Richard A. Layfield

Syntheses, structures and magnetic properties of Mn(II) dimers [CpMn(μ-X)]2(Cp = C5H5; X = RNH, R1R2N, CCR)

Manganocene, Cp2Mn, has been employed as a precursor in the synthesis of a range of Mn(II) dimers of the type [CpMn(μ-X)]2 [X = 8-NHC9H6N (1), N(Ph)(C5H4N) (2), N(4-EtC6H4)(C5H4N) (3) and CCPh (4)] as well as the bis-adduct [Cp2Mn{HNC(NMe2)2}2] (5). The solid-state structures of 1–5 are reported. Variable-temperature magnetic measurements have been used to assess the extent of Mn(μ-X)Mn communication within the dimers of 1–4 as a function of the bridging ligands (X).

Syntheses and magnetic properties of hexanuclear [Cp2Mn3(L1)4]2 and octanuclear [Mn8(L2)12(μ4-O)2] (L1 = 2-HNC5H5N, L2 = 2-NH-3-Br-5-MeC5H3N, Cp = C5H5)

The reactions of manganocene, Cp2Mn, with 2-aminopyridine (L1H) or 2-amino-3-bromo-5-methylpyridine (L2H) give the novel hexanuclear and octanuclear Mn(II) amido cage compounds [Cp2Mn3(L1)4]2 (1) and [Mn8(L2)12(mu 4-O)2] (2); magnetic measurements on which provide a rare insight into the magnetic properties of amido-bridged metal clusters.

Interpreting molecular crystal disorder in plumbocene, Pb(C5H5)2: Insight from theory

Plane-wave density functional theory has been applied in a novel way to help interpret the molecular crystal structure disorder observed in the orthorhombic zigzag phase of plumbocene, Pb(C5H5)2. A crystal lattice comprising uniformly staggered C5H5 rings was found to be lower in energy by 2.8 kJ mol-1 per unit cell, compared to a uniformly eclipsed packing arrangement. This energy difference has been attributed to the difference in the strength of intermolecular interactions between the Pb(C5H5)2 chains for the two different lattices. The calculations performed allowed the determination of the crystallographic occupancy factors by a quantum mechanical technique for the first time.

Transmetalation of chromocene by lithium-amide, -phosphide, and -arsenide nucleophiles.

The pnictogen-centered nucleophiles LiE(SiMe3)2 (E = N, P, or As) substitute a cyclopentadienide ligand of chromocene (Cp2Cr), with elimination of lithium cyclopentadienide, to give the series of pnictogen-bridged compounds [(μ:η(2):η(5)-Cp)Cr{μ-N(SiMe3)2}2Li] (1) and [(η(5)-Cp)Cr{μ-E(SiMe3)2}]2, with E = P (2) or E = As (3). Whereas 1 is a heterobimetallic coordination polymer, 2 and 3 are homometallic dimers, with the differences being due to a structure-directing influence of the hard or soft character of the bridging group 15 atoms. For compound 1, the experimental magnetic susceptibility data were accurately reproduced by a single-ion model based on high-spin chromium(II) (S = 2), which gave a g-value of 1.93 and an axial zero-field splitting parameter of D = -1.83 cm(-1). Determinations of phosphorus- and arsenic-mediated magnetic exchange coupling constants, J, are rare: in the dimers 2 and 3, variable-temperature magnetic susceptibility measurements identified strong antiferromagnetic exchange between the chromium(II) centers, which was modeled using the spin Hamiltonian H = -2J(S(CrA)·S(CrB)), and produced large coupling constants of J = -166 cm(-1) for 2 and -77.5 cm(-1) for 3.

A new, tetragonal, helical phase of plumbocene, Cp2Pb; variations on a molecular string (Cp=C5H5)

The isolation of a tetragonal, helical phase of plumbocene, Cp 2 Pb, provides a rare example of a helical metallocene and the opportunity to shed light on the relationship between the modes of aggregation found in its various polymorphs.

