Hydrosilylation of Aldehydes by a Manganese

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inorganics Article ArticleHydrosilylation of Aldehydes by a Manganese α- HydrosilylationDiimine Complex of Aldehydes by a Manganese α-Diimine Complex Veeranna Yempally 1, Azal Shahbaz 1, Wai Yip Fan 2, Sherzod T. Madrahimov 1,* and Ashfaq A. VeerannaBengali 1,* Yempally 1, Azal Shahbaz 1, Wai Yip Fan 2, Sherzod T. Madrahimov 1,* and Ashfaq A. Bengali 1,* 1 Science Department, Texas A&M University at Qatar, PO Box 23874 Doha, Qatar; [email protected] 1 (V.Y.);Science [email protected] Department, Texas A&M (A.S.) University at Qatar, PO Box 23874 Doha, Qatar; 2 [email protected] of Chemistry, (V.Y.); 3 Science [email protected] Drive, National University (A.S.) of Singapore, Singapore 117543, Singapore; 2 [email protected] of Chemistry, 3 Science Drive, National University of Singapore, Singapore 117543, Singapore; * Correspondence:[email protected] sherzod.madrahimov.qatar.tamu.edu (S.T.M.); [email protected] (A.A.B.) * Correspondence: [email protected] (S.T.M.); [email protected] (A.A.B.) Received: 21 October 2020; Accepted: 06 November 2020; Published: 10 November 2020 Received: 21 October 2020; Accepted: 6 November 2020; Published: 10 November 2020 Abstract: This paper describes the catalytic activity of air stable and easy to handle manganese Abstract:complexesThis towards paper describes the hydrosilylation the catalytic activity of aldehydes of air stable. andThese easy catalysts to handle incorporate manganese complexes a bulky towardsdiazabutadiene the hydrosilylation ligand and of exhibit aldehydes. good These functional catalysts group incorporate tolerance a andbulky chemoselectivity diazabutadiene ligand in the andhydrosilylation exhibit good of functional aldehydes group, utilizing tolerance primary and silanes chemoselectivity as the reducing in the agent. hydrosilylation The reactions of aldehydes, proceed utilizingwith turnover primary frequenci silaneses asapproaching the reducing 150 agent. h−1 in some The reactions instances, proceed similar withto those turnover observed frequencies for other 1 approachingmanganese-based 150 h −catalysts.in some instances,The conversion similar of to aromatic those observed aldehydes for other to the manganese-based corresponding catalysts.alcohols Thewas conversionfound to be of more aromatic efficient aldehydes than that to for the the corresponding analogous aliphatic alcohols systems. was found to be more eﬃcient than that for the analogous aliphatic systems. Keywords: catalysis; manganese; hydrosilylation Keywords: catalysis; manganese; hydrosilylation

1. Introduction 1. Introduction Hydrosilylation, the largest volume industrial reaction using homogenous catalysts, is an importantHydrosilylation, step in the the generation largest volume of valuable industrial building reaction blocks using for homogenous the synthesis catalysts, of organic is an importantchemicals stepand functional in the generation materials of valuable(Scheme building1) [1,2,3,4] blocks. The formation for the synthesis of silyl of ether organic protected chemicals alcohols and functionalin a single materialsstep with (Schemea high atom1)[ 1economy–4]. The using formation easy ofto silylhandle ether liquid protected hydrosilanes, alcohols makes in a singlethis reaction step with an aattractive high atom alternative economy to using hydrogenation easy to handle [5,6,7] liquid. The past hydrosilanes, decade has makes seen a this drama reactiontic rise an in attractive the base alternativemetal-catalyzed to hydrogenation hydrosilylation [5–7 ].of Thealkenes past and decade carbonyl has seen compounds a dramatic, with rise in efficiencies the base metal-catalyzed rivaling those hydrosilylationobserved for precious of alkenes metal and catalysts carbonyl [8,9,10,11,12] compounds,. Recently, with e ﬃmanganeseciencies rivaling-based catalysts those observed have been for preciousemployed metal in the catalysts hydrosilylation [8–12]. of Recently, carbonyl manganese-basedcompounds [13,14,15] catalysts. Manganese, have been the fourth employed most in earth the hydrosilylationabundant element of carbonyl by mass, compounds is inexpensive [13–15]. Manganese, and bio-compatible, the fourth most thus earth making abundant the resulting element byorganometallic mass, is inexpensive complexes and attractive bio-compatible, catalysts for thus organic making transformations the resulting, organometallic including hydrosilylation complexes attractivereactions. catalysts for organic transformations, including hydrosilylation reactions.

Scheme 1. Metal catalyzed hydrosilylation of organic carbonyls.carbonyls.

Several manganese complexes incorporating nitrogen nitrogen-based-based chelating and other ligands have been developed developed as as catalysts catalysts for for the theconversion conversion of aldehydes of aldehydes and ketones and ketones into alcohols into alcohols [14]. For [14] example,. For Luganexample, et al.Lugan demonstrated et al. demonstrated that manganese that complexesmanganese bearing complexes NHC bearing ligands NHC are eﬃ ligandscient photocatalysts are efficient forphotocatalysts the reduction for of the ketones reduction with diphenylsilanes of ketones with under diphenylsilanes UV irradiation under at room UV irradiation temperature at [ 16 room,17]. Similarly, Cutler and coworkers utilized manganese carbonyl complexes containing acyl ligands for Inorganics 2020, 8, x; doi: www.mdpi.com/journal/inorganics

