Differences in the η1-ligating properties of 2,4,6-tritertiarybutyl-phosphabenzene, PC5 H2 Bu3t and 2,4,6-tritertiarybutyl-1,3,5-triphosphabenzene, P3 C3 Bu3t</su

Article abstract of DOI:10.1016/j.jorganchem.2009.12.005

Several η¹-complexes of 2,4,6-tritertiarybutylphosphabenzene, PC₅ H₂ Bu₃^t, have been synthesised and structurally characterised including trans-[PtCl₂(PEt₃)(η¹-PC

Full text of DOI:10.1016/j.jorganchem.2009.12.005

Journal of Organometallic Chemistry 695 (2010) 717–720
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Differences in the
g
1-ligating properties of 2,4,6-tritertiarybutyl-phosphabenzene,
PC5H2But and 2,4,6-tritertiarybutyl-1,3,5-triphosphabenzene, P3C3But
3
3
*
Scott B. Clendenning, Peter B. Hitchcock, Gerard A. Lawless, John F. Nixon , Christopher W. Tate
Chemistry Department, School of Life Sciences, University of Sussex, Brighton, BN1 9QJ Sussex, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Several
g
1-complexes of 2,4,6-tritertiarybutylphosphabenzene, PC5H2Bu3t , have been synthesised and
structurally characterised including trans-[PtCl2(PEt3)(g -
1-PC5But3)], cis-[PdCl(C10H6CHMe(NMe2)(g1
Received 13 November 2009
Received in revised form 3 December 2009
Accepted 8 December 2009
Available online 28 December 2009
PC5H2But )] and [AuCl(
g
1-PC5H2But3)]. NMR spectroscopic evidence is presented for the partial isomerisa-
3
tion in solution of the 2,4,6-tritertiarybutyl-1,3,5-triphosphabenzene complexes trans-[PtCl2(PR3)(g1
-
P3C3But )], (PR3 = PMe3 and PMe2Ph), to the corresponding cis-isomers.
3
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Phosphabenzene
Triphosphabenzene
Complexes
Palladium
Platinum
Gold
1. Introduction
[AuCl(
g
1-PC5H2Bu3t )]. We also have carried out 31P NMR spectro-
scopic experiments which show that some trans-[PtCl2(PR3)(g1
-
Although previous studies have shown that the 2,4,6-trite-
P3C3But )] complexes undergo partial isomerisation to the cis-iso-
3
rtiarybutyl-1,3,5-triphosphabenzene ring 1 readily acts as a 6
electron donor towards metal centres [1–3], the only reported
examples of
1-bonded metal complexes are trans-
[PtCl2(PR3)(
1-P3C3But3)] (PR3 = PMe3, PEt3, PMe2Ph and PMePh2)
[4,5]. We therefore sought to investigate the relative -bonding
p
-
mer at ambient temperature.
g
g
r
properties of the structurally related 2,4,6-tritertiarybutylphos-
phabenzene, PC5But 2 and 1, in view of current interest in the cat-
3
alytic potential of these types of sp2-hybridised phosphorus-
containing rings and related phosphabarrelenes [6–14].
Recently, we showed for the first time that both 1 and 2 could
be protonated, alkylated and silylated at phosphorus using appro-
priate electrophilic reagents having halogenated carborane anions
[15], despite the extremely weakly basic and poor nucleophilic
nature of both ring systems [10,15–17]. We now report on the
2. Results and discussion
unexpected and strikingly different
rings 1 and 2 toward platinum metals. Although the coordination
chemistry of several monophosphinine ring systems has been
widely studied [10,12], only a few
r-ligating ability of the two
The attempted reaction of
1 with di-l-chloro-bis[(R)-di-
methyl(1-ethyl-
a
-naphthyl)-aminato-C2,N] palladium(II), 3 [19]
(Fig. 1), or [AuCl(tht)] only resulted in the recovery of unreacted
r-complexes of 2 have been
starting materials, as shown by 31P NMR spectroscopic monitoring,
structurally characterised [15,18]. We now describe the syntheses
the only observed resonance being that characteristic of
1
and single-crystal X-ray structural characterisation of the 2,4,6-
(dP = 232 ppm). Changing the solvent and/or varying the reaction
temperatures produced no observable effect. Complex 3 was cho-
sen in an attempt to obtain the first chiral triphosphabenzene com-
plex, while [AuCl(tht)] (tht = tetrahydrothiophen) normally readily
exchanges its very labile (tht) on treatment with tertiary
phosphines.
