7 research outputs found
Effect of Pyramidalization of the M<sub>2</sub>(SR)<sub>2</sub> Center: The Case of (C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>Ni<sub>2</sub>(SR)<sub>2</sub>
The effects of simple thiolates vs
chelating dithiolates on MāM
bonding, redox potentials, and synthetic outcomes have been probed
experimentally and computationally. Nickelocene (Cp<sub>2</sub>Ni)
has long been known to react with simple thiols to give diamagnetic
Cp<sub>2</sub>Ni<sub>2</sub>(SR)<sub>2</sub> with planar Ni<sub>2</sub>S<sub>2</sub> cores and long Ni-āÆ-āÆ-Ni distances. Ethane-
and propanedithiol (edtH<sub>2</sub> and pdtH<sub>2</sub>, respectively)
instead give di-, tri-, and pentanickel complexes, with nonplanar
Ni<sub>2</sub>S<sub>2</sub> cores. The 36e Cp<sub>2</sub>Ni<sub>2</sub>(pdt) (<b>1</b><sup><b>pdt</b></sup>) adopts a symmetrical
butterfly Ni<sub>2</sub>S<sub>2</sub> structure. Variable-temperature
NMR spectra indicate that <b>1</b><sup><b>pdt</b></sup> possesses a thermally accessible triplet state (Ī<i>G</i> = 2.65(5) kcal/mol) in equilibrium with a diamagnetic ground state.
DFT calculations indicate that the singletātriplet gap is highly
sensitive to the nonplanarity of the Ni<sub>2</sub>S<sub>2</sub> core.
The calculations further reveal that only the high-spin form of <b>1</b><sup><b>pdt</b></sup> features NiāNi bonding,
which is unprecedented. Cp<sub>2</sub>Ni<sub>3</sub>(pdt)<sub>2</sub> (<b>2</b><sup><b>pdt</b></sup>), which derives from <b>1</b><sup><b>pdt</b></sup>, crystallized as cis and trans
isomers, both with a central NiĀ(pdt)<sub>2</sub><sup>2ā</sup> unit that is S,S-chelated to two CpNi<sup>+</sup> centers. Reaction
of Cp<sub>2</sub>Ni with 1,2-ethanedithiol (H<sub>2</sub>edt) and
1,2-benzenedithiol (bdtH<sub>2</sub>) exclusively gave the trinickel
species <b>2</b><sup><b>edt</b></sup> and <b>2</b><sup><b>bdt</b></sup>, which are structurally analogous to <i>cis</i>-<b>2</b><sup><b>pdt</b></sup>. Solutions
of <b>2</b><sup><b>edt</b></sup> are unstable, depositing
crystals of Cp<sub>2</sub>Ni<sub>5</sub>(edt)<sub>4</sub> (<b>3</b><sup><b>edt</b></sup>). Cyclic voltammetric studies show that
the Ni<sub>2</sub> species oxidize readily to give the mixed-valence
cations [<b>1</b><sup><b>pdt</b></sup>]<sup>+</sup> and
[<b>1</b><sup><b>edt</b></sup>]<sup>+</sup>. Crystallographic
and EPR analyses indicate that these cations are delocalized mixed-valence
NiĀ(II)āNiĀ(III) species. Oxidation of Cp<sub>2</sub>Ni<sub>2</sub>(SEt)<sub>2</sub>, which features a planar Ni<sub>2</sub>S<sub>2</sub> core, afforded a mixed-valence cation, showing the pyramidal Ni<sub>2</sub>S<sub>2</sub> core observed in [<b>1</b><sup><b>pdt</b></sup>]<sup>0/+</sup> and [<b>1</b><sup><b>edt</b></sup>]<sup>0/+</sup>. Although not obtained from the Cp<sub>2</sub>Ni/H<sub>2</sub>edt reaction, the neutral complex <b>1</b><sup><b>edt</b></sup> was obtained by reduction of [<b>1</b><sup><b>edt</b></sup>]<sup>+</sup>. Variable-temperature NMR measurements
and DFT calculations indicate that the triplet is further stabilized
in this highly pyramidalized species
DFT Dissection of the Reduction Step in H<sub>2</sub> Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?
