Biferrocene NCN pincer metal-d complexes: Synthesis, reaction chemistry and cyclovoltammetric studies

The synthesis of biferrocene-bridged NCN pincer palladium and platinum complexes (NCN = [1-C₆H₂(CH₂NMe₂)₂-3,5]⁻) is discussed. Sonogashira cross-coupling of [(η⁵-C₅H₄)Fe(η⁵-C₅H₄CCH)]₂ (1) with I-1-NCN-4-X (2a, X = H; 2b, X = Br) produces [(η⁵-C₅H₄)Fe(η⁵-C₅H₄CC-1-NCN-4-X)]₂ (3a, X = H; 3b, X = Br). Homobimetallic 3b further reacts with [Pd₂(dba)₃ · CHCl₃] (4) or [Pt(tol)₂(SEt₂)]₂ (5) (dba = dibenzylidene acetone, tol = 4-tolyl), respectively, to give tetrametallic [(η⁵-C₅H₄)Fe(η⁵-C₅H₄CC-4-NCN-1-MBr)]₂ (6, M = Pd; 7, M = Pt) in which NCN-MBr fragments are connected by a biferrocene unit. Cyclovoltammetric studies show that the ferrocene moieties can independently be oxidized. The difference of the Fe(II)/Fe(III) redox couples amounts to ca. 300 mV and is not affected by the nature of the NCN pincer metal moities.     

Rhodium(III) NCN pincer complexes catalyzed transfer hydrogenation of ketones

Air-stable monomeric rhodium(III) NCN pincer complexes were synthesized via direct C-H bond activation of 1,3-bis(2-pyridyloxy)benzene, 3,5-bis(2-pyridyloxy)toluene and 3,5-bis(2-pyridyloxy)anisole with RhCl₃·3H₂O in ethanol under reflux. The synthesized complexes were characterized by elemental analysis and ¹H NMR. One of the complexes was structurally characterized by X-ray analysis. An investigation into the catalytic activity of the complex 1a as catalyst for transfer hydrogenation of ketones to alcohols at 82 °C in the presence of iPrOH/KOH was undertaken with the conversions up to 99%.     

Synthesis of achiral NCN pincer Pt(II) and Pd(II) complexes and catalytic application in the Suzuki-Miyaura reaction

A series of platinum and palladium pincer complexes supported by achiral 1,3-bis(2′-imidazolinyl)benzene-based NCN ligands have been prepared via direct C2 metalation. Meanwhile, ligand precursor 3b and Pt(II) complex 4b were characterized by crystallographic studies, which reveals that the platinum atom in 4b adopts a distorted-square-planar geometry. The Pd(II) pincer complexes 5b was found to be an efficient catalyst for Suzuki cross-coupling reaction of aryl bromides and phenylboronic acid under air. In the presence of 0.5?mol% of Pd(II) 5b in DMF/K₃PO₄·3H₂O for 8?h, the corresponding biaryl products could be obtained in 24–99% yields.     

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile manner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the precursors (3a-3e) with Pd(OAc)₂ in refluxing HOAc, followed by treatment with LiCl at room temperature provided convenient access to the corresponding pincer palladium(II) complexes (4a-4e). The molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlorides with phenylboronic acid.     

Synthesis of a new class of unsymmetrical PCP′ pincer ligands and their palladium (II) complexes: X-ray structure determination of PdCl{C6H3-2-CH2PPh2-6-CH2PBu2}

The unsymmetrical PCP′ pincer ligands {C₆H₄-1-CH₂PPh₂-3-CH₂PBu^t₂} and {C₆H₄-1-CH₂PPh₂-3-CH₂PPrⁱ₂} and the corresponding palladium complexes: PdCl{C₆H₃-2-CH₂PPh₂-6-CH₂PBu^t₂} and PdCl{C₆H₃-2-CH₂PPh₂-6-CH₂PPrⁱ₂} have been synthesized in good yields. The molecular structure of PdCl{C₆H₃-2-CH₂PPh₂-6-CH₂PBu^t₂} was determined through a single crystal X-ray diffraction study. The palladium center was found to be located into a slightly distorted square planar environment in which the {C₆H₄-1-CH₂PPh₂-3-CH₂PBu^t₂} ligand is coordinated as a tridentate, PCP pincer type chelate. The complex, PdCl{C₆H₃-2-CH₂PPh₂-6-CH₂PPrⁱ₂} catalyzes the Heck coupling of iodobenzene with styrene.     

Synthesis and structural studies of NCN diimine palladium pincer complexes bearing m-terphenyl scaffolds

The reaction of 2,6-[2-{RN=C(H)}C(6)H(4)](2)C(6)H(3)I [R = Ph (4), Cy (5), 2,6-Me(2)C(6)H(3) (6), 2,4,6-Me(3)C(6)H(2) (7), (S)-alpha-methylbenzyl (8)] with Pd(2)(dba)(3) afforded the NCN diimine pincer palladium complexes [2,6-[2-{RN=C(H)}C(6)H(4)](2)C(6)H(3)PdI] (9-13) by oxidative addition of the C-I bonds of the ligand precursors. Single-crystal X-ray diffraction analyses of complexes 9-13 reveal formal C(2)-symmetric environments. Variable-temperature NMR studies of complexes 11 and 12 show hindered rotation about the N-Ar bonds and also suggest that atropisomers of complexes 9-13 do not interconvert on the NMR time scale. Consistent with this proposal, isolation of the two possible isomers of 13 (13a and 13b) was possible, and their structures and NMR properties have been examined in detail.     

