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Platinum(II) and palladium(II) complexes of tetrakis(pyrazolyl)cyclotriphosphazenes
Polyhedron Vol. 16, No. 7, pp. 1003 101 I, 1997 Copyright ,(' 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387,'97 $17.00+0.00
~ Pergamon PII : S0277-5387(96)00385-3
Platinum(ll) and palladium(ll) complexes of tetrakis(pyrazolyl)cyclotriphosphazenes K. R. Justin T h o m a s , ~ * t V. Chandrasekhar, ~ Piero Zanello b and F r a n c o Laschi b ~Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India bDepartimento di Chimica dell' Universifft di Siena, Pian dei Mantellini 44. 53100 Siena. Italy
(Received 28 June 1996; accepted 16 August 1996) Abstract--Reactions of tetrakis(pyrazolyl)cyclotriphosphazene derivatives, gem-N3-P3Ph2Pz4 (2) and gemN3P3Ph2(dmpz)4 (4), with PdClz(PhCN)2 or PtCI:(PhCN)2 afforded complexes of the type MCI2L (L = 2 or 4). The detailed spectroscopic (proton, ~3C and 31p NMR) investigations revealed that in these complexes the phosphazenes act as bidentate N2-donor ligands via the two geminal pyrazolyl pyridinic nitrogen atoms, thus forming six-membered chelate rings. The platinum derivative N3P3Ph2Pz4" PtCI2 (6) reacted further with CuCI~ to yield a new heterobimetallic derivative, N3P3Ph2Pz4"PtCI2"CuC12 (9). The EPR spectra of 9 indicated a distorted five-coordinate geometry around copper. The electrochemistry of 9 is also reported. Copyright [ 1997 Elsevier Science Ltd Kevwords: cyclotriphosphazene ligand ; pyrazole ; platinum(II) ; palladium(ll), copper(lI) complexes: NMR.
Interest in the donor groups attached to cyclophosphazenes and their transition metal complexes arises from the fact that such small molecules can function as structural and functional models for their linear high molecular weight polymer analogs, which should exhibit unusual physical and chemical properties [1]. Ever since the pyrazolylcyclotriphosphazenes were introduced by Paddock and coworkers [2] into coordination chemistry, several novel pyrazolyl-substituted cyclotriphosphazene derivatives have been examined as ligands for transition metals [3 8]. Depending on the nature of the substituents other than pyrazolyl groups and the metals, four different modes of interaction between the pyrazolylcyclotriphosphazene and transition metals are observed (Fig. 1). In general they coordinate to the metal through the exocyclic pyrazolyl and the phosphazene skeletal nitrogens. The hexakis- and tetrakis(pyrazolyl)cyclotriphosphazenes N3P3(dmpz)6 and N~P~Ph2(dmpz)4 form five-coordinate complexes with copper(II) and nickel(II) halides [3-5], in which the metal is bound to the ligand via two nongeminal pyr-
R
N--N
R
X/
~P~ N
/X R
N --N
\/
/
M
N--N
N
"nongeminal N2 coordination'
N--N
"nongeminal N3 coordination'
R
=N~,
/P--
YoN--N\
N
* Author to whom correspondence should be addressed. tPresent address: Dr K. R. Justin Thomas, Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Madras 600 036, India.
,M
/X R
\ /
'geminal N2 coordination'
Fig.
1003
N 'geminal N~ coordination"
Coordination modes of triphosphazenes.
pyrazolylcyclo-
1004
K . R . Justin Thomas et al.
azolyl and one cyclotriphosphazene ring nitrogen. However, with platinum(II) and palladium(II) chlorides they produced mainly four-coordinate complexes with the N 2 donor set derived from two geminal pyrazolyl nitrogens [2,8]. The nongeminal coordination leads to five-membered chelate rings, while in the geminal (N2) mode the complexes possess six-membered chelates with the boat conformation. In the geminal pattern the ring phosphazene nitrogen involvement in coordination is scarce and is identified only with certain group VI metal carbonyl complexes [9]. The interaction of the cyclophosphazene skeletal nitrogen with the transition metal is of a-type and is generally weak. It was believed that the basicity of the cyclotriphosphazene ligand and the coordination number of the metals are the important factors affecting the ligating response of a particular pyrazolylcyclotriphosphazene. The basicity of a cyclophosphazene ligand may be affected by changing substituents other than donor groups with electronrich moieties and/or by modifying the substituents on the exocyclic donor groups. To unravel the basicity factor we have synthesized a new ligand N3P3Ph2Pz4 (2), which contains four simple pyrazolyl groups as donors. The difference between 2 and N3P3Ph2(dmpz)4 (4) is the presence or absence of methyl substituents at the pyrazole nucleus and the basicity of 2 would be comparatively low. Our aim here is to compare the electronic and steric effects in the spectral and electrochemical properties of the complexes. We describe here in particular the mononuclear Pt" and Pd Hchloride complexes of 2 and a heterobimetallic compound with Pt and Cu metal centers and an attempt is also made to compare with the related systems known in the literature.
EXPERIMENTAL Abbreviations Hdpctp : hexakis(3,5-dimethyl-l-pyrazolyl)cyclotriphosphazene, tdpctp: 2,2-diphenyl-4,4,6,6-tetrakis (3,5-dimethyl-1-pyrazolyl)cyclotriphosphazene, tpctp : 2,2-diphenyl-4,4,6,6-tetrakis (l-pyrazolyl) cyclotriphosphazene, dmpzH : 3,5-dimethylpyrazole, PzH : pyrazole.
Synthesis of the ligand 2 A solution of N3P3Ph2CI4 (4.31 g, l0 mmol) dissolved in benzene (50 cm 3) was added to a stirred mixture of pyrazole (2.72 g, 40 mmol) and triethylamine (6.06 g, 60 mmol) at 25°C. The reaction mixture was heated under reflux for 2 days, cooled and the precipitated triethylamine hydrochloride salt was filtered off. From the filtrate the solvent was stripped off in vacuo, leaving a residue. The residue was thoroughly washed with water (3 × 20 cm 3) and ether (3 x 20 cm 3) and subsequently dried in vacuo. A colorless solid obtained was crystallized from a 1 : 1 mixture of hexane and dichloromethane (4.18 g, Yield: 75%). M.p. : 219 'C. Found : C, 51.6 ; H, 3.9 ; N, 27.8. Calc. for C24H22NIIP3: C, 51.7; 3.9; N, 27.6%. IR (cm ~) : 1470 m, v ( C = N ) ; 1280 vs; 1220 vs, br; 1165 vs, br, v(P~-N). Synthesis of 3 The title compound was obtained in 60% yield by using a very similar procedure that of above except that N3P3Ph2C12 was substituted by N3P3[NH(CH2)3NH]C14 M.p. : 216°C. Synthesis of the complexes The complexes 5-8 were essentially made by following a same procedure. A general description is as follows: The ligand (1 mmol) and the metal precursor (1 mmol) were taken in benzene (20 cm 3) and heated to reflux for 2~4 h. The palladium complexes 5 and 7 were isolated as insoluble residues from the reaction mixture, while the platinum complexes 6 and 8 required the evaporation of the solvent followed by precipitation from diethyl ether. The solids obtained were crystallized from dichloromethane/benzene or hexane mixtures. The physical and analytical data presented in Table I. Synthesis of 9 To a stirred solution of 6 (0.1646 g) dissolved in dichloromethane (20 cm 3) was added anhydrous CuCI2 (0.030 g). The turbid mixture was stirred for 6 h. The light green precipitate formed was separated by filtration and dried in vacuo. Yield: 0.182 g (95%).
Materials
Physical measurements
The platinum and palladium precursors used in this study were obtained by known literature methods [10]. N3P3[NH(CH2)3NH]CI4, N3P3PhzCI4 and N3P3Phz(dmpz)4 synthesized by adopting the reported procedures [11, 12, 4]. Solvents dichloromethane, benzene, hexane and acetonitrile were distilled from P205 and stored over molecular sieves.
