Miao Meng

Perturbation of the Charge Density between Two Bridged Mo2 Centers: The Remote Substituent Effects

A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo2(ArNCHNAr)3]2(μ-O2CC6H4CO2) (Ar = p-XC6H4, with X = OCH3 (1), CH3 (2), F (3), Cl (4), OCF3 (5), and CF3 (6)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the para-substituents (X) being about 8 Å away from the Mo2 center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series 1 to 6, the metal-metal bond distances (d(Mo-Mo)) decrease slightly and continuously; the potential separations (ΔE(1/2)) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λ(max)) increase in order. More interestingly, the two types of methine protons, H(∥) on the horizontal and H(⊥) on the vertical ligands with respect to the plane defined by the Mo-Mo bond vectors and bridging ligand, display separate resonant signals δ(∥) and δ(⊥) in the NMR spectra. The displacements of the chemical shifts, Δδ(∥-⊥) = δ(∥) - δ(⊥), are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo2] units function to fine-tune the metal-metal interactions crossing the bridging ligand. The experimental parameters, ΔE(1/2), λ(max), and Δδ(∥-⊥), which are linearly related with the Hammett constants (σ(X)) of the X groups, can be used to probe the charge density on the two [Mo2] units and the electronic delocalization between them.

Crystal structure of bis(μ3-chlorido)-bis(μ2-chlorido)-tetrakis(μ2-2,6-bis (diphenylmethyl)-4-t-butylphenolato)tetralithium(I)dicobalt(II) toluene solvate, C75.5H70Cl2CoLi2O2

Abstract C75.5H70Cl2CoLi2O2, triclinic, P1̅ (no. 2), a = 13.1085(7) Å, b = 13.4704(7) Å, c = 18.8367(10) Å, α = 88.604(5)°, β = 88.255(4)°, γ = 68.282(5)°, V = 3088.3(3) Å3, Z = 1, Rgt(F) = 0.0666, wRref(F2) = 0.1743, T = 150 K.

Divergent Dynamic Kinetic Resolution of Racemic Mixture of Four Stereoisomers via N-Heterocyclic Carbene Organocatalysis

Racemic mixtures of four stereoisomers are easily formed via many fundamental organic transformations, but the direct utilities of these mixtures have been less studied and remain large challenges to date. In this work, we introduce a new method, i.e., divergent dynamic kinetic resolution, to achieve the separation of racemic mixtures of four stereoisomers. The hypothesis was proved by using a N-heterocyclic carbene-catalyzed benzoin reaction, which afforded two separable diastereomeric products bearing three consecutive stereocenters with good to excellent enantioselectivties. We believe that this resolution protocol will find applications in more transformations.