Tetracoordinate Complexes Research Articles

Mono-aluminum, di-magnesium and tri-zinc complexes supported by bisphenolate ligand: Synthesis, characterization and catalytic studies for ring-opening polymerization of cyclic esters

Mono-aluminum, di-magnesium and tri-zinc complexes coordinated by a Biphen2- ligand (Biphen-H2 = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethylbiphenyl-2,2′-diol) were synthesized and structurally characterized by X-ray single crystal determinations. The reaction of Biphen-H2 with AlMe3 (1.0 mol equiv.) in THF produced the tetra-coordinated monomeric aluminum complex [Al(Biphen)Me(THF)] (1), but treatment of Biphen-H2 with MgnBu2 under the same stoichiometric proportion in THF gave the dimeric magnesium complex [Mg(μ-Biphen)(THF)]2 (2). Tri-zinc [Zn3(μ, μ-Biphen)2Et2(THF)2] (3) resulted from treatment of Biphen-H2 with ZnEt2 (1.5 mol equiv) in THF; it consists of two different kinds of four-coordinated Zn centers. All of metal complexes are coordinated by at least one neutral THF coligand, indicating each metal center can behave as an active site to activate monomers during ring-opening polymerization (ROP). Catalysis for ROP of ε-caprolactone (ε-CL) or β-butyrolactone (β-BL) of complexes 1–3 are investigated, and the comparative studies of ε-CL polymerization are also discussed.

Regioselective, Borinic Acid-Catalyzed Monoacylation, Sulfonylation and Alkylation of Diols and Carbohydrates: Expansion of Substrate Scope and Mechanistic Studies

Synthetic and mechanistic aspects of the diarylborinic acid-catalyzed regioselective monofunctionalization of 1,2- and 1,3-diols are presented. Diarylborinic acid catalysis is shown to be an efficient and general method for monotosylation of pyranoside derivatives bearing three secondary hydroxyl groups (7 examples, 88% average yield). In addition, the scope of the selective acylation, sulfonylation, and alkylation is extended to 1,2- and 1,3-diols not derived from carbohydrates (28 examples); the efficiency, generality, and operational simplicity of this method are competitive with those of state-of-the-art protocols including the broadly applied organotin-catalyzed or -mediated reactions. Mechanistic details of the organoboron-catalyzed processes are explored using competition experiments, kinetics, and catalyst structure-activity relationships. These experiments are consistent with a mechanism in which a tetracoordinate borinate complex reacts with the electrophilic species in the turnover-limiting step of the catalytic cycle.

Origin for Fluxional Structure of Tetracoordinate PdIIComplexes

In the Pd-catalyzed cross-coupling reactions, the PdL2 complexes with the bulky and electron-rich L ligands (phosphines, NHCs, etc) are important compounds as a Pd(L)nprecusor. The catalytic activity of Pd(L)n-precusors was influenced by the electronic property of the strong σ-donor and the steric effect of the bulky L ligands. Due to the electron-transfer from the strong σ-donor L ligand to Pd, the oxidation state of Pd in the neutral [PdLnX] catalysts is zerovalent. In some X-ray crystallography results, the Pd([9]aneB2A)L2 complexes with mixed hard/soft tridentate ligand have been observed as a five-coordinate geometry with an apical (soft A...Pd) interaction. In Pd(η,η,ηC10H16N2Me2) complexes with an aza-macrocyclic ligand, the six coordination bonds of the three olefin units with Pd were symmetrically formed via three (olefin...Pd) interactions. Meanwhile, in hydration reactions on square planar [PdCl4] complexes, two vertical (H2O...Pd) and (HOH... Pd) interactions for the ligand exchange processes were investigated theoretically and experimentally. With increasing the strength of the fifth (solvent...Pd) interaction, the replacement of the equatorial (Pd-Cl) bond with (solvent...Pd) takes place via a five-coordinate transition states. Is there any relationship between the relative stability and fluxional configuration of the Pd isomers (exoand endotypes) and the apical (soft P...Pd) interaction in the Pd complexes? The unusual stereochemistry of square planar Pd complexes might be influenced by the fifth special (soft P...Pd) interaction, and the conformational rearrangement for the relative stability. The vacant bond on the zaxis allows the occupied dz2-orbital of Pd to interact with the Lewis acid ligand (σ-acceptor), but it is difficult for electron-rich substrates and Pd to interact apically (σ-donor ...filled dz2-orbital). Therefore, for Pd([9]aneB2A)L2, the apical (σ-donor...filled 4dz2-orbital) repulsion shown in Scheme 1 is suggested. In the present work, we have investigated the relative stabilities and extraordinary structures of the macrocyclic Pd([9]aneB2A)L2 complexes within the frameworks of the orbital interactions. The equilibrium geometric structures of tetracoordinate palladium [Pd([9]aneB2A)L2, Pd([9]aneBAB)L2 (A: P; B=N; L=Cl−, PH3)] complexes were fully optimized with the B3P86/6-311+G** (LanL2DZ for Pd) level using Gaussian 03. Optimized geometric structures of tetracoordinate Pd([9]aneN2P)L2 {Pd([9]aneN2P)L2} isomers including HOMO were represented in Figure 1 and their parameters were listed in Table 1. In the Pd complexes, Pd locates at the center of the tetracoordinate Pd complexes with a ([9]aneN2P) ligand and two L monodentates. In endo-Pd([9]aneN2P)Cl2

