The intramolecular rearrangement of phosphinohydrazides [R′2P-NR-NR-M] → [RN=PR′2-NR-M]: General rules and exceptions. Transformations of bulky phosphinohydrazines (R-NH-N(PPh2)2, R = t Bu, Ph2P)

Article abstract of DOI:10.1021/ic201617p

Reactions of diphosphinohydrazines R-NH-N(PPh₂)₂ (R = tBu (1), Ph₂P (3)) with some metalation reagents (Co[N(SiMe ₃)₂]₂, LiN(SiMe₃)₂, La[N(SiMe₃)₂

Full text of DOI:10.1021/ic201617p

Article
pubs.acs.org/IC
The Intramolecular Rearrangement of Phosphinohydrazides [R′2P−
NR−NR−M] → [RNPR′2−NR−M]: General Rules and Exceptions.
Transformations of Bulky Phosphinohydrazines (R−NH−N(PPh2)2, R =
tBu, Ph2P)
Alexander N. Kornev,*,† Vyacheslav V. Sushev,† Yulia S. Panova,† Natalia V. Belina,†
Olga V. Lukoyanova,† Georgy K. Fukin,† Sergey Y. Ketkov,† Gleb A. Abakumov,† Peter Lonnecke,‡
̈
and Evamarie Hey-Hawkins‡
†Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 49 Tropinin Street, 603950 Nizhny Novgorod,
Russia
‡Institut fur Anorganische Chemie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Reactions of diphosphinohydrazines R−NH−
N(PPh2)2 (R = tBu (1), Ph2P (3)) with some metalation
reagents (Co[N(SiMe3)2]2, LiN(SiMe3)2, La[N(SiMe3)2]3,
nBuLi, MeLi) were performed. Compound 1 was synthesized
by the reaction of Ph2PCl with tert-butylhydrazine hydro-
chloride in 83% yield. This compound reveals temperature-
dependent 31P NMR spectra due to hindered rotation about the
P−N bonds. Complicated redox reaction of 1 with Co[N-
(SiMe3)2]2 proceeds with cleavage of the P−N and N−N bonds
to form a binuclear cobalt complex [Co{HN(PPh2)2-κ2P,P′}2(μ-PPh2)]2 (2) demonstrating a short Co···Co distance of
2.3857(5) Å, which implies a formal double bond between the Co atoms. Strong nucleophiles (nBuLi, MeLi) cause
fragmentation of the molecules 1 and 3, while reactions of 3 with lithium and lanthanum silylamides give products of the NNP →
NPN rearrangement [Li{Ph2P(NPPh2)2-κ2N,N′}(THF)2] (4) and [La{Ph2P(NPPh2)2-κ2N,N′}{N(SiMe3)2}2] (5), respectively.
These complexes represent the first examples of a κ2N,N′ bonding mode for the triphosphazenide ligand [(Ph2PN)2PPh2]−. DFT
calculations showed large energy gain (52.1 kcal/mol) of the [NNP]− to [NPN]− anion rearrangement.
a triphosphazenide anion by oxidative scrambling of the amide,
which reacts further with cobalt(II) to give the spirocyclic
metallaphosphazene.8 During our study we described the
rearrangement of phosphinohydrazides,10−15 which may be
considered as a useful approach to the synthesis of
phosphazenides and phosphinoamides (Scheme 1).
INTRODUCTION
■
Heteroatom ligands containing a phosphorus(III)−nitrogen
bond have many applications in organometallic chemistry and
catalysis. Important examples are catalytic C−H bond
activation,1 hydroformylation of alkenes to give commercially
valuable alcohols instead of aldehydes,2 palladium catalyzed
Heck, Suzuki, and amination reactions of aryl chlorides.3 The
chemistry of phosphinoamides has lately received considerable
attention because of two possible resonance structures,
[R′2PNR]− and [R′2PNR]. These heterofunctional systems
often display unique dynamic features, such as hemilability,
which provides an efficient molecular activation procedure
under mild conditions that is very important for catalytic
applications.4 Simple phosphinoamides (R2P−NR′−) may be
synthesized very easily by the direct interaction of chlor-
ophosphines with secondary amines.5,6 The approaches to the
oligophosphazenides with R2P(III)-terminal groups are not so
easy. As an example, it is known that the triphosphazenide
ligand [Ph2P−NPPh2−NPPh2]− may be synthesized only in
the coordination sphere of transition metals by the
disproportionation of bis(diphenylphosphino)amide anion.7−9
So, cobalt(II) chloride and LiN(PPh2)2 in boiling toluene form
Like the Michaelis−Arbuzov rearrangement16 and its
modifications involving transition-metal centers,17 the major
driving force of these reactions is the formation of
tetracoordinated phosphorus(V) species instead of phosphorus-
(III). Meanwhile, there are a number of examples for complexes
in which the rearrangement of phosphinohydrazides does not
occur: (Ge(NPh−NPh−PPh2)2;10 Co(NPh−NPh−P(iPr)2 and
Ni(NPh−NPh−P(iPr)2).13 The tendency of the phosphinohy-
drazide system toward rearrangement depends on several
factors. These are the energies of the covalent N−N and
coordination M−P bonds that ought to be cleaved in the course
of the reaction. We have shown13,15 that the main factor, which
controls the P−N−N to NP−N rearrangement, is the charge
Received: July 27, 2011
Published: December 30, 2011
© 2011 American Chemical Society
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possibility of the P−N bond formation depends on the steric
properties of the substituents at nitrogen and phosphorus. In
order to check this assumption we have explored trans-
formations of phosphinohydrazines R−NH−N(PPh2)2 bearing
bulky substituents (R = tBu, Ph2P) upon metalation. Note that
the steric effect of the Ph2P group in phosphinoamides18 can
vary considerably (unlike tBu) since the P−N bond length is
strongly dependent on the charge localized on the
phosphinoamide group, so that a formal P−N single bond
demonstrates more π-type character in phosphinoamides.19
For metalation of phosphinohydrazines we used cobalt
silylamide, Co[N(SiMe3)2]2 (the model reagent for a number
experiments), and some nucleophiles {LiN(SiMe3)2, La[N-
(SiMe3)2]3, nBuLi, MeLi}, containing strongly coordinating
hard metal cations.
