Graeme Hogarth

Metal-dithiocarbamate complexes: chemistry and biological activity

Dithiocarbamates are highly versatile mono-anionic chelating ligands which form stable complexes with all the transition elements and also the majority of main group, lanthanide and actinide elements. They are easily prepared from primary or secondary amines and depending upon the nature of the cation can show good solubility in water or organic solvents. They are related to the thiuram disulfides by a one-electron redox process (followed by dimerisation via sulfur-sulfur bond formation) which is easily carried out upon addition of iodide or ferric salts. Dithiocarbamates are lipophilic and generally bind to metals in a symmetrical chelate fashion but examples of other coordination modes are known, the monodentate and anisobidentate modes being most prevalent. They are planar sterically non-demanding ligands which can be electronically tuned by judicious choice of substituents. They stabilize metals in a wide range of oxidation states, this being attributed to the existence of soft dithiocarbamate and hard thioureide resonance forms, the latter formally resulting from delocalization of the nitrogen lone pair onto the sulfurs, and consequently their complexes tend to have a rich electrochemistry. Tetraethyl thiuramdisulfide (disulfiram or antabuse) has been used as a drug since the 1950s but it is only recently that dithiocarbamate complexes have been explored within the medicinal domain. Over the past two decades anti-cancer activity has been noted for gold and copper complexes, technetium and copper complexes have been used in PET-imaging, dithiocarbamates have been used to treat acute cadmium poisoning and copper complexes also have been investigated as SOD inhibitors.

Multimetallic Assemblies Using Piperazine-Based Dithiocarbamate Building Blocks

Treatment of cis-[RuCl2(dppm)2] (dppm = bis(diphenylphosphino)methane) with dithiocarbamates, NaS2CNR2 (R = Me, Et) and [H2NC5H10][S2CNC5H10], yields cations [Ru(S2CNR2)2(dppm)2](+) and [Ru(S2CNC5H10)2(dppm)2](+), respectively. The zwitterions S2CNC4H8NHR (R = Me, Et) react with the same metal complex in the presence of base to yield [Ru(S2CNC4H8NR)(dppm)2](+). Piperazine or 2,6-dimethylpiperazine reacts with carbon disulfide to give the zwitterionic dithiocarbamate salts H2NC4H6(R2-3,5)NCS2 (R = H; R = Me), which form the complexes [Ru(S2CNC4H6(R2-3,5)NH2)(dppm)2](2+) on reaction with cis-[RuCl2(dppm)2]. Sequential treatment of [Ru(S2CNC4H8NH2)(dppm)2](2+) with triethylamine and carbon disulfide forms the versatile metalla-dithiocarbamate complex [Ru(S2CNC4H8NCS2)(dppm)2] which reacts readily with cis-[RuCl2(dppm)2] to yield [{Ru(dppm)2}2(S2CNC4H8NCS2)]. Reaction of [Ru(S2CNC4H8NCS2)(dppm)2] with [Os(CH=CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] (BTD = 2,1,3-benzothiadiazole), [Pd(C6H4CH2NMe2)Cl]2, [PtCl2(PEt3)2], and [NiCl2(dppp)] (dppp = 1,3-bis(diphenylphosphino)propane) results in the heterobimetallic complexes [(dppm)2Ru(S2CNC4H8NCS2)ML(n))](m+) (ML(n) = Os(CH=CHC6H4Me-4)(CO)(PPh3)2](+), m = 1; ML(n) = Pd(C,N-C6H4CH2NMe2), m = 1; ML(n) = Pt(PEt3)2, m = 2; ML(n) = Ni(dppp), m = 2). Reaction of [NiCl2(dppp)] with H2NC4H8NCS2 yields the structurally characterized compound, [Ni(S2CNC4H8NH2)(dppp)](2+), which reacts with base, CS2, and cis-[RuCl2(dppm)2] to provide an alternative route to [(dppm)2Ru(S2CNC4H8NCS2)Ni(dppp)](+). A further metalla-dithiocarbamate based on cobalt, [CpCo(S2CNC4H8NH2)(PPh3)](2+), is formed by treatment of CpCoI2(CO) with S2CNC4H8NH2 followed by PPh3. Further reaction with NEt3, CS2, and cis-[RuCl2(dppm)2] yields [(Ph3P)CpCo(S2CNC4H8NCS2)Ru(dppm)2](2+). Heterotrimetallic species of the form [{(dppm)2Ru(S2CNC4H8NCS2)}2M](2+) result from the reaction of [Ru(S2CNC4H8NCS2)(dppm)2] and M(OAc)2 (where M = Ni, Cu, Zn). Reaction of [Ru(S2CNC4H8NCS2)(dppm)2] with Co(acac)3 and LaCl3 results in the formation of the compounds [{(dppm)2Ru(S2CNC4H8NCS2)}3Co](3+) and [{(dppm)2Ru(S2CNC4H8NCS2)}3La](3+), respectively. The electrochemical behavior of selected examples is also reported.

