Stereochemistry and stereodynamics of η⁶-hexaethylbenzene transition metal complexes. Crystal and molecular structure of dicarbonyl(η⁶-hexaethylbenzene)(η²-maleic anhydride)molybdenum(0)

Abstract

The variable-temperature 90.1 -MHz 13C-{lH} and/or 145.87-MHz 31P-{1H} n.m.r. spectra of dicarbonyl ( qs-hexaet hyl benzene) (t hiocarbony1)chromium (o), (4), dicarbonyl(qs-hexaethyl benzene) -(trimethy1phosphine)c hromium (o), (5), dicarbonyl ( qs-hexaethylbenzene) (triet hyl phosphine)chromium (0) , (6), dicarbonyl (qs-hexaet hyl benzene) (triet hyl phosphine) molybdenum (o), ( 7 ) , dicarbonyl( qs-hexaethyl benzene) (triphenyl phosphite)chromium (o), (S), dicarbonyl (qs-hexaethylbenzene) (q2-maleic anhydride)chromium(o), (1 0), dicarbonyl(q"-hexaethylbenzene) ( q2-maleic anhydride) molybdenum(o), (1 1 ), and (qs-cyclopentadienyl) (qs-hexaethylbenzene) iron (11) hexafluorophosphate, (1 2), have been observed and subjected to lineshape analysis. In a number of cases, multiple heb stereoisomers are found to coexist in equilibrium in solution at low temperatures. For all the hexaethylbenzene (heb) complexes examined, the n.m.r. spectra can be satisfactorily analysed without the need t o invoke hindered rotation about the metal-arene bond and coalescence phenomena observed in the spectra are attributed to the onset of rapid rotation of the ethyl groups about the benzene ring and heb stereoisomerisation. Barriers to site exchange

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…In the solid state, the trimethylphosphine complex exists as an 80:20 mixture of the 1,3-proximal-2,4,5,6-distal (c)a nd 1-proximal-2,3,4,5,6-distal (e)i somers; [14] in contrast, the triethylphosphine complex exists as a5 0:50 mixture of the e and h isomers. [15] Moreover,asubsequent careful scrutinyo ft he triphenylphosphine compound 3 also revealed the presence of the 1-proximal-2,3,4,5,6-distal conformer e.T he existence of multiple con- formers in solution is also clearly evident from variable-temperature 31 PNMR data;t ypically,[ (HEB)Cr(CO) 2 {P(OPh) 3 }] exhibits resonances for at least three conformers, suggested to be c, e and h. [16] It is apparent from theser esults that the favoured conformer,orc onformers, in each case is markedly dependent on the cone angle of the ligand such that bulkier phosphines lead to structures with increasing numberso fdistal ethyl substituents. Moreover,w ith respect to the orientation of the tripod, for obvious steric reasons, eclipsing does not occur beneath a proximal methyl group.…”

“…Moreover, a subsequent careful scrutiny of the triphenylphosphine compound 3 also revealed the presence of the 1‐ proximal ‐2,3,4,5,6‐ distal conformer e . The existence of multiple conformers in solution is also clearly evident from variable‐temperature 31 P NMR data; typically, [(HEB)Cr(CO) 2 {P(OPh) 3 }] exhibits resonances for at least three conformers, suggested to be c , e and h …”

“…Interestingly, incorporation of the dinitrogen ligand yielded the picturesque bridged‐sandwich complex [(HEB)Cr(CO) 2 –N≡N–Cr(CO) 2 (HEB)] ( 11 ) in which both HEBs exhibit conformation a (Figure ); surprisingly, perhaps, [(HEB)Mo(CO) 2 (maleic anhydride)] ( 12 ) also adopts structure a , but low‐temperature NMR data revealed the existence of other isomers. The chromium analogue of 12 , that is, [(HEB)Cr(CO) 2 (maleic anhydride)], has also been prepared, but its dynamic behaviour has not been fully elucidated …”

“…Clearly, the balance between these conformers is delicate, and supports the observation by Hunter, Mislow et al. that multiple conformers of 20 co‐exist in solution …”

“…The chromium analogue of 12,t hat is, [(HEB)Cr-(CO) 2 (maleic anhydride)],h as also been prepared, but its dynamic behaviour has not been fully elucidated. [16] Reaction of 2 with elemental chlorine in CCl 4 produced the salt [(HEB)Mo(CO) 3 Cl][MoCl 6 ](13), thus generating yet another conformer ( Figure 6), the 1,4-proximal-2,3,5,6-distal structure (b)t hat optimises interactions with the square-pyramidal Mo(CO) 3 Cl moiety,b ut does not force all the ethyl groups onto the same faceasi nh. [21] As shown in Scheme1,t his result contrasts with the protonation of [(HEB)W(CO) 3 ](14) [22] with triflic acid to form [(HEB)W(CO) 3 H][CF 3 SO 3 ](15)t hat maintains its threefold symmetry as shown by the typical 1,3,5-proximal-2,4,6-distal NMR pattern observed at low temperature.…”