IR spectra were recorded on a Perkin-Elmer 1320 spectrophotometer. The samples were prepared as potassium bromide pellets and the wavenumber was calibrated using polystyrene as calibrant. ~H and ~3C N M R spectra were obtained on a Bruker spectrometer operating at 400 MHz and chemical shifts are reported with reference to internal tetramethylsilane. 3~p N M R
1005
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes Table 1. Physical data for the complexes"
Compound
Color
5 6~' 7 8 9'
Orange Yellow Orange Light yellow Light green
Melting point (dec.)
C
Elemental Analyses Found (Calc.) H
140 134 238 125 120
39.2[38.7] 42.1 [42.2] 45.4[45.1] 41.1 [41.2] 30.1 [30.2]
3.0[3.3] 3.3[3.4] 4.5[4.7] 4.1 [3.85] 2.3[2.5]
N
21.0[20.8] 16.4116.25] 18.2117.9] 16.5116.2] 16. I [16.0]
"The formulae of the complexes 4~9 are N 3P3Ph2Pz4"PdCI~, N3P~Ph2Pz4"PtCI2, N~P3Ph:(dmpz)4"PdC12, N3P~Ph2(dmpz)4" PtC12and N3P3Ph2Pz4"PtC12• CuCI2. hIncludes half a molecule of benzene. 'Cu: 6.6[6.45].
spectra were recorded on a Bruker spectrometer operating at 164.5 MHz. Phosphorus chemical shifts are reported with reference to external 85% H3PO4; upfield shifts are negative. Electrochemical measurements were performed with a BAS 100A electrochemical analyser [13]. The three-electrode cell consisted of a glassy-carbon working electrode, a platinum wire as a counter electrode and reference electrode saturated calomel electrode. All potential values are referred to the saturated calomel electrode. Under the present experimental conditions the oneelectron oxidation of ferrocene occurs at +0.38 V. RESULTS AND DISCUSSION
Synthesis The geminal tetrakis(pyrazolyl)cyclotriphosphazene, N3P3Ph2pz4 (2), is obtained from the reaction ofgem-N3P3Ph2C14 with pyrazole in the presence of triethylamine in refluxing benzene. Similarly, 3 is accessed by a facile nucleophilic substitution reaction of spiro-N3P3[NH(CH2)3NH]CI4 with pyrazole (Scheme 1). The phosphazenes 2 and 3 are obtained in relatively good yields. The pyrazolylcyclo-
triphosphazenes have been characterized by multinuclear N M R and IR spectroscopy and by C, H and N elemental analyses. The reaction of 2 and N3P3Phz(dmpz)4 (4) with PdClz(NCPh)2 or PtC12(NCPh)2 in benzene give the complexes of the general formula [MCI2"L], where L=2;M=Pd(5) o r P t (6) a n d L = 4 ; M = P d ( 7 ) or Pt (8), in good yields. The complexes are highly soluble in common ha]ogenated solvents, such as dichloromethane, chloroform, etc. In these complexes the two geminal pyrazolyl pyridinic nitrogen atoms are involved in coordination. The coordinating ability of these "metallo-ligands" has been illustrated by the preparation of a heterobimetallic compound 2" PtCI2" CuCI2 (9). All the complexes have been characterized by elemental analysis and multinuclear NMR spectroscopy. Electrochemical and ESR spectral studies have been performed on the heterobimetallic derivative 9.
NM R spectroscopy The ~H NMR spectra of 2 and 3 show that the four pyrazolyl groups are equivalent. They exhibit three
R~/R CI~/C1 Ph
II
I ~C1
PZ~/pz ~
ph.... II
ph..~p N ~ p ~ . c I 4EtaN/CsH. p h / p
R II I/R R/P'-,.. N~ P N S
R._ II I/R R/~N~P'~R
I/p~
N~P~pz
N\7" N 4pzH C 1 [] I / C 1 4Et3N/C6H~ p z ][ ]/pz C1/P'~ N~ P ~ C1 pz/P"-. N ~ P ~ pz 3
Scheme 1.
Ph~/Ph
R= dmpz (1)
R = pz (2) R = dmpz (4)
HN \
/
NH
P~\II l~pz ~)~N~p~ 3
Scheme 2.
1006
K. R. Justin Thomas e t al.
different signals due to the 4-C, 5-C and 3-C protons of pyrazole nucleus at c a 6.2, 7.7 and 7.9 ppm, respectively (Fig. 2). The unresolved multiplet at c a 6.2 ppm is assigned to the 4-C protons. Recent studies on the polypyrazolyl borates and their complexes indicate that the 3-C protons resonate at lower fields than that of 5-C [12,13]. Considering the criterion to be compatible with the case for these pyrazolylphosphazenes also, we attribute the doublet in the downfield region to the 3-C protons and the other doublet at c a 7.7 ppm to the 5-C protons. In addition, the pyrazolylcyclotriphosphazenes 2 and 3 show features due to the phenyl and 1,3-diaminopropane moieties, respectively. The proton N M R spectrum of 2 shows a cluster of signals in the region 7.21 7.81 ppm for the phenyl groups, while that of 3 exhibits a multiplet at 1.67, a triplet at 3.35 and a broad signal at 4.86 ppm due to the - - C H 2 - - , - - N C H 2 - - and the - - N H - - protons, respectively, of the 1,3-diamino propane s p i r o loop. After coordination to palladium (in complexes 5 and 7) all the four pyrazolyl groups become nonequivalent and give rise to approximately four sets of signals at r o o m temperature (Figs 3 and 4, Table 2). This is surprising to compare with the results reported by other authors who observe that the palladium derivatives of polypyrazolyl borate [14]
5-CH & Phenyl 3-CH
I
8.4
I
8.2
I
8.0
7.8
ppm Phenyl 4-CH
I
l
7.6
7.4
~1
I
6.6 ppm
6.4
I
6.2
Fig. 3. 'H NMR spectrum of the palladium complex 5 indicating the presence of four different pyrazole groups.
(a)
i
f
8
i
7 ppm
6
(b)
i
15o
I
140
I
I
130
120
I
110
]
100
I
3.0
I
I
ppm
2.5 ppm
2.0
Fig. 2. (a) 'H and (b) ~3C NMR spectra of 2, showing only one type of pyrazole group.
Fig. 4. ~H NMR spectrum of the palladium complex 7 in the methyl region.
1007
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes Table 2. Proton NMR data for the ligand and complexes"
No. 2 5 6 4 7 8 3
3-CH3 (or~ 3-CH
5-CH 3 (or) 5-CH
7.90 (d, 4H) 7.60, 7.78 7.81, 7.98 (d, 4H) 7.98 (4H) 2.26 (s, 12H) 2.75, 2.72, 2.68, 2.57 2.35, 2.22, 2.14 (s, 24H) ~' 2.19 (s, 12H) 8.10 (d, 4tt)
7.73 (d, 4H) 7.95, 8.16 8.19, 8.41 (t, 4H) 7.80 (4H) 2.15 (s, 12H)
2.08 (s, 12H) 7.75 (d, 4H)
4-CH
Phenyl
6.23 (t, 4H) 6.32, 6.37 6.41, 6.50 (q, 4H) 6.35 (4H) 5.88 (s, 4HI 5.79, 5.80 5.98.6.00 (s, 4H) 5.86 (br, 4H) 6.42 (t, 4H)
7.29 7.81 (m, 10H) 7.37 7.90 (m, 10H) 7.38 7.87 (m. 10H) 7.31 7.94(m, 10H) 7.37 8.24 (m, 10H) 7.3(~8.09 (m, 10H) 4.86 (br, NH, 2H) 3.35 (t, NCH> 4H) 1.63 (m, CH> 2H)
"Multiplicity given in parentheses. h Intensity ratio ~ : ~ :/~ : ~ :/~ : ct : 23.
or pyrazolylphosphazenes [8] are fluxional in nature at room temperature. This observation stimulated us to reinvestigate the spectra of the palladium complex 7, derived from tdpctp, which was earlier reported by Paddock and Gallicano [2]. They have observed only three signals for 3- and 5-methyl protons. Interestingly, the spectra recorded at 400 M H z shows seven signals in the ratio : 2 :/3 : e : 3 : ~ : 2/3 (Fig. 4). The observation of nonequivalent pyrazolyl groups are well explained by considering the two possible isomers for these complexes (Fig. 5). In isomer A the uncoordinated pyrazolyl groups are away fi'om the metal center and constitute a stereochemically equivalent set. However, in isomer B the uncoordinated pyrazolyl groups are stereo-
pz
Cl
"-%p
.( N//
Ph
(A) P.
