Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand
A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal pentadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-hexenyl group to the methylcyclopentyl derivative over the course of several hours. Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand as opposed to protonation of the methyl group. The produce of protonation at nitrogen is the dimeric species 4 which was prepared via separate synthesis and characterized by X-ray crystallography. Introduction.In Nature, two types of cobalamins, also known as Vitamin B12, exhibit a Co-C bond, methylcobalamin (MeCbl) 1 and adenosylcobalamin (AdoCbl), 2 featuring -CH3 and -CH2R groups in the axial position, respectively (Figure 1). In both stable forms of Vitamin B12, the cobalt center bears a +III oxidation state. During catalysis, however, the Co-C bond of MeCbl, can undergo heterolytic cleavage to give Co(I) and CH3 + , while for AdoCbl the bond can cleave homolytically to provide a source of Co(II) and organic radical. 3 Thus, the alkyl groups in these Co(III) alkyls exhibit divergent reactivity in the in vivo function of Vitamin B12. Although both MeCbl and AdoCbl bear the same functionalized corrin L3X supporting
<!>Synthesis and Characterization of 2-R<!>Properties of the Co-C bonds in 2-R<!>Conclusions.<!>Experimental Section.<!>Synthesis of 2-Me.<!>Synthesis of 2-CH2SiMe2Ph.<!>Synthesis of 2-CH2(c-C5H9).<!>Generation of 4.<!>Supporting Information Available.<!>Computational Details<!>Generation of (Cyclopentylmethyl)lithium<!>Generation of 5-Hexenyllithium<!>Generation of CpCo(I) cyclopentadiene complexes I-<!>Additional crystallographic data of 4:
<p>ligand, the side chain terminating in a benzimidazole function which serves as the second axial ligand 4 can dissociate or bind during the catalytic transformations mediated by Vitamin B12, modulating the behavior of the Co-C bond. 5 Since the discovery of the cobalamin coenzymes, many synthetic alkyl-cobalt(III) complexes have been reported in an effort to mimic and model the properties of Vitamin B12. A recent review by Detrembleur et al. codifies the main categories of reported synthetic alkyl-cobalt(III) complexes, sorting them according to the nature of the supporting ligand. 6 While the bulk of the compounds reported enlist porphyrin, cobaloxime, Schiff base or bis-(acetylacetonate) multidentate supporting ligands, other systems are potentially of interest. For example, tetrapodal pentadentate ligands, 7,8 can provide a well-defined platform around the metal center that mimics the five coordinate environment of the B12 cobalamin construct. 9 Such ligands have been deployed across the transition series and used for a variety of small molecule activation and catalytic transformations such as the reduction of water to hydrogen [10][11][12][13] or the oxidation of water to oxygen. 14,15 Despite the extensive chemistry of such ligands, only a single example of a Co(III) alkyl species supported by a pentadentate ligand has been structurally characterized, the pyN4 cobalt methyl complex I of Grohmann et al. shown in Figure 1. 16 However, its reactivity was not investigated, and the effect of the rigidity imposed by the pentadentate platform around the cobalt center remains unclear.</p><p>Based on the well-known pentapyridyl (PY5) system introduced 20 years ago, 17,18 and later on the work by Gardinier who substituted the equatorial pyridine arms with pyrazole donors, 19 our group has reported the synthesis of a B2Pz4Py ligand which features two borate moieties. [20][21][22] The introduction of the borate linkers renders the ligand dianionic and therefore more electron-rich. This stabilizes higher oxidation state intermediates so they can be studied in more detail. For Co, the B2Pz4Py ligand moves the Co(II)/Co(III) couple more than 0.5 V to more positive potentials, suggesting that it is a suitable platform for octahedral Co(III) alkyls. Herein we report the synthesis and properties of a family of neutral Co(III) alkyl complexes of the general formula shown in Figure 1.