Inorganics 2020, 8, 61; doi:10.3390/inorganics8110061 www.mdpi.com/journal/inorganics Inorganics 2020, 8, x 2 of 11 Inorganics 2020, 8, x 2 of 11 photocatalysts for the reduction of ketones with diphenylsilanes under UV irradiation at room photocatalysts for the reduction of ketones with diphenylsilanes under UV irradiation at room Inorganicstemperature2020, 8,[16,17]. 61 Similarly, Cutler and coworkers utilized manganese carbonyl complexes2 of 11 temperature [16,17]. Similarly, Cutler and coworkers utilized manganese carbonyl complexes containing acyl ligands for the efficient reduction of carbonyl compounds [18], while Chung et al. containing acyl ligands for the efficient reduction of carbonyl compounds [18], while Chung et al. reported the generation of a pre-catalyst from a manganese tricarbonyl complex with an indenyl thereported efficient the reduction generation of of carbonyl a pre-catalyst compounds from [18a manganese], while Chung tricarbonyl et al. reported complex the with generation an indenyl of a ligand for the hydrosilylation of ketones [19,20]. Currently, the highest reported turnover number (up pre-catalyst from a manganese tricarbonyl complex with an indenyl ligand for the hydrosilylation ligand for the−1 hydrosilylation of ketones [19,20].Currently, the highest reported turnover number (up to 76,800 h ) for a first-row metal-based hydrosilylation catalyst is for a bis(imino)pyridine1 ligated ofto 76,800 ketones h−1 [)19 for,20 a]. first-row Currently, metal-based the highest hydrosilylation reported turnover catalyst numberis for a bis(imino)pyridine (up to 76,800 h− )ligated for a manganese carbonyl complex [21]. first-rowmanganese metal-based carbonyl complex hydrosilylation [21]. catalyst is for a bis(imino)pyridine ligated manganese carbonyl Manganese-based carbonyl complexes with diazabutadiene (DAB) ligands are effective catalysts complexManganese-based [21]. carbonyl complexes with diazabutadiene (DAB) ligands are effective catalysts for the electrochemical reduction of CO2, and have been extensively studied for their promise as for theManganese-based electrochemical carbonyl reduction complexes of CO2, and with have diazabutadiene been extensively (DAB) studied ligands arefor etheirffective promise catalysts as visible light-assisted CO release agents (photo-CORM) [22–26]. However, these complexes have not forvisible the electrochemicallight-assisted CO reduction release ofagents CO2 ,(photo-COR and have beenM) extensively[22–26]. However, studied forthese their complexes promise ashave visible not yet been investigated as hydrosilylation catalysts for the reduction of aldehydes and ketones. Our light-assistedyet been investigated CO release as agentshydrosilylation (photo-CORM) catalyst [22s –for26 ].the However, reduction these of aldehydes complexes and have ketones. not yet beenOur interest in the application of these complexes towards the hydrosilylation of carbonyl compounds investigatedinterest in the as application hydrosilylation of these catalysts complexes for the reductiontowards the of aldehydes hydrosilylation and ketones. of carbonyl Our interest compounds in the stemmed from a recent report by Royo et al. that highlighted the use of the cationic [Mn(CO)3(bis- application of these complexes towards the hydrosilylation of carbonyl compounds stemmed from3 a stemmedMe from a recent report by Royo et al. that highlighted the use of the cationic [Mn(CO) (bis- NHC )(NCMe)]BF4 complex for this reaction [27]. The authors demonstrated that theMe NHC ligand recentNHCMe report)(NCMe)]BF by Royo4 complex et al. that for highlighted this reaction the use[27]. of The the authors cationic demonstrated [Mn(CO)3(bis-NHC that the )(NCMe)]BFNHC ligand4 enhanced catalytic performance, for the reduction of ketones and aldehydes by phenylsilane. An complexenhanced for catalytic this reaction performance, [27]. The for authors the reduction demonstrated of ketones that and the NHCaldehydes ligand by enhanced phenylsilane. catalytic An important role for the acetonitrile ligand was also proposed, since [Mn(CO)3(bis-NHCMe)(NCMe)]BF4 performance, for the reduction of ketones and aldehydes by phenylsilane. An3 importantMe role for the4 important role for the acetonitrile ligand was also proposed, since [Mn(CO)Me(bis-NHC )(NCMe)]BF exhibited improved catalytic activity compared to Mn(CO)3(bis-NHCMe )Br in the reduction of acetonitrileexhibited improved ligand was catalytic also proposed, activity since compared [Mn(CO) to3 (bis-NHCMn(CO)3(bis-NHC)(NCMe)]BFMe)Br 4inexhibited the reduction improved of acetophenone. Me catalyticacetophenone. activity compared to Mn(CO)3(bis-NHC )Br in the reduction of acetophenone. Our group recently synthesized several manganese carbonyl complexes containing a bulky DAB Our group recently synthesized several manganese carbonyl complexes containing a bulky Our group recently synthesized several manganese carbonyli complexes containing a bulkyi DAB ligand [R-DAB = (N,N-bis(2,6-R)-1,4-diaza-1,3-butadiene), R = Pr2Ph,i MePh], fac-Mn(Br)(CO)3( Pr2Ph- DAB ligand [R-DAB = (N,N-bis(2,6-R)-1,4-diaza-1,3-butadiene),i R2 = Pr Ph, MePh], fac-Mn(Br)(CO)3 i 2 ligand [R-DAB = (N,N-bis(2,6-R)-1,4-diaza-1,3-butadiene),i R = Pr Ph, MePh],i 2 fac-Mn(Br)(CO) ( Pr Ph-3 DAB)i (1a), [Mn(CO)4( Pr2Ph-DAB)][PFi 6] (1b), [Mn(CO)3(NCMe)( Pr2Ph-DAB)][PFi 6] (1c), and (DAB)Pr2Ph-DAB) (1a), [Mn(CO) (1a), [Mn(CO)4(iPr2Ph-DAB)][PF4( Pr2Ph-DAB)][PF6] (1b),6 ]([Mn(CO)1b), [Mn(CO)3(NCMe)(3(NCMe)(iPr2Ph-DAB)][PFPr2Ph-DAB)][PF6] (1c),6 ](and1c), [Mn(CO)3(NCMe)(Me2Ph-DAB)][PF6] (2b) for potential application as CO2 reduction catalysts and and[Mn(CO) [Mn(CO)3(NCMe)(Me3(NCMe)(Me2Ph-DAB)][PF2Ph-DAB)][PF6] (2b6) ](for2b potential) for potential application application as CO as2 COreduction2 reduction catalysts catalysts and photo-CORMs (Figure 1) [28]. Compound 1a showed remarkable photoreactivity, and efficient CO andphoto-CORMs photo-CORMs (Figure (Figure 1) [28].1)[ 28Compound]. Compound 1a showed1a showed remarkable remarkable photoreactivity, photoreactivity, and andefficient efficient CO release was observed upon visible light irradiation. The cationic complex 1b with a sterically COrelease release was was observed observed upon upon visible visible light light irradiation. irradiation. The The cationic cationic complex complex 1b1b withwith a a sterically encumbered DAB ligand was found to be air stable, easy to handle, and have an exceptionally labile encumbered DAB ligand was found to be air stable, easyeasy to handle, and have an exceptionally labile CO group such that room temperature dissolution in acetonitrile solvent resulted in the generation CO groupgroup suchsuch thatthat roomroom temperature temperature dissolution dissolution in in acetonitrile acetonitrile solvent solvent resulted resulted in in the the generation generation of of 1c (Figure 2) [28]. 1cof (Figure1c (Figure2)[ 282) ].[28].

iPr iPr iPr iPr iPr iPr Me Me NCMe iPr iPr Br iPr iPr CO iPr iPr Me Me NCMe N Br N CO N NCMe N NCMe N CO N CO N CO N CO Mn CO Mn CO Mn CO Mn CO N Mn N Mn N Mn N Mn CO CO CO N N CO N N iPr iPr CO CO iPr iPr CO CO iPr iPr CO CO Me Me CO CO iPr iPr CO iPr iPr CO iPr iPr CO Me Me CO

(1a) (1b) (1c) (2b) (1a) (1b) (1c) (2b) Figure 1. Manganese carbonyl catalysts employed in the hydrosilylation of aldehydes in this work. Figure 1. Manganese carbonyl catalysts employed in the hydrosilylation of aldehydesaldehydes in this work.

