Manganese–Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Abstract

Hydrogen-bonds (H-bonds) within the secondary coordination sphere are often invoked as essential noncovalent interactions that lead to productive chemistry in metalloproteins. Incorporating these types of effects within synthetic systems has proven a challenge in molecular design that often requires the use of rigid organic scaffolds to support H-bond donors or acceptors. We describe the preparation and characterization of a new hybrid tripodal ligand ([H2pout]3–) that contains two mono-deprotonated urea groups and one phosphinic amide: the urea groups serve as H-bond donors while the phosphinic amide group serves as a single H-bond acceptor. The [H2pout]3– ligand was utilized to stabilizes a series of Mn–hydroxido complexes in which the oxidation state of the metal center ranges from 2+ to 4+. The molecular structure of MnIII–OH complex demonstrates that three intramolecular H-bonds involving the hydroxido ligand are formed. Additional evidence for the formation of intramolecular H-bonds was provided by vibrational spectroscopy in which the energy of the O–H vibration supports its assignment as an H-bond donor. The step-wise oxidation of [MnIIH2pout(OH)]2- to its higher oxidized analogs was further substantiated by electrochemical measurements and results from electronic absorbance and electron paramagnetic resonance spectroscopies. Our findings illustrate the utility of controlling both the primary and secondary coordination spheres to achieve structurally similar Mn–OH complexes with varying oxidation states.

Effects of Noncovalent Interactions on High-Spin Fe(IV)–Oxido Complexes

“…This design principle was first incorporated in complexes of our tripodal ligands with sulfonamido groups ([RST] 3– , Figure D) that produced a variety of discrete heterobimetallic complexes with M III −(μ–OH)−LA II cores (M III = Mn, Fe, Co, Ga). − In these systems, two SO groups and the hydroxido ligand form an auxiliary site that is capable of facially binding a second metal ion. However, our studies found that sulfonamido groups do not provide a strong enough ligand field to stabilize more highly oxidized complexes. ,, We reasoned that redesign of the ligand to [poat] 3– would provide access to an Fe IV –oxido species capable of supporting a LA II at its auxiliary binding site and allow us to evaluate changes in the structural and spectroscopic properties of the Fe IV –oxido species upon LA II binding.…”

“…To further evaluate the effects of these intramolecular H-bonding interactions on Fe IV –oxido species, we prepared the ligand [poat] 3– , which contains deprotonated phosphinic amide groups instead of ureas. The design of this ligand was inspired by the work of Stephen and the ability of the ligand to stabilize high-valent complexes relies on the fact that deprotonated phosphinic amides and ureas produce comparable ligand fields . However, the local environments that surround the Fe–oxido centers in complexes of [poat] 3– are different from those in complexes of [H 3 buea] 3– because the [poat] 3– ligand lacks the intramolecular H-bond donors contained in [H 3 buea] 3– .…”

“…The design of this ligand was inspired by the work of Stephen 60 and the ability of the ligand to stabilize high-valent complexes relies on the fact that deprotonated phosphinic amides and ureas produce comparable ligand fields. 61 However, the local environments that surround the Fe−oxido centers in complexes of [poat] 3− are different from those in complexes of [H 3 buea] 3− because the [poat] 3− ligand lacks the intramolecular H-bond donors contained in [H 3 buea] 3− . Thus, our preparation of [Fe IV poat-(O)] − provides the ability to directly determine the influence of H-bonds on the properties of Fe IV −oxido centers by establishing an analog without these noncovalent interactions.…”

“…However, our studies found that sulfonamido groups do not provide a strong enough ligand field to stabilize more highly oxidized complexes. 61,64,65 We reasoned that redesign of the ligand to [poat] 3− would provide access to an Fe IV −oxido species capable of supporting a LA II at its auxiliary binding site and allow us to evaluate changes in the structural and spectroscopic properties of the Fe IV −oxido species upon LA II binding.…”

A guide to secondary coordination sphere editing

“…In a subsequent piece, Borovik and co-workers reported a ligand bearing two urea groups along with a phosphinic amide arm (Scheme 20, bottom right). 54 In this example, a {[Mn(III)]-OH} compound was accessed that bore three intramolecular H-bonding interactions where the phosphinic amide served as an acceptor and the urea arms served as donors. Such {[Mn(III)]-OH} compounds are rare, as they often react to form ill-defined multinuclear species.…”