Synthesis and structure of rhenium(IV) chloride complexes with acetonitrile

Cis and trans isomers of the tetrachlorodiacetonitrile rhenium(IV) complex were synthesized and isolated. The structure of cis-ReCl₄(N≡C-CH₃)₂ was studied by X-ray crystallography. It was shown that the coordination polyhedron of the central Re(IV) atom is formed by the four chlorine atoms and the nitrogen atoms of two coordinated acetonitrile molecules. The vibrational (IR and Raman) spectra of the cis isomer of the complex in solid state were interpreted by factor-group analysis. A comparison of the IR spectra of two isomers confirmed that they are cis and trans isomers.     

New synthetic routes to cationic rhenium tricarbonyl bipyridine complexes with labile ligands

Triflate abstraction from the complex [Re(OTf)(CO)(3)(bipy)] (1) using the salt NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane solution in the presence of L = PPh(3), NCMe, NCPh, imines, ketones, Et(2)O, THF, MeOH, and MeI affords cationic complexes [Re(L)(CO)(3)(bipy)](+) as their BAr'(4)(-) salts. The new complexes have been characterized spectroscopically and, for [Re(eta(1)-O=C(Me)R)(CO)(3)(bipy)]BAr'(4) (R = CH(3), 6a; R = Ph, 6b), and [Re(THF)(CO)(3)(bipy)]BAr'(4) (9), also by single-crystal X-ray diffraction. Compared with conventional methodologies, the route reported here allows the coordination of a broader range of weakly coordinating ligands and requires considerably milder conditions. On the other hand, the reactions of lithium acetylides with [Re(THF)(CO)(3)(bipy)]BAr'(4) (9) can be used for the high-yield syntheses of rhenium alkynyls [Re(Ctbd1;CR)(CO)(3)(bipy)] (R = Ph, 12; R = SiMe(3), 13). Complex 9 was found to catalyze the aziridination of benzylideneaniline with ethyl diazoacetate.     

Theoretical studies of ground and excited electronic States in a series of rhenium(i) bipyridine complexes containing diarylethynyl-based structure

The photophysical properties, which vary as X is varied, of Re(I)-halide complexes (X2-bpy)ReICl(CO)3 (where X=ph, DAE, DNE, and DPE; ph = phenyl (1); DAE = di(amineoethynylbenzene) (2); DPE = di(phenylethynylbenzene) (3); DNE = di(nitroethynylbenzene) (4); bpy=2,2'bipyridine), are investigated using density functional theory (DFT). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using B3LYP functional. Excited singlet and triplet states are examined using time-dependent density functional theory (TDDFT). The low-lying excited-state geometries are optimized at the ab initio configuration interaction singlets level. As shown, the diarylethynyl-based structure is an integral component of the bpy pi-conjugated network, which results in a good planar structure. The occupied orbitals involved in the transitions have a significant mixture of metal Re and group Cl, and the lowest unoccupied orbital is a pi orbital, which extends from ligand bpy to diarylethynyl-based substituents. The luminescence for each complex originates from the lowest triplet excited states and is assigned to the mixing of MLCT and LLCT characters. Significant insights on the effects of these diarylethynyl conjugated structure and ending substituents (NH2, ph, and NO2) on absorption and emission spectra are observed by analysis of the results of the TDDFT method. The diarylethynyl-based pi-conjugated network makes both the absorption and emission spectra red-shifted compared with simple complex (bpy)ReICl(CO)3. Furthermore, an electron-releasing group (NH2) makes absorption and emission spectra blue-shift and an electron-withdrawing group (NO2) makes them red-shift.     

Synthesis and characterization of novel rhenium(I) complexes towards potential biological imaging applications

Background

Re(I) tricarbonyl complexes exhibit immense potential as fluorescence imaging agents. However, only a handful of rhenium complexes have been utilized in biological imaging. The present study describes the synthesis of four novel rhenium complexes, their characterization and preliminary biological studies to assess their potential as biological imaging agents.

