Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex: Lessons from the Guanidine/Amine Series
Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex: Lessons from the Guanidine/Amine Series
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DOI：
10.1021/ja807809x
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发表时间：
2009-01-28
影响因子：
15
通讯作者：
Stack, T. Daniel P.
中科院分区：
文献类型：
--
作者：
Herres-Pawlis, Sonia;Verma, Pratilk;Stack, T. Daniel P.
报道了一种基于 1,3-丙二胺主链 (L-2) 及其 Cu-O-2 化学的新型杂化全甲基化胺-胍配体。 [(L-2)Cu-I(MeCN)](1+) 络合物在极性非质子溶剂中很容易在低温下氧化，形成双(mu-oxo)二铜(III) (0) 物质 (2b)，类似于母体双胍配体络合物 (1b) 和全甲基化二胺配体络合物 (3b)。 UV-vis 和 X 射线吸收光谱实验证实了 2b 是一种 O 物质，并且通过二茂铁-单羧酸 (FcCOOH) 的光学滴定证明了 2:1 Cu-O-2 络合物的完全形成。具有胍连接的 1b 和 2b 的紫外可见光谱显示出低强度可见特征，通过时间依赖性密度泛函理论 (TD-DFT) 计算，将其指定为胍 pi -> Cu2O2 核心跃迁。三个相关配合物 (1b-3b) 与酚盐在 195 K 下的反应性的比较特别具有洞察力，因为只有 2b 使 2,4-二叔丁基酚盐羟基化，生成 3,5-二叔丁基儿茶酚酸酯（> 95% 产率），其氧原子源自 O-2，让人想起酪氨酸酶的反应性。 1b 是不反应的，而 3b 产生 C-C 自由基偶联的双酚产物。 O配合物的外层氧化强度减弱和酚盐对Cu2O2核心的可及性增加是与2b中观察到的酚盐羟基化反应性相关的属性。 1b-3b 与 FcCOOH (O-H BDE 71 kcal mol(-1)) 形成双电子、双质子还原双(mu-羟基)二铜(II,II)络合物的相对低温反应性是定量的，并且可能先经过两个连续的质子耦合电子转移 (PCET) 步骤。光学滴定和 DFT 计算表明，第一步中形成的还原络合物是比母体 O 络合物更强大的氧化剂。这些机理见解有助于理解 2b 和 3b 所表现出的苯酚与双酚的反应性。
A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (L-2) and its Cu-O-2 chemistry is reported. [(L-2)Cu-I(MeCN)](1+) complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (0) 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-O-2 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-butylcatechol ate (> 95% yield) with the oxygen atom derived from O-2, 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.