Three-legged Piano-stool Complexes Research Articles

Phosphite Bearing [(μ-ADT)RFe2(CO)6] (ADT = Azadithiolate) Moieties: A Tool for the Building of Multimetallic [FeFe]-Hydrogenase Mimics

A new phosphite ligand having three [(μ-ADT)RFe2(CO)6] (R= p-HOC6H4) moieties (2) has been prepared in good yield by the reaction of complex [(μ-ADT)RFe2(CO)6] (R= p-HOC6H4) 1a with PCl3. Coordination of this phosphite to [PdCl2(MeCN)2] or [PtCl2(DMSO)2] forms heterometallic square planar complexes 5 (C84H48Cl2Fe12N6O42P2MS12) (M = Pt, Pd) in excellent yields. Three-legged piano stool complexes 6 (C52H39Cl2Fe6N3O21PMS6) (M = Rh, Ir) were obtained by the reaction of phosphite 2 with [MCl2Cp*]2 (M = Rh, Ir) in good yields. The formation of complexes 5 and 6 demonstrates the versatility of this new ligand for forming different heteropolymetallic complexes under mild reaction conditions. Moreover, the open-chain derivatives [(μ-ADT)RFe2(CO)6] (R= HOCH2CH2, o-HOC6H4) (1b and 1c, respectively) form cyclic complexes 4 by spontaneous intramolecular CO substitution by the P atom in one of the three [FeFe] fragments. The electrocatalytic behavior of complexes 2 and 4 upon the addition of AcOH is similar to that of related [(μ-ADT)Fe2(CO)6] derivatives. The successive additions of AcOH cause an increase in the current intensity in the wave at about −1.80 V for heteropolymetallic complexes 5 and 6. However, the appearance of a new wave around −1.40 V in complexes 5 points to an acid-promoted side reaction in the electrochemical process.

Dianionic Mononuclear Cyclo-P4 Complexes of Zero-Valent Molybdenum: Coordination of the Cyclo-P4 Dianion in the Absence of Intramolecular Charge Transfer.

Relative to other cyclic poly-phosphorus species (that is, cyclo-Pn ), the planar cyclo-P4 group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η4 -P4 complexes are presented that can be viewed as the simple coordination of the [cyclo-P4 ]2- dianion to a neutral metal fragment. Treatment of the neutral, molybdenum cyclo-P4 complexes Mo(η4 -P4 )I2 (CO)(CNArDipp2 )2 and Mo(η4 -P4 )(CO)2 (CNArDipp2 )2 with KC8 produces the dianionic, three-legged piano stool complexes, [Mo(η4 -P4 )(CO)(CNArDipp2 )2 ]2- and [Mo(η4 -P4 )(CO)2 (CNArDipp2 )]2- , respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η6 -benzene complex (η6 -C6 H6 )Mo(CO)3 regarding the metal-center valence state and electronic population of the planar-cyclic ligand π system.

N-Heterocyclic Carbene-Phosphinidene and Carbene-Phosphinidenide Transition Metal Complexes.

Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)PH] (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl2]2, which afforded the three-legged piano-stool complexes [LMCl2{HP(IPr)}] (9a/9b: M = Ru/Os, L = η6-p-cymene; 10a/10b: M = Rh/Ir, L = η5-C5Me5). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] (11a/11b: M = Ru/Os; 12a/12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)PTMS] (2) with [LMCl2]2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me3SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, which can be ascribed to the ability of the imidazole moiety to effectively stabilize a positive charge. Dehydrohalogenation of complexes 9/10 to 11/12 affords a significant increase of the metal-phosphorus bond order, with the carbene-phosphinidenide ligand acting as a strong 2σ,2π-electron donor.

