Fluoride ion capture from water with a cationic borane

The reaction of the [Li(THF)4]+ salt of dimesityl-1,8-naphthalenediylborate with [Me2NCH2]I affords a borane (1-(Mes2B)-8-(Me2NCH2)-C10H6) which can be converted into a cationic borane [3]+ ([1-(Mes2B)-8-(Me3NCH2)-C10H6]+) by methylation with MeOTf. This cationic borane promptly complexes fluoride to afford the corresponding zwitterionic ammonium/fluoroborate 3-F (1-(Mes2FB)-8-(Me3NCH2)-C10H6). Cation [3]+ fails to react with chloride, bromide, and iodide indicating that fluoride complexation is selective. Structural, spectroscopic, and computational studies carried out on 3-F show the existence of an unusual C-H...F-B hydrogen bond. Remarkably, [3]+ captures fluoride from water under biphasic conditions (H2O/CHCl3) to form 3-F. The high fluoride affinity of [3]+ can be correlated to the Coulombic forces which stabilize the B-F bond against heterolysis.     

Synthesis and structural characterization of mono- and bisfunctional o-carboranes

Functionalized o-carboranes are interesting ligands for transition metals. Reaction of LiC2B10H11 with Me2NCH2CH2Cl in toluene afforded 1-Me2NCH2CH2-1,2-C2B10H11 (1). Treatment of 1 with 1 equiv. of n-BuLi gave [(Me2NCH2CH2)C2B10H10]Li ([1]Li), which was a very useful synthon for the production of bisfunctional o-carboranes. Reaction of [1]Li with RCH2CH2Cl afforded 1-Me2NCH2CH2-2-RCH2CH2-1,2-C2B10H10 (R = Me2N (2), MeO (3)). 1 and 2 were also prepared from the reaction of Li2C2B10H10 with excess Me2NCH2CH2Cl. Treatment of [1]Li with excess MeI or allyl bromide gave the ionic salts, [1-Me3NCH2CH2-2-Me-1,2-C2B10H10][I] (4) and [1-Me2N(CH2=CHCH2)CH2CH2-2-(CH2=CHCH2)-1,2-C2B10H10][Br] (6), respectively. Interaction of [1]Li with 1 equiv. of allyl bromide afforded 1-Me2NCH2CH2-2-(CH2=CHCH2)-1,2-C2B10H10 (5). Treatment of [1]Li with excess dimethylfulvene afforded 1-Me2NCH2CH2-2-C5H5CMe2-1,2-C2B10H10 (7). Interaction of [1]Li with excess ethylene oxide afforded an unexpected product 1-HOCH2CH2-2-(CH2=CH)-1,2-C2B10H10 (8). 1 and 3 were conveniently converted into the corresponding deborated compounds, 7-Me2NHCH2CH2-7,8-C2B9H11 (9) and 7-Me2NHCH2CH2-8-MeOCH2CH2-7,8-C2B9H10 (10), respectively, in MeOH-MeOK solution. All of these compounds were characterized by various spectroscopic techniques and elemental analyses. The solid-state structures of 4 and 6-10 were confirmed by single-crystal X-ray analyses.     

Ammonium boranes for the selective complexation of cyanide or fluoride ions in water

With the recognition of aqueous fluoride and cyanide ions as an objective, we have investigated the anion binding properties of two isomeric ammonium boranes, namely [p-(Mes2B)C6H4(NMe3)]+ ([1]+) and [o-(Mes2B)C6H4(NMe3)]+ ([2]+). These cationic boranes, which could be obtained by reaction of the known 4- and 2-dimesitylboryl-N,N-dimethylaniline with MeOTf, have been investigated both experimentally and computationally. They both react with fluoride and cyanide ions in organic solvents to afford the corresponding fluoroborate/ or cyanoborate/ammonium zwitterions 1F, 1CN, 2F, and 2CN. In aqueous solution, however, these cationic boranes behave as remarkably selective receptors. Indeed, [1]+ only complexes cyanide ions while [2]+ only complexes fluoride ions. In H2O/DMSO 60:40 vol (HEPES 6 mM, pH 7), the cyanide binding constant of [1]+ and the fluoride binding constant of [2]+ are respectively equal to 3.9 (+/-0.1) x 108 and 910 (+/-50) M-1. Structural and computational studies indicate that both steric and electronic effects contribute to the unusual selectivity displayed by these cationic boranes. Owing to favorable Coulombic effects, the para-derivative [1]+ has a very high affinity for cyanide; yet these effects are not sufficiently intense to allow complexation of the more efficiently hydrated and less basic fluoride anion. In the case of the ortho-derivative [2]+, the proximity of the ammonium moiety leads to an increase in the Lewis acidity of the boron center thus making fluoride binding possible. However, steric effects prevent cyanide coordination to the boron center of [2]+. Finally, cation [1]+ and [2]+ bind their dedicated anions reversibly and show a negligible response in the presence of other common anions including Cl-, Br-, I-, NO3-, OAc-, H2PO4-, and HSO4-.     

