A. Hugo Klahn

cis–trans Isomerisation of CpRe(CO)2(H)(ArF)(ArF= C6FnH5−n; n= 0–5) is the rate determining step in C–H activation of fluoroarenes: a DFT study

Density functional calculations have been used to examine the reaction of {CpRe(CO)2} with fluorobenzenes C6FnH6−n (n = 0–5). Two classes of product have been observed experimentally (using Cp or Cp*): (a) coordination of the arene in an η2 fashion and (b) C–H activation to form a hydrido–aryl complex. Increasing the number of fluorines on the arene ring was shown to favour C–H activation. The thermodynamic and kinetic (reaction path) aspects of these transformations have been examined with DFT (B3PW91) calculations. For a given arene, the rhenium moiety is shown to exhibit the following order of thermodynamic preference for coordination: HCCH site > HCCF site > FCCF site. Binding energies to the different arenes do not follow a clear trend and span ca. 20 kJ mol−1. The Re–C bond energies in CpRe(CO)(H)(C6FnH5−n) span 55 kJ mol−1. Calculated structural parameters agree with the crystal structure of coordination of C6H6 and C6F6. Likewise the binding energy of C6H6 is in good agreement with experimental data. The calculated free energy difference between CpRe(CO)2(η2-C6FnH6−n) and CpRe(CO)2(H)(C6FnH5−n) shows that preference for the hydrido–aryl complex is determined principally by the bond dissociation energy of the C–H bond of the free arene. The binding energy to the η2-arene appears to be only a secondary factor. Three families of complexes are apparent. If there is no F on the carbon ortho to the Re–C bond that is formed, the η2-arene complex is energetically preferred. If there is one F at the ortho position, the energies of the products are similar. In the case of two ortho F substituents, the product of oxidative addition is significantly favoured. In agreement with the calculations, experimental evidence shows that benzene only coordinates to Cp*Re(CO)2, 1,4-C6F2H4 gives a mixture of products and 1,3-C6F2H4 gives only the hydrido–aryl complex. The arene with the stronger C–H bond is the one which gives more oxidative addition product because the Re–C bond energy increases with F substitution (and in particular with ortho F) more than twice as fast as the C–H bond dissociation energy. The reaction path for the overall transformation has been determined. The σ C–H complex is identified as an intermediate on the pathway for the oxidative addition. The initial product of oxidative addition is the cis hydrido–aryl isomer which subsequently isomerizes to the trans isomer. The rate determining step has been found to be the cis–trans isomerisation process and not the oxidation addition step. The cis–trans isomerisation proceeds via an unconventional concerted motion of H and the two COs. The variation of the Re–C bond energy is the dominant factor in determining the changes in the energy barrier between the different fluoroarenes, resulting in strong correlation between the thermodynamics and kinetics of reaction. The activation barriers are therefore also grouped in three families (0 F ortho, 1 F ortho, 2 F ortho).

An improved synthetic method and vibrational study of (pentamethylcyclopentadienyl) dicarbonylrhenium dihalides (η5-C5Me5)Re(CO)2X2 (X = C1, Br and I)

An improved synthetic method has been found for the preparation of the pentamethylcyclopentadienyl rhenium dicarbonyldihalide complexes. From the reaction of (η5-C5Me5)Re(CO)3 with Br2 or I2 in THF-H2O a mixture of cis and trans isomers of (η5-C5Me5)Re(CO)2X2 X = Br and I is formed. On the other hand, the reaction of [(η5-C5Me5)Re(CO)3C1][SbC16] in water gives the cis-(η5-C5Me5)Re(CO)2C12 complex. The solid IR spectra of the dicarbonyldihalide complexes are recorded and an assignment of the normal modes in terms of local symmetry is suggested by comparison with those observed in analogous molecules. A normal coordinate analysis performed using a modified general valence force field and considering simplified models, confirms most of the experimental assignments. The set of valence force constants reflects the structure of the isomers under study.

Reactions of rhenium trialkylphosphite complexes (η5-C5Me5)Re(CO)2{P(OR)3} (R Me or Et) with halogens and SbCl5: Reversible Arbuzov-like dealkylation to form rhenium dialkylphosphonate complexes (η5-C5Me5)ReX(CO)2{PO(OR)2}

