Cationic Low Oxidation State Phosphorus and Arsenic Compounds

Chapter 8

Cationic Low Oxidation State Phosphorus and Arsenic Compounds Bobby D. Ellis and Charles L . B. Macdonald*

Downloaded by PURDUE UNIV on May 27, 2016 | http://pubs.acs.org Publication Date: December 1, 2005 | doi: 10.1021/bk-2005-0917.ch008

Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, Canada

Compounds containing main group elements in unusually low oxidation states exhibit structural features and reactivities that are significantly different from those of analogous compounds containing the elements in their more typical oxidation states. This work summarizes the investigations of cationic univalent group 15 compounds, with a particular focus on the research derived from the seminal work of Schmidpeter concerning P(I) cations. These unusually stable "triphosphenium" cations consist of a P center stabilized by two phosphine donors and have the general form [R P-P-PR ]+. Recently, interest in this type of compound has been re-kindled because of the unique modes of reactivity they may display. Improved synthetic strategies to such compounds, and their arsenic analogues, have been developed and current research exploits the unique chemistry of Pn(I) cations to produce unprecedented chemicals and materials. +

3

108

3

© 2006 American Chemical Society Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

109

Introduction Heavier group 15 elements (pnictogens; Pn = P, As, Sb, Bi) are generally found in either of their typical oxidation states: +3 or +5. There exists only a handful of types of compounds containing pnictogen atoms in the +1 oxidation state. Neutral examples include transient pnictemdenes (Pn-R), which must be stabilized by either a Lewis base or transition metal complex to prevent oligomerization to (Pn-R) rings or double-bonded dimers (R-Pn=Pn-R) (7-3). Examples of charged species containing +1 oxidation state pnictogen atoms are even more rare, typically consisting of a Pn ion stabilized by two Lewis bases, which are usually phosphines (4). The electronic structure of these cations is described by the various canonical forms illustrated in Figure 1. The nature of the bonding implied by the different models in Figure 1 range from a base-stabilized Pn ion having formal single Pn-P bonds (a) to a situation in which there is a double bond between the Pn ion and each Lewis base (b); in theory, the actual bond order would depend on the degree of back-bonding from Pn to the stabilizing ligands. Structural and computational evidence both indicate that there is a significant amount of Pn to ligand back-bonding in these types of molecules and that canonical form (c) is the most adequate description of the electronic structure. Such a description suggests that back-bonding effectively "oxidizes" the Pn(I) center (5) by removing excess electron density and helps to explain the relative stability of such cations as to the point that some of these Pn(I) salts are even air stable. In contrast to +3 oxidation state pnictogen cations, pnictogenium cations, R Pn , in which a significant amount of the positive charge is located on the pnictogen center (6), the positive charge in the +1 oxidation state cations is localized on the substituents. The various canonical structures depicted in Figure 1 suggest many modes of reactivity for these P(I) cations, several of which are distinct from those of the higher oxidation state analogues. Some examples of unique chemistry that has been investigated recently are described herein. x


+

+

+

+

2

Ρη Θ ν

R P

R P' © PR 3

3

R P

PR

3

3

3

(c)

Θ PR

3

Rί :Ρ 3 3

^PR

N

3

R P^ ^PR. 3

(b)

Figure 1. Canonical Forms of Pn Cations Stabilized by Phosphines.

Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

110 The following account outlines research stemming from the seminal work of Schmidpeter in the development of synthetic strategies for the generation of cations containing phosphorus atoms in the +1 oxidation state and the initial investigation of their reactivity. Recently, interest in these types of compounds has been re-ignited, both in terms of their synthesis and potential uses as reagents and as sources of P ions. Research in this area has since been extended to include arsenic analogues, which are also discussed. +


Early Work The initial work of Schmidpeter and co-workers on +1 oxidation state pnictogen cations was done exclusively for Pn = P. The first example of such a cation was synthesized by the reduction of PCI3 by SnCl in the presence of an equimolar amount of a chelating diphosphine, bis(diphenylphosphino)ethane (dppe), as shown in Scheme 1 (7). The structure of this cation was confirmed by X-ray crystallography and the P-P bond distances, 2.122(1) Â and 2.128(2) Â, are found to fall between those of P-P single bonds, Ph P-PPh : P-P = 2.217 Â (S), and P-P double bonds, Mes*P=PMes*: P=P = 2.046 Â (9). Such intermediate bond lengths suggest a degree of multiple bonding in this structure. The investigators later discovered that an additional equivalent of dppe improves the reaction by sequestering the SnCU as a dppe-SnCl by-product (4). 2

2

2

4

2 PCI3 + 2 SnCl + 2 2

P h

2 ^ p

p

p

h

2

P h a P ^ P P h a [SnCl ] + SnCl 6

4

2

Scheme 1 Schmidpeter and co-workers also demonstrated that, in certain cases, one additional equivalent of phosphine may be used to reduce phosphorus trichloride, instead of tin(II) chloride, as shown in Scheme 2. The stability of the salt is enhanced by either the presence of a chloride acceptor, such as A1C1 , or by a concurrent metathesis anion exchange with, for example, [NaftBPhJ (10,11). The tetrachloroaluminate salt of the [Ph P-P-PPh ] cation was structurally characterized and again the average P-P bond distance of 2.132 Â is intermediate between those of single and double P-P bonds (10). 3

