The Direct Oxidative Addition of O₂ to a Mononuclear Cr(I) Complex Is Spin Forbidden

Mononuclear chromium­(I) alkyne complex (<i>i</i>-Pr₂Ph)₂nacnacCr­(η²-C₂(SiMe₃)₂) (<b>1</b>) reacts rapidly with dioxygen to yield chromium­(V) dioxo species (<i>i</i>-Pr₂Ph)₂nacnacCr­(O)₂ (<b>2</b>). 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

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)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

Chromium Mediated Reductive Coupling of Isonitrile Forms Unusual Heterocycles

The quintuply bonded α-diimine chromium dimer [^HL^iPrCr]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

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)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

The Direct Oxidative Addition of O₂ to a Mononuclear Cr(I) Complex Is Spin Forbidden

Mononuclear chromium­(I) alkyne complex (<i>i</i>-Pr₂Ph)₂nacnacCr­(η²-C₂(SiMe₃)₂) (<b>1</b>) reacts rapidly with dioxygen to yield chromium­(V) dioxo species (<i>i</i>-Pr₂Ph)₂nacnacCr­(O)₂ (<b>2</b>). 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

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,FcTl (hydrotris­(3-trifluoromethyl-5-ferrocenylpyrazolyl)­borate) and PhTp^Fc (phenyltris­(3-ferrocenylpyrazolyl)­borate) are also reported

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 <b>1</b>–<b>7</b> 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^II₂(μ-CH₃CO₂)₄]⁰ with 2,2′-bipyridine (bpy⁰) produced the complex [Cr^III(bpy⁰)­(bpy^•)­(CH₃CO₂)₂]⁰ (<i>S</i> = 1) (<b>1</b>), but in the presence of isopropylamine (ⁱPrNH₂) [Cr^III(bpy^•)­(ⁱPrNH₂)₂(CH₃CO₂)₂]⁰ (<i>S</i> = 1) (<b>2</b>) was obtained. Both <b>1</b> and <b>2</b> contain a Cr^III ion and a single (bpy^•)^1– ligand, so are <i>not</i> low-spin Cr^II species. One-electron oxidation of <b>1</b> and <b>2</b> yielded [Cr^III(bpy⁰)₂(CH₃CO₂)₂]­PF₆ (<i>S</i> = 3/2) (<b>3</b>) in both cases. In addition, the new neutral species [Cr^III(DAD^•)₃]⁰ (<i>S</i> = 0) (<b>4</b>) and [Cr^III(CF₃AP^•)₃]⁰ (<i>S</i> = 0) (<b>5</b>) 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-<i>tert</i>-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d³ 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 (PDI⁰) under anaerobic conditions affords dark brown microcrystals of [Cr^III(PDI⁰)­(PDI^•)]­(PF₆)₂ (<i>S</i> = 1) (<b>6</b>). 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 <b>6</b> possesses a localized electronic structure, such that one ligand is neutral (PDI⁰) and the other is a π-radical monoanion (PDI^•)^1–. Again, it should be highlighted that <b>6</b> is <i>not</i> a Cr^II species. Lastly, the structure of [Cr^III(^Mebpy^•)₃]⁰ (<i>S</i> = 0) (<b>7</b>, ^Mebpy = 4,4′-dimethyl-2,2′-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three (^Mebpy^•)^1– radical anions are present. To further validate our electronic structure assignments, complexes <b>1</b>–<b>6</b> 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 <b>2</b>, <b>4</b>, <b>5</b>, and <b>6</b> by Cr K-edge X-ray absorption spectroscopy