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ortho-Substituted and unsymmetrical 9-phospha-triptycenes were synthesizedviatwo synthetic approaches involving densely functionalizedortho-halogenated triarylmethane or phosphine precursors.ortho-Substituents imposed a considerable steric shielding due to the tricyclic cage-shaped structure with the aryl rings p-systems orthogonal to the phosphorus electron pair. A series of Au(i) and Rh(i) complexes were analysed in the solid state to determine Tolman electronic parameters, cone angles and buried volumes of these unprecedented functionalized phosphines. Quantum chemical calculations of electronic and steric descriptors revealed that these cage-shaped phosphines are electron-poor and that single methyl substituent is enough to provide the largest effect on steric shielding reported so far in triarylphosphines. An unsymmetrically substituted 9-phosphatriptycene was resolved by chiral HPLC, opening the avenue towards stableP-chirogenic triarylphosphines with unlimited configurational stability for new catalyst development in asymmetric transition-metal catalysis.

The metastable and thermodynamically favored phases of CuFeS2 are shown to be alternatively synthesized during partial cation exchange of hexagonal Cu2S using various phosphorus-containing ligands. Transmission electron microscopy and Energy dispersive spectroscopy mapping confirm the retention of the particle morphology and the approximate CuFeS2 stoichiometry. Powder X-ray diffraction patterns and refinements indicate that the resulting phase mixtures of metastable wurtzite-like CuFeS2 versus tetragonal chalcopyrite are correlated with the Tolman electronic parameter of the tertiary phosphorous-based ligand used during the cation exchange. Strong L-type donors lead to the chalcopyrite phase, and weak donors to the wurtzite-like phase. To our knowledge, this is the first demonstration of phase control in nanoparticle synthesis using solely L-type donors.

We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs., Highly charged: A new class of mesoionic N‐heterocyclic olefins is described featuring a highly ylidic double bond. The compounds can be utilized as ligands in main‐group and transition metal chemistry. Their donor properties exceed previously reported traditional N‐heterocyclic olefins.

Tertiary alkyl, aryl or amino phosphines PR3 (R = Me, nBu, C2H4CN, NEt2) and the bis[(2-diphenylphosphino)phenyl]ether (POP) were allowed to react with fluoroiodomethane to produce fluoromethyl phosphonium salts in yields between 60–99%. The compounds were characterized by vibrational and NMR spectroscopy and in most cases also by single crystal X-ray diffraction. Diphenyl(fluoromethyl) phosphine was synthesized as a mixed aryl–alkyl-phosphine and the TEP value (Tolman electronic parameter) was determined in order to explain its low reactivity. The molecular and crystal structures of the new fluoromethyl phosphonium salts [R3PCH2F]I with R = Me, C2H2CN and NEt2 as well as of the salt resulting from the fluoromethylation of POP provided additional information on the structural behavior of the bioisoster CH2F group bonded to phosphorus. Hydrogen bonding in the crystal is compared with that observed in the crystal structure of PPh3CH2FI. The toxicity of the sufficiently water soluble salt [Me3PCH2F]I was investigated and the toxicological effect of the CH2F group was compared to that of the bioisoster CH2OH group in THPS.

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet-triplet gaps (ΔEST =19.9 kcal mol-1 ; B3LYP/def2-TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield 77 Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm-1 ) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C-H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.

Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm-1 , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm-1 ).

Atomically precise gold superatoms offer useful templates to evaluate tunable properties via ligand engineering. Herein, the role of different linked N-heterocyclic carbene (NHC) protecting ligands ranging from strong to weak σ-donors was evaluated according to the Tolman electronic parameter (TEP), in species related to the classical [Au13Cl2(dppe)5]3+ nanocluster (1). Our results show a strong dependency on the nature of NHC, providing a useful design principle for the efficient tuning of the structural, optical, chiroptical and emission properties of the Au13Cl2 core. A sizable decrease is observed in the HOMO–LUMO gap for weaker σ-donor ligand cases, with a change in the LUMO nature from core-based orbitals in 1, to a π*-ligand nature. Furthermore, a shorter bridge results in interesting structural changes between the eclipsed ↔ staggered Au13Cl2 core unraveling the potential to convert light energy into mechanical work. Thus, the noticeable modulation of [Au13Cl2(NHC)5]3+ properties by different ligands underlies design rules for tunable clusters towards nanostructured materials, by taking advantage of the recent introduction of NHC-protected gold clusters.

