Catalytic Carbodiimide Guanylation by a Nucleophilic, High Spin Iron(II) Imido Complex

Yafei Gao, Veronica Carta, Maren Pink, and Jeremy M. Smith*

Communication

Catalytic Carbodiimide Guanylation by a Nucleophilic, High Spin

Iron(II) Imido Complex

Cite This: J. Am. Chem. Soc. 2021, 143, 5324 −5329

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sı Supporting Information

ABSTRACT: Reduction of the three-coordinate iron(III) imido [Ph B( BuIm) FeNDipp] (1) aﬀords [Ph B( BuIm) Fe

NDipp][K(18-C-6)THF ] (2), a rare example of a high-spin (S = 2) iron(II) imido complex. Unusually for a late metal imido

complex, the imido ligand in 2 has nucleophilic character, as demonstrated by the reaction with DippNH , which establishes an

equilibrium with the bis(anilido) complex [Ph B( BuIm) Fe(NHDipp) ][K(18-C-6)THF ] (3). In an unusual transformation,

formal insertion of PrNCN Pr into the FeN(imido) bond yields the guanidinate [Ph B( BuIm) Fe( PrN) CNDipp][K(18-

C-6)THF ] (4). Reaction of 4 with excess DippNH provides 3, along with the guanidine ( PrNH) CNDipp. As suggested by

these stoichiometric reactions, 2 is an eﬃcient catalyst for the guanylation of carbodiimides, converting a wide range of aniline

substrates under mild conditions.

s dictated by relative d-orbital energies, nucleophilic

imido ligands are typically associated with early transition

Scheme 1. Selected Examples of [2 + 2] Cycloaddition

Reactions Involving Late Transition Metal Imido

Complexes

−3

metals.

As a result, the reactivity of early transition metal

imido complexes is often distinct from that of their late metal

congeners, which are usually more electrophilic. For example,

the [2 + 2] cycloaddition between an MNR bond and an

unsaturated substrate hinges on the nucleophilicity of the

imido ligand, making it a characteristic reaction of early metal

imidos, but rarely observed for late metals.

The [2 + 2] cycloaddition reaction forms the basis of a range

of useful catalytic transformations for early metal imido

complexes, including alkyne carboamination, alkyne hydro-

amination, and the assembly of pyrroles from alkynes and

amines. Despite this utility, these catalysts often have limited

substrate scope, which may be partly related to the Lewis

acidity of the early transition metal. For example, catalytic

pyrrole formation by Ti-catalyzed [2 + 2 + 1] reactions fail for

alkynyl esters and tethered alkyl ethers, likely due to the

oxygen donors blocking access to the oxophilic metal.

Building on our insights from iron catalyzed alkene

isomerization, we hypothesize that this limitation can be

addressed by high spin late metal imido catalysts that weakly

bind Lewis basic functional groups. While there are now

multiple examples of isolable high spin late transition metal

This paper reports a rare example of a high-spin iron(II)

11−23

imido complex [Ph B( BuIm) FeNDipp][K(18-C-6)THF ]

imido complexes,

[2 + 2] cycloadditions involving these

(

2). This complex has nucleophilic character at the imido

ligand and reacts with a carbodiimide to yield a guanidinate

complex, likely via a formal [2 + 2] cycloaddition reaction. The

complexes are rare. Indeed, only a limited number of

stoichiometric [2 + 2] reactions have been observed, including

for the low spin complexes [(IMes)Fe(NDipp) ],

[

(PMe ) CoNDipp], and [( Bu PCH CH P Bu )Ni

Received: February 22, 2021

Published: April 1, 2021

NR] (Scheme 1). In addition, the noble metal imido

complexes [Cp*IrN Bu] and [( Bu PCHCHNCH

CHP Bu )IrN Bu] have been reported to undergo [2 + 2]

reactions with heterocumulenes. To the best of our knowledge,

there are no examples of [2 + 2] catalysis involving late metal

imido complexes.

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 5324−5329

324

Journal of the American Chemical Society

Communication

Scheme 2. Synthesis and Reactivity of Iron(II) Imido Complex 2

Figure 1. Molecular structures of complexes 2−4, as determined by single crystal X-ray diﬀraction. Ellipsoids are shown at 50% probability level.

Counterions, solvent molecules and most hydrogen atoms are omitted for clarity. Pink, dark gray, orange, and blue ellipsoids represent boron,

carbon, iron, and nitrogen atoms, respectively.

stoichiometric reactivity establishes 2 as a catalyst for

carbodiimide guanylation, which occurs for a range of

substrates and under mild conditions.

angles = 355°, Figure 1). Reduction leads to elongation of the

Fe−N bond (1.779(2) Å), which is over 0.1 Å longer than in

those of four-coordinate (S = 0) iron(II) imido com-

21,37−39

The dark green iron(III) imido complex Ph B( BuIm) Fe

plexes,

despite 2 having a lower coordination number.

