Michael A. Leech

Self-assembled polymetallic square grids ([2 × 2] M4, [3 × 3] M9) and trigonal bipyramidal clusters (M5)—structural and magnetic properties

New self-assembled grids and clusters are reported, with square [2 × 2] M4 (M = Mn(II)4, Cu(II)4), trigonal-bipyramidal Mn(II)5, and square [3 × 3] M9 (M = Mn(II), Cu(II)) examples. These are based on a series of ditopic and tritopic hydrazone ligands involving pyridine, pyrimidine and imidazole end groups. In all cases the metal centres are bridged by hydrazone oxygen atoms with large (>125°) bridge angles, leading to antiferromagnetic exchange for all the Mn systems (J = −2 to −5 cm−1), which results in S = 0 (Mn4), and S = 5/2 (Mn5, Mn9) ground states. The copper systems have a 90° alternation of the Jahn–Teller axes within the Cu4 and Cu8 grid rings (Cu9), which leads to magnetic orbital orthogonality, and dominant ferromagnetic coupling. For the Cu9 grid antiferromagnetic exchange between the ring and the central copper leads to a S = 7/2 ground state, while for the Cu4 grids S = 4/2 ground states are observed. The magnetic data have been treated using isotropic exchange models in the cases of the Cu4 and Cu9 grids, and the Mn5 clusters. However due to the enormity of a fully isotropic calculation a simplified model is used for the Mn9 grid, in which the outer Mn8 ring is treated as the equivalent of an isolated magnetic chain, with no coupling to the central metal ion.

New bi(tetrathiafulvalenyl) derivatives and their radical cations : synthetic and X-ray structural studies

A series of bi(tetrathiafulvalenyl) derivatives has been prepared from iodo-TTF precursors by Ullmann coupling (copper in refluxing N,N-dimethylformamide) or by reaction with copper(I) thiophene-2-carboxylate (CuTC) in 1-methylpyrrolidin-2-one at 20 °C. Solution electrochemical and UV-VIS spectroscopic studies suggest that there is no significant through-bond interaction between the two TTF units in these systems. The X-ray crystal structures are reported for 4,5,5′,5′′,4′′′,5′′′-hexakis(methylsulfanyl)-4′,4′′-bitetrathiafulvalene 9 and a semiconducting 1∶1 perchlorate salt of 4,5:4′′′,5′′′-bis(ethylenedithio)-5′,5′′-dimethyl-4′,4′′-bitetrathiafulvalene 8+··ClO4−. The torsion angle around the central bond is 89° in 9 and 77° in 8+··ClO4−. The crystal packing of 8+··ClO4− is characterised by puckered layers, parallel to the (001) plane, of cations contacting via their sulfur atoms; the anions occupy infinite channels, parallel to the z-axis and running through the cation motif.

Contrasting Nonclassical Silicon−Hydrogen Interactions in Niobium and Tantalum Half-Sandwich Complexes: Si−H···M Agostic versus M−H···Si−Cl Interligand Hypervalent Interactions

Reaction of [CpM(NAr)(PMe3)2] (M = Nb, Ta; Ar = 2,6-C6H3iPr2) with HSiClMe2 gives two remarkably different nonclassical Si···H···M products depending only on the identity of M; [CpTa(NAr)(H)(SiMe2Cl)(PMe3)] possesses an unusual electron-rich M−H···Si interligand hypervalent interaction while [CpNb{η3-N(Ar)SiMe2−H}Cl(PMe3)] is the first example of a β-agostic silylamine Si−H···M interaction showing a “stretched” Si−H bond.

A crystallographic, EPR and theoretical study of the Jahn–Teller distortion in [CuTp2](Tp−= tris{pyrazol-1-yl}hydridoborate)

Crystals of the title compound (1) contain two independent, centrosymmetric half-molecules per asymmetric unit. While both of these show Jahn-Teller elongated six-coordinate geometries, the lengths of the elongated Cu-N bonds in the two molecules differ by 0.117(2) A at 30 K. The structure of one of these molecules (molecule A) does not vary with temperature below 350 K. The other molecule (molecule B) shows Cu-N bond lengths that are temperature-dependent between 225 and 375 K, but do not vary further at lower temperature. This indicates a fluxional axis of Jahn-Teller elongation in this molecule at these higher temperatures. Consideration of the thermal parameters in these structures implies that the fluxionality in molecule B is frozen out near 150 K. This conclusion is supported by a Q-band powder EPR study. The d-d transition energies of molecules A and B have been calculated by several density function (DF) methods, including a time-dependent DF calculation. The crystallographic data have been reproduced using the vibronic coupling model of Burgi and Hitchman. This has shown that the different fluxionality regimes for molecules A and B are not a consequence of their different static molecular structures, but rather reflect their different local environments in the crystal.

Niobium η-cyclopentadienyl compounds with imido and amido ligands derived from tert-butylamine

The niobium η-cyclopentadienyl compounds with imido and amido ligands [Nb(η-C5H5)(NtBu)(NHtBu)Cl] 1,* [Nb(η-C5H5)(NtBu)(NHtBu)nBu] 2, [Nb(η-C5H5)(NtBu)(NHtBu)2] 3, [Nb(η-C5H5)(NtBu)(NHtBu)Me] 4, [Nb(η-C5H5)(η1-C5H5)(NtBu)(NHtBu)] 5, [Nb{(η-C5H4)CMe2(η1-C5H4)}(NtBu)(NHtBu)] 6, [Nb(η-C5H5){N(C6H3iPr2-2,6)}(NHtBu)Cl] 7,* [Nb(η-C5H5){N(C6H3iPr2-2,6)}(NHtBu)Me] 8, [Nb(η-C5H5)(NtBu)(NEt2)Cl] 9 and [Nb(η-C5H5)(NtBu)(NHtBu)(NEt2)] 10 have been prepared (* indicates the crystal structure has been determined). A correlation between the chemical shift of the NH proton and the value of Δδ measured between the α and β carbons of the tert-butyl groups of the amido ligands is discussed in relation to the degree of electron donation from the amido ligand to the niobium centre.

