Los medios de comunicación como generadores de cinismo político.

2}(CO)(PMe2Ph)(PPh3)] and

[Ru{3_{-B,S,S’-BH(mt)}

2}(CO)(PMe2Ph)2]

Ligand exchange investigations were continued with the range of electronically and sterically variant phosphines highlighted (purple) in Table 3.1. The reaction with an excess of PMe3 proceeded cleanly within one hour to give the mono-substitution product [Ru{3_{-B,S,S’-BH(mt)2}(CO)(PMe3)(PPh3)], the nature of which was inferred from} NMR spectroscopic data. The 31_P{1_{H} NMR spectrum revealed the downfield shift of} resonances for both phosphine environments relative to 3.1,where the broad peak at P = ‒29.6 suggests the triphenylphosphine ligand was located trans to the boron, with the doublet at P = 58.7 (1_{JPC = 11.3 Hz) corresponding to coordinated PMe3.}

Integration of the 31_P{1_{H} NMR resonances was indicative of the liberation of one} equivalent of triphenylphosphine. Despite the clean reaction observed by NMR spectroscopy, the inferred product [Ru{3_{-B,S,S’-BH(mt)2}(CO)(PMe3)(PPh3)] eluded} isolation, which could be owing to the lability and/or volatility of PMe3. Therefore, investigations were continued with the less volatile PMe2Ph, which has electronic and steric similarities to PMe3.

Similar to the reactivity observed with PMe3, complex 3.1 was treated with an excess of PMe2Ph at room temperature to afford the mono-substituted product, [Ru{3_-B,S,S’- BH(mt)2}(CO)(PMe2Ph)(PPh3)] 3.4 (Scheme 3.14). Interestingly, substitution of the second phosphine was not observed spectroscopically under these reaction conditions, despite the presence of excess PMe2Ph. The spectral data resemble those of [Ru{3_{-B,S,S’-BH(mt)2}(CO)(PMe3)(PPh3)]. The broad resonance at} _{P = ‒17.2 was} attributed to the trans (RuB) PPh3 ligand, while the relatively sharp doublet at P = 56.9 (1_{JPC = 11.3 Hz) was assigned as the smaller PMe2Ph ligand occupying the position trans} to sulfur. The use of PMe2Ph over the more symmetrical PMe3 is advantageous in that the two methyl groups may provide an indication of the local symmetry of the complex. The asymmetric nature of 3.4 was noted by 1_{H NMR spectroscopy with the} diastereotopic methyl groups from PMe2Ph resonating as chemically inequivalent doublets at H = 1.28 (2_{JHP = 5.5 Hz) and 1.43 (}2_{JHP = 5.3); whilst the remaining} 1_{H signals} are similar to that of 3.1. Although crystals of 3.4 suitable for X-ray diffraction studies were not obtained, the formulation of 3.4 was further supported by mass spectrometry and elemental analysis.

Following the overnight acquisition of a pure sample of 3.4, the 13_C{1_{H} NMR spectrum} revealed several (≈6) resonances in the methyl region where the methimazolyl unit is typically located (C = 33.5–34.2). The corresponding resonances in the 1_{H NMR} spectrum were distinct and the three products 3.4, isomer 3.4a and the product of double substitution [Ru{3_{-B,S,S’-BH(mt)2}(CO)(PMe2Ph)2] 3.5 were formulated} (Scheme 3.14). The decomposition of 3.4 occurred within 10 minutes of solvation in CDCl3 and stabilised to a relative ratio of 2:3:5 for 3.4, 3.5, 3.4a at room temperature. The formation of these products was initially suspected to be due to the residual acidity of CDCl3. However, a similar ratio of products formed when the less acidic CD2Cl2 was used as the characterisation solvent.

