Olefin metathesis featuring ruthenium indenylidene complexes with a sterically demanding NHC ligand

The synthesis and characterization of two new ruthenium indenylidene complexes [RuCl(2)(SIPr)(Py)(Ind)] 6 and [RuCl(2)(SIPr)(3-BrPy)(Ind)] 7 featuring the sterically demanding N-heterocyclic carbene 1,3-bis(2,6-di isopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) are reported. Remarkable activity was observed with these complexes in ring closing, enyne, and cross metathesis of olefins at low catalyst loadings. The performance of SIPr-bearing complexes 6 and 7 as well as [RuCl(2)(SIPr)(PCy(3))(Ind)] 5 in ring opening metathesis polymerization is also disclosed. This work highlights the enormous influence of the neutral "spectator" ligands on catalyst activity and stability.     

Yttrium alkyl complexes with a sterically demanding benzamidinate ligand: synthesis,structure and catalytic ethene polymerisation

The benzamidinate yttrium dialkyl complexes [PhC(NAr)2]Y(CH2SiMe3)2(THF)n (Ar = 2,6-diisopropylphenyl; n = 1, 2) were prepared; when activated with [PhNMe2H][B(C6F5)4], the species with n = 1 polymerises ethene to polyethene with a narrow polydispersity.     

Boron and aluminum complexes of sterically demanding phosphinimines and phosphinimides

Reactions of sterically demanding phosphinimines R3PNH [R=i-Pr (1), t-Bu (2)] were examined. Reactions with B(C6F5)3 formed the adducts (R3PNH)B(C6F5)3 [R=i-Pr (3), t-Bu (4)] in high yield. On the other hand, 2 reacts with HB(OBu)2, evolving H2 to give t-Bu3PNB(OBu)2 (5). The reaction of 2 equiv of 2 with BH3.SMe2 affords the species (t-Bu3PN)2BH (6). In contrast, the reaction of n-Bu(t-Bu)2PNH with BH3.SMe2 results in the formation of the robust adduct n-Bu(t-Bu)2PNH.BH3 (8). An alternative route to borane-phosphinimide complexes involves Me3SiCl elimination, as exemplified by the reaction of BCl2Ph with n-Bu3PNSiMe3, which gives the product n-Bu3PNBCl(Ph) (9). The corresponding reactions of the parent phosphinimines 1 and 2 with AlH3.NMe2Et give the dimers [(mu-i-Pr3PN)AlH2]2 (10) and [(mu-t-Bu3PN)AlH2]2 (11). Species 11 reacts further with Me3SiO3SCF3 to provide [(mu-t-Bu3PN)AlH(OSO2CF3)]2 (12). The reaction of the lithium salt [t-Bu3PNLi]4 (13) with BCl3 proceeds smoothly to give t-Bu3PNBCl2 (14), which is readily alkylated to give t-Bu3PNBMe2 (15). Subsequent reaction of 15 with B(C6F5)3 results in methyl abstraction and the formation of [(mu-t-Bu3PN)BMe]2[MeB(C6F5)3]2 (16). The reaction of 13 in a 2:1 ratio with BCl3 gives the salt [(t-Bu3PN)2B]Cl (17). This species can be methylated to give (t-Bu3PN)2BMe (18), which undergoes subsequent reaction with [Ph3C][X] (X=[B(C6F5)4], [PF6]) to form the related salts [(t-Bu3PN)2B][B(C6F5)4] (19) and [(t-Bu3PN)2B][PF6] (20), respectively. Analogous reactions with [Ph3C][BF4] afforded [t-Bu3PNBF2]2 (21). Compounds 3, 4, 6, 8, 11, 12, 17, 19, and 21 were characterized by X-ray crystallography.     

Mono- and dinuclear molybdenum complexes with sterically demanding cycloheptatrienyl ligands

