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

Abstract

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