Reactivity of an Unsaturated Iridium(III) Phosphoramidate Complex, [Cp*Ir{κ2-N,O}][BArF4

The three-legged piano stool complex [Cp*Ir(κ2-N,O-Xyl(N)P(O)(OEt)2)(Cl)], [1] (Cp* = η5-C5Me5, Xyl = 2,6-dimethylphenyl), was prepared from reaction of 0.5 equiv of [Cp*IrCl2]2 with the sodiated phosphoramidate ligand Na[Xyl(N)P(O)(OEt)2]. Treatment of [1] with Na[BArF4], [BArF4] = [B(C6H3(CF3)2)4], led to the formation of the 16-electron two-legged piano stool species [Cp*Ir(κ2-N,O-Xyl(N)P(O)(OEt)2)][BArF4], [2][BArF4], which was characterized in both solution and solid state. Reactivity screening revealed that complex [2][BArF4] undergoes addition of a variety of Lewis bases to afford the corresponding 18-electron adducts with concomitant movement of the phosphoramidate ligand from κ2-N,O to κ1-N, [Cp*Ir(κ1-N-Xyl(N)P(O)(OEt)2)(L)2][BArF4]; L = CNtBu, [3][BArF4], CNXyl, [4][BArF4], MeCN, [7][BArF4], bipy, [8][BArF4]; bipy = 2,2′-bipyridine. For complex [7][BArF4], variable-temperature 31P{1H} NMR spectroscopy revealed that MeCN coordination was reversible between 238 and 190 K. To probe E–H (E = Si, B) ...

Toward a Broadly Applicable Force Field for d(6)-Piano Stool Complexes.

Three-legged piano stool complexes are prototypical organometallic complexes relevant to a wide range of chemically relevant questions. Force field parametrization of transition-metal complexes is difficult and underdeveloped, and metal-specific force fields and software are required. Here we report our efforts to derive parameters for the conventional CHARMM and the Valbond-CHARMM force fields for d(6)-piano stool complexes. In Valbond-CHARMM, the usual angular term is replaced with hybrid orbital strength functions. These functions describe the energy not only of distorted bond angles around the minimum but also at very large distortions. Structure optimizations led to a good agreement between the calculated force field and the X-ray structures. They were comparable to RMSDs obtained between X-ray and DFT structures. In addition, and contrary to treating the systems with DFT, molecular dynamics simulations on the multiple nanosecond time scale are possible and allow to compute meaningful structural and energetic observables. Explicit solvent simulations of the complexes in methanol and water allow to determine the solvent distribution around the complexes. The parametrization presented here will be a useful starting point for dynamics investigations of catalysts in structurally more demanding environments.

Imidazolin-2-iminato Complexes of Rare Earth Metals with Very Short Metal−Nitrogen Bonds: Experimental and Theoretical Studies

The reactions of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH, 1-H) with trimethylsilylmethyl lithium (LiCH(2)SiMe(3)) and anhydrous rare earth metal trichlorides MCl(3) afforded the imidazolin-2-iminato complexes [(1)MCl(2)(THF)(3)] (2a, M = Sc; 2b, M = Y; 2c, M = Lu) and [(1)GdCl(2)(THF)(2)] x [LiCl(THF)(2)] (2d). Treatment of complexes 2 with dipotassium cyclooctatetradienide, K(2)(C(8)H(8)) resulted in the formation of two- or three-legged piano-stool complexes of the type [(eta(8)-C(8)H(8))M(1)(THF)(n)] (3a, M = Sc, n = 1; 3b, M = Y, n = 2; 3c, M = Lu, n = 2; 3d, M = Gd, n = 2). X-ray diffraction analyses of all eight complexes 2 and 3 revealed the presence of very short metal-nitrogen bonds, which are among the shortest ever observed for these elements. [(eta(8)-C(8)H(8))Sc(1)(THF)] (3a) reacted with 2,6-dimethylphenyl isothiocyanate (Xy-NCS) to form the [2 + 2]-cycloaddition product 4, which contains a thioureato-N,N' moiety. The related COT-titanium complex [(eta(8)-C(8)H(8))TiCl(1)] (6) could be obtained from [(1)TiCl(3)] (5) by reaction with K(2)(C(8)H(8)) and was structurally characterized. As a theoretical analysis of the nature of the metal-nitrogen bond, density functional theory (DFT) calculations have been carried out for complexes 3a and 6 and also for the model complexes [(eta(8)-C(8)H(8))Sc(NIm(Me))] (7), [(eta(8)-C(8)H(8))Ti(NIm(Me))](+) (8), and [(eta(8)-C(8)H(8))Ti(NXy)] (9), revealing a marked similarity of the bonding in imidazolin-2-iminato and conventional imido metal complexes.

