tmc literature highlights 2

Transition Met. Chem. 10, 31-35 (1985)

TMC Literature Highlights 2

TMC literature highlights - 2 31

Bond dissociation energies of metal-alkyls

Considering the dearth of thermochemical data on organometall ic systems, it is gratifying to see bond dissociation energies appearing for o metal-carbon (alkyl) bonds (Table 1). They are not all approximately equal as is sometimes asserted. The instinctive feeling that an M-Ph is stronger than an M-Me bond is confirmed. While replacement of H by F appears to stabilize the M - C bond in neutral molecules, has it a reverse effect in positively charged species? In the Co(dmgH)2 systems, electronic effects by the trans ligand, L, are less signifi- cant than changes in its size.

Table 1. Carbon-metal bond dissociation energies.

System D (kcal mol -t) Ref.

(@_MesCs)2Th(_R) 2 1)

R = Me 77.2, 79.3 ~ n-Bu 71.6, 73.9 Ph 90.3, 95.1 CH2CMe3 72.2, 77.0 CH2SiMc 3 80.3, 82.7

Cp2Th(_Ph) 2 74 b) 2)

(OC)sMn-R

R = M e 36 ~ Ph 40 CH2 21 CF3 41 COMe 31 COPh 21 COCF3 35

(OC)sMn+-Me ca. 33 3)

(OC)sMn+-CF3 25

Ta(-Me)5 62 b) 4~

W(_Me) 6 38 b) 4)

Fe+-Me 69 5~

Co+-Me 61

Ni+-Me 48

Fe+=CH2 (c.f.) 96

Co+=CH2 85

Ni+=CH2 86

Ni+=CF: 47

trans-py(saloph)Co-R 6) R = n-Pr 25

i-Pr 20 CH2CMe3 18 CH2Ph 22

trans-L(dmgH) 2Co-CH(Me)Ph L = various phosphines 17_24d~ 7~ L = imidazole and various pyridines 17.9-21.2 ~) 8)

"~ First and second D, respectively, b~ Mean value. ~/Assuming D[(OC)sMn-Mn(CO)s] = 22 kcal mol -j. d) Strongly dependent on cone angle of phosphine, e/Weakly dependent on basicity of L.

Activation of C-H and C-O bonds

The development of techniques for generating and studying positively charged gas phase species, particularly by Beauchamp,s(5, 9) and by Freiser 's 0~ groups, has increased our understanding of the reactions of metal ions with hydrocarbons. Species are produced not unlike those formed in more conventional processes, e.g. HCoMe +, Ni0qZ-CzH4)J -, and FeCH2CH2CH~-, and there are similarities too in types of reaction: e.g. C - H insertion, [3-hydrogen elimination and a-hydrogen abstraction. Methane reacts with Fe +, Co + and Ni + to give HMMe + by C - H insertion, and MH +, perhaps by direct abstraction of hydrogen. While all three ions can insert into both C - H and C-C bonds of higher alkanes, Rh + only attacks the first. In longer alkanes selectivity increases towards internal as opposed to terminal C-C bonds in the sequence: Fe + < Co + < Ni +.

Scheme 1 H/M~/~J - - M ) + I-I2

Scheme 2

The reactions of Fe +, Co p and Ni + with cycloalkanes and cycloalkanones have recently been studied by Jacobson and Freiser (11). With cyclobutane the reaction probably follows Scheme 1 and perhaps Scheme 2 as well. (The steps of all the Schemes are worth analysing carefully because of their resemblance to more conventional organometallic processes.) In contrast, when cyclopentane and cyclohexane are attacked the main pathway follows Scheme 3, ring cleavage not being the predominant process (although some cleavage does occur in the case of Co + and Ni + perhaps according to Scheme 4). Reactions of cyclopentene and cyclohexene have also been studied. Fe + reacts with cyclobutanone to give CO and fer- racyclobutane +, but the other two metal ions yielded MCO + and propene, which suggests that their metallacyclobutane rings may be considerably less stable. Cyclopentanone com- bines with all three ions to yield the metallacyclopentanes which then react according to Scheme 1 and perhaps Scheme 2.

