Bis(4-benzhydryl-benzoxazol-2-yl)methane - from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes

Abstract

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (^4-BzhH2 BoxCH₂ ) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(^4-BzhH2 BoxCH)] (M=Na (7), K (8₁ )) and dimeric [{M(^4-BzhH2 BoxCH)}₂ ] (M=K (8₂ ), Rb (9), Cs (10)) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)₂ K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11), including the energetically favoured monoanionic (E,E)-(^4-BzhH2 BoxCH) ligand. Further reaction of ^4-BzhH2 BoxCH₂ with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)₂ K@(18-crown-6)}{K@(18-crown-6)K(^4-Bzh BoxCH)}]_n (n→∞) (12) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.

Keywords: Ligand design; Metalation; Methanides; NacNac; Potassium; s-Block chemistry.

Scheme 1

Symmetrical bis(heterocyclo)amine (A), ‐phosphane (B), and ‐methane (C+D) ligands. Bis(benzoxazol‐2‐yl)methanes C (I, II) are (un)substituted in ortho‐position to imine group whereas D is ⁱPr (III) or ^tBu (IV) substituted in ortho‐ and para‐position. Asymmetrical and unsubstituted bis(heterocylo)methanes (E).

Figure 1

Molecular structure of bis(4‐benzhydryl‐benzoxazol‐2‐yl)methane (6, ^4‐BzhH2BoxCH₂). Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity except for the bridging (methylene) and benzylic ones. Selected bond lengths [Å] and bond angles [°]: N1−C7 1.289(2), N2−C9 1.291(2), C7−C8 1.491(2), C8−C9 1.497(2), C7−C8−C9 115.71(14), N1/2−C7−C8−C9=103.5(2)°/−26.38(9)°; O1/2−C7−C8−C9=31.47(10)°/−71.05(19)°.

Scheme 2

Synthesis of monomeric complexes 7 to 10 were carried out in toluene and ambient temperature under the following conditions (yield=YLD): a) 1.04 eq. Na, 5 d, YLD: 98 %, b) 1.15 eq. KH, 1 d, YLD: 89 %, c) 1.10 eq. Rb, 1 d, YLD: 76 %, d) 1.10 eq. Cs, 6 h, YLD: 74 %.

Figure 2

Molecular structure of {bis(4‐benzhydryl‐benzoxazol‐2‐yl)methanide} sodium (7, [Na(^4‐BzhH2BoxCH)]). Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity except for the bridging (methylene) and benzylic ones.

Figure 3

Molecular structure of monomeric {bis(4‐benzhydryl‐benzoxazol‐2‐yl)methanide} potassium (8₁ , [K(^4‐BzhH2BoxCH)]). Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity except for the bridging (methylene) and benzylic ones.

Figure 4

Molecular structure of dimeric {bis(4‐benzhydryl‐benzoxazol‐2‐yl)methanide} alkali metal complexes ([{M(^4‐BzhH2BoxCH)}₂], M=K (8₂ ), Rb (9), Cs (10)). Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity. Superposition of 8₂ ‐10 showing NacNac‐like C₃N₂ unit of (E,E)‐ (^4‐BzhH2BoxCH) (A) as well as twisted (E,E)‐(^4‐BzhH2BoxCH) (B) and their coordinating alkali metal ion, respectively.

Scheme 3

Reaction of monomeric complexes 8 to solvent separated ion [(THF)₂K(18‐crown‐6)]⁺[^4‐BzhH2BoxCH]⁻ (11) via addition of one equivalent 18‐crown‐6. In solution permanent rearrangement of the (Z,Z)‐, (Z,E/E,Z)‐, and (E,E)‐(^4‐BzhH2BoxCH) isomers is ubiquitous.

Figure 5

Molecular structure of {bis(4‐benzhydryl‐benzoxazol‐2‐yl)methanide} anion from 11, [(THF)₂K(18‐crown‐6)]⁺[^4‐BzhH2BoxCH]⁻. Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity except for the bridging (methylene) and benzylic ones. Selected bond lengths [Å] and bond angles [°]: N1−C7 1.329(2), N2−C9 1.327(2), C7−C8 1.390(2), C8−C9 1.389(2), C9−C8−C7 131.34(15).

Scheme 4

Optimised synthesis of polymeric [{(THF)₂K(18‐crown‐6)}{K(18‐crown‐6)K(^4‐BzhBoxCH)}]_n (n→∞) (12) by addition of 3.33 equivalents of KH and two equivalents of 18‐crown‐6.

Figure 6

Molecular structure of [{(THF)₂K@(18‐crown‐6)}{K@(18‐crown‐6) K‐(^4‐BzhBoxCH)}]_n (n→∞) (12). Anisotropic displacement parameters are depicted at the 50 % probability level. All hydrogen atoms are omitted for clarity except for the bridging (methylene) one. K1−N1 2.686(3), K1−C16 2.930(3), K1−C17 3.084(3), K1−C18 3.255(3), K1−C19 3.291(3), K1−C20 3.154(3), K1−C21 2.993(3), N1−C7 1.324(4), C7−C8 1.393(4), C2−C9 1.478(4), C9−C10 1.459(5), C9−C16 1.426(4), C4−K2 3.329(3) Å, C5−K2 3.245(8) Å, N1−K1−N1A 72.78(11), C1−C2−C3 114.2(3), C6−C5−C4 115.1(4), C15−C10−C11 115.7(3), C17−C16−C21 114.4(3).