Yi-Chou Tsai

Reactions of metal-metal quintuple bonds with alkynes: [2+2+2] and [2+2] cycloadditions.

Benzene, C6H6, is one of the most important molecules in organic chemistry. The term aromaticity was developed from the study of benzene and has become an important and fundamental concept in contemporary chemistry since Kekul proposed a ringlike structure for benzene in 1865. Today, aromatic compounds are not only limited to carbocycles, but also extended to heterocycles containing maingroup elements. For example, one or two CH groups of benzene can be replaced by group 14 and 15 elements, such as silicon, nitrogen, and phosphorus, to give heterocyclic aromatics. Furthermore, an aromatic compound can be a transition-metal-containing organometallic species. Hoffmann et al. predicted that a metallabenzene could be possible by replacing one CH group with a transition-metal fragment. On the experimental side, since the recognition of the first metallabenzene, osmabenzene, in 1982, many metallabenzenes have been subsequently synthesized and structurally characterized. There are many ways to synthesize aromatic compounds. Among the methods, the [2+2+2] cycloaddition reaction of alkynes is a powerful tool to construct highly substituted homoand heterocyclic aromatic molecules. Such a reaction is thermally allowed, but it does not proceed at ambient temperature without transition-metal catalysts. It is however worth noting that Sekiguchi et al. have demonstrated a catalyst-free [2+2+2] cycloaddition reaction for the synthesis of 1,2-disilabenzene by reacting a kinetically stable disilyne RSi SiR (R= SiiPr[CH(SiMe3)2]2) and phenylacetylene at room temperature. The chemistry of transition-metal multiple bonding was recently reawakened by the recognition the first isolable quintuply bonded dimeric chromium complex Cr2Ar’2 (Ar’= C6H3-2,6-(C6H3-2,6-iPr2)2 by Power and co-workers. [10] Since then, several quintuply bonded dinuclear group VI metal complexes stabilized by nitrogen donors have been reported. Besides interesting bonding, the chromium derivatives have recently been shown to display novel reactions with AlMe3, [11f] N2O, RN3, [14] and P4. [15] Reactions of the chromium dimers with alkynes through a [2+2] cycloaddition process to give 1:1 adducts were recently explored as well. In light of the bonding analogy between the C C p components of alkynes and metal–metal d components in the quintuply bonded species (Figure 1), we became inter-

Synthesis and reactions of β-diketiminato divanadium(I) inverted-sandwich complexes

Reduction of VCl(2)(Nacnac) (Nacnac = HC(C(Me)NC(6)H(3)-iPr(2))(2)) with KC(8) in toluene leads to the formation of a toluene-bridged inverted-sandwich divanadium(I) complex, (mu-eta(6):eta(6)-C(7)H(8))[V(Nacnac)](2), which behaves as a source of V(Nacnac) and a multi-electron reductant in the two reactions studied in this report.

Stepwise Construction of the CrCr Quintuple Bond and Its Destruction upon Axial Coordination

Give me five! Terdentate 2,6-diamidopyridyl ligands were used to stabilize the Cr-Cr quintuple bond and have made it possible to isolate and characterize not only the Cr-Cr quintuple-bonded complex, but also the mixed-valent intermediates (Cr(I) and Cr(II)), which are important species in the formation of type I quintuple-bonded complexes.

An Electron‐Rich Molybdenum–Molybdenum Quintuple Bond Spanned by One Lithium Atom

Take five: A unique quintuply bonded dimolybdenum complex [Mo(2)(μ-Li){μ-HC(N-2,6-Et(2)C(6)H(3))(2)}(3)] (see picture) was synthesized and characterized. The Mo-Mo interaction includes an unexpected bridging Li(+) ion. Calculations indicate the bridging Li(+) ion does not perturb the Mo-Mo bond length (2.0612(4) Å), but results in a relatively small effective Mo-Mo bond order of 3.67.