Development of an efficient solvent free one-pot Heck reaction catalyzed by novel palladium (II) complex-via green approach

A novel 1-glycyl-3-methyl imidazolium chloride–palladium (II) complex [[Gmim]Cl–Pd (II)] was found to be a heterogeneous catalyst for an efficient Heck reaction with good to excellent yield under solvent free condition. Tetra-coordinated palladium complex was prepared by reacting PdCl2 with 1-glycyl-3-methyl imidazolium chloride and its catalytic function invented for the CC bond formation. Spectroscopic evidence of complex has been proved by powder XRD, SEM, FT-IR and AFM. This protocol provides a simple strategy for the generation of a variety of new CC bonds under environmentally benign condition.

Syntheses and photophysical properties of new iminopyrrolyl boron complexes and their application in efficient single-layer non-doped OLEDs prepared by spin coating

Efficient non-doped OLEDs have been achieved using new binuclear tetracoordinate organoboron complexes containing 2-(N-aryl)formiminopyrrolyl ligands.

Zinc and magnesium complexes incorporated by bis(amine) benzotriazole phenoxide ligand: Synthesis, characterization, photoluminescent properties and catalysis for ring-opening polymerization of lactide

A new bis(amine) benzotriazole phenoxide ligand, (C8NN)BTP-H (1) was prepared from the Mannich condensation of 2-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol with the mixtures of excess paraformaldehyde and N,N,N-trimethylethylenediamine under reflux conditions. Zinc and magnesium complexes bearing the N,N,O-tridentate (C8NN)BTP ligand were synthesized and fully characterized. The reaction of (C8NN)BTP-H with ZnEt(2) (1.2 molar equiv.) in toluene gives the monomeric tetra-coordinated zinc complex [((C8NN)BTP)ZnEt] (2). The homoleptic and monomeric complexes [((C8NN)BTP)(2)M] (M = Zn (3) and M = Mg (4)) result from treatment of ZnEt(2) or Mg(n)Bu(2) with (C8NN)BTP-H (two equiv.), in which the metal center is hexa-coordinated by two tridentate (C8NN)BTP ligands. Luminescent properties and catalysis for lactide (LA) polymerizations of complexes 2-4 are studied. Complexes 2-4 produce bright green fluorescence with emission maxima ranging from 510 to 520 nm in the solid state. Experimental results indicate that complex 2 catalyzes the ring-opening polymerization of LA in the presence of 9-anthracenemethanol (9-AnOH) with efficient catalytic activities in a controlled fashion, yielding poly(rac-lactide) with a slight isotactic predominance (P(m) = 0.59).

