Christopher J. Sleigh

Utilization of SABRE-Derived Hyperpolarization To Detect Low-Concentration Analytes via 1D and 2D NMR Methods

The characterization of materials by the inherently insensitive method of NMR spectroscopy plays a vital role in chemistry. Increasingly, hyperpolarization is being used to address the sensitivity limitation. Here, by reference to quinoline, we illustrate that the SABRE hyperpolarization technique, which uses para-hydrogen as the source of polarization, enables the rapid completion of a range of NMR measurements. These include the collection of (13)C, (13)C{(1)H}, and NOE data in addition to more complex 2D COSY, ultrafast 2D COSY and 2D HMBC spectra. The observations are made possible by the use of a flow probe and external sample preparation cell to re-hyperpolarize the substrate between transients, allowing repeat measurements to be made within seconds. The potential benefit of the combination of SABRE and 2D NMR methods for rapid characterization of low-concentration analytes is therefore established.

Detection of unusual reaction intermediates during the conversion of W(N2)2(dppe)2 to W(H)4(dppe)2 and of H2O into H2.

W(N(2))(2)(dppe-κ(2)P)(2) reacts with H(2) to form WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P) and then W(H)(4)(dppe-κ(2)P)(2). When para-hydrogen is used in this study, polarized hydride signals are seen for these two species. The reaction is complicated by the fact that trace amounts of water lead to the formation of H(2), PPh(2)CH(2)CH(2)Ph(2)P(O) and W(H)(3)(OH)(dppe-κ(2)P)(2), the latter of which reacts further via H(2)O elimination to form W(H)(4)(dppe-κ(2)P)(2) and [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)]. These studies demonstrate a role for the 14-electron intermediate W(dppe-κ(2)P)(2) in the CH activation reaction pathway leading to [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-k(2)P}(dppe-k(2)P)]. UV irradiation of W(H)(4)(dppe-κ(2)P)(2) under H(2) led to phosphine dechelation and the formation of W(H)(6)(dppe-k(2)P)(dppe-k(1)P) rather than H(2) loss and W(H)(2)(dppe-κ(2)P)(2) as expected. Parallel DFT studies using the simplified model system W(N(2))(2)((Ph)HPCH(2)CH(2)PH(2)-κ(2)P)(H(2)PCH(2)CH(2)PH(2)-κ(2)P) confirm that ortho-metalation is viable via both W(dppe-κ(2)P)(2) and W(H)(2)(dppe-κ(2)P)(2) with explicit THF solvation being necessary to produce the electronic singlet-based reaction pathway that matches with the observation of para-hydrogen induced polarization in the hydride signals of [WH(3){Ph(C(6)H(4))PCH(2)CH(2)PPh(2)-κ(2)P}(dppe-κ(2)P)], W(H)(3)(OH)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) during this study. These studies therefore reveal the existence of differentiated and previously unsuspected thermal and photochemical reaction pathways in the chemistry of both W(N(2))(2)(dppe-κ(2)P)(2) and W(H)(4)(dppe-κ(2)P)(2) which have implications for their reported role in N(2) fixation.

Structure and dynamics in metal phosphine complexes using advanced NMR studies with para-hydrogen induced polarisation

The iridium and rhodium phosphine complexes IrCl(CO)(PPh3)2 1 (Vaska’s complex), Rh(PMe3)4Cl 2, and Rh(PMe3)3Cl 3, add H2 to form the corresponding dihydrides. Exchange with para-hydrogen (p-H2) provides a means of observing 1H NMR signals due to the metal bound hydrides at significantly enhanced levels of sensitivity. We show that monitoring these metal hydride complexes can be achieved by a range of 2D NMR methods, based on standard experiments, which have been modified to achieve optimum signal. The assignment of heteronuclei, including low sensitivity nuclei such as 103Rh, determination of heteronuclear coupling constants and measurement of their relative signs, is described for these systems using p-H2 derived starting magnetisation. In the case of Vaska’s complex the dihydride addition product contains a trans labilised carbonyl ligand, and substitution with appropriate phosphines brings about the formation of metal phosphine complexes with new ligand spheres. Appropriately modified NOESY experiments are demonstrated to rapidly probe structural arrangements, and monitor dihydride exchange. For Ir(H)2Cl(PPh3)3 dihydride exchange is shown to proceed mainly via Ir(H)2Cl(PPh3)2, which is shown to contain inequivalent hydrides. The reactivity of the arsine complex IrCl(AsPh3)3 9 towards H2 is examined, and the NOESY approach used to make structural assignments in the reaction product.