The first observation of the [Cp3Mn]− anion; structures of hexagonal [(η2-Cp)3MnK·1.5thf] and ion-separated [(η2-Cp)3Mn]2[Mg(thf)6] ·2thf

Hexagonal [(η2-Cp)3MnK·1.5thf] 1 and ion-separated [(η2-Cp)3Mn]2[Mg(thf)6] ·2thf 2 are obtained from reactions of CpK and Cp2Mg, respectively, with manganocene, Cp2Mn; they are the first complexes to be structurally characterised containing the [Cp3Mn]− anion.

Synthesis, structure and paramagnetism of manganese(II) iminophosphate complexes

The coordination chemistry of the bidentate bis(imino)bis(amino)phosphate ligands [Me(3)SiN═P{NR}{N(H)R}(2)](-), where R = n-propyl is [L(1)H(2)](-), R = cyclohexyl is [L(2)H(2)](-), and R = tert-butyl is [L(3)H(2)](-), with manganese(II), is described. The bis(imino)bis(amino)phosphate-manganese(II) complexes [(η(5)-Cp)Mn(μ-L(1)H(2))](2) (1), [Mn(L(2)H(2))(2)]·THF (2·THF), and [(η(5)-Cp)Mn(L(3)H(2))] (3) were synthesized by monodeprotonation of the respective pro-ligands by manganocene, Cp(2)Mn. The molecular structures of 1-3 reveal that the steric demands of the ligand N-substituents play a dominant role in determining the aggregation state and overall composition of the manganese(II) complexes. The coordination geometries of the Mn(II) centers are six-coordinate pseudotetrahedral in 1, four-coordinate distorted tetrahedral in 2, and five-coordinate in 3, resulting in formal valence electron counts of 17, 13, and 15, respectively. EPR studies of 1-3 at Q-band reveal high-spin manganese(II) (S = 5/2) in each case. In the EPR spectrum of 1, no evidence of intramolecular magnetic exchange was found. The relative magnitudes of the axial zero-field splitting parameter, D, in 2 and 3 are consistent with the symmetry of the manganese environment, which are D(2d) in 2 and C(2v) in 3.

Applications of manganocene in the synthesis of Mn(II) amide and imide cages

The reactions of manganocene, Cp2Mn (Cp = C5H5), with 2-aminopyrimidines give similar, octameric amido/imido cage complexes of general formulae [{CpMnNHR}{MnNR}]4 [R = 4,6-Me2pm (1a), 4-MeO-6-Mepm (1b), 4,6-(MeO)2pm (1c); pm = pyrimidinyl]. The structures of these complexes can be visualised as being composed of central Mn4N4 imido cubane units that are coordinated at their peripheries by four [CpMnNHpm] monomers. In contrast, less acidic 2-aminopyridine (2-NH2py) and 8-aminoquinoline (8-NH2quin) are only singly deprotonated under similar conditions, giving the unusual hexanuclear amido complex [Cp2Mn3(NHpy)4]2 (2) and the dimer [CpMn(μ-8-HNquin)]2 (3). N,N′-Dibenzylethylenediamine [(BnNHCH2)2] only forms the simple adduct [(η1-Cp)(η5-Cp)Mn{(BnNHCH2)2}] (4) (Bn = CH2Ph) with Cp2Mn, in which no deprotonation of the organic acid has occurred. The X-ray structures of the new complexes 1b–c, 3 and 4 are reported (1a and 2 having been communicated previously).

17. A New, tetragonal, helical phase Of plumbocene, Cp2Pb; variations on a molecular string

The isolation of a tetragonal, helical phase of plumbocene, Cp 2 Pb, provides a rare example of a helical metallocene and the opportunity to shed light on the relationship between the modes of aggregation found in its various polymorphs.

Group 14: Silicon, Germanium, Tin and Lead

Rather than providing an exhaustive survey of Group 14 organometallic chemistry published in 2004–2005, this chapter takes the form of a ‘highlight’ review and will focus primarily on low-coordinate, multiply bonded organo-derivatives of silicon, germanium, tin and lead. Transition metal complexes i...