Pd
N
N
P\/
Ph
.,
cl
r~
P "/
Pd N N Fig. 5. Isomeric structures of the complexes with geminal N2 coordination.
chemically nonequivalent. These differences, due to the spatial arrangement of pyrazolyl groups lead to a multitude of signals. Similar and somewhat less pronounced effects have been noticed in the ~3C N M R spectra of the compounds also (Table 3). The 31p N M R spectra of 2 and 3 display a AX~ pattern with a doublet at - 1.99 and 1.15 and a triplet at 24.10 and 14.58, respectively. The upfield doublet is readily assigned to the Ppz2 groups by comparison with the value for N3P3(dmpz)4 and 4. The two bond coupling observed for Ppz2 and PPh2 o r Pspiro centers in 2 and 3 are larger than that observed for PCI~ and PPh2 or P~p~rogroups in their parent compounds N3P3[NH(CHz)3NH]CI4 [12] and N3P3Ph2CI4 [15], respectively. The chemical shift of a particular phosphorus atom depends on several factors, such as the nature of the substituents (steric and electronegatively), extent of 7r-bonding with the substituents, bond angles at phosphorus, etc. The substituents on the adjacent phosphorus may also influence the chemical shift of the phosphorus nucleus under consideration [18]. It is clearly evident from Table 4 that the bulkiness of the pyrazolyl substituent is responsible for the drastic upfield shift when compared with the other aminocyclotriphosphazenes. A slight down-field shift for the Ppz2 group in 3 may be attributed to the electronic effect exerted by the electron rich spiro loop. The 3~p N M R spectra of palladium complexes 5 and 7 show an A B X pattern (see Table 4). The two ppz2 phosphorus nuclei which are equivalent in I and 4 have become nonequivalent and the phosphorus bearing the coordinated pyrazole are shielded and phosphorus nucleus associated with the uncoordinated pyrazolyl groups are slightly deshielded. The PPh: phosphorus nuclei in the palladium complexes are also deshielded. The platinum complexes show only two broad signals at r o o m temperature, suggestive of fluxional motions in solution. However. the nature of the dynamic processes are not clear at this stage.
1008
K. R. Justin T h o m a s et al. Table 3. ~3C NMR data for the ligand and complexes
Compound
3-C
4-C
5-C
Methyl
2
145.9
107.8
134.8
5 6
146.1 142.0 146.1
107.9 106.6 107.9
136.2 135.0 135.3
4
152.1
109.4
146.7
13.7, 12.8
7
111.0, 110.9 110.7 108.8
149.2
8
157.5, 157.4 153.7, 153.6 153.4
136.1
15.4, 13.8 13.5, 12.5 13.4, 12.1
3
145.0
107.4
135.0
Phenyl 134.5, 133.1, 131.8 130.5, 128.4 132.0, 130.7, 130.1 128.6, 128.5 135.0, 133.8, 132.0 130.7, 128.6 135.7, 134.3, 131.6 131.4, 127.6 131.6, 131.1, 130.9 128.6, 128.4 134.5, 132.8, 130.3 128.6, 127.1 40.0 (NCH2) 25.6 (CH2)
Table 4.31p NMR data for the ligand and complexes Compound
Pattern
6P(Ph2)
6P(pz2)
3j(p_p)
2 5
AX2 ABX
24.10 25.00
51.4 71.5 33.4, 24.6
6 4 7
AX2 AX2 ABX
24.08(br) 20.35 21.05
8 3 N3P3[H(CH2)3NH]CI4 ~ N3P3Ph2C14a N3P3(NHBut)zC12 ' N3P3(Az)2C14]t
AX2 AX2 AX2 AX2 AXz AX2
22.73 (br) 14.58" 12.3" 19.5 2.3" 34.2 t
- 1.99 -0.41 -3.52 - 1.95(br) -5.41 - 1.82 - 5.97 - 3.61 (br) 1.15 23.1' 17.1 ' 19.6' 21.9 ~
Hz
58.8 68.9 45.6, 22.3 50.7 43.7 12.0 44.7 30.0
"6P(spiro). ~From ref. [12]. ' 6 P (C12).
'~From ref. [15]. "6P ((NHBut)2), from ref. [16]. t f P (Az2), from ref. [17].
E P R spectroscopy The E P R spectrum o f 9 recorded at r o o m temperature as polycrystalline p o w d e r sample was isotropic a n d do n o t exhibit any metal or ligand splitting features. However, in solution it displayed informative spectra. Figure 6(b) shows the X - b a n d E P R spectrum o f tpctp'PtC12.CuC12 in acetonitrile solution recorded at 100 K. T h e well resolved lineshape is indicative o f a S = 1/2 p a r a m a g n e t i c species in an axial symmetry Call > 9 - ) with hyperfine resolution in the parallel region; a poorly but informative resolution o f nine superhyperfine lines is also detectable in the perpendicular region. The overall lineshape is a c c o u n t e d for by magnetic coupling o f the S = 1/2 unpaired electron with the c o p p e r nucleus (I = 3/2) ;
the m i n o r superhyperfine multiplet is a t t r i b u t a b l e to the electron spin interaction with four nitrogen nuclei ( 1 = 1; theoretical intensity of ratio = 1 : 5 : 10: 14 : 19 : 14 : 10 : 5 : 1). The second derivative analysis affords a better resolution of the a b s o r p t i o n pattern a n d gives the experimental values o f the A , ( N ) superhyperfine coupling constant. The p o o r resolution of the spectrum in the perpendicular region a n d corresponding linewidth are consistent with the n o n equivalence of the four c o o r d i n a t i n g nitrogen nuclei, as confirmed by c o m p u t e r simulation. This further supports the proposed tetragonal a r r a n g e m e n t [19] o f the copper e n v i r o n m e n t with the three nitrogens originating from two geminal pyrazolyl groups and one acetonitrile molecule lying in the equatorial plane (higher A , ( N ) ) a n d the fourth o f the cyclo-
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes
i100 G[
,
H
,~
•
DPPH
(b)
H
"
DPPH
1009
the electrochemical behavior of it. The redox pattern exhibited by the heterodimetal complex is intriguing. As illustrated in Fig. 7, in acetonitrile solution it undergoes three subsequent cathodic steps at peaks A, B and C, respectively. Controlled potential coulometric tests served to elucidate overall reduction path. The catho-anodic peak system A/G ( E = + 0.46 V) proved to involve a chemically reversible electron transfer, in that controlled potential electrolysis (E, = 0.0 V) consumes one electron per molecule. Concommitantly the initially EPR-active (see the previous section), yellow green solution turns colorless and E P R silent. Such a final solution exhibits a cyclic voltammetric profile quite complementary to that illustrated in Fig. 7(c). Subsequent exhaustive one-electron reduction at the second cathodic step (E,~ = - 1.0 V) causes deposition of copper metal on the microelectrode surface. The resulting solution displays only the reduction process C, which we confidently assign to the reduction of the platinum(If) center, in analogy with the results on the platinum complexes of tpctp and tdpctp, which exhibit an irreversible P t " / P t reduction at very negative potential values (in the range from - 1.5 V to - 1.6 V). Upon reoxidation of the electrodeposited copper metal
Fig. 6. EPR spectra of 9 (a) at room temperature (298 K) and (b) at 100 K in acetonitrile solution. 500~lxA triphosphazene nitrogen occupying an apical position. The A~ values suggest that the equatorial plane is more tetrahedrally distorted [20]. The involvement of acetonitrile is further supported by the fact that when the spectrum was recorded in D M F the super hyperfine splitting collapsed. The parameters of the simulated spectrum are :
(a)
0.9 Y 0.5
0
.:
F
,qi = 2.270 9± = 2.059 .G, = 1/3(9, +29,) = 2.130 AI = 148.0 G A l = 10.0 G A~, = l/3(Ali+2A±) = 56.0 G AI~(3N) = 4.0 G A±(3N) = 13.8 G A ( N ) = 3.7 G A I ( N ) = 12.7 G~. Figure 6(a) shows the room-temperature X-band EPR spectrum of 9 in acetonitrile solution. Both the first and the second derivative spectra are poorly resolved and do not exhibit superhyperfine resolution. The relevant isotropic parameters ( 9 ~ o = 2 . 1 3 2 ; A,~o = 58.3 G) well fit the low temperature (100 K) ones. It must be noted that no absorption attributable to paramenetic platinum centered species have been detected neither in the original complex nor in solution from exhaustive electrolysis.