Synthesis and properties of a cationic bidentate Lewis acid

As part of our efforts to increase the fluoride affinity of bidentate Lewis acids, we have set out to determine if the F(-) anion chelation occurring in such systems can be complemented by favorable Coulombic attractions. To explore this idea, the neutral B/Hg bidentate Lewis acid 1-{Mes(2)B}-8-{(2,6-Me(2)-4-Me(2)NC(6)H(2))Hg}C(10)H(6) (3) and its cationic analogue [1-{Mes(2)B}-8-{(2,6-Me2-4-Me(3)NC(6)H(2))Hg}C(10)H(6)]+ ([4]+) have been synthesized and studied. Compound 3 as well as the triflate salt of [4]+ react with [S(NMe(2))3][Me(3)SiF(2)] to afford the corresponding fluoride complexes [3-micro(2)-F]- and [4-micro(2)-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic centers in both [3-micro(2)-F]- and [4-micro(2)-F]. UV-vis titration experiments carried out in tetrahydrofuran/water (9/1, v/v) mixtures indicate that the fluoride binding constants of 3 and [4]+ are clearly differentiated and are equal to 1.3 (+/-0.1) x 10(2) M(-1) and 6.2 (+/-0.2) x 10(4) M(-1), respectively. The enhanced fluoride binding constant of [4]+, when compared to 3, confirms that the chelate effect occurring in these types of fluoride receptors can be combined with favorable Coulombic attractions to strengthen the host-guest interaction. Cation [4]+ remains highly selective for F- over other environmentally abundant anions including Cl-, Br-, NO(3)(-), H(2)PO(4)(-), and HSO(4)(-) and shows only a weak response to OAc(-). Finally, the addition of an aqueous solution of Al3+ to a solution containing [4-micro(2)-F] leads to complete regeneration of [4]+, showing that F(-) binding is reversible.     

Synthesis and structures of zirconium complexes [Et2H2N]+2[ZrCl6]2–, [Me3NCH2Ph]+2[ZrCl6]2–•MeCN, [Ph3PC6H4(CHPh2-4)]+2[ZrCl6]2–•2 MeCN,and [Ph4Sb]+2[ZrCl6]2–

Russian Chemical Bulletin - The complexes [Et2H2N]+2[ZrCl6]2– (1), [Me3NCH2Ph]+2[ZrCl6]2–?MeCN (2), [Ph3PC6H4(CHPh2-4)]+2[ZrCl6]2–?2 MeCN (3), and...     

Chemical reactivity and electrochemistry of metal-metal-bonded zincocenes

Attempts to prepare mixed-ligand zinc-zinc-bonded compounds that contain bulky C(5)Me(5) and terphenyl groups, [Zn(2)(C(5)Me(5))(Ar')], lead to disproportionation. The resulting half-sandwich Zn(II) complexes [(η(5)-C(5)Me(5))ZnAr'] (Ar' = 2,6-(2,6-(i)Pr(2)C(6)H(3))(2)-C(6)H(3), 2; 2,6-(2,6-Me(2)C(6)H(3))(2)-C(6)H(3), 3) can also be obtained from the reaction of [Zn(C(5)Me(5))(2)] with the corresponding LiAr'. In the presence of pyr-py (4-pyrrolidinopyridine) or DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), [Zn(2)(η(5)-C(5)Me(5))(2)] reacts with C(5)Me(5)OH to afford the tetrametallic complexes [Zn(2)(η(5)-C(5)Me(5))L(μ-OC(5)Me(5))](2) (L = pyr-py, 6; DBU, 8), respectively. The bulkier terphenyloxide Ar(Mes)O(-) group (Ar(Mes) = 2,6-(2,4,6-Me(3)C(6)H(2))(2)-C(6)H(3)) gives instead the dimetallic compound [Zn(2)(η(5)-C(5)Me(5))(OAr(Mes))(pyr-py)(2)], 7, that features a terminal Zn-OAr(Mes) bond. DFT calculations on models of 6-8 and also on the Zn-Zn-bonded complexes [Zn(2)(η(5)-C(5)H(5))(OC(5)H(5))(py)(2)] and [(η(5)-C(5)H(5))ZnZn(py)(3)](+) have been performed and reveal the nonsymmetric nature of the Zn-Zn bond with lower charge and higher participation of the s orbital of the zinc atom coordinated to the cyclopentadienyl ligand with respect to the metal within the pseudo-ZnL(3) fragment. Cyclic voltammetric studies on [Zn(2)(η(5)-C(5)Me(5))(2)] have been also carried out and the results compared with the behavior of [Zn(C(5)Me(5))(2)] and related magnesium and calcium metallocenes.     

