Some examples of the solvatochromic coordination compounds that have been most extensively studied are discussed here. Before discussion of true solvatochromic behavior, we comment on some examples where the solvent binds to a coordination site on the metal center, generating apparent solvatochromic behavior.
2.27.3.1 Solvatochromism Resulting from Coordination by the Solvent
2.27.3.1.1 Solvatochromism of coordination compounds with nickel(II) or copper(II)
Mixed-ligand coordination compounds with transition-metal ions such as nickel(II) and copper(II) show solvent-dependent color changes.69–76Recent reviews7,52,77,78discuss their features
356 Solvatochromism
in some depth and so only a summary is presented here. [M(diket)(diam)]: diketẳ-diketone, diamẳN-alkylated diamine, for instance, form square-planar complexes with NiIIand also with CuII, in weakly donating solvents. With increasing solvent donor ability, two solvent molecules coordinate octahedrally. In the case of nickel(II) there is a change in spin state from square planar Sẳ0 nickel(II) through formation of the six-coordinateSẳ1 species, thereby causing an abrupt change in the electronic absorption spectrum; solvents of intermediate donor ability can yield equilibrium mixtures of both stereochemistries (see Figure 3, ref.78). Intermediate five-coordinate symmetries may also be involved. The formation of four-, five-, and six-coordinate copper(II)d9 complexes upon solvent coordination does not involve a spin-state change, and generates a continuous change in spectrum rather than an abrupt one. In the case of copper, the coordination of anionic ligands can also cause significant changes in spectrum, analogous to the solvent effects.
Generally, the square-planar structure is changed to square pyramidal with an anionic donor, or to octahedral with a donor solvent having a high donor strength, as seen fromFigure 1in ref.79.
Highly sterically hindered diamines, such as N,N,N0,N0-tetramethylethylenediamine (tmen) and 1,2-dipiperidinoethane, form stable, mixed-ligand complexes such as M(diket)(tmen)]nþwhich often exhibit dramatic solvatochromic effects. For this reason they have been intensively studied.52,77,78,80,81
This is reminiscent of the very early work by Lifschitz and co-workers,5 who employed the hindered stilbenediamine ligand.
Copper(II) complexes with mixed ligands are also useful when quantitatively estimating the donating or accepting abilities of solvents. In the case of d9 [Cu(acac)(tmen)]2þ (acacẳacetylacetonate), in the presence of a noncoordinating anion such as perchlorate, the shift in electronic spectrum with solvent reflects the formation of six-coordinate species with increasing donicity of the solvent.78 Indeed this molecule is a useful reagent for assessing the donicity of solvents or solvent mixtures. Conversely, if a nondonor solvent such as dichloroethane is employed, one may ascertain the relative binding of various anions of differing donicity which influence the electronic spectrum in a manner similar to solvents of differing donicity.77,79 Interest- ingly, if one studies solvent effects in the presence of the strongly binding chloride ion, the dominant factor then is the ability of the solvent to displace chloride, i.e., the ability of the solvent to solvate chloride, and the formation of six-coordinate di-solvent complexes then reflects the acceptor ability of the solvent rather than its donicity.78
2.27.3.1.2 Solvatochromism of coordination compounds with oxo-vanadium(IV)
Oxovanadium diketonate species have long been associated with solvatochromism involving the electronic absorption spectrum and also the (VẳO) infrared stretching vibration.82–92The initial five-coordinate OV(acac)2 species becomes solvated to form a six-coordinate species with con- comitant shift in absorption transition energies and color change from blue to green. Formation constants can be calculated from the integral ratio of the most intense IR band originating from the (VẳO) (A1) stretching vibration of the free [VO(acac)2] and the coordinated [VO(acac)2] with solvent species. In pyridine (py) solution, a far IR band near 469 cm1in [VO(acac)2] splits into two frequencies, an observation interpreted in terms of an equilibrium mixture betweencis and transgeometric isomers of [VO(acac)2(py)].
2.27.3.2 Solvatochromism Resulting from Intermolecular Interactions with the Solvent 2.27.3.2.1 Tris(bipyridine)ruthenium(II)
The title complex has no ground-state dipole moment, and shows the small degree of solvato- chromism typical of a nonpolar complex.19However, the electronic spectrum of this complex has been debated for many years, the question being whether the visible region Ru(d)!bpy(*) MLCT transitions give rise to localized or delocalized excited states.93 The solvatochromism of this band correlates with the (1Dop)/(2Dopþ1) function of McRae’s equation, shifting by about 300 cm1between nitrobenzene and water as solvents. Meyer’s group19 concluded that the shift was too large to be consistent with differences in dispersion forces and that theD3symmetry of the ground state must be broken in the F-C excited state, i.e., that the excited electron is localized on one of the three bipyridine ligands. The shift is then attributed to both changes in dispersion forces and in dipole–induced dipole forces.19 Milder, however, has shown that the solvatochro- mism is less than that of one of the internal bpy(!*) transitions, in the complex and in the
bpy ligand.94 Since these are not expected to give polar excited states, it seems reasonable to conclude that the solvatochromism is due to the ‘‘general red shift,’’ i.e., to differences in dispersion forces between the ground and excited states. The greater solvatochromism of the bpy(!*) transition is consistent with the larger oscillator strength of this transition, using Bayliss’ expression39for calculation of the dispersion force contribution.40
2.27.3.2.2 Carbonyl complexes
There have been many investigations95 of the solvatochromism of [M(CO)4(diimine)] complexes (where MẳCr, Mo, or W, but most commonly Mo) since the earliest report by Saito et al.96 These complexes are particularly well suited to the study of solvatochromism because they are neutral and yet highly polar (up to about 10 D),97and therefore are soluble in an exceptionally wide range of solvents—from hydrocarbons, such as hexane, to DMSO. They usually display strong negative solvatochromism,95and the visible-region, metal-to-diimine charge-transfer band may shift more than 3,000 cm1to the blue upon going from a nonpolar to a polar solvent. This main band is believed to be az-polarized b2!b2* (inC2vsymmetry) transition; a second MLCT transition which is also solvatochromic is often seen at higher energies.
