Neutral Dihydride Research Articles

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion.

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

Hydrosilylation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic η1-Silane Iridium(III) Complex

Hydrosilation of a variety of ketones and aldehydes using the cationic iridium catalyst, (POCOP)Ir(H)(acetone)(+), 1, (POCOP = 2,6-bis(di-tert-butyl phosphinito)phenyl) is reported. With triethyl silane, all but exceptionally bulky ketones undergo quantitative reactions employing 0.5 mol% catalyst in 20-30 min at 25 °C. Hydrosilation of esters and amides results in over-reduction and cleavage of C-O and C-N bonds, respectively. The diastereoselectivity of hydrosilation of 4-tert-butyl cyclohexanone has been examined using numerous silanes and is highly temperature dependent. Using EtMe(2)SiH, analysis of the ratio of cis:trans hydrosilation products as a function of temperature yields values for ΔΔH(‡) (ΔH(‡) (trans) - ΔH(‡) (cis)) and ΔΔS(‡) (ΔS(‡) (trans) - ΔS(‡)(cis)) of -2.5 kcal/mol and -6.9 e.u., respectively. Mechanistic studies show that the ketone complex, (POCOP)Ir(H)(ketone)(+), is the catalyst resting state and is in equilibrium with low concentration of the silane complex, (POCOP)Ir(H)(HSiR(3))(+). The silane complex transfers R(3)Si(+) to ketone forming the oxocarbenium ion, R(3)SiOCR'(2) (+), which is reduced by the resulting neutral dihydride 3, (POCOP)Ir(H)(2), to yield product R(3)SiOCHR'(2) and (POCOP)IrH(+) which closes the catalytic cycle.

Neutral and Cationic Hydridoruthenium Tetrakiscarbene Complexes

AbstractStarting from the novel chlorido precursor trans‐[RuCl2(IMe)4] (1, IMe = 1,3,4,5‐tetramethylimidazol‐2‐ylidene), hydridoruthenium complexes trans‐[RuH2(IMe)4] (2) and [RuH(IMe)4][BEt4] (3BEt4) have been synthesized. Complex 2 was isolated from the reaction of 1 with LiAlH4, while ionic compound 3 was obtained when LiBHEt3 was used as the hydride source. Complexes 1–3 have been characterized by X‐ray crystallography, multinuclear NMR, IR, UV/Vis spectroscopy and mass spectrometry. Neutral dihydride 2 displays a tetragonal bipyramidal geometry with four carbene groups coordinated to ruthenium in equatorial positions and two apical hydrogen atoms. DFT calculations suggest that the trans structure observed for 2 is preferred over a cis arrangement for steric reasons. The ruthenium atom of 3 has a tetragonal pyramidal coordination environment with a vacant coordination site sterically protected by the Me substituents of the ligands. Thus, compound 3 should be an attractive target for future coordination studies.

Inactivity of iridium–triphenylphosphine complexes for catalytic imine hydrogenation: formation of neutral, dihydrido-ortho-metallated species

Reactions of cis,trans,cis-[Ir(H)2(PPh3)2(MeOH)2]PF6 in MeOH with the imines Ph(R′)C=NR under 1 atm H2 precipitate the neutral, Ir(III)-dihydrido complexes [Ir(H)2{RN=C(R′)(o-C6H4)}(PPh3)2], where R = alkyl or benzyl, and R′ = Ph, H, or Me; the dihydrides are well characterized through elemental analysis, NMR and IR data and, in one case, with the imine Ph2C=NCH2Ph, an X-ray structure. These products are formed via intermediate ortho-metallated species, exemplified by [Ir(H){PhCH2N=CPh(o-C6H4)}(PPh3)2(MeOH)]PF6 in the case of the Ph2C=NCH2Ph reaction; the intermediate then reacts with H2 (via net heterolytic cleavage: H2 → H− + H+) to give the neutral dihydride and HPF6 as co-product. The ‘unique’ imine PhCH=NPh, under the same conditions, does not form a dihydride and instead is readily catalytically hydrogenated to PhCH2N(H)Ph; remarkably, we have shown previously that this imine is ‘unique’ within the corresponding Rh system in that no catalytic hydrogenation occurs because the amine product ‘poisons’ the Rh centre by coordination through the N-phenyl ring.

