Σ-donor Properties Research Articles

Cyclic (Amino)(aryl)carbenes (CAArCs) as Strong σ-Donating and π-Accepting Ligands for Transition Metals.

Cyclic (amino)(aryl)carbenes (CAArCs) result from the replacement of the alkyl substituent of cyclic (alkyl)(amino) carbenes (CAACs) by an aryl group. This structural modification leads to enhanced electrophilicity of the carbene center with retention of the high nucleophilicity of CAACs, and therefore CAArCs feature a small singlet-triplet gap. The isoindolium precursors are readily prepared in good yields, and deprotonation at low temperature, in the presence of [RhCl(cod)]2 and [(Me2S)AuCl] lead to air-stable rhodium and gold CAArC-supported complexes, respectively. The rhodium complexes promote the [3+2] cycloaddition of diphenylcyclopropenone with ethyl phenylpropiolate, and induce the addition of 2-vinylpyridine to alkenes by CH activation. The gold complexes allow for the catalytic three-component preparation of 1,2-dihydroquinolines from aniline and phenyl acetylene. These preliminary results illustrate the potential of CAArC ligands in transition-metal catalysis.

Pancreatic cystic lesions, more than 10 years of experience

X-Ray crystal structures of six complexes M3(CO)11(L) (M = Ru, L = PPh(OMe)2, P(OCH2CF3)3, P(OCH2)3CEt and AsPh3; M = Os, L = PPh3 and PPh(OMe)2) have been determined. Consideration of these results, together with other published data, allowed several conclusions to be drawn concerning the effect of substituting one CO group in M3(CO)12 (M = Ru, Os) by a P, or As-donor ligand. The most important are that L occupies an equatorial site, the MM separation cis to L increases with increasing cone angle of L, while the COeq ligand cis to L on the same metal atom is more tighly bound. Distortions of the M3L12 molecule from D3h to D3 geometry appear to be related to the σ-donor properties of L. The reactions between Ru3(CO)12 and P(OCH2CF3)3, and between Os3(CO)12 and PPh3 or PPh(OMe)2, to give M3(CO)12−n(L)n(n = 1–3), are described. Crystal data: Ru3(CO)11{PPh(OMe)2}, monoclinic, P2/c, a 9.647(3), b 20.529(7), c 14.692(5) Å, β 119.93(2)°, U 2522(1) Å3, Z = 4, N0 (observed data, with > 3σ(I)) = 5829, R = 0.035, R′ = 0.037; Ru3(CO)11{P(OCH2CF3)3}, triclinic, P1, a 12.920(1), b 11.530(2), c 10.189(1) Å, a 85.93(1), β 86.57(1), γ 70.66(1)°, U 1427(1) Å3, Z = 2, No = 4061, R = 0.038, R′ = 0.054; Ru3(CO)11{P(OCH2)3CEt}, monoclinic P21/c, a 13.57(1), b 14.779(1), c 12.362(3) Å, β 98.34(4)°, U 2453(1) Å3, Z = 4, No = 5950, R = 0.039, R′ = 0.054; Ru3(CO)11(AsPh3), monoclinic, C2/c, a 22.496(6), b 16.348(4), c 17.345(5) Å, β 103.65(2)°, U 6198(3) Å3, Z = 8, No = 6071, R = 0.032, R′ = 0.028; Os3(CO)11(PPh3, monoclinic, C2/c, a 22.196(5), b 16.265(3), c 17.370(5) Å, β 103.86(2)°, U 6088(3) Å3 Z = 8, No = 4125, R = 0.031, R′ = 0.033; Os3(CO)3(CO)11{PPh(OMe)2}, monoclinic, P21/c, a 10.817(3), b 14.993(4), c 16.174(3) Å, β 101.87(2)°, U 2567(1) Å, Z = 4, No = 5506, R = 0.047, R′ = 0.057.

