PCP Pincer Ligand Research Articles

Ruthenium polyhydrides supported by rigid PCP pincer ligands: dynamic behaviour and reactions with CO2.

Two rigid β-elimination immune PCcarbeneP pincer ligands, differing in their electron donor properties by variation of the substitution pattern on the aromatic linker arms, were complexed to ruthenium to form the dichlorides LRRuCl2 (R = H or NMe2). These compounds were converted to hydrido chlorides by treatment with dihydrogen (H2) and a base. By converting to tert-butoxide derivatives in situ under an atmosphere of H2, the poly hydride PCalkylP complexes LHRRu(H)3 compounds were generated. In these complexes, H2 has added across the RuC bond in the PCcarbeneP starting materials. The polyhydrides are dynamic in solution and extensive NMR studies helped to elucidate the speciation and fluxional processes operative in this dynamic system. The polyhydride complexes react rapidly with CO2 to give the PCcarbeneP formato hydride complexes LRRu(H)-κ2-O2CH. For R = H, the 1,2-hydride shift from the anchoring alkyl of the PCalkylP carbon to the metal is reversible, but for R = NMe2 it is irreversible. The CO2 incorporated into the formato ligand of these compounds exchanges with free CO2via a bimolecular mechanism that is more rapid for R = NMe2 than for R = H; plausible explanations for this observation are proffered. Experiments designed to evaluate the efficacy of the R = NMe2 formato hydride complex as a catalyst precursor for CO2 hydrogenation to formate salts reveal poor performance in comparison to state-of-the-art ruthenium-based catalysts. This is due primarily to the precipitation of a dimeric μ-κ2-κ1-CO3 carbonate complex that is not an active catalyst for the reaction.

Machine Learning-Enabled Predictions of Condensed Fukui Functions and Designing of Metal Pincer Complexes for Catalytic Hydrogenation of CO2.

This research showcases the machine learning (ML)-enabled homogeneous catalyst discovery to be employed in carbon dioxide hydrogenation. To achieve the desired turnover frequency (TOF), the electrophilicity of the central metal atom is a crucial factor in transition metal pincer complexes. The condensed Fukui function is a direct measure of the catalytic performance of these pincer complexes. Herein, we demonstrate that machine learning is a convenient and effiecient method to calculate condensed Fukui functions of the central metal atom. The electrophilicity values of 202 pincer complexes were calculated by using density functional theory (DFT) to train the ML model. The test data of the experimentally established pincer complexes show a direct linkage between calculated electrophilicity and experimental TOF. Further, this data was used to develop an ML protocol to screen 2,84,062 catalyst complexes to get the electrophilicity values of the Mn, Fe, Co, and Ni transition metals encompassing various permutation combinations of PNP, PNN, NNN, and PCP pincer ligands. These findings validate the efficacy of machine learning in the rapid screening of metal pincer catalysts based on condensed Fukui functions.

Synthesis, characterization, and liquid injection field desorption ionization mass spectrometry analysis of pincer ligated group 6 (Cr, Mo, W) carbonyl complexes.

We report the synthesis of molybdenum and tungsten bromo dicarbonyl complexes (POCOPtBu)MIIBr(CO)2 (M = Mo or W; POCOPtBu = κ3-C6H3-1,3-[OP(tBu)2]2) supported by an anionic PCP pincer ligand, and the chromium complex (PNPtBu)Cr0(CO)3 (PNPtBu = 2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) bearing a neutral PNP pincer scaffold. The three group six complexes described in this study have been characterized by Liquid Injection Field Desorption Ionization Mass Spectrometry (LIFDI-MS), NMR, and IR spectroscopy. Single crystal X-ray diffraction studies show the MoII and WII complexes adopt a six-coordinate distorted trigonal prismatic geometry, whereas the Cr0 complex exhibits a distorted octahedral geometry.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand.

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

A High-Valent Ru-PCP Pincer Catalyst for Hydrogenation of Carbonyl and Carboxyl Compounds under Molecular Hydrogen.

