Synthesis of Backbone <i>P</i>‑Functionalized Imidazol-2-ylidene Complexes: <i>En Route</i> to Novel Functional Ionic Liquids

1-Alkyl-3-methyl-4-diphenylphosphoryl-imidazolium hydrogensulfate (4a,b) (a: R1 = R2 = Me; b: R1 = iPr, R2 = Me) and 1-alkyl-3-methyl-4,5-bis­(diphenylphosphoryl)­imidazolium hydrogensulfate (6a,c) (c: R1 = nBu, R2 = Me) were obtained selectively and in good yields by oxidative desulfurization of 1-alkyl-3-methyl-4-diphenylphosphino-imidazole-2-thiones (2a,b) and 1-n-butyl-3-methyl-4,5-bis­(diphenylphosphoryl)­imidazole-2-thione (3c) or 1,3-dimethyl-4-diphenylthiophosphoryl-5-diphenylphosphino-imidazole-2-thione (5a), respectively, with hydrogen peroxide. Synthesis of phosphoryl functionalized imidazol-2-ylidene complexes of group VI metal pentacarbonyls (7a–9a) and (10b–12b) and bis­(phosphoryl) functionalized imidazol-2-ylidene complexes of group VI metal pentacarbonyls (13c–15c) and (16a) with low steric demand (methyl, isopropyl, n-butyl) at both N-centers was achieved through deprotonation of imidazolium salts (4a,b) and (6a,c), respectively,having HSO4– as a counterionwith potassium tert-butoxide followed by rapid addition of metal pentacarbonyl acetonitrile complexes [M­(CO)5(CH3CN)] (M = Cr, Mo, W). The products were unambiguously characterized by elemental analyses, spectroscopic and spectrometric methods, and in addition, by single-crystal X-ray structure studies in the cases of 4b, 8a, 15c, and 16a; the latter two reveal imidazole ring bond distance alternation in contrast to 8a.