ChemInform Abstract: Design and Development of Ligands for Palladium‐Catalyzed Carbonylation Reactions

AbstractReview: [135 refs.

Design and Development of Ligands for Palladium-Catalyzed Carbonylation Reactions

The palladium-catalyzed carbonylation reaction remains a challenging and significant research field in organic chemistry, and has emerged as a powerful and straightforward protocol for the preparation of various bioactive carbonyl compounds under quite mild reaction conditions. The achievements in this area are correlated to the design and development of versatile ligands that not only facilitate the catalytic transformation, but also provide additional control over the selectivity of the reactions. In this context, a variety of rationally designed ligands with different electronic and steric properties have been synthesized and applied in palladium-catalyzed carbonylation reactions in recent decades. This review focuses mainly on the strategy of ligand design and the results obtained with representative ligands that have different σ-donor properties in the intra- and intermolecular palladium-catalyzed carbonylation reactions of (pseudo)haloarenes with gaseous carbon monoxide and numerous types of nucleophiles. The current limitations and potential trends for further development of palladium-catalyzed carbonylation reactions are also highlighted. 1 Introduction 2 Phosphine Ligands 2.1 Monodentate Phosphines 2.1.1 Triphenylphosphine and Analogues 2.1.2 Di(1-adamantyl)-n-butylphosphine 2.1.3 Biaryl Monophosphines 2.2 Bidentate Phosphine Ligands 2.2.1 Alkyl-Bridged Diphosphines 2.2.2 Ferrocene-Based Bidentate Phosphines 2.2.3 Xantphos and Analogues 2.2.4 BINAP and Analogues 3 N-Heterocyclic Carbenes (NHCs) 4 Other Ligands 4.1 Nitrogen Ligands 4.2 Thiourea-Type Ligands 5 Summary and Outlook

Synthesis, crystal structure and spectroscopic properties of trans-difluoro(1,4,7,11-tetraazaundecane)chromium(III) perchlorate

The structure of trans-[Cr(2,2,3-tet)F2]ClO4 (2,2,3-tet=1,4,7,11-tetraazaundecane) has been determined by single-crystal X-ray diffraction. The complex crystallizes in the space group P1‾ of the triclinic system with two mononuclear formula units in a cell of dimensions a=5.6741(11), b=9.971(2), c=13.222(3) Å, α=73.76(3)o, β=81.08(3)° and γ=85.12(3)°. The chromium(III) atom is in a slightly distorted octahedral environment coordinated by four N atoms of 2,2,3-tet and two F ligands in a mutually trans (meso RS) geometry. The mean Cr-N(2,2,3-tet) and CrF bond lengths are 2.064(9) and 1.885(7) Å, respectively. The crystal packing is stabilized by several hydrogen bonds. The 5K sharp-line absorption spectrum and the 298K FT-infrared and UV–visible spectra have been measured. Using the electronic transitions, a ligand field optimization has been performed to determine the metal-ligand bonding properties of the coordinated atoms. It is found that the fluoride is a strong σ- and π-donor and the nitrogen atoms of the 2,2,3-tet ligand also have strong σ-donor properties toward the chromium(III).

Aryl-based ferrocenyl phosphine ligands in the rhodium(I)-catalyzed hydroformylation of olefins

Ferrocenyl-based phosphine ligands [Fe{1-PPh2(spacer)-2-NMe2CH2C5H3}(C5H5)] (spacer=1,4-phenylene (rac-1), 1,3-phenylene (rac-2), 4,4′-biphenylene (rac-3), 2,5-thienylene (rac-4)) were applied in the rhodium(I)-catalyzed hydroformylation of various olefins (styrene, allylbenzene and 1-hexene) with higher chemo- and regioselectivity than a rhodium(I) catalyst precursor alone. The different σ-donor properties of rac-1–4 were elucidated by 31P{1H} NMR spectroscopy of the corresponding selenides.

