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Electrochemical approach to Fischer carbene complexes
Current Opinion in
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Electrochemistry
ScienceDirect Review Article
Electrochemical approach to Fischer carbene complexes Irena Hoskovcová1 and Jirí Ludvík2 Abstract
Although the chemistry of Fischer carbene complexes is recently a growing direction of research, a review summarizing their electrochemical investigations is still missing. In the present, text evaluation and comparison of experimental data allow insight on how the structural changes of Fischer carbene complexes influence their redox properties. It is evident that the respective reduction and oxidation potentials depend on multiple parameters. To shed light on this problem, the available data were systematized in several series, where effects of individual structural changes were evaluated as shifts of oxidation and reduction potentials with respect to a reference compound. The changes in electron delocalization caused by different substitutions were rationalized using quantum chemical calculations. Addresses 1 Department of Inorganic Chemistry, Institute of Chemical Technology Prague, Prague, Czech Republic 2 J. Heyrovský Institute of Physical Chemistry, Academy of Science of the Czech Republic, Prague, Czech Republic Corresponding author: Hoskovcová, Irena ([email protected]), ([email protected])
Carbene complexes are generally classified into three types with regard to the M]C bond properties: 1. ‘Electrophilic’ Fischer carbene complexes, FCCs, with electrophilic carbene carbon atom, M(d) =C(dþ); 2. ‘Nucleophilic’ Schrock carbenes with nucleophilic carbene carbon M(dþ)=C(d); 3. N-heterocyclic carbenes, NHC, derived by Cdeprotonation of imidazolium salts, with practically single bond MC and no specific carbene carbon reactivity, and also modern NHC complexes bearing cyclic alkyl amino carbene ligands, triazolylidene and other ring expanded ligands. Recently, new types of mixed NHCeFCC carbene complexes are synthesized [1]. This article is devoted to the Fischer type of carbene complexes. It is the first review summarizing the electrochemical approach in the current carbene research, focused on a deeper understanding of the relationship between structure and redox abilities of the title complexes.
Current Opinion in Electrochemistry 2019, 15:165–174 This review comes from a themed issue on Organic and Molecular Electrochemistry Edited by Marc Robert For a complete overview see the Issue and the Editorial Available online 22 June 2019 https://doi.org/10.1016/j.coelec.2019.06.005 2451-9103/© 2019 Elsevier B.V. All rights reserved.
Keywords Fischer carbene complexes, Oxidation potential, Reduction potential, Relationship structure – redox properties.
Introduction Carbenes R2C: were originally found as short-lived reaction intermediates characterized by two unshared valence electrons and an empty p-orbital. These reactive species can be stabilized when bonded to a transition metal atom under formation of a carbene complex of general formula LnM:CR(R0 ). The formal carbon-metal double bond is typical for these complexes. www.sciencedirect.com
The central metal atom in FCCs M is in its low oxidation state which is stabilized by coordination with pelectron acceptor ligands L, usually carbonyls. Owing to the polarization of the formal double bond between the metal and carbene carbon, the latter has electrophilic character. Its partly positive charge is compensated by the attached p-donor X (Figure 1a), typically alkoxy e OR or aminoeNR2 groups (X]N or O). According to the type of heteroatom X, Fischer carbenes are subdivided into aminocarbenes and alkoxycarbenes. This combination of atoms enables a mesomeric shift of
p-electrons, which is crucial for the stability of redox intermediates as well as for possible involvement of the carbene moiety in a larger delocalized system where the C]X bond has partly character of a double bond. M(d) = C(dþ) e X 4 M(d) e C = X(dþ)
The second substituent on the carbene carbon, A, is very often an aromate or another unsaturated grouping which can be also involved in an extended delocalized Current Opinion in Electrochemistry 2019, 15:165–174
166 Organic and molecular electrochemistry
Figure 1
(a) General structure of the discussed compounds, (b) calculated HOMO and (c) LUMO of the compound (CO)5M]C(NMe2)Ph.
system (X-C-A) to stabilize the molecule. The substitution of A and on A (R; namely in p-position) can be then used for a fine systematic tuning of the reduction potential. Electrochemical approach
FCCs can be used namely as homogeneous catalysts [2e 4] or as precursors in organic synthesis [5e7]. A general overview of electron transfer processes and subsequent reactivity is given in Ref. [8]. Owing to the polarization of the M]C bond, the molecule has (at least) two redox active centres: electrophilic (reducible) carbene carbon atom and nucleophilic (oxidizable) metal centre. Redox properties of both centres can be tuned independently and are decisive for their specific application. The principle of action of FCC as a catalyst lies in the reduction of the parent complex, resulting in the splitting of the metal-carbene carbon bond and thus in releasing the desired reactive carbene intermediate R2C:. For organic synthesis, oxidation of the parent compounds is also important resulting, for example, in binuclear complexes. In accordance with the aforementioned electron distribution, reduction potential depends above all on the nature of the heteroatom X and on the properties, both electronic and steric, of the substituent A, whereas differences in ligands L or the nature of the central metal are less significant. Oxidation process is directed to the metal region, and the most important shift of oxidation potential can be achieved by replacing the central metal atom M or a ligand L. For illustration, calculated shape of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, of the compound (CO)5M]C(NMe2) Ph is presented in Figure 1. Evidently, it is properly molecular electrochemistry which can give information about the relationship between structure and reactivity of Fischer carbene complexes or about the stability of their redox intermediates. The most significant electrochemically acquired data are the first reduction and/or the first oxidation potential, their difference (proportional to Current Opinion in Electrochemistry 2019, 15:165–174
the HOMOeLUMO gap), number of electrons involved, possible reversibility of these primary steps and presence of radical species. In the latter case, an in-situ combination of ultravioletevisible and/or electron paramagnetic resonance spectrometry with simultaneous electrochemical generation is very useful[9**], [10*]. The detailed interpretation of homologous series of electrochemical data can give information about mechanism of the reduction (oxidation) process, about ‘anomalous’ behaviour of some derivative and about distribution/delocalization of electrons within the molecule [11,12]. All this knowledge is important in the design of new generations of catalysts or synthetic precursors and in tuning their properties. The present text aims to summarize how changes in FCC composition (namely change of the central metal atom, ligand L, carbonyl atom substituent A and heteroatom X) are reflected in their electrochemical behaviour and redox properties. For the sake of clarity, we decided to compare not the redox potential values themselves generally but their relative shifts within a homologous series with regard to a selected reference compound DE = E(derivative) e E(reference). In this way, positive DE means a shift to more positive values (i.e. more difficult oxidation, easier reduction) and vice versa. This approach enables to compare data from various sources and gives us a direct measure for the importance of the change of composition. Replacement of M
The only data available for comparison are those concerning group 6 (Cr, Mo, W) metals. Carbene complexes of Mn, Re, Fe and Rh have different stoichiometry, and there are no available data on pairs of molecules that would differ only by the central metal atom. Data in Table 1 show that the potential shift which accompanies the central metal replacement is up to 10 times higher for the oxidation potential than for reduction, where the changes are very small, usually in tens of mV. This is experimental proof that the centre of oxidation is the metallic part (cf. Figure 1). www.sciencedirect.com
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Table 1 Replacement of M. Reference compounds: Cr complexes. Compound
M = Mo
(CO)5M]C(OEt)thi (CO)5M]C(OEt)fur (CO)5M]C(OEt)pyr (CO)5M]C(NHcy)fur (CO)4PPh3M]C(OEt)fur (CO)3dppeM]C(OEt)thi (CO)3dppeM]C(OEt)fur (CO)5M]C(NMe2)Ph (CO)4M]C(N{CH2CH]CH2}{h2-CH2CH]CH2})Ph (CO)5M]C(OEt)BODIPY (CO)5M]C(NH{4-mor})CH3 (CO)5M]C(NH{4-mor})C10H19]CH2
M=W
References
DE(ox)
DE(red)
DE(ox)
DE(red)
0.15a 0.14a 0.14a 0.14a 0.35a X 0.26 X X X X X
0.03a 0.02 0.07a 0.02a 0.02a X 0.08 X X X X X
0.19a 0.20a 0.17a 0.19a 0.41a 0.24a 0.31a 0.18 0.29 0.12 0.11 0.09
0.07 0.08 0.02 0.08a 0.06a 0.09 0.082 0.025 0.057 X X X
[13–18] [13–18] [13–18] [13–18] [13–18] [13–18] [13–18] [19] [19] [20] [21] [21]
thi, 2-thienyl; fur, 2-furyl; pyr, 1-methyl-2-pyrrolyl; BODIPY, boron-dipyrromethene; 4-mor, 4-morpholinyl. X: no data available. Potential shifts: DE - potential difference between reference and assessed compound: DE(Mo) = E(Mo) – E(Cr); DE(W) = E(W) – E(Cr). All data in V. (CO)5Cr=C(OEt)(thi): Eox = 0.496 V, Ered = −1.625 V, vs. Fc/Fc+ [15]. a
Peak potential difference.
