Cyclic Voltammetry Study of Closo -Ruthenacarboranes

: Electrochemical properties of transition metal complexes are important parameters that should be considered for the successful application of these compounds in catalytic reactions. The proper choice of ligands and the type of its coordination allow the construction of a catalyst with high performance. The reversibility of complex oxidation is a prerequisite for successful participation in redox catalysis, while the potential values correlate with the rate of the process and necessary catalyst loading. This work summarizes the results of the exploration of a series of ruthenium carborane complexes based on the nido -C 2 B 9 ligand obtained in our group by cyclic voltammetry and describes the found correlations. The knowledge of the electrochemical properties of the studied ruthenacarboranes is required for the optimization of its structure for successful catalysis of Atom Transfer Radical Polymerization or other applications. It was found that the value of the potential of reversible Ru(II)-Ru(III) transition may vary from − 0.501 to 0.389 V versus Fc|Fc + couple, depending on the nature of auxiliary phosphine, halogen or nitrile ligand, natural bite angle of κ 2 -diphosphine ligand and the presence of alkyl substituents in the carborane cage. The further oxidation towards formal Ru(IV) may be reversible or not depending on the complex structure. The found trends are in good agreement with the earlier performed ﬁndings in the ﬁeld of coordination chemistry and should be considered as a tool for obtaining of complexes suitable for catalytic applications.


Introduction
Carborane complexes of transition metals based on nido-C 2 B 9 ligand are isolobal analogs of cyclopentadienyl derivatives that have found a wide application in catalysis, including isomerization, hydrogenation, radical addition [1][2][3] and other spheres of chemistry [4][5][6]. Due to the double negative charge and delocalization of electron density, carborane ligand is capable to stabilize transition metals in high oxidation states and makes possible the formation of intermediates in catalytic cycles [1,4]. The introduction of substituents at carbon or boron atoms of carborane cage affects steric and electronic parameters of metallacarborane and may be considered as a tool for tuning a complex for its application in catalysis. Electrochemical properties of complexes of transition metals are used for predicting their catalytic performance, as the reversibility of the electron transfer and the value of potential are parameters that usually correlate with the results of catalytic tests [7,8]. It was shown that the redox potential of ruthenium ethylenediaminetetraacetate complexes determined by auxiliary ligands correlates with its electrocatalytic performance [9]. A similar observation was done for Ru(II)-polypyridine complexes with alkynyl Schiff base ligands [10]. The potential of Ru(II)-Ru(III) transition plays a key role in the activation of the ruthenium anticancer prodrugs in vivo [11] as well in light-harvesting applications [12] and light-emitting diodes [13]. Thus, electrochemical properties of complexes are widely explored and their correlations with structure and properties are carefully analyzed in numerous reviews and original papers [14][15][16][17][18].   Electrochemical measurements were performed with an IPC Pro-M potentiostate (Volta, Saint-Petersburg, Russia) with the digital recording of the results. The studies were performed in 1,2-dichloroethane with 0.2 M n-Bu 4 NPF 6 as the supporting electrolyte. The measurements were carried out under Ar atmosphere in a conventional three-electrode cell (3 mL) with a Pt disk (0.8 mm in diameter) as the working electrode, a Pt wire as the counter electrode and Ag/0.01 M AgNO 3 plus 0.2 M n-Bu 4 NPF 6 in acetonitrile as the reference electrode [36]. Pt electrodes were treated with concentrated sulfochromic mixture and washed with deionized water before use.
In a typical experiment 232 mg (0.6 mmol) of n-Bu 4 NPF 6 were dissolved in 3 mL of 1,2-dichloroethane, and a baseline signal was recorded to certain the absence of impurities on the electrode surface and in solution. After that 0.009 mmol (ca 6-9 mg) of ruthenacarborane was added and the CV curves were recorded after full dissolution of the complex.
Ferrocene was used as an internal standard [32]. A 1-2 mg of ferrocene was added to the electrochemical cell after the study of each complex. In the case of exploration compounds 11a-e external calibration of electrode relative ferrocene was performed.