NDipp (1) is formed in 66% yield from the reaction of iron(I)

The Fe−N distance is also longer than that in the two

Trip

complex Ph B( BuIm) Fe(η -toluene) with 1 equiv of

coordinate (S = 2) iron(II) imido complex [(IPr)Fe(NAr )]

Trip

DippN₃(Scheme 2). The molecular structure of 1 , as

determined by single crystal X-ray diﬀraction (Figure S1),

reveals a trigonal planar iron center (sum of angles = 359°)

with the Fe−N (1.708(2) Å) bond distance and Fe−N−C

angle (172.3(2)°) that are comparable to three-coordinate S =

(Ar

= 2,6-bis(2′,4′,6′-triisopropylphenyl)-phenyl)

(1.7151(16) Å). These structural data suggest a high spin

(S = 2) iron(II) formulation for 2. Indeed, the Fe−C(carbene)

bond distances in 2 (2.077(3)−2.087(2) Å) are similar to

10,29,40−42

other S = 2 iron(II) bis(carbene)borate complexes.

30−35

/2 iron(III) imido complexes.

The solution magnetic

The zero-ﬁeld Fe Mossbauer spectrum supports the

oxidation and spin state assignment, with spectral parameters

moment, as determined by Evans’ method (μ = 4.2(2) μ_B),

eff

combined with the zero-ﬁeld ⁵⁷Fe Mo

ssbauer spectrum (δ =

.25 mm/s and ΔE = 1.32 mm/s at 200 K), supports the S =

(δ = 0.46 mm/s and ΔE = 1.45 mm/s at 80 K) that compares

well with the three-coordinate S = 2 iron(II) complexes

/2 iron(III) formulation.

[LFeCH₃] (L = β-diketiminate, δ = 0.48 mm/s and ΔE =

Complex 1 is readily reduced by KC in the presence of 18-

1.74 mm/s at 4.2 K), [Ph B( BuIm) FeCH Bu] (δ = 0.34

2 2 2

crown-6 to provide red [Ph B( BuIm) FeNDipp][K(18-C-

mm/s and ΔE = 1.41 mm/s at 80 K) and [(IPr Me )Fe(σ-

)THF ] (2) in 80% yield (Scheme 2). Complex 2 crystallizes

CPhCPh ) ] (δ = 0.33 mm/s and ΔE = 2.20 mm/s at

2 2

with two independent molecules in the asymmetric unit, both

200 K). As expected for this spin state, the solution H NMR

of which have similar bond metrics. The solid-state

molecular structure of 2 reveals an anionic iron imido complex

in which the trigonal planar iron center is retained (sum of

spectrum is paramagnetically shifted, and the Evans’ solution

magnetic moment (μ_e_f_f= 5.0(1) μ ) is consistent with S = 2

iron(II). In contrast to 1, complex 2 is insoluble in nonpolar

325

J. Am. Chem. Soc. 2021, 143, 5324−5329

Journal of the American Chemical Society

Communication

solvents such as pentane and toluene, consistent with its ionic

formulation.

The electronic structure of the anionic iron complex in 2 has

been probed by DFT calculations (B3LYP/def2-TZVP/SVP).

Structural optimization of the full complex provides bond

metrics that reproduce those found in the solid-state structure

Fe−N bond distances (2.050(2) and 2.024(2) Å) are similar to

those in related S = 2 iron(II) complexes. The Fe

ssbauer spectral parameters (δ = 0.76 mm/s and ΔE =

3.13 mm/s at 80 K) also support the S = 2 iron(II)

formulation. The complex has also been characterized in

solution by H NMR spectroscopy and Evans’ magnetic

Table S2 ). Natural orbital analysis of the electronic structure

moment (μ_e_f_f= 5.4(2) μ ). Complex 3 can be independently

reveals doubly occupied orbitals with signiﬁcant σ- and π-

bonding character between iron and the imido ligand (Figure

synthesized by reaction of the iron(II) chloride complex

[Ph B( BuIm) FeCl(THF)] with 2 equiv of DippNHK in

2 2

, Figure S48 ). The highest, singly occupied orbitals have

the presence of 18-crown-6.

Interestingly, DippNH can be clearly observed in the H

NMR spectrum of crystalline samples of 3, suggesting the

equilibrium:

⇆ 2 + H NDipp

Using mesitylene as the internal standard, the equilibrium

constant was determined by H NMR spectroscopy to be K

−

≈

8 × 10 mol/L at room temperature. While common for

49−51

early metal complexes,

the formation of an imido ligand

by amine loss from the corresponding diamido complex is

39,52

unusual for iron.

