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Donald J. Darensbourg

Publications and source records attributed to Donald J. Darensbourg.

11 recordsLinked to original sources
PubMed1999

Structural and Spectroscopic Studies of 16-Electron, Unsaturated Derivatives of Low-Valent, Group 6 Carbonyl Complexes Containing pi-Donor Ligands.

Several eighteen- and sixteen-electron derivatives of tungsten(0), molybdenum(0), and chromium(0) carbonyl complexes, including [PPN](2)[Cr(CO)(3)(O,S-C(6)H(4))] (2c), [PPN](2)[W(CO)(3)(NH,S-C(6)H(4))] (5c), [PPN](2)[W(CO)(3)(O,S-C(6)H(4))] (6c), [PPN](2)[W(CO)(4)(S,S-C(6)H(4))] (7b) have been synthesized from the reaction of photochemically generated M(CO)(5)THF with a series of doubly deprotonated 1,2-disubstituted benzene rings with the appropriate oxygen, nitrogen, and sulfur donor atoms. These complexes have been characterized in the solid state by X-ray crystallography and in solution by IR and (13)C NMR spectroscopies. The crystal of 2c (C(84)H(71)N(3)O(5)P(4)SCr) is triclinic P&onemacr;, a =13.869(3) Å, b = 23.128(5) Å, c = 12.056(2) Å, alpha = 104.84(3) degrees, beta = 106.91(3) degrees, gamma = 95.29(3) degrees, Z = 2; that of 5c (C(89)H(77)N(7)O(3)P(4)SW) is monoclinic P2(1), a = 11.054(2) Å, b = 28.140(6) Å, c = 12.566(2) Å, beta = 90.58(1) degrees, Z = 2; that of 6c (C(85)H(70)N(4)O(4)P(4)SW) is triclinic P&onemacr;, 12.236(2) Å, b = 14.419(2) Å, c = 22.748(4) Å, alpha = 76.44(1) degrees, beta = 75.98(2) degrees, gamma = 70.98(1) degrees, Z = 2; that of 7b (C(82)H(64)N(2)O(4)P(4)S(2)W) is triclinic P&onemacr;, a = 12.650(1) Å, b = 14.810(1) Å, c = 21.053(2) Å, alpha = 77.182(7) degrees, beta = 78.334(7) degrees, gamma = 66.579(7), Z = 2; and that of 8 (C(10)H(8)O(4)P(2)W) is monoclinic P2(1)/c, a = 11.582(1) Å, b = 10.791(1) Å, c = 10.449(1) Å, beta = 100.867(7) degrees, Z = 2. The average nu(CO) frequencies for each tricarbonyl species reported are compared to those related dianions previously reported in order to gauge the pi-donor character of the different ligands. The (13)C NMR spectrum for each tricarbonyl derivative consists of a single sharp peak for the three inequivalent carbonyls as a result of a low-energy, fast intramolecular exchange process. Both inter- and intramolecular CO-exchange processes have been probed via variable temperature (13)C NMR. In the case of the 16-electron species the geometry of the metal dianion is that of a distorted trigonal bipyramid consisting of three carbonyl ligands and a five-membered chelate ring bound through the pi-donor atoms at an equatorial and an axial position, with the stronger pi-donor atom in the equatorial site. The equatorial site for the most effective pi-donor is preferred over the axial position because the unoccupied d(xy)() orbital lies in the equatorial plane, and may be stabilized via a pi-donor ligand in the equatorial position. The axial position exhibits a filled/filled repulsion as both orbitals available for pi-bonding are filled.

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PubMed1999

Coordination Chemistry, Structure, and Reactivity of Thiouracil Derivatives of Tungsten(0) Hexacarbonyl: A Theoretical and Experimental Investigation into the Chelation/Dechelation of Thiouracil via CO Loss and Addition.

