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«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»

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31. Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem.

Soc. Perkin Trans. 2 1987, S1-S19.





5.1 Introduction fac-[Re(CO)3L(L′)]n complexes, containing bisimine ligands, have recently found use as cell imaging agents in fluorescence microscopy.1 Complexes of the type, fac-[Re(CO)3L(L′)]n, in which L′ = dimethyl sulfoxide (DMSO), pyridine, or pyridine derivatives with various charges and lipophilicities and in which L = 2,2′-bipyridine (bipy) or a substituted bipyridine, have also sparked much interest.2 This interest arises because the excited state is localized on one bipyridine unit, making such complexes excellent candidates as probes responsive to their environment.1 These complexes are also important as precursors in supramolecular chemistry.3,4 fac-[Re(CO)3(bipy)(DMSO)]+ has found use as a precursor for binding the luminescent [Re(CO)3(bipy)]+ fragment to polytopic ligands for the construction of more elaborate assemblies.5 A desirable feature in Re complexes designed for use in cell imaging is lipophilicity, which favors cell membrane permeability.1 While investigating the properties of facRe(CO)3(N–N)L]+ complexes to be utilized as fluorescent probes, we noted that ligands needed to prepare fluorescent compounds are often only sparingly soluble in water, the solvent in which the common precursor is prepared.6 In addition, various pH conditions lead to different linkage isomers or mixtures of isomers for complicated ligands.7,8 Also, monitoring reaction mixtures is

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[Re(CO)3(CH3CN)3]+ precursor as a means of synthesizing Re compounds in organic solvents. In a previous study,9 we reported several novel compounds, which arose from reaction of the coordinated nitrile with ligand terminal amines upon treatment of fac-[Re(CO)3(CH3CN)3]+ with various tridentate amine ligands. Treatment of fac-[Re(CO)3(CH3CN)3]+ with bidentate aromatic N-donor L using acetonitrile or benzene as solvent gave the desired fac-[Re(CO)3(N– N)(CH3CN)]+ complexes in excellent yield. We discovered that synthetic reactions in methanol led to addition of solvent to a bound acetonitrile, giving iminoether complexes (Scheme 5.1).

Scheme 5.1.

Synthesis of [Re(CO)3L(HNC(CH3)OR)]BF4 complexes Iminoether metal complexes (MHNC(R)OR′) are formed by the reaction of M-N≡CR with R'OH, and two configurations of the iminoether ligand (E and Z) are possible with the OR' group being cis or trans to rhenium (Chart 5.1). Specifically when R'OH = methanol and R = methyl, we will refer to this ligand (HNC(CH3)OCH3) as acetimidate. Although there have been no previous reports of ReI iminoether complexes, the synthesis, characterization, and stereochemistry of iminoether complexes of platinum are well documented.10 Some of these platinum iminoether complexes are known to have antitumor activity.11 Natile and co-workers reported that the addition of alcohols to coordinated nitriles takes place under basic conditions, initially forming the Z isomer; subsequently the Z isomer isomerizes to the E isomer.10 A large steric bulk of the alcohol R′ group was reported to stabilize E over Z and increasing steric bulk of the nitrile R group was reported to stabilize the Z isomer.10 Chart 5.1. Configurations of the iminoether ligand in [Re(CO)3L(HNC(CH3)OR)]BF4 complexes In a recent review,12 Bokach et al. noted that the conditions needed for the reactions of alcohols with coordinated nitriles depend substantially on the oxidation state of the metal and that the addition of ROH to RCN in metal complexes of low to moderate oxidation state such as PtII,13 PdII,14 NiII,15 IrIII,16 and CuII 17 requires alkali or base. Bokach also pointed out the need for investigating more examples of E/Z configuration addition/transformation at different metal centers18 before a general conclusion as to the factors influencing the relative stability of the E or Z configuration of bound iminoether ligands can be reached.12 The low oxidation state of ReI, the formation of iminoether complexes in the absence of base, and the lack of a complete understanding of the factors influencing the E/Z ratio all prompted us to expand our study.