Scheme 1. Rearrangements of Mono-, Bis-, and
Trisphosphinohydrazides
a
RESULTS AND DISCUSSION
■
1-tert-Butyl-2-bis(diphenylphosphino)hydrazine (1) was pre-
pared in 83% yield as colorless crystals by the reaction of tert-
butylhydrazine hydrochloride with 2 equiv of Ph2PCl in THF:
a
M = metal atom.
In the solid state compound 1 exists as a mixture of
stereoisomers 1a,b (two crystallographic independent mole-
cules) that differ in the orientation of the tert-butyl group with
respect to the unsymmetrical NP2 fragment:
at the hydrazido nitrogen. A strong negative charge promotes
elongation of the N−N bond and its cleavage in the first step of
the reaction. Tentatively, the rearrangement proceeds via
several intermediate steps, depicted in the Schemes 2 and 3.
Scheme 2. Proposed Mechanism for the Rearrangement of
Monophosphinohydrazides
Both stereoisomers have very similar structural parameters.
The molecular structure of 1a is shown in Figure 1 with
selected bond lengths and angles. The crystallographic data and
the details of the structure determination are given in Table 1.
The corresponding data for 1b are given in Figure S1 and Table
S1 in the Supporting Information.
Formation of stereoisomers is possible due to hindered
rotation around the P−N and N−N bonds. The molecule has a
nearly planar P2NN core. The sum of bond angles at N(1) is
357.6°. Steric demand of the tert-butyl group results in
elongation of the N−N bond (1.445(1) Å) as compared to
the known phosphinohydrazines (1.401(2) Å in Ph2P−NPh−
NPh−H,10 1.418(1) in 8-quinolyl-NH−N(PPh2)2).15 This
value is very close to that found in the other sterically hindered
hydrazine (Ph2P)2N−NH−PPh2 (1.437(3) Å).12 There is a
variation between P−N bond lengths in the crystal structure of
1 (1.718(1) Å and 1.729(1) Å in 1a; 1.731(1) Å and 1.719(1)
Å in 1b). In general, the P−N bond lengths and P−N−P bond
angles (120.38(6)° and 118.86(6)° for 1a and 1b respectively)
are typical for this kind of ligand.20
Scheme 3. Proposed Mechanism for the Rearrangement of
Diphosphinohydrazides
The product A (Scheme 2), having a single phosphorus(V)
atom, can be formed only after cleavage of the P→M
coordination bond. Cleavage of the P→M bond is not
necessary for the formation of B, having two phosphorus
atoms (Scheme 3). Both schemes include formation of a novel
P−N bond by the interaction of positively charged phosphorus
and negatively charged nitrogen atoms. According to these
schemes, electron-withdrawing substituents at phosphorus and
electron-donating groups at nitrogen will favor the rearrange-
ment. On the other hand, it is reasonable to propose that the
The 31P NMR spectrum of 1 reveals a broad singlet (δ 71.0
ppm) at ambient temperatures, but forms two spin-coupled
doublets (AB spin system) at 193 K (Figure 2).
The signals for the two Ph2P groups at 76.0 and 67.5 ppm
coalesced at 273 K. Changes in the multiplicity of the 31P
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Figure 1. The molecular structure of 1a with ellipsoids of 30%
probability. Hydrogen atoms except H(2) were omitted for clarity.
Selected bond lengths (Å) and angles (deg) for 1: P(1)−N(1)
1.718(1), P(2)−N(1) 1.729(1), N(1)−N(2) 1.445(1), N(2)−C(25)
1.494(2), N(2)−H(2) 0.89(2); N(2)−N(1)−P(1) 118.9(1), N(2)−
N(1)−P(2) 118.3(1), P(1)−N(1)−P(2) 120.4(1), N(1)−N(2)−
C(25) 116.6(1), N(1)−N(2)−H(2) 106(1).
