Photochemical Unmasking of 1,3‐Dithiol‐2‐ones: An Alternative Route to Heteroleptic Dithiolene Complexes from Low‐Valent Molybdenum and Tungsten Precursors

Abstract

Mono-dithiolene complexes [Mo(CO) 2 (dt)(dppe)] and [W(CO) 2 (dt)(dppe)] {dt = cyclohex-1-ene-1,2-dithiol; 5,6-dihydro-2H-pyran-3,4-dithiol and dppe= 1,2-bis(diphenylphosphino)ethane} were synthesized by a photochemical procedure. The typical basic de-protection of the dithiolene ligand precursor was replaced by a light-induced opening of the 1,2-dithiole-2-one moiety. Advantages of this targeted approach comprise higher yields, cleaner transformations, and the possibility to continuously and precisely monitor the reaction progress. The [a] Scheme 1. Literature known pathways towards 1,2-dithiones and their tautomers: i. Nickel transfer metalation [7] ii. Chelotropic reaction of 1,3-dithiol-2ones initiated by UV-light [8] or heat [9] iii. Sulfuration of acetylenic compounds [10] iv. Sulfuration of dioxamides [11] v. Synthesis from thiirenes-1-oxides and Lawesson's Reagent. [12] tainly feasible approach towards metal-dithiolene complexes would be the direct use of 1,2-dithiones or their valence tautomers, [15] the 1,2-dithietes. This strategy was applied successfully to Pd, Ni, Co, Ru and Fe carbonyl precursors yielding bis-and trisdithiolene complexes. [15,16] With respect to the metals of our interest (Mo, W), in one case the reaction between the easily accessible bis(trifluoromethyl)-dithiete with hexacarbonyl molybdenum was described to yield 55 % of [Mo(S 2 C 4 F 6 ) 3 ]. [17] 1,2-Dithietes are commonly prepared by the reaction of elemental sulfur with the appropriate alkyne (Scheme 1; iii.) while not any alkyne leads to a wanted dithiete or dithione, though. [10,16b,18] The equilibrium between both tautomers is highly dependent on the backbone. Thus, electron-donating substituents usually stabilize dithiones [S 2 C 2 R 2 ; R = (p-dimethylamino)phenyl, 1-adamantyl] [8,19] while electron-withdrawing substituents favor dithietes which was established for the cases S 2 C 2 (CF 3 ) 2 , S 2 C 2 (CO 2 Me) 2 , and S 2 C 2 (CN) 2 . [9,20] Both, dithiones and dithietes are, however, undoubtedly unstable unless stabilized either electronically or sterically as they constitute reactive transient intermediates. [12] Comparably stable 1,2-dithiones are available from dioxamides upon thionation with Lawesson's Reagent (Scheme 1; iv.). [11] These specific species were first tested for the oxidative addition to molybdenum by Dieck and Form followed by only two related reports more than two decades later. [21] Electron donating dithiolenes which are not sterically hindered as for instance cyclic ones [e.g. cylohex-1-ene-1,2-dithiol (cydt) [22] or 5,6-dihydro-2H-pyran-3,4-dithiol (pydt) [23] ] or the well known S 2 C 2 Me 2 (mdt) [13] are neither isolable as dithiones/ dithietes nor as their unprotected alkali salts. Instead, precursors for this type of dithiolene ligand are commonly prepared in their 1,3-dithiol-2-one form. In order to coordinate such dithiolenes to metal carbonyl precursors their corresponding dithiones can be prepared in situ via a photochemically driven chelotropic decarbonylation (Scheme 1; ii.). ...

Fine‐Tuning Redox Properties of Heteroleptic Molybdenum Complexes through Ligand‐Ligand‐Cooperativity

“…The first reduction step is also associated with a change in coordination geometry from trigonal prismatic to octahedral, as was shown before by DFT calculations and reported for related monodithiolene complexes [13e, 14, 23] . Such a torsion is evident also here from the Bailar‐Twist angles [24] of the “stacked” (52.2°) and “non‐stacked” (45.2°) structures of 2 1− compared to the parent complex ( 2 ) (3.1°; Table S1).…”

Feinabstimmung der Redoxeigenschaften von heteroleptischen Molybdän‐Komplexen mittels Ligand‐Ligand‐Kooperativität

“…Der erste Reduktionsschritt geht mit einer Änderung der Koordinationsgeometrie von trigonal‐prismatisch zu oktaedrisch einher. Das wurde zuvor durch DFT‐Berechnungen und auch experimentell für verwandte Monodithiolenkomplexe gezeigt [13e, 14, 23] . Eine solche Änderung der Geometrie der “gestapelten” (52.2°) und der “nicht‐gestapelten” (45.2°) theoretischen Struktur von 2 1− geht auch hier aus den Bailar‐Twist ‐Winkeln im Vergleich mit dem Ausgangskomplex ( 2 ) (3.1°; Tabelle S1) hervor [24] .…”

“…Das wurde zuvor durch DFT-Berechnungen und auch experimentell für verwandte Monodithiolenkomplexe gezeigt. [13e, 14,23] Eine solche Änderung der Geometrie der "gestapelten" (52.2°) und der "nicht-gestapelten" (45.2°) theoretischen Struktur von 2 1À geht auch hier aus den Bailar-Twist-Winkeln im Vergleich mit dem Ausgangskomplex (2) (3.1°; Tabelle S1) hervor. [24] Alle vier in dieser Arbeit untersuchten Komplexe weisen bei der theoretischen Reduktion die gleiche Geometrieänderung auf.…”

Inspired by Nature—Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

“…Tris(dithiolene) complexes can be easily synthesized, but are generally rather unreactive and therefore not commonly of interest. Mono(dithiolene) complexes are only rarely found as functional analogues in the existing literature due to their difficult syntheses and limited stabilities [8,35,36]. In contrast, bis(dithiolene) complexes represent a class of model compounds which are relatively reliably synthesized and concomitantly suitable synthetic analogues for OAT reactivity.…”