Manganese-Catalyzed Hydroboration of Terminal Olefins and Metal-Dependent Selectivity in Internal Olefin Isomerization-Hydroboration

Subhash Garhwal, Asja A. Kroeger, Ranjeesh Thenarukandiyil, Natalia Fridman, Amir Karton,*

Article

Manganese-Catalyzed Hydroboration of Terminal Oleﬁns and Metal-

Dependent Selectivity in Internal Oleﬁn Isomerization−

Hydroboration

†

and Graham de Ruiter *

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c03451

Read Online

ACCESS

Metrics & More

Article Recommendations

sı Supporting Information

ABSTRACT: In the past decade, the use of earth-abundant metals in homogeneous catalysis has ﬂourished. In particular, metals

such as cobalt and iron have been used extensively in reductive transformations including hydrogenation, hydroboration, and

hydrosilylation. Manganese, on the other hand, has been considerably less explored in these reductive transformations. Here, we

report a well-deﬁned manganese complex, [Mn( BDI)(OTf) ] (2a; BDI = bipyridinediimine), that is an active precatalyst in the

hydroboration of a variety of electronically diﬀerentiated alkenes (>20 examples). The hydroboration is speciﬁcally selective for

terminal alkenes and occurs with exclusive anti-Markovnikov selectivity. In contrast, when using the analogous cobalt complex

[

Co( BDI)(OTf) ] (3a), internal alkenes are hydroborated eﬃciently, where a sequence of isomerization steps ultimately leads to

their hydroboration. The contrasting terminal versus internal alkene selectivity for manganese and cobalt was investigated

computationally and is further discussed in the herein-reported study.

INTRODUCTION

have received much attention in recent years. In contrast,

reductive transformations with manganese have been under-

■

The continuous eﬀort to present a more sustainable outlook

for future generations has led to a renaissance in the use of

earth-abundant metals in homogeneous catalysis. Their

natural abundance, low toxicity, and ready availability have

been strong incentives to replace their less environmentally

friendly noble-metal counterparts. Furthermore, the smaller

ionic radii and typically high-spin electronic structures of earth-

abundant metals have resulted in a unique reactivity that can

represented in the literature. Typically, manganese-mediated

transformations are reported for manganese(I), which avoids

the unfavorable high-spin (S = / ) conﬁguration of

Indeed, the groups of Milstein,

Kempe, Beller, Kirchner, and others have demonstrated

that manganese(I) can be an eﬀective catalyst in several

organic transformations. Yet, the application of manganese in

,10

manganese(II).

be otherwise hard to realize with their heavier congeners. The

success of earth-abundant metals results not only from their

unique reactivity in diﬀerent spin states but also from their

the hydroboration of alkenes has rarely been reported.

Received: November 23, 2020

ability to engage in reliable two-electron chemistry via metal−

ligand cooperativity. As a result, during the past few years,

earth-abundant metals have extensively been utilized as

catalysts in many organic transformations.

From among the earth-abundant metals such as manganese,

iron, and cobalt, catalysts that are based on iron and cobalt

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

2a or 3a in acetonitrile (Figure 2 ). Both complexes are similar

Article

Figure 1. Metal-dependent (manganese versus cobalt) selectivity in the hydroboration of terminal and internal alkenes, leading to their

isomerization/hydrofunctionalization with cobalt.

The hydroboration of alkenes is an eﬀective and atom-

Structural details for complexes 2a and 3a were obtained by

X-ray crystallographic studies on crystals formed via the slow

vapor diﬀusion of diethyl ether into a concentrated solution of

economic method for generating alkylboronate esters, which

are versatile reagents in many organic transformations,₁

particularly in metal-mediated cross-coupling reactions.

Traditionally, catalytic alkene hydroboration has been

catalyzed by precious metals such as rhodium, ruthenium,

and iridium. At present, alkene hydroboration with earth-

abundant metals such as cobalt, iron, nickel, and

copper is well established, while for manganese, only a

few reports have appeared in the recent literature.

To better understand the diﬀerences between manganese

and cobalt in the hydroboration of alkenes and to expand the

limited use of manganese in this useful transformation, we

prepared isostructural complexes [Mn( BDI)(OTf) ] (2a;

BDI = bipyridinediimine) and [Co( BDI)(OTf) ] (3a). As

we will demonstrate, the manganese complex 2a is an active

precatalyst for the hydroboration of terminal alkenes. The

hydroboration occurs with exclusive anti-Markovnikov regio-

selectivity (Figure 1) and with good functional group tolerance

in both linear oleﬁns and styrenes. When the metal center is

changed from manganese (2a) to cobalt (3a), the reactivity

toward internal alkenes can be turned on, resulting in their

isomerization−hydroboration (Figure 1), which is a topic of

Figure 2. Solid-state structures of (A) 2a and (B) 3a. Thermal

ellipsoids are shown at the 30% probability level. Hydrogen atoms and

cocrystallized solvent molecules are omitted for clarity.

much current interest. The observed metal-based divergence

between manganese and cobalt was investigated computation-

ally, suggesting that the metal-dependent selectivity results

from oleﬁn migratory insertion into the metal hydride, which is

more favorable for cobalt than for manganese. Furthermore,

the eﬀect of the spin state was also investigated, revealing that

high-spin manganese(II) complexes can be beneﬁcial for

catalysis.

in structure, with two axially bound triﬂates and the four

nitrogen atoms of the BDI ligand (N1−N4) in the equatorial

plane. On the basis of charge balance, the manganese and

cobalt metal centers are assigned as manganese(II) and

^c^o^b^a^l^t⁽^I^I⁾^,^r^e^s^p^e^c^tⁱ^v^e^l^y^.^T^h^e^a^v^e^r^a^g^e^Cimine−N

distances

imine

of 1.282(7) Å (2a) and 1.281(4) Å (3a) and the average

distances of 1.480(8) Å (2a) and 1.499(4) Å (3a)

^Cipso−C

imine

are also indicative of an M(II) (M = metal) center that is

supported by a neutral BDI ligand (Table S2).

RESULTS AND DISCUSSION

■

The paramagnetic nature of complexes 2a and 3a was

further conﬁrmed by SQUID magnetometry via temperature-

dependent analysis of the magnetic moment (Figure 3). For

the manganese complex 2a, the magnetic moment is nearly

temperature-independent between 300 and 6 K (μ_e_f_f= 6.05−

6.11) and decreases below 6 K to 5.72 (Figure 3, red trace).

Synthesis of Manganese and Cobalt Metal Com-

plexes. We commenced our studies by synthesizing the

corresponding manganese and cobalt complexes 2a and 3a.

Stirring an acetonitrile solution of the BDI ligand with an

anhydrous metal triﬂate salt resulted in the clean formation of

complexes 2a and 3a. The H NMR spectra of 2a and 3a only

The room temperature value of μ_e_f_f= 6.05 is close to the

show (broad) proton resonances between −20 and 150 ppm,

consistent with the formation of paramagnetic species (Figures

S15 and S19 ). However, their high-resolution mass spectrom-

etry spectra are consistent with the formation of 2a and 3a. We

note that, on the basis of ¹⁹F NMR spectroscopy, it is highly

likely that the triﬂates in complexes 2a and 3a are not bound to

the metal center in solution.

expected spin-only value of 5.92 for an S = / system,

consistent with an assignment of [Mn (BDI) (OTf) ]

containing a high-spin Mn d metal center. For cobalt,

the room temperature value of the magnetic moment of μ_e_f_f

4.82 (3a) is higher than the spin-only value of 3.87 expected

for a spin / system, but deviation is commonly observed for

27d,28

high-spin Co d metal complexes.

Below 130 K, the

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

a hydroborates a variety of alkenes with excellent yields

(

>99%) and with exclusive anti-Markovnikov regioselectivity

Table 1). The functional group tolerance includes halogens

4d), ethers (4e and 4g), internal alkenes (4i), and silyl

substituents (4f), although the (silyl)ether substituent in 4g

does give somewhat of a diminished yields (Table 1; 60−70%).

Aside from linear alkenes, various electronically and sterically

diﬀerentiated styrenes are also borylated with high eﬃciency

and regioselectivity. For example, para-substituted styrenes

bearing electron-withdrawing (e.g., −F, −Cl, −Br, or −CF ) or

electron-donating (e.g., −OMe or −Me) substituents are

hydroborated with excellent yields (85−99%) and with

exclusive anti-Markovnikov regioselectivity (Table 1; entries

l−4q). Changing the substitution pattern from para to either

ortho or meta aﬀects neither the yield nor the regioselectivity

of the reaction (Table 1; entries 4r−4t). These results

demonstrate that the manganese complex 2a is an eﬀective

precatalyst for the hydroboration of a wide variety of

substituted alkenes, with observed anti-Markovnikov regiose-

lectivities (>98%) that are comparable to the state-of-the-art for

Figure 3. Temperature dependence of the eﬀective magnetic moment

(μ_e_f_f) for complexes 2a (red circles) and 3a (blue circles).