tritertiarybutylphosphabenzene complexes trans-[PtCl2(PEt3)(g1
-
PC5H2But )],
cis-[PdCl(C10H6CHMe(NMe2)(
g
1-PC5H2But3)]
and
3
* Corresponding author.
E-mail address: J.Nixon@sussex.ac.uk (J.F. Nixon).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.12.005

718
S.B. Clendenning et al. / Journal of Organometallic Chemistry 695 (2010) 717–720
This surprising lack of reactivity of 1 (which we had also noted
mation of [PdCl(C10H6CHMeNMe2)(
NMR spectrum of 6 exhibits a single resonance (dP = 167.6 ppm)
corresponding to the
1-Pt-bonded phosphorus atom. Interest-
g
1-PC5H2Bu3t )] 6. The 31P
previously [5] towards a variety of metal halides in different oxida-
tion states e.g. TiCl4, SnCl4, RhCl3, PdCl2 and PtCl2), no doubt re-
flects the significant s-character of the phosphorus lone-pair
electrons in 1.
g
ingly, the 31P resonance of 2 was often observed in solution (dP
= 180 ppm), indicating that an equilibrium exists between complex
6 and the free ring. In support of these observations, recrystallisa-
tion of the reaction mixture often yielded crystals of both 6 and 3
even if 2 were present in excess. A single-crystal X-ray diffraction
study revealed the molecular structure of 6, which is presented in
Fig. 3, together with selected bond lengths and angles.
On the other hand, treatment of 2 with [Pt2Cl4(PR3)2] (R = PEt3)
in CH2Cl2 readily resulted in the formation of trans-[PtCl2(PEt3)(g1
-
PC5H2But )] 4 whose 31P NMR spectrum shows the characteristi-
3
cally large trans P–P coupling constant; (ring phosphorus
dP = 161.4 ppm; (2J(P2P1) 526.8 Hz; 1J(PtP1) 2520 Hz; PEt3 dP
= 10.9 ppm (2J(P2P1) 526.9 Hz, 1J(PtP2) 2935 Hz). Comparable cou-
pling constant data found in the related triphosphabenzene com-
The facile reaction of 2 with [AuCl(tht)] in CH2Cl2 affords 7
whose 31P NMR spectrum shows
a single resonance (dP
plex trans-[PtCl2(PEt3)(
g
1-P3C3But3)]
5
are 2J(P2P1) 508.5 Hz;
= 154.6 ppm), which is appreciably shifted to low frequency com-
pared to the resonance observed for the free ring. A single-crystal
X-ray diffraction study confirms the molecular structure of 7
shown in Fig. 4, together with selected bond length and angle data.
As expected, the geometry around the metal centre is linear and
the Au–P (2.2194(15) Å) distance is not unusual for this type of
complex. Interestingly the AuCl fragment projects out from the
sp2-hybridised ring P and above the plane of the aromatic ring with
a ring plane–Au–P angle of approximately 12.3°. A detailed com-
parison of bond lengths and other structural features is not possi-
ble since the molecular structure of the free ring 2 is yet to be
determined.
1J(PtP1) 2378 Hz; and 1J(PtP2) 2884 Hz) [4]. The slightly higher
1J(PtP1) ring couplings for 4 probably reflect the slightly greater
s-character in the monophosphabenzene system 1 than in 2,
reflecting the greater overall electron withdrawing properties of
phosphorus compared with carbon, which we previously estab-
lished by both PE measurements and DFT calculations [20].
