Strong Σ-donor Research Articles

Synthesis of Cyclic Alkyl(amino) Carbene Stabilized Silylenes with Small N-Donating Substituents.

Lewis base cAACs stabilized monomeric silylenes with halogen or methyl substituents at the silicon center have not been reported due to the strong σ-donor and π-acceptor character of cAAC. To prepare these monomeric silylenes, we used the silicon(IV) precursors 5 and 6 with a nitrogen donor group L (L=o-C6 H4 NMe2 ). The cAAC-stabilized (cAAC=C(CH2 )(CMe2 )2 N-Ar, Ar=2,6-iPr2 C6 H3 ) silylenes LSiCl(cAAC) (7) and LSiMe(cAAC) (8) were synthesized by reduction of LSiCl3 and LSiMeCl2 with two equivalents of KC8 in the presence of one equivalent of cAAC, respectively. Compounds 7 and 8 were characterized by single-crystal X-ray crystallography, NMR spectroscopy, and elemental analysis. Compounds 7 and 8 are stable in the solid state as well as in solution at room temperature for at least four months under inert conditions.

B-B Coupling and B-B Catenation: Computational Study of the Structure and Reactions of Metal-Bis(borylene) Complexes.

A detailed molecular orbital analysis of the metal-bis(borylene) complex [Fe(CO)3 {B(Dur)B(N(SiMe3 )2 )}] (Dur=2,3,5,6-tetramethylphenyl) (1 a) serves as a focal point of recent developments in this area of chemistry, such as B-B coupling and B-B catenation reactions. There is strong a π delocalization between the Fe(CO)3 and (B-Dur)(B-N(SiMe3 )2 ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe3 )2 on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B σ-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong σ-donating ligands, such as PMe3 or PCy3 , induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to [Fe(CO)2 {B(Dur)B(N(SiMe3 )2 )}]2 (3 a) with a short Fe-Fe distance of 2.355 Å. A detailed mechanism for the reaction of 3 a with CO to give the B-B catenation product 2 f is presented. The bonding of all intermediates is compared to their isolobal main-group analogues.

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer.

In this work, the effect of the electronically different ancillary ligands on the overall properties of the RuIIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl]0/+ was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CH3CN)Cl2] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]+; bpy: 2,2'-bipyridyl (moderate π-acceptor), [3]+; acac-: acetylacetonate (strong σ-donor), 4; and DTBCat2-: 3,5-di- tert-butyl catecholate (strong π-donor), 5. The complexes [2]+, [3]+, 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2]+ (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. ( J. Am. Chem. Soc., 2015, 137, 12330). Aerial O2 regenerates the catalyst via oxidation of the hydrazo intermediate.

Essential Role of Ancillary Ligand in Color Tuning and Quantum Efficiency of Ir(III) Complexes with N-Heterocyclic or Mesoionic Carbene Ligand: A Comparative Quantum Chemical Study.

Tuning photoluminescence properties is of prime importance for designing efficient light emitting diode (LED) materials. Here, we perform a computational study on the effect of normal N-heterocyclic carbene (NHC) and abnormal mesoionic carbene (MIC) ligands on the photoluminescence properties of some Ir(III) complexes, which are very promising LED materials. We find MIC as the privileged ligand in designing triplet emitters. The strong σ-donating and moderate π-accepting properties of MIC render a lower access to the nonemissive triplet metal-centered state (3MC), resulting in lowering the nonradiative rate constant ( knr) and correspondingly achieving higher quantum efficiency. We also demonstrate that the judicial choice of ancillary ligand can improve the efficiency of these materials even further. This quantum chemical investigation focuses on the importance of MIC as cyclometalating ligand and the substantial effects of ancillary ligands in controlling the color tuning and quantum efficiency for optoelectronic applications.

4-Phosphino-1,2,3-triazol-5-ylidenes: Stable Bridging Ligands for Coinage Metal Complexes

The synthesis and coordination of 4-phosphino-1,2,3-triazol-5-ylidenes have been developed. This ligand, which is thermally stable and a strong σ-donor, shows diverse coordination modes with coinage metals.

