Ruthenium bis(phosphinite) pincer complexes

Resorcinol-based ruthenium bis(phosphinite) complexes were synthesized. Complexes RuCl(CO)[2,6-(Bu^t ₂PO)₂C₆H₃] (9) and RuH(CO)[2,6-(Bu^t ₂PO)₂C₆H₃] (10) were obtained by cyclometallation of 1,3-(Bu^t ₂PO)₂C₆H₃ with RuCl₂(DMSO)₄ in 2-methoxyethanol in the presence of Hünig’s base. The interconversion of complexes 9 and 10 was studied. The addition of carbon monoxide to complexes 9 and 10 yielded 18e adducts, RuCl(CO)₂[2,6-(Bu^t ₂PO)₂C₆H₃] (11) and RuH(CO)₂[2,6-(Bu^t ₂PO)₂C₆H₃] (12), respectively. In the case of complex 9, this reaction is reversible. Reaction of complex 10 with trifluoroacetic acid resulted in complex Ru(CF₃COO)(CO)[2,6-(Bu^t ₂PO)₂C₆H₃] (13), which reacted with carbon monoxide to give complex Ru(CF₃COO)(CO)₂[2,6-(Bu^t ₂PO)₂C₆H₃] (14). Based on the IR spectral data, the TFA ligand in complexes 13 and 14 is bound in a bi- and monodentate fashion, respectively. The structure of compound 9 was determined by X-ray diffraction analysis.     

Mechanistic studies of the transfer dehydrogenation of cyclooctane catalyzed by iridium bis(phosphinite) p-XPCP pincer complexes

Reaction of bis(phosphinite) PCP iridium pincer complexes (p-XPCP)IrHCl (5a-f) [X = MeO (5a), Me (5b), H (5c), F (5d), C(6)F(5) (5e), Ar(F)(= 3,5-bis(trifluoromethyl)phenyl) (5f)] with NaOtBu in neat cyclooctane (COA) generates 1:1 mixtures of the respective (p-XPCP)IrH(2) complexes 4a-f and the cyclooctene (COE) olefin complexes (p-XPCP)Ir(COE) (6a-f) at 23 degrees C. At higher temperatures, complexes 4 and 6 are equilibrated because of the degenerate transfer dehydrogenation of COA with free COE (6 + COA right harpoon over left harpoon 4 + 2COE), as was shown by temperature-dependent equilibrium constants and spin saturation transfer experiments at 80 degrees C. At this temperature, the COE complexes 6 exchange with free COE on the NMR time scale with the more electron-deficient complexes 6 exchanging COE faster. The exchange is dissociative and zero order in [COE]. Further analysis reveals that the stoichiometric hydrogenation of COE by complex 4f, and thus the separated back reaction 4f + 2COE --> 6f + COA proceeds at temperatures as low as -100 degrees C with the intermediacy of two isomeric complexes (p-Ar(F)PCP)Ir(H)(2)(COE) (8f, 8f'). COE deuteration with the perdeuterated complex 4f-d(38) at -100 degrees C results in hydrogen incorporation into the hydridic sites of complexes 8f,8f'-d(38) but not in the hydridic sites of complex 4f-d(38), thus rendering COE migratory insertion in complexes 8f,8f' reversible and COE coordination by complex 4f rate-determining for the overall COE deuteration.     

Reactions of Pd-PEPPSI complexes with protic acids

The Pd-PEPPSI complexes widely used to catalyze numerous reactions eliminate the pyridine ligand on treatment with protic acids to give binuclear complexes [Pd(NHC)X₂]₂ with the Pd–X–Pd (X = Cl, Br, I) bridging bonds. The reaction proceeds with high yields (78–98%) and can be regarded as a preparative approach to binuclear complexes. A prolonged heat treatment of either Pd-PEPPSI complexes or binuclear [Pd(NHC)X₂]₂ complexes in the presence of strong protic acids results in the Pd–NHC bond cleavage to give azolium salts (proligands) and palladium salts.     

Iridium bis(phosphinite) p-XPCP pincer complexes: highly active catalysts for the transfer dehydrogenation of alkanes

A series of new bis(phosphinite) p-XPCPIrHCl pincer complexes ([PCP = eta(3)-5-X-C(6)H(2)[OP(tBu)(2)](2)-1,3], X = MeO (4a), Me (4b), H (4c), F (4d), C(6)F(5) (4e), and Ar(F) [=3,5-bis(trifluoromethyl)phenyl] (4f)) have been synthesized. Treatment of compounds 4a-f with NatOBu in cyclooctane (COA)/tert-butylethylene (TBE) mixtures generates species with unprecedented catalytic activity for the catalyzed transfer dehydrogenation of COA with TBE as acceptor to form cyclooctene (COE) and tert-butylethane (TBA). With substrate:precatalyst ratios of 3030COA:3030TBE:1p-XPCPIrHCl (4):1.1NaOtBu, turnover numbers (TONs) between 1400 and 2200 (up to 72% conversion in TBE) and initial turnover frequencies (TOFs) between 1.6 and 2.4 s(-1) have been observed at 200 degrees C.     

