Biomedical subjects

F Albert Cotton

Publications and source records attributed to F Albert Cotton.

At least 19 recordsLinked to original sources

PubMed2007

Enantioseparation of extended metal atom chain complexes: unique compounds of extraordinarily high specific rotation.

Extended metal atom chains (EMACs) contain a linear metal chain wrapped by various ligands. Most complexes are of the form M(3)(dpa)(4)X(2), where M = metal, dpa = 2,2'-dipyridylamide, and X = various anions. The ligands form helical coils about the metal chain, which results in chiral EMAC complexes. The EMACs containing the metals Co and Cu were partially separated in polar organic mode using a vancomycin-based chiral stationary phase. Under similar conditions, two EMACs with Ni metal and varying anions could be baseline separated. The polar organic mode was used because of the instability of the compounds in aqueous mobile phases. Also, these conditions are more conducive to preparative separations. Polarimetric measurements on the resolved enantiomers of Ni(3)(dpa)(4)Cl(2) indicate that they have extraordinarily high specific rotations (on the order of 5000 deg cc/g dm).

Anti-Bacterial Agents↗

PubMed2006

Molecular pairs and a propeller containing quadruply bonded dimolybdenum units linked by polyamidate ligands.

Two molecular pairs [Mo2(DAniF)3]2[N,N'-diethylterephthalamidate] (1) and [Mo2(DAniF)3]2{1,3-C6H4[C(O)NP]} (2) where DAniF = N,N'-di-p-anisylformamidinate) and the propeller ([Mo2(DAniF)3]3{1,3,5-C6H3[C(O)NPh]3} (3)) have been prepared in good yield and high purity by directly combining Mo2(DAniF)3(O2CCH3) with the corresponding polyamidates. Electrochemical measurements of these complexes show unresolved redox waves, which indicate that the dimetal centers are only electronically weakly coupled. Compound 1 was chemically oxidized by ferrocenium tetrafluoroborate to the two-electron oxidation product 4, where one electron was removed from each of the [Mo2] units. The hyperfine coupling in the EPR spectrum (A = 22 x 10-4 cm-1 ) suggests that 4 is an electron-trapped species with one electron residing on each of the two dimolybdenum units, as suggested also by electrochemical measurements.

Journal Article↗

PubMed2006

Strong electronic communication by direct metal-metal interaction in molecules with halide-bridged dimolybdenum pairs.

Reactions of [cis-Mo2(DAniF)2(NCCH3)4](BF4)2, DAniF = N,N'-di-p-anisylformamidinate, with an excess of anhydrous Bu(n)4NX (X = Cl, Br, I), produced the halide-bridged tetranuclear clusters, [cis-Mo2(DAniF)2]2(mu-X)4, X = Cl (1), Br (2), and I (3). All three compounds show two reversible one-electron oxidation processes with potential separations (DeltaE(1/2)) between the two oxidation processes of 540, 499, and 440 mV, respectively. These DeltaE(1/2) values show that the strength of the electronic coupling between the dimetal units decreases as the Mo2...Mo2 distance increases from 1 to 2, and then to 3. The structures, EPR spectra, and near-IR (NIR) spectra of the corresponding mixed-valence species (1-PF6, 2-PF6, and 3-PF6) indicate that the clusters are electronically delocalized. Calculations at the DFT level indicate that the strong electronic communication is principally due to a direct overlap between the delta orbitals from the adjacent dimetal units.

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PubMed2006

An isomeric pair of fluoride-bridged cyclic dimolybdenum triads.

A pair of isomeric cyclic triads containing three quadruply bonded [Mo2] units, [Mo2(cis-DAniF)2]2+ (DAniF = N,N'-di-p-anisylformamidinate), bridged by six fluoride anions, has been synthesized and crystallographically characterized. For the alpha isomer, the three [Mo2] units are oriented in two orthogonal directions. Two of them are structurally equivalent and parallel to each other, but oriented perpendicular to the third one. The beta isomer is a triangle with three geometrically identical [Mo2] units, parallel to each other, as the vertices. Thus, the beta isomer possesses idealized D3h symmetry while the alpha isomer only has C2v symmetry. These two isomers do not interconvert in boiling THF or toluene or under irradiation with ultraviolet light, but oxidation of the alpha isomer first generates an alpha+ species that changes to beta+. The two isomers have very similar electrochemical behavior, both showing three reversible one-electron redox processes for the [Mo2] centers and similar potential separations (DeltaE(1/2)). The first and second redox couples are well separated (ca. 390-410 mV), while the second and third ones are separated by only about 150 mV.

Journal Article↗

PubMed2006

A rare and highly oxidized Mo(2)5.5+ unit stabilized by oxo anions and supported by formamidinate bridges.

