Terminal Iron Carbyne Complexes Derived from Arrested CO2 Reductive Disproportionation

Article abstract of DOI:10.1002/anie.201705877

The encumbered tetraisocyanide dianion Na₂[Fe(CNAr (Formula presented.))₄] reacts with two molecules of CO₂ to effect reductive disproportionation to CO and carbonate ([CO₃]^2?). When the reaction is performed in the presence of silyl triflates, reductive disproportionation is arrested by silylative esterification of a mono-CO₂ adduct. This results in the formation of four-coordinate terminal iron carbynes possessing an aryl carbamate substituent owing to the direct attachment of an C(O)OSiR₃ group to an isocyanide nitrogen atom. Crystallographic, spectroscopic, and computational analyses of these iron–carbon multiply bonded species reveal electronic structure properties indicative of a conformationally locked iron carbyne unit.

Full text of DOI:10.1002/anie.201705877

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Accepted Article
Title: Terminal Iron Carbyne Complexes Derived from Arrested CO2
Reductive Disproportionation
Authors: Charles C. Mokhtarzadeh, Curtis E. Moore, Arnold L.
Rheingold, and Joshua S. Figueroa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705877
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10.1002/anie.201705877
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Terminal Iron Carbyne Complexes Derived from Arrested CO2
Reductive Disproportionation
Charles C. Mokhtarzadeh, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa*
Dedication ((optional))
Abstract:
The
encumbered
tetraisocyanide
dianion,
Me3C6H2)2C6H3),[15] as an encumbered analogue of Na2[Fe(CO)4]
by virtue of possessing four m-terphenyl isocyanide ligands.[16-18]
We reasoned that the large steric profile provided by four
CNArMes2 ligands in this system[19-21] might allow for the
stabilization and/or kinetic trapping of key intermediates in CO2
reductive disproportionation by a formally dianionic Fe center.[15]
Accordingly, herein, we show that Na2[Fe(CNArMes2)4] is indeed
effective at mediating CO2 reductive disproportionation. However,
in the presence of silyl electrophiles, the initial CO2 adduct in this
process can be trapped by formal addition to isocyanide, affording
unique examples of four-coordinate iron terminal carbyne
complexes. These carbynes possess structural and electronic
features suggesting the terminal carbyne ligand is
conformationally locked into a three-fold, equatorial plane when
multi-dentate ligands are absent from the system. In addition, this
work highlights the beneficial secondary-reactivity patterns
available to isocyanide ligands in highly-reduced environments
and provides a contrast to CO2 reductive disproportion systems
featuring homoleptic carbonyl coordination spheres.[3-4,7]
Na2[Fe(CNArMes2)4], reacts with two molecules of CO2 to effect
reductive disproportionation to CO and carbonate ([CO3]2–). When the
reaction is performed in the presence of silyl triflates, reductive
disproportionation is arrested via silylative esterification of a mono-
CO2 adduct. This results in the formation of four-coordinate terminal
iron carbynes possessing an arylcarbamate substituent due to the
direct attachment of an C(O)OSiR3 group to an isocyanide nitrogen
atom. Crystallographic, spectroscopic and computational analyses of
these iron-carbon multiply-bound species reveal electronic structure
properties indicative of a conformationally-locked Fe-carbyne unit.
Carbon dioxide reductive disproportionation to carbon monoxide
and carbonate (CO + [CO3]2–) has long been investigated as a
potentially useful synthetic CO2 fixation process.[1-11] In contrast to
other CO2 reduction processes that produce CO as feedstock,[12-
13] reductive disproportionation is unique in that CO2 itself serves
as the ultimate oxygen-atom acceptor en route to the production
of inorganic carbonate.[7,10,11] Many electron-rich transition metal
complexes, especially those that can readily function as two-
electron reductants, have been shown to mediate stoichiometric
and/or catalytic CO2 reductive disproportionation.[2-4,7-11] Of these
complexes, highly-reduced Group 6 and 8 metal carbonyl
dianions have been established as the most operationally simple
CO2 reductive disproportionation archetypes due to their
production of discrete alkali or alkaline-earth metal carbonate and
a volatile neutral transition-metal carbonyl upon addition of two
equivalents of CO2.[3-4,7] In such reductive disproportionation
schemes, the reduced carbonyl metalate salts Na2[W(CO)5] and
Na2[Fe(CO)4] have been extensively examined and have been
proposed to share common intermediates with respect to the
activated CO2 unit.[7] Chief among these is a proposed dianionic
metal-bound dioxycarbene (i.e. [LnM=C(O)2]2–),[14] which is
thought to add a second equivalent of CO2 to begin the O-atom
transfer process.[4,7] However, due to the highly-reduced and
sterically unencumbered nature of these carbonyl complexes,
such intermediates have not been spectroscopically or
structurally authenticated.
