Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappalli, 620 024, India.
Dalton Trans. 2011 Sep 7;40(33):8444-58. doi: 10.1039/c1dt10495k. Epub 2011 Jul 22.
A series of iron(III) complexes 1-4 of the tripodal tetradentate ligands N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L1), N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxy- propyl)amine H(L2), N,N-bis(pyrid-2-ylmethyl)-N-ethoxyethanolamine H(L3), and N-((pyrid-2-ylmethyl)(1-methylimidazol-2-ylmethyl))-N-(2-hydroxyethyl)amine H(L4), have been isolated, characterized and studied as functional models for intradiol-cleaving catechol dioxygenases. In the X-ray crystal structure of [Fe(L1)Cl(2)] 1, the tertiary amine nitrogen and two pyridine nitrogen atoms of H(L1) are coordinated meridionally to iron(III) and the deprotonated ethanolate oxygen is coordinated axially. In contrast, [Fe(HL3)Cl(3)] 3 contains the tertiary amine nitrogen and two pyridine nitrogen atoms coordinated facially to iron(III) with the ligand ethoxyethanol moiety remaining uncoordinated. The X-ray structure of the bis(μ-alkoxo) dimer {Fe(L5)Cl}(2)(2)5, where HL is the tetradentate N(3)O donor ligand N,N-bis(1-methylimidazol-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L5), contains the ethanolate oxygen donors coordinated to iron(III). Interestingly, the Fe(HL)(DBC) and [Fe(HL3)(HDBC)X] adducts, generated by adding ∼1 equivalent of piperidine to solutions containing equimolar quantities of iron(III) complexes 1-5 and H(2)DBC (3,5-di-tert-butylcatechol), display two DBC(2-)→ iron(III) LMCT bands (λ(max): 1, 577, 905; 2, 575,915; 3, 586, 920; 4, 563, 870; 5, 557, 856 nm; Δλ(max), 299-340 nm); however, the bands are blue-shifted (λ(max): 1, 443, 700; 2, 425, 702; 3, 424, 684; 4, 431, 687; 5, 434, 685 nm; Δλ(max), 251-277 nm) on adding 1 more equivalent of piperidine to form the adducts [Fe(L)(DBC)] and [Fe(HL3)(HDBC)X]. Electronic spectral and pH-metric titration studies in methanol disclose that the ligand in Fe(HL)(DBC) is protonated. The [Fe(L)(DBC)] adducts of iron(III) complexes of bis(pyridyl)-based ligands (1,2) afford higher amounts of intradiol-cleavage products, whereas those of mono/bis(imidazole)-based ligands (4,5) yield mainly the auto-oxidation product benzoquinone. It is remarkable that the adducts Fe(HL)(DBC)/[Fe(HL3)(DBC)X] exhibit higher rates of oxygenation affording larger amounts of intradiol-cleavage products and lower amounts of benzoquinone.
一系列铁(III)配合物 1-4 是由三脚架四齿配体 N,N-双(吡啶-2-基甲基)-N-(2-羟乙基)胺 H(L1)、N,N-双(吡啶-2-基甲基)-N-(2-羟丙基)胺 H(L2)、N,N-双(吡啶-2-基甲基)-N-乙氧基乙醇胺 H(L3)和 N-((吡啶-2-基甲基)(1-甲基咪唑-2-基甲基))-N-(2-羟乙基)胺 H(L4)分离得到的，这些配合物被用作间二酚裂解儿茶酚双加氧酶的功能模型进行了研究。在 [Fe(L1)Cl(2)]1 的 X 射线晶体结构中，H(L1)的叔胺氮和两个吡啶氮原子以赤道方式与铁(III)配位，去质子化的乙醇酸盐氧以轴向方式配位。相比之下，[Fe(HL3)Cl(3)]3 含有与铁(III)facially 配位的叔胺氮和两个吡啶氮原子，配体乙氧基乙醇部分保持未配位。双(μ-烷氧基)二聚体 {Fe(L5)Cl}(2)(2)5 的 X 射线结构，其中 HL 是四齿 N(3)O 供体配体 N,N-双(1-甲基咪唑-2-基甲基)-N-(2-羟乙基)胺 H(L5)，含有与铁(III)配位的乙醇酸盐氧供体。有趣的是，通过向含有等摩尔量的铁(III)配合物 1-5 和 H(2)DBC(3,5-二叔丁基儿茶酚)的溶液中加入约 1 当量的哌啶，生成了 Fe(HL)(DBC)和[Fe(HL3)(HDBC)X]加合物，它们显示出两个 DBC(2-)→铁(III)LMCT 带(λ(max)：1, 577, 905; 2, 575,915; 3, 586, 920; 4, 563, 870; 5, 557, 856 nm;Δλ(max), 299-340 nm)；然而，当再加入 1 当量的哌啶时，这些带发生蓝移(λ(max)：1, 443, 700; 2, 425, 702; 3, 424, 684; 4, 431, 687; 5, 434, 685 nm;Δλ(max), 251-277 nm)，形成加合物 [Fe(L)(DBC)]和[Fe(HL3)(HDBC)X]。甲醇中的电子光谱和 pH 滴定研究表明，Fe(HL)(DBC)中的配体被质子化。双吡啶基配体(1,2)的铁(III)配合物的[Fe(L)(DBC)]加合物提供更多的间二酚裂解产物，而单/双(咪唑)基配体(4,5)的加合物主要生成自氧化产物苯醌。值得注意的是，加合物Fe(HL)(DBC)/[Fe(HL3)(DBC)X]表现出更高的氧化速率，提供更多的间二酚裂解产物和更少的苯醌。
