Phosphorus Atoms in Unusual Environments - American Chemical

Chapter 4 Phosphorus Atoms in Unusual Environments Alan H. Cowley, Richard A. Jones, and Miguel A. Mardones

Downloaded by UNIV OF CINCINNATI on May 19, 2016 | http://pubs.acs.org Publication Date: April 7, 1992 | doi: 10.1021/bk-1992-0486.ch004

Department of Chemistry, University of Texas, Austin, TX 78712

Phosphorus-containing cubanes of the type (RMPR')4 (M=Al, Ga, In) have been prepared by alkane elimination or salt elimination methods. The new compounds have been characterized by X-ray crystallography, NMR, and mass spectroscopy. Conceptually, the group 13/15 cubanes can be regarded as tetramers of RM=PR'. If the steric bulk of the substituents R and R' is increased it is possible to isolate dimers, (RMPR')2. For several years, our laboratory has been interested in the chemistry of main group elements in low coordination number environments (7). Some low coordinate species from Group 15 are shown below. They include two-coordinate cations (phosphenium ions), two-coordinate radicals (phosphinyl radicals), as well as stable double-bonded and even triple-bonded neutral molecules. For many years it was thought that

Phosphenium ions

Diphosphenes

Phosphaarsenes

Phosphinyl radicals

Phosphaalkynes

Diarsenes

compounds with multiple bonds between heavier main group elements would not be isolable because of the inherent weakness of π-bonds when the principal quantum number exceeds 2. However, despite the unfavorable thermodynamic features, it is possible to effect kinetic stabilization of these low-coordinate species. 0097-6156/92/0486-0056$06.00/0 © 1992 American Chemical Society Walsh et al.; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

4. COWLEY ET AL

Phosphorus Atoms in Unusual Environments

57


Kinetic stabilization is accomplished principally by the use of bulky hgands. As shown below, the bulky groups can be alkyl (e.g. 1 and 2) or or aryl (e.g. 3). In some cases, such as the electrophilic phosphenium ions, steric bulk plus conjugaUve stabilization is necessary. Amido groups (4) have proved to be very useful in this context (2).

Special interest is associated with compounds that feature multiple bonding between heavier group 13 and 15 elements. As shown below, isolobal analogues of alkenes and alkynes can be envisioned. In addition to possessing intrinsically

R

R

R

.

R'

(M=A1, Ga, In; E=P, As, Sb)

(M=AI, Ga, In; E=P, As, Sb)

Isolobal with alkenes

Isolobal with alkynes

interesting bonding schemes and stereochemical features, these two classes of compounds and their oligomers are potentially important as precursors to compound semiconductors. Devices fabricated from gallium arsenide, indium

Walsh et al.; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

PHOSPHORUS CHEMISTRY

58

phosphide, and related ternary compounds have a wide range of uses including field effect transistors, light emitting diodes, and photodetectors. However, the chemistry which underlies the organometallic chemical vapor deposition synthesis of GaAs and InP is approximately 30 years old (J). M e M + E H -» M E + 3CH M=Ga, In; E=P, As


3

3

4

Apart from toxic nature of PH3 and ASH3 and the pyrophoric character of Me3Ga, there are also problems with stoichiometry control and the relatively high reaction temperatures. In an effort to overcome some of these problems we have devised the concept of single source precursors (4). The single source precursors are typically rings or clusters in which the 13-15 bonds are already formed. The precursors also feature hydrocarbyl groups such as f-butyl, isopropyl, or ethyl that undergo facile alkene elimination via a β-hydride shift mechanism. So far, most of the GaAs film work has been carried out with 5 (5). Mass spectral studies on this compound indicate that the Ga:As stoichiometry is retained (6) and that hydrocarbyl ligand elimination starts as low as 375 C. Mass spectroscopic and isotopic labelling studies indicate that monomer formation takes place in the vapor phase. e

