Metal Hydrido Complexes Research Articles

Activation of H–H, HO–H, C(sp2)–H, C(sp3)–H, and RO–H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)2(μ-Cl)2] (Cp* = η5-C5Me5) with Ph2PCH2CH2NC(NH(p-Tolyl))2 (H2L) in the presence of NaSbF6 affords the chlorido complexes [Cp*MCl(κ2N,P-H2L)][SbF6] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ3N,N′,P-HL)][SbF6] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ3N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ2N,P-H2L)][SbF6] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H2O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ3C,N,P-H2L-H)][SbF6] [M = Rh, 7; Ir, 8, H2L-H = Ph2PCH2CH2NC(NH(p-Tolyl))(NH(4-C6H3Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

Synthesis, Characterization, and Reactivity of a Hydrido- and Imido-Bridged Dinuclear Ytterbium(III) Complex.

The trivalent rare-earth metal hydrido and imido complexes are of versatile reactivity, and many such complexes have been synthesized. However, no example of a rare-earth metal complex bearing both hydrido- and imido-ligands has been reported. Herein, we report the first rare-earth metal complex bearing both hydrido- and imido-ligands, namely a hydrido- and imido-bridged dinuclear ytterbium(III) complex. The complex was synthesized via an unprecedented redox reaction of divalent rare-earth metal hydrido complex with azido compound. DFT calculation indicated that the N2 release from azido compound in the presence of ytterbium(II) is a kinetically facile process because of the cooperative effects of the two metal centers. The reactivity of the hydrido- and imido-bridged dinuclear ytterbium(III) complex was also explored, which showed the redox, addition and σ-bond metathesis reactivities.

Synthesis, Characterization, and Reactivity of a Hydrido‐ and Imido‐Bridged Dinuclear Ytterbium(III) Complex

AbstractThe trivalent rare‐earth metal hydrido and imido complexes are of versatile reactivity, and many such complexes have been synthesized. However, no example of a rare‐earth metal complex bearing both hydrido‐ and imido‐ligands has been reported. Herein, we report the first rare‐earth metal complex bearing both hydrido‐ and imido‐ligands, namely a hydrido‐ and imido‐bridged dinuclear ytterbium(III) complex. The complex was synthesized via an unprecedented redox reaction of divalent rare‐earth metal hydrido complex with azido compound. DFT calculation indicated that the N2 release from azido compound in the presence of ytterbium(II) is a kinetically facile process because of the cooperative effects of the two metal centers. The reactivity of the hydrido‐ and imido‐bridged dinuclear ytterbium(III) complex was also explored, which showed the redox, addition and σ‐bond metathesis reactivities.

Divalent Ytterbium Hydrido Complex Supported by a β-Diketiminato-Based Tetradentate Ligand: Synthesis, Structure, and Reactivity.

While the chemistry of trivalent rare-earth metal hydrido complexes has been well developed in the past 40 years, that of the divalent rare-earth metal hydrido complexes remains in its infancy because of the synthetic challenge of such complexes. In this paper, we report the synthesis and structural characterization of a divalent ytterbium hydrido complex supported by a bulky β-diketiminato-based tetradentate ligand. This hydrido complex is a dimer containing two μ-hydrogen ligands, and it easily undergoes a hydrido shift reaction to form a new divalent ytterbium hydrido complex that contains only one hydrido bridge. Furthermore, this hydrido complex reacts with pyridine and pyridine derivatives, showing versatile reactivity [Yb-H addition to pyridine, hydrido shift to ancillary ligand, and ytterbium(II)-center-induced redox reaction with bipyridine]. This hydrido complex reacts with Ph3P═O, resulting in a P-CPh cleavage of Ph3P═O and an elimination of C6H6; on the other hand, the reaction with Ph3P═S is a hydrido coupling-based redox reaction. The reactions of this hydrido complex with 1 and 2 equiv of PhSSPh clearly indicate that the hydrido coupling-based redox reaction is prior to the ytterbium(II) oxidation-based redox reaction.

Ligands Based on Phosphine-Stabilized Aluminum(I), Boron(I), and Carbon(0).

