Mixed-ligand Rhenium Research Articles

Synthesis and structural studies of mixed-ligand rhenium(V) complexes anchored by tridentate pyrazole-based ligands

The novel oxorhenium dichlorides mer-[ReO(L1)Cl 2] ( 1) and fac-[ReO(L2)Cl 2] ( 2) (L1 = 2-[2-(pyrazol-1-yl)ethyliminomethyl]phenolate; L2 = 2-[2-(pyrazol-1-yl)ethylaminomethyl]phenolate) were synthesized by reacting [NBu 4][ReOCl 4] with L1H and L2H, respectively. X-ray structural analysis of 1 and 2 has shown that L1 and L2 act as (N,N,O)-tridentate chelators coordinating to the Re(V) centre in a meridional and in a facial fashion, respectively. The reactivity of 2 towards potential bidentate/dianionic substrates is strongly dependent on the donor atom set, being observed that the presence of sulphur favours the displacement of the ancillary ligand (L2). By contrast, complex 2 reacted with (O,O)-bidentate substrates (1,2-ethanediol and oxalic acid) providing the mixed-ligand complexes fac-[ReO(L2)(OCH 2CH 2O)] ( 3) and fac-[ReO(L2)(C 2O 4)] ( 4). Complexes 3 and 4 are air and water-stable and have been characterized by the common spectroscopic techniques (IR, 1H and 13C NMR) and by X-ray diffraction analysis.

Preparative Routes to Luminescent Mixed-Ligand Rhenium(I) Dicarbonyl Complexes

A series of mixed-ligand 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) rhenium(I) dicarbonyl complexes that are emissive in fluid solution has been prepared, which includes a new class of the type cis-[Re(CO)2(P-P)(N-N)]+ (where P-P is a chelating diphosphine and N-N is a chelating polypyridine ligand). The four synthetic routes that have been developed rely on either reactive triflate displacement or abstraction of labile chloro ligands, followed by the use of the strong trans-labilizing effect of P donors or direct use of the trans effect of P donors. The spectroscopic, photophysical, and electrochemical properties of these new complexes systematically vary with the net donor ability of the ligands in the coordination sphere, as shown by correlations with Lever's E(L) parameters. Lifetimes and quantum yields of the bipyridine complexes encompass a broad range, 25-1147 ns and ca. 0.002-0.11, respectively.

Synthesis and biological evaluation of silylated mixed-ligand 99mTc complexes with the [PNS/S] donor atom set

New oxotechnetium complexes of general formula [ 99mTc(O)(PNS)(S(CH 2) n OSiR 3)] ( 4– 6) were synthesized by direct reduction of [ 99mTcO 4] − with stannous chloride, in the presence of the tridentate heterofunctionalized phosphine H 2PNS and of the monodentate silylated thiols [HS(CH 2) n OSiR 3] ( n=2, R=Ph ( 1); n=3, R=Ph ( 2); n=3, R=Et ( 3)). The mixed-ligand rhenium and technetium complexes of general formula [M(O)(PNS)(S(CH 2) n OH)] ( n=2: M= 99mTc, ( 7), M=Re, ( 7a); n=3: M= 99mTc, ( 8), M=Re, ( 8a)) were also prepared. All the 99mTc complexes were obtained with high radiochemical purity (>95%), after purification by HPLC, and were characterized by comparison of their HPLC profiles with the ones obtained for the corresponding Re compounds. The silylated compounds 4– 6 are stable in phosphate saline buffer (PBS) pH 7.4, rat plasma, human serum and whole blood, and do not bind to plasmatic proteins, and also do not challenge with glutathione. The biological behavior of [ 99mTc(O)(PNS)(S(CH 2) n OH)] ( 7, 8) and [ 99mTc(O)(PNS)(S(CH 2) n OSiR 3)] ( 4– 6) was studied. The effect of the pH on the cleavage of the O-Si bond in complexes 4– 6 was also evaluated.