+

3

3


Ill Cations stabilized by PPh may be converted to other cations by displacement of PPh by more basic phosphines to produce new symmetric and asymmetric cations (10). Schmidpeter and Lochschmidt also showed that cations such as these can undergo substitution of the phosphines by more basic anions, X", such as CN~, SnPh ", PPh " and POPh ", which generate either neutral R P=PX or anionic PX " molecules with P(I) centers (4,12). The symmetric P(I) stabilized cations contain a characteristic A X spin system, which allows for easy identification of their structure. The dicoordinate Ρ atom exhibits a triplet splitting pattern that has a chemical shift range of approximately δ = -156 to -261, depending on the identity of the stabilizing phosphines. The tetracoordinate Ρ atoms give rise to a doublet splitting pattern and range from δ = +12 to +104. The Jp. coupling constants range from 347 to 566 Hz for both symmetrical and unsymmetrical examples, and the Jp. coupling constants range from 15 to 41 Hz in the unsymmetrical cases (10,13). 3

3

3

3

2

2

2

2

l

P


2

PC1 + 3 P R + 2A1C1 3

3

3

P

^ [R P-P-PR ][A1C1 ] + [R PC1][A1C1 ] 3

R = Ph,NMe

3

4

3

4

2

PC1 + 3 PR + 2 [Na][BPh ] — — * [R P-P-PR ][BPh4] + [ R ^ a p P l ^ ] — 2 NaCl 3

3

4

3

3

Scheme 2 Acyclic "triphosphenium" cations may be oxidized by the addition of an organic chloride (or HC1) in the presence of an equivalent of A1C1 to produce dications as tetrachloroaluminate salts, in which die dicoordinate P(I) atom is oxidized from +1 to +3, as shown in Scheme 3A (14). With the exception of oxidation by HC1, cyclic cations were thought to be resistant to oxidization. The analogous cyclic dications may be produced, however, through chloride abstraction by two equivalents of A1C1 of a dichlorophosphine in the presence of a chelating phosphine, as shown in Scheme 3B (14). The oxidation of the dicoordinate Ρ atom results in a deshielding of about 100 ppm for the chemical shift of the central P(I) atom, ranging form δ = -23 to -157, and a shielding of 5 to 10 ppm of the tetracoordinate Ρ atoms in the stabilizing ligands. The % . coupling constants decreased to rangefrom239 to 358 Hz. It should be noted that the salt [Ph P-P(H)-PPh ][AlCl4]2 was characterized by X-ray crystallography and the most important change in the metrical parameters upon oxidation is the increase in the P-P bond lengths to 2.205(1) Â and 2.224(1) Â, which are indicative of single bonds (14). 3

3

Ρ

3

3


112 fR3P-P(R')-PR3][AlCl4]

[ R 3 P - P - P R 3 H A I C I 4 ] + R'Cl + AICI3

A

2

R' PH

P

RTC1 + 2 AICI3 + 2 2 ^

1

j™*

2

Ph P

PPh

2

[A1C1 ]

Β

4 2

2

\__/ Scheme 3 Schmidpeter and co-workers also demonstrated the nucleophilicity of the P(I) cations through coordination of the cation to A1C1 (11,15). Coordination of the Lewis acid results in a deshielding of about 50 ppm for the resonance of the P(I) atom, and broadening of the signal. The chemical shift of the P(III) atoms were also shielded by an additional 15 ppm One of these P(I) cations has been suggested to act as a source of P , as shown by the insertion of P into a C=C double bond, to generate 2phosphaallylic cations (16). It was speculated that the P(I) cation undergoes electrophilic attack by an electron-rich olefin, followed by sequential loss of the two phosphines to result in a phosphaallylic cation, as shown in Scheme 4 (R = NMe , R' = Me, Et). R' P(NMe ) R' Ν ^ N R . [BPh ] £ >=< +[R P-P-PR ][BPh ] P R Ν R' R'


3

+

+

2

2

3

3

3

4

4

3

R'

("ΥΎΛ

[BPh ] 4

V - N R ' R'N-—/ Scheme 4

Recent Developments Almost 10 years following Schmidpeter's initial synthesis of the first P(I) cation, in 1993 Gamper and Schmidbaur extended the SnCl reduction reaction to arsenic. By mixing equimolar amounts of a chelating diphosphine, tin(II) chloride and either arsenic or phosphorus trichloride, they synthesized both the 2


113 As(I) and P(I) cations (77). The arsenic compound was structurally characterized by single crystal X-ray diffraction. As with the phosphorus analogues, the As-P bond lengths of 2.250(1) Λ and 2.244(1) Â are intermediate between those of As-P single bonds, P(As(C(0) Bu)2)3: As-P = 2.305 Â (75), and double bonds, Mes*P=AsCH(SiMe ) : As=P = 2.124 Â (19). The phosphorus analogue may be deprotonated to form a zwitterionic species analogous to cyclic carbodiphosphoranes, as illustrated in Scheme 5. l