16a,b

manganese.

Recently, there has been considerable interest in alkene

isomerization/hydrofunctionalization of oleﬁns. In this

context, several studies have shown that cobalt complexes are

good catalysts for this synthetically useful transforma-

magnetic moment gradually decreases to a value of μ_e_f_f= 4.07

at 2K, most probably because of weak intermolecular

antiferromagnetic interactions at low temperature (Figure 3,

blue trace).

20d,n,o,q

tion.

We thus envisioned that changing the metal

center from manganese (2a) to cobalt (3a) might give us

Catalytic Hydroboration. With complexes 2a and 3a in

hand, we explored their initial reactivity in the hydroboration

of unactivated alkenes. The hydroboration of alkenes is an

important reaction that gives facile access to alkylboronate

esters frequently used as intermediates in the ﬁne chemical

industry. Using complex 2a (5 mol %), in the presence of

access to this highly useful reaction via the selective

16c

isomerization of internal alkenes.

Using catalytic conditions similar to those for manganese

(

Table 1), we explored the activity of the cobalt complex 3a in

the hydroboration of alkenes (Table 2). To our delight, the

cobalt catalyst 3a aﬀords good yields and exclusive anti-

Markovnikov selectivity in the hydroboration of terminal

alkenes. (Table 2, 4a−4c, 4f, 4h, and 4i). Styrene, on the other

hand, gives a mixture of α- and β-substituted products, with a

fair degree of hydrogenation of the double bond (Table 2; 4k).

The results obtained for 3a and styrene are in direct contrast to

pinacolborane (HBpin; 3 equiv) with NaO Bu (15 mol %) as

the activator, 1-hexene was selectively converted to the

corresponding terminal alkylboronate ester. The reaction

occurs under mild conditions (60 °C, N ) and typically takes

4 h to reach complete conversion. Under these conditions,

further investigation into the substrate scope demonstrates that

Table 1. Substrate Scope of the Manganese-Catalyzed (2a) Hydroboration of Terminal Alkenes

Reactions were performed with alkene (0.5. mmol), HBpin (1.5 mmol, 3.0 equiv), NaO Bu (0.075 mmol, 15 mol %), and catalyst 2a (5 mol %) at

0 °C for 24 h (Table 1). Yields were determined by H NMR spectroscopy in the presence of an internal standard. In all substrates, hydroboration

occurs with almost exclusive anti-Markovnikov regioselectivity (≥98%).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

precedent (Figure 4 ). The catalytic cycle starts with the

Article

Table 2. Substrate Scope of the Cobalt-Catalyzed (3a)

Hydroboration of Terminal Alkenes

diﬀerent from those for borane are observed (Table 2; 4k);

(ii) for both cobalt and manganese, the hydroboration is

sensitive toward modiﬁcation of the imine substituent on the

ligand (Figure 1), indicating a strong catalyst dependence

(

Table S1); (iii) the addition of TMEDA does not lead to

complete inhibition of the observed catalysis (Figure S67).

Computational Mechanistic Investigations. In order to

gain insights into the origins of the experimentally observed

selectivity diﬀerences between complexes 2a and 3a, we

subsequently studied the mechanism of the hydroboration

reaction using density functional theory (DFT) methods.

Hereby, we took into consideration the possibility of diﬀerent

spin states in the modeled complexes and the possibility for

reversible metal −ligand coordination of the imine substituents

(

Tables S7 −S12 ). Such structural ﬂexibility has been

previously observed by Solan and co-workers in related

complexes with nickel and zinc.

To guide the computational studies, we ﬁrst deﬁned a

plausible reaction mechanism that is based on literature

Reactions were performed with alkene (0.5. mmol), HBpin (0.6

mmol, 1.2 equiv), NaO Bu (0.045 mmol, 9 mol %), and catalyst 3a (2

mol %) in tetrahydrofuran (0.5 mL) at 60 °C for 24 h (Table 1).

Yields were determined by H NMR spectroscopy in the presence of

an internal standard. In all substrates, the hydroboration occurs with

almost exclusive anti-Markovnikov regioselectivity (≥98%).

the observed regioselectivity with 2a, which exclusively gives

the linear β-substituted (anti-Markovnikov) borylation product

(

vide supra). An additional diﬀerence between 2a and 3a is

that while the manganese catalyst 2a does not show any

reactivity toward internal alkenes, 3a eﬃciently borylates

internal alkenes to produce the terminal alkylboronate esters in

good yields (Table 2). The hydroboration of internal alkenes

occurs irrespective of the position of the double bond (Table

, e.g., 4a; 2-octene vs 4-octene), although shorter alkenes are

generally borylated more eﬃciently than longer ones (Table 2;

compare 4a with 4b). Aside from the position of the double

bond, their geometry is also important. For example, when

using cis-3-hexene, yields are lower than those for trans-3-

hexene (compare 4b and 4u). The observed terminal anti-

Markovnikov borylation most likely occurs via consecutive

alkene insertion and β-hydride elimination steps, providing a

Figure 4. Plausible mechanism for the manganese- and cobalt-

catalyzed hydroboration of alkenes.

formation of a metal hydride that is generated upon activation

of the precatalyst with NaO Bu and HBpin. The importance

of metal hydride intermediates in alkene hydroboration has

16c

also been demonstrated by Chirik,

Turculet,

and

convenient method for alkene isomerization−hydroboration.

others. Coordination of the alkene and subsequent insertion

into the metal hydride generates a metal alkyl species. Finally,

borylation can occur either via a stepwise oxidative addition/

reductive elimination pathway or via a concerted σ-bond

metathesis pathway (Figure 4). The validity of the proposed

mechanism was investigated for both cobalt and manganese in

order to unravel the origin of the observed selectivity

diﬀerences for internal and terminal alkenes (vide infra). As

we will demonstrate, decoordination of one the ligand side

arms herein is crucial in order to provide a vacant coordination

site.

Cobalt-Catalyzed Oleﬁn Hydroboration. Because com-

plex 3a was able to eﬃciently catalyze the hydroboration of

both terminal and internal alkenes, we will ﬁrst discuss the

computed mechanistic aspects of the cobalt-catalyzed hydro-

boration (Tables S9, S10, and S12). Hereafter, we will contrast

the calculated mechanism with that calculated for manganese

Indeed, isotope-labeling experiments are consistent with

consecutive insertion/elimination steps (Figures S5 −S8).

The marked diﬀerences in selectivity with cobalt and

manganese highlight the importance of the metal center in

either (i) enabling β-hydride elimination necessary for alkene

isomerization or (ii) their ability to generate a secondary

metal−alkyl species upon insertion of the internal alkene into

the M−H bond.

Before the origin of the diﬀerence in reactivity between 2a

and 3a is discussed in more detail, it is important to note that,

recently, it was reported that, under the experimental reaction

conditions, the combination NaO Bu/HBpin generates a good

hydroboration catalyst (BH₃). Notwithstanding, we believe

that in this study complexes 2a and 3a are the active

precatalysts based on the following observations: (i) for the

cobalt-catalyzed hydroboration of styrene, product selectivities

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 5. Calculated equilibrium structures for the corresponding cobalt (A) and manganese (B) hydrides in the singlet (cobalt) and quintet

manganese) states.

(

and explain the diﬀerences, leading to the observed selectivity

for terminal and internal alkenes (vide infra).

center or (ii) a low-spin cobalt(II) metal center that is

antiferromagnetically coupled to a ligand-based radical.

Because the activation protocol and reaction conditions

provide an overall reducing environment, redox noninnocence

of the BDI ligand should not be ruled out.

Analysis of the calculated geometry of Co-IA reveals that the

For cobalt, hydroboration is initiated by formation of the

corresponding hydride (Figure 4). The calculated geometries

of the ﬁve- and four-coordinate cobalt hydride species Co-IA

and Co-IB are shown in Figure 5A and feature a cobalt metal

center in square-pyramidal and distorted “butterﬂy” geo-

metries, respectively. For all cobalt complexes, the singlet,

triplet, and quintet spin states were considered (Tables 3 and

S9 ). Our calculations indicate that, for Co-IA, the singlet state

26a,34

imine−Cimine and Cimine−Cipso bond lengths of 1.361 and 1.430

Å are indicative of a ligand-based radical, which necessitates a

low-spin Co d metal center in Co-IA. Our calculations also

indicate that, for hydroboration to proceed, ligand dissociation

is necessary (Figures 4 and 6). Ligand dissociation from Co-IA

to give Co-IB is energetically uphill by 25.1 kcal mol⁻and

does not have a marked eﬀect on the calculated bond distances

(Table 3 and Figure 5). After formation of the cobalt hydride

Co-IB, oleﬁn coordination is exergonic by 1.8 kcal mol⁻(Co-

IIB ) on the singlet potential energy surface (PES; Figure 6 and

Table S9 ). Migratory insertion of the oleﬁn into the Co−H

bond occurs with an overall barrier of 25.5 kcal mol⁻(TS1)

and provides the secondary cobalt alkyl species Co-IIIB. The

−

is 15.1 kcal mol lower in energy than the triplet state, while

for Co-IB, the singlet and triplet states are nearly equal in

energy (Table 3). The calculated singlet state implies that

the electronic structure features either (i) a genuine Co metal

Table 3. Relative Energies for Diﬀerent Spin States of the

Cobalt and Manganese Metal Hydrides upon Coordination

(

IA) and Decoordination (IB) of One of the Imine Side

Arms

−

small activation energy (2.2 kcal mol ) required to transition

from Co-IIB to Co-IIIB indicates that migratory insertion

readily occurs once a single imine-ligand dissociates. A similar

trend is observed for the triplet PES (Figure 6, blue trace).

