Ruthenium(II) thiol and H2S complexes: synthesis, characterization, and thermodynamic properties

Plight of CORMs: The unreliability of four commercially available CO-releasing molecules, CORM-2, CORM-3, CORM-A1, and CORM-401, in studying CO biology

Carbon monoxide (CO) is an endogenously produced gaseous signaling molecule with demonstrated pharmacological effects. In studying CO biology, three delivery forms have been used: CO gas, CO in solution, and CO donors of various types. Among the CO donors, four carbonyl complexes with either a transition metal ion or borane (BH3) (termed CO-releasing molecules or CORMs) have played the most prominent roles appearing in over 650 publications. These are CORM-2, CORM-3, CORM-A1, and CORM-401. Intriguingly, there have been unique biology findings that were only observed with these CORMs, but not CO gas; yet these properties were often attributed to CO, raising puzzling questions as to why CO source would make such a fundamental difference in terms of CO biology. Recent years have seen a large number of reports of chemical reactivity (e.g., catalase-like activity, reaction with thiol, and reduction of NAD(P)+) and demonstrated CO-independent biological activity for these four CORMs. Further, CORM-A1 releases CO in an idiosyncratic fashion; CO release from CORM-401 is strongly influenced or even dependent on reaction with an oxidant and/or a nucleophile; CORM-2 mostly releases CO2, not CO, after a water-gas shift reaction except in the presence of a strong nucleophile; and CORM-3 does not release CO except in the presence of a strong nucleophile. All these beg the question as to what constitutes an appropriate CO donor for studying CO biology. This review critically summarizes literature findings related to these aspects, with the aim of helping result interpretation when using these CORMs and development of essential criteria for an appropriate donor for studying CO biology.

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Catalytic semihydrogenation of internal alkynes using H2 is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H2-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

Accelerated reduction and solubilization of elemental sulfur by 1,2-aminothiols

Nucleophilic 1,2-aminothiol compounds readily reduce typically-insoluble elemental sulfur to polysulfides in both water and nonpolar organic solvents. The resulting anionic polysulfide species are stabilized through hydrogen-bonding interactions with the proximal amine moieties. These interactions can facilitate sulfur transfer to alkenes.

31P Chemical Shifts in Ru(II) Phosphine Complexes. A Computational Study of the Influence of the Coordination Sphere

The NMR chemical shift has been the most versatile marker of chemical structures, by reflecting global and local electronic structures, and is very sensitive to any change within the chemical species. In this work, Ru(II) complexes with the same five ligands and a variable sixth ligand L (none, H₂O, H₂S, CH₃SH, H₂, N₂, N₂O, NO⁺, C═CHPh, and CO) are studied by using as the NMR reporter the phosphorus P_A of a coordinated bidentate P_A-N ligand (P_A-N = o-diphenylphosphino-N,N'-dimethylaniline). The chemical shift of P_A in RuCl₂(P_A-N)(PR₃)(L) (R = phenyl, p-tolyl, or p-FC₆H₄) was shown to increase as the Ru-P_A bond distance decreases, an observation that was not rationalized. This work, using density functional theory (DFT) calculations, reproduces reasonably well the observed ³¹P chemical shifts for these complexes and the correlation between the shifts and the Ru-P_A bond distance as L varies. An interpretation of this correlation is proposed by using a natural chemical shift (NCS) analysis based on the natural bonding orbital (NBO) method. This analysis of the principal components of the chemical shift tensors shows how the σ-donating properties of L have a particularly high influence on the phosphine chemical shifts.

How feasible is the reversible S-dissociation mechanism for the activation of FeMo-co, the catalytic site of nitrogenase?

