PCP-Bis(phosphinite) pincer complexes: new homogeneous catalysts for α-arylation of ketones

Two new p-alkoxycarbonylated palladium bis(phosphinite) PCP pincer complexes are easily prepared and for the first time evaluated as homogeneous catalysts in α-arylation of ketone enolates. Apart from the total absence of phenyl-aryl exchange by-products and significantly low catalyst loadings, the general α-arylation protocols described in this letter feature not only a broad applicability to a range of ketones and aryl bromides with marked electronic and steric differences but also the possibility to generate mono-diarylated products.     

Ruthenium(II) pincer complexes with oxazoline arms for efficient transfer hydrogenation reactions

Well-defined P^NN^CN pincer ruthenium complexes bearing both strong phosphine and weak oxazoline donors were developed. These easily accessible complexes exhibit significantly better catalytic activity in transfer hydrogenation of ketones compared to their PN³P analogs. These reactions proceed under mild and base-free conditions via protonation-deprotonation of the ‘NH’ group in the aromatization-dearomatization process.     

Novel unsymmetrical PCP′ pincer ligands and their palladium(II) complexes

Synthetic routes towards novel PCP′ pincer ligands were devised. Ligand 1-(Pr₂ⁱPOCH₂)-3-(Bu₂^tPCH₂)(C₆H₄) is prepared in a three step synthesis from 1,3-benzenedimethanol and 1-(Pr₂ⁱPO)-3-(Bu₂^tPCH₂)(C₆H₄) is accessible in three steps from 3-hydroxybenzylalcohol. Both their palladium(II) complexes are prepared in good yields but are distinctly different since [PdCl{(C₆H₃)(OPPr₂ⁱ)-2-(CH₂PBu₂^t)-6}] possesses two five-membered palladacycles, whereas [PdCl{(C₆H₃)(CH₂PBu₂^t)-2-(CH₂OPPr₂ⁱ)-6}] is unusual for a pincer complex in that it contains both five- and six-membered palladacycles. Both compounds also represent the first examples of pincer complexes where one donor is a phosphinite and the other is a phosphine. The X-ray structures of these complexes were solved and are discussed. The data reveal that an increase in the metallacycle ring-size leads to changes in bond lengths, but more importantly to significant increases in the bond angles.     

Kinetic investigation of a PC(sp)P pincer palladium (II) complex in the Heck reaction

An investigation of the kinetics of the Heck reaction between 4-iodoanisole and styrene catalysed by {cis-1,3-bis[(di-tert-butylphosphino)methyl]-cyclohexane} palladium (II) iodide (1) has been performed in DMF-d₇ solution. Based on mercury poisoning experiments a heterogeneous palladium catalyst formed from the pre-catalyst is proposed. Saturation behaviour with respect to the olefin concentration suggests a mechanism consisting of a pre-equilibrium association of the olefin followed by a rate determining reaction with aryl halide. The equilibrium constant for the olefin association, K₁, and the rate constant for the subsequent oxidative addition step, k₂, were determined to (5.7 ± 2.5) × 10⁻³ and 18.4 ± 2.7 M⁻¹ s⁻¹, respectively.     

Self-assembly of para-OH functionalized ECE-metalated pincer complexes

Various para-OH functionalized ECE-pincer metal complexes [MX(ECE-OH)L_n] (ECE-OH = [C₆H₂(CH₂E)₂-2,6-OH-4]⁻, E = NMe₂, PPh₂ and SPh) were synthesized. The X-ray crystal structures of neutral [PdCl(SCS-OH)], [PdCl(NCN-OH)], and cationic [Pd(PCP-OH)(MeCN)](BF₄) are reported. The neutral halide complexes exhibit self-assembly to form polymeric chains via H-bonding involving the para-OH group as donors and the halide ligand on the metal as acceptors. Moreover, the halide ligand can be replaced by a monomeric aryloxy-O ligand leading to the formation of a covalently bonded dimer. The crystal structure of such a dimer derived from [PdI(NCN-OH)] is reported. Furthermore, these pincer-metal complexes were tethered through a carbamate linker to a siloxane functionality with the aim to be immobilized on a silica support. The crystal structure of a siloxane-functionalized [PtI(NCN-Z)] complex exemplifies how other H-bonding interactions not involving the metal-halide groupings can lead to polymeric networks as well.     

Application of iridium pincer complexes in hydrogen isotope exchange reactions

Iridium pincer complex catalyzed hydrogen to deuterium exchange could be achieved using aromatic and heteroaromatic substrates. The reactions proceed under mild conditions and with high regioselectivity. The efficiency of the hydrogen isotope exchange reaction depends on the electronic properties of the pincer complex catalyst.     

