Harry
Adams
a,
Wassim Z.
Alsindi
b,
Graham M.
Davies
a,
Martin B.
Duriska
a,
Timothy L.
Easun
a,
Hazel E.
Fenton
a,
Juan-Manuel
Herrera
a,
Michael W.
George
*b,
Kate L.
Ronayne
c,
Xue-Zhong
Sun
b,
Michael
Towrie
c and
Michael D.
Ward
*a
aDepartment of Chemistry, University of Sheffield, Sheffield, UK S3 7HF. E-mail: m.d.ward@Sheffield.ac.uk
bSchool of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD. E-mail: mike.george@Nottingham.ac.uk
cCCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK OX11 0QX
First published on 14th September 2005
A series of complexes of the type K2[Ru(NN)(CN)4] has been prepared, in which NN is a diimine ligand, and were investigated for both their structural and photophysical properties. The ligands used (and the abbreviations for the resulting complexes) are 3-(2-pyridyl)pyrazole (Ru-pypz), 2,2′-bipyrimidine (Ru-bpym), 5,5′-dimethyl-2,2′-bipyridine (Ru-dmb), 1-ethyl-2-(2-pyridyl)benzimidazole (Ru-pbe), bidentate 2,2′:6′,2‴-terpyridine (Ru-tpy). The known complexes with NN = 2,2′-bipyridine (Ru-bpy) and 1,10-phenathroline (Ru-phen) were also included in this work. A series of crystallographic studies showed that the [Ru(NN)(CN)4]2− complex anions form a range of elaborate coordination networks when crystallised with either K+ or Ln3+ cations. The K+ salts are characterised by a combination of near-linear Ru–CN–K bridges, with the cyanides coordinating to K+ in the usual ‘end-on’ mode, and unusual side-on π-type coordination of cyanide ligands to K+ ions. With Ln3+ cations in contrast only Ru–CN–Ln near-linear bridges occurred, affording 1-dimensional helical or diamondoid chains, and 2-dimensional sheets constituted from linked metallamacrocyclic rings. All of the K2[Ru(NN)(CN)4] complexes show a reversible Ru(II)/Ru(III) couple (ca. +0.9 V vs. Ag/AgCl in water), the exception being Ru-tpy whose oxidation is completely irreversible. Luminescence studies in water showed the presence of 3MLCT-based emission in all cases apart from Ru-bpym with lifetimes of tens/hundreds of nanoseconds. Time-resolved infrared studies showed that in the 3MLCT excited state the principal C–N stretching vibration shifts to positive energy by ca. 50 cm−1 as a consequence of the transient oxidation of the metal centre to Ru(III) and the reduction in back-bonding to the cyanide ligands; measurement of transient decay rates allowed measurements of 3MLCT lifetimes for those complexes which could not be characterised by luminescence spectroscopy. A few complexes were also examined in different solvents (MeCN, dmf) and showed much weaker emission and shorter excited-state lifetimes in these solvents compared to water.
Firstly, the strong solvatochromism of [Ru(bpy)(CN)4]2−, which arises from interaction of the externally-directed electron lone pairs on the cyanide ligands, means that its 3MLCT energy and its ground and excited state redox potentials are highly solvent dependent; for example the 3MLCT luminescence maximum varies from 640 nm in water to 818 nm in dmf, and the Ru(II)/Ru(III) redox potential varies from +0.77 V to −0.28 V (vs. ferrocene/ferrocenium) between the same solvents.1 This also applies in the solid state: Mann and co-workers showed how a crystalline salt of [Ru(bpy)(CN)4]2− could act as a humidity sensor because of its reversible colour change from purple to yellow in the presence of humidity, which is associated with water molecules forming hydrogen-bonds to the externally-directed lone pairs of the cyanide ligands.5 We6 and others7 have exploited this effect by using changes in solvent to alter the ability of [Ru(bpy)(CN)4]2− to act as an energy-donor in simple dyad molecules: changing the solvent alters the energy available to the [Ru(bpy)(CN)4]2− energy-donor unit without significantly affecting the receiving energy level on the acceptor component, so the gradient for photoinduced energy-transfer can be finely controlled.