Planar-chiral phosphino alkenylferrocenes – Synthesis, solid-state structure and electrochemistry

The synthesis and characterization of phosphino alkenylferrocenes of type [Fe(η5-C5H3–2-PR′2–(E)-CH = CHR)(η5-C5H5)] (R = C6H5, 4-Cl–C6H4, 4-OMe–C6H4, 4-Me–C6H4, 4-CN–C6H4, 4-Ac–C6H4, Fc; R′ = Ph, MeFur, Cy, o-Tol, 3-Cl–C6H4; Fc = Fe(η5-C5H5)(η5-C5H4), MeFur = 5-methylfuran-2-yl, o-Tol = 2-tolyl) and the molecular structure of representative examples in the solid state are discussed. The alkenyl unit has been built up either by Heck–Mizoroki C,C cross coupling or Horner–Wadsworth–Emmons olefination. The electrochemical characterization of the sulphur-protected phosphino (phenylvinyl)-ferrocenes revealed a linear correlation between the redox potentials of the metallocenyl units (E°′) and the σpara Hammett parameters of the phenylene-bonded substituents. In contrast, the σ-donor properties of the phosphine moieties are not affected as could be demonstrated by analysis of the 1J(31P,77Se) coupling constants of the respective selenophosphines. In diferrocenyl ethylene [Fe(η5-C5H3–2-P(S)Ph2–(E)-CHCHFc)(η5-C5H5)] both ferrocenyl units are oxidized separately. In situ spectroelectrochemical measurements revealed that inter-valence charge transfer interactions contribute to the observed redox splitting of 495 mV and that the formed mono-cation can be classified as weakly coupled class II system according to Robin and Day.

A Carbone‐Stabilized Two‐Coordinate Phosphorus(III)‐Centered Dication

A coordinatively unsaturated dicationic phosphorus compound has been synthesized and characterized (see structure; P pink, N blue, C gray). DFT calculations revealed that the polycation should show superior Lewis acid and σ-donor properties to the corresponding bis(dimethylamino)phosphenium cations. Preliminary investigations with PMe3 confirmed unusual reactivity patterns for the dication.

Pushing the σ‐Donor Strength in Iridium Pincer Complexes: Bis(silylene) and Bis(germylene) Ligands Are Stronger Donors than Bis(phosphorus(III)) Ligands

Breaking the donor limits: The remarkable coordination chemistry of ECHE (E=Si, Ge) ligands with Group 9 metals (Ir, Rh) and their application in catalytic CH borylation of arenes was investigated. The spectroscopic and structural features of the first [ECE] iridium complexes show the stronger σ-donating properties of the divalent Si and Ge pincer ligands compared to analogous PIII-based ligands.

Robust Acenaphthoimidazolylidene Palladium Complexes: Highly Efficient Catalysts for Suzuki–Miyaura Couplings with Sterically Hindered Substrates

Robust acenaphthoimidazolylidene palladium complexes have been demonstrated as highly efficient and general catalysts for the sterically hindered Suzuki-Miyaura cross-coupling reactions in excellent yields even with low catalyst loadings under mild reaction conditions. The high catalytic activity of these complexes highlights that, besides the "flexible steric bulky" concept, σ-donor properties of the NHC ligands are also crucial to accelerate the transformations.

Facile Synthesis of Gold(III) Aryl–Carbene Metallacycles

New gold(III) metallacyclic complexes with unprecedented aryl–carbene bidentate ligands have been prepared by the reaction of the cycloaurated gold(III) complex [AuCl2(pap-C1,N)] (pap = 2-(2-pyridylamino)phenyl) with isocyanide CNR (R = 2-naphthyl (1), cyclohexyl (2), or 2,6-dimethylphenyl (3)). Complexes have been spectroscopically and structurally characterized, showing in some complexes aurophilic gold(III)···gold(III) and π–π interactions. In contrast with the starting material these complexes are emissive in the solid state, probably because of the better σ-donor properties of the new bidentate ligands.

