Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H2 Activation and Transfer Hydrogenation

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Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase

(Hmd): Eﬀect of Dicarbonyl versus Monocarbonyl Motifs in H₂

Activation and Transfer Hydrogenation

Zhu-Lin Xie, Wenrui Chai, Spencer A. Kerns, Graeme A. Henkelman, and Michael J. Rose*

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ABSTRACT: A set of bioinspired carbamoyl CNP pincer

complexes are reported that are relevant to [Fe]-hydrogenase

(Hmd). The dicarbonyl species [(C^NHN^NHP^R2)Fe(CO)₂I] [R = Ph,

i

1; R = Pr, 2] undergoes ligand deprotonation, resulting in the

dearomatized complexes of formulas [(C^NHN^N=P^R2)Fe(CO)₂] (5

1

and 6). The crystal structure and H{³¹P} NMR spectroscopy of

the iodide-bound dearomatized species [Na(18-crown-6)]-

[(C^NHN^N=P^Ph2)Fe(CO)₂I] (7) showed that the deprotonated

moiety was the phosphoramine N(H) linkage. Separately, the

monocarbonyl complexes [(C^NHN^NHP^R2)Fe(CO)(MeCN)₂](BF₄)

(8 and 9) synthesized, as well as deprotonated and dearomatized in

similar fashion. Reactivity studies revealed that the parent dicarbonyl

complexes require more forceful conditions for H₂activation,

compared with the monocarbonyl complexes. The ligand backbone was not found to participate in H₂activation and H₂→ hydride

transfer to an organic substrate was not observed in either case. Density functional theory calculations revealed that the higher

reactivity of the monocarbonyl complex in H₂splitting could be attributed to its higher aﬃnity for H₂. This behavior is attributed to

two key points related to the requisite d_π(Fe) → σ*(H₂) back-bonding interaction in a conventional M−H₂Kubas interaction: (i)

generally, the weaker π donor capacity of the dicarbonyls, and (ii) speciﬁcally, the detrimental eﬀect of a strongly π acidic CO ligand

(versus weakly π acidic MeCN ligand) trans to the H₂activation site. The higher reactivity of the monocarbonyl complex is also

evidenced by the catalytic transfer hydrogenation by monocarbonyl 8, whereas dicarbonyl 1 was ineﬀective. Overall, the results suggest

that Nature uses the dicarbonyl motif in [Fe]-hydrogenase to diminish the interaction between the Fe center and dihydrogen,

thereby preventing premature H₂activation prior to substrate (H₄MPT⁺) binding and any resulting nonspeciﬁc hydride transfer

reactivity.

genase.⁷In contrast to the thorough understanding of the

bimetallic hydrogenases, studies of [Fe]-hydrogenase remain

relatively nascent. The enzyme [Fe]-hydrogenase catalyzes a

INTRODUCTION

■

As the availability and environmental cost of fossil fuels make

their extraction increasingly diﬃcult, the world’s energy sources

are slowly shifting from traditional energies to renewable

non-redox hydride transfer from H₂to the substrate methenyl-

tetrahydromethanopterin (H₄MPT⁺), which serves as a C₁

energies.¹⁻³One promising alternative renewable energy source

carrier in methanogenic carbon dioxide (CO₂) reduction (see

is dihydrogen (H₂), which has many advantages: zero carbon

Scheme 1).⁷⁻⁹This metalloenzyme (also called Hmd: H₂-

emission, high mass energy density (120 MJ/kg), and high

energy eﬃciency in combustion.⁴Despite the beneﬁts of

dihydrogen, its generation limits its applications, because 95%

forming H₄methylene-PT dehydrogenase) plays an obligate role

in “nickel-free” CO₂→ CH₄metabolism in the absence of

bioavailable nickel (and thereby, [NiFe] hydrogenase).⁷⁻⁹The

of H₂production is derived from carbon-intensive methods,

such as steam reforming and water−gas shift.⁵Although

active site of [Fe]-hydrogenase (FeGP cofactor) exhibits a

unique array of nonproteinaceous ligands (except for Cys₁₇₆),

including a cis-dicarbonyl motif, a bidentate pyridone-acyl unit

methods such as biomass fermentation and direct water-splitting

have drawn substantial attention from researchers, the eﬃciency

of H₂generation remains an issue.⁶This motivates the

development of more-eﬃcient catalysts for both H₂generation

and utilization derived from Earth-abundant elements.

Received: December 4, 2019

In Nature, H₂is both generated and utilized by hydrogenases.

To date, three types of hydrogenases have been discovered:

[NiFe]-hydrogenase, [FeFe]-hydrogenase, and [Fe]-hydro-

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A

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Scheme 1. Reaction of Methenyl-H₄MPT⁺to Methylene-H₄MPT by [Fe]-Hydrogenase and the Structure of FeGP Cofactor

that presents a unique (to biology) organometallic Fe−C bond,

and a substrate binding site occupied by a weakly bound H₂O in

the resting state.^10,11

2). Later, Hu and Shima²⁶reported semisynthetic [Fe]-

hydrogenases comprised of apoenzyme reconstituted with

pyridine- and pyridinol-containing iron complexes. The

construct containing 2-hydroxypyridine (conjugate base:

pyridone) exhibited detectable turnover frequencies in the

forward and reverse reactions of [Fe]-hydrogenase (2 s⁻¹and 1

s⁻¹, respectively). In contrast, the methoxy analogue did not

exhibit any catalytic activity above the detection limit. These

results revealed the importance of the 2-hydroxy group

presumably as a pendant base that accepts H⁺during H₂

cleavage. The above reports also supported the second

mechanism¹³(proposed by QM/MM) in which the protein

scaﬀold is imperative for the catalytic activity.

Computational studies of [Fe]-hydrogenase have evaluated

several plausible mechanisms of H₂splitting. Hall and co-

workers¹²performed density functional theory (DFT) calcu-

lations on a simpliﬁed active site. This study suggested that the

active site binds and activates H₂in a stepwise fashion without

the participation of substrate (H₄MPT⁺). The coordination site

trans to the acyl unit is ideal for binding and cleavage of H₂, as it

requires the minimum structural change on the ligand backbone

during the catalytic circle. The main pathway of H₂heterolytic

splitting utilized metal−ligand bifunction between iron(II) and

the (deprotonated) pyridone-oxygen. The resulting iron-

hydride intermediate was calculated along the reaction

trajectory and provides hydride transfer to H₄MPT⁺with an

accessible activation barrier (15.2 kcal/mol). In 2014, Reiher

and co-workers¹³reported a theoretical study based on the full

protein structure using a multiscale modeling method (quantum

mechanics/molecular mechanics (QM/MM)). In contrast to

the previously proposed mechanism, Reiher suggested that the

protein scaﬀold that harbors the FeGP cofactor provides the

necessary conformation change for the substrate to approach the

active site. As such, the coordinated H₂can be cleaved by the

orbital push−pull eﬀect between the pyridone oxygen and the

cationic carbon on the substrate imidazolium ring (ΔE_a= 1.0

kcal/mol). As a result, no formal iron-hydride intermediate was

implicated.

