Precursor conversion kinetics and the nucleation of cadmium selenide nanocrystals

The kinetics of cadmium selenide (CdSe) nanocrystal formation was studied using UV-visible absorption spectroscopy integrated with an automated, high-throughput synthesis platform. Reaction of anhydrous cadmium octadecylphosphonate (Cd-ODPA) with alkylphosphine selenides (1, tri-n-octylphosphine selenide; 2, di-n-butylphenylphosphine selenide; 3, n-butyldiphenylphosphine selenide) in recrystallized tri-n-octylphosphine oxide was monitored by following the absorbance of CdSe at λ = 350 nm, where the extinction coefficient is independent of size, and the disappearance of the selenium precursor using {(1)H}(31)P NMR spectroscopy. Our results indicate that precursor conversion limits the rate of nanocrystal nucleation and growth. The initial precursor conversion rate (Q(o)) depends linearly on [1] (Q(o)(1) = 3.0-36 μM/s) and decreases as the number of aryl groups bound to phosphorus increases (1 > 2 > 3). Changes to Q(o) influence the final number of nanocrystals and thus control particle size. Using similar methods, we show that changing [ODPA] has a negligible influence on precursor reactivity while increasing the growth rate of nuclei, thereby decreasing the final number of nanocrystals. These results are interpreted in light of a mechanism where the precursors react in an irreversible step that supplies the reaction medium with a solute form of the semiconductor.     

Mechanistic study of precursor evolution in colloidal group II-VI semiconductor nanocrystal synthesis

The molecular mechanism of precursor evolution in the synthesis of colloidal group II-VI semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M-OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd-ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd-OA and Cd-ODPA can be fit to a single-exponential decay (kobs = (1.30 +/- 0.08) x 10-3 s-1, Cd-ODPA, 260 degrees C, and kobs = (1.51 +/- 0.04) x 10-3 s-1, Cd-OA, 117 degrees C). The reaction approaches completion at 70-80% conversion of TOPSe under anhydrous conditions and 100% conversion in the presence of added water. Activation parameters for the reaction between TBPSe and Cd-OA in n-nonane-d20 were determined from the temperature dependence of the TBPSe decay over the range of 358-400 K (deltaH++ = 62.0 +/- 2.8 kJ.mol-1, deltaS++ = -145 +/- 8 J.mol-1.K-1). A reaction mechanism is proposed where trialkylphsophine chalcogenides deoxygenate the oleic acid or phosphonic acid surfactant to generate trialkylphosphine oxide and oleic or phosphonic acid anhydride products. Results from kinetics experiments suggest that cleavage of the phosphorus chalcogenide double bond (TOP=E) proceeds by the nucleophilic attack of phosphonate or oleate on a (TOP=E)-M complex, generating the initial M-E bond.     

键长和电子效应共同调控的β-二酮Dy(Ⅲ)配合物的合成、结构与磁性

以N-乙基-3-吲哚三氟甲基β-二酮(EIFD)为主配体,分别以乙二醇单甲醚(EM)、乙二醇二甲醚(EDM)、二缩三乙二醇(TEG)为辅助配体,与DyCl3·6H2O反应合成了一系列Dy(Ⅲ)配合物[Dy(EIFD)3(EM)]·CH2Cl2(1)、[Dy(EIFD)3(EDM)]·CH2Cl2(2)和[Dy(EIFD)3(TEG)](3)。X射线单晶衍射分析表明,3个配合物都是八配位的单核结构,配位构型分别为双帽三棱柱、正十二面体和双帽三棱柱,分别具有C2v、D2d和C2v对称性。磁学性质显示了配合物1~3具有慢弛豫现象,能垒分别为95.1 K (1)、40.5 K (2)、53.8和13.4 K (3),且配合物1和3有明显的蝴蝶状磁滞回线。进一步讨论了配合物中Dy-O键长和含氧辅助配体的电子效应对配合物有效翻转能垒的影响。     

Synthesis and spectroscopy of N3P3X5OCH=CH2 (X = Cl, F, OCH3, OCH2CF3, N(CH3)2) and N3P3X4(OCH=CH2)2 (X = Cl, N(CH3)2). Correlations of ultraviolet photoelectron spectroscopy and nuclear magnetic resonance data to electronic and geometrical structure