C
-0.5/
-1.0
-l.g
G
(b)
0.8 " 0.5
T 500 ~A
-o.5
-1.o
(c)
L,
0.9 0.5/
/
0
/
0
,
,
,
,
i
,
-0.5
,
,
,
,,
i
-1.0
,
,
,
211.8
E (volt)
Electrochemistry The unusual E P R spectral observations of the complex t p c t p ' P t C l 2 " C u C l 2 prompted us to investigate
Fig. 7. Cyclic voltammograms of 9 in acetonitrile solution. Scan rates: (a), (c) 0.2 V s ~: (b) 0.1 V s ~. Direct current voltammetric scan rate: (a) 0.01 V s ~.
1010
K. R. Justin Thomas et al.
(E~,. = + 0.6 V), the solution restores the original voltammetric response shown in Fig. 7. In summary, the overall redox propensity of complex tpctp" PtCl2" CuC12 is so schematizable : +e(A)
~-c(B)
[LCuIIptH] -~(G)[LCu'Pt"]- ~ ) [ L C u ° p t l ' ] 2
Cu °
+
(C) + + 2e
- e + (E)
Cu I -e~ Cu"
[PtUL] 2
+
[Pt°L] 4 (F){ degradation
Finally a few further points deserve comment. (i) The thermodynamically easy access to the Cu'Pt"L (L = ligand) congener is quite rare [21], particularly when compared with the corresponding complex hdpctp'CuCl2"PtCl2, which under the same experimental conditions undergoes the [Cu" PtnL]/[Cu°PtnL] 2 reduction process through a single-stepped irreversible process (Ep = - 0.55 V), without any stabilization of the intermediate mixed-valent species [22]. This is likely attributable to the electronic factors arising from the substitution of two pyrazolyl groups for two electron-withdrawing phenyl groups and removal of methyl groups from 3- and 5-positions of pyrazole ring, which makes easier the addition of electrons. (ii) The analysis of the peak system A/G with scan rates varying from 0.02 to 2 V s- ' shows that the peak-to-peak separation progressively increases from 140 to 315 mV. This high departure from the constant values of 59 mV expected for an electrochemically reversible one-electron transfer denotes that a considerable geometrical strain accompanies the [Cu"PtHL]/[Cu°PtnL] 2- redox change [21].
CONCLUSION The new pyrazolylcyclophazene ligand 1 reacts in a facile manner with the platinum and palladium salts to give the corresponding mononuclear complexes. The possibility of using these complexes as ligands for the assembly of heterobimetallic compounds has been demonstrated for the platinum complex 6. It is also interesting to note that the absence of methyl groups in the pyrazolyl nucleus affects mainly the electrochemical property of the heterobimetallic derivative. However, the final conclusion on the role of substituents on pyrazolyl and cyclophosphazene moieties awaits more elaborate studies with more sensitive metals such as copper, cobalt, etc., and various suitably modified pyrazolylcyclotriphosphazenes. Work in this direction is in progress at our laboratories.
REFERENCES
1. Allcock, H. R., Polymer, 1980, 21,673 ; Potin, Ph. and De Jaeger, R., Eur. Polym. J. 1991, 27, 341 ; Chandrasekhar, V. and Justin Thomas, K. R., Appl. Orqanomet. Chem. 1993, 7, 1 ; Allcock, H. R., Desorcie, J. L. and Riding, G. H., Polyhedron, 1987, 6, 119. 2. Gallicano, K. D., Paddock, N. L., Rettig, S. J. and Trotter, J., lnorg. Nucl. Chem. Lett. 1979, 15, 417; Gallicano, K. D. and Paddock, N. L., Can. J. Chem. 1982, 60, 521. 3. Justin Thomas, K. R., Chandrasekhar, V., Pal, P., Scott, S. R., Hallford, R. and Cordes, A. W., hwry. Chem. 1993, 32, 606: Justin Thomas, K. R., Chandrasekhar, V., Scott, S. R., Hallford, R. and Cordes, A. W., J. Chem. Soc., Dalton Trans. 1993, 2589. 4. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V. and Tiekink, E. R. T., J. Chem. Sot., Dalton Trans. 1994, 1301. 5. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V., Scott, S. R. and Cordes, A. W., Polyhedron, 1995, 14, 977. 6. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V., Bryan, C. D. and Cordes, A. W., Inory. Chem. 1994, 34, 5382. 7. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., Curr. Sci. 1991,60, 700. 8. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., lnor9. Chem. 1993, 32, 6102. 9. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., J. Chem. Sot., Dalton Trans. 1994, 63. 10. Kharasch, M. S., Seyler, R. C. and Mayo, F. R., J. Am. Chem. Soc. 1938, 60, 882. 11. McBee, E. T., Okuhara, K. and Morton, C. J., hwrg. Chem, 1965, 4, 1672. 12. Chandrasekhar, V., Krishnamurthy, S. S., Manohar, H., Vasudevamurthy, A, R., Shaw, R. A, and Woods, M., J. Chem. Soc., Dalton Trans. 1984, 621. 13. Zanello, P., Opromolla, G., Casarin, M., Herberhold, M. and Xeitner, P., J. Or.qanomet. Chem. 1993, 443, 199. 14. Onishi, M. and Hirachi, K., Inorg. Chim. Acta, 1994, 224, 131; Reger, D. L., Swift, C. A. and Leboida, L., lnor9. Chem. 1984, 23, 249; Trofimenko, S., Prog. lnor9. Chem. 1986, 34, 115: Cabildo, P., Claramont, R. M. and Elguero, J., Or#. Magn. Res. 1984, 22, 603. 15. Keat, R., Shaw, R. A. and Woods, M., J. Chem. Sot., Dalton Trans. 1976, 1582. 16. Allen, C. W., Brown, D. E., Cordes, A. W. and Craig, S. C., J. Chem. Soe., Dalton Trans. 1988, 1405. 17. Van der Huizen, A. A., Van de Grampel, J. C., Rusch, J. W., Witting, T., Bolhuis, E. and Meetsma, A., J. Chem. Soc., Dalton Trans. 1986, 1317. 18. Chandrasekhar, V. and Justin Thomas, K. R., Struct. Bonding (Berlin), 1993, 81, 43. 19. Hathaway, B. J. and Billing, D. E., Coord. Chem. Rev. 1970, 5, 143. 20. Additon, A. W., Rao, T. N., Reedijk, J., van Rijn,
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes ,,,J. and Verschoor, G. C., J. Chem. Soc., Dalton Trans. 1984, 1349. 21. Zanello, P., Stereochemistry of Organometallic and Inorganic Compounds, Vol. 4, ed. I. Bernal. Elsevier, Amsterdam 1990, p. 181.