Figure 2. Facile substitution of a carbonyl ligand in 1b by acetonitrile at 298 K. Figure 2. Facile substitution of a carbonyl ligand in 1b by acetonitrile at 298 K. The facile generation of a vacant coordination site (implied by the lability of the CO ligand), together with the similarity between 1c and Royo’s catalyst prompted us to investigate the applicability

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The facile generation of a vacant coordination site (implied by the lability of the CO ligand), together with the similarity between 1c and Royo’s catalyst prompted us to investigate the applicability of these complexes in the catalytic hydrosilylation of aldehydes and ketones. It was envisioned that a rapid substitution of the CO or MeCN ligand in these complexes by phenylsilane or substrate would generate an easily accessible pre-catalyst, which could subsequently reduce aldehyde or ketone groups to yield silyl ethers. Additionally, the unique redox properties of the DAB ligand were expected to support changes in the oxidation state of the metal center in this process. In the current study, the ability of these complexes to promote the hydrosilylation of aldehydes and ketones is reported as a function of the ancillary ligands in the primary (1a–c) and secondary (2b) coordination sphere of the metal center. Complexes 1c and 2b were found to have activities similar to those reported for Royo’s catalyst, and provide another example for the application of manganese- based catalysts in the reduction of aldehydes to alcohols. Inorganics 2020, 8, 61 3 of 11

2. Results and Discussion of theseThe complexes synthesis, in characterization, the catalytic hydrosilylation and photochemical of aldehydes reactivity and ketones. of compounds It was envisioned 1a–c has thatbeen a reportedrapid substitution previously of theby our CO orgroup MeCN [28] ligand. In this in these paper, complexes these studies by phenylsilane were extended or substrate to explore would the catalyticgenerate an activity easily accessible of complexes pre-catalyst, 1a–c towards which could the subsequentlyhydrosilylation reduce of aldehyde aldehydes. or ketoneAdditionally, groups to yield silyl ethers. Additionally, the unique redox properties of the DAB ligand were expected compound 2b, with the sterically less crowded Me2Ph-DAB ligand, was synthesized from the to support changes in the oxidation state of the metal center in this process. In the current study, precursor compound Mn(Me2Ph-DAB)(CO)3Br, 2a, as shown in Scheme 2, to investigate the role of stericthe ability influence of these in catalytic complexes activity. to promote Both compounds the hydrosilylation, 2a and 2b of, were aldehydes isolated and and ketones characterized is reported by singleas a function crystal ofX-ray the diffraction, ancillary ligands FT-IR inand the 1H primary NMR spectroscopy. (1a–c) and secondary The X-ray (2b structures) coordination of 2a and sphere 2b areof the shown metal in center.Figures Complexes 3 and S1, respectively.1c and 2b were found to have activities similar to those reported for Royo’sThe catalyst, FTIR spectrum and provide of anothercomplex example 2b in dichloromethane for the application solvent of manganese-based displays characteristic catalysts infacial the carbonylreduction stretching of aldehydes frequencies to alcohols. of 2053 cm−1, 1978 cm−1 and 1963 cm−1, almost identical to those of compound 1c [28]. Compound 2b is stable in air, and unlike 1c, can be manipulated under visible 2. Results and Discussion light conditions without noticeable decomposition. Similar to other reported cationic manganese complexesThe synthesis, with an aceto characterization,nitrile ligand [24] and, photochemical2b has an octahedral reactivity structure of compounds, and its overall1a–c geometryhas been isreported similar to previously that of 1c. byHowever, our group reduced [28]. steric In this crowding paper, theseresults studies in the greater were extended C(2)–Mn(1) to explore–N(3) bond the anglecatalytic of 175.99 activity in of 2b complexes compared1a to–c 170.59towards in the 1c. hydrosilylationConsistent with of a aldehydes. closer approach Additionally, of the acetonitrile compound ligand2b, with towards the sterically the less less sterically crowded encumbered Me2Ph-DAB metal ligand, center, was synthesized the Mn1-N3 from bond the length precursor in 2b compound (2.009 Å ) isMn(Me slightly2Ph-DAB)(CO) shorter than in3Br, 1c 2a,(2.03as0 shownÅ ). These in findings Scheme2 are, to consistent investigate with the the role expectation of steric inﬂuencethat, relative in tocatalytic iPr2Ph- activity.DAB, the Both reduced compounds, steric profile2a and of2b the, wereMe2Ph isolated-DAB ligand and characterized should result by in single a more crystal accessible X-ray 1 Mndiﬀ raction,center in FT-IR 2b compared and H NMR to 1c spectroscopy.. As discussed The later, X-ray the structuresobserved structural of 2a and 2bdifferencesare shown inin 2b Figure and 1c3 canand be Figure correlated S1, respectively. with differences in the catalytic activities of the two complexes.

Inorganics 2020, 8, x 4 of 11 Scheme 2 2.. SynthesisSynthesis of of manganese manganese carbonyl carbonyl complex complex 2b..

Figure 3. Thermal ellipsoid (50% probability level) plot of the solid-state structure of 2b with select atom labeling. Hydrogen atoms and the PF ion are omitted for clarity. Selected bond lengths (Å): Figure 3. Thermal ellipsoid (50% probability 6level) plot of the solid-state structure of 2b with select Mn1–C1 1.824, Mn1–C2 1.832, Mn1–C3 1.822, Mn1–N1 2.046, Mn1–N2 2.045. Selected bond angles atom labeling. Hydrogen atoms and the PF6 ion are omitted for clarity. Selected bond lengths (Å): (deg): C2–Mn1–N3 175.99, C1–Mn1–N1 94.45, C3–Mn1–N1 174.99, C1–Mn1–N2 172.12. Mn1–C1 1.824, Mn1–C2 1.832, Mn1–C3 1.822, Mn1–N1 2.046, Mn1–N2 2.045. Selected bond angles (deg):The FTIRC2–Mn1–N3 spectrum 175.99, of C1–Mn1–N1 complex 2b 94.in45, dichloromethane C3–Mn1–N1 174.99, solvent C1–Mn1–N2 displays 172.12. characteristic facial 1 1 1 carbonyl stretching frequencies of 2053 cm− , 1978 cm− and 1963 cm− , almost identical to those of 2.1. Catalyst Optimization compound 1c [28]. Compound 2b is stable in air, and unlike 1c, can be manipulated under visible lightDue conditions to the previously without noticeable determined decomposition. lability of the Similar CO ligand to other in 1b reported, the initial cationic screening manganese and optimization was performed with this catalyst by monitoring the reaction of trans-cinnamaldehyde with phenylsilane. The reactions were conducted at 80 °C, with progress monitored by GC-MS at regular time intervals. Under these conditions, the hydrosilylation reaction proceeded with an optimal catalyst loading of 2.5 mol % and with high chemoselectivity, as side products arising from the reduction of the alkene group were not observed. The aldehyde group was selectively reduced to the corresponding alcohol (following hydrolysis with 2M aqueous NaOH), as indicated by 1H NMR and GC-MS analysis, with 98% conversion after 22 h with a moderate TON = 40 in acetonitrile solvent (Table 1, entry 2). Surprisingly, the alcohol was the major product, even without hydrolysis, with minor amounts of the silylether observed under these conditions, along with some unidentified species. A control experiment performed without the metal catalyst showed no conversion, thereby confirming the involvement of 1b in the catalytic process (Table 1, entry 7). The use of tertiary and secondary silanes also resulted in the reduction of trans-cinnamaldehyde, but as observed previously for other manganese-based systems [13], complete conversion required significantly longer reaction times (e.g., 92 h for Ph2SiH2).

Table 1. Hydrosilylation of trans-cinnamaldehyde catalyzed by 1a–c, 2b a.

