Dioxo-, Oxo-imido-, and Bis-imido-Molybdenum(VI) Complexes with a Bis-phenolate-NHC Ligand

Diverse Post-Metathesis Reactivities of Mixed Amido-Isothiocyanato Molybdenum Complexes

Attempts to prepare mixed isothiocyanato-bis(imido) MoVI complexes led to the discovery of post-metathesis rearrangements toward three distinct products (1-3), which feature the NCS-derived chelators [N(NMe2)CS]2- (L1 in dinuclear 1 and 2) and [N(SiMe3)(NMe2)CS]- (L2 in mononuclear 3). Notably, the preparation of bidentate ligand L1 and its coordination chemistry are unprecedented. Together with computational studies, it is proposed that the putative "mono-substituted" intermediate [Mo(NtBu)2(NMe2)(NCS)] serves as the common starting point for the observed molecular transformations. Construction of the [Mo(NtBu)2(NCS)2] core was ultimately possible in the presence of additional stabilizing donors (THF or PMe3), which yielded the complexes [Mo(NtBu)2(NCS)2(THF)2] (4) and [Mo(NtBu)2(NCS)2(PMe3)2] (5).

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

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.

Origin of Decomposition in a Family of Molybdenum Precursor Compounds

The bis(tert-butylimido)-molybdenum(VI) framework has been used successfully in the design of vapor-phase precursors for molybdenum-containing thin films, so understanding its thermal behavior is important for such applications. Here, we report the thermal decomposition mechanism for a series of volatile bis(alkylimido)-dichloromolybdenum(VI) adducts with neutral N,N'-chelating ligands, to probe the stability and decomposition pathways for these molecules. The alkyl groups explored were tert-butyl, tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine). We also report the synthesis of the new tert-octyl imido adducts, (^tOctN)₂MoCl₂·L (L = N,N,N',N'-tetramethylethylenediamine or 2,2'-bipyridine), which have been fully characterized by spectroscopic techniques as well as single-crystal X-ray diffraction and thermal analysis. We found that the decomposition of all compounds follows the same general pathway, proceeding first by the dissociation of the chelating ligand to give the coordinatively unsaturated species (RN)₂MoCl₂. Subsequent dimerization results in either an imido bridged adduct, [(RN)Mo(μ-NR)Cl₂]₂, or a chloride bridged adduct, [(RN)₂Mo(μ-Cl)Cl]₂, depending on the size of the R group. The dimeric species then likely undergoes an intramolecular γ-hydrogen transfer to yield a nitrido-amido adduct, (RHN)MoNCl₂, and an alkene. Ultimately, the resulting molybdenum species appears to decompose into free tert-alkylamine and Mo₂N or Mo₂C. The thermolysis reactions have been monitored using ¹H NMR spectroscopy, and the volatile decomposition products were analyzed using gas chromatography-mass spectrometry. A key intermediate has also been detected using electron ionization high-resolution mass spectrometry. Finally, a detailed computational investigation supports the mechanism outlined above and helps explain the relative stabilities of different N,N'-chelated bis(alkylimido)-dichloromolybdenum(VI) adducts.

Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations

Molybdenum(VI) bis(imido) complexes [Mo(NtBu)₂ (L^R )₂ ] (R=H 1 a; R=CF₃ 1 b) combined with B(C₆ F₅ )₃ (1 a/B(C₆ F₅ )₃ , 1 b/B(C₆ F₅ )₃ ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H₂ and Et₃ SiH affords ion pairs [Mo(NtBu)(NHtBu)(L^R )₂ ][HB(C₆ F₅ )₃ ] (R=H 2 a; R=CF₃ 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H₂ O leads to [Mo(NtBu)(NHtBu)(L^R )₂ ][(HO)B(C₆ F₅ )₃ ] (R=H 3 a; R=CF₃ 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B-H bond of [HB(C₆ F₅ )₃ ]^- . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L^R )₂ ][{PhCH₂ O}B(C₆ F₅ )₃ ] (R=H 4 a; R=CF₃ 4 b). Catalysis occurs at [HB(C₆ F₅ )₃ ]^- while [Mo(NtBu)(NHtBu)(L^R )₂ ]⁺ (R=H or CF₃ ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).

