CATION-INDUCED STRUCTURAL ALTERATIONS IN THE ORGANO ALKALI-METAL DERIVATIVES OF TRIPHENYLMETHANE - COMBINED X-RAY AND NMR-STUDY OF THE K-CS SALTS

The set of alkali metal solid-state structures of Ph3CM.nL [M = K, Rb, Cs; L = (ligands) PMDTA (N,N,N',N'',N''-pentamethyldiethylenetriamine), THF (tetrahydrofuran)] provides instructive comparisons. Ph3CK-THF.PMDTA (1) crystallizes as a monomeric contact ion pair: the K+ cation is symmetrically eta6-coordinated to one of the phenyl rings, but not to the deprotonated central carbon. Both [Ph3CRb-PMDTA], (2) and [Ph3CCs-PMDTA]n (3) form one-dimensional polymers and eschew THF. The Rb cations in 2 bridge the triphenylmethyl moieties by eta6-coordination to separate phenyl rings. This gives rise to a zigzag chain. In 3, each Cs cation also bridges two carbanions, but in a somewhat different fashion. While Cs+ is located rather symmetrically (eta6) above the phenyl ring of one trityl moiety, a ''propeller''-like coordination to a second trityl anion involves contacts with all three phenyl rings as well as with the deprotonated central carbon. The structure of 3 in THF-d8 solution was elucidated by Cs-133, H-1 HOESY spectroscopy, which reveals agostic interactions between cesium and hydrogen. This provides strong evidence that 3 is a contact ion pair in this solvent with the cation located above the central carbon. The recently proposed MNDO K parameters err in predicting Ph3CK.nL (1) to be ''oversolvated''; K-C distances and nonbonded repulsion energies are overestimated.