Phenanthrolines basicity

The same first-order replacements are seen when M is Mo or W, somewhat slower than in the case of Cr, but still much faster than for the hexacarbonyls. The rate increases with the pK of the inert ligand (N-N) and Fig. 9 shows the linear free-energy relation between log ki and pK . The relative orders would not have been expected on the basis of any 7t-bonding effects since increasing back-donation to CO would increase the M-C bond order. This increase in M-C bond order is supported by a decrease in Vco with increasing o-phenanthroline basicity. The same consideration applies for the pentacarbonyl halide anions where the first-order rates decrease (Cl > Br > I), unexpectedly as the halide polarizability increases. 

As with EDTA, which we encountered in Chapter 9, o-phenanthroline is a ligand possessing acid-base properties. The formation of the Fe(o-phen)3 + complex, therefore, is less favorable at lower pH levels, where o-phenanthroline is protonated. The result is a decrease in absorbance. When the pH is greater than 9, competition for Fe + between OH and o-phenanthroline also leads to a decrease in absorbance. In addition, if the pH is sufficiently basic there is a risk that the iron will precipitate as Fe(OH)2. 

Table 13 Relative basicities (log )rei of bimacrocyclic (concave) and mono-macrocyclic 1,10-phenanthrolines [29].

The dimethylplatinum(II) complexes having chelate nitrogen ligands, [PtMe2(N-N)] (N-N = 2,2 -bipyridyl or 1,10-phenanthroline) (46) reacted with alcohols (ROH, R = Me, Et, Pr) to give hygroscopic and basic platinum(IV) complexes. 

The oxidation of alcohols in a basic solution catalyzed by Cu(II) o-phenanthroline complexes has been recently studied by Sakharov and Skibida [305-309], The copper-phe-nanthroline complex is stable in a basic solution and appears to be a very efficient catalyst for the oxidation of alcohols to carbonyl compounds. The reaction rate increases with an increase in the partial pressure of dioxygen. The solvent dramatically influences the reaction rate (conditions 348 K, [MeOH] = 20%vol, [Cu—(o—phm)] = 0.01 mol L-1). 

The synthesis of thieno[2, 3 4,5]thieno[2,3-f][l,10]phenanthroline 361 has been described <1996JHC119>. The compound was obtained in four steps from known starting materials. The basic skeleton of the molecule of the phenanthroline ring was formed via photocyclization. 

In addition, the basicities (or acidities) of the concave 1,10-phenanthrolines 11 (s. Scheme 2) and 21 (s. Scheme 4) [18, 20] and the concave benzoic acids 38 (s. Scheme 6) [27, 33] have been measured. Also in these compounds, the acid/base properties of the parent compounds 1,10-phenanthroline and benzoic acid are at least conserved. Surprisingly, in the concave 1,10-phenanthroline cyclophanes 21 distinctly higher basicities than for 2,9-unsubstituted 1,10-phenanthrolines were found (ca. two orders of magnitude). Substitution in 4,7-position by aryloxy-substituents 21e (s. Structures 3) further increased the basicity [20]. 

When the Connolly-routine was applied to the concave 1,10-phenanthroline 21a (Structures 3), spheres with a radius up to 2.7 A were able to touch the nitrogen atoms whereas larger spheres were not. This means that spheres with a diameter of more than 5.5 A cannot react with the basic nitrogen atoms of 21a anymore [20]. 

Besides concave pyridines, concave 1,10-phenanthrolines and concave benzoic acids, a large number of other concave adds and bases are conceivable containing other acidic or basic groups in the concave position. The central question for such new concave reagents is how can the new functionality be incorporated into the bimacrocyclic structure, and how can a concave orientation be assured ... 

The triplet state acidities, pkl, are found to be not much different from ground state pK s. But feterocyclics like o-phenanthroline and quinoline are more basic in triplet state than in ground state, although less basic than in the first excited singlet state. It follows that the singlet transition in these molecules has considerably more charge transfer character than the transition in the triplet state. 

The routes for controlling the synthesis of mono- (903) and dinuclear (904) structure is, evidently, related to the donor properties of additional ligands [Scheme (4.62) L and L1]. In case of highly basic N-donors (L = py, bipy, o-phen), the mononuclear complexes 903 are generally formed [11,229,232], while the weakly basic solvents (L = MeOH [233]) lead to binuclear chelates of type 904. At the same time, the only example for 805 (L = o-phen) of a stable dimeric adduct with sufficiently high-basic o-phenanthroline is known and was shown above (Sec. 3.4.2). 

Addition of four methyl substituents to the 1,10-phenanthroline ligand increases the basicity of its N atoms, enhancing the propensity for hydrogen bonding with the water... 

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