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. (tBuN)2Cr(NHtBu)Cl can be deprotonated by KN(SiMe3)2, yielding K[(tBuN)3CrCl]. This tris(imido) anion undergoes nucleophilic substitution by PPh3 and tBuNH2 to form (tBuN)3Cr(PPh3) and (tBuN)2Cr(NHtBu)2, respectively. (tBuN)2Cr(NHtBu)2 loses one amido proton to form K[(tBuN)3Cr(NHtBu)] upon reaction with KN(SiMe3)2. The imido ligands of K[(tBuN)3CrCl] and (tBuN)3Cr(PPh3) are attacked by the electrophile MeI to produce (tBuN)2Cr(NMetBu)Cl and (tBuN)2Cr(NMetBu)I, respectively. An alternate way to make tris(imido) anions is deprotonation of (tBuN)2Cr(NHtBu)Cl by an alkyl lithium reagent, e.g., Me3SiCH2Li. The resulting Li[(tBuN)3CrCl] was alkylated by a second equivalent of Me3SiCH2Li to form Li[(tBuN)3Cr(CH2SiMe3)]. Reactivity studies of tris(imido) complexes show cycloaddition with PhNCO or CO2 to form metallacycles.

A molecular substitutional disorder case study suitable for instruction: L2CrII(THF)/L2[(trimethylsilyl)methyl]CrIII (L is 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide).

A solution of CrII and CrIII 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(C30H38N3)2(C4H8O)]0.88[Cr(C30H38N3)2(C4H11Si)]0.12 or L2CrII(THF)/L2[(trimethylsilyl)methyl]CrIII (L = 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide and THF is tetrahydrofuran), in pentane crystallizes in the monoclinic space group P21/c. The structure obtained shows most of the atoms coincident but with THF disordered with the (trimethylsilyl)methyl ligand. Structures with this disorder, involving more than two or three atoms, seem to appear rarely in the literature; however, in this case, the data set is ideal for the crystallographic instruction of molecular substitution disorder.

Structure and Reactivity of Chromium(VI) Alkylidenes.

Bis(arylimido)Cr(VI) dialkyls lacking ß-hydrogen decompose by α-hydrogen abstraction and, upon trapping with triphenylphosphine, yield isolable alkylidene complexes. Two such complexes, namely (ArN)2Cr═CHR(PPh3) (R = tBu, SiMe3), 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. Furthermore, the cis isomer has to be transformed into the trans,trans form and has to be esterified before it is suitable for terephthalate synthesis. To overcome these challenges, we investigated the synthesis of dialkyl muconates by cross-metathesis. The Ru-catalyzed cross-coupling of sorbates with acrylates, which can be bioderived, proceeded selectively to yield diester muconates in up to 41 % yield by using very low catalyst amounts (0.5-3.0 mol %) and no solvent. In the optimized procedure, the muconate precipitated as a solid and was easily recovered from the reaction medium. Analysis by GC-MS and NMR spectroscopy showed that this method delivered exclusively the trans,trans isomer of dimethyl muconate. The Diels-Alder reaction of dimethyl muconate with ethylene was studied in various solvents to obtain 1,4-bis(carbomethoxy)cyclohexene. The cycloaddition proceeded with very high conversions (77-100 %) and yields (70-98 %) in all of the solvents investigated, and methanol and tetrahydrofuran were the best choices. Next, the aromatization of 1,4-bis(carbomethoxy)cyclohexene to dimethyl terephthalate over a Pd/C catalyst resulted in up to 70 % yield in tetrahydrofuran under an air atmosphere. Owing to the high yield of the reaction of dimethyl muconate to 1,4-bis(carbomethoxy)cyclohexene, no separation step was needed before the aromatization. This is the first time that cross-metathesis is used to produce bioderived trans,trans-muconates as precursors to renewable terephthalates, important building blocks in the polymer industry.

FcTp(R) (R = iPr or tBu): 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. A series of thallium(I) and cobalt(II) complexes of the ferrocenyltris(3-R-pyrazolyl)borate ligand [FcTpR; R = isopropyl (iPr) or tert-butyl (tBu)] have been synthesized in order to expand the range of redox-active third-generation scorpionates. These are [ferrocenyltris(3-tert-butylpyrazol-1-yl-κN2)borato]thallium(I), [FeTl(C5H5)(C26H37BN6)], [ferrocenyltris(3-isopropylpyrazol-1-yl-κN2)borato]thallium(I), [FeTl(C5H5)(C23H31BN6)], chlorido[ferrocenyltris(3-tert-butylpyrazol-1-yl-κN2)borato]cobalt(II), [CoFe(C5H5)(C26H37BN6)Cl], [ferrocenyltris(3-tert-butylpyrazol-1-yl-κN2)borato]iodidocobalt(II) benzene disolvate, [CoFe(C5H5)(C26H37BN6)I]·2C6H6, and [ferrocenyltris(3-isopropylpyrazol-1-yl-κN2)borato]iodidocobalt(II), [CoFe(C5H5)(C23H31BN6)I]. The structures demonstrate that the metal coordination site can easily be modified by using bulkier substituents at the pz 3-position.

