Methylidyne Transfer Reactions with Niobium

Modular Synthesis of Monoanionic PN Ligands Leads to Unexpected Structural Diversity in Lanthanum Chemistry

We report a new synthetic entry to a series of N-substituted anilidophosphine ligands (short HPNR, R = pTol (HPNTol), 3,5-dimethylphenyl (HPN3,5Me), 3,5-bis(trifluoromethyl)phenyl (HPN3,5CF3), 2-methoxyphenyl (HPNOMe), diisopropylphenyl (HPNDipp), and adamantyl (HPNAd)), allowing a detailed tuning of their steric (and electronic) properties. HPNR could be converted into their lithium salts LiPNR, which are effective precursors for salt metathesis reactions. The new ligands are used for the synthesis of an array of lanthanide complexes using LaCl3(THF)1.2 as a precursor. Depending on the steric bulk of the anilidophosphine ligand, either chloride-bridged dimers of the general formula [(PNR)2La(μ-Cl2)La(PNR)2] (R = pTol (3f), 3,5-dimethylphenyl (3d) and adamantyl (3a)) or mononuclear complexes of the general formula (PNR)2LaCl (R = diisopropylphenyl (3e)) are observed, if the complexation reaction is carried out in toluene. Contrary, if salt metathesis reactions are carried out in dimethoxyethane (DME) as a coordinating solvent, -ate complexes of the general formula [(PNR)2La(μ-Cl2)Li(DME)] (R = Adamantyl (4a) and R = 2-methoxyphenyl (4d)) or [Li(DME)3][(PNR)2LaCl2] (R = 3,5-bis(trifluoromethyl)phenyl (4b), R = pTol (4f) and R = 3,5-dimethylphenyl (4d)) are observed. All ligands and complexes have been thoroughly characterized by 1D and 2D NMR spectroscopy, IR, and X-ray crystallography. Finally, the steric demand of the new anilidophosphine ligands is evaluated using SambVca simulations.

Probing σ-Aromaticity-Driven Ring Contraction of Metallabenzocyclobutadiene to Metallabenzocyclopropene

Ring contraction of metallacyclobutadiene to metallacyclopropene is rare because of the increasing strain from a four-membered ring to a three-membered one. Here we demonstrate a new series of reactions of metallabenzocyclobutadiene to metallabenzocyclopropene via density functional theory calculations. The results suggest that these reactions are thermodynamically favorable ranging from -17.4 to -29.4 kcal mol-1, and a low reaction barrier (10.3 kcal mol-1) is achieved when the metal center is Ru and the ligands are one cyanide and one chloride. Further analysis suggests that a strengthened binding energy helps stabilize the transition state in the protonation process. The aromaticity during the reaction was investigated using the electron density of delocalized bonds (EDDB), isomerization stabilization energy, and isodesmic reactions. The EDDB shows that the π-conjugation is disrupted in the intermediate, and then σ-aromaticity is generated and dominant in the products. Our findings could be helpful for experimentalists in developing novel ring contraction reactions driven by aromaticity.

Origin of Reactivity Trends of an Elusive Metathesis Intermediate from NMR Chemical Shift Analysis of Surrogate Analogues

Olefin metathesis has become an efficient tool in synthetic organic chemistry to build carbon-carbon bonds, thanks to the development of Grubbs- and Schrock-type catalysts. Olefin coordination, a key and often rate-determining elementary step for d0 Schrock-type catalysts, has been rarely explored due to the lack of accessible relevant molecular analogues. Herein, we present a fully characterized surrogate of this key olefin-coordination intermediate, namely, a cationic d0 tungsten oxo-methylidene complex bearing two N-heterocyclic carbene ligands─[WO(CH2)Cl(IMes)2](OTf) (1) (IMes = 1,3-dimesitylimidazole-2-ylidene, OTf-triflate counteranion), resulting in a trigonal bipyramidal (TBP) geometry, along with its neutral octahedral analogue [WO(CH2)Cl2(IMes)2] (2)─and an isostructural oxo-methylidyne derivative [WO(CH)Cl(IMes)2] (3). The analysis of their solid-state 13C and 183W MAS NMR signatures, along with computed 17O NMR parameters, helps to correlate their electronic structures with NMR patterns and evidences the importance of the competition among the three equatorial ligands in the TBP complexes. Anchored on experimentally obtained NMR parameters for 1, computational analysis of a series of olefin coordination intermediates highlights the interplay between σ- and π-donating ligands in modulating their stability and further paralleling their reactivity. NMR spectroscopy descriptors reveal the origin for the advantage of the dissymmetry in σ-donating abilities of ancillary ligands in Schrock-type catalysts: weak σ-donors avoid the orbital-competition with the oxo ligand upon formation of a TBP olefin-coordination intermediate, while stronger σ-donors compromise M≡O triple bonding and thus render olefin coordination step energy demanding.

