Karl Kirchner

Modularly Designed Transition Metal PNP and PCP Pincer Complexes based on Aminophosphines: Synthesis and Catalytic Applications

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.

Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride Mn(I) and Fe(II) PNP Pincer Complexes.

Herein, we describe an efficient coupling of alcohols and amines catalyzed by well-defined isoelectronic hydride Mn(I) and Fe(II) complexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alcohol dehydrogenation concept. A range of alcohols and amines including both aromatic and aliphatic substrates were efficiently converted in good to excellent isolated yields. Although in the case of Mn selectively imines were obtained, with Fe-exclusively monoalkylated amines were formed. These reactions proceed under base-free conditions and required the addition of molecular sieves.

Chemistry of coordinatively unsaturated organoruthenium amidinates as entry to homogeneous catalysis

The chemistry of coordinatively unsaturated organoruthenium complexes is reviewed in this article. In particular, the subject is focused on neutral and cationic organoruthenium amidinates, which formally have 16 valence electrons and show signs of coordinative unsaturation. The ruthenium amidinates, ( 5 -C5Me5)Ru(-amidinate) (1), and their isoelectronic analogues, [( 6 -arene)Ru(-amidinate)] + (2), are synthesized and characterized; a possible stabilizing factor of the unsaturated metal center is weak coordination of -electrons in the amidinates ligands. Reactions of various two-electron donor ligands with 1 or 2 reveal the strong -donor property of 1 and Lewis acid nature of 2. One or two-electron redox processes of 1 in the reactions with organic halides are studied by isolation of the corresponding Ru(III) and Ru(IV) products; the results lead to their catalysis for the Tsuji–Trost reaction and the intramolecular Kharasch reaction. The treatment of 2 with trimethylsilyldiazomethane results in the formation of cationic amidinato-carbene complexes, which involve unusual reversible metal-to-carbon silyl group migration.

Efficient Hydrogenation of Ketones and Aldehydes Catalyzed by Well-Defined Iron(II) PNP Pincer Complexes: Evidence for an Insertion Mechanism

We have prepared and structurally characterized a new class of Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)]n (L = Br–, CH3CN, pyridine, PMe3, SCN–, CO, BH4–; n = 0, +1) based on the 2,6-diaminopyridine scaffold where the PiPr2 moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]n with labile ligands (L = Br–, CH3CN, BH4–) and NH spacers are efficient catalysts for the hydrogenation of both ketones and aldehydes to alcohols under mild conditions, while those containing inert ligands (L = pyridine, PMe3, SCN–, CO) are catalytically inactive. Interestingly, complex [Fe(PNPMe-iPr)(H)(CO)(Br)], featuring NMe spacers, is an efficient catalyst for the chemoselective hydrogenation of aldehydes. The first type of complexes involves deprotonation of the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide cooperation, but no reversible aromatization/deprotonation of the PNP ligand. In the case of the N-methylated complex the mechanism remains unclear, but obviously does not allow bifunctional activation of dihydrogen. The experimental results complemented by DFT calculations strongly support an insertion of the C=O bond of the carbonyl compound into the Fe–H bond.

HYDRIDOTRIS(PYRAZOLYL)BORATE RUTHENIUM COMPLEXES : PROPERTIES AND APPLICATIONS

Abstract This review deals with ruthenium complexes of the hydrotris(pyrazolyl)borate (Tp) ligand and derivatives thereof. The period covered is from 1993 to 1998 (along with a few earlier references) including more than 40 new references, the majority of which have been published in the last 3 years. Among the co-ligands are hydride, dihydrogen, dinitrogen, CO, phosphines and amines. Particular emphasis is on complexes containing metal–carbon single and double bonds. Noteworthy is the synthesis of highly reactive vinylidene complexes and their involvement in stoichiometric CC coupling reactions with activated alkanes and olefines. Most recent developments include the application of RuTp complexes in catalytic transformations of organic molecules, such as the dimerization and polymerization of acetylenes, and the hydrogenation of ketones.

Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal–Ligand Cooperation

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)2Cl]+ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)2]+ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H2 to give cis-[Fe(PNP-iPr)(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)2Cl]+. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNPMe-iPr)(CO)2Cl]+ was prepared, showing no reactions with Zn and H2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)2H]+ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)2]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)2Cl]+ or [Fe(PNP-iPr)Cl2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)2] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)2H]+. X-ray structures of both cis-[Fe(PNP-iPr)(CO)2H]+ and [Fe(PNP-iPr)(CO)2] are presented.

Highly Efficient and Selective Hydrogenation of Aldehydes: A Well-Defined Fe(II) Catalyst Exhibits Noble-Metal Activity

The synthesis and application of [Fe(PNPMe-iPr)(CO)(H)(Br)] and [Fe(PNPMe-iPr)(H)2(CO)] as catalysts for the homogeneous hydrogenation of aldehydes is described. These systems were found to be among the most efficient catalysts for this process reported to date and constitute rare examples of a catalytic process which allows selective reduction of aldehydes in the presence of ketones and other reducible functionalities. In some cases, TONs and TOFs of up to 80000 and 20000 h–1, respectively, were reached. On the basis of stoichiometric experiments and computational studies, a mechanism which proceeds via a trans-dihydride intermediate is proposed. The structure of the hydride complexes was also confirmed by X-ray crystallography.