Theory‐Guided Experiments on the Mechanistic Elucidation of the Reduction of Dinuclear Zinc, Manganese, and Cadmium Complexes

Since the recognition of the first Zn Zn bond in the dinuclear sandwich decamethyldizincocene [(h-C5Me5)Zn Zn(h-C5Me5)], [1] the chemistry of Zn Zn-bonded species has grown so rapidly that many complexes of the type LZn ZnL have been characterized and studied. Regardless of the denticity of the supporting ligands L, they all coordinate to Zn in a terminally chelating mode. However, formation of these dinuclear compounds has not been mechanistically examined. We recently described the characterization of dinuclear Zn Zn-bonded species [{k-Me2Si(NDipp)2}Zn Zn{k-Me2Si(NDipp)2}] 2 (2) (Dipp= 2,6-iPr2C6H3) from KC8 reduction of dinuclear zinc complex [Zn2(m-k -Me2Si(NDipp)2)2] (1), whereby the coordination mode of the diamido ligands dramatically changes from bridging to chelating. We thus became interested in the structural preference and the formation mechanism of Zn Zn-bonded complexes. Elaborate calculations were performed to understand the reduction of 1, and a plausible mechanism was then proposed (Scheme 1). On two-electron reduction of 1, two intermediates, Ia and Ib, are generated, and the energy difference between them is only 0.3 kcalmol . The Zn Zn-bonded mixed-valent intermediate Ia is produced by one-electron reduction of 1, and subsequently undergoes a dramatic structural rearrangement to give Ib, in which one threecoordinate and one one-coordinate Zn atoms are proposed. The exact valence of the Zn atoms in Ib is still not clear. Although the application of quantum chemical methods (ab initio molecular orbital and density functional theory) to elucidate reaction mechanisms has been very successful, most of the time it is difficult to prove the theoretically developed reaction mechanisms by experiments. This is indeed the case for the transformation from 1 to 2. Attempts to probe both intermediates Ia and Ib failed. To this end, we turned our attention from zinc to manganese and cadmium, because they not only show structural similarity in the reported M M-bonded dinuclear complexes [(k-Nacnac)M M(k-Nacnac)] (M=Zn, Mn; Nacnac= HC[C(Me)NDipp]2) and [Ar’M MAr’] (M=Zn, Cd; Ar’= 2,6-(2,6-iPr2C6H3)2C6H3), but also feature an identical M M s-bonding scheme. Herein we report structural transformations on reduction of dinuclear manganese and cadmium complexes [Mn2{k -Me2Si(NDipp)2}2] (3) and [Cd2{mk-Me2Si(NDipp)2}2] (4). Characterization of the products supports the computed mechanism shown in Scheme 1. As shown in Scheme 2, reactions of the dilithiated diamido ligand and 1 equiv of anhydrous MnCl2 and CdCl2 in diethyl ether and THF, respectively, yielded the corresponding dimeric compounds 3 and 4 in good yields. The dinuclear nature of 3 and 4 was deciphered by single-crystal X-ray crystallography, and their molecular structures are provided in Figures S1 and S2 of the Supporting Information. Complex 3 is essentially composed of two MnN2Si fourmembered rings, which are brought together by two Mn N bonds, and consequently exhibit a boat conformation with two manganese atoms at the stern and two Si atoms at the bow. Each Mn atom is embraced by three nitrogen atoms and adopts a distorted T-shaped geometry. The central Mn2N2 Scheme 1. Calculated mechanism of transformation of 1 into 2.

Reductive N–N bond cleavage and coupling of organic azides mediated by chromium(I) and vanadium(I) β-diketiminate

Reactions of organic azides RN3 with two univalent inverted-sandwich complexes (μ-η6:η6-C6H5CH3)[M(Nacnac)]2 (M = Cr (1) and V (2); Nacnac = HC(C(Me)NC6H3iPr2)2) have been investigated. Where R = p-tolyl, reaction with 1 results in reductive N2 elimination and the formation of a diimido product Cr(NR)2(Nacnac) and a cycloaddition tetrazene product Cr[N(R)NNN(R)](Nacnac), which can be converted to Cr(NR)2(Nacnac) upon thermolysis. However, reaction of 1 with Me3SiN3 does not produce a diimido complex, but a monomeric three-coordinate amido product Cr[N(SiMe3)2](Nacnac) and an azide-bridged dinuclear compound [Cr(μ-N3)(Nacnac)]2. On the other hand, where R = p-tolyl, 1-adamantyl, or SiMe3, reaction with the more reducing 2 gives solely a diimido product V(NR)2(Nacnac). The symmetric diimido complex V(NSiMe3)2(Nacnac) (10) can be converted to an asymmetric diimido complex V(NSiMe3)(N-2,6-C6H3iPr2)[Me3SiNC(Me)C(H)C(Me)N(2,6-C6H3iPr2)] (11) upon heating.

