Bisalkyne complexes

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. 

Few monomeric d4 bisalkyne complexes with only monodentate ligands in the coordination sphere have been reported. The only molybdenum(II) complex in this category is Mo(PhC=CR)2(CO)(NCMe)I2 (R = Ph, Me), which was included in a reaction scheme illustrating products accessible from cleavage of the iodide bridges in dimeric [Mo(PhC=CR)(/r-I)-(I)(CO)(CNMe)]2 reagents (51). Efforts to convert Mo(RC=CR)(CO)-L2X2 complexes to bisalkyne derivatives were not successful (46). 

Molybdenum bisalkyne complexes form more readily in the pyrrole-/V-carbodithioate ligand system ( pyrroledithiocarbamate ) than in the corresponding dialkyldithiocarbamate systems (88). The pyrrole nitrogen is reluctant to share electron density with the attached CS2 moiety since the aromatic stabilization of the five-membered NC4 ring is lost in resonance form ii. As a result of decreased electron donation from the... 

Mixed bisalkyne complexes were prepared from monoalkyne precursors in several cases by room temperature reactions [Eq. (34)] (93). Higher temperatures produced bisalkynes via replacement of the original alkyne ligand. 

Another route to neutral bisalkyne complexes is from the trifluoro-methylacyl precursor which deinserts carbon monoxide to yield trifluoro-methyl molybdenum products. Photolysis of CpMo(CO)3[C(0)CF3] in the presence of CF3C=CCF3 forms CpMo(CF3C=CCF3)2(CF3) CpMo-(DMAC)2CF3 is formed without photolysis (98). Addition of hexafluoro-butyne to CpMo(CO)(MeC=CMe)(CF3) forms the mixed bisalkyne via CO substitution. 

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). 

Relatively few 13C-NMR data have been reported for bisalkyne complexes. The range of 13C alkyne chemical shifts is roughly 170-190 ppm for Mo(RC=CR)2(S2CNEt2)2 complexes (87). Terminal alkynes exhibit VCh values near 210 Hz in these three-electron donor roles, similar to values reported for four-electron donor alkynes. For cationic cyclopentadienyl bisalkyne derivatives an alkyne carbon 13C range of 140-180 is seen in Table VI. 

Rotational barriers have been probed for a number of bisalkyne complexes (Table VII). Cationic [CpM(RC=CR)2(CO)]+ complexes exhibit relatively high barriers (16-21 kcal/mol). Both standard variable-temperature NMR techniques (94) and two-dimensional methods (162) have been used to elucidate isomer interconversion schemes with two unsymmetrical alkynes in the coordination sphere. The plane of symmetry present when two symmetrical alkynes bind to a CpMX fragment is not retained in all isomers with RC=CH ligands. The availability of distal and proximal alkyne termini locations relative to the adjacent cis ligand leads to two cis isomers (R and R near one another) and one trans isomer (Fig. 25). Rotation of only one alkyne ligand converts cis to trans and vice versa, but direct cis to cis conversion is not possible unless both alkynes rotate simultaneously. 

Dynamic NMR studies indicate that the barrier to alkyne rotation in dithiocarbamate bisalkyne complexes is near 15 kcal/mol (87). For unsym-metrical alkyne ligands, as in Mo(PhC=CH)2(S2CNMe2)2, several isomers are possible with like substituents either adjacent ( cis ) or opposite ( trans ). Analysis of the NMR properties follows the logic presented by Faller and Murray for CpMo(RC=CR)2Cl (94). The C2 molecular symmetry dictated by the chelates can produce two different trans isomers with the two alkyne protons of PhC=CH in each isomer equivalent by C2 rotation (Fig. 26). Only one unique cis isomer is possible, but the two... 

Pyrrole-ZV-carbodithioate bisalkyne complexes display two distinct flux-ional processes. Rotation around the C—N bond of the S2C—NC4H4 ligand equilibrates both halves of the pseudoaromatic NC4H4 ring (AG = 10.7 kcal/mol) (88). Alkyne rotation exchanges both ends of the alkyne ligands at somewhat higher temperatures (AG =13.7 kcal/mol for MeC CMe, 13.8 kcal/mol for HC=CH). 

Although numerous alkyne insertion products have been reported for Mo(II) and W(II), simple dimerization to form cyclobutadiene or tri-merization to form an arene ligand is rare. One brief report of cyclobutadiene formation from a bisalkyne complex (206) has been followed by a full paper which suggests that an rj2-vinyl complex may be the precursor to CpM(S2CNR2) [ 174-C4(CF3)4] (207). 

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