Stereochemistry of coordination compounds. From alfred werner to the 21st century.

As a contribution to the scientific symposium, November 22nd, 2013, commemorating the Nobel Prize awarded to Alfred Werner in 1913, a presentation of the development of stereochemistry of coordination compounds during the past 120 years was given. Stereochemistry was fundamental to Werner's theory of coordination compounds. After Werner's death in 1919, stereochemistry in this field did not progress much further for almost 20 years, but then developed continuously. It was realized that stereochemical features of elements showing coordination numbers larger than four are responsible for an almost unlimited number of stereochemical possibilities, thus opening a molecular world of new structures. In the beginning of the 21st century, interest in the field rose again considerably, mainly due to the potential of stereoselective catalysis, and the self-assembly of supramolecular structures. An end of these developments is not in sight. Here an abbreviated version of the lecture is given. A PowerPoint(®) file, or a video of the presentation, can be downloaded.

Stereoselective Synthesis of Cyclometalated Iridium (III) Complexes: Characterization and Photophysical Properties.

The stereoselective synthesis of a highly luminescent neutral Ir(III) complex comprising two bidentate chiral, cyclometalating phenylpyridine derivatives, and one acetylacetonate as ligands is described. The final complex and some intermediates were characterized by X-ray structural analysis, NMR-, CD-, and CPL-spectroscopy.

Novel HEXOL-type cyclometallated iridium(III) complexes: stereoselective synthesis and structure elucidation.

Two diastereoisomers of tetranuclear cyclometallated iridium complexes, either having an inner core of HEXOL-type [Ir(IrCl2)3]6+ unit and a surface of six chiral, didentate, cyclometallated ligands, are stereoselectively synthesized from an enantiopure pinenopyridine derivative.

First diastereoselective formation of lanthanide triple helical complexes with a terdentate chiral C2 symmetric ligand.

Steric interactions between three enantiopure terdentate ligands leads to the diastereoselective formation of luminescent triple helical lanthanide complexes.

Octahedral Complexes with Predetermined Helical Chirality: Xylene-Bridged Bis([4,5]-pineno-2,2'-bipyridine) Ligands (Chiragen[o-, m-, p-xyl]) with Ruthenium(II).

Tetradentate ligands are obtained by joining two optically active [4,5]-pineno-2,2'-bipyridine molecules in a stereoselective reaction, where two new stereogenic centers are created. These ligands are new members of the chiragen family that form OC-6 complexes with predetermined helical chirality. Ru(II) complexes with 4,4'-dimethyl-2,2'-bipyridine occupying the remaining coordination sites have been synthesized with all three new ligands. Characterization of the ruthenium complexes by NMR spectroscopy confirm C(2)-symmetric structures in solution. CD spectra show that the complexes are composed of only one helical diastereomer with the expected absolute configurations. In addition, a strong chiral amplification is observed, if precursors of low enantiomeric purity are used. This is due to the inability of ligands that are heterochiral in the two bpy moieties to coordinate to one center. X-ray structural data were obtained for the complex Delta-[RuCG[o-xyl](4,4'-DMbpy)](PF(6))(2). Crystal data (Mo Kalpha, 298 K): trigonal, space group R3, a = 52.986(4) Å, c = 10.545(1) Å, V = 25639(4) Å(3), Z = 18, R1 = 0.087, and wR2 = 0.0986 for 2609 observed reflections.

Chiral Cyclometalated Platinum(II) and Palladium(II) Complexes with Derivatives of Thienylpyridine as Ligands: Helical Distortion of the Square Planar (SP-4) Geometry.

