Mercury- and Cadmium-Assisted [2 + 2] Cyclodimerization of <i>tert</i>-Butylselenium Diimide
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The complexes [MCl2{N,N′-tBuNSe(μ-NtBu)2SeNtBu}] [M = Cd (1), Hg (2)] were obtained in high yields by the reaction of tert-butylselenium diimide SeIV(NtBu)2 with CdCl2 or HgCl2 in tetrahydrofuran. Recrystallization of 1 and 2 from acetonitrile (MeCN) afforded yellow crystals of 1·MeCN and 2·MeCN, respectively. Isomorphic 1·MeCN and 2·MeCN contain an unprecedented dimeric selenium diimide ligand, which is N,N′-chelated to the metal through exocyclic imido groups. In addition to the complexes 1 and 2, the 77Se NMR spectra of acetonitrile solutions of 1·MeCN and 2·MeCN indicated the presence of the dimeric tBuNSe(μ-NtBu)2SeNtBu, monomeric SeIV(NtBu)2, and cyclic selenium imides. Density functional theory calculations at the PBE0/def2-TZVPP level of theory were used to assign the 77Se resonances of the dimer. A comparison of Gibbs energies of formation of some metal dichloride complexes [MCl2{N,N′-SeIV(NtBu)2}] and [MCl2{N,N′-tBuNSe(μ-NtBu)2SeNtBu}] (M = Zn, Cd, Hg) indicated that the formation of complexes containing a dimeric selenium diimide ligand is favored over those containing a monomeric ligand for the group 12 metals. In the case of the group 10 metal halogenides (M = Ni, Pd, Pt), the Gibbs energies of the complexes with monomeric SeIV(NtBu)2 ligands are close to those containing dimeric tBuNSe(μ-NtBu)2SeNtBu ligands. A plausible reaction pathway with a low activation energy involves the initial formation of [MCl2{N,N′-SeIV(NtBu)2}] (M = Zn, Cd, Hg), which then reacts with another molecule of Se(NtBu)2, leading to the final [MCl2{N,N′-tBuNSe(μ-NtBu)2SeNtBu}] complex. Without the presence of group 12 metal halogenides, the [2 + 2] cyclodimerization of SeIV(NtBu)2 is virtually thermoneutral, but the activation energy is relatively high, which accounts for the kinetic stability of tBuNSe(μ-NtBu)2SeNtBu in solution. A minor byproduct, [Cd7Cl14{N,N′-SeII(NHtBu)2}6]·4CH2Cl2, was identified by X-ray crystallography as a heptanuclear cluster with selenium(II) diamide ligands N,N′-chelated to the cadmium centers.
