Tricarbonyl complexes of rhenium(I) and technetium(I) with thiourea derivatives

fac-[M(CO)₃X₃]²⁻ complexes (M=Re, X=Br; M=Tc, X=Cl) react with thiourea derivatives under formation of stable rhenium(I) and technetium(I) complexes. The composition of the products can be controlled by the steric requirements of the ligands and their ability to form chelates.The products of reactions with tetramethylthiourea, Me₄tu (I), N,N-diethylthiocarbamoylbenzamidine, H₂Et₂tcb (II), and morpholinylthiocarbamoylbenzamidine, H₂morphtcb (III), have been studied by X-ray crystallography showing that the products belong to three different structural types. A mononuclear complex of the composition fac-[Re(CO)₃Br(Me₄tu)₂] has been isolated with tetramethylthiourea, whereas the thiocarbamoylbenzamidines deprotonate and act as N,S-chelating ligands. This results in the formation of a dimeric [Tc(CO)₃(HEt₂tcb-N,S)]₂ complex with a central, almost square Tc₂S₂ unit and a monomeric compound of the composition [Tc(CO)₃(Hmorphtcb-N,S)(H₂morphtcb-S)]. The latter compound contains a neutral, S-bonded morpholinylthiocarbamoylbenzamidine in the unusual imine form in addition to a chelate-bonded Hmorphtcb⁻ ligand.     

Rhenium(I)- and technetium(I) tricarbonyl complexes anchored by bifunctional pyrazole-diamine and pyrazole-dithioether chelators

The novel pyrazolyl containing ligands 4-(HOOC)pz(CH₂)₂NH(CH₂)₂NH₂ (L¹) and 4-(HOOCCH₂)-3,5-Me₂pz(CH₂)₂NH(CH₂)₂NH₂ (L²), and 3,5-Me₂pz(CH₂)₂S(CH₂)₂SCH₂CH₃ (L³), 3,5-Me₂pz(CH₂)₂S(CH₂)₂SCH₂COOEt (L⁴) and 3,5-Me₂pz(CH₂)₂S(CH₂)₂SCH₂COOH (L⁵) were synthesized, and their ability to stabilise complexes with the fac-[M(CO)₃]⁺ (M = Re,^99mTc) moiety was evaluated. Reactions of L¹-L⁵ with the Re(I) tricarbonyl starting materials (NEt₄)₂[Re(CO)₃Br₃] and/or [Re(CO)₅Br] afforded complexes fac-[Re(CO)₃(κ³-L)] (L = L¹-L⁵ (1-5)), which contain the pyrazolyl ancillary ligands coordinated in a tridentate fashion. Complexes 1-5 were characterized by the common analytical techniques, which included single crystal X-ray diffraction analysis in the case of 4. The structural analysis of 4 confirmed the tridentate coordination mode of the pyrazole-dithioether ligand, which is facially coordinated to the Re(I) centre through the nitrogen from the pyrazole ring and the two thioether sulphur atoms, without involvement of the terminal ester functional group. The distorted octahedral coordination environment around the metal is completed by the three facial carbonyl ligands. The radioactive congeners of complexes 1, 3 and 4, fac-[^99mTc(CO)₃(κ³-L)]⁺ (L = L¹ (1a), L³ (3a), L⁴ (4a)), have been prepared by reacting the precursor fac-[^99mTc(CO)₃(H₂O)₃]⁺ with the corresponding ligands, and their identity confirmed by HPLC comparison with the rhenium surrogates. Complexes 1a and 3a have been challenged in the presence of a large excess of histidine or cysteine, in order to evaluate their in vitro stability. Only a negligible displacement was observed, indicating that pyrazole-diamine and pyrazole-dithioether chelators provide a high kinetic inertness and/or stability to organometallic complexes with the fac-[^99mTc(CO)₃]⁺ moiety.     

