Synthesis and coordination chemistry of tri-substituted benzamidrazones

Abstract

A series of N¹,N¹,N³-tri-substituted benzamidrazones of the general formula [PhC(NHR)NNMe₂] (R = Me, n-Pr, i-Pr, n-Bu, Bn, Ph; 1a–f) was synthesized via condensation of 1,1-dimethylhydrazine with the corresponding imidoyl chloride, [PhC(Cl)NR]. Multinuclear NMR data, and zero-point energy DFT calculations conducted with the B3LYP functional and 6–31G+(d,p) basis set, suggest that these compounds exist as a single tautomer in solution; possessing a weak intramolecular hydrogen bond and a structure dominated by the localised resonance structure ArC(NHR)N–NMe₂. An X-ray crystallographic study upon PhC(NHPh)NNMe₂ (1f) demonstrated that this compound adopts an identical tautomer in the solid state. Reactions of [PhC(NHMe)NNMe₂] (1a) with [LMCl₂]₂ (M = Ru, L = cymene; M = Rh, Ir, L = Cp*) results in the stoichiometric formation of products of the formula [LM{PhC(NMe)NHNMe₂}Cl]⁺Cl⁻ (2a–c) in which the amidrazone chelates the metal in a κ²-N¹,N³-coordination mode. Formation of this five-membered chelate occurs with a concomitant tautomerisation of the amidrazone ligand to an alternative tautomer, i.e. [PhC(NMe)NHNMe₂], the latter tautomer is expected to be readily energetically accessible based upon the aforementioned DFT calculations. This series of salts may be deprotonated with lithium hexamethyldisilazide to form the corresponding charge neutral complexes [LM{PhC(NMe)NNMe₂}] (3a–c). In contrast, the reaction of N¹,N¹,N³-tri-substituted benzamidrazones with [(cymene)RuCl₂]₂ in the presence of NaOAc yielded a mixture of cyclometallation (C–H activation) and amidrazone chelation/deprotonation (N-H activation) products. Reaction of 1a yielded an inseparable mixture of products, whilst the reaction of 1c resulted in formation of the cyclometallated product [LM{C₆H₅C(NⁱPr)NHNMe₂}] (L = cymene, M = Ru; 4a) in a modest 62% yield. This latter complex could be isolated as a crystalline orange solid, full characterisation including single crystal X-ray diffraction demonstrated that the amidrazone coordinates in a κ²-N²,C-coordination mode.