The synthesis and coordination chemistry of a saturated analogue of a 'bulky-yet-flexible' N-heterocyclic carbene (NHC) ligand are described. "SIPaul" is a 4,5-dihydroimidazol-2-ylidene ligand with unsymmetrical aryl N-substituents, and is one of the growing class of 'bulky-yet-flexible' NHCs that are sufficiently bulky to stabilise catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the metal centre. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (%Vbur), while the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δSe of the corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.

Palladium terminal imido complexes hold promise as catalysts for the amination of CH bonds. This computational study elucidates the electronic structure of palladium(II) and palladium(IV) terminal imido complexes by CASSCF as well as DFT calculations. In search of stable palladium terminal imido complexes, a series of synthetically relevant ancillary ligands are evaluated computationally. The thermodynamic stability of the palladium(II) complexes is correlated with the singlet/triplet gap of the compounds, whereas the palladium(IV) complexes are stabilized by strong electron donor ligands as indicated by the Tolman electronic parameter.

A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N‐Alkylation, deprotonation and metalation with CuI/AgI/AuI synthons affords the heteroleptic triazolylidene complexes. Due to the combination of neutral, electron‐donating ferrocenyl substituents and cationic, strongly electron‐withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting “push–pull”, “pull–push” and “pull–pull” metalloligands with further switchable redox states based on their fully reversible FeII/FeIII, (ferrocene/ferrocenium) and CoIII/CoII, (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron‐poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the AuI complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.

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.

Cyclic (alkyl)(amino)carbenes cAACcy (1b) and cAACmenthyl (1c) react with [Ni(CO)4] to give the 18 VE complexes [Ni(CO)3(cAACcy)] (2b) and [Ni(CO)3(cAACmenthyl)] (2c). With these in hand, the donor-strength and the steric profile of the respective cAAC ligands were evaluated. CAACcy and cAACmenthyl possess similar overall-donating properties (Tolman electronic parameter (TEP) = 2046 (1b) and 2042 (1c)) as common NHCs, though they are also known to be better π-acceptors. 3,3-Diamino-2-aryloxyacrylimidamide 3b, arising from the reaction of cAACcy (1b) with released CO molecules, was obtained as side-product of CO substitution reactions at nickel carbonyls. In contrast to cAACmenthyl (%Vbur = 42), the sterically less encumbered cAACcy (%Vbur = 38) undergoes a subsequent CO substitution at [Ni(CO)3(cAACCy)] (2b) to afford the 16 VE complex [Ni(CO)(cAACcy)2] (4b). Treatment of both [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with allyl bromides led to the formation of cAAC-stabilized allyl nickel...

We present a convenient three-step synthesis of amino substituted phosphazenyl phosphines of the general formula (R2N)3P=N–P(NR2)2 [NR2 = N(CH2)4, N(CH2)5, N(CH2)6]. These easily accessible mixed valent compounds display a surprisingly high proton affinity and basicity in the same range as the corresponding Schwesinger diphosphazene (Me2N)3P=N–P=NEt(NMe2)2 (Et-P2) and Verkade’s proazaphosphatrane superbases. Within the central [PIII–N=PV] scaffold, the phosphine PIII and not the phosphazene NIII atom is the center of highest proton affinity, basicity and donor strength. As P-bases, the title compounds display calculated proton affinities between 265.8 (NR2 = NMe2) and 274.7 kcal·mol–1 [NR2 = N(CH2)4] and pKBH+ values between 26.4 (NR2 = NMe2) and 31.5 [NR2 = N(CH2)4] on the acetonitrile scale. As P- nucleophiles, they are key intermediates in the synthesis of hyperbasic bis(diphosphazene) proton sponges, chiral bis(diphosphazene) proton pincers, bisphosphazides, and superbasic P2- bisylides. Their Staudinger reactions as nucleophile towards 1, 8-diazidonaphthalene leading to 1, 8-naphthalene-bisphosphazides is described in detail. The donor strength of the title compounds towards fragments [Se] and [Ni(CO)3] is in the same range as that of N-heterocyclic carbenes.

