We report a ruthenium complex containing an N,N'-diimine ligand for the selective decomposition of formic acid to H and CO in water in the absence of any organic additives. A turnover frequency of 12 000 h and a turnover number of 350 000 at 90 °C were achieved in the HCOOH/HCOONa aqueous solution. Efficient production of high-pressure H and CO (24.0 MPa (3480 psi)) was achieved through the decomposition of formic acid with no formation of CO. Mechanistic studies by NMR and DFT calculations indicate that there may be two competitive pathways for the key hydride transfer rate-determining step in the catalytic process.

An unsymmetrically protonated PN(3) -pincer complex in which ruthenium is coordinated by one nitrogen and two phosphorus atoms was employed for the selective generation of hydrogen from formic acid. Mechanistic studies suggest that the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover number over 1 million was achieved.

A series of novel constrained-geometry-configuration (CGC) rare-earth metal complexes (RCH 2 −Py)Ln-(CH 2 SiMe 3 ) 2 (THF) n (Py = pyridyl; Ln = Y, n = 1, R = C 5 Me 4 (Cp′) (1); Ln = Y, n = 1, R = C 9 H 6 (Ind) (2); Ln = Y, n = 1, R = C 13 H 8 (Flu) (3a); Ln = Lu, n = 1, R = C 13 H 8 (Flu) (3b); Ln = Sc, n = 0, R = C 13 H 8 (Flu) (3c)) have been synthesized by treating rare-earth metal trisalkyls with PyCH 2 −Cp′, PyCH 2 − Ind, and PyCH 2 −Flu compounds, respectively, and fully characterized by NMR and X-ray diffraction analyses. Complexes 1, 2, and 3a−b are monomeric THF solvates while the scandium complex 3c is solvent-free, in which all the CGC ligands adopt a η 5 /κ 1 bonding mode via coordination of five carbon atoms from the cyclopentadienyl fragment and the pyridyl nitrogen atom with the central metals. Upon activation with Al i Bu 3 and [Ph 3 C][B(C 6 F 5 ) 4 ], these complexes showed different performances toward styrene polymerization. The Cp′CH 2 −Py and IndCH 2 −Py ligated yttrium complexes 1 and 2 showed very low activity to afford syndiotactic enriched polystyrene. Strikingly, the bulky FluCH 2 −Py supported complexes 3a−c displayed outstanding activities up to 1.56 × 10 7 g/(mol Ln ·h) and perfect syndioselectivity (rrrr > 99%), giving high molecular weight sPS; in particular, thus excellent performance was independent of the central metal type for the first time. In addition, the relationship of the ligand structure with the catalytic performances toward specific selective polymerization of styrene was reasonably revealed by comparison with the other Flu-based rare-earth metal catalysts reported previously by us. This might open a new pathway for designing catalysts for specifically selective polymerizations. ■ INTRODUCTIONSyndiotactic polystyrene (sPS), discovered first by Ishihara at Idemitsu in 1986 by using unlinked half-sandwich titanium catalysts, 1 is a promising engineering plastic due to its high melting point (T m = 270°C), rapid crystallization ability, high tensile modulus, low dielectric constant, and excellent resistance to heat and chemicals. 2 Since this breakthrough, a large number of titanium analogues generally formulated as [LTiX 3 ], where L is an alkyl or alkoxyl-and amino-substituted cyclopentadienyl (Cp), indenyl (Ind), or fluorenyl (Flu) moiety, while X is halogen or alkoxyl group, have been extensively investigated, which exhibited obvious improvements in both catalytic activity and syndioselectivity for styrene polymerization. 3 In contrast, rare-earth metal catalysts have been noted for their inertness or low specific selectivity toward styrene polymerization. 4 In 2004, great successes were achieved by using two type of catalytic systems based on the unlinked half-sandwich cationic scandium alkyl species 5 and neutral lanthanidocene bearing Flu−CMe 2 −Cp ligand, 6 both displayed good activity to provide perfect sPS. Later on some derivatives supported by the substituted Cp′, hetero(B, N, P)−Cp′, oxygen-linked Cp′ and Ind′ ligands have also been reported as efficient c...