Results
In the present work, a cyclic voltammetry study of 60 closo-ruthenacarboranes is discussed, and the correlations between complex structures and their electrochemical properties are established. The structures of the studied compounds are presented in Figure 1.
ties on the electrode surface and in solution. After that 0.009 mmol (ca 6-9 mg) of ruthenacarborane was added and the CV curves were recorded after full dissolution of the complex.
Ferrocene was used as an internal standard [32]. A 1-2 mg of ferrocene was added to the electrochemical cell after the study of each complex. In the case of exploration compounds 11a-e external calibration of electrode relative ferrocene was performed.

Results
In the present work, a cyclic voltammetry study of 60 closo-ruthenacarboranes is discussed, and the correlations between complex structures and their electrochemical properties are established. The structures of the studied compounds are presented in Figure 1. Complexes 1a-1d are air-stable 18-e compounds. The formal oxidation state of ruthenium is +4. These compounds undergo irreversible oxidation with Epa 1.0-1.1 V versus ferrocene. The CVA curve recorded for complex 1a is represented in Figure 2. The other curves are similar. According to the data provided in Table 1, the presence of alkyl substituents in the lower belt of the carborane cage results in a decrease in the value of the potential at which oxidation starts. A bromine-containing complex 1d is oxidized at a slightly lower potential than its chlorine-based analog. High values of oxidation potentials correlate with the high stability of these compounds in air or solution observed ear- Complexes 1a-1d are air-stable 18-e compounds. The formal oxidation state of ruthenium is +4. These compounds undergo irreversible oxidation with Ep a 1.0-1.1 V versus ferrocene. The CVA curve recorded for complex 1a is represented in Figure 2. The other curves are similar. According to the data provided in Table 1, the presence of alkyl sub-stituents in the lower belt of the carborane cage results in a decrease in the value of the potential at which oxidation starts. A bromine-containing complex 1d is oxidized at a slightly lower potential than its chlorine-based analog. High values of oxidation potentials correlate with the high stability of these compounds in air or solution observed earlier [19]. Complexes 1a-1d are air-stable 18-e compounds. The formal oxidation state of ruthenium is +4. These compounds undergo irreversible oxidation with Epa 1.0-1.1 V versus ferrocene. The CVA curve recorded for complex 1a is represented in Figure 2. The other curves are similar. According to the data provided in Table 1, the presence of alkyl substituents in the lower belt of the carborane cage results in a decrease in the value of the potential at which oxidation starts. A bromine-containing complex 1d is oxidized at a slightly lower potential than its chlorine-based analog. High values of oxidation potentials correlate with the high stability of these compounds in air or solution observed earlier [19]. . Some of them may be obtained by abstracting hydrogen atoms from complexes 1a-1d [19,26]. The reversible reduction of complex 2d is observed in Figure 2, while the CVA curves for other compounds have similar nature. Due to the presence of a semi-occupied orbital, these compounds attach additional electrons in the reversible reduction process. In this case, stable 18-e anions similar to the known [3-Cl-3,3-(PPh3)2-closo-C2B9H11] − [37] are formed on the electrode surface. These anions may be converted back to 17-e species in the reverse oxidation process. . Some of them may be obtained by abstracting hydrogen atoms from complexes 1a-1d [19,26]. The reversible reduction of complex 2d is observed in Figure 2, while the CVA curves for other compounds have similar nature. Due to the presence of a semi-occupied orbital, these compounds attach additional electrons in the reversible reduction process. In this case, stable 18-e anions similar to the known [3-Cl-3,3-(PPh 3 ) 2 -closo-C 2 B 9 H 11 ] − [37] are formed on the electrode surface. These anions may be converted back to 17-e species in the reverse oxidation process.
The analysis of the data provided in Table 2 allows us to conclude that the increase in the number of methylene units in the diphosphine ligand results in a gradual increase of potential value. This tendency is observed as for chlorine-containing complexes 2a-2e, so for its bromine-containing counterparts 2g-2j. It should be noted that the difference between E 1/2 of complexes distinguished in one CH 2 unit increases with the decrease of the length of methylene chain. Taking into account that donating abilities of the studied bis(diphenylphospino)alkane ligands are similar, the observed differences may be explained by steric factors and bite angles of diphosphine ligands. This proposition is confirmed by similarity of E 1/2 values measured for 2f based on 1,2-bis-(diphenylphosphino)benzene and 2b containing 1,2-bis-(diphenylphosphino)ethane with close bite angles [38]. The similar tendency is observed for complexes 2m and 2r.
