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Fe spin crossover complexes [Fe(L)] (L = 2-(6-R-pyridin-2-yl)-1,10-phenanthroline with R = H, methoxy, bromo, -(1H-pyrazol-1-yl) or L = 2-(3-methoxy-pyridin-2-yl)-1,10-phenanthroline) were prepared. These air stable and durable complexes show SCO behaviour with very different transition temperatures T ranging from 130 K to 600 K depending on the substitution pattern. The use of H NMR spectroscopy to elucidate the thermodynamics and kinetics of SCO in a solution of this series is described in detail. By introduction of an additional pyrazole donor (R) in the ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. This leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R of the proton spins. The larger R values were ascribed to different electronic structures with non-orbital degenerate quintet ground states and a larger energetic separation from the first excited state. These results are also supported by Mössbauer spectroscopy. The N8 coordination sphere stabilizes the complex in the high spin state and no indication for SCO was found. DFT calculations confirmed the experimentally obtained order of T and allowed the calculation of the complex structure in experimentally non-accessible spin states. Complexes of this series can be oxidized to the Fe complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.

Condensation and subsequent reduction of 2,2′‐diaminobiphenyls 5 with 6′‐ and 5′‐substituted 6‐carbaldehyde‐2,2′‐bipyridines 4 yielded N,N′‐bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenyl‐enediamines 7, which were employed as hexadendate ligands with N6 donor sets in the synthesis of dicationic [Fe2+(7‐κ6N)] complexes 8. Dependent on the substitution pattern the respective complexes are found in the HS state (8b and 8c) or show SCO behaviour. By means of temperature‐dependent susceptibility measurements, usingEvans' method, the thermodynamic parameters ΔH, ΔS and T1/2 for the SCO have been determined. T1/2 as well as ΔH are remarkably susceptible to substitution next to the central C–C bond of the biphenyl bridge.

The novel hexadentate nitrogen based ligand N,N'-bis-(2-(1H-pyrazol-1-yl)pyridine-6-ylmethyl)-2,2'-biphenylenediamine (3) was synthesized and used for the preparation of iron Spin Crossover (SCO) complexes [Fe(3)][BF4]2 (4) and [Fe(3)][ClO4]2 (5), which differ only by the respective counter ion. These complex salts show different spin transition temperatures T(1/2) (135 and 157 K, respectively). This effect was studied by the investigation of the solid state structure of different low- and high-spin isomers. All complexes of this series show closely related crystal packing regardless of the counter ion, metal (Zn/Fe) and spin state. The isomer exhibiting the lower transition temperature (4) was also investigated in respect to its photomagnetic behaviour. The LIESST process could be monitored for this complex, but no reverse-LIESST was observed. The relaxation of the photo-induced state occurs at ca. 80 K, showing a complex, three-state relaxation mechanism.

The novel biphenyl‐based ligand N,N′,N″,N″′‐tetrakis(2,2′‐bipyridin‐6‐ylmethyl)‐4,4′‐dimethyl‐1,1′‐biphenyl‐2,2′,6,6′‐tetramine (6) with 12 nitrogen donor atoms has been synthesized. Ligand 6 is able to host two metal ions in an octahedral coordination sphere. This was shown by the synthesis of the dinuclear iron(II) complex 5 ([Fe2(6)]4+). Furthermore, the corresponding mononuclear iron(II) complex 4 ([Fe(6)]2+), where one coordination site is free, was obtained. Both FeII complexes show spin‐crossover behaviour, which was shown by 1H NMR spectroscopy and Evans' measurements. A comparison of the two complexes gives evidence of a sterically induced cooperative behaviour in the dinuclear complex 5.

The series of biferrocenyl-functionalized phosphines Bfc(PR 2 )/Bfc(SePR 2 ) (R = C 6 H 5 (6/14), C 6 H 4 -2-CH 3 (7/15), c C 4 H 3 O (8/16), c C 6 H 11 (9/17); Bfc = 1′-biferrocenyl, Fe(η 5 -C 5 H 5 )(η 5 -C 5 H 4 )Fe(η 5 -C 5 H 4 ) 2 ) and biferrocenyl diphosphines bfc(PR 2 ) 2 /bfc(SePR 2 ) 2 (R = C 6 H 5 (10/18), C 6 H 4 -2-CH 3 (11/19), c C 4 H 3 O (12/20), c C 6 H 11 (13/21); bfc = 1′,1‴biferrocenyl, (Fe(η 5 -C 5 H 4 ) 2 ) 2 ) have been prepared by consecutive synthesis methodologies. The reaction of 6−9 with [Pd(Et 2 S) 2 Cl 2 ] (22) gave the appropriate palladium dichloride complexes trans-[Pd(Bfc(PR 2 )) 2 Cl 2 ] (R = C 6 H 5 (23), C 6 H 4 -2-CH 3 (24), c C 4 H 3 O (25), c C 6 H 11 ( 26)). The structures of 15, 16, 21, 23, and 25 in the solid state were determined by single-crystal X-ray diffraction studies, showing that the structural parameters of these molecules correspond to those of related seleno phosphines and phosphino palladium dichloride complexes. Additionally, all complexes were characterized by cyclic voltammetry using [ n Bu 4 N][PF 6 ] and [ n Bu 4 N][B(C 6 F 5 ) 4 ] as supporting electrolytes. Phosphines 6−9 and seleno phosphines 14−17 show mostly irreversible redox processes involving the phosphorus and the selenium atom, both being able to form radicals leading to dimerization and other follow-up reactions. In contrast, palladium complexes 23−26 show in both electrolyte solutions a reversible behavior, although the iron centers were oxidized at more positive potentials in comparison to free Bfc or bfc phosphines. UV/vis/near-IR spectroelectrochemical measurements were carried out with 25 and 26. At potentials between 300 and 700 mV IVCT bands typical for [Bfc] + are observed, reflecting intermetallic communication between the two ferrocene moieties within the biferrocenyl phosphine units, two of which are present. No further bands were found, indicating that no electronic communication between the biferrocenyl moieties along the P−Pd−P unit exists in the mixed-valent species. The palladium complexes are suitable catalysts in the Suzuki reaction of 2-bromotoluene (27) or 4′-chloroacetophenone (28) with phenylboronic acid (29). They can also be applied in the Heck C,C cross-coupling of iodobenzene (32) with tert-butyl acrylate (33). Depending on the steric (estimated by the Tolman cone angle) and electronic properties (estimated by 1 J 31 P 77 Se ) of the phosphine ligands, the activity of the corresponding palladium complexes can be predicted. It was found that bulky and electron-rich cyclohexyland o-tolyl-containing complexes are the most active catalysts in the appropriate Suzuki and Heck reactions.

