Mansfield Natalie E, Grundy Joanna, Coles Martyn P, Avent Anthony G, Hitchcock Peter B
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, UK.
J Am Chem Soc. 2006 Oct 25;128(42):13879-93. doi: 10.1021/ja064212t.
Spectroscopic, crystallographic, and computational studies of the substituent distribution about the "NCN" unit in a series of phospha(III)- and phospha(V)-guanidines, R(2)PC{NR'}{NHR'} and R(2)P(E)C{NR'}{NHR'} (R = Ph, Cy; R' = (i)Pr, Cy; E = S, Se), are reported. In the phosphorus(III) systems, the P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be the E(syn)-(alpha) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E(syn)-(alpha) and Z(anti) in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the alpha- or beta-conformer relative to rotation about the P-C(amidine)() bond; however, DFT calculations indicate a low-energy structure for the N,N'-dimethyl model complex in the beta-conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the E(syn)-(beta) and Z(syn)-(beta) isomers in solution ( approximately 3:2 ratio), whereas for the P-dicyclohexyl analogues, the latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement about the imine double bond) is the dominant form. Intramolecular E...HN (E = S, Se) interactions are observed in solution for the Z(syn)-(beta) configuration of both P-substituted species, characterized by J(SeH) coupling in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the infrared spectrum. An X-ray crystallographic analysis of representative Ph(2)P(E)- and Cy(2)P(E)-substituted species shows exclusively the E(syn)-(beta) configuration for the P-diphenyl substituted compounds and the Z(syn)-(beta) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents. Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the observed preferences in substituent orientation, suggesting that steric factors play an important role in determining the subtle energetic differences at work in these systems.
报道了一系列磷（III）-和磷（V）-胍R₂PC{NR'}{NHR'}和R₂P（E）C{NR'}{NHR'}（R = Ph，Cy；R' = （i）Pr，Cy；E = S，Se）中“NCN”单元周围取代基分布的光谱、晶体学和计算研究。在磷（III）体系中，二苯基磷取代的化合物仅以一种异构体形式存在，核磁共振光谱显示其为E（顺）-（α）构型。相比之下，相应的二环己基衍生物在溶液中以E（顺）-（α）和Z（反）的混合物形式存在。光谱技术无法确定后一种异构体相对于围绕P-C（脒基）键的旋转是以α-还是β-构象存在；然而，密度泛函理论计算表明N，N'-二甲基模型配合物在β-构象中有一个低能量结构。在其氧化的磺基和硒基形式中，二苯基磷化合物在溶液中以E（顺）-（β）和Z（顺）-（β）异构体的相互转化平衡混合物形式存在（比例约为3:2），而对于二环己基类似物，后一种构型（其中氮取代基在亚胺双键周围处于空间上更不利的顺式排列）是主要形式。对于两种磷取代物种的Z（顺）-（β）构型，在溶液中观察到分子内E…HN（E = S，Se）相互作用，对于磷（V）-硒化合物，其特征在于核磁共振光谱中的J（SeH）耦合以及红外光谱中NH吸收的红移。对代表性的Ph₂P（E）-和Cy₂P（E）-取代物种的X射线晶体学分析表明，二苯基磷取代的化合物仅为E（顺）-（β）构型，二环己基衍生物为Z（顺）-（β）形式，与硫族元素和氮取代基无关。模型化合物的密度泛函理论分析结果未能确定观察到的取代基取向偏好的令人信服的电子论据，这表明空间因素在确定这些体系中起作用的细微能量差异方面起着重要作用。
