"Catch-Store-Release" Strategy for the Stabilization of 4,4'-Methylene Diphenyl Diisocyanate (4,4'-MDI).

4,4'-Methylene diphenyl diisocyanate (MDI) is an industrially crucial compound, being one of the most utilized linkers in the polyurethane industry. However, its long-term stability is limited due to dimerization to form insoluble uretdione. Herein we demonstrate an organometallic "catch-store-release" concept aiming at improving the long-term chemical stability of MDI. Treatment of MDI with two equivalents of selected N-heterocyclic carbenes (NHC) forms stable MDI-NHC adducts. Treatment of the adducts with CuCl forms metastable di-CuI complexes that undergo decomposition to re-form MDI (up to 85 %), along with Cu-NHC complexes. The yield of re-formed MDI can be improved (up to 95 %) by the release of the NHC ligands in the form of thiourea; this prevents subsequent MDI dimerization/polymerization by the carbenes. Furthermore, the need to separate MDI from the reaction mixture can be eliminated by the direct reaction of MDI-NHC complexes with alcohols (as models for diols), that form dicarbamate (as a model for polyurethane) quantitatively.

Combining [MoVIO3] and [M0(CO)3] (M = Mo, Cr) Fragments within the Same Complex: Synthesis and Reactivity of the Single Oxo-Bridged Heterobimetallics Supported by Xanthene-Based Heterodinucleating Ligands.

A functional model of Mo-Cu carbon monoxide dehydrogenase (CODH) enzyme requires the presence of an oxidant (metal-oxo) and a metal-bound carbonyl in close proximity. In this work, we report the synthesis, characterization, and reactivity of a heterobimetallic complex combining Mo(VI) trioxo with Mo(0) tricarbonyl. The formation of the heterobimetallic complex is facilitated by the xanthene-bridged heterodinucleating ligand containing a hard catecholate chelate and a soft iminopyridine chelate. A catechol-coordinated square-pyramidal [MoVIO3] fragment interacts directly with the iminopyridine-bound [Mo0(CO)3] fragment via a single (oxo) bridge, with the overall disposition being related to the proposed first step in the CODH mechanism, where square-pyramidal [MoVIO2S] interacts with the [Cu-CO] via a single sulfido bridge. Our attempt to obtain a sulfido-bridged analogue (using [MoO3S]2- precursor) led to a mixture of products possibly containing different (oxo and sulfido) bridges. Despite a direct interaction between Mo(VI) and Mo(0) segments, no internal redox is observed, with the high lying occupied MOs being mostly d-π orbitals at Mo0(CO)3 and the low lying unoccupied MOs being d-π orbitals at MoVIO3. Due to the overall rigid structure, the heterobimetallic complex was found to be stable up to 100 °C in DMF-d7 (based on 1H NMR). The decomposition of the complex above this temperature does not produce CO2 (based on gas chromatography), dissociating stable Mo(CO)3(DMF)3 instead (based on IR). We also synthesized and studied the reactivity of the Mo(VI)/Cr(0) analogue. While this complex demonstrated more facile decomposition, no CO2 production was observed. Density functional theory calculations suggest that the formation of [CO2]2- and its subsequent reductive elimination is endergonic in the present system, likely due to the stability of fac-Mo0(CO)3 and the relative nucleophilic character of the carbonyl carbon engendered by back donation from Mo(0). The calculations also indicate that the replacement of one oxo by sulfido (both terminal and bridging), replacement of catechol with dithiolene, and replacement of Mo(0) with Cr(0) does not affect significantly the energetics of the process, likely requiring the use a less stable and less π-basic CO anchor.

Studies Relevant to the Functional Model of Mo-Cu CODH: In Situ Reactions of Cu(I)-L Complexes with Mo(VI) and Synthesis of Stable Structurally Characterized Heterotetranuclear MoVI2CuI2 Complex.

