Synthesis, Dynamics, and DFT Studies of Rhenium Dicarbonyl PNN

Mar 18, 2014 - with Re(CO)5X (X = Br, Cl) in toluene. One-electron oxidation of 1 by [(4-BrC6H4)3N][SbCl6] gave 2, which when oxidized by. PhIO afford...
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Synthesis, Dynamics, and DFT Studies of Rhenium Dicarbonyl PNN Pincer Complexes in Three Different Oxidation States Kothanda Rama Pichaandi,† Michael G. Mazzotta,† John S. Harwood,† Phillip E. Fanwick,† and Mahdi M. Abu-Omar*,†,‡ †

Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States School of Chemical Engineering, Forney Hall of Chemical Engineering, Purdue University, 480 Stadium Drive, West Lafayette, Indiana 47907, United States

‡

S Supporting Information *

ABSTRACT: The synthesis and characterization of ReI and ReII dicarbonyl halides 1 and 2 and the ReIII dicarbonyl dihalide 3 supported by a PNN pincer ligand are described. Complex 1 (1a, X = Br; 1b, X = Cl) was synthesized by refluxing the PNN ligand with Re(CO)5X (X = Br, Cl) in toluene. One-electron oxidation of 1 by [(4-BrC6H4)3N][SbCl6] gave 2, which when oxidized by PhIO afforded 3. X-ray and IR analysis of 1−3 revealed a systematic increase of Re−C(O) and decrease of CO bond lengths and increase in the corresponding CO stretching frequency. All of these results are consistent with weakening in the Re−C(O) bond from 1 to 3 upon increasing the oxidation state from +1 in 1 to +3 in 3, with 2 being in between. The heptacoordinate complex 3 exhibited temperature-dependent fluxional behavior, caused by the pseudorotation of the rhenium center. This phenomenon was observed by NMR and supported computationally by density functional theory (DFT) calculations.

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INTRODUCTION Metal carbonyl complexes of tridentate pincer ligands of Ru, Rh, Ir, and Os are known for their excellent catalytic activity. Notable examples include Milstein’s PNN and PNP ligands. Metal−ligand cooperativity in these systems via aromatization− dearomatization of the pincer ligand have found several applications in renewable energy research and green chemistry.1 Rhenium carbonyl complexes have been studied prominently with bidentate ligands for their photochemical applications.2 Only a few examples of mer-coordinated tridentate ligand complexes of rhenium are known.2a,3 Frenzel et al. reported recently on the photophysical properties of ReI dicarbonyl complexes with terpyridine coordination,3a a ligand otherwise studied extensively with other metals for its rich photochemistry.4 Richeson et al. have also pointed out that the full potential of rhenium carbonyl complexes of tridentate noninnocent pincer ligands has yet to be realized.2a In this context, rhenium carbonyl chemistry with Milstein’s ligands is an unexplored area. While this work was in progress, Milstein and co-workers published the synthesis of rhenium carbonyl pincer complexes with PNP ligands and their metal−ligand cooperation.3b Herein we report a series of mer-coordinated species [PNNRe(CO)2Xn]y+ (where X = Br, Cl and n = 1, y = 0 (1), n = 1, y = 1 (2), n = 2, y = 1 (3)) with three different oxidation states, rhenium(I), -(II), and -(III). The seven-coordinate complex 3 showed temperature-dependent dynamic behavior in solution and was investigated in detail by NMR and by DFT computations. The dearomatization of complex 1a was investigated and realized. © 2014 American Chemical Society

RESULTS AND DISCUSSION

Synthesis of Complexes 1−3. The synthetic route for complexes 1−3 is given in Scheme 1. Complex 1 (1a, X = Br; 1b, X = Cl) was prepared in 90% (1a) and 84% (1b) yields by refluxing ReX(CO)5 (X = Br, Cl) with the PNN ligand in toluene. One-electron oxidation of 1 with [(4-BrC6H4)3N][SbCl6] gave complex 2 quantitatively. Compounds 2a,b are paramagnetic; their EPR spectra are given in the Supporting Information. While complex 2b was stable both in the solid state and in solution, complex 2a was stable only in the solid state. Upon standing in solution overnight at ambient temperature, complex 2a gave 3a in 40% yield. This route failed to give 3b from 2b. However, when PhIO was used as an oxidant, both 3a and 3b were formed from 2a and 2b, respectively. In both methods, the source of the chloride is the SbCl6− anion. While the former method gave a pure compound in the case of 3a with SbCl6− as the counterion, the latter method employing PhIO generated an oxorhenium species (not characterized). 1H, 13C{1H}, and 31 1 P{ H} NMR spectra, including those with 13CO-labeled complexes, were identical for both synthetic methods and no major impurities were detected. However, a band at 910 cm−1 in the IR and an 17O{1H} NMR signal at 517 ppm when 17Oenriched PhI17O was used as an oxidant were observed. These Received: January 7, 2014 Published: March 18, 2014 1672

