PCP Pincer Complexes Research Articles

Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways.

A stereo- and regioselective Mn(I)-catalyzed hydroboration of terminal alkynes with pinacolborane (HBPin) is described. The hydroboration reaction is highly Z-selective in the case of aryl alkynes and E-selective in the case of aliphatic alkynes. The reaction requires no additives or solvents and proceeds with a catalyst loading of 1 mol % at 50-70 °C. The most active precatalyst is the bench-stable alkyl Mn(I) complex cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)]. The catalytic process is initiated by the migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate. This species undergoes C-H and B-H bond cleavage of the alkyne (aromatic alkynes) and HBPin (in the case of aliphatic alkynes) forming the active Mn(I) alkynyl and boryl catalysts [Mn(PCP-iPr)(CO)(C≡CR)] and [Mn(PCP-iPr)(CO)(BPin)], respectively. A broad variety of aromatic and aliphatic alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations. The functionalized alkenes can be used for further applications in cross-coupling reactions.

Solvothermal synthesis of cobalt PCP pincer complexes from [Co2(CO)8

The online version contains supplementary material available at 10.1007/s00706-023-03123-x.

PCP Pincer Complexes of Titanium in the +3 and +4 Oxidation States.

Ti(IV) and Ti(III) complexes using the tBuPCP ligand have been synthesized (tBuPCP = C6H3-2,6-(CH2PtBu2)2). The [tBuPCP]Li synthon can be reacted with TiCl4(THF)2 to form (tBuPCP)TiCl3 (1) in limited yields due to significant reduction of the titanium synthon. The Ti(III) complex (tBuPCP)TiCl2 (2) has been further characterized. This can have half an equivalent of halide abstracted to form [{(tBuPCP)TiCl}2{μ-Cl}][B(C6F5)4] (3) and can also be methylated, forming (tBuPCP)TiMe2 (4). All the Ti(III) complexes have been characterized using EPR and X-ray crystallography, giving insight into their electronic structures, which are further supported by DFT calculations.

Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes.

Two new bisphosphine [PCP] pincer cobalt(III) hydrides, [(L1)Co(PMe3)(H)(Cl)] (L11, L1 = 2,6-((Ph2P)(Et)N)2C6H3) and [(L2)Co(PMe3)(H)(Cl)] (L21, L2 = 2,6-((iPr2P)(Et)N)2C6H3), as well as one new bissilylene [SiCSi] pincer cobalt(III) hydride, [(L3)Co(PMe3)(H)(Cl)] (L31, L3 = 1,3-((PhC(tBuN)2Si)(Et)N)2C6H3), were synthesized by reaction of the corresponding protic [PCP] or [SiCSi] pincer ligands L1H, L2H, and L3H with CoCl(PMe3)3. Despite the similarities in the ligand scaffolds, the three cobalt(III) hydrides show remarkably different performance as catalysts in alkene hydrosilylation. Among the PCP pincer complexes, L11 has higher catalytic activity than complex L21, and both catalysts afford anti-Markovnikov selectivity for both aliphatic and aromatic alkenes. In contrast, the catalytic activity for alkene hydrosilylation of silylene complex L31 is comparable to phosphine complex L11, but a dependence of regioselectivity on the substrates was observed: While aliphatic alkenes are converted in an anti-Markovnikov fashion, the hydrosilylation of aromatic alkenes affords Markovnikov products. The substrate scope was explored with 28 examples. Additional experiments were conducted to elucidate these mechanisms of hydrosilylation. The synthesis of cobalt(I) complex (L1)Co(PMe3)2 (L17) and its catalytic properties for alkene hydrosilylation allowed for the proposal of the mechanistic variations that occur in dependence of reaction conditions and substrates.

Pincer‐Supported Perfluororhodacyclopentanes: High Nucleophilicity of the M−CF Bond

AbstractPerfluororhodacyclopentane supported by PBP pincer ligand PBP‐RhIII(CF2)4 (2) was prepared from PBP‐RhI‐PPh3 (1) and tetrafluoroethylene (TFE). By scXRD analysis, it was revealed that the Rh−CF bond trans to the boryl ligand was largely elongated compared to the one in the apical position. Although M−CF bonds of perfluorometallacyclopentanes are known to be very stable, 2 shows high reactivity toward hydrogenolysis, iodinolysis and protonolysis, thanks to the strong trans‐effect of the highly σ‐donating boryl ligand. Analogous PCP pincer complex PCP−RhIII(CF2)4 (4) showed slower reaction rate both in oxidative cyclization with TFE and hydrogenolysis of Rh−CF bond. Catalytic production of 1,1,2,2,3,3,4,4‐octafluorobutane from two TFE and hydrogen in one pot was achieved via repetitive oxidative cyclization of TFE and Rh followed by the hydrogenolysis of Rh−CF bonds. This report demonstrates the possible application of PBP‐pincer skeleton to the preparation of fluorinated compounds from TFE.

Pincer-Supported Perfluororhodacyclopentanes: High Nucleophilicity of the M-CF Bond.

