Re(CO)3 Metallopolymers with Complete Metal Monomer

Article pubs.acs.org/Macromolecules

Re(CO)3 Metallopolymers with Complete Metal Monomer Incorporation: Synthetic, Spectroscopic, Electrochemical, and Computational Studies Abed Hasheminasab,† Mahesh B. Dawadi,† Hamideh Shokouhi Mehr,† Richard S. Herrick,‡ and Christopher J. Ziegler*,† †

Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601, United States Department of Chemistry, College of the Holy Cross, Box C, Worcester, Massachusetts 01610- 2395, United States

‡

S Supporting Information *

ABSTRACT: A series of main chain organometallic polymers (MCOPs) containing Re(CO) 3 Cl(diimine) cores were synthesized. Three different types of polymerization reactions, including Yamamoto coupling, Heck coupling, and a new metal-mediated Schiff base formation/condensation reaction, allowed for the formation of metal polymers with 100% metal complex incorporation and average molecular weights (Mn) ranging from 25 to 850 kDa. Absorption spectroscopy, electrochemistry, and microscopy studies revealed that the type of polymerization effectively governs the physical and chemical properties of the target polymers. The electronic structures of monomeric and dimeric building units of these polymers backbones were probed by DFT and TDDFT computational methods. DFT calculations and electrochemical studies indicate that in all polymers the oxidation and reduction processes take place on Re(I) metal centers and polymer backbones, respectively. Incorporation of metal as an electron donor (D) and organic conjugated backbone as an electron acceptor (A) provides the D−A architecture.

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used in light harvesting applications.11,12 Strongly conjugated systems exhibit narrower optical band gaps (Egopt), resulting in broader absorption spectra in both the red and near-infrared (NIR) range, which covers most the visible spectrum and maximizes photon collection. When transition metals are incorporated into type II π-conjugated polymers, the photophysical properties can change significantly. Polymers modified with organometallic complexes exhibit absorption bands that are extended to lower energies and have increase extinction coefficients. The incorporation of metal complexes also provides opportunities to tune the photochemical and electrochemical properties which can also be suitable for photovoltaic applications. Ru(II), Os(II), Pt(II), and Ir(III) based metallopolymers are actively being investigated due to their rich and tunable coordination chemistry and electrochemical properties.13−17 Re(L)(CO)3(diimine) (L = monodendate ligand) organometallic complexes and their analogues have received much attention due to their unusual properties and facile synthesis. These compounds with octahedral coordination geometries and low-spin d6 electronic configurations have been used in

INTRODUCTION Metal-containing polymers have received an immense amount of attention by materials scientists since the first reports of πconjugated vinylferrocene polymers.1 The incorporation of organometallic complexes into polymer backbones provides for the combination of the chemical, physical, electronic, magnetic, and optical properties of both the organic and organometallic moieties in these hybrid materials.2,3 With regard to the location of metal complexes in the polymer itself, organometallic polymers can be categorized into two broad types: side group organometallic polymers (SGOPs) in which metal complex is present as a pendant group to the polymer chain (type I) and main chain organometallic polymers (MCOPs) with a directly embedded metal-complex core in the polymer backbone (type II, both types shown in Figure 1).4 In both types, the selection of monomers, the identity of the metal complexes, the degree of metal complex loading, and the respective amounts of conjugated and nonconjugated segments all help define the final desired physical and chemical characteristics of the polymer. Potential applications for these materials include photoswitches, sensors, nonlinear optics, and light-emitting diodes.5−10 There has also been considerable interest in the development of π-conjugated polymers. Polymers with conjugated π-electron systems can exhibit novel photophysical properties and can be © XXXX American Chemical Society

Received: February 16, 2016 Revised: April 4, 2016

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DOI: 10.1021/acs.macromol.6b00343 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Side group and main chain organometallic polymers (A) and the polymers presented in the current study (B).

to maximize its solubility. We have fully characterized our monomer compounds, including via X-ray structure elucidation and cyclic voltammetry. The resultant polymers in the numberaverage molecular weights (Mn) from 25 to 850 kDa, and they exhibit bathochromically shifted UV−vis absorption bands. Since they are soluble, we can spin-coat all of the polymers and have characterized them as films via scanning electron microscopy (SEM) and atomic force microscopy (AFM). Finally, we have carried out DFT/TDDFT calculations to help elucidate the electronic structures of our polymer systems.

many applications including as emission sensitizers, photosensitizers, photocatalysts, and electrocatalysts.18−24 The photophysical and electrochemical properties of Re(L)(CO)3(diimine) compounds are finely tunable by changing either the diimine or monodendate ligands. These complexes generally show two types of absorption bands: ligand centered (LC) π−π* transitions and d−π* metal-to-ligand charge transfer (MLCT) transitions. The MLCT generated excited states can often have slow charge recombination rates and thus long-lived lifetimes. The most studied Re(L)(CO)3(diimine) systems are those where the diimine is either bipyridine or phenanthroline.25−28 Because of their promising photophysical properties as well as their excellent thermal and chemical stabilities, ReI(CO)3 complexes have been incorporated into metal-containing polymers and macromolecules as emitters in light-emitting diodes (LEDs), photosensitizers in photovoltaic devices (PVs), and luminescent probes in biological systems.29−37 For polymeric compounds, Re(L)(CO)3(diimine) systems can be used in either type I or II metallopolymers. However, in many cases, it can be challenging to control either the molecular weight of these polymers or the percentage of metal momomer incorporation. In this report, we present several strategies for generating Re(L)(CO)3(diimine) polymers with 100% metal complex incorporation. The structures of the component monometallic monomers, dimetallic monomers, and the resultant polymers are shown in Figure 1. We can make soluble, large molecular weight Re(L)(CO)3(diimine) macromolecules using a variety of polymerization reactions, including Yamamoto coupling, Heck coupling, and a new metal-mediated Schiff base formation/condensation reaction. For the Yamamoto and Heck reactions, the dimetallic monomers are produced via Schiff base condensation reaction starting with readily available reagents, and we have functionalized the linker