Secondly, the cyanide ligands can act as bridging ligands to additional metal ions, allowing formation of Ru–CN–M bridged coordination oligomers and polymers by simple combination of [Ru(bpy)(CN)4]2− with other metal cations. We have recently described coordination polymers based on combination of [Ru(bpy)(CN)4]2− with lanthanide(3+) salts8,9 in which excitation of the Ru-based MLCT transition results in energy-transfer to, and sensitised luminescence from, lanthanide cations such as Yb(III), Nd(III), Er(III) and Pr(III) which have low-lying emissive levels and display sensitised near-infrared luminescence.9 Scandola and co-workers described a while ago energy-transfer between [Ru(bpy)2(CN)2] and a Cr(III)-macrocycle unit which were connected by Ru–CN–Cr bridges in a similar way.10
Thirdly, the IR-active cyanide ligands of [Ru(bpy)(CN)4]2− allow the movement of excitation energy and electrons in polynuclear complexes to be monitored by transient IR spectroscopy, since the CN vibrations are sensitive to redistribution of electron density in the complex; this technique is well known for carbonyl ligands in complexes such as [Re(bpy)(CO)3Cl] and its derivatives,11 but has been applied to luminescent cyanometallate complexes in very few cases.12
Despite these advantages, analogues and derivatives of [Ru(bpy)(CN)4]2− have been scarcely studied, beyond simple replacement of bpy by phenanthroline or addition of methyl substituents.13 This is in marked contrast to the vast number of analogues of [Ru(bpy)3]2+ that have been investigated in the last few decades.4 As part of a program of research into the photophysical properties of complexes of this type we have prepared a series of analogues in which the bpy ligand is replaced by other diimines. In the course of this work we have found that these [Ru(NN)(CN)4]2− complexes (NN is a diimine ligand) form a varied and attractive series of coordination networks with s- and f-block metal counter-ions, and the structural chemistry of these complexes is of as much interest as their photophysics. Heterometallic coordination networks based on cyanometallates have of course been of enduring interest for many years. In many cases the cyanometallate anion on which the network is based is an octahedral hexacyanometallate;14 however cyanometallates with different structures such as octadentate [W(CN)8]3–/4−,15C2v-symmetric tetradentate [Fe(bpy)(CN)4]− and [Fe(phen)(CN)4]−,16 square planar [M(CN)4]2− (M = Ni, Pd),17 and linear bidentate [M(CN)2]− (M = Ag, Au)18 have also been used recently as the basis of 1-, 2- or 3-dimensional networks based on cyanide bridges spanning two different metal ions.
This paper, therefore, describes the syntheses and structural characterisation of several coordination networks based on [Ru(NN)(CN)4]2− species with either potassium or lanthanide cations. We note that structural studies on [Ru(bpy)(CN)4]2− and its relatives are almost unknown, with the only examples currently on the Cambridge Structural Database being those referred to above.5,8,9 In addition, this paper describes the solution spectroscopic, redox and photophysical properties of the new [Ru(NN)(CN)4]2− species, including time-resolved IR studies of the 3MLCT states of some members of the series.
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Scheme 1 |
Of the complexes, Ru-bpy and Ru-phen are already known.1,13Ru-pypz, Ru-dmb and Ru-PBE are simple analogues of [Ru(bpy)(CN)4]2− but based on different diimine ligands available from the literature.19,20Ru-bpym was prepared to see how the presence of a vacant bipyrimidine-based coordination site, in addition to the cyanide-based coordination sites, would alter the structural properties. Ru-terpy was the unexpected result of an attempt to prepare [Ru(terpy)(CN)3]−, a known compound21 but whose synthesis requires the use of potassium cyanide. We were interested to see if a more user-friendly synthesis could be achieved from hexacyanoruthenate and terpyridine; however this yielded Ru-terpy in which the NMR spectrum clearly showed that the terpyridine was asymmetrically coordinated as a bidentate ligand, with all 11 protons inequivalent—something which terpyridine is known to do in other complexes.22 Under the conditions of the reaction we found no evidence for formation of [Ru(terpy)(CN)3]− by this route.
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Fig. 1 ORTEP diagram of the asymmetric unit of K2[Ru(phen)(CN)4]·4H2O, with some additional symmetry-equivalent atoms included to complete the coordination spheres around the metal atoms. |
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Fig. 2 View of the coordination chain in K2[Ru(phen)(CN)4]·4H2O (Ru = brown; K = purple; N = blue, O = red, C = black). |
K2[Ru(bpym)(CN)4]·3H2O has an entirely different structure. The [Ru(bpym)(CN)4]2− complex anion has a K+ cation occupying the second NN-chelating site of the bipyrimidine ligand. This atom [K(2)] is further coordinated by three water molecules, all of which are involved in bridging interactions to other K+ ions, and the N atoms of two cyanide ligands from separate [Ru(bpym)(CN)4]2− complex anions (Fig. 3). The coordination geometry about K(2) is approximately pentagonal bipyramidal, with N(8C) and O(3) being the axial ligands [N–K–O angle, 172.7°] and the sum of the five angles in the plane being 360.2°. K(1) in contrast is coordinated in a side-on manner by five cyanide ligands from two different [Ru(bpym)(CN)4]2− complex anions, in addition to three water molecules (which are all involved in bridging to another K+ ion). These interactions are slightly longer than in the structure of K2[Ru(phen)(CN)4], with the K–N and K–C side-on interactions averaging 3.22 and 3.25 Å respectively; the coordination environment of K(1) is included in Fig. 3. The combination of multiply bridging cyanide ligands, as well as bridging water ligands between K+ ions, results in propagation of the structure into a three-dimensional network. This consists of 2-D sheets, as shown in Fig. 4, which are connected in the third dimension by water ligands [O(1)] which cross-link the sheets by connecting K(1) in one sheet with K(2) in the next. In addition, the bipyrimidine ligands of sheets interpenetrate such that there is an alternating π-stacking interaction between them. It is noticeable from Fig. 4 how some of the cyanide ligands can interact with four different metal ions, being C-bound to the ruthenium centre, involved in side-on binding with two different K+ ions, and also acting as a terminal N-donor ligand to a third K+ centre.
