Phosphaguanidines as scaffolds for multimetallic complexes containing metal-functionalized phosphines

Article abstract of DOI:10.1021/ic7024457

Phospha(III)guanidines, R₂PC{NR′}{NHR′}, have been used to synthesize multimetallic compounds containing combinations of aluminum with platinum or copper, in which the main-group metal is N,N′-bound by an amidinate moiety, thereby generating a metal-functionalized phosphine that bonds to the transition metal through phosphorus.

Full text of DOI:10.1021/ic7024457

Inorg. Chem. 2008, 47, 2258-2260
Phosphaguanidines as Scaffolds for Multimetallic Complexes Containing
Metal-Functionalized Phosphines
Joanna Grundy, Natalie E. Mansfield, Martyn P. Coles,* and Peter B. Hitchcock
Department of Chemistry, UniVersity of Sussex, Falmer, Brighton BN1 9QJ, U.K.
Received December 18, 2007
Phospha(III)guanidines, R2PC{NR′}{NHR′}, have been used to
synthesize multimetallic compounds containing combinations of
aluminum with platinum or copper, in which the main-group metal
is N,N′-bound by an amidinate moiety, thereby generating a metal-
functionalized phosphine that bonds to the transition metal through
phosphorus.
Figure 1.
potential exists for metal bonding either through the amidine
Tandem catalysis offers significant advantages over the
sequential performance of individual reactions, including
circumventing the need for isolation and purification of
intermediates (typically leading to loss of material) and
reducing the time required to produce the final product.1 The
design of such systems remains a challenge, however, with
important factors that need consideration including (i)
compatibility of individual catalysts with all components of
the reaction, (ii) assurance that neither of the individual
catalytic cycles predominates, and (iii) control of the
sequence of the reactivity.
The development of single molecular species that behave
as tandem catalysts involves the incorporation of different
metals in the same compound (or the same metal in different
chemical environments), each of which is able to perform a
separate chemical transformation. This, in turn, depends on
the design of ligands that support metal fragments in distinct
coordination environments, achieved using multifunctional
ligands. Recent work has been presented by ourselves2–6 and
others7 on the synthesis and coordination chemistry of phospha-
guanidines, R2PC{NR}{NHR′}. As a ligand precursor, the
component (usually achieved through conversion to the corre-
sponding amidinate anion) or with participation from the
phosphine group. We report herein two systems in which the
phosphaguanidine supports multiple metal fragments, including
the first structurally characterized example of the κP,κN,N′ mode
for a mixed Al/Cu system.
Despite the obvious potential for neutral phospha-
(III)guanidines I-H and II-H to serve as κP-phosphine ligands
at a metal center (A; Figure 1), this bonding mode has only
been reported on the basis of spectroscopic data for cyclo-
pentadienylmolybdenum complex B.8 It should be noted that
the reaction of I-H with M(CO)5(THF) in an attempt to
generate P-bound phosphaguanidines led to elimination of
a second 1 equiv of CO and additional contributions to the
bonding through the Nimine atom (C).3
Previous work from our group has shown that the isolated
aluminum phosphaguanidinate Al(I)Me2 behaves as a metal-
functionalized phosphine that will coordinate to platinum,
affording the cis-square-planar complex PtMe2{Al(I)Me2}2.2
To develop a more general route to these “metal-function-
alized phosphines”, the reaction with PtMe2(cod) was
investigated prior to metal binding through the amidine
component. Colorless crystals that analyzed as PtMe2-
(R2PC{NiPr}{NHiPr})2 (1, R ) Ph; 2, R ) Cy) were isolated
in good yield upon crystallization from toluene. 31P NMR
data were consistent with a cis arrangement for the phos-
phines, with JPtP values of 1800 and 1682 Hz for 1 and 2,
*
To whom correspondence should be addressed. E-mail:
m.p.coles@sussex.ac.uk.
(1) Wasilke, J.-C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Chem. ReV.
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P. B. Dalton Trans. 2005, 2833–2841. Mansfield, N. E.; Coles, M. P.;
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respectively. The H NMR spectrum showed inequivalent
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2258 Inorganic Chemistry, Vol. 47, No. 7, 2008
10.1021/ic7024457 CCC: $40.75  2008 American Chemical Society
Published on Web 02/23/2008

COMMUNICATION
substituted platinum dimethyl compounds with related groups
at phosphorus.10,11
The most notable structural difference between 1 and 2 is
the relative position of the nitrogen substituents within the
phosphaguanidine. Considering the -C{NiPr}{NHiPr} com-
ponent, the P-diphenyl derivative adopts an Esyn configura-
tion, while the corresponding Z isomer is observed in 2. This
difference is consistent with the lower barrier to isomerization
about the CdN bond noted for noncoordinated II-H, in which
both the Esyn and Zsyn isomers were observed in solution.6
In both cases ∆CN values12 [1, 0.10 and 0.08 Å; 2, 0.11 and
0.10 Å] are consistent with localized bonding.
