p -coordination

1. p-coordination Tribenzylaluminum: p-coordination to the ortho-carbon of the aromatic ring on an adjacent species. Cyclopentadienyl Al compounds h2 coordination in gas phase polymeric in solid state (Al-Me = 1.95Å)

2. More CpAl Compounds  decamethyl analogue (C5Me5)2Al+, is similar Cp2AlMe (Shapiro et al Organometallics1994, 13, 3324)

3. Low Valent Group 13 Compounds Thallium and indium monovalent compounds have been known for quite some time (inert pair effect) For example, both CpTl and CpIn have been known since the 1950’s. CpTl and CpIn are polymeric in the solid state (Organometallics 1988, 7, 1051.)

4. Low Valent Group 13 Compounds • (C5R5)M (M = In, Tl) exhibit diverse structures • R = CH2Ph dimers (M-M = 3.63Å) • R = Me hexameric cluster M = In, Tl Schumann, Janiak, Pickhardt, Borner, Angew. Chem., Int. Ed. Engl. 1987, 26, 789. (note the Cp substituents have been omitted in these diagrams)

5. Low Valent Group 13 Organometallics Recently, oxidation state +1 has taken a higher profile in group 13. This oxidation state is stabilized by bulky, basic ligands. Actually (Cp*Ga)6 by single crystal X-ray.

6. Low Valent Group 13 Organometallics Metathesis from the M(I) starting material MCl + MgCp*2 (-78oC, ether) MgCl2 + 2 MCp* The bonding in the tetrahedron can be viewed as M-M s bonds, and the Cp can donate to an empty sp3 orbital as a Lewis base with its spherically symmetric aromatic orbital. this is not a face-capped tetrahedron as observed for MeLi

7. Lewis Basicity in Low Valent Group 13 Recent work by Cowley and group at the University of Texas shows that these compounds can be considered to have a lone pair (i.e. singlet state). (Cp*Al)4 + B(C6F5)3 Cp*Al-B(C6F5)

8. Oxidation of Low Valent Group 13 Interestingly, the same reaction with InIII(C6F5)3 resulted oxidation of Al to the +3 oxidation state. Now the aluminum is h3 bonded to the Cp and has s bonds to two pentafluorophenyl rings.

9. triplet singlet Singlet vs. Triplet It was suggested that this compound forms with the Lewis base adduct as an intermediate. (Cp*Al)4 + In(C6F5)3 Cp*Al-In(C6F5)  Cp*Al(C6F5)2 + In(C6F5) Suggests that Al can react as either singlet or triplet. J. Am. Chem. Soc. 2000, 122(5), 951.

10. Mid-valent Group 13 Compounds With intermediate steric bulk, it is possible to isolate M2+ compounds of group 13 (done in 1988). (note one less SiMe3 group on the carbon) 2 (Me3Si)2CHLi + AlCl3 ((Me3Si)2CH)2AlCl 2 K + ((Me3Si)2CH)2AlCl  [((Me3Si)2CH)2Al]2 + 2 KCl Al-Al 2.66Å (rcov = 1.30Å) planar Al2C4 framework

11. Mid-valent Group 13 Compounds Related compound: (2,4,6-iPr3C6H2)4Al2 Al-Al 2.65A Non-planar Al2C4 framework (angle between the AlC2 planes = 45 degrees)

12. Mid-valent Group 13 Compounds These species are known for Al, Ga, and In. Ga and In analogues were made by metathesis with M2Br4.2 L and the lithium salt of the ligand. Bonding model: Single bond between the M centers Metal is sp2 with an empty p orbital perpendicular to the plane of the molecule.

13. Reaction with A Lewis Base These compounds will react with methyllithium to stabilize the methyl carbanion, and with LiBr to stabilize the the bromide anion.

14. Hydride Abstraction Interestingly, if the same reaction is attempted with EtLi or tBuLi, a hydride is abstracted and an alkene is formed

15. Reduction The unfilled p system allows reduction. Reduction with alkali metals produces radical anion species (with both the CH(SiMe3)2 and aryl groups) [((Me3Si)2CH)2M]2 + M’ + n L  [((Me3Si)2CH)2M]2-. M’(L)n+ Formal bond order of 1.5 Al-Al shortened to 2.53Å and 2.47Å for CH(SiMe3)2 and aryl respectively. Both are planar Al2C4 structures.

