Isocyanide insertion reactivity of dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [{Nb(η5-C5H4SiMe3)(CH2Ph)2}2(μ-1,4-NC6H4N)]

Derivatization of Niobium Complexes Bearing Imido and Acetophenone Imine Ligands

“…In the 1 H NMR spectrum, the methylene protons of this alkyl group were diastereotopic and appeared at vastly different chemical shifts (3.926 and −0.888 ppm; 2 J HH = 9.2 Hz). Most previous reports of niobium complexes with the (trimethylsilyl)methyl ligand do not display inequivalent methylene protons (even in chiral species where these protons are expected to be diastereotopic), although the chemical shifts reported for these protons span a wide spectral range (−1.5 to 3.6 ppm). − A limited number of reports do show inequivalent proton signals for this methylene group, and in these cases, the two protons are generally widely separated in the spectrum (often ∼3 ppm apart). ,− Green and co-workers found a 1.5 ppm separation between these two diastereotopic protons in related hydrazido complexes and attributed the chemical shift difference to ring current effects from π interactions with adjacent ligands . In contrast, for more extreme cases (∼3 ppm chemical shift difference), this effect has been attributed to α-agostic interactions between the methylene group and the highly electrophilic metal center .…”

“…Although much work has been accomplished using cyclopentadienyl supporting ligands, non-cyclopentadienyl niobium complexes of this type with both imido and alkyl groups, such as methyl and (trimethylsilyl)methyl, or alkynyl ligands, such as (trimethylsilyl)acetylide and phenylacetylide, are quite rare and to date few examples of niobium complexes of this sort have been reported in the literature. 8,[35][36][37][38][39][40][41][42][43][44] Treatment of 15 with MeLi in ether at -77 °C resulted in a red material that NMR spectroscopy revealed to be a complex mixture of products. Alternatively, the choice of MeLi 3 LiBr as the alkylating agent gave better control over the derivatization reaction, most likely due to the attenuated reactivity of the carbanion in this reagent.…”

“…[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] A limited number of reports do show inequivalent proton signals for this methylene group, and in these cases, the two protons are generally widely separated in the spectrum (often ∼3 ppm apart). 41,[61][62][63][64][65] Green and co-workers found a 1.5 ppm separation between these two diastereotopic protons in related hydrazido complexes and attributed the chemical shift difference to ring current effects from π interactions with adjacent ligands. 62 In contrast, for more extreme cases (∼3 ppm chemical shift difference), this effect has been attributed to R-agostic interactions between the methylene group and the highly electrophilic metal center.…”

“…However, in four cases, 15a − d , derivatization reactions successfully resulted in low to moderate yields of complexes containing new Nb−C bonds. Although much work has been accomplished using cyclopentadienyl supporting ligands, non-cyclopentadienyl niobium complexes of this type with both imido and alkyl groups, such as methyl and (trimethylsilyl)methyl, or alkynyl ligands, such as (trimethylsilyl)acetylide and phenylacetylide, are quite rare and to date few examples of niobium complexes of this sort have been reported in the literature. ,− …”

“…Successful alkylation of complex 15 was also achieved upon its treatment with ((trimethylsilyl)methyl)lithium. The bulky (trimethylsilyl)methyl ligand has seen broad use in early-transition-metal chemistry, and there are many previous reports of its application in niobium complexes. − Its popularity is undoubtedly linked to current commercial availability and the kinetic stability provided by the sterically bulky trimethylsilyl group. On the basis of 1 H NMR spectroscopy, it was clear that an alkylated product ( 15b ) was produced as a single isomer in the reaction of 15 with LiCH 2 SiMe 3 .…”