Synthesis,characterization, cytotoxicity,and DNA binding of some new platinum(II) and platinum(IV) complexes with benzimidazole ligands

In this study, two Pt(II) and three Pt(IV) complexes with the structures of [PtL₂Cl₂] (1), [PtL₂I₂] (2), [PtL₂Cl₂(OH)₂] (3), [PtL₂Cl₂(OCOCH₃)₂] (4), and [PtL₂Cl₄] (5) (L = benzimidazole as carrier ligand) were synthesized and evaluated for their in vitro antiproliferative activities against the human MCF-7, HeLa, and HEp-2 cancer cell lines. The influence of compounds 1–5 on the tertiary structure of DNA was determined by their ability to modify the electrophoretic mobility of the form I and II bands of pBR322 plasmid DNA. The inhibition of BamH1 restriction enzyme activity of compounds 1–5 was also determined. In general, it was found that compounds 1–5 were less active than cisplatin and carboplatin against MCF-7 and HeLa cell lines (except for 1, which was found to be more active than carboplatin against the MCF-7 cell line). Compounds 1 and 3 were found to be significantly more active than cisplatin and carboplatin against the HEp-2 cell line.     

Trans-Pd/Pt(II) saccharinate complexes with a phosphine ligand: Synthesis,cytotoxicity and structure-activity relationship

New trans-[Pd(sac)₂(PPhMe₂)(DMSO)]·H₂O (Pd) and trans-[Pt(sac)₂(PPhMe₂)₂]·H₂O (Pt) complexes (sac = saccharinate and PPhMe₂ = dimethylphenylphosphine) were synthesized and characterized by elemental analysis, IR, NMR, ESI-MS spectral analyses and X-ray diffraction. The complexes were evaluated for their in vitro cytotoxicity against breast (MCF-7), colon (HCT116) and lung (A549) human cancer cell lines. The ATP viability assay displayed that Pd was biologically inactive, but Pt showed significant anticancer potency on MCF-7 cancer cells, similar to cisplatin. The results suggested that Pt targeted DNA, whereas Pd displayed higher binding affinity towards human serum albumin (HSA). Mechanism of action studies of Pt suggested apoptotic cell death due to significant increase in intracellular ROS (reactive oxygen species) levels, mitochondrial damage and formation of DNA double-strand breaks. Finally, this work represents a new example of potent transplatin anticancer complexes.     

Thiolato-bridged “Linear” trinuclear platinum complexes [Pt3(μ-SR)4(dppm)2]

Thiolato-bridged tri- and dinuclear platinum complexes of the types [Pt₃(μ-SR)₄(dppm)₂]²⁺ (1) and [Pt₂(μ-ER)₂(dppm)₂]²⁺ (2) (E=S or Se; R=alkyl or aryl; dppm=bis(diphenylphosphino)methane) have been prepared using the mononuclear precursors [Pt(ER)₂(dppm)]. The complexes have been characterized by NMR (¹H, ¹³C, ³¹P, ¹⁹⁵Pt), FT-IR and FAB mass spectral data. The structure of [Pt₃(μ-SC₆H₄CH₃-4)₄(dppm)₂][CF₃SO₃]₂ · 6CH₂Cl₂ (1d), has been established through X-ray crystallography, revealing a zig-zag arrangement of the three coordination spheres around the platinum atoms.     

The arylation of [Pt2(μ-S)2(PPh3)4]

Routes to the synthesis of the mixed sulfide-phenylthiolate complex [Pt₂(μ-S)(μ-SPh)(PPh₃)₄]⁺ have been explored; reaction of [Pt₂(μ-S)₂(PPh₃)₄] with excess Ph₂IBr proceeds readily to selectively produce this complex, which was structurally characterised as its PF₆⁻ salt. Reactions of [Pt₂(μ-S)₂(PPh₃)₄] with other potent arylating reagents (1-chloro-2,4-dinitrobenzene and 1,5-difluoro-2,4-dinitrobenzene) also produce the corresponding nitroaryl-thiolate complexes [Pt₂(μ-S){μ-SC₆H₂(NO₂)₂X}(PPh₃)₄]⁺ (X = H, F). The complex [Pt₂(μ-S)(μ-SPh)(PPh₃)₄]⁺ reacts with Me₂SO₄ to produce the mixed alkyl/aryl bis-thiolate complex [Pt₂(μ-SMe)(μ-SPh)(PPh₃)₄]²⁺, but corresponding reactions with the nitroaryl-thiolate complexes are plagued by elimination of the nitroaryl group and formation of [Pt₂(μ-SMe)₂(PPh₃)₄]²⁺. [Pt₂(μ-S)(μ-SPh)(PPh₃)₄]⁺ also reacts with Ph₃PAuCl to give [Pt₂(μ-SAuPPh₃)(μ-SPh)(PPh₃)₄]²⁺.     