101 I
22. Justin Thomas, K. R., Zanello, P. and Chandrasekhar, V., Manuscript in preparation.
~ Pergamon PII : S0277-5387(96)00385-3
Platinum(ll) and palladium(ll) complexes of tetrakis(pyrazolyl)cyclotriphosphazenes K. R. Justin T h o m a s , ~ * t V. Chandrasekhar, ~ Piero Zanello b and F r a n c o Laschi b ~Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India bDepartimento di Chimica dell' Universifft di Siena, Pian dei Mantellini 44. 53100 Siena. Italy
(Received 28 June 1996; accepted 16 August 1996) Abstract--Reactions of tetrakis(pyrazolyl)cyclotriphosphazene derivatives, gem-N3-P3Ph2Pz4 (2) and gemN3P3Ph2(dmpz)4 (4), with PdClz(PhCN)2 or PtCI:(PhCN)2 afforded complexes of the type MCI2L (L = 2 or 4). The detailed spectroscopic (proton, ~3C and 31p NMR) investigations revealed that in these complexes the phosphazenes act as bidentate N2-donor ligands via the two geminal pyrazolyl pyridinic nitrogen atoms, thus forming six-membered chelate rings. The platinum derivative N3P3Ph2Pz4" PtCI2 (6) reacted further with CuCI~ to yield a new heterobimetallic derivative, N3P3Ph2Pz4"PtCI2"CuC12 (9). The EPR spectra of 9 indicated a distorted five-coordinate geometry around copper. The electrochemistry of 9 is also reported. Copyright [ 1997 Elsevier Science Ltd Kevwords: cyclotriphosphazene ligand ; pyrazole ; platinum(II) ; palladium(ll), copper(lI) complexes: NMR.
Interest in the donor groups attached to cyclophosphazenes and their transition metal complexes arises from the fact that such small molecules can function as structural and functional models for their linear high molecular weight polymer analogs, which should exhibit unusual physical and chemical properties [1]. Ever since the pyrazolylcyclotriphosphazenes were introduced by Paddock and coworkers [2] into coordination chemistry, several novel pyrazolyl-substituted cyclotriphosphazene derivatives have been examined as ligands for transition metals [3 8]. Depending on the nature of the substituents other than pyrazolyl groups and the metals, four different modes of interaction between the pyrazolylcyclotriphosphazene and transition metals are observed (Fig. 1). In general they coordinate to the metal through the exocyclic pyrazolyl and the phosphazene skeletal nitrogens. The hexakis- and tetrakis(pyrazolyl)cyclotriphosphazenes N3P3(dmpz)6 and N~P~Ph2(dmpz)4 form five-coordinate complexes with copper(II) and nickel(II) halides [3-5], in which the metal is bound to the ligand via two nongeminal pyr-
R
N--N
R
X/
~P~ N
/X R
N --N
\/
/
M
N--N
N
"nongeminal N2 coordination'
N--N
"nongeminal N3 coordination'
R
=N~,
/P--
YoN--N\
N
* Author to whom correspondence should be addressed. tPresent address: Dr K. R. Justin Thomas, Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Madras 600 036, India.
,M
/X R
\ /
'geminal N2 coordination'
Fig.
1003
N 'geminal N~ coordination"
Coordination modes of triphosphazenes.
pyrazolylcyclo-
1004
K . R . Justin Thomas et al.
azolyl and one cyclotriphosphazene ring nitrogen. However, with platinum(II) and palladium(II) chlorides they produced mainly four-coordinate complexes with the N 2 donor set derived from two geminal pyrazolyl nitrogens [2,8]. The nongeminal coordination leads to five-membered chelate rings, while in the geminal (N2) mode the complexes possess six-membered chelates with the boat conformation. In the geminal pattern the ring phosphazene nitrogen involvement in coordination is scarce and is identified only with certain group VI metal carbonyl complexes [9]. The interaction of the cyclophosphazene skeletal nitrogen with the transition metal is of a-type and is generally weak. It was believed that the basicity of the cyclotriphosphazene ligand and the coordination number of the metals are the important factors affecting the ligating response of a particular pyrazolylcyclotriphosphazene. The basicity of a cyclophosphazene ligand may be affected by changing substituents other than donor groups with electronrich moieties and/or by modifying the substituents on the exocyclic donor groups. To unravel the basicity factor we have synthesized a new ligand N3P3Ph2Pz4 (2), which contains four simple pyrazolyl groups as donors. The difference between 2 and N3P3Ph2(dmpz)4 (4) is the presence or absence of methyl substituents at the pyrazole nucleus and the basicity of 2 would be comparatively low. Our aim here is to compare the electronic and steric effects in the spectral and electrochemical properties of the complexes. We describe here in particular the mononuclear Pt" and Pd Hchloride complexes of 2 and a heterobimetallic compound with Pt and Cu metal centers and an attempt is also made to compare with the related systems known in the literature.
EXPERIMENTAL Abbreviations Hdpctp : hexakis(3,5-dimethyl-l-pyrazolyl)cyclotriphosphazene, tdpctp: 2,2-diphenyl-4,4,6,6-tetrakis (3,5-dimethyl-1-pyrazolyl)cyclotriphosphazene, tpctp : 2,2-diphenyl-4,4,6,6-tetrakis (l-pyrazolyl) cyclotriphosphazene, dmpzH : 3,5-dimethylpyrazole, PzH : pyrazole.
Synthesis of the ligand 2 A solution of N3P3Ph2CI4 (4.31 g, l0 mmol) dissolved in benzene (50 cm 3) was added to a stirred mixture of pyrazole (2.72 g, 40 mmol) and triethylamine (6.06 g, 60 mmol) at 25°C. The reaction mixture was heated under reflux for 2 days, cooled and the precipitated triethylamine hydrochloride salt was filtered off. From the filtrate the solvent was stripped off in vacuo, leaving a residue. The residue was thoroughly washed with water (3 × 20 cm 3) and ether (3 x 20 cm 3) and subsequently dried in vacuo. A colorless solid obtained was crystallized from a 1 : 1 mixture of hexane and dichloromethane (4.18 g, Yield: 75%). M.p. : 219 'C. Found : C, 51.6 ; H, 3.9 ; N, 27.8. Calc. for C24H22NIIP3: C, 51.7; 3.9; N, 27.6%. IR (cm ~) : 1470 m, v ( C = N ) ; 1280 vs; 1220 vs, br; 1165 vs, br, v(P~-N). Synthesis of 3 The title compound was obtained in 60% yield by using a very similar procedure that of above except that N3P3Ph2C12 was substituted by N3P3[NH(CH2)3NH]C14 M.p. : 216°C. Synthesis of the complexes The complexes 5-8 were essentially made by following a same procedure. A general description is as follows: The ligand (1 mmol) and the metal precursor (1 mmol) were taken in benzene (20 cm 3) and heated to reflux for 2~4 h. The palladium complexes 5 and 7 were isolated as insoluble residues from the reaction mixture, while the platinum complexes 6 and 8 required the evaporation of the solvent followed by precipitation from diethyl ether. The solids obtained were crystallized from dichloromethane/benzene or hexane mixtures. The physical and analytical data presented in Table I. Synthesis of 9 To a stirred solution of 6 (0.1646 g) dissolved in dichloromethane (20 cm 3) was added anhydrous CuCI2 (0.030 g). The turbid mixture was stirred for 6 h. The light green precipitate formed was separated by filtration and dried in vacuo. Yield: 0.182 g (95%).
Materials
Physical measurements
The platinum and palladium precursors used in this study were obtained by known literature methods [10]. N3P3[NH(CH2)3NH]CI4, N3P3PhzCI4 and N3P3Phz(dmpz)4 synthesized by adopting the reported procedures [11, 12, 4]. Solvents dichloromethane, benzene, hexane and acetonitrile were distilled from P205 and stored over molecular sieves.
