Water-induced rearrangement of a platinacyclic carbene produces a platinacyclic carbaphosphazene with an intraannular Pt-C bond in a Pt-N-P-N-P-C ring

Article abstract of DOI:10.1002/anie.200461525

(Chemical Equation Presented) An unusual synthetic route to the first example of a six-membered M-N-P-N-P-C metallacycle (M = metal atom) is reported which involves a water-induced rearrangement of the methylated Pt-N-P-C-P-N carbene 1 to the unique Pt-N-P-N-P-C platinacyclocarbaphosphazene 2. In the presence of a Lewis acid (LA), 2 converts into its orthometalated isomer 3 quantitatively. (OTf=CF₃SO₃).

Full text of DOI:10.1002/anie.200461525

Angewandte
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Metallacycles
previous reports of MK -N-P-N-P-CL ring systems (V). Herein,
we present an unusual synthetic route to the first example of
such a metallacycle, which involves a water-induced rear-
rangement of our previously reported PtK-N-P-CL -P-N carbene
[1][OTf] (OTf = CF3SO3 ) into the unique PtK-N-P-N-P-CL
platinacyclophosphazene [2][OTf] (Scheme 2). In the pres-
Water-Induced Rearrangement of a Platinacyclic
Carbene Produces a Platinacyclic
À
À
Carbaphosphazene with an Intraannular Pt C
À À À À À
Bond in a Pt N P N P C Ring**
Min Fang, Nathan D. Jones, Michael J. Ferguson,
Robert McDonald, and Ronald G. Cavell*
The cyclophosphazenes are a key family of inorganic hetero-
cycles. These rings, which contain alternating four-coordinate
PV and two-coordinate NIII atoms (R2P N)n (n ꢀ 3), have
=
attracted considerable attention, principally because they are
precursors to polyphosphazenes of high molecular weight.[1]
These polyphosphazenes constitute the largest and one of the
most important classes of inorganic polymers. A large body of
synthetic phosphazine chemistry has furnished a wide array of
Scheme 2. The syntheses of 2 and 3 (LA=AgOTf and MeOTf, all
anions=OTf).
ence of Lewis acids (LA), [2][OTf] converts quantitatively to
its orthometalated isomer, [3][OTf] (Scheme 2; LA =
À
organocyclophosphazenes with exocyclic P E bonds (E = O,
S, N, and C).[2] More recently, the cyclophosphazenes have
served as ligands for transition metals,[3,4] and their complexes
have found application in catalysis,[5] dendrimers,[6] and cancer
therapeutics.[7] Cyclophosphazenes containing metal centers
that are covalently bonded in the ring (e.g., I–IV; Scheme 1)
MeOTf, AgOTf). Both metallacarbaphosphazenes 2 and 3,
+
= =
which contain the [P N P] fragment, have been fully
characterized spectroscopically and by X-ray crystallography.
Recently, we reported the synthesis, characterization, and
reactivity of the phosphoranimine-stabilized carbene complex
4 (Scheme 3),[10,11] which, when heated to 658C in the
presence of H2O, gave the orthometalated methanide 5
quantitatively within minutes. No orthometalation occurred
Scheme 1. Known cyclic metallaphosphazene structures (I–IV) and the
metallacarbaphosphazene ring obtained in this work (V).
are also well established,[8] but have a shorter history: the first
example was reported by Roesky et al. in 1986 (I, M = WCl3,
R = Ph).[9] Although a variety of these compounds are now
known, to the best of our knowledge, there have been no
[*] Dr. M. Fang, Dr. N. D. Jones, Dr. M. J. Ferguson, Dr. R. McDonald,
Prof. Dr. R. G. Cavell
Department of Chemistry, University of Alberta
Edmonton, Alberta T6G2G2 (Canada)
Fax: (+1)780-492-8321
E-mail: ron.cavell@ualberta.ca
[**] We thank the Natural Sciences and Engineering Research Council of
Canada (NSERC) and the American Chemical Society Petroleum
Research Fund (ACS-PRF) for financial support. N.D.J. acknowl-
edges the Killam Trust for a post-doctoral fellowship.