</p><!><p>The Co(III) bromide complex 1 22 is a convenient starting material for a family of Co(III) alkyls (Scheme 1). Treatment of 1 in toluene with the corresponding alkyllithium reagent in a 1:1 ratio affords the alkyl-Co(III) complexes 2-R (Scheme 1) in moderate to excellent yield. Various alkyl groups can be introduced, ranging from methyl (2-Me), to bulker groups (2-CH2SiMe3 and 2-CH2SiMe2Ph) and an alkyl group with a beta-hydrogen (2-i Bu).</p><p>The reaction proceeds quickly at room temperature, as observed by a color change from dark orange (1) to light pink (2-R), accompanied by the formation of a white precipitate (LiBr). The products are all diamagnetic and fully characterizable by NMR spectroscopy (vide infra). Although these reactions proceed cleanly without reduction of 1, the organometallic products are prone to reduction to Co(II) by excess RLi. Additional equivalents of RLi resulted in the disappearance of the diamagnetic peaks in the 1 H NMR spectrum and emergence of the distinct signal at + 75 ppm in the 11 B NMR spectrum, attributed to the previously reported Co(II) complex. 22 Therefore, a strict 1:1 ratio of 1 and alkyllithium was used for the syntheses of compounds 2-R.</p><p>The low spin, d 6 octahedral alkyl-cobalt(III) complexes 2-R are stable at room temperature under ambient atmosphere for a few hours, which is surprising since many alkyl-cobalt(III)</p><p>complexes have been shown to react with oxygen to form alkylperoxo complexes. 23,24 In addition, 2-R are thermally stable (tested up to 110 ºC in toluene). They were fully characterized using multi-nuclei NMR spectroscopy, mass-spectrometry and elemental analysis, and all the characterization data can be found in the experimental section.</p><p>Through spectral analysis of the 2D NMR spectra of 2-R, the peaks ranging from 4.13 (2-Me) to 4.10 (2-CH2SiMe3), 4.27 (2-CH2SiMe2Ph) and 4.88 (2-i Bu) were assigned to the protons located on the α carbon of the ligating alkyl group (Table inset, Scheme 1), which are close to the range of chemical shift of reported peaks of other alkyl-Co(III) complexes. 16,25 The 1 JCH coupling constants for the α C-H bonds are in good agreement with the values reported for the free alkanes, 26 consistent with classical, non-agostic structures for these compounds. In addition, three of the 2-R derivatives were characterized using X-ray crystallography (Figure 2). From the structures, the complexes exhibit an overall octahedral geometry with a Co-C bond length similar to other alkyl-Co(III) complexes. 16,25,27,28 In particular, the metrical data of 2-Me is comparable to that of MeCbl with a Co-C bond length of 1.987 (4) vs 1.99 for MeCbl and Co-N1 of 2.072 (3) vs 2.19 in MeCbl. 29 Notably, the axial Co-N1 bond of 2-Me is shorter than the Co-Npy bond using the pyN4  2)). This is attributed to the stronger trans influence of the alkyl group vs. the bromo ligand. 30 As in other structures involving this ligand environment, 20-22, 31, 32 the bulkier silylalkyl groups orient themselves along the vector bisecting the narrower N-Co-N angle, which is more sterically open than the perpendicular vector bisecting the wider N-Co-N angles. Nevertheless, rotation about the Co-C bonds is rapid on the NMR timescale, averaging the environments of the four pyrazolyl rings at all accessible temperatures.</p><!><p>The relatively electron richness of the dianionic B2Pz4Py ligand environment contributes to the apparent stability of these octahedral Co(III) alkyl complexes. Co(III)-C bonds tend to be fairly weak (14-40 kcal/mol) and subject to thermal homolysis via k1 (Scheme 2). 