Schiff bases spectroscopy

Ni111 Schiff base complexes have been produced by electrochemical oxidation of Ni11 complexes derived from naphthaldehyde, and identified by EPR spectroscopy.88... 

NMR spectroscopy has been widely used for identification of optically active Schiff bases.15-18 It is also a valuable tool for determination of their conformations, suggesting the reaction pathway or simply for verification of their purity.19-22 Selected examples are given below. 

Structure of the chiral poly-Schiff base [14] has been studied by NMR spectroscopy.29 It was shown that poly-Schiff bases have an interesting feature their chiral forms can be self-assembled into nanometer-size fibrous architecture, while their racemic analogues did not show any self-assembling properties. 

The NMR spectroscopy has been widely used in the studies of different types of equilibria like ring-chain tautomerism, racemisation or stereomutation and proton transfer equilibrium in Schiff bases. 

The dynamic NMR (DNMR) spectroscopy has been used in studies of stereomutations of non-symmetrical di-Schiff bases [18].39 It was shown that the hindered Schiff bases exist in DMSO in two chiral conformations. The presence of a pair of conformers being in equilibrium was explained by the existence of two stereogenic axes a g (aligned to Cl—N8 bond) and a 7 (aligned to C6—N7 bond) due to restricted rotation around two Ar—N bonds. The trans to cis interconversion as well as enantio- or diastereoisomerisation barriers for the compounds studied have been established using line shape analysis. 

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

The interactions between the chiral Schiff bases being derivatives of 2-aminoalcohols and substituted salicylic aldehydes and titanium (IV) isopropoxide [33] have been monitored by aH NMR spectroscopy.81... 

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

Fox et al.101-103 have studied the structure of Ni(II) complexes being derivatives of frans-l,2-diaminocyclohexane, using 1H NMR spectroscopy. Chemical shift differences (A<5) between di-Schiff base and its nickel (II) complexes observed for aromatic protons were attributed to the ring currents [40]. 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

Organotin(IV) compounds are Lewis acids. The crystalline structure of Lewis acid-base adducts between compounds R SnCl4- (/ = 0,..., 3) and Ni complexes with Schiff bases was determined by XRD, 119Sn Mossbauer and IR spectroscopy. Adducts are formed in 1 1, 1 2 and 2 1 ratios121. 

Using two-dimensional NMR spectroscopy, the spatial location of various carboxylate anions relative to the polyene chain of the protonated Schiff base of all-fraws-retinal was determined. The observed intermolecular NOE cross-peaks between a proton on the counterion and a proton near the nitrogen atom indicate the existence of ion-pair formation between the protonated retinal Schiff base and various counterions in chloroform. The results suggest that the most likely site of the carboxylate group of the counterion is in the immediate vicinity of the positively charged nitrogen atom of the retinal Schiff base. 

Binding of pyridoxal phosphate to peptide PP-42 also appears to be selective for lysine 30. As was indicated by NMR spectroscopy and UV/vis experiments, only one of three potential lysine Schiff bases appeared to form. To determine the site or sites of attachment, the aldimine peptide intermediates were reduced, proteolytically cleaved, and the fragments analyzed by mass spectroscopy. This... 

The cyclizations of cis- and lrans-2-hydroxymethyl-l-cyclohexylamine and cis- and lram-2-aminomethyl-l-cyclohexanol with 4-nitrobenzaldehyde have been studied by means of H NMR spectroscopy in CDCI3 solution (90ACSA364 91T2229). The time-dependent spectra confirmed that the reactions of all these amino alcohols proceeded via Schiff bases. With the exception of cis-2-hydroxymethyl-l-cyclohexylamine, the thermodynamically more stable perhydrobenzoxazine epimer is also the kinetically favored product. In the former case, from amino alcohol 21 (R = H), the Schiff base 37 with N-outside predominant conformation is formed first due to kinetic control, the less stable epimeric ring form 38 is obtained with N-outside predominant conformation. The thermodynamically controlled product 33 is formed subsequently, via the less stable open-chain form 37, in a slow equilibration process (90ACSA364). 

Only the most reactive organolithium compound, e.g. f-BuLi, is able to attack the carbon-nitrogen triple bond at temperatures below —20°C. For the other less reactive species, namely PhLi and n-BuLi, higher temperatures in ordinary solvents like pentane would be more appropriate to synthesize their lithiated Schiff bases. Flowever, for their detection IR spectroscopy seems to be unsuitable, since the relevant C=N stretching mode will be hidden by strong CH deformation modes of the solvent. 

Ru(PPh3)(H2O)j(SB "0 constimtes a series of complexes, made from RuClj(PPh3)3 and a chiral Schiff base SB obtained from salicylaldehyde and the L-forms of alanine, valine, serine, arginine, cysteine and aspartic acid. They were characterised by IR, circular dichroism, H and C[ H] NMR spectroscopies and by cyclic voltammetry. The supposed structure of one is shown in Fig. 1.41 [917]. 

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