However, the overall energy barrier of 35.1 kcal mol⁻is

signiﬁcantly higher than that calculated for the singlet PES

Energy (kcal mol⁻¹)

Spin state

Co-IA

Mn-IA

Singlet

Triplet

Quintet

0.0

15.1

25.5

−2.3

0.0

−

(

25.5 kcal mol ). Hereafter, isomerization of the internal

Energy (kcal mol⁻¹)

cobalt alkyl species Co-IIIB to the terminal cobalt alkyl species

Co-IIID occurs via a sequence of β-hydride elimination and

migratory insertion steps (Figure 6) with activation energies

Spin state

Co-IB

Mn-IB

Singlet

Triplet

Quintet

25.1

24.5

50.6

19.3

12.9

−1

between 4 and 6 kcal mol . Overall, there is very little

thermodynamic preference for the internal versus terminal

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

−

Figure 6. Calculated Gibbs free PES (ΔG , kcal mol ) for the hydroboration of 2-butene with complex 3a with a singlet (red trace) or a triplet

(

blue trace) spin state. For computational details, see the Supporting Information.

cobalt alkyl complexes given their similar energies (21.9 vs 23.9

Table 4. Calculated Gibbs Free Energies (ΔG₂₉₈, kcal

kcal mol ). The small diﬀerence of 2 kcal mol⁻¹is indicative

of a Curtin−Hammett-type scenario, where the selectivity for

internal versus terminal hydroboration is determined by the

diﬀerences in the energy barriers for borylation, provided that

the borylation step is rate-limiting for both the internal and

terminal alkenes. Similar ﬁndings were reported by Turculet et

al. for three-coordinate manganese, iron, and cobalt hydride

−

mol ) for the Borylation of Internal and Terminal Cobalt

−1

Alkyl Species Co-IIIB and Co-IIIB

Internal borylation

Energy

Terminal borylation

Energy

(

kcal mol⁻¹)

(kcal mol⁻¹)

Complex Singlet Triplet

Complex

Co-IIID

Singlet Triplet

complexes.

Co-IIIB

TS4Ox

21.9

35.2

29.0

31.8

23.9

20.2

28.5

11.7

23.9

33.7

29.2

31.0

23.3

21.0

Finally, for the migratory insertion pathway shown in Figure

, it is insightful to mention that ligand recoordination in Co-

TS4Ox

Co-IVB

TS5Red

Co-VB

Co-IVD

IIIB and Co-IIID is strongly exergonic and provides a stable

oﬀ-cycle intermediate. In a related study, we were able to

isolate such a ﬁve-coordinate cobalt alkyl (Co-Me) species that

exhibits a singlet ground state. The observed singlet state is

consistent with the preferred singlet PES calculated for the

migratory insertion step in the present study. Overall, these

results demonstrate that hydrometalation of internal alkenes is

feasible for cobalt and that a series of alkene isomerization

steps ultimately leads to the borylation of a terminal cobalt

alkyl species (vide infra). Because the direct migratory

insertion of 1-butene will follow the same mechanism, these

results also conﬁrm the observed reactivity of 3a toward

terminal alkenes (Figure S76 and Table S10).

Having examined the mechanism of alkene migratory

insertion, we subsequently consider the following borylation

pathways: (i) a stepwise pathway involving oxidative addition

of the borane, followed by reductive elimination of the

borylated product, or (ii) a concerted σ-bond metathesis

pathway resulting in formation of the borylated product and

metal hydride (Table S12). Our calculations show that,

starting from Co-IIIB, borylation occurs via a stepwise

pathway involving oxidative addition of the borane (Table

TS5Red (or TSσ‑bond

Co-VD

)

27.9

11.2

For computational details, see the Supporting Information.

Calculations indicate a direct σ-bond metathesis pathway.

singlet PES the borylation occurs in a stepwise fashion, the

triplet PES oﬀers a low-energy concerted σ-bond metathesis

pathway to the borylated product (Table 4). This suggests that

the observed borylation beneﬁts from a spin-crossover event,

which is common for ﬁrst-row transition-metal catalysts.

Overall, all of the calculated transition structures pertaining to

the borylation steps are higher in energy than those calculated

for the alkene isomerization processes, indicating that the ﬁnal

borylation step is indeed rate-limiting, in accordance with a

Curtin−Hammett-type scenario. While the calculated diﬀer-

ence between the energies of the transition structures on the

−

triplet PES for internal (28.5 kcal mol ) and terminal (27.9

−

kcal mol ) borylation might be underestimated, the

calculations qualitatively agree with our experimental results.

Namely, both the experimental and computational results

demonstrate that, for cobalt, migratory insertion of internal

and terminal alkenes is feasible and that subsequent tandem

oleﬁn isomerization/borylation leads to formation of the

experimentally observed terminal borylation product (Table

2).

). Hereby, the triplet PES is clearly favored over the singlet

−

PES by 6.7 kcal mol (Table 4).

However, when starting from the terminal cobalt alkyl

species Co-IIID, a diﬀerent picture emerges. While on the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Manganese-Catalyzed Oleﬁn Hydroboration. Compu-

tational studies regarding the alkene hydroboration with

manganese were initiated by evaluating the diﬀerent spin-

states for the manganese hydride complexes Mn-IA and Mn-

IB. As is evident from Table 5, both the triplet and quintet spin

−

Table 5. Calculated Gibbs Free Energy (ΔG , kcal mol )

298

for the Migratory Insertion of 2-Butene

Energy (kcal mol⁻¹)

Complex

Triplet

Quintet

Mn-IA

Mn-IB

Mn-IIB

TSI

−2.3

19.3

34.5

25.2

0.0

12.9

18.3

34.2

7.3

Mn-IIIB

For computational details, see the Supporting Information.

−

states are close in energy (ΔΔG₂₉₈= 2.3 kcal mol ); however,

upon ligand decoordination, the quintet state is clearly

preferred (Table 5). The quintet spin state implies a high-

−1

Figure 7. Calculated Gibbs free PES (ΔG , kcal mol ) for the

spin Mn d metal center that is antiferromagnetically coupled

to a ligand-based radical. The diﬀerence with the analogous

cobalt complex is that, whereas Co-IB is low-spin, Mn-IB is

high-spin. Aside from the diﬀerences in the favored spin states,

there are no distinct structural diﬀerences between the cobalt

and manganese hydride complexes M-IA and M-IIB, which is

also reﬂected by their similar geometries and bond lengths.

In order to be able to explain the experimentally observed

selectivity of the manganese catalyst 2a for terminal alkenes,

we begin by considering the mechanism and energetics of the

migratory insertion of 2-butene (Tables 5 and S7). As is

evident from Table 5, the computational results show that, on

both the triplet and quintet PESs, the migratory insertion has a

hydroboration of 1-butene with complex 2a with a triplet (red trace)

or quintet (blue trace) spin state. For computational details, see the

Supporting Information .

Table 6. Calculated Gibbs Free Energies (ΔG₂₉₈, kcal

−

mol ) for the Borylation of Terminal Manganese Alkyl

a b

Species Mn-IIID

Energy (kcal mol⁻¹)

Complex

Mn-IIID

Triplet

Quintet

4.4

22.5

30.8

12.8

31.2

12.8

TS4Ox

Mn-IVD

−

high activation energy of 34.5 and 34.2 kcal mol , respectively

Table 5; TS1). On the other hand, the migratory insertion of

-butene into the Mn−H bond has an activation energy that is

TS5Red (or TSσ‑bond

Mn-V

)

20.5

3.2

(

For computational details, see the Supporting Information.

−

lower by almost 3.0 kcal mol (Figure 7 and Table S8).

