Dieter Sellmann

Transition metal complexes with sulfur ligands: Part CXLIV. Square planar nickel complexes with NiS4 cores in three different oxidation states: synthesis, X-ray structural and spectroscopic studies

Abstract The reaction of ‘buS2’2−=3,5-ditertiarybutyl-1,2-benzenedithiolate(2−) with Ni(ac)2·4H2O and subsequently AsPh4Cl, yielded (AsPh4)2[Ni(‘buS2’)2] (1). Complex 1 is extremely air sensitive and oxidized rapidly to give (AsPh4)[Ni(‘buS2’)2] (2). The anion of 2 could be further oxidized by iodine to give neutral [Ni(‘buS2’)2] (3). Complexes 1, 2 and 3 contain nickel in the formal oxidation states +2, +3 and +4, and could be completely characterized by X-ray structure determination and spectroscopic methods. The results indicate that the oxidation of 1→2→3 concerns electrons residing in molecular orbitals having [NiS4] character. No evidence could be obtained for an involvement of the benzene rings or an electron delocalization beyond the [NiS4] core. This conclusion corresponds with the reactivity of 3 versus 2 and 1 towards protons and CO. While 3 is inert to the attack of H+ and CO, 2 and 1 exhibit rapid reactions to give ‘buS2’H2 and either nickel–chloro complexes or Ni(CO)4.

[Ru(HNO)(′pybuS4′)], the First HNO Complex Resulting from Hydride Addition to a NO Complex (′pybuS4′2−=2,6-Bis(2-mercapto- 3,5-di-tert-butylphenylthio)dimethylpyridine(2−))

Treatment of the nitrosyl complex [Ru(NO)(′pybuS4′)]Br (1 a) with NaBH4 in CH3OH yielded [Ru(HNO)(′pybuS4′)](2), which could be completely characterized. The X-ray structure determination of 2 confirmed the N coordination of the HNO ligand. Density functional theory calculations enabled us to assign the ν(NO) IR band of 2, which appears in KBr at 1358 cm−1 and in THF at 1378 cm−1. The unprecedented hydride addition to nitrosyl complexes yielding HNO complexes fills a white spot on the map of chemical reactions, represents the as yet unknown counterpart to the well-established formyl complex formation from CO complexes and hydrides, and distinctly differs from the formation reaction of [Os(HNO)(CO)Cl2(PPh3)2], the only other HNO complex characterized structurally. The HNO complex 2 is oxidized stepwise by [Cp2Fe]PF6 in the presence of NEt3 and directly by Bronsted acids to give [Ru(NO)(′pybuS4′)]+ in 2e− oxidations. H+/D+ exchange indicates acidity of the HNO proton.

On the function of nitrogenase FeMo cofactors and competitive catalysts: chemical principles, structural blue-prints, and the relevance of iron sulfur complexes for N2 fixation

Abstract This article tries to rationalize why low molecular weight complexes have not yet been able to copy nitrogenase catalyzed reactions or to act as competitive catalysts with nitrogenase-like activity. An answer is sought in that such complexes must rather fulfil the principles governing FeMoco function than duplicate its structure. Such principles, e.g. metal sulfur bonds, reversible M–S bond dissociation, Bronsted basicity, vacant sites, redox activity, are illustrated with metal complexes of multidentate thioether thiolate ligands. A structure–function relationship of metal sulfur [MS] centers is described, revealing that [MS] centers can stay structurally invariable in spite of considerable electronic changes. Complexes with [MS] centers further accomplish strong coupling of H + /e − fluxes, the heterolytic activation of H 2 , and the stabilization of the N 2 reduction key intermediate diazene N 2 H 2 . Low-spin states of Fe(II) centers can be enforced by sterical constraints. These coordination chemistry results, combined with X-ray structural and biochemistry findings, form the basis of a model for the FeMoco function. It proposes the breakage of one Fe–S–Fe bridge of FeMoco and the formation of two unique five-coordinate low-spin Fe(II) centers when the enzyme passes from the resting into the turnover state.

[Ru(N2)(PiPr3)(`N2Me2S2')]: Coordination of Molecular N2 to Metal Thiolate Cores under Mild Conditions

N2 coordination to the nitrogenase Fe7 MoS9 cofactor is considered a key step of biological N2 fixation, however, modeling this step with metal-sulfur complex fragments under mild conditions has remained a long-standing challenge. The title complex 2 represents a first example, forming from N2 and the precursor CH3 CN complex 1 under standard conditions [Eq. (1)].

