Proton-Transfer Reactions to Half-Sandwich Ruthenium Trihydride Complexes Bearing Hemilabile P,N Ligands: Experimental and Density Functional Theory Studies†Dedicated to Prof. Serafin Bernal in recognition of his contribution to inorganic chemistry, on the occasion of his retirement

Operationally unsaturated ruthenium complex stabilized by a phosphine 1-azaallyl ligand

Coordinatively unsaturated transition-metal compounds stabilized by supplemental electron donation from π-basic ligands are described as "operationally unsaturated". Such complexes are useful analogues of active catalyst structures that readily react with substrate molecules. We report that [Ph₂P(C₆H₄)NCHC(CH₃)₂]^- (L1) effectively stabilizes Ru(II) in an operationally unsaturated form. In the absence of Lewis bases, the 1-azaallyl group of L1 dominantly coordinates through a κ¹-N mode, but can readily and reversibly isomerize to an η³-NCC coordination mode to stabilize the metal. As an operationally unsaturated complex, Ru(Cp*)(L1) dimerizes at low temperature. At ambient temperature it rapidly reacts with pyridine or PPh₃ to form an adduct. These findings with L1 demonstrate that changes in the hapticity of a 1-azaallyl fragment offer an alternative means to stabilize low-coordinate metals.

Effects of Coordinating a Hemilabile Ligand to 14e Cp*M(NO) Scaffolds (M = Mo, W)

This article describes the differing chemical properties imparted by the two ligands, hemilabile 2-[(diisopropylphosphino)methyl]-3-methylpyridine (ⁱPr₂PN) and the related 1,2-bis(dimethylphosphino)ethane (dmpe), when attached to the 14e Cp*M(NO) scaffolds (Cp* = η⁵-C₅Me₅; M = W, Mo). For instance, the treatment of [Cp*W(NO)Cl₂]₂ with 2 or 1 equiv of dmpe in C₆H₆ affords excellent yields of [Cp*W(NO)(κ²-dmpe)Cl]Cl (1) or [Cp*W(NO)Cl₂]₂[μ-dmpe] (2). In contrast, the treatment of [Cp*W(NO)Cl₂]₂ with 1 equiv of ⁱPr₂PN in C₆H₆ does not produce the complex analogous to 1 but rather affords orange [Cp*W(NO)(κ²-P-N-ⁱPr₂PN)Cl][Cp*W(NO)Cl₃] (3) in 90% yield. Furthermore, subsequent reduction of 1 or 2 with 2 or 4 equiv of Cp₂Co in tetrahydrofuran (THF), respectively, results in the production of orange Cp*W(NO)(κ²-dmpe) (4) in good yields. However, a similar treatment of 3 with 1 equiv of Cp₂Co in THF does not result in the production of Cp*W(NO)(κ²-P,N-ⁱPr₂PN), the analogue of 4, but rather generates a 1:1 mixture of the novel complexes Cp*W(NO)(H)(κ¹-P-ⁱPr₂PN)Cl (5) and Cp*W(NO)(κ²-P,N-ⁱPr₂PCH-2-(3-Me-C₅H₃N))Cl (6), which are separable by crystallization from pentane and diethyl ether solutions, respectively. The divergent reactivity imparted by the dmpe and ⁱPr₂PN proligands is a unique demonstration of the unusual properties of a mixed-donor ligand. In the case of molybdenum, the reaction of [Cp*Mo(NO)Cl₂]₂ with 2 equiv of ⁱPr₂PN in C₆H₆ first forms Cp*Mo(NO)(κ¹-P-ⁱPr₂PN)Cl₂, which then converts to [Cp*Mo(NO)(κ²-P,N-ⁱPr₂PN)Cl][Cp*Mo(NO)Cl₃], the analogue of 3. Reduction of the Cp*Mo(NO)(κ¹-P-ⁱPr₂PN)Cl₂ intermediate complex with 2 equiv of Cp₂Co affords dark-green Cp*Mo(NO)(κ²-P,N-ⁱPr₂PN) (7). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Exploring the Ruthenium-Ligands Bond and Their Relative Properties at Different Computational Methods

We report some experimental bond distances and computational models of six ruthenium bonds obtained from DFT to higher computational methods like MP2 and CCSD. The bonds distances, geometrical RMSD, and the thermodynamic properties of the models from different computational methods are similar. It is observed that optimization of molecules of many light atoms with different functional methods results in significant geometrical variation in the values and order of the computed properties. The values of the hyperpolarizabilities, HOMO, LUMO, and isotropic and anisotropic shielding are found to depend greatly on the type of the functional used and the geometrical variation rather than on the nature of basis set used. However, all the methods rated modelled Ru-S, Ru-Cl, and Ru-O bonds as having the highest hyperpolarizabilities values. The infrared spectra data obtained from the different computational methods are significantly different from each other except for MP2 and CCSD which are found to be very similar.