Synthesis and characterisation of adducts of [Pt2(μ-S)2(PPh3)4] with organo-palladium and platinum-hydride substrates

The reactions of [Pt₂(μ-S)₂(PPh₃)₄] towards a range of palladium(II) complexes containing organometallic ligands (cyclopalladated N-donor ligands, η³-allyl, phenyl) have been explored, leading to the formation of a series of cationic, trinuclear sulfido-bridged aggregates containing {Pt₂PdS₂} cores. [Pt₂(μ-S)₂(PPh₃)₄] also reacts with the platinum(II) hydride complex trans-[PtHCl(PPh₃)₂] giving the adduct [Pt₂(μ-S)₂(PPh₃)₄PtH(PPh₃)]⁺. X-ray crystal structure determinations on the complexes [Pt₂(μ-S)₂(PPh₃)₄PdPh(PPh₃)]PF₆ and [Pt₂(μ-S)₂(PPh₃)₄PtH(PPh₃)]PF₆ are reported, and show the expected bis μ₃-sulfido aggregates with three square-planar metal centres.     

Coordination chemistry of the metalloligand [Pt2(μ-S)2(PPh3)4] with nickel(II) complexes - an electrospray mass spectrometry directed synthetic study

The reactivity of [Pt₂(μ-S)₂(PPh₃)₄] towards a range of nickel(II) complexes has been probed using electrospray ionisation mass spectrometry coupled with synthesis and characterisation in selected systems. Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with [Ni(NCS)₂(PPh₃)₂] gives [Pt₂(μ-S)₂(PPh₃)₄Ni(NCS)(PPh₃)]⁺, isolated as its BPh₄ ⁻ salt; the same product is obtained in the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with [NiBr₂(PPh₃)₂] and KNCS. An X-ray structure determination reveals the expected sulfide-bridged structure, with an N-bonded thiocyanate ligand and a square-planar coordination geometry about nickel. A range of nickel(II) complexes NiL₂, containing β-diketonate, 8-hydroxyquinolinate, or salicylaldehyde oximate ligands react similarly, giving [Pt₂(μ-S)₂(PPh₃)₄NiL]⁺ cations.     

Transition metal complexes with pyrazole derivatives as ligands

Degradation reactions of scorpionates were observed in the presence of transition metal salts MX₂ to give complexes of transition metal and pyrazole derivatives. Otherwise, pyrazolato complexes of transition metals and weakly coordinating anions such as nitrates have been synthesized from transition metal nitrates and 3-phenyl- and 4-phenyldiazo-pyrazole. A number of complexes with pyrazole derivatives as ligands, [Zn(3-tBupzH)₂Cl₂], [Fe₂(3-Phpz)₆Cl₄], [Cu(pzH)₄Br₂], [Ni(py)₂(pzH)₂Cl₂], [Li(THF)₄][Ti₂(μ-pz)₃Cl₄(NMe₂)₂], [Zn₂(μ-3-Phpz)₂(3-PhpzH)₂][(NO₃)₂], [M(3-PhpzH)₄(NO₃)₂] (M = Co, Ni, Cu, Zn, Cd), [Zn(3-PhpzH)₂(NO₃)₂], [Zn(4-PhNNpzH)₂(NO₃)₂](H₂O), and [Cd(4-PhNNpzH)₂(NO₃)₂(H₂O)₂], have been crystallized and characterized by single-crystal X-ray diffraction.     

