SQUID Magnetic Measurements on#1 -z+1,x-1/2,-y+1/2 Positional parameters and B(eq) for 2.62(5)2.40(5) 2.1(4) 2.4(4) 2.4(4) 2.3(4) 2.0(4) 2.0(4) 2.1(5) 2.7(6) 2.5(5) 3.2(6) 3.0(6) 3.9(7) 3.1(6) 4.3(7) 2.5(5) 3.7(6) 2.8(6) 3.1(6) 3.8(6) 3.3(6) 2.2(5) 2.2(5) 3.0(6) 2.6(5) 2.9(6) 3.5(6) 3.1(6) 3.7(7) 2.8(6) 3.3(6) 3.0(6) 2.5(5) 2.8(6) 3.0(6) 2.4(5) 2.6(5) 2.9(6) 2.5(5) 3.8(7) 3.3(6) 4.0(7) 3.7(7) 3.2(6) 3.2(6) 2.4(5) 2.9(6) 2.6(5) 0 1996 American Chemical Society J. Am. Chem. Soc. VI 18 Page8623 Laplaza Supplemental Page 9Positional parameters and B(eq) for 0.044(7) 0.038 (7 1.960 (7) 1.964 (7) 1.977 (7) 1.970 (7) 1.968 (7) 1.956 (7) 1.42 (1) 1.50(1)1.42 (1) 1.48 (1) 1.44 (1) 1.49 ( 122.8(9)122 (1) 117 ( 120 (1) 120 (1) 120 (

Cleavage of the relatively inert dinitrogen (N(2)) molecule, with its extremely strong N identical withN triple bond, has represented a major challenge to the development of N(2) chemistry. This report describes the reductive cleavage of N(2) to two nitrido (N(3-)) ligands in its reaction with Mo(NRAr)(3), where R is C(CD(3))(2)CH(3) and Ar is 3,5-C(6)H(3)(CH(3))(2'), a synthetic three-coordinate molybdenum(III) complex of known structure. The formation of an intermediate complex was observed spectroscopically, and its conversion (with N identical withN bond cleavage) to the nitrido molybdenum(VI) product N identical withMo(NRAr)(3) followed first-order kinetics at 30 degrees C. It is proposed that the cleavage reaction proceeds by way of an intermediate complex in which N(2) bridges two molybdenum centers.

Of great si&icance in both biological and synthetic systems are two-electron processes in which a divalent atom such as oxygen is completely transferred between two reaction partners.' Until now, the endogenous three-electron atom transfer process has been limited to examples of intermetallic nitrogen atom transfer as exemplified by the reaction (TTP)Cr + N=Mn(TTP) -(TTP)CFN 4-Mn(TTP).2 In the present work we establish a three-electron redox process in which a nitrogen atom from nitrous oxide is transferred to a molybdenum(II1) coordination complex.The complex Mo(NRAr)3 (1; R = C(CD3)2CH3, Ar = 33-C6H3Me2) was prepared for this work since d3 1 could conceivably engage in three-electron redox processes. Of the various possibilities, N-atom transfer was a particularly attractive target since stable nitrido complexes of the kind N=MoX3 (X = alkyl,3 amide? or &oxide5) are known. In a typical preparation, M o C~~( T H F )~~ (4.164 "01) and Li(NRAr)(OEt2)7 (8.315 m o l ) were added to 70 mL of cold (-100 "C) ether, and the mixture was stirred for 2.5 h after warming to 28 "C. The precipitated LiCl and excess MoC13(THF)3 were removed by filtration. Analysis of the filtrate by 2H NMR spectroscopy showed only one major product, with a relatively sharp (Avll2 = 35 Hz) signal at 64.6 ppm corresponding to the 2H-labeled tert-butyl groups in paramagnetic M O ( N R A~)~.~ The filtrate was concentrated and cooled to -35 "C under an argon atmosphere to produce orange-red, crystalline Mo(NRAr)3 (mp 126-128 "C, yield 70%)? MO(NR&)3 (1) is extremely oxygen-and (1) Holm, R. H.; Donahue,

In two recent reports we showed that the three-coordinate niolybdenum(rr1) complex [Mo(NRAr),] (1. R = C(CD,),CH,, Ar = 3,5-C,H3Me,) is an avid nitrogen-atom acceptor; the reactions of 1 with nitrous oxide ['] and dinitrogenC2l both occur with N -N bond cleavage to give the molybdenum(v1) complex [Mo(N)(NRAr),] having a terminal nitrido ligand. To determine whether the tendency of 1 to abstract a trivalent atom would extend to a heavier homologue, we investigated the reaction of [Mo(NRAr),] ( I ) with white phosphorus (P,). Accordingly, we present herein the synthesis and structure of the monomeric phosphido (P3-) complex [Mo(P)(NRAr),] (2), which contains a molybdenum-phosphorus triple bond. Doubly and triply bridging bonding modes for P, ligands in bimetallic or trimetallic systems, respectively, are well-do~umented,[~~ but terminal PI ligation has not been established previously.

