Synthetic, structural, spectroscopic and aging studies conclusively show that the main colorant of historical iron gall ink (IGI) is an amorphous form of Fe(III) gallate• xH 2 O (x = ∼1.5−3.2). Comparisons between experimental samples and historical documents, including an 18th century hand-written manuscript by George Washington, by IR and Raman spectroscopy, XRD, X-ray photoelectron spectroscopy, and Mossbauer spectroscopy confirm the relationship between the model and authentic samples. These studies settle controversy in the cultural heritage field, where an alternative structure for Fe(III) gallate has been commonly cited.P rior to the 20th century, historical iron gall ink (IGI) was by far the most common writing material of the western world, and a plethora of recipes from which to produce the ubiquitous dark, brown-black ink can be found starting from at least the Middle Ages. This ink has been used to pen many of the most important documents and drawings in human history, including unique, hand-written works such as Thomas Jefferson's original draft of the Declaration of Independence, Abraham Lincoln's first draft of the Emancipation Proclamation, and Beethoven's original scores. While the virtue of IGI lies in its relative permanence, the great vice of this medium lies in its well-known tendency to degrade paper and parchment substrates. 1−7 Despite its historical importance, there is little consensus on the chemical structure or composition of the iron-gallate complex, the main species responsible for the color of the IGI. We describe here a series of synthetic, structural, spectroscopic, and aging studies, which unequivocally demonstrate that the primary colorant in IGI is an amorphous form of an octahedral Fe(III) gallate metal organic framework structure that has previously been described by Wunderlich 8−10 and Feller. 11 Unlike the majority of prior studies, we use authentic IGI precursors to prepare both crystalline and amorphous forms of the IGI precipitate and study the crystal-to-amorphous transition by way of XRD, thermal gravimetric analysis (TGA), IR and Raman, Mossbauer, and X-ray photoelectron spectroscopy (XPS). Spectroscopic comparisons with historical documents prove the relevance of the synthetic crystalline and amorphous forms of the model IGI materials to those found in the authentic manuscripts.
For investigation of structure-property relationships in copper phosphine halide complexes, treatment of copper(I) halides with chiral bis(phosphines) gave dinuclear [Cu((R,R)-i-Pr-DuPhos)(μ-X)] [X = I (1), Br (2), Cl (3)], [Cu(μ-((R,R)-Me-FerroLANE)(μ-I)] (5), and [Cu((S,S)-Et-FerroTANE)(I)] (6), pentanuclear cluster CuI((S,S)-Et-FerroTANE) (7), and the monomeric Josiphos complexes Cu((R,S)-CyPF-t-Bu)(I) (8) and Cu((R,S)-PPF-t-Bu)(I) (9); 1-3, 5, and 7-9 were structurally characterized by X-ray crystallography. Treatment of iodide 1 with AgF gave [Cu((R,R)-i-Pr-DuPhos)(μ-F)] (4). DuPhos complexes 1-4 emitted yellow-green light upon UV irradiation at room temperature in the solid state. This process was studied by low-temperature emission spectroscopy and density functional theory (DFT) calculations, which assigned the luminescence to (M + X)LCT (CuX to DuPhos aryl) excited states. Including Grimme's dispersion corrections in the DFT calculations (B3LYP-D3) gave significantly shorter Cu-Cu distances than those obtained using B3LYP, with the nondispersion-corrected calculations better matching the crystallographic data; other intramolecular metrics are better reproduced using B3LYP-D3. A discussion of the factors leading to this unusual observation is presented.