A systematic quantum chemical study of the bonding in d6 -transition-metal complexes, containing phosphine-stabilized, main-group-element fragments, (R3 P)2 E, as ligands (E=AlH, BH, CH+ , C), is reported. By using energy decomposition analysis, it is demonstrated that a strong M-E bond is accompanied by weak P-E bonds, and vice versa. Although the Al-M bond is, for example, found to be very strong, the weak Al-P bond suggests that the corresponding metal complexes will not be stable towards phosphine dissociation. The interaction energies for the boron(I)-based ligand are lower, but still higher than those for two-carbon-based ligands. For neutral ligands, electrostatic interactions are the dominating contributions to metal-ligand bonding, whereas for the cationic ligand a significant destabilization, with weak orbital and even weaker electrostatic metal-ligand interactions, is observed. Finally, for iron(II) complexes, it is demonstrated that different reactivity patterns are expected for the four donor groups: the experimentally observed reversible E-H reductive elimination of the borylene-based ligand (E=BH) exhibits significantly higher barriers for the protonated carbodiphosphorane (CDP) ligand (E=CH) and would proceed through different intermediates and transition states. For aluminum, such reaction pathways are not feasible (E=AlH). Moreover, it is demonstrated that the metal hydrido complexes with CDP ligands might not be stable towards reduction and isomerization to a protonated CDP ligand and a reduced metal center.

ChemInform Abstract: Rhodium‐ and Iridium‐Catalyzed Asymmetric Addition of Optically Pure P‐Chiral H‐Phosphinates to Aldehydes Leading to Optically Active α‐Hydroxyphosphinates.

Optically active α-hydroxyphosphinates with both C- and P-stereogenic centers are obtained by rhodium- or iridium-catalyzed substrate-directed stereoselective addition of the optically pure H-phosphinates to aldehydes. The reaction most probably proceeds by a transition-metal-catalyzed mechanism with hydridometal complexes as key intermediates in the catalytic cycle.

Rhodium- and Iridium-Catalyzed Asymmetric Addition of Optically Pure P-Chiral H-Phosphinates to Aldehydes Leading to Optically Active α-Hydroxyphosphinates.

Optically active α-hydroxyphosphinates with both C- and P-stereogenic centers are obtained by rhodium- or iridium-catalyzed substrate-directed stereoselective addition of the optically pure H-phosphinates to aldehydes. The reaction most probably proceeds by a transition-metal-catalyzed mechanism with hydridometal complexes as key intermediates in the catalytic cycle.

Hydride Reduction of NAD(P)+ Model Compounds with a Ru(II)–Hydrido Complex

In order to better understand the regioselective hydride transfer of metal hydrido complexes to NAD(P)+ model compounds, reactions of [Ru(tpy)(bpy)H]+ (Ru-H: tpy = 2,2′:6″,2″-terpyridine, bpy = 2,2′-bipyridine) with various substituent NAD(P)+ model compounds were investigated in detail. All of the NAD(P)+ model compounds accepted hydride from Ru-H, yielding 1:1 adducts, where the dihydro form(s) of the model compounds coordinated with the carbamoyl group to the Ru(II) center of [Ru(tpy)(bpy)]2+, with very different reaction rates. Some reactions produced the adduct with only the 1,4-dihydro structure, whereas others produced a mixture of two adducts, with a 1,4- or 1,2-dihydro structure. In particular, temperature-dependent adduct formation kinetics studies provided important information on the transition state(s) of the hydride transfer reactions and factors for determining the regioselectivity. Most adducts were cleaved to the corresponding free dihydro product(s) with the same distribution of the regi...

ChemInform Abstract: Structural Characterization of Sr4Mg4H4[CoH5]3 Shows the Importance of Support from Polarizing Counter Ions to 3d Transition Metal Hydrido Complexes.

AbstractThe title compound is prepared by high‐pressure solid state reaction of a 4:1:2 mixture of SrH2, Co, and Mg2CoH5 (autoclave, 60 bar H2, 873 K, 12 h).

ChemInform Abstract: Transition Metal Hydrido‐Complexes: Electronic Structure and Bonding Properties

AbstractReview: 45 refs.

Structural characterization of Sr4Mg4H4[CoH5]3 shows the importance of support from polarizing counter ions to 3d transition metal hydrido complexes

The structure of the title compound was refined from neutron powder diffraction data in the cubic space group P-43m (215). The unit cell contains one formula unit with 3 structurally equivalent [Co(I)H5]4− complexes as well as 4 interstitial hydride (H−) ions. The presence of the larger and less polarizing Sr2+ ions weakens the bond in the complexes and probably also the stability of the structure. Attempts to synthesize the corresponding Ba analogue failed in contrast to using smaller and more polarizing Ca2+ and Yb2+ counterions.