Exposure of a THF solution of Na2[Fe(CNArMes2)4] (Na2[1])
to an excess of CO2 (1 atm) resulted in a rapid color change from
dark red to dark orange (Scheme 1). Analysis of the reaction
1
mixture by H NMR spectroscopy indicated the clean production
of a new diamagnetic species with a single CNArMes2 environment.
Crystallographic structure determination revealed this product to
be the five-coordinate monocarbonyl species Fe(CO)(CNArMes2
)
4
(2; Figure S4.1), which is isostructural to the mono-dinitrogen
complex Fe(N2)(CNArMes2)4.[15] Conversion of Na2[1] to
monocarbonyl 2 indicated that a CO2 reductive disproportionation
process is likely operative, as the analogous reaction between two
equivalents of CO2 and Na2[Fe(CO)4] produces Fe(CO)5 as the
exclusive iron-containing product.[3-7] Treatment of Na2[1] with an
excess of isotopically labeled 13CO2 followed by analysis of 2 by
13C{1H} NMR spectroscopy confirmed that the 13C-label was
incorporated exclusively into the Fe-bound carbonyl position. This
result also suggests that [O]-for-[NArMes2] metathetical exchange,
as is known to take place upon reaction of some homoleptic
isocyanide complexes with CO2,[22] is not operative. In addition,
extraction of this reaction mixture with alkaline D2O followed by
13C{1H} NMR spectroscopic analysis revealed a single intense
peak at 169 ppm,[23] which is indicative of [CO3]2– and an overall
CO2 reductive disproportionation process (Figure S1.1). It is also
noteworthy that treatment of Na2[1] with 1.0 equiv of CO2 results
in a 1:1 mixture of monocarbonyl 2 and unreacted Na2[1], as
determined by 1H NMR spectroscopy. This product distribution
indicates that the reaction between a mono-CO2/Fe intermediate
and a second equivalent of CO2 occurs at a rate
Recently we reported the disodium tetraisocyanoferrate(2–),
Na2[Fe(CNArMes2)4]
(Na2[1];
ArMes2
=
2,6-(2,4,6-
[*]
Dr. C. C. Mokhtarzadeh, Dr. C. E. Moore, Prof. A. L. Rheingold,
Prof. J. S. Figueroa
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive MC 0358, La Jolla, CA 92093 (USA)
E-mail: jsfig@ucsd.edu
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201705877
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Scheme 2. Synthesis of the terminal carbyne Na[3].
Scheme 1. Carbon dioxide reductive disproportionation mediated by Na2[1] and
proposed intermediates.
greater than that between Na2[1] and CO2.
Given these observations and the steric profile of Na2[1], we
reasoned that an intermediate Fe/mono-CO2 adduct could be
trapped by a substrate with greater electrophilicity than that of
another CO2 molecule. Previously, it has been shown that
tetraisocyanide Na2[1], and its monoanionic derivatives, display
Fe-based nucleophilicity toward both protic and electrophilic
methylating reagents.[15] However, we found that trimethylsilyl
triflate (Me3SiOTf; OTf = O3SCF3) does not react with this salt at
room temperature in toluene solution. While we attribute this
finding to the inaccessibility of the Fe center in Na2[1] to
encumbered silyl reagents, such reagents were considered an
appropriate choice to potentially intercept a nucleophilic, metal-
bound CO2 intermediate. Accordingly, treatment of a 1:1 mixture
of Na2[1] and Me3SiOTf with an excess of CO2 resulted in the
Figure 1. Full molecular structure of terminal carbyne Na[3] (left). Alternate view
of the primary coordination sphere of Na[3] with the m-terphenyl periphery in
wireframe (right). Thermal ellipsoids are drawn at the 30 % probability level.[44]
terminal carbynes.[25-26] For Na[3], the Ccarbyne-N bond distance of
1.448(12) Å (i.e. d(C1-N1)) is reflective of a single bond and
indicates complete reduction of the isocyanide CºN group to
which the silylated-CO2 group was added. Additionally, the N-C
bond distance in the carbamate N-C(O) unit of Na[3] (d(N1-C5)
= 1.371(10) Å) is reflective of partial double-bond character,
thereby indicating that the carbonyl group is a competitive p-
acceptor relative to the carbyne carbon. Therefore, it is likely that
the short Fe-Ccarbyne bond length in Na[3] partially originates from
the unique electronic environment provided by the carbamate-
carbyne functionality.
