Bis-guanidinate magnesium amide-catalyzed hydroboration of nitriles and isocyanides via a substrate-assisted pathway

Abstract

The bis-guanidinate amido magnesium(II) compound [LMgN(SiMe₃)₂; L = {(ArHN)(ArN)–C=N–C = (NAr)(NHAr)}; [Ar = 2,6-Et₂-C₆H₃] (Mg-1) has been employed as a pre-catalyst for the dihydroboration of nitriles, and as a catalyst for the dihydroboration of isocyanides, affording N,N-bis(boryl)amines and N,C-di(boryl)amines in good yields. The protocol introduces the first examples of nitrile and isocyanide reduction via a substrate-assisted pathway. The active catalyst for nitrile hydroboration, LMg-N(SiMe₃)₂·3-Me-C₆H₄CN (Mg-2), and the intermediate for isocyanide reduction, LMg-N(SiMe₃)₂·2,6-Me₂-C₆H₃NC (Mg-3), were isolated and thoroughly characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Based on these findings, plausible mechanistic cycles for both nitrile and isocyanide dihydroboration reactions have been proposed.

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Introduction

The reduction of nitriles (R–C≡N) and isocyanides (R–N=C) plays a pivotal role in synthetic chemistry.^1–3 This process yields primary amines from nitriles and secondary amines from isocyanides, both of which are highly valuable due to their extensive applications. These compounds serve as essential feedstocks for agrochemicals, pharmaceuticals, dyes, and drug production, making them vital to industry and a key focus for synthetic chemists.^4–13

Traditional ways for this kind of reduction include treatment of nitriles with metal hydrides,¹⁴ which may involve potential drawbacks such as use of pyrophoric and expensive reagents, poor selectivity,^15,16 and direct hydrogenation with combustible hydrogen gas,¹⁷ which requires harsh reaction conditions¹⁸ often concerning about safety issues. In this context, metal-catalyzed hydroboration has proved to be more advantageous over traditional stoichiometric synthetic procedures by producing higher selectivity, milder reaction conditions, and reduced waste generation, and has emerged as a new alternative towards the reduction of various unsaturated organic moieties.^13,19–24 Metal-catalyzed B–H or Si–H additions to the unsaturated organic functionalities are highly dominated by molecular metal hydrides following a hydride pathway;^14,25 however, a few examples of hydride-free pathway are also reported.^26–28 Despite extensive studies on transition,^1,13,29–48 main group,^1,48–65 and rare earth metal catalyzed^1,48,66 hydroboration of nitriles and isocyanides, reports on magnesium-catalyzed hydroboration of the same, which remained scarce.^56,57,63

Magnesium, being eco-friendly, non-toxic, earth-abundant, and biocompatible,^67–71 offers significant potential in this domain. However, despite numerous advancements in the catalytic as well as stoichiometric applications of magnesium-based complexes,^{1,21,28,72,73} only three studies have explored its catalytic application in hydroboration of nitriles⁵⁷ and isocyanides,⁶³ underscoring a critical gap in this field.

The first report on metal-catalyzed nitrile dihydroboration was documented by the Nikonov group in 2012 using a molybdenum-based catalyst, (2,6-ⁱPr₂-C₆H₃N)Mo(H)(Cl)(PMe₃)₃.⁴³ In 2016, the Hill group introduced a β-diketiminate (Nacnac) magnesium complex, [CH{C(Me)NAr}₂Mg-nBu] (Ar = 2,6-ⁱPr₂C₆H₃), as a pre-catalyst for nitrile hydroboration, representing the first example with magnesium (Fig. 1). Notably, the same group reported the first metal-catalyzed hydroboration of isocyanides in 2015 using the same pre-catalyst, with both cases involving in situ generation of magnesium hydride species as the active catalyst. In 2018, Ma and coworkers reported unsymmetrical-diketiminate-stabilized Mg(I) dimer catalyzed double hydroboration of nitriles, though they couldn't mention any mechanistic insights on the catalytic transformation.⁵⁶

Fig. 1 Magnesium-catalyzed dihydroboration of nitriles and isocyanides.

Recently, the Trovitch group reported cobalt-catalyzed nitriles dihydroboration,³⁸ in which authors introduced a chelate-assisted hydroboration pathway. To our knowledge, there have been no reports on the magnesium catalyzed hydroboration of nitriles and isocyanides involving chelate- assisted or substrate-driven pathway. This scarcity of studies on magnesium-catalyzed hydroboration of nitriles and isocyanides via substrate-assisted pathway, coupled with limited mechanistic insights, underscores the need to design innovative magnesium complexes and explore alternative pathways to unlock the full potential of this catalytic transformation.

