INTERACTIONS OF CPM'(CO)3-(M' = CR, MO, W) WITH CATIONS - EFFECTS ON CO EXCHANGE AND RX ADDITION-REACTIONS
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Solution structures of Li+, Na+, K+, Me4N+, PPN+, and M+(HMPA)x salts of CpM(CO)3-(M = Cr, Mo, W) in tetrahydrofuran have been defined by analysis of their v(CO) IR spectra. Interaction of alkali cations with one carbonyl oxygen is apparent. For the large bis(triphenylphosphine)irainium and HMPA solvated cations, which conductivity measurements showed to exist as associated ion pairs in THF, a noncation-perturbed, C3, CpM(CO)3 ion structure exists in solution. The Me4N+ cation strongly interacts with the carbonylates; however, the symmetry of the perturbed CpM(CO)3 could not be established. Small amounts (ca. 20 equiv) of the good alkali cation complexing agent HMPA were required to solvent-separate ion pairs of M+CpM(CO)3 in THF, whereas 400 equiv of CH3CN was required to achieve a similar environment for the CpM(CO)3. Force constant calculations showed that Li+ caused the greatest reorganization of electron density in the contact ion paired CpMo(CO)2(COLi+) as contrasted to symmetrically solvated CpMo(CO)3; however, the lifetime of such ion-pair interactions is short on the NMR time scale as shown by both C-13 and O-17 NMR studies. Carbon-13 labeled CO exchange into CpMo(CO)3 was found to be counterion dependent with rates decreasing as Li+ > Na+ > PPN+. The presumed intermediate, coordinatively unsaturated [CpMo(CO)2], was readily scavenged by 13CO but not by phosphine ligands. The nonactivated RX molecules n-BuI and n-BuBr were found to undergo halide displacement by symmetrically solvated CpMo(CO)3, in typical SN2 fashion, more rapidly than with the contact ion pairs CpMo(CO)2CoNa+. For RX = BzlCl, the opposite counterion effect was observed. Finding no evidence for radical intermediates, a cation-assisted transition-state formation was asserted for the reaction Na+CpMo(CO)3 + BzlCl CpMo(CO)3Bzl. 1982, American Chemical Society. All rights reserved.
