Philip A. W. Dean

A multinuclear (1H, 13C, 113Cd) nuclear magnetic resonance and magnetic circular dichroism spectroscopic study of thiolate complexes of cadmium

Appropriate combinations of 1H, 13C and 113Cd NMR have been used to study ca. 0.05 M solutions of a wide range of 113Cd-enriched thiolatocadmates formed from the corresponding poorly soluble cadmium thiolates in the presence of excess thiolate. In addition, MCD spectra have been recorded for dilute, ca. 10−5M, solutions of a representative set of thiolate complexes of cadmium. Cadmium-113 chemical shifts are given for the complexes formed by 31 different thiolates individually in water at ambient probe temperature, and slow-exchange 1H and/or 13C and/or 113Cd NMR spectra are reported for several typical complexes at reduced temperature in CD3OD. In aqueous solution at ambient probe temperature, ligands with vicinal thiolate groups and −SCH2CHRS− (R = H, Me, Et, CH2OH, CH2SO−3 or CH2S−)) form bis(chelate) complexes which have δCd ⪖ 778 ppm, less shielded than the range for tetrahedral cd(SR)2−4 (R = Me, Et, Pr, Me2CH, Bu, EtMeHC, Ph, PhCH2, CH2CH2X (X = OH, NH2, or CO−2, CH2CH(OH)Me, CH2CH(OH)CH2OH, LCH2CH(NH2)CO−2, or CH2CO−2 for which δCd < 666 ppm. Increasing alkyl substitution kin the α-position causes increasing shielding of the cadmium resonance for both the chelate complexes and the complexes of monodentate thiolates. Separate 113Cd and, in some cases, 1H resonances have been found for members of the series [Cd(SR)n(SR′)4−n]2− (R = Me, R′ = CHMe2; R = Ph, R′ = CH2Ph; n = 0−4) and [Cd(S(CH22S)n/2(SCHMe2)4−n2− (n = 0,2, or 4) at low temperature: 113Cd chemical shifts change additively or near-additively in these series. The MCD data indicate that in most of the complexes formed at the ca. 10−5M concentrations used for the optical studies, the local microsymmetry about the cadmium approaches tetrahedral. Specifically, the complexes [Cd(SR)2.5]0.5n−n (R = Me, Et, or Pr) exhibited well-resolved, symmetric A terms under the S → Cd charge transfer band in the 250 nm region. Complexes of the dithiolates −S(CH2nS− (n = 2,3, or 4) and −SCH2CHMeS− gave MCD spectra which were much broader and less well-resolved. The spectra of the systems containing the vicinal dithiolates showed the formation of two complexes, as the band centre of the MCD signal blue-shifted when L/M was increased from 1 to 2; at L/M = 2, bis(dithiolate) complexes are most likely formed. With the ligand −SCH2CHCH2OH (BAL), the corresponding S− spectra were similar at L/M = 2 but quite different at L/M = 1. A B term centred on the 250 nm charge transfer band was observed for the solution where BAL/Cd = 1, demonstrating loss of tetrahedral symmetry about cadmium, probably due to ligation by the ligand hydroxyl group. These studies provide 113Cd NMR and MCD ‘fingerprint’ data necessary for the interpretation of the corresponding spectra of related and more complicated, biologically important cadmium complexes.

The metal NMR spectra of thiolate and phenylselenolate complexes of zinc(II) and mercury(II)