Azobenzene-based protonated N-heterocyclic carbenes (NHCs), N,N'-bis(azobenzene)imidazolium chlorides (IAz-X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p-methoxy-azobenzene derivative get transformed from all-trans isomer to nearly all-cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p-methoxy-azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, [(IAz-OMe)IrCl(CO)2 ] was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop-casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all-trans isomer ( ν ˜ t t av ) and all-cis isomer ( ν ˜ c c a v ) indicates a significant decrement of Δtt-cc ν ˜ av =2.7 cm-1 (film) and 3.8 cm-1 (ATR). Therefore, moderate to excellent enhancement of electron density (Δtt-cc TEP=2.3 cm-1 [film] and 3.2 cm-1 [ATR]) at the C-2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron-donating ability at the catalytic center would pave the way to control NHC/NHC-metal catalyzed organic transformations through external stimuli.

Electron‐rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1‐alkylpyridin‐4‐ylidenamino and 1‐alkylpyridin‐2‐ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N‐heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine‐mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2. In addition, the coordination chemistry of the new phosphines towards CuI, AuI, and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air‐stable phosphonium salt is described., Highly electron‐rich phosphines decorated with 1‐alkylpyridinylidenamino substituents (PyAPs) are accessible in one or two steps starting from inexpensive commercially available aminopyridines and chlorophosphines. The most basic PyAPs were used for the reversible CO2 fixation and the formation of electron‐rich metal complexes relevant to catalysis.

A series of substituted phosphinines, 1-phosphabarrelenes and 5-phosphasemibullvalenes were synthesized and evaluated for their potential application as ligands in homogeneous catalytic reactions. While their buried volume (%Vbur) was calculated to get insight into the steric properties, [LNi(CO)3] complexes were prepared in order to determine the corresponding Tolman electronic parameter. ETS-NOCV (extended-transition-state natural orbital for chemical valence) calculations on [LAuCl] complexes further allowed an estimation of the σ‐ and π‐contributions to the L-M interaction. AuI coordination compounds of selected examples were prepared and characterized by single crystal X-ray diffraction. Finally, the three classes of PIII compounds were successfully used in the AuI-catalyzed cycloisomerization of N-2-propyn-1-ylbenzamide, showing very good activities and selectivities, which are comparable with the reported data of cationic phosphorus-based gold catalysts.

Synthesis and coordination of 4-halo-1,2,3-triazolylidenes have been developed. These novel ligands featured the character with σ-donation at carbon and a σ-hole at the halogen. Halogen bonding was observed by single crystal X-ray diffraction in their coinage metal complexes. The electronic properties of 4-iodo-1,2,3-triazolylidene were studied by both Ir–CO frequencies of the Tolman electronic parameter (TEP) and Huynh's electronic parameter (HEP) method, which suggested similar electronic properties to those of imidazolylidenes. During HEP tests, an interesting tunability was observed when different electron donors were employed.

The use of different N-heterocyclic carbene (NHC) as dative ligands benefits from their capabilities ranging from strong to weak σ-donor according to the Tolman electronic parameter (TEP), well spread in the formation of monometallic complexes, and recently extended to metallic nanocluster and surfaces. Here, we set to explore the role of relativistic effects in determining the stabilization of the dative carbon–metal bond on coinage metal complexes given by NHC-MCl species (M = Cu, Ag, Au), by quantifying their contribution to the different terms related to the bond formation by comparing scalar relativistic and non-relativistic DFT calculations. Our results show a strong contribution to the stabilization of the NHC-M bond, increasing along with the group, from the lighter Cu atom (7.9%) to Ag (15.3%), and most notoriously for the heavier member, Au (39.9%). This observation is similar along the different stabilizing terms related to the C-M bond, where the acceptor character of the metal center is affected, resulting in a stronger σ-donor interaction from the involved ligand. Thus, the theoretical evaluation of carbon–metal bonds requires the equal footing treatment of relativistic effects along with the group, which is beneficial for the theoretical evaluation of ligand capabilities prior to the engaging exploration of the efficient formation of related complexes involving lighter to heavy metallic centers.

