Pentacarbonyl Chromium Complexes Research Articles

Isolation and Diverse Reactivity of an Unsymmetrical 1,2-Bis(silylene)-Stabilized Pentacarbonyl Chromium(0) Species.

The construction of the unsymmetrical 1,2-bis(silylene) pentacarbonyl chromium(0) complex 1 was achieved through the reaction of chlorosilylene with half an equivalent of K2Cr(CO)5. X-ray diffraction analysis of 1 confirms the formation of the Si-Si bond and the coordination of one of the silicon atoms to the Cr center. Density functional theory (DFT) calculations disclose that highest occupied molecular orbital (HOMO) mainly corresponds to the lone pair of electrons on the silicon atom and the σ-bonding interaction between two Si atoms. Based on its unique electronic structure, its diverse reactivity toward the transition metal compounds and small molecules was investigated in detail. The reactions of 1 with Fe2(CO)9 or CuCl yielded the 1,2-bis(silylene)-stabilized heterobimetallic complex 2 or oxidized product 3, respectively. Additionally, treatments of 1 with selenium, CO2, or Me3SiN3 led to the formation of the corresponding selenium-, oxo-, and nitrogen-bridged complexes 4-7. All compounds were characterized by multinuclear NMR spectroscopy and X-ray crystallography.

An unexpected pentacarbonyl chromium complexation of a cyano group of the ABC core of cephalotaxine

A new penta-carbonyl chromium(0) complex of the type [Cr(CO)5(L)] (L = tetracyclic pyrrolobenzazepine unit 3) was surprisingly obtained by reacting [Cr(CO)3(naphthalene)] or [Cr(CO)3(tmtach)] with the tetracyclic pyrrolobenzazepine unit 3 in octane-ether/THF-solvent mixtures or acetone under ambient temperature or reflux. The new complex 13 has been characterized by spectral analysis including IR, 1H and 13C NMR data. For comparison purposes, the safrole-tricarbonyl chromium(0) complex 12 was prepared and characterized. X-ray diffraction analyses of both complexes were determined. Based on the above data, an octahedral structure has been assigned to the new complex 13.

Deamination of N→Sn‐Coordinated Organotin(II) Hydroxide: Formation of a New C–O Covalent Bond

AbstractThe synthesis of the pentacarbonylchromium complex [{2,6‐(Me2NCH2)2C6H3}(Cl)SnCr(CO)5] is reported. Starting with this compound, organotin(II)pentacarbonylchromium methoxide [{2,6‐(Me2NCH2)2C6H3}(OMe)SnCr(CO)5] was prepared and used for the synthesis of dimeric N→Sn intramolecularly coordinated organotin(II)pentacarbonylchromium hydroxide [{2,6‐(Me2NCH2)2C6H3}Sn(μ‐OH)Cr(CO)5]2 by easy hydrolytic protocol. In addition, acid–base‐type reaction of this latter compound with HOTf provided unprecedented cyclization product [6‐(Me2NCH2)‐2,3‐(C6H3CH2OSn)Cr(CO)5]2. The cyclization process is accompanied by the formation of a new C–O covalent bond along with the elimination of Me2NH.

Synthesis and DFT calculations of spirooxaphosphirane complexes

In situ formed Li/Cl phosphinidenoid complexes [Li(12-crown-4)][M(CO)5(ClPC5Me5)] 3a-c (M = Cr, Mo, W) reacted with cyclobutanone (4), cyclopentanone (5) and cyclohexanone (6) in Et2O to yield the first P-C5Me5 substituted C(3)-spirofused oxaphosphirane complexes 7a-c, 8a and 9a,a'. In the case of cyclopentanone and 1a the outcome of the reaction in THF was different: here the formation of 8a along with (anionic) phosphinoate complexes 14a and 15a was observed, the latter possess an unusual ring-opened oxaphosphirane and 2-cyclopentylidenecyclopentanone as co-ligands to the lithium cation. NMR, IR and MS data as well as single-crystal X-ray structures in the case of 7a-c, 8a, 9a and 15a are reported. DFT calculations on the parent 1-oxa-2-phosphaspiro[2.n]alkane pentacarbonylchromium(0) complexes 10 (a: n = 2; b: n = 3; c: n = 4; d: n = 5) revealed that both ring strain energies and G(r) values decrease significantly as the spiroring size increases. This is caused by an increase in the exocyclic α bond angle at the oxaphosphirane C(3) atom, hence decreasing the s-character of the corresponding orbitals involved in endocyclic bonds at C(3) and thus becoming better suited for accommodation of small ring angles.

