Carbamate Complex Research Articles

Pendant Arm Functionalized Benzamidinate Titanium Imido Compounds: Experimental and Computational Studies of Their Reactions with CO2

The syntheses of cyclopentadienyl titanium tert-butylimido complexes containing pendant arm functionalized amidinate ligands are reported, together with their reactions with CO2. Mechanistic and DFT computational studies are also described. Reaction of [Ti(η-C5R4Me)(NtBu)Cl(py)] (R = Me, H) with Li[Me3SiNC(Ph)N(CH2)nNMe2] (n = 2, 3) afforded the corresponding (dimethylamino)ethylidene or -propylidene functionalized benzamidinate complexes [Ti(η-C5R4Me)(NtBu){Me3SiNC(Ph)N(CH2)nNMe2}] (R = Me, n = 2 (1), 3 (2); R = H, n = 2 (3), 3 (4)). The n-propyl functionalized amidinate complex [Ti(η-C5H4Me)(NtBu){Me3SiNC(Ph)NCH2CH2Me}] (5) was also prepared for comparative purposes. The NMe2 donors in 3 and 4 coordinate relatively weakly, and in solution the activation Gibbs free energy for NMe2 dissociation for 3 is ca. 10 kJ mol-1 higher than that for 4. All five compounds react with CO2 to form the corresponding N,O-bound carbamate complexes [Ti(η-C5R4Me){NtBuC(O)O}{Me3SiNC(Ph)NR‘}] (R = H, Me; R‘ = CH2CH2NMe2, CH2CH2CH2NMe2, CH2CH2Me), which, in turn, extrude tBuNCO (above −25 °C for R = H and at room temperature for R = Me) to form the μ-oxo-bridged dimeric complexes [Ti2(η-C5R4Me)2(μ-O)2{Me3SiNC(Ph)NR‘}2], two of which have been structurally characterized. Only the C5Me5 carbamate complexes [Ti(η-C5Me5){NtBuC(O)O}{Me3SiNC(Ph)N(CH2)nNMe2}] can be isolated and (for n = 3) structurally characterized. The reaction of 4 and 5 with CO2 is spontaneous at −78 °C, whereas the corresponding reaction of 3 does not occur at significant rates below −35 °C. The rate of extrusion of tBuNCO from [Ti(η-C5H4Me){NtBuC(O)O}{Me3SiNC(Ph)NR‘] at −25 °C is twice as fast for R‘ = CH2CH2NMe2 as for CH2CH2CH2NMe2 and CH2CH2Me. DFT calculations have modeled the cycloaddition/extrusion reactions of 3−5 in conjunction with mechanistic and variable-temperature NMR experiments. The results show that the favored route is for the cycloaddition reaction to take place on 16-valence-electron, non pendant arm chelated isomers of 3 and 4 and that the relative ease of accessing these intermediates in fact controls the TiNtBu/CO2 coupling reaction.

Evidence for unimolecular CO2elimination in C–N bond metathesis reactions of basic carbamatozinc complexes Zn4O(O2CAm)6(Am = N-diethylamino, N-piperidyl, N-pyrrolidyl)

The tetranuclear basic zinc carbamates Zn4O(O2CAm)6(1, Am =N-diethylamino; 2, Am =N-piperidyl; 3, Am =N-pyrrolidyl) were shown by transient FTIR spectroscopy to undergo C-N bond metathesis reactions that result in exchange of the carboxyl group with bulk carbon dioxide and exchange of the amino group with bulk secondary amine (transamination). The net rate for CO2 exchange was measured by monitoring the uptake of 13CO2 and the concomitant release of 12CO2. This revealed a CO2-dependent and CO2-independent component to the CO2 exchange process. The CO2-dependent process was interpreted in terms of preassociation of the incoming CO2 with the complex prior to the N-CO2 bond cleavage process while the CO2-independent process was interpreted in terms of a unimolecular elimination of CO2 from the complex. The transamination reaction gave rates that are independent of the concentration of the incoming amine. Furthermore, the transamination rate for each complex was within a factor of four of the CO2-independent CO2 exchange rate for that complex. These data were interpreted in terms of a common rate-limiting process for CO2 exchange and transamination that involves unimolecular elimination of CO2. Rate-limiting unimolecular dissociation of intact carbamato ligands was eliminated as a possible pathway by showing that ligand exchange dynamics is rapid relative to the overall rate of C-N bond metathesis. The ability of these metal carbamate complexes to undergo facile C-N bond metathesis reactions of this type has implications in heterocumulene metathesis and CO2 fixation chemistry.

