Structural and spectroscopic properties of rhenium nitrosyl complexes

An oxidorhenium(v) complex with an electron-withdrawing ligand: benefits and drawbacks for a dual role catalyst

One very unique feature of oxidorhenium(v) complexes is their dual catalytic activity in both reduction of stable oxyanions like perchlorate ClO4 - and nitrate NO3 - as well as epoxidation of olefins. In our ongoing research efforts, we were interested to study how an electron-withdrawing ligand would affect both these catalytic reactions. Hence, we synthesized the novel bidentate dimethyloxazoline-dichlorophenol ligand HL1 and synthesized oxidorhenium(v) complex [ReOCl(L1)2] (1). Then, catalytic experiments were conducted showing that non-redox epoxidation activity is indeed enhanced, but redox catalysis via oxygen atom transfer (OAT) activity was reduced for ClO4 - and NO3 - reductions. From one nitrate reduction experiment, a small amount of the singly-oxidized dioxidorhenium(vi) complex [ReO2(L1)] (2) could be isolated, confirming the successful reduction sequence of nitrate to nitrite NO2 - (2e- reduction) to NO (1e- reduction). Furthermore, ligand L1 displayed a richer than usually observed coordination chemistry, allowing for the isolation of complexes [ReOCl2(SMe2)(L1)] (trans-3a), [ReOCl2(OPPh3)(L1)] (3b) and [ReCl3(OPPh3)(L1)] (3c). Complexes 1 and 3a-b were tested in cyclooctene epoxidation, 1 was additionally investigated as an oxyanion reduction catalyst of perchlorate and nitrate. All compounds HL1, 1, 2 and 3a-c could be characterized by single-crystal X-ray diffraction, besides other routine analyses.

Nitrosyl and Thionitrosyl Complexes of Technetium and Rhenium and Their Reactions with Hydrotris(pyrazolyl)borates

The very limited number of structurally known thionitrosyl complexes of technetium was increased by the synthesis of [TcII(NS)Cl3(PPh3)2] (3) and [TcII(NS)Cl3(PPh3)(OPPh3)] (4) and their reaction products with hydrotris(pyrazolyl)borates, {HB(pzR)3}-. Similar reactions were conducted with [TcI(NO)Cl2(PPh3)2(CH3CN)] and related rhenium thionitrosyls. Remarkably, most such reactions result in a rapid cleavage of the boron-nitrogen bonds of the ligands and the formation of pyrazole complexes of the two group 7 metals. Only one compound with an intact {HB(pzR)3}- ligand could be isolated: the technetium(I) complex [TcI(NO)Cl(PPh3){HB(pz)3}] (2). Other products show the coordination of one or four neutral pyrazole ligand(s) in the coordination spheres of technetium generated by thermal decomposition of the pyrazolylborates [TcI(NO)Cl2(PPh3)2(pzH)] (1) and [TcI(NS)Cl(pzHMe2)4]+ (5). Reactions with the corresponding thionitrosylrhenium complex [ReII(NS)Cl3(PPh3)2] require higher temperatures and only compounds with one pyrazole ligand, [ReI(NS)Cl2(PPh3)(pzHR)] (6a-6c), were isolated. The products were studied spectroscopically and by X-ray diffraction.

The reaction of rhenium nitrosyl with a sterically hindered NHC-carbene

In this article, we present the serendipitous synthesis of the unknown Re(I) complex [(OPPh3)Re(NO)2Cl3] (3) that we obtained reacting the Re(V) complex trans-[(PPh3)2ReOCl3] (1) with NO gas in presence of CH3COOH. We found that 3 reacts with 1,3-bis (2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes) to yield a stable oximate-Re(III) complex [(OPPh3)Re(NO)(ONIMes)Cl3] (4). We speculate that the IMes reacts with a bent NO, because the DFT calculations excluded the formation of both dimeric and η2-NO complexes in solution. The reactivity of the NO toward the carbene is probably due to an internal fluxional process in which the NO passes from linear to bent, triggered by the π-electrons given by the three chlorides to the Re through the mesomeric effect.

