Superbulky Ligands and Trapped Electrons: New Perspectives in Divalent Lanthanide Chemistry

Abstract

electron localization · lanthanides · lanthanum · samarium · superbulky ligands have been known since the early decades of the 20th century. EuCl 2 , SmCl 2 , and YbCl 2 were the first to be reported. [1,2] For the elements europium, samarium, and ytterbium all twelve MX 2 halides are known. This is not the case for the elements thulium, dysprosium, and neodymium for which only the halides of the triad chlorine, bromine, and iodine have been synthesized and crystallographically characterized. They structurally bear close resemblance to the respective alkaline-earth metal halides. [3, 4] The electronic configurations of the M 2+ ions of these six elements are 6s 0 5d 0 4f n with n = 4 (Nd), 6 (Sm), 7 (Eu), 10 (Dy), 13 (Tm), and 14 (Yb).These halides are produced as solids either by comproportionation reactions (2 MX 3 + M) [4] or by Wöhlers metallothermic reduction from the trihalides with alkali metals.[4c]The reduction potentials for the reactions M 3+ + e À ! M 2+ range between À0.35 V (M = Eu) and À2.6 V (Nd), [5] the highest values being similar to that of the half cell K/K + (À2.92 V).[6] With the proper choice of ligand, it should be possible to produce these six lanthanides in solution in the oxidation state + 2 by alkali metal (potassium) reduction from trivalent precursors.There were two major discoveries in the outgoing 20th century that boosted the solution chemistry of divalent lanthanides: First, the synthesis of [Sm(C 5 Me 5 ) 2 ] [7] and the discovery that it reduces molecular nitrogen to form the dimeric [Sm 2 (C 5 Me 5 ) 4 N 2 ].[8] Second, the discovery that THF or DME would not be reduced by divalent thulium. [9] [TmI 2 (dme) 3 ][10] followed by [DyI 2 (dme) 3 ] [11] and [NdI 2 -(thf) 5 ] [12] were the first molecular complexes of divalent thulium, dysprosium, and neodymium that could be handled in solution under argon (!) and crystallized. Although these latter three complexes were not organometallic compounds, their existence has stimulated vigorous research with organic ligands and, meanwhile, there are organometallic examples for all of these six lanthanides in the oxidation state + 2. [13] Two strategies have proved successful: 1) The ligands should preferably be (super)bulky organic ligands. 2) The formation of an anionic complex in combination with a bulky cation enhances the stability by its gain in lattice energy. Two examples for the use of superbulky ligands alone were first reported in lectures at a conference on rare-earth metals ("Tage der Seltenen Erden 2007") in Bonn.

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…106 However, chemists never stopped pushing synthetic limits and, recently, low valent lanthanide chemistry has emerged as a fast developing field. [107][108][109] Recent advancements feature the synthesis and characterization of divalent lanthanide complexes for the complete lanthanide series, [110][111][112][113][114][115][116] reduced dinitrogen complexes, [117][118][119][120][121] reduced arene complexes, 26-29, 57, 122-126 and bimetallic reductive cleavage of aromatic C-H bonds. 40,44 Our group focused on the reduction chemistry of rare-earth metal complexes with non-innocent  ligands, followed by a small molecule activation study.…”

Instability of some divalent rare earth ions and photochromic effect

“…It recently has been shown that Ln 2+ ions are accessible for all of the lanthanides except Pm, which was not investigated because of its radioactivity. Complexes of nine new +2 ions (non-classical), La 2+ , Ce 2+ , Pr 2+ , Gd 2+ , Tb 2+ , Ho 2+ , Er 2+ , Y 2+ , and Lu 2+ , were synthesized ( [4,5,7] and references there in). These new complexes had major structural differences compared to complexes of the traditional six +2 ions in that the difference in bond distances between a +2 ion complex and its +3 ion analog was small.…”

“…While for the non-classical Figure 7. Interpolation literary results [64,65,66,67,68,69,70,71,72] are used to estimate the impurity-fluoride distance for the divalent (2,4,6) and trivalent (1,3,5) rare earth ions in CaF 2 (1,2), SrF 2 (3,4) and BaF 2 (5,6) crystals divalent ions distortion is estimated by interpolation data from cubic Ce 3+ , Gd 3+ and Yb 3+ in CaF 2 and shift them upward by 0.02Å ( Figure 7). It should be noted that the slopes of the curves for the divalent and trivalent ions of the same crystal are similar to each other and the slope in BaF 2 is close to the ordinary dependence of the ionic radius of rare earth element (Re 3+ ).…”

“…It was thus a great surprise when it was reported in 2008 that the 5d 1 configuration could be preserved in an organometallic coordination compound [4,5]. It recently has been shown that Re 2+ ions are accessible for all of the lanthanides except Pm, which was not investigated because of its radioactivity.…”

“…It recently has been shown that Re 2+ ions are accessible for all of the lanthanides except Pm, which was not investigated because of its radioactivity. Complexes of nine new +2 ions, La 2+ , Ce 2+ , Pr 2+ , Gd 2+ , Tb 2+ , Ho 2+ , Er 2+ , Y 2+ , and Lu 2+ , were synthesized ( [4,5,6,7] and references there in). The synthesis of these compounds opens a new area of organometallic chemistry of lanthanides as d elements.…”