Higher‐Valent Derivatives of the d‐Metal Acids, 13. — Homoscorpionates as Tripodal Anchoring Ligands of Chloro Functionalized Oxo and Imido Complexes of Elements of Group 5–7 Hydridotris(pyrazolyl)borate complexes of elements of group 5–7 have been prepared by reaction of d0‐metalloyl chlorides [M(Y)nClm]x (Y = O, NR) with KTp* (Tp* = κ3‐HB(3,5‐Me2pz)3). Thus [NbOCl3]x and [TaOCl3]x are converted into monomeric and well‐characterized oxo complexes [Tp* Nb(O)Cl2] (1) and [Tp*Ta(O)Cl2] (2). In a similar way [Tp* Mo(O)2Cl] (3) and [Tp*W(O)2Cl] (4) have been obtained from the oxo chlorides [MoO2Cl2]x and [WO2Cl2]x, respectively. tert‐Butylamine reacts with NbCl5 and TaCl5 in the presence of pyridine to give the imido pyridine complexes [Nb(NtBu)Cl3(py)2] (5) and [Ta(NtBu)Cl3(py)2] (6) in excellent yield. Imido complexes [Tp*V(NtBu)Cl2] (7), [Tp* Nb(NtBu)Cl2] (8), and [Tp*Ta(NtBu)Cl2] (9) are formed by the reaction of KTp* with the precursers [V(NtBu)Cl3]x or 5 and 6, respectively. The corresponding imido complexes of group 6, namely [Tp*Cr(NtBu)2Cl] (10), [Tp*Mo(NtBu)2Cl] (11), and [Tp*W(NR)2Cl] (R = tBu 12a, 2,4,6‐Mesityl 12b, SO2Ph 12c) are obtained directly from the imidoyl chlorides or their complexes with Lewis bases. 11 and 12 are prone to protonation at the imido ligand to yield cationic amido species [Tp*M(NtBu)(NHtBu)Cl]BF4 (M = Mo 13, W 14). Finally the preparation of cationic imido rhenium complex [Tp*Re(NtBu)2Cl]PF6 (15), isoelectronic with 10–12, is described. The molecular structure of 11 has been determined by X‐ray diffraction. Despite of a maximum electron count of 20 valence electrons, all three pyrazolyl groups of the tripodal ligand are facially coordinated. In a way similar to the well‐known ring slippage of analogous cyclopentadienyl complexes, the weakest bound pyrazolyl ligand is asymmetrically attached to molybdenum deviating from the positions expected for local C3v symmetry.

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The synthesis of new tris(pyrazolyl)borato-substituted alkenyltungsten complexes and their oxyfunctionalization was investigated. The derivatives 2a−e of the title compounds were prepared in 40−70% yield by Grignard reaction of [Tp*W(O)2Cl] (1) [Tp* = hydridotris(3,5-dimethyl-1-pyrazolyl)borato] and subsequent treatment with molecular oxygen. These alkenyltungsten complexes, except the homoallyl complex 2b with a monosubstituted double bond, displayed a high reactivity toward singlet oxygen (1O2) to result in the corresponding allylic hydroperoxides by the Schenk ene reaction. While in the photooxygenation of the homoallyl complex 2a no special influence of the [Tp*W(O)2] fragment was observed, the allyl complexes 2c,d afforded stereoselectively the Z-configured products. Stereocontrol derives presumably from hyperconjugative stabilization by the W−C bond (β effect) of the perepoxide-like transition state. The allylic hydroperoxide, derived from the 1O2 ene reaction of the vinyl complex 2e, led by Hock-type cleavage to methacrolein and the tungstic acid derivative 9e. Upon photooxygenation in the presence of titanium tetraisopropoxide, only the homoallylic complex 2a underwent smoothly hydroxy-epoxidation. The other alkenyl complexes were sterically too hindered due to the bulky Tp* ligand. While the homoallyl and methallyl complexes 2a,d were quantitatively epoxidized by dimethyldioxirane (DMD), the vinyl complex 2e was again too sterically hindered because of the large [Tp*W(O)2] moiety, but its epoxide was obtained with m-chloroperbenzoic acid (mCPBA) as oxidant. In contrast, when equimolar amounts of methyl(trifluoromethyl)dioxirane (TFD) were used, the vinyl and methyl complexes 20e,f were regioselectively hydroxylated by C−H insertion into a pyrazolyl ring of the Tp* ligand. The present results demonstrate that the selective oxyfunctionalization of the alkenyltungsten complexes 2 by singlet oxygen and dioxiranes is controlled by electronic and steric factors of the [Tp*W(O)2] fragment.