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Fig. 3 ORTEP diagram of the asymmetric unit of K2[Ru(bpym)(CN)4]·3H2O, with some additional symmetry-equivalent atoms included to complete the coordination spheres around the metal atoms. |
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Fig. 4 View of the two-dimensional sheet structure of K2[Ru(bpym)(CN)4]·3H2O (Ru = brown;, K = purple; N = blue, O = red, C = black). |
The structure of K2[Ru(κ2-terpy)(CN)4]·MeOH·0.5H2O, in which the terpy ligand is bidentate, is likewise dominated by side-on cyanide⋯K+ interactions (Fig. 5, 6). K(2) is coordinated in this manner by two cyanide ligands from each of two {Ru(CN)4}2− units, with average K⋯C and K⋯N distances of 3.18 and 2.96 Å respectively; it also has one coordinated methanol ligand and a pyridyl ligand, which is the pendant 2-pyridyl group from a terpyridine whose other two donors are coordinated to ruthenium. K(1) in contrast is coordinated by two O atoms from (bridging) methanol ligands and the N atoms from three end-on cyanide ligands (Fig. 5). The combination of the bridging interactions involving cyanide and methanol ligands results in formation of a two-dimensional network. This is made up from 1-D chains which are based on alternating [Ru(terpy)(CN)4]2− and K(2) units, which are disposed such that the Ru⋯K⋯Ru⋯K sequence forms a zig-zag chain (Fig. 6) which is held together principally by the side-on cyanide⋯K(2) bonding interactions. These chains are linked side-by-side to make a two-dimensional sheet by the K(1) centres which interact with the externally-directed O and N(cyanide) donors, as shown in Fig. 5. As with the other two structures of potassium salts, the cyanide bridges are both simultaneously side-on coordinated and terminally coordinated to different K+ ions.
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Fig. 5 (a) ORTEP diagram of part of the asymmetric unit of K2[Ru(κ2-terpy)(CN)4]·MeOH·0.5H2O, emphasising the interaction of the cyanoruthenate anions with K(2) via side-on π-type cyanide coordination to K(2); (b) diagram of the coordination environment around K(1). |
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Fig. 6 View of the one-dimensional coordination network K2[Ru(κ2-terpy)(CN)4]·MeOH·0.5H2O formed by interaction of the cyanoruthenate anion with K(2); these chains are connected side-by-side by bridging cyanide interactions with K(1) (as in Fig. 5a) (Ru = brown; K = purple; N = blue, O = red, C = black). |
The crystal structure of KEr[Ru(PBE)(CN)4]2·10H2O (Fig. 7, 8) shows a combination of the side-on cyanide⋯K+ interactions seen in the earlier structures, as well as terminal Ru–CN–Er bridges. The structure is a one-dimensional chain consisting of Ru2Er2(μ-CN)4 diamonds in which the Er centres are shared between successive diamonds (Fig. 7). Each Er(III) centre is 8-coordinate, from four cyanide N atoms and four water ligands. The situation is more complex than is apparent from Fig. 7 because two of the water ligands [O(3), O(7)] are disordered over two sites with 50% occupancy in each; only one component of the disorder is shown in the figure. The Er–N and Er–O distances average 2.37 and 2.42 Å respectively, and the Ru⋯Er separations across the cyanide bridges are 5.417 and 5.498 Å. Given the disorder in the coordination sphere of Er(1) a description of its coordination geometry is inappropriate. The propensity of the K+ ions to become involved in side-on interactions with cyanide ligands is demonstrated by the environment of K(1) (Fig. 8). These ions are located above and below the Ru2Er2(μ-CN)4 diamonds, with 50% occupancy in each site, such that they interact approximately equally with all four cyanide ligands in the diamond; the average K–C and K–N distances are 3.22 and 3.10 Å. Each K+ ion also has a terminal water ligand [O(4); K(1)–O(4), 2.39 Å] and two additional water molecules [O(3) and O(7); 50% site occupancy each; K–O separations 2.90 Å] which are bridging to Er(1).
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Fig. 7 View of the one-dimensional diamondoid chain of KEr[Ru(PBE)(CN)4]2·10H2O, with potassium atoms omitted for clarity (Ru = brown, Er = green; N = blue, O = red, C = black). |
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Fig. 8 A view of part of the structure of KEr[Ru(PBE)(CN)4]2·10H2O illustrating the disorder of the potassium atom and its associated water ligands (Ru = brown, Er = green; K = purple; N = blue, O = red, C = black). |
The structure of KEu[Ru(pypz)(CN)4]2·10.5H2O (Fig. 9, 10) is quite different from this [and from the earlier-described complexes of [Ru(bpy)(CN)4]2− with lanthanide cations].8 It is a remarkably elaborate structure whose two main structural motifs are shown in Fig. 9 and 10. The tetracyanoruthenate anions and Eu(III) cations form a 24-membered ring based on four of each unit linked in a {Ru–CN–Eu–NC–}4 cyclic sequence. Pendant from each Eu vertex is an additional [Ru(pypz)(CN)4]2− unit which is connected to the Eu(III) by a single cyanide bridge. Accordingly, the [Ru(pypz)(CN)4]2− units in the ring [based on Ru(2) and Ru(3)] have two cyanides involved in bridging interactions to adjacent Eu(III) sites, whereas the ‘exocyclic’ [Ru(pypz)(CN)4]2− units [based on Ru(1) and Ru(4)] have only one cyanide involved in bridging to Eu(III). Each Eu(III) centre is accordingly eight-coordinate, from three bridging cyanides (two within the macro-ring and one from outside it) and five water ligands. The Ru⋯Eu separations across the cyanide bridges are in the range 5.32–5.58 Å. These rings are associated by cyanide bridging interactions to the K+ ions, as shown in Fig. 10, which illustrates how four [Ru(pypz)(CN)4]2− units, each from a different Ru–Eu ring, are associated by a combination of end-on Ru–CN–K+ and side-on cyanide/K+ interactions. K(1) is coordinated by four cyanides in a side-on manner, as well as one end-on cyanide [N(28)] and one water molecule [O(11)]. K(2) has three side-on cyanides, two of which are ‘end-on’ (although the C–N–K angles are far from linear) and two water molecules.
