Group 10 phosphinite POCOP pincer complexes derived from 4-n-dodecylresorcinol: An alternative way to produce non-symmetric pincer compounds

Diverse structural reactivity patterns of a POCOP ligand with coinage metals

We have utilised the 4,6-di-tert-butyl resorcinol bis(diphenylphosphinite) (POCOP) ligand for exploring its coordination ability towards group 11 metal centres. The treatment of the bidentate ligand 1 with various coinage metal precursors afforded a wide range of structurally diverse complexes 2-12, depending upon the metal precursors used. This furnishes several multinuclear Cu(I) complexes with dimeric (2) and tetrameric cores (3, 4, and 5). The tetrameric stairstep complex 4 shows thermochromic behaviour, whereas the dimeric complex 2 and tetrameric complex 3 show luminescence properties at cryogenic temperatures. Interestingly, the halide substitution reaction of the dimeric complex 2 with KPPh2 produces a unique mixed phosphine-based tetrameric Cu(I) complex, 5. Treatment of the POCOP ligand with [CuBF4(CH3CN)4] in the presence of 2,2'-bipyridine afforded heteroleptic complex 6, consisting of tri- and tetra-coordinated cationic Cu(I) centres. Furthermore, we could also isolate cubane (8) and stairstep (9) complexes of Ag(I). The cationic Au(I) complex (12) was obtained from the dinuclear Au(I) complex of POCOP, 11. Complex 12 revealed the presence of a strong intramolecular aurophilic interaction with an Au⋯Au bond distance of 3.1143(9) Å. Subsequently, the photophysical properties of these complexes have been studied. All the complexes were characterised by single-crystal X-ray diffraction studies, routine NMR techniques, and mass spectroscopy.

Synthesis and Characterization of Two Isostructural POCOP Ni(II) Pincer Complexes Containing Fluorothiophenolate Ligands: [Ni(SC6F4-4-H){C6H2-3-(C2H3O)-2,6-(OPiPr2)2}] and [Ni(SC6F5){C6H2-3-(C2H3O)-2,6-(OPiPr2)2}]

Among their many applications, metal pincer complexes are of interest for their properties as catalysts in cross-coupling reactions. Pincer ligands exhibit tridentate coordination to the metal center and occupy the meridional positions forming two chelate rings. The two Ni(II) POCOP pincer complexes with a fluorothiophenolate ligand reported herein, with formulas [Ni(SC6F4-4-H){C6H2-3-(C2H3O)-2,6-(OPiPr2)2}] (2) and [Ni(SC6F5){C6H2-3-(C2H3O)-2,6-(OPiPr2)2}] (3), are isostructural. Additionally, they are prepared in a facile manner from the chloride compound [NiCl{C6H2-3-(C2H3O)-2-6-(OPiPr2)2}] (1). The complexes exhibited slightly distorted square planar geometries around the metal. The fluorothiophenolate ligands are responsible of the C—H···F, C—F···π and C=O···πF interactions that contribute to stabilize the crystal structure arrays.

Novel meta-benzothiazole and benzimidazole functionalised POCOP-Ni(ii) pincer complexes as efficient catalysts in the production of diarylketones

Novel meta-benzothiazole and benzimidazole functionalised POCOP-Ni(ii) pincer complexes were synthesized and used as effcient catalysts for the production of diarylketones.

Bifunctional colorimetric chemosensing of fluoride and cyanide ions by nickel-POCOP pincer receptors

Three Ni(ii)-POCOP pincer complexes [NiCl{C₆H₂-4-OH-2,6-(OPPh₂)₂}], 1; [NiCl{C₆H₂-4-OH-2,6-(OPtBu₂)₂}], 2 and [NiCl{C₆H₂-4-OH-2,6-(OPiPr₂)₂}], 3 were studied as bifunctional molecular sensors for inorganic anions and acetate. In CH₃CN, fluoride generates a bathochromic shift with a colorimetric change for 1-3 with a simultaneous fluorescence turn on, this optical effect is based on deprotonation of the para-hydroxy group of the POCOP ligand. On the other hand, in a neutral aqueous solution of 80 vol% CH₃CN, additions of cyanide produce a distinct change of color by forming very stable complexes with the nickel-based receptors 1-3 with log K_a in the range of 4.38-5.03 M^-1 and pronounced selectivity over other common anions such as iodide, phosphate, and acetate. Additionally, bromide shows a modest spectral change and affinity, but lower than those observed for cyanide. On the basis of ¹H NMR experiments, UV-vis titrations, ESI-MS experiments, and the crystal structure of the neutral bromo complex of 1, it is proposed that the colorimetric change involves an exchange of chloride by CN^- on the Ni(ii) atom. The Ni(ii)-based sensor 1 allows the fluorescent selective detection of fluoride with a limit of 5.66 μmol L^-1 and colorimetric sensing of cyanide in aqueous medium in the micromolar concentration range.

Non-symmetric pincer ligands: complexes and applications in catalysis

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

A cyclodiphosphazane based pincer ligand, [2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3I]: Ni(II), Pd(II), Pt(II) and Cu(I) complexes and catalytic studies

Synthesis and late-transition metal complexes of pincer capable cyclodiphosphazane, 2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3I (1) are described. The condensation of 2-iodoresorcinol with cis-{ClP(μ-N(t)Bu)2PN(H)(t)Bu} produced a difunctional derivative 1 in good yield. The treatment of Ni(COD)2, Pd2(dba)3·CHCl3 or Pt(PPh3)4 with 1 afforded pincer complexes [2,6-{μ-((t)BuN)2P((t)BuHN)PO}2C6H3MI] (2 M = Ni; 3 M = Pd and 4 M = Pt). The reaction of complex 3 with copper halides resulted in the formation of heterobimetallic complexes bridged by rhombic {Cu(μ-X)}2 units, [{{Cu(μ-X)}2}{μ-((t)BuN)2P((t)BuHN)PO}2C6H3PdI] (5 X = I and 6 X = Br). The crystal structures of 1-3, and 6 were established by single X-ray diffraction studies. The palladium complex 3 was tested for catalytic P-arylation of diphenylphosphine oxide (Ph2P(O)H) under microwave irradiation. Moderate to good catalytic activity was observed with aryl bromides.

From bis(silylene) and bis(germylene) pincer-type nickel(II) complexes to isolable intermediates of the nickel-catalyzed Sonogashira cross-coupling reaction

The first [ECE]Ni(II) pincer complexes with E = Si(II) and E = Ge(II) metallylene donor arms were synthesized via C-X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide → CuBr} intermediates revealed an unusual E-Cu-Ni three-center-two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.

[2-(Diphenylphosphanyl)benzenethiolato-κ2P,S](pyridine-2-thiolato-κS)(triphenylphosphine-κP)palladium(II)

In the title compound, [Pd(C₅H₄NS)(C₁₈H₁₄PS)(C₁₈H₁₅P)], the Pd^II atom has a slightly distorted square-planar environment. Two coordination sites are occupied by a P,S-chelating 2-(diphenyl­phosphan­yl)benzene­thiol­ate ligand and the other two by a P atom from a triphenyl­phosphine ligand and an S atom from a pyridine-2-thiol­ate ligand, exhibiting a trans arrangement of the two P-donor atoms. In the crystal structure, weak intra- and inter­molecular C—H⋯π and π–π inter­actions are observed. The pyridyl ring is equally disordered over two positions.