Iron pincer complex and its graphene oxide composite as catalysts for Suzuki coupling reaction

We report the synthesis of Fe-NCN pincer complex as homogenous catalyst and its composite by immobilizing the complex on amino functionalized graphene oxide as a heterogeneous catalyst for Suzuki coupling reactions. Both the complex and the composite were employed in catalyzing the Suzuki-Miyaura cross-coupling reaction between the aryl halide and phenyl boronic acid in acetonitrile solvent media with Cs₂CO₃ as a base. Effect of substitution over aryl halide was also investigated. Immobilization of the pincer complex had advantageous recovery and reuse of the catalyst as compared to its homogenous analog with no significant decrease in the catalytic efficiency.     

Synthesis,characterization, and crystal structures of organonickel(II) complexes coordinated to novel 1‐bromo‐2,6‐bis{[(λ5‐phosphanylidene)imino]methyl}benzene NCN‐pincer ligands

A novel family of four 1‐bromo‐2,6‐bis{[(λ⁵‐phosphanylidene)imino]methyl}benzene ligands has been synthesized and characterized. The phosphiniminomethyl substituents are decorated with either three phenyl groups, two phenyl and one cyclohexyl group, one phenyl and two cyclohexyl groups, or three cyclohexyl groups. Each ligand was metallated using zero‐valent nickel through an oxidative addition to form a family of organonickel(II) complexes, namely (2,6‐bis{[(triphenyl‐λ⁵‐phosphanylidene)imino]methyl}phenyl‐κ³N,C¹,N′)bromidonickel(II) dichloromethane hemisolvate, [NiBr(C₄₄H₃₇N₂P₂)]·0.5CH₂Cl₂, (2,6‐bis{[(cyclohexyldiphenyl‐λ⁵‐phosphanylidene)imino]methyl}phenyl‐κ³N,C¹,N′)bromidonickel(II) diethyl ether hemisolvate, [NiBr(C₄₄H₄₉N₂P₂)]·0.5C₄H₁₀O, (2,6‐bis{[(dicyclohexylphenyl‐λ⁵‐phosphanylidene)imino]methyl}phenyl‐κ³N,C¹,N′)bromidonickel(II), [NiBr(C₄₄H₆₁N₂P₂)], and (2,6‐bis{[(tricyclohexyl‐λ⁵‐phosphanylidene)imino]methyl}phenyl‐κ³N,C¹,N′)bromidonickel(II), [NiBr(C₄₄H₇₃N₂P₂)]. This family of complexes represents a useful opportunity to investigate the impact of incrementally changing the steric characteristics of a complex on its structure and reactivity.     

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.     

Synthesis and crystal structures of the first C2-symmetric bis-aldimine NCN-pincer complexes of platinum and palladium

(S,S)-2,6-bis[(N-α-methylbenzyl)imino]phenylpalladium bromide was synthesised by oxidative addition of palladium(0) to (S,S)-1-bromo-2,6-bis[(N-α-methylbenzyl)imino]benzene. In contrast, (S,S)-2,6-bis[(N-α-methylbenzyl)imino]phenylplatinum chloride was synthesised by direct C-H activation from the reaction of potassium tetrachloroplatinate with (S,S)-1,3-bis[(N-α-methylbenzyl)imino]benzene. The X-ray crystal structures of both pincer complexes were obtained. Treatment of both complexes with silver hexafluoroanimonate gave effective but not stereoselective catalysts for a Michael reaction between methyl vinyl ketone and methyl 2-cyanopropanoate.     

Trimetallic FePd2 and FePt2 4-ferrocenyl-NCN pincer complexes

Ferrocene-bridged NCN pincer complexes of structural type Fe(η⁵-C₅H₄-4-NCN-1-MX)₂ (X = I: 6, M = Pd; 7, M = Pt; X = Cl: 8, M = Pt; NCN = [4-C₆H₂(CH₂NMe₂)₂-2,6]⁻) are accessible by the subsequent reaction of Fe(η⁵-C₅H₄-4-NCNH)₂ (4) with ⁿBuLi and [PtCl₂(SEt₂)₂] (synthesis of 8) or treatment of Fe(η⁵-C₅H₄-4-NCN-1-I)₂ (5) with [Pd₂(dba)₃] (synthesis of 6) or [Pt(tol)₂(SEt₂)]₂ (synthesis of 7) (dba = dibenzylidene acetone, tol = 4-tolyl). In addition, the Sonogashira cross-coupling of Fe(η⁵-C₅H₄I)₂ (1) with HCC-4-NCNH (2) gives Fe(η⁵-C₅H₄-CC-4-NCNH)₂ (3). The reaction behavior of 3 towards ^tBuLi is reported as well.Cyclovoltammetric studies show that the ferrocene entity can be oxidized reversibly. The Fe(II)/Fe(III) potential decreases with increasing electron density at the NCN pincer units due to the presence of the M-halide moiety (M = Pd, Pt).The solid state structure of Fe(η⁵-C₅H₄-4-NCN-1-PdI)₂ (6) is presented. In 6 the Fe(η⁵-C₅H₄)₂ unit connects two NCN-PdI pincer entities with palladium in a square-planar environment. The cyclopentadienyl ligands show a staggered conformation. The C₆H₂ rings are tilted by 23.5(3)° towards the C₅H₄ entities and the C₆H₂ plane is almost coplanar with the coordination plane (10.3(3)°).     