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Fig. 9 A view of the large metallamacrocyclic ring in KEu[Ru(pypz)(CN)4]·10.5H2O, showing only the Ru and Eu atoms and the bridging cyanide ligands and the terminal water ligands (Ru = brown, Eu = green; N = blue, O = red, C = black). |
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Fig. 10 A view of part of the lattice of KEu[Ru(pypz)(CN)4]·10.5H2O showing how four cyanoruthenate units which are ‘spokes’ from the Ru4Eu4 rings (see Fig. 9) are connected by a combination of end-on and side-on cyanide coordination to K+ ions (Ru = brown, Eu = green; K = purple; N = blue, O = red, C = black). |
In complete contrast, the structure of [Eu(NO3)(H2O)5][Ru(bpym)(CN)4] is a relatively simple one-dimensional helical chain (Fig. 11, 12). In this case there is a 1 : 1 ratio of Eu(III) and [Ru(bpym)(CN)4]2− centres, with charge balance being provided by a nitrate ion attached to Eu(III). The Eu(III) centres are 9-coordinate from five water ligands, a bidentate nitrate [although one Eu–O bond from the nitrate is considerably longer than the other: Eu(2)–O(5), 2.477 Å; Eu(2)–O(6), 2.774 Å] and two N-donor cyanide ligands, each from a separate [Ru(bpym)(CN)4]2− unit. The inset to Fig. 12 shows how the ⋯Ru–CN–Eu–NC–Ru⋯ sequence forms a spiral, with a pitch of 12.45 Å. The space group is chiral (Pna21) so this complex has spontaneously resolved on crystallisation to give optically pure individual crystals.
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Fig. 11 ORTEP diagram of the asymmetric unit of [Eu(NO3)(H2O)5][Ru(bpym)(CN)4], with some additional symmetry-equivalent atoms included to complete the coordination spheres around the metal atoms. |
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Fig. 12 Two views of [Eu(NO3)(H2O)5][Ru(bpym)(CN)4] illustrating formation of a one-dimensional helical coordination polymer; the top view shows all atoms, whereas the lower shows only Ru, Eu and the bridging cyanide ligands (Ru = brown, Eu = green; N = blue, O = red, C = black). |
Complex | UV/Vis absorptions λmax/nm (10−3ε/M−1 cm−1) | λ em/nm | τ/ns | Φ | E d/V |
---|---|---|---|---|---|
a Figures in square brackets refer to measurements made in MeCN; extinction coefficients of absorption spectra not measured due to poor solubility. b Lifetime determined from transient IR spectra rather than luminescence as luminescence in solution was very weak. c Figures in square brackets refer to measurements made in dmf from transient IR spectra. d Redox potential for Ru(II)/Ru(III) couple in water/0.1 M KCl. For comparison the known complexes [Ru(bpy)(CN)4]2− and [Ru(phen)(CN)4]2− have redox potentials (measured with the same experimental setup) of +0.89 and +0.88 V vs. Ag/AgCl. e Irreversible (no return wave); the quoted potential is the peak potential on the outward sweep. f Reversibility difficult to judge as this process is very close to the rapidly-rising background associated with the solvent cutoff. | |||||
Ru-dmb | 261 (9.6), 291 (15), 393 (2.1) [372, 514]a | 621 [758]a | 370 [36]a | 0.03 [0.002]a | +0.85 |
Ru-tpy | 296 (22), 411 (2.7) | 647 | 106 | 0.025 | +0.70e |
Ru-bpym | 342 (5.8), 437 (2.2) [415, 575]a | — | 3.4b[0.25]a,b | — | +1.05f |
Ru-PBE | 242 (15), 313 (17), 404 (3.6) | 665 | 471 [4.5]c | 0.007 | +0.79 |
Ru-pypz | 229 (23), 272 (20), 353 (6.9) | 554 | 17 | 0.001 | +0.83 |
The expected ligand-centred reductions were not accessible within the limits of the potential window provided by water as solvent.
In every case, in D2O solution, excitation into the 1MLCT absorption results in luminescence in the range 550–670 nm which is believed to originate from the 3MLCT excited state. Kovács and Horváth recently showed that using D2O as solvent afforded considerably longer-lived luminescence from complexes of this type than H2O due to attenuation of non-radiative decay pathways in D2O, a consequence of hydrogen-bond formation with the cyanide lone pairs.13 In addition to examining all of the new complexes, for comparison purposes we also re-measured the properties of Ru-bpy and Ru-phen and found a satisfactory agreement between our measurements and those published previously.1,13 A few effects are worth pointing out.