Saito96used McRae’s equation to correlate with the solvatochromism, but later studies have used various empirical parameters. Burgess usedETfor a number of compounds which give separate linear correlations for hydroxylic and nonhydroxylic solvents.98,99Burgess99,100and Baxter and Connor101 have also demonstrated the change in solvent sensitivity brought about by changing the substituents on the diimine ligand, and Kaim and co-workers have discussed in detail the factors affecting solvatochromism of carbonyl complexes with bidiazine ligands102and with other N-donor ligands.103 In an early report, tom Dieck and Renk observed that substitution of two of the carbonyls (in the axial positions, trans to each other) by phosphine ligands could reverse the direction of the solvatochromism.104 This was interpreted as a change in the direction of charge transfer, from MLCT to LMCT, though alternative interpretations have been proposed.1A simpler explanation is that the increased metal–diimine mixing which occurs upon phosphine substitution, due to the increased electron richness of the metal, changes the nature of the transition to one with more metal–ligand bonding to antibonding character.95With a smaller change in dipole moment between the ground and excited states there may be little change in dipole–dipole forces, and the red shift resulting from the change in dispersion forces may be seen. This is consistent with the small changes in peak position seen for the complex in which the reversed solvatochromism is reported.
Investigations by Lees’ group into the solvent effects on the lowest-energy excited states of dinuclear [(OC)5WLW(CO)5] complexes (Lẳpz, 4,40-bpy and 1,2-bis(4-pyridyl)ethane) showed that, like similar mononuclear complexes, the MLCT absorption bands exhibited strong negative solvatochromism.105,106 Kaim and co-workers have also studied related dinuclear species.103,107Since these species have no net ground-state dipole moment, the solvatochromism was attributed by both groups to differences in the polarizability between the ground and excited states, causing differences in induced-dipolar and dispersion interactions. However, Dodsworth and Lever have proposed that these explanations are incompatible with the results of fitting data for various dinuclear systems to McRae’s equation.40,108These correlations (e.g., see Figure 1in ref. 108) show that dipole–dipole interactions are important despite the lack of a formal ground- state dipole moment. They suggest that the solvatochromism derives from the individual dipoles of each half of the molecule interacting with the solvent.40,108 This conclusion is supported by work, including extended Hu¨ckel calculations, on related complexes by Granifo et al.109
2.27.3.2.3 Platinum-group complexes
The solvatochromism of square-planar complexes of the Ni group containing a diimine and a dithiolate ligand was first reported in 1973 by Miller and Dance, for Ni complexes.110 These complexes have an intense band in the visible region, lying around 12–20,000 cm1 depending upon the specific ligands present, and showing strong negative solvatochromism. There has been some uncertainty about the assignment of this band, which does not occur in the spectra of the corresponding bis(diimine) or bis(dithiolate) species. Most of these complexes formally contain a dithiolate dianion and are therefore neutral and soluble in a variety of solvents.
358 Solvatochromism
A plot of the solvatochromism of [Ni(tfd)(phen)] (tfdẳS2C2(CF3)2; phenẳ1,10-phenanthro- line) against the product of solvent dipole moment and solvent concentration gave an obvious, though not completely linear, correlation for a variety of polar solvents, including alcohols and acetic acid. Miller and Dance concluded that the ground state is markedly dipolar and that one of the lower excited states is less so, i.e., that the solvatochromism reduces, reverses, or rotates the dipole. The effects of electron-withdrawing substituents on the two ligands on the redox poten- tials and spectra were also studied. The strong band was assigned to a strongly allowed b2!b2* LLCT transition, from an orbital mainly on the dithiolene, with some metalpzcharacter, to one mainly on the diimine, but with an estimated 20% metald character and a significant dithiolene contribution. A more recent study of solvatochromism of related Ni complexes has been pub- lished by Chenet al.111
Eisenberg and co-workers have focused mainly on the related Pt complexes, which are similarly solvatochromic.24,112–116For example, the band in [Pt(Me2bpy)(met)], where Me2bpy is 4,40-dimethyl- 2,20-bipyridine and met is cis-1,2-dicarbomethoxyethylene-1,2-dithiolate, shifts about 2,000 cm1 to the blue on going from chloroform to acetonitrile: see Figure 1.113 The solvatochromism of many of these complexes correlates well with Lees’E*MLCTparameter,55demonstrating that this parameter is a measure of solvent polarity rather than a reflection of specific interactions between polar solvents and the carbonyl groups in the [W(CO)4bpy] from which it was derived. These authors assign the solvatochromic transition as being from a mixed Pt(d)/S(p) orbital to one which is mainly*(diimine), and characterize it as a mixed MLCT/LLCT transition.114,117Many of these complexes also emit in fluid solution, but the emission is generally less solvatochromic24 or nonsolvatochromic113 and may arise from a different state.118,119 Transient DC photocurrent (TDCP) measurements, which give information about the change in dipole moment between the ground and excited states, have led Hupp to suggest that dimers are formed in the excited state.119