Sm~osiumon Catalvsis and Oraanometallic Chemistw
Aldehyde and Ketone Ligands in Organometallic Complexes and Catalysis Yo-Hsin Huang and J. A. Gladyszl University of Utah, Salt Lake City, UT 841 12 Aldehydes and ketones are two of the most important functional groups in organic chemistry. Their synthetic and mechanistic chemistry has been studied in detail (1). I t has also been known for some time that aldehydes and ketones can serve as ligands in transition metal complexes (2-33). However, this area has not received agreat deal of systematic study. Indeed, the frequent use of acetone as a reaction or recrystallization solvent has likely led to the occasional serendipitous synthesis of acetone complexes-or a t least it has once in the authors' laboratory. There are numerous transition metal catalyzed reactions that involve aldehyde or ketone starting materials or intermediates (34-37). Two industrially important examples are given in Figure 1. Clearly, the systematic study of stable aldehyde and ketone complexes is necesary to provide insight into the key steps of these catalytic reactions. The purpose of this article is to acquaint the nonspecialist with this interface of organometallic chemistry and catalysis. In other words, what is the fundamental coordination chemistry of aldehydes and ketones? As a logical starting point, this review is limited to aldehydes and ketones bound to a single metal center. It is also limited to complexes that have been isolated in pure form, or can be generated in spectroscopically pure form in solution. Related species, such as ketone, COz, v4-enone, and acetylacetonate (acac) complexes, are not discussed. A more comprehensive treatment is planned for a later date.
A. CO Hydrogenation (ref 34)
COH*
-- H-jiH- -.
U
Hz
M-me,
H2
HOCW,
H
B. 0 x 0 Alcohol Synthesis (Hydroformylation)(ref 35)
Figure 1. Aldehyde complex Intermediates in metal-catalyzed reactions.
Formaldehyde Complexes Synthesis Interestingly, isolable formaldehyde complexes were unknown prior to Roper's synthesis of Os(CO)z(PPh&(v2HzC=O) (1) in 1979 (2). Now many examples, spanning nearly the entire transition metal series, are known, and preparations are summarized in eas 1-10, Figure 2. Most ot'the syntheses involve reaction of a-coordinatively saturated complex bearina an easily displaced liaand (or a coordinativelymsaturated complex) with formaidehyde or an oligomeric form thereof (eqs 1-6, 9b). One synthesis, developed in the authors' laboratory, entails oxygen atom transfer to a methylidene ligand. Thus, reaction of iodosobenene (CsH51f-0-1 with methylidene complex [(.n5CsHdRe(NO)(PPh3)(=CHz)lfPF6- gives formaldehyde complex 7 (eq 7). The remaining syntheses model steps in the catalytic processes are outlined in Figure 1.The mild CO hydrogenation effected by the tantalum dihydride in eq 9a spectacularly mimics a series of proposed steps in homogeneous CO reduction. In eqs 8 and 10, 6-hydride elimination is believed to occur from precursor methoxide complexes. The microscopic reverse would be a key step in methanol synthesis from CO/Hz and aldehyde hydrogenation. Formaldehyde complexes do not possess any intrinsic thermal instability relative to aldehyde and ketone complex-
' Author to whom correspondence should be addressed. 298
Journal of Chemical Education
es. Hence, the somewhat late development of formaldehyde complex chemistry probably reflects an unfamiliarity with the protocols for generating formaldehyde in solution. Structure All of the formaldehyde ligands in Figure 2 bind v2 (a) in solution and in the solid state. There are two limiting resonance forms for .nZ complexes, as shown in eq 11, Figure 3. One (I) can be considered a simple adduct of the C=O a cloud, whereas the other (II), an "oxametallacyclopropane", involves suhstantial donation from occupied metal d orbitals to the emptv C=O s ' orbital Ybackbondine"). Also.. n1 , (a) .. complexes should, like most carbonyl compo&d derivatives, also possess two limiting resonance forms (ea 12. Fie. 3). One (111jcontains a carbon-oxygen double bond, ahereas the other (IV) contains a carhon-oxygen single bond. The formal charges assigned to the atoms in I11 and IV are, as with many organometallic structures, somewhat arbitrary. There are several diagnostic spectroscopic assays for the binding mode of aldehyde and ketone ligands. For example, the IR vc=o of uncomplexed aldehydes and ketones are seldom lowered by more than 100 cm-' upon v' (a) coordination (15b, 166, 17, 18). However, .n2 (a) binding effects a suhstantial reduction in carbon-oxygen bond order. Accordingly, u c a in 1,2, and 3 have been assigned as 1017, 1220, and 1160 cm-I respectively (2a,b, 3,4a,c).
Figure 2. Syntheses of formaldehyde complexes.
Further, '3C NMFt spectra of q1 aldehyde and ketone ligands show C=O resonances in the normal downfield range for organic carbonyl groups (166,30). However, C=O resonances in q2 complexes appear upfield at 45-111 ppm (6,7a, 8,9a, 10,12,14b, 16a,20). Finally, the 'H NMR spectra of q' aldehvde lieands show resonances in the normal range for I? aldehydieprotons U 5 h , 171.~owever,aldehydeproton~in rom~lexesare shifted significantly upfield (26,3,46, 6,7a, 8,
most neutral complexes to give L,M(X)CHzOE species, as shown in eqs 13and 14,Figure 4. Similar reactions have been observed with iron complex 2 (36, c). Nucleophiles can add to the formaldehyde carbon of cationic complex 7, as shown in eq 15. The tantalum formaldehyde hydride complex 10 is likely in equilibrium with the coordinativelyunsaturated methoxide complex (75-CsHs)zTaOCH3 (see eq 10). Additions of nucleophiles to 10 trap this reactive intermediate, as shown
carbon-oxveen .- bond order n2-formaldehvde comdexes is closer to one than two. Reactions
Formaldehyde complexes have been found to undergo several different types of reactions. First, the formaldehyde ligand can be displaced by nucleophiles such as PPhs (1, refluxing methanol) (Zb), HzC=CH2 (2,70 atm) (3c), CH3CN (7,18 days, 25 'C) (66), and I- (7, CHsOH, 25 OC, A*,..
The chemistry described above illustrates an active, dynamic field in transition. Interest in large-scale catalytic reactions such as those in Figure 1 leads in turn to the develo~mentof new. smaller scale catalvtic reactions such as asymketric aldehyde and ketone hydrogenation (37).In all cases, there are abundant opportunities to address fundamental mechanistic questions, such as the effect of the aldehydeketone ligand binding mode upon reactivity. The careful study of stoichiometric reactions will play a key role in elucidatine many of these questions. ~ l d e h ~ d e n a nketonesalso d undergo a numher of catalytir reactions that do not conventionally involve transition metals, such as acid-catalyzed and base-promoted aldol-type condensations. There are additional important questions t o he considered here. How does the oresence of transition metals affect conventional catalysts or reaction conditions? Can transition metals take the place of conventional catalysts? Finally, can chiral transition metals complexes effect asymmetric reactions of aldehydes and ketones? The answers to the last two questions are yes (16,44). Hence, it can be ex~ectedthat the study of aldehyde and ketone complexes wiil be a most fertile area for future research. Acknowledgement
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198527,5517 44. :jato, S.; Matsuda, I.:Izurni, Y. Tetrahedron kff.
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