Synthesis, in vitro and in silico assessment of organometallic Rhenium(I) and Technetium(I) thymidine complexes

Thymidine kinases have been identified as suitable targets for non-invasive imaging of gene therapy and cancer. Thus, there is a high interest in new, reliable and inexpensive radiolabeled thymidine analogues for these applications. In this study we present the synthesis and in vitro evaluation of M(CO)₃-complexes of thymidine (M = ^99mTc, Re) for potential use in SPECT tumor imaging. 5′-amino-5′-deoxythymidine was derivatized at position C5′ with spacers of various lengths (∼0-30 Å) carrying tridentate metal chelating entities such as iminodiacetic acid and picolylamine-N-monoacetic acid. The nucleoside derivatives were reacted with the precursors [ReBr₃(CO)₃]²⁻ and [^99mTc(OH₂)₃(CO)₃]⁺, respectively. The organometallic thymidine complexes have been fully characterized by means of IR, NMR and mass spectrometry. Enzyme kinetic studies revealed mixed inhibition of the human cytosolic thymidine kinase with K_i values ranging from 4.4 to 334 μM for all thymidine complexes. Competitive inhibition of herpes simplex virus type 1 thymidine kinase was only achieved when thymidine and the metal core were separated by a spacer of approximately 30 Å length. These findings were supported by in silico molecular docking and molecular dynamic experiments.     

Synthesis, spectral studies, crystal structures and TDDFT studies of the rhenium(I) complexes of 2,4-dihydroxy-N′-(4-hydroxybenzilidene) benzohydrazide

The rhenium(I) carbonyl bromide complex, [ReBr(CO)₃(HL)], of the ligand derived from 2,4-dihydroxybenzaldehyde and 4-hydroxybenzoic acid hydrazide (HL), has been prepared. HL and its complex have been characterized by elemental analysis, MS, IR, UV-Vis and ¹H NMR spectroscopic methods. The structure of HL and the aqua-complex [Re(OH₂)(CO)₃(L)] where the ligands are monodeprotonated have been elucidated by X-ray diffraction. The structure of [ReBr(CO)₃(HL)] has been calculated from conformational parameters found in the aqua-complex. DFT and TDDFT calculations have been performed to obtain the IR spectra and UV-Vis absorption and emission spectra. The calculated spectra agree with the experimental results.     

Synthesis and structural studies of rhenium(I) tricarbonyl complexes with thione containing chelators

Sodium dihydrobis(2-mercaptothiazolyl)borate, Na[H₂B(tiaz)₂], reacts with (NEt₄)₂[Re(CO)₃Br₃] in water to afford fac-[Re{κ³-H(μ-H)B(tiaz)₂}(CO)₃] (1). In a similar manner, treatment of the same Re(I) starting material with bis(2-mercaptoimidazolyl)methane, H₂C(tim^Me)₂, yields fac-[ReBr{κ²-H₂C(tim^Me)₂}(CO)₃] (2). The organometallic complexes 1 and 2 have been characterized by IR, ¹H and ¹³C NMR spectroscopy, and also by X-ray crystallographic analysis. X-ray diffraction analysis revealed the presence of a short B-H?Re interaction in the case of 1, and the absence of C-H?Re interactions in the crystal structure of 2. For both compounds the rhenium atom adopts a slightly distorted octahedral coordination with a facial arrangement of the carbonyl ligands. The three remaining coordination positions are occupied by the two thione sulfur atoms from the anchor ligands, and by an agostic hydride (1) or a bromide ligand (2). Compound 1 is highly stable either in the solid state or in solution. In particular, its B-H?Re interaction is retained in solution, even in coordinating solvents, namely acetonitrile, dimethylsulfoxide and tetrahydrofuran. Unlike 1, compound 2 is only moderately stable in acetonitrile, undergoing a slow release of the bis(2-mercaptoimidazolyl)methane.     

The new facial tripod ligand 3,3-bis(1-methylimidazol-2-yl)propionic acid and carbonyl complexes thereof containing manganese and rhenium

The new facially coordinating tripod ligand 3,3-bis(1-methylimidazol-2-yl)propionate (bmip) has been studied. A synthetic route to sodium 3,3-bis(1-methylimidazol-2-yl)propionate (Na[bmip], 2a) and its hydrochloride (Hbmip · 2HCl, 2b) is reported. The electronic properties of Hbmip were calculated by DFT methods and are compared to those of structurally similar bis(pyrazol-1-yl)acetic acids. The ligand was applied in the synthesis of the two tricarbonyl complexes [Re(bmip)(CO)₃] (3) and [Mn(bmip)(CO)₃] (4). Methyl 3,3-bis(1-methylimidazol-2-yl)propionate (bmipme) (1), which is the precursor of Hbmip, and the complexes [Re(bmip)(CO)₃] (3) and [Mn(bmip)(CO)₃] (4) were characterised by single crystal X-ray analysis.     