2.5 mol % catalyst O OH 0 PhSiH3, 80 C, MeCN Entry Catalyst Solvent Time (h) Conversion(%) b

1 1a CH3CN 18 (2) 2 (5) 2 1b CH3CN 22 (2) 98 (98) 3 1c CH3CN 22 (2) 98 (98) 4 2b CH3CN 3 (2) 98 (98) 5 1c c Neat 2.0 90 6 2b c neat 2.0 95 7 no catalyst neat 24 n.r. Values in parenthesis refer to neat conditions. a Reaction conditions: silane (1.2 mmol), substrate (1.0 mmol). b Determined by 1H NMR on the basis of cinnamaldehyde consumption (MeOH as internal standard) after hydrolysis of the reaction mixture with 2 M NaOH. c Reaction performed with 1.0 mol % of catalyst.

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Inorganics 2020, 8, 61 4 of 11 complexes with an acetonitrile ligand [24], 2b has an octahedral structure, and its overall geometry is similar to that of 1c. However, reduced steric crowding results in the greater C(2)–Mn(1)–N(3) bond angle of 175.99 in 2b compared to 170.59 in 1c. Consistent with a closer approach of the acetonitrile ligandFigure towards 3. Thermal the less ellipsoid sterically (50% encumbered probability metallevel) center,plot of the the solid-state Mn1-N3 bondstructure length of 2b in with2b (2.009 select Å) is slightlyatom shorter labeling. than Hydrog in 1c (2.030en atoms Å). and These the findingsPF6 ion are are omitted consistent for clarity. with the Selected expectation bond lengths that, relative (Å): to i Pr2Ph-DAB,Mn1–C1 the1.824, reduced Mn1–C2 steric 1.832, profile Mn1–C3 of the 1.822, Me 2Mn1–N1Ph-DAB 2.046, ligand Mn1–N2 should 2.045. result Selected in a more bond accessible angles Mn center(deg): in 2b C2–Mn1–N3compared to175.99,1c. As C1–Mn1–N1 discussed 94. later,45, C3–Mn1–N1 the observed 174.99, structural C1–Mn1–N2 differences 172.12. in 2b and 1c can be correlated with differences in the catalytic activities of the two complexes. 2.1. Catalyst Optimization 2.1. Catalyst Optimization Due to the previously determined lability of the CO ligand in 1b, the initial screening and optimizationDue to thewas previously performed determined with this ca labilitytalyst by of monitoring the CO ligand the reaction in 1b, theof trans initial-cinnamaldehyde screening and optimizationwith phenylsilane. was performed The reactions with were this catalystconducted by monitoringat 80 °C, with the progress reaction ofmonitoredtrans-cinnamaldehyde by GC-MS at withregular phenylsilane. time intervals. The Under reactions these were conditions, conducted the at hydrosilylation 80 ◦C, with progress reaction monitored proceeded by with GC-MS an atoptimal regular catalyst time intervals.loading of Under 2.5 mol these % and conditions, with high the chemoselectivity, hydrosilylation as reaction side products proceeded arising with from an optimalthe reduction catalyst of the loading alkene of group 2.5 mol were % and not withobserv highed. chemoselectivity,The aldehyde group as sidewas productsselectively arising reduced from to the reductioncorresponding of the alcohol alkene (following group were hydrolysis not observed. with 2M The aqueous aldehyde NaOH), group as was indicated selectively by 1 reducedH NMR 1 toand the GC-MS corresponding analysis, with alcohol 98% (following conversion hydrolysis after 22 h with with a 2Mmoderate aqueous TON NaOH), = 40 in asacetonitrile indicated solvent by H NMR(Table and 1, entry GC-MS 2). analysis,Surprisingly, with 98%the alcohol conversion was afterthe major 22 h with product, a moderate even without TON = 40hydrolysis, in acetonitrile with solventminor amounts (Table1, entryof the 2). silylether Surprisingly, observed the alcohol under wasthese the conditions, major product, along even with without some unidentified hydrolysis, withspecies. minor A control amounts experiment of the silylether performed observed without under the these metal conditions, catalyst showed along withno conversion, some unidentified thereby species.confirming A control the involvement experiment of performed 1b in the catalytic without process the metal (Table catalyst 1, entry showed 7). The no conversion, use of tertiary thereby and confirmingsecondary silanes the involvement also resulted of in1b thein reduction the catalytic of trans process-cinnamaldehyde, (Table1, entry 7).but The as observed use of tertiary previously and secondaryfor other manganese-based silanes also resulted systems in the [13], reduction complete of trans conversion-cinnamaldehyde, required significantly but as observed longer previously reaction fortimes other (e.g., manganese-based 92 h for Ph2SiH2). systems [13], complete conversion required significantly longer reaction times (e.g., 92 h for Ph2SiH2). Table 1. Hydrosilylation of trans-cinnamaldehyde catalyzed by 1a–c, 2b a. Table 1. Hydrosilylation of trans-cinnamaldehyde catalyzed by 1a–c, 2b a. 2.5 mol % catalyst O OH 0 PhSiH3, 80 C, MeCN Entry Catalyst Solvent Time (h) Conversion(%) b Entry Catalyst Solvent Time (h) Conversion(%) b 1 1a CH CN 18 (2) 2 (5) 1 1a CH3CN3 18 (2) 2 (5) 2 1b CH3CN 22 (2) 98 (98) 2 1b CH3CN 22 (2) 98 (98) 3 1c CH3CN 22 (2) 98 (98) 3 1c CH3CN 22 (2) 98 (98) 4 2b CH3CN 3 (2) 98 (98) 54 1c2bc CH3NeatCN 3 (2) 2.098 (98) 90 65 1c2b cc Neatneat 2.0 2.0 90 95 76 no2b catalyst c neatneat 2.0 24 95 n.r.

Values in parenthesis7 refer tono neatcatalyst conditions. neata Reaction conditions:24 silane (1.2n.r. mmol), substrate (1.0 mmol). bValuesDetermined in parenthesis by 1H NMR refer on the to basis neat of conditions. cinnamaldehyde a Reaction consumption conditions: (MeOH silane as internal (1.2 standard) mmol), aftersubstrate hydrolysis (1.0 of the reaction mixture with 2 M NaOH. c Reaction performed with 1.0 mol % of catalyst. mmol). b Determined by 1H NMR on the basis of cinnamaldehyde consumption (MeOH as internal c Severalstandard) solvents, after hydrolysis including of the 1,2-dichloroethane, reaction mixture with toluene 2 M NaOH. and Reaction THF, were performed used, but with a 1.0 signiﬁcant mol improvement% of catalyst. in catalytic activity was not observed in comparison to performing the reaction in acetonitrile. However, thereactions conducted under neat (solvent free) conditions proceeded at a signiﬁcantly faster rate (2 h, 98% for 1b, Table1, entry 2) under similar catalytic conditions. These results suggest that acetonitrile inhibits the reaction, most likely by competing with silane and substrate for binding to the Mn center, so that in the absence of solvent the reactions proceed at a faster rate.

2.2. Catalyst Comparison In acetonitrile solvent and under identical reaction conditions, 2b showed superior catalytic activity compared to 1b and 1c, whereas negligible product formation was observed with catalyst Inorganics 2020, 8, x 5 of 11

Several solvents, including 1,2-dichloroethane, toluene and THF, were used, but a significant improvement in catalytic activity was not observed in comparison to performing the reaction in acetonitrile. However, thereactions conducted under neat (solvent free) conditions proceeded at a significantly faster rate (2 h, 98% for 1b, Table 1, entry 2) under similar catalytic conditions. These results suggest that acetonitrile inhibits the reaction, most likely by competing with silane and substrate for binding to the Mn center, so that in the absence of solvent the reactions proceed at a faster rate.