Synthesis of the sterically constrained ligand precursor cyclopentadienyl-2,6-diphenylbenzene and structure of [(2,6-Ph 2–C 6H 3-η 5-C 5H 4)Zr(NEt 2) 3

A Pd-catalysed Stille coupling of 2,6-diphenyliodobenzene with tributylcyclopentadienyltin gave the sterically constrained ligand precursor cyclopentadienyl-2,6-diphenylbenzene ( 1). Treatment of this precursor with tetrakis(diethylamido)zirconium(IV) gave the three-legged piano-stool complex [(2,6-Ph 2–C 6H 3-η 5-C 5H 4)Zr(NEt 2) 3] ( 2). Complex 2 was characterised by 1H- and 13C{ 1H}-NMR spectroscopy as well as by X-ray crystallography which showed significant distortions of the three-legged piano-stool geometry as a result of steric interactions with the bulky aryl substituent.

Chloro(1-{3-[2-(diphenylphosphanyl-κP)ethyl]-η6-benzyl}-3,5-dimethyl-1H-pyrazole-κN2)ruthenium(II) trifluoromethanesulfonate

To address the question of the role of chirality at the metal in enantioselective catalysis, a pseudo-tetrahedral three-legged piano-stool complex has been prepared, i.e. [RuCl(C26H27N2P)](CF3SO3). Anchoring a phosphine and a pyrazole tether to an arene (PArN) yields, after η6:η1:η1 coordination to ruthenium, [{η6:η1:η1-(PArN)}RuCl]+ as a 1:1 mixture of enantiomers. Unfortunately, all attempts to resolve the enantiomers failed. The structure solution revealed the presence of racemic crystals.

Synthesis of a Configurationally Stable Three-Legged Piano-Stool Complex

Synthesis of a Configurationally Stable Three-Legged Piano-Stool Complex

Synthesis of a Configurationally Stable Three-Legged Piano-Stool Complex.

A transition metal Lewis acid with metal-centered chirality is obtained by η6 :η1 :η1 coordination of a 1,3-disubstituted arene with a phosphane and a pyrazole tether (PArN) to ruthenium. The three-legged piano-stool complex [{η6 :η1 :η1 -(PArN)}Ru(H2 O)]2+ (structure depicted) displays remarkable configurational stability. Its planar chiral, neutral precursor [{η6 :η1 -(PArN)}RuCl2 ] can be resolved by preparative HPLC.

Bimetallic Complexes with Chiral Molybdenum Centers and Bis(.eta.5-cyclopentadienyl) Bridges: Interchange between Legs in Three-Legged Piano Stool Complexes

Bimetallic Complexes with Chiral Molybdenum Centers and Bis(.eta.5-cyclopentadienyl) Bridges: Interchange between Legs in Three-Legged Piano Stool Complexes

Intramolecular C-X activation as a synthetic route to bidentate cyclopentadienyl-alkoxide ligands: preparation and molecular structure of [3-(2,3,4,5-tetramethylcyclopentadienyl)propoxy]titanium dichloride

Cp'TiCl3 (1; Cp' = eta-5-C5Me4(CH2)3OMe) reacts with 2 equiv of the phosphorous ylide PPh3 = CH2 (2) to yield Cp'TiCl2(CHPPh3) (3) plus the phosphonium salt (MePPh3)Cl. Complex 3 decomposes thermally to give [eta-5:eta-1-C5Me4(CH2)3O]TiCl2 (4), containing a Cp* -alkoxide-functionalized ligand. The molecular structure of 4 shows it as a pseudo three-legged piano-stool complex.