Jürgen Heck

Mono- and dinuclear sesquifulvalene complexes, organometallic materials with large nonlinear optical properties

Abstract Currently, the most important class of nonlinear optical (NLO) compounds providing second harmonic generation (SHG) comprises of electron donating and accepting groups which are electronically coupled. These kinds of dipolar compounds have potential in photonic applications. In this article the synthesis and the NLO activities of dipolar mono- and dinuclear sandwich-like complexes are reviewed. These structures are composed of sesquifulvalenes and metal–ligand fragments ML n and M′L′ n which stabilize the aromatic units of the sesquifulvalene moiety. In these complexes the five-membered ring of the sesquifulvalene complexes is coordinated to ML n =Fe(C 5 H 5 ), Fe(C 5 Me 5 ), Ru(C 5 H 5 ) and Co(C 4 Ar 4 ) and acts as an electron donor; hence, its electron donating capability is varied by changing the ML n entity. The electron withdrawing capability of the seven-membered ring is changed by the coordination of Cr(CO) 3 and RuCp + or by keeping it uncoordinated. In addition, cationic borabenzene complexes are introduced as electron accepting groups. The electronic communication between the donor and acceptor is facilitated by a direct coupling or by introducing acetylenic and olefinic bridges between the donor and acceptor, e.g. E -1,2-ethendiyl, E , E -1,3-diendiyl, E , E , E ,-1,5-triendiyl, 2,5-thiendiyl, 2,5′-dithiendiyl. Structural as well as UV–vis spectroscopic studies indicate a distinct mutual influence of the donor and the acceptor of the sesquifulvalene complexes. The NLO properties of these mono- and dinuclear sandwich-type complexes are studied by means of hyper-Rayleigh scattering experiments. They prove that most of the ferrocenyl derivatives demonstrate fluorescence in the region of the SHG due to two-photon absorption whereas all of the ruthenium congeners show SHG which is quantified by determination of the first hyperpolarizability. The measured β -values are large and their magnitude can be partly explained by resonance enhancement. Taking the two-level model into account the calculated values of the static hyperpolarizability are still very large indicating that mono- and bimetallic sesquifulvalene complexes are powerful NLO-chromophores.

Second Harmonic Generation and Two-Photon Fluorescence as Nonlinear Optical Properties of Dipolar Mononuclear Sesquifulvalene Complexes

In order to investigate nonlinear optical properties, in particular second harmonic generation (SHG) of organometallic complexes, numerous dipolar monocationic sesquifulvalene complexes of the general form [Mc–Z–C7H6]+ have been synthesised (Mc = metallocenyl), wherein the metallocenyl moiety Mc acts as an electron donor, and the tropylium cation C7H6+ as an electron acceptor. The mutual electronic influence of the donor and the acceptor groups is warranted by a linking spacer Z, which is a single bond [Mc = CpFeC5H4 (4a); Mc = CpRuC5H4 (4b)], an unsaturated bridge containing olefins [Z = (E-CHCH)n: n = 1, Mc = CpFeC5H4 (25a), Mc = CpRuC5H4 (25b); n = 2, Mc = CpFeC5H4 (26); n = 3, Mc = CpFeC5H4 (27)], or thiophene units [Z = 2,5-C4H2S, Mc = CpFeC5H4 (28); Z = 5,5′-(2,2′-C4H2S)2, Mc = CpFeC5H4 (29); Z = 2-(5-E-CHCH)C4H2S, Mc = CpFeC5H4 (30)]. For the salts of 4a·BF4, 4b·PF6 and 25a·PF6 X-ray structure determinations have been performed [4a·BF4: orthorhombic, Pnma, = 21.75(2), b = 9.900(2), c = 6.881(3) A, V = 1482.0(15) A3, Z = 4; 4b·PF6: monoclinic, P2(1)/c, a = 8.104(3), b = 18.206(14), c = 11.228(4) A, β = 107.59(3), V = 1579.1(15) A3, Z = 4; 25a·PF6: triclinic, P1bar, a = 10.067(6), b = 10.496(6), c = 11.418(6) A, α = 94.07(4), β = 110.96(4), γ = 102.88(5)°, V = 1083.1(11) A3, Z = 2]; the solid state structures indicate an almost coplanar arrangement of the organic π-system. Cyclic voltammetry studies reveal an irreversible one-electron reduction and an electrochemically reversible one-electron oxidation step for the ferrocenyl derivatives, whereas the ruthenocenyl derivatives demonstrate an irreversible two-electron oxidation. The redox potentials clearly indicate that the oxidation occurs at the metallocene unit, and the reduction is localised on the tropylium entity. In the electronic absorption spectra two intense bands are observed in the region 400 < λ < 900 nm which undergo strong negative solvatochromic shifts. The origin of the high-energy absorption band is assumed to be an interligand charge-transfer (LL–CT) transition, and the low-energy absorption band is assigned to a donor-acceptor charge-transfer (DA–CT) transition. Whereas the LL–CT is continuously shifted to lower energy with increasing spacer length, the energy of the DA–CT approaches a limiting value. A comparable phenomenon is also observed for the difference between the oxidation and reduction potentials obtained from electrochemical studies. Investigations concerning the nonlinear optical properties of the dipolar cationic sesquifulvalene complexes by means of hyper-Rayleigh scattering (HRS) indicate that the ferrocenyl derivatives fluoresce due to two-photon absorption whereas the ruthenocenyl congeners exhibit second harmonic generation with considerably larger first hyperpolarizability which is partly resonance-enhancement based.