The [2,5,12-C3B8H15]- anion, the first representative of the eleven-vertex hypho family of tricarbaboranes

In one synthetic step from the readily available 9-Me(2)SCH(2)-nido-7,8-C(2)B(9)H(11) (compound 1), the first representative of the eleven-vertex hypho family of tricarbaboranes, [2,5,12-C(3)B(8)H(15)][X] (X=[NMe4]+ or [PPh4]+) (compound 2), has been isolated in 32% yield and structurally characterised by single-crystal X-ray diffraction, multi-nuclear NMR spectroscopy, mass spectrometry, and computational methods. Both [NMe4]+ or [PPh4]+ salts of anion 2 were found to undergo degradative conversion to the [hypho-6,7-C(2)B(6)H(13)]- anion (anion 3) in alkaline medium. The [PPh4]+ salt of anion 2 converted quantitatively to the [6-CH3-arachno-5,10-C(2)B(8)H(12)]- anion (anion 4) if passed through a silica column or to the neutral 5-CH3-arachno-6,9-C(2)B(8)H(13) (compound 5) on treatment of its [NMe4]+ salt with dilute HCl. Moreover, the reaction of compound 2 with [RhCl2(C(5)Me(5))]2 afforded the eleven-vertex ruthenadicarbaborane [1-C(5)Me(5)-4-CH(3)-closo-1,2,3-RhC(2)B(8)H(9)] (compound 8). All these reactions resulted in an extrusion of one of the cluster carbon atoms into an exoskeletal position.     

Eta6-mesityl,eta1-imidazolinylidene-carbene-ruthenium(II) complexes: catalytic activity of their allenylidene derivatives in alkene metathesis and cycloisomerisation reactions

The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me(3)C(6)H(2)CH(2))NCH(2)CH(2)N(R)Cdbond;](2) (2a: R=CH(2)CH(2)OMe, 2 b R=CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R=CH(2)Mes) group on the nitrogen atom, with [RuCl(2)(arene)](2) (arene=p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(II) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta(1)-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene,eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl(2)(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl(2)(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HCtbond;CCPh(2)OH, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl(2)[eta(1)-CN(CH(2)C(6)H(2)Me(3)-2,4,6)CH(2)CH(2)N- (CH(2)CH(2)OMe)](C(6)Me(6))] 3 with a monodentate carbene ligand and [RuCl(2)[eta(1)-CN[CH(2)(eta(6)-C(6)H(2)Me(3)-2,4,6)]CH(2)CH(2)N-(CH(2)C(6)H(2)Me(3)-2,4,6)]] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.     

Proton induced P-H and Mo-H bond activation at the phosphide bridged dimolybdenum complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4](R = Cy, 2,4,6-C6H2R'3; R'= H, Me, tBu)

The new hydride complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4] having bulky substituents (R = 2,4,6-C(6)H2tBu3= Mes*, R = 2,4,6-C6H2Me3= Mes) have been prepared in good yield by addition of Li[PHR] to the triply bonded [Mo2Cp2(CO)4] and further protonation of the resulting anionic phosphide complex [Mo2Cp2(mu-PHR)(CO)4]-. Protonation of the Mes* compound with either [H(OEt2)2][B{3,5-C6H3(CF3)2}4] or HBF4.OEt2 gives the cationic phosphinidene complex [Mo2Cp2(mu-H)(mu-PMes*)(CO)4]+ in high yield. In contrast, protonation of the analogous hydride compounds with Mes or Cy substituents on phosphorus give the corresponding unsaturated tetracarbonyls [Mo2Cp2(mu-PHR)(CO)4]+, which are unstable at room temperature and display a cis geometry. Decomposition of the latter give the electron-precise pentacarbonyls [Mo2Cp2(mu-PHR)(mu-CO)(CO)4]+, also displaying a cis arrangement of the metal fragments. In the presence of BF4- as external anion, fluoride abstraction competes with carbonylation to yield the neutral fluorophosphide hydrides [Mo2Cp2(mu-H)(mu-PFR)(CO)4]. Similar results were obtained in the protonation reactions of the hydride compounds having a Ph substituent on phosphorus. In that case, using HCl as protonation reagent gave the chloro-complex [Mo2ClCp2(mu-PHPh)(CO)4] in good yield. The structures and dynamic behaviour of the new compounds are analyzed on the basis of solution IR and 1H, 31P, 19F and 13C NMR data as well as the X-ray studies carried out on [Mo2Cp2(mu-H)(mu-PHMes)(CO)4](cis isomer), [Mo2Cp2(mu-H)(mu-PFMes)(CO)4](trans isomer), [Mo2Cp2(mu-PHCy)(mu-CO)(CO)4](BF4) and [Mo2ClCp2(mu-PHPh)(CO)4].     

