Weak Agostic Interaction Research Articles

Synthesis, characterization, DFT studies and biological assessment of palladium(II) complexes with imidazole derivatives

Six palladium(II) complexes (1–6) with 2-phenylimidazole (L1) and 1,2-dimethylimidazole (L2), were synthesized and characterized by elemental analyses, IR and NMR spectroscopy. The crystal structure of one of them ([Pd(2-phenylimidazole)2Br2], 1) was determined by X-ray crystallography. The palladium(II) ion in complex 1 is coordinated by two nitrogen atoms of 2-phenylimidazole and two bromide ions at trans positions, adopting a nearly regular square planar geometry. The structures of PdBr2 complexes {[Pd(2-phenylimidazole)2Br2] (1) and [Pd(1,2-dimethylimidazole)2Br2] (4)} as well as their chloride analogues, {[Pd(2-phenylimidazole)2Cl2] (1a) and [Pd(1,2-dimethylimidazole)2Cl2] (4a)} were predicted by DFT calculations. Complexes 1 and 1a appeared to be stabilized by a weak agostic interaction between palladium and hydrogen atoms. The energy of Pd⋯H bond was estimated to be 1.6 kcal/mol for both complexes that was comparable to the strength of Cl⋯H hydrogen bond in 1a (1.7 kcal/mol). The complexes were examined for various biological activities including; antimicrobial, antioxidant and anti-diabetic assays. The complexes exhibited moderate antimicrobial activities but they were not good as antioxidants. However, the alpha amylase inhibition assay showed the excellent anti-diabetic effect of the complexes.

Olefin coordination versus weak agostic C[sbnd]H[sbnd]M interactions in Sandwich-Type complexes of the split (3 + 2) Five-Electron donor hydrocarbon ligand Tricyclo[5.2.1.0]-deca-3,8-dien-5-yl (dicp) related to cyclopentadiene dimer

Sandwich-type complexes (dicp)MCp (M = Ti, V, Cr, Mn, Fe, Co, Ni; Cp = cyclopentadienyl) of the split (3 + 2) five-electron donor ligand tricyclo[5.2.1.0]-deca-3,8-dien-5-yl (dicp) related to cyclopentadiene dimer have been investigated by density functional theory. In the low-energy structures of the transition metal complexes (dicp)MCp (M = Ti, V, Cr, Mn, Fe), all of the split (3 + 2) five electrons from the dicp ligand are donated to the central transition metal atom leading to 14-, 15-, 16-, 17, and 18-electron configurations for the singlet (dicp)TiCp, quartet (dicp)VCp, triplet (dicp)CrCp, doublet (dicp)MnCp, and singlet (dicp)FeCp structures. For the late transition metals Co and Ni, only the allyl unit of the dicp ligand is strongly coordinated to the transition metal atom, in contrast to the Cp2Co and Cp2Ni in which all ten carbon atoms of the two cyclopentadienyl ligands are coordinated to the central metal atom. The potential energy surface for the manganese complex (dicp)MnCp is much more complicated than that of manganocene Cp2Mn, in which the doublet, quartet, and sextet structures are nearly degenerate in energy, with the quartet and sextet low-energy structures having only have part of the allyl or olefin unit coordinated to the central manganese atom. Weak agostic CHM interactions between the dicp ligand and the metal atoms, typically with M—H distances of ∼2.7 Å, are found in many of the (dicp)MCp complexes in contrast to the Cp2M (M = Ti, V, Cr, Mn, Fe, Co, Ni).

