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Ligand-ligand inter-valence charge-transfer absorption in reduced ruthenium(II) bipyridine complexes
Volume 92, number 6
CHEMICAL PHYSICS LETTERS
12 November 1982
LIGAND-LIGAND INTER-VALENCE CHARGE-TRANSFER ABSORPTION IN REDUCED RUTHENIUM(II) BIPYRIDINE COMPLEXES G.A. HEATH, UJ. YELLOWLEES Chemistry Department, King's Buildings, University of Edinburgh, Edinburgh EH9 3JJ, UK
and P.S. BRATERMAN Chemistry Department, University of Glasgow, GlasgowG12 8QQ, UK
Received 18 August 1982
An electronic absorption band at 4000 cm-1 in incompletely reduced Ru(II)-bipyridine and Ru(II)-bipyride-pyridine complexes is characteristic of co-existingbpy0 and bpy- ligands and assigned to bpy-fopy0 inter-valencecharge transfer.
1. Introduction
Recent spectro-electrochemical studies [1 ] have provided convincing evidence that stepwise reduction of [Ru(bpy)3 ] 2+ (1) according to
[Ru(bpy)3]2+ *e~ [Ru(bpy)3]+ 1
!-
-
_+e-
+e ~ [Ru(bpy)3 ] 0 ~ 12-
[Ru(bpy)3 ] 13
(1)
-
yields complexes with localised charge distributions, viz. [Ru(II) (bpy°)2 (bpy-)] + (I-), [Ru(lI)(bpy 0) (bpy-)] 0 (i2-), and ultimately [Ru(II)(bpy-)3 ] (13-) *. Thus electronic transitions are observed which are due separately to coordinated bpy ° and b p y - (1-, 12 -), or to coordinated b p y - alone (13 -), and these conclusions are supported by independent ESR data for I - and 13- [2]. The simultaneous presence of discrete bpy ° and b p y - groups implies the possibility of ligand-ligand inter-valence charge transfer (IVCT) phenomena. In~:bpy = 2, 2 '-bipyridine; py = pyridine. 646
deed Motten et al. [2] have attributed the notably temperature-dependent line broadening found in their ESR spectrum of I - , but absent for 13-, to just such a process, with an estimated thermal barrier to electron hopping of ~1000 cm- 1. This barrier Eth corresponds to the intersection of the potential energy curves of equivalent valence isomers; Hush [3] has shown that where the curves are quadratic in form and there is negligible interaction between the redox active centres the vertical (IVCT) transition energy Eop should be 4Eth. Accordingly, we have extended our spectro-electrochemical studies to encompass the near-IR absorption spectra (7000-3500 cm- 1) of the sequence I to 13 - , and other, closely related, species.
2. Results and discussion A weak band near 4000 cm- 1 (see table 1) is detected for the species [Ru(II)(bpy0)3_n (bpy-)n ] 2-n when n = 1 or 2 (I-, 12-), but not when n = 0 or 3 (1, 13 -), and we ascribe this to the bpy/bpy- IVCT transition. As far as we know, this is the first reported direct observation of such a transition between identical ligands. 0 009-2614/82/0000-0000/$ 02.75 © 1982 North-Holland
Volume 92, number 6
12 November 1982
CHEMICAL PHYSICS LETTERS
Table 1 Intervalence charge-transfer bands in [Ru(II)(bpy°)x_n (PY)6--2x] 2 --n complexes (x = 2, 3; 0 < n < x) Species
t, (cm-1)
e (2 mol-l cm-1)
104f a)
[Ru(bpy)3 ]+ [Ru(bpy)3 ] 0 cis-[Ru(bpy) 2 (PY)2 ] ÷ trans-[Ru(bpy)2 (PY)2 ] ÷
4500 4500 4350 4090
210 345 121 100
19.3 31.7 8.6 3.2
a) Takingf = 4.6 × 10 --9 e × fwhm.
