Synthesis, Structures, and Reactivities of Iron Complexes Bearing an Isoindoline‐Based, Polyprotic Pincer‐Type Pyrazole Ligand

Abstract

The synthesis and properties of polyprotic NNN pincer‐type bis(pyrazole) iron complexes derived from a 1,3‐bis(pyrazol‐3‐ylimino)isoindoline 1 were investigated. When 1 was treated with iron(II) chloride, the paramagnetic, square‐pyramidal complex [FeCl2(LH3)] (2; LH3=3‐(5‐tert‐butylpyrazol‐3‐ylamino)‐1‐(5‐tert‐butylpyrazol‐3‐ylimino)‐1H‐isoindole) was obtained. Reaction of 2 with two equiv. of silver triflate followed by addition of two equiv. of trimethylphosphine yielded the diamagnetic, octahedral complex [Fe(OTf)(PMe3)2(LH3)]OTf (3), which was converted to the carbonyl complex [Fe(CO)(PMe3)2(LH3)](OTf)2 (4). Treatment of 4 with triethylamine led to deprotonation of the chelate backbone to give the isoindolin‐2‐yl complex [Fe(CO)(PMe3)2(LH2)]OTf (5). The structures of the LH3 and LH2 ligands were compared in details. The NH groups in the pincer ligand in 2–5 are accompanied by hydrogen bonds, showing their Brønsted acidic nature. In addition, 3 catalyzed disproportionation of hydrazine, although the catalytic activity was lower than that of a related pyridine‐based pincer complex 6 a.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

“…The reaction of iron(II) chloride with 1,3-bis(1H-pyrazol-3-ylimino)isoindoline results in tautomerization of the ligand to give the 3-amino-1-imino-1H-isoindole complex 44 (Scheme 14). This dichloridoiron(II) complex exhibits reactivities similar to those of the tBu LH 2 complex 19 shown in Scheme 7 [62]. The CO stretching frequency of the carbonyl complex 46 (1963 cm −1 ) is much lower than that of the tBu LH 2 analogue 24a (2005 cm −1 ) [49], indicating the donating nature of the central isoindole unit.…”

“…Importantly, 46 undergoes deprotonation of the chelate backbone rather than the pyrazole groups to afford the isoindolin-2-yl complex 47 bearing a monoanionic bis(pyrazole) pincer ligand. chloridoiron(II) complex exhibits reactivities similar to those of the tBu LH2 comple shown in Scheme 7 [62]. The CO stretching frequency of the carbonyl complex 46 cm −1 ) is much lower than that of the tBu LH2 analogue 24a (2005 cm −1 ) [49], indicatin donating nature of the central isoindole unit.…”

“…The pivotal role of the pyrazole NH groups is suggested by the reactions of the N-methylated derivatives, 20a-Me and 20a-Me2, which are sluggish and much more complicated. The catalytic activities of the amide-substituted complex 70 [72] as well as the isoindoline-based pincer complex 45 [62] are also lower than that of 20a. Control experiments and theoretical calculations [73] led to the proposed mechanism summarized in Scheme 21, featuring multiple and bidirectional proton-coupled electron transfer (PCET) between the metal-ligand bifunctional platform and the hydrazine substrate.…”