Steric tuning of OH-functionalised N,N-manganese(I) complexes for the transfer hydrogenation of carbonyl compounds

Abstract

A new family of manganese(I) carbonyl complexes, [2-OH-6-{CMe=N(2,6-R¹₂-4-R²C₆H₂)}C₅H₃N]MnBr(CO)₃ (R¹ = ⁱPr, R² = H Mn1a, R¹ = ⁱPr, R² = Br Mn1b, R¹ = H, R² = ⁱPr Mn1d, R¹ = H, R² = Me Mn1e), bearing 6-(arylimino)pyridine N,N-chelating ligands that are appended with a 2-hydroxyl group, have been synthesised and evaluated as catalysts in the transfer hydrogenation (TH) of acetophenone and its derivatives. This set of complexes, differing in their steric and electronic properties of their N-aryl groups, reveal distinct catalytic performances with the least sterically hindered N-4-isopropylphenyl derivative, Mn1d, the most active allowing TON's of up to 1030. Moreover, Mn1d proved highly effective for an assortment of aryl-methyl ketones with the steric/electronic profile of the substrate having an observable effect on conversion. Mechanistic studies involving DFT calculations point towards an outer-sphere mechanism that is distinctly affected by the steric properties of the N-aryl groups of the manganese catalyst.

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Introduction

Within the field of transition metal (TM) catalysis, there is an on-going drive to develop more sustainable alternatives to precious metal systems that have long formed the cornerstone of homogeneous catalysis.^1,2 Notably, base metal catalysts drawn from the first row of the TMs have seen the emergence of molecular systems that are starting to approach the levels of performance displayed by their precious metal counterparts. One such area that has attracted attention in recent years is the use of manganese(I) complexes as these have shown promise as catalysts for a number of organic transformations.^3,4

With a view to improving the efficacy of such low valent Mn catalysis, the installation of non-innocent or functionalised ligands has been increasingly developed to access lower energy transition states in catalytic transformations or to imbue the complex with desirable properties.^1,5–10 In terms of the functionalised ligand itself, this can assist the metal centre through the use of pendant groups to help direct substrates or to act as a redox active unit. These redox active ligands work in concert with the metal centre to promote reversible transformations during the catalysis such as protonation/deprotonation of the ligand so as to accelerate catalysis.⁸

While much progress has been reported with NH-functionalised P,N,P and P,N,N ligands on manganese,⁹ research directed towards smaller and simpler bidentate systems incorporating a range of potentially redox non-innocent donor units including imines, has only recently started to emerge.¹⁰ Of note, a number of pyridine-containing examples functionalised with OH groups have displayed great potential (Fig. 1),^11–20 whereby the synergistic pairing of the hydroxy functionality and a manganese(I) centre has seen catalysts capable of facilitating a wide range of transformations under increasingly mild conditions.

Fig. 1 Previously reported manganese(I) complexes incorporating hydroxyl-functionalised pyridine-based chelating ligands, along with the target of this work.

In the context of transfer hydrogenation (TH), manganese(I) catalysis has witnessed some marked improvements^9,13,17–26 with TONs of up to 17 300 reported.^10d,g Notwithstanding these developments the mechanistic details of the reaction pathways followed remain uncertain with both inner- and outer-sphere mechanisms postulated.^4,9,27–33 Furthermore, the impact of steric effects and, to a lesser degree, electronic factors of the catalyst on the mechanistic pathway remains relatively unexplored.³⁴

Herein we report the synthesis of a novel series of manganese(I) carbonyl bearing 2-hydroxy-6-(arylimino)pyridine ligands differing in the steric and electronic profile of the N-aryl groups (Fig. 1). These complexes are then investigated as catalysts for the TH of firstly acetophenone and then a series of derivatives to explore the role of these steric/electronic variations on catalyst performance. To complement the experimental work, DFT calculations are performed to probe the mechanistic pathway and the role played by these steric effects.

Results and discussion

Synthesis of 6-arylimino-2-pyridone

The 6-arylimino-2-pyridones, 6-{CMe=N(2,6-R¹₂-4-R²C₆H₂)}C₅H₃NH-2-O (R¹ = ⁱPr, R² = H HL1a, R¹ = ⁱPr, R² = Br HL1b, R¹ = R² = ⁱPr HL1c, R¹ = H, R² = ⁱPr HL1d, R¹ = H, R² = Me HL1e), were synthesised in moderate to good yield via the condensation reaction of 6-acetyl-pyrid-2-one with the corresponding ortho- and para-substituted anilines in n-butanol at elevated temperature in the presence of an acid catalyst (Scheme 1). The 6-acetyl-pyrid-2-one starting material is not commercially available but could be readily obtained in two steps from 2-bromo-6-methoxypyridine. Confirmation that HL1a–HL1e adopt the 2-pyridone tautomer over the 2-pyridinol one was provided by single crystal X-ray diffraction studies performed on HL1a and HL1e (see SI). Indeed, free 6-substituted 2-pyridones generally have a tendency to adopt this tautomer.^35,36 Some variation in the solubility of HL1a–HL1e was seen with HL1a–HL1c displaying good solubility in hexane likely due to the increase in solubilising isopropyl groups on the N-aryl ring, whilst HL1d and HL1e proved noticeably less soluble.

Scheme 1 Synthetic route to the 6-arylimino-2-pyridones, HL1a–HL1e.

Syntheses of Mn1a–Mn1e and Mn2

The orange crystalline complexes, [2-OH-6-{CMe=N(2,6-R¹₂-4-R²C₆H₂)}C₅H₃N]MnBr(CO)₃ (R¹ = ⁱPr, R² = H Mn1a, R¹ = ⁱPr, R² = Br Mn1b, R¹ = H, R² = ⁱPr Mn1d, R¹ = Me, R² = Me Mn1e) were typically prepared in good yield by reacting HL1a, HL1b, HL1d and HL1e with a stoichiometric amount of manganese pentacarbonyl bromide in dichloromethane or diethyl ether overnight at 30 °C (Scheme 2). Conversely, [2-OH-6-{CMe=N(2,4,6-ⁱPr₃C₆H₂)}C₅H₃N]MnBr(CO)₃ (Mn1c), owing to the greater solubility of HL1c, was prepared using hexane at reflux to afford Mn1c as a bright orange powder.

Scheme 2 Preparation of Mn1a–Mn1e from HL1a–HL1e.

Single crystals of Mn1a–Mn1e suitable for X-ray diffraction studies were grown from methanol upon standing for 48 hours at room temperature. A view of representative Mn1d is shown in Fig. 2, while selected bond distances and angles are given in the caption (see SI for Mn1a, Mn1b, Mn1c and Mn1e). The structures of Mn1a–Mn1e are isostructural and based on a distorted octahedral arrangement in which the bromide and the two N-donor atoms of the chelating N,N-ligand are trans to carbonyl groups, while the third CO is trans to a bromide. In terms of the N,N-ligand, this now adopts the 2-pyridinol form following coordination to the Mn(I) centre. Interestingly, the bulkier 2,6-substituted N-aryl groups in Mn1a, Mn1b and Mn1c impart some distortion along the Br(1)–Mn(1)–CO axis with corresponding angles of 169.76(8)°, 170.53(7)° and 171.40(12)°, respectively. By contrast in Mn1d the corresponding Br–Mn–CO bond angle was found to be 179.64(6)° highlighting the minimal effect of the N-4-ⁱPrC₆H₅ group on this axis. Furthermore, the Mn(1)–N(2)_imine bond distance provides a valuable indicator on the steric properties imparted by the N-aryl ortho substituents with this bond length increasing in the order: Mn1a (2,6-ⁱPr₂) > Mn1e (2,6-Me₂) > Mn1d (2,6-H₂). In all cases, the O(1)–H proton is involved in intermolecular hydrogen bonding leading to some form of multimetallic assembly. For Mn1a, Mn1b, Mn1c and Mn1e, lattice methanol is additionally incorporated into the assembly, while Mn1d makes use of solely O(1)H⋯Br interactions.