Our own foray into this area commenced with the

development of carbamoyl-pyridine-thioether (CNS) iron

carbonyl complexes (Scheme 2).²⁷The dicarbonyl bromide

species (j) bears a tridentate ligand backbone with C, N, S

ligating in meridional mode. With this pincer-type mimic, we

observed the ﬁrst Fe−H species relevant to [Fe]-hydrogenase by

treating the bromide salt with KHBEt₃. However, the thermal

instability of the hydride species resulted in the isolation of

desulfurized complex (k) with a [Fe₂S^Me₂] core derived from C−

S bond cleavage. Similarly, treating the bromide complex with a

strong base, KO^tBu, led to C−S bond cleavage, forming a CNC

pentacoordinate Fe complex (l). It is likely that KO^tBu

generated a putative deprotonated carbamoyl intermediate

[(^O=C^N=NS^Me)Fe(CO)₂], which triggered the desulfurization

process. We have also reported model systems based on an

“anthracene scaﬀold”.^28,29The anthracene scaﬀold tethers the

C,N,S donors to provide the facial motif found in the active site.

Reactivity studies showed that the complexes [(Anth·

In a nonbiomimetic synthetic system (Scheme 2), Kirchner¹⁴

reported P^NHN^NHP iron(II) carbonyl complexes, in which the

dicarbonyl complex [(P^NHN^=NP)Fe(CO)₂]⁺cleaves H₂via a

metal−ligand bifunctional mechanism, leading to the hydride

complex [(P^NHN^NHP)Fe(CO)₂(H)] (a; L = CO). Later, the

same group¹⁵reported that the monocarbonyl complexes

[(P^NHN^NHP)Fe(CO)(L)(H)] (a), with labile ligands (L =

C

^NHNS^Me)Fe(CO)₂(THF)]⁺(m) and [(Anth·C^NHNS)Fe-

(CO)₂(PhCN)] (n) are functional for H₂activation.

Br⁻, MeCN, BH₄) trans to the hydride, are eﬃcient catalysts for

−

In this work, we report a bioinspired “hybrid” system that

contains elements of both the biological motif (iron-acyl

bonding) and proven catalytic platforms (phosphine pincer),

while maintaining a pendant base at the pyridine ortho position.

We aspired to “connect” the reactivity patterns of purely

bioinspired complexes (fac-CNS chelates) with purely synthetic

systems (mer-PNP chelates), as well as to clearly understand the

discrepancy between the dicarbonyl motif found in [Fe]-

hydrogenase and the monocarbonyl motif found in synthetic

phosphine catalysts. Herein, we report a bioinspired CNP pincer

complex with a biomimetic pendant base and carbamoyl group.

The experimental and computational studies were performed

regarding the synthesis, dearomatization, H₂activation, and

transfer hydrogenation within this model system.

hydrogenation of ketones and aldehydes to alcohols. In parallel,

methylene-spaced PNP iron monocarbonyl complexes (b)

developed by Milstein have been applied to hydrogenation of

CO₂,¹⁶ketones^17,18and triﬂuoroacetic esters¹⁹with high

eﬃciency. Relatedly, Hu synthesized a third type of PNP iron

complex with an oxygen spacer (c) that performed methanol-

assisted H/D scrambling and hydrogenation and transfer

hydrogenation of aldehydes.²⁰

Returning to [Fe]-hydrogenase, the mechanisms proposed in

DFT calculations warrant experimental investigation by

synthesizing model complexes that faithfully mimic the active

site. In this vein, Hu²¹⁻²³and (separately) Pickett^24,25reported

a series structural models containing acyl-methylpyridine (d, e,

and f) and “carbamoyl”-pyridine ligands (g, h, and i) (Scheme

B

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Scheme 2. Non-biomimetic and Biomimetic Iron Carbonyls for H₂Reactivity and Hydride Transfer: (i) PNP Pincer Iron

Carbonyls,¹⁴⁻²⁰(ii) Acyl-Methylpyridine Mimics,²¹⁻²³(iii) Carbamoyl-Pyridine Mimics,^24,25(iv) Carbamoyl-Pyridine-

Thioether Mimics,²⁷(v) Scaﬀold-Based Functional Mimics,^28,29and (vi) Structures Obtained in This Work

C

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Scheme 3. Synthesis of Monosubstituted Phosphine Ligands (L_Phand L_iPr) and Dicarbonyl Iron(II) Complexes (1, 2, 3, and 4)

Table 1. Selected Bond Lengths and Bond Angles

complex

3

4-H₂O

7

cis-8

trans-8

1.955(4)

2.2930(11)

1.963(3)

1.366(6)

1.696(4)

1.793(4)

cis-9

Bond Lengths (Å)

1.959(4)

Fe−(C)carbamoyl

Fe−P

1.969(5)

2.3037(14)

1.947(4)

1.372(6)

1.690(4)

1.792(5)

1.751(7)

1.149(6)

1.146(7)

1.962(10)

2.288(3)

1.940(8)

1.377(14)

1.711(9)

1.809(12)

1.722(13)

1.109(14)

1.150(17)

1.972(7)

2.2729(18)

1.957(5)

1.343(9)

1.658(6)

1.778(7)

1.963(2)

2.2769(10)

1.937(3)

1.372(5)

1.701(4)

2.2976(5)

1.9456(16)

1.362(3)

Fe−N^Py

N(−P)−C^Py

N−P

1.7012(17)

Fe−C(O)^in‑plane

Fe−C(O)^{out‑of‑plane}

CO^in‑plane

CO^{out‑of‑plane}

Fe−I

a

1.831(8), 1.793(7)

1.089(9)

1.118(8), 1.198(7)

1.770(5)

1.143(5)

1.765(2)

1.139(5)

1.147(3)

a

2.4984(15), 2.4929(14)

b

Fe−NCMe

Fe−OH₂

1.974(4), 1.932(4)

1.916(3), 1.908(4)

1.9717(17), 1.9291(17)

2.001(9)

Fe−OTf

2.064(5)

Bond Angles (deg)

164.94(14)

(O)C−Fe−P

N^Py−Fe−P

MeCN−Fe−NCMe

163.60(16)

81.86(12)

165.0(3)

82.4(2)

161.3(2)

79.74(17)

164.01(13)

81.81(10)

175.76

164.29(6)

82.24(5)

87.15(7)

82.23(9)

86.11

a

b

Substitutional disorder of CO and I. trans to CO.

isopropyl substituents. In the ³¹P{H} NMR spectra, complex 1

displays a singlet at 90.8 ppm, whereas complex 2 gives rise to a

singlet at 121.2 ppm. The IR spectrum of 1 shows two carbonyl

stretches at 2032 and 1976 cm⁻¹, and for 2 the carbonyl peaks

are red-shifted to 2019 and 1967 cm⁻¹, because of the stronger

(ⁱPr)₂P donor. The CO stretching frequencies are slightly blue-

shifted, relative to the native enzyme data (Hmd: 1996, 1928

cm⁻¹; FeGP cofactor: 2004, 1934 cm⁻¹) and comparable to the

anthracene-based model complexes ([(Anth·C^NHNS^Me)Fe-

(CO)₂I]: 2031, 1981 cm^{− 1}; [(Anth·C^{N H}NS)Fe-

(CO)₂(PhCN)]: 2016, 1956 cm⁻¹).

