Dicationic Nature Research Articles

Synthesis of bimetallic rhodium phosphinine complexes with enhanced catalytic activity towards alkyne hydrosilylation

Reaction of alkynes with PCcarbeneP complex 1 (featuring an alkylidene anchored pincer ligand) results in the formation of η3-benzo-λ5-phosphinine rhodium complexes 2. Reacting two equivalents of 1 with diynes allows easy access to bimetallic η3-benzo-λ5-phosphinine rhodium complexes 3. Using this approach, compounds 2a-c,e and 3a-c are synthesized and characterized. Testing these compounds as catalysts in the hydrosilylation of alkynes reveals that bimetallic complexes 3a-c show enhanced activity to monometallic and monocationic analogues (i.e. 2d) with improved yields of up to 13 times and reaction rates of over 4 times for the same rhodium concentration. We show that the dicationic monometallic η3-benzo-λ5-phosphinine rhodium complex 2e also displays enhanced activity and that the higher activity in bimetallic complexes 3a-c likely arises from their dicationic nature.

Thermal Growth of Au-Fe Heterometallic Carbonyl Clusters Containing N-Heterocyclic Carbene and Phosphine Ligands.

The thermal reactions of [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes ([NEt4][1]) or IPr ([NEt4][2]); IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2], Fe(CO)4(AuNHC)2 [NHC = IMes (3) or IPr (4)], Fe(CO)4(AuIMes)(AuIPr) (5), and Fe(CO)4(AuNHC)(AuPPh3) [NHC = IMes (6) or IPr (7)] were investigated in different solvents [CH2Cl2, CH3CN, dimethylformamide, and dimethyl sulfoxide (dmso)] and at different temperatures (50–160 °C) in an attempt to obtain higher-nuclearity clusters. 1 and 2 completely decomposed in refluxing CH2Cl2, resulting in [Fe2(CO)8(AuNHC)]− [NHC = IMes (10) or IPr (11)]. Traces of [Fe3(CO)10(CCH3)]− (12) were obtained as a side product. Conversely, 6 decomposed in refluxing CH3CN, affording the new cluster [Au3{Fe(CO)4}2(PPh3)2]− (15). The relative stability of the two isomers found in the solid state structure of 15 was computationally investigated. 4 was very stable, and only after prolonged heating above 150 °C in dmso was limited decomposition observed, affording small amounts of [Fe3S(CO)9]2– (9), [HFe(CO)4]− (16), and [Au16S{Fe(CO)4}4(IPr)4]n+ (17). A dicationic nature for 17 was proposed on the basis of density functional theory calculations. All of the other reactions examined led to species that were previously reported. The molecular structures of the new clusters 11, 12, 15, and 17 were determined by single-crystal X-ray diffraction as their [NEt4][11]·1.5toluene, [Au(IMes)2][15]·0.67CH2Cl2, [NEt4][12], and [17][BF4]n·solvent salts, respectively.

Dianionic Phase-Transfer Catalyst for Asymmetric Fluoro-cyclization

Inspired by the dicationic nature of the electrophilic fluorinating reagent, Selectfluor (1), we rationally designed a series of dicarboxylic acid precatalysts (2), which, when deprotonated, act as anionic phase-transfer catalysts for asymmetric fluorination of alkenes. Among them, 2a having the shortest linker moiety efficiently catalyzed unprecedented 6-endo-fluoro-cyclization of various allylic amides, affording fluorinated dihydrooxazine compounds with high enantioselectivity (up to 99% ee). In addition to cyclic substrates, acyclic trisubstituted alkenes underwent the reaction with good diastereoselectivity, whereas low diastereoselectivity was observed for linear disubstituted alkenes. Results suggest that the reaction proceeds via a fluoro-carbocation intermediate.

Bis(cyclopropenium)phosphines: Synthesis, Reactivity, and Applications.

