Inorganic Chemistry, Vol. 16, No. 3, 1977 545
Pentacyanoferrate(I1) Complexes
Contribution from the Instituto de Quimica, University of Sao Paulo, Si0 Paulo, Brazil, and the Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973
Pentacyanoferrate(I1) Complexes: Evaluation of Their Formal Potentials and Mechanism of Their Quenching of Tris(2,2'-bipyridine)ruthenium(II) Luminescence HENRIQUE E. TOMA*la and CAROL CREUTZlb
Received August 9, 1976
AIC60559H
Formal reduction potentials ( E O ' ) for complexes of the pentacyanoferrate ion with a number of unsaturated ligands have been measured by cyclic voltammetry and potentiometry. From the association constants for the complexes in the iron(I1) form ( K I I )and the Eo' values, the corresponding constants for the iron(II1) complexes have been calculated. The KIIIvalues are found to increase with the basicity of the ligand, while the opposite trend is found for the KIIvalues. These opposing trends are discussed in terms of the bonding properties of the pentacyanoferrate ion. The quenching of the luminescence of the excited state of tris(2,2'-bipyridine)ruthenium(II) by pentacyanoferrate(I1) complexes has also been investigated. For most of the complexes, including several previously reported hexacyanometalates, the quenching rate constants parallel their reduction potentials as is expected for a quenching mechanism involving reduction of the ruthenium excited state by iron(I1).
Introduction Although the dependence of the redox properties of transition metal complexes on the nature of the donor atom and the ligand is now well understood,*there are only a few systems displaying a range of potentials great enough for systematic studies. The immine complexes of transition metal ions, in particular those of the 1,lO-phenanthroline and 2,2'-bipyridine derivatives, form one such series. The strong affinity of the pentacyanoferrate(I1) ion for unsaturated ligands can be used to make another very interesting series of complexes with i m i d a ~ o l e , ~pyridine, " p y r a ~ i n e ,sulfoxide^,^^ ~~ CO, or NO as t h e sixth ligand. In this series, the u-donor and n-acceptor powers of the ligands are expected to vary considerably thus providing large differences in the stability constants for the complexes in their oxidized and reduced forms. This should result in large differences in redox potentials, as predicted from the cycle Fe(CN) H 0'+e-'Jt
KIII + L == Fe(CN),LZ- + H,O
'
Fe(CN),H,03-
+L
KII
11
ce-
Fe(CN),L3- t H,O
where
Since the equilibrium constants KIIare known for many of the pentacyanoferrate(I1) c o m p l e x e ~ ,measurement ~ of the reduction potentials makes it possible to calculate K,I[and, from this information, t h e relationships between KIIand KIIIand the properties of the ligand may be evaluated. The use of the pentacyanoferrate(I1) complexes in the quenching of the luminescence of the excited state (*Ru( b p ~ ) ~ ~of' )tris(2,2'-bipyridine)ruthenium(II) was suggested by the recent observation that f e r r ~ c y a n i d eand ~ , ~ other transition metal hexacyanides6 are efficient quenchers via energy-transfer or electron-transfer mechanisms. If the quenching occurs by an electron-transfer mechanism, the regularities in the redox properties of the pentacyanoferrates are expected t o be reflected in the rate constants for the quenching process. In fact, such correlations were observed for most of the cyano complexes reported here, but with some interesting exceptions. For some of these cases, other mechanisms, including electron-transfer reactiaps of the coordinated ligands, are considered. Experimental Section Materials. The pentacyanoferrate(I1) complexes were prepared from sodium amminepentacyanoferrate(I1) trihydrate, as previously
de~cribed.~ Solutions of the carbonylpentacyanoferrate(I1) complex M) were prepared as follows: in a flask provided with bubblers, pure carbon monoxide was passed through the yellow solution of the amminepentacyanoferrate(I1) complex for ca. 2 h. During the reaction, the free ammonia released in the substitution process was carefully neutralized with dilute sulfuric acid containing traces of ascorbic acid' to reduce eventual oxidizing impurities. When the reaction was complete, a colorless solution was obtained. Bluish impurities are formed in the absence of ascorbic acid. The presence of the residual aquopentacyanoferrate(I1) ion was tested by adding N-methylpyrazinium ion (eman 1.25 X lo4 M-' cm-' at 658 nm fer the pentacyano(N-methylpyrazinium)ferrate(II) complex). Cyclic Voltammetry Measurements. A Princeton Applied Research Corp. system consisting of a Model 173 potentiostat and a Model 175 universal programmer was employed in the cyclic voltammetry measurements. Reversible cyclic voltammograms were obtained at sweep rates varying from 20 to 200 mV/s, using platinum wires as auxiliary and working electrodes vs. saturated calomel electrode. All of the measurements were carried out under argon, with millimolar solutions of the pentacyanoferrate(I1) complexes at 25 OC, = 1.00 M (NaCl), pH 4.5 (acetate buffer, M). For the imidazole and pyridine complexes, the ligand buffers (rather than acetate) were employed in order to prevent the dissociation of the coordinated ligand. Absorption and Emission Spectra. A Cary 17 spectrophotometer was used for the absorption measurements in the visible and UV region. The emission spectrum and emission intensity of tris(2,2'-bipyridine)ruthenium(II) complex, in the presence or absence