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Synthesis of N-Heterocycle Substituted Silyl Ligands within the Coordination Sphere of Iron.

ABSTRACT: N-Heterocycle-substituted silyl iron complexes have been synthesized by nucleophilic substitution at trichlorosilyl ligands bound to iron. The homoleptic (tripyrrolyl)- and tris(3-methylindolyl)silyl groups were accessed from (Cl₃Si)CpFe(CO)₂ (Cl₃SiFp) by substitution of chloride with pyrrolide or 3-methylindolide, respectively. Analogously, nucleophilic substitution of Cl with pyrrolide on the anionic Fe(0) synthon Cl₃SiFe(CO)₄ ^- generates the (tripyrrolyl)silyl ligand, bound to the iron tetracarbonyl fragment. The bulkier 2-mesitylpyrrolide substitutes a maximum of 2 chlorides on Cl₃SiFp under the same conditions. The tridentate, trianionic nucleophile tmim (tmimH₃ = tris(3-methylindol-2-yl)methane) proves reluctant to perform the substitution in a straightforward manner; instead, ring-opening and incorporation of THF occurs to form the tris-THF adduct tmim(C₄H₈O)₃SiFe(CO)₄ ^-. The bidentate, monoanionic nucleophile 2-(dipp-iminomethyl)pyrrolide (^DippIMP, dipp = 2,6-diisopropylphenyl) shows chloride displacement and addition of a second ^DippIMP moiety on the imine backbone. The heterocycle-based silyl ligands were shown to be sterically and electronically tunable, moderately electron-donating ligands. The presented approach to new silyl ligands avoids strongly reducing conditions and potentially reactive hydrosilane intermediates.

SUBMITTER: Witteman L

PROVIDER: S-EPMC6158677 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature

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Project description:A series of N(1),N(1),N(3)-tri-substituted benzamidrazones of the general formula [PhC(NHR)=NNMe(2)] (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-NMe(2). An X-ray crystallographic study upon PhC(NHPh)=NNMe(2) (1f) demonstrated that this compound adopts an identical tautomer in the solid state. Reactions of [PhC(NHMe)=NNMe(2)] (1a) with [LMCl(2)](2) (M = Ru, L = cymene; M = Rh, Ir, L = Cp*) results in the stoichiometric formation of products of the formula [LM{PhC(=NMe)NHNMe(2)}Cl](+)Cl(-) (2a-c) in which the amidrazone chelates the metal in a ?(2)-N(1),N(3)-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)NHNMe(2)], 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)=NNMe(2)}] (3a-c). In contrast, the reaction of N(1),N(1),N(3)-tri-substituted benzamidrazones with [(cymene)RuCl(2)](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{C(6)H(5)C(=N(i)Pr)NHNMe(2)}] (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 ?(2)-N(2),C-coordination mode.

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Synthesis of N-Heterocycle Substituted Silyl Ligands within the Coordination Sphere of Iron.

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