0 �9

Scheme 3

�9 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985 0340-4285/85/0101-0031502.50/0

32 TMC literature highlights- 2 Transition Met. Chem. 10, 31-35 (1985)

Scheme 4

+@ ~,+m +m H 3 C-- ~ ~ M~/.J + C H 4

H3C/M~/j

/ __ ii_ - k

Normally C-H activation is associated with complexes of metals such as rhenium 02), rhodium (13' 14) and iridium (15- is), all of which are in the Transition Series and thus capable of vary- ing easily their oxidation states. Thus Watson's 09~ observation that lutecium compounds will react with hydrocarbons is interesting. Cp{LuH reacts with Phil and SiMe4 to give Cp~LuPh and Cp~LuCH2SiMe3, while Cp{LuMe reacts with CH 4 and Phil as in Equations (1) and (2) (Cp* = vlS-CsM%). The rate equations for (1) and (2) are of the form of (3). Cp~YMe also reacts according to Equation (1).

Cp{LuCH3 + 13CH4 ~ Cp~Lu13CH3 + ca4 (1)

Cp~LuCH3 + C6H6 ~ Cp{LuC6H5 + CH 4 (2)

rate = ka[Lu] + kb[Lu][hydrocarbon] (3)

It was recently reported (2~ that the thorium complex (1) readily undergoes a cyclometalation reaction, in which extru- sion of a saturated hydrocarbon molecule occurs to yield the thoriacyclobutane (2). Consideration of thermochemical data (zl) raised the fascinating question that perhaps complex (2) might react with hydrocarbons. In fact this hypothesis has

Me Me Me Me

M e - ~ M e M e - - ~ M e Me ~ /CH2Bu-t 5o~ Me ~ .~,,,,N e

CH2Bu-t e M e ~ - M e

Me Me Me Me (1) (2)

Me Me Me

M e - ~ M e M @ ~ M e

Me Me~Th~?e +6~176 Me ~T~Me

M ~ " Me c~ e~ ( \CH~Bu-t

Me Me /'b--Me M

/ ~ 30/+ SiMe4

Me Me ~T /CH2Bu-t

Me / h"C H2SiMe3

MeMe@~/e

been confirmed using methane and tetramethylsilane as sub- strates.

These results demonstrate that it is possible to design isol- able organoactinides of sufficiently high energy content that the stoichiometric (as opposed to catalytic) activation of saturated hydrocarbon molecules becomes thermodynamically favourable.

When the allylic rhodium species (3) is treated with H2, hydridorhodium species (4) result (22). Similarly, when (5) is treated with methane (2 days, 100~ a mixture of hydrido- complexes (4) and (5) is obtained (23). Propylene and small amounts of butane and butene were detected. That methane activation and incorporation were indeed responsible for these observations was proven by use of 13CH4.

[Si]-O-Rh + CI~ ~ [Si]-

(3) ~ "~"

[Si]-O--Rh + ~ [Si] -'O-R) \Me H

[Si] -O--Rh T �9 [Si] -O--RhH 2 r/

(SJ

Devising effective strategies for the selective, metal-medi- ated, homogeneous activation of C-H bonds on activated hydrocarbons represents a considerable challenge. In a detailed full paper the work of the Berkeley group on the ~l-pen-