l-Bu

f-Bu

However,fromthe point of view of a synthetic chemical challenge it is the alkyne-analogous 13/15 compounds that are the most interesting. As shown below, there is a potentially rich variety of structural forms and bonding modes that includes alkyne-analogous (6), cyclobutadiene-analogous (7), and benzene-analogous (8) species, as well as higher oligomers. Previous compounds of empirical formula RMER' were confined to the light atoms B, Al, and Ν (7). There were no examples of compounds where both partners are heavier main group elements. Our initial attempts to prepare the desired compounds werefrustratedby the high Lewis acidities of the products. Our next attempt, which involved intramolecular base stabilization, proved to be successful. For this experiment we decided to use the 2,6bis(dimethylaminomethyl) benzene ligand which has been developed by van Koten and coworkers (8) in the Netherlands for the stabilization of unusual transition metal oxidation states and intermediates. A metathetical reaction of the lithium salt of this ligand with GaCl3 affords the pentacoordinate gallium derivative (9). In turn, treatment of this gallium dichloride derivative with the dilithium salt of Ph3SiPH2 affords the first base-stabilized diphosphagalletane (10) (9). As revealed by an X-ray crystallographic


4. COWLEY ET AL.

59


study, one amino group of each ligand is uncoordinated, hence the geometry at gallium is approximately tetrahedral. The central Ga2P2 core is planar and resides on a center of symmetry. The Ga-P bond distance is relatively short (2.338(1)A) (10). However, the phosphorus geometry is pyramidal, thus if there is any Ρπ-Ρπ delocalization in the ring, it is slight. R'

M

R

R


R' R

ir ι

Μ

R'

/

Ε

/

Ε

M

R

\

R

Jl

I

R

\

χ Γ ν

N

E

R

R

R

8

10

Our next objective was to prepare a base-free example of a diphosphadigalletane. To achieve this goal, it was necessary to develop a new synthetic approach. The reaction of (r-BuGaCl2)2 with the monolithium salt of (2,4,6-f-Bu C6H2)PH affords the bis(phosphido)gallane (11) (77). Sublimation of compound (11) or heating in toluene solution results in elimination of the primary phosphine and conversion to the first base-free diphosphagalletane (12). Compounds (11) and (12) have been characterized by X-ray crystallography. The yellow bis(phosphido)gallane adopts a trigonal planar geometry at gallium and the galliumphosphorus bond lengths, which average 2.324(5)Λ, are quite short (10). Although the P-H hydrogens were not detected (72), it is clear that the geometry at each 3

2



60

phosphorus atom is pyramidal. The observed conformation is the one which minimizes the steric interactions between the bulky aryl groups. The base-free diphosphadigalletane possesses a planar Ga2P2ringand a Ga-P bond distance that is extremely short (2.274(4)Â) (10). There is some flattening of the phosphorus pyramid - the sum of angles being 314.7°. However, the structure is not indicative of extensive Ρπ-Ρπ bonding.

/-Bu

(/-BuGaCI ) + 2ArPHLi Downloaded by UNIV OF CINCINNATI on May 19, 2016 | http://pubs.acs.org Publication Date: April 7, 1992 | doi: 10.1021/bk-1992-0486.ch004

2

2

• Ρ

/ Ar'

/

Ga

\

\

/ Η

Η

Ρ

\ Ar'

ί-Bu 11 Ga

/ ΑγΤ

^

/-Bu

\ Ρ-Αγ'

