Subphthalocyaninato Boron(III) Hydride: Synthesis, Structure and Reactivity

Abstract Subphthalocyanine (SubPc) chemistry has been limited so far by their high sensitivity toward strong nucleophiles. In particular, the substitution of the axial chlorine atom by a nucleophilic group in the case of less‐reactive SubPcs, such as those bearing electron‐withdrawing peripheral substituents, presents some limitations and requires harsh conditions. By taking advantage of the electrophilic character of DIBAL‐H, it has been possible to prepare for the first time SubPc‐hydride derivatives that exhibit high reactivity as hydroboration reagents of aldehydes. This hydride transfer requires using a typical carbonyl activator (trimethylsilyl triflate) and only one equivalent of aldehyde, affording SubPcs with an axial benzyloxy group in good yield. This transformation has proven to be a useful alternative method for the axial functionalisation of dodecafluoroSubPc, a paradigmatic SubPc derivative, by using electrophiles for the first time. Considering the increasing interest in SubPcs as electron‐acceptor semiconductors with remarkable absorption in the visible range to replace fullerene in organic photovoltaic (OPV) devices, it is of the utmost importance to develop new synthetic methodologies for their axial functionalisation.


General methods
Chemical reagents were purchased from Aldrich or TCI Europe and were used without further purification. Synthetic grade solvents were used for chemical reactions and column chromatography purifications and anhydrous grade for reactions under dry conditions. Starting SubPcs 1a, 1b, 1c and 1d were prepared by previously described methods. 1 The monitoring of the reactions has been carried out by thin layer chromatography (TLC), employing aluminium sheets coated with silica gel type 60 F254 (0.2 mm thick, Merck). The analysis of the TLCs was carried out with an UV lamp of 254 and 365 nm. Purification and separation of the synthesized products was performed by normal-phase column chromatography, using silica gel (230-400 mesh, 0.040-0.063 mm, Merck). Eluents along with the relative ratio in the case of solvent mixtures are indicated for each case.
Melting points were measured in open-end capillary tubes by using a Büchi B-540 apparatus and are uncorrected.
Nuclear magnetic resonance spectra ( 1 H-, 13 C-, 19 F-and 11 B-NMR) were recorded on a Bruker AV-300 or a Bruker DRX-500 spectrometer either in the Organic Chemistry Department or in the

S3
Interdepartmental Investigation Service of UAM. Deuterated solvent employed in each case is indicated in brackets, and its residual peak was used to calibrate the spectra using literature reference δ ppm values. 2 Mass spectra (MS) and high-resolution mass spectra (HRMS) were recorded in the Interdepartmental Investigation Service of UAM, employing Atmospheric Pressure Chemical ionization (APCI) using a Bruker-MAXIS II or Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF), using Bruker-Ultraflex-III spectrometer, with a Nd:YAG laser operating at 355 nm. The matrixes and internal references employed are indicated for each spectrum.
Ultraviolet and visible (UV-Vis) spectra were recorded using solvents in the spectroscopic grade in the Organic Chemistry Department of UAM employing a JASCO-V660 spectrophotometer. The logarithm of the molar extinction coefficient (ε) is indicated in brackets for each maximum. Likewise, fluorescence measurements were carried out with a JASCO-V8600 spectrofluorometer. Fluorescence quantum yields (фF) of SubFlcs were determined in toluene and calculated by using the following equation: 3 Scripts R and S indicate reference and sample, respectively. Grad is the gradient from the plot of the integrated fluorescence intensity (at exc. λ= 520 nm) versus the absorption (at the same wavelength), and η is the refractive index of the solvent. Chloro-dodecafluoroSubPc (Cl-F12SubPc) in benzonitrile (фF = 0.58) was used as reference. 4 Infrared spectra were recorded in the Interdepartmental Investigation Service of UAM on a Bruker IFS 66v vacuum FT-IR spectrometer.
Electrochemical measurements were performed in the Organic Chemistry Department of UAM on an Autolab PGStat 30 equipment using a three-electrode configuration system. The measurements were carried out in argon saturated THF solutions containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6). A platinum electrode (3 mm diameter) was used as the working electrode, and a platinum wire and a Ag/AgNO3 (0.01 M in acetonitrile) electrode were employed as the auxiliary and the reference electrode, respectively. Fc was used as an external reference and all the potentials were given relative to the Fc/Fc + couple.
Single-crystal X-ray diffraction data collection for structure determinations were collected in the Interdepartmental Investigation Service of UAM at Bruker KAPPA APEX II CCD area-detector X-ray diffractometer operating with graphite-monochromated and Mo Kalpha radiation (λ = 0.71073 Å). The data are absorption corrected with the program SADABS. Intensities are calculated with the SAINT software, which also incorporates polarization and Lorentz effect corrections. The structures were solved and refined using the Bruker SHELXTL Software Package.

Experimental procedures and characterization of
SubPc 1b (50 mg, 0.062 mmol) was dissolved in freshly distilled toluene (1.5 ml) under an argon atmosphere. The solution was degassed by freeze-pump-thaw method. After that, the reaction mixture was cooled down at -55 °C, and then DIBAL-H was added dropwise (1 M in THF, 0.62 ml). The reaction mixture was stirred for 3 min and filtered through a short silica plug.

Boron (III) Subphthalocyanine hydride 2c
SubPc 1c (50 mg, 0.062 mmol) was dissolved in freshly distilled toluene (1.5 ml) under an argon atmosphere. The solution was degassed by freeze-pump-thaw method. After that, the reaction mixture was cooled down at -55 °C, and then DIBAL-H was added dropwise (1 M in THF, 1.9 ml, 30 eq ). The reaction mixture was stirred for 3 min and filtered through a short silica plug.

