Homoleptic Imidophosphorane Stabilization of Tetravalent Cerium

Abstract

The homoleptic complexes of cerium with the tris­(piperidinyl)­imidophosphorane ligand, [NP­(pip)3]−, present the most negative Ce3+/4+ redox couple known (<−2.64 V vs Fc/Fc+). This dramatic stabilization of the cerium tetravalent oxidation state [>4.0 V shift from the Ce3+/4+ couple in 1 M HClO4(aq)] is established through reactivity studies. Spectroscopic studies (UV–vis, electron paramagnetic resonance, and Ce L3-edge X-ray absorption spectroscopy), in conjunction with density functional theory studies, reveal the dominant covalent metal–ligand interactions underlying the observed redox chemistry and the dependence of the redox potential on the binding of potassium in the inner coordination sphere.

Comparison of tetravalent cerium and terbium ions in a conserved, homoleptic imidophosphorane ligand field

“…The Ce-N bond lengths are similar to those of the previously reported [Ce 4+ (NP(pip) 3 ) 4 ] (2-Ce(PN)), whose Ce-N bond lengths averaged 2.20(3) A. 31 The degree of structural rearrangement during oxidation is reected in changes in the M-N-P angles for the trivalent and tetravalent complexes. The Ce-N-P bond angles in 2-Ce(PN*) are 163.02 (11) , about 5 smaller than in 1-Ce(PN*).…”

“…For the previously reported Ce 3+ homoleptic imidophosphorane compound, [K][Ce 3+ (NP(pip) 3 ) 4 ] (1-Ce(PN)), the bond lengths were 2.37(1) A and 2.31(1) A for the potassium capped and terminal ligands, respectively. 31 The Ce-N-P bond angles in 1-Ce(PN*) are 168.2(2) on average for both the potassium capped ligands and the terminal ligands. There is no difference on average in deection of the angle for the terminal or potassium bound ligands in 1-Ce(PN*).…”

“…However, this solution behaviour has been shown to occur previously for 1-Ce(PN) and may be attributable to a low barrier to intramolecular potassium exchange within the complex. 31 Comparative Voronoi-Dirichlet polyhedral analysis of 2-Ce(PN*) and 2-Tb(PN*)…”

Dicerium letterbox-shaped tetraphenolates: f-block complexes designed for two-electron chemistry

“…39 The couples can be compared to a range of related O-ligated Ce IV complexes such as [Ce(O t Bu) 4 ( py) 2 ] (E pc = −1.99 V vs. Fc/ Fc + in DCM), 40 Ce(2-( t BuNO)py) 4 (E pc = −1.95 V vs. Fc/Fc + in DCM), 12,40 CeL(O t Bu) 4 (E pc = −2.39 V vs. Fc/Fc + in THF) 9 and imidophosphorane supported complexes Ce(NPPip 3 ) 4 (reduction range of −2.30 < E pc < −2.47 V vs. Fc/Fc + in THF). 10 In order to obtain a more chemically accurate view on the Ce complex oxidation state, HERFD-XAS (high energy resolution fluorescence detection X-ray absorption spectroscopy) was employed. HERFD-XAS provides a method to probe the 5d density of states in detail.…”

“…V vs. Fc/Fc + in [CeL(O t Bu) 2 (THF) 2 ], [L = 1, 10-di(2-tert-butyl-6-diphenylphosphiniminophenolate)ferrocene]. 9 In coordination and organometallic chemistry, various results have shown that the redox potential of the couple is tuneable by introducing different anionic ligands to the cerium ion [10][11][12][13] or by forming ionic 'ate' complexes. 7,14 For example, the alkali metal Ce III ate complexes, [M 3 (THF) n ][Ce(BINOLate) 3 ] (M = Li, Na, K, and Cs, BINOLate = 1,1′-binaphtholate) are readily oxidised to form two types of stable Ce IV complexes.…”