Solid-state interconversions: unique 100 % reversible transformations between the ground and metastable states in single-crystals of a series of nickel(II) nitro complexes.

The solid-state, low-temperature linkage isomerism in a series of five square planar group 10 phosphino nitro complexes have been investigated by a combination of photocrystallographic experiments, Raman spectroscopy and computer modelling. The factors influencing the reversible solid-state interconversion between the nitro and nitrito structural isomers have also been investigated, providing insight into the dynamics of this process. The cis-[Ni(dcpe)(NO2)2] (1) and cis-[Ni(dppe)(NO2)2] (2) complexes show reversible 100 % interconversion between the η(1)-NO2 nitro isomer and the η(1)-ONO nitrito form when single-crystals are irradiated with 400 nm light at 100 K. Variable temperature photocrystallographic studies for these complexes established that the metastable nitrito isomer reverted to the ground-state nitro isomer at temperatures above 180 K. By comparison, the related trans complex [Ni(PCy3)2(NO2)2] (3) showed 82 % conversion under the same experimental conditions at 100 K. The level of conversion to the metastable nitrito isomers is further reduced when the nickel centre is replaced by palladium or platinum. Prolonged irradiation of the trans-[Pd(PCy3)2(NO2)2] (4) and trans-[Pt(PCy3)2(NO2)2] (5) with 400 nm light gives reversible conversions of 44 and 27 %, respectively, consistent with the slower kinetics associated with the heavier members of group 10. The mechanism of the interconversion has been investigated by theoretical calculations based on the model complex [Ni(dmpe)Cl(NO2)].

Photoactivated linkage isomerism in single crystals of nickel, palladium and platinum di-nitro complexes--a photocrystallographic investigation.

Low temperature, single crystal photocrystallographic studies have been carried out on four square planar Group 10 complexes [Ni(PEt(3))(2)(NO(2))(2)] 1, [Pd(PPh(3))(2)(NO(2))(2)] 2, [Pd(AsPh(3))(2)(NO(2))(2)] 3 and [Pt(PPh(3))(2)(NO(2))(2)] 4, in which the two nitro groups adopt the trans configuration. Irradiation with UV light, at 100 K, of single crystals of complexes 1-3 photoisomerise from the η(1)-NO(2) nitro form to the η(1)-ONO nitrito form occurred. Complex 1 underwent 25% conversion to the nitrito form before crystal decomposition occurred. 2 and 3 underwent 46% and 39% conversion, respectively, to the nitrito form when a photostationary state was reached. While under the same experimental conditions 4 showed no isomerisation. The photocrystallographic results can be correlated with the results of DFT calculations and with the observed trends in the solution UV/visible absorption spectroscopy obtained for these complexes. The results suggest that while steric factors in the isomerization processes are important there may also be a kinetic effect relating to the lability of the metal involved.

Photocrystallographic identification of metastable nitrito linkage isomers in a series of nickel(II) complexes.

Single crystal photocrystallographic experiments and solid state Raman spectroscopy have been used to determine the low temperature, metastable structures of the nickel(ii) nitrito complexes [Ni(aep)(2)(η(1)-ONO)(2)] 1#O (aep = 1-(2-aminoethyl)piperidine), [Ni(aem)(2)(η(1)-ONO)(2)] 2#O (aem = 1-(2-aminoethyl)morpholine), and [Ni(aepy)(2)(η(1)-ONO)(2)] 3#O (aepy = 1-(2-aminoethyl)pyrrolidine and where the #O denotes the oxygen-bound nitrito metastable molecule). These linkage isomers of the equivalent nitro complexes [Ni(aep)(2)(η(1)-NO(2))(2)] 1, [Ni(aem)(2)(η(1)-NO(2))(2)] 2 and [Ni(aepy)(2)(η(1)-NO(2))(2)] 3 are formed by LED irradiation at temperatures below 120 K. The behavior of the three complexes upon irradiation is generally similar, but some subtle differences have been observed. From the crystallographic studies all three complexes 1-3 exhibit the endo-nitrito linkage isomer upon irradiation, however, for 3# (a crystal structure that contains components of both 3 and 3#O) an exo-nitrito isomer is also observed. Under conditions of 90-100 K, with blue light, the conversion percentages to the nitrito isomers, 1#O, 2#O and 3#O were 16%, 22% and 30%, respectively. At temperatures below 110 K all three nitrito isomers were stable for over four hours but while 2#O and 3#O could be detected at temperatures down to 30 K, at temperatures below 60 K the metastable structure 1#O appeared to be quenched and only the nitro isomer 1 was identified in the crystal. The solid state Raman spectra for 1#, 2# and 3# confirmed the photocrystallographic results with the nitrito isomers being identified from the O-N-O deformation vibrations.

