Non-innocent ligand reservoirs for reducing or oxidizing equivalents in carbonylrhenium(I) complexes: 1,1 0 -Bis(diphenylphosphino)ferrocene (dppf) and bis-triazinyl-pyridine (BTP) Sayak Roy, Travis Blane, Alyssia Lilio, Clifford P. Kubiak ⇑ Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, 0358, La Jolla, CA 92093-0358, United States article info Article history: Available online 13 February 2011 Dedicated to Prof. Wolfgang Kaim on the occasion of his 60th birthday. Keywords: Re(I)–carbonyl complexes Ferrocene Electrochemistry Spectroelectrochemistry abstract Organometallic complexes of Re(I) with ligands having opposite redox properties have been synthesized and structurally characterized. X-ray crystal structures of the complexes show typical fac-Re I (CO) 3 coor- dination to the redox active ligands. Complete electrochemical and spectroelectrochemical studies on the ligands and the metal complexes were performed. The IR-spectroelectrochemical responses were moni- tored using the fac-Re(CO) 3 unit as a probe. The 15–20 cm 1 hypsochromic or bathochromic shift of the m CO bands upon reduction or oxidation is attributed to ligand-centered processes. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Carbonylrhenium(I) complexes of polypyridyl ligands have been widely investigated due to their applications in homogeneous CO 2 reduction catalysis, light harvesting systems and intermolecu- lar electronic communication [1–3]. Drawing inspiration from Lehn’s catalyst, our lab has recently developed a Re(I)(di-tert- butyl-bipyridine) complex with greatly enhanced electrochemical and photoelectrochemical CO 2 reduction ability [3,4]. The details of the electrocatalytic pathway are still under investigation so new derivatives were synthesized that might offer insight into the catalytic cycle. The ligand 1,1 0 -bis(diphenylphosphino)ferrocene (dppf) has shown widespread utility in organic chemistry, homogeneous catalysis and materials chemistry (Fig. 1, right) [5]. The use of dppf as a co-ligand in stabilizing Cu I with weakly basic quinones and azo-carbonyl is now well established [6]. Bis-triazinyl pyridines are currently being examined as possible ligands for lanthanide and actinide separation in nuclear wastes (Fig. 1, left) [7]. These ligands have widespread potential in coordi- nation chemistry because of the small size of the p system and availability of much lower energy p ⁄ orbitals [7d], but surprisingly few articles have been published on the redox activity of triazinyl pyridines in metal complexes. With this in mind, we carried out a systematic electrochemical and spectroelectrochemical (SEC) investigation of Re(I)carbonyl complexes with ligands possessing strong r-donor abilities like dppf and strong p-accepting abilities like bis-triazinyl pyridines. In this paper, we report four tricarbonylrhenium(I) complexes with bis-(5,6-dimethyl-[1,2,4]triazinyl) pyridines (dtm), bis-(5,6-diphe- nyl-[1,2,4]triazinyl) pyridines (dtb) (Fig. 2) and 1,1 0 -bis(diphenylphos- phino)ferrocene (dppf) (Fig. 2). All complexes were fully characterized and their complete spectroscopic study is reported here. 2. Experimental 2.1. Physical measurements 1 H, 31 P{ 1 H} and 19 F NMR spectra were measured with a 300 MHz Varian spectrometer. UV–Vis–NIR absorption spectra were recorded with a Shimadzu UV-3600 spectrometer. Electro- chemistry was performed on a BAS CV-50W in dried deoxygenated CH 2 Cl 2 or CH 3 CN using 0.1 M Bu 4 NPF 6 (recrystallized from metha- nol and dried under vacuum at 80 °C) as supporting electrolyte. The working electrode was glassy carbon, counter was platinum wire and a Ag/AgCl wire was used as pseudo-reference. The ferro- cene/ferrocenium couple served as an internal reference. Infrared spectra were collected on Bruker Equinox 55 FTIR spectrometer. IR-spectroelectrochemistry was done with a home-built sixth gen- eration cell mounted on a specular reflectance unit. A detailed description of the cell is reported elsewhere [8]. Elemental analysis 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.02.005 ⇑ Corresponding author. E-mail address: [email protected](C.P. Kubiak). Inorganica Chimica Acta 374 (2011) 134–139 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Transcript
Inorganica Chimica Acta 374 (2011) 134–139
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier .com/locate / ica
Non-innocent ligand reservoirs for reducing or oxidizing equivalents incarbonylrhenium(I) complexes: 1,10-Bis(diphenylphosphino)ferrocene (dppf)and bis-triazinyl-pyridine (BTP)
Sayak Roy, Travis Blane, Alyssia Lilio, Clifford P. Kubiak ⇑Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, 0358, La Jolla, CA 92093-0358, United States
a r t i c l e i n f o
Article history:Available online 13 February 2011
Dedicated to Prof. Wolfgang Kaim on theoccasion of his 60th birthday.