Density
functional theory has been used to study diiron dithiolates
[HFe<sub>2</sub>(xdt)Ā(PR<sub>3</sub>)<sub><i>n</i></sub>(CO)<sub>5ā<i>n</i></sub>X] (<i>n</i> =
0, 2, 4; R = H, Me, Et; X = CH<sub>3</sub>S<sup>ā</sup>, PMe<sub>3</sub>, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt
= azadithiolate; pdt = propanedithiolate). These species are related
to the [FeFe]-hydrogenases catalyzing the 2H<sup>+</sup> + 2e<sup>ā</sup> ā H<sub>2</sub> reaction. Our study is focused
on the reduction step following protonation of the Fe<sub>2</sub>(SR)<sub>2</sub> core. FeĀ(H)Ās detected in solution are terminal (t-H) and
bridging (Ī¼-H) hydrides. Although unstable versus Ī¼-Hs,
synthetic t-Hs feature milder reduction potentials than Ī¼-Hs.
Accordingly, attempts were previously made to hinder the isomerization
of t-H to Ī¼-H. Herein, we present another strategy: in place
of preventing isomerization, Ī¼-H could be made a stronger oxidant
than t-H (<i>E</i>Ā°<sub>Ī¼āH</sub> > <i>E</i>Ā°<sub>tāH</sub>). The nature and number of
PR<sub>3</sub> unusually affect Ī<i>E</i>Ā°<sub>tāHāĪ¼āH</sub>: 4PEt<sub>3</sub> models
feature a Ī¼-H with a milder <i>E</i>Ā° than t-H,
whereas the 4PMe<sub>3</sub> analogues behave oppositely. The correlation
Ī<i>E</i>Ā°<sub>tāHāĪ¼āH</sub> ā stereoelectronic features arises from the steric strain
induced by bulky Et groups in 4PEt<sub>3</sub> derivatives. One-electron
reduction alleviates intramolecular repulsions only in Ī¼-H species,
which is reflected in the loss of bridging coordination. Conversely,
in t-H, the strain is retained because a bridging CO holds together
the Fe<sub>2</sub> core. That implies that <i>E</i>Ā°<sub>Ī¼āH</sub> > <i>E</i>Ā°<sub>tāH</sub> in 4-PEt<sub>3</sub> species but not in 4PMe<sub>3</sub> analogues.
Also determinant to observe <i>E</i>Ā°<sub>Ī¼āH</sub> > <i>E</i>Ā°<sub>tāH</sub> is the presence
of a Fe apical Ļ-donor because its replacement with a CO yields <i>E</i>Ā°<sub>Ī¼āH</sub> < <i>E</i>Ā°<sub>tāH</sub> even in 4PEt<sub>3</sub> species. Variants
with neutral NHC and PMe<sub>3</sub> in place of CH<sub>3</sub>S<sup>ā</sup> still feature <i>E</i>Ā°<sub>Ī¼āH</sub> > <i>E</i>Ā°<sub>tāH</sub>. Replacing pdt
with
(Hadt)<sup>+</sup> lowers <i>E</i>Ā° but yields <i>E</i>Ā°<sub>Ī¼āH</sub> < <i>E</i>Ā°<sub>tāH</sub>, indicating that Ī¼-H activation
can occur to the detriment of the overpotential increase. In conclusion,
our results indicate that the electron richness of the Fe<sub>2</sub> core influences Ī<i>E</i>Ā°<sub>tāHāĪ¼āH</sub>, provided that (i) the R size of PR<sub>3</sub> must be greater
than that of Me and (ii) an electron donor must be bound to Fe apically
Electron-Rich, Diiron Bis(monothiolato) Carbonyls: CāS Bond Homolysis in a Mixed Valence Diiron Dithiolate
The synthesis and
redox properties are presented for the electron-rich bisĀ(monothiolate)Ās
Fe<sub>2</sub>(SR)<sub>2</sub>Ā(CO)<sub>2</sub>Ā(dppv)<sub>2</sub> for R = Me ([<b>1</b>]<sup>0</sup>), Ph ([<b>2</b>]<sup>0</sup>), CH<sub>2</sub>Ph ([<b>3</b>]<sup>0</sup>).
