Molybdenum and tungsten η1- and η2-aldehyde complexes

A series of molybdenum(II) and tungsten(II) aldehyde complexes of the type [{Tp{M(CO)(RC=CMe)(eta(1)/eta(2)-O=CHR')][BAr'(4)] ({Tp} = Tp', Tp; R = Ph, Me) (5-8) has been prepared for the purpose of investigating sigma/pi-aldehyde binding. The aldehyde complexes were generated by protonation of the methyl complexes {Tp}M(CO)(RC=CMe)Me (1-4) with HBAr'(4). OEt2 (HBAr'(4) = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), loss of methane, and addition of aldehyde (acetaldehyde, propionaldehyde, pivaldehyde, acrolein, trans-crotonaldehyde, methacrolein, benzaldehyde, p-tolualdehyde, or p-anisaldehyde). The aldehyde complexes exhibit varying sigma/pi-aldehyde coordination depending on the aldehyde, the metal, and the metal's ancillary ligands. Within the series, pi-binding is observed only in the electron-rich and sterically less encumbered Tp tungsten complexes (7), where the pi/sigma-isomer ratio is dependent on the nature of the aldehyde. In contrast, aldehydes are sigma-bound in the less electron-rich Tp molybdenum complexes (8) and in the more sterically encumbered Tp' tungsten and molybenum complexes 5 and 6. Variable-temperature FTIR measurements show that the pi/sigma-isomer ratio in the Tp tungsten complexes (7) increases as the temperature is lowered.