2.27.4.1 Cyano Complexes of the Iron Group
Solvatochromism of cyano complexes, particularly of the Fe group, has been investigated for many years.4,6,11,99 Most of the complexes studied contain diimines as co-ligands, with the [MII(diimine)2(CN)2] complexes having the advantage that they are neutral and soluble in a wide range of solvents. Indeed, the complex [FeII(phen)2(CN)2] has been proposed as a universal solvent polarity indicator by Burgess,120 and has been used extensively to measure acceptor properties of solvents and cations.4,7,50,121 In addition to its solubility, its usefulness stems from the combination of ligands that provide a strong visible region MLCT band—from thed6iron(II)
Absorbance
1.0 0.8 0.6 0.4 0.2 0.0
400 500 600 700
Wavelength (nm)
Figure 1 Absorption spectra of [Pt(Me2bpy)(met)] in solvents of different polarity (adapted fromref. 113).
Solvents: –chloroform; ---dichloromethane; acetonitrile.
centre to the diimine ligand—coupled with the presence of cyanide ligands which interact strongly with the solvent or with other acceptors (e.g., BF3, Hþ, simple metal ions).122,123 Acceptor solvents stabilize the ground state by withdrawing electron density, either through a simple donor–acceptor interaction with the cyanide lone pair of electrons, or through hydrogen bonding at the same site. This allows an increase in back-bonding to the cyanide ligands which stabilizes the metal orbitals, so that the MLCT energy increases with increasing acceptor ability. In some of these complexes there are two solvatochromic MLCT bands in the visible/near UV region, terminating on different * levels of the ligand.124
Burgess’ group performed a number of early studies of [FeII(bpy)2(CN)2] and related complexes.99,125,126 The frequencies of maximum absorption of the visible region MLCT bands were plotted against Reichardt’sETvalue, giving two correlation lines in each case, for potentially hydrogen-bonding and nonhydrogen-bonding solvents. Correlation with solvent AN gives a single straight line, the band shifting to higher energy with increasing AN (Figure 2).7,124,127,128 The iron(III) complex [FeIII(phen)2(CN)2]þ also exhibits pronounced solvatochromism, but in this case for an LMCT band,4,126 with an AN dependence of opposite slope to that for the MLCT band of the iron(II) complex.129
Burgess,126and later Toma,124noted the greater solvent sensitivity of the tetra- and pentacyano complexes compared to the dicyano species. [Fe(CN)5(2,6-dimethylpyrazine)]3 is particularly solvatochromic, with the main band shifting from 22,700 cm1 in water to 16,250 cm1 in DMA.124 Similar behavior is observed for the related ruthenium complexes,23,130–132 and a thorough study of solvent effects on the absorption and emission spectra and the RuIII/II potentials of a series of complexes containing 1–5 cyanide ligands has been carried out by Meyer and co-workers.23 The solvatochromism of both absorption and emission was found to correlate well with AN rather than with dielectric continuum functions. The solvent sensitivity increases with the number of cyanides present, with the effects being additive, and it is greater for absorption than for emission. Where the absorption and emission involve the same excited state, and where dielectric continuum theory is applicable, the inner and outer reorganization energies are expected to be approximately equal for the two processes.23However, if there are strong specific interactions such as hydrogen bonding, the solvent–solute interactions may influence the nature of the ground state, and theoterms for the absorption and emission processes are different; in fact, for the cyano complexes,o,gsmay be as much as twice the magnitude ofo,es. The implications of vibronic structure, and the multiple states which exist in absorption spectra and are usually neglected when solvatochromism is studied, are also discussed in this paper.23
The polypyridyl ligand!* bands of these complexes were also observed to be solvatochro- mic, though to a lesser extent than the MLCT bands. This is due to orbital mixing giving the !* transitions some MLCT character, and to configuration interaction mixing transitions of the same symmetry.23
Sizova et al. have calculated the spectrum of [Ru(CN)5(pz)]3 (pzẳpyrazine) taking into account the solvent effect.63
Figure 2 Linear relationship between the lower energy MLCT absorption maximum of [Fe(phen)2(CN)2] and the acceptor number, AN. A plot of the same values againstET(30) is given in the insert (adapted from ref.7).
360 Solvatochromism
2.27.4.2 Ruthenium Ammine Complexes
The LMCT and MLCT transitions in a variety of RuIIand RuIIIpolypyridyl-ammines are exceedingly solvent dependent,11,133with [Ru(NH3)4(bqdi)]2ỵ(bqdiẳbenzoquinonediimine) being an interesting exception.134 Meyer and co-workers have studied a series of RuII and RuIII penta- and tetra- ammine complexes with pyridine-based co-ligands.133Neither the dielectric continuum model nor solvent dipole moment correlate with the charge-transfer energies of these complexes but, rather, parameters such as Gutmann’s donor number or the Taft–Kamletparameter, which reflect the hydrogen-bond-accepting ability of the solvent, must be used. These give reasonable linear correlations for the complexes, with the slopes showing some tendency to increase with the number of coordinated ammines. The origin of these solvent effects was investigated in detail and found to lie largely in the solvent effect upon the RuIII/IIredox potential. The donor solvents interact with the ammine protons, causing increased electron density on the ruthenium with increasing donor ability of the solvent. This effect is greater for RuIII than for RuIIcomplexes because of the greater acidity of the NH groups. The donor–acceptor interactions are strong enough to cause preferential outer-sphere solvation in binary solvent mixtures, with the stronger donor being in closer proximity to the ion.133
Calculations have been performed on a number of these species; their MLCT energies are sig- nificantly overestimated by semi-empirical calculations, such as INDO/S, in which the solvent is not included.11,60–67,135
Shin et al. used INDO in one of the earlier studies of [Ru(NH3)5L]nþ, where Lẳpy, pz, pzHỵ.61Inclusion of 15H2O molecules was found to lower the calculated charge-transfer energies significantly, but not as low as the experimental values. Sizovaet al.used a mixed INDO/
CNDO method.63Zerneret al. have calculated the spectra of [Ru(NH3)5py]2þ, [Ru(NH3)5py]3þ, and some related species in several solvents,60,66 using INDO/S and the self-consistent reaction field (SCRF) method. The INDO method tends to produce rather short hydrogen bonds,60 so a hybrid quantum mechanics–molecular mechanics method was used to obtain structures for the later study.66Spectra were calculated quantum mechanically for the complex and first solvent shell, with a further solvent layer being modeled by point charges. The solvent structure was obtained by Monte Carlo simulations. A spherical SCRF was then used to include the longer-range interactions.