Results

Four facial rhenium tricarbonyl complexes containing a pyridyl triazine core, (L1 = 5,5′(3-(2-pyridyl)-1,2,4-triazine-5,6-diyl)-bis-2-furansulfonic acid disodium salt and L2 = (3-(2- pyridyl)-5,6-diphenyl-1,2,4-triazine-4′,4′′-disulfonic acid sodium salt) have been synthesized by utililzing two different Re metal precursors, Re(CO)₅Br and [Re(CO)₃(H₂O)₃]OTf in an organic solvent mixture and water, respectively. The rhenium complexes [Re(CO)₃(H₂O)L1]⁺ (1), Re(CO)₃L1Br (2), [Re(CO)₃(H₂O)L2]⁺ (3), and Re(CO)₃L2Br (4), were obtained in 70–85% yield and characterized by ¹H NMR, IR, UV, and luminescence spectroscopy. In both H₂O and acetonitrile, complexes display a weak absorption band in the visible region which can be assigned to a metal to ligand charge transfer excitation and fluorescent emission lying in the 650–710 nm range. Cytotoxicity assays of complexes 1, 3, and 4 were carried out for rat peritoneal cells. Both plant cells (Allium cepa bulb cells) and rat peritoneal cells were stained using the maximum non-toxic concentration levels of the compounds, 20.00 mg ml^?1 for 1 and 3 and 5.00 mg ml^?1 for 4 to observe under the epifluorescence microscope. In both cell lines, compound concentrated specifically in the nuclei region. Hence, nuclei showed red fluorescence upon excitation at 550 nm.

Conclusions

Four novel rhenium complexes have been synthesized and characterized. Remarkable enhancement of fluorescence upon binding with cells and visible range excitability demonstrates the possibility of using the new complexes in biological applications.

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Graphical abstract Micrograph of rat peritoneal cells incubated with novel rhenium complex under epifluorescence microscope.

     

Synthesis and crystal structure of two copper complexes with one-dimensional chain structures bridged by bipyridine ligands

Two copper complexes {[Cu(phen)(azpy)(H₂O)(ClO₄)](ClO₄)}_n1 (phen=1,10-phenthroline, azpy=4,4′-azobispyridine) and {[Cu(phen)(bpe)(H₂O)(ClO₄)](ClO₄)}_n2 (bpe=trans-1,2-bis(4-pyridyl)ethylene), have been synthesized and characterized. The X-ray analysis reveals that copper ion has distorted square pyramidal coordination environments in the complexes 1 and 2. The copper is coordinated by two N atoms of phen, two N atoms from two bridging ligand azpy in 1 and from two bridging ligand bpe in 2, one O atom of coordinated water. Due to Jahn Teller distortion the sixth site in 1 and 2 is occupied by one O atom from one perchlorate anion. Copper ions are linked to each other through bridging ligand azpy in 1 and bridging ligand bpe in 2 to form one-dimensional chain. Variable-temperature magnetic susceptibility studies show that there is a weak antiferromagnetic interaction between the Copper ions in 1 and 2.     

Synthesis,structure, and luminescence of rhenium(I) complexes with substituted bipyridines

Two new rhenium(I) complexes chelated by a substituted 2,2′-bipyridine with general formula Re(CO)₃LCl, where L?=?6?-(2″-methoxyphenyl)-2,2′-bipyridine (L₁ ) and 6?-(4″-diphenylaminophenyl)-2,2′-bipyridine (L₂ ), are synthesized and characterized by IR, NMR, and elemental analysis. Structure of 1 was determined by single-crystal X-ray crystallography, revealing that rhenium is six-coordinate octahedral. The electrochemical, photophysical, and thermal properties of the two rhenium(I) complexes were investigated. Electroluminescent devices were fabricated by doping 1 in polymer blend host of poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole using simple solution spin-coating technique. The device exhibits a maximum current efficiency of 2.97?cd?A^?1 and peak brightness in excess of 2390?cd?m^?2.     

Catalytic and asymmetric cyclopropanation of alkenes catalysed by rhenium(I) bipyridine and terpyridine tricarbonyl complexes

Re(I) tricarbonyl bipyridine and terpyridine complexes catalyse stereospecific cyclopropanation of alkenes; high selectivity of cyclopropane vs coupling and an ee of 73% and 62% for cis- and trans-cyclopropanes of styrene respectively were achieved with the [Re(L)(CO)(3)(MeCN)]OTf complex (L = chiral C(2)-symmetric terpyridine ligand).     