Thallium(III) complexes of the metalloligands [Pt2(μ-S)2(PPh3)4] and [Pt2(μ-Se)2(PPh3)4]

Reactions of [Pt₂(μ-S)₂(PPh₃)₄] with the diarylthallium(III) bromides Ar₂TlBr [Ar = Ph and p-ClC₆H₄] in methanol gave good yields of the thallium(III) adducts [Pt₂(μ-S)₂(PPh₃)₄TlAr₂]⁺, isolated as their salts. The corresponding selenide complex [Pt₂(μ-Se)₂(PPh₃)₄TlPh₂]BPh₄ was similarly synthesised from [Pt₂(μ-Se)₂(PPh₃)₄], Ph₂TlBr and NaBPh₄. The reaction of [Pt₂(μ-S)₂(PPh₃)₄] with PhTlBr₂ gave [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]⁺, while reaction with TlBr₃ gave the dibromothallium(III) adduct [Pt₂(μ-S)₂(PPh₃)₄TlBr₂]⁺[TlBr₄]⁻. The latter complex is a rare example of a thallium(III) dihalide complex stabilised solely by sulfur donor ligands. X-ray crystal structure determinations on the complexes [Pt₂(μ-S)₂(PPh₃)₄TlPh₂]BPh₄, [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]BPh₄ and [Pt₂(μ-S)₂(PPh₃)₄TlBr₂][TlBr₄] reveal a greater interaction between the thallium(III) centre and the two sulfide ligands on stepwise replacement of Ph by Br, as indicated by shorter Tl-S and Pt?Tl distances, and an increasing S-Tl-S bond angle. Investigations of the ESI MS fragmentation behaviour of the thallium(III) complexes are reported.     

Synthesis and characterisation of [Pt2(μ-S)(μ-I)(PPh3)4] - A cationic iodo analogue of the metalloligand [Pt2(μ-S)2(PPh3)4]

Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with a number of transition metal-iodo complexes leads to the formation of the cationic iodo analogue [Pt₂(μ-S)(μ-I)(PPh₃)₄]⁺, identified using electrospray ionisation mass spectrometry (ESI MS). Synthetic routes to this complex were developed, using the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with either [PtI₂(PPh₃)₂] or elemental iodine. The complex was characterised by NMR spectroscopy, ESI MS and an X-ray structure determination, which reveals the presence of a planar, disordered {Pt₂SI}⁺ core. Monitoring the iodine reaction by ESI MS allows the identification of various iodine species, including the short-lived intermediate [Pt₂(μ-S)₂(PPh₃)₄I]⁺, which allows a mechanism for the reaction to be proposed.     

Triphos as a bidentate ligand: the reactivity of the dangling phosphorus in [PtMe2(triphos-P,P′)]

Complex [PtMe₂(triphos-P,P′)], (1) where the linear triphosphine triphos [=bis(diphenylphosphinoethyl)phenylphosphine] acts as a bidentate ligand, can be easily converted in a variety of new complexes due to the reactivity of the free phosphorus donor. The selective oxidation of the uncoordinated phosphorus gave [PtMe₂(triphosPO-P,P′)] whose X-ray crystal structure is here reported; from the reactions of 1 with platinum and non platinum precursors homotrimetallic [Pt₃Me₄XY(triphos)₂] (X=Y=Me, Cl, I, X=Me, Y=Cl) and heterotrimetallic ([Pt₂PdMe₄Cl₂(triphos)₂] and [Pt₂RhMe₄(cod)(triphos)₂]PF₆) complexes were obtained where triphos acts as a chelating/bridging ligand. When 1 was treated with triflic acid in the presence of a neutral electron donor L (L=SMe₂, pyridine, PPh₃), complexes [PtL(triphos)]²⁺ were rapidly recovered in high yields. The protonolysis of 1 in the presence of CO and methanol gave the new organometallic complex [Pt(COOMe)(triphos)]OTf.     

Dithiolate and diselenolate complexes [Pt2(μ-ECH2CHCHCH2E)(PPh3)4] (E = S, Se): Synthesis, characterisation and mass spectrometric formation of the dichalcogenide species [Pt2(μ-E2)(PPh3)4]

The reactions of [Pt₂(μ-E)₂(PPh₃)₄] (E = S, Se) with cis-1,4-dichlorobut-2-ene (cis-ClCH₂CHCHCH₂Cl) give the dichalcogenolate complexes [Pt₂(μ-ECH₂CHCHCH₂E)(PPh₃)₄]²⁺; an X-ray structure determination on the thiolate complex was carried out. The complexes give the expected dications in ESI mass spectra recorded at very low cone voltages, but at moderate cone voltages undergo facile fragmentation via a retro-Diels-Alder reaction and loss of 1,3-butadiene, giving the dichalcogenide species [Pt₂(μ-E₂)(PPh₃)₄]²⁺. Analogous species containing bidentate phosphine or arsine ligands have been previously generated electrochemically, and studied theoretically.     