of quenchers, were determined with a Perkin-Elmer Model MPF4 sptrofluorimeter, equipped with a 150-W xenon lamp. The measurements were carried out at 25 OC, with samples always freshly prepared under an argon atmosphere. With the exception of the pyrazinecarboxylate and N-methylpyrazinium complexes, all of the quenchers have no appreciable absorption at the excitation (450 nm) and emission (607 nm) wavelengths. Only minor corrections* have been applied, when necessary. Lifetime Measurements. Solutions of tris(2,2'-bipyridine)ruthenium(I1) containing strongly absorbing quenchers such as the pentacyano(N-methylpyrazinium)ferrate(II) complex could not be readily studied by intensity measurements. For these systems, the quenching rate constants were obtained by directly determining the excited-state lifetime r as a function of quencher concentration. The lifetimes were determined by monitoring the decay in emission intensity after excitation by a -30-11s pulse of 530-nm light. A neodymium laser (Korad Model K1500) which lases at 1060 nm was employed as the excitation ~ o u r c e .Frequency ~ doubling was accomplished by a potassium deuterated dihydrogen phosphate crystal. Excitation intensities at 530 nm were typically 10 einsteins cm-* s-]. The emitted light passed through a Bausch and Lomb high-intensity grating monochromator (605 nm, 10 nm band-pass) and was detected with a photomultiplier (Hitachi R446) and a preamplifier having a combined bandwidth in excess of 30 MHz. The signals from the preamplifier were displayed and photographed on a Tektronix 7633 oscilloscope equipped with a Tektronix 7B71 time base and 7A13
Henrique E. Toma and Carol Creutz
546 Inorganic Chemistry, Vol. 16, No. 3, I977
- .4
- .3
-.2
1
0
.l
POTENTIAL, V
Figure 1. Cyclic voltammograms for the pentacyano(imidazo1e)ferrate complex at several scan rates at p = 1.0 M (sodium chloride) and 25 "C with imidazole buffer (lo-* M). amplifier. Values of I,, the emission intensity at time t , were read from the photographs and plots of log I, vs. time were constructed. The lifetimes were obtained from the slopes of the semilog plots. The to measurements were made at 25 'C on deaerated solutions M in Ru(bpy)32+. Neutral density filters were used to attenuate the emission intensity at low quencher concentrations.
Results and Discussion Formal Potentials and Cyclic Voltammetry Measurements. Formal reduction potentials ( E O ' ) for a number of substituted pentacyanoferrate complexes were evaluated by direct potentiometry or by cyclic voltammetry. Most of the complexes, e.g., those derived from pyridine and pyrazine, are sufficiently stable and inert in both the iron(I1) and iron(II1) forms to permit the application of direct potentiometry to equimolar solutions in order to evaluate E O ' . The junction potentials in these cases were estimated to be in the range of f 5 mV by checking the quinhydrone electrode against SCE. The oxidized form of the N-methylpyrazinium complex aquates slowly in aqueous solution" but without any appreciable changes in the potentials measured. For the dimethyl sulfoxide and carbon monoxide complexes, decomposition takes place rapidly, and cyclic voltammetry was employed instead of direct potentiometry. For all the complexes, well-behaved and reversible cyclic voltammograms have been obtained, as shown in Figures 1 and 2. The cathodic and anodic peak separation was typically 60-65 mV, as expected for a one-electron, reversible process. Since the diffusion coefficients for the oxidized and reduced pentacyanides are very similar," the values can be regarded as good estimates of E O ' . The values of the formal reduction potentials of the pentacyanoferrates are collected in Table I, Also included are the values of the association constants for the complexes (KI1 and ICIII) and, for comparison purposes, the basicity constants for the heterocyclic ligands and KII/KIIIratios. Except for that of the imidazole complex, the values of KII have been previously reported? The KII1values tabulated were calculated from eq 1 using the KII and Eo'III,II values given in Table I and +0.39 V as the potential for reduction of Fe(CN)sH202to Fe(CN)5H203-. This approach neglects (a) the medium change from 0.5 M perchlorate (in which most of the KI,values were determined) to 1.O M chloride (in which most of the Eo' values were measured) and (b) differences in the activity coefficients of the Fe(CN)5H20and Fe(CN)& complexes. Although the resulting KrrIvalues are consequently only es-
Figure 2. Cyclic voltammograms for the pyridine, dimethyl sulfoxide, and carbonyl complexes of pentacyanoferrate ion at p = 1.O M (sodium chloride) and 25 "C showing the shifts in El,* as a function of the nature of the ligands.
Inorganic Chemistry, Vol. 16, No. 3, 1977 547
Pentacyanoferrate(I1) Complexes
Table I. Formal Potentials and Association Constants for Pentacyanoferrate(I1) and -(III) Complexes at 25 "C
L in Fe(CN),L H,O Imidazole (imid) 7-Picoline Pyridine (py) Isonicotinamide Pyrazine (pz) N-Methylpyrazinium (NMPz) Dimethyl sulfoxide (Me,SO) Carbonyl
E"'II1IIIP
v
0.39 (Zot.) 0.409 0.35 (CV) 0.45 (pot.) 0.47 (pot.)f 0.48 (CV) 0.50 (p0t.f 0.55 (pot.)f 0.78 (pot.)f 0.79 (CV) 0.89 (CV) 1.18 (CV)
K11,b M"
KIII,M-'
pKaC
1
1
1.8 x 105e 3.1 x 105 3.3 x 105
8.3 x 105 2.9 x 104 9.4 x 103
7 .O 6.1 1 5.30
x 103 x 103 x 10-l
3.65 0.65 -5.8
4.0 9.0 2.0
x 105 x 105 x 106g
3.3 x 1068 >io7
5.2 1.7 4.7
KIIIKIII
1 2 x 10-l 1.1 x 10 3.5 x 10 7.1 X 10 5.3 x l o 2 4.3 x lo6
1.1 x
3