Synthesis and reactivity of germyl complexes of manganese and rhenium

Trichlorogermyl complexes M(GeCl₃)(CO)_nP_{5− n} (1–4) [M = Mn, Re; n = 2, 3; P = PPh(OEt)₂ (a), P(OEt)₃ (b)] were prepared by allowing chloro compounds MCl(CO)_nP_{5− n} to react with an excess of GeCl₂•dioxane in 1,2-dichloroethane. Treatment of compounds 1–4 with LiAlH₄ in thf yielded trihydridegermyl derivatives M(GeH₃)(CO)_nP_5−n (5–8), whereas treatment of the same complexes with NaBH₄ in ethanol afforded triethoxygermyl derivatives M[Ge(OEt)₃](CO)_nP_5−n (9–11). Trimethylgermyl compounds M(GeMe₃)(CO)_nP_5−n (12, 13) and the alkynylgermyl derivative Mn[Ge(CCPh)₃](CO)₃[PPh(OEt)₂]₂ (14a) were also prepared by allowing trichlorogermyl compounds 1–4 to react with either MgBrMe or Li⁺CCPh⁻, respectively, in thf. Treatment of compound Re(GeCl₃)(CO)₃[PPh(OEt)₂]₂ (4a) with SnCl₂•2H₂O gave the stannyl-germyl derivative Re[GeCl₂(SnCl₃)](CO)₃[PPh(OEt)₂]₂ (15a). The complexes were characterised by spectroscopy and X-ray crystal structure determination of 4a, 5a, and 13a.     

Synthesis and reactions of isocyanate and carbamoyl complexes of rhenium

Re(CO)₂(NO)(PPh₃)₂ reacts with aroyl azides RCON₃ (R = C₆H₅, p-CH₃C₆H₄) in benzene to form isocyanate complexes of formula Re(CO)(NO)-(PPh₃)₂(RCONCO) (I). When the reaction is carried out in protic solvents such as ethanol, carbamoyl derivatives of formula Re(NCO)(NO)(PPh₃)₂-(CONHCOR) (II) are obtained, which give Re(NCO)(NO)(PPh₃)₂(CO)(NHCOR) when dissolved in chloroform, a terminal carbonyl ligand being formed from the carbamoyl group.I can be transformed into II by reaction with gaseous HCl, via [Re(CO)-(NO)(PPh₃)₂ {C(OH)=NCOR}]⁺Cl^- followed by anion exchange with NaN₃. II reacts with mineral acids HX (X = Cl, BF₄) to give amide derivatives of formula [Re(NCO)(NO)(PPh₃)₂(CO)(NH₂COR)]⁺ X^- which when X = Cl can be easily transformed into Re(NCO)(NO)(PPh₃)₂(CO)Cl, the amide ligand being removed. Both the protonation reactions of I and II are reversible. IR and ¹H NMR data of the new compounds and the mechanisms of formation of I and II are reported and discussed.     

Synthesis and reactivity of germyl complexes of manganese and rhenium

Trichlorogermyl complexes M(GeCl₃)(CO)_nP_{5? n} (1–4) [M = Mn, Re; n = 2, 3; P = PPh(OEt)₂ (a), P(OEt)₃ (b)] were prepared by allowing chloro compounds MCl(CO)_nP_{5? n} to react with an excess of GeCl₂?dioxane in 1,2-dichloroethane. Treatment of compounds 1–4 with LiAlH₄ in thf yielded trihydridegermyl derivatives M(GeH₃)(CO)_nP_5?n (5–8), whereas treatment of the same complexes with NaBH₄ in ethanol afforded triethoxygermyl derivatives M[Ge(OEt)₃](CO)_nP_5?n (9–11). Trimethylgermyl compounds M(GeMe₃)(CO)_nP_5?n (12, 13) and the alkynylgermyl derivative Mn[Ge(CCPh)₃](CO)₃[PPh(OEt)₂]₂ (14a) were also prepared by allowing trichlorogermyl compounds 1–4 to react with either MgBrMe or Li⁺CCPh^?, respectively, in thf. Treatment of compound Re(GeCl₃)(CO)₃[PPh(OEt)₂]₂ (4a) with SnCl₂?2H₂O gave the stannyl-germyl derivative Re[GeCl₂(SnCl₃)](CO)₃[PPh(OEt)₂]₂ (15a). The complexes were characterised by spectroscopy and X-ray crystal structure determination of 4a, 5a, and 13a.     

