Protonation of the amido groups of M(NMe 2 ) 5 (M = Ta or Nb) with trialkylguanidines, (RNH) 2 CNR (R = i-Pr or Cy), directly produced a series of five-co-ordinated complexes, M(NMe 2 ) 3 [(RN) 2 CNR] 1-4. Single crystal X-ray analysis confirmed that 1 contained a dianionic N,NЈ,NЉ-triisopropylguanidinate ligand which was co-ordinated in a chelating bidentate mode. In contrast, protonation of the amido groups of Ta(NMe 2 ) 4 Cl with triisopropylguanidine gave the six-co-ordinated complex Ta(NMe 2 ) 3 Cl[(i-PrN) 2 CNHi-Pr] 5 which possessed a bidentate monoanionic guanidinate ligand. Complex 5 can be converted into 1 by reaction with either LiNMe 2 or MeMgBr.
The protonation of two metal-amido groups of M(NMe(2))(5) with trialkylguanidines yielded a series of novel complexes with formulas [RNC(NR)NR]M(NMe(2))(3) (1-4) (M = Ta, Nb; R = isopropyl, cyclohexyl). These complexes contained dianionic N,N',N' '-trialkylguanidinate ligands which were coordinated in a chelating bidentate mode. A single-crystal X-ray study of [CyNC(NCy)NCy]Ta(NMe(2))(3) (3) (C(25)H(51)N(6)Ta, triclinic, P&onemacr;, a = 9.4155(2) Å, b = 13.3188(2) Å, c = 13.5215(2) Å, alpha = 117.075(1) degrees, beta = 101.744(1) degrees, gamma = 98.507(1) degrees, Z = 2) confirmed the connectivity of these species. These guanidinate ligands exhibited both planarity of the central CN(3) group and the correct orientation of the three NR substituents to allow for pi conjugation within the ligand core.
The preparation and characterization of guanidinate-containing complexes of Nb and Ta is described. The direct reactions of M(NMe(2))(5) with either dicyclohexylcarbodiimide (CyN=C=NCy) and diisopropylcarbodiimide ((i)PrN=C=N(i)Pr) proceeded smoothly at room temperature under nitrogen to yield [RNC(NMe(2))NR]M(NMe(2))(4) (M = Ta, Nb; R = Cy, (i)Pr). The spectroscopic characterization of these materials is consistent with a symmetrical chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. Confirmation of these details was provided by a single-crystal X-ray diffraction study in the case of [CyNC(NMe(2))NCy]Ta(NMe(2))(4) (1). Delocalization of the lone pair of electrons on the guanidinate NMe(2) group into the ligand N-C-N pi system does not appear to be significant in these species.
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