Phosphinines versus mesoionic carbenes: a comparison of structurally related ligands in Au(i)-catalysis

Highly selective, reversible water activation by P,N-cooperativity in pyridyl-functionalized phosphinines

Tetrapyridyl-functionalized phosphinines were prepared and structurally characterized. The donor-functionalized aromatic phosphorus heterocycles react highly selectively and even reversibly with water. Calculations reveal P,N-cooperativity for this process, with the flanking pyridyl groups serving to kinetically enhance the formal oxidative addition process of H2O to the low-coordinate phosphorus atom via H-bonding. Subsequent tautomerization forms 1,2-dihydrophosphinine derivatives, which can be quantitatively converted back to the phosphinine by applying vacuum, even at room temperature. This process can be repeated numerous times, without any sign of decomposition of the phosphinine. In the presence of CuI·SMe2, dimeric species of the type ([Cu2I2(phosphinine)]2) are formed, in which each phosphorus atom shows the less common μ2-bridging 2e--lone-pair-donation to two Cu(i)-centres. Our results demonstrate that fully unsaturated phosphorus heterocycles, containing reactive P[double bond, length as m-dash]C double bonds, are interesting candidates for the activation of E-H bonds, while the aromaticity of such compounds plays an appreciable role in the reversibility of the reaction, supported by NICS calculations.

A mesoionic carbene stabilized nickel(II) hydroxide complex: a facile precursor for C-H activation chemistry

We report the synthesis of a new nickel(II) hydroxide complex 2 supported by a rigid, tridentate triazolylidene-carbazolid ligand. The complex can be accessed in high yields following a simple and stepwise extraction protocol using dichloromethane and aqueous ammonium chloride followed by aqeous sodium hydroxide solution. We found that complex 2 is highly basic, undergoing various deprotonation/desilylation reactions with E-H and C-H acidic and silylated compounds. In this context we synthesized a variety of novel, functionalized nickel(II) complexes with trimethylsilylolate (3), trityl sulfide (4), tosyl amide (5), azido (6), pyridine (7), acetylide (8, 9), fluoroarene (10 & 11) and enolate (12) ligands. We furthermore found that 2 reacts with malonic acid dimethyl ester in a knoevennagel-type condensation reaction, giving access to a new enolate ligand in complex 13, consisting of two malonic acid units. Furthermore, complex 2 reacts with acetonitrile to form the cyanido complex 14. The formation of complexes 13 and 14 is particularly interesting, as they underline the potential of complex 2 in both C-C bond formation and cleavage reactions.

Digold Phosphinine Complexes Are Stable with a Bis(Phosphinine) Ligand but Not with a 2-Phosphinophosphinine

The reaction of [bis{3-methyl-6-(trimethylsilyl)phosphinine-2-yl}dimethylsilane] (19) with one and two equivalents of [AuCl(tht)] was attempted in order to selectively form the mono and digold species, respectively. The digold species [(AuCl)2(19)] (21) was synthesized in 32% yield and comprehensibly characterized (multinuclear NMR spectroscopy, elemental analysis, mass spectrometry and single-crystal X-ray diffraction). The monogold species showed no 31P nuclear magnetic resonance at 25 °C but two resonances at −70 °C due to rapid exchange of AuCl between the phosphinine donors at 25 °C and was also susceptible to redistribution reactions to form the digold species. Analogous reactions of [AuCl(tht)] with 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine (22) revealed preferential coordination of the AuCl unit to the PPh2 donor first, with coordination to the phosphinine achieved upon reaction with the second equivalent of [AuCl(tht)]. Unexpectedly, the digold complex was not stable, undergoing decomposition to give an unidentified black precipitate. Structural information could only be obtained on the digold hydrolysis product [(AuCl)2(1-OH-2-PPh2-3-MePC5H4)], which showed an aurophilic interaction.

A mesoionic carbene complex of manganese in five oxidation states

Reaction between a carbazole-based mesoionic carbene ligand and manganese(II) iodide results in the formation of a rare air-stable manganese(IV) complex after aerobic workup. Cyclic voltammetry reveals the complex to be stable in five oxidation states. The electronic structure of all five oxidation states is elucidated chemically, spectroscopically (NMR, high-frequency EPR, UV-Vis, MCD), magnetically, and computationally (DFT, CASSCF).