Second-Order Nonlinear Optical Activity of Dipolar Chromophores Based on Pyrrole-Hydrazono Donor Moieties

A series of highly efficient and thermally stable second-order nonlinear optical (NLO) dipolar donor-auxiliary donor-acceptor chromophores have been synthesised in which a hydrazono group and a pyrrole ring act as donor and auxiliary donor components, respectively, in combination with different aromatic and heteroaromatic acceptors. The new dyes have been systematically investigated by NMR spectroscopy, absorption spectroscopy, NLO measurements and thermal stability studies. NLO properties have been studied in detail by electro-optical absorption (EOA) and hyper-Rayleigh scattering (HRS) measurements in 1,4-dioxane and DMSO, respectively. The results originating from the two different methods have been compared and analysed in detail. We found that the NLO properties measured by the EOA and HRS methods correlate with each other and converge to reveal the dye with the acceptor 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran as the most efficient system. The unprecedented combination of a strong donor hydrazono group and the auxiliary donor effects of pi-excessive heteroaromatic rings afforded NLO chromophores with very high values (mu(g)beta0(EOA) up to 2038 x 10(-48) esu and beta(HRS) up to 3980 x 10(-30) esu at 1.5 microm).

μ-[Bis(η5-cyclopentadienyl)dimethylsilyl]bis(tricarbonylwolfram), ein über cyclopentadienylliganden verbrücktes bis(η5-cyclopentadienyltricarbonylwolfram)

Abstract The reaction of W(CO)3(EtCN)3 with (C5H5)2Si(CH3)2 leads to the Cp-bridged, binuclear hydrido complex [W(CO)3H]2-μ-[(η5-C5H4)2Si(CH3)2] as the main product, whereas only [W(CO)3]2-μ-(η5-C5H4)2Si(CH3)2] (I) is formed as the minor product. Much better yields of I can be obtained in the reaction of W(CO)3(EtCN)3 with the lithium salt Li2[(C5H4)2Si(CH3)2] and subsequent oxidation. As pointed out by X-ray structure analysis complex I has a cis-configuration with C2-symmetry due to the linked Cp ligands. In the chiral complex a topomerism can be proved by temperature-dependent NMR spectroscopy.

Kooperative Wirkung in π-ligandverbrückten Zweikernkomplexen: V. C5Me5 anstatt C5H5: Ergebnisse aus Versuchen zur Synthese zweikerniger μ-cot-komplexe vom typ (CpCr)2-μ-cot sowie aus untersuchungen zur moleküldynamik in (CpCr)2-μC8H8 und CpCrCot☆

The known dinuclear complex (CpCr)2-μ-C8H8 (Cp  cyclopentadienyl) is prepared by a one-pot reaction of CrCl2(TMEDA) (TMEDA  bis(N, N-dimethylamino) ethane) with NaCp and K2Cot (Cot  cyclooctatetraene), Thermolysis of the product in mesitylene leads to the Cot-bridged compound (CpCr)2-μ-Cot. From a detailed temperature-dependent 1H NMR study the molecular fluxionality of (CpCr)-μ-C8H8 is interpreted as a singlet-triplet equilibrium: ΔH = 21.0 kJ mol−, ΔS = 31.4 J mol− K−. Whereas the mononuclear species CpCrCot is only a low-yield by-product in the former reaction, its methylated derivative Cp*CrCot (Cp*  C5Me5) becomes the main product if LiCp* is used instead of NaCp. Complementary ESR and NMR investigations demonstrate that both of the mononuclear compounds can be regarded as classical mixed sandwich complexes of the type (η5-Cp)Cr(η6-cyclopolyolefine) which do not show any structural fluxionality; the hapticity of the Cot ligands is definitely fixed upon the time scale of the applied spectroscopic methods. The permethylation of the Cp ligands shows no appreciable influence on the ESR parameter of the Cot ligand.

