Bis Bidentate Research Articles

A Genuine Trivalent Bis‐Acylphosphide (BAP) Complex of Uranium

AbstractThe genuine trivalent uranium complex, K[UIII(mesBAP)4], employing four bidentate bis(2,4,6‐trimethylbenzoyl)phosphide (mesBAP) chelating ligands, is reported and obtained by the reduction of the literature‐known tetravalent analog [UIV(mesBAP)4]. Hence, the bis(acyl)phosphide mesBAP ligand allowed to establish a fully reversible redox couple with uranium in the oxidation states +III and +IV. In both complexes [U(mesBAP)4]0/−, the uranium ions are coordinated to four mesBAP ligands in a square antiprismatic geometry. All new compounds have been characterized by single‐crystal XRD analysis, 1H and 31P NMR, and UV/Vis/NIR electronic absorption spectroscopy, as well as SQUID magnetization and electrochemical measurements, and CW X‐band EPR in the case of the trivalent complex.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes in Catalytic N-Formylation/N-Methylation of Amines Using Carbon Dioxide and Phenylsilane.

A series of six Mn(I) complexes with general formula [MnBr(bisNHC)(CO)3 ], having a bidentate bis(N-heterocyclic carbene) ligand (bisNHC), has been developed by varying the bridging group between the NHC donors, the nitrogen wingtip substituents and the heterocyclic ring. The synthesis of the complexes has been accomplished by in situ transmetalation of the bisNHC from the corresponding silver(I) complexes. Removal of the bromide anion affords the corresponding solvento complexes [Mn(bisNHC)(CO)3 (CH3 CN)](BF4 ). The influence of the bisNHC structure on its electron donor ability has been evaluated by FTIR and 13 C NMR spectroscopy, both in the neutral and cationic complexes. Finally, the isolated Mn(I)-bisNHC complexes have been employed as homogeneous catalysts in the reductive N-formylation and N-methylation of amines with CO2 as C1 source and phenylsilane as reducing agent, showing a high selectivity for the N-methylated product. Preliminary mechanistic investigations suggest that, in the adopted reaction conditions, the formylated product can be formed via different reaction pathways, either metal-catalyzed or not, while the methylation reaction requires the use of the Mn(I) catalyst.

Experimental and Computational Studies on Cobalt(I)‐Catalyzed Regioselective Allylic Alkylation Reactions

AbstractHere, we report the development of cobalt(I)‐catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3‐dicarbonyl compounds. A family of well‐defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3)Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3)Cl] (dppp=1,3‐Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt‐catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions.

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3 )Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3 )Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

Structure Elucidation of Lithium Compounds Using 7 Li Residual Quadrupolar Couplings.

A new NMR method for the structure elucidation of lithium compounds under solution-like conditions is presented. It is based on the measurement of 7 Li residual quadrupolar couplings (RQCs) in a stretched polystyrene (PS) gel, and comparison to RQCs predicted from crystal or DFT-derived model structures in combination with alignment tensors derived from one-bond 1 H,13 C residual dipolar couplings (RDCs). The method was applied to five lithium model complexes containing monoanionic, bidentate bis(benzoxazole-2-yl)methanide, bis(benzothiazole-2-yl)methanide and bis(pyridyl)methanide ligands, of which two are first introduced in this work. In agreement with the crystalline state, four complexes are monomeric with Li coordinated fourfold by two additional THF molecules, whereas in one complex bulky tBu groups only provide space for one additional THF molecule.

Control in Advanced Biofuels Synthesis via Alcohol Upgrading: Catalyst Selectivity to n‐Butanol, sec‐Butanol or Isobutanol

AbstractRuthenium complexes with tetradentate PNNP donor ligands demonstrate a marked change in selectivity compared to analogous bis bidentate PN complexes in Guerbet catalysis, producing mixtures of n‐butanol (17 %), sec‐butanol (14 %) and ethyl acetate (66 %) rather than the usual 90 %+ selectivity to n‐butanol. Tridentate PNP ruthenium complexes such as [Ru(H)(Cl)(CO)(Ph2PCH2CH2NHCH2CH2PPh2)] also produce sec‐butanol and, in optimized conditions (120 °C, 10 mol% NaOEt base), achieve 71 % selectivity to this butanol isomer. The same tri‐ and tetradentate complexes are efficient catalysts for the conversion of methanol/ethanol mixtures to isobutanol (up to 97 % selectivity). In this way, judicious choice of ligand within this general catalyst family allows selectivity to three butanol isomers of interest as fuel molecules.