(24) tamethylcyclopentadienyliridium system is described . Heat- ing caused reductive elimination of alkane from (6) (R = Me, pentyl, or cyclohexyl) leading to an intermediate capable of undergoing oxidative addition to the C-H bonds in other alkanes. This reversible process equilibrated hydridoalkyl- iridium complexes formed by attack on the C-H bonds of different hydrocarbons. Thus, the equilibrium constant for interconversion of (6) (R = pentyt) and cyclohexane with (6) (R = cyclohexyl) and pentane was 10.8 at 140~ Using this Keq and some reasonable assumptions, the primary Ir-C bond in (6) (R = pentyl) was found to be 5.5 Kcal/mol, stronger than the secondary Ir-C bond in (6) (R = cyclohexyl). Ther- mal oxidative addition of CH4 (20 atm) to (6) (R = cyclohexyl) at 140-150~ for 14 h gave 58% (6) (R = Me), which

< L ICI Mesp I R Me3pIIr" Me

(6) (7)

Transition Met. Chem. 10, 31-35 (1985) TMC literature highlights - 2 33

was converted into the corresponding chloro(methyl)iridium complex (7) by treatment with CHC13.

One of the products of the reaction of the carbyne complex [W-=CC6H4Me-p)(CO)z(~I-Cp)] with [Os3(g-H)2(CO)10] is the dihapto bridging acyl complex (8) (25). When (8) is heated (toluene, 110 ~ scission of the acyl C-O bond occurs forming the oxo-alkylidyne complex (9). These observations have interesting implications for Fischer-Tropsch hydrocarbon synthesis. Thus, since (8) is derived from the carbyne [W~-CC6-

CH2CsH4Me-p @ -C H2C6H4Me -p

(8) (9) (C0)3

HeMe-p] fragment the formation of (9) regenerates a carbyne moiety, but with an added methylene group. Taken together, these reactions form the homologation sequence illustrated. This work demonstrates C-O bond scission directly

+ 2H + CO -O RC ~ RCH2 ~ RCH2C(=O) ~ RCH2C

from an ~12-acyl complex, which suggests that the sequence might form the basis of a catalytic chain-growth cycle.

Alkene metathesis

Transfer of ethylidene from [~ILCp(CO)2Fe= CHMe] + to p- RC6HnCH=CH2 (R = H, Me, MeO, F, CO, or CF3) gave cis- and trans-(methyl)cyclopropanes (26). Use of the labelled alkene c/s-p-MeOC6H4CH=CHD showed that the abnormally low cis-trans ratio for R=p-MeO (0.9) is due to Ca-C~ bond rotation, which occurs in an intermediate formed after the transition state. This is the first reported case of loss of alkene stereochemistry upon cyclopropane formation from an elec- triphilic carbene complex.

As part of an interest in olefin metathesis (27) the photolysis of [W(CO)5(=CPh2)] has been examined in the presence of CH2=CPh(C6HaOMe-p). This results in metathetical exchange of the carbene ligand with the aromatic substituted part of the alkene to form the new carbene complex [W(CO)5{=CPh(C6H4OMe-p)}]. The quantum yield for the photochemical metathesis is lower at 12-14~ than at 27-29 ~ Experiments with filters showed that the reaction is wavelength-dependent and follows irradiation of the [W(CO)5(=CPh2)] absorption at 375 nm, which is assigned to a ligand-field excitation in which the dx2_y2 orbital is occupied. It is suggested that a [W(CO)4(=CPh2)] fragment is formed. Coordination of the alkene results in metallacyclobutane formation and subsequent metathetical exchange.

Miscellaneous catalytic processes

In a bimetallic compound (28), both metals can act simulta- neously as centres for reactions involving catalytic processes. It is therefore of interest to know how bimetallic systems func- tion in oxidative addition and reductive elimination. Pudde- phatt's (29) group has made some progress on elucidating how the latter process occurs from bis(biphosphine) bridged bi- platinum species.

