A conformational study of phospha(III)- and phospha(V)-guanidine compounds

Published on Web 10/03/2006

A Conformational Study of Phospha(III)- and

Phospha(V)-guanidine Compounds

Natalie E. Mansfield, Joanna Grundy, Martyn P. Coles,* Anthony G. Avent, and

Peter B. Hitchcock

Contribution from the Department of Chemistry, UniVersity of Sussex, Falmer,

Brighton BN1 9QJ, UK

Received June 15, 2006; E-mail: m.p.coles@sussex.ac.uk

Abstract: Spectroscopic, crystallographic, and computational studies of the substituent distribution about

the “NCN” unit in a series of phospha(III)- and phospha(V)-guanidines, R₂PC{NR′}{NHR′} and R₂P(E)C-

i

{NR′}{NHR′} (R ) Ph, Cy; R′ ) Pr, Cy; E ) S, Se), are reported. In the phosphorus(III) systems, the

P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be

the E_syn-(R) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of

E_syn-(R) and Z_antiin solution. Spectroscopic techniques are unable to determine whether the latter isomer

exists as the R- or â-conformer relative to rotation about the P-C_amidinebond; however, DFT calculations

indicate a low-energy structure for the N,N′-dimethyl model complex in the â-conformation. In their oxidized

sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture

of the E_syn-(â) and Z_syn-(â) isomers in solution (∼3:2 ratio), whereas for the P-dicyclohexyl analogues, the

latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement

about the imine double bond) is the dominant form. Intramolecular E‚‚‚HN (E ) S, Se) interactions are

observed in solution for the Z_syn-(â) configuration of both P-substituted species, characterized by J_SeHcoupling

in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the

infrared spectrum. An X-ray crystallographic analysis of representative Ph₂P(E)- and Cy₂P(E)-substituted

species shows exclusively the E_syn-(â) configuration for the P-diphenyl substituted compounds and the

Z_syn-(â) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents.

Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the

observed preferences in substituent orientation, suggesting that steric factors play an important role in

determining the subtle energetic differences at work in these systems.

Introduction

Amidines and guanidines comprise two closely related

families of organic molecules with general formula R⁽¹⁾C-

{NR⁽²⁾}{NR⁽³⁾R⁽⁴⁾} (amidine: R⁽¹⁾) hydrogen, alkyl, aryl;

guanidine: R⁽¹⁾) amido; R^(2)-(4)) hydrogen, alkyl, aryl, silyl)

which, by virtue of their multiple functionality and high level

of substituent tunability, have attained prominence in a number

of different fields of chemistry. Much of the drive to develop

new substitution patterns around the amidine and guanidine

frameworks derives from their widespread application as ligands

in coordination chemistry.¹In this context, significant work has

focused on the tri- and tetra-substituted amidine and guanidine

compounds, respectively (i.e., R⁽⁴⁾) H), where retention of a

single NH functionality facilitates conversion to the correspond-

ing amidinate and guanidinate anions, which readily coordinate

to metals from throughout the periodic table.²

Figure 1. The four configurations of N-substituents within a trisubstituted

amidine unit.

arrangements and, when a hydrogen substituent is present,

tautomeric arrangements about the ligand framework (Figure

1).³As current interest is moving toward the application of these

families of compounds as reagents in asymmetric systems (e.g.,

organocatalysis,⁴anion recognition⁵), factors which determine

the relative stability of these different conformers are likely to

play increasingly important contributions in the intelligent design

of new reagents during the development of these areas.

As part of our continued interest in the coordination chemistry

of neutral amidines and guanidines,^6,7we have incorporated

additional phosphorus-based functionality within the general

One of the often overlooked yet fundamental features

associated with both classes of neutral compounds is the ability

of the nitrogen substituents, R^(2)-(4), to adopt different isomeric

(3) Ha¨felinger, G.; Kuske, F. K. H. In The Chemistry of Amidines and Imidates;

Patai, S., Rappoport, Z., Eds. Wiley: Chichester, 1991; Vol. 2.

(4) Ko¨hn, U.; Gu¨nther, W.; Go¨rls, H.; Anders, E. Tetrahedron: Asymmetry

2004, 15, 1419.

(1) Coles, M. P. Dalton Trans. 2006, 985.

(2) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219-300; Edelmann,

F. T. Coord. Chem. ReV. 1994, 137, 403; Bailey, P. J.; Pace, S. Coord.

Chem. ReV. 2001, 214, 91.

(5) Schug, K. A.; Lindner, W. Chem. ReV. 2005, 105, 67.

9

J. AM. CHEM. SOC. 2006, 128, 13879-13893

13879

A R T I C L E S

Mansfield et al.

Scheme 1. Synthetic Procedures Employed during This Study (a,

Scheme 2. Schematic Description of the (R) and (â) Conformers

in Phospha(III)guanidines (X ) Lone Pair, Chalcogen)

i

R′ ) Pr; b, R′ ) Cy; { } Denotes Previously Reported

Compounds That Are Included in This Report for Comparative

Purposes)^a

^aReagents and conditions: (i) ⁿBuLi, THF, -78 °C; (ii) R′NdCdNR′,

THF, 0 °C; (iii) [HNEt₃][Cl], THF, 25 °C; (iv) elemental E (E ) S, Se),

toluene, 25 °C

amidine framework at the central carbon position. A series of

compounds with general formula R₂PC{NR′}{NHR′} have been

prepared,^8,9which are referred to as phospha(III)guanidines by

analogy to their nitrogen-substituted guanidine relatives, R₂-

NC{NR′}{NHR′} (Scheme 1). We have recently extended this

work to prepare di(phospha(III)guanidine) compounds that have

been used in the formation of bimetallic species and have been

shown to function as a chelating diphosphine-type ligand at late

transition metals.¹⁰

Conversion of the phosphaguanidines to the corresponding

P(V)-chalcogeno analogues has been achieved in a straightfor-

ward manner by employing elemental sulfur and selenium to

give the corresponding sulfo- and selenophospha(V)guanidines

(Scheme 1).¹¹During studies of the oxidized species {3a,b}

and {4a,b}, variation in the geometric arrangement of the

nitrogen substituents in solution was noted. It was established,

using multinuclear NMR spectroscopy, that at room temperature

two interchanging species were present that corresponded to

an equilibrium mixture of the E_synand Z_synconfigurational

isomers. This observation was rationalized in terms of the

formation of an attractive, “nonbonded” interaction between the

chalcogen and the amino NH, locking the amidine in position

by restricting rotation about the P-C bond. As a result, steric

conflict between the imine and phosphorus substituents becomes

a major factor in determining the relative stability of the different

configurations, causing the CdN double bond to isomerize to

relieve unfavorable interactions. Consideration of the electronic

contribution to this rearrangement was not probed in detail,

although reference was made to the zwitterionic nature of

the phosphorus-chalcogen bond facilitating formation of the

NH‚‚‚EdP interaction.

atom under consideration. The resultant value gives an estima-

tion of the deviation of the three bond angles from the full angle

(normalized to 90°) ranging from 0% (planar) to 100% for

maximum pyramidalization, achieved when the sum of the three

angles is equal to 270°, corresponding to three mutually

perpendicular 2p_Natomic orbitals. Examples of structurally

characterized guanidines in which the tertiary amino substituents

do not form part of a heterocycle (thus imposing steric

restrictions on the displacement of these groups) are rare.¹³The

bond parameters for these compounds, in addition to examples

in which the guanidine is coordinated to a metal center,^7,14all

indicate a strong tendency for the formation of planar tertiary

amino nitrogen atoms [DP values in the range 0-7.9%],

consistent with significant delocalization of electron density into

the “NCN” component of the molecule. The equivalent overlap

between the phosphorus lone pair and the sp²-carbon in phospha-

(III)guanidines is energetically unfavorable, resulting in a

pyramidal arrangement of P-substituents as noted in the solid-

state structures of {1a} [DP ) 62.7%] and {1b} [DP ) 60.1%].

When considered alongside the localized carbon-nitrogen

bonding within the “NCN” unit, which renders the nitrogen

groups inequivalent, this pyramidal geometry introduces the

possibility of conformational isomers within the phosphaguani-

dine compounds, denoted R and â in this work (Scheme 2).

The occupation of the fourth vertex of the pyramidal phosphorus

by the chalcogen atom in the P(V) derivatives {3a,b} and {4a,b}

leads to the same geometrical considerations when discussing

these systems.

To be distinguishable from each other using standard analyti-

cal techniques, certain limitations regarding fluxional processes

need to be met: (a) restricted (or slow) rotation about the R₂P-

CN₂bond; (b) a high barrier to inversion for phosphorus in

the P(III) derivatives; (c) a slow rate of hydrogen exchange

between the different nitrogen atoms, particularly relevant in

the case of the E_anticonfiguration. The energetics of interchange

between these forms will be influenced by not only the steric

A convenient parameter that may be used to compare the

(non)planarity of three-coordinate atoms is the degree of

pyramidalization DP(%),¹²defined as:

3

[360 - R]

∑

i)1

DP(%) )

0.9

(10) Mansfield, N. E.; Coles, M. P.; Avent, A. G.; Hitchcock, P. B. Organo-

metallics 2006, 2470.

where the sum covers the three smallest bond angles, R_i, of the

(11) Grundy, J.; Coles, M. P.; Avent, A. G.; Hitchcock, P. B. Chem. Commun.

2004, 2410.

(6) Oakley, S. H.; Coles, M. P.; Hitchcock, P. B. Inorg. Chem. 2003, 42, 3154;

Oakley, S. H.; Coles, M. P.; Hitchcock, P. B. Inorg. Chem. 2004, 43, 5168;

Oakley, S. H.; Soria, D. B.; Coles, M. P.; Hitchcock, P. B. Polyhedron

2006, 25, 1247.

(7) Oakley, S. H.; Soria, D. B.; Coles, M. P.; Hitchcock, P. B. Dalton Trans.

2004, 537.

(8) Coles, M. P.; Hitchcock, P. B. Chem. Commun. 2002, 2794.

(9) Grundy, J.; Coles, M. P.; Hitchcock, P. B. Dalton Trans. 2003, 2573.

(12) Maksiæ, Z. B.; Kovae`evic, B. J. Chem. Soc., Perkin Trans. 2 1999, 2623.

(13) Heinisch, G.; Matuszczak, B.; Rakowitz, D.; Mereiter, K. J. Heterocycl.

Chem. 2002, 39, 695; Ghosh, R.; Samuelson, A. G. Chem. Commun. 2005,

2017.

(14) Birch, S. J.; Boss, S. R.; Cole, S. C.; Coles, M. P.; Haigh, R.; Hitchcock,

P. B.; Wheatley, A. E. H. Dalton Trans. 2004, 3568; Coles, M. P.;

Hitchcock, P. B. Eur. J. Inorg. Chem. 2004, 2662.

9

13880 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

corresponding phosphorus resonance at δ -21.2 (Figure 2b).

Selective decoupling of the other septet at 4.39 ppm, corre-

sponding to the N_aminemethine of the major species, results in

the collapse of the NH resonance at δ 3.59 to a single broad

resonance (Figure 2c), indicating the origin of this doublet is a

³J_HHcoupling rather than coupling to phosphorus. It was also

observed during this experiment that the NH signal of the minor

component, present as a pseudotriplet corresponding to an

unresolved doublet of doublets at δ 3.91, collapsed to a doublet

indicating the N_aminemethine resonance of the minor species is

coincident with the septet at 4.39 ppm. The additional compo-

nent that contributes to the splitting of the NH resonance was

shown to be a ³J_HPcoupling of similar magnitude, demonstrated

by decoupling the minor phosphorus resonance at δ -4.8 (

Figure 2d).