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Scheme 1. Synthesis of Complexes 1−3a

to their 13C{1H} (CO ligand) and 31P{1H} NMR chemical shifts. (1) The two characteristic sharp signals in the mid-IR region with equal intensity conform to the cis stereochemical arrangement of the CO ligands in all complexes. (2) The systematic increase in the wavenumber for the two CO ligands from 1 to 2 and then to 3 is indicative of weakening in the Re− CO bond, coinciding with the increase in the oxidation state of rhenium from +1 to +2 and then to +3, respectively. As the oxidation state increases, participation of the d shell electrons in π back-bonding decreases. These results are supported by DFT results, discussed in a later section. X-ray Structure Comparison of 1a,b, 2b, and 3a. X-ray structures of complexes 1b (the structure of 1a is given in the Supporting Information), 2b, and 3a are given in Figure 1, and a comparison of their bond lengths and bond angles is given in Table 2. Complexes 1a,b and 2b have a distorted-octahedral Table 2. Comparison of Bond Lengths and Bond Angles in 1b, 2b, and 1c

a Legend: (i) toluene, 110 °C, 12 h; (ii) CH2Cl2, −78 °C; (iii) CH2Cl2, 25 °C, overnight (3a); (iv) CH2Cl2, PhIO, −78 to +20 °C, 30 min (3a,b).

bond length (Å) or angle (deg)a

1b

2b

3a

Re−Cax(O) Re−Ceq(O) CaxO CeqO Re−N(Py) Re−N Re−P Re−Cl ∠C(O)ReC(O) ∠Ceq(O)ReCl

1.91(3) 1.903(8) 1.15(3) 1.16(1) 2.159(5) 2.327(8) 2.378(2) 2.512(9) 90.3(7) 96.9(3)

1.97(1) 1.95(1) 1.11(2) 1.13(2) 2.152(9) 2.30(1) 2.452(3) 2.402(3) 83.1(6) 93.7(4)

1.94(1) 2.01(1) 1.06(2) 1.09(2) 2.13(1) 2.34(1) 2.500(4) 2.450(3) 76.9(6) 110.8(4)

are characteristic of Re−O bonds,5 consistent with a rhenium oxo species. IR Studies on Complexes 1−3. A comparison of the CO signals in the IR spectra for 1−3 is given in Table 1 in addition Table 1. Comparison of 13C{1H} NMR (CO Ligand) and 31 1 P{ H} NMR Chemical Shifts as well as IR CO Stretching Frequencies for Complexes 1−3

a

Subscripts indicate the following: ax, axial; eq, equatorial.

NMR, δ (d, J(C−P) in Hz) complex 1a 1b 3a 3b 2a 2b

13

C{1H} for CO

210.6 211.0 194.7 195.9 nda nda

(5), 202.5 (7) (4), 203.3 (6) (51), 189.4 (5) (56), 188.8 (3)

31

P{1H}

68.8 70.0 67.4 (51)b 70.2 nda nda

IR, cm−1 1895, 1880, 2005, 2014, 1990, 1988,

geometry, and compound 3a has a pentagonal-bipyramidal geometry around the central Re atom. All of these complexes possess the halogen (Br in 1a and Cl in 1b, 2b, and 3a) and one of the CO ligands occupying the axial positions, leaving the CO ligands cis with respect to each other. Notable comparisons among 1b, 2b, and 3a are (1) a systematic increase of Re−Ceq(O) and Re−P bond lengths and a decrease of CO bond lengths when the oxidation state of Re changes from +1 (1b) to +2 (2b) and then to +3 (3a), consistent with the observed νCO values in the IR spectra, and (2) ∠C(O)ReC(O) bond angle decreased from 1b to 3a, with the value of 2b being intermediate.

1805 1789 1927 1936 1891 1890

a

Not determined; 2 is paramagnetic. NMR was taken in CD2Cl2 and IR was taken using ATR mode. bCoupling constant derived from 31 1 P{ H} NMR of 3a*.

