Dynamic Disorder and Fluxionality in M₃(CO)₁₂Clusters:  Variable-Temperature X-ray Diffraction Studies on Fe_nRu_3-n(CO)₁₂(n= 1, 2) and the Low-Temperature Phase of Fe₃(CO)₁₂

Abstract

The metal atom disorder in the crystalline clusters Fe2Ru(CO)12 (1) and FeRu2(CO)12 (2) has been shown to be dynamic in origin. At and below 313 K, 1 crystallizes in the noncentrosymmetric space group Pn with two independent molecules in the asymmetric unit, both molecules having approximate C 2 v symmetry with small C 2 distortions. Above 313 K, crystals of 1 undergo a phase transition to the centrosymmetric space group P21/n, becoming isomorphous and isostructural with the room-temperature phase of Fe3(CO)12 (3). These reversible changes are accompanied by an increase in the metal atom disorder, from a completely ordered structure at 223 K to a statistically disordered (1:1) “Star of David” structure at 323 K. Crystals of 2 show similar behavior, belonging to the noncentrosymmetric space group C2cb below 228 K and undergoing a phase transition to the centrosymmetric space group Ccmb above this temperature. An increase in the extent of disorder is observed in 2, from an ordered structure at 173 K to the extended Star of David disorder above the phase transition temperature. Compound 2 has an all-terminal carbonyl arrangement, with a strong D 3 distortion, and provides the first example of the D 3 structural type for a homoleptic M3(CO)12 cluster. In contradiction to our earlier studies, Fe3(CO)12 (3) has now been shown to undergo a phase transition at ∼210 K to a second monoclinic phase with a partial ordering of the metal atom triangles. The asymmetric unit comprises four complete and one half-molecule of 3, and one of these molecules (3a) is completely ordered. The thermal motion in 1 and the structures of 1−3 have been examined in the context of the various proposed fluxional mechanisms in M3(CO)12 clusters.

Fe-only hydrogenase active site mimics: Fe2(CO)6(μ-ADT) (ADT=azadithiolate) clusters bearing pendant 2,2′:6′,2″-terpyridine domains and containing alkynylthienylene or alkynylphenylene spacers

“…3). This results in relatively short intermolecular carbonyl OÁ Á ÁO contacts of 3.0 Å, similar to those observed in Fe 2 (CO) 9 [42] and Fe 3 (CO) 12 [43]. In contrast, molecules of 3 are packed so that there is partial interdigitation of organic and iron sulfur cluster domains (Fig.…”

On the nature of [Fe 3 (CO) 12 ] in the solid-state: The Ligand Polyhedral Model approach to the observed disorder

“…Of primary interest has been the mechanism by which these fluxional processes occur and numerous ideas have been put forward. These have also been reviewed elsewhere [6,7]. In this paper we wish to put forward the view that this fluxional behaviour is best understood in terms of the LPM [1] and, because of its nature, is considered to be directly responsible for the disorder observed in the crystalline state.…”

“…In this paper we wish to put forward the view that this fluxional behaviour is best understood in terms of the LPM [1] and, because of its nature, is considered to be directly responsible for the disorder observed in the crystalline state. We would emphasise that as far as we can establish this approach is the first to fully lay down the origin and nature of the 'Star of David' and other factors responsible for this disorder and would point out that the ideas presented here are applicable not only to [Fe 3 (CO) 12 ] and its close relatives [7] but to all cluster carbonyls such as [Co 4 (CO) 12 ], [Rh 4 (CO) 12 ] and [Rh 6 (CO) 16 ]. They would allow the type and origin of the disorder to be established and for example remove the difficulties in resolving the structure of [FeRu 2 (CO) 12 ] when the workers were obliged to state that 'The breakthrough came with a correct lucky guess that the metal triangle was situated on an inversion centre'.…”

“…The fluxional behaviour that this carbonyl undergoes in both solution and the solid state has also been the subject of intensive investigation and speculation [7] over a considerable time and these investigations and speculations have also been summarised by the same and other authors [6,7]. Of primary interest has been the mechanism by which these fluxional processes occur and numerous ideas have been put forward.…”

“…libration and polyhedral inter-conversion, for the fluxional behaviour these systems show. This fluxional behaviour is closely related to the disorder observed in single crystal X-ray analysis of these carbonyls [6,7]. In this paper the LPM is applied to provide a clearer understanding of the nature and origin of the disorder ('Star of David') observed for [Fe 3 (CO) 12 ] and its close relatives [Fe 2 Ru(CO) 12 ] and [FeRu 2 (CO) 12 ].…”