A straightforward route for the preparation of a set of bis(cyclopropenium)-substituted phosphines is reported. Due to their dicationic nature, these ligands depict an excellent π-acceptor character. The effect of the ligand substituent pattern on the catalytic activity of the metal complexes thereof derived is also studied. Whereas sterically demanding biaryl groups directly attached to the phosphorus atom seem to facilitate elementary steps such as the product release from the catalyst, long chain dialkylamino groups on the cyclopropenium units maximize the catalysts solubility and, thus, allow the use of typical apolar solvents such as toluene. Importantly, all new ligands prepared can be easily handled in air. Finally, the impact of the newly prepared dicationic phosphines in hydroarylation reactions is demonstrated. In particular, their use in the synthesis of several naphtho[1,2-b]furanes and naturally occurring naphthalene derivatives such as Calanquinone C is reported.

Lowering the pKa of a bisimidazoline lead with halogen atoms results in improved activity and selectivity against Trypanosoma brucei in vitro

Diphenyl-based bis(2-iminoimidazolidines) are promising antiprotozoal agents that are curative in mouse models of stage 1 trypanosomiasis but devoid of activity in the late-stage disease, possibly due to poor brain penetration caused by their dicationic nature. We present here a strategy consisting in reducing the pKa of the basic 2-iminoimidazolidine groups though the introduction of chlorophenyl, fluorophenyl and pyridyl ring in the structure of the trypanocidal lead 4-(imidazolidin-2-ylideneamino)-N-(4-(imidazolidin-2-ylideneamino)phenyl)benzamide (1). The new compounds showed reduced pKa values (in the range 1–3 pKa units) for the imidazolidine group linked to the substituted phenyl ring. In vitro activities (EC50) against wild type and resistant strains of T. b. brucei (s427 and B48, respectively) were in the submicromolar range with four compounds being more active and selective than 1 (SI > 340). In particular, the two most potent compounds (3b and 5a) acted approximately 6-times faster than 1 to kill trypanosomes in vitro. No cross-resistance with the diamidine and melaminophenyl class of trypanocides was observed indicating that these compounds represent interesting leads for further in vivo studies.

Self-assembly of α,ω-aliphatic diamines on Ag nanoparticles as an effective localized surface plasmon nanosensor based in interparticle hot spots

The adsorption and self-assembly of alpha,omega-aliphatic diamines on silver nanoparticles is studied in this work by surface-enhanced Raman scattering (SERS) spectroscopy and plasmon resonance. These bifunctional diamines can act as linkers of metal nanoparticles (NPs) inducing the formation of hot spots (HS), i.e. interparticle junctions or gaps between metal NPs, which are points where a huge intensification of the electromagnetic field occurs. In addition, the dicationic nature of these diamines leads to the formation of cavities just at the induced hot spots which can be applied to molecular recognition of analytes. The influence of the surface coverage and the aliphatic chain length in diamines on their self-assembly was tested by the vibrational spectra and correlated to the different plasmon resonances of the dimers detected in the extinction spectra. These factors can be used for tuning the plasmon resonance of dimers formed by two metal nanoparticles where interparticle hot spots are formed. Finally, the analytical potential of these functionalized Ag nanoparticles is demonstrated for the trace detection of the pesticide aldrin.

Enhanced Detection of Sphingoid Bases via Divalent Ruthenium Bipyridine Complex Derivatization and Electrospray Ionization Tandem Mass Spectrometry

Sphingoid bases, such as unsaturated sphingosine (So) and its corresponding dihydro-saturated species sphinganine (Sa), are present in cell samples in low abundance. This fact combined with their low-to-moderate electrospray ionization (ESI) potential, compared to other sphingolipids such as sphingomyelins, limits their detection and quantitation by liquid chromatography-tandem mass spectrometry (LC-MS(2)). To enhance the ESI efficiency of sphingoid bases, a novel procedure to generate stably derivatized analytes that enhance the LC-MS(2) detection of sphingoid bases when analyzed using LC-MS(2) was developed. In this method, a ruthenium complex, [4-(N-succimidyloxycarbonyl propyl)-4'-methyl-2,2'-bipyridine] bis(2,2'-bipyridine) Ru(II) dihexafluorophosphate, is added directly to a cell extract. This complex reacts with and covalently binds to an amino group within the sphingoid bases. The dicationic nature of the ruthenium ion enhances the compound's ionization efficiency resulting in increased LC-MS(2) signals for the derivatized sphingoid bases. Consequently, the detection and quantitation of sphingoid bases are greatly improved.

