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

Abstract

Well-defined dinuclear lanthanide complexes: stabilisation of both CeIII and CeIV.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

“…Recently, tetraphenolate ( p TP = [{2‐(OC 6 H 2 R 2 ‐2,4) 2 CH}‐C 6 H 4 ‐1,4] 4− ) ligands were used to access the metallacyclic 2+2 [Ce IV 2 ( p TP R ) 2 (THF) 4 ] ( 35 ) (R = Me, t Bu) with an unusual “letterbox” topology. This was achieved via the oxidation of the Ce III analogue [K(THF) n ][Ce 2 ( p TP R ) 2 (THF) 4 K] ( 36 ) with a range of oxidants (CuX 2 [X = Cl, OTF), I 2 , XeF 2 , HgX 2 (X = Cl, I, OAc)], involving a rare concerted two‐electron redox process within a single molecule [90] . Solution electrochemistry (THF with 0.1 m [ n Bu 4 N][PF 6 ]) indicates significant stabilization of the Ce IV cation (−1.83 V vs. Fc/Fc + ).…”

Dinuclear Ce(IV) Aryloxides: Highly Active Catalysts for Anhydride/Epoxide Ring-Opening Copolymerization

“…The syn conformation is drawn for simplicity for each cerium complex in Scheme 1, although both conformers have been observed by us in X-ray diffraction studies of related metal complexes. 46,47,52 X-ray structural analyses of some of the bimetallic group 1, 5, and 13 complexes of the phenols H 4 (pTP t ) and H 4 (mTP t ) also confirm the rotational freedom of the central arene bridge, although to our knowledge, no solution-state studies of their behavior have been reported. 46,49,50,53 1 H− 1 H NOESY NMR spectroscopic experiments were conducted to investigate the ambient solution-state conformation for the siloxide and aryloxide complexes.…”

“…Only a few solid state structures exist of TP complexes in the syn conformation, in the letterbox-shaped di-U IV complex [U 2 (mTP) 2 ], the M-M distance is 6.57 Å, 52 and in the open di-Ce III complex [Ce 2 (BH 4 ) 2 (pTP t )], the M-M distance is 10.85 Å. 47 For example, there is a maximum separation in the anti-conformation of 9.32 Å in [U 2 I 4 (mTP t )(THF) 4 ] and 10.40 Å in [U 2 I 4 (pTP t )(THF) 4 ]. 46,67 To further test the effect of metal-metal distance on catalytic activity, we have also made the new terphenyl bridged complex 4 in which the Ce-Ce distance is far too large for cooperativity.…”

“…We have recently shown that a tetraphenolate (TP) ligand platform is capable of stabilizing f-block metal centers in multiple oxidation states and particularly stabilizes the Ce IV oxidation state relative to Ce III . 46,47 Bimetallic complexes of the phenols H 4 (pTP t ) and H 4 (mTP t ) have been demonstrated to be active ring-opening polymerization initiators for monomers, including lactide (by H 2 K 2 (pTP t ) and H 2 K 2 (mTP t ) adducts), 48 epoxide (by bis-Al III adducts of pTP t ), 49 and εcaprolactone (by [MCl 2 (OEt)(NCMe)] 2 (pTP t ), where M = Nb, Ta). 50 Herein, we report the synthesis of a series of new dinuclear cerium(IV) complexes supported by this TP platform that are even more strongly stabilizing of the Ce IV oxidation state, their performance as rapid, single-component catalysts for the ROCOP of cyclic anhydrides and epoxides, and how the polymerization is fastest when the two metal centers can approach each other closely.…”

Intermediate Valence States in Lanthanide Compounds

“…Very recently, high‐energy‐resolution fluorescence detection X‐ray absorption spectroscopy (HERFD‐XAS) has been used to analyse a formal di‐cerium(IV) phenolate complex and confirmed mixed valency with considerable 4f 0 character [138] . In addition, the multiconfigurational ground state of formal Ce IV complexes stabilised by imidophosphorane ligands or imidophosphorane‐functionalised guanidinate ligands has been established through Ce L 3 ‐edge XANES studies [139–141] .…”