Synthesis of a hafnocene disilene complex

Reaction of the magnesium transmetalation product of a 1,2-dipotassiodisilane with hafnocene dichloride gives a disilene hafnocene complex. X-ray crystallography of the respective trimethylphosphane adduct provides structural proof for this assignment.     

Dinitrogen coordination and cleavage promoted by a vanadium complex of a sigma,pi,sigma-donor ligand

The deprotonation of the tripyrrole MeTPH(2) [MeTPH(2) = 2,5-[(2-pyrrolyl)(C(6)H(5))2C](2)(MeNC(4)H(2))], containing one N-methylated pyrrolyl ring, was carried out with 2 equiv of KH. The corresponding dipotassium salt reacted with VCl(3)(THF)(3) to afford the complex [(MeTP)VCl(THF)].THF (1). While the two lateral pyrrolide rings are sigma-bonded, the central one is perpendicularly oriented in a sort of pi-fashion. However, the bond distances clearly indicated that only the quaternized N atom is forming a bonding contact. Subsequent reduction of 1 with Na yielded the corresponding divalent complex [(MeTP)V(THF)].(C(7)H(8))(0.5) (2) where the central N-methylated ring adopted a more regular pi-orientation. When treated with a strong Lewis acid (AlMe(3)), THF was extracted from the vanadium coordination sphere, forming the dinuclear dinitrogen complex [(MeTP)V(mu-N(2))](2).(C(7)H(8))(2.9) (3). Reduction of 3 with potassium graphite gave cleavage of dinitrogen, affording the mixed-valent nitride-bridged complex [(MeTP)V(mu-N)](2).(THF) (4).     

Double strand DNA cleavage with a binuclear iron complex

Covalently linking two single strand DNA cleaving agents resulted in a new biomimetic binuclear iron complex capable of effecting oxidative double strand DNA cleavage.     

N-C bond formation promoted by a hafnocene dinitrogen complex: comparison of zirconium and hafnium congeners

Nitrogen-carbon bond formation from coordinated dinitrogen has been observed upon addition of 2 equiv of PhNCO to the hafnocene dinitrogen complex, [(eta5-C5Me4H)2Hf]2(mu2,eta2,eta2-N2). The resulting product most likely arises from initial N=C cycloaddition of the first equivalent of heterocumulene, followed by carbonyl insertion of a second equivalent into the newly formed hafnium-nitrogen bond. The resulting product has considerable hafnium imido character, as evidenced by the metrical parameters determined from the solid-state structure as well as reactivity studies, whereby PhNCO, p-tolyl isocyanate, and t-BuCCH each undergo cycloaddition with the hafnium-nitrogen bond. The origin of the nitrogen-carbon-forming chemistry is likely derived from the reluctance of the hafnocene dinitrogen complex to undergo ligand-induced N2 isomerization, as was observed with the zirconium congener.     

Rhodium-catalyzed silylation and intramolecular arylation of nitriles via the silicon-assisted cleavage of carbon-cyano bonds

A rhodium-catalyzed silylation reaction of carbon-cyano bonds using disilane has been developed. Under these catalytic conditions, carbon-cyano bonds in aryl, alkenyl, allyl, and benzyl cyanides bearing a variety of functional groups can be silylated. The observation of an enamine side product in the silylation of benzyl cyanides and related stoichiometric studies indicate that the carbon-cyano bond cleavage proceeds through the deinsertion of silyl isocyanide from eta(2)-iminoacyl complex B. Knowledge gained from these studies has led to the development of a new intramolecular biaryl coupling reaction in which aryl cyanides and aryl chlorides are cross-coupled.     

