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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 12.
Published in final edited form as:
Inorganica Chim Acta. 2000 August; 306(1): 112–115.
doi:  10.1016/S0020-1693(00)00145-6
PMCID: PMC2901877
NIHMSID: NIHMS214559

Structural characterizations of an Re(IV) complex [ReCl4(OPPh3)2] and of an imino species [ReOCl2(PPh3)(η2-OC6H4-2-CH=NH)] prepared from the reaction of [ReOCl3(PPh3)2] with salicylaldoxime

Abstract

The reaction of [ReOCl3(PPh3)2] with salicylaldoxime unexpectedly yields [ReCl4(OPPh3)2] (1) and [ReOCl2(PPh3)(η2-OC6H4-2-CH=NH)] (2). The crystal structure of the Re(IV) complex 1 consists of discrete mononuclear units. The Re site is bound to four chlorides and two trans oxygens of the OPPh3 groups. The crystal structure of 2 is also mononuclear with the geometry of the Re(V) center defined by a terminal oxo-group, two cis chlorides, the phosphorus of the PPh3, and the nitrogen and oxygen donors of the phenoxyimino ligand [OC6H4-2-CH=NH].

Keywords: Crystal structures, Rhenium complexes, Oxo complexes, Imino ligand complexes

1. Introduction

The widespread contemporary interest in the development of technetium and rhenium radiopharmaceuticals [15] has fostered a dramatic expansion of the coordination chemistry of these metals. One persistent strategy has focused on developing effective chelating ligands for the robust and ubiquitous {MO}3+ core (M = Tc, Re).

A well-developed approach to the structural modification of complexes with the {MO}3+ core has been dubbed ‘3+1’ chemistry [622] and exploits a dian-ionic tridentate ligand with XYZ donor groups (X=Z=S, O; X=S and Z=O; Y=–NR, O, S) and a monodentate thiol. However, it has been noted that such ‘3+1’ systems may undergo further reactions to achieve six coordination and/or closed shell configurations [22]. In an effort to increase the stability of the {MO}3+ core complexes, a new strategy has been devised recently which replaces the monodentate thiol with a bidentate mononegative ligand and which has been appropriately dubbed ‘3+2’ chemistry [22,23].

As part of our continuing investigations of ‘3+2’ chemistry of {MO}3+ cores, we have considered the use of oxime ligands as the bidentate component. During the course of these investigations, it was observed that upon reacting with [ReOCl3(PPh3)2], salicylaldoxime underwent cleavage of the N–O bond to yield an unusual imino species [ReOCl2(PPh3)-(OC6H4CHNH)] (2). Curiously, a Re(IV) species [ReCl4(OPPh3)2] (1) was isolated as a minor product.

2. Experimental

2.1. General considerations

NMR spectra were recorded on a Bruker DPX 300 (1H 300.10 MHz) spectrometer in CDCl3 (δ 7.27 ppm). IR spectra were recorded as KBr pellets with a Perkin–Elmer series 1600 FT IR. Elemental analysis for carbon, hydrogen and nitrogen were carried out by Oneida Research Services, Whitesboro, NY. Ammonium perrhenate and salicylaldoxime were purchased from Aldrich and used without further purification. Reagent grade solvents, distilled and dried by standard methods, were used in all cases. [ReOCl3(PPh3)2] was prepared according to the literature [24].

2.2. Preparation of [ReCl4(OPPh3)2] (1)

To a solution of salicylaldoxime (13.7 mg, 0.1 mmol) in chloroform (30 cm3) was added [ReOCl3(PPh3)2] (83 mg, 0.1 mmol). The green solution thus formed was refluxed for 40 min. The volume was condensed to 5 cm3 and upon slow cooling to 4°C, some pink microcrystalline solid was isolated from the chloroform solution. X-ray quality crystals were obtained by recrystallization from CH2Cl2–MeOH (yield, 10 mg, 11%). IR (KBr, cm−1): 1438(m), 1123(s), 727(m), 534(m). 1H NMR (CDCl3, ppm): 7.9 (br s, PPh3). Anal. Calc. for C36H30Cl4O2P2Re: C, 48.87; H, 3.39. Found: C, 48.59; H, 3.45%.

2.3. Preparation of [ReOCl2(PPh3){η2-OC6H4-2-CH=NH}] (2)

The mother liquor of the above reaction was subjected to a silica gel column. The eluent was first treated with chloroform to remove PPh3 and a trace amount of unreacted oxime ligand, then a gradient eluent of CHCl3–acetone (from 100/0 to 50/50) was applied. X-ray quality crystals for compound 2 were grown by slow diffusion of ethyl ether into a solution of the compound dissolved in minimum amount of CH2Cl2 (yield, 40 mg, 65%). IR (KBr, cm−1) 1616(m), 1482(m), 1437(m), 1275(m), 1096(m), 966(s), 903(m), 748(m), 693(m), 529(w), 508(m). 1H NMR (CDCl3, ppm): 6.8–7.7 (m, 19H, ArH), 9.26 (s, 1H, –CH=NH). Anal. Calc. for C25H21Cl2NO2PRe: C, 45.80; H, 3.21; N, 2.14. Found: C, 45.68; H, 3.33; N, 2.29%.

