Tolman Electronic Parameters Research Articles

Pyridinium‐abgeleitete mesoionische N‐heterocyclische Olefine (py‐mNHOs)

AbstractMesoionische Polarisation ermöglicht den Zugang zu elektronenreichen Olefinen, die als Organokatalysatoren, Liganden oder Nucleophile Anwendung finden. Wir berichten hier über die Synthese und Charakterisierung einer Reihe von 3‐Methylpyridinium‐abgeleiteten mesoionischen Olefinen (py‐mNHOs). Wir verwendeten ein mit DFT‐Rechnungen unterstütztes Designkonzept, das zeigte, dass Arylgruppen an den 1‐, 2‐, 4‐ und 6‐Positionen des heterocyclischen Kerns die kinetische Stabilisierung der neuartigen mesoionischen Verbindungen ermöglicht. Die elektronischen Tolman‐Parameter zeigen, dass py‐mNHOs bemerkenswert starke σ‐Donor‐Liganden für Übergangsmetalle und Hauptgruppen‐Lewis‐Säuren sind. Außerdem gehören sie zu den stärksten Nucleophilen auf der Mayr‐Reaktivitätsskala. Bei Reaktionen von py‐mNHOs mit elektronenarmen π‐Systemen wurde experimentell ein sukzessiver Übergang von der Bildung zwitterionischer Addukte über schrittweise zu konzertierten 1,3‐dipolaren Cycloadditionen beobachtet und durch quantenchemische Rechnungen analysiert.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs).

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Characterization of the Ligand Properties of Donor-stabilized Pnictogenyltrielanes.

A general synthesis and the characterization of novel alkyl-substituted NHC-stabilized pnictogenylboranes NHC ⋅ BH2 ER2 (NHC=N-heterocyclic carbene, E=P, As; R2 =Me2 , Ph2 , t BuH, Cy2 , (SiMe3 )2 ) are reported. These compounds were reacted with Ni(CO)4 to the corresponding complexes of the type [(NHC ⋅ BH2 ER2 )Ni(CO)3 ] to determine their donor strength by Tolman Electronic Parameters (TEPs) and their steric demand as ligands compared to classical phosphines, superbasic phosphines and other commonly applied donor systems. The results show that the NHC-stabilized pnictogenyltrielanes can be considered as being highly basic, while their steric influence depends strongly on the organic residues as well as the donor attached to the {BH2 } moiety. Although weaker than commonly used superbasic phosphines, the donor strength of pnictogenyltrielanes in general can be classified as of similar strength as NHCs. The steric and electronic properties can easily be modified by alkyl substitution as evident from the TEP trends.

PKaH values and θH angles of phosphanes to predict their electronic and steric parameters.

Phosphanes play an important role in various applications, serving as a class of organic bases with basicities spanning more than 30 orders of magnitude. Accessing comprehensive basicity data for phosphanes has been challenging due to scattered information across multiple sources and notable gaps in the existing data. In this report, we present basicities (pKaH values) of a diverse set of phosphanes, both newly measured or calculated and collected from the literature. We demonstrate that pKaH values can serve as an alternative to Tolman electronic parameters (TEP values) in evaluating the electronic properties of phosphanes. Additionally, we suggest parameters for assessing the steric properties of phosphanes without the need for preparation or calculation of metal-ligand complexes.

On the Edge of the Known: Extremely Electron-Rich (Di)Carboranyl Phosphines.

The syntheses of the first B9-connected carboranylphosphines (B9-Phos) featuring two carboranyl moieties as well as access to B9-Phos ligands with bulky electron-donating substituents, previously deemed unattainable, is reported. The electrochemical properties of the B9-Phos ligands were investigated, revealing the ability of the mesityl derivatives to form stabilized phosphoniumyl radical cations. The B9-Phos ligands display an extremely electron-releasing character surpassing that of alkyl phosphines and commonly used N-heterocyclic carbenes. This is demonstrated by their very small Tolman electronic parameters (TEPs) as well as extremely low P-Se coupling constants. Cone angles and buried volumes attest to the high steric demand exerted by the (di)carboranyl phosphines. The dicarboranyl phosphine AuI complexes show superior catalytic performance in the hydroamination of alkynes compared to the monocarboranyl phosphine analogs.

