Ting-Shen Kuo

Remarkably Short Metal–Metal Bonds: A Lantern‐Type Quintuply Bonded Dichromium(I) Complex

The field of quadruply bonded dinuclear complexes in which two metal atoms are embraced by eight ligands has been considered mature. The bonding and electronic structures of these compounds have been well understood, ever since the discovery of the first dimetal species containing a quadruple bond, [Re2Cl8] 2 , over 40 years ago. The quest for thermally stable and isolable dinuclear complexes with higher bond orders is one aim of chemists in this field. From the synthetic point of view, quintuply bonded dinuclear complexes have become the focus in the past few years. On the theoretical side, the hypothetical RMMRmolecules (M= Cr, Mo, W, U; R=H, F, Cl, Br, CN, Me) have been highlighted, particularly the trans-bent structures. These feature a quintuple bond between two metal atoms, and for RCrCrR, the calculated Cr–Cr bond lengths are in the range of 1.64–1.78 4. Experimentally, a recent landmark advance was the isolation of a quintuply bonded dichromium(I) complex supported by two monodentate bulky carbyl ligands [Ar’CrCrAr’] (Ar’=C6H3-2,6-(C6H3-2,6iPr2)2), [12] in which, coincidentally, a trans-bent (C2h) geometry is adopted and the two chromium atoms share five electron pairs in five bonding molecular orbitals according to computational analyses. 14] Moreover, D2h-symmetric [Cr2(m-h L)2] ( L=N,N’-bis(2,6-diisopropylphenyl)-1,4-diazadiene) was recently reported to feature a very short Cr–Cr distance of 1.8028(9) 4 and calculated to display some degree of fivefold bonding. We have been interested in the pursuit of low-coordinate and multiply bonded dinuclear complexes since our first report on the unconventional quadruply bonded dimolybdenum complex [Mo2{m-h -(DippN)2SiMe2}2] (Dipp= 2,6iPrC6H3), in which each Mo atom is ligated by only two nitrogen atoms. Accordingly, we were interested in the preparation of multiply bonded dinuclear complexes supported by ancillary ligands which can minimize the metal– ligand p-bonding interaction and maximize the metal–metal interaction. Inspired by the hypothetical eclipsing molecule M2L6, proposed by Hoffmann et al., [17] in which M M could be a quintuple bond, we set out to explore the possibility of synthesizing such complexes. Here we report the use of amidinate ligand ArNC(H)NAr (Ar= 2,6-C6H3(CH3)2) to stabilize mixed-valent Cr2 3+ complex [Cr2(ArNC(H)NAr)3] (2) with formal Cr Cr bond order of 4.5 and its one-electron reduced Cr2 2+ species [Cr2(ArNC(H)NAr)3] (3 with formal Cr Cr bond order of 5.0). Both of these species have very short Cr Cr bonds; the bonding in these two compounds was studied theoretically. Reduction of dichromium bis(amidinate) dichlorido complex [{Cr(thf)}2(m-Cl)2{m-h -(ArNC(H)NAr)}2] (1), [18] in which the Cr–Cr distance is 2.612(1) 4, with KC8 gave redbrown mixed-valent dichromium tris(amidinate) compound 2 in 47% yield (Scheme 1). Compound 2 is paramagnetic, as

Journey from Mo−Mo Quadruple Bonds to Quintuple Bonds

Heating K(4)Mo(2)Cl(8) and 2 equiv of Li[RC(NDipp)(2)] (R = H, Ph; Dipp = 2,6-i-Pr(2)C(6)H(3)) in tetrahydrofuran (THF) at 60 degrees C gives two paddlewheel type quadruply bonded dimolybdenum complexes, Mo(2)(mu-Cl)[Cl(2)Li(OEt(2))][mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (1), Ph (2)). The Mo-Mo bond lengths of 1 and 2 are 2.0875(4) and 2.0756(8) A, respectively, indicating typical Mo-Mo quadruple bonds. Reduction of 1 and 2 by two electrons results in the isolation of their corresponding Mo-Mo quintuple bonded complexes, Mo(2)[mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (3), Ph (4)), and the Mo-Mo bond lengths dramatically decrease to 2.0187(9) A (3) and 2.0157(4) A (4), a consequence of the formation of the second delta bond and representing the shortest metal-metal bonds beyond the first row elements. The Mo-Mo quintuple bonding characters are corroborated by DFT calculations at the level of BP86/def2-TZVP and BP86/def2-TZVPP.

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.

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.

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.