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Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, ND 58202, USA
Author to whom correspondence should be addressed.
Catalysts 2023, 13(4), 665;
Submission received: 27 February 2023 / Revised: 22 March 2023 / Accepted: 27 March 2023 / Published: 29 March 2023
(This article belongs to the Special Issue The Role of Catalysts in Functionalization of C-H and C-C Bonds II)


Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

1. Introduction

There has been an emerging shift towards first-row transition metals in catalysis [1,2,3,4]. Conventionally, the precious metal-based catalysts, represented by palladium, have been a dominating force, particularly in the pharmaceutical industry. However, the toxicity and the increasing scarcity of these elements have raised health and sustainability concerns and stimulated the search for alternative catalytic systems. Consequently, first-row metals such as Fe [5,6,7,8], Cu [9,10], and Zn [11,12,13,14] have attracted growing interest in various catalytic reactions due to their abundance and low toxicity. The distinct reactivity of the first-row metals may enable new transformations inaccessible with the second- and third-row metals. As one of the most abundant transition metals, Mn has seen a recent resurgence of research beyond traditional oxidation/oxygenation catalysis [15,16]. A wide variety of chemical transformations have been achieved with manganese catalysis, such as hydrogenation of alkenes [17], CO2, esters, ketones, nitriles [18,19,20,21,22,23,24], hydroboration of carbonyls, nitriles carboxylic acids and CO2 [25,26,27,28,29,30], electrochemical hydrogen production [31], dehydrogenative olefination of alkyl-substituted heteroarenes and sulfones with alcohols [32,33,34,35], alkylation of nitriles, esters, and amides [36,37], dehydrogenation of alcohols [38,39,40] and its application in the synthesis of pyrroles and imides from amines and diols [41,42], and dehydrogenative cross-coupling of hydrosilanes and alcohols [43,44,45,46].
Among these developments, manganese-catalyzed hydrosilylation occupies a special place, thanks to the early work by the Cutler group, showing that simple manganese carbonyl complexes were effective catalysts for the hydrosilylation of various unsaturated substrates [47,48,49,50,51,52]. Hydrosilylation of C=O-containing compounds represents a mild and versatile approach for the reduction of these compounds and various metal and nonmetal-catalyzed reactions have been reported using commercial hydrosilane reagents [53,54,55,56]. Not surprisingly, recent research in manganese also provides a wide range of catalysts for the hydrosilylation of aldehydes, ketones, esters, amides, and carboxylic acids [57,58,59,60,61,62,63,64,65,66,67,68,69,70]. Efficient hydrosilylation of C=C double bonds by manganese catalysts has also been reported [71,72]. A few representative examples of manganese (pre)catalysts for hydrosilylation are shown in Scheme 1. Although a direct comparison of the catalytic activity of these catalysts across different groups is difficult due to the varied reaction conditions and the choice of substrates, a brief summary of typical carbonyl substrates is compiled in Table S1.
Metals supported by salen ligands have been widely reported as catalysts in different transformations. Our previous work has shown a manganese salen complex is highly efficient in catalyzing the hydrosilylation and hydroboration of carbonyl groups with high yields and broad tolerance of functional groups [28,68]. Mechanistic investigations suggest that the availability of two cis-coordination sites at the Mn center is important for the catalytic activity. Although such a cis-coordination mode has been observed with salen complexes [73], it is conceivably difficult to achieve without some transformation on the salen ligand, since such ligands typically need four planar coordination sites on the metal center. Thus our attention has turned to the reduced salen, or salan ligands (where H2salan = N,N′-dimethyl-N,N′-bis(o-hydroxybenzyl)-1,2-diaminoethane), which are derived from the hydrogenation of the imine bonds in salen. The replacement of the C=N double bonds by the C-N single bonds provides a more flexible coordination environment around the metal center, and this may allow ready access to the cis-coordination mode and facilitate the catalytic reaction. It has been illustrated that salan ligands are a good platform that could support various transition metals and provide catalytic reactivity as well as selectivity [74,75,76]. Herein we report our studies of manganese salan complexes for catalytic hydrosilylation of carbonyl compounds.