In this context, we employed bis-guanidinate Mg(II) amide [LMgN(SiMe₃)₂] (Mg-1)⁷⁴ as a pre-catalyst for the hydroboration of the nitriles and a catalyst for the dihydroboration of isocyanides with good to excellent yields. We have isolated and characterized the active species LMg-N(SiMe₃)₂·3-Me-C₆H₄CN (Mg-2), and LMg-N(SiMe₃)₂·2,6-Me₂-C₆H₃NC (Mg-3) with various spectroscopic techniques (¹H, ¹³C{¹H}, ²⁹Si{¹H}) and XRD analysis. Moreover, based on the above findings, we established the first example of amido magnesium complex-catalyzed hydroboration of nitriles and isocyanides, with a mechanistic insight into the substrate-assisted pathway.

Results and discussion

Dihydroboration of nitriles

Our group previously developed a “Nacnac” analogue – specifically, a tetra-N-aryl substituted bis-guanidine ligand and its coordination chemistry.^64,75,76 Recently, we synthesized bis-guanidinate magnesium amide complex (Mg-1) and employed it as a catalyst for the C–C coupling of terminal alkynes with carbodiimide (CDI) (Fig. 2).⁷⁴

Fig. 2 Bis-guanidinate stabilized Mg(II) amide (Mg-1).

Inspired by the work of the Hill group on magnesium-catalyzed dihydroboration of nitriles,⁵⁷ we investigated the use of Mg-1 as a pre-catalyst for nitrile dihydroboration. Our initial experiment began with m-tolunitrile (1c) as a model substrate and pinacolborane (HBpin) as a hydroborating agent, to optimize the catalytic transformation. A control reaction without a catalyst, performed at 80 °C for 24 h under neat conditions, showed no conversion. However, under identical conditions, the presence of 5 mol% of Mg-1 afforded quantitative conversion to the corresponding N,N-bis(boryl)amine (2c), confirmed by ¹H NMR spectroscopy (Table 1, entry 2). Reducing the catalyst loading to 1 mol% under neat conditions at 60 °C achieved full conversion within 12 h (Table 1, entry 4). Further decrease in the reaction time led to reduced NMR conversions (Table 1, entries 5 and 6).

Table 1 Optimization table of magnesium catalyzed dihydroboration of m-tolunitrile^a

Entry	Cat.	mol%	Solvent	Temp. (°C)	Time (h)	Conv.^b (%)
a Reaction conditions: m-tolunitrile (0.1 mmol, 1.0 equiv.), pinacolborane (HBpin) (0.2 mmol, 2.0 equiv.), catalyst Mg-1, Mg-2 and Mg[N(SiMe3)2]2 (x mol%).b Conversions for the reduction of m-tolunitrile to N,N′-bis-borylated amine were examined by ^1H and ^13C{^1H} NMR spectroscopy based on the consumption of starting material and formation of characteristic new proton resonance for {CH2N(Bpin)2} moiety.
1	—	—	Neat	80	24	—
2	Mg-1	5	Neat	80	24	>99
3	Mg-1	3	Neat	70	12	>99
4	Mg-1	1	Neat	60	12	>99
5	Mg-1	1	Neat	60	11	94
6	Mg-1	1	Neat	60	6	55
7	Mg-1	1	Neat	rt	24	30
8	Mg-1	0.5	Neat	60	12	75
9	Mg-1	0.5	Neat	60	24	75
10	Mg-1	0.5	Neat	80	24	75
11	Mg-1	1	C6D6	60	14	>99
12	Mg-2	1	C6D6	60	10	>99
13	Mg[N(SiMe3)2]2	1	Neat	60	12	10

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The same reaction produced quantitative conversion upon heating at 60 °C for 14 h in C₆D₆ (Table 1, entry 11). Additionally, we tested the activity of Mg-2 (vide infra) for the dihydroboration reaction, which produced quantitative conversion at 80 °C, within 10 h in C₆D₆ (Table 1, entry 12). We also tested the catalytic activity of magnesium bis(trimethylsilyl)amide [Mg{N(SiMe₃)₂}₂]⁷⁷ reagent, which showed only 10% conversion to the diborylated product (Table 1, entry 13), highlighting the superior activity of a heteroleptic molecular magnesium complex like Mg-1, over a homoleptic magnesium complex [Mg{N(SiMe₃)₂}₂]. Further reduction in catalyst loading led to decreased conversions, even at elevated temperature and prolonged reaction time (Table 1, entries 7–10).

