David V. Partyka

Homoleptic, Four-Coordinate Azadipyrromethene Complexes of d10 Zinc and Mercury

Tetraarylazadipyrromethenes, and especially their boron chelates, are a growing class of chromophores that are photoactive toward red light. The coordination chemistry of these ligands remains to be explored. Reported here are four-coordinate zinc(II) and mercury(II) complexes of tetraarylazadipyrromethene ligands. The new complexes contain two azadipyrromethenes bound per d(10) metal center and are characterized by (1)H NMR, optical absorption spectroscopy, X-ray diffraction crystallography, and elemental analysis. Solid-state structures show that these bis-chelate complexes distort significantly from idealized D2d symmetry. AM1 geometry optimizations indicate relaxation energies in the range of 6.8-15.2 kcal mol(-1); interligand pi-stacking provides an added energetic impetus for distortion. The absorption spectra show a marked increase in the absorption intensity in the red region and, in the case of the zinc(II) complexes, the development of a second distinct absorption band in this region, which is red-shifted by ca. 40-50 nm relative to the free ligand. Semiempirical INDO/S computations indicate that these low-energy optical absorptions derive from allowed excitations among ligand-based orbitals that derive from the highest occupied molecular orbital and lowest unoccupied molecular orbital of the free azadipyrromethene.

Synergistic Binding of Both Lewis Acids and Bases to Phosphinidenes

Free phosphinidenes or phosphanylidenes (RP), the heavier analogues of carbenes, remain unisolable owing to their extreme reactivity. Although a significant body of stable transition metal complexes having terminal PR functionalities now exists, simple main group adducts of phosphinidenes are by comparison rare. Phosphanylidene-s-phosphoranes (RP= PR’3) are a fundamental type of base-stabilized adducts of phosphinidenes First recognized in 1961 with the synthesis of CF3P=PMe3, [3] and later studied in more detail, these early derivatives were thermally unstable and not structurally characterized. Derivatives bearing additional phosphorus atoms and lone pairs, such as R2P P=P(X)R’2 are also an important class of such materials, and they have received attention as phosphinidene precursors and transfer reagents. 5] The more recent discovery of thermally stable ArP=PR3, where Ar is a sterically demanding aryl group, allowed for structural identification and more detailed studies. Phosphanylidene-s-phosphoranes can be pictorially represented by various resonance forms (Scheme 1, top)

Phosphine- and Carbene-Ligated Silver Acetate: Easily-Accessed Synthons for Reactions with Silylated Nucleophiles

The useful synthon tricyclohexylphosphinesilver(I) acetate is easily prepared on gram scale by the reaction of silver(I) acetate and tricyclohexylphosphine in a 1:1 ratio in toluene. (PCy(3))Ag(OAc) (1) reacts with a wide range of silylated nucleophiles (Me(3)Si-X; product with X = N(3), 2; Cl, 3; SCN, 4; 1,2,4-triazol-1-yl, 5; trifluoromethanesulfonate (OTf), 6; SPh, 8; Br, 9) to effect room temperature Ag-X bond formation at the expense of the Ag-OAc bond. All new products were characterized by multinuclear NMR spectroscopies, IR spectroscopy, microanalysis, and X-ray crystallography. X-ray crystallography indicated a variety of coordination geometries at silver(I) are accessible, as di- and tetranuclear complexes were observed in all cases except 1, which forms a three-coordinate, mononuclear complex. In the case of 8, NMR and mass spectrometric data suggest fluxional species of variable nuclearity (but with empirical formula [(PCy(3))Au(SPh)](n)) exist in solution. To provide more definitive evidence of Ag-S bond formation, the ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) was used to synthesize a new starting material, (IPr)AgOAc (10), the Ag-OAc bond of which is amenable to silylation by Me(3)Si-X (complex with X = N(3), 11; Cl, 12; SPh, 14). Complex 14 was characterized crystallographically, and provided definitive evidence for Ag-S bond formation via silylation with PhS-SiMe(3). Me(3)SiBr and Me(3)SiI are also competent in the silylation of 10 to yield 13 and 15, but these compounds were more cleanly synthesized by the reaction of 12 and KBr/KI in a biphasic CH(2)Cl(2)/H(2)O mixture. In a preliminary exploration of reactivity, it was determined that azidosilver(I) complexes 2 and 11 react rapidly and quantitatively with (NO)(SbF(6)) (as was previously demonstrated in a related azidogold(I) system) to yield cationic silver(I) species (detected by mass spectrometry). In acetonitrile solution, ligand rearrangements of these cationic silver(I) species yield cationic bis(phosphine) or bis(carbene) complexes, the identities of which were authenticated by X-ray crystallography.