The MoMo Quintuple Bond as a Ligand to Stabilize Transition-Metal Complexes†

Herein, we report the employment of the Mo-Mo quintuple bonded amidinate complex to stabilize Group 10 metal fragments {(Et3P)2M} (M=Pd, Pt) and give rise to the isolation of the unprecedented δ complexes. X-ray analysis unambiguously revealed short contacts between Pd or Pt and two Mo atoms and a slight elongation of the Mo-Mo quintuple bond in these two compounds. Computational studies show donation of the Mo-Mo quintuple-bond δ electrons to an empty σ orbital on Pd or Pt, and back-donation from a filled Pd or Pt dπ orbital into the Mo-Mo δ* level (LUMO), consistent with the Dewar-Chatt-Duncanson model.

Reversible Cleavage/Formation of the Chromium–Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ2 -HC(N-2,6-i Pr2 C6 H3 )(N-2,6-R2 C6 H3 )}]2 (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η6 :η6 -1,3,5-(Me3 Si)3 C6 H3 )[Cr{κ2 -HC(N-2,6-i Pr2 C6 H3 )(N-2,6-R2 C6 H3 )}]2 (R=i Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me3 C6 H2 CN, the bridging arene 1,3,5-(Me3 Si)3 C6 H3 in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.

A Family of Multiply Bonded Dimolybdenum Boraamidinates with the Formal Mo−Mo Bond Orders of 3, 4, 4.5, and 5

A boraamidinato ligand [PhB(N-2,6-(i) Pr2 C6 H3 )2 ](2-) was employed to stabilize a new family of multiply bonded dimolybdenum complexes [MoCl(μ-κ(2) -PhB(N-2,6-(i) Pr2 C6 H3 )2 )]2 (4) and [Mo(μ-κ(2) -PhB(N-2,6-(i) Pr2 C6 H3 )2 )]2 (n-) (n=0 (5), 1 (6), 2 (7)), with the respective formal Mo-Mo bond orders of 3, 4, 4.5, and 5. Each metal center in 5-7 is two-coordinate with respect to the ligands. Of particular interest is the quadruply bonded dimolybdenum complex 5, featuring an unprecedented angular conformation. The bent Mo2 N4 core of 5 distorts toward planarity upon reduction. As a result, compound 7 features a planar Mo2 N4 core, while that of 6 is still bent but less significantly than that of 5. Additionally, the Mo-Mo bond lengths of 4-7 systematically decrease as the valency of the central Mo2 units decreases. Complex 7 features the shortest Mo-Mo bond length (2.0106(5) Å) yet reported.

Synthesis and Characterization of an Eclipsed Digermylene as a Building Block to Construct a Cyclic Octagermylene

The preparation of an unprecedented GeI -GeI bonded digermylene [K2 {Ge2 (μ-κ2 :η2 :η4 -2,6-(2,6-i Pr2 C6 H3 -N)2 -4-CH3 C5 H2 N)2 }] in an eclipsed conformation stabilized by two bridging diamidopyridyl ligands is presented. Although it exhibits an eclipsed conformation, the Ge-Ge bond length is 2.5168(6) Å, which is shorter than those in the trans-bent and gauche digermylenes. In combination with two pendant amido groups, the GeI2 motif is employed as a building block to assemble the first example of octagermylene [Ge4 (μ-κ2 :κ1 -2,6-(2,6-i Pr2 C6 H3 -N)2 -4-CH3 C5 H2 N)2 ]2 showing a cyclic configuration and containing three distinct types of GeI -GeI bonds.