The ferrocenylthiophenes 2,3-Fc 2 -c C 4 H 2 S (9), 2,4-Fc 2 -c C 4 H 2 S (10), and 2,3,4-Fc 3 -c C 4 HS (11) have been prepared by a 2-or 3-fold Negishi cross-coupling reaction of the appropriate bromo thiophenes 5−7 with FcZnCl (8; Fc = Fe(η 5 -C 5 H 4 )(η 5 -C 5 H 5 )) in the presence of either [Pd(PPh 3 ) 4 ] or [Pd(CH 2 CMe 2 P t Bu 2 )(μ-Cl)] 2 as catalyst. Concerning electron transfer studies on ferrocenyl-substituted aromatic heterocycles, the electrochemistry as well as in situ UV−vis−near-IR spectroelectrochemistry highlight the electrochemical properties of these compounds in a series of mono-, di-, tri-, and tetraferrocenylthiophenes, including 2-Fc- 12), and 2,3,4,5-Fc 4 -c C 4 S (13). These organometallic compounds display one (1, 2), two (3, 4, 9, 10), three (11, 12), or four (13) well-resolved electrochemically reversible one-electron-transfer processes using [N n Bu 4 ][B(C 6 F 5 ) 4 ] as the supporting electrolyte. The spectroelectrochemical studies reveal that ferrocenyl units placed in the α-position of the thiophene ring interact more strongly with the heterocycle than those in the β-position. Thus, the intensity of the ligand-to-metal charge transfer (LMCT) absorptions, caused by interactions between the thiophene core and the ferrocenyl moieties, decreases from 1 + to 2 + . Furthermore, in the series of diferrocenylthiophenes the interaction between the iron centers in the mono-oxidized compounds decreases in the series 3 + > 9 + > 10 + > 4 + . The structural properties of 10 were investigated by single-crystal X-ray diffraction studies, indicating that 10 possesses a syn conformation in the solid state with respect to the orientation of the two ferrocenyl units along the central thiophene core. Compound 10 is isomorphic with 3.

A series of Fe(2+) spin crossover (SCO) complexes [Fe(5/6)](2+) employing hexadentate ligands (5/6) with cis/trans-1,2-diamino cyclohexanes (4) as central building blocks were synthesised. The ligands were obtained by reductive amination of 4 with 2,2'-bipyridyl-6-carbaldehyde or 1,10-phenanthroline-2-carbaldehyde 3. The chelating effect and the rigid structure of the ligands 5/6 lead to exceptionally robust Fe(2+) and Zn(2+) complexes conserving their structure even in coordinating solvents like dmso at high temperatures. Their solution behavior was investigated using variable temperature (VT) (1)H NMR spectroscopy and VT Vis spectroscopy. SCO behavior was found for all Fe(2+) complexes in this series centred around and far above room temperature. For the first time we have demonstrated that the thermodynamics as well as kinetics for SCO can be deduced by using VT (1)H NMR spectroscopy. An alternative scheme using a linear correction term C(1) to model chemical shifts for Fe(2+) SCO complexes is presented. The rate constant for the SCO of [Fe(rac-trans-5)](2+) obtained by VT (1)H NMR was validated by Laser Flash Photolysis (LFP), with excellent agreement (1/(kHL + kLH) = 33.7/35.8 ns for NMR/LFP). The solvent dependence of the transition temperature T1/2 and the solvatochromism of complex [Fe(rac-trans-5)](2+) were ascribed to hydrogen bond formation of the secondary amine to the solvent. Enantiomerically pure complexes can be prepared starting with R,R- or S,S-1,2-diaminocyclohexane (R,R-trans-4 or S,S-trans-4). The high robustness of the complexes reduces a possible ligand scrambling and allows preparation of quasiracemic crystals of [Zn(R,R-5)][Fe(S,S-5)](ClO4)4·(CH3CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.

Within a series of α,α′-diferrocenylthiophenes, the influence of electron-withdrawing substituents at the ferrocenyl moieties and electron-donating functionalities at the thiophene bridge on the electronic behavior was studied. The synthesis of the appropriate 2,5-fc 2 -c C 4 H 2 S and 2,5-fc 2 -3,4- CHO (3,7), CN (4, 8)) was carried out using the Negishi C,C cross-coupling protocol to effect the coupling of the thiophene and the ferrocenyl moieties. The corresponding diferrocenylthiophenes were characterized spectroscopically. Within this context, the structural properties of 2−5, [5][B(C 6 F 5 ) 4 ], 6, and 8 in the solid state were investigated by single-crystal X-ray diffraction studies. Electrochemical investigations of thiophenes 2−8 demonstrated an increasing redox separation and hence thermodynamic stability of the corresponding mixed-valent species in the series 1 < 3 < 5 < 4 ≈ 2 < 6 < 7 < 8. Moreover, UV/vis/near-IR and infrared spectroelectrochemical studies verify these observations. These studies further reveal a valence trapped situation in corresponding mixed-valent species and allow a class II classification according to Robin and Day.

3,3′,4,4′,5,5′-Hexaferrocenyl-2,2′-bithiophene (4) has been prepared using a straightforward synthesis protocol that follows the palladium-promoted Negishi cross-coupling of 2-Br-3,4,5-Fc3-cC4S (2) with 3,4,5-Fc3-2-ZnCl-cC4S (3) (Fc = Fe(η5-C5H4)(η5-C5H5)). The electronic and structural properties of 4 were investigated by UV–vis spectroscopy and single-crystal X-ray diffraction studies. Comparison of the appropriate bond distances in the 2,2′-bithiophene moiety shows electron delocalization. Cyclic and square-wave voltammetry and in situ UV–vis/NIR spectroelectrochemistry highlight the electrochemical properties of 4. The ferrocenyls in 4 can be reversibly and separately oxidized in dichloromethane and anisole solutions using [N n Bu4][B(C6F5)4] as supporting electrolyte. Generally, these studies prove electrostatic interactions among the ferrocenyl termini as oxidation progresses. The atropisomerism of 4 caused by the inner rotation barrier about the 2,2′-bithiophene bond was investigated by dynamic 1H NMR spectroscopy.

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1''-biferrocenyl; bfc = 1',1''-biferrocen-1,1'''-diyl) mono-and biscarbene tungsten(0) complexes of the type [(CO) 5 W=C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO) 5 W=C(OMe)-R'-(OMe)C=W(CO) 5 ] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO) 6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1 -4. The ferrocenyl and biferrocenyl moieties in 1 -4 show reversible one electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1 -4 by the use of in situ UV-Vis-NIR and infrared spectroelectrochemical techniques. During the UV-Vis-NIR investigations typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by DFT calculations. The structural properties of 1 -4 in the solid state were investigated by single-crystal Xray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit synconformation of the ferrocenyl and carbene building blocks.