Firstly, for Ru-tpy the 1MLCT absorption and 3MLCT luminescence are red-shifted compared to Ru-bpy, despite the donor set being nominally identical. This we ascribe to the long bond Ru(1)–N(8) [2.190(5) Å], involving the central ring of the terpyridyl ligand, which is lengthened by virtue of having a sterically bulky group—the pendant pyridyl residue—at its C2 position, ortho to the site of coordination. Lengthening this bond will weaken the ligand field of the coordinated bipyridyl fragment compared to normal un-encumbered bpy (Ru–N separation ca. 2.10 Å), which will slightly raise the d(π) orbital set and hence reduce the MLCT absorption and emission energies. This phenomenon has been observed in derivatives of [Ru(bpy)3]2+ in which the bpy ligands have a bulky substituent at the C2/C6 position.25
Secondly, variation in the nature of the diimine ligand results in significant changes to the 1MLCT absorption and 3MLCT luminescence energies for electronic reasons. The highest-energy absorption and luminescence in the series come from Ru-pypz (luminescence in water at 554 nm), implying that the pypz ligand has a higher-energy LUMO than the other diimines. [The alternative possible explanation, that the d(π) orbitals are lower in energy than in the other complexes, is not borne out by the electrochemical studies, as the Ru(II)/Ru(III) couple is not at an unusually positive potential compared to the other complexes]. This is also borne out by the short luminescence lifetime (17 ns) which is a consequence of the high-lying 3MLCT state being close in energy to the metal-centred d–d state, which would provide a facile thermally-activated route for radiationless deactivation. Despite the relatively short luminescence lifetime, the high 3MLCT energy of Ru-pypz will make it of particular value as an energy-donor in polynuclear assemblies.26
Thirdly, the luminescence of Ru-bpym is too weak to allow the excited-state lifetime to be measured by luminescence methods. Replacement of bpy by bipyrimidine has been shown in other complexes to result in much weaker, shorter-lived emission: for example [Re(bpy)(CO)3Cl] has λem = 642 nm, τ = 39 ns and ϕ = 3 × 10−3 (2-methyl-thf, fluid solution)27 whereas [Re(bpym)(CO)3Cl] has λem = 689 nm, τ = 2.1 ns and ϕ = 10−4 (CH2Cl2, fluid solution).28 A similar effect is seen when bpy is replaced by bipyrimidine in Pt-diimine-diacetylides [Pt(NN)(CCR)2],28,29 and is a consequence of the energy-gap law; the lower energy of the 3MLCT excited state of Ru-bpym, arising from the lower energy of the bpym π* orbitals compared to bpy, means that the excited state is more efficiently quenched by molecular vibrations.
Finally, we note that for Ru-dmb, which is slightly soluble in MeCN, the emission maximum (like the lowest-energy MLCT absorption maximum) is substantially red-shifted compared to the situation in water, from 621 to 758 nm, and is much weaker—actually barely detectable—and short lived (see Table 1), in keeping with expectations based on the known solvatochromic behaviour of complexes of this type.1
Complex | ν CN (ground state)a/cm−1 | Main bleach in TRIRb/cm−1 | Main new transient in TRIRb/cm−1 | Δ/cm−1 |
---|---|---|---|---|
a Solid-state spectra measured on powdered sample using diamond-ATR cell. b Figures in square brackets refer to measurements made in D2O unless stated otherwise. c Measured in MeCN. | ||||
Ru-bpy | 2032(s), 2047(s), 2056(s), 2089(m) | 2051 | 2103 | 52 |
Ru-dmb | 2034(s), 2040(s), 2054(s), 2090(m) | 2049 [2068]c | 2095 [2104]c | 46 [36]c |
Ru-phen | 2032(s), 2038(s), 2047(s), 2088(m) | 2051 | 2098 | 47 |
Ru-tpy | 2041(s), 2047(s), 2091(m) | 2051 | 2100 | 49 |
Ru-bpym | 2046(s), 2057(s), 2065(s), 2102(m) | 2057 [2068]c | 2105 [2098]c | 48 [30]c |
Ru-PBE | 2028(s), 2044(s), 2085(m) | 2048 | 2095 | 47 |
Ru-pypz | 2028(s), 2045(s), 2052(s), 2092(m) | 2050 | 2098 | 48 |
In aqueous solution these are not all resolved; the three closely-spaced peaks merge to give a strong peak at 2050–2060 cm−1, and a weaker one appearing as a shoulder about 20–30 cm−1 higher. Fig. 13 shows the time-resolved difference IR spectrum of Ru-bpym obtained 100 ps after 400 nm excitation into the 1MLCT absorption band. The parent ν(CN) vibrations are bleached, and weaker new bands appear at higher energy. Assuming that the most intense new transient corresponds to the most intense peak in the ground-state spectrum, there is a shift to higher energy of 52 cm−1 of this ν(CN) vibration when the complex is in the 3MLCT excited state, as a consequence of loss of electron density on the Ru centre [which is transiently oxidised to Ru(III)] and the concomitant decrease in Ru(dπ) → CN(π*) back-bonding. The weakness of the transient ν(CN) vibration compared to the ground state vibration is consistent with our previous observations12b and suggests that the dipole of the C–N bonds is reduced in the 3MLCT excited state. This is reasonable, as cyanide ligands formally have the negative charge on the C atom: transient oxidation of Ru(II) to Ru(III) will reduce the partial negative charge on the coordinated C atom and hence diminish the C–N dipoles, leading to weaker IR absorptions. The inherent weakness of the transient ν(CN) signals is exacerbated by the fact that the transient signal at ≈ 2100 cm−1 will be partially overlapping with the bleach of the weak, highest-energy ν(CN) vibration in the ground state which will reduce the apparent intensity of the transient peak still further. A consequence of this is that only the most intense ν(CN) transient peak can clearly be seen in the difference spectra.