Simple protocol for the synthesis of the asymmetric PCP pincer ligand [C6H4-1-(CH2PPh2)-3-(CH(CH3)PPh2)] and its Pd(II) derivative [PdCl{C6H3-2-(CH2PPh2)-6-(CH(CH3)PPh2)}]

The asymmetric PCP pincer ligand [C₆H₄-1-(CH₂PPh₂)-3-(CH(CH₃)PPh₂)] (4) has been synthesized in a facile manner in three simple steps in high yield. Metallation of PCP pincer ligand (4) with [Pd(COD)Cl₂] affords complex [PdCl{C₆H₃-2-(CH₂PPh₂)-6-(CH(CH₃)PPh₂)}] (7) in good yield.     

Synthesis and structural studies on the chiral phosphine-NHC rhodium and palladium complexes for their performances in the metal-catalyzed reactions

Diverse chiral rhodium and palladium complexes ligated with phosphine-functionalized N-heterocyclic carbene ligands based on 1,1′-binaphthyl backbone have been synthesized. The structures of these phosphine-NHC rhodium and palladium complexes have been confirmed by X-ray diffraction analysis. The different sizes of the N-substituents from the NHC-P rhodium complexes had an inverse relationship with their ability of chiral induction, which was accounted by the Rh-catalyzed asymmetric hydrosilylation of acetophenone to afford corresponding chiral alcohol with up to 72% ee. The NHC-P palladium complexes connected with different kinds of coordination anions were also applied in the Suzuki and Heck reactions. The acetate-coordinated NHC-P palladium complex exhibited better catalytic activity to give the products in excellent yield under mild conditions.     

Synthesis and characterization of cyclopalladated complexes of benzylamine by IR and NMR spectroscopy studies

The chloro-bridged dimer [Pd(μ-Cl)(C₆H₄CH₂NH₂-κ²-C,N)]₂ reacts with PPh₂Et, P(p-tolyl)₃, AsPh₃, piper (piper =?C₅H₁₀N) and Py in dichloromethane at room temperature for 24 h in a one-to-two molar ratio and undergoing bridge-splitting reactions to give [PdCl(C₆H₄CH₂NH₂–κ²-C,N)L] (L =?PPh₂Et (1a), P(p-tolyl)₃ (1b), AsPh₃ (1c), piper (1d), C₆H₄CH₂NH₂ (3e) and Py (1f)). Complex 1f in THF at room temperature reacts with a stoichiometric amount of TlTfO (thallium triflate, TfO=CF₃SO₃) and Py (molar ratio 1 : 1 : 1) to afford [Pd(C₆H₄CH₂NH₂)(Py)₂]TfO (2). Infrared and NMR spectroscopies allow unambiguous characterization of these products.     

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.     

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.     

A highly active two six-membered phosphinite palladium PCP pincer complex [PdCl{C6H3(CH2OPPr)2-2,6}]

The diphosphinite 1,3-bis[(di-isopropylphosphinite)methyl]benzene (2) has been synthesized and its complexation in palladium chemistry investigated. Complex 3 represents the first example of a two six-member PCP pincer bis(phosphinite) species. A catalytic study of 3 in the Heck reaction, revealed this specie to have an outstanding activity in the olefination of iodo-, bromo- and chloro-benzenes. When compared with other PCP pincer complexes, the results show that both by increasing the ring size from five to six-membered and using phosphinite instead of phosphine groups lead to a more active catalyst.     

Synthesis and structural studies of mixed-ligand complexes

Seven mixed-ligand complexes of cobalt(II), nickel(II) and copper(II) containing benzoylacetone andL-proline (HL¹), 2-pyrrolidone-5-carboxylic acid (HL²) orL-thioproline (HL³) were prepared and characterized by means of elemental analysis, IR, electronic spectra, magnetic moment measurements and molar conductance. Both HL¹ and HL² coordinate with these metal ions in a neutral zwitterionic form (-NH₂-CH-COO^–), whereas HL³ coordinates as a monobasic chelating agent (O/N). The continuous thermochromism of the nickel(II) complex of HL¹ (2) was attributed to a geometry change; it was investigated by DTA, TG, electronic spectra and X-ray powder diffraction techniques.