The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

A General Concept for the Electronic and Steric Modification of 1-Metallacyclobuta-2,3-dienes: A Case Study of Group 4 Metallocene Complexes

The synthesis of group 4 metal 1-metallacyclobuta-2,3-dienes as organometallic analogues of elusive 1,2-cyclobutadiene has so far been limited to SiMe3 substituted examples. We present the synthesis of two Ph substituted dilithiated ligand precursors for the preparation of four new 1-metallacyclobuta-2,3-dienes [rac-(ebthi)M] (M=Ti, Zr; ebthi=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl)). The organolithium compounds [Li2(RC3Ph)] (1 b: R=Ph, 1 c: R=SiMe3) as well as the metallacycles of the general formula [rac-(ebthi)M(R1C3R2)] (2 b: M=Ti, R1=R2=Ph, 2 c: M=Ti, R1=Ph, R2=SiMe3; 3 b: M=Zr, R1=R2=Ph; 3 c: M=Zr, R1=Ph, R2=SiMe3) were fully characterised. Single crystal X-ray diffraction and quantum chemical bond analysis of the Ti and Zr complexes reveal ligand influence on the biradicaloid character of the titanocene complexes. X-band EPR spectroscopy of structurally similar Ti complexes [rac-(ebthi)Ti(Me3SiC3SiMe3)] (2 a), 2 b, and 2 c was carried out to evaluate the accessibility of an EPR active triplet state. Cyclic voltammetry shows that introduction of Ph groups renders the complexes easier to reduce. 13C CPMAS NMR analysis provides insights into the cause of the low field shift of the resonances of metal-bonded carbon atoms and provides evidence of the absence of the β-C-Ti interaction.

Solution and solid-state studies of hydrogen and halogen bonding with N-heterocyclic carbene supported nickel(II) fluoride complexes

Nickel fluoride complexes of the type [Ni(F)(L)2(ArF)] (L = phosphine, ArF = fluorinated arene) are well-known to form strong halogen and hydrogen bonds in solution and in the solid state. A comprehensive study of such non-covalent interactions using bis(carbene) complexes as acceptors and suitable halogen and hydrogen bond donors is presented. In solution, the complex [Ni(F)(iPr2Im)2(C6F5)] forms halogen and hydrogen bonds with iodopentafluorobenzene and indole, respectively, which have formation constants (K300) an order of magnitude greater than those of structurally related phosphine supported nickel fluorides. Co-crystallisation of this complex and its backbone-methylated analogue [Ni(F)(iPr2Me2Im)2(C6F5)] with 1,4-diiodotetrafluorobenzene produces halogen bonding adducts which were characterised by X-ray analysis and 19F MAS solid state NMR analysis. Differences in the chemical shifts between the nickel fluoride and its halogen bonding adduct are well in line with data that were obtained from titration studies in solution.

Mechanochemical Oxidative Degradation of Thienopyridine Containing Drugs: Toward a Simple Tool for the Prediction of Drug Stability

The long-term stability of an active-pharmaceutical ingredient and its drug products plays an important role in the licensing process of new pharmaceuticals and for the application of the drug at the patient. It is, however, difficult to predict degradation profiles at early stages of the development of new drugs, making the entire process very time-consuming and costly. Forced mechanochemical degradation under controlled conditions can be used to realistically model long-term degradation processes naturally occurring in drug products, avoiding the use of solvents, thus excluding irrelevant solution-based degradation pathways. We present the forced mechanochemical oxidative degradation of three platelet inhibitor drug products, where the drug products contain thienopyridine. Model studies using clopidogrel hydrogen sulfate (CLP) and its drug formulation Plavix show that the controlled addition of excipients does not affect the nature of the main degradants. Experiments using drug products Ticlopidin-neuraxpharm and Efient show that significant degradation occurs after short reaction times of only 15 min. These results highlight the potential of mechanochemistry for the study of degradation processes of small molecules relevant to the prediction of degradation profiles during the development of new drugs. Furthermore, these data provide exciting insights into the role of mechanochemistry for chemical synthesis in general.

P,N-type phosphaalkene-based Ir(i) complexes: synthesis, coordination chemistry, and catalytic applications

Iridium P,N phosphaalkene complexes show a rich coordination chemistry with unusual twofold C–H activation. The Ir(i) chloride complex can be applied for C–N coupling and alcohol upgrading reactions.

