1.10- phenanthroline formation constant

In analogy to the situation for bipyridine, the blue tris(l,10-phenanthroline)iron(3+) ion [1347949-7], [Fe(phen)2], must be obtained by oxidation of the corresponding iron(II) ion. [Fe(phen)2] has an absorption maximum at 590 nm, an absorptivity of 600 (Mem), and a formation constant of 10 . In solutions of pH > 4, this species is reduced to the iron(II) complex. The reduction is instantaneous in alkaline solution. At pH < 2, protons compete with iron(III) for the phenanthroline nitrogens and coordination is incomplete. [Fe(phen)2] is used most often in solution as an oxidant, but the trichloride [40273-22-1] and the triperchlorate monohydrate [20774-81-6] salts have been prepared. 

Complexation of Cd with a series of polyamine macrocycles, but also related open-chain polyamines, comprising or attached to the 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) moieties, has been studied by combined UV/vis spectrometry and potentiometry.24 Formation constants and distribution diagrams of the species present have been evaluated. As a result the thermodynamic stabilities, i.e., the formation constants, are lower for the bipy- and phen-contain-ing ligands than those for Cd complexes with aliphatic oligoaza macrocycles containing the same number of N donors. The probable reason is loss of flexibility of the ligands caused by the size and stiffness of the inserted heteroaromatic moieties. 

There is abundant evidence that steric hindrance to coordination will decrease the formation constant of any complex formed and may even inhibit complex formation altogether. Numerical data are available for transition metals and substituted ethylenediamines,27 or substituted oxines.-3 for 1,10-phenanthroline and its 2- and 2,9-methyl-substituted derivatives28 and for other systems.29 Since the introduction of a 2-methyl substituent into oxine diminishes 0 while increasing pK2, it is not surprising that the effective concentration [Al(2-MeOxine)3] may not be great enough to cause precipitation, although this is achieved in non-aqueous media such as ethanol. 

Metal ions have quite marked effects on the hydrolysis of methyl 8-hydroxyquinoline-2-carboxylafe (50)218 and ethyl l,10-phenanthroline-2-carboxylate (51).219 Base hydrolysis of the 8-hydroxyquinoline derivative (50) was studied over the pH range 9.2-12.1 and values of fcoH determined for HA and the anion A-. Formation constants KMA+ were determined at 25 °C for the equilibrium M2+ + A MA+, as were the rate constants fcOH for the base hydrolysis of the MA+ complexes (Table 19). Quite large rate accelerations are observed (103-106) when comparisons are made with the base hydrolysis of A . The charge carried by the complex does not appear to be a major factor in determining base hydrolysis rates. Thus at 25 °C the complex CuA+ undergoes base hydrolysis (kOH= 6.3xl05 s 1) at a very similar rate to the corresponding... 

Table 12 Successive Formation Constants for Iron(lI) 2,2 -Bipyridine and 1,10-Phenanthroline Complexes ...

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

A different approach is the metal-based template method [2cj, 10], for example starting from a wire (11) and a macrocycle (12), both containing a phenanthroline unit, and using Cu+ as a template (Figure 3c) [2cj. Species of this kind are essentially metal complexes, and may have very large formation constants. 

The formation constant of the tris(l,10-phenanthroline)iron(II) complex has been determined by a number of methods, including potentiometry, a competitive spectrophotometric method, and a method involving partition between aqueous and organic solvents. The typical value for log P3 is about 21.3 ... 

Figure 22-17. Spreadsheet for the determination of the overall formation constant of tris(l,10-phenanthroline)iron(II). (Student data, from E. J. Billo.)...

The fluorescence properties of several europium and samarium ) -diketonates have been measured and assignments of the transitions made. Rare-earth element hexafluoroacetylacetonates with amino-acids have also been reported to fluoresce. The luminescence of the heptafluoroheptane-2,4-dione complexes of Sm, Eu, and Tb has been measured in dilute ethanol at pH8 and 610nm mixed-ligand complexes with 1,10-phenanthroline exhibited an enhanced luminescence. Photolysis of the Tb chelate of 2,2,6,6-tetramethylheptane-3,5-dione has been examined at 311 nm in various alcohols, and loss of one -diketone ligand found to be the primary photochemical step. A linear correlation was demonstrated between the quantum yield of dissociation of the complex and the formation constant of the complex-alcohol adduct. 

In solution yellow iron(II) species containing one a,a -diimine ligand can be obtained from an excess of Fe + in an acidified solution of the ligand (which controls the amount of available free ligand via protonation). Formation constants for many such complexes of substituted 1,10-phenanthrolines have been reported and tabulated. The rate of formation of [Fe(phen)(H20)4] is rapid and has a very unfavourable entropy of activation ( = 53.6 kJ mol , A5 = — 66.9 J K mor Kinetics of the corresponding reactions with 2,2 -bipyridine have also... 

As expected from crystal field considerations the low-spin iron(II) complexes are substitution inert, and thermodynamically very stable with respect to Fe " and the free ligand overall formation constants (jffj) can be as large as 10 The consecutive formation constants for iron(II) 2,2 -bipyridine and 1,10-phenanthroline complexes derived by Irving and Mellor are not in the usual order K > Ki> K, but rather K > (Table 12) and this was attributed to spin pairing on... 

The pseudo-contact shifts recordedfor the 1 1 adduct of [M(dpm)3] (M = Eu, Pr, or Yb) with o-phenanthroline or 2,2 -bipyridyl were not in accord with the Robertson-McConnell expression, which assumed axial symmetry about the metal Lewis-base bond. The spectra of the phen complexes were relatively insensitive to variations in temperature, whereas the temperature variations of the spectrum of [M(dpm)3(bipy)] could be explained by biphenyl-type rotational isomerism of unidentate bipy. Further co-ordination by substrates added to [M(dpm)3(phen)] and its bipy analogues was not observed. In addition to 1 1 and 1 2 adducts, the system [M(fod)3]- 7t-C5H5)Fe(CO)2-CN] (M = Pr, Eu, Ho, or Yb) also contained 1 3 adducts. However, the relative successive formation constants for the 1 2 adducts decreased in the order Pr > Eu > Ho > Yb, indicating greater steric crowding with the heavier metal complexes. 

Complex formation of Ga with 2,2 -bipyridyl and 1,10-phenanthroline has been studied by the competing reaction with Ag+ ions in an acetate buffer. Formation constants of complexes are as follows (bipy), (3.45 0,73) X 10 i a (5.08 d= 0.86) x 10 (phen), / i (3.84 0.24) x 10 , (1,74 0.31) X 10 . Measurements of the same constants by competition reactions with Mn gave results which agreed with these within experimental error. 

The formation constants of indium(m)-l,10-phenanthroline and -2,2 -bipyridyl complexes have been determined, as follows (1,10-phen)... 

---