Sterically demanding cycloheptatrienylium (tropylium) salts of the type (1,3,5-C7H4R3)BF4 [R = t-Bu, (3a)BF4; R = SiMe3, (3b)BF4] have been prepared from the corresponding 1,3,5-trisubstituted benzene derivatives 1 by ring expansion with diazomethane followed by hydride abstraction with triphenylcarbenium tetrafluoroborate, (Ph3C)BF4. Complexation can be achieved by arene exchange and Mo(CO)3 group transfer employing [(eta6-p-xylene)Mo(CO)3] (4) to yield the cationic complexes (5)BF4. In refluxing mesitylene, [(eta7-C7H4t-Bu3)Mo(CO)3]BF4, (5a)BF4, undergoes CO substitution to furnish the mesitylene sandwich complex (6a)BF4. A cyclic voltammetric study reveals that this complex exhibits a reversible one-electron oxidation to the dicationic 17e complex 6a2+, which can also be accessed by chemical oxidation with AgBF4. On the contrary, the reduction of 6a+ is irreversible and does not yield a stable 19e complex 6a. To study the fate of the reduced 19e form, (5a)BF4 was treated with Na2Hg to diastereoselectively afford the C-C coupled bicycloheptatriene complex 7a. Paramagnetic, dinuclear complexes of the type [(eta7-C7H4R3)Mo(mu-Cl)3Mo(eta7-C7H4R3)] (8) have been obtained from the reaction of (5)BF4 with Me3SiCl. These can be regarded as mixed-valence Mo(0)/Mo(+I) compounds with a metal-metal bond order of 0.5. Cyclic voltammetric studies reveal that both complexes 8a and 8b undergo reversible one-electron oxidation as well as reduction. Treatment with one equivalent of ferrocenium hexafluorophosphate leads to removal of the unpaired electron and formation of the diamagnetic complexes (8)PF6. Theoretical DFT calculations have been carried out to further elucidate the bonding in these systems. In addition, the X-ray crystal structures of (5b)BF4, (6a)BF4 x CH2Cl2, (6a)(BF4)2 x (acetone)2, 7a x CH2Cl2, 8a x 0.5C6H14, and (8a)PF6 x Et2O are reported.     

Palladium-catalyzed borylation of sterically demanding aryl halides with a silica-supported compact phosphane ligand

Immobile but active: A silica-supported "compact" phosphane, Silica-SMAP, can be used in the Pd-catalyzed borylation of aryl chlorides or bromides with bis(pinacolato)diboron. The Silica-SMAP/Pd system significantly expands the substrate scope of the borylation toward sterically and electronically challenging aryl halides.     

A sterically demanding iminopyridine ligand affords redox-active complexes of aluminum(III) and gallium(III)

The combination of an electrophilic metal center with a redox active ligand set has the potential to provide reactivity unique from transition metal redox chemistry. In this report, substituted iminopyridine complexes containing monoanionic and dianionic (Me)IP(Mes) ligands have been characterized structurally and electronically. Green ((Me)IP(Mes)(-))AlCl(2) (1), ((Me)IP(Mes)(-))AlMe(2) (2), and ((Me)IP(Mes)(-))GaCl(2) (5) have a doublet spin state which results from the anion radical form of (Me)IP(Mes). Purple ((Me)IP(Mes)(2-))AlCl(OEt(2)) (3), ((Me)IP(Mes)(2-))AlMe(OEt(2)) (4), and ((Me)IP(Mes)(2-))GaCl(OEt(2)) (6) are each diamagnetic. We have also investigated the solvent dependence of the decomposition of the (Me)IP(Mes) anion radical. Complexes 1 and 2 can be obtained from benzene and hexanes whereas the use of ether solvents results in the formation of undesirable ((CH2)IP(Mes)(-))AlCl(2) (1a) and ((CH2)IP(Mes)(-))AlCl(2) (2a) formed by loss of a hydrogen atom from the (Me)IP(Mes)(-) ligand. Electrochemical measurements indicate that 1, 2, and 5 are redox active.     

Alkali metal complexes of sterically demanding amino-functionalized secondary phosphanide ligands

The reaction between {(Me(3)Si)(2)CH}PCl(2) (4) and one equivalent of either [C(6)H(4)-2-NMe(2)]Li or [2-C(5)H(4)N]ZnCl, followed by in situ reduction with LiAlH(4) gives the secondary phosphanes {(Me(3)Si)(2)CH}(C(6)H(4)-2-NMe(2))PH (5) and {(Me(3)Si)(2)CH}(2-C(5)H(4)N)PH (6) in good yields as colourless oils. Metalation of 5 with Bu(n)Li in THF gives the lithium phosphanide [[{(Me(3)Si)(2)CH}(C(6)H(4)-2-NMe(2))P]Li(THF)(2)] (7), which undergoes metathesis with either NaOBu(t) or KOBu(t) to give the heavier alkali metal derivatives [[{(Me(3)Si)(2)CH}(C(6)H(4)-2-NMe(2))P]Na(tmeda)] (8) and [[{(Me(3)Si)(2)CH}(C(6)H(4)-2-NMe(2))P]K(pmdeta)] (9) after recrystallization in the presence of the corresponding amine co-ligand [tmeda = N,N,N',N'-tetramethylethylenediamine, pmdeta = N,N,N',N',N'-pentamethyldiethylenetriamine]. The pyridyl-functionalized phosphane 6 undergoes deprotonation on treatment with Bu(n)Li to give a red oil corresponding to the lithium compound [{(Me(3)Si)(2)CH}(2-C(5)H(4)N)P]Li (10) which could not be crystallized. Treatment of this oil with NaOBu(t) gives the sodium derivative [{[{(Me(3)Si)(2)CH}(2-C(5)H(4)N)P]Na}(2) x (Et(2)O)](2) (11), whilst treatment of with KOBu(t), followed by recrystallization in the presence of pmdeta gives the complex [[{(Me(3)Si)(2)CH}(2-C(5)H(4)N)P]K(pmdeta)](2) (12). Compounds 5-12 have been characterised by (1)H, (13)C{(1)H} and (31)P{(1)H} NMR spectroscopy and elemental analyses; compounds 7-9, and 12 have additionally been characterised by X-ray crystallography. Compounds 7-9 crystallize as discrete monomers, whereas 11 crystallizes as an unusual dimer of dimers and 12 crystallizes as a dimer with bridging pyridyl-phosphanide ligands.     

