Protonation transition metal complexes

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

The scopes outlined above limit the purpose of this chapter, which will not cover (1) the transition metal complexation by protonated or functionalized forms of calix[4]arenes (2) the metalation of calix[4]arenes using non-transition metals and (3) chemical curiosities derived from the metalation of calix[4]arenes (some recent reviews cover these areas very well).1 In addition, the authors have been particularly careful to report only those compounds which have a well-established synthesis and a full spectroscopic and structural characterization. 

Transition metal complexes of bisimidazolepyrazine 28 show pronounced downfield shifts of the ring protons in their 1H NMR spectra due to the lowering of electron density <1999IJA350>. 

In transition metal complexes, proton hfs are normally < 20 MHz so that the corresponding second order contributions, which amount to < 10 kHz, may usually be neglected. For nitrogen ligands, however, the second order corrections produce frequency shifts up to 200 kHz. Since hf interactions of central ions can amount to several hundred megacycles, the terms in AE become very important for a correct description of the ENDOR spectra. 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln < T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln < Tj,i is not always fulfilled. 

Since the unpaired electron in transition metal complexes is generally localized near the central ion and the ligand atoms in the first coordination sphere, summation in (5.5) over these nuclei is often sufficient. In this approximated form, the point-dipole model has frequently been applied in ENDOR studies of transition metal complexes to determine the proton positions from their hfs tensors (Sect. 6). In some cases the accuracy of this method has turned out to be significantly higher than that of an X-ray diffraction analysis62,130 131). 

The proton ENDOR study of the chromyl ethyleneglycolate anion in ethanol reported by Mohl et al.293 presents the first successful adaptation of the ENDOR technique to a transition metal complex in liquid solutions. The aim of this work was to characterize the ENDOR relaxation behavior and to find optimum conditions for ENDOR detection. Two proton ENDOR lines with a hf splitting of ajj, = 1.74 MHz were observed. This is in agreement with a previous EPR study294 which had shown that all eight protons are equivalent. The optimum microwave and rf fields are both proportional to (Tr(g - geO rAj 2)1/2, where A denotes the dipolar part of the proton hfs tensor. For the chromyl ethyleneglycolate anion these two values have been calculated to — 8 10-6T and B = 2.7 mT. According to Mohl et al.293, successful proton... 

ENDOR on transition metal complexes in solutions is only attainable if no other nuclei possessing a much larger hf anisotropy than the protons are present. Moreover, the deviation of the g tensor principal values from g should be small, so that Tr(g - gel)2 < 3 10-3. Solvent and temperature, however, appear to have minor influence on optimum ENDOR detection conditions. 

R. J. Angelici. Basicities of Transition Metal Complexes from Studies of their Heats of Protonation A Guide to Complex Reactivity. Acc. Chem. Res. 1995, 28, 51-60. 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

Figure 10.10 Energy profiles of proton transfer to a hydride ligand of a transition metal complex in solution AEi = + 3 to 4kcal/mol, AE2 = — 5 to — 7 kcal/mol, A 3= + 10 to 14 kcal/mol, and A 4 = —7 kcal/mol the energy is a function of the proton-hydride distance, varying from an initial state (2.5 A) to the final product (0.9 A) conversion of the intimate ion pair to the solvent-separated ion pair is shown as a function of the H+- O" distance. (Reproduced with permission from ref. 29.)...

CgQ with this Zr complex, a red solution is formed, unlike the green solution ofr transition metal complexes of Cjq. The structure of the air-sensitive Cp2ZrClC5oH was confirmed by NMR spectroscopy. The hydrogen transferred from the Zr to CgQ resonates at 5 = 6.09, a typical value for fullerenyl protons [83]. Hydrolysis of Cp2ZrClC5oH with aqueous HCl provides access to the simplest hydrocarbon C5QH2 (30, Scheme 7.14). Spectroscopic characterization of CggH2 showed that the compound is the isomerically pure 1,2-addition product. 

The free electron pair(s) in the concave pyridines 3 (Table 1), 13 (s. Scheme 3) and 29 (s. Scheme 5) and especially in the concave 1,10-phenanthrolines 11 (s. Scheme 2) and 21 (Structures 3) are not only able to bind a proton, they may also be used to coordinate a metal ion. For concave 1,10-phenanthrolines 11 and 21, transition metal complexes 87 (Structure 11) have already been generated [18, 55]. They form readily in acetonitrile solution with binding constants of 10 10 and larger. Of great importance is the nature of the chains X in the concave 1,10-phenanthrolines 21 (Structures 3). Pure aliphatic chains lead to smaller association constants than polyether chains. 

Proton NMR spectra of some V1" and Cr1" complexes indicated a facial octahedral configuration with all three sulfur atoms cis.234 More recent 13C and 19F NMR data on a wide range of transition metal complexes of fluorinated monothio-/ -diketones support the assignment of cis square-planar and facial octahedral geometries.235,236 X-Ray structural data have established the cis square-planar configuration for a Pd" and a Pt" complex237 and four Ni" complexes,238,239 the tetrahedral configura-... 

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