In this study, we report the synthesis, characterization, and reactions of Cu(I) complexes of the general form Cu(L)(LigH2) (LigH2 = xanthene-based heterodinucleating ligand (E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol); L = PMe3, PPh3, CN(2,6-Me2C6H3)). New complexes [Cu(PMe3)(LigH2)] and [CuCN(2,6-Me2C6H3)(LigH2)] were synthesized by treating [Cu(LigH2)](PF6) with trimethylphosphine and 2,6-dimethylphenyl isocyanide, respectively. These complexes were characterized by multinuclear NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry (HRMS), and X-ray crystallography. In contrast, attempted reactions of [Cu(LigH2)](PF6) with cyanide or styrene failed to produce isolable crystalline products. Next, the reactivity of these and previously synthesized Cu(I) phosphine and isocyanide complexes with molybdate was interrogated. IR (for isocyanide) and 31P NMR (for PPh3/PMe3) spectroscopy demonstrates the lack of oxidation reactivity. We also describe herein the first example of a structurally characterized multinuclear complex combining both Mo(VI) and Cu(I) metal ions within the same system. The heterobimetallic tetranuclear complex [Cu2Mo2O4(µ2-O)(Lig)2]·HOSiPh3 was obtained by the reaction of the silylated Mo(VI) precursor (Et4N)(MoO3(OSiPh3)) with LigH2, followed by the addition of [Cu(NCMe)4](PF6). This complex was characterized by NMR spectroscopy, high-resolution mass spectrometry, and X-ray crystallography.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)2bpy vs mes2bpy ligands) and Lewis basic ((Ph)2bpy vs (MeOPh)2bpy vs (MeSPh)2bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO2 reduction mechanisms with [MnI(R2bpy)(CO)3(CH3CN)]+ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)2bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO2 binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO2-saturated electrolyte for the (Ph)2bpy-based precatalyst [2-CH3CN]+ confirms that CO2 binding occurs at the two-electron-reduced activated catalyst [2]- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO2 binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]0 dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO2 binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH3CN]+ maintains a lower overpotential than [2-CH3CN]+ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOFmax is observed at its reduction-first pathway with an almost identical overpotential as [2-CH3CN]+. Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH3CN]+ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH3CN]+ and [2-CH3CN]+. In fact, [3-CH3CN]+ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO2 with 2.8 M (5%) H2O establishes [2-CH3CN]+ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]0 dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH3CN]+ possesses an increased steric presence relative to [2-CH3CN]+, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand.

Treatment of Mn(N(SiMe3)2)2(THF)2 with bulky chelating bis(alkoxide) ligand [1,1':4',1''-terphenyl]-2,2''-diylbis(diphenylmethanol) (H2[O-terphenyl-O]Ph) formed a seesaw manganese(II) complex Mn[O-terphenyl-O]Ph(THF)2, characterized by structural, spectroscopic, magnetic, and analytical methods. The reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various nitrene precursors was investigated. No reaction was observed between Mn[O-terphenyl-O]Ph(THF)2 and aryl azides. In contrast, the treatment of Mn[O-terphenyl-O]Ph(THF)2 with iminoiodinane PhINTs (Ts = p-toluenesulfonyl) was consistent with the formation of a metal-nitrene complex. In the presence of styrene, the reaction led to the formation of aziridine. Combining varying ratios of styrene and PhINTs in different solvents with 10 mol% of Mn[O-terphenyl-O]Ph(THF)2 at room temperature produced 2-phenylaziridine in up to a 79% yield. Exploration of the reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various olefins revealed (1) moderate aziridination yields for p-substituted styrenes, irrespective of the electronic nature of the substituent; (2) moderate yield for 1,1'-disubstituted α-methylstyrene; (3) no aziridination for aliphatic α-olefins; (4) complex product mixtures for the ß-substituted styrenes. DFT calculations suggest that iminoiodinane is oxidatively added upon binding to Mn, and the resulting formal imido intermediate has a high-spin Mn(III) center antiferromagnetically coupled to an imidyl radical. This imidyl radical reacts with styrene to form a sextet intermediate that readily reductively eliminates the formation of a sextet Mn(II) aziridine complex.

Synthesis and Cu(I)/Mo(VI) Reactivity of a Bifunctional Heterodinucleating Ligand on a Xanthene Platform.

