Pyridyl unit

A number of dimers featuring an orthogonal disposition of the porphyrin planes have been prepared. The perpendicular geometry is achieved using a 4-pyridyl unit linked through the porphyrin meso posi-... 

Nevertheless, positions and differences of Ej and E2 allow some qualitative interpretations. The vinylene bridge in 5,6-position seems to be inert not only in ii, which resembles the 4,4 -bipyridyl system i/(c.f. 3.1) but also 13B2 and 13B3 which behave similar to the 2,2 -bipyridyl systems 2B2 and 2B3. In 13B3 the vinylene group prevents the two pyridyl units from distortion by the Ca-brit e between the nitrogen atoms compared to 2B3. Thus its Ksem stays closer to that of 13B2. 

The incorporation of a 2,6-pyridyl unit also prevents formation of the dimer-dimer catenane4, but gives the macrocyclic dimer 20 and tetramer 21 in 15 and 33% yield respectively (Figure 9) [22]. 

The ability of a linear template to orient two identical pyridyl units in a face-to-face stacked arrangement suggested that a linear template might be used to assemble two unsymmetrical reactants for a head-to-head photodimerization. Since different combinations of hydrogen-bond acceptor sites may be employed for the reaction (i.e. I runs-1 -( -pyndyl)-2-(n/-pyridyl)e(hylene (where n,m =2, 3, or 4 n f m)), a general means to establish regiocontrol of the cycloaddition could be achieved. 

Fig. 19 Formation of the dimeric coordination capsule 28 based on the calixarene ligand 27. The helical twist results from the orientation of the meta-substituted pyridyl units upon coordination to palladium(II) ions (represented by grey spheres). Adapted with permission from [141]. Copyright 2001 American Chemical Society...

Ochai and Busch (101) briefly mention the synthesis of a macrocycle containing a pyridyl unit (XXXII) from the condensation of 2,6-diacetylpyridine with bis(3-aminopropyl)imine in the presence of Ni(II) ions. A similar condensation (38), using the same pyridyl source, with tetraethylaminetetramine in the presence of Fe(II) ions produces macrocycles XXXIII and XXXIV, containing five and six donor... 

Other macrocycles containing pyridyl units (XXXV-XXXVIII) have been synthesized on Co (II) and Ni(II) templates by Lewis and Wainwright (84). In the case of Ni(II) the free macrocycle XXXVIII can be generated it reacts almost quantitatively with Fe(II) ions to give an air-stable high-spin complex. However, most macrocycles... 

ESTMS of 22-2 and also 22-3 showed the expected cryptates, but also peaks corresponding to cryptand dimers <2004CC2670> Table 2 summarizes the reported data. Further exploration with X-ray crystallography of the cryptates showed strong hydrogen bonding and n-n stacking between the substituted pyridyl units. 

Chan et al. reported this class of highly effective atropisomeric, heteroaromatic ligands consisting of two interconnected 2,6-dimethoxy-pyridyl units. The MeO substituents in an ortho position to the nitrogen in the pyridyl rings are introduced very logically to block the access of the pyridyl ring to the metal center which may exert adverse effects, as it... 

That subtle variations in the ligand system can have a large influence on the overall structure of the copper compound is also attested to by the different structures of [Cu(MesSiCH(Py-2))]4 (Py-2 = 2-pyridyl) and [Cu((MejSi)2C(Py-2))]2. In both compounds, the 2-methylpyridyl group is -bonded to a copper atom, while a linear coordination geometry at the copper center is achieved throu inter-molecular coordination of the nitrogen atom of an adjacent pyridyl unit. However [Cu(MejSiCH(Py-2))]4 exists as a tetramer in solution and in the solid state [68], whereas [Cu((MejSi)2C(Py-2))]2 has a dimeric structure (see Fig. 1.11) [68, 69]. This difference is probably a consequence of the presence of a second bulky Me Si substituent at the carbon atom bound to copper in [Cu((Me5Si)2C(Py-2))]2. 

A mixed organic-inorganic 3D porphyrin network was obtained by cocrystallization of palladium mc n-tetrapyridylporphyrinate and cadmium nitrate. All four pyridyl units are bound to octahedral cadmium centres which are each coordinated by two nitrate, water and pyridyl ligands (Figure 7.27). One half of the py-Cd-py connections is linear, the other half bent (103°), having two cis pyridine ligands. As is common in mc o-tetraarylporphyrins,... 

The mechanism for the SPAN layer changing the emission properties of the PPy VPV polymer is attributed to the formation of new emissive species due to protonation of the pyridyl units by SPAN. These species was identified by both absorption and PL experiments. Figure 9.15 shows the absorbance spectra of a PPy VPV layer, a SPAN layer, and a bilayer of PPy VPV/SPAN. SPAN is a self-doped, water-soluble conducting polymer with a room-temperature conductivity of 10-2 S/cm.18 It has a wide optical window from green to near infrared PPy VPV... 

Double deprotonation of the bifunctional mono-CpH/alcohol racemic ligand linked by the 2,6-pyridyl unit followed by salt metathesis with ZrGL yielded the ansa-Cp/oxo zirconium dichloride complex 555, but with the concomitant formation of the undesired complex 556 incorporating the mono-anionic form of the ligand and the simple adduct of the neutral ligand and Z1CI4 0 (Scheme 126). The latter two undesired species have been structurally characterized however, complex 555 obtained forms an insoluble oligomeric species after the loss of THF upon purification. 

He et al. [68] examined the effect of adding flexibility to the hydrogen bond acceptor bis-pyridyl units. They attached stilbazole derivatives to either end of differently sized oligo(ethylene oxide) spacers (4) (Fig. 7). Mixing... 

The synthesis of platinum polyynes with heteroaromatic organic spacer groups has also been readily accomplished, for example, species 188 with skeletal pyridyl units that make use of the dehydrohalogenation method (Scheme 16(a)) or an McsSnCl elimination procedure analogous to that in Equation (64) (Scheme 17)." The resultant polymers could be quaternized at nitrogen with methyl iodide and triflate. Materials such as 189 with a bithiazole spacer have also been prepared by the dehydrohalogenation method. 

Nitrogen can be incorporated in the macroring by inclusion of a small-ring nitrogen heterocycle such as pyridine or pyrimidine. Three approaches should be noted. If the pyridyl unit is incorporated as a 2,6-bis(methyleneoxy)pyridine derived from lutidine, the precursor will normally be a lutidine dihalide or diol. In the case of sulfur, the diol would be a dithiol. If the pyridyl unit is to be attached by 2,6-aminomethyl groups, amide formation followed by reduction is a possibility. In the event that the heterocycle will be attached directly to a macroring heteroatom, nucleophilic aromatic substitution may be useful. 

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