Reductions of a Rhenium(III) Terminal Oxo Complex by Isocyanides and Carbon Monoxide

We report on the reductive reactivity of the d4 rhenium(III) oxo olefin complex ORe(η2-DHF)(BDI) (DHF = dihydrofulvalene, BDI = N,N′-bis(2,6-diisopropylphenyl)-2,4-dimethyl-β-diketiminate) with isocyanides, R–NC (R = tBu, 2,6-xylyl), as well as carbon monoxide (CO). These reactions all proceed by an oxygen atom transfer (OAT) to generate rhenium(I) BDI complexes containing 4 equiv of the respective π-acidic reagent. In the case of isocyanides, the products (R–NC)4Re(BDI) (1, R = tBu; 2, R = 2,6-xylyl) were isolated and characterized. While these tetrakis(isocyanide) complexes are stable at room temperature, the tetracarbonyl complex (CO)4Re(BDI) quickly degraded upon formation, leading to demetalation of the BDI ligand. However, inclusion of a strong σ-donor in the reaction of ORe(η2-DHF)(BDI) with CO afforded the tricarbonyl complex fac-(CO)3Re(DMAP)(BDI) (3). In addition to 3, the oxidation product (O)2Re(DMAP)(BDI) (4) was also consistently isolated from this reaction in low yield. We hypothesize compl...

Blue-Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons.

Room-temperature luminescent CoIII complexes (1 and 2) are presented that exhibit intense ligand-to-metal and ligand-to-ligand charge transfer absorption in the low-energy UV region (λabs ≈360-400 nm) and low-negative quasi-reversible reduction events (E1/2(red) =-0.58 V and -0.39 V vs. SCE for 1 and 2, respectively). The blue emission of 1 and 2 at RT is due to the large bite angles and strong σ-donation of the ligands, the combined effect of which helps to separate the emissive 3 LMCT (triplet ligand-to-metal charge transfer) and the non-emissive 3 MC (triplet metal-centered) states. 1 and 2 were found to be powerful photo-oxidants (ECoIII*/CoII =2.26 V and 2.75 V vs. SCE of 1 and 2, respectively) and were used as inexpensive photoredox catalysts for the regioselective mono(trifluoromethylation) of polycyclic aromatic hydrocarbons (PAHs) in good yields (ca. 40-58 %).

Synthesis of Stable Pentacoordinate Silicon(IV)–NHC Adducts: An Entry to Anionic N-Heterocyclic Carbene Ligands

This work features the previously undescribed interactions of Martin’s spirosilane with different types of N-heterocyclic carbenes (NHCs). The level of interaction proved to be strongly dependent on the size of the Lewis base and could vary from the formation of isolable classical Lewis adducts to abnormal Lewis adducts, as evidenced by X-ray diffraction structure analyses and NMR studies. It has been found that abnormal adducts could be used as precursors for the synthesis of anionic NHCs bearing a weakly coordinating siliconate component. Complexation of these new types of carbenes with gold(I) and copper(I) has been efficiently accomplished. DFT calculations performed on the siliconate-based anionic NHC ligands revealed a high-lying HOMO and therefore a strong σ-donor character.

Oxidative P-P Bond Addition to Cobalt(-I): Formation of a Low-Spin Cobalt(III) Phosphanido Complex.

The first homoleptic cobalt phosphanido complex [K(thf)4 ][Co{1,2-(PtBu2 )2 C2 B10 H12 }2 ] (1) was prepared by an unprecedented oxidative P-P bond addition of an ortho-carborane-substituted 1,2-diphosphetane to cobalt(-I) in [K(thf)0.2 ][Co(η4 -cod)2 )] (cod=1,5-cycloctadiene). Compound 1 is a rare distorted tetrahedral 3d6 complex with a low-spin ground state configuration. Magnetic measurements revealed that the complex is diamagnetic between 2 to 270 K in the solid state and at 298 K in [D8 ]THF solution. Based on DFT calculations, the unusual singlet ground state is caused by the strong σ-donor and moderate π-donor properties of the bis(phosphanido) ligand.

