C–Cl bond activation and catalytic hydrodechlorination of hexachlorobenzene by cobalt and nickel complexes with sodium formate as a reducing agent

Abstract

A benzyne cobalt complex, Co(η(2)-C6Cl4)(PMe3)3 (2), was generated from the reaction of hexachlorobenzene with 2 equiv. of Co(PMe3)4 through selective activation of two C-Cl bonds of hexachlorobenzene. Meanwhile, the byproduct CoCl2(PMe3)3 was also confirmed by IR spectra. The cobalt(II) complex, CoCl(C6Cl5)(PMe3)3 (1), as an intermediate in the formation of aryne complex 2, was also isolated by the reaction of hexachlorobenzene with the stoichiometric amount of Co(PMe3)4. Complex 2 could be obtained by the reaction of 1 with Co(PMe3)4. Under similar reaction conditions, the reaction of Ni(PMe3)4 with hexachlorobenzene afforded only a mono-(C-Cl) bond activation nickel(II) complex, NiCl(C6H5)(PMe3)2 (5). The expected benzyne nickel complex was not formed. The structures of complexes 2 and 5 were determined by X-ray single crystal diffraction. Successful selective hydrodechlorinations of hexachlorobenzene were studied and in the presence of Co(PMe3)4 or Ni(PMe3)4 as catalysts and sodium formate as a reducing agent pentachlorobenzene and 1,2,4,5-tetrachlorobenzene were obtained. The catalytic hydrodechlorination mechanism is proposed and discussed.

Monoanionic bis(carbene) pincer complexes featuring cobalt(I–III) oxidation states

“…The two Co-C NHC bond lengths of 1.963(4) and 1.948(4) Å for ( DIPP CCC)CoClpy (2a) and 1.948 (3) and 1.930(3) for ( Mes CCC)CoClpy (2b) are very similar to those reported for [trans-(CNC)Co(κ 1 -CF 3 -SO 3 ) 2 ( py)], 1.942(6) and 1.941(6) Å, 16 and typical for Co II -C NHC bonds (1.845-2.127 Å). 20 In contrast, the Co-C Ar bond lengths of 1.871(3) Å and 1.872(3) Å for 2a and 2b, respectively, are noticeably shorter than typical Co II -C Ar bonds reported in the literature (1.9020-2.0570 Å) 20,30,31 and shorter than those observed in the [Co II (PCP Mei Pr)] complexes recently reported by Kirchner and coworkers (1.919 to 1.953 Å) 22 or the R POCOP R CoI complex reported by Heinekey (1.924(4) Å). 15 The Co-Cl bond lengths of 2.4465(9) Å for 2a and 2.4561 (10) Å for 2b also differ considerably from the bond lengths in the [Co II (PCP Mei Pr)] 22 and [Co(PCP-t Bu)Cl] 32 complexes (2.234(1)-2.3103(4) Å) and 2.260(1) Å, respectively.…”

“…Resonances located at 1.66, 2.00 and 2.01 ppm and integrating to 6H each, were assigned to the methyls of the mesityl moiety. Characterization by 31 P NMR spectroscopy revealed no resonances, precluding a more detailed analysis of the compound with this method and prompting our synthesis of a ( DIPP CCC) analogue. Addition of an equivalent of triphenylphosphine to ( DIPP CCC)CoN 2 , 3a, resulted in an immediate color change to deep red, affording ( DIPP CCC)CoN 2 (PPh 3 ) (3a-PPh 3 ) in excellent yields (89%).…”

Gas-phase synthesis and structure of thorium benzyne complexes

“…[1][2][3][4][5][6][7][8][9][10][11][18][19][20] In addition to these reactions, the [2+2+2]-cycloaddition with alkynes is also very common for late transition metal benzyne complexes. [12][13][14][15][16][17][21][22][23][24] In particular, good regiochemical control has been realized in these reactions. Since the first fascinating benzyne complex of transition metal (Z 2 -C 6 H 4 )Ta(Z 5 -C 5 Me 5 )Me 2 was discovered, 7 the syntheses, structures and reactivities of group 4 (Ti and Zr), 1-6 group 5 (V, Nb and Ta), [7][8][9][10][11] group 10 (Ni, Pd and Pt) [12][13][14][15][16][17] and other (Mo, Re, Ru, Co and Ge) [18][19][20][21][22][23][24][25][26] metal/metalloid benzyne complexes have been exhaustively investigated.…”

“…Transition metal complexes of benzyne (1,2-didehydrobenzene) and substituted benzynes are very important and highly reactive intermediates in organic synthetic chemistry. 1–24 Early transition metal benzyne complexes generally undergo insertion reactions with electrophiles such as alkenes, alkynes, aldehydes, ketones and nitriles. 1–11,18–20 In addition to these reactions, the [2+2+2]-cycloaddition with alkynes is also very common for late transition metal benzyne complexes.…”

“…12–17,21–24 In particular, good regiochemical control has been realized in these reactions. Since the first fascinating benzyne complex of transition metal (η 2 -C 6 H 4 )Ta(η 5 -C 5 Me 5 )Me 2 was discovered, 7 the syntheses, structures and reactivities of group 4 (Ti and Zr), 1–6 group 5 (V, Nb and Ta), 7–11 group 10 (Ni, Pd and Pt) 12–17 and other (Mo, Re, Ru, Co and Ge) 18–26 metal/metalloid benzyne complexes have been exhaustively investigated.…”