An Extended Study of the Molybdenum(IV) Octacyanide Anion: Comparison of Dodecahedral versus Square Antiprismatic Structural Forms via Solid-State 95Mo Nuclear Magnetic Resonance Spectroscopy
EMSL Project ID
13893
Abstract
This proposal involves the study of the diamagnetic molybdenum(IV) octacyanide anion, Mo(CN)84-, using solid-state 95Mo nuclear magnetic resonance (NMR) spectroscopy and is an extension of research previously initiated at the Pacific Northwest National Laboratories (PNNL). Molybdenum(IV) octacyanide is a diamagnetic eight-coordinate transition metal anion which adopts two known geometries in the solid state, approximately described by the following polytopes: dodecahedral (DD), as in K4Mo(CN)8·2H2O, or square antiprismatic (SA), as in Tl4Mo(CN)8. During a previous visit to PNNL, we obtained high-quality solid-state 95Mo NMR spectra for DD K4Mo(CN)8·2H2O on the 17.6 T NMR system. Subsequently, we acquired solid-state 95Mo NMR spectra of K4Mo(CN)8·2H2O on our 11.75 T NMR spectrometer and carried out both relativistic and non-relativistic density functional theory (DFT) calculations of the Mo magnetic shielding and electric field gradient (EFG) tensors. Our calculated results for the DD Mo(CN)84- anion are in very good agreement with experiment and indicate a substantial EFG at Mo, as well as a large magnetic shielding anisotropy (Ω). For completeness, we performed calculations on the SA form which, according to the point-charge model, should have a zero EFG at Mo. Surprisingly, the DFT calculations indicate that the Mo EFG and magnetic shielding tensors for SA Mo(CN)84- are very similar in magnitude to those for DD Mo(CN)84-, but their relative orientations are drastically different. In the current proposal, we wish to investigate Tl4Mo(CN)8, a typical Mo(CN)84- anion with SA structure, by solid-state 95Mo NMR spectroscopy to experimentally-characterize the molybdenum EFG and magnetic shielding tensors and compare these results to those obtained for DD Mo(CN)84-. The proposed experiments are necessary to finalize this project. Since 95Mo is a low-frequency, half-integer spin, quadrupolar nucleus with unfavorable nuclear properties (I = 5/2; N.A. = 15.92 %; Ξ = 6.517 MHz; Q = -0.022 barn) high applied magnetic fields, as well as signal-to-noise enhancement techniques, are necessary to obtain high-quality solid-state 95Mo NMR spectra in reasonable times. For the current study, we plan to use the quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence, which has proven to be an extremely robust technique in previous NMR studies of low-frequency half-integer quadrupolar nuclei in the solid state.
Project Details
Project type
Capability Research
Start Date
2005-04-11
End Date
2006-04-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Forgeron MA, and RE Wasylishen. 2006. "A Solid-State 95Mo NMR and Computational Investigation of Dodecahedral and Square Antiprismatic Octacyanomolybdate(IV) Anions: Is the Point-Charge Approximation an Accurate Probe of Local Symmetry?." Journal of the American Chemical Society 128(24):7817-7827.