D4 Alkyne complexes

A second populous class of T/2-vinyl complexes has resulted from addition of phosphines, phosphites, isonitriles, and thiolates to neutral d4 alkyne complexes. In particular, recent extensive work by the Davidson group with CpM(CF3C=CCF3)2X (97,176) and other hexafluorobutyne reagents has clarified much of this chemistry. We turn to this realm of Tj2-vinyl chemistry after first considering the one class of dithiocarbamate alkyne complexes known to form tj2-vinyl ligands. 

For other systems the transformation is still energetically accessible but die barrier may be higher if the alkyne complex is less destabilized. In d2 and d4 complexes, the four-electron repulsion with the alkyne can be avoided (the alkyne behaves like a 4-electron donor) and this stabilizes the alkyne complex. The intraligand 1,2 shift is then associated to a higher activation barrier. This has been illustrated in a study by Fledos et al. on [Cp2Nb(HC=CH)(L)]+ and [Cp2Nb(C=CH2)(L)]+ (L = CO, PH3) where DFT calculations show that the barrier to isomerization is +29.2 kcal.mof1 (resp. 

Syntheses of simple d4 alkyne monomers of molybdenum and tungsten with no chelating or cyclopentadienyl ligands in the coordination sphere were first reported in 1982 (44). During the past several years a variety of M(CO)(RC=CR)L2X2 complexes has been prepared, but reaction chemistry of the bound alkyne in these complexes has not yet been developed. 

For comparative purposes the syntheses and properties of formally seven-coordinate d4 biscyclopentadienyl alkyne complexes will be presented. The absence of a vacant dir orbital places these d4 complexes unambiguously in the two-electron alkyne category, i.e., N = 2 applies where N is defined as the formal electron donor number for the alkyne ligand. As in the porphyrin case a reductive ligation route provided access to these molecules. Sodium amalgam reduction of Cp2MoQ2 in the presence of alkyne yields Cp2Mo(RC=CR) [Eq. (28)] (82). Note that, in... 

Alt has prepared a cationic cis-carbene alkyne derivative, CpW(CO)-(HC=CH)[C(OEt)Me] + (77), but the presence of both an alkyne ligand and a carbonyl ligand in the d4 coordination sphere as well as a heteroatom on the carbene complicates orientational predictions relative to W(CHPh)-(RC=CR)L2X2. No structural data are yet available for this cationic carbene alkyne complex. 

Complex D4 is considered as the active species for both alkyne and olefin coordinations. Starting from the olefin coordinated complex (D5 ), the olefin insertion into the Pt-B bond is unfavorable because of a high activation barrier (22.9 kcal/mol). On the contrary, the acetylene insertion from the acetylene coordinated complex (D5) occurs easily with a small reaction barrier (9.0 kcal/mol). This significant difference in the reaction barriers has been used to explain the inertness of olefins for diborafion reactions. The smaller barrier from D5 to D6 coincides with the highly stable insertion product D6. In contrast, the olefin insertion product D6 is relatively unstable with respect to the olefin coordinated species D5 . 

The general description of this oxidative coupling reaction is given by Reaction 19 in Table II. These reactions are involved in the metathesis of olefins or alkynes (J9). We have not found any rules allowing or forbidding the reaction. Matrix 5 shows only the structures of complexes found in the literature which give such reactions d10-M (M = Ni) (40), d8-MLs (41), d6-ML6 (42), and d2-MLs (43). For the d° and d4 systems we were unable to find any examples. 

Bisalkyne derivatives have been crucial to the development of a comprehensive model for n donation in d4 monomers. The presence of two equivalent alkynes in the coordination sphere allows unambiguous interpretation of certain bonding properties, and in particular a formal donor number of three applies for each alkyne (N = 3). Bisalkyne complexes have been exploited to prepare monoalkyne monomers as well as for alkyne coupling reactions and ligand based transformations. 

Octahedral olefin-alkyne d4 complexes are characterized by a one-to-one match of each of the three metal dir orbitals with a ligand tt function as mentioned above. Three constructive two-center-two-electron metal-ligand tt bonds result. Extended Huckel calculations on W(H2C=CH2)-(HC=CH)(S2CNH2)2 produce the dv level ordering shown in Fig. 16... 

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

Both combinations of alkyne n orbitals find filled dir orbital symmetry matches in these d4 complexes. Extended Huckel calculations on Mo(HC=CH)2(S2CNH2)2 indicate a large HOMO-LUMO gap of 1.62 eV. These octahedral complexes have proved to be quite robust and resist exchange and substitution reactions in accord with a substantial frontier orbital energy gap (153). 

Bisalkyne d4 monomers, with N = 3 by symmetry, exhibit proton and carbon chemical shifts at higher fields than those of monoalkynes with N = 4. The proton chemical shift of 10.45 ppm for Mo(PhC=CH)2-(S2CNEt2)2 (52) falls nicely between the four-electron donor Mo(CO)-(PhC=CH)(S2CNEt2)2 case (12.6 ppm) and the two-electron donor (7r-C5H5)2Mo(HC=CH) case [7.68 ppm (Table II)]. Additional data for bisalkyne complexes, including pyrrole-N-carbodithioate derivatives, support a correlation of H chemical shifts with alkyne ttj donation, with three-electron donors typically near 10.0 0.5 ppm. Similar H values are found for cyclopentadienyl bisalkyne complexes with terminal alkyne ligands. Chemical shifts between 8.5 and 10.5 ppm characterize all the neutral and cationic bisalkynes listed in Table V except for [CpMo-(RC=CH)2(MeCN)]+ where one isomer has S near 11 ppm for the acetylenic proton (72). 

Bisalkyne bisdithiocarbamate derivatives are both harder to reduce and harder to oxidize than their carbonyl analogs (53). Factors discussed in the Section VI and used to rationalize visible absorption spectra are also applicable here. The potentials required for oxidation and reduction reflect the strength of the alkyne-metal dn interactions. Their properties, as well as chemical behavior, no doubt reflect the complementary nature of the complete set of a and 77 metal-ligand bonds in these happy d4 M(RC=CR)2(S2CNEt2)2 complexes. 

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