M═CR(2) (M = U, Y) Diphosphonioalkylidene and U≡O Terminal Oxo Multiple Bond Interactions Encoded in (13)C and (17)O Nuclear Magnetic Resonance Chemical Shift Anisotropies: Correlations of M═C Bond Order to Chemical Shift Tensors.

内容摘要

Diphosphonioalkylidene ligands have been central to developing f-element M═CR2 double bond chemistry, but their wide range of Ccarbene 13C NMR isotropic chemical shift (δiso) values has rendered their nature open to qualitative debate. Here, we report the use of 13C and 17O NMR spectroscopies to quantify the chemical shift anisotropies (CSAs) of two diphosphonioalkylidene complexes─[U(BIPMTMS)(O)(Cl)2] (1, J. Am. Chem. Soc. 2012, 134, 10047-10054; BIPMTMS = {C(PPh2NSiMe3)2}2-) and [Y(BIPMTMS)(I)(THF)2] (2; Organometallics 2009, 28, 6771-6776)─that exhibit disparate solution/solid-state Ccarbene 13C δiso values (386.2/401.9 and 61.0/54.9 ppm, respectively) and for 1 a highly deshielded solution oxo 17O NMR δiso (1333.6 ppm). CSA analysis reveals encoded chemical shift tensor (CST) signatures for the U═C and U≡O bonds of 1 due to strong σ ↔ π* and π ↔ σ* magnetic shielding (σ) couplings; the U═C bond exhibits 13C δ11 and span (Ω) values that are both ∼ 50 ppm larger than any alkylidene complex, and the U≡O 17O δiso is deshielded >200 ppm compared to uranyl 17O NMR data. CSA analysis confirms dominant 5f-orbital bonding and is consistent with an inverse-trans-influence in the C═U≡O linkage of 1. By contrast, the 13C NMR data for 2 exhibit signatures of Y═C double bonding tensioned with C═P ylidene contributions. For the M═CR2 bonds, we find that the 13C δiso (σiso), δ11 (σ11), Ω, and the paramagnetic contribution to the shielding (σp) CSTs correlate strongly to bond order for a range of transition metals, rare earths, and actinides. This work demonstrates that CSA analysis is a powerful method for probing actinide chemical bonding, and it brackets the spectroscopic and bonding variance of diamagnetic diphosphonioalkylidene complexes.