Heterobimetallic, Cubane-like Mo₃S₄M‘ Cluster Cores Containing the Noble Metals M‘ = Ru, Os, Rh, Ir. Unprecedented Tri(μ-carbonyl) Bridge Between Ruthenium Atoms in [{(η⁵-Cp‘)₃Mo₃S₄Ru}₂(μ-CO)₃]²⁺

Reaction of the methylcyclopentadienyl (Cp‘) cluster compound [(η5-Cp‘)3Mo3S4][pts] (pts = p-toluenesulfonate) with noble metal alkene complexes resulted in the formation of four new heterobimetallic cubane-like Mo3S4M‘ cluster cores (M‘ = Ru, Os, Rh, Ir). Thus, reaction with [(1,5-cod)Ru(CO)3] or [(1,3-cod)Os(CO)3] (cod = cyclooctadiene) afforded [(η5-Cp‘)3Mo3S4M‘(CO)2][pts] (M‘ = Ru:  [1][pts]; M‘ = Os:  [2][pts]). When [1][pts] was kept in CH2Cl2/pentane solution, partial loss of carbonyl ligands occurred and the carbonyl-bridged dicubane cluster [{(η5-Cp‘)3Mo3S4Ru}2(μ-CO)3][pts]2 was isolated. An X-ray crystal structure revealed the presence of the hitherto unobserved Ru(μ-CO)3Ru structural element. The formation of cluster compounds containing Mo3S4Rh and Mo3S4Ir cores was achieved in boiling methanol by reacting [(η5-Cp‘)3Mo3S4][pts] with [M‘Cl(cyclooctene)2]2 (M‘ = Rh, Ir) in the presence of PPh3. In this way [(η5-Cp‘)3Mo3S4M‘Cl(PPh3)][pts] (M‘ = Rh, Ir) could be isolated. An alternative route to the Mo3S4Rh cluster core was found in the reaction of [(η5-Cp‘)3Mo3S4][pts] with [RhCl(1,5-cod)]2, which yielded [(η5-Cp‘)3Mo3S4Rh(cod)][pts]2 ([7][pts]2). Substitution of the cod ligand in [7][pts]2 by 1,3-bis(diphenylphosphanyl)propane (dppp) gave [(η5-Cp‘)3Mo3S4Rh(dppp)][pts]2

Molecular Metal Sulfide Cluster Model for Substrate Binding to Oil-Refinery Hydrodesulfurization Catalysts

Reaction between [(η5-Cp‘)3Mo3S4]+ and [Ni(1,5-cod)2] (Cp‘ = methylcyclopentadienyl; 1,5-cod = 1,5-cyclooctadiene) in THF at ambient temperature yielded a coordinatively unsaturated cubane-like cluster cation, [(η5-Cp‘)3Mo3S4Ni]+. The ligand sphere at the Ni atom could be saturated by coordinating dimethyl sulfide, diethyl sulfide, di(tert-butyl) sulfide, tetrahydrothiophene, thiochroman-4-ol, 1,4-dithiane, pyridine, quinoline, or 4,4‘-bipyridine. The products structurally model a mode of substrate coordination on proposed binding sites of heterogeneous MoNi sulfide hydrotreating catalysts. No stable coordination compounds could be isolated for thiophene derivatives. X-ray crystal structures are reported for the ligand-bridged dicluster compounds [{(η5-Cp‘)3Mo3S4Ni}2(μ-C4H4S2)][pts]2 (C4H8S2 = 1,4-dithiane) and [{(η5-Cp‘)3Mo3S4Ni}2(μ-bipy)][pts]2 (bipy = 4,4‘-bipyridine)

Oxazolidin-2-one-Containing Pseudopeptides That Fold into β-Bend Ribbon Spirals

Three sets of oligomers containing the 4-carboxy-5-methyloxazolidin-2-one (Oxd) moiety have been synthesized with the aim of checking whether these molecules are able to fold in ordered structures:  A set [Boc-(l-Ala-l-Oxd)n-OR], B set [Boc-(l-Ala-d-Oxd)n-OR], and C set [Boc-(Aib-l-Oxd)n-OR] preferential conformations have been analyzed with IR absorption, NMR, and CD. We have noticed that in these oligomers three stabilizing effects are active:  (i) the rigid Oxd −CO−N(CH<)−CO− moiety, which always tend to assume a trans conformation; (ii) the formation of Oxd CO···HαC intramolecolar H-bonds; (iii) the alternate formation of 1 ← 4 intramolecular CO···HN H-bonds. Through the analysis of the experimental data, we could demonstrate that only the oligomers of the B set are able to meet all three requirements listed above. By a deeper insight into the CD spectra, we gathered that the secondary structure adopted by the B set oligomers is a β-bend ribbon spiral, which is a subtype of the 310-helix

Geometry, Energy, and Vibrational Frequencies of the Bis(dicyanomethylene)squarilium Dianion^†

Similarly to its 1,2-isomer, the bis(dicianomethylene)squaraine (BCSQ) atom connectivity can be a dianion, a radical anion, and a neutral, electron-poor species. This makes it attractive as a constituent unit of new materials. In the present paper we investigate the stable dianion, using theoretical calculation results to connect the geometry determined by X-ray diffraction of the single crystal of Na2BCSQ·4H2O and the vibrational frequencies determined by solution- and solid-state infrared and Raman spectroscopy of various neutral salts

Molecular Metal Sulfide Cluster Model for Substrate Binding to Oil-Refinery Hydrodesulfurization Catalysts