IR spectra were recorded on a Perkin-Elmer 1320 spectrophotometer. The samples were prepared as potassium bromide pellets and the wavenumber was calibrated using polystyrene as calibrant. ~H and ~3C N M R spectra were obtained on a Bruker spectrometer operating at 400 MHz and chemical shifts are reported with reference to internal tetramethylsilane. 3~p N M R
1005
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes Table 1. Physical data for the complexes"
Compound
Color
5 6~' 7 8 9'
Orange Yellow Orange Light yellow Light green
Melting point (dec.)
C
Elemental Analyses Found (Calc.) H
140 134 238 125 120
39.2[38.7] 42.1 [42.2] 45.4[45.1] 41.1 [41.2] 30.1 [30.2]
3.0[3.3] 3.3[3.4] 4.5[4.7] 4.1 [3.85] 2.3[2.5]
N
21.0[20.8] 16.4116.25] 18.2117.9] 16.5116.2] 16. I [16.0]
"The formulae of the complexes 4~9 are N 3P3Ph2Pz4"PdCI~, N3P~Ph2Pz4"PtCI2, N~P3Ph:(dmpz)4"PdC12, N3P~Ph2(dmpz)4" PtC12and N3P3Ph2Pz4"PtC12• CuCI2. hIncludes half a molecule of benzene. 'Cu: 6.6[6.45].
spectra were recorded on a Bruker spectrometer operating at 164.5 MHz. Phosphorus chemical shifts are reported with reference to external 85% H3PO4; upfield shifts are negative. Electrochemical measurements were performed with a BAS 100A electrochemical analyser [13]. The three-electrode cell consisted of a glassy-carbon working electrode, a platinum wire as a counter electrode and reference electrode saturated calomel electrode. All potential values are referred to the saturated calomel electrode. Under the present experimental conditions the oneelectron oxidation of ferrocene occurs at +0.38 V. RESULTS AND DISCUSSION
Synthesis The geminal tetrakis(pyrazolyl)cyclotriphosphazene, N3P3Ph2pz4 (2), is obtained from the reaction ofgem-N3P3Ph2C14 with pyrazole in the presence of triethylamine in refluxing benzene. Similarly, 3 is accessed by a facile nucleophilic substitution reaction of spiro-N3P3[NH(CH2)3NH]CI4 with pyrazole (Scheme 1). The phosphazenes 2 and 3 are obtained in relatively good yields. The pyrazolylcyclo-
triphosphazenes have been characterized by multinuclear N M R and IR spectroscopy and by C, H and N elemental analyses. The reaction of 2 and N3P3Phz(dmpz)4 (4) with PdClz(NCPh)2 or PtC12(NCPh)2 in benzene give the complexes of the general formula [MCI2"L], where L=2;M=Pd(5) o r P t (6) a n d L = 4 ; M = P d ( 7 ) or Pt (8), in good yields. The complexes are highly soluble in common ha]ogenated solvents, such as dichloromethane, chloroform, etc. In these complexes the two geminal pyrazolyl pyridinic nitrogen atoms are involved in coordination. The coordinating ability of these "metallo-ligands" has been illustrated by the preparation of a heterobimetallic compound 2" PtCI2" CuCI2 (9). All the complexes have been characterized by elemental analysis and multinuclear NMR spectroscopy. Electrochemical and ESR spectral studies have been performed on the heterobimetallic derivative 9.
NM R spectroscopy The ~H NMR spectra of 2 and 3 show that the four pyrazolyl groups are equivalent. They exhibit three
R~/R CI~/C1 Ph
II
I ~C1
PZ~/pz ~
ph.... II
ph..~p N ~ p ~ . c I 4EtaN/CsH. p h / p
R II I/R R/P'-,.. N~ P N S
R._ II I/R R/~N~P'~R
I/p~
N~P~pz
N\7" N 4pzH C 1 [] I / C 1 4Et3N/C6H~ p z ][ ]/pz C1/P'~ N~ P ~ C1 pz/P"-. N ~ P ~ pz 3
Scheme 1.
Ph~/Ph
R= dmpz (1)
R = pz (2) R = dmpz (4)
HN \
/
NH
P~\II l~pz ~)~N~p~ 3
Scheme 2.
1006
K. R. Justin Thomas e t al.
different signals due to the 4-C, 5-C and 3-C protons of pyrazole nucleus at c a 6.2, 7.7 and 7.9 ppm, respectively (Fig. 2). The unresolved multiplet at c a 6.2 ppm is assigned to the 4-C protons. Recent studies on the polypyrazolyl borates and their complexes indicate that the 3-C protons resonate at lower fields than that of 5-C [12,13]. Considering the criterion to be compatible with the case for these pyrazolylphosphazenes also, we attribute the doublet in the downfield region to the 3-C protons and the other doublet at c a 7.7 ppm to the 5-C protons. In addition, the pyrazolylcyclotriphosphazenes 2 and 3 show features due to the phenyl and 1,3-diaminopropane moieties, respectively. The proton N M R spectrum of 2 shows a cluster of signals in the region 7.21 7.81 ppm for the phenyl groups, while that of 3 exhibits a multiplet at 1.67, a triplet at 3.35 and a broad signal at 4.86 ppm due to the - - C H 2 - - , - - N C H 2 - - and the - - N H - - protons, respectively, of the 1,3-diamino propane s p i r o loop. After coordination to palladium (in complexes 5 and 7) all the four pyrazolyl groups become nonequivalent and give rise to approximately four sets of signals at r o o m temperature (Figs 3 and 4, Table 2). This is surprising to compare with the results reported by other authors who observe that the palladium derivatives of polypyrazolyl borate [14]
5-CH & Phenyl 3-CH
I
8.4
I
8.2
I
8.0
7.8
ppm Phenyl 4-CH
I
l
7.6
7.4
~1
I
6.6 ppm
6.4
I
6.2
Fig. 3. 'H NMR spectrum of the palladium complex 5 indicating the presence of four different pyrazole groups.
(a)
i
f
8
i
7 ppm
6
(b)
i
15o
I
140
I
I
130
120
I
110
]
100
I
3.0
I
I
ppm
2.5 ppm
2.0
Fig. 2. (a) 'H and (b) ~3C NMR spectra of 2, showing only one type of pyrazole group.
Fig. 4. ~H NMR spectrum of the palladium complex 7 in the methyl region.
1007
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes Table 2. Proton NMR data for the ligand and complexes"
No. 2 5 6 4 7 8 3
3-CH3 (or~ 3-CH
5-CH 3 (or) 5-CH
7.90 (d, 4H) 7.60, 7.78 7.81, 7.98 (d, 4H) 7.98 (4H) 2.26 (s, 12H) 2.75, 2.72, 2.68, 2.57 2.35, 2.22, 2.14 (s, 24H) ~' 2.19 (s, 12H) 8.10 (d, 4tt)
7.73 (d, 4H) 7.95, 8.16 8.19, 8.41 (t, 4H) 7.80 (4H) 2.15 (s, 12H)
2.08 (s, 12H) 7.75 (d, 4H)
4-CH
Phenyl
6.23 (t, 4H) 6.32, 6.37 6.41, 6.50 (q, 4H) 6.35 (4H) 5.88 (s, 4HI 5.79, 5.80 5.98.6.00 (s, 4H) 5.86 (br, 4H) 6.42 (t, 4H)
7.29 7.81 (m, 10H) 7.37 7.90 (m, 10H) 7.38 7.87 (m. 10H) 7.31 7.94(m, 10H) 7.37 8.24 (m, 10H) 7.3(~8.09 (m, 10H) 4.86 (br, NH, 2H) 3.35 (t, NCH> 4H) 1.63 (m, CH> 2H)
"Multiplicity given in parentheses. h Intensity ratio ~ : ~ :/~ : ~ :/~ : ct : 23.
or pyrazolylphosphazenes [8] are fluxional in nature at room temperature. This observation stimulated us to reinvestigate the spectra of the palladium complex 7, derived from tdpctp, which was earlier reported by Paddock and Gallicano [2]. They have observed only three signals for 3- and 5-methyl protons. Interestingly, the spectra recorded at 400 M H z shows seven signals in the ratio : 2 :/3 : e : 3 : ~ : 2/3 (Fig. 4). The observation of nonequivalent pyrazolyl groups are well explained by considering the two possible isomers for these complexes (Fig. 5). In isomer A the uncoordinated pyrazolyl groups are away fi'om the metal center and constitute a stereochemically equivalent set. However, in isomer B the uncoordinated pyrazolyl groups are stereo-
pz
Cl
"-%p
.( N//
Ph
(A) P.