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Fig. 13 IR difference spectrum of K2[Ru(bpym)(CN4)] in D2O obtained 100 ps after 400 nm excitation into the 1MLCT absorption manifold. Inset are kinetic traces showing decay of the transient (top) and recovery of the bleach (below) from which excited-state lifetimes were derived. |
Analogous behaviour is shown by all of the complexes in D2O, with a shift to higher energy of the most intense ν(CN) vibration of between 46 and 52 cm−1 in the 3MLCT excited state, as shown by a bleach of the ground-state vibrations and appearance of a much weaker transient peak at higher energy (Table 2). These values are the same within the limits of accuracy of the experiment, given a combination of 4 cm−1 resolution for the spectra and the low intensity of the transients. The fast timescale of this technique allowed us to determine accurately the lifetime of Ru-bpym, which could not be determined by luminescence methods as the luminescence was too weak and short-lived. Fig. 13 shows the IR difference spectrum for this complex and the kinetics of transient decays and parent recovery. The rates derived from these data match closely and give τ = 3.4 ns for the 3MLCT excited state in D2O.
Two of the complexes were sufficiently soluble in MeCN to allow study of the effect of changing solvent on their transient IR behaviour. Ru-dmb and Ru-bpym showed similar difference spectra shortly (picoseconds) after excitation to those seen in water, with the exception that the shift to higher energy of the main ν(CN) vibration was less, being 36 cm−1 for Ru-dmb (cf. 46 cm−1 in D2O) and 30 cm−1 for Ru-bpym (cf. 48 cm−1 in D2O). This appears to be principally a consequence of the fact that the most intense ν(CN) vibration in the ground state is at higher energy in MeCN than in water [for Ru-dmb, 2068 vs. 2049 cm−1; for Ru-bpym, 2068 vs. 2057 cm−1], an effect which must arise from differences in hydrogen-bonding with the solvent. By observing transient IR spectra at a range of delays after excitation we measured the lifetime of the 3MLCT excited state of Ru-bpym in MeCN to be 250 ps, an order of magnitude shorter than the value observed in water, in keeping with the general trend expected for complexes of this series.1 Similarly, the 3MLCT lifetime of Ru-PBE plummeted from 471 ns (measured by luminescence) to 4.5 ns (measured from decay of the transients in the excited-state IR spectrum) on changing the solvent from D2O to dmf (Table 1).
CCDC reference numbers 276244–276249.
See http://dx.doi.org/10.1039/b509042c for crystallographic data in CIF or other electronic format.
a The bond lengths and angles around the Ru(II) centre unit are unremarkable [Ru–C, 1.98–2.06 Å; Ru–N, 2.10–2.11 Å]. b Symmetry operations for generating equivalent atoms: A: x, −y + 1, z − 1/2. B: x, −y + 1, z + 1/2. C: x, y, z + 1. | |||
---|---|---|---|
K(2)–O(103) | 2.716(8) | K(3)–N(6C) | 2.775(11) |
K(2)–O(103B) | 2.751 (8) | K(3)–N(6B) | 2.788(13) |
K(2)–N(5A) | 2.791(11) | K(3)–N(3) | 3.017(14) |
K(2)–N(5) | 2.908(13) | K(3)–N(5) | 3.029(12) |
K(2)–N(6B) | 2.983(13) | K(3)–C(13) | 3.077(14) |
K(2)–N(2B) | 3.005(13) | K(3)–C(15) | 3.133(15) |
K(2)–C(16B) | 3.046(16) | K(3)–O(100) | 3.229(7) |
K(2)–C(14B) | 3.073(14) | K(3)–O(101B) | 3.025(9) |