Fluoride ion complexation by a B(2)/Hg heteronuclear tridentate Lewis acid

The reaction of [Li(THF)(4)][1,8-mu-(Mes(2)B)C(10)H(6)] with HgCl(2) affords [1,1'-(Hg)-[8-(Mes(2)B)C(10)H(6)](2)] () or [1-(ClHg)-8-(Mes(2)B)C(10)H(6)] (), depending on the stoichiometry of the reagents. These two new compounds have been characterized by (1)H, (13)C, (11)B and (199)Hg NMR, elemental analysis and X-ray crystallography. The cyclic voltammogram of in THF shows two distinct waves observed at E(1/2) -2.31 V and -2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride titration experiments monitored by electrochemistry suggest that binds tightly to one fluoride anion and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration experiment which indicates that the first fluoride binding constant (K(1)) is greater than 10(8) M(-1) while the second (K(2)) equals 5.2 (+/- 0.4) x 10(3) M(-1). The fluoride binding properties of have been compared to those of [1-(Me(2)B)-8-(Mes(2)B)C(10)H(6)] () and [1-((2,6-Me(2)-4-Me(2)NC(6)H(2))Hg)-8-(Mes(2)B)C(10)H(6)] (). Both experimental and computational results indicate that its affinity for fluoride anions is comparable to that of but significantly lower than that of the diborane . In particular, the fluoride binding constants of , and in chloroform are respectively equal to 5.0 (+/- 0.2) x 10(5) M(-1), 1.0 (+/- 0.2) x 10(3) M(-1) and 1.7 (+/- 0.1) x 10(3) M(-1). Determination of the crystal structures of the fluoride adducts [S(NMe(2))(3)][-mu(2)-F] and [S(NMe(2))(3)][-mu(2)-F] along with computational results indicate that the higher fluoride binding constant of arises from a strong chelate effect involving two fluorophilic boron centers.     

Ruthenium(II) polypyridyl complexes: potential precursors, metalloligands, and topo II inhibitors

Neutral and cationic mononuclear complexes containing both group 15 and polypyridyl ligands [Ru(kappa3-tptz)(PPh3)Cl2] [1; tptz=2,4,6-tris(2-pyridyl)-1,3,5-triazine], [Ru(kappa3-tptz)(kappa2-dppm)Cl]BF4 [2; dppm=bis(diphenylphosphino)methane], [Ru(kappa3-tptz)(PPh3)(pa)]Cl (3; pa=phenylalanine), [Ru(kappa3-tptz)(PPh3)(dtc)]Cl (4; dtc=diethyldithiocarbamate), [Ru(kappa3-tptz)(PPh3)(SCN)2] (5) and [Ru(kappa3-tptz)(PPh3)(N3)2] (6) have been synthesized. Complex 1 has been used as a metalloligand in the synthesis of homo- and heterodinuclear complexes [Cl2(PPh3)Ru(micro-tptz)Ru(eta6-C6H6)Cl]BF4 (7), [Cl2(PPh3)Ru(mu-tptz)Ru(eta6-C10H14)Cl]PF6 (8), and [Cl2(PPh3)Ru(micro-tptz)Rh(eta5-C5Me5)Cl]BF4 (9). Complexes 7-9 present examples of homo- and heterodinuclear complexes in which a typical organometallic moiety [(eta6-C6H6)RuCl]+, [(eta6-C10H14)RuCl]+, or [(eta5-C5Me5)RhCl]+ is bonded to a ruthenium(II) polypyridine moiety. The complexes have been fully characterized by elemental analyses, fast-atom-bombardment mass spectroscopy, NMR (1H and 31P), and electronic spectral studies. Molecular structures of 1-3, 8, and 9 have been determined by single-crystal X-ray diffraction analyses. Complex 1 functions as a good precursor in the synthesis of other ruthenium(II) complexes and as a metalloligand. All of the complexes under study exhibit inhibitory effects on the Topoisomerase II-DNA activity of filarial parasite Setaria cervi and beta-hematin/hemozoin formation in the presence of Plasmodium yoelii lysate.     