Figure 2. Series of variable-temperature 31P NMR spectra for 1.
variable-temperature 31P NMR spectra for 1 are very similar to
those reported for bis(diphenylphosphino)amines R−N-
(PPh2)2 (R = iPr,20 CHMePh).21
Earlier we have reported reactions between a similar ligand,
(Ph2P)2N−NH−Ph, and transition metal silylamides Co[N-
(SiMe3)2]2 and [NiN(SiMe3)2(PPh3)2], which result in
signals can be interpreted in terms of restricted rotation about
the P−N bonds. The activation energy of the dynamic process
was found to be 10.9 kcal mol−1. The appearance of two spin-
coupled doublets at low temperatures indicates the presence of
conformers with Cs symmetry in solution. The observed
coupling constant (JP,P = 25 Hz) and general appearance of the
Table 1. Crystal and Structure Refinement
1
2
4
5
empirical formula
formula wt
C28H30N2P2
456.48
C80H78Co2N2O2P6
1403.12
C44H46LiN2O2P3
734.68
C48H66LaN4P3Si4
1043.23
temp, K
130(2)
100(2)
150(2)
100(2)
cryst syst
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/c
triclinic
space group
a, Å
P1
̅
12.3019(1)
31.7222(3)
12.9120(2)
90
16.1309(8)
21.1279(10)
19.9712(9)
90
10.0645(3)
25.9844(9)
15.3793(5)
90
11.5932(4)
b, Å
12.2192(4)
c, Å
20.8318(6)
α, deg
86.390(1)
β, deg
90.004(1)
90
94.437(1)
101.985(1)
90
84.327(1)
γ, deg
vol, Å−3
90
63.190(1)
5038.8(1)
8
6786.0(6)
3934.3(2)
2620.4(2)
Z
4
4
2
cryst size, mm3
abs coeff, mm−1
abs correction
max./min transmission
density (calcd), Mg/m3
reflns collected
indep reflns
data/restraints/params
R indices [I > 2σ(I)]
R indices (all data)
0.53 × 0.39 × 0.28
0.191
0.30 × 0.15 × 0.08
0.681
0.35 × 0.35 × 0.35
0.190
0.25 × 0.23 × 0.15
1.034
SCALE3 ABSPACK
SADABS
SADABS
SADABS
0.9475/0.8218
1.373
0.9364/0.9364
1.240
0.8604/0.7822
1.322
1.203
120353
57210
23285
25274
15400 [R(int) = 0.0429]
15400/0/818
13284 [R(int) = 0.1296]
13284/26/847
R1 = 0.0595, wR2 = 0.0956
R1 = 0.1202, wR2 = 0.1075
0.664 and −0.513
0.998
7728 [R(int) = 0.0281]
7728/0/469
R1 = 0.0519, wR2 = 0.1433
R1 = 0.0693, wR2 = 0.1540
1.045 and −0.340
1.067
11901 [R(int) = 0.0204]
11901/9/757
R1 = 0.0305, wR2 = 0.0718
R1 = 0.0374, wR2 = 0.0745
1.149 and −0.444
1.059
R1 = 0.0391, wR2 = 0.0869
R1 = 0.0489, wR2 = 0.0915
0.310 and −0.259
1.19
−3
largest diff. peak and hole, e·Å
GOF
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formation of spirocyclic complexes [M{NPh−PPh2N−
PPh2}2] (M = Co, Ni), demonstrating a rearrangement of the
ligand.11 Following these observations, we have explored ligand
1 containing the bulkier tBu group at nitrogen in the reaction
with cobalt(II) bis(trimethylsilyl)amide. The reaction proceeds
at room temperature in toluene solution for 48 h to give the
only isolable product 2 as red-brown crystals:
4. NBO analysis reveals that the metal−metal bond is formed
mainly by the 3d and 4p electrons of cobalt. An MO isosurface
study shows that the HOMO corresponds to the π-type Co−
Co wave function overlapping while HOMO−1 and HOMO−
7 are responsible for the σ- and δ-bonding, respectively (Figure
5).
Additional evidence for the existence of a direct metal−metal
bond in 2 is provided by analysis of the electron density
topology performed within Bader’s AIM theory.26 Our AIM
study reveals a Co−Co (3,−1) critical point corresponding to
the chemical bond (Figure S4 in the Supporting Information).
It is quite obvious that the reaction of 1 with Co[N-
(SiMe3)2]2 proceeds with N−N and P−N bond cleavage
caused by the presence of the tBu group. We have no intention
to study the mechanistic aspects of this transformation in detail,
but some tentative conclusions are given here. The first step of
the reaction, undoubtedly, is coordination of 1 to the cobalt
atom via the Ph2P group. The NH proton is unlikely to be
abstracted by the (Me3Si)2N group due to steric reasons.
Instead, formation of tetrakis(trimethylsilyl)hydrazine was
registered by GLC. This is in agreement with the redox
scheme of the reaction, since no compounds with cobalt in high
oxidation states (namely, Co3+) were found in the reaction
mixture. Liberation of (Me3Si)2NN(SiMe3)2 is known to
proceed in some reactions of transition metal silylamides.27,28
It may be supposed also that the formation of the spirocyclic
complex [Co{NR−PPh2N−PPh2}2] (R = tBu) is not easy
because of steric hindrance as compared to related compounds
{M{NR−PPh2N−PPh2}2] (R = Ph, M = Co, Ni).11 Instead,
fragmentation of the molecule is observed.
