-coordination tribenzylaluminum: -coordination to the ortho-carbon of the aromatic ring on an...

-coordinationTribenzylaluminum: -coordination to the ortho-carbon of the aromatic ring on

an adjacent species.

AlPh

Ph

Ph

Al

Ph

Ph

2 coordination in gas phase

polymeric in solid state (Al-Me = 1.95Å)

Al

Me Me

Al2Me6 + CpH

Al

Me Me

Al

Me Me

2.20 2.25

Cyclopentadienyl Al compounds

More CpAl Compounds

Cp2AlMe (Shapiro et al Organometallics 1994, 13, 3324)

AlAlMeB(C6F5)3

MeB(C6F5)3

decamethyl analogue (C5Me5)2Al+ is similar

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.)

In

In

Low Valent Group 13 Compounds

• (C5R5)M (M = In, Tl) exhibit diverse structures

R = CH2Ph dimers (M-M = 3.63Å)

R = Me hexameric cluster

InIn

In

In

In In

M

M

(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

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.

GaI3 + Cp*SiMe3 Cp*GaI2Ga2 K

-2 KI

hexanes

- Me3SiI

Me SiMe3

Me

MeMe

MeActually (Cp*Ga)6 by single crystal X-ray.

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 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

AlAl

Al Al

Lewis Basicity in Low Valent Group 13Recent 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)

Oxidation of Low Valent Group 13Interestingly, the same reaction with InIII(C6F5)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.

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.

Al Al

triplet singlet

J. Am. Chem. Soc. 2000, 122(5), 951.

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

CH

CH

CH

CH

Si

Si

Si

Si Si

Si

Si

Si

Al-Al 2.66Å (rcov = 1.30Å) planar Al2C4 framework


Related compound: (2,4,6-iPr3C6H2)4Al2 Al-Al 2.65ANon-planar Al2C4 framework

(angle between the AlC2 planes = 45 degrees)

Al

iPr

iPr

iPr

iPr

iPr

iPrAl

iPr

iPr

iPr

iPr

iPr

iPr


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.

CH(SiMe3)2(Me3Si)2HC

CH(SiMe3)2(Me3Si)2HC

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

Al Al

CH(SiMe3)2

CH(SiMe3)2

(Me3Si)2HC

(Me3Si)2HC+ LiR

Al AlCH(SiMe3)2

CH(SiMe3)2

(Me3Si)2HC

(Me3Si)2HC

R

Li(TMEDA)2+

TMEDA

Hydride AbstractionInterestingly, if the same reaction is attempted with EtLi or tBuLi, a hydride is abstracted and an alkene is formed

Al Al

CH(SiMe3)2

CH(SiMe3)2

(Me3Si)2HC

(Me3Si)2HC+ LiCR2CH3

Al AlCH(SiMe3)2

CH(SiMe3)2

(Me3Si)2HC

(Me3Si)2HC

CR2CH3

Li(TMEDA)2+

TMEDA

Al AlCH(SiMe3)2

CH(SiMe3)2

(Me3Si)2HC

(Me3Si)2HC

H

Li(TMEDA)2+

H2C CR2

R = H, CH3

Reduction

The unfilled 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.

(Me3Si)2HC CH(SiMe3)2

(Me3Si)2HC CH(SiMe3)2

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

Low Valent Group 13 OrganometallicsUtilization 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

M

C

SiMe3

Me3Si SiMe3

M

C

SiMe3

SiMe3Me3Si

M C SiMe3

SiMe3

SiMe3

MCMe3Si

Me3Si

Me3Si

“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)

“Bulking up” the Ga Reduction with Na leads to 2,6-Mes2C6H3)Ga]3

2- (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!)

Metalloaromatic?Several arguments suggest that (2,6-Mes2C6H3)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

Mes

Mes

Ga

Mes

Mes

Ga

Mes

MesGa

2-


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.


(ArGaGaAr)2-(Na+)2, a Ga-Ga triple bond?

iPr

iPr

iPr

iPr

iPr

iPr

Ga

iPr

iPr

iPr

iPr

iPr

iPr

Ga

??

A proposal for the “trans-bent” triple bond

R

R

p-p bond

donor bond

donor bond

two sp2 hybridized Ga(I)R fragments

Ga-Ga

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.

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

Another Recent Ligand System used with Ga(I)

N LiN

iPr Pri

iPriPrGaI +

KN N

iPr Pri

iPriPr

Ga

LiI

The steric demand on this compound is so great as to preclude oligomerization in the crystalline phase

Reactivity of Ga(I) – -diketiminate

N N

iPr Pri

iPriPr

Ga+ B(C6F5)3

Ga-B donor acceptor bond by reaction with B(C6F5)3

Reactivity of Ga(I) – -diketiminate

N N

iPr Pri

iPriPr

M + N3(2,4,6-triisopropylphenyl)phenyl

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.

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 monomerTlAr* (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.


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 = 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

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).


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 removedFirst observation of this function for group 13 (note that anions are known)

-coordination tribenzylaluminum: -coordination to the ortho-carbon of the aromatic ring on an...

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