Synthesis, Characterization, and Reactivity of Tris(imido)chromium(VI) Complexes.

Multiple tris(imido)chromium(VI) complexes, including neutral and ionic compounds, have been synthesized and characterized. (BuN)Cr(NHBu)Cl can be deprotonated by KN(SiMe), yielding K[(BuN)CrCl]. This tris(imido) anion undergoes nucleophilic substitution by PPh and BuNH to form (BuN)Cr(PPh) and (BuN)Cr(NHBu), respectively.

A molecular substitutional disorder case study suitable for instruction: LCr(THF)/L[(trimethylsilyl)methyl]Cr (L is 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide).

A solution of Cr and Cr complexes, bis(2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ido)(tetrahydrofuran)chromium(II)-bis(2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ido)[(trimethylsilyl)methyl]chromium(III) (0.88/0.12), [Cr(CHN)(CHO)][Cr(CHN)(CHSi)] or LCr(THF)/L[(trimethylsilyl)methyl]Cr (L = 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide and THF is tetrahydrofuran), in pentane crystallizes in the monoclinic space group P2/c.

Structure and Reactivity of Chromium(VI) Alkylidenes.

Bis(arylimido)Cr(VI) dialkyls lacking β-hydrogen decompose by α-hydrogen abstraction and, upon trapping with triphenylphosphine, yield isolable alkylidene complexes. Two such complexes, namely (ArN)Cr═CHR(PPh) (R = Bu, SiMe), have been structurally characterized. The coordinatively unsaturated alkylidene intermediates are highly reactive; they effect C-H activation of saturated hydrocarbons and they react with olefins to produce metallacyclobutanes.

Bioderived Muconates by Cross-Metathesis and Their Conversion into Terephthalates.

Polyethylene terephthalate that is 100 % bioderived is in high demand in the market guided by the ever-more exigent sustainability regulations with the challenge of producing renewable terephthalic acid remaining. Renewable terephthalic acid or its precursors can be obtained by Diels-Alder cycloaddition and further dehydrogenation of biomass-derived muconic acid. The cis,cis isomer of the dicarboxylic acid is typically synthesized by fermentation with genetically modified microorganisms, a process that requires complex separations to obtain a high yield of the pure product.

FcTp(R) (R = Pr or Bu): third-generation ferrocenyl scorpionates.

Scorpionate (or trispyrazolylborate) ligands have seen much structural variation due to the relative ease of modifying their electronic and steric effects. Second-generation scorpionates were created by increasing the bulk in the 3-position of the pyrazole (pz) ring. A new class of third-generation scorpionates was obtained by modifying the remaining boron substituent.

Cr-Cr Quintuple Bonds: Ligand Topology and Interplay Between Metal-Metal and Metal-Ligand Bonding.

Chromium-chromium quintuple bonds seem to be approaching the lower limit for their bond distances, and this computational density functional theory study tries to explore the geometrical and electronic factors that determine that distance and to find ways to fine-tune it via the ligand choice. While for monodentate ligands the Cr-Cr distance is predicted to shorten as the Cr-Cr-L bond angle increases, with bridging bidentate ligands the trend is the opposite, since those ligands with a larger number of spacers between the donor atoms favor larger bond angles and longer bond distances. Compared to Cr-Cr quadruple bonds, the quintuple bonding in Cr2L2 compounds (with L a bridging bidentate N-donor ligand) involves a sophisticated mechanism that comprises a positive pyramidality effect for the σ and one π bond, but a negative effect for one of the δ bonds.

Dioxygen Activation by Non-Adiabatic Oxidative Addition to a Single Metal Center.

A chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [Tp(tBu,Me)Cr(V)(O)2] (Tp(tBu,Me) = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp(tBu,Me)Cr(III)(S2)]. The transformation of the putative peroxo intermediate [Tp(tBu,Me)Cr(III)(O2)] (S = 3/2) into [Tp(tBu,Me)Cr(V)(O)2] (S = 1/2) is spin-forbidden. The minimum-energy crossing point for the two potential energy surfaces has been identified.

Mechanism-based design of labile precursors for chromium(I) chemistry.