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EXPERIMENTAL SECTION

Materials and Methods. All reagents were purchased from Strem, Acros Organics, TCI AMERICA, or Sigma-Aldrich and used as received without further purification. All solvents were purified by alumina and copper columns in the Pure Solve solvent system (Innovative Technologies, Inc.) and were stored over molecular sieves. Syntheses were performed under a nitrogen atmosphere with a Schlenk line apparatus attached to a predrying column to minimize exposure to air and water. NMR spectra were recorded on Varian Mercury 300 MHz instrument. Chemical shifts were reported with respect to residual solvent peaks as internal standard (1H: CDCl3, δ = 7.26 ppm; 13C: CDCl3, δ = 77.2 ppm). Infrared spectra were collected on Thermo Scientific Nicolet iS5 instrument which was equipped with iD5 ATR. Electronic absorption spectra were recorded on Hitachi U2000 UV−vis spectrophotometer. Elemental analyses were performed by Atlantic Microlab (Norcross, GA). Mass spectrometric analyses were carried out at the Mass Spectrometry center at The University of Akron (Akron, OH). Gel permeation chromatography (GPC) measurements were performed at the Applied Polymer Research Center, Goodyear Polymer Center (Akron, OH). The GPC measurements were obtained by using a Waters GPC II liquid chromatograph with a Model 590 pump and a R401 differential refractometer using Styragel HR2 and HR4 columns. Poly(methyl methacrylate) (PMMA) samples were used as standards to determine molecular weight. DMF was used as the solvent with a flow rate of 0.7 B


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J = 6.0 Hz, 4H, O−CH2), 3.78 (s, 4H, NH2), 1.81 (m, 4H, alkyl chain), 1.47 (m, 4H, alkyl chain), 1.30 (m, 16H, alkyl chain), 0.85 (d, 6H, CH3). 13C NMR (75 MHz CDCl3, δ (ppm)): 147.2, 135.2, 132.8, 119.3, 115.3, 110.5, 68.6, 32.0, 29.7, 29.6, 29.4, 26.4, 14.3 ppm. CHN Anal. Calcd for C28H44O2N2: C, 76.31; H, 10.06; N, 6.35. Found: C, 75.43; H, 9.87; N, 6.98. IR (stretch, cm−1): 3481, 3372 (−NH2) and 1240 (C−O−C alkyl aryl ether). MS (ESI): m/z = calcd for C28H44O2N2: 440.3; found [M + H]+ = 441.5 and [M + Na]+ = 463.5. Synthesis of 4 and 5. The procedure for the synthesis of 4 is representative for both compounds. 4: Re(CO)5Cl (0.050 g, 0.141 mmol), pyridine-2-carboxaldehyde (0.0151 g, 0.0141 mmol), and 3 (0.031 g, 0.0705 mmol) were refluxed in 25 mL of THF for 12 h. After cooling to room temperature, the solvent volume was reduced to 2 mL, and then the red solid was collected by filtration and washed with 50:50 (volume) hexane/diethyl ether solution. Yield: 104 mg (60%). 1H NMR (300 MHz, CDCl3, δ (ppm)): 9.40 (d, 2H, NCH), 9.14 (d, J = 3.0 Hz, 2H, H on py), 8.36 (d, J = 3.0 Hz, 2H, H on py), 8.08 (m, 4H, H on py), 7.81 (m, 2H, H on Ar−H), 7.74 (m, 2H, Ar−H), 7.39 (m, 2H, Ar−H), 4.37 (m, 4H, O−CH2−), 1.95 (m, 4H, alkyl chain), 1.50−1.30 (m, 20H, alkyl chain), 0.84 (s, 6H, CH3). 13C NMR (75 MHz CDCl3, δ (ppm)): 197.6, 196.2, 186.9, 169.2, 155.6, 153.5, 149.8, 142.1, 139.5, 139.3, 129.1, 128.9, 125.9, 119.9, 112.5, 70.1, 31.9, 29.5, 29.4, 29.2, 26.3, 22.8, 14.3 ppm. CHN Anal. Calcd for Re2C46H50O8N4Cl2: C, 44.91; H, 4.09; N, 4.55. Found: C, 37.08; H, 3.59; N, 3.50. IR (CO stretch, cm−1): 2016, 1880. MS (ESI): m/z = calcd for Re2C46H50O8N4Cl2Na: 1253.2; found 1253.3. UV−vis spectrum in DMF λmax 283 nm (ε = 27.0 × 103 M−1 cm−1) and λmax 402 nm (ε = 13.0 × 103 M−1 cm−1). 5: Crystals suitable for X-ray diffraction appeared after a month in DMSO. Yield: 127 mg (65%). 1H NMR (300 MHz, CDCl3, δ (ppm)): 9.14 (s, 2H, NCH), 8.95 (d, J = 3.0 Hz, 2H, H on py), 8.09 (m, 2H, H on py), 7.98 (m, 2H, H on py), 7.62 (m, 2H, Ar−H), 7.27−7.12 (two set of m, 4H, Ar−H), 4.19 (m, 4H, O−CH2), 1.80 (m, 4H, alkyl chain), 1.54 (s, 4H, alkyl chain), 1.4−1.26 (m, 16H, alkyl chain), 0.85 (s, 6H, CH3). 13C NMR (75 MHz, CDCl3, δ (ppm)): 197.9, 195.7, 185.3, 168.4, 154.4, 154.0, 149.7, 142.3, 141.8, 139.3, 129.7, 125.8, 120.0, 112.6, 70.0, 31.9, 29.47, 29.39, 29.24, 26.27, 22.83, 14.28. CHN Anal. Calcd for Re2C46H48O8N4Cl2Br2: C, 39.80; H, 3.48; N, 4.03. Found: C, 39.75; H, 3.33; N, 4.09. IR (CO stretch, cm−1): 2017, 1899. MS (ESI): m/z = calcd for Re2C46H48O8N4Cl2Br2Na: 1411.0; found 1411.1. Synthesis of P1. Ni(COD)2 (0.05 g, 0.172 mmol), 1,5-cyclooctadiene (0.019 g, 0.172 mmol), and bpy (0.027 g, 0.172 mmol) were dissolved in 2−4 mL of DMF under a nitrogen atmosphere. When the color turned dark pink, compound 5 (0.16 g, 0.114 mmol) was added. The mixture was stirred for 30 min at room temperature and then heated to 100 °C for 24 h under nitrogen. The mixture was cooled to room temperature, and 5−10 mL of EtOH containing 7−8 mg of NaCl and 2−3 drops of acetic acid was added to remove the nickel catalyst. The mixture was aerated for 12 h. A greenish solution was decanted. The dark red precipitate was removed and washed numerous times with cold EtOH/H2O, EtOH, chloroform, and acetone. The solid was washed with ethanol several times followed by gradual fractional precipitation. The final residue was dissolved in DMF for further characterization. Yield: 100 mg (60%). GPC (DMF, poly(methyl methacrylate) Mn = 850 kDa, Mw = 2865 kDa (PDI = 3.4). IR (CO stretch, cm−1): 2017, 1888. Synthesis of P2. Divinylbenzene (0.011 g, 0.079 mmol), compound 5 (0.10 g, 0.079 mmol), palladium(II) acetate (1.4 mg, 7.8 mol %), trio-tolylphosphine (19 mg, 0.78 equiv), and tri-n-butylamine (0.11 mL) were added to 5 mL of DMF. The mixture was purged by nitrogen and heated up to 100 °C for 24 h under nitrogen. Then, the solvent was almost completely removed, and the concentrated mixture was poured into methanol. The dark red precipitate was removed, and it was washed several times with cold MeOH, chloroform, and acetone. The final residue was dissolved in DMF for further characterization. Yield: 59 mg (52%), GPC (DMF, poly(methyl methacrylate) Mn = 646 kDa, Mw = 3985 kDa (PDI = 6.2). IR (CO stretch, cm−1): 2016, 1883. Synthesis of P3. Compound 3 (0.062 g, 0.141 mmol) was mixed with glyoxal (0.0151 g, 0. 141 mmol) for 30 min. Re(CO)5Cl (0.050 g,

