Anaïs
Pitto-Barry
,
Amy
South
,
Alison
Rodger
and
Nicolas P. E.
Barry
*
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. E-mail: N.Barry@warwick.ac.uk
First published on 24th December 2015
The functionalisation of the 16-electron complex [Os(η6-p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)] (1) with a series of Lewis bases to give the 18-electron complexes of general formula [Os(η6-p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)(L)] (L = pyridine (2), 4-dimethylaminopyridine (3), 4-cyanopyridine (4), 4-methoxypyridine (5), pyrazine (6), pyridazine (7), 4,4′-bipyridine (8) and triphenylphosphine (9)) is reported. All 18-electron complexes are in equilibrium in solution with the 16-electron precursor, and thermochromic properties are observed in some cases (2, 3, 5, 8, and 9). The binding constants and Gibbs free energies of the equilibria are determined using UV-visible titrations and their stabilities investigated. Synthetic routes for forcing the formation of the 18-electron species are proposed, and analytical methods to characterise the equilibria are described.
Fig. 1 Aromatic region of the 1H NMR spectra of 5 mM solutions of complexes 1–9 in CDCl3, 298 K, and assigned 1H NMR spectrum of complex 2. |
Variable temperature 1H NMR spectra were measured from 228 K to 298 K in CDCl3 for complex 3 (Fig. 2). The peaks for the p-cymene ligand become sharper as the temperature decreases, whilst the dimethylaminopyridine (DMAP) resonances are upfield shifted upon coordination compared to the free ligand, and become sharper and more defined as the temperature decreases. The peaks shift even further upfield as the temperature decreases, indicating that there is more DMAP coordinating to the metal centre at lower temperatures and that the equilibrium shifts towards the 18-e complex as the temperature decreases.
UV-visible absorption spectra were measured for complexes 2–9 in dichloromethane solutions (10−4 M) at 298 K (Fig. 3; Fig. S1† for the spectrum of complex 1). All spectra present two bands, one at 520 nm and a second one of lower intensity at 460 nm. Based on calculations reported on Ru analogues,4 we hypothesise that these bands are due to a mixture of ligand-to-metal charge-transfer (LMCT) from sulfur σ and π orbitals to osmium, plus d–d transitions, plus metal-to-ligand charge-transfer (MLCT) from Os–S π orbitals to Os-p-cymene δ* molecular orbitals. Calculations will be performed in future work to fully assign these various transitions.
The effect of temperature on the equilibrium between the 16- (1) and 18-e complexes with nitrogen donors (2–8) was investigated by variable temperature UV-visible spectroscopy from 263 to 293 K in dichloromethane. The UV-visible spectra of a solution of complex 2 in a dichloromethane solution (10−4 M) at four different temperatures (263 K to 293 K; Fig. 4) clearly highlight the decreases of absorption at 520 nm with temperature decrease. The ratio of intensities between the two bands of the spectra (I460/I520) evolves from 0.83 at 293 K to 1.33 at 263 K, following the equilibrium shifts towards the 18-e complex as the temperature decreases. The intensity of the absorption band at 360 nm also significantly increases as temperature increases. These changes in the absorption spectra of the solution are reversible and result in thermochromic properties as illustrated in Fig. 5 (colours of the solutions of complex 8 at four different temperatures).
Fig. 4 UV-visible absorbance spectra of a solution of complex 2 in dichloromethane (10−4 M) at different temperatures. |
Fig. 5 Images showing colour of solution of complex 8 in dichloromethane (10−3 M) at 258 K (a), 268 K (b), 278 K (c) and 288 K (d). |
The colours of complexes 1–9 in dichloromethane solutions (10−3 M) at 298 K, and whether they exhibit thermochromic properties are shown in Table 1. The thermochromic properties of complexes 2, 3, 5, and 9 are illustrated in Fig. S2–S5.†
Colour in solution 298 K, CH2Cl2 | Thermo-chromism | K 103 M−1 | ΔG° kcal mol−1 | pKa ligand30 | |
---|---|---|---|---|---|
2 | Red | Yes | 6.6 ± 1.5 | −5.2 ± 0.2 | 5.2 |
3 | Orange | Yes | 2.9 ± 1.1 | −4.7 ± 0.2 | 9.2 |
4 | Purple | No | 38.4 ± 5.6 | −6.2 ± 0.1 | 1.9 |
5 | Orange | Yes | 1.1 ± 0.5 | −4.1 ± 0.4 | 6.6 |
6 | Purple | No | 17.7 ± 2.8 | −5.8 ± 0.1 | 0.6 |
7 | Purple | No | 24.8 ± 3.9 | −6.0 ± 0.1 | 2.3 |
8 | Red | Yes | 4.3 ± 2.0 | −5.0 ± 0.3 | 4.8 |
9 | Orange | Yes | 51.2 ± 7.0 | −6.4 ± 0.1 | 2.7 |
The stoichiometry of the equilibrium in dichloromethane was determined by UV-visible absorption spectroscopy, with the method of continuous variations (Job's plot). The value of X(A − A0) was plotted against X where X is the mole fraction of complex 1, A is the absorbance and A0 is the absorbance when X = 0. All of the Job's plots had a maximum at X = 0.5 demonstrating that the stoichiometry is 1:1 mol equiv. for 1:ligand. Unexpectedly the 1:1 stoichiometry even holds true for the bidentate ligands 4,4′-bipyridine and pyrazine (Fig. S6†). An attempt was made to synthesise the 2:1 complex with 4,4′-bipyridine by changing the ratio of 1:4,4′-bipyridine in the reaction mixture to 2:1 instead of 1:1, but this was unsuccessful.
