Mixed-valence, tetranuclear cobalt(iii,iv) complexes: preparation and properties of [Co4O4(O2CR)2(bpy)4]3+ salts

Katerina Dimitrou, Angelica D. Brown, George Christou, Thomas E. Concolino, Arnold L. Rheingold
2001 Chemical Communications  
Cyclic voltammetric examination of [Co 4 O 4 (O 2 CR) 2 -(bpy) 4 ](ClO 4 ) 2 cubane complexes 1 (R = various; bpy = 2,2A-bipyridine) in MeCN reveals a one-electron reversible oxidation in the range 0.68-0.86 V vs. ferrocene; the oneelectron oxidized clusters can be obtained in analytical purity by either controlled potential electrolysis to give [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ](ClO 4 ) 3 2 or oxidation with (NH 4 ) 2 Ce(NO 3 ) 6 to give [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ][Ce(NO 3 ) 6 ] 3. The ability
more » ... 6 ] 3. The ability of hard oxide (O 22 ) ions to stabilize unusually high metal oxidation states has long been recognized in such textbook species as MnO 4 2 and CrO 4 22 . More recently, some interesting new examples have been identified in which bridging O 22 ions stabilize rare oxidation states of Fe and Cu in non-mononuclear chemistry: apart from their intrinsic importance, such species offer insights into the mode of action of certain metalloenzymes. Thus, the Fe III Fe IV oxidation level, thought to correspond to species X of ribonucleotide reductase, 1,2 has been obtained in oxide-bridged Fe 2 complexes, 2,3 whereas a 2Fe IV species is believed to be species Q of methane monooxygenase. 1 Similarly, the [Cu 2 (m-O) 2 ] 2+ core (2Cu III ) has been structurally characterized, 4,5 as well as a related species containing the [Cu 3 (m 3 -O) 2 ] 3+ core (2Cu II , Cu III ). 6 Such species offer valuable insights into Cu-mediated O 2 -activation biochemistry. We have now discovered that the cubane [Co 4 (m 3 -O 4 ) 4 ] 4+ core containing 4Co III can be oxidized to a 3Co III , Co IV form, and we describe the preparation and characterization of several such novel species. As reported previously, 7 the D 2d symmetry complex [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ](ClO 4 ) 2 1 (R = Me; bpy = 2,2Abipyridyl) can be prepared by deprotonating [Co 2 (OH) 2 (O 2 C-Me) 3 (bpy) 2 ](ClO 4 ) with Li 2 O 2 in DMSO. A series of [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ](ClO 4 ) 2 1 complexes has now been prepared by ligand substitution with RCO 2 H in refluxing MeCN, and electrochemical examination of these [Co 4 O 4 ] 4+ complexes by cyclic voltammetry (CV) reveals irreversible reductions at @21.4 V vs. ferrocene and a reversible oneelectron oxidation at ca. 0.7 V vs. ferrocene. The CV scan for the R = C 6 H 4 NO 2 -4 complex is shown in Fig. 1 , and the oxidation potentials as a function of R group are listed in Table 1 . The peak separations are comparable with that of the ferrocene/ferrocenium couple used as an internal reference (Fig. 1) . Controlled potential coulometry at 0.90 V vs. ferrocene for the R = C 6 H 4 Me-4 and C 6 H 4 OMe-4 species gave 0.9-1.0 e 2 / Co 4 transferred, confirming a one-electron oxidation process. The CV are thus consistent with oxidation to [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ] 3+ , which would contain 3Co III ,Co IV if the oxidation were metal-based. Metal/bpy complexes can often exhibit bpy-based reversible reductions but not oxidations. 8 Analytically-pure oxidized clusters † were prepared in bulk by two methods: (i) controlled potential electrolysis in MeCN or MeCN-CH 2 Cl 2 containing 0.4 M LiClO 4 at 0.90-1.00 V vs. ferrocene, which caused precipitation of [Co 4 O 4 (O 2 CR) 2 -(bpy) 4 ](ClO 4 ) 3 2 in ! 85% yield as microcrystalline powders; and (ii) oxidation with (NH 4 ) 2 Ce(NO 3 ) 6 in MeCN, which caused precipitation of sparingly soluble microcrystalline [Co 4 O 4 (O 2 CR) 2 (bpy) 4 ][Ce(NO 3 ) 6 ] 3. ‡ Complexes 2 can be recrystallized from MeCN-Et 2 O. EPR spectra at 5 and 75-100 K of the R = C 6 H 4 Me-4 and C 6 H 4 OMe-4 derivatives as powders (3) or MeCN-toluene (1+1) glasses (2) show a broad (ca. 800 G) signal at g = 2.20 with no resolved hyperfine, consistent with a S = 1 ⁄ 2 species and a Co-based oxidation. Solid-state magnetic susceptibility studies at room temperature on the same two derivatives of 2 gave m eff ≈ 2.2 m B , consistent with one unpaired electron. 1 H NMR spectra show paramagnetically shifted and broadened signals for 2, again consistent with S = 1 ⁄ 2 oxidized clusters whereas complexes 1 are diamagnetic. Comparison of the spectra for 1 and 2 (R = C 6 H 4 Me-4), for example, show that only four bpy and three carboxylate signals are observed and that the bpy signals are much more shifted on oxidation than the carboxylate signals, Fig.
doi:10.1039/b102008k fatcat:jbcawdjrujfrvpbc5f3tuvjshi