Seven-coordinate rhenium(III) complexes of 1-(2-pyridylazo)-2-naphtholate: X-ray studies,spectroscopic characterization and DFT calculations

The reactions of [ReX₃(MeCN)(PPh₃)₂] (X = Cl or Br) with 1-(2-pyridylazo)-2-naphthol (HPAN) have been examined and the [ReBr(PAN)₂] · 2CHCl₃ (1) and [ReCl(PAN)₂] (2) complexes have been obtained. The both complexes have been structurally and spectroscopically characterized, and compound 1 has been additionally studied by magnetic measurements. The magnetic behavior is characteristic of mononuclear seven-coordinated Re(III) complex with d⁴ low-spin configuration, which gives diamagnetic ground state.     

Synthesis, X-ray studies, spectroscopic characterization and DFT calculations of p-tolylimido rhenium(V) complexes bearing an imidazole-based ligand

Novel p-tolylimido rhenium(V) complexes [Re(p-NC₆H₄CH₃)X₂(hpb)(PPh₃)] and [Re(p-NC₆H₄CH₃)(hpb)₂(PPh₃)]X (X = Cl, Br) have been obtained in the reactions of [Re(p-NC₆H₄CH₃)X₃(PPh₃)₂] with 2-(2-hydroxyphenyl)-1H-benzimidazole (Hhpb). The compounds were identified by elemental analysis IR, UV-Vis spectroscopy and X-ray crystallography. The electronic structures of the complex [Re(p-NC₆H₄CH₃)Cl₂(hpb)(PPh₃)] and the cation [Re(p-NC₆H₄CH₃)(hpb)₂(PPh₃)]⁺ have been calculated with the density functional theory (DFT) method. Additional information about binding in the [Re(p-NC₆H₄CH₃)Cl₂(hpb)(PPh₃)] and [Re(p-NC₆H₄CH₃)(hpb)₂(PPh₃)]⁺ has been obtained by NBO analysis. The electronic spectra of [Re(p-NC₆H₄CH₃)Cl₂(hpb)(PPh₃)] and [Re(p-NC₆H₄CH₃)(hpb)₂(PPh₃)]Cl were investigated at the TDDFT level employing B3LYP functional in combination with LANL2DZ.     

Novel p-tolylimido rhenium(V) complexes of imidazole-derived ligand - Synthesis, X-ray studies, spectroscopic characterization and DFT calculations

Three novel imido rhenium complexes of biologically relevant ligand 2-hydroxymethylbenzimidazole: [Re(p-NC₆H₄CH₃)Cl₂(hmbzim)(PPh₃)]·CHCl₃ (1), [Re(p-NC₆H₄CH₃)Br₂(hmbzim)(PPh₃)] (2) and [Re(p-NC₆H₄CH₃)(hmbzim)₂(PPh₃)]ReO₄·MeOH (3) have been synthesized and characterized spectroscopically and structurally (by single-crystal X-ray diffraction). The electronic spectra of 1 and 3 were investigated at the TDDFT level employing B3LYP functional in combination with LANL2DZ. Additional information about binding in the complexes 1 and 3 was obtained by NBO analysis, which confirms a linear coordination mode of the p-NC₆H₄CH₃ ligand and triple bond between the rhenium and imido ligand.     

Photophysical properties and computational investigations of tricarbonylrhenium(I)[2-(4-methylpyridin-2-yl)benzo[d]-X-azole]L and tricarbonylrhenium(I)[2-(benzo[d]-X-azol-2-yl)-4-methylquinoline]L derivatives (X = N-CH3, O, or S; L = Cl, pyridine)