Pyrazolylborate-zinc alkoxide complexes. 1. Basic properties, methylations, and heterocumulene insertions.

While a solution of TpPh,MeZn-OH in methanol contains only traces of TpPh,MeZn-OMe, according to the equilibrium constant K = 5.8 x 10(-4), the reactions of TpPh,MeZn-OH with the electronegative alcohols trifluoroethanol and hexafluoro-2-propanol easily yield TpPh,MeZn-OCH2CF3 and TpPh,MeZn-OCH(CF3)2. The extremely hydrolytically sensitive TpPh,MeZn-OR complexes, with R = Me, Et, i-Pr, and CH2CH2F, as well as TpCum,MeZn-OR, with R = Me and i-Pr, are accessible from the Tp*Zn-hydride complexes and the corresponding alcohol. Alkylations with methyl iodide have revealed the high nucleophilicity of TpPh,MeZn-OMe by conversion to dimethyl ether and TpPh,MeZn-I. This conversion occurs rapidly not only with pure TpPh,MeZn-OMe but also with TpPh,MeZn-OH (as such or in the presence of methanol) and with TpPh,MeZn-OCOOMe. A relation of the Tp*Zn-alkoxides to the function of the zinc enzyme alcoholdehydrogenase exists in the reaction of TpPh,MeZn-OCH(CH3)2 with aromatic aldehydes, which yields acetone and the corresponding benzyl oxides TpPh,MeZn-OH2Ar. The heterocumulenes carbon dioxide, carbon disulfide, isothiocyanates, and one isocyanate are inserted into the Zn-OR bonds, yielding one alkyl carbonate complex (TpPh,MeZn-OC(O)OMe), two xanthogenate complexes (TpPh,MeZn-SC(S)OR), three iminothiocarbonate complexes (TpPh,MeZn-SC(NR')OR), and one alkyl carbamate complex (TpPh,MeZn-NR-COOMe). All insertion reactions can be described by a common mechanism involving a four-center intermediate in which the most basic heteroatom of the heterocumulene is attached to zinc.

Efficient and Clean Synthesis of N‐Alkyl Carbamates by Transcarboxylation and O‐Alkylation Coupled Reactions Using a DBU—CO2 Zwitterionic Carbamic Complex in Aprotic Polar Media.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Efficient and clean synthesis of N-alkyl carbamates by transcarboxylation and O-alkylation coupled reactions using a DBU–CO 2 zwitterionic carbamic complex in aprotic polar media

N-Alkyl carbamates were obtained with good to excellent yields by clean and mild transcarboxylation of several amines with the previously synthesized DBU–CO 2 complex and subsequent O-alkylation. Transcarboxylation was found to be selective, as only carbamate was formed from 1-hydroxy-2-aminobutanol.

Vanadium nitride functionalization and denitrogenation by carbon disulfide and dioxide.

A dramatic difference in behavior is observed for the dithiocarbamate and carbamate complexes [Ar(But)N]3V(NCE2)Na(THF)2(E = S or O, respectively), prepared from the corresponding nitride species ([Ar(But)N]3V identical to NNa)2 by way of a nucleophilic addition reaction involving carbon disulfide or dioxide, and is rationalized with the aid of DFT calculations.

An experimental and theoretical investigation of the carbon dioxide insertion process into the tungsten-nitrogen bond of an anionic W(0) complex.

The pyridine bound 2-aminopyridine (2APH) derivative of tungsten pentacarbonyl has been prepared from photogenerated W(CO)5THF and 2APH. Deprotonation of the distal amine group by sodium hydride has provided two complexes, [Na][W(CO)5(2AP)] and [Na]2[W(CO)4(2AP)]2. Both complexes have been characterized by X-ray crystallography with the monomeric derivative being crystallized as its [Na2(18-crown-6)][W(CO)5(2AP)]2 salt which exhibits strong Na+...-NH interactions. Photolysis of W(CO)6 in the presence of excess 2-aminopyridine in THF has led to an efficient synthesis of the chelated neutral derivative, W(CO)4(2APH).2APH, where the extra equivalent of 2APH is hydrogen bonded to its bound counterpart. The 2-aminopyridine molecule of solvation was almost quantitatively removed via aqueous washings. Deprotonation of W(CO)4(2APH) with NaH afforded the amidopyridine derivative which was shown to rapidly undergo reaction with CO2 to yield the chelated carbamate complex, W(CO)4(OC(O)2AP)-. Nevertheless, because of the presence of small quantities of free 2-aminopyridine during the reactions with CO2, we have not been able to conclusively rule out participation by a ligand substitution process involving NC5H4NHCOOH. Ab initio computations were found to substantiate many of these experimental observations. That is, in the monodentate bound W(CO)5(2APH) derivative, binding through the pyridine nitrogen atom is favored by about 29 kJ/mol over the amine nitrogen atom, whereas the opposite site for binding is preferred for the deprotonated amido analogue, W(CO)5(2AP)-. Furthermore, both forms of W(CO)5(2AP)- were found to be more stable than the chelated tungsten tetracarbonyl anion plus CO. On the other hand, CO2 insertion into the W(CO)4(2AP)- anion to provide the chelated carbamate, W(CO)4(OC(O)2AP)-, was thermodynamically favored by >110 kJ/mol. Finally, both experimental and theoretical studies were inconclusive with regard to identifying reaction intermediates during the CO2 insertion pathway which involve prior interactions of CO2 at the amido nitrogen center.

