η⁵–η¹ Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR ¹<i>J</i>_Yb–P Coupling Constants

This paper reports the synthesis of a series of complexes based on the bis­(pentamethylcyclopentadienyl)­ytterbium­(II) (<b>1</b>; Cp*₂Yb) and bis­(tetramethylphospholyl)­ytterbium­(II) (<b>2</b>; Tmp₂Yb) fragments bearing an additional neutral bis­(methyliminophosphoranyl)­pyridine ligand (<b>L</b>) on which the steric demand is modulated at the phosphorus position (triethyl, <b>L</b>^<b>Et</b>; triphenyl, <b>L</b>^<b>Ph</b>; tricyclohexyl, <b>L</b>^<b>Cy</b>) to yield the original complexes Cp*₂Yb<b>L</b>^<b>Et</b> (<b>1-L</b>^<b>Et</b>), Cp*₂Yb<b>L</b>^<b>Ph</b> (<b>1-L</b>^<b>Ph</b>), Tmp₂Yb<b>L</b>^<b>Et</b> (<b>2-L</b>^<b>Et</b>), Tmp₂Yb<b>L</b>^<b>Ph</b> (<b>2-L</b>^<b>Ph</b>), and Tmp₂Yb<b>L</b>^<b>Cy</b> (<b>2-L</b>^<b>Cy</b>), while no reaction occurs between <b>1</b> and <b>L</b>^<b>Cy</b>. The crystal structures of these sterically crowded complexes are reported as well as room-temperature NMR data for all the complexes. The solid-state coordination mode of <b>L</b>^<b>R</b> differs depending on the nature of the fragments <b>1</b> and <b>2</b> and on the steric bulk of <b>L</b>^<b>R</b>. The crystal structure of the divalent Tmp₂Yb­(py)₂ (<b>3</b>) is also reported for structural and spectroscopic comparisons. Interestingly, in both <b>2-L</b>^<b>Et</b> and <b>2-L</b>^<b>Cy</b>, one of the two Tmp ligands coordinates in an η¹ rather than in an η⁵ fashion, a relevant coordination mode for the study of sterically induced reductions. The behavior of those complexes in solution varies with the sterics and electronics of the ligands, as demonstrated by variable-temperature NMR experiments. In solution, the ¹<i>J</i>_Yb–P coupling is used to track the coordination mode of the Tmp ligand and a large difference in the ¹<i>J</i>_Yb–P coupling constant allows the distinction between an η⁵ coordination mode and a dynamic η⁵–η¹ switch

Multiple One-Electron Transfers in Bipyridine Complexes of Bis(phospholyl) Thulium

The synthesis of original neutral bis­(phospholyl) thulium complexes, Dtp₂Tm­(L), where L is tetramethylbiphosphinine (tmbp) and bipyridine (bipy), is reported. The electronic structures of these complexes have been investigated and it appears that, in both cases, an electron transfer occurs from the divalent metal to the ligand, a consequence of the strong reduction potential of the bis­(phospholyl) thulium fragment, Dtp₂Tm. When 1 equiv of bipyridine is added to the Dtp₂Tm­(tmbp) complex, another electron transfer occurs to form the Dtp₂Tm­(bipy) complex along with free tmbp ligand. Astonishingly, despite the apparent trivalent nature of the thulium center, the Dtp₂Tm­(bipy) complex is still reactive toward neutral bipyridine to form a new complex in which one phospholyl ligand is replaced by a bipyridine radical anion. An experimental kinetic analysis is reported to rationalize this unprecedented redox reaction with thulium and reveals an associative type of mechanism

Multiple One-Electron Transfers in Bipyridine Complexes of Bis(phospholyl) Thulium

The synthesis of original neutral bis­(phospholyl) thulium complexes, Dtp₂Tm­(L), where L is tetramethylbiphosphinine (tmbp) and bipyridine (bipy), is reported. The electronic structures of these complexes have been investigated and it appears that, in both cases, an electron transfer occurs from the divalent metal to the ligand, a consequence of the strong reduction potential of the bis­(phospholyl) thulium fragment, Dtp₂Tm. When 1 equiv of bipyridine is added to the Dtp₂Tm­(tmbp) complex, another electron transfer occurs to form the Dtp₂Tm­(bipy) complex along with free tmbp ligand. Astonishingly, despite the apparent trivalent nature of the thulium center, the Dtp₂Tm­(bipy) complex is still reactive toward neutral bipyridine to form a new complex in which one phospholyl ligand is replaced by a bipyridine radical anion. An experimental kinetic analysis is reported to rationalize this unprecedented redox reaction with thulium and reveals an associative type of mechanism