We report a combined experimental and computational study of new rhenium tricarbonyl complexes based on the bidentate heterocyclic N-N ligands 2-(4-methylpyridin-2-yl)benzo[d]-X-azole (X = N-CH₃, O, or S) and 2-(benzo[d]-X-azol-2-yl)-4-methylquinoline (X = N-CH₃, O, or S). Two sets of complexes are reported. Chloro complexes, described by the general formula Re(CO)₃[2-(4-methylpyridin-2-yl)benzo[d]-X-azole]Cl (X = N-CH₃, 1; X = O, 2; X = S, 3) and Re(CO)₃[2-(benzo[d]-X-azol-2-yl)-4-methylquinoline]Cl (X = N-CH₃, 4; X = O, 5; X = S, 6) were synthesized heating at reflux Re(CO)₅Cl with the appropriate N-N ligand in toluene. The corresponding pyridine set {Re(CO)₃[2-(4-methylpyridin-2-yl)benzo-X-azole]py}PF₆ (X = N-CH₃, 7; X = O, 8; X = S, 9) and {Re(CO)₃[2-(benzo[d]-X-azol-2-yl)-4-methylquinoline]py}PF₆ (X = N-CH₃, 10; X = O, 11; X = S, 12) was synthesized by halide abstraction with silver nitrate of 1-6 followed by heating in pyridine and isolated as their hexafluorophosphate salts. All complexes have been fully characterized by IR, NMR, electrochemical techniques and luminescence. The crystal structures of 1 and 7 were obtained by X-ray diffraction. DFT and time-dependent (TD) DFT calculations were carried out for investigating the effect of the organic ligand on the optical properties and electronic structure of the reported complexes.     

Synthesis, characterization, reactivity and computational studies of new rhenium(I) complexes with thiosemicarbazone ligands derived from 4-(methylthio)benzaldehyde

N-thioamide thiosemicarbazone derived from 4-(methylthio)benzaldehyde (R = H, HL¹; R = Me, HL² and R = Ph, HL³) have been prepared and their reaction with fac-[ReX(CO)₃(CH₃CN)₂] (X = Br, Cl) in methanol gave the adducts [ReX(CO)₃(HLⁿ)] (1a X = Cl, n = 1; 1a′ X = Br, n = 1; 1b X = Cl, n = 2; 1b′ X = Br, n = 2; 1c X = Cl, n = 3; 1c′ X = Br, n = 3) in good yield.All the compounds have been characterized by elemental analysis, mass spectrometry (ESI), IR and ¹H NMR spectroscopic methods. Moreover, the structures of HL², HL³, HL³·(CH₃)₂SO and 1b′·H₂O were also elucidated by X-ray diffraction. In 1b′, the rhenium atom is coordinated by the sulphur and the azomethine nitrogen atoms (κS,N³) forming a five-membered chelate ring, as well as three carbonyl and bromide ligands. The resulting coordination polyhedron can be described as a distorted octahedron.The structure of the dimers is based on rhenium(I) thiosemicarbazonates [Re₂(L¹)₂(CO)₆] (2a), [Re₂(L²)₂(CO)₆] (2b) and [Re₂(L³)₂(CO)₆] (2c) as determined by X-ray studies. Methods of synthesis were optimized to obtain amounts of these thiosemicarbazonate complexes. In these compounds the dimer structures are achieved by Re-S-Re bridges, where S is the thiolate sulphur from a κS,N³-bidentate thiosemicarbazonate ligand.Some single crystals isolated in the synthesis of 2b contain [Re(L⁴)(L²)(CO)₃] (3b) where L⁴ (=2-methylamine-5-(para-methylsulfanephenyl)-1,3,4-thiadiazole) is originated in a cyclization process of the thiosemicarbazone. Furthermore, the rhenium atom is coordinate by the sulphur and the thioamidic nitrogen of the thiosemicarbazonate (κS,N²) affording a four-membered chelate ring.     

Novel Re(I) tricarbonyl complexes of chelating ligands with aromatic N-heterocycle ring and aliphatic amine donor - Synthesis, spectroscopic characterization, X-ray structure and DFT calculations

Three novel tricarbonyl rhenium(I) complexes of chelating ligands with aromatic N-heterocycle ring and aliphatic amine donor (2-aminomethylpyridine (ampy), 2-(2-aminophenyl)-1H-benzimidazole (apbi) and 2-(2-aminophenyl)benzothiazole (apbt)) have been synthesized and characterized. The compounds [Re(CO)₃(ampy)Cl] (1), [Re(CO)₃(apbi)Cl] (2) and [Re(CO)₃(apbt)Cl] (3) have been identified by IR, UV-Vis spectroscopy and X-ray analysis. The experimental studies on the complexes 1, 2 and 3 have been accompanied computationally by the density functional theory (DFT) calculations.     