Cluster chemistry in the Noughties: new developments and their relationship to nanoparticles

Over the past decade, the chemistry of low-valent transition metal clusters has again come to the fore, primarily as a result of the development of nanochemistry and the realization that large clusters are on the cusp of the nano-domain. This perspective focuses on these recent developments in low-valent transition metal cluster chemistry, specifically looking at cluster-nanoparticles, the use of small and medium sized clusters as nanoparticle precursors, the development of clusters as homogeneous catalysts and hydrogen uptake and storage systems, together with fundamental discoveries relating to novel transformations that can take place within the cluster framework.

Models of the iron-only hydrogenase: Structural studies of chelating diphosphine complexes [Fe2(CO)4(µ-pdt)(κ2P,P′-diphosphine)]

Six chelating diphosphine complexes, [Fe2(CO)4(µ-pdt)(κ2P,P′-diphosphine)], have been crystallographically characterised allowing differences between basal–apical and dibasal conformations to be analysed.

Models of the iron-only hydrogenase: a comparison of chelate and bridge isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as proton-reduction catalysts

Reactions of Fe2(CO)6(μ-pdt) (pdt = SCH2CH2CH2S) with aminodiphosphines Ph2PN(R)PPh2 (R = allyl, (i)Pr, (i)Bu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me3NO·2H2O, dibasal chelate complexes Fe2(CO)4{κ(2)-Ph2PN(R)PPh2}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe2(CO)4{μ-Ph2PN(R)PPh2}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, (i)Pr) have been crystallographically characterised together with Fe2(CO)4{μ-Ph2PN(H)PPh2}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P-Fe-P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe2(CO)4{Ph2PN(Me)PPh2}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal-metal bonding character, supporting a site-specific protonation of the iron-iron bond upon treatment with acid. Addition of HBF4·Et2O to the Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-H)(μ-pdt)][BF4], with spectroscopic measurements revealing the intermediate formation of a basal-apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe2(CO)4{Ph2PN(allyl)PPh2}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe-Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. -1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron-iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron-iron bond leads to faster catalysis for models of the Fe-Fe hydrogenase active site.

para-Ethynyl aniline as a building block for fully π-conjugated ligands and acetylide complexes: crystal structures of trans-[Pt(PPh3)2(CCC6H4NH2)2] and [(μ-H)Ru3(CO)9(μ3-CCC6H4NH2)]

para-Ethynyl aniline has been prepared, structurally characterised and investigated as a building block towards fully pi -conjugated multifunctional ligands and complexes. Palladium-copper catalysed coupling with aryl halides affords a number of new amino-substituted aryl acetylenes, while using [Ni(CO)(2)(PPh3)(2)] as a catalyst, cyclotrimerisation and dimerisation to give an ene-yne were competitive. Reaction of para-ethynyl aniline with low-valent metal centres affords acetylide complexes trans[Pt(PR3)(2)(C=CC6H4NH2)(2)] (R = Ph, Bu-n), cis-[Pt(eta (2)-dppe)(C=CC6H4NH2)(2)], all trans-[Ru(CO)(2)(PEt3)(2)(C=CC6H4NH2)(2)] and [(muH)Ru-3(CO)(9)(mu C-3=CC6H4NH2)]. The bis(acetylide) trans-[Pt(PPh3)(2)(C=CC6H4NH2)(2)] has been used to prepare extended chain complexes with amide, imine, imino-phosphorane and ferrocenyl imine units being generated. Attempts to prepare polymers via reaction with terephthaloyl chloride lead only to the formation of oligomers with an average of four monomer units

Bifunctional dithiocarbamates: a bridge between coordination chemistry and nanoscale materials

A dithiocarbamate-based methodology is employed to prepare linked heteromultimetallic complexes and then further exploited in the surface functionalisation of gold nanoparticles.