Abstract The cationic complexes of general formula [(η 5 -C 5 Me 5 )ReX(CO) 2 {P(OR) 3 }] + (R  Me; X  Cl ( 2a ), Br ( 3a ) or I ( 4a )) and (R  Et; X  Cl ( 2b ), Br ( 3b ) an I ( 4b )) have been synthesized by reactions of (η 5 -C 5 Me 5 )Re(CO) 2 {P(OR) 3 } with either halogens X 2 or SbCl 5 . Each is obtained as a mixture of cis and trans isomers of a typical four-legged piano-stool geometry and has been fully characterized in solution by a combination of IR and 1 H, 13 C and 31 P nuclear magnetic resonance spectroscopy. It is observed that under appropriate conditions these cationic complexes undergo transformation to the corresponding neutral dialkylphosphonate complexes of general formula trans -(η 5 -C 5 Me 5 )ReX(CO) 2 {PO(OR) 2 } ( 5a–7a and 5b–7b ) where R  Me ( a ) or Et ( b ) and X  Cl ( 5 ), Br ( 6 ) and I ( 7 ). This is ascribed to a Michaelis—Arbuzov-type dealkylation reaction involving a nucleophilic attack at the alkyl group by the counter-anion, and it is possible to render the cation more (or less) stable to this transformation by suitable choice of the counter-anion and the phosphite. The cationic triethylphosphite complexes exhibit a decreased tendency to undergo the dealkylation reaction. Most interestingly, the product ( i.e. cationic phosphite or neutral dialkylphosphonate) in the reaction of (η 5 -C 5 Me 5 )Re(CO) 5 {P(OR) 3 } with Br 2 at −78°C is dependent on the molar ratio of the reactants; when examined in detail, this is found to result from a counter-ion dependent reversible alkylation-dealkylation process. Thus the dialkylphosphonate is unchanged by treatment with alkylbromide alone but is completely transformed back to the cationic trialkylphosphite complex by alkyl bromide and bromine .

The Infrared Spectra of Rhenium Pentamethyl Cyclopentadienyl Complexes: (n 5-C5Me5)Re(CO3) and [(n 5-C5Me5)Re(CO)3X]+ (X=Cl, Br, I)

Abstract The IR spectra of the title compounds are recorded and an assignment of the normal modes in terms of local symmetry is suggested by comparison with those observed in analogous molecules. A set of force constants calculated for simplified models confirms most of the experimental assignments and reflects the symmetry of the complexes under study. The f CO of the two different CO groups in the cationic complexes indicated that CO groups cis to the halide ligand should be more selective towards nucleophiles.

Synthesis, reactivity and molecular structure of phosphino tetramethyl cyclopentadienyl complex (η5: η1-C5Me4CH2PPh2)Re(CO)2

The fulvene complex (eta(6)-C(5)Me(4)CH(2))Re(C(6)F(5))(CO)(2) reacts at the exocyclic methylene carbon with potassium diphenylphosphide to yield the anionic species [(eta(5)-C(5)Me(4)CH(2)PPh(2))Re(C(6)F(5))(CO)(2)](-) (). Protonation of with HCl at 0 degrees C produces the hydride complex trans-(eta(5)-C(5)Me(4)CH(2)PPh(2))Re(C(6)F(5))(H)(CO)(2) (). Thermolysis of a hexanes solution of , under nitrogen atmosphere, produces the chelated complex (eta(5):eta(1)-C(5)Me(4)CH(2)PPh(2))Re(CO)(2) () in good yield. The thermolysis under a CO atmosphere affords a mixture of the complexes (eta(5):eta(1)-C(5)Me(4)CH(2)PPh(2))Re(CO)(2) () and (eta(5)-C(5)Me(4)CH(2)PPh(2))Re(CO)(3) (). The reaction of with two electron donor ligands yields (eta(5)-C(5)Me(4)CH(2)PPh(2))Re(CO)(2)(L) (, L = CO; , L = PMe(3); , L = (t)BuNC). Complex also reacts with I(2), HBF(4) and MeOTf to yield the cationic compounds trans-[(eta(5):eta(1)-C(5)Me(4)CH(2)PPh(2))Re(R)(CO)(2)](+) (, R = I; , R = H; , R = Me). Upon treatment with chloroform, the hydride complex converts to the corresponding chloro derivative . The trans stereochemistry for complexes have been assigned on basis of nu(CO) IR intensities and (13)C-NMR chemical shifts. The reaction of the cationic complexes (, ) with KI and Me(3)NO.2H(2)O yields the neutral species cis-(eta(5):eta(1)-C(5)Me(4)CH(2)PPh(2))Re(I)(R)(CO) (, R = I, , R = Me). The molecular structure of and have been determined by X-ray crystallography.

Reactions of the tetramethylfulvene ligand coordinated to rhenium with hydrohalic acids and halogens: synthesis of dicarbonyl complexes with a ‘four-legged piano-stool’ structure containing mixed-halide ligands

Abstract The new tetramethylfulvene complexes of rhenium (η 6 -C 5 Me 4 CH 2 )Re(CO) 2 X, (X=Cl, Br, I) have been synthesized from (η 5 -C 5 Me 5 )Re(CO) 2 X 2 by a two step procedure and fully characterized by elemental analysis, IR and NMR spectroscopies. Reaction of (η 6 -C 5 Me 4 CH 2 )Re(CO) 2 X (X=Br, I) with HX′ regenerated the (η 5 -C 5 Me 5 ) ligand with the formation of the mixed-halide complexes cis -(η 5 -C 5 Me 5 )Re(CO) 2 XX′ (X=I, X′=Br; X=I, X′=Cl; X=Br, X′=Cl), whereas, the reaction with halogens (X′ 2 ) gave analogous complexes containing a halide substituted tetramethylcyclopentadienyl ligand, cis -(η 5 -C 5 Me 4 CH 2 X′)Re(CO) 2 XX′ (X=Br, X′=I; X=I, X′=Br). In both types of complexes the cis stereochemistry have been assigned on the basis of ν (CO) IR intensities and δ (CO) 13 C-NMR. 4

