Physical Organic Chemistry of Transition Metal Carbene Complexes. 14.1 Thermodynamic Acidity Measurements of Fischer Carbene Complexes in Acetonitrile

The p K a values of five Fischer carbene complexes of the general structure (CO) 5 Cr C(OR)CHR‘R‘‘, of two (CO) 4 (PR 3 )Cr C(OCH 3 )CH 3 type complexes and of one aminocarbene complex, were determined in acetonitrile by a spectrophotometric method. These p K a values span a range from 22.2 for (CO) 5 Cr C(OCH 3 )CH 3 to 27.6 for Deprotonation of the carbene complexes was achieved with strong amine bases such as TMG, DBU, and BEMP. Attempts at measuring the p K a of some other carbene complexes failed either because of instability of the respective anions (e.g., (CO) 5 Cr C(OCH 3 )CH(CH 3 ) 2 ) or because the p K a was too high for deprotonation to occur ((CO) 5 Cr C(N(CH 3 ) 2 )CH 3 ). Similarities and differences in the structural dependence of these acidities in acetonitrile compared to other solvents such as 50% acetonitrile−50% water, DMSO, and THF are discussed.