Carbon Dioxide Complexation: Infrared Spectroscopy of Iron and Ruthenium η5-Cyclopentadienyl Carbonyl Metallocarboxylates

The metallocarboxylates FpCO 2 - ( 1 -Mg 2+, -Li +, -Na +, -K + ), Cp*(CO) 2 FeCO 2 - K + ( 3 -K + ), and Cp(CO) 2 RuCO 2 - ( 4 -Na +, -K + ) were generated in THF at −78 °C by treating the CpM(CO) 2 - -Li +, -Na +, -K +, or -Mg 2+ metalate with 1.0−1.2 equiv of CO 2 (or 13 CO 2 ). The results of studies that were carried out on Me 3 SiCl trapping of 1 -Mg 2+, -Li +, -Na +, -K +, - n Bu 4 N +, and -(dibenzo-18-crown-6)K + as Cp(CO) 2 FeC(O)OSiMe 3 ( 5 ) indicated that 1 -Mg 2+, -Li +, -Na +, and -K + are stable at room temperature for at least 1 h, and with proper precautions their IR spectra can be obtained. The 13 C NMR and IR spectral υ(OCO) and υ(CO) assignments (−15 °C to room temperature) are consistent with (a) ionic or (η 1 -C:η 2 -O,O‘) metallocarboxylate structures that are symmetrically chelated to the metallic counterion and (b) increasing charge density on the Fp moiety through the counterion sequence 1 -Mg 2+, -Li +, - Na +, and -K + . This counterion sequence reflects an attenuation of carboxylate−counterion ion pairing that influences the (a) exchange of the 13 C label between the carboxylate and carbonyl ligands on Cp(CO) 2 M 13 CO 2 - [⇔ Cp(CO)( 13 CO)MCO 2 - ] and (b) reversible dissociation of carbon dioxide from these metallocarboxylates. A net effect of these coupled reactions is that Cp(CO) 2 M 13 CO 2 - converts residual (unbound) 13 CO 2 to unlabeled CO 2 . Both of the reactions were observed only with the more reactive 1 -Na + and 1 -K +, which less tightly bind their Na + and K + counterions to the carboxylate oxygens. An example of the postulated metalloanhydride intermediate Cp(CO)M 13 C(O)OC(O) - for the carboxylate−carbonyl label exchange may have been detected for M = Ru. Cp(CO) 2 RuCO 2 - ( 4 -Na + ) isomerized to (the tentatively assigned) Cp(CO)Ru−C(O)OC(O) - above ∼−10 °C, which then degraded upon further warming above 0 °C to Cp(CO) 2 RuH.