Treatment of 1 with 2 equiv of AlMe3 cleanly converts
the amidine functional groups to the amidinate via proto-
nolysis of one of the methyl ligands of aluminum. The
product was identified spectroscopically as the previously
reported trimetallic compound, PtMe2{Al(I)Me2}2 (3).2 This
alternative route demonstrates for the first time that a
coordinated amidine can be deprotonated, extending the
scope of this system to further development in the synthesis
of multimetallic compounds.
Figure 2. ORTEPs of 1 and 2. Selected bond lengths (Å) and angles (deg).
1: Pt-P1 2.293(1), Pt-P2 2.298(1), Pt-C39 2.109(4), Pt-C40 2.114(4),
C1-N1 1.369(5), C1-N2 1.271(5), C20-N3 1.276(5), C20-N4 1.360(5);
P1-Pt-P2 107.66(3), P1-Pt-C39 165.11(11), P1-Pt-C40 85.46(12),
P2-Pt-C39 85.57(11), P2-Pt-C40 165.53(12), C39-Pt-C40 82.26(16).
2: Pt-P1 2.3164(7), Pt-P2 2.3203(7), Pt-C39 2.101(3), Pt-C40 2.104(3),
C13-N1 1.273(4), C13-N2 1.383(4), C32-N3 1.280(4), C32-N4 1.375(4);
P1-Pt-P2 106.80(2), P1-Pt-C39 86.89(8), P1-Pt-C40 165.68(8),
P2-Pt-C39 165.86(8), P2-Pt-C40 87.20(8), C39-Pt-C40 79.34(11).
isopropyl groups, suggesting that localized C–N single and
CdN double bonds were retained within the amidine, as
observed in the noncoordinated phosphaguanidines.3
Previous analysis of I-H and II-H has shown that the NH
doublet is caused by coupling to the methine hydrogen of
the Namino substituent, with no observable 3JPH.3,6 However,
when the P,N-bonding mode is adopted, in which a Zanti
configuration of amidine substituents is enforced (C; Figure
1), the coupling to phosphorus is resolved and the NH proton
resonates as a pseudotriplet.3 The observed doublets in 1
and 2 at δ 4.46 and 5.39, respectively, are therefore consistent
with a κP-bonding mode in which the Nimino atom is not
involved in bonding to the metal.
Despite our best efforts, we were unable to isolate crystals
of 3 suitable for X-ray analysis, and because we considered
this an important technique for evaluating differences in
bonding between I-H and Al(I)Me2, an alternative system
was examined. Copper(I) was selected as a suitable metal
to replace the platinum because the coordination of phos-
phines is well-known and solution-state NMR data are readily
available. The reaction between 2 equiv of I-H and CuBr
afforded colorless crystals of CuBr(I-H)2 (4) in reasonable
yield. As for the platinum compounds, inequivalent isopropyl
Crystallographic analysis9 of 1 and 2 confirmed the cis-
square-planar geometry at platinum, in which the amidine
component of the phosphaguanidines is not contributing to
the bonding, thereby remaining available for interaction with
additional metal substrates (Figure 2). The P–Pt–P angles
[1, 107.66(3) Å; 2, 106.80(2) Å] are consistent with the
phosphaguanidines being considered as a “bulky” phosphine-
type ligand, being close in value to that found for the
analogous bis(tricyclohexylphosphine) compound, PtMe2-
(PCy3)2 [P–Pt–P ) 108.60(5) Å].10 The Pt–P bond lengths
differ slightly between 1 (ave 2.296 Å) and 2 (ave 2.318 Å)
but are consistent with other examples of cis-diphosphine-
