Coordination Chemistry, Structure, and Reactivity of Thiouracil Derivatives of Tungsten(0) Hexacarbonyl: A Theoretical and Experimental Investigation into the Chelation/Dechelation of Thiouracil via C
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- 作者：Donald J. Darensbourg ; Brian J. Frost ; Agnes Derecskei-Kovacs ; and Joseph H. Reibenspies
- 刊名：Inorganic Chemistry
- 出版年：1999
- 出版时间：October 18, 1999
- 年：1999
- 卷：38
- 期：21
- 页码：4715 - 4723
- 全文大小：161K
- 年卷期：v.38,no.21(October 18, 1999)
- ISSN：1520-510X
文摘
The synthesis of 2-thiouracil and 6-methyl-2-uracil derivatives of tungsten carbonyl from the reaction ofphotogenerated W(CO)5(solvent) (solvent = MeOH or THF) and the corresponding [Et4N][thiouracilate] isdescribed. The crystal structure of the [Et4N][W(CO)5(2-thiouracilate)], 1, derivative is reported where thethiouracilate is found to be bound to the tungsten center via the exocyclic sulfur atom. In the solid-state structureof 1 two anions are associated by means of two hydrogen bonds between the metal bound nucleobases. Thesepentacarbonyl complexes stereoselectively lose cis carbonyl ligands, as is apparent from 13C-labeling studies, toprovide the endocyclic nitrogen N(1)-chelated tungsten tetracarbonyl derivatives, e.g., [Et4N][W(CO)4(2-thiouracilate)], 3. The kinetics of the loss of CO from the pentacarbonyl anions to afford the metal tetracarbonyls,and the reverse of that process, were monitored by means of in situ infrared spectroscopy in the 
(CO) region asa function of temperature. These studies reveal that the tetracarbonyl anions in CO-saturated acetonitrile ([CO]
6 × 10-3 M) are unstable with respect to the formation of the pentacarbonyl derivatives, i.e., the equilibrium1 
3 + CO lies to the left under an atmosphere of carbon monoxide. From the activation parameters determinedfor the dissociative CO loss process (
H
= 82.0 ± 3.6 kJ mol-1 and S = -44.9 ± 9.6 J mol-1 K-1 forcomplex 1) it is apparent that the sulfur-bound thiouracilate ligand is serving as a 
-donor during CO dissociation,i.e., behaving as a cis-labilizing ligand. Ab initio geometry optimizations carried out for the process 1 3 + COat the Hartree-Fock and DFT levels support these experimental observations. For example, complex 1 is shownto be more stable than 3 + CO and chelation via the endocyclic N(1) donor is favored over N(3) binding. Finally,the "16-electron" intermediate resulting from CO dissociation in 1 was found to possess a significantly shortenedW-S interaction, presumably due to the -donating ability of the thiouracilate ligand.
(CO) region asa function of temperature. These studies reveal that the tetracarbonyl anions in CO-saturated acetonitrile ([CO] 6 × 10-3 M) are unstable with respect to the formation of the pentacarbonyl derivatives, i.e., the equilibrium1 3 + CO lies to the left under an atmosphere of carbon monoxide. From the activation parameters determinedfor the dissociative CO loss process (H = 82.0 ± 3.6 kJ mol-1 and S = -44.9 ± 9.6 J mol-1 K-1 forcomplex 1) it is apparent that the sulfur-bound thiouracilate ligand is serving as a -donor during CO dissociation,i.e., behaving as a cis-labilizing ligand. Ab initio geometry optimizations carried out for the process 1 3 + COat the Hartree-Fock and DFT levels support these experimental observations. For example, complex 1 is shownto be more stable than 3 + CO and chelation via the endocyclic N(1) donor is favored over N(3) binding. Finally,the "16-electron" intermediate resulting from CO dissociation in 1 was found to possess a significantly shortenedW-S interaction, presumably due to the -donating ability of the thiouracilate ligand.