Group 1 and group 2 metal complexes supported by a bidentate bulky iminopyrrolyl ligand: synthesis, structural diversity, and ε-caprolactone polymerization study

Abstract

We report here a series of alkali and alkaline earth metal complexes, each with a bulky iminopyrrolyl ligand [2-(Ph3CN=CH)C4H3NH] (1-H) moiety in their coordination sphere, synthesized using either alkane elimination or silylamine elimination methods or the salt metathesis route. The lithium salt of molecular composition [Li(2-(Ph3CN=CH)C4H3N)(THF)2] (2) was prepared using the alkane elimination method, and the silylamine elimination method was used to synthesize the dimeric sodium and tetra-nuclear potassium salts of composition [(2-(Ph3CN=CH)C4H3N)Na(THF)]2 (3) and [(2-(Ph3CN=CH)C4H3N)K(THF)0.5]4 (4) respectively. The magnesium complex of composition [(THF)2Mg(CH2Ph){2-(Ph3CN=CH)C4H3N}] (5) was synthesized through the alkane elimination method, in which [Mg(CH2Ph)2(OEt2)2] was treated with the bulky iminopyrrole ligand 1-H in 1 : 1 molar ratio, whereas the bis(iminopyrrolyl)magnesium complex [(THF)2Mg{2-(Ph3CN=CH)C4H3N}2] (6) was isolated using the salt metathesis route. The heavier alkaline earth metal complexes of the general formula {(THF)nM(2-(Ph3CN=CH)C4H3N)2} [M = Ca (7), Sr (8), and n = 2; M = Ba (9), n = 3] were prepared in pure form using two synthetic methods: in the first method, the bulky iminopyrrole ligand 1-H was directly treated with the alkaline earth metal precursor [M{N(SiMe3)2}2(THF)n] (where M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent at ambient temperature. The complexes 7-9 were also obtained using the salt metathesis reaction, which involves the treatment of the potassium salt (4) with the corresponding metal diiodides MI2 (M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent. The molecular structures of all the metal complexes (1-H, 2-9) in the solid state were established through single-crystal X-ray diffraction analysis. The complexes 5-9 were tested as catalysts for the ring-opening polymerization of ε-caprolactone. High activity was observed in the heavier alkaline earth metal complexes 7-9, with a very narrow polydispersity index in comparison to that of magnesium complexes 5 and 6.

Alkali Blues: Blue‐Emissive Alkali Metal Pyrrolates

“…The centrosymmetric arrangement of the dimeric core [Li( t Bu L)] 2 is similar to the few reported structures of alkali metal‐pyrrolate complexes (Scheme ) and to the solid‐state structure of the parent ligand H t Bu L, which crystallizes as a centrosymmetric hydrogen‐bridged dimer (H t Bu L) 2 (Scheme ) . These centrosymmetric dimers, [Li( t Bu L)] 2 and (H t Bu L) 2 , differ from the C 2 ‐symmetric copper(I) dimer [Cu( t Bu L)] 2 (Scheme ) .…”

“…The K1 ⋅⋅⋅ centroid(C1,C2,C3,C4,N1) distance to the η 5 ‐coordinating pyrrolate amounts to 2.876 Å (Table S4). All potassium ⋅⋅⋅ donor‐atom distances agree with those of reported potassium 2‐imino‐pyrrolates . The preference of potassium ions for π coordination and large coordination numbers is a consequence of their soft character and large ionic radius…”

“…Similar p coordination of neighboring pyrrolates to sodium ions has been observed in [(NaL) 2 ] n ,acomplex lacking the tert-butyl group (but with h 5 instead of h 2 coordination;S cheme 1) [31] andi n[ (NaL') 2 ] 2 with sterically demanding aryl substituents (h 3 coordination). [33] The p coordination reflects the much softer character of sodium cations as compared with lithium cations, which prefer N donor ligandss uch as nitriles…”

“…Only a few solid‐state structures of lithium, sodium, and potassium 2‐iminopyrrolates have been reported, revealing chelating ligands, solvent coordination, or oligomerization up to polymerization to complete the coordination sphere of the metal ions (e.g. [(NaL) 2 ] n or (K 2 L) n ; Scheme ) …”

“…[(NaL) 2 ] n or (K 2 L) n ;Scheme 1). [33][34][35] To the best of our knowledge,t he photophysical properties of 2-iminopyrroles HL and alkali metal 2-iminopyrrolates ML have not yet been studied. However,f luorescenta lkali metal complexes of 8-hydroxyquinolinates [(Liq) n ;S cheme 1] are well known and these dyesa re successfully used as emitters and electron injectors in organic light-emitting diodes.…”