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. Moreover, the shorter Cr-Cr distances produce a mismatch of the bridging ligand lone pairs and the metal acceptor orbitals, which results in a negative correlation of the Cr-Cr and Cr-N bond distances in both experimental and calculated structures.

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. Although the dinuclear complex [(Tp(tBu,Me)Cr)2(µ-O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

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. These ligands display a reversible three-electron oxidation. The preparations of Tp(CF3,Fc)Tl (hydrotris(3-trifluoromethyl-5-ferrocenylpyrazolyl)borate) and PhTp(Fc) (phenyltris(3-ferrocenylpyrazolyl)borate) are also reported.

An electron transfer series of octahedral chromium complexes containing a redox non-innocent α-diimine ligand.

An electron-transfer series of octahedral α-diimine complexes [((H)L(Cy))3Cr](n+)(BARF)n (n = 2, 1, 0) has been synthesized and crystallographically characterized. Cyclic voltammetry indicated additional formation of [((H)L(Cy))3Cr](3+). The molecular structures suggested that all redox processes were ligand-based. Magnetic moments were consistent with spin ground states of S = 0 for [(H)L(Cy)3Cr](0), S = 1/2 for [(H)L(Cy)3Cr](+1), and S = 1 for [(H)L(Cy)3Cr](+2). The experimental data is consistent with chromium maintaining the +III oxidation state throughout, while being coordinated by varying numbers of neutral diimines ((H)L(Cy)) and diimine radical anions ((H)L(Cy)˙(-)).

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. [C4(NCy)4](2-), whereas hexamerization produces a substituted 1,4-diaza-bicyclo[3.3.0]octadiene dianion. These unprecedented transformations complement the coupling reactions of isoelectronic CO, and they may prove synthetically useful.

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. Instead, the reaction likely proceeds via an unsymmetric binuclear chromium bis(µ-oxo) complex. The latter has been independently prepared and structurally characterized. Its reactivity with O2 is consistent with the proposed mechanism.

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. Their spectroscopic properties and structures are compared with those of related compounds. The molecules of (II)-(V) show hydrogen bonding to either solvent molecules or neighboring complex molecules via amide groups. The title compounds feature the ability to engage other ligands in hydrogen bonding and they show strong electron-withdrawing character. Compound (V) displays voids of ca 800 Å(3) in the crystal structure.

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. One-electron oxidation of 1 and 2 yielded [Cr(III)(bpy(0))2(CH3CO2)2]PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [Cr(III)(DAD(•))3](0) (S = 0) (4) and [Cr(III)(CF3AP(•))3](0) (S = 0) (5) have been synthesized. Both complexes contain three π-radical anion ligands, which derive from one electron reduction of 1,4-bis(cyclohexyl)-1,4-diaza-1,3-butadiene and one electron oxidation of 2-(2-trifluoromethyl)-anilino-4,6-di-tert-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d(3) Cr(III) gives the observed singlet ground states. Reaction of [Cr(II)(CH3CN)6](PF6)2 with 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (PDI(0)) under anaerobic conditions affords dark brown microcrystals of [Cr(III)(PDI(0))(PDI(•))](PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [Cr(III)(PDI)2](3+/2+/1+/0), in which all one-electron transfer processes are ligand-based. By X-ray crystallography, it was shown that 6 possesses a localized electronic structure, such that one ligand is neutral (PDI(0)) and the other is a π-radical monoanion (PDI(•))(1-). Again, it should be highlighted that 6 is not a Cr(II) species. Lastly, the structure of [Cr(III)((Me)bpy(•))3](0) (S = 0) (7, (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three ((Me)bpy(•))(1-) radical anions are present. To further validate our electronic structure assignments, complexes 1-6 were investigated computationally using density functional theory (DFT) and found in all cases to contain a Cr(III) ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.

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. Turning thus to mononuclear metallacycles featuring Cr(III), a chromacyclopentane, chromacyclopentene and chromacyclopentadiene have been prepared as models of catalytic intermediates. Of these, the latter also catalyzes the trimerization of ethylene. These results support the proposal that selective ethylene oligomerization catalysis involves an interplay between Cr(I) ethylene complexes and mononuclear Cr(III) metallacycles.

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 2.931 (5) Å. The alkyl chains form the rungs of a ladder with hydrogen-bonding inter-actions forming the side rails.

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 (1.919-1.962 Å) and elongated C-C bonds (1.315-1.436 Å), consistent with [2+2] cycloaddition reactions. The hydrocarbon adducts are 4-membered metallacycles, the bridging alkynes of which are progressively skewed with respect to the Cr-Cr axis. In contrast, perfluoroalkyne adds across the metal ligand moiety.

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. All of these complexes were fully characterized by conventional spectroscopic methods and most were characterized by single-crystal X-ray crystallographic analysis. Magnetic measurements for all complexes agreed with a high-spin (d(6), S=2) electronic configuration. The halide series enabled the estimation of the covalent radius of iron in these complexes as 1.24 Å.