Diastereoselective coordination of racemic secondary phosphines (PHRR′) to Cu(I) precursors containing chiral bis(phosphines) (diphos*) was explored as a potential route to P-stereogenic phosphido complexes. Reaction of [Cu(NCMe) 7); two of the three expected isomers of the bis(secondary phosphine) complexes [Cu((R,R)i-Pr-DuPhos)(PhHP(CH 2 ) n PHPh)][PF 6 ] (n = 2 (8); n = 3 (9)) were formed preferentially in related reactions. Reaction of the halide-bridged dimers [Cu((R,R)-i-Pr-DuPhos)(X)] 2 or [Cu((R,R)-Me-FerroLANE)(I)] 2 with PHMe(Is) gave the labile adducts Cu((R,R)-i-Pr-DuPhos)(PHMe(Is))(X) (X = Cl (10), Br (11), I (12)) and Cu((R,R)-Me-FerroLANE)(PHMe-(Is))(I) (13). Complexes 1, 6, and 8−11 were structurally characterized by X-ray crystallography. Variable temperature NMR studies of 6 and 8 showed that the secondary phosphine ligands underwent reversible dissociation. Deprotonation of 6 or 7 generated the P-stereogenic phosphido complexes Cu(diphos*)(PMeIs) (diphos* = (R,R)-i-Pr-DuPhos ( 14) or (R,R)-Me-FerroLANE) (17)), observed by 31 P NMR spectroscopy, but decomposition also occurred. Density functional theory calculations were used to characterize the diastereomers of thermally unstable 17 and the inversion barrier in a model copperphosphido complex. These observations provided structure−property relationships which may be useful in developing catalytic asymmetric reactions involving secondary phosphines and P-stereogenic copper phosphido intermediates.
Tandem alkylation/arylation of primary phosphines PH 2 R (R = Ph, Cy, Fc, FcCH 2 ; Fc = ferrocenyl) with 5-bromo-6chloromethylacenaphthene (1) and 2 equiv of NaOSiMe 3 using the catalyst precursor Cu(IPr)(Cl) gave a series of 1-phosphapyracenes (R-PyraPhos, 2a−d), which were isolated as borane adducts 3a−d. Similar reactions of the chiral air-stable primary phosphines PH 2 Ar* (Ar* = (S)-binaphthyl ( 4), (R)-MeO-binaphthyl ( 5)) to yield 2e,f and 3e,f were diastereoselective (dr = 2:1 and 1.2:1, respectively), and chromatography gave a highly enriched sample of one diastereomer of 3f. The mechanism of catalysis was investigated by NMR monitoring and independent syntheses of potential intermediates. The phosphido complexes Cu(IPr)(PHAr′) (Ar′ = Ph, (R)-MeO-binaphthyl) were generated in equilibrium mixtures, along with Me 3 SiOH, from Cu(IPr)(OSiMe 3 ) and PH 2 Ar′. They reacted with benzyl chloride 1 to yield Cu(IPr)(Cl) and the secondary phosphines PHAr′(CH 2 Ar) (Ar = Br-acenaphthyl); addition of NaOSiMe 3 yielded PyraPhos derivatives 2a,f. Deprotonation of the cations [Cu(IPr)(PHAr′CH 2 Ar)][OTf] (Ar′ = Ph, (R)-MeO-binaphthyl) was investigated as a route to the secondary phosphido complexes Cu(IPr)(PAr′CH 2 Ar) (13). We propose that C−Br oxidative addition in the Cu(I)−phosphido intermediates 13 followed by P−C reductive elimination from Cu(III)−phosphido complexes forms the PyraPhos ring, with diastereoselection arising from rapid pyramidal inversion of Cu−phosphido groups.
A known ligand precursor, prepared by an improved method from 1,3bis(bromomethylbenzene), a base, and (R,R)-2,5-dimethylphospholane-borane, was deprotected with DABCO and used to prepare chiral m-xylene-based PCP pincer complexes. Reaction with Ni(DME)Br 2 and NEt 3 gave square planar Ni(DuPinPhos)(Br) (9), which was crystallographically characterized. Heating with [Ir(COE) 2 Cl] 2 yielded the dinuclear hydride complex [Ir(DuPinPhos)(H)(Cl)] 2 (μ-C 6 H 4 (CH 2 -dimethylphospholanyl) 2 ] ( 10), which contained both pincer chelates and a bridging bis(phospholane). Nickel complex 9 was a catalyst precursor for enantioselective alkylation of secondary phosphines with 2bromomethylnaphthalene using the base NaOSiMe 3 . The dynamic behavior of a catalytic intermediate, the P-stereogenic terminal phosphido complex Ni(DuPinPhos)(PPhIs) (Is = 2,4,6-(i-Pr) 3 C 6 H 2 ), was characterized by variable temperature NMR spectroscopy.
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