994 Inorganic Chemistry, Vol. 16, No. 5, 1977
Thomas, Schultz, and Parry Contribution from the Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Synthesis and Characterization of Dicoordinate Phosphorus Cations. Compounds of the Type [(R2N)2P]'[Y]- and Their Congeners MICHAEL G . THOMAS, CHARLES W. SCHULTZ, and R. W. PARRY'
Received August 9, 1976
AIC60580J
On the basis of 'H and 3'P NMR spectroscopy, infrared data, measurements of electrical conductivity, and chemical information, the compound ((CH3)2N)2PC1.A1C13is assigned the ionic structure [((CH3)2N)2P]t[A1C14]-. The related compound (CH3)2NPC12.A1C13is assigned the structure [(CH3)2NPC1]+[AlC14]-.Salts of ((CH3),N)2Pt containing counterions such as PF;, B2Ff, GaCl,, and FeC1; have been prepared along with the GaC1, salt of the (CH3)2NPC1+cation. The phosphorus in the cation (CH3)2NPCltis the most deshielded phosphorus atom yet recorded. It has a chemical shift of -325 ppm from H3P04. Both dicoordinate phosphorus cations are strong Lewis acids combining with a base such as (R2N)3Pto give previously described cations such as [(R2N)3P-P(NR2)Y]t where Y is NR2 or C1. The dicoordinate cations can also serve as ligands toward the metal atoms of metal carbonyls. Evidence for an N-P p r - p ~bond is found with (R2N)2PC.
In an investigation of the acid-base interactions of chloQuestions now remain relative to the structure of the two ro(dimethylamido)phosphorus(III) with aluminum(II1) remaining monoadducts: (CH3)2NPCl2.A1Cl3and ((Cchloride some anomalies of both stoichiometry and structure H3)2N)2PCl.AlC13.Traditional acid-base theory suggests a were encountered. The following compounds could be predirect N-AI or P-A1 bond. In fact, the comparable phosphorus pared:' (CH3)2NPC12.AlC13, ((CH3)2N)2PC1.A1C13, 2trichloride-aluminum chloride addition compound Cl3PA1Cl3 is known4 to have the expected P-A1 donor-acceptor bond. ((CH3)2N)2PC1*A1C13,(((CH3)zN)2PC1)(((CH3)2N)3PJ'A1C13, and ((CH3)2N)3P.A1C13.It was significant that the 2:l adAnother possibility exists, however. By analogy to the structure ducts 2((CH3)2N)3P-A1C13 and 2(CHJ2NPCl2.A1Cl3could assigned to ((CH3)2N)3P.A1C13one might write ((CH3)2not be prepared even though the intermediate 2:l adduct N)2PCl-A1C13as [((CH3),N)3P-P(N(CH3),),1+[A12C1,1-. 2((CH3)2N)2PC1.A1C13 was a stable well-defined species. This structure, involving chloride transfer, suggests still a The foregoing facts raised interesting structural questions. different possibility. Transfer of the chloride ion from In formulating these molecules, normal AlC13addition to either ((CH3)2N)2PClto A1Cl3 would generate a dicoordinate the phosphorus or nitrogen lone pair of electrons would be phosphorus cation which can be represented as anticipated. Indeed, Sollott and Peterson2 invoked a nitrogen-aluminum bond in their description of (CH3)2NPC12. AlC13. Such a representation did, however, pose real structural F C N (1.P: Hl LAICI41problems in describing the 2:1 adduct 2((CH3)2N)2PC1.A1C13 unless a five-coordinate aluminum atom were postulated. If this postulate were made, it was difficult to understand why the compound 2((CH3)2N)3P.A1C13could not be made. "A The structural puzzle of the 2: 1 adducts was solved3when it was found that 2((CH3)2N)2PCl.AlC13had the unexpected For the related complex (cH3)2NPC12-A1C13 the structure ionic structure would be
+
L
[
(CH3)zNP-
1
1
~
~ [AIC1433 7
while the mixed diadduct (((CH3)2N)2PCl)(((CH3)2N)3PJ' [The symbol : next to the phosphorus atom in the dicoordinate AlC13 had the related structure cation indicates a free electron pair which would be stereochemically active.] In this paper data obtained from conductivity measurements, NMR spectroscopy, and infrared spectroscopy are used to support the representation involving the dicoordinate phosI phorus cations ((CH,),N),P+ and (CH3)2NPCl+. The N(CH3)z A ( C H , d structure given for ((CH,),N),P+ is very similar to that of the cyclic cation The monoadduct ((CH3)2N)3P..41C13 was found3 to contain a similar cation but a quite different anion which accounted for our failure to make the 2:l adduct with this base. The structure of the monoadduct was
L
The above anion appears to be unstable and decomposes to AlClL and [Cl2A1NR2I,.
first identified as the PFL salt in an excellent and definitive study by Fleming' and her students. While structural similarities are pronounced at low temperatures, the noncyclic compound is significantly more reactive than its cyclic counterpart. This is particularly true at ambient temperatures
Dicoordinate Phosphorus Cations
Inorganic Chemistry, Vol. 16, No. 5, I977 995
Table I. Equivalent Conductance of lo-' M Solutions of Compounds in Methylene Chloride at 25 "C
Compd
,
AlCl ((CH, ), N),PCI
Equiv conductance, cm2
Compd
Equiv conductance, cm2
0.21 f 0.01"