16. Similar Chemistry with Ga and In As with Al, E2R4 and E2R4- can be prepared for Ga and In (same two R groups) In the aryl Ga case, reduction leads to bond distance change from 2.52 to 2.34Å consistent with increase in bond order form 1 to 1.5

17. Low Valent Group 13 Organometallics Utilization of a similar synthetic procedure but employing a more sterically demanding R group, C(SiMe3)3, produces an unexpected family of clusters – M4{C(SiMe3)3}4 (M = Ga, In) For M = Ga mean M-M = 2.688Å Dynamic system

18. “Bulking up” the Ga (2,6-Mes2C6H3)GaCl2 synthesized via metathesis reaction. Measure of sterics – C-Ga-C = 153.5º and C-Ga-Cl = 103º (T-shaped)

19. “Bulking up” the Ga Reduction with Na leads to 2,6-Mes2C6H3)Ga]32- (Na+)2 This species exhibits symmetrical Ga3 ring “bicapped” with Na+. K+ analogue also reported. Ga-Ga of 2.441 and 2.42Å respectively (short!)

20. Metalloaromatic? Several arguments suggest that (2,6-Mes2C6H3)Ga]32- is metalloaromatic. a planar cyclic structure that is a 2p-electron system. NMR evidence of ring currents J. Am. Chem. Soc. 1996, 118, 10635

21. “Bulking up” the Ga Increasing the steric demand by replacing Me with iPr groups. (2,4,6-triisopropylphenyl)phenyl ligands Reduction of the ArGaCl2 leads to (ArGaGaAr)2-(Na+)2 A Ga-Ga triple bond? 2.32Å bond length non-linear C-Ga-Ga arrangement.

22. “Bulking up” the Ga (ArGaGaAr)2-(Na+)2, a Ga-Ga triple bond? A proposal for the “trans-bent” triple bond

23. Ga-Ga Questions: importance of factors such as Na-Aryl and Na-Ga interactions in shortening the Ga-Ga bond? The reduction of GaAr* with potassium instead of sodium to afford the ring compound K2Ar*Ga4Ar* demonstrated the critical importance of the size of the alkali metal to the stability of Na2Ar*GaGaAr*. Power, et al Angew. Chem., Int. Ed. 2000, 39, 3500. More of this chemistry can be found in Power et al J. Am. Chem. Soc. 2003, 125, 2667

24. Another Recent Ligand System used with Ga(I) The steric demand on this compound is so great as to preclude oligomerization in the crystalline phase

25. Reactivity of Ga(I) – b-diketiminate Ga-B donor acceptor bond by reaction with B(C6F5)3

26. Reactivity of Ga(I) – b-diketiminate Stable monomeric imidos of Ga and Al Ga-N = 1.74 vs. 1.92Å Ga-N-C 134.6º Angew. Chem. Int. Ed.2001, 40, 2172.

27. Steric Bulk and M-M bonding in the heavier Group 13 Compounds single Tl-Tl bonds observed in R2Tl-TlR2 (R = Si(SiMe3)3, 2.914 Å; SitBu3, 2.97 Å; and SitBu2Ph, 2.881(2) Å which result from overlap of formally sp2 orbitals like we saw before. Related work with terphenyl ligand which was sufficiently large gave the monomer TlAr* (Ar* = C6H3-2,6(C6H2-2,4,6-iPr3)2), with one-coordinate thallium, could be isolated. Power et al Angew. Chem., Int. Ed. 1998, 37, 1277.

28. Steric Bulk and M-M bonding in the heavier Group 13 Compounds Power et al J. Am. Chem. Soc. 2005, 127, 4794 terphenyl ligands with slightly less crowding aryl rings led to isolation of the first neutral, metal-metal bonded gallium or indium dimers; i.e., ArGaGaAr and ArInInAr = C6H3-2,6(C6H3-2,6-iPr2)2). Power, P. P. J. Am. Chem. Soc. 2003, 125, 2667. Power, P. P. J. Am. Chem. Soc. 2002, 124, 8538 first “dithallene” analogue of these, ArTlTlAr Tl(1)-Tl(1A) = 3.0936(8); Tl(1)-C(1) ) 2.313(5); C(1)-Tl(1)-Tl(1A) ) 119.74(14); C(2)-C(1)-Tl(1) ) 117.6(4); C(6)-C(1)-Tl(1) ) 122.9(4).

29. Steric Bulk and M-M bonding in the heavier Group 13 Compounds Power et al J. Am. Chem. Soc. 2005, 127, 4794 less crowded ligand Ar¢¢ (Ar¢¢ = C6H3-2,6- (C6H3-2,6-Me2)2) results in the synthesis of the first neutral trimeric group 13 metal ring derivative (TlAr¢¢)3 hydrogens and flanking 2,6-dimethylphenyl groups removed First observation of this function for group 13 (note that anions are known)