Reactivity of a Borane-Appended 1,1′-Bisphosphinoferrocene Ligand: Transition Metal and Lewis Acid Coordination, Cyclopentadienyl Ring Borylation, and Boronium Cation Generation

The reaction of a borane-appended analogue of 1,1'-bis (phosphino)ferrocene, [Fe(eta(5)-C5H4PPh2)(eta(5)-(C5H4PBu)-Bu-t{(o-BPh2)-C6H4})] (FcPPB), with [W(CO)(6)] under photochemical conditions lead to [W(CO)(4)(FcPPB*)] (1) (FcPPB* = [Fe(eta(5)-C5H4PPh2){eta(5)-C5H3P(Bu-t)C6H4-o-BPh}]), in which the borane has added to a cyclopentadienyl ring of the ligand backbone and benzene has been eliminated. Formation of 1 did not occur under thermal conditions, and benzene solutions of the free FcPPB ligand were unreactive under both photochemical and refluxing conditions. Boranes of greater Lewis acidity than the pendent triarylborane of FcPPB were introduced to free FcPPB in an effort to promote intramolecular electrophilic borylation of the ferrocene backbone by eliminating the borane-phosphine interaction in FcPPB. However, addition of B(C6F5)(3) provided [Fe(eta(5)-C5H4PPh2{B-(C6F5)(3)})(eta(C5H4PBu)-C-5-Bu-t{C6H4(BPh2)-o})], FcPPB{B-(C6F5)(3)} (2), in which B(C6F5)(3) forms an adduct with the -PPh2 group, and the intramolecular phosphine-borane interaction present in free FcPPB is maintained in 2. Furthermore, addition of two equiv of BF3(OEt2) to FcPPB yielded a single diastereomer of [Fe(eta(5)-C5H4PPh2)(eta(5)-(C5H4PBu)-Bu-t{C6H4(BPh)-o})][BF4] (3), in which one B-phenyl substituent has been abstracted to generate a boronium cation {BR2L2+}. Reaction of FcPPB with [AuCl(PPh3)]/GaCl3 afforded dimetallic [{Au(FcPPB)}(2)}[GaCl4](2) (4) as a mixture of diastereomers featuring mu-1 kappa P:2 kappa P'-coordination of each FcPPB ligand and a near linear geometry at each gold(I) center. The pendent borane in 4 interacts only weakly or not at all with gold, but electrophilic addition of the borane to the ferrocene backbone was not observed upon UV irradiation.