Steric and Electronic Manipulation of the Agostic and π‐Syndetic Donations in a Known Iminophosphane Ni(II) Complex Containing a Rotatable In‐Plane Aromatic Ring

AbstractDensity functional theory (DFT) calculations show that in a known nickel(II) complex containing a chelating iminophosphane ligand, where a pendant aromatic ring forms a weak agostic interaction with the metal centre, significant π‐syndetic donation is developed which establishes this kind of donation in a complex derived from experiment. Electron withdrawing and donating groups added to the carbon para to the agostic carbon do not cause the aromatic ring to rotate. Natural bond orbital (NBO) analysis shows that electron withdrawing groups cause little change to the agostic and π‐syndetic donations but electron donation switches off the π‐syndetic donation forming η1‐complexes of varying strength but which still retain a mostly unchanged agostic component. Increasing the size of the substituent ortho to the agostic carbon sequentially, cause rotation of the ring whereby the agostic donation declines slowly, the π‐syndetic donation rises followed by a rapid decline in both donations. The study shows that both donations may be manipulated where a rotatable aromatic ring is present.

Recent developments in meta-benziporphodimethene: A new porphyrin analogue

The article underscores a unique class of benziporphyrin analogues, in which two of its meso‑carbon atoms are sp3 hybridized, leading to the formation of a new molecule: meta-benziporphodimethene (meta-BPDM). This molecule exhibits intriguing structural, spectroscopic and chemical properties. meta-benziporphodimethene was first discovered 16 years ago. Although, initially it was an accidental product obtained while synthesizing meta- and para- benziporphyrin, but the molecule stood-out when acted as a chemo-sensor for Zn2+ ions. The discussion here, starts with outlining the importance of porphyrin molecules, followed by the progress in the field of its synthetic modifications. Special emphasis has been given to the new porphyrin analogue: meta-BPDM. We have herein covered some recent applications reported like metal-ion sensing, breast cancer detector, UV- turn-on behavior for toxic metals of meta-BPDM. Owing to its low synthetic yield, not much research has been done on these systems, despite their beneficial properties. We have also inculcated the factors that may lead to enhancement of product yield of meta-benziporphodimethene systems. The X-ray and NMR analysis indicating weak agostic interactions has also been discussed. This article shall provide a better understanding of this class of carbaporphyrinoid family, to its readers and may open new paths for exploring these systems in future.

Rhodium(III) and Iridium(III) Complexes of a NHC-Based Macrocycle: Persistent Weak Agostic Interactions and Reactions with Dihydrogen.

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) 2,2′-biphenyl complexes [M(CNC-12)(biph)][BArF4] (M = Rh (1a), Ir (1b)), featuring the macrocyclic lutidine- and NHC-based pincer ligand CNC-12 are reported. In the solid state these complexes are notable for the adoption of weak ε-agostic interactions that are characterized by M···H–C contacts of ca. 3.0 Å by X-ray crystallography and ν(CH) bands of reduced wavenumber by ATR IR spectroscopy. Remarkably, these interactions persist on dissolution and were observed at room temperature using NMR spectroscopy (CD2Cl2) and solution-phase IR spectroscopy (CCl4). The associated metrics point toward a stronger M···H–C interaction in the iridium congener, and this conclusion is borne out on interrogation of 1 in silico using DFT-based NBO and QTAIM analyses. Reaction of 1 with dihydrogen resulted in hydrogenolysis of the biaryl and formation of fluxional hydride complexes, whose ground state formulations as [Rh(CNC-12)H2][BArF4] (2a″) and [Ir(CNC-12)H2(H2)][BArF4] (2b‴) are proposed on the basis of inversion recovery and variable-temperature NMR experiments, alongside a computational analysis. Reactions of 1 and 2 with carbon monoxide help support their respective structural properties.

Approaches to Sigma Complexes via Displacement of Agostic Interactions: An Experimental and Theoretical Investigation

A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) has been prepared and characterized. These complexes exhibit agostic interaction in the sixth coordination site. The binding of various X–H (X = H, Si, B, and C) bonds in small molecules such as H2, silanes, tetracoordinate boranes, and CH4 to the ruthenium center via displacement of the relatively weak agostic interaction in these complexes has been studied. Structures of [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) complexes and the sigma-borane complex trans-[RuH(CO)(η1-H-BH2·NMe3)(PNRP)]+ (R = PhCH2 (15)) have been established by X-ray crystallography. The relative binding strengths of the X–H bonds to ruthenium center in these complexes has been studied using computational methods.

