Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo- and Regioselectivity Impacted by the Lewis Acidity and Size of the Support.

Bimetallic catalysts of nickel(0) with a trivalent rare-earth ion or Ga(III), NiML3 (where L is [iPr2PCH2NPh]-, and M is Sc, Y, La, Lu, or Ga), were investigated for the selective hydrogenation of diphenylacetylene (DPA) to (E)-stilbene. Each bimetallic complex features a relatively short Ni-M bond length, ranging from 2.3395(8) Å (Ni-Ga) to 2.5732(4) Å (Ni-La). The anodic peak potentials of the NiML3 complexes vary from -0.48 V to -1.23 V, where the potentials are negatively correlated with the Lewis acidity of the M(III) ion. Three catalysts, Ni-Y, Ni-Lu, and Ni-Ga, showed nearly quantitative conversions in the semihydrogenation of DPA, with NiYL3 giving the highest selectivity for (E)-stilbene. Initial rate studies were performed on the two tandem catalytic reactions: DPA hydrogenation and (Z)-stilbene isomerization. The catalytic activity in DPA hydrogenation follows the order Ni-Ga > Ni-La > Ni-Y > Ni-Lu > Ni-Sc. The ranking of catalysts by (Z)-stilbene isomerization initial rates is Ni-Ga ≫ Ni-Sc > Ni-Lu > Ni-Y > Ni-La. In operando 31P and 1H NMR studies revealed that in the presence of DPA, the Ni bimetallic complexes supported by Y, Lu, and La form the Ni(η2-alkyne) intermediate, (η2-PhC≡CPh)Ni(iPr2PCH2NPh)2M(κ2-iPr2PCH2NPh). In contrast, the Ni-Ga resting state is the Ni(η2-H2) species, and Ni-Sc showed no detectable binding of either substrate. Hence, the mechanism of Ni-catalyzed diphenylacetylene semihydrogenation adheres to two different kinetics: an autotandem pathway (Ni-Ga, Ni-Sc) versus temporally separated tandem reactions (Ni-Y, Ni-Lu, Ni-La). Collectively, the experimental results demonstrate that modulating a base-metal center via a covalently appended Lewis acidic support is viable for promoting selective alkyne semihydrogenation.

Multiple Bonds in Uranium-Transition Metal Complexes.

Novel heterobimetallic complexes featuring a uranium atom paired with a first-row transition metal have been computationally predicted and analyzed using density functional theory and multireference wave function based methods. The synthetically inspired metalloligands U{(iPr2PCH2NAr)3tacn} (1) and U(iPr2PCH2NPh)3 (2) are explored in this study. We report the presence of multiple bonds between uranium and chromium, uranium and manganese, and uranium and iron. The calculations predict a 5-fold bonding between uranium and manganese in the UMn(iPr2PCH2NPh)3 complex, which is unprecedented in the literature.

Bimetallic nickel-lutetium complexes: tuning the properties and catalytic hydrogenation activity of the Ni site by varying the Lu coordination environment.

We present three heterobimetallic complexes containing an isostructural nickel center and a lutetium ion in varying coordination environments. The bidentate iPr2PCH2NHPh and nonadentate (iPr2PCH2NHAr)3tacn ligands were used to prepare the Lu metalloligands, Lu(iPr2PCH2NPh)3 (1) and Lu{(iPr2PCH2NAr)3tacn} (2), respectively. Reaction of Ni(COD)2 (where COD is 1,5-cyclooctadiene) and 1 afforded NiLu(iPr2PCH2NPh)3 (3), with a Lu coordination number (CN) of 4 and a Ni-Lu distance, d(Ni-Lu), of 2.4644(2) Å. Complex 3 can further bind THF to form 3-THF, increasing both the Lu CN to 5 and d(Ni-Lu) to 2.5989(4) Å. On the other hand, incorporation of Ni(0) into 2 provides NiLu{(iPr2PCH2NAr)3tacn} (4), in which the Lu coordination environment is more saturated (CN = 6), and the d(Ni-Lu) is substantially elongated at 2.9771(5) Å. Cyclic voltammetry of the three Ni-Lu complexes shows an overall ∼410 mV shift in the Ni(0/I) redox couple, suggesting tunability of the Ni electronics across the series. Computational studies reveal polarized bonding interactions between the Ni 3d z 2 (major) and the Lu 5d z 2 (minor) orbitals, where the percentage of Lu character increases in the order: 4 (6.0% Lu 5d z 2 ) < 3-THF (8.5%) < 3 (9.3%). All three Ni-Lu complexes bind H2 at low temperatures (-30 to -80 °C) and are competent catalysts for styrene hydrogenation. Complex 3 outperforms 4 with a four-fold faster rate. Additionally, adding increasing THF equivalents to 3, which would favor build-up of 3-THF, decreases the rate. We propose that altering the coordination sphere of the Lu support can influence the resulting properties and catalytic activity of the active Ni(0) metal center.