Lutidine adducts of urea: molecular mechanisms for twinning effects on cooling

Abstract

The structures of three adducts between urea and 2,6-or 3,5-lutidine have been determined: urea : 2,6-lutidine (1 : 1) (1), urea : 2,6-lutidine (2 : 1) (2) and urea : 3,5-lutidine (2 : 1) (3). Adducts 1 and 3 each crystallize as different polymorphs depending on the temperature. In each of the five structures, the lutidine nitrogen acts as a hydrogen bond acceptor for both NH groups of a urea molecule, and the lutidine molecules form infinite stacks. The known structure of 1 in space group C2/c (1m), previously known only from photographic data in projection, was redetermined at 200 K. Both residues display crystallographic twofold symmetry. The urea substructure is a ribbon of [R 2 2 (8)] rings. Further cooling results in a reduction in symmetry to P1 (structure 1t); crystals are non-merohedrally twinned by 180°rotation about (b*-c*), which is the same direction as the b axis of 1m. The qualitative structure remains almost unchanged, but the lutidine rings tilt slightly away from the twofold axis (which is lost on twinning). The three structures involving 2 : 1 adducts all display a urea substructure in the form of a corrugated layer, from which urea molecules project and form hydrogen bonds to the lutidines. The urea layers are formed from ribbons of [R 2 2 (8)] rings that are linked by lateral hydrogen bonds. However, the linkages are in all three cases subtly different. Adduct 2 crystallizes in Pnma with the lutidine and both ureas displaying mirror symmetry. Adduct 3 crystallizes in space group Abm2 (3o) at 173 K, with two lutidines and four ureas all displaying crystallographic mirror symmetry; on cooling to 100 K, the space group changes to Cc (3m) and the crystals form reticular pseudo-merohedral twins by 180°rotation about a*, probably as a consequence of the mutual displacement of the urea ribbons, parallel to their direction of propagation, to accommodate a change in hydrogen bonding.

Highly Selective and Catalytic Oxygenations of C−H and C=C Bonds by a Mononuclear Nonheme High‐Spin Iron(III)‐Alkylperoxo Species

“…Thes elective oxidations of substrates are performed by am etal-based oxidant from [Fe III (t-BuL Urea )(OOCm)-(OH 2 )] 2+ mediated by ag eneral base.T he NH groups of urea and nitrogen center of 2,6-lutidine are known to form hydrogen bonds. [35] Theh ydrogen-bonded pyridine base subsequently takes up ap roton from the iron-coordinated urea group.T he conjugate acid thus formed facilitates the heterolytic OÀObond cleavage through ahydrogen-bonding interaction [24] with the distal oxygen of the peroxide unit (see Scheme S4), resulting in the generation of aputative iron(V)oxo oxidant, which performs substrate oxidation. In the absence of the base,t he minor pathway involving spontaneous deprotonation of the urea group likely forms the active oxidant displaying oxidations but with low selectivity.H owever,f urther mechanistic studies are required to gain information about the active oxidant involved in the reaction pathway.…”

Highly Selective and Catalytic Oxygenations of C−H and C=C Bonds by a Mononuclear Nonheme High‐Spin Iron(III)‐Alkylperoxo Species

“…The selective oxidations of substrates are performed by a metal‐based oxidant from [Fe III ( t ‐BuL Urea )(OOCm)(OH 2 )] 2+ mediated by a general base. The NH groups of urea and nitrogen center of 2,6‐lutidine are known to form hydrogen bonds . The hydrogen‐bonded pyridine base subsequently takes up a proton from the iron‐coordinated urea group.…”

Two polymorphs of 4-hydroxypiperidine with different NH configurations

“…We attempted in an earlier publication to give a tentative explanation for phase changes on cooling in two urea-lutidine adducts, in which the degree of rearrangement was 40 however limited. 8 Further studies were therefore carried out to address three problems: (1) Is it possible to obtain a reliable supply of the less common form 1o? (2) Can the solid state forms 1o and 1t be interconverted?…”