The molecular structure of 2 is shown in Figure 3 with
selected bond lengths and angles.
Each Co atom in the dimeric molecule is coordinated by one
bis(diphenylphosphino)amine ligand (DPPA) and two bridging
PPh2 groups. The Co2(μ-P)2 core is rigorously planar; notable
are the acute Co(1)−P(3)−Co(2) and Co(1)−P(4)−Co(2)
bond angles of 66.56(2)° and 66.04(2)°, respectively. Both
P(1)Co(1)P(2) and P(5)Co(2)P(6) planes form dihedral
angles with the Co2(μ-P)2 core of 67.3° and 76.1°, respectively.
The Co−Co distance in 2 (2.3857(5) Å) is much shorter than
the sum of the covalent radii (2.52 Å),22 which implies a formal
double bond between the Co atoms, as required to achieve 18
valence electrons about each metal. For comparison, a
nonbonding Co···Co distance of 3.573(2) Å found in
[Co2{μ-PPh2}2(CO)6],23 a single Co−Co bond of 2.56(1) Å
in [(CpCo)2{μ-PPh2}2]24 and a double bond of 2.343(2) Å in
[Co2{μ-PPh2}2(CO)2(PEtPh)2]25 are in agreement with our
data. The unit cell contains two THF molecules per one
molecule of 2, which form hydrogen bonds with NH group of
both DPPA ligands (Figure S2 in the Supporting Information).
Computational optimization of the molecular structure of 2
at the B3LYP/6-31G(d) level of theory also resulted in a short
Co−Co distance (2.310 Å) (Figure S3 in the Supporting
Information). Molecular orbitals for 2 are presented in Figure
Interestingly, the lithium salt of 1 was not obtained at
ambient conditions while many other lithium phosphinohy-
drazides were characterized in crystalline form or in
solution.10−15 Thus, unexpectedly the reaction between 1 and
MeLi in toluene/Et2O solution gave a mixture of products,
LiPPh2, Ph2P−PPh2, Ph2PMe and tBu−PPh2 according to the
31P NMR spectra and chromatography−mass spectrometry
measurements (Supporting Information, page 8). Interaction of
1 with n-BuLi proceeds in a similar way to give Ph4P2 and
Figure 3. The molecular structure of 2 with ellipsoids of 30% probability. Phenyl groups were omitted for clarity. Selected bond lengths (Å) and
angles (deg) for 2: Co(1)−Co(2) 2.3857(5), Co(1)−P(3) 2.1671(8), Co(1)−P(1) 2.1750(8), Co(1)−P(4) 2.1878(8), Co(1)−P(2) 2.1896(8),
P(1)−N(1) 1.687(2); P(3)−Co(1)−P(4) 114.00(3), P(1)−Co(1)−P(2) 72.52(3), P(6)−Co(2)−P(5) 72.45(3), P(3)−Co(2)−P(4) 113.36(3),
P(4)−Co(1)−P(2) 115.06(3), N(1)−P(1)−Co(1) 93.93(8), N(1)−P(2)−Co(1) 93.52(8), Co(1)−P(4)−Co(2) 66.04(2), P(1)−N(1)−P(2)
100.0(1), P(5)−N(2)−P(6) 97.9(1).
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Figure 4. Molecular orbitals for 2 and their energies in electronvolts are provided.
Figure 5. Contour maps for the HOMO, HOMO−1 and HOMO−7 of the 2 molecule in the plane orthogonal to the Co−Co bond and bearing two
bridging P atoms. The contours start at 0.01/−0.01 Eh, and the step is 0.01/−0.01 Eh.
LiPPh2 as the main products. At the same time, more sterically
hindered LiN(SiMe3)2 does not react with 1 at all. Thus, the
behavior of 1, containing a bulky tBu group, in metalation
reactions is quite different from that of known phosphinohy-
drazines.10−15
In view of these results, the behavior of the related ligand,
tris(diphenylphosphino)hydrazine (3) (R = Ph2P), deserves
detailed consideration. Earlier we have reported the reactions of
3 with Co[N(SiMe3)2]2 and NiCp2 which gave six-membered
cyclic metallophosphazenes12 as a result of the rearrangement
The reaction proceeds in toluene solution at ambient
conditions; the product was crystallized by addition of THF
and cooling to 0 °C. The molecular structure of 4 with selected
bond lengths and angles is shown in Figure 6. The
crystallographic data and details of the structure determination
are given in Table 1.
of this ligand in the coordination sphere of cobalt(II) and
nickel(II):
Meanwhile, our attempts to synthesize the lithium salt of 3 or
its rearrangement product by reaction with nBuLi were
unsuccessful.12 Instead, a complex mixture of products was
formed. Among them (Ph2P)2NH (31P NMR δ 44.5 ppm) was
found, which is indicative of the N−N bond cleavage in 3.