Four cis-bis-cyclometalated complexes of platinum(II) and palladium(II) have been synthesized with chiral substituted thienylpyridine ligands. These ligands are (5aR,6aS)-5,5a,6a,7-tetrahydro-6,6-dimethyl-3-(2'-thienyl)cyclopropa[g]isoquinoline (th4,5capy) (1), (6aR,7aS)-5,6,6a,7a-tetrahydro-7,7-dimethyl-2-(2'-thienyl)cyclopropa[h]quinoline (th5,6capy) (2), (6R,8R)-5,6,7,8-tetrahydro-9,9-dimethyl-2-(2'-thienyl)-6,8-methanoquinoline (th5,6betappy) (3), and (6R,8R)-5,6,7,8-tetrahydro-9,9-dimethyl-3-(2'-thienyl)-6,8-methanoisoquinoline (th4,5ppy) (5). Two complexes have a well-defined chirality at the metal center, due to the sterical interaction of the ligands. These complexes are Delta-Pt(th5,6capy)(2) (8) and Lambda-Pt(th5,6betappy)(thpy) (9). The crystal structure of 8 shows a strong distortion from the ideal square planar geometry. Crystals of Delta-Pt(th5,6capy)(2) (8) are tetragonal, space group P4(3)2(1)2, a = 13.407(1) Å, b = 13.407(1) Å, c = 14.860(2) Å, alpha = beta = gamma = 90 degrees, and Z = 4. Final R = 0.0237 and R(w) = 0.0450 for 2353 observed reflections. Compound 8 possesses a crystallographic C(2) symmetry.

Molecular Architecture of Polynuclear Ruthenium Bipyridyl Complexes with Controlled Metal Helicity.

The synthesis of di- and trinuclear ruthenium(II) complexes is reported, where each metal center has a tris(bidentate) octahedral coordination sphere with predetermined stereochemistry. New members of the "Chiragen" ligand series, consisting of two linked chiral 4,5-pineno-2,2'-bipyridine groups, have been prepared, with small spacer units between the coordination centers (-(CH(2))(n)() {n = 0, 3} and -CH(2)(bpy)CH(2)-). X-ray structural data were obtained for the ligand Chiragen[3]. (Crystal data: orthorhombic, space group P2(1)2(1)2(1), a = 12.229(1) Å, b = 12.790(1) Å, c = 20.215(1) Å, V = 3161.8(4) Å(3), Z = 4.) Combination of the ligands with Ru(bpy)(2)Cl(2) (where bpy is 2,2'-bipyridine) led to a mixture of diastereomers, while the use of enantiomerically pure Delta- or Lambda-[Ru(bpy)(2)(py)(2)](dibenzoyltartrate) or Delta-Ru(CG[m-xyl])Cl(2) led to almost complete stereoselectivity in the products. Circular dichroism spectra show that the complexes are composed of one helical diastereomer, with the expected absolute configuration predetermined by the chiral building block used. Additionally, (1)H-NMR spectroscopy indicates C(2) point group symmetry for the structures in solution, confirming the absence of DeltaLambda diastereomers.

Cyclometalated Complexes of Palladium(II) and Platinum(II): cis-Configured Homoleptic and Heteroleptic Compounds with Aromatic C&arcraise;N Ligands.