Oxidation potentials of the W compounds are the most positive, Mo complexes are oxidized always at slightly less positive potentials and the Cr compounds are
oxidized most easily. In all cases, the difference of oxidation potential between Cr and Mo is several times larger than that between Mo and W. This is in
Table 2 Replacement of CO. (a) Reference compounds: (CO)5M]C(OEt)(fur); derived compounds: (CO)4LM]C(OEt)(fur). DE(ox) M L Cr Mo W Mo W Mo W
PPh3 PPh3 PPh3 AsPh3 AsPh3 SbPh3 SbPh3
−0.51 −0.31 −0.31 −0.45 −0.29 −0.44 −0.26
(b) Reference compounds: (CO)5M]C(OEt)(fur), derived compounds: (CO)3LM]C(OEt)(fur) M bidentate L DE(ox) Cr Mo W
dppe dppe dppe
−0.84 −0.76 −0.75a
DE(red)
Reference
−0.18 −0.19 −0.20 −0.17 −0.17 −0.17 −0.14
[15] [16] [27] [28*] [28*] [29*] [29*]
DE(red)
Reference
−0.55 −0.49 −0.58
[15] [16,18] [17,30]
(c) Reference compounds: (CO)5M]C(NMe2)(Ph); derived compounds: (CO)4M]C (N{CH2CH]CH2}{h2-CH2CH]CH2})(Ph) (chelating ligand h2-CH2CH=CH2} M L DE(ox) DE(red) Cr W Feb
2
{h -CH2CH]CH2} {h2-CH2CH]CH2} {h2-CH2CH]CH2}
−0.33 −0.22 0.05
0.0 −0.03 −0.12
Reference [24**]
fur, 2-furyl; dppe, 1,2-bis(diphenylphosphinoethane). Potential shift DE = Ederived – Ereference. All data in V. (CO)5Cr=C(OEt)(fur): Eox = 0.454 V, Ered = −1.719 V, vs. Fc/Fc+ [15]. a
Peak potential difference.
b
One CO ligand less in both reference and derived compound. Reference compound (CO)4Fe=C (NMe2)(Ph), derived compound (CO)3Fe=C (N {CH2CH=CH2}{h2-CH2CH=CH2})(Ph).
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agreement with general trends in the reactivity of dmetals (similarity between 4d and 5d metals). Replacement of L
CO ligand is mostly used in FCC as a very good pacceptor, stabilizing metals in their low oxidation states. Replacement of CO by another ligand leads usually to lower p-acceptor properties of the set of ligands L and, consequently, to higher electron density on the metal centre and easier oxidation process (as the oxidation process is metal-centred). At the same time, more difficult (more negative) reduction process can be expected. Nonequivalence of the Eox and Ered shifts shows that molecules with less CO substituents tend to lower chemical stability as they have a narrower zone of redox stability DE = jEox e Eredj. Available data published for structurally identical complexes of Cr, Mo and W where one CO ligand was replaced by PPh3 (or AsPh3 or SbPh3) are summarized in Table 2. As expected, the oxidation potential is influenced always more markedly than the reduction potential. The highest shifts of Eox (towards more positive values) were observed in Cr compounds; hence, they are more sensitive towards changes of ligand(s) L than Mo and W complexes. Oxidation potential shifts caused by the substitution of CO by another ligand can be treated either by Pickett’s ligand parameters PL [22] or by Lever’s parameters EL, IM and SM [23]. The set of Lever’s parameters is more general, taking into account also properties of the metal by means of IM and SM. The predicted value of redox potential E: E = (SMSEL) þ IM.
The ligand parameters are additive in principle, and the trend of the observed Eox changes is in agreement with their relative value [24]. For central Cr atom, change of CO for h2-allyl group (Figure 2) shifts Eox by 0.3 V and change of CO for PPh3 group shifts Eox by 0.5 V to less positive values (Table 2a), whereas tabulated PL values are 0.21 and 0.35, respectively [25]. Replacing one Figure 2
CO, effect of PPh3, AsPh3 and SbPh3 appeared to be rather similar in W complexes (change within 0.05 V). In Mo complexes, the oxidation potential of the PPh3 derivative is lower by 0.31 V, whereas those of the AsPh3 and SbPh3 derivatives are shifted by 0.44 V towards less positive values (Table 2a). Substitution of the second CO group in diphosphine (1,2bis(diphenylphosphinoethane), dppe) complexes further emphasizes the trend we have described in phosphines (Table 2b). Substitution of one methyl in the diethylamino group by allyl leads to another structural type. The allyl group is able to be coordinated through its double bond under the formation of a chelate, replacing one carbonyl ligand of the central metal (Table 2c, Figure 2). Again, chromium compounds are more sensitive to the change than tungsten compounds. Their reduction potentials are nearly equal because the coordinated allyl and methyl have approximately the same substituent effect. The shift of the oxidation potential towards less positive values caused by the substitution of CO by allyl is less pronounced than in the case of PPh3 ligands according to the respective electron donating properties. The new formed new chelate ring represents another bridge between the carbene and the metal part of the molecule compensating partly potential differences. A very different picture is seen in the pair of complexes with central Fe atom [24]. These molecules, in agreement with the 18-electron rule, have no more pseudooctahedral geometry; their coordination number is lower by one. Substitution of CO by h2-allyl ligand does not change Eox significantly but Ered by 120 mV more negatively. This can be explained [26] by a better orbital overlap of ligands with Fe atom enabled by lower interligand repulsion, which leads to better back-donation to the allyl p* orbitals (Eox) and to carbene moiety (Ered) (Table 2c). When nitrogen of the morpholinyl side group in (CO)5Cr]C(NH{4-mor})C10H19]CH2 coordinates to Cr instead of one CO ligand, a chelate (CO)4Cr] C(NH{4-mor})C10H19]CH2 is formed and its oxidation potential is shifted by 0.34 V to less positive values [21]. This behaviour is in agreement with the aforementioned analogous cases. The tetracoordinated rhodium carbenes described in Ref. [31] behave very similarly like the group 6 carbenes. In contrast to the five-coordinated Fe complex (d8, 5 ligands), these two sets of molecules are well stabilized electronically (from the central metal point of view) being either d6 octahedral (Group 6) or d8 squareplanar (Rh) arrangements.
Chelate ring formation by a side-chain allyl group [24**]. Current Opinion in Electrochemistry 2019, 15:165–174
When 2 CO groups in (CO)2ClRh]C(Fc)(NHPr) are replaced by cyclooctadiene (COD), yielding (COD) www.sciencedirect.com
Electrochemical approach to Fischer carbene Hoskovcová and Ludvík
Figure 3
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Table 3 Replacement of X Reference compounds: (CO)3L2M]C(OEt)(A); Derived compounds: CO)3L2M]C(amine)(A).
Rh(I) carbenes studied by Ramollo et al [31].
ClRh]C(Fc)(NHPr) (Figure 3) [19], its oxidation is more easy, as in compounds mentioned in Table 2. Because both of these molecules contain two oxidizable centres, oxidation occurs in two steps and both steps will be influenced by COD: the first one was identified as Fc/Fcþ, and after the substitution, its oxidation potential is shifted by 0.11 V less positively. The second step corresponding to the oxidation of the central atom, Rh(I/II), is shifted after the substitution by 0.35 V in the same direction. The larger shift of Rh(I/II) than that of ferrocene is caused by the shorter distance between the oxidized centre and the site at which the structural change occurred. Replacement of X: alkoxy vs. aminocarbenes
The amino group is more electron donating than alkoxy substituent; therefore, replacement of alkoxy group by amino group always shifts both oxidation and reduction potentials to less positive (= more negative) values. Owing to the closer proximity of X to the carbene carbon reduction centre than to the metal-centred site of oxidation, the two redox centres are unevenly sensitive to this structural change: the reduction potential is more affected (typically by 0.3e0.6 V) than the oxidation one (typically by 0.1e0.2 V). The shift of oxidation potentials corresponds with a trend predicted by Pickett’s model of ligand parametrization. This model [32] is based on [Cr(CO)6] and describes changes of oxidation potential defining an electrochemical ligand parameter PL = E1/2[Cr(CO)5L] e E1/2[Cr(CO)6]. PL values were found to be 0.51 to 0.64 V for oxocarbenes and 0.69 to 0.80 V for aminocarbenes [33]. Although the range is rather broad (E depends also on substituents of the carbene, especially on their ability to take part in p-conjugated systems), it fits well with the observed data: all aminocarbenes are oxidized at less positive potentials (more easily) comparing with the corresponding alkoxy carbenes. The upper part of Table 3 collects data of potential shifts when the ethoxy group in the reference www.sciencedirect.com
M
Amine
A
L2
DE(ox)
DE(red)
Reference
Cr Cr Mo W W Cr Cr Cr Cr Mo W W
NHcy NHcy NHcy NHcy NHcy NHBu NHFc NHcy NHcy NHcy NHcy NHcy
thi fur fur thi fur Fc Fc thi fur fur thi fur
2 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 CO dppe dppe dppe dppe dppe
−0.12 −0.10 −0.06a −0.17a −0.08a −0.09 X −0.22 −0.19 −0.12 −0.08a −0.10a
−0.52a −0.56a −0.56 −0.54 −0.57 Out of range −0.15 Out of range −0.35a −0.35 −0.33 −0.26
[15,34] [15] [16] [17] [17] [35] [36] [15] [15] [16] [17] [17]
cy, cyclohexyl; thi, 2-thienyl; fur, 2-furyl; Fc, ferrocene; dppe, 1,2bis(diphenylphosphinoethane). X: no data available. Potential shift DE = Ederived - Ereference. All data in V. (CO)5W]C(OEt) (thi): Eox = 0.728 V, Ered = −1.521 V, vs. Fc/Fc+ [9]. a
Peak potential difference.
