Ligand effects in chromium diphosphine catalysed olefin co-trimerisation and diene trimerisation

Reactivity of Ir(I)-aminophosphane platforms towards oxidants

The iridium(I)-aminophosphane complex [Ir{κ3C,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}2; SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}2). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(η3-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(η3-C3H5)(CNtBu)(SiNP)]Cl via the observed intermediate [IrCl2(η1-C3H5)(CNtBu)(SiNP)]. On the other hand, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{κ3C,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2]+ (1 : 2) in acetonitrile affording [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)3]2+ isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with I2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I, containing the η2,η3-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir2{κ3C,P,P'-(SiNP-H)}2(μ-I)3]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{κ3C,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the η2,η3-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

Highly Enantioselective Binaphthyl-Based Chiral Phosphoramidite Stabilized-Palladium Nanoparticles for Asymmetric Suzuki C-C Coupling Reactions

The optically pure binaphthyl-based phosphoramidite ligands and their perfluorinated analogs have been first used for the preparation of chiral palladium nanoparticles (PdNPs). These PdNPs have been extensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, ³¹P NMR, and thermogravimetric analysis techniques. The circular dichroism(CD) analysis of chiral PdNPs exhibited negative cotton effects. Perfluorinated phosphoramidite ligands provided smaller (2.32-3.45 nm) and well-defined nanoparticles, in comparison with the nonfluorinated analog (4.12 nm). The catalytic behavior of binaphthyl-based phosphoramidite stabilized chiral PdNPs has been investigated in the asymmetric Suzuki C-C coupling reactions for the formation of sterically hindered binaphthalene units, and high isolated yields (up to 85%) were achieved with excellent enantiomeric excesses (>99% ee). Recycling studies revealed that chiral PdNPs could be reused over 12 times without significant loss in activity and enantioselectivity (>99% ee). The nature of the active species was also investigated with a combination of poisoning and hot filtration tests and found that catalytically active species is the heterogeneous nanoparticles. These results indicate that the use of phosphoramidite ligands as a stabilizer for developing efficient and unique chiral nanoparticles could open up a field for many other asymmetric organic transformations promoted by chiral catalysts.

A Self-Separating Multiphasic System for Catalytic Hydrogenation of CO2 and CO2 -Derivatives to Methanol

Catalytic conversion of CO₂ and hydrogen to methanol was achieved in a self-separating multiphasic system comprising the tailor-made complex [Ru(CO)ClH(MACHO-C₁₂ )] (MACHO-C₁₂ =bis{2-[bis(4-dodecylphenyl)phosphino]ethyl}amine) in n-decane as the catalyst phase. Effective catalyst recycling was demonstrated for the carbonate and the amine-assisted pathway from CO₂ to methanol. The polar products MeOH or MeOH/H₂ O generated from the catalytic reactions spontaneously formed a separate phase, allowing product isolation and catalyst separation without the need for any additional solvent. In the amine-assisted hydrogenation of CO₂ , the catalyst phase was recycled over ten subsequent runs, reaching a total turnover number to MeOH of 19200 with an average selectivity of 96 %.

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated PIII donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (-NH-, -NR-) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand

The iridium(III) hydride compound [IrH{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (1PF₆) was obtained by reaction of [Ir(SiNP)(cod)][PF₆] with CN^tBu as the result of the intramolecular oxidative addition of the SiCH₂-H bond to iridium(I) [SiNP = Si(CH₃)₂{N(4-tolyl)PPh₂}₂, SiNP-H = CH₂Si(CH₃){N(4-tolyl)PPh₂}₂]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆) forms in the first place and that further reacts with CN^tBu, affording the square planar intermediate [Ir(SiNP)(CN^tBu)₂][PF₆] (3PF₆) that finally undergoes the intramolecular oxidative addition of the SiCH₂-H bond. The reactivity of 1PF₆ was investigated. On one hand, the reaction of 1PF₆ with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (X = Cl, 4PF₆; Br, 5PF₆), and the reaction of 5PF₆ with AgPF₆ in the presence of acetonitrile affords the solvato species [Ir{κ³C,P,P'-(SiNP-H)}(CH₃CN)(CN^tBu)₂]²⁺ (6²⁺) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF₆ with HBF₄ gives the iridium(III) compound [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄) as the result of the formal addition of hydrogen fluoride to the Si-N bonds of 1⁺ [HNP = HN(4-tolyl)PPh₂]. A similar outcome was observed in the reaction of 1PF₆ with CF₃COOH rendering 7PO₂F₂. In this case the intermediate [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si-N-P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF₆, 4PF₆ and 7BF₄ have been determined by X-ray diffraction measurements.

