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

Data were retrieved from the Cambridge Structural Database for crystal stru ctures containing metal complexes of ethylenediamine (en), tetramethylethyl enediamine (tmeda), bis(dimethylphosphino)ethane (dmpe), bis( diphenylphosp hino) 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-memb ered chelate ring systems [complexes of diethylenetriamine, M(dien) (91 str uctures, 108 fragments), tris( 2-aminoethyl) amine, M(nn(3)) (49 structures , 54 fragments), tris(2-diphenylphosphinoethyl) phosphine, M(pp(3)), and tr is(2-diphenylphosphinoethyl) amine, M(np(3)) (54 structures, 56 fragments)] , and have been analysed using principal component analysis (PCA) of the in tra-ring torsion angles. A limited number of preferred conformers is observ ed for each system: enantiomeric twist conformations (delta or lambda) for M(en), M( tmeda), M( dppe) and M(dmpe); three unique conformers: delta delt a (and its enantiomer lambda lambda), lambda delta or delta lambda for M(di en); and two types for M(nn(3)) or M(xp(3)) (x = n or p): two enantiomeric C-3 symmetric A (lambda lambda lambda,delta delta delta) and six equivalent B (delta lambda delta, delta delta lambda, lambda delta delta, delta lambd a lambda, lambda lambda delta and lambda delta lambda) conformers of lower symmetry. The pseudorotation pathway for delta <-> lambda interconversion i n the single ring systems is clear for M( dppe) and also for M(dmpe) but le ss so for M(en) and, M(tmeda). The delta lambda and lambda delta conformers of M(dien) apparently interconvert through delta delta (or lambda lambda) intermediates with one ring at a time inverting. Similarly the interconvers ion of conformers in the M(xp(3)) and M(nn(3)) systems seems likely, on the basis of the distribution of structures in conformation space, to follow a sequence delta delta delta <-> delta lambda delta <-> lambda lambda delta <-> lambda lambda lambda (or its equivalent) but not delta delta delta <-> delta lambda delta <-> delta lambda lambda <-> lambda lambda lambda. In the M(dien), M(nn(3)) and M(xp(3)) systems the conformation preferred is linke d to the metal coordination geometry. In particular, the presence of an N-M -N angle approaching 180 degrees in a mer-octahedral or square-based pyrami d stereochemistry in M(dien) species enforces a delta lambda conformation. Similarly, related mer-like stereochemistry in M(nn(3)) and M(xp(3)) comple xes leads to a B-type conformation. Longer M-N (or M-P) distances enforce m ore puckered, symmetrical, twist conformations of five-membered rings. A ne w pseudo-principal component analysis method is introduced which allows qua ntitative comparison of conformations in analogous but not identical ring s ystems. Conformations in M(en) are compared with M(dien) or M(nn(3)), with the single-ring system exhibiting less distorted conformations. Phosphine s ystems exhibit greater variation of conformation than their amine counterpa rts. In particular M(dmpe) systems show considerably more variation than M( tmeda) whereas M(dppe) species are more varied in conformation than are M(d mpe).