Dihapto Coordination of Aromatic Molecules by the Asymmetric π-Bases {TpRe(CO)(L)} (Tp = hydridotris(pyrazolyl)borate; L = ^tBuNC, PMe₃, pyridine, 1-methylimidazole, or NH₃)

Abstract

A variety of rhenium asymmetric π-basic dearomatization agents of the general formula TpRe(CO)(L)(L π ) (Tp ) hydridotris(pyrazolyl)borate; L ) t BuNC, pyridine, PMe 3 , 1-methylimidazole, or NH 3 ; L π ) dihapto-coordinated ligand) have been synthesized via three different routes. By varying the ligand, L, the steric and electronic properties of these complexes can be tuned, and thus the stability and selectivity of the η 2 -aromatic systems can be adjusted. Comparisons among the various rhenium complexes are presented as well as comparisons to the established pentaammineosmium(II) system. Scheme 2. Synthesis of Re(I) η 2 -Aromatic Complexes

Crystal structure of dichlorido[tris(pyrazolyl)borate]triphenylphosphinerhenium(III), C27H25BCl2N6PRe

“…Furthermore, some mononuclear rhenium(III), rhenium(V) and rhenium(VII) complexes containing the Tp -ligand have been synthesized [8,9]. To our knowledge, the crystal structures of rhenium complexes are relatively limited.…”

Charge donation to and dearomatization of benzene attending complexation: DFT estimates of binding energies of TpMXO(L) with benzene, for Tp = hydridotris(pyrazolyl) borate, MXO = MoNO, ReCO, and WNO, and L = ammonia, N‐methylimidazole, pyridine, phosphine, methyl isocyanide, and carbon monoxide

“…Each method captures trends in behavior for binding energies and conformational changes. 11 Neither method captures the full severity of the bond alternation of the "butadiene" segment of the dearomatized benzene displayed in the X-ray reports.…”

“…Use of the metals Re, W, and Mo with the tridentate scorpionate ligand hydridotris(pyrazolyl)borate (called tp) promises greater economy and safety as well a route to enantiomeric selectivity. 2 Here we report our characterization of bonding, structure, and charge transfer in tpReCO(L), tpWNO(L), and tpMo-NO(L) and their benzene complexes. Of particular value to the designer would be a way to link properties of L to the dearomatizion efficacy of a system.…”

Development of 2nd Row Molybdenum Dearomatization Agents

“…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).…”

“…Initial attempts were made with nitrile, phosphine, and pyridine based ancillary ligands (L) (entries 1 -10, Table 2), which provided good electronic agreement with the previous osmium system (18-22, Table 2). [63][64] However, despite the close electronic match to the {Os(NH3)5} 2+ fragment, these complexes provided a limited scope of aromatic coordination, likely due to increased sterics exhibited by the rhenium ligand set. In order to overcome the increased sterics of the Tp -based motif, more-donating ligands were explored.…”