Synthesis and photophysical properties of new chromium(III) complexes of N-derivatized 1,4,8,11-tetraazacyclotetradecane ligands cis-[Cr(1,8-R(2)cyclam)Cl(2)]Cl,where R is a pendant chromophore. exclusive formation of the cis isomer

Several new chromium(III) complexes have been synthesized utilizing derivatives of the macrocyclic ligand cyclam (1,4,8,11-tetraazacyclotetradecane) with various N-substituted chromophores in the 1 and 8 positions (1,8-R(2)cyclam) where R = CH(3), CH(2)Ph, CH(2)Nph (Nph = naphthyl), and CH(2)Anth (Anth = anthracyl). X-ray crystal structures were determined for all four complexes, and these are formed exclusively in the cis configuration with the two tertiary amines in the "hinge" positions (i.e., along the folding axis) of the coordinated ligand. As a result, the cis isomers appear to be inert to isomerization under conditions dramatically more forcing than needed to effect the cis to trans isomerization of the unsubstituted Cr(cyclam)Cl(2)(+) ion. Photophysical studies demonstrated that emission occurs solely from the metal-centered ligand field doublet excited states regardless of whether initial excitation is into the quartet ligand field bands or into the pi-pi bands of the pendant chromophore. Thus, excitation of the pendant chromophore results in efficient intramolecular energy transfer to the metal centered ligand field excited states.     

Spectroscopic characterization of chromium(III), manganese(II) and nickel(II) complexes with a nitrogen donor tetradentate, 12-membered azamacrocyclic ligand

The complexes of Cr(III), Mn(II) and Ni(II) were synthesized with macrocyclic ligand i.e. 5,11-dimethyl-6,12-diethyl-dione-1,2,4,7,9,10-hexazacyclododeca -1,4,6,10-tetraene. The ligand (L) was prepared by [2+2] condensation reaction of 2,3-pentanedione and semicarbazide hydrochloride. These complexes were found to have the general composition [Cr(L)X(2)]X and [M(L)X(2)] (where M=Mn(II) and Ni(II); X=Cl(-), NO(3)(-), (1/2)SO(4)(2-), NCS(-) and L=ligand [N(6)]). The ligand and its transition metal complexes were characterized by the elemental analysis, molar conductance, magnetic susceptibility, mass, IR, electronic and EPR spectral studies. On the basis of IR, electronic and EPR spectral studies, an octahedral geometry has been assigned for these complexes except sulphato complexes which are of five coordinated geometry.     

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles. Chemistry and excited-state behavior of 1,4-C2-cyclam complexes

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported. The ligand appears to form only trans complexes, and the structure of trans-[Cr(1,4-C(2)-cyclam)Cl(2)]PF(6) is presented. The constraint imposed by the additional C(2) linkage distorts the bond angles significantly away from the ideal values of 90 and 180 degrees. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) (k(obs) for trans-[Cr(1,4-C(2)-cyclam)(H(2)O)(2)](3+) formation = 6.5 x 10(-)(2) s(-)(1), 0.01M HNO(3), 25 degrees C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl(2)](+). The complexes trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) and trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) are found to have extinction coefficients four to five times higher than their cyclam analogues, owed to the lack of centrosymmetry caused by the steric constraint. The trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) complex is a very weak emitter in aqueous solution with a broad room-temperature emission centered at 735 nm (tau = 0.24 micros). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) in aqueous solution results in CN(-) ligand loss. This is in stark contrast to its unconstrained cyclam analogue, which is photoinert and has a room-temperature emission lifetime of 335 micros.     

Syntheses of 8-quinolinolatocobalt(III) complexes containing cyclen based auxiliary ligands as models for hypoxia-activated prodrugs

