A General Route to Labile Niobium and Tantalum d0 Monoimides. Discussion of Metal−Nitrogen Vibrational Modes

Reaction of TaCl 5 with 2 equiv of an amine in the presence of sodium silicate and pyridine affords Ta(NR)Cl 3 (py) 2 in good yield. Reaction of NbCl 5 with ZnCl 2 followed by addition of an amine RNH 2 and pyridine affords M(NR)Cl 3 (dme) (dme is 1,2-dimethoxyethane). For niobium this reaction proceeds smoothly regardless of the amine but is ineffective with tantalum and alkyl amines. An alternative route involves reaction of TaCl 5 with 3 equiv of RNH 2 to form [RNH 3 ] 2 [Ta(NR)Cl 5 ], followed by reaction of this salt with ZnCl 2 in the presence of dme. The molecular structure of Nb(N t Bu)Cl 3 (dme) (formula C 8 H 19 Cl 3 NNbO 2 ) was determined by X-ray crystallography (monoclinic space group Cc with a = 30.565(4) Å, b = 7.2406(13) Å, c = 13.915(2) Å, β = 90.626(7)°, V = 3079.4(8) Å 3, Z = 8). The Nb−N bond length is 1.72 Å with a Nb−N−C bond angle of 177° in a distorted octahedral structure. In order to characterize the M−N stretching frequencies in these compounds, IR data for each compound are compared with calculated stretching frequencies using the commercially available Spartan calculation package. These experiments reveal that there is no real M−N stretching frequency in these imidos. Rather, the M−N modes are strongly coupled to N−C and C−H or C−C modes in these imidos. IR active modes are observed at ∼1260 cm - 1 for tantalum alkyl imidos and ∼1350 cm - 1 for tantalum aryl imidos. These correspond to a Ta−(N−C) stretch coupled to the CR 3 umbrella deformation.