The solid-state structure of Na[3] also reveals that the Fe
center does not adopt a C3v-symmetric coordination geometry
with an apical carbyne unit.[24] Rather, the geometry around the
Fe center in Na[3] is best approximated as trigonal
monopyramidal (Houser t4 Index = 0.84),[28] with an axial
CNArMes2 ligand (C4) and an equatorial set comprising two
CNArMes2 ligands (C2 and C3) and the carbyne unit (C1). This
overall Cs-symmetric structure follows from the fact that the sum
of the angles between C1, C2 and C3 (i.e. 340.8˚) lacks the
degree of pyramidalization normally expected for tetrahedral-type
structures. In addition, the average of three angles surrounding
the C4 carbon are most acute in the structure (104.6˚), which
suggests an axial-type positioning relative to the other ligands.
Significantly, the Cs-symmetric environment displayed by Na[3] in
the solid state is retained in solution as determined by both 1H and
13C{1H} NMR spectroscopy. In the 1H NMR spectrum of Na[3]
(C6D6; Figures S1.2 and S1.3), twelve distinct chemical
environments are found for the CNArMes2 methyl groups, thereby
reflecting a significant reduction in symmetry from a canonical C3v
form. Additionally, the 13C{1H} NMR spectrum (C6D6) of Na[3]
gives rise to a 1:2:1 pattern of resonances at 225.9, 214.8 and
201.2 ppm, respectively, the most downfield of which is assigned
to the FeºCcarbyne unit.[24-26] When Na[3] is prepared using
isotopically-enriched 13CNArMes2, the 225.9 and 214.8 ppm
resonances split into a triplet (2J = 5.0
formation
of
the
red-orange
contact-ion
pair,
Na[FeºCN((ArMes2)(C(O)OSiMe3))(CNArMes2)3] (Na[3]; Scheme 2),
as determined by X-ray crystallographic analysis (Figure 1). Most
importantly, four-coordinate Na[3] is best described as an
arylcarbamate-substituted iron terminal carbyne and formally
results from silylative trapping of a molecule of CO2 on an
isocyano nitrogen atom followed by electronic rearrangement.
Notably, only three iron terminal-carbyne complexes have been
reported previously[24-26] despite their long-standing implications
for homogeneous Fischer-Tropsch-type reactions.[27]
There are several noteworthy structural features of the
terminal carbyne complex Na[3]. The Fe-C bond distance to the
carbamate-carbyne unit is 1.658(10) Å (Fe1-C1) and is indicative
of significant multiple-bond character. Indeed, this Fe-C distance
in Na[3] is similar to that found in the five-coordinate carbyne
complex (SiP3)Fe(COSiMe3) (d(FeºC) = 1.671(2) Å; SiP3 = [Si(o-
(i-Pr)2PC6H4)3),[25] and those found in the four-coordinate
dicarbyne (DPB)Fe(COSiMe3)2 (d(FeºC) = 1.639 Å and 1.676 Å;
DPB = PhB(o-(i-Pr)2PC6H4)2),[26] which were both prepared by
Peters and co-workers. However, the Fe-carbyne distance in
Na[3] is significantly shorter than that of the five-coordinate
terminal Fe aminocarbyne cation, [Fe(CN(i-Pr)2)(CO)3(PPh3)]+
((d(FeºC) = 1.734(6) Å) prepared by Fischer.[24] In the latter
complex, p-donation from the diisopropylamino group to the
carbyne carbon atom, which is reflected by a Ccarbyne-N bond
distance of 1.267(6) Å, was proposed as the electronic origin for
partial eradication of the Fe-C triple bond. Consequently the Fe-
Ccarbyne bond in this cation more closely matched iron terminal
carbene complexes (i.e. LnFe=CR2).[24] Such p-donor interactions
are not present to a significant extent in Peter’s alkoxy-substituted
Fe
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10.1002/anie.201705877
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COMMUNICATION
to the carbyne. Most significantly, the HOMO for [3m]– is Fe dz2
in character and projects into the void trans to the apical
isocyanide ligand. Such an electronic structure arrangement
conforms to the classic trigonal monopyramidal splitting pattern
for metal centers in p-acidic ligand fields, where dz2 is the highest
lying metal-based orbital.[33] In addition, this electronic
environment suggests that the addition of electrophilic substrates
to
a four-coordinate, carbyne anion similar to Na[3], or
alternatively, addition of nucleophilic substrates to cations of the
type [Fe(CR)L3]+ (L = 2e– donor ligand), would likely proceed to
the position cis to the carbyne unit.
