Quaterpyridines

The Boekelheide reaction has been applied to the synthesis of non-natural products with the preparation of quaterpyridines serving as an example. The sequence began with the 2,4-linked bipyridyl-N-oxide 25. Execution under the typical reaction conditions produced the expected bis-pyridone 26. Treatment with POCI3 afforded the corresponding dichloride that was submitted to a palladium-catalyzed coupling with 2-stannyl pyridine to produce the desired quaterpyridine 27. 

Pyridines which failed to produce detectable quantities of 2,2 -bipyridines include 2-aminopyridine, 3-aminopyridine, 3,5-dibromo-pyridine, and ethyl isonicotinate. 2,2 -Bipyridine failed to give any 2,2 6, 2 6",2" -quaterpyridine, and this is discussed in a later section. 

The crude 2,2 -bipyridine obtained from the reaction of pyridine and degassed Raney nickel was found to contain 1.5% of 2 6, 2"-terpyridine, but no 2,2 2, 2" 6 ",2 "-quaterpyridine could be detected. Moreover, experiments with 2,2 -bipyridine and Raney nickel have failed to yield quaterpyridine, and the amount of terpyridine formed in experiments with mixtures of pyridine and 2,2 -bipyridine was found to be no higher than in the reaction with pyridine itself. " ... 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

Photophysical studies have been performed on dendrimers 41 [49], built around a [Ru(bpm)3]2+ core (bpm=2,2 -bipyrimidine),and42 [59],built around a [Ru(QP)3]2+ core (QP = 2,2 3, 2" 6",2" -quaterpyridine). In both compounds energy transfer from the peripheral Re(I)-based chromophores to the central Ru(II)-based unit occurs with unitary efficiency. 

Quadrupole splitting constants, 37 129 Quantum mechanical methods, 38 424 Quantum mechanical predication, lattice energies and, 1 181-186 2,2 6, 2 6",2"-Quaterpyridine coordination compounds, 30 104-106 Structure, 30 70 Quenching center, 35 321 organometallics, 19 93-98 chemical, 19 98... 

Lehn, J. M., Sauvage, J. P., Simon, J., etal, Synthesis and metal-complexes of a conformationally restricted quaterpyridine -crystal-structure of its dimeric dinuclear Cu (I) complex, [Cu2(pqp)2]2+. Nouv. J. Chim. - New J. Chem. 1983, 7, 413-420. 

The strain introduced into the ligand will be even higher in complexes of the higher oligopyridines, and it was at one time suggested that they could not exhibit their maximum denticity in monodentate complexes. It is now clear that quaterpyridine may act as a quaterdentate ligand, albeit with very unsymmetrical M—N distances. 

C—C bond enables it to act as a bis bidentate. Each copper(I) is bound to two bidentate quaterpyridine moieties. 

The planar quaterpyridine ligand (190) can coordinate in a somewhat strained square-planar arrangement with Cn, Co , Co , Ni , Cr , and Pd but with potentially tetrahedral metal centers such as Cu or Ag a subtle interplay of steric factors (metal and ligand) allows the formation of dinuclear dibridged helical structures (see Section 8). ... 

The tetradentate quaterpyridine and hexadentate sexipyri-dine ligands form 1 1 complexes with yttrium aud europium nitrates, respectively the first-named ligand generates nine-coordinate [Y(qtpy)(N03)2(H20)]+ cations while the latter produces 10-coordinate [Eu(spy)(N03)2] cations where the ligand twists itself helically round the metal. ... 

In an early study by Lehn et al., it was also demonstrated that quaterpyridine (36) forms a monomeric copper(I) species (that is not helical), whereas with cop-per(II) a dimeric species is obtained these complexes may be interconverted elec-trochemically. The above monomeric copper(I) complex was shown by X-ray diffraction to have a tetragonal pyramidal structure [four basal nitrogens and an axial oxygen (water)] in the solid state. This ligand also yields a dinuclear complex with ruthenium(II) of type [Ru2L2] . ... 

In a related investigation to that just discussed, the tetramethyl-substituted quaterpyridine 37, which was designed to promote co-ordination in a twisted configuration, yielded complexes with 1 1 and 2 1 (metal ligand) stoichiometries with manganese(II) and cobalt(II). With copper(I), a dimeric helical complex of... 

The non-symmetrical alkyl-substituted quaterpyridines 39 and 40 (Figure 6.21) also react with an excess of [Cu(MeCN)4](PF ) to yield the corresponding complexes of type [Cu2L2](PF )2. ° An analysis of the NMR spectra of each product indicated that, while the ligand derivative with R = Me (39) yields a 1 1 mixture of head-to-head and head-to-tail dimers (see Figure 6.21), the presence of the... 

The bis(methylthio)-substituted quarterpyridine, 4, 4"-bis(methylthio)-2,2 6, 2" 6, 2" -quaterpyridine (44), yields a binuclear copper(I) complex, but a mononuclear species with copper(II). ° A spectroelectrochemical study indicated that a redox-induced transformation occurs between these two complex species. The X-ray structure of the 2 2 [copper(I) ligand] helical complex confirmed that each copper is tetrahedrally co-ordinated to four pyridyl nitrogens from two ligand strands, with a short Cu-Cu distance of 3.32 A being present. 

The interaction of the chiral quaterpyridine 75, bearing fused chiral groups in the 5,6- and 5", 6" -positions, with copper(I) has been investigated. The respective ligand isomers were demonstrated to form structurally characterised P or M dinuclear double helicates in approximately 99% diasteromeric excess upon coordination to the above ion. ... 

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