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N−N Bond Cleavage of Organohydrazines by Molybdenum(II) and Tungsten(II) Complexes Containing a Linear Tetraphosphine Ligand. Formation of Nitrido or Imido Complexes and Their Reactivities

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journal contribution
posted on 03.04.2020, 16:25 by Daisuke Watanabe, Sumie Gondo, Hidetake Seino, Yasushi Mizobe
The Mo(II) tetraphosphine−dihalide complexes [MoX24-P4)] (X = Cl (2a), Br (2b); P4 = meso-o-C6H4(PPhCH2CH2PPh2)2) having uncommon trigonal-prismatic geometry cleaved the N−N bond in Me2NNH2 and PhNHNH2 at room temperature to form the nitrido complexes trans-[MoX(N)(κ4-P4)] (X = Cl (3a), Br (3b)). In contrast, similar reactions of the W(II) congener [WBr24-P4)] afforded the imido complex trans-[WBr(NH)(κ4-P4)]+ (4c+), which was isolated as a PF6 or BPh4 salt. Addition of HBF4 converted 3 into the imido complexes trans-[MoX(NH)(κ4-P4)][BF4] (X = Cl (4a[BF4]), Br (4b[BF4])), while deprotonation of 4a+ and 4b+ by NEt3 readily regenerated 3a and 3b. At elevated temperatures, 2a also reacted with RNHNHR (R = Me, Ph), giving the corresponding organoimido complexes trans-[MoCl(NR)(κ4-P4)][PF6] (6[PF6]) after anion metathesis with K[PF6]. The mechanism has been proposed for the N−N bond cleavage, which involves the proton shift in the end-on coordinated organohydrazine followed by the two-electron reduction by the M(II) center to give the M(IV) imido complex and the amine. The nitrido ligand in 3a is susceptible to electrophilic attack by methyl trifluoromethanesulfonate to form 6[CF3SO3] (R = Me), while reactions of 3a with acid chlorides RCOCl (R = Me, Ph, p-tolyl (Tol)) afforded the neutral acylimido complexes cis,mer-[MoCl2(NCOR)(κ3-P4)] (7). Treatment of 7 with K[PF6] yielded the cationic complexes trans-[MoCl(NCOR)(κ4-P4)][PF6] (8[PF6]). On the other hand, imido complex 4a[BF4] smoothly reacted with isocyanates RNCO (R = Ph, Tol) in the presence of a catalytic amount of Et3N to give the carbamoylimido complexes trans-[MoCl(NCONHR)(κ4-P4)][BF4] (9[BF4]), and the thiocarbamoyl analogue trans-[MoCl(NCSNHPh)(κ4-P4)][PF6] (10[PF6]) was also obtained from 4a[PF6] and PhNCS. Although hydrolysis of 7 (R = Tol) in a THF/aqueous KOH mixture resulted in the formation of p-toluamide only in low yield, reduction of the same complex with NaBH4 and LiAlH4 generated p-toluamide and 4-methylbenzylamine, respectively, in moderate yields. Molecular structures of 3a, 4a[BF4], 4c[BPh4], 6[PF6] (R = Me), 6[BF4] (R = Ph), 7 (R = Me), 8[PF6] (R = Me), and 9[BF4] (R = Tol) were determined crystallographically.

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