Effects of a Tridentate Pincer Ligand on Parahydrogen Induced Polarization.

The role of ligands in rhodium- and iridium-catalyzed Parahydrogen Induced Polarization (PHIP) and SABRE (signal amplification by reversible exchange) chemistry has been studied in the benchmark systems, [Rh(diene)(diphos)]+ and [Ir(NHC)(sub)3 (H)2 ]+ , and shown to have a great impact on the degree of hyperpolarization observed. Here, we examine the role of the flanking moieties in the electron-rich monoanionic bis(carbene) aryl pincer ligand, Ar CCC (Ar=Dipp, 2,6-diisopropyl or Mes, 2,4,6-trimethylphenyl) on the cobalt-catalyzed PHIP and PHIP-IE (PHIP via Insertion and Elimination) chemistry that we have previously reported. The mesityl groups were exchanged for diisopropylphenyl groups to generate the (Dipp CCC)Co(N2 ) catalyst, which resulted in faster hydrogenation and up to 390-fold 1 H signal enhancements, larger than that of the (Mes CCC)Co-py (py=pyridine) catalyst. Additionally, the synthesis of the (Dipp CCC)Rh(N2 ) complex is reported and applied towards the hydrogenation of ethyl acrylate with parahydrogen to generate modest signal enhancements of both 1 H and 13 C nuclei. Lastly, the generation of two (Mes CCC)Ir complexes is presented and applied towards SABRE and PHIP-IE chemistry to only yield small 1 H signal enhancements of the partially hydrogenated product (PHIP) with no SABRE hyperpolarization.

Fluorescence and Metal-Binding Properties of the Highly Preorganized Tetradentate Ligand 2,2'-Bi-1,10-phenanthroline and Its Remarkable Affinity for Cadmium(II).

The metal-ion-complexing properties of the tetradentate ligand 2,2'-bi-1,10-phenanthroline (BIPHEN) in 50% CH3OH/H2O are reported for a variety of metal ions. BIPHEN (with two reinforcing benzo groups in the backbone) was compared to other tetrapyridyls, 2,9-di(pyrid-2-yl)-1,10-phenanthroline (DPP; with one benzo group) and 2,2':6',2″:6″,2‴- quaterpyridine (QPY; with no benzo groups), with levels of preorganization BIPHEN > DPP > QPY. Formation constants were determined by following the variation of the intense π → π* transitions in the absorbance spectra of BIPHEN in the presence of metal ion as a function of the pH. The log K1 values show that the increased level of preorganization produced by the two benzo groups, reinforcing the backbone of the BIPHEN ligand, leads to increased complex stability with large metal ions (an ionic radius greater than 0.9 Å) compared to the less preorganized tetrapyridines DPP and QPY. In particular, the large CdII ion [log K1(BIPHEN) = 12.7] shows unusual selectivity over the small ZnII ion [log K1(BIPHEN) = 7.78]. The order of levels of preorganization BIPHEN > DPP > QPY leads to enhanced selectivity for SmIII over GdIII with increased preorganization, which is of interest in relation to separating AmIII from GdIII in the treatment of radioactive waste. AmIII is very close in ionic radius to SmIII, so that the size-based selectivity produced by the enhanced preorganization of BIPHEN should translate into enhanced AmIII/GdIII selectivity. The chelation-enhanced fluorescence (CHEF) effect in BIPHEN complexes is discussed. The CHEF effect in the ZnII complex is somewhat smaller than that for CdII, which is discussed in terms of decreased overlap in the Zn-N bonds formed by the too small ZnII, leading to a partial photoinduced-electron-transfer quenching of fluorescence. The structure of the complex [Cd(BIPHEN)2](ClO4)2 is reported and shows that the Cd-N bonds are largely normal for the unusual 8-coordination observed, except that steric clashes between the terminal pyridyl groups of each of the BIPHEN ligands, and the rest of the orthogonal BIPHEN ligand, lead to some stretching of the outer Cd-N bonds.

Controlling the Fluorescence Response of PET Sensors via the Metal-Ion π-Contacting Ability of the Fluorophore: Coumarin, a Weaker π Contacter.