Whereas related derivatives adopt <i>C</i><sub>2</sub>-symmetric
Fe<sub>2</sub>(CO)<sub>2</sub>P<sub>4</sub> cores, [<b>1</b>]<sup>0</sup>ā[<b>3</b>]<sup>0</sup> have <i>C</i><sub>s</sub> symmetry resulting from the unsymmetrical steric properties
of the axial vs equatorial R groups. Complexes [<b>1</b>]<sup>0</sup>ā[<b>3</b>]<sup>0</sup> undergo 1e<sup>ā</sup> oxidation upon treatment with ferrocenium salts to give the mixed
valence cations [Fe<sub>2</sub>(SR)<sub>2</sub>Ā(CO)<sub>2</sub>Ā(dppv)<sub>2</sub>]<sup>+</sup>. As established crystallographically,
[<b>3</b>]<sup>+</sup> adopts a rotated structure, characteristic
of related mixed valence diiron complexes. Unlike [<b>1</b>]<sup>+</sup> and [<b>2</b>]<sup>+</sup> and many other [Fe<sub>2</sub>Ā(SR)<sub>2</sub>L<sub>6</sub>]<sup>+</sup> derivatives, [<b>3</b>]<sup>+</sup> undergoes CāS bond homolysis, affording
the diferrous sulfido-thiolate [Fe<sub>2</sub>Ā(SCH<sub>2</sub>Ph)Ā(S)Ā(CO)<sub>2</sub>Ā(dppv)<sub>2</sub>]<sup>+</sup> ([<b>4</b>]<sup>+</sup>). According to X-ray crystallography,
the first coordination spheres of [<b>3</b>]<sup>+</sup> and
[<b>4</b>]<sup>+</sup> are similar, but the Feāsulfido
bonds are short in [<b>4</b>]<sup>+</sup>. The conversion of
[<b>3</b>]<sup>+</sup> to [<b>4</b>]<sup>+</sup> follows
first-order kinetics, with <i>k</i> = 2.3 Ć 10<sup>ā6</sup> s<sup>ā1</sup> (30 Ā°C). When the conversion
is conducted in THF, the organic products are toluene and dibenzyl.
In the presence of TEMPO, the conversion of [<b>3</b>]<sup>+</sup> to [<b>4</b>]<sup>+</sup> is accelerated about 10Ć, the
main organic product being TEMPO-CH<sub>2</sub>Ph. DFT calculations
predict that the homolysis of a CāS bond is exergonic for [Fe<sub>2</sub>Ā(SCH<sub>2</sub>Ph)<sub>2</sub>Ā(CO)<sub>2</sub>Ā(PR<sub>3</sub>)<sub>4</sub>]<sup>+</sup> but endergonic for
the neutral complex as well as less substituted cations. The unsaturated
character of [<b>4</b>]<sup>+</sup> is indicated by its double
carbonylation to give [Fe<sub>2</sub>Ā(SCH<sub>2</sub>Ph)Ā(S)Ā(CO)<sub>4</sub>Ā(dppv)<sub>2</sub>]<sup>+</sup> ([<b>5</b>]<sup>+</sup>), which adopts a bioctahedral structure
Preparation and Protonation of Fe<sub>2</sub>(pdt)(CNR)<sub>6</sub>, Electron-Rich Analogues of Fe<sub>2</sub>(pdt)(CO)<sub>6</sub>
The complexes Fe<sub>2</sub>(pdt)Ā(CNR)<sub>6</sub> (pdt<sup>2ā</sup> = CH<sub>2</sub>(CH<sub>2</sub>S<sup>ā</sup>)<sub>2</sub>) were prepared by thermal substitution
of the hexacarbonyl complex with the isocyanides RNC for R = C<sub>6</sub>H<sub>4</sub>-4-OMe (<b>1</b>), C<sub>6</sub>H<sub>4</sub>-4-Cl (<b>2</b>), Me (<b>3</b>). These complexes represent
electron-rich analogues of the parent Fe<sub>2</sub>(pdt)Ā(CO)<sub>6</sub>. Unlike most substituted derivatives of Fe<sub>2</sub>(pdt)Ā(CO)<sub>6</sub>, these isocyanide complexes are sterically unencumbered and
have the same idealized symmetry as the parent hexacarbonyl derivatives.