Scheme 3. Structures discussed in the text. The structure 2’ is the
unrearranged isomer of the title compound with a carbodiphosphorane
structure. The bracketed structures are unobserved species.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461525
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2005

Communications
over a period of several hours in the absence of H2O. Methyl
triflate (MeOTf) reacted with 4 to give [1][OTf]. However,
when this complex was heated in the presence of H2O
(1 equiv), neither of the anticipated N-methyl analogues of 5
([6][OTf] and [7][OTf]—which are shown in brackets to
indicate that they are not observed) were formed, but instead
the unusual platinacyclophosphazene [2][OTf] resulted
(Scheme 2). In the absence of H2O, [1][OTf] was stable. To
ensure a high yield of [2][OTf], a solution of H2O (1 equiv) in
THF was added dropwise to a solution of [1][OTf] in THF;
when neat H2O was added, unidentified side products
resulted and yields were low.[12] The reaction was complete
within 24 h. Typical conversions of [2][OTf], as determined in
situ by 31P{1H} NMR spectroscopy, were 60–70%, with the
best conversion being about 90%; yields were approximately
60%. The platinacyclic carbaphosphazene [2][OTf] is insen-
sitive to and insoluble in H2O; heating a THF solution that
contained [2][OTf] and H2O to 658C for several days had no
effect. In the solid state, [2][OTf] was also stable in air.
The structures of [2][OTf] (monoclinic or triclinic poly-
morph depending on crystal growing methods)[13] were
deduced by using a combination of X-ray crystallography
and NMR spectroscopy. Although the crystallographic data
for the monoclinic form of [2][OTf] gave a good fit (R1 =
2.3%) when it was assumed that there was no rearrangement
of the backbone (as the structure of the unrearranged isomer
of 2, 2’), this structure was not compatible with NMR data.
Heteronuclear correlation through multiple quantum coher-
ence (HMQC) NMR studies (see Supporting Information)
revealed no NH2 protons. In addition to the cross signals
arising from the CH2 protons in 1,5-cyclooctanediene (cod)
ligand, one additional CH2 signal was found. Moreover, the
Figure 1. ORTEP diagram of the structure of [2][OTf] (monoclinic poly-
morph). Non-hydrogen atoms are represented by Gaussian ellipsoids
at the 50% probability level. Hydrogen atoms on methylene [C(1)] and
methyl [C(2)] centers are shown with arbitrarily small thermal parame-
ters, those on the cod and phenyl rings are not shown. Selected bond
lengths [ꢀ] and angles [8]: Pt–N(2) 2.051(2), Pt–C(1) 2.072(3), P(1)–
N(1) 1.593(2), P(1)–C(1) 1.777(3), P(1)–C(11) 1.807(3), P(1)–C(21)
1.800(3), P(2)–N(1) 1.599(2), P(2)–N(2) 1.627(2), N(2)–C(2) 1.482(4);
angles: N(2)-Pt-C(1) 90.92(10), Pt-C(1)-P(1) 108.5(1), C(1)-P(1)-N(1)
112.2(1), P(1)-N(1)-P(2) 123.2(2), N(1)-P(2)-N(2) 114.7(1), P(2)-N(2)-
Pt 123.2(1); P(2)-N(2)-C(2) 117.4(2), Pt-N(2)-C(2) 119.4(2); dihedral
angles: P(2)-N(1)-P(1)-C(1) 2.6(2), P(2)-N(2)-Pt-C(2) 177.1(3), N(2)-
P(2)-N(1)-P(1) À45.5(2), Pt-C(1)-P(1)-N(1) 57.6(2).
+ [16]
PPh3]+ and 1.572(3) ꢀ in [Ph2(NH2)P N P(CH3)Ph2] .