2. Results and Discussion

2.1. Synthesis and Characterization of (Salan)Mn Complexes

The salan ligands were obtained by following the literature procedures [77]. Briefly, the reaction of salicyaldehyde derivatives with diamine afforded the parent H2salen ligand, which was first reduced with excess NaBH4 and subsequently methylated by reductive amination with NaBH4/CH2O. The metallation was carried out with MnCl2 or Mn(OAc)2/LiCl to first obtain the chloride compounds. The substitution with NaN3 in the presence of AgClO4 afforded the azido complex (Scheme 2) [78]. The presence of the azido ligand in (salan-tBu2)Mn(N3) (5b) was confirmed by the peak at 2050 cm−1 in the FT-IR spectrum (Figure 1). Similarly, the parent (salan)Mn(N3) (5a) featured an azide-stretching peak at 2037 cm−1. For comparison, the starting NaN3 shows an IR peak at 2104 cm−1. The known salen analogue, (salen-tBu2)Mn(N3) (6) [79], was also obtained in a similar procedure, and the IR peak for the azido ligand as a thin film is observed at 2063 cm−1. The vibrational mode of a series of (salen)Mn(N3) derivatives appeared in the range of 2046–2048 cm−1 in a solution phase determination, typical for the apical azido group in five-coordinate square pyramidal Mn(III) complexes. Efforts were made to prepare the nitrido manganese salan complexes since this would offer a direct comparison with the salen analogue [68]. However, experimental attempts, either by photolysis [80,81,82,83,84] of the azido precursor (salan-tBu2)Mn(N3) or by bleach oxidation of (salan-tBu2)MnCl in the presence of NH4OH [85], were not successful. Thus, we focused on the (salan)Mn(N3) complexes which showed reasonable activity in the hydrosilylation reaction.

2.2. Catalytic Hydrosilylation

Having a series of manganese (III) complexes in hand, we first examined the efficacy of complex 5b in the hydrosilylation under a variety of reaction conditions, with benzaldehyde PhCHO as a model substrate and phenylsilane PhSiH3 as the hydrogen source, and the results are compiled in Table 1. Screening different solvents in the reaction illustrated that benzene was an optimal solvent in this system, while the hydrosilylation in other common solvents such as CD3CN and CDCl3 was slower or with low conversions. The reaction with benzene as a solvent at elevated temperature exhibited a high conversion of >99 % in less than 1 h with 0.5 mol % loading of the catalyst (entry 1, Table 1). Reaction in CD3CN under identical conditions resulted in a 94 % conversion within 6.5 h (entry 2). Unlike C6D6 and CD3CN, CDCl3 as a solvent showed negligible conversion after a long reaction time (entry 4), though increasing the catalyst loading to 1 mol % at a longer reaction time did lead to some conversion of PhCHO (entry 5). Reaction in toluene-d8 showed comparable reactivity, as in benzene, reaching a >99% conversion within 40 min (entry 3). Other reaction parameters were also explored. The reaction with C6D6 could be carried out at a lower temperature (80 °C or at room temperature), and longer reaction times were needed for reasonable conversion (entries 6 & 7). For example, the reaction could still reach a 90% conversion after a few days at 80 °C. However, when the reaction was run under air, only minimal conversion was obtained within 5 h (entry 8), which was less active than the (salen)MnN system in the presence of air [68]. So the rest of the reactions were carried out under N2. Among other common hydrosilanes, a tertiary hydrosilane, triethoxysilane, was also active for the hydrosilylation in CD3CN, and a 54% conversion of PhCHO was observed after 8 h of reaction (entry 9). At this point, the triethoxysilane was completely consumed, though the side reaction product was not identified. When the loading of the triethoxysilane was increased to two equivalents of PhCHO, the complete conversion of PhCHO could be achieved within 8 h (entry 10). Under the same conditions, a reaction with a ketone, acetophenone, was slower but reached a 99% conversion within 25 h (entry 11). Secondary silanes, such as Ph2SiH2, showed lower activity than PhSiH3 and tertiary silanes such as Et3SiH showed no activity.
After exploration of the reaction conditions, we examined the hydrosilyation of PhCHO with different salan Mn(III) complexes (entries 12–15, Table 1). The parent salen azido complex 5a was capable of catalyzing the reaction, though with somewhat lower activity (entry 14). On the other hand, the chloride complex 4b on its own showed little activity in the hydrosilylation of PhCHO (entry 12). However, it could be activated by treatment with an equimolar amount of AgClO4. The hydrosilylation of PhCHO took place after an induction period and up to 90% conversion was achieved after 48 h (entry 13). Figure 2 depicts the conversion-time profiles of these catalytic systems and it is notable that the (salan-tBu2)Mn(N3) complex (5b) was the most active among these complexes. For further comparison, the salen analogue (salen-tBu2)Mn(N3) (6) was also tested under the same conditions, though it displayed lower activity than 5a and 5b (entry 15).
With the reaction condition established above, we next examined the scope of carbonyl substrates with 5b. Hydrosilylations of a variety of aldehydes and ketones occurred at 120 °C with low loadings of the catalyst 5b (0.5 mol %) in benzene-d6 (as it is cheaper than toluene-d8) in the presence of PhSiH3 (Table 2). The purified corresponding alcohols were obtained after an acidic workup of the reaction mixtures. High conversions and good isolated yields were observed in most cases. The reaction generally proceeded faster with aldehydes when compared to ketones, as typically observed in the hydrosilylation of carbonyl compounds [53,54,55,56]. In an intermolecular competition reaction between PhCHO and PhCOMe with PhSiH3 (1:1:1 molar ratio), PhCHO conversion reached 89% within 3 h while only <5% PhCOMe reacted. Common functional groups, including halides, nitro, and methoxy, were tolerated under the reaction conditions. The benzaldehyde derivatives bearing the electron-withdrawing groups Br and NO2 tended to react more rapidly than those with electron-donating OMe (entries 2–3 vs. 4). The progress of the reaction of 4-methoxyacetophenone (entry 7, Table 2) monitored by 1H NMR spectroscopy was shown in Figure S5. In addition, the reaction of 4-nitroacetophenone was peculiar under this condition in that no further conversion was observed at 76% after 72 h (entry 6, Table 2). The reaction of cinnamaldehyde led to the reduction of the carbonyl group without any reduction of the C=C double bond, suggesting that the reaction was selective toward carbonyl groups over alkenes (entry 9). The nearly exclusive 1,2-hydrosilylation of the α,β-unsaturated carbonyl substrates was different from that of the (salen)MnN-catalyzed reaction, in which the 1,4-addition product seemed to dominate [68]. Furthermore, aliphatic carbonyl substrates also underwent hydrosilylation reactions under the same conditions (entries 10–11), and the conversions were generally high (>95%).
In a reusability test of catalyst 5b, we carried out the hydrosilylation of PhCHO with PhSiH3 under standard conditions. After the complete consumption of PhCHO, a second batch of PhCHO and PhSiH3 (1:1 molar ratio) was added and the complete reaction of PhCHO was observed within 2 h. A third and a fourth batch were added in the subsequent runs and gave comparable results. These observations indicated that the catalyst could be reused without much loss of reactivity. At a practical level, a gram-scale reaction with PhCHO was carried out with 0.5 mol% of 5b under solvent-free conditions, and >97% conversion of PhCHO was observed in 4 h. After hydrolysis and purification, a benzyl alcohol product was obtained in good yield.

