistry
Vienna, Vienna, Austria
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