Interactions of hafnocene and zirconocene dihydrides with acetylenes. Synthesis of the first acetylene complex of hafnium

The first acetylene complex of hafnium, Cp₂Hf[Me₃SiC=CHf(H)Cp₂], was synthesized by the reaction of hafnocene dihydride Cp₂HfH₂ with bis(trimethylsilyl)acetylene in benzene. The reaction is accompanied by elimination of the Me₃Si group from the molecule of the initial acetylene, as a result of which the acetylenide derivative of hafnium Cp₂Hf(C=CSiMe₃)(H) acts as an acetylene ligand in the complex. Under analogous conditions, the reaction of zirconocene dihydride Cp₂ZrH₂ with bis(trimethylsilyl)acetylene affords an analogous acetylene complex of zirconium Cp₂Zr(M₃SiC=CZr(H)Cp₂]. Reactions of Cp₂HfH₂ with tolane and 3-hexyne proceed differently than the reaction with bis(trimethylsilyl)acetylene. Here the corresponding hafnacyclopentadiene metallacycles are the final products. For preliminary communication, see Ref. 3. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 853–856, April, 1997.     

A phenyl-substituted germole dianion and its reaction with hafnocene dichloride

The reaction of phenyl-substituted dipotassium germacyclo-pentadienediide with one equivalent of hafnocene dichloride at low temperature provides a bicyclic germylene. With two equivalents of hafnocene dichloride, a dinuclear hafnium complex with a mmm-coordinating germolyl dianion ligand is formed.     

Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N₂ to nitrides (N³⁻). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N₂ cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N₂. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N₂. Two heteroleptic diuranium nitride complexes, [K{U^IV(OSi(O^tBu)₃)(N(SiMe₃)₂)₂}₂(μ-N)] (1) and [Cs{U^IV(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (3-Cs), containing different combinations of OSi(O^tBu)₃ and N(SiMe₃)₂ ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K₂{U^IV/III(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e⁻ reduction of N₂, yielding the N₂⁴⁻complex [K₃{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)(μ-η²:η²-N₂)], 7. Parallel N₂ reduction pathways were also identified, leading to the isolation of N₂ cleavage products, [K₃{U^VI(OSi(O^tBu)₃)₂(N(SiMe₃)₂)( N)}(μ-N)₂{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}]₂, 8, and [K₄{(OSi(O^tBu)₃)₂U^V)( N)}(μ-NH)(μ-κ²:C,N-CH₂SiMe₂NSiMe₃)-{U^V(OSi(O^tBu)₃)₂][K(N(SiMe₃)₂]₂, 9. These complexes provide the first example of N₂ cleavage to nitride by a uranium complex in the absence of reducing alkali metals.

Combinations of ligands were used to tune U N U complexes yielding a U(iii)/U(iii) nitride, which activates N₂. Parallel N₂ reduction pathways were identified, leading to the first example of N₂ cleavage by U without external alkali reducing agents.     

Interaction of carbon dioxide with zirconocene and hafnocene dihydrides

Zirconocene and hafnocene dihydrides Cp₂MH₂ (M = Zr, Hf) are capable of rapid absorbing carbon dioxide at room temperature and atmospheric pressure in a THF medium. In the case of Cp₂ZrH₂, the reaction results in the cleavage of the C=O bond of a CO₂ molecule to form cyclic trimeric zirconocene oxide [Cp₂ZrO]₃, whose structure was confirmed by analytical and spectral methods as well as by X-ray diffraction study. Small amounts of formaldehyde and methyl formate are found in the organic products of the reaction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2366–2368, September, 1996.     

Tetrametallic Reduction of Dinitrogen: Formation of a Tetranuclear Samarium Dinitrogen Complex

The cooperative attack of four (dipyrromethanyl)Sm(II) units on dinitrogen resulted in a novel tetranuclear samarium dinitrogen complex (shown schematically). The presence of halogen atoms inhibited reactivity with dinitrogen through the assembly of divalent samarium clusters. dipyrr=diphenylmethyldipyrrolide dianion.     

Aryl-oxygen bond cleavage by a trihydride-bridging ditantalum complex

We have described the synthesis of the cyclometalated trihydride ditantalum(V) complexes supported by the aryloxide tridentate ligand. According to variable-temperature NMR studies, these dimers could provide a masked form of Ta(IV)-Ta(IV) and/or Ta(III)-Ta(III). In addition, these complexes were found to undergo hydrodeoxygenation of the aryloxide ligand.     