The active site of the enzyme nitrogenase (N₂→ NH₃) is a Fe₇MoS₉C cluster that contains three doubly-bridging μ-S atoms around a central belt. A vanadium nitrogenase variant has a slightly different cluster, containing two μ-S atoms. Recent crystal structures have revealed substitution of one μ-S (S2B, bridging Fe2 and Fe6), by CO in Mo-nitrogenase and an uncertain light atom in V-nitrogenase. These systems retained catalytic activity, and were able to recover the lost μ-S atom. Electron density attributed to the dissociated S is displaced by 7 Å in the crystal structure of the non-standard V-protein. The hypothesis arising from these observations is that the chemical mechanism of nitrogenase involves reversible dissociation of S2B, leaving Fe2 and Fe6 seriously under-coordinated and reactive in trapping N₂ and binding reaction intermediates. Accumulated experimental evidence points to the Fe2-S2B-Fe6 domain as the centre of catalytic hydrogenation of N₂. Using DFT simulations of a large model (>488 atoms) containing all relevant surrounding protein residues, I have investigated the chemical steps that could allow dissociation of S2B. The participation of H atoms is crucial, as is involvement of the nearby side chain of His195 that can function as proton donor to S2B and hydrogen-bonding supporter of displaced S2B. A significant result is that after ingress and binding of N₂ at Fe2 the breaking of the Fe2-S2B bond can be strongly exergonic with negligible kinetic barrier. Subsequent extension of the Fe6-S2B bond and dissociation as H₂S (or SH^-) is endergonic by 20-25 kcal mol^-1, partly because the separating H₂S is restricted by surrounding amino-acids. I present a number of reaction sequences and energy landscapes, and derive thirteen chemical principles relevant to the postulated S-dissociation mechanism. A key conclusion is that unhooking of S2BH or S2BH₂ from Fe2 is favourable, likely, and propitious for subsequent H transfer to bound N₂ or reaction intermediates. The space between Fe2 and Fe6 supports two bridging ligands, and another H atom on Fe6 can move without kinetic barrier to occupy the bridging position vacated by S2B.

An N-Acetyl Cysteine Ruthenium Tricarbonyl Conjugate Enables Simultaneous Release of CO and Ablation of Reactive Oxygen Species

We have designed and synthesised a [Ru(CO)3 Cl2 (NAC)] pro-drug that features an N-acetyl cysteine (NAC) ligand. This NAC carbon monoxide releasing molecule (CORM) conjugate is able to simultaneously release biologically active CO and to ablate the concurrent formation of reactive oxygen species (ROS). Complexes of the general formulae [Ru(CO)3 (L)3 ](2+) , including [Ru(CO)3 Cl(glycinate)] (CORM-3), have been shown to produce ROS through a water-gas shift reaction, which contributes significantly, for example, to their antibacterial activity. In contrast, NAC-CORM conjugates do not produce ROS or possess antibacterial activity. In addition, we demonstrate the synergistic effect of CO and NAC both for the inhibition of nitric oxide (formation) and in the expression of tumour-necrosis factor (TNF)-α. This work highlights the advantages of combining a CO-releasing scaffold with the anti-oxidant and anti-inflammatory drug NAC in a unique pro-drug.

Ruthenium(III) complexes containing bi- and tridentate phosphorus−nitrogen ligands

Air-stable RuIII complexes containing a bidentate aminophosphine ligand (PN) of the type mer-RuCl3(PN)(PR3) are made from the precursors RuCl3(PR3)2(DMA)·DMA (PN = o-diphenylphosphino-N,N′-dimethylaniline (P−N) and (R)-N,N′-dimethyl-1-[o-diphenylphosphinophenyl]ethylamine ((R)-AMPHOS); R = Ph, p-tolyl; DMA = N,N′-dimethylacetamide). With the tridentate bis[o-(N,N′-dimethylamino)phenyl]phenylphosphine (BNP), the product is mer-RuCl3(BNP) (3), while tris[o-(N,N′-dimethylaminophenyl)phosphine (TNP) is unreactive toward the precursor. Crystal structures of mer-RuCl3(PN)(PPh3), where PN is P−N (2a), (R)-AMPHOS (4a), and 3·CHCl3 are reported as well as those of (R)-AMPHOS, BNP, and TNP. The RuIII−aminophosphine complexes are the first monomeric RuIII species to be formed via the useful, easily synthesized, air-stable RuIII precursors RuCl3(PR3)2(DMA)·DMA (1a and 1b); complex 2a is formed also via reaction of HCl with trans-RuCl2(P−N)(PPh3). A crystal structure of mer,cis-RuCl3(DMA)2(PPh3)·DMA (1c), a side-product from the synthesis of the RuIII precursor, is also presented and is the first-reported complex of DMA with RuIII. Preliminary data show that the RuIII−aminophosphine complexes in DMA (a proton-accepting solvent) are reduced by H2 to RuII species that can react further to form an η2-H2 adduct and then a RuII-hydridochloro species.