Structural characterisation of 4 was established by a single-
crystal X-ray diffraction study and its molecular structure is shown
in Fig. 2, together with selected bond lengths and angles.
In contrast to the unreactivity of 1. treatment of 2 with 0.5
equivalents of the palladium complex 3 readily results in the for-
In summary, we have show that the
2,4,6-tritertiarybutyl-1,3,5-triphosphabenzene ring
r
-donor properties of the
towards
1
transition metals are significantly less than those of the corre-
sponding 2,4,6-tritertiarybutylphosphabenzene ring 2 despite the
P lone-pair electrons in both aromatic rings being flanked by two
bulky CtBu substituents. Furthermore, although the structurally
confirmed trans-stereochemistry of 4 is in full agreement with that
originally proposed by us [4] for the analogous 2,4,6-tritertiarybu-
tyl-1,3,5-triphosphabenzene systems, we propose that both are the
kinetic rather than thermodynamic products in view of 31P NMR
spectroscopic studies (vide infra).
Cl
Pd
N
H
Me
Me
Me
2
3
Fig. 1. Di-l-chloro-bis[(R)-dimethyl(1-ethyl-a
-naphthyl)-aminato-C2,N] palladium(II).
Fig. 3. Molecular structure of cis-[PdCl(C10H6CHMeNMe2)(g
1-PC5H2But3)] 6.
Fig. 2. Molecular structure of trans-[PtCl2(PEt3)(
g
1-PC5H2But3)] 4. Selected bond
Selected bond lengths (Å) and angles (°): Pd–C(18) 2.008(4), Pd–N 2.112(3), Pd–P
2.2358(11), Pd–Cl 2.4057(11), P–C(1) 1.716(4), P–C(5) 1.729(4), C(1)–C(2) 1.386(6),
C(4)–C(5) 1.392(6), C(3)–C(4) 1.400(6), C(2)–C(3) 1.401(6). C(18)–Pd–N 81.27(14),
C(18)–Pd–P 93.23(12), N–Pd–P 172.91(9), C(18)–Pd–Cl 175.31(12), N–Pd–Cl
94.88(9), P–Pd–Cl 90.80(4), C(1)–P–C(5) 106.6(2), C(1)–P–Pd 126.11(14), C(5)–P–
Pd 126.35(15), C(2)–C(1)–P 120.2(3), C(4)–C(5)–P 119.3(3), C(5)–C(4)–C(3)
127.0(4), C(1)–C(2)–C(3) 126.6(4), C(4)–C(3)–C(2) 120.2(4).
lengths (Å) and angles (°): Pt–P(2) 2.285(2), Pt–Cl(2) 2.309(2), Pt–Cl(1) 2.320(2), Pt–
P(1) 2.335(2), P(1)–C(5) 1.726(8), P(1)–C(1) 1.741(8), C(4)–C(5) 1.397(11), C(2)–
C(3) 1.391(11), C(3)–C(4) 1.394(11), C(1)–C(2) 1.391(11). C(5)–P(1)–C(1) 107.3(4),
C(5)–P(1)–Pt 126.5(3), C(1)–P(1)–Pt 125.9(3), C(2)–C(1)–C(6) 121.2(7), C(2)–C(1)–
P(1) 118.8(6), C(6)–C(1)–P(1) 120.0(6), C(1)–C(2)–C(3) 127.4(7), C(2)–C(3)–C(4)
120.1(7), C(4)–C(5)–P(1) 118.3(6), C(3)–C(4)–C(5) 128.0(7).

S.B. Clendenning et al. / Journal of Organometallic Chemistry 695 (2010) 717–720
719
After stirring for 12 h, the solvent was removed to give the product
as a yellow solid. n-Pentane (5 ml) was added to a sample of the
product ꢀ80 mg and the resulting suspension was filtered hot into
a round-bottomed flask. After 1 month at room temperature yel-
low crystals of the product were obtained. Yield = 180 mg, 73%.