mL/min. Samples were prepared by dissolving 15−20 mg in 5 mL of DMF. X-ray Data Collection and Structure Determination. X-ray intensity data for all compounds were collected on a CCD-based diffractometer with dual Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.541 78 Å). Crystals were mounted on a cryoloop using Paratone oil and placed under a steam of nitrogen at 100 K. The detector was placed at a distance of 5.009 cm from the crystal. Cyclic Voltammetry. Cyclic voltammograms were obtained by BAS 100B electrochemical analyzer from Bioanalytical Systems equipped with a standard three-electrode cell. 0.79 mm2 gold working electrode was polished first with 3 μm fine diamond and then 0.05 μm alumina. The electrode was rinsed with ethanol and deionized water after each polishing and wiped with a KimWipe. The platinum wire was used as an auxiliary electrode. Samples were prepared in dry DMF in the presence of 0.1 M TBAPF6 as a supporting electrolyte. All approximated formal potentials and reversible redox couples [E°′ = (Epc + Epa)/2], ΔEp = Epa − Epc, were reported against Ag/AgCl reference electrode at 298 K, and currents were calculated by use of an extrapolated baseline current. Formal potentials have shown independency of different scan rate. At a 0.10 V s−1 scan rate, the Fc/Fc+ occurs at 0.46 ± 0.005 V (ΔEp = 80 mV; ipa/ipc = 0.98). SEM and AFM. Scanning electron microscopy (SEM) measurements were performed using a JEOL JSM-1230 microscope wtih an accelerating voltage of 0.5−35 kV. Atomic force microscopy (AFM) was performed using a Veeco Instruments Multimode AFM device. Samples were prepared by spin-coating of polymeric solutions on silicon wafers. Silicon wafers were cleaned according the procedure was reported by Jacobs et al.38 Peroxymonosulfuric acid solutions consisting of 50% H2SO4 and 50% H2O2 was used to oxidize and remove organic residues on the surface. Wafers were rinsed and soaked with hot DI water and then dried with a jet of N2 before coating. DFT and TDDFT Calculations. The starting geometries of compounds 6, 7,39 and 4 were taken from X-ray structures. All compounds were optimized using the B3LYP exchange-correlation functional40,41 coupled with the relativistic DZP basis set for the Re atoms42 and the 6-311G(d)43 basis set for the remaining atoms. Energy minima in optimized geometries were confirmed by frequency calculations. DMF was used as a solvent in all of the single point DFTPCM and TDDFT-PCM calculations; solvent effects were calculated using the polarized continuum model (PCM).44 All DFT calculations were conducted using the Gaussian 09 software package.45 Synthetic Procedures. Compound 1. The synthesis of compound 1 was previously reported.46 Compound 2. 5.0 g (10 mmol) of 1, 3.5 g (25 mmol) of potassium carbonate, and 4.8 g (25 mmol) of 1-bromooctane were mixed, degassed, and refluxed in DMF for 24 h. The mixture was filtered, solvent was removed, and the remaining solid was washed with cold methanol several times. The product was recrystallized in ethanol and washed with diethyl ether. Yield: 3.15 g (45%). 1H NMR (300 MHz, CDCl3, δ (ppm)): 7.95 (m, 4H, Ar−H), 7.76 (m, 4H, Ar−H) 7.33 (d, 2H, J = 9.0 Hz, Ar−H), 7.18 (d, J = 6.0 Hz, 4H, Ar−H), 4.03 (t, J = 6.0 Hz, 4H, O−CH2), 1.63 (t, 4H, J = 9.0 Hz, alkyl chain), 1.20 (m, 4H, alkyl chain), 1.16 (m, 16H, alkyl chain), 0.85 (d, 6H, J = 6.0 Hz, CH3). 13 C NMR (75 MHz CDCl3, δ (ppm)): 167.6, 155.2, 143.8, 134.3, 132.5, 130.3, 123.8, 120.3, 120.0, 112.6, 69.0, 31.8, 29.4, 29.3, 29.1, 26.0, 22.8, 14.24 ppm. IR (stretch, cm−1): 1710 (CO imide), 1377 (C−N−C imide), and 1230 (C−O−C alkyl aryl ether). MS (ESI): m/ z = calcd for C44H48O6N2: 700.38; found [M + Na]+ = 723.3. Compound 3. 5.0 g (7.0 mmol) of 2 was dissolved in acetonitrile and heated to 80 °C, 9.0 g (180 mmol) of hydrazine monohydrate was added dropwise, and this mixture was stirred for 2 h. The resultant mixture was then filtered. The solvent was evaporated from the filtrate, and the resultant solid was washed with cold chloroform several times. Further purification was done by column chromatography with basic alumina and 5% methanol/dichloromethane solution. The crystalline product was collected by slow evaporation of solvent. Crystals suitable for X-ray diffraction were prepared by slow vapor diffusion of hexane into chloroform. Yield: 1.50 g (50%). 1H NMR (300 MHz, CDCl3, δ (ppm)): 6.94 (m, 4H, Ar−H), 6.72 (d, J = 6.0 Hz, 2H, Ar−H), 4.03 (t, C


