STERICALLY HINDERED [TRIS(IMIDAZOLYL)PHOSPHINE]COPPER COMPLEXES - FORMATION AND REACTIVITY OF A PEROXO-DICOPPER(II) ADDUCT AND STRUCTURE OFA DINUCLEAR CARBONATE-BRIDGED COMPLEX

Copper(I) complexes of the new tripodal ligands tris[2-(1,4-diisopropy limidazolyl)]phosphine (Pim(iPr2)) and -[2-(1-isopropyl-4-tert-butylim idazolyl)]phosphine (Pim(iPr,tBu)) have been prepared and characterize d by H-1 NMR and elemental analysis. The dioxygen reactivity of the ps eudotetrahedral complexes [Cu(Pim(iPr2))CH3CN]BF4 and [Cu(Pim(iPr,tBu) )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 (lambda = 343 nm, epsilon = 19 500 M(-1) cm(-1); lambda = 549 nm, epsilon = 790 M(-1) cm(-1)) and O-O str etching frequency (v(O-O) = 750 cm(-1)) of this adduct approximate tho se of oxyhemocyanin. Warning a mixture of the peroxide complex and tri phenylphosphine to room temperature affords [Cu(Pim(iPr2))PPh(3)]BF4 a nd produces no O=PPh(3). In methanol at -78 degrees C, the peroxide co mplex is unreactive toward acid, carbon dioxide, 1-hexene, and triphen ylphosphine. Thermal decomposition of the dioxygen adduct in the prese nce of water yields a bis(hydroxide)-bridged dicopper(II) dimer, [CU(P im(iPr2))](2)(OH)(2)(BF4)(2), which further reacts with atmospheric CO 2 to produce the carbonate-bridged complex [CU(Pim(iPr2))]2CO3(BF4)(2) . X-ray crystallography has been used to characterize the carbonate co mplex. Crystal data: orthorhombic, space group Pbca, with a = 16.8705( 17) Angstrom, b = 17.4731(14) Angstrom, c = 51.358(4) Angstrom, V = 15 139.4(22) Angstrom(3), and Z = 8. The structure of a prototypical mono nuclear 5-coordinate complex, [Cu(Pim(iPr2))-OAc]BF4, has also been de termined. Crystal data: monoclinic, space group P2(1)/n, with a = 9.65 7(3) Angstrom, b = 17.001(5) Angstrom, c = 24.642(6) Angstrom, beta = 100.987(23)degrees, V = 3971.4(19) Angstrom(3), and Z = 4. The geometr y about copper(II) in both structures is approximately square pyramida l, indicating that Pim(iPr2) can accommodate side-on binding of dioxyg en to copper. The more sterically hindered copper(I) complex [Cu(Pim(i Pr,tBu))CH3CN]BF4 is inert to O-2, but it reacts readily with CO to fo rm the isolable adduct [Cu(Pim(iPr,tBu))CO]BF4 with nu(CO) = 2086 cm(- 1). In accord with its lack of dioxygen reactivity, [Cu(Pim(iPr,tBu))C H3CN]BF4 undergoes a quasi-reversible redox process at very high poten tial, with E(1/2) (in CH3CN) of + 1.08 V vs Ag/AgCl.