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 -coordination Tribenzylaluminum:  -coordination to the ortho-carbon of the aromatic ring on an adjacent species.  2 coordination in gas phase polymeric.

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Presentation on theme: " -coordination Tribenzylaluminum:  -coordination to the ortho-carbon of the aromatic ring on an adjacent species.  2 coordination in gas phase polymeric."— Presentation transcript:

1  -coordination Tribenzylaluminum:  -coordination to the ortho-carbon of the aromatic ring on an adjacent species.  2 coordination in gas phase polymeric in solid state (Al-Me = 1.95Å) Cyclopentadienyl Al compounds

2 More CpAl Compounds Cp 2 AlMe (Shapiro et al Organometallics 1994, 13, 3324) decamethyl analogue (C 5 Me 5 ) 2 Al +  is similar

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 (C 5 R 5 )M (M = In, Tl) exhibit diverse structures R = CH 2 Ph dimers (M-M = 3.63Å) R = Me hexameric cluster (note the Cp substituents have been omitted in these diagrams) Schumann, Janiak, Pickhardt, Borner, Angew. Chem., Int. Ed. Engl. 1987, 26, 789. M = In, Tl

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 (-78 o C, ether)  MgCl MCp* The bonding in the tetrahedron can be viewed as M-M  bonds, and the Cp can donate to an empty sp 3 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(C 6 F 5 ) 3  Cp*Al-B(C 6 F 5 )

8 Oxidation of Low Valent Group 13 Interestingly, the same reaction with In III (C 6 F 5 ) 3 resulted oxidation of Al to the +3 oxidation state. Now the aluminum is  3 bonded to the Cp and has  bonds to two pentafluorophenyl rings.

9 Singlet vs. Triplet It was suggested that this compound forms with the Lewis base adduct as an intermediate. (Cp*Al) 4 + In(C 6 F 5 ) 3  Cp*Al-In(C 6 F 5 )  Cp*Al(C 6 F 5 ) 2 + In(C 6 F 5 ) Suggests that Al can react as either singlet or triplet. tripletsinglet J. Am. Chem. Soc. 2000, 122(5), 951.

10 Mid-valent Group 13 Compounds With intermediate steric bulk, it is possible to isolate M 2+ compounds of group 13 (done in 1988). (note one less SiMe 3 group on the carbon) 2 (Me 3 Si) 2 CHLi + AlCl 3  ((Me 3 Si) 2 CH) 2 AlCl 2 K + ((Me 3 Si) 2 CH) 2 AlCl  [((Me 3 Si) 2 CH) 2 Al] KCl Al-Al 2.66Å (r cov = 1.30Å) planar Al 2 C 4 framework

11 Mid-valent Group 13 Compounds Related compound: (2,4,6- i Pr 3 C 6 H 2 ) 4 Al 2 Al-Al 2.65A Non-planar Al 2 C 4 framework (angle between the AlC 2 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 M 2 Br 4.2 L and the lithium salt of the ligand. Bonding model: Single bond between the M centers Metal is sp 2 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 t BuLi, a hydride is abstracted and an alkene is formed

15 Reduction The unfilled  system allows reduction. Reduction with alkali metals produces radical anion species (with both the CH(SiMe 3 ) 2 and aryl groups) [((Me 3 Si) 2 CH) 2 M] 2 + M’ + n L  [((Me 3 Si) 2 CH) 2 M] 2 -. M’(L) n + Formal bond order of 1.5 Al-Al shortened to 2.53Å and 2.47Å for CH(SiMe 3 ) 2 and aryl respectively. Both are planar Al 2 C 4 structures.

16 Similar Chemistry with Ga and In As with Al, E 2 R 4 and E 2 R 4 - 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(SiMe 3 ) 3, produces an unexpected family of clusters – M 4 {C(SiMe 3 ) 3 } 4 (M = Ga, In) For M = Ga mean M-M = 2.688Å Dynamic system

18 “Bulking up” the Ga (2,6-Mes 2 C 6 H 3 )GaCl 2 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-Mes 2 C 6 H 3 )Ga] 3 2- (Na + ) 2 This species exhibits symmetrical Ga 3 ring “bicapped” with Na +. K + analogue also reported. Ga-Ga of and 2.42Å respectively (short!)

20 Metalloaromatic? Several arguments suggest that (2,6-Mes 2 C 6 H 3 )Ga] 3 2- is metalloaromatic. a planar cyclic structure that is a 2  -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 i Pr groups. (2,4,6-triisopropylphenyl)phenyl ligands Reduction of the ArGaCl 2 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 The reduction of GaAr* with potassium instead of sodium to afford the ring compound K 2 Ar*Ga 4 Ar* demonstrated the critical importance of the size of the alkali metal to the stability of Na 2 Ar*GaGaAr*. Power, et al Angew. Chem., Int. Ed. 2000, 39, Questions: importance of factors such as Na - Aryl and Na - Ga interactions in shortening the Ga - Ga bond? 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) –  -diketiminate Ga-B donor acceptor bond by reaction with B(C 6 F 5 ) 3

26 Reactivity of Ga(I) –  -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 R 2 Tl-TlR 2 (R = Si(SiMe 3 ) 3, Å; Si t Bu 3, 2.97 Å; and Si t Bu 2 Ph, 2.881(2) Å which result from overlap of formally sp 2 orbitals like we saw before. Related work with terphenyl ligand which was sufficiently large gave the monomer TlAr* (Ar* = C 6 H 3 - 2,6(C 6 H 2 -2,4,6- i Pr 3 ) 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 first “dithallene” analogue of these, ArTlTlAr 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 = C 6 H 3 -2,6(C 6 H 3 -2,6- i Pr 2 ) 2 ). Power, P. P. J. Am. Chem. Soc. 2003, 125, Power, P. P. J. Am. Chem. Soc. 2002, 124, 8538 Tl(1) - Tl(1A) = (8); Tl(1) - C(1) ) 2.313(5); C(1) - Tl(1) - Tl(1A) ) (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  = C 6 H 3 -2,6- (C 6 H 3 -2,6- Me 2 ) 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)


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