Figure 2. DFT calculated molecular orbitals for model [3m]–.
Scheme 3. Proposed mechanisms for the formation of the terminal carbyne
Na[3].
Hz) and doublet of doublets (2J = 5.0 Hz, 2.5 Hz), respectively
(Figure S1.6). This coupling pattern signifies two-bond 13C-13C
coupling between the carbyne carbon and the equatorial
isocyanides, in addition to coupling between the equatorial and
axial CNArMes2 ligands,[29] and suggests strongly that the low-
symmetry observed for Na[3] in the solid state is an inherent
feature of the system.
With respect to the formation of carbyne Na[3], we note that
intermediate species are not observed when the reaction
sequence between Na2[1], CO2 and Me3SiOTf is monitored by 1H
NMR spectroscopy between –50 ˚C and 20 ˚C. To date, we have
been unable to determine whether CO2 is activated and trapped
on the Fe center followed by migration to the isocyanide nitrogen
atom (Scheme 3, Path A), or the alternative process where
activation and silylation occur directly on the isocyanide ligand
(Scheme 3, Path B).[22] We currently favor the mechanism
proceeding by initial Fe–based CO2 activation. This contention is
based on the known Fe-centered reactivity of Na2[1] with protic
sources,[15] and also circumstantial evidence derived from an
apparent size-dependency on silylative CO2-trapping in this
system. For example, treatment of Na2[1] with CO2 in the
presence of PhMe2SiOTf results in the corresponding carbamate-
Given that terminal Fe carbynes are rare, we believe that
the molecular structure of Na[3] reveals an important electronic-
structure analogy with Fischer’s original cationic example,
[Fe(CN(i-Pr)2)(CO)3(PPh3)]+.[24]
Five-coordinate
[Fe(CN(i-
Pr)2)(CO)3(PPh3)]+ adopts a trigonal bipyramidal structure with an
axial phosphine ligand and an equatorial carbyne. Accordingly,
four-coordinate Na[3] can be viewed straightforwardly as an
analogue of [Fe(CN(i-Pr)2)(CO)3(PPh3)]+ stemming from removal
of the axial phosphine followed by a 2e_ reduction of the resulting
coordination site.[30] However, geometric preservation of the
remaining ligands indicates that the FeºC triple bond of terminal
Fe carbynes is configurationally locked in the equatorial plane
when multi-dentate ancillaries are not present to enforce an
altered relative geometry.[25] DFT calculations on the truncated
model complex [FeºCN((Xyl)(C(O)OSiMe3))(CNXyl)3]– ([3m]–; Xyl
= 2,6-Me2C6H3) are consistent with this notion and resulted in an
optimized structure that matches the coordination geometry of
Na[3] (Figure S2.1), despite lacking encumbering substituents
and contact-ion pairing to the Na+ counterion.[31-32] More
importantly, the molecular orbitals calculated for [3m]– conform to
a distinctive trigonal-monopyramidal ligand field with the FeºC
triple bond located in the equatorial plane. As shown in Figure 2,
the HOMO-3 and HOMO-4 calculated for [3m]– represent the p-
bonding framework between the Fe center and the carbyne
carbon and correspond to dxz and dxy metal orbitals. The HOMO-
2 calculated for [3m]– is a p-backbonding interaction between the
dyz orbital and the apical isocyanide ligands, while HOMO-1
corresponds to a dx2-y2 component that is p-backbonding with
respect to the equatorial isocyanide ligands and s* with respect
carbyne
derivative,
Na[FeºCN((ArMes2)(C(O)OSiMe2Ph))(CNArMes2)3] (Na[4]) as the
exclusive product. However, when the more encumbering silane
Ph2MeSiOTf
is
used,
the
carbamate-carbyne,
Na[FeºCN((ArMes2)(C(O)OSiMePh2))(CNArMes2)3]
(Na[5]) is
produced in only 10% yield along with substantial quantities of the
five-coordinate monocarbonyl 2. We interpret this observation as
reflective of the steric profile of Ph2MeSiOTf, which renders it
kinetically inefficient for trapping a mono-CO2 adduct of Na2[1]. It
is most probable that in the absence of a silane trap, this mono-