compounds of general formula (CO)5M]C(A)(OEt) is substituted by the aminocyclohexyl group (NHcy). When only 3 CO groups are present and the two remaining coordination positions are occupied by dppe (lower part of Table 3), the shift of Ered caused by the change from alkoxy-to aminocarbonyls is not so pronounced. The explanation is based on the fact that a lower number of CO ligands (strong p-acceptors) in combination with electron donating dppe has a lower capacity to stabilize the partial negative charge on the metal centre; therefore, electron density is partly moved to the carbene carbon (both in the ‘reference’ as well as in the derived compounds). Hence, their reduction potentials are, consequently, shifted to more negative values and the sensitivity towards the oxy-to-amino change is lower (cf. Replacement of L). Another example concerning a less frequent central metal can be taken from Ref. [31] which deals with rhodium carbenes (discussed already above) with carbene substituent A = ferrocene (Fc). The amino carbene (CO)2ClRh]C(Fc)(NHPr) is reduced by 0.35 V more negatively comparing with its ethoxy analogue (CO)2ClRh]C(Fc)(OEt): the trend of the shift fits well with the data for group 6 metals. The first oxidation wave of the amino carbene appears by 0.12 V lower than that of ethoxy derivative; however, like in the case of COD ligand discussed above, the first oxidation process is centred on the ferrocene substituent instead of rhodium (similar effect was observed also in the W case discussed below), and the data of ferrocene oxidation cannot, therefore, be compared with metal centred oxidation potentials. Current Opinion in Electrochemistry 2019, 15:165–174
170 Organic and molecular electrochemistry
Replacement of A and substitution on A
The substituent A is directly attached to the carbene carbon; therefore, any of its changes affects primarily LUMO constitution, thus reduction potential, whereas influence on oxidation potential is always much lower (DEred >> DEox). Replacement of whole A—direct induction effect
There are several examples in the literature both for aminocarbenes (X = NMe2) and alkoxycarbenes (X = OEt) where A = H, Me, (substituted) Ph, ferrocene (Fc) and five-membered heterocycles thienyl (thi), furyl (fur) and 1-methylpyrrolyl (pyr) attached in position 2 or 3. Cr-complexes with A = 2-thi were selected as reference compounds. Because changes in oxidation potentials are nearly negligible, only influence on reduction potentials will be discussed. In case of Cr-complexes of aminocarbenes substituted by five-membered heterocycles, 1methylpyrrolyl derivative is reduced much more negatively than thienyl and furyl derivatives (by ca. 300 mV in 2-hetaryl and by ca. 150 mV in 3-hetaryl derivatives) because of the electron donating methyl group (upper part of Table 4). However, the main systematic effect consists in that all 2-hetaryl compounds are reduced ca by 300 mV more easily (less negatively) than the 3hetaryl isomers; the interpretation will be discussed in the next section. Table 4 Potential shifts due to replacement of A. M
A
DE(ox)
DE(red)
reference
Reference compound: A = 2-thi, (CO)5Cr]C(NMe2)-2-thi Cr H −0.04 −0.53 [19,24**] CH3 −0.06 −0.66 Ph −0.03 −0.35 [37*] 2-fur +0.04 +0.01 2-pyr −0.03 −0.29 3-thi −0.02 −0.31 3-fur +0.02 −0.31 3-pyr −0.09 −0.44 Reference compounds: (CO)5M]C(OEt)-2-thi Cr 2-fur −0.04 −0.10 [14] 2-pyr −0.07 −0.39 Mo 2-fur −0.06a −0.10a [16] 2-pyr −0.09a −0.48a [18] W 2-fur −0.03a −0.08 [13] 2-pyr −0.10a −0.44 Cr Fc −0.21 −0.56 [35] W Fc (+0.08a b) −0.56 [38,39]
Similar behaviour, that is, more negative reduction potentials in 1-methylpyrrolyl derivatives then in furyl and thienyl ones, is observed also in analogous Cr-, Mo- and W-alkoxycarbenes substituted only by 2-hetaryl substituents. The reduction potential is here more sensitive to direct substitution than in aminocarbenes (lower part of Table 4): 2-fur derivatives are reduced by ca. 100 mV and 2-pyr derivatives are reduced by ca. 390 mV more negatively than the reference 2-thi compounds [13,14,16,18]. The influence of substituents directly attached to the carbene carbon on the Ered is very strong and depends on the electron donating/withdrawing properties of A. It is difficult to quantify them, nevertheless, the most reasonable is to use the substituent’s constants smeta [40,41] because they reflect only the induction effect. Taking into account three molecules where smeta of A is available, (CO)5Cr]C(NMe2)A, A = Ph, Me, H [23,31], the plot of the three reduction potentials vs. smeta is linear with the slope (r-valuedsee below) equal to rred = 2.12, hence extremely high (for comparison, rox = 0.38). When A = 2-thi is replaced by ferrocene, Ered is shifted strongly to more negative values (Table 4). Similarly, electrochemical behaviour of Mn(I) carbene complex derived from cymantrene (MnI(CO)3Cp; Cp = cyclopentadienyl) where one CO ligand is formally replaced by a double bond to a alkoxycarbene ligand, where A = 2-thi, 2-fur or Fc ((CO)2(Cp)MnI] C(OEt)A, Figure 4), should be mentioned [42]: reduction of thienyl and furyl derivatives takes place at similar potential values, unlike the ferrocene derivative reduction which moves out of the potential window due to large negative shift. These are other examples of how strong influence on the reduction potential of a carbene complex has a direct substitution of A. Oxidation of these Mn(I) compounds is much easier than oxidation of cymantrene, in accordance with the lower number of CO ligands (see above). The aforementioned comparison of Mn(I) FCC with Cr(0) or other group 6 FCC is based on the electronic and Figure 4
2-fur, 2-furyl; 2-thi, 2-thienyl; 2-pyr, 1-methyl-2-pyrrolyl; 3-fur, 3-furyl; 3-thi, 3-thienyl; 3-pyr, 1-methyl-3-pyrrolyl; Fc, ferrocene. Potential shift DE ]Ederived – Ereference All data in V. (CO)5Cr]C(NMe2)2-thi, Eox = 0.855 V, Ered = −1.600V, vs. SCE [30]. a
Peak potential difference.
b
Oxidation of the ferrocene moiety precedes oxidation of Wcomplex.
Current Opinion in Electrochemistry 2019, 15:165–174
Mn(I) carbenes derived from cymantrene investigated by Bezuidenhout et al [42]. www.sciencedirect.com
Electrochemical approach to Fischer carbene Hoskovcová and Ludvík
coordination similarity of these atoms: they all have d6 electronic configuration and 6 ligands (h5-Cp is equivalent to 3 CO).