Organometallic complexes of Re(I) with ligands having opposite redox properties have been synthesizedand structurally characterized. X-ray crystal structures of the complexes show typical fac-ReI(CO)3 coor-dination to the redox active ligands. Complete electrochemical and spectroelectrochemical studies on theligands and the metal complexes were performed. The IR-spectroelectrochemical responses were moni-tored using the fac-Re(CO)3 unit as a probe. The 15–20 cm�1 hypsochromic or bathochromic shift of themCO bands upon reduction or oxidation is attributed to ligand-centered processes.
� 2011 Elsevier B.V. All rights reserved.
1. Introduction
Carbonylrhenium(I) complexes of polypyridyl ligands havebeen widely investigated due to their applications in homogeneousCO2 reduction catalysis, light harvesting systems and intermolecu-lar electronic communication [1–3]. Drawing inspiration fromLehn’s catalyst, our lab has recently developed a Re(I)(di-tert-butyl-bipyridine) complex with greatly enhanced electrochemicaland photoelectrochemical CO2 reduction ability [3,4]. The detailsof the electrocatalytic pathway are still under investigation sonew derivatives were synthesized that might offer insight intothe catalytic cycle.
The ligand 1,10-bis(diphenylphosphino)ferrocene (dppf) hasshown widespread utility in organic chemistry, homogeneouscatalysis and materials chemistry (Fig. 1, right) [5]. The use of dppfas a co-ligand in stabilizing CuI with weakly basic quinones andazo-carbonyl is now well established [6].
Bis-triazinyl pyridines are currently being examined as possibleligands for lanthanide and actinide separation in nuclear wastes(Fig. 1, left) [7]. These ligands have widespread potential in coordi-nation chemistry because of the small size of the p system andavailability of much lower energy p⁄ orbitals [7d], but surprisinglyfew articles have been published on the redox activity of triazinylpyridines in metal complexes.
ll rights reserved.
With this in mind, we carried out a systematic electrochemicaland spectroelectrochemical (SEC) investigation of Re(I)carbonylcomplexes with ligands possessing strong r-donor abilities likedppf and strong p-accepting abilities like bis-triazinyl pyridines. Inthis paper, we report four tricarbonylrhenium(I) complexes withbis-(5,6-dimethyl-[1,2,4]triazinyl) pyridines (dtm), bis-(5,6-diphe-nyl-[1,2,4]triazinyl) pyridines (dtb) (Fig. 2) and 1,10-bis(diphenylphos-phino)ferrocene (dppf) (Fig. 2). All complexes were fully characterizedand their complete spectroscopic study is reported here.