Transition metal hydrido-complexes: Electronic structure and bonding properties

A coherent overview of the physical and chemical properties of the family of transition metal hydrido complexes is addressed from ab initio through an exhaustive treatment from the solid state at different levels to the molecular state, taking a promising class of compounds for potential applications, the nonahydrido complexes as an illustrative case study. A new chemical vision is presented, pertaining to structure–property relationships such as the different chemical behaviors of the tricapped trigonal prismatic {TH9} (T = Tc, Re) complex anions at the two distinct lattice sites. From energy differences different stability and binding of the Tc versus Re based compounds are underlined whereby the latter is found more tightly bonded. Such solid state results are also supported by calculations at the molecular level with larger infra-red frequencies within the ReH9 complex anion versus TcH9 corresponding to both bending and stretching T–H modes in agreement with experiment. Electronic density of states show both compounds to be insulating with large band gaps and narrow valence bands which are differentiated for the two sites of the transition element (T1 and T2): T1H9 sub-motifs at the corners and T2H9 ones forming a honeycomb arrangement as it is illustrated from the electron localization function (ELF) plots and chemical bonding. ELF maps are also significant of largely ionic 2K+(TH9)2− compound within which covalent-like (TH9)2− show electron localization on hydrogen and free electron behavior within the tricapped trigonal prism.

Transition Metal Complexes Coordinated by an NAD(P)H Model Compound and their Enhanced Hydride‐Donating Abilities in the Presence of a Base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.

Reaction of an NAD(P) + model compound coordinated to a transition metal complex with ruthenium and rhenium hydrido complexes

Ruthenium(II) and rhenium(I) complexes with a deprotonated 1-benzylamidium cation as ligand are reduced by metal hydrido complexes. The products are binuclear complexes with a deprotonated 1-benzyl-1,4-dihydronicotinamide (BNAH) ligand, such as [{Ru(tpy)(bpy)} 2(BNAH-H +)] 3+. The reducing ability of [{Ru(tpy)(bpy)} 2(BNAH-H +)] 3+ is similar to that of free BNAH but less than for the corresponding mononuclear [Ru(tpy)(bpy)(BNAH-H +)] +.

Insights into the mechanism of chemical vapor generation of transition and noble metals

The mechanisms of chemical vapor generation (CVG) of transition and noble metals by reaction with tetrahydroborate(III) have been investigated in an effort to reconcile fundamental properties of compounds containing metal–hydride bonds with the experimental evidence collected in the present work and in the recent analytical literature on CVG. Silver, gold and rhodium were investigated in detail. Various materials constituting the wetted surfaces of the reaction apparatus (Ryton, glass, silanized glass) were examined for their effect on the overall process, as was the presence of surface-active agents (Triton X-100TM, antifoam B). Two types of reactions contribute to the formation of volatile species. The first (primary reaction) occurs through homogeneous liquid phase reactions between aqueous analyte complexes (MLn) and hydroboron species (BH4− and its hydrolysis products). The second (secondary reaction) is a surface mediated reaction between reaction intermediates, mostly formed in the primary reaction and chemisorbed by active sites on surfaces, and hydroboron species. The participation of active surface sites generally improves the efficiency of CVG compared with use of an inert surface, but this feature cannot always be usefully employed in analytical applications due to slow reaction kinetics producing memory effects. Reaction intermediates which can be classified as hydrido metal complexes, LnMHx, possess complex chemistry, making interpretation of experimental results difficult. Hydrido metal complexes, in addition to free atoms and nanoparticles, may constitute the volatile species leaving the solution and/or reaching the atomizer.

Formation of Novel 1:1 Adducts Accompanied by Regioselective Hydride Transfer from Transition-Metal Hydrido Complexes to NAD(P) Models

Important intermediates in regioslective hydride reduction of NAD(P) model compounds were detected by various spectroscopic methods. The metal hydrido complexes [Ru(tpy)(bpy)H]+ and [Re(bpy)(CO)3H] react with the NAD(P) models to give 1:1 adducts, which are cleaved to the 1,4-dihydro form of the models and the solvento complexes, quantitatively.

Raman spectroscopic studies on palladium and platinum hydrido complexes

Raman spectra of platinum metal hydrido complexes, Na2PtH4 (I4/mmm), K2PtH4, K2PtD4, Rb2PtH4, Rb2PtD4, Rb2PdD4 (Fm3m) and K3PtH5 (P4/mbm), are presented and discussed with respect to the influence of the different strengths of Pt—H and Pd—H bonds and of the relative volume of the reduced (per formula unit) unit cells. The ‘antisymmetric’ MD (M = Pt,Pd) stretching mode (species B1g of free, square MX4 units) decreases from 1436 to 1291 cm−1 on going from K2PtD4 to Rb2PdD4. Likewise, the PtH stretch of Na2PtH4 at 2026 cm−1 decreases to 2001 cm−1 for K2PtH4 but increases to 2039 cm−1 for K3PtH5. The totally symmetric MH stretches (A1g), which are of lower intensity, are shifted by about 50 cm−1 to higher wavenumbers compared with the asymmetric stretches. The results of a group theoretical treatment of the respective site group and unit-cell group modes are given. Copyright © 1999 John Wiley & Sons, Ltd.

