Phenolate hydroxylation in a bis(mu-oxo)dicopper(III) complex: lessons from the guanidine/amine series.
A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (2L) and its Cu-O2 chemistry is reported. [(2L)CuI(MeCN)]1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UV-vis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O2 complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine pi --> Cu2O2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O2, reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu2O2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol(-1)) to form the two-electron, two-proton reduced bis(mu-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.
Herres-Pawlis
Sonja
S
Department Chemie, Anorganische Chemie, Universität Paderborn, 33098 Paderborn, Germany. shp@mail.uni-paderborn.de
Verma
Pratik
P
Haase
Roxana
R
Kang
Peng
P
Lyons
Christopher T
CT
Wasinger
Erik C
EC
Flörke
Ulrich
U
Henkel
Gerald
G
Stack
T Daniel P
TD
D000588:Amines / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D019791:Guanidine / Q000737:chemistry*; D006900:Hydroxylation; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D013053:Spectrophotometry; D013696:Temperature; D017186:Titrimetry
D000588:Amines; D008024:Ligands; D009942:Organometallic Compounds; C501836:bis-mu-oxodicopper(III)-phenolate complex; D019791:Guanidine; D010100:Oxygen