Fig. 2 ORTEP representation of Mn1d with the thermal elipsoids set to 50% probability. All protons except for H(1) have been removed for clarity. Selected bond distances (Å): Br(1)–Mn(1) 2.5470(3), Mn(1)–N(1) 2.0736(14), Mn(1)–N(2), 2.0300(14). Selected bond angles (°): N(1)–Mn(1)–Br(1) 84.57(4), N(2)–Mn(1)–N(1) 77.69(5), C(18)–Mn(1)–Br(1), 179.64(6). Selected torsion angle (°): N(2)–C(8)–C(9)–C(10) 178.70(15).

The IR data for Mn1a–Mn1e reveal three strong carbonyl absorption bands with modest variation between complexes. For the 2,6-diisopropyl-containing Mn1a, Mn1b and Mn1c, the average carbonyl stretching frequency is the highest for para-bromo Mn1b (1958 cm⁻¹ vs. 1953 cm⁻¹ for Mn1a and Mn1c) which is in line with the least back-donation into the CO ligands on account of the electron withdrawing bromide. This trend is also reflected in the ¹³C NMR spectra in which the average of the chemical shifts for the three carbonyl resonances for Mn1b is the most downfield for this series of complexes. In the ¹H NMR spectra, the OH protons can be seen in a narrow downfield range between δ 12.45–13.31 in keeping with their involvement in hydrogen bonding as seen in the X-ray structures.

For comparative purposes the related N,O-manganese complex, [2-OH-6-(CMe=O)C₅H₃N]MnBr(CO)₃ (Mn2), was also synthesised from 6-acetyl-pyrid-2-one and structurally characterised (Fig. 3).³⁷ As expected, the introduction of the N,O-ligand has a notable effect on the carbonyl resonances in the ¹³C NMR spectrum with a downfield shift clearly evident from that seen for Mn1a–Mn1e suggesting a sizable difference in back-bonding, which is also reflected by a dramatic increase in the average carbonyl bond stretching frequency (Table 1).

Fig. 3 ORTEP representation of Mn2 with thermal ellipsoids set to 50% probability. All protons, apart from H1, have been removed for clarity. Selected bond distances (Å) Br(1)–Mn(1) 2.5477(9), Mn(1)–N(1) 2.091(4), Mn(1)–O(2) 2.033(4), Mn(1)–C(8) 1.805(5), Mn(1)–C(9) 1.806(6), Mn(1)–C(10) 1.808(6). Selected bond angles (°): O(2)–Mn(1)–Br(1) 87.24(10), O(2)–Mn(1)–N(1) 77.51(15), C(8)–Mn(1)–Br(1) 176.76(18), C(18)–Mn(1)–Br(1) 179.64(6).

Table 1 ¹³C{¹H} NMR signals for the Mn–CO's and the CO stretching frequencies in Mn1a–Mn1e and Mn2

Complex	^13C NMR carbonyl signals^a (δ, ppm)	ν(CO) stretching frequency^b (cm^−1)
a Recorded in DMSO-d6 at 298 K.b FT-IR data recorded in the solid state.
Mn1a	220.9, 222.3, 222.9	2018, 1937, 1903
Mn1b	220.9, 222.5, 222.9	2019, 1954, 1901
Mn1c	220.4, 221.6, 222.4	2018, 1937, 1903
Mn1d	220.6, 221.8, 222.7	2024, 1940, 1904
Mn1e	221.3, 222.4, 222.6	2020, 1940, 1897
Mn2	219.3, 223.4, 225.6	2037, 1950, 1926

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Derivatisation and solvent dependent deprotonation

During attempts at growing crystals of the manganese complexes for X-ray diffraction, it became evident that the type of crystallising solvent and the time spent in the solvent could lead to aggregation, following deprotonation and partial oxidation of the metal centre. For example, attempted crystallisation of Mn1d from a dimethylformamide (DMF) solution layered with diethyl ether gave after several weeks red blocks of the trinuclear species, [[2-O-6-{CMe=N(4-ⁱPrC₆H₄)}C₅H₃N]MnBr(CO)₃]₂Mn(DMF)₄ (Mn1d′) in which a Mn(II)(DMF)₄ fragment in sandwiched between two (N,N)Mn(I) units (Fig. 4). Evidently, the deprotonation of the 2-pyridinol unit is a facile process which we believe is pivotal to the function of the catalyst (see later). Curiously, partial deprotonation and scavenging of a CO was observed when Mn1b was crystallised from a mixture of dichloromethane and hexane forming yellow blocks of co-crystallised Mn1b⋯[2-O-6-{CMe=N(2,6-ⁱPr₂-4-BrC₆H₂)}C₅H₃N]Mn(CO)₄ (Mn1b′) which associate through hydrogen-bonding to form a dimeric pair (Fig. 5). Interestingly, comparison of the C(1)–O(1) and C(23)–O(5) bond lengths in these neighbouring molecules highlight the presence of a 2-pyridinol C–O bond (1.323(7) Å) and a 2-pyridonate C=O bond (1.266(7) Å), respectively; an observation that elegantly highlights the proton responsiveness of the OH functional group.¹¹

Fig. 4 ORTEP representation of Mn1d′ with the thermal ellipsoids set to 50% probability. Grown for a ‘half’ asymmetric unit through symmetry operation −x, −y + 1, −z + 1. All protons have been removed for clarity. Selected bond distances (Å): Br(1)–Mn(1) 2.5295(4), Mn(1)–N(1) 2.0535(16), Mn(1)–N(2), 2.0332(16), C(1)–O(1) 1.274(2), O(1)–Mn(2) 2.1485(14). Selected bond angles (°): N(1)–Mn(1)–Br(1) 86.57 (4), N(2)–Mn(1)–N(1) 77.91(6), C(18)–Mn(1)–Br(1), 178.36(11), O(5)–Mn(2)–O(6) 92.31(5).

Fig. 5 ORTEP representation of Mn1b′ with thermal ellipsoids set to 50% probability. All protons, apart from H1, have been removed for clarity. Selected bond distances (Å): Mn(1)–N(1) 2.053(4), Mn(1)–N(2) 2071(4), Mn(1)–Br(1) 2.5343(11), Mn(1)–C(20) 1.815(6), Mn(1)–C(21) 1.813(8), Mn(1)–C(22) 1.805(6), C(1)–O(1) 1.323(7), Mn(2)–N(3) 2.081(4), Mn(2)–N(4) 2.078(4), Mn(2)–C(42) 1.805(7), Mn(2)–C(43) 1.859(8), Mn(2)–C(44) 1.799(7), Mn(2)–C(45) 1.854(9), C(23)–O(5) 1.266(7). Selected bond angles (°): N(2)–Mn(1)–N(1) 77.67(15), C(21)–Mn(1)–Br(1) 166.73(19), N(3)–Mn(2)–N(4) 76.83(17), C(45)–Mn(2)–C(43) 167.2(3).

Transfer hydrogenation catalysis

To explore the effectiveness of Mn1a–Mn1e and Mn2 as catalysts for transfer hydrogenation, the conversion of acetophenone (1a) to 1-phenyl ethanol (2a) was chosen as the test transformation (Table 2).^11,17 Typically, these reactions were performed in isopropanol as the hydrogen source, potassium tert-butoxide as the base with the run temperature maintained at 80 °C over a 24-hour run time. An initial screen, based on a [Mn] : [KO^tBu] : [acetophenone] ratio of 1 : 5 : 100, showed a range in levels of catalytic activity with Mn1d > Mn1e > Mn1c ~ Mn1b > Mn1a ≫ Mn2 (entries 1–6, Table 2). Of those tested, Mn1d showed the highest activity (S/C = 100/1, 96% conversion, entry 4, Table 2), while 6-acetyl-2-pyridone Mn2 was almost inactive (13% conversion, entry 6, Table 2). Within the 6-arylimino-2-pyridone series, Mn1d was the most active which would suggest the reduced steric properties of the N-4-isopropylphenyl group are highly influential. As a control, Mn(CO)₅Br which is known to have some catalytic ability in other transformations,³⁸ MnBr₂ as well as free HL1d were also tested (entries 7–9, Table 2); the conversions were however significantly lower than any of the catalysts synthesised in this work.

