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

Abstract

In the field of metal-metal multiple bonding, there are many factors that affect the metal-metal bond lengths. The most determinant cause is the structural configuration of complexes, [1] and this has been illustrated by the Group 6 metalmetal quadruple bonds. In the large number of tetragonal quadruply bonded complexes [M 2 X 8 ], [1] the increase in the internal twist angle between two MX 4 fragments leads to elongation of the metal-metal quadruple bond. [1,2] For example, those quadruply bonded dimolybdenum complexes with the torsion angles about the MoÀMo bond of 26-408 possess long MoÀMo bonds in the range of 2.18-2.19 , which is because a major part of the d bonding has been abolished.In contrast to the relatively mature metal-metal tripleand quadruple-bond chemistry, [1, 3] quintuple bonding is in its infancy. In contrast to the trigonal triply bonded and tetragonal quadruply bonded dinuclear complexes, the geometry of a quintuply bonded compound has been controversial. Theoretically, the most accepted configuration of a quintuple bonded complex is the type I trans-bent geometry (Scheme 1). [4] On the experimental side, the first quintuple bond, in [Cr 2 Ar' 2 ] (Ar' = 2,6-[2,6-(iPr 2 ) 2 C 6 H 3 ] 2 C 6 H 3 ) [5a] and derivatives, [5b] was reported by Power et al. It is worthy noting these remarkable complexes in fact adopt the type II geometry, although the central Cr 2 unit is stabilized by two monoanionic terphenyl ligands. Since then, many efforts have been dedicated to isolating dinuclear species featuring quintuple-bond character. For example, the groups of Theopold, [6] Tsai, [7] and Kempe [8] reported several type II quintuply bonded Cr 2 and Mo 2 complexes with extremely short metal-metal quintuple bonds spanned by two bidentate nitrogen-based ligands. The existence of the CrÀCr quintuple bond of the type II complex was recently corroborated by charge-density studies. [9] Preliminary reactivity studies on the type II complexes indicate these univalent and low-coordinate dinuclear species are reactive towards unsaturated organic functionalities and small molecules. [10,11] Compared to the many type II complexes, only one example of the type III complex, [Cr 2 {m-HC(N-2,6-Me 2 C 6 H 3 ) 2 } 3 ] À (1), was recently reported, although such type of species was proposed by Hoffmann et al. in 1979. [12] Unexpectedly, compound 1 possesses an ultrashort CrÀCr quintuple bond (1.74 ), [7a] which has also been supported by theoretical calculations. [7a, 13] The quintuply bonded Group 6 M 2 (M = Cr, Mo) complexes reported to date have very short M À M bond distances. Theoretical analysis on the type II Cr 2 complexes [6][7][8] implied that these short CrÀCr distances are associated with the steric bulk of the ligands. [13] Furthermore, computations on the quintuply bonded Group 6 model compounds M 2 X 2 (M = Cr, Mo, W; X = H, F, Cl, Br, CN, Me) revealed the MÀM bond distances vary with the structural conformation. [4f] Since the recognition of the structure of 1, we became interested in characterizin...

Interconversion of Quadruply and Quintuply Bonded Molybdenum Complexes by Reductive Elimination and Oxidative Addition of Dihydrogen

“…[1a] Following decades of intense scrutiny of double, triple, and quadruple metal-metal bonds, in 2005 Power and co-workers made an outstanding discovery with the characterization of the first stable molecule with fivefold bonding between two chromium atoms. [2] This report encouraged the search for further examples of quintuply bonded dimetal compounds [1b] and was followed by numerous experimental [3][4][5][6][7][8][9][10][11] and computational studies. [12][13][14][15][16][17][18] By and large, these studies have focused on chromium compounds.…”

“…[9] Recently, another compound with the Mo I 2 central unit reinforced by three amidinate ligands and a bridging lithium cation has also been reported by the same group of researchers. [10] Identification of the above complexes with fivefold metalmetal bonding has prompted research directed toward under-standing their bonding characteristics [12][13][14][15][16][17][18] and the chemical reactivity of their central MÀM quintuple bond. So far, only a few reports have appeared in the literature concerning mainly the chemical properties of the Cr À Cr quintuple bond.…”

Interconversion of Quadruply and Quintuply Bonded Molybdenum Complexes by Reductive Elimination and Oxidative Addition of Dihydrogen

“…2–8, 11 However, Tsai and co‐workers have extended their investigation of the CrCr quintuple bond6, 7, 11 to analogous molybdenum complexes and have isolated two closely related compounds supported by monoanionic amidinate ligands, N , N ′‐disubstituted with 2,6‐diisopropylphenyl groups 9. Recently, another compound with the Mo I 2 central unit reinforced by three amidinate ligands and a bridging lithium cation has also been reported by the same group of researchers 10…”