Synthesis of zirconium, hafnium, and tantalum complexes with sterically demanding hydrazide ligands

The bulky hydrazine t-BuN(H)NMe2 was synthesized via hydrazone and t-BuN(H)N(H)Me intermediates as the major component in a 90:5:5 mixture consisting of t-BuN(H)NMe2, t-BuN(Me)N(H)Me, and t-BuN(Me)NMe2. Reacting the mixture with n-BuLi followed by distillation and fractional crystallization led to the isolation of the ligand precursor LiN(t-Bu)NMe2. Lithium hydrazides, LiN(R)NMe2, were reacted with metal chlorides to afford the hydrazide complexes M(N(Et)NMe2)4 (M = Zr or Hf), MCl(N(R)NMe2)3 (M = Zr, R = i-Pr or t-Bu; M = Hf, R = t-Bu), and TaCl3(N(i-Pr)NMe2)2. The X-ray crystal structures of [LiN(i-Pr)NMe2]4, [LiN(t-Bu)NMe2.THF]2, ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu), and TaCl3(N(i-Pr)NMe2)2 were determined. The structural analyses revealed that the hydrazide ligands in ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu) and TaCl3(N(i-Pr)NMe2)2 are eta2 coordinated.     

Synthesis and structural characterization of tris(2-mercapto-1-adamantylimidazolyl)hydroborato complexes: a sterically demanding tripodal [S3] donor ligand

The tris(2-mercapto-1-adamantylimidazolyl)hydroborato ligand, [Tm(Ad)], has been synthesized via the reaction of 1-adamantyl-2-mercaptoimidazole with MBH(4) (M = Li, K). [Tm(Ad)]M has been used to synthesize a variety of compounds of the main-group and transition elements, including [Tm(Ad)]ZnI, {[Tm(Ad)]GaI}[GaI(4)], {[Tm(Ad)]GaCl}[GaCl(4)], {[Tm(Ad)]GaGa[Tm(Ad)]}[GaCl(4)](2), {[Tm(Ad)](2)In}[InI(4)], [Tm(Ad)]In(κ(2)-mim(Ad))Cl, [Tm(Ad)]Ga→B(C(6)F(5))(3), [Tm(Ad)]In→B(C(6)F(5))(3), and [Tm(Ad)]Re(CO)(3). Structural characterization of [Tm(Ad)]Re(CO)(3) demonstrates that the [Tm(Ad)] ligand is more encapsulating than other [Tm(R)] ligands, including [Tm(Bu(t))], while IR spectroscopic studies indicate that the [Tm(Ad)] and [Tm(Bu(t))] ligands have very similar electron-donating properties.     

Synthesis of aluminium complexes bearing a piperazine-based ligand system

Aluminium complexes bearing the N,N-chelating ligand 1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine (1) have been synthesised. Both monometallic and bimetallic aluminium methyl complexes (2 and 3, respectively) were prepared by treatment of 1 with the appropriate amount of AlMe₃. Complex 2 can be converted to 3 by addition of excess AlMe₃. Bimetallic aluminium-ethyl complex 4 was also prepared. Treatment of 1 with AlEt₂Cl afforded the monometallic chloride complex 5. Treatment of this latter complex with potassium alkoxides (KOR, R = Me, Et, ⁱPr, ^tBu) or AgOTf afforded the corresponding aluminium alkoxide complexes (6, R = Et; 7, R = Me; 8, R = ⁱPr; 9, R = ^tBu; 10, R = OTf) in good yields. Aluminium ethoxide complex 6 was also synthesised by treatment of 1 with AlEt₂OEt. All of these complexes were tested as potential catalysts in the ring-opening polymerisation of rac-lactide and caprolactone with limited success.     