Dinitrogen complexes of the type Tp(R,R)Cr-N2-CrTp(R,R) are not the most labile precursors for Cr(i) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex - Tp(tBu,Me)Cr(η(2)-C2(SiMe3)2) - proved to be much more reactive.

Ferrocenyl-substituted tris(pyrazolyl)borates--a new ligand type combining redox activity with resistance to hydrogen atom abstraction.

The low-temperature syntheses of ferrocenyl-substituted tris(pyrazolyl)borate ligands Tp(Fc*) (hydrobis(3-ferrocenylpyrazolyl)mono(5-ferrocenylpyrazolyl)borate), Tp(Fc,Me*) (hydrobis(3-ferrocenyl-5-methylpyrazolyl)mono(5-ferrocenyl-3-methylpyrazolyl)borate), and Tp(Fc,iPr) (hydrotris(3-ferrocenyl-5-isopropylpyrazolyl)borate) are reported. The Tl salts of Tp(Fc*) and Tp(Fc,Me*) can be thermally isomerized to the symmetric Tp(Fc) (hydrotris(3-ferrocenylpyrazolyl)borate) and Tp(Fc,Me) (hydrotris(3-ferrocenyl-5-methylpyrazolyl)borate) species, respectively. Conversely, upon heating, the thermal isomerization of Tp(Fc,iPr) results in the generation of a mixture of regioisomers.

Chromium mediated reductive coupling of isonitrile forms unusual heterocycles.

The quintuply bonded α-diimine chromium dimer [(H)L(iPr)Cr]2 reductively couples cyclohexyl isocyanide to produce various novel nitrogen heterocycles. Tetramerization yielded, inter alia, the aromatic squaramidinate, i.e.

Binding and activation of small molecules by a quintuply bonded chromium dimer.

The quintuply bonded [(H)L(iPr)Cr]2 reacts with various small molecules, revealing a pattern of two kinds of transformations. Unsaturated molecules that are neither polar nor oxidizing form binuclear [2+n] cycloaddition products retaining Cr-Cr quadruple bonds. In contrast, polar or oxidizing molecules effect the complete cleavage of the Cr-Cr bond.

The direct oxidative addition of O2 to a mononuclear Cr(I) complex is spin forbidden.

Mononuclear chromium(I) alkyne complex (i-Pr2Ph)2nacnacCr(η(2)-C2(SiMe3)2) (1) reacts rapidly with dioxygen to yield chromium(V) dioxo species (i-Pr2Ph)2nacnacCr(O)2 (2). The mechanism of this oxygen cleavage has been studied experimentally and computationally. Isotope labeling studies rule out a direct four-electron oxidative addition of O2 to one chromium atom, which involves a spin-forbidden transformation.

Improved syntheses, and structural and electronic characterization of carboxamide-substituted Tp(CONHPh,Me) and Tp(CONHt-Bu,Me) ligands.

Improvements in the syntheses of the carboxamide-substituted tris(pyrazolyl)borate ligands Tp(CONHPh,Me) [tris(3-anilinocarbonyl-5-methylpyrazol-1-yl)borate] and Tp(CONHt-Bu,Me) [tris(3-tert-butylaminocarbonyl-5-methylpyrazol-1-yl)borate] are reported. Their Tl(I) salts, namely [tris(3-anilinocarbonyl-5-methylpyrazol-1-yl-κN(2))borato]thallium(I), [Tl(C33H31BN9O3)], (II), and [tris(3-tert-butylaminocarbonyl-5-methylpyrazol-1-yl-κN(2))borato]thallium(I), [Tl(C27H43BN9O3)], (III), as well as the Cu(I) carbonyl complexes (Tp(CONHPh,Me))Cu(CO), namely carbonyl[tris(3-anilinocarbonyl-5-methylpyrazol-1-yl-κN(2))borato]copper(I) tetrahydrofuran trisolvate, [Cu(C33H31BN9O3)(CO)]·3C4H8O, (IV), and (Tp(CONHt-Bu,Me))Cu(CO), namely carbonyl[tris(3-tert-butylaminocarbonyl-5-methylpyrazol-1-yl-κN(2))borato]copper(I) tetrahydrofuran hemisolvate, [Cu(C27H43BN9O3)(CO)]·0.5C4H8O, (V), have been prepared.