2. Experimental
2.1. Physical measurements
1H, 31P{1H} and 19F NMR spectra were measured with a300 MHz Varian spectrometer. UV–Vis–NIR absorption spectrawere recorded with a Shimadzu UV-3600 spectrometer. Electro-chemistry was performed on a BAS CV-50W in dried deoxygenatedCH2Cl2 or CH3CN using 0.1 M Bu4NPF6 (recrystallized from metha-nol and dried under vacuum at 80 �C) as supporting electrolyte.The working electrode was glassy carbon, counter was platinumwire and a Ag/AgCl wire was used as pseudo-reference. The ferro-cene/ferrocenium couple served as an internal reference. Infraredspectra were collected on Bruker Equinox 55 FTIR spectrometer.IR-spectroelectrochemistry was done with a home-built sixth gen-eration cell mounted on a specular reflectance unit. A detaileddescription of the cell is reported elsewhere [8]. Elemental analysis
S. Roy et al. / Inorganica Chimica Acta 374 (2011) 134–139 135
was performed by Midwest Micro Lab, LLC, Indianapolis, IN. AThermoFinnigan LCQdeca mass spectrometer with an electrosprayionization source was used for mass spectrometry analysis.
2.2. Materials
The 1,10-bis(diphenylphosphino)ferrocene and Re(CO)5Cl werepurchased from Aldrich Chemical Company and used withoutfurther purification. The ligands dtm, dtb and the complex [Re(d-ppf)(CO)3Cl] were synthesized using reported procedures[7e,9,10]. All solvents were dried and degassed prior to use by puri-fication via passage through alumina under argon.
2.3. Synthesis
2.3.1. Synthesis of [Re(dtm)(CO)3Cl] (1-Cl)Re(CO)5Cl (145 mg, 0.4 mmol) was dissolved in 50 mL of dry
toluene. An equimolar amount of dtm (100 mg, 0.4 mmol) wasadded to the solution and the reaction mixture was stirred and re-fluxed overnight. The orange precipitate was collected andrecrystallized from a mixture of CH2Cl2/hexanes 1:3. Yield:138 mg (66%), C13H12N6ReO3Cl1 (521.93), Anal. Calc. C, 29.91; H,
Fig. 2. Complexes 1-Cl, 2
2.29; N, 16.10. Found: C, 28.90; H, 2.35; N, 16.01%. 1H NMR [CD3CN,d (ppm)]: 2.16 (s, 6H), 2.33 (s, 6H); IR [CH3CN, mco (cm�1)]: 2031,1935, 1909; ESI-MS (m/z): 539.66 [M+NH4]+.
2.3.2. Synthesis of [Re(dtb)(CO)3Cl] (2-Cl)Re(CO)5Cl (145 mg, 0.4 mmol) was dissolved in 50 mL of dry
toluene. An equimolar amount of dtb was added (180 mg,0.180 mmol) to the solution and the reaction mixture was allowedto reflux overnight. The solution was evaporated to yield a red res-idue, which was recrystallized from a mixture of CH2Cl2/hexanes1:3. Yield: 150 mg (48%), C33H20N6ReO3Cl1 (770.21), Anal. Calc. C,51.45; H, 2.59; N, 10.90. Found: C, 51.35; H, 2.65; N, 11.06%. 1HNMR [CD3CN, d (ppm)]: 7.45–7.77 (6 aromatic H); IR [CH3CN, mco
2.3.3. Synthesis of [Re(dppf)(CO)3(OTf)] (3-OTf)[Re(dppf)(CO)3Cl] (31.4 mg, 0.036 mmol) and a small excess of
silver triflate (20 mg, 0.0781 mmol) were stirred at room tempera-ture under a nitrogen atmosphere for 12 h. The precipitate was fil-tered out and the solvent was removed under reduced pressure.The yellow solid was recrystallized from the mixture of CH2Cl2/pentane 1:3. Yield: 327 mg (92%), C38H28Re1Fe1P2F3S1O6 (942.00),Anal. Calc. C, 48.44; H, 2.97. Found: C, 48.31; H, 2.94%. 1H NMR[CD2Cl2, d (ppm)]: 4.44 (s, 4H), 4.50 (s, 2H), 4.56 (s, 2H), 7.43–7.60 (m, 20H); 31P{1H} NMR [CD2Cl2, d (ppm)]: 12.42; 19F NMR[CDCl3, d (ppm)]: �79.5; IR [CH2Cl2, mco (cm�1)]: 1916, 1970,2048; ESI-MS (m/z): 824.87 (M-OTf).
