Unique tripodal S-donor capping agents with an attached carboxylate are found to bind tightly to the surface of CdSe nanocrystals (NCs), making the latter water soluble. Unlike that in similarly solubilized CdSe NCs with one-sulfur or two-sulfur capping agents, dissociation from the NC surface is greatly reduced. The impact of this behavior is seen in the photochemical generation of H 2 in which the CdSe NCs function as the light absorber with metal complexes in aqueous solution as the H 2 -forming catalyst and ascorbic acid as the electron donor source. This precious-metalfree system for H 2 generation from water using [Co(bdt) 2 ]− (bdt, benzene-1,2-dithiolate) as the catalyst exhibits excellent activity with a quantum yield for H 2 formation of 24% at 520 nm light and durability with >300,000 turnovers relative to catalyst in 60 h.photochemistry | solar energy | catalysis | water splitting | quantum dots A rtificial photosynthesis (AP) represents an important strategy for energy conversion from sunlight to storage in chemical bonds (1-4). Unlike natural photosynthesis in which CO 2 + H 2 O are converted into carbohydrates and O 2 , the key energy-storing reaction in AP is the splitting of water into its constituent elements of hydrogen and oxygen (5-16). As a redox reaction, water splitting can be divided into two half-reactions, of which the light-driven generation of H 2 is the reductive component. Many systems for the photogeneration of H 2 have been described over the years and they typically consist of a light absorber, a catalyst for H 2 formation, and sources of protons and electrons. For systems that function in aqueous media, the protons are provided by water, whereas for nonaqueous systems, the protons are provided by weak, generally organic acids. The source of electrons in these photochemical systems is generally a sacrificial electron donor-that is, a compound that decomposes following one electron oxidation.Reports of the light-driven generation of hydrogen date back more than 30 y, beginning with a multicomponent system containing [Ru(bpy) 3 ] 2+ (where bpy is 2,2′-bipyridine) as the chromophore or photosensitizer (PS) and colloidal Pt as the catalyst for making H 2 from protons and electrons (17). In these and many subsequent systems, electron mediators were used to accept an electron from the excited chromophore, PS*-thereby serving as an oxidative quencher-and transfer it to the catalyst. Whereas two of the initial mediators were bpy complexes of rhodium and cobalt (17, 18), the overwhelming majority of electron mediators in these systems were dialkylated 2,2′-and 4,4′-bipyridines and their derivatives (19)(20)(21)(22). The most extensively used of these mediators was methyl viologen (MV 2+ , dimethyl-4,4′-bipyridinium, usually as its chloride salt). These mediators were subsequently found to undergo deactivation in their role by hydrogenation (23, 24). The sacrificial electron donors used in these studies depended on system pH and were generally based on compounds having tertiary amine func...

The cyclometalated complexes [Pt(ppy)Ar(SMe 2 )] or [Pt(bhq)Ar(SMe 2 )], where ppyH = 2-phenylpyridine, bhqH = benzo[h]quinoline, and Ar = 4-tolyl or 4-anisyl, react with bis(diphenylphosphino)methane, dppm, in a 1:1 ratio to give the corresponding complexes [Pt(ppy)Ar(κ 1 -dppm)] and [Pt(bhq)Ar(κ 1 -dppm)], in which the dppm ligands are monodentate, or in a 2:1 ratio to give the symmetrical binuclear complexes [{Pt(ppy)Ar} 2 (μ-dppm)] and [{Pt(bhq)Ar} 2 (μ-dppm)], in which the dppm ligands are bridging bidentate. Most remarkably, the reaction of [Pt(ppy)Ar(SMe 2 )] with [Pt(bhq)Ar 0 (κ 1 -dppm)] or of [Pt(bhq)Ar 0 (SMe 2 )] with [Pt(ppy)Ar(κ 1 -dppm)] occurs selectively to give the unsymmetrical bridged complexes [(ppy)ArPt(μ-dppm)PtAr 0 (bhq)]. An example of each structural type has been characterized crystallographically, and it is shown that some of the bhq complexes undergo supramolecular self-assembly through π-stacking. † Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth for his pioneering research in organometallic chemistry and his outstanding service to Organometallics.

Reaction of each of the known starting complexes [PtR(C^N)(SMe2)], 1, in which R = Me or p-MeC6H4 and C^N is either ppy (deprotonated 2-phenylpyridine) or bhq (deprotonated benzo[h]quinoline), with one equivalent of CF3CO2H, gave the complexes [Pt(C^N)(CF3CO2)(SMe2)], 3 (C^N = ppy, 3a; bhq, 3b). The bis-chelate complexes [Pt(C^N)(P^P)](CF3CO2), 4, were obtained by reaction of complexes 3 with one equivalent of either of the P^P bisphosphine reagents, dppf = 1,1'-bis(diphenylphosphino)ferrocene or dppe = bis(diphenylphosphino)ethane. Complexes 4 were alternatively made by reaction of the complexes [PtMe(κ(1)C-C^N)(P^P)], 2, with one equivalent of CF3CO2H. When the complex 3b was reacted with 0.5 equivalents of dppe, 0.5 equivalents of the related bis-chelate product, 4d, formed along with 0.5 equivalents of the unreacted starting complex 3b. In contrast, when the complex 3b was reacted with 0.5 equivalents of dppf, then the dimeric complex [Pt2(bhq)2(CF3CO2)2(μ-dppf)], 5, formed in pure form. In all the above-mentioned acid reactions, the M-R bond rather than the M-C bond of the cycloplatinated complex is cleaved. When the PPh3 analogues of complexes 1, i.e. the complexes [PtR(C^N)(PPh3)], 6, in which C^N is ppy or tpy = deprotonated 2-p-tolylpyridine, were reacted with one equivalent of CF3CO2H, the course of the reaction reversed and the M-C bonds of the cycloplatinated complexes are cleaved rather than the M-R bonds. The latter reaction gave [PtR(κ(1)N-HC^N)(PPh3)(CF3CO2)], as an equilibrium mixture of two isomers 7 and 8. Crystal structures of the typical complexes show a variety of extensive intermolecular hydrogen bonding involving C-H bonds from the different ligands and electronegative atoms (O or F) from the CF3CO2 moiety. On the basis of data obtained from kinetic studies (using (1)H NMR spectroscopy), a dissociative mechanism is proposed for the case of the 7c/8c isomerization process, involving dissociation of the κ(1)N-Htpy neutral ligand, rather than the alternative route of PPh3 or CF3CO2 ligand dissociation.

The cyclometalated complexes [Pt(ppy)R(SMe(2))] or [Pt(bhq)R(SMe(2))], where ppyH = 2-phenylpyridine, bhqH = benzo[h]quinoline and R = methyl or p-tolyl, react with bis(diphenylphosphino)ethane, dppe, in a 1:1 ratio to give the corresponding complexes [Pt(κ(1)-C-ppy)R(dppe)] or [Pt(κ(1)-C-bhq)R(dppe)], in which the ppy or bhq ligands are monodentate and dppe is chelating. The similar reaction in a 2:1 ratio gives the binuclear complexes [{Pt(ppy)R}(2)(μ-dppe)] or [{Pt(bhq)R}(2)(μ-dppe)], in which the dppe ligands are in the unusual bridging bidentate bonding mode.

Cycloplatinated(ii) complexes 1–4 were synthesized and characterized. All complexes exhibit strong luminescence except 3.