The exchange of chlorine for bromine atoms leads to the increase of Ru(II)-Ru(III) transition potential by 0.02-0.04 V. The introduction of n-butyl substituent results in decrease of redox potential of 2k by 0.08 V relative its analogue 2d indicating donating ability of butyl substituent.
Oxidation of 17-e species 2a-2k proceeds fully irreversible as shown for 2d in Figure 2. The value of Ep a tends to increase with the increase of the number of methylene units in diphosphine ligand, the bromine-containing complexes are characterized by higher Ep a values, similarly as it was observed for Ru(II)-Ru(III) transition.
Compounds 2l-2r contain neutral monodentate ligand instead of halogen and are diamagnetic 18-e complexes of ruthenium (II). Thus, the corresponding Ru(II)/Ru(III) transition should be considered as oxidation. It also proceeds reversible but at higher potential values (0.1-0.4 V versus ferrocene) in comparison with the earlier discussed 17-e species. The increase of the oxidation potential is observed in row 2l-2n with the increase of the length of the methylene bridge in the diphosphine ligand and its bite angle. Interestingly complexes 2n-2p with different nitrile ligands (acetonitrile, benzonitrile, and acrylonitrile) have similar redox potentials which do not depend on the nature of the alkyl or aryl radical. At the same time the exchange of nitrile by triphenylphosphine results in the increase of the potential of Ru(II)/Ru(III) transition by 0.23 V (2l and 2q). The redox behavior of 2l and 2q can be explained by the predominant tendency of phosphine ligands on the redox potential of ruthenacarborane.
Further oxidation of 2m-2p to Ru(IV) species proceeds irreversibly. The measured Ep a values are higher than for the similar 17-e species. In the cases of compounds 2l, 2q, and 2r no second oxidation waves were observed in the studied range of potentials. It should be noted that the Ep a value for Ru(III)-Ru(IV) transition for nitrile-based complexes 2n-2p depends on the structure of the auxiliary ligand. The acrylonitrile-based complex 2p with a double bond in its structure is characterized by the lowest oxidative stability, while the highest potential was observed for its saturated acetonitrile-based analog.
A key feature of compounds 3a-3n and 4a-4b is the presence of covalent bond between carborane and diphosphine ligands. This bond should increase the stability of complexes and prevent its full decomposition. Complexes 3a-3h are paramagnetic 17-e species while compounds 3i-3n are diamagnetic. Electrochemical properties of such complexes are generally similar to those discussed earlier for compounds 2a-2r. These compounds are characterized by reversible Ru(II)-Ru(III) transitions. Further oxidation to Ru(IV) is irreversible. An increasing number of methylene groups in diphosphine ligand or exchange of chlorine by bromine results in the increase of E 1/2 value similarly as observed for 2.
CVA curves recorded for complex 3b at different scan rates, provided in Figure 3, show that no reduction signal is observes after oxidation at 0.8 V versus Fc|Fc + even with higher scan rates. The dependences of anodic and cathodic currents on a square root scan rate are linear (see Figure 3b) confirming the reversibility of a Ru(II)-Ru(III) transition.  Note, a weak reverse peak of reduction after irreversible oxidation to Ru(IV) state has appeared at 0.1 V. The detailed investigation of the complexes 3a-3h has shown that this peak probably corresponds to the formation of novel complexes capable of reversible transition. According to the CVA curve depicted in Figure 4, irreversible oxidation of 3c results in the appearance of a reduction peak at 0.13 V. The subsequent oxidation results in the appearance of a new oxidation peak at 0.18 V, which has not been observed on the oxidation wave in the first cycle. When the potential of the working electrode was kept at 0.8 V for 30 s, the current of the redox wave at 0.18 V has become larger confirming this proposition. Thus, we may conclude that the presence of covalent linkage prevents full decomposition of complexes 3 and 4 after deep oxidation. Note, a weak reverse peak of reduction after irreversible oxidation to Ru(IV) state has appeared at 0.1 V. The detailed investigation of the complexes 3a-3h has shown that this peak probably corresponds to the formation of novel complexes capable of reversible transition. According to the CVA curve depicted in Figure 4, irreversible oxidation of 3c results in the appearance of a reduction peak at 0.13 V. The subsequent oxidation results in the appearance of a new oxidation peak at 0.18 V, which has not been observed on the oxidation wave in the first cycle. When the potential of the working electrode was kept at 0.8 V for 30 s, the current of the redox wave at 0.18 V has become larger confirming this proposition. Thus, we may conclude that the presence of covalent linkage prevents full decomposition of complexes 3 and 4 after deep oxidation.