More importantly, lithium silylamide, LiN(SiMe3)2, reacts with
3 to form the rearrangement product 4 only in 78% yield:
Figure 6. The molecular structure of 4 with ellipsoids of 30%
probability. Phenyl groups, atoms C of the THF molecule and
hydrogen atoms were omitted for clarity. Selected bond lengths (Å)
and angles (deg) for 4: P(1)−N(1) 1.673(1), P(2)−N(2) 1.601(1),
P(2)−N(1) 1.601(1), P(3)−N(2) 1.680(1), Li(1)−N(2) 2.061(3),
Li(1)−N(1) 2.067(3), Li(1)−O(2S) 1.918(3,) Li(1)−O(1S)
1.955(3); O(2S)−Li(1)−O(1S) 100.1(2), N(2)−Li(1)−N(1)
75.4(1), N(2)−P(2)−N(1) 104.10(7), P(2)−N(1)−P(1) 125.44(9),
P(2)−N(1)−Li(1) 90.1(1), P(2)−N(2)−P(3) 122.80(9), P(2)−
N(2)−Li(1) 90.3(1).
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The open PNPNP phosphazenide chain in 4 is coordinated
to the lithium atom by two nitrogen atoms to form an entirely
planar four-membered metallacycle. The geometry about the
lithium atom (additionally coordinated by two THF molecules)
is distorted tetrahedral with O(2S)−Li(1)−O(1S) and N(2)−
Li(1)−N(1) angles of 100.1(2)° and 75.4(1)°, respectively.
The terminal P−N bonds, P(1)−N(1) of 1.673(1) Å and
P(3)−N(2) of 1.680(1) Å, which exert single-bond character
are longer than the internal P−N bonds P(2)−N(2) of
1.601(1) Å and P(2)−N(1) of 1.601(1) Å, that participate in
the delocalized bonding scheme.
It is well-known that the ligand [Ph2PNP(Ph2)NPPh2] forms
six-membered metallacycles with transition metals (Co, Ni,7,8,12
Fe9) by κ2P,P′-complexation. To the best of our knowledge,
compound 4 is the first example of a κ2N,N′ bonding mode for
this ligand.
Bearing in mind the differences between the reagents nBuLi
and LiN(SiMe3)2 in the reaction with 3, we have explored
related reactions of 3 with lanthanum and sodium silylamides.
As expected, both silylamides react with 3, but lanthanum
silylamide, La[N(SiMe3)2]3, reacts very cleanly. The reaction
proceeds in toluene solution at 115 °C in 5 h with substitution
of one (Me3Si)2N group to give 5 in 92% yield. Using an excess
of 3 in the reaction does not result in substitution of a second
silylamide group. Apparently, the formation of the tight chelate
increases the strength of the remaining La−N(SiMe3)2 bonds.
Steric hindrances also should be taking into consideration.
Note that at room temperature the process was not
observed; on the other hand, at temperatures above 130 °C
the starting material converts into another products,
(Ph2P)2NH and (Ph2PN)4, as we have described earlier.12
Figure 7. The molecular structure of 5 with ellipsoids of 30%
probability. Phenyl and methyl groups and hydrogen atoms were
omitted for clarity. Selected bond lengths (Å) and angles (deg) for 5:
La(1)−N(3) 2.374(1), La(1)−N(4) 2.377(1), La(1)−N(1) 2.518(1),
La(1)−N(2) 2.583(1), P(2)−N(1) 1.623(1), P(2)−N(2) 1.624(1),
P(1)−N(1) 1.725(1), P(3)−N(2) 1.717(1); N(3)−La(1)−N(4)
116.58(5), N(1)−La(1)−N(2) 60.00(4), N(1)−P(2)−N(2)
103.59(7), P(1)−N(1)−P(2) 115.83(8), P(2)−N(2)−P(3)
116.24(8).
It is surprising to see that the P−N bonds in the lanthanum
complex are notably longer than the corresponding P−N bonds
in the lithium derivative. This difference may reflect the greater
steric bulk of the molecule 5 due to the presence of (Me3Si)2N
group. Note that the known six-membered metallacycles with a
[(Ph2PN)2PPh2-κ2P,P′]− ligand exhibit averaged N−P bond
lengths (1.59−1.62 Å).7,9,12
The molecular structure of 5 with selected bond lengths and
angles is shown in Figure 7. The crystallographic data and
details of the structure determination are given in Table 1.
