Two new programs have been developed for searching the Cambridge Structural Database (CSD) and visualizing database entries: ConQuest and Mercury. The former is a new search interface to the CSD, the latter is a high-performance crystal-structure visualizer with extensive facilities for exploring networks of intermolecular contacts. Particular emphasis has been placed on making the programs as intuitive as possible. Both ConQuest and Mercury run under Windows and various types of Unix, including Linux.

The crystallographically determined bond length, valence angle, and torsion angle information in the Cambridge Structural Database (CSD) has many uses. However, accessing it by means of conventional substructure searching requires nontrivial user intervention. In consequence, these valuable data have been underutilized and have not been directly accessible to client applications. The situation has been remedied by development of a new program (Mogul) for automated retrieval of molecular geometry data from the CSD. The program uses a system of keys to encode the chemical environments of fragments (bonds, valence angles, and acyclic torsions) from CSD structures. Fragments with identical keys are deemed to be chemically identical and are grouped together, and the distribution of the appropriate geometrical parameter (bond length, valence angle, or torsion angle) is computed and stored. Use of a search tree indexed on key values, together with a novel similarity calculation, then enables the distribution matching any given query fragment (or the distributions most closely matching, if an adequate exact match is unavailable) to be found easily and with no user intervention. Validation experiments indicate that, with rare exceptions, search results afford precise and unbiased estimates of molecular geometrical preferences. Such estimates may be used, for example, to validate the geometries of libraries of modeled molecules or of newly determined crystal structures or to assist structure solution from low-resolution (e.g. powder diffraction) X-ray data.

Abstract:The complexes [ (P,)Rh(hfacac)] 1 [P, = R,P-(X)-PR,] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(PJRh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable-temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X-ray crystallography. The steric repulsion of the PR, groups within the chelating fragment was found to significantly influence the coordination geometry of [(P,)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P-Rh-P angles in the solid state and the lo3Rh Keywords: carbon dioxide activation * homogeneous catalysis ligand effects * molecular modeling * rhodium chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [ (PJRh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO, to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.

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Experimental and theoretical methods have been employed to investigate the influence of the chelating phosphine ligand on the 103Rh chemical shift in complexes containing the [(P2)Rh] fragment (P2 = chelating bidentate phosphine). The δ(103Rh) values obtained by 2D(31P,103Rh{1H}) NMR spectroscopy for a series of neutral rhodium complexes [{R2P(CH2) n PR2}Rh(hfacac)] (R = Ar, Ph, Cy, Me, n = 1−4, hfacac = hexafluoroacetylacetonate) have been compared. Systematic variation of the phosphine ligand has allowed separation of electronic and geometrical effects. The purely electronic influence of para substituents in complexes [{(p-XC6H4)2P(CH2)4P(p-XC6H4)2}Rh(hfacac)] correlates directly with the Hammett σP constants of X, but leads to variations in the chemical shift of less than 80 ppm between X = CF3 and X = OMe. In contrast, geometrical changes in complexes [(P2)Rh(hfacac)] lead to variations in the chemical shift over a range of approximately 800 ppm. The individual contributions of various structural parameters on the δ(103Rh) values have been assessed by density-functional-based calculations for suitable model compounds. The same approach has been extended to the rationalization of the trends in 103Rh chemical shifts of cationic complexes with four P donor ligands around a Rh(+I) center and selected anionic Rh(−I) complexes [(P2)2Rh]-. This analysis allows for the first time a direct corroboration of geometrical variations and their effect on 103Rh chemical shifts, demonstrating that correlations of reactivity with 103Rh chemical shifts can give valuable information on structure/reactivity relationships.

Light-induced cyclizations of suitably functionalized polyalkene terpenoids, such as geranyl, all-tvatzs-farnesyl, and all-trans-geranylgeranyl derivatives, via formation of radical cations are proven to be a powerful method for the single-step synthesis of mono-and mostly all-trans-fused polycyclic compounds from readily available precursors. Whereas some of these highly stereo-and chemoselective transformations required the use of micellar media, they can now be conveniently performed in homogeneous solutions upon suitable choice of the electron acceptors and of the functionality pattern of the polyalkene substrates. Moreover, the mode of cyclization, i.e., 6-vs. 5-membered ring formation and termination of the cyclization cascades, are steered efficiently by the substituents of the polyalkenes (polyalkenyl acetate vs. cc, P -unsaturated ethyl polyalkenoate and polyalkene-1,ldicarbonitrile). At the same time, the protic solvents used add highly stereoselectively to the w-alkene sites of the polyalkens in anti-Markovnikov sense which strongly suggests that radical cations are intercepted. Interestingly, the transformations achieved here upon photoelectron transfer parallel the biosynthetic paths of non-oxidative terpene cyclizations which are thought to occur purely by protonation of the isoprenoid polyalkenes.

