The effect of P-cyclohexyl groups on the coordination chemistry of phosphaguanidinates

Article abstract of DOI:10.1039/b601235c

The effect of P-cyclohexyl groups on the coordination chemistry of phosphaguanidinates, is discussed. Chelating ligands which combine disparate donor-groups are attractive in catalytic chemistry as they offer a degree of selectivity due to inherent differences in metal-ligand bonding parameters. Many areas of applied chemistry have been explored, focusing mainly on the late transition elements with asymmetric C-C bond forming reactions and copolymerization of olefins and carbon monoxide. The reactivity with metal halide was studied and it was found that the phosphine group was more likely to participate in the bonding.

Full text of DOI:10.1039/b601235c

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The effect of P-cyclohexyl groups on the coordination chemistry of
phosphaguanidinates
Natalie E. Mansfield, Martyn P. Coles* and Peter B. Hitchcock
Received 25th January 2006, Accepted 2nd March 2006
First published as an Advance Article on the web 15th March 2006
DOI: 10.1039/b601235c
The anion of P-dicyclohexylphosphaguanidine, Cy2PC-
a P,N-chelate at lithium and
cobalt(II); bridging through the Nimine atom forms cyclic
compound was sparingly soluble in C6D6 enabling limited NMR
{NiPr}{NHiPr}, forms
data to be obtained, which confirmed the presence of lithium
(7Li NMR: broad singlet at d 1.87), and indicated the absence
of THF. In agreement with these data, elemental analysis was
consistent with the base-free formula, [Li(Cy2PC{NiPr}2)]n.†
Single crystal X-ray diffraction of 1‡ revealed two distinct
molecules within the unit cell which differ slightly in their
bond lengths and angles. Each molecule is comprised of the
cyclic hexamer, [Li(Cy2PC{NiPr}2)]6 (Fig. 2); four disordered
molecules of toluene are also present in the unit cell. Both
hexameric molecules consists of three unique forms of the basic
“Li(Cy2PC{NiPr}2)” component (referred to as A, B and C) with
the remaining three units being symmetry generated equivalents.
In each case the lithium is P,N-bound by the ligand, as noted in
II,6 and the spectroscopically characterized rhodium compound,
Rh(j1-N,P-Ph2PC{NAr}2)(PPh3)2.8 However, in contrast to these
monomeric compounds, the imine group of 1 is bonded to the
lithium atom of an adjacent unit in a unique j1-N,P-j2-Nꢀ-
bridging mode, with Nimine–Li distances in the range 1.974(12)–
hexamers in the former complex.
Chelating ligands which combine disparate donor-groups are
attractive in catalytic chemistry as they offer a degree of selec-
tivity due to the inherent differences in metal–ligand bonding
parameters. Amongst the most widely applied combination of
atoms is that based around the j1-P,N-donor set,1 with a major
feature of interest being the potential for hemilability within the
system.2 Many areas of applied chemistry have been explored,
focussing mainly on the late transition elements with, for example,
recent reports on systems active for alkene hydrocarboxylation,3
asymmetric C–C bond forming reactions4 and copolymerization
of olefins and carbon monoxide.5
Previous work has shown that the Li-salt of N,Nꢀ-diisopropyl-P-
diphenylphospha(III)guanidine crystallizes from THF as the dimer
[Li(Ph2PC{NiPr}2)(THF)]2 (I), which deaggregates to afford the
monomeric species Li(Ph2PC{NiPr}2)(TMEDA) (II) on reaction
with the appropriate base (Fig. 1).6 In I, the ligand adopts a
j1,2-N-j1-Nꢀ-bonding mode to generate a central ‘Li2N2’ core. In
contrast, within compound II the ligand adopts a j1-N,P-chelating
mode, with the resultant monomeric compound containing an
uncomplexed imine group.
˚
1.987(12) A. Examination of the bond lengths of each unit (Fig. 3)
show that these values are largely indistinguishable from the Namine
–
Libondwithinthemetallacycle, suggestingthatthehexamericcore
is tightly bound into a cohesive entity. The remaining bond lengths
associated with each unit are comparable to those of the TMEDA
adduct, despite the distorted trigonal planar metal centres in 1
ꢀ
[
at Li: A, 352.8◦; B, 359.1◦; C, 356.8◦] compared to the
angles
four-coordinate lithium in compound II.