SYNTHESES AND REACTIONS OF THE COMPLEXES (η5-C5Me5)Re(CO)2(C6CL5–nHn)Cl (n = 2,3): COMPOUNDS DERIVED FROM INITIAL C—CI ACTIVATION

Abstract The UV irradiation of (η5-C5Me5)Re(CO)3 in the presence of 1,2,4,5-C6Cl4H2 and 1,3,5-C6Cl3H3 (λ = 350 nm, hexane solution) effected intramolecular C—Cl activation, generating the complexes trans-(η5-C5Me5)Re(CO)2(2,4,5-C6Cl5-nHn)Cl, ((1), n = 2; (2), n = 3), respectively. Complex (1) dissolved in polar organic solvents produces, an equilibrium mixture with its cis isomer. The reaction of (1) with AgBF4, in acetonitrile, led to formation of the cationic complex [cis-(η5-C5Me5)Re(CO)2(2,4,5-C6Cl3H2)(MeCN)]+. The tetramethylfulvene complex (η6-C5Me4CH2)Re(CO)2(2,4,5-C6Cl3H2) (3) was obtained by reacting the cationic complex with the fluorinating agent Et3N′3HF.

Neutral phosphine and phosphite derivatives of the fragment (η5-C5Me5)Re(CO)2 and the cationic cis-[(η5-C5Me5)Re(CO)2{(P(OPh)3}I]+ complex

Abstract The phosphine complexes, (η5-C5Me5)Re(CO)2PR2R′ [R = R′= Ph (1); R = Me, R′ = Ph (2)] and the phosphite complexes, (η5-C5Me5)Re(CO)2[P(OR)3] [R = Et (3); R = Ph (4)] have been prepared in 38–70% yield by reaction of the THF complex (η5-C5Me5)Re(CO)2(THF) with the corresponding phosphine or by direct irradiation of (η5-C5Me5)Re(CO)3 in the presence of the phosphite ligand. From the reaction of the neutral triphenylphosphite complex (4) with I2 in hexane the cationic complex cis-[(η5-C5Me5)Re(CO)2{P(OPh)3}I]+ (5) could be isolated in almost quantitative yield. The cis orientation of the CO groups for the latter complex has been assigned on the basis of 2J(CP) coupling constants and by the intensity ratio of the symmetric and antisymmetric stretching frequencies of the CO ligands.

Dialkyldicarbonyl rhenium complexes (η5-C5Me5)Re(CO)2R2, R = Me and Et. X-ray structure of trans-(η5-C5Me5)Re(CO)2Et2

The compounds cis-(η5-C5Me5)Re(CO)2Me2 and trans-(η5-C5-Me5)Re(CO)2Et2 were prepared by alkylation of cis-(η5-C5Me5)Re(CO)2Cl2 using the corresponding organocopper (RCu). Photolysis of cis-(η5-C5Me5)Re(CO)2Me2 in frozen toluene-d8 readily produces the trans-(η5-C5Me5)Re(CO)2Me2. All the compounds were characterized by spectroscopy and analyses, and the stereochemistry assigned from examination of v(CO) IR intensities; additionally trans-(η5-C5Me5)Re(CO)2Et2 was studied by X-ray crystallography. Crystal data: monoclinic P21/m, a = 8.034(8) A, b = 11.991(8) A, c = 8.69(2) A, β = 91.38°, V = 837(2) A3, Z = 4, λ(Mo K α) = 0.7107 A, RF = 0.0285 and wRI = 0.071 for 1670 measured reflections with 5 < 2ϑ < 50°.

Stepwise substitution reactions of the (η5-C5Me5)Re(CO)2I2 complex: aryl-iodo and aryl-methyl derivatives trans-(η5-C5Me5)Re(CO)2(Ar) X, X = I and Me. X-ray structure of trans-(η5-C5Me5)Re(CO)2(Ph)I

Abstract The diiodo complex cis -( η 5 -C 5 Me 5 )Re(CO) 2 I 2 undergoes monosubstitution in reaction with arylcopper (ArCu) to produce the corresponding aryl-iodo complexes trans -( η 5 -C 5 Me 5 )Re(CO) 2 (Ar)I, Ar = phenyl and tolyl. These complexes have been fully characterized by using a combination of elemental analyses and IR, 1 H and 13 C NMR spectroscopy; additionally trans -( η 5 -C 5 Me 5 )Re(CO) 2 (Ph)I was studied by X-ray crystallography. This complex crystallizes in the monoclinic space group P 2 1 / m with a = 8.191(2), b = 17.471(4), c = 12.995(3) A , β = 93.97(2)°, V = 1832.6(7) A 3 and D calc = 2.107 g cm −3 for Z = 4. The refined structure gave R = 2.74% and wR = 2.98% for 2858 observed reflections. Further reaction of the aryl-iodo complexes with MeLi yields the corresponding aryl-methyl derivatives trans -( η 5 -C 5 Me 5 )Re(CO) 2 (Ar)Me. The full spectroscopic characterization of these complexes is also described in this paper.