Predetermined Chirality in Mono- and Dinuclear Cyclometalated Rhodium(III) Complexes

Abstract

Dinuclear and polynuclear metal complexes with octahedral centers coordinated to di‐ or polydentate ligands are often obtained as complicated mixtures of various stereoisomers. Stereospecific synthesis of such species is therefore of high current interest. Chiral derivatives of pyridine can be used for this purpose. Dinuclear μ‐chloro‐bridged RhIII complexes with two didentate, cyclometalated thienylpyridine‐type ligands at each metal center are formed stereoselectively when pinene groups are fused to the pyridine rings. The two octahedral RhIII centers have homochiral configurations, ΔΔ and ΛΛ. The heterochiral diastereomer ΔΛ is not observed. With (8R,10R)‐2‐(2′‐thienyl)‐4,5‐pinenopyridine [Hth4,5‐(R,R)ppy] the ΔΔ to ΛΛ ratio is 9:1 when the separation eluent contains NaCl. Modeling the ΛΛ and the ΔΛ isomers of the dinuclear species shows crowding of the pinene groups in both cases; however, the strain can be released by relatively small distortions only in the case of the ΛΛ isomer. NO3− cleaves the dichloro bridge, yielding the mononuclear species Δ[Rh(L2)2(NO3)] (2) in a completely stereoselective manner when NaCl is replaced by KNO3 in the eluent mixture. The molecular structure has been determined by X‐ray structure analysis for both the ΔΔ and the mononuclear complex Δ[Rh(L2)2(NO3)] (2) in order to confirm the configuration at the metal center. 1H‐NMR, 13C‐NMR and CD spectra were measured and the latter shows that the CD activity is solely due to the chirality at the metal center.

Syntheses, Spectroscopy, and Structural Analyses of Dinuclear Chiral‐at‐Metal μ‐Aqua‐tetrakis[(R or S)‐N‐1‐(Ar)ethylsalicylaldiminato]di‐Λ‐ or ‐Δ‐nickel(II) Complexes

“…This phenomenon has also been observed in bridged dinuclear octahedral complexes using suitable chiral building blocks in the solid state 2a,2b,4. A pronounced diastereoselectivity was reported in dinuclear ΔΔ[Rh(L) 2 (μ‐Cl)] 2 [L = th4,5( R , R )ppy] with ΔΔ/ΛΛ = 9:1 and in ΛΛ[Rh(L′) 2 (μ‐Cl)] 2 [L′ = th4,5( S , S )ppy] with ΛΛ/ΔΔ = 9:1, according to the results of X‐ray studies 17a…”

Asymmetric Halogeno-Bridged Complexes of the Late Transition Metals − A Computational and Experimental Study

“…The homochiral complexes are energetically favoured by 7.77 kcal/mol at HartreeϪFock level. On the basis of molecular modelling studies of [(ppy) 2 Rh(µ-Cl)] 2 [19] and closely related complexes, [20] it has previously been suggested that the meso form is less stable due to interligand steric repulsion. Accordingly, for this class of compounds only ∆∆/ΛΛ complexes have been structurally characterized.…”

“…Accordingly, for this class of compounds only ∆∆/ΛΛ complexes have been structurally characterized. [20,21] A common feature of all the symmetrical complexes is a planar M(µ-Cl) 2 M unit ( Table 2, Entries 1Ϫ11), the only exception being [(PPh 3 ) 2 Pd(µ-Cl)] 2 2ϩ , which displays a slightly bent chloro bridge in the crystal. This characteristic is also observed for most other halo-bridged complexes of d 8 transition metal ions.…”

Asymmetric Halogeno-Bridged Complexes of the Late Transition Metals − A Computational and Experimental Study

“…[16] In Table 2, selected structural data for some of the computed species are compared with the experimental data. On the basis of molecular modelling studies of [(ppy) 2 Rh(µ-Cl)] 2 [19] and closely related complexes, [20] it has previously been suggested that the meso form is less stable due to interligand steric repulsion. For [(H 3 P)MCl(µ-Cl)] 2 (M ϭ Pd, Pt), two different isomers (cis and trans) were obtained.…”

“…The molecular structures of all the species were fully optimized at the B3LYP level and characterized as minima by a frequency analysis. [20,21] A common feature of all the symmetrical complexes is a planar M(µ-Cl) 2 M unit ( Table 2, Entries 1Ϫ11), the only exception being [(PPh 3 ) 2 Pd(µ-Cl)] 2 2ϩ , which displays a slightly bent chloro bridge in the crystal. The trans isomer was found to be stabilized by ca.…”