In the presence of added phosphine, [HPt(~-H)(~- dppm)2PtH] +, (10) and [MePt(g-H)(~t-dppm)2PtH] +, (11),

both eliminate hydrogen thermally. (They also do so photo- chemically(3~ The latter does not give methane, while [MePt(g-H)(g-dppm)zPtMe] + does not react at all. The reaction is intramolecular as is demonstrated by first-order kinetics in the Pt2 species and, in the first complex, by H/D crossover type experiments. Kinetic studies on the first complex conform to an equilibrium followed by a rate-determining step, n.m.r. studies at reduced temperatures indicating that a species is formed which has the structure shown in Equation (4), so that it is reasonable to postulate (4) followed by (5). K4 and k5 correlate with different phosphines which provides further evi- dence for this point.

[HPt(g-H)(~dppm)zPtH] + + L JI (4)

[HPt (~t-H) (~t-dppm)2PtHL] +

[HPt(~-H)(g-dppm)2PtHL] + ~ [HPt(~t-dppm)zPtL] + + H2 (5)

For two phosphines activation parameters have been mea- sured for (5):

2~H; ~ AS~ (kJ mol -I) (J K -1 mo1-1)

PPh 92 + 5 45 + 10 P(C6H4CI-p) 3 85 + 5 17 + 13

The entropies of activation are not particularly positive for a dissociative process. Values of k5 at 298 K for these phosphines and for PPhz(C6H4Me-o) differ by no more than a fac- tor of eight, that of the last, the most bulky species, not being significantly larger than the others. This suggests that at the rate-determining step, the hydrogens to be eliminated are somewhat remote from the phosphine ligand. Large H/D isotope effects of 3.5 are observed for the overall process, which stress the role of the hydrogen in the rate-determining step.

In the case of the second complex, (11), no pre-equilibrinm is observed in the kinetics, but (13) can be identified by n.m.r. at low temperatures, ks/K4 for this complex, (11), (when PPh3 is used) is only 0.7 that for the first system [i.e. involving (10)], which may indicate that the Me group, like the phosphine, is some distance away from the hydrogen at the point of elimination.

As regards the mechanism it is just possible that, in the methyl system, (13) is an artefact and that the genuine inter- mediates (14) (as the authors admit in a footnote), this species reacting too quickly to be detected. Then, (15) is formed readily during the equivalent of step (5), elimination occurs from just one platinum, and it is easy to see why H2 is produced rather than CH 4 and also to account for much of the data mentioned above. However, the activation energy for reductive elimination of H2 from cis-[PtH2(PMe3)2] has been calcu- lated (31) to be ca. 100 kJ mol -I. Although this agrees nicely with the observed values of AH~, some enthalpy in addition will be required to go from (14) to (15).

The authors prefer to assume that (13) is a genuine intermediate and favour (though by no means conclusively) the formation of (16) as an intermediate. Thus the loss of H2 involves cis-elimination from both platinum centres, so that (5) should be rewritten as Equation (5').

(13) or (12) ~ (16) or the H3 analogue --* products (5')

34 TMC literature highlights- 2 Transition Met. Chem. 10, 31-35 (1985)

(IO) R = H (12) R = H (14) R = Me ( l l ) R = Me (13) R = Me

p t ~ p ~ t]~ L @ + H'" [. Me/~ H tIL

(15) (16)

There is considerable interest in carbon dioxide fixation, and it is considered likely that the first step in an activation process involves coordination to a transition metal centre (for a recent review see ref. 34). Reaction of cis-[Mo(Nz)z(PMe3)a] with CO~ (Fischer-Porter vessel) affords (80% yield) the complex (17), which is assigned on the basis of spectroscopic and

(33) chemical reactions the illustrated structure . Complex (17) is

�9 o,,c.~. Me3P~ i ,-'~PMe3

M . o ~ IV[esP/C ~ O PMe3

II 0 (17)

a yellow, moderately air-stable solid that can be heated in vacuo at ca. 50 ~ for 4-5 h without decomposition. It is suggested that this stability partly derives from the stability of the Mo-CO2 bonds due to extensive back-bonding from the molybdenum to the CO2, and to the oxophilic nature of molybdenum.