Stable cyclopropenylvinyl ligands via insertion into a transient cyclopropenyl metal bond

Treatment of the rhodium pincer complexes [RhCl(RPm)] (RPm = N,N'-bis(di-R-phosphinomethyl)perimidinylidene, R = Ph, Cy) with triphenylcyclopropenium hexafluorophosphate affords rhodacyclobutadiene complexes. These in turn react with activated alkynes (RCCCO2Me, R = H, CO2Me) to afford unusually stable cyclopropenylvinyls, implicating the intermediacy of σ-cyclopropenyl isomers. In contrast, treatment of [RhCl{py(NHPtBu2)2-2,6}] with the same reagent instead results in double functionalisation (SEAr) at the pincer backbone.

Evidence for Carbene Intermediates in Isocyanide Homologation by Aluminium(I)

The C-C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer-Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral AlI complex (Me NacNac)Al (1, Me NacNac=HC[(CMe)(NDipp)]2 , Dipp=2,6-diisopropylphenyl) and various isocyanides. The step-by-step coupling mechanism was studied in detail by low-temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6-bis(benzhydryl)-4-Me-phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri-, tetra-, and even pentamerization products were isolated with the sterically less congested phenyl and p-methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT-type chemistry of aluminium(I) and isocyanides.

Metal-Ligand Cooperativity to Assemble a Neutral and Terminal Niobium Phosphorus Triple Bond (Nb≡P)

Decarbonylation along with P-atom transfer from the phosphaethynolate anion, PCO^- , to the Nb^IV complex [(PNP)NbCl₂ (N^t BuAr)] (1) (PNP=N[2-Pⁱ Pr₂ -4-methylphenyl]₂ ^- ; Ar=3,5-Me₂ C₆ H₃ ) results in its coupling with one of the phosphine arms of the pincer ligand to produce a phosphanylidene phosphorane complex [(PNPP)NbCl(N^t BuAr)] (2). Reduction of 2 with CoCp*₂ cleaves the P-P bond to form the first neutral and terminal phosphido complex of a group 5 transition metal, namely, [(PNP)Nb≡P(N^t BuAr)] (3). Theoretical studies have been used to understand both the coupling of the P-atom and the reductive cleavage of the P-P bond. Reaction of 3 with a two-electron oxidant such as ethylene sulfide results in a diamagnetic sulfido complex having a P-P coupled ligand, namely [(PNPP)Nb=S(N^t BuAr)] (4).

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L ¹ ) with the general formula [NbL ¹ (N ^t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H ₃ L ¹ ][Cl] and subsequent reaction with [Nb(N ^t Bu)Py ₂ Cl ₃ ]. The pyridine ligand in 1-Py can be removed by the addition of B(C₆F₅)₃ as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL ¹ (N ^t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC₈, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including ¹H-, ¹³C{¹H}-, and ¹H-¹⁵N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon−Carbon Bond Cleavage

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [^t BuOCO]W≡C^t Bu(THF)₂ (1) with phosphaalkyne (10) results in the formation of [O₂ C(^t BuC=)W{η² -(P,C)-P≡C-Ad}(THF)] (13-^t Bu_THF ) and [O₂ C(AdC=)W{η² -(P,C)-P≡C-^t Bu}(THF)] (13-Ad_THF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-C_arene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [^t BuOCO]W[κ² -C(^t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-^t Bu_THF or 13-Ad_THF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-^t Bu/Ad_THF and 14.

Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions

Transition metals form a variety of alkylidyne complexes with either a d⁰ metal center (high-valent) or a non-d⁰ metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR⁺. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

Synthesis and reactivity of titanium ‘POCOP’ pincer complexes

Titanium ‘POCOP’ complexes have been made, and their ability to support further reactivity investigated, giving a rare isolable titanium chlorohydride.

