New structural forms of organostannoxane macrocycle networks

The reaction of pyrazole-3,5-dicarboxylic acid with dibenzyltin dichloride in the presence of potassium hydroxide affords a novel 2D network containing rectangular box type hexatin units interconnected by two Bz<SUB>2</SUB>SnCl bridging groups. Hydrolysis of the latter affords a polymeric tape containing alternate hexatin macrocycle and tetratin ladder motifs

Di- and trinuclear complexes derived from hexakis(2-pyridyloxy)cyclotriphosphazene. Unusual P-O bond cleavage in the formation of [{(L'CuCl)<SUB>2</SUB>(Co(NO<SUB>3</SUB>)}Cl] (L' = N<SUB>3</SUB>P<SUB>3</SUB>(OC<SUB>5</SUB>H<SUB>4</SUB>N)<SUB>5</SUB>(O))

Hexakis(2-pyridyloxy)cyclotriphosphazene (L) is an efficient multisite coordination ligand which binds with transition metal ions to produce dinuclear (homo- and heterometallic) complexes [L(CuCl)(CoCl3)], [L(CuCl)(ZnCl3)], [L(CoCl)(ZnCl3)], and [L(ZnCl2)2]. In these dinuclear derivatives the cyclophosphazene ligand utilizes from five to six nitrogen coordination sites out of the maximum of nine available sites. Further, the spacer oxygen that separates the pyridyl moiety from the cyclophosphazene ring ensures minimum steric strain to the cyclophosphazene ring upon coordination. This is reflected in the near planarity of the cyclophosphazene ring in all the dinuclear derivatives. In the dinuclear heterobimetallic derivatives one of the metal ions [Cu(II) or Co(II)] is hexacoordinate and is bound by the cyclophosphazene in a &#951; 5-gem-N5 mode. The other metal ion in these heterobimetallic derivatives [Co(II) or Zn(II)] is tetracoordinate and is bound in an &#951; 1-N1 fashion. In the homobimetallic derivative, [L(ZnCl2)2], one of the zinc ions is five-coordinate (&#951; 3-nongem-N3), while the other zinc ion is tetracoordinate(&#951; 2-gem-N2). The reaction of L with CuCl2 followed by Co(NO3)2&#183;6H2O yields a trinuclear heterobimetallic complex [{(L'CuCl)2Co(NO3)}Cl] [L' = N3P3(OC5H4N)5(O)]. In the formation of this compound an unusual P-O bond cleavage involving one of the phosphorus-pyridyloxy bonds is observed. The molecular structure of [{(L'CuCl)2Co(NO3)}Cl] [L' = N3P3(OC5H4N)5(O)] reveals that each of the two the P-O-cleaved L' ligands is involved in binding to Cu(II) to generate the motif L'CuCl. Two such units are bridged by a Co(II) ion. The coordination environment around the bridging Co(II) ion contains four oxygen (two P-O units, one chelating nitrate) and two nitrogen atoms (pyridyloxy nitrogens)

Dichlorosilylene: A High Temperature Transient Species to an Indispensable Building Block

Ghadwal R, Azhakar R, Roesky HW. Dichlorosilylene: A High Temperature Transient Species to an Indispensable Building Block. Accounts of Chemical Research. 2012;46(2):444-456.Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R2C:) such as silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl2 and SiCl2 are highly reactive intermediates and play an important role in many chemical transformations. GeCl2 can be stabilized as a dioxane adduct, whereas SnCl2 and PbCl2 are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl3 at 1150 °C, chemists proposed dichlorosilylene (SiCl2) as an intermediate, which further dissociates to Si and SiCl4. Similarly, base induced disproportionation of HSiCl3 or Si2Cl6 to SiCl2 is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl2)n. However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl2 was the isolation of monochlorosilylene PhC(NtBu)2SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl2 with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl2 without using any hazardous reducing agents. Dehydrochlorination of HSiCl3 with NHC under mild reaction conditions produces (NHC)SiCl2. We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl2 primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl2 produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl2 and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offer the possibility to synthesize interesting compounds with low-valent silicon by further reduction. Coordination of NHC to the silicon increases the acidity of the backbone protons on the imidazole ring, and therefore (NHC)SiCl2 can functionalize NHC at the C-4 or C-5 position

Selective functionalization of a bis-silylene

Ghadwal R, Azhakar R, Pröpper K, Dittrich B, John M. Selective functionalization of a bis-silylene. Chemical Communications. 2013;49(53): 5987.Functionalization of N-heterocyclic carbenes (NHCs) has an important influence on their stability, Lewis donor, and acceptor properties. In this study, we report on the selective functionalization of a four-membered N-heterocyclic bis-silylene (2,6-Ar2C6H3NSi:)2 (1) (Ar = 2,4,6-iPr3C6H2) with mono-oxygen sources N2O and Me3NO. Treatment of 1 with N2O results in the selective formation of mono-silylene (2,6-Ar2C6H3NSi(OH)2)(2,6-Ar2C6H3NSi:) (2) as a major product, along with a small amount of further oxidized product (2,6-Ar2C6H3NSi(OH)2)2 (3). Compound 2 is the first four-membered mono-silylene with a di-coordinate silicon atom

Facile Access to Transition-Metal-Carbonyl Complexes with an Amidinate-Stabilized Chlorosilylene Ligand

