N-Oxide Coordination to Mn(III) Chloride

Abstract

We report on the synthesis and characterization of Mn(III) chloride (Mn^IIICl₃) complexes coordinated with N-oxide ylide ligands, namely trimethyl-N-oxide (Me₃NO) and pyridine-N-oxide (PyNO). The compounds are reactive and, while isolable in the solid-state at room temperature, readily decompose into Mn(II). For example, "[Mn^IIICl₃(ONMe₃)_n]" decomposes into the 2D polymeric network compound complex salt [Mn^II(µ-Cl)₃Mn^II(µ-ONMe₃)]_n[Mn^II(µ-Cl)₃]_n·(Me₃NO·HCl)_3n (4). The reaction of Mn^IIICl₃ with PyNO forms varied Mn(III) compounds with PyNO coordination and these react with hexamethylbenzene (HMB) to form the chlorinated organic product 1-cloromethyl-2,3,4,5,6-pentamethylbenzene (8). In contrast to N-oxide coordination to Mn(III), the reaction between [Mn^IIICl₃(OPPh₃)₂] and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) resulted in electron transfer-forming d⁵ manganate of the [TEMPO] cation instead of TEMPO-Mn(III) adducts. The reactivity affected by N-oxide coordination is discussed through comparisons with other L-Mn^IIICl₃ complexes within the context of reduction potential.

Keywords: C–H chlorination; Mn(III); N-oxide ligands; coordination chemistry.

Scheme 1

Synthesis of Mn(III) chloride complexes.

Scheme 2

Synthesis of 3a, 3b, and 4.

Figure 1

MicroED structure of [Mn^II(µ-Cl)₃Mn^II(µ-ONMe₃)]_n[Mn^II(µ-Cl)₃]_n·(Me₃NO·HCl)_3n (4). The grey box contains the unit cell and is viewed down the b-axis (left) and c-axis (right). Selected bond lengths (Å) for the cationic chain: Mn1–Cl1 = 2.56(3); Mn1–O1 = 2.19(3); N1–O1 = 1.397(19); Mn1(µ-Cl)···Mn1(µ-O)···Mn1 = 3.27, 3.11. Selected bond lengths (Å) for the anionic chain: Mn2–Cl2 = 2.54(3); Mn2···Mn2 = 3.19. Color scheme: cyan polyhedra = [Mn^IICl₃]⁻; green polyhedra = [Mn^II₂Cl₃(ONMe₃)]⁺; green sphere = Cl; magenta spheres = Mn; blue spheres = N; red spheres = O; grey spheres = C.

Figure 2

(Left) FTIR spectra of 3a (red) and 4 (black). (Right) Molecular structure of 3b with the outer sphere Cl counter anion shown (hydrogen atoms and MeCN are omitted for clarity). Selected bond lengths (Å) and angles (deg.) for 3a: Mn1–Cl1 = 2.3225(6); Mn1–O1 = 1.901(2); Mn1–O2 = 1.896(2); Mn1–O3 = 1.991(3); Cl1–Mn1–Cl1 = 145.87(4); O1–Mn1–O2 = 177.33(11).

Scheme 3

Synthesis of 5, 6, and 7. Small changes in reaction conditions give different products.

Figure 3

Molecular structures of Mn(III) centers in (a) 5, (b) 6·7, and (c) 7 (full crystal structures are presented in the SI). Selected bond lengths (Å) and angles (deg.) for 5: Mn1–Cl1 = 2.5535(4); Mn1–O1 = 1.9389(12); Mn1–O2 = 1.9293(11); Mn1–O3 = 1.9301(12); Mn1–O4 = 1.9255(11); Mn1–O5 = 2.2340(13); O1–Mn1–Cl1 = 93.54(4); O1–Mn1–O4 = 92.03(5); O4–Mn1–Cl1 = 89.60(4). Selected bond lengths (Å) and angles (deg.) for 6·7: Mn1–Cl1 = 2.2804(6); Mn1–Cl2 = 2.3687(6); Mn1–Cl3 = 2.2790(6); Mn1–O1 = 1.9218(15); Mn1–O2 = 1.9194(16); Mn2–Cl4 = 2.5172(6); Mn2–Cl5 = 2.5390(6); Mn2–O3 = 1.9397(15); Mn2–O4 = 1.9309(15); Mn2–O5 = 1.9493(15); Mn2–O6 = 1.9465(15); Cl1–Mn1–Cl2 = 116.23(2); Cl2–Mn1–Cl3 = 105.08(2); Cl1–Mn1–Cl3 = 138.69(3); O1–Mn1–Cl1 = 90.58(5); O2–Mn1–Cl1 = 84.49(5); O3–Mn2–Cl4 = 91.04(5); O3–Mn2–O4 = 89.02(6); O4–Mn2–Cl4 = 91.28(5). Selected bond lengths (Å) and angles (deg.) for 7: Mn1–Cl1 = 2.3172(8); Mn1–Cl2 = 2.3091(8); Mn1–Cl3 = 2.2961(8), Mn1–O1 = 1.912(2); Mn1–O2 = 1.916(2); Cl1–Mn1–Cl2 = 109.02(3); O1–Mn1–O2 = 168.97(9).

Scheme 4

Conditions for C–H chlorination reactivity of HMB with Mn(III) chloride compounds.

Scheme 5

Reaction of 1 with TEMPO.

Figure 4

Molecular structure (ellipsoids 50%) of 9 determined with XRD (H atoms and one part of disorder omitted, only one of the three identical subunits in the unit cell shown for clarity) Selected bond lengths (Å) and angles (deg.) for 9: Mn1–Cl1 = 2.3737(5); Mn1–Cl2A = 2.3549(18); Mn1–Cl3A = 2.3728(10); Mn1–Cl4A = 2.4025(15); N1–O1 = 1.1922(17); N6–O6 = 1.191(2); Cl1–Mn1–Cl2A = 108.62(6); Cl3A–Mn1–Cl4A = 103.78(6).