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Macromolecules Scheme 1. Synthesis of Compounds 1−5 and Polymers P1−P3

corresponding monometallic complexes.51 Because of the presence of MLCT bands, irradiation of Re(CO)3 diimine complexes can induce electron transfer and charge separation, where an electron is injected from a metal-based orbital onto the diimine ligand. With regard to their electrochemistry, DFT and TDDFT calculations revealed that reduction occurs on a ligand-based orbital rather than on the metal. On the basis of our prior work, we were able to successfully synthesize new Re(I) dimetallic complexes (4 and 5) as potential monomers for polymerization reactions. These compounds were made via metal-mediated Schiff base one pot syntheses and were produced in high yield by using the bridge precursor 3 (Scheme 1). All of the resultant compounds are stable molecules, and we have characterized them completely. For the spacer, we used a benzidine-3,3′-diol modified with octyl chains (compound 3). Diamine 3 was synthesized via a threestep reaction depicted in Scheme 1. The synthetic route was started by protecting the diamine groups by converting them to the phthalimides under basic conditions.52 Subsequently, octyl groups were incorporated into the spacer via Williamson synthesis by using 1-bromooctane as an alkylating reagent. The etherified product was obtained as a pure material via crystallization in ethanol. Then, deprotection of the amines was carried out by adding excess hydrazine hydrate in acetonitrile solution. Finally, purification by basic column chromatography provides the desired modified diamine reagent 3 in high purity. All reaction steps including protection, etherification, and deprotection were monitored by IR spectroscopy for detection of imide, ether, and primary diamine signals as illustrated in Figure S1. The 1H NMR spectrum in chloroform of 3 reveals the two terminal amine (−NH2) groups. For the one pot syntheses of 4 and 5, mixtures of compound 3, Re(CO)5Cl, and either pyridine-2-carboxaldehyde or 5bromo-2-pyridinecarboxaldehyde were refluxed in THF solution. Gradually, a dark orange color appeared in the reaction solutions, which is a sign of metal-mediated Schiff base formation. The reaction was continued for 12 h, and upon reduction of the solvent volume, orange solids precipitated out

0.141 mmol) was dissolved in hot DMF. The solution was kept warm at 40 °C while adding dropwise to the glyoxal solution via an addition funnel. The mixture was then heated to 100 °C for 24 h. A dark redpink solid separated from the deep pink solution, as expected for a high molecular weight fraction. The dark-pink solution was cooled down to room temperature. After several minutes, some solid began to coagulate and was collected immediately and washed several times with cold hexane, chloroform, and acetone. The product was dissolved in Pro-analysis DMF for GPC analysis. Yield: 32 mg (30%, selected fraction). GPC (DMF, poly(methyl methacrylate) Mn = 24 kDa, Mw = 104 kDa (PDI ∼ 4.2). IR (CO stretch, cm−1): 2018, 1895. GPC test of the remaining pink solution shows a highly polydisperse mixture which we were not able to fractionize.