The synthesis, structure, and full characterization of a redox-switchable germylene based on a [3]ferrocenophane ligand arrangement, [Fc(NMes)2Ge] (4), is presented. The title compound is readily available from Fc(NHMes)2 (2) and Ge{N(SiMe3)2}2, or from the dilithiated compound Fc(NLiMes)2 · 3 Et2O (3) and GeCl2. Cyclic voltammetry studies are provided for 4 confirming the above mentioned view of a redox-switchable germylene metalloligand. Although several 1:1 Rh(I) and Ir(I) complexes of 4 (5 - 7) are cleanly formed in solution, all attempts to isolate them in pure form failed due to stability problems. However, crystalline solids of [Mo(κ1Ge-4)2(CO)4] (8) and [W(κ1Ge-4)2(CO)4] (9) were isolated and fully characterized by common spectroscopic techniques. DFT calculations were performed on a series of model compounds in order to elucidate a conceivable interplay between the metal atoms in neutral and cationic bimetallic complexes of the type [Rh(κ1E-qE)(CO)2Cl]0/+ (qE = [Fc(NPh)2E] with E = C, Si, Ge). The bonding characteristics of the coordinated Fc-based metalloligands (qE/qE+) are strongly affected upon in silico oxidation of the calculated complexes. The calculated Tolman electronic parameter (TEP) significantly increase by ca. 20 cm-1 (E = C) to 25 cm-1 (E = Si, Ge) upon oxidation. The change in the ligand donating abilities upon oxidation can mainly be attributed to Coulombic effects, while an orbital-based interaction appears to have only a minor influence.

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.

A new rhenium complex, cis-[ReO(btmp)PPh3Cl2] (2a) with 2-benzothiazolyl-5-methoxyphenol (Hbtmp) has been synthesized and characterized by means of FT-IR, UV–Vis, 1H NMR and elemental analyses. The structural parameters of 2a as obtained from XRD studies have been compared with the calculated [PBEPBE/STMIDI] values. The observed results are in good agreement. The vibrational frequencies, electronic transitions of 2a have been calculated and compared with the experimental results. The global chemical reactivity descriptors based on conceptual density functional theory have also been calculated for a set of analogous systems derived from 2a varying the central metal center (Re or Tc) and substituted phosphine moieties. A good relationship between the calculated hardness (η) and Tolman’s cone angle (θ) of phosphines is observed both for Re and Tc complexes. Similar harmony between electronic chemical potential (μ) and Tolman’s electronic parameter (ν) of phosphines is also noted.

Achieving chemo- and regioselectivity simultaneously is challenging in organic synthesis. Transition metal-catalyzed reactions are effective in addressing this problem by the diverse ligand effect on the catalyst center. Ligand-controlled regioselective Pd-catalyzed carbonylation of styrenes with aminophenols was realized, chemoselectively affording amides. Using a combination of boronic acid and 5-chlorosalicylic acid as the additives, linear amides were obtained in high yields and selectivity using tris(4-methoxyphenyl)phosphine (L3) in acetonitrile, while branched amides were obtained in high yields and selectivity in butanone by changing the ligand to 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane (L5). Further studies show that the nature of the ligand is key to the regioselectivity. Cone angle and Tolman electronic parameter (TEP) have been correlated to the reactivity and regioselectivity. Studies on the acid additives show that different acids act as the proton source and the corresponding counterion can help enhance the reactivity and selectivity.

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.

t 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 pKurn:x-wiley:14337851:media:anie201903342:anie201903342-math-0001 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.

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength-related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.

The applicability of two types of transition-metal carbonyl complexes as appropriate candidates for computationally derived Tolman&rsquo, s ligand electronic parameters were examined with density functional theory (DFT) calculations employing the B97D3 functional. Both Pd(0)L2(CO) and HRh(I)L2(CO) complexes correlated well with the experimental Tolman Electronic Parameter scale. For direct comparison of the electronic effects of diphosphines with those of monophosphines, the palladium-containing system is recommended. The t r a n s influence of various phosphines did not show a major difference, but the decrease of the H-Rh-P angle from linear can cause a significant change.

Reactions of the homoleptic and heteroleptic antimony ligands SbiPr3, SbiPr2Ph, 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), e ≈ 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(SbiPr3)2 reacts with CO to form the TBP complex Ni(I)2(SbiPr3)2(CO). Lastly, the complexes were investigated for nickel metal deposition on Si|Cu(100 nm) substrates. The complexes with the strongest donating ligand, SbiPr3, deposited the purest ...

