Carbodiimide insertion reactions of homoleptic heavier alkaline earth amides and phosphides

Article abstract of DOI:10.1039/c0dt00089b

A series of homoleptic guanidinate-type complexes of the heavier alkaline earth (Ae) metals calcium and strontium have been prepared. The six-coordinate compounds [Ae{ⁱPrNC(NPh₂)CNⁱPr} ₂(THF)₂] (Ae =

Full text of DOI:10.1039/c0dt00089b

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Carbodiimide insertion reactions of homoleptic heavier alkaline earth amides
and phosphides†‡
Anthony G. M. Barrett,c Mark R. Crimmin,a,c Michael S. Hill,*a Peter B. Hitchcock,b Sarah L. Lomas,a
Mary F. Mahona and Panayiotis A. Procopioud
Received 8th March 2010, Accepted 18th June 2010
First published as an Advance Article on the web 7th July 2010
DOI: 10.1039/c0dt00089b
A series of homoleptic guanidinate-type complexes of the heavier alkaline earth (Ae) metals calcium
i
and strontium have been prepared. The six-coordinate compounds [Ae{ PrNC(NPh2)CNiPr}2(THF)2]
(Ae = Ca and Sr) were synthesised through reactions of the appropriate THF-solvated
hexamethyldisilazide [Ae{N(SiMe3)2}2(THF)2] with two molar equivalents of diphenylamine and
1,3-di-iso-propylcarbodiimide. Both compounds were shown to crystallise with a cisoid arrangement of
the two THF molecules at the metal centres. In contrast, the (Me3Si)2N-substituted calcium
guanidinate, [Ca{CyNC{N(SiMe3)2}CNCy}2(THF)2], contains two coordinated THF molecules with a
trans disposition. Further reactions of the free amidine [{(2-FC6H4)N}C(NHiPr)2] with either
[Ca{N(SiMe3)2}2(THF)2] or its dimeric unsolvated analogue provided monomeric or dimeric
derivatives respectively in which the ligands had tautomerised to an anisobidentate form. A further
reaction of the phosphaguanidine [CyN C(PPh2)N(H)Cy] with [Sr{N(SiMe3)2}2(THF)2] provided the
first example of a phosphaguanidinate complex of this heavier alkaline earth metal. This compound has
also been characterised in the solid state and shown to exist with a transoid configuration of the
coordinated THF molecules.
guanidinato,15 phosphaguanidinato16 and propargylamidinato17
anions (Scheme 1). Although much of this work has utilized
Introduction
The coordination chemistry of the heavier alkaline earth (Ae)
elements Ca, Sr and Ba has advanced enormously over the
heteroleptic derivatives of the sterically demanding b-diketiminate
ligand I,10 cumulene hydroelementation, to form the protonated
last decade.1 Many potential applications, from homogeneous
E–C coupled urea, guanidine, phosphaguanidine and propargy-
catalysis2–8 to precursor chemistry for high technology coatings
prepared by chemical vapour deposition methods,9 require the
provision and maintenance of a low nuclearity environment.
Central to this effort, therefore, has been the selection of a wide
lamidine products, may be achieved through the use of alkaline
earth silylamides [Ae{N(SiMe3)2}2] under catalytic conditions
(Scheme 2). We have previously proposed that these catalytic
reactions most likely occur via the intermediacy of homoleptic
array of sterically demanding ligand sets such as the b-diketiminate
(I),10 triazenido (II)2d and tris(imidazolin-2-ylidene-1-yl)borate
(III)11 anions to impose the requisite level of kinetic control over
the otherwise ionic and completely labile coordination chemistries
of the group 2 elements. We, and others, have previously re-
ported that, in common with similarly d0 complexes of group
1 and group 3/lanthanide elements,12,13 a variety of cumulenic
substrates such as isocyanates and N-N-dialkylcarbodiimides
are highly susceptible toward insertion into the polar Ae–E
bonds of heavier group 2 amides (i.e. E = NR2/NAr2), phos-
phides (E = PR2/PAr2) and acetylides (E = C CR/C CAr)
to form a wide variety of coordinated alkaline earth ureido,14
resting state species which are then protonated under a kinetic
regime rationalised by the Curtin-Hammet principle.14–17 As a
continuation of this study we now illustrate the generality of this
insertion chemistry through a series of reactions of dialkylcarbodi-
imides with the Ae–E bonds of a variety of group 2 amides and
phosphides.
aDepartment of Chemistry, University of Bath, Claverton Down, Bath, UK,
BA2 7AY. E-mail: msh27@bath.ac.uk
bThe Chemistry Laboratory, University of Sussex, Falmer, Brighton, BN1
9QJ
cDepartment of Chemistry, Imperial College London, Exhibition Road,
South Kensington, London, U.K, SW7 2AZ
dGlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Steve-
nage, Hertfordshire, U.K., SG1 2NY
† Dedicated to Professor David Rankin on the occasion of his retirement.
‡ CCDC reference numbers 768276–768281 . For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00089b
Scheme 1
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Scheme 2
Results and discussion
Guanidination and phosphaguanidination of carbodiimides
Fig. 1 ORTEP representation of Compound 2 (thermal ellipsoids at
30% probability). Hydrogen atoms and atom fragments from the minor
disordered component are omitted for clarity.