Table 2 Catalytic evaluation for the TH of acetophenone and subsequent optimisation of Mn1d^a

Entry	Catalyst (mol%)	Base (mol%)	Conversion (%)
a Reaction conditions: catalyst (x mol%), base (x mol%), acetophenone (2 mmol) in ^iPrOH (5 mL) at 80 °C for 24 h in darkness. Spectroscopic conversions (%) were measured using quantitative ^1H NMR spectroscopy.b Reaction time 48 h.c Reaction time 4 h.
1	Mn1a (1)	K^tBuO (5)	58
2	Mn1b (1)	K^tBuO (5)	65
3	Mn1c (1)	K^tBuO (5)	65
4	Mn1d (1)	K^tBuO (5)	96
5	Mn1e (1)	K^tBuO (5)	80
6	Mn2 (1)	K^tBuO (5)	13
7	Mn(CO)5Br (1)	K^tBuO (5)	8
8	MnBr2 (1)	K^tBuO (5)	8
9	HL1d	K^tBuO (5)	5
10	Mn1d (0.5)	K^tBuO (5)	94
11	Mn1d (0.5)	K^tBuO (3)	98
12	Mn1d (0.5)	K^tBuO (1)	56
13	Mn1d (0.5)	K^tBuO (10)	72
14	Mn1d (0.5)	K^tBuO (20)	75
15	Mn1d (0.5)	Na^tBuO (3)	60
16	Mn1d (0.5)	KOH (3)	33
17	Mn1d (0.5)	NaOH (3)	56
18	Mn1d (0.5)	None	0
19^b	Mn1d (0.03)	K^tBuO (0.18)	34
20^c	Mn1d (0.5)	K^tBuO (3)	96

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Subsequently, Mn1d was then used alone to identify an optimised set of conditions using acetophenone as the test substrate (entries 10–19, Table 2). Firstly, by systematically varying the [Mn1d] : [KOBu^t] : [1a] ratio, it was found that a ratio of 1 : 6 : 200 gave the highest conversion of 98% (entry 11, Table 2). Secondly, the type of base was screened by additionally employing Na^tBuO, KOH and NaOH with the result that KOBu^t remained the most effective (entries 11 and 15–17, Table 2); an observation that has some literature precedent.^{11,17,39–42} As a control experiment, the absence of base gave 0% conversion (entry 18, Table 2). Notably with the substrate to catalyst ratio (S/C) increased to 3000/1, a 34% conversion could still be attained, which corresponds to a TON of 1030 (entry 19, Table 2).

Given that the initial TH testing had all been conducted over 24 hours, it was decided to probe the catalytic performance of these manganese catalysts over shorter run times.^11,40 Hence, by monitoring the reaction progress of Mn1d under optimised conditions at regular intervals it became clear that there was a rapid conversion of 81% after just 0.5 h. This then increased to 96% after 4 h before reaching a plateau (entry 20, Table 2). It is possible that the diffusion of the catalyst and substrate within the solution could justify the slower second stage of the reaction.

To probe the steric effects observed in the catalysis, the percentage buried volume (V_bur) around the potential active catalytic site was computed for Mn1c, Mn1d, Mn1e and Mn2 (Fig. 6). It can be noted that in the southeastern (SE) quadrant in the figure, a trend in decreasing free volume is seen: Mn1d (40%) > Mn1e (29.4%) > Mn1c (23.9%). This decrease in free volume between Mn1c and Mn1d suggests reduced access to the active site supporting the experimentally determined conversions. As expected, ketone-containing Mn2 exhibits an even greater value for V_bur in the SE quadrant (69.4%).

Fig. 6 SambVca generated steric maps of the (N,N)Mn(CO)₃ or (N,O)Mn(CO)₃ fragments in Mn1c, Mn1d, Mn1e and Mn2 showing the buried volume (V_bur) around the catalytic centre.⁴³ Map radius set to 5 Å, bromine atom removed with positional representation of molecules orientation. For Mn1a and Mn1b see SI.

With Mn1d identified as the most effective catalyst, we then set about exploring the substrate scope of this manganese complex. In particular, a selection of (hetero)aryl-methyl ketone substrates ere explored in order to gain a deeper understanding of the tolerance of this catalyst to differing electronic and steric properties (Scheme 3).

Scheme 3 TH of a selection of substituted (hetero)aryl-methyl ketones. Optimised reaction conditions as follows: catalyst (0.01 mmol), base (0.06 mmol), ketone (2 mmol) in ⁱPrOH at 80 °C for 24 h in darkness. Spectroscopic conversions (%) were measured using quantitative ¹H NMR spectroscopy.

Besides the initially tested substrate (1a: 98% conversion to 2a), para-alkyl substituted derivatives 1b and 1j were also shown to give good conversions to the corresponding alcohols (2b and 2j), whereas the mesityl-substituted substrate 1h showed inferior performance, an observation that is likely due to the additional steric properties of the ortho-methyl groups. A clear favourability was identified for fluorine-containing aryl substrates where high conversions to the corresponding alcohol were observed (>98% for 1d–1f). This could be, in part, due to the powerful electron withdrawing effect of these fluorine atoms across the aryl ring making the carbonyl bond more susceptible to addition of two hydrogen atoms (see later). It is notable that this is not the case for all the halogenated substrates tested with bromide containing substrate 1c proving rather sluggish. In addition, the one non-methyl aryl ketone examined, CF₃-containing 1m, showed a considerably lower conversion. Moreover, the para-amino bearing 1i afforded less than 10% conversion which can likely be attributed to the amino-nitrogen undergoing coordination to the catalyst and, hence inhibiting turnover. Perhaps of wider applicability of this catalysis, is its tolerance towards different heterocycles. Notably, the current catalyst saw excellent to moderate conversions to the corresponding alcohols using pyridyl ketone 1k and thiophene-containing ketone 1n, respectively. As a more challenging substrate, the ferrocenyl substrate (1l) allowed only low conversions to 2l (7%).

Mechanistic studies

In an attempt to shed light on the catalytic steps followed in these transformations, DFT calculations were employed using Mn1d in order to model both (i) an outer-sphere concerted reaction pathway and (ii) a stepwise inner-sphere mechanism based on the conversion of 1a to 2a; the former generally being considered the most favoured.^{11,28,29,32,33,41,44–47} Scheme 4 shows the proposed outer-sphere pathway using Mn1d-I (generated following loss of HBr from Mn1d) as the starting point (set at ΔG = 0.0 kcal mol⁻¹). Following interaction of Mn1d-I with an incoming isopropanol molecule, Mn1d-TS2-OUT is formed as a transition state. Concerted protonation and hydride formation ensues to form hydride intermediate, Mn1d-III, which is tentatively supported by a broad hydride resonance at ca. δ −5 ppm in the ¹H NMR spectrum. Mn1d-III can then undergo interaction with acetophenone (1a) to form two transition states, Mn1d-TS3-OUT-R (21.3 kcal mol⁻¹) in which the phenyl ring of the substrate is positioned above the pyridinol ring and a higher energy transition state (Mn1d-TS3-OUT-S, 22.5 kcal mol⁻¹) in which the phenyl group is remote from the pyridinol ring. This small differences in energy can likely be credited to the assistive effect of π–π stacking of the aromatic rings as well as the steric hindrance with the N-aryl group. Interestingly the relative energy level of Mn1d-TS2-OUT suggests that the largest energy barrier is not in the hydrogen transfer to 1a (ΔG = 13.3, 12.1 kcal mol⁻¹) but rather the initial formation of the Mn–H species (ΔG = 24.7 kcal mol⁻¹).

