Are the carbon monoxide complexes of Cp(2)M (M = Ca, Eu, or Yb) carbon or oxygen bonded? An answer from DFT calculations

The Scandium(II) Carbonyl Complex (C5H2tBu3)2Sc(CO) and Its Isocyanide Analog (C5H2tBu3)2Sc(CNC6H3Me2-2,6)

Treatment of the scandium(II) metallocene Cpttt2Sc (Cpttt = C5H2tBu3) with CO or the isocyanide CNXyl (Xyl = C6H3Me2-2,6) yields the carbonyl complex Cpttt2Sc(CO), 1, or the isocyanide complex Cpttt2Sc(CNXyl), 2, which were identified by X-ray crystallography. Isotopic labeling with 13CO shows the CO stretch of 1 at 1875 cm-1 shifts to 1838 cm-1 in 1-13CO. The CN stretch in 2 is shifted to 1939 cm-1 compared to 2118 cm-1 for the free isocyanide. The 80.1 MHz (28.7 G) 45Sc hyperfine coupling in 1 and 74.7 MHz (26.8 G) in 2 are similar to the 82.6 MHz (29.6 G) coupling constant in Cpttt2Sc and indicate that 1 and 2 are Sc(II) complexes. A comprehensive analysis of the electronic structures of 1 and 2 using DFT calculations is reported.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

Effect of secondary ligands' size on energy transfer and electroluminescent efficiencies for a series of europium(III) complexes, a density functional theory study

In this paper, a quantum chemistry method was used to investigate the effect of different sizes of substituted phenanthrolines on absorption, energy transfer, and the electroluminescent performance of a series of Eu(TTA)(3)L (L = [1,10] phenanthroline (Phen), Pyrazino[2,3-f][1,10]phenanthroline (PyPhen), 2-methylprrazino[2,3-f][1,10]phenanthroline(MPP), dipyrido[3,2-a:2',3'-c]phenazine(DPPz), 11-methyldipyrido[3,2-a:2',3'-c]phenazine(MDPz), 11.12-dimethyldipyrido[3,2-a:2',3'-c]phenazine(DDPz), and benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (BDPz)) complexes. Absorption spectra calculations show that different sizes of secondary ligands have different effects on transition characters, intensities, and absorption peak positions. The larger secondary ligands DPPz, MDPz, DDPz and BDPz lead to incomplete energy transfer from the triplet states of ligands to the (5)D(0) of the Eu(3+) ion compared with smaller ones (PyPhen and MPP) due to their lower S(1) or T(1) state energy levels than that of TTA or (5)D(0) of Eu(3+). "Small polaron" stabilization energy (SPE) results reveal that electron trapping is the dominant electroluminescence (EL) mechanism in these materials due to their lower LUMO energies than 4,4'-N,N'-dicarbazolebiphenyl (CBP). Reorganization energy (l) values show that these materials have better electron than hole transporting properties. In addition, the reasons for the origin of the 500 nm emission in Eu-PyPhen- and Eu-MPP-based OLED devices were investigated, and we suppose this emission may result from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), not from Alq(3).

Reactions of Laser-Ablated La and Y Atoms with CO: Matrix Infrared Spectra and DFT Calculations of the M(CO)x and MCO+ (M = La, Y; x = 1−4) Molecules

Reactions of laser-ablated lanthanum and yttrium atoms with carbon monoxide molecules in solid neon have been investigated using matrix-isolation infrared spectroscopy. The M(CO)x and MCO+ (M = La, Y; x = 1-4) molecules have been formed and identified on the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments. Density functional theory calculations have been performed on these lanthanum and yttrium carbonyls. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts substantiates the identification of these carbonyls from the matrix infrared spectrum. The present study reveals that the C-O stretching vibrational frequencies of MCO+ decrease from Sc to La, which indicates an increasing in metal d orbital --> CO pi* back-donation in this series.

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

Addition of CO to [1,2,4-(Me3C)3C5H2]2CeH,Cp'2 CeH, in toluene yields the cis-(Cp'2Ce)2(mu-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100 degrees C over 7 months. When the solvent is pentane, Cp'2Ce(OCH2)CeCp'2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'2Ce(OCH2)CeCp'2, generating Cp'2CeOCH2COCeCp'2. The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H2) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H2. The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H2 to give Cp2CeOMe is exoergic by 50 kcal mol-1. The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta2-CHO), is formed by further reaction with H2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H2 by CH4 increases the activation energy by 17 kcal mol-1.

