Synthesis of tetraphenylporphyrinate manganese(III) siloxides by silyl group transfer from silanethiols

Abstract Reaction of [Mn(OAc)(TPP)] (TPP = dianion of meso-tetraphenylporphine) with both HSSiiPr3 and HSSiPh3 in the presence of air leads to formation of the corresponding siloxide complexes, [Mn(OSiR3)(TPP)] (R = iPr, Ph), via silyl group transfer from S to O. The new compounds have been fully characterized in solution and the solid state and represent rare examples of Mn(III) porphyrinates containing Lewis basic axial ligands. X-ray crystallographic analyses of both complexes reveal very short Mn–O bond distances consistent with the presence of a siloxide ligand. Investigations of the reaction pathway are consistent with initial reduction of [Mn(OAc)(TPP)] to [MnII(TPP)] by the silanethiol. Subsequent aerobic oxidation of the reaction mixture is proposed to generate a Mn(III) porphyrinate and the corresponding silanol, which combine to yield the observed siloxide complex. These findings stand in sharp contrast to those of iron(III) porphyrinates, where silanethiolate complexes are stable.


Introduction
The chemistry of manganese in porphyrin and porphyrinoid ligand scaffolds has been exploited to both model iron-containing heme proteins and to develop new synthetic methodologies, especially those involving oxidation of C-H bonds [1][2][3][4][5][6][7][8][9][10][11][12]. Despite this intense interest, the coordination chemistry of sulfur-containing ligands with manganese pophyrinates is underdeveloped. This scarcity is somewhat surprising given the important role that thiolate ligands play in shaping the electronic structure and reactivity of iron porphyrinates [13][14][15]. The only prior work in this area is that by Mu and coworkers who reported the synthesis and spectroscopic characterization of arylthiolate-containing Mn(III) porphyrinates, [Mn(SPh)(TPP)] (TPP = meso-tetraphenylporphyrinate) and [Mn(SC 6 H 4 -p-NO 2 )(TPP)] [16][17][18]. These complexes were prepared by addition of methanolic solutions of the corresponding sodium thiolate salts to [MnCl(TPP)]. No crystal structures for these complexes were reported and the authors noted that each compound was sensitive to reduction to manganese(II) in the presence of Lewis bases. Along these same lines, yu and coworkers reported that Mn(III) porphyrinates could serve as catalysts for the oxidative dimerization of thiols [19].
The propensity toward reduction of porphyrin-bound Mn(III) thiolate complexes is reminiscent of the reactivity displayed by the analogous fe(III) species, [fe(SR)(por)] [20]. In an effort to suppress this reduction chemistry, we recently reported the synthesis and characterization of a series of fe(III) porphyrinates containing silanethiolate ligands (scheme 1) [21]. This class of thiolate ligand possesses unique electronic and steric properties that permits isolation and detailed study of the corresponding fe(III) porphyrinate complexes. Given this success with iron, we were curious as to whether the silanethiolate ligand could be used in manganese chemistry to further expand the scope of known sulfur compounds [22]. In this contribution we describe our attempts to prepare silanethiolate complexes of Mn(III) tetraphenylporphyrinate. unexpectedly, reaction of silanethiols with [MnX(TPP)] (X = Cl, OAc) did not result in formation of the desired silanethiolate species, but rather isolation of the corresponding siloxide complex via silyl group transfer. The solid-state and solution characterizations of these siloxide compounds are reported as are investigations into their routes of formation.