Amino-Acrylamido Carbenes: Modulating Carbene Reactivity via Decoration with an α,β-Unsaturated Carbonyl Moiety

Two 6-methoxy-4-oxo-1,3-diaryl-3,4-dihydropyrimidinium salts (2a and 2b) have been prepared as precursors to novel amino-acrylamido carbenes. Treatment of 2a (where the aryl groups are mesityl groups) with one equivalent of sodium hexamethyldisilazide in aromatic hydrocarbons affords the persistent amino-acrylamido carbene 3a, which has been characterized spectroscopically. This novel carbene has been trapped with Ir(I) transition metal fragments as well as electrophilic carbon disulfide and nucleophilic isocyanides. Infrared spectroscopic studies carried out on 3a-Ir(CO)2Cl (5a) indicated that this carbene exhibits a Tolman electronic parameter (TEP) of 2049 cm–1, a value that suggests that 3a is a stronger donor than both diamidocarbenes (DACs) and a recently reported amino-ureido carbene (DAC TEPs ≈ 2057 cm–1; amino-ureido TEP = 2058 cm–1), but similar σ-donating properties to a monoamido-amino carbene (TEP = 2050 cm–1). This result has been corroborated by DFT analyses carried out on all four species,...

A Cyclic Diaminocarbene with a Pyramidalized Nitrogen Atom: A Stable N‐Heterocyclic Carbene with Enhanced Electrophilicity

Since their discovery,[1,2] thousands of papers have dealt with the preparation and use of stable carbenes. Arguably, among them,[3–6] the N-heterocyclic carbenes (NHCs) A have been the most studied.[7–12] Despite their widespread applications, NHCs still suffer from low modularity, which confines them to a narrow range of strong σ-donor ligands and nucleophiles. In recent years, several strategies have successfully been developed for tuning their σ-donor properties, which are mainly linked to the σ-effect of the carbene substituents.[8] However, the modulation of the π-acceptor properties of NHCs is more challenging since the high-energy LUMO is ruled by the strong π-donation of the two amino groups to the carbene vacant orbital.[13] As shown in Figure 1, the replacement of a π-electron donating and σ-withdrawing amino substituent by a σ-donating alkyl group (B) leads to an increase of electrophilicity but also nucleophilicity.[14,15] Carbonyls can be introduced in the backbone in order to compete with the carbene center for the nitrogen π-donation. The so-called diamidocarbenes C are very electrophilic, but at the expense of their nucleophilicity.[16–19] Here we report the synthesis and single crystal X-ray diffraction study of a cyclic diaminocarbene, which retains the nucleophilicity of classical NHCs, but displays enhanced electrophilicity.

A Stable Two-Coordinate Acyclic Silylene

Simple two-coordinate acyclic silylenes, SiR(2), have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B(NDippCH)(2) substituent (Dipp = 2,6-(i)Pr(2)C(6)H(3)), an isolable monomeric species, Si{B(NDippCH)(2)}{N(SiMe(3))Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C-H bonds, consistent with a low singlet-triplet gap (103.9 kJ mol(-1)), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems.

Synthesis, Structure, and Properties of a T-Shaped 14-Electron Stiboranyl-Gold Complex

A cyclic stiboranyl-gold complex (1) supported by two 1,8-naphthalenediyl linkers has been synthesized and structurally characterized. The gold atom of this complex adopts a T-shaped geometry and is separated from the antimony center by only 2.76 Å. Surprisingly, the trivalent gold atom of this complex is involved in an aurophilic interaction, a phenomenon typically only observed for monovalent gold complexes. This phenomenon indicates that the stiboranyl ligand possesses strong σ-donating properties making the trivalent gold atom of 1 electron rich. This view is supported by DFT calculations as well as Au L(3)- and Sb K-edge XANES spectra which reveal that 1 may also be described as an aurate-stibonium derivative. In agreement with this view, complex 1 shows no reactivity toward the halides Cl(-), Br(-), and I(-). It does, however, rapidly react with F(-) to form an unprecedented anionic aurate fluorostiborane complex ([2](-)) which has been isolated as the tetra-n-butylammonium salt. The increased coordination number of the antimony center in this anionic complex ([2](-)) does not notably affect the Au-Sb separation (2.77 Å) or the geometry at the gold atom which remains T-shaped.

