A systematic study of steric control of the thiocyanate's bonding mode in [Pd(substituted diamine)(thiocyanate)2] and [Pd(substituted diamine)2] (SCN)2 complexes

Article abstract of DOI:10.1016/S0020-1693(00)81895-2

Steric control of the thiocyanate's bonding mode in Pd(II) complexes has been systematically studied by the synthesis and characterization (elemental analysis, IR (solid state and solution), molar conductance, ₁₃C NMR and electronic spectral measurements) of 14 complexes containing bidentate substituted diamines representing a wide range of steric requirements. In the solid state, the diamines producing the least steric hindrance (1,2-diamino-2-methylpropane, 1,2-bis(methylamino)ethane, 1-amino-2-dimethylaminoethane, 2-aminomethylpyridine) were found to form ionic complexes of the type [Pd(diamine)₂-(SCN)₂. Diamines having intermediate steric demands (1,2-bis(dimethylamino)ethane, 1,2-bis(phenylamino)-ethane, 1-amino-2-morpholinoethane, 1-amino-2-pyrrolidinoethane, 1-amino-2-piperidinoethane) produced S-bound [Pd(diamine)(SCN)₂] complexes, whereas those exhibiting the greatest steric requirements (1,2-dipiperidinoethane, 1,2-dipyrrolidinoethane, 1,3-bis(dimethylamino)propane) yielded N-bound [Pd(diamine)(NCS)₂] complexes. Two diamines did not allow these trends: the sterically undemanding 1,2-diaminobenzene produced a neutral S-bound complex, as did the bulky 1,2-dimorpholinoethane. The structure of the 1,2-diamino-2-methylpropane complex was confirmed by the results of a single crystal X-ray structure determination [PdS₂N₆C₁₀H₂₄: a = 6.834(2), b = 10.304(2), c = 11.704(2) A, β = 91.59(2)°, P2₁/c, Z = 2], as was the structure of the [Pd(1-amino-2-pyrrolidinoethane)₂] (SCN)₂ complex resulting from the attempted recrystallization of [Pd(1-amino-2-pyrrolidinoethane)(SCN)₂] in hot DMF [PdS₂N₆C₁₄H₂₈; a = 6.805(9), b = 16.765(31), c = 8.964(10) A, β = 111.22(9)°, P2₁/c, Z = 2]. In DMF solution, all of the neutral S-bound complexes were found to undergo partial or full linkage isomerization to the N-bound forms, with the exception of those containing 1,2-bis(phenylamino)ethane and 1,2-diaminobenzene.