The Conversion of Superoxide to Hydroperoxide on Cobalt(III) Depends on the Structural and Electronic Properties of Azole-Based Chelating Ligands.

Conversion from superoxide (O2−) to hydroperoxide (OOH−) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported by facially-capping tridentate tris(3,5-dimethyl-4-X-pyrazolyl)hydroborate ([HB(pzMe2,X)3]−; TpMe2,X) and bidentate bis(1-methyl-imidazolyl)methylborate ([B(ImN-Me)2Me(Y)]−; LY) ligands toward H-atom donating reagent (2-hydroxy-2-azaadamantane; AZADOL) has been explored. The oxygenation of the cobalt(II) precursors give the corresponding cobalt(III)-superoxido complexes, and the following reaction with AZADOL yield the hydroperoxido species as has been characterized by spectroscopy (UV-vis, resonance Raman, EPR). The reaction of the cobalt(III)-superoxido species and a reducing reagent ([CoII(C5H5)2]; cobaltocene) with proton (trifluoroacetic acid; TFA) also yields the corresponding cobalt(III)-hydroperoxido species. Kinetic analyses of the formation rates of the cobalt(III)-hydroperoxido complexes reveal that second-order rate constants depend on the structural and electronic properties of the cobalt-supporting chelating ligands. An electron-withdrawing ligand opposite to the superoxide accelerates the hydrogen atom transfer (HAT) reaction from AZADOL due to an increase in the electrophilicity of the superoxide ligand. Shielding the cobalt center by the alkyl group on the boron center of bis(imidazolyl)borate ligands hinders the approaching of AZADOL to the superoxide, although the steric effect is insignificant.

Spectral, crystallographic, theoretical, and catalytic activity studies of the PdII complexes in different coordination modes of benzoylthiourea ligand

Syntheses of neutral monodentate dichloro bis[N‐(diethylcarbamothioyl)‐4‐fluorobenzamido-κS] palladium(II), [PdIICl2(LH-κS)2] and anionic bidentate bis[N‐(diethylcarbamothioyl)‐4‐fluorobenzamido-κ2O,S] palladium(II), [PdII(L-κ2S,O)2] complexes of N‐(diethylcarbamothioyl)‐4‐fluorobenzamide (LH) ligand is reported. While [PdIICl2(LH-κS)2] was obtained by reaction between PdCl2 and LH in acetonitrile solution as a single crystal form, [PdII(L-κ2S,O)2] was formed recrystallization of [PdIICl2(LH-κS)2] in dichloromethane: ethanol solvent mixture as powder form. The PdII complexes were characterized by elemental analysis, FT-IR, 1H NMR, and 13C NMR spectral measurements. Furthermore, the molecular structure of [PdIICl2(LH-κS)2] was defined by single crystal X-ray diffraction, and the result obtained was also confirmed by spectral data. The optimized molecular geometry of the ligand and [PdIICl2(LH-κS)2] complex was obtained by density functional theory calculation at the B3LYP/6-311G(d,p) and LanL2DZ level of theory, respectively. We found that the optimized geometrical parameters agree very well with the experimental one. Therefore, we used the same theoretical level (B3LYP/LanL2DZ) for the [PdII(L-κ2S,O)2] which is a powder form, not a single crystal form. The energies of frontier molecular orbitals (EHOMO and ELUMO), energy band gap, and quantum molecular descriptors of the prepared compounds were calculated. Information about the site of chemical reactivity, charge density distribution, the size, and shape of the molecules was obtained by mapping with molecular electrostatic potential (MEP). NBO analysis was applied for the investigation of intra and inter-molecular bonding and conjugative interaction in molecular systems. The Mulliken and natural population analyses were used to predict positive and negative charges on every atom in the molecules. Hirshfeld surface analysis and 2D fingerprint plots were applied to visualize and quantify intermolecular interactions and packing modes in the supramolecular structure. The Coulomb interaction, dispersion, and total energy calculations were used to visualize and analyze the three-dimensional figures of the crystal lattice. Furthermore, the catalytic efficiencies of both palladium complexes have been confirmed in the Suzuki-Miyaura C-C reaction using a very low 0.01 mol% catalyst loading.

