Understanding the CO Dissociation in [Fe(CN)2(CO)2(dithiolate)]2

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Understanding the CO Dissociation in [Fe(CN)(CO)(Dithiolate)]Complexes with Quantum Chemical Topology Tools Alexandre Lebon, Pierre-Yves Orain, and Antony Memboeuf J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05399 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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The Journal of Physical Chemistry

Understanding the CO Dissociation in [Fe(CN)2 (CO)2 (dithiolate)]2 Complexes with Quantum Chemical Topology Tools Alexandre Lebon,

∗

Pierre-Yves Orain, and Antony Memboeuf

Laboratoire de chimie électrochimie moléculaire et chimie analytique, UMR, CNRS 6521, 6, Avenue Le Gorgeu, 29285 Brest Cedex, France

E-mail: [email protected]

∗

To whom correspondence should be addressed 1

ACS Paragon Plus Environment


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Abstract

The active site of the [NiFe]-hydrogenase contains a pentacoordinated iron atom, therefore a vacant coordination site is available for substrate binding. Nonetheless, most organometallic models of the [NiFe]-hydrogenase failed to reproduce this key feature of the active site. In order to rationalize such behaviour, quantum chemical calculations were carried out on a series of [Fe(CN)2 (CO)n (dithiolate)]2 n=1,2 complexes, where dithiolate denotes the ligands (CF3 )2 C2 S22 , (CO2 Me)2 C2 S22 , Ph2 C2 S22 , C6 Cl2 H2 S22 , C6 H4 S22 , C2 H4 S22 , C3 H6 S22 . Structural and energetic features are discussed and, a topological analysis based on two scalar elds the one electron density and the electron localization function (E.L.F.) has been attempted to describe the nature of the metal-ligand bonds. The present approach contributes to better understand the ability of non-innocent dithiolene to strongly labilize one CO whereas innocent dithiolate cannot. The methodology developed throughout the paper could be useful in the eld of the CO releasing molecules.

2


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Introduction Hydrogenases constitute a class of enzyme capable of catalyzing, in a reversible fashion, the proton reduction into H 2 . Even though the active site of [FeFe]-hydrogenase is well modelled by organometallics complexes, 1 there is, to the best of our knowledge, very few models mimicking the key features of the [NiFe]-hydrogenase active site. 2 This latter consists of an organometallic entity in which the iron atom is pentacoordinated ( i.e. a square pyramid), whereas for all the organometallics models, the iron atom is embedded in an octahedral environment site, as in the Ni II FeII state. The [NiFe]-hydrogenase can be reversibly inhibited by carbon monoxide but the exogenous CO is bound to the nickel atom. 3 Therefore, these octahedral complexes cannot be regarded as viable models of the CO-inhibited form of the [NiFe]-hydrogenase active site. The [Fe(bdt)(CN)2 (CO)]2 complex reported by Rauchfuss 4 perfectly models the environment of the [NiFe]-hydrogenase active site. It is worth mentioning that the dicarbonyl intermediate [Fe(bdt)(CN) 2 (CO)2 ]2 was evidenced by IR spectroscopy but could not be isolated. Nevertheless, the stability of the dicarbonyl complex depends on the nature of the dithiolate ligand. In fact, the propanedithiolate analogue can easily be isolated as a dicarbonyl complex: [Fe(pdt)(CN) 2 (CO)2 ]2 either in

trans

-CN 5 or

cis,cis

conguration. 6

In this context, we have endeavored a study to understand the inuence of the dithiolate ligand on the ability of the complex to release one carbonyl and stabilize the monocarbonyl complex. We are aiming at gaining a better understanding of the [NiFe]-hydrogenase active site through a thorough theoretical study at the DFT level. To perform such a study we investigated complex of general formula [Fe(CN) 2 (CO)2 (dithiolate)]2 with seven dithiolate ligands namely 1= (CF 3 )2 C2 S22 , 2=(CO2 Me)2 C2 S22 , 3=Ph2 C2 S22 , 4= C6 Cl2 H2 S22 , 5=C6 H4 S22 , 6=C2 H4 S22 , 7=C3 H6 S22 . The dithiolate ligands are ordered from the more electroattractor complex 1 to the less electroattractor complex 7. The chemical structures of the dithiolate ligands are sketched in Fig. 1 with the possible decarbonylation reactions. Among the six possible reactions, the decarbonylation 3

cis,cis


→

axial

-CO will be discussed


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at length. In the hydrogenases, [FeFe]-hydrogenases or [NiFe]-hydrogenases one Fe atom the proximal Fe is linked to a cubane [4Fe-4S] cluster via a bridging cysteine sulfur. The proximal Fe of the [FeFe] subcluster is coordinated by one CO and one CN . The distal Fe, the site we are concerned with, is also coordinated by one CO and one CN . A bridging CO ligand coordinates to both iron atoms. 7

Figure 1: The [Fe(CN) 2 (CO)2 (dithiolate)2 ] and [Fe(CN)2 (CO)(dithiolate)]2 complexes. Most of the descriptions of the bonding of the M-CO, where M is a transition metal, rely on the traditional picture of Dewar, Chatt and Duncanson. 8,9 This model is based on a balance between σ donation from the carbonyl (the carbon lone pair) to the vacant orbital of the metal atom and π back-donation from the metal to the CO π ∗ orbital. However as already pointed out by Frenking

et al.

, 10 the correlation was proven to be questionable either be-

tween the M-C bond length and ligand → metal donation, or between C-O bond length and metal → ligand back donation. Therefore the classical model of the chemical bonding (donation/ back-donation on the ground of the individual molecular orbitals) should be regarded as a qualitative scheme to understand some metal-ligand interactions, but not as a quantitative 4


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model. In the present study we performed extensive DFT calculations to investigate trends in the structural, electronic properties of [Fe(CN) 2 (CO)2 (dithiolate)]2 iron complexes which might release one CO. All the computations are performed in the gas phase, and no implicit or explicit addition of a solvent has been considered. In addition to a structural study, we focus on the behavior of the bonding in the complexes [Fe(CN) 2 (CO)2 (dithiolate)]2 around the Fe atom and the two sulfur atoms. The exploitation of the topology of the electron density

11

and of the electron localization function (ELF) 1214 gives physical insights to un-

derstand the decarbonylation process. This approach that is complementary to the more popular analysis of the orbitals provides a quantitative view of the chemical bonding within the family of dianion complexes under scrutiny. Analysis around bond critical points (BCPs) of the electron density and partition of the ELF into basins related to core and valence attractors aords a powerful insight in the chemistry of the selected systems. A precise count of the number of electrons involved in the pairs as well as the uctuations between the basins enables to discuss the strength and nature of the bond. To our knowledge no equivalent study has been carried out for this family of complexes. Computational details are rst introduced. The Results and Discussion part contains structural and energetic features. It then focuses on the assessment of the Bader charges followed by an analysis of the electron density at bond critical points. It ends with the examination of the topology of the electron pair distribution.