2.3. Mechanistic Consideration

In our previous study with a (salen)MnVN catalyst, [68] it was observed that the reduction of MnVN by hydrosilanes, as indicated by the color change and the NMR observations, was the first step of catalysis. Though the exact nature of the reduced Mn species was not confirmed, it was thought to be a Mn(II) or Mn(III) species that could interact with and activate hydrosilanes for the subsequent reactions. There was also evidence that the salen ligand might have undergone some transformation. In the current study, we employed Mn(III)-salan species as the (pre)catalyst so the reduction of Mn might not be necessary, though the reduction to a Mn(II) species as the active catalyst was possible under the current conditions. It has been shown that several manganese(III) catalysts could be efficient catalysts in hydrosilylation reactions [61,62,63,64,65,66,67,68,69,70]. However, (salan)MnCl 4b showed minimum catalytic activity in the hydrosilylation of PhCHO, indicating that Mn(III) alone was not a determining factor. Activation of 4b by AgClO4 suggested that it was critical to open up the coordination site around the metal center for catalysis. The comparatively higher activity of 5b (salan)Mn(N3) vs. 4b (salan)MnCl could be attributed to the weaker azido-metal interaction than M-Cl due to the larger size of N3. In an analogous case, (salan)Cr(N3) was active in the ring-opening reaction of epoxides, while (salan)CrCl was considered a precatalyst that required further activation [79]. By the same consideration, the reduction of MnVN was supposedly required to open up the coordination site in the (salen)MnN catalysis. It should also be pointed out that the metal center in (salen)MnN and (salan)MnCl presumably adopts a square pyramidal geometry with an open site trans to the nitrido or chloride ligands. This indicated that a second open site, preferably cis to the first one, around the metal center, might be needed for hydrosilylation, i.e. to accommodate the coordination of both hydrosilane (or a hydride) and the carbonyl substrate. The observation that (salan-tBu2)Mn(N3) is more active than (salen-tBu2)Mn(N3) in catalysis (Table 1, entry 1 vs. 13) lends support to this notion since the salan ligands are more flexible to accommodate two cis open sites. As to the further activation of the hydrosilanes, a radical mechanism seems unlikely, since no ring-opening product was observed with cyclopropyl phenyl ketone [86,87]. The reaction progress of the cyclopropyl phenyl ketone monitored by the NMR spectroscopy was shown in Figure 3, and the straight conversion of the ketone to silyl ethers could be noted with the cyclopropyl ring intact during the process. The electronic effect of 4-substituted benzaldehydes indicated that the electron-withdrawing groups tended to give faster reactions than the electron-donating groups, which was comparable to the (salen)MnN-catalyzed hydrosilylation reactions [68], suggesting a similar activation mechanism. Taken together, we speculated a reaction pathway involving the formation of a hydrosilane-Mn adduct that generated an electrophilic silicon center, the coordination of a carbonyl substrate cis to the silane, followed by a subsequent nucleophilic attack of oxygen on the silicon for a silyl ether bond, though further details may vary.