Thermal Dihydrogen Elimination from Cp*₂Yb(4,5-diazafluorene)

The reaction of 4,5-diazafluorene with Cp*₂Yb­(OEt₂), where Cp* is pentamethylcyclopentadienyl, affords the isolable adduct Cp*₂Yb­(4,5-diazafluorene) (<b>1</b>), which slowly eliminates H₂ to form Cp*₂Yb­(4,5-diazafluorenyl) (<b>2</b>); the net reaction is therefore <b>1</b> → <b>2</b> + H^•. The ytterbium atom in <b>1</b> is shown to be intermediate valent by variable-temperature L_III-edge X-ray absorption near-edge (XANES) spectroscopy, consistent with its low effective magnetic moment (μ_eff). The experimental studies are supported by complete active space self-consistent field (CASSCF) calculations, showing that two open-shell singlets lie below the triplet state. The two open-shell singlets are calculated to be multiconfigurational and closely spaced, in agreement with the observed temperature dependence of the XANES and χ data, which are fit to a Boltzmann distribution. A mechanism for dihydrogen formation is proposed on the basis of kinetic and labeling studies to involve the bimetallic complex (Cp*₂Yb)₂(4,5-diazafluorenyl)₂, in which the heterocyclic amine ligands are joined by a carbon–carbon bond at C(9)–C(9′)

Thermal Dihydrogen Elimination from Cp*₂Yb(4,5-diazafluorene)

The reaction of 4,5-diazafluorene with Cp*₂Yb­(OEt₂), where Cp* is pentamethylcyclopentadienyl, affords the isolable adduct Cp*₂Yb­(4,5-diazafluorene) (<b>1</b>), which slowly eliminates H₂ to form Cp*₂Yb­(4,5-diazafluorenyl) (<b>2</b>); the net reaction is therefore <b>1</b> → <b>2</b> + H^•. The ytterbium atom in <b>1</b> is shown to be intermediate valent by variable-temperature L_III-edge X-ray absorption near-edge (XANES) spectroscopy, consistent with its low effective magnetic moment (μ_eff). The experimental studies are supported by complete active space self-consistent field (CASSCF) calculations, showing that two open-shell singlets lie below the triplet state. The two open-shell singlets are calculated to be multiconfigurational and closely spaced, in agreement with the observed temperature dependence of the XANES and χ data, which are fit to a Boltzmann distribution. A mechanism for dihydrogen formation is proposed on the basis of kinetic and labeling studies to involve the bimetallic complex (Cp*₂Yb)₂(4,5-diazafluorenyl)₂, in which the heterocyclic amine ligands are joined by a carbon–carbon bond at C(9)–C(9′)

Influence of the Torsion Angle in 3,3′-Dimethyl-2,2′-bipyridine on the Intermediate Valence of Yb in (C₅Me₅)₂Yb(3,3′-Me₂‑bipy)

The synthesis and X-ray crystal structures of Cp*₂Yb­(3,3′-Me₂bipy) and [Cp*₂Yb­(3,3′-Me₂bipy)]­[Cp*₂YbCl_1.6I_0.4]·CH₂Cl₂ are described. In both complexes, the NCCN torsion angles are approximately 40°. The temperature-independent value of <i>n</i>_f of 0.17 shows that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yields a <i>n</i>_f value of 0.27; that is, the two configurations in the wave function are f¹³(π*₁)¹ and f¹⁴(π*₁)⁰ in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (<i>n</i>_f accounts for f-hole occupancy; that is, <i>n</i>_f = 1 when the configuration is f¹³ and <i>n</i>_f = 0 when the configuration is f¹⁴). A correlation is outlined between the value of <i>n</i>_f and the individual ytterbocene and bipyridine fragments such that, as the reduction potentials of the ytterbocene cation and the free x,x′-R₂-bipy ligands approach each other, the value of <i>n</i>_f and therefore the f¹³:f¹⁴ ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases

Carbon–Hydrogen Bond Breaking and Making in the Open-Shell Singlet Molecule Cp*₂Yb(4,7-Me₂phen)