Activation of small molecules by a rhodium bis(phosphinite) pincer complex

Rhodium hydrido chloride pincer complex RhH(Cl)[2,6-(Bu^{pt b2}PO)^b2C^b6H^b3] was synthesized and used for the preparation of new complexes with labile two-electron ligands Rh(L)[2,6-(Bu^{pt b2}PO)^b2C^b6H^b3] (L = MeCN or S(CH^b2)^b4) and complexes with small molecules, such as CO, O^b2, H^b2, and N^b2.     

Reactions of iridium hydride pincer complexes with dioxygen: new dioxygen complexes and reversible O2 binding

The reaction of molecular oxygen with iridium pincer hydride complexes, ((tBu)PCP)Ir(H)(X) [(tBu)PCP = kappa(3)-C(6)H(3)(CH(2)P(t)Bu(2))(2), X = Ph, H, CCPh], results in O(2) induced reductive elimination and formation of the novel dioxygen complexes ((tBu)PCP)Ir(O(2))(n) [n = 1 (), 2 ()].     

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 characterization of new (pyrazolyl)aryl phosphinite PCN pincer palladium(II) complexes

Two unsymmetrical PCN pincer Pd(II) complexes 3a–3b which are based on (pyrazolyl)aryl phosphinite ligands and contain two fused six-membered palladacycles have been synthesized from 3-(3,5-dimethylpyrazol-1-ylmethyl)benzyl alcohol (2) by one-pot phosphorylation/palladation reaction via C–H bond activation of the related ligands. The pyrazole-coordinated phosphine-free Pd(II) complex (4) was also isolated in the preparation of pincer complex 3a. The new complexes were characterized by elemental analysis, ¹H NMR, ¹³C NMR, ³¹P {¹H} NMR (for pincer complexes) and IR spectra. And the molecular structures of 3b and 4 have been further determined by X-ray single-crystal diffraction. The pincer Pd complexes 3a and 3b exhibited rather low activity in the allylation of benzaldehyde.     

Square-planar iridium(II) and iridium(III) amido complexes stabilized by a PNP pincer ligand

Squaring the circle: the novel dienamido pincer ligand N(CHCHPtBu(2))(2)(-) affords the isolation of the unusual square-planar iridium(II) and iridium(III) amido complexes [IrCl{N(CHCHPtBu(2))(2)}](n) (n=0 (1), +1 (2)). In contrast, the corresponding iridium(I) complex of the redox series (n=-1) is surprisingly unstable. The diamagnetism of 2 is attributed to strong N→Ir π donation.     

Substituent effects on Ni-S bond dissociation energies and kinetic stability of nickel arylthiolate complexes supported by a bis(phosphinite)-based pincer ligand

Pincer complexes of the type [2,6-(R(2)PO)(2)C(6)H(3)]NiSC(6)H(4)Z (R = Ph and i-Pr; Z = p-OCH(3), p-CH(3), H, p-Cl, and p-CF(3)) have been synthesized from [2,6-(R(2)PO)(2)C(6)H(3)]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni-S bond length (approx. 2.20 ?) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with "[2,6-(Ph(2)PO)(2)C(6)H(3)]Ni" rather than "[2,6-(i-Pr(2)PO)(2)C(6)H(3)]Ni", and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH(2)Br, CH(3)I, CCl(4), and Ph(3)CCl have been examined and several mechanistic pathways have been explored.     

Decarbonylative ether dissection by iridium pincer complexes

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C–C bond, one C–O bond, and two C–H bonds, along with formation of two new C–H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups. Intermolecular decarbonylation of cyclic and linear ethers was observed when diethylaminophenylphosphinite iridium(i) dinitrogen or norbornene complexes were employed. Mechanistic studies reveal the nature of key intermediates along a pathway involving initial iridium(i)-mediated double C–H bond activation.

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported.     

Prototropic rearrangements in cycloheptatrienyl PCP pincer iridium complexes

When the cycloheptatriene iridium(iii) pincer complex (PCP)Ir(CO)(H)(Cl) (3) (PCP = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)) is treated with the bases NaH, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) under various conditions different products are obtained. At elevated temperatures and with DBU or LiTMP as a base the trans dihydride (PCP')Ir(CO)(H)(2) (PCP' = 2-(CHP(t)Bu(2))-7-(CH(2)P(t)Bu(2))C(7)H(4)) (5) is formed where the pi-system extends into one of the phosphine bridges. This compound loses H(2) to give the square-planar iridium(I) carbonyl complex (PCP'IrCO). The dihydride 5 can also rearrange to the new isomeric iridium(I) carbonyl 6 (PCP'IrCO, PCP' = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)). Thus the two hydrides have moved into the ligand backbone creating a methylene group in the 3-position of the cycloheptatriene ring. Alternatively, 6 is formed by a rearrangement from 6a which differs from 6 by having the methylene group in the 4-position of the cycloheptatriene ring. The iridium(I) carbonyl 6a in turn is made from 3 by treatment with DBU at room temperature. Interestingly, when compound is heated to reflux in THF the hydrogen bound at the metal carbon is shifted to a carbon atom in the cycloheptatriene ring generating a ring methylene group (3a). From this complex HCl is eliminated upon chromatography forming 6 as the final product. Quantum chemical calculations at various levels of theory illustrate the relative energetic stabilities of all iridium complexes.     