Structure−Property Dependence of the First Hyperpolarisabilities of Organometallic Merocyanines Based on the μ-Vinylcarbynediiron Acceptor and Ferrocene Donor

In order to investigate the structure−property relationship of nonlinear optical materials, a series of organometallic chromophores were synthesised utilising the [Fe2(η-C5H5)2(CO)2(μ-CO)(μ-C−)]+ electron-accepting moiety and the ferrocenyl group, Fc, as the electron donor. The π-linker between these two termini was systematically modified and the mutual electronic communication between them was determined using IR, NMR, and electronic absorption spectroscopy. An X-ray structure determination of [Fe2(η-C5H5)2(CO)2(μ-CO)(μ-C−CH=CH−CH=C(Cl)−Fc)][BF4] confirmed the strong electronic interaction between the donor and the acceptor with reduced π-bridge bond-length alternation. The nonlinear optical properties of these complexes were examined using the hyper Rayleigh scattering technique. The experimental first hyperpolarisabilities are some of the highest obtained for ferrocenyl chromophores and, significantly, no enhancement was found due to two-photon absorption fluorescence. When polyene linkers −(CH=CH)n− are used, the values for β0 increase with a ca. n1.5 dependence with no sign of saturation up to n = 4. However, the highest values for β and βo were obtained for linkers which contained an aromatic ring as opposed to pure polyenes and in this respect a benzene ring was more effective than a thiophene or furan. Consequently, the higher β and β0 are not exhibited by those merocyanines with the highest values for λmax. It is concluded for these compounds that a low excitation energy Eeg and a large transition moment M for the electronic excitation are less important than a large change in the dipole moment Δµeg. Furthermore, a chloro substituent on the olefinic double bond proximate to the ferrocenyl group has a dramatic effect on the β and β0 values.

Pentacarbonyl(cyclobutenyliden)chrom-Komplexe — Synthese, Struktur und nichtlineare optische Eigenschaften

Abstract The pentacarbonyl(vinylidene)chromium complexes [(CO) 5 Cr=C=C(R)R′] ( 1 ) [C(R)R′ = CPh 2 ( a ), C(CH 2 ) 5 ( b ), CMe 2 ( c ), C(Me)H ( d )] react with 1-diethylamino-1-propyne, MeC≡CNEt 2 , by regiospecific addition of the C≡C bond of the alkyne to the C=C bond of the vinylidene ligand to form the cyclobutenylidene complexes ( 2a–d ). The analogous reaction of [(CO) 5 Cr=C=C(Ph)H] with MeC≡CNEt 2 affords via cycloaddition and formal [1,3]-H shift the complex ( 3 ). The cycloadditions of Me 2 NC≡CNMe 2 and MeC≡COEt to 1b give ( 4 ) and ( 5 ) respectively. The addition of ( S )-2-methoxymethyl( N -1-propynyl)pyrrolidine ( 6 ) and ( R )-methyl(1-propynyl)(1-phenylethyl)amine ( 8 ) with 1a–c leads to E/Z mixtures of [R * = ( S )-2-methoxymethyl pyrrolidinyl ( 7a–c ), R * = ( R )-N(Me)[CH(Me)Ph] ( 9a–c )]. In the reactions of 1a–c with 6 predominantly the E isomer, in the reactions of 1a–c with 8 predominantly the Z isomer is formed. All 3-amino substituted complexes exhibit a considerable solvatochromic behavior. The first hyperpolarizability β of 2b was determined by hyper-Rayleigh scattering ( β = 22 × 10 −30 esu). The Kurtz powder method was used to investigate the effectiveness in second harmonic generation of 2d , 3 , 7a , 7b , and 9c . Their efficiencies range from 4 to 35 times that of urea. The structures of 2b and Z- 9b were established by X-ray structural analyses. Together with the results of spectroscopic investigations, they reveal a strongly delocalized π -system [(CO) 5 Cr=C=C=C=N].

Donor−Acceptor Interaction in Cationic Archetype Mono- and Dinuclear Sesquifulvalene Complexes [(η5-C5H5)Fe{µ-(η5-C5H4)(η7-C7H6)}M′L′]n+ (n = 1, 2)