Nickel(II) and copper(II) complexes of aza macrocycles derived from trichloromethane

Syntheses of nickel(II) complexes of the tetraaza macrocycles 2,7-dichloro-1,3,6,8-tetraazacyclodecane (DCCD) and 2,8-dichloro-1,3,7,9-tetraazacyclododecane (DICD) and a copper(II) complex of 2,6,8,12,13,17-hexaazabicyclo[5.5.5]heptadecane (HBCH) are reported in the template condensation of trichloromethane with 1,2-diaminoethane or 1,3-diaminopropane. Formulation of the synthesized products [Ni(DCCD)(H₂O)₂]Cl₂, [Ni(DICD)(H₂O)₂]Cl₂?·?H₂O, and [Cu₃(HBCH)(H₂O)₆]Cl₆, and the metal-free ligand hydrochloride HBCH?·?6HCl has been confirmed by elemental analyses, conductivity measurements, and spectral studies. Potentiometric studies of nickel(II) and copper(II) complexes of HBCH and structurally similar 2,5,8,10,13,16,17,20,23-nonaazabicyclo[7.7.7]tricosane (NACT, earlier derived from trichloromethane and diethylenetriamine) have also been performed in the structural support of HBCH. In 1?:?1, metal?:?HBCH solution, copper(II) is coordinated to four N-donors of two-HN(CH₂)₃NH– groups of the ligand in a non-planar tetraaza cavity. The equilibrium constant value (log?K?=?15.41) for the reaction Cu^2+?+?A???CuA²⁺ (A?=?HBCH) is in favor of the cyclic structure of the ligand. A high value (log?K?=?23.27) for corresponding reaction in the NACT system is due to conformational change in the ligand, where copper(II) organizes the macrocycle to form a nearly planar cavity in which the cation fits well.     

Palladium(II) aryl-amido complexes of diphosphinoazines in unsymmetrical PNP’ pincer-type configuration

Square planar palladium(II) aryl-amido complexes of diphosphinoazines in monoanionic unsymmetrical PNP’ pincer-type coordination were prepared by reactions of phenyl-, o-tolyl-, or 2,6-dimethylphenyllithium with previously described chloro-amido complexes of diphosphinoazines having isopropyl, cyclohexyl and tert-butyl substituents on phosphorus atoms. The compounds were characterized by NMR showing free rotation around metal-aryl bond in the complexes; the presence of C_ipso-Pd bond was detected by two-dimensional experiments. In addition to that, crystal and molecular structure of one phenyl-amido complex, [Pd(C₆H₅){P(C₆H₁₁)₂CHC(Bu^t)NNC(Bu^t)CH₂P(C₆H₁₁)₂}], was determined by X-ray diffraction together with the structure of a chloro-amido complex [PdCl{PBu^t₂CHC(Bu^t)NNC(Bu^t)CH₂PBu^t₂}]. In both structures the ligand trans to the amide nitrogen is well surrounded by substituents on phosphorus atoms, the former complex showing significant interactions between two cyclohexyl hydrogen atoms and the π-system of the phenyl ring. The values od Pd-C and Pd-N bond distances in this complex are the same as those in a monodentate analog [Pd(PMe₃)₂(C₆H₅)(NHC₆H₅)] which contrasts with the different values in a similar PNP symmetrical pincer complex reported in the literature.     

Dinuclear macrocyclic palladium complexes having pincer coordinating groups and their catalytic properties in Mizoroki-Heck reactions

An improved synthetic method of palladium(II) dinuclear macrocyclic complexes have been described. Each of the two isomers of the complexes [Pd₂LBr₂] has a macrocyclic ligand L in which two 2,6-bis(diaminomethyl)phenyl units coordinate to the Pd(II) centers with N, C, N donor atoms. Substitution of the bromo ligands of one of the isomer of the complexes with acetonitrile ligands affords a new dinuclear complex. Catalytic activities of these complexes were studied for the Mizoroki-Heck type reactions of iodobenzene and styrene. High turnover number up to 30,000 was achieved using one of the isomer of the complexes.     

