CARBON-DIOXIDE COMPLEXATION - INFRARED-SPECTROSCOPY OF IRON AND RUTHENIUM ETA(5)-CYCLOPENTADIENYL CARBONYL METALLOCARBOXYLATES

The metallocarboxylates FpCO(2)(-) (1-Mg2+, -Li+, -Na+, -K+), CP(CO)( 2)FeCO2-K+ (3-K+), and Cp(CO)(2)RuCO2- (4-Na+, -K+) were generated in THF at -78 degrees C by treating the CpM(CO)(2)(-)-Li+, -Na+, -K+, or -Mg2+ metalate with 1.0-1.2 equiv of CO2 (or (CO2)-C-13). The results of studies that were carried out on Me(3)SiCl trapping of 1-Mg2+, -Li, -Na+, -K+, -(n)Bu(4)N(+), and -(dibenzo-18-crown-6K+ as Cp(CO)(2)FeC (O)OSiMe(3) (5) indicated that 1-Mg2+, -Li+, -Na+, and -K+ are stable at room temperature for at least 1 h, and with proper precautions thei r IR spectra can be obtained. The C-13 NMR and IR spectral upsilon(OCO ) and upsilon(CO) assignments (-15 degrees C to room temperature) are consistent with (a) ionic or (eta(1)-C:eta(2)-O,O') metallocarboxylate structures that are symmetrically chelated to the metallic counterion and (b) increasing charge density on the Fp moiety through the counte rion sequence 1-Mg2+, -Li+, -Na+, and -K+. This counterion sequence re flects an attenuation of carboxylate-counterion ion pairing that influ ences the (a) exchange of the C-13 label between the carboxylate and c arbonyl ligands on Cp(CO)(2)M(13)CO(2)(-) [double left right arrow Cp( CO)((CO)-C-13)MCO(2)(-)] and (b) reversible dissociation of carbon dio xide from these metallocarboxylates. A net effect of these coupled rea ctions is that Cp(CO)(2)M(13)CO(2)(-) converts residual (unbound) (CO2 )-C-13 to unlabeled CO2. Both of the reactions were observed only with the more reactive 1-Na+ and 1-K+, which less tightly bind their Na+ a nd K+ counterions to the carboxylate oxygens. An example of the postul ated metalloanhydride intermediate Cp(CO)M(13)C(O)OC(O)(-) for the car boxylate-carbonyl label exchange may have been detected for M = Ru. Cp (CO)(2)RuCO2- (4-Na+) isomerized to (the tentatively assigned) Cp(CO)R u-C(O)OC(O)(-) above similar to-10 degrees C, which then degraded upon further warming above 0 degrees C to Cp(CO)(2)RuH.