In an effort to probe the feasibility of a model of Mo-Cu CODH (CODH = carbon monoxide dehydrogenase) lacking a bridging sulfido group, the new heterodinucleating ligand LH2 was designed and its Cu(I)/Mo(VI) reactivity was investigated. LH2 ((E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol) features two different chelating positions bridged by a xanthene linker: bis(pyridyl)amine for Cu(I) and catecholate for Mo(VI). LH2 was synthesized via the initial protection of one of the amine positions, followed by two consecutive alkylations of the second position, deprotection, and condensation to attach the catechol functionality. LH2 was found to exhibit dynamic cooperativity between two reactive sites mediated by H-bonding of the catechol protons. In the free ligand, catechol protons exhibit H-bonding with imine (intramolecular) and with pyridine (intermolecular in the solid state). The reaction of LH2 with [Cu(NCMe)4]+ led to the tetradentate coordination of Cu(I) via all nitrogen donors of the ligand, including the imine. Cu(I) complexes were characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), X-ray crystallography, and DFT calculations. Cu(I) coordination to the imine disrupted H-bonding and caused rotation away from the catechol arm. The reaction of the Cu(I) complex [Cu(LH2)]+ with a variety of monodentate ligands X (PPh3, Cl-, SCN-, CN-) released the metal from coordination to the imine, thereby restoring imine H-bonding with the catechol proton. The second catechol proton engages in H-bonding with Cu-X (X = Cl, CN, SCN), which can be intermolecular (XRD) or intramolecular (DFT). The reaction of LH2 with molybdate [MoO4]2- led to incorporation of [MoVIO3] at the catecholate position, producing [MoO3(L)]2-. Similarly, the reaction of [Cu(LH2)]+ with [MoO4]2- formed the heterodinuclear complex [CuMoO3(L)]-. Both complexes were characterized by multinuclear NMR, UV-vis, and HRMS. HRMS in both cases confirmed the constitution of the complexes, containing molecular ions with the expected isotopic distribution.

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes.

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]Ph (H2[OO]Ph = [1,1':4',1''-terphenyl]-2,2''-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]Ph(THF)2 (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr2([OO]Ph)2 (in CH2Cl2/THF). The electronic structure and bonding in Cr[OO]Ph(THF)2 were probed by density functional theory calculations. Isolated Cr2([OO]Ph)2 undergoes facile reaction with 4-MeC6H4N3, 4-MeOC6H4N3, or 3,5-Me2C6H3N3 to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]Ph(N(4-MeC6H4))2 was confirmed by X-ray crystallography. The reaction of Cr2([OO]Ph)2 with bulkier azides N3R (MesN3, AdN3) forms paramagnetic products, formulated as Cr[OO]Ph(NR). The attempted formation of a Cr-alkylidene complex (using N2CPh2) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]Ph(NNCPh2)2. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr2([OO]Ph)2 or H2[OO]Ph with NO+ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.

Catalytic Nitrene Homocoupling by an Iron(II) Bis(alkoxide) Complex: Bulking Up the Alkoxide Enables a Wider Range of Substrates and Provides Insight into the Reaction Mechanism.

The reaction of HOR' (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR')2(THF)2 (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR')2(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent 57Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR')2(ArNNNNAr) complexes contain high-spin ( S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR')2(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR')2(ArNNNNAr) with a different azide (N3Ar') produces all three possible products ArNNAr, ArNNAr', and Ar'NNAr'. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.

Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex.

The formally Co(IV) carbene Co(OR)2(═CPh2) is formed upon the reaction of diphenyldiazomethane with the cobalt bis(alkoxide) precursor Co(OR)2(THF)2. Structural, spectroscopic, and theoretical studies demonstrate that Co(OR)2(═CPh2) has significant high-valent Co(IV)═CPh2 character with non-negligible spin density on the carbene moiety.

Synthesis and Reactions of 3d Metal Complexes with the Bulky Alkoxide Ligand [OC(t)Bu2Ph].

