Formyl complex, neutral

Figure 5. An isolable, crystalline neutral formyl complex...

Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis (29). One reason is that formylation of a transition metal monoanion would afford a neutral formyl complex. Insofar as comparisons are valid, neutral formyl complexes tend to be kinet-ically less stable than anionic formyl complexes. In cases where neutral formyl complexes are stable (vide infra), the corresponding transition metal monoanions are unknown. Whereas formic acetic anyhydride might be of greater use for the preparation of anionic formyl complexes from transition metal dianions, only a limited number of transition metal dianions [i.e., (CO)5Cr2", (t7-C5H5)(CO)3V2 J are known (49). These appear to... 

As mentioned above, appropriate hydride nucleophiles are capable of attacking coordinated CO [Eq. (3)]. This route to anionic formyl complexes was reported in 1976 by Casey, Gladysz, and Winter (29-34). All of the anionic formyl complexes in Table 1, including 22 [Eq. (4)], can be prepared by hydride attack on neutral metal carbonyl precursors. 

Reagents such as LiAlH4 and KH are not effective for the synthesis of formyl complexes. LiAlH4 does react with many metal carbonyl compounds, but it can transfer more than one and usually effects the formation of metal hydride products (50). Similar results are usually found with NaBH4(50), although some neutral formyl complexes (vide infra) can be obtained under special conditions. KH will also react with some metal carbonyls. However, rates are not very rapid, and any formyl intermediates are likely to decompose faster than they form (51). 

All neutral transition metal formyl complexes described in the literature through the end of 1980 (52-68) are compiled in Table II. General comments made in the previous section regarding Table 1 apply. 

Roper has been able to isolate another osmium formyl by rearrangement of an rj2-formaldehyde complex, as shown in Eq. (9) (54). Because of the unavailability of such precursors, this reaction also does not provide a general entry into neutral formyl complexes. However, Eq. (9) does lend support to the claim that the related ruthenium formyl, Ru(H)(solv) (PPh3)3(CHO) (40), can be isolated as an impure solid, contaminated with substantial quantities of an rf -formaldehyde precursor (55). 

IR spectra of both anionic and neutral formyl complexes show tv=(> between 1530 and 1630 cm 1 which are medium in intensity relative to... 

As shown in Eqs. (17) and (18), the isolated formyls 19 and 24 are capable of reducing aldehydes and ketones (37, 38, 42. 47, 66). Thus there is no doubt that hydride transfer is an intrinsic chemical property of anionic formyl complexes. One reaction of a neutral formyl complex with an aldehyde has been reported addition of benzaldehyde to (i7-C5H5)Re(NO)(CO)(CHO) (38) yields the alkoxycarbonyl complex (i7-C5H5)Re(NO)(CO)(C02CH2C6Hs) (62). This transformation, which appears to require catalysis by adventitious acid, can be viewed as occurring via attack of initially formed benzyl alcohol upon the intermediate carbonyl cation [(i -C5H5)Re(NO)(CO)2]+. 

Formyl transfers" involving neutral formyl complexes, such as shown in Eq. (22), have recently been reported (68). By precipitation of the metal carbonyl cation by-product, quite pure solutions of kinetically labile neutral formyl complexes may be obtained. 

Reactions of neutral formyl complexes with alkylating agents can follow different courses. Roper has observed the O-methylation reaction shown in Eq. (23) (54). Cationic methoxymethylidene complex 53 was obtained in excellent yield. 

Two reports of H2 formation upon acidification of anionic formyls 6 (31) and 19 (38) could not be reproduced (32, 47). Thus there are no documented examples of H2 evolution upon protonation of anionic formyl complexes. It is clear, however, that rapid reactions ensue in all cases (32, 47, 66) and that good yields of neutral metal carbonyl (H loss) products are obtained. 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

The years 1978 and 1979 have witnessed continuing activity on the catalytic reduction of CO and models for it. Both Casey and Gladysz have established that the neutral formyl complex (C5H5)Re(CO)(NO)(CHO) which they synthesized (59b,c) is the first intermediate in the borohydride reduction of coordinated CO to methyl as reported by Graham and co-workers (55). When the neutral formyl complex is reacted with BH3 THF, the species (C5H5)Re(CO)(NO)(CH3) results. A similar reduction does not occur when H2 is used as the reductant, however (59b). While the previous report by Nesmeyanov et al. (86) of a hydroxymethyl species in the BH4 reduction process is now viewed as incorrect (59b,e), Casey has recently described (59e) unequivocal characterization of this species, and has shown how the formyl complex (C5H5)Re(CO)(NO)(CHO) can lead to its formation as shown in (23a). 

