Robert Stranger

Organometallic complexes for nonlinear optics. Part 27. Syntheses and optical properties of some iron, ruthenium and osmium alkynyl complexes

The syntheses of the alkynyl complexes M(4-CCC6H4NO2)(dppe)(η-C5H5) [M=Fe (1), Ru (2), Os (3)], Os(4-CCC6H4NO2)(PPh3)2(η-C5H5) (4) and Ru(4-CCC6H4NO2)(CO)2(η-C5H5) (5) are reported. Structural studies reveal a decrease in RuC(1) distance on proceeding from 5 to 2, consistent with greater back-donation of electron density to the alkynyl ligand from the more electron-rich metal center in 2. Electrochemical data show that the MII/III couple for the dicarbonyl complex 5 is at a significantly more positive potential than that of the related diphosphine complex 2, consistent with ligand variation modifying the electron richness and hence donor strength of the metal center. Time-dependent density functional calculations on model complexes M(4-CCC6H4NO2)(PH3)2(η-C5H5) (M=Fe, Ru, Os) have been employed to assign the intense low-energy optical transition in these complexes as MLCT in character, the higher energy band being phenyl–phenyl* in nature. Molecular quadratic optical nonlinearities have been measured using the hyper-Rayleigh scattering procedure at 1064 nm. β values vary as Fe≤Ru≤Os for metal variation and CO<phosphines for co-ligand variation, the latter consistent with the variation in donor strength of the metal center inferred from electrochemical and crystallographic data. The observed trend in β on metal variation follows the trend in backbonding energies calculated by DFT.

Rationalizing the 1.9 Å Crystal Structure of Photosystem II—A Remarkable Jahn–Teller Balancing Act Induced by a Single Proton Transfer

Green plants and algae oxidize water to molecular oxygen in photosystem II (PS II) within a calcium/tetramanganese site known as the water-oxidizing complex (WOC). Oxygen is generated by the WOC in a four-electron process involving a series of intermediate states (S states, labeled S0...S4) of increasingly higher mean oxidation level. Over the past decade, X-ray crystallographic (XRD) structures of PS II at progressively improved resolution have revealed much detail of the WOC. At present, only PS II from thermophilic cyanobacteria has been crystallized for XRD study and the enzyme is presumed to be in the dark stable S1 state. The first PS II structure (at 3.5 resolution) to resolve side chain positions was presented by Barber and co-workers. Consistent with subsequent studies at higher resolution, it revealed the compact Mn3Ca “cube” structure of the WOC connected more distantly to a single Mn, referred to as the “dangler”. More recent improved structures at 3.0 and 2.9 , substantially clarified the metaland proteinsupplied ligand positions within the WOC, but were still of insufficient resolution to reveal the positions of bridging oxo groups and water molecules (including the substrate water molecules). Finally, Umena et al., using a new crystallization method, produced an atomic resolution structure at 1.9 , the most resolved to date. Despite this remarkable achievement, revealing, for the first time, the positions of bridging O atoms within the Mn4Ca core of the WOC, aspects of the new structure have been met with scepticism. 4] Central concerns over this structure involve 1) the identity and unexpected placement of the O(5) moiety (Figure 1), which appears to be either a weakly bound oxo, hydroxo, or water ligand at distances of 2.4–2.7 from four of the metal atoms in the WOC, and 2) the disparity in some key metal– metal distances when compared with earlier, high-precision extended X-ray absorption fine structure (EXAFS) results and the previous lower-resolution XRD structures (see Table 1). Although the Mn EXAFS data do not unambigu-

Application of computational chemistry to understanding the structure and mechanism of the Mn catalytic site in photosystem II--a review.

Applications of Density Functional Theory (DFT) computational techniques to studies of the molecular structure and mechanism of the oxygen evolving, water oxidising Mn(4)/Ca catalytic site in Photosystem II are reviewed. We summarise results from the earlier studies (pre 2000) but concentrate mainly on those developments which have occurred since publication of several PS II crystal structures of progressively increasing resolution, starting in 2003. The work of all computational groups actively involved in PS II studies is examined, in the light of direct PS II structural information from X-ray diffraction crystallography and EXAFS on the metals in the catalytic site. We further address the consistency of the various computational models with results from a range of spectroscopic studies on the PS II site, in all of those functionally intermediate states (S-states) amenable to study. Experimental data considered include Mn K-edge XANES studies, hyperfine coupling of Mn nuclei and various ligand nuclei (including those from substrate water) seen by several EPR techniques applied to the net spin half intermediates, S(0) and S(2), at low temperatures. Finally we consider proposed catalytic mechanisms for the O-O bond formation step, from two groups, in the light of the available experimental evidence bearing on this process, which we also summarise.