Übergangsmetallkomplexe mit Schwefelliganden CXXIV. Koordination von C- und S-Liganden an Metall—Carben—Thiolat-Fragmente [M(‘S2C’)] [M = NiII, PdII, PtII; ‘S2C’2− = 1,3-Imidazolidinyl-N,N′-bis(2-benzolthiolat)(2 −)]. [M(L)(‘S2C’)]-Komplexe mit L = CN−, CH3−, COCH3−, CNBu, CR2, SH− und SR− (R = Et, Ph, o-C6H4NH2)

Abstract A series of [M(L)(‘S 2 C’)] complexes (M = Ni II , Pd II , Pt II ) containing the carbene dithiolate ligand ‘S 2 C’ 2− = 1,3-imidazolidinyl- N,N ′-bis(2-benzenethiolate)(2 −) and various C and S co-ligands have been synthesized. When treated with KCN, n -butylisonitrile, the electron-rich olefin R 2 C=CR 2 [=bis(1,3-diphenylimidazolidine-2-ylidene)], LiMe, thiolates such as SEt − , SPh − or o -SC 6 H 4 NH 2 , and hydrogen sulfide, the parent complex [Ni(‘S 2 C’)] 2 · DMF ( 1 ) yields the corresponding anionic or neutral [Ni(L)(‘S 2 C’)] complexes which were isolated as [K 2 (Kryptofix5)(THF) 2 (μ-OH 2 )][Ni(CN)(‘S 2 C’)] ( 2 ), [Ni(CNBu)(‘S 2 C’)] ( 3 ), [Ni(CR 2 )(‘S 2 C’)] ( 4 ), [Li(12-crown-4) 2 ][Ni(CH 3 )(‘S 2 C’)] ( 5 ), [NBu 4 ][Ni(SEt)(‘S 2 C’)] · THF ( 6 ), [Na(15-crown-5)][Ni(SPh)(‘S 2 C’)] ( 7 ), [Na(15-crown-5)][Ni( o -SC 6 H 4 NH 2 )(‘S 2 C’)] · 0.5THF ( 8 ), and [Na(15-crown-5)][Ni(SH)(‘S 2 C’)] ( 9 ). Analogous complexes of 9 have also been obtained with Pd ( 10 ) and Pt ( 11 ). The complexes were characterized by elemental analyses and the usual spectroscopic methods. X-ray structure determinations of 2 , 3 , 4 , 6 , 9 and 10 revealed that the [M(‘S 2 C’)] fragments are stereochemically very rigid, being little influenced by the different co-ligands L. The four-coordinate metal centers exhibit an approximately square-planar coordination geometry. The ‘S 2 C’ 2− ligands show a characteristic propeller-like twist resulting from positioning the C 6 rings of the ‘S 2 C’ unit above and below the coordination plane. As evidenced by the molecular structures of 9 vs. 10 , this twist can vary, and the ‘S 2 C’ 2− ligand is flexible enough to accommodate also larger metal ions than Ni II . Complexes 9–11 belong to the rare cases of isolable SH complexes. The complexes 2–11 exhibit a remarkable thermal stability ( 4 is stable up to 220°C), and the [M(‘S 2 C’)] fragments so far proved inert towards decomposition. When treated with Bronsted acids, the general reactivity feature of all anionic [M(L)(‘S 2 C’)] complexes is release of HL and regeneration of the parent complexes [M(‘S 2 C’)] 2 . The methyl complex [Li(4-crown-4) 2 ][Ni(CH 3 )(‘S 2 C’)] ( 5 ) is one of the rare examples in which a methyl ligand binds to an [NiS] center. While the parent complex [Ni(‘S 2 C’)] 2 · DMF ( 1 ) proved unreactive towards CO, 5 readily inserts CO yielding the highly reactive acetyl derivative [Ni(COCH 3 )(‘S 2 C’)] − . This complex could not be isolated, but its formation was established by spectroscopic methods and by its subsequent reaction with PhSH yielding, among other products, the thioester CH 3 COSPh. The model character of this reaction sequence for acetyl—CoA synthesis catalyzed by CO dehydrogenases is discussed.