Bonding and structure of disilenes and related unsaturated group-14 element compounds

Structure and properties of silicon-silicon doubly bonded compounds (disilenes) are shown to be remarkably different from those of alkenes. X-Ray structural analysis of a series of acyclic tetrakis(trialkylsilyl)disilenes has shown that the geometry of these disilenes is quite flexible, and planar, twist or trans-bent depending on the bulkiness and shape of the trialkylsilyl substituents. Thermal and photochemical interconversion between a cyclotetrasilene and the corresponding bicyclo[1.1.0]tetrasilane occurs via either 1,2-silyl migration or a concerted electrocyclic reaction depending on the ring substituents without intermediacy of the corresponding tetrasila-1,3-diene. Theoretical and spectroscopic studies of a stable spiropentasiladiene have revealed a unique feature of the spiroconjugation in this system. Starting with a stable dialkylsilylene, a number of elaborated disilenes including trisilaallene and its germanium congeners are synthesized. Unlike carbon allenes, the trisilaallene has remarkably bent and fluxional geometry, suggesting the importance of the π-σ* orbital mixing. 14-Electron three-coordinate disilene-palladium complexes are found to have much stronger π-complex character than related 16-electron tetracoordinate complexes.

ChemInform Abstract: Rendering Schrock‐Type Molybdenum Alkylidene Complexes Air Stable: User‐Friendly Precatalysts for Alkene Metathesis.

AbstractThe decomplexation of phenanthroline or bipyridine ligands from several molybdenum alkylidene complexes using ZnCl2 as activation agent releases tetracoordinate molybdenum complexes, which catalyze high‐yielding metathesis reactions under mild conditions (the activation is carried out at temperatures of 60 to 100°C, the metathesis reaction in the most cases at 25°C).

Neutral Nickel Oligo- and Polymerization Catalysts: The Importance of Alkyl Phosphine Intermediates in Chain Termination

An unconventional chain termination reaction has been explored for the SHOP (Shell higher olefin process)-type, anilinotropone, and salicylaldiminato nickel-based oligo- and polymerization catalysts by using density functional theory (DFT). Starting from the tetracoordinate alkyl phosphine complex, the termination reaction was found to involve a rearrangement of the alkyl chain to form a pentacoordinate β-agostic complex, β-hydride elimination, and olefinic chain dissociation and to compete with propagation at sufficiently high phosphine concentration and/or basicity. It provides the first complete and convincing mechanistic rationale for the decreasing chain lengths observed upon increasing phosphine concentration and basicity. The unconventional reaction was found to be a major termination pathway for the SHOP-type catalyst and is very unlikely to lead to branching and olefin isomerization, which is critical for explaining why the SHOP catalyst, in contrast to the anilinotropone and salicylaldiminato catalysts, tends to lead to the oligomerization of ethylene to form linear α-olefins. Based on our results we have proposed a new and extended catalytic cycle for the SHOP-type ethylene oligomerization catalyst. Finally, the importance of the new termination reaction for the SHOP-type catalyst suggests that this reaction may also operate with other ethylene oligomerization nickel catalysts. This prediction was confirmed for a pyrazolonatophosphine catalyst, for which the new termination route was found to be even more facile, which explains the short oligomers produced by this catalyst.

Novel N,O,N-Supported Tetracoordinate Aluminum Complexes for the Highly Controlled and Immortal ROP of Trimethylene Carbonate (TMC) under Mild Conditions: Access to Narrowly Disperse poly-TMC and Derived Copolymers

The novel N,O,N-supported tetracoordinate amidoaluminum complexes {η3(N,O,N}-(C5H9)N-C6H4}2OAlNMe2 (2a, R = C5H9; 2b, R = C6H11) have been synthesized and structurally characterized. In the solid state, as determined from X-ray crystallographic studies, complex 2a consists of a four-coordinate Al species in which the Al center adopts a distorted-trigonal-monopyramidal (tmp) geometry with the nitrogens of the three amido groups (defining the pyramidal base) being nearly coplanar with Al. Such Al species, when combined with an alcohol source such as benzyl alcohol, effectively polymerize trimethylene carbonate (TMC) at room temperature in a highly controlled manner to yield narrowly disperse poly(trimethylene carbonate), as deduced from polymer characterizations with various kinetic studies. The high degree of molecular chain length control of the present system was further exploited to access narrowly dispersed PEG-functionalized amphiphilic copolymers. The attractive features of the system lie in the combination of an excellent activity, a high level of chain length control, and mild reaction conditions.

