Porphyrin(2.1.2.1) rhenium(I) complexes: Synthesis, structures, properties, and dipyrrin act as bipyridyl-like ligand

Abstract Two new rhenium(I) porphyriniod complexes (H2PY: 3,3-rhenium(I) tricarbonyl chloride-(31 E,32 Z,71 E,72 Z)-4,8-bis(perfluorophenyl)-11 H,32 H,72 H-51 λ2 ,71 λ4 -1,5(2,5),3,7(5,2)-tetrapyrrola-2,6(1,2)-dibenzenacyclooctaphane, and H2PZ: 3,3-rhenium(I) tricarbonyl acetate-(31 E,32 Z,71 E,72 Z)-4,8-bis(perfluorophenyl)-11 H,32 H,72 H-51 λ2 ,71 λ4 -1,5(2,5),3,7(5,2)-tetrapyrrola-2,6(1,2)-dibenzenacyclooctaphane) containing two different Re(I)−N π-bonding networks were prepared from the saddle-shaped porphyrin(2.1.2.1) bearing pentafluorophenyl substituents at the two meso-positions (H2Por: 31 E,32 Z,71 E,72 Z)-4,8-bis(perfluorophenyl)-11 H,32 H,51 H,72 H-1,5(2,5),3,7(5,2)-tetrapyrrola-2,6(1,2)-dibenzenacyclooctaphane). Despite the tetrapyrrolic nature of the porphyrin(2.1.2.1) scaffold, this is the first case of a pyrrole-based macrocycle which acts as a bipyridyl-like complexing ligand in the formation of an octahedral Re(I) complex. X-ray analysis and 1H NMR studies show that both Re(I) complexes (H2PY and H2PZ) consist of one octahedral Re(I) ion π-bonded to a dipyrrin unit and one protonated dipyrrin moiety linked together through o-phenylene bridges. This bonding behavior is compared to related metal complexes of porphyrin(2.1.2.1) macrocycles as well as other Re(I) porphyrins and demonstrates that the distorted conformation and, by virtue, the electronics of H2Por serve as a novel platform for examining structure property relationships which differ than those of other related planar porphyrin macrocycles. Graphical Abstract


Instruments and materials
The UV-vis absorption spectra were measured with a JASCO UV/VIS/NIR Spectrophotometer V-670. 1 H NMR and 19 F NMR spectra were recorded on a JNM-ECX 400 spectrometer (operating as 400 MHz for 1 H, 100 MHz for 13 C, and 376 MHz for 19 F) using the residual solvent as the internal reference for 1 H (d ¼ 7.26 ppm in CDCl 3 ), 13 C (d ¼ 76.8 ppm in CDCl 3 ), and CF 3 COOH as the external reference for 19 F (d ¼ À76.5 ppm). The high-resolution matrix-assisted laser desorption/ionization mass (HR-MALDI-MS) spectra were recorded on a Bruker Daltonics autoflex MALDI-TOF MS spectrometer. Cyclic voltammetry was conducted in a solution of 0.1 M TBAPF 6 in dry-CH 2 Cl 2 with a scan rate of 0.1 V s À1 in an argon-filled cell which consisted of a glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Infrared spectra were measured as KBr pellets using an FT-IR (Thermo Nicolet, NEXUS, TM) spectrometer. The rhenium(I) pentacarbonyl chloride was supplied by Tokyo Chemical Industry. Toluene, n-hexane, dichloromethane (CH 2 Cl 2 ), sodium acetate, silica gel, and tetrabutylammonium hexafluorophosphate (TBAPF 6 ) were supplied by Sinopharm chemical reagents company. All solvents and chemicals were used without purification except as noted. H 2 Por was synthesized according to literature procedures [25][26][27].

