Requirements for Lewis Acid-Mediated Capture and N–N Bond Cleavage of Hydrazine at Iron

Abstract

An iron complex bearing a pyridine(dicarbene) pincer was designed to probe the requirements of Lewis acid enabled N 2 H 4 capture and subsequent N-N bond cleavage. Appended boron Lewis acids were installed by two methods to circumvent the incompatibilities associated with Lewis acid/base quenching of free carbenes and boranes. N 2 H 4 capture by borane Lewis acids is dependent on both Lewis acidity and the steric profile about boron. A substitutionally inert primary coordination sphere at iron prevents an Fe-N 2 H 4 interaction as well as N-N bond homolysis upon reduction.

A guide to secondary coordination sphere editing

“…Exemplar of this notion, Szymczak and co-workers showed that iron could be coordinated to a pyridine(dicarbene) pincer ligand and subsequently hydroborated. 96 The scope of hydroboration was found to be modular using a series of monohydroboranes (HBR 2 ) where BR 2 = 9-BBN (9-borabicyclo[3.3.1]nonyl), Bsia 2 (sia = siamyl), BCy 2 (Cy = cyclohexyl), and BPin (Pin = pinacol). Exposure of the Bsia 2 -and BCy 2 -compounds, [( BR 2 CNC)Fe(CO) 2 ] to 1 equiv.…”

Requirements for Late-Stage Hydroboration of Pyridine N-Heterocyclic Carbene Iron(0) Complexes: The Role of Ancillary Ligands

“…The differing reactivities for late-stage hydroboration by the set of complexes 1 , 3-PR 3 , and 4-R inform on the requirements for this process and provide a reasonable outline for making reaction predictions. Previously, our work with 1 demonstrated that, in the zerovalent state, both the N-heterocyclic carbenes and the carbonyl ligands were substitutionally inert . In contrast, divalent 2 was incompatible with hydroboration conditions, consistent with an increased lability of the N-heterocyclic carbene and/or acetonitrile ligands.…”

“…Boron Lewis acids provide high steric and electronic tunability and can be incorporated into a metal–ligand construct through a variety of methods. While the addition of hydro- or haloboranes to metallacycles − and diol/boronic acid condensation have been used to install boron groups postmetalation, hydroboration strategies offer the greatest ease, versatility, and generality (Figure , top). , The majority of reports using late-stage hydroboration strategies overcome the compatibility hurdles by using metals that form robust M–L bonds (limiting intramolecular Lewis pair formation), − ancillary ligands that have low affinity for boron Lewis acids (limiting ligand abstraction), or metals that do not readily react with HBR 2 reagents (limiting metal-centered reactivity). , Examples that lack the above properties are rare.…”

“…Previously, we described the successful late-stage hydroboration of ( allyl CNC)­Fe­(CO) 2 ( 1 ; allyl H 2 CNC = 2,6-bis­(3-allyl-1 H -imidazolium)­pyridine)a zerovalent iron complex with substitutionally inert ancillary carbonyl ligands . The hydroborated products marked rare examples of low-valent iron complexes with acids appended in the secondary coordination sphere.…”

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene−π Interactions: Ligand Binding and Ethylene Polymerization

“…1 Incorporation of hydrogen bond donor groups has been reported to support metal oxo and hydroxo ( A , Figure 1), 1a,1b,2 nitrite activation, 3 CO 2 reduction, 4 and other reactivity. 5 Ligands appended with Lewis acidic moieties have also been reported to promote hydrazine bonding at Fe, 6 reductive CO coupling ( B ), 7 selective alkyne hydrogenation, 8 and other small molecule activation. 9 Lewis base incorporation into the secondary coordination sphere of Fe complexes for N 2 reduction, 10 Ni catalysts for proton reduction ( C ), 11 and Co water oxidation catalysts 12 has reported to change catalyst selectivity and activity.…”