Low-valent metals traditionally dominate the domain of catalytic hydrogenation. However, metal-ligand cooperating (MLC) catalytic systems, operating through heterolytic H-H bond splitting by a Lewis acidic metal and a basic ligand site, do not require an electron-rich metal. On the contrary, high-valent metals that induce weaker back donation facilitate heterolytic bond activation. Here we report, for the first time, the efficient hydrogenation of carbonyl and carboxyl compounds under molecular hydrogen catalyzed by a structurally well-defined RuIV catalyst bearing a bifunctional PCP pincer ligand. The catalyst exhibits reactivity toward molecular hydrogen superior to that of the low-valent analog and allows hydrogen activation even at room temperature.

Current Frontiers in Pincer Chemistry

Ever since the introduction of the archetypal (bisphosphinomethyl)phenyl PCP pincer ligand by Shaw in 1976, the use of this class of tridentate ligands with a central phenyl moiety has found widespread utility in coordination chemistry and homogeneous catalysis. Initially, van Koten's definition of pincer ligands, which was inspired by the claws of the crab, only included rigid, meridionally binding, monoanionic ligands. The rapid growth in popularity of pincer chemistry in the 1990s can be associated with the modular tunability of the steric and electronic properties of these ligands, which can give pincer supported complexes enhanced chemical stability and well-defined reactivity. Consequently, the application of pincer supported complexes has had a significant impact on the advancement of key areas in small molecule activation and catalysis, such as C–H activation, olefin polymerization and cross-coupling. As of today, the term pincer ligand is used more broadly and now comprises a wide range of more or less flexible, neutral and charged, tridentate ligands with a large variety of central and flanking donor groups. While this development means that there is no longer a clear definition of a pincer ligand, the broadening of the field has allowed for the combination of new, chemically active functional groups with the beneficial structural properties of pincer ligands. Accordingly, the utility of pincer ligands was enhanced by the introduction of functional pincer ligands, which adopt active roles in chemical transformations, such as reversible electron and/or proton transfer steps, and extend pincer ligands beyond traditional inert auxiliary ligands. This conceptual advance has contributed to recent progress in many fields, like the renaissance of base metal catalysis, the use of non-innocent, redox active and cooperating ligands in catalysis, or the development of acceptorless de-/hydrogenation strategies for organic synthesis. This themed issue of the Zeitschrift für Anorganische und Allgemeine Chemie dedicated to Pincer Chemistry addresses three current focal areas in the field with contributions from leading groups in Asia, America, and Europe: (a) The synthesis of new pincer-type ligands, (b) the electronic structures of pincer supported complexes, and (c) the application of pincer ligated complexes in chemical transformations and catalysis. The work from Caulton et al., Huang et al., Langer and Vogt et al., Nishibayashi et al., Ozerov et al. and Walter et al. focuses on the development of synthetic methodologies towards new pincer platforms with respect to the central donor site, the connecting backbone, and the flanking “wings” of the ligands. These studies offer insight into the tunability of donor strength, rigidity and steric properties in pincer ligands. Contributions from Anderson et al., Kirchner et al., Milsmann et al. and Ray and Swart et al. disclose detailed spectroscopic and computational studies of iron and copper pincer complexes. Particular focus lies on their electronic structures, redox state assignments and control of accessible spin states and reactivity. Guan et al., Kuwata et al., Morris et al., Jones and Nachtigall et al. and Schneider et al. examined the proton/electron transfer model chemistry of complexes with proton responsive and redox active pincer ligands. These contributions are complemented by a systematic computational study from Pidko et al. that evaluates the effects of pincer substitution on metal-ligand cooperativity. Finally, Goswami et al. utilize such functional pincer ligands as hydrogen relays in transfer hydrogenation catalysis. These contributions reflect the current frontiers and the state-of-the-art in Pincer Chemistry and intend to stimulate the use pincer ligands in coordination chemistry and catalysis.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity.

