The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

The cationic iridium complex [Ir(COD)(PCy(3))(py)]BF(4) (1) is shown to react with dihydrogen in the presence of pyridine (py) to form the dihydride complex fac,cis-[Ir(PCy(3))(py)(3)(H)(2)]BF(4) (2). Complex 2 undergoes rapid exchange of the two bound pyridine ligands which are trans to hydride with free pyridine; the activation parameters for this process in methanol are DeltaH(double dagger) = 97.4 +/- 9 kJ mol(-1) and DeltaS(double dagger) = 84 +/- 31 J K(-1) mol(-1). When parahydrogen is employed as a source of nuclear spin polarization, spontaneous magnetization transfer proceeds in low magnetic field from the two nascent hydride ligands of 2 to its other NMR active nuclei. Upon interrogation by NMR spectroscopy in a second step, signal enhancements in excess of 100 fold are observed for the (1)H, (13)C and (15)N resonances of free pyridine after ligand exchange. The degree of signal enhancement in the free substrate is increased by employing electronically rich and sterically encumbered phosphine ligands such as PCy(3), PCy(2)Ph, or P(i)Pr(3) and by optimizing the strength of the magnetic field in which polarization transfer occurs.

Coordinatively unsaturated Pt(II) complex [Pt(I(t)Bu')(I(t)Bu)](+) stabilized by N-heterocyclic carbene (NHC) ligands dehydrogenates N,N-dimethylamineborane through a mechanism that involves hydride abstraction, assisted by an amine, to yield a platinum-hydride complex [PtH(I(t)Bu')(I(t)Bu)] with concomitant formation of the boronium cation [(NHMe2)2BH2](+). This latter species is very likely in equilibrium with the THF stabilized borenium cation [(NHMe2)(THF)BH2](+), bearing an acidic NH group that is able to protonate the platinum hydride [PtH(I(t)Bu')(I(t)Bu)] releasing H2, the amino borane H2B-NMe2 and regenerating the catalytic [Pt](+) species.

The cationic iridium complexes [Ir(COD)(PR3)2]BF4 (1a-c) (a, R = Ph; b, R = p-tolyl; c, R = p-C6H4-OMe) react with parahydrogen in the presence of pyridine to give trans, cis, cis-[Ir(PR3)2(py)2(H)2]+ (2a-c) and small amounts of fac, cis-[Ir(PR3)(py)3(H)2]+ (3a-c), each of which exhibit polarized hydride resonances due to the magnetic inequivalence associated with the resultant AA"XX" spin system when 15N-labeled pyridine is employed. The pyridine ligands in 2 are labile, exchanging slowly into free pyridine with a rate constant of 0.4 s(-1) for 2a at 335 K in a dissociative process where DeltaH(double dagger) = 134 +/- 1 kJ mol(-1) and DeltaS(double dagger) = 151 +/- 5 J mol(-1) K(-1). Pyridine ligand exchange in 2 proves to be slower than that determined for 3. Parahydrogen induced polarization (PHIP) based on the hydride ligands of 2 and 3 is transferred efficiently to the 15N nuclei of the bound pyridine ligand by suitable insensitive-nuclei-enhanced-by-polarization-transfer (INEPT) based procedures. Related methods are then used to facilitate the sensitization of the free pyridine 15N signal by a factor of 120-fold through ligand exchange even though this substrate does not contain parahydrogen. This therefore corresponds to the successful polarization of an analyte by parahydrogen induced polarization methods without the need for the actual chemical incorporation of any parahydrogen derived nuclei into it.