Impact of Weak Agostic Interactions in Nickel Electrocatalysts for Hydrogen Oxidation

To understand how H2 binding and oxidation is influenced by [Ni(PR2NR′2)2]2+ catalysts with H2 binding energies close to thermoneutral, two [Ni(PPh2NR′2)2]2+ (R = Me, C14H29) complexes with phenyl substituents on phosphorus and varying alkyl chain lengths on the pendant amine were studied. In the solid state, [Ni(PPh2NMe2)2]2+ exhibits a weak agostic interaction between the Ni(II) center and the a C–H bond of the pendant N–CH3 group. DFT computations and variable-temperature 31P NMR experiments suggest that the agostic interaction persists in solution. The equilibrium constants for H2 addition to these complexes were measured by 31P NMR spectroscopy, affording free energies of H2 addition (ΔG°H2) of −0.8 kcal mol–1 in benzonitrile and −1.7 to −2.7 kcal mol–1 in THF. The agostic interaction contributes to the low driving force for H2 binding by stabilizing the four-coordinate Ni(II) species prior to binding of H2. The pseudo-first-order rate constants for addition of H2 (1 atm) were measured by variable-sc...

Computation provides chemical insight into the diverse hydride NMR chemical shifts of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H-, F- and Cl-) and their [Ru(R2PCH2CH2PR2)2(L)H]+ congeners.

Relativistic density functional theory calculations, both with and without the effects of spin-orbit coupling, have been employed to model hydride NMR chemical shifts for a series of [Ru(NHC)4(L)H]0/+ species (NHC = N-heterocyclic carbene; L = vacant, H2, N2, CO, MeCN, O2, P4, SO2, H-, F- and Cl-), as well as selected phosphine analogues [Ru(R2PCH2CH2PR2)2(L)H]+ (R = iPr, Cy; L = vacant, O2). Inclusion of spin-orbit coupling provides good agreement with the experimental data. For the NHC systems large variations in hydride chemical shift are shown to arise from the paramagnetic term, with high net shielding (L = vacant, Cl-, F-) being reinforced by the contribution from spin-orbit coupling. Natural chemical shift analysis highlights the major orbital contributions to the paramagnetic term and rationalizes trends via changes in the energies of the occupied Ru dπ orbitals and the unoccupied σ*Ru-H orbital. In [Ru(NHC)4(η2-O2)H]+ a δ-interaction with the O2 ligand results in a low-lying LUMO of dπ character. As a result this orbital can no longer contribute to the paramagnetic shielding, but instead provides additional deshielding via overlap with the remaining (occupied) dπ orbital under the Lz angular momentum operator. These two effects account for the unusual hydride chemical shift of +4.8 ppm observed experimentally for this species. Calculations reproduce hydride chemical shift data observed for [Ru(iPr2PCH2CH2PiPr2)2(η2-O2)H]+ (δ = -6.2 ppm) and [Ru(R2PCH2CH2PR2)2H]+ (ca. -32 ppm, R = iPr, Cy). For the latter, the presence of a weak agostic interaction trans to the hydride ligand is significant, as in its absence (R = Me) calculations predict a chemical shift of -41 ppm, similar to the [Ru(NHC)4H]+ analogues. Depending on the strength of the agostic interaction a variation of up to 18 ppm in hydride chemical shift is possible and this factor (that is not necessarily readily detected experimentally) can aid in the interpretation of hydride chemical shift data for nominally unsaturated hydride-containing species. The synthesis and crystallographic characterization of the BArF4- salts of [Ru(IMe4)4(L)H]+ (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; L = P4, SO2; ArF = 3,5-(CF3)2C6H3) and [Ru(IMe4)4(Cl)H] are also reported.

