Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation.

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

Tetranuclear Copper(I) and Silver(I) Pyrazolate Adducts with 1,1'-Dimethyl-2,2'-bibenzimidazole: Influence of Structure on Photophysics.

A reaction of a cyclic trinuclear copper(I) or silver(I) pyrazolate complex ([MPz]3, M = Cu, Ag) with 1,1'-dimethyl-2,2'-bibenzimidazole (L) leads to the formation of tetranuclear adducts decorated by one or two molecules of a diimine ligand, depending on the amount of the ligand added (0.75 or 1.5 equivalents). The coordination of two L molecules stabilizes the formation of a practically idealized tetrahedral four-metal core in the case of a copper-containing complex and a distorted tetrahedron in the case of a Ag analog. In contrast, complexes containing one molecule of diimine possess two types of metals, two- and three-coordinated, forming the significantly distorted central M4 cores. The diimine ligands are twisted in these complexes with dihedral angles of ca. 50-60°. A TD-DFT analysis demonstrated the preference of a triplet state for the twisted 1,1'-dimethyl-2,2'-bibenzimidazole and a singlet state for the planar geometry. All obtained complexes demonstrated, in a solution, the blue fluorescence of the ligand-centered (LC) nature typical for free diimine. In contrast, a temperature decrease to 77 K stabilized the structure close to that observed in the solid state and activated the triplet states, leading to green phosphorescence at ca. 500 nm. The silver-containing complex Ag4Pz4L exhibited dual emission from both the singlet and triplet states, even at room temperature.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°H- or kH-) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans-[L2Mn(CO)3H] (1; L = P(OPh)3, 2; L = PPh3) and fac-[(L-L')Mn(CO)3H] (3, L-L' = Ph2PCH2PPh2 (dppm); 4, L-L' = Ph2PCH2-NHC) with organic bases and Lewis acid (B(C6F5)3) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG≠298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH2-bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1-4 increases in the order mer,trans-[(P(OPh)3)2Mn(CO)3H] (1) < mer,trans-[(PPh3)2Mn(CO)3H] (2) ≈ fac-[(dppm)Mn(CO)3H] (3) < fac-[(Ph2PCH2NHC)Mn(CO)3H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.

Versatile Reactivity of Half-Sandwich Rhodium(III) Iminophosphonamide Complexes.

Novel 18e̅ and 16e̅ pentamethylcyclopentadienyl rhodium(III) complexes [(η5-C5Me5)RhX(NPN)] (1a,b, X = Cl; 2a-c, X = PF6, BAr4F) with chelating zwitterionic iminophosphonamide (NPN) ligands (Ph2P(NR)(NR'); a, R = R' = p-Tol; b, R = p-Tol, R' = Me; c, R = R' = Me) were synthesized and characterized by single-crystal X-ray diffraction. In the 16e̅ complexes 2, the rhodium (Rh) atom is efficiently stabilized by π-donation of unshared N electrons, thus hampering coordination of the external ligands and rendering the 18e̅ complexes labile. Due to low coordination enthalpy, the cationic 18e̅ monocarbonyl and pyridine adducts 2a·L are stable only at low temperatures. At room temperature, 2·CO adducts readily give stable carbonyl-carbamoyl complexes [(η5-C5Me5)Rh(CO){(CO(NR')Ph2P(NR)}]+ (4) formed as a result of CO insertion into the Rh-N bond, thus showing high nucleophilicity of the N atoms in 18e̅ complexes. High basicity of the Na+NPN- precursors caused side deprotonation of the η5-C5Me5 ligand during the synthesis of 1 that yields unstable fulvene Rh(I) complexes [(η4-C5Me4CH2)Rh{Ph2P(NR)(NR')2}] (3a,b). Complex 3a undergoes a facile reaction with isoprene to yield an unusual [(η5:η1-C5Me4(CH2)C(Me)═CHCH2)Rh(NPN)] complex─the first example of intermolecular 1,4-metallacycloaddition of diene to the Rh-fulvene complex.

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes.

The lithiation of the NHC ligand backbone in Cp(CO)2Mn(IMes) followed by transmetalation on the C4 carbenic position with Cp(CO)2FeI led to the heterobimetallic complex Cp(CO)2Mn(µ-dIMes)Fe(CO)2Cp bearing the anionic ditopic imidazol-2,4-diylidene dIMes ligand. Subsequent treatment of the later with TfOH induced a selective decoordination of the [Cp(CO)2Mn] fragment to form the cationic abnormal NHC complex [Cp(CO)2Fe(aIMes)](OTf), which was further derivatized to the bis(iron) dIMes complex [Cp(CO)2Fe(µ-dIMes)Fe(CO)2Cp](OTf) by reaction with tAmOK and Cp(CO)2FeI. The effect of the metalation at the C4 or C2 position on the imidazole ring on the electronic donation properties of the associated C2 and C4 carbenic centers in the dIMes ligand was quantified through systematic experimental and theoretical studies of IMes, aIMes, and dIMes complexes. The evaluation of the catalytic activity of the series of cationic Fe(II) complexes based on IMes, aIMes, and dIMes ligands in a benchmark ketone hydrosilylation showed the superiority of the bimetallic derivative.