Displacement of the ethene ligand in (dippe)Pd(C2H4) (dippe = iPr2PC2H4PiPr2) by 1-alkynes RC⋮CH affords the mononuclear complexes (dippe)Pd(RC⋮CH) (R = Me (2a), Ph (3a), CO2Me (4), SiMe3 (5)). The molecular structure of 3a has been determined by X-ray crystallography. Mononuclear 2a and 3a have been reacted with stoichiometric amounts of (dippe)Pd(η1-C3H5)2 as a source for [(dippe)Pd0] to yield the dinuclear derivatives {(dippe)Pd}2(μ-RC⋮CH) (R = Me (2b), Ph (3b)). By the reaction of (dippe)Pd(C2H4) with difunctional vinylacetylene the mononuclear complex (dippe)Pd{(1,2-η2)-RC⋮CH} (R = CHCH2 (6a)) is formed, which is in equilibrium with isomeric (dippe)Pd{(3,4-η2)-H2CCHC⋮CH} (6b). Addition of [(dippe)Pd0] to 6a,b yields dinuclear {(dippe)Pd}2(μ-RC⋮CH) (R = CHCH2 (6c)). Reaction of (dippe)Pd(C2H4) with butadiyne affords (dippe)Pd(η2-HC⋮CC⋮CH) (7c). From dippe, Pt(cod)2, and C4H2 the Pt homologue has also been synthesized and thus, together with the already known Ni derivative, the series (dippe)M(η2-HC⋮CC⋮CH) (M = Ni (7a), Pd (7c), Pt (7f)) is now complete. When 7c and [(dippe)Pd0] are combined, the dinuclear complex {(dippe)Pd}2(μ-RC⋮CH) (R = C⋮CH (7e)) is formed in solution, whereas isomeric {(dippe)Pd}2{μ-(1,2-η2):(3,4-η2)-HC⋮CC⋮CH} (7d) is present in the solid state. The preparation of the Pd0−1-alkyne complexes refutes the conventional wisdom that this type of compound is inherently unstable. By reaction of (dippe)Pd(C2H4) with internal alkynes C2R2 the complexes (dippe)Pd(RC⋮CR) (R = Me (8a), Ph (9), CO2Me (10), SiMe3 (11)) have also been prepared. Combining 8a with [(dippe)Pd0] affords dinuclear {(dippe)Pd}2(μ-MeC⋮CMe) (8b). Finally, solution thermolysis of 2b and 8b gives rise to dinuclear alkyne-free Pd2(dippe)2 (12).

The synthesis and characterization of complexes [(P∩P)2Rh][hfacac] (P∩P = chelating bidentate phosphine ligand R2P(CH2)nPR2 (2a-g), hfacac = hexafluoroacetylacetonate anion) (4) is reported. The molecular structures of 4a (R = Ph, n = 1) and 4f (R = Cy, n = 2) in the solid state were determined by single-crystal X-ray diffraction. The complexes crystallize in the monoclinic space groups C2/c (No. 15) and P21/n (No. 14), respectively. No coordinative interaction between the rhodium center of the cation [(P∩P)2Rh]+ (4a+, 4f+) and the hfacac anion is evident in either cases. In the crystal structure of 4a, hydrogen bonds between the oxygen atoms of the hfacac anion and methylene protons of the CH2 bridges of the phosphine ligand lead to highly symmetric chains of regularly alternating cations and anions. The coordination geometry around the rhodium center in 4a+ is ideally square-planar, whereas 4f+ is significantly distorted towards a tetrahedron with an angle between the two P2Rh moieties of 18.6°. The cation 4b+ (R = Cy, n = 1) was investigated in form of the tetrafluoroborate salt for comparison. The compound [{Cy2P(CH2)PCy2}2Rh][BF4] crystallizes as a THF solvate (4b′) in the triclinic space group P[Formula: see text] (No. 2) containing ideally square-planar [(P∩P)2Rh]+ cations. Key words: rhodium, chelating ligands, coordination modes, 1,3-diketonates, phosphorus ligands.