Rhodium(I) and Iridium(I) Complexes of the Conformationally Rigid IBioxMe4 Ligand: Computational and Experimental Studies of Unusually Tilted NHC Coordination Geometries

Computational methods have been used to analyze distorted coordination geometries in a coherent range of known and new rhodium(I) and iridium(I) complexes containing bioxazoline-based NHC ligands (IBiox). Such distortions are readily placed in context of the literature through measurement of the Cnt(NHC)–CNCN–M angle (ΘNHC; Cnt = ring centroid). On the basis of restricted potential energy calculations using cis-[M(IBioxMe4)(CO)2Cl] (M1; M = Rh, Ir), in-plane (yawing) tilting of the NHC was found to incur significantly steeper energetic penalties than orthogonal out-of-plane (pitching) movement, which is characterized by noticeably flat potential energy surfaces. Energy decomposition analysis (EDA) of the ground-state and pitched structures of M1 indicated only minor differences in bonding characteristics. In contrast, yawing of the NHC ligand is associated with a significant increase in Pauli repulsion (i.e., sterics) and reduction in M→NHC π back donation, but is counteracted by supplemental stabilizing bonding interactions only possible due to the closer proximity of the methyl substituents with the metal and ancillary ligands. Aided by this analysis, comparison with a range of carefully selected model systems and EDA, distorted coordination modes in trans-[M(IBioxMe4)2(COE)Cl] (M2; M = Rh, Ir) and [M(IBioxMe4)3]+ (M3; M = Rh, Ir) have been rationalized. Steric interactions were identified as the major contributing factor and are associated with a high degree of NHC pitching. In the case of Rh3, weak agostic interactions also contribute to the distortions, particularly with respect to NHC yawing, and are notable for increasing the bond dissociation energy of the distorted ligands. Supplementing the computational analysis, an analogue of the formally 14 VE Rh(I) species Rh3 bearing the cyclohexyl-functionalized IBiox6 ligand ([Rh(IBiox6)3]+, Rh3-Cy) was prepared and found to exhibit an exceptionally distorted NHC ligand (ΘNHC = 155.7(2)°) in the solid state.

Synthesis of a 14-electron iridium(III) complex with a xanthene-based bis(silyl) chelate ligand (xantsil): A distorted seesaw-shaped four-coordinate geometry and reactions leading to 16-electron complexes

Synthesis, structure determination, and reactions of a 14-electron four-coordinate iridium(III) complex bearing a xanthene-based bis(silyl) chelate ligand, i.e., Ir{κ2(Si,Si)-xantsil}(PCy3)Cl (1a, xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)), are reported. A precursor of 1a, the 16-electron (dihydrido)iridium(V) complex Ir{κ2(Si,Si)-xantsil}(H)2(PCy3)Cl (2), was prepared by the reaction of [IrCl-(coe)2]2 (coe = cyclooctene) with 4,5-bis(dimethylsilyl)-9,9-dimethylxanthene (xantsilH2) and PCy3. Dehydrogenation reaction of 2 with 3,3-dimethylbut-1-ene, a hydrogen acceptor, afforded a mixture of 1a and its isomer 1b, abbreviated as 1a + 1b, together with 2,2-dimethylbutane. Single crystals obtained from a CH2Cl2 solution of 1a + 1b contained only isomer 1a. X-ray crystal structure analysis of one of the crystals revealed that 1a adopts a distorted seesaw-shaped four-coordinate geometry where the coordinatively unsaturated metal center is stabilized by weak agostic interaction of two γ-C–H bonds of the PCy3 ligand. On the other hand, NMR spectroscopic analysis of 1a + 1b demonstrated that 1a is in fast equilibrium with a minor amount of 1b in solution. Replacement of the chloro ligand in complexes 1a + 1b by a triflato ligand with AgOTf (OTf = OSO2CF3) afforded quantitatively a single product, i.e., the 16-electron iridium–triflato complex Ir{κ3(Si,O,Si)-xantsil}(PCy3)(OTf) (3). 1a + 1b and 16-electron complex 2 are interconvertible via oxidative addition of dihydrogen (from 1a + 1b to 2) and alkene hydrogenation (from 2 to 1a + 1b). Complexes 1a + 1b and 2 were found to catalyze hydrogenation of 3,3-dimethylbut-1-ene.

