A stable silicon congener of highly strained bicyclo[3.2.0]hepta-1,3,6-triene

A silicon-containing fused bicyclic compound with a highly strained bridgehead double bond, 2,3,6,7-tetra-tert-butyl-4-(tert-butyldimethylsilyl)-5-(tert-butyldimethylsiloxy)-5-silabicyclo[3.2.0]hepta-1,3,6-triene (2), was synthesized quantitatively by the reaction of 1,2-bis-tert-butyl-4,4-bis(tert-butyldimethylsilyl)-4-silatriafulvene (3) with di-tert-butylcyclopropenone (4) at 80 degrees C. An X-ray crystallographic analysis for 2 not only confirmed a bicyclic structure having a silacyclopentadiene (silole) ring fused with a silacyclobutene ring but also the remarkable deformation around the double bonds; the sum of the bond angles around the unsaturated bridgehead carbon was 333 degrees . The strain energy of a model 5-silabicyclo[3.2.0]hepta-1,3,6-triene was calculated at the MP2/6-31+G(d,p)//B3LYP/6-31+G(d) level (30.2 kcal/mol) to be comparable to that for parent bicyclo[3.2.0]hepta-1,3,6-triene (30.7 kcal/mol). Despite the high steric strain, 2 was stable enough to be kept intact for several months in the air. The high stability is ascribed to the effective steric protection of the ring system by the bulky substituents.     

Photochemical and thermal reactions of intermediates in the phenylnitrene rearrangement inside a hemicarcerand

Broadband irradiation (lambda > 320 nm) of hemicarceplex H(.)1 between -74 degrees C and -84 degrees C, produces encapsulated didehydroazepine (2), triplet phenylnitrene ((3)PN), 2-azabicyclo[3.2.0]hepta-1,3,6-triene (6), and 4-azaspiro[2.4]hepta-1,4,6-triene (7). The highly strained anti-Bredt imine 6 is formed from 2 via a photochemical four-electron electrocyclization. Under the irradiation conditions, 6 rearranges further to azaspirene 7. In addition, 6 thermally rearranges to 7 via a 1,5-sigmatropic shift (DeltaG(267K) = 20.0 +/- 0.5 kcal/mol), yielding a final equilibrium composed of [7]/[6] = 5 at room temperature. The observation of a photochemical rearrangement of 2 to 6 contrasts earlier results of narrow band irradiations (lambda = 334 nm) of matrix-isolated 2, which gave (3)PN (Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879-5881). Encapsulated (3)PN is remarkably stable due to the prevention of its dimerization by the surrounding hemicarcerand. Above 255 K, it slowly decays with a rate constant k = 10(7.7+/-0.4) s(-1) x exp {(13300 +/- 500 cal/mol)/RT}. The isolation of substantial amounts of a hemicarcerand lacking one acetal spanner suggests that (3)PN decays preferentially by inserting into an inward-pointing acetal C-H bond of H.     

Synthesis of 1-azaspiro[2.4]hepta-1,4,6-trienes and azaspiroconjugation studied by photoelectron spectroscopy

1-Azaspiro[2.4]hepta-1,4,6-trienes 3a-c have been prepared by photolysis or thermolysis of 6-azidofulvenes 5a-c, which were accessible by nucleophilic substitution reactions of the precursors 4a,b or by nucleophilic addition of hydrazoic acid to ethenylidene-cyclopentadiene (6c). The UV photoelectron spectrum of 2-methyl-1-azaspiro[2.4]hepta-1,4,6-triene (3c) has been recorded and analyzed by making use of density functional theory (DFT) B3LYP calculations. Substantial homoconjugative interactions have been determined. The lone-pair orbital n(N) of the 2H-azirine nitrogen atom interacts with the pi 1 orbital of the cyclopentadiene ring. The energies of these orbitals are lowered or increased by 0.95 or 0.91 eV with respect to the two parent compounds cyclopentadiene (7) and 3-methyl-2H-azirine (9), respectively. In addition, in compound 3c the pi (C=N) orbital of the three-membered ring interacts with a sigma orbital of the cyclopentadiene unit and is destabilized by 0.47 eV by this effect.     

