Reactions of (Butadiene)tantalocene Cation with Alkyl Isocyanides

Abstract

Treatment of (η 4 -s-trans-butadiene)tantalocene cation (3, with methyltris(pentafluorophenyl)borane anion) with tert-butyl isocyanide (4 h, at 45 °C in bromobenzene) yields the [(η 2 -butadiene)(CtNCMe 3 )TaCp 2 + ] complex 4a (70% isolated, ν ˜(CtNR) 2164 cm -1 ). Likewise treatment with n-butyl or cyclohexyl isocyanide gives the analogous products 4b and 4c, respectively. The cyclohexyl isocyanide adduct 4c was characterized by X-ray diffraction. It shows the η 2 -butadiene ligand oriented at the front of the Cp 2 Ta + bent metallocene wedge with the noncoordinated butadiene vinyl group located in the "vinyl-inside" position. The Ta-CtN-R unit is almost linear (angles Ta-C-N ) 177.2(5)°, CtN-R ) 174.0(8)°; d(CtN) ) 1.152(6) Å). Photolysis of [(butadiene)TaCp 2 + ] (3) with excess cyclohexyl isocyanide followed by thermal treatment gave rise to the formation of [bis(κC-cyclohexyl isocyanide)-TaCp 2 + ] (8, with [CH 3 B(C 6 F 5 ) 3 -] counteranion; >50% isolated, ν ˜(CtNR) 2134 and 2088 cm -1 ). Complex 8 was characterized by an X-ray crystal structure analysis. It shows a pseudotetrahedral [Cp 2 Ta(CtNR) 2 + ] cation (R ) C 6 H 11 ). The Ta-CtN-R units are both slightly bent at nitrogen (angles CtN-R ) 169.0(13) and 163.7(8)°; the corresponding Ta-C-N angles are 179.4(9) and 177.4(7)°). The bonding situations of complexes 4 and 8 were analyzed by DFT calculations. The computational study shows that metal to isonitrile back-bonding does not contribute significantly to the Cp 2 Ta + -CtNR bonding interaction in these complexes. The bonding between Cp 2 Ta + and the CtNR ligands is dominated by electrostatic effects.

Theoretical studies of transition metal complexes with nitriles and isocyanides

“…(CNMe)(:CCH=CH 2 )] + complexes containing one carbyne ligand and two carbonyl ligands (strong p-acceptors) along with the isocyanide ligand are composed predominantly of the orbitals of the metal atom and the carbonyl and carbyne ligands with a very small contribution of the AOs of the nitrogen atom of isocyanide (Figs 3 and 4). 38 The nature of the coordination bond in the isocyanide complexes [Cp 2 Ta(CNMe)(Z 2 -CH 2 =CH7CH=CH 2 )] + (Cp is the cyclopentadienyl ligand) (5), [Cp 2 Ta(CNMe) 2 ] + (6) and [Cp 2 M(CNMe)] 0/+ (M = Ta, Zr) was examined based on the results of DFT calculations 39 with full geometry optimisation of different isomers of these complexes. Analysis of the theoretical and experimental structural data (the linearity of the isocyanide molecule, the equal CN bond lengths in free and coordinated isocyanides) and examination of the energy components of the coordination bond showed that p-back donation is of secondary importance in the interaction between isocyanide and tantalum in the complex 5, which contains other strong p-acceptors.…”

“…In contrast, when p-back donation becomes dominant, the multiplicity of the M7L bond increases, whereas the multiplicity of the C:N bond decreases, which could cause a bent distortion of the isocyanide ligand. 39,53 Actually, according to the X-ray diffraction data, 54 ± 58 the isocyanide ligand has a linear structure in most of isocyanide complexes of transition metals in high oxidation states in which p-back donation is manifested to only a small extent, if at all. At the same time, the isocyanide ligand has an angular structure in complexes of transition metals in low oxidation states in which efficient electron density transfer occurs from the d-orbitals of metal to the p*-MO of the ligand, 59,60 the C7N7C angle being contracted to 140 8.…”

“…In the general case, the relative length of the C:N multiple bond in transition metal complexes with nitriles and isocyanides also cannot be considered as a reliable criterion for the degree or even existence of p-back donation because the length of this bond is substantially influenced not only by two opposite effects, viz., s-donation and p-back donation, but also by the Coulombic component. 39,51 A comparison of a series of related complexes showed that the M7L bond length is a more reliable criterion for the degree of p-back donation.…”

“…Actually, lowering of the n(CO) and n(CN) vibrational frequencies on going from the free ligand to the complex was predicted theoretically and observed experimentally for many carbonyl complexes as well as for a series of transition metal complexes with isocyanides (predominantly, with metals in low oxidation states). 39,59,63,64 However, complex formation with isocyanides possessing weaker p-acceptor properties as compared to CO often leads to an increase in the n(CN) frequency. Thus theoretical analysis of the force constants and n(CN) vibrational frequencies for the [Mn(CNMe) n (CO) 57n Br] (n = 0 ± 4), [Mn(CNMe) n (CO) 67n ] + (n = 0 ± 6) and [Fe(CNMe) 6 ] 2+ complexes demonstrated that the n(CN) frequency for the complexes is higher than that for the free ligand, which indicates that s-donation dominates over p-back donation.…”

11 Vanadium, niobium and tantalum

“…85 Activation of this complex with methylalumoxane yields an effective Ziegler^Natta catalyst, and it also reacts stoichiometrically with ketones to give non-planar seven-membered metallacycles, such as 2 formed with adamantanone. 86 Reactions of the metallacycles with various alkyl isocyanides have been studied, 87 and the preparations, crystal structures and reactions of some related tantalum systems containing the {(butadiene)(cot)(Cp)} ligand set have also been reported. 88 The ¢rst homoleptic isocyanides of niobium and tantalum, [M(CNXyl) 7 ](BF 4 ), have been prepared, and converted to [MI(CNXyl) 6 ].…”