Experimental charge density study into C-C σ-interactions in a Binor-S rhodium complex.

Transition-metal complexes containing (C-C)→M σ-interactions have potential applications in both catalysis and the activation and cleavage of C-C bonds. Fully characterising the bonding and interactions in complexes containing such (C-C)→M σ-interactions is vital to understand their chemical behaviour. As a result a high-resolution experimental X-ray charge density study has been undertaken on [Rh(Binor-S)(PCy(3))][HCB(11)Me(11)] (Binor-S = 1,2,4,5,6,8-dimetheno-s-indacene) which contains a (C-C)→Rh interaction. The data are analysed using Bader's "Atoms in Molecules" (AIM) approach with particular attention paid to the interactions around the rhodium centre. The results provide clear evidence for the σ(C-C)→Rh interaction in the solid-state which is classified as a weak covalent interaction. These results are supported by theoretical calculations.

Fluorescent gallium and indium bis(thiosemicarbazonates) and their radiolabelled analogues: synthesis, structures and cellular confocal fluorescence imaging investigations.

New fluorescent and biocompatible aromatic Ga(III)- and In(III)-bis(thiosemicarbazonato) complexes for dual mode optical and PET or SPECT molecular imaging have been synthesised via a synthetic method based on transmetallation reactions from Zn(II) precursors. Complexes have been fully characterised in the solid state by single crystal X-ray diffraction and in solution by spectroscopic methods (UV/Vis, fluorescence, (1)H and (13)C{(1)H} NMR). The bis(thiosemicarbazones) radiolabelled rapidly in high yields under mild conditions with (111)In (a gamma and Auger emitter for SPECT imaging and radiotherapy with t(1/2) = 2.8 d) and (68)Ga (a generator-available positron emitter for PET imaging with t(1/2) = 68 min). Cytotoxicity and biolocalisation studies using confocal fluorescence imaging and fluorescence lifetime imaging (FLIM) techniques have been used to study their in vitro activities and stabilities in HeLa and PC-3 cells to ascertain their suitability as synthetic scaffolds for future multimodality molecular imaging in cancer diagnosis and therapy. The observation that the indium complexes show certain nuclear uptake could be of relevance towards developing (111)In therapeutic agents based on Auger electron emission to induce DNA damage.

Highly efficient visible-light driven photochromism: developments towards a solid-state molecular switch operating through a triplet-sensitised pathway.