Treatment of NiCl2(dme) and NiBr2(dme) (dme = dimethoxyethane) with 2 equiv of LiOR (OR = OC(t)Bu2Ph) forms the distorted trigonal planar complexes [NiLiX(OR)2(THF)2] (THF = tetrahydrofuran) 5 (X = Cl) and 6 (X = Br). The reaction of CuX2 (X = Cl, Br) with 2 equiv of LiOR affords the Cu(I) product Cu4(OR)4 (7). The same product can be obtained using the Cu(I) starting material CuCl. NMR studies indicated that the reduction of Cu(II) to Cu(I) is accompanied by the oxidation of the alkoxide RO(-) to form the alkoxy radical RO(•), which subsequently forms tert-butyl phenyl ketone by ß-scission. Treatment of compounds 1-4 ([M2Li2Cl2(OR)4], M = Cr-Co) with thallium hexafluorophosphate allowed the isolation of the distorted tetrahedral complexes of the form M(OR)2(THF)2 for M = Mn (8), Fe (9), and Co (10). Cyclic voltammetry performed on compounds 8-10 demonstrated irreversible oxidations for all complexes, with the iron complex 9 being the most reducing. Complex 9 shows a reactivity toward PhIO and Ph3SbS to form the corresponding dinuclear iron(III) complexes Fe2(O)(OR)4(THF)2 (11) and Fe2(S)(OR)4(THF)2 (12), respectively. X-ray structural studies were performed, showing that the Fe-O-Fe angle for complex 11 is 176.4(1)° and that the Fe-S-Fe angle for complex 12 is 164.83(3)°.

Efficient carbene transfer reactivity mediated by Fe(II) complexes supported by bulky alkoxides.

Herein we describe the stoichiometric and catalytic carbene-transfer reactivity of iron(II) alkoxide complexes with iodonium ylide precursors. Treatment of PhIC(CO2Me)2 with styrene in the presence of catalytic amounts of several different Fe(OR)2(THF)2 precursors results in efficient cyclopropanation for a variety of styrenes. Computational and reactivity studies suggest a novel remote metallocarbene/vinyl radical intermediate, Fe(OR)2(κ2-(OC(OMe))2C), which could be responsible for the reactive nature of the catalyst.

Novel alkoxide cluster topologies featuring rare seesaw geometry at transition metal centers.

Caution! Chemists playing: Novel clusters of the form [M2Li2Cl2(OR)4] featuring rare seesaw geometry at the transition metal centers were synthesized for M=Cr-Co. The use of sterically hindering alkoxide ligands, as well as the inclusion of lithium ions in the structures enforces this highly unusual configuration.

Iron in a trigonal tris(alkoxide) ligand environment.

Mononuclear Fe(II) and Fe(III) complexes residing in a trigonal tris(ditox) (ditox = (t)Bu2(Me)CO(-)) ligand environment have been synthesized and characterized. The Fe(III) ditox complex does not react with oxidants such as PhIO, whereas NMe3O substitutes a coordinated tetrahydrofuran (THF) in the apical position without undergoing oxo transfer. In contrast, the Fe(II) ditox complex reacts rapidly with PhIO or Me3NO in THF or cyclohexadiene to furnish a highly reactive intermediate, which cleaves C-H bonds to afford the Fe(III)-hydroxide complex. When generated in 1,2-difluorobenze, this intermediate can be intercepted to oxidize phosphines to phosphine oxide. The fast rates at which these reactions occur is attributed to a particularly weak ligand field imparted by the tris(alkoxide) ancillary ligand environment.

Reductive coupling of azides mediated by an iron(II) bis(alkoxide) complex.

The iron(III) hexazene complex (RO)2Fe(µ-κ(2):κ(2)-AdN6Ad)Fe(OR)2 (3) was synthesized via reductive coupling of 1-azidoadamantane at the iron(II) bis(alkoxide) complex Fe(OR)2(THF)2 (2). The X-ray crystal structure depicts electron delocalization within the hexazene moiety. Density functional theory studies propose the formation of an iron azide dimer on the route to hexazene, in which each azide is monoreduced and the iron centers are oxidized to the 3+ oxidation state.

Simple magnesium alkoxides: synthesis, molecular structure, and catalytic behaviour in the ring-opening polymerization of lactide and macrolactones and in the copolymerization of maleic anhydride and propylene oxide.

