Bipyrimidine complexes

Oxidation of Methane. A variety of new catalyst systems have been disclosed, and new reagents were developed with the aim to perform selective transformation of methane to methanol, methyl esters, and formaldehyde. Much work was carried out in strongly acidic solutions, which enhances the electrofilicity of the metal ion catalyst, and the ester formed is prevented from further oxidation. An important advance in the selective oxidation of methane to methanol is Periana s 70% one-pass yield with high selectivity in sulfuric acid solution under moderate conditions.1073 The most effective catalyst is a Pt-bipyrimidine complex. Pt(II) was shown to be the most active oxidation state generating a Pt-methyl intermediate that is oxidized to yield the product methyl ester. A density functional study... 

The aerobic oxidation of methane in water catalyzed by [Pt(Mebipym)Cl2] [PV2Mo1004o]5 (Mebipym = N-methy-2,2 -bipyrimidine) complex supported on Si02 was reported [149]. The conjugation of [PV2Mo1004o]5 to a known Pt2 + -bipyrimidine complex by electrostatic interaction could fadlitate the oxidation of the Pt2 + intermediate to a Pt4 + intermediate by 02, resulting in the catalytic aerobic oxidation of methane to methanol in water and then surprisingly further to acetaldehyde via a carbon-carbon coupling reaction. 

A later variant involved incorporation of an oxidant, Pt(IV), which led to formation of functionalized species, RX, from alkane, RH. In the typical chloride-rich Shilov systems, X is commonly Cl and OH. The Pt(IV) oxidant is reduced to Pt(II) during the reaction, but it has proved hard to replace this expensive oxidant by a cheaper one while retaining activity. A remarkable system of this type discovered by Periana [109] uses cone. H2SO4 as both oxidant and solvent and a Pt(II) 2,2 -bipyrimidine complex as catalyst with the result that CH3OSO3H, a methanol derivative, is formed from methane. 

Ard " and Gladkov and Solovyov propose, from normal coordinate calculations performed for the copper porphin complex, that bands near 368, 234 and 206 cm contain important contributions of the vCuN mode. From a vibrational study on the non-macrocycle phenantroline copper complexes it has been proposed the vCuN modes near 410 and 288 cm In bipyridine complexes of Cu(II), the vCuN mode is proposed at 297cm " a similar assignment is proposed in bipyrimidine complexes. ... 

These 2,2 -bipyrimidine complexes are relatively unstable, and attack by azide and thiocyanate, not established for, e.g., the [Fe(bipy)3] cation, has been observed for the [Fe(bipym)3] + cation. The ferrozine [fz=(3)] complex [Fe(fz)s] " is very stable and one of the most inert di-imine-iron(ii) complexes. There is here ample time to demonstrate the rapid formation of an intermediate, and to follow the decay of the [Fe(fz)3] " and the intermediate by conventional spectroscopic techniques. The use of aqueous methanol as solvent proved advantageous, as this increased the chemical potential of the hydroxide ion and thence increased formation of the intermediate. The 4— charge on the complex here, in contrast to the great majority of di-imine complexes studied, rules out significant ion-pairing between complex and hydroxide. ... 

The proposed mechanism for this methane oxidation is shown in Scheme 18.2 and follows steps discussed in more detail in the next section. One remarkable feature of this platinum system in this highly acidic medium at high temperatures is the thermodynamic stability of platinum(II) boimd by the bipyrimidine ligand. Addition of platinum metal to bipyrimidine in sulfuric acid leads to generation of the platinum(II) bipyrimidine complex (Equation 18.8). 

Following up on their earlier report of surprisingly selective mercury-catalyzed oxidation of methane to methyl bisulfate by sulfuric acid (which was also the reaction medium) [52], Periana and coworkers discovered that a bipyrimidine complex of Pt(II) worked even better, generating the same product in over 70% yield - a remarkable achievement, given that selective oxygenation of methane to methanol or derivatives thereof rarely surpasses yields of a few percent. A mechanism closely akin to that of the Shilov system was proposed (Scheme 15), with SO3 replacing Pt(IV) as the oxidant to convert RPt(II) to RPt(IV) whether the initial C-H activation involved an RPt(IV)H intermediate or not was left an open question [52]. 

The study of exchange interactions between transition-metal ions and copper(II) in particular has been an active field of research (Section 6.6.3.6).254-256 In this context, Verdaguer and co-workers investigated complexes (292) and (293), supported by flexible 2,2 -bipyrimidine.257... 

As observed in other oxo-centered triruthenium-acetate complexes, the electronic absorption spectra of 36 and dimer 37 with rt/zr -metal latcd 2,2 -bipyrimidine are... 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

Computational results were also obtained, in a different study for the possibility that the chloride rather than the ammonia in cis-(NH3)2PtCl2 was substituted by methane (148). In the same contribution, an analogous study examined the reactivity of the (bipyrimidine)PtCl2 complex (Fig. 4). 

It is also noteworthy that complexes containing ligands such as TAP, HAT, bpz (2,2 -bipyrazine) or bipym (2,2 -bipyrimidine) (Fig. 2), have free non-chelated nitrogen atoms. It has been shown that all these compounds are all more basic in the excited state than the ground state [75,93,94], so that the excited states are already protonated at pH 5-6 on the non-chelated nitrogen atoms. 

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

The emission from [Ru(bpz)3] is quenched by carboxylic acids the observed rate constants for the process can be rationalized in terms of the protonation of the non-coordinated N atoms on the bpz ligands. The effects of concentration of carboxylate ion on the absorption and emission intensity of [Ru(bpz)3] have been examined. The absorption spectrum of [Ru(bpz)(bpy)2] " shows a strong dependence on [H+] because of protonation of the free N sites the protonated species exhibits no emission. Phosphorescence is partly quenched by HsO" " even in solutions where [H+] is so low that protonation is not evidenced from the absorption spectrum. The lifetime of the excited state of the nonemissive [Ru(Hbpz)(bpy)2] " is 1.1ns, much shorter than that of [Ru(bpz)(bpy)2] (88 nm). The effects of complex formation between [Ru(bpz)(bpy)2] and Ag on electronic spectroscopic properties have also been studied. Like bpz, coordinated 2,2 -bipyrimidine and 2-(2 -pyridyl)pyrimidine also have the... 

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