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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Inorganica Chim Acta. Author manuscript; available in PMC 2010 July 6.
Published in final edited form as:
Inorganica Chim Acta. 2000 September 1; 307(1-2): 160–163.
doi:  10.1016/S0020-1693(00)00207-3
PMCID: PMC2897098
NIHMSID: NIHMS214544

Synthesis and crystal and molecular structure of a tetranuclear cluster based on the rhenium(III)-bisorganohydrazino core: [Re(HNNC4H3N2)(NNC4H3N2)(OCH3)2]4

Abstract

Reaction of NH4ReO4 with excess 2-hydrazinopyrimidine in methanol yields [Re(η1-NNC4H3N2H)(η2-HNNC4H3N2)Cl3] (1). Attempts to recrystallize 1 by slow diffusion of methanol into DMF after 8 months produced black crystals of [Re(HNNC4H3N2)(NNC4H3N2)(OCH3)2]4 (2). The structure of 2 consists of isolated tetranuclear clusters, constructed from {Re(η2-HNNC4H3N2)(η1-NNC4H3N2)(OCH3)2} units linked through the β-nitrogen of the chelating organodiazene ligand of adjacent units into a box-like aggregate.

Keywords: Crystal structures, Rhenium complexes, Organohydrazino complexes

1. Introduction

The continuing expansion of technetium- and rhenium-organohydrazine chemistry reflects not only an interest in the fundamental coordination chemistry and bonding of an unusual and robust functional group [1], but also practical applications to the development of radiopharmaceuticals [2,3]. In this latter respect, the chemistry of technetium with 2-hydrazino-nicotinamide derivatized peptides has received considerable attention [46]. In modeling the chemistry of the Tc-organohydrazino core of these radiopharmaceuticals, we have developed the chemistry of a class of compounds derived from the rhenium bis-pyridinodiazene core, {Re(η2-HNNC5H4N)(η1-NNC5H4NH)}3+, shown schematically [7] (Scheme 1). An unusual feature of these compounds is the presence of the organohydrazine ligands as the chelating, neutral organodiazine (A) and the neutral pyridinium-diazenido (B) in the parent compound [Re(η2-HNNC5H4N)(η1-NNC5H4-NH)Cl3] and in the neutral chelating form (A) and as the mononegative anionic organodiazenido ligand (C) in the derivatives of the type [Re(η2-HNNC5H4N)(η1-NNC5H4N)(L)(LL)] (where L is a monodentate mononegative ligand and LL is a bidentate mononegative ligand) [8] (Scheme 2).

Since hydrazinopyrimidine offers some advantages over hydrazinopyridine in terms of solubility and derivatization, we undertook an investigation of the chemistry of technetium- and rhenium 2-hydrazinopyrimidine complexes as models for potential bifunctional chelate-metal cores. The chemistry appeared analogous to that of 2-hydrazinopyridine in the facile synthesis of [Re(η2-HNNC4H3N2)(η1-NNC4H3N2H)Cl3] (1) from NH4ReO4. However, attempts to recrystallize 1 from dimethylformamide–MeOH yielded the unusual tetranuclear cluster [Re(HNNC4H3N2)(NNC4H3N2)(OCH3)2]4 (2).

2. Results and discussion

The reaction of perrhenate with 2-hydrazinopyrimidine in MeOH–HCl involves four-electron reduction of the rhenium from the Re(VII) to the Re(III) form with concomitant two-electron oxidation and two proton elimination of each of two hydrazinopyrimidine ligands.

ReO4+2H2NNH(C4H3N2)+4HCl[Re(η2-HNNC4H3N2)(η1-NNC4H3N2H)Cl3]+4H2O+Cl
(1)

The reaction chemistry is analogous to that previously described for [Re(η2-HNNC5H4N)(η1-NNC5H4NH)-Cl3] [9].

Compound 1 was insoluble in most common organic solvents, with the exception of DMF. A DMF–MeOH solution, left to stand for 8 months, yielded black crystals of the tetranuclear cluster [Re(η2-HNNC4-H3N2)(η1-NNC4H3N2)(OCH3)2]4·2MeOH (2·2MeOH). The synthesis of 2 from 1 requires in a formal sense the substitution of two chloride ligands on each Re site by two methoxy groups and the elimination of an HCl molecule from each Re site, which necessitates deprotonation of one of the organodiazene ligands of each [Re(η2-HNNC4H3N2)(η1-NNC4H3N2H)}3+ core. In view of the facile deprotonation of the pyridinium-diazenido site (B) observed in previous studies, it is anticipated that the pyrimidinium diazimido site of 1 provides the proton.

4[Re(HNNC4H3N2)(NNC4H3N2H)Cl3]+8MeOH[Re(HNNC4H3N2)(NNC4H3N2)(OMe)2]4+12HCl

As shown in Fig. 1, the structure of 2·2MeOH consists of tetranuclear clusters and isolated MeOH molecules of crystallization. The cluster exhibits a square arrangement of Re sites linked through the pyrimidinodiazene ligands which chelate to one Re center through the α-nitrogen of the hydrazine and one pyrimidine ring nitrogen and bridge to a second Re site through the β-nitrogen of the hydrazine. The coordination geometry at the Re centers is distorted octahedral, defined by the chelating nitrogen donors of the pyrimidinodiazene ligand, the α-nitrogen of a terminal pyrimidinodiazenido ligand, the oxygen donors of two methoxy groups, and the β-nitrogen of an organodiazene ligand of a neighboring {Re(HNNC4H3N2)-(NNC4H3N2)}2+ subunit.