Pd
N
N
P\/
Ph
.,
cl
r~
P "/
Pd N N Fig. 5. Isomeric structures of the complexes with geminal N2 coordination.
chemically nonequivalent. These differences, due to the spatial arrangement of pyrazolyl groups lead to a multitude of signals. Similar and somewhat less pronounced effects have been noticed in the ~3C N M R spectra of the compounds also (Table 3). The 31p N M R spectra of 2 and 3 display a AX~ pattern with a doublet at - 1.99 and 1.15 and a triplet at 24.10 and 14.58, respectively. The upfield doublet is readily assigned to the Ppz2 groups by comparison with the value for N3P3(dmpz)4 and 4. The two bond coupling observed for Ppz2 and PPh2 o r Pspiro centers in 2 and 3 are larger than that observed for PCI~ and PPh2 or P~p~rogroups in their parent compounds N3P3[NH(CHz)3NH]CI4 [12] and N3P3Ph2CI4 [15], respectively. The chemical shift of a particular phosphorus atom depends on several factors, such as the nature of the substituents (steric and electronegatively), extent of 7r-bonding with the substituents, bond angles at phosphorus, etc. The substituents on the adjacent phosphorus may also influence the chemical shift of the phosphorus nucleus under consideration [18]. It is clearly evident from Table 4 that the bulkiness of the pyrazolyl substituent is responsible for the drastic upfield shift when compared with the other aminocyclotriphosphazenes. A slight down-field shift for the Ppz2 group in 3 may be attributed to the electronic effect exerted by the electron rich spiro loop. The 3~p N M R spectra of palladium complexes 5 and 7 show an A B X pattern (see Table 4). The two ppz2 phosphorus nuclei which are equivalent in I and 4 have become nonequivalent and the phosphorus bearing the coordinated pyrazole are shielded and phosphorus nucleus associated with the uncoordinated pyrazolyl groups are slightly deshielded. The PPh: phosphorus nuclei in the palladium complexes are also deshielded. The platinum complexes show only two broad signals at r o o m temperature, suggestive of fluxional motions in solution. However. the nature of the dynamic processes are not clear at this stage.
1008
K. R. Justin T h o m a s et al. Table 3. ~3C NMR data for the ligand and complexes
Compound
3-C
4-C
5-C
Methyl
2
145.9
107.8
134.8
5 6
146.1 142.0 146.1
107.9 106.6 107.9
136.2 135.0 135.3
4
152.1
109.4
146.7
13.7, 12.8
7
111.0, 110.9 110.7 108.8
149.2
8
157.5, 157.4 153.7, 153.6 153.4
136.1
15.4, 13.8 13.5, 12.5 13.4, 12.1
3
145.0
107.4
135.0
Phenyl 134.5, 133.1, 131.8 130.5, 128.4 132.0, 130.7, 130.1 128.6, 128.5 135.0, 133.8, 132.0 130.7, 128.6 135.7, 134.3, 131.6 131.4, 127.6 131.6, 131.1, 130.9 128.6, 128.4 134.5, 132.8, 130.3 128.6, 127.1 40.0 (NCH2) 25.6 (CH2)
Table 4.31p NMR data for the ligand and complexes Compound
Pattern
6P(Ph2)
6P(pz2)
3j(p_p)
2 5
AX2 ABX
24.10 25.00
51.4 71.5 33.4, 24.6
6 4 7
AX2 AX2 ABX
24.08(br) 20.35 21.05
8 3 N3P3[H(CH2)3NH]CI4 ~ N3P3Ph2C14a N3P3(NHBut)zC12 ' N3P3(Az)2C14]t
AX2 AX2 AX2 AX2 AXz AX2
22.73 (br) 14.58" 12.3" 19.5 2.3" 34.2 t
- 1.99 -0.41 -3.52 - 1.95(br) -5.41 - 1.82 - 5.97 - 3.61 (br) 1.15 23.1' 17.1 ' 19.6' 21.9 ~
Hz
58.8 68.9 45.6, 22.3 50.7 43.7 12.0 44.7 30.0
"6P(spiro). ~From ref. [12]. ' 6 P (C12).
'~From ref. [15]. "6P ((NHBut)2), from ref. [16]. t f P (Az2), from ref. [17].
E P R spectroscopy The E P R spectrum o f 9 recorded at r o o m temperature as polycrystalline p o w d e r sample was isotropic a n d do n o t exhibit any metal or ligand splitting features. However, in solution it displayed informative spectra. Figure 6(b) shows the X - b a n d E P R spectrum o f tpctp'PtC12.CuC12 in acetonitrile solution recorded at 100 K. T h e well resolved lineshape is indicative o f a S = 1/2 p a r a m a g n e t i c species in an axial symmetry Call > 9 - ) with hyperfine resolution in the parallel region; a poorly but informative resolution o f nine superhyperfine lines is also detectable in the perpendicular region. The overall lineshape is a c c o u n t e d for by magnetic coupling o f the S = 1/2 unpaired electron with the c o p p e r nucleus (I = 3/2) ;
the m i n o r superhyperfine multiplet is a t t r i b u t a b l e to the electron spin interaction with four nitrogen nuclei ( 1 = 1; theoretical intensity of ratio = 1 : 5 : 10: 14 : 19 : 14 : 10 : 5 : 1). The second derivative analysis affords a better resolution of the a b s o r p t i o n pattern a n d gives the experimental values o f the A , ( N ) superhyperfine coupling constant. The p o o r resolution of the spectrum in the perpendicular region a n d corresponding linewidth are consistent with the n o n equivalence of the four c o o r d i n a t i n g nitrogen nuclei, as confirmed by c o m p u t e r simulation. This further supports the proposed tetragonal a r r a n g e m e n t [19] o f the copper e n v i r o n m e n t with the three nitrogens originating from two geminal pyrazolyl groups and one acetonitrile molecule lying in the equatorial plane (higher A , ( N ) ) a n d the fourth o f the cyclo-
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes
i100 G[
,
H
,~
•
DPPH
(b)
H
"
DPPH
1009
the electrochemical behavior of it. The redox pattern exhibited by the heterodimetal complex is intriguing. As illustrated in Fig. 7, in acetonitrile solution it undergoes three subsequent cathodic steps at peaks A, B and C, respectively. Controlled potential coulometric tests served to elucidate overall reduction path. The catho-anodic peak system A/G ( E = + 0.46 V) proved to involve a chemically reversible electron transfer, in that controlled potential electrolysis (E, = 0.0 V) consumes one electron per molecule. Concommitantly the initially EPR-active (see the previous section), yellow green solution turns colorless and E P R silent. Such a final solution exhibits a cyclic voltammetric profile quite complementary to that illustrated in Fig. 7(c). Subsequent exhaustive one-electron reduction at the second cathodic step (E,~ = - 1.0 V) causes deposition of copper metal on the microelectrode surface. The resulting solution displays only the reduction process C, which we confidently assign to the reduction of the platinum(If) center, in analogy with the results on the platinum complexes of tpctp and tdpctp, which exhibit an irreversible P t " / P t reduction at very negative potential values (in the range from - 1.5 V to - 1.6 V). Upon reoxidation of the electrodeposited copper metal
Fig. 6. EPR spectra of 9 (a) at room temperature (298 K) and (b) at 100 K in acetonitrile solution. 500~lxA triphosphazene nitrogen occupying an apical position. The A~ values suggest that the equatorial plane is more tetrahedrally distorted [20]. The involvement of acetonitrile is further supported by the fact that when the spectrum was recorded in D M F the super hyperfine splitting collapsed. The parameters of the simulated spectrum are :
(a)
0.9 Y 0.5
0
.:
F
,qi = 2.270 9± = 2.059 .G, = 1/3(9, +29,) = 2.130 AI = 148.0 G A l = 10.0 G A~, = l/3(Ali+2A±) = 56.0 G AI~(3N) = 4.0 G A±(3N) = 13.8 G A ( N ) = 3.7 G A I ( N ) = 12.7 G~. Figure 6(a) shows the room-temperature X-band EPR spectrum of 9 in acetonitrile solution. Both the first and the second derivative spectra are poorly resolved and do not exhibit superhyperfine resolution. The relevant isotropic parameters ( 9 ~ o = 2 . 1 3 2 ; A,~o = 58.3 G) well fit the low temperature (100 K) ones. It must be noted that no absorption attributable to paramenetic platinum centered species have been detected neither in the original complex nor in solution from exhaustive electrolysis.