We have synthesized a series of fac-[Re(CO)3L(HNC(CH3)OR')]BF4 complexes bearing the substituted bipyridine moiety and iminoether ligands. We report here the first examples of an iminoether ligand bound to a ReI center, as well as the first solid-state evidence of the configuration of the iminoether ligand bound to ReI. However, ReIV compounds (cisRe(HNC(CH3)OCH3)2Cl4] and [Re(HNC(CH3)OCH2CH3)2Cl4]) were prepared in 1968 by Rouschias et al.,19 and the crystal structure of cis-[Re(HNC(CH3)OCH3)2Cl4], formed during an attempted crystallization of cis-[ReIV(CH3CN)2Cl4] in methanol, was reported recently.20 We have utilized a series of dimethyl-2,2'-bipyridine ligands to evaluate distortions in planarity of fac-[Re(CO)3L(HNC(CH3)OCH3)]BF4. Our study thus also provides a structural comparison of ReI complexes bearing the same ligand system, in which only the position of the methyl substituent in the bipyridine moiety is varied. Below, we do not use the fac- designation when discussing specific compounds because all the new compounds have this geometry.

5.2 Experimental Section Starting Materials. Re(CO)5Br was synthesized as described in the literature.21 Re2(CO)10, 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bipy), 5,5′-dimethyl-2,2′bipyridine (5,5′-Me2bipy), 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me2bipy), AgPF6 and AgBF4 were obtained from Aldrich. [Re(CO)3(CH3CN)3]PF6 and [Re(CO)3(CH3CN)3]BF4 were synthesized

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spectrometer. Peak positions are relative to TMS or solvent residual peak, with TMS as reference. All NMR data were processed with TopSpin and Mestre-C software.

X-ray Data Collection and Structure Determination. Single crystals were placed in a cooled nitrogen gas stream at 90 K on a Nonius Kappa CCD diffractometer fitted with an Oxford Cryostream cooler with graphite-monochromated Mo Kα (0.71073 Å) radiation. Data reduction included absorption corrections by the multi-scan method, with HKL SCALEPACK.23 All X-ray structures were determined by direct methods and difference Fourier techniques and refined by full-matrix least squares by using SHELXL97.24 All non-hydrogen atoms were refined anisotropically. All H atoms were visible in difference maps, but were placed in idealized positions. A torsional parameter was refined for each methyl group.

[Re(CO)3(CH3CN)3]PF6 (1a). [Re(CO)3(CH3CN)3]PF6 was synthesized by a slight modification of a known procedure,22 in which AgPF6 was used instead of AgClO4. A solution of Re(CO)5Br (1.62 g, 4 mmol) and AgPF6 (1.02 g, 4 mmol) in acetonitrile (20 mL) was heated at reflux for 16 h. After the reaction mixture was filtered, the solvent was removed by rotary evaporation to give a white solid, which was recrystallized by adding diethyl ether (∼30 mL) to a solution of the solid in acetonitrile (∼ 5 mL) and allowing the mixture to stand undisturbed for 1 h (1.75 g, 81% yield). X-ray quality crystals were obtained from acetonitrile/isopropyl ether solution. 1H NMR signal (ppm) in CDCl3: 2.52 (s, CH3), in DMSO-d6; 2.65 ppm, in acetone-d6;

2.66 ppm, in acetonitrile-d3; 2.43 ppm.

[Re(CO)3(CH3CN)3]BF4 (1b). Substituting AgBF4 for AgPF6 in the above procedure produced [Re(CO)3(CH3CN)3]BF4 as a white crystalline precipitate (1.62 g, 84% yield). X-ray quality crystals were obtained from a solution of the compound in acetonitrile/isopropyl ether.