Computational Details Computational methods were employed to further understand the decarbonylation process under discussion. Our calculations were performed with the Gaussian 09 program package . 15 The structures of all the complexes were optimized without symmetry constraint with ve functionals. In addition to the widely used B3LYP , 16 we carried out calculations with the BP86 , 17 with the GGA of Perdew, Burke and Ernzerhof , 18 and with the most recent M062X 5



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of Truhlar

19

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and the B97D functional that includes the correction of dispersion forces. 20 Op-

timization of the geometry of the structures was performed with low level of forces inferior to 0.01 eV.Å−1 , for the optimization of geometry the 6-31g(d,p) basis sets are used. Computations were resumed for more precise single point energy calculations using the more extended 6-311++g(2df,2pd) basis set. Only for the B97D, the single point energy calculations were carried out with another functional the M06L . 21 Here we followed a successful approach developed for charge systems in a cycloaddition reaction . 22 The character of the stationary points was identied using normal mode frequency analysis. According to Bruschi

et al.

, 23

the exchange functional BP86 17 provides good structural results for the [NiFe]-hydrogenase active site but more recent functionals have been developed to take into account the eect of dispersion forces e.g. the B97D functional

20

or the M062X. 19 Hence, structural features are

compared with experimental ndings. Carbonyl binding energies to the metal complex have been assessed following the formula. Eb = E (Complex − CO) + E (CO) − E (Complex). For the monocarbonyl complex, the CO ligand can either stay in an equatorial position or move to an apical position, see Fig. 1. The two congurations of the complex has been computed as well. The number of electrons on each atomic basin is evaluated using Bader's method, 11 which is based on partitioning the molecule into atomic volumes by locating the zero-ux surfaces of the one-electron density eld. The number of electron was then computed with the TopMod suite of programs. 26 The sum of all the electrons on each atomic basin recovers the total number of electrons in the molecule. The one-electron density is obtained after post processing the wave function given as output by Gaussian with TopMod. An analysis of the critical points

11

is further carried out with the program CRITIC2. 27 Special attention is given to

the value of the one-electron density at the bond critical points (BCPs) and to the value of the Laplacian for the same points. These values provide information on the nature of the chemical bonding. 28,29 The Laplacian of the one electron density gives a useful information to locate the regions of molecular space where there exists a depletion or a concentration of 6


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the electronic density. A negative Laplacian is associated with a concentration of the charge whereas a positive value corresponds to a depletion of the charge. 11 The Electron Localization Function (ELF) was originally introduced by Becke and Edgecombe within the context of Hartree-Fock calculations. 12 The ELF is a local function which describes how ecient the Pauli repulsion is at a given point of the molecular space. The denition of the ELF used herein comes from the approach developed by Savin

et al.

, 13 The

ELF arises from the local excess kinetic energy density, due to Pauli principle. It is dened as ELF(~r) =

1 1+x2

, with x(~r) =

T −TvW TT F

. Here, T is the electronic kinetic energy obtained

from the self-consistent resolution of the KS equations N

T =

e 1X | ∇ψi (~r) |2 , 2 i=1

(1)

with Ne the number of electrons and ψi the one-electron orbitals. TvW is the von Weizsacker functional, which gives the exact kinetic energy for a bosonic system, but also for the ground state of 1- and 2-electron systems, as in these cases the ground state wave function is bosonic-like, i.e. nodeless:

TvW =

1 | ∇ρ(~r) |2 , 8 ρ(~r)

(2)

with ρ the self-consistent electron density. The true kinetic energy T reduces to TvW in those cluster regions where a single orbital contributes. The dierence TP = T − TvW ≥ 0 is known as the Pauli kinetic energy, and it is the correction to TvW due to the fact that electrons are fermions. It thus represents the eects of Pauli repulsion. Finally, TT F is the Thomas-Fermi functional, which gives the correct kinetic energy for a homogeneous electron gas:

TT F =

3 (3π 2 )2/3 ρ(~r)5/3 . 10

(3)

With these denitions, it becomes clear that the ELF is normalized between zero and 7



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unity. Other denitions of the ELF are possible. For instance, the reference to the homogeneous electron gas can be avoided according to some authors. 30 For a simple purely covalent bond (two electrons with unpaired spins occupying the same bonding molecular orbital), only one orbital contributes to the electron density in the inter-nuclear region, so T = TvW and ELF=1 in that region. An ELF value close to 1 corresponds to a high probability of nding a pair of electrons. It is the opposite for an ELF value close to 0. Nevertheless, it is important to emphasize that what the ELF is really measuring is the magnitude of the Pauli repulsion. The ELF was designed to provide a rigorous basis for the analysis of the wave function and of the bonding in molecules and crystals. In 1994, Silvi and Savin

14

proposed to use

the gradient eld of the ELF in order to perform a topological analysis of the molecular space. The ELF partition of the molecular space can be divided in three types of domains. Bonding and non-bonding valence domains correspond to bonds and lone pairs respectively. The third type of domain is associated to core basins. Attractors are special points in domains where the value of the ELF reaches a maximum. Further on, we will refer to basin or domain indiscriminately. Domains are dened around attractors and the gradient of the ELF within a domain converges toward the attractor. The core basins are organized around nuclei (with Z ≥ 2) and the valence basins occupy the remaining space. The core basins are labeled by C(X) in which X is the atomic label of the atom to which the core belongs. The core basins closely match the inner shell structure of the atoms. A valence basin is characterized by its synaptic order, which is the number of core basins with which it shares a common boundary. Monosynaptic basins (labeled V(A)) usually correspond to lone pair regions, whereas disynaptic and polysynaptic basins (labeled V(A,B,C,...)) characterize the covalent bonds. Note that there is no need to associate synapticity to a core basin. Overall, the spatial distribution of the valence basins closely matches the non-bonding and bonding domains of the Valence Shell Electron Pair Repulsion (VSEPR) model. In the data presented hereafter, all the valence basins are mono or disynaptic. The anal8


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ysis of the basins around the attractors enabled to extract the population of these basins and their mutual interactions with the help of the TopMod09 suite of programs. 26 The partition of the molecular space enables basin-related properties to be calculated by integrating the density of a certain property over the volume of the basins. For a basin i encompassing the volume Ωi , the average population reads as: Z N (Ωi ) =

ρ(~r)d~r

(4)

Ωi

and its variance: σ 2 (Ωi ) = (N (Ωi ) − N (Ωi ))2

(5)

The variance of the basin population is a measure of the quantum mechanical uncertainty of the basin population, which can be interpreted as a consequence of the electron delocalization. The variance, σ 2 (Ωi ) can also be splitted in terms of contributions from other basins: σ 2 (Ωi ) = −

X

V (Ωi , Ωj )

(6)

i6=j

where V (Ωi , Ωj ) is the population covariance, that is provided by the TopMod 26 suite of programs. According to Matito and Solà, 31 the relative uctuation measures the ratio of electrons delocalized in a given basin with respect to the population of that basin. This positive quantity is expressed as a percentage of electron delocalization:

λF (Ωi ) =

σ 2 (Ωi ) N (Ωi )

(7)

For all the calculations with Gaussian09 no pseudo-potentials were included to model the core electrons of the iron atom. In fact, it is claimed by Kozlowski and Pilmé 32 that the pseudopotential had some eects on the valence shells when calculating the polarization in e.g.

CH3 OH.

9



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Results and Discussion F3C

MeO2C

CF3

-S

CO2Me

-S

S-

1

Ph

Ph

-S

S-

S-

2

Cl

3

Cl -S

S-

-S

4

-S

S-

5

S-

-S

6

S-

7

Figure 2: Representation of the dithiolate ligands to be complexed with Fe(CN) 2 (CO)2 .

Structures and Binding Energies. The complex [Fe(CN)2 (CO)2 (dithiolate)]2 can adopt three dierent congurations referred to as

trans

-CO,

trans

-CN and

cis,cis

(Fig. 1).