Lossen rearrangement by Rh(III)-catalyzed C-H activation/annulation of aryl hydroxamates with alkynes: access to quinolone-containing amino acid derivatives.

A convenient and robust method for the preparation of new CF3-containing 2-quinolones has been developed via a Rh(III)-catalyzed C-H activation/Lossen rearrangement/annulation cascade of N-pivaloyloxy-arylamides with internal alkynes bearing an α-CF3-α-amino acid moiety on the triple bond. This work expands the scope of valuable products that are available through C-H activation/annulation reactions of arylamides in organic synthesis.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion.

The mechanism of the consecutive halogenation of the tetrahydroborate anion [BH4]- by hydrogen halides (HX, X = F, Cl, Br) and hexahydro-closo-hexaborate dianion [B6H6]2- by HCl via electrophile-induced nucleophilic substitution (EINS) was established by ab initio DFT calculations [M06/6-311++G(d,p) and wB97XD/6-311++G(d,p)] in acetonitrile (MeCN), taking into account non-specific solvent effects (SMD model). Successive substitution of H- by X- resulted in increased electron deficiency of borohydrides and changes in the character of boron atoms from nucleophilic to highly electrophilic. This, in turn, increased the tendency of the B-H bond to transfer a proton rather than a hydride ion. Thus, the regularities established suggested that it should be possible to carry out halogenation more selectively with the targeted synthesis of halogen derivatives with a low degree of substitution, by stabilization of H2 complex, or by carrying out a nucleophilic substitution of B-H bonds activated by interaction with Lewis acids (BL3).

Mononuclear Copper(I) 3-(2-pyridyl)pyrazole Complexes: The Crucial Role of Phosphine on Photoluminescence.

A series of emissive Cu(I) cationic complexes with 3-(2-pyridyl)-5-phenyl-pyrazole and various phosphines: dppbz (1), Xantphos (2), DPEPhos (3), PPh3 (4), and BINAP (5) were designed and characterized. Complexes obtained exhibit bright yellow-green emission (ca. 520-650 nm) in the solid state with a wide range of QYs (1-78%) and lifetimes (19-119 µs) at 298 K. The photoluminescence efficiency dramatically depends on the phosphine ligand type. The theoretical calculations of buried volumes and excited states explained the emission behavior for 1-5 as well as their lifetimes. The bulky and rigid phosphines promote emission efficiency through the stabilization of singlet and triplet excited states.

Synthesis of Bis(Carboranyl)amides 1,1'-µ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 (n = 0, 1) and Attempt of Synthesis of Gadolinium Bis(Dicarbollide).

Bis(carboranyl)amides 1,1'-µ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 (n = 0, 1) were prepared by the reactions of the corresponding carboranyl acyl chlorides with ethylenediamine. Crystal molecular structure of 1,1'-µ-(CH2NH(O)C-1,2-C2B10H11)2 was determined by single crystal X-ray diffraction. Treatment of bis(carboranyl)amides 1,1'-µ-(CH2NH(O)C(CH2)n-1,2-C2B10H11)2 with ammonium or cesium fluoride results in partial deboronation of the ortho-carborane cages to the nido-carborane ones with formation of [7,7'(8')-µ-(CH2NH(O)C(CH2)n-7,8-C2B9H11)2]2-. The attempted reaction of [7,7'(8')-µ-(CH2NH(O)CCH2-7,8-C2B9H11)2]2- with GdCl3 in 1,2-dimethoxy- ethane did not give the expected metallacarborane. The stability of different conformations of Gd-containing metallacarboranes has been estimated by quantum-chemical calculations using [3,3-µ-DME-3,3'-Gd(1,2-C2B9H11)2]- as a model. It was found that in the most stable conformation the CH groups of the dicarbollide ligands are in anti,anti-orientation with respect to the DME ligand, while any rotation of the dicarbollide ligand reduces the stability of the system. This makes it possible to rationalize the design of carborane ligands for the synthesis of gadolinium metallacarboranes on their base.

Dichotomous Si-H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes.