RESULTS AND DISCUSSION

■

Synthesis, Spectroscopy, and X-ray Structure of

Dicarbonyl Complexes. The CNP pincer complexes herein

are derived from monosubstituted phosphine ligands (L_Phand

L_iPr; see Scheme 3), which were synthesized by a nucleophilic

substitution reaction of 2,6-diaminopyridine and chlorodiphe-

nylphosphine or chlorodiisopropylphosphine (1 equiv) by

modiﬁcation of a reported procedure.³⁰⁻³²The metalation of

L_Phand L_iPrproceeded at room temperature with

[Fe(CO)₄(I)₂] in DCM, which generated the dicarbonyl

complexes (1 and 2) with a bioinspired carbamoyl unit related

to Hmd. Complexes 1 and 2 were characterized by nuclear

magnetic resonance (¹H NMR, ³¹P{¹H} NMR) and infrared

(IR) spectroscopy. The ¹H NMR spectrum of 1 exhibits features

at 9.64 and 8.74 ppm that correspond to carbamoyl-NH and

phosphino-NH, respectively. For 2, the resonances of NH

protons are shifted upﬁeld (8.70 ppm for carbamoyl-NH, 6.93

ppm for phosphino-NH), indicating the increased electronic

density at the Fe center in 2 resulting from the electron-donating

To provide structural characterization and provide a more

labile coordinate site for reactivity studies, the thermodynami-

cally formed dicarbonyl complexes were derivatized by halide

abstraction with silver triﬂate, forming the triﬂate-bound

complex and AgI precipitate. Treatment of a 1,2-dicholoro-

ethane (DCE) solution of complex 1 or 2 with Ag(OTf)

resulting in the formation of either the adduct [(C^NHN^NHP^Ph2)-

Fe(CO)₂(OTf)] (3) or the triﬂate salt [(C^NHN^NHP^iPr2)Fe-

i

(CO)₂(OTf)] (4), depending on the R group (R = Ph or Pr,

D

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Figure 1. ORTEP diagrams for (a) [(C^NHN^NHP^Ph2)Fe(CO)₂(OTf)] (3) (30% thermal ellipsoids) and (b) [(C^NHN^NHP^iPr2)Fe(CO)₂(H₂O)](OTf)

(4-H₂O) (50% thermal ellipsoids). The H atoms except for the NH protons are omitted for clarity.

Scheme 4. Synthesis of Dearomatized Complexes (5, 6)

respectively). The IR spectra exhibited the two carbonyl peaks at

higher wavenumber (2049, 1993 cm⁻¹for 3; 2045, 1994 cm⁻¹

for 4) and the ³¹P resonance of 4 was observed as a singlet at

122.8 ppm. However, the ³¹P NMR of 3 showed no resonance

due to paramagnetic impurity. The crystal structures of the

triﬂate salts were obtained upon recrystallization of the products

(vapor diﬀusion of cyclohexane into the THF solution of

complexes). Surprisingly, complex 4 underwent ligand exchange

of OTf⁻by ambient H₂O during crystallization (the

crystallization was set up outside the N₂box), generating

dicarbonyl complexes; the in-plane CO is further from the Fe

center than the axial CO ligand (1.792(5) Å vs 1.751(7) Å in 3;

1.809(12) Å vs 1.722(13) Å in 4), which can be ascribed to the

stronger trans inﬂuence of pyridine versus halide/H₂O. This is

comparable to the Fe−CO bond length in cis-[Fe(PNP-ⁱPr)-

(CO)₂(Br)]⁺, where the CO in the plane of the ligand backbone

has a longer distance to the Fe center, compared with the CO

perpendicular to the plane (1.772(1) Å vs 1.758(2) Å).³⁹Both

complexes display an intermolecular hydrogen bonding motif on

the amide proton linkers in the solid state (Figures S37 and S38

in the Supporting Information). The NH−P proton interacts

with carbamoyl oxygen with bond metrics of O···H = 1.93 Å,

O···N = 2.781(7) Å, bond angle = 163° for 3, and O···H = 1.98

Å, O···N = 2.854(10) Å, bond angle = 174° for 4. In 4, the outer

sphere triﬂate anion displays hydrogen bonding with the NH−

carbamoyl proton with O···H = 2.07 Å, O···N = 2.932(7) Å,

bond angle = 168° for 1, and O···H = 2.10 Å, O···N = 2.913(15)

Å, bond angle = 154°.

Activation via Deprotonation/Dearomatization of

Dicarbonyl Complexes. Treatment of dicarbonyl complex 1

in THF with 1 equiv of the bulky phenolate, sodium 2,6-di-tert-

butyl-4-methoxyphenolate (NaDBHA), yielded an orange

solution containing the deprotonated/dearomatized product

formulated as [(C^NHN^N=P^Ph2)Fe(CO)₂] (5) (Scheme 4). The

ν(CO) peaks were shifted to lower energy at 2006 and 1952

cm⁻¹, indicating deprotonation of the ligand backbone. While

the precursor 1 possesses two acidic NH protons on the side

arms, ¹H NMR after deprotonation provides no information on

determining the deprotonation site, as the spectrum shows the

disappearance of both NH resonances. The ³¹P{¹H} NMR

spectrum of 5 exhibits a singlet at 90.6 ppm, which is a trivial

shift, compared to that of 1 (90.8 ppm). These results may be

H₂O-bound complex [(C^NHN^{NH iPr2})Fe(CO)₂(H₂O)](OTf)

P

(4-H₂O). Selected bond parameters and crystal information are

tabulated in the Table 1 and Table S1 in the Supporting

Information. Complex 3 crystallized as a neutral complex

(Figure 1), wherein the Fe center is coordinated with a

carbamoyl-C, pyridine-N, and diphenyl-P in the conventional

“pincer” equatorial fashion. The CO ligands are arranged in cis

fashion, and the sixth coordination site is occupied by triﬂate

ligand. The bond distances are unremarkable, except that the

Fe−P bond distance (2.3037(14) Å) is longer than regular range

of bond length for iron(II) carbonyl phosphine complexes

(2.16−2.25 Å).^{14,15,17,18,33−36}This is attributed to the trans

inﬂuence of the strongly σ-donating carbamoyl unit.²¹Complex

4 crystallized as a cationic complex, wherein the Fe center is

coordinated to the CNP ligand, cis CO ligands, and a water

moleculewith the charge balance provided by the outer-

sphere triﬂate anion. Presumably, the stronger σ-donating eﬀect

of isopropyl-substituted phosphine ligand than diphenylphos-

phine^37,38gives rise to (i) the more labile triﬂate anion due to

increased electronic density at the Fe center, and (ii) the Fe−P

bond of 4 is slightly shortened (2.288(3) Å), compared with 3

(2.3037(14) Å). Two types of Fe−CO bond are presented in the

E

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Figure 2. CO region in the IR spectra (drop-cast) of (a) 1 only, (b) reaction of 1 + NaDBHA producing 5, (c) reaction of 1 + [NEt₄][DBHA] +

TlBArF producing 5, (d) reaction of 1 + NaDBHA + 18-crown-6 ether producing 7, and (e) reaction of 1 + [NEt₄][DBHA] producing 7.

ascribed to a fast proton exchange process the NH proton with

protic species. Similarly, Kirchner et al. reported a fast proton

exchange process between the two N-sites of deprotonated

[Fe(PNP-BIPOL)(CH₃CN)₃]BF₄in MeCN, giving rise to a

singlet in ³¹P{¹H} NMR.⁴⁰Since NMR results were ambiguous

in determining the exact deprotonation site of 1, DFT

calculations of P−NH-deprotonated 5 and carbamoyl NH-

deprotonated 5′ were performed at PBE0 theory level. The P−

NH-deprotonated 5_DFTis shown to be 12.3 kcal/mol lower than

5′_DFTin energy (see Table S2 in the Supporting Information),

indicating that the NH on the phosphine side arm is

deprotonated. The reversibility of the deprotonation was

demonstrated by treating 5 with pyridinium bromide in THF,

as ¹H NMR and IR spectroscopies revealed the regeneration of

complex 1^Br(Br-bound). For complex 2, similar deprotonation

was observed upon the treatment of NaDBHA and the IR

spectrum of the deprotonated complex 6 demonstrated

analogously red-shifted CO bands at 2001 and 1942 cm⁻¹,

again consistent with deprotonation of the ligand backbone. The

³¹P{¹H} NMR spectrum exhibits a singlet at 116.3 ppm.