With optimized reaction conditions in hand, we explored the substrate scope. A variety of nitriles (1a–1p), including electron-donating (1a–1g), electron-withdrawing (1h–1n) substituted aryl nitriles, as well as alkyl nitriles (1o, 1p), were successfully dihydroborated with HBpin in the presence of Mg-1. The corresponding N,N-bis(boryl)amines (2a–2p) were obtained with quantitative conversion (Table 2) except for 2p (88%). Nitrile 1a underwent complete conversion to 2a within 6 h. To probe substituent effects, we examined 1c (electron-donating) and 1m (electron-withdrawing) under shorter reaction times (<12 h), which resulted in comparatively lower conversions (11 h, ∼90%). Based on these observations, subsequent substrates were screened under standardized overnight heating (up to 12 h), ensuring quantitative conversions. Furthermore, to assess the effect of electronic substitution on the initial rate, we monitored the hydroboration of 1c and 1m at 3 h under optimal conditions in C₆D₆ (see SI, Fig. S20 and S42). Both substrates displayed nearly identical NMR conversions (32–34%), indicating comparable initial reaction rates. These observations suggest that electronic effects of the substituents have little influence on the reaction rate, and the reduced rates in the substituted nitriles are instead attributed to steric hindrance during catalyst–substrate complex formation (see stoichiometric reactions). The aliphatic nitrile 1o gave >99% NMR conversion, while its aryl analogue 1p gave a slightly lower yield (88%). All products were characterized by ¹H and ¹³C{¹H} NMR spectroscopy. New signals in the range of 2.86–4.38 ppm in the ¹H NMR spectra, corresponding to the CH₂N(Bpin)₂ moiety, confirmed successful conversion. The yield of 2p was determined using mesitylene as an internal standard [see SI, Fig. S47 and S48].

Table 2 Substrate scope for Mg-1 catalyzed dihydroboration of nitriles^a,b,c

a Reaction conditions: nitriles (0.2 mmol, 1.0 equiv.), pinacolborane (HBpin) (0.4 mmol, 2.0 equiv.), catalyst Mg-1 (1 mol% %), heating at 60 °C, 12 h (6 h for 1a) stirring under neat conditions.b Conversion for the product was examined by ¹H and ¹³C{¹H} NMR spectroscopy based on the consumption of starting material and formation of characteristic new proton resonance for the (CH₂N(Bpin)₂) moiety of products 2a–2p.c For compounds 2i and 2p, mesitylene is used as an internal standard.

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Dihydroboration of isocyanides

Encouraged by our success with nitriles and noting the limited reports on isocyanide hydroboration,^62,63 we evaluated the catalytic activity of Mg-1 for the dihydroboration of isocyanide. Using 2,6-dimethylphenyl isocyanide (3a) as a model substrate, a catalyst-free reaction with HBpin at 80 °C for 24 h in benzene-d₆ yielded no conversion (Table 3, entry 1). In contrast, using 10 mol% Mg-1 resulted in quantitative formation of N,C-di(boryl)amine (4a) (Table 3, entry 2), as confirmed by ¹H NMR spectroscopy. Decreasing the catalyst loading to 6 mol% still provided full conversion at 70 °C within 12 h (Table 3, entry 4). The same reaction showed no change in the NMR conversion under neat conditions (Table 3, entry 5). Further reduction in the catalyst load decreased the conversion significantly (Table 3, entry 6). The reagent Mg[N(SiMe₃)₂]₂⁷⁷ produced only 30% conversion under standard reaction conditions (Table 3, entry 7).

Table 3 Optimization table of magnesium catalyzed hydroboration of isocyanide^a,b,c

Entry	Cat	mol%	Solvent	Temp. (°C)	Time (h)	Conv.^b (%)
a Optimization reaction conditions: 2,6-dimethylphenyl isocyanide (3a) (0.1 mmol, 1.0 equiv.), pinacolborane (HBpin) (0.21 mmol, 2.1 equiv.), catalyst Mg-1 or Mg[N(SiMe3)2]2 (x mol%).b Conversion of 3a to the corresponding 1,2-diborylated amine (4a) was examined by ^1H and ^13C{^1H} NMR spectroscopy based on the consumption of starting material and formation of characteristic new proton resonance for the {CH2N(Bpin)2} moiety.c Reactions were performed with 3b and 3c (0.1 mmol, 1.0 equiv.), pinacolborane (HBpin) (0.21 mmol, 2.1 equiv.), catalyst Mg-1 (6 mol%), heated at 70 °C, for 12 h. Conversions to 1,2-diborylated amines (4b, 4c)were examined by ^1H and ^13C{^1H} NMR spectroscopy based on the formation of a characteristic new proton resonance for {CH2(Bpin)N(Bpin)} moiety.
1	—	—	C6D6	80	24	—
2	Mg-1	10	C6D6	80	24	>99
3	Mg-1	7	C6D6	70	12	>99
4	Mg-1	6	C6D6	70	12	>99
5	Mg-1	6	Neat	70	12	>99
6	Mg-1	5	C6D6	70	24	80
7	Mg[N(SiMe3)2]2	6	C6D6	70	12	30

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Next, we investigated other isocyanides: 2-naphthyl isocyanide (3b) showed 70% conversion to 4b, while cyclohexyl isocyanide (3c) underwent complete conversion to 4c (Table 3). These products were confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. New singlets in the 2.74–3.61 ppm range, attributed to the N(Bpin)CH₂Bpin moiety, confirmed successful formation (see SI). We attempted dihydroboration of isocyanide substrates 3d, 3e, and 3f; however, the corresponding borylated products 4d, 4e, and 4f were obtained in low conversions (SI, Table S1).