Nonetheless, the energy barrier remains high and qualitatively

explains why for manganese the hydroboration of terminal

alkenes remains challenging. A survey across the reported

manganese-catalyzed hydroboration reactions reveals that most

of them require higher temperatures and longer reaction times,

Calculations indicate a direct σ-bond metathesis pathway.

limiting step for cobalt is the borylation via σ-bond metathesis,

while for manganese, it is the migratory insertion of the alkene

(Table 4 and Figure 7). The energy diﬀerence between the

transition structures of the rate-limiting steps for cobalt (27.9

16c,36

−1

especially when compared to cobalt.

kcal mol ) and manganese (31.5 kcal mol ) indicates that

the hydroboration of terminal alkenes should be more facile for

cobalt than for manganese. These computational results are in

qualitative agreement with the experimentally obtained results

(Table S2 ). Under identical reaction conditions (60 °C, 3 mol

% catalyst, and 1.2 equiv of HBpin), the cobalt catalyst 3a

converts 1-octene into the corresponding alkylboronate ester

in 99% yield, while for the manganese catalyst 2a, only 25%

yield was observed. These observations are in line with other

reported examples that demonstrate that, for manganese

catalysis, typically higher temperatures and longer reaction

Having explored the manganese-catalyzed alkene migratory

insertion, we turn to the subsequent borylation step. First, we

note that, because complex 2a did not show any reactivity

toward internal alkenes, the mechanistic aspects of their

borylation will not be discussed. Notwithstanding, a full PES

for the hydroboration of 2-butene is provided in Figure S75.

For the borylation of terminal manganese alkyl species Mn-

IIID, calculations show that the lowest-energy pathway lies on

the quintet PES (Tables 6 and S11). The borylation proceeds

via a concerted σ-bond metathesis pathway with an activation

−

16c,36

energy of 20.5 kcal mol . In contrast, on the triplet PES,

times are required.

borylation occurs via a stepwise oxidative addition−reductive

Second, the herein-presented computational studies also

point toward a diﬀerence in regioselectivity between

manganese and cobalt complexes 2a and 3a. In particular,

the computational results show that the manganese-catalyzed

migratory insertion of an internal alkene is unlikely (34.2 kcal

−

elimination pathway with an energy barrier of 30.8 kcal mol

(Table 6). Because the energy barrier for σ-bond metathesis is

much lower than that calculated for migratory insertion (31.5

−

kcal mol ; Figure 7), migratory insertion is rate-limiting in the

manganese-catalyzed hydroboration.

−

mol ), while for cobalt, there is no clear diﬀerence between

−

Upon comparison of these computational results with those

obtained for cobalt, a few diﬀerences are noted. First, the rate-

the migratory insertion of an internal (25.5 kcal mol ) or a

−

terminal (24.4 kcal mol ) alkene. This explains why, in

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

https://pubs.acs.org/10.1021/acs.inorgchem.0c03451

Article

experiments, the cobalt complex 3a is able to catalyze the

hydroboration of both internal and terminal alkenes, while the

manganese catalyst 2a is selective for only terminal alkenes.

Author Contributions

†

S.G. and A.A.K. contributed equally.

SUMMARY AND CONCLUSIONS

■

Notes

In summary, we have demonstrated that complexes of the

types 2a and 3a are eﬀective precatalysts for the hydroboration

of terminal alkenes. The hydroboration reaction occurs under

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

mild conditions (60 °C, N ) and proceeds with exclusive anti-

This research was supported by the Azrieli Foundation (Israel)

and the Technion EVPR Fund−Mallat Family Research Fund.

G.de.R. is an Azrieli young faculty fellow and a Horev Fellow

supported by the Taub Foundation. R.T. thanks the Council

for Higher Education in Israel for an incoming PBC fellowship.

A.K. acknowledges resources from the National Computational

Infrastructure, supported by the Australian Government, and

system administration support provided by the Faculty of

Science at the University of Western Australia to the Linux

cluster of the Karton group. A.K. and A.A.K. gratefully

acknowledge the provision of a Forrest Research Foundation

Scholarship and an Australian Government Research Training

Program Stipend (to A.A.K.) and an Australian Research

Council Future Fellowship (to A.K.; Project FT170100373).

Markovnikov selectivity. When using internal alkenes, however,

marked diﬀerences in the reactivities of the manganese and

cobalt catalysts were observed. Whereas for manganese no

reactivity was observed for internal alkenes, the cobalt complex

a was able to perform a tandem alkene isomerization/

hydroboration reaction. To understand these diﬀerences in the

reactivities, the reaction mechanism of the hydroboration

reaction was investigated computationally, taking into

consideration the diﬀerent spin states and structural ﬂexibilities

of complexes 2a and 3a. We demonstrate that the observed

metal-based divergence is due to diﬀerences in the energy

barriers toward the initial migratory insertion of the internal

−

alkene, which is much higher for manganese (34.2 kcal mol )

−

than for cobalt (25.5 kcal mol ).

REFERENCES

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03451.

■

(

1) Gebbink, R. J. M. K.; Moret, M. E. Non-Noble Metal Catalysis:

Molecular Approaches and Reactions, 1st ed.; Wiley-VCH: Weinheim,

Germany, 2019.

2) (a) Hu, M.; Ge, S. Versatile Cobalt-Catalyzed Regioselective

Synthetic procedures, characterization data, catalysis,

and computational data (PDF)

Chain-Walking Double Hydroboration of 1,N-Dienes to Access Gem-

Bis(Boryl)Alkanes. Nat. Commun. 2020 , 11 , 765. (b) Zhou, Y.-Y.;

Uyeda, C. Catalytic Reductive [4 + 1]-Cycloadditions of Vinylidenes

and Dienes. Science 2019, 363, 857. (c) Tondreau, A. M.; Atienza, C.

C. H.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.; Chirik, P.

J. Iron Catalysts for Selective Anti-Markovnikov Alkene Hydro-

silylation Using Tertiary Silanes. Science (Washington, DC, U. S.)

2012, 335, 567−570.

Accession Codes

CCDC 2027374 −2027377 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

(3) (a) Holland, P. L. Reaction: Opportunities for Sustainable

Catalysts. Chem. 2017 , 2 , 443 −444. (b) Holland, P. L. Distinctive

Reaction Pathways at Base Metals in High-Spin Organometallic

Catalysts. Acc. Chem. Res. 2015, 48 , 1696 −1702.

AUTHOR INFORMATION

Corresponding Authors

Amir Karton − School of Molecular Sciences, The University of

Western Australia, 6009 Perth, WA, Australia; orcid.org/

000-0002-7981-508X ; Email: amir.karton@uwa.edu.au

Graham de Ruiter − Schulich Faculty of Chemistry,

TechnionIsrael Institute of Technology, Technion City

■

(4) (a) Arevalo, R.; Chirik, P. J. Enabling Two-Electron Pathways

with Iron and Cobalt: From Ligand Design to Catalytic Applications.

J. Am. Chem. Soc. 2019 , 141 , 9106 −9123. (b) Karunananda, M. K.;

Mankad, N. P. Cooperative Strategies for Catalytic Hydrogenation of

Unsaturated Hydrocarbons. ACS Catal. 2017, 7 , 6110 −6119.

Coupling Reactions Implying Ligand-Based Redox Changes. Chem-

CatChem 2016 , 8 , 3310 −3316. (d) Zell, T.; Milstein, D. Hydro-

genation and Dehydrogenation Iron Pincer Catalysts Capable of

Metal-Ligand Cooperation by Aromatization/Dearomatization. Acc.

Chem. Res. 2015 , 48 , 1979 −1994. (e) Luca, O. R.; Crabtree, R. H.

Redox-Active Ligands in Catalysis. Chem. Soc. Rev. 2013, 42, 1440−

200008, Haifa, Israel; orcid.org/0000-0001-6008-

86X ; Email: graham@technion.ac.il

Authors

Subhash Garhwal − Schulich Faculty of Chemistry, Technion

Israel Institute of Technology, Technion City 3200008, Haifa,

Israel; orcid.org/0000-0002-0093-9224

Asja A. Kroeger − School of Molecular Sciences, The

University of Western Australia, 6009 Perth, WA, Australia;

orcid.org/0000-0001-7699-541X

1459. (f) Darmon, J. M.; Stieber, S. C. E.; Sylvester, K. T.; Fernandez,

I.; Lobkovsky, E.; Semproni, S. P.; Bill, E.; Wieghardt, K.; DeBeer, S.;

Chirik, P. J. Oxidative Addition of Carbon-Carbon Bonds with a

Redox-Active Bis(imino)pyridine Iron Complex. J. Am. Chem. Soc.

012 , 134 , 17125 −17137. (g) Lyaskovskyy, V.; de Bruin, B. Redox

Non-Innocent Ligands: Versatile New Tools to Control Catalytic

Reactions. ACS Catal. 2012, 2, 270 −279.

Ranjeesh Thenarukandiyil − Schulich Faculty of Chemistry,

TechnionIsrael Institute of Technology, Technion City

(5) (a) Wu, C.; Liao, J.; Ge, S. Cobalt-Catalyzed Enantioselective

Hydroboration/Cyclization of 1,7-Enynes: Asymmetric Synthesis of

Chiral Quinolinones Containing Quaternary Stereogenic Centers.