The synthesis of 2-thiouracil and 6-methyl-2-uracil derivatives of tungsten carbonyl from the reaction of photogenerated W(CO)(5)(solvent) (solvent = MeOH or THF) and the corresponding [Et(4)N][thiouracilate] is described. The crystal structure of the [Et(4)N][W(CO)(5)(2-thiouracilate)], 1, derivative is reported where the thiouracilate is found to be bound to the tungsten center via the exocyclic sulfur atom. In the solid-state structure of 1 two anions are associated by means of two hydrogen bonds between the metal bound nucleobases. These pentacarbonyl complexes stereoselectively lose cis carbonyl ligands, as is apparent from (13)C-labeling studies, to provide the endocyclic nitrogen N(1)-chelated tungsten tetracarbonyl derivatives, e.g., [Et(4)N][W(CO)(4)(2-thiouracilate)], 3. The kinetics of the loss of CO from the pentacarbonyl anions to afford the metal tetracarbonyls, and the reverse of that process, were monitored by means of in situ infrared spectroscopy in the nu(CO) region as a function of temperature. These studies reveal that the tetracarbonyl anions in CO-saturated acetonitrile ([CO] approximately 6 x 10(-)(3) M) are unstable with respect to the formation of the pentacarbonyl derivatives, i.e., the equilibrium 1 right harpoon over left harpoon 3 + CO lies to the left under an atmosphere of carbon monoxide. From the activation parameters determined for the dissociative CO loss process (DeltaH() = 82.0 +/- 3.6 kJ mol(-)(1) and DeltaS() = -44.9 +/- 9.6 J mol(-)(1) K(-)(1) for complex 1) it is apparent that the sulfur-bound thiouracilate ligand is serving as a pi-donor during CO dissociation, i.e., behaving as a cis-labilizing ligand. Ab initio geometry optimizations carried out for the process 1 right harpoon over left harpoon 3 + CO at the Hartree-Fock and DFT levels support these experimental observations. For example, complex 1 is shown to be more stable than 3 + CO and chelation via the endocyclic N(1) donor is favored over N(3) binding. Finally, the "16-electron" intermediate resulting from CO dissociation in 1 was found to possess a significantly shortened W-S interaction, presumably due to the pi-donating ability of the thiouracilate ligand.

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PubMed1997

Structural Diversity in Monomeric Cadmium Phenoxides.

Three monomeric Cd(II) phenoxide complexes have been prepared by reacting Cd[N(SiMe(3))(2)](2) with 2 equiv of 2,6-disubstituted phenols bearing sterically bulky tert-butyl or phenyl groups. The strongly coordinating solvents THF, tetrahydrothiophene (THT), and pyridine used for these reactions were incorporated into the metal's coordination sphere, leading to complexes with a general formulation of Cd(O-2,6-R(2)C(6)H(3))(2)(solv)(2)(-)(3). The Cd complexes obtained have been characterized crystallographically and have been found to adopt differing solid-state geometries. The X-ray crystal structure of Cd(O-2,6-(t)BuC(6)H(3))(2)(THF)(2), 1, previously reported by Buhro, displayed unusual square-planar coordination of the metal center. Complex 2, Cd(O(t)BuC(6)H(3))(2)(THT)(2), has been found to take on the same square-planar geometry, even with the more strongly donating thioether ligand. Alternatively, complex 3, Cd(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), has been found to adopt distorted-tetrahedral geometry, quite similar to its zinc congener. The O(1)-Cd-O(2) angle between the phenoxide ligands in 3 is 150.1(2) degrees, and the angle between the ether ligands is 83.1(3) degrees. When the strongly basic solvent pyridine was used, a five-coordinate complex 4, Cd(O-2,6-(t)BuC(6)H(3))(2)(py)(3), was isolated. This complex 4 is best described as having trigonal bipyramidal geometry with the phenoxide ligands and one pyridine defining the equatorial plane and two axial pyridine ligands having an angle of 169.7(2) degrees. The angle between the phenoxide ligands in 4 is 156.1(2) degrees. These complexes 1-4 have also been characterized in noncoordinating solvent solutions by (1)H, (13)C, and (113)Cd NMR spectroscopy and have been found to contain labile donor ligands. Preliminary studies indicate that, in a noncoordinating solvent, a rapid equilibrium exists between species with and without coordinated donor solvent ligands.