Stoichiometric reactions for the hydroboration of nitriles and isocyanides

To gain mechanistic insights into the bis-guanidinate magnesium amide-catalyzed hydroboration reactions of nitriles and isocyanides, we executed a series of control experiments. Previously, the Hill group reported β-diketiminate (Nacnac) stabilized magnesium alkyl as a pre-catalyst for the hydroboration of organic nitriles,⁵⁷ involving the in situ formation of magnesium hydride. It is worthy to note, that molecular magnesium hydrides can be accessed by the reaction between magnesium amide and silane reagents,^14,78–84 however, a 1 : 3 stoichiometric reaction of Mg-1 and HBpin at 90 °C for 24 h indicated the formation of the Mg1-HBpin adduct in the ¹H NMR spectrum (see SI, Fig. S7) (Scheme 1i). In addition, we observed no conversion of HBpin into BH₃ in the presence of Mg-1 (see SI, Fig. S13) which rules out the any hidden catalysis^85,86 for the hydroboration reactions. A 1 : 1 stoichiometric reaction between Mg-1 and m-tolunitrile (1c) in C₆D₆ produced the monomeric nitrile coordinated adduct LMg-N(SiMe₃)₂·3-Me-C₆H₄CN Mg-2 within 5 min of the reaction at room temperature (Scheme 1ii). The compound Mg-2 was isolated and characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction analysis. The ¹H NMR spectrum of the compound Mg-2 displayed a downfield shift of the methyl groups of the N(SiMe₃)₂ moiety at 0.35 ppm than its parent precursor Mg-1 at 0.01 ppm in C₆D₆,⁷⁴ indicating the coordination of the electron-withdrawing nitrile group. A considerable downfield shift has been observed for the Ar–NH moiety of the ligand backbone at 5.03 ppm (Mg-1; Ar–NH: ¹H NMR δ 4.89 ppm) in the ¹H NMR spectrum, indicating the Mg-2 formation. ¹³C{¹H} NMR spectrum also displayed a significant shift of the N(SiMe₃)₂ carbons at 5.7 ppm than the Mg-1 at 4.4 ppm in C₆D₆ [see SI, Fig. S2]. Additionally, one singlet signal at −7.93 ppm in the ²⁹Si{¹H} NMR spectrum shows a slight shift from its parent precursor at −7.22 ppm in C₆D₆, indicating the formation of the new species Mg-2 [see SI, Fig. S1, S2, and S3].

Scheme 1 Control experiments for dihydroboration of nitrile.

Though some isocyanide adducts of ligated magnesium complexes are reported in the literature,⁸⁷ to our knowledge, Mg-2 is the first example of a nitrile adduct of a bis-guanidinate-stabilized magnesium amide complex. The single crystals suitable for X-ray diffraction analysis were grown inside a J. Young valve NMR tube in C₆D₆ at room temperature. The X-ray diffraction study shows that the compound crystallizes in a monoclinic system with a P2₁/n space group. The molecular structure shows a monomeric unit with a magnesium centre surrounded by four N atoms: two from the chelated bis-guanidinate ligand, and others from the N(SiMe₃)₂ and the nitrile (CN) group, attaining a distorted tetrahedral geometry (Fig. 3). The Mg1–N6 bond distance of the compound Mg-2 is 2.0036 (14) Å, significantly longer than the Mg–N(SiMe₃)₂ bond distance of 1.9503 (14) Å in Mg-1,⁷⁴ which is due to the enhanced steric hindrance at the magnesium centre in Mg-2 than Mg-1.

Fig. 3 Molecular structure of Mg-2. The thermal ellipsoids are shown at probability 50%, and H atoms except H4 and H5, and the ethyl substitutions on the N-aryl groups of the bis-guanidinate ligand are omitted for clarity. The selected bond lengths (Å) and bond angles (°): Mg1–N6 2.0036 (14), Mg1–N2 2.0427 (13), Mg1–N1 2.0447 (13), Mg1–N7 2.1546 (14), Si1–N6 1.7014 (13), Si2–N6 1.7039 (13), N7–C3 1.140 (2), N1–Mg1–N2 89.64 (5), N1–Mg1–N6 119.13 (6), N2–Mg1–N6 134.40 (6), Mg1–N7–C3 156.68 (14), Si1–N6–Mg1 120.66 (7), Si2–N6–Mg1 117.55 (7), Si1–N6–Si2 121.56 (8).