Reaktionen an komplexgebundenen liganden : XXI. Eisen-komplexe mit distickstoff- und stickstoffwasserstoff-liganden; synthese und eigenschaften von [C5H5Fe(dppe)L]X (L = N2, N2H4, NH3, CO, aceton, THF, H−, Cl−; X = Cl−, PF6−)☆

Abstract ]The [C5H5Fe(CO)dppe]+ cation, [dppe = (C6H5)2PC2H4P(C6H5)2], is synthesized conveniently by a novel method. Adding a few drops of tetrahydrofuran to a solid mixture of C5H5Fe(CO)2Cl and dppe yields [C5H5Fe(CO)dppe]Cl in a spontaneous and quantitative reaction; reprecipitation with NH4PF6 leads to [C5H5Fe(CO)dppe]PF6, which on UV irradiation splits off the CO ligand in solution yielding the highly reactive complex ion [C5H5Fe(dppe)]+. [C5H5Fe(dppe)]+ coordinates a large variety of molecules, especially N2, N2H4 and NH3. UV irradiation of [C5H5Fe(CO)dppe]Cl in aceton yields via the isolable intermediate [C5H5Fe(acetone)dppe]Cl the complex [C5H5Fe(Cl)dppe]. Attempts to reduce the N2-complex [{C5H5Fe(dppe)}2N2](PF6)2 lead only with NaBH4 to a definite product, the hydride complex [C5H5Fe(H)dppe]. IR and 1H NMR data show that the σ—π ligand N2 influences the [C5H5Fe(dppe)]+ system in practically the same way as the σ-ligands NH3 and N2H4 do; they show further that in the N2 complex the electron density at the iron center is higher than in the CO complex.

Elementary reactions, structure-function relationships, and the potential relevance of low molecular weight metal-sulfur ligand complexes to biological N2 fixation

Abstract This article tries to rationalize the shortcomings of various model compounds and discusses requirements that a low-molecular compound must fulfill in order to become a potentially competitive catalyst for nitrogenases. For fundamental reasons, such a synthetic catalyst cannot be a precise structural duplicate of the active centers of nitrogenase. Results obtained with iron-sulfur carbonyl and diazene complexes further indicate that (1) the coupling and chronology of proton and electron transfer steps, (2) invariance of iron-sulfur distances within a wide range of electron density changes at the iron centers, and (3) Brönsted basic thiolate donors favoring the protonation of metal-sulfur cores and the formation of N–H···S bridges may be essential in order to reduce N2 via N2H2 and N2H4 to NH3 under mild conditions.

The nitrogenase catalyzed N2 dependent HD formation: a model reaction and its significance for the FeMoco function

Abstract The first completely characterized model for the nitrogenase catalyzed ‘N 2 dependent HD formation’ from D 2 and protons is described. This key reaction of nitrogenases is most plausibly rationalized by the ‘open-side’ FeMoco model, which enables us to explain the severe constraints imposed on the N 2 dependent HD formation as well as the noncompetitive inhibition of N 2 fixation by CO.

Übergangsmetallkomplexe mit schwefelliganden: X. Substitutions- und redox-reaktionen von molybdän(0)-carbonyl-komplexen mit den mehrzähnigen thioetherthiol-liganden 1,2-methylthiobenzolthiol, 1,4,7-trithiaheptan, 23,8,9-dibenzo-1,4,7,10-tetrathiadecan sowie 1,2-benzoldithiol☆