Synthesis and characterization of chiral bidentate bis(N-heterocyclic carbene)-carboxylate palladium and nickel complexes

Complexes [Pd{(S)-NHCMes,R}2] (R = Me, 2a; iPr, 2b; iBu, 2c) and [Pd{(R)-NHCMes,Me}2] (2a’) were prepared by reaction of the corresponding imidazolium precursor [ImMes,R] (namely (S)–2-alkyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a-c, and (R)-2-methyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a’), with Ag2O and subsequent interaction with Pd(OAc)2, under appropriate conditions. Alternatively, complexes 2 were also obtained by treating compounds [ImMes,R], 1, with an in-situ prepared lithium diisopropylamide solution and then reaction with [Pd(OAc)2]. On the other hand, the nickel complex [Ni{(S)-NHCMes,iPr}2], 3b, was synthesized by the interaction of the complex [NiCp2] (Cp = η5-C5H5) with [ImMes,iPr], 1b, in THF. If this reaction was carried out in acetonitrile, a mixture of complex 3b and the cyclopentadienyl monocarbene complex [NiCp{(S)-NHCMes,iPr}], 4b, was obtained. Similarly to 2b, compound 3b was also prepared by the in-situ formation of the carbene [(S)–NHCMes,iPr]- and then reaction with [NiBr2(dme)] (dme = 1,2-dimethoxyethane). Complexes 2, 3 and 4 were obtained with good yields and fully characterized by analytical (HR-ESI and microanalysis), spectroscopic (IR, 1H and 13C NMR and polarimetry) and X-ray methods (2c). In the solid state, 2c is a square planar complex containing two [(S)-NHCMes,iBu]- ligands coordinated in a κ2-C,O fashion, where the carbene and carboxylate donors have the expected trans configuration. The existence of two possible conformers of complexes 2 and 3 was analyzed using DFT methods.

Synthesis, structures, and photophysical properties of three-coordinate copper(I) complexes bearing bidentate bis[(2-diphenylphosphino)phenyl]ether (POP) ligand and monodentate substituted-quinoline ligand

The three-coordinate Cu(I) complexes [Cu(POP)(X)]PF6, X = quinoline (P1), 2-(4-methoxyphenyl)quinoline (P2), 2-(4-(trifluoromethyl)phenyl)quinoline (P3), POP = bis[(2-diphenylphosphino)phenyl] ether were synthesized and characterized. The photophysical properties of these Cu(I) complexes were tuned by the addition of organic groups including methoxyphenyl and trifluoromethylphenyl groups into the quinoline ligand. The resulting complexes exhibit the metal-to-ligand charge transfer (MLCT) band in the lower energy. The emissions for all the complexes in solid state are in the range of 542–558 nm with higher quantum yields of 11.4–14.2% and long-lived excited lifetimes of 179–481 µs at room temperature. The photophysical properties were further explained by DFT and TDDFT methods.

Highly active and thermostable camphyl α‐diimine–nickel(II) catalysts for ethylene polymerization: Effects of N‐aryl substituting groups on catalytic properties and branching structures of polyethylene

A group of nickel bromide complexes containing the rigid bidentate bis(arylimino)camphane ligands (Ar‐BIC) with different N‐aryl substituents, [ArN═C(camphyl)C(camphyl)═NAr]NiBr2 (Aryl = 2,6‐Me2C6H3 [Ni1], 2,4‐Me2C6H3 [Ni2], 2,4,6‐Me3C6H2 [Ni3], 2,6‐Me2‐4‐{(C6H4F)2CH}‐C6H2 [Ni4], 2,4‐Me2‐6‐{(C6H4F)2CH}‐C6H2 [Ni5, with the corresponding ligand as L5], and 2‐Me‐4,6‐{(C6H4F)2CH}2‐C6H2 [Ni6]), has been synthesized and characterized. The molecular structures of L5, Ni1, Ni3, Ni4, and Ni6 were elucidated with single‐crystal X‐ray diffraction. The nickel centers displayed a four‐coordinate geometry that can be best described as a distorted tetrahedral geometry. Upon treatment with either MMAO or Me2AlCl, Ni1–Ni6 showed moderate to high activities for ethylene polymerization, producing polyethylenes (PEs) with low to high molecular weights (0.02–23.7 × 105 g mol−1) and low polydispersity indices (PDI: 1.6–2.6). In comparison with reference nickel precatalysts ligated by bis(arylimino)acenaphthene (Ar‐BIAN) and bis(arylimino)butane (Ar‐BIB), the title complexes displayed much better thermal stability and higher catalytic activities (up to 11.2 × 106 g PE (mol Ni)−1 h−1 at 70°C), delivering polyethylenes with higher molecular weights. All the resultant polyethylenes are moderately to highly branched with the branching content and type of branching strongly affected by the type of N‐aryl substituting groups. Notably, the polymers produced with ortho‐hydrogen Ni2/Me2AlCl possessed the highest branching density and unique terminal vinyl (CH═CH2) and internal vinylene (CH═CH) structure. On the other hand, ortho‐di(p‐fluorophenyl)methyl‐containing Ni5 and Ni6 just gave 30 and 19 branches per 1000 Cs due to the reduced chain walking capability as a result of the bulkier substituting groups.