UV-visible titrations in dichloromethane were then carried out to determine the relative binding strengths of the 16-e complex and the ligands for complexes 2–9. For each titration a solution of the ligand was gradually added (0 to 20 mol equiv.) to a solution of complex 1 with a constant concentration (10−4 M). In order to minimise the dilution of the solution of complex 1, the ligand solution was prepared by adding 20 mol equiv. of the ligand to a 10−4 M solution of complex 1. The titration of complex 1 by 4,4′-bipyridine in dichloromethane, forming 8, is shown in Fig. 6. The UV-visible absorption spectra were characterised by two main bands at 520 nm and 360 nm as noted above. The band at 520 nm decreased in intensity upon addition of the pyridine whereas the band at 360 nm increased in intensity upon addition of the ligand.
Fig. 6 UV-visible titration of complex 1 by 4,4′-bipyridine (0–20 mol equiv.) in dichloromethane (10−4 M) at 298 K. |
From the UV-visible titrations, the binding constants K between complex 1 and the ligands were calculated using the non-linear ThordarsonFittingProgram (Table 1).25 All the titrations were repeated three times and the standard deviation for the calculated values of K are given in Table 1. The magnitude of the binding constants (103–104 M−1) is low as compared to the usually observed complexation constants in coordination chemistry (≫106 M−1),26 and is in the range of binding constants observed in host–guest inorganic chemistry (e.g. via non-covalent interactions between a metalla-cage and an aromatic planar guest molecule27–29), which is also consistent with the presence of equilibria. The experimental Gibbs free energy (ΔG°) was obtained from the Gibbs equation using the calculated value of K. The calculated values of the binding constants are reported in Table 1, along with the pKa (acid dissociation constant) of the corresponding ligand.
The choice of solvent can have a significant effect on the equilibrium between two species and on the reactions of the complexes in solution. For example the alkyne–azide cycloaddition using [RuCp*(PiPr3)Cl] has a higher conversion in dichloromethane than in tetrahydrofuran.2 The effect of the solvent on the stability of complexes 2–8 (pyridine derivatives) was investigated. We found that reversible thermochromism also occurs when tetrahydrofuran is used as the solvent for the same complexes as in dichloromethane (Table S1† and Fig. 7). The colours are very similar to the dichloromethane solution (with very similar UV-vis absorption spectra, see Fig. S7†), suggesting a very weak or non-existent solvatochromism.
Fig. 7 Images showing the colour of complex 2 in tetrahydrofuran (10−3 M) at 263 K (a), 268 K (b), 273 K (c) and 278 K (d). |
As in dichloromethane, the stoichiometry of the complexes in tetrahydrofuran solutions was determined by the method of continuous variations for complexes 2–8 so that the effect of the solvent on the binding constants could be determined. The titrations were carried out using the same procedure as above (Fig. S7† for the UV-visible titration of complex 1 by 4-dimethylaminopyridine in tetrahydrofuran). The binding constants for complex 1 with the ligands were then estimated from these titrations and are shown in Table S1.† We were particularly interested in complexes 6 and 8 which both contain potentially bidentate ligands, in order to investigate whether the unexpected 1:1 stoichiometry observed in dichloromethane holds true in tetrahydrofuran. Both Job's plots confirms that the choice of solvent makes no difference to the 1:1 stoichiometry. UV-visible titrations were also carried out in tetrahydrofuran.
The formation of the mononuclear Os organometallic complexes 6 and 8 by addition of bidentate pyridinic ligands (pyrazine and bipyridine) is another surprising result, since the two binding sites for these ligands are expected to be identical, and therefore the formation of a dinuclear complex was expected to take place. More surprisingly, the 2:1 complexes had been previously reported with the Ru analogue of the Os 16-electron complex 1 (with pyrazine and bipyridine acting as bridging ligands between two Ru metal centres6). Hence, it seems clear that possible electrostatic repulsion between the two carborane ligands can be ruled out. Ruthenium and osmium possess similar atomic radii (178, and 185 pm, respectively – the lanthanide contraction),30,31 so the difference of reactivity between the Ru/Os analogues does not seem to arise from steric constraints. Relativistic effects (stronger with Os than with Ru – Os being heavier) are of importance in metal–metal bonds, but are less significant for metal–nitrogen bonds; furthermore, if existing, they should favour the formation of an Os-bridging ligand–Os complex analogous to a Ru-bridging ligand–Ru complex. We hypothesise that differences in electron distribution in the complexes [Os(η6-p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)(pyrazine/bipyridine)] and [Ru(η6-p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)(pyrazine/bipyridine)] possibly lead to a difference in acidity for the second nitrogen site of the bidentate ligand and might explain the different stoichiometries observed with both metals. Density functional theory calculations will be performed on both systems in future work to determine the pKa of the second nitrogen atom after coordination of the metal centre to the first nitrogen site of the bidentate ligands in order to test this hypothesis.
Interestingly, the Gibbs free energies of the complexes in tetrahydrofuran are of the same magnitude as in dichloromethane. This confirms that the solvent seems to have very little effect on the strength of the binding in the complexes and the stability of the complexes, and that only weak interaction (as compared to N-donor and P-donor ligands) between metal centre and solvent take place with solvents we had chosen assuming they were non-coordinative solvents (as compared to dimethylsulfoxide or acetonitrile for example which do coordinate the metal centre to form the 18-e solvate-adduct). We also plotted the Gibbs free energy against the pKa of the ligand for complexes 2–8 in tetrahydrofuran (Fig. S8†), and obtained a similar reasonably linear trend as observed in dichloromethane, demonstrating that the Mn+/H+ analogy offers a similar rationalisation in both solvents.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5dt04398k |
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