After the release of one CO, the remaining carbonyl can either sit in an apical or equatorial position as depicted in Fig. 1. Fig. 2 displays the dithiolate ligands. The complexes are further identied on the basis of those ligands. Experimental data indicate that for 7 two isomers

trans

-CN

5

and the

cis,cis

6

can be

obtained depending on the synthetic procedure. No inter-conversion between these two isomers was reported which suggest a high energetic barrier associated to it. The dicarbonyl complexes could easily be isolated, facile decarbonylation was not reported for those compounds. On the contrary, dicarbonyl complex 5 could not be isolated, easy decarbonylation leads to the formation of the monocarbonyl complex 5-CO with the remaining CO in apical position. Spectroscopic data suggests a

cis,cis

conguration 4 for the dicarbonyl complex 5.

Energies have been compared for the trans -CN and the cis,cis conguration with respect 10


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-1

∆E (kcal.mol )

15

(a) M062X

10

5

B3LYP B97D

BP86

0

trans-CO

PBE

-5 35 (b)

PBE

30 -1

Eb (kcal.mol )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BP86 25

BP86*

20

B3LYP B97D

15 M062X 10 5

1

2

3

4 Complex

5

6

7

Figure 3: (color on line) Energy characteristics: (a) energy dierence relative to the trans CO conguration for all the complexes in full lines the cis,cis conguration and in dashed lines the trans -CN conguration and (b) the binding energies of the CO apical.

11



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to the trans -CO conguration. Fig. 3a exhibits the energy dierences computed with dierent levels of theory. It is evidenced that the M062X exchange correlation functional favors in all cases a trans -CO conguration that is not reported experimentally. The same trend is obtained with the widely used B3LYP. However it is more appropriate for the prediction with complex 7. In return, the exchange correlation functionals PBE, B97D and BP86 give satisfactory results since the

cis,cis

conguration (solid lines) are either more stable for complex

7 or at slightly higher energy by about 1kcal.mol −1 than the trans -CO conguration. The trans

-CN conguration appears as much less stable for complexes 1 to 5 and is stabilized

for complexes 6 and 7. This is consistent with the experimental observations of two possible congurations for complex 7 by two research groups. 5,6

Table 1: Comparison of the computed data for three exchange correlation functionals with the reference data obtained from X-ray diraction. d1 = dF e−CO and d in degrees for the two ligands. d2 = dF e−S in Å and SFeS XC R 5-CO 7

PBE d1 1.67

1.75 1.72

d2 2.26 2.26 2.45 2.44

B97D d SFeS

d1 1.67

90.8

1.76 1.73

88.8

d2 2.30 2.30 2.49 2.51

BP86 d SFeS

d1 1.69

90.6

1.77 1.75

88.4

d2 2.26 2.26 2.42 2.44

exp. d SFeS

89.1

d1 1.694

91.8

1.79

d2 2.20 2.21 2.34

d SFeS

89.4

To select a functional among the PBE, BP86 and B97D, computed angles and interatomic distances have been compared to available structural data. Fe-CO, Fe-S distances d angle are reported in Table 1 for the two experimentally isolated complexes, and the SFeS

that is 5-CO 4 and 7. 6 From now on, only

cis,cis

congurations will be examined for the

dicarbonyl complexes. The examination of the structural data suggests that the best prediction are provided by the BP86 functional for which the Fe-CO and Fe-S distances exhibit an excellent agreement with the experimental data. For the monocarbonyl complex 5-CO, d angle. the deviation is smaller than one percent both for the Fe-CO distance and the SFeS

The Fe-S distances are also best reproduced with this functional even though there is some discrepancies with the experimental results. The same conclusions can be drawn from the 12


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data for the dicarbonyl complex 7. Only the Fe-S distance (2.26Å) is 0.05 Å larger than the experimental one (2.21 Å), Bruschi

et al.

23

already reported the same deviation for the

Ni-S distance with the same functional. The CO binding energies have been computed for all the dithiolate ligands at dierent levels of theory. For the selected functional, the BP86, calculations have been resumed with the TZVP basis set for structure relaxation and the QZVP basis set for a subsequent single point energy calculation (see the curve with BP86*). These energies are plotted in Fig. 3 b). It is reported that the CO binding energy displays a global increase of approximately 60% from complex 1 to complex 7. Unsaturated dithiolate ligands (dithiolene) are prone to easy decarbonylation as suggested by both binding and Gibbs free energies. A smooth increase of both quantities is reported as evidenced by table S1 of the Supporting Information. The reaction is endothermic but a small amount of energy induces the decarbonylation. In return, The CO ligand is more rmly attached to the iron atom in the case of saturated dithiolate ligands. An other way to evaluate the M-CO bond strength relies on the infrared CO stretching band frequencies. Frequencies of these specic bands can be extracted from the calculations, the evolution of the infrared band frequencies, in the present study, are correlated to the electoattractor character of the dithiolate ligand. The C-O separation in the meantime does not evolve in a monotonic fashion throughout the set of complexes (see Table S2 of Supporting information) It is worth saying that the CO stretching mode might couple to lower frequency modes and in such a case could not provide a reliable measure of the CO bond strength 24 as it was proven theoretically. The environment of iron and sulfur atoms have been followed before and after dissociation. This analysis relies on the data gathered in Table 2. In all cases, the Fe-CO bonds shorten for the monocarbonyl complexes by almost 0.1 Å. In the dicarbonyl complexes, the Fe-CO separation is 0.03 Å shorter for the equatorial carbonyl. This Fe-CO distance depends upon the position of the carbonyl ligand and the number of them but no straightforward relation exists between these distances and the dithiolate ligand. If we turn to the sulfur atom 13



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Table 2: Distances around the iron and sulfur atoms for the dicarbonyl and monocarbonyl complexes. Distances are given in Å. d (Å) Fe-COap Fe-COeq C-S Fe-S d (Å) Fe-CO C-S Fe-S

1

2

3

4

5

6

7

1.78 1.75 1.77 2.36

1.79 1.75 1.76 2.37

1.79 1.75 1.78 2.37

1.81 1.77 1.76 2.37

1.78 1.75 1.77 2.38

1.80 1.76 1.83 2.42

1.77 1.75 1.85 2.43

1-CO

2-CO

3-CO

4-CO

5-CO

6-CO

7-CO

1.69 1.77 2.22

1.69 1.76 2.22

1.69 1.78 2.22

1.69 1.75 2.24

1.69 1.77 2.26

1.68 1.86 2.28

1.68 1.86 2.28

environment the dependence of the C-S and Fe-S distances as a function of the dithiolate ligand can be clearly established. For the 1 to 5 complexes these distances amount to approximately 1.77 and 2.37 Å, for dC−S and dF e−S , respectively. It then jumps to 1.85 and 2.42 Å, for dC−S and dF e−S for the last two groups namely the complexes 6 and 7. The same conclusions can be drawn for the monocarbonyl complexes. Interestingly, one can note that the dithiolate ligand appears as more distant from the Fe(CN) 2 (CO)2 moiety for complexes

