A structure-activity study was carried out for Ni catalyzed alkyl-alkyl Kumada-type cross coupling reactions. A series of new nickel(II) complexes including those with tridentate pincer bis(amino)amide ligands ((R)N(2)N) and those with bidentate mixed amino-amide ligands ((R)NN) were synthesized and structurally characterized. The coordination geometries of these complexes range from square planar, tetrahedral, to square pyramidal. The complexes had been examined as precatalysts for cross coupling of nonactivated alkyl halides, particularly secondary alkyl iodides, with alkyl Grignard reagents. Comparison was made to the results obtained with the previously reported Ni pincer complex [((Me)N(2)N)NiCl]. A transmetalation site in the precatalysts is necessary for the catalysis. The coordination geometries and spin-states of the precatalysts have a small or no influence. The work led to the discovery of several well-defined Ni catalysts that are significantly more active and efficient than the pincer complex [((Me)N(2)N)NiCl] for the coupling of secondary alkyl halides. The best two catalysts are [((H)NN)Ni(PPh(3))Cl] and [((H)NN)Ni(2,4-lutidine)Cl]. The improved activity and efficiency was attributed to the fact that phosphine and lutidine ligands in these complexes can dissociate from the Ni center during catalysis. The activation of alkyl halides was shown to proceed via a radical mechanism.

In search of structure–activity relationships for polyoxometalate (POM)‐based water reduction catalysts, nickel‐monosubstituted Keggin‐type POMs ([Ni(H2O)XW11O39]n−; XP, Si, Ge) were compared with respect to their activity in photochemical hydrogen evolution. The title compound series was characterized by single‐crystal X‐ray diffraction methods and a wide range of spectroscopic and electrochemical techniques. Nickel substitution was identified as a crucial feature for catalytic activity through comparison with nickel‐free reference POMs. Furthermore, turnover number (TON) and turnover frequency strongly depended on the heteroatom X, and the highest TON among the series was recorded for [Ni(H2O)GeW11O39]6−. Photochemical hydrogen evolution activity was compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends were correlated with electronic structure properties derived from density functional theory calculations.

A series of new gallium‐containing silicotungstates was obtained from the one step reaction of the lacunary Na10[α‐SiW9O34]·15H2O Keggin‐type building block (SiW9) with gallium nitrate in aqueous solution. The sandwich‐type polyoxotungstate [Ga6(H2O)x{α‐SiW9O34(OH)y}2]n– (1) crystallizes first as a mixture of two similar polyoxometalates (1a and 1b), while [Ga4(H2O)2{α‐SiW10O38}2]12– (2) is generated from the mother liquor in a second step. Both newly obtained polyoxometalates with tetra‐ and hexanuclear gallium cores were structurally characterized with a wide range of methods encompassing single crystal and powder X‐ray diffraction techniques, FT‐IR spectroscopy, and thermal analysis. The solution stability of both dimeric POMs K10[Ga6(H2O)3{α‐SiW9O35(OH)2}2]·35.5H2O (1a) and K12[Ga4(H2O)2{α‐SiW10O38}2]·30H2O (2) was investigated with 183W NMR and HR ESI mass spectrometry, and the electrochemical properties of both compounds were determined by cyclic voltammetry.

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Nickel-Containing Keggin-Type Polyoxometalates as Hydrogen EvolutionCatalysts: Photochemical Structure-Activity Relationships. -The Ni-monosubstituted Keggin-type polyoxometalates (POMs) K 5[Ni(H2O)PW11O39]·12H2O (I), K6[Ni(H2O)GeW11O39]·14H2O (II), and K6[Ni(H2O)SiW11O39]·14H2O (III) are prepared according to a modified published method. As an example, compound (I) is prepared from an acidified (HCl) aqueous 1:3 molar mixture of Na 2WO4 and Na2HPO4 (100 C) which is subjected to solvent extraction by Et 2O. The organic fraction then is heated to 95 C, and a solution of Ni(NO3)3 in acetate buffer (pH 7) is added (2 min stirring) followed by cooling to 65 C and further addition of MeOH. A green precipitate is isolated after keeping the mixture at 8 C overnight (52% yield based on Na 2WO4·2H2O). The compounds crystallize in the tetragonal space group P4n2 (Z = 2, single crystal XRD) and show Ni/W disorder at one of the three symmetry-unique W sites. Nickel substitution is identified as a crucial feature for catalytic activity through comparison with nickel-free reference POMs. Furthermore, turnover numbers (TON) and turnover frequencies strongly depended on the heteroatom (P, Ge, Si) with the highest TON recorded for [Ni(H 2O)GeW11 O39] 6-. Photochemical H2 evolution activity is compared with redox and onset potentials obtained from electrochemical analyses. Furthermore, activity trends are correlated with electronic structure properties derived from DFT calculations. -(VON ALLMEN, K.; MORE, R.; MUELLER, R.; SORIANO-LOPEZ, J.; LINDEN, A.; PATZKE*, G. R.; ChemPlusChem 80 (2015) 9, 1389-1398, http://dx.doi.org/10.1002/cplu.201500074 ; Inst. Chem., Univ. Zuerich, CH-8057 Zuerich, Switz.; Eng.) -J. Schramke 46-010