120411-06-5Relevant academic research and scientific papers
Monocyclopentadienyl complexes of niobium(III) and tantalum(III). Structures of [Cp′NbCl(CO)2]2(μ-Cl)2 and Cp′NbCl2(ArCCAr) (Ar = p-tolyl)
Curtis, M. David,Real, Julio,Kwon, Daekeun
, p. 1644 - 1651 (2008/10/08)
The compounds [CpRMCl(μ-Cl)(CO)2]2 (1, R = H (C5H5), 1a, R = Me (C5H4Me), M = Nb; 2, R = H, M = Ta) were prepared by the Al reduction of CpRMCl4 under CO. Internal alkynes displace two CO ligands to give complexes of the type CpM(RCCR)Cl2. Phosphines displace only one CO group and form compounds, e.g., CpNbCl2(CO)(PMe3)2. A tris(phosphine) complex, CpTaCl2(PMe3)2, was prepared by reduction of CpTaCl4 by Na/Hg in the presence of Me3P. Acrylate esters displace CO to give metallacycles, CpNb(CH2CHC(OR)O)Cl2. Likewise, butadiene displaces both carbonyls from Cp*Ta(CO)2Cl2(THF) with the formation of the previously reported tantalacyclopentene, Cp*Ta(CH2CHCHCH2)Cl2 (Cp* = C5Me5). The niobium alkyne adduct Cp′NbCl2(tolan) was also prepared by direct reduction of Cp′NbCl4 with Al in the presence of the alkyne. An EHMO analysis revealed that the alkyne adducts are best viewed as Nb(V) metallacyclopropenes with additional donation of electron density from π⊥ (see A below). The (Matrix Presented) EHMO analysis also explained why the alkyne adducts form stable 16e complexes, whereas phosphines add an additional ligand to form 18e complexes. The structures of 1a and Cp′NbCl2(tolan) (3c) were determined by single-crystal X-ray diffractometry. 1a: a = 7.117 (1) A?, b = 11.902 (2) A?, c = 11.892 (2) A?, β = 101.79 (1)° space group P21/c, Z = 2, V = 986.0 (2) A?3, ρcalcd = 2.01 g/mL, R = 0.027, Rw = 0.038 based on 1965 reflections with I > 3σ(I). 3c: a = 9.793 (3) A?, b = 13.353 (3) A?, c = 15.677 (4) A?, β = 93.15°, space group P21/n, Z = 4, V= 2047 (1) A?3, ρcalcd = 1.46 g/mL, R = 0.037, Rw = 0.036 based on 1999 reflections with I > 3σ(I).