Soluble thiolato-zincates and -mercurates(II) of the general type M(SR)2−4 and the phenylselenolato complexes M(SePh)2−4 (M = Zn or Hg) have been prepared in situ from the appropriate metal nitrate and excess thiolate or selenolate. The high field metal NMR spectra of these complexes have been measured, at 25.0 and 71.5 MHz for 67Zn and 199Hg respectively, with isotopic enrichment to 89.7 atom% for 67Zn, but at natural abundance for 199Hg. The zinc complexes studied in D2O solution, were Zn(SR)2−4 (R = Me, Pri, or Ph), Zn???SS])22 and Zn(SePh)2−4. The variation of δZn with ligand was found to be −SePh < −SPh < −Spri < −SMe s −S(CH2)2S−, as has been found earlier for 113Cd also; the shifts for solutions in which [Zn]total = 0.15 M, to higher frequency than the resonance of 2m Zn(ClO4)2 at 297 K range from 224 to 418 ppm, and the approximate linewidths from ca. 55 Hz for Zn(SMe)2−4 at 368 K to ca. 1200 Hz for Zn(SPri)2−4 at 297 K or Zn???SS])2−2 at 363 K. The line widths appear to be the major factor limiting the application of 67Zn NMR towards complexes of less symmetrical thiolates; rapid chemical exchange is not a problem, as 1H and 13C NMR studies of the zinc complexes show them to be less labile than their cadmium analogs. Conversely the mercury complexes studied here (R = H, Me, Et, or CH2OH), Hg(SR)4 (R = M=, Et, Pri, or Ph) and Hg(SePh)2−4) are more labile than their cadmium counterparts, and this lability reduces the utility of 199Hg NMR in cases where multiple mercury-binding sites are available. The overall variation of δHg with ligand , −SePh < −SPh < −SR (R = alkayl) < chelating vicinal dithiolates, is the same as found in the metal NMR of the corresponding zinc and cadmium complexes, but the (relatively small) effects of alkyl substitution are dissimilar. For ca. 0.1 M solutions in 1:1 (ν:ν) H2O: D2O at 297 K, the 199Hg chemical shifts to higher frequency than the signal of 0.1 M Hg(ClO4)2 in 5% (ν:ν) HClO4 range from 1275 ppm in Hg(SePh)2−4 to 2234 ppm for the linewidths are relatively sharp (e.g. ???4 Hz for Hg(SMe)2−4, ???75 Hz for Hg(SPh)2−4).

A selenium-77 and cadmium-113 NMR spectroscopic study of phenylselenolate complexes of cadmium

Abstract The reduced-temperature slow-exchange 77Se and 113Cd NMR spectra of the complexes [Cd(SPh)n(SePh)4−n]2− have been measured. At 213 K, the 113Cd chemical shifts change linearly from ca. 541 ppm (n = 0) to ca. 590 ppm (n = 4) relative to external 0.1 M Cd(ClO4)2(aq) at ambient probe temperature. Also 1J(113Cd-77Se) changes linearly from 126 ± 3 Hz to 46 ± 4 Hz and the 77Se complexation shifts (= δSe(complex) - δSe(free)) decrease monotonically from ca. −22 ppm to ca. −30 ppm as n is increased from 0 to 3. In the ternary mixtures Cd2+PhS−PhSe−, the selenolate binds preferentially, the ratio of the overall formation constants K([Cd(SePh)4]2−)/K([Cd(SPh)4]2−) being of the order 103–104; the significance of this finding to the well-known antagonism of selenium compounds towards cadmium intoxication is discussed.

Synthesis, NMR (13C, 111Cd, 199Hg) spectra and structures of (Ph4As)2[Hg2Cl4(SC(O)Ph)2] and (Ph4As)2[CdCl4Hg(SC(O)Ph)2]

Abstract Combination of Ph4AsCl · H2O, Hg(SC(O)Ph)2 and HgCl2 or CdCl-· 2.5H2O in the required stoichiometric ratio gives (Ph4As)2[MCl4Hg(SC(O)Ph)2], M = Hg (1) or Cd (2). Crystals of 1 were also isolated in an unsuccessful attempt to prepare the complex (Ph4As)[Na{Hg(SC(O)Ph)3}2]. X-ray crystallographic analyses of 1 and 2 show both to contain a Hg(II) bonded to two thiobenzoate ligands with a near-linear SHgS skeleton. The SHgS angle is 174.9(2)° in 1 and 175.4(1)° in 2. In each, the Hg(SC(O)Ph)2 moiety is involved in weak interactions with two chlorine atoms from [MCl4]2−. Mercury-199 and 111Cd NMR spectral studies indicate that the Hg(SC(O)Ph)2 remains intact in solution, and that scrambling or transfer of ligands between the linear and tetrahedral metal centres does not occur. Carbon-13 and 199Hg NM R data are reported for Hg(SC(O)Ph)2.