High intensity ultrasound-assisted reduction of sterically demanding nitroaromatics

Sterically demanding nitroaromatic compounds have been prepared and reduced to their corresponding amines with high intensity ultrasound using hydrazine in the presence of a Raney nickel catalyst. These reactions were dependent on catalyst quality, solvent and ultrasonic amplitude and, in comparison to their silent reactions, proceeded much faster and afforded higher yields.     

Synthesis of novel sterically demanding carbo- and heterocyclic beta-ketoesters

We present an easy method for the synthesis of beta-ketoesters starting from various carbocyclic and heterocyclic carboxylic acids and esters. The beta-ketoester side-chain was introduced by a sequence involving alpha-deprotonation and quenching with CO(2), conversion to the corresponding acid chloride and subsequent chain elongation using deprotonated ethyl acetate.     

Structural and functional mimic of galactose oxidase by a copper complex of a sterically demanding [N2O2] ligand

A structural and functional mimic of the galactose oxidase (GOase) enzyme active-site by a copper complex supported over a sterically demanding ligand having [N2O2] donor sites is reported. Specifically, the binding of the histidine (496 and 581) and tyrosine (272 and 495) residues to the copper center in a square-pyramidal fashion in the active-site of galactose oxidase (GOase) enzyme has been modeled in a copper complex, ([(3-tert-butyl-5-methyl-2-hydoxybenzyl)(3'-tert-butyl-5'-methyl-2'-oxobenzyl)(2-pyridylmethyl)]amine)Cu(OAc)) (1b), stabilized over a sterically demanding ligand in which the two phenolate-O atoms mimicked the tyrosine binding while an amine-N and pyridyl-N atoms emulated the histidine binding to the metal center, similar to that in the enzyme active-site. Furthermore, the copper complex 1b is found to be an effective functional model of the enzyme as it efficiently catalyzed the chemoselective oxidation of primary alcohols to aldehydes in high turnover numbers under ambient conditions. An insight into the nature of the active-species was obtained by EPR and CV studies, which in conjunction with the DFT studies, revealed that the active-species is an anti-ferromagnetically coupled diamagnetic radical cation, (1)1b+, obtained by one electron oxidation at the equatorial phenolate-O atom of the ligand in the 1b complex.     

Preparation and properties of sterically demanding and chiral distibine ligands

A series of new rigid distibines, 1,8-bis(R(2)Sb)naphthalene (R = Me: (); R = Ph: ()), and chiral distibines, 2,2'-bis(R(2)Sb)-1,1'-binaphthyl (R = Me: (); R = Ph: () obtained as racemic mixtures) and the discrete enantiomers of 4,5-bis((R(2)Sb)methyl)-2,2-dimethyl-1,3-D/L-dioxolane (R = Me: () (l), () (d); R = Ph: () (l), () (d)) have been obtained in high yields, using either electrophilic halostibine reagents with di-lithium reagents (()-()) or nucleophilic stibide reagents with dibromo-derivatives (()-()). The distorted octahedral complexes [Mo(CO)(4)(L)], L = ()-(), planar [PtCl(2)(L)], L = (), (), (), (), and neutral, five-coordinate [RhCl(cod)(L)], L = (), (), (), are reported and trends in the spectroscopic data are discussed in terms of the ligand donor properties. Crystal structures of () and [Mo(CO)(4)()] reveal significant structural changes occur upon coordination, and these are also reflected in the solution NMR spectroscopic parameters. Changes in the C-Sb-C angles and C-Sb bond distances upon coordination of () are discussed in term of increased s/p orbital mixing. Air oxidation of () forms a very unusual stibine oxide, the structure of which shows a distorted Sb(4)O(4) cubane core (bridging O atoms) with two orthogonal naphthalene units.     

A new coupling reaction for the synthesis of ruthenium half-sandwich complexes with sterically demanding cyclopentadienyl ligands

The reaction of RuCl(3)(solv.)(n) with tert-butylacetylene in methanol or ethanol leads to the formation of chloro-bridged half-sandwich complexes with sterically demanding cyclopentadienyl ligands, which are of high interest as starting materials for the synthesis of novel Ru catalysts.     

