Christopher J.
Adams
*a,
Stuart L.
James
a,
Xiaoming
Liu
b,
Paul R.
Raithby
*a and
Lesley J.
Yellowlees
*b
aDepartment of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW
bDepartment of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ
First published on 24th December 1999
A new class of platinum–bipyridyl compounds has been synthesized by the dehydrohalogenative reaction of [4,4′-bis(tert-butyl)-2,2′-bipyridyl]platinum dichloride [PtCl2(tBu2bipy)] 1 with terminal alkynes HCCR, in the presence of copper(I) iodide and diisopropylamine. The products [Pt(C
CR)2(tBu2bipy)] (R = C6H4NO2-p2, C6H53, C6H4CH3-p4 or SiMe35), have been characterised by spectroscopic and analytical methods, and a single crystal molecular structure determination has been carried out on 4. Extended Hückel molecular orbital calculations have also been carried out, and the results are used to help rationalise the voltammetric, EPR and spectroelectrochemical properties of the new compounds. These show that compounds 3, 4 and 5 undergo a one-electron bipyridyl based redox process, but that 2 has an unresolved two-electron process located on the nitro groups.
There is also interest in platinum–bipyridyl complexes, either with regard to their solid state structure and how this is related to their luminescence properties,3 the formation of d9 metal centres,4 or their ability to undergo oxidative addition to form platinum(IV) complexes.5 An underlying problem has been the relative insolubility of platinum–bipyridyl complexes in organic solvents, and recent work has started to utilise substituted bipyridines to overcome this problem.6
We report here the synthesis and characterisation of a new class of platinum–bipyridyl compounds, containing a substituted bipyridyl ligand and σ-bonded acetylides in the remaining co-ordination sites. A preliminary communication on related work has appeared,7 and since this work was started a report containing another platinum–acetylide complex with a diimine ligand has been published.8 We know of only four other published platinum–acetylide structures which also contain nitrogen donors, three of which are square planar platinum(II) compounds such as those detailed here,2,9 and one of which is an octahedral platinum(IV) compound.10
From 1, the synthesis of the acetylide complexes was as previously described.7 Stirring overnight with an excess of acetylene in the presence of a catalytic amount of copper(I) iodide and a small amount of diisopropylamine leads to formation of the acetylide complexes [Pt(CCR)2(tBu2bipy)] 2–5 in good yield. Purification is easily effected by alumina column chromatography (for 3–5) and/or recrystallisation, leading to isolation of the products as air stable yellow solids (Table 1).
Compound | R | Colour |
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Yield (%) |
---|---|---|---|---|
2 | C6H4NO2-p | Yellow | 2111, 2123 | 54 |
3 | C6H5 | Yellow | 2115, 2124 | 83 |
4 | C6H4CH3-p | Yellow | 2114, 2126 | 69 |
5 | SiMe3 | Yellow-green | 2040, 2056 | 57 |
The UV/visible spectra of compounds 1–5 in the 250–1000 nm region have been recorded (Table 2). All except 2 show similar spectra consisting of two major bands (ε ≈ 104 dm3 mol−1 cm−1), one at 380 to 400, and one at 280 to 290 nm. These show a solvatochromic shift, moving to shorter wavelengths with increasing solvent polarity, and, in common with Gidney et al.,16 the bands have been assigned to the first and second metal to ligand charge transfer (MLCT) respectively. These correspond to excitation of an electron from the highest occupied molecular orbital (HOMO) (mainly platinum dx2 − y2 in nature) to the lowest and second lowest unoccupied molecular orbitals (LUMO and SLUMO), which are mainly bipyridyl in nature. The spectra of 1 and 5 also show a double transition at 307 and 320 nm that is not solvatochromic; this may be assigned to a bipyridyl π to π* transition. It appears as a shoulder on the second MLCT band of 4.
Solvent | ||||
---|---|---|---|---|
Compound | R | Aceto-nitrile | Dichloro-methane | Ethyl acetate |
2 | C6H4NO2-p | 277 (1.5) | 283 (1.4) | 286 (1.5) |
371 (2.9) | 369 (2.3) | 369 (2.5) | ||
3 | C6H5 | 284 (3.0) | 284 (4.6) | 287 (4.0) |
384 (0.5) | 395 (1.2) | 407 (0.7) | ||
4 | C6H4CH3-p | 282 (5.0) | 286 (7.9) | 287 (4.7) |
388 (0.9) | 399 (1.1) | 412 (0.7) | ||
5 | SiMe3 | 282 (9.1) | 287 (2.4) | 290 (3.5) |
375 (2.6) | 384 (0.7) | 398 (0.9) |
The spectrum of compound 2 is slightly different. The 280 nm MLCT band is still visible, as is some structure due to the bipyridyl π to π* transition at around 320 nm. However, there is now one dominant absorption at around 370 nm that is much less solvatochromic than the first MLCT band of compounds 3–5. The band has an absorption coefficient approximately four times that of the other complexes, and this spectral difference indicates that the origin of this band may be different from that in 3–5.