Similar to the lithium salt 4, the PNPNP phosphazenide
chain in 5 is coordinated to lanthanum via two nitrogen atoms
to form a nearly planar four-membered metallacycle. The
geometry about the lanthanum atom (additionally coordinated
by two (Me3Si)2N ligands) is distorted tetrahedral with N(1)−
La(1)−N(2) and N(3)−La(1)−N(4) bond angles of 60.00(4)°
and 116.58(5)°, respectively. The terminal P−N bonds of
1.725(1) and 1.717(1) Å indicate single-bond character, while
the shorter internal P−N bonds of 1.623(1) and 1.624(1) Å
participate in the delocalized bonding scheme. The P−N bond
lengths in 4 and 5 are represented for comparison in a single
scheme:
Since complexes 4 and 5 have notably different P−N bond
lengths, the corresponding JP,P coupling constants in the 31P
NMR spectra are expected to be rather different. The proton-
decoupled 31P NMR spectrum of the lanthanum complex 5
(A2X spin system) consists of a doublet (δ 41.7, 2JP,P = 36 Hz)
2
and a triplet (δ 34.2, JP,P = 36 Hz) at room temperature in a
ratio of 2:1 (Figure S5 in the Supporting Information). The
lithium complex 4 demonstrates similar chemical shifts (δ 42.7
and 34.1 ppm) but a more complex 31P{1H} NMR spectrum,
which was assigned to an A2B spin system with an AB coupling
constant of 120 Hz (Figure S6 in the Supporting Information).
Apparently, shortening P−N bonds in 4 (as compared to those
in 5) makes s-orbital character on phosphorus atoms more
2
considerable, which determines a larger JP,P coupling constant
observed for 4.
We have also explored the similar reaction of 3 with sodium
silylamide, NaN(SiMe3)2, in toluene/THF solution. The 31P
NMR spectrum of the reaction mixture showed a broad
2
resonance (a doublet at 42.1, JP,P = 98 Hz) and a triplet (δ
2
34.5, JP,P = 98 Hz) at room temperature. Unfortunately,
attempts to crystallize the sodium salt only gave a solid of
insufficient purity.
DFT calculations showed that a rearrangement of the neutral
molecule (Ph2P)2N−NH−PPh2 to the iminophosphorane
879
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Article
solved by direct methods and were refined on F2hkl using SHELXL-97
packages.34 All non-hydrogen atoms in 1, 2, 4, and 5 were refined
anisotropically. All H atoms in 1, H(N) atoms in 2 and all H atoms
except methyl hydrogens at C(37), C(40), C(42), C(47) in 5 were
found from Fourier syntheses of electron density and were refined
isotropically. Other hydrogen atoms in 2, 4, and 5 were placed in
calculated positions and were refined in the riding model. SCALE3
ABSPACK35 (for 1) and SADABS36 were used to perform absorption
corrections in 2, 4, and 5. The crystal for 1 was found to be slightly
twinned (twin law by rows, −1 0 0, 0 −1 0, 0 0 1; twin domain ratio
0.947(1), 0.053(1). The main crystallographic data and structure
refinement details for 1, 2, 4, and 5 are presented in Table 1.
DFIX Instructions were used for the fix of geometry of THF
molecules in 2 and some objectively detected the C−H distances in 5.
CCDC 819234 (1), 819235 (2), 819236 (4), and 819237 (5)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge at www.ccdc.cam.ac.uk/data_
request/cif from the Cambridge Crystallographic Data Centre.
Computational Details. DFT calculations performed in this work
were carried out at the B3LYP/6-31G(d) level of theory with the
Gaussian 03 package (Supporting Information). The optimized
geometries of 2 and the anions [(Ph2PN)2PPh2]− and [(Ph2P)2N−
NPPh2]− correspond to energy minima. Frequency calculations for
every mentioned structure were performed. There are no imagery
frequencies in every case. For the geometry optimization of 2 we used
the full structure without simplification. The NBO analysis37 was
performed to analyze the charge distribution in the molecules
investigated. The electron density topology was investigated within
the frames of Bader’s atom in molecules theory26 with use of the
AIMAll package.38
Ph2P−NPPh2−NH−PPh2 is energetically favorable by 31.5
kcal/mol. The total energies of the two isomers were compared
without modeling the rearrangement process (Table S2 in the
Supporting Information). However, more importantly, the
rearrangement of the anion [(Ph2P)2N−N(PPh2)]− (model
compound without metal counterion) to the iminophosphora-
nate anion [(Ph2PN)2PPh2]− is a much more favorable process
(by 52.1 kcal/mol) (Table S3 in the Supporting Information).
The possibility of enhanced charge delocalization in the
iminophosphoranate anion as compared to the isomeric
phosphinohydrazide (Tables S4−S7 in the Supporting
Information) gives an additional ∼20 kcal/mol, which makes
the NNP → NPN rearrangement extremely favorable.
CONCLUSION
■
In summary, considering the ligands R−NH−N(PPh2)2 (R =
tBu (1), Ph2P (3)) with respect to NNP → NPN rearrange-
ment, the electron-donating, bulky tBu group apparently
encourages N−N bond cleavage, but its steric demand may
prevent the P−N coupling reaction and rearrangement of the
ligand. Instead, fragmentation of the molecule is observed to
give HN(PPh2)2 and diphenylphosphanido fragments as new
building blocks as was observed in the binuclear cobalt complex
[Co{HN(PPh2)2-κ2P,P′}2(μ-PPh2)]2 (2). The phosphinohy-
drazines 1 and 3 react with nBuLi to give decomposition
products. More intriguing results were obtained while exploring
the reactivity of 3 toward lithium and lanthanum silylamides.
Both reactions gave products resulting from rearrangement of
phosphinohydrazide to the iminophosphoranate,
[(Ph2PN)2PPh2]−, which exhibits κ2N,N′ coordination to the
metal (Li or La).