We have previously described the facile insertion of the cyclohexyl-
substituted carbodiimide, CyN
C NCy (Cy = C6H11) into the
Ca–NPh2 bond of a b-diketiminato calcium diphenylamide.15a
In a similar manner, addition of a hexane solution of two
equivalents of diphenylamine and 1,3-di-isopropylcarbodiimide
to the group 2 silylamides [Ae{N(SiMe3)2}2(THF)2] resulted in the
compounds and selected bond length and angle data are, however,
provided in Tables 1–3 respectively. Both compounds feature
a single pseudo-octahedral group 2 centre with a coordination
sphere provided by two bidentate guanidinate ligands and two
molecules of coordinated THF. The coordinated ether molecules
rapid formation of the corresponding homoleptic THF-solvated
i
guanidinato
complexes,
[Ae{ PrNC(NPh2)NiPr}2(THF)2],
1
compounds 1 (Ae = Ca) and 2 (Ae = Sr) (Scheme 3). The H
1
and 13C{ H} NMR spectra of both isolated compounds were
consistent with the expected formulations and the formation
of the guanidinate complex was characterised for compound
1 by the quaternary carbon resonance of the N3C core at
demonstrate a
cisoid geometry [O(1)–Ca–O(2) 80.87(4)◦]
reminiscent of that observed in the previously described calcium
i
complex [Ca{ PrNC(NHiPr)NPh}2(THF)2],15b the strontium
160.8 ppm.15a Although an analogous signal in the 13C{ H}
triazenide complex [Sr{Dipp2N3}2(THF)2] (Dipp
= 2,6-di-
1
NMR spectrum of compound 2 could not be observed, the
spectra of neither compound displayed the level of complexity
that we have previously observed at ambient temperatures for
isopropylphenyl)2d and a homoleptic strontium formamidinate
[Sr{(DippN)2CH}2(THF)2] bearing the same sterically demanding
N-aryl substituents.18 Although broadly indicative of
a
i
[Ca{ PrNC(NHiPr)NPh}2(THF)2], the product of an analogous
similar donation of anionic charge, compound 1 displays a,
reaction with [Ca{N(SiMe3)2}2(THF)2] employing aniline rather
than diphenylamine.15b In this latter case variable temperature
NMR studies evidenced the establishment of fluxional processes
indicative of monomer/dimer equilibration via THF dissociation
somewhat surprisingly, wider range of N–Ca bond lengths
˚
[2.3962(13)–2.4428(14) A] than that observed within the similarly
i
six-coordinate [Ca{ PrNC(NHiPr)NPh}2(THF)2] [2.4179(10)–
˚
2.4291(11) A] which contains two anisobidentate guanidinate
2
and exchange of k -terminal and bridging interactions of the
units.15b The bite angles of the N–Ca–N chelates within 1 and
this previously reported complex are, however, consistent [for 1
56.28(5)◦ and 55.80(5) versus 56.13(3)◦] and are slightly more
obtuse compared to those within a five-coordinate homoleptic
guanidinate complex bearing a more sterically demanding
(Me3Si)2N- substituent, [Ca{CyNC(N(SiMe3)2)NCy}2(OEt2)],
[53.19(9)◦].15c The bond lengths and angles about the N3C
tautomerised [iPrNC(NHiPr)NPh]- guanidinate ligand. For both
compounds 1 and 2 we infer from the much simpler NMR
spectra a greater level of coordinative stability as a result, most
likely, of both the greater steric demands of the Ph2N- unit
and the inability of the as-formed guanidinate ligand to exist
in similar alternative tautomeric forms. Crystallisation from
hexane solution provided crystals of both compounds 1 and
2 suitable for X-ray crystallographic analysis. Although the
compounds are not isostructural, both are broadly analogous
in terms of the gross features of their coordination geometries
and, consequently, only the structure of the strontium compound
2 is illustrated in Fig. 1. Details of the analyses of both
core of the guanidinate ligands within
1 are effectively
identical to those within the b-diketiminate complex,
i
[{ArNC(Me)CHC(Me)NAr}Ca{ PrNC(NPh2 )CNiPr}(THF)],
the only previous calcium complex containing the
[iPrNC(NPh2)CNiPr]- anion to have been structurally
characterised.15b
Scheme 3
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Table 1 Crystallographic data for compounds 1–6
1
2
3
4
5
6
Molecular formula
C46H64CaN6O2
C46H64SrN6O2
820.65
C46H96CaN6O2Si2
917.72
C34H54CaF2N6O2
656.91
C52H76Ca2F4N12
1025.41
C58H80N4O2P2Sr
1014.82
Formula weight/g mol-1 773.11
Crystal system
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
C2/m
Triclinic
P1
Triclinic
P1
Monoclinic
P21/a
¯
¯