To verify the dissociation of iodide from the Fe center in

complex 5 (lost as NaI), we substituted NaDBHA with other

bases and compared the changes of CO stretches in the solid-

state IR spectra recorded via the drop-cast method (see the

Supporting Information for details regarding the physical

measurements). As shown in Figure 2, treatment of 1 with the

corresponding ammonium salt, tetraethylammonium 2,6-di-tert-

butyl-4-methoxyphenolate ([NEt₄][DBHA]) and a halide

abstracting reagent, thallium tetrakis(3,5-bis(triﬂuoromethyl)-

phenyl)borate (TlBAr^F), aﬀords an IR spectrum that displays

two CO bands at 2004 and 1949 cm⁻¹similar to those of 5.

This observation means that, in the reaction of NaDBHA and 1,

F

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iodide dissociates from the Fe center, forming a pentacoordi-

nate/deprotonated species 5. Interestingly, the addition of a

reagent, [(18-crown-6)Na][DBHA] (Figure 2d) or [NEt₄]-

[DBHA] (Figure 2e) to complex 1 gave rise to two CO peaks

red-shifted much further in the solid IR (1992, 1936 cm⁻¹and

1991, 1934 cm⁻¹, respectively). The diﬀerence in the CO region

can be explained by the formation of an iodide-bound

deprotonated species 7, as evidenced by the crystal structure

(vide infra). It is noteworthy that the IR spectra in Figure 2 were

obtained by drop casting the sample solution that was taken

directly from the reaction. These results indicate that the iodide

is so tightly bound to the Fe center that, in solid state, without

Tl⁺or Na⁺cation, it ligates the iron, regardless of the

deprotonation of the ligand backbone.

While attempts to obtain the X-ray structure of the

dearomatized species 5 or 6 were not fruitful, the reaction of 1

with NaDBHA and 1 equiv of 18-crown-6 (Figure 2d) did aﬀord

[(18-crown-6)Na][(C^NHN^N=P^Ph2)Fe(CO)₂] (7) as orange

crystals. The structure of complex 7 exhibits an Fe center

coordinated to the CNP ligand, two cis-COs and I⁻anion,

displaying a pseudo-octohedral geometry (Figure 3). Interest-

Figure 4. Comparison of ¹H NMR spectra of 7 and 5 in THF-d⁸. Peaks

corresponding to residual solvent impurity and phenol are labeled with

an asterisk (*).

suggest that, in solution, complex 7 loses iodide and converts to

the pentacoordinate complex 5. Similar dehydrohalogenation

has been observed or proposed in PNP iron complexes; the

deprotonation/dearomatization of the ligand backbone pro-

motes halide dissociation from the Fe center.^14,17Importantly,

such dearomatized and dehalogenated intermediate has been

proposed to be the active species to cleave H₂by the fashion of

metal−ligand cooperation.⁴¹It is promising that the CNP pincer

complex could also function as a H₂activation complex;

however, with an anionic carbamoyl unit, the reactivity might be

diﬀerent from the neutral PNP complexes. Since the iodide

anion is still dissolved in the solution, upon crystallization, it

1

binds with the Fe center, forming 7. While the H NMR

spectrum of complex 7 matches that of 5, the ¹H NMR spectrum

of 7 exhibits the NH resonance at 8.90 ppm, which was not seen

in the original spectrum of 5. This can be attributed to the lack of

protic solvent to facilitate proton exchange of NH, whereas in 5,

phenol is present, leading to the extinction of the NH peak.

Synthesis of Monocarbonyl Complexes. To study the

eﬀect of the number of CO ligands on tuning hydrogen

activation reactivity, we synthesized the corresponding mono-

carbonyl CNP complexes. Direct addition of stoichiometric

amounts of the decarbonylating reagent trimethylammonium N-

oxide (Me₃NO) to 1 resulted in no reaction, and no change in

the CO bands was observed in the IR spectroscopy. Meanwhile,

treatment of 1 with excessive Me₃NO led only to complete

decarbonylation of both CO ligands. The high electron density

on the Fe center produces strong backbonding from Fe to CO,

which stabilizes CO ligands on Fe. Previously, we reported that

decarbonylation can be facilitated by replacing the anionic

halide with neutral L-type ligand.⁴²Therefore, to attain the

monocarbonyl complex 8, we utilized a two-step synthesis

shown in Scheme 5. Starting with 1, the halide abstracting

reagent AgBF₄was added to the acetonitrile solution of 1,

generating the acetonitrile-bound dicarbonyl species. Then,

Me₃NO was used to selectively eliminate one CO ligand,

aﬀording the monocarbonyl species with two acetonitrile

molecules bound in trans or cis fashion, trans-8 and cis-8. The

IR spectrum of 8 (dropcast) exhibits a single CO band at 1987

Figure 3. ORTEP diagram (30% thermal ellipsoids) for [Na(18-crown-

6)][(C^NHN^N=P^Ph2)Fe(CO)₂I] (7). H atoms except for the NH proton

are omitted for clarity.

ingly, the iodide anion shows substitutional disorder, because of

its coordination from both sides of the CNP plane (see Figure

S39 in the Supporting Information). The occupations of the two

isomers are ∼50% for each. The 18-crown-6-encapsulated Na⁺

cation binds with carbamoyl-O, at a distance of 2.309(5) Å.

Notably, the NH on the phosphine side arm was verifiably

deprotonated. As a result, the N(−P)−C^Pyand N−P distances

are shortened to 1.343(9) Å and 1.658(6) Å, respectively (∼1.37

Å and ∼1.70 Å for 3 and 4, respectively). The Fe−P bond length

also decreases (2.2729(18) Å), with respect to 3 (2.3037(14) Å)

and 4 (2.288(3) Å). The bond angle of P−N−C in 7 is more

acute (114°) than those of 3 and 4 (120° and 119°,

respectively). In addition, hydrogen bonding is observed

between carbamoyl-NH and crown-O (O···H = 2.24 Å, O···N

= 3.028(9) Å, bond angle = 153°). Importantly, no sign of

hydrogen bonding is observed on the phosphine side arm, which is

consistent with the deprotonation state of the phosphine-NH

linker.

Surprisingly, the dissolution of 7 in d⁸-THF gives rise to a ¹H

NMR spectrum equivalent to that of 5 (Figure 4), and the

³¹P{¹H} NMR spectrum of 7 displays a singlet peak at 91.4 ppm,

almost the same to the³¹P resonance of 5 (90.6 ppm). These

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Scheme 5. Synthesis of Monocarbonyl Complexes (trans-8 and cis-8)

1

cm⁻¹. In H NMR, the cis and trans isomers can be readily

were recorded along with the reaction. As shown in Figure 6, the

¹H NMR resonances derived from trans-8 gradually decreased,

as those of cis-8 increased; the same trend was observed in the

³¹P NMR spectrum. These results indicate that trans-8 is the

kinetic product and cis-8 is the thermodynamic product.

