special feature, perspective. For the article “Speculative synthetic chemistry and the nitrogenase problem,” by Sonny C. Lee and Richard H. Holm, which appeared in issue 7, April 1, 2003, of Proc. Natl. Acad. Sci. USA (100, 3595–3600; first published March 17, 2003; 10.1073/pnas.0630028100), the figures and schemes should have been published in color. The corrected illustrations and their legends appear below.
Fig. 1.
Other [MFe3S4] cubane motifs in biology: void-filled 3-Fe clusters (Left) and the C-cluster in a Ni-Fe-S carbon monoxide dehydrogenase (Right). A different structure is available (at lower resolution) for the C-cluster in an analogous enzyme from a different organism that contains the [NiFe3S4] cubane cluster in a less-distorted form (15). M = Mn, Co, Ni, Cu, Zn, Cd, Ga, Tl; N*, histidine; S*, cysteinate.
Fig. 2.
Selected higher-nuclearity Fe-S clusters. (Y, z) = (N2H4, 4), (OH/N3/CN, 5), (S, 6); X, monoanions (halide/thiolate/aryloxide).
Fig. 3.
Metalloclusters in the nitrogenase MoFe protein as derived from protein crystallography (pre-2002). N*, histidine; O*, serine/serinate; S*, cysteinate.
Fig. 4.
Revised FeMo-cofactor structure derived from the most recent, high-resolution macromolecular structure determination. N*, histidine; S*, cysteinate.
Fig. 5.
Representative nitride-containing iron-carbonyl clusters: [Fe4(μ4-N)(CO)12]- (Left), [Fe5(μ5-N)(μ-CO)2(CO)12]- (Center), and [Fe6(μ6-N)(μ-CO)3(CO)12]3- (Right). See Table 1 for corresponding Cambridge Structural Database refcodes.
Fig. 6.
Representative weak-field iron-imide clusters. X, monoanions (halide/thiolate).
Scheme 1.
Direct synthesis of a single [MoFe3S4] heterometallic cubane cluster.
Scheme 2.
Synthesis of a [NiFe3S4] heterometallic cubane cluster.
Scheme 3.
Synthesis of [MFe4S6] heterometallic clusters. (M, x) = (Mo, 4), (V, 7).
Scheme 4.
Selected reactions of edge-bridged double-cubane clusters. LnM = (Et3P)(Cl4cat)Mo for Cl4cat-ligated products or TpM for Tp-ligated products; Cl4cat, tetrachlorocatecholate(2-); Tp, hydrotris(pyrazolyl)borate(1-); (M, x = z) = (Mo, 3), (V, 4).