Copper-Doped CdSe/ZnS Quantum Dots: Controllable Photoactivated Copper(I) Cation Storage and Release Vectors for Catalysis†

The first photoactivated doped quantum dot vector for metal-ion release has been developed. A facile method for doping copper(I) cations within ZnS quantum dot shells was achieved through the use of metal-dithiocarbamates, with Cu+ ions elucidated by X-ray photoelectron spectroscopy. Photoexcitation of the quantum dots has been shown to release Cu+ ions, which was employed as an effective catalyst for the Huisgen [3+2] cycloaddition reaction. The relationship between the extent of doping, catalytic activity, and the fluorescence quenching was also explored.

Multimetallic Arrays: Bi-, Tri-, Tetra-, and Hexametallic Complexes Based on Gold(I) and Gold(III) and the Surface Functionalization of Gold Nanoparticles with Transition Metals

Reaction of [AuCl(PPh(3))] with the zwitterion S(2)CNC(4)H(8)NH(2) yields [(Ph(3)P)Au(S(2)CNC(4)H(8)NH(2))]BF(4). Treatment of this species with NEt(3) and CS(2) followed by [AuCl(PPh(3))] leads to [{(Ph(3)P)Au}(2)(S(2)CNC(4)H(8)NCS(2))], which can also be obtained directly from [AuCl(PPh(3))] and KS(2)CNC(4)H(8)NCS(2)K. A heterobimetallic variant, [(dppm)(2)Ru(S(2)CNC(4)H(8)NCS(2))Au(PPh(3))](+), can be prepared by the sequential reaction of [(dppm)(2)Ru(S(2)CNC(4)H(8)NH(2))](2+) with NEt(3) and CS(2) followed by [AuCl(PPh(3))]. Reaction of the same ruthenium precursor with [(dppm)(AuCl)(2)] under similar conditions yields the trimetallic complex [(dppm)(2)Ru(S(2)CNC(4)H(8)NCS(2))Au(2)(dppm)](2+). Attempts to prepare the compound [(dppm)Au(2)(S(2)CNC(4)H(8)NH(2))](2+) from [(dppm)(AuCl)(2)] led to isolation of the known complex [{(dppm)Au(2)}(2)(S(2)CNC(4)H(8)NCS(2))](2+) via a symmetrization pathway. [{(dppf)Au(2)}(2)(S(2)CNC(4)H(8)NCS(2))](2+) was successfully prepared from [(dppf)(AuCl)(2)] and crystallographically characterized. In addition, a gold(III) trimetallic compound, [{(dppm)(2)Ru(S(2)CNC(4)H(8)NCS(2))}(2)Au](3+), and a tetrametallic gold(I) species, [{(dppm)(2)Ru(S(2)CNC(4)H(8)NCS(2))Au}(2)](2+), were also synthesized. This methodology was further exploited to attach the zwitterionic (dppm)(2)Ru(S(2)CNC(4)H(8)NCS(2)) unit to the surface of gold nanoparticles, which were generated in situ and found to be 3.4 (+/-0.3) and 14.4 (+/-2.5) nm in diameter depending on the method employed. Nanoparticles with a mixed surface topography were also explored.

Dimolybdenum oxo-imido complexes: crystal structures of [(MeC5H4)2Mo2O2(μ-O)(μ-NPh)] and [(MeC5H4)MoO(μ-NPh)]2

Abstract Oxidation of [(MeC5H4)Mo(CO)3]2 (1) by nitrobenzene in refluxing toluene affords three dimolybdenum oxo-imido complexes [(MeC5H4)2Mo2O2(μ-O)(μ-NPh)] (2), [(MeC5H4)MoO(μ-NPh)2 (3), and [(MeC5H4)2Mo2O(NPh)(μ-O)(μ-NPh)] (4). Complexes 3 and 4 are isomers but do not interconvert. Complex 4 undergoes facile hydrolysis in hydrocarbonsolvents giving 2 and aniline. The X-ray crystal structures of compounds 2 and 3 have been determined. Both structures display a trans-disposition of methylcyclopentadienyl ligands and terminal oxo ligands. The Mo2 (μ-X)2 metallacore geometries are rigorously planar with the phenyl substituent(s) of the imido ligands being titled slightly out of this plane. The MoMo distances correspond to normal single bonds.