Reactivity Studies of Phosphinines: The Selenation of Diphenyl-Phosphine Substituents and Formation of a Chelating Bis(Phosphinine) Palladium(II) Complex

Phosphinines and donor-substituted phosphinines are of recent interest due to their use in homogeneous catalysis. In this article, a Pd(II) bis(phosphinine) complex was characterised and phosphorus–selenium coupling constants were used to assess the donor properties of the diphenylphosphine substituents of phosphinine ligands to promote their further use in catalysis. The selenation of 2,5-bis(diphenylphosphino)-3,6-dimethylphosphinine (5) and 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine (6) gave the corresponding phosphine selenides 8 and 9, respectively, leaving the phosphinine ring intact. Multinuclear NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction confirmed the oxidation of all the diphenylphosphine substituents with 1JP-Se coupling constants determined to be similar to SePPh3, indicating that the phosphinine rings were electronically similar to phenyl substituents. Solutions of 6 were found to react with oxygen slowly to produce the phosphine oxide 10 along with other by-products. The reaction of [bis{3-methyl-6-(trimethylsilyl)phosphinine-2-yl}dimethylsilane] (4) with [PdCl2(COD)] gave the chelating dichloropalladium(II) complex, as determined by multinuclear NMR spectroscopy, mass spectrometry and an elemental analysis. The molecular structure of the intermediate 2 in the formation of 4,6-di(tert-butyl)-1,3,2-diazaphosphinine (3) was also determined, which confirmed the structure of the diazaphosphacycle P(Cl){N=C(tBu)CH=C(tBu)-N(H)}.

Au(I)-mediated N2-elimination from triazaphospholes: a one-pot synthesis of novel N2P2-heterocycles

Novel tosyl- and mesitylsulfonyl-substituted triazaphospholes were synthesized and structurally characterized. In an attempt to prepare the corresponding Au(I)-complexes with stoichiometric amounts of AuCl·S(CH₃)₂, cyclo-1,3-diphospha(III)-2,4-diazane-AuCl-complexes were obtained instead. Our here presented results offer a new strategy for preparing such coordination compounds selectively in a one-pot approach.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II)

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

We report the synthesis of vanadium(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L1 and the general formula [VOCl(L1)]. A comparison of the structural (SC-XRD), electronic (UV-vis), and electrochemical (cyclic voltammetry) properties of 1 with the benzimidazolinylidene congener 2 (general formula [VOCl(L2)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium(V) but also for the vanadium(IV) oxidation state by isolating the reduced vanadium(IV) complexes [Co(Cp*)2][1] and [Co(Cp*)2][2] ([Co(Cp*)2] = decamethylcobaltocenium). The electronic structures of the compounds were investigated by computational methods. Complex 1 was found to be a moderate precursor for salt metathesis reactions, showing selective reactivity toward phenolates or secondary amides, but not toward primary amides and phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with electron-rich phosphines failed to give the desired vanadium(III) complex. However, treatment of the deprotonated ligand precursor with vanadium(III) trichloride resulted in the clean formation of the corresponding MIC vanadium(III) complex 6, which undergoes a clean two-electron oxidation with organic azides yielding the corresponding imido complexes. The reaction with TMS-N3 did not afford a nitrido complex, but instead the imido complex 10. This study reveals that, contrary to popular belief, MICs are capable of supporting early transition-metal complexes in a variety of oxidation states, thus making them promising candidates for the activation of small molecules and redox catalysis.

A ruthenium cis-dihydride with 2-phosphinophosphinine ligands catalyses the acceptorless dehydrogenation of benzyl alcohol

The first ruthenium dihydride complex featuring a phosphinine ligand cis-[Ru(H)₂(2-PPh₂-3-Me-6-SiMe₃-PC₅H₂)₂] was synthesised exclusively as the cis-isomer. When formed in situ from the reaction of cis-[Ru(Cl)₂(2-PPh₂-3-Me-6-SiMe₃-PC₅H₂)₂] with two equivalents of Na[BHEt₃], as demonstrated by ³¹P and ¹H NMR spectroscopy, the catalysed acceptorless dehydrogenation of benzyl alcohol was observed leading to benzyl benzoate in up to 70% yield.