The syntheses of the vinyloxycyclotriphosphazene derivatives N3P3X5OCH=CH2 (X = OMe, OCH2CF3) and the N3P3(NMe2)4(OCH=CH2)2 isomeric mixture along with improved preparations of N3P3X5OCH=CH2 (X = F, NMe2) are reported. The interactions between the vinyloxy function and the cyclophosphazene in these and the previously reported N3P3Cl5 (OCH=CH2) and N3P3F6-n(OCH=CH2)n (n = 1-4) have been examined by ultraviolet photoelectron spectroscopy (UPS) and NMR spectroscopy. The UPS data for the chloro and fluoro derivatives show a strong electron-withdrawing effect of the phosphazene on the olefin that is mediated with decreasing halogen substitution. The 1H and 13C NMR data for N3P3X5OCH=CH2 (X = F, Cl, OMe, OCH2CF3, NMe2) show significant changes as a function of the phosphazene substituent. There is a linear correlation between the beta-carbon chemical shift on the vinyloxy unit and the phosphorus chemical shift at the vinyloxyphosphorus centers. The chemical shifts of the different phosphorus centers on each ring are also related in a linear fashion. These relationships may be understood in terms of the relative electron donor-acceptor abilities of the substituents on the phosphazene ring. The 1H NMR spectra of the N3P3(NMe2)4(OCH-CH2)2 isomeric mixture allow for assignment of the relative amounts of cis and trans isomers. A model for the observed cis preference in the formation of N3P3Cl4(OCH=CH)2 is presented.     

含“CCl_3”或“C≡C”基团的酯类化合物的合成与表征

本文合成了六个含“ＣＣｌ３”或“ＣＣ”基团的酯类化合物（［Ｃｌ３ＣＣ（Ｏ）ＯＣＨ２］２１;ｐ－（Ｃｌ３ＣＣ（Ｏ）Ｏ）２Ｃ６Ｈ４２;（Ｃｌ３ＣＣ（Ｏ）Ｏ）３Ｃ３Ｈ５３;（Ｃｌ３ＣＣ（Ｏ）ＯＣＨ２）４Ｃ４;Ｃｌ３ＣＣ（Ｏ）ＯＣＨ２ＣＣＨ５;Ｃ６Ｈ５Ｃ（Ｏ）ＯＣＨ２ＣＣＨ６）并对其进行了Ｃ／Ｈ分析、ＩＲ、１ＨＮＭＲ等项表征,对化合物５、６进行了质谱分析。     

Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.     

Synthesis, structure, electrochemistry, and spectroelectrochemistry of hypervalent phosphorus(V) octaethylporphyrins and theoretical analysis of the nature of the PO bond in P(OEP)(CH(2)CH(3))(O)

A variety of phosphorus(V) octaethylporphyrin derivatives of the type [P(OEP)(X)(Y)](+)Z(-) (OEP: octaethylporphyrin) (X = CH(3), CH(2)CH(3), C(6)H(5), F; Y = CH(3), CH(2)CH(3), OH, OCH(3), OCH(2)CH(3), On-Pr, Oi-Pr, Osec-Bu, NHBu, NEt(2), Cl, F, O(-); Z = ClO(4), PF(6)) were prepared. X-ray crystallographic analysis of eleven compounds reveals that the degree of ruffling of the porphyrin core becomes greater and the average P-N bond distance becomes shorter as the axial ligands become more electronegative. Therefore, the electronic effect of the axial substituents plays a major role in determining the degree of ruffling although the steric effect of the substituents plays some role. A comparison of the (1)H NMR chemical shifts for the series of [P(OEP)(CH(2)CH(3))(Y)](+)Z(-) complexes with those of the corresponding arsenic porphyrins, which possess a planar core, indicates a much smaller ring current effect of the porphyrin core in the severely ruffled phosphorus porphyrins. The electrochemistry, spectroelectrochemistry and ESR spectroscopy of the singly reduced compounds are also discussed. The OH protons of [P(OEP)(X)(OH)](+) are acidic enough to generate P(OEP)(X)(O) by treatment with aq dilute NaOH. X-ray analysis of P(OEP)(CH(2)CH(3))(O) reveals that the PO bond length is very short (1.475(7) A) and is comparable to that in triphenylphosphine oxide (1.483 A). The features of the quite unique hexacoordinate hypervalent compounds are investigated by density functional calculation of a model (Por)P(CH(2)CH(3))(O) and (Por)P(F)(O) (Por: unsubstituted porphyrin).     