Reactions of (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline with diethylzinc: Synthesis, characterization and catalytic studies for ring-opening polymerization of ε-caprolactone

Abstract Zinc complexes supported by NN-bidentate AABn ligand (AABn-H = (E)-N-[2-(benzyliminomethyl)phenyl]-2,6-diisopropylaniline) are synthesized and fully characterized. The reaction of diethyl zinc (ZnEt2) with AABn-H (1.1 mol equiv) yields three-coordinated mono-adduct zinc complex [(AABn)ZnEt] (1). However, treatment of AABn-H with ZnEt2 in the same stoichiometric proportion in the presence of 4-dimethylaminopyridine (DMAP, one equiv) affords [(AABn)Zn(DMAP)Et] (2) as the tetra-coordinated monomeric zinc complex. Bis-adduct complex [(AABn)2Zn] (3) results from treatment of ZnEt2 with AABn-H (two equiv) in refluxing toluene in high yield. Complexes 1 and 3 are efficient catalysts for the ring-opening polymerization (ROP) of e-caprolactone (e-CL) in the presence of alcohol. The “controlled” character of Zn complex 1 also enabled preparation of the PEG-b-PCL copolymer.

Histamine Molecule and Dianion Oxalate are Efficient Blocks for Building 2D Supramolecular Networks

One salt and two Cu(II) complexes (H2hsm)(ox), 1, [Cu(hsm)(ox)], 2, and [Cu(hsm)(ox)H2O], 3, have been synthesized and X-ray characterized (hsm is histamine and ox2− is the oxalate dianion). Starting from the prochiral tetracoordinated complex 2, pentacoordinated complex 3 crystallizes as a racemic mixture of the enantiomeric Δ and Λ isomers, in space group P21/c. In all cases, the side chain of the hsm group is gauche, allowing the formation of strong hydrogen bonds in the salt 1, and to chelate the metal center in complexes 2 and 3. The combination hsm/ox seems to favor the formation of 2D supramolecular structures (planes or wavy planes), through efficient networks of N–H···O hydrogen bonds. Cell parameters: 1, P21/c, a = 6.260 (2) A, b = 11.500 (4) A, c = 12.525 (4) A, β = 104.047 (17)o; 2, C2/c, a = 10.7966 (13) A, b = 15.5622 (16) A, c = 11.3996 (15) A, β = 106.261 (11)o; 3, P21/c, a = 7.0627 (6) A, b = 7.1323 (6) A, c = 20.0296 (19) A, β = 91.529 (7)o. Crystal structures based on histamine, histaminium and oxalate revealed that 2D supramolecular networks are stabilized with these building blocks, both in an organic salt and in two Cu(II) coordination complexes.

Synthesis and structural characterization of zinc complexes supported by amino-benzotriazole phenoxide ligands: Efficient catalysts for ring-opening polymerization of ε-caprolactone and β-butyrolactone

Two amino-benzotriazole phenol ligands, 2-(2H-benzotriazol-2-yl)-6-((diethylamino)methyl)-4-alkyl-phenol (alkyl = CH3, C1DEABTP-H, and alkyl = C8H17, C8DEABTP-H) were prepared through the Mannich condensation of 4-alkyl-2-(2H-benzotriazol-2-yl)phenol with the mixtures of excess paraformaldehyde and diethylamine under reflux conditions. Zinc complexes supported by amino-benzotriazole phenoxide ligands (C1DEABTP− and C8DEABTP−) were synthesized and fully characterized. The reaction of ZnEt2 with DEABTP-H (1 equiv.) produces the tetra-coordinated dimeric zinc complexes [(μ-C1DEABTP)ZnEt]2 (1) and [(μ-C8DEABTP)ZnEt]2 (2). Experimental results indicate that complex 1 catalyzes the ring-opening polymerization of ε-caprolactone and β-butyrolactone with good catalytic activities in a controlled character.