π-Arene complexes of the main group elements. II. Complexes of cadmium(II) and zinc(II)

Abstract π-Arene complexes of cadmium(II) and zinc(II) have been prepared from the first time. The 1:1 complexes Cd(AsF6)2. Arene(Arene=hexaethylor hexamethylbenzene, pentamethylbenzene, durene, p -xylene or benzene), Cd(SbF6)2. Arene(Arene = hexamethylbenzene, toluene or benzene) and Zn(SbF6)2. Arene(Arene = hexamethylbenzene or pentamethylbenzene) are synthesized from the strong acid salt and arene in liquid sulfur dioxide. 1H and 13C NMR spectra are consistent with localized bonding of the arene to the metal cation. Exchange-averaged vn]13C chemical shifts for the systems Cd(AsF6)2-arene-SO2 confirm the 1:1 stoichiometry in solution and suggest that the stabilities of the complexes are in the approximate range 0.48 – 2.1 M−1 for the series benzene-hexamethylbenzene. For the system Cd(AsF6)2-C6Me6-SO2, a detailed 113Cd NMR study is consistent with the solution stoichiometry and stability determined from 13C NMR. In general, complexation to an arene produces deshielding of the 113Cd resonance of Cd(AsF6)2.

Syntheses, characterization and thermal properties of [M(Spy)2(SpyH)2] (M=Cd and Hg; Spy−=pyridine-4-thiolate; SpyH=pyridinium-4-thiolate)) and [M(SpyH)4](ClO4)2 (M=Zn, Cd and Hg)

Abstract The complexes [M(Spy)2(SpyH)2] (Spy−=pyridine-4-thiolate; M=Cd (1) and Hg (2)), of a hitherto unknown type, were prepared by reacting the corresponding metal perchlorates with pyridine-4-thiol in the ratio 1:6 in DMSO–MeOH. The cations [M(SpyH)4]2+ (M=Zn, Cd and Hg) were prepared by a similar reaction, but with M–pySH=1:4 in MeOH, and isolated as their perchlorate salts, 3, 4 and 5. The structures of 1–5 were determined by X-ray crystallography. Both compounds 1 and 2, and the cations in 3–5, are mononuclear complexes in which the metal centre adopts a distorted MS4 kernel. Intermolecular NH⋯N hydrogen bonds in 1 and 2 lead to one dimensional polymeric chain structures, and similarly NH⋯O⋯HN hydrogen bonds between the cations and anions in 3–5 result in chain structures. Thermogravimetric and pyrolysis experiments demonstrate that 1 and 2 decompose to give the corresponding metal sulfides. Metal (113Cd/199Hg (as appropriate)) NMR spectra have been measured for solutions of 4 and 5 with and without incremental addition of NaOMe. Separate signals were not observed for [M(SpyH)n(Spy)4−n](n−2)+, even at the lowest accessible temperature. The changes in metal NMR chemical shift as [M(SpyH)4]2+ is converted into [M(Spy)4]2− are surprisingly small.

A preparative and 1H and 13C NMR spectroscopic study of mercury(II)-Arene complexes

Abstract A wide range of new arene complexes of mercury(II) have been isolated from the appropriate arene and a strong acid salt of mercury (Hg(SbF 6 ) 2 , HgF(AsF 6 ) or Hg(O 2 CCF 3 ) 2 ) in liquid SO 2 . Low-temperature, slow-exchange (except when Arene = C 6 H 6 ), 1 H and 13 C NMR spectra of the systems Hg(SbF 6 ) 2 AreneSO 2 have provided evidence for very stable Hg(Arene) 2+ and Hg(Arene)2 2+ complexes. Intramolecular exchange is rapid at the lowest accessible temperatures: the intramolecular exchange-averaged couplings to 199 Hg suggest strongly that the arenes are bound to mercury in an η 1 -manner.