Novel rhenium(III) complexes with the picolinate ligand: Synthesis,spectroscopic investigations,X-ray structures and DFT calculations for [ReX2(pic)(PPh3)2] complexes

The paper presents a combined experimental and computational study of novel rhenium(III) complexes with the picolinate ligand – [ReCl₂(pic)(PPh₃)₂] (1) and [ReBr₂(pic)(PPh₃)₂] (2). Both complexes 1 and 2 have been characterised spectroscopically and structurally (by single-crystal X-ray diffraction). Complex 1 has been additionally studied by magnetic measurement. The magnetic behavior is characteristic of a mononuclear d⁴ low-spin octahedral Re(III) complex (³T_1g ground state) and arises because of the large spin–orbit coupling (ζ = 2500 cm⁻¹), which gives a diamagnetic ground state. DFT and time-dependent (TD)DFT calculations have been carried out for complex 1, and UV–vis spectra of the [ReX₂(pic)(PPh₃)₂] compounds have been discussed on this basis.     

Structural and theoretical studies of (E,E)-benzaldehyde azine and its rhenium(IV) complex

The azine bridged dicatechol ligand (E,E)-benzaldehyde azine (H₄L) was fully characterized by X-ray analysis. The reaction of [ReCl₆]²⁻ with this compound was studied and the novel Re(IV) complex (HNEt₃)(NBu₄)[ReCl₄(H₂L)] was prepared and characterized. The structure and spectroscopy of the compound H₄L and its Re(IV) complex were studied experimentally and by means of density functional calculations.     

Syntheses and structural characterizations of rhenium carbonyl complexes of a bitopic ferrocene-linked bis(pyrazolyl)methane ligand

The reaction between Fe[C₅H₄CH(pz)₂]₂ (pz = pyrazolyl ring) and two equivalents of Re(CO)₅Br in refluxing toluene produces Fe[C₅H₄CH(pz)₂Re(CO)₃Br]₂ (1) in high yield. A similar reaction with a ligand/rhenium ratio of slightly greater than one yields mainly 1 and a low yield of Fe[C₅H₄CH(pz)₂Re(CO)₃Br][C₅H₄CH(pz)₂] (2). The compound H₂C(pz)₂Re(CO)₃Br (3) was prepared by the reaction of H₂C(pz)₂ and Re(CO)₅Br. Compounds 1 and 2 show a reversible oxidation at ca. 0.9 V (Ag/AgCl) that can be assigned to the oxidation of the ferrocene moiety and one irreversible oxidation at ca. 1.4 V assigned to the oxidation of the rhenium metal center. The solid-state structures of 1 · CH₃NO₂, 1 · 2CH₃NO₂, 1 · 2CH₃CN and 2 · 1/2Et₂O · 1/2C₃H₆O have been determined, with 1 · 2CH₃NO₂ and 1 · 2CH₃CN being isomorphous. All four are organized into supramolecular structures by the interactions of the acidic hydrogens of the pyrazolyl and methine groups with either the bromine atoms or carbonyl ligand oxygen atoms, and in 2 the lone pairs on the uncomplexed bis(pyrazolyl)methane units.     

Synthesis and characterization of thiosemicarbazone derivatives of 2-ethoxy-3-methoxy-benzaldehyde and their rhenium(I) complexes

The ligands (HL¹, HL² and HL³) have been prepared and their reaction with fac-[ReX(CO)₃(CH₃CN)₂] (X = Br, Cl) in chloroform gave the adducts [ReX(CO)₃(HL)] (1a X = Cl, R = H; 1a′ X = Br, R = H; 1b X = Cl, R = CH₃; 1b′ X = Br, R = CH₃; 1c X = Cl, R = Ph; 1c′ X = Br, R = Ph) in good yield. All the compounds have been characterized by elemental analysis, mass spectrometry (FAB), IR and ¹H NMR spectroscopic methods, and the structures of the ligands have been elucidated by X-ray diffraction. In the case of HL¹, we have tried the reaction with [ReX(CO)₅] (X = Br, Cl) in toluene and we proved the formation of the adduct also by this way by the isolation of single crystals of 1a′ · ½C₇H_8.     