A large, positive nOe between the amino proton and the

P-cyclohexyl substituents in the major isomer is consistent with

the E_syn-(R) configuration predominating in solution. Similar

experiments indicate an interaction between the NH and the

N_imineisopropyl group in the minor isomer, suggesting a Z_anti

arrangement of substituents, although assignment to the specific

R- or â-conformation form is not possible solely on the basis

of these data. These observations differ to those made for the

P-diphenyl compounds {1a,b}, where only the E_syn-(R) is

observed in solution over the temperature range 298-333 K,

giving the first indication that, despite being remote from one

another, the phosphorus substituents exert an influence on the

relative energies of the different configurations of the amidine

unit.

Irradiation of the N_iminemethine resonance of 2a induces a

large negative nOe of the corresponding methine resonance of

the N_aminesubstituent, signifying an exchange process between

these two groups. Since the barrier to inversion at phosphorus

is a relatively high-energy process (typically in the range 35-

38 kcal mol^-1),¹⁵a mechanism by which this is occurring is

proposed involving a hydrogen exchange between the two

N-atoms. While it is possible that an intermolecular process may

be taking place involving two (or more) molecules of 2a, no

concentration dependence was observed for this interconversion,

suggesting an intramolecular process. The corresponding hy-

drogen-exchange process in P-diphenyl derivative {1a} is only

observed upon heating the sample to 333 K, consistent with a

higher-energy process. Variable-temperature NMR spectroscopy

performed on 2a shows a change in the ratio of 14:1 noted at

298 K to 9:1 at 343 K. Van’t Hoff analysis of the data gave a

value for ∆H of 1.9 kcal mol^-1, with the corresponding entropy

value of +1.2 eu. Interpretation of these data is, however,

complicated by the existence of greater than one fluxional

process taking place within these compounds (vide infra).

Scheme 3 summarizes proposed mechanisms for the fluxional

processes observed for 2a in solution. In Pathway 1 {proton

exchange}, rotation of the C-N single bond initially converts

the E_syn-(R) form to the E_anti-(R), followed by proton transfer

between the two nitrogen atoms. This mechanism is reasonable,

considering steric terms, as it has been shown that the anions

of 2a,b are able to adopt an N,N′-chelating coordination mode

Figure 2. Results from selective decoupling experiments in ¹H NMR

spectroscopy for 2a, showing the isopropyl methine resonances of the major

isomer, and NH resonances for both species present in solution: (a) full

spectrum; (b) major phosphorus resonance decoupled; (c) N_aminemethine

resonance decoupled; (d) minor phosphorus resonance decoupled.

properties of the substituents, but also the NH‚‚‚EdP nonbonded

interactions previously noted in the phospha(V)guanidine

system.

This paper describes the previously unreported P-dicyclohexyl

derivatives of the P(III)- and P(V)phosphaguanidines, enabling

detailed structural comparisons to be made with the P-diphenyl

compounds. The results from a combined spectroscopic and

crystallographic study have enabled further understanding of

the key features that influence the configurational preferences

to be established. In addition, DFT studies have been conducted

to probe the electronic factors involved in determining the

relative stabilities of different arrangements of substituents.

Results and Discussion

Synthesis and Spectroscopic Analysis of Phospha(III)-

guanidines, R₂PC{NR′}{NHR′}. The general synthetic meth-

odology that we have employed in the synthesis of P-diphe-

nylphospha(III)guanidines,⁹has been extended to encompass

the corresponding P-dicyclohexyl analogues, Cy₂PC{NR′}-

i

{NHR′} [2a, R′ ) Pr; 2b, R′ ) Cy, Scheme 1]. The neutral

compounds were isolated as white, crystalline solids in high

yield (75-80%) by crystallization from concentrated hexane

at -30 °C. In contrast to the P-diphenyl analogues {1a,b},

multinuclear NMR analysis of analytically pure samples indi-

cated the presence two isomers in solution, most evident from

³¹P{¹H} NMR data [2a (14:1 ratio), δ_major-21.2; δ_minor-4.8.

2b (12:1 ratio), δ_major-21.1; δ_minor-4.1]. The NⁱPr substituted

compound, 2a, was selected for a detailed NMR spectroscopic

analysis due to the simplified spectra in comparison to the per-

cyclohexyl derivative, 2b; comparable solution-state behavior

is, however, observed for each compound and it is therefore

reasonable to assume similar fluxional processes operating for

both compounds.

The room temperature ¹H NMR spectrum of 2a indicates that

each isomeric form has inequivalent nitrogen substituents,

consistent with localized CdN double and C-N single bonds

within the amidine unit [(a), Figure 2]. The septet corresponding

to the N_iminemethine proton of the major isomer at δ 4.68

exhibits additional coupling to phosphorus, demonstrated by

simplification of this multiplet upon selective decoupling of the

(15) Lehn, J. M.; Munsch, B. J. Chem. Soc., Chem. Commun. 1969, 1327;

Baechler, R. D.; Farnham, W. B.; Mislow, K. J. Am. Chem. Soc. 1969, 91,

5686; Baechler, R. D.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 3090;

Mislow, K.; Baechler, R. D. J. Am. Chem. Soc. 1971, 93, 773; Fryzuk, M.

D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928.

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13881

A R T I C L E S

Mansfield et al.

Scheme 3. Possible Mechanisms Allowing Interconversion of the Different Isomeric Forms of Phospha(III)guanidines in Solution

Table 1. Crystal Structure and Refinement Data for 2a, {3b}, and {4b}

2a

{3b

}

{4b}

formula

C₁₉H₃₇N₂P

324.48

C₂₅H₃₃N₂PS

424.56

C₂₅H₃₃N₂PSe

471.46

formula weight

temperature (K)

wavelength (Å)

crystal size (mm)

crystal system

space group

a (Å)

173(2)

223(2)

0.71073

0.25 × 0.20 × 0.15

orthorhombic

Pbca (No.61)

9.9734(3)

24.3803(7)

16.6163(4)

90

0.35 × 0.30 × 0.25

monoclinic

P2₁/c (No.14)

14.3934(2)

9.6078(2)

17.3407(3)

90

0.30 × 0.30 × 0.30

triclinic

P1h (No.2)

8.8687(1)

9.6892(1)

14.9910(2)

82.172(1)

82.934(1)

69.345(1)

1190.24(2)

2

b (Å)

c (Å)

R (deg)

â (deg)

90

4040.3(2)

8

1.07

0.14

3.46 to 26.02

21806

3964

100.495(1)

90

2357.91(7)

4

1.20

0.22

3.93 to 25.03

23714

4118

γ (deg)

V (Å³)

Z

D_c(Mg m^-3

)

1.32

1.66

3.73 to 27.88

15459

5599

absorption coefficient (mm^-1

)

θ range for data collection (deg)

reflections collected

independent reflections

[R_int) 0.074]

[R_int) 0.065]

[R_int) 0.030]

reflections with I > 2σ(I)

data/restraints/parameters

goodness-of-fit on F²

2902

3539

5129

3964/0/203

1.002

4118/0/266

1.031

5599/0/266

1.040

final R indices [I > 2σ (I)]

R1 ) 0.056

wR2 ) 0.136

R1 ) 0.082

wR2 ) 0.153

1.15 and -0.22

R1 ) 0.035

wR2 ) 0.082

R1 ) 0.044

wR2 ) 0.087

0.29 and -0.27

R1 ) 0.025

wR2 ) 0.056

R1 ) 0.030

wR2 ) 0.059

0.34 and -0.32

R indices (all data)

largest diff. peak and hole (e Å^-3

)

at early transition-metal elements,¹⁶which necessarily involves

formation of an E_antiarrangement of substituents. Rotation about

the C-N and C-P bonds regenerates the initial configuration

via the corresponding â-conformer. Pathway 2 {E_syn- Z_anti

interconversion} involves redistribution of electron density and

formation of a zwitterionic intermediate that can conceivably

occur from a number of starting configurations. As an illustra-

tion, Scheme 3 shows the process proceeding via the E_anti-(R)

conformer, leading to the Z_antiisomer, which is able to convert

between the R- or â-conformers via P-C rotation. Using

spectroscopic techniques alone, it has not been possible to

identify from which configuration such an electronic rearrange-

ment is likely to occur, or indeed to determine whether Pathways

1 and 2 are independent from one another.

Crystallographic Characterization of Cy₂PC{NⁱPr}{NHⁱPr}

(2a). The molecular structure of 2a is illustrated in Figure 3,

crystal structure and refinement data are collected in Table 1,

and bond lengths and angles, along with those of {1a} and {1b}

for comparison, are presented in Table 2.

As observed previously in the structurally characterized

P-diphenyl examples {1a,b},⁹compound 2a exists as the

monomeric species in the solid state, in which a carbon-bound

(16) Mansfield, N. E.; Grundy, J.; Coles, M. P.; Hitchcock, P. B. 2006.

Manuscript in preparation.

9

13882 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

{1b}, respectively. In our previous communication we postu-

lated that this seemingly sterically unfavorable twist was due

to an attractive interaction between the amino proton and the

aromatic π-cloud of the phenyl substituent. In an attempt to

probe the causes behind this structural characteristic, and other

features associated with this class of molecules, DFT calcula-

tions have been carried out on representative model complexes.

DFT Studies on Model Phospha(III)guanidine Com-

pounds. Examples of model phospha(III)guanidine compounds

have been examined using the B3LYP density functional theory

and the 6-31g(d) basis set, implemented by Gaussian 03.¹⁷

Initial calculations were performed on the E_syn-(R) isomers of

the N-methyl substituted models Ph₂PC{NMe}{NHMe} (A) and

Cy₂PC{NMe}{NHMe} (B), where the reduced complexity of

the nitrogen substituents was employed to lower their steric

impact, thus highlighting any significant electronic contributions

to the calculated geometries. Discrepancies between the bond

lengths and angles of the fully optimized geometries and the

observed crystal structure data were typically within a 2%

margin of error. It was noted, however, that the computed

geometry at the N_amineatom indicated a greater pyramidalization

than observed in the crystallographic data, as shown by the DP-

(%) values¹²[A ) 5.5 cf. 1a ) 0.4 and 1b ) 0.3; B ) 9.6 cf.

2a ) 1.9], which may reflect an underestimation in the

delocalization of the nitrogen lone pair within the amidine

unit.