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).

Deoxygenation reactions in organic synthesis catalyzed by dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes have been applied as efficient, inexpensive and benign catalysts to deoxygenation reactions of a diverse number of compounds in the last two decades. Dioxomolybdenum complexes have demonstrated wide applicability to the deoxygenation of sulfoxides into sulfides and reduction of N-O bonds. Even the challenging nitro functional group was efficiently deoxygenated, affording amines or diverse heterocycles after reductive cyclization reactions. More recently, carbon-based substrates like epoxides, alcohols and ketones have been successfully deoxygenated. Also, dioxomolybdenum complexes accomplished deoxydehydration (DODH) reactions of biomass-derived vicinal 1,2-diols, affording valuable alkenes. The choice of the catalytic systems and reductant is decisive to achieve the desired transformation. Commonly found reducing agents involved phosphorous-based compounds, silanes, molecular hydrogen, or even glycols and other alcohols.

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.

Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N₂ reduction, which may result in the generation of coordinatively unsaturated iron sites with two sulfur donors and a carbon donor. In an effort to synthesize and study small-molecule model complexes with a one-carbon/two-sulfur coordination environment, we have designed two new SCS pincer ligands containing a central NHC donor accompanied by thioether- or thiolate-functionalized aryl groups. Metalation of the thioether ligand with Fe(OTf)₂ gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate ligand with Fe(HMDS)₂ gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the amide complex with phenylhydrazine gives a product with a rare κ¹-bound phenylhydrazido group which undergoes N-N cleavage to give a phenylamido complex.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

The bis(imido) complexes (BDI)Nb(N t Bu)2 and (BDI)Nb(N t Bu)(NAr) (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes via 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react via insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N t Bu)2 with tert-butylacetylene yielded an acetylide-amido-imido complex. In contrast to these results, many related mono(imido) Nb BDI complexes do not exhibit 1,2-addition reactivity, suggesting that π-loading plays an important role in activating the Nb-N π-bonds toward addition. The same bis(imido) complexes were also shown to engage in [2 + 2] cycloaddition reactions with oxygen- and sulfur-containing heteroallenes to give carbamate- and thiocarbamate-imido complexes: some of these complexes readily dimerized to give bis-μ-sulfido, bis-μ-iminodicarboxylate, and bis-μ-carbonate complexes. The mononuclear carbamate imido complex (BDI)Nb(NAr)(N( t Bu)CO2) (12) could be induced to eject tert-butylisocyanate to generate a four-coordinate terminal oxo imido intermediate, which could be trapped as the five-coordinate pyridine or DMAP adduct. The DMAP adducted oxo imido complex (BDI)NbO(NAr)(DMAP) (16) was shown to engage in 1,2-addition of silanes across the Nb-oxo π-bond; this represents a new reaction pathway in group 5 chemistry.

Molybdenum(VI) bis-imido Complexes of Dipyrromethene Ligands

We report the synthesis of high-valent molybdenum(VI) bis-imido complexes 1-4 with dipyrromethene (DPM) supporting ligands of the general formula (DPM^R)Mo(NR')₂Cl (R, R' = mesityl (Mes) or tert-butyl (^tBu)). The electrochemical and chemical properties of 1-4 reveal unexpected ligand noninnocence and reactivity. ¹⁵N NMR spectroscopy is used to assess the electronic properties of the imido ligands in the tert-butyl complexes 1 and 3. Complex 1 is inert toward ligand (halide) exchange with bulky phenolates such as KOMes or amides (e.g., KN(SiMe₃)₂), whereas the use of the lithium alkyl LiCH₂SiMe₃ results in a rare nucleophilic β-alkylation of the DPM ligand. While the reductions of the complexes occur at molybdenum, the oxidation is centered at the DPM ligand. Quantum-chemical calculations (complete active space self-consistent field, density functional theory) suggest facile (near-infrared) interligand charge transfer to the imido ligand, which might preclude the isolation of the oxidized complex [1]⁺ in the experiment.

