Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein (Part II): From Li_nCrPh_3+n(thf)_x (n = 1, 2, 3) to Dimeric Triphenylchromate(II) Complexes

Abstract

Polyphenylchromium(III) organometallics with various phenylation degrees and stabilized by diverse Lewis bases with various donor strengths and denticity were investigated in order to better understand the formation of (η 6 -arene)chromium complexes according to the procedure of

Contributions of Erhard Kurras to Organochromium Chemistry – From a Historical Perspective

“…The molecular structure of [Li 2 Cr(C 4 H 8 ) 2 (THF) 2 ] 2 was described later (see Chapter 2.4 on Cr−Cr bonds). Since that time, there have been several other studies of such complexes, [1e] however, as a general rule for the existence of σ‐alkyl‐chromium compounds, it became clear that they precipitate when well‐suited alkyl groups such as methyl or alkylene are used, which can prevent β ‐hydride abstraction. This issue was later described in detail in a series of reviews [1]…”

“…Binuclear organochromium(II) compounds were intensively studied by Hein, [12f] Kurras [6g,7a,8c] and others, all mentioned in one published review [1e] and a book article [1g] including information about the question of whether a Cr−Cr quadruple bond exists or not. A Cr−Cr quadruple bond can be described in the MO picture by the interactions of the 3 (σ‐bond), the 3 and 3 (two π‐bonds) and the 3 atomic orbitals (δ‐bond) between the two chromium(II) atoms (Figure 4).…”

“…Ultimately, all of Kurras′ novel dichromium(II) compounds were studied by X‐ray diffraction and indeed, they all possess very strong Cr−Cr interactions and can be described via a Cr−Cr quadruple bond [13] . For example, tetrallyldichromium(II) compounds, [13c–e] and the later synthesized similar alkyl and phenyl compounds feature also very strong Cr−Cr bonds, [1d,e] or [Li 4 Cr 2 (CH 3 ) 8 (C 4 H 8 O) 4 ], [13a] [Li 4 Cr 2 (C 4 H 8 ) 4 (C 4 H 8 O) 4 ], [13b] and tetraallyldichromium(II) Cr 2 (C 3 H 5 ) 4 [13c–e] In 1978, four years after their synthesis by Kurras and co‐workers in 1974, the dinuclear complexes of Cr(II) and Mo(II) with bridging dimethylphosphonium bis(methylide) ligands (Figure 3C) were studied by Cotton and co‐workers, showing a “supershort” Cr−Cr quadruple bond of 1.895(3) Å [14a,b] …”

“…Many of these complexes have been described in excellent reviews as documents of progress since then [1] . There are many examples of their use in various stoichiometric and catalytic reactions [1d,e] . Nevertheless, it is worth pointing out that some of the first steps in this area of chemistry were influenced by single important contributions of Erhard Kurras.…”

Phenylchromium(III) Chemistry Revisited 100 Years afterFranz Hein(Part III): From (Ar)3–nCrCln(L)x(Ar = Ph,p-F-C6H4, C6D5;n= 0, 1, 2; L = thf, dme, tmeda, 12C4) to Bis(π-arene)chromium Complexes

“…In the first part of this miniseries, which appeared 100 years after Hein ’s initial report, we discussed syntheses and structures of arylchromium­(III) complexes with varying numbers of σ-bound phenyl groups . The second part dealt with the chemistry of these compounds, finally leading to bis­(π-arene)chromium derivatives . In his first publication on “polyphenylchromium” compounds in January 1919, Franz Hein reported the synthesis of the raw bromide “(C 6 H 5 ) 5 CrBr” besides plenty of biphenyl .…”