SPECTROSCOPIC STUDIES OF SIDE-ON PEROXIDE-BRIDGED BINUCLEAR COPPER(II) MODEL COMPLEXES OF RELEVANCE TO OXYHEMOCYANIN AND OXYTYROSINASE
SPECTROSCOPIC STUDIES OF SIDE-ON PEROXIDE-BRIDGED BINUCLEAR COPPER(II) MODEL COMPLEXES OF RELEVANCE TO OXYHEMOCYANIN AND OXYTYROSINASE
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DOI：
10.1021/ja00052a043
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发表时间：
1992-12-16
影响因子：
15
通讯作者：
SOLOMON, EI
中科院分区：
文献类型：
--
作者：
BALDWIN, MJ;ROOT, DE;SOLOMON, EI
介绍了两侧对mu-eta2:eta2过氧化物桥接铜二聚体[Cu(HB(3,5- r2pz)3)]2（O2）的光谱研究，其中（HB(3,5- r2pz)3）是三（吡唑基）硼酸盐配体，R = i-Pr或Ph，并与我们之前对端对结合（eta1）过氧化物铜单体和桥接二聚体（反式mu-1,2）配合物的研究进行了比较。350 nm （epsilon = 26 000 M-1 cm-1）和538 nm （epsilon = 2000 M-1 cm-1）的两个跃迁被指定为过氧化物到铜的电荷转移跃迁。本文提出了一种利用配体到金属电荷转移跃迁的强度来量化从配体到金属中心的电子给能密度的方法。研究发现，在过氧化物pi*轨道上，侧对桥接的过氧化物比端对结合的单体和二聚体中的过氧化物给铜的电子密度要大得多。电荷的捐赠量与过氧化物和铜之间的sigma键数成正比（侧对桥接二聚体= 4，端对桥接二聚体= 2，端对单体= 1）。增加的电荷捐赠导致侧对络合物中负的过氧化物减少，原则上应该产生更强的O-O键。根据R = i-Pr配合物在763 （rR， O-18为723 cm-1(2)）、331 （IR， O-18为321 cm-1(2)）、284 （rR， O-18无移位(2)）和572 cm-1（基于rR为1144 cm-1， O-18为1098 cm-1(2)）的同位素位移，在共振拉曼光谱中划分了4种振动模式，对应的峰值在R = i-Pr配合物的共振拉曼光谱中为749、285和1055 cm-1。正坐标分析表明，侧接式桥接配合物的氧-氧力常数k(O-O) （2.4 mdyn/埃）小于端接式单体（2.9 mdyn/埃）或端接式桥接二聚体（3.1 mdyn/埃），因此，尽管侧接式配合物的过氧化物电荷供能更大，但O-O键却比端接式配合物弱。这是这种侧桥结构中过氧化物的π受体能力的直接证据，它涉及一些过氧化物sigma*特征混合到最高能量占据的分子轨道中，正如Xalpha计算所预测的那样[Ross， P. K.；所罗门，e.i.j. Am。化学。社会科学学报，1994,13(3):346 - 359。利用电荷转移强度与配体电荷捐赠的相关性，氧血青素的电荷转移强度表明它可能具有四个铜-过氧化物键，因此具有侧对过氧化物桥接结构。氧血青素中这种侧面桥接几何结构的电子结构解释了其独特的光谱特征，包括345nm吸收带的高强度和高能量，低O-O拉伸频率，以及在共振拉曼光谱的预期能量范围内缺乏对称的Cu-O拉伸。这种电子结构也为了解血青素和酪氨酸的氧结合和激活机制提供了新的思路。
Spectrosopic studies on two side-on mu-eta2:eta2 peroxide-bridged cupric dimers, [Cu(HB(3,5-R2pz)3)]2(O2), where (HB(3,5-R2pz)3) is a tris(pyrazolyl)borate ligand and R = i-Pr or Ph, are presented and compared to our previous studies of end-on bound(eta1) peroxide-coppper monomer and bridged dimer (trans-mu-1,2) complexes. Two transitions at 350 nm (epsilon = 26 000 M-1 cm-1) and 538 nm (epsilon = 2000 M-1 cm-1) are assigned as peroxide-to-copper charge-transfer transitions. A method has been developed to quantitate electron density donation from the ligand to the metal center using the intensity of the ligand-to-metal charge-transfer transitions. It is found that the side-on bridging peroxide donates significantly more electron density from the peroxide pi* orbitals to the coppers than does,peroxide in end-on bound monomer and dimer complexes. The amount of charge donation is proportional to the number of sigma bonds between the peroxide and the coppers (side-on bridged dimer = 4, end-on bridged dimer = 2, end-on monomer = 1). The increased charge donation results in a less negative peroxide in the side-on complex and should in principle produce a stronger O-O bond. Four vibrational modes are assigned in the resonance Raman and infrared spectra on the basis of isotopic shifts at 763 (rR, 723 cm-1 with O-18(2)), 331 (IR, 321 cm-1 with O-18(2)), 284 (rR, no shift with O-18(2)), and 572 cm-1 (based on an overtone at 1144 cm-1 in rR, 1098 cm-1 with O-18(2)) in the R = Ph complex, with corresponding peaks at 749, 285, and 1055 cm-1 in the resonance Raman spectra of the R = i-Pr complex. A normal coordinate analysis shows that the oxygen-oxygen force constant, k(O-O), is smaller in the side-on bridged complex (2.4 mdyn/angstrom) than in the end-on monomer (2.9 mdyn/angstrom) or end-on bridged dimer (3.1 mdyn/angstrom), and thus the O-O bond is weaker in the side-on complex than in the end-on, despite its greater peroxide charge donation. This is direct evidence for the pi acceptor ability of the peroxide in this side-on bridged structure, which involves some peroxide sigma* character mixing into the highest energy occupied molecular orbital, as predicted by Xalpha calculations [Ross, P. K.; Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 3246-3259]. Using the correlation of charge-transfer intensities to ligand charge donation, the charge-transfer intensity of oxyhemocyanin indicates that it likely has four copper-peroxide bonds and thus a side-on peroxide bridging structure. The electronic structure of this side-on bridging geometry in oxyhemocyanin explains its unique spectroscopic features, including the high intensity and energy of the 345-nm absorption band, the low O-O stretching frequency, and the lack of a symmetric Cu-O stretch in the expected energy range of the resonance Raman spectrum. This electronic structure also provides insight into the mechanisms of oxygen binding and activation in hemocyanin and tyrosine.