6 and 7. This feature will be later on accounted for a stronger dative bonding. Analysis derived from the electron density. The charge on atoms can be deduced from the electron density ρ. In the present study we have used the partitioning of Bader to derive the atomic charge. δQ designs the excess or decit of charge with respect to the neutral atom, in other words population minus Z. For all the investigated ligands, Fig. 4 plots these δQ. Panel (a) displays a negative δQ for the iron atom, panel (b) exhibits the positive average δQ for the two sulfur atoms and panel (c) the slight positive δQ for the two carbon atoms adjacent to the sulfur atoms. It also plots the charge behaviour after CO dissociation, see the red curves for the monocarbonyl complexes. In the monocarbonyl complex, the carbonyl is assumed to sit in the more stable apical position. Fig. 4 (a) clearly demonstrates the trend of the transition metal to donate charge to its octahedral environment. The charge decit on the iron atom is decreasing after CO dissociation. This charge donation is estimated to approximately 0.8 electron, a value close to 14


Page 15 of 35

-0.80

δQFe(e)

(a) -0.85

-0.90 0.5

(b)

δQS(e)

0.4

dicarbonyl monocarbonyl

0.3 0.2 0.1

(c)

0.15 δQC(e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10 0.05

1

2

3

4 5 Complex

6

7

Figure 4: (color on line). Population minus nucleus charge on (a) the iron atom, on (b) the two sulfur atoms and on (c) the two carbon atoms adjacent to the sulfur atoms. one electron that is expected for the Fe(I) oxidation state. This oxidation state is encountered on the iron centers of [FeFe]-hydrogenases. 25 It is worth mentioning that overall the ligand behind the dithiolate ligand seems to have little inuence on the charge transfer undergone by the iron atom. Conversely, the charge excess on the sulfur atom δQS seems drastically aected by the ligand. In fact, for the sulfur atoms, a global excess of charge is evidenced whatever the ligand. δQS jumps by 0.2 electron per sulfur atom for the two complexes 6 and 7. This nding is also noted for the monocarbonyl complexes as the charge excess has exactly the same amount. Finally δQC testies of a small charge excess on the two carbon atoms that turns out to be constant throughout all the investigated dithiolate ligands. The partitioning of Bader enables to separate atomic volume and computes the charge within the atomic basin. The addition of the charge of all atomic basins gives an excess of two electrons. These 15



excess electrons are shared among two entities the Fe(CN) 2 (CO)2 moiety that presents a charge excess of about 0.8 electrons and the dithiolate moiety that gathers 1.2 electrons. This charge separation is general for all the dithiolate ligands. Fe

BCP

S

Value of ρ at BCP

S BCP

C

Laplacian of ρ at BCP

BCP

C

0.32

-0.5 ρC(C-C)

∆ρC(C-C)

0.30

-0.6

0.28

-0.7

0.26

(d)

(a)

-0.8 -0.9

ρC(S-C)

∆ρC(S-C)

-0.25

0.19 0.18

-0.30

0.17

(e)

(b)

Population (e)

0.24 0.20

Population (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

-0.35

0.16 0.068

ρC(Fe-S)

∆ρC(Fe-S)

0.14

0.066 0.12

0.064 0.062

(c)

(f)

0.060

1

2

3

4

5

6

7

Complex

1

2

0.10

3

4

5

6

7

Complex

Figure 5: Populations at bond critical points around the dithiolate ligands. Left values of the one-electron density and right values of the Laplacian of the density. Same labels as Fig. 2. Let us come to the analysis at the bond critical points (BCPs). Coordinates of BCPs and values of the electron density and Laplacian at these specic points were extracted from the CRITIC2 program. 27 A small value of the electron density at a BCP implies a ionic or polar bond whereas a large value is associated with a covalent bond. Fig. 5 gathers the results collected around the dithiolate ligand. Left part of Fig. 5 display the value of the electron density at the BCPs. Fig. 5 (a) plots the value of ρ at the BCP connecting the two carbon atoms adjacent to the dithiolate ligand, this BCP is labeled as ρC . It amounts to 0.32 electron for a C-C double bond with complexes 1, 2 and 3. It then falls to 0.28 electron for complexes 4 and 5 and nally to 0.25 for the simple C-C bond in complexes 6 16


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and 7. The same analysis is then driven for the C-S bond and the Fe-S bond, Fig. 5 (b) and (c) respectively. The value of the electron density at BCPs for the C-S bond presents a constant value of 0.19 for complexes from 1 to 5 before dropping to 0.17 for complexes

6 and 7. Finally the Fe-S bond (Fig 5 (c)) appears as much weaker since the value at the BCP amounts to 0.068 electron and drops to 0.06 for complexes 6 and 7. This latter characteristic dierentiates the Fe-S bonds with the other bonds implied in the dithiolate ligand. This 0.068 value is typical of closed-shell interaction, as the complexes can be splitted into two moieties the dithiolate ligand and the Fe(CN)2 (CO)2 moiety. The Fe-S bonds can be regarded as polar bonds. This treatment can be complemented with the assessment of the Laplacian at this BCPs. A negative Laplacian at these BCPs corresponds to a covalent bond. Fig. 5(d) and (e) testify of a covalent bonding between the C-C atoms and the S-C atoms. The C-S bonding weakens for the two complexes 6 and 7. This yields again an evidence of the isolation of dithiolate ligand that was already emphasized by the evolution of the S-C distance in the former section. The ionic or polar character of the Fe-S bonding is quantied by the positive values of the Laplacian at the BCP, as sketched in Fig 5 (f). The variation of the electron density at the BCP for the Fe-C bond (not shown here) does not depend signicantly upon the dithiolate ligand. In fact, it amounts to 0.151 electron for the apical CO and the complexes from 1 to 5 and gain hardly 2% to reach 0.154 electron for complexes 6 and 7. This value is intermediate between a pure covalent bond as the one reported for the C-C bond and a polar bond exemplied by the one observed between the iron and sulfur atoms. This Fe-C bond will be termed later as a dative bond on the basis of the analysis of the ELF. The value of ρC for the Fe-C bond has also been evaluated for the equatorial CO and amounts to ρC = 0.163 electron. This latter value conrms that the equatorial CO ligand is more tightly bound to the Fe atom. The value of the Laplacian at ρC are related to the position of the carbonyl ligand but both values are positive in agreement with the expectation of a dative bond . 29 The Laplacian of the electron density has been plotted in Fig. 6 for the complexes 1, 5 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and 7. The positive value of this Laplacian taken for the isosurface level of ∆ρ = 0.3 depicts the region of depletion of the density where the chemical bonding occurs. It is visible from these set of plots that the iron atom is so to say electronically isolated from the rest of the complex. The Laplacian of the density enables to visualize the non bonding regions 28 as the lone pairs of the sulfur atoms or the lone pairs of nitrogen and oxygen atoms. The negative value of the same quantity not shown here indicates that the region of density accumulation are surrounding the atomic nuclei.