Scheme 4 DFT calculated energy pathway for the proposed outer sphere mechanism for the transfer hydrogenation of 1a using Mn1d in isopropanol with geometry-optimised coordinates of transition states (a) Mn1d-TS2-OUT, (b) Mn1d-TS3-OUT-R and (c) Mn1d-TS3-OUT-S shown above. The solvent corrected energy values are shown in italics, while associated bond distances/interactions in the (a), (b) and (c) are in Å.

The alternative inner sphere pathway is shown in the SI (see Fig. S67) and proceeds in a stepwise manner involving three Mn intermediates. Firstly, Mn1d-I is transformed into INT-II via transfer of the O–H proton from 1a to the pyridonate unit leading to pyridinol (via aromatisation) and isopropoxide formation. β-H elimination and acetone dissociation then gives hydride intermediate, Mn1d-III; also found in the outer sphere pathway. Subsequently, transfer of the Mn-hydride in Mn1d-III to the carbonyl carbon in 1a leads to 1-phenylethoxide INT-IV (as R and S forms) that can regenerate Mn1d-I by loss of 2a.

Overall, comparison of the two computed pathways reveals a modest difference in the highest energy transition states of 1.5 kcal mol⁻¹ (viz. Mn1d-TS2-IN vs. Mn1d-TS2-OUT, Fig. S68), with only a minor bias towards the outer sphere pathway. Consequently, an inner-sphere pathway remains plausible; similar conclusions have been reported elsewhere.²⁸ However, it should be pointed out that solvent-assisted pathways and the requirement for excess base have not been fully accounted for in this mechanistic analysis.^48,49

As noted earlier, the experimental results indicate that manganese complexes with N-aryl groups bearing more sterically bulky ortho-substituents were less effective in this transformation (98% for Mn1d vs. 56% for Mn1c). To further probe this observation, DFT calculations were additionally employed using Mn1c (N-2,4,6-triisopropylphenyl) to compute the corresponding outer-sphere TH pathway. Scheme 5 shows a catalytic cycle that incorporates the highly exergonic initiation step and shows the relative energies for Mn1-TS2-OUT and Mn1-TS3-OUT for both Mn1c and Mn1d. In both cases, the values determined for Mn1c exceed those found for Mn1d, with Mn1c-TS2-OUT higher in energy by 3.3 kcal mol⁻¹, while Mn1c-TS3-OUT-R and Mn1c-TS3-OUT-S are higher by 5.8 and 5.4 kcal mol⁻¹ respectively; findings that would suggest that Mn1-TS2-out is the rate determining step. Similarly, the ground state for hydride Mn1-III is 1.5 kcal mol⁻¹ higher in energy for Mn1c.

Scheme 5 Catalytic cycle (top) for the transfer hydrogenation of acetophenone (1a) for both Mn1d and Mn1c along with the computed energy profiles (bottom); all ΔG values are in kcal mol⁻¹.

In short, the higher energy barriers for Mn1-TS2-OUT and Mn1-TS3-OUT for Mn1c over Mn1d support the experimental findings and highlight the importance of steric factors in the TH. Evidently, the catalytic pocket created by Mn1d is more accessible thereby leading to a more effective approach by the substrate.

Conclusions

In summary, five examples of 6-arylimino-2-pyridones, HL1a–HL1e, have been successfully synthesised using a series of straightforward synthetic steps. Upon reaction with Mn(CO)₅Br, the Mn(I) complexes Mn1a–Mn1e have been isolated in good yield and fully characterised and reveal the N,N-chelating ligand to adopt the 2-pyridinol form. Likewise, the related N,O-complex Mn2 has shown a preference for the 2-pyridinol form. All six complexes exhibited distinct variations in the level of activity for the TH of acetophenone with the least sterically hindered 4-isopropylphenyl-containing Mn1d the standout performer and N,O-bound Mn2 the poorest. Furthermore, Mn1d could mediate the TH of a wide range of substituted (hetero)aryl-methyl ketones with the conversions to the corresponding secondary alcohols influenced by steric/electronic properties of the ketonic substrate. DFT calculations highlight firstly a modest bias toward an outer-sphere mechanistic pathway in line with that found for related Mn(I) catalysts. Secondly, these calculations highlight the detrimental role played by the steric properties on the efficiency of this hydrogenative transformation; a finding that is consistent with the empirical results. Overall, we feel that these results could provide important insights in future catalyst design that may be of relevance to the wider field of (de)hydrogenation.

Experimental details

General methods

All reactions were carried out under standard Schlenk conditions under an atmosphere of nitrogen unless otherwise stated. All solvents used were dried over appropriate drying agents, distilled and stored over molecular sieves and degassed prior to use. All NMR spectra (¹H, ¹³C) were recorded on a Bruker Avance III 500 MHz spectrometer with a 5 mm BBO probe, Bruker Avance III HD 400 MHz spectrometer with a 5 mm BBO probe or with a Bruker Avance NEO 500 MHz spectrometer with either a 5 mm Prodigy BBO cryoprobe or a 5 mm BBFO probe. All deuterated solvents were purchased from Goss Scientific and Sigma-Aldrich. Infra-red spectra were obtained using a Bruker ALPHA II compact FT-IR spectrometer. High resolution mass spectra were obtained using a Waters G2 XS QTof mass spectrometer equipped with Waters Acquity classic UPLC system. Samples for elemental analysis were weighed using Mettler Toledo high precision scale and analysed using ThermoFlash 2000 at London Metropolitan University. All reagents were obtained from commercial suppliers while the compound 6-bromo-2-methoxypyridine was prepared as described previously.⁵⁰

Synthesis of 6-acetyl-2-methoxypyridine

A 3-neck oven dry round bottom flask equipped with a dropping funnel and charged with a stir bar was placed under an atmosphere of nitrogen. 6-Bromo-2-methoxypyridine (2.906 g, 15.5 mmol, 1 eq.) was added via syringe along with THF (40 mL) and cooled to −78 °C. n-BuLi (13 mL, 1.6 M in hexanes, 1.3 eq.) was then slowly added to the solution and the mixture stirred for 1 h. Dry dimethylacetamide (1.7 mL, 1.1 eq.) was then added dropwise to the solution and stirred for a further 1 h at −78 °C. After warming to room temperature, the reaction was quenched with aqueous NH₄Cl (25 mL, 20% w/v) and the aqueous layer was subsequently separated and washed with diethyl ether (5 × 25 mL). The combined organic layers were then washed with de-ionised water (2 × 25 mL) and brine (25 mL). The organic layer was then dried over MgSO₄ and all volatiles removed under reduced pressure to afford a crude yellow oil. This oil was purified using silica gel chromatography using an eluent system based on 2 : 98 ethyl acetate : petroleum ether to afford the product as a white crystalline material (1.926 g, 79%). The ¹H NMR data obtained were consistent with that reported in the literature.^{51 1}H NMR (500 MHz, CDCl₃, 298 K): δ 7.69 (dd, J = 7.3, 8.2 Hz, 1H, Py–H), 7.63 (dd, J = 0.9, 7.2 Hz, 1H, Py–H), 6.93 (dd, J = 0.9, 8.2 Hz, 1H, Py–H), 4.00 (s, 3H, OCH₃), 2.68 (s, 3H, CCH₃). HRMS (ESI+, MeOH): calc. for C₈H₁₀NO₂ [M + H] 152.0712, found 152.0712.