X-Ray analysis
X-ray crystallographic data for H 2 PY (CCDC 2158069) and H 2 PZ (CCDC 2158070) were recorded on a Bruker D8 Venture diffractometer. The crystals were kept at 213 K during data collection. Data were collected using a Bruker APEX-II CCD diffractometer, and the data reduction was performed using SAINT [v8.37A] (Bruker, 2015). SADABS-2016/2 (Bruker, 2016) was used for a multi-scan absorption correction. The wR 2 (int) for H 2 PY was 0.1470 before and 0.0923 after correction and the ratio of minimum to maximum transmission was 0.5372. The wR 2 (int) for H 2 PZ was 0.1443 before and 0.0882 after correction and the ratio of minimum to maximum transmission was 0.4755. It should be noted that the k/2 correction factor is not present for either structure. Using Olex2 [28], the structure was solved with the SHELXT [29] structure solution program using Intrinsic Phasing and refined with the SHELXL [29] refinement package using Least Squares minimization. Two disordered pentafluorophenyl groups of H 2 PY consisting of C40-C45 and F1-F5, C40A-C45A and F1A-F5A, C34-C39 and F6-F10, and C34A-C39A and F6A-F10A were restrained by appropriate instructions of SADI and SIMU during refinement. One disordered pentafluorophenyl group of H 2 PZ consisting of C37-C42 and F6-F10, and C37A-C42A and F6A-F10A was also restrained by appropriate instructions of SADI, ISOR, and SIMU during refinement. More detailed information on the analysis is provided in the CIFs (CCDC 2158069-2158070).

Ligand exchange
H 2 PY (20 mg, 0.018 mmol) and NaOAc (15 mg, 0.18 mmol) were dissolved in 10 mL toluene and stirred under refluxing conditions in a nitrogen atmosphere overnight. After removal of the solvent, the obtained solid was purified by silica gel column chromatography (n-hexane/CH 2 Cl 2 ¼ 1/2, and CH 2 Cl 2 ), the brown portion was collected to give H 2 PZ in a 61% yield (12 mg, 0.011 mmol) and the red portion was collected to recover H 2 PY starting material in a 32% yield (6.4 mg, 0.0060 mmol).

Results and discussion
Insertion of the Re(I) ion into the porphyrin(2.1.2.1) scaffold bearing pentafluorophenyl substituents at the two meso-positions, H 2 Por, was attempted with 5 eq. of Re(CO) 5 Cl in refluxed toluene. H 2 Por was purposely chosen for this study as the meso-pentafluorophenyl substituents stabilize the free base form, as has been shown previously [25][26][27]. The resulting Re(I) complex, H 2 PY, from the reaction between H 2 Por and Re(CO) 5 Cl was isolated in a 72% yield. When 10 eq. of NaOAc were added to the reaction mixture, two Re(I) complexes were isolated as shown in Scheme 1. These reaction conditions afforded H 2 PY, possessing a Re(CO) 3 Cl unit, in a 27% yield and H 2 PZ, possessing a Re(CO) 3 OAc unit, in a 38% yield. The observed replacement of the Clligand by OAcprompted us to examine the ligand exchange reaction by treating H 2 PY with excess NaOAc in refluxing toluene overnight. These reaction conditions afforded H 2 PZ in a 61% yield while the starting material, H 2 PY, was recovered in a 32% yield (Scheme S1, Supporting Information, SI). The high-resolution mass spectra (HR-MS) of both H 2 PY and H 2 PZ indicated only one monovalent rhenium ion was coordinated to H 2 Por (Figures S1 and S2, SI). The presence of CO ligands on H 2 PY and H 2 PZ was confirmed by infrared (IR) spectroscopy where the CO stretching vibrations were found to be located at 2013.25, 1893.75 cm À1 for H 2 PY, and 2013.80, 1900.02 cm À1 for H 2 PZ, values essentially identical to those observed for tricarbonylrhenium porphyrins [12]. Moreover, an additional CO stretching vibration was observed for H 2 PZ at 1731.28 cm À1 corresponding to the presence of the OAcligand in the complex. Additional noteworthy peaks were observed at ca. 3000 cm À1 for both complexes ( Figures S3 and S4, SI) and identified as N-H vibrations of the uncomplexed dipyrrin subunit on the basis of previously reported IR spectra of mono rhenium porphyrins [12].