Synthesis, characterization, interaction with DNA and cytotoxicity in vitro of dinuclear Pd(II) and Pt(II) complexes dibridged by 2,2'-azanediyldibenzoic acid

The dinuclear complexes [Pd₂(L)₂(bipy)₂] (1), [Pd₂(L)₂(phen)₂] (2), [Pt₂(L)₂(bipy)₂] (3) and [Pt₂(L)₂(phen)₂] (4), where bipy = 2,2′-bipyridine, phen = 1,10-phenanthroline and L = 2,2′-azanediyldibenzoic dianion) dibridged by H₂L ligands have been synthesized and characterized. The binding of the complexes with fish sperm DNA (FS-DNA) were investigated by fluorescence spectroscopy. The results indicate that the four complexes bound to DNA with different binding affinity, in the order complex 4 > complex 3 > complex 2 > complex 1, and the complex 3 binds to DNA in both coordination and intercalative mode. Gel electrophoresis assay demonstrates the ability of the complexes to cleave the pBR 322 plasmid DNA. The cytotoxic activity of the complexes was tested against four different cancer cell lines. The four complexes exhibited cytotoxic specificity and significant cancer cell inhibitory rate.     

Dinuclear platinum complexes with designer thiolate ligands from the monoalkylation of [Pt2(μ-S)2(PPh3)4]

Alkylation reactions of the nucleophilic platinum(II) sulfide complex [Pt₂(μ-S)₂(PPh₃)₄] with functionalised alkylating agents have been investigated as a versatile synthetic route to dinuclear, cationic sulfide-thiolate complexes of the type [Pt₂(μ-S)(μ-SR)(PPh₃)₄]⁺, extending the range of thiolate complexes that can be prepared using this methodology. A wide range of functional groups can be incorporated, using appropriate alkylating agents, and include ketone, ester, amide, hydrazone, semicarbazone, thiosemicarbazone, oxime, guanidine, urea and thiourea groups.     

Synthesis of new platinum(II) complexes containing hybrid thioether-pyrazole ligands: Structural analysis by H and C{H} NMR spectroscopy and X-ray crystal structures

Treatment of the ligands 1,8-bis(3,5-dimethyl-1-pyrazolyl)-3,6-dithiaoctane (bddo), 1,9-bis(3,5-dimethyl-1-pyrazolyl)-3,7-dithianonane (bddn), and 1,6-bis(3,5-dimethyl-1-pyrazolyl)-2,5-dithiahexane (bddh) with several platinum starting materials as K₂PtCl₄, PtCl₂, [PtCl₂(CH₃CN)₂] and [PtCl₂(PhCN)₂] was developed under different conditions. The reactions did not yield pure products. The ratio of the NSSN, NS, SS, NN, and 2NS isomers has been calculated through NMR experiments. Treatment of the mixtures of complexes with NaBPh₄ affords [Pt(NSSN)](BPh₄)₂ (NSSN = bddo, bddn). These Pt(II) complexes have been characterised by elemental analyses, conductivity measurements, IR and ¹H and ¹³C NMR spectroscopy. The X-ray structures of the complexes [Pt(NSSN)](BPh₄)₂ (NSSN = bddo, bddn) have also been determined. In these complexes, the metal atom is tetracoordinated by the two azine nitrogen atoms of the pyrazole rings and two thioether sulfur atoms. When the [Pt(NSSN)](BPh₄)₂ (NSSN = bddo, bddn) complexes were heated under reflux in a solution of Et₄NBr in CH₂Cl₂/CH₃OH (1:1), a mixture of isomers was obtained.     

