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Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes
Author(s) -
Robertson Alasdair P. M.,
Reckziegel Alexander,
Jones John J.,
Rosair Georgina M.,
Welch Alan J.
Publication year - 2017
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201700492
Subject(s) - chemistry , steric effects , ligand (biochemistry) , phosphine , triphenylphosphine , trans effect , intramolecular force , bond length , stereochemistry , electronic effect , crystallography , lone pair , medicinal chemistry , molecule , crystal structure , organic chemistry , catalysis , biochemistry , receptor
Analysis of the literature structures [(CO)(PPh 3 ) 2 MC 2 B 9 H 11 ] and [(CO) 2 (PPh 3 )MC 2 B 9 H 11 ] suggests that in [L 3 MC 2 B 9 H 11 ] metallacarboranes the trans influence of CO is greater than that of PPh 3 . Extending this study to the [L 4 MC 2 B 9 H 11 ] system the new molybdacarboranes [3,3,3‐(CO) 3 ‐3‐PPh 3 ‐3,1,2‐ closo ‐MoC 2 B 9 H 11 ] ( 2 ), [1,2‐Me 2 ‐3,3,3‐(CO) 3 ‐3‐PPh 3 ‐3,1,2‐ closo ‐MoC 2 B 9 H 9 ] ( 3 ) and trans ‐[3,3‐(CO) 2 ‐3,3‐(PPh 3 ) 2 ‐3,1,2‐ closo ‐MoC 2 B 9 H 11 ] ( 4 ) were prepared and fully characterised. Consideration of the exopolyhedral ligand orientations (ELO) in 2 confirms that, in terms of trans influence, CO > PPh 3 in [L 4 MC 2 B 9 H 11 ] also. The ELO is effectively reversed in 3 through intramolecular steric crowding between the cage CH 3 groups and the PPh 3 ligand. The dicarbonylbis(triphenylphosphine) compound 4 has effective C s symmetry with one CO ligand trans and the other CO ligand cis to the cage C–C connectivity. Unexpectedly the Mo–CO bond lengths are equal. DFT calculations on 4 reproduce this unusual result, but suggest that in the less‐crowded PH 3 analogue, the Mo–CO bond length trans to cage C would be about 0.2 Å shorter than that trans to cage B. To test this prediction, the analogous PEt 3 complex was prepared as cis and trans structural isomers 5 and 6 . The cis isomer 5 is quantitatively converted into the trans isomer 6 when heated to reflux in THF. In 6 the Mo–CO bond more trans to cage C is about 0.2 Å shorter than that which is more trans to cage B, in line with the DFT prediction.