The detailed Re(I) coordination geometries of H 2 PY and H 2 PZ were unambiguously determined by single crystal X-ray analysis ( Figure 2 and Table 1). The Re(I) coordination models of H 2 PY and H 2 PZ are similar with our reported mono-Rh(I) porphyrin(2.1.2.1) complexes ( Figure S5, SI) [27], where only one dipyrrin subunit of H 2 PY and H 2 PZ is involved in coordination. As seen in the figure, the Re(I) ion of H 2 PY is coordinated with three CO groups, one Clligand, and two imino-type N atoms of one dipyrrin. Similar coordination is observed for H 2 PZ where the only difference is that the Clligand has been replaced by OAc -. The bond distances between the a-carbons of pyrrole rings and the meso-carbons are 1.393(12), 1.377(13), 1.388(14) and 1.393(13) Å for H 2 PY, and 1.397(9), 1.374(10), 1.384(9) and 1.402(10) Å for H 2 PZ (Table 2) and indicate only small alternations, relative to free base H 2 Por [25][26][27], upon complexation of the Re(I) ion. The most important structural finding comes from the essentially equidistant Re(I)ÀN bond lengths (2.236(7) and 2.202(7) Å for H 2 PY or 2.210(6) and 2.194(5) Å for H 2 PZ), an observation which is in sharp contrast to that observed for monometallic tricarbonylrhenium(I) porphyrins which show Re(I)ÀN bond lengths that differ by $0.150 Å (Table 2, Figure S6, SI) [31]. The observation of the essentially  identical Re À N bond distances in the structures of H 2 PY and H 2 PZ suggest two p-bonds between Re(I) and N atoms of the dipyrrin stemming from the donation of an electron pair from the nitrogen atom into an empty d orbital of the Re(I) [31][32][33][34]. This feature, and the two equidistant Re(I)ÀN distances of H 2 PY and H 2 PZ, are more analogous to that of Re(I) bipyridine complexes containing two Re(I)ÀN p-bonds having essentially identical bond distances ( Figure S6, SI). Therefore, the characteristic Re(I)ÀN bonds distances found in these new complexes strongly suggest that the dipyrrin subunit of H 2 Por acts as bipyridyl-like ligand when coordinating Re(CO) 3 -L (L ¼ Clor OAc -) [8][9][10][31][32][33][34]. On the other side of the molecule, the two uncoordinated pyrrole units form a non-planar structure where the angle between the planes of the two pyrroles are 16.3 and 33.5 for H 2 PY and H 2 PZ, respectively. The nonplanarity of this dipyrrin subunit likely can be ascribed to the steric repulsion between the two N-H atoms which are separated by 1.960 and 2.103 Å for H 2 PY and H 2 PZ, respectively ( Figure 2 and Table 2). Moreover, the Clligand of H 2 PY and two oxygen atoms of the OAcligand of H 2 PZ exhibit hydrogen bonding with two pyrrolic N À H atoms ( Figure S7, SI). The cavity shapes of the porphyrin(2.1.2.1) scaffold still keep their rectangular shape, similar to that observed for the free base derivative, H 2 Por (Figure 2 (Figure 2 and Table 2) [25][26][27]. This N À N distances character indicated the octahedral coordination geometry of Re(I), relative to the square planar coordination geometry of nickel(II) [25], palladium(II) [25], platinum(II) [25] and copper(II) ( Figure S5, SI) [26]. Given the results of the X-ray analysis discussed above, it is important to address a resonance structure that would afford such observations. All previously reported metalloporphyrin(2.1.2.1) complexes have been shown to possess typical r-bonds between pyrrole nitrogen atoms and chelated metal ions [25][26][27]; however, as evidenced by the equidistant Re(I)-N distances which are more comparable to rheniumbipyridine derivatives, the currently investigated Re(I) complexes appear to have only p bonds between the metal ion and porphyrin(2.1.2.1) ligand. As shown in Scheme 2, several resonance structures are possible; however, the contribution of the tautomer D is likely negligible as this mesomeric form requires macrocyclic aromaticity and previous studies have shown porphyrin(2.1.2.1) derivatives to be nonaromatic molecules possessing two separate, discrete dipyrrin-systems [25,26]. Instead, the resonance of zwitterionic structures B and C is more likely. Further support for the presence of Re(I)ÀN p-bonding in H 2 PY and H 2 PZ comes from orbital analyses by DFT calculations (Figures 3 and S8, SI) [35]. In the highest occupied molecular orbitals (HOMOs) of H 2 PY and H 2 PZ, antibonding d-p interactions were observed and the corresponding d-p interactions appeared in HOMO-5 and HOMO-2 of H 2 PY and H 2 PZ, respectively [35].