Rac-{[2-(Diphenyl-thio-phosphan-yl)ferrocen-yl]meth-yl}trimethyl-ammonium iodide chloro-form monosolvate.

The title compound, [Fe(C5H5)(C21H24NPS)]I·CHCl3, is built up from a (ferrocenylmeth­yl)trimethyl­ammonium cation, a iodine anion and a chloro­form solvent mol­ecule, all residing in general positions. The N atom of the ammonium group is displaced by 1.182 (2) Å from the plane of the substituted cyclo­penta­dienyl (Cp) ring towards the Fe atom, whereas the C atom attached to the same Cp ring is slightly below this plane by −0.128 (2) Å. These deviations might result from weak agostic interactions between the two H atoms of the CH2 group and the Fe atom.

Supramolecular Architecture in Gold(I) and Gold(III) 2-Pyridineformamide Thiosemicarbazone Complexes by Secondary Interactions: Synthesis, Structures, and Luminescent Properties

Reaction of the thiosemicarbazone ligands 2-pyridineformamide thiosemicarbazone (HAm4DH) or 2-pyridineformamide 3-hexamethyleneiminylthiosemicarbazone (HAm4hexim) with the gold(I) compound [AuCl(tdg)] (tdg = thiodiglycol = 2,2′-thiodiethanol) gives the complexes [Au(H2Am4DH)2]Cl3·H2O (1) and [Au(HAm4hexim)2]Cl (2). The room temperature oxidation of solutions of 2 in water/methanol leads to the crystallization of a gold(III) complex of formula [Au(Am4hexim)Cl]Cl·2H2O (3). Single-crystal X-ray diffraction analysis revealed that in compounds 1 and 2 gold(I) is linearly coordinated to sulfur atoms from two monodentate thiosemicarbazone molecules. Packing architectures and hydrogen bonding networks obtained using graph set notations are discussed for all complexes. Weak agostic interactions exist in both 1 [(N–H)···Au; H···Au = 2.70 and 2.82 Å] and 2 [(C–H)···Au; H···Au = 2.79 Å], and gold–gold contacts are present in the solid state in 1 (Au···Au = 3.83 Å) but not in 2 (Au···Au = 5.46 Å). The fluorescence of the ligands and complexes of gold(I) was studied. In addition, elemental analysis data and spectroscopic properties in the solid state and in solution are also described for complexes 1 and 2.

Microwave synthesis of bis(tetrazolato)-Pd II complexes with PPh 3 and water-soluble 1,3,5-triaza-7-phosphaadamantane (PTA). The first example of C–CN bond cleavage of propionitrile by a Pd II Centre