6,[33][34][35][36] Typically, the equilibrium lies towards the alkyl and k-1 is essentially diffusion controlled such that the R• radical only slowly escapes the solvent cage; the presence of a radical trapping agent can accelerate this process by drawing the equilibrium to the right. Bond homolysis may also be accelerated by one electron reduction (via k R• ) since addition of an electron populates an antibonding orbital and lowers the Co-C bond order. However, in situations where Co(I) is less accessible, loss of Rvia k R-(Scheme 2) becomes a possibility although this has not been directly observed to our knowledge. Therefore, given the historical importance of such compounds as models for the behavior of cobalamin alkyls, it is of interest to probe the reactivity of these species towards radical trapping agents, oneelectron reducing agents and protons. for the four synthesized compounds. These are consistent with experimental and computed values 6 for other six coordinate Co(III)-C complexes (perhaps towards the high end of the range) but follow a general trend of weaker bonds for sterically more imposing R groups. 33 Based on these values, it might be expected that the homolytic bond cleavage equilibrium in Scheme 2 can be apprehended using radical trapping agents such as 1,4-cyclohexadiene (CHD) or 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), Scheme 3. While compounds 2-R are stable towards bond homolysis up to 110˚C, in the presence of the hydrogen atom donor CHD, (BDE = 71 kcal/mol), 37 all of the compounds except 2-Me slowly released R-H</p><p>at 80˚C, along with the known five coordinate (B2Pz4Py)Co(II) complex 3, identifiable via its characteristic 11 B NMR chemical shift of 75 ppm (Figures S1-S7). 21 Protio benzene was also detected in these reactions. This suggests that under these conditions, the R• formed via bond homolysis can be trapped when it is sterically large enough to escape the solvent cage; in the case of 2-Me, this does not occur. More efficient trapping of R• from all of the cobalt (III) alkyls is seen when TEMPO is employed (Scheme 3, bottom). Here, the reactions occur smoothly at room temperature over ≈24 hrs to produce 3 and the TEMPO-R products in excellent NMR yields (Figures S8-S15). In these TEMPO trapping reactions, once bond cleavage and cage escape occurs, reaction of R• with TEMPO is essentially diffusion controlled, whereas when CHD is the trapping agent, there is an added barrier involving H atom transfer, accounting for the harsher conditions necessary to effect those reactions.  reagent was generated in situ from the requisite iodo-alkane and t BuLi and added to a solution of 1 in toluene; it was not possible to prevent some cyclization to methylcyclopentyl lithium even at low temperature 39 and so an 2:1 mixture of 2-(CH2)4CH=CH2 and 2-CH2(c-C5H9) was produced in this synthesis (Figure S16). The after 24 h at room temperature in C6D6 we observed its full consumption and the formation of 2-CH2(c-C5H9) (Figures S17-18). The cyclization is rather slow and a small amount of 3 is observed to form by 11 B NMR spectroscopy, suggesting that some R• escapes during this process. Nonetheless, the experiment indicates that reversible bond homolysis is occurring in this system.</p><p>The above observations suggest that for the K1 equilibrium of Scheme 2, the ratio of k1/k-1 is very low and that trapping of alkyl radicals by Co(II) complex 3 is rapid, contributing to the overall macroscopic stability of these alkyls. Martin and Finke have shown that one electron reduction of MeCbl results in significantly enhanced rates of bond homolyis. 34 Indeed, cyclic voltammetric measurements on compounds 2-R indicate that all of them undergo an irreversible one-electron reduction at potentials of -0.89 V (2-Me), -0.86 V (2-CH2SiMe3), -0.88 V (2-CH2SiMe2Ph) and -0.85 V (2-i Bu), all vs Fc/Fc+. Figure 3 shows the CV of 2-Me at various scan rates and the data fo the other 2-R compounds are shown in Figures S19-21). This suggests that the addition of one electron to 2-R leads to a highly unstable radical anion [Co(II)-R] •-. However, interpretation of the cyclic voltammogram suggests that this radical anion does not decompose via bond homolyis (via k R• , Scheme 2) but rather via dissociative loss of R -, i.e. heterolytic cleavage of Co-C. As seen in Figure 3, for 2-Me, when scanned in the range of 0.1 to -1.6 V at variable scan rates (0.02 to 0.5 V/s), a reduction