Figure 3. ORTEP representation of the molecular structure of Cy₂PC{Nⁱ-

Pr}{NHⁱPr} (2a). Ellipsoids drawn at the 30% probability level; hydrogens

except NH omitted.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for {1a,b}

and 2a (labels from Figure 3)

{

1a}

{1b

}

2a

Bond Lengths

1.882(3)

P-C(1)

1.878(2)

1.286(2)

1.364(2)

1.833(2)

1.831(2)

0.078

1.882(2)

1.278(3)

1.375(3)

1.871(2)

1.874(2)

0.097

C(1)-N(1)

C(1)-N(2)

P-C(R₁)

P-C(R₂)

1.268(3)

1.372(3)

1.835(3)

1.835(2)

0.104

∆

CN

Bond Angles

N(1)-C(1)-N(2)

N(1)-C(1)-P

N(2)-C(1)-P

C(1)-P-C(R₁)

C(1)-P-C(R₂)

C(R₁)-P-C(R₂)

120.2(2)

120.56(19)

122.97(14)

116.46(14)

102.05(9)

100.25(9)

103.44(9)

118.39(18)

125.40(16)

116.21(14)

99.66(9)

99.04(9)

104.71(9)

124.07(19)

115.63(18)

99.92(12)

101.80(11)

101.81(11)

In both cases A and B, the HOMO contains considerable lone-

pair character at the phosphorus atom, in agreement with our

recent experimental results in which it has been shown that these

ligands will coordinate as Lewis bases via the phosphorus to

late transition-metal centers.⁸Despite the apparent underestima-

tion in the delocalization of the N_aminelone pair inferred from

the geometry at this atom, a significant stabilization arises from

overlap with the CdN_imineπ*-orbital in both models, while the

main stabilizing influence associated with the N_iminelone pair

is from overlap with the P-C σ*-orbital. In A, stabilization of

the phosphorus lone pair arises predominantly through delo-

calization into the antibonding orbitals of the phenyl substituents,

which accounts for the observed difference in natural charge

on the phosphorus atom of A [+0.83] compared with B [+0.78],

where no such delocalization is possible in this system.

amidine unit is present with an E_syn-(R) configuration of

substituents. The hydrogen atom on the amino nitrogen was

located and refined. There are no significant differences noted

in the bond lengths upon replacement of the P-phenyl substit-

uents with cyclohexyl groups, with retention of the localized

C-N single and CdN double bonds (∆_CN) 0.097 Å),³

consistent with the solution-state structure. In 2a the amidine

unit is rotated such that it lies in the sterically most favorable

position with respect to the phosphorus substituents, with

N(1)-C(13)-P-Cy torsion angles of +54.8° and -51.9° and

the amino substituent located between the two cyclohexyl

substituents (Figure 4). This is in contrast to the corresponding

rotational position of the amidine group in the P-diphenyl

analogues, which are aligned with one of the P-Ph bonds

[N_amine-C-P-Ph torsion angles: {1a} 0.0° and -104.4°; {1b}

10.6° and -95.7°] (Figure 4), with short intramolecular

NH‚‚‚C_ipsodistances of 2.33 Å and 2.42 Å noted for {1a} and

The origin of the preferentially canted amidine unit in the

P-diphenyl derivatives (Figure 4) was examined using model

compound A. Using an initial conformation in which the

amidine unit had been rotated about the P-C bond such that it

bisected that Ph-P-Ph angle [N_amine-C-P-C_ipsotorsion

angle: (53.14°] and allowing full optimization, the molecule

minimized to a geometry in which the N_aminegroup eclipsed

one of the phenyl substituents [final torsion angles: -9.31° and

-114.92°], indicating that the preference for this orientation

cannot be solely a result of crystallographic forces in the solid

state. NBO analysis of the minimized structure did not indicate

any significant interaction between the NH and the phenyl

substituents in A, as postulated in the original publication.

However, it is noted that molecular orbitals 54a and 53a

(corresponding to the HOMO-14 and HOMO-15) indicate

overlap between the amino group and the π-system of the

Figure 4. View of the core atoms of compounds {1a,b} and 2a showing

the relative rotation of the amidine unit about the P-C vector.

(17) Frisch, M. J.; et al. Gaussian 03, revision 03W; Gaussian, Inc.: Wallingford

CT, 2004.

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13883

A R T I C L E S

Mansfield et al.

N-Cy compound {4b} was selected for a detailed NMR study.

1

The H NMR spectrum of {4b} (Figure 7) contains two low-

field multiplets (a and b), corresponding to the ortho C₆H₅

protons of two distinct species that were shown, through spin

saturation transfer experiments, to form an equilibrium mixture.

Further evidence for these two species is given by four resolved

resonances for the R-H of the cyclohexyl groups (e-h),

consistent with inequivalent N-substituents within the amidine

unit of each form. The major NH resonance (d) at δ 5.42 is

present as an unresolved doublet of doublets, exhibiting three-

bond coupling of similar magnitude to both phosphorus and

the R-CH of the N_amine-substituent; spin saturation transfer

experiments indicate this peak to be in exchange with an

additional pseudotriplet at δ 6.96 (c), obscured by the aromatic

resonances. Selective decoupling experiments were implemented

to pair the N_imineand N_amineR-CH resonances, and while

selective irradiation of the multiplet at δ 3.45 gave a signal

enhancement for the corresponding multiplet at δ 3.65, no such

enhancement was observed between the remaining two signals,

due to more pronounced localization of the carbon-nitrogen

single and double bonds in this isomer. nOe experiments showed

that, upon irradiation of the ortho phenyl protons (a), the only

enhancement observed corresponded to other aromatic reso-

nances, while irradiation of (b) resulted in enhancement of both

(d) and (f), corresponding to NH and R-CH groups, respectively.

This suggests free rotation about the P-CN₂bond in one of

the solution-state species and a “locked” geometry in the other.

The ³¹P{¹H} NMR spectrum of {4b} shows two well-

separated resonances at δ 36.0 and δ 24.3 in 1.0:1.3 ratio (Table

3). Each peak displays selenium satellites with J_SePvalues of

-721 and -753 Hz, respectively; the corresponding doublets

in the ⁷⁷Se{¹H} NMR spectrum resonate at δ -223.6 (-721

Hz) and δ -310.9 (-753 Hz). It is noted that in the ¹H coupled

⁷⁷Se experiment, the lower frequency resonance exhibits a

further coupling of 6 Hz, which is removed upon selective

decoupling of the NH resonance (c). The magnitude of this

coupling is less than predicted for a direct Se-H bond (typical

Figure 5. MO54a and MO53a of A (constructed with isovalues at 0.02)

indicating positive overlap (*) between the amino component of the amidine

and the π-system of an aromatic substituents on phosphorus.

proximate phenyl substituents that may contribute to the

stabilization of this arrangement (Figure 5).

To help further our understanding of the fluxional processes

determined to be occurring in solution (vide supra), it was of

interest to compare the relative energies the eight configurations

illustrated in Scheme 2. Full geometry optimizations were

performed for model compounds A and B in which the torsion

angle for the amidine unit was arbitrarily set to approximate

the (R) and (â) conformations (Figure 6). In agreement with

the experimental data, the E_syn-(R) conformer proved to be the

minimum energy structure for both A and B, with the highest

energy calculated for the sterically unfavorable Z_syn-(R) ar-

rangement. Assuming the interchange of the nitrogen substit-

uents observed in the ¹H NMR spectra proceeds via the proton

exchange mechanism proposed in Scheme 3 [i.e. E_syn-(R) T

1

2

values: J_SeH≈ 50 Hz; J_SeH/³J_SeH≈ 10-20 Hz)¹⁹suggesting

a hydrogen-bonding interaction between the selenium atom and

the amino proton. Similar intramolecular contacts have afforded

E

_anti-(R) T E_anti-(â) T E_syn-(â)], a pathway involving higher-

J

_SeHvalues of around 12-13 Hz,²⁰indicating that this is likely

energy isomers is calculated for the P-diphenyl model A, in

agreement with the observation that this process only occurs at

elevated temperature. Interestingly, relatively low-energy struc-

tures are noted for the Z_syn-(â) and Z_anti-(â) configurations of

B, suggesting that the minor isomer of 2a, spectroscopically

identified as containing the amidine unit in the Z_antiform, is

likely to be the â-conformer.

Synthesis and Structure of Phospha(V)guanidines, R₂P-

(E)C{NR′}{NHR′}. Oxidation of the phospha(III)guanidines

{1a,b} and 2a,b to the corresponding P(V)sulfide and -selenide

derivatives proceeded smoothly via reaction with the elemental

chalcogen, to afford Ph₂P(E)C{NR′}{NHR′} [{3a,b} E ) S;

{4a,b} E ) Se]¹¹and Cy₂P(E)C{NR′}{NHR′} [5a,b E ) S;

6a,b E ) Se] (Scheme 1).¹⁸Compounds 3-6 were purified by

crystallization from hot heptane, affording the target species in

good to excellent yields (61-95%).

to be a weak Se‚‚‚H interaction in {4b}. It is noted, however,

that this interaction is evidently strong enough to slow rotation

about the P-C_amidinebond on the NMR time scale to generate

the “locked” conformation, as described above. In summary,

these spectroscopic data for {4b} present strong evidence for

the existence of E_synand Z_synamidine configurations in solution,

which are both present as the â-conformers.

Variable-temperature NMR studies of {4b} show that below

248 K the relative amounts of E_syn-(â):Z_syn-(â) isomers in

solution are constant at 2.5:1.0, representing the thermodynamic

ratio between these two isomers. Upon raising the temperature

above 248 K, the relative amount of Z_syn-(â) increases to give

the 1.3:1.0 ratio observed at 298 K. Van’t Hoff analysis of the

data between 248 and 298 K gives a value of ∆H of 2.7 kcal

(18) Attempted synthesis of the corresponding phosphine oxides of 1b using

tert-butyl peroxide resulted in cleavage of the phosphorus-carbon bonds

and isolation of the hydrogen-bonded ion-pair, [HC{NHCy}₂][Ph₂PO₂].

Mansfield, N. E.; Coles, M. P.; Hitchcock, P. B. Unpublished results.

(19) McFarlane, W.; Wood, R. J. J. Chem. Soc., Dalton Trans. 1972, 1397.

(20) Wu, R.; Herna´ndez, G.; Odom, J. D.; Dunlap, R. B.; Silks, L. A. Chem.

Commun. 1996, 1125.

Comparable NMR spectroscopic data are observed for each

of the P-diphenyl derivatives, irrespective of the chalcogen and

nitrogen substituents, and it is therefore assumed that similar

solution-state behavior occurs in all cases. In this instance, the

9

13884 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

Figure 6. Graph of the relative energies of the optimized structures of Ph₂PC{NMe}{NHMe} (A) and Cy₂PC{NMe}{NHMe} (B) considering the eight

different configurations, as described in Scheme 2.

mol^-1with the corresponding entropy value calculated at +9.1

eu. Assuming that a mechanistic pathway similar to that outlined

in Scheme 3 is operating for the E_syn-to-Z_synisomerization

(involving generation of a zwitterionic intermediate), the

increased energy assoicated with this process compared to that

associated with 2a may reflect a decreased tendency for the

amino nitrogen to assume a positive charge, due to overlap with

the lone pairs of the chalcogen atom (vide infra).

As observed for the P(III) systems described above, changing

the phosphorus substituents from phenyl to cyclohexyl has a

large impact on the configuration of the amidine unit, despite

the distance between these groups within the molecule. To

illustrate these differences, the spectroscopic features associated

with the corresponding P-dicyclohexyl complex, Cy₂P(Se)C-

{NCy}{NHCy}, 6b, are described. The ³¹P{¹H} NMR spectrum

of 6b also shows two resonances [δ 62.7 and δ 66.4], although

for this compound the ratio of the two species is heavily biased

toward one isomer, with a ratio of ∼19:1. As for {4b}, the

⁷⁷Se resonance corresponding to the major isomer exhibits a

large J_SePcoupling of -690 Hz and a small J_SeHcoupling of

6 Hz, which disappears on selective decoupling of the NH

1

resonance (Figure 8).

1

The H NMR spectrum of 6b reflects this change in the

relative ratio of species present in solution, with resonances for

the major isomer dominating the spectra, such that the low-

intensity signals of the minor isomer were difficult to assign.

The NH resonance of the major species is observed as a poorly

defined doublet of doublets at δ 6.90, with two resolved

multiplets for the R-CH of the N-cyclohexyl substituents, again

consistent with localized bonding in the amidine. nOe’s are

observed between both N-cyclohexyl groups, and between the

N_aminesubstituent and the NH proton, consistent with the major

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13885

A R T I C L E S

Mansfield et al.

