Polyhedron 30 (2011) 697–701

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Polyhedron

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Synthesis and characterization of rhenium(I) complexes with the polypyridinicquinone functionalized electron acceptor ligand [3,2-a:20,30-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen

Ramiro Díaz a,⇑, Angélica Francois a, Bárbara Loeb b

a Facultad de Recursos Naturales, Universidad Católica de Temuco, Avda. Rudecindo Ortega 02950, Temuco, Chileb Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 September 2009Accepted 1 December 2010Available online 7 December 2010

Keywords:Acceptor ligandQuinoneRe(I) complexes

0277-5387/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.poly.2010.12.001

⇑ Corresponding author. Tel.: +56 45 205413; fax: +E-mail address: ramdiazh@uct.cl (R. Díaz).

In this work, the synthesis and characterization of fac-[Re(CO)3(Nqphen)(L)]PF6 complexes is reported.Nqphen is the quinone substituted acceptor ligand [3,2-a:20 ,30-c]-benzo[3,4]-phenazine-11,16-quinone,and L represents the donor monodentate pyridine substituted ligands 4-tert-butylpyridine (t-Bupy),4-methoxypyridine (MeO-py) or 10-(4-picolyl)phenothiazine (py-PTZ). The complexes were synthesizedby refluxing in methanol the metal precursor fac-Re(CO)3(Nqphen)TfO (TfO = trifluoromethanesulpho-nate anion) with the corresponding L ligand. The UV–Vis spectra of the complexes are dominated byintense intraligand (IL) bands, and less intense metal ligand charge transfer (MLCT) bands with maximain the 380–400 nm region. The IR shows the typical pattern for tricarbonyl Re complexes with facial (fac)geometry. An additional v(CO) stretching band, attributed to the quinone fragment of Nqphen, isobserved.

Electrochemical data indicate that the acceptor capacity of Nqphen is increased in the complexeswith regard to the free ligand. This effect is sensitive to the nature of the L ligand, following the order:MeO-py < t-Bupy < py-PTZ, indicating therefore that the donor capacity of L affects the rest of themolecule. The results obtained for the fac-[Re (CO)3(Nqphen)(pyPTZ)]PF6 complex here reported werecompared with those observed for the homologous complex fac-[Re(CO)3(Aqphen)(L)]0/+, withAqphen = 12,17-dihydronaphtho[2,3-h]dipyrido[3,2-a:20 ,30-c]-phenazine-12,17-dione, and L = Cl�, TfO�,py-PTZ.

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1. Introduction

In inorganic chromophores as polypyridine d6 metal complexes,the presence on the ligands of strong electron acceptor groups [1],such as quinones [2], permits to introduce directionality to thephotoinduced electron transfer thus favouring the generation of acharge separate excited state. As a consequence, exited state nonradiative decay should be diminished, and consequently anenhancement in the excited state lifetime should be observed [2–7]. Different 2,20-bipyridine (bpy) and 1,10 phenanthroline (phen)ligands with incorporated quinone residues, and their correspond-ing complexes with Ru(II) and Re(I), have been reported [3]. Thelack of emission in the photophysical studies of some of thesecomplexes reflects the quencher capacity of the quinone residue[2,4–7]. If additionally a ligand with electron donor properties isincorporated into the complex, D–C–A (D = electron donor,C = cromophore, A = electron acceptor) systems are formed. Oneexample in this direction is the fac-[Re(CO)3(Aqphen)(py-PTZ)]PF6,

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56 45 205285.

complex, with Aqphen = 12,17-dihydronaphtho[2,3-h]dipyrido[3,2-a:20,30-c]-phenazine-12,17-dione and py-PTZ = 4-phenothiazine-pyridine, Scheme 1 [2c,7]. The quencher capacity of the quinoneAqphen ligand was evidenced by the absence of emission at RTand at 77 K. The presence of a charge separated excited state withs = 300 ns was fully proved by advanced spectroscopic techniques,such as TA (Transient Absorbance) and TRIR (Time ResolvedInfrared).

As can be seen in Scheme 1, the structural geometry of Aqphenmay allow some through space charge recombination when form-ing a complex. In a former work, [8] the ‘‘linear’’ ligand [3,2-a:20,30-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen, Scheme 1, wasproposed. Its synthesis and characterization, as well as the corre-sponding for the neutral fac-Re(CO)3 (Nqphen)L (were L = Cl�,TfO�) complexes, was reported [8]. The spatial linear structure ofNqphen should improve directionality in the complexes, and atthe same time diminish the probability of excited state throughspace charge recombination.