General features of the ELF. Let us turn to the presentation of the results of the electron localization function that are sketched in Fig. 7. The complex 5 has been chosen for the representation. The color code enables to distinguish between the core, valence and non bonding valence localization basins. The ELF is plotted for η = 0.6 which is a relatively low value for this family of compound. At this value the domains are merging which explains the continuity of the basins that seems to encompass several atoms, as for example around the carbon skeleton of the benzene cycle. The general feature that can be derived from these two pictures at η = 0.6 is that the complex can be splitted in two moieties. This trend is found for all the studied complexes. On the one hand, all the bonds for the dithiolate ligand are covalent. On the other hand, the moiety centered on the transition metal ion reveals electronically isolated carbonyl and cyanide ligands. The basins surrounding these latter ligands eventually merge at very low ELF value, η = 0.25, at this small value the iron atom is still not connected to the ligands that complete the octahedron. Chemical intuition would suggest an ionic interaction between the iron atom and the carbonyl and the cyanide ligands. The ELF interpretation is however a bit more subtle as it exists valence basins between the iron and carbon atoms of the cyanide and carbonyl ligands. It is more clearly evidenced on the lower panel of Fig. 7 for which η = 0.85 and where the V(Fe,C) are valence basins connecting the iron atom and the carbonyl or cyanide ligands. Later on we will see that the bonding nature between the iron atom and carbon atoms of the carbonyl ligand can be considered as dative. That is the two 18


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Figure 6: (color on line). Laplacian of the electron density at ∆ρ = 0.3 for the dicarbonyl complexes 1, 5 and 7, same label as Fig. 2.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7: (color on line). ELF domains for complex 5 at η =0.6 and η =0.85. Color code for the basins green= core, red= monosynaptic, grey= disynaptic, pink= protonated disynaptic. Lower panel is an assignment of the basins. Green dots feature the attractor positions of the V(S), V(S,C) and V(S,Fe) ELF domains. 20


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electrons of the bond are provided by the carbon atom. A valence bond can be exemplied between the carbon and the nitrogen atoms by the valence basin V(C,N) in the CN . Other valence basins are apparent on the ligand, an example is between the two carbon atoms connected to sulfur atoms. The population of these valence domains V(C,C) that are observed in the dithiolate ligand side of the complex amounts to 3.80, 2.64 and 1.88 for complexes 1,5 and 7, respectively, in keep with the bond order expected in such a case. Not surprisingly, the valence basin V(C,C) gathers almost two electron pairs for complex 1, one and a half pair for complex 5 and only one electron pair for complex 7. In fact for complex 1, the two carbon atoms are connected by a double bond. For the complex 5, the carbon atoms are connected to each other by a simple bond and there is an additional contribution from delocalized electrons, that implies the aromaticity of this ligand. With the help of electronic and geometry-based indexes 33,34 of aromaticity or decomposing the ELF according to molecular orbitals, we could assess the evolution of aromaticity in the C6 H4 S22 and C6 Cl2 H2 S22 ligands. The indexes of aromaticity suggest negligible changes during decarbonylation, as is reported in table S3 of the Supporting Informations. Finally for the complex 7, the C-C bond is a simple bond. At last, around the sulfur atoms a strong overlapping of the V(S) basins-lone pairs occurs with another V(S) basin of the same atom together with overlapping with the V(S,Fe) basin. The shape of the non bonding basins around the sulfur atom is reminiscent of the basins of the Laplacian of the electron density that are sketched in Fig. 6 for three representative complexes.

Localization around the dithiolate ligand. Let us now analyze in more details the behavior of the ELF for the dithiolate ligands that has a key role during the dissociation process. In addition to the basin encompassing the core electrons C(S) each sulfur atom has two non-bonding basins V(S) and two bonding basins V(S,Fe) and V(S,C). The attractors of these basins are depicted by green dots on the lower panel of Fig. 7. The volume occupied by the non-bonding pairs is large and even at η = 0.85 the two V(S) basins are merging whereas all other valence basins are well separated. These non bonding basins common to all 21



the complexes under study deserves a more careful analysis based on the topological analysis of the ELF function.

V(S)(e)

4.8

(a)

4.4 4.0

V(S,Fe)(e)

2.0 1.8

(b)

S1

Fe-octahedron Fe-pyramidal

1.6 1.4 1.2

S2

1.0 2.2

(c) V(S,C)(e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

2.0 1.8 1.6

1 2 1-CO 2-CO

3 3-CO

4 5 4-CO 5-CO Complex

6 6-CO

7 7-CO

Figure 8: (color online). Population of the localization domain of the ELF in the vicinity of the dithiolate ligand. Same label as Fig. 2. In Fig. 8 are plotted all the domain populations found around the sulfur atoms. The populations of the non bonding domains are displayed in Fig. 8 (a) whereas the bonding basins V(S,Fe) and V(S,C) are presented in Fig. 8 (b) and (c) respectively. The order of binding energy established in Fig. 3 is followed. Each sulfur atom bears two lone pairs. For these lone pairs, the V(S) basins appear as saturated since a value of 2 electrons is expected for each lone pair. On each sulfur atom the count of non bonding electrons exceeds 4, reaching even a value of 4.7 per sulfur atom for the complexes 6 and 7. This saturation is evidenced whatever the investigated ligand but to a lower extent as it amounts to 4.4 for the ve former ligands. This saturation is observed even after the carbonyl has been released, see red curves. Again, the behaviour seems to be opposite for the complexes 6 and 7 with respect 22


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to the other complexes from 1 to 5. In fact, we notice that the average V(S) population slightly increases for complexes 6 and 7. To summarize in the decarbonylation process the population of the lone pairs are sensitive to the change from a hexa to a pentacoordinated environment. This nding points out again the special role of the dithiolate ligand as it mediates the charge transfer between the ligand side and the iron complex moiety. Overall one can notice that the lone pairs of the sulfur atoms show a trend to localize the excess of charge. Fig. 8 (b) and (c) can be examined with the same viewpoint. In fact the valence basins V(S,Fe) and V(S,C) exhibit an increase of their population once the carbonyl is released for

1, 2, 3, 4, 5, whereas no such eect occurs for 6 and 7. The accumulated charge around the lone pair is back-donated to the bonding localization domain V(S,Fe) and V(S,C) after decarbonylation. We also notice in the three panels that the values V(S), V(S,Fe) and V(S,C) vary appreciably for the two sulfur atoms of the dithiolate ligand, it is particularly obvious in complexes 2 and 3. This can be accounted for the asymmetry of these ligands. In fact, the two sulfur atoms are facing part of the ligands whose orientation diers. The population of the V(S,Fe) and V(S,C) domains give also other relevant information. The C-S bond is almost saturated as it approaches two electrons per C-S bond, as shown in Fig. 8(c). This is veried for all the complexes except 6 and 7. This C-S localization basin acquires charge as the iron atom environment change from hexa to pentacoordinated with even some amount of saturation. The Fe-S bond appears as less covalent and evolves in much the same way during the transition however the Fe-S bond conserves its non saturated character. To conclude this section, we can analyze the results collected in Table 3, where the coecient λF measures the ratio of electrons delocalized in a basin with respect to the population of that basin . 31 Values as high as 40% are testifying of a high degree of delocalization, that even appears to fall somehow for the two complexes 6 and 7. At the same time, the electronic transfer between the non-bonding V(S) basins toward the bonding V(S,Fe) and 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3: Population (N) and uctuation ( λF ) of the ELF domains for the sulfur atoms lone pairs. Populations are in number of electrons and relative uctuation in %. NV (S)⇐⇒V (S) number of electron owing between two neighboring V(S) basins and N←V (S)→ number of electron owing toward the neighboring carbon and iron atom. Complex

1 2 3 4 5 6 7

N 4.35 4.30 4.50 4.11 4.38 4.30 4.32 4.28 4.45 4.41 4.76 4.85 4.74 4.80

λF

42.4 41.0 39.8 42.5 40.4 42.4 41.7 41.6 41.3 41.5 39.2 38.7 38.8 39.2

NV (S)⇐⇒V (S) N←V (S)→ 0.26 0.31 0.25 0.31 0.25 0.29 0.26 0.33 0.26 0.31 0.26 0.32 0.26 0.33 0.25 0.32 0.26 0.32 0.27 0.32 0.27 0.27 0.28 0.27 0.27 0.28 0.27 0.28

V(S,C) basins follows the same trend, since it presents an average of 0.31 electron for the complexes 1 to 5 that slightly drops to 0.27 electron for complexes 6 and 7. All the non negligible electron transfers reported in Table 3 suggest a large spatial spread of the electron from the sulfur atom lone pairs. The Laplacian of the density in these specic regions already pointed a depletion of the density, which can nally be related with the present analysis of the ELF basins that emphasizes the high delocalization of the electron in these regions.