Tuning the locus of oxidation in Cu-diamido-diphenoxo complexes: From Cu(III) to Cu(II)-phenoxyl radical

While N2O2 tetraanionic ligands containing a strong N-amidate σ-donor are generally assumed to stabilise metal high valence states, we herein have shown that, in dianionic Cu(II)-diamido-diphenoxo complexes, H-bonding and electronic effects on the phenolate groups may modulate the electronic structure of their oxidised species from Cu(III) to Cu(II)-phenoxyl radical complexes; and so in the negative potential range. We observe that electron-poor phenolate complexes 22− and 32− oxidise to Cu(III) species, whereas electron rich phenolate complex 12− oxidises to a Cu(II)-phenoxyl radical. Our DFT results suggest that π-electron-rich phenolate rings in 12− are responsible for an increase of the HOMO orbital energy, bringing the HOMO-SOMO gap small enough to favour a ligand-based oxidation process. Further DFT-calculations have also shown that upon changing the o,p-phenol substituent from electron-widthdrawing groups (NO2) to electron-donating ones (OMe), the favoured oxidised state switches from Cu(III) to Cu(II)-radical. These results emphasize the use of the versatile diamido-diphenoxo backbone asa promising way to novel GO-chemical models, as well as molecular switches.

Carbodicarbenes: Unexpected π-Accepting Ability during Reactivity with Small Molecules.

An investigation of carbodicarbenes, the less explored member of the carbenic complex/ligand family has yielded unexpected electronic features and concomitant reactivity. Observed 1,2-addition of E-H bonds (E = B, C, Si) across the carbone central carbon and that of the flanking N-heterocyclic carbene (NHC) fragment, combined with single-crystal X-ray studies of a model Pd complex strongly suggests a significant level of π-accepting ability at the central carbon of the NHC moiety. This feature is atypical of classic NHCs, which are strong σ-donors, with only nominal π-accepting ability. The unanticipated π-acidity is critical for engendering carbodicarbenes with reactivity more commonly observed with frustrated Lewis pairs (FLPs) rather than the more closely related NHCs and cyclic (alkyl)(amino)carbenes (CAACs).

Exploring the Hydrolytic Behavior of the Platinum(IV) Complexes with Axial Acetato Ligands.

Platinum(IV) complexes are generally thought to be kinetically inert, and are expected to be stable enough to resist premature aquation before entering the cancer cells. Nevertheless, in this work, complex 2 with axial acetato ligands can hydrolyze relatively quickly under biologically relevant conditions with a half-life of 91.7 min, resulting in the loss of the equatorial chlorido ligand. Further study indicated that the fast hydrolysis of complex 2 may be attributed to the strong σ-donor ability of N-isopropyl-1R,2R-diaminocyclohexane, and an increasing σ-donor ability of the amine group can promote the hydrolysis rate of the corresponding platinum(IV) complex. The experiment results were proven by the corresponding DFT calculation. Our study can help to re-evaluate the aqueous properties of the platinum(IV) complexes with axial acetate, which may be less inert to hydrolysis than expected under biologically relevant conditions.

A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic Carbene Sites, for the Efficient Hydrogenation of CO2 to Formate

Functionalizing the recently developed porous materials such as porous organic frameworks and coordination polymer networks with active homogeneous catalytic sites would offer new opportunities in the field of heterogeneous catalysis. In this regard, a novel covalent triazine framework functionalized with an Ir(III)-N-heterocyclic carbene complex was synthesized and characterized to have a coordination environment similar to that of its structurally related molecular Ir complex. Because of the strong σ-donating and poor π-accepting characters of the N-heterocyclic carbene (NHC) ligand, the heterogenized Ir-NHC complex efficiently catalyzes the hydrogenation of CO2 to formate with a turnover frequency of up to 16 000 h–1 and a turnover number of up to 24 300; these are the highest values reported to date in heterogeneous catalysis for the hydrogenation of CO2 to formate.