the supporting electrolyte at 25 °C and different scan rates. [Ru] = 0.003 M. Scan rate: 1-0.1 V/s, 2-0.2 V/s, 3-0.4 V/s, 4-0.8 V/s; (b)-the dependence of anodic current on the square root of scan rate (ν) for complex 3b in the same conditions. Note, a weak reverse peak of reduction after irreversible oxidation to Ru(IV) state has appeared at 0.1 V. The detailed investigation of the complexes 3a-3h has shown that this peak probably corresponds to the formation of novel complexes capable of reversible transition. According to the CVA curve depicted in Figure 4, irreversible oxidation of 3c results in the appearance of a reduction peak at 0.13 V. The subsequent oxidation results in the appearance of a new oxidation peak at 0.18 V, which has not been observed on the oxidation wave in the first cycle. When the potential of the working electrode was kept at 0.8 V for 30 s, the current of the redox wave at 0.18 V has become larger confirming this proposition. Thus, we may conclude that the presence of covalent linkage prevents full decomposition of complexes 3 and 4 after deep oxidation.  The oxidation of 18-e complexes of Ru(II) 3i-3n to Ru(III) is also reversible but the potentials of Ru(II)-Ru(III) transition are higher in comparison with 17-e species. Its further oxidation to Ru(IV) is irreversible, while a small reverse peak is observed at 0.6 V (see curve 3i in Figure 5). Note that triphenylphosphine derivatives of Ru(II) have higher values of redox potentials in comparison with nitrile-based complexes (see curves 3i and 3m of Figure 5). Complex 3n is characterized by a surprisingly low value measured for Ru(II)-Ru(III) transition. It is probably caused by its decomposition with the formation of a dioxygen-containing derivative upon dissolution [28].
Physchem 2023, 3, FOR PEER REVIEW 8 The oxidation of 18-e complexes of Ru(II) 3i-3n to Ru(III) is also reversible but the potentials of Ru(II)-Ru(III) transition are higher in comparison with 17-e species. Its further oxidation to Ru(IV) is irreversible, while a small reverse peak is observed at 0.6 V (see curve 3i in Figure 5). Note that triphenylphosphine derivatives of Ru(II) have higher values of redox potentials in comparison with nitrile-based complexes (see curves 3i and 3m of Figure 5). Complex 3n is characterized by a surprisingly low value measured for Ru(II)-Ru(III) transition. It is probably caused by its decomposition with the formation of a dioxygen-containing derivative upon dissolution [28]. Complexes 3g, 3h, 4a and 4b contain methyl substituents in the carborane cage. A comparison of the data obtained for these compounds and their unsubstituted analogs shows that the presence of methyl groups in the structure of carborane ligand results in a decrease of E1/2 value. The introduction of the first group results in the decrease of potential by 0.068 V (3b and 3g) while the introduction of the second one decreases the potential only by 0. 029 V (3g and 3h). It should be noted that the position of the methyl Complexes 3g, 3h, 4a and 4b contain methyl substituents in the carborane cage. A comparison of the data obtained for these compounds and their unsubstituted analogs shows that the presence of methyl groups in the structure of carborane ligand results in a decrease of E 1/2 value. The introduction of the first group results in the decrease of potential by 0.068 V (3b and 3g) while the introduction of the second one decreases the potential only by 0.029 V (3g and 3h). It should be noted that the position of the methyl group in the cage has no significant influence on the electrochemical parameters. The introduction of two methyl groups to carbon atoms in the upper belt of the cage results in a similar decrease of the potential approximately by 0. 1 V (4a-4b and 3a-3b).