We have also examined the behaviour of partly 'su'f~stituted analogues of I, cis-[Ru(bpy)2(py)2] 2+ (cis-lI) and [Ru(bpy)(py)4 ] 2+ (III), and of the structurally distinct trans-[Ru(bpy)2(py)2 ] 2+ (trans-ll), and their corresponding reduced forms, according to [Ru0opy)2]2+ +e, [Ru(bpy)2 ] + II
II-
+e . [Ru(bpy)2(py)210
(2)
II[Ru(bpy)(py)4 ] 2+ ~ 1II
[Ru(bpy)(py)4 ] +
(3)
III-
These species are all inert to solvolysis and geometric isomerism on our electrochemical timescales, and the voltammetric data and UV-visible absorption spectra [4] are again fully consistent with separately absorbing coordinated bpy ° and b p y - chromophores. We Fmd (see table 1 )that only those complexes containing both bpy 0 and bpy (i.e. I - , 12-, cis-ll- and trans-ll-) exhibit the characteristic near-infrared band. For example, I I I - is featureless in this region in contrast to isovalent I I - and I - , and 1I 2 is featureless in contrast to 12-. The IVCT band is in all cases relatively broad (fwhm ~. 2000 c m - 1) and weak. It is decidedly more intense in F and 1 2 - than in cis- or trans-H-, which have only one donor and one acceptor centre. The slight but significant differences between cis- and trans-H- are presumably due to the different mutual arrangements of the chelating ligands. We conclude that the incompletely reduced complexes [Ru(bpy)3 ] +,0 and [Ru(bpy)2(py)2 ] + do indeed contain Ru(II) and distinct co-existing bpy 0
and b p y - ligands and exhibit low-intensity IVCT transitions in accord with this formulation. These complexes are therefore mixed-valence compounds showing electron transfer between negligibly interacting ligand sites, in general agreement with Hush's theory [3] for such systems, which has been more usually applied to adjacent metal sites in binuclear or polynuclear compounds. We have recently shown [5] that the absorption spectrum of the thermally equilibrated excited state, I*, in the range 2 5 0 - 6 5 0 nm can be understood in terms of the formulation [Ru(III)(bpyO)2(bpy-)] 2+ with distinct bpy ° and b p y - chromophores. Accordingly there is an analogy between electrochemical and MLCT reduction of the ligand array, and our detection of the intervalence process in I - lends quafified support to Woodruffs hypothesis [6] that I* could show such a band. By characterising typical IVCT transitions near 4000 cm -1 , with e ~200 £ mo1-1 cm -1 (and particularly since Ir(III)-reduced-bipyridyl complexes give similar results [4] ), the present data make clear that observation of the intervalence absorption band in I* itself will present extreme difficulty.
3. Experimental Ru(bpy)3C12 was purchased from G.F. Smith Inc., converted to the BF~- salt, and recrystallised from acetonitrile, while trans-ll and III perc~orates were prepared by the method of Krause [7], and pure cis-lI perchlorate by treatment of cis-[Ru(bpy)2C12] [8] with pyridine followed by aqueous NaC10 4. Spectroelectrochemical data were collected as in our earlier work [1], using a Metrohm E506 potentiostat and an optically transparent thin layer electrode cell. Careful
647
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CHEMICAL PHYSICS LETTERS
correction for solvent background is critical, because o f solvent vibrational combination bands, and was performed b y direct subtraction. All compounds were studied in dimethylsulphoxide; in addition, I and its reduction products were studied in dimethylformamide and in acetonitrile, with indistinguishable results.
Acknowledgement We thank Napier College, Edingburgh for access to their Beckman 5270 spectrometer, and SERC for a postgraduate studentship (to LJY) and provision o f electrochemical equipment.
648
12 November 1982
References [1] G.A. Heath, L.J. Yellowlees and P.S. Braterman, J. Chem. Soc. Chem. Commun. (1981) 287. [2] A.G. Motten, K. Hanck and M.K. DeArmond, Chem. Phys. Letters 79 (1981) 541. [3] N.S. Hush, Progr. Inorg. Chem. 8 (1967) 391. [4] G.A. Heath, L.J. Yellowlees and P.S. Braterman, to be published. [5] P.S. Braterman, A. Harriman, G.A. Heath and L.J. Yellowlees, in preparation. [6] P.G. Bradley, N. Kress, B.A. Hornberger, R.F. Dallinger and W.H. Woodruff, J. Am. Chem. Soc. 103 (1981) 7441. [7] R.A. Krause, lnorg. Chim. Aeta 22 (1977) 209. [8] B.P. Sullivan, D.J. Salmon and T.J. Meyer, Inorg. Chem. 17 (1978) 3334.
