Polymorphs, enantiomorphs, chirality and helicity in [Rh{N,O}(eta4-cod)] complexes with {N,O}=salicylaldiminato Schiff base or aminocarboxylato ligands.

The dimeric complex acetato(eta4-cycloocta-1,5-diene)rhodium(I), [Rh(O2CMe)(eta4-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(4-cycloocta-1,5-diene)rhodium(I) complexes [{Rh(eta4-cod)}2(salen)] (1), [{Rh(eta4-cod)}2(salophen)] (2), [Rh((S)-N-phenylglycinato)(eta4-cod)] (3S), [Rh(rac-N-phenylglycinato)(eta4-cod)] (3rac), [Rh((R)-N-(4-methoxphenyl)ethyl-2-oxo-1-naphthaldiminato)(eta4-cod)] (4) and [Rh(N-(o-tolyl)-2-oxo-1-naphthaldiminato)(eta4-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...pi 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)(eta4-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 group P41 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-HO 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 group Cc where all molecules show the same orientation.