1H NMR (400 MHz, CD2Cl2): 8.10, 8.05 (m, ring H’s), 1.96 (m, 6H,
P{CH2CH3}3), 1.79 (s, 18H, But), 1.39 (s, 9H, But), 1.27, 1.23 (2 over-
lapping t, 9H, P{CH2CH3}3, 7.61 Hz). 31P NMR (CD2Cl2, 161.9 MHz):
(dP = 161.4 (d, ring P, 2J(PAPX) 526.8, 1J(PtPA) 2520 Hz), (dP = 10.9 (d,
PEt3, 2J(PXPA) 526.9, 1J(PtPx) 2935 Hz).
Crystal data for 4: C23H44Cl2P2Pt, M = 648.51, monoclinic, space
group
P21/n
(No.14),
a = 11.9240(3) Å,
b = 15.9942(5) Å,
c = 15.1677(3) Å, b = 107.503(2)°, V = 2758.78(12) Å3, T = 173(2) K,
Z = 4, Dc = 1.56 Mg m3,
l
= 5.40 mmÀ1, k = 0.71073 Å, crystal size
0.15 Â 0.10 Â 0.10 mm3, 20 506 measured reflections, 5394 inde-
Fig. 4. Molecular structure of [AuCl(
g
-PC5H2But )]. 7. Selected bond lengths (Å) and
pendent reflections, 4472 reflections with I > 2r(I), Final indices
3
angles (°): Au–P 2.2194(15), Au–Cl 2.2654(17), P–C(1) 1.726(4), C(1)–C(2) 1.387(5),
C(3)–C(2) 1.398(5). P–Au–Cl 177.76(6), C(1)’–P–C(1) 108.0(3), C(1)–P–Au
125.29(13), C(2)–C(3)–C(2)’ 120.8(5), C(1)–C(2)–C(3) 127.0(4), C(2)–C(1)–P
118.5(3).
R1 = 0.043, wR2 = 0.097 for I > 2r(I), R1 = 0.057, wR2 = 0.102 for all
data. Data collection: KappaCCD, Program package WINGX, Abs cor-
rection MULTISCAN Refinement using SHELXL-97, Drawing using OR-
TEP-3 for Windows. There are two residual peaks of ca. 2 e ÅÀ3
which make no chemical sense and are assumed to be artifacts
Cl
PR3
Cl
Cl
Cl
tBu
tBu
Pt
Pt
3.2. Preparation of [PdCl(C10H6CHMe(NMe2)(g
1-PC5H2But3)] 6
P
P
PR3
P
P
PC5H2But
(0.035 g, 1.3 Â 10À4 mol) and
5
(0.044 g,
3
6.5 Â 10À5 mol) were combined, dissolved in toluene and stirred
vigorously for 24 hours. The solution was reduced to 2 ml in vol-
ume and filtered hot into a round-bottomed flask. Storage at
À50 °C for 1 week resulted in yellow crystals. Yield = 28 mg, 35%.
31P NMR (CD2Cl2, 161.9 MHz): (dP = 167.6 (s, ring P).
tBu
tBu
tBu
tBu
P
P
trans-
cis-
Fig. 5. Trans- to cis-isomerisation of [PtCl2(PEt3)(P3C3But )].