The coordination versatility of a 1,3,5-triphenylbenzene-tris-mesoionic carbene ligand is illustrated by the preparation of complexes with three different metals: rhodium, iridium and nickel. The rhodium and iridium complexes contained the [MCl(COD)] fragments, while the nickel compound contained [NiCpCl]. The preparation of the tris-MIC (MIC = mesoionic carbene) complex with three [IrCl(CO)2] fragments, allowed the estimation of the Tolman electronic parameter (TEP) for the ligand, which was compared with the TEP value for a related 1,3,5-triphenylbenzene-tris-NHC ligand. The electronic properties of the tris-MIC ligand were studied by cyclic voltammetry measurements. In all cases, the tris-MIC ligand showed a stronger electron-donating character than the corresponding NHC-based ligands. The catalytic activity of the tri-rhodium complex was tested in the addition reaction of arylboronic acids to α,β-unsaturated ketones. We gratefully acknowledge financial support from MINECO of Spain (CTQ2014-51999-P), UJI (P11B2014-02), Generalitat Valenciana (GV/2015/097) and FCT-Fundação para a Ciência e a Tecnologia (UID/Multi/04551/2013 and PTDC/QEQ- QIN/ 0565/2012). C. M. is thankful to the FPI program for a fellowship, B. R. thanks FCT for IF/00346/2013 and G. G.-B. thanks the MINECO for a postdoctoral grant (FPDI-2013-16525). The authors are grateful to the Serveis Centrals d’Instrumentació Científica (SCIC) of the Universitat Jaume I for providing with spectroscopic and X-ray facilities, and to FCT (RECI/BBBBQB/ 0230/2012).

4-Phenylsemicarbazide and 1,5-diphenylcarbazide are suitable starting materials for the syntheses of N-heterocyclic carbene (NHC) compounds with new backbone structures. In the first case, cyclisation and subsequent methylation leads to a cationic precursor whose deprotonation affords the triazolon-ylidene 2, which was converted to the corresponding sulfur and selenium adducts and a range of metal complexes. In contrast, cyclisation of diphenylcarbazide affords a neutral betain-type NHC-precursor 7, which is not in equilibrium with its carbene tautomer 7a. Precursor 7 can either be deprotonated to give the anionic NHC 8 or methylated at the N or O atom of the backbone resulting in two isomeric cationic species 16 and 20. Deprotonation of the latter two provides neutral NHC compounds with a carboxamide or carboximidate backbone, respectively. The ligand properties of the new NHC compounds were evaluated by IR and (77) Se NMR spectroscopy. Tolman electronic parameter (TEP) values range from 2050 to 2063 cm(-1) with the anionic NHC 8 being the best overall donor.

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-t-BuC5H3)NpN}2C:] (A'-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3'-positions. A'-Np was synthesized in nine steps starting from 1,1'-di-tert-butylferrocene (1), the first step being its 3,3'-dilithiation to afford rac-[Fe(η(5)-t-BuC5H3Li)2] (rac-fc'Li2, 2). The structures of rac-fc'(SiMe3)2 (3), rac-fc'Br2 (4), rac-fc'(N3)2 (5), and the immediate carbene precursor [A'-NpH]BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A'-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. -4.0 eV) and low singlet-triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm(-1) was derived from IR data of cis-[RhCl(A'-Np)(CO)2], indicating the high donicity of A'-Np as a ligand. Consistent with its ambiphilic nature, A'-Np was found to react readily with carbon monoxide, affording the betainic enolate (A'-Np)2CO as four stereoisomers, viz. (RpRp-A'-Np)═C(O(-))(RpRp-A'-Np(+)), (SpSp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), (RpRp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), and (SpSp-A'-Np)═C(O(-))(RpRp-A'-Np(+)). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A'-Np was found to react swiftly with dichloromethane, affording the addition product A'-NpH-CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH-CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A'-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of -0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A'-Np were found to be reversibly oxidized at half-wave potentials of -0.418 and -0.437 V, respectively.

The 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.

