Dinuclear Complexes of Chiral Tetradentate Pyridylimine Ligands:Diasteroeoselectivity,Positive Cooperativity,anion Selectivity,Ligand Self-Soring Based on Chirality,and Magnetism

摘要

The synthesis and coordination chemistry of two chiral tetradentate pyridylimine Schiff base ligands ae reported.The ligand were prepared by the nucleophilic displacement of both bromides of 1,3-(bromoethyl)benzene (2) or 3,5-bis(bromomethyl)toluene(3) by the anion of (s)-valinol,followed by capping of both amine groups iwth pyridien020carboxaldehyde.both lignads react withCoCl_2 and NiCl_2 to give [M_2L_2Cl_2]~2+ complexes.Remarkably,neither fluoride nor bromide ions can act as bridging ligands.The formation of [Co_2((S)-3)_2Cl_2]~2+ is highly diastereoselective,and X-ray crystallography shows that both metal centers in the [Co_2((S)-3)_2Cl_2](CoCl_4) complex adopt the LAMBDA configuration (crystal data:[Co_2(C_31H_40N_4O_2)_2Cl_2](CoCl_4)(CH_3CN)_3,monoclinic,P2_1,a=11.595(2) A,b=22.246(4)A,C=15.350(2)A,V=3705(1)A~3,bta=110.643(3)deg,Z=2).Structurally ,the dinuclear ocjplex can be viewed as a helicate with the helical saxis running perpendicular to the [Co_2Cl_2] plane.the reaction of racemic 2 with CoCl_2 was shown by ~1H NMR spectroscopy to yield a racemic mixture of LAMBDA,LAMBDA-[Co_2((S)-2)_2Cl_2]~2+ and DELTA,DELTA0[Co_2((R)-2)_2Cl_2]~2+ complexes;that is,a homochiral recognition process takes place.spectrophotometric titrations were performed by titrating (S)-3 with Co(ClO_4)_2 followed by Bu_4NCl,and theglobal stability constnats of [Co_2((S)-3)]~2+ (log beta-110=5.7),[Co_2((S)-3)_2]~2+ (logbeta_120=11.6),and [Co_2((S)-3)_2Cl_2]~2+(log beta_110=23.8)were calcualted.The results revealed a strong positive cooperativity in theformation of [Co_2((S)-3)_2Cl_2]~2+.Variable-temperature magnetic susceptibility curves for [Co_2((S)-3)_2Cl_2](BPh_4)_2 and [Co_2((S)-3)_2Cl_2](BPh_4)_2 are very similar and indicate that there are no significant magnetic interactions between the cobalt(II) centers.