Heavy-Atom Tunneling in Bicyclo[4.1.0]hepta-2,4,6-trienes

Heavy-atom tunneling limits the lifetime and observability of bicyclo[4.1.0]hepta-2,4,6-triene, a key intermediate in the rearrangement of phenylcarbene. Bicyclo[4.1.0]hepta-2,4,6-triene had been proposed as the primary intermediate of the rearrangement of phenylcarbene, but despite many efforts evaded its characterization even in cryogenic matrices. By introducing fluorine substituents into the ortho-positions of the phenyl ring of phenylcarbene, the highly strained cyclopropene 1,5-difluorobicyclo[4.1.0]hepta-2,4,6-triene becomes stable enough to be characterized in argon matrices. However, even at 3 K this cyclopropene is only metastable and rearranges via heavy-atom tunneling to the corresponding cycloheptatetraene. Calculations suggest that fluorination is necessary to slow down the tunneling rearrangement of the bicycloheptatriene. The parent bicycloheptatriene rapidly rearranges via heavy-atom tunneling and therefore cannot be detected under matrix isolation conditions.     

Access to the naphthylcarbene rearrangement manifold via isomeric benzodiazocycloheptatrienes

Irradiation (lambda = 670 or >613 nm) of 4,5-benzodiazocycloheptatriene (15), matrix isolated in argon at 10 K, produces primarily 2,3-benzobicyclo[4.1.0]hepta-2,4,6-triene (9) accompanied by small amounts of triplet 4,5-benzocycloheptatrienylidene (2) and 2-naphthylcarbene (10). A reversible photoequilibrium is established in which 9 is converted to 10 at lambda = 290 nm and then regenerated at lambda = 360 nm. Similarly, matrix-isolated 2,3-benzodiazocycloheptatriene (16) produces 4,5-benzobicyclo[4.1.0]hepta-2,4,6-triene (11) at lambda = 670 or >613 nm, but without detection of 2,3-benzocycloheptatrienylidene (4). Irradiation of 11 at lambda = 290 nm induces ring opening to triplet 1-naphthylcarbene (12), which, in turn, cyclizes back to 11 at lambda = 342 or >497 nm. The diazo compounds and photoproducts are characterized by IR, UV/visible, and ESR spectroscopy, where appropriate, and by comparison of the experimental and B3LYP/6-31G calculated IR spectra for each species. Alternate rearrangement products such as allenes 6, 7, and 8 are not detected in the photolysis of either diazo compound.     

含吡啶环氮(王)冠的合成

本文报道了新的含吡啶环的氮(王)冠的合成。以活泼双功能团化合物2,6-二(溴四基)-吡啶和N-对甲苯磺酰基取代的多乙撑基多胺(二乙撑三胺,三乙撑四胺)二钠盐直接缩合成环,得到含吡啶环的氮(王)冠化合物:3,6,9-三对甲苯磺酰基-3,6,9,15-四氮双环[9.3.1]十五环-1(15),11,13-三烯(A)和3,6,9,12-四对甲苯磺酰基-3,6,9,12,18-五氮双环[12.3.1]十八环-1(18),14,16-三烯(B)。(A)和(B)经用30%的HBr-CH_3COOH溶液处理,得到3,6,9,15-四氮双环[9.3.1]十五环-1(15),11,13-三烯(C)和3,6,9,12,18-五氮双环[12.3.1]十八环-1(18),14,16-三烯(D)。以上化合物均经元素分析、IR和1~H NMR.等鉴定。这种直接缩合的方法,具有反应条件温和、设备简单、操作简便等优点。     

Interplay of orbital symmetry and nonstatistical dynamics in the thermal rearrangements of bicyclo[n.1.0]polyenes.

CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.     

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C(7)H(6) Energy Surface

Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C(7)H(6). Fulvenallene (6) is the most stable C(7)H(6) isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol(-)(1), respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet ((3)A") ground state, 16 kJ mol(-)(1) more stable than the singlet state ((1)A'). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol(-)(1)) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the (3)A(2) and (3)B(1) triplet states of cycloheptatrienylidene ((3)3) are predicted to lie very close in energy. The singlet "aromatic" cycloheptatrienylidene ((1)3, (1)A(1)) is found to be a transition structure interconverting two chiral cyclic allenes (4) and it lies approximately 25 kJ mol(-)(1) below the triplet states. Bicyclo[3.2.0]hepta-1,3,6-triene (5) is predicted to be a stable equilibrium structure, lying in a significant energy well. Rearrangement of 4 to 5 constitutes the rate-determining step for the rearrangement of phenylcarbene to fulvenallene (6), the ethynylcyclopentadienes (9), and spiro[2.4]heptatriene (7). Rearrangement of 9A to 6, via a 1,4-H shift, requires a large barrier of 325 kJ mol(-)(1). Rearrangement of benzocyclopropene (10) to 6 involves a methylenecyclohexadienylidene intermediate (27) and is associated with an energy barrier of 223 kJ mol(-)(1). The calculated mechanisms and energetics for the interconversions of various C(7)H(6) isomers are in good accord with experimental results to date.     