We introduce a new highly efficient photochromic organometallic dithienylethene (DTE) complex, the first instance of a DTE core symmetrically modified by two Pt(II) chromophores [Pt(PEt(3))(2)(C≡C)(DTE)(C≡C)Pt(PEt(3))(2)Ph] (1), which undergoes ring-closure when activated by visible light in solvents of different polarity, in thin films and even in the solid state. Complex 1 has been synthesised and fully photophysically characterised by (resonance) Raman and transient absorption spectroscopy complemented by calculations. The ring-closing photoconversion in a single crystal of 1 has been followed by X-ray crystallography. This process occurs with the extremely high yield of 80%--considerably outperforming the other DTE derivatives. Remarkably, the photocyclisation of 1 occurs even under visible light (>400 nm), which is not absorbed by the non-metallated DTE core HC≡C(DTE)C≡CH (2) itself. This unusual behaviour and the high photocyclisation yields in solution are attributed to the presence of a heavy atom in 1 that enables a triplet-sensitised photocyclisation pathway, elucidated by transient absorption spectroscopy and DFT calculations. The results of resonance Raman investigation confirm the involvement of the alkynyl unit in the frontier orbitals of both closed and open forms of 1 in the photocyclisation process. The changes in the Raman spectra upon cyclisation have permitted the identification of Raman marker bands, which include the acetylide stretching vibration. Importantly, these bands occur in the spectral region unobstructed by other vibrations and can be used for non-destructive monitoring of photocyclisation/photoreversion processes and for optical readout in this type of efficiently photochromic thermally stable systems. This study indicates a strategy for generating efficient solid-state photoswitches in which modification of the Pt(II) units has the potential to tune absorption properties and hence operational wavelength across the visible range.

When is an imine not an imine? Unusual reactivity of a series of Cu(II) imine-pyridine complexes and their exploitation for the Henry reaction.

In this paper we report the synthesis and solid-state structures for a series of pyridine based Cu(II) complexes and preliminary data for the asymmetric Henry reaction. Interestingly, the solid-state structures indicate the incorporation of an alcohol into one of the imine groups of the ligand, forming a rare α-amino ether group. The complexes have been studied via single crystal X-ray diffraction, EPR spectroscopy and mass spectrometry. Intriguingly, it has been observed that the alcohol only adds to one of the imine moieties. Density functional theory (DFT) calculations have also been employed to rationalise the observed structures. The Cu(II) complexes have been tested in the asymmetric Henry reaction (benzaldehyde + nitromethane or nitroethane) with ee's up to 84% being achieved as well as high conversions and modest diastereoselectivities.

Synthesis and structure of 6-aminofulvene-2-aldiminate complexes.

We report here a synthetic route to bis(N,N'-aryl)-6-aminofulvene-2-aldimine (AFA) ligand systems, specifically Ph(2)-AFAH and Dip(2)-AFAH. The synthesis and structural characterization of a series of Cu(I) complexes [(Ph(2)-AFA)Cu(CNPh)(2)] (2), [(Ph(2)-AFA)Cu(CN(i)Pr)] (3), and [(Dip(2)-AFA)Cu(CN(i)Pr)] (4), from the reaction of the corresponding lithiated AFA systems with Cu-Cl derivatives are reported; notably in the case of [(Ph(2)-AFA)Cu(CNPh)(2)] studies have revealed the existence of two structural isomers (2a and 2b), both of which can be isolated and structurally characterized. Density functional theory (DFT) calculations suggest that the two crystal forms are comparatively close in energy, and geometry optimization reveals a convergence of these two forms to a geometry that more closely resembles the solid-state structure of isomer 2b, having a CH···π interaction. The reactions of the AFA compounds Ph(2)-AFAH and Dip(2)-AFAH with ZnMe(2) and AlMe(3) have also been investigated, and the results of these reactions are described here.

[Rh(C7H8)(PPh3)Cl]: an experimental charge-density study.

In order to gain a deeper understanding into the bonding situation in rhodium complexes containing rhodium-carbon interactions, the experimental charge-density analysis for [Rh(C(7)H(8))(PPh(3))Cl] (1) is reported. Accurate, high-resolution (sin theta/lambda = 1.08 A(-1)), single-crystal data were obtained at 100 K. The results from the investigation were interesting in relation to the interactions between the rhodium metal centre and the norbornadiene fragment and illustrate the importance of such analyses in studying bonding in organometallic complexes.

Intermolecular alkene and alkyne hydroacylation with beta-S-substituted aldehydes: mechanistic insight into the role of a hemilabile P-O-P ligand.