Synthesis of 6-acetyl-pyridin-2-one

2-Methoxy-6-acetyl pyridine (2.035 g, 14.8 mmol) was added to a 100 mL round bottom flask equipped with a condenser and a stir bead. Concentrated HBr (36 mL, 33% in AcOH) was added and the mixture heated to 110 °C for 4 h. After cooling to room temperature, the solution was carefully neutralised with aqueous NaOH (4 M) and washed with diethyl ether (4 × 125 mL). The aqueous layer was then dried under reduced pressure and the resulting residue ground into a powder. This was then extracted with acetone (6 × 75 mL) and dried under reduced pressure to give a beige solid (1.815 g, 99%). The ¹H NMR data obtained were consistent with that reported in the literature.^{52 1}H NMR (500 MHz, CDCl₃, 298 K): δ 9.57–9.44 (br s, 1H, NH), 7.48 (dd, J = 6.7, 9.3 Hz, 1H, Py–H), 6.88 (d, J = 6.6 Hz, 1H, Py–H), 6.83 (d, J = 9.3 Hz, 1H, Py–H), 2.55 (s, 3H, CCH₃). ¹³C NMR (126 MHz, CDCl₃, 298 K) δ 190.4 (1C, C=O), 161.6 (1C, C–OH), 139.5 (1C, NCC), 138.6 (1C, CH), 128.1 (1C, CH), 109.7 (1C, CH), 24.5 (1C, CCH₃). HRMS (ESI+, MeOH): calc. for C₇H₈NO₂ [M + H] 138.0555, found 138.0555, calc. for C₇H₈NO₂Na [M + Na] 160.0376, found 160.374. FT-IR v_max/cm⁻¹ 3094br (NH), 2989w (CH), 1685m (CO), 1643s (CO), 1599s (CC), 1433m (CH), 1245m (CO), 798 (CC).

Synthesis of 6-(1-(arylimino)ethyl)pyridin-2(1H)-ones (HL1a–HL1e)

General procedure: 6-acetyl-pyridin-2-one (0.494 g, 3.60 mmol) was added to a 50 mL round bottom flask containing the corresponding aniline (5.36 mmol, 1.5 eq.), p-toluene-sulfonic acid (0.01 eq.) and n-butanol (10 mL, ca. 0.35 M of 6 acetyl-pyridin-2-one). The flask was then fitted with a Dean Stark apparatus and condenser. The mixture was stirred and heated to reflux for 72 h. Once cooled to room temperature, all volatiles were removed under reduced pressure and the resulting oil purified via silica column chromatography eluting with 100% ethyl acetate to give the product as a yellow solid.

Aryl = 2,6-diisopropylphenyl (HL1a). Following the general procedure described above with the same molar ratios, HL1a was isolated as a yellow powder (0.672 g, 63%). The ¹H NMR data obtained were consistent with that reported in the literature.^{53 1}H NMR (400 MHz, CDCl₃, 298 K): δ 10.34 (br s, 1H, OH), 7.50 (dd, J = 6.7, 9.1 Hz, 1H, Py–H), 7.19–7.10 (m, 3H, Ar–H), 6.77 (d, J = 9.2 Hz, 1H, Py–H), 6.66 (d, J = 6.7 Hz, 1H, Py–H), 2.57 (quin, J = 6.9 Hz, 2H, CH), 2.01 (s, 3H, CCH₃), 1.12 (d, J = 6.9 Hz, 12H, CH₃). ¹³C NMR (101 MHz CD₃SOCD₃, 298 K) δ (162.5, 1C, N=CCH₃), (145.1, 1C, C=O), (141.1, 1C, NCC), 140.5 (1C, NCC), 139.1 (2C, CH), 136.0 (1C, CH), 128.5 (1C, CH) 124.7 (1C, CH), 123.5 (2C, CH), 114.4 (1C, CH), 28.1 (1C, CCH), 26.0 (1C, CCH), 23.6 (2C, CH₃), 23.2 (2C, CH₃), 16.9 (1C, N=CCH₃). HRMS (ESI+, MeOH): found: 297.1967, calc. for C₁₉H₂₅N₂O [M + H] 297.1967 FT-IR v_max/cm⁻¹ 3128br (OH), 2959w (CH), 1650vs (C=N) 1597s (CH), 1450w (CH), 1431w (CH), 1285m (CO), 978 m (CN), 801s (CC), 562 m (CC).

Aryl = 4-bromo-2,6-diisopropylphenyl (HL1b). Following the general procedure described above with the same molar ratios, HL1b was isolated as a yellow powder (1.120 g, 85%). ¹H NMR (400 MHz, CDCl₃, 298 K): δ 10.17 (s, 1H, OH), 7.42 (t, J = 8 Hz, 1H, Py–H), 7.19 (s, 2H, Ar–H), 6.70 (1H, d, J = 8.6 Hz, Py–H), 6.59 (d, J = 6.8 Hz, 1H, Py–H), 2.46 (sept, J = 6.8 Hz, 2H, CH), 1.94 (s, 3H, CCH₃), 1.04 (d, J = 6.9 Hz, 12H, CH₃). ¹³C NMR (101 MHz, CD₃SOCD₃, 298 K): δ 162.2 (1C, N=CCH₃), 144.5 (1C, C=O), 140.9 (2C, NCC), 139.0 (3C, CH), 126.4 (2C, CH), 117.3 (1C, CH), 109.0 (1C, CH), 28.3 (2C, CCH), 23.3 (2C, CH₃), 22.9 (2C, CH₃), 16.9 (1C, N=CCH₃). HRMS (TOF + ESI, MeOH), found: 375.1066, calc. for C₁₉H₂₃N₂OBr [M] 375.1072. FT-IR v_max/cm⁻¹ 3090br (OH), 2963w (CH), 1630vs (C=N) 1592s (CH), 1456w (CH), 1440w (CH), 1283m (CO), 980w (CN), 863s (CC) 797s (CBr), 562 m (CC).

Aryl = 2,4,6-triisopropylphenyl (HL1c). Following the general procedure described above with the same molar ratios, HL1c was isolated as a yellow powder (1.024 g, 84%). ¹H NMR (400 MHz, CDCl₃, 298 K): δ 10.34 (s, 1H, OH), 7.49 (dd, J = 6.8, 9.3 Hz, 1H, Py–H), 6.99 (s, 2H, Ar–H), 6.75 (dd, J = 1.5, 9.2 Hz, 1H, Py–H), 6.65 (d, J = 7.1 Hz, 1H Py–H), 2.89 (sept, J = 6.9 Hz, 1H, CH) 2.54 (sept, J = 6.9 Hz, 2H, CH), 2.01 (s, 3H, CCH₃), 1.27 (d, J = 7.0 Hz, 6H, CH₃), 1.11 (dd, J = 0.9, 6.9 Hz, 12H, CH₃). ¹³C NMR (100 MHz; CDCl₃, 298 K): δ 162.8 (1C, N=CCH₃), 158.3 (1C, CH), 145.3 (1C, C=O), 142.3 (1C, NCC), 141.2 (1C, CCH), 140.5 (1C, CH), 136.1 (2C, NCC), 125.3 (2C, CH), 121.4 (1C, CH), 107.1 (1C, CH), 34.4 (1C, CHCH₃), 28.8 (2C, CHCH₃), 24.6 (2C, CH₃), 23.8 (2C, CH₃), 23.3 (2C, CH₃), 16.3 (1C, N=CCH₃). HRMS (TOF): calculated for C₂₂H₃₁N₂O [M + H], 339.2436, found: 339.2430. FT-IR v_max/cm⁻¹ 3175br (OH), 2957w (CH), 1656vs (C=N) 1602vs (CH), 1452m (CH), 1431w (CH), 1289w (CO), 1007w (CN), 801s (CC), 562 m (CC).

