Molybdenum−Iron Sulfide-Bridged Double Cubanes

Abstract

A new class of clusters, sulfide-bridged double cubanes containing the units MFe3(μ3-S)4 (M = Fe, Mo), has been investigated as possible synthetic precursors to the iron−molybdenum cofactor (FeMoco) of nitrogenase. Clusters containing the symmetric core structures [Fe4Q4-Q-Fe4Q4]2+ (11−13, Q = S, Se) have been prepared by the coupling of separate cubane clusters ([Fe4S4Cl4]2-, [Fe4Q4(LS3)Cl]2-, where LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−)). Similarly, the cuboidal cluster [VFe4S6(PEt3)4Cl] was coupled to form [VFe4S6(PEt3)4]2S (14). The cluster [(Fe4S4Cl3)2S]4- (11) is the first structurally proven example of the class (Challen, P. R.; Koo, S.-M.; Dunham, W. R.; Coucouvanis, D. J. Am. Chem. Soc. 1990, 112, 2455). Bridged structures have also been established crystallographically for {[Fe4Se4(LS3)]2Se}4- (13) and 14. Other criteria for identification of this structure developed using 11−14 are coupled redox processes in cyclic voltammetry and the detection of intact double cubane ions by electrospray mass spectrometry. For the coupling of heterometal cubanes, the Mo site was protected by chelation with Meida (N-methylimidodiacetate(2−)) as in [(Meida)MoFe3S4Cl3]2- (6), thereby directing the bridging reaction to the Fe sites. The structures of two [VFe3S4]2+ cubane clusters containing the tricoordinate (Meida)V fragment are reported. The reaction system 6/Li2S afforded {[(Meida)MoFe3S4Cl2]2S}4- (15). The symmetrical sulfide-bridged double cubane structure of 15 has been established by the electrochemical and mass spectrometric criteria and by the existence of the four isomers consistent with this structure. The equimolar reaction system 6/[Fe4S4Cl4]2-/Li2S produced a mixture of {[(Meida)MoFe3S4Cl2]S(Fe4S4Cl3)}4- (17), 15, and 11. Cluster 17 is also formed in the system 15/[Fe4S4Cl4]2-. The unsymmetrical sulfide-bridged double cubane structure of 17 was established by mass spectrometry and detection of the two isomers consistent with this structure. In the first reaction system, the product mole ratio 17:15 ≈ 3:1 is explained in terms of differential steric hindrance of conformations arising from rotation around the Fe−S−Fe bridge. The core composition of double cubane 15 (Mo2Fe6S9) approaches that of FeMoco (MoFe7S9). The core composition of 17 is exactly the same as FeMoco; 17 is the first synthetic cluster with this property.

Photoluminescent Metal-Sulfur Clusters Derived from Tetrathiometalates: Metal-to-Metal Charge-Transfer Excited States of d0-d10 Heterobimetallic Sulfido Clusters with Bulky Phosphine Ligands

“…Hence, our quest for a MS 4 nÀ -derived heterometallic cluster that is emissive in both the solid state and solution media continues. In this context, we draw attention to the d 0 ± d 10 2 ). Remarkably, clusters 5´(ClO 4 ) 2 and 6´(ClO 4 ) 2 are photoluminescent both in the solid state and in solution, which, to our knowledge, constitute the first MS 4 nÀ -derived metal ± sulfur clusters that have this property.…”

“…Remarkably, clusters 5´(ClO 4 ) 2 and 6´(ClO 4 ) 2 are photoluminescent both in the solid state and in solution, which, to our knowledge, constitute the first MS 4 nÀ -derived metal ± sulfur clusters that have this property. Further, the electronic spectra of 5´(ClO 4 ) 2 and 6´(ClO 4 ) 2 show lowenergy bands that probably originate from metal-to-metal charge-transfer (MMCT) transitions, a type of CT band that is rarely observed for metal ± sulfur clusters [6,7] although well documented for other types of polynuclear metal complexes. [8] Results Syntheses: Treatment of the ammonium salts of tetrathiometalates MS 4 2À (M Mo/W) with two equivalents of M'(PCy 3 )X (M' Ag/Au, X ClO 4 /Cl) in dichloromethane at room temperature for several hours readily afforded clusters 2 ± 4 in 43 ± 78 % yields [Reaction (1) in Scheme 1].…”

“…[8] Results Syntheses: Treatment of the ammonium salts of tetrathiometalates MS 4 2À (M Mo/W) with two equivalents of M'(PCy 3 )X (M' Ag/Au, X ClO 4 /Cl) in dichloromethane at room temperature for several hours readily afforded clusters 2 ± 4 in 43 ± 78 % yields [Reaction (1) in Scheme 1]. Reaction of the same tetrathiometalates with $ 2 equivalents of [Cu 2 (dcpm) 2 (MeCN) 2 ](ClO 4 ) 2 in acetonitrile at room temperature under nitrogen for 24 h led to isolation of clusters 5´(ClO 4 ) 2 or 6´(ClO 4 ) 2 in $ 88 % yields [Reaction (2) in Scheme 1]. Clusters 2 ± 4, 5´(ClO 4 ) 2 , and 6´(ClO 4 ) 2 contribute the first examples of heterobimetallic sulfido clusters formed from MoS 4 2À /WS 4 2À and coinage metal ions that bear bulky phosphine ligands.…”

“…Previously, a number of their analogues have been reported, including [M' 2 L y (MS 4 )] (y 2 ± 4), [1a,d, 9] [Cu 3 (PPh 3 ) 3 Cl(MS 4 )], [1d] [M' 4 L y (MS 4 ) 2 ] (y 4, 8), [1d, 10] (nBu 4 N)[Cu 3 (dppm) 2 Br 2 (WS 4 )], [11] 1´X (X PF 6 , Br), [11] and [Cu 4 (dppm) 4 (MS 4 )](PF 6 ) 2 (M Mo: 5'´(PF 6 ) 2 , W: 6'´(PF 6 ) 2 ), [11] as well as 1´ClO 4 ; [6] however, all the phosphine ligands in these clusters have considerably smaller cone angles. [12] The formation of [Cu 4 (dcpm) 4 (MS 4 )] 2 (M Mo: 5 2 , W: 6 2 ) from Reaction (2) in fairly high yields is particularly interesting. First, we have demonstrated that a similar reaction of WS 4 2À with [Cu 2 (dppm) 2 (MeCN) 4 ](ClO 4 ) 2 affords the tetranuclear cluster 1´ClO 4 in 25 % yield [Reaction (3) in Scheme 1]; [6] no [Cu 4 (dppm) 4 (WS 4 )] 2 (6' 2 ), the dppm counterpart of the pentanuclear cluster 6 2 , is obtained from the reaction.…”

Unravelling seemingly complex chemistry of reactions using electrospray ionization mass spectrometry

“…The products of several electrochemically mediated syntheses were detected using reversed-phase chromatography and on-line coupling to an ESI-MS instrument. A series of molybdeniumiron sulfide bridged double cubanes, MFe 3 (m 3 -S) 4 (M = Fe, Mo), considered as synthetic precursors to the Fe-Mo cofactor (FeMoco) of nitrogenase, were studied using ESI-MS. 184 ESI-MS has also proven to be useful in studies involving the electrochemical behavior of cobalt tris(dithiocarbamates) and tris(diselenocarbamates) in CH 3 CN, 185 ferrocene and ferrocene derivatives, 186,187 mercury dithiophosphate complexes 188 and the detection of iodine in electrochemical reduction studies. 189 On-line electrochemistry-ESI-MS has been reported by two laboratories.…”