Negatively charged metallacarborane redox couples with both members stable to air.

The metallacarborane [3,3'-Co(1,2-closo-C2B9H11)2](-) has been synthesized. This species allows the formation of redox couples in which both partners are negatively charged. The E1/2 potential can be tuned by adjusting the nature and number of substituents on B and C. The octaiodinated species [3,3'-Co(1,2-closo-C2B9H7I4)2](-) is the most favorable, as it is isolatable and stable in air. A DFT study on stability and redox potentials of complexes has been performed.


Introduction
The cobaltabisdicarbollide, [3,3'-Co(1,2-closo-C2B9H11)2]-, [1]- [1], is a remarkable anion: it is chemically and thermally stable in a diverse number of situations; [2] it can be substituted at carbon atoms or at boron atoms, [3] and in the latter ones regioselectively at different sites of each one of the two globes. [4] The central core of this anion, "Co(C2B3)2", is very similar 7 throughout the paper, based on experimental evidence, the computational studies also indicate strong E½ dependence on the site of the substituent. As it can be seen in Figure 4 for 15, the iodine orbital contribution is larger at the 8 and 10 positions (2nd and 4rt planes in Figure 2) than in 9 and 12 (3rd plane). Hence, the existence of substituent in the 2nd and 4th planes should be more efficient in terms of E½ tuning than substituent on the 3rd plane. This is in agreement with the values in Table 2 and with Figure 2 for the 2nd and 3rd planes. The mismatch with the 4th plane may have been due to a certain degree of shadowing of the E½ tuning at position 10 caused by the substituent on the 3rd level.
It is clear from Table 2  In [1]-, the Co is Co3+, and as the dicarbollide is a high field ligand, the 6 electrons d are paired, thus [1]-shall be a diamagnetic species. In [1]2-, there are 7 electrons d, and a paramagnetic species is expected. For [1]-, the independent oxidized and reduced species would be difficult to be observed, indeed the reduced form has never been reported nor isolated, but in I8- [1]-the chances to observe both redox partners should be much higher.
Furthermore, tests on the stability of both forms, oxidized and reduced, in not highly strict anaerobic conditions would be a good indication of possible applications of these complexes. In a typical reaction the [NMe4]+ salt of I8- [1]-was dissolved in THF and mixed with freshly prepared Na [C10H8]. Immediately the color of the solution turned from light 8 orange to dark red. Progress of the reaction was followed by NMR analysis directly from the crude and the 11B{1H}-NMR spectrum gave four well defined NMR resonances with intensities 2:6:4:6 in a wide field range (+28 / -95 ppm), clearly suggesting that the generated species was paramagnetic. Following oxidation by air, the sample returned to the original Co3+ color and its 11B{1H}-NMR spectrum to the expected range (-2 / -20 ppm).
Both 11B{1H}-NMR spectra for I8- [1]-(in red) and I8- [1]2-(in black) are shown in Figure 5. To this objective, the crude of the reaction between I8-[1]-and Na[C10H8] was filtered to remove the existing solid whose IR did not display any B-H absorption. The solvent was evaporated and the naphthalene was recovered by low pressure sublimation. The final product was collected as a dark brown solid and the further 11B{1H}-NMR analysis showed that it was not altered during the purification process. Moreover, the stability in both air and inert gas (N2 and Ar) conditions was also checked. The I8- [1]2-species in solid state was perfectly air-stable for 7 days, while under inert conditions its stability increased to more than 1 month (Figure 7).
The THF solution of the reduced I8-[1]2-species was stable for several hours in air. After one day, some re-oxidation was observed.
Stable paramagnetic compounds derivatives of neutral C2B10 and anionic [CB11]-clusters have been recently reported [25] but this reduced I8- [1]2-species is the first isolated and stable paramagnetic cobaltabisdicarbollide, which is the most commonly used and widely studied metallacarborane.

Conclusions
The wide pattern of possible substitutions in [3,3'-Co(1,2-closo-C2B9H11)2]-has facilitated the preparation of a set of regioselective polyiodinated derivatives, which in turn, by a Kumada inspired B-C cross coupling, has led to several regioselective polymethylated derivatives. A singular property of [3,3'-Co(1,2-closo-C2B9H11)2]-is that it is redox reversible and permits tailoring the redox potential very accurately. No other platform allows for such an adjustment. Boron dehydromethylation shifts the E½ value to more negative potential values, whereas boron dehydroiodination does the opposite effect. The redox potential shift can be large, even larger than 1 V, because of the cumulative individual effects, that are also site dependent. The effect of the B-I unit on the E½ value is inversely proportional to the distance. The air exposure of [3,3'-Co(1,2-closo-C2B9H11)2]2is totally impractical, however from the E½ shift to more positive potentials of the couple [3,3'-Co(8,9,10,12-I4-1,2-closo-C2B9H7)2]-1/-2 caused by 8 dehydroiodinations, the E½ has shifted from -1.80 to -0.68 V vs Fc+/Fc. This E½ shift allows both the oxidized and reduced forms of the couple [3,3'-Co(8,9,10,12-I4-1,2-closo-C2B9H7)2]-1/-2 standing in air for several hours and days. These results open an unexplored way to adjust the desired E½ value in devices in which E½ modification will not imply a sharp molecular modification. In this paper, the redox potential of a series of related complex of Co3+ has allowed for the quantification of the ligands electron acceptor/donor properties, which can then be applied as an important characterization parameter.

Synthesis of [NMe4][3,3'-Co-(1-Ph-8,9,10,12-I4-1,2-closo-C2B9H6)2]
The same procedure as for the obtaining of compound  EPR spectra were obtained in an X-Band Bruker ELEXYS E500 spectrometer equipped with a TE102 microwave cavity, a Bruker variable temperature unit and a field frequency lock system Bruker ER 033 M. The signal to noise ratio of spectra was increased by accumulation of scans using the F/F lock accessory to guarantee large field reproducibility.
Line positions were determined with an NMR Gaussmeter Bruker ER 035 M. The modulation amplitude was kept well below the line width, and the microwave power was well below saturation.

Computational Details
Calculations were performed using the Gaussian09 (revisión D01)[32] with the hybrid B3LYP functional [33] together with LANL2DZ basis set (Hay-Wadt effective core potentials for the iodine and cobalt atoms [34]). In order to improve the convergence of the  Table 3. B3LYP calculated free energies (in a.u.) for the three optimized isomers (a, b and c in Figure 3) for the fifteen Co2+ and Co3+ compounds 1-15. The values in parenthesis correspond to the free energy difference between the isomers (in kcal/mol) taking as reference the usually most stable c isomer. Only for the compound 12, the c isomer is significantly less stable than the other two isomers due to the simultaneous 4,4',7,7' substitution on each C2B3 face.     Table 2).