PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES - 14- THERMODYNAMIC ACIDITY MEASUREMENTS OF FISCHER CARBENE COMPLEXES IN ACETONITRILE

The pK(a) values of five Fischer carbene complexes of the general stru cture (CO)(5)Cr=C(OR)CHR'R '' of two (CO)(4)(PR3)Cr=C(OCH3)CH3 type co mplexes and of one aminocarbene complex, (CO)(5)Cr=C(NCH- (CH3)CH2)CH3 , were determined in acetonitrile by a spectrophotometric method. Thes e pK(a) values span a range from 22.2 for (CO)(5)Cr=C(OCH3)CH3 to 27.6 for (CO)(5)Cr=C(NCH(CH3)CH2)CH3. Deprotonation of the carbene complex es was achieved with strong amine bases such as TMG, DBU, and BEMP. At tempts at measuring the pK(a) of some other carbene complexes failed e ither because of instability of the respective anions (e.g., (CO)(5)Cr =C(OCH3)CH(CH3)(2)) or because the pK(a) was too high for deprotonatio n to occur ((CO)(5)Cr= C(N(CH3)(2))CH3), Similarities and differences in the structural dependence of these acidities in acetonitrile compar ed to other solvents such as 50% acetonitrile-50% water, DMSO, and THF are discussed.