Michael C. Perry

Some hydridotris(3,5-dimethylpyrazolyl)borate complexes of zinc, cadmium and mercury—synthetic, structural and NMR investigations

Abstract Attempts to prepare 1 : 1 complexes, L*CdX, where L* = hydridotris(3,5-dimethylpyrazoyl)borate and X = Cl, I, met with limited success. Thus, with X = Cl, CdL*2 was a usual product, although the reactions were not always clean. An L*CdI compound was prepared. By contrast, reactions of KL* with HgCl2 gave L*HgCl cleanly. The reactions of [(Ph3P)2CdCl2] or [(di-2-pyridylamine)CdCl2] with KL* gave “L*Cd(OH) · H2O” in alcoholic media, whereas [(1,2-phenylenediamine)CdI2] on reaction with KL* in Ch2Cl2 gave CdL*2. KL* removed 18-crown-6 from [(18-crown-6(CdCl2] as [(18-crown-6)K]L*. PhHgCl reacted with KL* in THF to give a product of stoichiometry “PhHgL* · 0.5 KCl”. Variable-temperature 1H and 199Hg NMR studies in CDCl3 gave data consistent with the equilibrim with low temperatures favouring the covalent form. Other NMR data are presented and discussed. The crystal structure of CdL*2 is reported, but severe disorder, with the molecule taking up two orientations about a 3 axis, reduced the accuracy of the structure determination. The environment around cadmium is essentially octahedral with a slight trigonal distortion in one molecule (A), but not in the other (B). The CdN distances [2.36(1) in A, 2.43(2) A in B] are in good agreement with those found in analogous cadmium complexes.

New method for NMR signal enhancement by polarization transfer, and attached nucleus testing

A new NMR pulse sequence that permits signal enhancement of insensitive nuclei by polarization transfer from sensitive nuclei is presented and is illustrated by its application to 13C NMR spectroscopy where resonances from 13C in all chemical environments (including quaternary carbon nuclei) can be detected simultaneously.

Molecular complexes. Part 18.—A nuclear magnetic resonance adaptation of the continuous variation (job) method of stoicheiometry determination

An n.m.r. counterpart of the classical Job method for determining the stoicheiometry of molecular complexes by spectrophotometry is derived. It is applied to the fluoranil–hexamethylbenzene and acetonitrile–benzene systems, for which stoicheiometries consistent with those deduced classically are obtained, although for the latter system an additional complex is identified. The method is used to reveal the dependence of the measured stoicheiometry on the solution concentration of the interacting species. It is demonstrated that 1:1 and 1:2 complexes can be identified separately when they coexist, and that the equilibrium constant for the formation of the former can be measured readily.

Reaction of phosphorylating reagents with substituted phenols and coals, using microwave heating

Abstract Attempts have been made to accelerate the direct reaction of coal OH groups with phosphorylating reagents, e.g. Ph 2 PCl, (PhO) 2 PCl and (EtO) 2 PCl, by use of microwave heating. Results for coals and model compounds, e.g. 2,4,6-tri- t -butylphenol and 2-phenylphenol, show that various reactions are accelerated, e.g. oxidation, P(III)→P(V), hydrolysis, PCl→POH, solvolysis, PEtO→POH, and in some cases replacement of -OEt by -OR, where R = 2,4,6-tri- t -butylphenyl. Surprisingly, under these circumstances some progress could be made identifying the origin of some 31 P MAS n.m.r. resonances from coals phosphorylated with (EtO) 2 PCl under microwave heating. These data are discussed. No possibility was found of developing quantitative methods of analysis based on rapid ‘one-pot’ derivation using phosphorylating reagents followed by in situ quantitative 31 P MAS n.m.r. spectroscopy.

The synthesis and NMR spectroscopic examination of coordination complexes of 2-telluraindane with rhodium

Rhodium complexes of 2-telluraindane were synthesised and the nature of metal-ligand interactions in solution were studied using NMR spectroscopy. Two complexes were prepared; [(Cp*Rh)(C8H8Te)][O3SCF3]2 (1) was prepared by the reaction of [Cp*RhCl2]2 with 2-telluraindane in the presence of silver triflate while [(Cp*RhCl2)(C8H8Te)] (2) was synthesised by a direct reaction between the rhodium dimer and the ligand. Complex 1 gives a 125Te coordination shift of +260 ppm and a 125Te–103Rh coupling constant of 65.9 Hz implying that the metal is directly bonded to the tellurium in the ligand and not to the aryl ring (i.e η1 not η6 coordination is envisaged). The observation of an AB quartet for the methylene protons (JAB=16 Hz) is as expected for 2-telluraindane coordinated in this manner. In contrast, complex 2 shows a broad singlet for these methylene protons at room temperature (r.t.) which sharpens on heating to 323 K and resolves into the expected AB quartet (JAB=13.5 Hz) on cooling to 223 K, however the 125Te spectrum is a singlet at r.t. but resolves into a doublet at 238 K (J103Rh–125Te=108 Hz). It is argued that these data are only reconcilable in terms of a dissociative mechanism in which the equilibrium populations of the free species are small and where the rate of exchange at r.t. lies between that required to average the 125Te chemical shifts and that required to effectively decouple the 125Te–103Rh interaction.

Measurement of -OH groups in coals of different rank using microwave methodology, and the development of quantitative solid state n.m.r. methods for in situ analysis

Abstract Experiments with both model compounds (substituted phenols) and with 11 coals (nine British and two American) have established that microwave heating will greatly accelerate silylation reactions of the phenolic -OH groups, e.g. for Creswell coal complete silylation of -OH groups occurs in 35 min in the microwave oven, whereas 24 h is required using a bench reflux technique. Microwave reaction times for coals vary from 35 min to 3 h for more dense coals such as Cortonwood. The above observations have allowed the development of a ‘one pot’ silylation of coal, followed by an in situ analysis of the added Me3Si- groups by quantitative 29Si magic angle spinning nuclear magnetic resonance (MAS n.m.r.) spectroscopy. The development of a quantitative n.m.r. method required the determination of 29Si spin lattice relaxation times, T1, e.g. for silylated coals T1 ∼ 8 s; for silylated phenols, T1 ∼ 25 s; for the synthetic smectite clay laponite, T1 ∼ 25 s; and for Ph3SiH, T1 ∼ 64 s. Inert laponite was selected as the standard. The requirement to wait for five T1max between pulses, together with the relatively low natural abundance of 29Si (4.71%), results in rather long accumulation times to obtain spectra of analytical quality (8–48 h). However, in comparison with other methods, even in the most unfavourable case, the total time from commencement of analysis to result may be described as ‘rapid’. The results for O OH O total obtained are compared with other literature data. Comparison with ketene data, for example, shows agreement to vary from excellent (Creswell) through satisfactory (Cortonwood) to poor (Pittsburgh). Even in cases where agreement with ketene data is less good, the silylation result may be close to estimates made via other acetylation methods. Possible reasons for the variations observed are discussed.

6 Li magic angle spinning nuclear magnetic resonance spectroscopy: a powerful probe for the study of lithium-containing materials

A narrow chemical-shift range has been established for a variety of lithium compounds via study of their 6Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. Comparative studies of 6Li and 7Li spectra (MAS, and ‘off-angle’ spinning) establish that, for solid-state (and even solution) analytical purposes, 6Li is the preferred nucles, since the advantage of narrow absorption lines outweighs the poorer sensitivity of 6Li relative to 7Li. 6 Li MAS NMR spectra have been obtained for laponite clay, which had been thermally treated at 200, 400, 600, 800 and 1300 °C; at the lower temperatures (<800 °C) both dehydrated and rehydrated specimens were considered. The data are consistent with mobility of lithium ions from the trioctahedral clay sites at 600 °C.Both conventional and microwave methods were used to prepare lithium-exchanged laponite. The superior resolution achievable in 6Li MAS NMR in comparison with 7Li MAS NMR is demonstrated with the microwave specimen where use of 6Li spectroscopy revealed two lithium sites. On storage of the sample for 3 months, the two sites give way to a single lithium environment. Possible causes are discussed.

Bis-(8-oxo quinoline)diorgano tellurium(IV) compounds; structural and spectroscopic studies

Abstract Three new compounds of the type R2Te(OR′)2 are reported in which R′ bears a potentially co-ordinating group: bis-(8-hydroxo quinoline)dimethyltellurium (I) bis-(8-oxo-2-methyl quinoline)dimethyltellurium (II), and bis-(8-oxo-quinoline) di-(p-tolyl)tellurium (III). The crystal structures of II and III have been determined. The primary geometry around tellurium in both cases can be described as ψ-trigonal bipyramidal but long Te ⋯ N contacts in the range 2.840(6)–2.899(4) A which lie well within the van der Waals distance imply extension of the co-ordination sphere. Variable temperature multi-nuclear (1 H, 13C, 125Te) studies of the compounds I, II, and III in solution indicate the presence of a single species over the range 216–343 K. The data do not distinguish between the presence of a single 14-Te-6 pertellurane seen in the crystallographic studies, or that of such a species in equilibrium, rapid on the 1H and 125Te timescales, with the 10-Te-4 tellurane.

Do the measurements of 27Al and 7Li Knight shifts have any analytical role in studies of freshly prepared lithium aluminium alloys

Abstract 27Al and 7Li Knight shifts have been measured for a series of freshly prepared Li/Al alloys. The phase diagram implies that three phases should be accessible for the compositions (up to 6% Li) considered and it is shown that, particularly in the 27Al spectra, three Knight-shifted resonances are observable for several compositions. It is suggested that these might correspond to the three phases (α, δ, δ′). The resonance believed to be associated with the δ′ phase is sensitive to the lithium content, but changes discontinuously when precipitation of the cubic δ phase (LiAl) occurs.

Enhancement of the NMR spectra of insensitive nuclei using PENDANT with long-range coupling constants

The recently reported NMR pulse sequence PENDANT, that enables the simultaneous detection of all insensitive nuclei (S), of a given type, in a compound and the enhancement of the signals arising from those nuclei coupled to sensitive nuclei (I), is developed further. The sequence has been modified to enable the detection of spin–spin coupled multiplets without phase distortions. Its normal form, for use with short range coupling constants, has been adapted to facilitate the S–N enhancement of insensitive nuclei through the use of long range coupling constants. The developments are illustrated by reference to {1H}–13C and {1H}–29Si spectra.