Fig. 1 Structurally characterized phosphaguanidinate anions.
We have recently extended the family of phospha(III)guanidines
to include the P-dicyclohexyl derivatives, Cy2PC{NRꢀ}{NHRꢀ}
i
(Rꢀ = Cy, Pr).7 Deprotonation of the neutral compound was
achieved with nBuLi in THF and◦the product of the reaction was
crystallised from toluene at −30 C, to afford colourless crystals
(1). Full characterisation by NMR spectroscopy was not possible
due to difficulty in redissolving the isolated crystals. However, the
Fig. 2 ORTEP representation of one of the independent molecules of
[Cy2PC{NiPr}2Li]6 in compound 1 (ꢀ = −x, −y, −z; ꢀꢀ = − x + 1, −y + 1,
−z + 1; ellipsoids at the 30% probability level; hydrogen atoms omitted).
The Department of Chemistry, University of Sussex, Falmer, Brighton, UK
BN1 9QJ. E-mail: m.p.coles@sussex.ac.uk; Fax: +44 (0)1273 677196;
Tel: +44 (0)1273 877339
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to the relatively electron rich cyclohexyl substituent, the electron
density within the PCN2-core becomes substantially greater,
increasing the likelihood that the Nimine will become involved in
bonding.
The targeted application of the lithium phosphaguanidinate
described is as a ligand transfer reagent. The reactivity of 1
(typically generated in situ to facilitate homogeneous reaction
conditions) with metal halide salts has therefore been studied,
concentrating on the mid- to late-transition elements where it is
felt likely that the phosphine group is more likely to participate
in the bonding. Accordingly, reaction of two equiv. of the Li-
salt with CoCl2 and subsequent work-up afforded the homoleptic
compound Co(Cy2PC{NiPr}2)2 (2) as forest green crystals.† The
solution magnetic moment (Evan’s method9) was determined in
C6D6 (298–333 K), affording data consistent with a single unpaired
electron (leff = 1.85 lB), indicative of a square planar metal centre.
It is known that homoleptic amidinate complexes of cobalt(II)
adopt N,Nꢀ-bonding with a highly distorted tetrahedral geometry
in the solid-state,10 with bond angles in the range ∼66◦ (amidinate
bite angle) to 137◦; where reported, the magnetic moments of
these compounds are consistent with three unpaired electrons,
corresponding to the retention of a tetrahedral geometry in
solution.
˚
3 Selected bond lengths (A) within the three ‘Li(Cy2PC-
Fig.
{NiPr}2)’ units of 1, together with corresponding values from comp-
ound II.
=
The ‘N CNPLi’ atoms of each unit form an approximate
˚
˚
˚
plane (max. deviation: A, 0.02 A; B, 0.20 A; C, 0.11 A) with a
small bite angle for the ligand in each case [P–Li–N: A, 66.5(4)◦;
B, 64.5(4)◦, C, 66.3(3)◦]. These metallacycles form a ‘puckered’
system throughout the hexamer, with the phosphine moieties
arranged alternatively above and below the plane defined by the six
lithium atoms (Fig. 4); the interplanar angles are +26.9◦, −60.5◦
and +48.7◦ for units A, B and C, respectively, where positive and
negative values indicate the phosphine moiety positioned above
and below the Li6-plane. The angle at which the Nimine atom
from each unit bonds to the lithium, as measured by the N–
Li–N angle, varies considerably from 151.2(7)◦ [N(4)–Li(1)–N(5)]
to 139.2(7)◦ [N(1)–Li(2)–N(3)]. Both of these features contribute
towards generating the curvature necessary to form the observed
cyclic structure.
X-Ray diffraction analysis of 2‡ was in agreement with the
solution-state data, revealing a trans-square planar cobalt(II)
complex with a j1-N,P-chelating mode (Fig. 5). This geometry
closely matches that of the recently reported homoleptic cobalt(II)
bis(diphenylphosphino)anilide, Co(Ph2PC6H4NH)2,11 with the ex-
pected reduction in bite angle in 2 [72.65(4)◦] resulting from
the smaller, four-membered metallacycle. The carbon–nitrogen
distances within 2 are more localized than in 1 (DCN values12: 1 unit
˚
A = 0.020; 1 unit B = 0.030; 1 unit C = 0.027; 2 = 0.094 A) and as a
result of increased electron density at the amido nitrogen the Co–N
˚
distance [1.8729(14) A] is notably shorter than in the homoleptic
˚
amidinate complexes [av. 2.00 A]. However, no intermolecular
close contacts were noted in this case. Further studies of the
coordination chemistry of the P-dicyclohexylphosphaguanidinate
Fig. 4 Hexameric core of one of the independent molecules of 1.