Impact of the coordination of multiple Lewis acid functions on the electronic structure and vn configuration of a metal center

The covalent bond classification (CBC) method represents a molecule as ML_lX_xZ_z by evaluating the total number of L, X and Z functions interacting with M. The CBC method is a simplistic approach that is based on the notion that the bonding of a ligating atom (or group of atoms) can be expressed in terms of the number of electrons it contributes to a 2-electron bond. In many cases, the bonding in a molecule of interest can be described in terms of a 2-center 2-electron bonding model and the ML_lX_xZ_z classification can be derived straightforwardly by considering each ligand independently. However, the bonding within a molecule cannot always be described satisfactorily by using a 2-center 2-electron model and, in such situations, the ML_lX_xZ_z classification requires a more detailed consideration than one in which each ligand is treated in an independent manner. The purpose of this article is to provide examples of how the ML_lX_xZ_z classification is obtained in the presence of multicenter bonding interactions. Specific emphasis is given to the treatment of multiple π-acceptor ligands and the impact on the vⁿ configuration, i.e. the number of formally nonbonding electrons on an element of interest.

Rhenium-Mediated Conversion of Dinitrogen and Nitric Oxide to Nitrous Oxide

Reductive splitting of N₂ is an attractive strategy towards nitrogen fixation beyond ammonia at ambient conditions. However, the resulting nitride complexes often suffer from thermodynamic overstabilization hampering functionalization. Furthermore, oxidative nitrogen atom transfer of N₂ derived nitrides remains unknown. We here report a Re^IV pincer platform that mediates N₂ splitting upon chemical reduction or electrolysis with unprecedented yield. The N₂ derived Re^V nitrides undergo facile nitrogen atom transfer to nitric oxide, giving nitrous oxide nearly quantitatively. Experimental and computational results indicate that outer‐sphere ReN/NO radical coupling is facilitated by the activation of the nitride via initial coordination of NO.

Crystal structure and magnetic study of the complex salt [RuCp(PTA)2-μ-CN-1κC:2κN-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5]

A new RuII-ReII complex salt, μ-cyanido-κ2 C:N-bis-[(η5-cyclo-penta-dien-yl)bis(3,5,7-tri-aza-phosphaadamantane-κP)ruthenium(II)] tetra-bromido-(ethanol/methanol-κO)nitrosylrhenate(II), [Ru(CN)(C5H5)2(C6H12N3P)4][ReBr4(NO)(CH4O)0.5(C2H6O)0.5] or [RuCp(PTA)2-μ-CN-1κC:2κ2 N-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5] (PTA = 3,5,7-tri-aza-phosphaadaman-tane) was obtained and characterized by single-crystal X-ray diffraction, elemental analysis and infrared spectroscopy. The title salt was obtained by liquid-liquid diffusion of methanol/DMSO solutions of (NBu4)[Re(NO)Br4(EtOH)] and [(PTA)2CpRu-μ-CN-1κC:2κ2 N-RuCp(PTA)2](CF3SO3). The RuII and ReII independent moieties correspond to a binuclear and mononuclear complex ion, respectively. A deep geometrical parameter analysis was performed, and no significant differences were found with earlier reports containing similar mol-ecules. The magnetic properties were investigated in the temperature range 2.0-300 K, and the complex behaves as a quasi-magnetically isolated spin doublet with weak anti-ferromagnetic inter-actions.

Understanding of [RuL(ONO)]n+ acting as nitric oxide precursor, a theoretical study of ruthenium complexes of 1,4,8,11-tetraazacyclo- tetradecane having different substituents: How spin multiplicity influences bond angle and bond lengths (Ru-O-NO) in releasing of NO

Generation of nitric oxide has been a great interest in cell biology as it involves a wide range of physiological functions including the blood pressure control; thus the exploitation of ruthenium chemistry has been motivated in biochemical and clinical points of view. Herein, the structural and electronic properties of ruthenium(II) complexes of 1,4,8,11-tetraazacyclotetradecane containing pyridyl, imidazole and benzimidazole (L¹, L², L³) were analyzed theoretically in the context of how spin multiplicity plays a crucial role influencing the NO release from the LRu-ONO moiety. The results show that β-cleavage of nitrito in the complex motivates the release of NO as it depends highly on total spin multiplicity of metal ion altering significantly the geometrical parameters; particularly, a decrease of bond length of Ru-ONO is highly associated with an increase of RuO-NO bond distance that correlates with the decrease of the Ru-O-NO bond angle ultimately leading to the release of NO; apparently, the bending nature of Ru-O-NO defines its release from the complex. This is consistent with orbital energy (d_x²-_y²) where the stabilization of axial Ru-O bond in the complex was observed, and proved by molecular orbital studies. In the excitation of the complex (singlet to triplet or singlet to quintet), the NO release has been facilitated, agreeing with the Gibbs free energy data where a lower energy for NO release was obtained compared to other types of excitations. In the calculated electronic spectra, a visible broad band with relatively high intensity for [RuL¹ONO]⁺ was observed, agreeing approximately with reported experimental results.