171
Figure 5
The ferrocene moiety introduces a new, electron rich, redox active substructure into the complex with its own Eox. Its presence shifts reduction as well as oxidation potentials to a more negative value. The negative shift of Ered was discussed previously. A similar effect is observed in the oxidation of Cr and Mn complexes, where their Eox is lower; hence, the first oxidation process is directed to Cr or Mn atom. In W complexes, thanks to their high oxidation stability, oxidation of ferrocene proceeds first (cf. Table 4). Substitution of A: linear free energy relationship approach
Another set of molecules [19] [24**], with A = p-R-Ph: (CO)5Cr]C(NMe2)p-R-Ph, R = OCH3, CH3, H, Cl, CF3, COOEt and COOH, represents a homogeneous series where the substituent in p-position influences through the phenyl ring the reducibility (and also very slightly the oxidizability) of the carbene complexes by means of their substituent’s constants spara of the Hammett type (in contrast to smeta, they reflect the sum of induction and resonance effects). The shift of Ered can be in this case evaluated quantitatively using linear free energy relationship (LFER) by plotting a dependence of Ered(ox) on the Hammett constant spara: [16,34,37]. Ered(ox) = rred(ox) $ s þ c
The constants spara reflect the electron donating (negative values of spara) and withdrawing (positive values of spara) properties of the substituents; the slope r represents the sensitivity of Ered(ox) towards the influence of substituents which depends on the (i) distance of the substitution from the redox centre and/or (ii) extent of p-delocalization between the substituent and redox centre. In the aforementioned series, for R = OCH3, CH3, H, Cl, CF3, the plots are linear with the rred = 0.344 (R2 = 0,957) and rox = 0.082 (R2 = 0, 908) that means that Eox is ca. 4 times less sensitive to the substitution than Ered because of longer distance and low level of delocalization [19], [24**] Figure 5. Although the linearity could be expected, the main message is that all compounds in this series are reduced according to the same mechanism, the LUMO involves the same atoms and has the same shape (cf Figure 1c); hence, the reduction centre is the same in all molecules. In addition to this, the plot of observed Ered vs. calculated LUMO energy is also linear confirming the experimental results and their interpretation and simultaneously proving the reasonability of calculations. www.sciencedirect.com
Hammett plot of the reduction potentials of (CO)5Cr]C(NMe2)p-R-Ph, R = OCH3, CH3, H, Cl, CF3, COOEt (:) and COOH (B).
Role of electron delocalization
After adding the last two compounds of the above series, where A = para-R-phenyl (R = COOEt and COOH), to the plot in Figure 5, their values of Ered do not fit the linear dependence and are by about 150e170 mV less negative than expected from the plot. This anomaly undoubtedly points to the different mechanism of the electron transfer. The quantum chemical calculations showed that the carbonyl in p-position completely changes the LUMO: together with the partial C]N double bond, a quinoidal structure is formed, and due to the more extended p-system, the reduction takes place more easily. To present the crucial difference, the LUMOs of carbene complexes with -OMe and eCOOEt is depicted in Figure 6. The same effect of p-carbonyl substituents was observed also in Fe complexes [24**]. This finding shows the importance of the LFER treatment for revealing of different mechanisms during the evaluation of electrochemical data of a series of compounds. Influence of the more delocalized p-electron system on the Ered can be shown also in case of the molecules where A is represented by five-membered heterocycles. It was observed (and mentioned above - Table 4) that 2hetaryl derivatives are reduced remarkably (by 150e 300 mV) easier than their 3 isomers [37*]. The reason is that the heterocycle attached in position 2 is included in conjugation and extends the p-system. This interpretation was confirmed by quantum chemical calculation of the respective LUMOs e Figure 7. Another similar series is formed by compounds where A represents a five-membered ring attached to the carbene carbon in position 2 (2-fur, 2-thi or 2-pyr) which Current Opinion in Electrochemistry 2019, 15:165–174
172 Organic and molecular electrochemistry
Figure 6
LUMO of (CO)5Cr]C(NMe2)p-R-Ph, where R = –OMe (left) and –COOEt (right).
Figure 7
LUMO of (CO)5Cr]C(NMe2)-2-thi (left) and of (CO)5Cr]C(NMe2)-3-thi (right) [37*].
is further substituted in its position 5 by 2-thienyl [13,14,18]. The experiments showed that these two heterocycles interconnected through positions 2- and 5further extend the delocalized system; therefore, their reduction potentials are shifted to less negative values by ca. 100e130 mV in comparison to their ‘parent’ compounds substituted by a single heterocycle. Oxidation is also slightly (by 10e30 mV) shifted to less positive potentials. It is another example that substitution extending the conjugated system results in easier reduction as well as oxidation. Current Opinion in Electrochemistry 2019, 15:165–174
Conclusion Evaluation and comparison of experimental data allow for a brighter insight into how the structural changes of FCCs influence their redox properties. Systematizing the available data into several series leads to general rules concerning redox potentials of FCC: The oxidation process is directed to the metal region. It depends first of all on the metal nature; dependence on the CO substitution is very important, too. Replacing a CO ligand always results in easier www.sciencedirect.com
Electrochemical approach to Fischer carbene Hoskovcová and Ludvík
oxidation. The impact of heteroatom X nature or of carbene carbon substituent A is small. The reduction potential of these molecules depends prevalently on the carbene carbon substitution. o Replacing an alkoxy group with an amino group results in more difficult reduction (and easier oxidation, the effect on reduction is more pronounced). o Substitution on the carbene carbon affects reduction potential in accordance with the electron donating/withdrawing properties of the substituent A. o The role of p-conjugation between the carbene carbon moiety and A has been revealed using LFER approach. Better p-conjugation makes the reduction process easier by stabilizing of the primary product.
173
7.
Santamaria J: Fischer-type group 6 carbene complexes in the synthesis of optically active molecules. Curr Org Chem 2009, 13:31–46. https://doi.org/10.2174/138527209787193800.
8.
Sierra MA, Gómez-Gallego M, Martínez-Álvarez R: Fischer carbene complexes: beautiful playgrounds to study single electron transfer (SET) reactions. Chem Eur J 2007, 13: 736–744. https://doi.org/10.1002/chem.200601470.
9. **
Weststrate NA, Bouwer S, Hassenrück Ch, van Jaarsveld NA, Liles DC, Winter RF, Lotz S: Synthesis and properties of Fischer carbene complexes of N,N-dimethylaniline and anisole p-coordinated to chromium tricarbonyl. J Organomet Chem 2018, 869:54–66. https://doi.org/10.1016/j.jorganchem. 2018.05.022. An excellent example of multiple experimental approaches to molecule characterization. 10. Metelková R, Hoskovcová I, Polá sek M, Urban J, David T, * Ludvík J: Stereoisomeric products of electrochemical reduction of heterocyclic Fischer aminocarbene Cr(0) complexes. Development of the electrochemistry-mass spectrometry tandem approach using biphasic (acetonitrile-hexane) preparative electrolysis. Electrochim Acta 2015, 162:17–23. https:// doi.org/10.1016/j.electacta.2014.10.110. Biphasic preparative electrolysis with direct MS analysis of products.
These rules enable to understand specific behaviour of some derivatives, to predict redox properties of new compounds and to tune their abilities in the next synthetic generations. Interpretation of the observed redox behaviour is supported by quantum chemical calculations.
11. Anjali BA, Suresh Ch H: Electronic effect of ligands vs. reduction potentials of Fischer carbene complexes of chromium: a molecular electrostatic potential analysis. New J Chem 2018, 42:18217–18224. https://doi.org/10.1039/ C8NJ04184A.
Conflict of interest
13. Landman M, Buitendach BE, Conradie MM, Fraser R, van Rooyen PH, Conradie J: Fischer mono- and biscarbene complexes of tungsten with mono- and dimeric heteroatomic substituents. J Electroanal Chem 2015, 739:202–210. https:// doi.org/10.1016/j.jelechem.2014.12.019.
Nothing declared.
Acknowledgement Financial support was provided by the Czech Science Foundation, Czechia, (grant No. 17-21770S). The authors are grateful to specific university research program (MSMT No 21-SVV/2019) and to institutional support RVO 61388955.