1
methyl groups and a doublet for the NH proton in the H
NMR spectrum suggested that bonding was through the P
atom only, despite our previous studies in which we have
shown that amidine and guanidine compounds readily
coordinate to copper(I) centers through the Nimine atom.13
The molecular structure of 4 (Figure 3) shows the expected
distorted trigonal-planar geometry [Σangles ) 359.92°] with
both phosphaguanidines bonding as κP ligands. The largest
angle is found between the two phosphaguanidine ligands
[P–Cu–P ) 127.59(2)°], although for this metal fragment,
the value is significantly less than that in the corresponding
bis(tricyclohexylphosphine) compound, CuBr(PCy3)2 [P–Cu–P
) 135.6(1) Å],14 being much closer to that in the bis(triph-
enylphosphine) analogue [P–Cu–P ) 126.0(1) Å].15 In
addition, a significant difference is noted in the two P–Cu–Br
angles [∆PCuBr ) 8.82°], indicating a degree of coordinative
(9) Crystallographic data. 1: C40H56N4P2Pt·C7H8, M ) 942.05, T ) 173(2)
K, monoclinic, space group P21/c (No. 14), a ) 13.3845(2) Å, b )
22.7704(3) Å, c ) 15.5458(2) Å, ꢀ ) 104.780(1)°, U ) 4581.13(11)
Å3, Z ) 4, Dc ) 1.37 Mg m-3, µ(Mo KR) ) 3.17 mm-1, independent
reflections ) 7728 [Rint ) 0.058], R1 [for 6438 reflections with I >
2σ(I)] ) 0.030, wR2 (all data) ) 0.063. 2: C40H80N4P2Pt, M ) 874.11,
T ) 173(2) K, monoclinic, space group P21/c (No. 14), a ) 11.8881(2)
Å, b ) 17.9465(3) Å, c ) 21.0820(3) Å, ꢀ ) 94.286(1)°, U )
4485.26(12) Å3, Z ) 4, Dc ) 1.29 Mg m-3, µ(Mo KR) ) 3.23 mm-1
,
independent reflections ) 8767 [Rint ) 0.052], R1 [for 7609 reflections
with I > 2σ(I)] ) 0.023, wR2 (all data) ) 0.053. 4: C38H50BrCuN4P2,
M ) 768.21, T ) 173(2) K, monoclinic, space group P21/n (No. 14),
a ) 10.4275(1) Å, b ) 17.1857(2) Å, c ) 22.3749(3) Å, ꢀ )
97.210(1)°, U ) 3977.96(8) Å3, Z ) 4, Dc ) 1.28 Mg m-3, µ(Mo
KR) ) 1.66 mm-1, independent reflections ) 9060 [Rint ) 0.054],
R1 [for 7135 reflections with I > 2σ(I)] ) 0.038, wR2 (all data) )
0.081. 5: C42H60Al2BrCuN4P2, M ) 880.29, T ) 173(2) K, monoclinic,
space group P21/n (No. 14), a ) 14.0207(2) Å, b ) 22.0117(3) Å, c
) 16.2740(2) Å, ꢀ ) 110.996(1)°, U ) 4689.01(11) Å3, Z ) 4, Dc )
1.25 Mg m-3, µ(Mo KR) ) 1.46 mm-1, independent reflections )
8218 [Rint ) 0.087], R1 [for 5932 reflections with I > 2σ(I)] ) 0.049,
wR2 (all data) ) 0.103.
(11) Palcic, J. D.; Baughman, R. G.; Peters, R. G. J. Coord. Chem. 2005,
58, 521–527. Wisner, J. M.; Bartczak, T. J.; Ibers, J. A. Organome-
tallics 1986, 5, 2044–2050. Durran, S. E.; Smith, M. B.; Slawin,
A. M. Z.; Gelbrich, T.; Hursthouse, M. B.; Light, M. E. Can. J. Chem.
2001, 79, 780–791.
(12) Häfelinger, G.; Kuske, F. K. H. The Chemistry of Amidines and
Imidates; Wiley: Chichester, U.K., 1991; Vol. 2, Chapter 1.
(13) Oakley, S. H.; Coles, M. P.; Hitchcock, P. B. Inorg. Chem. 2003, 42,
3154–3156. Oakley, S. H.; Soria, D. B.; Coles, M. P.; Hitchcock, P. B.
Dalton Trans. 2004, 537–546.
(14) Bowmaker, G. A.; Boyd, S. E.; Hanna, J. V.; Hart, R. D.; Healy, P. C.;
Skelton, B. W.; White, A. H. Dalton Trans. 2002, 2722–2730.