Preparation of siloxide complexes
In similar fashion to our previously published approach to iron(III) porphyrinates containing silanethiolate ligands, we reasoned that a protonolysis route to the putative [Mn(SSiR 3 )(TPP)] complexes would be most effective in mitigating reduction to manganese(II). Selecting a starting material posed a challenge with this system, however, as manganese(III) porphyrinates containing anionic oxygen ligands are not readily available. The simplest such compound, [Mn(OH)(TPP)], has been reported, yet neither a solid-state structure nor a reliable synthesis exists [23][24][25][26][27]. Attempts to prepare [Mn(OH)(TPP)] in our hands proved difficult, and material could not be isolated in high purity as it was always accompanied by the presence of Mn(IV) impurities. Routes to the related methoxide species, [Mn(OMe)(TPP)], are likewise complicated, in this case due to reduction to manganese(II) [24]. Given the limited number of trivalent manganese porphyrinates containing Brønsted basic axial ligands, we elected to employ the acetate complex, [Mn(OAc)(TPP)].
Stoichiometric reaction of [Mn(OAc)(TPP)] with the silanethiol, HSSi i Pr 3 , under ambient conditions proceeded to a new compound after 12 h as judged by 1 H NMR and uV-vis spectroscopy. Surprisingly, this new species was not the desired thiolate complex, [Mn(SSi i Pr 3 )(TPP)], but rather the siloxide species, [Mn(OSi i Pr 3 )(TPP)] (scheme 2). The formation of this siloxide complex indicates that a silyl group transfer has occurred from the silanethiol sulfur to an oxygen bound to manganese. Whether the oxygen originates from ambient oxygen or water is not known at present, but control experiments indicate that air is an essential component of the reaction mixture (vide infra). Reactions performed in this fashion did not go to completion even in the presence of excess HSSi i Pr 3 (> 2 eq.) as judged by the observation of a persistent resonance for [Mn(OAc)(TPP)] in the 1 H NMR spectrum of crude material. Nonetheless, the compound could be isolated in sufficient purity by recrystallization from pentane. Silyl transfer reactions Scheme 1. synthesis of silanethiol complexes of iron(iii) porphyrinates. between S and O are known for several organic compounds, but examples involving transition metal complexes are scarce [28][29][30][31][32][33][34].
Consistent with other Mn(III) meso-tetraarylporphyrinates, the 1 H NMR spectrum of [Mn(OSi i Pr 3 ) (TPP)] features significantly broadened peaks, with a resonance for the pyrrolic hydrogens of the porphyrin ring appearing upfield of 0 ppm (see Supplementary Information) [35]. Notably, this pyrrolic resonance appears at −8.6 ppm, which is greater than 10 ppm downfield from the corresponding resonance in [Mn(OAc)(TPP)]. The arylthiolate-bound Mn(III) porphyrinates reported by Mu and coworkers were reported to display pyrrolic resonances below −20 ppm, providing further evidence for coordination of oxygen versus sulfur in the present compounds [17]. The electronic absorption spectrum of [Mn(OSi i Pr 3 )(TPP)] in toluene displays an apparent split Soret band with a broad pre-Soret feature (see Supplementary Information). Although the complexity of the spectrum suggests the presence of possible impurities, repeated syntheses produced identical spectra even when an alternate preparative route was employed (vide infra).
Crystals of [Mn(OSi i Pr 3 )(TPP)] suitable for X-ray diffraction were grown by slow evaporation from pentane. The solid-state structure displayed in figure 1 confirms the presence of the siloxide group, with a short Mn-O bond distance of 1.918(2) Å. To the best of our knowledge, this distance represents the shortest Mn-O contact of any known Mn(III) porphyrinate with the exception of Valentine's η 2 -bound peroxo anion [36]. Moreover, [Mn(OSi i Pr 3 )(TPP)] is one of only a handful of structurally characterized manganese(III) porphyrinates containing basic oxygen-bound axial ligands [37][38][39][40][41].
To provide additional evidence for formation of [Mn(OSi i Pr 3 )(TPP)], the complex was prepared directly from NaOSi i Pr 3 and [MnCl(TPP)] (scheme 3). The reaction was performed under an inert atmosphere due to the moisture sensitivity of NaOSi i Pr 3 . The 1 H NMR spectrum of the resulting material matched well with that observed from the reaction of HSSi i Pr 3 with [Mn(OAc)(TPP)]. High-resolution mass spectra of material prepared by this method displayed a prominent peak for the parent ion consistent with [Mn(OSi i Pr 3 )(TPP)] (see Supplementary Information).
Given that salt metathesis proved a successful means of preparing [Mn(OSi i Pr 3 )(TPP)], we also investigated the analogous procedure with NaSSi i Pr 3 in a further attempt to prepare [Mn(SSi i Pr 3 )(TPP)]. Such a strategy is more akin to that originally utilized by Mu to prepare [Mn(SPh)(TPP)] [17]. upon addition of NaSSi i Pr 3 to [MnCl(TPP)], however, rapid reduction to [Mn II (TPP)] was observed by uV-vis spectroscopy. Therefore, the putative silanethiolate complex of Mn III (TPP) appears unstable with respect to reduction (scheme 4).
The reaction of [Mn(OAc)(TPP)] and HSSiPh 3 was examined to determine if this silyl group transfer could be observed with a different silanethiol (scheme 5). Material isolated from the reaction demonstrated a 1 H NMR spectrum with a pyrrolic resonance downfield from that of the acetate, similar to

Silyl group transfer pathway
Having confirmed that the reaction of [Mn(OAc)(TPP)] with both HSSi i Pr 3 and HSSiPh 3 forms the corresponding siloxide complex, we next sought to better understand the silyl group transfer. Control reactions of [Mn(OAc)(TPP)] with the silanethiols under nitrogen established that reduction to the divalent manganese porphyrinate occurs as an initial step with presumed formation of the silyldisulfide, (R 3 Si) 2 S 2 (scheme 6). The formation of [Mn II (TPP)] can be followed conveniently by uV-vis spectroscopy owing to a sharp Soret feature at 23,000 cm −1 [42,43]. exposure of the [Mn II (TPP)] solution to air leads immediately to uV-vis absorbances consistent with oxidation back to Mn(III) porphyrinate species [44].
In order to establish that air is a necessary component of the reaction, a control reaction was performed with [Mn II (TPP)] and the silyldisulfide, ( i Pr 3 Si) 2 S 2 , under an atmosphere of nitrogen. under these conditions, no reaction was observed. upon exposure to the ambient atmosphere, however, uV-vis absorbances consistent with [Mn(OSi i Pr 3 )(TPP)] were observed to form.
Given our findings, silyl group transfer appears most likely the result of reactivity between an oxygen-bound Mn(III) porphyrinate and the corresponding silyldisulfide. The first step of the process involves reduction of [Mn(OAc)(TPP)] by silanethiol to give [Mn II (TPP)] and (R 3 Si) 2 S 2 . We then propose that aerobic oxidation of [Mn II (TPP)] affords a putative Mn(III) species, such as [Mn(OH)(TPP)]. In concert with oxidation of the manganese center, the silyldisulfide is converted to a mixture of the corresponding silanol and the thiosulfate in similar fashion to that reported by Pladzyk and coworkers for the related tri-tert-butoxysilyldisulfide [34]. The in situ generated silanol then reacts with the Mn(III) porphyrinate to produce the corresponding Mn(III) siloxide complex (scheme 7). The driving force for this process is likely the much stronger Si-O versus Si-S bond strength [45,46].