The activation of silanes in dehydrogenative coupling with alcohols under general base catalysis was studied experimentally (using multinuclear NMR, IR, and UV-visible spectroscopies) and computationally (at DFT M06/6-311++G(d,p) theory level) on the example of Ph4-nSiHn (n = 1-3) interaction with (CF3)2CHOH in the presence of Et3N. The effect of the phenyl groups' number and H- substitution by the electron-withdrawing (CF3)2CHO- group on Si-H bond hydricity (quantified as hydride-donating ability, HDA) and Lewis acidity of silicon atom (characterized by maxima of molecular electrostatic potential) was accessed. Our results show the coordination of Lewis base (Y = Me3N, ROH, OR-) leads to the increased hydricity of pentacoordinate hypervalent Ph4-nSi(Y)Hn complexes and a decrease of the reaction barrier for H2 release. The formation of tertiary complexes [Ph4-nSi(Y)Hn]···HOR is a critical prerequisite for the dehydrocoupling with alkoxides being ideal activators. The latter can be external or internal, generated by in situ HOR deprotonation. The mutual effect of tetrel interaction and dihydrogen bonding in tertiary complexes (RO-)Ph4-nSiHn···HOR leads to dichotomous activation of Si-H bond promoting the proton-hydride transfer and H2 release.

Stereoisomerism as an Origin of Different Reactivities of Ir(III) PC(sp3)P Pincer Catalysts.

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp3)P pincer ligand (1,8-bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV-visible) studies show the existence of two isomers of hexacoordinate complexes 1·MeCN, 2·MeCN, and 2·Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp3), whereas only the equatorial isomer is found for 1·Py. These complexes are stabilized by various intramolecular noncovalent C-H···Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1·L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

Steric and Electronic Effect of Cp-Substituents on the Structure of the Ruthenocene Based Pincer Palladium Borohydrides.

Ruthenocene-based PCPtBu pincer ligands were used to synthesize novel pincer palladium chloride RcF[PCPtBu]PdCl (2a) and two novel palladium tetrahydroborates RcF[PCPtBu]Pd(BH4) (3a) and Rc*[PCPtBu]Pd(BH4) (3b), where RcF[PCPtBu] = κ3-{2,5-(tBu2PCH2)2-C5H2}Ru(CpF) (CpF = C5Me4CF3), and Rc*[PCPtBu] = κ3-{2,5-(tBu2PCH2)2C5H2}Ru(Cp*) (Cp* = C5Me5). These coordination compounds were characterized by X-ray, NMR and FTIR techniques. Analysis of the X-ray data shows that an increase of the steric bulk of non-metalated cyclopentadienyl ring in 3a and 3b relative to non-substituted Rc[PCPtBu]Pd(BH4) analogue (3c; where Rc[PCPtBu] = κ3-{2,5-(tBu2PCH2)2C5H2}Ru(Cp), Cp = C5H5) pushes palladium atom from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom. This displacement increases in the order 3c < 3b < 3a following the order of the Cp-ring steric volume increase. The analysis of both X-ray and IR data suggests that BH4 ligand in both palladium tetrahydroborates 3a and 3b has the mixed coordination mode η1,2. The strength of the BH4 bond with palladium atom increases in the order Rc[PCPtBu]Pd(BH4) < Rc*[PCPtBu]Pd(BH4) < RcF[PCPtBu]Pd(BH4) that appears to be affected by both steric and electronic properties of the ruthenocene moiety.

Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes.

Thermodynamic hydricity (HDAMeCN) determined as Gibbs free energy (ΔG°[H]-) of the H- detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [BnHn]2-, and their lithium salts Li2[BnHn] (n = 5-17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B2H6 (HDAMeCN = 82.1 kcal/mol) and its dianion [B2H6]2- (HDAMeCN = 40.9 kcal/mol for Li2[B2H6]) can be selected as border points for the range of borane clusters' reactivity. Borane clusters with HDAMeCN below 41 kcal/mol are strong hydride donors capable of reducing CO2 (HDAMeCN = 44 kcal/mol for HCO2-), whereas those with HDAMeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B2H6, B4H10, B5H11, etc.). The HDAMeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDAMeCN.

Synthesis and Structure of Methylsulfanyl Derivatives of Nickel Bis(Dicarbollide).

Symmetrically and unsymmetrically substituted methylsulfanyl derivatives of nickel(III) bis(dicarbollide) (Bu4N)[8,8'-(MeS)2-3,3'-Ni(1,2-C2B9H10)2], (Bu4N)[4,4'-(MeS)2-3,3'-Ni(1,2-C2B9H10)2], and (Bu4N)[4,7'-(MeS)2-3,3'-Ni(1,2-C2B9H10)2] were synthesized, starting from [Ni(acac)2]3 and the corresponding methylsulfanyl derivatives of nido-carborane (Bu4N)[10-MeS-7,8-C2B9H11] and (Bu4N)[10-MeS-7,8-C2B9H11]. Structures of the synthesized metallacarboranes were studied by single-crystal X-ray diffraction and quantum chemical calculations. The symmetrically substituted 8,8'-isomer adopts transoid conformation stabilized by two pairs of intramolecular C-H···S hydrogen bonds between the dicarbollide ligands. The unsymmetrically substituted 4,7'-isomer adopts gauche conformation, which is stabilized by two nonequivalent C-H···S hydrogen bonds and one short chalcogen B-H···S bond (2.53 Å, -1.4 kcal/mol). The gauche conformation was found to be also preferred for the 4,7'-isomer.