Iron-molybdenum charge-transfer hybrids containing organometallic and inorganic fragments bridged by aryldiazenido ligands in a mu-eta(6):eta(1) coordination mode: syntheses, characterization, X-ray structures, electrochemistry, and theoretical investigation

Representative members of a new family of covalently bonded charge-transfer molecular hybrids, of general formula [(eta5-C5H5)Fe(mu,eta6:eta1-p-RC6H4NN)Mo(eta2-S2CNEt2)3] +PF6- (R: H, 5+PF6-; Me, 6+PF6-; MeO, 7+PF6-) and [(eta5-C5Me5)Fe(mu,eta6:eta1-C6H5NN)Mo(eta2-S2CNEt2)3]+PF6-, 8+PF6-, have been synthesized by reaction of the corresponding mixed-sandwich organometallic hydrazines [(eta5-C5H5)Fe(eta6-p-RC6H4NHNH2)]+PF6- (R: H, 1+PF6-; Me, 2+PF6-; MeO, 3+PF6-) and [(eta5-C5Me5)Fe(eta6-C6H5NHNH2)]+PF6-, 4+PF6-, with cis-dioxomolybdenum(VI) bis(diethyldithiocarbamato) complex, [MoO2(S2CNEt2)2], in the presence of sodium diethyldithiocarbamato trihydrate, NaSC(=S)NEt2.3H2O, in refluxing methanol. These iron-molybdenum complexes consist of organometallic and inorganic fragments linked each other through a pi-conjugated aryldiazenido bridge coordinated in eta6 and eta1 modes, respectively. These complexes were fully characterized by FT-IR, UV-visible, and 1H NMR spectroscopies and, in the case of complex 7+PF6-, by single-crystal X-ray diffraction analysis. Likewise, the electrochemical and solvatochromic properties were studied by cyclic voltammetry and UV-visible spectroscopy, respectively. The electronic spectra of these hybrids show an absorption band in the 462-489 and 447-470 nm regions in CH2Cl2 and DMSO, respectively, indicating the existence of a charge-transfer transition from the inorganic donor to the organometallic acceptor fragments through the aryldiazenido spacer. A rationalization of the properties of 5+PF6--8+PF6- is provided through DFT calculations on a simplified model of 7+PF6-. Besides the heterodinuclear complexes 5+PF6--8+PF6-, the mononuclear molybdenum diazenido derivatives, [(eta1-p-RC6H4NN)Mo(eta2-S2CNEt2)3] (R: H, 9; Me, 10; MeO, 11), resulting from the decoordination of the [(eta5-C5H5)Fe]+ moiety of complexes 5+PF6--7+PF6-, were also isolated. For comparative studies, the crystalline and molecular structure of complex 10.Et2O was also determined by X-ray diffraction analysis and its electronic structure computed.     

Studies on the properties of organoselenium(IV) fluorides and azides

The reaction of organoselenides and -diselenides (R2Se and (RSe)2) with XeF2 furnished the corresponding organoselenium(IV) difluorides R2SeF2 (R=Me (1), Et (2), iPr (3), Ph (4), Mes (=2,4,6-(Me)3C6H2) (5), Tipp (=2,4,6-(iPr)3C6H2) (6), 2-Me 2NCH2C6H4 (7)), and trifluorides RSeF3 (R=Me (8), iPr (9), Ph (10), Mes (11), Tipp (12), Mes* (=2,4,6-(tBu) 3C6H2) (13), 2-Me2NCH2C6H4 (14)), respectively. In addition to characterization by multinuclear NMR spectroscopy, the first molecular structure of an organoselenium(IV) difluoride as well as the molecular structures of subsequent decomposition products have been determined. The substitution of fluorine atoms with Me3SiN3 leads to the corresponding organoselenium(IV) diazides R2Se(N3)2 (R=Me (15), Et (16), iPr (17), Ph (18), Mes (19), 2-Me 2NCH2C6H4 (20)) and triazides RSe(N3)3 (R=Me (21), iPr (22), Ph (23), Mes (24), Tipp (25), Mes* (26), 2-Me2NCH2C6H4 (27)), respectively. The organoselenium azides are extremely temperature-sensitive materials and can only be handled at low temperatures.     