Conclusion
In this contribution, we have described the synthesis and characterization of the first examples of siloxide-containing Mn(III) porphyrinates. The complexes are prepared via a silyl group transfer from sulfur to oxygen upon treatment of [Mn(OAc)(TPP)] with R 3 SiSH (R = i Pr or Ph) in the presence of air. The siloxide complexes feature very short Mn-O bonds consistent with the pronounced Lewis basicity of the anionic R 3 SiO − unit. Investigations into their formation suggest that initial reduction of Mn(III) by the silanethiols produces [Mn II (TPP)], which reacts with oxygen in the presence of (R 3 Si) 2 S 2 to afford the siloxide complexes. Consequently, the chemistry of Mn III (TPP) with silanethiols appears to differ significantly from that of fe III (TPP), where silanethiolate complexes are both stable and isolable.

General comments
Manipulations requiring the exclusion of air and moisture were performed under an atmosphere of nitrogen gas in a Vacuum Atmospheres glovebox. Tetrahydrofuran, toluene, and pentane were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves. Benzene and benzene-d 6 were sparged with nitrogen and stored over 4 Å molecular sieves prior to use. 1 H NMR spectra were recorded on a Varian spectrometer operating at 500 MHz ( 1 H) and referenced to the residual proton resonance of benzene-d 6 (δ 7.16 ppm versus TMS). Due to the paramagnetism of each complex, all NMR resonances appeared as broad singlets. uV-vis spectra were recorded in toluene on a Cary-60 spectrophotometer in air-tight Teflon-capped quartz cells. High-resolution mass spectra were recorded using atmospheric pressure chemical ionization (APCI) in negative ion mode on a finnigan TSQ 70/700 triple quadrupole mass spectrometer.

Crystallography
Crystals suitable for X-ray diffraction were mounted in Paratone oil onto a glass fiber. Diffraction data for [Mn(OSiR 3 )(TPP)] were collected using a Rigaku AfC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å) at 293(2) K (R = Ph) or 98(2) K (R = i Pr). Low temperature data collection was accomplished with a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. Data collection and unit cell refinement were performed using Crystal Clear software [47]. Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR [48], respectively. All structures were solved by direct methods and refined on F 2 using full-matrix, least-squares techniques with SHeLXL-97 [49,50]. All carbon bound hydrogen positions were determined by geometry and refined by a riding model. Crystallographic data and refinement parameters for each structure can be found in the Supplementary Material.

Materials
HSSi i Pr 3 , HSSiPh 3 , and HOSi i Pr 3 were obtained from commercial sources and used as received. [MnX(TPP)] (X = Cl, OAc) was prepared according to literature procedures [51]. NaOSi i Pr 3 , NaSSi i Pr 3 , and ( i Pr 3 SiS) 2 were prepared as described below. [Mn II (TPP)] was prepared by reduction of [MnCl(TPP)] with excess Zn powder in THf.

NaOSi i Pr 3
In the glovebox, 100 μL (504 μmol) of HOSi i Pr 3 was added to 2 mL of THf. To the resulting solution was added 11.8 mg (482 μmol) of NaH. The reaction mixture was allowed to stir at room temperature for 2 h. After this, all volatiles were removed under reduced pressure. The remaining residue was dissolved in 5 mL of pentane and evaporated under reduced pressure to afford 78.4 mg (99% yield) of white solid. The material was used in subsequent reactions without purification.

( i Pr 3 Si) 2 S 2
In a modified literature procedure [52], a vial was charged with 115 μL (536 μmol) of HSSi i Pr 3 , 75 μL (540 μmol) of et 3 N, and 1 mL of toluene. After briefly stirring the contents of the solution, 63.8 mg (503 μmol) of I 2 was added as solid in one portion. The resulting mixture was allowed to stir at room temperature for 15 min during which time a light brown solid appeared. The mixture was filtered through a pad of Celite to afford a pale brown solution. The solution was passed through a plug of silica gel eluting with additional toluene to afford a colorless solution. The solution was evaporated to dryness under reduced pressure and the resulting residue was re-dissolved in pentane. Removal of the pentane afforded 79.1 mg (45% yield) of a colorless solid.

Supplementary material
Additional spectra, crystallographic details, and crystallographic information files (cif ).