Reaction of the iminophosphorane Ph3PNC6H4Me-4 (1a) with Hg(OAc)2 and LiCl gives the mercurated iminophosphorane [Hg{C6H3(NPPh3)-2-Me-5}Cl] (2). The latter reacts with NaBr to give [Hg{C6H3(NPPh3)-2-Me-5}Br] (3). 2 reacts with MeC6H4NCO-4 or CX2 (X = O, S) to give [Hg{C6H3(NCNC6H4Me-4‘)-2-Me-5}Cl] (4) or [Hg{C6H3{NCNC6H3(HgCl)-1‘-Me-5‘}-2-Me-5}Cl] (5), respectively. Iminophosphoranes Ph3PNC6H4R-4 (1b) react with Pd(OAc)2 to give the complexes [Pd{κ2-C,N-C6H4(PPh2NC6H4R-4‘)-2}(μ-OAc)]2 (R = Me (6a), MeO (6b)), in which the palladation takes place at one of the phenyl substituents of the PPh3 group. Complex 6b reacts with NaBr or t BuNC to give [Pd{κ2-C,N-C6H4(PPh2NC6H4OMe-4‘)-2}(μ-Br)]2 (7) or [Pd{κ2-C,N-C6H4(PPh2NC6H4OMe-4‘)-2}(OAc)(CN t Bu)] (8), respectively. Complexes 6a,b react with NaClO4 and N,N,N ‘,N ‘-tetramethylethylenediamine (tmeda), yielding [Pd{κ2-C,N-C6H4(PPh2NC6H4R-4‘)-2}(tmeda)]ClO4 (R = Me (9a), MeO (9b)). The compound Ph3PNC6H4I-2 (1c) adds oxidatively to [Pd2(dba)3]·dba (dba = dibenzylideneacetone) in the presence of tmeda, resulting in the formation of complex [Pd{C6H4(NPPh3)-2}I(tmeda)] (10). The complex 10 reacts (i) with PPh3 and TlOTf (TfO = CF3SO3) to give [Pd{C6H4(NPPh3)-2}(tmeda)(PPh3)]TfO (11·TfO), (ii) with XyNC (Xy = C6H3Me-2,6) (1:3 molar ratio) to give [Pd{κ2-C,N-C(NXy)C6H4(NPPh3)-2}I(CNXy)] (12), and (iii) with XyNC and TlOTf (1:3:1) to give [Pd{κ2-C,N-C(NXy)C6H4(NPPh3)-2}(CNXy)2]TfO (13). An excess of the alkyne MeO2CC⋮CCO2Me reacts with 10 and AgClO4 (4:1:1) to give the inserted compound [Pd{κ2-C,N-C(CO2Me)C(CO2Me)C6H4(NPPh3)-2}(tmeda)]ClO4 (14·ClO4). The crystal structures of 2, 6a·CH2Cl2, 9a, 11·TfO, and 14·ClO4 have been determined by X-ray diffraction studies.

An NMR method is reported for the efficient removal of signals derived from nuclei with thermally equilibrated spin state populations whilst leaving, intact, signals derived from para-hydrogen induced polarisation (PHIP) through gradient assisted coherence selection.

Mixtures of “Pd(dba)2” (dba = dibenzylideneacetone) and 2,2‘-bipyridine (bpy; 1:2) or N,N,N‘,N‘-tetramethylethylenediamine (tmeda; 1:1) react with 2-bromo-4-nitroaniline to give [Pd{C6H3NH2-2-NO2-5}Br(N−N)] (N−N = bpy (1b), tmeda (1b‘)). Reactions of 2-iodoaniline with mixtures of “Pd(dba)2” and isonitriles RNC (R = C6H3Me2-2,6 (Xy), 2:1:2 molar ratios; R = tBu, 2.9:1:2 molar ratios) result in the formation of the complexes [Pd{κ2 C,N-C(NXy)C6H4NH2-2}I(CNXy)] (2a) and trans-[Pd{C(NtBu)C6H4NH2-2}I(CNtBu)2] (3a*). The reactions of [Pd{C6H4NH2-2}I(bpy)] and 1b‘ with RNC give the complexes trans-[Pd{C(NR)C6H3NH2-2-Y-5}}X(CNR)2] (Y = H, X = I, R = Xy (3a), tBu (3a*); Y = NO2, X = Br, R = Xy (3b), tBu (3b*)). Complexes 3 react with Tl(TfO) (TfO = CF3SO3) to give decomposition products, with the exception of 3a, which gives the cyclopalladated complex cis -[Pd{κ2 C,N-C(NXy)C6H4NH2-2}(CNXy)2]TfO (4a). Complex 2a or 3 reacts with Tl(TfO) in the presence of the corresponding ligand, L or L2, to give the cationic complex [Pd{C(NR)C6H3NH2-2-Y-5}(CNR)L2]TfO (L = tBuNC, Y = H (5a*), NO2 (5b*); L2 = bpy, Y = H, R = Xy (6a), tBu (6a*)). When L = PPh3, the resulting complexes trans-[Pd{C(NR)C6H3NH2-2-Y-5}(CNtBu)2(PPh3)]TfO (Y = H (7a*), NO2 (7b*)) decompose easily to the Pd(I) complex [Pd2(CNtBu)4(PPh3)2](TfO)2 (8). The reaction of 2a with Tl(TfO) affords [{Pd[κ2 C,N-C(NXy)C6H4NH2](CNXy)}2(μ-I)]TfO (9a), and that with a mixture of bpy and Tl(TfO) in acetone with subsequent bubbling of CO through the solution gives [Pd(R)(CNXy)(bpy)](TfO)2 (10), where R is 2-(xylylamino)-3-xylylquinazolinium-4-yl. The crystal structures of 2a, 3a, 4a, 6a, 8, 9a·CH2Cl2, and 10·1.5CH2Cl2 have been determined by X-ray diffraction studies. Some hydrogen bond interactions (Csp 3 −H···Pd, N−H···π−arene, N−H···I−Pd; eight-membered rings ···O−S−O···H−N−C−C−H··· and ···I−Pd−N−H···I−Pd−N−H···) lead to interesting supramolecular structures.