Highly stable neutral and positively charged dicarbollide sandwich complexes

Novel sandwich metallacarboranes commo-[3,3'-Ni(8-SMe2-1,2-C2B9H10)2] (1), commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2]+ (2+), commo-[3,3'-Ru(8-SMe2-1,2-C2B9H10)2] (4) and commo-[3,3'-Fe(8-SMe2-1,2-C2B9H10)2] (5) have been prepared by reaction of [10-SMe2-7,8-nido-C2B9H10]- with NiCl2 x 6 H2O, CoCl2, [RuCl2(dmso)4] and [FeCl2(dppe)], respectively. Reduction of 2+ with metallic Zn leads to the neutral and isolable complex commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2] (3). Theoretical calculations using the ZINDO/1 semiempirical method show three energy minima for complexes 1-3 and 5 that agree with the presence of three different rotamers in solution at low temperature, while four relative energy minima have been found for 4. The calculated rotational energy barriers for complexes 1-5 have been found in the range 5.2+/-0.2 and 11.5+/-0.2 kcal mol(-1). These values are in agreement with the experimental data calculated for complexes 2+ and 5. Only one rotamer is found in the X-ray crystal structure of complexes 1-3, while two are observed for 4. Neutral complexes 1, 3 and 4 exhibit a gauche conformation, whereas a cisoid conformation is found for the 2+ ion. Rotamers evident from X-ray diffraction studies are in agreement with the global energy minimum calculated by the ZINDO/1 method. The electrochemical studies conducted on 1, 3, 4 and 5 support the proposal that the charge-compensated ligand [10-SMe2-7,8-nido-C2B9H10]- stabilises lower oxidation states in metals than the dianionic [7,8-nido-C2B9H11]2- and even the [C5H5]- ligands.     

Unusual 9-->10 rearrangement of the substituted cage carbon in the ferratricarbollide series. Synthesis of the isomeric complexes [2-eta 5-(C5H5)-10-X-closo-2,1,7,10-FeC3B8H10] (where X = H2N, MeHN, Me2N, and ButHN)

Treatment of the zwitterionic amine tricarbollides of general formula 7-L-nido-7,8,9-C3B8H10 (1) (where L = Me2HN (1c) and ButH2N (1d)) with [(eta 5-C5H5)Fe(CO)2]2 in refluxing mesitylene resulted in the formation of a mixture of the known compounds [2-(eta 5-C5H5)-9-X-closo-2,1,7,9-FeC3B8H10] (2) (where X = H2N (2a), Me2N (2c), and ButHN (2d)) and a series of new, isomeric ferratricarbollylamines [2-(eta 5-C5H5)-10-X-closo-2,1,7,10-FeC3B8H10] (3) (where X = H2N (3a), Me2N (3c), and ButHN (3d)) in moderate yields. Complexes of type 3 (where X = H2N (3a), MeHN (3b), Me2N (3c), and ButHN (3d)) were also obtained readily by heating complexes of type 2 (where X = H2N (2a), MeHN (2b), Me2N (2c), ButHN (2d), and Bu(t)(Me)N (2e)) at ca. 300 degrees C for 10 min. All the complexes of type 3 contain reactive amine functions in meta positions with respect to the metal center. The observed 9-->10 rearrangement of the substituted cluster carbon is quite unexpected and is believed to result from higher thermodynamic stability of the 10-substituted isomers. The structures of all compounds of type 3 were established by high-field NMR spectroscopy and mass spectrometry, and that of 3d was determined by an X-ray diffraction study.     

Stable exo-nido-metallacarboranes (M = Os(IV)) that incorporate meta-carborane-based dicarbollide ligands