A single crystal of complex 4·3CHCl3 suitable for an X-ray diffraction study was grown by evaporation of a chloroform–hexane solution. This confirmed the proposed geometry of the compound, revealing it to be essentially planar (apart from the two tert-butyl groups) (Fig. 1). The average length of the Pt–C bond is 1.944 Å, a little shorter than is generally found in bis(acetylide) complexes of platinum with phosphine ligands17 but in good agreement with the other published bis(acetylide) structure containing a diimine ligand.8 Conversely, the average carbon–carbon distance in the triple bond of 1.23(2) Å is longer than is generally seen. The two acetylide groups bend away from each other, with the deviation from linearity at the acetylenic carbons averaging around 13°. Selected bond lengths and angles are presented in Table 3. There are hydrogen-bonding interactions between the chloroform solvate molecules, but the only such interaction involving the molecules of 4 is shown in Fig. 2. This involves the platinum atom Pt(1) and one of the hydrogen atoms H(27b) of the tolyl group of a neighbouring molecule (related by the symmetry operation x, y, z + 1), at a distance of 2.8 Å.18 The effect is to link the molecules of 4 into chains that run through the crystal lattice in the c direction.
Pt(1)–C(28) | 1.940(19) | Pt(1)–C(19) | 1.947(17) |
Pt(1)–N(2) | 2.040(5) | Pt(1)–N(1) | 2.065(14) |
N(1)–C(5) | 1.35(2) | N(2)–C(6) | 1.34(2) |
C(5)–C(6) | 1.44(3) | C(19)–C(20) | 1.25(2) |
C(20)–C(21) | 1.38(2) | C(28)–C(29) | 1.20(2) |
C(29)–C(30) | 1.45(3) | ||
C(28)–Pt(1)–C(19) | 91.7(7) | C(28)–Pt(1)–N(2) | 94.7(7) |
C(19)–Pt(1)–N(2) | 173.6(6) | C(28)–Pt(1)–N(1) | 173.1(7) |
C(19)–Pt(1)–N(1) | 95.0(6) | N(2)–Pt(1)–N(1) | 78.6(6) |
C(6)–N(2)–Pt(1) | 114.7(13) | C(20)–C(19)–Pt(1) | 172.1(15) |
N(2)–C(6)–C(5) | 117.1(17) | N(1)–C(5)–C(6) | 113.6(16) |
C(19)–C(20)–C(21) | 175.0(18) | C(28)–C(29)–C(30) | 173(2) |
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Fig. 1 The crystal structure of compound 4. Three molecules of CHCl3 have been omitted for clarity. |
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Fig. 2 Part of the crystal structure of compound 4, showing the intermolecular interaction between H(27b) of one molecule and the platinum atom of a neighbouring molecule. |
In order to rationalise the differences in the spectra of compounds 2 and 3–5, some simple MO calculations based on the structures of 2 and 4 have been performed with the CACAO software package.19 The results show that for 4 the HOMO is largely metal based, with in-phase overlap of the dx2 − y2 orbital with the in-plane π-bonding combinations of the two acetylide triple bonds. (The compound is lying in the xy plane with the acetylide ligands pointing between these axes, in common with the axis system used in ref. 4.) The LUMO is approximately 91% bipyridyl based, in agreement with previous calculations.4 In 2, however, there are two degenerate LUMOs, each being based largely on the pz orbitals of one of the two nitro groups. The third lowest unoccupied MO is the first to be bipyridine based, but is only approximately 72% bipyridyl in nature and contains a large contribution from the nitro groups of the acetylide ligands.
In situ reduction of compound 3 to the radical monoanion [Pt(CCC6H5)2(tBu2bipy)]−3−− at −1.5 V at 240 K is shown in Fig. 3. The spectrum of the electrogenerated species shows a marked resemblance to that previously reported for [Pt(bipy)L2]−,4 and so the bands centred at 769, 500 and 370 nm are assigned to π → π* transitions of the co-ordinated bipyridyl anion, bipy−, and the band at 425 nm is assigned to a charge transfer transition. It may be concluded therefore that the one-electron reduction product 3−− should be formulated as [PtII(C
CC6H5)2(tBu2bipy−)]. Note that there are no isosbestic points in the spectra of 3/3−− because the spectrum of the latter has a greater absorption coefficient at every wavenumber than that of the former. However, the spectrum of the parent species is completely regenerated if the electrogeneration potential is reset to 0 V after conversion into the anion is complete. The spectra obtained by reducing 2 under the same conditions are quite different from those obtained on reducing 3. The two absorptions at around 320 nm that are assigned as intraligand bipy π → π* transitions remain relatively unaffected, but the more intense band at 370 nm undergoes a considerable change (Fig. 4). None of the bands associated with the reduced bipyridyl ligand that are seen in Fig. 3 are generated, and so it may be inferred that the co-ordinated bipyridyl ligand is not the site of redox activity in this compound, and that the 370 nm absorption is not the same MLCT band seen for compounds 1 and 3–5.