Applications in synthesis

Dicarbonyl(q4-cyclohexa-l,3-diene) (~l-cyclopentadienyl) - molybdenum hexafluorophosphate reacts with the stabilised enolate nucleophile NaCH(COzMe)2 specifically to give the z~- allyl complex (18). The major problem of controlled decom- plexation of this and related products was overcome using a novel and potentially useful iodolactonization demetalation procedure (35). Thus, (18) gave 90-95% of the lactone (19).

+ Mo(CO)z(r~-Cp)

NaCH(CO~Me)2 >

Mo(CO)2(r,, -Cp) COa Me

(MeOzC)2CH"" ~ �9 ~__J~O (18) (19)

The regioselectivity in addition of nucleophiles to metal- coordinated polyene systems is of interest both for practical synthesis and theory. While the theoretical analyses assume kinetically controlled reactions, this is shown not to be the case

in the addition of carbon nucleophiles to (1,3-diene)Fe(CO)3 complexes. Reactive C nncleophiles add at -78~ to (1,3- diene)Fe(CO)3 complexes with a strong preference for an internal position to give intermediate [(homoallyl)Fe(CO)3]- complexes, which are stable for many hours at this tempera- ture, and can be protonated to give mono-olefins, However, warming to 0 ~ reverses the anion addition, and the anion unit adds at the terminal position to give an [(allyl)Fe(CO)3]- intermediate, apparently the more stable product. The half- preferential addition at an internal position, reversal, equilib- ration and product formation from addition at the terminal position was shown for simple substituted 1,3-diene ligands.

Activation of benzylic positions to proton abstraction in basic media is very regioselective when induced by complexa- tion of Cr(CO)3 to the benzene ring (3~ Condensing m-alkyl- anisole complexes (20) (R = H; R ' = Me, Et or i-Pr) with H C H O in DMSO containing Me3COR gave > 63% condensa- tion products, whereas (21) (R = Me, Et or i-Pr) were inert or very unreactive under these reaction conditions.

MeO R' MeO MeO" " ~ Cr(CO)3 / , ~ i~ s Cr(CO)s Cr(CO)s

(20) (21) (22)

C@H2OH M OSiM%Bu-t

Cr(CO)~ PhCH20 ~ / f~3 (24)

Cr(CO)3 (23)

Thus, condensing (20) (R = H or R' = Me) with HCHO gave 10% styrene complex (21) (R 2 = H) and 53% (hydroxy- methyl)styrene complex (21) (R 2 = CH2OH), whereas (20) (R = Me; R ' = H) gave only 10% (23). Similarly (20) (R ,R ' = n-Pr) gave 95% indan (23). Similar hydroxymethyla- tion of an estradiol complex gave (24).

References

(1) j. W. Bruno, T. J, Marks and R. L. Morss, J. Am. Chem. Soc., 105, 6824 (1983). - (21 A. R. Dias, M. S. Salema and J. A. Martinho Simoes, Organometallics, 1, 971 (1982). - (3) A. Connor, M. T. Zafa- rani-Moattar, J. Bickerton, N. I. El Saied, S. Suradi, R. Carson, G. A1 Takhin and H. A. Skinner, Organometallics, 1, 1166 (1982). - (4) F. A. Adedeji, J. A. Connor, H. A. Skinner, L. Galyer and G. Wilkin- son, J. Chem. Soc., Chem. Comm., 159 (1976). - (5~ L. F. Halle, P. B. Armentrout and J. L. Beauchamp, Organometallics, 1, 963 (1982); L. F. Halle, P. B. Armentrout and J. L. Beauchamp, Organometallics, 2, 1829 (1983). - (6) T.-T. Tsou, M. Loots and J. Halpern, J. Am. Chem. Soc., 104, 623 (1982). - (7) F. T. Ng, G. L. Rempel and J. Halpern, Inorg. Chim. Acta, 77, L 165 (1983). - (8) F. T. Ng, G. L. Rempel and J. Halpern, J. Am. Chem. Soc., 104, 621 (1982). - (9) L. F. Halle, R. Houriet, M. M. Kappes, R. H. Staley and J. L. Beauchamp, J. Am. Chem. Soc., 104, 6293 (1982); R. Houriet, L. F. Halle and J. L. Beauchamp, Organometallics, 1, 1818 (1983). - (,0) D. B. Jacobson and B. S. Freiser, J. Am. Chem. Soc., 105, 736 (1983); D. B. Jacobson and B. S. Freiser, J. Am. Chem. Soc., 105, 5197 (1983); G. B. Byrd and B. S. Freiser, J. Am. Chem. Soc., 104, 5944 (1982).

