Single-Crystal (31)P NMR and X-ray Diffraction Study of a Molybdenum Phosphine Complex: (5-Methyldibenzophosphole)pentacarbonylmolybdenum(0)

Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

Solid-state NMR of flavins and flavoproteins

Why apply solid-state NMR (SSNMR) to flavins and flavoproteins? NMR provides information on an atom-specific basis about chemical functionality, structure, proximity to other groups, and dynamics of the system. Thus, it has become indispensable to the study of chemicals, materials, catalysts, and biomolecules. It is no surprise then that NMR has a great deal to offer in the study of flavins and flavoenzymes. In general, their catalytic or electron-transfer activity resides essentially in the flavin, a molecule eminently accessible by NMR. However, the specific reactivity displayed depends on a host of subtle interactions whereby the protein biases and reshapes the flavin's propensities to activate it for one reaction while suppressing other aspects of this cofactor's prodigious repertoire (Massey et al., J Biol Chem 244:3999-4006, 1969; Müller, Z Naturforsch 27B:1023-1026, 1972; Joosten and van Berkel, Curr Opin Struct Biol 11:195-202, 2007). Thus, we are fascinated to learn about how the flavin cofactor of one enzyme is, and is not, like the flavin cofactor of another. In what follows, we describe how the capabilities of SSNMR can help and are beginning to bear fruit in this exciting endeavor.

A Chelate-Stabilized Ruthenium(σ-pyrrolato) Complex: Resolving Ambiguities in Nuclearity and Coordination Geometry through 1H PGSE and 31P Solid-State NMR Studies

Reaction of RuCl2(PPh3)3 with LiNN' (NN' = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a sigma-pyrrolato complex, [Ru(NN')(PPh3)2]2(mu-Cl)2 (3b), rather than mononuclear RuCl(NN')(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, sigma-bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit sigma --> pi isomerization of 3b to a piano-stool, pi-pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31P NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31P CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (1J(99Ru,31P) = 244 +/- 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes.

Electronic Properties of Furyl Substituents at Phosphorus and Their Influence on 31P NMR Chemical Shifts

The electronic properties of 2-furyl and 3-furyl substituents attached to phosphanes and phosphonium salts were studied by means of IR spectroscopy and experimental and computational (31)P NMR spectroscopy. The heteroaromatic systems proved to be electron withdrawing with respect to phenyl substituents. However, phosphorus atoms with attached furyl substituents are strongly shielded in NMR. The reason for this phenomenon was studied by solid state (31)P MAS NMR experiments. The chemical shift tensor was extracted, and the orientation within the molecules was determined. The tensor component sigma(33), which is effected the most by furyl systems, is oriented perpendicular to the P-C bonds of the substituents. P-furyl bonds are shorter than P-phenyl bonds. We assume therefore a lower ground-state energy of the molecules, because of the electron withdrawing properties of the 2-furyl systems. The sigma(para) component of the (31)P NMR magnetic shielding is therefore smaller, which results in an overall increase of the magnetic shielding.

Magnitudes and Orientations of 31P Chemical Shielding Tensors in Pt(II)−Phosphine Complexes and Other Four-Fold Coordinated Phosphorus Sites

31P MAS and double-quantum filtered 31P MAS NMR experiments at and near the n = 0 rotational resonance condition, as well as off-magic angle spinning 31P NMR experiments on two polycrystalline samples of Pt(II)-phosphine thiolate complexes are reported. Numerical simulations yield complete descriptions of the two 31P spin pairs. 195Pt MAS NMR spectra are straightforward to obtain but sensitively reflect only some parameters of the 195Pt(31P)2 three-spin system. Based on the 31P NMR results obtained and in conjunction with a large body of literature data and irrespective of the chemical nature of the specimen, a unified picture of the dominating motif of 31P chemical shielding tensor orientations of phosphorus sites with 4-fold coordination is identified as a local (pseudo)plane rather than the directions of P element bond directions.

Phosphorus magnetic shielding tensors for transition-metal compounds containing phosphine, phosphido, and phosphinidene ligands: Insights from computational chemistry

The present study examines the quality of the restricted Hartree–Fock (RHF) ab initio, B3LYP hybrid density functional theory (DFT), and relativistic zeroth-order regular approximation (ZORA) DFT methods for the calculation of phosphorus chemical shift (CS) tensors in phosphine, phosphido, and phosphinidene transition-metal complexes. A detailed comparison of calculated and experimental 31P CS tensors allows us to identify the characteristic advantages of each computational method. The results from B3LYP and ZORA-DFT calculations indicate that a double-ζ quality basis set reproduces experimental values of the principal components of the 31P CS tensor in many of the phosphorus-containing transition-metal complexes investigated, whereas the RHF method requires a triple-ζ doubly polarized basis set, yet fails in the case of the terminal phosphido group. Inclusion of the spin-orbit relativistic correction with the ZORA-DFT formalism requires a triple-ζ quality basis set to reproduce the experimental data. We demonstrate the merit of modern computational methods for investigating theoretically the effect of geometric variations upon the phosphorus CS tensor by systematically altering the W—P bond length and the W-P-CMe bond angle in W(CO)5(PMe3). Additionally, a previously reported correlation, determined experimentally, relating the 31P CS tensor to the Fe-P-Fe bond angle in a series of iron phosphido-bridging compounds, has been reproduced with calculations using the model compound Fe2(CO)6(µ2-PPh2)(µ2-Cl). The results presented demonstrate the value of modern computational techniques for obtaining a greater understanding of the relationship between phosphorus chemical shifts and molecular structure.Key words: 31P chemical shift, phosphine, phosphido, phosphinidene, RHF, B3LYP, relativistic, ZORA-DFT.