Extending the coordination chemistry of cobalt with the metalloligand [Pt2(μ-S)2(PPh3)4]: Synthesis of the first cobalt(III) derivatives

The coordination chemistry of the metalloligand [Pt₂(μ-S)₂(PPh₃)₄] towards cobalt(II) and cobalt(III) centres has been explored using an electrospray ionisation mass spectrometry (ESI MS)-directed methodology. Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with CoCl₂·6H₂O in methanol gave a green-yellow suspension of the known adduct [Pt₂(μ-S)₂(PPh₃)₄CoCl₂], and the CoBr₂ adduct could be similarly prepared. When in situ-generated [Pt₂(μ-S)₂(PPh₃)₄CoCl₂] is reacted with 8-hydroxyquinoline (HQ) and base, the initial product is the cobalt(II) adduct [Pt₂(μ-S)₂(PPh₃)₄CoQ]⁺, which is then converted in air to the cobalt(III) adduct [Pt₂(μ-S)₂(PPh₃)₄CoQ₂]⁺, isolated as its hexafluorophosphate salt. The corresponding picolinate (Pic) derivative [Pt₂(μ-S)₂(PPh₃)₄Co(Pic)₂]⁺ was similarly prepared, however reaction of [Pt₂(μ-S)₂(PPh₃)₄], CoCl₂·6H₂O and 8-(tosylamino)quinoline (HTQ) produced only the cobalt(II) adduct [Pt₂(μ-S)₂(PPh₃)₄CoTQ]⁺. Reactions of [Pt₂(μ-S)₂(PPh₃)₄], CoCl₂·6H₂O and dithiocarbamates gave cobalt(III) complexes [Pt₂(μ-S)₂(PPh₃)₄Co(S₂CNR₂)₂]⁺ [R = Et or R₂ = (CH₂)₄], and proceeded much more rapidly, consistent with the known ability of the dithiocarbamate ligand to stabilize cobalt in higher oxidation states. A study of the fragmentation of cobalt(III) adducts by positive-ion ESI mass spectrometry indicated that [Pt₂(μ-S)₂(PPh₃)₄CoQ₂]⁺ fragments to form the radical cation [Pt₂(μ-S)₂(PPh₃)₄]⁺, which could also be generated by ESI MS analysis of [Pt₂(μ-S)₂(PPh₃)₄] in methanol-NaOH solution. In contrast, the corresponding indium(III) derivative [Pt₂(μ-S)₂(PPh₃)₄InQ₂]⁺, and the cobalt(III) dithiocarbamate [Pt₂(μ-S)₂(PPh₃)₄Co(S₂CN(CH₂)₄)₂]⁺ are much more reluctant to fragment under analogous conditions, and the differences are discussed in terms of cobalt(III) redox chemistry.     

Influence of chain length on mono- versus di-alkylation in the reactivity of [Pt2(μ-S)2(PPh3)4] towards α,ω-dihalo-n-alkanes; a synthetic route to platinum(II) ω-haloalkylthiolate complexes

The reactions of [Pt₂(μ-S)₂(PPh₃)₄] with α,ω-dibromoalkanes Br(CH₂)_nBr (n = 4, 5, 6, 8, 12) gave mono-alkylated [Pt₂(μ-S){μ-S(CH₂)_nBr}(PPh₃)₄]⁺ and/or di-alkylated [Pt₂(μ-S(CH₂)_nS}(PPh₃)₄]²⁺ products, depending on the alkyl chain length and the reaction conditions. With longer chains (n = 8, 12), intramolecular di-alkylation does not proceed in refluxing methanol, with the mono-alkylated products [Pt₂(μ-S){μ-S(CH₂)_nBr}(PPh₃)₄]⁺ being the dominant products when excess alkylating agent is used. The bridged complex [{Pt₂(μ-S)₂(PPh₃)₄}₂{μ-(CH₂)₁₂}]²⁺ was accessible from the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with 0.5 mol equivalents of Br(CH₂)₁₂Br. [Pt₂(μ-S){μ-S(CH₂)₄Br}(PPh₃)₄]⁺ can be cleanly isolated as its BPh₄⁻ salt, but undergoes facile intramolecular di-alkylation at −18 °C, giving the known species [Pt₂(μ-S(CH₂)₄S}(PPh₃)₄]²⁺. The reaction of I(CH₂)₆I with [Pt₂(μ-S)₂(PPh₃)₄] similarly gives [Pt₂(μ-S){μ-S(CH₂)₆I}(PPh₃)₄]⁺, which is fairly stable towards intramolecular di-alkylation once isolated. These reactions provide a facile route to ω-haloalkylthiolate complexes which are poorly defined in the literature. X-ray crystal structures of [Pt₂(μ-S){μ-S(CH₂)₅Br}(PPh₃)₄]BPh₄ and [Pt₂(μ-S(CH₂)₅S}(PPh₃)₄](BPh₄)₂ are reported, together with a study of these complexes by electrospray ionisation mass spectrometry. All complexes fragment by dissociation of PPh₃ ligands, and the bromoalkylthiolate complexes show additional fragment ions [Pt₂(μ-S){μ-S(CH₂)_n−2CHCH₂}(PPh₃)_m]⁺ (m = 2 or 3; m ≠ 4), most significant for n = 4, formed by elimination of HBr.     