In order to gain a deeper insight into structure-property relationships of the donor (D)−acceptor (A) interactions in cationic sesquifulvalene complexes the mono- and dicationic complexes [(η5-C5H5)Fe{µ-(η5-C5H4)(η7-C7H6)}M′L′]+ were synthesized as BF4 and PF6 salts: M′L′ = Cr(CO)3 (3PF6); M′L′ = none (4BF4); M′L′ = Ru(η5-C5H5)+ [5(PF6)2]; M′L′ = Ru(η5-C5Me5)+ [6(PF6)2]. The ferrocenyl substituent works as the electron donor and the tropylium moiety as the electron acceptor. For the electronic ground state the NMR spectroscopic results demonstrate an increasing charge delocalization in the order M′L′ = Cr(CO)3 < Ru(η5-C5Me5)+ < Ru(η5-C5H5) < none. The same order of the acceptor moieties is observed for the difference between the oxidation and reduction potentials, which are related to an electrochemically reversible and a complete irreversible one-electron transfer, respectively. A comparison of the structural data of 3−6 reveals a small but distinct contribution of the cross-conjugated mesomeric form to the bis(aromatic) mesomeric form in the ground state of the sesquifulvalene complexes, which increases, when the electron-withdrawing ability of the acceptor becomes stronger. The UV/Vis spectra are dominated by two intense absorption bands in the visible and near-UV region, which are assigned to a low-energy donor(metal)−acceptor charge-transfer (D−A CT) transition and to a high-energy donor(ligand)−acceptor charge-transfer (L−A CT) transition. For these archetype sesquifulvalene complexes the first hyperpolarizability β was determined by means of hyper-Rayleigh scattering (HRS). Whereas for 3 no second harmonic signal could be detected and 4 shows fluorescence due to two-photon absorptions, 5 and 6 possess appreciable β values.

Synthesis and Properties of a Novel Series of Organometallic Merocyanines Combining the Potent Electron‐Donating [(CpFeCO)2(μ‐CO)(μ‐C=CH−)] Fragment with Tropylium‐Type Acceptors

The potent electron-donating group [(CpFeCO)2(µ-CO)(µ-C=CH−)] was combined with different tropylium-based acceptors yielding the push-pull complexes [(CpFeCO)2(µ-CO)(µ-C=CH−CH=A)][BF4] where =A represents [=CH−(η7-C7H6)Cr(CO)3]+ (5), [={C10H7}]+ (azulenylium) (7) [={C10H4Me2iPr}]+ (guaiazulenylium) (8). Crystal structures of the precursor complexes [(CpFeCO)2(µ-CO)(µ-C=CH−CH=CH−C7H7)] (3), [(CpFeCO)2(µ-CO){µ-C=CH−CH=CH−(η6-C7H7)Cr(CO)3}] (4) and the push-pull derivative 5 are presented. Whereas complexes 3 and 4 demonstrate localised carbon−carbon double and single bonds in the π-bridge between the diiron moiety and the seven-membered ring a small but distinct π-bond delocalisation can be observed in the π-linker of the cationic derivative 5. From the NMR spectroscopic results a correlation between the 13C NMR shift of the bridging carbon atom µ-C of the diiron unit and the coupling constant J(1H-1H) of the β- and γ-protons of the π-bridge was found. This correlation indicates an increased π-bond delocalisation throughout the conjugated bridge with respect to the electron-accepting capability in the order tricarbonyl(η7-cycloheptatrienylium)chromium < guaiazulenylium < azulenylium implying a predominantly charge-delocalised form in the case of the azulenylium complex 7. Considerably large first hyperpolarisabilities β were determined for 5 and 8 by means of hyper-Rayleigh scattering. Based on the two-level model β0 values were calculated and related to the bond-length alternation in the π-linker. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Organometallic Supramolecular Chemistry with Monosaccharides: Triethylammoniumμ-Chloro-bis{chloro(η5-cyclopentadienyl)(methyl 4,6-O-benzylidene-β-D-glucopyranosidato-1κO2,1:2κO3)zirconate}

The reaction of [CpZrCl3(thf)2] with methyl 4,6-O-benzylidene-beta-D-glucopyranoside (beta-MeBGH2, 1) in the presence of Et3N results in the formation of the zirconate complex [Et3NH] [(CpZrCl)2(mu-Cl) (mu-(beta-MeBG)]2] (2). X-ray structure analyses were performed from the ligand precursor beta-MeBGH2 1 as well as from 2. Compound 1 crystallizes in the monoclinic chiral space group P2(1). The molecules show a flat arrangement including the benzylidene protecting group, and are packed in columns. The columns are held together in pairs by the formation of hydrogen bonds between the hydroxy functions in positions 2 and 3. Compound 2 crystallizes in the orthorhombic space group P2(1)2(1)2(1). The beta-MeBG ligands are chelating the Zr atoms through the oxygen atoms in positions 2 and 3 of the glucopyranosidato ligand revealing a 1-zircona-2,5-dioxolane moiety each; the oxygen atom in position 3 is linked to both of the Zr atoms. Additionally one chloro ligand is bridging the two Zr centers. Two terminally bound chloro ligands stick out from the two Zr atoms into a chiral U-shaped cavity constructed by the two beta-MeBG ligands. The cavity incorporates the tertiary ammonium cation [Et3NH]+ which is bound to one of the terminal chloro ligands through a hydrogen bond. The inclusion of the [Et3NH]+ cation in the U-shaped cavity, even in solution, is demonstrated by NMR spectroscopic data.