K(2)⋯K(3A) | 4.367(6) | K(3)–O(101) | 2.714(8) |
K(2)⋯K(2A) | 3.968(4) | K(3)⋯K(3A) | 3.923(4) |
a The bond lengths and angles around the Ru(II) centre unit are unremarkable [Ru–C, 1.98–2.05 Å; Ru–N, 2.11 Å]. b Symmetry operations for generating equivalent atoms: A: −x + 1, y − 1/2, −z + 3/2. B: x, −y + 3/2, z + 1/2. C: −x + 1, −y + 2, −z + 2. D: −x + 1, y + 1/2, −z + 3/2. E: −x + 2, −y + 2, −z + 2. | |||
---|---|---|---|
K(2)–O(3) | 2.673(2) | K(2)–O(1E) | 2.914(2) |
K(2)–O(2) | 2.741(2) | K(2)–N(2) | 2.919(2) |
K(2)–N(8C) | 2.776(3) | K(2)–N(3) | 2.941(2) |
K(2)–N(7D) | 2.880(2) | ||
K(1)–O(1) | 3.235(2) | K(1)–C(12) | 3.214(3) |
K(1)–O(3A) | 2.727(2) | K(1)–N(8) | 3.320(3) |
K(1)–O(2A) | 2.910(2) | K(1)–C(9B) | 3.418(3) |
K(1)–C(11B) | 3.232(3) | K(1)–N(5B) | 3.277(2) |
K(1)–N(7B) | 2.938(3) | K(1)–C(11) | 3.284(3) |
K(1)–C(10) | 3.116(3) | K(1)–N(7) | 3.443(3) |
K(1)–N(6) | 3.133(3) | ||
K(2)⋯K(1D) | 4.0473(9) | K(2)⋯K(1C) | 4.2049(9) |
a Symmetry operations for generating equivalent atoms: A: x, y + 1, z. B: −x, y + 1/2, −z + 1/2. C: −x + 1/2, −y + 3/2, −z + 1/2. D: −x, y − 1/2, −z + 1/2. | |||
---|---|---|---|
Ru(1)–C(1) | 1.970(7) | Ru(1)–C(3) | 2.057(7) |
Ru(1)–C(4) | 1.977(6) | Ru(1)–N(11) | 2.105(5) |
Ru(1)–C(2) | 2.045(7) | Ru(1)–N(21) | 2.190(5) |
K(1)–N(1A) | 2.681(6) | K(1)–N(2C) | 2.895(5) |
K(1)–N(4B) | 2.769(5) | K(1)–O(101C) | 3.176(8) |
K(1)–N(2) | 2.827(6) | K(1)–O(101) | 3.395(8) |
K(2)–N(4) | 2.866(6) | K(2)–C(1B) | 3.086(6) |
K(2)–N(31D) | 2.881(6) | K(2)–C(4) | 3.134(6) |
K(2)–N(4B) | 2.908(6) | K(2)–N(2) | 3.144(5) |
K(2)–N(1B) | 2.943(6) | K(2)–C(4B) | 3.168(6) |
K(2)–O(101) | 2.948(7) | K(2)–C(2) | 3.330(6) |
K(1)⋯K(2) | 4.238(2) | K(1)⋯K(2C) | 4.934(2) |
K(1)⋯K(2B) | 4.254(2) | K(1)⋯K(1C) | 3.454(3) |
a Bond distances around K+ are not quoted due to extensive disorder involving both the K+ ions themselves and some of the coordinated water ligands. b Symmetry operations for generating equivalent atoms (A): −x, 1 − y, −z. c 50% site occupancy. | |||
---|---|---|---|
Ru(2)–C(13) | 1.94(2) | Ru(2)–C(7) | 1.964(17) |
Ru(2)–C(10) | 1.93(2) | Ru(2)–N(2) | 2.104(12) |
Ru(2)–C(14) | 1.95(2) | Ru(2)–N(8) | 2.129(14) |
Er(1)–N(4) | 2.365(12) | Er(1)–O(3)c | 2.44(3) |
Er(1)–N(6A) | 2.370(18) | Er(1)–O(7)c | 2.51(3) |
Er(1)–O(1) | 2.374(12) |
a Symmetry operations for generating equivalent atoms: A 1 − x, 1 − y, 1 − z. B 1 − x, 1 − y, −z. C 1 − x, −y, −z. D x, −1 + y, z. | |||
---|---|---|---|
Eu(1)–O(4) | 2.363(4) | Eu(2)–O(10) | 2.351(4) |
Eu(1)–O(1) | 2.382(4) | Eu(2)–O(9) | 2.359(4) |
Eu(1)–O(3) | 2.431(4) | Eu(2)–O(7) | 2.390(4) |
Eu(1)–O(2) | 2.432(4) | Eu(2)–O(6) | 2.473(4) |
Eu(1)–O(5) | 2.448(4) | Eu(2)–O(8) | 2.495(4) |
Eu(1)–N(7) | 2.467(5) | Eu(2)–N(14) | 2.496(4) |
Eu(1)–N(21) | 2.507(4) | Eu(2)–N(20) | 2.498(4) |
Eu(1)–N(13A) | 2.511(4) | Eu(2)–N(27) | 2.499(4) |
Ru(1)–C(11) | 1.966(5) | Ru(3)–C(35) | 1.969(5) |
Ru(1)–C(12) | 1.977(5) | Ru(3)–C(36) | 1.980(5) |
Ru(1)–C(10) | 2.034(5) | Ru(3)–C(34) | 2.036(5) |
Ru(1)–C(9) | 2.035(5) | Ru(3)–C(33) | 2.043(5) |
Ru(1)–N(3) | 2.112(4) | Ru(3)–N(17) | 2.107(4) |
Ru(1)–N(1) | 2.146(4) | Ru(3)–N(15) | 2.150(4) |
Ru(2)–C(23) | 1.973(5) | Ru(4)–C(48) | 1.968(5) |
Ru(2)–C(24) | 1.974(5) | Ru(4)–C(47) | 1.981(5) |
Ru(2)–C(22) | 2.019(5) | Ru(4)–C(45) | 2.022(5) |
Ru(2)–C(21) | 2.038(5) | Ru(4)–C(46) | 2.033(5) |