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RESULTS AND DISCUSSION Syntheses of Monomers and Polymers. In order to design conjugated type II Re(I) metallopolymers that are potentially useful for LEDs and PV fabrication, a series of palladium-mediated termolecular polymerization reactions have been previously reported.37,47−49 The use of these methods requires exact control of feeding ratios in order to manage the degree of conjugation, metal complex loading, and solubility. However, in these termolecular palladium-mediated reactions, simultaneous control of the metal complex loading in the polymer backbone and polymer chain lengths can be challenging. Charge carrier mobility is directly related to the degree of metal complex content in these polymers, so the degree of metal complex loading can be an important factor in the design of PVs for optimizing and controlling their efficiencies.50 Previously, we reported the high yield synthesis of some monometallic and dimetallic Re (I) complexes via one pot methods, followed by investigations into their photophysical and electrochemical properties.39 Compared to analogously structured monomeric compounds, these dimetallic Re(I) complexes exhibit higher extinction coefficients in their LC and MLCT absorption bands. The bridging ligands as spacers have significant effects on the dimetallic photophysical, photochemical, redox potentials, electron transfer, and intervalence charge transfer (IVCT) properties relative to the D


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relative to a typical imine bond, due to the metal to ligand backbonding. Also, the structure of 5 exhibits a small angle of ∼24° between the two benzene rings and a slight elongation of the phenyl−phenyl C−C bond (∼1.52 Å). The elongation of the bond can be ascribed to the disruption of the π overlap between the two rings upon adopting a nonplanar orientation. Some disorder is observed in one of the peripheral alkyl chains in the structure of 5, which is not unexpected. The IR spectra compounds 4 and 5 exhibit diagnostic metal carbonyl stretching bands that correspond to the pseudo-C3v, a1, and e modes, as expected for the facial Re(CO)3 unit and in agreement with the observed crystal structure. Using compounds 4 and 5, we were also able to synthesize type II conjugated metallopolymers (P1 and P2) with 100% enrichment of the polymer backbone with Re(CO)3 diimines. Since the solubilizer spacer bis(octyloxy)-1,1′- biphenyl is embedded within each monomer (Scheme 1), both homo- and heteropolymerization reactions can be accomplished readily. Polymerization of 5 results in P1, and heteropolymerization of 5 with divinylbenzene produces polymer P2. We were able to characterize the polymers via gel permeation chromatography (GPC) using poly(methyl methacrylate) (PMMA) as a standard to determine the molecular weight and dispersity. P1 was synthesized in DMF by homopolymerization via direct Yamamoto coupling of complex 5 by using Ni(cod)2, a reductive coupling agent. Cyclooctadiene and bipyridine were used as neutral ligands for nickel according to the procedure reported by Yamamoto et al.53,54 A dehalogenation polycondensation reaction using the zero as valent nickel complex under a nitrogen atmosphere over 24 h allowed the C−C bond formation between Re(I) dimetallic complexes via an aryl−aryl cross-coupling reaction. We slightly modified the work-up and purification of the resultant polymer to make it compatible with its physical and chemical characteristics.55 Upon completion of the reaction, the resultant mixture was poured into a cold ethanolic solution containing acetic acid and sodium chloride (to remove the nickel catalyst), and a dark red solid began to precipitate. The ethanolic mixture was aerated under stirring in order to accelerate the conversion of Ni(0) to Ni(II), which is soluble in ethanol. After the solid fraction settled down, the greenish supernatant solution was removed. The remaining solid was washed several times with cold EtOH/H2O, absolute EtOH, chloroform, and acetone in order to remove unreacted and low molecular weight species via gradual fractional precipitation. We observed that after drying the solid fraction was no longer soluble. To maintain solubility of our desired polymer, we decided to redissolve the product in DMF while still wet (after the primary solvent wash) for subsequent analysis. When still wet with solvent, this polymer remains readily soluble in DMF. For the P1 system, we observed Mn up to 850 kDa. Polymer P2 was synthesized by a palladiumcatalyzed Heck coupling reaction using a modified reaction presented previously by Chan et al.48,56 and Schanze et al.57 This hetropolymerization was carried out by reacting the dimeric metal complex (5) directly with 1,4-divinylbenzene as the second spacer (Scheme 1). As in the synthesis of P1, the presence of bis(octyloxy)-1,1′ biphenyl as an solubilizing internal spacer in complex 5 not only provides good solubility for the monomers but also provides facile bimolecular reaction and coupling efficiency. Our bimolecular method achieves a high molecular weight polymer by minimizing undesired side reactions and polymerizations. The 1,4-divinylbenzene unit is

that were isolated as pure products. Because of the presence of the octyl chains on the spacer, these dimetallic complexes exhibited high solubility in chloroform, THF, and acetonitrile, in marked contrast to similarly structured dimetallic Re(I) complexes without aliphatic chains. However, these new dimetallic compounds are only slightly soluble in low molecular weight alcohols such as methanol and ethanol. In the 1H NMR spectra of these complexes, the diagnostic CH resonance of the imine (NCH) around 9.1 ppm confirms the presence of the desired compounds 4 and 5. As we have noted in previous reports, there is some isomerization in these dimetallic compounds, since the halides on Re(CO)3 centers can be placed on the same side or on opposite sides of the diimine ligand. This is observed in the imine CH signal in the NMR spectra, which exhibit two closely spaced imine proton peaks.39 We were able to grow single crystals of precursor 3 as well as rhenium dimetallic compound 5 and elucidated their structures by single crystal X-ray diffraction. The X-ray data collection and structure parameters are listed in Table 1, and Figure 2 shows Table 1. X-ray Crystal Data and Structure Parameters for Compounds 3 and 5 compound

3

5

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dc (mg/m3) μ (mm−1) F(000) reflns collected indep reflns GOF on F2 R1 (on Fo2, I > 2σ(I)) wR2 (on Fo2, I > 2σ(I)) R1 (all data) wR2 (all data)

