Imidazol-2-ylidene-N′-phenylureate ligands in alkali and alkaline earth metal coordination spheres - heterocubane core to polymeric structural motif formation

The synthesis and isolation of two potassium, one lithium and two calcium complexes of imidazol-2-ylidene-N'-phenylureate ligands [Im(R)NCON(H)Ph] [(R = tBu (1a); Mes (1b) and Dipp (1c); Mes = mesityl, Dipp = 2,6-diisopropylphenyl] are described. Potassium complexes, [{kappa(2)-(Im(Mes)NCONPh)K}(4)](2b) and[{kappa(3)-(Im(Dipp)NCONPh)K}(2){KN(SiMe3)(2)}(2)](n) (2c), were prepared in good yields by the reactions of 1b and 1c, respectively, with potassium bis(trimethyl) silyl amide at ambient temperature in toluene. Lithium complex [{(2,6-tBu(2)-4-Me-C6H2O) Li(Im(tBu)NCON(H)Ph)}(2){Im(tBu)NCON(H)Ph}] (3a) was isolated by a one-pot reaction between 1a and LiCH2SiMe3, followed by the addition of 2,6-tBu(2)-4-Me-C6H2OH in toluene. Calcium complex [{kappa(2)-(Im(tBu)NCONPh)Ca{N(SiMe3)(2)}-{KN(SiMe3)(2)}] n (4a) was isolated by the one-pot reaction of 1a with [KN(SiMe3)(2)] and calcium diiodide in THF at ambient temperature. The solid-state structures of ligand 1a and complexes 2b, 2c, 3a and 4a were confirmed by single-crystal X-ray diffraction analysis. It was observed that potassium was coordinated to the oxygen atom of urea group and to the nitrogen atom of the imidazolin-2-imine group, in the solid-state structure of 2b. In complex 4a, the calcium ion was ligated to the monoanionic imidazol-2-ylidene-N'-phenylureate ligand in a bi-dentate (kappa(2)) fashion through the oxygen and nitrogen atoms of the isocyanate building block leaving the imidazolin-2-imine fragment uncoordinated. In the solid state of the potassium complex 2c, tri-dentate (kappa(3)) coordination from the imidazol-2-ylidene-N'-phenylureate ligand was observed through the oxygen and nitrogen atoms of the isocyanate building block and of the imidazolin-2-imine fragment. In contrast, in the dimeric lithium complex 3a, the neutral imidazol-2-ylidene-N'-phenylureate ligand was bound to the lithium centre in a mono-dentate fashion (kappa(1)) through an oxygen atom of the isocyanate moiety. It is to be noted that in each complex thus observed, the elongated carbon-nitrogen bond distances indicate substantial electron delocalisation from the imidazole ring to the ureate group present in ligand 1.