The synthesis of sterically demanding amino acid-derived cyclic phosphonamides

The preparation and utilization of C(2)-symmetric 1,4-diamines in the synthesis of amino acid-derived cyclic phosphonamides 1-3 are described. The 1,4-diamines are synthesized via three methods: (i) amino acid/fumaryl chloride coupling followed by amide reduction, (ii) amino acid/1,4-diamine coupling followed by amide reduction, and (iii) a template-supported ring-closing metathesis/hydrolysis sequence. The pseudo C(2)-symmetric cyclic phosphonamides 1-3 are prepared by condensation of the C(2)-symmetric 1,4-diamines to P(III) centers, followed by oxidation.     

A new binding motif of sterically demanding thiolates on a gold cluster

A gold cluster, Au(41)(S-Eind)(12), was synthesized by ligating the bulky arenethiol 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacene-4-thiol (Eind-SH) to preformed Au clusters. Extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and the fragmentation pattern in the mass spectrometry analysis indicated that formation of gold-thiolate oligomers at the interface was suppressed, in sharp contrast to conventional thiolate-protected Au clusters.     

Neutral and cationic dicarbonyl complexes of manganese (I) with diphosphines

The reaction of BrMn(CO)₅ with dppm in refluxing toluene gives the neutral compunds cis-cis-BrMn(CO)₂(dppm)₂ which has been shown by ³¹P NMR spectroscopy to have one dppm monodentate and the other bidendate. This complex reacts with TIPF₆ in dichloromethane solution to give the salt cis-[Mn(CO)₂-(dppm)₂]PF₆ or, if the reaction is carried out in the presence of CO, the salt mer-[Mn(CO)₃(dppm)₂]PF₆ which also has one monodentate dppm (by ³¹P NMR). The cationic complex cis-[Mn(CO)₂(dppm)₂]⁺ isomerizes to the transisomer when irradiated with UV light, while heating of the latter gives back the cis-isomer. The perchlorate salts of the cation cis-[Mn(CO)₂(dppm)₂⁺ can be prepared by reacting fac-O₃ClOMn(CO)₃(dppm) withdppm in refluxing toluene, and trans-[Mn(CO)₂(diphos)(diphos)′]⁺, diphos or diphos′ being dppm or dppe, by treating the fac-O₃ClMn(CO)₃(diphos) with dppm or dppe under UV irradiation.     

Preparation of copper ion complexes of sterically congested diaryldiazomethanes having a pyridine ligand and characterization of their photoproducts

To realize fairly stable high-spin polycarbenes by utilizing heterospin systems comprising 2p spins of organic radicals and 3d spins of magnetic metal ions, we prepared dianthryldiazomethanes having two pyridyl groups at the 2,2'- or 2,7-positions, that is, bis[10-(4-tert-butyl-2,6-dimethylphenyl)-2-(4-pyridyl)-9-anthryl]diazomethane (2,2'-DPy-1-N(2)) and [10-(4-tert-butyl-2,6-dimethylphenyl)-9-anthryl][(10-(4-tert-butyl-2,6-dimethylphenyl)-2,7-di(4-pyridyl)-9-anthryl]diazomethane (2,7-DPy-1-N(2)). The triplet carbene DPy-(3)1 generated by photolysis of DPy-1-N(2) was characterized by ESR and UV-vis spectroscopy in a matrix at low temperature as well as by time-resolved UV-vis in solution at room temperature. The results showed that the triplet carbene DPy-(3)1 was destabilized to some extent as opposed to the parent triplet carbene before pyridination, but it was still fairly persistent, having a half-life of more than 30 min in solution at room temperature. Photoproducts from the complex between DPy-1-N(2) and Cu(hfac)(2) were characterized in a similar manner, and the results suggested that the generated carbene centers interacted magnetically with the Cu(II) ion to form a high-spin species with significant thermal stability. The fact that no significant signals due to the isolated triplet carbene DPy-(3)1 were observed suggested that the pyridine moiety binds with Cu(hfac)(2) in a nearly quantitative manner under these cryogenic conditions. Magnetic measurements of the photoproduct using a superconducting quantum interference device (SQUID) magneto/susceptometer were performed to determine the spin state of the complex. The temperature dependence of the molar paramagnetic susceptibility indicated the presence of ferromagnetic interaction. The field dependences of magnetization for the complexes, expressed using M versus H/T plots, were analyzed in terms of the two-component Brillouin function to be S = 3.18 (F = 0.66) and S = 0.02 (F = 0.23) for the 1:1 complex of 2,7-DPy-1 and Cu(hfac)(2) and S = 2.70 (F = 0.33) and S = 0.49 (F = 0.11) for the 1:1 complex of 2,2'-DPy-1 and Cu(hfac)(2).