The energy diﬀerence between cis-8 and trans-8 was obtained

through DFT calculation by geometry optimization at PBE0/

opt (see the Supporting Information for details of the basis set)

level of theory. The converged structures (see Figures S44 and

S45 in the Supporting Information) precisely simulated the

structural characteristics of 8, indicating the accuracy of PBE0 in

structural calculation with transition-metal complexes. Impor-

tantly, the electronic energy of cis-8 is lower than trans-8 by

∼1.5 kcal/mol (see Table S2 in the Supporting Information).

This result is consistent with the NMR study of conversion of

the trans isomer to the cis isomer.

The isopropyl analogue complex 9 was synthesized by the

similar method, in which we only isolated the cis-9 in 75% yield.

A small amount (∼8%) of trans-9 complex could be observed

from the crude NMR (see Figures S33 and S34 in the

Supporting Information). The ¹H NMR displays the NH

protons at 8.75 and 6.86 ppm, which are assignable to carbamoyl

NH and phosphine NH, respectively. The signals of isopropyl

protons exhibit in an asymmetric pattern, because the cis

acetonitrile ligands make the two isopropyl groups chemical

unequivalent (methine: 2.82, 2.69 ppm; methyl: 1.46−1.27

ppm). The ³¹P NMR shows a singlet at 110.6 ppm, lower than

cis-8. The CO stretching frequency of cis-9 appears at 1969

cm⁻¹, red-shifted relative to 8, indicating a higher electron

density on the Fe of cis-9. The crystal structure of cis-9 is shown

in Figure 7, and the selected bond parameters are tabulated in

Table 1. The Fe atom coordinates with the CNP ligand, one CO

ligand, and two cis-acetonitrile molecules in a pseudo-octahedral

geometry. The Fe−C(O) bond distance is comparable with 4

(1.963(2) Å for cis-9 and 1.962(10) Å for 4). Note that, in cis-9,

the Fe−P bond length (2.2976(5) Å) is the longest among all

the structures.

identiﬁed at a ratio of ∼3:5. The assignments for each feature

were based on the ¹H−³¹P HMBC NMR on the isolated product

in MeCN-d₃(see Figure S32 in the Supoorting Information).

The cis isomer exhibits two singlets at 8.85 and 7.88 ppm, which

correspond to the carbamoyl-NH and phosphino-NH, respec-

tively. The trans isomer displays the speciﬁc NH signals at 8.80

and 8.05 ppm. The protons at 5- and 3-positions on pyridine ring

give rise to two doublets at 6.47, 6.34 ppm for cis-8 and 6.72,

6.52 ppm for trans-8. In the aliphatic region, two singlet

resonances at 2.31 and 1.74 ppm are attributed to the methyl

protons of the coordinated acetonitrile of cis-8, and the singlet

observed at 1.56 ppm corresponds to the methyl protons of

trans-8. These methyl proton resonances persisted for about 1

day and disappeared upon ligand exchange with the deuterated

acetonitrile solvent. In ³¹P{¹H} NMR, a singlet is observed at

89.4 ppm for cis-8, and trans-8 gives rise to a singlet at 100.5

ppm.

The crystal structure of 8 was obtained upon diﬀusion of

diethyl ether to the acetonitrile solution, and, interestingly,

trans-8 and cis-8 co-crystallized in the crystal lattice. The

structure is shown in Figure 5, and the selected bond parameters

Figure 5. Asymmetric part of the unit cell for 8: ORTEP diagrams (30%

thermal ellipsoids) for cis-[(C^NHN^NHP^Ph2)Fe(CO)(MeCN)₂](BF₄)

(cis-8) and trans-[(C^NHN^NHP^Ph2)Fe(CO)(MeCN)₂](BF₄) (trans-8).

Both complexes were found co-crystallized in the same unit cell. H

atoms except for the NH protons are omitted.

H₂Reactivity. Having proven that the phosphoramide NH of

CNP complexes can be deprotonated, the possibility of the CNP

pincers performing H₂cleavage was investigated. This would be

consistent with numerous literature examples of organometallic

iron pincer complexes. Kirchner et al. reported that the

dearomatized P^NHN^NHP−Fe pincer complex cleaves H₂, the

proton of which protonates the ligand backbone and the hydride

binds to the Fe center.¹⁴Milstein et al., synthesized a series of

methylene-derived P^CH2N^CH2P−Fe complexes, which perform

hydrogenation of aldehydes, ketones, and triﬂuoroacetic esters

with high eﬃciency.^17,19,46The active species of methylene-

deprotonated intermediate is also proven spectroscopically and

computationally to be critical for hydrogen splitting and hydride

transfer. Note that the organometallic pincer complexes are

mostly constructed via neutral phosphine tridentate ligands. It is

likely that, in our CNP complexes, the increase of the negative

charge of the ligand backbone from neutral to −1 would increase

are tabulated in Table 1. Both Fe atoms exhibit pseudo-

octahedral geometry and coordinate with the CNP ligand, one

CO ligand, and two acetonitrile molecules. The charge of the

−

complex is balanced by BF₄anion. The Fe−C(O) bond

distances are shortened (1.959(4) Å for cis-8 and 1.955(4) Å for

trans-8), compared with dicarbonyl complexes. It is noteworthy

that, in cis-8, the orientation of acetonitrile applies the weakest

trans-inﬂuence on pyridine, leading to the shortest Fe−N^pybond

length (1.937(3) Å) among all the structures. The Fe−NCMe

bond distances are among comparable range of iron(II)

acetonitrile bond distance,^40,43−45except for the longest

Fe(2)−N(9) bond (1.974(4) Å).

While room-temperature synthesis of the monocarbonyl

complexes aﬀorded trans-8 as the major product, conversion of

trans-8 to cis-8 was observed at elevated temperature. The

solution of 8 in MeCN-d₃was heated at 55 °C and NMR spectra

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Figure 6. ¹H NMR and ³¹P{¹H} NMR spectra showing interconversion of trans-[(C^NHN^NHP^Ph2)Fe(CO)(MeCN)₂](BF₄) (trans-8) to cis-

[(C^NHN^NHP^Ph2)Fe(CO)(MeCN)₂](BF₄) (cis-8) in MeCN-d³solution incubated at 55 °C.

Figure 7. ORTEP diagrams (30% thermal ellipsoids) for cis-

[(C^NHN^NHP^iPr2)Fe(CO)(MeCN)₂](BF₄) (cis-9). H atoms except for

the NH protons are omitted.

the reactivity in H₂activation. Moreover, in relation to the

bioinspiration for Hmd, we hypothesized that the non-

biomimetic monocarbonyl motif would provide a reactivity

advantage over the dicarbonyl motif found in the enzyme,

because of the higher electron density on the Fe center.

Figure 8. ²H NMR spectra of D₂activations with dicarbonyl complex 1

or monocarbonyl complex 8. Reaction conditions: (a) 1 + D₂(7 atm) +

KO^tBu (1 equiv), THF, RT, 1 d; (b) 1 + D₂(7 atm) + KO^tBu (2 equiv),

THF, RT, 1 d; (c) 8+ D₂(4 atm) + NaDBHA (1 equiv), THF, RT, 2 d;

(d) 8 + D₂(4 atm) + NaDBHA (1 equiv) + EtOH, THF, RT, 2 d. The

identity of the asterisk (*) feature in spectrum (c) remains unknown.