[2 + 3] Cycloaddition reactions of the di(azido)-Pd II complex trans-[Pd(N 3) 2(PPh 3) 2] ( 1) with an organonitrile RCN ( 2), under heating for 12 h, give the bis(tetrazolato) complexes trans-[Pd(N 4CR) 2(PPh 3) 2] ( 3) [R = Me ( 3a), Ph ( 3b), 4-ClC 6H 4 ( 3c), 4-FC 6H 4 ( 3d), 2-NC 5H 4 ( 3e), 3-NC 5H 4 ( 3f), 4-NC 5H 4 ( 3g)]. The reaction of trans-[Pd(N 3) 2(PPh 3) 2] ( 1) with propionitrile ( 2h) also affords, apart from trans-[Pd(N 4CEt) 2(PPh 3) 2] ( 3h), the unexpected mixed cyano-tetrazolato complex trans-[Pd(CN)(N 4CEt)(PPh 3) 2] ( 3h′) which is derived from the reaction of the bis(tetrazolato) 3h with propionitrile, with concomitant formation of 5-ethyl-1 H-tetrazole, via a suggested unusual oxidative addition of the nitrile to Pd II. The [2 + 3] cycloadditions of [Pd(N 3) 2(PTA) 2] ( 4) (PTA = 1,3,5-triaza-7-phosphaadamantane) with RCN ( 2), under heating for 12 h, give the bis(tetrazolato) complexes trans-[Pd(N 4CR) 2(PTA) 2] ( 5) [R = Ph ( 5a), 2-NC 5H 4 ( 5b), 3-NC 5H 4 ( 5c), 4-NC 5H 4 ( 5d)]. All these reactions are greatly accelerated by microwave irradiation (1 h, 125 °C, 300 W). Taking advantage of the hydro-solubility of PTA, a simple liberation of 5-phenyl-1 H-tetrazole from the coordination sphere of trans-[Pd(N 4CPh) 2(PTA) 2] ( 5a) was achieved. The complexes were characterized by IR, 1H, 13C{ 1H} and 31P{ 1H} NMR spectroscopies, ESI +-MS, elemental analyses and, for 3b, also by X-ray structure analysis. Weak agostic interactions between the CH groups of the triphenylphosphines and the palladium(II) centre were found.

Synthesis, Structures, and Reactions of Manganese Complexes Containing Diphosphine Ligands with Pendant Amines

Addition of the pendant amine ligand PNRP (PNRP = Et2PCH2NRCH2PEt2; R = Me, Ph, n-Bu) to Mn(CO)5Br gives fac-Mn(PNRP)(CO)3Br. Photolysis of fac-Mn(PNRP)(CO)3Br with dppm [dppm = 1,2-bis(diphenylphosphino)methane] provides mixed bis(diphosphine) complexes, trans-Mn(PNRP)(dppm)(CO)(Br). Reaction of trans-Mn(PNRP)(dppm)(CO)(Br) with LiAlH4 leads to trans-Mn(PNRP)(dppm)(CO)(H). The crystal structure of trans-Mn(PNMeP)(dppm)(CO)(H) determined by x-ray diffraction shows an unusual distortion of the Mn-H towards one C-H of the dppm ligand, resulting in an H Mn CO angle of 155(1)° and C H • • • H Mn distance of 2.10(3) A. Mn(P2PhN2Bn)(dppm)(CO)(H) [P2PhN2Bn = 1, 5-diphenyl-3,7-dibenzyl-1,5-diaza-3,7-diphosphacyclooctane] can be prepared in a similar manner; its structure has one chelate ring in a chair conformation and the second in a boat conformation. The boat-conformer ring directs the nitrogen of the ring towards the carbonyl ligand, and the N • • • C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) A, indicating a weak interaction between the N of the pendant amine and the CO ligand. Reaction of NaBArF4 (ArF = = 3,5-bis(trifluoromethyl)phenyl) with Mn(P P)(dppm)(CO)(Br) produces the cations [Mn(P P)(dppm)(CO)]+. The crystal structure of [Mn(PNMeP)(dppm)(CO)][BArF4] shows two very weak agostic interactions between C-H bonds on the phenyl ring and the Mn. The cationic complexesmore » [Mn(P P)(dppm)(CO)]+ react with H2 to form dihydrogen complexes [Mn(H2)(P P)(dppm)(CO)]+ (Keq = 1 - 90 atm-1 in fluorobenzene, for a series of different P P ligands). Similar equilibria with N2 produce [Mn(N2)(P P)(dppm)(CO)]+ (Keq generally 1-3.5 atm-1 in fluorobenzene). This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

