Sterically Hindered [Tris(imidazolyl)phosphine]copper Complexes: Formation and Reactivity of a Peroxo-Dicopper(II) Adduct and Structure of a Dinuclear Carbonate-Bridged Complex

Abstract

Copper (1) complexes of the new tripodal ligands tris[2-( 1,4-diisopropylimidazolyl)]phosphine (PimiR2) and tris-[2-( 1 -isopropyl-4-terr-butylimidazolyl)]phosphine (PimiRJBu) have been prepared and characterized by 'H NMR and elemental analysis. The dioxygen reactivity of the pseudotetrahedral complexes [Cu(Pimik2)CH3CN]BF4 and [Cu(PimiRJBu)CH3CN]BF4 has been examined. The former complex forms a violet peroxo-dicopper(II) species upon reaction with dioxygen at low temperature; the electronic absorption spectrum (A = 343 nm, E = 19 500 M-' cm-I; ;1 = 549 nm, E = 790 M-I cm-l) and 0-0 stretching frequency (~( 0 -0 ) = 750 cm-l) of this adduct approximate those of oxyhemocyanin. Warming a mixture of the peroxide complex and triphenylphosphine to room temperature affords [Cu(PimiR2)PPh3]BF4 and produces no O-PPh3. In methanol at -78 OC, the peroxide complex is unreactive toward acid, carbon dioxide, 1 -hexene, and triphenylphosphine. Thermal decomposition of the dioxygen adduct in the presence of water yields a bis(hydroxide)-bridged dicopper(II) dimer, [Cu(PimiR2)]2(OH)2(BF4)2, which further reacts with atmospheric C02 to produce the carbonate-bridged complex [ C U ( P~~' "~) ]~C O~( B F~)~. X-ray crystallography has been used to characterize the carbonate complex. Crystal data: orthorhombic, space group Pbca, with a = 16.8705(17) A, b = 17.4731(14) A, c = 51.358(4) A, V = 15139.4(22) A3, and Z = 8. The structure of a prototypical mononuclear 5-coordinate complex, [Cu(PimiR2)-OAcIBF4, has also been determined. Crystal data: monoclinic, space group P21ln, with a = 9.657(3) A, b = 17.001(5) A, c = 24.642(6) A, p = 100.987(23)', V = 3971.4(19) A3, and Z = 4. The geometry about copper-(11) in both structures is approximately square pyramidal, indicating that PimiR2 can accommodate side-on binding of dioxygen to copper. The more sterically hindered copper(1) complex [Cu(PimiRJB")CH3CN]BF~ is inert to 0 2 , but it reacts readily with CO to form the isolable adduct [Cu(PimiRJBU)CO]BF4 with v(C0) = 2086 cm-I. In accord with its lack of dioxygen reactivity, [Cu(PimiRJBU)CH3CN]BF4 undergoes a quasi-reversible redox process at very high potential, with ,5112 (in CH3CN) of +1.08 V vs AglAgC1.

Biomimetic Copper(I)-CO Complexes: A Structural and Dynamic Study of a Calix[6]arene-Based Supramolecular System

“…The only model complexes that exhibit such low values are those derived from nonchelating imidazoles (2059 ± 2067 cm À1 ), [68] or from the anionic tris(pyrazolyl)borate ligand. [57,58,62,63] On the other hand, for complexes obtained with the neutral tris(imidazolyl)methoxymethane (n CO 2080 cm À1 ) [59] and tris(imidazolyl)phosphane (n CO 2083 ± 2086 cm À1 ) [60] tripods, the CO stretches are more energetic. Complexes 4, although structurally very similar to the biological systems, display n CO values that are % 40 cm À1 higher.…”

Soluble Gold and Palladium Complexes Heterogenized on MCM‐41 Are Effective and Versatile Catalysts

“…The complexes show bands due to intraligand πǞπ* and nǞπ* transitions in the aromatic ring and azomethine-group and charge-transfer transitions. [21] The bands in the 400-450 nm region correspond to the d-d transitions expected for planar complexes [22] and MLCT bands. The diffuse-reflectance spectra of the complexes are almost identical before and after heterogenization, thereby indicating that there is no significant distortion in the geometry and electronic environment of these complexes upon heterogenization.…”

Structures, Mechanisms, and Results in Bifunctional Catalysis and Related Species Involving Proton Transfer

“…Here we describe some fundamental studies of bifunctional catalysts and related complexes, with a focus on phosphines bearing an imidazolyl or pyridyl group and the roles the basic nitrogen substituent can play. Though many pyridyl [13,14] and fewer imidazoylphosphine complexes are known (examples [15][16][17][18][19][20][21][22]), as far as we are aware, relatively few reports of enhanced catalysis exist. In pioneering work at Shell in the 1990s [23,24], a pyrid-2-yl or 6-methyl-2-pyrid-yl phosphine led to very significant enhancements in rate and selectivity for Pd-catalyzed methoxycarbonylation of propyne.…”