3
Crystal data for 6: C31H45ClNPPd.(C7H8), M = 696.63, orthorhom-
bic, space group P212121 (No. 19), a = 12.1168(3) Å, b =
13.9137(2) Å, c = 21.5795(5) Å, V = 3638.08(13) Å3, T = 173(2) K,
Thus, as previously reported,[4], the reaction of the
chloro-bridged dimers [Pt2Cl4(PR3)2] with triphosphabenzene 1
immediately quantitatively afforded the trans-g
1-complexes
Z = 4, Dc = 1.27 Mg m3,
l
= 0.65 mmÀ1, k = 0.71073 Å, crystal size
[PtCl2(PR3)(P3C3But )] (PR3 = PEt3 5, PMe3 8, PMe2Ph 9 and PMePh2
0.25 Â 0.25 Â 0.10 mm3, 23 598 measured reflections, 7120 inde-
3
10) which were fully spectroscopically characterised by 31P and
pendent reflections, 6164 reflections with I > 2r(I), Final indices
195Pt NMR spectroscopy as the sole products.
R1 = 0.039, wR2 = 0.083 for I > 2r(I), R1 = 0.052, wR2 = 0.090 for all
In more detailed solution NMR studies we have found that a
slow isomerisation process takes place in some (but not all) of
the above systems. In the case of the trans-complexes 8 and 9 a
spontaneous partial isomerisation to the cis-isomers 8a and 9a is
observed at room temperature (Fig. 5), the relatively slow rate of
isomerisation being easily followed by comparison of the relative
intensities of the peaks in the 31P NMR spectrum corresponding
to each isomer (data are listed in Section 3). The ratio of isomers
observed at different times were found to be (i) 8: cis:trans = 0:100
(40 min); 2:98 (5 h); 20:80 (20 h); 26:74 (22 h). (ii) 9: cis:-
trans = 3:97 (30 min); 47:53 (17 h); 57:43 (41 h) whence it can
be seen that both the rate of formation and degree of conversion
from trans- to cis-isomers are higher for 9 than for 8. Thus,
although the trans-isomer is formed initially as the kinetic product
of the bridge splitting reactions, the thermodynamically favoured
product is the cis-isomer.
data. Data collection: KappaCCD, Program package WINGX, Abs cor-
rection MULTISCAN Refinement using SHELXL-97, Drawing using OR-
TEP-3 for Windows. The disordered methyl C atoms for the C(6) tBu
group were left isotropic.
3.3. Preparation of [AuCl(g
1-PC5H2But3)] 7
PC5H2But (0.03 g, 1.1 Â 10À4 mol) and [AuCl(tht)] (0.036 g,
3
1.1 Â 10À4 mol) were combined and dissolved in toluene (20 ml).
After stirring for 20 h, the solvent was removed and the remaining
solid dissolved in n-pentane (5 ml). The solution was filtered hot
into a round-bottomed flask and stored at room temperature for
1 week. Colourless crystals of the product were obtained.
Yield = 45 mg, 80%. 1H NMR (400 MHz, C6D6): 7.95, 7.89 (s, ring
H’s), 1.41 (s, 18H, But), 1.07 (s, 9H, But). 31P NMR (C6D6,
161.9 MHz): (dP = 154.6 (s, ring P).
3. Experimental
Crystal data for 7: C17H29AuClP, M = 496.79, monoclinic, space
group P21/m (No. 11), a = 6.1103(1) Å, b = 15.5516(5) Å,
c = 10.2639(3) Å, b = 104.442(2)°, V = 944.51(4) Å3, T = 173(2) K,
Standard Schlenk tube procedures were employed and all sol-
vents were rigorously dried and redistilled before use. Starting
materials and metal complexes were made by published literature
procedures.
Z = 2, Dc = 1.75 Mg m3,
l
= 8.01 mm–1, k = 0.71073 Å, crystal size
0.25 Â 0.20 Â 0.10 mm3, 14 613 measured reflections, 1912 inde-
pendent reflections, 1805 reflections with I > 2r(I), Final indices
R1 = 0.028, wR2 = 0.072 for I > 2r(I), R1 = 0.030, wR2 = 0.074 for all
3.1. Preparation of trans-[PtCl2(PR3)(
g
1-PC5H2But3)] 4
data. Data collection: KappaCCD, Program package WINGX, Abs cor-
rection MULTISCAN Refinement using SHELXL-97, Drawing using OR-
TEP-3 for Windows. The molecule lies on a crystallographic mirror
plane.