Silver-Free Au(I) Catalysis Enabled by Bifunctional Urea- and Squaramide-Phosphine Ligands via H-Bonding

A library of gold(I) chloride complexes with phosphine ligands incorporating pendant (thio)urea and squaramide H-bond donors was prepared with the aim of promoting chloride abstraction from Au(I) via H-bonding. In the absence of silver additives, complexes bearing squaramides and trifluoromethylated aromatic ureas displayed good catalytic activity in the cyclization of N-propargyl benzamides, as well as in a 1,6-enyne cycloisomerization, a tandem cyclization-indole addition reaction and the hydrohydrazination of phenylacetylene. Kinetic studies and DFT calculations indicate that the energetic span of the reaction is accounted by both the chloride abstraction step, facilitated by the bidentate H-bond donor via an associative mechanism, and the subsequent cyclization step.

Gold(I) Complexation of Phosphanoxy-Substituted Phosphaalkenes for Activation-Free LAuCl Catalysis

The phosphanoxy-substituted phosphaalkene bearing the P=C-O-P skeleton can be prepared from diphosphene Mes*P=PMes* (Mes*=2,4,6-tBu₃ C₆ H₂ ), and their use for catalysis is of interest. In this paper, complexation of the phosphanoxy-substituted phosphaalkenes with gold are investigated, and the catalytic activity of the mono- and bis(chlorogold) complexes are subsequently evaluated. Reaction of the P=C-O-P compound with (tht)AuCl (tht=tetrahydrothiophene) showed dominant coordination on the sp³ phosphorus, and complete coordination on the sp² phosphorus required removal of tetrahydrothiophene. Atoms In Molecules (AIM) analysis based on the X-ray structure of the mono(chlorogold) complex indicated a pseudo coordinating interaction between the gold center and the P=C unit. The bis(chlorogold) complexes displayed conformational isomerism, and catalyzed the cycloisomerization/alkoxycyclization of 1,6-enyne and for hydration of terminal alkyne without activation treatment. Even the mono(chlorogold) complexes catalyzed the alkoxycyclization reactions without a silver co-catalyst, indicating that the alcohols were effective in activating the AuCl unit.

The core of the matter - arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1'-ferrocenylene)arene gold(I) complexes

Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

The modular syntheses of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes and their homotrinuclear gold(i) complexes are reported. Choosing the arene core allows fine-tuning of the exact oxidation potentials and thus tailoring of the electrochemical response. The tris[chloridogold(i)] complexes were investigated in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, showing cooperative behaviour vs. a mononuclear chloridogold(i) complex. Adding one, two, or three equivalents of 1,1′-diacetylferrocenium[tetrakis(perfluoro-tert-butoxy)aluminate] as an oxidant during the catalytic reaction (in situ) resulted in a distinct, stepwise influence on the resulting catalytic rates. Isolation of the oxidised species is possible, and using them as (pre-)catalysts (ex situ oxidation) confirmed the activity trend. Proving the intactness of the P–Au–Cl motif during oxidation, the tri-oxidised benzene-based complex has been structurally characterised.

Trinuclear gold(i) complexes of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes with four accessible oxidation states catalyse the ring-closing isomerisation of N-(2-propyn-1-yl)benzamide with different rates depending on their redox state.

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

This paper describes a novel synthetic methodology for the preparation of 1,2,3-triazole based phosphines and their transition metal chemistry and preliminary catalytic studies.

Making the unconventional μ2-P bridging binding mode more conventional in phosphinine complexes

As compared to the normal η¹-P σ-complexes or η⁶-phosphinine π-complexes, the rare μ²-P bridging binding mode of phosphinines can be tuned by employing electron donating substitute.

Phosphinines, as aromatic heterocycles, usually engage in coordination as η¹-P σ-complexes or η⁶-phosphinine π-complexes. The μ²-P bridging coordination mode is rarely observed. With the aim to study the effect of different electronic configurations of phosphinines on the coordination modes, a series of anionic phosphinin-2-olates and neutral phosphinin-2-ols were prepared with moderate to high yield. Then the coordination chemistry of these two series was studied in detail towards coinage metals (Au(i) and Cu(i)). It is observed that the anionic phosphinin-2-olates possess a higher tendency to take a bridging position between two metal centers compared to the neutral phosphinin-2-ols. Based on these experimental findings bolstered by DFT calculations, some insight is gained on how the unconventional μ²-P phosphinine bridging coordination mode can be made more conventional and used for the synthesis of polynuclear complexes.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

2-Phosphinophosphinine ligands generate Rh catalysts for the hydroboration of ketones and imines in contrast to standard phosphine ligands.