SYNTHESIS tetraarylborates

Barlow et al. reported the synthesis of cationic (T) -cyclopentadienyl)(r -fulvene)ruthenium(ll) salts from ruthenocenylmethanol (57) either by treatment with trimethylsilyl triflate to achieve 58 in 79% yield. Alternatively, the reaction of 57 with tetrafluoroboric acid or with hexafluorophosphoric acid gave the tetrafluoroborate or hexafluorophosphate salts in 59 and 60 in 92% and 52% yield, respectively. Treatment of 59 with sodium tetrakis[3,5-bis(trifluoromethyl)]borate afforded the tetraarylborate salt 61 in 69% yield. 

It was already pointed out in the introductory remarks that non-nucleophilic reaction conditions are one prerequisite for the synthesis of silylium ions in the condensed phase. The use of weakly coordinating anions is particularly decisive. Halogenated closo-carborate monoanions and - more recently - closo-borate dianions have found the most widespread use besides fluorinated tetraarylborates. While with bulky substituents these anions form silylium ions salts with well-separated cations and borate anions [10, 20, 22, 42], silylium ions with smaller alkyl substituents tend to coordinate directly to the anion forming zwitterionic compounds [7, 31, 32, 47]. For example, upon reaction of trimethylsUane with trityl-cZoso-carborate 86 in excess trimethylsUane the trimethylsUylium carborate 87 is formed (Scheme 27) [44]. 

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