Heterometallic Cu2Fe and Zn2Fe2Complexes Derived from [Fe(CO)4]2-and Cu/Fe Bifunctional N2O Activation Reactivity

Article abstract of DOI:10.1021/acs.organomet.0c00212

The synthesis and characterization of two heterometallic clusters, [(IPr)Cu]2Fe(CO)4 (1) and [(IPr)ZnFe(CO)4]2 (2), derived from Collman's reagent are reported. Methylation of 1 with CH3I to produce (IPr)CuI and (IPr)CuFe(Me)(CO)4 (3) was used to calibrate the relative reactivity of 1 to the previously studied analogue (IPr)Cu-FeCp(CO)2. Bifunctional N2O activation by 1 resulted in oxidation of a CO ligand to carbonate rather than the more typically observed CO2, producing [(IPr)Cu]2(μ-CO3) (4) stoichiometrically along with an iron byproduct that was trapped with PPh3 as trans-Fe(CO)3(PPh3)2.

Full text of DOI:10.1021/acs.organomet.0c00212

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Heterometallic Cu2Fe and Zn2Fe2 Complexes Derived from
[Fe(CO)4]2− and Cu/Fe Bifunctional N2O Activation Reactivity
Yutthana Lakliang and Neal P. Mankad*
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ABSTRACT: The synthesis and characterization of two hetero-
metallic clusters, [(IPr)Cu]2Fe(CO)4 (1) and [(IPr)ZnFe(CO)4]2
(2), derived from Collman’s reagent are reported. Methylation of 1
with CH3I to produce (IPr)CuI and (IPr)CuFe(Me)(CO)4 (3) was
used to calibrate the relative reactivity of 1 to the previously studied
analogue (IPr)Cu-FeCp(CO)2. Bifunctional N2O activation by 1
resulted in oxidation of a CO ligand to carbonate rather than the
more typically observed CO2, producing [(IPr)Cu]2(μ-CO3) (4)
stoichiometrically along with an iron byproduct that was trapped
with PPh3 as trans-Fe(CO)3(PPh3)2.
he ongoing effect of climate change motivates the pursuit
reduction selectively was accomplished with (NHC)Cu-[M]
catalysts (M = Fe, W, Mo) when they were activated by a H-
[B] reductant.24 However, we still have not succeeded in
bimetallic CO2 activation using this approach, whereas “early−
late” heterobinuclear catalysts that do activate CO2 using early-
transition-metal Lewis acids (e.g., ZrIV) are not catalytically
active due to the formation of strong metal−oxygen
bonds.7,25,26
In this context, we hypothesized that the heterobimetallic
catalysts might have higher reactivity if the metal−metal bond
polarity were further enhanced by replacing monoanionic
[Mco]− with a dianionic metal−carbonyl nucleophile. Coll-
man’s reagent, Na2Fe(CO)4, is a well-known compound for
organic and organometallic synthesis.27 Because Collman’s
reagent features a highly reduced iron formally in its Fe(2−)
oxidation state, we hypothesized that its incorporation into
heterobimetallic architectures could increase Cu−Fe bond
polarity and perhaps enhance reactivity in comparison to
(NHC)Cu-Fp derived from [Fp]− with a formally Fe(0)
center. Herein, we report the synthesis and structural
characterization of two new heterometallic complexes,
trinuclear [(IPr)Cu]2Fe(CO)4 (1) and tetranuclear [(IPr)-
ZnFe(CO)4]2 (2), and the small-molecule activation chemistry
of the Cu derivative which can be compared directly to that of
its (IPr)CuFp predecesor (IPr = N,N′-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene). Related complexes
T
of technologies that capture1 greenhouse gases and
convert them into useful chemicals or fuels.2 Achieving such
transformations of gaseous small molecules requires catalysts
that mediate their efficient activation in order to overcome
their thermodynamic stability and/or kinetic inertness.3 With
biological systems as an inspiration, one strategy is to activate
inert small molecules using a bifunctional approach, wherein
Lewis acidic and basic transition-metal sites cooperate to
achieve bond activation. For example, the Ni/Fe cluster of
anaerobic carbon monoxide dehydrogenase has been structur-
ally characterized with a bound CO2 molecule bifunctionally
activated by nucleophilic attack at carbon and Lewis acidic
stabilization of negative charge accumulation at oxygen.4
Similarly, N2O can be converted to N2 when it is activated
by adjacent Cu sites that act in a bifunctional manner, as seen
in the CuZ cluster of nitrous oxide reductase or the binuclear
active sites of Cu-ZSM-5.5,6 Mimicking this cooperative
strategy for small-molecule activation is currently an active
area of research.7−16
Previously, our group reported the various heterobimetallic
catalyst systems (NHC)M-[Mco] (NHC = N-heterocyclic
carbene; M = Cu, Ag, Au, ZnCl; [Mco] = e.g. FeCp(CO)2,
Mn(CO)5, WCp(CO)3, Co(CO)4, etc.), which assemble upon
mixing (NHC)MCl and [Mco]− precursors.17−19 These metal−
metal bonds feature polarity that we established through
computational methods,19,20 chemical reactivity,17,19,21 and
spectroscopic analysis.22 For example, we established that the
Cu−Fe bond in (NHC)Cu-Fp (Fp = FeCp(CO2)) is
polarized: Cu retains positively charged (electrophilic)
character, whereas Fe retains negatively charged (nucleophilic)
character. This bifunctionality enables cooperative activation of
small molecules such as H2, CS2, and N2O under mild
conditions.20,21,23 Additionally, catalyst control over CO2
Received: March 27, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00212
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such as [(Ph3P)2Cu]2Fe(CO)4,28 [(PPh3)Au2Fe(CO)4],29 and
[(py)3ZnFe(CO)4]30 have been characterized before, but
reactivity studies were not performed.
