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

Abstract

The Mo(II) tetraphosphine−dihalide complexes [MoX2(κ4-P 4)] (X = Cl (2a), Br (2b); P 4 = 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-P 4)] (X = Cl (3a), Br (3b)). In contrast, similar reactions of the W(II) congener [WBr2(κ4-P 4)] afforded the imido complex trans-[WBr(NH)(κ4-P 4)]+ (4c + ), which was isolated as a PF6 or BPh4 salt. Addition of HBF4 converted 3 into the imido complexes trans-[MoX(NH)(κ4-P 4)][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-P 4)][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-P 4)] (7). Treatment of 7 with K[PF6] yielded the cationic complexes trans-[MoCl(NCOR)(κ4-P 4)][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-P 4)][BF4] (9[BF4]), and the thiocarbamoyl analogue trans-[MoCl(NCSNHPh)(κ4-P 4)][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.

Reactions of Mo(II)-tetraphosphine complex [MoCl2{meso-o-C6H4(PPhCH2CH2PPh2)2}] with a series of small molecules

“…The 31 P{ 1 H} NMR spectrum of a C 6 D 6 solution shows four signals including one high-field doublet at d À10.4, which is characteristic of the free P atom of P4. This spectral feature is in good agreement with that of the crystallographically characterized 3a [5], suggesting that 4a also has the octahedral geometry with the j 3 -P4 ligand as observed for 3a. By the donation of 4 electrons from the side-on bound alkyne, 18-electron count of the Mo center may be satisfied by this six-coordinate, octahedral geometry.…”

“…In the 31 P{ 1 H} NMR spectra of the CDCl 3 and C 6 D 6 solutions, four signals with same intensities were observed, which is in agreement with the solid state structure containing four inequivalent P atoms. Based on the previous findings [1][2][3][4][5], two signals in the lower field are assignable unambiguously to the inner P atoms P 2 and P 3 , while those in the higher field to the outer P atoms P 1 and P 4 . However, further assignments of either P 2 or P 3 and either P 1 or P 4 were done tentatively as shown in Section 3, where the P atoms P 2 and P 1 having the longer Mo-P bonds in each pair were assigned to the resonances observed in the higher field.…”

“…Interestingly, upon dissolution into CDCl 3 , 4a is readily transformed into a cationic mono(alkyne) complex [MoCl(PhC"CH)(j 4 -P4)]Cl (4b), as indicated by the 31 P{ 1 H} NMR spectrum exhibiting only two signals comparable to that of the X-ray analyzed 3b [5]. The alkynic proton appears in the 1 H NMR spectrum at d 12.4 as the triplet of triplets.…”

“…Quite recently, it has also been demonstrated that 2 facilitates the N-N bond cleavage of organohydrazines to give nitrido and imido complexes [MoCl(N)(j 4 6 ] (R = Me, Ph). From the former nitrido complex, acylimido complexes of two types [MoCl 2 (NCOMe)(j 3 -P4)] (3a) and [MoCl(NCOMe)(j 4 -P4)][PF 6 ] (3b) have been derivatized by treatment with MeCOCl and fully characterized by Xray diffraction [5]. In this paper, we wish to describe the details of the reactions of 2 with other selected small molecules, forming a series of new P4 complexes …”

Facile Synthesis of Hyper‐Branched Tetraphosphanes and Tetraphosphane Chalcogenides

“…[7c,9] Also, such dinuclear organometallic complexes attract special attention due to their possible applications as building blocks for new types of catalysts and materials with magnetic, nonlinear optical (NLO), or liquid crystalline properties. [10] Tetrapodal phosphanes, including chiral ones, [9a,9b] act as ligands in Rh-catalyzed asymmetric hydrogenation of arylenamides, [11] highly regioselective isomerization of olefins, [12] alkylation [13] and amination, [14] Mo-or W-catalyzed N-N bond cleavage of organohydrazines, [15] and Heck, [16] Negishi, [17] Sonogashira, [18] and Suzuki [19] couplings and for other important syntheses. [20] Particularly promising are the branched tetraphosphanes with longer and flexible spacers containing other heteroatoms between the phosphane moieties and bulky hydrophobic substituents that make them more adjustable and hemilabile ligands in the metallocomplex formation as well as convenient scaffolds for supramolecular nanostructured materials.…”