To further prove the existence of the two NH protons on the uncoordinated dipyrrin unit, 1 H NMR spectra of free-base H 2 Por and two Re(I) complexes, H 2 PY and H 2 PZ, were compared as shown in Figure 4. As seen in the figure, H 2 Por shows the expected two NH protons at 12.19 ppm, a resonance which is shifted far downfield as compared to that observed for porphyrin NH protons due to the nonaromatic nature of such porphyrin(2.1.2.1) molecules [25,26]. As compared to the chemical shift for the two NH protons in the free base derivative (H 2 Por), H 2 PY and H 2 PZ also show resonances in this region which integrate to give two protons but are more downfield shifted. These signals (at 13.09 H 2 PY and 13.97 ppm H 2 PZ) are assigned to the two NH protons on the uncoordinated dipyrrin unit and the shift of these protons to a lower field is ascribed to the secondary H À Cl and H À O hydrogen bonding interactions induced, consistent with X-ray crystal analysis (Figures 4, S7, S9 and S10, SI). This data can be compared to the reported mono-Rh(I) complex of porphyrin(2.1.2.1) which displayed a resonance at 11.96 ppm corresponding to only one NH proton on the uncoordinated dipyrrin [27], further confirming that the two Re(I) complexes possess two NH protons. Moreover, the 1 H NMR spectra of the two unsymmetrical rhenium complexes displayed four sets of peaks from 6.6 to 5.8 ppm corresponding to the b-protons of pyrrole protons and can be compared to the two signals observed for the same protons of H 2 Por. The 13 C NMR spectra ( Figures S11 and S12   field signal corresponding to the CO groups and the higher-field chemical shift to the OAcligand on H 2 PZ. To investigate the impact of the unique Re(I) coordination geometries, the optical and electronic properties of H 2 PY and H 2 PZ were investigated with the help of theoretical calculations. The optical absorption profiles of H 2 PY and H 2 PZ showed more red-shifted absorptions than the free base H 2 Por (Figure S15, SI). The major bands of H 2 PY and H 2 PZ are located at 486 and 483 nm, respectively, and the absorption edges are located around 650 nm. The HOMO-LUMO gap for H 2 Por was obtained as 2.97 eV by DFT calculation [26,27]. The HOMO-LUMO gaps of H 2 PY and H 2 PZ were 2.10 and 1.98 eV, which are remarkably smaller than that of H 2 Por. This result is supportive for the red-shifted absorption spectra of the two complexes. The time-dependent DFT (TD-DFT) calculations of H 2 PY and H 2 PZ were carried out in a vacuum and predicted that the absorption spectra of the two Re(I) complexes have intramolecular charge transfer (ICT) transitions arising from donor-acceptor characteristics where the coordinated dipyrrin moiety serves as a donor and the non-coordinated dipyrrin unit acts as an acceptor in both monometallic Re(I) complexes (Figures S8, S16 and S17; Tables S1 and S2, SI) [8][9][10]. This property is best described by the frontier molecular orbitals of H 2 PY and H 2 PZ which indicate that the HOMO is localized on the Re(I)-dipyrrin moiety while the LUMO is localized on the non-coordinated dipyrrin. The discrete localization of these two orbitals is also consistent with our previous work regarding the detailed electrochemistry of copper porphyrin(2.1.2.1) derivatives, which indicated that complexes of this type are nonaromatic and possess two separate dipyrrin p systems [26]. Moreover, TD-DFT calculation for H 2 PY predicted that the absorption at 753.25 nm is composed of the HOMO-1 to LUMO and HOMO to LUMO transition (oscillator strength, f ¼ 0.0002). This predicted ICT absorption for H 2 PY is similar to that predicted for the Rh(I) complex of porphyrin(2.1.2.1) [27].
