Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes

• Published in: European journal of inorganic chemistry Vol. 2017; no. 38-39; pp. 4581 - 4588
• Main Authors: Robertson, Alasdair P. M., Reckziegel, Alexander, Jones, John J., Rosair, Georgina M., Welch, Alan J.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 25-10-2017
• Subjects: Cages; Carbonyl ligands; Carbonyls; Carboranes; Crowding; Electronic effects; Inorganic chemistry; Isomers; Ligands; Phosphane ligands; Steric effects
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201700492

Analysis of the literature structures [(CO)(PPh3)2MC2B9H11] and [(CO)2(PPh3)MC2B9H11] suggests that in [L3MC2B9H11] metallacarboranes the trans influence of CO is greater than that of PPh3. Extending this study to the [L4MC2B9H11] system the new molybdacarboranes [3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H11] (2), [1,2‐Me2‐3,3,3‐(CO)3‐3‐PPh3‐3,1,2‐closo‐MoC2B9H9] (3) and trans‐[3,3‐(CO)2‐3,3‐(PPh3)2‐3,1,2‐closo‐MoC2B9H11] (4) were prepared and fully characterised. Consideration of the exopolyhedral ligand orientations (ELO) in 2 confirms that, in terms of trans influence, CO > PPh3 in [L4MC2B9H11] also. The ELO is effectively reversed in 3 through intramolecular steric crowding between the cage CH3 groups and the PPh3 ligand. The dicarbonylbis(triphenylphosphine) compound 4 has effective Cs 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 PH3 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 PEt3 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.
The balance between steric and electronic preferences for the exopolyhedral ligand orientation (ELO) in molybdacarboranes is explored by synthetic and structural studies.