3. Materials and Methods

All the substrates and reagents were obtained commercially, and the liquid substrates were degasified and dried over activated molecular sieves (4 Å) prior to the reaction. The 1H and 13C NMR spectra were recorded on a Bruker AVANCE 500 (Billerica, MA, USA) or AVANCE NEO 400 (Billerica, MA, USA) NMR spectrometer and referenced to the residue peaks in CDCl3 (7.26), CD3CN (1.94), or C6D6 (7.16).
General procedures for hydrosilylation. To a J Young NMR tube, the catalyst (0.5 mol %), substrate (one equivalent), C6D6 (0.35 mL), and PhSiH3 (one equivalent) were added in order under nitrogen in a glovebox. The sealed tube was heated in an oil heating bath preset at 120 °C and the reaction was monitored by the NMR spectroscopy. When the reaction was completed, the reaction mixture was transferred to a vial with diethyl ether (2 mL). The mixture was then hydrolyzed using HCl (2 mL, 1 M) and extracted with diethyl ether. The organic phase was combined and dried over anhydrous Na2SO4. After the removal of the solvent, the alcohol product was isolated by column chromatography using silica with hexane-EtOAc as an eluent. The final products were confirmed by 1H NMR and comparison with the literature’s data.

4. Conclusions

A series of well-defined manganese salan complexes have been synthesized and shown to exhibit catalytic activity in the hydrosilylation of various aldehydes and ketones. Decent-to-high yields of the corresponding alcohols were obtained after an acidic workup, and a variety of functional groups in the carbonyl compounds, such as the methoxy, halides, and nitro groups, were tolerated. Future efforts will focus on the mechanistic elucidation of the silane activation and on additional ligand systems that may improve the catalytic activity of manganese compounds.

Supplementary Materials

The following supporting information can be downloaded at:, Detailed experimental procedural, and characterization data for catalysts and the hydrosilylation products; Figures S1–S5: 1H NMR spectra of catalytic hydrosilylation reactions of selected substrates with PhSiH3. Table S1: Comparison of catalytic activity of selective Mn complexes [76,77,88,89,90,91,92].

Author Contributions

Conceptualization, S.V. and G.D.; investigation, N.A. and S.V.; writing—original draft preparation, N.A.; writing—review and editing, G.D.; supervision, G.D.; funding acquisition, G.D. All authors have read and agreed to the published version of the manuscript.


This research was based upon work supported by the National Science Foundation under Grant No. NSF EPSCoR Award IIA-1355466. We thank the NSF MRI award # 2117699 for the acquisition of the NMR instrument. N.A. thanks SACM for a scholarship.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.


This manuscript is based on the M.S. thesis work of N.A. at the University of North Dakota (, accessed on 27 February 2023).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.