Facile Activation of Triarylboranes by Rhenium(V) Oxo Imido Complexes

We report the synthesis and reactivity studies of a pair of rhenium(V) oxo imido complexes. Oxidation of the rhenium(III) terminal oxo ORe(η²-DHF)(BDI) (DHF = dihydrofulvalene, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) with organic azides R-N₃ (R = ^tBu, 2,6-diisopropylphenyl) yields the title complexes. Computational studies confirm that the rhenium oxo moieties of these complexes are polarized and correspondingly nucleophilic, owing to the preferential π bonding of the imido ligand to the Re center. This asymmetry in the metal-ligand multiple bond electronic structure facilitates the ready activation of B-C bonds in triarylboranes (BPh₃ and B(C₆F₅)₃), yielding rhenium(V) aryl borinate complexes. In the case of BPh₃, subsequent cyclometalation of the 1,2-addition products was found to take place upon heating, ejecting benzene to form bidentate diphenylborinate complexes.

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

The chemistry of NHCcore pincer ligands of LX₂ type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R₃P⁺-CR₂^-) or by a heteroatom through amide (R₂N^-), oxide (RO^-), or thio(seleno)oxide (RS^-, RSe^-) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species.

An Iron Pincer Complex in Four Oxidation States

The synthesis and characterization of a series of homoleptic iron complexes [Fe(^benzNHCOCO)₂]^2-/1-/0/1+ supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand ^benzNHCOCO^2- (II) is presented. While the reaction of 2 equiv of free ligand II with a ferrous iron precursor leads to the isolation of the coordination polymer [Fe(^benzNHCOCOK)₂]_n (1), treatment of II with ferric iron salts allows for the synthesis and isolation of the mononuclear, octahedral bis-pincer compound K[Fe(^benzNHCOCO)₂] (2) and its crown-ether derivative [K(18c6)(THF)₂][Fe(^benzNHCOCO)₂] (3). Electrochemical studies of 2 suggested stable products upon further one- and two-electron oxidation. Hence, treatment of 2 with 1 equiv of AgPF₆ yields the charge-neutral species [Fe(^benzNHCOCO)₂] (4). Similarly, the cationic complex [Fe(^benzNHCOCO)₂]PF₆ (5) is obtained by addition of 2 equiv of AgPF₆. The characterization of complexes 1, 3, and 4 reveals iron-centered reduction and oxidation processes; thus, preserving the dianionic, closed-shell structure of both coordinated ^benzNHCOCO pincer chelates, II. This implies a stabilization of a highly Lewis acidic iron(IV) center by four phenolate anions rather than charge distribution across the ligand framework with a lower formal oxidation state at iron. Notably, the overall charge-neutral iron(IV) complex undergoes reductive elimination of the pincer ligand, providing a metal-free compound that can be described as a spirocyclic imidazolone ketal (6). In contrast, the ligand-metal bonds in 5, formally an iron(V) complex, are considerably covalent, rendering the assignment of its oxidation state challenging, if not impossible. All compounds are fully characterized, and the complexes' electronic structures were studied with a variety of spectroscopic and computational methods, including single-crystal X-ray diffraction (SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field ⁵⁷Fe Mössbauer spectroscopy, variable-field and variable-temperature superconducting quantum interference device (SQUID) magnetization measurements, and multi-reference ab initio (NEVPT2/CASSCF) as well as density functional theory (DFT) studies. Taken altogether, the electronic structure of 5 is best described as an iron(IV) center antiferromagnetically coupled to a ligand-centered radical.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.