C
-0.5/
-1.0
-l.g
G
(b)
0.8 " 0.5
T 500 ~A
-o.5
-1.o
(c)
L,
0.9 0.5/
/
0
/
0
,
,
,
,
i
,
-0.5
,
,
,
,,
i
-1.0
,
,
,
211.8
E (volt)
Electrochemistry The unusual E P R spectral observations of the complex t p c t p ' P t C l 2 " C u C l 2 prompted us to investigate
Fig. 7. Cyclic voltammograms of 9 in acetonitrile solution. Scan rates: (a), (c) 0.2 V s ~: (b) 0.1 V s ~. Direct current voltammetric scan rate: (a) 0.01 V s ~.
1010
K. R. Justin Thomas et al.
(E~,. = + 0.6 V), the solution restores the original voltammetric response shown in Fig. 7. In summary, the overall redox propensity of complex tpctp" PtCl2" CuC12 is so schematizable : +e(A)
~-c(B)
[LCuIIptH] -~(G)[LCu'Pt"]- ~ ) [ L C u ° p t l ' ] 2
Cu °
+
(C) + + 2e
- e + (E)
Cu I -e~ Cu"
[PtUL] 2
+
[Pt°L] 4 (F){ degradation
Finally a few further points deserve comment. (i) The thermodynamically easy access to the Cu'Pt"L (L = ligand) congener is quite rare [21], particularly when compared with the corresponding complex hdpctp'CuCl2"PtCl2, which under the same experimental conditions undergoes the [Cu" PtnL]/[Cu°PtnL] 2 reduction process through a single-stepped irreversible process (Ep = - 0.55 V), without any stabilization of the intermediate mixed-valent species [22]. This is likely attributable to the electronic factors arising from the substitution of two pyrazolyl groups for two electron-withdrawing phenyl groups and removal of methyl groups from 3- and 5-positions of pyrazole ring, which makes easier the addition of electrons. (ii) The analysis of the peak system A/G with scan rates varying from 0.02 to 2 V s- ' shows that the peak-to-peak separation progressively increases from 140 to 315 mV. This high departure from the constant values of 59 mV expected for an electrochemically reversible one-electron transfer denotes that a considerable geometrical strain accompanies the [Cu"PtHL]/[Cu°PtnL] 2- redox change [21].
CONCLUSION The new pyrazolylcyclophazene ligand 1 reacts in a facile manner with the platinum and palladium salts to give the corresponding mononuclear complexes. The possibility of using these complexes as ligands for the assembly of heterobimetallic compounds has been demonstrated for the platinum complex 6. It is also interesting to note that the absence of methyl groups in the pyrazolyl nucleus affects mainly the electrochemical property of the heterobimetallic derivative. However, the final conclusion on the role of substituents on pyrazolyl and cyclophosphazene moieties awaits more elaborate studies with more sensitive metals such as copper, cobalt, etc., and various suitably modified pyrazolylcyclotriphosphazenes. Work in this direction is in progress at our laboratories.
REFERENCES
1. Allcock, H. R., Polymer, 1980, 21,673 ; Potin, Ph. and De Jaeger, R., Eur. Polym. J. 1991, 27, 341 ; Chandrasekhar, V. and Justin Thomas, K. R., Appl. Orqanomet. Chem. 1993, 7, 1 ; Allcock, H. R., Desorcie, J. L. and Riding, G. H., Polyhedron, 1987, 6, 119. 2. Gallicano, K. D., Paddock, N. L., Rettig, S. J. and Trotter, J., lnorg. Nucl. Chem. Lett. 1979, 15, 417; Gallicano, K. D. and Paddock, N. L., Can. J. Chem. 1982, 60, 521. 3. Justin Thomas, K. R., Chandrasekhar, V., Pal, P., Scott, S. R., Hallford, R. and Cordes, A. W., hwry. Chem. 1993, 32, 606: Justin Thomas, K. R., Chandrasekhar, V., Scott, S. R., Hallford, R. and Cordes, A. W., J. Chem. Soc., Dalton Trans. 1993, 2589. 4. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V. and Tiekink, E. R. T., J. Chem. Sot., Dalton Trans. 1994, 1301. 5. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V., Scott, S. R. and Cordes, A. W., Polyhedron, 1995, 14, 977. 6. Justin Thomas, K. R., Tharmaraj, P., Chandrasekhar, V., Bryan, C. D. and Cordes, A. W., Inory. Chem. 1994, 34, 5382. 7. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., Curr. Sci. 1991,60, 700. 8. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., lnor9. Chem. 1993, 32, 6102. 9. Chandrasekaran, A., Krishnamurthy, S. S. and Nethaji, M., J. Chem. Sot., Dalton Trans. 1994, 63. 10. Kharasch, M. S., Seyler, R. C. and Mayo, F. R., J. Am. Chem. Soc. 1938, 60, 882. 11. McBee, E. T., Okuhara, K. and Morton, C. J., hwrg. Chem, 1965, 4, 1672. 12. Chandrasekhar, V., Krishnamurthy, S. S., Manohar, H., Vasudevamurthy, A, R., Shaw, R. A, and Woods, M., J. Chem. Soc., Dalton Trans. 1984, 621. 13. Zanello, P., Opromolla, G., Casarin, M., Herberhold, M. and Xeitner, P., J. Or.qanomet. Chem. 1993, 443, 199. 14. Onishi, M. and Hirachi, K., Inorg. Chim. Acta, 1994, 224, 131; Reger, D. L., Swift, C. A. and Leboida, L., lnor9. Chem. 1984, 23, 249; Trofimenko, S., Prog. lnor9. Chem. 1986, 34, 115: Cabildo, P., Claramont, R. M. and Elguero, J., Or#. Magn. Res. 1984, 22, 603. 15. Keat, R., Shaw, R. A. and Woods, M., J. Chem. Sot., Dalton Trans. 1976, 1582. 16. Allen, C. W., Brown, D. E., Cordes, A. W. and Craig, S. C., J. Chem. Soe., Dalton Trans. 1988, 1405. 17. Van der Huizen, A. A., Van de Grampel, J. C., Rusch, J. W., Witting, T., Bolhuis, E. and Meetsma, A., J. Chem. Soc., Dalton Trans. 1986, 1317. 18. Chandrasekhar, V. and Justin Thomas, K. R., Struct. Bonding (Berlin), 1993, 81, 43. 19. Hathaway, B. J. and Billing, D. E., Coord. Chem. Rev. 1970, 5, 143. 20. Additon, A. W., Rao, T. N., Reedijk, J., van Rijn,
Complexes of tetrakis(pyrazolyl)cyclotriphosphazenes ,,,J. and Verschoor, G. C., J. Chem. Soc., Dalton Trans. 1984, 1349. 21. Zanello, P., Stereochemistry of Organometallic and Inorganic Compounds, Vol. 4, ed. I. Bernal. Elsevier, Amsterdam 1990, p. 181.
101 I
22. Justin Thomas, K. R., Zanello, P. and Chandrasekhar, V., Manuscript in preparation.