Bipyridyls steric effects

Bipyridyl,4-methyl-4 -vinyl-electrochemical polymerization, 6,25 electropolymerization, 6,16 Bipyridyls bis(ZV-oxide) metal complexes, 2, 496 metai complexes, 2, 89, 90,93 steric effects, 2, 90 2,2 -Biquinolyl... 

Chelate complexes. Chelate rings can be formed by pyridines containing -substituents such as carboxyl or CH=NR. Important bicyclic chelating agents are 2,2-bipyridyl 68 (R = H), o-phenanthroline 69 and 8-hydroxyquinoline 70, which all form bis and tris complexes with many metals <1994CSR327>. This type of complex formation has many analytical applications. Overlap between the d-orbitals of the metal atom and the pyridine -orbitals is believed to increase the stability of many of these complexes. Steric effects can hinder complex formation as in 68 (R = Me). 

Now that the regular features of affinity for metals have been reviewed, some interesting irregular features can be discussed. Selectivity for particular ions is dependent on such irregularities. The strong tendency of cupric ions to form planar complexes means that copper is uniquely sensitive to a type of steric effect imposed by bulky groups. Thus, in Table 11.2, the stability constants for the cupric complexes of bipyridyl, phenan-throline, and folic acid lie below those of the corresponding nickel complexes, which is contrary to the natural order discussed in Section 11.2. 

A recent study822 of the effect of steric interactions on the luminescence of 2,2 -bipyridyl complexes of Rh found luminescence from both ligand and metal-centered orbitals of [Rh(dmbpy)3]3+ (dmbpy = 3,3 -dimethyl-2,2 -bipyridyl). At 77 K the metal-centered luminescence dominates, but in fluid solution the ligand-centered emission is enhanced. As in the unsubstituted analogs, ligand-centered luminescence dominates under all conditions for [Rh(dmbpy)2Cl2]+. Luminescence from fluid solutions of [Rh(phen)3]3+ and [Rh(bipy)3]3+ is consistent with the 77K behavior,816 as ligand-centered emission is observed.822... 

Other bipyridyl-related systems (7 and 8) have been found to be less effective for the oxidation of al-cohols.i 2,2 6, 2 -Terpyridinium hydrochloride chlorochromate (9) has also been prepared and found to be inert under standard conditions. It has been suggested that the steric bulk prevents effective electron transfer. 

Biaryl synthesis through Kumada coupling has employed a recyclable catalyst in which PdCl2 is anchored in a mesoporous silica with an appended bipyridyl unit." It also has addressed the steric hindrance issue. Choice of ligands appears to be important. One report describes the use of a PdCl2 complex of both l,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene and 3-chloropyridine, another indicates the effectiveness of (t-Bu2POH)2PdCl2. ... 

Detailed structural, spectroscopic, and computational studies on [Ru([n]aneS4)(2,2 -bipyridyl)][PF6]2 have concluded that the conformations of the macrocycles are determined by steric rather than electronic effects in these crowded cations and that the dynamic processes in solution previously attributed to reversible dissociation are in fact because of pyramidal sulfur inversion. ... 

Substitution. The kinetics of reactions of /ra/t -CPdClgCR SR ) ] complexes with a series of amines of various basicities and steric requirements have been studied in 1,2-dimethoxyethane at 298 K. In fact the first step is fast, and it is the second step whose kinetics have been examined here. Amine basicity is an important factor logarithms of second-order rate constants correlate linearly with amine pKa values in the absence of steric hindrance. Steric factors have a large effect when the incoming amine e.g. 2-methylpyridine) or the leaving sulphide (e.g. phenyl isopropyl sulphide) is bulky. The kinetic pattern for the reaction of [PdCl2(cod)] with 2,2 -bipyridyl in aqueous methanol has been interpreted by the reaction sequence shown in Scheme 3. The mechanism is dissociative with respect to the palladium centre. The rate law for the reaction of the [PdBr(dien)]+ cation with inosine has already been discussed in the introductory section to this chapter. 

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