Migratory insertion of isocyanide into a ketenyl-tungsten bond as key step in cyclization reactions

Treatment of the side-on tungsten alkyne complex of ethinylethyl ether [Tp*W(CO)2(η2-C,C'-HCCOCH2CH3)]+ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} (2a) with n-Bu4NI afforded the end-on ketenyl complex [Tp*W(CO)2(κ1-HCCO)] (4a). This formal 16 ve complex bearing the prototype of a ketenyl ligand is surprisingly stable and converts only under activation by UV light or heat to form a dinuclear complex [Tp*2W2(CO)4(μ-CCH2)] (6). The ketenyl ligand in complex 4a underwent a metal template controlled cyclization reaction upon addition of isocyanides. The oxametallacycles [Tp*W(CO)2{κ2-C,O-C(NHXy)C(H)C(Nu)O}] {Nu = OMe (7), OEt (8), N(i-Pr)2 (9), OH (10), O1/2 (11)} were formed by coordination of Xy-NC (Xy = 2,6-dimethylphenyl) at 4a and subsequent migratory insertion (MI) into the W-ketenyl bond. The resulting intermediate is susceptible to addition reactions with protic nucleophiles. Compounds 2a-PF6, 4a/b, and 7-11 were fully characterized including XRD analysis. The cyclization mechanism has been confirmed both experimentally and by DFT calculations. In cyclic voltammetry, complexes 7-9 are characterized by a reversible W(ii)/W(iii) redox process. The dinuclear complex 11 however shows two separated redox events. Based on cyclic voltammetry measurements with different conducting electrolytes and IR spectroelectrochemical (SEC) measurements the W(ii)/W(iii) mixed valent complex 11+ is assigned to class II in terms of the Robin-Day classification.

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.

Construction of an iminoketenylidene

The new isonitrile-μ-carbido complexes [WPt(μ-C)Br(CNR)(PPh₃)(CO)₂(Tp*)] (R = C₆H₂Me₃-2,4,6, C₆H₃Me₂-2,6; Tp* = hydrotris(dimethylpyrazolyl)borate) rearrange irreversibly in polar solvents to provide the first examples of iminoketenylidene (CCNR) complexes.

Tethered Tungsten-Alkylidenes for the Synthesis of Cyclic Polynorbornene via Ring Expansion Metathesis: Unprecedented Stereoselectivity and Trapping of Key Catalytic Intermediates

This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand. Treating the tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 (1) with isocyanates (RNCO; R = tBu, Cy, and Ph) leads to cycloaddition occurring exclusively at the C═N bond to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery of [(O2CtBuC═)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(tBuOCO)W(≡NR)(tBuCCO)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = tBu (11-tBu) and is isolated and characterized by single-crystal X-ray diffraction. Finally, adding to the short list of catalysts capable of ring expansion metathesis polymerization (REMP), complexes 6-NR and 11-tBu are active for the stereoselective synthesis of cyclic polynorbornene.

Monoanionic Anilidophosphine Ligand in Lanthanide Chemistry: Scope, Reactivity, and Electrochemistry

We present the synthesis of a series of new lanthanide(III) complexes supported by a monoanionic bidentate anilidophosphine ligand (N-(2-(diisopropylphosphanyl)-4-methylphenyl)-2,4,6-trimethylanilide, short PN^-). The work comprises the characterization of a variety of heteroleptic complexes containing either one or two PN ligands as well as a study on further functionalization possibilities. The new heteroleptic complexes cover selected examples over the whole lanthanide(III) series including lanthanum, cerium, neodymium, gadolinium, terbium, dysprosium, and lutetium. In case of the two diamagnetic metal cations lanthanum(III) and lutetium(III), we have furthermore studied the influence of the lanthanide ion (early vs. late) on the reactivity of these complexes. Thereby we found that the radius of the lanthanide ion has a major influence on the reactivity. Using sterically demanding, multidentate ligand systems, e.g., cyclopentadienide (Cp^-), we found that the lanthanum complex La(PN)₂Cl (1-La) reacts well to the corresponding cyclopentadienide complex, while for Lu(PN)₂Cl (1-Lu) no reaction was observed under any conditions tested. On the contrary, employing monodentate ligands such as mesitolate, thiomesitolate, 2,4,6-trimethylanilide or 2,4,6-trimethylphenylphosphide, results in the clean formation of the desired complexes for both lanthanum and lutetium. All complexes have been studied by various techniques, including multi nuclear NMR spectroscopy and X-ray crystallography. ³¹P NMR spectroscopy was furthermore used to evaluate the presence of open coordination sites on the complexes using coordinating and noncoordinating solvents, and as a probe for estimating the Ce-P distance in the corresponding complexes. Additionally, we present cyclic voltammetry (CV) data for Ce(PN)₂Cl (1-Ce), La(PN)₂Cl (1-La), Ce(PN)(HMDS)₂ (8-Ce) and La(PN)(HMDS)₂ (8-La) (with HMDS = hexamethyldisilazide, (Me₃Si)₂N^-) exploring the potential of the anilidophosphane ligand framework to stabilize a potential Ce(IV) ion.