Photochemical pump and NMR probe: chemically created NMR coherence on a microsecond time scale.

We report pump-probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan (1)H NMR spectrum that can be recorded after a pump-probe delay of just 10 µs. The evolution of the spectra can be followed as the pump-probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial (31)P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.

Contrasting photochemical and thermal reactivity of Ru(CO)2(PPh3)(dppe) towards hydrogen rationalised by parahydrogen NMR and DFT studies.

The synthesis, characterisation and thermal and photochemical reactivity of Ru(CO)2(PPh3)(dppe) 1 towards hydrogen are described. Compound proved to exist in both fac (major) and mer forms in solution. Under thermal conditions, PPh3 is lost from 1 in the major reaction pathway and the known complex Ru(CO)2(dppe)(H)2 2 is formed. Photochemically, CO loss is the dominant process, leading to the alternative dihydride Ru(CO)(PPh3)(dppe)(H)2 3. The major isomer of 3, viz. 3a, contains hydride ligands that are trans to CO and trans to one of the phosphorus atoms of the dppe ligand but a second isomer, 3b, where both hydride ligands are trans to distinct phosphines, is also formed. On the NMR timescale, no interconversion of 3a and 3b was observed, although hydride site interchange is evident with activation parameters of DeltaH(double dagger) = 95 +/- 6 kJ mol(-1) and DeltaS(double dagger) = 26 +/- 17 J K(-1) mol(-1). Density functional theory confirms that the observed species are the most stable isomeric forms, and suggests that hydride exchange occurs via a transition state featuring an eta2-coordinated H2 unit.

Applications of the parahydrogen phenomenon in inorganic chemistry.

The study of reaction mechanisms by NMR spectroscopy normally suffers from limitations in sensitivity that arise from the physical constraints of the detection method. An overview is presented of how chemical reactions can be studied using parahydrogen assisted NMR spectroscopy where detected signal strengths can exceed those normally seen by factors of over 28,000.

New insights into catalytic hydrogenation by phosphido-substituted triruthenium clusters: confirmation of intact cluster catalysis by parahydrogen NMR.

The phosphido-substituted triruthenium cluster Ru(3)(CO)(9)(mu-H)(micro-PPh(2)) is shown to react with H(2) to form the trihydride cluster Ru(3)(CO)(9)(H)(mu-H)(2)(mu-PPh(2)), which undergoes a number of re-arrangement reactions on heating to yield other phosphido-substituted triruthenium clusters. In the presence of alkyne substrates, heating the system leads to catalytic hydrogenation via CO loss and the formation of a Ru(3)(eta(2)-PhC[double bond, length as m-dash]CHPh)(CO)(8)(micro-H)(PHPh(2)) resting state, in a reaction affected by the polarity of the solvent. No mononuclear fragments are observed in the catalytic transformation, confirming directly that the phosphido ligand is able to exert a stabilising influence on the cluster core.

Catalytic hydrogenation by triruthenium clusters: a mechanistic study with parahydrogen-induced polarization.

The reactivity of the cluster family [Ru(3)(CO)(12-x)(L)(x)] (in which L=PMe(3), PMe(2)Ph, PPh(3) and PCy(3), x=1-3) towards hydrogen is described. When x=2, three isomers of [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] are formed, which differ in the arrangement of their equatorial phosphines. Kinetic studies reveal the presence of intra- and inter-isomer exchange processes with activation parameters and solvent effects indicating the involvement of ruthenium-ruthenium bond heterolysis and CO loss, respectively. When x=3, reaction with H(2) proceeds to form identical products to those found with x=2, while when x=1 a single isomer of [Ru(3)(H)(mu-H)(CO)(10)(L)] is formed. Species [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] have been shown to play a kinetically significant role in the hydrogenation of an alkyne substrate through initial CO loss, with rates of H(2) transfer being explicitly determined for each isomer. A less significant secondary reaction involving loss of L yields a detectable product that contains both a pendant vinyl unit and a bridging hydride ligand. Competing pathways that involve fragmentation to form [Ru(H)(2)(CO)(2)(L)(alkyne)] are also observed and shown to be favoured by nonpolar solvents. Kinetic data reveal that catalysis based on [Ru(3)(CO)(10)(PPh(3))(2)] is the most efficient although [Ru(3)(H)(mu-H)(CO)(9)(PMe(3))(2)] corresponds to the most active of the detected intermediates.

A combined parahydrogen and theoretical study of H2 activation by 16-electron d8 ruthenium(0) complexes and their subsequent catalytic behaviour.

The photochemical reaction of Ru(CO)(3)(L)(2), where L = PPh(3), PMe(3), PCy(3) and P(p-tolyl)(3) with parahydrogen (p-H(2)) has been studied by in-situ NMR spectroscopy and shown to result in two competing processes. The first of these involves loss of CO and results in the formation of the cis-cis-trans-L isomer of Ru(CO)(2)(L)(2)(H)(2), while in the second, a single photon induces loss of both CO and L and leads to the formation of cis-cis-cis Ru(CO)(2)(L)(2)(H)(2) and Ru(CO)(2)(L)(solvent)(H)(2) where solvent = toluene, THF and pyridine (py). In the case of L = PPh(3), cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) is shown to be an effective hydrogenation catalyst with rate limiting phosphine dissociation proceeding at a rate of 2.2 s(-1) in pyridine at 355 K. Theoretical calculations and experimental observations show that H(2) addition to the Ru(CO)(2)(L)(2) proceeds to form cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) as the major product via addition over the pi-accepting OC-Ru-CO axis.