Comparative study of lactide polymerization by zinc alkoxide complexes with a β-diketiminato ligand bearing different substituents

A series of β-diketiminate zinc complexes has been synthesized and their reactivity for the ring-opening polymerization (ROP) of lactide has been studied. The reaction of β-diketimines (LH) with diethyl zinc forms the monomeric [LZnEt] complexes which further react with benzyl alcohol (BnOH) in toluene/hexane yielding dinuclear or trinuclear zinc complexes. These complexes have been characterized by single crystal X-ray diffraction, which showed the tri- and tetra-coordinated zinc complexes in which zinc atoms exhibit trigonal and tetrahedral geometry, respectively. All complexes have been tested as initiators for the ring-opening polymerization of lactide; they are all highly active. The rate of polymerization is heavily dependent on the N-aryl substituents with the order: alkyl group ∼ alkoxy > halide group > nitro group. The β-diketiminate zinc complexes allow controlled ring-opening polymerization as shown by the linear relationship between the percentage conversion and the number-average molecular weight. On the basis of literature reports, a mechanism for ROP of lactide has been proposed.

Synthesis and characterization of copper(I) complexes from triphenylphosphine and isatin Schiff bases of semi- and thiosemicarbazide

The reaction of isatin-3-thiosemicarbazone (ITC, 1) or isatin-3-semicarbazone (ISC, 2) with nitrato bis(triphenylphosphine)copper(I) gave the four coordinate copper(I) complexes [Cu(PPh3)2(ITC)]NO3 (3) and [Cu(PPh3)2(ISC)]NO3 (4). The synthesized complexes were characterized by FT-IR, UV–VIS, Raman and elemental analysis. The crystal structure of 3 was investigated by single crystal X-ray diffraction. The ITC coordinates to the copper(I) ion in a bidentate fashion via the N(imine) and S atoms which along with two triphenylphosphine ligands form a tetracoordinate complex. The complex has a distorted tetrahedral coordination environment. Crystal data at 150.0 K: space group P21/c with a=12.5777(4), b=15.2062(5), c=21.9057(7) Å, β=95.628(3)o, Z=4, R 1=0.049.

Electron‐Rich Vaska‐Type Complexestrans‐[Ir(CO)Cl(2‐Ph2PC6H4COOMe)2] andtrans‐[Ir(CO)Cl(2‐Ph2PC6H4OMe)2]: Synthesis, Characterisation and Reactivity

AbstractThe in‐situ‐generated dimeric precursor [Ir(CO)2Cl]2reacts with four molar equivalents of the ligands 2‐Ph2PC6H4COOMe (a) and 2‐Ph2PC6H4OMe (b) to afford tetracoordinated complexes of the typetrans‐[Ir(CO)ClL2] (1a,1b), where L =aandb. The IR spectra of1aand1bin CHCl3solution show the terminal υ(CO) bands at around 1957 and 1959 cm–1, respectively, which are significantly lower in frequency compared to Vaska's complex,trans‐[Ir(CO)Cl(PPh3)2] (1965 cm–1) and substantiate the enhanced electron density at the metal centre. The single‐crystal X‐ray structure of1aindicates iridium–oxygen (ester group) distances [Ir···O(2) 3.24 Å, Ir···O(5) 3.29 Å] and angle [O(5)···Ir···O(2) 157.25°] suggesting a long‐range intramolecular “secondary” Ir···O interaction resulting in a pseudo‐hexacoordinated complex. Complex1breacts with O2to generate [Ir(O2)(CO)Cl(2‐Ph2PC6H4OMe)2] (2b), while1aremains unreactive. Complex2bshows a distorted octahedral structure with peroxo O–O linkage (O2–O3 1.47 Å). The kinetic study of the reaction of1band Vaska's complex towards dioxygen addition reveals that the rate of dioxygen addition to1bis about three times faster than Vaska's complex. Complexes1aand1bundergo oxidative addition with small molecules like CH3I and I2to produce IrIIIcarbonyl species of the type [Ir(CO)Cl(CH3)IL2] (3a,3b) and [Ir(CO)ClI2L2] (4a,4b), where L =a,b.