Perfluororhodacyclopentane supported by PBP pincer ligand PBP-RhIII (CF2 )4 (2) was prepared from PBP-RhI -PPh3 (1) and tetrafluoroethylene (TFE). By scXRD analysis, it was revealed that the Rh-CF bond trans to the boryl ligand was largely elongated compared to the one in the apical position. Although M-CF bonds of perfluorometallacyclopentanes are known to be very stable, 2 shows high reactivity toward hydrogenolysis, iodinolysis and protonolysis, thanks to the strong trans-effect of the highly σ-donating boryl ligand. Analogous PCP pincer complex PCP-RhIII (CF2 )4 (4) showed slower reaction rate both in oxidative cyclization with TFE and hydrogenolysis of Rh-CF bond. Catalytic production of 1,1,2,2,3,3,4,4-octafluorobutane from two TFE and hydrogen in one pot was achieved via repetitive oxidative cyclization of TFE and Rh followed by the hydrogenolysis of Rh-CF bonds. This report demonstrates the possible application of PBP-pincer skeleton to the preparation of fluorinated compounds from TFE.

Nitronyl-nitroxide PCP-pincer complexes of palladium and nickel: synthesis and an EPR study

Nitronyl-nitroxide PCP-pincer complexes of palladium and nickel: synthesis and an EPR study

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.

Carbocyclic pincer carbene complexes of ruthenium: syntheses and reversible hydrogenation.

The ruthenium carbene pincer complex 2 was synthesized treating the benzo annulated cycloheptatriene bisphosphine 1 with RuCl3. Addition of three equivalents of hydrogen to the carbocyclic carbene complex 2 was achieved in reaction of 2 with hydrogen at elevated temperatures. Hydrogenated complex 4, exhibiting a rigid chair conformation in solution, was dehydrogenated by heating a toluene solution of complex 4 to reflux for 5-7 d. In reaction with ethylene, complex 4 transfers one equivalent of hydrogen, forming ethane and alkyl complex 5.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes.

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCPNMe-iPr)Cl], [Co(PCPO-iPr)Cl] or [Co(PCPCH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF4 led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]+ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO+. Hence, these systems can be described as {CoNO}8 according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCPNMe-iPr)(NO)]+ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}8 hydride species.

New 1,2,3-triazole based bis- and trisphosphine ligands: synthesis, transition metal chemistry and catalytic studies.

The syntheses and transition metal chemistry of triazole-based bis- and tris-phosphines, 5-(diphenylphosphanyl)-1-(2-(diphenylphosphanyl)phenyl)-4-phenyl-1H-1,2,3-triazole (2), 5-(diphenylphosphanyl)-4-(2-(diphenylphosphanyl)phenyl)-1-phenyl-1H-1,2,3-triazole (5), 1,4-bis(2-(diphenylphosphanyl)phenyl)-1H-1,2,3-triazole (6) and 5-(diphenylphosphanyl)-1,4-bis(2-(diphenylphosphanyl)phenyl)-1H-1,2,3-triazole (7), are described. Bisphosphines 5 and 6 show versatile coordination behavior due to the presence of at least four donor atoms. The reactions of 5 and 6 with group VI metal carbonyl derivatives were found to be highly sensitive to the reaction conditions. Bisphosphine 5 upon treatment with [M(CO)4(piperidine)2] (M = Mo and W) yielded both P,P and P,N coordinated complexes [M(CO)4(5)] [M = Mo-κ2-P,N (8); W-κ2-P,N (9); Mo-κ2-P,P (10); W-κ2-P,P (11)], whereas 6 afforded only P,N coordinated complexes [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))CH}-κ2-P,N}Mo(CO)4] (12) and [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))CH}-κ2-P,N}W(CO)4] (13). The reactions of 5 with [M(COD)Cl2] (M = Pd and Pt) yielded κ2-P,P chelate complexes 14 and 15, respectively, whereas the treatment of 6 with [Pd(COD)Cl2] at ambient temperature resulted in the formation of a rare fused six-membered PCP pincer complex [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))C}-κ3-P,C,P}PdCl] (16). Similar reactions of 6 with [NiCl2(DME)] and [Pt(COD)Cl2] in the presence of LiHMDS yielded [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))C}-κ3-P,C,P}NiCl] (17) and [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))C}-κ3-P,C,P}PtCl] (18), respectively. The reaction between 6 and [M(COD)Cl]2 (M = Rh and Ir) produced cationic complexes [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))CH}-κ2-P,N}Rh(C8H12)]Cl (19) and [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))CH}-κ2-P,N}Ir(C8H12)]Cl (20), respectively, whereas the reaction with [Rh(acac)(CO)2] resulted in a pincer complex [{o-Ph2P(C6H4){1,2,3-N3C(o-Ph2P(C6H4))C}-κ3-P,C,P}Rh(CO)] (21). The structures of most of the compounds have been determined by single crystal X-ray analyses. The fused six-membered PCP palladium pincer complex 16 is found to be an excellent catalyst for the Mizoroki-Heck coupling reaction.

Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity

In the current investigation, the reaction of [Cr(CO)6] with the ligand precursor PO(C-Br)OP-tBu (1a) was investigated. When a suspension of [Cr(CO)6] and 1a in toluene was transferred into a seale...

A Practical Synthesis of Ammonia from Nitrogen Gas, Samarium Diiodide and Water Catalyzed by a Molybdenum–PCP Pincer Complex

A practical method for ammonia synthesis is described. The reaction of atmospheric pressure of nitrogen gas with samarium diiodide as a reducing reagent and water as a proton source in the presence of a catalytic amount of a molybdenum trichloride complex bearing a PCP [1,3-bis(di-tert-butylphosphinomethyl)benzimidazol-2-ylidene]-type pincer ligand occurs under ambient conditions to afford ammonium sulfate after treatment with sulfuric acid.

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

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...

Iron PCP Pincer Complexes in Three Oxidation States: Reversible Ligand Protonation To Afford an Fe(0) Complex with an Agostic C-H Arene Bond.

In the current investigation, the reaction of Fe2(CO)9 with the ligand precursor 2-chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (P(C-Cl)PNEt- iPr) (1) was investigated. When a suspension of Fe2(CO)9 and 1 in CH3CN was transferred in a sealed microwave glass vial and stirred for 18 h at 110 °C the complex [Fe(PCPNEt- iPr)(CO)2Cl] (2) was obtained. In an attempt to prepare the hydride Fe(II) complex [Fe(PCPNEt- iPr)(CO)2H] (3), 2 was reacted with 1 equiv of Li[HBEt3] in THF. Instead of ligand substitution, this complex underwent a one electron reduction which led to the formation of the low-spin d7 Fe(I) complex [Fe(PCPNEt- iPr)(CO)2] (4). Exposure of a benzene solution of 4 to NO gas (1 bar) at room temperature affords the diamagnetic complex [Fe(PCPNEt- iPr)(CO)(NO)] (5). This is the first iron PCP nitrosyl complex. Protonation of 5 with HBF4·Et2O affords the cationic Fe(0) complex [Fe(κ3 P,CH,P-P(CH)PNEt- iPr)(CO)(NO)]BF4 (6) which features an η2-Caryl-H agostic bond. Even with relatively weak bases such as NEt3 the agostic C-H bond can be deprotonated with reformation of the starting material 5. Therefore, protonation of 5 is completely reversible.

Chiral Bis(Phospholane) PCP Pincer Complexes: Synthesis, Structure, and Nickel-Catalyzed Asymmetric Phosphine Alkylation

A known ligand precursor, prepared by an improved method from 1,3-bis(bromomethylbenzene), a base, and (R,R)-2,5-dimethylphospholane-borane, was deprotected with DABCO and used to prepare chiral m-xylene-based PCP pincer complexes. Reaction with Ni(DME)Br2 and NEt3 gave square planar Ni(DuPinPhos)(Br) (9), which was crystallographically characterized. Heating with [Ir(COE)2Cl]2 yielded the dinuclear hydride complex [Ir(DuPinPhos)(H)(Cl)]2(μ-C6H4(CH2-dimethylphospholanyl)2] (10), which contained both pincer chelates and a bridging bis(phospholane). Nickel complex 9 was a catalyst precursor for enantioselective alkylation of secondary phosphines with 2-bromomethylnaphthalene using the base NaOSiMe3. The dynamic behavior of a catalytic intermediate, the P-stereogenic terminal phosphido complex Ni(DuPinPhos)(PPhIs) (Is = 2,4,6-(i-Pr)3C6H2), was characterized by variable temperature NMR spectroscopy.

Reversible Ligand Protonation of a Mn(I) PCP Pincer Complex To Afford a Complex with an η2-Caryl–H Agostic Bond

The first manganese PCP pincer complex, [Mn(PCPNEt-iPr)(CO)3], was obtained by treating [Mn2(CO)10] and 2-chloro-N1,N3-bis(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (P(C-Cl)PNEt-iPr) in CH3CN under solvothermal conditions in a sealed microwave glass tube for 8 h at 140 °C. Protonation of this complex with HBF4·Et2O leads to the formation of the cationic Mn(I) complex [Mn(κ3P,CH,P-P(CH)PNEt-iPr)(CO)3]BF4, which features an η2-Caryl–H agostic bond. There was no evidence for the formation of a Mn(III) hydride species. The agostic C–H bond is comparatively acidic, and even relatively weak bases, such as NEt3, deprotonate this bond to re-form the starting material [Mn(PCPNEt-iPr)(CO)3]; protonation is fully reversible. Substitution of one CO ligand was achieved upon treatment with NO+, affording the cationic complex [Mn(PCPNEt-iPr)(CO)2(NO)]+, where the two CO ligands are trans to one another.

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...

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (tBuPCP)IrH(Cl) (1; tBuPCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be moderately active in NHMe2·BH3 (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (tBuPCP)IrHCl(η1-BH3·NHMe2) (3) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3·NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of 1) the h...