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Scheme 1. Selected examples of Mn (pre)catalysts for hydrosilylation.
Scheme 1. Selected examples of Mn (pre)catalysts for hydrosilylation.
Catalysts 13 00665 sch001
Scheme 2. Synthesis of salan Mn complexes. (a) MnCl2 or Mn(OAc)2/LiCl; (b) AgClO4/NaN3.
Scheme 2. Synthesis of salan Mn complexes. (a) MnCl2 or Mn(OAc)2/LiCl; (b) AgClO4/NaN3.
Catalysts 13 00665 sch002
Figure 1. Comparison of FT-IR spectra of (salan)Mn(N3) (5a) (top), (salan-tBu2)Mn(N3) (5b) (middle), and NaN3 (bottom).
Figure 1. Comparison of FT-IR spectra of (salan)Mn(N3) (5a) (top), (salan-tBu2)Mn(N3) (5b) (middle), and NaN3 (bottom).
Catalysts 13 00665 g001
Figure 2. The conversion-time profile with different catalysts (0.5 mol%) in the hydrosilylation of PhCHO.
Figure 2. The conversion-time profile with different catalysts (0.5 mol%) in the hydrosilylation of PhCHO.
Catalysts 13 00665 g002
Figure 3. Hydrosilylation of cyclopropyl phenyl ketone monitored by the NMR spectroscopy.
Figure 3. Hydrosilylation of cyclopropyl phenyl ketone monitored by the NMR spectroscopy.
Catalysts 13 00665 g003
Table 1. Hydrosilylation of PhCHO under different conditions a.
Table 1. Hydrosilylation of PhCHO under different conditions a.
Catalysts 13 00665 i001
EntryCat (Equiv.)Silane (Equiv.)SolventTemp (°C)TimeConvn (%)
15b (0.5%)PhSiH3(1)C6D612040 min>99%
25b (0.5%)PhSiH3(1)CD3CN1206.5 h94%
35b (0.5%)PhSiH3(1)C7D812040 min>99%
45b (0.5%)PhSiH3(1)CDCl31203 d<1%
55b (1%)PhSiH3(1)CDCl31204 d10%
65b (1%)PhSiH3(1)C6D6RT67 h20%
75b (1%)PhSiH3(1)C6D68013 h90%
8 b5b (1%)PhSiH3(1)C6D61203 h4.8%
95b (0.5%)(EtO)3SiH(1)CD3CN1208 h53.6%
105b (0.5%)(EtO)3SiH(2)CD3CN1208 h>99%
11 c5b (0.5%)(EtO)3SiH(2)CD3CN12025 h>99%
12 d4b (0.5%)PhSiH3(1)C6D612048 h9%
13 e4b (0.5%)PhSiH3(1)C6D612048 h87%
14 f5a (0.5%)PhSiH3(1)C6D61202.3 h99%
15 g6 (0.5%)PhSiH3(1)C6D612012 h>99%
a Reaction conditions: PhCHO (1 equivalent). The temperature refers to the heating bath temperature. b The reaction was run under air. c Acetophenone as substrate. d Reaction with complex 4b. e Combination of 4b with AgClO4 (1:1 ratio). f Reaction with complex 5a. g Reaction using (salen-tBu2)Mn(N3) 6.
Table 2. Hydrosilylation of Aldehydes and Ketones Catalyzed by (salan)MnN3 a.
Table 2. Hydrosilylation of Aldehydes and Ketones Catalyzed by (salan)MnN3 a.
EntrySubstrateTime (h)%Conv (Yield)Product
1Catalysts 13 00665 i0020.67100 (76)Catalysts 13 00665 i003
2Catalysts 13 00665 i0045.3100 (60)Catalysts 13 00665 i005
3Catalysts 13 00665 i0062.3100 (70)Catalysts 13 00665 i007
4Catalysts 13 00665 i00861100 (95)Catalysts 13 00665 i009
5Catalysts 13 00665 i0104193 (73)Catalysts 13 00665 i011
6Catalysts 13 00665 i0127276Catalysts 13 00665 i013
7Catalysts 13 00665 i01440100 (62)Catalysts 13 00665 i015
8Catalysts 13 00665 i01615100 (49)Catalysts 13 00665 i017
9Catalysts 13 00665 i01813100 (97)Catalysts 13 00665 i019
10Catalysts 13 00665 i02036>95 (88)Catalysts 13 00665 i021
11Catalysts 13 00665 i02218>96 (94)Catalysts 13 00665 i023
a Reaction conditions: the reactions were performed with ~1 mmol of the substrates, 1.0 equivalent of PhSiH3 0.5 mol% of catalyst 5b in benzene heated at ~120 °C. The conversions were estimated from the 1H NMR spectroscopy, and the yields in parentheses were isolated yields of alcohols after acidic workup.
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Almutairi, N.; Vijjamarri, S.; Du, G. Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones. Catalysts 2023, 13, 665.

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Almutairi N, Vijjamarri S, Du G. Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones. Catalysts. 2023; 13(4):665.

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Almutairi, Nora, Srikanth Vijjamarri, and Guodong Du. 2023. "Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones" Catalysts 13, no. 4: 665.

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