2.2. Catalyst Comparison Inorganics 2020, 8, 61 5 of 11 In acetonitrile solvent and under identical reaction conditions, 2b showed superior catalytic activity compared to 1b and 1c, whereas negligible product formation was observed with catalyst 1a 1a(Table(Table 1,1 entries, entries 1–4). 1–4). For For example, example, the the reaction reaction turnover turnover frequency frequency (TOF) (TOF) in in the case ofof 2b was 1 1 significantlysignificantly higher thanthan forfor 1b1b andand 1c1c(13 (13 h h−−1 vsvs 1.81.8 hh−−1).). Furthermore, Furthermore, unlike unlike the the other complexes, catalytic activity was also observed at room temperature.temperature. The higher reactivity of 2b is most likely i i related to the reduced steric bulk of the the Me Me22Ph-DABPh-DAB ligand ligand compared compared to to PrPr2-DAB,2-DAB, which which results results in in a aless less sterically sterically congested congested site site at at the the metal metal center center fa facilitatingcilitating the the binding binding of of silane and aldehyde at didifferentfferent stagesstages inin thethe reduction reduction reaction. reaction. However, However, the the observed observed di differencefference in in reactivity reactivity between between1b ,1b1c, and1c and2b disappeared2b disappeared when when the reactionsthe reactions were were conducted conduc underted under neat conditions, neat conditions, resulting resulting in an almost in an completealmost complete reduction reduction within twowithin hours two in hours all three in all cases three (Table cases1, entries(Table 1, 2–4). entries For 2–4).1c and For2b 1c, catalytic and 2b, activitycatalytic with activity almost with complete almost conversion complete toconversion the alcohol to under the neatalcohol conditions under wasneat maintainedconditions withwas 1 catalystmaintained loadings with catalyst as low asloadings 1 mol %as (Tablelow as1 1, entriesmol % (Table 5 and 6),1, entries resulting 5 and in TOF6), resulting values ofin 45TOF h −valuesand 1 −1 −1 48of 45 h− h, respectively. and 48 h , respectively. In acetonitrileacetonitrile solvent, solvent, both both1b and1b 1candselectively 1c selectively reduced reduced the aldehyde the aldehyde group, with group, 98% conversionwith 98% intoconversion the alcohol into afterthe alcohol 22 h. However, after 22 ah. higher However, conversion a higher to theconversion alcohol wasto the observed alcohol during was observed the first twoduring hours the of first the two reaction hours in of the the case reaction of 1c. As in mentionedthe case of earlier, 1c. As previousmentioned studies earlier, have previous shown thatstudies the COhave group shown in 1bthatis the exceptionally CO group labile,in 1b is and exceptionally is replaced bylabile, acetonitrile and is replaced at room temperatureby acetonitrile within at room two hours.temperature Thus, itwithin is reasonable two hours. to expect Thus, thatit is inreasonable the early to stages expect of thethat reaction, in the early1b is stages converted of the to reaction,1c prior to1bsilane is converted activation, to 1c resulting prior to in silane a lower activation, amount ofresulting alcohol in production a lower amount during theof alcohol initial phase production of the reactionduring the in thisinitial case. phase of the reaction in this case. In all instances instances (except (except 1a1a),), the the GC-MS GC-MS spectra spectra obtained obtained after after ten ten minutes minutes of reaction of reaction showed showed the thepresence presence of both of both the E the and E andZ isomers Z isomers of the of alcoho the alcohol,l, with withonly onlythe thermodynamically the thermodynamically favored favored E form E formisolated isolated upon uponreaction reaction completion completion (Figure (Figure 4). Intrig4). Intriguingly,uingly, the distribution the distribution ratio ratio of isomers of isomers at this at thistime time depends depends upon upon the thecatalyst’s catalyst’s identity identity with with E:Z E:Z ratios ratios of of38:62 38:62 (1b (1b), ),67:33 67:33 (1c (1c),), and and 70:30 70:30 ( 2b) (Figures(Figures S2–S4).S2–S4). Observation ofof thethe Z form of the alcoholalcohol during the early stages of the reaction raises the interesting possibility that catalysts 1b1b,, 1c1c andand 2b2b initiallyinitially promote promote the the transtrans → ciscis isomerizationisomerization → of transtrans-cinnamaldehyde, following which which the the cis isomer is hydrosilylated to yield the Z formform ofof cinnamyl alcohol.alcohol. BecauseBecause ofof restricted restricted rotation rotation about about the the C =C=CC bond bond in intrans trans-cinnamaldehyde,-cinnamaldehyde, such such an isomerizationan isomerization is unlikely is unlikely without withou disruptiont disruption of the of conjugated the conjugatedπ system, π system, which which may occur mayupon occur alkene upon bindingalkene binding to the Mnto the center. Mn center. The observed The observed variation variat in theion initialin the distributioninitial distribution of the E:Zof the ratios E:Zpoints ratios topoints the existenceto the existence of subtle of subtle kinetic kinetic effects effects arising arising from from differences differences in the in ligand the ligand structure structure around around the Mnthe Mn center. center. While While it isit diis ffidifficultcult to to further further rationalize rationalize this this trend trend with with the the preliminarypreliminary datadata at hand, these observations areare the subject of further on-going experimental and theoretical investigations into the mechanismmechanism ofof thethe reactions.reactions.

Figure 4. Formation of E and Z isomers of cinnamyl alcohol after 10 min of reaction followed by full conversion to the E isomer at reaction completion.

2.3. Substrate Scope Since 2b was found to be the most eﬀective catalyst (in acetonitrile solvent), substrate scope was assessed with this catalyst and the results are shown in Table2. Under solvent-free conditions, 2b exhibited a high activity towards the reduction of benzaldehyde (Table2, entry 1). Even at a relatively low catalyst loading of 0.1 mol %, 90% conversion was observed within a few hours, resulting 1 in a respectable TOF value of 150 h− . Similarly, the hydrosilylation of the carbonyl group of p-nitro-, p-bromo-, and p-methoxybenzaldehyde, to yield the corresponding alcohol, proceeded with high InorganicsInorganics 2020,, 8,, xx 6 of 11

Figure 4. Formation of E andand Z isomersisomers ofof cinnamylcinnamyl alcoholalcohol afterafter 1010 min of reaction followed by full conversion to the E isomer at reaction completion.

