Hydroxide-promoted core conversions of molybdenum-iron-sulfur edge-bridged double cubanes: oxygen-ligated topological PN clusters.

The occurrence of a heteroatom X (C, N, or O) in the MoFe7S9X core of the iron-molybdenum cofactor of nitrogenase has encouraged synthetic attempts to prepare high-nuclearity M-Fe-S-X clusters containing such atoms. We have previously shown that reaction of the edge-bridged double cubane [(Tp)2Mo2Fe6S8(PEt3)4] (1) with nucleophiles HQ- affords the clusters [(Tp)2Mo2Fe6S8Q(QH)2](3-) (Q = S, Se) in which HQ- is a terminal ligand and Q(2-) is a mu2-bridging atom in the core. Reactions with OH- used as such or oxygen nucleophiles generated in acetonitrile from (Bu3Sn)2O or Me3SnOH and fluoride were examined. Reaction of 1 with Et4NOH in acetonitrile/water generates [(Tp)2Mo2Fe6S9(OH)2]3- (3), isolated as [(Tp)2Mo2Fe6S9(OH)(OC(=NH)Me)(H2O)](3-) and shown to have the [Mo2Fe6(mu2-S)2(mu3-S)6(mu6-S)] core topology very similar to the P(N) cluster of nitrogenase. The reaction system 1/Et4NOH in acetonitrile/methanol yields the P(N)-type cluster [(Tp)2Mo2Fe6S9(OMe)2(H2O)](3-) (5). The system 1/Me3SnOH/F- affords the oxo-bridged double P(N)-type cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}5- (7), convertible to the oxidized cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}4- (6), which is prepared independently from [(Tp)2Mo2Fe6S9F2(H2O)](3-)/(Bu3Sn)2O. In the preparations of 3-5 and 7, hydroxide liberates sulfide from 1 leading to the formation of P(N)-type clusters. Unlike reactions with HQ-, no oxygen atoms are integrated into the core structures of the products. However, the half-dimer composition [Mo2Fe6S9O] relates to the MoFe7S9 constitution of the putative native cluster with X = O. (Tp = hydrotris(pyrazolyl) borate(1-)).

Cationic zinc enolates as highly active catalysts for acrylate polymerization.

Unprecedented cationic zinc enolates have been generated by a novel activation route involving the amido to imino ligand transformation with B(C6F5)3, structurally characterized, and utilized as highly active catalysts for the production of high molecular weight polyacrylates at ambient temperature.

Dizinc alkoxides and amides supported by binucleating bis(amidoamine) ligands.

Several new dizinc complexes that are supported by dianionic bis(amidoamine) ligands are reported. Reaction of N,N'-bis(2-dimethylaminoethyl)dibenzofuran-4,6-diamine ((Me)LH(2)) with 2 equiv of EtZn(O(i)Pr) forms the dizinc bis(alkoxide) (Me)LZn2(O(i)Pr)2 (1), which was isolated in 76% yield. Similarly, (Me)LH2 reacts cleanly with EtZn(OPh) and EtZn(OCHPh2) to form (Me)LZn2(OPh)2 (2) and (Me)LZn2(OCHPh2)2 (3), respectively. The solid-state structures of 1 and 2 feature puckered [Zn2(mu-OR)2]2+ cores, with short intermetal separations (2.81-2.88 Angstroms). Overall, the molecules have approximate (noncrystallographic) C2v symmetry. The use of the more-hindered (i)Pr-substituted ligand N,N'-bis(2-diisopropylaminoethyl)dibenzofuran-4,6-diamine (i(Pr)LH2) to prepare zinc alkoxides gave similar results. Thus, reaction of i(Pr)LH2 with 2 equiv of EtZn(OPh), EtZn(OMe), EtZn(OCHPh2), and EtZn(OCH2Ph) forms i(Pr)LZn2(OPh)2 (4), i(Pr)LZn2(OMe)2 (5), i(Pr)LZn2(OCHPh2)2 (6), and i(Pr)LZn2(OCH2Ph)2 (7), respectively (isolated yields 48-63%). At 70 degrees C, C6D6 solutions of 6 undergo beta-hydride transfer with 2 equiv of benzaldehyde to form 7 and benzophenone in quantitative yield (according to 1H NMR spectroscopy). Benzene solutions of 1 react with 1 equiv of trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) to form (Me)LZn2(O(i)Pr)(OTf) (8) in 70% isolated yield. In the solid state, 8 features a bridging alkoxide donor as well as a 1,3-bridging triflate group. The previously reported dinuclear organozinc species (Me)LZn2Ph2 (9) reacts with 1 equiv of tert-butylamine to form the protonolysis product (Me)LZn2(Ph)(NH(t)Bu) (10) in 66% isolated yield. The solid-state structure of 10 (two independent molecules) reveals a somewhat asymmetric [Zn2(mu-Ph)(mu-NH(t)Bu)]2+ core with short Zn-Zn separations [2.6761(5) and 2.6518(5) Angstroms]. In CD2Cl2 solution, the Ph bridge of 10 undergoes rapid reversible cleavage. Cleavage of this bridging interaction followed by rotation about the Zn-Ph bond and re-formation of the bridging interaction results in exchange of the inequivalent ortho (and meta) protons of the phenyl ligand. Variable-temperature 1H NMR spectroscopic data indicate that this exchange occurs with DeltaG = 12.7(1) kcal.mol(-1) (-27 degrees C). At 75 degrees C, toluene solutions of (Me)LH2 react with 2 equiv of EtZnNH(t)Bu to form the dizinc bis(amido) product (Me)LZn2(NH(t)Bu)2 (11) in 46% isolated yield. The solid-state structure of 11 (two independent molecules) features a puckered and fairly symmetric [Zn2(mu-NH(t)Bu)2]2+ core with short intermetal separations [2.775(1), 2.760(1) Angstroms].

The direct alpha-zincation of amides, phosphonates and phosphine oxides by H-Zn exchange.

Stoichiometric or catalytic quantities of simple 2 degrees amines greatly increase the rate of H-Zn exchange between ZnPh2 and a range of relatively non-acidic substrates, allowing for the convenient and direct preparation of alpha-functionalized organozincs.

New dibenzofuran-bridged bis(amidoamine) and bis(ethylenediamine) ligands and their dinuclear zinc and aluminium complexes.

A new class of dibenzofuran-bridged bis(amidoamine) and bis(ethylenediamine) ligands are used to prepare structurally-characterized dinuclear zinc and aluminium complexes.