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn2(CO)10] with the ligands (P(C-Br)PCH2-iPr) (1a) and (P(C-Br)PO-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCPCH2-iPr)(CO)3] (2a) and [Mn(-PCPO-iPr)(CO)3] (2b) were obtained. Protonation of 2a with HBF4·Et2O led to the formation of [Mn(κ3P,CH,P-P(CH)PCH2-iPr)(CO)3]BF4 (3) featuring an η2-Caryl-H agostic bond. The solvothermal reaction of 1a with [Fe2(CO)9] afforded the Fe(II) PCP pincer complex [Fe(PCPCH2-iPr)(CO)2Br] (4). Treatment of 4 with Li[HBEt3] afforded the Fe(I) complex [Fe(PCPCH2-iPr)(CO)2] (5a). When using the sterically more demanding ligands (P(C-Br)PCH2-tBu) (1c) and (P(C-Br)PO-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn2(CO)10], the reaction with [Fe2(CO)9] and [Fe(CO)5] led to the formation of the Fe(I) complexes [Fe(PCPCH2-tBu)(CO)2] (5b) and [Fe(PCPO-tBu)(CO)2] (5c). X-ray structures of representative complexes are provided.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands.

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of MI(CO) and MIIICl2(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Synthesis and characterization of xylene-based group-six metal PCP pincer complexes

In the present study, the Cr(III) complex trans-[Cr(PCPCH2-iPr)(CH3CN)(Br)2] as well as seven-coordinate cationic bromo carbonyl Mo(II) and W(II) complexes of the type [M(PCPCH2-iPr)(CO)3Br] (M=Mo, W) featuring PCP pincer ligands based on a xylene scaffold were prepared and characterized. The seven-coordinate bromo tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. The mechanism of this process was studied by means of DFT calculations. Structures of representative complexes were determined by single-crystal X-ray analyses.Graphical abstract

Crystal structures of [IrCl2(NHCHPh)((dppm)(C(N2dppm))-κ3 P,C,P')]Cl·5.5MeCN and [IrI(NHCHPh)(((dppm)C(N2))-κ2 P,C)(dppm-κ2 P,P')]I(I3)·0.5I2·MeOH·0.5CH2Cl2: triazene fragmentation in a PCN pincer iridium complex.

The structure of [IrCl2(C58H51N3P4)]Cl·5.5CH3CN or [IrCl2(NHCHPh)(((dppm)C(N2dppm))-κ 3P,C,P)]Cl·5.5CH3CN [3, dppm = bis-(di-phenyl-phos-phino)methane; systematic name: di-chlorido(1,1,3,3,7,7,9,9-octa-phenyl-4,5-di-aza-1,3λ5,7λ4,9-tetra-phosphanona-3,5-dien-6-yl-κ2 P 1,P 9)-(phenyl-methanimine-κN)iridium(III) chloride aceto-nitrile hemihendeca-solvate], resulting from an oxygen-mediated cleavage of a triazeneyl-idene-phospho-rane ligand producing a diazo-methyl-ene-phospho-rane and a nitrene moiety, which in turn rearrange via a Staudinger reaction and a 1,2-hydride shift to the first title complex, involves a six-coordinate IrIII complex cation coordinated by a facial PCP pincer ligand, a benzaldimine and two chlorido ligands. The pincer system features a five- and a seven-membered ring, with the central divalent carbon of the PCP pincer ligand being connected to a phosphine and a diazo-phospho-rane. The chlorido ligands are positioned trans to the central carbon atom and to the phospho-rus donor of the seven-membered ring of the pincer system, respectively. A chloride ion serves as counter-ion for the monocationic complex. The structure of [IrI(C26H22N2P2)(C26H22P2)(C6H7N)]I(I3)·0.5I2·CH3OH·0.5CH2Cl2 or [IrI(NHCHPh)((dppm)C(N2)-κ 2P,C)(dppm-κ 2P,P')]I(I3)·0.5I2·CH3OH·0.5CH2Cl2 {4, systematic name: (4-diazo-1,1,3,3,-tetra-phenyl-1,3λ4-diphosphabutan-4-yl-κP 1)iodido[methyl-enebis(di-phenyl-phosphine)-κ2 P,P'](phenyl-methanimine-κN)iridium(III) iodide-triiodide-di-chloro-methane-iodine-methanol (2/2/1/1/2)}, accessed via treatment of the triazeneyl-idene-phospho-rane complex [Ir((BnN3)C(dppm)-κ 3P,C,N)(dppm-κ 2P,P')]Cl with hydro-iodic acid, consists of a dicationic six-coordinate IrIII complex, coordinated by a bidentate diazo-methyl-ene-phospho-rane, a benzaldimine, a chelating dppm moiety and an iodido ligand. The phospho-rus atoms of the chelating dppm are trans to the central carbon atom of the diazo-methyl-ene-phospho-rane and the iodide ligand, respectively. Both an iodide and a triiodide moiety function as counter-ions. The aceto-nitrile solvent mol-ecules in 3 are severely disordered in position and occupation. In 4, the I3 - anion is positionally disordered (ratio roughly 1:1), as is the I- anion with a ratio of 9:1. The di-chloro-methane solvent mol-ecule lies near a twofold rotation axis (disorder) and was refined with an occupancy of 0.5. Another disorder occurs for the solvent methanol with a 1:1 ratio.