To test these hypotheses, we performed the H₂activation

studies using D₂and ²H NMR spectroscopy. To a high-pressure

NMR tube were charged the THF solution of complexes (1 or 8,

dicarbonyl and monocarbonyl complexes, respectively), base

(bulky phenolate or KO^tBu), and D₂gas (4−7 atm); after a

speciﬁed time period, ²H NMR spectra were obtained to identify

the formation of deuterated products. As shown in Figure 8a,

treatment of dicarbonyl complex 1 with D₂and 1 equiv of either

NaDBHA (weaker phenolate base) or KO^tBu (stronger alkoxide

base) did not result in any product peak, as evidenced by the ²H

NMR spectrum. However, inclusion of 2 equiv of KO^tBu

resulted in a new feature at 3.16 ppm assigned to the tert-butanol

OD resonance (Figure 8b), indicating heterolytic D₂cleavage.

By comparison, complex 8 was found to readily activate D₂gas

(4 atm) in the presence of only 1 equiv of the weaker base

2

NaDBHA (Figure 8c). The H NMR spectrum exhibited two

product peaks at 5.56 and 2.56 ppm, assigned to deuterated 2,6-

di-tert-butyl-4-methoxyphenol (PhOD) and D₂O, respectively,

as well as a feature at 3.27 ppm, which was derived from an as-

yet-unidentiﬁable alcohol −OD resonance, while the D₂

splitting was not observed in the control experiments without

complex (same concentration of NaDBHA + 7 atm D₂in THF).

However, attempts to detect the putative iron deuteride species

with NMR were also not fruitful, even at lower temperature

(−40 °C). It is likely that the low concentration, as well as the

transient nature of the deuteride species, prohibited its

detection.

t

The BuOD formation suggests that D₂is cleaved by the

exogenous tert-butanoxide base. It is evident that the

deprotonated ligand backbone does not participate in the

2

reaction, because of the absence of ND signal in H NMR. In

Previously, it has been proposed that ethanol can promote H₂

addition, the putative Fe−D intermediate was not detected.

cleavage catalyzed by a P^NHN^NHP iron complex by forming a

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proton shuttle between the ligand-backbone pendant base and

the iron-bound dihydrogen molecule, thus decreasing the

reaction barrier for H₂splitting.^15,47To examine this possibility,

20 μL of ethanol (∼65 equiv) was added to the reaction

containing 8 (Figure 8d). Surprisingly, the reactivity of 8

a base-assisted mechanism by calculating the enthalpy change

(ΔH) of two subsequent steps involving dearomatized ligand

backbone and external bases: (i) H₂binding to the

pentacoordinate dearomatized complexes (A), generating H₂

adducts (B); and (ii) H₂cleavage by external bases, forming

hydride intermediates (C). The ΔH via the reaction trajectory is

illustrated in Figure 9 and the enthalpies of base molecules have

2

decreased, as the H NMR spectrum only showed a negligible

peak attributed to EtOD signal and no features assignable to

PhOD or D₂O was observed. The same result was also obtained

when NaDBHA was substituted by KO^tBu. The disappearance

of PhOD and D₂O peaks indicates that the smaller-sized

ethoxide, generated by NaDBHA or KO^tBu, was involved in D₂

cleavage, instead of the bulkier phenolate or tert-butoxide,

because it was easier for ethoxide to approach the Fe center.

Furthermore, the decreased reactivity of 8 is likely due to the

coordination of ethanol molecule to the Fe center, leading to a

lower amount of the active species cleaving D₂. Overall, the

adverse eﬀect of ethanol toward D₂activation suggests that the

ligand−metal cooperation pathway in which D₂is cleaved by Fe

center and the deprotonated NH moiety is not feasible. It is

external bases, e.g., tert-butoxide, bulky phenolate, or ethoxide,

that actually facilitate the reaction.

The aforementioned reactions were also investigated in

MeCN as solvent in lieu of THF. The ²H NMR spectra display

no resonances for deuterated species, except for those derived

from solvent and D₂. This result indicates that D₂is activated

upon binding to the Fe center; with MeCN as a solvent, the

vacant site of Fe center is occupied by MeCN, which precludes

any reactivity with D₂.

Next, we investigated the D₂/deuteride transfer reactivity of 1

and 8 to assess the possibility of CNP complexes to perform the

complete Hmd-relevant process. Complexes 1 or 8 were treated

with D₂, base and a deuteride acceptorsuch as an imidazolium

cation (1,3-bis(2,6-diﬂuorophenyl)-2-(p-tolyl)imidazolium,

Im⁺), or the more strongly deuteride-accepting acridinium

cation (9-phenyl-10-methylacridinium, Ac⁺).⁴⁸However, nei-

ther ²H NMR spectroscopy nor mass spectrometry could detect

the corresponding deuteride-transferred product, Im−D or Ac−

D. The lack of deuteride transfer could be attributed to the high

Lewis acidity of the Fe center. Bound with only one anionic

donor, carbamoyl unit, as well as strong π-accepting CO ligand,

the Fe center functions as a deuteride acceptor, rather than a

deuteride donor.

DFT Study of H₂Activation. Based on the above reactivity

study, it is evident that the monocarbonyl complex exhibits

higher reactivity in H₂activation than the dicarbonyl complex.

To obtain theoretical insight, DFT was used to investigate H₂

binding and activation. Three putative pentacoordinate active

species were considered (Scheme 6): the dicarbonyl complex

(A_diCO), the monocarbonyl complex with MeCN trans to the

vacant site (A_MeCN), and the monocarbonyl complex with CO

trans to the vacant site (A_CO). According to the experimental

evidence, the basic amide linker on the ligand backbone does not

participate in the reaction (vide infra). We therefore considered

Figure 9. Calculated relative enthalpies in the H₂binding and cleavage

reactions with external bases of A_CO+ NaDBHA (red), A_diCO

+

NaDBHA (solid green), A_MeCN+ NaDBHA (blue) and A_diCO+ KO^tBu

(dashed green) proceeding from the corrected iso-energetic state A

through B and forming C.

been taken into account in the total energy calculation, as shown

in the equation in Figure 9. In addition, we must point out that

the ΔG values of the reactions are dominated by ΔH as the

entropy change is negligible (TΔS_A→B= −4.96 kcal/mol), and,

therefore, it is convenient to only consider the enthalpy change

in these reactions.