A series of iridium complexes containing phenanthro[4,5-abc]phenazino[11,12-d]imidazol-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidazol-2-ylidene ligands have been obtained and fully characterized. These complexes display highly extended polyaromatic systems attached to the backbone of the N-heterocyclic carbene. The presence of this extended polyaromatic system makes the electron-donating character of these ligands sensitive to the presence of π-stacking additives, such as pyrene and hexafluorobenzene. The computational studies predict that the addition of pyrene affords an increase of the electron-donating character of the polyaromatic ligand (TEP decreases), while the addition of hexafluorobenzene has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR spectroscopy, by measuring the shift of the CO stretching bands of a series of IrCl(NHC)(CO)2 complexes, where NHC is the N-heterocyclic carbene ligand with the polyaromatic system. Finally, the energy of the π-stacking interaction of one of the key Ir(I) complexes with pyrene and hexafluorobenzene has been estimated by using the Benesi-Hildebrand treat-ment, based on the δ-shifts observed by 1H NMR spectroscopy. MEC of Spain (CTQ2011-24055/BQU)

The catalytic activity of transition metal complexes (R)nM–L can be predicted utilizing the metal–ligand electronic parameter (MLEP) that is based on the local stretching force constant of the M-L bond. Vibrational spectroscopy is an excellent tool to accurately determine vibrational mode properties such as stretching frequencies. These correspond, because of mode–mode coupling, to delocalized vibrational modes, which have to be first converted into local vibrational modes, and their properties. Each bond of a molecule can be uniquely characterized by the local stretching force constant and frequency. The former is ideally suited to set up a scale of bond strength orders, which identifies weak M–L bonds with promising catalytic activity. It is shown how the MLEP replaces the TEP (Tolman Electronic Parameter), which is based on the CO stretching frequencies of a (CO)nM–L complex and which is now exclusively used in hundreds of investigations. However, the TEP is at best a qualitative parameter that suffers from relatively large mode–mode coupling errors and the basic deficiency of most indirect descriptors: They cannot correctly describe the intrinsic M–L bond strength via the CO stretching frequencies.

Synthesis and structural characterization of novel Rh(I) complexes bearing 1,2,4-triazol-3-ylidene or imidazol-2-ylidene substituted with an N-2-nitrobenzenesulfonyl (N-nosyl) group are described. The complexes were characterized by infrared (IR), NMR, and single-crystal X-ray diffraction analyses. The Tolman electronic parameter (TEP) values of both of the 1,2,4-triazol-3-ylidene (2064 cm−1) and imidazol-2-ylidene (2059 cm−1), as determined from the average carbonyl (CO) stretching frequencies for the corresponding RhCl(NHC)(CO)2 (NHC = N-heterocyclic carbene) complexes, were higher than that of RhCl(IMes)(CO)2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) (2050 cm−1). As the TEP values of the NHCs increased, the 13C NMR signal of the carbenic carbon in the corresponding RhCl(NHC)(cod) (cod = 1,5-cyclooctadiene) and RhCl(NHC)(CO)2 complexes shifted downfield. Moreover, the crystallographic analysis of the RhCl(NHC)(cod) complexes revealed that one of the S O bonds in the sulfonyl group could be conjugated with the NHC frameworks, while the remaining S O bond conjugated with the 2-nitrophenyl groups of the N-nosyl substituent.

This paper focuses on ferrocene-based expanded-ring N-heterocyclic carbenes (NHCs) of the type [Fe(C5H4–NR–C–NR′–C5H4)] (1-R/R′), which contain two different N-substituents. Three combinations were addressed, with R = neopentyl (Np) in each case and R′ being either 2-adamantyl (Ad), phenyl (Ph) or 9-anthracenylmethyl (Acm). The NHCs were generated by reaction of the corresponding formamidinium tetrafluoroborates [H–1-R/R′][BF4] with lithium diisopropylamide (LDA). While only 1-Np/Ad was sufficiently stable for isolation, 1-Np/Ph and 1-Np/Acm could be efficiently trapped in situ by complexation reactions. Two series of RhI complexes were prepared, viz. [RhCl(cod)(1-R/R′)] (cod = 1,5-cyclooctadiene) by reacting [{Rh(μ-Cl)(cod)}2] with 1-R/R′ and cis-[RhCl(CO)2(1-R/R′)] by reacting [RhCl(cod)(1-R/R′)] with CO. All complexes exhibit pronounced anagostic α-CH⋯Rh interactions, both in solution and in the solid state, in accord with a strong influence of the N-substituents on the steric ligand properties, as is chemically illustrated by the huge reactivity difference of [RhCl(cod)(1-Ad)] (R = R′ = Ad) and [RhCl(cod)(1-Np/Ad)] towards CO, the former complex being inert. Tolman electronic parameter (TEP) values are 2050 ± 1 cm−1 for the unsymmetrical NHCs studied, indicating only a weak influence of the N-substituents on the electronic ligand properties.

The 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.