Electron pairing around the metal center. Next the rst steps of the CO ligand detachment has been followed for three complexes 1, 5 and 7. The apical CO has been moved apart from the iron atom by steps of 0.1 Å. During this process, the population of the V(C,C) basin of the C-C bond close to the dithiolate ligand did not manifest any variation. Other basins however do present marked variation of their population such as the V(Fe,C), the V(C,O) and the V(O) basins. The population of these basins are displayed in Fig. 9 (a), (b) and (c), respectively. It is reminded that V(Fe,C) is the localization domain of the ELF

24


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Page 25 of 35

function between the iron atom and CO or CN ligands (see Fig. 7). Its existence points toward a shared-shell interaction in this part of the complex. The V(C,O) basins is related to the covalent bond between the carbon and the oxygen atom of CO. The V(O) basins are related to the two lone pairs on the oxygen atom. These domains of the ELF are illustrated in Fig. 7 for an isosurface value of η = 0.85. The populations of the domains appear in full lines for the apical CO and in dashed lines for the equatorial CO. As the apical CO is moved apart from the complex, the V(Fe,C) basin population decreases from a value of about 3.0 electrons to 2.6. In return, the V(Fe,C) basin population related to equatorial CO gains population. 0.0 V(Fe,C) (e)

3.2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

(a) 1 5 7

3.0 V(C)

2.8

V(C) V(C)

2.6

V(C,O) (e)

(b) 2.8

2.6 5.0

V(O) (e)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

4.8 4.6 4.4 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

∆d (Å)

Figure 9: (color online). Population of valence ELF domains as a function of the Fe-C distance increase. Dashed lines stand for an equatorial CO, continuous lines to the apical CO. Turning back to the apical CO it is worth noting that the V(Fe,C) basin becomes a V(C) basin after a small variation of the separation distance of about 0.5 Å. This decrease 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of synaptic order from disynaptic to monosynaptic is typical of a dative bond where the electrons are provided by only one atom of the bond. The population of the V(C) basins combined with the carbon lone pair then tends gradually toward its value for the CO molecule that was calculated to 2.55 electrons. A diminution of the C-O distance is observed from the carbonyl ligand to the CO molecule, as the C-O bond length varies from 1.17 to 1.14Å. This V(Fe,C) basin connects the iron atom to the carbonyl group. Calculations for this basin projected on the molecular orbitals show that the Fe atom donates 0.5 electron whereas the carbonyl group contributes to about 1.2 electrons. The back-donation appears thus as the dominant factor over the donation as it was already evidenced on monocarbonyl metal-CO complexes by Pilmé

et al.

35

Details of the calculation are presented in gure S1 and table

S4 of the Supporting information. Last but not least, the V(Fe,C) basin is observed to be larger with the equatorial CO. This result can be ascribed to a stronger binding energy of the equatorial CO. A similar trend was reported for the Fe 3 (CO)12 isomers for both C2v and D3h structures as the V(Fe,C) domains were always more populated of 0.1 e for the equatorial CO. 36 As to the V(C,O) basin population, one can distinguish again between the behaviour for the apical and equatorial CO. The former recovers part of the population for this basin since the population amount to more than 2.80. It is nevertheless smaller than the 3.0 electrons computed for the CO molecule. The V(C,O) basins associated with equatorial CO, because of the shortening of the Fe-CO bond continues to lose part of its population. Finally, the two lone pairs of the oxygen atoms V(O) at 2.9 Åfrom the iron atom also display some specic behaviour. It is noted that the V(O) basin populations diers for the 7 complex. In fact in complex 7 (continuous green line) a larger delocalization of the charge is calculated for the apical CO. As for the V(S) basins the non-bonding pairs for all the complexes show a trend to be over saturated, this feature for both V(O) and V(S) is reported to be stronger with propane and ethane dithiolate (not shown here). Let us turn to the analysis of the population for V(Fe,C) basins. V(Fe,C) basins connect 26


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Table 4: Population analysis at the V(Fe,C) basins. N is the population of the V(Fe,C) basin, δ the number of delocalized electron of that basin, and → C(F e) the number of electrons of the V(Fe,C) basins delocalized toward the iron atom. complex

1 2 3 4 5 6 7

molecule free CO free CN−

N 2.98 3.00 2.99 3.00 3.00 3.02 3.03 NV (C)

2.55

apical CO δ

1.10 1.11 1.10 1.10 1.08 1.10 1.12

→ C(Fe)

N 3.09 3.09 3.09 3.08 3.10 3.13 3.10

0.50 0.51 0.50 0.50 0.50 0.50 0.51

δ

NV (C)

0.81

2.55

equatorial CO δ

1.18 1.18 1.19 1.18 1.19 1.19 1.16 δ

0.81

→ C(Fe)

0.57 0.57 0.58 0.57 0.58 0.58 0.57

apical CN N 2.47 2.47 2.47 2.48 2.48 2.50 2.52 NV (C)

2.84

the carbonyl or cyanide ligands to the iron atom. It is the purpose of table 4 to list the populations for each basins. As previously stated, the V(Fe,C) basin of equatorial CO is 0.1 electron more populated than the one corresponding to the apical CO. Table 4 also gives the parameter δ that numbers the electrons of the V(Fe,C) basins shared between the C(Fe) basin and the CO ligand basins. δ is obtained after summing the population covariance of the V(Fe,C) basin with the C(Fe) basin and the basins of the CO ligand . 26 δ amounts to 1.18 electron for the equatorial CO against 1.10 electron for the apical carbonyl. This illustrates the stronger electronic (chemical) interaction in the plane. Among the delocalized electrons a larger part is shared with the core basin of the iron atom for the equatorial CO, 0.58 against 0.50. In return, 0.6 electron are delocalized within the ligand. As a consequence, the position of the CO ligand modies the electron transfer toward the iron atom but let almost invariant the delocalization from the CO basins (V(C,O),V(O)) toward the V(Fe,C) basin. The population of the ELF localization domains related to the cyanide ligands are listed in the last columns of Table 4 for the apical cyanide. The population of the V(Fe,C) basin that is to be compared with the population of the V(C) basin for the free entity, decreases from 2.84 electrons in the free anion to about 2.50 electrons for the apical cyanide ligand. This behaviour contrasts with the 0.4 electron increase from V(C) to V(Fe,C) between the 27



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Page 28 of 35

free CO molecule and the carbonyl ligand. The CO ligand was demonstrated to be linked by a dative bond to the metal atom center. In view of such an evolution of the population of the cyanide ligand, the bonding between this ligand and the Fe atom cannot be considered as a dative bond. It is known on the basis of experimental data that the bonding of the cyanide on metals is quite strong as the recent ndings of Wang

et al.

claim it. In fact,

the PES spectra of [M(CN)2 ]− (M=Cu,Ag,Au) exhibit three detachment photon energies at 6.424 eV (193 nm), 7.866 eV (157 nm), and 10.488 eV (118 nm) for Cu, Ag and Au, respectively. 37 In addition, with the help of quantum chemical topology tools, we came to the same conclusion than Pilmé

et al.