Fig. 1
(a) A view of the asymmetric unit of 2, showing the atom-labeling scheme and 50% thermal ellipsoids. (b) A view of the tetranuclear cluster. Re sites are large speckled spheres; nitrogen atoms are dotted spheres; carbon atoms are intermediate, lined spheres. ...

The final difference Fourier maps clearly exhibit the protonation of the α-nitrogens of the chelating pyrimidinodiazene ligands. There are no excursions of electron density associated with any of the other nitrogen sites of the cluster, confirming the deprotonation of the pyrimidine nitrogen of the parent compound. This deprotonation pattern is consistent with the decreasing basicity of the nitrogen sites: α-hydrazine nitrogen > pyrimidine nitrogen > β-hydrazine nitrogen. It is also noteworthy that substitution chemistry of 1 follows the same pattern as that of [Re(HNNC5H4N)(NNC5-H4NH)Cl3] wherein the overall charge of the core is adjusted by proton exchange:

{Re(HNNC5H4N)(NNC5H4NH)}3++H+H+{Re(HNNC5H4N)(NNC5H4N)}2+

The metrical parameters associated with the {Re(HNNC4H3N2)(NNC4H3N2)}2+ cores of 2 are unexceptional. The nearly planar arrangement of this core unit and the syn disposition of the pyrimidine rings with respect to the N(3)−Re−N(5) vector are also common features of this core.

The isolation of 2 upon prolonged exposure of 1 in DMF/methanol confirms that cores of the type {M(η2-HNNR)(η1-NNRHx)}n+ (x = 0, 1) are chemically robust, surviving both reactions at ancillary sites and treatment in organic and mixed organic–aqueous media. However, the structural characterization also reveals a complex coordination chemistry which often results in complications upon extending the chemistry to radiopharmaceutical applications.

3. Experimental

3.1. Preparation of [Re(η2-HNNC4H3N2)(η1-NNC4H3N2H)Cl3] (1)

In a 25 ml Schlenk flask was placed [NH4][ReO4] (0.015 g, 0.056 mmol), 2-hydrazinopyrimidine (0.050 g, 0.450 mmol) and 2.0 ml methanol. The reaction was stirred briefly and 0.01 ml of aqueous 36% HCl (0.30 mmol) was added dropwise. The reaction was heated to reflux for 20 min. The dark red solid was washed with 5 ml portions of MeOH and dried (0.0141 g, 49.5%). Anal. Calc. for C8N8H8Cl3Re: C, 18.9; H, 1.58; N, 22.0. Found: C, 18.9; H, 1.62; N, 21.5%

3.2. Preparation of [Re(η2-HNNC4H3N2)(η1-NNC4H3N2)(OMe)2]4 ·2MeOH (2 ·2MeOH)

Compound 1 was dissolved in N,N-dimethylformamide. The solution was carefully layered with methanol. After 8 months, black crystals of 2 were observed on the sides of the test tube (Yield: 0.0012 g, 15%). Anal. Calc. for C10.5H15N8O2.5Re: C, 26.3; H, 3.13; N, 23.4. Found: C, 26.1; H, 3.06; N, 23.1%

3.3. X-ray crystallography

Data were collected on a Bruker SMART diffractometer system using graphite monochromated Mo Kα radiation (λ(Mo Kα)=0.71073 Å). Data collection was carried out at low temperature (85–94 K). The crystal parameters and other experimental details of the data collection are summarized in Tables 13. A complete description of the details of the crystallographic methods is given in the supporting information. The structure was solved by direct methods [10]. Neutral atom scattering factors were taken from Cromer and Waber [11] and anomalous dispersion corrections were taken from those of Creagh and McAuley [12]. All calculations were performed using SHELXTL [10]. Non-hydrogen atoms were refined anisotropically. No anomalies were encountered in the refinements of any of the structures. Atomic positional parameters for the structures have been deposited with the Cambridge Structural Database.

Table 1
Summary of crystal data for the structural study of [Re(η2-HNNC4H3N2)(η1-NNC4H3N2)(OMe)2]4·2MeOH (2·2MeOH)
Table 3
Selected bond lengths (Å) and angles (°) for 2 a

4. Supplementary material

All atomic and thermal parameters and all interatomic angles are available from the authors upon request. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as publication no. CCDC 139541. Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: ku.ca.mac.cdcc @tisoped or www: http://www.ccdc.cam.ac.uk).

Table 2
Atomic positional parameters (× 104) and isotropic thermal parameters (Å2 × 103) for 2

Acknowledgements

This work was supported by a grant from the Department of Energy Office of Health and Environmental Research (D2-FG02-99ER62791).

References

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