New Structural Motifs of Lithium N-Heterocyclic Carbene Complexes with Bis(3-tert-butylimidazol-2-ylidene)dialkylborate Ligands

Abstract

Employment of bidentate, anionic bis(3-tert-butylimidazol-2-ylidene)dialkylborate N-heterocyclic ligands [BIMtBuBR2]− (R = Ph or Me) led to the isolation of two unprecedented lithium carbene complexes: monomeric chelating [(BIMtBuBPh2)Li(OEt2)] (4a) and nonchelating, dimeric μ2-κC 2:κC 2′ -[(BIMtBuBMe2)2Li2] (4b). Both have been characterized by spectroscopic methods and by single-crystal X-ray diffraction.

Boron‐Centered Scorpionate‐Type NHC‐Based Ligands and Their Metal Complexes

“…Neutral heteroleptic bis(NHC)borates and phosphane complexes of Pd II and Pt II of the general formula [H 2 B(Im R ) 2 ]MI(PEt 3 ) (complexes 12 – 17 ) resulted from treatment of [H 2 B(Im R ) 2 ]Li (R = Me, Et, or i Pr) with [M(µ‐Cl)Cl(PEt 3 )] 2 (M = Pd or Pt) in a 2:1 ratio (Figure ) . This class of ligands was later used to coordinate to several metals such as Ag I , Ca II , Sr II , Ni II , Fe II , Ir I , and Rh I …”

“…For the synthesis of bulky bis(NHC)borate ligands, characterized by the presence of bulkier wingtip groups such as mesityl and tert ‐butyl, a new synthetic route was developed. Treatment of dialkyl‐ and diarylboron halides (R 2 BX: R = Ph or Me; X = Br or Cl) with N ‐ tert ‐butylimidazole smoothly afforded the corresponding imidazolium salts [Ph 2 B(HIm t Bu ) 2 ]Cl (L 4 Cl) and [Me 2 B(HIm t Bu ) 2 ]Br (L 5 Br) . The ligand precursor salts [H 2 B(HIm Mes ) 2 ]X (L 6 X, X = I or Br) and [H 2 B(HIm t Bu ) 2 ]X (L 7 X, X = I or Br) were synthesized by treatment of tert ‐butylimidazole or mesitylimidazole with Me 3 N·BH 2 X .…”

“…Interestingly, L 4 Cl was unreactive toward n BuLi, and the carbene 18 could be successfully generated from L 4 Cl only with t BuLi. On the other hand, deprotonation of L 5 Br with n BuLi proceeded easily, to yield 19 without incorporation of LiBr (Figure ) …”

Copper(I) Complexes of Anionic Tridentate CNC Pincer Ligands

“…This is not surprising as N‐heterocyclic carbenes are expected to be better ligands for Cu I than for Li(I). Structurally characterized bis(NHC) lithium adducts like [5a]Li are not common , . The [H 2 C(NHC Dipp ) 2 ]LiN(SiMe 3 ) 2 is perhaps the closest analog in the literature for a comparison, which has longer Li–C (2.253 Å) and similar Li–N (1.948 Å) distances to that of [5a]Li .…”

“…Structurally characterized bis(NHC) lithium adducts like [5a]Li are not common. [24,27] The [H 2 C(NHC Dipp ) 2 ]LiN(SiMe 3 ) 2 is perhaps the closest analog in the literature for a comparison, which has longer Li-C (2.253 Å) and similar Li-N (1.948 Å) distances to that of [5a]Li. However, the donor CNC sites of this molecule is not in a pincer arrangement.…”