Scope and Mechanism of the Iridium-Catalyzed Cleavage of Alkyl Ethers with Triethylsilane

The cationic iridium pincer complex [(POCOP)Ir(H)(acetone)](+)[B(C(6)F(5))(4)](-) {1, POCOP = 2,6-[OP(tBu)(2)](2)C(6)H(3)} was found to be a highly active catalyst for the room-temperature cleavage and reduction of a wide variety of unactivated alkyl ethers including primary, secondary, and tertiary alkyl ethers as well as aryl alkyl ethers by triethylsilane. Mechanistic studies have revealed the full details of the catalytic cycle with the catalyst resting state(s) depending on the basicity of the alkyl ether. During the catalytic reduction of diethyl ether, cationic iridium silane complex, [(POCOP)Ir(H)(eta(1)-Et(3)SiH)](+)[B(C(6)F(5))(4)](-) (3), and Et(2)O are in rapid equilibrium with neutral dihydride, (POCOP)Ir(H)(2) (5) and diethyl(triethylsilyl)oxonium ion, [Et(3)SiOEt(2)](+)[B(C(6)F(5))(4)](-) (7), with 5 + 7 strongly favored. Species 7 has been isolated from the reaction mixture and fully characterized. The turnover-limiting step in this cycle is the reduction of 7 by the neutral dihydride 5. The relative rates of reduction of 7 by dihydride 5 and Et(3)SiH were determined to be approximately 30,000:1. In the cleavage of the less basic ethers anisole and EtOSiEt(3), the cationic iridium silane complex, 3, was found to be the catalyst resting state. The hydride reduction of the intermediate oxonium ion EtO(SiEt(3))(2)(+), 9, occurs via attack by Et(3)SiH. In the case of anisole, the intermediate PhMeOSiEt(3)(+), 10, is reduced by 5 and/or Et(3)SiH.

A para-Hydrogen Investigation of Palladium-Catalyzed Alkyne Hydrogenation

The complexes [Pd(bcope)(OTf)2] (1a), where bcope is (C8H14)PCH2-CH2P(C8H14), and [Pd(tbucope)(OTf)2] (1b), where tbucope is (C8H14)PC6H4CH2P(tBu)2, catalyze the conversion of diphenylacetylene to cis- and trans-stilbene and 1,2-diphenylethane. When this reaction was studied with para-hydrogen, the characterization of [Pd(bcope)(CHPhCH2Ph)](OTf) (2a) and [Pd(tbucope)(CHPhCH2Ph)](OTf) (2b) was achieved. Magnetization transfer from the alpha-H of the CHPhCH2Ph ligands in these species proceeds into trans-stilbene. This process has a rate constant of 0.53 s-1 at 300 K in methanol-d4 for 2a, where DeltaH = 42 +/- 9 kJ mol-1 and DeltaS = -107 +/- 31 J mol-1 K-1, but in CD2Cl2 the corresponding rate constant is 0.18 s-1, with DeltaH = 79 +/- 7 kJ mol-1 and DeltaS = 5 +/- 24 J mol-1 K-1. The analogous process for 2b was too fast to monitor in methanol, but in CD2Cl2 the rate constant for trans-stilbene formation is 1.04 s-1 at 300 K, with DeltaH = 94 +/- 6 kJ mol-1 and DeltaS = 69 +/- 22 J mol-1 K-1. Magnetization transfer from one of the two inequivalent beta-H sites of the CHPhCH2Ph moiety proceeds into trans-stilbene, while the other site shows transfer into H2 or, to a lesser extent, cis-stilbene in CD2Cl2, but in methanol it proceeds into the vinyl cations [Pd(bcope)(CPh=CHPh)(MeOD)](OTf) (3a) and [Pd(tbucope)(CPh=CHPh)(MeOD)](OTf) (3b). When the same magnetization transfer processes are monitored for 1a in methanol-d4 containing 5 microL of pyridine, transfer into trans-stilbene is observed for two sites of the alkyl, but the third proton now becomes a hydride ligand in [Pd(bcope)(H)(pyridine)](OTf) (5a) or a vinyl proton in [Pd(bcope)(CPh=CHPh)(pyridine)](OTf) (4a). For 1b, under the same conditions, two isomers of [Pd(tbucope)(H)(pyridine)](OTf) (5b and 5b') and the neutral dihydride [Pd(tbucope)(H)2] (7) are detected. The single vinylic CH proton in 3 and the hydride ligands in 4 and 5 appear as strong emission signals in the corresponding 1H NMR spectra.