The synthesis of two chiral bulky alkoxide pro-ligands, 1-adamantyl-tert-butylphenylmethanol HOCAdtBuPh and 1-adamantylmethylphenylmethanol HOCAdMePh, is reported and their coordination chemistry with magnesium(II) is described and compared with the coordination chemistry of the previously reported achiral bulky alkoxide pro-ligand HOCtBu2Ph. Treatment of n-butyl-sec-butylmagnesium with two equivalents of the racemic mixture of HOCAdtBuPh led selectively to the formation of the mononuclear bis(alkoxide) complex Mg(OCAdtBuPh)2(THF)2. 1H NMR spectroscopy and X-ray crystallography suggested the selective formation of the C2-symmetric homochiral diastereomer Mg(OCRAdtBuPh)2(THF)2/Mg(OCSAdtBuPh)2(THF)2. In contrast, the less sterically encumbered HOCAdMePh led to the formation of dinuclear products indicating only partial alkyl group substitution. The mononuclear Mg(OCAdtBuPh)2(THF)2 complex was tested as a catalyst in different reactions for the synthesis of polyesters. In the ROP of lactide, Mg(OCAdtBuPh)2(THF)2 demonstrated very high activity, higher than that shown by Mg(OCtBu2Ph)2(THF)2, although with moderate control degrees. Both Mg(OCAdtBuPh)2(THF)2 and Mg(OCtBu2Ph)2(THF)2 were found to be very effective in the polymerization of macrolactones such as ω-pentadecalactone (PDL) and ω-6-hexadecenlactone (HDL) also under mild reaction conditions that are generally prohibitive for these substrates. The same catalysts demonstrated efficient ring-opening copolymerization (ROCOP) of propylene oxide (PO) and maleic anhydride (MA) to produce poly(propylene maleate).

Catalytic synthesis of azoarenes via metal-mediated nitrene coupling.

Various valuable properties of azoarenes ("azo dyes"), including their vivid colors and their facile cis-trans photoisomerization, lead to their wide use in the chemical industry. As a result, ∼700 000 metric tons of azo dyes are produced each year. Most currently utilized synthetic methods towards azoarenes involve harsh reaction conditions and/or toxic reagents in stoichiometric amounts, which may affect selectivity and produce significant amounts of waste. An efficient alternative method towards this functional group includes transition metal catalyzed nitrene coupling. This method is generally more sustainable compared with most stoichiometric methods as it uses only catalytic amounts of co-reactants (metal catalysts), requires easily synthesizable organoazide precursors, and forms only dinitrogen as a by-product of catalysis. During the last decade, several catalytic systems were reported, and their reactivity was investigated. This perspective article will review these systems, focusing on various nitrene coupling mechanisms, and the substrate scope for each system. Particular attention will be devoted to the iron-alkoxide catalytic systems investigated in the PI's laboratory. The design and structural features of several generations of iron bis(alkoxide) complexes will be discussed, followed by the structure-activity studies of these catalysts in nitrene homo- and heterocoupling.

Synthesis and structure of a new bulky bis-(alkoxide) ligand on a terphenyl platform.

A new sterically bulky chelating bis-(alkoxide) ligand 3,3'-([1,1':4',1''-terphen-yl]-2,2''-di-yl)bis-(2,2,4,4-tetra-methyl-pentan-3-ol), (H2[OO]tBu), was prepared in a two-step process as the di-chloro-methane monosolvate, C36H50O2·CH2Cl2. The first step is a Suzuki-Miyaura coupling reaction between 2-bromo-phenyl-boronic acid and 1,4-di-iodo-benzene. The resulting 2,2''-di-bromo-1,1':4',1''-terphenyl was reacted with t BuLi and hexa-methyl-acetone to obtain the desired product. The crystal structure of H2[OO]tBu revealed an anti conformation of the [CPh2(OH)] fragments relative to the central phenyl. Furthermore, the hydroxyl groups point away from each other. Likely because of this anti-anti conformation, the attempts to synthesize first-row transition-metal complexes with H2[OO]tBu were not successful.

Synthesis, electronic structures, and reactivity of mononuclear and dinuclear low-valent molybdenum complexes in iminopyridine and bis(imino)pyridine ligand environments.