Reactions of the [K]+ salts of the [nido-7,9-C2B9H12]- anion (2) and its C-phenylated derivative [7-Ph-nido-7,9-C2B9H11]- (4) with [OsCl2(PPh3)3] (3) proceed in benzene at ambient temperature with the formation of 16-electron chlorohydrido-Os(IV) exo-nido complexes, [exo-nido-10,11-{(Ph3P)2OsHCl}-10,11-(mu-H)2-7-R-7,9-C2B9H8] (5: R = H; 6: R = Ph), along with the small amounts of the charge-compensated nido-carboranes [nido-7,9-C2B9H11PPh3] (7) and [7-Ph-nido-7,9-C2B9H10PPh3] (8) as byproducts. However, when carried out under mild heating in ethanol, the reaction of 2 with 3 selectively afforded a 16-electron dihydrido-Os(IV) exo-nido complex [exo-nido-10,11-{(Ph3P)2OsH2}-10,11-(mu-H)2-7,9-C2B9H9] (9). Structures of both complexes 5 and 9 have been confirmed by single-crystal X-ray diffraction studies, which revealed that nido-carboranes in these species function as a bidentate dicarbollide ligands [7-R-nido-7,9-C2B9H10]2- linked to the Os(IV) center via two B-H...Os bonds involving adjacent B-H vertices in the upper CBCBB belt of the carborane cage. Thus, compounds 5 and 9 represent the first structurally characterized exo-nido-metallacarboranes based on meta-dicarbollide-type ligands. Variable-temperature 1H and 31P{1H} NMR experiments indicate that complex 9 is fluxional in solution and shows an unusual exchange between terminal Os-(H)2 and bridging {B-H}2...Os hydrogen atoms. Upon heating in d8-THF at 65 degrees C, complex 9 converts irreversibly to its closo isomer [2,2-(PPh3)2-2,2-H2-closo-2,1,7-OsC2B9H11] (13), which could thus be obtained as a pure crystalline solid. The structure of 13 has been established on the basis of analytical and multinuclear NMR data and a single-crystal X-ray diffraction study.     

The structural characteristics of organozinc complexes incorporating N,N'-bidentate ligands

Dimethylzinc reacts with an excess of N-2-pyridylaniline 6 to give the homoleptic species, Zn[PhN(2-C(5)H(4)N)](2) 8. Single crystal X-ray diffraction reveals a solid-state dimer based on an 8-membered (NCNZn)(2) core motif. Zn[CyN(2-C(5)H(4)N)]Me (Cy =c-C(6)H(11)) 10, prepared by the combination of ZnMe(2) with the corresponding cyclohexyl-substituted pyridylamine, is also dimeric in the solid state but reveals a central (ZnN)(2) metallacycle. Employment of (p-Tol)NH(2-C(5)H(4)N)(p-Tol = 4-MeC(6)H(4)) 11 yielded the tris(zinc) adduct Zn(3)[(p-Tol)N(2-C(5)H(4)N)](4)Me(2) 12, which incorporates a central chiral molecule of 'Zn[(p-Tol)N(2-C(5)H(4)N)](2)' 12a, that bridges two 'Zn[(p-Tol)N(2-C(5)H(4)N)]Me' 12b units. A similar trimetallic structure is noted when the pyridylaniline substrate 11 is replaced with the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH), affording Zn(3)(hpp)(4)Me(2) 13. Spectroscopic studies point to retention of the solid-state structure of in hydrocarbon solution. Reaction of 13 with dimesityl borinic acid, Mes(2)BOH (Mes = mesityl), affords Zn(3)(hpp)(4)(OBMes(2))(2) 14 in which the trimetallic core is retained. This reactivity is in contrast to the closely related reaction of dimeric Zn[Me(2)NC[N(i)Pr](2)]Me 15 with Mes(2)BOH, which yielded Zn[Me(2)NC[N(i)Pr](2)][OBMes(2)].Me(2)NC[N(i)Pr][NH(i)Pr] 16 as a result of protonation at the guanidine ligand in addition to the Zn-Me bond.     

ortho-Metallated complexes of platinum(II) and diplatinum(I) containing the carbanions (2-diphenylphosphino)phenyl and (2-diphenylphosphino)-n-tolyl (n = 5, 6)