CHEMICAL PHYSICS LETTERS
12 November 1982
LIGAND-LIGAND INTER-VALENCE CHARGE-TRANSFER ABSORPTION IN REDUCED RUTHENIUM(II) BIPYRIDINE COMPLEXES G.A. HEATH, UJ. YELLOWLEES Chemistry Department, King's Buildings, University of Edinburgh, Edinburgh EH9 3JJ, UK
and P.S. BRATERMAN Chemistry Department, University of Glasgow, GlasgowG12 8QQ, UK
Received 18 August 1982
An electronic absorption band at 4000 cm-1 in incompletely reduced Ru(II)-bipyridine and Ru(II)-bipyride-pyridine complexes is characteristic of co-existingbpy0 and bpy- ligands and assigned to bpy-fopy0 inter-valencecharge transfer.
1. Introduction
Recent spectro-electrochemical studies [1 ] have provided convincing evidence that stepwise reduction of [Ru(bpy)3 ] 2+ (1) according to
[Ru(bpy)3]2+ *e~ [Ru(bpy)3]+ 1
!-
-
_+e-
+e ~ [Ru(bpy)3 ] 0 ~ 12-
[Ru(bpy)3 ] 13
(1)
-
yields complexes with localised charge distributions, viz. [Ru(II) (bpy°)2 (bpy-)] + (I-), [Ru(lI)(bpy 0) (bpy-)] 0 (i2-), and ultimately [Ru(II)(bpy-)3 ] (13-) *. Thus electronic transitions are observed which are due separately to coordinated bpy ° and b p y - (1-, 12 -), or to coordinated b p y - alone (13 -), and these conclusions are supported by independent ESR data for I - and 13- [2]. The simultaneous presence of discrete bpy ° and b p y - groups implies the possibility of ligand-ligand inter-valence charge transfer (IVCT) phenomena. In~:bpy = 2, 2 '-bipyridine; py = pyridine. 646
deed Motten et al. [2] have attributed the notably temperature-dependent line broadening found in their ESR spectrum of I - , but absent for 13-, to just such a process, with an estimated thermal barrier to electron hopping of ~1000 cm- 1. This barrier Eth corresponds to the intersection of the potential energy curves of equivalent valence isomers; Hush [3] has shown that where the curves are quadratic in form and there is negligible interaction between the redox active centres the vertical (IVCT) transition energy Eop should be 4Eth. Accordingly, we have extended our spectro-electrochemical studies to encompass the near-IR absorption spectra (7000-3500 cm- 1) of the sequence I to 13 - , and other, closely related, species.
2. Results and discussion A weak band near 4000 cm- 1 (see table 1) is detected for the species [Ru(II)(bpy0)3_n (bpy-)n ] 2-n when n = 1 or 2 (I-, 12-), but not when n = 0 or 3 (1, 13 -), and we ascribe this to the bpy/bpy- IVCT transition. As far as we know, this is the first reported direct observation of such a transition between identical ligands. 0 009-2614/82/0000-0000/$ 02.75 © 1982 North-Holland
Volume 92, number 6
12 November 1982
CHEMICAL PHYSICS LETTERS
Table 1 Intervalence charge-transfer bands in [Ru(II)(bpy°)x_n (PY)6--2x] 2 --n complexes (x = 2, 3; 0 < n < x) Species
t, (cm-1)
e (2 mol-l cm-1)
104f a)
[Ru(bpy)3 ]+ [Ru(bpy)3 ] 0 cis-[Ru(bpy) 2 (PY)2 ] ÷ trans-[Ru(bpy)2 (PY)2 ] ÷
4500 4500 4350 4090
210 345 121 100
19.3 31.7 8.6 3.2
a) Takingf = 4.6 × 10 --9 e × fwhm.