Molybdenum in redox non-innocent ligand environments features prominently in biological inorganic systems. While Holm and coworkers, along with many other researchers, have thoroughly investigated formally high-oxidation-state molybdenum (Mo(IV)-Mo(VI)) ligated by dithiolenes, less is known about molybdenum in other formal oxidation states and/or different redox-active ligand environments. This work focuses on the investigation of low-valent molybdenum in four different redox non-innocent nitrogen ligand type environments (mononucleating and dinucleating iminopyridine, mononucleating and dinucleating bis(imino)pyridine). The reaction of iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L1) with Mo(CO)3(NCMe)3 produced Mo(L1)(CO)3(NCMe). Mo(L1)(CO)3(NCMe) undergoes transformation to Mo(L1)(CO)4 upon treatment with CS2 or prolonged stirring in dichloromethane. The reaction of the open-chain dinucleating bis(iminopyridine) ligand N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine) (L2) similarly produced an hexacarbonyl complex Mo2(L2)(CO)6(NCMe)2 which also underwent transformation to the octacarbonyl Mo2(L2)(CO)8. Both complexes featured anti-parallel geometry of the chelating units. The oxidation of Mo(L1)(CO)3(NCMe) with I2 resulted in Mo(L1)(CO)3I2. The reaction of mononucleating potentially tridentate bis(imino)pyridine ligand (L3) (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) with both Mo(CO)3(NCMe)3 and Mo(CO)4(NCMe)2 produced complexes Mo(L3)(CO)3(NCMe) and Mo(L3)(CO)4 in which L3 was coordinated in a bidentate fashion, with one imino sidearm unbound. The reaction of dinucleating macrocyclic di(bis(imino)pyridine) analogue (L4) led to the similar chemistry of Mo2(L4)(CO)6(NCMe)2 and Mo2(L4)(CO)8 complexes. Treatment of Mo(L3)(CO)3(NCMe) with I2 formed a mono(carbonyl) complex Mo(L3)(CO)I2 in which molybdenum was formally oxidized and L3 underwent coordination mode change to tridentate. The electronic structures of formally Mo(0) complexes in iminopyridine and bis(imino)pyridine ligand environments were investigated by density functional theory calculations.

Synthesis, characterization, and alkoxide transfer reactivity of dimeric Tl2(OR)2 complexes.

Reaction of LiOCtBu2Ph with TlPF6 forms the dimeric Tl2(OCtBu2Ph)2 complex, a rare example of a homoleptic thallium alkoxide complex demonstrating formally two-coordinate metal centers. Characterization of Tl2(OCtBu2Ph)2 by 1H and 13C NMR spectroscopy and X-ray crystallography reveals the presence of two isomers differing by the mutual conformation of the alkoxide ligands, and by the planarity of the central Tl-O-Tl-O plane. Tl2(OCtBu2Ph)2 serves as a convenient precursor to the formation of old and new [M(OCtBu2Ph)n] complexes (M = Cr, Fe, Cu, Zn), including a rare example of T-shaped Zn(OCtBu2Ph)2(THF) complex, which could not be previously synthesized using more conventional LiOR/HOR precursors. The reaction of [Ru(cymene)Cl2]2 with Tl2(OCtBu2Ph)2 results in the formation of a ruthenium(ii) alkoxide complex. For ruthenium, the initial coordination of the alkoxide triggers C-H activation at the ortho-H of [OCtBu2Ph] which results in its bidentate coordination. In addition to Tl2(OCtBu2Ph)2, related Tl2(OCtBu2(3,5-Me2C6H3))2 was also synthesized, characterized, and shown to exhibit similar reactivity with iron and ruthenium precursors. Synthetic, structural, and spectroscopic characterizations are presented.

Pseudotetrahedral d(0), d(1), and d(2) metal-oxo cores within a tris(alkoxide) platform.

Low-coordinate first-row metal complexes of d(0) [vanadium(V)], d(1) [chromium(V)], and d(2) [chromium(IV)] assume the unusual ligand field of a pseudotetrahedron when supported by a tripodal tBu(2)(Me)CO(-) alkoxide framework. Structural, spectroscopic, and reactivity studies, supported by density functional theory calculations, indicate that the d electrons in the chromium(V) and -(IV) oxo complexes reside in metal-oxygen antibonding orbitals, engendering disparate reactivity of the metal-oxo, depending on the number of d electrons present.