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Anisotropic NMR interaction tensors in the decamethylaluminocenium cation
Robert W Schurko1, Ivan Hung, Charles L B Macdonald
1Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4. rschurko@uwindsor.ca
Journal of the American Chemical Society
|October 31, 2002
Summary
The bis(pentamethylcyclopentadienyl)aluminum cation, [Cp(2)Al](+), exhibits high symmetry, similar to ferrocene. NMR and computational methods reveal its structure, dynamic ring rotation, and anisotropic NMR interactions, confirming its unique properties.
Area of Science:
- Organometallic Chemistry
- Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy
- Computational Chemistry
Background:
- Main group metallocenes, such as the bis(pentamethylcyclopentadienyl)aluminum cation ([Cp(2)Al](+)), are structurally analogous to transition metal metallocenes like ferrocene.
- Understanding the molecular structure and electronic properties of these compounds is crucial for advancing main group chemistry.
- Anisotropic NMR interaction tensors provide detailed insights into the local electronic environment and molecular symmetry.
Purpose of the Study:
- To comprehensively characterize the molecular structure and anisotropic NMR interaction tensors of the bis(pentamethylcyclopentadienyl)aluminum cation ([Cp(2)Al](+)).
- To investigate the dynamic behavior, specifically ring rotation, within the [Cp(2)Al](+) cation.
- To correlate experimental NMR data with theoretical calculations to understand the origin of observed NMR interactions.
Main Methods:
- Solid-state Nuclear Magnetic Resonance (NMR) experiments, including 27Al[(1)H] magic-angle spinning (MAS) and static NMR, and 13C CPMAS NMR.
- Single-crystal X-ray diffraction for structural determination of [Cp(2)Al][AlCl(4)].
- Ab initio self-consistent field (SCF) and hybrid density functional theory (DFT) calculations, alongside analytical and numerical simulations of NMR powder patterns.
Main Results:
- The single-crystal X-ray diffraction structure of [Cp(2)Al][AlCl(4)] was determined.
- Solid-state 27Al NMR revealed axially symmetric chemical shielding and electric field gradient tensors with a large chemical shielding span (Omega = 83(3) ppm) and a small nuclear quadrupole coupling constant (C(Q)((27)Al) = 0.86(10) MHz).
- 13C NMR experiments and calculations indicated dynamic rotation of the pentamethylcyclopentadienyl rings with a low internal rotational barrier.
Conclusions:
- The bis(pentamethylcyclopentadienyl)aluminum cation ([Cp(2)Al](+)) possesses high molecular symmetry, consistent with its structural analogy to ferrocene.
- Experimental and theoretical data are in excellent agreement, providing insights into the molecular origins of NMR interactions.
- The observed NMR parameters, including the large chemical shielding span and small quadrupole coupling constant, are rationalized by the high symmetry of the [Cp(2)Al](+) cation.
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