Single Electron Transfer-Mediated Selectiveendo- andexocyclic Bond Cleavage Processes in Azaphosphiridine Chromium(0) Complexes: A Computational Study

Azaphosphiridines κP pentacarbonylchromium(0) complexes 2a,b (2a: R = H; 2b: R = Me) exhibit an average ring strain ranging from 24.2 to 25.7 kcal mol(-1) as obtained from homodesmotic reactions at the LPNO-NCEPA1/def2-TZVPP//BP86/def2-TZVP level. Parent azaphosphiridine chromium complex 1 is more stable than the ylidic P-iminiumphosphanide chromium complex isomer 6, which is obtained from (formal) endocyclic P-C bond cleavage. Computational evidence is provided for an insertion of carbon monoxide into the P-N bond of 1 to form 1,3-azaphosphetidin-2-one chromium complex 11, as the reaction was exergonic by -15.1 kcal mol(-1). The VBSD (variation of bond strength descriptors) methodology unveiled that SET (single electron transfer) oxidation of trimethyl-azaphosphiridine chromium complex 2b results in selective endocyclic P-C bond cleavage to afford the trimethyl-iminiumphosphanyl radical cation complex 13(•+). SET reduction of a wide variety of differently P-substituted azaphosphiridine complex derivatives (2a: R = H; 2b: R = Me; 2c: R = Cp; 2d: R = Cp*; 2e: R = CHTms(2); 2f: R = CMe(3); 2g: R = CMe(2)Ph; 2h: R = CMePh(2); 2j: R = Ph; 2k: R = C(6)F(5); Cp*: pentamethylcyclopentadienyl; Tms: trimethylsilyl) lead to selective decomplexation and thus to the corresponding unligated azaphosphiridines 14. Only in case of the P-trityl substituted azaphosphiridine complexes 2i does the SET reduction preferably cleave the exocyclic P-C bond thus affording azaphosphiridinide complex 12(-) and the triphenylmethyl radical.

Characterization of pentacarbonyl(4-methylpyridine)chromium(0) complex using density functional theory (DFT) and Hartree–Fock (HF) computational methods

The optimized molecular structure, atomic charges, vibrational frequencies, thermodynamic properties, nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectral data of pentacarbonyl(4-methylpyridine)chromium(0) complex have been investigated by performing ab initio Hartree–Fock (HF) and density functional theory, B3LYP, B3PW91 and BE1PBE methods with 6-311G, 6-311+G(3d,3p) and 6-31G(d,p) basis set. The calculated NMR data at 6-311G basis set, vibrational frequencies at 6-311+G(3d,3p) basis set and the optimized geometric bond lengths and bond angles at 6-31G(d,p) basis set are in good agreement with the corresponding experimental values. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been simulated. In addition, the transition state and energy band gap and infrared intensities have also been reported.

Oligomerization of Terminal Phosphinidene Complexes

The thermolysis of a 7-phenyl-7-phosphanorbornadiene pentacarbonylchromium complex yields a mixture of the cyclic trimer and tetramer of [PhP-Cr(CO)5]. In the presence of CuCl and W(CO)6 at 60 °C, ...

ChemInform Abstract: A New Approach to N-Aryl- and N-(2-Heteroaryl)imidates from Chromium Carbenes and Sulfilimines.

Irradiation of pentacarbonyl(alkoxymethylcarbene)chromium(0) complexes in the presence of a number of N-aryl- or N-heteroaryl-substituted sulfilimines gives N-aryl- or N-heteroarylimidates in fair to excellent yields. © 1991, American Chemical Society. All rights reserved.

Synthesis, Crystal Structure, Thermal Behavior and IR Characterization of Pentacarbonyl(4-methylpyridine)chromium(0) Complex

Pentacarbonyl(4-methylpyridine)chromium(0) complex was isolated from n-hexane solution as yellow plate-like crystals and characterized by using X-ray crystallography. It crystallizes in the orthorhombic system with the space group Cmcm and Z = 4. The unit cell parameters are a = 11.737(1) A, b = 12.857(2) A, c = 8.465(1) A. The single crystal X-ray structure of the complex shows that the coordination sphere around the chromium central atom is slightly distorted octahedron, involving the 4-methylpyridine (4-mp), ligand as a monodentate N-donor ligand and five carbonyl groups. The four equatorial CO groups in the complex, with the Cr–C2 distance of 1.886 Ǻ, are slightly bent away from the 4-methylpyridine ligand with the N–Cr–C2 angle of 91.69°. The pyridine ring plane makes an angle of 135.17° with the Cr–N–CO bond axis. The thermal analysis (differential thermal analysis and thermal gravimetry) and IR spectra of the complex indicated that the compound undergoes complete decomposition to form the Cr2O3 as the final decomposition product. The crystal structure of pentacarbonyl(4-methylpyridine)chromium(0) complex has been determined and its thermal behavior has also been studied.

