Platinum , inert complexes

Since platinum(IV) complexes are also kinetically inert, optical diastereomers of Pt(en)2(L-2,3-diaminopropionic acid)4+ have been prepared.1028 The first synthetic procedure involves the chlorine oxidation of PtCl2(L-2,3-diaminopropionic acid) followed by reaction with ethylenedi-amine. Resolution is achieved through the (+)-tartrate salt. Alternatively the resolved complex can be prepared directly from the reaction of L-2,3-diaminopropionic acid on optically active cis-[PtCl2(en)2]Cl2. (... 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

The NMR method for studying the rates of moderately fast reactions has found little use with the strictly inorganic complexes of the platinum group metals, since they include many of the most inert complexes known. There are, however, two types of compounds of these metals which often are rather labile—i.e., the organo and hydrido derivatives. For such compounds, the NMR method, although less useful for stereochemical studies, is proving very valuable for studying reaction rates. 

The metallointercalation reagents are a class of heavy metal derivatives that bind to double-stranded polynucleotides by inserting between adjacent base pairs in the helix.1 2 Prototype members of this class of intercalators are (2,2 6, 2"-terpyridine)(thiolato)platinum(II) complexes.3 These may be synthesized from chloro(2,2 6, 2"- terpyridine)platinum(II), which can both intercalate and bind covalently by losing chloride ion. Covalent binding of the thiolato complexes is much slower owing to the more inert character of the Pt—S bond. Metallointercalation reagents also have the potential to bind to proteins that have natural receptor sites for nucleic acid bases. They may therefore also be used to provide isomorphous heavy atom derivatives for X-ray analysis. 

However, it is kinetically inert and requires a catalyst for this and many other of its reactions. Generally, heterogeneous catalysts such as CuO and Rh203 are used, though homogeneous catalysis has been reported for some reactions of nitric oxide. For example, a number of platinum metal complexes, as well as metals and metal oxides, can catalyze the oxygen transfer reaction ... 

Hexamethyldisilane, which is inert to the phosphine-platinum(O) complex, can react with an isonitrile-platinum(O) complex to give bis(trimethylsilyl)Pt(II) complex 25 in high yield (Eq. 9) [23]. 

Many catalysts are metals, metal oxides or other simple salts, or metal complexes. For example, formation of platinum(IV) complexes involving ligand substitution is an extremely slow process, due to the kinetic inertness of this oxidation state. However, the addition of small amounts of a platinum(II) complex to the reaction mixture leads to excellent catalysis of the reaction, assigned to mixed oxidation state bridged intermediates that promote ligand transfer. 

Platinum. In marked contrast to PdIV, platinum(iv) forms many thermally stable and kinetically inert complexes. So far as is known, Ptiv complexes are invariably octahedral and, in fact, Ptlv has such a pronounced tendency to be six-coordinated that in some of its compounds quite unusual structures are adopted. An apparent exception to the rule is /i5-C5H5Pt(CH3)3 but, as with other /i5-CsHs complexes, the ring can be considered as occupying three positions of an octahedron. Several interesting examples of this tendency of Ptlv to be 6-coordinate exist where novel bonding is required for this to be achieved (see below). 

The substitution reactions of amine platinum(IV) complexes is appreciably slower than those of most other inert metal amines, at least in acidic solutions. Consequently, the availability of a relatively labile leaving group, such as trifluoro-methanesulfonate, may have advantages where substitution is required at the sixth site about the pentaammineplatinum(IV) ion. In parallel with reports of other second- and third-row complexes in this chapter, the synthesis of [Pt(NH3)5(0S02Cp3)] from the [Pt(NH3)5Cl]Cl3 precursor is readily achieved. Both are described below. 

The subsequent functionalization of an M-C bond present in the resulting organometallic intermediate is the next reaction step. Not much is known about the ways that would allow for an oxy-functionalization of transient monohydrocarbyl palladium(ll) and platinum(II) complexes. Both Pd -C and Pt -C bonds are relatively inert toward electrophiles. In particular, oxygen, the most abundant and one of the most practically attractive oxidants, is usually unreactive toward such species. As a result, dioxygen activation with organoplatinum(II) and organopalla-dium(II) complexes is of significant current interest [5-7]. 

We should note the fact that in the bacterial systems, the substitutionally inert complexes which are mutagenic and active in the repair assay all have effects similar to those of the platinum complexes known to be anti-tumor drugs. Since repair effects are closely correlated with activity of the platinum compounds (17), then the anti-tumor activity could be related to the substitutional inertness of our complexes. We have noted that many of the complexes we have studied are far less bacteriocidal than the platinum compounds, and yet comparable in repair activity. This suggests that further study of substitutionally inert metal complexes may yield anti-tumor drugs which are as effective as the platinum compounds, and yet lack their undesirable toxic side effects. 