Dication species stabilized by heteroazulenes: synthesis and properties of 1,3- and 1,4-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyliumyl]-, bis[bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]pyrrol-3-yl)methyliumyl]benzene, and their related dications

A general synthetic route to a novel type of heteroazulene analogue of exceptionally stable dimethyliumbenzenes (16a–c·2PF666−−− and 17a–c·2PF666−−−) bearing 1,3-di- and 1,4-dimethylium groups substituted with four 2H-cyclohepta[b]furan-2-one 9a, four 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 9b, and two 9a and two 9b, is reported. The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 9a and 9b with isophthalaldehyde and terephthalaldehyde to afford the corresponding 1,3- and 1,4-dimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF6 solution. In spite of the dicationic nature of 16a–c and 17a–c, they exhibited high stability with large pKR+ values due to the stabilizing effect of the heteroazulene units; however, we could not determine pKR+ and pKR++ values separately. Thus, the pKR+ values correspond to the average of the pH used to form neutralized dications and half-neutralized monocations. The electrochemical reduction of the cations exhibits irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the basis of the comparison with those of the related monocation species.

Resolution of topologically chiral molecular objects

The enantiomers of topologically chiral molecular objects cannot be interconverted by a continuous deformation. It must be noted that this behavior is opposite to that of molecules showing classical or Euclidian chirality. Interlocked oriented rings and the trefoil knot are prototypical topologically chiral objects. We have been designing a transition-metal templated route to the corresponding real molecules (i.e., [2]-catenanes and molecular knots) using copper(I) as the template and 2,9-diphenyl-1,10-phenanthroline (dpp) based ligands. The precursor to the catenane species was a tetrahedral Cu(dpp)2+ complex in which the two ligands fit in around Cu(I), thereby allowing the interlocking process. In the case of the knot, the precursor was a double-stranded helical complex in which two bis-phenanthroline ligands wrap around two Cu(I) ions. The compounds were obtained as racemates and were resolved as their Cu(I) complexes either by HPLC on chiral stationary phases or by diastereoselective crystallization. For the trefoil knot (K · 2Cu+), the latter method proved to be the most efficient. It took advantage of the dicationic nature of the species to be resolved. The original triflate anion was exchanged by an optically pure anion (S)-(+)-1,1′-binaphthyl-2,2′-diyl phosphate ((+)-BNP-) and the diastereomer (+)-K · 2Cu+ · 2(+)BNP- was selectively crystallized in a mixture of nitromethane and benzene. Removal of the Cu(I) ions by treatment with cyanide afforded an enantiomerically pure molecular knot (K), showing an optical rotatory power close to +2,000° · mol-1 · L · dm-1. Chirality 10:125–133, 1998. © 1998 Wiley-Liss,Inc.

Chlorhexidine: is it still the gold standard?

After 20 years of use by the dental profession, chlorhexidine is recognized as the gold standard against which other antiplaque and gingivitis agents are measured. Chlorhexidine's antiplaque effect is a result of the dicationic nature of the chlorhexidine molecule, which affords the agent the property of persistence of antimicrobial effect at the tooth surface, through both bactericidal and bacteriostatic effects. Although other antiplaque agents may show either purely immediate effect, or limited persistence, the degree of chlorhexidine's persistence of effect at the tooth surface is the basis of its clinical efficacy. Similarly, the cationic nature of the chlorhexidine molecule is the basis of the most common side effect associated with the use of the agent--extrinsic tooth staining. Such tooth staining seems to be the result of a local precipitation reaction between tooth-bound chlorhexidine and chromogens found within foodstuffs and beverages. The cationic nature of the chlorhexidine molecule also means that the activity of the agent is rapidly reduced in the presence of anionic agents, specifically those found within certain types of toothpaste; thus care is required when using normal toothbrushing alongside chlorhexidine. By understanding how the chemical properties of the chlorhexidine molecule can explain the plethora of clinical efficacy and safety data, the use of chlorhexidine can be optimally aimed towards the patient groups who would most benefit from the superior therapeutic effect of the agent. Specifically, chlorhexidine would seem to be of most value to patients in whom the ability to perform adequate oral hygiene procedures has been compromised. In these individuals the delivery of the correct dose of chlorhexidine to the tooth surface can be optimized through the judicial use of the several different chlorhexidine formulations now available. Thus, by understanding the properties and limitations of the chlorhexidine molecule, the dental profession can ensure that the efficacy of the agent is maximized, and the side effects associated with the agent are minimized, allowing chlorhexidine to rightly remain the gold standard against which other antiplaque agents are measured.

