Fu-Tyan Lin

Anodic oxidation mechanism of a spiropyran

The modulation of chemical events using light is attractive for applications in many technologies including displays, information storage and sensors. Spiropyrans are the most well studied of the technologically relevant photochromic compounds. Their oxidation is important from at least two perspectives: oxidative failure of spiropyrans limits their lifetime, and electrochemical reactions can be expected to alter their photochemical properties leading to new uses. Thus, we have undertaken the first study of the oxidation of a spiropyran. Spiropyrans contain an aniline-like moiety so they are expected to be oxidizable. We have chosen a particular compound for detailed investigation, 6-hydroxy-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline]2. A 6-hydroxy functionalization creates a hydroquinone analogue, which will lead to the possibility of reversible electrochemistry. Cyclic voltammetry of 2 at a glassy carbon electrode in acetonitrile shows two anodic waves with a single cathodic wave on the reverse sweep. The first one-electron wave is due to the oxidation of the indoline moiety. This radical cation converts to a semiquinone radical which disproportionates, leading to a quinone and a hydroquinone. The hydroquinone is oxidized to the quinone at higher potentials leading to the second oxidation wave. Reduction of the quinone to the hydroquinone gives the reduction wave. Bulk electrolysis, 1H NMR, 13C NMR, chronoamperometry, UV–VIS spectroscopy and chemical experiments support the proposed mechanism. The electrochemistry of these compounds is compared to the simpler electrochemistry of another class of photochromics: the diphenylchromenes.

STEREOCHEMICALLY-RIGID AND FLUXIONAL PYRIMIDINES WITHIN THE BIS-RU(HEDTA)(PYM)2- COMPLEX

Abstract The formation of the bis-pyrimidine complex [RuII(hedta)(pym)2]− occurs by sequential additions of pym to [RuII(hedta)(H2O)]− (hedta3− = N-hydroxyethylethylenediaminetriacetate; pym = pyrimidine). The second step is rate limited by the dissociation of an in-plane carboxylato donor of the hedta3− ligand; k = 1.57 × 10−4 s−1 at 22°C. E1/2 values for the RuII/III waves in 0.10 M NaCl, T = 22°C are 0.18 V for [RuII(hedta)(pym)]− and 0.54 V for [RuII(hedta)(pym)2]−. The latter is similar to the bidentate 2,2′-bipyridine complex, [Ru(hedta)(bpy)]−, with E1/2 = 0.48 V, illustrative of the influence of two-coordinated N-heterocyclic π-acceptor ligands. 1H NMR spectra of the [RuII(hedta)(pym)2]− complex reveal a differentiation in the coordination behavior of the two bound pyrimidines. One pyrimidine is stereochemically-rigid, exhibiting resonances at 9.25 (H2), 8.82 (H6), 8.68 (H4) and 7.45 (H5) ppm, respectively. The second pyrimidine is fluxional with rapid migration of the RuII center between N-1 and N-3 sites. This motion interchanges the H4 and H6 protons which resonate at 8.82 ppm; the H2 resonance appears at 9.15 ppm and H5 at 7.61 ppm. The weaker bonding exhibited by the fluxional pyrimidine also allows for rapid exchange of the fluxional pyrimidine with any free pyrimidine in the solution. The difference in the two pyrimidines is consistent with assisted displacement of the fluxional pyrimidine, promoted by associative attack by the pendant carboxylate arm which is displaced in forming the bis complex. The stereochemically-rigid pyrimidine is nearer the −CH2CH2OH (alcohol) arm of hedta3− which does not associate strongly and provide an assisted ligand exchange path. Acidification of the [RuII(hedta)(pym)2]− complex to pD ≅ 1.0 results in formation of η2(1,2)-[Ru(hedta)(pym)]− as the major product in 1.0 Da, rather than the previously established (Y. Chen, F.-T. Lin and R.E. Shepherd, Inorg. Chem., 36 (1997) 818) 2:43:22:33 distribution for N-1:η2(1,2):η2(5,6):η2(1,6) isomers which is generated spontaneously from the initially N-1-bound [Ru(hedta)(pym)]− 1:1 complex after 14 days.

Optical control over Pb2+ binding to a crownether-containing chromene

Naphthochromene 1 binds Pb 2+ in the dark, and the complex is photodissociated by a novel mechanism.

Stabilization of the merocyanine form of photochromic compounds in fluoro alcohols is due to a hydrogen bond

Fluoroalcohols [1,1,1,3,3,3-hexafluoropropan-2-ol (HFP), 2,2,2-trifluoroethanol (TFE) and 2-fluoroethanol (FE)], acting as Lewis acids, stabilize the π-conjugated, colored merocyanine forms of spiropyran and spirooxazine photochromic compounds as metal ions do.