Figure 1. ORTEP drawings of compounds 1b, 2b, and 3a. Ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. 1673

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trans P atom on this PNN ligand, we synthesized fac(CO)3ReBr(η2-P,N[pyridine]-PNN) (see the Supporting Information for synthesis and characterization) and found a P−C coupling of 69 Hz. This value is 18 Hz higher than the value observed for structure A of 3a. To confirm the coupling of the CO ligand to P, we used 13C-enriched CO gas to synthesize Re(13CO)5Br and used it in the preparation of 13CO-enriched complex 3a*. The 31P{1H} NMR chemical shift of 3a* having 13 C-labeled CO (Figure 2) appeared as a doublet with a J(C− P) coupling constant of 51 Hz, matching the value observed in the 13C{1H} NMR. The 31P−13C 2D NMR of 3a* (Figure 2) possessing 13C-labeled CO further established the bonding between the phosphorus of the pincer ligand and CO. This is the first observation, to our knowledge, of a large coupling constant for a CO group present cis to P without having a direct covalent bond to the phosphorus. For 3b the J(C−P) coupling value is 56 Hz. A typical coupling constant for a covalent phosphino carbonyl group (R2PC(O)R′) is 113 Hz.8 We attribute this to the short distance (2.626 Å found by X-ray, 2.71 Å by DFT) between the carbon atom of the CeqO group and the P atom. Though Kirchner and co-workers reported pincer complexes of Mo and W having similar structures and dynamic behaviors, they observed J(C−P) coupling constants of ∼10 Hz at room temperature.6a However, we believe those complexes may have higher coupling constants at lower temperatures. The X-ray analysis of 3a has the structure A, and DFT also predicted A to be lower in energy than B, supporting the NMR observation of A at lower temperature. At higher temperatures the signals became very weak in 31P{1H} NMR and are absent in 13C{1H} NMR and new signals were observed in 1H NMR, reflecting the dynamic behavior. For complex 3b structure A was positively identified by comparison with the NMR of 3a under the same conditions. Here complete transformation to A was not observed at −50 °C and additional signals were detected in the 1H NMR. Further lowering of temperature was not attempted due to instrumental limitation. DFT results supporting the pseudo rotational behavior of 3 are described next. DFT Studies. DFT calculations were performed to understand the effect of increase in oxidation state in the weakening/elongation of Re−C(O) in compounds 1−3 as well as to support the fluxional behavior in 3. We took chlorinesubstituted 1b−3b for the purpose of calculation. As the oxidation state of Re increases from +1 to +3 from 1b to 3b, the CO group’s π* orbital net contribution to the bonding orbitals responsible for back bonding with d orbitals in Re decreases, whereas the antibonding orbitals of the corresponding orbitals increase (Table 3). This clearly supports the weakening cum elongation of Re−C(O) bond lengths from 1 to 3, as observed by IR and X-ray analysis. Kirchner and co-workers reported DFT studies on a fluxional process similar to that exhibited by 3. They located a transition

The structures of 1a,b revealed a mild disorder arising from the positional scrambling between one of the CO ligands and the halogen atom. Despite this disorder, the refinement gave satisfactory R values of 0.033 and 0.0491, respectively. Fluxional Behavior of 3. Heptacoordinate complexes are known to exhibit rotational behavior around the metal center, resulting in dynamic NMR behavior in solution.6 In line with that, complex 3, a seven-coordinate species, exhibited temperature-dependent fluxional behavior in solution, observed by NMR. We attribute this to pseudorotation around the Re center, as depicted in Scheme 2. Scheme 2. Fluxional Behavior of 3

For complex 3a, near-complete transformation to structure A (Scheme 2) was observed in solution at ≤−50 °C. Structure A was identified by 1H, 13C{1H}, and 31P{1H} NMR as well as 31 P−13C 2D NMR spectra (Figure 2) using 13C-enriched Clabeled species (3a*) at −50 °C. The details of the characterization of structure A are given below.

Figure 2. 31P{1H}−13C{1H} 2D NMR of 3a* in CD2Cl2 at 224 K. The single-INEPT polarization transfer method9 and 13C detection were used; 1H decoupling was carried out using WALTZ-16 modulation. The J(C−P) splitting is not seen in the cross-peaks in the 31P (vertical) dimension because the pulse program removes the splitting in that dimension.

The 1H NMR showed the stereospecific proton signals observed for the PNN ligand with the exclusion of the aromatic ring in 3a, and are consistent with the η3 coordinated PNN-Re having cis-dicarbonyl geometry similar to 1a and 1b. In the 13 C{1H} NMR, the chemical shifts corresponding to the CO ligands (Table 1) are doublets with coupling constants of 51 and 4 Hz. While higher coupling constant values are common observations for a CO group trans to a P atom,7 such a configuration is not possible here, as the nitrogen arm of the PNN ligand occupies the trans position evidenced by the 1H NMR. To verify the coupling constant value for a CO group

Table 3. Comparison of Contribution of π* Orbitals of CO to the Back-Bonding in 1b−3b

