Al-HC Chemistry - American Chemical Society

Chapter 2

Al-H-C Chemistry

Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 29, 2013 | http://pubs.acs.org Publication Date: December 1, 2005 | doi: 10.1021/bk-2005-0917.ch002

Herbert W. R o e s k y Institut für Anorganische Chemie der Universität Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany

The synthesis of LAl(OH) , LAl(SH) , LAl(SeH) , and LAl(NH ) (L = HC(CMeNAr) , Ar = 2,6-iPr C H ) is described. For the preparation of LAl(SH) from L A l H in the presence of sulfur a catalyst is required, whereas LAl(OH) and LAl(NH ) are obtained in the presence of a N— heterocyclic carbene as a HCl acceptor. The starting material for the latter reaction is LAlCl . The organometallic hydroxides are environmentally friendly, in comparison to the metal halides, due to their OH functionality resembling that of water. The compounds are interesting precursor for the preparation of heterobimetallic compounds e.g. LΑl(μS) ZrCp . 2

2

2

2

2

2

2

2

6

3

2

2

2

2

2

2

2

The most fundamental and lasting objective of synthesis is not product new compounds but production ofproperties. George S. Hammond

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© 2006 American Chemical Society

In Modern Aspects of Main Group Chemistry; Lattman, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

21 Introduction In nature there are a number of hydrated forms of alumina (AI 0 ) known with the composition AIOOH and AI(OH) . Prominent examples of composition AlOOH are the minerals boehmite and diaspore. The natural aluminum trihydroxides are gibbsite (γ-Α1(ΟΗ) ) and bayerite (α-Α1(ΟΗ) ) (/). All the polymorphs of alumina can also be prepared using various synthetic methods. Gibbsite, boehmite, and bayerite are obtained from aluminum salts by precipitation from aqueous solutions depending on temperature and pH conditions. Another route for the preparation of the aluminum hydroxides is the hydrothermal method. The vapor pressure of water and the temperature are the key factors for the nature of the aluminum polymorphs prepared by the latter method. The different polymorphs of aluminum have in common a well defined (2) oxygen network with interstices. The aluminum atoms are located in these interstices and are octahedrally coordinated. The arrangement of the oxygen network governs the type of product formed after dehydration. The hydrogen atoms play an important role in the cohesion of the layered structures. Moreover the positions of the hydrogen atoms also determine the space group of the aluminum hydroxides although their positions in many systems remain controversial. Characteristic of all aluminum hydroxides is the arrangement of the OH groups in a bridging rather than in a terminal position (3). Furthermore compounds with the valence isoelectronic functional groups such as SH, SeH, and N H are not known in nature. Herein we report on soluble molecular organoaluminum compounds containing the OH, SH, SeH, and N H functionalities. 2

3

3


3

3

2

2

Organoaluminum Dihydroxide Hydroxides of aluminum can be considered as reactive intermediates in the controlled hydrolysis of organoaluminum compounds for the preparation of alumoxanes with the general formula [RA10] and [R A10AlR ] . In recent years there have been a few reports on the preparation of organoaluminum hydroxides with bridged or capped OH groups {4-9) that have been isolated and structurally characterized. Although the arylboronic acids RB(OH) have found interesting application in Suzuki (70) coupling, the corresponding organoaluminum dihydroxide was prepared only recently (11). In the meantime, however, several routes for the preparation of LAl(OH) 1 have been developed. They are summarized in Scheme 1, with L = HC(CMeNAr) (Ar = 2,6-iPr C H ). The reaction of LA1I with KOH required the application of the two phase system toluene and liquid ammonia at -78 °C. Moreover, KH has to be added to remove the amount of water present in the KOH (77). n

2

2

n

2

2

2

2

6

3

2


22

LA1C1 + 2 H 0 + 2C(Nt-BuCH) 2

2

2

LAH +2KOH

-+>

2

LA1(0H) 1

LA1(SH) +2H 0 Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 29, 2013 | http://pubs.acs.org Publication Date: December 1, 2005 | doi: 10.1021/bk-2005-0917.ch002

2

2

^

2

Ar = 2,6-iPr C H 2

6

Scheme 1. Alternative routes for the preparation of LAl(OH)