2.3.4. Synthesis of [Re(dppf)(CO)3(CH3CN)](OTf) (4-OTf)[Re(CO)3(dppf)(OTf)] (32 mg, 0.0328 mmol) was dissolved in
dry CH3CN and refluxed in a nitrogen atmosphere for 6 h. The sol-vent was removed under vacuum leaving a yellow solid. The com-pound was recrystallized from CH2Cl2/pentane 1:3. Yield: 309 mg(93%), C40H31N1Re1Fe1P2F3S1O6 (1015.00), Anal. Calc. C, 47.33; H,3.05; N, 1.37. Found: C, 47.26; H, 3.15; N, 1.22%. 1H NMR [CD2Cl2,d (ppm)]: 31P{1H} NMR [CDCl3, d (ppm)]: 9.96; 19F NMR [CDCl3, d(ppm)]; IR [CH2Cl2, mco (cm�1)]: 1946, 1971, 2050; ESI-MS (m/z):865.69 (M-OTf).
-Cl, 3-OTf and 4-OTf.
Table 1Crystal data and structure refinement of 1-Cl and 4-OTf.
136 S. Roy et al. / Inorganica Chimica Acta 374 (2011) 134–139
2.4. Crystallography
Slow diffusion of hexane into the solution of 1-Cl in CH3CN re-sulted in needle and plate shaped crystals. Yellow needles of 4-OTfwere obtained by diffusion of pentane into CH2Cl2 solution at roomtemperature. Single-crystal X-ray structure determinations arecarried out at 100(2) K on either a Bruker P4 or Platform diffrac-tometer using Mo Ka radiation (k = 0.71073 Å) in conjunction witha Bruker APEX detector. All structures were solved by direct meth-ods using SHELXS-97 and refined with full-matrix least-squares pro-cedures using SHELXL-97 [11]. The non-hydrogen atoms were refinedwith anisotropic thermal parameters. The crystal data and
refinement details are presented in Table 1 for all complexes andselected bond lengths and bond angles are tabulated in Table 2.The structure of 4-OTf contains a disordered triflate anion.
3. Results and discussion
3.1. Synthesis and characterization
The complexes 1-Cl and 2-Cl were synthesized by refluxingcommercially available Re(CO)5Cl with the ligands, dtm and dtb,respectively. Three recrystallizations from CH3CN/Et2O 1:3 re-sulted in pure crystalline compounds. A typical three-band (Eand A) pattern was observed in the IR spectrum, confirming afac-[Re(CO)3] group. Stirring of the reported complex [Re(d-ppf)(CO)3Cl] (3-Cl) [10] with AgOTf in CH2Cl2 followed by filtrationof insoluble AgCl, afforded 3-OTf. The complex 4-OTf was synthe-sized by refluxing 3-OTf in CH3CN. The IR spectrum of 3-OTf showsthree well-separated CO bands at 2050, 1970 and 1915 cm�1
where as 4-OTf exhibits relatively coalescenced E bands (1947and 1972 cm�1) attributed to solvent coordination, along with asharp A band (2050 cm�1).
3.2. Crystallography
The IR, NMR and mass spectroscopic results were confirmed bydetermination of X-ray crystal structure of 1-Cl (Fig. 3). Crystallo-graphic data, bond lengths and bond angles are listed in Tables 1and 2. There is no reported structure for the crystal structure ofthe free ligand and we were unable to grow crystals so we couldnot compare the bond lengths in free ligand with the lengths in1-Cl. The structure 1-Cl shows typical facial binding of theRe(CO)3Cl unit by the dtm ligand leading to a neutral coordinationsphere. Significant contraction along the N1–Re–N2 bond angle re-sults in a distortion of the octahedral geometry. The triazinyl ringsare slightly displaced from the coordination plane created by theRe–N1–C6–C7–N2 atoms. The coordination of Re(I) by the 6,60-nitrogens of dtm is sterically hindered relative to the 2,20-nitrogensdue to substituted methyl groups at the 5,50 positions. The crystalstructure shows axial disorder along Cl–Re–CO. A shorter axial CObond length in comparison to equatorial CO indicates better Re(I)to p⁄ back donation in the equatorial direction.