We have also examined the behaviour of partly 'su'f~stituted analogues of I, cis-[Ru(bpy)2(py)2] 2+ (cis-lI) and [Ru(bpy)(py)4 ] 2+ (III), and of the structurally distinct trans-[Ru(bpy)2(py)2 ] 2+ (trans-ll), and their corresponding reduced forms, according to [Ru0opy)2]2+ +e, [Ru(bpy)2 ] + II
II-
+e . [Ru(bpy)2(py)210
(2)
II[Ru(bpy)(py)4 ] 2+ ~ 1II
[Ru(bpy)(py)4 ] +
(3)
III-
These species are all inert to solvolysis and geometric isomerism on our electrochemical timescales, and the voltammetric data and UV-visible absorption spectra [4] are again fully consistent with separately absorbing coordinated bpy ° and b p y - chromophores. We Fmd (see table 1 )that only those complexes containing both bpy 0 and bpy (i.e. I - , 12-, cis-ll- and trans-ll-) exhibit the characteristic near-infrared band. For example, I I I - is featureless in this region in contrast to isovalent I I - and I - , and 1I 2 is featureless in contrast to 12-. The IVCT band is in all cases relatively broad (fwhm ~. 2000 c m - 1) and weak. It is decidedly more intense in F and 1 2 - than in cis- or trans-H-, which have only one donor and one acceptor centre. The slight but significant differences between cis- and trans-H- are presumably due to the different mutual arrangements of the chelating ligands. We conclude that the incompletely reduced complexes [Ru(bpy)3 ] +,0 and [Ru(bpy)2(py)2 ] + do indeed contain Ru(II) and distinct co-existing bpy 0
and b p y - ligands and exhibit low-intensity IVCT transitions in accord with this formulation. These complexes are therefore mixed-valence compounds showing electron transfer between negligibly interacting ligand sites, in general agreement with Hush's theory [3] for such systems, which has been more usually applied to adjacent metal sites in binuclear or polynuclear compounds. We have recently shown [5] that the absorption spectrum of the thermally equilibrated excited state, I*, in the range 2 5 0 - 6 5 0 nm can be understood in terms of the formulation [Ru(III)(bpyO)2(bpy-)] 2+ with distinct bpy ° and b p y - chromophores. Accordingly there is an analogy between electrochemical and MLCT reduction of the ligand array, and our detection of the intervalence process in I - lends quafified support to Woodruffs hypothesis [6] that I* could show such a band. By characterising typical IVCT transitions near 4000 cm -1 , with e ~200 £ mo1-1 cm -1 (and particularly since Ir(III)-reduced-bipyridyl complexes give similar results [4] ), the present data make clear that observation of the intervalence absorption band in I* itself will present extreme difficulty.
3. Experimental Ru(bpy)3C12 was purchased from G.F. Smith Inc., converted to the BF~- salt, and recrystallised from acetonitrile, while trans-ll and III perc~orates were prepared by the method of Krause [7], and pure cis-lI perchlorate by treatment of cis-[Ru(bpy)2C12] [8] with pyridine followed by aqueous NaC10 4. Spectroelectrochemical data were collected as in our earlier work [1], using a Metrohm E506 potentiostat and an optically transparent thin layer electrode cell. Careful
647
Volume 92, number 6
CHEMICAL PHYSICS LETTERS
correction for solvent background is critical, because o f solvent vibrational combination bands, and was performed b y direct subtraction. All compounds were studied in dimethylsulphoxide; in addition, I and its reduction products were studied in dimethylformamide and in acetonitrile, with indistinguishable results.
Acknowledgement We thank Napier College, Edingburgh for access to their Beckman 5270 spectrometer, and SERC for a postgraduate studentship (to LJY) and provision o f electrochemical equipment.
648
12 November 1982
References [1] G.A. Heath, L.J. Yellowlees and P.S. Braterman, J. Chem. Soc. Chem. Commun. (1981) 287. [2] A.G. Motten, K. Hanck and M.K. DeArmond, Chem. Phys. Letters 79 (1981) 541. [3] N.S. Hush, Progr. Inorg. Chem. 8 (1967) 391. [4] G.A. Heath, L.J. Yellowlees and P.S. Braterman, to be published. [5] P.S. Braterman, A. Harriman, G.A. Heath and L.J. Yellowlees, in preparation. [6] P.G. Bradley, N. Kress, B.A. Hornberger, R.F. Dallinger and W.H. Woodruff, J. Am. Chem. Soc. 103 (1981) 7441. [7] R.A. Krause, lnorg. Chim. Aeta 22 (1977) 209. [8] B.P. Sullivan, D.J. Salmon and T.J. Meyer, Inorg. Chem. 17 (1978) 3334.