3

2

The conversion of LA1I to 1 mainly proceeds at the interface of toluene/ammonia due to the different solubility of the starting materials. The total yield of 1 is 48%. In summary, this route is rather complicated and needs expensive precursors. The synthesis of 1 using LA1C1 and water in the presence of the N heterocyclic carbene 1,3-di-t-butylimidazol-2-ylidene in benzene resulted in a 65% yield of the desired product (12). The N-heterocyelic carbene C(Nt-BuCH) functions as an HC1 acceptor to yield the imidazolium chloride, which can be easily recovered by filtration and recycled, to the free carbene using a strong base such as KOtBu and NaH, respectively (12). Finally, treatment of LAl(SH) 2

2

2

2


2

23 2 with water yields 1 under elimination of H S (13). The latter route to 1 is quite facile due to the easy conversion of LA1H to LA1(SH) . The reaction of LA1I with KOH in a two phase system liquid ammonia/toluene with the appropriate stoichiometry and the addition of KH for the 10-15% content of H 0 in KOH resulted in the formation of an alumoxane 3 with terminal hydroxide groups (14) (Scheme 2). 2

2

2

2

2


2LAU

K O H , H Q, K H

2

LAI—Ο—AIL

2

OH

OH 3

Scheme 2 Compound 3 is formed in a 67% yield. The reaction of 3 with Me AlH proceeds under elimination of hydrogen and methane to yield the trimeric alumoxane 4 (14) (Scheme 3). 2

LAI 3 +

AIL

Me AlH 2

Al I

Me 4

Scheme 3 Compound 4 contains two four-coordinate Al and one three-coordinate Al linked by μ-οχο bridges to form a highly distorted six-membered A1 0 ring. The coordination sphere around the four coordinated aluminum atoms consists of two nitrogen and two oxygen atoms for each aluminum. The coordination sphere of the three coordinated aluminum is completed by two oxygen and one carbon of a methyl group. Compound 4 can be considered to contain a trapped monomeric MeAlO (MAO) (14). 3

3


24

Organoaluminum Monohydroxide In a quite different manner an aluminum monohydroxide 5 was obtained by the reaction of LA1H 6 with tBuN=C=0 in a molar ratio of 2:1 in the presence of water (Scheme 4) (15). tBuN=C=0 and LAIH without water yields the 1:1 insertion product 7. 2


2

2 L A H + tBu-N=C=0 2

2

H

*°

>

V 0

^ H

°v

.N-tBu I

H

LAIH + tBu-N=C=0 2

*-

L-Al^

N-tBu H

7 Scheme 4

Compound 7 is obviously an intermediate in the formation of 5. Finally the reaction of LA1H and 7 in the presence of water yields 5. However, an excess of water results in the decomposition of 5. Compound 5 was characterized by a single X-ray crystal structural analysis (15). Compound 5 contains a bent Al-OAl unit (112.35(9)°) with two tetrahedral distorted aluminum centers. The AlOH bond length (1.727(2) A) is comparable to those in LAl(OH) (11,16) (1.6947(15) and 1.7107(16) A). 2

2


25 !

Hydrolysis of trimesitylaluminum and -gallium was monitored by H NMR spectroscopy in the temperature range -60 °C to room temperature using deuterated THF as a solvent. As a result of this reaction the hydrolysis of trimesitylaluminum leads to (Me A10H )nTHF 8 a water adduct, and the monohydroxide (Mes A10H) -2THF 9. The structure of 9 shows a central A1 0 four-membered ring. Two THF molecules are coordinated to the bridging μ-ΟΗ groups with short μ-ΟΗ· O(THF) distances of 1.912 and 1.861 A respectively (77). In the case of gallium the water adduct Mes GaOH -2THF 10 was isolated and characterized at low temperature by a single crystal structural analysis (7 7). (Figure 1) The relative stability of 10 is assigned to the influence of two THF molecules forming hydrogen bonds with the water protons 3

2

2

2

2


3

2

2

Figure 1. The molecular structure of the water adduct Mes GaOH -2THF (10). Hydrogen atoms have been omitted for clarity. 3

2


26 At elevated temperature elimination of mesitylene leads to the dimeric hydroxide (Mes GaOH) THF 11 whereas excess of water gives MeSoGaôO^OHMTHF 12 (17). The molecular structure of 12 is given in Figure 2.