First, the H₂binding processes are all exothermic. A_MeCN

shows the highest aﬃnity for H₂(ΔH_A→B= −22.7 kcal/mol)

and A_COhas the lowest aﬃnity for H₂(ΔH_A→B= −13.6 kcal/

mol) with A_diCOexhibiting a moderate aﬃnity (ΔH_A→B= −15.2

kcal/mol) (Figure 9). While all three Fe complexes exhibit some

aﬃnity for H₂, the diﬀerence in reactivities does not solely lie on

the number of CO ligandsi.e., the electron density at the Fe

center. The lowest ΔH_A→Bfor MeCN and the similar ΔH_A→B

values between diCO and CO suggest that the ligand trans to the

H₂binding site determines the H₂aﬃnity. According to Kubas

principles,⁴⁹⁻⁵¹the Fe−H₂interaction resembles the classic

Dewar−Chatt−Duncanson model, in which the ﬁlled σ(H₂)

2

donates electron density to the empty d_{x −y}(Fe) orbital, forming

a σ-bond, and the ﬁlled d_xy(Fe) back-donates electron density to

σ*(H₂), forming a π-interaction. By examining the MO diagram

of B (see Figure S57 in the Supporting Information), we

identiﬁed the σ bonding orbital of Fe−H₂for the complexes in

low energy orbitals (HOMO−40 for B_CO; HOMO−27 for

B

_diCO; MOHO−45 for B_MeCN) and the π-interaction emerging

from back-donation are closer to the frontier orbitals: HOMO−

5 for B_CO; HOMO−8 for B_diCO; MOHO−4 for B_MeCN. The

weak π-accepting ligand MeCN trans to the vacant site is less

competitive (versus trans CO) with H₂for back-donation from

d_xz(Fe)which is an orbital that the two trans ligands (H₂and

MeCN/CO) must share.⁵²Therefore, the trans MeCN case

provides the strongest Fe−H₂bonding interaction in the set.

Notably, the identity of the external base does not have an

apparent inﬂuence on ΔH of the H₂binding process. For the

dicarbonyl complex A_diCO, changing the base from NaDBHA

Scheme 6. Dearomatized Pentacoordinate Complexes

Considered in the DFT Calculations

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a

Table 2. Reaction Optimization for Transfer Hydrogenation of ⁱPrOH to Benzylaldehyde

c

Yield (%)

catalyst

time, t

base

temperature,

conversion

turnover number,

d

entry

(10 mol %)

(h)

base

equiv

T (°C)

solvent

ⁱPrOH

ⁱPrOH+H₂O

ⁱPrOH

(%)

a

b

c

TON

1

2

8

1

−

9

20

1

NaDBHA

DBU

1

2

3

60

40

RT

40

60

80

40

80

0

3

0

1

22

21

2

0

6

3

87

33

4

53

3

2

4

7

5

0

2

6

0

7

KO^tBu

38

34

53

84

3

17

18

35

31

3

11

7

9

2

3

8

KOH

8

9

KOH

7

11

2

10

11

12

13

14

15

16

17

KOH

51

0

b

NaDBHA

KO^tBu

KOH

0

26

2

8

11

0

6

2

0

KOH

83

40

37

58

29

18

13

21

47

22

23

32

7

3

2

1

2

20

KOH

1

KOH

1

KOH

5

a

b

General reaction conditions: 0.0157 mmol of base, 0.0523 mmol of benzylaldehyde, 10 mol % of catalyst, 1 mL of solvent. 0.9 mL ⁱPrOH + 0.1

c

d

mL H₂O. Yields calculated from the integration of GC-MS data. TON = (mmol of benzyl alcohol)/(mmol of catalyst).

(solid green line in Figure 9) to KO^tBu (dashed green line)

slightly increases the enthalpy of the system (from −15.2 kcal/

mol to −14.7 kcal/mol). This is reasonable in that the base is not

directly participating in this step.

three. Importantly, although the number of CO ligands does

aﬀect the electron density of the Fe center, it seems that the

identity of the trans ligand to the H₂binding site is more pivotal

in our pincer system. The trans MeCN ligand features weak σ-

donating and weak π-accepting ability, which promotes the

bonding of H₂to Fe center. This strategy has also been utilized

by [Fe]-hydrogenase, as the acyl moiety is a strong σ-donor and

weak π-acceptor and unquestionably enables the enzyme to

exhibit a stronger aﬃnity for H₂. Therefore, we believe this is a

crucial way to modulate trans inﬂuence to tune the activity of

catalysts for H₂activation.

Transfer Hydrogenation. The aﬃnity of CNP complexes

to bind alcohols (vide supra) prompted us to test their function

as transfer hydrogenation (TH) catalysts to demonstrate the

functional reactivity of the CNP system. The selected reaction

was transfer hydrogenation of benzylaldehyde using ⁱPrOH as a

hydrogen source. A complete screening of the base, temperature,

solvent, and catalysts was performed (see Table 2). Starting with

the monocarbonyl complex 8 and NaDBHA as a base at 60 °C,

increasing to the amount of NaDBHA to 3 equiv led to higher

reactivity with 87% of conversion of benzylaldehyde (see Table

2, entry 3). However, GC-MS results revealed the major product

to be the dienone b (53%) with the minor product as enone c

(6%). The desired hydrogen-transferred product benzyl alcohol

a formed with yield of 22%. The side products b and c were

produced from the Claisen−Schmidt condensation in which

acetone reacted with benzylaldehyde under basic conditions.⁵³

Decreasing the temperature provided smaller amounts of side

products (see Table 2, entries 4 and 5), but the yield of benzyl

alcohol also decreased. Screening of diﬀerent bases revealed that

the neutral nitrogen-based base, 1,8-diazabicyclo[5.4.0]undec-

7-ene (DBU), is not suitable to promote the reaction, most likely

Next, we considered the energy change of the H₂cleavage step

via base-assisted mechanism, in which H₂on B is heterolytically

cleaved, producing hydride intermediate C and conjugate acid.

When NaDBHA is chosen as a base (Figure 9, solid lines), all

three complexes give rise to exothermic results in which C_MeCN

shows the lowest energy (−31.9 kcal/mol), C_diCOin the middle

(−26.4 kcal/mol), and C_COis the highest in energy (−18.2 kcal/

mol). Although the ΔH_B→Cvalues are comparable for all three

complexes, the total ΔH_A→Cfor all three reactions shows clear

trend in that the monocarbonyl complex A_MeCNis the most

reactive species in H₂activation and, signiﬁcantly, A_COis the

least active species among them, indicating that A_COis not

responsible for H₂activation. This is consistent with the

experimental evidence that monocarbonyl complex 8 is more

easily activated (1 equiv of NaDBHA is suﬃcient for H₂

heterolysis). Interestingly, substitution of NaDBHA for KO^tBu

led to a drastic decrease in enthalpy for diCO (−52.1 kcal/mol),

making the H₂cleavage with A_diCOcomplex more feasible. This

is in agreement with the experimental results showing that

dicarbonyl complex 1 was able to activate H₂in the presence of 2

equiv KO^tBu. These results demonstrate that the propensity of

H₂cleavage is highly dependent on the basicity of the external

base.

The calculated energy proﬁle demonstrates the additive

eﬀects of the trans inﬂuence in the primary coordination sphere

and the basicity of external bases in H₂activation. Both

theoretically and experimentally, we have showed that the

monocarbonyl complex A_MeCNis the most reactive among the

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because of its weaker basicity (Table 2, entry 6). Instead, using

the strong inorganic base KOH (Table 2, entry 8) aﬀorded a

higher yield of benzyl alcohol and lower formation of side

products, compared with other organic bases (NaDBHA, DBU,

and KO^tBu). The optimal temperature with KOH was

determined to be 60 °C, with the highest yield of a at 35%

(Table 2, entries 9 and 10). Water has also been reported to

beneﬁt the TH reaction with PNP complexes, because of its

formation of a proton bridge, which facilitates the proton

transfer between the pendant base and the substrate.⁴⁷However,

complexes. Conditions for reactions (1 equiv of PhONa for 8

and 2 equiv of KO^tBu for 1) show that monocarbonyl complex 8

exhibits higher reactivity for H₂activation than dicarbonyl

complex 1.