2.3. Substrate Scope Since 2b waswas foundfound toto bebe thethe mostmost effectiveeffective catalystcatalyst (in acetonitrile solvent), substrate scope was assessed with this catalyst and the results are shown in Table 2. Under solvent-free conditions, 2b exhibited a high activity towards the reduction of benzaldehyde (Table 2, entry 1). Even at a relatively low catalyst loading of 0.1 mol %, 90% conversion was observed within a few hours, resulting in a −1 respectable TOF value of 150 h−1.. Similarly,Similarly, thethe hydrosilylatiohydrosilylation of the carbonyl group of p-nitro-, p- bromo-, and p-methoxybenzaldehyde, to yield the corresponding alcohol, proceeded with high yield and selectivity, demonstrating good functional group tolerance (entries 2–4). Under similar 3 conditions, p-benzaldehyde with an electron-donating group (–OCH3) was reduced more readily 2 than with an electron withdrawing group (–Br, –NO2). Given that ortho andand para substituentssubstituents exhibitexhibit similar directing effects, thethe comparablecomparable TOFTOF valuesvalues forfor thethe reductionreduction ofof p- and o-nitrobenzaldehyde (entries 4 and 5) to the respective alcohols further indicates that the electronic properties of the substituentsInorganics 2020 on, 8 the, 61 aromatic ring, and not their steric profile,profile, influenceinfluence thethe efficiencyefficiency ofof thethe reaction.reaction.6 of 11 Heteroatom substituted aromatic aldehydes, such as 2-pyridinecarboxaldehyde, 2- thiophenecarboxaldehyde, 2-pyrrolecarboxyaldehyde and chlorofurfural (entries 10–13), were also yield and selectivity, demonstrating good functional group tolerance (entries 2–4). Under similar reduced to the corresponding alcohols by employing catalyst 2b,, butbut withwith lowerlower yields.yields. TheseThese sub-sub- conditions, p-benzaldehyde with an electron-donating group (–OCH3) was reduced more readily than optimal catalytic results can be rationalized by the expected deactivation of the catalyst following with an electron withdrawing group (–Br, –NO2). Given that ortho and para substituents exhibit similar substrate coordination to the Mn center through the heteroatom in competition with silane or directing effects, the comparable TOF values for the reduction of p- and o-nitrobenzaldehyde (entries aldehyde binding. 4 and 5) to the respective alcohols further indicates that the electronic properties of the substituents To examine the influence of the aromatic ring upon the hydrosilylation reaction, the reduction on the aromatic ring, and not their steric profile, influence the efficiency of the reaction. Heteroatom of aliphatic aldehydes, such as 1-heptanal (entry 14) and 2-methylpentanal, was attempted. While the substituted aromatic aldehydes, such as 2-pyridinecarboxaldehyde, 2-thiophenecarboxaldehyde, GC-MS spectrum after 3 h in both cases indicated the formation of multiple unidentified side 2-pyrrolecarboxyaldehyde and chlorofurfural (entries 10–13), were also reduced to the corresponding products, evidence for the formation of either the silyl ether or alcohol (upon hydrolysis) was not alcohols by employing catalyst 2b, but with lower yields. These sub-optimal catalytic results can be obtained even after prolonged reaction times (24 h). This difference in reactivity between the aliphatic rationalized by the expected deactivation of the catalyst following substrate coordination to the Mn and aromatic systems is likely due to conjugation in the latter case, which serves to weaken the center through the heteroatom in competition with silane or aldehyde binding. aldehyde C=O bond, thereby facilitating hydrosilylation. a Table 2. Hydrosilylation of aldehydes and ketones catalyzed by 2b with PhSiHa . 3.a 3 Table 2. Hydrosilylation of aldehydes and ketones catalyzed by 2b withwith PhSiHPhSiH3.a b Entry Substrate mol % Time (h) Conversion (%)b Entry Substrate mol % Time (h) Conversion (%) b 2.52.5 3.0 3.0 99 99 1 1 1.01.0 3.0 3.0 94 94 0.10.1 6.0 6.0 90 * 90 * 0.1 6.0 90 *

2 2 1.01.0 3.0 3.097 97

3 3 1.01.0 5.0 5.099 99

1.01.0 19 19 77 77 4 4 2.52.5 7.0 7.0 95 * 95 *

5 5 2.52.5 7.0 7.0 8787 ** 87 *

6 2.5 41 50 InorganicsInorganics 2020,, 8,, xx 6 6 2.52.5 41 4150 50 7 of 11

7 7 2.52.5 41 4155 55

8 8 2.52.5 41 41n. r. n. r.

2.52.5 3.0 3.0 99 99 9 9 1.01.0 3.0 3.0 98 98

10 2.5 6.5 71

11 2.5 6.0 87

12 2.5 6.5 15

13 2.5 6.5 53

14 2.5 24 n.r.

aa Reaction conditions: silane (1.2 mmol), substrate (1.0 mmol). bb Determined by 11H NMR on the basis of consumption of substrate (MeOH as internal standard)rd) afterafter hydrolysishydrolysis ofof ththe reaction mixture with 2 M NaOH. All reactions were performed under a nitrogen atmosphere and neat conditions.* isolated yield after purification.

The results also demonstrate that catalyst 2b isis significantlysignificantly moremore efficientefficient atat reducingreducing aldehydesaldehydes inin comparisoncomparison toto ketones.ketones. ForFor example,example, underunder simisimilarlar conditionsconditions andand inin dramaticdramatic contrastcontrast toto thethe reduction of benzaldehyde, the conversion of acetophenone to the corresponding alcohol proceeded with only a 55% yield after 41 h (entry 7). Similarly reduced reactivity was also observed for benzophenone reduction (entry 6). Furthermore, 2b waswas unableunable toto catalyzecatalyze thethe reductionreduction ofof 2-2- methoxyacetophenone even after 41 h.

2.4. Proposed Mechanism For many systems, the mechanism of hydrosilylation has been thoroughly investigated, and the initialinitial stepssteps includeinclude thethe oxidativeoxidative additionaddition ofof anan SiSi–H bond across the metal center. To confirm that Si–H bond activation is also a key step in the current system, experiments were conducted to detect thethe presencepresence ofof aa Mn–HMn–H bondbond uponupon thethe reactionreaction ofof catalystcatalyst withwith silane.silane. TheThe stoichiometricstoichiometric (1:1)(1:1) 1 reaction of catalyst 1b withwith PhSiHPhSiH33 inin thethe absenceabsence ofof aldehydealdehyde waswas followedfollowed byby 1H NMR in acetonitrile-d33.. AtAt roomroom temperature,temperature, nono changechange inin thethe spspectrum was observed after the addition of phenylsilane. However, upon heating the mixture to 8080 °C,°C, thethe colorcolor ofof thethe solutionsolution changedchanged fromfrom dark red to dark yellow, and a peak at δ == −5.68 ppm, assigned to the hydride resonance of the putative Mn(H)(SiH22Ph) intermediate, was observed (Figure S6). The position of the hydride resonance is in a region similar to that observed for the related Mn–silane complexes [17,27]. Unfortunately, attempts to isolate this intermediate were unsuccessful. While an Ojima like hydrosilylation mechanism is likely in the present case, further mechanistic investigations with supporting DFT modeling are ongoing and will be reported in due course.

