Lanthanide(III)/Actinide(III) Differentiation in the Cerium and Uranium Complexes [M(C₅Me₅)₂(L)]^0,+ (L=2,2′‐Bipyridine, 2,2′:6′,2′′‐Terpyridine): Structural, Magnetic, and Reactivity Studies

Abstract

Treatment of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of bipy (Cp*=C(5)Me(5); bipy=2,2'-bipyridine) in THF gave the adducts [M(Cp*)(2)I(bipy)] (M=Ce (1 a), M=U (1 b)), which were transformed into [M(Cp*)(2)(bipy)] (M=Ce (2 a), M=U (2 b)) by Na(Hg) reduction. The crystal structures of 1 a and 1 b show, by comparing the U-N and Ce-N distances and the variations in the C-C and C-N bond lengths within the bidentate ligand, that the extent of donation of electron density into the LUMO of bipy is more important in the actinide than in the lanthanide compound. Reaction of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of terpy (terpy=2,2':6',2''-terpyridine) in THF afforded the adducts [M(Cp*)(2)(terpy)]I (M=Ce (3 a), M=U (3 b)), which were reduced to the neutral complexes [M(Cp*)(2)(terpy)] (M=Ce (4 a), M=U (4 b)) by sodium amalgam. The complexes [M(Cp*)(2)(terpy)][M(Cp*)(2)I(2)] (M=Ce (5 a), M=U (5 b)) were prepared from a 2:1 mixture of [M(Cp*)(2)I] and terpy. The rapid and reversible electron-transfer reactions between 3 and 4 in solution were revealed by (1)H NMR spectroscopy. The spectrum of 5 b is identical to that of the 1:1 mixture of [U(Cp*)(2)I(py)] and 3 b, or [U(Cp*)(2)I(2)] and 4 b. The magnetic data for 3 and 4 are consistent with trivalent cerium and uranium species, with the formulation [M(III)(Cp*)(2)(terpy(*-))] for 4 a and 4 b, in which spins on the individual units are uncoupled at 300 K and antiferromagnetically coupled at low temperature. Comparison of the crystal structures of 3 b, 4 b, and 5 b with those of 3 a and the previously reported ytterbium complex [Yb(Cp*)(2)(terpy)] shows that the U-N distances are much shorter, by 0.2 A, than those expected from a purely ionic bonding model. This difference should reflect the presence of stronger electron transfer between the metal and the terpy ligand in the actinide compounds. This feature is also supported by the small but systematic structural variations within the terdentate ligands, which strongly suggest that the LUMO of terpy is more filled in the actinide than in the lanthanide complexes and that the canonical forms [U(IV)(Cp*)(2)(terpy(*-))]I and [U(IV)(Cp*)(2)(terpy(2-))] contribute significantly to the true structures of 3 b and 4 b, respectively. This assumption was confirmed by the reactions of complexes 3 and 4 with the H(.) and H(+) donor reagents Ph(3)SnH and NEt(3)HBPh(4), which led to clear differentiation of the cerium and uranium complexes. No reaction was observed between 3 a and Ph(3)SnH, while the uranium counterpart 3 b was transformed in pyridine into the uranium(IV) compound [U(Cp*)(2){NC(5)H(4)(py)(2)}]I (6), where NC(5)H(4)(py)(2) is the 2,6-dipyridyl(hydro-4-pyridyl) ligand. Complex 6 was further hydrogenated to [U(Cp*)(2){NC(5)H(8)(py)(2)}]I (7) by an excess of Ph(3)SnH in refluxing pyridine. Treatment of 4 a with NEt(3)HBPh(4) led to oxidation of the terpy(*-) ligand and formation of [Ce(Cp*)(2)(terpy)]BPh(4), whereas similar reaction with 4 b afforded [U(Cp*)(2){NC(5)H(4)(py)(2)}]BPh(4) (6'). Th...

Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide and Lanthanoid-Actinide Complexes Possessing Redox-Active Bridging Ligands

“…Inasmuch as there are only minor differences in the structural parameters between the Th IV and U IV congeners, this effect clearly implicates the role of 5f-and 6d-orbital participation from the actinide ion in enhancing communication between the peripheral lanthanide metal centers. This observation is consistent with our studies of monometallic (C 5 Me 5 ) 2 An IV (An = Th and U) ketimide complexes in which the participation of the metal 5f-orbitals in metal-ketimide bonding is clearly manifest in both spectroscopic and electrochemical properties and is greater for uranium than thorium. 7,8,[29][30][31] The role of the bridging actinide moiety in enhancing communication in these heterotrimetallic systems is acutely illustrated by comparison to the behavior of the structurally analogous {(C 5 Me 5 ) 2 Yb} 2 (µ-1-Me-3,5-tpy 2 -C 6 H 2 ) (15) system in which the (C 5 Me 5 ) 2 An(ketimide) 2 bridge is replaced with a simple phenyl bridge.…”

“…Such differences, specifically the presence of 5f/6d-orbitals in uranium for back-bonding with tpy-π* orbitals, have been noted previously on comparison of isostructural Ce III -and U III -tpy complexes. 2 Overall, the structural data establish the similarity of the complexes across a variety of ion types in both the tetravalent and trivalent ion positions. This similarity allows for meaningful comparison between the other types of characterization for the complexes.…”

“…With substantial extension of 5f-orbitals from the core, early actinide compounds (Th-Pu) have shown signatures of significant covalent metal-ligand bonding, giving rise to unique chemical and physicochemical properties. [1][2][3][4][5][6][7][8] The introduction of multiple metal ions into structural frameworks known to exhibit f-element metal-ligand covalency creates the possibility for metal-metal electronic delocalization and mixed-valency. The ability of f-element materials to form mixed-valent species has direct consequences on their corrosion, speciation, and environmental chemistry.…”

“…[25][26][27][28] As part of these efforts, we reported the synthesis and comprehensive physicochemical characterization of uranium(IV)-and thorium(IV)-bis(ketimide) complexes (Chart 2). Complexes 4a-b and 5a-b were prepared by insertion of benzonitrile and 4'-cyano-2,2':6',2''-terpyridine, respectively, into the actinide-C benzyl bonds of (C 5 Me 5 ) 2 An(CH 2 C 6 H 5 ) 2 (An = Th, U). 6,25 We have observed that 4a-b and 5a-b and related systems show distinct characteristics of metal-ketimide ligand covalency in bonding as judged experimentally through ground-and excited-state electronic absorption spectroscopy, 8,[29][30][31][32] electrochemistry, 30 and magnetic susceptibility studies, 30 complimented by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations.…”

“…We also report our initial efforts to use X-ray absorption spectroscopy for this class of (C 5 Me 5 ) 2 An organometallic actinide complexes for the elucidation of metal oxidation state.…”