38

According to this last study, the analysis of

bifurcation of the ELF localization domain around the V(Fe,C) basin is consistent with a M 0 − CN − bonding scheme, it contradicts the picture of the dative bond for the cyanide

ligand. Therefore the bonding here stems from an electrostatic interaction. Finally, this section can be concluded by mentioning a correlation between the V(Fe,C) populations for the apical CO and CN− . Indeed the coupled evolution of their populations points toward a stabilization eect of the CO ligand by the cyanide ligand in

trans

. For complex 5, we

note 3.0 electrons on the V(Fe,C) basin bonding CO to the iron atom and 2.48 electrons for the apical cyanide. For the complex 7 both basins gain electron population, see Table 4. This stabilization seems to be tuned by the dithiolate ligand, such a mechanism was already proposed by Rauchfuss. 4

Conclusions Dicarbonyl and monocarbonyl complexes of general formula [Fe(CN) 2 (CO)2 (dithiolate)]2 and [Fe(CN)2 (CO)(dithiolate)]2 have been analyzed by mean of quantum topology tools after DFT calculations. Dithiolate ligands are (CF 3 )2 C2 S22 , (CO2 Me)2 C2 S22 , Ph2 C2 S22 , C6 Cl2 H2 S22 , C6 H4 S22 , C2 H4 S22 and C3 H6 S22 . According to structural and energy considerations the BP86 functional provided the best agreement with the existing experimental 28


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data. On the one hand binding energies are weaker for complexes 1 to 5 with unsaturated dithiolate ligands (dithiolene) than for the complexes 6 and 7 with saturated dithiolate ligands. On the other hand, structural parameters such as the Fe-S and the C-S bond length exhibit noticeable increase for the saturated dithiolate ligands corresponding to complexes

6 and 7. Unsaturation of the carbon atoms next to the sulfur atoms appears as a key factor to achieve the delivery of one CO in this family of complexes. Quantum topological tools conrm this trend and complete the understanding by unveiling the unusual electronic environment around the sulfur atoms. The structural data are reinforced by complementary computations relying on the topological analysis of the electron density and of the Electron Localization Function (ELF). In all cases, the sulfur atoms gain electrons with respect to the neutral sulfur atom. Analysis at BCPs around the dithiolate ligands points toward a clear electronic separation of the Fe(CN)2 (CO)2 moiety and the dithiolate ligand. In fact, a closed-shell interaction is reported between the iron and sulfur atoms. The Fe(CN) 2 (CO)2 moiety and the dithiolate ligand are connected by a polar bond. For complexes 6 and 7, the electronic values indicate that less electrons are engaged in the C-S and Fe-S bonds. As far as the analysis of the ELF population is concerned, the sulfur atom lone pairs are marked by over-saturation, particularly for the complexes 6 and 7. The high degree of uctuation around the sulfur atom lone pairs diminishes for the complexes 6 and 7. This latter feature has to be associated with the larger Fe-S and C-S distances and the stronger polar character of the bonding between the Fe(CN) 2 (CO)2 moiety and the dithiolate ligand. It is also worth stating that from complexes 1 to 5 little variation are noticed on the structural Fe-S and C-S bond lengths and electronic features at BCPs. The topology of the one-electron density and of the ELF were examined around the metal center. The Fe(I) oxidation state corresponds to the one reported enzymatic process of [FeFe]hydrogenases, this oxidation state is not modied after decarbonylation. The population of the V(Fe,C) basin is larger for equatorial CO which corroborates the idea of a stronger 29



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bonding of this carbonyl ligand to the Fe atom. Both apical and equatorial carbonyl are bonded to the metal by a dative bond. The existence of a stabilization eect of the apical CO by the CN is mediated to some extent by the dithiolate ligand has also been established. The methodology that we have developed throughout this study appears of great relevance in the eld of the CO releasing molecules(CORMs) to better assess or predict the features related to the releasing of a carbonyl by organometallics complexes. For instance it could be applied to understand the process of releasing a carbon monoxide molecule in some molecules of pharmacological potential. 3942

Supporting Information: Binding energies and thermochemistry (Table S1), the infrared CO-stretching mode (Table S2), the index of aromaticity of complexes 4 and 5 (Table S3, Figure S1) and the topological assessment of donation/back-donation (Table S4, Figure S2)

Acknowledgement. A.L thanks Julien Pilmé of LCT at Université Pierre et Marie Curie (Paris) and Vincent Tognetti of COBRA at Université de Rouen for answering his questions.

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References (1) Schilter D., Camara J. M., Huynh M. T., Hammes-Schier S., and Rauchfuss T. B., Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides, Rev.

Chem.

, 2016, 116, 8693-8749

(2) Chambers G. M., Huynh M. T., Li Y., Hammes-Schier S., Rauchfuss T. B., Reijerse E. and Lubitz W., Models of the Ni-L and Ni- (SI)a States of the [NiFe]-Hydrogenase Active Site,

Inorg. Chem.

, 2016, 55, 419-431

(3) Ogata H., Mizoguchi Y., Mizuno N., Miki K., Adachi S., Yasuoka N., Yagi T., Yamauchi O., Hirota S., and Higuchi Y., Structural Studies of the Carbon Monoxide Complex of [NiFe]hydrogenase from Desulfovibrio vulgaris Miyazaki F: Suggestion for the Initial Activation Site for Dihydrogen,

J. Am. Chem. Soc.

2002, 124, 11628-11635

(4) Rauchfuss T. B., Contakes S. M., Hsu S. C. N., Reynolds M. A., and Wilson S. R., The Inuence of Cyanide on the Carbonylation of Iron(II): Synthesis of Fe-SR-CN-CO Centers Related to the Hydrogenase Active Sites,

J. Am. Chem. Soc.

2001, 123, 6933-4

(5) Li Z., Ohki Y., and Tatsumi K., Dithiolato-Bridged Dinuclear Iron- Nickel Complexes [F e(CO)2 (CN )2 (µ − SCH2 CH2 CH2 S)N i(S2 CN R2 )] - Modeling the Active Site of

[NiFe] Hydrogenase,

J. Am. Chem. Soc.

2005, 127, 8950-1

(6) Jiang J., Maruani M., Solaimanzadeh J., Lo W., Koch S. A., and Millar M., Synthesis and Structure of Analogues for the Ni-Fe Site in Hydrogenase Enzyme,

Inorg. Chem.

2009, 48, 6359-6361 (7) Lubitz W., Ogata H., Rüdiger O., and Reijerse E., Hydrogenases, 114

Chem. Rev.

2014,

, 4081-4148

(8) Dewar M.J.S., A Review of π Complex Theory,

31

Bull. Soc. Chim. Fr.


1951 18 C79


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

(9) Chatt J. and Duncanson J.S., Olen Co-ordination Compounds. Part III. Infra-red Spectra and Structure: Attempted Preparation of Acetylene Complexes,

J. Chem. Soc

1953, 2929-2935 (10) Ehlers A. W., Dapprich S., Vyboishchikov S. F., and Frenking G., Structure and Bonding of the Transition Metal Carbonyl Complexes M(CO) 5 L (M=Cr,Mo,W) and M(CO)3 L (M=Ni,Pd,Pt;L=CO,SiO,CS,N 2 ,NO+ ,CN ,NC ,HCCH,CCH2 ,CH2 ,CF2 ,H2 ) Organomet.

