Table S2. Crystal data and structure refinement for (PPN)2[Fe(phen)(CN-BPh3)4]. ______________________________________________________________________________ Empirical formula C161 H130 B4 Cl2 Fe N8 P4 Formula weight 2470.59 Temperature 100(2) K Wavelength 1.54178 Å Crystal system Monoclinic Space group C2/c Unit cell dimensions a = 21.782(2) Å a= 90°. b = 17.4783(19) Å b= 94.841(7)°. c = 35.354(5) Å g = 90°. Volume 13412(3) Å 3 Z 4 Density (calculated) 1.224 Mg/m 3 Absorption coefficient 2.151 mm -1 F(000) 5168 Crystal size 0.300 x 0.200 x 0.050 mm 3 Theta range for data collection 3.246 to 74.772°. Index ranges -27 ≤ h ≤ 26, -21 ≤ k ≤ 21, -44 ≤ l ≤ 43 Reflections collected 112475 Independent reflections 13749 [R(int) = 0.0780] Completeness to theta = 67.679° 100.0 % Absorption correction Semi-empirical from equivalents Max. and min. transmission 1.

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Reactions of α- and β-diimine quinoline aldoximes with Mn­(I) and Re­(I) tricarbonyl halides afford quinoline oxime complexes. Both Mn­(I) and Re­(I) complexes experience severe geometric strain due to ligand steric interactions: 6-membered metallocycles exhibit more pronounced distortions than 5-membered ones, consistent with density functional theory structural analyses. Such distortions likely also affect reactivity patterns, as evidenced by Re­(I)-induced deoximation of a quinoline variant containing a CF3-ketoxime.

Local proton sources capable of interacting with catalytic intermediates have been shown to affect proton-dependent reactions. Herein we report the synthesis of a Re(diimine)(CO)3Cl catalyst analog containing a trifluoroethanol-appended bipyridine ligand (bpy-CF3OH), and the corresponding methyl-ether (bpy-CF3OMe) to study the role of the pendent proton donor in electrocatalytic CO2 reduction. Compared to the parent catalyst, Re(bpy)(CO)3Cl, and Re(bpy-CF3OMe)(CO)3Cl, the alcohol analog revealed additional electrocatalytic features and a 200 mV anodic shift in catalytic onset potential. We also show evidence of proton-coupled electron transfer or hydrogen bond-assisted catalysis in all analogs, detailed by a progressive anodic shift in the main catalytic wave with increasing [H2O] which displays slopes of ~120 mV log[H2O]–1. Multiple catalytic mechanisms appeared to occur simultaneously in all the analogs, and attempts were made to differentiate their dependence on [H2O] and [CO2]. The effect of a local proton source is significant under conditions where [H2O] < 1 M but becomes negligible at high [H2O]. These results suggest that the appended alcohol affects the solvation of the molecule, and may act directly as a proton source or as part of a proton shuttle.