Ru(2)–N(10) | 2.114(4) | Ru(4)–N(24) | 2.115(4) |
Ru(2)–N(8) | 2.149(4) | Ru(4)–N(22) | 2.145(4) |
K(1)–N(28C) | 2.827(5) | K(2)–N(6) | 2.986(5) |
K(1)–N(4D) | 2.981(5) | K(2)–N(19) | 3.408(5) |
K(1)–N(6D) | 3.107(5) | K(2)–N(21) | 3.371(4) |
K(1)–N(11) | 3.141(5) | K(2)–N(26B) | 3.155(5) |
K(1)–N(13) | 3.025(5) | K(2)–N(27B) | 3.263(5) |
K(1)–C(23) | 3.193(5) | K(2)–C(48B) | 3.318(5) |
K(1)–C(9D) | 3.276(5) | K(2)–C(46B) | 3.322(5) |
K(1)–C(21) | 3.327(5) | K(2)–C(11) | 3.372(5) |
K(1)–C(11D) | 3.329(5) | K(2)–O(12) | 2.808(5) |
K(1)–O(11) | 2.733(5) | K(2)–O(13) | 2.730(5) |
a Symmetry transformations used to generate equivalent atoms: A: 1 − x, 2 − y, 1/2 + z. | |||
---|---|---|---|
Ru(1)–C(11) | 1.968(4) | Ru(1)–C(13) | 2.052(4) |
Ru(1)–C(12) | 1.982(4) | Ru(1)–N(1) | 2.116(3) |
Ru(1)–C(10) | 2.037(4) | Ru(1)–N(3) | 2.117(3) |
Eu(2)–O(2) | 2.375(3) | Eu(2)–O(5) | 2.479(3) |
Eu(2)–O(8) | 2.407(3) | Eu(2)–N(7A) | 2.494(3) |
Eu(2)–O(1) | 2.438(3) | Eu(2)–O(3) | 2.577(3) |
Eu(2)–O(4) | 2.444(2) | Eu(2)–O(6) | 2.774(3) |
Eu(2)–N(6) | 2.449(3) |
Complex | K2[Ru(phen)(CN)4]·4H2O | K2[Ru(bpym)(CN)4]·3H2O | [Ru(κ2-terpy)(CN)4]·MeOH·0.5H2O | KEr[Ru(PBE)(CN)4]2·10H2O | KEu[Ru(pypz)(CN)4]2·10.5H2O | [Eu(NO3)(H2O)5] [Ru(bpym)(CN)4] |
---|---|---|---|---|---|---|
a All measurements made on a Bruker SMART diffractometer using Mo-Kα radiation. b PLATON suggests that C2/c is also possible but this results in a poorer refinement. The Flack parameter of 0.29(8) for this structure reflects the presence of racemic twinning. c R1 value is based on selected data [with I > 2σ(I)]; wR2 value is based on all data. | ||||||
Formula | C16H16K2N6O4Ru | C12H12K2N8O3Ru | C20H16K2N7O1.5Ru | C36H46ErKN15O9Ru2 | C48H70Eu2K2N28O21Ru4 | C12H16EuN9O8Ru |
M | 535.62 | 495.57 | 557.67 | 1241.38 | 2161.72 | 667.37 |
T/K | 150 | 150 | 173 | 173 | 150 | 150 |
Crystal system, space group | Monoclinic, Ccb | Monoclinic, P21/c | Monoclinic, I2/a | Monoclinic, C2/c | Triclinic, P![]() |
Orthorhombic, Pna21 |
a/Å | 10.2914(16) | 6.5658(7) | 17.648(3) | 28.636(6) | 14.0042(16) | 24.791(2) |
b/Å | 30.116(5) | 23.435(2) | 8.9779(16) | 14.487(3) | 16.3639(19) | 6.5425(6) |
c/Å | 6.6628(11) | 11.7021(12) | 29.053(6) | 15.487(3) | 18.520(2) | 12.4486(12) |
α/° | 90 | 90 | 90 | 90 | 113.599(2) | 90 |
β/° | 101.862(3) | 93.836(2) | 105.468(3) | 103.05(3) | 103.578(2) | 90 |
γ/° | 90 | 90 | 90 | 90 | 91.274(2) | 90 |
V/Å3 | 2020.9(6) | 1796.6(3) | 4436.5(14) | 6259(2) | 3748.2(7) | 2019.1(3) |
Z | 4 | 4 | 8 | 4 | 2 | 4 |
D calc/Mg m−3 | 1.760 | 1.832 | 1.670 | 1.317 | 1.915 | 2.195 |
μ/mm−1 | 1.224 | 1.367 | 1.112 | 1.921 | 2.630 | 3.888 |
Reflections collected | 11781 | 19966 | 14915 | 30015 | 42204 | 21936 |
Independent reflections | 4359 (Rint = 0.0847) | 4104 (Rint = 0.0587) | 4950 (Rint = 0.0650) | 5504 (Rint = 0.1580) | 16650 (Rint = 0.0360) | 4622 (Rint = 0.0299) |
Data/restraints/parameters | 4359/63/262 | 4104/0/235 | 4950/0/285 | 5504/0/331 | 16650/0/946 | 4622/1/281 |
Final R1, wR2 indicesc | 0.0548, 0.1191 | 0.0297, 0.0671 | 0.0586, 0.1476 | 0.0915, 0.3235 | 0.0396, 0.0952 | 0.0185, 0.0425 |
Largest diff. peak and hole/ e Å−3 | 1.021, −0.983 | 0.636, −0.617 | 1.167, −0.916 | 1.877, −1.897 | 2.191, −0.967 | 0.808, −0.411 |
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