Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

Abstract

KEYWORDS N-heterocyclic Phosphenium complexes -NO complexes -π-acceptor ligandselectronic structure -ligand centered reactivity ABSTRACT. N-heterocyclic phosphenium (NHP) and nitrosonium (NO + ) ligands are often viewed as isolobal analogues which share the capability to switch between different charge states and display thus redox 'non-innocent' behavior. We report here on mixed complexes [(NHP)M(CO)n(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)3(NO)] or PPN[Cr(CO)4(NO)], respectively, and fully characterized. The structural and spectroscopic (IR, UV-VIS) data support the presence of carbene-analogue NHP ligands with overall positive charge state and π-acceptor character. Even the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO -) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/NO-complexes is closely akin to that of the Hieber anion, [Fe(CO)3(NO)] -. The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)2(NO)](4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange, or trapped with electrophiles (H + , SnPh3 + ) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.

Reversible Silylium Transfer between P‐H and Si‐H Donors

“…Cis ‐ and trans ‐ 2 a , b show diagnostic downfield 31 P{ 1 H} chemical shifts for signals due to the new phosphenium ligand ( δ =440–441 ppm). The observed preference for trans versus cis geometry in 2 a , b is consistent with the established strong π‐acidity of phosphenium ligands, [15, 17b, 18a, 19, 20] since a trans structure ensures that PR 2 + does not compete with CO ligands for π‐backbonding at a Mo‐based d‐orbital [21] . X‐ray diffraction analysis of crystals of 2 b confirmed the solid state structure of the trans isomer (Figure 1), with bond angles at the phosphenium (P2) consistent with planarity at this sp 2 ‐hybridized center, and a short Mo‐P2 distance of 2.29 Å, relative to the Mo‐phosphine (P1) distance of 2.52 Å.…”

N‐Heterocyclic Phosphido Complexes of Rhodium Supported by a Rigid Pincer Ligand

“…[14][15][16][17][18] The possibility to describe NHP + /NHPligands in these two states makes them comparable to nitrosyls (NO + /NO -) and makes assigning formal oxidation states to the metals and exact electronic descriptions to the redox active ligand abstruse, culminating in a covalent bonding situation where formal oxidation states have lost their significance. [19,20] The notable advantage of NHP + ligands over nitrosyls is that they are sterically and electronically tunable, by changing the N-substituents or the backbone. Due to the inverted binding properties of NHP + ligands, the tunability of their steric and electronic properties and the potential interconversion to the formally NHPstate, NHP's bound to metal fragments could display different reactivities and open many new potential applications if studied in more depth.…”

“…[14][15][16][17][18] The possibility to describe NHP + /NHPligands in these two states makes them comparable to nitrosyls (NO + /NO -) and makes assigning formal oxidation states to the metals and exact electronic descriptions to the redox active ligand abstruse, culminating in a covalent bonding situation where formal oxidation states have lost their significance. [19,20] The nota-dimer [(PPP)Rh(CN t Bu)] 2 (5). Analysis of the solid-state structures and computational studies have provided insight into the electronic structure and bonding in 3-5, revealing that the NHP +/fragments in 3, 4, and 5 are best described using an NHP -/Rh I description.…”

A Series of Dimeric Cobalt Complexes Bridged by N-Heterocyclic Phosphido Ligands

“…Alternatively, a pyramidal geometry about the phosphorus center may indicate two-electron reduction of the NHP + ligand to an anionic NHP – phosphido moiety, leading to a more covalent M–P NHP bond. The two binding modes of NHP +/− s and ambiguity in metal formal oxidation states that ensues draw to mind an interesting analogy to nitrosyl ligands (NO +/0/‑ ) ligands in their coordination chemistry, allowing NHP +/− s to be considered as tunable versions of NO +/0/– . , …”