Structural, molecular mechanics, and DFT study of cadmium(II) in its crown ether complexes with axially coordinated ligands, and of the binding of thiocyanate to cadmium(II)

摘要

The role of relativistic effects (RE) in the structures of Cd(II) complexes with crown ethers, and the reason the ‘soft’ Cd(II) strongly prefers to bind to SCN through N, are considered. The synthesis and structures of [Cd(18-crown-6)(thiourea)2] (ClO4)2.18-crown-6 (1) and [Cd(Cy2-18-crown-6)(NCS)2] (2) are reported. (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane; Cy2-18-crown-6 = cis_anti_cis-2,5,8,15,18,21-hexaoxatricylo[ 20.4.0.0(9,14)]hexacosane). In 1 Cd is coordinated in the plane of the crown which has close to D3d symmetry, with long Cd_O bonds averaging 2.688 _. The two thiourea molecules form relatively short Cd_S bonds that average 2.468 _, with an S_Cd_S angle of 164.30 . This structure conforms with the idea that Cd(II) can adopt a near-linear structure involving two covalently-bound donor atoms (the S-donors) with short Cd_S bonds, which resembles gas-phase structures for species such as CdCl2. The structure of 2 is similar, with the two SCN ligands N-bonded to Cd, with short Cd_N bonds of 2.106 _, and N_Cd_N angle of 180 . The crown in 2 forms long Cd_O bonds that average 2.698 _. Molecular mechanics calculations suggest that a main reason Cd(II) prefers to bind to SCN through N is that when bound through S, the small Cd_S_C angle, which is typically close to 100 , brings the ligand into close contact with other ligands present, and causes steric destabilization. In contrast, the Cd_N_C angles for SCN coordinated through N are much larger, being 171.4 in 2, which keeps the SCN groups well clear of the crown ether. DFT (density functional theory) calculations are used to generate the structures of [Cd(18-crown-6)(H2O)2]2+ (3) and [Cd(18-crown-6)Cl2] (4). In 3, the Cd(II) is bound to only three O-donors of the macrocycle, with Cd_O bonds averaging 2.465 _. The coordinated waters form an O_Cd_O angle of 139.47 , with Cd_O bonds of 2.295 _. In contrast, for 4, the Cd is placed centrally in the cavity of the D3d symmetry crown, with long Cd_O bonds averaging 2.906 _. The Cl groups form a Cl_Cd_Cl angle of 180 , with short Cd_Cl bonds of 2.412 _. With ionically bound groups on the axial sites of[Cd(18-crown-6)X2] complexes, such as with X =H2O in 3, the Cd(II) does not adopt linear geometry involving the two X groups, with long Cd_O bonds to the O-donors of the macrocycle. With covalently-bound X = Cl in 4, short Cd_Cl bonds and a linear [Cl_ Cd_Cl] unit results, with long Cd_O bonds to the crown ether.