2-Iodophenyl)-3-p-tolylurea (1) reacts with Pd(dba) 2 ([Pd 2 (dba) 3 ]‚dba; dba ) dibenzylideneacetone) in the presence of the appropriate ligands to give the ortho-palladated arylureasThe latter reacts with PPh 3 to yield [Pd{C 6 H 4 NHC-(O)NHTo-2}(tmeda)(PPh 3 )]OTf (4b). Treatment of 2b with CO gives the corresponding acyl derivative [Pd{C(O)C 6 H 4 NHC(O)NHTo-2}I(tmeda)] (5b). If Tl(TfO) is added after the bubbling of CO, the C,N coupling product 3-p-tolyl-1H-quinazoline-2,4-dione (6) is formed. Complex 5b reacts with XyNC (Xy ) 2,6-dimethylphenyl) to yield trans-[Pd{C(O)C 6 H 4 NHC-(O)NHTo-2}I(CNXy) 2 ] (7). Both 5b and 7 decompose in solution to give 6. Complex 2b reacts with isocyanides to give the iminoacyl derivatives trans-[Pd{C(dNR)C 6 H 4 NHC(O)NHTo-2}I(CNR) 2 ] (R ) Xy (8x), t Bu (8t)). The complex 8x gives the C,N coupling product 4-(xylylimino)-3-p-tolyl-3,4-dihydro-1H-quinazolin-2-one ( 9) after treatment with TlOTf. 8x also reacts with bases (K 2 CO 3 and Tl 2 CO 3 ) to yield the iminoacyl amido carbene C,N,C pincer palladium complex [Pd{κ 3 C,N,C-C(dNXy)C 6 H 4 NC(O)NToC(NHXy)-2}(CNXy)] (10); this complex is an active precatalyst in Heck and Suzuki reactions. The reaction of 2b with R′Ct CR′′ and TlOTf gives [Pd{κ 2 C,N-CR′dCR′′C 6 H 4 NHC(O)NHTo-2}(tmeda)]OTf (R′ ) R′′ ) Et (11be), CO 2 Me (11bm), Ph (11bp); R′ ) Ph, R′′ ) CO 2 Me (11bq)). These complexes decompose in solution, and in the case of 11bp, 2,3-diphenylindole-1-carboxylic acid p-tolylamide (12p) was isolated. 11bq reacts with PPh 3 to give 3-phenyl-1H-indole-2carboxylic acid methyl ester (13q). 11bp and 11bq decompose in the presence of CO to yield 3,4-diphenyl-2-quinolone (14po) and 2-oxo-4-phenyl-1,2-dihydroquinoline-3-carboxylic acid methyl ester (14qo), respectively. Similarly, when 11bq is reacted with XyNC, the C,N coupling compound 2-xylyl-4-phenyl-1,2-dihydroquinoline-3-carboxylic acid methyl ester (14qnx) is formed. The crystal and molecular structures of 3b, 10‚0.5CDCl 3 , 11bp‚OEt 2 , 11bq, and 14po have been determined.