New ligands H(2)L2-H(2)L6 comprise the cyclen macrocycle which is N,N'-dialkylated at the 1,7-nitrogen atoms by three- and four-carbon alkyl chains bearing terminal sulfonic (C(3) H(2)L2), phosphonic (C(3) H(2)L3, C(4) H(2)L4) or carboxylic acid (C(3) H(2)L5, C(4) H(2)L6) groups, and HL7 is N-monoalkylated by a four-carbon sulfonic acid group. The ligands were prepared by alkylation of a bridged bisaminal intermediate. The syntheses of cobalt(III) complexes containing a tetradentate cyclen, N,N'-1,7-Me(2)cyclen, cyclam or L2-L7 ligand together with the bidentate 8-quinolinato (8QO(-)) ligand, of interest as it is a model for a more potent cytotoxic analogue, were investigated. Coordination of ligands (L) cyclen, N,N'-1,7-Me(2)cyclen or cyclam to cobalt(III) was achieved using Na(3)[Co(NO(6))] to form [Co(L)(NO(2))(2)](+). HOTf (trifluoromethansulfonic acid) was used to prepare the triflato complexes [Co(L)(OTf)(2)](+), followed by substitution of the labile triflato ligands to yield [Co(L)(8QO)](ClO(4))(2) isolated as the perchlorate salts. One further example containing cyclam and the 5-hydroxymethyl-8-quinolinato ligand was also prepared by this method. Complexes containing the pendant arm ligands L2-L6 were prepared from the cobalt precursor trans-[Co(py)(4)Cl(2)](+). Reaction of this complex with H(2)L2·4HCl and 8QOH produced [Co(L2)(8QO)] in one step and contains two deprotonated sulfonato pendant arms. The reaction of H(2)L3·4HBr with [Co(py)(4)Cl(2)](+) gave [Co(L3)]Cl in which L3 acts as a hexadenate ligand with the three-carbon phosphonato side chains coordinated to cobalt. H(2)L5·4HCl bearing three-carbon carboxylic acid pendant arms gave a similar result. The four-carbon ligands were coordinated to cobalt by reaction of [Co(py)(4)Cl(2)](+) with H(2)L4·4HBr or H(2)L6·4HCl to give [Co(HL4)Cl(2)] or [Co(H(2)L6)Cl(2)]Cl, which in turn with 8QOH gave the 8QO(-) complexes [Co(L4)(8QO)] bearing anionic phosphate pendant arms or [Co(H(2)L6)(8QO)]Cl(2) containing neutral carboxylic acid side chains. The reaction of Na(3)[Co(CO(3))(3)] with the mono-N-alkylated ligand HL7·4HCl and then HOTf gave [Co(L7)(CO(3))] and then in turn [Co(L7)(OTf)(2)]. The carbonato complex [Co(L7)(CO(3))] with [8QO](2)[SO(4)] produced [Co(L7)(CO(3))]. All complexes containing L7 bear an anionic sulfonato group on the side chain. The synthesis and characterisation of the six new ligands based on N-alkylated cylen ligand and the cobalt complexes outlined above are described, along with cyclic voltammograms of the 8QO(-) complexes and the molecular structures determined by X-ray crystallography of [Co(cyclen)(H(2)O)(2)](OTf)(3) (formed by aquation of the triflato complex), [Co(cyclen)(8QO)](ClO(4))(2), Co(L2)(8QO)·2H(2)O, Co(L4)(8QO)·6H(2)O and [Co(H(2)L6)Cl(2)]Cl·H(2)O. These demonstrate the coordination of the cyclen ligand in the folded anti-O,syn-N configuration with the N-alkylated nitrogens occupying apical positions.     

Synthesis of cis and trans bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: a spectroscopic investigation of the M(III)-alkynyl interaction

Alkynyl complexes of the type [M(cyclam)(CCR)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M = Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluoromethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthrenyl, and cyclohexyl) were prepared in 49% to 93% yield using a one-pot synthesis involving the addition of 2 equiv of RCCH and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)(2)]OTf complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the stereochemistry was characterized using infrared spectroscopy of the CH(2) rocking and NH bending region. All of the trans-[M(cyclam)(CCR)(2)]OTf complexes exhibit strong Raman bands between 2071 and 2109 cm(-1), ascribed to ν(s)(C≡C). The stretching frequencies for the Cr(III) complexes are 21-28 cm(-1) lower than for the analogous Rh(III) complexes, a result that can be interpreted in terms of the alkynyl ligands acting as π-donors. UV-vis spectra of the Cr(III) and Rh(III) complexes are dominated by strong charge transfer (CT) transitions. In the case of the Rh(III) complexes, these CT transitions obscure the metal centered (MC) transitions, but in the case of the Cr(III) complexes the MC transitions are unobscured and appear between 320 and 500 nm, with extinction coefficients (170-700 L mol(-1) cm(-1)) indicative of intensity stealing from the proximal CT bands. The Cr(III) complexes show long-lived (240-327 μs), structureless, MC emission centered between 731 and 748 nm in degassed room temperature aqueous solution. Emission characteristics are also consistent with the arylalkynyl ligands acting as π-donors. The Rh(III) complexes also display long-lived (4-21 μs), structureless, metal centered emission centered between 524 and 548 nm in degassed room temperature solution (CH(3)CN).     

