Development of Group 6 Dearomatization Agents

Abstract

A series of complexes haVing the general formulas [TpM(NO)(L)Br] and [TpmM(NO)(L)Br]BF 4 (M ) Mo or W and L ) a moderate σ-donor) haVe been prepared. Variations in L proVide access to M(I) complexes haVing a broad range of electronic and steric properties. Reduction of a limited number of these complexes in the presence of aromatic molecules produces noVel complexes with the general formula [TpM(NO)(L)(η 2 -aromatic)] n+ (n ) 0, 1).

Development of 2nd Row Molybdenum Dearomatization Agents

“…66 However, despite the range of TpMo I (NO)(L)(X) complexes synthesized, and their more reducing nature than the {TpRe(MeIm)(CO)} fragment (Table 3); the molybdenum based dearomatization agent was found to have a limited coordination scope (naphthalene, furan, and thiophene). 7,66,68,72 This result is likely due to the less effective backbonding ability of the 2 nd row molybdenum center compared to the 3 rd row osmium and rhenium forerunners. coordination offered by the tungsten system is access to a variety of aromatic ligands with biologically interesting cores.…”

“…Moreover, the {TpRe(MeIm)(CO)} fragment posed synthetic challenges, which made it difficult to synthesize on a large scale. 2,7,[66][67] Given these disadvantages, earth abundant group 6 molybdenum and tungsten systems were proposed. Unlike the difficulties that were inherent in the development of the 2 nd generation rhenium dearomatization agent, 3 rd generation group 6 dearomatization agents could be built upon the foundation laid by the rhenium ligand set ({TpM(L)(CO)}).…”

“…63,66,68,72 The coordination of the aromatic ligand is achieved in a manner similar to that of the 2 nd generation rhenium system (Method A, Scheme 6), by reduction in the presence of the desired ligand (Scheme 8). 7,66,68 In some cases these η 2 -aromatic complexes are also available via a onepot reaction from TpMo(NO)(X)2, when L is amendable to the highly reducing reaction conditions required (Method C, Scheme 8). 66 However, despite the range of TpMo I (NO)(L)(X) complexes synthesized, and their more reducing nature than the {TpRe(MeIm)(CO)} fragment (Table 3); the molybdenum based dearomatization agent was found to have a limited coordination scope (naphthalene, furan, and thiophene).…”

“…7,66,68 In some cases these η 2 -aromatic complexes are also available via a onepot reaction from TpMo(NO)(X)2, when L is amendable to the highly reducing reaction conditions required (Method C, Scheme 8). 66 However, despite the range of TpMo I (NO)(L)(X) complexes synthesized, and their more reducing nature than the {TpRe(MeIm)(CO)} fragment (Table 3); the molybdenum based dearomatization agent was found to have a limited coordination scope (naphthalene, furan, and thiophene). 7,66,68,72 This result is likely due to the less effective backbonding ability of the 2 nd row molybdenum center compared to the 3 rd row osmium and rhenium forerunners.…”

“…10-11, 13, 80-81 Another limitation is the relatively low stability of the resulting W I species, which is prone to undergoing a second oxidation. 2,8,66 This low oxidative stability results in an irrecoverable metal, whereas the previous Os III and Re II systems offered recyclable d 5 complexes (vide supra).…”