Probing the Steric and Electronic Characteristics of a New Bis-Pyrrolide Pincer Ligand

Abstract

A new pincer ligand is synthesized to be dianionic, with the potential to be redox active. It has pyrrrole rings attached to both ortho sites of a pyridine, as the linking element. This H2L can be doubly deprotonated and then used to replace two chloride ligands in MCl2(NCPh)2, to form LM(NCPh) for M = Pd, Pt. The acid form H2L reacts with ZnEt2 with elimination of only 1 mol of ethane to yield (HL)ZnEt, a three-coordinate species with one pendant pyrrole NH functionality. This molecule binds the Lewis base p-dimethylaminopyridine (DMAP) to give first a simple 1:1 adduct that eliminates ethane on heating to form four-coordinate LZn(DMAP), which has an unusual structure due to the strong preference of the pincer ligand to bind in a mer (planar) geometry. A molecule with two HL(-) ligands each bonded in a bidentate manner to FeCl2 is synthesized and shown to contain four-coordinate iron with a flattened-tetrahedral structure. The electrochemistry of LM(NCPh) and (L)Zn(DMAP) shows three oxidation processes, which is interpreted to involve at least two oxidations of the pyrrolide arms.

A cis‐Divacant Octahedral and Mononuclear Iron(IV) Imide

“…The FeN py (Fe1N1=2.021(2) Å) and FeN pyrr (Fe1N2=1.987(1) Å) distances are unexceptional and comparable to those of recently reported [(Hpyrr 2 py) 2 Fe] (e.g. FeN py =2.077(2) Å, FeN pyrr =2.012(2) Å) 8. Notably, the iron center in 1 exhibits a non‐planar coordination geometry, with a deviation of 0.57 Å from the plane defined by the three pyrr 2 py nitrogens, and with the coordinated Et 2 O found cis (N1‐Fe1‐O1=102.37(8)°, N2‐Fe1‐O1=106.03(4)°) to the pyrr 2 py ligand.…”

“…Notably, the iron center in 1 exhibits a non‐planar coordination geometry, with a deviation of 0.57 Å from the plane defined by the three pyrr 2 py nitrogens, and with the coordinated Et 2 O found cis (N1‐Fe1‐O1=102.37(8)°, N2‐Fe1‐O1=106.03(4)°) to the pyrr 2 py ligand. The most valid geometric description of 1 is between trigonal pyramidal and cis ‐divacant octahedral ( τ 4 =0.80),10 reminiscent of that found for the Zn II analogue [(pyrr 2 py)Zn(DMAP)] (DMAP= para ‐(dimethylamino)pyridine) 8. A space‐filling model9 of 1 suggests that the tert ‐butyl groups of the pyrrole α‐carbons effectively block the site trans to the pyridyl nitrogen, preventing adoption of a square‐planar geometry.…”

“…Recently, we reported the synthesis of a bulky bis(pyrrolyl)pyridine pincer (H 2 pyrr 2 py, Scheme ) and its coordination chemistry with a variety of late transition metals, including Fe II 8. Herein, we describe the synthesis of a novel Fe II complex supported by the dianionic pyrr 2 py pincer, namely [(pyrr 2 py)Fe(OEt 2 )], and discuss its clean and direct conversion into the Fe IV imide [(pyrr 2 py)FeNAd].…”

Synthesis and characterization of bis(imino)pyridine complexes of divalent Mg and Zn

“…Additionally, a new redox active ligand system developed by Caulton and coworkers, involving a bis-pyrrolide pincer type ligand, has been used to prepare redox active N-donor Zn complexes. 19 Another approach to synthesize Zn complexes with reduced ligands is chemical reduction of the ligand, followed by salt metathesis with a Zn salt such as ZnCl 2 , and this approach was employed successfully by Wieghardt and coworkers in the synthesis of iminopyridine (IP) complexes of Zn, such as tetrahedral (IP − ) 2 Zn. 20 In a related approach, a Zn macrocyclic complex of the biquinazoline ligand (Mabiq) was obtained, Zn(Mabiq)Cl, and subsequently reduced to Zn(Mabiq − ).…”

Nickel(II) carbonyl, ammonia, and acetonitrile complexes supported by a pyridine dipyrrolide pincer ligand

“…1). The pyridine dipyrrolide is a particularly attractive ligand framework, as several examples of pyridine dipyrrolide pincers with different substituents on the ligand backbone (e.g., Me, t-Bu, Ph, Mes) are known and have already been successfully used with both early and late transition-metal ions such as Ti (Zhang et al, 2016), Zr (Zhang et al, 2016(Zhang et al, , 2020, Cr (Gowda et al, 2018), Mo (Gowda et al, 2018), Fe (Sorsche et al, 2020;Hakey et al, 2019), Co (Grant et al, 2016), Pt (Komine et al, 2014), Pd (Yadav et al, 2018) and Zn (Komine et al, 2014) to form well-defined metal complexes.…”