To further investigate the electronic properties of H 2 Por, H 2 PY and H 2 PZ, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed in CH 2 Cl 2 containing 0.1 M TBAPF 6 and the results are shown in Figures 5 and S18. The free-base H 2 Por has three oxidations at 1.62, 1.39 and 1.15 V and three reductions at À1.14, À1.36 and À1.58 V versus SCE ( Figure S18, SI). This behavior is quite different from that observed for H 2 PY and H 2 PZ which, due to their similar structures, display quite similar voltammograms. As seen in Figure 5, the first reduction of either rhenium(I) complex is much more facile (E 1/2 ¼ À0.35 V for H 2 PY and E 1/2 ¼ À0.40 V for H 2 PZ) than the first reduction of the free base derivative. The facile electron addition to the two Re(I) complexes is consistent with the reduction of the positively charge uncoordinated dipyrrin unit. The 0.74-0.79 V negative shift in the reduction potential is similar to the differences between the first reduction of free base porphyrins and their protonated forms [36]. Moreover, the first electron addition to the protonated uncoordinated dipyrrin unit is consistent with DFT calculations which revealed that the LUMO is localized on this portion of the molecule. The second reductions of both Re(I) complexes are quasi-reversible and proposed to occur at the metal-center where ligand dissociation following electron addition occurs as has been seen for other quasi-reversible Re(I)/Re(0) reductions [37,38]. Moreover, the first reversible oxidations of the Re(I) derivatives are located at E 1/2 ¼ 1.20 and 1.18 V for H 2 PY and H 2 PZ, respectively. The potentials corresponding to the first reversible electron abstraction from H 2 PY and H 2 PZ are slightly shifted from those reported for monometallic rhenium tricarbonyl tetraphenylporphyrins, [(CO) 3 Re](HTPP) [39,40], due to the electron-withdrawing mesopentaflurophenyl on the currently investigated compounds. Nonetheless the first oxidation of [(CO) 3 Re](HTPP) was proposed to be metal-centered and this is also proposed here given the findings from our DFT calculations. Overall, the difference between the first reversible reduction and first reversible oxidation gives an electrochemically measured HOMO-LUMO gaps of 1.55 and 1.58 V. These values are smaller than those found in the theoretical calculations, an observation attributed to both solvation effects and the previously observed fact that calculations at the B3LYP level underestimate the HOMO-LUMO energy gap of the porphyrin-like compounds [41,42].

Conclusion
Two new Re(I) complexes were synthesized from a porphyrin(2.1.2.1) scaffold. Results showed that H 2 PY can be converted to H 2 PZ by a ligand exchange reaction. The 1 H NMR spectra, crystal structures, and DFT calculations revealed that the two isolated complexes contain a Re(I) ion coordinated by two Re À N p-bonds with a dipyrrin subunit in the porphyrin(2.1.2.1) moiety. To the best of our knowledge, these are the first cases of porphyrinoid macrocycles acting as bipyridyl-like ligands for the complexation of metal cations. The theoretical calculations of H 2 PY and H 2 PZ predicted an ICT absorption due to a donor-accepter system in which the coordinated Re(I)-dipyrrin moiety serves as a donor and the non-coordinated dipyrrin unit which acts as an acceptor. These findings were consistent with the electrochemical results which reaffirmed the presence of a protonated, non-coordinated dipyrrin subunit. The Re(I) coordination revealed in this work along with the results on porphyrin(2.1.2.1) complexes in our previous work results highlight the unique nature of this bent tetrapyrrolic chelating ligand as an outstanding platform to investigate new structure property relationships.

Disclosure statement
No potential conflict of interest was reported by the authors.