A series of tetranuclear compounds consisting of two {Mo2[(m-CF3C6H4)NC(H)N(m-CF3C6H4)]3}n+ moieties linked by two OH- or two O2- ions has been characterized. Abbreviating the dimolybdenum plus three spectator bridging ligands as [Mo2], the following three compounds have been made-[Mo2](mu-OH)2[Mo2] (1), [Mo2](mu-O)2[Mo2] (2), and {[Mo2](mu-O)2[Mo2]}SbF6 (3). Compound 1, which is diamagnetic and contains quadruply bonded Mo2(4+) units, is converted to diamagnetic 2 by oxidation with O2. Compound 2, which has Mo2(5+) units, is oxidized by NOSbF6 to 3 which has a rare Mo2(5.5+) core and an odd electron delocalized over the two dimolybdenum units.

Electron Spin Resonance Spectroscopy↗

PubMed2006

High yield syntheses of stable, singly bonded Pd2(6+) compounds.

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.

Journal Article↗

PubMed2006

Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo2(4+) compounds and their bicyclic guanidinate precursors.

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo2(4+) units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 +/- 0.03 eV and an ionization onset at the very low energy of 3.93 +/- 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (eta5-C9Me7)2Co and (eta5-C5Me5)2Cr.

Journal Article↗

PubMed2006

A trimetal chain cocooned by two heptadentate polypyridylamide ligands.

Compounds of nickel in which a chain of three metal atoms is closely embraced by two interlocking heptadentate dianions derived from a chain of five pyridyl groups linked at the 2, or 2 and 6, positions by four amide nitrogen atoms are reported. This new type of extended metal atom chain (EMAC) compound differs from earlier ones in that ligand exchange at the axial positions cannot occur, because the axial ligands are part of the entire ligand. Four such compounds, all crystallographically characterized, are reported. This work is a proof-of-concept project that will be extended to other metals with these and other homologous ligands.

Journal Article↗

PubMed2006

Strong reducing agents containing dimolybdenum Mo2(4+) units and their oxidized cations with Mo2(5+/6+) cores stabilized by bicyclic guanidinate anions with a seven-membered ring.

The syntheses of two analogues of the bicyclic guanidinate ligand hpp (hpp = the anion of the guanidine-type compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine) which contain two fused rings are reported. Each compound contains one seven-membered ring while the other is either a five (Htbd) or a six (Htbu) membered ring. In THF/Bu4NPF6, the dimolybdenum compounds Mo2(tbd)4 and Mo2(tbu)4 are easily oxidized and they have signals in the differential pulse voltammograms at -1.059 and -1.009 V (vs. Ag/AgCl), respectively and for the Mo2(5+/6+) couples and in the same order -0.242 and -0.312 V for the Mo2(6+/5+) couples. The two compounds produce the corresponding Mo2(bicyclic guanidinate)4Cl compounds immediately upon dissolution in CH2Cl2 and these easily form species with Mo2(6+) cores. In Mo2(tbd)4Cl there are two crystallographically independent molecules with Mo-Mo distances of 2.1711(7) and 2.1690(7) A. The distance between metal atoms increases to 2.206(1) A upon oxidation to Mo2(tbd)4Cl2 which has a triply bonded Mo2(6+) core. For the diamagnetic compound Mo2(tbu)4 this distance is 2.0677(9) A and it increases to 2.133(2) A upon reduction of the bond order from 4 to 3.5 in the paramagnetic compound Mo2(tbu)4Cl.

Journal Article↗

PubMed2006

Transition from a nonbonding to a bonding interaction in a tetranuclear [Mo2]2(mu-OR)4 cluster.

Tetranuclear Mo4 clusters with two quadruply bonded Mo2(4+) units, [Mo2(cis-DAniF)2] (DAniF = N,N'-di-p-anisylformamidinate), linked by alkoxides (OCH3 for 1 and OC2H5 for 4) have been prepared. The nonbonding separation between the midpoints of the quadruply bonded units, ca. 3.24 A, is the shortest among compounds having two linked Mo2(4+) units. Electrochemical measurements show two redox waves for each compound with large DeltaE(1/2) values (554 and 587 mV for 1 and 4, respectively) that correspond to K(C) values on the order of 10(9). The large electronic communication is attributed to the short separation between dinuclear units that favor direct delta-to-delta orbital interactions between the two dimetal centers. Compound 1 was chemically oxidized using stoichiometric amounts of ferrocenium salts to a one-electron oxidation product 2 (in which the counteranion is PF6-) and a two-electron oxidation product 3 (which contains two BF4- anions). Upon oxidation there are significant decreases in the distance between the two [Mo2] units to 3.100 A and then to 2.945 A. The mixed-valence species 2 shows two broad absorption bands at 5900 and 7900 cm(-1) in the NIR region which are assigned to the HOMO-1 --> SOMO and HOMO-2 --> SOMO transitions. Compound 3 is fluxional in solution, as shown by variable-temperature 1H NMR spectra. The sharp signals in the NMR spectrum at -50 degrees C and the lack of an EPR signal suggest that this species is diamagnetic and that a four-center, two-electron bond is formed in the cyclometallic Mo4 cluster. To a first-order approximation, an average bond order of 0.25 is assigned to the bonding interaction between the two Mo atoms along the long edges of the rectangle defined by the four Mo atoms.