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orbital

1b

2b

3b

HOMO HOMO-1 HOMO-2 LUMO LUMO+1

0.18 0.22 0.26 0 0

0.23 0.2 0 0.2 0

0.09 0.15 0 0.2 0.13

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upfield chemical shifts for the pyridinyl protons and the stereospecific nature for the alkyl protons of the PNN ligand was absent in comparison with its precursor 1a. The 31P{1H} NMR exhibited an upfield shift of 5.7 ppm in comparison with 1a. These NMR characteristics are consistent with the dearomatized metal complexes reported by Milstein.1

state for CO and hydride ligands in the PNP plane exchanging positions in a single step.6a Hence, we used DFT to drive the Cleq−Re−N1−N2 dihedral angle in 3b by 10° increment between the two bookend structures A and B (Scheme 2). The potential energy was then calculated for the fully optimized structure at each dihedral angle. The barrier for such rotation was found to be ∼17 kcal/mol (Figure 3). This value is in reasonable agreement with the transition state module (18.6 kcal/mol) reported by Kirchner.6a

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CONCLUSION We reported herein the synthesis and characterization of a series of dicarbonyl halides of rhenium complexed with Milstein’s PNN ligand, possessing oxidation states +1 to +3. The weakening of the Re−C(O) bond along the increase in oxidation states was evident from X-ray and IR results, with an increase in the corresponding bond lengths and the CO stretching frequencies, respectively. The temperature-dependent dynamic NMR behavior exhibited by heptacoordinate complexes 3a,b was attributed to their pseudorotational behavior and was supported by DFT calculations. The synthesis of dearomatized complex 6 from 1a was achieved by first removal of the halogen by a silver metathesis reaction and subsequent dearomatization with LiHMDS.

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Figure 3. Potential energy of 3b as a function of the dihedral angle Cleq−Re−N(Py)−N predicted by DFT calculations (B3LYP density functional model and LANL2DZ basis set within the Gaussian 09 suite program). Constrained geometrical optimizations were done by freezing the Cleq−Re−N(Py)−N Cartesian coordinates with the corresponding bond angles with increment of 10° between the bookend structures A and B.

Dearomatization of 1a. Investigation into the dearomatizaton chemistry of these complexes is important as metal complexes with PNN ligands are known to undergo dearomatization during catalysis.1 Therefore, we explored the dearomatization of 1a (Scheme 3). Initial trials with addition of Scheme 3. Dearomatization Chemistry of 1aa

a

EXPERIMENTAL SECTION

All reactions were performed in a nitrogen-filled glovebox or using standard Schlenk techniques under argon. Solvents were degassed and purified with a solvent purification system (Anhydrous Engineering Inc.) prior to use. CD2Cl2 was dried with CaH2, distilled under argon, and stored over molecular sieves. PhI17O was prepared using H2O containing 75% enriched oxygen atoms of 17O according to the procedure reported by Hill and co-workers.10 Re(13CO)5 was prepared using labeled 13CO gas (98%) following the procedure of Rheingold and co-workers.11 The PNN ligand and all other chemicals were purchased from Aldrich and used as received. One-dimensional and variable-temperature NMR spectra were recorded on a Bruker DRX-500 NMR spectrometer equipped with a 5 mm broad-band (BBO) probe. Two-dimensional 13C−31 P correlation experiments were acquired using a four-channel Bruker DRX-500 NMR spectrometer equipped with a 5 mm triple-resonance probe comprising one fixed 13C coil and one tunable (broad-band) coil. Bruker TopSpin software (version 1.3) was used for data acquisition and processing. For our experiments we used the singleINEPT polarization transfer method9 and 13C detection. The 13C splitting of the cross peaks was removed through the use of a 180° pulse during the evolution time, but 31P decoupling was not applied during data acquisition, resulting in cross peaks showing the J(13C−31P) splitting in the F2 dimension. 1H decoupling using WALTZ-16 modulation was applied during the entire experiment. The spectrum acquired for 3b was obtained with the sample in CD2Cl2 at 220 K using sweep widths of 15 ppm in both dimensions, 76 increments of 1K complex data points each, and 16 scans per increment. The data acquisition was optimized for a J(13C−31P) value of 53 Hz. The data matrix was zero-filled during processing to a size of 256 × 1K real points. The data were acquired and processed in phasesensitive mode using States-TPPI methodology. Infrared spectra were recorded on a Thermo-Nicolet Nexus FT-IR spectrometer, operated in ATR mode as well as in solution (CH2Cl2) using a NaCl cell. EPR spectra were recorded on a Bruker ESP 300E EPR spectrometer equipped with an HP 5350B microwave frequency counter, an Oxford ITC4 temperature controller, and a VC40 gas flow controller (for liquid He) and a Eurotherm temperature control unit (for liquid N2). IR spectra and UV−vis spectra were obtained using Thermo-Nicolet Nexus FT-IR and Shimadzu 2501-A spectrophotometers, respectively. Mass spectrometry was performed by the Purdue University Campus Wide Mass Spectrometry Center using a Hewlett-Packard Engine mass spectrometer (GC/MS). All electrospray ionization analyses were carried out on a FinniganMAT LCQ Classic (ThermoElectron Corp, San Jose, CA) mass spectrometer

Legend: (i) THF, 25 °C; (ii) CH2Cl2, −78 °C; (iii) THF, −78 °C.