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* (1, py = 2-pyridyl, Mes* = 2,4,6-tBu-C₆H₂) and the novel quin-CH═PMes* (2, quin = 2-quinolinyl) is described. The reaction with [Rh(μ-Cl)cod]₂ produces Rh(I) bis(phosphaalkene) chlorido complexes 3 and 4 with distorted trigonal bipyramidal coordination environments. Complexes 3 and 4 show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o-tBu groups of the Mes* substituents to yield the cationic bis(phosphaindane) Rh(I) complex 5, which could not be observed for the pyridine-based analogue 3. Using sub- or superstoichiometric amounts of AgOTf the C-H bond activation at an o-tBu group of one or at both Mes* was detected, respectively. Density functional theory (DFT) studies suggest an oxidative proton shift pathway as an alternative to a previously reported high-barrier oxidative addition at Rh(I). The Rh(I) mono- and bis(phosphaindane) triflate complexes 6 and 7, respectively, undergo deprotonation at the benzylic CH₂ group of the phosphaindane unit in the presence of KOtBu to furnish neutral, distorted square-planar Rh(I) complexes 8 and 9, respectively, with one of the P,N ligands being dearomatized. All complexes were fully characterized, including multinuclear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.

Iridium(III) bis(thiophosphinite) pincer complexes: synthesis, ligand activation and applications in catalysis

Iridium(III) bis(thiophosphinite) complexes of the type [(^RPSCSP^R)Ir(H)(Cl)(py)] (^RPSCSP^R = κ³-(2,6-SPR₂)C₆H₃) (R = tBu, iPr, Ph) can be prepared from the ligand precursors 1,3-(SPR₂)C₆H₄ by C-H activation at Ir using [Ir(COE)₂Cl]₂ or [Ir(COD)Cl]₂. Optimisation of the protocol for complexation showed that direct cyclometallation in the absence or presence of pyridine, as well as C-H activation in the presence of H₂ are viable options that, depending on the phosphine substituent furnish the five-coordinate Ir(III) hydride chloride complexes 2-R or the base stabilised species 3-R in good yields. In case of the ^PhPSCSP^Ph ligand, P-S activation results in the formation of a thiophosphine stabilised Ir(III) hydride complex [(^PhPSCSP^Ph)Ir(H)(Cl)(PPh₂SH)] (4). Reaction of 2-tBu with H₂ in the presence of base furnishes an Ir(III) dihydride complex (5) via a labile Ir(III) dihydride-dihydrogen complex (6). All complexes are inactive for transfer dehydrogenation of cyclooctane in the presence of NaOtBu and tert-butylethylene, likely due to decomposition of the Ir complex in the presence of base at higher temperature.

Model-based signal tracking in the quantitative analysis of time series of NMR spectra

Hard modeling of NMR spectra by Gauss-Lorentz peak models is an effective way for dimensionality reduction. In this manner high-dimensional measured data are reduced to low-dimensional information as peak centers, amplitudes or peak widths. For time series of spectra these parameters can be assumed to be smooth functions in time. We suggest to model these time-dependent parameter functions by cubic spline functions, which makes a stable quantitative analysis of NMR series possible even for crossing, highly overlapping peaks. Applications are presented for the batch distillation of methanol and diethylamine, and the reaction of acetic anhydride with 2-propanol.

Molecular Catalysts for the Reductive Homocoupling of CO 2 towards C 2+ Compounds

The conversion of CO₂ into multicarbon (C₂₊) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create “carbon‐neutral” fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo‐ and electrochemical reductive homocoupling of CO₂ toward C₂₊ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO₂ reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Catalytic and mechanistic studies of a highly active and E-selective Co(II) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Herein we report the application of a Co(II) PNNH pincer catalyst system (PNNH = 2-(5-(t-butyl)-1H-pyrazol-3-yl)-6-(dialkylphosphinomethyl)pyridine) for the highly E-selective transfer semihydrogenation of internal diaryl alkynes using methanol and ammonia borane...