2

2

Figure 2. The X-ray single crystal structure of Mes Ga60 (OH)4-4THF (12) is shown. The mesityl substituents are replaced by six C atoms and the four THF molecules are represented by 4 oxygen atms. 6

4

Organoaluminum Dithiol The SH functionality plays an important role as an intermediate in the formation of metal sulfides such as ZnS, PbS, and FeS . There are few examples of structurally characterized transition metal complexes known containing two terminal hydrogen sulfide groups (18). Among the main group metals only recently the LA1(SH) 2 (19) was prepared. LAIH 13 prepared from L H and 2

2

2


27 ΑΙΗ3·ΝΜβ3 reacts with elemental sulfiir to give a mixture of products, where 2 is formed only in small amounts. The addition of P(NMe2)3 as a catalyst increases the yield of 2 to 90% (Scheme 5) P(NMe ) 2

l

LA1H + A S 13


2

3

> LA1(SH) 2

8

2

Scheme 5 The proposed mechanism for the insertion of sulfur into the Al-H bonds is given in Scheme 6. 1/8 S P(NMe ) 2

1/8 S

8

> S=P(NMe )

3

2

LA1H + S P(NMe ) 13 LAl(H)-S~P(NMe ) SH 2

2

2

2

2

2

3

3

> LA1(H)(SH) + SP(NMe ) 14 2

3

3

> LA1(SH) ~SP(NMe ) 2 2

2

2

3

2

LA1(H)SH

> S P(NMe )

3

> LAI(H)-S-P(NMe ) (SH)

3

S P(NMe )

8

2

3

Scheme 6 To investigate the role of the P(NMe2)3 in this reaction the Ή and P NMR spectra were monitored. From the H and P NMR studies it is obvious that P(NMe ) and S=P(NMe ) do not function as a catalyst. We assume that S P(NMe ) plays the important role as a reactive intermediate. The intermediate 14 was identified in the Ή NMR spectrum. Compound 2 forms pale yellow crystals which were suitable for an X-ray structural analysis. The S-H bonds (1.2 Â) fall in the range of other metal complexes containing S-H groups (18). The latent acidic nature of the SH protons in 2 makes them prone to react with Cp ZrMe to generate LAI^-S) ZrCp 15 (Scheme 7) (13). 31

l

2

2

3

2 3

2

2

31

3

2

2

2


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SH \

Cp + Cp ZrMe

LAI,

2

SH

2

LAI

-2 CH4

Cp

2

15


Scheme 7 Compound 15 is a crystalline solid which was characterized by an X-ray structural analysis (13). The reaction shown in Scheme 7 opens up an interesting new field for the preparation of heterobimetallic disulfides.

Organoaluminum Diselenol For the preparation of LAl(SeH) 16 compound LA1H 13 and elemental selenium were used in toluene at room temperature. The diselenol 16 was obtained in 58% yield. Both the sulfur in the reaction of LA1(SH) 2 as well as the selenium for preparing LAl(SeH) 16 function as oxidizing agents and convert the hydridic hydrogens in 13 to the protonic ones in 2 and 16. Compound 16 is stable in the solid state but undergoes condensation in solution under elimination of H Se to yield L(HSe)A!-Se-Al(SeH)L 17 (Scheme 8, (20)). 2

2

2

2

2

2LAl(SeH) 16

2

-H Se 2

» L(HSe)Al-Se-Al(SeH)L 17

Scheme 8 The X-ray structures of both, 16 and 17, determined from their single crystals, clearly show them to be well-separated monomers excluding the presence of any intermolecular hydrogen bonds. This observation is in contrast to the oxygen congener 1 which forms a dimer in the solid state with Ο H-0 bonds. Obviously the Br0nsted acidity decreases in the order LAl(OH) < LAl(SH) LA1(NH ) -2HC1 18 2

LA1C1 + 2NH 2

3

2

2

Scheme 9 Therefore, using an amine instead of the N-heterocyclic carbene causes side reactions due to the equilibrium of free base and protons within the amine system. Moreover, the resulting imidazolium chloride is sparingly soluble in hydrocarbon solvents and allows an easy separation from the reaction mixture by filtration (12). Compound 18 was characterized by an X-ray single crystal structural analysis. 18 is monomeric in the solid state and the N H groups are not involved in any hydrogen bonding. 2

Summary Organometallic hydroxides of aluminum containing terminal OH groups are easily available. They are promising precursors for the preparation of heterobimetallic systems of the type Al-O-M (M = metal). So far compounds with the Al-O-M skeleton are rare. However, the reported synthetic strategies allow an access to this new class of compounds. The synthesis of LA1(SH) , LAl(SeH) , and LA1(NH ) are likely to change the way in which some of the "unknown" might be accessible using the reported new synthetic methods. 2

2

2

2

Acknowledgment Financial support by the Deutsche Forschungsgemeinschaft and the Gôttinger Akademie der Wissenschaften is highly acknowledged. I am very


30

thankful to my coworkers for their contributions, the names are given in the references.


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Al-HC Chemistry - American Chemical Society

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