Isolation of a stable 1-iridabicyclo[3.2.0]hepta-1,3,6-triene and its reversible transformation into an iridacycloheptatriene

Reaction of the iridacyclopentadiene complex 2 with 2-butyne yields a mixture in equilibrium of the iridacycloheptatriene 3 and a novel 1-iridabicyclo[3.2.0]hepta-1,3,6-triene, 4.     

Copper complexes of new benzodioxotetraaza macrocycles with potential applications in nuclear medicine

Two novel benzodioxotetraaza macrocycles [2,9-dioxo-1,4,7,10-tetraazabicyclo[10.4.0]1,11-hexadeca-1(11),13,15-triene (H2L1) and 2,10-dioxo-1,4,8,11-tetraazabicyclo[11.4.0]1,12-heptadeca-1(12),14,16-triene (H2L2)] were synthesized by a [1 + 1] crablike cyclization. The protonation constants of both ligands were determined by 1H NMR titration and by potentiometry at 25.0 degrees C in 0.10 M ionic strength in KNO3. The latter method was also used to ascertain the stability constants of their copper(II) complexes. These studies showed that the CuL1 complex has a much lower thermodynamic stability than the CuL2, and the H2L2 displays an excellent affinity for copper(II), due to the good fit of copper(II) into its cavity. The copper complexes of the novel ligands were characterized by electronic spectroscopy in solution and by crystal X-ray diffraction. These studies indicated that the copper center in the CuL1 complex adopts a square-pyramidal geometry with the four nitrogen atoms of the macrocycle forming the equatorial plane and a water molecule at axial position, and the copper in the CuL2 complex is square-planar. Several labeling conditions were tested, and only H2L2 could be labeled with 67Cu efficiently (>98%) in mild conditions (39 degrees C, 15 min) to provide a slightly hydrophilic radioligand (log D = -0.19 +/-0.03 at pH 7.4). The in vitro stability was studied in the presence of different buffers or with an excess of diethylenetriamine-pentaethanoic acid. Very high stability was shown under these conditions for over 5 days. The incubation of the radiocopper complex in human serum showed 6% protein binding.     

Pentafluorophenylation of Perfluorinated Benzocyclobutene,Indan, and Tetralin by Reaction with Pentafluorobenzene in SbF5

The reactivity of perfluorinated benzocyclobutene, indan, and tetralin in reaction with pentafluorobenzene in SbF₅ medium, and also the relative stability of generated therewith perfluoro-1-phenylbenzocycloalkenyl cations decrease with increasing alicyclic fragment in the benzocycloalkene. Treating the solutions of salts of the above cations with anhydrous HF results in the corresponding perfluoro-1-phenylbenzocycloalkenes, and the hydrolysis of salts furnishes their 1-hydroxy derivatives. In a reaction of 1-hydroxyperfluoro-1-phenylbenzocyclobutene, -indan, and -tetralin with SOCl₂ the hydroxy group is replaced by chlorine. Besides with indan and tetralin derivatives form respectively 7-pentafluorophenyloctafluoro-3-chlorobicyclo[4.3.0]hepta-1,4,6-triene and 7-pentafluorophenyldecafluoro-3-chlorobicyclo[4.4.0]octa-1,4,6-triene.     

P=N bond formation via incomplete N-atom transfer from a ferrous amide precursor

Incomplete N-atom transfer from Fe to P is observed when the ferrous amide complex (PNP)Fe(dbabh) (PNP-=N[2-P(iPr)2-4-methylphenyl]2, dbabh=2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene), prepared from salt metathesis of (PNP)FeCl and Li(dbabh), is thermolyzed at 70 degrees C over 48 h in C6D6. Several plausible reaction pathways resulting from the transformation of (PNP)Fe(dbabh) are discussed, including the possibility of an Fe(IV) nitride as an intermediate.     