Synthesis and Catalytic Reactivity of Cobalt Pincer Nitrosyl Hydride Complexes

The synthesis, characterization, and catalytic activity of low-spin {CoNO}⁸ pincer complexes of the type [Co(PCP)(NO)(H)] are described. These compounds are obtained either by reacting [Co(PCP)(κ²-BH₄)] with NO and Et₃N or, alternatively, by reacting [Co(PCP)(NO)]⁺ with boranes, such as NH₃·BH₃ in solution. The five-coordinate, diamagnetic Co(III) complex [Co(PCP^NMe-iPr)(NO)(H)] was found to be the active species in the hydroboration of alkenes with anti-Markovnikov selectivity. A range of aromatic and aliphatic alkenes were efficiently converted with pinacolborane (HBpin) under mild conditions in good to excellent yield. Mechanistic insight into the catalytic reaction is provided by means of isotope labeling, NMR spectroscopy, and APCI/ESI-MS as well as DFT calculations.

Isomers in chlorido and alkoxido-substituted oxidorhenium(v) complexes: effects on catalytic epoxidation activity

The syntheses and characterizations of oxidorhenium(v) complexes trans-dichlorido [ReOCl2(PPh3)(L1a)] (trans-2a), cis-dichlorido [ReOCl2(PPh3)(L1b)] (cis-2b) and ethoxido-complex [ReO(OEt)(L1b)2] (4b), ligated with the dimethyloxazoline-phenol ligands HL1a and HL1b are described. The bidentate ligand HL1a (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol) is unsubstituted on the phenol ring; ligand HL1b (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol) contains a nitro group in para-position to the hydroxy group. In the reaction of precursor complex [ReOCl3(PPh3)2] and HL1a the two stereoisomers cis/trans-2a, with respect to chlorido ligands, are formed. The solid state structures of both isomers cis- and trans-2a were determined by single crystal X-ray diffraction analysis. In contrast, with ligand HL1b, only the cis-isomer cis-2b was obtained. Ethoxido-complex 4b is exclusively obtained when precursor [ReOCl3(OPPh3)(SMe2)] is reacted with 2 equiv. of HL1b in ethanol in the presence of the base 2,6-dimethylpyridine (lutidine). If no lutidine is added, chlorido-complex [ReOCl(L1b)2] (3b) is obtained. Complexes [ReOCl2(PPh3)(L1a)] (cis/trans-2a), [ReOCl2(PPh3)(L1b)] (cis-2b), [ReO(OMe)(L1a)2] (4a) and [ReO(OEt)(L1b)2] (4b) were tested as homogeneous catalysts in the benchmark reaction of cyclooctene epoxidation. The influence of isomerism and effects of ligand substitutions on catalytic activity was investigated. Based on the time-conversion plots it can be concluded that cis/trans-isomerism does not influence catalytic activity, but electron-withdrawing substituents, as in cis-2b, 3b and 4b, show a beneficial effect.

Synthesis and properties of anionic ruthenium thionitrosyl and selenonitrosyl complexes that contain tetraanionic 2-hydroxybenzamidobenzene ligands

Although transition-metal complexes that contain thiocarbonyl (CS) and selenocarbonyl (CSe) ligands have been well studied, only three neutral or cationic selenonitrosyl (NSe) complexes have been reported, while anionic NSe complexes remain elusive. Herein, we report the first examples of anionic NSe-ligated ruthenium complexes, which were obtained from the reaction of anionic ruthenium nitrido complexes, elemental selenium, and 4-(N,N-dimethylamino)pyridine (DMAP). The structures of one of these ruthenium NSe complexes, as well as of the corresponding thionitrosyl (NS) and nitrosyl (NO) complexes, were systematically examined by X-ray diffraction analyses and theoretical calculations. In contrast to previous reportes, the NSe ligand in these complexes is a better π-acceptor than the NO and NS ligands and exhibits a stronger trans influence.

