The N-heterocyclic carbene–ytterbium(II) amides (NHC)₂Yb[N(SiMe₃)₂]₂ (1: NHC: 1,3,4,5-tetramethylimidazo-2-ylidene (IMe₄); 2: NHC: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr)) and the NHC-stabilized rare-earth phosphide (IMe₄)₃Yb(PPh₂)₂ (3) have been synthesized and fully characterized. Complexes 1–3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC-free amide (THF)₂Yb[N(SiMe)₂]₂. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare-earth complex that can be recovered after catalytic reactions.

The novel boron persulfide 2 LB(η²-S₂) (L=[ArNC(R)CHC(R)]⁻; Ar=2,6-Me₂C₆H₃, R=tBu) was obtained by the reaction of the 2-chloro-azaborolyl anion 1 (LBCl)K(THF) with 0.25 equiv of elemental sulfur (S₈). Persulfide 2 is labile in solution and could be converted to the cyclic tetrasulfide LBS₄ (3) and hexasulfide LBS₆ (4) in the presence of sulfur at room temperature and 50 °C, respectively. Desulfination of 2 with triphenylphosphine resulted in the formation of the thioxoborane LB=S (5). Alternatively, 3 and 4 could be obtained by the reaction of 1 with an excess of sulfur. Structural analysis of 2 disclosed the relatively long S−S bond of 2.1004(8) Å due to the lone-pair repulsions of the two sulfur atoms, as disclosed by DFT calculations.

Cs₂CO₃ has been found to be an efficient and chemoselective catalyst for reduction of aldehydes and ketones to alcohols with one equivalent of Ph₂SiH₂ as the reductant under solvent-free conditions. Most of the aldehydes employed can be effectively hydrosilated quantitatively to give the corresponding silyl ethers in 2 h at room temperature, whereas the hydrosilylation of ketones proceeded smoothly at 80 °C. The catalyst system tolerates a number of functional groups including halogen, alkoxyl, olefin, ester, nitro, cyano, and heteroaromatic groups; the selective hydrosilylation of aldehydes in the presence of ketone can be effectively controlled by temperature; and hydrosilylation of α,β-unsaturated carbonyls resulted in the 1,2-addition products. The catalytic hydrosilylation of suitable dicarbonyls can be applied to the synthesis of poly(silyl ether)s with a high molecular weight and narrow molecular distribution.

Azaborolyl anions, the five-membered BN heterocycles, have attracted a considerable attention due to their aromaticity and isoelectronic relationship with ubiquitous cyclopentadienyl ligands. Besides their syntheses and applications in the preparation of metal complexes, the other aspects of their chemistry have been virtually unexplored. Reduction of the azabutadienyl chelate boron dichloride [ArNC(R)CHC(R)]BCl₂ (2, Ar=2,6-Me₂C₆H₃, R=tBu) with two equivalents of potassium yielded the novel 2-chloro-azaborolyl anion [ArNC(R)CHC(R)BCl]K(thf) (3) as a stable product in good yield. Reaction of 3 with 1,3,4,5-tetramethylimidazol-2-ylidene (NHC) yielded the first NHC–azaborole adduct with the elimination of KCl. The salt elimination reaction was also observed in the reactions with H₂O and the organic azide ArN₃, leading to the formation of an oxo-bridged 3H-1,2-diazaborole and an intramolecular donor-stabilized iminoborane, demonstrating that 3 is a source of the unexplored 1,2-azaborole isosteric to cyclopentadienylidene.

The synthesis, characterization and methyl methacrylate polymerization behaviors of 2-(N-arylimino)pyrrolide nickel complexes are described. The nickel complex [NN]₂Ni (1, [NN] = [2-C(H)NAr-5-tBu-C₄H₂N]⁻, Ar = 2,6-iPr₂C₆H₃) was prepared in good yield by the reaction of [NN]Li with trans-[Ni(Cl)(Ph)(PPh₃)₂] in THF. Reaction of [NN]Li with NiBr₂(DME) yielded the nickel bromide [NN]Ni(Br)[NNH] (2). Complexes 1 and 2 were characterized by ¹H NMR and IR spectroscopy and elemental analysis, and by X-ray single crystal analysis. Both complexes, upon activation with methylaluminoxane, are highly active for the polymerization of methyl methacrylate to give high molecular weight polymethylmethacrylate with narrow molecular distributions. Copyright © 2009 John Wiley & Sons, Ltd.