Re(V) Complexes with an Open-Chain Quadridentate Ligand Containing Two Amine and Two Amido Donors. Synthesis, Characterization, and Solution Equilibria of Re₂O₃(dioxo-tetH₄)₂ and [ReO(H₂O)(dioxo-tetH₄)]Cl (dioxo-tetH₆ = 1,4,8,11-tetraazaundecane-5,7-dione)

Abstract

We are interested in identifying mononuclear cationic [M(V)=O]3+ (M = Tc, Re) complexes for radiopharmaceutical applications. The open-chain ligand, 1,4,8,11-tetraazaundecane-5,7-dione-(dioxo-tetH6) with two amine and two amide donors, was selected for investigation since the literature led us to expect that a five-coordinate [Re(V)=O(dioxo-tetH4)]+ cation would dominate. Instead, the neutral mu-oxo bridged dinuclear complex, Re2O3(dioxo-tetH4)2 (1), and a salt of the six-coordinate mononuclear cation, [ReO(H2O)(dioxo-tetH4)]+ (2), were isolated; the structure of each was determined by X-ray crystallography. The cation (2) is unusual because it has a trans-oxo/aqua core. Such aqua compounds are rarely isolated, and the Re-OH2 distance is relatively short (2.185 A). The cation has two pKa values, 4.1 and 8.7, determined with visible spectroscopy. Since the Re-OH2 bond is short, the coordinated water is likely to be acidic. Thus the two pKa's are assigned to the stepwise deprotonation of the water ligand to give a trans-oxo/hydroxo neutral form and a trans-dioxo anion. Although 1 was the first product isolated following ligand exchange of ReOCl3(Me2S)(OPPh3) with dioxo-tetH6 under neutral conditions, it probably formed from the hydroxo mononuclear complex. Under concentrated conditions (approximately 300 mM) the dinuclear complex deposited from solution, but the 1H NMR spectra of 2 (approximately 20 mM) were consistent with the presence of only monomeric forms in D2O, pH 3-12. 1H NMR experiments demonstrated that in DMSO-d6 2 converts to 1 upon addition of base, consistent with the proposal that two units of the hydroxo monomer condense to give the dinuclear form. In addition, all spectra of pure 1 dissolved in DMSO-d6 included extra low intensity signals that were characteristic of the monomer. Thus, although 1 is favored over the neutral monomer in DMSO-d6, the two complexes exist as a mixture of equilibrating forms. Our results do not support the previous findings for the Re(V) complex with a macrocyclic diamine-diamide ligand related to dioxo-tetH6. The data indicate that the ability of an amido group to donate electron density to a Re(V) center is moderately greater than the donating ability of a neutral amine group.

Synthesis and Characterization of Novel Rhenium(V) Tetradentate N2O2Schiff Base Monomer and Dimer Complexes

“…The structure of 1 is crystallographically analogous to the Tc(V) analogue, with virtually identical bond angles and distances reported in the X-ray structures . A water coordinated trans to the rhenium oxo was previously thought not to exist for the tetradentate Schiff base complexes, although trans aquo Re(V) oxo complexes with other equatorial ligands are known. ,, …”

“…The ORTEP representations of compounds 1 − 6 are shown in Figures −7, respectively. The ReO bond distances in these complexes range from 1.68 to 1.711 Å (Table ), typical for mono-oxo Re(V) complexes. ,,− , The ReO bond distances observed for the complexes with a methoxide ( 4 ) or μ-oxo species ( 5 , 6 ) coordinated trans to the oxo group (Table ) are slightly longer, but still within the range typical for these types of complexes. ,,, The typical bond length for a Re(V) oxo double/triple bond is approximately 1.68−1.70 Å, whereas the trans rhenium dioxo bond lengths average 1.75 Å. , The Re−N and Re−O bond distances in the equatorial positions of the complexes are as expected (1.9−2.1 Å). ,,− ,, The group trans to the ReO bond is long for compounds 1 , 2 , and 3 , as expected from trans labilization of this position and consistent with other octahedral mono-oxo Re(V) reported structures . In general, the trans influence of the oxo group results in Re−X bond distances longer than a similar species in an equatorial position. − ,

2 ORTEP representation of trans -[ReO(acac 2 en)OH 2 ]Cl, 1 , with 30% thermal ellipsoids.

3 ORTEP representation of trans -μ-oxo-[ReO(acac 2 en)OReO 3 ], 2 , with 30% thermal ellipsoids.

4 ORTEP representation of trans -[ReO(acac 2 pn)Cl], 3 , with 30% thermal ellipsoids.

5 ORTEP representation of trans -[ReO(acac 2 pn)OMe], 4 , with 30% thermal ellipsoids.

6 ORTEP representation of trans -[μ-oxo-Re 2 O 3 (acac 2 en) 2 ], 5 , with 40% probability thermal ellipsoids.

7 ORTEP representation of trans -[μ-oxo-Re 2 O 3 (acac 2 pn) 2 ],...

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A hydrolytically stable oxo-rhenium(v) antitumor agent for synergistic combination therapy with cisplatin: from synthesis and mechanistic studies to toxicity assessment in zebrafish

“…[35][36][37] It is important to note here that the Re(V)-Cl bond is known to undergo rapid and irreversible hydrolysis in aqueous media, which will predominantly remain as Re(V)-OH at physiological pH 7.4 due to the high acidity of H 2 O bound to Re(V) (pK a ∼ 4.1). 41 By contrast, hydrolysed species of a Re (I)-Cl system will remain as Re(I)-OH 2 (pK a ∼ 9) at physiological pH. 42 Therefore, hydrolysis is expected to be suppressed for the Re(I)-Cl system, but not for Re(V)-Cl systems, by Cl − present in the plasma or intestinal fluid (∼100 mM) before reaching the target.…”

Synthesis, characterization and structural investigation of new rhenium-oxo complexes containing bidentate phosphine ligands: an exploration of chirality and conformation in chelate rings of small and large bite angle ligands

“…3,4 The more recent application of rhenium complexes as radiopharmaceutical agents, [5][6][7][8] as well as their role as models for technetium complexes, which are also extensively used as radiopharmaceuticals, 9,10 has given rise to a significant area of research. Oxorhenium(V) complexes of the type [ReOX 3 L 2 ] (X = halogen; L = triaryl or alkyl phosphine, arsine or stibine ligands) are versatile precursors in the synthesis of a wide variety of rhenium complexes [11][12][13][14][15][16] and also display activity as oxygen transfer agents. [17][18][19][20][21][22][23][24][25][26] Organophosphorus compounds continue to gain in popularity as ligands in synthetic inorganic chemistry since, by varying the substituents on the phosphorus, the chemical behavior of both the ligand and the resultant complex can be altered.…”