Styrene underwent unprecedented coordination-insertion copolymerization with naked polar monomers (ortho-/meta-/para-methoxystyrene) in the presence of a pyridyl methylene fluorenyl yttrium catalyst. High activity (1.26×10 g mol h ) and excellent syndioselectivity were observed, and high-molecular-weight copolymers (24.6×10 g mol ) were obtained. The insertion rate of the polar monomers could be adjusted in the full range of 0-100 % simply by changing the loading of the polar styrene monomer. Strikingly, the copolymers had tapered, gradient, and even random sequence distributions, depending on the position of the polar methoxy group on the phenyl ring and thus on its mode of coordination to the active metal center, as shown by tracking the polymerization process and DFT calculations.

Because of the steric bulkiness and the η 5 /κ 1 -constrained-geometry-configuration (CGC) geometry, the entire range of pyridyl-methylene-fluorenyl-stabilized rare earth metal bisalkyl complexes, (Flu-CH 2 -Py)Ln(CH 2 SiMe 3 ) 2 (THF) x (Flu = fluorenyl; Py = pyridyl; for 1, Ln = Sc and x = 0; for 2−11, Ln = Lu, Tm, Er, Ho, Y, Dy, Tb, Gd, Nd, or Pr and x = 1), and monoalkyl complex, (Flu-CH 2 -Py) 2 La(CH 2 SiMe 3 ) (THF) (12), has been successfully achieved for the first time via the sequential salt metathesis reactions. Activated by [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 , complexes 1−9 showed high activity and perfect syndioselectivity for styrene polymerization, while the large Nd-and Pr-attached precursors 10 and 11 exhibited slightly decreased syndioselectivity but rather low activity; the monoalkyl La precursor 12 was completely inert. The activity increased with the decrease in the rare earth metal size, in striking contrast to the literature that has shown that a large metal facilitates a high activity, which was also not a result of an enthalpic effect (ΔH ⧧ ) or an entropic effect (ΔS ⧧ ) according to Eyring plots. The types of organoborates and the aluminum alkyls, the electron donors, and the polarity of the reaction medium, which affected the coordination of styrene to the active species, aroused significantly different catalytic activity, indicating that styrene coordination played the key role in the polymerization process. On the basis of this, the density functional theory calculation of the active species in the model of [(Flu-CH 2 -Py)Ln-nC 17 H 19 ] + revealed whenever the orbitals of the pyridyl-methylene fluorenyl ligand overlapped with those of the rare earth metals, the LUMO energy of the active species was lowered and thus the catalytic activity was high. Therefore, the LUMO energy of the active species could be adopted as a potential criterion to estimate the activity of a catalytic system for styrene polymerization. This work reveals for the first time the power of the pyridyl-methylene fluorenyl ligand and the nature of the factors influencing the catalytic performance.

Formic acid (FA) has been extensively studied as one of the most promising hydrogen energy carriers today. The catalytic decarboxylation of FA ideally leads to the formation of CO2 and H2 that can be applied in fuel cells. A large number of transition‐metal based homogeneous catalysts with high activity and selectivity have been reported for the selective FA dehydrogentaion. In this review, we discussed the recent development of C,N/N,N‐ligand and pincer ligand‐based homogeneous catalysts for the FA dehydrogenation reaction. Some representative catalysts are further evaluated by the CON/COF assessment (catalyst on‐cost number)/(catalyst on‐cost frequency). Conclusive remarks are provided with future challenges and opportunities.

Rhodium(III)- and cobalt(III)-catalyzed C-H activation of indoles and coupling with 1,6-enynes is discussed. Under rhodium(III) catalysis, the alkyne insertion follows 2,1-regioselectivity with a subsequent type-I intramolecular Diels-Alder reaction (IMDA) to afford [6,5]-fused cycles. When catalyzed by the cobalt(III) congener, 1,2-insertion of the alkyne is preferred, and followed by a rare type-II IMDA, thus leading to bridged [3,3,1]-cycles. This selectivity of the alkyne insertion was mainly tuned by the steric sensitivity of the catalyst.

Effective conversion of carbon dioxide from air into formate by a phosphorus-nitrogen PN3P-Ru(ii) pincer complex.