Recently, the synthesis of hexadentate ligands based on N,N′‐bis‐(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenylenediamine (1a–d) and the corresponding iron(II) complexes (3a–d) was reported by our group. In this contribution we present the synthesis of the analogous cobalt(II) complexes 4a–d. Together with the iron(II) complex the electrochemical behavior of the complexes 3 and 4 was investigated by cyclic voltammetry (CV). The aminomethyl substructure was identified as the main source of ligand degradation caused by chemical oxidation with air. Upon exposure to air the amine group in complexes 3 and 4 is oxidized to imine and even amide groups. Some examples (8, 9) of the oxidation products were characterized by X‐ray structure analysis. In order to increase the robustness of the FeII and CoII complexes towards oxidation, the ligand scaffold of 1a was modified by N‐methylation of the amino group yielding the tertiary amine 2. The corresponding iron(II) and cobalt(II) complexes employing 2 as ligand were synthesized {[Fe(2)][PF6]2 (5), [Co(2)][PF6]2 (6)} and fully characterized, their redox behavior and spin‐state was investigated by CV and Evans' method. It was found, that the introduced N‐methyl group leads to a substantial anodic shift of the MII–MIII redox potential and the stabilization of FeII and CoII high‐spin state. By means of X‐ray structure analysis these effects could be explained by repulsive steric effects of the methyl group.

New 6,6Ј-dibromo-and 6,6Ј-bis(dimethylamino)-substituted 2,2Ј-diphosphanylbiphenyl ligands 11-14 were prepared starting from 2,2Ј-dibromo-4,4Ј-dimethyl-6,6Ј-dinitro-1,1Ј-biphenyl (4). Depending on the phosphane groups [diphenylphosphanyl (11, 13) or diisopropylphosphanyl (12,14)] the palladium dichloride complexes show different coordination symmetry. Whereas the smaller diphenylphosphanyl groups lead to C 2 -symmetric complexes, the respective bis(diisopropyl)phosphanes 12 and 14 form C 1 -symmetric complexes that show fluxional behavior due to the restricted rotation of the isopropyl groups as well as the exchange of atom positions within the C 1 -symmetric conformer. All complexes have[a] TU Chemnitz, 4858 been tested as precatalysts in the Suzuki-Miyaura cross coupling reaction of 2-bromotoluene and phenylboronic acid. The activity of the catalytic system increases with the size of the diphosphanes and the donating ability of the ligand. In contrast to C 2 -symmetric palladium complex 15, platinum complex 19 was found to be C 1 -symmetric in the solid state despite the fact that both complexes have the small bis-(diphenylphosphanyl)-substituted diphosphane ligand 11 in common. NiBr 2 adduct 20 with a similar diphosphane 13 exists as a mixture of distorted square-planar and tetrahedral coordination sphere geometries in equilibrium with each other.Scheme 1. 1,1Ј-Biphenyl-2,2Ј-diyl-bridged diphosphanes discussed below and the numbering for relevant positions.phanes are of interest, as the large bromo substituents in the ortho positions lead to high activation barriers for the rotation around the central C-C bond. [22] Additionally, the dimethylamino groups can increase the electron density of the biphenyl system, [23] act as additional hemilabile donors [24] or form proton sponge systems. [25][26][27] Results and DiscussionLigands based on 2-phosphanyl-and 2,2Ј-diphosphanylbiphenyls are accessible by various methods employing electrophilic, [28] nucleophilic, [29] and radical chemistry [30] as well as transition-metal-catalyzed [31,32] reactions. A particularly effective method has been the treatment of a 2,2Ј-di-www.eurjic.org FULL PAPER lithiated biphenyl with a chlorophosphane. [28,33,34] Dilithiated biphenyl species are easily prepared by halogen/lithium exchange from the 2,2Ј-diiodo-/2,2Ј-dibromobiphenyls. We prepared diiodobiphenyl 6 and dibromobiphenyl 7 by the route depicted in Scheme 2. [35,36] The known diamine derivative 5 can be prepared from inexpensive and commercially available 4-methyl-2-nitroaniline (1). The methyl group in the 4-position blocks the C-4 atom from electrophilic attack by bromine in the first step to derivative 2; consequently, 2 is the only isolable regioisomer. [37] Moreover, the 4-methyl group improves the solubility of all compounds and simplifies workup procedures. The key step in the synthesis is an Ullmann [35,38] homo-coupling of iodo compound 3 to form the central C-C bond in 4. This reaction is highly selective and proceeds smoothly under relatively mild conditions in ...