Hexafluorothioacetone based synthesis of fluorinated heterocycles

Cycloadducts of hexafluorothioacetone (HFTA) were prepared in high yield by a CsF catalyzed reaction between readily available 2,2,4,4-tetrakis-(trifluoromethyl)-1,3-dithietane (as a source of HFTA) with conjugated electron-rich hydrocarbon dienes, such as cyclopentadiene, 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene or (1Z,3Z)-cyclohepta-1,3-diene. Cyclohexa-1,4- and (1Z,5Z)-cycloocta-1,5-dienes, also undergo the reaction with in situ generated HFTA, but form the products of insertion of HFTA into the C-H bond of the diene as a result of ene-reaction. The highly selective reaction of HFTA with (1Z,3Z,5Z)-cyclohepta-1,3,5-triene and (1Z,3Z,5Z,7Z)-cycloocta-1,3,5,7-tetraene leads to the formation of cycloadducts derived from exclusive addition of thioacetone to the corresponding bicyclic isomers—bicyclo[4.1.0]hepta-2,4-diene or bicyclo[4.2.0]octa-2,4,7-triene, respectively. The corresponding cycloadducts of HFTA with 2,3-dimethylbutadiene-1,3-cyclohexa-1,3-cyclohexa-1,4-dienes and (1Z,3Z,5Z)-cyclohepta-1,3,5-triene were also prepared by direct reaction of sulfur/hexafluoropropene/KF and the corresponding hydrocarbon substrate at 35-45 °C in DMF.     

Exploring the interaction of mercury(II) by N(2)S(2) and NS(3) anthracene-containing macrocyclic ligands: photophysical, analytical, and structural studies

The complexation properties toward Hg(II) of six macrocyclic ligands, 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1), 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L2), 7-(10-methyl-9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L3), 7,7'-[9,10-anthracenediylbis(methylene)]bis-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L4), 1,4,7-trithia-11-azacyclotetradecane (L5), and 11,-(anthracen-9-ylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L6), were studied. The stoichiometries of the formed species were determined from absorption and fluorescence titrations. In these anthracene-containing macrocycles, a fluorescent quenching of the emission was found upon Hg(II) addition. The X-ray crystal structure of [HgCl2(L2)] x 1/2CH2Cl2 was determined. The asymmetric unit contains two independent [HgCl2(L2)] molecules and one dichloromethane molecule. Each Hg(II) ion is coordinated by the pyridine nitrogen, the two sulfur atoms of one L2 molecule, and two chloride ions. Analytical studies using solvent extraction separation of Hg(II) from aqueous solutions were performed to determine the Hg(II) extraction capability of ligands L1, L2, and L5.     

Comparative study of the photochemistry of the azidopyridine 1-oxides

The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.     

UV-mediated decomposition of diazomalonates in benzene: Unexpected access to functionalized bicyclo[3.2.0]heptane skeleton

Upon irradiation with 300-nm UV light, the photolysis of diazomalonates in benzene unexpectedly affords 2,6-dicarboxylate bicyclo[3.2.0]hepta-2,6-dienes in low yields. These products are proven to be derived from cyclohepta-1,3,5-triene intermediates presumably via a tandem 1,5-carboxylate migration/[2+2] cycloaddition sequence.     

Bicyclo[4.2.0]octa-1,3,5-triene: 2-Mono- and 2,5-Disubstituted Derivatives via Highly Regioselective Lithiation of Its Cr(CO)3 Complex and via Reductive Silylation/Oxidation

Treatment of [η⁶-(bicyclo[4.2.0]octa-1,3–5 triene)]tricarbonylchromium(0) ( 2 ) with BuLi or lithium 2,2,6,6-tetramethylpiperidinide (TMPLi) gives rise to a highly regioselective deprotonation at C(2). Subsequent reaction with electrophiles (6 examples) gives [η⁶-(2-R-bicyclo[4.2.0]octa-1,3,5-triene)]tricarbonylchromiurn complexes 3 and 5 – 9 in moderate (R?I, 50%; R?CHO, 67%) to good (R?Me, D, SiMe₃, CO₂Me, > 80%) yield (Scheme 1). Analogous reactions with tricarbonyl (η⁶-indane)chromium ( 10 ) give mixtures of complexes substituted at C(4) and C(5) (Scheme 2). In 10 , deprotonation β to the ring junction is strongly favoured with the bulky base TMPLi. Double lithiation/electrophile additions to 2 give access to [η⁶-(2-R′-5-R″-bicyclo[4.2.0]octa-1,3,5-triene)]tri-carbonylchromium complexes (e.g. 13 (R′?R″?Me₃Si) and 14 (R′?Me₃Si, R″?CHO)) as single products. The Cr(CO)³ group can be easily removed by oxidation (I₂, Ce(IV), O₂/light; 2 examples each) to give the free arenes. Base-catalyzed (C_sF, DMF/D₂O) deuterodesilylation of 13 yields the [(2,5-²H₂)bicyclo[4.2.0]octa-1,3,5-triene]chromium complex 15 , and treatment of 2,5-bis(trimethylsilyl) compound 16 with CF₃COOD gives the 2,4-dideuterated 17 . Compound 16 is also accessible more directly via reductive silylation/oxidation of bicyclo[4.2.0]octa-1,3,5-triene ( 1 ). Stereoselective base-catalyzed (t-BuOK.) H/D exchange of the benzylic H-atoms. opposite to the Cr(CO)₃ moiety in 2 takes place rapidly in (D₆)DMSO, but benzylic functionalization via this route remains elusive.     

Bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation: a theoretical validation of a bishomoaromatic radical cation intermediate

[structure: see text] A natural bond orbital analysis of the distonic bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation interprets its structure and radical character by a three-center two-electron bond between C2, C3, and C7 (a bishomoaromatic stabilization) and a singly occupied orbital on C5, n(5). Moreover, B3LYP/6-311+G(d,p) ESR parameters, which agree excellently with experiment, are interpreted in terms of spin polarization in the natural hybrids of sigma(C5-H5), and a dual hyperconjugative effect involving n(5), sigma(C1-H1a), sigma(C1-H1b), and antibonding counterparts.     

Slowdown of H/D exchange reaction rate and water dynamics in ionic liquids: deactivation of solitary water solvated by small anions in 1-butyl-3-methyl-imidazolium chloride

The H/D exchange reaction and the rotational dynamics of heavy water (D2O) are studied at 50 degrees C in the ionic liquid, 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), in the [D2O] range of 3-55 M. The initial H/D exchange rates are observed as 1.0 x 10(-7), 4.5 x 10(-6), 1.0 x 10(-5), 4.1 x 10(-5), 1.1 x 10(-4), and 3.7 x 10(-4) s(-1), respectively, at [D2O] of 2.8, 7.1, 8.1, 11, 15, and 25 M. The rate is very slow and less than 10(-5) s(-1) at [D2O] below approximately 7 M. It steeply increases to the order of 10(-4)s(-1) for 7 M < [D2O] < 10 M, and linearly increases with [D2O] in the more water-rich region. The intercept of the linear region at [D2O] = approximately 9 M is interpreted by considering that each chloride anion deactivates 1.6 equiv water molecules due to the strong solvation. Correspondingly, the rotational correlation time of D2O at [D2O] < 7 M is 1 order of magnitude larger than that in water-rich conditions.     

The solution structures and dynamics and the solid-state structures of substituted cyclopentadienyltitanium(IV) trifluorides

Organotitanium fluorides (C5Me4R)TiF3 (R = H, Me, Et) sublimate with formation of crystalline dimers. From solution, we obtained crystals of dimers and tetramers. The tetramer [{(C5Me5)TiF3}4] irreversibly dissociates in the solid state to dimers (DeltaH = 8.33 kcal mol(-1)). The variable-temperature (1)H and (19)F NMR spectroscopy measurements of the toluene-d(8) solution of [{(C5Me5)TiF3}2] revealed at 202 K one monomeric, two dimeric (with C2h and Cs symmetry), two tetrameric (with D2 and C2v symmetry), and two trimeric (both C2 symmetry) molecules. With the increase in temperature and dilution of the solution, the composition of the solution shifts to the smaller molecules. The thermodynamic and activation parameters for the reversible dissociation of dimers to monomers in the solution are DeltaH = 9.2 kcal mol(-1), DeltaS = 24.2 cal mol(-1) K(-1), DeltaH(double dagger) = 12.2 kcal mol(-1), DeltaS(double dagger) = 9.7 cal mol(-1) K(-1). The dissociation path with a weakly double-bridged transition-state dimer was proposed. The thermodynamic parameters for the reversible dissociation of the C2v tetramer to the dimers in solution are DeltaH = 7.9 kcal mol(-1) and DeltaS = 26.8 cal mol(-1) K(-1). From both tetramers, the D2 molecule is 0.34(5) kcal mol(-1) lower in enthalpy and 6.5(5) cal mol(-1) K(-1) lower in entropy than the C2v molecule. The structures of both trimers were proposed. The low-temperature 19F NMR spectra of the CDCl3 solution of [{(C5Me5)TiF3}2] are consistent with equilibria of a monomer, two dimers (with C2h and Cs symmetry), and a trimer. The vapor pressure osmometric molecular mass determination of CDCl3 solution of [{(C5Me5)TiF3}2] at 302 K is consistent with the equilibrium of the dimer and the monomer.