1R,3S-1,2,2-三甲基-1,3-环戊二胺为配体的铂髤配合物的合成、表征和抗肿瘤活性

本文合成了9种含有由天然D( )鄄樟脑衍生的1R,3S鄄1,2,2鄄三甲基鄄1,3鄄环戊二胺(A)为配体的铂髤配合物[Pt髤AX]{其中,X=(CH2)3C(COO-)2(1,1鄄环丁烷二羧酸根),2CH3OCH2COO-,2CH3CH2OCH2COO-,2CH3(CH2)3OCH2COO-,[OCH(CH3)COO]2-(乳酸根),(OCH2COO)2-(乙醇酸根),2CH3OCH2CH2OCH2COO-,2CH3CH2OCH2CH2OCH2COO-和2CH3(CH2)3OCH2CH2OCH2COO-}。通过元素分析、热重分析、红外光谱、1H核磁共振谱和电喷雾质谱等对配合物进行了表征。体外生物活性测试表明,部分配合物对A549人肺癌细胞和HCT鄄116人结肠癌细胞具有较强的抗肿瘤活性。     

An investigation into the allylic imidate rearrangement of trichloroacetimidates catalysed by cobalt oxazoline palladacycles

Dimeric palladacycles, di-mu-X-bis[{eta(5)-(S)-((p)R)-2-[2'-(4'-methylethyl)oxazolinyl]cyclopentadienyl,1-C,3'-N}(eta(4)-tetraphenylcyclobutadiene)cobalt]dipalladium (COP-X), containing bridging groups X=OAc, Cl, Br, I, O(2)CCF(3), p-O(2)CC(6)H(4)F, were synthesised and compared as catalysts for the asymmetric allylic imidate rearrangement of (E)-Cl(3)CC(=NH)OCH(2)CH=CHR with R=nPr. The enantiomeric excess of the product (S)-Cl(3)CC(=O)NHCHRCH=CH(2) was essentially invariant of X (93-96%) and the yield increased in the sequence I相似文献   

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

Stoichiometric reactions of methylparathion with a palladium aryl oxime metallacycle

The reaction of [Pd(3)(OAc)(6)] with (E)-acetophenone oxime and pyridine in CHCl(3) under reflux affords the metallacycle [Pd(OAc)[C,N-(C(6)H(4)C(CH(3))=NOH)-2](py)] (1) as a yellow air-stable complex. The same reaction carried out at room temperature in the absence of pyridine affords the trinuclear oximato complex [Pd(mu-(E)-ON=C(CH(3))Ph)(mu-OAc)](3) (2), which can be converted into 1 upon heating in the presence of pyridine. As indicated by (1)H and (31)P NMR spectroscopy, complex 1 reacts with methylparathion in acetone-d(6)-D(2)O solutions to afford [Pd(SP(=O)(OCH(3))(2))[C,N-(C(6)H(4)C(CH(3))=NOH)-2](py)] (3) and [Pd(mu-SP(=O)(OCH(3))(2))[C,N-(C(6)H(4)C(CH(3))=NOH)-2]](2) (4) as well as free p-nitrophenol. Compounds 1-4 have been characterized by single-crystal X-ray analysis, NMR and EA. Compounds 1 and 3 are mononuclear complexes with the acetate and dimethylthiophosphate ligand, respectively, trans from the phenyl group. Compound 2 is a trinuclear complex whose structure can be derived from that of [Pd(3)(OAc)(6)] by replacing three of the acetate ligands on one side of Pd(3) plane by three N,O-coordinated oximate ligands. Complex 4 is a dinuclear complex in which the two square-planar palladium moieties are linked by the sulfur atoms of the bridging dimethylthiophosphate ligands.     

Synthesis of heterometallic bismuth/molydenum alkoxides and their behavior on silica surfaces