Like the hexacarbonyls, the stereodynamics of <b>1</b>ā<b>3</b> involve both turnstile rotation of the FeĀ(CNR)<sub>3</sub> as well as the inversion of the chair conformation of the pdt ligand.
Structural studies indicate that the basal isocyanide has nonlinear
CNC bonds and short FeāC distances, indicating that they engage
in stronger FeāC Ļ-backbonding than the apical ligands.
Cyclic voltammetry reveals that these new complexes are far more reducing
than the hexacarbonyls, although the redox behavior is complex. Estimated
reduction potentials are <i>E</i><sub>1/2</sub> ā
ā0.6 ([<b>2</b>]<sup>+/0</sup>), ā0.7 ([<b>1</b>]<sup>+/0</sup>), and ā1.25 ([<b>3</b>]<sup>+/0</sup>). According to DFT calculations, the rotated isomer of <b>3</b> is only 2.2 kcal/mol higher in energy than the crystallographically
observed unrotated structure. The effects of rotated versus unrotated
structure and of solvent coordination (THF, MeCN) on redox potentials
were assessed computationally. These factors shift the redox couple
by as much as 0.25 V, usually less. Compounds <b>1</b> and <b>2</b> protonate with strong acids to give the expected Ī¼-hydrides
[H<b>1</b>]<sup>+</sup> and [H<b>2</b>]<sup>+</sup>. In
contrast, <b>3</b> protonates with [HNEt<sub>3</sub>]ĀBAr<sup>F</sup><sub>4</sub> (p<i>K</i><sub>a</sub><sup>MeCN</sup> = 18.7) to give the aminocarbyne [Fe<sub>2</sub>(pdt)Ā(CNMe)<sub>5</sub>Ā(Ī¼-CNĀ(H)ĀMe)]<sup>+</sup> ([<b>3</b>H]<sup>+</sup>). According to NMR measurements and DFT calculations, this
species adopts an unsymmetrical, rotated structure. DFT calculations
further indicate that the previously described carbyne complex [Fe<sub>2</sub>(SMe)<sub>2</sub>(CO)<sub>3</sub>Ā(PMe<sub>3</sub>)<sub>2</sub>(CCF<sub>3</sub>)]<sup>+</sup> also adopts a rotated structure
with a bridging carbyne ligand. Complex [<b>3</b>H]<sup>+</sup> reversibly adds MeNC to give [Fe<sub>2</sub>(pdt)Ā(CNR)<sub>6</sub>(Ī¼-CNĀ(H)ĀMe)]<sup>+</sup> ([<b>3</b>HĀ(CNMe)]<sup>+</sup>). Near room temperature, [<b>3</b>H]<sup>+</sup> isomerizes
to the hydride [(Ī¼-H)ĀFe<sub>2</sub>Ā(pdt)Ā(CNMe)<sub>6</sub>]<sup>+</sup> ([H<b>3</b>]<sup>+</sup>) via a first-order pathway
Preparation and Protonation of Fe<sub>2</sub>(pdt)(CNR)<sub>6</sub>, Electron-Rich Analogues of Fe<sub>2</sub>(pdt)(CO)<sub>6</sub>
The complexes Fe<sub>2</sub>(pdt)Ā(CNR)<sub>6</sub> (pdt<sup>2ā</sup> = CH<sub>2</sub>(CH<sub>2</sub>S<sup>ā</sup>)<sub>2</sub>) were prepared by thermal substitution
of the hexacarbonyl complex with the isocyanides RNC for R = C<sub>6</sub>H<sub>4</sub>-4-OMe (<b>1</b>), C<sub>6</sub>H<sub>4</sub>-4-Cl (<b>2</b>), Me (<b>3</b>). These complexes represent
electron-rich analogues of the parent Fe<sub>2</sub>(pdt)Ā(CO)<sub>6</sub>. Unlike most substituted derivatives of Fe<sub>2</sub>(pdt)Ā(CO)<sub>6</sub>, these isocyanide complexes are sterically unencumbered and
have the same idealized symmetry as the parent hexacarbonyl derivatives.
Like the hexacarbonyls, the stereodynamics of <b>1</b>ā<b>3</b> involve both turnstile rotation of the FeĀ(CNR)<sub>3</sub> as well as the inversion of the chair conformation of the pdt ligand.