In this paper the synthesis and characterization of the series ofcationic D–C–A complexes with donor L ligands fac-[Re(CO)3(Nq-phen)(L)]PF6, (with L the donor ligand 4-tert-butylpyridine

N

N N

N

O

O

N

N N

N

O

O

NN

S

Nqphen

Aqphen

py-PTZ

Scheme 1.

Re(CO)5Cl fac- Re(CO)3(Nqphen)Cl

fac-Re(CO)3(Nqphen)TfO fac-[Re(CO)3(Nqphen)(L)]PF6

i ii

iii iv

Scheme 2.

698 R. Díaz et al. / Polyhedron 30 (2011) 697–701

(t-Bupy), 4-methoxypyridine (MeO-py) or 4-phenothiazinepyririne(py-PTZ)), is reported. In the case of L = py-PTZ, the properties ofthe corresponding complex are compared with the above men-tioned fac-[Re(CO)3(Aqphen)(py-PTZ)]PF6 [7] complex.

2. Experimental

All complexes were characterized by conventional spectro-scopic techniques (1H NMR, IR, UV–Vis), elemental analysis andelectrochemistry.

2.1. Measurements

UV–Vis spectra were recorded on a Shimadzu UV 3101PC. 1HNMR spectra were recorded on a Bruker AC/200 MHz spectrometeror a Bruker AVANCE 400 FT-NMR spectrometer with TMS as refer-ence. Infrared spectra were recorded in KBr pellets in a Bruker Vec-tor-22 FTIR spectrometer. Cyclic voltammetry was performed byusing a BAS CV-50W 2.3 MF-9093 equipment. The measurementswere recorded in acetonitrile or 1,2-dichloroethane. Tetrabuthy-lammonium hexafluorophosphate (Aldrich) was used aselectrolyte.

2.2. Materials

The solvents employed for synthesis were p.a. grade, while forelectrochemical measurements spectroscopic grade solvents wereused. All reagents (Aldrich) were p.a. grade and were used withoutfurther purification. The starting material Re(CO)5Cl was kindlyprovided by Dr. Klahn [9]. Nqphen, fac-Re(CO)3(Nqphen)Cl, fac-Re(CO)3(Nqphen)TfO and 10-(4-picolyl)-phenothiazine (py-PTZ)were prepared according to published procedures [8,10]. The li-gands MeO-py and t-Bupy were obtained from Aldrich.

2.3. Synthesis

The complexes were synthesized according to the diagram ofScheme 2, where i, ii, iii and iv represent additional reactantsand experimental conditions: i = Nqphen, THF, N2, reflux 1 h;ii = AgTfO, toluol, N2, 2 h reflux; iii = L = t-Bupy, MeO-py or py-PTZ, MeOH, 1.5 h reflux; iv = NH4PF6, 12 h stirring.

Stages i and ii were done according to reference [8]. The com-plete procedure for stages iii and iv is detailed in what followsfor the complex with L = py-PTZ. The remaining compounds weresynthesized by similar procedures: 100 mg (0.15 mmol) of fac-Re(-CO)3(Nqphen)TfO were dissolved in anhydrous MeOH (20 mL) and50 mg (1.15 equiv.) of py-PTZ were added. The mixture was re-fluxed for 1.5 h under inert atmosphere. The solution was cooledto room temperature, and excess of NH4PF6 was added. The mix-ture was stirred for 12 h. A yellow precipitate was formed. The sol-vent was removed by decantation and the solid purified by columnchromatography (Al2O3; petroleum ether and MeCN/CHCl3). Thesolution was concentrated by rotary evaporation, and the productprecipitated with Et2O, filtered, and dried under high vacuum.

L = MeO-py 108.4 mg, 95.6%. Anal. Calc. for C31H17N5O6R-ePF6�0.15CH3CN: C, 42.11; N, 8.08; H, 1.97. Found: C, 42.35; N,8.04; H, 2.09%. TOF MS FD+: m/z 742.072 [M�PF6

�]+ (C31H17N5O6187Re), 740.069 [M�PF6

�]+ (C31H17N5O6185Re).

L = py-PTZ 100.1 mg, 76.5%. Anal. Calc. for C40H24N6O5SP-F6Re�0.2CHCl3: C, 46.56; N, 8.14; H, 2.34; S, 3.10. Found: C, 45.65;N, 8.09; H, 2.19; S, 3.01%.

L = t-Bupy 107.8 mg, 92.4%. Anal. Calc. for C34H23N5O5P-F6Re�0.15CHCl3: C, 44.74; N, 7.67; H, 2.54. Found: C, 43.58; N,7.99; H, 2.89%.