The palladium(II) and platinum(II) bis-homoleptic complexes M(C&arcraise;N)(2), where C&arcraise;N is benzo[h]quinoline (H-bhq), 2-phenylpyridine (H-phpy), 2-(2'-benzothienyl)pyridine (H-bthpy), 2-(2'-thienyl)quinoline (H-thq), and 2-(2'-thienyl)pyridine (H-thpy), were prepared by metal exchange of the lithiated ligands C&arcraise;N with M(Et(2)S)(2)Cl(2). The palladium(II) bis-heteroleptic complexes, Pd(C&arcraise;N)(C'&arcraise;N'), were synthesized from Pd(C&arcraise;N)(2) bis-homoleptic complexes, which were converted by HCl into the dichloro-bridged compounds [Pd(C&arcraise;N)Cl](2). By addition of Et(2)S, the Pd(C&arcraise;N)(Et(2)S)Cl complexes were formed, which were allowed to react with Li(C'&arcraise;N'), yielding M(C&arcraise;N)(C'&arcraise;N') compounds. An alternative way for obtaining the bis-heteroleptic molecules is by ligand exchange, according to the equilibrium M(C&arcraise;N)(2) + M(C'&arcraise;N')(2) = 2M(C&arcraise;N)(C'&arcraise;N'). The crystal structures of Pt(bhq)(2) (1) and Pt(thq)(2) (3) present an important distortion of the square planar (SP-4) geometry toward a two-bladed helix. Bis-homoleptic and some bis-heteroleptic complexes of palladium(II) have also been synthesized. In both cases, the steric interactions between the two ligands cause again a helical distortion rather than yielding trans compounds. For cis-bis(benzo[h]quinoline)platinum(II) (1), in the crystal (monoclinic, space group P2(1)/n, a = 13.728(3) Å, b = 6.9537(15) Å, c = 19.701(5) Å, beta = 106.17(2) degrees, Z = 4, rho(calcd) = 2.028 g.cm(-)(3); diffractometer measurements, block-matrix least-squares refinement, R = 0.043, R(w) = 0.047) the average Pt-N and Pt-C distances are 2.151(6) and 1.988(7) Å, respectively. One benzo[h]quinoline ligand is significantly less planar than the other. For cis-bis[2-(2'-thienyl)quinoline]platinum(II) (3), in the crystal (trigonal, space group P3(2)21, a = b = 9.373(1) Å, c = 20.152(3) Å, Z = 3, rho(calcd) = 2.022 g.cm(-)(3); diffractometer measurements, full-matrix least-squares refinement, R = 0.010, R(w) = 0.010) the molecule has C(2) symmetry and is chiral. The Pt-N and Pt-C bond lengths are 2.156(2) and 1.984(3) Å, respectively. The quinoline moitey is not planar but bent about the fused bond by 6.8 degrees. The thiophene moiety is inclined to the best plane through the quinoline moiety by 24.4 degrees.

Square Planar (SP-4) and Octahedral (OC-6) Complexes of Platinum(II) and -(IV) with Predetermined Chirality at the Metal Center.

cis-Bis-homoleptic platinum(II) complexes, with predetermined helical chirality at the metal center, can be obtained by using strongly sterically interacting ligands. With this aim, two new ligands, (8R,10R)-2-(2'-thienyl)-4,5-pinenopyridine, th4,5ppy (2), and (8R,10R)-2-(2'-thienyl)-5,6-pinenopyridine, th5,6ppy (4), were synthesized and coordinated to platinum. The structures of the resulting complexes, Pt(th4,5ppy)(2) (5) and Pt(th5,6ppy)(2) (6), were determined by X-ray diffraction, and it was found that they both crystallize with a Delta-cis configuration. Thermal oxidative additions (TOA) of alkyl halides were performed with both complexes leading, in the case of 5, to a mixture of isomers and, in the case of 6, to isomerically pure products. The predetermination of chirality at the metal center is therefore preserved in the octahedral (OC-6) platinum(IV) complexes. Crystals of Pt(th4,5ppy)(2) (5) are orthorhombic, of space group P2(1)2(1)2(1), with a = 12.973(1) Å, b = 13.619(2) Å, c = 17.665(2) Å, alpha = beta = gamma = 90 degrees, and Z = 4. Final R = 0.0268 and R(w) = 0.0424 for 3101 observed reflections. Crystals of Pt(th5,6ppy)(2) (6) are hexagonal, of space group P6(1), with a = 11.5465(4) Å, b = 11.5465(4) Å, c = 35.356(3) Å, alpha = beta = 90 degrees, gamma = 120 degrees, and Z = 6. Final R = 0.0424 and R(w) = 0.0845 for 2660 observed reflections. Neither molecule possesses a crystallographic C(2) symmetry.

Predetermined Chirality at Metal Centers.

Asymmetric synthesis in coordination chemistry was described very clearly by Smirnoff in 1920, but, contrary to the development in organic chemistry, it was almost completely neglected for several decades. The interest in chirality in coordination chemistry (see the stereoview of [Ru(bpy)3 ]2+ ) has increased rapidly in recent times as a consequence of developments in several fields where chirality is important (polynuclear systems, supramolecular structures, and enantioselective catalysis). Here we show many examples of how, through the choice of ligand, the configuration of a metal center or the chirality of a helicate can be predetermined.

