ETA(3) AND ETA(6) BRIDGING CATIONS IN THE ,N',N'',N''-PENTAMETHYLDIETHYLENETRIAMINE-SOLVATED COMPLEXES OF BENZYLPOTASSIUM AND BENZYLRUBIDIUM - AN X-RAY, NMR, AND MO STUDY

The metalation of toluene (PhCH3) with a 1:1 mixture of n-BuLi/MO(t)Bu (M = K,Rb) at ambient temperature affords orange-red powders of benzy lpotassium or benzylrubidium. On addition of N,N,N,N'',N''-pentamethyl diethylenetriamine (PMDTA) the precipitates dissolve to give burgundy- red solutions, from which needles of [PhCH2K.PMDTA.0.5PhCH3]n (1) or [ PhCH2Rb.PMDTA]n (2) are obtained. X-ray analysis reveals that 1 incorp orates half a molecule of toluene per asymmetric unit. In both 1 and 2 , the eta3 and eta6 interaction of one M+ cation with two benzyl fragm ents gives rise to the formation of polymeric chains. The coordination sphere of the cations is completed by one chelating triamine ligand. MO calculations predict multihapto metal carbon interactions to become more important from Li to Cs and substantiate the solid-state results . Ab initio calculations on three different polymers suggest that the array present in the crystal structure is the most stable. The gated d ecoupled C-13 NMR spectrum of 1 in THF-d8 solution reveals a 1J(C-13,H -1) coupling constant for the benzylic carbon atom which may be interp reted in terms of a mainly planar configuration of the CH2- group in s olution.