CO2 adduct reacts with another equivalent of CO2 by the
traditional, metal-centered reductive disproportionation route to
generate monocarbonyl 2.[3-4,7]
Despite the larger steric
properties associated with the silyl groups Na[4] and Na[5], both
complexes exhibit structural features in the solid-state that are
nearly identical to those observed for the SiMe3-derivative, Na[3]
(Figure 3; See the ESI). These findings suggest that the
electronic/geometric structure correlations outlined above for
Na[3] are general for this class of Fe terminal carbynes.
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10.1002/anie.201705877
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COMMUNICATION
the production of CO and [CO3]2– using isolable Fe-C multiply
bound species as intermediates. Studies aimed at exploring this
potential are in progress.
Acknowledgments
We are grateful to the U.S. Department of Energy, Office of
Science, Basic Energy Sciences for support of this work (DE-
SC0008058) and the U.S. National Science Foundation for a
Graduate Research Fellowship to C.C.M. The W. M. Keck
Foundation is gratefully acknowledged for support of computing
resources at the W. M. Keck Laboratory for Integrated Biology II
at UCSD. J.S.F is a Camille Dreyfus Teacher-Scholar (2012-
2017).
Keywords: Multiple Bonds • Carbynes • Iron • Carbon Dioxide •
Isocyanide
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should display metal-centered nucleophilic reactivity, this
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treatment of Na[3] with 2.0 equiv of Na[HBEt3] cleanly
regenerates Na2[1] along with the formation of trimethylsilane
(HSiMe3), as assayed by 1H NMR spectroscopy. Quenching of
this mixture with D2O, followed by 13C{1H} NMR analysis also
indicated the presence of ca. 1.0 equivalent of formate anion
([HCO2]–). We believe this reaction proceeds by hydride attack on
the carbamate SiMe3 group, leading to decarboxylation and re-
formation of Na2[1]. In addition, the liberated CO2 is likely then
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trapped by
a second equivalent of Na[HBEt3] to form a
[25]
[26]
formatoborate.[43] Indeed, the use of 1.0 equiv. of Na[HBEt3] for
this reaction results in a equimolar mixture of Na2[1] and
unreacted Na[3]. While unusual, the nucleophilic desilylation of an
ester-type functionality by hydride is most plausibly enabled by
the electronic saturation of the carbamate carbonyl group by p-
donation from nitrogen, as well as a driving force provided by the
loss of CO2. Nevertheless, this reactivity profile potentially opens
an avenue to redirecting CO2 reductive disproportion away from
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2
The JC-C between the axial and equatorial isocyanides is not
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See Figure S1.6.
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[30]
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COMMUNICATION
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These data can be obtained free of charge from the Cambridge
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[35]
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10.1002/anie.201705877
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COMMUNICATION
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COMMUNICATION
Locked in place: Carbon dioxide
reductive disproportionation by an iron
tetraisocyanide dianion can be
arrested by silylative trapping. This
results in the formation of unique
C. C. Mokhtarzadeh, C. E. Moore, A. L.
Rheingold, J. S. Figueroa *
Page No. – Page No.
Terminal Iron Carbyne Complexes
Derived from Arrested CO2 Reductive
Disproportionation
trigonal
monopyramidal
terminal
carbynes in which the FeºC triple bond
is conformationally locked within the
equatorial plane.
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