Polymorphs, enantiomorphs, chirality and helicity in [Rh{N,O}(η4-cod)] complexes with {N,O} = salicylaldiminato Schiff base or aminocarboxylato ligands

Abstract

The dimeric complex acetato(η⁴-cycloocta-1,5-diene)rhodium(I), [Rh(O₂CMe)(η⁴-cod)]₂ (cod = cycloocta-1,5-diene) reacts with N,O-chelating Schiff-base ligands or with N-phenylglycine to afford the diminato- or aminocarboxylato(η⁴-cycloocta-1,5-diene)rhodium(I) complexes [{Rh(η⁴-cod)}₂(salen)] (1), [{Rh(η⁴-cod)}₂(salophen)] (2), [Rh((S)-N-phenylglycinato)(η⁴-cod)] (3S), [Rh(rac-N-phenylglycinato)(η⁴-cod)] (3rac), [Rh((R)-N-(4-methoxphenyl)ethyl-2-oxo-1-naphthaldiminato)(η⁴-cod)] (4) and [Rh(N-(o-tolyl)-2-oxo-1-naphthaldiminato)(η⁴-cod)] (5) [salen²⁻ = N,N′-ethylene-bis(salicylaldiminato), salophen²⁻ = N,N′-(1,2-phenylene)-bis(salicylaldiminato)]. The complexes are characterized by IR-, UV/Vis-, ¹H/¹³C-NMR- and mass-spectroscopy. Complexes 1, 2, 4 and 5 contain six-membered metallaaromatic Rh–(N–CCC–O)-chelate rings which accept C–H⋯π contacts. The crystal structure of 2 presents a polymorph (dimorph) (2a) to a previously reported structure (2b, CSD refcode SCLIRB10). Polymorphic forms 2a and 2b are traced to a different interlocking of adjacent dinuclear molecules with their corrugated van der Waals surface. The achiral N-phenylglycine ligand gives a chiral N-phenylglycinato complex [Rh(O₂C–CH₂–NHPh)(η⁴-cod)] (3) with the nitrogen atom becoming the stereogenic center upon metal coordination. Complex 3 can crystallize as the enantiomorph 3S in the tetragonal, chiral space groupP4₁ in a spontaneous resolution of the racemic mixture into homo-chiral helix-enantiomers due to inter-molecular N–H⋯O hydrogen bonding which connects only molecules of the same (S-) configuration into (right-handed or P-) 4₁-helical chains. Variation of the crystallization conditions gives 3 as a racemic polymorphic 3rac. R- and S-complexes 3 assemble in the polymorph 3rac in parallel chains along the 2₁-axes through N–H⋯O hydrogen bonding. Again, only molecules of the same configuration are combined into a chain, albeit neighboring chains have complexes of opposite configuration. The chiral enantiomeric naphthaldiminato complex 4 displays a herring-bone arrangement. Achiral compound 5 crystallizes in the non-centrosymmetric polar space groupCc where all molecules show the same orientation.