Two aminobenzothiazole derivatives for Pd(II) and Zn(II) coordination

Two derivatives of 2-aminobenzothiazole are reported: an imine potentially able to generate on deprotonation a mononegative ligand moiety and an amide suited to metal coordination. Both have been used for producing tetracoordinated complexes of zinc (II) and palladium (II). The thermodynamic and optical properties of ligands and complexes were analysed by calorimetry, polarizing microscopy and an X-ray technique. In particular, the fluorescence properties of ligands and complexes were explored. Preparation and properties of two 2–aminobenzothiazole derivatives are reported: one imine potentially able to generate a mononegative ligand moiety and one amide suited to metal coordination. Both have been used for producing tetracoordinated complexes of zinc (II) and palladium (II). Fluorescence properties of ligands and complexes were analysed.

Zinc-Induced Precipitation of Milk Fat Globule Membranes: A Simple Method for the Preparation of Fat-Free Whey Protein Isolate

A simple method to isolate milk fat globule membrane (MFGM) from cheese whey has been developed. The method was based on the premise that divalent cations should be able to form insoluble complexes with MFGM under certain solution conditions; this ability, however, is dependent on their coordination chemistry. Incubation of cheese whey with 0-50 mm CaCl2 or MgCl2 at 30 °C did not cause precipitation of MFGM, whereas incubation with Zn(Ac)2 under similar conditions induced selective precipitation of MFGM in a concentration-dependent manner at pH 5.2, with complete precipitation occurring above 20 mm Zn(Ac)2. The whey proteins remained soluble in the supernatant under these conditions. The ability or inability of a cation to induce precipitation of MFGM is related to the ionic radius as well as its coordination geometry. Calcium and magnesium ions have a strong tendency to form hexa-coordinated (n = 6) complexes in a regular octahedral geometry, whereas zinc prefers to form a tetra-coordinated complex in a tetrahedral geometry with MFGM phosphate groups. It is proposed that the tetrahedral geometry of zinc coordination in the zinc-MFGM complex permits hydrophobic interaction between MFGM particles, resulting in precipitation at 30 °C. Further processing of the supernatant using membrane ultrafiltration/diafiltration resulted in a fat-free whey protein isolate.

Molecular structure of N, N′- o-phenylene-bis(salicylideneaminato)zinc(II), Zn(saloph), according to gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of N, N′- o-phenylene-bis(salicylideneaminato)zinc(II) has been determined by synchronous gas-phase electron diffraction (GED) and mass spectrometric experiment at 626(5) K and by density functional theory calculations (B3LYP/Stuttgart ECP(Zn), TZV (O, N, C, H) with the addition of d-polarization functions; in case of N, O with the addition of p-diffuse functions; B3LYP/Stuttgart ECP(Zn), cc-pVTZ (O, N, C, H)). Both experimental and theoretical approaches yielded the structure of C s-symmetry with a planar ZnN 2O 2 coordination site. The overall ligand conformation is also planar with small distortion towards an umbrella-shape. The most important structure parameters are r h1(Zn–N) = 2.072(12) Å; r h1(Zn–O) = 1.926(7) Å; ∠NZnN = 77.4(1.4)°; ∠NZnO = 90.4(1.4)°; ∠OZnO = 101.9(2.5)°. Structural features of tetra-coordinate zinc complexes are discussed.

Synthesis and structure of photoluminescent di- and trinuclear copper(I) complexes assembled from alkane-bridged bis-imidazole

[Cu 2Br 2L 1(PPh 3) 4] 2 · 2CH 2Cl 2 · 3H 2O ( 1) (L 1 = bis( N-imidazolyl)butane) and [Cu 3I 2(PPh 3) 6(L 2) 2] · 2DMSO · Br · H 2O ( 2) (L 2 = bis( N-imidazolyl)methane) have been synthesized and characterized by X-ray diffraction analysis and elemental analysis. Compound 1 is a tetracoordinated dinuclear complex, it consists of two Cu(PPh 3) 2 units bridged by a bis( N-imidazolyl)butane ligand. While compound 2 is a trinuclear copper(I) complex, in the compound copper atoms are located in two kinds of coordination spheres, for the two terminal copper(I) atoms, they are surrounded by two P, one N, and one iodine atoms to complete their tetrahedron geometry. While the middle copper(I) atom is surrounded by two P and two nitrogen atoms, the bromide anion exists as free counter anion. The photoluminescent studies show that the two complexes display intense blue emissions.