Compounds 5a-5g are characterized by two covalent bonds between carborane and diphosphine ligands which significantly increase its stability. The presence of the second ortho-cycloboronated fragment surprisingly leads to the reversibility of Ru(III)-Ru(IV) transition. This reversibility is observed for 17-e species 5a-5f (see curve 5b in Figure 2) and for 18-e complex 5g ( Figure 5). Other dependencies between the complex structure and redox potential values are the same as discussed in the cases of compounds 2-4. The increase of the length of the methylene chain expectedly leads to the increase of E 1/2 for Ru(II)-Ru(III) transition and also increases E 1/2 of reversible oxidation to Ru(IV). Methyl substituents in the carborane cage decrease values of E 1/2 for both transitions.
Comparison of E 1/2 values measured for complexes 2c, 3a, and 5a shows that the appearance of the first and the second ortho-cycloboronated fragments in ruthenacarborane complex decreases its redox potential. A similar tendency is observed for other rows of relative compounds (see Figure 2). Formation of this fragment may be considered as intramolecular introduction of ortho-phenylene substituent in the carborane cage. Thus, it correlates with the discussed ability of alkyl substituents to decrease the oxidation potential of ruthenacarboranes.
Compounds 6a-6f have similar structures to the earlier discussed 2a-2r but contain chelate P-O-P ligands which coordinate to ruthenium by two phosphorus atoms. Compounds 6a-6c are 17-e Ru(III) species that undergo reversible reduction to Ru(II) at potential values close to the same of the corresponding bis(diphenylphosphino)alkane derivatives 2b and 2c. Its CVA curves are similar to the same of 2d. Its oxidation to Ru(IV) proceeds irreversible and the Ep a values significantly depend on the nature of the ligand. The Ni-Xantphos-based complex 6b has the lowest oxidative stability, while the XantPhos based 6c is the most stable. Among the 18-e derivatives of Ru(II) 6d-6f only two compounds are capable of reversible oxidation. Its E 1/2 is typical for complexes with nitrile ligands. Oxidation of 6e to Ru(III) is fully irreversible due to the low oxidative stability of the Ni-Xantphos ligand.
Ruthenacarboranes 7-9 are 18-e complexes of Ru(II) with triphosphine ligands. Despite the similarity of their structures, they are characterized by different electrochemical behavior. Complex 7 is characterized by two reversible transitions, while the oxidation of 8 and 9 is irreversible ( Figure 5). It should be noted that Ep a values for complex 7 are close to the values observed in the discussed tris-phosphine derivatives 3l, 3m, and 2q (see curves 3m and 7 in Figure 5).
Compound 10 is a product of interaction 9 with carbon tetrachloride [34]. It is a 17-e complex of trivalent ruthenium. It can also be considered as a derivative of 2a, where one hydrogen atom is substituted by the PPh 2 group. Its electrochemical properties are similar to the same of 2a. It also undergoes reversible reduction to Ru(II) and irreversible oxidation to Ru(IV) but at lower potential values than 2a. The lower E 1/2 values correlate with donating ability of −PPh 2 substituent in the alkyl fragment.
Ruthenacarboranes 11a-11e exhibit 18-e species and contain tridentate nitrogen-based ligands in their structure. Another peculiarity of the mentioned compounds is a so-called pseudocloso conformation of carborane ligand with enlarged C-C distance exceeding the sum of covalent radii [35]. These compounds are capable of reversible reduction at potentials close to E 1/2 of the Fc|Fc + couple. It should be noted that the introduction of an alkyl substituent to the nitrogen atom results in a change in the value of redox potential. Still, no direct correlation between the nature of the substituent and E 1/2 value can be observed.

Discussion
The analysis of the obtained results allowed us to clearly recognize some patterns and dependences of electrochemical parameters of ruthenacarboranes on their structures which are in good agreement with the earlier mentioned ones.