The N atom of the nitrile is attached to the magnesium through a coordinate bond, which is evident from the longer Mg1–N7 bond distance of 2.1546 (14) Å than the purely covalent Mg1–N6 bond distance of 2.0036 (14) Å. The C3–N7 bond distance is 1.140 (2) Å, which is well in agreement with the C≡N moiety of the nitrile functional group, which confirms that the nitrile group is intact and coordinated to the magnesium centre. The N1–Mg1–N2 bite angle in the Mg-2 is 89.64 (5)°, which is acute than the bite angle of both the bis-guanidinate [94.36 (6)°]⁷⁴ as well as Nacnac stabilized amido magnesium complexes [95.11(6)°].⁸⁸

Attempts to isolate N-borylimine intermediates from catalytic reactions of 1c with HBpin were unsuccessful, likely due to their high reactivity than nitriles, leading to full reduction of N,N-bis(boryl) amine.^89,90 Notably, there have been a few reports on monohydroboration of nitriles.^44,91–96

Surprisingly, in a 1 : 1 stoichiometric reaction of Mg-2 and HBpin at 60 °C in C₆D₆ for 30 min, we observed the formation of an entirely new magnesium N-boryl imine compound Int-1 (Scheme 1iii), where a N-borylated imine part is stabilized by the guanidinate magnesium amide counterpart. The Int-1 was obtained with 50% NMR yield and was identified through ¹H NMR spectroscopy [see SI, Fig. S9]. A diagnostic singlet signal at 8.72 ppm in the ¹H NMR spectrum of Int-1, in C₆D₆, indicates the Mg-N(Bpin)=C–H moiety of the newly formed borylated imine. This is comparable to the Hill group's reported Nacnac stabilized magnesium aldimidoborohydride, where the imine N=C–H proton resonates near 9.27 ppm in the ¹H NMR spectrum.⁵⁷ Further heating of the reaction mixture started to produce the fully reduced N,N-bis(boryl) amine. A very careful addition of m-tolunitrile (1c) to the mixture of Mg-2 and Int-1, at room temperature slowly formed the N-borylated imine (2c′) and regenerated Mg-2, within 12 h (Scheme 1iii) [see SI, Fig. S11] with a quantitative conversion, which implies that the Mg-2 and Int-1 are in equilibrium in the reaction mixture. Further isolation of the 2c′ was unsuccessful due to the extreme reactivity of the compound; it couldn't be stabilized in its imine form. Despite several attempts, we were unable to isolate the Int-1 in its solid-state form. All the sets of stoichiometric reactions are summarized in the stacked ¹H NMR representation in Fig. 4.

Fig. 4 Stacked ¹H NMR spectra for the stoichiometric reactions.

The overall study suggests that the reaction proceeds via an in situ formation of an active magnesium hydride species by Mg–N/H–B metathesis upon treatment of HBpin with Mg-2 (Scheme 3, vide infra). The formation of the adduct is a crucial step, as it weakens the Mg–N(SiMe₃)₂ bond [Mg-2; Mg1–N6: 2.0036(14) Å, Mg-1; Mg–N: 1.9503(14) Å], making it more susceptible to metathesis with HBpin. This process drives the overall reaction forward, indicating a substrate-assisted pathway.

Next, a 1 : 2 stoichiometric reaction of Mg-2 and HBpin in the presence of m-tolunitrile at 60 °C in C₆D₆ formed the N,N-bis(boryl) amine 2c, and along with the regeneration of the compound Mg-2 (Scheme 1iv) within 12 h [see SI, Fig. S10]. The formation of the compound 2c was confirmed by ¹H NMR spectroscopy. The disappearance of the singlet signal at 8.72 ppm in the ¹H NMR spectrum in C₆D₆ and the formation of a new signal at 4.63 ppm indicate the CH₂ protons of the N(Bpin)₂-CH₂-Ar moiety, which confirms the production of the compound 2c.

A 1 : 1 stoichiometric reaction between Mg-1 and 2,6-dimethylphenyl isocyanide (3a) in C₆D₆ readily formed the monomeric isocyanide adduct LMg-N(SiMe₃)₂·2,6-Me₂-C₆H₃NC Mg-3 within 5 min at room temperature (Scheme 2i).

Scheme 2 Control experiments for dihydroboration of isocyanide.