Scheme 3.14: Temperature dependent substitution of triphenylphosphine in 3.1. To encourage conversion of 3.4 to complexes 3.5 and 3.4a that were previously observed in CDCl3, an aliquot of the crude reaction mixture of 3.4 in THF was briefly heated to reflux. The NMR spectra showed the clean partial conversion of 3.4 to complex 3.5, with no evidence for the other product (3.4a) or the Cs-symmetric isomer of 3.5. Full conversion of 3.4 to 3.5 could be achieved by performing the same reaction under THF reflux for 18 hours (Scheme 3.14). The formation of 3.5 over this time period was accompanied by an increased complexity of the methyl region in the 1_{H NMR spectrum} (H = 1.27–1.48). This is due to the four chemically inequivalent methyl environments of the two PMe2Ph ligands, which resonate as doublets within this region, whereas two doublets would be expected for Cs-3.5. Four distinct methyl resonances from PMe2Ph were also present in the 13_C{1_{H} NMR spectrum. Two of the PMe2Ph methyl groups} couple to the chemically inequivalent phosphines in a doublet of doublet multiplicity at

C = 15.6 (1_{JCP = 31.2,} 3_{JCP = 3.3 Hz) and 18.6 (}1_{JCP = 32.3,} 3_{JCP = 4.2 Hz), while the other} two environments appear only as doublets at C = 16.3 (1_{JCP = 13.6 Hz) and 19.4 (}1_{JCP =} 16.2 Hz). This effect may be attributed to the relative angle of the methyl carbon to the other phosphine atom (Karplus-type relationship, Figure 3.9b). Although rotation about the Ru–P bond in solution is expected, the steric properties of the phenyl moiety could dictate conformational preferences (Figure 3.9c). Coordination of two PMe2Ph ligands was further ascertained by the respective sharp doublet and broad singlet phosphine environments at P = 14.5 (2_{JPC = 12.3 Hz) and –16.4 in the} 31_P{1_{H} NMR spectrum.}

Figure 3.9: a) Molecular structure of [Ru{3_{-B,S,S’-BH(mt)}

2}(CO)(PMe2Ph)2] 3.5 (organic hydrogen atoms omitted, phenyl groups simplified, displacement ellipsoids shown at

50% probability). Selected bond lengths (Å) and angles (°): B1–Ru1 2.253(4), B1–H1 1.09(6), Ru1–P1 2.4095(9), Ru1–P2 2.2876(9), Ru1–C1 1.821(4), H1–B1–Ru1 120(3), B1–Ru1–P1 171.51(11), B1–Ru1–P2 88.00(11), B1–Ru1–C1 88.78(16), P1–Ru1–P2

99.57(3). b) PMe2Ph groups emphasisedand c)space-filling diagram.

The formulation of the isomerised complex 3.4a was further confirmed through a process of elimination in the 1_{H and} 31_P{1_{H} NMR spectra of resonances corresponding} to 3.4 and 3.5 (Figure 3.10). The methyl environments of 3.4a in Figure 3.10 consist of two distinct doublets for the PMe2Ph ligand (H = 1.23, 1.45, 2_{JHP = 8.5 Hz), whereas the} downfield resonances (H = 3.19, 3.24) correspond to the chemically inequivalent N-methyl groups of the methimazolyl backbone. The olefinic signals of all three products reside in a similar chemical shift range of 6.40–6.58 ppm, which occluded the unambiguous assignment of the resonances that result from 3.4a. The two dominant

resonances in the 31_P{1_{H} NMR spectrum further support the assignment of 3.4a.} Consistent with the ruthenaboratrane complexes discussed thus far in this chapter, the two phosphine environments resolved as a broadened signal at P = 22.0 and a sharp doublet at P = 12.9 (2_{JPP = 11.4 Hz), which were postulated as PMe2Ph and PPh3} respectively.

Figure 3.10: Comparison of the methyl regions in the 1_{H NMR spectra of 3.5, 3.4 and} the decomposition spectrum of 3.4 in CDCl3 to identify isomer 3.4a.