Monomeric alkoxide and alkylcarbonate complexes of nickel and palladium stabilized with the iPrPCP pincer ligand: a model for the catalytic carboxylation of alcohols to alkyl carbonates.

Monomeric alkoxo complexes of the type [(iPrPCP)M-OR] (M = Ni or Pd; R = Me, Et, CH2CH2OH; iPrPCP = 2,6-bis(diisopropylphosphino)phenyl) react rapidly with CO2 to afford the corresponding alkylcarbonates [(iPrPCP)M-OCOOR]. We have investigated the reactions of these compounds as models for key steps of catalytic synthesis of organic carbonates from alcohols and CO2. The MOCO-OR linkage is kinetically labile, and readily exchanges the OR group with water or other alcohols (R'OH), to afford equilibrium mixtures containing ROH and [(iPrPCP)M-OCOOH] (bicarbonate) or [(iPrPCP)M-OCOOR'], respectively. However, [(iPrPCP)M-OCOOR] complexes are thermally stable and remain indefinitely stable in solution when these are kept in sealed vessels. The constants for the exchange equilibria have been interpreted, showing that CO2 insertion into M-O bonds is thermodynamically more favorable for M-OR than for M-OH. Alkylcarbonate complexes [(iPrPCP)M-OCOOR] fail to undergo nucleophilic attack by ROH to yield organic carbonates ROCOOR, either intermolecularly (using neat ROH solvent) or in intramolecular fashion (e.g., [(iPrPCP)M-OCOOCH2CH2OH]). In contrast, [(iPrPCP)M-OCOOMe] complexes react with a variety of electrophilic methylating reagents (MeX) to afford dimethylcarbonate and [(iPrPCP)M-X]. The reaction rates increase in the order X = OTs < IMe ≪ OTf and Ni < Pd. These findings suggest that a suitable catalyst design should combine basic and electrophilic alcohol activation sites in order to perform alkyl carbonate syntheses via direct alcohol carboxylation.

Syntheses and crystal structures of [IrIII{C(CHCO2Et)(dppm)2-κ4 P,C,C',P'}ClH]Cl·2.75CH2Cl2 and its derivatives, [IrIII{C(CHCO2Et)(dppm)2-κ4 P,C,C',P'}(CH2CO2Et)Cl]Cl·CH3OH·0.5H2O, [IrIII{C(CHCO2Et)(dppm)2-κ4 P,C,C',P'}Cl2]Cl·CH3OH·2H2O and [IrIII{C(CHCO2Et)(dppm)2-κ4 P,C,C',P'}(CH2CO2Et)(CO)]Cl2·2CH2Cl2·1.5H2O.