Space group
˚
a/A
20.9359(3)
10.5703(2)
21.4465(4)
90
107.570(1)
90
4524.67(14)
4
0.18
10.2465(3)
17.1561(5)
25.8067(5)
90
92.401(2)
90
4532.6(2)
4
1.23
14.2500(5)
26.3080(9)
10.1110(5)
90
132.16(2)
90
2809.8(2)
2
0.239
8.3450(2)
12.1410(3)
18.4510(4)
87.950(1)
82.074(1)
77.667(1)
1808.77(7)
2
0.221
1.206
3.58 to 27.55
0.0527, 0.1603
0.0715, 0.1742
10.7164(2)
14.7870(2)
18.3215(3)
85.925(1)
80.722(1)
79.709(1)
2816.54(8)
2
0.260
1.209
3.61 to 27.53
0.0493, 0.1200
0.0727, 0.1340
13.1977(2)
16.0704(3)
13.9234(3)
90
113.2530(1)
90
2713.17(9)
2
1.097
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
m/mm-1
r/g cm-3
1.14
1.20
1.12
1.242
q range (◦)
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
3.44 to 26.02
0.045, 0.114
0.056, 0.123
3.44 to 26.17
0.0438, 0.0864
0.0732, 0.0977
3.86 to 27.52
0.0731, 0.1702
0.0855, 0.1759
5.63 to 27.52
0.0340, 0.0747
0.0514, 0.0835
Measured/independent 31099/8671/0.032 35153/8844/0.072 21929/3278/0.0633 31965/8218/0.0433 51547/12871/0.0496 36348/6172/0.0621
reflections/Rint
˚
Table 2 Selected bond lengths (A) for compounds 1–6
1
2
3
4
5
6
Ae–N(1)
Ae–N(2)
Ae–N(4)
Ae–N(5)
Ae–O(1)
Ae–O(2)
N(1)–C(1)
N(2)–C(1)
N(4)–C(20)
N(5)–C(20)
C(1)–N(3)
C(20)–N(6)
2.3962(13)
2.4428(14)
2.4278(14)
2.4272(13)
2.4360(13)
2.4359(12)
2.348(2)
1.321(2)
1.325(2)
1.318(2)
1.456(2)
1.462(2)
2.575(2)
2.593(2)
2.562(2)
2.602(2)
2.582(2)
2.572(2)
1.322(3)
1.324(3)
1.328(3)
1.321(3)
1.467(3)
1.464(3)
2.420(2)
—
2.4430(15)
2.4319(15)
2.4404(16)
2.4198(15)
2.4018(13)
2.3796(14)
1.360(2)b
1.315(2)c
1.362(2)d
1.312(2)e
1.389(2)f
1.393(2)g
2.3639(16)
2.4096(16)
2.3982(16)
2.4518(16)h
2.4711(13)i
—
2.5579(15)
2.5693(15)
—
—
—
2.460(3)
—
1.334(3)
—
—
2.5447(14)
—
1.359(3)b
1.310(3)c
1.301(2)j
1.396(2)k
1.366(2)l
1.373(2)m
1.330(2)n
1.330(2)o
—
—
—
—
1.455(5)a
—
1.8999(18)p
—
a C(1)–N(2). b N(1)–C(7). c N(2)–C(7). d N(4)–C(24). e N(5)–C(24). f C(7)–N(3). g C(24)–N(6). h Ca(1)–N(6). i Ca(1)–F(1). j N(4)–C(17). k N(6)–C(17)¢.
l N(3)–C(7). m N(5)–C(17). n N(1)–C(5). o N(2)–C(5). p C(5)–P.
Table 3 Selected bond angles (◦) for compounds 1–6
1
2
3
4
5
6
N(1)–Ae–N(2)
N(4)–Ae–N(5)
O(1)–Ae–O(2)
N(1)–Ae–O(1)
N(1)–Ae–O(2)
N(5)–Ae–O(2)
N(4)–Ae–O(2)
N(5)–Ae–N(2)
N(4)–Ae–N(2)
N(2)–Ae–O(2)
N(1)–C(1)–N(2)
N(1)–C(1)–N(3)
N(2)–C(1)–N(3)
56.28(5)
55.80(5)
80.87(4)
90.71(5)
152.20(5)
88.32(5)
98.25(5)
118.02(5)
162.72(5)
97.67(5)
119.51(14)
119.83(14)
120.65(15)
52.34(7)
52.58(7)
83.10(8)
88.92(8)
149.96(7)
89.43(7)
96.13(7)
115.28(7)
160.76(7)
98.74(7)
119.0(2)
120.5(2)
120.4(2)
55.99(10)a
—
55.42(5)
55.75(5)
86.36(5)
88.31(5)
104.88(5)
96.43(5)
83.59(5)
56.71(6)
54.94(5)j
—
—
—
52.40(5)
—
180.0b
90.35(7)
—
180.0l
94.61(5)
—
—
—
—
—
—
—
124.01(10)c
—
95.27(5)
106.79(6)k
133.55(6)
—
127.60(5)m
—
116.41(5)
160.00(6)
115.90(15)g
121.43(17)h
122.66(17)i
89.65(7)d
116.8(3)e
121.59(16)f
—
85.94(5)
116.65(15)n
126.10(13)o
117.24(13)i
116.28(17)g
120.23(18)h
123.45(18)i
a N(1)–Ca(1)–N(1)¢. b O(1)–Ca(1)–O(1)¢. c N(1)–Ca–N(1)“. d N(1)–Ca(1)–O(1)¢. e N(1)–C(1)–N(1)¢. f N(1)–C(1)–N(2). g N(1)–C(7)–N(2). h N(1)–C(7)–
N(3). i N(2)–C(7)–N(3). j N(4)–Ca(1)–N(6)¢. k N(1)–Ca(1)–N(4). l O–Sr–O¢. m N(1)–Sr–N(2)¢. n N(1)–C(5)–N(2). o N(1)–C(5)–P. p N(2)–C(5)–P
18
˚
˚
The Sr–N bond lengths [range, 2.562(2)–2.602(2) A] within com-
[2.537(3)–2.619(3) A] but are lengthened in comparison with
pound 2 are also commensurate with those reported for the sim-
the mononuclear [Sr{CyNC(N(SiMe3)2)NCy}2(OEt2)] [2.534(3),
15c
˚
ilarly six-coordinate triazenide complex, [Sr{Dipp2N3}2(THF)2]
2.514(3) A] and the Sr–N distances to the terminal ligands
within two recently reported dimeric strontium derivatives of
2d
˚
[2.596(16), 2.604(2) A]. These distances are, however, within the
range observed for the formamidinate, [Sr{(DippN)2CH}2(THF)2]
the [iPrNC(NMe2)NiPr]- [range, 2.5067(17)–2.5577(18) A] and
˚
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[iPrNC(NiPr2)NiPr]- [range, 2.562(2)–2.568(2) A] anions as a
˚
result of the reduced formal coordination number of these five◦-
coordinate species.19 The N–Sr–N bond angles within 2 [52.34(7)
and 52.58(7)◦] are, somewhat surprisingly, more acute than those
observed for 1 despite the 16% increase in M2+ radius for
the heavier group 2 element. This observation implies that the