C28H44N2O2 440.65 triclinic P-1 5.7826(7) 14.5704(17) 15.6201(18) 80.241(6) 86.825(6) 82.751(6) 1285.9(3) 2 1.138 0.071 484 17365 4546 1.057 0.0438 0.1131 0.0550 0.1215

C46H48Br2Cl2N4O8Re2 1388.00 triclinic P-1 8.7144(4) 14.4078(6) 20.3607(8) 98.579(2) 94.190(2) 92.762(2) 2516.51(18) 2 1.832 12.539 1340 28533 7762 1.093 0.0445 0.1184 0.0467 0.1235

the structures of both 3 and 5. Elucidation confirmed the expected structure for the modified benzidine-3,3′-bis(octyloxy) 3, and the structure reveals that the modified benzene rings are coplanar. The C−C bond length between the phenyl rings measures ∼1.49 Å, and the C−N amine bond length is observed to be ∼1.39 Å, which is slightly shorter than a typical C−N σ bond (∼1.47 Å) and implies some conjugation between the amine groups and the phenyl ring. Single crystals of compound 5 grew from DMSO solution over the course of a month. Unfortunately, all efforts at growing a suitable crystal for diffraction of compound 4 failed. The elucidated structure for 5 exhibits the expected dimeric structure, where each pyridine imine group binds a facial Re(CO)3 unit. The CN bond distance in the metal−bond chelate measures ∼1.27−1.29 Å, which is slightly lengthened E


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Figure 2. Structures of complexes 3 and 5 with 35% thermal ellipsoids. H atoms have been omitted for clarity.

able to both increase the conjugation and flexibility of organometallic polymer backbone. The polymer was isolated by gradual fractional precipitation using cold MeOH, chloroform, and acetone as wash solvents. The resultant dark red solid behaved as P1 and could be redissolved in DMF solution only as a wet material. GPC analysis of P2 exhibited Mn close to 650 kDa. On the basis of our success with the use of 4 and 5 in the synthesis of P1 and P2, we decided to develop a third, new method for the synthesis of a 100% Re(CO)3-enriched polymer. As described above and in our previous work, we have used metal-mediated Schiff base formation to produce Re(CO)3 dimetallic complexes. We extended this chemistry to produce a polymer via the same reaction using glyoxal rather than pyridine-2-carboxyaldehdye as the aldehyde component. As shown in Scheme 1, this results in a condensation polymer (P3) from the one pot reaction of Re(CO)5Cl, compound 3, and glyoxal. A DMF solution of Re(CO)5Cl was added slowly to a mixture of glyoxal and compound 3 over the course of the reaction. Although the mechanism of the reaction remains unclear, metal coordination stabilizes the resultant Schiff base units and allows for efficient polymerization. The viscosity of the mixture increases over the course of the reaction, and the color changes to a deep purple hue. A dark solid precipitated during the reaction which did not show good solubility in DMF, DMSO, or NMP and thus could be a very high molecular weight fraction. FTIR analysis of the solid shows CO stretching bands confirming the presence of rhenium. A second fraction of dark solid starts coagulating out of the solution when it is cooled to room temperature. This fraction was collected and washed several times with cold hexane, dichloromethane, and acetone and then dried in a vacuum oven for a day. After drying, this second fraction could not be redissolved in a solvent, as seen in P1 and P2. When still wet, the second fraction was soluble in DMF. A third fraction remained in solution upon completion and cooling of the reaction. GPC analysis of this third fraction reveals a highly polydisperse mixture of polymers for this final fraction. Thus, the second fraction represented our desired condensation polymer P3. As in the previous polymers, P3 was characterized by using GPC which showed a Mn close to 25 kDa. All three polymers show good stabilities up to 300 °C as measured by TGA (Supporting Information). Spectroscopic and Electrochemical Studies. The absorption spectra of dimetallic complex 4 and the three

polymers in DMF solution are shown in Figure 3. The absorption spectrum of 4 exhibits a ligand centered (LC,

Figure 3. Absorption spectra for compound 4 and polymers P1−P3. The inset spectra show normalized band structures above 350 nm for greater clarity.

π−π*) transition at 283 nm and a broad metal to ligand charge transfer (MLCT, d−π*) transition centered around 402 nm. All polymers show π−π* bands at approximately the same wavelengths observed for the dimetallic complex 4, but the appearance of the MLCT bands for P1−P3 reveals significant changes depending on the type of polymerization. Polymers P1 and P2 exhibit MLCT transitions in the same region (∼400 nm) as 4, while P1 shows a significant broadening of the visible band up to 700 nm. In contrast, P2 does not exhibit such a broadened MLCT pattern. The MLCT band for P3 shows a bathochromic shift (λmax = 467 nm) with a broad shoulder ending near 700 nm. We hypothesize that the observed differences result from two factors. First, P3 has a different repeat unit than P1 or P2, so not surprisingly it exhibits a different absorption spectrum. This is supported by our calculations (vide inf ra). Second, all of the polymers exhibit some degree of electronic coupling or conjugation along the backbone, which results in the formation of red-shifted shoulders on the MLCT bands. F


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Macromolecules The electrochemical properties of complex 4 and polymers P1−P3 were studied by cyclic voltammetry (Figure 4).