3. Results and discussion

The characterization of the complexes by elemental analysisgave satisfactory results, with a higher degree of deviation for com-pound with L = t-Bupy. Solvent occlusion was observed in all cases.The nature of the complex with L = MeO-py was additionallychecked by mass spectroscopy, where the presence of the expectedmolecular fragments was observed for the most abundant Re iso-topes, 185Re and 187Re M, with peak intensities in accordance withtheir natural abundance.

3.1. IR and UV–Vis spectra

Table 1 summarizes the IR and UV–Vis data for all the com-plexes. Information for the free Nqphen ligand and neutral com-plexes is also included for comparison reasons.

The IR spectra of the Re complexes were characterized by twovery intense bands at ca 2030 and 1920 cm�1, inherent to a facialCO coordination in Re(I) complexes [11,12]. The wider band at low-er energy, ascribed to the E mode, appears as a doublet in somecases. For all the cationic complexes herein reported, both bandsappear at higher energy than for the neutral fac-Re(CO)3(Nq-phen)Cl precursor (2020 and 1897 cm�1), due to the change insymmetry from Cs (L = Cl�) to pseudo C3v, (L = py) [13]. For theband at ca 2030 cm�1, the shift (ca 11 cm�1) was less than that(ca 24 cm�1) for the band at ca 1920 cm�1, related to the E mode.

Table 1IR and UV–Vis spectroscopic data for the cationic fac-[Re(CO)3(Nqphen)(L)]PF6 series of complexes. Data for the free Nqphen ligand, the neutral precursorcomplexes, and fac-[Re(CO)3(Aqphen)(L)]PF6 are additionally given for comparison.

Complexa IR (cm�1)b kabs (nm) c

m(CO)d m(CO)e IL TCML

Nqphenf – 1686 241, 274, 317fac-Re(CO)3(Nqphen)Cl 2020, 1897, 1871 1694 220, 276, 306 390fac-Re(CO)3(Nqphen)TfO 2040, 1935 1692 254, 273, 305 385fac-[Re(CO)3(Nqphen)(t-Bupy)]+ 2033, 1933, 1920 1695 215, 270, 305 380fac-[Re(CO)3(Nqphen)(MeO-py)]+ 2030, 1931, 1921 1697 215, 270, 304 382fac-[Re(CO)3(Nqphen)(py-PTZ)]+ 2031, 1930, 1921 1697 244 (sh), 267, 302 391fac-[Re(CO)3(Aqphen)(py-PTZ)]+g 2040, 1938, 1923 1678 383 405

a Cationic complexes as salt of PF6�.

b KBr pellet.c In acetonitrile.d Carbonyl ligands.e Quinone.f Free ligand, UV–Vis in CHCl3.g Ref. [7].

R. Díaz et al. / Polyhedron 30 (2011) 697–701 699

The latter reflects more the effect on the Re-CO back-bonding ofsubstituting Cl� by a neutral pyridine type ligand. However, theposition of the v(CO) bands is not noticeably affected by the natureof the donor substituent on py.

In Table 1 data for the fac-[Re(CO)3(Aqphen)(py-PTZ)]PF6 com-plex have also been included. In this complex, the m(CO) signalsfor the carbonyls in the fac position appear at higher energy thanfor fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6. This can be interpreted asa consequence of a reduced Re-CO retrodonation because of thegreater electron acceptor capacity of Aqphen.

As observed for the Aqphen complexes [7], the quinone m(CO)signal of Nqphen appears at higher energies (ca 10 cm�1) in thecomplexes compared to the free ligand, reflecting the influenceof the metal center on the p conjugated framework of Nqphen.On the other hand, no meaningful difference is observed for thisband when comparing the cationic with the neutral complexes.

Finally, the band at 840 cm�1, assigned to m(P–F), indicates thepresence of the PF6

� counter ion in the cationic complexes.The UV–Vis spectra of the complexes are dominated by intense

intraligand bands (IL, p ? p⁄ and n ? p⁄), which hide the TCML(d ? p⁄) lower energy bands. The maxima of the latter whereestimated by difference spectra taking free Nqphen ligand asreference. For example, Fig. 1 shows the UV–Vis spectrum for

200 250 300 350 400 450 500

0.0

0.2

0.4

0.6

0.8

Abs

orba

nce

Wavelength (nm)

Fig. 1. UV-Vis spectrum of fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6 in MeCN.

fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6. The lower energy band of thefree ligand appears at 317 nm and is assigned to the n ? p⁄ transi-tion. In the complexes this band appears displaced to higher energyreflecting again the metal donor effect that raises the energy of thep⁄ level. As for IR, no meaningful difference among the differentcationic and neutral complexes is observed.