The common feature of the four iridium(III) salt complexes, (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}(eth-oxy-oxoethanyl-idene)methane-κ4 P,C,C',P')chlorido-hydridoiridium(III) chloride methyl-ene chloride 2.75-solvate (4), (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}(eth-oxy-oxoethanyl-idene)methane-κ4 P,C,C',P')chlorido-(eth-oxy-oxoethanido)iridium(III) chloride-methanol-water (1/1/0.5) (5), (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}(eth-oxy-oxoethanyl-idene)methane-κ4 P,C,C',P')di-chlorido-iridium(III) chloride-methanol-water (1/1/2) (6) and (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}(eth-oxy-oxoethanyl-idene)methane-κ4 P,C,C',P')carbon-yl(eth-oxy-oxoethanide)iridium(III) dichloride-meth-yl-ene chloride-water (1/2/1.5) (7) or in terms of their formulae [Ir(C55H50O2P4)ClH]Cl·2.75CH2Cl2 (4), [Ir(C4H7O2)(C55H50O2P4)Cl]Cl·CH3OH·0.5H2O (5), [Ir(C55H50O2P4)Cl2]Cl·CH3OH·2H2O (6) and [Ir(C4H7O2)(C55H50O2P4)(CO)]Cl2·2CH2Cl2·1.5H2O (7) is a central IrIII atom coordin-ated in a distorted octa-hedral fashion by a PCCP ligand system and two additional residues, such as chlorides, a hydride, a carbonyl or an alkyl unit. Thereby, the PCP pincer ligand system and the residue trans to the carbodi-phospho-rane (CDP) C atom surround the iridium(III) transition metal in the equatorial plane under the formation of two five-membered dissimilar chelate rings [C-CCDP-P (4, 5, 6 and 7) for the first ring: 120.2 (3), 121.9 (5), 111.2 (3) and 121.7 (2) °; for the second ring: 112.1 (3), 113.5 (5), 120.5 (3) and 108.3 (2)°]. A cyclo-propane-like heterocycle is positioned approximately orthogonal (84.21-88.85°) to the equatorial plane, including an alkyl-idene bridge connecting the IrIII atom and the coordinating CDP atom of the PCP subunit. In general, the neutral PCCP ligand system coordinates the metal in a tetra-dentate way via three Lewis acid/base bonds and by an alkyl-idene unit presenting strengthened inter-actions. In all the crystal structures, (disordered) solvent mol-ecules are present in the voids of the packed mol-ecules that inter-act with the positively charged complex and its chloride counter-ion(s) through weak hydrogen bonding.

Synthesis of Ruthenium Complexes Bearing PCP-Type Pincer Ligands and Their Application to Direct Synthesis of Imines from Amines and Benzyl Alcohol

Ruthenium complexes bearing N-heterocyclic carbene- and phosphine-based PCP-type pincer ligands are synthesized and characterized by X-ray crystallography. The ruthenium–PCP complexes have catalytic activity toward direct synthesis of imines from reactions of amines and benzyl alcohol. The lifetime of the ruthenium complex bearing the PCP pincer ligand is longer than that of the ruthenium complex bearing a pyridine-based PNP-type pincer ligand.

Synthesis and Reactivity of Group Six Metal PCP Pincer Complexes: Reversible CO Addition Across the Metal–Caryl Bond

In the present study, Cr(III) complexes of the type trans-[Cr(PCPNEt-iPr)(solvent)Cl2] (solvent = CH3CN, THF), the Cr(0) complex [Cr(κ3P,CH,P-P(CH)PNEt-iPr)(CO)3] which features an η2-Caryl–H agostic bond as well as seven coordinate cationic chloro carbonyl Mo(II) and W(II) complexes of the type [M(PCPNEt-iPr)(CO)3Cl] featuring PCP pincer ligands based on a 1,3-diaminobenzene scaffold were prepared and characterized. The seven coordinate chloro tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. Another interesting aspect is a rapid and reversible addition of one CO ligand across the metal–Cipso bond. The mechanism of the dynamic process of the chloro carbonyl complexes was investigated by means of DFT calculations. The Mo(II) and W(II) tricarbonyl complexes could be reduced to the respective anionic Mo(0) and W(0) complexes [Mo(PCPNEt-iPr)(CO)3]− and [Mo(PCPNEt-iPr)(CO)3]−. These air sensitive compounds are readily protonated by MeOH to form agostic and h...

Crystal structures of four new iridium complexes, each containing a highly flexible carbodi-phos-phorane PCP pincer ligand.