Structures, Energies, and Bonding Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues

In this work, we computationally investigated from quantum chemical calculations (DFT) at the BP86 level with the various basis sets def2-SVP, def2-TZVPP, and TZ2P+, chemical bonding issues of the recently described carbene-analogues gold(I) complexes AuCl-NHEMe (Au1-NHE) with E = C – Pb. The optimized structures and the metal-ligand bond dissociation energy (BDE) were calculated, and the nature of the E→Au bond was studied with charge and energy decomposition methods. The equilibrium structures of the system showed that there were major differences in the bonded orientation from the ligands NHC-NHPb to gold(I) complex between the lighter and the heavier homologues. The BDEs results showed that the metal-carbene analogues bonds were very strong bonds and the strongest bond was calculated for Au1-NHC which had the bond strength De = 79.2 kcal/mol. Bonding analysis of Au1-NHE showed that NHE ligands exhibited donor-acceptor bonds with the σ lone pair electrons of NHE donated into the vacant orbital of the acceptor fragment (AuCl). The EDA-NOCV results indicated that the ligand NHE in Au1-NHE complexes were strong σ-donors and very weak π donor and the bond order in complexes was Au1-NHC > Au1-NHSi > Au1-NHGe > Au1-NHSn > Au1-NHPb. We also realised that the gold-ligand bond was characterized by a π back-donation component from the Au to the ligand. All investigated complexes in this study were suitable targets for synthesis and gave a challenge in designing Au nano-crystals of narrow size distribution from gold(I) complexes that carried versatile N-heterocyclic carbene-analogues NHE.

A DFT Study on the Stabilization of the B≡B Triple Bond in a Metallaborocycle: Contrasting Electronic Structures of Boron and Carbon Analogues.

The electronic structure of (η5 -Cp)2 Zr(NH2 -BB-NH2 ) (3 b) suggests that it could be a candidate for having a boron-boron triple bond in the cyclic system; however, computational studies shows that 3 b is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η5 -Cp)2 Zr[B(PH3 )2 -BB-B(PH3 )2 ] (3 c) reduces the relative energy dramatically. The B≡B triple bond arises through the donation of two electrons from the metal fragment, ZrCp2 , to the in-plane π-bonding orbital of the central B-B unit. Strong σ-donating and chelating bis-phosphine ligands (Me2 P(CH2 )n PMe2 ), which stabilize donor-acceptor bonding interaction in gem-diborene L2 B-BBr2 (10), would be a good choice along the synthetic path towards 3 d, (η5 -Cp)2 Zr[B4 (Me2 P(CH2 )3 PMe2 )2 ]. A comparison of the energetics of the reaction leading to a cyclic boryne system (3 d), with the linear boryne isomer [(B2 NHCPh )2 ] shows that the angle strain from cyclization is compensated by stabilization from the metal.

Acenaphthoimidazolylidene Gold Complex-Catalyzed Alkylsulfonylation of Boronic Acids by Potassium Metabisulfite and Alkyl Halides: A Direct and Robust Protocol To Access Sulfones

A robust acenaphthoimidazolylidene gold complex is demonstrated as a highly efficient catalyst in the direct alkylsulfonylation of boronic acids. Remarkably, a wide range of highly reactive and unreactive C-electrophiles were well-tolerated to produce various (hetero)aryl-alkyl, aryl-alkenyl, and alkenyl-alkyl sulfones in satisfactory yields with 5 mol % catalyst loading. Along with the steric properties of NHC ligands, the high catalytic activity of this gold complex suggests that the strong σ-donation of acenaphthoimidazolylidene also played a role in promoting this challenging redox-neutral catalytic process.

Theoretical insights into the nature of PtSn bond: Reevaluating the bonding/back-bonding properties of trichlorostannate with comparison to the cyano ligand.