A comparison of the solution behavior of Pt(II) complexes of N,N′- and N,N-ethylenediaminediacetate (edda and uedda)

Abstract Pt(II) complexes of N , N ′-ethylenediaminediacetate (edda) and N , N -ethylenediaminediacetate (uedda) have been prepared from K 2 PtCl 4 by stepwise addition of the nitrogen backbone donors at pH ∼ 2.9 (50–60 °C, 60 h) and further coordination of the deprotonated carboxylate donors at pH ≥ 4 (65–75 °C, 24 h). Coordination of the glycinato donors was shown by 1 H and 13 C NMR and IR methods. The symmetrical edda ligands form 38.3% ( R , R )/( S , S )-[Pt(edda)] isomers and 61.7% meso ( R , S )/( S , R )-[Pt(edda)] isomers. All four forms of [Pt(edda)] undergo aquation of one in-plane glycinato donor in 72 h as detected by the appearance of a 13-line 1 H NMR pattern which may be deconvoluted into four AB glycinato sets. These results are indicative of pendant or ion-paired glycinato donor for [Pt(edda) (H 2 O)] which is placed either on the same side, or the opposite side, of the PtN 2 O 2 plane and coordinated glycinato donor. 195 Pt NMR shows that H 2 O is actually replaced by Cl − , i.e.[Pt(edda)Cl] − . The unsymmetrical [Pt II (uedda) X] (X = H 2 O, Cl − , OH − ) complex exhibits no major change over long time intervals (≥ 10 days, pD ∼ 6). The presence of a minor species at 15% abundance may be a similarly structured species as for [Pt(edda)(H 2 O)] with a pendant glycinato functionality. The major complex in solution is shown by the 1 H NMR with [NaCl] and [NaClO 4 ]-dependence studies to be [Pt(uedda)(H 2 O)] at low [Cl − ] and [Pt(uedda)Cl] − at 1.0 M Cl − . 195 Pt NMR confirms the formulation of X = H 2 O at low [Cl − ]. 1 H and 13 C NMR evidence supports one axially associated and one in-plane coordinated glycinato donor each for the major [Pt(uedda)(H 2 O)] complex. The 13 C NMR shows only one type of glycinato donor with a chemical shift of 189.3 ppm for the major species, and two types for the 15% species (185.6 and 170.5 ppm). The major species of [Pt(uedda)(H 2 O)] has only one type of carboxylate stretch in the IR spectra (1661 cm −1 ; shoulder feature at 1639 cm −1 ) which compares favorably with the fully-coordinated pair of glycinato donors of [Pt(edda)] (1640 cm −1 ). It is proposed that the structures of [Pt(uedda)(H 2 O] and [Pt(uedda)Cl] − are pseudo-square pyramids which illustrates the capacity of Pt(II) to adopt five-coordinate, 18-electron complexes when a suitable chelate ligand offers a fifth associable donor. These species are similar to the five-coordinate intermediates of ligand substitution reactions of typical square-planar Pt(II) complexes.

pH-dependent coordination of the glycinato donors of nitrilotriacetatoplatinate(II), [PtII(nta)]−

Abstract Two main species are formed when nitrilotriacetic acid (H3nta) displaces Cl− ligands from PtCl42−. The resultant [PtII(nta)Cl]2− and [Pt(ntaH, Cl]− complexes were examined by 1H and 13C NMR methods. The 1:1 complex is responsive to changes in pH, indicating a titratable glycinato arm in [PtII(nta)Cl]2− rather than a [PtII(nta)], polymer previously prepared by Smith and Sawyer. The major species (∼82% at pH 4.5) has two in-plane glycinato donors exhibiting an AB 1 NMR quartet (Ha = 4.40 ppm, Hb = 4.25 ppm, Jab = 16.3 Hz, area 2) and a singlet for the third glycinato donor (3.85 ppm, area 1). The latter is assigned as a weakly axially-associated glycinato donor which renders [PtII(nta)Cl]2− as a five-coordinate entity. The minor component (∼ 17%) is the dichloro derivative having two pendant glycinato arms (AB quartet: Ha = 3.91 ppm, Hb = 3.83 ppm, Jab = 16.7 Hz, area 2) and a lone in-plane glycinato donor (singlet, 4.03 ppm, area 1). At pH=3.0 the major species exhibits proton exchange-induced shifts of the glycinato singlet as well as near coalescence of the in-plane glycinato donors, indicative of an associative/dissociative equilibrium of the axial glycinato donor which alters the coordination number of the PtII center. This exchange is frozen out at pH 1.2 where the axial donor is fully removed by protonation as a pendant group in [Pt(ntaH)Cl]− (1H NMR singlet, 4.08 ppm, area 1, pendant glycinato; AB quartet, Hb = 4.36 ppm, Hb = 4.26 ppm, Jab = 16.4 Hz area 2, in-plane pair of glycinato donors). Further proton-induced dechelations of the in-plane glycinato donors occur over a 14 day period, forming a monodentate N-bound species thought to be [PtII(ntaH3)Cl3]− with one glycinato 1H NMR singlet (4.30 ppm). The existence of one chloride in [PtII(nta)Cl]2−, the major (82%) species, and two chlorides in the lesser (17%) species was confirmed by 195Pt NMR. The 195Pt NMR spectra indicate that the axial interaction in [PtII(nta)Cl]2− is very weak, with δPt shifts of −1309 ppm compared to −1317 ppm for mer-[Pt(mida)Cl]−, an authentic four-coordinate analogue.