Constrained Digold(I) Diaryls: Syntheses, Crystal Structures, and Photophysics

A series of di(gold(I) aryls), L(AuR)(2) (L = DPEphos, DBFphos, or Xantphos; R = 1-naphthyl, 2-naphthyl, 9-phenanthryl, or 1-pyrenyl), have been prepared. The complexes were characterized by multinuclear NMR spectroscopy, static and time-dependent optical spectroscopy, mass spectrometry, microanalysis, and X-ray crystallography. In addition, DFT calculations on model dinuclear gold complexes have been used to examine the electronic structures. Photophysical properties of the dinuclear complexes have been compared to mononuclear analogues. Low-temperature excited-state lifetimes for both the mononuclear and dinuclear complexes in toluene indicate triplet-state emission. Time-resolved DFT calculations suggest that emission originates from aryl-ligand transitions, even if the LUMO resides elsewhere.

Arylgold(I) Complexes from Base-Assisted Transmetalation: Structures, NMR Properties, and Density-Functional Theory Calculations

The synthesis of gold(I) complexes of the type LAuR (L = PCy(3), IPr; R = aryl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) starting from LAuX (X = Br, OAc) and boronic acids in the presence of Cs(2)CO(3) has been investigated. The reactions proceed smoothly in good to excellent yields over the course of 24-48 h in isopropyl alcohol at 50-55 °C. The aryl groups include a variety of functionalities and steric bulk, and in two cases, are heterocyclic. All of the products have been characterized by multinuclear NMR spectroscopy and elemental analysis and most by X-ray crystallography. This work affirms that, almost without exception, base-assisted auration is a useful and reliable way to form gold-carbon bonds.

Gold(I) halide complexes of bis(diphenylphosphine)diphenyl ether ligands: a balance of ligand strain and non-covalent interactions

A series of bis(gold(I) halide; halide = Cl, Br, I) complexes of di(phosphino)diphenyl ether derivatives (L = DPEphos, DBFphos, Xantphos, tBuXantphos) have been synthesized. The new complexes have been characterized by X-ray crystallography, multinuclear NMR, and elemental analysis. The compounds luminesce at room temperature in dichloromethane solution. Many such complexes undergo aurophilic Au...Au bonding, and have chiral structures as a result. In complexes of the tBuXantphos ligand, X-ray crystallography indicates that an ion pair forms where the diphosphine ligand chelates one gold atom, and the other is part of an [AuX(2)](-) counterion (X = Cl, Br, I). It appears that the observed conformations of the metal-coordinated ligands are a balance of ligand strain and non-covalent interactions, including aurophilicity, intramolecular pi-stacking, halide-halide repulsion, and intramolecular Au-O interactions. Together with previous investigations, this research shows that Xantphos and its derivatives form a robust set of coordination complexes with gold that are stable in air and amenable to further synthetic manipulation. It is anticipated that these materials will be suitable precursors for gold-carbon coupling reactions and gold-based catalysis.

A porphyrin complex of Gold(I): (Phosphine)gold(I) azides as cation precursors

A silver- and Brönsted acid-free protocol for generating the (tricyclohexylphosphine)gold(I) cation from the corresponding azide complexes is disclosed. The gold(I) cations so liberated are trapped by complexation with octaethylporphyrin. The first structurally authenticated gold(I) porphyrin complex crystallizes with formula C72H112Au2F12N4P2Sb2, space group C2/c, a = 21.388 (4), b = 19.679 (4), c = 19.231 (3) Å; β = 111.030 (3)°. Solution spectroscopic studies indicate that the di-gold complex fragments on dissolution in organic solvents. Approximate density-functional theory calculations find an electrostatic origin for the binding of two gold(I) centers to the unprotonated nitrogen atoms, despite greater orbital density on the porphyrin meso carbons.