The infrared spectra of {1b}, {4b}, and 6b have also been

recorded in CDCl₃solution (Figure 9). The υ(N-H) of {1b},

in which no hydrogen bonding involving the NH is observed

using other spectroscopic or solid-state analytical techniques,

was identified as a strong absorption at 3427 cm^-1(Figure 9a).

In agreement with NMR data, two isomeric forms of {4b} have

been identified (Figure 9b), with broad absorptions at 3304 and

3262 cm^-1assigned to υ(N-H‚‚‚Se) and a sharp signal observed

at 3428 cm^-1for the terminal υ(N-H). To corroborate these

assignments, the corresponding IR of the cyclohexyl derivative

6b, in which the predominant isomer involves intramolecular

N-H‚‚‚Se interactions, only shows the bathochromically shifted

NH absorption at 3251 cm^-1(Figure 9c). Compared with

calculated²¹[∼2550 cm^-1] and observed²²[2240-2320 cm^-1

]

Se-H absorptions, these values are too high to be considered

as originating from a direct Se-H bond, consistent with the

postulated intramolecular hydrogen-bonding situation.

Figure 7. ¹H NMR spectrum of {4b} (500 MHz, [²H]₈-toluene, 293 K)

showing the aromatic and cyclohexyl R-CH regions.

Crystallographic Study of P(V) Sulfide and Selenide

Derivatives and the Borane Adduct, Ph₂P(BH₃)C{NCy}-

{NHCy}. Single-crystal X-ray diffraction studies were per-

formed on representative compounds {3b}, {4b}, 5a, 6a, and

6b (Figures10 and 11); crystal structure and refinement data

are collected in Table 4 and bond lengths and angles in Table

5. The molecular structure of each compound conforms to the

expected formula of R₂P(E)C{NR′}{NHR′}, and all derivatives

are monomeric with no significant intermolecular interactions.

As for the P(III) derivatives discussed above, the hydrogen atom

on the amino nitrogen was located and refined in each case.

For all compounds, the phosphorus atom is in a distorted

tetrahedral geometry, with P ) E bond lengths consistent with

related R₃PdS sulfides (R ) Ph, 1.949-1.955 Å;²³R ) Cy,

1.966-1.961 Å²⁴) and R₃PdSe selenides (R ) Ph, 1.105-2.113

Å;²⁵R ) Cy, 2.108 Å²⁶). Localized CdN double and C-N

single bonds are noted within the “NCN” component, with ∆_CN

values ∼0.08 Å.

Table 3. Summary of ³¹P NMR Data (121.4 MHz, [²H]₆-benzene,

298 K) for 3-6

major resonance

(J_PSe

minor resonance

(J_PSe

relative ratio

(293 K)^a

compound

)

{3a}

{3b}

{4a}

{4b}

5a

δ 33.6

δ 34.0

δ 23.2 (-755 Hz)

δ 23.9 (-753 Hz)

δ 63.3

δ 63.7

δ 62.4 (-692 Hz)

δ 62.7 (-690 Hz)

δ 38.2

δ 38.6

1.6:1.0

1.2:1.0

1.5:1.0

1.3:1.0

8.4:1.0

18.0:1.0

14.6:1.0

19.0:1.0

δ 35.2 (-721 Hz)

δ 35.5 (-721 Hz)

δ 67.4

5b

δ 67.0

6a

6b

δ 67.0^b

δ 66.4^b

^aRelative ratio of isomers calculated from integration of ³¹P NMR

b

resonances. J_PSecould not be detected due to the low relative intensity of

the signal.

isomer in this case being the Z_syn-(â). These data therefore

indicate an almost complete shift in the equilibrium for the

P-dicyclohexylphospha(V)guanidines to the theoretically most

sterically unfavorable Z_synisomer in solution.

The structurally characterized P-diphenyl derivatives, {3b}

and {4b}, exist as the E_syn-(â) isomer in the solid state,

corresponding to the major component identified in solution.

In this configuration, the NH proton is located in a position

that will enable interaction with the chalcogen lone pairs, with

NH‚‚‚EdP distances of 2.45 Å and 2.63 Å noted for {3b} and

{4b}, respectively (within the sum of their van der Waals radii:

²⁷S ) 1.80 Å; Se ) 1.90 Å; H ) 1.20 Å). These distances are

within the range observed in related structures composed of five-

membered rings containing such intramolecular interactions

[sulfides: 2.25-2.76 Å; selenides: 2.60-2.79 Å].^28,29To

1

It is worthwhile noting that in the H NMR spectra of the

P(V) system, the NH resonance is present as a (poorly resolved)

pseudotriplet in both the E_syn-(â) and Z_syn-(â) isomers of {4b}

and the observable Z_syn-(â) isomer of 6b. Comparing these data

to the spectra of the P(III) derivatives {1a,b}⁹and 2a,b (vide

supra), the corresponding NH resonance for the E_syn-(R)

configuration is observed as a doublet, with no coupling to the

phosphorus. Given that the NH resonance of the Z_anticonfigu-

ration of 2a is present as a well-defined (albeit broad) pseudo-

3

triplet with resolvable J_HPcoupling (Figure 2), and that the

DFT analysis indicated an energy profile of in favor of the

â-conformation of the Z_antiisomer, we assign the minor isomer

in the P-dicyclohexylphospha(III)guanidines as corresponding

to the â-conformation.

(21) Leszczyn˜ski, J.; Kwiatkowski, J. S. J. Phys. Chem. 1993, 97, 1364.

(22) Smith, D. M.; Ibers, J. A. Polyhedron 1998, 17, 2105; Di Vaira, M.;

Peruzzini, M.; Stoppioni, P. Inorg. Chem. 1991, 30, 1001; Cui, C.; Roesky,

H. W.; Hao, H.; Schmidt, H.-G.; Noltemeyer, M. Angew. Chem., Int. Ed.

2000, 39, 1815.

NMR spectroscopic data also indicate differences in the

distribution of electron density depending on the configuration

of the amidine, indicated by changes in the J_PCvalues. An

unusually large ¹J_PCcoupling of 123 Hz is observed to the PCN₂

in the Z_syn-(â) isomer of {4b}, approximately 3 times the

corresponding value in the E_syn-(â) isomer [43 Hz] and the

P-diphenyl phospha(III)guanidines [32 Hz]; a similarly large

value of 107 Hz is noted in 6b. Also, ³J_PCcoupling is observed

to the R-CH of both N-substituents in the Z_synisomers of {4b}

and 6b, while this coupling is only observed through the CdN

bond in the corresponding P(III) analogues.

(23) Codding, P. W.; Kerr, K. A. Acta Crystallogr., Sect. B 1978, 34, 3785;

Foces-Foces, C.; Llamas-Saiz, A. L. Acta Crystallogr., Sect. C 1998, 54,

IUC9800013; Ziemer, B.; Rabis, A.; Steinberger, H.-U. Acta Crystallogr.,

Sect. C 2000, 56, e58.

(24) Kerr, K. A.; Boorman, P. M.; Misener, B. S.; van Roode, J. G. H. Can. J.

Chem. 1977, 55, 3081; Reibenspies, J. H.; Draper, J. D.; Struck, G.;

Darensbourg, D. J. Z. Kristallogr. 1996, 211, 400.

(25) Codding, P. W.; Kerr, K. A. Acta Crystallogr., Sect. B 1979, 35, 1261;

Jones, P. G.; Kienitz, C.; Thone, C. Z. Kristallogr. 1994, 209, 80.

(26) Davies, J. A.; Dutremez, S.; Pinkerton, A. A. Inorg. Chem. 1991, 30, 2380.

(27) Bondi, A. J. Phys. Chem. 1964, 68, 441.

(28) Siasios, G.; Tiekink, E. R. T. Z. Kristallogr. 1992, 200, 295; Siasios, G.;

Tiekink, E. R. T. Z. Kristallogr. 1993, 207, 59; Siasios, G.; Tiekink, E. R.

T. Z. Kristallogr. 1995, 210, 619; Andrieu, J.; Camus, J.-M.; Poli, R.;

Richard, P. New J. Chem. 2001, 25, 1015; Zhang, Q.; Ly, T.; Slawin, A.

M. Z.; Woolins, J. D. ReV. Roum. Chim. 2002, 47, 1015.

9

13886 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

1

Figure 8. ⁷⁷Se NMR spectra of 6b (95.3 MHz, [²H]₆-benzene, 298 K). (a) ⁷⁷Se{¹H} spectrum. (b) H-coupled ⁷⁷Se spectrum. (c) Selective decoupling of

1

the H resonance at δ 6.90.

Figure 10. ORTEP representation of the molecular structures of Ph₂P-

(S)C{NCy}{NHCy} {3b} and Ph₂P(Se)C{NCy}{NHCy} {4b}. Ellipsoids

drawn at the 30% probability level; hydrogen atoms, except NH, omitted.

Figure 9. Solution-state IR spectra of (a) Ph₂PC{NCy}{NHCy} {1b}; (b)

Ph₂P(Se)C{NCy}{NHCy} {4b}; (c) Cy₂P(Se)C{NCy}{NHCy} 6b.

corresponding increase in the P-CdN_imineangle [av {3b} and

{4b} ) 128.56°; av {1a} and {1b} ) 123.52°] representing a

“pitch” of the amidine unit toward the chalcogen atom. This

observation may be explained considering two complimentary

perspectives: using a purely steric argument, the shift in the

ground-state configuration from E_syn-(R) to E_syn-(â) that occurs

upon oxidation places the N_iminesubstituent in an unfavorable

position with respect to the phenyl groups, causing steric

repulsion to force the amidine unit to reposition itself. The

alternative perspective is to consider an electronic attraction

between the amine proton and the chalcogen which “pulls” the

amidine unit away from the phosphorus substituents.

generate a favorable alignment for this intramolecular interaction

while maintaining a planar amidine unit, a small twist angle is

adopted between the “EdP-C” and “N-C-N” planes, with

values of 13.9° and 12.0° for {3b} and {4b}, respectively.

The sp²-carbon of the amidine unit is planar for each of the

structurally determined P(III)- and P(V)-diphenyl derivatives,

with experimentally equivalent (within 3σ) “N-CdN” angles

of ∼120°. However, the P(V)-C-N_amineangle is considerably

smaller than in the corresponding P(III) compounds [av {3b}

and {4b} ) 110.36°; av {1a} and {1b} ) 116.05°] with a

(29) Siasios, G.; Tiekink, E. R. T. Z. Kristallogr. 1995, 210, 868.

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13887

A R T I C L E S

Mansfield et al.

Figure 11. ORTEP representation of the molecular structures of Cy₂P(S)C{NⁱPr}{NHⁱPr} (5a), Cy₂P(Se)C{NⁱPr}{NHⁱPr} (6a) and Cy₂P(Se)C{NCy}-

{NHCy} (6b). Ellipsoids drawn at the 30% probability level; hydrogen atoms, except NH, omitted.