Reaction between [(η5-Cp‘)3Mo3S4]+ and [Ni(1,5-cod)2] (Cp‘ = methylcyclopentadienyl; 1,5-cod = 1,5-cyclooctadiene) in THF at ambient temperature yielded a coordinatively unsaturated cubane-like cluster cation, [(η5-Cp‘)3Mo3S4Ni]+. The ligand sphere at the Ni atom could be saturated by coordinating dimethyl sulfide, diethyl sulfide, di(tert-butyl) sulfide, tetrahydrothiophene, thiochroman-4-ol, 1,4-dithiane, pyridine, quinoline, or 4,4‘-bipyridine. The products structurally model a mode of substrate coordination on proposed binding sites of heterogeneous MoNi sulfide hydrotreating catalysts. No stable coordination compounds could be isolated for thiophene derivatives. X-ray crystal structures are reported for the ligand-bridged dicluster compounds [{(η5-Cp‘)3Mo3S4Ni}2(μ-C4H4S2)][pts]2 (C4H8S2 = 1,4-dithiane) and [{(η5-Cp‘)3Mo3S4Ni}2(μ-bipy)][pts]2 (bipy = 4,4‘-bipyridine)

Oxazolidin-2-one-Containing Pseudopeptides That Fold into β-Bend Ribbon Spirals

Three sets of oligomers containing the 4-carboxy-5-methyloxazolidin-2-one (Oxd) moiety have been synthesized with the aim of checking whether these molecules are able to fold in ordered structures:  A set [Boc-(l-Ala-l-Oxd)n-OR], B set [Boc-(l-Ala-d-Oxd)n-OR], and C set [Boc-(Aib-l-Oxd)n-OR] preferential conformations have been analyzed with IR absorption, NMR, and CD. We have noticed that in these oligomers three stabilizing effects are active:  (i) the rigid Oxd −CO−N(CH<)−CO− moiety, which always tend to assume a trans conformation; (ii) the formation of Oxd CO···HαC intramolecolar H-bonds; (iii) the alternate formation of 1 ← 4 intramolecular CO···HN H-bonds. Through the analysis of the experimental data, we could demonstrate that only the oligomers of the B set are able to meet all three requirements listed above. By a deeper insight into the CD spectra, we gathered that the secondary structure adopted by the B set oligomers is a β-bend ribbon spiral, which is a subtype of the 310-helix

Asymmetric Synthesis of 8-Aminoindolizidine from Chiral 2-Pyrroleimines

1-Allyl-2-pyrroleimines obtained from (S)-valinol and (S)-phenylglycinol underwent highly diastereoselective addition of allylmagnesium chloride, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to (Z)-1-propenyl isomerization of the 1-pyrrole substituent. Transformation of the 2-amino alcohol moiety to an oxazolidinone, or its cleavage and subsequent N-protection, followed by ring-closing metathesis of the two alkene groups gave the unsaturated bicyclic compound. Full hydrogenation of the alkene function and the aromatic rings afforded the indolizidine derivative as a mixture of two or three diastereomers with a ratio which was dependent on the nature of both the N-substituent and the catalyst. The two prevalent diastereomers were isolated, and their configuration was determined by X-ray crystallographic analysis

Asymmetric Synthesis of 8-Aminoindolizidine from Chiral 2-Pyrroleimines

1-Allyl-2-pyrroleimines obtained from (S)-valinol and (S)-phenylglycinol underwent highly diastereoselective addition of allylmagnesium chloride, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to (Z)-1-propenyl isomerization of the 1-pyrrole substituent. Transformation of the 2-amino alcohol moiety to an oxazolidinone, or its cleavage and subsequent N-protection, followed by ring-closing metathesis of the two alkene groups gave the unsaturated bicyclic compound. Full hydrogenation of the alkene function and the aromatic rings afforded the indolizidine derivative as a mixture of two or three diastereomers with a ratio which was dependent on the nature of both the N-substituent and the catalyst. The two prevalent diastereomers were isolated, and their configuration was determined by X-ray crystallographic analysis

Asymmetric Synthesis of 8-Aminoindolizidine from Chiral 2-Pyrroleimines

1-Allyl-2-pyrroleimines obtained from (S)-valinol and (S)-phenylglycinol underwent highly diastereoselective addition of allylmagnesium chloride, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to (Z)-1-propenyl isomerization of the 1-pyrrole substituent. Transformation of the 2-amino alcohol moiety to an oxazolidinone, or its cleavage and subsequent N-protection, followed by ring-closing metathesis of the two alkene groups gave the unsaturated bicyclic compound. Full hydrogenation of the alkene function and the aromatic rings afforded the indolizidine derivative as a mixture of two or three diastereomers with a ratio which was dependent on the nature of both the N-substituent and the catalyst. The two prevalent diastereomers were isolated, and their configuration was determined by X-ray crystallographic analysis

Asymmetric Synthesis of 8-Aminoindolizidine from Chiral 2-Pyrroleimines

1-Allyl-2-pyrroleimines obtained from (S)-valinol and (S)-phenylglycinol underwent highly diastereoselective addition of allylmagnesium chloride, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to (Z)-1-propenyl isomerization of the 1-pyrrole substituent. Transformation of the 2-amino alcohol moiety to an oxazolidinone, or its cleavage and subsequent N-protection, followed by ring-closing metathesis of the two alkene groups gave the unsaturated bicyclic compound. Full hydrogenation of the alkene function and the aromatic rings afforded the indolizidine derivative as a mixture of two or three diastereomers with a ratio which was dependent on the nature of both the N-substituent and the catalyst. The two prevalent diastereomers were isolated, and their configuration was determined by X-ray crystallographic analysis