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Fig. 3 The spectral changes seen during the reduction of compound 3 to 3−−. |
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Fig. 4 The spectral changes seen during the reduction of compound 2 to 2−−. |
The in situ generated solution EPR spectrum of compound 3−− is shown in Fig. 5, along with an excellent simulation. In the spectrum of the monoreduced anion hyperfine coupling of the unpaired electron to the platinum nucleus and superhyperfine coupling to the nuclei is observed, which may be modelled using the data given in Table 4. The spectrum shows a great similarity to the previously reported solution EPR spectrum of [Pt(bipy)(CN)2]−,4 and the coupling constants (also given in Table 4) are in remarkable agreement. Thus, the conclusions reached for [Pt(bipy)(CN)2]− must also apply in this system; that is, that the unpaired electron in 3−− resides on the co-ordinated tBu2bipy, in agreement with the predictions of the EHMO calculations and the UV/visible results given above. The simpler EPR spectrum for 3−− as opposed to the cyanide complex confirms that the protons in the 4 and 4′ positions in bipy couple to the unpaired electron in [Pt(bipy)(CN)2]−, since by substituting those positions with tert-butyl groups one set of superhyperfine coupling is removed. We therefore assign the 14N couplings in 3−− to the bipyridyl nitrogens, and the 1H couplings to one of the bipyridyl hydrogen atoms, though it is not possible to decide which. In addition, the similar EPR behaviours of 3−− and [Pt(bipy)(CN)2]− indicate that the phenylacetylide ligands are electronically analogous to cyanide ligands.
195Pt coupling | 14N coupling | 1H coupling | |||||
---|---|---|---|---|---|---|---|
Compound | No. | A iso/G | No. | A iso/G | No. | A iso/G | ΔHpp |
22−2− | 1 | 15.8 | 1 | 10.1 | 2 | 3.37 | 3.00 |
3−− | 1 | 21.0 | 2 | 3.37 | 2 | 2.85 | 3.2 |
[Pt(bipy)(CN)2]−![]() |
1 | 20.5 | 2 | 3.4 | 2 | 2.8 |
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Fig. 5 The observed and simulated EPR spectra of compound 3−−. |
The solution EPR spectrum of compound 22−2− and an excellent simulation of the same are shown in Fig. 6; the simulation parameters are also included in Table 4. The EPR spectra of 22−2− and 3−− are obviously very different, and hence require very different coupling constants to model them. The spectrum of 22−2− has significantly smaller coupling to the platinum nucleus and a very much larger coupling to 14N. Furthermore note that now the unpaired electron couples to only one 14N nucleus, whereas in 3−− it couples to two such nuclei. Thus we suggest that the site of redox activity in 2 is the nitrophenyl ligand, and more specifically the nitro group which carries much of the electron density when the compound is in the reduced state. It therefore has a degenerate pair of LUMOs based on the nitro groups which are orthogonal to each other and spatially well separated, meaning that the two electrons are not interacting with each other and the direduced complex can satisfactorily be modelled using an S = 1/2 system. The 1H coupling is presumably to two equivalent protons on the phenyl ring.
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Fig. 6 The observed and simulated EPR spectra of compound 22−2−. |
These findings indicate that the transition forming the 370 nm absorption band of compound 2 is based at least partly upon the nitro groups. The almost non-existent solvatochromism would seem to rule out this transition being charge transfer in nature, and therefore this band may tentatively be assigned as an intra-ligand feature.
Compound 3 was a yellow solid obtained using phenylacetylene instead of TMSA, followed by recrystallisation from CH2Cl2–hexane. Yield: 0.156 g (0.234 mmol, 83%) (C34H34N2Pt.CH2Cl2 requires C, 56.06; H, 4.84; N, 3.74%; M 666. Found: C, 56.17; H, 4.80; N, 3.40%; M+, m/z 666.2). 1H NMR (CDCl3): δ 1.43 [s, 18 H, tBu], 7.1–7.3 [m, 6 H, phenyl CH], 7.5–7.6 [m, 6 H, phenyl CH and H5], 7.93 [d, 2 H, 4J(H3H5) = 1.8, H3] and 9.70 [d, 2 H, 3J(H5H6) = 5.3 Hz, H6].
Compound 4 was a yellow solid obtained using p-tolylacetylene instead of TMSA, followed by recrystallisation from CH2Cl2–hexane. Yield: 0.255 g (0.37 mmol, 69%) (C36H38N2Pt requires C, 62.30; H, 5.52; N, 4.04%; M 693. Found: C, 62.04; H, 5.57; N, 4.03%; M+, m/z 693.3). 1H NMR (CDCl3): δ 1.43 [s, 18 H, tBu], 2.31 [s, 6 H, tolyl CH3], 7.04 [d, 4 H, 3J(HH) = 8, tolyl CH], 7.42 [d, 4 H, tolyl CH], 7.54 [dd, 2 H, 3J(H5H6) = 5.9, 4J(H3H5) = 1.8 Hz, H5], 7.92 [d, 2 H, H3] and 9.72 [d, 2 H, H6].
CCDC reference number 186/1736.
See http://www.rsc.org/suppdata/dt/a9/a907028a/ for crystallographic files in .cif format.
This journal is © The Royal Society of Chemistry 2000 |