In spite of disorder in the phenyl rings and an unusually highunit cell parameter (Z = 30) for the crystal of 4-OTf (Fig. 3), thecoordination sphere around Re(I) is clear. Re(I) shows distortedoctahedral geometry due to the considerably smaller bite angleof the dppf ligand (ca. 100�). The solvent molecule CH3CN coordi-nates the metal in an axial position. Re(I) to ligand back donationresults in the elongation of the C–O and C–N bond lengths of coor-dinated CO and CH3CN.
3.3. Optical spectra
The UV–Vis spectra of the ligands dtm and dtb, and the com-pounds 1-Cl and 2-Cl were measured in CH3CN. The complexesshow a higher energy band (415 nm in 1-Cl and 460 nm in 2-Cl)due to electronic transition from dp of Re(I) to the p⁄ of the bis-triazyl pyridine ligand. The lower energy bands (240, 270 and330 nm for 1-Cl and 235, 290 and 340 nm for 2-Cl) in the complexesare attributed to intra-ligand (p–p⁄) transitions (Fig. 4). The spectraof the free ligands have the intra-ligand transition bands in thesame region (230–310 nm). The bathochromic shift of these bandsin the complexes is of the result of coordination to Re(I).
The yellow colored complexes 3-OTf and 4-OTf exhibit spectralfeatures very similar to those of 3-Cl. All the complexes show a
Fig. 3. Molecular structures of 1-Cl and 4-OTf with ellipsoids at 50%. The H atoms for both structures and the OTf in 4-OTf are removed for clarity.
Fig. 4. Optical spectra of 1-Cl (black) and 2-Cl (red, inset) in CH3CN. (Forinterpretation of the references in color in this figure legend, the reader is referredto the web version of this article.)
S. Roy et al. / Inorganica Chimica Acta 374 (2011) 134–139 137
weak absorption band at 430–440 nm due to an intra-ligand tran-sition in the coordinating dppf [10,12].
Fig. 5. Electrochemistry of 1-Cl (left) and 3-OTf (right) in 0.1 M Bu4NPF6/CH3CN and 0.
3.4. Electrochemistry
Cyclic voltammetry experiments on the ligands, dtm and dtb,and the complexes 1-Cl and 2-Cl were compared. The electrochem-istry of the free ligands was performed in 0.1 M Bu4NPF6/CH2Cl2
due to poor solubility in CH3CN. As a result of the electron donatingeffects of the methyl substituents in dtm, the ligand is reduced at amore negative potential than dtb (supporting information, S1).Complexes of 1-Cl and 2-Cl exhibit almost identical electrochemi-cal features with one reversible reduction leading to occupancy inthe p⁄ orbitals of ligands. Ligand coordination to the metal causesthe ligand-centered reductions to shift anodically due to chargetransfer from ligand nitrogens to the metal. The first reduction isfollowed by a quasi-reversible and an irreversible step. This kindof behavior is generally due to chloride loss and reduction of chlo-ride-free Re species [13]. The complexes each show a comparablethird reduction step, confirmed by SEC as ligand centered process(Fig. 5, left and supporting information S2).
The electrochemical responses of the triflate complexes 3-OTfand 4-OTf are comparable in CH2Cl2. Both complexes have the re-dox active ferrocenyl phosphine as coligand, which shows a revers-ible oxidation. Oxidation of 4-OTf occurs at a higher positivepotential compared to 3-Cl and 3-OTf because of overall accumula-tion of positive charge (supporting information S4) [13]. The
1 M Bu4NPF6/CH2Cl2, respectively (At RT, 100 and 50 mV/s scan rate, respectively).
Table 3Oxidation and reduction potentials from cyclic voltammetry.a
a In 0.1 M Bu4NPF6/CH3CN at RT.b In 0.1 M Bu4NPF6/CH2Cl2.c In 0.1 M Bu4NClO4/CH2Cl2 at RT, Ag/Ag+ as reference.d Half wave potential corresponding to a reversible wave.e Cathodic peak potential corresponding to an irreversible wave.