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PubMed1997

Amino Acid Complexes of Metal Carbonyls: Mechanistic Aspects of the CO-Labilizing Ability of Glycinate Ligands in Zero-Valent Chromium and Tungsten Derivatives.

The amino and phosphino acid derivatives of chromium(0) and tungsten(0), [Et(4)N][Cr(CO)(4)(O(2)CCH(2)NH(2))] (1), [Et(4)N][Cr(CO)(4)(O(2)CCH(2)NHMe)] (2), [Et(4)N][Cr(CO)(4)(O(2)CCH(2)NMe(2))] (3), [Et(4)N][W(CO)(4)(O(2)CCH(C(CH(3))(3))NH(2))] (4), [Et(4)N][W(CO)(4)(O(2)CCH(C(6)H(5))NH(2))], [Et(4)N][W(CO)(4)(O(2)CCH(2)PPh(2))] (5), and [Et(4)N][Cr(CO)(4)(O(2)CCH(2)PPh(2))] have been synthesized from the reaction of the M(CO)(5)THF adduct with the tetraethylammonium salt of the corresponding amino or phosphino acid in THF solution. The complexes have been characterized in solution by (13)C NMR and infrared spectroscopies and in the solid state by X-ray crystallography. The geometry of the metal anion is, in each case, that of a distorted octahedron consisting of four carbonyl ligands and a puckered five-membered glycinate chelate ring, bound through the nitrogen atom and one of its oxygen atoms. Notable about complex 1 is that the crystal obtained exhibited both a different morphology and a different space group than its tungsten analogue. Examination of the packing diagram reveals that this change is due to the different orientation of the chelate ring in 1 relative to the corresponding orientation in the W(CO)(4)(O(2)CCH(2)NH(2))(-) anion. Complexes 1 and 2 exhibit intermolecular hydrogen-bonding interactions between the amine N-H group and the distal oxygen on an adjacent molecule, with N.O distances of 2.828 and 2.821 Å, respectively. Investigations of the lability of the carbonyl ligands have been carried out. The lability is proposed to be due to base-assisted removal of a proton from the amine ligand leading to a substitutionally labile amide transient species. The tungsten analogue of complex 1 was used to obtain evidence in support of this mechanism. The isotope effect (k(H)/k(D)) was measured for W(CO)(4)(O(2)CCH(2)NH(2))(-) using d(5)-glycine and was found to be 2.34. The activation parameters for the intermolecular exchange of CO in the [Et(4)N][W(CO)(4)(O(2)CCH(2)NH(2))] salt were determined and found to be DeltaH() = 15.4 +/- 1.0 kcal/mol and DeltaS() = -23.2 +/- 3.2 eu, values consistent with the proposed mechanism. In addition, the effect of substitution of electron-donating (C(CH(3))(3)) and electron-withdrawing (C(6)H(5)) substituents on the methylene carbon was evaluated. There was little change in the rate of CO exchange observed for W(CO)(4)(O(2)CCH(C(CH(3))(3))NH(2))(-) (5) and W(CO)(4)(O(2)CCH(C(6)H(5))NH(2))(-) vs W(CO)(4)(O(2)CCH(2)NH(2))(-), showing that steric or electronic effects away from the N center are not responsible for the observed CO lability. As anticipated on the basis of the proposed substitutional pathway, the phosphino acid metal carbonyl derivatives did not exhibit facile intermolecular CO exchange.

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PubMed1997

Syntheses, Structures, and Binding Constants of Cyclic Ether and Thioether Adducts of Soluble Cadmium(II) Carboxylates. Intermediates in the Homopolymerization of Oxiranes and Thiiranes and in Carbon Dioxide Coupling Processes.