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and 1-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO(4)] (DPEphos=bis(2-diphenylphosphinophenyl)ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH(2)CH(2)SMe or ortho-HCOC(6)H(4)SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)(2)(DPEphos)][X] [X=closo-CB(11)H(6)Cl(6) or [BAr(F) (4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3))] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)(DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH(2)CH(2)SMe)H][X] or [Rh(DPEphos)(COC(6)H(4)SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB(11)H(6)I(6)] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC(2)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X] and [Rh(DPEphos)(eta(1)-MeSC(6)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH(2)CH(2)CO(2)Me)(COC(2)H(4)SMe)(DPEphos)][X] and [Rh(CH(2)CH(2)CO(2)Me)(COC(6)H(4)SMe)(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC(2)H(4)CO)H(MeCN)][X] and [Rh(DPEphos)(MeSC(6)H(4)CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH(2) linkage, [Ph(2)P(C(6)H(4))](2)CH(2), gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph(2)P(C(6)H(4))](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.

Reversible addition of water to the high-hydride-content cluster [Rh6(PiPr3)6H12][BArF4]2. Synthesis and Structure of [Rh6PiPr3)6H11(OH)][BArF4]2.

The hydroxyhydrido salt [Rh(6)(P(i)Pr(3))(6)H(11)(OH)][BArF(4)](2) results from the addition of water to [Rh(6)(P(i)Pr(3))(6)H(12)][BArF(4)](2). This reaction is reversible, and the addition of dihydrogen to [Rh(6)(P(i)Pr(3))(6)H(11)(OH)][BArF(4)](2) results in the elimination of water and the regeneration of the hydride cluster.

Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine).

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.

Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6]- (X = H, Br). Synthesis, structures and reactivity with alkenes.

Addition of the new phosphonium carborane salts [HPR(3)][closo-CB(11)H(6)X(6)] (R = (i)Pr, Cy, Cyp; X = H 1a-c, X = Br 2a-c; Cy = C(6)H(11), Cyp = C(5)H(9)) to [Rh(nbd)(mu-OMe)](2) under a H(2) atmosphere gives the complexes Rh(PR(3))H(2)(closo-CB(11)H(12)) 3 (R = (i)Pr 3a, Cy 3b, Cyp 3c) and Rh(PR(3))H(2)(closo-CB(11)H(6)Br(6)) 4 (R = (i)Pr 4a, Cy 4b, Cyp 4c). These complexes have been characterised spectroscopically, and for 4b by single crystal X-ray crystallography. These data show that the {Rh(PR(3))H(2)}(+) fragment is interacting with the lower hemisphere of the [closo-CB(11)H(6)X(6)](-) anion on the NMR timescale, through three Rh-H-B or Rh-Br interactions for complexes 3 and 4 respectively. The metal fragment is fluxional over the lower surface of the cage anion, and mechanisms for this process are discussed. Complexes 3a-c are only stable under an atmosphere of H(2). Removing this, or placing under a vacuum, results in H(2) loss and the formation of the dimer species Rh(2)(PR(3))(2)(closo-CB(11)H(12))(2) 5a (R = (i)Pr), 5b (R = Cy), 5c (R = Cyp). These dimers have been characterised spectroscopically and for 5b by X-ray diffraction. The solid state structure shows a dimer with two closely associated carborane monoanions surrounding a [Rh(2)(PCy(3))(2)](2+) core. One carborane interacts with the metal core through three Rh-H-B bonds, while the other interacts through two Rh-H-B bonds and a direct Rh-B link. The electronic structure of this molecule is best described as having a dative Rh(I) --> Rh(III), d(8)--> d(6), interaction and a formal electron count of 16 and 18 electrons for the two rhodium centres respectively. Addition of H(2) to complexes 5a-c regenerate 3a-c. Addition of alkene (ethene or 1-hexene) to 5a-c or 3a-c results in dehydrogenative borylation, with 1, 2, and 3-B-vinyl substituted cages observed by ESI-MS: [closo-(RHC[double bond, length as m-dash]CH)(x)CB(11)H(12-x)](-)x = 1-3, R = H, C(4)H(9). Addition of H(2) to this mixture converts the B-vinyl groups to B-ethyl; while sequential addition of 4 cycles of ethene (excess) and H(2) to CH(2)Cl(2) solutions of 5a-c results in multiple substitution of the cage (as measured by ESI-MS), with an approximately Gaussian distribution between 3 and 9 substitutions. Compositionally pure material was not obtained. Complexes 4a-c do not lose H(2). Addition of tert-butylethene (tbe) to 4a gives the new complex Rh(P(i)Pr(3))(eta(2)-H(2)C=CH(t)Bu)(closo-CB(11)H(6)Br(6)) 6, characterised spectroscopically and by X-ray diffraction, which show coordination of the alkene ligand and bidentate coordination of the [closo-CB(11)H(6)Br(6)](-) anion. By contrast, addition of tbe to 4b or 4c results in transfer dehydrogenation to give the rhodium complexes Rh{PCy(2)(eta(2)-C(6)H(9))}(closo-CB(11)H(6)Br(6)) 7 and Rh{PCyp(2)(eta(2)-C(5)H(7))}(closo-CB(11)H(6)Br(6)) 9, which contain phosphine-alkene ligands. Complex has been characterised crystallographically.

[Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF4]: a well characterised transition metal dichloroethane complex.

Reaction of [Ir(PPh(3))(2)(COD)][BAr(F)(4)] with H(2) in dichloroethane solution results in [Ir(PPh(3))(2)(H)(2)(ClCH(2)CH(2)Cl)][BAr(F)(4)], which has been fully characterised by X-ray crystallography, NMR spectroscopy and ESI-MS. Its activity towards alkene hydrogenation has been compared with analogous CH(2)Cl(2) complexes.

A DFT based investigation into the electronic structure and properties of hydride rich rhodium clusters.

Density functional theory has been used to investigate the structures, bonding and properties of a family of hydride rich late transition metal clusters of the type [Rh(6)(PH(3))(6)H(12)](x) (x = 0, +1, +2, +3 or +4), [Rh(6)(PH(3))(6)H(16)](x) (x = +1 or +2) and [Rh(6)(PH(3))(6)H(14)](x) (x = 0, +1 or +2). The positions of the hydrogen atoms around the pseudo-octahedral Rh(6) core in the optimized structures of [Rh(6)(PH(3))(6)H(12)](x) (x = 0, +1, +2, +3 or +4) varied depending on the overall charge on the cluster. The number of semi-bridging hydrides increased (semi-bridging hydrides have two different Rh-H bond distances) as the charge on the cluster increased and simultaneously the number of perfectly bridging hydrides (equidistant between two Rh centers) decreased. This distortion maximized the bonding between the hydrides and the metal centers and resulted in the stabilization of orbitals related to the 2T(2g) set in a perfectly octahedral cluster. In contrast, the optimized structures of the 16-hydride clusters [Rh(6)(PH(3))(6)H(12)](x) (x = +1 or +2) were similar and both clusters contained an interstitial hydride, along with one terminal hydride, ten bridging hydrides and two coordinated H(2) molecules which were bound to two rhodium centers in an eta(2):eta(1)-fashion. All the hydrides were on the outside of the Rh(6) core in the lowest energy structures of the 14-hydride clusters [Rh(6)(PH(3))(6)H(14)] and [Rh(6)(PH(3))(6)H(14)](+), which both contained eleven bridging hydrides, one terminal hydride and one coordinated H(2) molecule. Unfortunately, the precise structure of [Rh(6)(PH(3))(6)H(14)](2+) could not be determined as structures both with and without an interstitial hydride were of similar energy. The reaction energetics for the uptake and release of two molecule of H(2) by a cycle consisting of [Rh(6)(PH(3))(6)H(12)](2+), [Rh(6)(PH(3))(6)H(16)](2+), [Rh(6)(PH(3))(6)H(14)](+), [Rh(6)(PH(3))(6)H(12)](+) and [Rh(6)(PH(3))(6)H(14)](2+) were modelled, and, in general, good agreement was observed between experimental and theoretical results. The electronic reasons for selected steps in the cycle were investigated. The 12-hydride cluster [Rh(6)(PH(3))(6)H(12)](2+) readily picks up two molecules of H(2) to form [Rh(6)(PH(3))(6)H(16)](2+) because it has a small HOMO-LUMO gap (0.50 eV) and a degenerate pair of LUMO orbitals available for the uptake of four electrons (which are provided by two molecules of H(2)). The reverse process, the spontaneous release of a molecule of H(2) from [Rh(6)(PH(3))(6)H(16)](+) to form [Rh(6)(PH(3))(6)H(14)](+) occurs because the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital in [Rh(6)(PH(3))(6)H(16)](+) is 0.9 eV, whereas in [Rh(6)(PH(3))(6)H(14)](+) the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital is only 0.3 eV. At this stage the factors driving the conversion of [Rh(6)(PH(3))(6)H(14)](+) to [Rh(6)(PH(3))(6)H(12)](2+) are still unclear.