1 equiv of potassium tert-butoxide as a base failed to give the desired dearomatized complex 6; instead, they gave 7 with hydroxide replacing the bromine. Formation of 7, we believe, could be the result of the reaction of dearomatized species with the tert-butyl alcohol formed as a byproduct in this reaction. Complex 7 was characterized by NMR, mass spectrometry, and X-ray diffraction (see the Supporting Information). However, when silver triflate was used to remove the halogen first, subsequent reaction of 5 with lithium hexamethyldisilazide (LiHMDS) gave the desired dearomatized complex 6, which was isolated as a dark red powder. 1H NMR of 6 exhibited 1675

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Py), 7.97 (d, 1H, Py), 7.83 (d, 1H, Py), 5.19 (dd, 1H, 12, 16 Hz, PyCH2P), 4.94 (d, 1H, 16 Hz, PyCH2N), 4.71 (d, 1H, 16 Hz, PyCH2N), 4.22 (m, 1H, NCH2CH3), 4.00 (dd, 1H, 12, 16 Hz, PyCH2P), 3.30 (m, 2H, NCH2CH3), 2.75 (m, 1H, NCH2CH3), 1.70 (d, 18H, 16 Hz, PC(CH3)3), 1.32 (t, 3H, NCH2CH3), 1.26 (t, 3H, NCH2CH3). 13C{1H} NMR (125 MHz, CD2Cl2, −50 °C): δ 194.8 (d, 52 Hz, CO), 189.4 (d, 5 Hz, CO), 161.8, 159.8, 144.4, 124.8 (d, 11 Hz), 124.3, 70.4 (PyCH2N), 56.4 (NCH2CH3), 53.4 (NCH2CH3), 43.4 (d, 17 Hz, PyCH2P), 42.7 (d, 17 Hz, PC(CH3)3), 37.3 (d, 26 Hz, PC(CH 3 ) 3 ), 31.2 (PC(CH 3 ) 3 ), 26.9 (br, PC(CH 3 ) 3 ), 11.6 (NCH2CH3), 9.7 (NCH2CH3). 31P{1H} NMR (202 MHz, CD2Cl2, −50 °C): δ 67.4. IR ν (solid, cm−1): 2005 (s), 1927 (s). Anal. Found (calcd): C, 24.79 (24.86); H, 3.48 (3.48); N, 2.79 (2.76). Synthesis of [(PNN)ReXCl(CO)2][SbCl6] (3) using PhIO. To a pale yellow solution of 1 (0.2 mmol) in dichloromethane (10.0 mL), cooled to −78 °C, was added a suspension of [(4-BrC6H4)3N][SbCl6] (138.0 mg, 0.2 mmol) in dichloromethane (10.0 mL) in 15 min. For 3a the reaction mixture turned reddish orange and was stirred for another 10 min at −78 °C. For 3b the temperature was raised to ambient conditions and the reaction mixture turned deep yellow. Then the reaction solution was transferred via cannula to a suspension of PhIO (170.0 mg, 0.8 mmol) in dichloromethane (10.0 mL) in a 50 mL Schlenk flask at −78 °C. The temperature of the reaction mixture was raised to −15 °C, and the reaction mixture turned pale yellow. It was further brought to 20 °C gradually, stirred for 20 min, and filtered to remove excess iodosobenzene. The solvent from the clear solution was removed under vacuum. The dark brown oily solid that was obtained was washed with toluene and ether (20 mL each) and dried under vacuum to give 3 as a pale yellow powder. Yield of 3a: 131.0 mg (65%). Yield of 3b: 126.0 mg (65%). 