The evidence for a nucleophilic imido ligand in 2 prompted

us to investigate its activity toward substrates known to be

active in [2 + 2] cycloaddition reactions. In an initial

demonstration, we ﬁnd that 2 reacts with equimolar PrN

CN Pr to provide the yellow complex [Ph B( BuIm) Fe-

(

PrN) CNDipp][K(18-C-6)THF ] (4) in 92% yield (Scheme

). The molecular structure of 4 reveals a four-coordinate iron

center (τ = 0.8) that is coordinated by the bis(carbene)-

borate and a newly formed guanidinate ligand (Figure 1). The

Fe−N (2.031(2) and 1.998(2) Å) and Fe−C (2.126(2) and

.119(2) Å) bond distances are similar to those for S = 2

iron(II) complexes. The S = 2 assignment is further

supported by the zero-ﬁeld Fe Mossbauer spectrum (δ =

also been characterized in solution by H NMR spectroscopy

and Evans’ magnetic moment (μ_e_f_f= 5.0(1) μ_B).

.62 mm/s and ΔE = 4.20 mm/s at 80 K). The complex has

The molecular structure of 4 can be described as the result

of PrNCN Pr insertion into the iron imido bond. This

contrasts with early transition metal imido complexes, which

react with carbodiimides to provide the corresponding [2 + 2]

,53

azametallocyclobutanes.

Nonetheless, the formation of 4

Figure 2. Selected orbitals showing the FeN σ and π interactions

that are in the C−Fe−C plane; (a) π* HOMO; (b) π-bonding

HOMO−6; (c) σ-bonding HOMO−7. Natural orbitals shown with

isodensity = 0.05.

can be rationalized according to a reaction sequence in which

an initial formal [2 + 2] cycloaddition reaction between 2 and

PrNCN Pr is followed by [1,3]-Fe migration. It is

worth noting that, with the exception of [(IMes)Fe-

NDipp)₂], iron imido complexes are typically unreactive

(

iron−nitrogen π* character, which serves to decrease the

multiple bond character of the iron−nitrogen bond. The

remaining singly occupied orbitals are predominantly iron

based (Figure S48). As a consequence of this electronic

structure, the computed spin density is mainly located on the

iron center (Mulliken spin density +3.58), with non-negligible

spin density (+0.22) on the imido nitrogen atom (Figure S47).

In light of the negative charge on the complex, we

anticipated that the imido ligand in 2 to have nucleophilic

character. In accord with this expectation, 2 reacts with 1 equiv

toward carbodiimides. Indeed, carbodiimides are often formed

as the products of nitrene transfer from iron imido complexes

to isonitriles.

As expected from the equilibrium between 3 and 2, complex

4 can also be formed from the reaction of 3 with excess PrN

CN Pr. Moreover, 3 is formed from the reaction of excess

DippNH

with 4, which also provides equimolar N-2,6-

diisopropylphenyl-N′,N″-diisopropylguanidine, as character-

ized by H NMR spectroscopy and GC-MS.

Together, the stoichiometric reactions suggest 2 as a catalyst

for carbodiimide guanylation. Guanidines ﬁnd widespread

application, e.g. building blocks toward complex organic

compounds, superbase catalysts and versatile supporting

of DippNH to provide yellow [Ph B( BuIm) Fe(NHDipp) ]-

[

K(18-C-6)THF ] (3) in 88% yield (Scheme 2). The

molecular structure of 3, as determined by single crystal X-

ray diﬀraction (Figure 1), reveals a four-coordinate iron center

(

5,56

ligands.

We observe smooth conversion of equimolar

τ = 0.9) bound by the bis(carbene)borate ligand and two

PrNCN Pr and DippNH

to N-2,6-diisopropylphenyl-

anilido ligands. The Fe−C (2.133(2) and 2.125(2) Å) and

N′,N″-diisopropylguanidine in the presence of 5 mol % 2, with

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J. Am. Chem. Soc. 2021, 143, 5324−5329

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.1c02068.

Communication

Table 1. Catalytic Carbodiimide Guanylation

Scheme 3. Proposed Mechanism of Catalytic Carbodiimide

Guanylation

intermediate C. From here, there are two possible routes to

the guanidine product. In path a, internal proton transfer in C

releases the guanidine while regenerating A. In path b, C is

protonated by another equivalent of ArNH₂to form

bis(anilido) D concomitant with guanidine release. The

imido complex A is then regenerated by the proton transfer

Reaction conditions: aniline (1 mmol), carbodiimide (1.1 mmol)

with 5 mol % 2 at room temperature in 2 mL of THF. The yields are

isolated yields based on aniline. The formation of DippNH is also

observed by GC-MS when other anilines are used. NMR yields using

mesitylene as the internal standard.

equilibrium involving ArNH . Since the rate of catalysis is

slower when 3 is used as the catalyst instead of 2 or 4, this

suggests that path a is more likely. We have so far been unable

to prepare C, as the reaction of 4 with 1 equiv of DippNH₂

results in 50% conversion to 3.