Dodecanuclear‐Ellipse and Decanuclear‐Wheel Nickel(II) Thiolato Clusters with Efficient Femtosecond Nonlinear Absorption

Thiolate ligands have been of longstanding interest for a variety of reasons: 1) their diverse binding modes give rise to an array of bonding motifs that are of fundamental importance in coordination chemistry, 2) metal–thiolate interactions are key elements of numerous metalloproteins and play a crucial role in the broader field of bioinorganic chemistry, and 3) thiolate-mediated magnetic coupling is an essential component in novel molecular magnets. Amongst the range of metal thiolate-derived structures, the synthesis, structure, and magnetic properties of toroidal (or tiara-like) architectures have raised considerable interest. However, most of the known tiara-like [M(m-SR)]n (M = Ni, Pd) clusters to date have been constructed by single thiolate ligands and possess geometrically similar ring systems, which has restricted, to some extent, the abundance of examples and structural diversity of the tiara family. The development of high-performance molecular materials with optimized nonlinear optical (NLO) properties has also been the focus of much current research. 7] Previous studies have demonstrated that the presence of large pelectron delocalization and a symmetrical planar structure play crucial roles in determining the properties of nonlinear chromophores. 8] Curiously, though, despite the quasiaromatic nature of the bonding that has been proposed in nickel toroidal species, their optical properties are little explored; in particular, no study of the NLO properties is extant. We present here the synthesis of the largest tiara-like nickel(II)–thiolate cluster thus far by a novel route that employs two different thiolate bridges, structural studies that reveal an unprecedented elliptical structure for [Ni(m-StBu)(m-etet)]12 (etet = 2-ethylthioethanethiolate) and two new decanuclear-wheel nickel(II)–thiolato clusters [Ni(m-StBu)(m-pyet)]10 (pyet = 2-(2-mercaptoethyl)pyridine) and [Ni(mStBu)(m-atet)]10 (atet = 2-aminoethanethiol), and the first NLO studies of examples from this important class of molecules, together with time-dependent DFT studies that shed light on the optical behavior. The reaction of NiCl2·6 H2O with 1 equivalent K(etet) gave, after work-up, separable [(CH3C6H5) {Ni(m-StBu)(metet)}10] (1a) and [Ni(m-StBu)(m-etet)]12·(CH3C6H5)2 (1 b), whereas similar reactions with K(pyet) or K(atet), instead of K(etet), afforded [(CH3C6H5) {Ni(m-StBu)(m-pyet)}10]·(CH3C6H5)4 (2) and [(0.5CH3C6H5) {Ni(m-StBu)(m-atet)}10]·(CH3C6H5)2 (3), respectively. The atomic arrangements and stoichiometries of 1 a, 1b, 2, and 3 were unequivocally established from low-temperature CCD area-detector X-ray diffraction studies. The single-crystal X-ray analysis of 1b reveals a heretofore unknown dodecagonal-ellipse Ni12S24 framework, as displayed in Figure 1. The top view (Figure 1 a) of the cyclic Ni12S24 architecture shows that edge-fusion of the 12 localized planar [NiS4] subunits along opposite nonbonding S–S edges gives rise to a triple-layer elliptical geometry that approximately conforms to pseudo-D2 symmetry. The transannular Ni···Ni distances of 1 b are in the range of 11.343(6)–13.528(7) , while the dihedral angles between adjoining [NiS2] planes vary from 140.32 to 157.848 because of the unsymmetrical elliptical geometry of this unique 12membered Ni ring. The side view (Figure 1b) of the toroid 1b [*] Prof. Dr. C. Zhang, Dr. S. C. Meng, Dr. J. F. Zhang Molecular Materials Research Center, Scientific Research Academy, School of Chemistry and Chemical Engineering, Jiangsu University Zhenjiang 212013 (P.R. China) Fax: (+ 86)511-8879-7815 E-mail: chizhang@ujs.edu.cn