Structural studies indicate that the basal isocyanide has nonlinear
CNC bonds and short FeāC distances, indicating that they engage
in stronger FeāC Ļ-backbonding than the apical ligands.
Cyclic voltammetry reveals that these new complexes are far more reducing
than the hexacarbonyls, although the redox behavior is complex. Estimated
reduction potentials are <i>E</i><sub>1/2</sub> ā
ā0.6 ([<b>2</b>]<sup>+/0</sup>), ā0.7 ([<b>1</b>]<sup>+/0</sup>), and ā1.25 ([<b>3</b>]<sup>+/0</sup>). According to DFT calculations, the rotated isomer of <b>3</b> is only 2.2 kcal/mol higher in energy than the crystallographically
observed unrotated structure. The effects of rotated versus unrotated
structure and of solvent coordination (THF, MeCN) on redox potentials
were assessed computationally. These factors shift the redox couple
by as much as 0.25 V, usually less. Compounds <b>1</b> and <b>2</b> protonate with strong acids to give the expected Ī¼-hydrides
[H<b>1</b>]<sup>+</sup> and [H<b>2</b>]<sup>+</sup>. In
contrast, <b>3</b> protonates with [HNEt<sub>3</sub>]ĀBAr<sup>F</sup><sub>4</sub> (p<i>K</i><sub>a</sub><sup>MeCN</sup> = 18.7) to give the aminocarbyne [Fe<sub>2</sub>(pdt)Ā(CNMe)<sub>5</sub>Ā(Ī¼-CNĀ(H)ĀMe)]<sup>+</sup> ([<b>3</b>H]<sup>+</sup>). According to NMR measurements and DFT calculations, this
species adopts an unsymmetrical, rotated structure. DFT calculations
further indicate that the previously described carbyne complex [Fe<sub>2</sub>(SMe)<sub>2</sub>(CO)<sub>3</sub>Ā(PMe<sub>3</sub>)<sub>2</sub>(CCF<sub>3</sub>)]<sup>+</sup> also adopts a rotated structure
with a bridging carbyne ligand. Complex [<b>3</b>H]<sup>+</sup> reversibly adds MeNC to give [Fe<sub>2</sub>(pdt)Ā(CNR)<sub>6</sub>(Ī¼-CNĀ(H)ĀMe)]<sup>+</sup> ([<b>3</b>HĀ(CNMe)]<sup>+</sup>). Near room temperature, [<b>3</b>H]<sup>+</sup> isomerizes
to the hydride [(Ī¼-H)ĀFe<sub>2</sub>Ā(pdt)Ā(CNMe)<sub>6</sub>]<sup>+</sup> ([H<b>3</b>]<sup>+</sup>) via a first-order pathway
Mechanistic Insight into Electrocatalytic H<sub>2</sub> Production by [Fe<sub>2</sub>(CN){Ī¼-CN(Me)<sub>2</sub>}(Ī¼-CO)(CO)(Cp)<sub>2</sub>]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models
DFT has been used
to investigate viable mechanisms of the hydrogen evolution reaction
(HER) electrocatalyzed by [Fe<sub>2</sub>(CN)Ā{Ī¼-CNĀ(Me)<sub>2</sub>}Ā(Ī¼-CO)Ā(CO)Ā(Cp)<sub>2</sub>] (<b>1</b>) in AcOH. Molecular
details underlying the proposed ECEC electrochemical sequence have
been studied, and the key functionalities of CN<sup>ā</sup> and amino-carbyne ligands have been elucidated. After the first
reduction, CN<sup>ā</sup> works as a relay for the first proton
from AcOH to the carbyne, with this ligand serving as the main electron
acceptor for both reduction steps. After the second reduction, a second
protonation occurs at CN<sup>ā</sup> that forms a FeĀ(CNH) moiety:
i.e., the acidic source for the H<sub>2</sub> generation. The hydride
(formally 2e/H<sup>+</sup>), necessary to the heterocoupling with
H<sup>+</sup> is thus provided by the Ī¼-CNĀ(Me)<sub>2</sub> ligand
and not by Fe centers, as occurs in typical L<sub>6</sub>Fe<sub>2</sub>S<sub>2</sub> derivatives modeling the hydrogenase active site. It