3.2. 1H NMR

The 1H NMR data for all complexes are shown in Table 2. TheNqphen proton signals assignment was based on the previously re-ported 1H–13C-Cosy for fac-Re(CO)3(Nqphen)TfO [8]. Due to coordi-nation, the Nqphen pattern is displaced to lower field in thecationic complexes in relation to the free ligand. The same effectis observed when comparing the cationic with the neutral com-plexes. In this case, this is probably an effect of the replacementof an anionic ligand by a neutral ligand, enhancing therefore thepositive charge on the metal center.

Fig. 2 shows the aromatic region of the 1H NMR spectrum of fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6. The proton signals correspondingto the Nqphen and py-PTZ ligands can be clearly observed. The pro-tons signals of the py-PTZ ligand were assigned on the basis of peakintegration, coupling constants and by comparison with similar re-ported complexes [14,15]. As for all the cationic complexes here re-ported, the integrals for the L ligands protons reflect a 1:1 relationof these ligands with respect to Nqphen.

The L ligands, due to their differences in donor capacity, affectin a different way the extent that coordination to Re deshieldsthe Nqphen protons. This effect is mainly observed on the a, b,and c protons, and is more marked for L = MeO-py. A similar effectwas observed in the series fac-[Re(CO)3(ppl)(L)]PF6, (whereppl = pyrazino[2,3-f][1,10]-phenanthroline; L = t-Bupy, MeO-py opy-PTZ) [15], although in this case L = py-PTZ showed the lowestdeshielding effect. The d an e protons of Nqphen are not affected,because of the dominant effect of the adjacent quinone residue.As observed by IR and UV–Vis, the latter group is not noticeably af-fected by the nature of L.

When comparing the Nqphen complexes with the correspond-ing Aqphen complexes, e.g. fac-[Re(CO)3(Nqphen)(L)]0/+ and fac-[Re(CO)3(Aqphen)(L)]0/+, with L = Cl�, TfO�, py-PTZ, a similar pat-tern is observed for protons a, b, and c. In general, protons a andc appear very close, and noticeably more deshielded than protonb. The metal deshielding effect on these protons is similar for bothtype of complexes e. For example, when L = Cl�, the chemical shiftsof protons a, b and c in the Nqphen complex are fairly moredeshielded (ca 0.1 ppm) than in the Aqphen complex.

Table 21H MR chemical shifts (ppm) in CD3CN for the fac-[Re(CO)3(Nqphen)(L)]+/0 complexes (200 or 400 MHz).

Position Nqphena Cl� TfO� t-Bupy Py-PTZ MeO-py

Ha 9.41 (dd) 9.58 (dd) 9.59 (dd) 10.15 (d) 10.10(dd) 9.81 (dd)Hb 7.92 (dd) 8.14 (dd) 8.29 (dd) 8.69 (dd) 8.64 (dd) 8.39 (dd)Hc 9.80 (dd) 10.00 (dd) 9.94 (dd) 10.18 (d) 10.13 (d) 9.96 (dd)Hd 8.54 (dd) 8.58 (dd) 8.47 (dd) 8.55 (dd) 8.54 (dd) 8.51 (dd)He 8.00 (dd) 8.03 (dd) 8.05 (dd) 8.18 (dd) 8.17 (dd) 8.10 (dd)H1 – – – 8.66 (d) 8.70 (d) 8.16 (d)H2 – – – 7.47 (d) 7.46 (d) 6.76 (d)H4 – – – – 7.15 (dd) –H5 – – – – 6.92 (td) –H6 – – – – 6.90 (td) –H7 – – – – 6.66 (dd) –CH3 – – – 1.20 (s) – 3.75 (s)CH2 – – – – 5.21 (s) –

a Free ligand, in CDCl3.

6.6ppm

7.0 7.47.88.28.69.09.49.810.2

10.1 15.3 5.2 5.1 5.1 10.0 5.1

2 4

6 5

7

1 e

c a

db

N

NS

ReCON

NNN

O

O

CO

CO

ab

ce

d1

24

56

7

PF6

Fig. 2. 1H NMR for the aromatic region of fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6, at 200 MHz in CD3CN.

Table 3Electrochemical data of fac-[(Nqphen)Re(CO)3(L)]PF6 complexes in MeCN. Potentialsgiven in V versus SCE; supporting electrolyte 0.1 M Bu4NPF6.