Substitution of inosine for chloride in [Pt2(hdta)Cl2]2−(hdta4− = 1,6-hexanediamine-N,N,N′,N′-tetraacetate)

The substitution reaction of inosine (ino) with the displacement of Cl− from [Pt2 11(hdta)Cl2]2−, (hdta4 = 1,6-hexanediamine-N,N,N′,N′-tetraacetate) has been studied in D2O at 35.0°C. Each Pt11 center has initially the coordination comparable to mer-[Pt(mida)Cl]−, (ida2 = N-methyliminodiacetate), as shown by the 195Pt NMR signal at - 1330 ppm for mer, mer-[Pt2(hdta)Cl2]2− and −1317 ppm for mer-[Pt(mida)Cl]. By operating under conditions wherein the ratio of [inosine]:available Pt11 binding sites = 0.25, (excess Pt11), the first ligand addition of inosine was followed by monitoring with time the changes in the 500 MHz NMR spectrum of [Pt2(hdta)Cl2]2−. The H-8 proton resonance of inosine (site of Pt11-coordination) appears at 8.82 ppm for the coordinated [Pt2(hdta)(ino)Cl]− complex, replacing the 8.32 ppm H-8 resonance of free inosine. A substitution rate of 3.48 × 10−4s−1, controlled by the loss of Cl− was determi comparison, the guanosine bases of DNA displace Cl from cis-[Pt(NH3)2Cl2] about 3.4 times more slowly. Time-dependent spectra of the tether CH2 protons of the hdta4 backbone also exhibit characteristics downfield shifts as Cl− is displaced by ino, with the second CH2 outward from the Pt11N bond experiencing the largest influence (Δδ=−0.21 ppm). The 13C NMR spectrum of the product mixture of [Pt2(hdta)-(ino)Cl] [Pt2(hdta)Cl2]2 showed 27% of the ino-substituted complex (25% theoretical). Also no free carboxylate resonances appear near 175 ppm, but rather only the new resonance at 188.6 ppm for the inosine complex and the 188.4 ppm resonance of the [Pt2(hdta)Cl2]2 excess reagent are detected. This identifies the first inosine addition step as occurring only with Cl− displacement. A second addition step further displaces a coordinated glycinate group at a slower rate.

A five-coordinate Pt(II) complex of pyridylmethylaminediacetate

Preparation et caracterisation spectrometrique IR et RMN du complexe du platine (II) avec un compose derive de la pyridine

Bivalent metal ion-dependent photochromism and photofluorochromism from a spiroquinoxazine

Illumination of solutions of spiroquinoxazine 1 yields both a coloured species and a fluorescent species, both of which are reverse photochromic and both of which are trapped by bivalent metal ion complexation.

A temperature-dependent dynamic rearrangement of cis-(R,S)-[Pd(egta)]2– (egta4–=glycine, N,N′-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) in aqueous solution

The cis-(R,S)-[Pd(egta)]2– complex, egta4–=glycine, N,N′-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]), has been examined by 1H- and 13C-n.m.r. methods over the 18.0 to 95.0°C range in D2O. A dynamic process occurs above 65°C which makes the protons on the NCH2 functionalities of the egta tether become 1H-n.m.r. equivalent. The two states that interconvert coalesce at 81°C. Evidence from 13C-n.m.r. spectra obtained at 81°C show that the in-plane coordinated carboxylates are not lost, but rather a pendant carboxylate becomes attached with loss of the central imino donor. The resultant palladium(II)NO3 intermediate is able to reform cis-(R,S)-[Pd(egta)]2– or, presumably, give trans-(R,R)-[Pd(egta)]2–. The rate limiting step occurs with a rate constant of 178s–1 at 81°C and an activation energy of 20.5kJ/mol. However, competitive aquation of glycinato donors above 85°C prevents isolation of a stable trans-(R,R)-[Pd(egta)]2– isomer.