Table 4. Crystal Structure and Refinement Data for 5a, 6a, 6b and 7

5a

6a

6b

7

formula

C₁₉H₃₇N₂PS

356.54

173(2)

C₁₉H₃₇N₂PSe

403.44

173(2)

C₂₅H₄₅N₂PSe

483.56

173(2)

C₂₅H₃₆BN₂P

406.34

173(2)

formula weight

temperature (K)

wavelength (Å)

crystal size (mm)

crystal system

space group

a (Å)

0.71073

0.20 × 0.20 × 0.20

0.30 × 0.30 × 0.20

0.40 × 0.40 × 0.35

0.20 × 0.15 × 0.10

monoclinic

P2₁/n (No.14)

9.1464(4)

15.3363(4)

17.7735(7)

90

104.714(2)

90

2411.36(16)

4

triclinic

P1h (No.2)

8.9912(3)

10.0100(3)

12.4892(5)

77.228(2)

77.769(2)

76.842(2)

1051.90(6)

2

P1h (No.2)

8.9350(3)

10.0808(3)

12.5565(4)

77.397(2)

78.020(2)

77.321(2)

1061.57(6)

2

P1h (No.2)

9.9173(2)

10.1444(3)

14.1035(4)

105.398(1)

97.112(1)

104.885(1)

1293.67(6)

2

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

Z

D_c(Mg m^-3

)

1.13

0.23

1.26

1.85

1.24

1.53

1.12

0.13

absorption coefficient (mm^-1

)

θ range for data collection (deg)

reflections collected

3.46 to 26.10

14392

3.45 to 26.01

14137

3.52 to 26.08

21177

3.80 to 25.04

20602

independent reflections

reflections with I > 2σ(I)

data/restraints/parameters

goodness-of-fit on F²

4138 [R_int) 0.049]

4158 [R_int) 0.039]

5083 [R_int) 0.042]

4223 [R_int) 0.061]

3401

3695

4421

3253

4138/0/212

1.016

4158/0/212

1.040

5083/0/266

0.944

4223/0/278

1.040

final R indices [I > 2σ (I)]

R1 ) 0.037

wR2 ) 0.083

R1 ) 0.050

wR2 ) 0.089

0.25 and -0.21

R1 ) 0.029

wR2 ) 0.065

R1 ) 0.036

wR2 ) 0.067

0.40 and -0.42

R1 ) 0.030

wR2 ) 0.071

R1 ) 0.039

wR2 ) 0.075

0.33 and -0.29

R1 ) 0.045

wR2 ) 0.094

R1 ) 0.068

wR2 ) 0.104

0.20 and -0.23

R indices (all data)

largest diff. peak and hole (e Å^-3

)

In contrast to the P-diphenyl derivatives, structural charac-

terization of 5a, 6a, and 6b indicates that the Z_syn-(â) form is

more stable in the solid state (Figure 11). This configuration is

observed for examples containing sulfide and selenide groups

(with the configuration in the latter derivatives shown to be

independent of nitrogen substituent) indicating that the origin

of this difference derives from the P-cyclohexyl groups.

Structurally characterized examples of the Z_synform of an

amidine are unusual, usually being restricted to those in which

the C-substituent of the NCN unit is bulky, such as in the

C-triptycenyl³⁰and C-ter-phenyl derivatives.^30,31

Å],³²they lie toward the long end of the range for the

corresponding selenides [2.37-2.65 Å].^29,33

To accommodate the close proximity of the two nitrogen

substituents in the Z_synform, several structural differences are

noted compared to the E_synisomers observed for the P-diphenyl

compounds (Figure 12). The amine nitrogen generally exhibits

a more pronounced pyramidalization in the P-dicyclohexyl

compounds, with a increased DP(%) at the N_amineatom [5a )

7.5; 6a ) 8.6; 6b ) 2.5] compared with the P-diphenyl

analogues [{3b} ) 1.4; {4b} ) 1.2]. This is most clearly

observed in the displacement of the N_aminesubstituent from the

NCN plane, with the resultant angle between the C-N_amine-R

and CdN_imine-R planes showing considerable variation through

the series of compounds [{3b} ) 6.2°; {4b} ) 19.4°; 5a )

34.1°; 6a ) 38.2°; 6b ) 23.4°]. The different substituent

arrangement also manifests itself in an increased N-C-N angle

In this configuration, the chalcogen and amine protons of the

phospha(V)guanidines are still able to interact, with intramo-

lecular NH‚‚‚EdP distances of 2.54, 2.68, and 2.70 Å for 5a,

6a, and 6b, respectively. While these are within the range

previously noted for related P-dicyclohexyl sulfides [2.40-2.63

(32) Siasios, G.; Tiekink, E. R. T. Z. Kristallogr. 1994, 209, 885; Horn, E.;

Kurosawa, K.; Tiekink, E. R. T. Acta Crystallogr., Sect. E 2001, 57, o321.

(33) Kramolowsky, R.; Sawluk, J.; Siasios, G.; Tiekink, E. R. T. Z. Kristallogr.

- New Cryst. Struct. 1998, 213, 45.

(30) Baker, R. J.; Jones, C. J. Organomet. Chem. 2006, 691, 65.

(31) Schmidt, J. A. R.; Arnold, J. J. Chem. Soc., Dalton Trans. 2002, 2890.

9

13888 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

Table 5. Selected Bond Lengths (Å) and Angles (deg) for {3b},

{4b}, 5a, and 6a,b (labels from Figure 10)

{

3b

}

{4b

}

5a

6a

6b

Bond Lengths

1.9536(6) 2.1183(4) 1.9690(6) 2.1237(5) 2.1207(5)

PdE

E‚‚‚H

2.45

2.63

2.54

2.68

2.70

P-C(1)

C(1)-N(1)

C(1)-N(2)

P-C(R₁)

P-C(R₂)

1.8909(16) 1.8811(14) 1.8601(16) 1.864(2)

1.279(2)

1.361(2)

1.8103(15) 1.8108(14) 1.8273(16) 1.831(2)

1.8061(16) 1.8131(14) 1.8357(15) 1.839(2)

0.082

1.862(2)

1.273(2)

1.358(2)

1.839(2)

1.822(2)

0.085

1.2785(18) 1.270(2)

1.3602(18) 1.356(2)

1.270(3)

1.348(3)

∆

0.082

Bond Angles

N(1)-C(1)-N(2) 121.18(15) 120.95(13) 133.27(16) 133.07(19) 134.89(18)

0.086

0.078

CN

Figure 13. (a) ORTEP representation of the molecular structure of 7.

Ellipsoids drawn at the 30% probability level; hydrogen atoms except NH

and BH₃, omitted. (b) View of the core atoms of 7 showing the relative

rotation of the amidine unit about the P-C vector.

E-P-C(1)

111.05(6) 112.37(4) 111.58(5) 111.45(6) 112.67(6)

111.17(6) 110.19(5) 114.36(6) 114.11(6) 111.59(6)

112.55(6) 111.98(4) 112.59(5) 112.45(7) 114.32(6)

129.16(12) 127.96(11) 112.45(11) 111.93(13) 111.02(13)

109.64(12) 111.07(12) 114.17(12) 114.82(15) 114.05(13)

106.09(7) 106.32(6) 104.14(7) 104.31(8) 106.46(9)

108.47(7) 105.19(6) 106.02(7) 106.23(9) 103.02(8)

E-P-C(R₁)

E-P-C(R₂)

N(1)-C(1)-P

N(2)-C(1)-P

C(1)-P-C(R₁)

C(1)-P-C(R₂)

Table 6. Selected Bond Lengths (Å) and Angles (deg) for 7

P-B

1.905(3)

1.280(3)

1.8154(19)

0.087

P-C(13)

C(13)-N(1)

P-C(7)

1.8737(19)

1.367(3)

1.8195(19)

C(R₁)-P-C(R₂) 107.22(7) 110.55(6) 107.48(7) 107.68(8) 108.14(8)

C(13)-N(2)

P-C(1)

∆

CN

N(1)-C(13)-N(2)

B-P-C(1)

N(1)-C(13)-P

C(13)-P-C(1)

N(1)-C(13)-N(2)

120.51(18)

B-P-C(13)

B-P-C(7)

N(2)-C(13)-P

C(13)-P-C(7)

C(1)-P-C(7)

114.06(11)

110.98(11)

125.37(15)

104.41(9)

105.61(9)

117.31(11)

114.10(14)

103.29(8)

120.51(18)

of 133.74° (av) for the P-dicyclohexyl species compared with

121.06 (av) in the P-diphenyl derivatives, with a corresponding

decrease in the N_iminedC-P angle noted in the former com-

pounds.

adduct Ph₂P(BH₃)C{NCy}{NHCy} (7) was prepared and struc-

turally characterized. It was reasoned that the BH₃group at

phosphorus would exert a steric role in this position, while not

being involved in any attractive intramolecular interactions with

the amino NH group. The compound was synthesized by the

combination of the P(III) starting reagent {1b} with a solution

of the THF adduct of BH₃, and the product was crystallized

from hexane in low yield. While IR (Nujol mull, CsI), elemental

analysis, and X-ray diffraction studies were consistent with the

formulation of 7 as the borane adduct [υ(B-H) 2395 cm^-1(br);

υ(P-B) 565 cm^-1(sh)], all attempts at acquiring solution-state

spectroscopic data were unsuccessful, with apparent loss of the

borane and regeneration of {1b}, precluding any comparison

of solution-state behavior between 7 and the phospha(V)-

guanidines.

The observed change in the configuration of the amidine unit,

which is dependent on the nature of the phosphorus substituents,

is likely to derive from a combination of repulsive steric

interactions with the N_iminegroup and an increase in the attractive

force between the amine proton and the chalcogen. The latter

electronic effect may be rationalized in terms of an increased

stabilization of the zwitterionic P⁽⁺⁾-E^(-)resonance form by

the electron-donating cyclohexyl substituents, augmenting the

negative charge at the chalcogen which in turn enhances the

NH‚‚‚EdP interaction.

To help evaluate the steric factors that contribute to these

structural changes in the amidine configuration, the borane

The molecular structure of 7 determined by X-ray crystal-

lography (Figure 13, Tables 4 and 6) indicates that the amidine

component of compound 7 adopts an E_syn-(R) configuration in

the solid state, as noted for the previously reported P-diphenyl

phospha(III)guanidines {1a} and {1b}. Once again localized

bonding is observed in the amidine component (∆_CN) 0.087

Å), with the amine NH orientated toward the ipso-carbon of

one of the phenyl substituents [N_amine-C-P-Ph torsion angles:

-2.4° and 107.9°] and an intramolecular NH‚‚‚C_ipsodistance

of 2.38 Å.

The estimated cone angles of the PdE and P-B-H groups

for {3b}, {4b}, and 7, calculated using bond lengths from

crystallographic data and van der Waals radii taken from ref

27, are 134°, 128°, and 118°, respectively, indicating that the

steric influence of the borane is less than that of the sulfide or

selenide substituents. This may be insufficient to cause a

conformational change of the amidine to change from E_syn-(R)

to E_syn-(â) that, in addition to the lack of any attractive

intramolecular forces, explains the observed configuration in

7. As for the P(III) compounds described above, DFT calcula-

Figure 12. View of the core of P(V) phosphaguanidine compounds along

the P-C vector, illustrating the structural variation in the position of the

nitrogen substituents.

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13889

A R T I C L E S

Mansfield et al.

Figure 14. Orbital energy diagram for the model complexes, Ph₂P(S)C{NMe}{NHMe} (C) and Cy₂P(S)C{NMe}{NHMe} (D) [E_syn-(â) and Z_syn-(â)

conformations], illustrating the HOMO and the HOMO-2 (constructed with isovalues at 0.02).

Figure 15. Spacefilling models of the phosphine moiety of phospha(III)- and phospha(V)guanidines, viewed along (left) and perpendicular (right) to the

P-C vector (coordinates taken from X-ray crystallographic data). (a) Ph₂PC{NⁱPr}{NHⁱPr}, {1a}; (b) Cy₂PC{NⁱPr}{NHⁱPr}, 2a; (c) Ph₂P(Se)C{NCy}-

{NHCy}, {4b}; (d) Cy₂P(Se)C{NCy}{NHCy}, {6b}.

tions have been utilized to help gain further insight into these

structural preferences.

were typically within a 2% threshold, indicating a good

correlation between the experimental and theoretical data.