Table 3. HOMO, LUMO, and Band Gap Energies Based on Electrochemical and Optical Onset Data

Observed potentials and relevant electrochemical parameters are listed in Table 2. Generally, all compounds possess Table 2. Reduction and Oxidation Potentials and Relevant Parameters for All Compounds Ered°′ (V) (ΔEp, mV)

ipa/ipc

4 P1 P2 P3

−0.86 (30) −0.84 (52)

0.84 0.97

Ered (V)

Eox (V)

−0.63 −0.63, −0.96

1.39 1.44 1.38 0.87, 1.32

Eredonset

Eoxonset

4 P1 P2 P3

−0.48 −0.37 −0.38 −0.40

1.24 1.17 1.07 0.53

LUMO/HOMO58 −3.9, −4.0, −4.0, −4.0,

−5.6 −5.6 −5.5 −4.9

EgCV 58

λonset (nm)

Egopt 59,60

1.7 1.6 1.5 0.9

556 690 650 718

2.2 1.8 1.9 1.7

show a decreasing pattern from dimetallic monomer 4 to P3 while results from optical calculation followed the same pattern with the exception of P2. However, we believe that the small discrepancy results from the fundamental errors associated with these experiments. Bulk and Surface Morphologies. Scanning electron microscopy (SEM) was performed on compound 4 and polymers P1−P3 in order to probe their morphologies as bulk materials. Samples were dispersed in ethanol and sonicated and then were mounted on stubs grounded with conductive tape. The samples were then coated with a thin layer of silver by sputtering. Comparing the image obtained from the polymers to that from compound 4 reveals significant morphology changes in the bulk phase after polymerization takes place (Figure 5). Compound 4 shows individual particles with an average size of 20−80 nm. The image for polymer P1 shows a continuous amorphous solid with features that indicate the presence of crystallinity causing phase separation and granulation. This can result from the higher rigidity in polymer backbone of P1. However, adding 1,4-divinylbenzene as a spacer into the P2 polymer chain provides a polymer image that shows a homogeneous amorphous due to the higher flexibility which was given by divinylbenzene moiety. Polymer P3 exhibits a morphology different from both P1 and P2 but shares some noncrystalline attributes with P2. In order to study the surface morphologies of thin films of these polymers, films were prepared by spin-coating of small aliquot of polymeric solutions (DMF/CH2Cl2: 60/40) on silicon (Si) wafers (1.0 × 1.0 cm2) as a substrate. The surfaces of the silicon substrates were first treated with peroxymonosulfuric acid to oxidize and remove organic residues on the surface. This solution does not roughen the wafer surface but provides a fresh clean silicon dioxide substrate with a high surface energy that lets the solvent system easily wet the surface.61 The tapping mode was used for AFM scans. 2D height images, pseudo-3D height images, phase contrast images, and amplitude errors are shown in Figure 6 and Figures S17− S19. Based on the AFM images, all polymers are able to form films on the surface. In prior work, DMF is not considered a favorable solvent for film formation due to its lack of volatility, so chloroform or 1,2-dichloroethane is typically used for spincoating.47−49 For our systems, we found that a solvent mixture of DMF and CH2Cl2 is adequate for spin-coating these polymers. 2D and 3D microscopy of P1 show a rough surface which confirms the behavior observed by SEM. It appears that the rigidity of the polymer backbone causes granulation and phase separation at the film surface which shows rough features in the topography. The polymers P3 and P2 both exhibit uniform thin layers. Regardless of the presence of some voids that can result from using DMF as a less than ideal solvent, polymer P2 shows a smoother surface due to the presence of the divinylbenzene spacer.

Figure 4. Cyclic voltammograms of monomer and polymers in 0.1 M TMAPF6/DMF at 0.10 V and 10.0 μAV−1 sensitivity versus AgCl. All CVs were started at 1.6 V, scanned to −1.2 V, and reversed for collecting cathodic and anodic currents.

compd

compd

irreversible oxidations in the range of ∼1.39−1.45 V which can be ascribed to oxidation of the ReI−carbonyl centers. Polymer P3 has shown an additional small oxidation wave around ∼0.87 V which could result from oxidation of the organic polymer backbone.52 With regard to reduction of the monomer, a single reversible wave at −0.86 V is observed for complex 4. We also observe a small feature in the reduction at ∼−0.5 V, which we surmise might result from a ligand bridge based reduction. P1 also shows a reversible reduction wave at ∼−0.84 V, close to that of dimetallic building block 4, while P2 exhibits around −0.63 V. Two separated quasi-reversible reduction waves are observed for P3 at −0.63 and −0.96 V. Based on the observed cathodic scans and our prior work on the electrochemistry of ReI(CO)3 dimetallic compounds, all reduction processes could be assigned to the addition of electron to the polymer backbone based LUMO orbitals.48 We can calculate the HOMO potential energies, LUMO potential energies, and band gap energies (Eg) for all compounds based on onset reduction/oxidation potentials.58 In addition, the band gap energies can be measured optically (Egopt) by using the onset wavelength (λonset)59,60 and compared with electrical band gaps (EgCV) calculated from voltammetry. These results are listed in Table 3. Eg values extracted by cyclic voltammetry G


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Figure 5. Scanning electron microscopy (SEM): (a) 4, (b) P1, (c) P2, and (d) P3.

Figure 6. Atomic force microscopy (AFM) of P1−P3.

Computational Studies. In addition to the abovedescribed experiments, we completed theoretical calculations using density functional theory (DFT) and relativistic spin− orbit time-dependent density functional theory (TD-DFT) in order to estimate theoretical band gap energies (Egtheo) of compounds 4 and P1−P3 to help probe the origin of their spectroscopic and redox properties.62−64 Since our aim was to achieve an improved understanding of the nature of the electronic communication between each Re(CO)3 diimine center and the geometrical structure of the polymeric

backbones and properties of the resultant polymers, we decided to model our polymers based on the distance structures of the polymeric subunits (P1a−P3a). In addition, to highlight the differences with the monomer components, we started our modeling system based on compounds 6a and 7a as the smallest building unit present in polymers P1−P3. A scheme showing all of the model structures is shown at the top of Figure 8. The octyloxy functional groups were replaced by −OH groups in all models to simplify the calculations, since these changes should not significantly affect the equilibrium H