Mono- and dinuclear zirconocene(iv) amide complexes for the catalytic dehydropolymerisation of phenylsilane

Mono- and dinuclear zirconocene amide complexes were tested as catalysts for the dehydropolymerisation of phenylsilane. The dinuclear complex is surprisingly stable, producing mixtures of polysilanes and cyclic oligomers.

How solvents affect the stability of cationic Rh(I) diphosphine complexes: a case study of MeCN coordination

Cationic rhodium(I) diphosphine complexes, referred to as Schrock-Osborn catalysts, are privileged homogeneous catalysts with a wide range of catalytic applications. The coordination of solvent molecules can have a significant influence...

Correction: Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Correction for ‘Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes’ by David Decker et al., Inorg. Chem. Front., 2022, DOI: 10.1039/d1qi00998b.

Catalytic dehydrocoupling of methylamine borane using Yamashita's [Ir(PBP)] boryl complex - characterisation of a novel highly fluxional Ir tetrahydride

We report the synthesis and characterisation of two iridium pincer monocarbonyl complexes possessing Yamashita’s PBP boryl ligand. Both complexes can be prepared by reaction of the parent hydride chloride complex...

1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene, unprecedented intramolecular C-H activation, and reactivity studies

The structure, bonding, and reactivity of small, highly unsaturated ring systems is of fundamental interest for inorganic and organic chemistry. Four-membered metallacyclobuta-2,3-dienes, also referred to as metallacycloallenes, are among the most exotic examples for ring systems as these represent organometallic analogs of 1,2-cyclobutadiene, the smallest cyclic allene. Herein, the synthesis of the first examples of 1-zirconacyclobuta-2,3-dienes of the type [Cp'₂Zr(Me₃SiC₃SiMe₃)] (Cp'₂ = rac-(ebthi), (ebthi = 1,2-ethylene-1,1'-bis(η⁵-tetrahydroindenyl)) (2a); rac-Me₂Si(thi)₂, thi = (η⁵-tetrahydroindenyl), (2b)) is presented. Both complexes undergo selective thermal C-H activation at the 7-position of the ansa-cyclopentadienyl ligand to produce a new type of "tucked-in" zirconocene system, 3a and 3b, that possesses a η³-propargyl/allenyl ligand. Both types of complexes react with carbonyl compounds, producing enynes in the case of 2a and 2b, as well as η¹-allenyl complexes for 3a and 3b. Computational analysis of the structure and bonding of 2a and 3a reveals significant differences to a previously described related Ti complex. All complexes were fully characterised, including X-ray crystallography and experimental results were supported by DFT analysis.

Ball milling - a new concept for predicting degradation profiles in active pharmaceutical ingredients

A method for forced oxidative mechanochemical degradation of active pharmaceutical ingredients (APIs) using clopidogrel hydrogensulfate as a model compound is presented. Considerable and selective formation of degradants occurs already after very short reaction times of less than 15 minutes and the nature of the products is strongly dependent on the used oxidant.

Mechanistic insights into dehydrocoupling of amine boranes using dinuclear zirconocene complexes

Catalytic dehydrocoupling of H₃B·NMe₂H using Cp₂Zr(Cl)(μ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied. Spectroscopic monitoring and stoichiometric experiments show the formation and interconversion of several catalytically active Zr species.

Dehydropolymerisation of methylamine borane using highly active rhodium(iii) bis(thiophosphinite) pincer complexes: catalytic and mechanistic insights

The bis(thiophosphinite) pincer complexes [(^RPSCSP^R)Rh(py)(H)(Cl)] (^RPSCSP^R = C₆H₄–2,6-(SPR₂)₂ with R = ⁱPr, 2a and R = Ph, 2b) are highly active precatalysts for the dehydropolymerisation of methylamine borane.

Reactivity of phospha-Wittig reagents towards NHCs and NHOs

Phospha-Wittig reagents, RPPMe3 (R = Mes* 2,4,6-tBu3-C6H2; MesTer 2,6-(2,4,6-Me3C6H2)-C6H3; DipTer 2,6-(2,6-iPr2C6H3)-C6H3), can be considered as phosphine-stabilized phosphinidenes. In this study we show that PMe3 can be displaced by NHCs or NHOs. Interestingly, phosphinidene-like reactivity results in a subsequent C(sp2)-H activation of the exocyclic CH2 group in NHOs. This concept was further extended to allyl-apended NHOs, which resulted in phosphine-substituted allyl species.