Molybdenum and Tungsten Nitrosyl Complexes in Hydrogen Activation

AbstractTwo transition‐metal hydride complexes of the type [M(dippe)2(NO)(H)] [M = W (2a), Mo (2b); dippe = 1,2‐bis(diisopropylphosphanyl)ethane] have been prepared by the reaction of [M(dippe)2(NO)(Cl)] [M = W (1a), Mo (1b)] with LiBH4. The nitrosyl groups of the tungsten complexes 1a and 2a are capable to coordinate a LiBH4 molecule to form the stable adducts W(dippe)2(Cl)(NO···LiBH4) (1c) and W(dippe)2(H)(NO···LiBH4) (2c). Addition of ethylenediamine to a toluene solution of 2c led to rupture of the 2c adduct and afforded 2a in good yield. After the interaction of 2a with [H(Et2O)][BF4], the stable seven‐coordinated cationic dihydride [W(dippe)(H)2(dippe)(NO)][BF4] (6a) was isolated. The reaction between 2b and [H(Et2O)][BF4] led to the formation of [Mo(dippe)2(NO)(FBF3)] (6b) in which BF4– was found to be coordinated to the metal centre. The [H(Et2O)2][BArF4] [ArF = 3,5‐(CF3)2C6H3] acid interacted with 2a to yield the seven‐coordinated complex [W(dippe)(H)2(dippe)(NO)][BArF4] (5a) similar to 6a. In the reaction between 2b and [H(Et2O)2][BArF4], the 16e– five‐coordinated complex [Mo(dippe)2(NO)][BArF4] (4b) was formed. X‐ray diffraction revealed that 4a has a weak agostic interaction trans to the NO ligand. Complex 4b was found to react rapidly with hydrogen gas under ambient conditions to form the dihydride complex [Mo(dippe)(H)2(dippe)(NO)][BArF4] (5b), which is unstable in the absence of a hydrogen atmosphere. The equilibrium constant for the reversible reaction of 4b with hydrogen was found to be K = 2.6 bar–1 at 25 °C. Complex 4b was tested as a catalyst for acetone hydrogenation; a maximum TON of 7 was found.

Synthesis and Structural Characterization of Novel Thulium(III) Cyclic Organohydroborate Complex with a Weak Agostic Interaction (THF)3Tm{(μ-H)2(BC8H14) }3

Preparation of (THF) 3Tm{(µ-H)2BC8H14} 3. In the drybox 275.3 mg (1.0 mmol) of TmCl3 and 480.3 mg (3.0 mmol) of K(H2BC8H14) were put into a flask. After degassing, 30 mL of THF was transferred into the flask at -78 o C. The solution was warmed to room temperature and stirred for 24 hours. During this process the solution turned cloudy due to the formation of KCl which was removed by filtration. Volatile components were removed by means of dynamic high vacuum leaving a white solid. The solid was redissolved in ether and the solution was filtered to remove impurities. A white solid (THF)3Tm- {(µ-H)2BC8H14}3 was obtained in 65% yield upon removal of the solvent under vacuum. Crystal, suitable for X-ray diffrac- tion, was obtained by crystallization from toluene. IR (KBr) 2922(s), 2856(s), 1702(m), 1660(m), 1446(m), 1012(m), 670(m) cm -1 .

The reaction of the bis(6,6-dimethylcyclohexadienyl)zirconium fragment with PhC 2SiMe 3 – A 5 + 2 + 2 ring construction

The reaction of the edge-bridged open zirconocene, Zr(6,6-dmch) 2(PMe 3) 2 (dmch = dimethylcyclohexadienyl) with two equivalents of PhC 2SiMe 3 leads to a 5 + 2 + 2 ring construction, resulting in a 4.3.1 bicyclodecadienyl skeleton. The resulting complex, with η 3-allyl, η 4-diene, and η 5-dienyl coordination, has a formal 16 electron configuration, and contains close contacts between the metal center and two lengthened C–C bonds, indicative of weak agostic interactions.