NMR studies on ligand exchange at [IrH2Cl(CO)(PPh3)2] and [IrH2Cl(PPh3)3] by para-hydrogen induced polarisation

Enhancement of NMR signals by para-hydrogen induced polarisation allows the rapid characterisation of [IrH2Cl(CO)(PPh3)2] and [IrH2Cl(PPh3)3]: these complexes undergo ligand exchange via a 16-electron complex, [IrH2Cl(PPh3)2], which has a square-based pyramidal structure.

New products in an old reaction: isomeric products from H2 addition to Vaska’s complex and its analogues

para-Hydrogen enhanced NMR signals aid detection of minor isomers of complexes IrH2(L)2(CO)Cl (L = PPh3, PMe3, PPh2Cl and AsPh3) containing magnetically inequivalent hydride ligands that are produced via addition across the L–Ir–L axis of Ir(L)2(CO)Cl: in the case of L = PPh3, reaction with CO and H2 is shown to yield the substitution product IrH2(CO)2(PPh3)Cl which reacts further via HCl transfer to form IrH(CO)(PPh3)2Cl2 and thereby enables the detection of IrH3(CO)2(PPh3).

Activation of H2 by halocarbonyl bis-phosphine and bis-arsine iridium(I) complexes. The use of parahydrogen induced polarisation to detect species present at low concentration and investigate their reactivity

The iridium phosphine complexes Ir(CO)Cl(L)2 [L = PPh3, PMe3, AsPh3 and PPh2Cl, and L2 = (PPh2Cl)(PPh3)] add H2 to form the corresponding dihydrides IrH2(CO)Cl(L)2. These products are detected at enhanced levels of sensitivity through the 1H NMR signatures of their hydride resonances via para-hydrogen (p-H2) based spin state synthesis. Products corresponding to addition across both the Cl–Ir–CO and L–Ir–L axes are detected. For L = PPh3, there is a 100 fold preference for the former pathway at 295 K, while for L = AsPh3 the second product is favoured by a factor of 2.85. At elevated temperatures a third product corresponding to addition over the Cl–Ir–L axis is detected for L = AsPh3 and PPh2Cl. Under these conditions, the CO and HCl transfer products Ir(H)3(CO)2(AsPh3), and IrH(CO)Cl2(AsPh3)2 are also formed in a thermal reaction. When IrH2(CO)Cl(L)2 is warmed or photolysed with H2 and CO, the corresponding products are produced for L = PPh3 and PMe3. However after photolysis with H2 alone Ir(H)3(CO)(L)2 is the favoured product. Additional products detected during the photochemical studies include Ir(H)2(PPh3)(PPh2C5H4CO), an unusual orthometallation product containing an η2-acyl ligand, and the binuclear products H(Cl)Ir(PMe3)2(μ-H)(μ-Cl)Ir(PMe3)(CO) and (H)2Ir(PMe3)2(μ-Cl)2Ir(PMe3)(CO).

NMR detection of thermal and photochemical dihydrogen addition products of mono- and tri-nuclear ruthenium complexes containing carbonyl and triphenylphosphine ligands through para-hydrogen induced polarisation

Enhancement of NMR signals by para-hydrogen induced polarisation is shown to facilitate the detection of isomers of Ru(CO)2(H)2(PPh3)2 and Ru(CO)3(H)2(PPh3) which contain inequivalent hydride ligands, and to demonstrate that Ru3(CO)9(PPh3)3 adds H2 to form Ru3(CO)8(H)(µ-H)(PPh3)3, as well as undergoing fragmentation to form Ru(CO)3(H)2(PPh3).