No abstract

Irradiation of Tp‘(CO)3WH in the presence of alkynes or unconjugated dienes generates η3-allyl complexes, Tp‘(CO)2W(η3-CHRCHCHR‘). A kinetic preference for anti-alkyl substitution due to η2-vinyl intermediates exists for the reactions with terminal alkyne substrates. The monoalkyl-substituted allyl complexes are configurationally stable on the NMR time scale but show a thermodynamic preference for syn-substitution upon heating in solution. Separate crystal structures of the syn and anti isomers of the 1-methylallyl complex, Tp‘(CO)2W(η3-CH2CHCHCH3), have revealed isomers which differ by approximately 90° in allyl orientation (θ). The term meso is introduced to emphasize the orthogonal relationship between the anti (θ = 30°, exo) and the syn (θ = 120°, meso) isomers. The change in orientation of the allyl is accompanied by a change in the OC−W−CO angle from acute to obtuse in order to maximize donation from the π-nonbonding orbital of the allyl into the dπ metal orbitals. EHMO (extended Hückel molecular orbital) calculations on a model complex [H3W(CO)2(η3-C3H5)]2- show electronic minima at θ = 30° and 115° corresponding to the two orientations.

Chloride displacement from Tp‘(CO)2M⋮CCl (M = Mo, W (1); Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) with LiMe2Cu affords the methylcarbyne complex Tp‘(CO)2M⋮CCH3 (M = Mo (2), W (3)). Deprotonation of this methylcarbyne forms [Tp‘(CO)2M=CCH2]- (M = Mo, W (4)), a nucleophilic vinylidene anion which reacts with a variety of substrates. Reaction of 4 with CH3I yields Tp‘(CO)2W⋮CCH2CH3 (5), while PhC(O)H yields Tp‘(CO)2W⋮CCH2CH(OH)(Ph) (8) and PhC(O)CH3 yields Tp‘(CO)2W⋮CCH2C(OH)(CH3)(Ph) (10). In the presence of base complex 8 undergoes elimination to form a conjugated vinyl carbyne complex Tp‘(CO)2W⋮CCHCH(Ph) (11). Reaction of 4 with PhC(O)Cl and then base forms Tp‘(CO)2W⋮CCH2C(O)Ph (14), which rearranges in solution to a metallafuran complex, (15). Spectroscopic data suggests that a dinuclear intermediate, Tp‘(CO)2W⋮CCH2CPh(OH)CH2C⋮W(CO)2Tp‘ (13), is first formed from 4 and PhC(O)Cl before the addition of base produces the expected monomeric product, Tp‘(CO)2W⋮CCH2C(O)Ph (14).

The electrochemical behavior of a series of bis-carbyne complexes, Tp‘(CO)2M⋮(C m H n )⋮M(CO)2Tp‘ (Tp‘ = hydridotris(3,5-dimethyl-1-pyrazolyl)borate; M = Mo, W; m = 3, n = 2; m = 4, n = 0, 2, 4), has been studied. These complexes exhibit two oxidation waves between 0 and 0.5 V (vs ferrocene). The difference in E 1/2 for the two oxidation waves is a function of the hydrocarbon spacer with the ⋮C−C⋮C−C⋮ spacers (m = 4, n = 0) exhibiting considerably larger ΔE 1/2 values than the corresponding ⋮C−CHCH−C⋮ spacers (m = 4, n = 2), which were in turn more strongly coupled than the saturated analogues (m = 4, n = 4). While the trend in electronic coupling was quantitatively as expected for the degree of unsaturation in the bridge, the absolute magnitude of the coupling as indicated by the comproportionation constants (K c = 100−104) was much smaller than that in isologous dimers where the bridge is connected to the metal centers via single bonds (K c = 108−1012). A qualitative model based on relevant orbital occupancy as a function of metal oxidation state has been developed to account for the experimental results.

Irradation of Tp′(CO) 3 WH in the presence of alkynes produces a diverse array of products, including η 2 -vinyl, η 2 -acyl, metallafuran, and carbyne complexes. A common mechanistic feature consistent with the observed distribution of products is cis insertion of alkynes into a photogenerated "Tp′(CO) 2 WH" species. Initial cis 2,1-insertion of the alkynes leads to η 2vinyl and η 3 -allyl products, while cis 1,2-insertion ultimately produces η 2 -acyl, metallafuran, and carbyne products. An X-ray structure of a metallafuran product, Tp′(CO) 2 W(η 2 (C,O)-C(Bu n )CHC(dO)CHdCHBu n ), shows trans stereochemistry for the double bond of the pendant 1-hexenyl group, which indicates that cis 1,2-insertion of the first equivalent of alkyne forms an intermediate η 1 -vinyl complex. Additional support for η 1 -vinyl intermediates is present in the η 2 -acyl product Tp′(CO) 2 W(η 2 (C,O)-C(dO)CHdCHC(CH 3 ) 3 ), which evinces a trans double bond as assayed via 1 H NMR. Irradiation of Tp′(CO) 3 WH in the presence of Me 3 SiCtCCH 3 produces two isomers of Tp′(CO) 2 W(η 3 -syn-CH 2 CHCHSiMe 3 ) which differ in the orientation of the allyl fragment relative to the metal center. Heating of the allyl complex results in an unusual η 3 -allyl to carbyne rearrangement.