The compound Mg-3 was isolated and characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Mg-3 has shown a significant downfield shift of the 18H of the N(SiMe₃)₂ group at 0.18 ppm (Mg-1; N(SiMe₃)₂: δ 0.01 ppm) in the ¹H NMR spectrum in C₆D₆, indicating the coordination of the electron-withdrawing N=C group. A singlet signal at 2.00 ppm, in the ¹H NMR spectrum, corresponding to the methyl groups of the 2,6-dimethylphenyl isocyanide. Additionally, the ²⁹Si{¹H} NMR spectrum displays a singlet signal at −7.69 ppm in C₆D₆, significantly shifted from its parent precursor Mg-1 at −7.22 ppm, confirming the formation of the Mg-3 [see SI, Fig. S4–S6]. Diffractable single crystals of the compound Mg-3 were grown inside the J. Young valve NMR tube at room temperature in C₆D₆. The X-ray analysis shows the compound Mg-3 crystallizes in a monoclinic system with a P-1 space group. The molecular structure displayed a monomeric structure where one molecule of 2,6-dimethylphenyl isocyanide is coordinated to the magnesium centre of Mg-1, through the carbon atom, making it a four-coordinated magnesium complex with a distorted tetrahedral geometry (Fig. 5). To the best of our knowledge, Mg-3 is the first example of an isocyanide adduct of a bis-guanidinate magnesium complex. The Mg1–N6 bond distance is 2.0077 (13) Å, significantly longer than the parent precursor Mg-1 (Mg-N(SiMe₃)₂; 1.9503 (14) Å), corresponding to the enhanced steric hindrance due to the increased coordination number to the magnesium centre. The C1–N7 bond distance in the coordinated isocyanide counterpart is 1.159 (2) Å, slightly longer than the C–N bond distance [N3–C38 1.145 (3) Å] in the isocyanide adduct of the analogous Nacnac stabilized magnesium siloxide,⁹⁷ reported by Hill and coworkers. The C1–N7–C2 bond angle is 178.53 (16)°, which implies a close to linear framework of the isocyanide moiety. The longer C1–N7 bond distance indicates the ease of the B–H addition to the isocyanide R–N=C: bond. Similar to the Mg-2, the N1–Mg1–N2 bite angle in the compound Mg-3 is 90.53 (5)°, which is acute than the bite angles of both the Mg-1⁷⁴ as well as the analogous Nacnac magnesium amide complex (vide supra).⁸⁸

Fig. 5 Molecular structure of compound Mg-3. The thermal ellipsoids are shown at probability 50%, and H atoms except H4 and H5 and the ethyl substitutions on the N-aryl groups of the bis-guanidinate ligand are omitted for clarity. The selected bond lengths (Å) and bond angles (°): Mg1–N6 2.0077 (13), Mg1–N2 2.0492 (12), Mg1–N1 2.0681 (12), Mg1–C1 2.3599 (16), Si1–N6 1.7067 (13), Si2–N6 1.7132 (13), N7–C1 1.159 (2), N1–Mg1–N2 90.53 (5), N1–Mg1–N6 132.50 (5), N2–Mg1–N6 120.99 (5), Mg1–C1–N1 176.39 (12), Si1–N6–Mg1 120.17 (7), Si2–N6–Mg1 120.89 (7), Si1–N6–Si2 118.88 (7).

A subsequent stoichiometric reaction of compound Mg-3 with 2 equiv. of HBpin at 70 °C for 12 h formed the N,C-di(boryl)amine 4a and Mg-1 with a quantitative conversion (Scheme 2ii), verified through ¹H NMR spectroscopy [see SI, Fig. S12].

Kinetic experiments and discussions

To justify the mechanistic implications of the stoichiometric outcomes, a kinetic study via comparison of concentration profiles⁹⁸ for the Mg-1-catalyzed nitrile's dihydroboration was conducted. The m-tolunitrile(1c) was chosen as a standard substrate because of its indicative signals for its consumption and the formation of the product in the ¹H NMR spectrum. All the reactions were carried out at 343 K and were monitored by ¹H NMR spectroscopy up to 80% conversion with respect to mesitylene used as an internal standard, using 10 mol% of the Mg-1 with various ratios of m-tolunitrile to HBpin.

A 1 : 2 stoichiometric ratio of m-tolunitrile (1c) (0.16 M) and HBpin (0.33 M), with varying catalyst load of Mg-1 adjusted at 10 and 7 mol%, apparently displayed a first order kinetics, (Fig. 6A) consistent with other Nacnac stabilized alkaline earth metal complexes mediated catalysis reactions.^57,99–101 This observation indicates involvement of a single isolated magnesium center, which is involved in the rate determination of the reaction.

Fig. 6 (A) ln[m-tolunitrileConcⁿ] vs. Time plot, at different loadings of Mg-1. (B) [Product Concⁿ] vs. Time plot, at pseudo-first order condition with respect to m-tolunitrile.

