Moumita Majumdar

Role of axial donors in the ligand isomerization processes of quadruply bonded dimolybdenum(II) compounds.

Quadruply bonded dimolybdenum(II) complexes with NP-R (2-(2-R)-1,8-naphthyridine; R = thiazolyl (NP-tz), furyl (NP-fu), thienyl (NP-th)) and 2,3-dimethyl-1,8-naphthyridine (NP-Me 2) have been synthesized by reactions of cis-[Mo2(OAc)2(CH3CN)6][BF4]2 with the corresponding ligands. The products cis-[Mo2(NP-tz)2(OAc)2][BF4]2 (1), trans-[Mo2(NP-fu)2(OAc)2][BF4]2 (2), trans-[Mo2(NP-th)2(OAc)2][BF4]2 (3), and trans-[Mo2(NP-Me2)2(OAc)2][BF4]2 (4) were isolated and characterized. The NP-R ligands with stronger R = pyridyl and thiazolyl donors result in cis isomers whereas the weaker furyl and thienyl appendages lead to compounds having a trans orientation of the ligands. The use of NP-Me2 leads to a trans structure with a tetrafluoroborate anion occupying one of the axial sites. Complete replacement of two acetate groups by acetonitrile in 1 and 2 resulted in the cis isomers [Mo2(NP-tz)2(CH3CN)4][OTf]4 (5) and [Mo2(NP-fu)2(CH3CN)4][OTf]4 (6) respectively. The combination of one acetate and two acetonitriles as ancillary ligands, however, yields trans-[Mo2(NP-tz)2(OAc)(CH3CN)2][BF4]3 (7) in the solid state as determined by X-ray crystallography. (1)H NMR spectra of the products are diagnostic of the cis and trans dispositions of the ligands. Solution studies reveal that the ligand arrangements observed in the solid state are mostly retained in the acetonitrile medium. The only exception is 7, for which a mixture of cis and trans isomers are detected on the NMR time scale. The isolation of trans compounds 2- 4 from the cis precursor [Mo2(OAc)2(CH3CN)6][BF4]2 indicates that an isomerization process occurs during the reactions. The mechanism involving acetate migration through axial coordination has been invoked to rationalize the product formation. Compounds 1- 7 were structurally characterized by single-crystal X-ray methods.

Transmetallation reactions of a lithium disilenide

The first magnesium, copper and zinc disilenides were prepared via transmetallation reactions of a lithium disilenide and structurally characterised. The copper and zinc derivatives show red-shifted UV/vis absorptions due to admixture of metal d-orbitals to the highest occupied molecular orbital.

Reductive Cleavage of Carbon Monoxide by a Disilenide

The complete reductive cleavage of the triple bond in carbon monoxide was achieved using a lithium disilenide at room temperature. The C-C-coupled product can be regarded as a silanone dimer with pending alkyne and silirene moieties and incorporates two equivalents of CO per disilenide unit. A formation mechanism via ketenyl intermediates is proposed on the basis of DFT calculations and elucidated experimentally by employing Group 6 metal carbonyls as both stabilizing entity and source of CO in the reaction with disilenide. The isolation of cyclic silylene complexes with weakly donating ketenyl donor groups further supports the mechanistic scenario.

The Cp*Si+ cation as a stoichiometric source of silicon

The Cp*Si(+) cation acts as a stoichiometric source of silicon in the reaction with the disilenide Tip(2)Si=Si(Tip)Li (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) affording known neutral unsaturated silicon clusters. It thereby provides a conceptually different approach to this novel class of compounds. The proposed mechanism involves a Cp*-substituted cyclotrisilene in which Cp*(-) acts as a leaving group upon single electron reduction or in a nucleophilic substitution step.

σ-π conjugated organosilicon hybrid polymers from copolymerization of a tetrasiladiene and 1,4-diethynylbenzene.

A catalyst- and by-product-free protocol for the synthesis of σ-π conjugated organosilicon polymers is reported. The regiospecific [2+2] cycloaddition of C≡C triple bonds to Si=Si double bonds allowed the preparation of air-stable ethynyl-terminated extended monomers from 1,4-bis(ethynyl)benzene and the para-phenylene bridged tetrasiladiene, Tip2 Si=SiTip-pC6H4-SiTip=SiTip2 (Tip = 2,4,6-iPr3C6H2). The polymer obtained from the extended monomer and further tetrasiladiene exhibits pronounced σ-π conjugation, as was evident from the red-shift in the absorption spectrum compared to model systems. We show that the thermal stability of the employed bis(alkyne) co-monomer is translated into this polymer.

Cyclometalated Ir–Sn Construct for Cyanosilylation

Two cyclometalated compounds [IrIIICl{(2-biphenylene-1,8-naphthyridine-κC,N}(η5-pentamethylcyclopentadienyl)] (1) and [IrIIICl{(2-(2-N-Methyl-pyrrolyl-1,8-naphthyridine-κC,N}(η5-pentamethylcyclopentadienyl)] (2) containing naphthyridine based ligands have been synthesized in high yield. Insertion of SnCl2 to a terminal Ir–Cl bond of 1 affords the mixed Ir–SnCl3 compound [IrIIISnCl3{(2-biphenylene-1,8-naphthyridine-κC,N}(η5-pentamethylcyclopentadienyl)] (3). The heterobimetallic compound 3 is shown to be an excellent catalyst for a variety of cyanosilylation reactions. A cooperative mechanism has been proposed which involves the simultaneous activation of aldehyde and cyanide precursor by Sn and unbound naphthyridine nitrogen.