Previously, we reported the syntheses of (IPr)CuFp and
(IPr)(Cl)ZnFp by reactions between (IPr)CuCl and (IPr)-
ZnCl2(THF) synthons and KFp.17 Analogous preparations
were used to synthesize 1 and 2 from K2Fe(CO)4. The solid-
state structure of 1 determined by X-ray crystallography (one
of four molecules from the asymmetric unit) is shown in
Figure 1a. Unlike [(Ph3P)Cu]2Fe(CO)4, which features a trans
2.373(7) Å; Cu(1)···C(53), 2.442(6) Å; Cu(2)···C(54),
2.47(1) Å; Cu(2)···C(53), 2.404(8) Å. The corresponding
asymmetry parameters33 indicate that all are in the semi-
bridging regime.34 The C(53)−O(2) unit can be considered a
semibridging μ3-CO between Fe(1) and both Cu(1) and
Cu(2); the C(53)−O(2) distance of 1.25(1) Å indicates
significant CO activation. The C(54)−O(3) and C(55)−O(4)
units can be considered semibridging μ2-CO ligands between
Fe(1) and Cu(2) or Cu(1), respectively; the corresponding
C−O distances of 1.194(8) and 1.183(9) Å, respectively,
indicate weaker activation. The fourth C(52)−O(1) unit is a
terminal carbonyl group with a correspondingly short C−O
distance of 1.15(1) Å. The solid-state IR spectrum of 1 has a
feature at 1952 cm−1 assigned to the terminal CO ligand,
features at 1874 and 1833 cm−1 assigned to the semibridging
μ2-CO ligands, and a feature at 1814 cm−1 assigned to the
more activated μ3-CO semibridging carbonyl. The 13C{1H}
NMR spectrum exhibits a single CO resonance at 216.5 ppm,
indicating that the semibridging interactions are fluxional such
that the four CO sites exchange with each other in solution on
the NMR time scale.
The solid-state structure of 2 is shown in Figure 1b. Unlike
coordinatively saturated, binuclear L3ZnFe(CO)4 complexes,30
the unsaturated “(IPr)ZnFe(CO)4” species in 2 apparently
dimerizes to form tetranuclear [(IPr)ZnFe(CO)4]2. The Zn−
Fe distances in 2 of 2.5221(7) and 2.5293(8) Å are much
longer than that in (IPr)(Cl)Zn−FeCp(CO)2 (2.3714(4)
Å).17 To compare the M−Fe distances, Cotton’s formal
shortness ratio (FSR) calculations can be used to correct for
metal sizes.35 The average FSR value for Cu−Fe in 1 (1.004) is
smaller than that for Zn−Fe in 2 (1.048), which might be due
to the differences in Pauling’s atomic radii for Cu (1.173 Å)
and Zn (1.249 Å).36 The Fe centers in 2 have nearly ideal
octahedral environments, possibly due to the lack of any close
Zn···CO contacts to create structural distortions as in 1. The
cross-cluster Fe···Fe (4.196(1) Å) and Zn···Zn (2.8124(5) Å)
distances are too long for any significant interaction.