Protonation reactions of dinuclear pyrazolato iridium(I) complexes.

The complex [[Ir(mu-Pz)(CNBu(t))(2)](2)] (1) undergoes double protonation reactions with HCl and with HO(2)CCF(3) to give the neutral dihydride complexes [[Ir(mu-Pz)(H)(X)(CNBu(t))(2)](2)] (X = Cl, eta(1)-O(2)CCF(3)), in which the hydride ligands were located trans to the X groups and in the boat of the complexes, both in the solid state and in solution. The complex [[Ir(mu-Pz)(H)(Cl)(CNBu(t))(2)](2)] evolves in solution to the cationic complex [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]Cl. Removal of the anionic chloride by reaction with methyltriflate allows the isolation of the triflate salt [[Ir(mu-Pz)(H)(CNBu(t))(2)](2)(mu-Cl)]OTf. This complex undergoes a metathesis reaction of hydride by chloride in CDCl(3) under exposure to the direct sunlight to give the complex [[Ir(mu-Pz)(Cl)(CNBu(t))(2)](2)(mu-Cl)]OTf. Protonation of both metal centers in [[Ir(mu-Pz)(CO)(2)](2)] with HCl occurs at low temperature, but eventually the mononuclear compound [IrCl(HPz)(CO)(2)] is isolated. The related complex [[Ir(mu-Pz)(CO)(P[OPh](3))](2)] reacts with HCl and with HO(2)CCF(3) to give the neutral Ir(III)/Ir(III) complexes [[Ir(mu-Pz)(H)(X)(CO)(P[OPh](3))](2)], respectively. Both reactions were found to take place stepwise, allowing the isolation of the intermediate monohydrides. They are of different natures, i.e., the metal-metal-bonded Ir(II)/Ir(II) compound [(P[OPh](3))(CO)(Cl)Ir(mu-Pz)(2)Ir(H)(CO)(P[OPh](3))] and the mixed-valence Ir(I)/Ir(III) complex [(P[OPh](3))(CO)Ir(mu-Pz)(2)Ir(H)(eta(1)-O(2)CCF(3))(CO)(P[OPh](3))].

New osmium hydrides containing the bulky phosphine 1,2-bis(diisopropylphosphino)ethane (dippe)