A Stable Acyclic Ligand Equivalent of an Unstable 1,3‐Dithiol‐5‐ylidene

A Stable Acyclic Ligand Equivalent of an Unstable 1,3‐Dithiol‐5‐ylidene

ChemInform Abstract: Stable Cyclic Carbenes and Related Species Beyond Diaminocarbenes

AbstractReview: 265 refs.

Influence of CH 3 substituents on tetrahydropyrimidin-2-ylidene: σ-donating properties and in situ catalytic activities of precursor salts/Pd(OAc) 2 system for C–C coupling reactions

In this study, symmetrical and non-symmetrical 1,3-dialkyltetrahydropyrimidinium salts ( 1·HCl and 2·HBr, respectively), bearing two methyl groups at C 5 position of the pyrimidine ring were synthesized and characterized. One of the new salts 1,3-bis(2,4,6-trimethylbenzyl)-5,5-dimethyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate ( 1a·HBF 4, 1a = 6-Bn∗- Me 2) was converted to cis-[RhCl(6-Bn∗- Me 2)(CO) 2] to determine the σ-donating properties in comparison to those of their 5-membered NHC complexes. Furthermore, the in situ catalytic activities of all salts, in combination with Pd(OAc) 2, were tested for the Heck–Mizoraki and Suzuki–Miyaura type C–C coupling reactions. In contrast to the expectation, only the substituents on N,N′-atoms were found to play an important role in the catalytic activity.

The Development and Catalytic Uses of N-Heterocyclic Carbene Gold Complexes

Gold has emerged as a powerful synthetic tool in the chemist's arsenal. From the early use of inorganic salts such as AuCl and AuCl(3) as catalysts, the field has evolved to explore ligands that fine-tune reactivity, stability, and, more recently, selectivity in gold-mediated processes. Substrates generally contain alkenes or alkynes, and they usually involve straightforward protocols in air with solvents that can often times be of technical grade. The actual catalytic species is the putative cationic gold(I) complex [Au(L)](+) (where L is a phosphorus-based species or N-heterocyclic carbene, NHC). The early gold systems bearing phosphine and phosphite ligands provided important transformations and served as useful mechanistic probes. More recently, the use of NHCs as ligands for gold has rapidly gained in popularity. These two-electron donor ligands combine strong σ-donating properties with a steric profile that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the gold-NHC complexes have been used as well-defined precatalysts and have permitted the isolation of reactive single-component systems that are now used instead of the initial [Au(L)Cl]/silver salt method. Because some are now commercially available, NHC-containing gold(I) complexes are gathering increasing interest. In this Account, we describe the chronological development of this chemistry in our laboratories, highlighting the advantages of this family of gold complexes and reviewing their synthesis and applications in catalysis. We first outline the syntheses, which are straightforward. The complexes generally exhibit high stability, allowing for indefinite storage and easy handling. We next consider catalysis, particularly examining efficacy in cycloisomerization, other skeletal rearrangements, addition of water to alkynes and nitriles, and C-H bond activation. These processes are quite atom-economical, and in the most recent C-H reactions the only byproduct is water. State-of-the-art methodology now involves single-component catalysts, precluding the need for costly silver co-catalysts. Remarkably, the use of an NHC as a supporting ligand has permitted the isolation of [Au(L)(S)](+) species (where S is a solvent molecule such as a nitrile), which can act as single-component catalysts. Some improvements are still needed, as the single components are most often synthesized with a silver reagent. Owing to the stabilizing effect of NHC coordination, some NHC-containing systems can catalyze extremely challenging reactions (at temperatures as high as 140 °C) and react at very low loadings of gold (ppm levels). Our latest developments deal with C-H bond functionalization and hold great promise. We close with a selection of important developments by the community with gold-NHC complexes. As demonstrated by the turns and twists encountered during our own journey in the gold-NHC venture, the chemistry described here, combining fundamental organometallic, catalytic, and organic methodology, remains rich in opportunities, especially considering that only a handful of gold(I) architectures has been studied. We hope this Account will encourage young researchers to explore this emerging area, as the adage "the more you do, the more you have to do" surely holds true in gold-mediated catalysis.