Aryl = 4-isopropylphenyl (HL1d). Following the general procedure described above with the same molar ratios, HL1d was isolated as a yellow powder (0.639 g, 70%). ¹H NMR (400 MHz, CDCl₃, 298 K) δ 10.30 (br s, 1H, OH), 7.46 (dd, J = 7.0, 8.8 Hz, 1H, Py–H), 7.24 (d, J = 8.1 Hz, 2H, Ar–H), 6.73 (m, J = 8.3 Hz, 1H, Py–H, 2H Ar–H), 6.64 (d, J = 7.0 Hz, 1H, Py–H), 2.85 (spt, J = 6.9 Hz, 1H, CH), 2.21 (s, 3H, CH₃), 1.19 (d, J = 6.9 Hz, 6H, CH₃). ¹³C NMR (101 MHz, CDCl₃, 298 K): δ 155.7 (1C, N=CCH₃), 145.2 (1C, C=O), 144.6 (1C, NCC), 140.5 (1C, C–CH), 139.1 (1C, CH), 126.0 (2C, CH), 123.6 (2C, CH), 119.0 (1C, CH), 114.4 (1C, CH), 105.8 (1C, CH), 32.6 (1C, CCH), 23.2 (1C, CH₃), 23.0 (1C, CH₃), 14.1 (1C, N=CCH₃). HRMS (ESI+, MeOH): calc. for C₁₆H₁₈N₂O [M + H] 255.1497, found 255.1498. Calc. for C₁₆H₁₈N₂ONa [M + Na] 277.1317, found 277.1319. FT-IR v_max/cm⁻¹ 3314br (OH), 2955w (CH), 1648vs (C=N) 1598s (CH), 1450w (CH), 1440w (CH), 1291m (CO), 1011m (CN), 795s (CC), 550m (CC).

Aryl = 2,4,6-trimethylphenyl (HL1e). Following the general procedure described above with the same molar ratios, HL1e was isolated as a dark yellow powder (0.867 g, 90%). ¹H NMR (400 MHz, CDCl₃, 298 K): δ 10.35 (br s, 1H, OH), 7.48 (dd, J = 6.8, 9.3 Hz, 1H, Py–H), 6.88 (s, 2H, Ar–H), 6.74 (d, J = 9.2 Hz, 1H, Py–H), 6.64 (d, J = 6.7 Hz, 1H, Py–H), 2.29 (s, 3H, Ar–H), 1.98 (s, 3H, CCH₃), 1.93 (s, 6H, Ar–H). ¹³C NMR (101 MHz, CDCl₃, 298 K): δ 157.9 (1C, C=N), 144.2 (1C, C–OH), 140.8 (1C, C–N), 140.0 (1C, C–N), 133.4 (1C, CH), 128.7 (3C, CH), 125.4 (1C, CCH₃), 125.0 (1C, CCH₃), 106.6 (1C, CH), 20.7 (1C, CH₃), 17.9 (1C, CH₃), 15.3 (1C, N=CCH₃). HRMS (ESI+, MeOH): calc. for C₁₆H₁₈N₂O [M + H] 255.1497, found 255.1497. Calc. for C₁₆H₁₈N₂ONa [M + Na] 277.1319, found 277.1317. FT-IR v_max/cm⁻¹ 3144br (OH), 2972w (CH), 1633vs (C=N) 1600s (CH), 1475w (CH), 1454w (CH), 1287w (CO), 1007m (CN), 980m (CN), 816s (CC), 554m (CC).

Synthesis of [2-OH-6-{CMe=N(Ar)}C₅H₃N]MnBr(CO)₃ (Mn1a–Mn1e)

General procedure: a small oven dried Schlenk flask, equipped with a stir bead, was evacuated and backfilled with nitrogen. The flask was charged with an equimolar amounts of HL1 and Mn(CO)₅Br and dissolved in the corresponding solvent (0.1 M solution). This solution was then stirred in darkness at 30 °C for 18 h. The resulting precipitate was filtered and washed with hexane to give the product as an orange solid.

Ar = 2,6-ⁱPr₂C₆H₃ (Mn1a). Following the general procedure described above, HL1a (88.0 mg, 0.3 mmol) and Mn(CO)₅Br (82.0 mg, 0.3 mmol) were dissolved in diethyl ether (3 mL). The resulting precipitate was washed with diethyl ether (1 × 10 mL) and hexane (2 × 5 mL) affording Mn1a as an orange solid (81.3 mg, 53%). ¹H NMR (400 MHz, CD₃SOCD₃, 298 K) δ 12.76–13.75 (m, 1H, OH), 8.05 (t, J = 7.84 Hz, 1H, Py–H), 7.79 (d, J = 7.52 Hz, 1H, Py–H), 7.34 (t, J = 7.10 Hz, 3H, Ar–H), 7.16 (d, J = 8.25 Hz, 1H, Py–H), 3.67 (quin, J = 6.80 Hz, 1H, CH), 2.80 (quin, J = 7.40 Hz, 1H, CH), 2.30 (s, 3H N=CCH₃), 1.25 (br d, J = 6.60 Hz, 3H, CCH₃), 1.22 (br d, J = 6.42 Hz, 3H, CCH₃), 1.01 (br d, J = 6.69 Hz, 6H, CCH₃). ¹³C NMR (126 MHz, CD₃SOCD₃, 298 K): δ 222.9 (1C, C≡O), 222.3 (1C, C≡O), 220.9 (1C, C≡O), 177.2 (1C, C=N), 165.1 (1C, C–OH), 153.5 (1C, C–N), 145.5 (1C, C–N), 141.0 (1C, CH), 139.9 (1C, CH), 138.3 (1C, CCH), 127.3 (1C, CH), 124.8 (1C, CH), 124.3 (1C, CH), 119.7 (1C, CH), 114.0 (1C, CH), 27.4 (2C, CCH), 25.0 (1C, CH₃), 24.9 (1C, CH₃), 24.6 (1C, CH₃), 24.3 (1C, CH₃), 24.2 (1C, CH₃), 19.7 (1C, N=CCH₃). HRMS (ESI−, MeOH): calc. for C₂₂H₂₃N₂O₄Mn⁷⁹Br [M − H]⁻ 513.0220, found 513.0220. Calc. for C₁₉H₂₃N₂OMn⁷⁹Br [M − H–3(CO)]⁻ 429.0374, found 429.0376. FT-IR v_max/cm⁻¹ 3062w (OH), 2961w (CH), 2018vs (CO), 1937vs (CO), 1903vs (CO), 1633m (CN), 1592s (CH), 1458w (CH), 1440w (CH), 1260m (CO), 1096m (CN), 1013s (CN) 801vs (CC), 630s (CC). Elemental analysis calc. for C₂₂H₂₃N₂O₄MnBr (0.25Et₂O) C, 51.8; H, 5.0; N 5.3 found C, 51.5; H, 5.3; N, 5.2.