Modification of bidentate bis(N-heterocyclic imine) ligands for low-valent main group complexes

The synthesis of modified neutral bis-NHI (NHI is N-heterocyclic imine) ligands and their application for the stabilization of tetryliumylidenes are reported. The ligands’ scaffolding consists of either saturated or methylated imidazoline backbones, and the bridge alternated from flexible ethylene to more rigid o-phenylene. Transmetalation reactivity of the cationic SnII compounds was tested towards LiAlH4 and IDipp→SiCl2 [IDipp is 1,3-bis(2,6-diisopropyl- phenyl)imidazol-2-ylidene] affording the respective aluminium and silicon complexes.

Insights into Formate Oxidation by a Series of Cobalt Piano-Stool Complexes Supported by Bis(phosphino)amine Ligands.

A series of (cyclopentadienyl)cobalt(III) half-sandwich complexes (1-4) supported by bidentate bis(phosphino)amine ligands was synthesized and characterized by NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The CoIII-hydride complex 4-H bearing the bis(cyclohexylphosphine) ligand derivative was successfully isolated via protonation of the neutral reduced CoI complex 5 with a weak acid. Experimental and computational methods were used to determine the thermodynamic hydride accepting ability of these CoIII centers and to evaluate their reactivity toward the oxidation of formate. We find that the hydride accepting ability of 1-4 ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating carbon dioxide and the CoIII-hydride complex 4-H in 72% yield.

Synthesis, structural characterization, and density functional theory calculations of the two new Zn (II) complexes as antibacterial and anticancer agents with a neutral flexible tetradentate pyrazole‐based ligand

Two new mono‐ and dinuclear zinc (II) complexes with a formula of [Zn (LN)OMeCl] (1) and [Zn2(LN)(N3)2Cl2] (2) [where LN = 1,1,3,3‐tetrakis(3,5‐dimethyl‐1‐pyrazolyl)propane)] have been synthesized in methanol as a solvent and fully characterized by the single crystal X‐ray diffraction, elemental analysis, FT‐IR, UV–Vis, density functional theory (DFT) calculations, antibacterial, and anticancer properties. In both of them, zinc ion is tetrahedrally coordinated to two nitrogen atoms from one of the chelating bidentate bis(3,5‐dimethylpyrazolyl)methane units of the LN ligand and one terminal chlorine. The forth coordination site was occupied by methoxy group in 1 and azide co‐ligand in complex 2. In contrast to 1, in the structure of 2 two metal centers are bridged by the tetradentate chelating LN ligand to form a dinuclear complex. Interestingly, the environment around the central atom in these complexes are mimicked with the core structure of the zinc metalloproteinases, Thermolysin {[NNO]ZnX} and Carbonic anhydrase {[NNN]ZnX} for complexes 1 and 2, respectively. Data analysis of the antibacterial activities demonstrated that the gram positive species showed more sensitivity toward the target title agents. Although, both complexes exhibit antibacterial activities and kill bacteria at low concentration, but complex 1 has higher DNA cleavage activity than 2. Additionally, the cytotoxicity of the free ligand and its complexes was evaluated against the human carcinoma cell line (Caco‐2) by the MTT method and the IC50 values were calculated for all of them. Due to the presence of pyrazole rings in the structure of the ligand and its complexes, these materials have a substantial potential to inhibit cancer cell metastasis. The energetic stabilization due to donor‐acceptor interactions in terms of the natural bond orbital (NBO) analyses and nonlinear optical properties were also examined by using the DFT calculations.

A Bidentate Iodine(III)-Based Halogen-Bond Donor as a Powerful Organocatalyst*.

In contrast to iodine(I)‐based halogen bond donors, iodine(III)‐derived ones have only been used as Lewis acidic organocatalysts in a handful of examples, and in all cases they acted in a monodentate fashion. Herein, we report the first application of a bidentate bis(iodolium) salt as organocatalyst in a Michael and a nitro‐Michael addition reaction as well as in a Diels–Alder reaction that had not been activated by noncovalent organocatalysts before. In all cases, the performance of this bidentate XB donor distinctly surpassed the one of arguably the currently strongest iodine(I)‐based organocatalyst. Bidentate coordination to the substrate was corroborated by a structural analysis and by DFT calculations of the transition states. Overall, the catalytic activity of the bis(iodolium) system approaches that of strong Lewis acids like BF3.

