Influence of Ligand Geometry in Bimetallic Ytterbocene Complexes of Bridging Bis(bipyridyl) Ligands
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- 作者：Christin N. Carlson ; Christopher J. Kuehl ; Linda Ogallo ; David A. Shultz ; J. D. Thompson ; Martin L. Kirk ; Richard L. Martin ; Kevin D. John ; David E. Morris
- 刊名：Organometallics
- 出版年：2007
- 出版时间：August 13, 2007
- 年：2007
- 卷：26
- 期：17
- 页码：4234 - 4242
- 全文大小：230K
- 年卷期：v.26,no.17(August 13, 2007)
- ISSN：1520-6041
文摘
Two new bimetallic complexes, [Cp*2Yb]2(
-1,3-(2,2'-bipyridyl)-5-tBu-C6H3) (1) and [Cp*2Yb]2(-1,4-(2,2'-bipyridyl)-C6H4) (2), and their corresponding two-electron oxidation products [1]2+ and [2]2+have been synthesized with the aim of determining the impact of the bridging ligand geometry on theelectronic and magnetic properties of these materials. Electrochemistry, optical spectroscopy, and bulksusceptibility measurements all support a ground-state electronic configuration of the type [(f)13-(
a*)1-(b*)1-(f)13]. Density functional theory calculations on the uncomplexed bridging ligands as doubly reducedspecies also indicate that the diradical electronic configuration is the lowest lying for both meta- andpara-bis(bipyridyl) systems. The electrochemical and optical spectroscopic data indicate that the electroniccoupling between the metal centers mediated by the diradical bridges is weak, as evidenced by the smallseparation of the metal-based redox couples and the similarity of the f-f transitions of the associateddicationic complexes ([1]2+ and [2]2+) relative to those of the monometallic [Cp*2Yb(bpy)]+ analogue.The magnetic susceptibility data show no evidence for exchange coupling between the paramagneticmetal centers in the neutral complexes, but do indicate weak exchange coupling between YbIII and ligandradical spins on each of the effectively independent halves of the bimetallic complexes. These findingsare in contrast to those reported recently for CoIII/II dioxolene bimetallic complexes bridged by thesesame bis(bipyridyl) ligands. The difference is attributed in part to the dominant singlet diradical characterof the bridging ligands in the ytterbocene complexes. These experimental and theoretical results areconsistent with expectations for organic diradical spin orientations for meta versus para substituentsacross a phenylene linker, but this effect does not induce significant longer-range superexchange orelectronic interactions between the metal centers in these systems.
-1,3-(2,2'-bipyridyl)-5-tBu-C6H3) (1) and [Cp*2Yb]2(-1,4-(2,2'-bipyridyl)-C6H4) (2), and their corresponding two-electron oxidation products [1]2+ and [2]2+have been synthesized with the aim of determining the impact of the bridging ligand geometry on theelectronic and magnetic properties of these materials. Electrochemistry, optical spectroscopy, and bulksusceptibility measurements all support a ground-state electronic configuration of the type [(f)13-(a*)1-(b*)1-(f)13]. Density functional theory calculations on the uncomplexed bridging ligands as doubly reducedspecies also indicate that the diradical electronic configuration is the lowest lying for both meta- andpara-bis(bipyridyl) systems. The electrochemical and optical spectroscopic data indicate that the electroniccoupling between the metal centers mediated by the diradical bridges is weak, as evidenced by the smallseparation of the metal-based redox couples and the similarity of the f-f transitions of the associateddicationic complexes ([1]2+ and [2]2+) relative to those of the monometallic [Cp*2Yb(bpy)]+ analogue.The magnetic susceptibility data show no evidence for exchange coupling between the paramagneticmetal centers in the neutral complexes, but do indicate weak exchange coupling between YbIII and ligandradical spins on each of the effectively independent halves of the bimetallic complexes. These findingsare in contrast to those reported recently for CoIII/II dioxolene bimetallic complexes bridged by thesesame bis(bipyridyl) ligands. The difference is attributed in part to the dominant singlet diradical characterof the bridging ligands in the ytterbocene complexes. These experimental and theoretical results areconsistent with expectations for organic diradical spin orientations for meta versus para substituentsacross a phenylene linker, but this effect does not induce significant longer-range superexchange orelectronic interactions between the metal centers in these systems.