(15) Davis, P. H.; Belford, R. L.; Paul, I. C. Inorg. Chem. 1973, 12, 213–
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(10) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.;
Giering, W. P. Organometallics 1999, 18, 474–479.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2259

COMMUNICATION
Figure 4. Schematic representations of (a) 4 and (b) 5, viewed in the
P2CuBr plane through the P · · ·P vector.
consistent with a larger cone angle upon incorporation of
the AlMe2 groups, although it is noted that the Cu–P
distances are not significantly perturbed. Viewing the mol-
ecule along the P · · · P vector in the P2CuBr plane (Figure 4)
shows that the relative position of the phosphorus substituents
changes significantly from 4, in which an almost perfectly
staggered conformation is adopted, to 5, in which the
substituents are virtually eclipsed, maximizing the torsion
between the AlMe2 groups. The bonding within the amidinate
portion of the ligand is as expected,18 with a distorted
tetrahedral geometry at aluminum and a small bite angle for
the chelate [ave N–Al–N ) 68.61°].
In summary, we have shown spectroscopically and struc-
turally that the amidine group within κP-bonded phosph-
aguanidines does not interact with the metal in PtMe2 and
CuBr complexes and is therefore potentially available for
interaction with additional metal fragments. This mode of
reactivity has been confirmed by employing AlMe3, which
is known to cleanly convert the amidine to the corresponding
amidinate via loss of methane and formation of the N,N′-
chelate. Structural characterization of a trimetallic Al/Cu/
Al complex confirms phosphaguanidinate as a bridge be-
tween the disparate metals, providing a general route to
multimetallic complexes supported by this ligand framework.
Further work investigating the effects that this association
has on the chemical reactivity of the P- and N,N′-bonded
metals, toward developing single molecules as tandem
catalysts, is underway.
Figure 3. ORTEPs of CuBr(I-H)2 (4) and CuBr{Al(I)Me2}2 (5). Selected
bond lengths (Å) and angles (deg). 4: Cu-P1 2.2401(6), Cu-P2 2.2223(6),
Cu-Br 2.3434(3), P1-C13 1.881(2), C13-N1 1.278(3), C13-N2 1.369(3),
P2-C32 1.874(2), C32-N3 1.271(3), C32-N4 1.365(3); P1-Cu-P2
127.59(2), P1-Cu-Br 111.756(18), P2-Cu-Br 120.574(19). 5: Cu-P1
2.2246(10), Cu-P2 2.2379(10), Cu-Br 2.3646(6), P1-C13 1.865(3),
C13-N1 1.331(5), C13-N2 1.333(4), N1-Al1 1.933(3), N2-Al1 1.952(3),
P2-C34 1.869(4), C34-N3 1.328(4), C34-N4 1.326(5), N3-Al2 1.938(3),
N4-Al2 1.925(3); P1-Cu-P2 130.45(4), P1-Cu-Br 117.24(3), P2-Cu-Br
112.22(3), N1-Al1-N2 68.47(13), N3-Al2-N4 68.74(13).
flexibility for these ligands as phosphines, depending on the
orientation of the P substituents and the geometry of the
metal fragment to which they are coordinated. The Cu–P
distances are as expected,15,16 and the amidine component
is consistent with localized bonding [both ∆CN values 0.09
Å] with an Esyn configuration.
Reaction of 4 with AlMe3 (2 equiv) proceeds cleanly via
alkane elimination to afford the corresponding trimetallic
species CuBr{Al(I)Me2}2 (5). A slight shift in the 31P NMR
resonance was observed from δ –15.1 in 4 to δ –18.4 in 5,
and as expected for the N,N′-coordination of the amidinate
to the aluminum center, the isopropyl groups are equivalent
by NMR spectroscopy.
Acknowledgment. We gratefully thank the EPSRC and
the University of Sussex for financial support. The late Dr.
A. G. Avent is acknowledged for assistance with NMR
studies.
Crystals of 5 suitable for X-ray analysis were grown from
toluene (Figure 3), revealing the first example in which the
phosphaguanidinate anion serves as a bridge between dif-
ferent metal centers.17 The Cu atom retains a trigonal-planar
geometry in 5, with a slight increase in the P–Cu–P angle
Supporting Information Available: Experimental and full
characterization data for 1-5 and X-ray data for 1, 2, 4, and 5 in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(16) Bowmaker, G. A.; Engelhardt, L. M.; Healy, P. C.; Kildea, J. D.;
Papasergio, R. I.; White, A. H. Inorg. Chem. 1987, 22, 3533–3538.
(17) The P-dicyclohexylphosphaguanidinate anion was shown to bridge
between Li atoms via the Nimine atom in the cyclic hexamer
[Li(Cy2PC{NiPr}2)]6. See ref 5.
IC7024457
(18) Coles, M. P.; Swenson, D. C.; Jordan, R. F., Jr Organometallics 1997,
16, 5183–5194.
2260 Inorganic Chemistry, Vol. 47, No. 7, 2008