Catriona M. Spencer

The determination of individual rates from magnetization-transfer measurements

The rate equations for magnetization transfer between many sites have been solved for the general problem of one or many sites being perturbed by a selective pulse or pre-irradiation with a low power continuous wave frequency. Individual rate constants and relaxation times are derived, but no allowance is made for nuclear Overhauser effects. The analysis is applied to magnetization-transfer measurements made on the four-site 13C NMR problem, Cr(η6-C8H8(CO)3.

The synthesis and characterisation of dihydridobis-(trialkyltin)(pentamethylcyclopentadienyl)-rhodium(V) and iridium(V) complexes and related reactions

Abstract The dihydridobis(trialkylstannyl)-rhodium(V) and -iridium(V) complexes [C5-Me5M(H)2(SnR3)2] were made by reaction of R3SnH and [(C5Me5M)2Cl4] or [C5Me5M(C2H4)2]. An intermediate in the last reaction was spectroscopically identified as [C5Me5M(H)(SnR3)(C2H4)] (M ue5fc Ir, Rue5fc Me), but the rhodium analogues could not be detected. The hydrido-carbonyl complexes, [C5Me5M(H)-(SnR3)(CO)] were readily obtained by reaction of R3SnH with [C5Me5M(CO)2]; they were more stable than their silyl analogues, but the rhodium complexes slowly decomposed. The complexes [C5Me5M(H)2(SnR3)2] were very resistant to attack by nucleophiles but the rhodium complexes slowly reacted with PPh3 to give [C5Me5Rh(PPh3)(SnR3)2]; this behaviour was in marked contrast to that shown by [C5Me5Rh(H)2(SiR3)2]. The complex [C5Me5Rh(H)2(SnMe3)2] was deprotonated by n-butyllithium to give [C5Me5Rh(H)(SnMe3)2] ; this was reversed on addition of methanol.

Reactions of rhodium(V) hydrido–silyl complexes with butyl-lithium and with lithium triethylhydridoborate

Reaction of [(η5-C5Me5)Rh(H)2(SiR3)2](R, = Me3, Et3, or Me2Ph) with LiBun gave [(η5-C5Me5)-RhH(SiR3)2]–; these species regenerated [(η5-C5Me5)Rh(H)2(SiR3)2] on reaction with methanol. By contrast, [(η5-C5Me5)Rh(H)2(SiEt3)2] reacted with Li[BEt3H] slowly to give [(η5-C5Me5)Rh-(H)2(SiEt3)]–, which, with methanol gave the trihydride, [(µ5-C5Me5)Rh(H)2(SiEt3)]. The anion, [(η5-C5Me5)RhH(SiMe3)2]– also reacted with CO2 to give the labile [(η5-Me5)RhH(SiMe3)-(CO)], which was also made by reaction of [(µ5-C5Me5)Rh(CO)2] with SiMe3H. Syntheses of [(η5-C5Me5)Rh(H)2(SiR3)2](R3= Me3 Me2Ph) are described. The secondary isotope shift of 1H/2H on the 103Rh chemical shift on going from [(η5-C5Me5)Rh(H)3(SiEt3)] to [(η5-C5Me5)Rh(H)2D(SiEt3)] has been determined as –9 p.p.m.

The observation of four separate fluxional processes in [Re3(CO)10H3]2–, including an example of the roatation of an ReRe fragment on ReH(CO)4

Magnetization transfer 13C n.m.r. mesurements have been used to investigate the fluxionality of [Re3(CO)10H3]2– in CD3CN, the lowest energy process being the local scrambling of the carbonyls of the doubly hydride bridged H2Re(CO)3 unit; the next process can be viewed as ‘olefin’ ReH(CO)4, rotation about the ‘acetylene’[(CO)3Re(µ-H)2- Re(CO)3]2–, followed by the single hydride moving to the unbridged edge and finally local carbonyl scrambling on the ReH(CO)4.