Reactions of [Aryloxy(phenyl)carbene]pentacarbonylchromium(0) Complexes with Thiolate Ions. Decreasing Reactivity with Increasing Basicity of the Nucleophile

A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Brønsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (beta(nuc) approximately equal to 1.0) followed by a slightly descending leg with a negative slope (beta(nuc) approximately equal to -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pK(RSH)(a) to rate-limiting nucleophilic attachment at high pK(RSH)(a). The negative beta(nuc) values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.

Transfer of heterocyclic carbene ligands from chromium to gold, palladium and platinum

The reaction of pentacarbonyl(pyrazolin-3-ylidene)chromium complexes, ▪ ( 2a– c) (R = Ph ( a), C 6H 4NMe 2-4 ( b); (C 5H 4)FeCp ( c)), with [AuCl(SMe 2)], H[AuCl 4], [PdCl 2(NCPh) 2] and [PtCl 2(NCPh) 2] gives, by transfer of the heterocyclic carbene ligand, new chloro pyrazolin-3-ylidene complexes of gold(I) and gold(III), dichloro bis(pyrazolin-3-ylidene) palladium and dichloro bis(pyrazolin-3-ylidene) platinum in high yield. The chloride ligand in ▪ (Fc = (C 5H 4)FeCp) is readily displaced by trifluoroacetate. The analogous substitution of iodide for the chloride ligands in ▪ (M = Pd, Pt) give the corresponding diiodo complexes although in a much slower reaction. In contrast, the reaction of silver trifluoroacetate with ▪ affords a binuclear Pd–Ag complex containing two pyrazolin-3-ylidene and three trifluoroacetate ligands two of whom occupy bridging positions between Pd and Ag. The reactions of the pyrazolidin-3-ylidene complex ▪ (R = C 6H 4NMe 2-4) with [AuCl(SMe 2)] and [PdCl 2(NCPh) 2] yield chloro pyrazolidin-3-ylidene gold and dichloro bis(pyrazolidin-3-ylidene) palladium complexes. The related dichloro bis(tetrahydropyrimidin-4-ylidene) palladium complex is formed in the reaction of ▪ with the palladium complex [PdCl 2(NCPh) 2]. The solid-state structures of several of these heterocyclic carbene complexes including the structure of the binuclear Pd–Ag complex are established by X-ray structure analyses.

Transformation of a Pyrazolidene into a Tetrahydropyrimidinylidene Ligand by Deprotonation/Reprotonation

In an unusual ring enlargement reaction the heterocyclic ligand in a pentacarbonyl(pyrazolidene) chromium complex is transformed in high yield into the corresponding tetrahydropyrimidinylidene ligands by a deprotonation/protonation (or deprotonation/alkylation) sequence.

Kinetics and Mechanism of Alkaline Hydrolysis of [(Methoxy)(p-substituted styryl)-carbene] Pentacarbonyl Chromium(0) Complexes in Aqueous Acetonitrile

