Metallocene derivatives of early transition elements. Part 1. Niobium(IV) chlorides, chloroalkyls, and dialkyls [Nb(η-C5H4X)2RR′], and the crystal and molecular structure of [Nb(η-C5H5)2(CH2Ph)2]

Abstract

Reaction of [NbCl₄(thf)₂] with Li[C₅H₄X](X = H, Me, or SiMe₃) in tetrahydrofuran (thf) gives a high yield of the volatile [Nb(η-C₅H₄X)₂Cl₂], (1). Treatment of (1) with 2Li[CH₂SiMe₃] provides [Nb(η-C₅H₄X)₂(CH₂SiMe₃)₂], (2a)–(2c), whereas 2Mg(R)Cl (R = Ch₂SiMe₃ or CH₂CMe₃) affords the chloroalkyls [Nb(η-C₅H₄X)₂ClR], (3a)–(3e); using the less bulky Grignard reagent Mg(Me)l or Mg(CH₂Ph)Cl yields the dialkyls [Nb(η-C₅H₄X)₂R′₂], (2e)–(2h). The solution e.s.r. spectra of compounds (1)–(3) have been recorded and show a ten-line spectrum due to hyperfine coupling to ⁹³Nb but not α hydrogens. The g_av. values are in the range 1.97–2.00 [(1) < (3) < (2)] and a(⁹³Nb)(8.6–11.7 mT) decrease in the reverse sequence and in the order X = H > Me > SiMe₃. An X-ray crystal-structure determination (to R= 0.045, R′= 0.064) of [Nb(η-C₅H₅)₂(CH₂Ph)₂] shows it to possess roughly tetrahedral geometry, taking each C₅H₅ ring as unidentate, Nb–C(benzyl)_av. 2.304(3)Å, Nb–C(C₅H₅)_av. 2.410(28)Å, Nb–C–Ph_av. 120.0(5)°, and C–Nb–C_av.(C = benzyl C) 79.0(4)°. There is a relationship between a(⁹³Nb) and (a) electronic configuration of M, d⁰ > d¹, (b) electronegativity of ligand L, Cl^– > R^–, and (c) steric effects, X = H > Me > SiMe₃; X-ray data establish that L–M–L (e.g. C–Nb–C) also correlates with (a) and (b).