International audience; By analogy to the Tolman electronic parameter, a ligand electronic parameter, referred to as L2EP, is introduced here for estimating the donating ability of chelating ligands, featuring two coordinating extremities. It is based on the average of the computed infrared stretching frequencies of CO in a series of isostructural rhodium(I)-dicarbonyl complexes, that is linearly correlated to the number x of N-heterocyclic carbene coordinating ends (x = 0, 1, or 2). The L2EP values allow the design of an unified scale for the classification of the electron donation of chelating ligands, based on an ortho-phenylene bridge substituted by two coordinating extremities, which may have a different donating character. Strengths and limitations of the L2EP scale are illustrated for a large diversity of bidentate chelating ligands with coordinating ends ranging from extremely electron-rich phosphonium yldiides to extremely electron-poor amidiniophosphonites.

Understanding the nature and the strength of metal-ligand interactions in d- and f-block metal complexes has always been a central issue for both synthetic and theoretical chemists. These interactions are usually described according to the well accepted Dewar-Chatt-Duncanson model, and thus over the years numerous research groups directed their efforts to shed light on the role of σ- and π-contributions. Among others, the electronic parameter introduced by Tolman in the 1970s represents a milestone in this field. Herein we present a quantitative description of the nickel-phosphine bond in Tolman's nickel(0) carbonyl complexes. The combination of Natural Orbitals for Chemical Valence with Energy Decomposition Analysis resulted in the definition of a new parameter (Tphos) which comprises all the energetic contributions needed to describe the nickel-phosphine bond and thus stands as a reliable descriptor of the electronic properties of phosphines. Moreover, steric effects of phosphines (i.e. Tolman's cone angles) have been considered too, and a linear relation including Ni-P bond distances, Tphos and cone angle has been found.

The deprotonation of 1,3-dimesitylbenzimidazolium tetrafluoroborate with a strong base afforded 1,3-dimesitylbenzimidazol-2-ylidene (BMes), which was further reacted in situ with rhodium or ruthenium complexes to afford three new organometallic products. The compounds [RhCl(COD)(BMes)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BMes)] were used to probe the steric and electronic parameters of BMes. Comparison of the percentage of buried volume (%V(Bur)) and of the Tolman electronic parameter (TEP) of BMes with those determined previously for 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-dimesitylimidazolin-2-ylidene (SIMes) revealed that the three N-heterocyclic carbenes (NHCs) had very similar profiles. Nonetheless, changes in the hydrocarbon backbone subtly affected the stereoelectronic properties of these ligands. Accordingly, the corresponding [RuCl2(PCy3)(NHC)(=CHPh)] complexes displayed different catalytic behaviors in the ring-closing metathesis (RCM) of α,ω-dienes. In the benchmark cyclization of diethyl 2,2-diallylmalonate, the new [RuCl2(PCy3)(BMes)(=CHPh)] compound (1d) performed slightly better than the Grubbs second-generation catalyst (1a), which was in turn significantly more active than the related [RuCl2(PCy3)(IMes)(=CHPh)] initiator (1b). For the formation of a model trisubstituted cycloolefin, complex 1d ranked in-between catalyst precursors 1a and 1b, whereas in the RCM of tetrasubstituted cycloalkenes it lost its catalytic efficiency much more rapidly.

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.

The Tolman electronic parameter (TEP) derived from the A1-symmetrical CO stretching frequency of nickel-tricarbonyl complexes L-Ni(CO)3 with varying ligands L is misleading as (i) it is not based on a mode decoupled CO stretching frequency and (ii) a generally applicable and quantitatively correct or at least qualitatively reasonable relationship between the TEP and the metal-ligand bond strength does not exist. This is shown for a set of 181 nickel-tricarbonyl complexes using both experimental and calculated TEP values. Even the use of mode-mode decoupled CO stretching frequencies (L(ocal)TEPs) does not lead to a reliable description of the metal-ligand bond strength. This is obtained by introducing a new electronic parameter that is directly based on the metal-ligand local stretching force constant. For the test set of 181 nickel complexes, a direct metal-ligand electronic parameter (MLEP) in the form of a bond strength order is derived, which reveals that phosphines and related ligands (amines, arsines, stibines, bismuthines) are bonded to Ni both by σ-donation and π-back-donation. The strongest Ni-L bonds are identified for carbenes and cationic ligands. The new MLEP quantitatively assesses electronic and steric factors.