(4) DFT calculations show that the eﬃciency of H₂reactivity

follows the trend of CO < diCO < MeCN. This reactivity

diﬀerence is largely derived from the step of H₂binding, in which

electron density at the Fe center (minor eﬀect) and inﬂuence of

ligand trans to the H₂binding site (major eﬀect) jointly

modulate the aﬃnity of H₂of the pentacoordinate species (A).

Subsequently, the cooperation of the Fe center and external

bases leads to H₂cleavage. The eﬃciency should be related to

the pK_aof the base and the extent of H−H bond weakening,

which are consistent with the experimental results.

(5) The catalytic activity of the monocarbonyl CNP complex

8 toward transfer hydrogenation of benzylaldehyde is observed,

whereas the dicarbonyl complex 1 and −PⁱPr₂-substituted

complex 9 exhibit lower to no reactivity. This could be ascribed

to the latter species’ more-stable hydride species, leading to low

hydricity and hydride transfer.

i

the addition of ∼10% water (V_{H O}:V_PrOH= 1:9) to the reaction

2

mixture suppressed the reaction, regardless of the base identity

(Table 2, entries 11−13). Shortening the reaction time from 20

h to 1 h (Table 2, entry 14) resulted in only a slightly lower yield

of a (29%), indicating that benzylaldehyde was hydrogenated

within the ﬁrst hour of reaction. Lastly, the eﬀects of changing

complexes on TH were investigated. Exchanging the mono-

carbonyl complex 8 for the dicarbonyl complex 1 led to decrease

of the yield of a to 18% (Table 2, entry 15), indicating lower

catalytic activity of the dicarbonyl system. It is noteworthy that,

in the absence of Fe complex, the TH reaction still occurred,

with benzyl alcohol formed at 13% yield (Table 2, entry 16).

Indeed, it has been reported that quantitative reduction of

acetophenone to phenylethanol was observed in a concentrated

NaOH solution.⁵⁴Surprisingly, substituting the isopropyl

monocarbonyl analogue (with −PⁱPr₂) complex 9 for 8 (with

−PPh₂) again provided a lower yield (21%; see Table 2, entry

17). This is comparable to the reaction with 1, indicating that the

isopropyl substitution does not promote the TH reactivity in

this series of CNP complexes.

(6) These results highlight three key items regarding the [Fe]-

hydrogenase enzyme:

(i) The counterintuitive use of the less H₂-reactive

dicarbonyl motif may restrain the reactivity of [Fe]-

hydrogenase, thus providing more selectivity for hydride

transfer to the imidazolium unit of the H₄MPT⁺.

(ii) The methylene-acyl carbanion present in the active site

occurs trans to the site of H₂activation; this strong σ

donor and weak π acceptor likely drives signiﬁcant H−H

elongation in the putative Kubas intermediate.

Although some catalytic activity was observed with our CNP

complexes, the overall performance of the catalysts does not

compare favorably with other well-established catalysts.⁵⁵Such

low TH activity could be due to the low hydricity of the hydride

species (thermodynamic hydride donating ability),⁵⁶consider-

ing that the active species in TH is usually a metal-hydride

intermediate.⁵⁵For example, Hu reported the [Fe(H)(Br)-

(iii) The pyridone/pyridonate-O atom directly adjacent to the

H₂binding site in the enzyme likely provides both the

optimum basicity and orientation that promotes H₂

heterolysis (ΔE_a≈ 1 kcal/mol, as estimated via the

QM/MM method).¹³

(7) Future bioinspired catalysts (with or without phosphine)

should aspire to achieve such a second-coordination sphere

feature. In addition, new catalysts and complexes with strong σ-

donating and weak π-accepting ligand trans to the hydride (e.g.,

H⁻, phosphine, and cyclopentadiene) would increase reactivities

toward both H₂splitting and hydride transfer.

i

(ⁱPr₂PONOP Pr₂)] catalyzes the TH reaction to aldehydes

using formate as the hydride source, in which the dihydride

i

species [Fe(H)₂(ⁱPr₂PONOP Pr₂)] was proposed to be the

active species.²⁰Although we have not yet observed the hydride

intermediate in the present set of CNP complexes, it is

reasonable to hypothesize that the catalytic cycle involves the

formation of hydride species. In the previous section, we have

established the energy proﬁle of H₂cleavage with the CNP

complexes. The DFT calculations suggest high stability of the

hydride species (C), which likely results in weak hydricity and,

thus, TH activity.

ASSOCIATED CONTENT

* Supporting Information

■

sı

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03476.

Details of synthetic procedures, NMR spectra, IR spectra,

X-ray crystallography, and DFT calculations (PDF)

CONCLUSION

■

Accession Codes

The main conclusions of this work can be summarized as

follows:

CCDC 1907564−1907568 contain the supplementary crystallo-

graphic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-

ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road, Cam-

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

(1) Bioinspired iron(II) carbonyl pincers are synthetically

accessible via metalation of the apo-ligand with a ferrous

carbonyl salt, which forms the Fe−C(carbamoyl) bond in situ.

(2) Base addition results in deprotonation of the phosphor-

amide unit (analogous to the Kirchner system), rather than the

carbamoyl unit.

AUTHOR INFORMATION

Corresponding Author

(3) Incubation of the pentacoordinate dearomatized/

deprotonated complexes in D₂gas leads to the appearance of

■

t

the PhOD or BuOD signals in ²H NMR spectroscopy,

Michael J. Rose − Department of Chemistry, The University of

Texas at Austin, Austin, Texas 78712, United States;

indicating the heterolytic cleavage of D₂with the CNP

L

https://dx.doi.org/10.1021/acs.inorgchem.9b03476

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

orcid.org/0000-0002-6960-6639; Email: mrose@

pubs.acs.org/IC

Article

H δ +-O, Bond and Methenyl-H4MPT+ Triggered Hydride Transfer. J.

Am. Chem. Soc. 2009, 131 (31), 10901−10908.

cm.utexas.edu

(13) Finkelmann, A. R.; Senn, H. M.; Reiher, M. Hydrogen-Activation

Mechanism of [Fe] Hydrogenase Revealed by Multi-Scale Modeling.

Chem. Sci. 2014, 5 (11), 4474−4482.

Authors

Zhu-Lin Xie − Department of Chemistry, The University of Texas

at Austin, Austin, Texas 78712, United States; orcid.org/

0000-0002-1969-2831

Wenrui Chai − Department of Chemistry, The University of Texas

at Austin, Austin, Texas 78712, United States; orcid.org/

0000-0002-5801-0135

Spencer A. Kerns − Department of Chemistry, The University of

Texas at Austin, Austin, Texas 78712, United States;

orcid.org/0000-0001-6936-8098

Graeme A. Henkelman − Department of Chemistry, The

University of Texas at Austin, Austin, Texas 78712, United

States; orcid.org/0000-0002-0336-7153

̈

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Notes

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The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Z.X., S.A.K., and M.J.R. acknowledge funding from the National

Science Foundation (NSF) (No. CHE-1808311) and the Welch

Foundation (No. F-1822), and are grateful to Dr. Vince Lynch

for assistance in solving X-ray structures. W.C. and G.H.

acknowledge funding from the Welch Foundation (No. F-1841)

and computational resources from the Texas Advanced

Computing Center.

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Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H2 Activation and Transfer Hydrogenation

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