Investigating The Impacts Of Internal Electrostatic Fields On Homogeneous Transition Metal Complexes
The use of electrostatic fields to influence chemical reactivity is a rapidly growing field ofstudy and is increasingly invoked as a strong contributor to enzyme catalysis. Analogous research applied to homogeneous inorganic chemistry remains comparatively underexplored, however several recent examples of transition metal complexes containing rigidly oriented internal electrostatic fields have showcased promising advantages associated with this “non-classical” design strategy. Herein, we report a series of tris(2-aminoethyl)amine (tren)-based tris(phosphinimine) ligands (R3P3tren) and their coordination to transition metals. The phosphinimine moieties contained in these ligands are best represented by a zwitterionic resonance picture in which an anionic charge is positioned on nitrogen and a cationic charge is positioned on phosphorous. Coordinating these ligands to Cu(I) ions results in the formation of trigonal pyramidal complexes wherein the cationic phosphonium residues are held in the secondary coordination spheres in close proximity to an open coordination site. Detailed experimental and computational studies on these complexes revealed that the presence of the cationic charges impacts the electronic structures of the complexes to promote unique 2:1:2 splitting of the d-orbital manifolds, which subsequently impacts the experimentally determined Cu(I)/Cu(II) redox potentials. Treatment of these complexes with O2 at low temperature (-85 to -100 °C) in THF results in the formation of end-on (h1) cupric superoxide complexes, as evidenced by spectroscopic, reactivity, and computational studies. The cupric superoxide complex bound by the permethylated ligand derivative, Me3P3tren, is long-lived compared to the majority of literature analogs (t/12 = 10.4 h at -85 °C), and computational analysis implicates the charged phosphonium residues on the periphery of the O2 binding pocket as being important to this stability. Experimental corroboration for this postulation was provided through use of a new series of ligands, CF3-PhMe2P3tren, in which the identity of the substituent at the 4-positions of the phosphinimine phenyl groups was modulated (X = NMe2, H, CF3). The X-PhMe2P3tren ligands impart systematic adjustments to the electronic and secondary sphere electrostatic properties of the copper complexes while maintaining a consistent steric profile in the vicinity of the O2 binding pockets. Key differences in the thermal stabilities and hydrogen atom transfer (HAT) kinetics were observed across this series of h1 cupric superoxide complexes, which are discussed in the context of both intra- and inter-molecular electrostatic interactions. Notably, the most stable cupric superoxide complex of the series (bound by CF3-PhMe2P3tren) displays markedly improved thermal stability compared to the Me3P3tren-bound cupric superoxide complex and can be observed as high as room temperature on a multi-minute timescale. Finally, Me3P3tren was bound to Mo to form a facially bound molybdenum tricarbonyl Me3P3tren-Mo(CO)3, which is compared to several isostructural analogs that were prepared using literature-reported tetradentate ligands with N4 donor sets (tren, tris[2-(dimethylamino)ethyl]amine (Me6tren), and tris(2-pyridylmethyl)amine (TMPA)). Two-electron oxidation of Me3P3tren-Mo(CO)3 in the presence of various counter anions resulted in the formation of trigonal bipyramidal MoII complexes in which two of the three CO ligands dissociated ([Me3P3tren-Mo(CO)]2+). Large differences (ca. 50 cm-1) in the CO stretching frequency for [Me3P3tren-Mo(CO)]2+ were observed depending on the identity of counteranion employed, and computational analysis supports a model in which through-space electrostatic interactions between the anions and [Me3P3tren-Mo(CO)]2+ lead to varying degrees of CO activation.