Photodissociation of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)L and Fe(CO)(4)L: a theoretical study

The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)(6) and Fe(CO)(5). The lowest excited states of Cr(CO)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH(3) ligand. Analysis of their ground-state Cr-P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.     

Epoxidation of an alkene promoted by various nickel(II) multiaza macrocyclic complexes

The epoxidation of trans-β-methylstyrene promoted by various Ni(II) complexes of macrocyclic ligands (cyclam and 1–5) using PhIO as a terminal oxidant has been investigated. In terms of the rate of epoxide formation, the complexes of monocyclic ligands (cyclam, 1 and 2) are better catalysts than those of polycyclic ligands (3–5) and the cyclam complex without pendant arms is better catalyst than those (1 and 2) with pendant arms. However, a series of the complexes show remarkably similar reactivity in the transfer of oxygen from active high-valent intermediate to the alkene and they provide nearly the same final yield in certain reaction conditions. Therefore, the yield of epoxide produced in a given period depends mainly on the rate of reaction of the complex with PhIO, which is greatly affected by the ligand structure. In order to become a better catalyst, the complex should have low Ni(II)/Ni(III) oxidation potential and the macrocyclic ligand should exert less steric hindrance around the Ni(II) center to allow easy axial approach of the oxidant.     

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,11-C3-cyclam)Cl2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C3-cyclam complexes are compared with those of the corresponding cyclam (1,4,8,11 tetraazacyclotetradecane) and 1,4-C2-cyclam (1,4,8,11-tetraazabicyclo[10.2.2]hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C3-cyclam)Cl2]+ is similar to that of the corresponding 1,4-C2-cyclam complex and is more than 5 orders of magnitude faster than the cyclam counterpart. A monotonic increase in the extinction coefficient is observed on going from the cyclam complexes to the 1,11-C3-cyclam complexes to the 1,4-C2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans-[Cr(1,11-C3-cyclam)(CN)2]+ complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau=23 micros). Like the corresponding 1,4-C2-cyclam complex (tau=0.24 micros), the 1,11-C3-cyclam complex shows no deuterium-isotope effect in room-temperature solution. This is in marked contrast to the corresponding cyclam complex which has an emission lifetime of 335 micros and a significant deuterium isotope effect in room-temperature solution. Low temperature (77K) data are also presented in an attempt to understand the differences in photophysical behavior.     

Effects of steric constraint on Chromium(III) complexes of tetraazamacrocycles. 3. Insights into the temperature-dependent radiationless deactivation of the 2Eg (Oh) excited state of trans-[Cr(N4)(CN)2]+ complexes

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.     

Optimizing millisecond time scale near-infrared emission in polynuclear chrome(III)-lanthanide(III) complexes

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)(3)](6+), the connection of a second strong-field [CrN(6)] sensitizer in [CrLnCr(L2)(3)](9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)(3)](9+) under reasonable pumping powers.     

Synthesis,magnetic properties,and electronic spectra of Bis(beta-diketonato)chromium(III) and nickel(II) complexes with a chelated imino nitroxide radical: X-ray structures of [Cr(acaMe)(2)(IM2py)]PF(6) and [Ni(acac)(2)(IM2py)

Two new series of each of four Cr(III) and Ni(II) imino nitroxide complexes with various kinds of beta-diketonates, [Cr(beta-diketonato)(2)(IM2py)]PF(6), and [Ni(beta-diketonato)(2)(IM2py)] (IM2py = 2-(2'-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy)) have been synthesized, and their structures and magnetic and optical properties have been examined. The X-ray analysis demonstrated that a IM2py ligand coordinated to Cr(III) and Ni(II) acts as a five-membered bidentate chelate. The variable-temperature magnetic susceptibility measurements indicated the antiferromagnetic and ferromagnetic interaction of Cr(III) and Ni(II) with IM2py, respectively, giving a variety of the magnetic coupling constant J values with varying the beta-diketonato ligands. The UV-vis shoulders around (19-20) x 10(3) and (17-18) x 10(3) cm(-)(1) for the Cr(III) and Ni(II) complexes, respectively, characteristic of the IM2py complexes were assigned to the metal-ligand charge-transfer transitions, Cr(t(2g))-SOMO(pi*) and Ni(e(g))-SOMO(pi*) MLCT in terms of the resonance Raman spectra and the variable-temperature absorption spectra. The absorption components centered around (13-14) x 10(3) cm(-1) for the Cr(III) and Ni(II) complexes were due to the formally spin-forbidden d-d transition within the t(2g) and e(g) subshells, associated with the intensity enhancement. The spectroscopic behavior with varying the beta-diketonato ligands is discussed in connection with the antiferromagnetic or ferromagnetic coupling constant J values on the basis of the exchange mechanism along with the coligand effect.     