A synthetic methodology for the preparation of a large variety of eta(3)-HB(3-Phpz)(3)Cd(acetate) adducts is presented which involves replacement of toluene in the eta(3)-HB(3-Phpz)(3)Cd(acetate) solvate complex by the appropriate cyclic ether or cyclic thioether. In this manner, adducts of THF, dioxane, propylene oxide, cyclohexene oxide, and propylene sulfide were isolated. The solid-state structures of several of these complexes were determined by X-ray crystallography, revealing a six-coordinate complex where the acetate ligand is shown to be fairly symmetrically bonded to the cadmium center. In methylene chloride solution, the cyclic ether or thioether readily dissociates to afford the five-coordinate complex, as demonstrated by (113)Cd NMR. A quantitative assessment of the binding of these base adducts of eta(3)-HB(3-Phpz)(3)Cd(acetate) was determined by measuring the temperature dependence of the equilibrium constants for the five- and six-coordinate derivatives. The presence of one sharp (113)Cd resonance in this equilibrium mixture is indicative of rapid intermolecular exchange between the five- and six-coordinate complexes when compared to the chemical shift differences in these two species ( approximately 6600 Hz at 89 MHz). The order established for ether binding is THF > dioxane > propylene sulfide > cyclohexene oxide >/= propylene oxide, with DeltaH degrees and DeltaS degrees spanning the ranges -27.7 to 24.3 kJ/mol and -89.7 to -94.1 J/(mol K). The epoxide and thioepoxide adducts were shown to serve as models for the initiation step in the copolymerization of epoxides with carbon dioxide catalyzed by metal carboxylates. That is, the carboxylate ligand was shown to ring-open the epoxide or thioepoxide, subsequently affording polyethers or polythioethers with ester end groups. By way of contrast, in the presence of CO(2) and epoxides, this system led to cyclic carbonate production.

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PubMed1996

The Reversible Insertion Reaction of Carbon Dioxide with the W(CO)(5)OH(-) Anion. Isolation and Characterization of the Resulting Bicarbonate Complex [PPN][W(CO)(5)O(2)COH].

[Et(4)N][W(CO)(5)OH] (1) and [PPN][W(CO)(5)O(2)COH] (2) have been synthesized and characterized by (1)H and (13)C NMR and IR spectroscopies, and the X-ray crystal structure of 2 has been determined. Complex 2 crystallizes in the triclinic space group P&onemacr; with unit cell parameters a = 12.208(2) Å, b = 13.497(2) Å, c = 13.681(2) Å, alpha = 101.06(2) degrees, beta = 114.76(1) degrees, gamma = 98.45(2) degrees, V = 1942.6(5) Å(3), and Z = 2. The structure of the anion of complex 2 consists of a central W(0) bound to five carbonyl ligands, and the coordination around the metal is completed by a monodentate bound bicarbonate ligand located 2.19(1) Å away from the metal center. In the solid state, two anions are hydrogen bonded to one another via the bicarbonate ligands in the unit cell. Complex 1 inserts CO(2), COS, or CS(2) to rapidly afford the corresponding bicarbonate or thiocarbonate complexes. The lower limit for the rate constant for the carboxylation of complex 1 has been determined to be 4.2 x 10(-)(4) M(-)(1) s(-)(1) at -70.2 degrees C, and the lower limit for the rate constant for the decarboxylation of complex 2 has been found to be 2.5 x 10(-)(3) s(-)(1) at 20.0 degrees C. In addition, the rate constant for the decarbonylation of 2 was determined to be 7.60 x 10(-)(3) s(-)(1) at 36.0 degrees C, a value which is somewhat faster than anticipated on the basis of analogous data for a large variety of W(CO)(5)O(2)CR(-) derivatives. This is attributed to a diminution of the electron-withdrawing ability of the OH substituent in O(2)COH as a result of hydrogen bonding to solvent. Nevertheless, it is clear that the rate of decarboxylation of the anion from complex 2 is faster than the rate of CO dissociation. Concomitantly, carboxylation of complex 1 is faster than CO dissociation, since the W(CO)(5)OH(-) is inert toward (13)CO exchange on the time scale of carboxylation at -70.2 degrees C.

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PubMed1996

Coordinatively Unsaturated Derivatives of Group 6 Metal Carbonyls Containing the pi-Donating Ligand 3,5-Di-tert-butylcatecholate.