The reaction of [(C(3)H(5))Mo(CO)(2)(CH(3)CN)(2)Cl], 2, with [Bi(OCH(2)CH(2)OCH(3))(3)](2) on a large scale leads to the novel molybdenum/bismuth alkoxide [(C(3)H(5))Mo(CO)(2)(mu-kappa O,2 kappa O'-OCH(2)CH(2)OCH(3))(2)(mu-kappa O-OCH(2)CH(2)OCH(3))BiCl], 6, as the main product as well as to [(C(3)H(5))Mo(CO)(2)(mu-kappa O,2 kappa O'-OCH(2)CH(2)OCH(3))(2)(mu-Cl)BiCl], 4, as a byproduct. Both compounds were characterized by elemental analysis, IR, and NMR spectroscopy as well as by X-ray diffraction. If 6 is brought into contact with a large excess of silica gel, aggregation and condensation reactions are initiated, which led to clusters of ca. 200 nm size spread over the silica surface. When the resulting material is calcinated at 350 degrees C in the presence of O(2), all organic ligands are eliminated and the metal oxo units rearrange: SEM/EDX measurements showed afterward Mo-free bismuth oxo clusters with sizes between 30 and 1000 nm, which are distributed together with molybdenum oxo particles of lower nuclearity over the silica surface. If such a material is employed as a potential catalyst for the propene oxidation under technical conditions, no activity is observed. If, however, the process is performed under very low pressures, a conversion of 5% is found. This result is discussed in the context of the mechanism proposed for the technical oxidation of propene to acrolein on bismuthmolybdate catalysts.     

钛钼异双核配合物的合成及结构研究

合成了13个钛-钼异双核金属有机配合物,测定了它们的碳、氢、氮、钛及钼的含量,还测定了~1H、~(13)C、~(11)B及~(95)Mo NMR和红外、质谱、电导数据.确认该类配合物分子中存在Ti-Mo之间的金属-金属键和一个桥羰基键,并具有如下结构:其中R=H,Pz(吡唑基);R'=CH_3,OCH_3,OC_2H_5;X=H,CH_3;Y=H,CH_3.     

Tetraphosphinitoresorcinarene complexes: dynamic clusters with silver(I) and copper(I) halides

Silver(I) and copper(I) halide derivatives of several tetrakis(diphenylphosphinito)resorcinarene ligands are reported. The complexes [resorcinarene(O(2)CR)(4)(OPPh(2))(4)(M(5)X(5))], with resorcinarene = (PhCH(2)CH(2)CHC(6)H(2))(4), R = C(6)H(11), 4-C(6)H(4)Me, C(4)H(3)S, OCH(2)CCH, or OCH(2)Ph, M = Ag, X = Cl, Br, or I, M = Cu, and X = Cl or I, contain a crownlike [P(4)M(5)X(5)] metal halide cluster. These crown clusters were found to be dynamic in solution, as studied by variable-temperature NMR, and easily fragment to give the corresponding complexes containing [P(4)M(4)X(5)](-) and [P(4)M(2)(micro-X)](+) units. Reaction of pentasilver crown clusters with triflic acid gave the corresponding disilver complexes [resorcinarene(O(2)CR)(4)(OPPh(2))(4)]Ag(2)(micro-Cl)]]CF(3)SO(3). Thus, these resorcinarene-based ligands act as a platform for the easy and reversible assembly of copper(I) and silver(I) clusters with novel structures.     

Chlorophosphonates: inexpensive precursors for stereodefined chloro-substituted olefins and unsymmetrical disubstituted acetylenes

New chlorophosphonates bearing a 1,3,2-dioxaphosphorinane ring which are useful for the stereospecific synthesis of 5-chlorofurfuryl substituted olefins and chloro-substituted dienes have been obtained by an easy, inexpensive route. The utility of some of these in the synthesis of ferrocenyl- and anthracenyl-substituted unsymmetrical acetylenes has been explored. The structures of the phosphonates (OCH(2)CMe(2)CH(2)O)P(O)CH(2)(C(4)H(2)ClO) (4) and (OCH(2)CMe(2)CH(2)O)P(O)(CH=CHCH(Cl)Ph (7) have been determined; in addition, the stereochemistry of (5-chlorofurfuryl)CH=CH(4-ClC(6)H(4)) (13b) and 2, 4-Cl(2)C(6)H(3)-CH=CH-CH=C(Ph)Cl (14a) is unambiguously proved by the X-ray structure determination.     