Molecular Modeling Studies on Phospha(V)guanidines.

Using a similar approach to that employed for the P(III)

compounds described above, the representative models Ph₂P-

(S)C{NMe}{NHMe} (C) and Cy₂P(S)C{NMe}{NHMe} (D)

have been examined using DFT. Calculations were performed

on the E_syn-(â) and Z_syn-(â) isomers of the P-diphenyl and

P-dicyclohexyl sulfide models to engender further information

on the differences between these two isomers. Once again,

discrepancies between the bond lengths and angles of the fully

optimized geometries and the observed crystal structure data

Figure 14 compares selected molecular orbitals for C and D

in both the E_synand Z_synisomeric forms of the â-conformer.

The HOMOs are similar in all cases and show significant

electron density associated with a lone pair on the sulfur atom,

indicating the possibility of utilizing this atom in the coordina-

tion chemistry of these species, as previously observed with

the application of Ph₃PdS as a ligand.³⁴The HOMO-2 orbital

contains a significant overlap between a lone pair localized on

the sulfur atom and a π-symmetry component associated with

the N_aminegroup, suggesting that this orbital is important in

9

13890 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

stabilizing the (â) isomer favored in the P(V) system. This

orbital is comparable in its makeup for all four examples

investigated using this method, with the through space overlap

between these groups appearing to be slightly more pronounced

in the E_synconfiguration for both the P-diphenyl and P-

dicyclohexyl model compounds.

and cyclohexyl-substituted phosphorus exist as a mixture of E_syn-

(â) and Z_syn-(â) configurations in solution, with a strong bias

toward the latter arrangement in the P-dicyclohexyl species.

X-ray diffraction analyses reflect this trend, with examples of

P-diphenyl compounds crystallizing with the amidine in the E_syn

-

(â) configuration, while selected P-dicyclohexyl analogues are

shown to exist with the Z_syn-(â) in the solid state. In both cases,

nonbonding interactions between the chalcogen and the amino

hydrogen atom are noted that, on the basis of J_Se‚‚‚H coupling

in the ⁷⁷Se NMR spectra and the observation of multiple υ-

(N-H) absorptions, are shown to be present in solution. DFT

analysis indicates the presence of orbital overlap between the

two groups, although it does not indicate a strong electronic

preference for either the E- or Z-configuration of the imine

double bond.

As indicated above, the contribution from the zwitterionic

form of the phosphorus-chalcogen double bond was considered

as playing a potentially important role in determining the

strength of the nonbonding interaction to the amino hydrogen.

Given that the solution-state data indicate a strong preference

for the Z_syn-(â) configuration of the P-dicyclohexyl species in

solution, it is encouraging to note that this arrangement of

substituents in D gives the largest positive and negative values

for the natural charges at phosphorus [+1.40] and sulfur

[-0.66]. The charges associated with these atoms in the

remaining models, however, do not indicate any clear trends

that can be correlated to the experimental observations.

Overall the results from these DFT calculations are very

similar, irrespective of the phosphorus substituent and the

configuration of the amidine group. There does not appear to

be a compelling electronic argument derived from these models

as to why the P-dicyclohexyl compounds prefer to exist in the

sterically more encumbered Z_synconfiguration. It is likely,

therefore, that steric factors play an important role in determining

the subtle energetic differences that determine whether E_synor

Experimental Section

General Methods. All manipulations were carried out under dry

nitrogen using standard Schlenk-line and cannula techniques, or in a

conventional nitrogen-filled glovebox. Solvents were dried over ap-

propriate drying agent and degassed prior to use. NMR spectra were

recorded using a Bruker Avance DPX 300 MHz spectrometer at 300.1

(¹H), 75.4 (¹³C{¹H}), 121.4 (³¹P{¹H}) MHz and a Bruker AMX 500

MHz spectrometer at 500.1 (¹H), 125.7 (¹³C{¹H}), 202.4 (³¹P{¹H}) and

95.3 (⁷⁷Se{¹H}) MHz, from samples at 298 K in [²H₆]-benzene, unless

otherwise stated. Coupling constants are quoted in Hz. Proton and

carbon chemical shifts were referenced internally to residual solvent

resonances, phosphorus and selenium were referenced externally to H₃-

PO₄(aq) and SeMe₂(CDCl₃), respectively. Elemental analyses were

performed by S. Boyer at the London Metropolitan University.

Z_synis the predominant form in solution, illustrated in the space

filling models in Figure 15. It is, however, noted that the

aforementioned S‚‚‚N_amineoverlap in the HOMO-2 of C and

D is the most likely cause of the preferential adoption of the

(â)-conformation in the phospha(V)guanidines that are the

subject of this study.

i

The reagents Ph₂PH (Fluka), Cy₂PH (Strem), PrNdCdNⁱPr (Ald-

rich), CyNdCdNCy (Aldrich), S (Aldrich), Se (gray, Aldrich), BH₃‚

THF (1.0 M solution in THF, Aldrich), and ⁿBuLi (2.5 M in hexanes,

Acros) were purchased from commercial sources and used as received.

[HNEt₃][Cl] (Aldrich) was recrystallized from anhydrous chloroform,

dried by heating in vacuo for 1 h, and stored under N₂. The

phosphaguanidines Ph₂PC{NⁱPr}{NHⁱPr} {1a} and Ph₂PC{NCy}-

{NHCy} {1b} were made according to literature procedures.⁹

Summary

In this work we report the synthesis of the novel phospha-

i

(III)guanidines Cy₂PC{NR′}{NHR′} (R′ ) Pr, Cy) and their

conversion to the corresponding sulfo- and selenophospha(V)

derivatives. Analysis by NMR spectroscopy indicated that at

room temperature, these compounds exist as a mixture of the

n

Cy₂PC{NⁱPr}{NHⁱPr} (2a). From a solution of a 2.0 M BuLi in

hexanes (5.6 mmol) 2.80 mL was added dropwise to a solution of Cy₂-

PH (1.00 g, 5.0 mmol) in THF (∼20 mL) at 0 °C. The resultant pale-

E_syn-(R) configuration and a minor component in which the Z_syn

i

yellow solution was stirred for 20 min prior to the addition of PrNd

arrangement of substituents is present; this latter isomer cannot

be definitively assigned as the (R) or (â) conformer on the basis

of spectroscopy alone. A mechanism for the interconversion of

the different isomeric forms is proposed, and using model

systems, DFT analysis of the relative energies of the different

configurations is in agreement for a higher-energy process for

the P-diphenyl derivative. Evidence from computational studies

is also presented to explain the observation that in the P-

diphenyl-substituted compounds, the amidine unit is rotated such

that the NH amino group is in close proximity to one of the

aromatic rings.

CdNⁱPr (0.90 mL, 5.5 mmol) to afford a clear colorless solution. After

stirring at ambient temperature for 4 h, the reaction mixture was added

to a slurry of [HNEt₃][Cl] (0.80 g, 5.5 mmol) in THF (∼10 mL),

affording a clear colorless solution. Removal of volatiles after 16 h

yielded a crude white solid, from which the product was extracted by

filtration, using hexane as the solvent. Concentration and storage of

the resultant solution at -30 °C afforded analytically pure 2a as

colorless needles. Yield 1.23 g (75%).

Anal. Calcd for C₁₉H₃₇N₂P (324.48): C, 70.33; H, 11.49; N, 8.63.

Found: C, 70.32; H, 11.41; N, 8.79. ¹H NMR (500 MHz): Major

3

isomer (E_syn) δ 4.71 (m, 1H, CHMe₂), 4.41 (sept, J_HH) 6.5, 1H,

CHMe₂), 3.59 (d, ³J_HH) 6.8, 1H, NH), 1.99 (m, 2H, R-CH Cy), 1.79-

We have also demonstrated that amidine components of the

sulfo- and selenophospha(V)guanidines containing both phenyl-

3

0.96 (m, 20H, Cy), 1.37 (d, J_HH) 6.2, 6H, CHMe₂), 1.09 (d, J_HH

)

3

6.4, 6H, CHMe₂). Minor isomer (Z_anti) δ 4.41 (sept, J_HH) 6.5, 1H,

CHMe₂), 3.91 (dd, J ) 14.5, 1H, NH), 3.36 (sept, J_HH) 6.5, 1H,

3

(34) Garner, C. D.; Howlader, N. C.; Mabbs, F. E.; Boorman, P. M.; King, T.

J. J. Chem. Soc., Dalton Trans. 1978, 1350; Burford, N.; Royan, B. W.;

Spence, R. E. v. H.; Rogers, R. D. J. Chem. Soc., Dalton Trans. 1990,

2111; Fornies, J.; Navarro, R.; Sicilia, V.; Tomas, M. Inorg. Chim. Acta

1990, 168, 201; Stumpf, K.; Blachnik, R.; Roth, G.; Kastner, G. Z.

Kristallogr. - New Cryst. Struct. 2000, 215, 589; Hussain, M. S.; Isab, A.

A.; Saeed, A.; Al-Arfaj, A. R. Z. Kristallogr. - New Cryst. Struct. 2001,

216, 629; Chutia, P.; Kumari, N.; Sharma, M.; Woolins, J. D.; Slawin, A.

M. Z.; Dutta, D. K. Polyhedron 2004, 23, 1657; Cook, J. B.; Nicholson,

B. K.; Smith, D. W. J. Organomet. Chem. 2004, 689, 860.

CHMe₂), 2.22 (m, 2H, R-CH Cy), 1.79-0.96 (m, 20H + 6H, Cy, plus

3

CHMe₂), 0.94 (d, J_HH) 6.3, 6H, CHMe₂). ¹³C{¹H} NMR (125

1

MHz): Major isomer (E_syn) δ 154.0 (d, J_CP) 38, PCN₂), 51.5 (d,

1

³J_CP) 39, dNCH), 42.0 (-NHCH), 34.5 (d, J_CP) 18, R-CH Cy),

31.5 (d, J_CP) 18, Cy), 30.3 (d, J_CP) 9, Cy), 27.3 (d, J_CP) 10, Cy),

27.2 (d, J_CP) 14, Cy), 26.6 (Cy), 25.8 (CHMe₂), 22.9 (CHMe₂). Minor

isomer (Z_anti) δ 157.1 (d, ¹J_CP) 7, PCN₂), 51.8 (d, ³J_CP) 31, dNCH),

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13891

A R T I C L E S

Mansfield et al.

46.6 (-NHCH), 45.3 (d, ¹J_CP) 23, R-CH Cy), 34.0 (d, J_CP) 12, Cy),

29.9 (d, J_CP) 9, Cy), 27.7 (d, J_CP) 9, Cy), 27.5 (d, J_CP) 7, Cy),

27.1 (Cy), 24.6 (CHMe₂), 23.9 (CHMe₂). ³¹P{¹H} NMR: Major isomer

(E_syn) δ -21.2. Minor isomer (Z_anti) δ -4.8.

NMR data have not been individually assigned for each isomer and

are reported together.) ³¹P{¹H} NMR: Major isomer (Z_syn) δ 34.0.

Minor isomer (E_syn) δ 38.6.