Synthesis and Coordination Chemistry of a New Chiral Tridentate PCP N-Heterocyclic Carbene Ligand Based on a Ferrocene Backbone

The new chiral imidazolium salt 1,3-bis[(R)-1-((S)-2-diphenylphosphinoferrocenyl)ethyl]imidazolium iodide (4, (PCPH)I) was prepared in three steps from commercially available N,N-dimethyl-1-ferrocenylethylamine (1) in 55% overall yield. Salt 4 is the precursor for the in situ generation of a novel tridentate carbene ligand that could not be isolated upon deprotonation of 4 with NaOtBu. Reaction of 4 with [Pd(OAc)2]3 afforded (SP-4)-1,3-bis[(R)-1-((S)-2-diphenylphosphino-κP-ferrocenyl)ethyl]imidazol-2-ylideneiodopalladium(II) acetate, [PdI(PCP)]OAc (6). Treatment of 4 with NaOtBu and [RuCl2(PPh3)3] gave the mixed halide complex (SP-5)-1,3-bis[(R)-1-((S)-2-diphenylphosphino-κP-ferrocenyl)ethyl]imidazol-2-ylidenechloroiodoruthenium(II) [RuClI(PCP)] (7a), as a mixture of two isomers in 60% yield. Additional complexes that could be obtained from 4 are [PdCl(PCP)]PF6 (5), [RuCl2(PCP)] (7b), and [RuI(PCP)(NCCH3)2]PF6 (8). The crystal and molecular structures of 5 and 7a were determined by X-ray diffraction. The Ru(II) center undergoes a weak agostic interaction with one of the two stereogenic methine units.

Synthesis, Structure, Theoretical Studies, and Ligand Exchange Reactions of Monomeric, T-Shaped Arylpalladium(II) Halide Complexes with an Additional, Weak Agostic Interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.

Ruthenium complexes with N(SPR2)2− (R = Ph or Pri)

Reactions of [Ru(PPh3)3Cl2], [Ru(CO)2Cl2]x, or [Ru(dmso)4Cl2] (dmso = dimethyl sulfoxide) with KLR [LR = N(SPR2)2, R = Ph or Pri] afforded [Ru(LR)2(PPh3)] (R = Ph 1 or Pri2), cis-[Ru(LR)2(CO)2] (L = Ph 3 or Pri4), or cis-[Ru(LPh)2(dmso)2] 5, respectively. The crystal structures of complexes 1 and 2 have been determined. They show weak agostic interaction between Ru and LR with calculated Ru⋯H–C separations of 3.37 and 2.91 A, respectively. The Ru–P and average Ru–S distances in 1 are 2.218(1) and 2.400 A, respectively. The corresponding bond lengths for 2 are 2.210(2) and 2.404 A. Treatment of 2 with ButNC afforded trans-[Ru(LPr)2(ButNC)2] 6, the average Ru–S and Ru–C distances of which are 2.453 and 1.990(3) A, respectively. Reaction of RuCl3 with KLR in methanol gave the homoleptic complexes [Ru(LR)3] (L = Ph 7 or Pri8). The average Ru–S distance and S–Ru–S angle in 7 are 2.414 A and 97.41°, respectively. While complex 1 reacts with pyridine (py) to give [Ru(LPh)2(PPh3)(py)] 9, reaction of 2 with py led to isolation of structurally characterised [Ru(LPr)2(SO)] 10. The Ru–S(O) and S–O bond lengths in 10 are 2.0563(11) and 1.447(3) A, respectively, the Ru–S–O angle being 125.5(2)°. Treatment of 1 with SO2 afforded structurally characterised cis-[Ru(LPh)2(PPh3)(SO2)] 11. The SO2 ligand binds to Ru in 11 in a η1-S mode and the Ru–S(O) distance is 2.140(4) A. Complex 2 reacted with SO2 to give the μ-sulfato-bridged ruthenium(III) dimer [{Ru(LPr)(PPh3)}2(μ-SO4)2] 12, which has been characterised by X-ray crystallography. The Ru–P and average Ru–S and Ru–O distances in 12 are 2.294(2), 2.321 and 2.133 A, respectively. Complex 1 is capable of catalysing hydrogenation of styrene in the presence of Et3N presumably via a ruthenium hydride intermediate.