A range of hydride complexes of osmium(II) and osmium(IV) containing the bulky chelating phosphine dippe have been prepared and characterized. The cationic dichlorohydridoosmium(IV) [OsHCl 2(dippe) 2][BPh 4] 1 was obtained by reaction of five-coordinate [OsCl(dippe) 2][BPh 4] with HCl in dichloromethane, whereas the trihydride [OsH 3(dippe) 2][BPh 4] 2 was prepared by reaction of cis-[OsCl 2(dippe) 2] with NaBH 4/NaBPh 4 in ethanol. This compound has shown to be a ‘classical’ trihydride Os IV complex rather than an Os II hydrido(dihydrogen) derivative, being reversibly deprotonated by KOBu t to yield the neutral dihydride cis-[OsH 2(dippe) 2] 3. cis-[OsCl 2(dippe) 2] reacted with Li[HBEt 3] in THF affording the chlorohydride trans-[OsHCl(dippe) 2] 4. This species dissociates chloride furnishing the 16-electron cationic monohydride [OsH(dippe) 2] +, which was detected in solution, but not isolated due to its great tendency to react with traces of oxygen to yield the hydrido(dioxygen) complex trans-[OsH(O 2)(dippe) 2][BPh 4] 5, which was fully characterized.

Protonation of Metal−Metal Bonds in Dinuclear Iridium Complexes: Consequences for Structure and Reactivity

Protonation of the carbonyl-bridged dimer [Cp*Ir(μ-CO)]2 (Cp* = η5-C5Me5) (1) with 1 equiv of the strong acid HBAr‘4·2Et2O (Ar‘= 3,5-(CF3)2C6H3) generates the hydride- bridged species {[Cp*Ir(CO)]2(μ-H)}BAr‘4 (2). Addition of excess acid to 1 results in the formation of the dihydride dication [Cp*Ir(CO)]2(μ-H)22+ (3). The monoprotonated cation, 2, reacts rapidly with CO or H2 to afford [Cp*Ir(CO)]2(μ-CO)(μ-H)+ (4) and [Cp*Ir(CO)H]2(μ-H)+ (5), respectively. The structures of the cationic dimers 4 and 5 have been confirmed by X-ray diffraction studies. Deprotonation of complex 5 generates the neutral dihydride dimer [Cp*Ir(CO)H]2 (6), a compound with an unsupported Ir−Ir bond. An X-ray diffraction analysis of 6 reveals an Ir−Ir bond length of 2.730(1) A. Formal protonation of the Ir−Ir bond of 6 to give 5 results in an increase of approximately 0.198 A in the length of that bond.

Dinuclear dihydride complexes of iridium: a study of structure and dynamics

Reaction of the neutral dihydride complexes (η-C5R5)Ir(L)H2(L = P(OPh)3, CO; R = H, Me) with triflic acid (HO3SCF3) at ambient temperature or above affords hydrogen and the dimeric hydride bridged species {[(η-C5R5)Ir(L)H]2(µ-H)}O3SCF3. Deprotonation of the cationic complexes gives the neutral dimers of the form [(η-C5R5)Ir(L)H]2. Spectroscopic data are consistent with the presence of only terminal hydride ligands in these complexes. Variable temperature 1H and 31P NMR studies indicate that a rapid dynamic process exchanges the two terminal hydride ligands and that the complexes exist as unequal mixtures of the racemic and meso diastereomers. Synthesis of a lower symmetry derivative, [(η-C5Me4Et)Ir(CO)H]2, reveals that a rapid epimerization process occurs that interconverts the two diastereomers. Thermolysis of the carbonyl complex [(η-C5Me5)Ir(CO)H]2 affords [(η-C5Me5)Ir(CO)]2 and dihydrogen. Keywords: iridium, hydride, bimetallic, dynamics, epimerization.

Structural isomers of H 2Ru 4C(CO) 12; X-ray crystal structure of Ru 4C(CO) 13

The anionic hydrido cluster [HRu 4C(CO) 12] − and the neutral dihydride H 2Ru 4C(CO) 12 have been prepared in good yield. Spectroscopic studies of H 2Ru 4C(CO) 12 indicate that two isomers are formed, one with a structure similar to that of H 2Fe 4C(CO) 12 the other with metal-bonded hydrido ligands. The parent carbido cluster Ru 4C(CO) 13 has been synthesised from the hydrido anion, and has a structure similar to that of the analogous iron cluster Fe 4C(CO) 13.