, 1996 15 105-117.

(11) Bader R. F. W., Atoms in Molecules. A Quantum Theory (Clarendon, Oxford), 1990. (12) Becke A. D., Edgecombe K. E., A Simple Measure of Electron Localization in Atomic and Molecular Systems,

J. Chem. Phys.

1990,92, 5397-5403.

(13) Savin A., Jepsen O., Flad J., Andersen O.K., Preuss H., Von Schnering H. G., Electron Localization in Solid-state Structures of the Elements: the Diamond Structure, Int. Ed. Engl.

Angew.

1992, 31 187-188.

(14) Silvi B.and Savin A., Classication of Chemical Bonds based on Topological Analysis of Electron Localization Function,

Nature

1994, 371 683-686.

(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.

et al.

,

Gaussian, Inc.

,

Wallingford CT, 2009 (16) Becke A.D., Density-Functional Thermochemistry III. The Role of Exact Exchange, Chem. Phys.

J.

1993, 98, 5648-5652.

(17) Perdew J. P., Density-Functional Approximation for the Correlation Energy of the Inhomogeneous Electron Gas,

Phys. Rev. B

, 1986 33, 8822-4.

(18) Perdew J. P., Burke K., Ernzerhof M., Generalized Gradient Approximation Made Simple, Phys.

Rev. Lett.

1996, 77, 3865-8. 32


Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Zhao Y. and Truhlar D.G., The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of four M06class Functionals and 12 Other Functionals,

Theor. Chem. Acc.

2008, 120, 215-241.

(20) Grimme S., Semiempirical GGA-Type Density Functional Constructed with a LongRange Dispersion Correction,

J. Comput. Chem.

2006, 27, 1787-1799.

(21) Zhao Y. and Truhlar D.G., Comparative DFT Study of Van der Waals Complexes: Rare-gas Dimers, Alkaline-earth Dimers, Zinc Dimer, and Zinc-Rare-gas Dimers Phys. Chem. A

J.

2006, 110, 5121-5129.

(22) Kalvet I., Tammiku-Taul J., Mäeorg U., Tämm K., Burk P., and Sikk L., NMR and DFT Study of the Copper(I)-Catalyzed Cycloaddition Reaction: H/D Scrambling of Alkynes and Variable Reaction Order of the Catalyst,

ChemCatChem

2016, 8, 1-6

(23) Bruschi M., De Gioia L., Zampella G., Reiher M., Fantucci P. and Stein M.,A Theoretical Study of Spin States in N iS4 Complexes and Models of the [NiFe] Hydrogenase Active Site,

J. Biol. Inorg. Chem.

2004, 9, 873-884

(24) Setiawan D., Kalescky R., Kraka E., and Cremer D., Direct Measure of MetalâLigand Bonding Replacing the Tolman Electronic Parameter, Inorg. Chem. 2016 55 2332-2344. (25) Siegbahn P.E.M., Tye J.W., and Hall M.B., Computational Studies of [NiFe] and [FeFe] Hydrogenases

Chem. Rev.

, 2007, 107, 4414-4435.

(26) Noury S., Krokidis X., Fuster F. and Silvi B., Computational Tools for the Electron Localization Function Topological Analysis,

Comput. and Chem.

1999, 23 597-604

(27) Otero de la Roza A., Johnson E.R. and Luaña V., Critic2: A Program for Real-space Analysis of Quantum Chemical Interactions in Solids,

Comp. Phys. Comm.

1007-1018 33


2014, 185


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(28) Popelier P.L.,

Atoms in Molecules an Introduction

(29) Matta C.F., Chap 9 in Physics

Page 34 of 35

, Prentice Hall 1999

Challenges and Advances in Computational Chemistry and

Springer 2006 337

(30) Matito E., Silvi B., Duran M. and Solà M., Electron Localisation Function at the Correlated Level,

J. Chem. Phys.

2006 125 024301

(31) Matito E. and Solà M., The Role of Electronic Delocalization in Transition Metal Complexes from the Electron Localization Function and the Quantum Theory of Atoms in Molecules Viewpoints,

Coord. Chem. Rev.

2009, 255 647-665

(32) Kozlowski D. and Pilmé J., New Insights in Quantum Chemical Topology Studies Using Numerical Grid-Based Analysis,

J. Comput. Chem.

2011, 32, 3207-3217.

(33) Palusiak M. and Krygowski T.M., Application of AIM Parameters at Ring Critical Points for Estimation of π -Electron Delocalization in Six-Membered Aromatic and Quasi-Aromatic Rings,

Chem. Eur. J.

2007, 13 7996-8006

(34) Feixas F., Matito E., Poater J. and Solà M., Quantifying Aromaticity with Electron Delocalisation Measures,

Chem. Soc. Rev.

2015, 44 6434-6451.

(35) Pilmé J., Silvi B., and Alikhani M.E., Structure and Dtability of M-CO, M=rstTransition-Row Metal: An Application of Density Functional Theory and Topological Approaches,

J. Phys. Chem. A

2003, 107 4506-4513.

(36) Andrés J., Berski S., Feliz M., Llusar R., Sensato F., and Silvi B., The Nature of the Chemical Bond in Di and Polynuclear Metal Cluster Complexes as Depicted by the Analysis of the Electron Localization Function,

C.R. Chimie

2005, 8 1400-1412

(37) Wang X.-B., Wang Y.-L., Yang J., Xing X.-P.,Wang J. L and L.-S., Evidence of Significant Covalent Bonding in Au(CN )−2 , J.

Am. Chem. Soc.

34


2009 131 16368

Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Pilmé J., Silvi B., and Alikhani M.E., Comparative Study of the Bonding in the First Series of Transition Metal 1:1 Complexes M-L (M=Sc, ..., Cu; L=CO, N2 , C2 H2 , N H3 , H2 O and F ),

J. Phys. Chem. A

2005, 109 10028-10037.

(39) Motterlini R., Mann B.E., Johnson T.R., Clark J.E., Foresti R. and Green C.J., Bioactivity and Pharmacological Actions of Carbon Monoxide-Releasing Molecules, Pharmac. Design

Cur.

, 2003 9 2525-2539

(40) Alberto R. and Motterlini R., Chemistry and Biological Activities of CO-Releasing Molecules (CORMs) and Transition Metal Complexes,

Dalton Trans.

, 2007 1651-1660

(41) Motterlini R., Mann B.E. and Foresti R., Therapeutic Applications of Carbon Monoxide-Releasing Molecules,

Expert Opin. Investig. Drugs

2005 14 1-14

(42) Romanski S., Kraus B., Schatzschneider U.,Neudör J-M., Amslinger S. and. Schmalz H-G, Acyloxybutadiene Iron Tricarbonyl Complexes as Enzyme- Triggered COReleasing Molecules (ET-CORMs),

Angew. Chem. Int. Ed.

CHEMISTRY

TOPOLOGY

Dithiolene

S S

0.2− CN

CN Fe CO

S CO

S

2−

2−

CN

S

0.2−

S

CN Fe CN CO

CO

2011, 50, 2392-2396

S S

Dithiolate

0.5− 0.5−

CN Fe 0.8+ CO

CO CN

2−

CN Fe 0.8+ CO CO

35

2−


Understanding the CO Dissociation in [Fe(CN)2(CO)2(dithiolate)]2

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