Kinetics of cerium(IV) oxidation of macrocyclic complexes of chromium(III) and fate of chromium(IV) intermediates

Kinetics and mechanism of the cerium(IV) oxidation of Cr(III) complexes of a series of macrocyclic (or pseudomacrocyclic) ligands with [14]-membered intraligand ring-sizes have now been investigated at I = 1.0 M (LiClO₄) Temp. 30°C. The complexes of the formulation Cr(macrocycle)(X)(H₂O) where X = CHCl₂ and H₂O, n = 0 or 1 undergo oxidation to Cr(VI) with the formation of chromium(IV) intermediates. The observed kinetic parameters for the Ce(IV) oxidation of Cr(III) macrocyclic complexes have been discussed in terms of changes brought about by the macrocyclic ligands on the Cr(III)—Cr(IV) redox potentials and in specific rates for Cr(IV)—Cr(V) conversion. On the basis of this study, it has been suggested that the trapping of Cr(IV) is easier when a macrocyclic ligand having a symmetrical intra-ligand ring size and unsaturation in the cyclic structure is coordinated equatorially. Cyclic voltammetric studies indicate the formation of Cr(IV) transient in the case of electrochemical oxidation of trans-Cr(Me₄[14]tetraene)(H₂O).     

Mechanism of reaction of alkyl radicals with (Ni(II)L)2+ complexes in aqueous solutions

The reactions of methyl radicals, CH(3), with the macrocyclic complexes Ni(II)L(1-5) (L(1-5) = cyclam derivatives, vide infra) and Ni(II)edta in aqueous solutions were studied. Methyl radicals react with all these nickel complexes, forming intermediates with Ni(III)-C sigma-bonds. The L(m)Ni(III)-CH(3) complexes are formed in equilibria processes with relatively fast forward rate constants of k(f) > 1 x 10(8) M(-1) s(-1) (except in the case of NiL(2)-trans I cyclam, where the reaction is slower). In all cases the decomposition of the transient complexes occurs via the homolytic cleavage of the metal-carbon sigma-bond. When the homolysis is relatively slow, an isomerisation process of the transient is also observed with the exception of NiL(2), where no isomerisation was observed. The results suggest that the strength of the Ni(III)-CH(3)sigma-bond is mainly affected by steric hindrance.     

Spectroscopic approach in characterization of chromium(III), manganese(II), iron(III) and copper(II) complexes with a nitrogen donor tetradentate, 14-membered azamacrocyclic ligand

The complexes of Cr(III), Mn(II), Fe(III) and Cu(II) were synthesized with the macrocyclic ligand i.e. 2,3,9,10-tetraketo-1,4,8,11-tetraazacyclotetradecane. The ligand was prepared by the [2 + 2] condensation reaction of diethyloxalate and 1,3-diamino propane. These complexes were found to have the general composition M(L)X3 and M'(L)X2 [where M = Mn(II) and Cu(II), M' = Cr(III) and Fe(III), L = ligand (N4) and X = Cl-, NO3-, 1/2SO4(2-) and [CH3COO-]. The ligand and its transition metal complexes were characterized by the elemental analyses, molar conductance, magnetic susceptibility, mass, IR, electronic, and EPR spectral studies. On the basis of IR, electronic and EPR spectral studies an octahedral geometry has been assigned for Cr(III), Mn(II) and Fe(III) and a tetragonal geometry for Cu(II) complexes.     

