Terpyridine, bidentate

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. 

Reaction of 3-(2-pyridyl)pyrazole and 6-(3-pyrazolyl)-2,2 -bipyridine with zinc salts results in mononuclear pseudo-octahedral complexes [ZnL3](PF6)2 and [ZnL2](PF6)2 respectively. The ligands coordinate as neutral mononucleating chelates in a similar fashion to the bidentate 2,2 -bipyridine or tridentate 2,2 6, 2"-terpyridine respectively.153... 

The classic low spin [FeN6]2+ systems are those derived from the bidentate diimines 2,2/-bipyridine 1 (bpy) and 1,10-phenanthroline 2 (phen) and the tridentate terimine 2,2/ 6/,2//-terpyridine 3 (trpy). 

The model tridentate terimine system is terpyridine 3 and salts of its [Fe N6]2+ derivative are low spin, like those of the diimines 1 and 2, but there are important differences in the bidentate systems on the one hand and the tridentate on the other. Although the average Fe-N distance is virtually the same (-1.96 A) in all three species, in [Fe(trpy)2]2+ the Fe-Ncentrai distances are much shorter than the Fe-Ndistai and so the [Fe N6]2+ coordination unit... 

The cyclic voltammetry behavior of the Cu(II) rotaxane, 4(5)2+ (Fig. 14.8b), is very different from that of 4, t l +. The potential sweep for the measurement was started at - 0.9 V, a potential at which no electron transfer should occur, regardless of the nature of the surrounding of the central Cu(II) center (penta- or tetracoordinate). Curve i shows two cathodic peaks a very small one, located at + 0.53 V, followed by an intense one at —0.13V. Only one anodic peak at 0.59 V appears during the reverse sweep. If a second scan ii follows immediately the first one i, the intensity of the cathodic peak at 0.53 V increases noticeably. The main cathodic peak at —0.15 V is characteristic of pentacoordinate Cu(II). Thus, in 4(5)2+ prepared from the free rotaxane by metalation with Cu(II) ions, the central metal is coordinated to the terdentate terpyridine of the wheel and to the bidentate dpp of the axle. On the other hand, the irreversibility of this peak means that the pentacoordinate Cu(I) species formed in the diffusion layer when sweeping cathodically is transformed very rapidly and in any case before the electrode potential becomes again more anodic than the potential of the pentacoordinate Cu2 + /Cu+ redox system. The irreversible character of the wave at —0.15 V and the appearance of an anodic peak at the value of + 0.53 V indicate that the transient species, formed by reduction of 4(5)2 +, has undergone a complete reorganization, which leads to a tetracoordinate copper rotaxane. The second scan ii, which follows immediately the first one i, confirms this assertion. 

A second molecular shuttle 10(4) +, has been synthesized in our group38 and is represented in Fig. 14.15. The mobile ring is the nonhindering 39-membered ring 9 containing one 8,8 -diphenyl-3,3 -biisoquinoline chelate, and its linear part contains, as in previous molecular shuttle 7(4)+, one bidentate 1,10-phenanthroline and one tridentate terpyridine units. Another difference is the phenylene group as a linker, which makes this thread more rigid. 

One of the most common types of ligand in supra molecular photochemistry is the bidentate chelate 2,2 bipyridine (bpy, 11.1), which represents the first member of the homologous series of polypyridyls that includes 2,2 6, 2"-terpyridine (tpy, 11.3) and carries on up to the helicate-forming sexipyridine... 

The nonsymmetrical catenate Cu.25 consists of the two different rings 26 and 27 reported in Figure 32 one includes the classical bidentate dpp and a polyoxo-ethylene chain the other incorporates a dpp moiety and the terdentate ligand 2,2 6, 2"-terpyridine (tpy), linked by three methylene groups. Both macrocycles have a 33 atom-long internal rim to minimize the hindrance toward rotation of one ring over the other. 

The ligands discussed above are widely diversified certain of them can only serve as monodentate (imidazole, acetonitrile and isothiocyanate), bidentate (bipyridine, phenanthroline) or terdentate (terpyridine, TPTZ), others being the polyfunctional ligands with different donor sites may coordiate to the metal in various modes. In the latter case, the formation of chelate metallo-rings and/or the linking of the adjacent An coordination polyhedra is observed. 

Tridentate nitrogen ligands form 1 1 uncharged complexes with copper, in contrast to bidentate ligands that form tetrahedral ionic complexes.81 Well-controlled polymers of styrene and MA are obtained with a substituted terpyridine (L-18), whereas the unsubstituted derivative induced a heterogeneous and uncontrolled polymerization.111 A similar controlled polymerization of styrene was reported for a substituted terpyridine-based complex of Cu(II) (Cu-5) coupled with Al(O i-Pr)3.112 A tridentate ligand with two pyridines and one amine (L-19) gave narrow MWDs for styrene, MA, and MMA (MJMn = 1.1 —... 

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