The 1H NMR spectra were identical to that of the PF6– crystals given above. We report the crystallographic data of this compound obtained at 90 K in Supporting Information. (An X-ray structural characterization using data collected at room temperature has been reported.25) Synthesis of [Re(CO)3L(HNC(CH3)OCH3)]BF4 Complexes. Two methods (A and B) were employed to obtain [Re(CO)3L(HNC(CH3)OCH3)]BF4 complexes. Method A involved heating a benzene solution (10 mL) of [Re(CO)3(CH3CN)3]BF4 (0.1 mmol, 48 mg) and L (0.1 mmol) at reflux for 16 h. The solvent was removed by rotary evaporation, and the resulting solid was dissolved in ~1 mL of acetonitrile; diethyl ether (~25 mL) was then added to give [Re(CO)3L(CH3CN)]BF4 as a crystalline precipitate. This precipitate was washed well with diethyl ether and dried. [Re(CO)3L(CH3CN)]BF4, in 10 mL of a 1:1 acetonitrile:methanol mixture, was then heated at reflux for 24 h. The resulting solution was cooled to room temperature and taken to dryness by rotary evaporation. The residue was dissolved in acetonitrile;

addition of diethyl ether produced an orange precipitate, which was collected on a filter and washed with diethyl ether and dried. Method A resulted in high yields of crystalline material.

Method B, employed to obtain X-ray quality crystals, involved first stirring an acetonitrile solution (10 mL) of [Re(CO)3(CH3CN)3]BF4 (0.1 mmol, 0.0481 g) and L (0.1 mmol) at room temperature and then heating the solution at reflux for 2-3 days. The reaction mixture was monitored by NMR spectroscopy to ensure that the reaction went to completion. An equal volume of methanol was then added, and the mixture was heated at reflux for 1 day. The resulting solution yielded X-ray quality crystals upon slow evaporation. The yields reported below are based on the initial concentration of [Re(CO)3(CH3CN)3]BF4 (0.1 mmol, 48 mg).

[Re(CO)3(bipy)(HNC(CH3)OCH3)]BF4 (2). Method A just described yielded the intermediate product, [Re(CO)3(bipy)(CH3CN)]BF4, as a yellow crystalline precipitate (39 mg, 76% yield). 1H NMR signals (ppm) in CDCl3: 8.91 (d, H6/6′), 8.56 (d, H3/3′), 8.26 (t, H4/4′), 7.64 (t, H5/5′), 2.21 (s, CCH3). (The synthesis of [Re(CO)3(bipy)(CH3CN)]BF4 was reported by Amoroso et al., but synthetic details and NMR data were not provided and different starting materials were used.1 The synthesis of [Re(CO)3(bipy)(CH3CN)]PF6 has been reported,26 but NMR data were not provided.) Method A afforded [Re(CO)3(bipy)(HNC(CH3)OCH3)]BF4 as an orange-colored precipitate (39 mg, 63% yield). Method B afforded X-ray quality crystals (21 mg, 36% yield). 1H NMR (ppm) in CDCl3: 8.90 (d, H6/6′), 8.37 (d, H3/3′), 8.16 (t, H4/4′), 7.62 (b, NH), 7.56 (t, H5/5′), 3.92 (s, OCH3), 2.17 (s, CCH3).

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[Re(CO)3(4,4′-Me2bipy)(CH3CN)]BF4, was obtained as a yellow crystalline precipitate (41 mg, 69% yield). 1H NMR signals (ppm) in CDCl3: 8.71 (d, H6/6′), 8.35 (s, H3/3′), 7.39 (d, H5/5′), 2.65 (s, 4/4′-CH3), 2.20 (s, CCH3). ([Re(CO)3(4,4′-Me2bipy)(CH3CN)]PF6 has been synthesized previously, but details were not reported.27 No NMR data were provided.) Method A afforded [Re(CO)3(4,4′-Me2bipy)(HNC(CH3)OCH3)]BF4 as an orange-colored precipitate (38 mg, 62%

yield). Method B afforded X-ray quality crystals (21 mg, 34% yield). 1H NMR (ppm) in CDCl3:

8.69 (d, H6/6′), 8.14 (s, H3/3′), 7.41 (b, NH), 7.33 (d, H5/5′), 3.90 (s, OCH3), 2.59 (s, 4/4′-CH3), 2.14 (s, CCH3).