Mesoionic N-Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2.

An extended class of stable mesoionic N-heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO2 as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero-couplings between amide and amine using CO2 . These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal-free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

Mesoionic N‐Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

AbstractAn extended class of stable mesoionic N‐heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO2as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero‐couplings between amide and amine using CO2. These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal‐free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

Synthesis of Ni(dvtms) and Ni(CO)3 Complexes Ligated by an Isolable Two‐Coordinate Cyclic Alkylsilylene

Ni(CO)3 complexes ligated by an isolable cyclic (alkyl)(amino)silylene (CAASi) or an isolable cyclic dialkylsilylene (CDASi) were synthesized by the reaction of the corresponding Ni(dvtms) complex (dvtms = 1,1,3,3‐tetramethyl‐1,3‐divinyldisiloxane) with CO to estimate the donor strengths of CAASi and CDASi. The CO stretching frequencies (Tolman electronic parameters, TEP) of the Ni(CO)3 complexes indicate that the net donating ability of CDASi and CAASi to Ni(CO)3 are almost the same and in between PPh3 and P(tBu)3. The DFT calculations suggested that the TEP values are mainly correlated with the energy level of the lone pair orbital of the ligand but not strongly associated with those of the vacant π‐accepting orbitals.

Monosubstituted, Anionic Imidazolyl Ligands from N−H NHC Precursors and Their Activity in Pd‐Catalyzed Cross‐Coupling Reactions

AbstractWe report that treatment of several 2‐diphenylphosphinoimidazoles with Pd(II) salts generates monosubstituted N−H NHC−Pd complexes via insertion into the C−P bond. Removal of the N−H proton in situ leads to anionic (X‐type) or imidazolyl‐Pd complexes that are highly stable and catalytically active, achieving up to 340,000 turnovers at 1 ppm catalyst loading in Suzuki‐Miyaura reactions. DFT‐calculated Tolman electronic parameters for the sterically small ligands suggest that these ligands are significantly more donating than traditional NHCs, which provides a rationale for rapid cross‐coupling catalysis. Excellent reactivity is also demonstrated in Sonogashira reactions.magnified image

Tunable Rh(I) Fischer carbene complexes for application in the hydroformylation of 1-octene

The preparation of a series of rhodium(I) complexes coordinated by various electronically tuneable Fischer carbene (FC) ligands, is reported. The Rh(I) metal complexes’ electronic properties could readily be modulated by variation of a p-N,N-dimethylaniline moiety with a ruthenocenyl substituent, or alternatively, substituting the carbene O-heteroatom for an amino-group. The electronic properties of the complexes were evaluated, and it was determined from the Tolman electronic parameters that the donor-ability of the FC ligands are comparable to N-heterocyclic carbenes. Furthermore, the facile control of the electronic properties of the complexes was demonstrated by mild oxidation of a ferrocenyl aminocarbene rhodium(I) complex, yielding the corresponding ferrocenium rhodium(I) complex cation. Finally, the complexes were evaluated as catalyst precursors for the hydroformylation of 1-octene.

Phosphazenyl Phosphines: The Most Electron-Rich Uncharged Phosphorus Brønsted and Lewis Bases.

It was discovered that phosphazenyl phosphines (PAPs) can be stronger P-superbases than the corresponding Schwesinger type phosphazene N-superbases. A simple synthetic access to this class of PR3 derivatives including their homologization is described. XRD structures, proton affinities (PA), and gas-phase basicities (GB) as well as calculated and experimental pK values in THF are presented. In contrast to their N-basic counterparts, PAPs are also privileged ligands in transition metal chemistry. In fact, they are currently the strongest uncharged P-donors known, exceeding classical and more recently discovered ligands such as PtBu3 and imidazolin-2-ylidenaminophosphines (IAPs) with respect to their low Tolman electronic parameters (TEPs) and large cone angles.

Syntheses, Structures, and Characterization of Nickel(II) Stibines: Steric and Electronic Rationale for Metal Deposition.