Most of the studied compounds 2-11 undergo reversible Ru(II)-Ru(III) transition while the reversibility of the oxidation to the formal Ru(IV) state depends on the complex structure. The measured potentials of transitions lie in the area typical for Ru(II)|Ru(III) transition [39][40][41][42]. At the same time, a comparison of E 1/2 value for complex 2a (−0.332 V) with its counterparts containing cyclopentadienyl-anion (Cp, E 1/2 = −0.015 V) or pentamethylcyclopentadienyl (Cp*, E 1/2 = −0.190 V) [41] allows us to conclude that dicarbollide ligand is more donating than the mentioned carbocyclic ligands. This ability of parent [C 2 B 9 H 11 ] 2− anion is in good agreement with the observed for its substituted analogs [42] and may be used for the creation of catalysts for processes based on reversible oxidation.
It should be noted that the introduction of alkyl substituents in the carborane cage results in the decrease of E 1/2 value indicating the donating ability of the substituent. This trend is in good agreement with the observed behavior of sandwich pyrrolyl/dicarbollide cobalt complexes [43] or ruthenium complexes with charge-compensated carborane ligands [42]. The mentioned tendency correlates with the observed dependence of E pa values of ruthenium complexes with substituted η 6 -arene on the number of methyl groups [44].
The formation of ortho-phenylenecycloboronated fragments in complexes 3-5 leads to the decrease of E 1/2 for Ru(II)-Ru(III) transition. It may be considered as intramolecular substitution in carborane ligand and thus correlates with the previously discussed influence of alkyl substituents. Moreover, the formation of the second ortho-cycloboronated link results in the reversibility of the second wave corresponding to the Ru(III)-Ru(IV) transition. Such an effect is similar to the observed reversible oxidation of Ru(η 6 -C 6 Me 6 )-based complexes contrary to the irreversible process for the similar Ru(η 6 -C 6 H 6 ) one and may be caused by the decrease of electrode adsorption [44].
Basing on the results of the experiments we may conclude that the increase in the number of methylene units in the alkylidene fragment of the diphosphine ligand results in the increase of the E 1/2 value for Ru(II)-Ru(III) transition. A similar trend was observed for palladium diphosphine complexes [45] and is probably explained by structural distortions and stabilizing the LUMO in the case of compounds with large bite angles.
According to the provided data, the potential of Ru(II)-Ru(III) transition of the studied ruthenacarboranes 2 varies from −0.439 to +0.389 V versus ferrocene depending on the nature of auxiliary ligands. The comparison of E 1/2 for the relative Ru(III) complexes with different halogen atoms shows that chlorine-based complex usually displays a lower redox potential than similar complexes with bromine atoms. It should be noted that analogous observation was done for copper complexes with nitrogen-based ligands [46]. Complexes of Ru(II) with nitrile ligands have higher E 1/2 values in comparison with halogen-based complexes of Ru(II) similar to Cp-based complexes [41]. Ruthenacarboranes containing triphenylphosphine are characterized by the highest oxidation potentials.

Conclusions
We may conclude that closo-ruthenacarboranes, despite similar structures, are very diverse from the point of view of their electrochemical properties. Diamagnetic complexes of Ru(IV) undergo irreversible oxidation. 17-e complexes of Ru(III) undergo reversible reduction to Ru(II) species. Oxidation of 18-e complexes of Ru(II) to Ru(III) species also reverses. The further oxidation of the formed cations towards formal Ru(IV) may be reversible or not depending on the complex structure.
The provided results of electrochemical studies of ruthenacarboranes indicate that Ru(II)-Ru(III) transition is reversible for the major part of the complexes, while the value of potential varies in a wide range depending on the nature of auxiliary phosphine ligand and substituents in the carborane cage. Considering that dicarbollide ligand is more donating than carbocyclic ligands closo-ruthenacarboranes seem suitable for application as catalysts for ATRP and other redox processes. Among the studied complexes the compounds of types 3-5 with covalently bounded carborane and diphosphine moieties are the most stable and preferable for catalytic applications.
Despite the value of the potential of reversible Ru(II)-Ru(III) transition of ruthenacarboranes may vary in a wide range from −0.501 to 0.389, the found correlations allow proposing the ways of further expanding this range by modification of carborane cage or changing the auxiliary ligand. Moreover, the data provided in Table 2 and the found regularities may be successfully used for tuning other classes of ruthenium complexes for their practical applications.

Data Availability Statement:
The data that support the findings of this study are available on request from the corresponding author via e-mail.

Conflicts of Interest:
The authors declare no conflict of interest.