Nickel(II) complexes prepared from NNN type ligands and pseudohalogens

Six nickel(II) complexes, using azide and thiocyanate ions, have been synthesized from bis-2,6(pyrazol-1-yl)pyridine (pp) and some methyl derivatives, 2-(3,5-dimethyl(pyrazol-1-yl)-6-(pyrazol-1-yl)pyridine (app) and bis-2,6(3,5-dimethyl(pyrazol-1-yl) pyridine (dmpp) in non-aqueous media. The complex structures were analyzed using elemental analysis, IR spectroscopy and thermogravimetry. Appropriate crystals of complex, containing azide [Nipp(N₃)₂]·MeOH (I) and thiocyanate [Nidmpp(SCN)₂·MeOH] (VI) were prepared and the molecular structures determined using X-ray diffraction. Complex I was seen to be dinuclear as stated in literature, space group P2₁/n, monoclinic, a=10.503, b=10.681, c=13.291 Å, β=106.56° and Z=2 whereas complex VI was found to be mononuclear, space group P2₁/n, monoclinic, a=8.646, b=12.614, c=20.697 Å, β=97.18° and Z=2. The Ni(II) coordination in both complexes were octahedral. Thermogravimetric studies showed azide containing structures to resemble the characteristics of explosive materials. Coordinative MeOH were seen to leave the structure in thiocyanate containing complexes, followed by irregular degradation above 300°C.     

NNN pincer Ru(II)-catalyzed dehydrogenative coupling of 2-aminoarylmethanols with nitriles for the construction of quinazolines

An efficient NNN pincer Ru(II)-catalyzed preparation of quinazolines via acceptorless dehydrogenative strategy has been developed. Under the optimized conditions, a broad range of substituted o-aminobenzyl alcohols and (hetero)aryl or alkyl nitriles were well tolerated to afford various 2-substituted quinazolines in high yields. Subsequently, a set of control experiments have been performed to elucidate the reaction mechanism, which underwent alcohol oxidation, nitrile hydration, and cyclocondensation steps. The current protocol is featured with several advantages, such as environmental benignity, operational simplicity, broad substrate scope (compatible with aliphatic nitriles, up to 87% yield), and short reaction time (mostly in 2?h).     

Solid-phase thermal decomposition of polynuclear nickel(II) and cobalt(II) complexes

Solid-phase thermal decomposition of polynuclear Ni^II and Co^II pivalate complexes was studied by differential scanning calorimetry and thermogravimetry. The decomposition of the polynuclear (from bi-to hexanuclear) Co^II carboxylate complexes is accompanied by aggregation to form a volatile octanuclear complex. Thermolysis of the polynuclear NiII carboxylates results in their destructure, and the phase composition of the decomposition products is determined by the nature of coordinated ligands. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 250—260, February, 2006.     

Synthesis and Characterization of Two Trinuclear Nickel(II) Complexes

Two novel trinuclear nickel(II) complexes have been synthesized and characterized by X‐ray single crystal diffraction. Compound [Ni₃(ashz)₃(py)₂(DMF)₂]·(DMF)₂ ( 1 ) crystallizes in the monoclinic, space group C2/c, with a = 22.114(2), b = 10.509(9), c = 19.485(2) Å, β = 114.443(1)°, Z = 4; compound [Ni₃(acshz)₃(py)₂(DMF)₂]·(DMF)₂ ( 2 ) crystallizes in the monoclinic space group P2₁/n with a = 20.0620(2), b = 9.7017(6), c = 25.0533(2) Å, β = 97.0610(2)°, Z = 4, where ashz and acshz are deprotonated N‐acetylsalicylhydrazide (H₃ashz) and N‐acetyl‐5‐chlorosalicylhydrazide (H₃acshz), respectively. The crystal structure analysis of 1 and 2 showed that three Ni²⁺ ions in a linear arrangement are bridged by two ligands ((ashz)^3? or (acshz)^3?) to form a neutral nuclear with two four‐coordinate square‐planar nickel ions linked by a six‐coordinate octahedral central nickel ion.     

Synthesis, characterization and photoluminescent properties of platinum complexes with novel bis(imidazoline) pincer ligands

Chiral C₂-symmetric bis(imidazoline) pincer ligands 2a-d have been synthesized for the first time. Direct cycloplatination of these ligands with K₂PtCl₄ in dry acetic acid afforded the corresponding cycloplatinated pincer complexes 3a-d. The X-ray single-crystal structure of platinum complex 3d and the preliminary studies on the photoluminescent properties of 3 are reported.     

