A Conformational Study of Phospha(III)- and Phospha(V)-guanidine Compounds

Abstract

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.

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

“…The reaction product was characterized by single-crystal X-ray diffraction and shown to exist in the solid state as a mononuclear fivecoordinate calcium complex in which the phosphaguanidinate ligand binds via a symmetric−NCN−chelate with auxiliary coordination at calcium provided by the b-diketiminate spectator ligand and a single molecule of THF. Consistent with previous studies upon the coordination chemistry of phosphaguanidine ligands (Mansfield et al 2006), the phosphorus lone pair is not delocalized across the amidinate moiety, and there is a significant degree of pyramidalization at phosphorus.…”

Synthesis and conformational features of sym N,N′,N″-triarylguanidines

“…The Δ CN 0⋅077(4) Å value in 7 is shorter than the Δ CN′ 0⋅106(4) Å value. The amino nitrogen in 7 are planar (ΣN = 360°) that contrasts with the significantly pyramidilised amino nitrogens in A, 44 45 The aforementioned structural variation may be explained by invoking (i) n-π conjugation or negative hyperconjugative interaction involving the lone pair on the amino nitrogen with C=N π * orbital, 47 (ii) the interaction of the lone pair on the amino nitrogen with the antibonding orbital of the aryl substituent 48,49 and intermolecular N-H⋅⋅⋅N hydrogen bonding (see below). tion of o-OMe substituents with respect to the planar CN 3 unit.…”

Half-sandwich rare-earth metal tris(alkyl) ate complexes catalyzed phosphaguanylation reaction of phosphines with carbodiimides: an efficient synthesis of phosphaguanidines

“…Various carbodiimides could be applied to the present conditions. Although phosphaguanidines could, in principle, have eight 15 all of the 1 H, 13 C and 31 P NMR spectra of 9a-r indicated only one isomer in solution. In the case of N-ethyl-N 0 -tert-butylcarbodiimides (9c), this CPPC reaction need higher catalyst loadings and long reaction time probably due to the steric hindrance of the tertbutyl group.…”