INTERACTION OF CARBON-DIOXIDE WITH TRANSITION-METAL-SUBSTITUTED HETEROPOLYANIONS IN NONPOLAR-SOLVENTS - SPECTROSCOPIC EVIDENCE FOR COMPLEX-FORMATION

Tetraheptylammonium salts of various transition-metal-substituted hete ropolyanions with alpha-Keggin ([XW11O39M](n-)), alpha-Wells-Dawson ([ P2W17O61M](m-)), and Weakley and Finke structures ([P2W18O68Co4](10-)) were investigated with respect to their reactivity with CO2 in nonpol ar solvents. It was found that copper(II)- and manganese(III)substitut ed heteropolyanions do not react with CO2. Germano- and silicotungstat es with the alpha-Keggin structure do form complexes with CO2 when sub stituted with Co(II), Ni(II), and Mn(II). In contrast, boro- and phosp hotungstates substituted with Co(II), Ni(II), and Mn(II) are unreactiv e. The alpha(2) isomers of Wells-Dawson phosphotungstates show reactiv ity similar to that of alpha-Keggin silicotungstates-i.e., Co(II), Ni( II), and Mn(II) derivatives do react with CO2. On the other hand, the alpha(1) isomer of the Co(II)-substituted Wells-Dawson anion does not react with CO2, and neither does the Weakley and Finke cobaltotungstat e. When reactions do occur, they are completely reversible. An excess of water decomposes the complexes. Traces of water are, however, neces sary for the reactions to take place. The CO2 adducts were characteriz ed by UV/vis, IR, and C-13 NMR. The IR data could be explained as orig inating either from CO2 complexes with a direct eta(1) metal-carbon bo nd or from bicarbonato complexes. IR spectra with isotopically enriche d (CO2)-C-13 and (CO2)-O-18 support the presence of a eta(1) metal-car bon bond. The C-13 NMR spectra indicate the presence of two different kinds of paramagnetic CO2 complexes after the reaction of alpha-[SiW11 O39Co](6-) with CO2 (chemical shifts 792 and 596 ppm at 26 degrees C). The variable-temperature experiments are consistent with the chemical exchange between these two species. UV/vis, IR, and NMR studies in th e presence of controlled amounts of water or ethanol suggest the exist ence of H-bonding in the CO2 complexes, similar to that reported in th e past for complexes between heteropolyanions and dioxygen.