1H NMR (500 MHz, CD2Cl2, −50 °C): δ 8.24 (t, 1H, Py), 7.98 (d, 1H, Py), 7.82 (d, 1H, Py), 5.18 (dd, 1H, 12, 16 Hz, PyCH2P), 4.96 (d, 1H, 16 Hz, PyCH2N), 4.74 (d, 1H, 16 Hz, PyCH2N), 4.06 (dd, 1H, 12, 16 Hz, PyCH2P), remaining signals in the aliphatic region could not be identified due to the complexicity of NMR. 13C{1H} NMR (125 MHz, CD2Cl2, −50 °C): δ 195.9 (d, 56 Hz, CO), 188.8 (d, 3 Hz, CO), 161.7, 159.5, 144.4, 124.7 (d, 9 Hz), 124.3, 70.6 (PyCH2 N), 55.0 (NCH2CH3), 53.1 (NCH 2 CH 3 ), 43.4 (d, 18 Hz, PyCH 2 P), 42.7 (d, 18 Hz, PC(CH3)3), 37.3 (d, 27 Hz, PC(CH3)3), 31.0 (PC(CH3)3), 26.0 (PC(CH3)3), 11.5 (NCH2CH3), 8.8 (NCH2CH3). 31P{1H} NMR (202 MHz, CD2Cl2, −50 °C): δ 70.0. IR ν (solid, cm−1): 2014 (s), 1936 (s) Anal. Found (calcd): C, 26.68 (26.00); H, 3.58 (3.64); N, 2.72 (2.89). Synthesis of 6. To a solution of (PNN)Re(CO)2Br (112.0 mg, 0.17 mmol) in THF (10.0 mL) was added AgSO3CF3 (91.7 mg, 0.35 mmol), and the mixture was stirred for 20 min. The reaction mixture was filtered to remove the precipitated silver bromide. A precooled solution of LiHMDS (124.0 mg, 0.74 mmol) in THF (3.0 mL) was added to the reaction mixture at −40 °C and brought to room temperature in 20 min. The solvent was removed under vacuum, and the precipitate obtained was dissolved in toluene and filtered. The filtrate was layered with pentane at −40 °C, and the precipitate obtained was filtered and dried under vacuum to produce 6 as a dark red powder. Yield: 85.0 mg (87%). 1H NMR (500 MHz, CD2Cl2): δ 6.15 (m, 1H, Py), 6.06 (d, 1H, Py), 5.21 (d, 1H, Py), 3.66 (s, 2H, PyCH2N), 3.62 (d, 1H, PyCHP), 2.90 (m, 2H, NCH2CH3), 2.73 (m, 2H, NCH2CH3), 1.08 (d, 14 Hz, 18 H, PC(CH3)3), 1.06 (t, 6H, NCH2CH3) 13C{1H} NMR (125 MHz, CD2Cl2): δ 208.9 (d, 7 Hz, CO), 204.6 (CO), 158.1, 156.4, 131.5, 117.0 (d, 16 Hz), 97.4, 72.5 (d, 54 Hz, PyCHP), 66.0 (PyCH2N), 52.6 (NCH2CH3), 38.4, (d, 29 Hz, PC(CH3)3), 28.7 (d, 3 Hz, PC(CH3)3), 10.4 (NCH2CH3). 31P{1H} NMR (202 MHz, CD2Cl2): δ 63.1. ESI-MS (m/z): 563/565 (M+ + H). IR ν (in CD2Cl2 solution, cm−1): 1900 (s), 1819 (s). Elemental analysis of 6 could not be carried out due to its extreme sensitivity toward moisture. Computational Details. All the DFT calculations reported in this paper used the Gaussian0912 program system through Gauss View.13 Ground-state geometries for 1b, 2b and the two bookends A and B of compound 3b were optimized by implementing the B3LYP density functional model and LANL2DZ basis sets. Constrained geometrical optimizations were done by the freezing Cartesian coordinates of