Angew. Chem., Int. Ed. 2019, 58, 8882 −8886. (b) Wen, H.; Wang, K.;

Zhang, Y.; Liu, G.; Huang, Z. Cobalt-Catalyzed Regio- and

Enantioselective Markovnikov 1,2-Hydrosilylation of Conjugated

Dienes. ACS Catal. 2019, 9, 1612−1618. (c) Ghosh, C.; Kim, S.;

200008, Haifa, Israel

Natalia Fridman − Schulich Faculty of Chemistry, Technion

Israel Institute of Technology, Technion City 3200008, Haifa,

Israel

Complete contact information is available at:

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Trovitch, R. J. Efficient Cobalt Catalyst for Ambient-Temperature

Article

Mena, M. R.; Kim, J.-H.; Pal, R.; Rock, C. L.; Groy, T. L.; Baik, M.-H.;

Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted

Borylation Mechanism, and a New Synthetic Route to Amides. J.

Am. Chem. Soc. 2019, 141, 15327−15337. (d) Espinosa, M. R.;

Charboneau, D. J.; Garcia de Oliveira, A.; Hazari, N. Controlling

Selectivity in the Hydroboration of Carbon Dioxide to the Formic

Acid, Formaldehyde, and Methanol Oxidation Levels. ACS Catal.

Iron Pincer Catalysis. In Organometallics for Green Catalysis; Dixneuf,

P. H., Soule, J.-F., Eds.; Springer International Publishing: Cham,

Switzerland, 2019; pp 141 −174. (e) Wei, D.; Darcel, C. Iron Catalysis

in Reduction and Hydrometalation Reactions. Chem. Rev. 2019 , 119 ,

2550 −2610. (f) Raya-Baron, A.; Ona-Burgos, P.; Fernandez, I. Iron-

́ ́

Catalyzed Homogeneous Hydrosilylation of Ketones and Aldehydes:

Advances and Mechanistic Perspective. ACS Catal. 2019 , 9 , 5400 −

5417. (g) Zell, T.; Langer, R. From Ruthenium to Iron and

Manganese A Mechanistic View on Challenges and Design

Principles of Base-Metal Hydrogenation Catalysts. ChemCatChem

2018 , 10 , 1930 −1940. (h) Mukherjee, A.; Milstein, D. Homogeneous

Catalysis by Cobalt and Manganese Pincer Complexes. ACS Catal.

2019, 9, 301−314. (e) Hu, M.-Y.; He, Q.; Fan, S.-J.; Wang, Z.-C.; Liu,

L.-Y.; Mu, Y.-J.; Peng, Q.; Zhu, S.-F. Ligands with 1,10-phenanthro-

line scaffold for highly regioselective iron-catalyzed alkene hydro-

silylation. Nat. Commun. 2018, 9, 221. (f) Friedfeld, M. R.; Zhong, H.;

Ruck, R. T.; Shevlin, M.; Chirik, P. J. Cobalt-catalyzed asymmetric

hydrogenation of enamides enabled by single-electron reduction.

Science 2018, 360, 888. (g) Wang, C.; Teo, W. J.; Ge, S. Access to

stereodefined (Z)-allylsilanes and (Z)-allylic alcohols via cobalt-

catalyzed regioselective hydrosilylation of allenes. Nat. Commun.

2018 , 8 , 11435 −11469. (i) Furstner, A. Iron Catalysis in Organic

Synthesis: A Critical Assessment of What It Takes To Make This Base

Metal a Multitasking Champion. ACS Cent. Sci. 2016 , 2 , 778 −789.

(j) Sun, J.; Deng, L. Cobalt Complex-Catalyzed Hydrosilylation of

Alkenes and Alkynes. ACS Catal. 2016 , 6 , 290 −300. (k) Bauer, I.;

2017, 8, 1−9. (h) Hoyt, J. M.; Schmidt, V. A.; Tondreau, A. M.;

Knolker, H.-J. Iron Catalysis in Organic Synthesis. Chem. Rev. 2015,

Chirik, P. J. Iron-catalyzed intermolecular [2 + 2] cycloadditions of

unactivated alkenes. Science 2015 , 349 , 960. (i) Bleith, T.; Wadepohl,

H.; Gade, L. H. Iron Achieves Noble Metal Reactivity and Selectivity:

Highly Reactive and Enantioselective Iron Complexes as Catalysts in

the Hydrosilylation of Ketones. J. Am. Chem. Soc. 2015, 137, 2456−

115, 3170−3387. Nakazawa, H.; Itazaki, M. Fe−H Complexes in

Catalysis. In Iron Catalysis: Fundamentals and Applications; Plietker,

B., Ed.; Springer: Berlin, 2011; pp 27 −81. (m) Junge, K.; Schroeder,

K.; Beller, M. Homogeneous Catalysis using Iron Complexes. Recent

Developments in Selective Reductions. Chem. Commun. (Cambridge,

U. K.) 2011 , 47 , 4849 −4859. (n) Sherry, B. D.; Furstner, A. The

Promise and Challenge of Iron-Catalyzed Cross Coupling. Acc. Chem.

Res. 2008, 41, 1500−1511.

459. (j) Friedfeld, M. R.; Shevlin, M.; Hoyt, J. M.; Krska, S. W.;

Tudge, M. T.; Chirik, P. J. Cobalt Precursors for High-Throughput

Discovery of Base Metal Asymmetric Alkene Hydrogenation

Catalysts. Science 2013 , 342 , 1076. (k) Zuo, W.; Lough, A. J.; Li, Y.

F.; Morris, R. H. Amine(imine)diphosphine Iron Catalysts for

Asymmetric Transfer Hydrogenation of Ketones and Imines. Science

(8) (a) Kallmeier, F.; Kempe, R. Manganese Complexes for

(De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron

Catalysts. Angew. Chem., Int. Ed. 2018 , 57 , 46 −60. (b) Yang, X.;

Wang, C. Manganese-Catalyzed Hydrosilylation Reactions. Chem. -

Asian J. 2018 , 13 , 2307 −2315. (c) Gorgas, N.; Kirchner, K.

Isoelectronic Manganese and Iron Hydrogenation/Dehydrogenation

1558 −1569. (d) Garbe, M.; Junge, K.; Beller, M. Homogeneous

Catalysis by Manganese-Based Pincer Complexes. Eur. J. Org. Chem.

2017 , 2017 , 4344 −4362. (e) Carney, J. R.; Dillon, B. R.; Thomas, S.

P. Recent Advances of Manganese Catalysis for Organic Synthesis.

Eur. J. Org. Chem. 2016 , 2016 , 3912 −3929. (f) Liu, W.; Ackermann,

L. Manganese-Catalyzed C-H Activation. ACS Catal. 2016, 6, 3743−

3752. (g) Khusnutdinov, R. I.; Bayguzina, A. R.; Dzhemilev, U. M.

Manganese Compounds in the Catalysis of Organic Reactions. Russ. J.

Org. Chem. 2012, 48, 309−348.

(9) (a) Russell, S. K.; Bowman, A. C.; Lobkovsky, E.; Wieghardt, K.;

Chirik, P. J. Synthesis and Electronic Structure of Reduced

Bis(imino)pyridine Manganese Compounds. Eur. J. Inorg. Chem.

2012, 2012, 535 −545. (b) Reardon, D.; Aharonian, G.; Gambarotta,

S.; Yap, G. P. A. Mono- and Zerovalent Manganese Alkyl Complexes

Supported by the α ,α ’-Diiminato Pyridine Ligand: Alkyl Stabilization

at the Expense of Catalytic Performance. Organometallics 2002, 21,

786−788.

(10) Please note that, although manganese(I)-based catalysts are

more prevalent, both low- and high-spin manganese(II) complexes

can be highly active in catalysis. For example, see: Vasilenko, V.;

Blasius, C. K.; Gade, L. H. One-Pot Sequential Kinetic Profiling of a

Highly Reactive Manganese Catalyst for Ketone Hydroboration:

Leveraging σ -Bond Metathesis via Alkoxide Exchange Steps. J. Am.

Chem. Soc. 2018, 140, 9244−9254. Kelly, C. M.; McDonald, R.;

Sydora, O. L.; Stradiotto, M.; Turculet, L. A Manganese Pre-Catalyst:

Mild Reduction of Amides, Ketones, Aldehydes, and Esters. Angew.

Chem., Int. Ed. 2017, 56, 15901−15904. Mukhopadhyay, T. K.;

Flores, M.; Groy, T. L.; Trovitch, R. J. A Highly Active Manganese

Precatalyst for the Hydrosilylation of Ketones and Esters. J. Am.

Chem. Soc. 2014, 136, 882−885.

