The rapid colorimetric determination of molybdenum with dithiol in biological, geochemical and steel samples.

A rapid method for the determination of molybdenum in botanical, biological, geochemical and steel samples with dithiol, is described. Botanical and biological samples are ashed at 550 °C before leaching with 4 M hydrochloric acid, while geochemical samples are fused with potassium hydrogensulphate, and steels are decomposed with nitric and hydrochloric acids. The dithiol complex of molybdenum is formed by the addition of an alkaline solution of dithiol to the sample solution, and then extracted into isoamyl acetate. Ascorbic acid and citric acid are used to eliminate interferences from iron and tungsten, and the addition of potassium iodide gives the procedure very high tolerance to copper. Up to 150 geochemical samples or ashed botanical or biological samples can be analysed per man-day. Sensitivity of the method is 0.05, 0.5 and 10 p.p.m. for biological, geochemical and steel samples, respectively. The relative standard deviation is better than ±7% over the standard range used, and recovery of added molybdenum is complete.     

Photolysis of 7-(2,4,6-trialkylphenyl-)7-phosphanorbornene 7-oxides in the presence of protic species

Irradiation of 7-(2,4,6-trialkylphenyl-)7-phosphanorbornene 7-oxides at 254 nm in the presence of alcohols or water led to H-phosphinic acid derivatives. The experimental data are consistent with the mechanism established earlier for P-phenyl derivatives, involving a five-coordinate adduct from the interaction of the phosphanorbornene and the protic species which then fragments. There is no evidence that the langer substituent allowed a competing reaction to occur where the first step is unimolecular fragmentation to a two-coordinate phosphoryl species. © 1997 John Wiley & Sons, Inc.     

The characterization of NMR shielding in monocyclic phosphines

The phenyl-substituted saturated monocyclic phosphines, PhP(CH₂)_n, n = 2–5, show an interesting variation in their phosphorus NMR shieldings. The shielding does not vary uniformly with ring size, but rather the smallest ring (n = 2) has the highest shielding while the next smallest (n = 3) has the lowest shielding. Hartree–Fock calculations in the gauge-including atomic orbital (GIAO) approach on the related hydrogen derivatives have reproduced this trend in shielding and allow a qualitative understanding of the experimental observations. With respect to the relatively unstrained n = 4,5 ring systems, the unusual behavior of the n = 2 and 3 molecules can be understood in terms of the differences in the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gaps and the p-character of the phosphorus lone pair. The HOMO/LUMO gap is largest for phosphirane (n = 2) but smallest in phosphetane (n = 3). The hybrid character of the lone pair in phosphirane (n = 2) is almost sp while that for phosphetane (n = 3) is essentially sp². © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 451–457, 1997     

The important role of the phosphorus lone pair in phosphole aromaticity

Calculations on phosphole systems using the G3MP2B3 model chemistry show that the phosphorus lone pair is critical to the system's aromaticity. Protonation of the lone pair results in antiaromatic molecules as measured by homomolecular homodesmotic reactions. Attempts to separate out effects of hyperconjugation on the butadiene portion of the system are unsuccessful with current practices. Because these hyperconjugation effects will tend to cancel each other in the phosphole systems, analyses using the unmodified homomolecular homodesmotic reactions are considered reasonable measures of their aromaticity. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:754–758, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20364     

Fragmentation Reactions of Bridged Heterocyclic Phosphorus Compounds as a Source of Low-Coordination Species

Abstract

Several types of unsaturated, bridged structures have been prepared that can undergo retro-cycloaddition with the extrusion of phosphorus species having low coordination. By this technique, the species RO-PO₂, R-PO₂, R₂N-P=S, RO-P=S, and R-P=CH₂ have been generated and detected by trapping experiments.     

Phospholes with Reduced Pyramidal Character from Steric Crowding. 1. Synthesis and NMR Characterization of 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole

The sterically crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was synthesized by dehydrohalogenation of the corresponding 3,4-dibromophospholane, in order to probe the possibility that the steric congestion would cause some flattening of the phosphorus pyramid and an increase in electron delocalization. The phosphole was a recrystallizable solid with (31)P NMR delta 1.8. Semiempirical calculations indicated that the pyramidal shape was retained but was noticeably flatter than in 1-phenylphosphole. In the low energy conformation, the phosphole and phenyl ring planes are approximately orthogonal, with the 2-tert-butyl group in the less crowded position that is syn to the lone pair on phosphorus. The 6-methyl group is positioned under the phosphole ring. This conformational prediction was amply confirmed by several chemical shift and coupling effects in the (13)C NMR spectrum. The (1)H NMR spectrum displayed an unusually large four-bond coupling (6 Hz) of (31)P to the m-phenyl proton syn to the lone pair (and none to the anti-meta proton), consistent with the orthogonal conformation. The oxide of the phosphole showed more stability than that of less crowded phospholes and gave a (31)P NMR signal that was detectable over a several hour period at room temperature. The oxide proceeded to give the usual Diels-Alder dimer and also formed a cycloadduct with N-phenylmaleimide. The phosphoryl group of the latter was reduced with trichlorosilane to give the phosphine. This new 7-phosphanorbornene derivative gave the most downfield (31)P NMR shift (delta 153.3) of any member of this family, all of which are characterized by remarkable deshielding in the syn isomer.     

A study of NMR chemical shielding in 5-coordinate phosphorus compounds (phosphoranes)

Calculations of the phosphorus NMR chemical shielding in 5-coordinate phosphorus compounds have been carried out using the gauge-including-atomic-orbital (GIAO) 6-311+G(nd,p) basis set at both scaled density functional theory (sDFT) and estimated infinite order Møller–Plesset (EMPI) approaches. Results are generally in accord with previous studies on 3-coordinate phosphorus compounds but fail badly for compounds containing multiple chlorine atoms and indicate a need for a relativistic treatment of these species. We observe that some compounds with reported experimental ³¹P NMR chemical shifts far downfield of the calculated values are in fact in the range known from experiment and calculation to be in that expected for phosphonium ions; the reported structures need to be reconsidered.