Reactions of bis(diphenylphosphine)acetylene with metal carbonyl complexes

The reaction of [Co₂(CO)₈] with DPPA at room temperature yields a diphosphine bridged product [Co₄(CO)₁₂(μ-Ph₂-P-C≡C-P-Ph₂)₂] 1. Heating of 1 at 45°C promoted cleavage of the P-C_sp bond with the formation of binuclear, phosphido-bridged σ-π-acetylide isomer complexes [Co₂(CO)₅(μ-PPh₂) (μ-σ-π-C≡C-PPh₂ )] 2a, 2b. Heating (60°C) of the complex [CpFe(CO)₂CH₃] and DPPA affords mono and binuclear acetyl, P-coordinated diphenylphosphinoalkyne metal complexes [CpFe(Ph₂P-C≡C-PPh₂)CO(COCH₃)] 3, [CpFeCO(COCH₃)]₂-μ-(Ph₂P-C≡C-PPh₂) 4.     

Unsaturated iridium pyridinedicarboxylate pincer complexes with catalytic activity in borylation of arenes

Unsaturated σ,π-cyclooctenyl and hydrido Ir(III) complexes bearing an unusual tridentate dianionic ONO pincer-type ligand have been straightforwardly obtained from 2,6-pyridinedicarboxylic acid and standard Ir(I) starting materials. These complexes efficiently catalyzed the arene C-H borylation under thermal conditions.     

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.     

Temperature- and solvent-dependent binding of dihydrogen in iridium pincer complexes

Mixtures of deuterium labeled complexes (p-XPOCOP)IrH2-xDx (1-6-d0-2) {POCOP = [C6H2-1,3-[OP(tBu)2]2] X = MeO (1), Me (2), H (3), F (4), C6F5 (5), and ArF = 3,5-(CF3)2-C6H3 (6)} have been generated by reaction of (p-XPOCOP)IrH2 complexes with HD gas in benzene followed by removal of the solvent under high vacuum. Spectroscopic analysis employing 1H and 2D NMR reveals significant temperature and solvent dependent isotopic shifts and HD coupling constants. Complexes 1-6-d1 in toluene and pentane between 296 and 213 K exhibit coupling constants JHD of 3.8-9.0 Hz, suggesting the presence of an elongated H2 ligand, which is confirmed by T1(min) measurements of complexes 1, 3, and 6 in toluene-d8. In contrast, complex 6-d1 exhibits JHD = 0 Hz in CH2Cl2 or CDCl2F whereas isotopic shifts up to -4.05 ppm have been observed by lowering the temperature from 233 to 133 K in CDCl2F. The large and temperature-dependent isotope effects are attributed to nonstatistical occupation of two different hydride environments. The experimental observations are interpreted in terms of a two component model involving rapid equilibration of solvated Ir(III) dihydride and Ir(I) dihydrogen structures.     

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.     

Mixed-ligand iridium(IV) complexes with amino acids,adenine and hypoxanthine

The complexation in iridium(IV)-purine base (adenine, hypoxanthine)-amino acid (α-alanine, aspartic acid, lysine) systems was studied by pH titration. The stability constants of 1: 1: 1 complexes were determined. The stability of 1: 1: 1 mixed-ligand complexes with hypoxanthine and adenine increases in the series Ala < Lys < Asp. Reactions between aqueous solutions gave the following coordination compounds: [Ir(C₅H₄N₄O)(C₃H₆NO₂)Cl]Cl₂, [Ir(C₅H₄N₄O)(C₄H₅NO₄)]Cl₂, [Ir(C₅H₄N₄O)(C₆H₁₃N₂O₂)]Cl₃, [Ir(C₅H₅N₅)(C₃H₆NO₂)]Cl₃, [Ir(C₅H₅N₅)(C₄H₅NO₄)]C_l2, and [Ir(C₅H₅N₅)(C₆H₁₃N₂O₂)]Cl₃. The individual character of the complexes was established by chemical and thermogravimetric analyses and powder X-ray diffraction. The complexes were characterized by NMR, IR, and X-ray photoelectron spectroscopy. Alanine and lysine in mixed-ligand iridium(IV) complexes are bidentate (α-NH₂ and COO⁻ groups), aspartic acid is tridentate, and purine bases function as polydentate ligands through heterocycle N atoms and functional groups (NH₂ in adenine and C=O in hypoxanthine).