Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}(8-10) complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)](+/0/-). Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO pi-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)](-) complex, an example of a ls-{FeNO}(10) species, features a true reverse dative Fe -> B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}(8) system, with two additional electrons stored on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}(9) complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N-2 complex, [Fe(TPB)(N-2)](-), which is a key intermediate in Fe(TPB)-catalyzed N-2 fixation. Our present study finds that the Fe -> B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.