InorganicsInorganics 2020 2020, 8, ,8 x, x 7 7of of 11 11

7 7 2.52.5 4141 5555

8 8 2.52.5 4141 n.n. r. r. Inorganics 2020, 8, 61 7 of 11

Table2.52.5 2. Cont.3.03.0 9999 9 9 1.01.0 3.03.0 9898 Entry Substrate mol % Time (h) Conversion (%) b

1010 2.52.52.5 6.56.5 6.5 7171 71

1111 2.52.52.5 6.06.0 6.0 8787 87

1212 2.52.52.5 6.56.5 6.5 1515 15

1313 2.52.52.5 6.56.5 6.5 5353 53

1414 2.52.52.5 2424 24n.r.n.r. n.r.

a a b b 1 1 ReactionReactiona Reaction conditions: conditions: silane silane (1.2 (1.2 (1.2 mmol), mmol), mmol), substrate substrate substrate (1.0 mmol). (1.0 (1.0 mmol). mmol).b Determined DeterminedDetermined by 1H NMR by by onH H theNMR NMR basis on ofon consumptionthe the basis basis ofof consumptionof consumption substrate (MeOH of of substrate substrate as internal (MeOH (MeOH standard) as as internal internal after hydrolysis standa standard) ofrd) after the after reaction hydrolysis hydrolysis mixture of of th with the ereaction reaction 2 M NaOH. mixture mixture All reactions with with were performed under a nitrogen atmosphere and neat conditions.* isolated yield after purification. 2 2M M NaOH. NaOH. All All reactions reactions were were perfor performedmed under under a anitrogen nitrogen atmosphere atmosphere and and neat neat conditions.* conditions.* isolated isolated yield after purification. yieldyieldTo after examine after purification. purification. the influence of the aromatic ring upon the hydrosilylation reaction, the reduction of aliphatic aldehydes, such as 1-heptanal (entry 14) and 2-methylpentanal, was attempted. While the TheThe results results also also demonstrate demonstrate that that catalyst catalyst 2b 2b is is significantly significantly more more efficient efficient at at reducing reducing aldehydes aldehydes GC-MS spectrum after 3 h in both cases indicated the formation of multiple unidentified side products, inin comparison comparison to to ketones. ketones. For For example, example, under under simi similarlar conditions conditions and and in in dramatic dramatic contrast contrast to to the the evidence for the formation of either the silyl ether or alcohol (upon hydrolysis) was not obtained even reductionreduction of of benzaldehyde, benzaldehyde, the the conversion conversion of of acet acetophenoneophenone to to the the corresponding corresponding alcohol alcohol proceeded proceeded after prolonged reaction times (24 h). This difference in reactivity between the aliphatic and aromatic withwith only only a a55% 55% yield yield after after 41 41 h h (entry (entry 7). 7). Similarly Similarly reduced reduced reactivity reactivity was was also also observed observed for for systems is likely due to conjugation in the latter case, which serves to weaken the aldehyde C=O bond, benzophenonebenzophenone reduction reduction (entry (entry 6). 6). Furthermore, Furthermore, 2b 2b was was unable unable to to catalyze catalyze the the reduction reduction of of 2- 2- thereby facilitating hydrosilylation. methoxyacetophenonemethoxyacetophenone even even after after 41 41 h. h. The results also demonstrate that catalyst 2b is significantly more efficient at reducing aldehydes in comparison to ketones. For example, under similar conditions and in dramatic contrast to the reduction 2.4.2.4. Proposed Proposed Mechanism Mechanism of benzaldehyde, the conversion of acetophenone to the corresponding alcohol proceeded with only a 55%ForFor many yield many aftersystems, systems, 41 h the (entrythe mechanism mechanism 7). Similarly of of hydrosilylation hydrosilylation reduced reactivity has has been been was th th alsooroughlyoroughly observed investigated, investigated, for benzophenone and and the the initialinitialreduction steps steps include (entry include 6). the the Furthermore, oxidative oxidative addition addition2b was of unable of an an Si Si to–H–H catalyze bond bond across across the reduction the the metal metal of center. 2-methoxyacetophenonecenter. To To confirm confirm that that Si–HSi–Heven bond bond after activation activation 41 h. is is also also a akey key step step in in the the cu currentrrent system, system, experiments experiments were were conducted conducted to to detect detect thethe presence presence of of a aMn–H Mn–H bond bond upon upon the the reaction reaction of of catalyst catalyst with with silane. silane. The The stoichiometric stoichiometric (1:1) (1:1) 1 reactionreaction2.4. Proposed of of catalyst catalyst Mechanism 1b 1b with with PhSiH PhSiH3 3in in the the absence absence of of aldehyde aldehyde was was followed followed by by 1 H1H NMR NMR in in acetonitrile-d3. 3At room temperature, no change in the spectrum was observed after the addition of acetonitrile-acetonitrile-For manydd. .At At systems, room room temperature, temperature, the mechanism no no change ofchange hydrosilylation in in the the sp spectrumectrum has been was was thoroughlyobserved observed after after investigated, the the addition addition and of theof phenylsilane. However, upon heating the mixture to 80 °C, the color of the solution changed from phenylsilane.phenylsilane.initial steps However, includeHowever, the upon upon oxidative heating heating addition the the mixture mixture of an Si–Hto to 80 80 bond°C, °C, the the across color color the of of metalthe the solution solution center. changed Tochanged confirm from from that dark red to dark yellow, and a peak at δ = −5.68 ppm, assigned to the hydride resonance of the darkdarkSi–H red red bond to to dark activationdark yellow, yellow, is alsoand and a a key apeak peak step at at in δ the δ= =− current5.68−5.68 ppm, ppm, system, assigned assigned experiments to to the the werehydride hydride conducted resonance resonance to detect of of the the the putative Mn(H)(SiH2Ph)2 intermediate, was observed (Figure S6). The position of the hydride putativeputativepresence Mn(H)(SiHMn(H)(SiH of a Mn–HPh)Ph) bond intermediate,intermediate, upon the reaction waswas observed ofobserved catalyst (Figure with(Figure silane. S6).S6). TheThe stoichiometric positionposition ofof thethe (1:1) hydridehydride reaction resonanceresonance is is in in a aregion region similar similar to to that that obse observedrved for for the the related related Mn–silane Mn–silane1 complexes complexes [17,27]. [17,27]. of catalyst 1b with PhSiH3 in the absence of aldehyde was followed by H NMR in acetonitrile-d3. Unfortunately, attempts to isolate this intermediate were unsuccessful. While an Ojima like Unfortunately,Unfortunately,At room temperature, attemptsattempts no toto change isolateisolate in thisthis the intermediate spectrumintermediate was were observedwere unsuccessful.unsuccessful. after the addition WhileWhile anan of Ojima phenylsilane.Ojima likelike hydrosilylationhydrosilylation mechanism mechanism is is likely likely in in the the present present case, case, further further mechanistic mechanistic investigations investigations with with However, upon heating the mixture to 80 ◦C, the color of the solution changed from dark red to dark supporting DFT modeling are ongoing and will be reported in due course. supportingsupportingyellow, and DFT DFT a peak modeling modeling at δ = are are5.68 ongoing ongoing ppm, assigned and and will will tobe be the reported reported hydride in in resonance due due course. course. of the putative Mn(H)(SiH Ph) − 2 intermediate, was observed (Figure S6). The position of the hydride resonance is in a region similar to that observed for the related Mn–silane complexes [17,27]. Unfortunately, attempts to isolate this intermediate were unsuccessful. While an Ojima like hydrosilylation mechanism is likely in the present case, further mechanistic investigations with supporting DFT modeling are ongoing and will be reported in due course. Inorganics 2020, 8, 61 8 of 11

3. Materials and Methods

3.1. General Unless otherwise stated, all experiments were performed in the absence of light. Anhydrous solvents (MeCN, CH2Cl2, Et2O) and Schlenk techniques were used for the synthesis of all the complexes. Manganese pentacarbonyl bromide (Mn(CO)5Br) and thallium hexafluorophosphate (TlPF6) were i purchased from Strem chemicals, and the ligands Pr2Ph-DAB and Me2Ph-DAB were purchased from Sigma-Aldrich (St. Louis, MI, USA). All reagents were used as received without further purification. NMR spectra were recorded on a Bruker Avance II 400 spectrometer (Billerica, MA, USA) while GC-MS data were obtained using an Agilent GC-MS spectrometer (Santa Clara, CA, USA). 1H, 19F, and 13C NMR were referenced to residual solvent resonances. Infrared spectra were obtained on a Bruker vertex 80 FTIR spectrometer utilizing a 0.75 mm path length temperature-controlled cell (Harrick Scientific) with CaF2 windows. Compounds 1a, 1b, and 1c were synthesized according to the reported procedure [28].

