Cr−Cr Quintuple Bonds: Ligand Topology and Interplay Between Metal−Metal and Metal−Ligand Bonding

Publisher's PDF.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.University of Delaware. Center for Catalytic Science & Technology

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

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

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