New complexes of chromium(III) containing organic π-radical ligands: an experimental and density functional theory study.

The electronic structures of a series of chromium complexes 1-7 have been experimentally investigated using a combination of X-ray crystallography, magneto- and electrochemistry, and Cr K-edge X-ray absorption and UV-vis spectroscopies. Reaction of the dimer [Cr(II)2(μ-CH3CO2)4](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr(III)(bpy(0))(bpy(•))(CH3CO2)2](0) (S = 1) (1), but in the presence of isopropylamine ((i)PrNH2) [Cr(III)(bpy(•))((i)PrNH2)2(CH3CO2)2](0) (S = 1) (2) was obtained. Both 1 and 2 contain a Cr(III) ion and a single (bpy(•))(1-) ligand, so are not low-spin Cr(II) species.

A well-defined model system for the chromium-catalyzed selective oligomerization of ethylene.

The chromium(I) dinitrogen complex [(i-Pr2Ph)2nacnacCr]2(μ-η(2):η(2)-N2) catalyzes the selective trimerization of ethylene to 1-hexene at ambient pressure and temperature, and in the absence of any cocatalyst. After the conversion of the substrate, the catalyst cleanly converts to another chromium(I) species, namely [(i-Pr2Ph)2nacnacCr]2(μ-η(2):η(2)-C2H4), which is not catalytically active. Binuclear metallacycles containing Cr(II) have been prepared as candidates for catalytically active intermediates; however they are not kinetically competent to explain the catalysis.

Guanidinium 2-(myristoylsulfanyl)ethane-sulfonate.

In the title compound, CH(6)N(3) (+)·C(16)H(31)O(4)S(2) (-) [systematic name: guanidinium 2-(tetra-deca-noylsulfan-yl)ethane-sulfon-ate], each 2-(myristoyl-thio)-ethane-sulfonate ion displays hydrogen bonding to three guanidinium counter-ions, which themselves display hydrogen bonding to two symmetry-related 2-(myristoylthio)ethanesulfonate ions. Thus each cation forms six N-H⋯O bonds to neighboring anions, thereby self-assembling an extended ladder-type network. The average hydrogen-bond donor-acceptor distance is 2.

Reactions of a quintuply bonded chromium dimer with alkynes.

Quintuply bonded [(H)L(iPr)Cr](2) reacts with alkynes RC≡CR (R = Me, Et, Ph, CF(3)) to form exclusively 1 : 1 adducts [(H)L(iPr)Cr](2)(RCCR). All products feature relatively short Cr-Cr distances (1.919-1.

A family of four-coordinate iron(II) complexes bearing the sterically hindered tris(pyrazolyl)borato ligand Tp(tBu,Me).

A new family of 14-electron, four-coordinate iron(II) complexes of the general formula [Tp(tBu,Me)FeX] (Tp(tBu,Me) is the sterically hindered hydrotris(3-tert-butyl-5-methyl-pyrazolyl) borate ligand and X=Cl (1), Br, I) were synthesized by salt metathesis of FeX(2) with Tp(tBu,Me)K. The related fluoride complex was prepared by reaction of 1 with AgBF(4). Chloride 1 proved to be a good precursor for ligand substitution reactions, generating a series of four-coordinate iron(II) complexes with carbon, oxygen, and sulphur ligands.

Synthesis, characterization, and electronic structure of diimine complexes of chromium.

We have prepared and structurally characterized several complexes of chromium coordinated by diimine (or 1,4-diazadiene) ligands, that is, Ar-N=C(R)-(R)C=N-Ar (RL(Ar)) (where Ar = 2,6-diisopropylphenyl ("iPr") or 2,6-dimethylphenyl ("Me") and R = H or Me). The reaction of CrCl2 with HLiPr gave dinuclear [(HLiPr)Cr]2(mu-Cl)3(Cl)(THF) when isolated from Et2O; in THF solution, however, the product exists as mononuclear (HLiPr)CrCl2(THF)2. Two isostructural derivatives, (MeLMe)CrCl2(THF)2 and (HL(Me))CrCl2(THF)2, have also been prepared.