wave at -0.89 V (vs Fc/Fc+) is observed to be partially reversible at faster scan rates, but is essentially irreversible at slower scan rates. Anodic scanning shows a pseudo reversible oxidation at -0.33 V, which is identical to the potential observed for the Co(II)/Co(III) couple of five coordinate complex 3 under the same conditions, 22  Interestingly, despite having a similar reduction potential to the others, the same reaction with 2-Me appears to not to proceed; mixing 2-Me with Cp2Co results in no reaction at room temperature (Figure S29). We believe this stems from divergent reactivity of [Cp2Co] + with the methide anion in comparison to the bulkier alkide anions discussed Scheme 5. One electron reduction of 2-R (R ≠ Me) above. 40 Rather than add to one of the Cp rings, Mereduces the cobaltocinium cation back to cobaltocene, and the methyl radical that results dimerizes to ethane or scavenges a hydrogen atom to produce methane. Indeed, we reproduced this chemistry and observed  35). In the 1 H NMR spectrum both ethane and methane were observed according to the chemical shifts referenced against C6D6 (Figure S37). 41 Based on the 1 H NMR integration of C2H6 vs CH4, a rough estimation of a 4:1 ratio of C2H6 : CH4 was found (Figure S37). A small amount of the known diamagnetic compound [(B2Pz4Py)Co(III)] + [NTf2]was identified by 1 H and 19 F NMR spectroscopy, 22 but the bulk of the cobalt containing product was the paramagnetic purple precipitate, which was identified to be the dimeric Co(II) complex 4 by X-ray crystallography (Figure 4</p><p>The HOMO of 2-Me (Figure S39) is primarily localized along the Py-Co-CH3 vector and therefore has electron density associated with both the methyl group and the pyridyl nitrogen. We thus interpret these results in terms of two competing pathways involving protonation of the methyl group leading to CH4 and the contact ion pair  7); Co1'-N4', 1.982 (7). Further metrical data are given in Table S2.</p><p>results then dimerizes with ligand rearrangement to give the observed product 4. The structure of 4 is reminiscent of the coordination mode found in in the lithium salts of the ligands that we have observed previously; 22 a thermal ellipsoid representation of its structure is given in Figure 4, along with some selected metrical parameters, which are unremarkable. Interestingly, when 4 is treated with LiO i Pr in THF, rapid reformation of 3-THF is observed. Evidently the substitutional lability of Co(II) compounds enables facile conversion between dinuclear and mononuclear compounds in this system.</p><!><p>Cobalt (III) alkyl derivatives have a place of significance in the history of bioorganometallic chemistry due to their role as models for vitamin B12 chemistry. We therefore were interested in preparing alkyl complexes using our recently reported dianionic, electron rich pentadentate diborate platform 20 to examine their properties in this context but also as potential synthons for incorporating (B2Pz4Py)Co units into dinuclear structures. The compounds 2-R are straightforwardly synthesized in good to excellent yields via salt metathesis between Co(III) bromide 1 and alkyllithium reagents. The dianionic nature of the ligand stabilizes the Co(III) oxidation state and the alkyl compounds are remarkably robust, thermally and with respect to ambient atmosphere. Nonetheless, there is evidence for reversible bond homolysis in the reaction of compounds 2-R with hydrogen atom donors like 1,4-cyclohexadiene or radical traps like TEMPO, and also in the observed slow cyclization of the "radical clock" 5-hexenyl group to the methylcyclopentyl derivative. Most interestingly, reduction of the Co(III) alkyls to Co(II) anions does not accelerate bond homolysis as previously observed, 34 but leads to ejection of Rthrough heterolytic cleavage of the Co-C bond. This is a rare, possibly unprecedented, reactivity mode for Co(III) alkyls and again reflects the unusually electron rich nature of the B2Pz4Py ligand framework, which is not prone to supporting Co(I) centers. This study underscores the utility of this ligand platform for stabilizing higher oxidation state complexes and intermediates in metals from across the transition series.