Principal component analysis of conformations in fused ring chelate complexes: conformer identification, stereochemistry, and interconversion pathways

Abstract

Data were retrieved from the Cambridge Structural Database for crystal structures containing metal complexes of ethylenediamine (en), tetramethylethylenediamine (tmeda), bis(dimethylphosphino)ethane (dmpe), bis(diphenylphosphino)ethane (dppe) chelate complexes [M(en) (279 structures, 468 fragments), M(tmeda) (156 structures, 181 fragments), M(dmpe) (205 structures, 288 fragments) and M(dppe) (273 structures, 338 fragments)] and fused five-membered chelate ring systems [complexes of diethylenetriamine, M(dien) (91 structures, 108 fragments), tris(2-aminoethyl)amine, M(nn₃) (49 structures, 54 fragments), tris(2-diphenylphosphinoethyl)phosphine, M(pp₃), and tris(2-diphenylphosphinoethyl)amine, M(np₃) (54 structures, 56 fragments)], and have been analysed using principal component analysis (PCA) of the intra-ring torsion angles. A limited number of preferred conformers is observed for each system: enantiomeric twist conformations (δ or λ) for M(en), M(tmeda), M(dppe) and M(dmpe); three unique conformers: δδ (and its enantiomer λλ), λδ or δλ for M(dien); and two types for M(nn₃) or M(xp₃) (x = n or p): two enantiomeric C₃ symmetric A (λλλ, δδδ) and six equivalent B (δλδ, δδλ, λδδ, δλλ, λλδ, and λδλ) conformers of lower symmetry. The pseudorotation pathway for δ ↔ λ interconversion in the single ring systems is clear for M(dppe) and also for M(dmpe) but less so for M(en) and, M(tmeda). The δλ and λδ conformers of M(dien) apparently interconvert through δδ (or λλ) intermediates with one ring at a time inverting. Similarly the interconversion of conformers in the M(xp₃) and M(nn₃) systems seems likely, on the basis of the distribution of structures in conformation space, to follow a sequence δδδ ↔ δλδ ↔ λλδ ↔ λλλ (or its equivalent) but not δδδ ↔ δλδ ↔ δλλ ↔ λλλ. In the M(dien), M(nn₃) and M(xp₃) systems the conformation preferred is linked to the metal coordination geometry. In particular, the presence of an N–M–N angle approaching 180° in a mer-octahedral or square-based pyramid stereochemistry in M(dien) species enforces a δλ conformation. Similarly, related mer-like stereochemistry in M(nn₃) and M(xp₃) complexes leads to a B-type conformation. Longer M–N (or M–P) distances enforce more puckered, symmetrical, twist conformations of five-membered rings. A new pseudo-principal component analysis method is introduced which allows quantitative comparison of conformations in analogous but not identical ring systems. Conformations in M(en) are compared with M(dien) or M(nn₃), with the single-ring system exhibiting less distorted conformations. Phosphine systems exhibit greater variation of conformation than their amine counterparts. In particular M(dmpe) systems show considerably more variation than M(tmeda) whereas M(dppe) species are more varied in conformation than are M(dmpe).