Generation and Reactivity of Polychalcogenide Chains in Binuclear Cobalt(II) Complexes

Abstract

A series of six binuclear Co(II)-thiolate complexes, [Co₂(BPMP)(S-C₆H₄-o-X)₂]¹⁺ (X = OMe, 2; NH₂, 3), [Co₂(BPMP)(μ-S-C₆H₄-o-O)]¹⁺ (4), and [Co₂(BPMP)(μ-Y)]¹⁺ (Y = bdt, 5; tdt, 6; mnt, 7), has been synthesized from [Co₂(BPMP)(MeOH)₂(Cl)₂]¹⁺ (1a) and [Co₂(BPMP)(Cl)₂]¹⁺ (1b), where BPMP^1- is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol. While 2 and 3 could allow the two-electron redox reaction of the two coordinated thiolates with elemental sulfur (S₈) to generate [Co₂(BPMP)(μ-S₅)]¹⁺ (8), the complexes, 4-7, could not undergo a similar reaction. An analogous redox reaction of 2 with elemental selenium ([Se]) produced [{Co₂(BPMP)(μ-Se₄)}{Co₂(BPMP)(μ-Se₃)}]²⁺ (9a) and [Co₂(BPMP)(μ-Se₄)]¹⁺ (9b). Further reaction of these polychalcogenido complexes, 8 and 9a/9b, with PPh₃ allowed the isolation of [Co₂(BPMP)(μ-S)]¹⁺ (10) and [Co₂(BPMP)(μ-Se₂)]¹⁺ (11), which, in turn, could be converted back to 8 and 9a upon treatment with S₈ and [Se], respectively. Interestingly, while the redox reaction of the polyselenide chains in 9a and 11 with S₈ produced 8 and [Se], the treatment of 8 with [Se] gave back only the starting material (8), thus demonstrating the different redox behavior of sulfur and selenium. Furthermore, the reaction of 8 and 9a/9b with activated alkynes and cyanide (CN^-) allowed the isolation of the complexes, [Co₂(BPMP)(μ-E₂C₂(CO₂R)₂)]¹⁺ (E = S: 12a, R = Me; 12b, R = Et; E = Se: 13a, R = Me; 13b, R = Et) and [Co₂(BPMP)(μ-SH)(NCS)₂] (14), respectively. The present work, thus, provides an interesting synthetic strategy, interconversions, and detailed comparative reactivity of binuclear Co(II)-polychalcogenido complexes.

Chart 1. Abbreviations and Designations of the Binuclear Co(II) Complexes and Ligandsa

Scheme 1. Synthesis of the Binuclear Co(II) Complexes 2–7 from 1a/1b

Figure 1

Molecular structures of 1a (a), 1b (b), 2 (c), 3 (d), 4 (e), 5 (f), 6 (g), and 7 (h) with 30% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for clarity (except for the hydroxy group of the coordinated MeOH in 1a and amino groups in 3).

Figure 2

Molecular structures of 8 [for 8 (ClO₄)] with 30% probability thermal ellipsoids and partial atom labeling scheme (hydrogen atoms are omitted for clarity) (a) and XPS of 8 (ClO₄) showing the Co 2p (b) and S 2p (c) regions.

Figure 3

Molecular structures of 9a (a) and 9b (b) with 30% probability thermal ellipsoids and partial atom labeling scheme (hydrogen atoms are omitted for clarity) and XPS of 9b (BPh₄) showing the Co 2p (c), Se 3p (d), and Se 3d (e) regions. Note that only the Co₂Se₃ unit of [{Co₂(BPMP)(μ-Se₄)}{Co₂(BPMP)(μ-Se₃)}](ClO₄)₂ [9a (ClO₄)₂] is shown (a).

Figure 4

³¹P NMR (¹H decoupled, DMSO-d₆, 600 MHz) spectroscopic monitoring for the reaction of 8 (ClO₄) with varying equivalents of PPh₃. Ratio of 8 (ClO₄)/PPh₃ = 1:n, where n = 1, 2, 3, 4, 5 (a); n = 3, 3.25, 3.50, 3.75, 4 (b).

Figure 5

Molecular structure of 10 (a) and 11 (b) with 30% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for clarity.

Figure 6

³¹P NMR (¹H decoupled, DMSO-d₆, 600 MHz) spectroscopic monitoring for the reaction of 9b (BPh₄) with varying equivalents of PPh₃: (a) ratio of 9b (BPh₄)/PPh₃ = 1:n, where n = 1, 2, 3, 4 and (b) ratio of 9b (BPh₄)/PPh₃ = 1:n, where n = 2, 2.25, 2.50, 2.75, 3.

Scheme 2. Schematic Presentation for the Synthesis of Binuclear Cobalt(II)–Polychalcogenido Complexes and Their Interconversion and Reactivity with Phosphines

Figure 7

Molecular structures of 13a (a) and 13b (b) with 30% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for clarity.

Figure 8

IR spectra (ATR) for the solid products obtained from the reaction of varying equivalents of (Bu₄N)(CN) with 9b (a) and 8 (b).

Scheme 3. Schematic Presentation for the Reactivity of Binuclear Cobalt(II)–Polychalcogenido Complexes, 8, 9a, and 9b

Figure 9

Molecular structures of 14 (a) and 15 (b) with 30% probability thermal ellipsoids and partial atom labeling scheme. Hydrogen atoms are omitted for clarity.