The current state of the art carbon supported electrocatalyst is made using variants of the colloidal approach. A common approach is to dissolve the metal salt solution in an appropriate solvent followed by reduction to form a colloid. A wide variety of recipes using reducing agents, organic stabilizers, or shell-removing approaches have also been developed in recent years. The patents most frequently referred to in this field are from United Technologies by Petrow and Allen.Sols of the metal are obtained for instance by an initial formation of a metastable platinum-sulfito complex, which is inert at ambient temperature but decomposes and produces the small Pt-crystallites at temperatures in excess of 60°C. Thus a relatively well-defined crystal size between 2 to 6 nm can be obtained. 

Tetraphenylcyclobutadienepalladium and -nickel complexes and tetra-methylcyclobutadienenickel chloride react readily with nucleophilic reagents to give 7r-cyclobutenyl complexes 12, 30, 31, 65, 91), a reaction reminiscent of those described by Chatt et al. for diene-palladium and diene-platinum halide complexes (Section VI, F). Non-halogen-containing cyclobutadiene complexes, however, appear inert under similar conditions so that this reaction is very dependent on the other ligands present. Some similarity between cyclobutadiene-metal and diene-metal complexes appears to exist but how far the parallel can be drawn remains to be seen. The reactions are fully discussed in Section VI. 

As we have seen, square planar complexes usually occur with the metals such as Pt(II), Pd(II), Ni(II), and Au(III).Tlie platinum(II) complexes are particularly inert and, because these rather slow reactions can be followed by fairly traditional and straightforward methods, have been extensively studied and analyzed. 

Among the second- and third-row transition metal halide complexes, those of platinum have received considerable study. Both the octahedral platinum(IV) complex PtCli and the square planar platinum(II) complex PtCl have been investigated. Both complexes are substitution inert under thermal conditions, and kinetic studies of their substitution chemistry have been important in the development of a general understanding of the mechanisms of substitution reactions in transition metal chemistry. The photochemistry of PtCli" was one of the earliest such studies to be made, and the early discoveries of the photosensitive nature of platinum halides led to these salts being used in photographic processes. The subsequent decision to use a silver-based process was based more on economical rather than on technical reasons. 

Conversely, the rate of catalysed isomerization of platinum(n) complexes in inert solvents is decreased by the addition of polar solvents. The reduction in rate parallels the reciprocal of the concentration of the added polar solvent. Here the effect on the kinetics can be ascribed to competition between the catalyst and the polar solvent at the platinum(u). From these kinetic results one can estimate the solvation number of the platinum(n) complex with respect to the polar solvent. Surprisingly, in most cases this solvation number turns out to be one - only with methanol as added solvent does one get the expected solvation number of two. ... 

PtMe2(OR)(N-N)(OH2)] OH (47) (Eq. 6.16) [30, 31]. These complexes, likely resulting from an oxidative ROH addition, were characterized by elemental analysis, IR and NMR spectroscopy, conductivity measurements and conversion to derivatives containing weakly coordinating bulky anions. These reactions are of interest because they represent the first examples of oxidation of platinum(II) complexes with alcohols and provide the first stable alkoxoplatinum(IV) complexes. The alkoxo-platinum(IV) bond is inert against solvolysis by alcohols, water and even dilute perchloric acid. 

These auto-reduction processes of platinum tetrammine complexes, whether performed in vacuum or in inert atmosphere, lead to particle sizes within the range 1-4 run encapsulated in the zeolite grains. Different states of iridium dispersions obtained by auto-reduction of [IrfNHjljCl] in Y zeolites were reported by the same authors [116]. Auto-reduction processes are difficult to control and are not well suited to obtain well-defined and homogeneous states of metal dispersion in zeolites. 

Pairs of geometric isomers differ from each other in their standard free energies of formation. However, the differences are typically not large. In the case of labile complexes, one isomer will be transformed into the thermodynamically favored form as equilibrium is reached. However, in the case of inert complexes, the conversion of one isomer into the other can be inhibited. In such cases, it may prove possible to study redox equilibria without the problem of an accompanying isomerization. Platinum and ruthenium chemistry has provided many examples of such systems. 

For complexes like PtL2X2 (X = halogen L = NH3, PR3, etc.) where cis-and trans-isomers exist, the trans-isomer is usually thermodynamically more stable. The c/s-isomer may be formed first in a reaction and, in the case of platinum, may be relatively inert to substitution. (Thermodynamic data are relatively scarce trans-Pt(NH3)2Cl2 is some 13kJmol-1 more stable than the cis-isomer.)... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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