The coordinating properties of the trichlorostannate ligand in hydrido platinum trichlorostannato and platinum bis-trichlorostannato complexes, containing various phosphine ligands, have been elucidated by means of charge decomposition analysis, the Ziegler-Rauk Energy Decomposition with the Natural Orbitals for Chemical Valence, Domain-Averaged Fermi Hole, and natural bond orbital methods. Trichlorostannate has been found to be a strong σ-donor and a weak π-acceptor ligand with coordination properties not far from those of the cyano ligand. For back-bonding, the tin-chlorine σ∗ orbitals are mostly responsible. In contrast to previous assumptions, the 5d orbitals of tin play no role at all in the interaction with the platinum center. QTAIM calculations suggest, that the platinum-tin interaction should be interpreted as donor-acceptor, rather than covalent type. Trichlorostannate has been found to have weaker trans influence in comparison to the cyano ligand. © 2017 Wiley Periodicals, Inc.

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made. However, limited systems have succeeded in the catalytic formation of ammonia. Here we show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N2)2(PCP)}2(μ-N2)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. of ammonia are produced based on the catalyst. DFT calculations on 1 reveal that the PCP-pincer ligand serves as not only a strong σ-donor but also a π-acceptor. These electronic properties are responsible for a solid connection between the molybdenum centre and the pincer ligand, leading to the enhanced catalytic activity for nitrogen fixation.

Bonding Situation of Bis-gold Chloride Complexes with N-heterocyclic Carbene-Analogues [(AuCl)2-NHEMe] (E=C – Pb) based on DFT Calculations

Abstract We computationally investigated the structure and bonding situation of bis-gold chloride complexes, (AuCl)2, containing N-heterocyclic carbenes and analogues, called tetrylenes [(AuCl)2-NHEMe] (Au2-NHE) with E=C to Pb, using density functional theory (DFT) at the BP86 level with the basis sets def2-SVP, def2-TZVPP, and TZ2P+. The nature of the (AuCl)2-NHEMe bond in the Au2-NHE complexes was analyzed using charge and energy decomposition methods. The calculated equilibrium structures of the Au2-NHE complexes showed that the tetrylene ligands, NHEMe (E=C to Ge), are bonded in a head-on fashion to the bis-gold chloride fragment, (AuCl)2, and that the structure of Au2-NHSn contains a distorted head-on NHSn ligand, and the NHPb ligand is bonded in a side-on mode to the bis-gold chloride fragment, (AuCl)2. The theoretical calculation of bond dissociation energy (BDE) suggests that the bis-gold chloride-NHEMe bond strength increases from Au2-NHC to Au2-NHSi and then significantly decreases from Au2-NHSi to Au2-NHSn, and the strongest bond is exhibited for Au2-NHPb. The EDA-NOCV results and NOCV pairs indicate that the NHEMe ligand in the Au2-NHE complexes is a strong σ-donor and a weak π-acceptor, as well as a very weak π-donor (AuCl)2←{NHEMe}. The trend in the Au-E bond strength resulted from the increase in (AuCl)2←NHEMe donation. The results of this study may establish an interesting class of compounds worthy of further study.

Pseudo electron-deficient organometallics: limited reactivity towards electron-donating ligands.

Half-sandwich metal complexes are of considerable interest in medicinal, material, and nanomaterial chemistry. The design of libraries of such complexes with particular reactivity and properties is therefore a major quest. Here, we report the unique and peculiar reactivity of eight apparently 16-electron half-sandwich metal (ruthenium, osmium, rhodium, and iridium) complexes based on benzene-1,2-dithiolato and 3,6-dichlorobenzene-1,2-dithiolato chelating ligands. These electron-deficient complexes do not react with electron-donor pyridine derivatives, even with the strong σ-donor 4-dimethylaminopyridine (DMAP) ligand. The Ru, Rh, and Ir complexes accept electrons from the triphenylphosphine ligand (σ-donor, π-acceptor), whilst the Os complexes were found to be the first examples of non-electron-acceptor electron-deficient metal complexes. We rationalised these unique properties by a combination of experimental techniques and DFT/TDFT calculations. The synthetic versatility offered by this family of complexes, the low reactivity at the metal center, and the facile functionalisation of the non-innocent benzene ligands is expected to allow the synthesis of libraries of pseudo electron-deficient half-sandwich complexes with unusual properties for a broad range of applications.