A study of the acid-base properties of selected amines and fluoroboranes using displacement reactions

Base displacement reactions have been used to study fluoroborane Lewis acids. It was possible to isolate and characterize products from some displacement reactions including the new compounds, triethylenediamine:BH[lowered 2]F, :BHF[lowered 2] and :BF[lowered 3], piperazine:BH[lowered 2]F, :BHF[lowered 2] and :BF[lowered 3] and quinuclidine:BH[lowered 2]F and :BHF[lowered 2]. Some difunctional bases have been included in this work, making it possible to investigate the relative stabilities of mono- and bisborane adducts. Only monoborane adducts were Isolated from difunctional base displacement reactions and a rationale for this behavior is developed. The results of equilibrium base displacement reactions were Interpreted in terms of HSAB theory but it was necessary to modify Pearson's theory to properly account for Important observations. The modification may be expressed as, "hard acids prefer to bind to bases of equivalent hardness and soft acids prefer to bind to bases of equivalent softness." Using this theory it was possible to determine that the hardness-softness order of the Lewis acids considered is (hard to soft): BF[lowered 3]>BHF[lowered 2]>BH[lowered 2]F>BH[lowered 3]. Similarly, the order of softness for the Lewis bases considered was found to be (soft to hard): [diameter][lowered 3]P>(CH[lowered 3])[lowered 3]P>Hn(CH[lowered 2]CH[lowered 2])[lowered 2]NH>H[lowered 2]NCH[lowered 2]CH[lowered 2]NH[lowered 2]>HC(CH[lowered 2]CH[lowered 2])[lowered 3]N>N(CH[lowered 2]CH[lowered 2])[lowered 3]N>(CH[lowered 3])[lowered 2]NH>CH[lowered 3]NH[lowered 2].