Synthesis and structural characterization of isocyanate, amido and imido niobocene derivatives: crystal structures of [Nb(η5-C5H4SiMe3)2Cl(κ2N,C-OCNPh] and [{Nb(η5-C5H4SiMe3)2Cl}2(µ-1,3-N2C6H4)]†

Abstract

The niobium complex [{Nb(η⁵-C₅H₄SiMe₃)₂Cl}₂] 1 reacted with phenyl isocyanate, 1,3- and 1,4-phenylenediisocyanate to give the corresponding complexes [Nb(η⁵-C₅H₄SiMe₃)₂Cl(κ²N,C-OCNPh)] 2, [{Nb(η⁵-C₅H₄SiMe₃)₂Cl}₂{κ²N,C-µ-1,3-(OCN)₂C₆H₄}] 3 and [{Nb(η⁵-C₅H₄SiMe₃)₂Cl}₂{κ²N,C-µ-1,4-(OCN)₂C₆H₄}] 4, respectively. The spectroscopic data were not sufficient unequivocally to differentiate between the presence of a κ²C,N or κ²C,O co-ordination mode. However the crystal structure of 2 showed it to have κ²C,N co-ordination. Complexes 3 and 4 have an analogous structure in which two metal centres are linked through an aryl ring containing a diisocyanate ligand. The reactivity of these complexes under reduction and thermal decomposition conditions was examined. Complexes 2–4 with Na/Hg underwent a reduction at each metal centre followed by protonation to afford the corresponding [Nb(η⁵-C₅H₄SiMe₃)₂(OCHNPh)] 5, [{Nb(η⁵-C₅H₄SiMe₃)₂}₂{µ-1,3-(OCHN)₂C₆H₄}] 6 and [{Nb(η⁵-C₅H₄SiMe₃)₂}₂{µ-1,4-(OCHN)₂C₆H₄}] 7. The imido [Nb(η⁵-C₅H₄SiMe₃)₂Cl(NPh)] 8 and diimido derivatives [{Nb(η⁵-C₅H₄SiMe₃)₂Cl}₂(µ-1,3-N₂C₆H₄)] 9 and [{Nb(η⁵-C₅H₄SiMe₃)₂Cl}₂(µ-1,4-N₂C₆H₄)] 10 were isolated by heating 2, 3 or 4 or alternatively by treating 1 with the appropriate amount of the corresponding aniline, 1,3- or 1,4-phenylenediamine, respectively. The molecular crystal structure of 9 was determined by single-crystal diffractometry. It contains two bent niobocene units linked by a 1,3-diimidophenylene ligand, with the niobium atoms slightly deviating from the plane formed by this ligand. Complexes 8–10 reacted with Grignard reagents MgRCl (R = Me, Et or Prⁱ) or MgMe₂ in tetrahydrofuran solution to give the corresponding substitution products [Nb(η⁵-C₅H₄SiMe₃)₂R(NPh)] (R = Me 11, Et 12 or Prⁱ 13), [{Nb(η⁵-C₅H₄SiMe₃)₂R}₂(µ-1,3-N₂C₆H₄)] (R = Me 14, Et 15 or Prⁱ 16) and [{Nb(η⁵-C₅H₄SiMe₃)₂R}₂(µ-1,4-N₂C₆H₄)] (R = Me 17, Et 18 or Prⁱ 19). The reaction of 8–10 with Na/Hg in appropriate molar ratios afforded the corresponding hydride complexes [Nb(η⁵-C₅H₄SiMe₃)₂H(NPh)] 20, [{Nb(η⁵-C₅H₄SiMe₃)₂H}₂(µ-1,3-N₂C₆H₄)] 21 and [{Nb(η⁵-C₅H₄SiMe₃)₂H}₂(µ-1,4-N₂C₆H₄)] 22. Finally, the monoimido complexes 8 and 11–13 were easily protonated with 1 equivalent of HBF₄·OEt₂ to give, in one step, the corresponding amido cationic complexes [Nb(η⁵-C₅H₄SiMe₃)₂X(NHPh)]⁺ [BF₄]^– (X = Cl 23, Me 24, Et 25 or Prⁱ 26). The structures of the different families of complexes isolated were determined by spectroscopic methods.