Synthesis and luminescence properties of Cr(III) complexes with cyclam-type ligands having pendant chromophores, trans-[Cr(L)Cl(2)]Cl

The synthesis and spectroscopic properties of new cyclam-type ligands 5,7-dimethyl-6-R-1,4,8,11-tetraazacyclotetradecane (L), where R is a pendant chromophore such as an anthracene derivative, are reported. These ligands were prepared according to a nickel(II) template procedure, and the X-ray crystal structures of several Ni(II) intermediates are described. Reaction of the free base ligands L with CrCl(3)x3THF resulted in facile formation of trans-[Cr(L)Cl(2)]Cl complexes, and the structures and spectroscopic characterizations of these complexes are also described. Examination of the photophysical properties of trans-[Cr(L)Cl(2)]Cl solutions at 77 K demonstrated the emission spectra to be dominated by phosphorescence from the ligand field doublet of the chromium(III) center. This also applies to the Cr(III) complex trans-[Cr(mac)Cl(2)]Cl, where mac is the anthracene derivative 5,7-dimethyl-6-anthracenylcyclam. Excitation into the pi-pi(*) states of the anthracene leads to marked quenching of the fluorescence from this chromophore and sensitized phosphorescence from the metal-centered doublet state.     

Anionic Fischer-type carbene complexes as bidentate (N,O) ligands

New polynuclear complexes, (L1)3M2 [M2 = Cr(III) (4a,4b), Fe(III) (5), Co(III) (8)], (L1)2M2(L2)2 [M2 = Co(II) (7), Ni(II) (9)], (L1)2M2(O)L2 [M2 = V(IV) (6)] and L1M2Cp2 [M2 = Ti(III) (10)] with L1 = (CO)5M1=C[C=NC(CH3)=CHS](O-)(M1 = Cr or W) and L2 = 4-methylthiazole or THF, are described. The molecular structures of these complexes determined by X-ray diffraction show that the Fischer-type carbene complexes act as bidentate ligands towards the second metal centre, coordinating through C(carbene)-attached O-atoms and imine N-atoms of the thiazolyl groups to form five-membered chelates with the oxygen atoms in the mer configuration. Isostructural complexes have similar characteristic band patterns in their far-IR spectra. Cyclic voltammetry of selected complexes reveals the oxidation of the carbene complex ligand between 1.01 and 1.29 V. Oxidation of the central metal (M2) takes place at 0.56 and 0.86 V for 7 and 9, respectively. Three stepwise reductions of Cr(III) to Cr(0) occur for 4a and 4b in the region -0.51 to -1.58 V. These new ligand types and other variants thereof should find application in ligand design with the first metal -- and other ligands attached thereto -- in the carbene complex ligand, playing an important role.     

Photochemistry and photophysics of Cr(III) macrocyclic complexes

The phosphorescence and photochemical behavior of the macrocyclic complexes (1,4,7,10,13,16-hexaazacyclooctadecane)chromium(III) (Cr([18]-aneN(6))(3+); 1) and (4,4',4'-ethylidynetris(3-azabutan-1-amine)) chromium(III) (Cr(sen)(3+); 2) have been compared to each other and to the complex Cr(en)(3)(3+). For both macrocyclic complexes, phosphorescence from room temperature aqueous solutions is too weak to be observed, contrasting with Cr(en)(3)(3+), though both had somewhat longer 77 K lifetimes than Cr(en)(3)(3+). Phosphorescence lifetimes for these macrocyclics decreased with increasing temperature much faster than for Cr(en)(3)(3+) and a conventional extrapolation based on a fit of reciprocal lifetimes (corrected for the low-temperature contribution) to the Arrhenius equation gave estimated room temperature phosphorescence lifetimes of a few nanoseconds, consistent with the failure to observe room temperature emission. Fitting of the nonlinearity of the data seen in these plots suggested that two high-temperature processes were occurring with estimated activation parameters (E in kJ mol(-1) and A in s(-1)) for Cr([18]-aneN(6))(3+): E(1) = 40, A(1) = 1 x 10(16); E(2) = 24, A(2) = 1 x 10(14): Cr(sen)(3+); E(1) = 45, A(1) = 2 x 10(15); E(2) = 29, A(2) = 7 x 10(11). Cr([18]-aneN(6))(3+) was photochemically inert on irradiation. On irradiation into the lowest quartet ligand field absorption band, Cr(sen)(3+) photolyzes with a quantum yield of 0.098 +/- 0.001 at room temperature. Laser flash photolysis with conductivity detection showed that this photoreaction occurred faster than protonation of the liberated amine ligand at all practical proton concentrations. The quantum yield for irradiation directly into the doublet absorption band of Cr(sen)(3+) was 0.077 +/- 0.003. Photoaquation of Delta-Cr(sen)(3+) led to loss of optical activity and product analysis by capillary electrophoresis showed that both racemic and Delta-Cr(sen)(3+) photoaquate to a single main product, trans-Cr(sen-NH)(H(2)O)(4+). The product stereochemistry is shown to be consistent with predictions based on the angular overlap model for Cr(III) photochemistry, recognizing the additional constraints imposed by the ligand. The abnormally short room temperature solution lifetime of the doublet state is a result of a radiation-less process that competes with other processes depleting the doublet state. However, this doublet-state deactivation process does not lead to photoaquation but competes with BISC and photoaquation via the quartet state, resulting in an unprecedented reduction in photoaquation quantum yield on direct irradiation into the doublet state.     

