Crystal structure of bis(pentamethylcyclopentadienyl)-(4,4′-di-tert-butylbipyridyl)hafnium(IV) — hexane (1:0.5), Hf(C10H15)2(C18H24N2) · 0.5C6H14

Abstract

C 41 H 61 HfN 2 ,monoclinic, P2 1 /n (no. 14), a =13.4410(4) Å, b =13.9983(6) Å, c =21.1996(8) Å, b =98.144(3)°, V =3948.5 Å 3 , Z =4,Rgt(F) =0.051, wRref(F 2 ) =0.121, T =200 K. Source of materialCp* 2 HfCl 2 (1.198 g, 2.31 mmol), finely cut lithium (0.064 g, 9.22 .mmol) and 4,4'-di-tert-butyl-2,2'-bipyridine (0.619 g, 2.31 .mmol) were suspended in 20 mL toluene. The mixture was stirred at 60°Cfor two days, resulting in apurple solution. The volatiles were removed in vacuum, followed by extraction of the residue with 2 × 10 mL n-hexane. The dark blue filtrate was stored at -78°C, after 24 hours dark blue crystals had formed which were isolated, washed with cold n-hexane and dried in vacuum to give 0.748 g(45 %) of the title compound. DiscussionRecently we reported on the synthesis of hafnocene alkyne complexes and found the alkyne to be coordinated strongly to the metal resulting in ahafnacyclopropene structure [1]. We were interested to investigate the influence of the Lewis acidic metallocene center on other substrates such as THF [2] and bipyridines. Upon reaction of decamethylhafnocene dichloride with lithium and 4,4'-di-tert-butyl-2,2'-bipyridine the deep blue, highly air-and moisture sensitive title compound is formed. The bipyridine coordinates the metal with both Natoms. Structural data suggest the existence of a s,s complex rather than a p,p coordination mode. The bond distance C5-C6 of 1.387(8) Å is indicative of adouble bond, this value is at 1.494 Å in free 4,4'-ditert-butylbipyridine [3]. This significantly shorter bond distance and the existence of ab ent metallacycle (angle between the planes N1,Hf1,N2 and N1,C5,C6,N2: 33.6(2)°)allow the interpretation of this complex as a2,5-diazametallacyclopent-3-ene. The aromaticity of the ring system is reduced due to the donation of electron density from the hafnocene center into the antibonding p*orbitals of the ligand. Additionally, half amolecule of n-hexane as asolvent molecule was found in the asymmetric unit of the title compound.

Reactions of 2‐Substituted Pyridines with Titanocenes and Zirconocenes: Coupling versus Dearomatisation

“…A useful tool to distinguish the oxidation states of coordinated bipy is the bond distance of the ipso ‐carbon atoms linking the pyridines (C−C′). For example, in [Cp 2 Zr (N , N ′‐bipy)] ( E ) and [Cp* 2 Hf(4,4′‐di‐ tert ‐butylbipyridyl)], both of which are formal [bipy] 2− complexes, these bonds are significantly shortened as a consequence of strong π‐donation of the metal d 2 electrons into the π* orbitals of bipy . This π‐back‐donation capability is known to be increased with a rising methylation degree at cyclopentadienyl rings owing to the increasing electron density at the metal centre …”