Balancing Steric and Electronic Effects in Carbonyl–Phosphine Molybdacarboranes

Abstract

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.

What Can We Learn from the Crystal Structures of Metallacarboranes?

“…its C,C-dimethyl analogue [1,2-Me2-3,3,3-(CO)3-3-PPh3-closo-3,1,2-MoC2B9H9], Figure 13b. In the former, the lower STE PPh3 ligand has the smallest |θ|, 25.0°, as expected, whilst in the latter species steric congestion between PPh3 and the cage Me substituents pushes the phosphine round to the largest |θ|, 170.9° [55]. For this reason, all discussion of preferred exopolyhedral ligand orientation in terms of structural trans effects is best restricted to cases of unsubstituted carborane cages.…”

“…An understanding of the unexpected ELOs in TAKCUD and LICDOQ is afforded by a detailed examination of their structures, revealing intramolecular H-bonding between the O atom of the acetyl or methoxyalkylidene ligand and the protonic H atom(s) on cage carbon. In TAKCUD, Figure 12a, the H-bond is between O1 and H2 whilst in LICDOQ, Figure 12b ) and concluded that, at least in this class of compound, the STE of CO is greater than that of PPh3 [55]. A small number of 3,1,2-MC2B9 metallacarboranes are known having three different exopolyhedral ligands (Table 6).…”

“…Coe and Glenwright order their STEs as CO > PPh 3 on the basis of analysis of [M(CO) 5 PPh 3 ] structures [53], whilst See and Kozina conclude the reverse ordering, PPh 3 > CO, from analysis of the structures of complexes in which these ligands are trans to Cl [54]. We recently surveyed the preferred ELO in mixed carbonyl-phosphine 3,1,2-MC 2 B 9 metallacarboranes (of both types, [3,3- (PPh 3 ) 2 -3-COcloso-3,1,2-MC 2 B 9 H 11 ] and [3,3-(CO) 2 -3-PPh 3 -closo-3,1,2-MC 2 B 9 H 11 ]) and concluded that, at least in this class of compound, the STE of CO is greater than that of PPh 3 [55].…”