High-spin diimine complexes of iron(II) reject binding of carbon monoxide: Theoretical analysis of thermodynamic factors inhibiting or favoring spin-crossover
High-spin diimine complexes of iron(II) reject binding of carbon monoxide: Theoretical analysis of thermodynamic factors inhibiting or favoring spin-crossover
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DOI：
10.1021/ic050966h
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发表时间：
2005-11-14
影响因子：
4.6
通讯作者：
Kubas, GJ
中科院分区：
文献类型：
--
作者：
Hardman, NJ;Fang, XG;Kubas, GJ
合成了一系列新的Fe（II）配合物FeCl2{N(R)=C(Me)C(Me)=N(R)}，含有半可溶侧臂的二亚胺配体R (R = CH2(CH2)(2)NMe2, 1, CH2(CH2)（2)OMe, 2, CH2(CH2)(2)SMe), 3）。1的晶体结构为6配位，其中两个胺臂均连接，而2的晶体结构为5配位，其中一个甲氧基臂悬垂。大量的尝试将CO结合到这些物种合成二氢配合物的前体，但没有成功。在CO气氛下，I与1或2等量的AgOTf反应，只分离出一个6-座标的含双（三酸盐）的产物[Fe{N(R)=C(Me)C(Me)= N(R)}(OTf)(2)] (R = CH2(CH2)(2)NMe2), 5。5-配位2与AgSbF6在CO作用下的反应没有得到CO加合物，而是得到一个含弱结合SbF6的指示双核配合物[Fe{N(R)=C(Me)C(Me)=N(R)}(mu-Cl)](2)[SbF6](2) (R = CH2(CH2)(2)OMe), 4。因此，硬供体阴离子与铁的配位比CO的配位更有利。由于铁在该体系中处于高自旋状态，并且在能量上无法自旋交叉到低自旋状态，因此CO的结合被意外拒绝。对16e配合物及其CO加合物在自旋态S = 0、1和2时CO与Fe（II）中心相互作用的理论计算有助于进一步理解这一问题。结果表明，CO与高自旋五坐标铁模型二亚胺配合物的相互作用基本上是热中性的，但与低自旋二膦片段的相互作用约为48千卡/摩尔。因此，自旋交叉在热力学上是不利的，而不是动力学上的（例如，“自旋块”效应）；即，主要n给体基团的配体场强显然不足以产生低自旋CO加合物。
A new series of Fe(II) complexes, FeCl2{N(R)=C(Me)C(Me)=N(R)}, containing diimine ligands with hemilabile sidearms R (R = CH2(CH2)(2)NMe2, 1, CH2(CH2)(2)OMe, 2, CH2(CH2)(2)SMe), 3) were synthesized. The crystal structure of 1 showed 6-coordination where both amine arms were attached, whereas 2 was a 5-coordinate 16e species with one methoxy arm dangling free. Extensive attempts were made to bind CO to these species to synthesize precursors for dihydrogen complexes but were unsuccessful. Reaction of I with 1 or 2 equiv of AgOTf under CO atmosphere resulted in isolation of only a 6-coordinate bis(triflate)-containing product [Fe{N(R)=C(Me)C(Me)= N(R)}(OTf)(2)] (R = CH2(CH2)(2)NMe2), 5. Reaction of 5-coordinate 2 with AgSbF6 under CO did not give a CO adduct but afforded instead a dicationic dinuclear complex [Fe{N(R)=C(Me)C(Me)=N(R)}(mu-Cl)](2)[SbF6](2) (R = CH2(CH2)(2)OMe), 4, containing a weakly bound SbF6. Thus coordination of hard-donor anions to iron was favored over CO binding. The unexpected rejection of binding of CO is rationalized by the iron being in a high-spin state in this system and energetically incapable of spin crossover to a low-spin state. Theoretical calculations on CO interaction with Fe(II) centers in spin states S = 0, 1, and 2 for both the 16e complexes and their CO adducts aid further understanding of this problem. They show that interaction of CO with a high-spin 5-coordinate Fe model diimine complex is essentially thermoneutral but is exergonic by about 48 kcal/mol to a comparable but low-spin diphosphine fragment. Spin crossover is thus disfavored thermodynamically rather than kinetically (e.g. a "spin block" effect); i.e., the ligand field strengths of the primarily N-donor groups are apparently insufficient to give a low-spin CO adduct.