013 , 342 , 1080. (l) Langer, R.; Leitus, G.; Ben-David, Y.; Milstein,

D. Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer

Complex. Angew. Chem., Int. Ed. 2011, 50, 2120−2124. (m) Langer,

R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Ben-David, Y.;

Milstein, D. Low-Pressure Hydrogenation of Carbon Dioxide

Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal

Activity. Angew. Chem., Int. Ed. 2011, 50, 9948−9952. (n) Boddien,

A.; Mellmann, D.; Gartner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.;

Laurenczy, G.; Ludwig, R.; Beller, M. Efficient Dehydrogenation of

Formic Acid Using an Iron Catalyst. Science 2011, 333, 1733.

(

o) Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.; Dyson, P.

J.; Scopelliti, R.; Laurenczy, G.; Beller, M. A Well-Defined Iron

Catalyst for the Reduction of Bicarbonates and Carbon Dioxide to

Formates, Alkyl Formates, and Formamides. Angew. Chem., Int. Ed.

010 , 49 , 9777 −9780. (p) Bart, S. C.; Lobkovsky, E.; Chirik, P. J.

Preparation and Molecular and Electronic Structures of Iron(0)

Dinitrogen and Silane Complexes and Their Application to Catalytic

Hydrogenation and Hydrosilation. J. Am. Chem. Soc. 2004, 126,

3794 −13807. (q) Nakamura, M.; Matsuo, K.; Ito, S.; Nakamura, E.

Iron-Catalyzed Cross-Coupling of Primary and Secondary Alkyl

Halides with Aryl Grignard Reagents. J. Am. Chem. Soc. 2004, 126,

6) (a) Alig, L.; Fritz, M.; Schneider, S. First-Row Transition Metal

686−3687.

De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

Chem. Rev. 2019, 119, 2681−2751. (b) Fritz, M.; Schneider, S. The

Renaissance of Base Metal Catalysis Enabled by Functional Ligands.

In The Periodic Table II: Catalytic, Materials, Biological and Medical

Applications; Mingos, D. M. P., Ed.; Springer International Publishing:

Cham, Switzerland, 2019; pp 1−36. (c) Gandeepan, P.; Muller, T.;

Zell, D.; Cera, G.; Warratz, S.; Ackermann, L. 3d Transition Metals

for C-H Activation. Chem. Rev. 2019 , 119 , 2192 −2452. (d) Du, X.;

Huang, Z. Advances in Base-Metal-Catalyzed Alkene Hydrosilylation.

ACS Catal. 2017, 7, 1227−1243.

(

7) (a) Hapke, M.; Hilt, G. Cobalt Catalysis in Organic Synthesis:

Methods and Reactions ; Wiley, 2020. (b) Wen, H.; Liu, G.; Huang, Z.

Recent Advances in Tridentate Iron and Cobalt Complexes for

Alkene and Alkyne Hydrofunctionalizations. Coord. Chem. Rev. 2019 ,

(11) (a) Chakraborty, S.; Gellrich, U.; Diskin-Posner, Y.; Leitus, G.;

Avram, L.; Milstein, D. Manganese-Catalyzed N-Formylation of

Amines by Methanol Liberating H2: A Catalytic and Mechanistic

Study. Angew. Chem., Int. Ed. 2017, 56, 4229−4233. (b) Nerush, A.;

Vogt, M.; Gellrich, U.; Leitus, G.; Ben-David, Y.; Milstein, D.

Template Catalysis by Metal-Ligand Cooperation. C-C Bond

86 , 138 −153. (c) Junge, K.; Papa, V.; Beller, M. Cobalt-Pincer

Complexes in Catalysis. Chem. - Eur. J. 2019, 25, 122−143. (d) Jones,

W. D. Hydrogenation/Dehydrogenation of Unsaturated Bonds with

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Formation via Conjugate Addition of Non-activated Nitriles under

Article

Mild, Base-free Conditions Catalyzed by a Manganese Pincer

Complex. J. Am. Chem. Soc. 2016, 138 , 6985 −6997.

(20) (a) Wu, C.; Ge, S. Ligand-Controlled Cobalt-Catalyzed

Regiodivergent Hydroboration of Aryl,Alkyl-Disubstituted Internal

Allenes. Chem. Sci. 2020 , 11 , 2783. (b) Chen, X.; Cheng, Z.; Lu, Z.

Cobalt-Catalyzed Asymmetric Markovnikov Hydroboration of Styr-

enes. ACS Catal. 2019 , 9 , 4025 −4029. (c) Huang, Q.; Hu, M.-Y.;

Zhu, S.-F. Cobalt-Catalyzed Cyclization/Hydroboration of 1,6-Diynes

with Pinacolborane. Org. Lett. 2019, 21, 7883−7887. (d) Leonard, N.

G.; Palmer, W. N.; Friedfeld, M. R.; Bezdek, M. J.; Chirik, P. J.

Remote, Diastereoselective Cobalt-Catalyzed Alkene Isomerization-

Hydroboration: Access to Stereodefined 1,3-Difunctionalized In-

danes. ACS Catal. 2019, 9, 9034−9044. (e) Cruz, T. F. C.; Lopes, P.

S.; Pereira, L. C. J.; Veiros, L. F.; Gomes, P. T. Hydroboration of

Terminal Olefins with Pinacolborane Catalyzed by New Mono(2-

Iminopyrrolyl) Cobalt(II) Complexes. Inorg. Chem. 2018 , 57 , 8146 −

12) (a) Deibl, N.; Kempe, R. Manganese-Catalyzed Multi-

component Synthesis of Pyrimidines from Alcohols and Amidines.

Angew. Chem., Int. Ed. 2017, 56, 1663 −1666. (b) Kallmeier, F.;

Irrgang, T.; Dietel, T.; Kempe, R. Highly Active and Selective

Manganese C = O Bond Hydrogenation Catalysts: The Importance of

the Multidentate Ligand, the Ancillary Ligands, and the Oxidation

State. Angew. Chem., Int. Ed. 2016, 55, 11806−11809.

13) (a) Ryabchuk, P.; Stier, K.; Junge, K.; Checinski, M. P.; Beller,

M. Molecularly Defined Manganese Catalyst for Low-Temperature

Hydrogenation of Carbon Monoxide to Methanol. J. Am. Chem. Soc.

2019, 141, 16923−16929. (b) Elangovan, S.; Topf, C.; Fischer, S.;

159. (f) Zuo, Z.; Wen, H.; Liu, G.; Huang, Z. Cobalt-Catalyzed

Hydroboration and Borylation of Alkenes and Alkynes. Synlett 2018,

9 , 1421 −1429. (g) Cruz, T. F. C.; Veiros, L. F.; Gomes, P. T.

Jiao, H.; Spannenberg, A.; Baumann, W.; Ludwig, R.; Junge, K.;

Beller, M. Selective Catalytic Hydrogenations of Nitriles, Ketones, and

Aldehydes by Well-Defined Manganese Pincer Complexes. J. Am.

Chem. Soc. 2016, 138, 8809−8814.

Cobalt(I) Complexes of 5-Aryl-2-iminopyrrolyl Ligands: Synthesis,

Spin Isomerism, and Application in Catalytic Hydroboration. Inorg.

Chem. 2018, 57, 14671−14685. (h) Tamang, S. R.; Bedi, D.; Shafiei-

Haghighi, S.; Smith, C. R.; Crawford, C.; Findlater, M. Cobalt-

Catalyzed Hydroboration of Alkenes, Aldehydes, and Ketones. Org.

Lett. 2018 , 20 , 6695 −6700. (i) Bai, T.; Janes, T.; Song, D.

Homoleptic Iron(II) and Cobalt(II) Bis(Phosphoranimide) Com-

plexes for the Selective Hydrofunctionalization of Unsaturated

Molecules. Dalton Trans. 2017, 46, 12408−12412. (j) Zhang, G.;

Wu, J.; Wang, M.; Zeng, H.; Cheng, J.; Neary, M. C.; Zheng, S.

Cobalt-Catalyzed Regioselective Hydroboration of Terminal Alkenes.

Eur. J. Org. Chem. 2017, 2017, 5814−5818. (k) Peng, J.; Docherty, J.

H.; Dominey, A. P.; Thomas, S. P. Cobalt-Catalysed Markovnikov

Selective Hydroboration of Vinylarenes. Chem. Commun. 2017, 53,

14) (a) Weber, S.; Stoger, B.; Veiros, L. F.; Kirchner, K. Rethinking

Basic Concepts Hydrogenation of Alkenes Catalyzed by Bench-

Stable Alkyl Mn(I) Complexes. ACS Catal. 2019, 9, 9715−9720.

b) Mastalir, M.; Glatz, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K.

Sustainable Synthesis of Quinolines and Pyrimidines Catalyzed by

Manganese PNP Pincer Complexes. J. Am. Chem. Soc. 2016, 138,

(

5543−15546.

