Syntheses and Redox Properties of Carboxylate-Ligated Hexanuclear Ce(IV) Clusters and Their Photoinduced Homolysis of the Ce(IV)-Ligand Covalent Bond.

Oxo- and hydroxo-bridged hexanuclear Ce(IV) clusters surrounded by 12 carboxylate ligands, Ce6O4(OH)4(O2CR)12(L)n (R = 2,6-Me2-4-MeOC6H2 (1a), 2,6-Me2-4-tBuC6H2 (1b), 2,4,6-Me3C6H2 (1c), 2,6-Me2C6H3 (1d), 2,6-Me2-4-FC6H2 (1e), 2,6-Me2-4-ClC6H2 (1f), 9-anthracenyl (1g), and CH2tBu (1h), L = H2O or RCO2H), were synthesized by treating Ce(OtBu)4 with the corresponding carboxylic acids (2-3 equiv.) in acetone or toluene, and the molecular structures of 1d and 1g were clarified by X-ray diffraction studies. UV-vis analyses of the clusters showed broad absorption corresponding to the ligand-to-metal charge transfer (LMCT) in the ultraviolet A (315-400 nm) to blue light region; density functional theory (DFT) studies of the simplified Ce(IV) and related Zr(IV) clusters, M6O4(OH)4(O2CR)12 (M = Ce, Zr, R = Ph, Me), revealed that the low-lying vacant 4f-orbitals of the Ce(IV) were responsible for absorption in the ultraviolet A to blue light region. Irradiation of blue LED light to 1a-f under an argon atmosphere resulted in the formation of 7-methylisobenzofuran-1(3H)-one (2a-f), which involved the following four steps: photoinduced homolysis of the Ce(IV)-OCOR bond, intramolecular hydrogen atom transfer to generate the corresponding benzyl radical, oxidation to the benzyl cation, and intramolecular cyclization. Cyclic voltammetry of cerium clusters 1a-f having 2,6-dimethyl-4-substituted arylcarboxylate ligands showed electrochemically irreversible redox waves in the range of -0.79 to -0.38 V (vs [Cp2Fe]+/Cp2Fe for E1/2). The one-electron-reduced Ce(III)Ce(IV)5 clusters 3a-h were isolated by reducing 1a-h with Cp*2Co to give [Cp*2Co][Ce6O4(OH)4(O2CR)12(thf)n] (3a-h); cluster 3d was the first structurally determined hexanuclear cerium cluster containing a [Ce6O4(OH)4]11+ core.

Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids.

We found that in situ generated cerium(IV) carboxylate generated by mixing the precursor Ce(OtBu)4 with the corresponding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV) carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical. In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding peroxy lactone and γ-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).

Synthesis and Characterization of Alkoxide-Bridged Heterometallic Clusters of Cerium and Copper.

We prepared alkoxide-bridged heterometallic clusters of cerium and copper by the complexation of two metal alkoxides: treatment of Ce(OtBu)4 with [Cu(OtBu)]4 in a 1:1 metal ratio produced an alkoxide-bridged tetranuclear cluster, Ce2Cu2(OtBu)10 (1). Upon adding 4-substituted pyridine derivatives to complex 1, trinuclear clusters, Ce2Cu(OtBu)9(L) (2a: L = DMAP (4-dimethylaminopyridine); 2b: L = BPY (4,4'-bipyridine)), were obtained along with the release of 0.25 equiv of [Cu(OtBu)]4, in which a three-coordinated copper center was involved. In contrast, reaction of 1 with 4 equiv of 2,6-dimethylphenylisocyanide (XylNC) and 0.5 equiv of [Cu(OtBu)]4 resulted in the selective formation of CeCu2(OtBu)6(CNXyl)2 (3). In addition, Ce2K(OtBu)9 was used for complexation with CuCl2 by salt-elimination, giving Ce2CuCl(OtBu)9 (4) including a five-coordinated copper center. These complexes 1-4 were characterized by crystal structure determination as well as cyclic voltammetry of 1, 2a, and 4. The cyclic voltammogram of 4 in CH2Cl2 and THF suggested that reorganization of the coordination sphere around the copper center was observed for 4 during the Cu(I/II) redox processes assisted by the coordination of THF.

NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d0 group 4 metal complexes.

d0 metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the 13C NMR chemical shift tensors of the α-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the π-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. The 13C NMR chemical shifts, the α-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

Metal alkyls programmed to generate metal alkylidenes by α-H abstraction: prognosis from NMR chemical shift.