Organolanthanide mediated catalytic cycles: a computational perspective

In this perspective the contribution of recent theoretical studies to our understanding of lanthanide (Ln) catalysis is explored. In general, the results of computational studies have proven consistent with available experimental evidence. Considerable success has been obtained in elucidating the mechanisms for C-H bond activation (sigma-bond metathesis in particular) and the addition of C-X bonds across an unsaturated functionality (and the hydroamination of alkenes in particular). Ln catalysts are computationally challenging because relativistic effects are important, and large ligands are required to restrict high coordination numbers, in addition to limiting facile redistribution processes. Thus, key technical issues relating to the computational investigation of organolanthanide complexes are discussed. Increasing computational resources have seen studies expand from the optimisation of simple molecules to the study of catalytic cycles where the Ln is coordinated by larger and more complex ligands. The ability of theoretical studies to complement experimental developments by supplying a deeper understanding of the mechanistic process is reviewed with emphasis on the elucidation of transition state structures, intermediates, spectator ligand coordination, and negative entropy steps. Recent computational investigations of the catalytic cycle for Ln mediated hydroamination are a focus, as these have provided substantial and detailed rationalisations for the regio- and stereo-selectivity of inter- and intra-molecular hydroamination. Examination of transition state geometries and electronic structure appears to offer insights that could be used to facilitate the rational design of new Ln-based catalysts.

DFT studies of the methyl exchange reaction between Cp2M–CH3or Cp*2M–CH3(Cp = C5H5, Cp* = C5Me5, M = Y, Sc, Ln) and CH4. Does M ionic radius control the reaction?

The activation energies for the methyl exchange reactions between Cp2M-CH3 and H-CH3 have been calculated for M = Sc, Y and representative metals of the lanthanide family (La, Ce, Sm, Ho, Yb and Lu) with DFT(B3PW91) calculations with large-core pseudopotentials for M. The sigma-bond metathesis reactions are calculated to have lower activation energies for early lanthanides than for late lanthanides and any of group 3 metals. The relative activation barriers are analyzed using the NBO charge distributions in the reactant and in the transition states. It is shown that the methane needs to be polarized in the transition state as H((+delta))-CH3((-delta)) by the reactant, because this sigma-bond metathesis is best viewed as heterolytic cleavage of methane, leading to a proton transfer between two methyl groups in the field of an electropositive M metal. Early lanthanides, which are involved in strongly ionic metal-ligands bonds are thus associated with the lowest activation energies. The ionic radius and the steric effects influence the relative rates of reaction for the complexes of Sc, Y and Lu. In agreement with earlier works of Sherer et al., the experimental reactivity trends found by Tilley are reproduced best with Cp*2M-CH3 (Cp* = C5Me5) rather than Cp2M-CH3 (Cp = C5H5) because the steric bulk of C5Me5 deactivates most the complex where the metal has the smallest ionic radius (Sc). While the steric effects and the influence of the metal ionic radius cannot be neglected, these factors are not the only ones involved in determining the activation barriers of the sigma-bond metathesis reaction.

Comparative reactivity of sterically crowded nf3 (C5Me5)3Nd and (C5Me5)3U complexes with CO: formation of a nonclassical carbonium ion versus an f element metal carbonyl complex

Sterically crowded isoelectronic nf(3) (C(5)Me(5))(3)M complexes of neodymium and uranium, compounds which have unconventionally long metal ligand distances, are found to react very differently with CO as a substrate. The 4f(3) complex (C(5)Me(5))(3)Nd reacts with CO to form a nonclassical carbonium ion complex, (C(5)Me(5))(2)Nd(O(2)C(7)Me(5)), which contains a three-coordinate planar carbon. (C(5)Me(5))(3)U reacts with CO to form an even more crowded CO adduct through a reaction type never observed before for (C(5)Me(5))(3)M compounds. The rare uranium carbonyl complex, (C(5)Me(5))(3)U(CO), has nu(CO) = 1922 cm(-1) and a U-C(CO) distance of 2.485(9) A.