Novel oxorhenium complexes with 2-(2′-hydroxyphenyl)-2-benzothiazolinato ligand: X-ray studies,spectroscopic characterization and DFT calculations

The [ReOX₂(hbt)(EPh₃)] (X = Cl, Br; E = As, P) chelates have been prepared in the reactions of [ReOX₃(EPh₃)₂] complexes (X = Cl, Br; E = P, As) with 2-(2′-hydroxyphenyl)-2-benzothiazole (hbtH) in acetone. From the reactions of [ReOX₃(PPh₃)₂] with hbtH two kind of crystals [ReOX₂(hbt)(PPh₃)] · MeCN and [ReOX₂(hbt)(PPh₃)] with different arrangement of halide ions (cis and trans) were isolated, whereas the [ReOX₃(AsPh₃)₂] oxocompounds react with hbtH to give only cis-halide isomers. The complexes were structurally and spectroscopically characterised. The electronic structures of both [ReOBr₂(hbt)(PPh₃)] isomers have been calculated with the density functional theory (DFT) method. The TDDFT/PCM calculations have been employed to produce a hundred of singlet excited-states starting from the ground-state geometry optimized in the gas phase of cis- and trans-halide isomers of [ReOBr₂(hbt)(PPh₃)] and the UV–Vis spectra of these complexes have been discussed on this basis.     

New emissive fac-tricarbonylchlorobis(ligand)rhenium(I) complexes prepared from pyridine/thiophene hybrid ligands

Stille coupling between tributyl-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-stannane and 4-bromopyridine resulted in the preparation of the new pyridine/thiophene hybrid ligand 4-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-pyridine [4-py-EDOT] (1). Reaction of 1, 4-thiophen-2-yl-pyridine (2), or 4-[2,2^′]bithiophenyl-5-yl-pyridine (3) with ClRe(CO)₅ resulted in the isolation of complexes 4-6, ClRe(L)₂(CO)₃, where L=1, 2, or 3 respectively. The solid-state structure of 4 was determined by X-ray crystallography, which clearly shows the fac arrangement of the three CO ligands and the two 4-py-EDOT ligands arranged cis to one another. The metal complexes 4-6 have been characterized by ¹H and ¹³C NMR, ESI or FAB MS, FTIR, UV-Vis, fluorescence, and elemental analysis.     

3,3-Bis(3,5-dimethylpyrazol-1-yl)propionic acid: A tripodal N,N,O ligand for manganese and rhenium complexes - Syntheses and structures

The tripodal N,N,O ligands 3,3-bis(3,5-dimethylpyrazol-1-yl)propionic acid (Hbdmpzp) (1) and 3,3-bis(pyrazol-1-yl)propionic acid (Hbpzp) (2) form the “missing link” between the well-known bis(pyrazol-1-yl)acetic acids and related ligands with a longer “carboxylate arm”. To illustrate the reactivity of this ligand, manganese and rhenium complexes bearing the ligand bdmpzp are reported. The complexes are compared to related compounds bearing other tripod ligands such as bis(3,5-dimethylpyrazol-1-yl)acetate (bdmpza) and 3,3-bis(1-methylimidazol-2-yl)propionate (bmip). Spectroscopic and structural data are used as a basis for comparison, as well as DFT calculations. Both ligands 1 and 2 and the complexes fac-[Mn(bdmpzp)(CO)₃] (3) and fac-[Re(bdmpzp)(CO)₃] (4) were characterised by X-ray crystallography.     

New oxorhenium complexes with the 8-quinolinolato ligand: X-ray structure and DFT calculations for [ReOBr(hqn)2]

The reactions of [ReO(OEt)X₂(PPh₃)₂] (X = Cl or Br) with 8-hydroxyquinoline (Hhqn) have been examined and the [ReOX(hqn)₂] complexes have been obtained. The crystal and molecular structures of [ReOBr(hqn)₂] have been determined. The electronic structure of [ReOBr(hqn)₂] has been calculated with the density functional theory (DFT) method, and additional information about binding in the ReO³⁺ unit has been obtained by NBO analysis. The spin-allowed electronic transitions of [ReOBr(hqn)₂] have been calculated with the time-dependent DFT method, and the UV–Vis spectra of the [ReOX(hqn)₂] compounds have been discussed on this basis.