Reactivity, Structures, and NMR Spectroscopy of Half-Sandwich Pentamethylcyclopentadienyl Rhodium Amido Complexes Relevant to Transfer Hydrogenation

The reactivity, structures, and NMR spectroscopy of a series of compounds relevant to asymmetric transfer hydrogenation, of general formula Cp*RhCl(S,S-4-RC6H4SO2NCHPhCHPhNH2) (Cp* = η5-C5Me5, S,S-2a R = Me, S,S-2b R = tBu, S,S-2c R = F), have been studied. 1H/15N HMQC NMR spectra of 2a−2c were recorded with 15N in natural abundance making use of the coupling to the C5Me5 protons; the coupling constants JRhN were ca. 15 and 20 Hz for the amino and amido nitrogens, respectively. 1H/103Rh NMR spectra of 2a and 2c were recorded similarly. The chloride ligand in S,S-2a has been shown to be very labile; reaction with CO afforded the cationic complex [Cp*Rh(CO){C(O)N(Ts)CHPhCHPhNH2}][Cl] (3a) as a mixture of diastereomers, S,S,RRh and S,S,SRh, with opposite chirality at Rh; reaction with tBuNC gave [Cp*Rh(CNtBu)(S,S-TsCHPhCHPhNH2)][PF6] (4a), and reactions with LiBr and KI gave Cp*RhBr(S,S-TsNCHPhCHPhNH2) (6a) and Cp*RhI(S,S-TsNCHPhCHPhNH2) (7a), respectively. Complexes S,S-2c, S,S-6a, and S,S-7a have been characterized by X-ray crystallography; the amino N−Rh bond is significantly shorter than the amido N−Rh bond in all cases (difference Δr 0.058(2), 0.085(6), and 0.046(4) Å, respectively), and the complexes all possess intramolecular NH···X (X = Cl, Br, I) hydrogen bonds. The reaction of 2a with formic acid results in complete displacement of the chelating ligand and the formation of dinuclear [{Cp*Rh}2(μ-H)(μ-Cl)(μ-HCO2)][BPh4] (5a), which was also characterized crystallographically. Reaction of 2c with AgOTf resulted in the formation of [Cp*Rh(OH2)(4-FC6H4SO2NCHPhCHPhNH2)][OTf] ([8c·H2O][OTf]) or [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)]2[OTf]2 ([8c]2[OTf]2) depending upon the conditions employed. [8c·H2O][OTf] was characterized by X-ray crystallography, and the structure showed that both CHPh centers in the ligand had been racemized, converting the S,S isomer of the starting material into a mixture of both R,R and S,S isomers.