In summary, we have isolated a rare example of a high-spin

(S = 2) iron(II) imido complex. The imido ligand has

properties that are more akin to early metal imido complexes,

including nucleophilic character, as demonstrated by the

complete conversion occurring over hours at room temper-

ature. More generally, 2 catalyzes the room temperature

formation of a variety of guanidines from both PrNCN Pr

and CyNC=NCy (Table 1). We observe that anilines

bearing both electron-withdrawing and electron-donating

substituents are active in the reaction. Even anilines with

potentially coordinating groups are successful substrates for the

reaction. In all cases, high isolated yields of the guanidine

products are obtained. It worth noting that 1 does not catalyze

these guanylation reactions under the same conditions.

In addition to the remarkable substrate scope, the mild

conditions of the catalytic guanylation reaction are notable.

While a number of early transition metal imido complexes have

been reported to catalyze the guanylation of carbodiimides,

heating is required to drive most of these reactions.

also worth noting that while the simple iron salts have been

reported to catalyze certain guanylation reactions, this is only

above 120 °C, with no activity at lower temperatures. The

nature of the catalytically active species in these reactions is

unknown. By contrast, 2 catalyzes the guanylation reaction at

room temperature, even for very bulky substrates (e.g.,

DippNH₂).

proton transfer reaction with DippNH and a carbodiimide

insertion reaction, which likely involves a formal [2 + 2]

cycloaddition reaction. The imido complex is an eﬃcient

catalyst for the guanylation of carbodiimides, providing high

yields of product under mild conditions. The broad substrate

scope demonstrates the advantageous properties of a high spin,

late metal imido complex for this catalytic reaction.

,53,57,58

It is

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

Full experimental and computational details (PDF)

Atomic coordinate information (XYZ)

Accession Codes

In accord with the stoichiometric transformations described

CCDC 2064486−2064489 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.

above, both 3 and 4 catalyze the guanylation of PrNC

N Pr by DippNH . However, the rate of reaction is notably

slower when 3 is used as the catalyst, with full conversion only

occurring after 48 h at room temperature. This suggests that

catalysis is initiated by the formation of 2 from 3. In addition,

no catalysis by 2 is observed for a 20:1 mol ratio of DippNH

AUTHOR INFORMATION

Corresponding Author

to PrNCN Pr, suggesting that the carbodiimide does not

■

insert into the Fe−N bonds of 3.

Based on our observations, we propose a catalytic cycle for

carbodiimide guanylation (Scheme 3). Starting from imido A,

the carbodiimide inserts into the FeN bond to provide B.

Jeremy M. Smith − Department of Chemistry, Indiana

University, Bloomington, Indiana 47405, United States;

orcid.org/0000-0002-3206-4725; Email: smith962@

indiana.edu

Subsequent reaction with 1 equiv of ArNH provides

327

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Journal of the American Chemical Society

Authors

Yafei Gao − Department of Chemistry, Indiana University,

Bloomington, Indiana 47405, United States; orcid.org/

(12) Laskowski, C. A.; Miller, A. J.; Hillhouse, G. L.; Cundari, T. R.

Communication

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000-0001-7970-8535

Exhibiting Spin Crossover and C −H Bond Activation. J. Am. Chem.

Veronica Carta − Department of Chemistry, Indiana

University, Bloomington, Indiana 47405, United States

Maren Pink − Department of Chemistry, Indiana University,

Bloomington, Indiana 47405, United States

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14) Hennessy, E. T.; Liu, R. Y.; Iovan, D. A.; Duncan, R. A.; Betley,

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Cobalt(II) Imido Complex with NHC Ligation: Synthesis, Structure,

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https://pubs.acs.org/10.1021/jacs.1c02068

Complete contact information is available at:

Notes

The authors declare no competing ﬁnancial interest.

MacMillan, S. N.; Lancaster, K. M.; Betley, T. A. Direct Comparison

Imido Complexes Competent for C −H Bond Amination. J. Am.

ACKNOWLEDGMENTS

■

Chem. Soc. 2017, 139, 12043−12049.

This material is based upon work supported by the U.S.

Department of Energy, Oﬃce of Science, Oﬃce of Basic

Energy Sciences under Award Number DE-SC0019466.

Support for the acquisition of the Bruker Venture D8

diﬀractometer through the Major Scientiﬁc Research Equip-

ment Fund from the President of Indiana University and the

Oﬃce of the Vice President for Research is gratefully

acknowledged.

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of C −H Bond Amination Efficacy through Manipulation of Nitrogen-

Valence Centered Redox: Imido versus Iminyl. J. Am. Chem. Soc.

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