guanidinate bite angle is effectively invariant apart from minor
adjustments imposed by factors such as the steric demands of
the local coordination environment of the complexes and crystal
packing effects.20
The formation of compounds 1 and 2 in the presence of
both the amine and carbodiimide starting materials implies
that the protonolysis of the silylamide starting materials oc-
curs more rapidly than the potential insertion reaction into
the Ae–N(SiMe3)2 bonds. The homogeneous nature of the
reactions to form compounds 1 and 2 and the previously
noted reduced solubility of homoleptic calcium and strontium
diphenylamides in hydrocarbon solvents implies that the for-
mation of any new amido species is, at best, transient and
possibly contains coordinated carbodiimide during the pro-
cess of Ph2NH to silylamide proton transfer.21 This scenario
is supported by the observation that sequential addition of
amine or carbodiimide substrates to [Ca{N(SiMe3)2}2(THF)2]
in non-coordinating solvents resulted in either precipitation
of the presumed diphenylamidocalcium species or carbodi-
imide insertion into the Ca–N(SiMe3)2 bond (vide infra). Prece-
dent for this latter reactivity is provided by the reaction be-
Fig. 2 ORTEP representation of Compound 3 (thermal ellipsoids at 30%
probability). Hydrogen atoms are removed for clarity. Primed labelled
atoms related to those in the asymmetric unit by -x, y, -z + 1, while
double and triple primed labels are generated by the x, -y, z and -x, -y,
-z + 1 operations, respectively.
data are provided in Tables 1 and 2 respectively. Although the
calcium atom, which sits on an inversion centre, within 3 is again
six-coordinate the THF donors occupy positions with a trans
rather than the cis disposition which was observed within the
structure of compound 1. This geometrical adjustment does not,
however, result in any major changes to the observed bond lengths
and angles within 3, which show a close correspondence with the
similar contacts within compound 1.
tween CyN
C
NCy and [Ca{N(SiMe3)2}2]2 which resulted in
the guanidinate species, [Ca{CyNC(N(SiMe3)2)NCy}2(OEt2)].15c
An analogous reaction utilising the THF-solvated sily-
lamide precursor [Ca{N(SiMe3)2}2(THF)2] resulted in the
formation of the THF-coordinated calcium guanidinate
[Ca{CyNC(N(SiMe3)2)NCy} (THF)2], compound 3. Although
it was reported that [Ca{2CyNC(N(SiMe3)2)NCy}2(OEt2)] is
susceptible to reversible decomposition with extrusion of the
To further explore the influence of the presence of THF
upon the solution and solid-state structures of homoleptic
calcium guanidinates, reactions were carried out between the
previously reported guanidine base, [{(2-FC6H5)N}C(NHiPr)2]
and both THF-solvated and base-free calcium silylamides
(Scheme 4) to provide the corresponding monomeric THF-
solvated and dimeric base-free guanidinate compounds 4 and
5. Our previous NMR analysis of the interchange between the
carbodiimide,15c isolated samples of compound 3 provided a H
1
i
THF-solvated [Ca{ PrNC(NHiPr)NPh}2(THF)2] and unsolvated
NMR spectrum consistent with the formation of compound 1
and the free amidine [CyNC(N(SiMe3)2)N(H)Cy] when treated
with a two-fold excess of Ph2NH and di-iso-propylcarbodiimide.
This observation is consistent with both a rationale for initial
rapid protonolysis during the formation of 1 and 2 as well as
our previous observations of a lack of any crossover reactivity
in attempted catalytic reactions between anilines and preformed
guanidine substrates.15b
Complex 3 has been fully characterised in solution by 1H and 13C
NMR spectroscopy and in the solid state by an X-ray diffraction
analysis. The results of this latter study are illustrated in Fig. 2
while details of the analysis and selected bond length and angle
i
[Ca{ PrNC(NHiPr)NPh}2]2 had fallen short of a complete descrip-
tion of the observed dynamic behaviour.15b As for this previously
reported system, complexes 4 and 5 provided identical variable
1
temperature H NMR data in d8-toluene solution implying the
occurrence of similar monomer-dimer equilibria. Although it was
hoped that the inclusion of a 19F NMR label would enhance our
appreciation of the precise mode of exchange, further variable
temperature 19F NMR data recorded for 4 were uninformative.