15) (a) Erken, C.; Kaithal, A.; Sen, S.; Weyhermu

M.; Werle, C.; Leitner, W. Manganese-Catalyzed Hydroboration of

ller, T.; Holscher,

Carbon Dioxide and other Challenging Carbonyl Groups. Nat.

Commun. 2018 , 9 , 4521. (b) Kulkarni, N. V.; Brennessel, W. W.;

Jones, W. D. Catalytic Upgrading of Ethanol to n-Butanol via

Manganese-Mediated Guerbet Reaction. ACS Catal. 2018, 8, 997−

4726−4729. (l) Chu, W.-Y.; Gilbert-Wilson, R.; Rauchfuss, T. B.; van

002. (c) Widegren, M. B.; Harkness, G. J.; Slawin, A. M. Z.; Cordes,

Gastel, M.; Neese, F. Cobalt Phosphino-α -Iminopyridine-Catalyzed

Hydrofunctionalization of Alkenes: Catalyst Development and

Mechanistic Analysis. Organometallics 2016, 35, 2900−2914.

D. B.; Clarke, M. L. A Highly Active Manganese Catalyst for

Enantioselective Ketone and Ester Hydrogenation. Angew. Chem., Int.

Ed. 2017, 56, 5825−5828.

(m) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. A

Highly Chemoselective Cobalt Catalyst for the Hydrosilylation of

16) (a) Carney, J. R.; Dillon, B. R.; Campbell, L.; Thomas, S. P.

Alkenes using Tertiary Silanes and Hydrosiloxanes. ACS Catal. 2016,

Manganese-Catalyzed Hydrofunctionalization of Alkenes. Angew.

Chem., Int. Ed. 2018, 57, 10620−10624. (b) Zhang, G.; Zeng, H.;

Wu, J.; Yin, Z.; Zheng, S.; Fettinger, J. C. Highly Selective

Hydroboration of Alkenes, Ketones and Aldehydes Catalyzed by a

Well-Defined Manganese Complex. Angew. Chem., Int. Ed. 2016, 55,

, 3589 −3593. (n) Palmer, W. N.; Diao, T.; Pappas, I.; Chirik, P. J.

High-Activity Cobalt Catalysts for Alkene Hydroboration with

Electronically Responsive Terpyridine and α -Diimine Ligands. ACS

Catal. 2015, 5 , 622 −626. (o) Scheuermann, M. L.; Johnson, E. J.;

Chirik, P. J. Alkene Isomerization-Hydroboration Promoted by

Phosphine-Ligated Cobalt Catalysts. Org. Lett. 2015, 17, 2716−

4369−14372. (c) Macaulay, C. M.; Gustafson, S. J.; Fuller, J. T.;

Kwon, D.-H.; Ogawa, T.; Ferguson, M. J.; McDonald, R.; Lumsden,

M. D.; Bischof, S. M.; Sydora, O. L.; Ess, D. H.; Stradiotto, M.;

Turculet, L. Alkene Isomerization-Hydroboration Catalyzed by First-

Row Transition-Metal (Mn, Fe, Co, and Ni) N-Phosphinoamidinate

Complexes: Origin of Reactivity and Selectivity. ACS Catal. 2018, 8,

2719. (p) Ruddy, A. J.; Sydora, O. L.; Small, B. L.; Stradiotto, M.;

Turculet, L. (N-Phosphinoamidinate)Cobalt-Catalyzed Hydrobora-

tion: Alkene Isomerization Affords Terminal Selectivity. Chem. - Eur.

J. 2014 , 20 , 13918 −13922. (q) Obligacion, J. V.; Chirik, P. J.

Bis(imino)pyridine Cobalt-Catalyzed Alkene Isomerization-Hydro-

boration: A Strategy for Remote Hydrofunctionalization with

Terminal Selectivity. J. Am. Chem. Soc. 2013, 135, 19107−19110.

(

907−9925.

17) (a) Fernandez, E. Science of Synthesis: Advances in Organoboron

Chemistry Towards Organic Synthesis; George Thieme Verlag, 2019.

b) Hall, D. G. Boronic Acids: Preparation and Applications in Organic

Synthesis and Medicine, 1st ed.; Wiley-VCH: Weinheim, Germany,

005; pp 1−549.

18) (a) Suzuki, A. Cross-Coupling Reactions Of Organoboranes:

(

(21) (a) Su, W.; Qiao, R.-X.; Zhen, X.-L.; Tian, X.; Han, J.-R.; Fan,

S.-M.; Cheng, Q.-S.; Liu, S.-X. Ligand-Free Iron-Catalyzed

Regiodivergent Hydroboration of Unactivated Terminal Alkenes.

Chem. Rxiv 2020 , 1 −7. (b) Chen, X.; Cheng, Z.; Lu, Z. Iron-

Catalyzed, Markovnikov-Selective Hydroboration of Styrenes. Org.

Lett. 2017, 19, 969−971. (c) Ogawa, T.; Ruddy, A. J.; Sydora, O. L.;

Stradiotto, M.; Turculet, L. Cobalt- and Iron-Catalyzed Isomerization-

Hydroboration of Branched Alkenes: Terminal Hydroboration with

Pinacolborane and 1,3,2-Diazaborolanes. Organometallics 2017 , 36 ,

417 −423. (d) Espinal-Viguri, M.; Woof, C. R.; Webster, R. L. Iron-

Catalyzed Hydroboration: Unlocking Reactivity through Ligand

Modulation. Chem. - Eur. J. 2016, 22, 11605−11608. (e) MacNair,

A. J.; Millet, C. R. P.; Nichol, G. S.; Ironmonger, A.; Thomas, S. P.

Markovnikov-Selective, Activator-Free Iron-Catalyzed Vinylarene

Hydroboration. ACS Catal. 2016, 6, 7217 −7221. (f) Greenhalgh,

M. D.; Jones, A. S.; Thomas, S. P. Iron-Catalysed Hydro-

functionalisation of Alkenes and Alkynes. ChemCatChem 2015 , 7 ,

190−222. (g) Obligacion, J. V.; Chirik, P. J. Highly Selective

Chem., Int. Ed. 2011 , 50 , 6722 −6737. (b) Miyaura, N.; Suzuki, A.

An Easy Way To Construct C-C Bonds (Nobel Lecture). Angew.

Palladium-Catalyzed Cross-Coupling Reactions of Organoboron

Compounds. Chem. Rev. 1995, 95, 2457−2483.

(

19) (a) Cuenca, A. B.; Fernandez, E. Rhodium-Catalyzed C−B

Bond Formation. In Rhodium Catalysis; Claver, C., Ed.; Springer

International Publishing: Cham, Switzerland, 2018; pp 1−29.

b) Fernandez, E.; Segarra, A. M. Iridium-Catalyzed Boron-Addition.

(

In Iridium Complexes in Organic Synthesis; Oro, L. A., Claver, C., Eds.;

Wiley-VCH: Weinheim, Germany, 2009; pp 173 −194. (c) Miyaura,

N. Metal-Catalyzed Reactions of Organoboronic Acids and Esters.

Bull. Chem. Soc. Jpn. 2008 , 81 , 1535 −1553. (d) Vogels, C.; Westcott,

S. Recent Advances in Organic Synthesis Using Transition Metal-

Catalyzed Hydroborations. Curr. Org. Chem. 2005, 9, 687−699.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Bis(imino)pyridine Iron-Catalyzed Alkene Hydroboration. Org. Lett.

Article

Magnetic Characterization of Ni(II), Co(II), And Fe(II) Binuclear

Complexes on a Bis(Pyridyl-Triazolyl)Alkane Basis. Dalton Trans.

013 , 15 , 2680 −2683. (h) Zhang, L.; Huang, Z. Iron-Catalyzed

Alkene Hydroboration with Pinacolborane. Synlett 2013 , 24 , 1745 −

747. (i) Zhang, L.; Peng, D.; Leng, X.; Huang, Z. Iron-Catalyzed,

2019, 48, 10526 −10536. (b) Herchel, R.; Vah

ovska, L.; Potocnak, I.;

́ ́ ̌ ̌ ́

Travnícek, Z. Slow Magnetic Relaxation in Octahedral Cobalt(II)

Atom-Economical, Chemo- and Regioselective Alkene Hydroboration

Field-Induced Single-Ion Magnet with Positive Axial and Large

Rhombic Anisotropy. Inorg. Chem. 2014 , 53 , 5896 −5898. (c) Mishra,

V.; Lloret, F.; Mukherjee, R. Coordination Versatility of 1,3-bis[3-(2-

pyridyl)pyrazol-1-yl]propane: Co(II) and Ni(II) Complexes. Inorg.

Chim. Acta 2006 , 359 , 4053 −4062. (d) Chen, L.; Albert Cotton, F.

Cobalt(II) Complexes with the 1,5,9,13-Tetraazacyclohexadecane

Ligand. Inorg. Chim. Acta 1997, 263, 9−16.

with Pinacolborane. Angew. Chem., Int. Ed. 2013 , 52 , 3676 −3680.