Synthesis, Characterization, and Reactivity of Urea Derivatives Coordinated to Cobalt(III). Possible Relevance to Urease

The syntheses of a cobalt(III) complex, 2, containing N-(2-pyridylmethyl)urea and of six complexes, 3, containing phenyl-substituted N-2-pyridylmethyl-N'-(X)phenylureas (where X = 4-H, 4-CH(3), 4-Br, 3-Cl, 4-CF(3), and 4-NO(2)), have been accomplished by reaction of [(en)(2)Co(OSO(2)CF(3))(2)](CF(3)SO(3)) with the urea ligands in tetramethylene sulfone. The complexes have been characterized by UV-vis, FTIR, (1)H NMR, and (13)C NMR spectra along with elemental analysis. Also, X-ray crystallographic analysis of 2 confirms that the urea ligand chelates as a bidentate through the pyridyl nitrogen atom and the endo deprotonated, urea nitrogen atom to form a stable five-membered ring. Crystals of the perchlorate salt of 2 were monoclinic, space group P2(1)/c with a = 9.743(1) Å, b = 13.924(3) Å, c = 15.006(4) Å, beta = 97.07(1) degrees, and Z = 4. Reflection data (3454) with I = 3sigma(I) were refined to conventional R factors of 0.037 and 0.051. In acidic solution (0.05-1.00 M HCl at 55 degrees C), the phenyl-substituted complexes undergo hydrolysis to form the bis(ethylenediamine)(2-picolylamine-N,N')cobalt(III) ion, 4, aniline, and CO(2). The hydrolysis kinetics of the phenyl-substituted complexes were studied by UV-vis spectroscopy (I = 1.00 M HCl/LiCl). At 55 degrees C the observed rate constants fit the rate law k(obsd) = kK[H(+)]/(1 + K[H(+)]). It is proposed that the protonated urea eliminates aniline to give a coordinated isocyanate intermediate that hydrolyzes rapidly to the pyridyl methylamine complex and CO(2) via the carbamate complex. Since all of the studies of this kind to date appear to involve the NCO intermediate, it raises the prospect that urease also functions by a similar path and that urease should be tested with NCO(-) as a substrate.

Enantioselective Deprotonation/Electrophile Addition Reactions of Tricarbonyl(phenyl carbamate)chromium Complexes

Sequential reaction of the tricarbonyl[(η6-phenyl) carbamate]chromium complex 3 with chiral amide bases (see 4 and 5) and electrophiles yielded planar chiral ortho-substituted complexes 6 with up to 70% enantiomeric excess (ee) (Scheme 2, Table 1 and 2). The enantiomer purity could be increased to >90% ee by fractional crystallization. In all but one case the racemate crystallized selectively, leaving the enantiomerically enriched complex in solution. X-Ray crystal-structure analyses of rac-6a and (1R)-6a suggest that this can be ascribed to a more favorable packing of enantiomers of opposite configuration in the solid state than that of the enantiomerically pure solid. Increasing the temperature of the intermediate ortho-lithiated aryl carbamate complex induced an anionic ortho-Fries rearrangement: The 1,3-transposition of the carbamoyl group yielded the ortho-substituted (η6-benzamide)tricarbonylchromium complexes 10 in 65% yield, after exposure to the electrophile (Scheme 6), and the use of a chiral amide base 5 in the deprotonation step afforded the product with an ee of 54%.