Synthesis and biological evaluation of novel 99mTc-oxo and 99mTc-nitrido complexes with phenylalanine dithiocarbamate for tumor imaging

In this study, phenylalanine dithiocarbamate (PHEDTC) ligand was successfully synthesized and then radiolabeled with [^99mTcO]³⁺ core and [^99mTc≡N]²⁺ core to produce ^99mTcO–PHEDTC and ^99mTcN–PHEDTC, respectively. Both complexes were prepared with high radiochemical purity and had good stability. The partition coefficient results showed they were hydrophilic, while ^99mTcN–PHEDTC was more hydrophilic than ^99mTcO–PHEDTC. The biodistribution study in mice bearing S 180 tumor showed that ^99mTcO–PHEDTC and ^99mTcN–PHEDTC had high tumor uptake at 2 h post-injection, 1.91 and 1.21, respectively. The good uptake and retention in tumor together with favorable tumor-to-muscle ratios make them promising candidates for further evaluation as potential tumor imaging agents.     

Synthesis,characterization and reactivity of some rhenium phosphite complexes

The preparation and characterization of some Re(III), Re(IV) and Re(V) chloro phosphite complexes are reported. Both Re(III) and Re(IV) complexes react with sodium borohydride, yielding the corresponding polyhydrides, ReH₅[P(OEt)₃]₃ and ReH₇[P(OEt)₃]₂. The thermal and photochemical reactivity of these complexes is described.     

Synthesis of a novel macrocyclic ligand containing bipyridine units

The 6-hydroxymethyl-6'-tetrahydropyranyloxymethyl-2, 2'-bipyridine (2) was synthesized by the reaction of 6,6'-dihy-droxymethyl-2, 2'-bipyridine (1) with 3, 4-dihydropyran (DHP). 6-Tetrahydropyranyloxymethyl-6'-iodomethyl-2, 2'-bipyridine (5) was obtained from mesylate and iodizating reaction of compound 2. The coupling of 2 and 5 followed by hydrolysis gave bis(6'-hydroxymethyl-2,2'-bipyridme-6-methyl)ether (7) .The macrocydic ligand 8 was obtained by treating 7 and 6,6'-dibromomethyl-2,2'-bipyridine. The synthetic conditions of the intermediate 2 and macocyclic ligand 8 were discussed.     

Synthesis, structure, and LLCT transitions in terminal hydrazido(2-) bipyridine complexes of titanium

Addition of 2,2'-bipyridine and its derivatives to Ti(NMe2)2(dpma), where dpma is N,N-di(pyrrolyl-alpha-methyl)-N-methylamine, followed by various hydrazine derivatives was used to generate a series of terminal hydrazido(2-) complexes. Among the new complexes is Ti[=NN(H)Ph](But-bpy)(dpma), which was structurally characterized, where But-bpy is 4,4'-tert-butyl-2,2'-bipyridine. Other new complexes reported are Ti(NNMe2)(Me-bpy)(dpma), Ti(NNMe2)(bpy)(dpma), Ti(NNMe2)(Ph-bpy)(dpma), Ti[NN(Me)Ph](But-bpy)(dpma), Ti[NN(Me)p-tolyl](But-bpy)(dpma), and Ti[NN(Me)4-FC6H4](But-bpy)(dpma). Titanium hydrazido(2-) complexes bearing bpy substituents possess a low-energy transition, leading them to have blue or green colors, which is somewhat unusual for titanium(IV) species. Through absorption studies on the derivatives, it was determined that the low-energy transition is the result of an unusual ligand-to-ligand charge transfer where electron density residing on the hydrazido(2-) is transferred to the bpy pi* orbitals.     