Complexa Ep ox(Re(II/I))

E1/2 red

Nqphenb – �0.65c, �1.23, �1.53d

fac-Re(CO)3(Nqphen)TfO +1.71 �0.80, �1.29fac-[Re(CO)3(Nqphen)(MeO-py)]+ +1.24 �0.43c, �1.00, 1.19fac-[Re(CO)3(Nqphen)(t-Bupy)]+ +1.34 �0.54c, �1.04, �1.34d, �1.46fac-[Re(CO)3(Nqphen)(py-PTZ)]+ +1.11e �0.31c, �0.67c, �0.86, �1.25

a Cationic complexes as salt of PF6�.

b Free ligand [8].c Quinone(0/�1, �1/�2).d Ep red, irreversible wave.e Broad band Re(II/I) and py-PTZ(0/+1).

700 R. Díaz et al. / Polyhedron 30 (2011) 697–701

3.3. Cyclic voltammetry

The results for the cyclic voltammetry measurements are sum-marized in Table 3. Most of the redox processes were reversibleand were mainly assigned by comparison with the free ligand.The presence of the L ligand produces a displacement of the E1/2

reduction, in the order t-Bupy < MeO-py < py-PTZ. In the case of

py-PTZ, the processes centered on the quinone group splits, givingevidence for the presence of two processes, one assigned to the for-mation of the semiquinone and the other to the dianion. WhenL = t-Bupy, the reduction processes on the quinone group appearsas a single broad band, corresponding to both processes.

The oxidation observed is assigned to the ReI/II processes.Clearly, neutral donor L ligands facilitate this oxidation, comparedto anionic ligands, especially in the case of py-PTZ, in accordance toliterature reports for similar complexes with this ligand [15].Moreover, in complex fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6, the py-PTZ oxidation, commonly observed as a reversible process at ca0.9 V in related complexes [7,16] collapses with the ReI/II oxidationin a single broad irreversible peak. The behavior observed probablyreflects a certain degree of electronic delocalization between thepy-PTZ group and the rest of the molecule. The py-PTZ ligand re-reduction is precluded by the delocalization of the electron pro-duced by oxidation on the Nqphen framework. A similar effecthas been reported for a ruthenium complex that incorporates thePTZ and quinone fragments [17]. On the other hand, in the parentfac-[Re(CO)3(Aqphen)(py-PTZ)]PF6 complex, this effect was not ob-served [7]. In this complex the oxidation processes related to py-PTZ and to ReI are well separated and defined, the py-PTZ oxidationbeing reversible. Probably, the geometry of Aqphen (see Scheme 1),avoids this type of delocalization. The difference between the

R. Díaz et al. / Polyhedron 30 (2011) 697–701 701

Aqphen and Nqphen complexes is possibly assigned to theirstructural differences, where the linearity of Nqphen enhancesdelocalization possibilities. A similar conclusion was obtained fromESR and theoretical Studies [18].

Preliminar photophysical studies show the absence of emissionof fac-[Re(CO)3(Nqphen)(py-PTZ)]PF6 at RT. Based on the studiesdone for the fac-[Re(CO)3(Aqphen)(py-PTZ)]PF6 complex, where itwas experimentally demonstrated that the photoexcited MLCTstate is quenched by the formation of a charge separated excitedstate, where the photoexcited electron migrates to the quinoneportion of the ligand [7], the lack of emission for the Nqphen com-plex can be attributed to a similar deactivation process. Moreover,the effect should be enhanced due to the linearity of the Nqphenligand.

4. Conclusions

The synthesis and characterization of a new series of cationicrhenium(I) complexes with the general formula fac-[Re(CO)3(Nq-phen)(L)]PF6, where Nqphen is a quinone acceptor ligand and L isa 4-substituted pyridinic donor ligand, have been described.

The nature of L does not affect significantly the spectroscopicproperties of the complexes. On the contrary, the redox potentials,both for the metal center and for the ligand framework, varydepending on L. This indicates a certain degree of electronicinteraction between L and Nqphen through the metal center.The unexpected irreversibility of the py-PTZ(0/+1) process in thefac-[Re(CO)3(Nqphen)(py-PTZ)]PF6 complex, not observed for thefac-[Re(CO)3(Aqphen)(py-PTZ)]PF6 complex, would be a conse-quence of the greater electron delocalization on Nqphen with re-spect to Aqphen, due to the structural linearity of the former, asdiscussed previously. The described properties, and the lack ofemission at RT, make the Nqphen–Re-complexes interesting forfurther studies on long distance excited state charge separation.

Acknowledgements

Authors acknowledge at Fondecyt for support through grants1020517 and 1070799 and Dirección General de Investigación dela Universidad Católica de Temuco through grant DGIUCT 2006-3-11.

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