PC5H2But (0.1 g, 3.8 Â 10À4 mol) and [Pt2Cl4(PEt3)2] (0.145 g,
3
1.98 Â 10À4 mol) were combined and dissolved in CH2Cl2 (20 ml).

720
S.B. Clendenning et al. / Journal of Organometallic Chemistry 695 (2010) 717–720
3.4. Formation of cis-[PtCl2(PR3)(
g
1-P3C3But3)] (R = PMe3 8a, PMe2Ph
Appendix A. Supplementary data
9a) from the corresponding trans-isomers
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.12.005.
P3C3But and 0.5 molar equivalents of [PtCl2(PR3)]2 (PR3 = PMe3,
3
PMe2Ph) were combined, dissolved in a minimal volume of CH2Cl2
or CHCl3 and the solution stirred for 30 min to afford a yellow solu-
References
tion of trans-[PtCl2(PR3)(g
1-P3C3But3)] (PR3 = PMe3, PMe2Ph) in
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quantitative yield. Removal of the solvent in vacuo yielded a yellow
powder. Trans- to cis-isomerism was monitored by 31P NMR spec-
troscopy: Compound 8: cis:trans = 0:100 (40 min); 2:98 (5 h);
20:80 (20 h); 26:74 (22 h). Compound 9: cis:trans = 3:97
(30 min); 47:53 (17 h); 57:43 (41 h).
31P{1H} NMR data for trans-isomers 8 and 9: (dP in ppm; J in Hz)
(8): 31P{1H} NMR (121.5 MHz, CDCl3): AB2XY system, dP = 264.8 [d,
2
2
2JP(A)P(B) 36.3, P(B)]; dP = 203.0 [dt; JP(A)P(X) 543.3, JP(A)P(B) 36.3,
2
1
1JPtP(A) 2418; P(A)]; dP = À18.3 [d; JP(A)P(X) 543.3, JPtP(X) 2886;
P(X)]. 195Pt NMR (107.496 MHz, CDCl3): d = À3705 (dd; JPtP(A)
1
1
2
2418, JPtP(X) 2886). (9): dP = 265.4 [d, JP(A)P(B) 36.6, P(B)];
2
2
1
dP = 202.5 [dt; JP(A)P(X) 543.6, JP(A)P(B) 36.6, JPtP(A) 2487; P(A)];
dP = À11.4 [d; JP(A)P(X) 543.6, JPtP(X) 2920; P(X)]. 195Pt NMR
2
1
1
1
(107.496 MHz, CDCl3): d = À3710 (dd; JPtP(A) 2487, JPtP(X) 2884).
31P{1H} NMR data for cis-isomers 8a and 9a: (8a): 31P{1H} NMR
2
(121.5 MHz, CDCl3): AB2XY system, dP = 274.2 [d, JP(A)P(B) 42.0,
P(B)]; dP = 173.3 [td; JP(A)P(B) 42.0, JP(A)P(X) 25.3, JPtP(A) 4196;
2
2
1
2
1
P(A)]; dP = À23.2 [d; JP(A)P(X) 25.3, JPtP(X) 3261; P(X)]. (9a):
2
2
d = 275.9 [d, JP(A)P(B) 42.2, P(B)]; dP = 172.2 [td; JP(A)P(B) 42.2,
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481.
2JP(A)P(X) 25.0, JPtP(A) 4192; P(A)]; dP = À19.3 [d; JP(A)P(X) 25.0,
1
2
1JPtP(X) = 3352; P(X)].
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Inorg. Chem. 21 (1982) 1007.
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Acknowledgements
We thank the EPSRC for post-doctoral support (for CWT), NSERC
for a scholarship (for SBC) and Professor S.B. Wild (ANU, Canberra)
for providing the sample of compound 3.