At elevated temperature (353 K) compound 4 provided a single
19F resonance at -124.4 ppm, which appeared as an unresolved
multiplet. Although this coupling resolved to a clearly defined
Scheme 4
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Fig. 4 ORTEP representation of Compound 5 (thermal ellipsoids at 30%
probability). Hydrogen atoms, except those bonded to N3 and N6, and
isopropyl methyl groups are removed for clarity. Primed labelled atoms
are related to those in the asymmetric unit by the -x, -y + 3, -z + 2 #2 -x
+ 1, -y + 2, -z + 1 symmetry operations.
Fig. 3 ORTEP representation of Compound 4 (thermal ellipsoids at 30%
probability). Hydrogen atoms except those bonded to N3 and N6 are
removed for clarity.
AXX¢X¢¢ spin system [3JHF = 13.2, 4JHF = 8.7, 4JHF = 4.9 Hz] typical
of ortho-substituted fluoroarenes at reduced temperatures,22 no
deconvolution into separate signals indicative of THF dissociation
and compound dimerisation were apparent down to temperatures
at which the complex precipitated (< 243 K) from the d8-toluene
NMR solvent. Pure samples of compound 5 provided similar, but
unresolved 19F NMR chemical shift data and we, thus, conclude
that any monomer/dimer equilibration proceeds too rapidly to be
observed.
are rather scarce, the Ca–F distance of 5 is very similar to the
˚
C–F–Ca linkages [Ca–F 2.468(2), 2.489(3) A] observed within an
N,F-chelated b-diketiminate complex in which the fluorine donors
are provided by a trifluoromethyl substituent of the anisobidentate
ligands.23 In this latter case the C–F bonds undergo an apparent
anchimerically-assisted cleavage reaction with the formation of
a new calcium fluoride species. Although no such reactivity was
observed for 5 the Ca–F bond distance may be thus considered as
indicative of a reasonably strong interaction.
The structure of compound 4 is illustrated in Fig. 3 and details
of the X-ray analysis and selected bond length and angle data are
summarised in Tables 1, 2 and 3 respectively. The six-coordinate
geometry of the calcium centre within 4 is similar to that observed
for compound 1 and it is isostructural with its non-fluorinated
Phosphaguanidinates have recently been established as a novel
mono-anionic ligand class.24 Both computational and experi-
mental investigations have established that the phosphorus lone-
pair is not significantly delocalised across the amidinate moiety
but is localised upon the phosphorus itself.24c As a result, the
anion is considerably less stable than the analogous guanidinates.
Initial studies into the coordination chemistry of these potentially
tridentate ligands have demonstrated that both N,N and N,P
coordination modes may be observed, depending upon the nature
of the metal and auxiliary ligands.24 Hou and co-workers have
reported that group one amides, including [Li{N(SiMe3)2}],
[Na{N(SiMe3)2}] and [K{N(SiMe3)2}], act as highly active pre-
catalysts for the hydrophosphination of carbodiimides to form
the analogous phosphaguanidines.12a More recently, the same
group have documented the organo(III)lanthanum complex
[{Me2Si(MesN)(5-C5Me4)}La(2-CH2C6H4NMe2)(THF)] (Mes =
2,4,6-trimethylphenyl),13f as an active catalyst for this transfor-
mation. In 2008 we reported that heavier group 2 silylamides are
also highly competent catalysts for the hydrophosphination of
carbodiimides with diphenylphosphine to form the corresponding
phosphaguanidines.16a These reactions were proposed to occur
via carbodiimide insertion into the Ae–P bond of transient
diphenylphosphido complexes and subsequent reaction of these
catalyst resting state complexes with the Brønsted acidic phos-
phine by means of a series of Lewis acid/base equilibration steps.
The formation of a phosphaguanidinate complex was demon-
strated when Ae = Ca by isolation of a b-diketiminate derivative,
i
analogue [Ca{ PrNC(NHiPr)NPh}2(THF)2].15b The calcium to
ligand distances bear close comparison to analogous measure-
i
2
ments within [Ca{ PrNC(NHiPr)NPh}2(THF)2] and the k -N,N¢-
guanidinate is similarly anisobidentate having tautomerised to
the [iPrNC(NHiPr)NC6H4-2-F]- form. The asymmetric unit of
compound 5 comprises two S2-symmetric centrosymmetric dimers
which are essentially identical. Only the Ca1-containing molecule
is shown in Fig. 4 and subsequent discussion is restricted to this
molecule. Details of the solution and refinement are given in Table
1 and selected bond length and angle data are provided in Tables
2 and 3. The dimeric structure of 5 describes a motif in which each
2
Ca centre is coordinated by a single k -[iPrNC(NHiPr)NC6H4-
2
2
1
2-F]- ligand and two bridging m -k -k guanidinates. Although
not isostructural, the molecular constitution of 5 is again sim-
i
ilar to its non-fluorinated analogue, [Ca{ PrNC(NHiPr)NPh}2]2
2
2
˚
15b
[Ca–Nterminal, 2.3604(11), 2.3748(11) A, Ca-m -k -N, 2.4269(11),
1
˚
˚
2.5590(11) A, Ca-k -N 2.4009(11) A] and both the terminal
[Ca1–N1 2.3639(16), Ca1–N2 2.4096(16) A] and bridging Ca–
N [Ca1–N6 2.4518(16), Ca1–N6¢ 2.5811(16) A] bond distances
˚
˚
imply a similar level of charge donation from the ligands to