Synthesis and cytotoxic activities of transition-metal complexes of a binaphthyl-linked bipyridine ligand

A binaphthyl-linked bipyridyl compound, 1,1′-bis(6-methyl-6′-oxymethylenyl-2,2′-bipyridine)binaphthyl, (L) has been synthesised and used as a ligand for the formation of Cu(II), Ni(II), and Co(II) complexes. The ligand and its transition-metal complexes were characterized by physico-chemical and spectroscopic methods. The complexes were also investigated for cytotoxic activity. The cytotoxicity of complexes, CuL(ClO₄)₂, NiL(ClO₄)₂(H₂O), CoL(ClO₄)₂, were tested in vitro applying seven well-characterized human tumor cell lines, MCF7, EVSA-T, WIDR, IGROV, M19 MEL, A498, H226, and the microculture sulforhodamine B (SRB) test. All complexes show a very high cytotoxicity (ID₅₀ < 250 ng/ml) in these cell lines.     

Synthesis and electronic characterization of bipyridine dithiolate rhodium(III) complexes

Five new mixed diimine 1,1'-dithiolate or dithiocarbamate ligand complexes of the form [Rh(bpy)2(SS)][PF6]n, where bpy = 2,2'-bipyridine and SS = various substituted dialkyldithiocarbamates or 1,1'-dithiolates, were synthesized from cis-[Rh(bpy)2(OTf)2][OTf]. The triflate ligands are easily displaced by other ligands and allow these syntheses to proceed in high yields (80-90% overall) under relatively mild reaction conditions and to give high purity products. Electrochemistry shows irreversible two-electron reduction of Rh(III) to Rh(I) and a concomitant loss of one bipyridine ligand; this is followed by reversible one-electron reduction of the remaining 2,2'-bipyridine ligand. The electronic characterizations of these complexes are consistent with significant delocalization of the sulfur electron density onto the empty metal d orbitals. The 1,1'-dithiolate ligands induce larger red shifts in the absorption and emission spectra than the dithiocarbamates as the 1,1'-dithiolates have a more extensive conjugation system.     

Synthesis of N-heterocyclic carbene rhenium(I) carbonyl complexes

Reaction of aminophosphinimine [RHN(CH(2))(2)N[double bond, length as m-dash]PPh(3)] (R = H, Et) with Re(2)(CO)(10) provided the NH-functionalized carbene rhenium complex [Re(2)(CNHCH(2)CH(2)NR)(CO)(9)] (3a, R = H, 3b, R = Et). Treatment of 3 with Br(2) provided the mono nuclear [Re(CNHCH(2)CH(2)NR)(CO)(4)Br] (1, R = H, 2, R = Et). However, NH-functionalized carbene complexes 1-3 did not undergo N-alkylation with alkyl halides to yield the N-substituted NHC complexes. The direct ligand substitution of [Re(CO)(5)Br] with a carbene donor was employed to prepare [Re(IMes(2))(CO)(4)Br] (6a, IMes(2) = 1,3-di-mesitylimidazol-2-ylidene; 6b, IMes(2) = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene). Analyses of spectroscopic and crystal data of 6a and 6b show similar corresponding data among these complexes, suggesting the saturated and unsaturated NHCs have similar bonding with Re(I) metal centers. Reduction of 6a and 6b with LiEt(3)BH yielded the corresponding hydrido complexes 7a-b [ReH(CO)(4)(IMes(2))], but not 1 and 2. Ligand substitution of 1, 6a and 6b toward 2,2'-bipyridine (bipy) was investigated. Crystal structures of 1, 3a-b, 6a-b and 7b were determined for characterization and comparison.     

Chloronitrosyl complexes of rhenium

Preparation and properties of several highly stable nitroso complexes of rhenium viz. ReCl₃(NO), Re(OH)₃(NO)₃ M₂ReCl₅(NO) where M is Cs, K and NH₄ are described. These are analogous to the corresponding ruthenium complexes. The oxidation number of the ‘ReNO’ unit as a whole, is +3 but that of the rhenium atom cannot be determined. The nitric oxide stretching band lies within the range of 1710–1920 cm^?1. The cesium and potassium salts are weakly paramagnetic at room temperature.