The calculated gas-phase MLCT energy of 35,900 cm1 decreases when water is included in the calculation; two bands are predicted, at 24,500 cm1and 25,500 cm1, which agree well with the observed energy of 24,500 cm1. The calculations show that the solvent contributes about 1eto the Ru complex regardless of the quantum-mechanical method used, and this greatly improves the accuracy of the predicted MLCT band. The calculated spectrum changes little after the addition of 12 water molecules.66 Hush and Reimers have also performed calculations on ruthenium penta-ammine complexes using their model, ZHR-SS, which can be used for calculating the solvent-induced shift of absorp- tion or emission bands.12,62This is a four-step method in which the gas-phase electronic structure and spectroscopy of the solute are determined, the structure of the solution is modeled, and the effect of the solvent structure on the electronic transitions is calculated. The gas-phase calculation of the solute is obtained at the highest level of theory possible. Using this method, calculated MLCT energies for [RuII(NH3)5L]nỵ(LẳNH3, py, pz, pzHỵ) in aqueous solution are 3–6,000 cm1higher than observed, but significantly lower than the calculated energies in the absence of solvent.
More recently Ferretti and co-workers have used the polarizable continuum model (PCM) in conjunction withab initio calculations to study the absorption spectra of [RuII(NH3)5pz]2þand [RuII(NH3)54,40-bpy]2þin several solvents.64,65
2.27.4.2.1 Cyanamides of ruthenium
The Crutchley group have done extensive studies of ruthenium cyanamide complexes, particularly with ammine co-ligands, including detailed investigations of substituent and solvent effects on oscillator strengths as well as transition energies.136–140 These complexes have a well-defined LMCT band in the visible region which is assigned to a single transition, b1!b1*.136 For the complex [Ru(NH3)5(2,3-dichlorophenyl)cyanamide]2þ the solvatochromism of the visible region CT band correlates well with DN50,51for a range of polar solvents of widely varying DN.137
For a series of phenylcyanamides with different substituents the oscillator strengths of the phenylcyanamide!RuIIILMCT transitions correlate with the donor number of the solvent. The difference in oscillator strengths was found to be quite considerable, e.g., an increase infof over 50% as DN decreases from 30 to about 2. Two of the complexes show linear correlations, while the others show evidence of curvature at low donor number, which is attributed to ion-pairing
effects. Donor solvents are believed to interact with the ammine protons, allowing electron density to move from the NH bond to the Ruammine bond, thus increasing the electron density on the ruthenium. This in turn weakens the -overlap with the negatively charged cyanamide ligand and decreases the oscillator strength.139
2.27.4.3 Bis(bipyridine)(3,4-diamino-30,40-diimino-30,40-dihydrodiphenyl)ruthenium(II)
The complex bis(bipyridine)(3,4-diamino-30,40-diimino-30,40-dihydrobiphenyl)ruthenium(II), [Ru (bpy)2(dadib)]2þ, which comprises a donor diaminobenzene (opda) unit linked to an acceptor RuII(bqdi) unit, shows unusual solvatochromic behavior. In certain solvents, such as DMSO, two intense transitions are observed in the visible region, whereas in other solvents (e.g., water) there appears to be only one transition.141,142The solvent-induced shifts correlate fairly well with a dual- parameter fit involving the hydrogen-bond acceptor and donor parameters, and , of Taft, Kamlet, et al.141 AM1 calculations on a model system using BF2þ
as a simple acceptor unit in place of [Ru(bpy)2]2þ,141and extended Hu¨ckel calculations on the complex142were used to demon- strate that the changes in the spectrum were caused by changes in the dihedral angle of the biphenyl unit, driven by changes in solvent–solute hydrogen bonding to the unbound amino groups at the remote end of the molecule. In donor solvents the amino-group lone pairs tend to conjugate with the opda ring, the ligand tends towards planarity, and a pathway exists for donation of electron density from the donor to the ruthenium. The effective symmetry of the complex is lowered and more than one Ru(d)!* transition has significant intensity. In the presence of acceptors (e.g., water, Hþ), the amino groups interact with these in preference to donating into the opda ring, the dihedral angle increases, and the complex resembles [Ru(bpy)2(bqdi)]2þ, which has effective C2v symmetry and only one intense, visible-region transition.143