Moving to dioxaborolanes, with ArBPin/CsF combinations only the free ArBPin was visible by NMR spectroscopy in halocarbon solvents, although solid is present in these reactions. Assessing these mixtures by NMR spectroscopy using an internal standard revealed a significant decrease in the intensity of ArBPin resonances on addition of CsF for 1 and 2. This indicates the formation of poorly soluble (in halocarbon) fluoroborate salts derived from 1 and 2. Thus 1 and 2 do react with CsF consistent with their ability to catalyse fluorination using these solvents. In contrast, no evidence for formation of the fluoroborate was observed on combining CsF/PhBPin (by NMR spectroscopy versus an internal standard which showed no decrease in the amount of PhBPin present in halocarbon solutions). The disparity can be attributed to the lower fluoride affinity of PhBPin which will disfavour reaction with CsF and is presumably why PhBPin is a poor catalyst for nucleophilic fluorination of 6. The para-nitro derivative, 3, also showed no reaction with CsF in CDCl3 (by NMR spectroscopy versus an internal standard), despite 3 having an effectively identical calculated fluoride affinity to that for 1. This is consistent with the relatively poor catalytic performance of 3 in the fluorination of 6 (Table ). Furthermore, in MeCN while 1/2 are converted completely to soluble fluoroborates on reaction with CsF (e.g. Table , entry 7), combining 3 with excess CsF in MeCN led to only ca. 10% of Cs [3-F], with 3 being the dominant boron containing species observed in MeCN solution. Thus despite a similar calculated fluoride affinity to 1, borane 3 is much less disposed to react with CsF in a range of solvents. We propose that this is due to a sufficiently different (to effect reactivity) magnitude of interaction with the Cs + cation in the fluoroborates derived from 1-3. This is tentatively attributed to the C-F⋯Cs + interactions expected using 1 and 2 being stronger than NO2⋯Cs + interactions expected when using 3. Multiple short C-F⋯Cs contacts are present in the solid-state structure of the related salt Cs[FB(neop)(C6H3(CF3)2)] (B; Figure ), which may persist to some extent in solution. Notably, while B was crystallised from THF/pentane no THF was present in the structure coordinating to Cs + , instead a distorted Cs4F4 cubane is formed, with further ligation of Cs + by neopO⋯Cs and C-F⋯Cs interactions (Figure ). The tetrameric nature of the structure of B highlights the propensity of Cs[fluoroborates] to oligomerise in the absence of additional good ligands for caesium. Finally, borane 5 was studied as it is a triorganoborane with the same calculated fluoride affinity as BEt3 but a different environment around the boron centre, which significantly impacts its performance in catalysing nucleophilic fluorination (vide supra). Compound 5 showed no propensity to bind fluoride in halocarbon solvents (by NMR spectroscopy) in contrast to BEt3, consistent with the disparate catalytic nucleophilic fluorination performance observed in DCM. This further confirms that calculated fluoride affinity values must be used with caution for predicting reactivity when there is a coordinating cation present. Using DCM/MeCN mixtures or neat MeCN did enable formation of the fluoroborate, Cs[5-F] (11B = 4.1 ppm 1 JBF = 80 Hz, 19F = -153.6), consistent with the observation of fluorination using this borane in these solvents. This again indicates that interaction of Cs + with MeCN provides a significant contribution to the solubilisation of CsF. ) Å. This is consistent with the presence of a more Lewis acidic caesium centre more strongly interacting with the B-F unity, thereby reducing the B-F bond strength. This is consistent with the observed impact of caesium ligation on the ability of fluoroborates to transfer fluoride from boron to carbon electrophiles. The low formal coordination number of Cs + in Cs[5-F] may explain the disparity in reactivity between 5 and BEt3 towards CsF, particularly in halocarbon solvents. The larger hydrocarbyl groups in 5 may prevent additional interactions to Cs + (e.g. formation of higher CsnFn aggregates containing additional Cs⋯FB interactions) thus leading to unfavourable solvation energetics (and thus no reaction) when 5 is combined with CsF in halocarbon solvents. This again emphasises that appropriate ligation of caesium in Cs[F-BR3] is vital alongside the optimal borane fluoride affinity in enabling borane catalysed phase transfer fluorinations.