Phosphines vs. NHCsElectronic and Steric Differences in

Organometallic ChemistryAdrian Henle

Group Meeting 11/6/2013

Metals (and Salts) Are Boring

http://www.jinmabusiness.com/en/tiekuangshi.htmhttp://www.webelements.com/compounds/silver/silver_chloride.html

Ligands Make Chemistry Fun!

• Properties of coordination compounds determined by metal center, ligands

• Ligand properties:– Electronic– Steric

Phosphines

• Formula PR3

• Popular ligand type– Cross-coupling rxns– Olefin metathesis– DT Lab chemistry (sorry Justin)

• Range from simple/compact tocomplex/massive (e.g. XPhos)

http://en.wikipedia.org/wiki/XPhos

N-Heterocyclic Carbenes (NHCs)

• Arduengo, 1998• Increasingly popular:

– O.M. chemistry (e.g. Grubbs’ II)– Metal-free chemistry

http://en.wikipedia.org/wiki/Grubbs_catalystArduengo, et al. 1998 Angew. Chem. Int. Ed. 37 1963–1965Curran, et al. 2013 J. Am. Chem. Soc. 135 14433–14437

σ-Donation, π-Backbonding

• σ-DonationLigand orbital (lone pair)imparts e- density to dσ

• π-BackbondingMetal dπ orbital givese- density to ligand

Phosphine Electronics

• HOMOdσσ Donation

• dπLUMOπ Backbonding

NHC Electronics

• (HOMO-1)dσσ Donation

• HOMOdππ Donation

• dπLUMOπ Backbonding

Electronics Summary

• Phosphines and NHCs both donate e- density to metal centers

• NHCs donate more strongly, are poorer at accepting backbonding

• Qualitative MO diagram (chalkboard)• Result: NHC-metal complexes generally have

greater electron density at metal center

NHCs vs. Phosphines: Sterics

• Tolman cone angle– Great for symmetric tertiary phosphines– Useless for NHCs, other complex ligands

• Percent Buried Volume

M

P

Percent Buried Volume (%Vbur)

• Calculated from XRD data• For phosphines and NHCs, SaMBVCa (Salerno

Molecular Buried Volume Calculation)• Clavier 2010: ~700 calculations from

crystallographic data• Result:

NHC %Vbur = 33-66%PR3 %Vbur = 26-53%

https://www.molnac.unisa.it/OMtools/sambvca.phpClavier & Nolan 2010 Chem. Commun. 46 841-861

Problem: Metal-Ligand Distance

• All SaMBVCa calculations take Cartesian coordinates for ligand atoms only

• Clavier & Nolan set all bond distances at 2.00 and 2.28 Angstroms—actual results may vary

• Especially problematic for geometrically flexible NHCs

Conclusions

• NHCs are generally stronger e- donors– Better donation, poorer backbonding

• NHCs are generally more sterically congesting– But, both NHCs and PR3’s are diverse– NHC steric bulk much harder to quantify

• Arbitrary bond lengths draw easy criticism– Don’t do 700 calculations on possibly irrelevant

structures.