The series of group 6 metal tricarbonyl derivatives of di-tert-butylcatecholate have been synthesized from the reactions of M(CO)(5)THF (M = Cr, Mo, W) with 2 equiv of [Et(4)N][3,5-(t)Bu(2)OC(6)H(2)OH]. Subsequent removal of the free catechol was achieved by the addition of NaOMe. The complexes were shown by X-ray crystallography to exhibit coordinatively unsaturated M degrees centers. These metal dianions which have formally 16e(-) configurations are stabilized by pi-donation from the oxygen atoms of the catecholate ligand. This is evident from the short M-O bond distances, e.g., for M = W, 2.059(6) Å vs 2.151(4) Å for a single bond. The structures of these five-coordinate dianions can be loosely defined as trigonal bipyramidal with the more electron-rich oxygen donor of the catecholate (ortho to the electron-releasing tert-butyl substituent) occupying an equatorial site as indicated by a shorter M-O bond length. The tungsten derivative was shown to reversibly react with CO or phosphines to afford the 18e(-), saturated complexes. Although the molybdenum tricarbonyl derivative reacts with CO to partially provide the tetracarbonyl complex, the analogous process involving chromium did not occur. That is, the formation of an O-->M pi bond vs an additional M-CO bond is favored for M = chromium. Complex 2, [Et(4)N](2)[W(CO)(4)DTBCat], crystallizes in the monoclinic space group P2(1)/c with a = 10.013(5) Å, b = 43.921(14) Å, c = 9.113(4) Å, beta = 115.76(3) degrees, V = 3609(3) Å(3), and d(calc) = 1.429 g/cm(3), for Z = 4. Complex 4, [Et(4)N](2)[Mo(CO)(3)DTBCat], crystallized in the monoclinic space group C2 with a = 18.255(7) Å, b = 8.596(3) Å, c = 22.369(7) Å, beta = 91.05(6) degrees, V = 3510(2) Å(3), and d(calc) = 1.251 g/cm(3), for Z = 4. Similarly, complex 5, [Et(4)N](2)[Cr(CO)(3)DTBCat], crystallized in the monoclinic space group C2 with a = 18.09(2) Å, b = 8.553(3) Å, c = 21.927(11) Å, beta = 91.09(8) degrees, V = 3393(4) Å(3), and d(calc) = 1.208 g/cm(3), for Z = 4.

Journal Article↗
PubMed1996

Coordinatively and Electronically Unsaturated Tungsten(0) Carbonyl Complexes Stabilized by pi-Donating Amido Ligands.

Novel, coordinatively and electronically unsaturated tungsten tricarbonyl dianions of 2-aminophenol and 1,2-diaminobenzene have been synthesized from the reaction of photogenerated W(CO)(5)THF and [Et(4)N][OC(6)H(4)NH(2)] with subsequent deprotonation by [Et(4)N][OH] accompanied by facile CO dissociation, and the reaction of W(CO)(5)THF and 2 equiv of [Et(4)N][NHC(6)H(4)NH(2)], respectively. These air-sensitive derivatives have been fully characterized both in solution (nu(CO) and (13)C NMR) and in the solid-state (X-ray crystallography). These metal dianions which have formally 16e(-) configurations are stabilized by pi-donation from the amido groups of the chelating ligands, as evident from short W-N bond distances. Both solution (nu(CO) and (13)C NMR) and solid-state (W-N vs W-O bond distances) data on these derivatives indicate the amido ligand to be a better pi-donor than the oxo ligand. Complex 2 crystallized in the monoclinic space group P2(1)/n with a = 14.499(5) Å, b = 14.708(5) Å, c = 15.137(4) Å, beta = 114.13(2) degrees, V = 2946(2) Å(3), and d(calc) = 1.433 g/cm(3), for Z = 4. Complex 3 crystallized in the triclinic space group P&onemacr; with a = 11.479(6) Å, b = 11.786(8) Å, c = 13.148(7) Å, alpha = 102.41(5) degrees, beta = 91.27(4) degrees, gamma = 99.96(5) degrees, V = 1708(2) Å(3), and d(calc) = 1.444 g/cm(3), for Z = 2.

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