2.4. X-ray structural determinations

Crystallographic data for both compounds were collected with a Siemens P4 diffractometer equipped with the SMART CCD system [25] and using Mo Kα radiation (λ= 0.71073 Å). The data were collected at 80 K. Both data sets were corrected for Lorentz and polarization effects, and absorption corrections were made using SADABS [26].

The structure solutions and refinements were carried out using the SHELXL96 [27] software package. Both structures were solved using direct methods and all of the nonhydrogen atoms were located from the initial solution. After locating all of the nonhydrogen atoms in each structure, the models were refined against F2, initially using isotropic and later anisotropic thermal displacement parameters until the final value of Δ/σmax was less than 0.001. At this point the hydrogen atoms were located from the electron density difference map and a final cycle of refinements was performed until the final value of Δ/σmax was again less than 0.001 (Tables 13).

Table 1
Crystallographic data for 1 and 2
Table 3
Selected bond lengths (Å) and angles (°) for [ReOCl2(PPh3)(η2-OC6H4-2-CH=NH)] (2)

3. Discussion

The reaction of [ReOCl3(PPh3)2], and salicylaldoxime in chloroform yields a green solution from which pink crystals of a minor product [ReCl4(OPPh3)2] (1) and, after suitable workup, green crystals of the major product [ReOCl2(PPh3)(η2-OC6H4-2-CH=NH)] (2) were isolated. While the infrared spectrum of 1 was featureless in the 900–1000 cm−1 region, indicating the absence of the {ReO}3+ core, the spectrum of 2 was characterized by a strong band at 942 cm−1 associated with ν(Re–O). The 1H NMR spectrum of 2, in addition to aromatic protons in the 6.8–7.7 ppm region, exhibited a singlet at 9.26 ppm assigned to the methine proton. The 1H NMR spectrum of 1 was unexceptional.

The isolation of the phenoxyimino complex 2 as the major product suggests substitution of one chloride and one phosphine of the [ReOCl3(PPh3)2] precursor by the salicylaldoxime ligand and cleavage of the N–O bond to form phosphine oxide.

[ReOCl3(PPh3)2]+HOC6H4CH=NOH[ReOCl2(PPh3)(OC6H4CH=NH)](2)+OPPh3+HCl

The formation of 1 as the minor product requires reduction of the Re(V) core of [ReOCl3(PPh3)2] to Re(IV) and considerable rearrangement of the coordination sphere. The details of this conversion have not been addressed.

As shown in Fig. 1, the crystal structure of 1 consists of isolated molecules of [ReCl4(OPPh3)2]. The geometry about the Re(IV) center is defined by four chloride donors and two trans oxygens from the two phosphine oxide ligands. The metrical parameters for 1 are unexceptional.

Fig. 1
A view of the structure of [ReCl4(OPPh3)2] (1), showing the atom-labeling scheme and 50% thermal ellipsoids.

The crystal structure of 2, shown in Fig. 2, reveals isolated mononuclear species with a distorted octahedral Re(V) site. The coordination geometry about the rhenium is established by the terminal oxo-group, two chloride donors, the phosphorus atom of the triphenylphosphine, and the N and O donors of the bidentate phenoxyimino ligand. The C7–N1 bond distance of 1.309(6) Å is consistent with double bond character and hence the characterization of the ligand as the phenoxyimine. Other metrical parameters are unexceptional for the {ReO}3+ core.

Fig. 2
A view of the structure of [ReOCl2(PPh3)(OC6H4CHNH)] (2), showing the atom-labeling scheme and 50% thermal ellipsoids.

4. Conclusions

The products of the reaction of [ReOCl3(PPh3)2] with salicylaldoxime were unanticipated. Oximes are robust ligands, and N–O cleavage is not a common observation in reactions with metal-oxo species [28]. Similarly, reduction to the Re(IV) oxidation state in the minor product [ReCl4(OPPh3)2] (1) is uncharacteristic. The compound [ReOCl3(PPh3)2] is a common starting material for the preparation of complexes with the {ReO}3+ core; yet, this work represents the first report of this reduced complex. Investigation of other oximes is in progress in order to establish the generality of this chemistry.

Table 2
Selected bond lengths (Å) and angles (°) for [ReCl4(OPPh3)2] (1)a

Acknowledgments

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

Footnotes

5. Supplementary material

All atomic and thermal parameters and all interatomic angles are available from the authors upon request. Crystallographic data (excluding structure factor) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Center as publication Nos. CCDC 139797–139798. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; ku.ca.mac.cdcc@tisoped or www:http://www.ccdc.cam.ac.uk).

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