Fig. 7. IR spectroelectrochemical oxidation of 3-OTf to (3-OTf)+ in 0.1 M Bu4NPF6/CH2Cl2.
a 0.1 M Bu4NPF6 at RT.b In CH3CN.c In CH2Cl2.d 0.1 M Bu4NClO4.
138 S. Roy et al. / Inorganica Chimica Acta 374 (2011) 134–139
irreversible reduction step of 4-OTf at very negative potential isattributed to a Re(I) centered reduction (Table 3 and Fig. 5, right).
3.5. IR-spectroelectrochemistry
Additional information on the effects and location of electronswas obtained from IR-spectroelectrochemical (IR-SEC) studies.The IR-SEC measurements on compounds l-Cl and 2-Cl were per-formed in CH3CN at RT. Both l-Cl and 2-Cl show typical three bandspectrum due to the fac-Re(CO)3Cl moiety. Upon the first one elec-tron reversible reduction, a shift in the bands of about 20 cm�1 tolower energy is observed (Fig. 6, left). The shift of the bands is ex-pected due to electron uptake into the p⁄ level of bis-triazyl pyri-dine ligands, which in turn increases metal to CO back donation(metal dp to p⁄ of CO). As the more negative potentials are applied,a further shift to lower energy is observed. The amount of the shiftis much less than the shift in the first reduction. This feature is ex-plained by chloride loss and formation of the neutral solvated com-plex [Re(L-L)(CO)3(CH3CN)]. There is also the appearance of a smallpeak, which is tentatively assigned to the formation of a very smallquantity of dinuclear species, common among bipyridine com-plexes of Re(CO)3 (see supporting information, S3). The input ofthe 2nd electron into the compounds results in a considerable blueshift in the mCO stretch. Upon applying more voltage, the com-pounds show signs of decomposition (Fig. 6, right).
The compounds 3-OTf and 4-OTf show identical spectroelectro-chemical behavior in 0.1 M Bu4NPF6/CH2Cl2. Because of the ferro-cene backbone, dppf ligand can be oxidized reversibly. Uponoxidation, a uniform blue shift of the mCO bands reflects decreasedback donation from Re(I) to the CO p⁄ orbitals (Fig. 7, Table 4, andsupporting information S5).
Fig. 6. IR spectroelectrochemical reduction of 2-Cl to (2-Cl
4. Summary
The present work describes the synthesis and complete charac-terization of the set of complexes, 1-Cl, 2-Cl, 3-OTf and 4-OTf, bear-ing diverse redox active ligands. The solution behavior of thecomplexes was monitored by NMR, IR and UV–Vis spectroscopy,
)�� (left) and 2� to 2� (right) in 0.1 M Bu4NPF6/CH3CN.
S. Roy et al. / Inorganica Chimica Acta 374 (2011) 134–139 139
and redox properties were investigated by IR-spectroelectrochem-istry. In spite of disorder in the X-ray crystal structure for 1-Cl, thefacial binding of the Re(CO)3Cl unit to the ligand is clear. The struc-ture of complex 4-OTf exhibits direct coordination of the acetoni-trile solvent molecule to the Re(I) unit.
Acknowledgements
The authors thank Starla D. Glover, and John C. Goeltz for theirinsightful discussions. Jim Golen and Curtis Moore are greatlyappreciated for their help and guidance with crystallography.Authors also gratefully acknowledge Helios Solar Energy ResearchCenter, which is supported by the Director, Office of Science, Officeof Basic Energy Sciences of the US Department of Energy, under thecontract no. DE-AC02-05CH11231.
Appendix A. Supplementary material
CCDC 798826 and 798827 contain the supplementary crystallo-graphic data for 1-Cl and 4-OTf, respectively. These data can be ob-tained free of charge from The Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif. Supplementarydata associated with this article can be found, in the online version,at doi:10.1016/j.ica.2011.02.005.
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