Reactions of the homoleptic and heteroleptic antimony ligands Sb iPr3, Sb iPr2Ph, SbMe2Ph, and SbMePh2 with NiI2 generate rare NiII stibine complexes in either square planar or trigonal bipyramidal (TBP) geometries, depending on the steric size of the ligands. Tolman electronic parameters were calculated (DFT) for each antimony ligand to provide a tabulated resource for the relative strengths of simple antimony ligands. The electronic absorbance spectra of the square planar complexes exhibit characteristic bands [λmax ≈ 560 nm (17 900 cm-1), ε ≈ 4330 M-1 cm-1] at lower energies compared to the reported phosphine complexes, indicating the weak donor strength of the stibine ligands and resultant low-energy ligand field d→ d transitions. The square planar complex Ni(I)2(Sb iPr3)2 reacts with CO to form the TBP complex Ni(I)2(Sb iPr3)2(CO). Lastly, the complexes were investigated for nickel metal deposition on Si|Cu(100 nm) substrates. The complexes with the strongest donating ligand, Sb iPr3, deposited the purest layer of NiCu alloy according to the balanced reaction Ni(I)2(SbIII iPr3)2 → Ni0 + SbV( iPr3)I2; the iodinated SbV byproduct was unambiguously detected in the supernatant by 1H NMR and mass spectrometry. Complexes with weaker ligands (poor I2 acceptors/scavengers) resulted undesired deposition of iodine and CuI on the surface. This work thus serves as a guide for the design and synthesis of 3 d metal complexes with neutral, heavy main-group donors that are useful for metal deposition applications.

A closer look at the reactivity between N-heterocyclic carbenes and fluoroalkenes

The fundamental reactivity leading to N-heterocyclic fluoroalkene adducts is explored in detail, featuring a total of 15 N-heterocyclic carbenes (NHCs) with various electronic and steric environments. The activity of these carbenes towards tetrafluoroethylene (TFE), hexafluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene fluoride (VDF) is assessed in THF and toluene. Attempts were made to correlate the observed reactivity with electronic (Tolman Electronic Parameters) and steric (% buried volume) parameters unique to each NHC, but a trend has yet to be fully determined. However, the unique steric constraints of a cyclic (alkyl)(amino)carbene (CAAC) were shown to modify the initial point of nucleophilic attack on HTFE, providing selective transformation to a different adduct than has been observed to date with all reactions involving this fluoroalkene.

Heteromultimetallic Complexes with Redox-Active Mesoionic Carbenes: Control of Donor Properties and Redox-Induced Catalysis.

Mesoionic carbenes (MICs) are currently hugely popular as ligands, and triazolylidenes are arguably the most prominent classes of such MICs. Mesoionic carbenes with ferrocenyl substituents are presented that can act as metalloligands for the generation of heteromultimetallic iridium(I) and gold(I) complexes. The ferrocenyl substituents allow for reversible oxidation of these heteromultimetallic complexes, and these oxidation steps have a strong influence on the donor properties of the MICs. Tolman electronic parameters (TEP) determined from analysis of the iridium-carbonyl complexes show that the neutral ferrocenyl-MIC ligands are stronger donors than the imidazolylidene based carbenes, the one-electron oxidized ferrocenyl MICs are in the range of the tricyclohexyl phosphines and the two-electron oxidized forms, which are electron-poor, lie in the range of triphenyl phosphines. Taking advantage of the generation of these electron-poor MICs, we show their gold(I) complexes are potent catalysts for the synthesis of oxazolines, with complexes of the oxidized MIC ligands, without any additional additive, outperforming their neutral counterparts by almost a factor of ten. These results thus present the first examples of MIC ligands that are reversibly electronically tunable, and show the potential of the oxidized MIC ligands in types of catalysis where electron-poor ligands are necessary. The potential of MICs for molecular electroactive materials is also shown.

Electronic and steric Tolman parameters for proazaphosphatranes, the superbase core of the tri(pyridylmethyl)azaphosphatrane (TPAP) ligand.