(11) D. B. Jacobson and B. S. Freiser, J. Am. Chem. Soc., 105, 7492 (1983); D. B. Jacobson and B. S. Freier, Organometallics, I05, 513 (1984). - Oz) j. R. Sweet and W. A. G. Graham, J. Am. Chem. Soc., 105, 305 (1983); Organometallics, 2, 135 (1983).- 03) W. D. Jones and F. J. Feher, J. Am. Chem. Soc., 2, 562 (1983). - 04) R. A. Perlana and

Transition Met. Chem. 10, 31-35 (1985) TMC literature highlights - 2 35

R. G. Bergmann, Organometallics, 3, 508 (1984). - (15/j. K. Hoyano, A. D. McMaster and W. A. G. Graham, J. Am. Chem. Sot., 105, 7190 (1983). - (i6) M. J. Wax, J. M. Stryker, J. M. Buchanan and R. G, Bergmann, Chem. Eng. News, 61, 33 (1983). - (17) A. H. Janowicz and R. G. Bergman, J. Am. Chem. Soc., 105, 3929 (1983). - (as) M. J. Burk, R. H. Crabtree, C. P. Parnell and R. J. Uriate, Organometal- lics, 3, 816 (1984). - ( 1 9 ) p. L. Watson, J. Chem. Soc., Chem. Comm., 276 (1983); J. Am. Chem. Soc., 105, 6491 (1983). - (s0) j . W. Bruno, T. J. Marks and V. W. Day, J. Am. Chem. Soc., 104, 7357 (1982); J. Organometal. Chem., 150, 237 (1983).

(21) j . W. Bruno, T. J. Marks and L. R. Morss, J. Am. Chem. Sot., 105, 6824 (1983). - (22/M. D. Ward and J. Schwartz, J. Mol. Catal., 11, 397 (1981). - (23) N. Kitajima and J. Schwartz, J. Am. Chem. Soc., 166, 2220 (1984). - (24) M. J. Wax, J. M. Stryker, M. J. Buchanan, C.

A. Kovac and R. E. Bergman, J. Am. Chem. Soc., 106, 1121 (1984). - (25) j. R. Shapley, J. T. Park, M. R. Churchill, J. W. Ziller and L. R. Beanan, J. Am. Chem. Soc., 106, 1144 (1984). - (26) M. Brookhart, S. E. Kegley and R. E, Husk, Organometallics, 3, 650 (1984). - (27t L. K. Fong and N. J. Cooper, J. Am. Chem. Soc., 106, 2515 (1984). - (28) A useful summary of refs. is contained in: R. G. Finke, G. Gaughan, C. Pierpoint and J. H. Noordik, Organometallics, 2, 1481 (1983). - (29) R. H. Hill and R. J. Puddephatt, J. Am. Chem. Soc., 105, 5795 (1983); K. A. Azam, M. P. Brown, R. H. Hill, R. J. Puddephatt and A. Yavari, OrganometaUics, 3, 697 (1984). - (301 R. H. Hill and R. J. Puddephatt, Organometallics, 2, 1472 (1983).

(31/j. O. Noell and P. J. Hay, J. Am. Chem. Soc., 104, 4578 (1982).

M. G r e e n and M. G r e e n

tmc literature highlights 2

Documents