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Figure 7. Optimized structures of monomeric and dimetallic compounds and building blocks of the of polymer backbones.

geometry and the electron density distributions.65,66 The optimized structures are depicted in Figure 7. After full optimization in the ground state, the result for compound 4a indicates that two benzene rings are nonplanar (the inter-ring torsion angle was calculated about ∼37°). This nonplanarity decreases the conjugation degree and is evidence that most physical and chemical properties in the polymers might result from short-range interactions between 2 and 3 Re(CO)3Cl diimine centers. For polymers P1 and P2, the P1a and P2a units (without the alkyl chains) were used as models for the repeating unit inside the polymeric backbones. The optimized structure for P1a reveals the adjacent Re(CO)3Cl pyridineimine units are also slightly nonplanar (torsion angle is about ∼38°). We assume that the rigidity and granulation observed for P1 can be explained by these sequential nonplanar conformations in the polymer backbone. On the other hand, the optimized structure of P2a shows replacing the divinylbenzene affords a fully planar conjugate section between Re(CO)3Cl pyridineimines. Divinylbenzene typically exhibits a cis conformation, providing enough flexibility in agreement with the morphological aspects we observed in SEM and AFM. A small degree of nonplanarity (∼37°) was also observed between the central phenyl rings for P3a. All optimized models exhibit the CN bond lengths and phenyl−phenyl C−C bonds of ∼1.29 and ∼1.52 Å, respectively, in agreement with the data we observed in the crystallography. The effect of the −OH functional group on the Egtheo was compared between compounds 6 and 7 (as the smallest units in P1−P3) with 6a and 7a. For 6 and 7, their physical and chemical properties have been fully investigated in our previous work.39 Results show that due to adding the −OH groups the Egtheo of each Re(CO)3 diimine centers does not affect energy levels significantly, and only incremental changes were observed. Comparing Egtheo values reveals that adding the adjacent Re(CO)3 diimine centers via any kind of polymerization method will decrease the band gaps, as shown in the energy diagram in Figure 8. We used DFT methods to calculate the frontier molecular orbitals and their compositions for all modeled compounds depicted in Figure 9. As represented for 4a, 6a, 7a, and P1a the HOMO levels are ∼90−95% metal centered and located on Re(CO)3Cl centers with d-orbital

Figure 8. DFT predicted energy diagram for target rhenium compounds and polymeric models.

involvement. We have discussed such electronic structures in our previous work.51 In contrast, the LUMO levels are all ligand based. The HOMO and LUMO compositions are altered in polymers P2a and P3a. Polymer P2a shows ∼36% participation of the divinylbenzene organic π system in the HOMO level with the ∼44% of Re(CO)3Cl metal centered orbitals. At the same time, the divinylbenzene also shows a significant contribution (∼36%) in the LUMO, which can be considered as π*-type orbitals on the organic backbone. The P3a HOMO level exhibits ∼25% electron density on the biphenyl ring component, with an ∼15% to the LUMO level. I


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Figure 9. DFT predicted frontier MOs diagrams for the model compounds 4a, 6a, 7a, and P1a−P3a.

■

CONCLUSION We successfully used a dimetallic Re(CO)3Cl complex 5 as a precursor to synthesize air stable, main chain organometallic Re(I) metallopolymers via Yamamoto and Heck coupling reactions with 100% metal monomer incorporation. In addition, the high soluble modified diamine 3 let us develop a new metal-mediated Schiff base formation/condensation polymerization. Our studies indicate that the polymerizarion method can greatly affect the structures of the resultant polymers and thus control their chemical and physical properties including optical, electrochemical, band gaps (Eg), and morphological properties. DFT and TDDFT calculations were employed to understand the electronic structures and excitation energies of these 100% metal monomer incorporated polymers. They indicate that the reductions and oxidation take place on the polymers backbones and that the metal centers can inject electrons into the polymer backbone, resulting in the potential for donor−acceptor (D−A) type properties. We will present data on the photovoltaic properties of these materials in a separate publication and will continue our investigations into the development of highly metal enriched organometallic polymer systems, with a focus on increasing the degree of conjugation in these macromolecules.

We used TDDFT calculations to investigate the electronic structure of compound 4 (Figure 10) to make the tentative

Figure 10. TDDFT predicted UV−vis spectra for 4.

assignments of the observed UV−vis spectra of polymer blocks based on P1−P3a as a dimetallic units (Figure S20). As shown in Figure 10, the TDDFT predicted transitions and two major bands observed for compound 4 in agreement. The low energy region is dominated by a HOMO−1 → LUMO excitation with an energy of ∼2.9 eV and an oscillator strength f = 0.2488. The higher energy region of the spectrum has a major contribution from an intraligand π−π* transitions with an energy around 4.4 eV ( f = 0.4230). The TDDFT predicted spectra of P1−P3a were compared with normalized spectra of the analogous polymers. The P1a and P3a dimetallic models exhibit spectra that are in good agreement with their corresponding polymers. The P2a calculated spectrum is not completely in agreement with the P2 spectrum, however. Clearly, the presence of the larger organic spacer unit greatly alters the degree of electronic coupling across the polymer, which is not nearly as well modeled in our calculations as the P1 and P3 systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00343. Figures S1−S34; table of PCM-DFT optimized coordinates; table of PCM-TDDFT expansion coefficients (PDF) Structure of C28H44N2O2 (3) (CIF) Structure of C46H48Br2Cl2N4O8Re2 (5) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.J.Z.). J


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Macromolecules Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.J.Z. acknowledges The University of Akron for support of this research. REFERENCES

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Re(CO)3 Metallopolymers with Complete Metal Monomer

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