= =
+
= =
Therefore, it can be concluded that 2 contains a {P N P}
= =
putative “P C P” group in 2’ would be expected to show a
fragment. Atoms P(2), N(2), Pt, and C(2) are also nearly
nucleophilic character that is conspicuously absent in the
reactivity of 2 (see below). Refinement of the X-ray crystallo-
graphic data by assuming [2][OTf] gave a slightly better fit
(R1 = 2.2%).[13] A similar result was obtained for the refine-
ment of the triclinic polymorph of [2][OTf]. Similarly, the
identity of [3][OTf] was confirmed by using a combination of
X-ray crystallography and NMR spectroscopy.
Bond lengths and angles for 2 in the two determinations of
[2][OTf] (monoclinic or triclinic polymorph) are very similar.
A labeled ORTEP representation of the molecular structure
of the monoclinic isomorph of [2][OTf][13] is shown in Figure 1
(the structure of 2 in triclinic [2][OTf] is included in the
Supporting Information). The CF3SO3À ion is not coordinated
to the metal center and has only a few close contacts with
hydrogen atoms (the separations which are less than the sum
of van der Waals radii are for Ph–H: F(2)-H(13) 2.518(2),
F(2)-H(14) 2.631(2), O(1)-H(25) 2.683(3), O(2)-H(22)
2.491(3), O(2)-H(43) 2.633(3), and O(3)-H(34) 2.483(3) ꢀ;
for the methylene bridge hydrogen atom: O(2)-H(1B)
2.360(3) ꢀ). Within the six-membered ring, C(1), P(1),
N(1), and P(2) are essentially coplanar (dihedral angle
coplanar (f = (P(2)-N(2)-Pt-C(2)) = 177.1(3)8, and the sum
À
of angles around N(2) is 360.0(5)8). The P(2) N(2) bond
À
length of 1.627(2) ꢀ is between that of a P N single and
double bond, thus indicating that there is electronic delocal-
ization over P(1), N(1), P(2), and N(2), even though N(2) is
not coplanar with the P(2)-N(1)-P(1) plane (f = À45.5(2)8).
À
The Pt C(1) bond length (2.072(3) ꢀ) is within the typical
[17]
À
À
range for a Pt C single bond (2.039–2.085 ꢀ). The Pt N(2)
À
bond length (2.051(2) ꢀ) is close to those found for Pt N
dative bonds, such as 2.049 ꢀ found in PtII–NH3 complexes.[17]
The N-P-C-P-N to N-P-N-P-C rearrangement reported
herein is unique in that it gives a cyclic product but is not
without precedence in giving acyclic compounds. Two pre-
=
vious examples have been reported. (Me3SiN PPh2)2CH2, 8,
was converted to the acyclic cation [Ph2(NH2)PNP(CH3)Ph2]+
by treatment with aqueous HCl solution[18] or by treatment
with Ph3GeCl or Ph3SnCl in wet solvents.[16] Possible inter-
mediates for these rearrangements are shown in the Support-
ing Information. In these systems and in the conversion of 1
À
into 2 (Scheme 2), hydrolysis of the N Si bonds is promoted
by the formation of an iminium group, which presumably
renders the silicon much more electrophilic and susceptible to
attack by H2O. In contrast, complex 4, which bears no
iminium groups, does not undergo N-P-C-P-N to N-P-N-P-C
rearrangement in the presence of H2O. The key rearrange-
ment step is the attack of the basic phosphoranimine nitrogen
À
f(P(2)-N(1)-P(1)-C(1)) = 2.6(2)8). N P bond lengths of the
P(1)-N(1)-P(2) fragment are virtually identical (1.593(2) and
À
1.599(2) ꢀ, respectively) and are consistent with the P N
bond lengths determined for other compounds that contain a
+
[14–16]
= =
= =
[P N P] fragment,
for example, 1.57(1) ꢀ in [Ph3P N
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À
atom on an electrophilic phosphorus atom, which leads to P
C bond cleavage.