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14. Landman M, Liu R, van Rooyen PH, Conradie J: Electrochemistry of Fischer alkoxycarbene complexes of chromium: the use of density functional theory to predict and understand oxidation and reduction potentials. Electrochim Acta 2013, 114:205 –214. https://doi.org/10.1016/j.electacta. 2013.10.013. 15. Gostynski R, Fraser R, Landman M, Erasmus E, Conradie J: Electrochemical study of chromium(0) Fischer carbene complexes: trends in redox potential. Polyhedron 2017, 127: 323–330. https://doi.org/10.1016/j.poly.2017.02.018. 16. Landman M, Levell TJ, Buitendach BE, Conradie MM, Conradie J: Effect of CO substitution on the redox properties of Fischer Mo(0) carbene complexes Mo(CO)5[C(Y)(2-furyl), Y [ OEt, NHcy or NH2. Electrochim Acta 2015, 174:282–289. https://doi. org/10.1016/j.electacta.2015.05.191. 17. Landman M, Pretorius R, Fraser R, Buitendach BE, Conradie MM, van Rooyen PH, Conradie J: Electrochemical behaviour and structure of novel phosphine- and phosphite-substituted tungsten(0) Fischer carbene complexes. Electrochim Acta 2014, 130:104–118. https://doi.org/10.1016/j.electacta.2014.02. 127. 18. van Rensburg AJ, Landman M, Erasmus E, van der Westhuizen D, Ferreira H, Conradie MM, Conradie J: Electrochemical, and X-ray photoelectron spectroscopic insight into molybdenum(0) Fischer ethoxycarbene complexes. Electrochim Acta 2016, 219:204–213. https://doi.org/10.1016/j.electacta. 2016.09.143. 19. Hoskovcová I, Rohá cová J, Meca L, Tobrman T, Dvorák D, Ludvík J: Electrochemistry of chromium(0)–aminocarbene complexes. The use of intramolecular interaction LFER for characterization of the oxidation and reduction centre of the complex. Electrochim Acta 2005, 50:4911–4915. https://doi.org/ 10.1016/j.electacta.2004.12.047. 20. Chu GM, Guerrero-Martínez A, Ramíres de Arellano C, Fernández I, Sierra MA: Remote control by p-conjugation of the emissive properties of Fischer carbene-BODIPY dyads. Current Opinion in Electrochemistry 2019, 15:165–174
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Current Opinion in Electrochemistry 2019, 15:165–174
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Electrochemistry
ScienceDirect Review Article
Electrochemical approach to Fischer carbene complexes Irena Hoskovcová1 and Jirí Ludvík2 Abstract
Although the chemistry of Fischer carbene complexes is recently a growing direction of research, a review summarizing their electrochemical investigations is still missing. In the present, text evaluation and comparison of experimental data allow insight on how the structural changes of Fischer carbene complexes influence their redox properties. It is evident that the respective reduction and oxidation potentials depend on multiple parameters. To shed light on this problem, the available data were systematized in several series, where effects of individual structural changes were evaluated as shifts of oxidation and reduction potentials with respect to a reference compound. The changes in electron delocalization caused by different substitutions were rationalized using quantum chemical calculations. Addresses 1 Department of Inorganic Chemistry, Institute of Chemical Technology Prague, Prague, Czech Republic 2 J. Heyrovský Institute of Physical Chemistry, Academy of Science of the Czech Republic, Prague, Czech Republic Corresponding author: Hoskovcová, Irena ([email protected]), ([email protected])
Carbene complexes are generally classified into three types with regard to the M]C bond properties: 1. ‘Electrophilic’ Fischer carbene complexes, FCCs, with electrophilic carbene carbon atom, M(d) =C(dþ); 2. ‘Nucleophilic’ Schrock carbenes with nucleophilic carbene carbon M(dþ)=C(d); 3. N-heterocyclic carbenes, NHC, derived by Cdeprotonation of imidazolium salts, with practically single bond MC and no specific carbene carbon reactivity, and also modern NHC complexes bearing cyclic alkyl amino carbene ligands, triazolylidene and other ring expanded ligands. Recently, new types of mixed NHCeFCC carbene complexes are synthesized [1]. This article is devoted to the Fischer type of carbene complexes. It is the first review summarizing the electrochemical approach in the current carbene research, focused on a deeper understanding of the relationship between structure and redox abilities of the title complexes.
Current Opinion in Electrochemistry 2019, 15:165–174 This review comes from a themed issue on Organic and Molecular Electrochemistry Edited by Marc Robert For a complete overview see the Issue and the Editorial Available online 22 June 2019 https://doi.org/10.1016/j.coelec.2019.06.005 2451-9103/© 2019 Elsevier B.V. All rights reserved.
Keywords Fischer carbene complexes, Oxidation potential, Reduction potential, Relationship structure – redox properties.
Introduction Carbenes R2C: were originally found as short-lived reaction intermediates characterized by two unshared valence electrons and an empty p-orbital. These reactive species can be stabilized when bonded to a transition metal atom under formation of a carbene complex of general formula LnM:CR(R0 ). The formal carbon-metal double bond is typical for these complexes. www.sciencedirect.com
The central metal atom in FCCs M is in its low oxidation state which is stabilized by coordination with pelectron acceptor ligands L, usually carbonyls. Owing to the polarization of the formal double bond between the metal and carbene carbon, the latter has electrophilic character. Its partly positive charge is compensated by the attached p-donor X (Figure 1a), typically alkoxy e OR or aminoeNR2 groups (X]N or O). According to the type of heteroatom X, Fischer carbenes are subdivided into aminocarbenes and alkoxycarbenes. This combination of atoms enables a mesomeric shift of
p-electrons, which is crucial for the stability of redox intermediates as well as for possible involvement of the carbene moiety in a larger delocalized system where the C]X bond has partly character of a double bond. M(d) = C(dþ) e X 4 M(d) e C = X(dþ)
The second substituent on the carbene carbon, A, is very often an aromate or another unsaturated grouping which can be also involved in an extended delocalized Current Opinion in Electrochemistry 2019, 15:165–174
166 Organic and molecular electrochemistry
Figure 1
(a) General structure of the discussed compounds, (b) calculated HOMO and (c) LUMO of the compound (CO)5M]C(NMe2)Ph.
system (X-C-A) to stabilize the molecule. The substitution of A and on A (R; namely in p-position) can be then used for a fine systematic tuning of the reduction potential. Electrochemical approach
FCCs can be used namely as homogeneous catalysts [2e 4] or as precursors in organic synthesis [5e7]. A general overview of electron transfer processes and subsequent reactivity is given in Ref. [8]. Owing to the polarization of the M]C bond, the molecule has (at least) two redox active centres: electrophilic (reducible) carbene carbon atom and nucleophilic (oxidizable) metal centre. Redox properties of both centres can be tuned independently and are decisive for their specific application. The principle of action of FCC as a catalyst lies in the reduction of the parent complex, resulting in the splitting of the metal-carbene carbon bond and thus in releasing the desired reactive carbene intermediate R2C:. For organic synthesis, oxidation of the parent compounds is also important resulting, for example, in binuclear complexes. In accordance with the aforementioned electron distribution, reduction potential depends above all on the nature of the heteroatom X and on the properties, both electronic and steric, of the substituent A, whereas differences in ligands L or the nature of the central metal are less significant. Oxidation process is directed to the metal region, and the most important shift of oxidation potential can be achieved by replacing the central metal atom M or a ligand L. For illustration, calculated shape of the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, of the compound (CO)5M]C(NMe2) Ph is presented in Figure 1. Evidently, it is properly molecular electrochemistry which can give information about the relationship between structure and reactivity of Fischer carbene complexes or about the stability of their redox intermediates. The most significant electrochemically acquired data are the first reduction and/or the first oxidation potential, their difference (proportional to Current Opinion in Electrochemistry 2019, 15:165–174
the HOMOeLUMO gap), number of electrons involved, possible reversibility of these primary steps and presence of radical species. In the latter case, an in-situ combination of ultravioletevisible and/or electron paramagnetic resonance spectrometry with simultaneous electrochemical generation is very useful[9**], [10*]. The detailed interpretation of homologous series of electrochemical data can give information about mechanism of the reduction (oxidation) process, about ‘anomalous’ behaviour of some derivative and about distribution/delocalization of electrons within the molecule [11,12]. All this knowledge is important in the design of new generations of catalysts or synthetic precursors and in tuning their properties. The present text aims to summarize how changes in FCC composition (namely change of the central metal atom, ligand L, carbonyl atom substituent A and heteroatom X) are reflected in their electrochemical behaviour and redox properties. For the sake of clarity, we decided to compare not the redox potential values themselves generally but their relative shifts within a homologous series with regard to a selected reference compound DE = E(derivative) e E(reference). In this way, positive DE means a shift to more positive values (i.e. more difficult oxidation, easier reduction) and vice versa. This approach enables to compare data from various sources and gives us a direct measure for the importance of the change of composition. Replacement of M
The only data available for comparison are those concerning group 6 (Cr, Mo, W) metals. Carbene complexes of Mn, Re, Fe and Rh have different stoichiometry, and there are no available data on pairs of molecules that would differ only by the central metal atom. Data in Table 1 show that the potential shift which accompanies the central metal replacement is up to 10 times higher for the oxidation potential than for reduction, where the changes are very small, usually in tens of mV. This is experimental proof that the centre of oxidation is the metallic part (cf. Figure 1). www.sciencedirect.com
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167
Table 1 Replacement of M. Reference compounds: Cr complexes. Compound
M = Mo
(CO)5M]C(OEt)thi (CO)5M]C(OEt)fur (CO)5M]C(OEt)pyr (CO)5M]C(NHcy)fur (CO)4PPh3M]C(OEt)fur (CO)3dppeM]C(OEt)thi (CO)3dppeM]C(OEt)fur (CO)5M]C(NMe2)Ph (CO)4M]C(N{CH2CH]CH2}{h2-CH2CH]CH2})Ph (CO)5M]C(OEt)BODIPY (CO)5M]C(NH{4-mor})CH3 (CO)5M]C(NH{4-mor})C10H19]CH2
M=W
References
DE(ox)
DE(red)
DE(ox)
DE(red)
0.15a 0.14a 0.14a 0.14a 0.35a X 0.26 X X X X X
0.03a 0.02 0.07a 0.02a 0.02a X 0.08 X X X X X
0.19a 0.20a 0.17a 0.19a 0.41a 0.24a 0.31a 0.18 0.29 0.12 0.11 0.09
0.07 0.08 0.02 0.08a 0.06a 0.09 0.082 0.025 0.057 X X X
[13–18] [13–18] [13–18] [13–18] [13–18] [13–18] [13–18] [19] [19] [20] [21] [21]
thi, 2-thienyl; fur, 2-furyl; pyr, 1-methyl-2-pyrrolyl; BODIPY, boron-dipyrromethene; 4-mor, 4-morpholinyl. X: no data available. Potential shifts: DE - potential difference between reference and assessed compound: DE(Mo) = E(Mo) – E(Cr); DE(W) = E(W) – E(Cr). All data in V. (CO)5Cr=C(OEt)(thi): Eox = 0.496 V, Ered = −1.625 V, vs. Fc/Fc+ [15]. a
Peak potential difference.
