Sequential Reactions of Alkynes on an Iridium(III) Single Site.

Sequential insertion of terminal alkynes on IrIII cyclometalated complexes allow the formation of novel metallapolycycles in a controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, C(sp3 )-H activation, and reductive coupling) on a single IrIII center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure. The sequence is a multicomponent reaction in which the metal not only promotes the different steps but also determines their stereoselectivity. This is an elegant example of the synergy between a metal-promoting reaction and a symmetry-defined stereochemistry.

Effect of a κ1-Bonded-M-1,2,3-triazole (M = Co, Ru) on the Structure and Reactivity of Group 6 Alkoxy (Fischer) Carbenes.

The [3 + 2] cycloaddition of two different metal-bound azides, [(Me4cyclam)CoII(N3)]ClO4 and (η5-C5H5)(dppe)RuII(N3), (dppe = Ph2PCH2CH2PPh2) with Cr(0) and W(0) (ethoxy)(alkynyl) Fischer carbenes has been efficiently used for the preparation of polymetallic metal-carbene complexes. The presence of the κ1-bonded metal triazole causes a significant influence on the electronic properties, structure, and reactivity of this new class of Fischer alkoxycarbenes. For the Ru(II) derivatives, their chemical behavior is considerably influenced by the interaction of the (η5-C5H5)(dppe)RuII-triazole moiety with the empty p-carbene orbital that provokes a noticeable decrease in the electrophilicity of the M═C carbon (manifested by the shielding of the 13C NMR chemical shifts). In turn, in the Co(II) derivatives, the incorporation of the (Me4cyclam)CoII moiety diminishes the aromaticity of the triazole ring and has a marked effect on the energy and distribution of the LUSO orbital, mostly resident on the Co(II) fragment. The almost negligible participation of the carbene moiety in the LUSO makes this position unable to react with nucleophiles. The reactions reported in this work constitute the first examples of [3 + 2] cycloaddition of azides and alkynyl Fischer carbene complexes in solution.

An Entry to Mixed NHC-Fischer Carbene Complexes and Zwitterionic Group 6 Metal Alkenyls.

The addition of NHCs to α,ß-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel group 6 (Fischer) carbene complexes.

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,ß-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

Synthesis of Dendritic ZSM-5 Zeolite through Micellar Templating Controlled by the Amphiphilic Organosilane Chain Length.

The synthesis of ZSM-5 zeolites by hydrothermal crystallization of protozeolitic nanounits functionalized with amphiphilic organosilanes of different chain length (Cn-N(CH3)2-(CH2)3-Si-(OCH3)3, n = 10, 14, 18 and 22) has been investigated. Well-developed dendritic nanoarchitectures were achieved when using C14 and C18 organosilanes, exhibiting a radial and branched pattern of zeolitic nanounits aggregates. In contrast, although C10 and C22 organosilanes led to materials with hierarchical porosity, they lack of dendritic features. These differences have been linked to the formation of an amorphous mesophase at the gel preparation stage for the C14 and C18 samples, in which the surfactant micelles are covalently connected with the protozeolitic nanounits through siloxane bonds. The presence of the dendritic nanostructure positively impacts both the textural and catalytic properties of ZSM-5 zeolite. Thus, ZSM-5 (C14) and ZSM-5 (C18) samples exhibit the largest contribution of mesoporosity in terms of both surface area and pore volume. On the other hand, when tested as catalysts in the aldol condensation of furfural with cyclopentanone, which is an interesting reaction for the production of sustainable jet fuels, the highest catalytic activity is attained over the dendritic ZSM-5 materials due to their remarkable accessibility and balanced Brønsted/Lewis acidity.

Triazole vs. triazolium carbene ligands in the site-selective cyclometallation of o-carboranes by M(iii) (M = Ir, Rh) complexes.

Ir(iii) and Rh(iii)-mediated site-selective cage B-H and C-H bond activation in o-carboranylmethyl derivatives has been achieved. The selectivity of the reaction is related to the electron donating properties of the ligand. 1,2,3-Triazole-derivatives use the N2 position of the triazole ring to direct the selective o-carborane B-H bond activation, whereas the corresponding triazolylidene derivatives lead to the cage C-H bond activation with complete site-selectivity.