Ruthenium-mediated cyclotrimerization of alkynes utilizing the cationic complex [RuCp(CH3CN)3]PF6

The substitutionally labile cationic complex (RuCp(CH3CN)3) � (as the PFsalt) was tested with respect to its ability to catalyze the cyclotrimerization of terminal alkynes and diynes to afford benzene derivatives. Whereas (RuCp(CH3CN)3) � is in fact promoting the catalytic cyclotrimerization of alkynes, the formation of stable and inert sandwich complexes of the type (RuCp(h 6 - arene)) � deactivates the catalyst and thus quenches the catalytic cycle. All new sandwich complexes were isolated and characterized spectroscopically. A proposal for a plausible catalytic cycle including possible degradation pathways of the catalyst is presented based on DFT calculations. As key intermediates several novel carbene complexes have been identified including metallacyclo- pentatriene and metallaheptatetraene species. # 2003 Elsevier B.V. All rights reserved.

Selective CC Bond Formation between Alkynes Mediated by the [RuCp(PR3)]+ Fragment Leading to Allyl, Butadienyl, and Allenyl Carbene Complexes—An Experimental and Theoretical Study

The reaction of alkynes with [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Me, Ph, Cy) affords, depending on the structure of the alkyne and the substituent of the phosphine ligand, allyl carbene or butadienyl carbene complexes. These reactions involve the migration of the phosphine ligand or a facile 1,2 hydrogen shift. Both reactions proceed via a metallacyclopentatriene complex. If no alpha C[bond]H bonds are accessible, allyl carbenes are formed, while in the presence of alpha C[bond]H bonds butadienyl carbenes are typically obtained. With diphenylacetylene, on the other hand, a cyclobutadiene complex is formed. A different reaction pathway is encountered with HC[triple bond]CSiMe(3), ethynylferrocene (HC[triple bond]CFc), and ethynylruthenocene (HC[triple bond]CRc). Whereas the reaction of [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Ph and Cy) with HC[triple bond]CSiMe(3) affords a vinylidene complex, with HC[triple bond]CFc and HC[triple bond]CRc this reaction does not stop at the vinylidene stage but subsequent cycloaddition yields allenyl carbene complexes. This latter C[bond]C bond formation is effected by strong electronic coupling of the metallocene moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the Ru[double bond]C bond. The vinylidene intermediate appears only in the presence of bulky substituents of the phosphine coligand. For the small R=Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual allyl carbene complexes. X-ray structures of representative complexes are presented. A reasonable mechanism for the formation of both allyl and allenyl carbenes has been established by means of DFT calculations. During the formation of allyl and allenyl carbenes, metallacyclopentatriene and vinylidene complexes, respectively, are crucial intermediates.

The substitution chemistry of RuCp* (temeda)Cl

SummaryHalide abstraction from RuCp*(tmeda)Cl (1,tmeda=Me2NCH2CH2NMe2) with NaBPh4 in CH2Cl2 leads to the formation of the sandwich complex RuCp*(η6-C6H5BPh3) (2). In the presence of CH3CN (1 equiv.) and CO, however, the cationic complexes [RuCp*(tmeda)(CH3CN)]+ (3) and [RuCp*(temeda)(CO)]+ (5) are obtained. In CH3CN,tmeda is also replaced giving [RuCp*(CH3CN)3]+ (4). Complex1 reacts readily with terminal acetylenes HC≡CR, the products depending on the nature ofR (Ph, SiMe3,n-Bu, COOEt). Thus, withR=Ph the ruthenacyclopentatriene complex RuCp*(σ,σ′-C4Ph2H2)Cl (6), withR=SiMe3 the cyclobutadiene complex Ru(Cp*)(σ4-C4H2(1,2-SiMe3)2)Cl (7), and withR=n-Bu and COOEt the binuclear complexes (Cp*)RuCl2(η2:η4-μ2-C4H2(1,3-R)2)Ru(Cp*) (8,9) are obtained. Furthermore, with diethyl maleate in the presence of 1 equiv. of LiCl,1 transforms into the new anionic complex Li[Ru(Cp*) (η2-C2H2(COOEt)2)Cl2] (10). X-ray structures of2,3,4,7, and10 are included.ZusammenfassungChloridabspaltung von RuCp*(tmeda)Cl (1,tmeda=Me2NCH2CH2NMe2) mittels NaBPh4 in CH2Cl2 führt zur Bildung des Halbsandwich-Komplexes RuCp*(η6-C6H5BPh3) (2), während in Gegenwart von CH3CN oder CO die beiden kationischen Verbindungen [RuCp*(tmeda)(CH3CN)]+ (3) und [RuCp*(tmeda)(CO)]+ (5) entstehen. In CH3CN als Lösungsmittel wird sogartmeda unter Bildung von [RuCp*(CH3CN)3]+ (4) verdrängt. Komplex1 reagiert sehr leicht mit terminalen Alkinen HC≡CR, wobei die Produkte stark von der Natur des SubstituentenR (Ph, SiMe3,n-Bu, COOEt) abhängen. Im Fall vonR=Ph entsteht der Ruthenacyclopentatrien-Komplex RuCp*(σσ′-C4Ph2H2)Cl (6), mitR=SiMe3 der Cyclobutadien-Komplex Ru(Cp*)(η4-C4H2(1,2-SiMe3)2)Cl (7), und im Fall vonR=n-Bu und COOEt bilden sich die binuklearen Komplexe (Cp*)RuCl2(η2:η4-μ2-C4H2(1,3-R)2)Ru(Cp*) (8,9). Überdies reagiert1 mit Maleinsäurediethylester in Gegenwart von LiCl zum neuen anionischen Komplex Li[Ru(Cp*) (η2-C2H2(COOEt)2)Cl2] (10). Von2,3,4,7 und10 wurden die Kristallstrukturen bestimmt.