Effects of the halogenido ligands on the Kumada-coupling catalytic activity of [Ni{ t BuN(PPh2)2-κ2P}X2], X = Cl, Br, I, complexes

Novel nickel(ii) complexes bearing ( t butyl)bis(diphenylphosphanyl)amine and different halogenido ligands, [Ni(P,P)X2] = [Ni{ t BuN(PPh2)2-κ2P}X2], (X = Cl, Br, I) are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction of model substituted iodobenzenes and p-tolylmagnesium bromide. The data obtained together with DFT calculations indicate that these new catalysts operate in the Ni(i)-Ni(iii) mode. The highest catalytic activity and selectivity are exhibited by [Ni(P,P)Cl2], which is most easily reduced by the used Grignard reagent to the Ni(i) state. This process is much more energy demanding in the case of the bromido and iodido complexes, causing the appearance of the induction period. [Ni(P,P)Cl2] is also very active in the cross-couplings of substrates with iodine atoms sterically shielded by ortho substituents. The data obtained are in good accordance with the described positive effect of the increased electron-releasing power of N-substituents R' on the overall catalytic performance of [Ni{R'N(PPh2)2-κ2P}X2] complexes.

Ethylene oligomerisation chromium catalysts with unsymmetrical PCNP ligands

Chromium(iii) complexes of chelating diphosphines, with PNP or PCNCP backbones, are excellent catalysts for ethylene tetra- and/or trimerisations. A missing link within this ligand series are unsymmetric chelating diphosphines based on a PCNP scaffold. New bidentate PCNP ligands of the type Ph₂PCH₂N(R)PPh₂ (R = 1-naphthyl or 5-quinoline groups, 2a-d) have been synthesised and shown to be extremely effective ligands for ethylene tri-/tetramerisations. Three representative tetracarbonyl Cr⁰ complexes bearing a single PN(R)P (5), PCN(R)P (6), or PCN(R)CP (7) diphosphine (R = 1-naphthyl) have been prepared from Cr(CO)₄(η⁴-nbd) (nbd = norbornadiene). Furthermore we report a single crystal X-ray diffraction study of these compounds and discuss their structural parameters.

The effects of metals and ligands on the oxidation of n-octane using iridium and rhodium “PNP” aminodiphosphine complexes

Ir and Rh “PNP” complexes with different ligands are utilized for the oxidation of n-octane. Based on the obtained conversion, selectivity, and the characterized recovered catalysts, it is found that the combination of Ir and the studied ligands does not promote the redox mechanism that is known to result in selective formation of oxo and peroxo compounds [desired species for C(1) activation]. Instead, they support a deeper oxidation mechanism, and thus higher selectivity for ketones and acids is obtained. In contrast, these ligands seem to tune the electron density around the Rh (in the Rh-PNP complexes), and thus result in a higher n-octane conversion and improved selectivity for the C(1) activated products, with minimized deeper oxidation, in comparison to Ir-PNP catalysts.

Role of the ligand and activator in selective Cr–PNP ethene tri- and tetramerization catalysts – a spectroscopic study

The reaction of the ethene tetramerization catalyst ((C₆H₅)₂P)₂N^iPrCrCl₃(THF), and trimerization catalyst ((o-C₆H₄OMe)₂P)₂N^MeCrCl₃, with AlMe₃ and MMAO was investigated using spectroscopy (Cr K-edge XAS, X-band EPR and UV-vis) and catalytic studies.

Selective Reductions of N,N-Bis{chloro(aryl)-phosphino}-amines Yielding Three-, Five-, Six-, and Eight-Membered Cyclic Azaphosphanes

To date, a plethora of λ³ -P nitrogen-containing compounds is known. A large number of them are used as ligands in coordination chemistry and homogeneous catalysis. PN-containing compounds tend to build up cyclic moieties, which have received less attention in regard to their application as ligands in transition metal chemistry. Hence, different dehalogenation reactions of N,N-bis{chloro(aryl)-phosphino}-amines have been developed to synthesize different P-N-P containing cyclic compounds. Their coordination behavior to group VI transition metal carbonyls was explored.

Ligands with an NPNPN-framework and their application in chromium catalysed ethene tri-/tetramerization

Novel Cr(iii) catalysts supported by linear phosph(iii)azanes of the type R(1)R(2)N-P(Ph)-NR(3)-P(Ph)-NR(4)R(5) have been prepared, all of which, upon activation with MMAO-3A, are highly active for ethene tri-/tetramerization with considerable selectivity. The effect of ligand substitution as well as solvent on the catalytic performance has been examined.

Influence of the 5f Orbitals on the Bonding and Reactivity in Organoactinides: Experimental and Computational Studies on a Uranium Metallacyclopropene

The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η(5)-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-C2(SiMe3)2] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(IV) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(η(2)-C═C) moiety, leading to more covalent bonds between the (η(5)-C5Me5)2U(2+) and [η(2)-C2(SiMe3)2](2-) fragments than those in the related Th(IV) compound. Consequently, very different reactivity patterns emerge, e.g., 2 can act as a source for the (η(5)-C5Me5)2U(II) fragment when reacted with alkynes and a variety of heterounsaturated molecules such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.