The Tolman electronic parameters (TEP) and cone angles were experimentally measured for a series of substituted proazaphosphatrane ligands by synthesizing their respective Ni(L(R))(CO)3 complexes, where L = P(RNCH2CH2)3N and R = Me, iPr, iBu and Bz. The complexes Ni(L(Me))(CO)3 (), Ni(L(iPr))(CO)3 (), Ni(L(iBu))(CO)3 () and Ni(L(Bz))(CO)3 () display CO vibrational frequencies (A1 mode) at 2057.0, 2054.6, 2054.9 and 2059.1 cm(-1), respectively. The TEPs for the phosphine ligands in are among the lowest measured, with values close to P(tBu)3 the most donating phosphine measured by Tolman. The cone angles of L(R) measured in are 152, 179, 200 and 207° for R = Me, iPr, iBu and Bz, respectively. The substituted proazaphosphatranes have larger cone angles compared to the analogous trialkyl subsituted monophosphines. Our study demonstrates that while the cone angles have a significant dependence on R, all of the substituted proazaphosphatranes are strong electron donors. Additionally, in order to determine the electronic donor strength of our previously reported multidentate ligand, TPAP, Ni(TPAP)(CO)2 () (TPAP = tris(2-pyridylmethyl)azaphosphatrane) and Ni(L(Me))2(CO)2 () were also synthesized and evaluated in a similar fashion.

Determining the Ligand Properties of N‐Heterocyclic Carbenes from 77Se NMR Parameters

AbstractThe 1JCSe coupling constants for a range of NHC–selenium adducts have been measured and used to establish a correlation with the σ‐donor strength of the respective carbenes. For the subclass of amido‐carbenes, the 1JCSe values revealed a high donor capacity, very much in contrast to what the DFT‐calculated HOMO energies suggest. The 1JCH coupling constants for the C‐2 atoms in azolium‐type NHC precursors were more readily obtained and show the same trend as the 1JCSe coupling constants. In addition, the use of 77Se chemical shifts to determine π‐acidity has been extended to a broader range of derivatives, namely 1·Se–22·Se. The superior resolution of the δ(77Se) method in comparison with the Tolman electronic parameters derived from IR spectroscopy is demonstrated for the caffeine‐derived bis‐carbene 19.

Rhodium(I) and Silver(I) Complexes of 4, 5‐Dicyano‐1, 3‐dimesityl‐ and 4, 5‐Dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene

AbstractThe new N‐heterocyclic carbene (NHC) precursors 4, ‐dicyano‐1, ‐dimesityl‐ (9) and 4, 5‐dicyano‐1, 3‐dineopentyl‐2‐(pentafluorophenyl)imidazoline (14) were synthesized. The structure of 9 could be determined by X‐ray crystallography. With the 2‐pentafluorophenyl‐substituted imidazolines 9 and 14, the [AgCl(NHC)], [RhCl(COD)(NHC)], and [RhCl(CO)2(NHC)] complexes [NHC = 4, 5‐dicyano‐1, 3‐dimesitylimidazol‐2‐ylidene (3) and 4, 5‐dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene (4)] were obtained. Crystal structures of [AgCl(3)] (15), [RhCl(COD)(3)] (17), [RhCl(COD)(4)] (18), and [RhCl(CO)2(3)] (19) were solved and with the crystal data of 19, the percent buried volume ( %Vbur) of 31.8(±0.1) % was determined for NHC 3. Infrared spectra of the imidazolines 9 and 14 and of the complexes 15–20 were recorded and the CO stretching frequencies of complexes 19 and 20 were used to determine the Tolman electronic parameters of the newly obtained NHCs 3 (TEP: 2060 cm–1) and 4 (TEP: 2061 cm–1), thus proving that 1, 3‐substitution of maleonitrile‐NHCs does not have a significant effect for the high π‐acceptor strength of these carbenes.