Amidoamine-based dendrimers with end-grafted Pd–Fe units: Synthesis, characterization and their use in the Heck reaction

The synthesis and characterization of novel amidoamine-based metallodendrimers with heterobimetallic end-grafted amidoferrocenyl-palladium-allyl chloride units is described. Dendrimer (Fe((η 5-C 5H 4PPh 2)(η 5-C 5H 4))C(O)HNCH 2CH 2NHC(O)CH 2CH 2)N[CH 2CH 2N(CH 2CH 2C(O)NHCH 2CH 2NH-C(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2))) 2] 2 ( 9- Fe) and the corresponding metal species (Fe((η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl))(η 5-C 5H 4))C(O)HNCH 2CH 2NHC(O)CH 2CH 2)N[CH 2CH 2N(CH 2CH 2C(O)NHCH 2CH 2NHC(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl)))) 2] 2 ( 9- Fe– Pd) were prepared by a consecutive divergent synthesis methodology including addition–amidation cycles, standard peptide coupling, and coordination procedures. For comparative reasons also the monomeric and dimeric molecules (Fe(η 5-C 5H 4PPh 2)(η 5-C 5H 4C(O)NH n C 3H 7)) ( 5- Fe) and [Fe(η 5-C 5H 4PPh 2)(η 5-C 5H 4C(O)NHCH 2)] 2 ( 6- Fe) as well as N(CH 2CH 2C(O)NHCH 2CH 2NHC(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2))) 3 ( 7- Fe) and [CH 2N(CH 2CH 2C(O)NHCH 2CH 2NHC(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2))) 2] 2 ( 8- Fe) were prepared from Fe(η 5-C 5H 4PPh 2)(η 5-C 5H 4CO 2H) ( 3). Using [Pd(η 3-C 3H 5)Cl] 2 ( 4) as palladium source heterobimetallic metallodendrimers (Fe(η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl))(η 5-C 5H 4C(O)NH n C 3H 7)) ( 5- Fe– Pd), [Fe(η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl))(η 5-C 5H 4C(O)NHCH 2)] 2 ( 6- Fe– Pd), N(CH 2CH 2C(O)NHCH 2CH 2NHC(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl)))) 3 ( 7- Fe– Pd) and [CH 2N(CH 2CH 2C(O)NHCH 2CH 2NHC(O)(Fe(η 5-C 5H 4)(η 5-C 5H 4PPh 2(Pd(η 3-C 3H 5)Cl)))) 2] 2 ( 8- Fe– Pd) were synthesized. Additionally, seleno-phosphines of 5- Fe– Se and 9- Fe– Se, respectively, were prepared by addition of elemental selenium to 5- Fe or 9- Fe to estimate their σ-donor properties. The palladium-containing amidoamine supports are catalytically active in the Heck–Mizoroki cross-coupling of iodobenzene with tert-butyl acrylate. The catalytic data are compared to those obtained for the appropriate mononuclear and dinuclear compounds 5- Fe– Pd and 6- Fe– Pd. This comparison confirms a positive cooperative effect. The mercury drop test showed that (nano)particles were formed during catalysis, following on heterogeneous carbon–carbon cross-coupling.