Poly(imidazolyliden-yl)borato Complexes of Tungsten: Mapping Steric vs. Electronic Features of Facially Coordinating Ligands

A convenient synthesis of [HB(HImMe)3](PF6)2 (ImMe = N-methylimidazolyl) is decribed. This salt serves in situ as a precursor to the tris(imidazolylidenyl)borate Li[HB(ImMe)3] pro-ligand upon deprotonation with nBuLi. Reaction with [W(≡CC6H4Me-4)(CO)2(pic)2(Br)] (pic = 4-picoline) affords the carbyne complex [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}]. Interrogation of experimental and computational data for this compound allow a ranking of familiar tripodal and facially coordinating ligands according to steric (percentage buried volume) and electronic (νCO) properties. The reaction of [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}] with [AuCl(SMe2)] affords the heterobimetallic semi-bridging carbyne complex [WAu(μ-CC6H4Me-4)(CO)2(Cl){HB(ImMe)3}].

Tris(carbene)borates; alternatives to cyclopentadienyls in organolanthanide chemistry

The chemistry of the tris-carbene anion phenyltris(3-alkyl-imidazoline-2-yliden-1-yl)borate, [C3Me]- ligand, is initiated for f-block metal cations. Neutral, molecular complexes of the form Ln(C3)2I are formed for cerium(III), while a separated ion pair [Ln(C3)2]I forms for ytterbium(III). DFT/QTAIM computational analyses of the complexes and related tridentate tris(pyrazolyl)borate (Tp) - supported analogs demonstrates the anticipated strength of the σ donation and confirms greater covalency in the metal-carbon bonds of the [C3Me]- complexes in comparison with those in the TpMe,Me complexes. The DFT calculations demonstrate the crucial role of THF solvent in accurately reproducing the contrasting molecular and ion-pair geometries observed experimentally for the Ce and Yb complexes.

Mechanistic Study for the Reaction of B12 Complexes with m-Chloroperbenzoic Acid in Catalytic Alkane Oxidations

The oxidation of alkanes with m-chloroperbenzoic acid (mCPBA) catalyzed by the B₁₂ derivative, heptamethyl cobyrinate, was investigated under several conditions. During the oxidation of cyclohexane, heptamethyl cobyrinate works as a catalyst to form cyclohexanol and cyclohexanone at a 0.67 alcohol to ketone ratio under aerobic conditions in 1 h. The reaction rate shows a first-order dependence on the [catalyst] and [mCPBA] while being independent of [cyclohexane]; V_obs = k₂[catalyst][mCPBA]. The kinetic deuterium isotope effect was determined to be 1.86, suggesting that substrate hydrogen atom abstraction is not dominantly involved in the rate-determining step. By the reaction of mCPBA and heptamethyl cobyrinate at low temperature, the corresponding cobalt(III)acylperoxido complex was formed which was identified by UV-vis, IR, ESR, and ESI-MS studies. A theoretical study suggested the homolysis of the O-O bond in the acylperoxido complex to form Co(III)-oxyl (Co-O^•) and the m-chlorobenzoyloxyl radical. Radical trapping experiments using N-tert-butyl-α-phenylnitrone and CCl₃Br, product analysis of various alkane oxidations, and computer analysis of the free energy for radical abstraction from cyclohexane by Co(III)-oxyl suggested that both Co(III)-oxyl and the m-chlorobenzoyloxyl radical could act as hydrogen-atom transfer reactants for the cyclohexane oxidation.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

Activation of Molecular Oxygen by a Cobalt(II) Tetra-NHC Complex*

The first dicobalt(III) μ₂‐peroxo N‐heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16‐membered macrocyclic tetra‐NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end‐on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding Co^III acetate complex going along with H₂O₂ formation. In contrast, both Co^II and Co^III complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT‐calculations, suggesting a nucleophilic nature of the dicobalt(III) μ₂‐peroxo complex. All isolated compounds are characterized by NMR, ESI‐MS, elemental analysis, EPR and SC‐XRD.

Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex

The photofunctionality of the cobalt–hexacarbene complex [Co(III)(PhB(MeIm)₃)₂]⁺ (PhB(MeIm)₃ = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ∼ 10^–4) but remarkably long-lived (τ ∼ 1 μs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d⁶ complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.

New phosphorescent iridium(III) dipyrrinato complexes: synthesis, emission properties and their deep red to near-infrared OLEDs

A series of heteroleptic Ir(iii) complexes composed of two cyclometalated C^N ligands and one dipyrrinato ligand used as an ancillary ligand are synthesized and characterized. With the introduction of a fluorine atom, phenyl ring or diphenylamino group into both C^N ligands and by keeping the ancillary ligand unchanged, these Ir(iii) dipyrrinato phosphors do not show an obvious shift in their emission bands. They exhibit emissions extending well into the near-infrared region with an intense band located at around 685 nm in both photo- and electroluminescence spectra, and the deep red to near-infrared organic light emitting diodes (OLEDs) based on them afforded a maximum external quantum efficiency of 2.8%. Density functional theory (DFT) calculations show that both the electronic contributions on the lowest unoccupied molecular orbitals (LUMOs) and the highest energy semi-occupied molecular orbitals (HSOMOs) are mainly localized on the dipyrrinato ligand, indicating that the ancillary ligand, which remains unchanged in this series, exhibits a lower triplet state energy in the iridium phosphors than those involving the C^N ligands. Therefore a switch from "(C^N)2Ir" to dipyrrinato ligand-based emission is observed in these iridium(iii) complexes.