A series of pseudo-first-order reactions was conducted using a 20-fold excess of m-tolunitrile (3.33 M) and a catalyst loading of 0.0167 M while varying HBpin concentrations from 0.33 M to 1.33 M. The reaction rate displayed a clear first-order kinetics at lower HBpin concentrations (0.33 M and 0.66 M) but deviated to a near-zero-order dependence at higher concentrations (1.00 M, 1.16 M, and 1.33 M) (Fig. 6B). It was observed that the rate of product formation increased significantly from 0.33 M to 0.66 M HBpin, whereas only a marginal change was observed between 1.00 M and 1.33 M, indicating saturation kinetics, consistent with the formation of a catalyst–substrate intermediate species (designated as Int-1) similar to the Hill group's reported magnesium-catalyzed nitriles dihydroboration.⁵⁷ This saturation behavior suggests a Michaelis–Menten–type^102,103 kinetic profile, where the reaction of Mg-2 and HBpin proceeds through the formation of an active catalyst–HBpin intermediate (ca. Int-1), which precedes the rate-determining step.

Catalyst + HBpin ⇌ [Catalyst-HBpin] → Product

At lower concentrations of HBpin (0.33 M, 0.67 M), the active catalyst Mg-2 remains unsaturated, and the rate increases proportionally with HBpin concentration, indicating a first-order kinetics arising from the rate-limiting formation of Int-1; however, at higher concentrations of HBpin (1.00 M to 1.33 M), catalyst saturation occurs, and the rate becomes independent of the concentration of HBpin, showing zero-order kinetics. This transition highlights the key role of catalyst–substrate complexation in governing the observed rate behavior. A stacked ¹H NMR study (Fig. 7) of the reaction pseudo-first-order conditions with respect to nitrile (20 equiv.) revealed the formation of an N-boryl imine intermediate (2c′) at high nitrile concentrations. This observation aligns with outcomes from stoichiometric reactions. As the reaction proceeds and nitrile is consumed, 2c′ formation drops while the final product 2c starts to appear. The 2c′ is observed only under stoichiometric and kinetic conditions, not in catalytic runs, suggesting it is a short-lived intermediate.

Fig. 7 Stacked ¹H NMR spectra (400 MHz) for the reaction of m-tolunitrile (1c) (3.33 M, 20 equiv.), HBpin (0.33 M, 2 equiv.), and Mg-1 (10 mol%, 0.0167 M) in C₆D₆. Spectra were recorded at 70 °C, in different time intervals from 30 min to 360 min. 2c′; 3-CH₃-C₆H₄-CH=N(Bpin). 2c; 3-CH₃-C₆H₄-CH2-N(Bpin)₂.

To investigate the effect of nitrile concentration on the reaction rate, a series of catalytic reactions was conducted using 0.0167 M Mg-1 under pseudo-first-order conditions with respect to HBpin (3.33 M, 20-fold excess), while varying the concentration of m-tolunitrile (0.167–0.67 M).

At lower nitrile concentrations (0.167 and 0.33 M), a zero-order kinetics was observed, indicating that nitrile's concentration plays no direct role in the rate-determining step and is only involved in the formation of the catalytically active species, Mg-2. However, at higher nitrile concentration (0.67 M), the reaction rate decreased despite maintaining a zero-order kinetics, suggesting that excess nitrile concentration inhibits the reaction by saturation of the Mg-2 active site and impeding HBpin access (Fig. 8A).

Fig. 8 (A) [Product Concⁿ] vs. time plot, at pseudo-first order condition with respect to HBpin, linear fit. (B) ln[Nitrile Concⁿ] vs. Time plot, for electron withdrawing and electron donating substituted aryl nitriles, linear fit.

A further study examined the effect of electron-donating and electron-withdrawing groups on the reaction mechanism (Fig. 8B). The dihydroboration of 2,4-difluorobenzonitrile (1i) proceeded with a slightly decreased rate with respect to m-tolunitrile (1c), following first-order kinetics. The slightly reduced reaction rate for 1i is likely due to its strong electron-withdrawing nature and more steric hindrance, which decreases the availability of the nitrile lone pair for coordination with Mg-1. This in turn, inhibits the formation of [Mg-1 nitrile] adduct, a key step in the overall reaction pathway. Further study of dihydroboration of both the electron-withdrawing (1m) and the electron-donating (1c) substituted nitriles within initial 3 h, exhibited nearly similar reaction rate, highlighting the importance of the formation [Mg-1 nitrile] adduct.