Ar = 2,6-ⁱPr₂-4-BrC₆H₂ (Mn1b). Following the general procedure above, HL1b (113.0 mg, 0.3 mmol) and Mn(CO)₅Br (83.0 mg, 0.3 mmol) were dissolved in diethyl ether (3 mL). On completion of the reaction, the solvent was concentrated under reduced pressure and hexane (5 mL) added to induce precipitation. The solid was filtered and washed with diethyl ether (1 × 10 mL) and hexane (2 × 5 mL), to afford Mn1b as an orange solid (130 mg, 73%). ¹H NMR (400 MHz, CD₃SOCD₃, 298 K): δ 13.33 (br, s, 1H, OH), 8.04 (t, J = 7.91 Hz, 1H, Py–H), 7.80 (d, J = 7.41 Hz, 1H, Py–H), 7.49 (s, 2H, Ar–H), 7.17 (d, J = 8.31 Hz, 1H, Py–H), 3.64 (quin, J = 6.75 Hz, 1H), 2.76 (quin, J = 6.41 Hz, 1H), 2.31 (s, 3H, N=CCH₃), 1.23 (br d, J = 6.62 Hz, 3H, CCH₃), 1.19 (br d, J = 6.28 Hz, 3H, CCH₃), 1.01 (br d, J = 6.51 Hz, 6H, CCH₃). ¹³C NMR (126 MHz, CD₃SOCD₃, 298 K): δ 222.9 (1C, C≡O), 222.5 (1C, C≡O), 220.9 (1C, C≡O), 178.1 (1C, C=N), 165.2 (1C, C–OH), 153.4 (1C, C–N), 144.9 (1C, C–N), 143.2 (1C, CH), 141.6 (1C, CH), 141.2 (1C, CCH), 127.9 (1C, CH), 127.5 (1C, CH), 120.5 (1C, CH), 120.3 (1C, CH), 114.3 (1C, CH), 27.8 (1C, CCH), 27.8 (C, CCH), 24.8 (1C, CH₃), 24.7 (1C, CH₃), 24.4 (1C, CH₃), 24.1 (1C, CH₃), 20.0 (1C, N=CCH₃). HRMS (ESI−, MeOH): calc. for C₂₂H₂₃N₂O₄Mn⁷⁹Br [M − H]⁻ 513.0220, found 513.0220. Calc. for C₁₉H₂₃N₂OMn⁷⁹Br [M − H–3(CO)]⁻ 429.0374, found 429.0376. FT-IR v_max/cm⁻¹ 3062w (OH), 2961w (CH), 2018vs (CO), 1937vs (CO), 1903vs (CO), 1633m (CN), 1592s (CH), 1458w (CH), 1440w (CH), 1260m (CO), 1096m (CN), 1013s (CN) 801vs (CC), 630s (CC). Elemental analysis calc. for C₂₂H₂₃Br₂MnN₂O₄ (0.25CH₂Cl₂) C, 43.3; H, 3.9; N 4.6 found C, 43.2; H, 4.0; N, 4.3.

Ar = 2,4,6-ⁱPr₃C₆H₂ (Mn1c). Following the general procedure described above, HL1c (206.0 mg, 0.6 mmol) and Mn(CO)₅Br (166.0 mg, 0.6 mmol) were dissolved in hexane (6 mL) and the reaction mixture stirred and heated to reflux for 18 h. Once cooled to room temperature, a suspension was formed and collected by gravity filtration and washed with hexane (5 × 20 mL) affording Mn1c as an orange solid (264 mg, 78%). ¹H NMR (400 MHz, CD₃SOCD₃, 298 K): δ 13.17 (br s, 1H, OH), 8.03 (t, J = 7.84 Hz, 1H, Py–H), 7.80 (d, J = 7.20 Hz, 1H, Py–H), 7.16 (m, 3H, Ar–H, Py–H), 3.64 (quin, J = 6.64 Hz, 1H), 2.93 (quin, J = 6.84 Hz, 1H), 2.77 (quin, J = 6.41 Hz, 1H), 2.27 (s, 3H, N=CCH₃), 1.22 (br d, J = 5.84 Hz, 6H, CCH₃), 1.21 (br d, J = 6.07 Hz, 6H, CCH₃), 1.00 (br d, J = 6.84 Hz, 6H, CCH₃). ¹³C NMR (126 MHz, CD₃SOCD₃, 298 K): δ 222.4 (1C, C≡O), 221.6 (1C, C≡O), 220.4 (1C, C≡O), 176.0 (1C, C=N), 164.6 (1C, C–OH), 153.0 (1C, C–N), 146.1 (1C, C–N), 143.0 (1C, CCH), 140.3 (1C, CCH), 138.9 (1C, CCH), 137.3 (1C, CH), 121.6 (1C, CH), 121.2 (1C, CH), 119.5 (1C, CH), 113.3 (1C, CH), 32.4 (1C, CCH), 26.7 (1C, CCH), 24.4 (1C, CCH), 24.2 (1C, CH₃), 24.0 (1C, CH₃), 23.7 (1C, CH₃), 23.2 (2C, CH₃), 19.1 (1C, N=CCH₃). HRMS (ESI⁻, MeOH): calc. for C₂₅H₂₉N₂O₄Mn⁷⁹Br [M − H]⁻ 555.0691, found 555.0691. Calc. for C₂₂H₂₉N₂OMn⁷⁹Br [M − H–3(CO)]⁻ 471.0844, found 471.0847. FT-IR v_max/cm⁻¹ 3101br (OH), 2961w (CH), 2018s (CO), 1940s (CO), 1903s (CO), 1637m (CN), 1590m (CH), 1458w (CH), 1440w (CH), 1258m (CO), 1100w (CN), 1011w (CN) 803vs (CC), 626s (CC).

Ar = 4-ⁱPrC₆H₄ (Mn1d). Following the general procedure described above, HL1d (258.3 mg, 1.02 mmol) and Mn(CO)₅Br (278.1 mg, 1.02 mmol) were dissolved in dichloromethane (10 mL). The resulting solid was filtered and washed with dichloromethane (10 mL) and diethyl ether (3 × 10 mL) to afford Mn1d as an orange solid (349 mg, 74%). ¹H NMR (500 MHz, CD₃SOCD₃, 298 K): δ 12.45 (br s, 1H, OH), 8.02 (br t, J = 7.7 Hz, 1H, Py–H), 7.77 (br d, J = 7.1 Hz, 1H, Py–H), 7.41 (br d, J = 7.9 Hz, 2H, Ar–H), 7.28 (br s, 1H, Ar–H), 7.13 (br d, J = 8.3 Hz, 1H, Py–H), 7.04 (br s, 1H, Ar–H), 2.97 (quin, J = 6.2 Hz, 1H, CH), 2.29 (s, 3H, CCH₃), 1.26 (d, J = 6.7 Hz, 6H, CH₃) ¹³C NMR (126 MHz, CD₃SOCD₃, 298 K): δ 222.7 (1C, C≡O), 221.8 (1C, C≡O), 220.5 (1C, C≡O), 174.4 (1C, C=N), 165.3 (1C, C–OH), 154.0 (1C, C–N), 149.2 (1C, CCH), 146.6 (1C, C–N), 140.9 (2C, CH), 127.0 (2C, CH), 119.4 (1C, CH), 113.7 (1C, CH), 32.9 (1C, CCH), 23.8 (1C, CH₃), 23.7 (1C, CH₃), 18.0 (1C, N=CCH₃). HRMS (ESI⁻, MeOH): calc. for C₁₉H₁₇N₂O₄Mn⁷⁹Br [M − H]⁻ 470.9752, found 470.9755. Calc. for C₁₆H₁₇N₂OMn⁷⁹Br [M − H–3(CO)]⁻ 386.9905, found 386.9911. FT-IR v_max/cm⁻¹ 3114w (OH), 2959w (CH), 2024vs (CO), 1940vs (CO), 1904vs (CO), 1604m (CN), 1462w (CH), 1332w (CH), 1260m (CO), 1094m (CN), 1054m (CN), 1019m (CN) 809s (CC), 628s (CC). Elemental analysis calc. for C₁₉H₁₈N₂BrO₄Mn (0.25CH₂Cl₂) C, 46.8; H, 3.8; N 5.7 found C, 46.8; H, 3.6; N, 5.6.

