Investigation of structure and dynamics in the sodium metallocenes CpNa and CpNa·THF via solid-state NMR, X-ray diffraction and computational modelling

Abstract

Solid-state (23) Na NMR spectra of two organometallic complexes, cyclopentadienylsodium (CpNa) and the tetrahydrofuran (THF) solvate of CpNa (CpNa·THF), are presented. Analytical simulations of experimental spectra and calculated (23) Na electric-field gradient (EFG) tensors confirm that both complexes are present in microcrystalline samples of CpNa recrystallized from THF. For the solvate, (23) Na NMR experiments at 9.4 T and 11.7 T elucidate sodium chemical shielding (CS) tensor parameters, and establish that the EFG and CS tensor frames are non-coincident. Single-crystal X-ray diffraction (XRD) experiments are used to determine the crystal structure of CpNa·THF: Cmca (a = 9.3242(15) Å, b = 20.611(3) Å, c = 9.8236(14) Å, α = β = γ = 90° , V = 1887.9(5)Å(3) , Z = 8). For CpNa, (23) Na NMR data acquired at multiple field strengths establish sodium CS tensor parameters more precisely than in previous reports. Variable-temperature (VT) powder XRD (pXRD) experiments determine the temperature dependence of the CpNa unit cell parameters. The combination of (23) Na quadrupolar NMR parameters, pXRD data and calculations of (23) Na EFG tensors is used to examine various models of dynamic motion in the solid state. It is proposed that the sodium atom in CpNa undergoes an anisotropic, temperature-dependent, low frequency motion within the ab crystallographic plane, in contrast with previous models.

Sodium complexes containing 2-iminopyrrolyl ligands: the influence of steric hindrance in the formation of coordination polymers

“…The Na-Na distances inside the dimer units are similar in 1 and 4 tetramers, but the central Na-Na separation, between dimers connected by Na-pyrrolyl p-bonding, is slightly shorter in the case of 1 than in 4; the geometric features of tetramers 2 and 3 are intermediate between those of 1 and 4, which agree well with the trend obtained for d iso (Table 2). A similar downfield shift tendency was reported in the 23 Na NMR studies of sodium cyclopentadienyl (NaCp, d iso = -57.5 ppm) and its tetrahydrofuran solvate CpNa•THF (d iso = -45.5 ppm); 35 at room temperature, it was observed that C q increased with increasing substitution of the Cp ring, but a full molecular orbital analysis of the origin of chemical shielding in polymeric sodocenes was not performed in order to explain the unusually high shielding of sodium nuclei. 36 Table 2 also shows that the chemical shifts measured in CD 3 CN for compounds 1 to 3 follow a trend similar to solid state data.…”

Synthesis and Structures of s- and p-Block Metal Complexes Containing Sterically Demanding Pentaarylcyclopentadienyl Substituents

“…A comparable structure was observed for Cp BIG3,5‑di‑ t ‑Bu ­Li, in which the one-dimensional chain is separated into [Cp BIG 2 Li] − anions and [Cp BIG 2 ­Li 3­ (solv)] + /​[Li­(solv)] + cations . The formation of solvent separated ion pairs is typically observed for Cp complexes of lightweight alkali metals, but several Cp complexes including light-weighted alkali metals such as Na are known, in which the chainlike structure is retained upon coordination of additional solvent molecules. , …”

“…23 The formation of solvent separated ion pairs is typically observed for Cp complexes of lightweight alkali metals, but several Cp complexes including light-weighted alkali metals such as Na are known, in which the chainlike structure is retained upon coordination of additional solvent molecules. 48,49 Two different solvent-coordinated potassium complexes (13a, 13b) were structurally characterized. An inverse tripledecker potassocene cation [(Cp BIGt-Bu ) 2 K 3 (solv) n ] + as well as potassocene anions [Cp BIGt-Bu 2 K] − were observed in both structures, and 13a also indicates a neutral [Cp BIGt-Bu 2 K 2 -(solv) n ] moiety (see the Experimental Section for details).…”

Solution structures of alkali metal cyclopentadienides in THF estimated by ECC-DOSY NMR-spectroscopy (incl. software)

“…34,35 The 23 Na NMR resonance (À28 to À31 ppm) is shifted towards lower field compared to solid state MAS NMR findings for CpNa and [CpNaÁTHF] and can be shifted even lower upon cooling. 12 The same was observed for the 7 Li NMR signal, which shifts and also broadens upon cooling. In 1990 it has been reported by Paquette et al that CpLi undergoes a fast exchange process between monomeric and a ''sandwich'' dimeric species, which results in the splitting of the 6 Li signal at lower temperatures, but they could not ''tell whether the monomer-dimer equilibrium of ''CpLi'' is shifted to either side at room temperature''.…”

“…7,8 Crystal structure analysis shows that the donor free compounds form polymeric chains that are linear for [CpLi] N and [CpNa] N but bent for all higher homologues (for CpRb there are ambivalent structures, Scheme 1). [9][10][11] Various solvation states account for various excerpts from the solid state structures 12 17 Most of the crystal structures have to be classified as contact ion pairs (CIP), but there are a few examples of solvent-separated ion pairs (SSIPs) preferentially generated by the use of ammonia or crown ethers. 13,18 Aggregation of organometallic molecules in solution determines not only their reactivity but can also shed light on reaction mechanisms, and hence provide a handle to improve e.g.…”