The syntheses and characterisation of, and the determination of 1J(103Rh–103Rh) in [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](R = H or Me) and [(η5-C5Me5)2Rh2(µ-CH2)2{µ-Ph2P(CH2)nPPh2}]2+(n= 1 or 2)

Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2Cl2] with CH2CRCH2MgCl gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](3, R = H; 4, R = Me). Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2(MeCN)2]2+ with Ph2P(CH2)nPPh2gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ- Ph2P(CH2)nPPh2}]2+(5, n= 1; 6, n= 2). The complexes (3)–(6) were characterised largely by their n.m.r. spectra which also allowed the determination of 1J(103Rh–103Rh) as 13.5 for (3), 13.2 for (4), 12.4 for (5), and 11.9 Hz for (6). After allowance for gyromagnetic ratios and s-electron densities at the nuclei, these coupling constants are comparable with a 1J(l95Pt–195Pt) coupling of ca. 8 000 Hz.

Four separate fluxional processes in (η6-cyclo-octatetraene)(η4-cyclo-octa-1,5-diene)osmium. A novel example of a metal [1,5] shift on a π-co-ordinated polyene ligand

The 13C n.m.r. spectrum of Os(η6-C8H8)(η4-C8H12) in [2H8]toluene at -93 °C shows that the molecule possesses no symmetry, and a low energy twitch occurs with ΔG‡ 8.9 kcal mol–1 producing an apparent plane of symmetry; at higher temperature, the cyclo-octatetraene and cyclo-octadiene ligands mutually rotate, with ΔG‡ca. 11.5 kcal mol–1, exchange commencing at room temperature on the cyclo-octatetraene ring by a novel [1,5] shift mechanism with ΔG‡= 16.5 kcal mol–1 and at slightly higher temperature a general shift of the metal around the cyclo-octatetraene ring occuring with ΔG‡ca. 19.5 kcal mol–1(1 cal = 4.184 J).

The fluxional behaviour of Pt2(η1,η2-C3H5)2(acac)2

Abstract The 1 H NMR spectrum of Pt 2 (η 1 ,η 2 -C 3 H 5 ) 2 (acac) 2 has been examined and shown to be consistent with its solid state structure. Magnetization transfer measurements have shown pairwise exchange of the allylic protons, consistent with the platinum atoms reversing their bonding to each allyl group with Δ G ‡ 15.8 kcal mol −1 . An equilibrium is slowly established with Δ G 23.0 kcal mol −1 and Δ G ° 0.43 kcal mol −1 between the dimer and Pt(η 3 -C 3 H 5 )(acac), which was not detected in the original work.

Synthesis of [Fe(?4-C8H8)(CO)2L][L = MeNC, PriNC, ButNC, PhNC, 2,6-Me2C6H3NC, or P(OMe)3] and proof of the applicability of the Woodward?Hoffmann rules to the fluxionality of [Fe(?4-C8H8)(CO)2(CNPri)]

The compounds [Fe(η4-C8H8)(CO)2L][L = MeNC, PriNC, ButNC, PhNC, 2,6-Me2C6H3NC, or P(OMe)3] have been synthesised and their 1H, 13C, and 31P n.m.r. spectra reported. A detailed study of the fluxionality of [Fe(η4-C8H8)(CO)2(CNPri)] has been performed using the DANTE pulse sequence. The mechanism of fluxionality is shown to be consistent with the Woodward–Hoffmann rules and other mechanisms are rejected.

Trialkylsilyl(hydrido)(ethylene)pentamethylcyclopentadienylrhodium(III) complexes

The equilibria [(C5Me5)Rh(H)2(SiEt3)2]+ C2H4⇌ HSiEt3+[(C5Me5)Rh(H)(SiEt3)(C2H4)], and [(C5Me5)Rh(H)(SiEt3)(C2H4)]+ C2H4⇌ HSiEt3+[(C5Me5)Rh(C2H4)2], can be induced either thermally or photochemically starting from either side.

A determination of the activation energy of cyclopentadienyl group rotation and molecular tumbling in [Fe(η5-C5H5)(η5-C5H4R)](R = CMe2Et or Bun) using carbon-13 nuclear magnetic resonance relaxation measurements

From variable-temperature 13C spin-lattice relaxation time and nuclear Overhauser enhancement measurements in [2H8]toluene solution at 25 and 100 MHz, the activation energies for ring rotation and molecular tumbling have been determined for n-butylferrocene and t-pentylferrocene.