To understand why only certain borane/MF combinations are effective fluorination catalysts, their ability to form M[fluoroborate] salts was explored initially. With BEt3 and with 1/2 no change to the NMR spectra (including the amount of borane observed in solution vs. an internal standard) was observed on addition to KF suspended in CHCl3, consistent with the high lattice enthalpy of KF relative to CsF (KF = 194.4 kcal•mol -1 and CsF = 178.7 kcal•mol - 1 ) leading to no reaction and thus no fluorination of 6 using these boranes/KF. In contrast, combining BEt3 with CsF formed the fluoroborate in a range of solvents (Table ). Notably, the NMR spectra for Cs [FBEt3] were significantly different in DCM / CDCl3 (entries 1 and 2) compared to those in MeCN (entry 3), with this solvent dependence attributed to a different aggregation of the Cs[FBEt3] salt. This is supported by DOSY NMR studies which indicated [FBEt3] -was a monomer in MeCN, but exists in larger aggregates in DCM ([Cs(FBEt3)]n with n > 1, vide infra). This is attributed to MeCN being more effective at ligating Cs + than halocarbon solvents, breaking up Csn(-F)n (n > 1) units. A related process would explain the addition of [2.2.2]-cryptand (1.25 eq) to Cs[FBEt3] in halocarbon solvents resulting in a considerable shift in fluoroborate resonances (entries 2 and 4). The cryptand by binding Cs + will weaken the B-F⋯Cs interaction which will increase the B-F bond strength (vide infra). should be the most nucleophilic source of fluoride, due to the downfield shifted 11 B resonance (which is generally associated with less electron density located at boron which would correlate with a weaker B-F bond in this context). This is consistent with the catalytic fluorination results where halocarbon solvents gave better outcomes than using MeCN.