Ga

/

Ar' = /-Bu

/-Bu 12 The reaction of /-Bu2AlH with Ph3SiPH2 proved to be very interesting (13). The initially isolated product (13) resulted from the elimination of molecular hydrogen at room temperature. However, refluxing (13) in toluene for 12 hours caused isobutane elimination and conversion to thefirstaluminum-phosphorus cubane (14). The cubane structure was confirmed by X-ray crystallography which also revealed that the cube is slightiy distorted in the sense that the internal bond angles at phosphorus are < 90 , while those at aluminum slightly exceed 90*. The average aluminumphosphorus distance of 2.414(4)Â is slightly less than that observed in aluminumphosphorus dimers which span the range 2.433(4) to 2.475(1) (14). The aluminumphosphorus cubane is reactive to both electrophiles and nucleophiles. Preliminary thermolysis and X-ray photoelectron spectroscopic studies reveal that the aluminumphosphorus cubane is also a potentially useful, low temperature precursor to the wide band gap semiconductor, aluminum phosphide. Very recently, we have discovered that other cubanes can be formed via metathesis reactions. For example, the reaction of iPrlnl2 with the dilithium salt of Ph3SiPH2 affords thefirstindium-phosphorus cubane. Like gallium arsenide, indium phosphide crystallizes in the zinc blend (ZnS) structure. However, under high pressures indium phosphide undergoes a phase change to another, denser cubic modification. The new indium-phosphorus cubane might therefore serve as a model for the high pressure phase of indium phosphide. One final e


4. COWLEY ET AL


61

comment about the structure - like the aluminum-phosphorus cubane, the indiumphosphorus cubane is distorted. However, the average bond angle at phosphorus exceeds 90° in the indium compound.

H

SiPh

i-Bu

i-Bu

/

\/\

/ V \ Alf


3

Al

i-Bu

i-Bu Ph Si

Η

3

14

13

A characteristic feature of synthetic chemistry is the element of surprise. One such surprise was the formation of a novel gallium-phosphorus cluster in which a molecule of i-BuGaCl2 behaves as both a Lewis acid and a Lewis base (15). The reaction of the bis(tertiarybutyl)diphosphide anion with f-BuGaCl2 was expected to produce a three- or six-membered gallium-phosphorus ring, f-BuGa(P-f-Bu)2 or [f-BuGa(P-i-Bu)2]2- However, the mass spectrum indicated that the product contained an extra molecule of i-BuGaCl2. Moreover the P NMR spectrum showed two resonances of equal abundance. ί-Bu-Ga resonances in 2 : 1 abundance were evident in the H spectra. Elucidation of the structure needed an X-ray analysis. A convenient way to visualize the structure of (15) is to regard it as a complex of i-BuGaCl2 with the 3 1

l

R

R=/-Bu

15




62

six-membered gallium-phosphorusring.Two of the phosphorus atoms are involved in electron pair donation to the f-BuGaCl2 molecule and the bonding in the cluster is completed by the formation of CI bridges between r-BuGaCl2 and theringgallium atoms. In a formal sense, therefore, the r-BuGaCl2 molecule plays the role of both Lewis acid and Lewis base. In this context, the geometry around the Ga of the f-BuGaCh moiety is best described as approximately trigonal bipyramidal. It should be noted however that the Ga-Cl-Ga bridges are very unsymmetrical and that the Ga-Cl distances are very long (2.681(2)Â). The boat conformation of the Ga2P4ringis undoubtedly caused by the interations with the f-BuGaCl2 molecule. So far, much of the discussion has focussed on the bonding of phosphinidene units to aluminum, gallium, or indium. However, we are also interested in the coordination of phosphinidenes to transition metals - particularly in cases where the phosphinidene unit coordinates in a terminal fashion. As indicated below, terminal phosphinidene complexes, like imido complexes, can exist in angular or linear forms. LM n

p'