Oxidation potentials of the W compounds are the most positive, Mo complexes are oxidized always at slightly less positive potentials and the Cr compounds are
oxidized most easily. In all cases, the difference of oxidation potential between Cr and Mo is several times larger than that between Mo and W. This is in
Table 2 Replacement of CO. (a) Reference compounds: (CO)5M]C(OEt)(fur); derived compounds: (CO)4LM]C(OEt)(fur). DE(ox) M L Cr Mo W Mo W Mo W
PPh3 PPh3 PPh3 AsPh3 AsPh3 SbPh3 SbPh3
−0.51 −0.31 −0.31 −0.45 −0.29 −0.44 −0.26
(b) Reference compounds: (CO)5M]C(OEt)(fur), derived compounds: (CO)3LM]C(OEt)(fur) M bidentate L DE(ox) Cr Mo W
dppe dppe dppe
−0.84 −0.76 −0.75a
DE(red)
Reference
−0.18 −0.19 −0.20 −0.17 −0.17 −0.17 −0.14
[15] [16] [27] [28*] [28*] [29*] [29*]
DE(red)
Reference
−0.55 −0.49 −0.58
[15] [16,18] [17,30]
(c) Reference compounds: (CO)5M]C(NMe2)(Ph); derived compounds: (CO)4M]C (N{CH2CH]CH2}{h2-CH2CH]CH2})(Ph) (chelating ligand h2-CH2CH=CH2} M L DE(ox) DE(red) Cr W Feb
2
{h -CH2CH]CH2} {h2-CH2CH]CH2} {h2-CH2CH]CH2}
−0.33 −0.22 0.05
0.0 −0.03 −0.12
Reference [24**]
fur, 2-furyl; dppe, 1,2-bis(diphenylphosphinoethane). Potential shift DE = Ederived – Ereference. All data in V. (CO)5Cr=C(OEt)(fur): Eox = 0.454 V, Ered = −1.719 V, vs. Fc/Fc+ [15]. a
Peak potential difference.
b
One CO ligand less in both reference and derived compound. Reference compound (CO)4Fe=C (NMe2)(Ph), derived compound (CO)3Fe=C (N {CH2CH=CH2}{h2-CH2CH=CH2})(Ph).
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agreement with general trends in the reactivity of dmetals (similarity between 4d and 5d metals). Replacement of L
CO ligand is mostly used in FCC as a very good pacceptor, stabilizing metals in their low oxidation states. Replacement of CO by another ligand leads usually to lower p-acceptor properties of the set of ligands L and, consequently, to higher electron density on the metal centre and easier oxidation process (as the oxidation process is metal-centred). At the same time, more difficult (more negative) reduction process can be expected. Nonequivalence of the Eox and Ered shifts shows that molecules with less CO substituents tend to lower chemical stability as they have a narrower zone of redox stability DE = jEox e Eredj. Available data published for structurally identical complexes of Cr, Mo and W where one CO ligand was replaced by PPh3 (or AsPh3 or SbPh3) are summarized in Table 2. As expected, the oxidation potential is influenced always more markedly than the reduction potential. The highest shifts of Eox (towards more positive values) were observed in Cr compounds; hence, they are more sensitive towards changes of ligand(s) L than Mo and W complexes. Oxidation potential shifts caused by the substitution of CO by another ligand can be treated either by Pickett’s ligand parameters PL [22] or by Lever’s parameters EL, IM and SM [23]. The set of Lever’s parameters is more general, taking into account also properties of the metal by means of IM and SM. The predicted value of redox potential E: E = (SMSEL) þ IM.
The ligand parameters are additive in principle, and the trend of the observed Eox changes is in agreement with their relative value [24]. For central Cr atom, change of CO for h2-allyl group (Figure 2) shifts Eox by 0.3 V and change of CO for PPh3 group shifts Eox by 0.5 V to less positive values (Table 2a), whereas tabulated PL values are 0.21 and 0.35, respectively [25]. Replacing one Figure 2
CO, effect of PPh3, AsPh3 and SbPh3 appeared to be rather similar in W complexes (change within 0.05 V). In Mo complexes, the oxidation potential of the PPh3 derivative is lower by 0.31 V, whereas those of the AsPh3 and SbPh3 derivatives are shifted by 0.44 V towards less positive values (Table 2a). Substitution of the second CO group in diphosphine (1,2bis(diphenylphosphinoethane), dppe) complexes further emphasizes the trend we have described in phosphines (Table 2b). Substitution of one methyl in the diethylamino group by allyl leads to another structural type. The allyl group is able to be coordinated through its double bond under the formation of a chelate, replacing one carbonyl ligand of the central metal (Table 2c, Figure 2). Again, chromium compounds are more sensitive to the change than tungsten compounds. Their reduction potentials are nearly equal because the coordinated allyl and methyl have approximately the same substituent effect. The shift of the oxidation potential towards less positive values caused by the substitution of CO by allyl is less pronounced than in the case of PPh3 ligands according to the respective electron donating properties. The new formed new chelate ring represents another bridge between the carbene and the metal part of the molecule compensating partly potential differences. A very different picture is seen in the pair of complexes with central Fe atom [24]. These molecules, in agreement with the 18-electron rule, have no more pseudooctahedral geometry; their coordination number is lower by one. Substitution of CO by h2-allyl ligand does not change Eox significantly but Ered by 120 mV more negatively. This can be explained [26] by a better orbital overlap of ligands with Fe atom enabled by lower interligand repulsion, which leads to better back-donation to the allyl p* orbitals (Eox) and to carbene moiety (Ered) (Table 2c). When nitrogen of the morpholinyl side group in (CO)5Cr]C(NH{4-mor})C10H19]CH2 coordinates to Cr instead of one CO ligand, a chelate (CO)4Cr] C(NH{4-mor})C10H19]CH2 is formed and its oxidation potential is shifted by 0.34 V to less positive values [21]. This behaviour is in agreement with the aforementioned analogous cases. The tetracoordinated rhodium carbenes described in Ref. [31] behave very similarly like the group 6 carbenes. In contrast to the five-coordinated Fe complex (d8, 5 ligands), these two sets of molecules are well stabilized electronically (from the central metal point of view) being either d6 octahedral (Group 6) or d8 squareplanar (Rh) arrangements.
Chelate ring formation by a side-chain allyl group [24**]. Current Opinion in Electrochemistry 2019, 15:165–174
When 2 CO groups in (CO)2ClRh]C(Fc)(NHPr) are replaced by cyclooctadiene (COD), yielding (COD) www.sciencedirect.com
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Figure 3
169
Table 3 Replacement of X Reference compounds: (CO)3L2M]C(OEt)(A); Derived compounds: CO)3L2M]C(amine)(A).
Rh(I) carbenes studied by Ramollo et al [31].