Surprising diversity of non-classical silicon–hydrogen interactions in half-sandwich complexes of Nb and Ta: M–H ⋯ Si–Cl interligand hypervalent interaction (IHI) versus stretched and unstretched β-Si–H⋯M agostic bonding

Reaction of the niobium diphosphine compound [NbCp(NAr)(PMe3)2] (Ar = 2,6-C6H3Pri2) with HSiMe2Cl gives the formally d2 silylamido derivative [NbCp{η3-N(Ar)SiMe2-H}Cl(PMe3)] 6. X-Ray diffraction and NMR studies of this compound show that it has a stretched β-agostic Si–H → Nb interaction. Reaction of the related precursor [NbCp(NAr′)(PMe3)2] (Ar′ = 2,6-C6H3Me2) with HSiMe2Cl gives an isomeric structure [NbCp{η3-N(Ar′)SiMe2-H}(PMe3)(Cl)] 7 differing from 6 in that the phosphine rather than chloride lies trans to the co-ordinated Si–H bond. A preliminary X-ray study and large 1J(Si–H) coupling constant of 116 Hz suggest that this compound is best described as an unstretched β-agostic (Si–H⋯M) d2 silylamide complex. Reaction of the tantalum diphosphine compound [TaCp(NAr)(PMe3)2] with HSiMe2Cl affords the d0 silylhydride derivative [TaCp(NAr)(H)(SiMe2Cl)(PMe3)] 8 which, according to an X-ray diffraction study and NMR data, has an interligand hypervalent interaction (IHI) between the silyl and hydride ligands. Reactions of 6 and 8 with Me3SiX (X = I, OTf) lead to the corresponding iodido and triflate derivatives [NbCp{η3-N(Ar)SiMe2-H}X(PMe3)] (X = OTf 11 or I 12) and [TaCp(NAr)(H)(SiMe2X)(PMe3)] (X = OTf 14 or I 15). Reaction of 8 with AgOTf gives [TaCp(NAr)(PMe3)2Cl]OTf 13, the crystal structure of which has been determined. Density functional theory calculations on models of the compounds 6 and 7 showed that the experimental geometries are only correctly reproduced when the phosphine ligands are adequately modelled. The extent of oxidative addition of the Si–H bond to the metal in 6 mainly depends on the basicity of the phosphine ligand. With PH3 in place of PMe3 the calculated structures are better described as silanimine-hydrido derivatives. The formation of isomeric type 6versus7 is determined by an interplay of the steric and electronic effects of the ligand environment.

Combined single crystal neutron diffraction and solution NMR relaxation studies of mono- and bis(silyl) substituted niobocene hydrides with nonclassical interligand interactions

A neutron diffraction study of the bis(silyl) complex Cp2NbH(SiMe2Cl)2 (3) provides unambiguous localization of the hydride ligand in a central position in the bisecting plane of the niobocene moiety, which is thus in accordance with the molecular symmetry group and the results of recent density functional theory (DFT) calculations. This result is compared with the quantitative localization of the hydride ligands in solutions of the isomeric mono(silyl) complexes Cp2NbH2(SiMe2Cl) (1 and 2) and the bis(silyl) complex 3 by means of T1min, T1, T1sel and T1bis NMR measurements. A good agreement between the solution and solid state structural data is observed. It is found that the presence of a neighbouring SiMe2Cl ligand increases the Nb–hydride bond length remarkably, probably through the mechanism of Si–H interligand hypervalent interaction (IHI). This effect is especially pronounced in the mono(hydride) complex 3 containing two Si groups, where the Nb–H distance is determined as 1.781(1) A (NMR relaxation, solution) and 1.816(8) A (neutron diffraction, solid state).

A crystallographic and EPR study of the fluxional Cu(II) ion in [CuL2][BF4]2 (L = 2,6-dipyrazol-1-ylpyridine)

This paper reports a crystallographic and EPR study of pseudo-Jahn–Teller fluxionality in [Cu(L1)2][BF4]2 (1; L1 = 2,6-dipyrazol-1-ylpyridine). For 50 ≤ T ≤ 350 K, the Cu(II) ion in crystalline 1 is fluxional, with its axis of pseudo-Jahn–Teller elongation being disordered about the two N{pyrazole}–Cu–N{pyrazole} axes. The crystallographic data for 1 at these temperatures are well reproduced by a two-state model that neglects intermolecular interactions, but which yields an unusually small pseudo-Jahn–Teller radius (SpJT) for the compound. This was confirmed by measuring SpJT independently from the mean-square displacement amplitudes (MSDAs) in 1 and [Zn(L1)2][BF4]2 (2). At 41 K, 1 undergoes a phase transformation to a new polymorph containing three molecules per asymmetric unit, which have static structures and exhibit more normal SpJT values. Q-band EPR data show that the proportion of static spins in a powdered sample of 1 grows in relatively slowly as the temperature is lowered below 40 K.