Chemically reversible reactions of hydrogen sulfide with metal phthalocyanines

Hydrogen sulfide (H₂S) is an important signaling molecule that exerts action on various bioinorganic targets. Despite this importance, few studies have investigated the differential reactivity of the physiologically relevant H₂S and HS^– protonation states with metal complexes. Here we report the distinct reactivity of H₂S and HS^– with zinc(II) and cobalt(II) phthalocyanine (Pc) complexes and highlight the chemical reversibility and cyclability of each metal. ZnPc reacts with HS^–, but not H₂S, to generate [ZnPc-SH]⁻, which can be converted back to ZnPc by protonation. CoPc reacts with HS^–, but not H₂S, to form [Co^IPc]⁻, which can be reoxidized to CoPc by air. Taken together, these results demonstrate the chemically reversible reaction of HS^– with metal phthalocyanine complexes and highlight the importance of H₂S protonation state in understanding the reactivity profile of H₂S with biologically relevant metal scaffolds.

The protonation state of H₂S influences its reactivity with different metal phthalocyanine (Pc) complexes. Both ZnPc and CoPc react with H₂S in a chemically reversible manner, with redox-inactive ZnPc binding HS⁻ and redox-active CoPc undergoing reduction. The [ZnPc-SH]⁻ product can be reverted to ZnPc by protonation, and [Co^IPc]⁻ can be redoxidized to CoPc with air.

Large structural changes upon protonation of Fe4S4 clusters: the consequences for reactivity

Recognition that protonation of μ₃-S in [Fe₄S₄X₄]²⁻ clusters causes breaking of an S–Fe bond provides a kinetically consistent general mechanism for the acid-catalysed substitution reactions of [Fe₄S₄X₄]²⁻ clusters.

Studies of iron(III) porphyrinates containing silanethiolate ligands

The chemistry of several iron(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron(III) precursors, [Fe(OMe)(TPP)] and [Fe(OH)(H2O)(TMP)] (TPP = dianion of meso-tetraphenylporphine; TMP = dianion of meso-tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H2S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron(III) porphyrinates is discussed.

Comparative properties of coordinated H2 and H2S at a ruthenium(II) centre

Thermodynamic data for the reversible formation of cis-RuCl2(P-N)(PPh3)(η(2)-H2) () from trans-RuCl2(P-N)(PPh3) in C6D6 are determined by variable temperature (31)P{(1)H} and (1)H NMR spectroscopy; P-N = o-diphenylphosphino-N,N'-dimethylaniline. Values of ΔH° = -26 ± 4 kJ mol(-1), ΔS° = -40 ± 15 J mol(-1) K(-1), and ΔG° (at 25 °C) = -13.8 ± 0.2 kJ mol(-1) are compared with recently reported data for the corresponding H2S adduct (4a), where the exothermicity is greater by ~20 kJ mol(-1), but this is counteracted by a more unfavourable entropy change, and overall the K and ΔG° values at 25 °C are close. For loss of H2 from 2a in the solid state, whose X-ray structure is presented, ΔH° is 50 ± 3 kJ mol(-1) as measured by Differential Scanning Calorimetry. The pKa values of the coordinated H2 (~11) and H2S (~14) are estimated by reactions of 2a and 4a with proton sponge (1,8-bis(dimethylamino)naphthalene) in CD2Cl2 at 20 °C; the mono-hydrido and -mercapto products are identified in situ. A corresponding H2O adduct is not deprotonated under the same conditions. Related dihydrido, mercapto and hydroxy species are formed by in situ reactions of 1a with NaH, NaSH, and NaOH, respectively.

Reversible binding of water, methanol, and ethanol to a five-coordinate ruthenium(II) complex

The known green, five-coordinate, square-pyramidal trans-RuCl(2)(P-N)(PPh(3)) complex reversibly binds water, MeOH and EtOH in the vacant coordination site in the solid state and in CH(2)Cl(2) solution to give pink adducts (P-N = o-diphenylphosphino-N,N'-dimethylaniline). The adducts are well characterized, including X-ray analysis of the aqua complex, trans-RuCl(2)(P-N)(PPh(3))(H(2)O), which crystallizes in two different benzene-solvated forms. Comparison of the structural data with those determined previously for the binding of H(2)S, thiols, and H(2), which form cis-RuX(2)(P-N)(PPh(3))L products (X = Cl, Br; L = a S-ligand or H(2)) reveals the trans-influence trend P > H(2)S ~ thiols > H(2) > Cl ~ Br > H(2)O. Thermodynamic data for the binding of water were estimated in solution by UV-Vis spectroscopy, and ΔH(o) data for the aqua and alcohol adducts in the solid state were obtained by differential scanning calorimetry. Inclusion of published data for the S-ligand adducts reveals the thermal stability trend of the solid complexes as MeSH > MeOH > H(2)S > H(2)O > EtSH > EtOH.