Silylated mixed-ligand rhenium complexes with the [PNS/S] donor atom set

New oxorhenium complexes of the ‘3+1’ type with 2-(diphenylphosphanyl)- N-(2-thioethyl)benzamide (H 2PNS) and trimethyl-, triethyl- and triphenyl-hydroxyl silylated monodentate thiols are reported. These new complexes have been prepared by reacting [N nBu 4][Re(O)Cl 4] with the tridentate H 2PNS and the corresponding silylated thiol. The characterization of the complexes involved elemental analysis, 31P and 1H NMR spectroscopies and X-ray crystallographic analysis for the triethyl-silylated Re complex.

Synthesis and structural characterization of a novel Re[P][NN][S][SO] mixed ligand rhenium(III) complex

A novel mixed ligand hexacoordinated rhenium(III) complex of the general formula Re III[P][NN][S][SO] has been synthesized by reacting Re VOCl 3(PPh 3) 2 with 2,2′-bipyridine (NN), p-thiocresol (S) and N-acetyl- l-cysteine (SO) in a 1:1:1:1 ratio in methanol. During the reaction, the central rhenium atom undergoes reduction from Re(V) to Re(III). The complex was isolated as red crystals and characterized by elemental analysis, IR and NMR spectroscopies. X-ray crystallographic studies showed that the coordination geometry around rhenium is distorted octahedral. The equatorial plane is defined by the two nitrogen atoms of 2,2′-bipyridine, the sulfur of N-acetyl- l-cysteine and the sulfur of the aromatic thiol, while the phosphorous and the carboxylate oxygen of N-acetyl- l-cysteine occupy the apical positions.

Calix[4]arene rhenium(V) complexes as potential radiopharmaceuticals.

The calix[4]arene platform was used for the syntheses of novel rhenium(V) complexes, that may have potential applications as radiopharmaceuticals. The reaction of ReO(PPh3)2Cl3 with tetradentate N2O2-calix[4]arene ligand 8 in ethanol gave the novel mixed-ligand rhenium complex 9 with the structure ReO(N2O2-calix)OEt. The configuration was elucidated by using a number of 1H NMR techniques. In 9, the ethoxy ligand could be easily and quantitatively exchanged for another monodentate ligand to give complex 12. Tetradentate N2S2-calix[4]arene ligand 15 formed the rhenium complex 16 either via reaction with ReO(PPh3)2Cl3 in an organic solvent or by reaction with rhenium gluconate in an aqueous solution. Complex 16 showed good stability in phosphate-buffered saline solution (37 degrees C, 5 d). The crystal structures of a mono- and a bimetallic complex were determined. The bimetallic N2O2-calixarene complex dimer 11 crystallized in the monoclinic space group C2/c, with a = 38.963(5) A, b = 23.140(6) A, c = 27.382(6) A, beta = 128.456(10) degrees, V = 19,333(7) A3, Z = 8, and final R = 0.0519. The monometallic N2S2 model complex 17 crystallized in the monoclinic space group Cc, with a = 15.715(2) A, b = 12.045(2) A, c = 20.022(3) A, beta = 94.863(12) degrees, V = 3776.3(10) A3, Z = 4, and final R = 0.0342.

Rhenium(V)-Salen Complexes: Configurational Control and Ligand Exchange.