Cleavage of Carbon Dioxide by an Iridium-Supported Fischer Carbene. A DFT Investigation

The reaction of CO(2), OCS, and PhNCO with an iridium-supported Fischer alkoxycarbene has been investigated with density functional theory. We have confirmed the mechanism for the important CO(2) reaction and successfully rationalized the selective cleavage of the CS and CN bonds in OCS and PhNCO. Armed with this information we have used our model to predict that the same iridium system will preferentially cleave the CS bond in methyl thiocyanate (MeNCS) rather than the CN bond. The formation of the iridium-supported carbene itself has also been investigated and a fascinating autocatalytic mechanism has been discovered which nicely fits the observed experimental behavior.

Activation of CS2 and CS by ML3 Complexes

The aim of this study was to determine the best neutral ML3 metal complexes for activating and cleaving the multiple bonds in CS2 and CS. Current experimental results show that, so far, only one bond in CS2 can be cleaved, and that CS can be activated but the bond is not broken. In the work described in this paper, density functional theory calculations have been used to evaluate the effectiveness of different ML3 complexes to activate the C-S bonds in CS2 and CS, with M = Mo, Re, W, and Ta and L = NH2. These calculations show that the combination of Re and Ta in the L3Re/CS2/TaL3 complex would be the most promising system for the cleavage of both C-S bonds of CS2. The reaction to cleave both C-S bonds is predicted to be exothermic by about 700 kJ mol(-1) and to proceed in an almost barrierless fashion. In addition, we are able to rationalize why the breaking of the C-S bond in CS has not been observed experimentally with M = Mo: this reaction is strongly endothermic. There is a subtle interplay between charge transfer and pi back-donation, and it appears that the Mo-C and Mo-S bonds are not strong enough to compensate for the breaking of the C-S bond. Our results suggest that, instead, CS could be cleaved with ReL3 or, even better, with a combination of ReL3 and TaL3. Molecular orbitals and Mulliken charges have been used to help explain these trends and to make predictions about the most promising systems for future experimental exploration.

Organometallic complexes for nonlinear optics. 42. Syntheses, linear, and nonlinear optical properties of ligated metal-functionalized oligo(p-phenyleneethynylene)s.

A combination of UV-vis-NIR spectroscopy, femtosecond Z-scan measurements, and time-dependent density functional theory (TD-DFT) calculations have been used to comprehensively investigate the linear optical and nonlinear optical (NLO) properties of pi-delocalizable metal-functionalized oligo(phenyleneethynylene)s. A range of unsymmetrically or symmetrically end-functionalized mono-, di-, tri-, penta-, hepta-, and nona(phenyleneethynylene)s were synthesized, with larger examples bearing varying numbers of 2,5-di(hexyloxy)phenyl groups to ensure sufficient solubility of the metal complex derivatives. The effect of incorporating varying numbers of solubilizing substituents in the OPE bridge, peripheral group modification, OPE lengthening, coligand variation, and metal location in the OPE on the linear optical properties has been established, with the first three molecular modifications resulting in significant changes in the optical absorption maxima. TD-DFT calculations reveal that the most intense transition in the linear optical spectra is localized on the OPE bridge and involves excitation from acetylenic to cumulenic molecular orbitals that are not greatly spatially separated from one another. The nonlinear optical properties are dominated by two-photon absorption, which for all but 1,4-{trans-[RuCl(dppm)(2)]C[triple bond]C}(2)C(6)H(4) appears as a band around 11,400 cm(-1) and a sharp increase of nonlinear absorption at frequencies >17,000 cm(-1). Surprisingly, there is relatively little influence of the length of the OPE bridge on the magnitude of the two-photon absorption cross sections, which are in the range 300-1000 GM.