3

2

Ph Ph

Ph Ph 2 Base


,P

[SnCl ] 6

- [BaseH] SnCl 2

A

:Q©PPh C 2

e

Me, ^SiMe

2

ι

3

SiMe

LiCPRSi NaBPh Pn = P R = Ph

R = Ph

Pn = P

R = Me

PnCl

3

4

3

BP

Me P^PMe ^ Me Si— C© ®C-SiMe Me pl® ®PMe 2

2

3

2

ΚΡ °

2

Me Si-C^p(^C-SiMe Ρ Ρ Ph Ph 3

3

2

3

2

Scheme 6


114 Me 4 AsCl + 12 LiC(PMe ) (SiMe ) 3

2

2

Me P^ P^ ^ i As-As ι 2

-

3

_.,,

2

2

S l M e 3

C

Scheme 7 Ellermann and co-workers concurrently produced similar salts using an analogous nitrogen-based on the lithium reagent, LiN(PPh )2 (21). The reactions were performed using pnictogen triiodides and resulted in the successful synthesis of an eight-membered zwitterionic ring and a seven-membered cationic ring for arsenic. The cation was structurally characterized as the iodide salt and the distances between As(I) and P(III) were again consistent with partial multiple-bond character. Complete reduction to the element was also observed for Sb and Bi. In 2000, Dillon and co-workers continued to build on the seminal work of Schmidpeter. The group synthesized many new cyclic triphosphenium cations from P X (X = Cl, Br, I) based on P NMR data, and they structurally characterized the six-membered ring analogue of Schmidpeter's original fivemembered ring, also as the hexachlorostannate salt (22). They noted that the formation of the P(I) salt was observed whether or not a tin(II) halide was present in the reaction mixture. It was suggested that the reaction occurring involved the reduction of P X and the oxidation of dppe to either [dppeX][X] (Scheme 8A) or [dppeX ][X]2 (Scheme 8B) depending on the stoichiometry. The oxidized diphosphine is evident by peaks in P NMR spectra between δ = 66 to 30 ppm (for the chloro-systems) either as two doublets for [dppeX][X] or a singlet for [dppeX ][X] .


2

3I

3

3

2

31

2

2

P X + 2dppe

*

[(dppe)P]X + [dppeX][X]

2 P X + 3dppe

•

2 [(dppe)P]X + [dppeX ][X]

3

3

2

A 2

Β

Scheme 8 The following year, Dillon and co-workers reported additional P(I) cyclic cations and new cyclic As(I) cations (23). They again showed that the halide salts could be made through stirring of PnX (Pn = P, As) with diphosphines based on *P NMR data. However, only the hexachlorostannate salts of [(dppben)Pn] , Pn = P, As, dppben = l,2-bis(diphenylphosphino)benzene, and the unusual stannate C H (Ph POSnCl ) " salt of [(dppE)As] , dppE = cû-1,2bis(diphenylphosphino)ethene were structurally characterized. The Pn(I)-P(III) 3

3

+

2

2

2

2

5

+

2


115 bonds lengths all lie in the expected region intermediate between single and double Pn-P bonds. We have also recently reported a hexachlorostannate salt of a cyclic As(I), [(dppe)As] SnCl , the arsenic analogue to Schmidpter's first cyclic P(I) cation, prepared using his method (24). The X-ray crystal structure showed bond lengths or angles in good agreement with similar structures reported. The molecular structure is depicted in Figure 2.


2

6

Figure 2. Molecular Structure of [(dppe)As] [SnCl ]'. Hydrogen Atoms and One Molecule of CH2CI2 Have Been Removedfor Clarity. (AdaptedfromReference (24). Copyright 2004 Taylor & Francis.) 2

6

In our investigation of the synthesis of and chemistry iodide salts of P(I) and As(I) cyclic cations, however we have noticed that the oxidation of excess diphosphine is not part of the redox couple with the reduction of Pnl . Rather the interaction of Pnl with diphosphines, such as dppe, helps to promote the reduction of P(III) to P(I) and the oxidation of iodide to iodine, as shown in Scheme 9. P NMR spectra of the reaction mixtures exhibit no indication of either [dppel][l] or [dppel i[l] during the reaction (J). The iodine can be washed out of the system and colorless crystals of f(dppe)P][I] suitable for single crystal X-ray crystallography can be obtained by the slow evaporation of dichloromethane. The structure of one of the two independent molecules in the asymmetric unit is depicted in Figure 3. If the reaction is performed in donor solvents, such as THF or MeCN, there is evidence of the formation of the oxidized iodo-phosphonium iodide salt contaminants, presumably generated from the reactive iodine by-product. 3

3

31

2

2



116

Figure 3. Molecular Structure of [(dppe)P][I]. Hydrogen Atoms Have B Removedfor Clarity. (Adaptedfrom Reference (5). Copyright 2003 Ro Society of Chemistry.)

Pnl + Ph P^ 2

PPh

Ph P

2

3

2

Cationic Low Oxidation State Phosphorus and Arsenic Compounds

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