Expanding the Boxmi Ligand Family: Synthesis and Application of NON and NSN Ligands.

Two synthetic strategies for a new family of neutral NON ligands featuring a "bis(oxazolinylmethylidene)isobenzofuran" framework (boxman) are reported. A Pd-mediated cyclization reaction forming the isobenzofuran core constitutes the key reaction in the eight-step synthetic route to the nonbackbone-methylated target compound H,Rboxman. In contrast, the introduction of two additional methyl groups provides stereochemical control during backbone construction and thereby access to the methylated derivative Me,Rboxman, which was synthesized in five steps and improved yields. In addition, the synthetic sequence was transferred to the thio analogue, providing access to the NSN ligand H,Rboxmene. Subsequent complexation experiments with iron and cobalt chloride precursors afforded the four-coordinated chlorido complexes Me,RboxmanMCl2 (R = Ph, iPr; M = Fe, Co) and established the boxman family as trans-chelating, bidentate bis(oxazoline) ligands. Application of the latter in the nickel(II)- and zinc(II)-catalyzed α-fluorination of β-ketoesters and oxindoles (up to 98% yield and 94% ee) demonstrated their suitability for enantioselective catalysis.

Synthesis, X‐ray Structures, and Photoluminescence of the Octahedral Cu4I4 Cluster with Bulky Bidentate Bis(phosphanyl)amine Ligand

The reaction of C10H7‐1‐N(PPh2)2 (1) with two equivalents of CuI in acetonitrile resulted in the formation of octahedron Cu4I4[1]2 complex (2). The crystal structure of 2 showed it adopted a rare octahedral arrangement. The rectangular Cu4 plane is μ4‐capped by two of the iodides and is placed in axial positions above and below the Cu4‐plane form an octahedron, whereas the other two iodides are bonded to two copper atoms in a μ2‐fashion. The luminescence of complex 2 arises from a triplet halide‐to‐ligand charge transfer (3XLCT) excited state and 3CC (Cu4I4 cluster‐centered) excited state are not involved in the luminescence by the rigid bidentate ligand 1 in spite of the short CuI–CuI bond length. Complex 2 was identified and characterized by multinuclear NMR (1H, 13C, 31P NMR) and IR spectroscopy. Crystal structure determinations of 1 and 2 were carried out.

Catalytic Activity of trans-Bis(pyridine)gold Complexes.

Gold catalysis has become one of the fastest growing fields in chemistry, providing new organic transformations and offering excellent chemoselectivities under mild reaction conditions. Methodological developments have been driven by wide applicability in the synthesis of complex structures, whereas the mechanistic understanding of Au(III)-mediated processes remains scanty and have become the Achilles’ heel of methodology development. Herein, the systematic investigation of the reactivity of bis(pyridine)-ligated Au(III) complexes is presented, based on NMR spectroscopic, X-ray crystallographic, and DFT data. The electron density of pyridines modulates the catalytic activity of Au(III) complexes in propargyl ester cyclopropanation of styrene. To avoid strain induced by a ligand with a nonoptimal nitrogen–nitrogen distance, bidentate bis(pyridine)–Au(III) complexes convert into dimers. For the first time, bis(pyridine)Au(I) complexes are shown to be catalytically active, with their reactivity being modulated by strain.

Preorganization: A Powerful Tool in Intermolecular Halogen Bonding in Solution.

Preorganization is a powerful tool in supramolecular chemistry which has been utilized successfully in intra‐ and intermolecular halogen bonding. In previous work, we had developed a bidentate bis(iodobenzimidazolium)‐based halogen bond donor which featured a central trifluoromethyl substituent. This compound showed a markedly increased catalytic activity compared to unsubstituted bis(iodoimidazolium)‐based Lewis acids, which could be explained either by electronic effects (the electron withdrawal by the fluorinated substituent) or by preorganization (the hindered rotation of the halogen bonding moieties). Herein, we systematically investigate the origin of this increased Lewis acidity via a comparison of the two types of compounds and their respective derivatives with or without the central trifluoromethyl group. Calorimetric measurements of halide complexations indicated that preorganization is the main reason for the higher halogen bonding strength. The performance of the catalysts in a series of benchmark reactions corroborates this finding.

Gem-Dialkyl Effect in Diphosphine Ligands: Synthesis, Coordination Behavior, and Application in Pd-Catalyzed Hydroformylation

A series of palladium complexes with C3-bridged bidentate bis(diphenylphosphino)propane ligands with substituents of varying steric bulk at the central carbon have been synthesized. The size of the...