Synthesis and coordination chemistry of (PNEt2)2-bridged [2]ferrocenophanes

(Electrochemical) Properties and Computational Investigations of Ferrocenyl-substituted Fe3(μ3-PFc)2(CO)9 and Co4(μ4-PFc)2(CO)9 Clusters and Their Reduced Species

The formation of ferrocenyl-functionalized iron and cobalt carbonyl clusters is reported, based on a reaction of FcPCl₂ (3) (Fc = Fe(η⁵-C₅H₅)(η⁵-C₅H₄)) with Fe₂(CO)₉ and Co₂(CO)₈, respectively. Therein, nido-Fe₃(CO)₉(μ₃-PFc)₂ (4) and nido-Co₄(CO)₁₀(μ₃-PFc)₂ (5) clusters were obtained as the first diferrocenyl-substituted carbonyl clusters with a symmetrical cluster core. Cluster 4 shows two reversible one-electron processes within the anodic region, based on Fc/Fc⁺ redox events, as well as two processes in the cathodic region. In situ IR and electron paramagnetic resonance (EPR) measurements of all electronic states confirmed an Fc-based oxidation and a core-based reduction. On the basis of the results of a single-crystal X-ray analysis of structures of 4 and 5, computational studies of the highest occupied molecular orbital-lowest unoccupied molecular orbital energies, the spin density, quantum theory of atom-in-molecule delocalization indices, and the atomic charges were performed to explain the experimental results. The latter revealed a reorganization of the cluster core upon reduction and the existence of weak P···P interactions in 4 and 5. Ferrocenyl-related redox processes, occurring reversibly in case of 4, were absent for 5, due to a different distribution of the HOMO energies. EPR measurements furthermore confirmed the core-based radical anion and the formation of a decomposition product at potentials lower than [M]^2- (M = Fe, Co).

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

Stereochemical Alignment in Triphospha[3]ferrocenophanes

A series of triphospha[3]ferrocenophanes of the type Fe(C₅ H₄ -PtBu)₂ PX with X=H, F, Cl, Br, I, NEt₂ , tBu has been prepared and characterized by heteronuclear NMR spectroscopy and X-ray crystallography. Despite having three stereogenic centers, the selective formation of a reduced number of diastereomers (either one or two) has been observed for these ferrocenophanes. Theoretical calculations revealed that the inversion of the central stereogenic center inverts the frontier orbital sequence leading to either an iron or a phosphorus centered HOMO depending on the respective diastereomer. CV measurements supported these results. For the all-tert-butyl substituted [3]ferrocenophane Fe(C₅ H₄ )₂ (PtBu)₃ a chiral staggered conformation has been found in the solid state which differs substantially from the only other all-organo substituted [3]ferrocenophane, Fe(C₅ H₄ )₂ (PPh)₃ .