NMR Spectroscopy of the Complexes

The NMR spectrum of the complex was recorded at four different temperatures (298, 316, 333 and 343 K) to investigate the effect of temperature on this (proposed) isomer equilibrium. The intensities of the signals were temperature dependent the less intense signals gained intensity upon increase in temperature (Figs. 4.15 and 4.16). The effect was shown to be reversible upon cooling of the sample. 

Another experiment was conducted with [Zn2(CH3L2)(CH3COO)2](PF6). Here, the H-NMR spectrum was recorded in the more strongly coordinating solvent d -(CD3)2SO to shift the equilibrium between the isomers to the ether arms-ofF isomer. The change in intensities can be seen in Fig. 4.17. The integral ratio has changed substantially. 

In order to explore the mechanism employed by the Zn(II) complexes, the incorporation of into the reaction product was monitored by NMR. In addition to the Zn(II) complexes [Zn2(CH3L2)(CH3COO)2](PF6) and 

---

H NMR spectroscopy of the complexes of UCI4 with 18-crown-6 and dicyclohexyl-18-crown-6, and of U(Cp3C02)4 with these two ligands and with the cryptands [2.1.1], [2.2.1], [2.2.2] and [2.2.2.B] in nitromethane or methyl cyanide indicate that the metal atom is bonded to the oxygen atoms of the ligand. ... 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The tetradentate ligands (340) and (341) form 1 1 metakligand complexes with [IrCl(cod)]2.548 The complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. The multitopic ligands L, (342) and (343), bind to Ir1 to form [IrL] species which have been characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy.549 The complexes show enantioselectivities of up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions. 

The chemical shift differences of the diastereotopic hydrogens are listed in Table 17 they depend strongly on solvent effects, as expected for an ionic product. They are in the range of 8 = 0.01 -0.1, well suited for measurement of the enantiomeric purity of the phosphanes. An alternative method for the measurement of Horner phosphanes is by 13C-NMR spectroscopy of diastereomeric complexes formed with [>/3-( + )-0 7 ,57 )-pinenyl]nickel bromide dimer73. 

The studies of Wilkinson et al. included IR and H-l NMR spectroscopy of the intermediate species of this catalyst system (7). This led to recognizing tris(triphenylphosphine)rhodium(I) carbonyl hydride (D) as the key stable rhodium complex. The reactive trans-bis-(triphenylphosphine)rhodium(I) carbonyl hydride (E) resulting via the dissociation of this complex... 

The reader is referred to an earlier review2 for a discussion of the IR and NMR spectroscopy of metal complexes of arynes. 

HIV-protease/inhibitor complexes have a molecular weight of approximately 22 kDa. Although NMR spectroscopy is well suited to determination of the structure of molecules in this size range, efforts to determine the solution structure of the complex were hampered by the fact that the protease undergoes rapid autocatalysis in solution. It required the development of potent inhibitors before NMR studies of the complex became feasible. The first solution structure of HIV-protease bound to the cyclic urea inhibitor DMP-323 (Fig. 25) was reported by Yamazaki in 1996.133 The protease exists as a homodimer. Each 99-residue monomer contains ten /3-strands and the dimer is stabilized by a four-stranded antiparallel /3-sheet formed by the N- and C-terminal strands of each monomer. The active site of the enzyme is formed at the interface, where each monomer contributes a catalytic triad (Asp25-Thr26-Gly27) that is... 

In 1998, Christie and co-workers reported the first example where stereoselectivity was induced by a chiral metal complex.42 By making dia-stereomerically pure heterobimetallic Mo-Co alkyne complexes possessing a menthyl group (35 and 36), they were able to obtain diaster-eomeric excesses of >99% (determined by 13C NMR spectroscopy) of the resulting cyclopentenone (37 and 38 - Scheme 13). 

Organometallic complexes such as tris, bis and monocylopentadienyl complexes, cyclooctatetraenyl complexes, cyclopentadienyl-cyclooctatetraenyl complexes, indenyl complexes, fluorenyl complexes, complexes with other aromatic ligands, callixerene complexes, NMR spectroscopy of organometallic complexes, vibrational spectra, and catalytic applications form the theme of the sixth chapter. 

The 1,4-addition of 4-tert-butyl benzenethiol to 4-inethyl-4-phenyl-2,5-cyclohexanedienone catalyzed by cinchonidine is more complex as, in addition to a 1 1 adduct 9, the bis-addition product 10 is formed. The monoaddition product consists of trans- and cu-isomers in a 3 1 ratio with 17% ee and 52% ee, respectively (determined by 13C-NMR spectroscopy of the corresponding acetals with optically active alcohols)8. ... 

The effect of metal ion stereochemical preference in mediating the final structure of a self-assembling helix was examined by Williams and coworkers using the ligand (20) [26]. The formation of a double-hehcal structure is seen from the reaction of two equivalents of copper(I) and two equivalents of (20) (Figure 8). This structure, with pseudotetrahedral geometry around the metal centers, was found to exist in solution by UV-visible spectroscopy, H NMR and cyclic voltammetry. The double helix does not appear to be the only species existing in solution mononuclear [Cu(20)J+ was observed by H NMR spectroscopy. The H NMR spectrum of the complex between copper(I) and... 

It is important to note that complexes 1 and 5 are recovered unchanged from the reaction mixture following catalysis, and P NMR spectroscopy of the product solution showed no other phosphorus-containing species present. 

Natural product extracts are generally complex and comprise mixtures of neutral, acidic, basic, lipophilic, hydrophilic, and amphiphilic (e.g., amino acids) compounds and, as a consequence, there is rarely one method that will serve for all eventualities. It is sometimes worthwhile to carry out H or NMR spectroscopy of the extract or fraction to determine the class of compound(s) to be separated (1)—deuterated NMR solvents are cheap ( 1.00 for CDCI3) and 1 D NMR experiments are quicker to run than the extensive... 

Figure 7-11 H-NMR spectroscopy of the electron transfer complex of horse heart cytochrome c and CCP. Titrations involving the addition of P. denitrificans peroxidase to (A) horse heart cytochrome c and...

---