The feasibility of hydride attack at a coordinated CO, leading to meial-fuimyl complexes, has been demonstrated in a number of model reactions, from which examples of ionic, neutral and muUimelallic formyl complexes are listed in Scheme 3. 

Two neutral formyl complexes have been synthesized and isolated via equation (a), beginning with the cationic carbonyl compounds, CpRe(NO)(CO)L where L = CO and PPh3 . These compounds are intermediates in the BH4 reduction of coordinated... 

In compliance with this expectation, neutral formyl complexes have been synthetized ° by a reaction of Re carbonyl cations with hydrides. With very strong hydride donors, like trialkyl- or trialkoxy-borohydrides, anionic formyl complexes have been prepared even from neutral carbonyl compounds. 

A hydride is one of the simplest nucleophiles, and Casey ° and Gladysz have prepared kinetically stable formyl complexes by the direct attack of hydride on a number of neutral chromium-, molybdenum-, and iron-carbonyl complexes (Equation 11.2). Although these complexes are relatively electron rich, because they possess zero-valent metal centers, the negative charge in the product can be stabilized by the remaining -ir-ac-cepting CO ligands. 

Interest in the stepwise reduction of coordinated CO continues. Further work on determining whether surface methylenes could arise from formyl intermediates has been reported using 0s3(C0)i2 as a model system. On hydride reduction, 033(00)12 yields [OS3 (C0)u (GH0)] which can be converted into [OS3(C0)u(U-CH2)] by protonation. The methylene complex eliminates CHi, on heating in H2 gas, and forms [033112(00)9(113-000)] in the absence of H2. The first stable neutral formyl complex of a 3d-transltion metal has been claimed. Reduction of trans-[Mn(C0) 1,(P(0R) 3 2 ] or mer-[Mn(C0) 3(P(0R) 3 3 ] yields [Mn(CO)3(OHO)(P(0R)3 2], and the crystal structure of the complex with R = Ph was determined. Other reports on the reduction of Or-, Mo- or Fe-coordinated 00 are referenced below. ... 

Tam W, Wong WK, Gladysz JA Neutral metal formyl complexes generation, reactivity, and models for Fischer-Tropsch catalyst intermediates, J Am Chem Soc 101(6) 1589-1591, 1979. 

It is often the case that metal formyl complexes are thermally unstable and, even with slight increases in temperature, are subject to fast transformations into other products. For example, the reaction s of [Re CO)2NO(il5-Cp)][PF6] with K[BH(OPr-f)3] in THF at -78 °C yields the neutral formyl complex, [Re(CHO)(CO)(NO)(Ti5-Cp)], which decomposes at room temperature with formation of the hydride complex, [ReH(CO)NO(ii5-Cp)], Scheme 10.8 ... 

The Vilsmeier formylation of copper deuteroporphyrin dimethyl ester (6) in which unsubstituted /3-positions are present yields a complex mixture of mono- and disubstituted formylation products which can be partially separated by chromatography on neutral alumina.106... 

The formation constants of an actinium isopropyltropolonate complex were determined. Thermochemically relevant studies of thorium enolates generally involve bis(pentamethyl-cyclopentadienyl)thorium derivatives. Cp 2Th(Cl)(C(0)CFl2Bu-f) with an anionic acyl group that readily rearranges to the isomeric enolate Cp 2Th(Cl)OCH=CHBu-t. The Z-isomer is formed upon heating and the -isomer upon catalysis with Cp 2ThH2. Is the E or Z enolate thermodynamically more stable For the simple alkyl enolates MeCH=CHOR, the equilibration reaction of the Z- and E-isomers is nearly thermo-neutral . Consider the two species Cp 2Th(H)OCH(Bu-t)2 and Cp 2Th(H)0-2,6-C6H3 (Bu-f)2. The reversible addition of CO yields the rp- formyl derivative in reactions that are 19 4 and 25 6 kJmoR exothermic. These formyl species dimerize to form the classical enediolate, Cp 2Th(OR)OCH=CHO(OR)ThCp 2. This product is formed as the Z-isomer, plausibly thermodynamically preferred over the -isomer, much as (Z)-MeOCH=CHOMe is preferred over its E-counterpart by 6.0 0.2 kJmoR. ... 

---