Identification of reaction-relevant Cu complexes. To elucidate the function of reaction components and identify reactionrelevant Cu complexes, [Cu(OAc)2]2•2H2O was first treated with NaI; however, no reaction or denucleation of the paddlewheel was observed by EPR spectroscopy (not shown, see Fig. ). We therefore hypothesized that the reaction initiates by formation of a Cu(II)(phen)n complex. Treatment of [Cu(OAc)2]2•2H2O with phen (2 equiv) and analysis by EPR spectroscopy showed formation of a complex with a spectrum consistent with that of [3]OAc and less consistent with 4 (Figure ). This suggested that [3]OAc was the dominant species arising from reaction-relevant Cu:phen stoichiometry (1:2). Note that to avoid complications in data analysis, 1a was used to avoid off-cycle inhibitory processes that are associated with release of pinacol from reactions using ArBpin (i.e., 1b; see ESI for control experiments). In addition, while it is possible to prepare Cu(II)(phen)3 complexes, 27 these are not readily accessible under conditions relevant to the iododeboronation process and, as such, were discounted from this study. + is likely a key on-cycle species and exists in equilibrium with, or is a precursor to, [5] + , and (iii) iodide transfer is more efficient than chloride. This latter point begins to impact upon understanding of halodeboronation in general, in particular aligning with the more difficult fluorodeboronation process. In situ reaction monitoring and sequential addition of reagents showed productive reactivity was only observed with the addi-tion of NaI prior to 1a (not shown, see ESI S24, S25), suggesting transmetalation occurs after formation of complex [3]I. Attempts to identify Cu(II)(aryl) complexes were unsuccessful, consistent with transmetalation being rate-limiting as indicated for other reactions in this class, e.g., Chan-Lam processes. We hypothesized that Cu(I) complex 6 was produced following reductive elimination of the product from a Cu(III) intermediate. This complex was notable due to the 1:1 Cu:phen ratio, which implied loss of one phen ligand from the proposed intermediate [3] + . To enable catalysis, 6 would require oxidation to Cu(II) in the presence of air (the terminal oxidant used in the iododeboronation process). Oxidation of CuOAc under air was therefore monitored by UV-Vis (Figure ) and in the presence of other reaction-relevant additives. Oxidation was not observed in air or in the presence of NaOAc or substrate 1a. B(OH)3, which has been effective in promoting oxidation in Chan-Lam amination reactions, also had no effect; 17 however, addition of phen (Cu:phen = 1:2) and AcOH was effective at promoting the oxidation. The effect of AcOH is consistent with previous observations in Chan-Lam reactions. The dependence on a Cu:phen ratio of 1:2 for catalyst turnover was shown by the direct use of 10 mol% 4, when a yield of 6% was obtained. When an additional equivalent of phen was included (with respect to Cu loading), this enabled catalytic turnover, resulting in a 70% yield (Table ). The effect of phen stoichiometry on oxidation has was notable: oxidation of [Cu(phen)I] required an 18 h reflux, compared to [Cu(phen)2I], where oxidation was achieved within 20 mins at 70 °C. Heating to a minimum of 30 °C was also required for oxidation to occur (Fig. ), with a reaction yield of 9% (i.e., one catalytic cycle) at room temperature and 70% at 30 ºC (Table ). Moreover, the EPR spectrum of the oxidation in the presence of phen was consistent with that of the proposed [3]I (Figure ). The above data indicated that ligand speciation, specifically relating to phen stoichiometry, varied during the reaction and Cu required one or two phen for specific mechanistic events. To shed more light on this issue and to clarify the details of the iododeboronation reaction mechanism we turned to DFT calculations (Figure ). The geometry computed in solution for the [Cu(phen)2OAc] + cation, [3] + , displays a distorted square-pyramidal geometry (t = 0. Other pathways, including those in which HO -was considered as base, proved higher in energy; in addition, transmetalation with PhBpin gave a higher overall barrier of 26.9 kcal/mol (see ESI for full details).