CHARGE DISPERSAL IN IMINIUM-SUBSTITUTED ALKYNES - SYNTHESIS, SPECTRALCHARACTERIZATION, AND CRYSTAL-STRUCTURE DETERMINATION OF A CO-2(CO)(6)-COMPLEXED PROPYNE IMINIUM SALT AND STRUCTURAL COMPARISON WITH ITS UNCOMPLEXED COUNTERPART

In order to elucidate the charge dispersal from an iminium group onto an attached C=C bond, the crystal structures of a propyne iminium salt and its Co-2(CO)(6)-complexed analogue have been determined. For 1-(1 -methyl-2-butyn-1-ylidene)pyrrolidinium triflate (3), the bond lengths of the C=C-C=N+ unit agree well with those obtained earlier by ab ini tio calculations; they confirm the nature of these cations as alkynyl- substituted iminium salts, with no significant resonance contribution from a beta-aminoallenyl structure. The triflate counterion is closely associated with the iminium function, and a C+-O contact smaller than the sum of the van der Waals radii is found. The first Co-2(CO)(6) co mplex with a propyne iminium ligand has been prepared from 3 and Co-2( CO)(8), and after anion exchange, the crystal structure of (1-pyrrolid iniumylidene)-2-butyne)]dicobalt(Co-Co) hexafluorophosphate (4b) has b een determined. The iminium character is preserved in this complex (C= N+ bond length 1.302(8) Angstrom), and dispersal of the positive charg e onto one of the cobalt atoms is obviously not important. IR and NMR data confirm this conclusion. Salt 3, C10H14NF3O3S, crystallizes in th e monoclinic space group P2(1)/c with a = 9022(3) Angstrom, b = 12.183 (2) Angstrom, c = 12.013(2) Angstrom, beta = 97.73(3)degrees, and Z = 4. Cobalt complex 4b, C15H14-NCo2F6O6P, crystallizes in the orthorhomb ic space group Pca2(1) with a = 14.170(3) Angstrom b = 10.780(2) Angst rom, c = 13.718(3) Angstrom, and Z = 4.