Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f -shell. Here, we showcase the use of state-of-the-art magnetic resonance spec- troscopy (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the structure of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, that features a notable structure according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex; taken together, they provide a detailed understanding of this paramagnetic compound. Namely, this complex displays a three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interaction, whose NMR spectroscopic signature was acquired for the first time for a lanthanide paramagnetic species. The electronic configuration of Yb(III)[CH(SiMe3)2]3 is demonstrated to be close to the one of the free Yb(III) ion, with the partially filled f -shell of the Yb atom having little influence on its bonding properties and with minimal delocalization of f -electron density from Yb to the directly bonded carbons.

Designing highly active supported ethylene polymerization catalysts that do not require a co-catalyst to generate electrophilic metal alkyl species is still a challenge despite its industrial relevance. Described herein is the synthesis and characterization of well-defined silica-supported cyclopentadienyl LnII sites (Ln=Yb and Sm) of general formula [(≡SiO)LnCp*]. These well-defined surface species are highly activite towards ethylene polymerization in the absence of added co-catalyst. Initiation is proposed to occur by single electron transfer.

Significance The rational understanding and design of catalysts pose major challenges to chemists. While catalysts are involved in around 90% of industrial chemical processes, their discovery and development are usually based on screening and serendipity. Here, we show through a detailed analysis of the NMR chemical shift that the activity of olefin polymerization and oligomerization catalysts is directly related to the chemical shift of the carbon atom bound to the metal center. This relation is traced to specific frontier molecular orbitals, which induce π-character in the metal-alkyl bond, thereby favoring insertion. This result not only reveals a surprising analogy between olefin polymerization and metathesis, but also establishes chemical shift as a predictive descriptor for catalytic activity in these industrially relevant processes.

High yield syntheses of [(tmtaa)M2(CO)4] from [(tmtaaH)M(COD)] and [M2(COD)2(μ-OH)2] and CO, in toluene are described (tmtaa=6,8,15,17-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine). Crystal structures of the d8–d8 dimetal compounds [(tmtaa)M2(COD)2], M=Rh, and [(tmtaa)Ir2(CO)4] and the d7–d7 [(tmtaa)Ir2(CO)4I2] resulting from oxidative addition of I2 to [(tmtaa)Ir2(CO)4] are analyzed relative to the distortion within the macrocyclic ligand as the M−M bond order increases from zero to one. The oxidative addition reactions of MeI, PhSSPh, and pySSpy to [(tmtaa)Ir2(CO)4] are also described.

Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M-Cα/α' and M···Cβ distances, long Cα/α'-Cβ bond length, and isotropic 13C chemical shifts for both early d0 and late d4 transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have 13C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d0 systems, with upfield shifts to ca. -30 ppm for the α-carbon of metallacycles with higher d n electron counts (n = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ11 ≥ δ22 ≥ δ33) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π*(M-Cα/α') character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*2Yb are dramatically different from those of the 2,2'-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb(III), f(13), and (phen(•-) ) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent ( J. Am. Chem. Soc 2009 , 131 , 6480 ; J. Am. Chem. Soc 2010 , 132 , 17537 ). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy(•-) has only one π*1 orbital of b1 symmetry of accessible energy, but phen(•-) has two π* orbitals of b1 and a2 symmetry that are energetically accessible. The carbon pπ-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp*2Yb(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180-190 °C. When 3,8-Me2phenanthroline is used, the adduct Cp*2Yb(3,8-Me2phen) exists in the solution in a dimer-monomer equilibrium in which ΔG is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me2, and 5,6-Me2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and LIII-edge XANES spectroscopy as a function of temperature and variable-temperature (1)H NMR spectroscopy.

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me3C)3C5H2]2CeH, abbreviated as Cp'2CeH or [Ce]'-H, is reported. The reaction proceeds from Cp'2Ce(2-pyridyl), isolated from the reaction of pyridine with Cp'2CeH, to Cp'2Ce(4,5,6-trihydropyridyl), and then to Cp'2Ce(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp'2Ce(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H2 and temperature. The dependence of the rate on the H2 pressure is associated with the formation of Cp'2CeH, which increases the rate of the first and/or second additions of H2 but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H2 with a reasonably high energy, ΔG(⧧) = 20.5 kcal mol(-1), on Cp'2Ce(2-pyridyl), leading to Cp'2CeH(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp'2CeH to the C(4)═C(5) bond, followed by hydrogenolysis, giving Cp'2Ce(4,5,6-trihydropyridyl) and regenerating Cp'2CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp'2Ce(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp'2CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp'2Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C(4)═C(5) double bond. In the presence of H2, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp'2Ce(2-pyridyl) and piperidine. The structures of Cp'2CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature (1)H NMR spectroscopy.