Kinetic studies have been performed for alkaline hydrolysis of a series of [(methoxy)(p-substituted styryl)carbene]pentacarbonyl chromium(0) complexes (<TEX>$(CO)_5$</TEX>Cr=<TEX>$C(OCH_3)CH=CHC_6H_4X$</TEX>, X = p-<TEX>$OCH_3$</TEX>, p-<TEX>$CH_3$</TEX>, H, p-Cl, p-<TEX>$NO_2$</TEX>). Second-order rate constants <TEX>$(k_{{OH}^-})$</TEX> for the alkaline hydrolysis in 50% acetonitrile-water(v/v) were determined spectrophotometrically at various temperatures. At a low pH region (pH < 7.5), the observed rate constant <TEX>$(k_{obs})$</TEX> remained constant with a small value, while in a high pH region (pH > 9.5), <TEX>$k_{obs}$</TEX> increases linearly with increasing the pH of the medium. The second-order rate constants <TEX>$(k_{{OH}^-})$</TEX> increase as the substituent X changes from a strong electron donating group to a strong electron withdrawing group. The Hammett plot obtained for the alkaline hydrolysis is consisted of two intersecting straight lines. The nonlinear Hammett plot might be interpreted as a change in the rate-determining step. However, the fact that the corresponding Yukawa-Tsuno plot is linear with <TEX>$\rho$</TEX> and r values of 0.71 and 1.14, respectively indicates that the nonlinear Hammett plot is not due to a change in the rate-determing step but is due to ground-state stabilization through resonance interaction. The positive <TEX>$\rho$</TEX> value suggests that nucleophilic attack by <TEX>$OH^-$</TEX> to form a tetrahedral addition intermediate is the rate-determining step. The large negative <TEX>${\Delta}S^\neq$</TEX> value determined in the present system is consistent with the proposed mechanism.

Cascade Reactions of β‐Amino‐Substituted α,β‐Unsaturated Fischer Carbene Complexes with 1,5‐Dien‐3‐ynes as a Convenient Access to Ring‐Annelated Benzene Derivatives

AbstractUpon heating pentacarbonyl(3‐dimethylamino‐1‐ethoxypropenylidene)chromium complexes 1 with 1,5‐dien‐3‐ynes 2 in pyridine at 80 °C, benzene‐annelated cyclopentenones 8 and their regioisomers 9, resulting from a sequence of cocyclization, 6π‐electrocyclization and hydrolysis, were isolated in 12–75 % yield (13 examples). The more flexible the alkenyl substituents were in the dienynes 2, the longer were the reaction times needed to achieve good chemical yields. This new cascade reaction of Fischer carbene complexes provides a direct route to trindanone analogues under milder conditions than traditional methods, and is compatible with more functionalities. Compounds 14 and 15 with steroid‐like skeletons were thus prepared in 54–77 % yields (4 examples) from complex 1‐iPr and the bicyclic alkyne 2. Hexacycles 17 and 18 were accessible with high diastereoselectivity in the triquinane moiety by the same method in 45 % yield, (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)

Mass Spectrometric Detection and Fragmentation Patterns of Synthetically Useful Chromium and Tungsten Carbene Complexes

A novel method for the detection of synthetically useful pentacarbonyl chromium and tungsten carbene complexes by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry is presented which utilizes the addition of CN- anions to the electrophilic carbene carbon. This method is nondestructive, provided the ionization proceeds under soft conditions. The scope and limitations of this method are examined by utilizing different types of carbene complexes. Depending on the α-CH acidity of the carbene complexes, [M − H]- anions can be observed either in competition with the CN- adducts or as the only ions, even in the presence of cyanide. Secondary aminocarbene complexes exclusively form [M − H]- anions by deprotonation of the NH moiety. The ions under study have been mass-selected and subjected to collision-induced decay (CID) experiments in order to examine their fragmentation reactions. CO loss is not the only observable process. Most prominently, 1,2-eliminations and a retro-aldol reaction within a sugar carbene complex compete with decarbonylation and thus have rather low activation barriers. Some reactions can be attributed to coordinatively unsaturated chromium atoms formed through preceding CO losses.

Synthesis and oxidation of (benzimidazolylidene)Cr(CO) 5 complexes

A series of (benzimidazolylidene)pentacarbonylchromium(0) complexes were synthesized by the addition of lithium 1,2-phenylenediamides to Cr(CO) 6 in the presence of 12-crown-4, followed by the treatment of chlorotrimethylsilane. X-ray crystal structural analysis revealed a rather single-bond-like character between Cr and the carbene carbon. One reversible oxidation process was observed in the cyclic voltammograms of these complexes. Chemical oxidation using (4-BrC 6H 4) 3NSbCl 6 was studied by ESR to afford a hyperfine structure derived not from Cr but from N and H, indicating that the spin is delocalized on the benzimidazolidene moiety.