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H2 Activation Mode.

Deprotonation of the MnI NHC-phosphine complex fac-[MnBr(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (2) under a H2 atmosphere readily gives the hydride fac-[MnH(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO)3 (κ3 P,C,C-Ph2 PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO)3 (κ2 P,C-Ph2 P=CHNHC)] (6 b) as key intermediate able to activate H2 via a non-classical mode of metal-ligand cooperation implying a formal λ5 -P-λ3 -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the MnI series reported to date (TON up to 6200).

Z-H Bond Activation in (Di)hydrogen Bonding as a Way to Proton/Hydride Transfer and H2 Evolution.

The ability of neutral transition-metal hydrides to serve as a source of hydride ion H- or proton H+ is well appreciated. The hydride ligands possessing a partly negative charge are proton accepting sites, forming a dihydrogen bond, M-Hδ- ⋅⋅⋅δ+ HX (M=transition metal or metalloid). On the other hand, some metal hydrides are able to serve as a proton source and give hydrogen bond of M-Hδ+ ⋅⋅⋅X type (X=organic base). In this paper we analyse recent works on transition-metal and boron hydrides showing i) how formation of an intermolecular complex between the reactants changes the Z-H (M-H and X-H) bond polarity and ii) what is the implication of such activation in the mechanisms of hydrides reactions.

Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to trans-W(N2)2(dppe)2.

The interaction of trans-W(N2)2(dppe)2 (1; dppe = 1,2-bis(diphenylphosphino)ethane) with relatively weak acids (p-nitrophenol, fluorinated alcohols, CF3COOH) was studied by means of variable temperature IR and NMR spectroscopy and complemented by DFT/B3PW91-D3 calculations. The results show, for the first time, the formation of a hydrogen bond to the coordinated dinitrogen, W-N≡N···H-O, that is preferred over H-bonding to the metal atom, W···H-O, despite the higher proton affinity of the latter. Protonation of the core metal-the undesirable side step in the conversion of N2 to NH3-can be avoided by using weaker and, more importantly, bulkier acids.

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.

Oxidative Coupling of Anionic Abnormal N-Heterocyclic Carbenes: Efficient Access to Janus-Type 4,4'-Bis(2H-imidazol-2-ylidene)s.

The oxidative coupling of anionic imidazol-4-ylidenes protected at the C2 position with [MnCp(CO)2 ] or BH3 led to the corresponding 4,4'-bis(2H-imidazol-2-ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C-C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4'-bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus-type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4'-bis(IMes), favored by steric hindrance along with stabilizing intramolecular C-H⋅⋅⋅π aryl interactions, allows the alignment of the π-systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.

Coordinatively Labile 18-Electron Arene Ruthenium Iminophosphonamide Complexes.

The thermodynamics of chloride dissociation from the 18e arene ruthenium iminophosphonamides [(η6 -arene)RuCl{(R'N)2 PR2 }] (1 a-d) [previously known with arene=C6 Me6 , R=Ph, R'=p-Tol (a); R=Et, R'=p-Tol (b); R=Ph, R'=Me (c); and new with arene=p-cymene, R=Ph, R'=p-Tol (d)] was assessed in both polar and apolar solvents by variable-temperature UV/Vis, NMR, and 2D EXSY 1 H NMR methods, which highlighted the influence of the NPN ligand on the equilibrium parameters. The dissociation enthalpy ΔHd decreases with increasing electron-donating ability of the N- and P-substituents (1 a, 1 d>1 b>1 c) and solvent polarity, and this results in exothermic spontaneous dissociation of 1 c in polar solvents. The coordination of neutral ligands (MeCN, pyridine, CO) to the corresponding 16e complexes [(η6 -arene)Ru{(R'N)2 PR2 }]+ PF6- (2 a-d) is reversible; the stability of the 2⋅L adducts depends on the π-accepting ability of L. Carbonylation of 2 a and 2 d resulted in rare examples of cationic arene ruthenium carbonyl complexes (3 a, 3 d), while the monocarbonyl adduct derived from 2 c reacts further with a second equivalent of CO with conversion to carbonyl-carbamoyl complex 3 c, in which one CO molecule is inserted into the Ru-N bond. The new complexes 1 d, 2 d, 3 a, 3 c, and 3 d were isolated and structurally characterized.