Coordination chemistry of sterically encumbered pyrrolyl ligands to chromium(ii): mono(pyrrolyl)chromium and diazachromocene formation

Abstract

A series of diazachromocenes with sterically demanding pyrrolyl ligands, 2,5-(Me3C)2C4H2N (1), 2,5-(Me3C)2-3,4-Me2C4N (2) and 2,3,5-(Me3C)3C4HN (3), was prepared and investigated by various spectroscopic techniques, and in some cases by X-ray diffraction and magnetic susceptibility studies. The diazachromocenes exhibit an S = 1 ground state; no indication of a spin-equilibrium was obtained. With the same ligands mono(pyrrolyl)chromium(II) complexes are accessible, [{(κN-2,5-(Me3C)2C4H2N)Cr(thf)}2(μ-Cl)2] (1-CrCl(thf)), [(κN-2,5-(Me3C)2-3,4-Me2-C4H2N)Cr(Cl)(tmeda)] (2-CrCl(tmeda) and [(η(5)-2,3,5-(Me3C)3C4HN)Cr(μ-Cl)2] (3-CrCl), which show either η(5)- or η(1)-(κN) coordination depending on the substitution pattern. (1)H NMR spectroscopy serves as a valuable tool to distinguish between these coordination modes. The Cr(II) atoms in the mono(pyrrolyl) complexes adopt a high spin configuration (S = 2) and in dimeric species antiferromagnetic coupling between the spin carriers was observed. However, none of these mono(pyrrolyl)chromium complexes is an effective or selective ethylene oligomerization catalyst on activation with MAO or AlMe3, supporting the importance of a Cr(I)/Cr(III)-based catalytic cycle.

Paramagnetic Chromium(II) Complexes and Chromium(IV) Nitrides with Bulky Alkylcyclopentadienyl Ligands

“…distances of the high spin chromium(II) complexes 1a , 1c , 3a , 3b , and 4b interpreted as indicators for the high spin chromium(II) state in this manuscript. This interpretation is supported by the chloride [Cp′′′Cr(µ‐Cl)] 2 , which has been characterized as an antiferromagnetically coupled dimer with two high spin chromium(II) central ions by magnetic susceptibility data obtained at variable temperature and fits well into the data compiled in Table . The oxygen donor atoms of the sulfate bridges are slightly closer to the Cr III atoms Cr2 and Cr3 (194.6/199.3 pm and 195.2/197.2 pm) than to the Cr II atom Cr1 (199.9/201.2 pm), but this difference is much smaller than expected from Cr III and hs Cr II ionic radii (75.5 vs. 94 pm for C.N = 6).…”

“…The Cr II –Cr II distances below 230 pm (Table ) are associated with low magnetic moments and correspond to strong bonds . The 318.1 pm Cr ··· Cr distance seen in the tri( tert ‐butyl)cyclopentadienyl derivative [Cp′′′Cr(µ‐Cl)] 2 of 2 has been noted to fall into the range of Cr–Cr bonds . In our opinion the distances above 300 pm found in very similar dichromium(II) complexes with sterically more demanding alkylcyclopentadienyl ligands correspond to Cr ··· Cr interactions with almost vanishing bond order approaching complete rupture.…”

“…Organometallic complexes of chromium have shown intriguing metal‐metal bonding properties and are well‐known for catalytic activity in ethylene oligomerization, epoxidation, epoxide/carbon dioxide copolymerization,, alternating epoxide/phthalic anhydride ring‐opening copolymerization, dioxygen activation and other processes. Bulky alkylcyclopentadienyl ligands have been seen in a small number of chromocenes, and in one mono(cyclopentadienyl)chromium complex [Cp′′′Cr(µ‐Cl)] 2 . Chromium(II) half sandwich complexes without π acceptor ligands promise to be even more reactive than the nickel, or iron precedents studied before: The 94 pm radius of chromium(II) ions in their high spin state is surpassed among the M 2+ cations of the 3d row only by Ti 2+ and opens Cr II half sandwich complexes for chemical reactions.…”

Ethylene oligomerization promoted by chromium complexes bearing pyrrolide–imine–amine/ether tridentate ligands

“…12b,12d, 14 The average Cr-Cl av (2.318 Å for 3a and 2.325 for 3b) bond distances are comparable to those found in similar chromium(III) compounds having tridentate ligands such as CrCl 3 L (L = bis(carbene)pyridine, Cr-Cl av = 2.327 Å) and CrCl 3 {HN(CH 2 CH 2 SEt) 2 } (Cr-Cl av = 2.312 Å). The Cr-N(imine) bond distances in 3a and 3b (1.991 (1) (1) Ethylene Oligomerization Studies.…”

The Missing Link: Manganese Complexes with Sterically Demanding Pyrrolyl Ligands

“…In order to close this gap, we recently reported the preparation of several diazachromocenes and mono(pyrroyl)chromium(II) complexes 10. In these investigations the sterically demanding pyrrolyl ligands Pyr t Bu 2 ( 1 ) and Pyr t Bu 2 Me 2 ( 2 ) [Pyr t Bu 2 Me 2 = 2, 5‐(Me 3 C) 2 ‐3, 4‐Me 2 C 4 H 2 N] adopt η 5 ‐coordination in diazachromocenes [(η 5 ‐Pyr t Bu 2 ) 2 Cr] ( 1‐Cr ) and [(η 5 ‐Pyr t Bu 2 Me 2 ) 2 Cr] ( 2‐Cr ), but κ N coordination is realized in the mono(pyrrolyl) complexes [{(κ N ‐PyrBu 2 )Cr(thf)} 2 (μ‐Cl) 2 ] and [(κ N ‐Pyr t Bu 2 Me 2 )Cr(tmeda)Cl] 10. For manganese only azacymantrene complexes have been extensively studied 11.…”