Nickel versus copper: enhanced antibacterial activity in a series of new nickel(II) Schiff base complexes

Five new Ni(II) Schiff base complexes [NiL^x(Solv)₂] denoted by NiL^x, x = 1–5, were synthesized and characterized. The Schiff base ligands were synthesized from the condensation of 5-bromo-2-hydroxy-3-nitrobenzaldehyde with different aliphatic and aromatic diamines. The X-ray crystal structure of NiL³ was determined. The ligands and complexes were tested as antibacterial agents against two gram(+) and two gram(?) human pathogenic bacteria. The complexes showed moderate antibacterial activity against both gram type bacteria. The new Ni(II) complexes showed enhanced antibacterial activity compared to the previously reported Cu(II) complexes of the same ligands.     

Palladium and rhodium complexes containing 1,3-bis(oxazolinyl)propyl (Probox) ligands: Macrocycles and pincer compounds

New chiral 1,3-bis(4R-phenyl-2-oxazolinyl)propane (Probox^Ph) and achiral 1,3-bis(4,4-dimethyl-2-oxazolinyl)propane (Probox^Me2) ligands have been prepared by Cd(OAc)₂-catalyzed condensation reactions. These ligands, and the known isopropyl derivative Probox^iPr, react with [PdCl₂(NCPh)₂] and [RhCl(η²-C₈H₁₄)₂]₂ to form 16-membered bimetallic macrocycles. Additionally, Probox^Me2 and RhCl₃ react to form a new monoanionic NCN-type pincer complex (κ³-N,C,N-Probox^Me2)RhCl₂. The structures of new palladium and rhodium macrocycles with the Probox ligands are confirmed by X-ray crystallography, and natural abundance ¹⁵N 2D NMR experiments prove oxazoline coordination to the metal centers in solution. Addition of a weakly donating water ligand to (κ³-N,C,N-Probox^Me2)RhCl₂ gives a six-coordinate compound with a mer-Probox configuration, whereas PMe₃ coordination provides a single fac coordinated Probox isomer.     

Structural Consequences of Deacylation of Nickel(II) Cyclidenes in Acidic and Basic Conditions

Two nickel(II) complexes with deacylated unsaturated tetraaza macrocyclic cyclidene ligands were isolated and structurally characterized. Under acidic conditions, both acyl groups attached to the carbons in the precursor complex are cleaved off, yielding a symmetric dicationic complex (IIIc) which retains the usual saddle shaped conformation of the 16-membered cyclidene complexes. Under basic conditions, one acyl group remains attached to the cyclidene, while the other (deacylated) chelate ring becomes singly deprotonated. This asymmetric deacylation is observed for the first time in the family of cis-dimethyl cyclidenes. The presence of a deprotonated conjugated chelate ring in the 15-membered macrocycle (VIb) results in the overall planar shape of the molecule.     

Synthesis, structure, and quaternization and complexation reactions of κSCS pincer palladium complexes having 3,5-pyridinediyl unit

The cyclopalladation of 3,5-bis(diphenylphosphinothioyl)pyridine afforded new κ³S,C,S-pincer palladium complexes with a σ-bond between Pd and 4C of the centered 3,5-pyridinediyl unit. By utilizing the quaternization and complexation ability of the pyridine imine nitrogen (N_py) atom, various new pincer-type complexes, including hetero-binuclear complexes, have been synthesized.     

Nickel(II) 1,10-Phenanthroline complexes: cis-[Aqua(Bromo) bis (1,10-Phenanthroline)Nickel(II)] Bromide Trihydrate and ( tris (1,10-Phenanthroline)Nickel(II)] Bromide Octahydrate

[Ni(phen)₂(H₂O)Br]Br·3H₂O where phen is 1,10-phenanthroline, is a light-blue material which crystallizes in the monoclinic space group P2₁/c with Z = 4, a = 10.4300(4), b = 25.310(2), c = 9.7790(9)?Å and β = 102.932(6)°. The structure was determined at ambient temperature from 5161 reflections with R = 0.0643 and R _w = 0.1306. The structure consists of a complex cation, a bromide anion and three waters of hydration. The Ni atom is pseudo-octahedral with a cis arrangement of Br and H₂O. This cis geometry persists in solution, as evidenced by ¹H NMR spectroscopy, although the Br may be replaced by another H₂O. [Ni(phen)₃]Br₂·8H₂O is a light-red material which crystallizes in the monoclinic space group C2/m with Z = 8, a = 23.6320(11), b = 21.4880(13), c = 15.5470(9)?Å and β = 107.927(3)°. The structure was determined at 120?K from 6820 reflections with R = 0.0733 and R _w = 0.1022. The structure consists of a complex cation, two bromide anions and eight waters of hydration. The anions and waters are extensively disordered. The Ni atom is pseudo-octahedral.