the group 2 metal centre. The bonding at each calcium centre
within 5 is, however, augmented by an additional Ca–F [Ca1–
˚
F1 2.4711(13) A] interaction with the o-F substituent of the
bridging N-aryl guanidinate ligands, raising the overall coordi-
nation number at calcium to six. Although this interaction has
little notable effect upon the nitrogen to calcium binding of
the individual ligands and directly comparable structural data
i
[{ArNC(Me)CHC(Me)NAr}Ca{ PrNC(PPh2 )CNiPr}(THF)],
which was formed by reaction of 1,3-di-isopropylcarbodiimide
with the heteroleptic calcium phosphide.16b Although none of
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the products of the stoichiometric reactions with catalytically
active homoleptic amides were reported in our initial com-
munication of this reactivity, a subsequent report by West-
erhausen has shown that homoleptic calcium phosphaguani-
dinate complexes are readily available by insertion of ei-
ther 1,3-disopropylcarbodiimide or 1,3-dicyclohexylcarbodiimide
into the Ca–P bonds of [Ca(PPh2)2(THF)4], formed in situ
in THF solution by reaction of KPPh2 and CaI2.16b These
compounds were also prepared by variation of the order of
addition and metathesis of CaI2 with the preformed potas-
sium phosphaguanidinate species. In parallel studies we have
found that identical homoleptic calcium complexes may also
be prepared by reaction of the isolated phosphaguanidine
ligands [iPrN C(PPh2)N(H)iPr] and [CyN C(PPh2)N(H)Cy]
with [Ca{N(SiMe3)2}2(THF)2]. The compounds prepared by this
method provide solid state X-ray and solution state NMR data
identical to those reported by Westerhausen,16b and we now
report that this methodology may also be extended to the heavier
members of the series. Reaction of two molar equivalents of
[CyN C(PPh2)N(H)Cy] with [Sr{N(SiMe3)2}2(THF)2] in hexane
provided [Sr{CyNC(PPh2)NCy}2(THF)2], compound 6, which is,
to the best of our knowledge, the first example of a strontium phos-
phaguanidinate complex. Although crystallisation from hexane
solution provided single crystals suitable for an X-ray diffraction
analysis (vide infra) and isolated samples provided microanalytical
Fig. 5 ORTEP representation of Compound 6 (thermal ellipsoids at 30%
probability). Hydrogen atoms are removed for clarity. Primed labelled
atoms related to those in the asymmetric unit by the -x, -y, -z symmetry
operation.
delocalised across the amidinate moiety and the phosphorus atom
is significantly pyramidalised.24 The C–P bond within 6 [C5–P
˚
1.8999(18) A] is comparable to the same measurement in the
˚
analogous calcium species [1.906(2) A] and is thus typical of the
elongation of this parameter when phosphaguanidinate ligands
are bound to more electropositive metals.24
In conclusion, a number of homoleptic group 2 amides
and phosphides have been shown to react with 1,3-
dialkylcarbodiimides to form the corresponding guanidinate
and phosphaguanidinate complexes. The work reported herein,
therefore, documents stoichiometric reactivity equivalent to our
previously reported catalytic application of homoleptic group 2
amides to the guanidination and phosphaguanidination of 1,3-
dialkylcarbodiimides.15b,16a Each of these species can be envisaged
to react with a further equivalent of a protic amine or phosphine
to liberate the guanidine or phosphaguanidine products and
regenerate the intermediate amide and phosphide species. We are
continuing to study the reactivity of well defined heavier group
2 complexes and will elaborate upon these observations in future
publications.
1
data consistent with the expected formulation, 31P{ H} NMR
analysis provided evidence for the establishment of complex
equilibria with the observation of at least six resonances in variable
proportions across a narrow chemical shift range between -11.6
and -23.8 (major) ppm. Although precipitation of the complex at
reduced temperatures prevented variable temperature analysis of
1
complex 6, a 31P{ H} DOSY spectrum recorded in C6D6 solution
of 6 at 298 K provided evidence for this proposal. All of the
individual resonances provided comparable diffusion coefficients,
D, consistent with similar molecular volumes in solution, apart
from the major constituent at -23.8 ppm, which provided a
somewhat higher value for D. This is indicative of a reduced
molecular volume25 and the 31P chemical shift value is similar to
that reported previously for the analogous calcium phosphaguani-
dinate, [Ca{CyNC(PPh2)NCy}2(THF)2], (-21.5 ppm).16b We, thus,
propose that this major component in the spectrum of 6 is due to
the retention of a structure analogous to this bis-THF adduct
in solution and that the multiplicity of signals observed for 6 is
a result of the greater solution lability of the larger and more
electropositive strontium centre toward THF dissociation and
solution dimerisation. The results of an X-ray diffraction analysis
performed upon a single crystal of compound 6 are illustrated
in Fig. 5 while details of the analysis and selected bond length
and angle data are provided in Tables 1, 2 and 3 respectively.