Abstract During a study of the reactions of molybdenum(0) carbonyl complexes with multidentate organosulfur ligands, (NMe 4 ) 2 [Mo(CO) 3 (SC 2 H 4 SC 2 H 4 S)] was obtained from [Mo(CO) 3 (CH 3 CN) 3 ] and (NMe 4 ) 2 (SC 2 H 4 SC 2 H 4 S); subsequent alkylation with 1,2-dibromoethane gave [Mo(CO) 3 TTCN] (TTCN = 1,4,7-trithiacyclonane). TTCN, which up to now had been obtained only with difficulty, is liberated from [Mo(CO) 3 TTCN] by (NMe 4 ) 2 (SC) 2 H 4 SC 2 H 4 S) under regeneration of the starting complex (NMe 4 ) 2 [Mo(CO) 3 (SC 2 H 4 SC 2 H 4 S)]. TTCN and [MoCl 3 (THF) 3 ] yield [MoCl 3 (TTCN)]. [Mo(CO) 4 (norbornadiene)] and (NMe 4 ) 2 (dttd), (dttd 2 = 2,3,8,9-dibenzo-1,4,7,10-tetrathiadecane(−2)), give (NMe 4 ) 2 [μ-dttd{Mo(CO) 4 } 2 ], which reacts to form a Mo(CO) 3 species under heating; further CO substitution is not observed. Treatment of [Mo(CO) 2 (PPh 3 ) 2 (CH 3 CN) 2 ] with C 6 H 4 (SH) 2 and CH 3 SC 6 H 4 SH respectively, leads via redox reactions to the molybdenum(II) complexes [Mo(CO) 2 (PPh 3 ) 2 (C 6 H 4 S 2 )] and [Mo(CO) 2 (PPh 3 )(CH 3 SC 6 H 4 S) 2 ]. Under normal conditions, these complexes show no substitution of PPh 3 by C 6 H 4 S 2 2− or CH 3 SC 6 H 4 S − ; however, under thermally more vigorous conditions in refluxing methoxyethanol, [Mo(CO) 2 (PPh 3 ) 2 (C 6 H 4 S 2 )] and Na 2 (C 6 H 4 S 2 ) yield [Mo IV (C 6 H 4 S 2 ) 3 ] 2− in a redox reaction; the anion is isolated as (NMe 4 ) 2 [Mo(C 6 H 4 S 2 ) 3 ]. [Mo(CO) 3 (CH 3 CN) 3 ] and CH 3 SC 6 H 4 SH yield, at room temperature, the binuclear [Mo 2 (CO) 6 (CH 3 SC 6 H 4 S) 2 ]; with Na/THF, it leads to [Mo(CO) 4 (CH 3 SC 6 H 4 S)] − , with PPh 3 to [Mo(CO) 2 PPh 3 (CH 3 SC 6 H 4 S) 2 ], and with CH 3 SC 6 H 4 S − to [Mo(CO) 4 (CH 3 SC 6 H 4 S)] − as well as to [Mo(CO)(CH 3 SC 6 H 4 S) 3 ] − , which are isolated as NMe 4 + salts. In (NMe 4 )[Mo(CO)(CH 3 SC 6 H 4 S) 3 ], apart from one CO, only sulfur is coordinated to the Mo center; its CO ligand shows an extremely low IR absorption at 1775 cm −1 indicating high electron density at the Mo center. Photochemical substitution of CO by N 2 was not possible either with this CO or with the other CO complexes.

Übergangsmetallkomplexe mit schwefelliganden: XIV. (NEt4)2[Cr(CO)3(S2C6H4)], bis-tetraethylammoniumtricarbonyl-1,2-benzendithiolatochromat(0), ein 16e-konfigurierter chrom(0)-komplex mit fünffach koordiniertem chrom; röntgenstrukturanalyse und reaktionen

Abstract According to the X-ray structure analysis (NEt 4 ) 2 [Cr(CO) 3 S 2 C 6 H 4 ] contains mononuclear anions, in which the chromium is coordinated by three CO ligands and two sulfur atoms of the 1,2-benzenedithiolate ligand forming a distorted trigonal bipyramid; thus [Cr(CO) 3 S 2 C 6 H 4 ] 2− represents a five-coordinate chromium(0) complex with formal 16 e configuration. Due to the coordinatively as well as electronically unsaturated chromium center [Cr(CO) 3 S 2 C 6 H 4 ] 2− is highly reactive; the 18 e complexes [Cr(CO) 4 S 2 C 6 H 4 ] 2− and [Cr(NO) 2 (S 2 C 6 H 4 ) 2 ] 2− are obtained on reactionwith CO and NO + or NO, respectively. The C 6 H 4 S 2 ligands in the latter complex can be alkylated by 1,2-C 2 H 4 Br 2 yielding [Cr(NO) 2 dttd] (dttd 2 = 2,3,8,9-dibenzo-1,4,7,10-tetrathiadecan(−2)); reaction of [Cr(NO) 2 (MeCN) 4 (PF 6 ) 2 with Na(CH 3 SC 6 H 4 S) gives the binuclear complex [Cr(NO)(CH 3 SC 6 H 4 S) 2 ] 2 .