</p><!><p>General Procedures. Unless mentioned otherwise, all manipulations were performed under an inert gas (argon) in gloveboxes or using standard Schlenk techniques. All anhydrous solvents were prepared by passing through Mbraun SP-800 solvent purification system and were stored in 500 mL thick-walled vessels over sodium/benzophenone ketal or CaH2. All dried solvents were degassed, and vacuum distilled prior to use. Pentane, toluene and benzene solvents were dried over K/Na alloy and freshly distilled before use. Chemicals were obtained from common vendors and used as received. The syntheses of the dianionic pentadentate B2Pz4Py ligand, 1, 3 and [(B2Pz4Py)Co(III)] + [NTf2]have previously been reported. 22 LiCH2SiMe3, 43 and LiCH2SiMe2Ph, 44 were prepared according to literature procedures. Li i Bu was synthesized using the same procedure as for LiCH2SiMe3.</p><!><p>To a pressure tube equipped with a Teflon tap containing suspension of 1 (60 mg, 0.087 mmol) in 8 mL of toluene, LiCH3 (2 mg, 0.087 mmol) was added as a solid all at once. The resulting orange reaction mixture was stirred for 4 h at room temperature.</p><p>The solution was stored at -40˚C overnight and passed through a syringe filter. The solvent was removed in vacuo. The orange solid was washed with 5 mL of pentane and dried in vacuo to afford 38 mg of 2-Me (70%). Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in saturated solution of 2-Me in benzene. 1  3 JHH = 7.6 Hz, H6), 7.10 (1H, t, 3 JHH = 7.6 Hz, H3), 5.77 (4H, t, 3 JHH = 2.4 Hz, H10), 4.10 (2H, s, H12), 2.34 (6H, s, H8), -0.09 (9H, s, H13). 13</p><!><p>To a pressure tube equipped with a Teflon tap containing suspension of 1 (60 mg, 0.087 mmol) in 8 mL of toluene, LiCH2SiMe2Ph (14 mg, 0.087 mmol) dissolved in 2 mL of toluene was added dropwise. The resulting light pink reaction mixture was stirred for 1 h at room temperature. The solution was stored at -40˚C overnight and passed through a syringe filter. The solvent was removed in vacuo. The pink solid was washed with 5 mL of pentane and dried in vacuo to afford 47 mg of 2-CH2SiMe2Ph (71%). Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane in saturated solution of 2-CH2SiMe2Ph in benzene. 1</p><!><p>A suspension of 1 (60 mg, 0.087 mmol) in 8 mL of toluene in a swivel frit apparatus was cooled to -78˚C using an acetone/dry ice bath. 272 μL of (cyclopentylmethyl)lithium (0.32 M, 0.087 mmol) in 3:2 ratio of pentane:diethyl ether was added dropwise at -78 ºC under a flow of Ar. The acetone/dry ice bath was removed. The resulting brown reaction mixture was stirred for 1 h at room temperature. The solution was filtered to remove lithium salts. The solvent was removed in vacuo. The brown solid was washed with 5 mL of pentane and dried in vacuo to afford 54 mg of 2-CH2(c-C5H9) (90%). 1</p><!><p>To a pressure tube equipped with a Teflon tap containing suspension of 3 (60 mg, 0.099 mmol) in 10 mL of bromobenzene, HNTf2 (28 mg, 0.099 mmol) was added as a solid all at once at room temperature. The reaction was immediate, resulting in the 19 formation of a deep purple precipitate. The solvent was removed in vacuo to afford 75 mg of 4 (85%). Crystals suitable for X-ray diffraction were obtained from the precipitate formed in bromobenzene. 1</p><!><p>The following files are available free of charge.</p><p>Experimental and characterization details for all new compounds, including spectroscopic data, X-ray crystallographic data and computational details with the cartesian coordinates for calculated structures (PDF). Comparative study. J. Am. Chem. Soc. 1968 V vs SHE) was added during each experiment as an internal reference.</p><!><p>All the quantum-chemical calculations were conducted using the Gaussian16 program suite. 4 As functional we have used the Becke's 3-parameter hybrid one, 5 combined with the non-local correlation functional provided by Perdew/Wang , denoted as B3PW91. 