Elucidation of a Sc(I) Complex by DFT Calculations and Reactivity Studies

Monovalent lanthanide(I) in borozene complexes

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻ species changes from C_s to C_7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C_7v-LnB₈⁻ clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻ doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻ series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

The most common oxidation state for lanthanides is +3. Here the authors use photoelectron spectroscopy and theoretical calculations to study half-sandwich complexes where a lanthanide center in the oxidation state +1 is bound to an aromatic wheel-like B₈^2- ligand.

Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands

The controlled manipulation of the axial oxo and equatorial halide ligands in the uranyl dipyrrin complex, UO₂Cl(L), allows the uranyl reduction potential to be shifted by 1.53 V into the range accessible to naturally occurring reductants that are present during uranium remediation and storage processes. Abstraction of the equatorial halide ligand to form the uranyl cation causes a 780 mV positive shift in the U^V/U^IV reduction potential. Borane functionalization of the axial oxo groups causes the spontaneous homolysis of the equatorial U-Cl bond and a further 750 mV shift of this potential. The combined effect of chloride loss and borane coordination to the oxo groups allows reduction of U^VI to U^IV by H₂ or other very mild reductants such as Cp*₂Fe. The reduction with H₂ is accompanied by a B-C bond cleavage process in the oxo-coordinated borane.

Synthesis of rare-earth-metal-in-cryptand dications, [Ln(2.2.2-cryptand)]2+, from Sm2+, Eu2+, and Yb2+ silyl metallocenes (C5H4SiMe3)2Ln(THF)2

Ln²⁺-in-cryptand complexes (Ln = Sm, Eu, Yb) containing +2 ions surrounded only by neutral ligands are readily formed from silyl metallocenes.

Chemistry of soluble β-diketiminatoalkaline-earth metal complexes with M-X bonds (M=Mg, Ca, Sr; X=OH, Halides, H)

Victor Grignard's Nobel Prize-winning preparation of organomagnesium halides (Grignard reagents) marked the formal beginning of organometallic chemistry with alkaline earth metals. Further development of this invaluable synthetic route, RX+Mg→RMgX, with the heavier alkaline earth metals (Ca and Sr) was hampered by limitations in synthetic methodologies. Moreover, the lack of suitable ligands for stabilizing the reactive target molecules, particularly with the more electropositive Ca and Sr, was another obstacle. The absence in the literature, until just recently, of fundamental alkaline earth metal complexes with M-H, M-F, and M-OH (where M is the Group 2 metal Mg, Ca, or Sr) bonds amenable for organometallic reactions is remarkable. The progress in isolating various unstable compounds of p-block elements with β-diketiminate ligands was recently applied to Group 2 chemistry. The monoanionic β-diketiminate ligands are versatile tools for addressing synthetic challenges, as amply demonstrated with alkaline earth complexes: the synthesis and structural characterization of soluble β-diketiminatocalcium hydroxide, β-diketiminatostrontium hydroxide, and β-diketiminatocalcium fluoride are just a few examples of our contribution to this area of research. To advance the chemistry beyond synthesis, we have investigated the reactivity and potential for applications of these species, for example, through the demonstration of dip coating surfaces with CaCO(3) and CaF(2) with solutions of the calcium hydroxide and calcium fluoride complexes, respectively. In this Account, we summarize some recent developments in alkaline earth metal complex chemistry, particularly of Mg, Ca, and Sr, through the utilization of β-diketiminate ligands. We focus on results generated in our laboratory but give due mention to work from other groups as well. We also highlight the closely related chemistry of the Group 12 element Zn, as well as the important chemistry developed by other groups using the complexes we have reported. Although Mg and Ca are more abundant in living organisms, no other metal has as many biological functions as Zn. Thus Zn, the nontoxic alternative to the heavier Group 12 elements Cd and Hg, occupies a unique position ripe for further exploration.