We have begun to benchmark the reactivity of these new
clusters using reactions for which 1 can be compared directly
to its (IPr)CuFp analogue. We initially established the Cu−Fe
bond polarity of (IPr)CuFp through its reactivity with methyl
iodide.17,22 Similarly, here complex 1 was exposed to equimolar
CH3I. An analysis of the product mixture by 1H NMR
indicated quantitative formation of (IPr)CuI and a second
species assigned as (IPr)CuFe(Me)(CO)4 (3) (Scheme 1). No
evidence for (IPr)CuCH3 or I2Fe(CO)4 was detected,
consistent with the proposed Cu−Fe bond polarity of formally
Cu(I) Lewis acidic and Fe(2−) Lewis basic sites. Exposure of
the product mixture to additional CH3I did not result in any
further conversion of 3 to (IPr)CuI. Because we initially
Figure 1. Solid-state structures of (a) [(IPr)Cu]2Fe(CO)4 (1) and
(b) [(IPr)ZnFe(CO)4]2 (2) determined by X-ray crystallography. For
clarity, hydrogen atoms and cocrystallized solvent molecules are
omitted and NHC ligands are shown as wireframes. All other atoms
are shown as 50% probability ellipsoids. For 1, only one of the four
molecules from the asymmetric unit is shown.
orientation of Cu groups (168.7(2)°) about an octahedral Fe
center,28 complex 1 features a bent Cu−Fe−Cu angle
(106.43(5)°) and modestly bent CNHC−Cu−Fe angles
(168.1(2) and 172.1(2)°). The Cu−Fe distances (2.348(2)
̈
and 2.375(1) Å) are close to the sum of Pyykko’s single-bond
covalent radii of Cu and Fe (2.28 Å)31 and are among the
shortest Cu−Fe distances of any type in Cambridge Structural
Database (CSD). The Cu−Fe distances of 1 are similar to that
of (IPr)Cu−FeCp(CO)2 (2.3462(5) Å) and slightly longer
than the shortest known Cu−Fe distance found in (IPr)Cu−
FeCp(CO)(PPh2Me) (2.299(2) Å), which presumably has
enhanced dative Fe→Cu donation due to the electron-
donating phosphine ligand.32 The median Cu−Fe distance in
the CSD, 2.549 Å, is much longer than the Cu−Fe distances in
1. The Cu···Cu distance of 3.783(1) Å is too long for a
significant interaction. The [Fe(CO)4] unit contained within 1
has a near-perfect tetrahedral arrangement.
Scheme 1. Methyl Iodide Reactivity of 1
The solid-state structure of 1 contains four independent
molecules in the asymmetric unit; only one molecule is
discussed here but is representative of the isostructural set.
Four short Cu···CO distances are evident: Cu(1)···C(55),
B
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hypothesized that 1 would have more reactive Cu−Fe bonds
than (IPr)CuFp, we sought to measure rate constants under
pseudo-first-order conditions (excess CH3I), using 1H NMR to
monitor the appearance of Fe−CH3 resonances. The observed
rate constants at room temperature for 1 and (IPr)CuFp were
0.028 and 0.041 s−1, respectively. In other words, in contrast to
our initial hypothesis, the two complexes are about equally
reactive toward CH3I within error. This result highlights the
limitations of formal oxidation states in predicting reactivity
behavior.37
We previously reported that exposure of (NHC)CuFp
derivatives to N2O results in Cp− migration from Fe to Cu,
producing (NHC)CuCp products quantitatively with precip-
itation of an amorphous FexCyOz material.21 Thus, we were
curious to examine the behavior toward N2O of complex 1,
which lacks a Cp− group. Stirring a solution of 1 in toluene
under N2O (1 atm) at room temperature caused rapid
precipitation of a dark brown material. Analysis of the soluble
fraction by 1H NMR spectroscopy indicated the formation of a
single product, which was determined to be the carbonate
complex [(IPr)Cu]2(μ-CO3) (4) by X-ray crystallography. The
solid-state structure of 4 (Figure 2) shows a bridging μ-
No reaction between 1 and CO2 (1 atm) in C6D6 at room
temperature was observed from 1H NMR spectral data. Use of
isotopically enriched 13CO2 further indicated that no
equilibration was occurring between carbon dioxide and the
carbonyl groups, as has been observed in some early−late
heterobimetallic systems.25 Further work on activation of CO2
and other small molecules by 1 and 2 is ongoing in our
laboratories.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00212.
■
sı
Experimental procedures and spectral data (PDF)
Accession Codes
CCDC 1985380−1985382 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.
AUTHOR INFORMATION
Corresponding Author
■
Neal P. Mankad − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States;
orcid.org/0000-0001-6923-5164; Email: npm@uic.edu
Author
Yutthana Lakliang − Department of Chemistry, University of
Illinois at Chicago, Chicago, Illinois 60607, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00212
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Funding for this study was provided by the NSF (CHE-
1664632).
■
Figure 2. Stoichiometry of N2O activation by 1 (top) and solid-state
structure of [(IPr)Cu]2(μ-CO3) (4; bottom).
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D
https://dx.doi.org/10.1021/acs.organomet.0c00212
Organometallics XXXX, XXX, XXX−XXX