The synthesis, structural characterisation and coordination behaviour of mono- and ditopic p-hydroquinone-based bis(pyrazol-1-yl)methane ligands is described (i.e., 2-(pz2CH)C6H3(OH)2 (2a), 2-(pz2CH)-6-(tBu)C6H2(OH)2 (2b), 2-(pz2CH)-6-(tBu)C6H2(OSiiPr3)(OH) (2c), 2,5-(pz2CH)2C6H2(OH)2 (4)). Ligands 2a, 2b and 4 can be oxidised to their p-benzoquinone state on a preparative scale (2a ox, 2b ox, 4 ox). An octahedral Ni II complex [trans-Ni(2c)2] and square-planar Pd II complexes [Pd2bCl2] and [Pd2b ox Cl2] have been prepared. In the two Pd II species, the ligands are coordinated only through their pyrazolyl rings. The fact that [Pd2bC12] and [Pd2b oxC12] are isolable compounds proves that redox transitions involving the p-quinone substituent are fully reversible. In [Pd2b oxCl2], the methine proton is highly acidic and can be abstracted with bases as weak as NEt(3). The resulting anion dimerises to give a dinuclear macrocyclic Pd II complex, which has been structurally characterised. The methylated ligand 2-(pz2CMe)C6H3O2 (11 ox) and its Pd II complex [Pd11 oxCl2] are base-stable. A new class of redox-active ligands is now available with the potential for applications both in catalysis and in materials science.

Five pairs of perfluorophenyl-and phenyl-substituted tris(pyrazol-1-yl)borate complexes have been prepared and thoroughly investigated, namely, [(C 6 R′ 5 Bpz 3 ) 2 Fe], [(C 6 R′ 5 Bpz 3 ) 2 Cu], [(C 6 R′ 5 Bpz 3 )Mn(CO) 3 ], K[(C 6 R′ 5 Bpz 3 )Mo(CO) 3 ], and [(C 6 R′ 5 Bpz 3 )Mo(CO) 2 (2-methallyl)] (R′ = F, H; pz = pyrazol-1yl). X-ray crystal structure analyses and IR spectroscopical measurements reveal only a minor influence of the spectator substit- [a]

9-Ferrocenyl-9-borafluorene (FcBC12H8; 3) is readily accessible from FcHgCl and BrBC12H8. The 9-borafluorenyl substituent is bent out of the plane of the cyclopentadienyl ring toward the iron center with corresponding dip angles of α* = 25.5° and 17.1° for the two crystallographically independent molecules in the asymmetric unit of 3. The degree of ligand bending is one of the highest ever measured for a ferrocenylborane derivative, thereby indicating a pronounced Fe−B interaction. In line with that, oxidation of the Fe center to its FeIII state using AgBF4 results in a substantial decrease of the dip angle in the ferricinium species [3]BF4 (α* = 6.3°). Compound 3 can therefore be considered a rare example of a redox-switchable main group Lewis acid. The stability of the molecular framework of 3 is sufficiently high to allow for the isolation and structural characterization of its pyridine-N-oxide adduct (4). Moreover, reaction of 3 with 0.5 equiv of 1,1′-dilithioferrocene (1,1′-fcLi2 × 2/3 tmeda) and subsequent limited access of O2 leads to the formation of the trinuclear mixed-valence complex Li[Fc-(BC12H8)-fc-(BC12H8)-Fc], which has been structurally characterized as 12-crown-4 adduct (Li(12-c-4)2[6]).

TMEDA-free (TMEDA: tetramethylethylenediamine) LiCH(2)SMe is a suitable reagent for the selective introduction of (methylthio)methyl groups into PhBBr(2) and its p-silylated derivative Me(3)Si--C(6)H(4)--BBr(2). The resulting compounds, R*--C(6)H(4)--B(Br)(CH(2)SMe) (R*=H: 2; R*=SiMe(3): 7) and PhB(CH(2)SMe)(2) (3), form cyclic dimers through B--S adduct bonds in solution and in the solid state. Compounds 2 and 3 have successfully been used for preparing the (N(2)S) scorpionate [PhBpz(2)(CH(2)SMe)](-) ([5](-)) (pz: pyrazol-1-yl) and the (NS(2)) scorpionate [PhBpz(CH(2)SMe)(2)](-), respectively. Compound 7 proved to be an excellent building block for the heteroditopic poly(pyrazol-1-yl)borate p-[pz(3)B--C(6)H(4)--Bpz(2)(CH(2)SMe)](2-) ([10](2-)) that mimics the two ligation sites of the copper enzymes peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. Treatment of the monotopic tripod [5](-) with CuCl and CuBr(2) results in the formation of complexes K[Cu(5)(2)] and [Cu(5)(2)]. An X-ray crystallography study of K[Cu(5)(2)] revealed a tetrahedral (N(2)S(2)) coordination environment for the Cu(I) ion, whereas the Cu(II) ion of [Cu(5)(2)] possesses a square-pyramidal (N(4)S) ligand sphere (S-atom in the axial position). The remarkable redox properties of K[Cu(5)(2)] and [Cu(5)(2)] have been assessed by cyclic voltammetry and quantum chemical calculations. The reaction of K[Cu(5)(2)] with dry air leads to the Cu(II) species [Cu(5)(2)] and to a tetranuclear Cu(II) complex featuring [PhB(O)pz(2)](2-) ligands. Addition of CuCl to K(2)[10] gives the complex K(3)[Cu(10)(2)] containing two ligand molecules per Cu(I) center. The Cu(I) ion binds to both heteroscorpionate moieties and thereby establishes a coordination environment similar to that of the Cu(I) ion in K[Cu(5)(2)].

The reaction of (C 6 F 5 ) 2 B(OEt) with t Bu 2 P(CH 2 Li) yields the cyclic phosphonium ethoxyborateadduct formation and ortho-fluoride substitution. Treatment of [1]OEt with HCl in Et 2 O gives the chloroborate [1]Cl in almost quantitative yield. The reaction of [1]Cl with Li[AlH 4 ] leads not only to the reduction of the B-Cl bond but also to the selective substitution of one fluorine atom on the tetrafluorophenylene bridge ([4]H). [1]Cl is readily transformed into [1]F, [1]OAc, and [1]OTf upon reaction with Tl[PF 6 ], AgOAc, and AgOTf, respectively (OAc=acetate; OTf=triflate). [1]OTf represents a convenient storage form of [1] þ , because its B-O bond is highly labile. The free Lewis acid [1] þ was prepared in the form of its aluminate salt [1] þ [Al(O t Bu F ) 4 ] -. The phenyl derivative of [1]OEt, (C 6 H 5 )(OEt)B(CH 2 )(C 6 F 4 )P t Bu 2 ([5]OEt), is also accessible and serves as starting material for the preparation of [5]Cl and [5] þ [Al(O t Bu F ) 4 ] -.