We were able to identify an intermediate, 9 (Scheme 3),
by means of in situ NMR spectroscopic studies (31P NMR at
258C: d = 73.8 (brs, 1J(P,Pt) = 393 Hz), À7.2 ppm (brs, 1J-
(P,Pt) = 95 Hz)). The formation was further confirmed by
electrospray mass spectrometry (m/z calcd (observed)
802.24697 (802.24722) [M+] with an isotope-match score of
0.9482). The intermediate 9 was the major product formed
(calculated by in situ 31P NMR spectroscopy: 80% of P
atoms) when only about 0.5 equivalent H2O was added to
1
À
[1][OTf]. The H NMR data for Pt CH in 9 (d = 4.11 ppm,
dd, J(H,Pt) = 73 Hz, 2J(H,P) = 7.5 Hz, 2J(H,P) = 8.4 Hz) is
similar to data for the same feature in 5 (d = 4.00 ppm, dd,
2
2
2J(H,Pt) = 96, J(H,P) = 13, J(H,P) = 16 Hz).[10] Other char-
Figure 2. ORTEP diagram of the structure of [3][OTf]. Non-hydrogen
acterization data for 9 are given in the Supporting Informa-
atoms are represented by Gaussian ellipsoids at the 50% probability
level. Hydrogen atoms on methyl, methylene, and N(2) centers are
shown with arbitrarily small thermal parameters, those on the cod and
phenyl rings are not shown. For all phenyl rings, except that forming
the orthometalated center, only the ipso carbon atom is shown. The
dotted line indicates the interaction of the triflate oxygen atom with
the hydrogen atom on N(2). Selected bond lengths [ꢀ] and angles [8]:
Pt–C(1) 2.072(2), Pt–C(12) 2.040(2), P(1)–N(1) 1.607(2), P(1)–C(1)
1.775(3), P(1)–C(11) 1.786(3), P(2)–N(1) 1.583(2), P(2)–N(2)
1.635(2), N(2)–C(2) 1.465(4), C(11)–C(12) 1.411(3); angles: P(1)-
N(1)-P(2) 128.4(1), C(1)-Pt-C(12) 85.5(1); dihedral angles: C(1)-P(1)-
N(1)-P(2) 0.9(2), N(2)-P(2)-N(1)-P(1) 48.1(2), P(1)-C(11)-C(12)-Pt
À1.5(3), C(1)-P(1)-C(11)-C(12) À23.0(2).
À
tion. We surmise that the initial step is hydrolysis of the N Si
bond of the iminium moiety in 1, which then undergoes N-to-
C proton transfer. Coordination of the methylated N to Pt
displaces the other P N imine center to form 9. The resultant
=
Me3SiOH by-product of this reaction condenses to (Me3Si)2O
(as determined by 1H and 29Si NMR spectroscopies) and H2O.
The liberated silylated phosphimine moiety in 9 further reacts
with H2O to again form Me3SiOH (which condenses as
above) and a P NH group. Subsequent P C bond cleavage
and proton transfer from the N atom to the C atom completes
the rearrangement into 2.
=
À
In the presence of substoichiometric quantities of the
electrophiles (AgOTf and MeOTf, about 0.5 equivalent), 2
was quantitatively converted into 3 (Scheme 2); there was no
incorporation of the electrophile into the product.[19] Analysis
by NMR spectroscopy and mass spectrometry showed that
the products of the reactions mediated by either of these
Lewis acid catalysts were identical. Without catalyst, the
conversion of 2 into 3 occurred but was extremely slow;
heating a C6D6 solution of 2 to 658C for two weeks gave less
than 33% conversion. No intermediate was observed by
in situ 31P NMR spectroscopy for either the catalyzed or
uncatalyzed reactions. The precise role of the catalytic
electrophile in this reaction remains poorly understood. It is
possible that reversible association between electrophile and
the amido N atom in 2 (all the possible resonance forms 2a–d
are given in the Supporting Information) gives rise to a
coordinatively unsaturated Pt center, which then undergoes
orthometallation.[20]
In summary, the first example of a MK -C-P-N-P-NL metalla-
heterocycle, the platinacyclic carbaphosphazene [2][OTf] has
been synthesized in high yield through a unique four- to six-
membered ring expansion and rearrangement induced by
water. The synthetic method described herein for [2][OTf]
may provide routes for synthesizing new metal-containing
cyclophosphazenes. The mechanisms through which 2 and 3
were formed are currently under investigation.
