Electronic Structure of Six-Membered N-Heterocyclic Carbenes and Its Heavier Analogues: Reactivity of the Lone Pair versus the Exocyclic Double Bond

Electronic structure of the six-membered N-heterocyclic carbene, silylene, germylene, and stannylene having an exocyclic double bond at the C3 carbon atom as well as the relative reactivity of the lone-pair on the divalent group 14 element and the exocyclic double bond have been studied at the BP86 level of theory with a TZVPP basis set. The geometrical parameters, NICS values, and NBO population analysis indicate that these molecules can be best described as the localized structure 1X(a), where a trans-butadiene (C1-C2-C3-C4) unit is connected with diaminocarbene (N1-X-N2) via N-atoms having a little contribution from the delocalized structure 1X(b). The proton affinity at X is higher than at C4 for 1C, and a reverse trend is observed for the heavier analogues. Hence, the lone pair on a heavier divalent Group 14 element is less reactive than the exocyclic double bond. This is consistent with the argument that, even though the parent six-membered carbene and its heavier analogues are nonaromatic in nature, the controlled and targeted protonation can lead to either the aromatic system 3X having a lone pair on X or the nonaromatic system 2X with readily polarizable C3-C4 π-bond. The energetics for the reaction with BH(3) and W(CO)(6) further suggest that both the lone pair of Group 14 element and the exocyclic double bond can act as Lewis basic positions, although the reaction at one of the Lewis basic positions in 1X does not considerably influence the reactivity at the other. The protonation and adduct formation with BH(3) and W(CO)(5) at X lead to nonaromatic systems whereas similar reactions at C4 lead to aromatic systems due to π-bond polarization at C3-C4. The degree of polarization of the C3-C4 π-bond is maximum in the protonated adduct and reduces in the complexes formed with BH(3) and W(CO)(5).     

Imine hydrolysis and role of a rhodium(I)-imine-amine complex in homogeneous H2-hydrogenation of the imine and a rare example of inequivalent NH2 protons

A Rh-catalyzed, homogeneous hydrogenation of the imine, PhCH(2)N=CHPh, is shown to involve a Rh-imine-amine species that subsequently activates H(2), the amine (benzylamine) being formed via a Rh-catalyzed hydrolysis of the imine by adventitious water. The imine-amine complex, cis-(Rh[P(p-tolyl)(3)](2)(PhCH(2)N=CHPh)(NH(2)CH(2)Ph))PF(6) (2b), is structurally characterized, and the solution (1)H NMR data reveal inequivalent NH(2) protons.     

Cr(i)Cl as well as Cr+ are stabilised between two cyclic alkyl amino carbenes

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)₂CrCl₂ (1) with equivalent amount of KC₈ results in the formation of (cAAC)₂CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C₆H₃(CF₃)₂)₄] resulting in [(cAAC)₂Cr]⁺[B(C₆H₃(CF₃)₂)₄]^–, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.     

Back Cover: Manipulating Intermetallic Charge Transfer for Switchable External Stimulus-Enhanced Water Oxidation Electrocatalysis (Angew. Chem. Int. Ed. 44/2023)

Electrocatalytic processes involving the oxygen evolution reaction (OER) present a kinetic bottleneck due to the existence of linear-scaling relationships, which bind the energies of the different intermediates in the mechanism limiting optimization. Here, we offer a way to break these scaling relationships and enhance the electrocatalytic activity of a Co−Fe Prussian blue modified electrode in OER by applying external stimuli. Improvements of ≈11 % and ≈57 % were achieved under magnetic field (0.2 T) and light irradiation (100 mW cm⁻²), respectively, when working at fixed overpotential, η=0.6 V at pH 7. The observed enhancements strongly tie in with the intermetallic charge transfer (IMCT) intensity between Fe and Co sites. Density Functional Theory simulations suggest that tuning the IMCT can lead to a change of the OER mechanism to an external stimuli-sensitive spin crossover-based pathway, which opens the way for switchable electrocatalytic devices.