Boron (III) Subphthalocyanine hydride 2d
SubPc 1d (100 mg, 0.232 mmol) was dissolved in freshly distilled toluene (3ml) under an argon atmosphere. The solution was degassed by freeze-pump-thaw method. After that, the reaction was kept at room temperature and then DIBAL-H was added dropwise (1 M in THF, 4.7 ml). The reaction mixture was stirred for 40 min and filtered through a short silica plug. The solvent was removed under vacuum to yield SubPc 2d as a pink solid. (1.9 mg, 2%) S11 Figure S11. (exc. λ = 520 nm).

Experiment of decomposition of SubPc 3f by TMSOTf.
Scheme S1. Decomposition reaction of SubPc 3f by TMSOTf SubPc 3f (10 mg, 13.4 μmol) and trimethylsilyl trifluoromethanesulfonate (10 mol%) were dissolved in dry toluene-d8 (1 ml) under argon atmosphere. The reaction mixture was stirred at 50 °C and the complete disappearance of 3f was observed by TLC.          The reaction would be initiated by the coordination of the aldehyde to TMSOTf and then the hydride transfer from 2a to I1 would take place originating the corresponding silyl ether and OTf-SubPc. 8 Then, a transmetalation between OTf-SubPc and the silyl ether would yield 3 and the regeneration of TMSOTf.
1.4.2.2. Experiment to study the feasibility of the transmetalation step: SubPc 4 (13.5 μmol) and trimethylsilyl benzyl ether (13.7 μmol) were dissolved in dry toluene (1 ml) under argon atmosphere. The reaction mixture was stirred at 60 °C and the reaction progress monitored by TLC. After 24h, no consumption of 4 was detected. 9

Computational study
All the stationary points were optimized in the frame of Density Functional Theory (DFT), using the wB97X-D 10 functional and the def2svp 11 basis set. This functional was selected due to its good performance for treating similar systems 12 in part due to the inclusion of empirical dispersion, and also analogous reaction mechanisms. For each optimized structure, analytic harmonic frequencies were computed at the same level of theory to confirm the nature of the stationary points. Furthermore, transition states were connected with reactants and products, when possible, via Intrinsic Reaction Coordinate (IRC) calculations. Solvent (toluene) effects were also taken into account through the implicit Polarizable Continuum Model (PCM). 13 All the calculations were performed using the Gaussian09 program. 14

Reactants
Reverse IRC from the 3mTS ends in a stationary point in which both reactants are oriented along our reaction coordinate. However, due to the large size of our system, and its flexibility for instance on the relative orientation of the aldehyde and the SubPc, we have checked that our considered reaction coordinate is the most favoured path in solution. Relaxed scan along the  dihedral ( Figure S48) and the C-H distance confirmed that: 1) our reactants, where the aldehyde is stacked with the SubPc (~170°) and with a distance ~3 Å , are the most stable arrangement, 2) they are connected to the products through a TS characterized by a ~1.7 Å C-H distance and further revealed that 3) if the aldehyde is displaced towards more perpendicular arrangement (~100°) the energy barrier between reactants and products decreases, but this rotation also requires some energy. The resulting optimized geometry is characterized by an imaginary frequency (-262 cm -1 ) wherein the B-O bond and the B-H bond, respectively, form and break ( Figure S49). Figure S49. Displacement vectors associated to the imaginary frequency (-262 cm -1 ) characterizing the optimized constrained geometry of 4mTS c

Mechanism through 3mTS
The geometries for the optimized reactants and products are depicted in Figure S50a and c. The transition state 3mTS ( Figure S50b) is characterized by a single imaginary frequency (-336 cm -1 ) wherein the B-H bond breaks simultaneously to the formation of the C-H bond. At the same time, the C-O bond distance increases, as expected due to the rehybridization of the C atom. The connection between this 3mTS and reactants and products was confirmed performing IRC calculations. To get further insight on the larger stability of 3mTS with respect to 4mTS c , we have computed at the corresponding geometries the associated isodensity surface colour-coded with ESP and charges. As inferred from Figure S51, the boron atom in the 4mTS c has a significantly higher (~2.5 fold) positive charge compared to that of 3mTS, that would be stabilized by the presence of neighbour electron-rich atoms. However, the difference of the oxygen ESP charge between both TSs is small (+18%), because of its coordination to the trimethylsilyl fragment, suggesting that is not enough to achieve the boron-stabilization. At this point, the lowest energy pathway is, first, the formation of C-H thus the oxygen becomes more electron-rich and then, the formation of the B-O bond. In addition, we also believe that the steric hindrance provided by the trimethylsilyl group could hinder the stabilization of 4mTS c .
To further support this hypothesis, the optimization of 3mTS and 4mTS without trimethylsilyl fragment, i.e. the benzaldehyde hydroboration without activation, were performed (3mTS´and 4mTS´). Interestingly, unlike the previous case, only the four-member transition state 4mTS´ was located ( Figure S51), being characterized by a single imaginary frequency where the B-O and C-H bond are simultaneously formed. Contrary, attempts to optimize the 3mTS´ were unsuccessful even in the constrained optimization cases. The electrostatic potential map and charges of this 4mTS´ ( Figure S52) show an even larger positive character of the Boron atom (1.848) but in this case the electronic density in the oxygen atom also increases significantly (+70%) compared with that of 4mTS c , stabilizing enough the interaction, in agreement with our hypothesis.