Dehydropolymerisation of Methylamine Borane and an N-Substituted Primary Amine Borane Using a PNP Fe Catalyst

Dehydropolymerisation of methylamine borane (H3 B⋅NMeH2 ) using the well-known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH2 CH2 PiPr2 )2 ) (1) gives poly(aminoborane)s by a chain-growth mechanism. In toluene, rapid dehydrogenation of H3 B⋅NMeH2 following first-order behaviour as a limiting case of a more general underlying Michaelis-Menten kinetics is observed, forming aminoborane H2 B=NMeH, which selectively couples to give high-molecular-weight poly(aminoborane)s (H2 BNMeH)n and only traces of borazine (HBNMe)3 by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H3 B⋅NMe2 H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H3 B⋅N(CH2 SiMe3 )H2 was studied in homo- and co-dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

A Comparative Study on the Thermodynamics of Halogen Bonding of Group 10 Pincer Fluoride Complexes

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5‐R₂‐^tBuPOCOP^tBu)MF] (3,5‐R₂‐^tBuPOCOP^tBu=κ³‐C₆HR₂‐2,6‐(OPtBu₂)₂ with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1‐tBu (R=tBu, M=Ni), 2‐H (R=H, M=Pd), 2‐tBu (R=tBu, M=Pd), 2‐COOMe (R=COOMe, M=Pd) and 3‐tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2‐tBu with 1,4‐diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

Homogeneous catalytic transfer semihydrogenation of alkynes – an overview of hydrogen sources, catalysts and reaction mechanisms

Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks.

Intermolecular hydrogen bonding in isostructural pincer complexes [OH-( t-BuPOCOP t-Bu)MCl] (M = Pd and Pt)

In the crystal structure of the isostructural title compounds, namely {2,6-bis[(di-tert-butylphosphanyl)oxy]-4-hydroxyphenyl}chloridopalladium(II), [Pd(C22H39O3P2)Cl], 1, and {2,6-bis[(di-tert-butylphosphanyl)oxy]-4-hydroxyphenyl}chloridoplatinum(II), [Pt(C22H39O3P2)Cl], 2, the metal centres are coordinated in a distorted square-planar fashion by the POCOP pincer fragment and the chloride ligand. Both complexes form strong hydrogen-bonded chain structures through an interaction of the OH group in the 4-position of the aromatic POCOP backbone with the halide ligand.

1-Titanacyclobuta-2,3-diene - an elusive four-membered cyclic allene

The synthesis and characterisation of a 1-titanacyclobuta-2,3-diene complex, an organometallic analog of elusive 1,2-cyclobutadiene, is presented.

The synthesis of an unusual 1-metalla-2,3-cyclobutadiene complex [rac-(ebthi)Ti(Me₃SiC₃SiMe₃)] (rac-ebthi = rac-1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)), a formal metallacyclic analogue of a non-existent four-membered 1,2-cyclobutadiene, is described. By variation of the cyclopentadienyl ligand of the titanocene precursor it was possible to stabilise this highly exotic compound which selectively reacts with ketones and aldehydes to yield enynes by oxygen transfer to titanium. Analysis of the bonding and electronic structure of the metallacycle shows that the complex is best described as an unusual antiferromagnetically coupled biradicaloid system, possessing a formal Ti(iii) centre coordinated with a monoanionic radical ligand.

Nickel(ii) PE1CE2P pincer complexes (E = O, S) for electrocatalytic proton reduction

The catalytic activity of a series of nickel complexes with different phosphinite/thiophosphinite ligands for electrocatalytic proton reduction strongly depends on the nature of the pincer ligands.

A general benzylic C–H activation and C–C coupling reaction of zirconocenes mediated by C–N bond cleavage in tert-butylisocyanide – unusual formation of iminoacyl complexes

Reactions of the zirconocene alkyne complex [rac-(ebthi)Zr(η²-Me₃SiC₂SiMe₃)] (rac-(ebthi) = rac-1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)) with tert-butylisocyanide and methylbenzenes were investigated.

