Abstract
The reaction between diamido ligand, Li4[Me2Si(NDipp)2]2 (Dipp = 2,6-iPrC6H3), and MCl2 (M = Mn and Cd) yields Mn2{□2-Me2Si(NDipp)2}2 (3) and Cd2{□-□2-Me2Si(NDipp)2}2 (4). After stepwise reduction of Complex 3, we could separate [(THF)2K⊂18-crown-6][Mn2{□-□2-Me2Si(NDipp)2]2 (18-crwon-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane; THF = tetrahydrofuran ([(THF)2K⊂18-crown-6][5]), [K⊂222-crptand]2[Mn2{□2-Me2Si(NDipp)2]2 (222-cryptand = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) ([K⊂222-cryptand]2[6]) and [K2⊂6]. Both Complex [K⊂222-cryptand]2[6] and [K2⊂6] possess [Mn22+] core with Mn-Mn single bond. The addition of KH to Complex 4 leads the isolation of tetranuclear cadmium complex, [(THF)2K⊂18-crown-6]2[{□2-Me2Si(NDipp)2}Cd{□-Me2Si(NDipp)2}Cd]2 ([(THF)2K⊂18-crown-6]2[7]), with Cd-Cd single bond. The isolation of these compounds corroborate the existence of the intermediates anticipated in structural transformations between Zn2{□-□2-Me2Si(NDipp)2}2 (1) and [{□2-Me2Si(NDipp)2}Zn-Zn{□2-Me2Si(NDipp)2}]2- (2) by theoretical calculations. A series of stable molecules containing unprecedented three-center, two-electron M-A-M bonds (M = Zn, Cd; A = K, Rb) were prepared. Reduction of dinuclear Zn(II) and Cd(II) compounds, [Zn(THF)(□-□3-N2N)]2 (N2N = 2,6-(DippN)2-4-MeC5H4N) (8) and [Cd(□-□3-N2N)]2 (9), by potassium graphite or elemental rubidium with the presence of 18-crown-6 ether afforded thermally stable compounds [K(THF)n(18-crown-6)][(MKM)(N2N)2] (10: M = Zn, n = 0; 11: M = Cd, n = 1) and [Rb(THF)(18-crown-6)][(ZnRbZn)(N2N)2] (12). The A-M bond lengths are surprisingly short (Zn-Kavg = 2.474 Å, Cd-Kavg = 2.606 Å, and Zn-Rbavg = 2.532 Å). The M-Rb-M three-center, two-electron covalent bonding is also supported by the fact that the bridging alkali metals cannot be replaced by other Group 1 metal ions via ion exchange reactions. Complex 10 shows highly reducting potential in view of the reaction between 10 and MgI2. Treatment of pyridyl diamido ligand, Li2[N2N](OEt2), with CrCl3 and subsequently reduced by 2.5 equiv of potassium graphite gives a novel quintuply-bonded dichromium complex, {(OEt2)KCr(□1:□2-N2N)}2 (26). We also found the arene-philic and substitution effect on the formation of Cr-Cr quintuple bond. In addition, the reaction of NbCl3(DME) and different type of ligands, such as amidinates, pyridyl diamido ligand, and □-dimine ligand, leads to the isolation of (□-Cl)3[Li(THF)2(□-Cl)2][Nb(□2-HC(N-2,6-iPr2C6H3)]2 (30), Cl3Nb(□2-o,o’-iPr2C6H3-DAB) (o,o’-iPr2C6H3-DAB = 2,3-dimethyl-1,4-bis-(2,6-iPr2C6H3)-1,4-diaza-1,3-butadiene)) (31), and [ClNb (□-□3-N2N)]2 (32). Complex 30 and 32 have Nb=Nb doble bond. Redution of Complex 31 with zinc powder gives dinioubium complex, [ClNb(□-Cl)2Nb(THF)](□2-o,o’-iPr2C6H3-DAB)2 (33), with Nb-Nb single bond. However, addition of 0.25 equiv of amidinates ligand to the precursor, reduced by potassium graphite from NbCl3(DME), leads to the isolation of tetranuclear niobium(II) complex, [{(THF)Nb}(□-Cl) 2{Nb(THF)Cl}]2[□-□2-HC(N-2,6-iPr2C6H3)2]2 (29). It’s an unprecedented butter-fly conformation. We also present N-N coupling reactions mediated by univalent Zn–Zn and Mn–Mn bonds. Treatment of the Zn–Zn bonded complex K2[{□2-Me2Si(NDipp)2}Zn-Zn{□2-Me2Si(NDipp)2}]¬¬ (17) with 2 equiv of p-tolylazide or azidotrimethylsilane in presence of 18-crwon-6 ether gives [K(18-crown-6)(THF)]2{[□2-Me2Si(NDipp)2]Zn(□-□4-RNN2NR)Zn[□2-Me2Si(NDipp)2]} (R = p-tolyl) (19a) with a bridging trans-tetrazene ligand [(p-tolyl)NN2N(p-tolyl)] and [K(18-crown-6)(THF)2]2 {[□2-Me2Si(NDipp)2]Zn(□-NSiMe3)Zn[□2-Me2Si(NDipp)2]} (20), respectively. However, addition of 2 equiv of organic azides RN3 (R = 1-adamantyl, p-tolyl) to the Mn–Mn bonded complex [Mn(Nacnac)]2 (Nacnac = HC[C(Me)N(2,6-iPr2C6H3)]2 (18) also induces N-N coupling to give (□-□2:□2-RN6R)[Mn(Nacnac)]2 (20: R = adamantyl; 21: R = p-tolyl). Both 20 and 21 feature essentially the same core with a bridging hexazene ligand (RNN4NR). Interestingly, a trinuclear manganese complex [(Nacnac)Mn(μ-N3)]3 (22), where three manganese atoms are linked together via three bridging azido ligands, is obtained if 18 is treated with 2 equiv of azidotrimethylsilane. Furthermore, the reaction of Silyl-linked bis(amidinate) ligand, [Li(THF)4][Li3{□3-□4-Me2Si[NC(C6H5)N(2,6-iPr2C6H3)]2}2] and CuI leads to the isolation of Cu4{□-□4-Me2Si[NC(C6H5)N(2,6-iPr2C6H3)]2}2 (34) with the d10-d10 interactions between four copper atoms. Finally, addition of the same ligand to CrCl3 gives [Cl(□-Cl)(THF)Cr]2{□-□4-Me2Si[NC(C6H5)N(2,6-iPr2C6H3)]2} (35). [Cl(THF) Cr]2{□-□4-Me2Si[NC(C6H5)N(2,6-iPr2C6H3)]2} (36) is prepared by further reduced by 2 equiv of potassium graphite from complex 35. There is no bonding interactions between two dichormium atoms of Comples 34 and 35. All the synthesized products are characterized by NMR spectroscopy and elemental analysis and their molecular structures are determined by single crystal X-ray crystallography.