A Non-Trigonal Phosphorus Ligand with Secondary Sphere Fe(II) Binding: Experimental and Theoretical Investigations

Abstract

Metal Ligand Cooperative (MLC) chemistry implements dual active sites towards activation of small molecules. A Literature search through examples of this phenomenon reveals speciation trends regarding ligand active sites and the surrounding ligand scaffolds. As with seminal enzymatic cases of the phenomenon (ie. Fe-Fe hydrogenase), light pnictogen -especially N-atom derivatives serve as the main source of ligand active site(s). Furthermore, most synthetic examples employing 3d metals and amines towards an MLC-construct employ ancillary phosphines. These phosphines ligands serve as strong field supporters, leading toward the production of low-spin complexes and π-back bonding stabilization of metal center(s) in some incidences. Despite the auxiliary delineation for the phosphine ligand, recent reports display examples of standalone phosphorus species, through geometric distortions and supporting N-atom scaffold design, performing activation of substrates. Through such catalytic formalisms as oxidative addition and reductive elimination, a class of non-trigonal phosphorus (NTP) species activate an array of E-H (E=RNH, RO; R = alkyl; aryl) and, in some incidences, aryl C-F bonds. As a means of justification to this phenomenon, the symmetry descent from the more classically nucleophilic trigonal phosphine geometries to a more T-Shaped geometry reportedly decreases the energy gap between HOMO and LUMO frontier orbitals. It is through this low-lying LUMO energy that the NTP P-center is reputed to be if not electrophilic, ambiphilic (-having the capacity towards nucleophilic and electrophilic character). It is the contention of the current work that, through an MLC-inspired activation of substrates, the combination of a similar geometrically distorted PN3 phosphorus center with synthetic concessions made towards binding low-valent 3d metals, can effectively activate small molecules at two active sites through the formation of the subsequent complexes. Towards this end bis[(N-tert-butylureayl)N’-ethyl] amine (H23NPurea) was principally employed due to the 2° amine/ ureayl-N phosphorus binding lacuna and the remaining ureate anionic metal binding sites. The reactivity of the H23NPurea is explored towards phosphorus-ligand assisted activation of select aryl substrates, whereupon the P-atom center works along with the surrounding nucleophilic ureayl-N species towards substrate activation. Kinetics data and theoretical justifications for any claims made towards H23NPurea reactivity will similarly be explored. Through the utilization of documented phosphine selenide JP-Se coupling magnitudes and NBO derived Fukui indices for the corresponding PIII analogs, a comparative of the relative P-center electrophilicity/nucleophilicity of H23NPurea, previously reported NTP-species, and trigonal phosphines will be explored with the secondary effects of the surrounding ligand frameworks briefly considered. The synthesis and reactivity for the FeII congener of H23NPurea, K[Fe(3NPurea)OAc] will be compared/contrasted to those for the standalone phosphorous species (H23NPurea), with documented deviations in substrate activation and the subsequent binding moieties for the FeII and P-atom active sites justified through expanded window 1H-NMR. DFT-derived geometry and frequency optimizations will similarly be applied toward activated substrate binding geometries for K[Fe(3NPurea)OAc].