Azhakar R, Ghadwal R, Roesky HW, Hey J, Stalke D. Facile Access to Transition-Metal-Carbonyl Complexes with an Amidinate-Stabilized Chlorosilylene Ligand. Chemistry - An Asian Journal. 2012;7(3):528-533.Three transition‐metal–carbonyl complexes [V(L)(CO)3(Cp)] (1), [Co(L)(CO)(Cp)] (2), and [Co(L2)(CO)3]+[CoCO)4]− (3), each containing stable N‐heterocyclic‐chlorosilylene ligands (L; L=PhC(NtBu)2SiCl) were synthesized from [V(CO)4(Cp)], [Co(CO)2(Cp)], and Co2(CO)8, respectively. Complexes 1, 2, 3 were characterized by NMR and IR spectroscopy, EI‐MS spectrometry, and elemental analysis. The molecular structures of compounds 1, 2, 3 were determined by single‐crystal X‐ray diffraction

Reactions of Stable N-Heterocyclic Silylenes with Ketones and 3,5-Di-tert-butyl-o-benzoquinone

Azhakar R, Ghadwal R, Roesky HW, Hey J, Stalke D. Reactions of Stable N-Heterocyclic Silylenes with Ketones and 3,5-Di-tert-butyl-o-benzoquinone. Organometallics. 2011;30(14):3853-3858

A début for base stabilized monoalkylsilylenes

Azhakar R, Ghadwal R, Roesky HW, Wolf H, Stalke D. A début for base stabilized monoalkylsilylenes. Chemical Communications. 2012;48(38): 4561.The first base stabilized monoalkylsilylenes LSitBu (2) and LSi[C(SiMe3)3] (3) (L = PhC(NtBu)2) were synthesized by the facile metathesis reactions of LitBu and KC(SiMe3)3 with LSiCl (1). The reaction of LSitBu (2) with N2O afforded the dimer [LSitBu(μ-O)]2 (4) which contains a four-membered Si2O2 ring

Facile Access to the Functionalized N-Donor Stabilized Silylenes PhC(N Bu) SiX (X = PPh , NPh , NCy , N Pr , NMe , N(SiMe ) , O Bu)

Azhakar R, Ghadwal R, Roesky HW, Wolf H, Stalke D. Facile Access to the Functionalized N-Donor Stabilized Silylenes PhC(N Bu) SiX (X = PPh , NPh , NCy , N Pr , NMe , N(SiMe ) , O Bu). Organometallics. 2012;31(12):4588-4592.Reactions of silylenes with organic substrates generally lead to silicon(IV) compounds. Ligand substitution at the silicon(II) atom of silylene, without changing the formal +2 oxidation state, is very rare. We report herein a straightforward route to functionalized silylenes LSiX (L = PhC(NtBu)2 and X = PPh2 (1), NPh2 (2), NCy2(3), NiPr2 (4), NMe2 (5), N(SiMe3)2 (6), OtBu (7)). Silylenes 1–7 have been prepared in quantitative yield by a modified ligand exchange reaction of PhC(NtBu)2SiCl (LSiCl) with the corresponding lithium or potassium salts. Compounds 1–7 were characterized by spectroscopic and spectrometric techniques. Single-crystal X-ray structures of 1, 3, and 4 were determined

Double N–H bond activation of N,N′-bis-substituted hydrazines with stable N-heterocyclic silylene

Azhakar R, Ghadwal R, Roesky HW, Hey J, Stalke D. Double N–H bond activation of N,N′-bis-substituted hydrazines with stable N-heterocyclic silylene. Dalton Trans. 2012;41(5):1529-1533.The reaction of N-heterocyclic silylene (NHSi) L [L = CH{(C[double bond, length as m-dash]CH2)(CMe)(2,6-iPr2C6H3N)2}Si] with benzoylhydrazine, 1,2-dicarbethoxyhydrazine, 1,2-diacetylhydrazine and 1,2-bis(tert-butoxycarbonyl)hydrazine in 1 : 1 molar ratio resulted in compounds 1–4 with an almost quantitative yield and five coordinate silicon atoms. Compounds 1–4 were formed by double N–H bond activation by deliberate selection of N,N′-bis-substituted hydrazine compounds bearing the –C(O)NHNH– unit. Compounds 1–4 were characterized by NMR spectroscopy, EI-MS and elemental analysis. The molecular structures of compounds 1–3 were unambiguously established by single crystal X-ray structural analysis

Reactivity Studies of a Stable N-Heterocyclic Silylene with Triphenylsilanol and Pentafluorophenol

Azhakar R, Ghadwal R, Roesky HW, Granitzka M, Stalke D. Reactivity Studies of a Stable N-Heterocyclic Silylene with Triphenylsilanol and Pentafluorophenol. Organometallics. 2012;31(15):5506-5510.The reaction of the stable N-heterocyclic silylene [CH{(C═CH2)(CMe)(2,6-iPr2C6H3N)2}Si] (1) with triphenylsilanol and pentafluorophenol in a 1:2 molar ratio resulted in quantitative yields of the pentacoordinate silicon-containing compounds [CH{(CMe)2(2,6-iPr2C6H3N)2}Si(H){OSiPh3}2] (2) and [CH{(CMe)2(2,6-iPr2C6H3N)2}Si(H){OC6F5}2] (3), respectively. Compounds 2 and 3 were formed by O–H bond activation of triphenylsilanol and pentafluorophenol. They were characterized by elemental analysis, NMR spectroscopy, and EI-MS spectrometry. In their solid-state structures the silicon atom is tetracoordinate in 2, whereas it is pentacoordinate in 3