system. The electrospray needle voltage was set at 4.0 kV, the heated capillary voltage was set to 10 V, and the capillary temperature was 207 °C. Typical background source pressure was 1.2 × 10−5 Torr, as read by an ion gauge. The sample flow rate was approximately 8 mL/min. The drying gas was nitrogen. The LCQ is typically scanned to 2000 amu. The electrospray ionization high-resolution mass measurements were obtained in the peak matching mode using a FinniganMAT XL95 (FinniganMAT Corp., Bremen, Germany) mass spectrometer. Elemental analysis was done by Galbraith Laboratory. Synthesis of (PNN)ReX(CO)2 (X = Br (1a), Cl (1b)). To a suspension of Re(CO)5X (1.0 mmol) in toluene (25.0 mL) was added the PNN ligand (350.0 mg, 1.1 mmol), and the mixture ws kept under reflux for 48 h. Then the reaction mixture was cooled to room temperature. For 1a, the volume was reduced to 7−10 mL under vacuum. The solution mixture was further heated to reflux to dissolve any precipitated product and was cooled overnight. For 1b, no further steps were required. Pale yellow crystals obtained for both 1a and 1b were filtered and dried under vacuum. Yield of 1a: 568.0 mg (90%). 1H NMR (500 MHz, CD2Cl2): δ 7.71 (t, 1H, Py), 7.45 (d, 1H, Py), 7.26 (d, 1H, Py), 4.98 (d, 15 Hz, 1H, PyCH2N), 4.12 (d, 1H, 15 Hz, PyCH2N), 3.97 (dd, 9 Hz, 16 Hz, 1H, PyCH2P), 3.39 (dd, 9 Hz, 16 Hz, 1H, PyCH2P), 3.78 (m, 1H, NCH2CH3), 3.49 (m, 1H, NCH2CH3), 3.10 (m, 1H, NCH2CH3), 2.89 (m, 1H, NCH2CH3), 1.35 (d, 9H, 4 Hz, PC(CH3)3, 1.32 (d, 9H, 3.5 Hz, PC(CH3)3), 1.19 (t, 3H, NCH2CH3), 1.12 (t, 3H, NCH2CH3). 13 C{1H} NMR (125 MHz, CD2Cl2): δ 210.6 (d, 5 Hz, CO), 202.5 (d, 7.0 Hz, CO), 164.2, 160.9, 138.2, 122.5 (d, 7 Hz), 120.4, 65.9 (PyCH2N), 55.3 (NCH2CH3), 50.2 (NCH2CH3), 40.2 (PyCH2P), 40.0 (PC(CH3)3), 38.6 (d, 16 Hz, PC(CH3)3), 30.9 (d, 2.8 Hz, PC(CH3)3), 30.7, (d, 2.6 Hz, PC(CH3)3), 11.9, (NCH2CH3), 10.2 (NCH2CH3). 31P{1H} NMR (202 MHz, CD2Cl2): δ 68.8. IR ν (solid, cm−1): 1895 (s) 1805 (s). ESI-MS (m/z): 563/565 (M+ − Br). Anal. Found (calcd): C, 39.49 (39.13); H, 5.65 (5.47); N, 4.36 (4.35). Yield of 1b: 504.0 mg (84%). 1H NMR (500 MHz, CD2Cl2): δ 7.71 (t, 1H, Py), 7.44 (d, 1H, Py), 7.25 (d, 1H, Py), 4.90 (d, 15 Hz, 1H, PyCH2N), 4.10 (d, 15 Hz, 1H, PyCH2N), 3.92 (dd, 9 Hz, 15 Hz, 1H, PyCH2P), 3.35 (dd, 9 Hz, 15 Hz, 1H, PyCH2P), 3.63 (m, 1H, NCH2CH3), 3.47 (m, 1H, NCH2CH3), 3.14 (m, 1H, NCH2CH3), 2.86 (m, 1H, NCH2CH3), 1.32 (t, 18H, 12.7 Hz, PC(CH3)3), 1.19 (t, 3H, NCH2CH3), 1.14 (t, 3H, NCH2CH3). 13C{1H} NMR (125 MHz, CD2Cl2): δ 211.0 (d, 4 Hz, CO), 203.3 (d, 6.0 Hz, CO), 164.0, 160.9, 138.1, 122.3 (d, 7 Hz), 120.3, 65.8 (PyCH2N), 55.6 (NCH2CH3), 49.3 (NCH2CH3), 39.8 (d, 25 Hz, PyCH2P), 39.5 (d, 21 Hz, PC(CH3)3), 38.3 (d, 14 Hz, PC(CH3)3), 30.5 (t, 2 Hz, PC(CH3)3), 11.7 (NCH 2 CH 3 ), 10.0 (NCH 2 CH 3 ). 31 P{ 1 H} NMR (202 MHz, CD2Cl2): δ 69.5. IR ν (solid, cm−1): 1880 (s) 1789 (s). ESI-MS (m/z): 563/565 (M+ − Cl). Anal. Found (calcd): C, 42.35 (42.03); H, 5.92 (5.88); N, 4.56 (4.67). Synthesis of [(PNN)ReX(CO)2][SbCl6] (X = Br (2a), Cl (2b)). To a pale yellow solution of 1 (0.2 mmol) in dichloromethane (13 mL), cooled to −78 °C was added a suspension of [(4-BrC6H4)3N][SbCl6] (170.0 mg, 0.21 mmol) in dichloromethane (13 mL) in 15 min. For 2a, the reaction mixture turned reddish orange and was stirred for another 10 min at −78 °C. For 2b, the temperature was raised to −10 °C and the reaction mixture turned into deep yellow; then the temperature was raised to room temperature. Then the solvent was removed under vacuum and the precipitate was washed with toluene three times (30 mL each) and dried under vacuum to give 2. X-rayquality crystals were obtained by vapor diffusion of ether into a dichloromethane solution of 2b. Yield of 2a: 195.0 mg (quantitative). IR ν (KBr, cm−1): 1990 (s), 1891 (s). Anal. Found (calcd): C, 25.57 (25.76); H, 3.46 (3.60); N, 2.77 (2.86). Yield of 2b: 185.0 mg (quantitative). IR ν (KBr, cm−1): 1988 (s), 1890 (s). Anal. Found (calcd): C, 27.08 (26.99); H, 3.76 (3.77); N, 2.74 (3.00). Synthesis of [(PNN)ReBrCl(CO)2][SbCl6] (3a). Upon standing at ambient temperature overnight, the reaction mixture in dichloromethane from the synthesis of 2a gave 3a as pale yellow crystals. Yield: 80 mg (40%). 1H NMR (500 MHz, CD2Cl2, −50 °C): δ 8.24 (t, 1H, 1676