Compound 6 is isostructural to [Ca{CyNC(PPh2)NCy}2(THF)2]
and adopts a similar transoid configuration with respect to
the disposition of the THF molecules about the six-coordinate
strontium centre.16b The Sr–N bond distances within 6 [Sr–N1
Experimental
General procedures
All manipulations were carried out using standard Schlenk line
and glovebox techniques under an inert atmosphere of either
nitrogen or argon. NMR experiments were conducted in Youngs
tap NMR tubes made up and sealed in a Glovebox, NMR spectra
were recorded on either a Bruker AV-500 spectrometer (13C NMR
125.8 Hz) or a Bruker AV-400 spectrometer (13C NMR 100.6
MHz). Solvents (Toluene, Benzene, THF, Hexane) were dried by
distillation from sodium-benzophenone ketyl, under nitrogen and
stored in ampoules over molecular sieves. C6D6 and d8-toluene
were purchased from Goss Scientific Instruments Ltd. and dried
over molten potassium before distilling under nitrogen and storing
over molecular sieves. 1,3-dialkylcarbodiimides were purchased
from Sigma-Aldrich and used as received. The free guanidine
and phosphaguanidine bases, [{(2-FC6H5)N}C(NHiPr)2]15b and
[CyN C(PPh2)N(H)Cy],16a and the homoleptic calcium and
strontium silylamides, [Ae{N(SiMe3)2}2THF2], were synthesised
by literature procedures.26
˚
2.5579(15), Sr–N2 2.5693(15) A] are, as expected, significantly
elongated by comparison to the analogous calcium complex.
These measurements are, however, comparable to the related mea-
surements in the similarly six-coordinate guanidinate complex,
˚
2 [range, 2.563(2)–2.602(2) A]. Consistent with previous studies
of phosphaguanidinate ligands, the phosphorus lone pair is not
7398 | Dalton Trans., 2010, 39, 7393–7400
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i
Synthesis of [Ca{ PrNC(NPh2)NiPr}2(THF)2], 1. In a glove-
reduced to induce crystallisation. The product 5 was isolated as
a colourless crystalline solid (0.14 g, 38%) by slow cooling of a
box [Ca{N(SiMe3)2}2THF2] (1.00 g, 1.98 mmol) was dissolved
in hexane and a hexane solution of 1,3-di-isopropylcarbodiimide
(0.50 g, 3.96 mmol) and diphenylamine (0.67 g, 3.96 mmol) was
added. The solution was stirred for 1 h at room temperature and
the volume reduced to induce crystallisation. 1 was isolated as a
colourless crystalline solid (1.03 g, 67%). 1H NMR (C6D6, 298 K,
400 MHz): 1.05 (d, 24H, J = 6.0 Hz, CH3), 1.52 (m, 8H, THF), 3.84
(m, 8H, THF) 6.83 (m, 4H, p-C6H5), 7.12 (m, 8H, m-C6H5), 7.38
(m, 8H, o-C6H5). 13C NMR (C6D6, 298 K, 125.8 MHz) 25.5, 27.4,
57.0, 69.1, 118.2, 121.0, 121.2, 129.2, 145.8, 160.8. Anal. Calc. for
C46H64CaN6O2: % C 71.45, H 8.36, N 10.87. Found: C 71.30, H
8.22, N 10.80.
◦
hot toluene solution to 5 C. Multinuclear NMR data as above
but without resonances associated with THF. Anal. Calc. for
C52H76Ca2F4N12: % C 60.90, H 7.48, N 16.40. Found: C 60.79,
H 7.88, N 16.28.
Synthesis
of
[Sr{CyN C(PPh2)CN(H)Cy}2(THF)2],
solid mixture
6. Hexane (10 mL) was added to
a
of [Sr{N(SiMe3)2}2(THF)2] (0.14 g, 0.25 mmol) and
[CyN C(PPh2)N(H)Cy] (0.20 g, 0.50 mmol). The mixture
was stirred for 30 min and the solvent volume reduced to
induce crystallisation. The product 6 was isolated as a colourless
◦
1
i
Synthesis of [Sr{ PrNC(NPh2)NiPr}2(THF)2], 2. This com-
crystalline solid (0.20 g, 79%) by storage at -30 C. H NMR
(C6D6, 298 K, 400 MHz): 1.15–1.35 (m, 24H, C6H11, THF), 1.70
(m, 4H, C6H11), 1.69–1.89 (m, 16H, C6H11), 3.21 (m, 4H, NCH),
3.70 (m, 8H, THF), 6.99–7.08 (m, 12H), 7.36–7.58 (m, 8H). 13C
NMR (C6D6, 298 K, 125.8 MHz) 23.9, 25.7, 26.4, 26.9, 33.1, 35.9,
49.6, 129.5 (d, J = 6.6 Hz), 129.7, 132.9 (d, J = 19.2 Hz), 135.4
(d, J = 14.0 Hz), 151.9 (d, J = 31.8 Hz). 31P NMR (C6D6, 298
K, 202.5 MHz) major -23.8, minor resonances at -21.8, -20.4,
-18.1, -14.7, -12.3. Anal. Calc. for C58H80N4O2P2Sr: % C 68.64,
H 7.96, N 5.52. Found: C 68.50, H 7.78, N 5.35.
pound was made by the same general method as that employed
for 1 using [Sr{N(SiMe3)2}2THF2] (0.40 g, 0.72 mmol), 1,3-di-
isopropylcarbodiimide (0.18 g, 1.44 mmol) and diphenyl amine
(0.25 g, 1.44 mmol). Crystallisation from hexane provided 2 as
1
colourless crystals (0.39 g, 66%). H NMR (C6D6, 298 K, 400
MHz): 1.03 (d, 24H, J = 6.0 Hz, CH3), 1.49 (m, 8H, THF), 3.75
(m, 8H, THF), 3.85 (m, 4H, CH(CH3)), 6.82 (m, 4H, p-C6H5), 7.13
(m, 8H, m-C6H5), 7.37 (m, 8H, o-C6H5). 13C NMR (C6D6, 298 K,
125.8 MHz) 24.7, 25.6, 46.9, 68.5, 120.2, 121.1, 129.2, 145.8. Anal.
Calc. for C46H64SrN6O2: % C 67.32, H 7.88, N 10.24. Found: C
67.25, H 7.76, N 10.18.