6 For Co, the relativistic energy-consistent pseudopotential of the Stuttgart-Köln ECP library was used in combination with its adapted segmented basis. 7 For all other atoms, a standard 6-31G** basis set was used. 8,9 All stationary points have been identified as minima (number of imaginary frequencies Nimag=0).</p><p>Further Synthetic Details.</p><!><p>This procedure was done according to a modified literature procedure under Ar. 10 In a two-neck round bottom flask equipped with a Y-Joint was added iodo(methyl)cyclopentane (90 mg, 0.43 mmol). A 3:2 ratio of pentane (500 µL) and diethyl ether (340 µL) was added to the flask to have an overall 0.5 M solution of iodo(methyl)cyclopentane. The solution was cooled to -78 ºC using an acetone/dry ice bath and t BuLi (1.7M, 500 µL, 0.86 mmol) was added dropwise under vigorous stirring. The solution was stirred at -78 ºC for 1 h and the concentration of 0.32 M was found assuming 100 % yield.</p><!><p>This procedure was done according to a modified literature procedure under Ar. 10 In a two-neck round bottom flask equipped with a Y-Joint was added 6-iodo-1-hexene (90 mg, 0.43 mmol). A 3:2 ratio of pentane (500 µL) and diethyl ether (340 µL) was added to the flask to have an overall 0.5 M solution of 6-iodo-1-hexene. The solution was cooled to -78 ºC using an acetone/dry ice bath and t BuLi (1.7M, 500 µL, 0.86 mmol) was added dropwise under vigorous stirring. The solution was stirred at -78 ºC for 1 h and the concentration of  A J-Young tube equipped with a Teflon tap was charged with 5.0 mg of 2-R in toluene-d8. A 1 H NMR spectrum was recorded. CHD (5 eq) was added at room temperature and another 1 H NMR spectrum was recorded. The J-Young tube was then heated at 80 ºC for 24 h and another 1 H NMR spectrum was recorded. If any reactivity was observed, a 11 B NMR spectrum was recorded to detect complex 3.       A J-Young tube equipped with a Teflon tap was charged with 5.0 mg of 2-R in C 6 D 6 . A 1 H NMR spectrum was recorded. TEMPO (1.1 eq) was added as a solid at room temperature and another 1 H NMR spectrum was recorded after 24 h. If any reactivity was observed, a 11 B NMR spectrum was recorded to detect complex 3.       A J-Young tube equipped with a Teflon tap was charged with 5.0 mg of 2-(CH 2 ) 4 CH=CH 2 in C 6 D 6 . A 1 H NMR spectrum was recorded. The J-Young tube was stirred for 16 h and 24 h at room temperature and two 1 H NMR spectra were recorded. A 11 B NMR spectrum was recorded at room temperature after 24 h of stirring.      A J-Young tube equipped with a Teflon tap was charged with 5.0 mg of 2-R in C 6 D 6 . A 1 H NMR spectrum was recorded. CoCp 2 (1.1 eq) was added at room temperature and another 1 H NMR spectrum was recorded 15 mins after the addition of CoCp 2 . A 11 B NMR spectrum was recorded to detect complex 3.        Procedure for the reactivity of 2-R with HNTf 2 in C 6 D 6 .</p><!><p>A J-Young tube equipped with a Teflon tap was charged with 5.0 mg of 2-R in C 6 D 6 . A 1 H NMR spectrum was recorded. A stock solution of HNTf 2 in C 6 D 6 was prepared and 1 eq of HNTf 2 was added to the J-Young tube. A 1 H NMR spectrum was recorded at room temperature.</p><!><p>During the refinement, it was noticed that there was a large amount of residual electron density located between the Co atoms and the ellipsoids for Co were elongated in the same direction. Given the synthetic and recrystallization conditions used, the residual electron density was attributed to bromine from bromobenzene, the recrystallization solvent. Based on the bromine present and the elongated Co ellipsoids, the Co atom was split in to Co1A and Co1B. Initially, the occupancies of bromine and Co atoms were refined independently with the occupancy refining to 0.1326 (19) and 0.12(3) for Br and Co1B, respectively. Based on their similar occupancies, Co1B is assumed be the oxidized to Co(III) and charge balanced by Br(-I). For consistency, the occupancy of Br and Co1B were set equal and allowed to refine, reaching occupancies of 0.1325 (19).</p>