Development of ruthenium-based complexes as anticancer agents: toward a rational design of alternative receptor targets

In the search for novel anticancer agents, the development of metal-based complexes that could serve as alternatives to cisplatin and its derivatives has received considerable attention in recent years. This becomes necessary because, at present, cisplatin and its derivatives are the only coordination complexes being used as anticancer agents in spite of inherent serious side effects and their limitation against metastasized platinum-resistant cancer cells. Although many metal ions have been considered as possible alternatives to cisplatin, the most promising are ruthenium (Ru) complexes and two Ru compounds, KP1019 and NAMI-A, which are currently in phase II clinical trials. The major obstacle against the rational design of these compounds is the fact that their mode of action in relation to their therapeutic activities and selectivity is not fully understood. There is an urgent need to develop novel metal-based anticancer agents, especially Ru-based compounds, with known mechanism of actions, probable targets, and pharmacodynamic activity. In this paper, we review the current efforts in developing metal-based anticancer agents based on promising Ru complexes and the development of compounds targeting receptors and then examine the future prospects.

Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

In crystal structures of the molybdenum complexes [(1,2,4-C5H2(t)Bu3)Mo(PMe3)2H3] (Cp(t)Bu3) and [(C5H(i)Pr4)Mo(PMe3)2H3] (Cp(i)Pr4), the Mo-bound hydrogen positions were resolved for Cp(t)Bu3, but not for Cp(i)Pr4. NMR experiments revealed the existence of an unknown mechanism for hydrogen atom exchange in these molecules, which can be "frozen out" for Cp(t)Bu3 but not for Cp(i)Pr4. Density functional theory calculations of the most stable conformations for both complexes in the gas phase and in a continuum solvent model indicate that the H's of the Cp(i)Pr4 complex are unresloved because of their disorder, which does not occur for Cp(t)Bu3. A corresponding examination of alternative rearrangement pathways shows that the rearrangements of the H's could occur by two mechanisms: parallel to the cyclopentadienyl (Cp) ring in a single step and perpendicular to the Cp ring in two steps. The parallel pathway is preferred for both molecules, but it has a lower energy barrier for Cp(i)Pr4 than for Cp(t)Bu3.

Picolyl-NHC hydrotris(pyrazolyl)borate ruthenium(II) complexes: synthesis, characterization, and reactivity with small molecules

Ruthenium(II) hydrotris(pyrazolyl)borate chloro complexes bearing picolyl-functionalized N-heterocyclic carbenes [TpRu(κ(2)-C,N-picolyl-(R)I)Cl] (picolyl-(Me)I = 3-methyl-1-(2-picolyl)imidazol-2-ylidene) (1a), picolyl-(iPr)I = 3-isopropyl-1-(2-picolyl)imidazol-2-ylidene (1b), picolyl-(Me)45DClI = 3-methyl-1-(2-picolyl)-4,5-dichloroimidazol-2-ylidene (1c), picolyl-(Ph)I = 3-phenyl-1-(2-picolyl)imidazol-2-ylidene (1d), picolyl-(Me)BI = 3-methyl-1-(2-picolyl)benzoimidazol-2-ylidene (1e)) have been synthesized and characterized. Furthermore, cationic carbonyl derivatives 2a-e have been prepared, characterized, and used to study the donor properties of the picolylcarbene ligands (picolyl-(R)I) via infrared spectroscopy. Also, the reactivity of the 16-electron species [TpRu(κ(2)-C,N-picolyl-(R)I)](+), in situ generated using NaBAr(F)4 (Ar(F) = 3,5-bis(trifluoromethyl)phenyl) as a halide scavenger, toward N2, CH3CN, H2, CH2CH2, S8, and O2 was studied indicating a strong influence of the NHC wingtip and backbone substituents in the product distribution. The crystal structures of [TpRu(κ(2)-C,N-picolyl-(iPr)I)Cl] (1b), [TpRu(κ(2)-C,N-picolyl-(Me)I)CO][BAr(F)4] (2a), [TpRu(κ(2)-C,N-picolyl-(Ph)I)CO][BAr(F)4] (2d), [{TpRu(κ(2)-C,N-picolyl-(Me)I}2(μ-N2)][BAr(F)4]2 (3'a), [{TpRu(κ(2)-C,N-picolyl-(Ph)I)}2(μ-N2)][BAr(F)4]2 (3'd), [TpRu(κ(2)-C,N-picolyl-(iPr)I)(η(2)-CH2CH2)][BAr(F)4] (5b), and [{TpRu(κ(2)-C,N-picolyl-(Me)I)}2(μ-S2)][BAr(F)4]2 (6) are reported.