Cyclohexyl and isopropyl substituents, except the a-carbons, omitted.
Generation of 1 in the presence of TMEDA in an at-
tempt to isolate the P-dicyclohexyl analogue of compound II
failed, with the hexamer once again crystallizing preferentially
from the reaction solution. This indicates a strong prefer-
ence for the oligomerisation within 1, in contrast to the P-
diphenylphospha(III)guanidines previously reported. We believe
that through changing the phosphorus substituents from phenyl
Fig. 5 ORTEP representation of 2 (ꢀ = −x + 1, −y + 1, −z + 1; ellipsoids at
the 30% probability level; hydrogen atoms omitted). Selected bond lengths
◦
˚
(A) and angels ( ): Co–N(1) 1.8729(14), Co–P 2.2181(4), C(13)–N(1)
1.375(2), C(13)–N(2) 1.281(2), C(13)–P 1.8619(17); N(1)–Co–P 72.65(4),
N(1)–Co–Pꢀ 107.35(4), N(1)–Co–(N1ꢀ) 180.0, P–Co–Pꢀ 180.0.
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are ongoing in the research group and will be reported in due
course.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b601235c
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Notes and references
† Selected data for 1: Crystallized yield 67%. Anal. calc. for C19H36N2LiP:
C, 69.07; H, 10.98; N, 8.48%. Found: C, 69.00; H, 11.00; N, 8.32%. Selected
data for 2: Crystallized yield 53%. Anal. calc. for C38H72N4CoP2: C, 64.66;
H, 10.28; N, 7.94%. Found: C, 64.75; H, 10.32; N, 7.48%. Solution magnetic
moment (C6D6, 273–333 K): leff = 1.85 lB.
‡ Selected crystallographic data for 1: C114H216Li6N12P6·4(C7H8), M =
2350.98, T = 173(2) K, triclinic, space group P1 (No.2), a = 14.9993(4),
˚
b = 15.3885(3), c = 32.7501(7) A, a = 89.639(1), b = 85.970(1), c =
◦
87.721(1) , U = 7534.6(3) A , Z = 2, Dc = 1.04 Mg m−3, l (Mo-Ka) =
3
˚
0.12 mm−1, independent reflections = 20864 (Rint = 0.112), R1 [for 10921
reflections with I > 2r(I)] = 0.106, wR2 (all data) = 0.290 (NOTE very
weak limited diffraction. Carbon atoms for the toluene solvates were left
isotropic and for the two disordered solvate molecules the H-atoms were
omitted). Selected crystallographic data for 2: C38H72CoN4P2, M = 705.87,
10 J. Hagadorn and J. Arnold, Inorg. Chem., 1997, 36, 132; J. A. R. Schmidt
and J. Arnold, J. Chem. Soc., Dalton Trans., 2002, 3454; B. S. Lim, A.
Rahtu, J.-S. Park and R. G. Gordon, Inorg. Chem., 2003, 42, 7951.
11 H.-F. Klein, R. Beck, U. Florke and H.-J. Haupt, Eur. J. Inorg. Chem.,
2003, 240.
T = 173(2) K, monoclinic, space group P21/c◦(No.14), a = 11.9830(3),
3
˚
˚
b = 10.8349(3), c = 16.3276(4) A, b = 108.076(2) , U = 2015.26(9) A , Z =
2, Dc = 1.16 Mg m−3, l (Mo-Ka) = 0.54 mm−1, independent reflections =
3944 (Rint = 0.056), R1 [for 3215 reflections with I > 2r(I)] = 0.033,
wR2 (all data) = 0.080. CCDC reference numbers 296234 and 296235.
12 G. Hafelinger and F. K. H. Kuske, in The Chemistry of Amidines
and Imidates, ed. S. Patai and Z. Rappoport, Wiley, Chichester, 1991,
vol. 2.
¨
2054 | Dalton Trans., 2006, 2052–2054
This journal is
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©