Cadmium binding to metal-free metallothionein: a correlation of UV, CD and 113Cd NMR data and a 113Cd NMR characterization of the binding sites in the reconstituted protein

Abstract 113Cd nmr, circular dichroism and UV absorption spectra have been measured for rat liver 113Cd, Zn-MT formed in vivo following induction with 113CdCl2. These spectra of the native metallothionein are compared with corresponding sets of data for the same sample but first acidified to pH 1.6 and then brought back to pH 7.6. It is shown that at pH 1.6 the cadmium is dissociated from the protein binding sites. The 113Cd nmr spectrum of the sample returned to pH 7.6 demonstrates that when metal-free metallothioneins bind cadmium in vitro the cadmium ions occupy sites that are very similar, or may be even identical, to those occupied in vivo. Taken together, the nmr and CD results represent the first demonstration that metal-free metallothionein can form a metallated protein in vitro in which the geometry of the metal binding sites and the overall protein conformation are almost exactly the same as those found for the native metallated protein.

A multinuclear, (77Se, 119Sn, 125Te) nuclear magnetic resonance spectroscopic study of the series Sn(SPh)x(SePh)y(TePh)4−x−y

Abstract The complete series of fifteen species of the type Sn(SPh) x (SePh) y (TePh) 4−x−y , including thirteen new species, has been prepared in equilibrium mixtures in solution by reaction of equimolar amounts of Sn(EPh) 2 and Ph 2 E′ 2 (E = E′ = S; E = E′ = Se; E = S, E′ = Se; E = Se, E′ = S; E = S, E′ = Te; E = Se, E′ = Te; E = S/Se mixture, E′ = Te; E = S/Se mixture, E′ = Se/Te mixture) and characterized by 119 Sn and 77 Se and/or 125 Te NMR as appropriate. The reaction is thought to involve oxidative insertion followed by redistribution. Equilibrium mixtures of Sn(SPh) x (SePh) 4−x can also be prepared by redistribution of the known compounds Sn(SPh) 4 and Sn(SePh) 4 , and by the redox reaction of Sn(EPh) 2 with PhE′H (E = Se, E′ = S; E = S, E′ = Se). Protonation of Sn(EPh) 3 − (E = S or Se) with CF 3 SO 3 H or CF 3 CO 2 H is shown to produce Sn(EPh) 4 . The 119 Sn, 77 Se and 125 Te NMR chemical shifts and the one-bond 119 Sn 77 Se and 119 Sn 125 Te nuclear spin-spin coupling constants can be fitted satisfactorily using established pairwise additivity models. There are smooth correlations between the NMR chemical shifts of the different nuclei, between 1 J ( 119 Sn 77 Se) and 1 J ( 119 Sn 125 Te), and between the various sets of the chemical shift and spin-spin coupling data. The possible origins of these correlations are discussed.

The subtle tetramorphism of MePh3P+I3−

The crystallisation and crystal structures of three new crystal forms of MePh3P+I3− are described, bringing to four the number of polymorphs of this compound. Crystallisations were from alcohols, mixed alcohols, and dichloromethane, and are reproducible. There are strong similarities between all four of the crystal packing arrangements, which are solvent-free. In all structures the MePh3P+ cations associate in two-dimensional nets using edge-to-face, offset-face-to-face, and methyl-to-face motifs, and the I3− ions lie between these nets, with subtle variations in structure caused by sliding displacements of relatively constant cation-anion assemblies. The related compound EtPh3P+I3− has very similar crystal packing. The interactions between the I3− and MePh3P+ are mainly C–H⋯I, with some phenyl faces parallel to I3−. The major electrostatic component of the lattice energy is associated with the occurrence of I3− ions against the faces of nets of cations, consistent with this structural feature being the most invariant aspect of the crystal packing, while the structural variability is associated with the less stabilising cation⋯cation motifs. It appears that these crystals provide a good example of a system where shape is not a dominant feature in crystal packing, and that the tetramorphs represent almost equi-ergic local minima in a relatively deep section of the lattice energy hypersurface. The crystal packing of (MePh3P+)2I82− is similar.