Reactions of the cationic but-2-yne complex [WI(CO)(NCMe){Ph 2P(CH 2)PPh 2} (η 2-MeC 2Me)][BF 4] with bidentate nitrogen donor ligands

Reaction of the complex [WI(CO)(NCMe){Ph 2P(CH 2)PPh 2}(η 2MeC 2Me)][BF 4] with an equimolar quantity of NN {NN = 2,2′-bipyridyl, 1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, N, N′-cyclohexyldiazabutadiene, N, N′- tbutyldiazabutadiene or N, N′- p-methoxyphenyldiazabutadiene} at room temperature in CH 2Cl 2 smoothly affords good yields of the new dicationic mixed ligand complexes [W(CO)(NN){Ph 2P(CH 2)PPh 2}(η 2MeC 2Me)][BF 4]I ( 1– 6). The dicationic nature of these complexes was confirmed by reaction of [W(CO)(2,2′-bipy){Ph 2P(CH 2)PPh 2}(η 2MeC 2Me)][BF 4]I with two equivalents of Na[BPh 4] in acetonitrile to give the bistetraphenylborate) complex [W(CO)(2,2′-bipy){Ph 2P(CH 2)PPh 2}(η 2MeC 2Me)][BPH 4] 2 ( 7). 13C NMR spectroscopy indicates that the but-2-yne acts as a four-electron donor in these complexes.

The reactions of [WI 2(CO)(NCMe)(η 2-RC 2R) 2] (R = Me OR Ph) with OPPh 3 and SPPh 3

Equimolar quantities of [WI 2(CO)(NCMe)(η 2-RC 2R) 2] (R = Me or Ph) and L (L = OPPh 3 or SPPh 3) react in CH 2Cl 2 at room temperature to give [WI 2(CO)L(η 2-RC 2R) 2] ( 1)– 4). The reaction of [WI 2(CO)(NCMe)(η 2-RC 2R) 2] (R = Me or Ph) with 2 equivalents of L (L = OPPh 3 or SPPh 3) eventually affords the cationic complexes [WI(CO)L 2(η 2-RC 2R) 2]I ( 5– 8). The complexes [WI 2(CO)(NCMe)(η 2-RC 2R) 2] (R = Me or Ph) react with 3 equivalents of L (L = OPPh 3 or SPPh 3) to yield the dicationic complexes [W(CO)L 3(η 2-RC 2R) 2]2I ( 9– 12). The mono- and dicationic nature of 5– 8 and 9– 12 was established by preparing the tetraphenylborate complexes [WI(CO)(OPPh 3) 2(η 2-MeC 2Me) 2][BPh 4] ( 13) and [W(CO)(OPPh 3) 3(η 2-MeC 2Me) 2][BPh 4] 2 ( 14), respectively.