X-ray absorption spectroscopic studies of chromium nitroso complexes. Crystal and molecular structure of (Ph4P)3[Cr(NO)(NCS)5].2.4(CH3)2CO

X-ray absorption spectroscopy (XAS) provides a direct means of solving the controversy on Cr oxidation states in nitroso complexes. The first XAS studies of four known Cr-NO complexes, [Cr(NO)(OH(2))(5)](2+), [Cr(NO)(acac)(2)(OH(2))], [Cr(NO)(CN)(5)](3)(-), and [Cr(NO)(NCS)(5)](3)(-), have been performed, in comparison with the related Cr(III) complexes, [Cr(OH(2))(6)](3+), [Cr(acac)(3)], [Cr(CN)(6)](3)(-), and [Cr(NCS)(6)](3)(-). The X-ray absorption near-edge structure (XANES) spectra of the Cr-NO complexes are distinguished from those of the corresponding Cr(III) complexes by increased intensities of pre-edge absorbancies due to the 1s --> 3d transition, as well as with slight shifts (by 0.2-1.0 eV) of the edge positions to lower energies, with no major changes in the edge shape. These features, together with the available structural data on Cr-NO complexes, show that the effective Cr oxidation states in such complexes are close to Cr(III), due to the pi-back-bonding within the Cr-NO moiety. Multiple-scattering fitting of X-ray absorption fine structure (XAFS) spectra of [Cr(NO)(acac)(2)(OH(2))] supported the assignment of this complex as a trans-isomer (Keller, A.; Jezovska-Trzebiatowska, B. Polyhedron 1985, 4, 1847-1852). The first crystal structure of a Cr nitroso-isothiocyanato complex, (Ph(4)P)(3)[Cr(NO)(NCS)(5)].2.4(CH(3))(2)CO, has been determined.     

Synthesis and ligand-based mixed valency of cis- and trans-Cr(III)(X(4)SQ)(X(4)Cat)(L)(n) (X = Cl and Br,n = 1 or 2) complexes: effects of solvent media on intramolecular charge distribution and ligand dissociation of Cr(III)(X(4)SQ)(3)

The treatment of Cr(III)(X(4)SQ)(3) (SQ = o-semiquinonate; X = Cl and Br) with acetonitrile affords trans-Cr(III)(X(4)SQ)(X(4)Cat)(CH(3)CN)(2) (X = Cl (1) and Br (2)). In the presence of 2,2'-bipyridine (bpy) or 3,4,7,8-tetramethyl-1,10-phenanthrene (tmphen), the reaction affords Cr(III)(X(4)SQ)(X(4)Cat)(bpy).nCH(3)CN (X = Cl, n = 1 (3); X = Br, n = 0.5 (4)) or Cr(III)(X(4)SQ)(X(4)Cat)(tmphen) (X = Cl (5) and Br (6)), respectively. All of the complexes show a ligand-based mixed-valence (LBMV) state with SQ and Cat ligands. The LBMV state was confirmed by the presence of the interligand intervalence charge-transfer band. Spectroscopic studies in several solvent media demonstrate that the ligand dissociation included in the conversion of Cr(III)(X(4)SQ)(3) to 1-6 occurs only in solvents with relatively high polarity. On the basis of these results, the effects of solvent media were examined and an equilibrium, Cr(III)(X(4)SQ)(3) <--> Cr(III)(X(4)BQ)(X(4)SQ)(X(4)Cat) (BQ = o-benzoquinone), is proposed by assuming an interligand electron transfer induced by solvent polarity.     

Emissive chromium(III) complexes with substituted arylethynyl ligands

Arylethynylchromium(III) complexes of the form trans-[Cr(cyclam)(CCC(6)H(4)R)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R = H, CH(3), or CF(3) in the para position, and OTf = trifluoromethanesulfonate) have been prepared and characterized by IR spectroscopy and X-ray diffraction. The complexes are emissive with excited-state lifetimes in a deoxygenated fluid solution between 200 and 300 micros.