is remarkable, in this regard, that CN<sup>ā</sup> plays a
role more subtle than that previously expected (increasing electron
density at Fe atoms). In addition, the role of AcOH in shuttling protons
from CN<sup>ā</sup> to CNĀ(Me)<sub>2</sub> is highlighted. The
incompetence for the HER of the related species [Fe<sub>2</sub>{Ī¼-CNĀ(Me)<sub>2</sub>}Ā(Ī¼-CO)Ā(CO)<sub>2</sub>(Cp)<sub>2</sub>]<sup>+</sup> (<b>2</b><sup><b>+</b></sup>) has been investigated
and attributed to the loss of proton responsiveness caused by CN<sup>ā</sup> replacement with CO. In the context of hydrogenase
mimicry, an implication of this study is that the dithiolate strap,
normally present in all synthetic models, can be removed from the
Fe<sub>2</sub> core without loss of HER, but the redox and acidābase
processes underlying turnover switch from a metal-based to a ligand-based
chemistry. The versatile nature of the carbyne, once incorporated
in the Fe<sub>2</sub> scaffold, could be exploited to develop more
active and robust catalysts for the HER
Imine-Centered Reactions in Imino-Phosphine Complexes of Iron Carbonyls
Fundamental reactions of imino-phosphine
ligands were elucidated
through studies on Ph<sub>2</sub>PC<sub>6</sub>H<sub>4</sub>CHī»NC<sub>6</sub>H<sub>4</sub>-4-Cl (PCHNAr<sup>Cl</sup>) complexes of iron(0),
ironĀ(I), and ironĀ(II). The reaction of PCHNAr<sup>Cl</sup> with FeĀ(bda)Ā(CO)<sub>3</sub> gives FeĀ(PCHNAr<sup>Cl</sup>)Ā(CO)<sub>3</sub> (<b>1</b>), featuring an Ī·<sup>2</sup>-imine. DNMR studies, its optical
properties, and DFT calculations suggest that <b>1</b> racemizes
on the NMR time scale via an achiral N-bonded imine intermediate.
The <i>N</i>-imine isomer is more stable in FeĀ(PCHNAr<sup>OMe</sup>)Ā(CO)<sub>3</sub> (<b>1</b><sup><b>OMe</b></sup>), which crystallized despite being the minor isomer in solution.
Protonation of <b>1</b> by HBF<sub>4</sub>Ā·Et<sub>2</sub>O gave the iminium complex [<b>1</b>H]ĀBF<sub>4</sub>. The related
diphosphine complex FeĀ(PCHNAr<sup>Cl</sup>)Ā(PMe<sub>3</sub>)Ā(CO)<sub>2</sub> (<b>2</b>), which features an Ī·<sup>2</sup>-imine,
was shown to also undergo N protonation. Oxidation of <b>1</b> and <b>2</b> with FcBF<sub>4</sub> gave the FeĀ(I) compounds
[<b>1</b>]ĀBF<sub>4</sub> and [<b>2</b>]ĀBF<sub>4</sub>.
The oxidation-induced change in hapticity of the imine from Ī·<sup>2</sup> in [<b>1</b>]<sup>0</sup> to Īŗ<sup>1</sup> in
[<b>1</b>]<sup>+</sup> was verified crystallographically. Substitution
of a CO ligand in <b>1</b> with PCHNAr<sup>Cl</sup> gave FeĀ[P<sub>2</sub>(NAr<sup>Cl</sup>)<sub>2</sub>]Ā(CO)<sub>2</sub> (<b>3</b>), which contains the tetradentate diamidodiphosphine ligand. This
CāC coupling is reversed by chemical oxidation of <b>3</b> with FcOTf. The oxidized product of [FeĀ(PCHNAr<sup>Cl</sup>)<sub>2</sub>(CO)<sub>2</sub>]<sup>2+</sup> ([<b>4</b>]<sup>2+</sup>) was prepared independently by the reaction of [<b>1</b>]<sup>+</sup>, PCHNAr<sup>Cl</sup>, and Fc<sup>+</sup>. The CāC
scission is proposed to proceed concomitantly with the reduction of
FeĀ(II) via an intermediate related to [<b>2</b>]<sup>+</sup>