Aminophobanes: hydrolytic stability, tautomerism and application in Cr-catalysed ethene oligomerisation

9-Amino-9-phosphabicyclo[3.3.1]nonanes, (PhobPNHR'; R = Me or (i)Pr) are readily prepared by aminolysis of PhobPCl and are significantly less susceptible to hydrolysis than the acyclic analogues Cy2PNHR'. Treatment of Cy2PNHMe with Cy2PCl readily gave Cy2PNMePCy2. By contrast, treatment of PhobPCl with PhobPNHMe in the presence of Et3N does not afford PhobPNMePPhob but instead the salt [PhobP(= NMeH)PPhob]Cl is formed which, upon addition of [PtCl2(NC(t)Bu)2] gives the zwitterionic complex [PtCl3(PhobP(= NMeH)PPhob)]. The neutral PhobP(= NMe)PPhob is accessible from PhobNMeLi and is converted to the chelate [PdCl2(PhobPNMePPhob)] by addition of [PdCl2(cod)]. The anomalous preference of the PhobP group for the formation of PPN products is discussed. The unsymmetrical diphos ligands PhobPNMePAr2 (Ar = Ph, o-Tol) are prepared, converted to [Cr(CO)4(PhobPNMePAr2)] and shown to form Cr-catalysts for ethene oligomerisation, producing a pattern of higher alkenes that corresponds to a Schulz-Flory distribution overlaid on selective tri/tetramerisation.

Iridium and rhodium “PNP” aminodiphosphine complexes used as catalysts in the oxidation of styrene

New Ir and Rh “PNP” aminodiphosphine complexes are effective catalysts in styrene oxidation withtert-butyl hydroperoxide as the oxidant. The Ir catalysts were more active than the Rh catalysts with high yields to benzaldehyde in comparison to styrene oxide.

The novel [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}I2] complex: Structural features and catalytic reactivity in the oligomerization of ethylene

The novel [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}I2] complex (1) was synthesized and investigated by a variety of spectroscopic methods (IR, 1H and 31P NMR). The molecular structure of 1, determined by single crystal X-ray diffraction, was compared with those of the analogue [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}Cl2] complex (2) and the recently reported [Ni{iCPrP)2NH-P,P´}I2] complex. Complexes 1 and 2 were tested as homogeneous catalysts for the oligomerization of ethylene, leading to the formation of C4 and, to a smaller extent, C6 products, in moderate yields.

Intramolecular C-H oxidative addition to iridium(I) triggered by trimethyl phosphite in N,N'-diphosphanesilanediamine complexes

The reaction of [Ir(SiNP)(cod)][PF6] ([1][PF6]) and of IrCl(SiNP)(cod) (5) (SiNP = SiMe2{N(4-C6H4CH3)PPh2}2) with trimethyl phosphite affords the iridium(iii) derivatives of the formula [IrHClx(SiNP-H){P(OMe)3}2-x]((1-x)+) (x = 0, 3(+); x = 1, 6) containing the κ(3)C,P,P'-coordinated SiNP-H ligand (SiNP-H = Si(CH2)(CH3){N(4-C6H4CH3)PPh2}2). The thermally unstable pentacoordinated cation [Ir(SiNP){P(OMe)3}(cod)](+) (2(+)) has been detected as an intermediate of the reaction and has been fully characterised in solution. Also, the mechanism of the C-H oxidative addition has been elucidated by DFT calculations showing that the square planar iridium(i) complexes of the formula [IrClx(SiNP){P(OMe)3}2-x]((1-x)+) (x = 0, 4(+); x = 1, 7) should be firstly obtained from 2(+) and finally should undergo the C-H oxidative addition to iridium(i) via a concerted intramolecular mechanism. The influence of the counterion of 2(+) on the outcome of the C-H oxidative addition reaction has also been investigated.

Cobalt aminodiphosphine complexes as catalysts in the oxidation of n-octane

New cobalt “PNP” aminodiphosphine complexes have been prepared and characterised. They show good catalytic activity in the oxidation of n-octane to C8 oxygenates using tert-butyl hydroperoxide as an oxidant.

Spectral studies of a Cr(PNP)-MAO system for selective ethylene trimerization catalysis: searching for the active species

Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble Cr^IIICl₃(PNP) (PNP = bis(diarylphosphino)alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of Cr^IIICl₃(PNP) with MAO leads to first replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower-valent species. Reactivity studies demonstrate that the majority of the chromium species detected is not catalytically active.

Mechanistic Studies of Ethylene and α-Olefin Co-oligomerization Catalyzed by Chromium-PNP Complexes

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons from ethylene as a chemical feedstock, co-oligomerization of ethylene and linear α-olefins (LAOs) was investigated, using a previously reported chromium complex, [CrCl(3)(PNP(OMe))] (1, where PNP(OMe) = N,N-bis(bis(o-methoxyphenyl)phosphino)methylamine). Activation of 1 by treatment with modified methylaluminoxane (MMAO) in the presence of ethylene and 1-hexene afforded mostly C(6) and C(10) alkene products. The identities of the C(10) isomers, assigned by detailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprising of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO co-trimers, and that co-trimers derived from one ethylene and two LAO molecules are also generated. Complex 1/MMAO is also capable of converting 1-hexene to C(12) dimers and C(18) trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed and compared to previous reports of olefin co-trimerization.