The crystal structure and magnetic properties of 3-pyridinecarboxylate-bridged Re(II)M(II) complexes (M = Cu, Ni, Co and Mn)

The novel Re(II) complex NBu4[Re(NO)Br4(Hnic)] (1) and the heterodinuclear compounds [Re(NO)Br4(μ-nic)Ni(dmphen)2]·½CH3CN (2), [Re(NO)Br4(μ-nic)Co(dmphen)2]·½MeOH (3), [Re(NO)Br4(μ-nic)Mn(dmphen)(H2O)2]·dmphen (4), [Re(NO)Br4(μ-nic)Cu(bipy)2] (5) [Re(NO)Br4(μ-nic)Cu(dmphen)2] (5') (NBu4(+) = tetra-n-butylammonium cation, Hnic = 3-pyridinecarboxylic acid, dmphen = 2,9-dimethyl-1,10-phenanthroline, bipy = 2,2'-bipyridine) have been prepared and the structures of 1-5 determined using single crystal X-ray diffraction. The structure of 1 consists of [Re(NO)Br4(Hnic)](-) anions and NBu4(+) cations. Each Re(II) is six-coordinate with four bromide ligands, a linear nitrosyl group and a nitrogen atom from the Hnic molecule, in a distorted octahedral surrounding. The structures of 2-5 are made up of discrete heterodinuclear Re(II)M(II) units where the fully deprotonated [Re(NO)Br4(nic)](2-) entity acts as a didentate ligand through the carboxylate group towards the [Ni(dmphen)2](2+) (2), [Co(dmphen)2](2+) (3), [Mn(dmphen)(H2O)2](2+) (4) and [Cu(bipy)2](2+) (5) fragments, the Re-M separation across the nic bridge being 7.8736(8) (2), 7.9632(10) (3), 7.7600(6) (4) and 8.2148(7) Å (5). The environment of the Re(II) ion in 2-5 is the same as 1 that in and all M(II) are six-coordinate in highly distorted octahedral surroundings, the main source of the distortion being due to the reduced bite of the chelating carboxylate. The magnetic properties of 1-5' were investigated in the temperature range 1.9-300 K. 1 behaves as a quasi-magnetically isolated spin doublet with very weak antiferromagnetic interactions through space Br···Br contacts. Its magnetic susceptibility data were successfully modeled through a deep analysis of the influence of the ligand field, spin-orbit coupling, tetragonal distortion and covalence effects as variable parameters. Compounds 2-5' exhibit weak antiferromagnetic interactions. The intramolecular exchange pathway in this family being discarded because of the symmetry of magnetic orbitals of the Re(II) ion (d(xy)) precludes any spin delocalization on the bridging nic orbitals, the observed magnetic interactions are most likely mediated by π-π type interactions between the peripheral ligands which occur in them. Only in the case of 4, short through space Br···Br contacts of ca. 4.03 Å (values larger than 5.5 Å in 2, 3 and 5) could be involved in the exchange coupling.

Ru–NO and Ru–NO2bonding linkage isomerism in cis-[Ru(NO)(NO)(bpy)2]2+/+complexes – a theoretical insight

Calculated energy profile (kcal mol⁻¹) for linkage isomers relative to the ground state structure (GS)1aprior the monoelectronic reduction.

Probing the catalytic potential of chloro nitrosyl rhenium(I) complexes

The reduction of the mononitrosyl Re(II) salt [NMe(4)](2)[ReCl(5)(NO)] (1) with zinc in acetonitrile afforded the Re(i) dichloride complex [ReCl(2)(NO)(CH(3)CN)(3)] (2). Subsequent ligand substitution reactions with PCy(3), PiPr(3) and P(p-tolyl)(3) afforded the bisphosphine Re(i) complexes [ReCl(2)(NO)(PR(3))(2)(CH(3)CN)] (3, R = Cy a, iPr b, p-tolyl c) in good yields. The acetonitrile ligand in 3 is labile, permitting its replacement with H(2) (1 bar) to afford the dihydrogen Re(I) complexes [ReCl(2)(NO)(PR(3))(2)(η(2)-H(2))] (4, R = Cy a, iPr b). The catalytic activity of 2, 3 and 4 in hydrogen-related catalyses including dehydrocoupling of Me(2)NH·BH(3), dehydrogenative silylation of styrenes, and hydrosilylation of ketones and aryl aldehydes were investigated, with the main focus on phosphine and halide effects. In the dehydrocoupling of Me(2)NH·BH(3), the phosphine-free complex 2 exhibits the same activity as the bisphosphine-substituted systems. In the dehydrogenative silylation of styrenes, 3a and 4a bearing PCy(3) ligands exhibit high catalytic activities. Monochloro Re(I) hydrides [Re(Cl)(H)(NO)(PR(3))(2)(CH(3)CN)] (5, R = Cy a, iPr b) were proven to be formed in the initiation pathway. The phosphine-free complex 2 showed in dehydrogenative silylations even higher activity than the bisphosphine derivatives, which further emphasizes the importance of a facile phosphine dissociation in the catalytic process. In the hydrosilylation of ketones and aryl aldehydes, at least one rhenium-bound phosphine is required to ensure high catalytic activity.

