PCP Pincer Ligand Research Articles

Missing link: PCP pincer ligands containing P–N bonds and their Pd complexes

New PCP ligands in which the phosphine donor arms are connected to the central aromatic ring via NH moieties and their (PCP)PdCl complexes have been prepared. One such (PCP)PdCl complex was characterized by X-ray diffractometry in the solid state. The (PCP)PdCl complexes are exceptionally robust towards oxygen and water despite the presence of P–N bonds.

A Modular Approach to Achiral and Chiral Nickel(II), Palladium(II), and Platinum(II) PCP Pincer Complexes Based on Diaminobenzenes

The synthesis and characterization of a series of nickel, palladium, and platinum complexes containing new achiral and chiral PCP pincer ligands based on 1,3-diaminobenzene, 5-trifluoromethyl-1,3-diaminobenzene, and 3,5-diamino-4-chloroisobutylbenzoate are reported. The new PCP ligands are prepared conveniently in high yield by treatment of the respective diaminobenzene with 2 equiv of a variety of achiral and chiral R2PCl compounds in the presence of base. PCP complexes of Ni(II), Ni(PCP)Cl, were synthesized by the reaction of NiCl2·6H2O with 1 equiv of a PCP ligand. In similar fashion, treatment of M(COD)X2 (M = Pd, Pt; X = Cl, Br) with 1 equiv of a PCP ligand yields the square-planar complexes M(PCP)X. Palladium PCP complexes featuring a coordinated TFA ligand (TFA = CF3COO-) are obtained by the reaction of Pd(TFA)2 with 1 equiv of a PCP ligand. Alternatively, palladium PCP complexes can also be generated via an oxidative addition route. Addition of 2 equiv of PCP ligands based on 3,5-diamino-4-chloroisobutylbenzoate to Pd2(dba)3 affords the respective Pd(PCP)Cl pincer complexes in high yields. X-ray structures of representative Ni, Pd, and Pt PCP complexes have been determined. Finally, the use of the palladium complexes as catalysts for the Suzuki−Miyaura coupling of some aryl bromides and phenyl boronic acid has been examined.

A Ruthenium(II) Complex Stabilized by a Highly Fluorinated PCP Pincer Ligand

A novel Ru(II) complex containing an electron-poor, highly fluorinated PCPArF pincer ligand has been synthesized in good yield via a transcyclometalation reaction. The complex has been fully characterized by elemental analysis, 1D and 2D NMR techniques, UV−vis spectroscopy, and cyclic voltammetry. Single-crystal X-ray structural analysis and DFT calculations were performed. The structural features and electronic properties of the remarkably stable PCPArF−Ru(II) complex 4 have been investigated and show unanticipated differences compared to its protio analogue.

Synthesis, Crystal Structure, and Redox and Photophysical Properties of Novel Bisphosphinoaryl RuII-Terpyridine Complexes

Novel organometallic [Ru(PCP)(tpy)]Cl complexes, containing the terdentate coordinating monoanionic bisphosphinoaryl ligands [C6H3(CH2PR2)2-2,6]- (PCP) (R = Ph (11); iPr (12)) and 2,2‘:6‘,2‘ ‘-terpyridine, have been synthesized by two different synthetic pathways in good yields. The molecular structure in the solid state of [Ru{C6H2(CH2PPh2)2-2,6}(tpy)](OTf) (14) has been determined by X-ray crystallography. The spectroscopic and electrochemical properties of the [Ru(PCP)(tpy)]Cl complexes were compared with those obtained for [Ru{C6H3(CH2NMe2)2-2,6}(tpy)](Cl) (3) containing the monoanionic bisaminoaryl ligand [C6H3(CH2NMe2)2-2,6]- (NCN). The obtained results revealed that substitution of the NCN-pincer ligand by PCP-pincer ligands offers a powerful tool to tune the redox and photophysical properties as well as the reactivity of the ruthenium(II) metal centers in the resulting photoactive monomeric species.