Tricyclohexylphosphinecarbondisulphide seven-coordinate complexes of molybdenum(II) and tungsten(II)

The complexes [MI 2(CO) 3(NCMe) 2] (M = Mo or W) react with one equivalent of S 2CPCy 3 in CH 2Cl 2 to afford the new seven-coordinate complexes [MI 2(CO) 3(S 2CPCy 3)]. Reaction of [MI 2(CO) 3(NCMe) 2] (M = Mo or W), with two equivalents of S 2CPCy 3 afford the new dicationic complexes [M(CO) 2(S 2CPCy 3) 2]2I in good yield. The dicationic nature [W(CO) 3(S 2CPCy 3) 2]2I was confirmed by the formation of the complex [W(CO) 3(S 2CPCy 3) 2][BPh 4] 2 by iodide exchange with Na[BPh 4]. Further reaction in situ of [M(CO) 3(S 2CPCy 3) 2]2I with one equivalent of S 2CPCy 3 afforded the tris ligand complex [W(CO) 3(S 2CPCy 3) 3]2I. The reaction of [W(CO) 3(S 2CPCy 3) 3]2I with Na[BPh 4] in acetonitrile gave [W(CO) 3(S 2CPCy 3) 3][BPh 4] 2, which confirmed the dicationic nature of [W(CO) 3(S 2CPCy 3) 3]2I. The mixed ligand complexes [MI(CO) 3(PPh 3)(S 2CPCy 3)]I (M = Mo or W) and [WI(CO) 3L(S 2CPCy 3)]I (L = P(OPh) 3, AsPh 3 or SbPh 3) were synthesised by reaction of [MI 2(CO) 3-(NCMe) 2] with an equimolar quantity of L in CH 2Cl 2 followed by an in situ reaction with one equivalent of S 2CPCy 3. The cationic nature of [MI(CO) 3(PPh 3)(S 2CPCy 3)]I was confirmed by iodide exchange with Na[BF 4] to afford the complexes [MI(CO) 3(PPh 3)(S 2CPCy 3)][BF 4]. The reaction of two equivalents of [WI 2(CO) 3(NCMe)(PPh 3)] with S 2CPCy 3 afforded the ligand-bridged complex [W 2I 4(CO) 6(PPh 3) 2(μ-S 2CPCy 3)]. Treatment of [WI 2(CO) 3(NCMe) 2] with two equivalents of PPh 3 followed by an in situ reaction with one equivalent of S 2CPCy 3 afforded the dicationic complex [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I. The ionic nature of the complex was confirmed by iodide exchange with Na[BPh 4] to give [W(CO) 3(PPh 3) 2(S 2CPCy 3)][BPh 4] 2. Treatment of [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I with one further equivalent of S 2CPCy 3 afforded the complex [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I. The low temperature 13C NMR spectra (carbonyl region) of several-coordinate complexes are interpreted to suggest likely structures for these compounds.

Metal derivatives of 1,3-bis(2-pyridyl)-1,3-propanedione and N,N′-dimethyl-1,3-bis(2-pyridyl)-1,3-propanedione

Complexes of transition metal ions with the oxygen-nitrogen containing ligand 1,3-bis(2-pyridyl)-1,3-propanedione (bpypH) have been synthesized. These present stoichiometries which do not conform with the usual tendency manifested by β-diketones, since molecular formulas with the composition M(bpyp)X are found. A nitrogen methylated ligand N,N′-dimethyl-1,3-bis(2-pyridyl)-1,3-propanedione methyl sulphate [dimebpypH] (MeSO4)2 has also been synthesized and its complexes with transition metal ions studied. The stoichiometries now, resemble those of typical β-diketones, namely [dimepyp]2MX4 with four anions because of the dicationic nature of the original ligand. This, together with IR evidence suggests a dual type of coordination by bpip via the β-diketone and the pyridyl moieties.

Synthesis and characterisation of 2:1 guanosine and inosine derivatives of aromatic diamine platinum(II) complexes

The six platinum complexes of formula [Pt(LL)(NuO) 2]Cl 2, where LL is o-phenylenediamine, 4,5-dimethyl- o-phenylenediamine or 2,2′-bipyridyl and NuO is a nucleoside such as guanosine or inosine, have been prepared by interacting an appropriate dichlorodiamine-platinum(II) with an appropriate nucleoside in excess. The resulting complexes have been characterized by chemical analysis and by ultraviolet-visible, infrared and 1H NMR spectroscopy. The 1H NMR study has established the binding of guanosine and inosine to platinum through N 7. The dicationic nature of the complexes has been established by conductivity measurements which show them to be 1:2 electrolytes.