1. Addition-elimination across the M-CCarbene bond
Reversible reactions across the M-C bond of lanthanide NHC complexes to form new N-E and C-E bonds
A. I. Germerotha, Z. R. Turnera, I. J. Caselya, R. M. Bellabarbab, R. P. Toozeb, P. L. Arnolda,*
a School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK
bSasol Technology, Purdie Building, North Haugh, St. Andrews, KY16 9ST, UK
Many organometallic f-block complexes are highly effective at the activation of small molecules including alkanes, although subsequent functionalisation chemistry can be hampered by the difficulty of functional group incorporation chemistry for these metals.[1] We have been studying ligand systems that might bring in a second substrate.
N-heterocyclic carbene ligands (NHCs) are versatile, neutral, two-electron σ-donors which are used in a wide variety of organometallic chemistry. The carbene donor atom in an NHC is stabilised by the two adjacent heterocyclic nitrogen atoms. In recent years, bidentate saturated NHC-ligands bearing an anionic alkoxide tether have been developed by our group.[2] The tether allows the ligand to be ‘anchored’ to the metal centre, so that the hemilabile carbene can be retained in the metal’s coordination sphere. The syntheses of mono or bisligand f-block element carbene complexes are therefore possible.[3] We plan to use the combination of the Lewis acidic metal with the labile carbene to activate small molecules.
Carbene amido and alkyl complexes
Synthesis of the bright yellow monoligand and bisligand cerium complexes, colourless group 3 complexes and the dark blue monoligand uranium analogue have been accomplished.
The proligand HL can be made in five steps from ArNH2 in 68 % yield. HL
Addition-elimination across the M-CCarbene bond
X-ray structure of U(LSi)N’’2I
X-ray structure of
Li(thf)[(L)Sc(CH2Si’)3
X-ray structure of Ce(L)N’’2
For E = CPh3 the first C-C bond forming reaction yet on
organolanthanide complexes
X-ray structure of Y(L)(CH2Si’)2
Products formed
● N-Si ●N-B ●N-P ●N-Sn ●C-C ●C-Si
● Reaction of small molecules of the form E-X (E = CR3, SiR3, PR2, BR2, SnR3; X = halide, pseudohalide) with M(L)R2 (M = Y, Ce, U) results in the addition across the neutral metal carbene bond, adding E to the carbene moiety and X to the metal as schematically shown in M(LE)RX2[4]
● The metal carbene is then reformed by elimination of E-R to form M(L)XR or M(L)R2 thus incorporating additional groups such as boranes, silanes, phosphanes or stannanes into compounds that are active for σ-bond metathesis
● M(L)XR complexes can in the case of M = Y, Ce reform the starting material M(L)R2 by reacting it with 1 eq of KR or LiR
● When the [Li(thf)[(L)MR3] (M = Sc, Y) compound is treated with 1 eq of E-X (E = CR3, SiR3, X = halide, pseudohalide) the aforementioned complex M(LE)R2X is formed.
● E-X reagents that were used in our studies include Me3SiI, Me3SiCl, Me3SiN3, But3SnCl, (B-I)-9-BBN, ClCPh3 and ClPPh2
● All addition-elimination reactions of these substrates across the metal-carbene bond are reversible
Thanks to:
References
[1] P. L. Arnold, I. J. Casely, Chem. Rev.,2009, 109, [2] P. L. Arnold, I. J. Casely, Z. R. Turner, C. D. Carmichael, Chem. Eur. J., 2008, 14, [3] P. L. Arnold, I. J. Casely, Z. R. Turner, A. I. Germeroth, R. M. Bellabarba, R. P. Tooze, Dalton Trans., 2009, 39, [4] P. L. Arnold, Z. R. Turner, R. Bellabarba, R. P. Tooze, J. Am. Chem. Soc., 2010, 132, 4052.