Hexamethylbenzene ruthenium Complexes

Mesitylene and hexamethylbenzene cyclohexadienyl ruthenium complexes 236 have also been prepared in good yield by reduction of their corresponding bisarene ruthenium dications 235 with sodium borohydride in water (142,143). Cyclohexadienyl derivative 236c can be easily formed by treatment of bisarene ruthenium(O) complex 197 (arene = hexamethylbenzene) with a solution of hydrochloric acid in acetone. The structure of 236c and the presence of the endo C—H bond have been clearly resolved by infrared and 1H-NMR spectra (144,145) [Eq. (28)]. 

Another type of diene ruthenium(O) has been obtained by deprotonation of two hexamethylbenzene ruthenium(II) complexes. Complex 235c in... 

The crystal structure of (ij4-cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand60. For this complex and similar iron-, ruthenium- and osmium-(ij4-cyclooctatetraene)(arene) complexes, their XH and 13C NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be <6.6 kcal mol 1. 

Consequently, organometallic ruthenium(II) and osmium(II) arene complexes have recently attracted interest as anticancer agents [51]. The presence of a 7i-bonded arene in Ru11 (and Os11) complexes can have a dramatic influence on their chemical reactivity. There is a delicate balance between electron donation from the arene into the empty Ru 4d orbitals and back-donation from the filled 4d6 orbitals into vacant arene orbitals. This is influenced by the donor-acceptor power of the arene (e.g. hexamethylbenzene as a strong donor, in contrast to biphenyl which may act as acceptor) and by the other ligands on Ru11 which can influence the... 

The bisacetato ruthenium complex 28, on heating in 2-propanol, leads to the bridged hydrido dinuclear complexes 73 and 74. The bistrifluoroacetato complex 28 also leads to complex 73. The Tj2-acetato complex 39 was transformed in hot 2-propanol to another bridged hydrido derivative (75, arene = durene, mesitylene, p-cymene, hexamethylbenzene 60-70%). The introduction of alkyl substituents on the benzene ring is reflected by a shift of the p FI resonance toward high field (14,53). 

The cationic hexamethylbenzene ruthenium complex 254 can be depro-tonated by treatment with t-BuOK to give the ruthenium complexes 255 and 256 in which the r 4-o-xylylene ligand is coordinated via its exocyclic diene (Scheme 23). Protonation of 255 gives the dication 15c, whereas protonation of 256 produces the fluxional tj3-benzyl derivative 257 which is stabilized by an agostic C—H bond (156,157). 

Complexation of both benzene rings of 192 or 8 give the multidecker sandwich complexes 193 and 195 respectively [174]. Cyclic voltammetry of 193 clearly demonstrates two distinct 1-electron reduction steps [90]. On the basis of its rigid structure [175] //" -coordination is less easily adopted than in hexamethylbenzene, even though its redox potential is lower [89]. This implies that the phane geometry is widely preserved, and so 194 probably represents the first example of a 20-electron complex of ruthenium (Fig. 49) [90]. 

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