LM ^ = Ρ n

R

R Angular Linear Much elegant work has been done with terminal phosphinidenes as reactive intermediates - particularly by Mathey and coworkers (16). However, it was only in 1987 that Lappert and coworkers (17) reported thefirstangular terminal phosphinidene complex. We have recently succeeded in preparing thefirstlinear terminal phosphinidene complex WCl2(PMePh2)(CO)(sPAr') [(16), Ar'=2,4,6-fBU3C6H2]. US). The tungsten (II) phosphine complex, WCl2(PMePh2)4, undergoes oxidative addition via insertion into the phosphorus-carbon double bond of the phosphaketene (2,4,6-i-BuC6H)P=C=0. The 3lp chemical shift of (16) (δ+193.0) is relatively upfield, and the phosphorus-tungsten coupling constant is very large (649 Hz). These data suggest the presence of a triple bond between tungsten and phosphorus. This suggestion was confirmed subsequently by an X-ray analysis which revealed that the P-W bond distance is quite short (2.169(1)Â and the C-P-W angle is 168.2(2)°. 3

2

Acknowledgment We are grateful to the National Science Foundation and the Robert A. Welch Foundation for generous financial support.

Literature Cited (1) For a review, see Cowley, A.H.J.Organometal. Chem. 1990, 400, 71. (2) See, for example, Kopp, R.W.; Bond, A.C. Parry, R.W. Inorg. Chem. 1976,15,3042; Schultz, C.W.; Parry, R.W. Inorg. Chem. 1976, 15, 3046 and references therein. (3) Manasevit, Appl. Phys. Lett. 1968,12,156. (4) Cowley, A.H.; Jones. R.A. Angew. Chem. Int. Ed. Engl. 1989, 28, 1 (5) Cowley, A.H.; Benac, B.L.; Ekerdt, J.G.; Jones, R.A.; Kidd, K.B.; Lee, J.Y.; Miller, J.E. J. Am. Chem. Soc. 1988, 110, 6248.


63 4. COWLEY ET AL Phosphorus Atoms in Unusual Environments

(6) It is recognized that miniscule deviations from exact 1:1 stoichiometry can have major effects on the electrical and/or optical properties of the resulting semiconductor material. This aspect is under active investigation. (7) For representative examples, see Paetzold, P. Adv. Inorg. Chem. 1987, 31, 123; Nöth, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1603; Waggone K.M.; Hope, H.; Power, P.P. ibid. 1988, 27, 1699. (8) Van Koten, G. Pure Appl. Chem. 1989, 61, 1681. (9) Cowley, A.H.; Jones, R.A.; Mardones, M.A.; Ruiz, J.; Atwood, J.L.; Bott, S.G. Angew. Chem., Int. Ed. Engl. 1990, 29, 1150. (10) See reference 4 for a discussion of Ga-P bond distances. (11) Atwood, D.A.; Cowley, A.H.; Jones, R.A.; Mardones, M.A.J.Am. Chem. Soc. 1991, 113, 7050. (12) The presence of P-H bonds was confirmed by PNMR Spectroscopy: δρ=-110.3; J =203.5Ηz. (13) Cowley, A.H.; Jones, R.A.; Mardones, M.A.; Atwood, J.L.; Bott, S.G. Angew Chem, Int. Ed. Engl. 1990, 29, 1409. (14) Janik, J.F.; Duesler, E.N.; McNamara, W.F.; Westerhausen, M.; Paine, R.T. Organometallics 1989, 8, 506. For additional structural information on this class of compound, see Sangokoya, S.A.; Pennington, W.T.; Robinson, G.H.; Hrncir, D.C. J. Organomet. Chem. 1990, 385, 23. (15) Cowley, A.H.; Jones, R.A.; Mardones, M.A.; Atwood, J.L.; Bott, S.G. Angew. Chem., Int. Ed. Engl. 1991, 30, 1141. (16) For a review, see Mathey, F. Angew Chem. Int. Ed. Engl. 1987, 26, 275. (17) Hitchcock, P.B.; Lappert, M.F.; Leung, W.P. J. Chem Soc., Chem. Commun. 1987, 1282. (18) Cowley, A.H.; Pellerin, B.; Atwood, J.L.; Bott, S.G.J.Am. Chem. Soc. 1990,112,6734. RECEIVED December 10, 1991 31


1

PH


Phosphorus Atoms in Unusual Environments - American Chemical

Recommend Documents