Coordinately bonded pyridine

The structure and nature of acidity of sulphated zirconia has been recently investigated (ref. 8-10). Untreated zirconium hydroxide which is amorphous, crystallises 1n the monoclinic form at 350°C. On the contrary, the sulphate treated zirconium hydroxide crystallises at higher temperature (500ooC) into tetragonal form, which has significantly greater surface area as compared to the monoclinic zirconia. The infra red spectra of absorbed pyridine indicate presence of only coordinatively bonded pyridine and not the protonated pyridine implying that the zirconia is predominantly of Lewis... 

The IR spectra of pyridine adsorbed on SO4/ZrO2-l(650°C) are shown in Fig. 12, where both the pyridinium ion (at 1540 cm 1) and coordinately bonded pyridine (at 1440 cm-1) are observed, the decrease in the former band being seen together with the increase in the latter band after evacuation at high temperature (spectrum C) (150, 151). Spectrum D shows the changes that occur upon the addition of water to the sample the increase in the 1540-cm 1 band indicates that a considerable amount of Bronsted acid has been formed, and the decrease in the 1440-cm 1 band shows a... 

However, the behavior or function of spilt-over hydrogen on zeolite is not clear yet. Pyridine, which is a typical organic base, is chemisorbed on either Brpnsted (B) acid site or Lewis (L) acid site to form pyridinium ion or coordinately bonded pyridine complexes with B or L acid sites, respectively, and giving different IR-adsorption bands [7]. Therefore, the change in the IR band of adsorbed pyridine on B or L acid sites could be attributed to the change in the adsorbed pyridine on acid site. 

Hydrogen bonded pyridine Coordinately bonded pyridine Pyridinium ion ... 

Fig. 34. Compound 129 assembles to a supramolecular structure through coordinative bonds between the cobalt(III) ions and the pyridine residues of the bridging B(py)(OMe) group...

The chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

Fig.1. Structures of porphyrin 1, chlorophyll 2, and phthalocyanine 3. In the presence of metal salts M"+X (M=metal, X=counter anion, n=oxidation state or number of counter anions), porphyrins produce chelate complexes. Some metal chelates of the porphyrins, such as ZnPor, form further coordination bonds with other ligands such as pyridines...

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

Ligand self-assembly through coordinative bonding has been used to increase the bulkiness of a monodentate tris-3-pyridyl phosphine ligand employing the zinc porphyrin/pyridine interaction (Scheme 33) [95-97]. The corresponding rhodium catalyst allowed for regioselective hydroformylation of2-octene [95]. 

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

Three (or four, see following text) additional coordination bonds are established with two solvent molecules (pyridine, H20) and the anion X- (Cl- or N03-). The good behaviour of this reaction system led to the preparation and crystallographic characterization of a comprehensive quasi-isostructural series of... 

The chemistry of coordination compounds comprises an area of chemistry that spans the entire spectrum from theoretical work on bonding to the synthesis of organometallic compounds. The essential feature of coordination compounds is that they involve coordinate bonds between Lewis acids and bases. Metal atoms or ions function as the Lewis acids, and the range of Lewis bases (electron pair donors) can include almost any species that has one or more unshared pairs of electrons. Electron pair donors include neutral molecules such as H20, NH3, CO, phosphines, pyridine, N2, 02, H2, and ethyl-enediamine, (H2NCH2CH2NH2). Most anions, such as OH-, Cl-, C2042-, and 11, contain unshared pairs of electrons that can be donated to Lewis acids to form coordinate bonds. The scope of coordination chemistry is indeed very broad and interdisciplinary. 

Nonadienoate (73) is used for the synthesis of another royal jelly acid, 2-decenedioic acid (146) (130). The terminal double bond was car-bonylated with Co2(CO)8 coordinated by pyridine. Linear ester was obtained in 80% selectivity. Hydrolysis and double-bond migration promoted by strong base produced royal jelly acid (146) as a crystalline compound. 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

Parry (344) determined the infrared spectrum of pyridine adsorbed on rj-alumina dehydrated at 450°. Characteristic differences in the 1400-1700 cm region exist in the spectra of pyridine adsorbed via hydrogen bonds, pyridinium ions, and pyridine coordinately bonded to electrophilic sites. Pyridinium ions are characterized by a strong band at 1540 cm and a very strong band at 1485-1500 cm" coordinately bonded pj ridine has a strong absorption at 1447-1460 cm". No evidence was found for the existence of Bronsted sites on the alumina surface. 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or... 

Differential thermal analysis of the PVP-Co(III) complexes also supplied information about the strength of the coordinate bond between Co and FVP5. The dissociation temperature of the coordinate bond increases with degree of coordination (x) 198° (x = 0.17) < 204° (monomeric pyridine complex) < 224°... 

As discussed in the last section, the driving force for the self-assembly of cate-nane 13 is the efficient aromatic contact between two monomer rings. Therefore, we designed a rectangular molecular box 21 containing Pd(II)-pyridine coordinate bonds. 

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