Ph₂P(Se)C{NⁱPr}{NHⁱPr} (4a). This compound was prepared in a

manner analogous to that described for 3a, using the following

quantities: Ph₂PC{NⁱPr}{NHⁱPr} (0.31 g, 1.0 mmol) and gray selenium

(80 mg, 1.00 mmol). The compound was crystallized from heptane as

described for 3a. Yield (colorless crystals) 0.29 g (74%). Despite

repeated attempts, accurate elemental analysis could not be obtained

for this compound, with results consistent with the presence of residual

selenium.

Cy₂PC{NCy}{NHCy} (2b). This compound was prepared in a

manner analogous to that described for 2a, using the following

n

quantities: Cy₂PH (1.00 g, 5.0 mmol), BuLi (2.80 mL of a 2.0 M

solution in hexanes, 5.6 mmol), CyNdCdNCy (1.20 g, 5.5 mmol),

and [HNEt₃][Cl] (0.80 g, 5.5 mmol). The compound was isolated from

hexane at -30 °C as pale-yellow needles. Yield 1.65 g (81%).

Anal. Calcd for C₂₅H₄₅N₂P (404.61): C, 74.21; H, 11.21; N, 6.92.

Found: C, 74.18; H, 11.12; N, 6.93. ¹H NMR: Major isomer (E_syn) δ

4.37 (m, 1H, R-CH N-Cy), 4.19 (m, 1H, -CH, N-Cy), 3.74 (d, ³J_HH

) 6.9, 1H, NH), 2.07-0.84 (m, 42H, Cy). ¹³C{¹H} NMR: Major

¹H NMR: Major isomer (E_syn) δ 7.93 (m, 4H, o-C₆H₅), 6.97 (m,

6H, m- and p-C₆H₅), 5.29 (m, 1H, NH), 4.201 (m, 2H, -NHCH and

3

dNCH), 1.00 (d, J_HH) 6.1, 12 H, CHMe₂). Minor isomer (Z_syn) δ

1

3

isomer (E_syn) δ 153.9 (d, J_CP) 38, PCN₂), 59.7 (d, J_CP) 37, d

NCH), 48.8 (-NHCH), 36.3 (N-Cy), 34.8 (d, ¹J_CP) 17, R-CH P-Cy),

33.5 (N-Cy), 31.5 (d, J_PC) 18, P-Cy), 30.4 (d, J_CP) 9, P-Cy),

27.4 (d, J_CP) 12, P-Cy), 27.3 (d, J_CP) 16, P-Cy), 26.7 (N-Cy),

26.6 (Cy), 26.5 (Cy), 25.4 (Cy), 25.3 (Cy). (Only ¹H and ³¹C{¹H} NMR

data corresponding to the major isomer (E_syn) can be accurately reported

8.21 (m, 4H, o-C₆H₅), 7.02 (m, 6H, m- and p-C₆H₅), 6.87 (m, 1H, NH),

3

3.81 (sept, J_HH) 6.1, 1H, -NHCH), 3.60 (m, 1H, dNCH), 1.04 (d,

3

³J_HH) 6.1, 6H, CHMe₂), 0.85 (d, J_HH) 6.2, 6H, CHMe₂). ¹³C{¹H}

1

NMR: δ 145.9 (d, J_CP) 122, PCN₂), 141.6 (d, J_CP) 42, PCN₂),

133.3 (d, J_CP) 10, C₆H₅), 132.6 (d, J_CP) 11, C₆H₅), 132.6 (d, J_CP

)

76, C₆H₅), 132.7 (d, J_CP) 79, C₆H₅), 131.5 (d, J_CP) 39, C₆H₅), 132.4

(d, J_CP) 39, C₆H₅), 128.8 (d, J_CP) 12, C₆H₅), 128.0 (d, J_CP) 12,

C₆H₅), 50.7 (d, J_CP) 16, CHMe₂), 49.4 (d, J_CP) 21, CHMe₂), 46.8

(d, J_CP) 9, CHMe₂), 43.9 (d, J_CP) 5, CHMe₂), 24.3 (CHMe₂), 24.2

(CHMe₂), 23.7 (CHMe₂), 22.0 (CHMe₂). (¹³C{¹H} NMR data have not

been individually assigned for each isomer and are reported together.)

³¹P{¹H} NMR: Major isomer (E_syn) δ 23.2 (¹J_PSe) -755). Minor

isomer (Z_syn) δ 35.2 (¹J_PSe) -721). ⁷⁷Se{¹H} NMR ([²H]₈-toluene):

Major isomer (E_syn) δ -222 (¹J_SeP) -755). Minor isomer (Z_syn) δ

-304 (¹J_SeP) -721).

Ph₂P(Se)C{NCy}{NHCy} (4b). This compound was prepared in a

manner analogous to that described for 3a, using the following

quantities: Ph₂PC{NCy}{NHCy} (0.50 g, 1.3 mmol) and gray selenium

(100 mg, 1.27 mmol). The compound was crystallized from heptane

as described for 3a. Yield (colorless crystals) 0.39 g (65%).

due to the low intensity of the resonances from the minor species (Z_anti

)

and coincident chemical shifts of many signals.) ³¹P{¹H} NMR: Major

isomer (E_syn) δ -21.1. Minor isomer (Z_anti) δ -4.1.

Ph₂P(S)C{NⁱPr}{NHⁱPr} (3a). A suspension of elemental sulfur

(32 mg, 1.00 mmol) in toluene (∼20 mL) was added dropwise at room

temperature to a solution of Ph₂PC{NⁱPr}{NHⁱPr} (0.31 g, 1.0 mmol)

in toluene (∼10 mL), and the resultant solution was stirred for 8 h.

Removal of the volatiles in vacuo afforded an off-white oil, which was

dissolved in heptane and filtered to remove residual chalcogen.

Purification was achieved by slowly cooling the hot (∼75 °C) heptane

solution to ambient temperature. Yield (colorless crystals) 0.25 g (72%).

Anal. Calcd for C₁₉H₂₅N₂PS (344.46): C, 66.25; H, 7.31; N, 8.13.

1

Found: C, 66.00; H, 7.22; N, 7.80. H NMR: Major isomer (E_syn) δ

7.93 (m, 4H, o-C₆H₅), 6.98 (m, 6H, m- and p-C₆H₅), 5.20 (m, 1H, NH),

4.21 (m, 2H, -NHCH and dNCH), 1.02 (d, ³J_HH) 6.0, 6H, CHMe₂),

1.00 (d, ³J_HH) 6.5, 6H, CHMe₂). Minor isomer (Z_syn) δ 8.28 (m, 4H,

o-C₆H₅), 7.04 (m, 6H, m- and p-C₆H₅), 6.77 (m, 1H, NH), 3.82 (sept,

³J_HH) 5.9, 1H, -NHCH), 3.60 (m, 1H, dNCH), 1.07 (d, ³J_HH) 6.1,

6H, CHMe₂), 0.82 (d, ³J_HH) 6.3, 6H, CHMe₂). ¹³C{¹H} NMR: δ 147.5

(d, ¹J_CP) 131, PCN₂), 144.2 (d, ¹J_CP) 54, PCN₂), 133.0 (d, J_CP) 76,

C₆H₅), 132.9 (d, J_CP) 10, C₆H₅), 132.2 (d, J_CP) 11, C₆H₅), 131.5 (d,

J_CP) 43, C₆H₅), 131.4 (d, J_CP) 43, C₆H₅), 128.8 (d, J_CP) 12, C₆H₅),

Anal. Calcd for C₂₅H₃₃N₂PSe (471.48): C, 63.69; H, 7.05; N, 5.94.

1

Found: C, 63.61; H, 7.22; N, 5.74. H NMR: δ Major isomer (E_syn

)

δ 7.98 (m, 4H, o-C₆H₅), 7.02 (m, 6H, m- and p-C₆H₅), 5.59 (m, 1H,

NH), 4.06 (m, 1H, -NHCH), 3.85 (m, 1H, dNCH), 1.94-0.88 (m,

20H, Cy). Minor isomer (Z_syn) δ 8.26 (m, 4H, o-C₆H₅), 7.02 (m, 6H,

m- and p-C₆H₅), 6.96 (m, 1H, NH), 3.65 (m, 1H, dNCH), 3.45 (m,

1H, -NHCH), 1.94-0.88 (m, 20H, Cy). ¹³C{¹H} NMR ([²H]₈-toluene):

δ 145.6 (d, J_CP) 123, PCN₂), 141.3 (d, J_CP) 43, PCN₂), 132.7 (d,

J_CP) 77, C₆H₅), 132.5 (d, J_CP) 68, C₆H₅), 133.2 (d, J_CP) 10, C₆H₅),

128.0 (d, J_CP) 12, C₆H₅), 50.7 (d, J_CP) 16, CHMe₂), 49.3 (d, J_CP

)

21, CHMe₂), 46.7 (d, J_CP) 9, CHMe₂), 43.7 (d, J_CP) 5, CHMe₂),

24.4 (CHMe₂), 24.3 (CHMe₂), 23.8 (CHMe₂), 22.1 (CHMe₂). (¹³C{¹H}

NMR data have not been individually assigned for each isomer and

are reported together.) ³¹P{¹H} NMR: Major isomer (Z_syn) δ 33.6.

Minor isomer (E_syn) δ 38.2.

132.6 (d, J_CP) 11, C₆H₅), 131.6 (d, J_CP) 3, C₆H₅), 131.0 (d, J_CP

3, C₆H₅), 128.7 (d, J_CP) 12, C₆H₅), 127.8 (d, J_CP) 12, C₆H₅), 58.4

)

3

(d, J_CP) 15, R-CH), 57.4 (d, J_CP) 18, R-CH), 53.8 (d, J_CP) 10,

3

R-CH), 50.3 (d, J_CP) 6, R-CH), 34.4, 34.0, 32.1, 26.2, 26.1, 25.5,

24.4 (Cy). (¹³C{¹H} NMR data for both isomer are indistinguishable

from one another and are reported together.) ³¹P{¹H} NMR: Major

isomer (E_syn) δ 23.9 (¹J_PSe) -753). Minor isomer (Z_syn) δ 35.5 (¹J_PSe

) -721). ⁷⁷Se{¹H} NMR ([²H]₈-toluene): Major isomer (E_syn) δ -224

(¹J_SeP) -753). Minor isomer (Z_syn) δ -311 (¹J_SeP) -721, J_Se‚‚‚H )

6).

Ph₂P(S)C{NCy}{NHCy} (3b). This compound was prepared in a

manner analogous to that described for 3a, using the following

quantities: Ph₂PC{NCy}{NHCy} (0.47 g, 1.2 mmol) and elemental

sulfur (40 mg, 1.20 mmol). The compound was crystallized from

heptane as described for 3a. Yield (colorless crystals) 0.48 g (95%).

Anal. Calcd for C₂₅H₃₃N₂PS (424.58): C, 70.72 H, 7.83; N, 6.60.

Cy₂P(S)C{NⁱPr}{NHⁱPr} (5a). This compound was prepared in a

manner analogous to that described for 3a, using the following

quantities: Cy₂PC{NⁱPr}{NHⁱPr} (0.10 g, 0.3 mmol) and excess

elemental sulfur (20 mg, 0.62 mmol). The compound was crystallized

from heptane as described for 3a. Yield (colorless crystals) 0.067 g

(61%).