ClRh]C(Fc)(NHPr) (Figure 3) [19], its oxidation is more easy, as in compounds mentioned in Table 2. Because both of these molecules contain two oxidizable centres, oxidation occurs in two steps and both steps will be influenced by COD: the first one was identified as Fc/Fcþ, and after the substitution, its oxidation potential is shifted by 0.11 V less positively. The second step corresponding to the oxidation of the central atom, Rh(I/II), is shifted after the substitution by 0.35 V in the same direction. The larger shift of Rh(I/II) than that of ferrocene is caused by the shorter distance between the oxidized centre and the site at which the structural change occurred. Replacement of X: alkoxy vs. aminocarbenes
The amino group is more electron donating than alkoxy substituent; therefore, replacement of alkoxy group by amino group always shifts both oxidation and reduction potentials to less positive (= more negative) values. Owing to the closer proximity of X to the carbene carbon reduction centre than to the metal-centred site of oxidation, the two redox centres are unevenly sensitive to this structural change: the reduction potential is more affected (typically by 0.3e0.6 V) than the oxidation one (typically by 0.1e0.2 V). The shift of oxidation potentials corresponds with a trend predicted by Pickett’s model of ligand parametrization. This model [32] is based on [Cr(CO)6] and describes changes of oxidation potential defining an electrochemical ligand parameter PL = E1/2[Cr(CO)5L] e E1/2[Cr(CO)6]. PL values were found to be 0.51 to 0.64 V for oxocarbenes and 0.69 to 0.80 V for aminocarbenes [33]. Although the range is rather broad (E depends also on substituents of the carbene, especially on their ability to take part in p-conjugated systems), it fits well with the observed data: all aminocarbenes are oxidized at less positive potentials (more easily) comparing with the corresponding alkoxy carbenes. The upper part of Table 3 collects data of potential shifts when the ethoxy group in the reference www.sciencedirect.com
M
Amine
A
L2
DE(ox)
DE(red)
Reference
Cr Cr Mo W W Cr Cr Cr Cr Mo W W
NHcy NHcy NHcy NHcy NHcy NHBu NHFc NHcy NHcy NHcy NHcy NHcy
thi fur fur thi fur Fc Fc thi fur fur thi fur
2 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 CO dppe dppe dppe dppe dppe
−0.12 −0.10 −0.06a −0.17a −0.08a −0.09 X −0.22 −0.19 −0.12 −0.08a −0.10a
−0.52a −0.56a −0.56 −0.54 −0.57 Out of range −0.15 Out of range −0.35a −0.35 −0.33 −0.26
[15,34] [15] [16] [17] [17] [35] [36] [15] [15] [16] [17] [17]
cy, cyclohexyl; thi, 2-thienyl; fur, 2-furyl; Fc, ferrocene; dppe, 1,2bis(diphenylphosphinoethane). X: no data available. Potential shift DE = Ederived - Ereference. All data in V. (CO)5W]C(OEt) (thi): Eox = 0.728 V, Ered = −1.521 V, vs. Fc/Fc+ [9]. a
Peak potential difference.
compounds of general formula (CO)5M]C(A)(OEt) is substituted by the aminocyclohexyl group (NHcy). When only 3 CO groups are present and the two remaining coordination positions are occupied by dppe (lower part of Table 3), the shift of Ered caused by the change from alkoxy-to aminocarbonyls is not so pronounced. The explanation is based on the fact that a lower number of CO ligands (strong p-acceptors) in combination with electron donating dppe has a lower capacity to stabilize the partial negative charge on the metal centre; therefore, electron density is partly moved to the carbene carbon (both in the ‘reference’ as well as in the derived compounds). Hence, their reduction potentials are, consequently, shifted to more negative values and the sensitivity towards the oxy-to-amino change is lower (cf. Replacement of L). Another example concerning a less frequent central metal can be taken from Ref. [31] which deals with rhodium carbenes (discussed already above) with carbene substituent A = ferrocene (Fc). The amino carbene (CO)2ClRh]C(Fc)(NHPr) is reduced by 0.35 V more negatively comparing with its ethoxy analogue (CO)2ClRh]C(Fc)(OEt): the trend of the shift fits well with the data for group 6 metals. The first oxidation wave of the amino carbene appears by 0.12 V lower than that of ethoxy derivative; however, like in the case of COD ligand discussed above, the first oxidation process is centred on the ferrocene substituent instead of rhodium (similar effect was observed also in the W case discussed below), and the data of ferrocene oxidation cannot, therefore, be compared with metal centred oxidation potentials. Current Opinion in Electrochemistry 2019, 15:165–174
170 Organic and molecular electrochemistry
Replacement of A and substitution on A
The substituent A is directly attached to the carbene carbon; therefore, any of its changes affects primarily LUMO constitution, thus reduction potential, whereas influence on oxidation potential is always much lower (DEred >> DEox). Replacement of whole A—direct induction effect
There are several examples in the literature both for aminocarbenes (X = NMe2) and alkoxycarbenes (X = OEt) where A = H, Me, (substituted) Ph, ferrocene (Fc) and five-membered heterocycles thienyl (thi), furyl (fur) and 1-methylpyrrolyl (pyr) attached in position 2 or 3. Cr-complexes with A = 2-thi were selected as reference compounds. Because changes in oxidation potentials are nearly negligible, only influence on reduction potentials will be discussed. In case of Cr-complexes of aminocarbenes substituted by five-membered heterocycles, 1methylpyrrolyl derivative is reduced much more negatively than thienyl and furyl derivatives (by ca. 300 mV in 2-hetaryl and by ca. 150 mV in 3-hetaryl derivatives) because of the electron donating methyl group (upper part of Table 4). However, the main systematic effect consists in that all 2-hetaryl compounds are reduced ca by 300 mV more easily (less negatively) than the 3hetaryl isomers; the interpretation will be discussed in the next section. Table 4 Potential shifts due to replacement of A. M
A
DE(ox)
DE(red)
reference
Reference compound: A = 2-thi, (CO)5Cr]C(NMe2)-2-thi Cr H −0.04 −0.53 [19,24**] CH3 −0.06 −0.66 Ph −0.03 −0.35 [37*] 2-fur +0.04 +0.01 2-pyr −0.03 −0.29 3-thi −0.02 −0.31 3-fur +0.02 −0.31 3-pyr −0.09 −0.44 Reference compounds: (CO)5M]C(OEt)-2-thi Cr 2-fur −0.04 −0.10 [14] 2-pyr −0.07 −0.39 Mo 2-fur −0.06a −0.10a [16] 2-pyr −0.09a −0.48a [18] W 2-fur −0.03a −0.08 [13] 2-pyr −0.10a −0.44 Cr Fc −0.21 −0.56 [35] W Fc (+0.08a b) −0.56 [38,39]
Similar behaviour, that is, more negative reduction potentials in 1-methylpyrrolyl derivatives then in furyl and thienyl ones, is observed also in analogous Cr-, Mo- and W-alkoxycarbenes substituted only by 2-hetaryl substituents. The reduction potential is here more sensitive to direct substitution than in aminocarbenes (lower part of Table 4): 2-fur derivatives are reduced by ca. 100 mV and 2-pyr derivatives are reduced by ca. 390 mV more negatively than the reference 2-thi compounds [13,14,16,18]. The influence of substituents directly attached to the carbene carbon on the Ered is very strong and depends on the electron donating/withdrawing properties of A. It is difficult to quantify them, nevertheless, the most reasonable is to use the substituent’s constants smeta [40,41] because they reflect only the induction effect. Taking into account three molecules where smeta of A is available, (CO)5Cr]C(NMe2)A, A = Ph, Me, H [23,31], the plot of the three reduction potentials vs. smeta is linear with the slope (r-valuedsee below) equal to rred = 2.12, hence extremely high (for comparison, rox = 0.38). When A = 2-thi is replaced by ferrocene, Ered is shifted strongly to more negative values (Table 4). Similarly, electrochemical behaviour of Mn(I) carbene complex derived from cymantrene (MnI(CO)3Cp; Cp = cyclopentadienyl) where one CO ligand is formally replaced by a double bond to a alkoxycarbene ligand, where A = 2-thi, 2-fur or Fc ((CO)2(Cp)MnI] C(OEt)A, Figure 4), should be mentioned [42]: reduction of thienyl and furyl derivatives takes place at similar potential values, unlike the ferrocene derivative reduction which moves out of the potential window due to large negative shift. These are other examples of how strong influence on the reduction potential of a carbene complex has a direct substitution of A. Oxidation of these Mn(I) compounds is much easier than oxidation of cymantrene, in accordance with the lower number of CO ligands (see above). The aforementioned comparison of Mn(I) FCC with Cr(0) or other group 6 FCC is based on the electronic and Figure 4
2-fur, 2-furyl; 2-thi, 2-thienyl; 2-pyr, 1-methyl-2-pyrrolyl; 3-fur, 3-furyl; 3-thi, 3-thienyl; 3-pyr, 1-methyl-3-pyrrolyl; Fc, ferrocene. Potential shift DE ]Ederived – Ereference All data in V. (CO)5Cr]C(NMe2)2-thi, Eox = 0.855 V, Ered = −1.600V, vs. SCE [30]. a
Peak potential difference.
b
Oxidation of the ferrocene moiety precedes oxidation of Wcomplex.
