The Redox Chemistry of [Co₆C(CO)₁₅]^2–: A Synthetic Route to New Co-Carbide Carbonyl Clusters

Abstract

The oxidation and reduction reactions of [Co6C(CO)15](2-) have been studied in detail, leading to the isolation of several new Co-carbide carbonyl clusters. Thus, [Co6C(CO)15](2-) reacts in tetrahydrofuran (THF) with oxidants such as HBF4·Et2O and [Cp2Fe][PF6], resulting first in the formation of the previously reported [Co6C(CO)14](-); then, in CH2Cl2, the new dicarbide [Co11C2(CO)23](2-) is formed. The latter may be further oxidized, yielding the isostructural monoanion [Co11C2(CO)23](-), whereas its reduction with (cyclopentadienyl)2Co affords the unstable trianion [Co11C2(CO)23](3-), which decomposes during workup. Oxidation of [Co6C(CO)15](2-) in CH3CN with [C7H7][BF4] affords the same major products, and besides, the new monoacetylide [Co10(C2)(CO)21](2-) was obtained as side-product. Conversely, the reduction of [Co6C(CO)15](2-) in THF with increasing amounts of Na/naphthalene results in the following species: [Co6C(CO)13](2-), [Co11(C2)(CO)22](3-), [Co7C(CO)15](3-), [Co8C(CO)17](4-), [Co6C(CO)12](3-), and [Co(CO)4](-). The new [Co11C2(CO)23](-), [Co11C2(CO)23](2-), [Co10(C2)(CO)21](2-), [Co8C(CO)17](4-), [Co6C(CO)12](3-), and [Co7C(CO)15](3-) clusters were structurally characterized. Moreover, the paramagnetic species [Co11C2(CO)23](2-) and [Co6C(CO)12](3-) were investigated by means of electron paramagnetic resonance spectroscopy. Finally, electrochemical studies were performed on [Co11C2(CO)23](n-) (n = 1-3).

Synthesis of the Highly Reduced [Fe6C(CO)15]4– Carbonyl Carbide Cluster and Its Reactions with H+ and [Au(PPh3)]+

“…DFT calculations allowed us to study from a computational point of view the electronic features of the new compounds and of the possible reaction intermediates. This study was prompted by recent results on the reductions of simple Fe, Co, and Ni clusters under similar experimental conditions, which afforded highly reduced carbonyl clusters …”

Cobalt Carbonyl Clusters Enable Independent Control of Two Proton Transfer Rates in the Mechanism for Hydrogen Evolution

“…[20] Herein we describe the electrochemical behavior of [Co 11 C 2 (CO) 23 ] 2À (1 2À ) which has E1 = 2 (1 3À /4À ) = À 0.95 V vs. SCE compared with 2 4À which has E1 = 2 = À 1.06 V (Chart 1). [21] We show that 1 3À is an active HER catalyst and formation of the intermediate (H-1) 3À is fast, measured at k PT1 = 3 × 10 8 M À 1 s À 1 . The measured rate for PT2 is five orders of magnitude slower, at 3.7 × 10 3 M À 1 s À 1 , and this implies that the rate of PT2 is gated by the hydricity of (H-1) 3À even while PT1 remains diffusion limited.…”

“…Under 1 atm of N 2 reversible redox couples were observed with E 1/2 = À 0.2, À 0.57 and À 0.95 V vs. SCE and assigned as the 1 1À /2À , 1 2À /3À and 1 3À /4À couples, respectively (Figure 1 top). [21] Diffusion coefficients for 1 2À , 1 3À , and 1 4À were obtained from variable scan rate CV experiments and are D = 7.8 × 10 À 7 , 2.5 × 10 À 6 , and 5.4 × 10 À 6 cm 2 s À 1 , respectively (Calculation S1, Table S1, Figure S1). These diffusion coefficients are similar to those that have been reported for nanomaterials including 2 3À ,quantum dots, [22,23] and fullerenes, [24] but slower than the diffusion coefficients of smaller molecular species like metal coordination complexes.…”

“…It is known that a linear correlation exists between ν CO and z/M, where z is the cluster charge, and M is the number of cobalt atoms; and this relationship holds true over cobalt carbide carbonyl clusters with 2-13 cobalt atoms and charge À 1 to À 4 (Figure 1 bottom). [19,21] We generated 1 3À in situ using one equivalent of cobaltocene as a reductant in acetonitrile. The IR spectrum displayed absorption bands at 1974 and 1768 cm À 1 which are lower in energy, as expected for a more electron rich cluster.…”

“…The IR spectrum displayed absorption bands at 1974 and 1768 cm À 1 which are lower in energy, as expected for a more electron rich cluster. [21] Observation of 1 4À was achieved by addition of two equivalents of sodium benzophenone to 1 2À in dry THF. The IR spectrum of 1 4À at 1942 and 1760 cm À 1 is also consistent with the z/M plot.…”