Journal Article↗

PubMed2006

Homologues of the easily ionized compound Mo2(hpp)4 containing smaller bicyclic guanidinates.

Two bicyclic guanidinate ligands consisting of 5,5-membered (tbo) and 5,6-membered (tbn) rings have been used for the preparation of dimolybdenum compounds, such as Mo2(tbo)4 and Mo2(tbo)4Cl, and species containing Mo2(tbn)4(n+) with n = 0-2. The compounds with quadruply bonded Mo2(4+) species are strong reducing agents and have potentials of about -1 V (vs Ag/AgCl) for the Mo2(5+/4+) process. The structure of the THF solvate of Mo2(tbo)4 shows the longest Mo-Mo bond distance, 2.1453(4) A, for a quadruply bonded species, and this is due to a large divergent angle induced by the geometry of the ligand. This distance increases to 2.2305(8) A upon oxidation by CH2Cl2 to Mo2(tbo)4Cl. For the 5,6-membered-ring ligand tbn, even though the divergent angle is large compared to formamidinate ligands, it is not as large as that in tbo, and the Mo-Mo distance in Mo2(tbn)4, 2.082(1) A, is in the normal range for paddlewheel Mo2(4+) compounds. This distance increases to 2.2233(8) A upon oxidation by O2 in CH2Cl2, which forms Mo2(tbn)4Cl2 x CH2Cl2.

Journal Article↗

PubMed2006

Metal-metal bonding in mixed valence Ni2(5+) complexes and spectroscopic evidence for a Ni2(6+) species.

Dinickel(II) complexes of the ligands N,N'-di-p-anisylformamidinate (DAniF) and N,N',N''-triphenylguanidinate (TPG) have been synthesized and crystallographically characterized, along with their one-electron-oxidized analogues. In both systems, the Ni-Ni distances become shorter by approximately 0.1 A upon oxidation, in accord with the proposal that the resulting Ni2(5+) complexes are appropriately described as having one electron removed from a metal-based sigma orbital and an overall Ni-Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni2(HNCHNH)4 and [Ni2(HNCHNH)4]+ appear to predict that the lowest energy state of the latter species would have one unpaired electron in an essentially ligand-based orbital. A single-point calculation of Ni2(DAniF)4 employing the geometry of its crystal structure with the full ligand included reveals a reversal of the previously predicted order of the HOMO and HOMO-1, and suggests that the unpaired electron in [Ni2(DAniF)4]+ is in a metal-based orbital of sigma symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni2(DAniF)4 shows an unexpectedly rich cyclic voltammogram with four stepwise reversible oxidation waves. Coulometric experiments show that the doubly oxidized species has a significant lifetime at -25 degrees C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni2(6+) core with a single Ni-Ni sigma bond.

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PubMed2006

A diamagnetic dititanium(III) paddlewheel complex with no direct metal-metal bond.

Reaction of Ti[N(But)Ar]3 (Ar = 3,5-C6H3Me2 or Ar' = C6H5) with CO2 at -40 degrees C produces diamagmetic Ti(III) paddlewheel complexes with long Ti-Ti separations (>3.4 Angstrom), thus excluding direct Ti-Ti bonding. 1H NMR spectroscopy shows that the compounds are diamagnetic in solution in the temperature range of -65 to +70 degrees C. In the solid state, the diamagnetism was found to persist between 2 and 300 K. Calculations at the density functional theory level suggest that the diamagnetism results from antiferromagnetic coupling by superexchange through the ligand pi system.

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PubMed2006

Dynamic equilibrium between cyclic oligomers. Thermodynamic and structural characterization of a square and a triangle.