Reactions of ReO(PPh(3))(2)Cl(3) with tetradentate bis(salicylideneamine) ligands, H(2)salpd (1) and H(2)salbd (2), in different alcohols gave the novel mixed-ligand rhenium complexes ReO(sal)OAlk (OAlk = variety of alkoxy ligands). Configurational studies show that the rhenium complexes ReO(salpd)OAlk (1a-f) display either a symmetrical or a nonsymmetrical configuration, depending on the size of the alcohol and its boiling point. The rhenium complexes ReO(salbd)OAlk (2a-d) are all nonsymmetrical due to the number of carbons that bridge the imine nitrogens. In the case of the symmetrical ReO(salpd)OMe (1a) complex the methoxy ligand can be exchanged for a number of ligands of different types (OAlk, OPh, SAlk, OC(O)Alk). In the newly formed complexes the original configuration was retained except for the ReO(salpd)SAlk (1i,j) complexes which were isolated in the nonsymmetrical configuration. Starting from the nonsymmetrical ReO(salpd)OPr (2c) complex, ligand exchange led to a mixture of the symmetrical and nonsymmetrical complexes, with ratios depending on the reaction time. The crystal structures of ReO(salbd)OPr (2c), and ReO(salpd)OPhOMe (1g) have been determined. ReO(salbd)OPr crystallizes in the triclinic space group P&onemacr;, Z = 2, with a = 10.0344(16) Å, b = 10.647(2) Å, c = 11.481(2) Å, alpha = 86.551(15) degrees, beta = 86.998(14) degrees, gamma = 80.112(15) degrees, V = 1205.1(4) Å(3), and final R = 0.0460. Crystals of ReO(salpd)OPhOMe are orthorhombic, space group P2(1)2(1)2(1), Z = 4, with a = 10.6222(15) Å, b = 12.442(3) Å, c = 16.354(3) Å, V = 2161.4(7) Å(3), and final R = 0.0371. Under the influence of traces of water a number of symmetrical complexes react to a "dimeric" structure, consisting of two ReO(salpd) moieties bridged by an oxygen atom with the bridging Re-O-Re angle symmetrically imposed at 180 degrees. [ReO(salpd)](2)O (3) crystallizes in the monoclinic, space group P2(1)/c, Z = 4, with a = 14.860(2) Å, b = 12.545(2) Å, c = 16.5111(17) Å, beta = 95.030(10) degrees, V = 3066.1(7) Å(3), and final R = 0.0439.

Retropane — a new rhenium complex as a potential ligand to label the dopamine transporter

The synthesis of a new mixed-ligand rhenium complex containing the α-tropanol moiety is reported. The compound is a predecessor to the analogous technetium-99m complex for SPECT imaging of the dopamine transporter in the brain.

Mixed-ligand rhenium(V) oxo monomers with triphenylphosphine oxide and lopsided and symmetrical heterocyclic ligands. Putative intermediates in rhenium(V) oxo synthetic chemistry

Normally, ReOCl3 (Me2S) (OPPh3) (1) serves as a useful source of ReO3+ or ReOClx(3−x)+ for the synthesis of Re complexes by ligand exchange. Complexes of the type ReOCl3(OPPh3)(L) (L = 1,5,6-trimethylbenzimidazole (Me3Bzm) (2); 3,5-lutidine (3,5-lut) (3); pyridine (py) (4)) were prepared from 1 and one equivalent of L. Formation of these mixed-ligand complexes is unsual because the normally labile OPPh3 in 1 was retained. Addition of non-coordinating triethylamine (NEt3) gave [HNEt3][ReOCl4(OPPh3] (5). The anion in 5 has been populated to be an intermediate in some synthetic schemes. ReOCl3(OPPh3)(Me3Bzm) (2) was characterized by X-ray crystallography. Crystallographic data are: C28H27Cl3N2O2PRe, P21/c, a = 18.503(3), b = 9.780(2), c = 15.735(4) A, β = 97.56(2)°, Dcalc = 1.76 g cm−3, Z = 4, R = 0.041, Rw = 0.066 for 3193 independent reflections. In 2, the pseudo-octahedral Re has the OPPh3 ligand trans to the oxo ligand, and one of the OPPh3 phenyl rings lies nearly over the five-membered ring of Me3Bzm. Reaction of ReOCl3(OPPh3)(Me3Bzm) (2) with one equivalent of 3,5-lut or py resulted in the precipitation of mixed-ligand species best formulated as Re2O3Cl4(Me3Bzm)2(3,5-lut)2 (6) and Re2O3Cl4(Me3Bzm)2(py)2 (7), respectively.