A hexanuclear iridium complex with a novel binding mode of a PCP–pincer ligand

The hexanuclear iridium complex [η 4-(COD)Ir] 2{η 6-[κ 4-C 6H 2(CH 2P t Bu 2) 2]Ir 2H 2Cl 3} ( 1) has been prepared by the reaction of [Ir(COD)Cl] 2 with the “PCP” ligand precursor, 1,3-C 6H 4(CH 2P t Bu 2) 2 (PCP–H). Characterization by X-ray diffraction reveals that complex 1 has an unusual structure in which metalation (C–H addition) of the ligand has occurred at the 4- and 6-positions of the PCP aryl ring. This is in contrast to the widespread reactivity of this ligand precursor in which a single metalation typically occurs at the 2-position, thereby allowing coordination of both phosphino groups to a single metal center to give the κ 3-meridional pincer coordination. The complex can be viewed as being composed of a dimer of two bis-metalated (Ir(III)) PCP units, held together by two bridges of three chlorides each; the aryl ring of each of the PCP ligands of this hypothetical tetra-iridium dianion is then bound to a cationic [Ir(I)(COD)] + unit. The structure discussed here crystallizes as the tetra(acetone) solvate in the monoclinic space group P2 1/ n with lattice parameters a=14.4154(11) Å, b=15.4435(12) Å, c=19.5181(15) Å, b=99.996(1)° and V=4279.31(6) Å 3. Convergence to conventional R values of R( F)=0.041 and R w( F)=0.092 was obtained for 445 variable parameters and 8085 reflections with F>4 σ( F). Complex 1 also crystallizes as the octabenzene solvate in the orthorhombic space group Pbca, with lattice parameters a=16.4247(14) Å, b=24.1695(21) Å, c=28.0460(24) Å and V=11134(1) Å 3. Convergence of the orthorhombic phase to R( F)=0.038 and R w( F)=0.084 was obtained for 595 variable parameters and 12 885 reflections with F>4 σ( F).

EPR study of intramolecular dynamics in o-semiquinonic nickel complexes with PCP-pincer ligand in solution.

A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl were investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.

Synthesis and Catalytic Properties of Di- and Trinuclear Palladium Complexes with PCP-Pincer Ligands

Linear and branched conjugated pincer ligands having Ph2P groups were synthesized: 3,3',5,5'- tetrakis(diphenylphosphinomethyl)diphenylacetylene, 3,3',5,5'-tetrakis(diphenylphosphinomethyl)diphenyldi- acetylene, 1,3,5-tris[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene, and hexakis[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene. Palladation of these ligands by heating with Pd(BF4)2(MeCN)4 in boiling acetonitrile gave the corresponding di- and trinuclear ionic pincer palladium complexes. No individual complex was obtained from hexakis[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene. The ionic com- plexes were converted into the corresponding chloride complexes by treatment with sodium chloride in a mixture of water with methylene chloride. The structure of the ionic palladium complex with 3,3',5,5'-tetrakis(diphenylphosphinomethyl)diphenylacetylene was established by X-ray analysis. The obtained palladium complexes exhibited a considerable catalytic activity in the Heck reaction of iodobenzene with ethyl acrylate and in the Michael addition of ethyl cyanoacetate with methyl vinyl ketone. The catalytic activity per palladium atom decreases as the number of palladium atoms in the complex increases.

Synthesis and reactivity of a ferrocene-derived PCP-pincer ligand.

The 1,3-bis(diphosphinomethyl)ferrocene 3 readily reacts with [(C2H4)2RhCl]2 to form an equilibrating pair of diastereomers 8a and 8b by C-H insertion into the ferrocene.

Amine and Olefin Complexes of Pt(II) Having a PCP-Pincer Ligand

The relative stability of σ-donor amine versus π-olefiniccomplexes of Pt(II) can be evaluated by measuring equili-brium constants, which may provide valuable informationrelevant to catalytic hydroamination of olefin.Experimental SectionAll operations for air sensitive compounds were perform-ed on a standard Schlenk line or in an inert atmosphereglovebox under argon or nitrogen. All solvents wereproperly dried and deaerated by conventional manners.

Development of the First P-Stereogenic PCP Pincer Ligands, Their Metallation by Palladium and Platinum, and Preliminary Catalysis

The potentially tridentate P-stereogenic [P*CP*] ligands 1,3-{bis[(tert-butyl)(phenyl)phosphino]methyl}benzene and 1,3-{bis[(tert-butyl)(phenyl) phosphino]methyl}-2-bromobenzene have been synthesized as the protected phosphine-borane adducts. Deprotection with a secondary amine affords the free phosphine ligand which can be metallated by Pd and Pt with standard metal synthons. Two of the resultant [P*CP*] metal complexes have been characterized by X-ray crystallography. The complexes exhibit a C2 symmetric environment about the remaining binding site of the square-planar center, with t-Bu groups filling two quadrants of the open site. The Pd complexes can be converted by use of a Ag salt to the analogous aquo complex, which is catalytically active in the aldol condensation of methyl 2-isocyanoacetate and benzaldehyde. Preliminary results and comparisons with previously reported catalysts with more distal C-stereogenicity are presented.

Synthesis and Molecular Structure of a Perfluoroalkyl Complex of Platinum Containing a PCP Pincer Ligand

Treatment of (TMEDA)Pt(C3F7)Me (TMEDA = tetramethylethylenediamine) with 1,3-bis[(diphenylphosphino)methyl]benzene (PCPH) in mesitylene under reflux affords the first known pincer complex of a group 10 metal with a fluoroalkyl ligand.