David J. Burkey

Structural Lessons from Main-Group Metallocenes

Abstract The metallocenes of the Group 2 and Group 14 elements display a higher degree of structural and chemical similarity than would be expected based on the differences in their electronic configurations alone. A comparison of their structures suggests that the interplanar ring angles in both are primarily determined by the size and shape of the cyclopentadienyl ligands; the two additional valence electrons in the Group 14 compounds exert little stereochemical influence. Although the two metallocene families display differences in reactivity, the presence or absence of metal valence electrons is not always a useful criterion for distinguishing between them. In Group 14 metallocenes, the electrons are not efficient donors to electrophiles, and sufficiently bulky cyclopentadienyl rings can interfere with the extent of redox reactions. Examination of main-group metallocene melting points reveals a trend based on the symmetry and flexibility of the cyclopentadienyl ring. In rigorously monomeric species, c...

Cyclopentadienyl ring metathesis with bis(pentamethylcyclopentadienyl)calcium as a route to mixed ring organolanthanide complexes; the crystal structure of (C5Me5)2Nd(C5H5)

Abstract Bis(pentamethylcyclopentadienyl)calcium, (C 5 Me 5 ) 2 Ca, can be made by the reaction of Ca[N(SiMe 3 ) 2 ] 2 with C 5 Me 5 H in toluene. It undergoes cyclopentadienyl ring metathesis with tris(cyclopentadienyl)lanthanide complexes, Cp 3 Ln (Ln = La, Nd, Sm) in toluene to generate the mixed ring complexes (C 5 Me 5 ) 2 LnCp. The X-ray crystal structure of (C 5 Me 5 ) 2 NdCp shows that the complex is a sterically crowded monomer with η 5 -C 5 Me 5 and Cp rings. The average NdC distances for both the C 5 Me 5 and Cp rings are 2.76–2.79 A.

Effects of steric strain on the bonding in zinc metallocenes: the structure of [(C3H7)4C5H]2Zn

Abstract Zinc diiodide reacts with the potassium salts of 1,2,4-tri and 1,2,3,4-tetra(isopropyl)cyclopentadiene to form the corresponding zinc metallocenes in high yield. Both compounds are highly air-sensitive, but [(C 3 H 7 ) 4 C 5 H] 2 Zn is isolated as a high-melting crystalline solid, whereas [(C 3 H 7 ) 3 C 5 H 2 ] 2 Zn is a waxy solid that melts just above room temperature. Like other structurally characterized zincocenes, [(C 3 H 7 ) 4 C 5 H] 2 Zn has an asymmetric “slipped-sandwich” structure with one η 1 - and one approximately η 5 -ring. The ZnC distance for the η 1 -ring of 2.223(4) A is the longest yet observed in a zincocene. The ZnC distances for the η 5 -ring cover a larger than usual range (0.52 A), and include the shortest ZnC ring bond reported in a cyclopentadienyl zinc complex (1.991(4) A). These extremes in structural values reflect the increased slippage of the rings in [(C 3 H 7 ) 4 C 5 H] 2 Zn, which is a consequence of the exceptional steric bulk of the [(C 3 H 7 ) 4 C 5 H] − anion. Variable temperature NMR data for [(C 3 H 7 ) 4 C 5 H] 2 Zn are consistent with the existence of an asymmetric structure in solution.

Synthesis and structural characterization of Hexa(cyclohexyl)ferrocene, [1,2,4-(C6H11)3C5H2]2Fe

Abstract 1,2,4-tricyclohexylcyclopentadiene can be formed from the phase-transfer catalysed addition of cyclohexyl bromide to cyclopentadiene. Reaction of two equivalents of its lithium salt with FeCl 2 in THF produces the corresponding substituted ferrocene. Crystals of orange [1,2,4-(C 6 H 11 ) 3 C 5 H 2 ] 2 Fe grown from toluene are monoclinic, space group C 2/ c , with a = 16.386 (4) A, b = 12.142 (3) A, c = 21.003 (3) A, β = 107.38 (1), and D calcd = 1. 131 g cm −3 for Z = 4. Least- squares refinement based on 1291 observed reflections led to a final R value of 0.049. The molecule possesses a classic sandwich geometry with staggered rings and a crystallographically imposed two-fold rotation axis passing through the metal center. Despite the presence of six cyclohexyl rings, the cyclopentadienyl ligands can orient themselves around the iron to avoid unfavorable inter- ring steric interactions. As a result, the average Fe-C ring distance of 2.06 (1) A in hexa(cyclohexyl)ferrocene is not significantly longer than the corresponding distance in ferrocene itself.