Metal alkylidenes, which are key organometallic intermediates in reactions such as olefination or alkene and alkane metathesis, are typically generated from metal dialkyl compounds [M](CH2R)2 that show distinctively deshielded chemical shifts for their α-carbons. Experimental solid-state NMR measurements combined with DFT/ZORA calculations and a chemical shift tensor analysis reveal that this remarkable deshielding originates from an empty metal d-orbital oriented in the M-Cα-Cα' plane, interacting with the Cα p-orbital lying in the same plane. This π-type interaction inscribes some alkylidene character into Cα that favors alkylidene generation via α-H abstraction. The extent of the deshielding and the anisotropy of the alkyl chemical shift tensors distinguishes [M](CH2R)2 compounds that form alkylidenes from those that do not, relating the reactivity to molecular orbitals of the respective molecules. The α-carbon chemical shifts and tensor orientations thus predict the reactivity of metal alkyl compounds towards alkylidene generation.

Group 2 metal (Mg, Ca, Sr) silylamides supported by a cyclen-derived macrocyclic polyamine.

Heteroleptic bis(silyl)amides of magnesium and calcium [(L)M{N(SiMe3)2}] [M = Mg, Ca; LH = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane; (Me3TACD)H] were previously synthesized from LH and [M{N(SiMe3)2}2]. Strontium bis(silyl)amides [Sr{N(SiMe3)2}2(thf)2] and [Sr{N(SiHMe2)2}2(thf)2/3] reacted with LH to give different types of products, depending on the presence of the ß-SiH function. While the former underwent protonolysis to give the amido-bridged dimer [(L)Sr{N(SiMe3)2}]2 (1), the latter gave the adduct [(LH)Sr{N(SiHMe2)2}2] (2) as a stable solid. 2 slowly underwent an intramolecular Si-H/H-N dehydrocoupling in solution to give [{(L)SiMe2N(SiHMe2)}Sr{N(SiHMe2)2}] (3) by liberating H2. The results of transamination of 1 with HN(SiHMe2)2 depended on the relative stoichiometric ratio. A 1 : 1 mixture in n-pentane gave [{(L)SiMe2N(SiHMe2)}Sr{N(SiMe3)2}] (4) and H2, while excess HN(SiHMe2)2 gave the adduct 2 under similar conditions. Compounds 2 and 3 exhibit Sr↼H-Si interactions according to X-ray crystallography, NMR, and IR spectroscopy. Lighter congeners of elusive [(L)Sr{N(SiHMe2)2}] were isolable for Mg (5) and Ca (6).

Magnesium hydridotriphenylborate [Mg(thf)6][HBPh3]2: a versatile hydroboration catalyst.

Magnesium bis(hydridotriphenylborate), isolated as a solvent-separated ion pair [Mg(thf)6][HBPh3]2, effectively catalyzed the hydroboration of several unsaturated substrates including CO2.

Chemoselective Reduction of Tertiary Amides to Amines Catalyzed by Triphenylborane.

Triphenylborane (BPh3 ) was found to catalyze the reduction of tertiary amides with hydrosilanes to give amines under mild condition with high chemoselectivity in the presence of ketones, esters, and imines. N,N-Dimethylacrylamide was reduced to provide the α-silyl amide. Preliminary studies indicate that the hydrosilylation catalyzed by BPh3 may be mechanistically different from that catalyzed by the more electrophilic B(C6 F5 )3 .

Cerium-Complex-Catalyzed Oxidation of Arylmethanols under Atmospheric Pressure of Dioxygen and Its Mechanism through a Side-On µ-Peroxo Dicerium(IV) Complex.

A new Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)2] (1), bearing a dianionic pentadentate ligand with an N3O2 donor set, has been prepared by treating (NH4)2Ce(NO3)6 with the sodium salt of ligand L1. Complex 1 in the presence of TEMPO and 4 Å molecular sieves (MS4 A) has been found to serve as a catalyst for the oxidation of arylmethanols using dioxygen as an oxidant. We propose an oxidation mechanism based on the isolation and reactivity study of a trivalent cerium complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)(THF)] (2), its side-on µ-O2 adduct [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(µ-η(2):η(2)-O2) (3), and the hydroxo-bridged Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(µ-OH)2 (4) as key intermediates during the catalytic cycle. Complex 2 was synthesized by reduction of 1 with 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diazacyclohexadiene. Bubbling O2 into a solution of 2 resulted in formation of the peroxo complex 3. This provides the first direct evidence for cerium-catalyzed oxidation of alcohols under an O2 atmosphere.