The π-contact hypothesis, that quenching of the fluorescence of complexes of photoinduced electron transfer sensors with heavy diamagnetic metal ions may be caused by π contacts between the metal ion and the fluorophore of the sensor, is examined with a study of the fluorescent properties of the sensor 4-[[bis(2-pyridinylmethyl)amino]methyl]-6,7-dimethoxy-1-benzopyran-2-one (cdpa) and the structures of its complexes with some metal ions. The coumarin-type fluorophore of cdpa is a weaker π-contact former than the anthracenyl fluorophore of the analogue adpa (Inorg. Chem. 2014, 53, 9014): only Ag(I), the strongest π contact former, quenches the fluorescence of cdpa, apart from paramagnetic Cu(II) and Ni(II), which quench fluorescence by a redox mechanism not requiring π contacts. The structures of [Ag(cdpa)NO3] (1), [Pb(cdpa)(NO3)2] (2), [Zn(cdpa)(NO3)2] (3), [Cd(cdpa)Cl2]2 (4), [Cd(cdpa)2H2O](NO3)2 (5), and [Hg(cdpa)2H2O](NO3)2 (6) are reported. Structure 1 shows that Ag(I) is the only metal ion studied that forms π contacts with the fluorophore of cdpa in the solid state: Ag···C η(2) π contacts of 3.083 and 3.095 Å, in line with quenching of the fluorescence of the Ag(I)(cdpa) complex. In contrast, Pb(II), Zn(II), and Cd(II) show chelation-enhanced fluorescence in their cdpa complexes, and the structures of 2-4 show that the fluorophore of cdpa in each case forms no π contacts. By contrast, the adpa complexes of Pb(II) and Cd(II) show π contacts with its more strongly π-contacting fluorophore (Inorg. Chem. 2014, 53, 9014). The structures of 5 and 6 show bis-complexes of cdpa: the coordination geometries of Cd(II) and Hg(II) are discussed in relation to the number of covalently bound donor atoms present. The preferred hapticity of π-contacted metal ions is evaluated from the literature structures, suggesting that d(10) metal ions such as Ag(I) and Hg(II), and tetragonally distorted Cu(II) and Pd(II), prefer η(1) and η(2) π contacts, while more ionically bound metal ions such as K(I), Ba(II), and La(III), as well as d(10)s(2) metal ions such as Tl(I), Pb(II), and Bi(III), prefer η(6) contacts.

Acidity of the amidoxime functional group in aqueous solution: a combined experimental and computational study.

Poly(acrylamidoxime) adsorbents are often invoked in discussions of mining uranium from seawater. While the amidoxime-uranyl chelation mode has been established, a number of essential binding constants remain unclear. This is largely due to the wide range of conflicting pK(a) values that have been reported for the amidoxime functional group. To resolve this existing controversy we investigated the pK(a) values of the amidoxime functional group using a combination of experimental and computational methods. Experimentally, we used spectroscopic titrations to measure the pK(a) values of representative amidoximes, acetamidoxime, and benzamidoxime. Computationally, we report on the performance of several protocols for predicting the pK(a) values of aqueous oxoacids. Calculations carried out at the MP2 or M06-2X levels of theory combined with solvent effects calculated using the SMD model provide the best overall performance, with a root-mean-square deviation of 0.46 pK(a) units and 0.45 pK(a) units, respectively. Finally, we employ our two best methods to predict the pK(a) values of promising, uncharacterized amidoxime ligands, which provides a convenient means for screening suitable amidoxime monomers for future generations of poly(acrylamidoxime) adsorbents.

The effect of π contacts between metal ions and fluorophores on the fluorescence of PET sensors: implications for sensor design for cations and anions.

The idea that M···C π contacts between diamagnetic heavy metal ions such as Pb(II), Ag(I), Pd(II), or Hg(II) and the anthracenyl fluorophore of adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine) are responsible for quenching the fluorescence of the complexes of these metal ions with adpa is explored crystallographically. The structures of [Pb(adpa)(NO3)2] (1), [Ag(adpa)NO3] (2), [Pd(adpa)NO3]NO3 (3), [Zn(adpa)(NO3)2] (4), and [Cd(adpa)Br2] (5) are reported. The π contacts with the fluorophore are for 1 are a Pb···C π contact of 3.178 Å; for 2, an Ag···C π contact of 3.016 Å; and for 3, a Pd···C π contact of 2.954 Å on the axial site of the Pd(II) ion. The Zn(II) ion in 4 has no Zn···C π contact, with the anthracenyl fluorophore rotated completely away from the Zn(II) ion. These structures confirm that in the Pb(II), Ag(I), and Pd(II) complexes of adpa, which experience strong quenching of fluorescence, there are strong M···C π contacts, as expected if it is the π contacts that quench fluorescence. In contrast, for the Zn(II) adpa complex, which forms no π contact, there is a strong increase in fluorescence intensity. The structure of 5 shows a long Cd···C π contact at 3.369 Å, in contrast to a previously reported structure with two coordinated nitrates where the Cd···C π contact is 3.097 Å. The long Cd···C π contact in [Cd(adpa)Br2] suggests how coordination of Br(-), as well as other more covalently bound ligands such as Cl(-), SCN(-), and S2O3(2-), cause an increase in fluorescence intensity, reported for the Cd(II)adpa complex in 50% CH3OH/H2O. Coordination of covalently bound ligands to the Cd(II) weakens the Cd···C π contact and so enhances fluorescence, whereas more ionically bound ligands such as SO4(2-), NO3(-), or H2O produce a strong Cd···C π contact and weakened fluorescence. Complexes of the Cd(II)/adpa type may form the basis for a new type of anion/small molecule sensor. The tendency of metal ions to form π contacts with aromatic groups is analyzed in terms of the frequency of occurrence of π contacted structures in the literature, as well as by DFT calculations on the adpa complexes.

Mechanism of chelation enhanced fluorescence in complexes of cadmium(II), and a possible new type of anion sensor.

The Cd(II) complex of adpa (N-(9-anthracenylmethyl)-N,N-di-(picolyl)amine) in MeOH-H2O has increased fluorescence intensity with [Cl(-)], a new type of anion sensor. The structure of [Cd(adpa)(NO3)2] has a proposed fluorescence-quenching π-contact between Cd and the fluorophore, while the Cl in [Cd(adpa)Cl2] disrupts the π-contact, restoring fluorescence.