The adducts formed between the 4,7-Me₂-, 3,4,7,8-Me₄-, and 3,4,5,6,7,8-Me₆-phenanthroline ligands and Cp*₂Yb are shown to have open-shell singlet ground states by magnetic susceptibility and L_III-edge XANES spectroscopy. Variable-temperature XANES data show that two singlet states are occupied in each adduct that are fit to a Boltzmann distribution for which Δ<i>H</i> = 5.75 kJ mol^–1 for the 4,7-Me₂phen adduct. The results of a CASSCF calculation for the 4,7-Me₂phen adduct indicates that three open-shell singlet states, SS1–SS3, lie 0.44, 0.06. and 0.02 eV, respectively, below the triplet state. These results are in dramatic contrast to those acquired for the phenanthroline and 5,6-Me₂phen adducts, which are ground state triplets (J. Am. Chem. Soc. 2014, 136, 8626). A model that accounts for these differences is traced to the relative energies of the LUMO and LUMO+1 orbitals that depend on the position the methyl group occupies in the phenanthroline ligand. The model also accounts for the difference in reactivities of Cp*₂Yb­(3,8-Me₂phen) and Cp*₂Yb­(4,7-Me₂phen); the former forms a σ C–C bond between C(4)­C(4′), and the latter undergoes C–H bond cleavage at the methyl group on C(4) and leads to two products that cocrystallize: Cp*₂Yb­(4-(<i>CH</i>₂),7-Mephen), which has lost a hydrogen atom, and Cp*₂Yb­(4,7-Me₂-<i>4H</i>-phen), which has gained a hydrogen atom

Reversible Sigma C–C Bond Formation Between Phenanthroline Ligands Activated by (C₅Me₅)₂Yb

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), f¹³, 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 p_π-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 Δ<i>G</i> 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 L_III-edge XANES spectroscopy as a function of temperature and variable-temperature ¹H NMR spectroscopy

Reductive Disproportionation of CO₂ with Bulky Divalent Samarium Complexes

The base-free divalent samarium complex Cp^tt₂Sm (<b>1</b>; Cp^tt = 1,3-(^<i>t</i>Bu)₂(C₅H₃)) has been synthesized in diethyl ether by salt metathesis of SmI₂. Crystals of <b>1</b> suitable for X-ray study have been obtained by sublimation at 116 °C under reduced pressure. The dissolution of <b>1</b> in thf and pyridine solution leads to the solvent adducts Cp^tt₂Sm­(thf)₂ (<b>3</b>) and Cp^tt₂Sm­(py) (<b>4</b>), respectively, while drying <b>3</b> under reduced pressure yields Cp^ttSm­(thf) (<b>5</b>). The reaction of CO₂ with the base-free divalent samarium complexes Cp^tt₂Sm (<b>1</b>) and Cp^ttt₂Sm (<b>2</b>; Cp^ttt =1,2,4-(^<i>t</i>Bu)₃(C₅H₂)) leads to the clean formation of bridged carbonate samarium dimers [Cp^ttt₂Sm]₂(μ-CO₃) (<b>7</b>) and [Cp^tt₂Sm]₂(μ-CO₃) (<b>8</b>). This is indicative of the reductive disproportionation of CO₂ in both cases with release of CO. This contrasts with the formation of the oxalate-bridged samarium dimer reported from the reaction of CO₂ with the Cp*₂Sm­(thf)₂ complex. Otherwise, the reaction with CO does not proceed with the bulky complexes, while traces of O₂ have led to the formation of the original bridged peroxo samarium dimer [Cp^ttt₂Sm]₂(μ-O₂) (<b>6</b>). The mechanism for these reactions is studied herein by experiments and also by theoretical computations. The key result is that the different pathways are rather close in energy, which also explains why the nature of the final product, if only one is present, is difficult to <i>predict</i> a priori in this chemistry

Diniobium Inverted Sandwich Complexes with μ‑η⁶:η⁶‑Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure

Monometallic niobium arene complexes [Nb­(BDI)­(N^<i>t</i>Bu)­(R-C₆H₅)] (<b>2a</b>: R = H and <b>2b</b>: R = Me, BDI = <i>N</i>,<i>N</i>′-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)] (<b>7a</b>: R = H and <b>7b</b>: R = Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds <b>7a</b>-<b>b</b> showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H­(OEt₂)]­[B­(C₆F₅)₄], resulting in the monoprotonated cationic inverted sandwich complex <b>8</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]­[(BDI)­Nb­(N^<i>t</i>Bu)]­(μ-C₆H₅)]­[B­(C₆F₅)₄] and the dicationic complex <b>9</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)]­[B­(C₆F₅)₄]₂ (BDI^# = (ArNC­(Me))₂CH₂). NMR, UV–vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb–arene interactions. The X-ray crystal structures, UV–vis spectra, arene ¹H NMR chemical shifts, and large <i>J</i>_CH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L_3,2-edge XANES spectra and the lack of sp³ hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes