Transition Metal Chemistry, 19 (1994) 113 114

Solubilities and solvation of bis-(2,2'-bipyridyl)- phenanthroline)dicyanoiron(II) in alcohols John Burgess*, Duncan N. Drasdo and Kuldip Singh Chemistry Department, University of Leicester, Leicester LE1 7RH, U K

Solubilities and solvation of dicyanoiron(II) complexes 113

and bis-(1,10-

Summary

Solubilities of bis-(2,2'-bipyridyl)- and bis-(1,10-phenan- throline)dicyanoiron(II) are reported for a range of primary aliphatic alcohols, from M e O H to 1-decanol. The estab- lished trends are compared with those for other solutes, and for the title compounds in HzO-a lcoho l solvent mixtures.

Introduction

The complexes Fe(CN)z(LL)z [where LL = 2,2'-bipyridyl (bipy), 1,10-pbenanthroline (phen), a 1,4-diazabutadiene derivative or a pyridine-based Schiff base] are well estab- lished compounds of high stability m. Thanks to their low-spin t6o configuration and associated high crystal field stabilisation energy they are also substitution inert. Indeed the half-life for substitution at Fe(CN)2(bipy)2 is of the order of 70 h at 308 K in the presence of an appro- priate nucleophile~2); aqueous solutions of these com- pounds are indefinitely stable (years). These ternary complexes are of considerable interest in relation to solvation in binary aqueous solvent media, since they have two hydrophilic cyanide ligands and two diimine ligands which present a hydrophobic surface to the solvent. The markedly greater solubility of Fe(CN)z(bipy)/ in certain a l coho l -HzO mixtures than in either pure solvent separately has been ascribed to synergism between favourable hydration of the cyanides and favourable solvation of the bipyridyl ligands by the alcohol com- ponent (3). We were interested in establishing solubilities and solvation patterns for two complexes of this type in a series of monohydroxylic alcohols, in order to see how the hydroxylic/paraffinic balance affected solubilities and how the increasing difficulty of solvating the cyanide ligands on ascending the homologous series of alcohols was reflected in solubilities. We now report and discuss solubilities of Fe(CN)2(bipy)2 and Fe(CN)2(phen)2 in alcohols ranging from M e O H up to 1-deeanol.

Normally the solubilities would be measured by moni- toring absorbances of appropriately diluted samples of saturated solutions, but there is evidence that the extinction coefficients for these solvatochromic complexes may be significantly affected by the nature of the solvent, at least in t-butyl a l c o h o l - H 2 0 media C4). We have therefore monitored solubilities through atomic absorption spec- trometry, where difficulties associated with the use of a range of solvents are here easier to circumvent. We have also monitored charge-transfer absorbances to investigate the possible dependence of extinction coefficients on the nature of the alcohol.

* Author to whom all correspondence should be directed.

Experimental

The iron(II) complexes were prepared, purified and char- acterised by Schilt's methods m. The alcohols were dried by standard procedures <5).

Saturated solutions of the complexes were prepared by agitating a generous excess of solid compound with the appropriate solvent for several hours in thermostatted containers. After scttling, an aliquot of saturated solution was carefully removed from the supernatant liquid and diluted as appropriate for subsequent analysis.

Solubilities were established through atomic absorption spectrometry (Perkin-Elmer 1100B), with the instrument perforce calibrated afresh for each solvent when used in the flame mode. The lowest solubilities were determined using a graphite furnace at tachment (HGA 400). U.v.-vis. absorption spectra were obtained on a Shimadzu UV-160 spectrophotometer.

Results and discussion

Solubilities determined from atomic absorption spectro- metry and absorbances of saturated solutions as deter- mined from M L C T absorption bands are reported in Table l.