Further studies on the dialkylation chemistry of [Pt2(μ-S)2(PPh3)4] with activated alkyl halides RC(O)CH2X (X = Cl, Br)

Further studies have been carried out into the reactivity of [Pt₂(μ-S)₂(PPh₃)₄] towards a range of activated alkylating agents of the type RC(O)CH₂X (R = organic moiety, e.g. phenyl, pyrenyl; X = Cl, Br). Alkylation of both sulfide centers is observed for PhC(O)CH₂Br, 3-(bromoacetyl)coumarin [CouC(O)CH₂Br], and 1-(bromoacetyl)pyrene [PyrC(O)CH₂Br], giving dications [Pt₂{μ-SCH₂C(O)R}₂(PPh₃)₄]²⁺, isolated as their PF₆⁻ salts. The X-ray structure of [Pt₂{μ-SCH₂C(O)Ph}₂(PPh₃)₄](PF₆)₂ shows the presence of short Pt?O contacts. In contrast, the corresponding chloro compounds [typified by PhC(O)CH₂Cl] and imino analogues [e.g. PhC(NOH)CH₂Br] do not dialkylate [Pt₂(μ-S)₂(PPh₃)₄]. The ability of PhC(O)CH₂Br to dialkylate [Pt₂(μ-S)₂(PPh₃)₄] allows the synthesis of new mixed-alkyl dithiolate derivatives of the type [Pt₂{μ-SCH₂C(O)Ph}(μ-SR)(PPh₃)₄]²⁺ (R = Et or n-Bu), through alkylation of in situ-generated monoalkylated compounds [Pt₂(μ-S)(μ-SR)(PPh₃)₄]⁺ (from [Pt₂(μ-S)₂(PPh₃)₄] and excess RBr). In these heterodialkylated systems ligand replacement of PPh₃ occurs by the bromide ions in the reaction mixture forming monocations [Pt₂{μ-SCH₂C(O)Ph}(μ-SR)(PPh₃)₃Br]⁺. This ligand substitution can be easily suppressed by addition of PPh₃ to the reaction mixture. The complex [Pt₂{μ-SCH₂C(O)Ph}(μ-SBu)(PPh₃)₄]²⁺ was crystallographically characterized. X-ray crystal structures of the bromide-containing complexes [Pt₂{μ-SCH₂C(O)Ph}(μ-SR)(PPh₃)₃Br]⁺ (R = Et, Bu) are also reported. In both structures the coordinated bromide is trans to the SCH₂C(O)Ph ligand, which adopts an axial position, while the ethyl and butyl substituents adopt equatorial positions, in contrast to the structures of the dialkylated complexes [Pt₂{μ-SCH₂C(O)Ph}₂(PPh₃)₄]²⁺ and [Pt₂{μ-SCH₂C(O)Ph}(μ-SBu)(PPh₃)₄]²⁺ (and many other known analogues) where both alkyl groups adopt axial positions.     

An electrospray mass spectrometry-directed survey of the coordination chemistry of the metalloligand [Pt2(μ-S)2(PPh3)4] with platinum(II) and palladium(II) chloride substrates: influence of metal-ligand lability on product type