Synthesis and Structural Characterization of β-Diketiminate−Lanthanide Amides and Their Catalytic Activity for the Polymerization of Methyl Methacrylate and ε-Caprolactone

The synthesis and catalytic activity of lanthanide monoamido complexes supported by a beta-diketiminate ligand are described. Donor solvents, such as DME, can cleave the chloro bridges of the dinuclear beta-diketiminate ytterbium dichloride {[(DIPPh)2nacnac]YbCl(mu-Cl)3Yb[(DIPPh)2nacnac](THF)} (1) [(DIPPh)2nacnac = N,N-diisopropylphenyl-2,4-pentanediimine anion] to produce the monomeric complex [(DIPPh)2nacnac]YbCl2(DME) (2) in high isolated yield. Complex 2 is a useful precursor for the synthesis of beta-diketiminate-ytterbium monoamido derivatives. Reaction of complex 2 with 1 equiv of LiNPri2 in THF at room temperature, after crystallization in THF/toluene mixed solvent, gave the anionic beta-diketiminate-ytterbium amido complex [(DIPPh)2nacnac]Yb(NPri2)(mu-Cl)2Li(THF)2 (3), while similar reaction of complex 2 with LiNPh2 produced the neutral complex [(DIPPh)2nacnac]Yb(NPh2)Cl(THF) (4). Recrystallization of complex 3 from toluene solution at elevated temperature led to the neutral beta-diketiminate-lanthanide amido complex [{(DIPPh)2nacnac}Yb(NPri2)(mu-Cl)]2 (5). The reaction medium has a significant effect on the outcome of the reaction. Complex 2 reacted with 1 equiv of LiNPri2 and LiNC5H10 in toluene to produce directly the neutral beta-diketiminate-lanthanide amido complexes 5 and [{(DIPPh)2nacnac}Yb(NC5H10)(THF)(mu-Cl)]2 (6), respectively. These complexes were well characterized, and their crystal structures were determined. Complexes 4-6 exhibited good catalytic activity for the polymerization of methyl methacrylate and epsilon-caprolactone.

Reactions of Li- and Yb-coordinated N,N′-bis(trimethylsilyl)-β-diketiminates: one- and two-electron reductions, deprotonation, and C–N bond cleavage

The synthesis and characterisation of novel Li and Yb complexes is reported, in which the monoanionic beta-diketiminato ligand has been (i) reduced (SET or 2 [times] SET), (ii) deprotonated, or (iii) C-N bond-cleaved. Reduction of the lithium beta-diketiminate Li(L(R,R'))[L(R,R')= N(SiMe(3))C(R)CHC(R')N(SiMe(3))] with Li metal gave the dilithium derivative [Li(tmen)(mu-L(R,R'))Li(OEt(2))](R = R'= Ph; or, R = Ph, R[prime or minute]= Bu(t)). When excess of Li was used the dimeric trilithium [small beta]-diketiminate [Li(3)(L(R,R[prime or minute]))(tmen)](2)(, R = R'= C(6)H(4)Bu(t)-4 = Ar) was obtained. Similar reduction of [Yb(L(R,R'))(2)Cl] gave [Yb[(mu-L(R,R'))Li(thf)](2)](, R = R[prime or minute]= Ph; or, R = R'= C(6)H(4)Ph-4 = Dph). Use of the Yb-naphthalene complex instead of Li in the reaction with [Yb(L(Ph,Ph))(2)] led to the polynuclear Yb clusters [Yb(3)(L(Ph,Ph))(3)(thf)], [Yb(3)(L(Ph,Ph))(2)(dme)(2)], or [Yb(5)(L(Ph,Ph))(L(1))(L(2))(L(3))(thf)(4)] [L(1)= N(SiMe(3))C(Ph)CHC(Ph)N(SiMe(2)CH(2)), L(2)= NC(Ph)CHC(Ph)H, L(3)= N(SiMe(2)CH(2))] depending on the reaction conditions and stoichiometry. The structures of the crystalline complexes 4, 6x21/2(hexane), 5(C(6)D(6)), and have been determined by X-ray crystallography (and have been published).