Received: August 3, 2004
Revised: November 25, 2004
Published online: February 23, 2005
Keywords: carbenes · metallacycles · phosphorus · platinum ·
.
rearrangement
The PtK-C-C-P-CL metallacyclic structure of [3][OTf] is
shown in Figure 2.[13] As in [2][OTf], the CF3SO3À ion does not
coordinate to the metal center. The close O–H contact
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1035.
À
(2.145(2) ꢀ) between CF3SO3 and the HNMe fragment
suggests that the NH proton bears a significant positive
charge and that the resonance form 3a is an important
contributor to the structure of 3 (see the Supporting
Information for the remaining resonance forms).
Related crystal structures of complexes with similar rings
have been reported.[10,21,22,23] The C(1), P(1), N(1), and P(2)
À
atoms in 3 are coplanar as they are in 2. The P N bond lengths
in the {P N P} fragment (1.607(2) and 1.583(2) ꢀ) and P-N-
+
= =
P angle (128.4(1)8) are similar to those in 2 (1.593(2) and
1.599(2) ꢀ, and 123.2(2)8).
[7] H. R. Allcock, F. H. Allen, J. P. OꢁBrien, J. Am. Chem. Soc. 1977,
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[12] Synthesis of [2][OTf]: A THF solution containing H2O (4.94 mL,
0.0245m, 0.121 mmol) was added dropwise to a stirred solution
of [1][OTf] (130 mg, 0.127 mmol) in THF (10 mL). The mixture
was heated to 658C for 24 h. Removal of the solvent and
recrystallization from THF gave [2][OTf] (73 mg, 65%).
2
31P{1H} NMR (161.9 MHz, [D8]THF): d = 33.5 (d, J(P,P) = 6.1,
2J(P,Pt) = 122 Hz), 25.0 ppm (d, 2J(P,P) = 6.0, 2J(P,Pt) = 124 Hz);
13C{1H} NMR (100.6 MHz, [D8]THF) for CH2Pt: d = 15.3 ppm
(dd, 1J(P,C) = 46, 3J(P,C) = 8.4 Hz, 1J(C,Pt) not observed). Com-
plete synthetic and characterization data are given in the
Supporting Information.
[13] CCDC-242499 ([2][OTf], monoclinic form), CCDC-2425001
([2][OTf], triclinic form), and CCDC-242451 ([3][OTf]) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. The full crystal data and their completely labeled
diagrams are included in the Supporting Information.
[14] A. Mꢂller, M. Mꢃhlen, B. Neumꢂller, N. Faza, W. Massa, K.
Dehnicke, Z. Anorg. Allg. Chem. 1999, 625, 1748 – 1751.
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[17] A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G.
Watson, R. Taylor, J. Chem. Soc. Perkin Trans. 2 1989, S1 – S83.
[18] R. Appel, I. Ruppert, Z. Anorg. Allg. Chem. 1974, 406, 131 – 144.
[19] Synthesis of [3][OTf]: A mixture of MeOTf (5.3 mL, 47 mmol),
[2][OTf] (82 mg, 93 mmol) and C6H6 (4 mL) was stirred for 1 day
at room temperature and then heated to 68–708C for 7 h.
Removal of the solvent gave spectroscopically pure [3][OTf]
quantitatively. 31P{1H} NMR (161.9 MHz, C6D6): d = 21.7 (s),
37.7 ppm (s, 2J(P,Pt) = 217 Hz); 13C{1H} NMR (100.6 MHz,
[D8]THF) for CH2Pt: d = 26.1 ppm (dd, 1J(P,C) = 50, 3J(P,C) =
1.2 Hz, 1J(C,Pt) not observed); Microscope IR: n˜ = 3246 cmÀ1
(m, nNH). Complete synthetic and characterization data are given
in the Supporting Information.
[20] P. Foley, R. DiCosimo, G. M. Whitesides, J. Am. Chem. Soc.
1980, 102, 6713 – 6725.
[21] M. W. Avis, K. Vrieze, J. M. Ernsting, C. J. Elsevier, N. Veldman,
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