No abstract

Four helix-loop-helix 63mer peptides were designed and synthesized in order to assess the utility of peptides as scaffolds for the stabilization of complex metal sites in proteins. Bridged assembly [Ni(II)-(mu(2)-S.Cys)-Fe(4)S(4)], consistent with spectroscopic information on the A-cluster of carbon monoxide dehydrogenase, was chosen as the target assembly. The peptides consist of two helices with approximately 20 residues connected by a flexible loop containing the ferredoxin consensus sequence Cys-Ile-Ala-Cys-Gly-Ala-Cys to bind the Fe(4)S(4) cluster. A fourth cysteine was positioned to serve as the bridging ligand between the cluster and Ni(II). Three other binding residues were incorporated in appropriate positions to constitute a binding site for Ni(II). One of the peptides was designed with an N(3)S (His(3)Cys) site, and each of the other three with N(2)S(2) (His(2)Cys(2)) sites. A detailed account of the synthesis and characterization of the peptides and their metalloderivatives is presented. The four peptides were synthesized using an Fmoc/t-Bu-based solid-phase strategy, purified by reversed-phase HPLC, and characterized by ES-MS. On the basis of size-exclusion chromatography and circular dichroism spectropolarimetry, these peptides appear to dimerize in solution to form four-helix bundles of high helical contents. Reactions of the peptides with preformed cluster [Fe(4)S(4)(SCH(2)CH(2)OH)(4)](2)(-) and subsequent purification by column chromatography yield a product consistent with the incorporation of one [Fe(4)S(4)](2+) cluster per 63mer, as judged from absorption and Mössbauer spectra. Addition of a Ni(II) salt to the [Fe(4)S(4)]-peptides results in an apparent equilibrium between free Ni(II) and a peptide-bound nickel form, as established by column chromatography studies. Nickel EXAFS data (Musgrave, K. B.; Laplaza, C. E.; Holm, R. H.; Hedman, B.; Hodgson, K. O. Results to be published.) provide strong evidence that the peptide-bound nickel binds in the desired site in two of the metallopeptides. This work represents the first exploration of peptides as scaffolds for the support of biologically relevant bridged assemblies containing iron-sulfur clusters.

Aus dem Molybdänkomplex 1 und weißem Phosphor bildet sich in einer glatten Reaktion bei 28 °C der Komplex 2 mit terminalem Phosphido(P3‐)‐Liganden, der nach einer Einkristall‐Röntgenstrukturanalyse im Festkörper monomer vorliegt. Die Mo‐P‐Dreifachbindungslänge beträgt 2.119(4) Å. Reaktion von 2 mit Mesitylazid ergibt einen Komplex mit terminalem Iminophosphenium(PNR+)‐Liganden; mit Schwefel bildet sich ein Phosphormonosulfid(PS)‐Komplex. R  C(CD3)2CH3, Ar  3,5‐C6H3Me2.

In earlier work, de novo designed peptides with a helix-loop-helix motif and 63 residues have been synthesized as potential scaffolds for stabilization of the [Ni(II)-X-Fe(4)S(4)] bridged assembly that is the spectroscopically deduced structure of the A-Cluster in clostridial carbon monoxide dehydrogenase. The 63mers contain a consensus tricysteinyl ferredoxin domain in the loop for binding an Fe(4)S(4) cluster and Cys and His residues proximate to the loop for binding Ni(II), with one Cys residue designed as the bridge X. The metallopeptides HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M, containing three His and one Cys residue for Ni(II) coordination and two His and two Cys residues for binding M = Ni(II) and Co(II), have been examined by Fe-, Ni-, and Co-K edge spectroscopy and EXAFS. All peptides bind an [Fe(4)S(4)](2+) cubane-type cluster. Interpretation of the Ni and Co data is complicated by the presence of a minority population of six-coordinate species with low Z ligands, designated for simplicity as [M(OH(2))(6)](2+). Best fits of the data were obtained with ca. 20% [M(OH(2))(6)](2+) and ca. 80% M(II) with mixed N/S coordination. The collective XAS results for HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M demonstrate the presence of an Fe(4)S(4) cluster and support the existence of the distorted square-planar coordination units [Ni(II)(S.Cys)(N.His)(3)] and [Ni(II)(S.Cys)(2)(N.His)(2)] in the HC(4)H(2) and HC(5)H metallopeptides, respectively. In the HC(5)H metallopeptide, tetrahedral [Co(II)(S.Cys)(2)(N.His)(2)] is present. We conclude that the designed scaffolded binding sites, including Ni-(mu(2)-S.Cys)-Fe bridges, have been achieved. This is the first XAS study of a de novo designed metallopeptide intended to stabilize a bridged biological assembly, and one of a few XAS analyses of metal derivatives of designed peptides. The scaffolding concept should be extendable to other bridged metal assemblies.