Synthesis and interconversions of reduced, alkali–metal supported iron–sulfur–carbonyl complexes

Abstract

We report the synthesis, interconversions and X-ray structures of a set of [mFe–nS]-type carbonyl clusters (where S = S²⁻, S₂²⁻ or RS⁻; m = 2–3; n = 1–2). All of the clusters have been identified and characterized by single crystal X-ray diffraction, IR and ¹³C NMR. Reduction of the parent neutral dimer [μ₂-(SPh)₂Fe₂(CO)₆] (1) with KC₈ affords an easily separable ∼1 : 1 mixture of the anionic, dimeric thiolate dimer K[Fe₂(SPh)(CO)₆(μ-CO)] (2) and the dianionic, sulfido trimer [K(benzo-15-crown-5)₂]₂[Fe₃(μ₃-S)(CO)₉] (3). Oxidation of 2 with diphenyl-disulfide (Ph₂S₂) cleanly returns the starting material 1. The Ph–S bond in 1 can be cleaved to form sulfide trimer 3. Oxidation of sulfido trimer 3 with [Fc](PF₆) in the presence of S₈ cleanly affords the all-inorganic persulfide dimer [μ₂-(S)₂Fe₂(CO)₆] (4), a thermodynamically stable product. The inverse reactions to form 3 (dianion) from 4 (neutral) were not successful, and other products were obtained. For example, reduction of 4 with KC₈ afforded the mixed valence Fe(I)/Fe(II) species [((Fe^I₂S₂)(CO)₆)₂Fe^II]²⁻ (5), in which the two {Fe₂S₂(CO)₆}²⁻ units serve as bidendate ligands to a Fe(II) center. Another isolated product (THF insoluble portion) was recrystallized in MeCN to afford [K(benzo-15-crown-5)₂]₂[((Fe₂S)(CO)₆)₂(μ-S)₂] (6), in which a persulfide dianion bridges two {2Fe–S} moieties (dimer of dimers). Finally, to close the interconversion loop, we converted the persulfide dimer 4 into the thiolate dimer 1 by reduction with KC₈ followed by reaction with the diphenyl iodonium salt [Ph₂I](PF₆), in modest yield. These reactions underscore the thermodynamic stability of the dimers 1 and 4, as well as the synthetic and crystallization versatility of using the crown/K⁺ counterion system for obtaining structural information on highly reduced iron–sulfur–carbonyl clusters.