Quintuply‐Bonded Dichromium(I) Complexes Featuring Metal–Metal Bond Lengths of 1.74 Å

Abstract

The construction of a metal–metal quintuple bond has long been a challenge for chemists, since a large number of quadruple-bonded dinuclear complexes have been reported and their bonding and electronic structures extensively investigated and well understood. On the basis of theoretical work, many possible structures could be capable of accommodating a metal–metal quintuple bond, in contrast to the strict requirement of having twometal atoms embraced by eight ligands in a tetragonal geometry for a metal–metal quadruple bond. Of particular interest is that all of these model structures display a common feature: a low-coordinate environment around metal centers. From a practical point of view, the first quintuple-bonded dichromium complex [Ar’CrCrAr’] (Ar’=C6H3-2,6-(C6H3-2,6-iPr2)2, Cr Cr= 1.8351(4) ), which adopts a trans-bent geometry, was reported by Power and co-workers in 2005. More recently, in 2007, Theopold and co-workers reported an interesting dichromium complex supported by a-diimines, [Cr2(m-h -{C(H)N(C6H3-2,6-iPr2)}2)2], which was shown by computations to exhibit some degree of Cr Cr quintuplebond character. Since our first report on the characterization of an unconventional quadruple-bonded dimolybdenum complex [Mo2{m-h -(DippN)2SiMe2}2], where each Mo atom is ligated by only two nitrogen donors, we have been interested in the pursuit of low-coordinate and multiply-bonded dinuclear complexes. We recently characterized a mixed-valent dichromium complex stabilized by three amidinate ligands, [Cr2{ArNC(H)NAr}3] (Ar = 2,6-C6H3(CH3)2), and its oneelectron reduction partner [Cr2{Ar NC(H)NAr}3] , which exhibited the shortest metal–metal bond length of 1.7397(9) . Qualitatively, the latter is believed to incorporate a Cr Cr quintuple bond. In view of Power s and Theopold s complexes, wherein both Cr centers were coordinated by two donor atoms, we set out to prepare dichromium bis(amidinato) complexes, from which metal–metal quintuple bonds are expected. Herein we report a series of complexes of the form [Cr2{m-h -ArNC(R)NAr}2], which all exhibit very short Cr Cr quintuple-bond lengths of approximately 1.74 . Amidinate ligands, featuring substituents of different bulk, are used to stabilize these Cr Cr quintuple bonds. Prior to the synthesis of the target molecules, four green mononuclear complexes, 1a–d, of the form [CrCl2(thf)2{h ArNC(R)NAr}] were prepared in good yields by treatment of [CrCl3(thf)3] or CrCl3 with lithiated amidines (Scheme 1, see