Facile synthetic access to rhenium(II) complexes: activation of carbon-bromine bonds by single-electron transfer

The five-coordinated Re(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (R=Cy 1 a, iPr 1 b) were reacted with benzylbromide, thereby affording the 17-electron mononuclear Re(II) hydride complexes [Re(Br)(2)(H)(NO)(PR(3))(2)] (R=Cy 3 a, iPr 3 b), which were characterized by EPR, cyclic voltammetry, and magnetic susceptibility measurements. In the case of dibromomethane or bromoform, the reaction of 1 afforded Re(II) hydrides 3 in addition to Re(I) carbene hydrides [Re(=CHR(1))(Br)(H)(NO)(PR(3))(2)] (R(1)=H 4, Br 5; R=Cy a, iPr b) in which the hydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re(I) dibromide complexes [Re(Br)(2)(NO)(PR(3))(2)(eta(2)-H(2))] (R=Cy 2 a, iPr 2 b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re(II) complex salts [R(3)PCH(2)R'][Re(Br)(4)(NO)(PR(3))] (R'=-CH=CH(2) 6, Ph 7). The reduction of Re(II) complexes has also been examined. Complex 3 a or 3 b can be reduced by zinc to afford 1 a or 1 b in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3 a 150, 3 b 134 h(-1)) or allylbromide (TOF: 3 a 575, 3 b 562 h(-1)). The reaction of 6 a and 6 b with zinc in acetonitrile affords in good yields the monophosphine Re(I) complexes [Re(Br)(2)(NO)(MeCN)(2)(PR(3))] (R=Cy 8 a, iPr 8 b), which showed high catalytic activity toward highly selective dehydrogenative silylation of styrenes (maximum TOF of 61 h(-1)). Single-electron transfer (SET) mechanisms were proposed for all these transformations. The molecular structures of 3 a, 6 a, 6 b, 7 a, 7 b, and 8 a were established by single-crystal X-ray diffraction studies.

Highly selective dehydrogenative silylation of alkenes catalyzed by rhenium complexes

Choosy chemicals: Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), ethylene (3); R=iPr (a), cyclohexyl (b)) proved to be suitable catalyst precursors for the highly selective dehydrogenative silylation of alkenes. Two types of rhenium(I) hydride species, [ReBrH(NO)(PR(3))(2)] (4) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (5), were found in the [ReBr(2)(L)(NO)(PR(3))(2)]-catalyzed dehydrogenative silylation of alkenes.Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), and ethylene (3); R=iPr (a) and cyclohexyl (Cy; b)) catalyze dehydrogenative silylation of alkenes in a highly selective manner to yield silyl alkenes and the corresponding alkanes. Hydrosilylation products appear only rarely depending on the type of olefinic substituent, and if they do appear then it is in very minor amounts. Mechanistic studies showed that two rhenium(I) hydride species of type [ReBrH(NO)(PR(3))(2)] (R=iPr (4 a) and Cy (4 b)) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (R(1)=p-CH(3)C(6)H(4), R=iPr (5 a), Cy (5 b); R(1)=H, R=iPr (5 a'), Cy (5 b')) are involved in the initiation pathway of the catalysis. The rate-determining steps of the catalytic cycle are the phosphine dissociation from complexes of type 5 and the reductive eliminations to form the alkane components. The catalytic cycle implies that the given rhenium systems have the ability to activate C-H and Si-H bonds through the aid of a facile redox interplay of Re(I) and Re(III) species. The molecular structures of 4 b and 5 a were established by means of X-ray diffraction studies.