Novel binuclear doubly vinyl-bridged iridium complexes formed by olefinic C—H activation

Treatment of [C5H5IrX2]n (2: X = Cl; 3: X = Br) with CH2=CHtBu in the presence of Na2CO3–iPrOH gives, besides the bis(olefin) complex [C5H5Ir(CH2=CHtBu)2] (4), the doubly vinyl-bridged iridium(II) compound {[C5H5Ir(μ-η1,η2-CH=CHtBu)]2} (5). The related binuclear complex {[CH2(C5H4)2][Ir2(μ-η1,η2-CH=CHtBu)2]} (9) was obtained from {[CH2(C5H4)2][IrBr2]2}n (8) and CH2=CHtBu–Na2CO3 in ethanol and characterized by an X-ray structure analysis (monoclinic, C2/c (No. 15), a = 20.050(8), b = 8.843(1), c = 24.25(1) Å, β = 99.53(2)°, V = 4241 Å3, Z = 8, R = 0.032, Rw = 0.041). Protonation of both 5 and 9 with HBF4•OEt2 yields the BF4 salts of the unsymmetrical hydridometal cations 10 and 11 in which the vinyl bridges are still intact. The reaction of the binuclear bis(cyclooctene)metal compounds {[CH2(C5H4)2][Rh(C8H14)2]2} (14) and {[CH2(C5H4)2][Rh(C8H14)2][Ir(C8H14)2]} (17) with bromine yields almost quantitatively the polymeric (or oligomeric) dibromo derivatives 16 and 18, which could, however, not be converted to vinyl bridged complexes by treatment with CH2=CHtBu–Na2CO3–ROH. Keywords: iridium complexes, C-H activation, bridging vinyl ligands, cationic hydridometal complexes, heterometallic complex.

Regio- and stereoselective dimerization of 1-alkynes catalyzed by an Os(II) complex

The complex [(PP 3)OsH(N 2)]BPh 4 is a catalyst precursor for the regio- and stereoselective dimerization of HCCR (RPh, SiMe 3) to ( Z)-1,4-disubstituted-but-3-en-l-ynes (PP 3P(CH 2CH 2PPh 2) 3). In the presence of H 2O or C 2H 5OH, the catalytic reaction with HCCSiMe 3 selectively gives but-3-en-l-ynyl-trimethyisilane. A detailed study under different experimental conditions, the detection of some intermediates, and the use of isolated complexes in independent reactions, taken altogether, permit mechanistic conclusions which account for the observed products. A key-role is played by (vinylidene)σ-alkynyl complexes which transform into η 3-butenynyl derivatives via intramolecular CC bond formation. The Os(II) η 3-butenynyl complexes are likely reagents in the rate determining step of the catalytic cycle, and produce free ( Z)-1,4-disubstituted-but-3-en-l-ynes upon σ-bond metathesis reaction with HCCR. The 16-electron fragments [(PP 3)OsX] + (X  H, Cl, CCR) are capable of promoting the 1-alkyne to vinylidene tautomerism. In particular, the (vinylidene)hydride [(PP 3)OsH{CC(H)-SiMe 3}]BPh 4 has been isolated and properly characterized. Since the stoichiometric reaction of the latter compound with HCCSiMe 3 gives vinyltrimethylsilane, the formation of (vinylidene)hydride species is suggested to be an effective step, alternative to 1-alkyne insertion, in the reduction of 1-alkynes to alkenes assisted by hydrido metal complexes.

Amidinato complexes of the platinum metals and an avid carbonyl abstraction reaction

Platinum metal hydrido and trifluoroacetato complexes react with N,N′-diphenylamidines, PhNC(R)NHPh (R = H, Me, Et, Ph), giving the amidinato derivatives Ru{PhNC(R)NPh} 2(CO)(PPh 3), MX{PhNC(R)NPh}(CO)(PPh 3) 2 and IrX 2{PhNC(R)NPh}(PPh 3) 2 (M = Ru, Os; X = H, Cl; X 2 = H 2, Cl 2 or HCl). The reactions of RuH 2(PPh 3) 4 and OsH 4(PPh 3) 3 with amidines are accompanied by an avid alcohol decarbonylation reaction to form the carbonyls MH{PhNC(R)NPh}(CO)(PPh 3) 2.