Synthesis. tBuN(H)−N(PPh2)2 (1). A solution of Ph2PCl (11.00 g,
50 mmol) in 50 mL of THF was slowly added to a mixture of
tBuNHNH2(HCl) (3.79 g, 30 mmol) and 10.2 g of Et3N (100 mmol)
in 50 mL of THF at ambient conditions. The mixture was stirred for
24 h and then filtered. THF was distilled off, and a large amount of n-
hexane (∼100 mL) was added. Another portion of Et3N(HCl) was
filtered off. The remaining solution was concentrated and left
overnight at room temperature. Large colorless crystals of 1 were
formed, isolated by filtration and dried in vacuum. Yield: 11.37 g
(83%). Anal. Calcd for C28H30N2P2, %: C, 73.67; H, 6.62; P, 13.57.
Found, %: C, 73.59; H, 6.68; P, 13.63. 1H NMR (d8-toluene, 295 K) δ
(ppm): 8.0−6.7 (m, 21H, Ph and NH); 0.76 (s, 9H, tBu). 31P{1H}
NMR, δ (ppm): 72.6. IR (KBr pellet), ν/cm−1: 3422 (NH), 3053m,
2967m, 1477s, 1433s, 1386w, 1361w, 1304w, 1261m, 1203m, 1094s,
1025m, 880s, 794s, 740s, 720s, 694vs, 643m, 558m, 512m, 476m.
[Co{HN(PPh2)2-κ2P,P′}2(μ-PPh2)]2 (2). A solution of tBuN(H)-
N(PPh2)2 (0.27 g, 0.60 mmol) in 10 mL of toluene was added to a
solution of cobalt silylamide (0.15 g, 0.40 mmol) in the same solvent
(10 mL). The mixture was kept for 48 h at room temperature. Toluene
was removed in vacuum and replaced by THF. The mixture was kept
overnight at 0 °C. Red-brown crystals of 2 were formed, isolated by
filtration and dried in vacuum. Yield: 0.19 g (0.14 mmol, 68%). Anal.
Calcd for C80H78Co2N2O2P6 (two THF solvate molecules), %: C,
Computation of total energies for the triphosphinohydrazine
Ph2PNH−N(PPh2)2 and isomeric iminophosphoirane Ph2P−
NPPh2−NH−PPh2 has shown the energy difference of 31.5
kcal/mol. At the same time, the difference between total
energies of hydrazido anion [Ph2P−NN(PPh2)2]− and isomeric
iminophosphoranate [(Ph2PN)2PPh2]− is significantly higher
(ΔE = 52.1 kcal/mol). An additional ∼20 kcal/mol comes from
the possibility of enhanced charge delocalization in the
iminophosphoranate which makes the rearrangement of
phosphinohydrazide anion extremely favorable.
EXPERIMENTAL SECTION
■
General Remarks. Solvents were purified following standard
methods.29 Toluene was thoroughly dried and distilled over sodium
metal prior to use. Diethyl ether and THF were dried and distilled
over Na/benzophenone. The silylamides [Co{N(SiMe3)2}2]30,31 and
[La{N(SiMe3)2}3]32 were prepared according to known methods. All
manipulations were performed with rigorous exclusion of oxygen and
moisture, in a vacuum or under an argon atmosphere using standard
Schlenk techniques. The spectrophotometric determination of cobalt
in 2 was provided by the method described in ref 33. Infrared spectra
were recorded on a Bruker spectrometer Vertex 70 from 400 to 4000
cm−1 in Nujol mulls on KBr plates. NMR spectra were recorded in
CDCl3 or C6D6 solutions using a Bruker DPX-200 device. The identity
of the reaction products (Me3Si)2NH and (Me3Si)2NN(SiMe3)2 was
established by comparison of retention times with authentic samples
using gas chromatography analyses with a Tsvet-500 device, equipped
with 0.4 cm × 200 cm stainless steel columns, packed with 5% SE-30
on Chromatone N-Super, with a thermoconductivity detector and with
helium as a carrier gas.
1
68.48; H, 5.60; Co, 8.40 Found, %: C, 68.27; H, 5.72; Co, 8.32. H
NMR (THF- d8, 295 K) δ (ppm): 3.95 (s, 2H, NH), 6.5−9.5 (m, 60H,
Ph). 31P{1H} NMR, δ (ppm): 112.2 (m, Ph2PCo), 22.0 (m, Ph2PN).
IR (nujol), ν/cm−1: 3450w, 1307w, 1091m, 1025w, 973w, 842m, 735s,
695s, 610m, 554w, 532m. UV/vis spectrum (THF): λmax 435 (240),
515 (110) nm.
[Li{Ph2P(NPPh2)2-κ2N,N′}(THF)2] (4). A solution of LiN(SiMe3)2
(0.20 g, 1.2 mmol) in 10 mL of toluene was added to a solution of
(Ph2P)2N−NH−PPh2 (3) (0.71 g, 1.2 mmol) in the same solvent (10
mL) at 0 °C. The color of the solution turned yellow. After the
mixture was concentrated to 10 mL, 0.5 mL of THF was charged into
the system via a syringe, and the crystallization was initiated. The
mixture was left undisturbed overnight at 0 °C. Large colorless crystals
of 4 were formed, isolated by filtration and dried in vacuum. Yield:
0.68 g (78%). Anal. Calcd for C44H46LiN2O2P3, %: C, 71.93; H, 6.31.