Olefin Metathesis Catalysts Containing N,N′-Diamidocarbenes

A series of Ru-based olefin metathesis catalysts containing N,N′-diamidocarbenes (DACs) were synthesized and studied. X-ray crystallographic analysis revealed that the Ru−Ccarbene distances (1.938(5)−1.984(4) A) measured in the DAC-supported complexes were relatively short, particularly in comparison to the range of Ru−Ccarbene distances typically observed in analogous N-heterocyclic carbene (NHC) supported complexes (1.96−2.03 A). While the Tolman electronic parameters (TEP) of various DACs (2056−2057 cm−1) were calculated to be similar to that of PCy3 (2056 cm−1), the ring-closing metathesis (RCM) of diethyl diallylmalonate facilitated by DAC-supported Ru complexes proceeded at a relatively slow rate. However, unlike the phosphine-containing complexes, the DAC analogues catalyzed the RCM of diethyl dimethallylmalonate to its respective tetrasubstituted olefin. A series of electrochemical experiments revealed that the Ru complexes bearing a DAC ligand underwent oxidation at significantly higher potential...

Chalcogenides of the aminomethylphosphines derived from 1-methylpiperazine, 1-ethylpiperazine and morpholine: NMR, DFT and structural studies for determination of electronic and steric properties of the phosphines

Chalcogenide derivatives of three aminomethylphosphines: P(CH2N(CH2CH2)2NCH3)3 (1), P(CH2N(CH2CH2)2NCH2CH3)3 (2) and P(CH2N(CH2CH2)2O)3 (3) were prepared: oxides--OP(CH2N(CH2CH2)2NCH3)3 (4), OP(CH2N(CH2CH2)2NCH2CH3)3 (5), OP(CH2N(CH2CH2)2O)3 (6), sulfides--SP(CH2N(CH2CH2)2NCH3)3 (7), SP(CH2N(CH2CH2)2NCH2CH3)3 (8), SP(CH2N(CH2CH2)2O)3 (9) and selenides--SeP(CH2N(CH2CH2)2NCH3)3 (10), SeP(CH2N(CH2CH2)2NCH2CH3)3 (11), SeP(CH2N(CH2CH2)2O)3 (12). The spectroscopic NMR analyses, DFT (B3LYP/6-31G**) calculations together with crystallographic studies of compounds 5, 6, 9 and 12 demonstrate that the structures and spectroscopic properties are strongly influenced by the chalcogen atom and not entirely contingent on aliphatic rings in the molecules. TEPs (Tolman's electronic parameters), estimated with DFT methods equal 2059.7 cm(-1) for 1, 2059.8 cm(-1) for 2 and 2061.2 cm(-1) for 3. These values are similar to TEPs estimated experimentally for other aminomethylphosphines. Phosphines 1, 2 and 3, despite very large Tolman cone angles show rather low influence of molecular geometry on their electronic properties: S4' (symmetric deformation coordinate) (59.8-64.8) and S4 (63.4-63.7) parameters are moderate. Suresh's steric effect (S(eff)) parameters for phosphines 1, 2 and 3 (2.70, 2.73 and 2.66, respectively) indicate minor electron-donating effect. Electronic effect (E(eff)) parameters (4.92, 5.21 and -0.40, respectively) can be easily modified by changing the substituents. 1J(SeP) coupling constants in the selenides are low (10: 707.5 11: 707.5 12: 709.8 Hz), but do not correlate with the TEP values in a way typical for aliphatic phosphines. The examined compounds do not show mutagenic properties and their potential toxicity is low, which is relevant in the context of their possible medical applications.

1,2,3-Triazolylidenes as Versatile Abnormal Carbene Ligands for Late Transition Metals

The [3 + 2] cycloaddition of azides and acetylenes followed by nitrogen quaternization was applied for the generation of novel and highly modular triazolium salts. The selective substitution of the 1,3,4-substitution pattern presets such salts as precursors for a new class of abnormal carbene ligands, thus expanding the family of these high-impact ligands. Metalation of the triazolium salts is highly versatile and is illustrated by direct C-H bond activation as well as by applying a transmetalation protocol, thus providing access to Pd(II), Ru(II), Rh(I), and Ir(I) abnormal carbene complexes. The donor properties of these carbenes were analyzed by using Tolman electronic parameters and were found to be slightly stronger than those the most basic normal carbenes.