Determination of small quantities of heavy metals in water-soluble salts and natural water by X-ray fluorescence

Techniques for X-ray fluorescence (XRF) determination of heavy metals in natural waters and pure salts combined with preconcentration, are proposed. In these techniques, concentrates are small quantities of aqueous phase. Preconcentration of the said impurities was carried out either by extraction of their carbamate complexes with subsequent re-extraction of these elements in an aqueous phase or by directed crystallisation of aqueous eutectic salt solutions. For making specimens from the liquid concentrates their viscosity was essentially increased by the introduction of dry gelatine into the concentrates. Techniques for the determination of heavy metal impurities on the level of maximum permissible concentration (natural waters) and up to 1×10 −5–2×10 −5 mass% (pure salts) by extraction-XRF and crystallisation-XRF were developed.

A Likely Intermediate during the CO2-Induced Activation of 2-Alkylindoles toward Electrophilic Substitution: Structure of a Unique Tetramer Formed by Joining Two Boat-Shaped (LiOCO)2 Rings

Reaction of 2,3-dimethylindole in thf with nBuLi and CO2 gives a lithium carbamate complex (2·thf)n, which has a unique tetrameric solid-state structure (n = 4) containing two boat-shaped (−LiOCOLiOCO−) rings joined by four inter-ring Li−O connections. The two-coordinate O atoms of these rings are near to the 2-Me groups of the indolyl residues, suggesting an explanation for second lithiation (and subsequent electrophilic substitution) of 2-alkylindoles and related heterocyclics occurring specifically at the 2-alkyl position.

Speciation of butyltin compounds in sediments using gas chromatography interfaced with quartz furnace atomic absorption spectrometry

A method is presented for the determination of tri- and dibutyltin in sediments by gas chromatography—quartz furnace atomic absorption spectrometry. The compounds are released from the matrix by the action of acetic acid and extracted into hexane after formation of an extractable complex with diethyldithiocarbamic acid (DDTC). Pentylmagnesium bromide is used to derivatize the non-volatile organotin—carbamate complexes. A detection limit of 2.5 ng/g (as Sn) can be achieved. The optimization of the different parameters is discussed and the accuracy of the method was validated by the analysis of two certified reference materials. A first approach was made to determine tri- and dimethyltin compounds forming extractable complexes with NaDDTC under pH controlled conditions.

Planar chiral arene tricarbonylchromium complexes via enantioselective deprotonation / electrophile addition reactions

Sequential reaction of prochiral (η 6-arene)Cr(CO) 3 complexes with chiral amide bases and electrophiles yielded planar chiral complexes. Benzaldehyde acetal and phenyl carbamate complexes gave o-substituted products with 64 to 81% enantiomeric excess. With the benzaldehyde acetal complex, competitive benzylic deprotonation occurred. Enantiomeric purity of the substituted carbamate complexes could be increased to > 90% ee by fractional crystallization. The racemate crystallized selectively, leaving the enantiomerically enriched complex in solution.

Synthesis, characterization, and carbonylation reactions of methylpalladium amide, carbamate, and alkyl carbonate complexes

A series of trans- and cis-methylpalladium carbamate complexes PdMe(OCONRR′)L 2 (L = tertiary phosphine; R, R′= H, alkyl, and phenyl) was prepared by the reaction of dimethylpalladium complexes with primary or secondary amine and carbon dioxide. When diphenylamine was used, methylpalladium amide complexes trans-PdMe(NPh 2)L 2 (L = PMe 3 and PEt 2Ph) were isolated instead of carbamates. The carbamate and amide complexes thus prepared were characterized by IR and NMR spectroscopy and elemental analysis. The carbamate complex trans-PdMe(OCONEt 2)(PMe 3) 2 released one of the PMe 3 ligands on recrystallization to form a dimeric compound Pd 2Me 2 (μ-OCONEt 2) 2(PMe 3) 2, whose structure was determined by X-ray diffraction study. The carbamate complexes reacted with carbon monoxide under pressure to give corresponding α-keto amide (MeCOCONRR′) and amide (MeCONRR′). Reactions of dimethylpalladium complexes with alcohols and carbon dioxide gave methylpalladium alkyl carbonate complexes trans-PdMe(OCOOR)L 2 (R = Me, sBu; L = PMe 3, PPh 3). Treatment of alkyl carbonate complexes with carbon monoxide afforded the corresponding alkyl acetate as the sole carbonylation product.