Structural, Magnetic Coupling and Oxidation State Trends in Models of the CaMn4 Cluster in Photosystem II

Density functional theory calculations are reported on a set of isomeric structures I, II and III that share the structural formula [CaMn4C9H10N2O16]q+.(H2O)3 (q= -1, 0, 1, 2, 3). Species I has a skeletal structure, which has been previously identified as a close match to the ligated CaMn4 cluster in Photosystem II, as characterized in the most recent 3.0 angstroms crystal structure. Structures II and III are rearrangements of I, which largely retain that models bridging ligand framework, but feature metal atom positions broadly consistent with, respectively, the earlier 3.5 and 3.2 angstroms crystal structures for the Photosystem II water-oxidising complex (WOC). Our study explores the influence of the cluster charge state (and hence S state) on several important properties of the model structures; including the relative energies of the three models, their interconversion, trends in the individual Mn oxidation states, preferred hydration sites and favoured modes of magnetic coupling between the manganese atoms. We find that, for several of the explored cluster charge states, modest differences in the bridging-ligand geometry exert a powerful influence over the individual manganese oxidation states, but throughout these states the robustness of the tetrahedron formed by the Ca and three of the Mn atoms remains a significant feature and contrasts with the positional flexibility of the fourth Mn atom. Although structure I is lowest in energy for most S states, the energy differences between structures for a given S state are not large. Overall, structure II provides a better match for the EXAFS derived metal-metal distance parameters for the earlier S states (S0 to S2), but not for S3 in which a significant structural change is observed experimentally. In this S state structure III provides a closer fit. The implications of these results, for the possible action of the WOC, are discussed.

Organometallic complexes for nonlinear optics. Part 32: Synthesis, optical spectroscopy and theoretical studies of some osmium alkynyl complexes

Abstract The complexes trans -[Os(CCPh)Cl(dppe) 2 ] ( 1 ), trans -[Os(4-CCC 6 H 4 CCPh)Cl(dppe) 2 ] ( 2 ), and 1,3,5-{ trans -[OsCl(dppe) 2 (4-CCC 6 H 4 CC)]} 3 C 6 H 3 ( 3 ) have been prepared. Cyclic voltammetric studies reveal a quasi-reversible oxidation process for each complex at 0.36–0.39 V (with respect to the ferrocene/ferrocenium couple at 0.56 V), assigned to the Os II/III couple. In situ oxidation of 1 – 3 using an optically transparent thin-layer electrochemical (OTTLE) cell affords the UV–Vis–NIR spectra of the corresponding cationic complexes 1 + – 3 + ; a low-energy band is observed in the near-IR region (11 000–14 000 cm −1 ) in each case, in contrast to the neutral complexes 1 – 3 which are optically transparent below 20 000 cm −1 . Density functional theory calculations on the model compounds trans -[Os(CCPh)Cl(PH 3 ) 4 ] and trans -[Os(4-CCC 6 H 4 CCPh)Cl(PH 3 ) 4 ] have been used to rationalize the observed optical spectra and suggest that the low-energy bands in the spectra of the cationic complexes can be assigned to transitions involving orbitals delocalized over the metal, chloro and alkynyl ligands. These intense bands have potential utility in switching nonlinear optical response, of interest in optical technology.

Dinitrogen activation in sterically-hindered three-coordinate transition metal complexes.

Dinuclear metal systems based on sterically-hindered, three-coordinate transition metal complexes of the type ML3 where the ancillary ligands L comprise bulky organic substituents, hold great promise synthetically for the activation and scission of small, multiply-bonded molecules such as N2, NO and N2O. In this study we have employed density functional methods to identify the metal/ligand combinations which achieve optimum activation and/or cleavage of N2. Strong pi donor ligands such as NH2 and OH are found to produce the greatest level of activation based on N-N bond lengths in the intermediate dimer complex, L3Mo(mu-N2)MoL3, whereas systems containing the weak or non-pi donor ligands NH3, PH3, OH2 and SH2 are found to be thermodynamically unfavourable for N2 activation. In the case of the Mo-NH2 and W-NH2 systems, a fragment bonding analysis reveals that the orientation of the amide ligands around the metal is important in determining both the spin state and the extent of dinitrogen activation in the intermediate dimer. For both systems, an intermediate dimer structure where one of the NH2 ligands on each metal is rotated 90 degrees relative to the other ligands, is more activated than the structure in which the NH2 ligands are trigonally disposed around the metals. The level of activation is found to be very sensitive to the electronic configuration of the metal with d3 metal ions delivering the best activation along any one transition series. In particular, strong activation or cleavage of N2 was calculated for the third row d3 metals systems involving Ta(II), W(III) and Re(IV), with the level of activation decreasing as the nuclear charge on the metal increases. This trend in activation reflects the size of the valence 5d orbitals and consequently, the capacity of the metal to back donate into the dinitrogen pi* orbitals.