Current Opinion in Electrochemistry 2019, 15:165–174
Mn(I) carbenes derived from cymantrene investigated by Bezuidenhout et al [42]. www.sciencedirect.com
Electrochemical approach to Fischer carbene Hoskovcová and Ludvík
coordination similarity of these atoms: they all have d6 electronic configuration and 6 ligands (h5-Cp is equivalent to 3 CO).
171
Figure 5
The ferrocene moiety introduces a new, electron rich, redox active substructure into the complex with its own Eox. Its presence shifts reduction as well as oxidation potentials to a more negative value. The negative shift of Ered was discussed previously. A similar effect is observed in the oxidation of Cr and Mn complexes, where their Eox is lower; hence, the first oxidation process is directed to Cr or Mn atom. In W complexes, thanks to their high oxidation stability, oxidation of ferrocene proceeds first (cf. Table 4). Substitution of A: linear free energy relationship approach
Another set of molecules [19] [24**], with A = p-R-Ph: (CO)5Cr]C(NMe2)p-R-Ph, R = OCH3, CH3, H, Cl, CF3, COOEt and COOH, represents a homogeneous series where the substituent in p-position influences through the phenyl ring the reducibility (and also very slightly the oxidizability) of the carbene complexes by means of their substituent’s constants spara of the Hammett type (in contrast to smeta, they reflect the sum of induction and resonance effects). The shift of Ered can be in this case evaluated quantitatively using linear free energy relationship (LFER) by plotting a dependence of Ered(ox) on the Hammett constant spara: [16,34,37]. Ered(ox) = rred(ox) $ s þ c
The constants spara reflect the electron donating (negative values of spara) and withdrawing (positive values of spara) properties of the substituents; the slope r represents the sensitivity of Ered(ox) towards the influence of substituents which depends on the (i) distance of the substitution from the redox centre and/or (ii) extent of p-delocalization between the substituent and redox centre. In the aforementioned series, for R = OCH3, CH3, H, Cl, CF3, the plots are linear with the rred = 0.344 (R2 = 0,957) and rox = 0.082 (R2 = 0, 908) that means that Eox is ca. 4 times less sensitive to the substitution than Ered because of longer distance and low level of delocalization [19], [24**] Figure 5. Although the linearity could be expected, the main message is that all compounds in this series are reduced according to the same mechanism, the LUMO involves the same atoms and has the same shape (cf Figure 1c); hence, the reduction centre is the same in all molecules. In addition to this, the plot of observed Ered vs. calculated LUMO energy is also linear confirming the experimental results and their interpretation and simultaneously proving the reasonability of calculations. www.sciencedirect.com
Hammett plot of the reduction potentials of (CO)5Cr]C(NMe2)p-R-Ph, R = OCH3, CH3, H, Cl, CF3, COOEt (:) and COOH (B).
Role of electron delocalization
After adding the last two compounds of the above series, where A = para-R-phenyl (R = COOEt and COOH), to the plot in Figure 5, their values of Ered do not fit the linear dependence and are by about 150e170 mV less negative than expected from the plot. This anomaly undoubtedly points to the different mechanism of the electron transfer. The quantum chemical calculations showed that the carbonyl in p-position completely changes the LUMO: together with the partial C]N double bond, a quinoidal structure is formed, and due to the more extended p-system, the reduction takes place more easily. To present the crucial difference, the LUMOs of carbene complexes with -OMe and eCOOEt is depicted in Figure 6. The same effect of p-carbonyl substituents was observed also in Fe complexes [24**]. This finding shows the importance of the LFER treatment for revealing of different mechanisms during the evaluation of electrochemical data of a series of compounds. Influence of the more delocalized p-electron system on the Ered can be shown also in case of the molecules where A is represented by five-membered heterocycles. It was observed (and mentioned above - Table 4) that 2hetaryl derivatives are reduced remarkably (by 150e 300 mV) easier than their 3 isomers [37*]. The reason is that the heterocycle attached in position 2 is included in conjugation and extends the p-system. This interpretation was confirmed by quantum chemical calculation of the respective LUMOs e Figure 7. Another similar series is formed by compounds where A represents a five-membered ring attached to the carbene carbon in position 2 (2-fur, 2-thi or 2-pyr) which Current Opinion in Electrochemistry 2019, 15:165–174
172 Organic and molecular electrochemistry
Figure 6
LUMO of (CO)5Cr]C(NMe2)p-R-Ph, where R = –OMe (left) and –COOEt (right).
Figure 7
LUMO of (CO)5Cr]C(NMe2)-2-thi (left) and of (CO)5Cr]C(NMe2)-3-thi (right) [37*].
is further substituted in its position 5 by 2-thienyl [13,14,18]. The experiments showed that these two heterocycles interconnected through positions 2- and 5further extend the delocalized system; therefore, their reduction potentials are shifted to less negative values by ca. 100e130 mV in comparison to their ‘parent’ compounds substituted by a single heterocycle. Oxidation is also slightly (by 10e30 mV) shifted to less positive potentials. It is another example that substitution extending the conjugated system results in easier reduction as well as oxidation. Current Opinion in Electrochemistry 2019, 15:165–174
Conclusion Evaluation and comparison of experimental data allow for a brighter insight into how the structural changes of FCCs influence their redox properties. Systematizing the available data into several series leads to general rules concerning redox potentials of FCC: The oxidation process is directed to the metal region. It depends first of all on the metal nature; dependence on the CO substitution is very important, too. Replacing a CO ligand always results in easier www.sciencedirect.com
Electrochemical approach to Fischer carbene Hoskovcová and Ludvík
oxidation. The impact of heteroatom X nature or of carbene carbon substituent A is small. The reduction potential of these molecules depends prevalently on the carbene carbon substitution. o Replacing an alkoxy group with an amino group results in more difficult reduction (and easier oxidation, the effect on reduction is more pronounced). o Substitution on the carbene carbon affects reduction potential in accordance with the electron donating/withdrawing properties of the substituent A. o The role of p-conjugation between the carbene carbon moiety and A has been revealed using LFER approach. Better p-conjugation makes the reduction process easier by stabilizing of the primary product.
173
7.
Santamaria J: Fischer-type group 6 carbene complexes in the synthesis of optically active molecules. Curr Org Chem 2009, 13:31–46. https://doi.org/10.2174/138527209787193800.
8.
Sierra MA, Gómez-Gallego M, Martínez-Álvarez R: Fischer carbene complexes: beautiful playgrounds to study single electron transfer (SET) reactions. Chem Eur J 2007, 13: 736–744. https://doi.org/10.1002/chem.200601470.
9. **
Weststrate NA, Bouwer S, Hassenrück Ch, van Jaarsveld NA, Liles DC, Winter RF, Lotz S: Synthesis and properties of Fischer carbene complexes of N,N-dimethylaniline and anisole p-coordinated to chromium tricarbonyl. J Organomet Chem 2018, 869:54–66. https://doi.org/10.1016/j.jorganchem. 2018.05.022. An excellent example of multiple experimental approaches to molecule characterization. 10. Metelková R, Hoskovcová I, Polá sek M, Urban J, David T, * Ludvík J: Stereoisomeric products of electrochemical reduction of heterocyclic Fischer aminocarbene Cr(0) complexes. Development of the electrochemistry-mass spectrometry tandem approach using biphasic (acetonitrile-hexane) preparative electrolysis. Electrochim Acta 2015, 162:17–23. https:// doi.org/10.1016/j.electacta.2014.10.110. Biphasic preparative electrolysis with direct MS analysis of products.
These rules enable to understand specific behaviour of some derivatives, to predict redox properties of new compounds and to tune their abilities in the next synthetic generations. Interpretation of the observed redox behaviour is supported by quantum chemical calculations.
11. Anjali BA, Suresh Ch H: Electronic effect of ligands vs. reduction potentials of Fischer carbene complexes of chromium: a molecular electrostatic potential analysis. New J Chem 2018, 42:18217–18224. https://doi.org/10.1039/ C8NJ04184A.
Conflict of interest
13. Landman M, Buitendach BE, Conradie MM, Fraser R, van Rooyen PH, Conradie J: Fischer mono- and biscarbene complexes of tungsten with mono- and dimeric heteroatomic substituents. J Electroanal Chem 2015, 739:202–210. https:// doi.org/10.1016/j.jelechem.2014.12.019.
Nothing declared.
Acknowledgement Financial support was provided by the Czech Science Foundation, Czechia, (grant No. 17-21770S). The authors are grateful to specific university research program (MSMT No 21-SVV/2019) and to institutional support RVO 61388955.
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