dx.doi.org/10.1021/om500007m | Organometallics 2014, 33, 1672−1677

Organometallics

Article

Cleq−Re−N(Py)−N with the corresponding dihedral angle with an increment of 10° from A to B. % Contributions of MO on the optimized geometries of 1b-3b were calculated by Chemissian software.14

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Herrmann, W. A.; Lopes, A.; Pillinger, M.; Romão, C. C. Inorg. Chim. Acta 1998, 279, 44. (d) Herrmann, W. A.; Kuchler, J. G.; Weischselbaumer, G.; Herdtweck, E.; Kiprof, P. J. Organomet. Chem. 1989, 372, 351. (6) (a) Ö ztopcu, Ö .; Holzhacker, C.; Puchberger, M.; Weil, M.; Mereiter, K.; Veiros, L. F.; Kirchner, K. Organometallics 2013, 32, 3042. (b) Baker, P. K.; Al-Jahdali, M.; Meehan, M. M. J. Organomet. Chem. 2002, 648, 99. (c) Bradley, F. C.; Wong, E. H.; Gabe, E. J.; Lee, F. L.; Lepage, Y. Polyhedron 1987, 6, 1103. (d) Baker, P. K.; Fraser, S. G.; Keys, E. M. J. Organomet. Chem. 1986, 309, 319. (e) Curtis, M. D.; Shiu, K. B. Inorg. Chem. 1985, 24, 1213. (7) (a) Gibson, D. H.; Ding, Y.; Miller, R. L.; Sleadd, B. A.; Mashuta, M. S.; Richardson, J. F. Polyhedron 1999, 18, 1189. (b) O'Connor, J. M.; Uhrhammer, R.; Chadha, R. K.; Tsuie, B.; Rheingold, A. L. J. Organomet. Chem. 1993, 455, 143. (c) Simpson, R. D.; Bergman, R. G. Organometallics 1992, 11, 4306. (8) Weber, L.; Reizig, K.; Boese, R. Organometallics 1985, 4, 2097. (9) (a) Bast, P.; Berger, S.; Gunther, H. Magn. Reson. Chem. 1992, 30, 587. (b) The pulse sequence we used was a local modification of the “hxinepph” pulse sequence from the Bruker TopSpin pulse program library. (c) In contrast to ref 9a, we obtained better results observing 13 C rather than 31P. (10) Schardt, B. C.; Hill, C. L. Inorg. Chem. 1983, 22, 1563. (11) O’Connor, J. M.; Uhrhammer, R.; Rheingold, A. L.; Staley, D. L.; Chadha, R. K. J. Am. Chem. Soc. 1990, 112, 7585. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; ; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; ; Fox, D. J. Gaussian09; Gaussian, Inc., Wallingford, CT, 2009. (13) Dennington, R.; Keith, T.; Millam, J. GaussView, Version 5; Semichem Inc., Shawnee Mission, KS, 2009. (14) Leonid, S. Chemissian, 1.771. ed.; 2005−2010.

ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving details of the synthesis and characterization data for fac-(CO)3ReBr(η2-P,N[pyridine]PNN) and 7, spectral details, crystal data and structure and refinement details of 1a,b, 2b, 3a, and 7, DFT details, and X-ray crystallographic data for 1a,b, 2b, 3a, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.M.A.-O.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the DOE-BES, Grant No. DE-FG-02-06ER15794. We thank Dr. Huaping Mo and Mr. Zhi Cao of Purdue University for helping us with the NMR experiments and for useful discussions of DFT, respectively. This research was supported through computational resources provided by Information Technology at Purdue-Rosen Center for Advanced Computing, West Lafayette, IN.

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REFERENCES

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dx.doi.org/10.1021/om500007m | Organometallics 2014, 33, 1672−1677