Completely Stereospecific Self-Assembly of a Circular Helicate.

A nanoscale turbine wheel is an apt description of the structure of the enantiomerically pure helicate [Ag6 (L2)6 ]6+ (1), which precipitates as PF6 salt on mixing dissolved AgPF6 with the ligand L2 (a bis-bidentate ligand comprising two condensed α-pinene/bipyridine units linked by a xylylene bridge). The helicate has an outer diameter of about 3 nm and an inner diameter of 0.84 nm, and is a potential model for the study of stereospecific recognition processes.

Designed Molecules for Self-Assembly: The Controlled Formation of Two Chiral Self-Assembled Polynuclear Species with Predetermined Configuration.

Squaring the cycle: Ligand L1 has been developed for the fabrication of chiral molecular squares, which can also be considered as circular helicates. By using a chirally modified ligand, one of the two forms of the complex can be generated selectively. bpy=2,2'-bipyridin-6-yl.

Stereoselectivity in the formation of tris-diimine complexes of Fe(II), Ru(II), and Os(II) with a C2-symmetric chiral derivative of 2,2'-bipyridine.

A C2-symmetric enantiopure 4,5-bis(pinene)-2,2'-bipyridine ligand (-)-L was used to investigate the diastereoselectivity in the formation of [ML3]2+ coordination species (M = Fe(II), Ru(II), Os(II), Zn(II), Cd(II), Cu(II), Ni(II)), and [ML2Cl2] (M = Ru(II), Os(II)). The X-ray structures of the [ML3]2+ complexes were determined for Delta-[FeL3](PF6)2, Delta-[RuL3](PF6)2, Lambda-[RuL3](PF6)2, Delta-[OsL3](PF6)2, and Lambda-[OsL3](TfO)2. All of these compounds were also characterized by NMR, CD and UV/VIS absorption spectroscopy. The [FeL3]2+ diastereoisomers were studied in equilibrated solutions at various temperatures and in several solvents. The [RuL3]2+ complexes, which are thermally stable up to 200 degrees C, were photochemically equilibrated.

Helicates of chiragen-type ligands and their aptitude for chiral self-recognition.

Two (-)-5,6-pinene-bipyridine moieties connected by a para-xylylene bridge (so-called chiragen-type ligands), (-)-L1, undergo self-assembly upon reaction with equimolar amounts of CuI to form enantiopure circular hexanuclear P-helicates. If both enantiomers of L1 are used, mixtures of P and M hexanuclear helicates are exclusively obtained through a complete chiral recognition; that is, no mixing of the (+) and (-) ligands, respectively, occurs upon complexation. This was proven by a) NMR spectroscopy where identical spectra to those for complexes with the enantiomerically pure ligands were obtained and b) circular dichroism (CD) spectroscopy. The reaction is completely changed by the use of the corresponding meso-L1. Instead of well-defined species, oligomeric mixtures are observed, a result demonstrating the crucial role played by ligand chirality in self-assembly processes. Structural variations on the chiral ligand L1, such as a meta-xylylene bridge instead of a para-xylylene one (in L4) or four pinene groups instead of two (in L5 and L6), favor nondiscrete coordination assembly.

Chiral induction in self-assembled helicates: CHIRAGEN type ligands with ferrocene bridging units.

The aim of this work was the preparation of enantiomerically pure bis(pinene-bipyridine) ligands containing the ferrocenyl moiety. Several such ligands (1-3) were synthesized and completely characterized. These molecules can be diastereoselectively deprotonated at the acidic methylene group of the pinene moiety using a strong and sterically hindered base such as LDA. Subsequent reaction of the formed anion with alkyl halides yield the family of C(2)-symmetric enantiopure compounds (1a-c). Copper(I), silver(I), or zinc(II) complexes with several ligands (C1-C8) were prepared and structurally characterized in the solid state and in solution. Self-assembled helical species are formed in several cases. It became evident that the chiral groups present in the ligand do not completely determined the helical configuration of the assemblages. Diastereoselectivity is thus not complete with this type of ligands, contrary to other, similar ligands studied before.