When the ortho-metallated complexes cis-[Pt(kappa(2)-C6H3-5-R-2-PPh2)2] (R = H 1, Me 2) are either heated in toluene or treated with CO at room temperature, one of the four-membered chelate rings is opened irreversibly to give dinuclear isomers [Pt2(kappa(2)-C6H3-5-R-2-PPh2)2(mu-C6H3-5-R-2-PPh2)2] (R = H 10, Me 11). A single-crystal X-ray diffraction study shows the Pt...Pt separation in 10 to be 3.3875(4) A. By-products of the reactions of 1 and 2 with CO are polymeric isomers (R = H 13, Me 14) in which one of the P-C ligands is believed to bridge adjacent platinum atoms intermolecularly. In contrast to the behaviour of 1 and 2, when cis-[Pt(kappa(2)-C6H3-6-Me-2-PPh2)2] (cis-3) is heated in toluene, the main product is trans-3, and reaction of cis-3 with CO gives a carbonyl complex [Pt(CO)(kappa(1)-C-C6H3-6-Me-2-PPh2)(2-C6H3-6-Me-2-PPh2)] 15, in which one of the carbanions is coordinated only through the carbon. Formation of a dimer analogous to 10 or 11 is sterically hindered by the 6-methyl substituent. Comproportionation of 1 or 2 with [Pt(PPh3)2L] (L = PPh3, C2H4) gives diplatinum(I) complexes [Pt2(mu-C6H3-5-R-2-PPh2)2(PPh3)2] (R = H 16, Me 17). An X-ray diffraction study shows that 17 contains a pair of planar-coordinated metal atoms separated by 2.61762(16) A. There is no evidence for the formation of an analogue containing mu-C6H3-6-Me-2-PPh2. The axial PPh3 ligands of 16 are readily replaced by ButNC giving [Pt2(mu-2-C6H4PPh2)2(CNBut)2] 18, which is protonated by HBF4 to form a mu-hydridodiplatinum(II) salt [Pt2(mu-H)(mu-2-C6H4PPh2)2(CNBut)2]BF4 [21]BF4. The J(PtPt) values in [21]BF4 and 18, 2700 Hz and 4421 Hz, respectively, reflect the weakening of the Pt-Pt interaction caused by protonation. Similarly, 16 and 17 react with the electrophiles iodine and strong acids to give salts of general formula [Pt2(mu-Z)(mu-C6H3-5-R-2-PPh2)2(PPh3)2]Y (Y = Z = I, R = H 19+, Me 20+; Z = H, Y = BF4, PF6, OTf, R = H 22+; Z = H, Y = PF6, R = Me 23+). A single-crystal X-ray diffraction study of [23]PF6 shows that the cation has an approximately A-frame geometry, with a Pt-Pt separation of 2.7888(3) A and a Pt-H bond length of 1.62(1) A, and that the 5-methyl substituents have undergone partial exchange with the 4-hydrogen atoms of the PPh2 groups of the bridging carbanion. The latter observation indicates that the added proton of [23]+ undergoes a reversible reductive elimination-oxidative addition sequence with the Pt-C(aryl) bonds.     

Hafnocene catalysts for selective propylene oligomerization: efficient synthesis of 4-methyl-1-pentene by beta-methyl transfer

A series of hafnocene complexes (eta5-C5Me4R1)(eta5-C5Me4R2)HfCl2 with [R1, R2] = [H, H] (1), [Me, H] (2), [Me, Me] (3), [Et, Me] (4), [(i)Pr, Me] (5), [SiMe(3), Me] (6), [(t)Bu, Me] (7), [(n)Bu, Me] (8), [(i)Bu, Me] (9), [Et, Et] (10), [(n)Bu, (n)Bu] (11), [(i)Bu, (i)Bu] (12) was tested as catalyst precursors for propylene oligomerization. Upon activation with methylaluminoxane or [Ph(3)C][B(C(6)F(5))(4)]/Al(i)Bu(3), complexes 2-4 and 8-12 catalyzed the dimerization of propylene to produce 4-methyl-1-pentene with selectivities ranging from 23.9 to 61.6 wt % in the product mixture. The selectivity was dependent on the nature of the substituents R(1) and R(2), with the highest value found for (eta5-C5Me4(i)Bu)2HfCl2 (12). Rapid deactivation was observed for 5-7, whereas (eta5-C5Me4H)2HfCl2 (1) polymerized propylene. 4-Methyl-1-pentene is proposed to form by repeated 1,2-insertion of propylene into the hafnocene methyl cation, followed by selective beta-methyl elimination. Detailed analysis of the byproduct distribution (isobutene, 1-pentene, 2-methyl-1-pentene, 2,4-dimethyl-1-pentene, 4-methyl-1-heptene, 4,6-dimethyl-1-heptene), determined by gas chromatography, was performed with the aid of a stochastic simulation involving rate constants for the propagation by insertion, beta-hydride elimination, and beta-methyl elimination. The rate of termination is dependent on the structure of the growing chain of the active species as well as on the bulkiness of the cyclopentadienyl ligands. The selectivity highly depends on the reaction conditions (pressure, temperature, concentration of methylaluminoxane). The rates of beta-methyl elimination leading to 4-methyl-1-pentene were proportional to propylene pressure for 2-4 and 8-10 but practically independent from propylene pressure for the sterically bulkier derivatives 11-12.