In summary, kinetic studies show that the reaction follows first-order kinetics with respect to the magnesium catalyst, indicating the involvement of a single active species governing the reaction rate. Nitriles exhibit zero-order kinetics; however, they inhibit the reaction rate at higher concentrations due to catalyst saturation. HBpin shows first-order kinetics at low concentrations, shifting to zero-order at higher concentrations, consistent with the formation of a catalyst–substrate intermediate, indicating a Michaelis–Menten-type behavior. Electron-withdrawing nitriles, such as 2,4-difluorobenzonitrile, suppress the reaction rate slightly by weakening coordination to the magnesium center. Overall, the catalytic cycle is primarily governed by the formation of Mg-2, and its reaction with HBpin.

The rate of the reaction follows eqn (1) and (2).

(At a lower concentration of HBpin)

(At a higher concentration of HBpin)

Catalytic cycle for the dihydroboration of nitriles

Based on stoichiometric reactions and the previous literature reports,^38,104 we propose a (Scheme 3) a plausible mechanistic cycle for the Mg-1-catalyzed hydroboration of organic nitriles and isocyanides. As far as magnesium-catalyzed hydroboration is concerned, initially Mg-1 reacts with nitrile 1c to form the active species bis-guanidinate magnesium nitrile adduct Mg-2. The reaction of Mg-2 and HBpin, generates a short-lived magnesium imido intermediate Int-1′ via in situ hydride formation. This species subsequently undergoes rapid reaction with pinB-N(SiMe₃)₂ (Scheme 3) to afford magnesium amino-N-boryl imine intermediate (Int-1). The Int-1 simultaneously reacts with HBpin to form Int-2, which, upon reacting with m-tolunitrile (1c), produces the desired N,N′-bis-boryl amine (2c) and regenerates the active species Mg-2, hereby closing the catalytic cycle. Notably, at the outset, Int-1 remains in equilibrium with Mg-2, which, upon reacting with excess m-tolunitrile(1c), possibly produces N-boryl imine 2c′ along with reformation of Mg-2; however, this has not been observed in the catalytic reactions under optimal conditions. Unlike magnesium-catalyzed dihydroboration of nitriles, Mg-1 acts as an active catalyst for the dihydroboration of isocyanides. Initially, the Mg-1 reacts with 3a and forms intermediate Mg-3 via coordination of isocyanide to the magnesium centre, which reacts with 2 equiv. of HBpin to produce the 1,2-diborylated amine 4a, along with the regeneration of the active catalyst Mg-1, thereby closing the catalytic cycle (Scheme 3). The overall outcomes successfully established Mg-1 as a pre-catalyst for the nitriles dihydroboration; however, it acts as an active catalyst for the dihydroboration of isocyanides.

Scheme 3 Mechanistic cycle of the Mg-1-catalyzed dihydroboration of nitrile and isocyanide.

Conclusion

In conclusion, we have successfully synthesized the first heteroleptic bis-guanidinate magnesium amide adducts bearing coordinated nitrile and isocyanide groups. Furthermore, we have demonstrated the magnesium amide-catalyzed hydroboration of nitriles and isocyanides via a substrate-assisted pathway. This catalytic protocol enables the formation of N,N-bis(boryl)amines and N,C-di(boryl)amines in good to excellent yields. Notably, we observed that in contrast to the homoleptic magnesium bis-amide, the heteroleptic N,N′-chelated bis-guanidinate magnesium amide acts as a superior catalyst for both the dihydroboration of nitriles and isocyanides. Comprehensive mechanistic investigations were established through various stoichiometric reactions, which led to the successful isolation of catalytically relevant intermediates and the active magnesium species, which were unambiguously identified using multinuclear NMR spectroscopy and single-crystal X-ray diffraction analysis. Notably, we were able to generate a highly reactive N-boryl imine intermediate in a stoichiometric reaction. These findings not only expand the scope of magnesium catalysis in bond-forming transformations but also provide critical mechanistic insights into substrate-assisted activation pathways in main-group catalysis.

Author contributions

S. M. carried out most of the synthesis and characterization, drafted the manuscript, and contributed to its subsequent and final versions. S. R. P. managed the supplementary data and assisted in catalyst preparation and kinetic studies. A. D. conducted the catalysis studies and contributed to manuscript drafting. S. N. conceptualized the project, acquired funding, supervised the work, and contributed to the final manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Supplementary information (SI): all the materials and instrumentation, and supplementary figures and tables. See DOI: https://doi.org/10.1039/d5qo01184a.

CCDC 2476567 and 2476571 contain the supplementary crystallographic data for this paper.^105a,b

Acknowledgements

The authors thank the National Institute of Science Education and Research (NISER), Bhubaneswar, Odisha, Homi Bhabha National Institute (HBNI), Mumbai, Department of Atomic Energy (DAE), Govt. of India. The Science and Engineering Research Board (SERB), India (CRG/2021/007000) is acknowledged for providing financial support.

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