X-ray crystallography
Synthesis of [Sr{CyNC(N(SiMe3)2)NCy}2(THF)2], 3. A solu-
tion of 1,3-dicyclohexylcarbodiimide (0.41 g, 1.98 mmol) in hexane
was added to a solution of [Ca{N(SiMe3)2}2THF2] (0.50 g, 0.99
mmol) in the same solvent. The pale yellow solution was stirred for
1 h and the volume reduced to incipient crystallisation. Storage at
-30 ◦C for 16 h produced compound 3 as colourless crystals. (0.51
g, 56%). 1H NMR (C6D6, 298 K, 400 MHz): 0.35 (s, 36H, SiCH3),
1.35–1.42 (m, 24H, C6H11, THF), 1.70 (m, 4H, C6H11), 1.83–1.92
(m, 16H, C6H11), 3.33 (m, 4H, NCH), 3.67 (m, 8H, THF). 13C
NMR (C6D6, 298 K, 125.8 MHz) 2.8, 25.5, 26.7, 39.0, 55.0, 68.5,
164.4. Anal. Calc. for C46H96CaN6O2Si2: % C 60.20, H 10.56, N
9.16. Found: C 59.99, H 10.48, N 9.01.
Data for 1 and 2 were collected at 173 K and those of 3–6
at 150 K on Nonius Kappa CCD diffractometers equipped
with low temperature devices, using graphite monochromated
˚
Mo-Ka radiation (l = 0.71073 A). Data were processed using
the Nonius Software.27 Structure solution, followed by full-matrix
least squares refinement was performed using either the WinGX-
1.70 suite of programs28 or the programme suite X-SEED.29
semi-empirical absorption correction was applied in all cases.
The asymmetric unit of 3 consists of one quarter of a molecule,
with the central calcium located at a special position with 2/m
symmetry, which serves to generate the remainder of the molecule.
A
˚
For 4 H3 and H6 were located and refined at 0.90 A from the
i
Synthesis
of
[Ca{ PrNC(NHiPr)NC6H4-2-F}2(THF)2],
parent nitrogen atoms. C32 and C33 were disordered over 2
sites in a 60 : 40 ratio. The asymmetric unit of 5 consists of 2
crystallographically independent dimer halves. The fluorine atoms
on the non-ligated aromatic rings exhibited disorder for F21, F2A
(65% occupancy) and F22, F2B (35% occupancy). The associated
partial hydrogen atoms were not included in the refinement. Efforts
(FLAT instruction in SHELXL) to maintain planarity between the
aromatic rings and the minor fluorine fragments proved largely
ineffective, perhaps as a function of minimal electron density in
the areas of the electron density map. CCDC numbers for X-ray
structures of 1–6, 768276–768281.
4. Toluene (10 mL) was added to
a solid mixture of
[Ca{N(SiMe3)2}2THF2] (0.50 g, 0.99 mmol) and 2-(2-
fluorophenyl)-1,3-disopropylguanidine (0.41 g, 1.98 mmol).
The mixture was stirred for 30 min and the solvent volume
reduced to induce crystallisation. The product 4 was isolated as
a colourless crystalline solid (0.48 g, 73%) by slow cooling of a
hot toluene solution to 5 ◦C. 1H NMR (C6D6, 298 K, 400 MHz):
0.82 (d, 24H, J = 6.8 Hz), 1.41 (m, 8H, THF), 3.28 (m, 4H, CH),
3.40 (m, 8H, THF), 3.58 (broad s, 2H, NH), 6.75–6.79 (m, 2H),
6.77–7.14 (m, 6H).13C NMR (C6D6, 298 K, 125.8 MHz) 2.8, 23.1,
25.8, 43.3, 67.7, 116.2 (d, JFC = 20 Hz), 121.9, 126.3, 139.1 (d,
JFC = 13 Hz), 150.2, 156.1 (d, JFC = 242.4 Hz). 19F NMR (C6D6,
3
4
4
298 K, 376.5 MHz) -124.4 (ddd, JHF = 13.2, JHF = 8.7, JHF
=
Acknowledgements
4.9 Hz). Accurate microanalytical data could not be obtained for
this compound due to facile loss of THF.
We thank GlaxoSmithKline for the generous endowment (to
A.G.M.B.), the Royal Society for a University Research Fellowship
(M.S.H.) and Royal Society Wolfson Research Merit Award
(A.G.M.B.) and the Engineering and Physical Sciences Research
Council and GlaxoSmithKline for generous support of our studies.
i
Synthesis of [Ca{ PrNC(NHiPr)NC6H4-2-F}2]2, 5. Toluene
(10 mL) was added to a solid mixture of [Ca{N(SiMe3)2}2]2 (0.26 g,
0.36 mmol) and [iPrNC(NHiPr)NC6H4-2-F] (0.30 g, 1.45 mmol).
The mixture was stirred for 30 min, filtered and the solvent volume
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13 (a) W.-X. Zhang and Z. Hou, Org. Biomol. Chem., 2008, 6, 1720; (b)
W.-X. Zhang, M. Nishiura and Z. Hou, Synlett, 2006, 1213; (c) W.-X.
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