A Multitechnique Approach to Spin-Flips for Cp2Cr(II) Chemistry in Confined State

Paramagnetic solid-state NMR, extended X-ray absorption fine structure (EXAFS), and Raman spectroscopies, along with detailed quantum mechanical calculations performed with different density functional theory (DFT) functionals, are successfully applied to investigate the magnetic, structural, and vibrational properties of molecularly isolated chromocene (Cp2Cr, where Cp = C5H5) and of its Cp2Cr(CO) adduct. Paramagnetic solid-state NMR unequivocally demonstrates that a spin flip occurs by coming from the paramagnetic Cp2Cr (triplet state) to the diamagnetic Cp2Cr(CO) adduct (singlet state), thus clarifying the theoretical dilemma of the disagreement among different functionals in predicting the most stable spin state. EXAFS and Raman spectroscopies are able to experimentally discriminate between singlet and triplet states, because a different spill state corresponds to a different geometry of the molecule, and therefore to different vibrational features. The here reported multitechnique approach could have great relevance in establishing the occurrence of spill flip in the chemical reactivity of transition metal complexes in both homo- and heterogeneous catalysis

Structure and enhanced reactivity of chromocene carbonyl confined inside the cavities of NaY zeolite

Chromocene (Cp2Cr) hosted inside the supercage cavities of the NaY zeolite undergoes a structural distortion induced by the strong local electric fields generated by charge balancing counterions. This effect, clearly observed by in situ Cr K-edge EXAFS study, is the key factor in enhancing the reactivity of Cp2Cr towards CO. The Cp2Cr(CO) adducts initially formed are not as stable as when hosted in non-polar environments, such as toluene solution or polystyrene. The presence of strong anionic/cationic pairs (Y-/Na+) favors, in CO atmosphere, the loss of a Cp ring driven by an electron transfer mechanism (accompanied by ligand rearrangement) that results in the formation of charged [CpCr(CO)3]\uf02d and [Cp2Cr(CO)]+ carbonyl species that are stabilized by the Na+ and Y- pairs. The shape selectivity of the supercage cavity of the Y zeolite is necessary for this reaction, as it can host the two Cp2Cr molecules needed for the disproportionation. Fast FTIR spectroscopy, working in operando conditions, allows to follow the time evolution of the IR stretching modes peculiar of reactants and products and thus to infer a reaction mechanism. Combination of quantum mechanical calculation with in situ EXAFS study supports the hypothesis made on the basis of IR results. The work demonstrates that the zeolitic voids act as \u201cnanoscale reaction chambers\u201d, where the reactivity of guest organometallic complexes can provide molecular insights into the elementary steps of heterogeneous catalysis. In this context, the investigation of metallocenes reactivity inside a polar matrix can be extremely useful to understand their properties in the polymerization conditions, where they are usually found as part of an ion-pair together with the anionic form of the activator (e.g. MAO)

A Multitechnique Approach to Spin-Flips for Cp2Cr(II) Chemistry in Confined State

Paramagnetic solid-state NMR, extended X-ray absorption fine structure (EXAFS), and Raman spectroscopies, along with detailed quantum mechanical calculations performed with different density functional theory (DFT) functionals, are successfully applied to investigate the magnetic, structural, and vibrational properties of molecularly isolated chromocene (Cp2Cr, where Cp = C5H5) and of its Cp2Cr(CO) adduct. Paramagnetic solid-state NMR unequivocally demonstrates that a spin flip occurs by coming from the paramagnetic Cp2Cr (triplet state) to the diamagnetic Cp2Cr(CO) adduct (singlet state), thus clarifying the theoretical dilemma of the disagreement among different functionals in predicting the most stable spin state. EXAFS and Raman spectroscopies are able to experimentally discriminate between singlet and triplet states, because a different spin state corresponds to a different geometry of the molecule, and therefore to different vibrational features. The here reported multitechnique approach could have great relevance in establishing the occurrence of spin flip in the chemical reactivity of transition metal complexes in both homo- and heterogeneous catalysis

A Multitechnique Approach to Spin-Flips for Cp₂Cr(II) Chemistry in Confined State

Paramagnetic solid-state NMR, extended X-ray absorption fine structure (EXAFS), and Raman spectroscopies, along with detailed quantum mechanical calculations performed with different density functional theory (DFT) functionals, are successfully applied to investigate the magnetic, structural, and vibrational properties of molecularly isolated chromocene (Cp2Cr, where Cp = C5H5) and of its Cp2Cr(CO) adduct. Paramagnetic solid-state NMR unequivocally demonstrates that a spin flip occurs by coming from the paramagnetic Cp2Cr (triplet state) to the diamagnetic Cp2Cr(CO) adduct (singlet state), thus clarifying the theoretical dilemma of the disagreement among different functionals in predicting the most stable spin state. EXAFS and Raman spectroscopies are able to experimentally discriminate between singlet and triplet states, because a different spin state corresponds to a different geometry of the molecule, and therefore to different vibrational features. The here reported multitechnique approach could have great relevance in establishing the occurrence of spin flip in the chemical reactivity of transition metal complexes in both homo- and heterogeneous catalysis

Immobilized Furanone Derivatives as Inhibitors for Adhesion of Bacteria on Modified Poly(styrene- co

The ability of brominated furanones and other furanone compounds with 2(3H) and 2(5H) cores to inhibit bacterial adhesion of surfaces as well deactivate (destroy) them has been previously reported. The furanone derivatives 4-(2-(2-aminoethoxy)-2,5-dimethyl-3(2H)-furanone and 5-(2-(2-aminoethoxy)-ethoxy)methyl)-2(5H)-furanone were synthesized in our laboratory. These furanone derivatives were then covalently immobilized onto poly(styrene-co-maleic anhydride) (SMA) and electrospun to fabricate nonwoven nanofibrous mats with antimicrobial and cell-adhesion inhibition properties. The electrospun nanofibrous mats were tested for their ability to inhibit cell attachment by strains of bacteria commonly found in water (Klebsiella pneumoniae Xen 39, Staphylococcus aureus Xen 36, Escherichia coli Xen 14, Pseudomonas aeruginosa Xen 5, and Salmonella tymphimurium Xen 26). Proton nuclear magnetic resonance spectroscopy (1H NMR), electrospray mass spectroscopy (ES-MS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were used to confirm the structures of the synthesized furanones as well as their successful immobilization on SMA. To ascertain that the immobilized furanone compounds do not leach into filtered water, samples of water, filtered through the nanofibrous mats were analyzed using gas chromatography coupled with mass spectroscopy (GC-MS). The morphology of the electrospun nanofibers was characterized using scanning electron microscopy (SEM)