A dynamic equilibrium has been found in CDCl3 between a neutral molecular square, [cis-Mo2(DAniF)2]4(O2CC6F4CO2)4 (1) and triangle, [cis-Mo2(DAniF)2]3(O2CC6F4CO2)3 (2) (DAniF = the anion of N,N'-di-p-anisylformamidine). The two components have been crystallographically characterized and solution studies by 1H and 19F NMR spectra of the concentration- and the temperature-dependence of the equilibrium have been performed. The conversion of three moles of molecular squares 1 to four moles of molecular triangles 2 has an equilibrium constant of 1.98(7) x 10(-4) at 23.7 degrees C. At this temperature, the DeltaG(0) for the conversion of three moles of squares to four moles of triangles is 21.0 kJ mol(-1). The conversion is enthalpically disfavored (DeltaH(0) = 23.5 kJ mol(-1)), but entropically favored (DeltaS(0) = 8.2 J K(-1) mol(-1)).

Journal Article↗

PubMed2006

Dimolybdenum-containing molecular triangles and squares with diamidate linkers: structural diversity and complexity.

By employing cis-Mo2(DAniF)2(2+) (DAniF = N,N'-di(p-anisyl)formamidinate) as the vertex building block and terephthaloyldiamidate as the linker, four dimolybdenum-containing cyclic oligomers have been synthesized and structurally characterized. In these compounds, described by the general formula [cis-Mo2(DAniF)2((ArNOC)2C6H4)2]n, n = 3 and 4, the geometry and composition of the products are affected by the identity of the aromatic groups of the linker. When Ar = phenyl, n = 3 (1a and 1b); however, n = 4 for Ar = p-trifluoromethylphenyl (2) and when Ar = m-trifluoromethylphenyl (3). All these compounds have a central cavity, shaped by the diamidate linker, that is capable of serving as host to guest molecules in a selective manner. For compounds 2 and 3, self-assembly that takes place in the crystalline state entails intermolecular C-H...F-C interactions. Such interactions generate a one-dimensional network with a tunnel cross section of 10 x 10 A(2) in 2, whereas in 3, they result in a cage in which two THF molecules are encapsulated. The F...H distances vary in a broad range from 2.38 to 2.70 A.

Journal Article↗

PubMed2006

Uniquely strong electronic communication between [Mo2] units linked by dioxolene dianions.

Unprecedented strong electronic communication has been found in dimolybdenum pairs containing quadruply bonded Mo2(DAniF)3(+) (DAniF = N,N'-di-p-anisylformamidinate) units linked by dioxolene (C6X2O4(2-)) anions. The neutral compounds [Mo2(DAniF)3]2(C6X2O4) (1, X = H; 2, X = Cl; 3, X = NO2) and the singly oxidized products {[Mo2(DAniF)3]2(C6X2O4)Mo2}PF6 (4, X = H; 5, X = Cl) have been synthesized and characterized by X-ray crystallography and spectroscopic methods. Unusually short Mo-O distances (approximately 2.05 A for 1 and 2, and approximately 2.01 A for 4 and 5) are implicated in the remarkably strong interaction between [Mo2] units via the linkers. This leads to an extensive charge delocalization in the mixed-valence species, which is mediated by the dioxolene linker, as revealed by the large deltaE(1/2) values (763, 795, and 816 mV for 1, 2, and 3, respectively). Additional evidence for the strong electronic coupling is provided by UV-vis, NIR, and EPR spectroscopies and DFT calculations.

Journal Article↗

PubMed2006

Strong electronic interaction between two dimolybdenum units linked by a tetraazatetracene.

The large rigid dianion fluoflavinate, C(14)H(8)N(4)(2)(-), consisting of four fused and planar six-membered rings with four nitrogen donor atoms, has been used to link two metal-to-metal bonded and redox-active Mo(2)(n)()(+) units which are each locally bridged by three additional groups, collectively denoted [Mo(2)]. In 1, the [Mo(2)] units are Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate), and in 5, they are trans-Mo(2)(DAniF)(2)(O(2)CCH(3)) groups. These [Mo(2)](fluoflavinate)[Mo(2)] compounds show three reversible one-electron oxidation steps, one more than all other [Mo(2)](linker)[Mo(2)] species known to date. The first two redox processes are metal-based, and the third one has been assigned to a ligand oxidation by comparison to that of paddlewheel compound 4 which contains only one dimolybdenum unit with a monoanionic fluoflavinate ligand. Chemical oxidations of 1 produce the singly- and doubly-oxidized species 2 and 3, respectively. All compounds have been characterized by X-ray crystallography and, as appropriate, by various techniques such as NMR, EPR, near-IR, and UV-vis. The fluoflavinate ligand strongly mediates electronic communication between the dimetal units, and the mixed valence species 2 can be described as electronically delocalized. Calculations at the DFT level using a variety of functionals support such an assignment and indicate that a strong transition in the NIR for the singly oxidized species can be assigned to the HOMO-1 to SOMO transition.

Journal Article↗

PubMed2006

Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds.

A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.

Chlorides↗