Synthesis and Characterization of Cobalt NCN Pincer Complexes

Abstract

A series of cobalt complexes, stabilized by a monoanionic tridentate NCN pincer ligand, was synthetized and characterized. Preparation of the paramagnetic 15 VE complex [Co(NCN^CH2-Et)Br] (1) was accomplished by transmetalation of Li[2,6-(Et₂NCH₂)₂C₆H₃] with CoBr₂ in THF. Treatment of this air-sensitive compound with NO gas resulted in the formation of the diamagnetic Co(III) species [Co(NCN^CH2-Et)(NO)Br] (2) as confirmed by X-ray diffraction. This complex features a strongly bent NO ligand (Co-N-O∠135.0°). The ν_NO is observed at 1609 cm^-1 which is typical for a bent metal-N-O arrangement. Coordinatively unsaturated 1 could further be treated with pyridine, isocyanides, phosphines and CO to form five-coordinate 17 VE complexes. Oxidation of 1 with CuBr₂ led to the formation of the Co(III) complex [Co(NCN^CH2-Et)Br₂]. Treatment of [Co(NCN^CH2-Et)Br₂] with TlBF₄ as halide scavenger in acetonitrile led to the formation of the cationic octahedral complex [Co(NCN^CH2-Et)(MeCN)₃](BF₄)₂. A combination of X-ray crystallography, IR-, NMR- and EPR-spectroscopy as well as DFT/CAS-SCF calculations were used to characterize all compounds.

Keywords: Cobalt; DFT studies; EPR studies; Nitrosyl ligand; Pincer complexes.

Scheme 1

Literature known NCN cobalt pincer complexes.

Scheme 2

Synthesis of complex 1 via transmetalation.

Figure 1

Structural view of [Co(NCN^CH2−Et)Br] (1) showing 50 % displacement ellipsoids (H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: C1−Co1 1.850(3), Co1−Br1 2.438(3), Co1−N1 2.033(3), Co1−N2 2.044(3), N1−Co1−N2 167.76(9), C1−Co1−Br1 179.35(8), C1−Co1−N1 83.9(1).

Figure 2

EPR spectra of complexes 1 (top) and 3 a (bottom) in toluene glass at 100 K microwave frequency of 9.43 GHz and microwave power 15.9 mW. Associated simulations are depicted in red color.

Figure 3

BP86/def2‐TZVP computed frontier orbitals (d‐splitting) for [Co(NCN^CH2−Et)Br] (1).

Scheme 3

Synthesis of the nitrosyl complex [Co(NCN^CH2−Et)(NO)Br] (2).

Figure 4

Structural view of [Co(NCN^CH2−Et)(NO)Br] (2) showing 50 % displacement ellipsoids (H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1−C1 1.883(2), Co1−Br1 2.4915(4), Co1−N1 2.0753(18), Co1−N2 2.0641(19), Co1−N3 1.7400(17), N3−O1 1.178(2), C1−Co1−Br1 162.84(6), N1−Co1−N2 143.71(7), C1−Co1−N3 93.72(9), Co1−N3−O1 135.0(2).

Scheme 4

Synthesis of 17 VE complexes 3 a–d.

Scheme 5

Synthesis of the Co(III) species 4 and 5.