Compound [Ir(C8H12)(C51H45P4)]Cl2 or [Ir(cod)(CH(dppm)2-κ3P,C,P)]Cl2 (1a), was obtained from [IrCl(cod)]2 and the carbodi-phospho-rane (CDP) salt [CH(dppm)2]Cl [where cod = cyclo-octa-1,5-diene and dppm = bis-(di-phenyl-phosphino)methane]. Treatment of 1a with thallium(I) tri-fluoro-methane-sulfonate [Tl(OTf)] and subsequent crystallization gave complex [Ir(C8H12)(C51H45P4)](OTf)2·CH3CO2C2H5·CH2Cl2 or [Ir(cod)(CH(dppm)2-κ3P,C,P)](OTf)2·CH3CO2C2H5·CH2Cl2 (1b) [systematic name: (cyclo-octa-1,5-diene)(1,1,3,3,5,5,7,7-octa-phenyl-1,7-diphospha-3,5-di-phospho-niaheptan-4-yl)iridium(I) bis-(tri-fluoro-methane-sulfonate)-ethyl acetate-di-chloro-methane (1/1/1)]. This five-coordinate iridium(I) complex cation adopts a trigonal-bipyramidal geometry with the CDP carbon and one cod double bond in axial sites. Compound 1b represents the first example of a non-meridional coordination of the PCP pincer ligand [CH(dppm)2]+ with a P-Ir-P angle of 98.08 (2)°. Compound 2, [IrCl2H(C51H44P4)]·(CH3)2CO or [IrCl2H(C(dppm)2-κ3P,C,P)]·(CH3)2CO [systematic name: di-chlorido-hydrido(1,1,3,3,5,5,7,7-octa-phenyl-1,5λ5,7-triphospha-3-phospho-niahept-4-en-4-yl)iridium(III) acetone monosolvate], crystallizes as an acetone monosolvate. It is a six-coordinate IrIII coordination compound. Here, the PCP pincer ligand is coordinated in a meridional manner; one chlorido ligand is positioned trans to the carbon donor, the remaining two coordination sites being occupied by the second chlorido and a hydrido ligand trans to each other. Complex 3, [IrCl2H(C51H45P4)]Cl·5H2O or [IrCl2H(CH(dppm)2-κ3P,C,P)]Cl·5H2O [systematic name: di-chlorido-hydrido(1,1,3,3,5,5,7,7-octa-phenyl-1,7-diphospha-3,5-di-phospho-niaheptan-4-yl)iridium(III) chloride penta-hydrate], represents the conjugate CH acid of 2. The ligand [CH(dppm)2]+ is coordinated in a meridional manner. In the cationic six-coordinate IrIII complex 4, [IrClH(CO)(C51H44P4)]Cl·2CH3OH·H2O or [IrClH(CO)(C(dppm)2-κ3P,C,P)]Cl·2CH3OH·H2O [systematic name: carbonyl-chlorido-hydrido(1,1,3,3,5,5,7,7-octa-phenyl-1,5λ5,7-triphospha-3-phos-pho-niahept-4-en-4-yl)iridium(III) chloride-methanol-water (1/2/1)], the chlorido ligand is found in the plane defined by the Ir center and the meridional PCP ligand; the H and CO ligands are positioned axially to this plane and trans to each other.

Crystal structure of an iridium(III) complex of the [C(dppm)2] PCP pincer ligand system and its conjugate CH acid form.

After the successful creation of the newly designed PCP carbodi-phospho-rane (CDP) ligand [Reitsamer et al. (2012 ▸). Dalton Trans.41, 3503-3514; Stallinger et al. (2007 ▸). Chem. Commun. pp. 510-512], the treatment of this PCP pincer system with the transition metal iridium and further the analysis of the structures by single-crystal diffraction and by NMR spectroscopy were of major inter-est. Two different iridium complexes, namely (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}methane-κ3P,C,P')carbonyl-chlorido-hydridoiridium(III) chloride di-chloro-methane tris-olvate, [IrIII(CO){C(dppm)2-κ3P,C,P'}ClH]Cl·3CH2Cl2 (1) and the closely related (bis-{[(di-phenyl-phosphan-yl)meth-yl]di-phenyl-phosphanyl-idene}methanide(1+)-κ3P,C,P')carbonyl-chlorido-hy-dridoirid-ium(III) dichloride-hydro-chloric acid-water (1/2/5.5), [IrIII(CO){CH(dppm)2-κ3P,C,P')ClH]Cl}2 (2), have been designed and both complexes show a slightly distorted octa-hedral coordinated IrIII centre. The PCP pincer ligand system is arranged in a meridional manner, the CO ligand is located trans to the central PCP carbon and a hydride and chloride are located perpendicular above and below the P2C2 plane. With an Ir-CCDP distance of 2.157 (5) Å, an Ir-CO distance of 1.891 (6) Å and a quite short C-O distance of 1.117 (7) Å, complex 1 presents a strong carbonyl bond. Complex 2, the corresponding CH acid of 1, shows an additionally attached proton at the carbodi-phospho-rane carbon atom located anti-periplanar to the hydride of the metal centre. In comparison with complex 1, the Ir-CCDP distance of 2.207 (3) Å is lengthened and the Ir-C-O values indicate a weaker trans influence of the central carbodi-phospho-rane carbon atom.