3.2. fac-Mn(Br)(CO)3(Me2Ph-DAB) (2a)

A round-bottom flask was charged with 0.315 g of Mn(CO)5Br (1.15 mmol), 0.300 g of Me2Ph-DAB (1.14 mmol), and 20 mL of anhydrous diethyl ether under a nitrogen atmosphere. The solution was left overnight with constant stirring at room temperature. The solvent was evaporated to yield a solid dark blue-colored residue which was washed with cold hexane (3 2 mL) and dried under vacuum to afford × 0.496 g of a dark blue powder (90% yield). Dark blue-colored crystals of suitable quality for single crystal X-ray diffraction were grown from slow evaporation of the solvent (1:1 dichloromethane/hexane) 1 at 25 ◦C. IR data in dichloromethane (νCO in cm− ): 2035 (s), 1971 (s), 1930 (m). NMR data in CDCl3: 1 H δ 8.32 (s, 2 H), 7.13 (m, 6 H), 2.55 (s, 6 H of CH3), 2.26 (s, 6 H of CH3).

3.3. fac-[Mn(CO)3(Me2Ph-DAB)(CH3CN)]PF6 (2b)

A round-bottom flask was charged with 0.500 g of 2a (1.08 mmol), 0.725 g TlPF6 (2.08 mmol), and 50 mL of anhydrous dichloromethane under a nitrogen atmosphere. To the above reaction mixture, 1 mL of anhydrous acetonitrile was added via a syringe. The solution was left overnight with constant stirring at room temperature. The reaction mixture was filtered through a Celite pad to remove thallium bromide and evaporated to yield an orange residue which was washed several times with diethyl ether and dried in vacuo to afford 0.450 g (76% yield) of an orange solid, 2b. Orange crystals of suitable quality for X-ray diffraction were grown from the slow evaporation of a solvent mixture (hexane/dichloromethane (1:1) at 25 ◦C in an nitrogen-filled KIYON Glovebox (Kiyon Glovebox System, 1 Seoul, South Korea). IR data in dichloromethane (νCO in cm− ): 2053 (s), 1978 (m), 1963 (br). NMR data 1 in CD2Cl2: H δ 8.44 (s, 2 H), 7.17 (m, 6 H), 2.35 (s, 3 H of MeCN), 2.30 (s, 6 H of DAB-Me), 2.24 (s, 6 H of DAB-Me).

3.4. X-ray Crystallography Diffraction data for 2a and 2b were obtained by placing the crystals under streaming nitrogen (100 K) in a Bruker-AXS Smart Apex CCD single-crystal diffractometer. Data were collected at 100(2) K using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The space groups were determined based on systematic absences and intensity statistics. The structures were solved by direct methods and refined by fullmatrix least-squares on F2. Anisotropic displacement parameters were determined for all non-hydrogen atoms. Hydrogen atoms were placed at idealized positions and refined with fixed isotropic displacement parameters. The following is a list of programs used: data collection and cell refinement, APEX2 [29]; data reductions, SAINTPLUS version 6.63 [30]; absorption correction, SADABS [31]; structural solutions, SHELXS-97 [32]; structural refinement, SHELXL-97 [33]; graphics and publication materials, Mercury version 3.0 [34]. The CIF files, tables, and CHECKCIF outputs are available in the Supporting Information (Tables S1–S6). Inorganics 2020, 8, 61 9 of 11

3.5. General Procedure for Hydrosilylation of Carbonyl Compounds Employing Catalyst 2b In an inert air glove box, an 8 mL scintillation vial was charged with an appropriate amount of catalyst 2b (1.0 mol % or 2.5 mol %). The vial was charged with 1.0 mmol of PhSiH3 and 0.78 mmol of the carbonyl compound (aldehyde or ketone). The progress of the reaction was monitored by GC-MS. After completion of the reaction, the reaction mixture was treated with 5.0 mL of dichloromethane solvent. The resulting reaction mixture was then hydrolyzed with 2.0 M aqueous NaOH solution at room temperature for 2 h. The product was extracted from the dichloromethane layer and dried over anhydrous magnesium sulphate. The solvent was removed by rotary evaporation (37 ◦C) under reduced pressure to isolate the corresponding alcohol. The conversion yield was determined by NMR spectroscopy following the addition of 0.51 M MeOH in a capillary tube as an internal standard. A similar procedure was followed for hydrosilylation with catalysts 1a–1c. The product alcohol 1H NMR spectra for all the substrates used in this study were in agreement with the literature-reported data [21].

4. Conclusions Air stable and easy to handle manganese complexes incorporating a bulky DAB ligand were found to be attractive hydrosilylation catalysts. In the presence of these catalysts, the eﬃcient hydrosilylation of aldehydes with good functional group tolerance and chemoselectivity was observed with primary silanes as the reducing agent. The reactions proceeded without the need for an external activating agent. In some cases, direct alcohol product formation was observed by GC-MS even without hydrolysis. Aldehydes such as benzaldehyde and cinnamaldehyde were converted into corresponding alcohols 1 at the moderate catalytic loading of 0.1 mol %, with turn-over frequencies approaching 150 h− , comparable to other Mn-based catalysts. The conversion of aromatic aldehydes was more eﬃcient than for the aliphatic systems. Mechanistic studies are on-going but there has been an initial detection of a Mn–H intermediate, suggesting a classical Ojima-like mechanism for the catalytic reaction.

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/8/11/61/s1, Figure S1: X-ray crystal structure for compound fac-Mn(Br)(CO)3(MePh-DAB) (2a), Figure S2: GC-MS of the reaction mixture (Cinnamaldehyde + Silane) at 80 ◦C in the presence of catalyst 1b after 10 min, Figure S3: GC-MS of the reaction mixture (Cinnamaldehyde + Silane) at 80 ◦C in the absence of catalyst 1c after 10 min, Figure S4: GC-MS of the reaction mixture (Cinnamaldehyde + Silane) at 80 ◦C in the absence of catalyst 1d after 10 min, Figure S5: GC-MS of the reaction mixture (Cinnamaldehyde + Silane) at 80 ◦C in the absence of catalyst, Figure S6: 1 H-NMR spectrum of catalyst 1b with a stoichiometric amount of PhSiH3 at RT in acetonitrile-d3, Tables S1–S6: X-Ray crystallographic data, bond angles, and bond lengths for Compounds 2a and 2b. Author Contributions: Conceptualization, V.Y.; Formal analysis, V.Y.; Funding acquisition, W.Y.F.; Methodology, V.Y. and W.Y.F.; Supervision, A.A.B. and S.T.M.; Validation, A.S.; Writing—original draft, V.Y.; Writing—review & editing, A.A.B., S.T.M. and W.Y.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by a grant from the Qatar National Research Fund (QNRF) under the National Priorities Research Program, grant number 8-094-1-018. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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