Direct Synthesis of Ligand-Based Radicals by the Addition of Bipyridine to Chromium(II) Compounds

Abstract

The reaction of 2,2'-bipyridine (bpy) with monomeric chromium(II) precursors was used to prepare the S = 1 complexes Cr(tBu-acac)2(bpy) (1) and (η(5)-Cp)(η(1)-Cp)Cr(bpy) (3), as well as the S = 2 compound Cr[N(SiMe3)2]2(bpy) (4). The crystallographically determined bond lengths indicate that the bpy ligands in 1 and 3 are best regarded as radical anions, while 4 shows no structural evidence for electron transfer from Cr(II) to the neutral bpy ligand.

Evidence for Non‐Innocence of a β‐Diketonate Ligand

“…It is remarkable that the presence of a weak‐field ligand as THF is sufficient to promote the electron‐transfer. However, this result is paralleled by that of the {Cr II ‐bpy} (bpy=2,2′‐bipyridine) redox‐couple, which is always found as {Cr III ‐bpy .− } in octahedral complexes, but as {Cr II ‐bpy 0 } when the coordination geometry of Cr is square planar . While one may speculate if the apparent non‐innocence of the hfac − ligand has led to incorrectly assigned metal ion oxidation states in previously reported complexes, it is noteworthy to consider the electronic structure situation of 1 and 2 .…”

“…It is remarkable that the presence of a weak-field ligand as THF is sufficient to promote the electrontransfer.H owever,t his result is paralleled by that of the {Cr II -bpy} (bpy = 2,2'-bipyridine) redox-couple, which is always found as {Cr III -bpyC À }i no ctahedral complexes, [32] but as {Cr IIbpy 0 }w hen the coordination geometry of Cr is square planar. [33] While one may speculate if the apparent non-innocence of the hfac À ligand has led to incorrectly assigned metal ion oxidation states in previously reported complexes, it is noteworthy to consider the electronic structure situation of 1 and 2.I ndeed, the combination of ah igh-spind 4 configuration and am oderate reducing power leading to ah alf-filled, highenergy d z 2 orbital renders Cr II almostu nique. Thus, it seems possible that 1 and 2 are the only examples of non-innocence of simple b-diketonates in transition-metal complexes.…”

Hydrides, Halides, and Polymers: Some Unexpected Intermediates on the Routes to First-Row Transition Metal M{N(SiMe3)2}n (n = 2, 3) Complexes

“…The highly unstable divalent nickel amide may be prepared analogously . Although a few Lewis base-stabilized complexes of Cr­{N­(SiMe 3 ) 2 } 2 are known, − no M­{N­(SiMe 3 ) 2 } 2 complexes have been reported for the earlier transition metals titanium or vanadium. Our initial attempts to prepare V­{N­(SiMe 3 ) 2 } 2 by reaction of the vanadium­(II) salt VI 2 (THF) 4 with 2 equiv of LiN­(SiMe 3 ) 2 (OEt 2 ) were unsuccessful, giving only intractable oils as products.…”

“…During this work, two observations for these silylamide complexes became a focus of interest. First, it is notable that while divalent M­{N­(SiMe 3 ) 2 } 2 complexes have been described for the mid and late transition metals from chromium to nickel (although only as base-stabilized complexes in the case of chromium), no such complexes have been described for titanium or vanadium. ,,,,− Second, we noted that while the most recently reported trivalent M­{N­(SiMe 3 ) 2 } 3 (M = Mn, Co) complexes were obtained by the reaction of BrN­(SiMe 3 ) 2 with the corresponding divalent M­{N­(SiMe 3 ) 2 } 2 species, the fate of the bromine in these reactions is not well understood . Attempts to resolve these outstanding questions about the chemistry of these molecules are described here.…”

Fingerprinting the oxidation state of U(iv) by emission spectroscopy

“…The synthesis of 1 involved simply the reaction of bipy with [Et 4 N] 4 [U(NCS) 8 ], without additional reagents. 11 All other compounds listed above, with the exception of [Cp*(η 8 -C 8 H 8 )U(R 2 bipy)], are formed in the presence of a reducing agentsingle electron transfer reactivity of bipy is known for transition metals such as Cr(II), 13 but without the presence of a reducing agent neutral bipy species are commonly formed. In the [Cp*(η 8 -C 8 H 8 )U(R 2 bipy)] example it has been postulated that one of the COT dianions acts as the reducing agent.…”