Ar = 2,4,6-Me₃C₆H₂ (Mn1e). Following the general procedure described above, HL1e (25.4 mg, 0.1 mmol) and Mn(CO)₅Br (28.1 mg, 0.1 mmol) were dissolved in dichloromethane (1 mL). The resulting precipitate was filtered and washed with dichloromethane (2 mL) and hexane (3 × 10 mL) affording Mn1e as an orange solid (22.8 mg, 48%). ¹H NMR (500 MHz, CD₃SOCD₃, 298 K): δ 13.11 (br s, 1H, OH), 8.04 (t, J = 7.9 Hz, 1H, Py–H), 7.79 (d, J = 7.3 Hz, 1H, Py–H), 7.14 (d, J = 8.3 Hz, 1H, Py–H), 7.01 (s, 2H, Ar–H), 2.40 (s, 3H, CH₃), 2.29 (s, 3H, CH₃), 2.23 (s, 3H, CH₃), 2.04 (s, 3H, CH₃). ¹³C NMR (126 MHz, CD₃SOCD₃, 298 K): δ 222.6 (1C, C≡O), 222.4 (1C, C≡O), 221.3 (1C, C≡O), 176.9 (1C, C=N), 165.2 (1C, C–OH), 153.7 (1C, C–N), 146.5 (1C, C–N), 141.1 (1C, CH), 135.2 (1C, CCH₃), 129.3 (1C, CH), 129.3 (1C, CH), 129.2 (1C, CCH₃), 126.9 (1C, CCH₃), 119.6 (1C, CH), 113.8 (1C, CH), 20.5 (1C, CH₃), 20.3 (1C, CH₃), 18.0 (1C, CH₃), 17.7 (1C, N=CCH₃). HRMS (ESI−, MeOH): calc. for C₁₉H₁₇N₂O₄Mn⁷⁹Br [M − H]⁻ 470.9752, found 470.9747. Calc. for C₁₆H₁₇N₂OMn⁷⁹Br [M − H–3(CO)]⁻ 386.9905, found 386.9889 FT-IR v_max/cm⁻¹ 3093br (OH), 2979br (CH), 2020s (CO), 1940s (CO), 1897vs (CO), 1596m (CN), 1472w (CH), 1370m (CH),1326w (CH), 1260m (CO), 1015w (CN) 809s (CC), 680m (CC), 628s (CC). Elemental analysis calc. for C₁₉H₁₈N₂BrO₄Mn (0.75CH₃OH) C, 47.7; H, 4.3; N 5.6 found C, 47.9; H, 4.6; N, 5.4.

Synthesis of [2-OH-6-(CMe=O)C₅H₃N]MnBr(CO)₃ (Mn2)

A small oven dried Schlenk flask, equipped with a stir bead, was evacuated and backfilled with nitrogen. The flask was charged with an equimolar amounts of 6-acetyl-pyrid-2-one (41.7 mg, 0.3 mmol) and Mn(CO)₅Br (82.1 mg, 0.3 mmol) and dissolved in dichloromethane (3 mL). This solution was then stirred in darkness at 30 °C for 18 h. The resulting precipitate was then filtered, washed with dichloromethane (6 mL) and hexane (2 × 5 mL) and then dried under reduced pressure to afford Mn2 as a brick red solid (61.8 mg, 58%). ¹H NMR (500 MHz, CD₃SOCD₃, 298 K) δ (ppm) = 13.35 (br s, 1H, OH), 8.18–8.07 (m, 2H, Py–H), 7.28 (dd, J = 3.1, 6.3 Hz, 1H, Py–H), 2.83 (s, 3H, O=CCH₃). Mn2(DMSO) (40%): 11.03 (br s, 1H, OH), 7.65 (br s, 1H, Py–H), 7.18 (br s, 1H, Py–H), 6.74 (br s, 1H, Py–H), 2.83 (s, 3H, O=CCH₃). ¹³C NMR (126 MHz, CD₃SOCD₃): δ 225.6 (s, 1C, C≡O), 223.4 (1C, C≡O), 219.3 (1C, C≡O), 210.8 (1C OCCH₃), 165.5 (1C, CH), 149.5 (1C, CH), 141.3 (1C, CH), 122.1 (1C, CH), 117.1 (1C, CH), 25.9 (1C, CH₃). HRMS (ESI−, MeOH): calc. for C₁₀H₆NO₅Mn⁷⁹Br [M − H]⁻, 353.8810, found 353.8806; calc. for C₇H₆NO₂Mn⁷⁹Br [M − H–3(CO)]⁻ 269.8962, found 269.8961. Elemental analysis calc. for C₁₀H₆NO₅MnBr C, 33.7; H, 2.0; N 3.9 found C, 33.9; H, 2.1; N, 3.9.

Catalytic protocol employed for the transfer hydrogenation

(a) General procedure for screening Mn1a–Mn1e and Mn2 for the TH of acetophenone (1a). An oven dry 20 mL Schlenk flask, equipped with a micro stir bar, was evacuated and backfilled with nitrogen. To this a pre-determined amount of base was added and the flask re-evacuated before being backfilled with nitrogen. Dry ⁱPrOH (5 mL) was then added and the vessel stirred to dissolve/suspend the base in solution. The corresponding mol% of manganese complex was then added and the mixture stirred to form the active species. The substrate was then added (2 mmol) and the reaction surrounded in foil and placed into an oil bath set at 80 °C and the mixture left stir at this temperature for 24 h with the tap open to the bubbler. After cooling to room temperature, the mixture was diluted with dichloromethane (5 mL) and passed through a silica pipette filter eluting with dichloromethane. A sample of this solution was withdrawn and all volatiles removed under reduced pressure. The spectroscopic conversion of the substrate was then measured using ¹H NMR spectroscopy in CDCl₃.

(b) General procedure for screening substrates 1a–1n using Mn1a. An oven dry 20 mL Schlenk flask, equipped with a micro stir bar, was evacuated and backfilled with nitrogen. To this K^tBuO (7.0 mg, 0.06 mmol, 3 mol%) was added and the flask re-evacuated before being backfilled with nitrogen. Dry ⁱPrOH (5 mL) was then added and the vessel stirred to dissolve/suspend the K^tBuO in solution. Mn1d (4.7 mg, 0.01 mmol, 0.5 mol%) was then added and the mixture stirred to form the active species as a red solution. The substrate was then added (2 mmol) and the reaction surrounded in foil and placed into an oil bath set at 80 °C and the mixture left stir at this temperature for 24 h with the tap open to the bubbler. After cooling to room temperature, the mixture was diluted with dichloromethane (5 mL) and passed through a silica pipette filter eluting with dichloromethane. A sample of this solution was withdrawn and all volatiles removed under reduced pressure. The spectroscopic conversion of the substrate was then measured using ¹H NMR spectroscopy in CDCl₃.

Computational methods

All the calculations were performed at B3LYP-D3(BJ)^54,55 method using ORCA 6.0.1 software package.^56,57 For Mn, the def2-TZVP⁵⁸ with the auxiliary def2/J⁵⁹ basis sets were used. The Br atom was treated with the 6-311+g(d,p)^60–62 basis set, while the remaining atoms (C, H, N, and O) were described using the 6-31+G(d,p)^51–63 basis set. Structures were optimized using the SMD implicit solvation model, with isopropanol as the solvent.⁶⁴ Harmonic vibration frequency calculations were used to confirm the stationary points and to obtain thermodynamic data. The 3D representations of the optimized structures were generated by CYLview20.⁶⁵

Author contributions

S. J. Prettyman was responsible for the data curation, investigation validation and writing. M. H. Alhalafi was involved in some of the initial synthetic work. G. A. Solan was responsible for the supervision, funding acquisition, conceptualisation and writing – review and editing. K. Singh was responsible for the collection of the XRD data, while Dr S. Alqahtani performed the computational investigation.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: figures and additional characterisation data for ligands, complexes and products of catalysis; coordinates of all optimised structures by DFT calculations. See DOI: https://doi.org/10.1039/d6dt00709k.

CCDC 2540706–2540715 for Mn1a–Mn1e, Mn2, Mn1b′, Mn1d′, HL1a and HL1e contain the supplementary crystallographic data for this paper.^66a–j

Acknowledgements

The University of Leicester is thanked for financial support and the provision of a studentship to SJP. The EPSRC are acknowledged for grant EP/W02151X/1.

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