C-H cleavage and double alkyl additions to the disulfido ligand in dicyanido-bridged Ru2S2 complexes

Novel dicyanido-bridged dicationic RuIIISSRuIII complexes [{Ru(P(OCH3)3)2}2(mu-S2)(mu-X)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (4, X=Cl, Br) were synthesized by the abstraction of the two terminal halide ions of [{RuX(P(OCH3)3)2}2(mu-S2)(mu-X)2] (1, X=Cl, Br) followed by treatment with m-xylylenedicyanide. 4 reacted with 2,3-dimethylbutadiene to give the C4S2 ring-bridged complex [{Ru(P(OCH3)3)2}2{mu-SCH2C(CH3)=C(CH3)CH2S}(mu-X)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (6, X=Cl, Br). In addition, 4 reacted with 1-alkenes in CH3OH to give alkenyl disulfide complexes [{Ru(P(OCH3)3)2}2{mu-SS(CH2C=CHR)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3) (7: R=CH2CH3, 9: R=CH2CH2CH3) and alkenyl methyl disulfide complexes [{Ru(P(OCH3)3)2}2{mu-S(CH3)S(CH2C=HR)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (8: R=CH2CH3, 10: R=CH2CH2CH3) via the activation of an allylic C-H bond followed by the elimination of H+ or condensation with CH3OH. Additionally, the reaction of 4 with 3-penten-1-ol gave [{Ru(P(OCH3)3)2}2{mu-SS(CH2C=CHCH2OH)}(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3) (11) via the elimination of H+ and [{Ru(P(OCH3)3)2}2(mu-SCH2CH=CHCH2S)(mu-Cl)2{mu-m-C6H4(CH2CN)2}](CF3SO3)2 (12) via the intramolecular elimination of a H2O molecule. 12 was exclusively obtained from the reaction of 4 with 4-bromo-1-butene.     

Carboxylic‐Acid‐Passivated Metal Oxide Nanocrystals: Ligand Exchange Characteristics of a New Binding Motif

Ligand exchange is central in the processing of inorganic nanocrystals (NCs) and requires understanding of surface chemistry. Studying sterically stabilized HfO₂ and ZrO₂ NCs using ¹H solution NMR and IR spectroscopy as well as elemental analysis, this paper demonstrates the reversible exchange of initial oleic acid ligands for octylamine and self‐adsorption of oleic acid at NC surfaces. Both processes are incompatible with an X‐type binding motif of carboxylic acids as reported for sulfide and selenide NCs. We argue that this behavior stems from the dissociative adsorption of carboxylic acids at the oxide surface. Both proton and carboxylate moieties must be regarded as X‐type ligands yielding a combined X₂ binding motif that allows for self‐adsorption and exchange for L‐type ligands.     

Formation of a novel trinuclear spirostannoxane tin(IV) compound. Crystal and molecular structure of [Sn(nBu)(Cl)[(OCH2CH2S)2Sn(nBu)]2] and the stannolane [(nBu)Sn(SCH2CH2O)SCH2CH2OH

The reaction of nBuSnCl3 and the sodium salt of 2-mercaptoethanol (1:1) in ethanol gave the compound Sn(nBu)(Cl)[(OCH2CH2S)2Sn(nBu)]2 (1). [(nBu)Sn(SCH2CH2O)SCH2CH2OH] (2) was initially isolated from the reaction of 1 with nBuMgCl as a rearrangement product but was also synthesized from nBuSn(O)OH and two molar equivalents of 2-mercaptoethanol. Both compounds were characterized by means of IR, 119Sn, 13C, and 1H NMR, FAB mass spectroscopy, and elemental analyses. The structures were determined by single-crystal X-ray diffraction. 1 crystallizes in the monoclinic Cc space group (a = 18.492(3) A, b = 17.329(2) A, c = 10.787(1) A, beta = 111.88(1) degrees, Z = 4), while 2 crystallizes in the orthorhombic Pbca space group (a = 14.458(2) A, b = 10.393(1) A, c = 16.479(2) A, Z = 8). 1 is a trimetallic Tin(IV) compound in which the central atom is in 6-fold coordination, while the two remaining tin atoms show 5-fold coordination. Both pentacoordinated tin atoms are bonded to a butyl group and to the oxygen and the sulfur atoms from two [OCH2CH2S]2- ligands forming two stannolanes, which are fused with the hexacoordinated tin atom forming a distannoxane system. This arrangement is quite different from previous ladder or staircase structures. NMR data point to maintenance of this structure in solution. 2 consists of [(nBu)Sn(SCH2CH2O)(SCH2CH2OH)] units, which are associated via intermolecular Sn-O interactions building up a dimer. The tin atom forms two "stannolane" units by interaction with [OCH2CH2S]2- and [HOCH2CH2S]- ligands.     

Phosphine-free synthesis of CdSe nanocrystals

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).