Logarithms of solubilities of the two complexes are plotted in Figure 1. The trends on going from HzO to M e O H and then up the homologous series of primary alcohols are very similar for the two iron(II)-diimine- cyanide compounds. Indeed they are also similar to a number of organic compounds, e.g. several sulfa drugs <6) and diethylpyridinone <7), which have similarly balanced hydrophilic/lipophilic exteriors. However, the solubility trends for the iron(II) complexes differ from the predo- minantly hydrophobic ligands 2,2'-bipyridyl, 1,10-phenan- throline ~s) and triphenylphosphine (9), whose solubility increases steadily on going from H 2 0 to M e O H to higher alcohols.

For each of our ternary iron(II) compounds, solubilities in H 2 0 and 1-0ctanol are nearly equal, suggesting a close balance between hydrophilic and lipophilic contributions from the cyanide and diimine ligands. Higher solubilities in the lower alcohols suggest that these can solvate both cyanide and diimine ligands. Addition of H 2 0 to M e O H or EtOH, or of one of these alcohols to H20 , further increases the solubility of Fe(CN)2(bipy)2, as shown in Figure 1. The particularly high affinity of cyanide for H 2 0 is presumably responsible for the increase in solubility on going from M e O H to 90 and 80% M e O H - H 2 0 and from E tOH to solvent mixtures containing up to 50% H20. It is important to note that there is not a dramatic increase on adding a small quantity of HzO to M e O H or EtOH, as in practice our saturated alcohol solutions of these complexes will contain 0.01-0.1% H 2 0 introduced from the complexes since even after vacuum desiccation they are hydrates Fe(CN)2(bipy)2'3H20 and Fe(CN)2(phen)2" 2H20. This in itself indicates how hydrophilic are the

0340-4285 �9 1994 Chapman & Hall

114 Burgess et al. Transition Metal Chemistry, 19 (1994) 113-114

Table 1. Experimental results for the determination of solubilities of Fe(CN)z(bipy)2 and Fe(CN)/(phen) 2 in H20 and 1-alkanols, from atomic absorption spectrometry and from u.v.-vis, spectroscopy; all results at 298.2 K.

H/O MeOH EtOH n-PrOH n-BuOH n-HxOH n-OctOH n-DecOH

Fe(CN)z(bipy)z p.p.m. Fe a 310 21310 7430 1730 630 290 192 31

Fe(CN)/(phen)/ p.p.m. Fe a 16 1760 610 190 86 17 18 ASS b 0.10 13.4 4.1 1.4 0.47 0.13 0.11

~These values are p.p.m, iron in the respective saturated solutions, i.e. machine reading times dilution; babsorbance of saturated solution, i.e. measured absorbance times dilution.

-2

E -3

0 ~0

v

o

0 --4 _J

-5

s

i i i

0 I -1- -r 1- "1- -r -r 0 0 0 0 0 0 0

I ~ ~ ~" ~ X ~ 0 ,,, n rn "I- o

' , , 0 0 I

Figure 1. Logarithms of solubilities of Fe(CN)2(bipy)z (0) and Fe(CN)2(phen)z (~, O) in H20, primary alcohols and MeOH-H20 and EtOH H20 mixtures, in all cases at 298.2 K.

cyanide ligands. Since the solid forms of each compound in equilibrium with saturated solution will be hydrates in the case of HzO but alcohol solvates or anhydrous material in other media we are not able to calculate transfer chemical potentials. However, the solubilities reported in Table 1 suggest values of the order of - 10 to - 2 0 k J mol-~ for transfer from MeOH or EtOH into 1-0ctanol or 1-decanol.

Calculation of molar extinction coefficients from the absorbance data in Table 1 reveals no trends; indeed values are essentially equal within reasonable estimates of confidence limits. This is a little surprising in view of the significant changes in molar extinction coefficient reported for Fe(CN)2(bipy)2 in a series of t-butyl a l coho l -H20 solvent mixtures earlier ~4). It may be that the particularly marked effects of this alcohol on H 2 0 structure have a more pronounced effect on solvation than the changes on going from one pure solvent to another.

Acknowledgements

We are grateful to Suada Ahmed, De Montfort University, Leicester, for some confirmatory measurements.

References

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(Received 21 January 1993) TMC 2958