C-C sigma complexes of rhodium.

In this article, the complexes [Rh(Binor-S')(PR3)][BAr(4)(F)] (R = (i)Pr, Cy, C(5)H(9)) are described. A combination of x-ray crystallography, NMR spectroscopy, density functional theory, and "atoms in molecules" calculations unequivocally demonstrates that the complexes contain rare examples of metal...C-C agostic interactions. Moreover, they are fluxional on the NMR time scale, undergoing rapid and reversible C-C activation. Kinetic data and calculations point to a bismetallacyclobutane, Rh(V), intermediate.

Sequential reduction of high hydride count octahedral rhodium clusters [Rh6(PR3)6H12][BArF4]2: redox-switchable hydrogen storage.

Cyclic voltammetry on the octahedral rhodium clusters with 12 bridging hydride ligands, [Rh6(PR3)6H12][BArF4]2 (R = Cy Cy-[H12]2+, R = iPr iPr-[H12]2+; [BArF4]- = [B{C6H3(CF3)2}4]-) reveals four potentially accessible redox states: [Rh6(PR3)6H12]0/1+/2+/3+. Chemical oxidation did not produce stable species, but reduction of Cy-[H12]2+ using Cr(eta6-C6H6)2 resulted in the isolation of Cy-[H12]+. X-ray crystallography and electrospray mass spectrometry (ESI-MS) show this to be a monocation, while EPR and NMR measurements confirm that it is a monoradical, S = 1/2, species. Consideration of the electron population of the frontier molecular orbitals is fully consistent with this assignment. A further reduction is mediated by Co(eta5-C5H5)2. In this case the cleanest reduction was observed with the tri-isopropyl phosphine cluster, to afford neutral iPr-[H12]. X-ray crystallography confirms this to be neutral, while NMR and magnetic measurements (SQUID) indicate an S =1 paramagnetic ground state. The clusters Cy-[H12]+ and iPr-[H12] both take up H2 to afford Cy-[H14]+ and iPr-[H14], respectively, which have been characterized by ESI-MS, NMR spectroscopy, and UV-vis spectroscopy. Inspection of the frontier molecular orbitals of S = 1 iPr-[H12] suggest that addition of H2 should form a diamagnetic species, and this is the case. The possibility of "spin blocking" in this H2 uptake is also discussed. Electrochemical investigations on the previously reported Cy-[H16]2+ [J. Am. Chem. Soc. 2006, 128, 6247] show an irreversible loss of H2 on reduction, presumably from an unstable Cy-[H16]+ species. This then forms Cy-[H12]2+ on oxidation which can be recharged with H2 to form Cy-[H16]2+. We show that this loss of H2 is kinetically fast (on the millisecond time scale). Loss of H2 upon reduction has also been followed using chemical reductants and ESI-MS. This facile, reusable gain and loss of 2 equiv of H2 using a simple one-electron redox switch represents a new method of hydrogen storage. Although the overall storage capacity is very low (0.1%) the attractive conditions of room temperature and pressure, actuation by the addition of a single electron, and rapid desorption kinetics make this process of interest for future H2 storage applications.