Palladium-catalyzed silylation of alcohols with hexamethyldisilane

The combination of hexamethyldisilane and a catalytic amount of [PdCl(eta3-C3H5)]2-PPh3 was found to be effective for the trimethylsilylation of alcohols, where both of the two trimethylsilyl groups of hexamethyldisilane were transferred to alcohols without coproduction of any stoichiometric amount of byproduct but H2.     

Activation of hafnocene catalyzed polymerization of 1-hexene with MAO and borate

A study of 1-hexene polymerization with ethylene-bis(9-fluorenyl) hafnium dichloride has been carried out using two different cocatalyst systems, methyl-aluminoxane/trimethylaluminum (MAO/TMA) and tris-isobutyl-aluminum/N,N-dimethylanilinium tetrakis(pentafluorophenyl) borate (TIBA/borate). When MAO/TMA was used, 1-hexene polymerized into a low molar mass poly(1-hexene) with low catalytic activity. Activation with TIBA/borate increased polymerization activity drastically as well as the molar mass of the polymers. In order to analyze differences in the activity profiles, UV-Vis spectroscopy was employed to investigate ligand to metal charge transitions (LMCT) of the hafnocene dichloride during the activation process. The low catalytic activity and the fast chain transfer to the cocatalyst with MAO/TMA may originate from strong bonding between the metallocene cation and the MAO/TMA species thus obstructing monomer coordination and insertion.     

Borane-mediated silylation of a metal-oxo ligand

The addition of 1 equiv of HSiPh(3) to UO(2)((Ar)acnac)(2) ((Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)), in the presence of 1 equiv of B(C(6)F(5))(3), results in the formation of U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2) (1), via silylation of an oxo ligand and reduction of the uranium center. The addition of 1 equiv of Cp(2)Co to 1 results in a reduction to uranium(IV) and the formation of [Cp(2)Co][U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2)] (2) in 78% yield. Complexes 1 and 2 have been characterized by X-ray crystallography, while the solution-phase redox properties of 1 have been measured with cyclic voltammetry.     

Sequential N-O and N-N bond cleavage of N-heterocyclic carbene-activated nitrous oxide with a vanadium complex

Chemically induced bond cleavage of nitrous oxide typically proceeds by rupture of the N-O bond with concomitant O-atom transfer and liberation of dinitrogen. On a few occasions, N-N bond scission has been observed instead. We report a reaction sequence involving an N-heterocyclic carbene and a vanadium complex that results in cleavage of both the N-O bond and the N-N bond.     

Dinitrogen activation, partial reduction, and formation of coordinated imide promoted by a chromium diiminepyridine complex

Reduction of {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrCl (3) with NaH afforded the dinuclear dinitrogen complex {[{2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Cr(THF)]2(mu-N2)}.THF (5). Reaction carried in exclusion of dinitrogen afforded instead deprotonation of the ligand with the formation of {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(THF) (4). Further reduction of 5 with NaH yielded a curious dinuclear compound formulated as [{2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Cr(THF)][{2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(THF)](mu-N2 H)(mu-Na)2 (6) containing two sodium atoms only bound to the dinitrogen unit and the pi systems of the two diiminepyridine ligands. Subsequent reduction with NaH triggered a complex series of events, leading to the formation of a species formulated as {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(mu-NH)][Na(THF)] (7) on the basis of crystallographic, spectroscopic, isotopic labeling, and chemical degradation experiments.     

Nitrosation of Dimethylamine with Dinitrogen Trioxide

A two-step procedure for preparing N-nitrosodimethylamine by direct nitrosation of aqueous solutions of dialkylamines with dinitrogen dioxide was suggested. The first step involves preparation of dialkylammonium nitrite, and in the second step, on heating to 70–90°С in a weakly acidic solution, it transforms into N-nitrosodialkylamine. The yield of N-nitrosodialkylamines is 95–98% based on the converted dialkylamine. A low-waste process for N-nitrosodimethylamine production was developed.