Donor-substituted allenylidene(carbonyl)(XR 3)chromium complexes (X=P, As, Sb) – synthesis and properties

Photolysis of the allenylidene pentacarbonyl chromium complexes [(CO) 5CrCCC(R 1)R 2] (R 1=NMe 2, NPh 2; R 2=NMe 2, OMe, Ph) in THF in the presence of equimolar amounts of XR 3 (XR 3=various phosphanes, P(OMe) 3, AsPh 3, SbPh 3) affords cis-allenylidene tetracarbonyl XR 3 complexes, cis-[(CO) 4(XR 3)CrCCC(R 1)R 2]. When in the photolysis of [(CO) 5CrCCC(NMe 2)Ph], the phosphanes PR 3 (R=C 6H 4F- p, C 6H 4Cl- p, OMe) are used in excess (three equivalents) two carbonyl ligands are displaced and the mer-tricarbonyl complexes mer-[(CO) 3(PR 3) 2CrCCC(NMe 2)Ph] are formed both PR 3 ligands being mutually trans. The structure of the new complexes is established by IR, NMR, and UV–Vis spectroscopy, that of cis-[(CO) 4(PPh 3)CrCCC(NMe 2)Ph] additionally by an X-ray structural analysis. As indicated by the spectroscopic data of the compounds, these complexes are best described as hybrids of allenylidene and zwitterionic alkynyl complexes with delocalization of the electron pair at nitrogen bonded to the C γ atom of the allenylidene ligand towards the metal center. The relative contribution of the allenylidene and zwitterionic alkynyl resonance forms is influenced by XR 3. Increasing the donor properties of XR 3 favors the allenylidene resonance form.

A unusual bridging mode of vinylidene ligands in nonacarbonyl(vinylidene)dichromium complexes

Thermolysis of the pentacarbonyl(vinylidene)chromium complexes [(CO) 5CrCC(R 1)R 2] ( 1a– d) (C(R 1)R 2=CMe 2 ( a), C(CH 2) 5 ( b), C(Et)Me ( c), C( t Bu)Me ( d)), affords binuclear, vinylidene-bridged nonacarbonyldichromium complexes, [(CO) 5Cr(μ-η 1,η 2-CC(R 1)R 2)Cr(CO) 4] ( 2a– d). The solid-state structures of 2a and 2d have been established by X-ray structural analyses. The structural data prove the μ-η 1,η 2-CC(R 1)R 2 bonding mode. To account for the NMR spectroscopic features a dynamic process is proposed involving rapid coordination site exchange of the CC bond between Cr(1) and Cr(2) (windshield wiper mechanism) and of one CO ligand.

Physical organic chemistry of transition metal carbene complexes. 24. Thermodynamic and kinetic acidities of phenyl-substituted (benzylmethoxycarbene)pentacarbonylchromium(0) complexes. Is there a transition-state imbalance?

A kinetic study of the reversible deprotonation of phenyl-substituted (benzylmethoxycarbene)pentacarbonylchromium(0) complexes by OH(-) and by a series of primary aliphatic and a series of secondary alicyclic amines in 50% MeCN-50% water (v/v) at 25 degrees C is reported. Brønsted alpha(CH) values (dependence on carbene complex acidity) and beta(B) values (dependence on amine basicity) were determined. According to current notions about proton transfers involving carbon acids activated by pi-acceptors, alpha(CH) was expected to substantially exceed beta(B), the result of transition-state imbalances that are characteristic of such reactions. However we find that alpha(CH) and beta(B) have essentially the same values, which are close to 0.5. It is shown that these findings do not indicate the absence of an imbalance but rather suggest that the manifestation of the imbalance is masked by the pi-donor effect (3H-Z <--> 3H-Z(+/-)) of the methoxy group.

Efficient Syntheses of Angularly Fused Triquinanes via β-Amino-Substituted α,β-Unsaturated Fischer-Carbene Complexes

Formal [3+2] cycloadditions of the carbene ligand in the new (3-dimethylamino-1-alkoxyalkenylidene)pentacarbonylchromium complexes 5 and 14 both with cyclopropyl substituents in the 3-position, onto alkynes give 5-dimethylamino-3-ethoxycyclopentadienes 6 and 15a-f, the latter along with regioisomers 16a-d, in good yields (55-72%). Treatment of these with hydrochloric acid liberates two carbonyl groups in each of them, and the resulting 5-dimethylaminocyclopent-1-en-3-ones 7 and 17a-c/18a-c, under basic conditions, undergo a cascade of two cyclizations, to yield angularly fused triquinanes 10 (68%) and 23a-c (30-97%). The overall procedure consists of four operational steps and produces cyclopropane-annelated tricyclo[6.3.0.0 [1] [5] ]undecane skeletons from two different alkynes each in 22-47% overall yield.