The reactivity of the metalloligand [Pt₂(μ-S)₂(PPh₃)₄] towards a wide range of platinum(II) and palladium(II) chloride complex substrates [L₂MCl₂] has been explored, using the technique of electrospray ionisation mass spectrometry to directly analyse reaction solutions. In the majority of cases, products are formed by addition of the ML₂²⁺ fragment to the {Pt₂S₂} core, giving trinuclear species [Pt₂(μ-S)₂(PPh₃)₄ML₂]²⁺. The adducts with Pt(diene) [diene=cyclo-octa-1,5-diene (cod), norbornadiene], Pd(cod), Pd(bipy) (bipy=2,2^′-bipyridine), Pt(PMe₃)₂ and Pt(PTA)₂ (PTA=phosphatriaza-adamantane) moieties were synthesised and characterised on the macroscopic scale, with [Pt₂(μ-S)₂(PPh₃)₄Pt(cod)] (BF₄)₂ and [Pt₂(μ-S)₂(PPh₃)₄Pd(bipy)] (PF₆)₂ also characterised by X-ray diffraction studies. No metal scrambling was found to occur, as has been observed in some previous cases involving the related complexes [Pt₂(μ-Se)₂(PPh₃)₄] and [Pt₂(μ-S)₂(dppe)₂] (dppe=Ph₂PCH₂CH₂PPh₂). With cis-[PtCl₂(SOMe₂)₂] the species [Pt₂(μ-S)₂(PPh₃)₄PtCl(SOMe₂)]⁺ was formed, as a result of the lability of the SOMe₂ ligand. With palladium(II)-phosphine systems, the observed product species is dependent on the phosphine; the bulky PPh₃ ligand in [PdCl₂(PPh₃)₂] leads primarily to the analogous known species [Pt₂(μ-S)₂(PPh₃)₄PdCl(PPh₃)]⁺, and a small amount of the metal-scrambled species [PtPd₂S₂(PPh₃)₅Cl]⁺. In contrast, [PdCl₂(PTA)₂], containing the small PTA ligand gave [Pt₂(μ-S)₂(PPh₃)₄Pd(PTA)₂]²⁺.     

Synthesis and characterisation of palladium(II) and platinum(II) compounds containing pyrazole-derived ligands: crystal structure of [PdCl2(HL)] (HL = 3-phenyl-5-(2-pyridyl)pyrazole)

Reaction of the ligands 3-phenyl-5-(2-pyridyl)pyrazole (HL¹), 3,5-bis(2-pyridyl)pyrazole (HL²), 3-methyl-5-(2-pyridyl)pyrazole (HL³) and 3-methyl-5-phenylpyrazole (HL⁴) with [MCl₂(CH₃CN)₂] (M = Pd(II), Pt(II)) or [PdCl₂(cod)] gives complexes with stoichiometry [PdCl₂(HL)₂] (HL = HL¹, HL², HL³), [Pt(L)₂] (L = L¹, L², L³) and [MCl₂(HL⁴)₂] (M = Pd(II), Pt(II)). The new complexes were characterised by elemental analyses, conductivity measurements, infrared and ¹H NMR spectroscopies. The crystal and molecular structure of [PdCl₂(HL¹)] was resolved by X-ray diffraction, and consists of monomeric cis-[PdCl₂(HL¹)] molecules. The palladium centre has a typical square planar geometry, with a slight tetrahedral distortion. The tetra-coordinated metal atom is bonded to one pyridine nitrogen, one pyrazolic nitrogen and two chloro ligands in a cis disposition. The ligand HL¹ is not completely planar.     

N-Heterocyclic carbene gold(I) and silver(I) adducts of [Pt2(μ-S)2(PPh3)4]

Reaction of the metalloligand [Pt₂(μ-S)₂(PPh₃)₄] with the N-heterocyclic carbene (NHC) complexes IPrAuCl, IMesAuCl and IMesAgCl in methanol gave the first examples of metal adducts of [Pt₂(μ-S)₂(PPh₃)₄] that contain NHC ligands, namely [Pt₂(μ-S)₂(PPh₃)₄AuL]⁺ (L = IPr, IMes) and [Pt₂(μ-S)₂(PPh₃)₄AgIMes]⁺. The complexes were isolated as hexafluorophosphate salts. Reaction of [Pt₂(μ-S)₂(PPh₃)₄] with excess IPrAuCl in refluxing methanol yielded only the mono-adduct, in contrast to the behaviour with the gold(I) phosphine complex Ph₃PAuCl, which undergoes double addition giving [Pt₂(μ-SAuPPh₃)₂(PPh₃)₄]²⁺. The X-ray structure of [Pt₂(μ-S)₂(PPh₃)₄AuIPr]PF₆ was determined and reveals that the ‘free’ sulfide is substantially sterically protected by the IPr ligand, accounting for the low reactivity towards addition of a second AgIPr⁺ moiety.