X-ray Crystallography. The X-ray diffraction data were collected
on an Xcalibur-S (Agilent Technologies) for 1 and on a SMART
APEX diffractometer for 2, 4, and 5 (graphite-monochromated, Mo
Kα radiation, φ−ω-scan technique, λ = 0.71073 Å). All structures were
1
Found, %: C, 71.85; H, 6.34. H NMR (THF-d8, 295 K) δ (ppm):
2
7.1−8.1 m (Ph). 31P{1H} NMR, δ (ppm): 42.7 (m, JA,B = 120 Hz);
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Inorganic Chemistry
Article
2
34.1 (m, JA,B = 120 Hz). IR (Nujol), ν/cm−1: 1125sh, 1091vs, br,
(11) Sushev, V. V.; Kornev, A. N.; Min’ko, Y. A.; Belina, N. V.;
Kurskiy, Yu. A.; Kuznetsova, O. V.; Fukin, G. K.; Baranov, E. V.;
Cherkasov, V. K.; Abakumov, G. A. J. Organomet. Chem. 2006, 691,
879−889.
(12) Sushev, V. V.; Belina, N. V.; Fukin, G. K.; Kurskiy, Yu. A.;
Kornev, A. N.; Abakumov, G. A. Inorg. Chem. 2008, 47, 2608−2612.
(13) Kornev, A. N.; Belina, N. V.; Sushev, V. V.; Fukin, G. K.;
Baranov, E. V.; Kurskiy, Y. A.; Poddelskii, A. I.; Abakumov, G. A.;
1062sh, 913m, 897m, 785s, 742s, 727m, 697s, 673m, 542m, 531m,
503m, 474m.
[La{Ph2P(NPPh2)2-κ2N,N′}{N(SiMe3)2}2 (5). A mixture of 3 (0.29
g, 0.5 mmol) and La[N(SiMe3)2]3 (0.31 g, 0.5 mmol) in toluene (10
mL) was heated at 115 °C for 5 h. The resulting solution was
concentrated to 5 and 0.5 mL of Et2O was added. The mixture was left
overnight at 0 °C. Colorless crystals of 5 were obtained, isolated by
filtration and dried in vacuum. Yield: 0.48 g (92%). Anal. Calcd for
C48H66LaN4P3Si4, %: C, 55.26; H, 6.38; N, 5.37. Found, %: C, 55.21;
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1
H, 6.42; N, 5.33. H NMR (THF-d8, 295 K) δ (ppm): 7.0−8.2 (m,
30H, Ph); −0.51 (s, 36H, Me). 31P{1H} NMR (THF, 295 K), δ
(ppm): 41.7 (d, 2JP,P = 36 Hz); 34.2, (t, 2JP,P = 36 Hz). IR (Nujol), ν/
cm−1: 1242vs, 1180m, 1110s, 1070w, 980vs, br, 880w, 826s, 806s,
768m, 738m, 714m, 696m, 675m, 592s, 5322s, 506m, 479m.
(15) Kornev, A. N.; Belina, N. V.; Sushev, V. V.; Panova, J. S.;
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430.
ASSOCIATED CONTENT
■
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S
* Supporting Information
Crystallographic information for 1, 2, 4, and 5 in CIF format;
optimized structure of 2 at the B3LYP/6-31G(d) level and
selected bond distances and angles; molecular orbitals for 2,
contour maps for the HOMO, HOMO−1 and HOMO−7 and
electron density map of 2; 31P{1H} NMR spectra of 4 and 5;
selected Mulliken charges for 3 and related particles; B3LYP
total energy for isomeric ligands; interaction of 1 with MeLi
and n-BuLi. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Organomet. Chem. 2002, 651, 90−97.
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2008, 2832−2838.
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1983, 2, 383−387. (b) Harley, A. D.; Whittle, R. R.; Geoffroy, G. L.
Organometallics 1986, 5, 1283.
AUTHOR INFORMATION
■
(24) Coleman, J. M.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 542−
552.
(25) Harley, A. D.; Whittle, R. R.; Geoffroy, G. L. Organometallics
1983, 2, 60−63.
(26) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: Oxford, 1990.
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Corresponding Author
*E-mail: akornev@iomc.ras.ru. Fax: +7 (831) 4627497. Phone:
+7 (831) 4627795.
ACKNOWLEDGMENTS
■
This work was supported by The Russian President’s program
“Leading Scientific Schools” (No. 7065.2010.3), State Con-
tracts No. 16.740.11.0015 and No. P-337, and by The Deutsche
Forschungsgemeinschaft (HE 1376/29-1). We thank Dr. O. V.
Kouznetsova and N. M. Khamaletdinova for IR measurements.
(30) Andersen, R. A.; Faegri, K.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782.
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