1

Found: C, 70.95; H, 7.91; N, 6.55. H NMR: Major isomer (Z_syn) δ

7.99 (m, 4H, o-C₆H₅), 6.98 (m, 6H, m- and p-C₆H₅), 5.53 (m, 1H, NH),

4.09 (m, 1H, -NHCH), 3.88 (m, 1H, dNCH), 1.95-0.90 (m, 20H,

Cy). Minor isomer (E_syn) δ 8.33 (m, 4H, o-C₆H₅), 6.98 (m, 6H, m- and

p-C₆H₅), 6.90 (m, 1H, NH), 3.65 (m, 1H, dNCH), 3.44 (m, 1H,

-NHCH), 1.95-0.90 (m, 20H, Cy). ¹³C{¹H} NMR: δ 147.5 (d, J_CP

) 132, PCN₂), 144.0 (d, J_CP) 52, PCN₂), 133.8 (d, J_CP) 70, C₆H₅),

Anal. Calcd for C₁₉H₃₇N₂PS (356.54): C, 64.00; H, 10.46; N, 7.86.

1

Found: C, 63.91; H,10.35; N, 7.84. H NMR: Major isomer (Z_syn) δ

3

133.2 (d, J_CP) 57, C₆H₅), 132.9 (d, J_CP) 10, C₆H₅), 132.2 (d, J_CP

)

6.64 (dd br, J ) 7.3, 1H, NH), 3.85 (sept, J_HH) 6.0, 1H, CHMe₂),

3

11, C₆H₅), 131.7 (d, J_CP) 3, C₆H₅), 131.2 (d, J_CP) 3, C₆H₅), 128.4

3.60 (m, 1H, CHMe₂), 2.32-1.52 (m, 22H, Cy), 1.16 (d, J_HH) 6.1,

3

(d, J_CP) 12, C₆H₅), 127.7 (d, J_CP) 12, C₆H₅), 58.5 (d, J_CP) 24,

6H, CHMe₂), 0.92 (d, ³J_HH) 6.3, 6H, CHMe₂). ¹³C{¹H} NMR: Major

3

1

3

R-CH), 57.4 (d, J_CP) 8, R-CH), 53.8 (d, J_CP) 22, R-CH), 50.2,

isomer (Z_syn) δ 145.2 (d, J_CP) 114, PCN₂), 48.6 (d, J_CP) 20, d

34.8, 34.5, 34.2, 32.1, 26.3, 26.2, 26.1, 25.5, 24.8, 24.6 (Cy). (¹³C{¹H}

NCH), 46.6 (d, J_CP) 8, -NHCH), 38.2 (d, J_CP) 50, R-CH Cy),

3

1

9

13892 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Phospha(III)- and Phospha(V)guanidine Compounds

A R T I C L E S

26.9, 26.7, 26.6, 26.5, 26.4, 26.3, 26.1 (nonassigned resonances

corresponding to an overlapping series of doublets (J_CP) from the

R-CH Cy), 35.3, 34.4, 26.9, 26.8, 26.7, 26.6, 26.5, 26.3, 25.6, 24.9,

24.5 (nonassigned resonances corresponding to an overlapping series

of doublets (J_CP) from the methylene groups of the cyclohexyl

substituents). ³¹P{¹H} NMR: Major isomer (Z_syn) δ 62.7 (¹J_PSe) -688).

Minor isomer (E_syn) δ 66.4 (coupling to selenium not recorded due to

low intensity of the signal). ⁷⁷Se{¹H} NMR: Major isomer (Z_syn) δ

methylene groups of the cyclohexyl substituents) 24.9 (CHMe₂), 24.0

1

(CHMe₂). (Only the H and ¹³C{¹H} NMR data for the major (Z_syn

)

component can be reported with confidence, due to the low intensity

of the resonances corresponding to the minor (E_syn) species.) ³¹P{¹H}

NMR: Major isomer (Z_syn) δ 63.3. Minor isomer (E_syn) δ 67.4.

Cy₂P(S)C{NCy}{NHCy} (5b). This compound was pre-

pared in a manner analogous to that described for 3a, using

the following quantities: Cy₂PC{NCy}{NHCy} (0.10 g, 0.2 mmol)

and elemental sulfur (20 mg, 0.50 mmol). The compound was

crystallized from heptane as described for 3a. Yield (colorless crystals)

0.063 g, (58%). Anal. Calcd for C₂₅H₄₅N₂PS (436.68): C, 68.76; H,

10.39; N, 6.42. Found: C, 68.62; H, 10.26; N, 6.30. ¹H NMR: Major

isomer (Z_syn) δ 6.83 (dd br, J ) 7.4, 1H, NH), 3.63 (m, 1H, R-CH,

N-Cy), 3.42 (m, 1H, R-CH, N-Cy), 2.41-1.07 (m, 42H, Cy). ¹³C-

{¹H} NMR: Major isomer (Z_syn) δ 145.0 (d, ¹J_CP) 114, PCN₂), 57.1

1

-467 (¹J_SeP) -690, J_Se‚‚‚H ) 6). (Only the H, ¹³C{¹H}, and ⁷⁷Se-

{¹H} NMR data for the major (Z_syn) component can be reported with

confidence, due to the low intensity of the resonances corresponding

to the minor (E_syn) species.)

Ph₂P(BH₃)C{NCy}{NHCy} (7). A solution of BH₃‚THF (0.25 mL

of a 1.0 M solution in THF, 0.3 mmol) was added dropwise via syringe

to a solution of Ph₂PC{NCy}{NHCy} (0.10 g, 0.3 mmol) in THF at

room temperature. The resultant solution was stirred for 12 h, after

which time the volatiles were removed under reduced pressure,

affording a crude white oil. Crystallization from hexane at -30 °C

afforded analytically pure 7. Yield (colorless crystals) 0.022 g (22%).

Anal. Calcd for C₂₅H₃₆BN₂P (406.35): C, 73.89; H, 8.93; N, 6.89.

Found: C, 73.73; H, 8.64; N, 6.61. IR (Nujol mull, CsI, cm^-1): 3424m

υ(N-H), 2395br υ(B-H), 1600 υ(CdN), 565 υ(P-B). Compound 7

was unstable in solution with respect to loss of borane, precluding the

acquisition of reliable NMR data.

Details of the Crystallographic Study. Details of the crystal data,

intensity collection, and refinement for complexes 2a, {3b}, and {4b}

are collected in Table 1 and for 5a, 6a, 6b, and 7 in Table 2. Crystals

were covered in an inert oil, and suitable single crystals were selected

under a microscope and mounted on a Kappa CCD diffractometer. The

structures were refined with SHELXL-97.³⁵In all cases, the H-atom

on the amine nitrogen was refined with other H-atoms in riding mode;

the hydrogen atoms on boron in Ph₂P(BH₃)C{NCy}{NHCy} (7) were

refined.

Details of the Computational Study. All calculations were per-

formed using the Gaussian 03 suite of programs.¹⁷The model

compounds that were examined were constructed using the Cartesian

coordinates taken from the relevant X-ray structures. To simplify the

calculations and limit the steric influence of the nitrogen substituents,

the isopropyl and cyclohexyl groups were replaced with methyl groups

using the GaussView suite of programs. To examine the relative

energies of the R- and â-conformations, the amidine group was rotated

about the P-C bond while the rest of the molecule was fixed. In all

instances, full optimization and frequency calculations were performed

using the B3LYP density functional theory and the 6-31G(d) basis set.

Natural bond orbital (NBO) analyses³⁶were calculated on optimized

geometries.

3

1

(d, J_CP) 19, dNCH), 53.8 (d, J_CP) 8, -NHCH), 38.3 (d, J_CP

)

50, R-CH, Cy), 35.3, 34.5, 27.2, 27.0, 26.8, 26.6, 26.5 (×2), 26.4, 26.2,

25.6, 25.0, 24.6 (nonassigned resonances corresponding to an overlap-

ping series of doublets (J_CP) from the methylene groups of the

cyclohexyl substituents). (Only the ¹H and ¹³C{¹H} NMR data for the

major (Z_syn) component can be reported with confidence, due to the

low intensity of the resonances corresponding to the minor (E_syn

)

species.) ³¹P{¹H} NMR: Major isomer (Z_syn) δ 63.7. Minor isomer

(E_syn) δ 67.0.

Cy₂P(Se)C{NⁱPr}{NHⁱPr} (6a). This compound was prepared in a

manner analogous to that described for 3a, using the following

quantities: Cy₂PC{NⁱPr}{NHⁱPr} (0.10 g, 0.3 mmol) and elemental

selenium (50 mg, 0.63 mmol). The compound was crystallized from

heptane as described for 3a. Yield (colorless crystals) 0.084 g, (67%).

Despite repeated attempts, accurate elemental analysis could not be

obtained for 6a. ¹H NMR: Major isomer (Z_syn) δ 6.72 (dd br, J ) 7.0,

1H, NH), 3.83 (sept, ³J_HH) 6.1, 1H, CHMe₂), 3.60 (m, 1H, CHMe₂),

3

2.40-1.51 (m, 22H, Cy), 1.15 (d, J_HH) 6.1, 6H, CHMe₂), 0.93 (d,

³J_HH) 6.3, 6H, CHMe₂). ¹³C{¹H} NMR: Major isomer (Z_syn) δ 145.2

(d, ¹J_CP) 114, PCN₂), 48.7 (d, ³J_CP) 19, dNCH), 46.8 (d, ³J_CP) 9,

-NHCH), 38.1 (d, ¹J_CP) 43, R-CH Cy), 26.8 (×2), 26.6, 26.6, 26.4,

26.4, 26.3, 26.0 (nonassigned resonances corresponding to an overlap-

ping series of doublets (J_CP) from the methylene groups of the

1

cyclohexyl substituents), 24.8 (CHMe₂), 23.9 (CHMe₂). (Only the H

and ¹³C{¹H} NMR data for the major (Z_syn) component can be reported

with confidence, due to the low intensity of the resonances correspond-

ing to the minor (E_syn) species.) ³¹P{¹H} NMR: Major isomer (Z_syn) δ

62.4 (¹J_PSe) -692). Minor isomer (E_syn) δ 67.0 (coupling to selenium

not recorded due to low intensity of the signal).

Acknowledgment. We thank the University of Sussex for

financial support and Drs. M. S. Hill and J. R. Fulton for helpful

discussion during the preparation of this manuscript.

Cy₂P(Se)C{NCy}{NHCy} (6b). This compound was prepared in

a manner analogous to that described for 3a, using the following

quantities: Cy₂PC{NCy}{NHCy} (0.10 g, 0.2 mmol) and elemental

selenium (40 mg, 0.50 mmol). The compound was crystallized from

heptane as described for 3a. Yield (colorless crystals) 0.077 g, (64%).

Anal. Calcd for C₂₅H₄₅N₂PSe (483.57): C, 62.09; H, 9.38; N, 5.79.

Supporting Information Available: A complete citation list

for ref 17. This material is available free of charge via the

Internet at http://pubs.acs.org.

1

Found: C, 62.09; H, 9.25; N, 5.79. H NMR: Major isomer (Z_syn) δ

JA064212T

6.90 (dd br, J ) 7.2, 1H, NH), 3.63 (m, 1H, R-CH N-Cy), 3.45 (m,

1H, R-CH N-Cy), 2.46-0.94 (m, 42H, Cy). ¹³C{¹H} NMR (125

(35) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal

Structures; Go¨ttingen, 1997.

(36) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211.

1

MHz): Major isomer (Z_syn) δ 143.2 (d, J_CP) 107, PCN₂), 57.2 (d,

3

1

³J_CP) 18, dNCH), 53.9 (d, J_CP) 8, -NHCH), 38.2 (d, J_CP) 42,

9

J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13893

Article Doi

A conformational study of phospha(III)- and phospha(V)-guanidine compounds

DOI: 10.1021/ja064212t

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja064212t

Full text of DOI:10.1021/ja064212t

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