D.; Guzei, I. A.; Schomaker, J. M. Heteroleptic Nickel Complexes for

j) Wu, J. Y.; Moreau, B.; Ritter, T. Iron-Catalyzed 1,4-Hydroboration

of 1,3-Dienes. J. Am. Chem. Soc. 2009, 131, 12915−12917.

(

22) (a) Touney, E. E.; Van Hoveln, R.; Buttke, C. T.; Freidberg, M.

the Markovnikov-Selective Hydroboration of Styrenes. Organo-

metallics 2016, 35, 3436−3439. (b) Ulm, F.; Chetcuti, M. J.;

Ritleng, V.; Cornaton, Y.; Djukic, J.-P. Hydroboration of alkenes

catalysed by a nickel N-heterocyclic carbene complex: reaction and

mechanistic aspects. Chem. - Eur. J. 2020 , 26 , 8916 −8925.

(29) Docherty, J. H.; Peng, J.; Dominey, A. P.; Thomas, S. P.

Activation and Discovery of Earth-Abundant Metal Catalysts Using

Sodium Tert-Butoxide. Nat. Chem. 2017, 9, 595−600.

(30) Bage, A. D.; Hunt, T. A.; Thomas, S. P. Hidden Boron

Catalysis: Nucleophile-Promoted Decomposition of HBpin. Org. Lett.

2020, 22, 4107−4112.

c) Vijaykumar, G.; Bhunia, M.; Mandal, S. K. A phenalenyl-based

nickel catalyst for the hydroboration of olefins under ambient

conditions. Dalton Trans. 2019 , 48 , 5779 −5784. (d) Li, J.-F.; Wei, Z.-

Z.; Wang, Y.-Q.; Ye, M. Base-free nickel-catalyzed hydroboration of

simple alkenes with bis(pinacolato)diboron in an alcoholic solvent.

Green Chem. 2017, 19, 4498−4502.

(31) Considering computational cost, we use catalyst model systems

in which the isopropyl substituents on the imine are replaced with

methyl substituents. For full computational details, see the Supporting

Information.

(32) Griffith, G. A.; Al-Khatib, M. J.; Patel, K.; Singh, K.; Solan, G.

23) (a) Iwamoto, H.; Imamoto, T.; Ito, H. Computational design of

A. Solid and Solution State Flexibility of Sterically Congested

Bis(imino)bipyridine Complexes of Zinc(ii) and Nickel(ii). Dalton

Trans. 2009, 185−196.

high-performance ligand for enantioselective Markovnikov hydro-

boration of aliphatic terminal alkenes. Nat. Commun. 2018 , 9 , 2290.

b) Lee, Y.; Hoveyda, A. H. Efficient Boron-Copper Additions to

(33) We note that we could not locate equilibrium structures for the

Aryl-Substituted Alkenes Promoted by NHC-Based Catalysts.

Enantioselective Cu-Catalyzed Hydroboration Reactions. J. Am.

Chem. Soc. 2009, 131 , 3160 −3161. (c) Semba, K.; Fujihara, T.;

Terao, J.; Tsuji, Y. Copper-catalyzed borylative transformations of

non-polar carbon-carbon unsaturated compounds employing bor-

ylcopper as an active catalyst species. Tetrahedron 2015 , 71 , 2183 −

quintet metal hydride species. Because our subsequent investigation

of the reaction mechanism returned transition structures with

inaccessibly high energies, we will not further consider the quintet

PES. (Absolute energies and geometries of the transition structures

are given in the Supporting Information.)

(34) (a) Chirik, P. J. Electronic Structures of Reduced Manganese,

197. (d) Noh, D.; Chea, H.; Ju, J.; Yun, J. Highly Regio- and

Iron, and Cobalt Complexes Bearing Redox-Active Bis(imino)-

pyridine Pincer Ligands. In Pincer and Pincer-Type Complexes:

Applications in Organic Synthesis and Catalysis; Szabo, K. J., Wendt,

O. F., Eds.; Wiley-VCH: Weinheim, Germany, 2014; pp 189−212.

Enantioselective Copper-Catalyzed Hydroboration of Styrenes.

Angew. Chem., Int. Ed. 2009, 48, 6062−6064.

24) (a) Tamang, S. R.; Findlater, M. Emergence and Applications

of Base Metals (Fe, Co, And Ni) in Hydroboration and Hydro-

silylation. Molecules 2019 , 24 , 3194. (b) Obligacion, J. V.; Chirik, P. J.

Earth-Abundant Transition Metal Catalysts for Alkene Hydro-

silylation and Hydroboration. Nat. Rev. Chem. 2018, 2, 15−34.

b) Zhou, Y.-Y.; Hartline, D. R.; Steiman, T. J.; Fanwick, P. E.; Uyeda,

C. Dinuclear Nickel Complexes in Five States of Oxidation Using a

Redox-Active Ligand. Inorg. Chem. 2014, 53, 11770−11777. (c) Lu,

C. C.; Weyhermuller, T.; Bill, E.; Wieghardt, K. Accessing the

26) (a) Lu, C. C.; Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt,

25) Sommer, H.; Julia-

Metals for Remote Functionalization. ACS Cent. Sci. 2018, 4, 153−

65.

Hernandez, F.; Martin, R.; Marek, I. Walking

́ ́

Different Redox States of α -Iminopyridines within Cobalt Complexes.

Inorg. Chem. 2009, 48, 6055−6064. (d) de Bruin, B.; Bill, E.; Bothe,

E.; Weyhermuller, T.; Wieghardt, K. Molecular and Electronic

Structures of Bis(pyridine-2,6-diimine)metal Complexes [ML ](PF )

6 n

K. Neutral Bis(α -iminopyridine)metal Complexes of the First-Row

Transition Ions (Cr, Mn, Fe, Co, Ni, Zn) and Their Monocationic

Analogues: Mixed Valency Involving a Redox Noninnocent Ligand

System. J. Am. Chem. Soc. 2008, 130, 3181−3197. (b) Bart, S. C.;

Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.;

Wieghardt, K.; Chirik, P. J. Electronic Structure of Bis(imino)pyridine

Iron Dichloride, Monochloride, and Neutral Ligand Complexes: A

Combined Structural, Spectroscopic, and Computational Study. J. Am.

Chem. Soc. 2006, 128, 13901−13912.

with pyridinediimine ligands, see: Obligacion, J. V.; Chirik, P. J.

n = 0, 1, 2, 3; M = Mn, Fe, Co, Ni, Cu, Zn). Inorg. Chem. 2000, 39,

936−2947.

35) The singlet state, although possible, has exceedingly high

(

barriers for migratory insertion (Tables S6 and S7) and was not

considered any further.

(36) For a comparison between cobalt and manganese complexes

bis(imino)pyridine Cobalt-Catalyzed Alkene Isomerization-Hydro-

boration: A Strategy for Remote Hydrofunctionalization with

Terminal Selectivity. J. Am. Chem. Soc. 2013, 135, 19107 −19110.

Carney, J. R.; Dillon, B. R.; Campbell, L.; Thomas, S. P. Manganese-

Catalyzed Hydrofunctionalization of Alkenes. Angew. Chem., Int. Ed.

27) (a) Mukhopadhyay, T. K.; Flores, M.; Groy, T. L.; Trovitch, R.

J. A Highly Active Manganese Precatalyst for the Hydrosilylation of

Ketones and Esters. J. Am. Chem. Soc. 2014, 136, 882−885. (b) Paris,

S. I. M.; Laskay, U. A.; Liang, S.; Pavlyuk, O.; Tschirschwitz, S.;

2018, 57, 10620−10624.

Lonnecke, P.; McMills, M. C.; Jackson, G. P.; Petersen, J. L.; Hey-

Hawkins, E.; Jensen, M. P. Manganese(II) Complexes of Di-2-

pyridinylmethylene-1,2-diimine Di-Schiff Base Ligands: Structures

And Reactivity. Inorg. Chim. Acta 2010, 363, 3390−3398.

(

c) Ebralidze, I. I.; Leitus, G.; Shimon, L. J. W.; Wang, Y.; Shaik,

S.; Neumann, R. Structural Variability in Manganese(II) Complexes

of N,N ’-Bis(2-pyridinylmethylene) Ethane (and Propane) Diamine

Ligands. Inorg. Chim. Acta 2009, 362, 4713−4720. (d) Figgs, B. N.;

Lewis, J. The Magnetic Properties of Transition Metal Complexes. In

Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Wiley-VCH, 2007;

pp 37−239.

(

28) (a) Gusev, A.; Nemec, I.; Herchel, R.; Riush, I.; Titis,

J.; Boca,

̌ ̌

R.; Lyssenko, K.; Kiskin, M.; Eremenko, I.; Linert, W. Structural and