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[Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4, was obtained as a yellow crystalline precipitate (50 mg, ′ 70% yield). 1H NMR signals (ppm) in CDCl3: 8.68 (s, H6/6′), 8.35 (d, H3/3′), 8.01 (d, H4/4′), 2.53 (s, 5/5′-CH3), 2.22 (s, CCH3). X-ray quality crystals were obtained by slow evaporation from a chloroform solution. (Meyer et al. have reported the synthesis of [Re(CO)3(5,5′Me2bipy)(CH3CN)]PF6 from Re(CO)3(5,5′-Me2bipy)Cl, which was heated at reflux in acetonitrile with AgClO4•H2O. The product was obtained by addition of a saturated aqueous solution of NH4PF6. No NMR data were provided.26) Method A afforded [Re(CO)3(5,5′Me2bipy)(HNC(CH3)OCH3)]BF4 as an orange-colored precipitate (41 mg, 67% yield). Method B afforded X-ray quality crystals (23 mg, 37%)). 1H NMR signals (ppm) in CDCl3: 8.65 (s, H6/6′), 8.17 (d, H3/3′), 7.91 (d, H4/4′), 7.51 (b, NH), 3.92 (s, OCH3), 2.50 (s, 5/5′-CH3), 2.17 (s, CCH3).

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crystalline precipitate (42 mg, 72% yield). 1H NMR signals (ppm) in CDCl3: 8.43 (d, H3/3′), 8.09 (t, H4/4′), 7.54 (d, H5/5′), 3.06 (s, 6/6′-CH3), 2.22 (s, CCH3). Method A afforded [Re(CO)3(6,6′-Me2bipy)(HNC(CH3)OCH3)]BF4 as an orange-colored precipitate (39 mg, 63% yield). Method B afforded X-ray quality crystals (20 mg, 33%). These crystals contained

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Me2bipy)(HNC(CH3)OCH3)]BF4: 8.15 (d, H3/3′), 7.97 (t, H4/4′), 7.59 (b, NH), 7.49 (d, H5/5′), 3.81 (s, OCH3), 3.04 (s, 6/6′-CH3), 2.15 (s, CCH3) and for the 6,6′-Me2bipy of solvation: 8.18 (d, H3/3′), 7.66 (t, H4/4′), 7.14 (d, H5/5′), 2.62 (s, 6/6′-CH3).

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methanol in the general procedure described above for 4 afforded [Re(CO)3(5,5′Me2bipy)(HNC(CH3)OCH2CH3)]BF4 as an orange-colored precipitate (50 mg, 76% yield).

Method B afforded X-ray quality crystals (29 mg, 46%)). 1H NMR signals (ppm) in CDCl3: 8.66 (s, H6/6′), 8.17 (d, H3/3′), 7.90 (d, H4/4′), 7.45 (b, NH), 4.18 (q, OCH2), 2.49 (s, 5/5′-CH3), 2.17 (s, CCH3), 1.49 (t, CH3).

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[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)OCH3)]BF4 in DMSO-d6 (600 µL) was treated with increasing amounts of Cl– (1 to 150 mM) and the solution was monitored by 1H NMR spectroscopy upon each addition of Cl–. The complex concentration was kept constant throughout the titration by using a 5 mM solution of the complex in DMSO-d6 to prepare the Cl– stock solution.

5.3 Results and Discussion Structural Results. All complexes reported here exhibit a pseudo octahedral structure, with the three carbonyl ligands occupying one face. The remaining three coordination sites are

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[Re(CO)3(CH3CN)3]PF6 (1a) (Figure 5.1), or two nitrogen atoms of 2,2′-bipyridine or of a substituted 2,2′-bipyridine and a nitrogen of an acetimidate ligand for complexes having the general formula, [Re(CO)3L(HNC(CH3)OCH3)]BF4 (Figure 5.2). Crystal data and details of the structural refinement for all these complexes are summarized in Table 5.1. The atom numbering in Figure 5.2 is used to describe the solid-state data.

Figure 5.1.

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