1-D and 2-D phosphine coordination materials based on a palladium(II) PCP pincer metalloligand

Ancillary functionalization of a PCP pincer ligand with para-carboxylic acids provides a simple route to the subsequent preparation of crystalline coordination polymers using pre-cyclometallated complexes as polymer building blocks. This synthetic strategy is demonstrated here using air- and moisture-stable PCP-PdCl metalloligands to prepare new coordination polymers with both 1-D (chain) and 2-D (sheet) networks, named PCM-29 and PCM-30, respectively. The single crystal X-ray structures and solid-state gas sorption properties of both materials are presented. The Brunauer-Emmett-Teller (BET) surface area of PCM-30 was found to be 302m2g−1 using CO2 as the probe gas, indicating permanent porosity and reversible small molecule diffusion between the 2-D sheets.

Nickel and Platinum PCP Pincer Complexes Incorporating an Acyclic Diaminoalkyl Central Moiety Connecting Imidazole or Pyrazole Rings

This work explores avenues toward PCP pincer complexes with acyclic diaminocarbene central moieties that are suitable for ligand-assisted reactivity involving the carbene carbon. For this purpose, diphosphine chelating ligands 1 and 2, with bis(imidazolyl)methyl and bis(pyrazolyl)methyl backbones, respectively, were prepared in two high-yield synthetic steps. Nickel(II) and platinum(II) dihalide complexes 3 and 6, incorporating 1, were prepared and converted into PCP pincer ligands 5 and 7 upon deprotonation with KHMDS. Chelated nickel(I) complex 4 was obtained as a byproduct to 3. Although the solid-state structures of 5 and 7 presented geometric strain at the diaminomethyl carbon, this moiety could not be converted into a diaminocarbene by proton or hydride abstraction. 7 could be converted into platinum(IV) analog 8 by oxidation with PhICl2. Ligand 2 proved more versatile than 1, or less dependable in its behavior, generating P,N-chelating cobalt(II) dibromide complex 9. Easily accessible ligands 1 and...

Theoretical studies on the thermodynamics and kinetics of one-pot synthesis of aromatic PCP and PNP pincer ligands

In the present study, the kinetics and thermodynamics of the successful one-pot synthesis of arene-based PCP/PNP pincer ligands by less nucleophilic secondary halophosphines (mono/bis(phosphinomethyl)benzene/pyridine) via SN2 reactions, which have been recently developed by Shih and Ozerov (Organometallics 34:4591, 35), have been studied. By utilizing different substituents on the target molecules, the impact of such modifications has been investigated both in gas phase and in solution. Performed calculations showed clear dependence of the activation energy and reaction rates on the presence of lithium in the structure. Additionally, the nature of the leaving group was observed to influence the favorability of the reaction. Similarly, the type of the halogen on both substrate and nucleophile had impact on the reaction rates, with the use of iodide leading to the observation of the fastest reaction rates. The solvent used was also noted to exert considerable influence on the thermodynamics and kinetics of the reactions. However, the R group in the nucleophile (XPR2) showed no significant inductive effect on the activation energy, but a slight steric effect could be observed.

Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made. However, limited systems have succeeded in the catalytic formation of ammonia. Here we show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N2)2(PCP)}2(μ-N2)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. of ammonia are produced based on the catalyst. DFT calculations on 1 reveal that the PCP-pincer ligand serves as not only a strong σ-donor but also a π-acceptor. These electronic properties are responsible for a solid connection between the molybdenum centre and the pincer ligand, leading to the enhanced catalytic activity for nitrogen fixation.