Nitrogen- and oxygen-bonded urethane: hydrolysis and linkage isomerization of [(NH3)5Co(NH2CO2C2H5)]3+ and [(NH3)5CoOC(NH2)OCH2CH3]3+

Coordination of ethyl carbamate via N rather than O to the pentaamminecobalt(III) moiety results in activation toward ligand hydrolysis. The carbonyl oxygen of the carbamate anion and of its ethyl ester (urethane) are preferentially coordinated in kinetically controlled syntheses. Noncoordinating bases facilitate the thermal rearrangement of the O-bound urethane complex in solution to the N-bonded linkage isomer, which has been isolated as its unreactive deprotonated form [(NH 3 ) 5 CoNHCO 2 C 2 H 5 ] 2+ . Similarly, the N-bound carbamate complex, previously believed to be an intermediate in the acid-induced conversion of [(NH 3 ) 5 CoNCO] 2+ to [(NH 3 ) 6 Co] 3+ and CO 2 , has been detected for the first time in this reaction

Activation of carbon dioxide leading to a chemisorbed carbamate species at a Cu(100) surface

The formation of a strongly chemisorbed copper carbamate complex, tilted with respect to the surface, is shown to occur during the coadsorption of CO2 and NH3 at a Cu(100) surface at low temperatures; a mechanism is suggested involving the nucleophilic attack of ammonia at the positively polarised carbon atom of a transient CO2–(s) species.

ChemInform Abstract: Reactivity of Rhenium(I) Pseudo‐allyl Complexes with Heterocumulenes such as RNCS (R: Me or Ph, p‐MeC6H4) and PhNCO. Part 1. Crystal Structure Determination of Two Isostructural Monothio‐ and Dithio‐carbamate Rhenium(I) Complexes Cocrystallized as (Re(CO)2(PPh3)2(S=C(NHPh)‐S))·(Re(CO)2(PPh3)2(O‐C(NHPh)=S)).

AbstractThe compounds (I) react with the heterocumulenes (II), (IV) and (VII) by displacement of the starting chelate ligand and formation of the carbamate complexes as shown in the scheme.

Reactivity of rhenium(I) pseudo-allyl complexes with heterocumulenes such as RNCS (R = Me or Ph, p-MeC6H4) and PhNCO. Part 1. Crystal structure determination of two isostructural monothio- and dithio-carbamate rhenium(I) complexes co-crystallized as [Re(CO)2(PPh3)2{SC(NHPh)–S}]·[Re(CO)2(PPh3)2{O–C(NHPh)S}

The compounds [Re(CO)2(PPh3)2(OCHNC6H4Me-p)] and [Re(CO)2(PPh3)2(amt)][amt = anion of 2-(aminomethyl)thiazole] react with an excess of PhNCO under non-anhydrous conditions to give the displacement of the starting chelate ligand and the formation of the carbamate complex [Re(CO)2(PPh3)2{OC(NHPh)O}]. When the reaction is carried out under the same conditions with an excess of RNCS (R = Ph or p-MeC6H4) the monothio- and dithio-carbamate complexes [Re(CO)2(PPh3)2{O–C(NHR)S}](1) and [Re(CO)2(PPh3)2{SC(NHR)–S}](2) are obtained. The formation of complexes (2) proceeds via the intermediate formation of (1). A mixture of complexes (1b) and (2b)(R = Ph) gives rise to a crystallized compound, which X-ray analysis has allowed to be formulated as [Re(CO)2(PPh3)2{SC(NHPh)S}]·[Re(CO)2(PPh3)2{OC(NHPh)S}] where discrete molecules of two isostructural rhenium complexes are co-crystallized in the monoclinic cell, of space group P21, with a= 9.314(6), b= 22.976(4)c= 18.662(5)A, and β= 91.01(3)°. In both molecules the Re is six-co-ordinate in a distorted octahedral environment with trans PPh3 groups, cis CO groups, and one bidentate ligand. Distances in the co-ordination sphere are: Re–P 2.42 (mean), Re–C 1.90 (mean), Re–S 2.52 (mean), and Re–O 2.37 A. The ligand moieties are essentially planar in both complexes.

Structure of binuclear adducts of Cu(II) carbamates and phosphates with bases from ESR data

1. For adducts of copper(II) carbamate and phosphate complexes in solutions a binuclear structure of the half-open lantern type is realized in which the two Cu2+ ions are connected only By two bridge anions present in the cis position relative to one another. The parameters of the ESR spectra for this low-symmetry structure have been assigned by modeling the spectra on a computer. 2. The rhombic component of the fine structure in the ESR spectra has been interpreted on the basis of a model of dipole-dipole interactions for nonpunctate magnetic dipoles.

Reversible CO2 fixation by amino complexes of Cu(II)

1. The reversible combination of CO2 with ephedrinate and amino complexes of copper(II) consists in the formation of unstable mononuclear and polynuclear carbamate complexes of copper(II). 2. From ESR data, conclusions are drawn concerning the structure of the mononuclear carbamate complexes.