Dehydropolymerisation of methylamine borane using a dinuclear 1,3-allenediyl bridged zirconocene complex

The dinuclear zirconocene chloride complex 1 is a highly active precatalyst for the dehydropolymerisation of methylamine borane. Comparison with mononuclear Zr chlorides and related dinuclear complexes suggests that the nature of the bridging motif is essential for the unique reactivity of 1.

Synthesis of Rh(iii) thiophosphinito pincer hydrido complexes by base-free C-H bond activation at room temperature

Rhodium(iii) thiophosphinito pincer hydrido complexes were synthesised by C-H activation under exceptionally mild conditions at room temperature without additional base or irradiation and fully characterised by multinuclear NMR spectroscopy and X-ray crystallography. C-H activation under these mild conditions contrasts with the reactivity of related systems with POCOP ligands.

Synthesis and crystallographic characterization of [2,2-bis-(η5-penta-methyl-cyclo-penta-dien-yl)-3,4-bis(tri-methyl-sil-yl)-2-zircona-furan-5-one-κO5]triisobutyl-aluminium

The crystal structure of the title zwitterionic zirconocene complex containing a furan-one unit, [AlZr(C10H15)2(C4H9)3(C9H18O2Si2)], is reported. On reacting a zircona-furan-one with two equivalents of HAl(i-Bu)2, disproportionation of the Lewis acid results in the formation of a triiso-butyl-aluminium fragment, Al(i-Bu)3, which coordinates to the exocyclic carbonyl O atom of the zircona-furan-one ring. Single-crystal X-ray diffraction reveals that the zircona-furan-one ring remains intact with coordination of the aluminium to the exocyclic O atom. One of the i-butyl groups is disordered over two sets of sites, with an occupancy ratio of 0.731 (3):0.269 (3).

Synthesis and coordination chemistry of the PPN ligand 2-[bis(diisopropylphosphanyl)methyl]-6-methylpyridine

The synthesis and crystal structure of the multidentate PPN ligand 2-[bis(diisopropylphosphanyl)methyl]-6-methylpyridine (L), C19H35NP2, are described. In the isostructural tetrahedral Fe and Co complexes of type LMCl2 (M = Fe, Co), namely {2-[bis(diisopropylphosphanyl)methyl]-6-methylpyridine-κ2 P,N}dichloridoiron(II), [FeCl2(C19H35NP2)], and {2-[bis(diisopropylphosphanyl)methyl]-6-methylpyridine-κ2 P,N}dichloridocobalt(II), [CoCl2(C19H35NP2)], the ligand adopts a bidentate P,N-coordination, whereas in the case of the octahedral Mn complex {2-[bis(diisopropylphosphanyl)methyl]-6-methylpyridine-κ2 P,P′}bromidotricarbonylmanganese(I), [MnBr(C19H35NP2)(CO)3], the ligand coordinates via both P atoms to the metal centre.

Titanocene Silylpropyne Complexes: Promising Intermediates en route to a Four-Membered 1-Metallacyclobuta-2,3-diene?

Coordination of the alkyl-substituted alkynes Me₃ SiC₂ CH₂ R (1: R=SiMe₃ ; 2: R=N(SiMe₃ )₂ ) to titanocene centres [Cp'₂ Ti] (Cp'=Cp, Cp*) yields stable alkyne complexes of the type Cp'₂ Ti(η² -Me₃ SiC₂ CH₂ R) (3: Cp'=Cp, R=SiMe₃ ; 5: Cp'=Cp, R=N(SiMe₃ )₂ ; 6: Cp'=Cp*, R=SiMe₃ ) that are not prone to alkyne/allene isomerisation. When reacting alkyne 2 with Cp^*₂ TiCl₂ and Mg formation of the complex Cp*₂ Ti(III)(η³ -Me₃ SiC₂ CH₂ ) (7) which displays a propargylic unit coordinated to the Ti^III centre takes place. All complexes were fully characterised, the molecular structures for 5, 6, and 7 are discussed.

Formation of high-molecular weight polyaminoborane by Fe hydride catalysed dehydrocoupling of methylamine borane

The complex [(PNHP)Fe(H)(CO)(HBH₃)] (PNHP = HN(CH₂CH₂Pi-Pr₂)₂) serves as a catalyst precursor for the selective dehydrocoupling of methylamine borane at room temperature, tentatively via an off-metal polymerisation pathway.