Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 106, No. 1, February 1994, pp. 37--43.
9 Printed in India.
Organometallic chemistry of diphosphazanes. Part IX: Syntheses and
spectroscopic studies of molybdenum and tungsten tetracarbonyl
complexes of unsymmetrical diphosphazanes
R ' P K A M A L E S H B A B U and S S K R I S H N A M U R T H Y *
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
560012, India
MS received 1! October 1993
Abstract. The unsymmetrical diphosphazanes X2PN(Pri) PYY'( I a- I h) { X = Ph, YY' = 02
C6H, (la) or YY' = O2CIzH s {lb); X = Ph, Y = Ph. Y' = OC6H4Me-4- (lc), OC6H,Br-_4
(ld), OC6H3Me2-_3,_5(le), OCsH4N-2- (If), N2C,~HMe2-3,5 (lg) or CI (Ih)} react with
[ M (CO)~(NHC5 H io)2] (M = M o, W) to yield the cis-chelate complexes [M (CO),){X2PN (Pr')
PYY'i] {M = Mo (2a-2h): M W (3__f,3g)}. These complexes have been characterized by
IH, ~ap and ~3C NMR and IR spectroscopic studies.
=
Keywords. Unsymmetrical diphosphazane ligands; group 6 carbonyl complexes.
1.
Introduction
D i p h o s p h a z a n e s have attracted considerable attention in recent years as "short-bite"
ligands in transition metal organometallic chemistry (King 1980; M a g u e and Lin
1992; Balakrishna et al 1993, 1994; Field et al 1993; Rossi et al 1993). However,
studies with unsymmetrically substituted diphosphazanes are sparse ( C o l q u h o u n and
M c F a r l a n e 1977; Babu et al 1991, 1993). We had earlier established a correlation
between the 31p N M R chemical shifts of metal c a r b o n y l complexes of the type
c i s - [ M o ( C O h {X 2 P N ( R ) P X 2 } ] and the n-acceptor ability of the p h o s p h o r u s centres
(Balakrishna et al 1990). In order to extend the validity of the correlation, we have
synthesized g r o u p 6 m e t a l - t e t r a c a r b o n y t complexes of a series of unsymmetrically
substituted diphosphazanes of the type c i s - [ M ( C O h {X2 P N ( P r i ) P Y Y ' } ] and characterised them by IR and N M R spectroscopy. The results of these studies are presented
here. These unsymmetrically substituted diphosphazanes offer an a d v a n t a g e in that
the t w o - b o n d I ' - P coupling constants in the ligands as well as in the complexes can
be directly measured from their 31p N M R spectra.
2.
E x p e r i m e n t a l details
All manipulations were carried out under an atmosphere of dry dinitrogen using
standard Schlenk-tube technique (Shriver and D r e z d z o n 1986). Solvents were purified
* For correspondence
For Part VIII see, Babu et al (1993)
37
38
R P Kamalesh Babu and S S Krishnamurthy
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Or#anometallic chemistry of diphosphazanes
39
by standard methods. IR, and NMR spectra were recorded as reported previously
(Babu et al 1993). C, H, N, analyses were carried out with a Heraeus C H N - O Rapid
instrument. The diphosphazane ligands (lc, -If) were prepared by the reaction
Ph2PN(Pri)PPhCI (Cross et al 1976) with the corresponding phenol or secondary
amine in boiling benzene in the presence of triethylamine (Babu, unpublished results).
The diphosphazane (la) was prepared as reported earlier (Babu et al 1991); diphosphazane {lb) was prepared in a similar manner (Babu, unpublished results). The
precursor complex [Mo(CO)4(NHC 5H,0)2 ] was prepared by a published procedure
(Darensbourg and Kump 1978).
2.1
Preparationof cis-[Mo(CO)4{X2PN(pri)pyY'}] (2a-2h)
A mixture of cis-[Mo(CO)4(NHCsH~o)2] 15 x 10-4mol) and the diphosphazane
ligand (5 x 10-4mol) was dissolved in 25ml of dichloromethane and the solution
was stirred for 30 min. The resultant solution was filtered through silica gel and the
solvent evaporated under reduced pressure. Crystailisation of the residue from a
dichloromethane-petrol mixture (I:I), yielded the tetracarbonyl complex as a pale
yellow solid. Yield 80-90%.
2.2 Preparation of cis-[W(CO), {X2 PN(Pri)P YY' }] (3_f,3.__~g)
The procedure is similar to that of the molybdenum complex. In this case, the reaction
mixture was heated under reflux for 3 h. Yield 75-80%.
The analytical and spectroscopic data are summarised in tables 1-3.
Table 2. '~C {'H} NM R data (carbony{ resonances only) for some diphosphazane
complexes'.
'3C NMR values
Diphosphazane complex
6(ppm)
2Jec(Hz)
[Mo(CO), {Ph 2 PN(Pr ~)PPh (OC 6 H, Me--4..)}]
218-61dd)
218.3 (dd)
213.0(t)
208-1 (t)
25-0, 10-5
30-5, 10.0
8.1
9.0
[Mor
218.4(dd)
217-8(ddl
212-7(0
208.2 (t)
25-5, lif0
30-0, 9.5
8.5
lif0
[M o(CO)4 {Ph zP N (Pr~)PPh (OC 6H 3Me2 -3, 5_)}]
220.4 (t)
219.9 (dd)
215.0(t)
209.6 (t)
15.0
15.0, 11.6
8<1
9-7
[Mo(CO)( { Ph 2 PN(Pri)PPh(N2 C s HMe2-3 ., 5_)} ]
219-1 (dd)
218-1 (dd)
25-5, {0.0
29"0, 9.9
4 {Ph 2 PN(Pr') PPh (OC6 H, Br-4_)}]
(2d)
(2._g)
b
214.4(t)
207'2 (t)
"Recorded at 50-32 MHz, internal standard TMS; bBabu et al 1993
dd - doublet of doublets; t - triplet
9"0
8-0
40
R P K a m a l e s h Babu and S S Krishnamurthy
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Organometallic chemistry of diphosphazanes
3.
41
Results and discussion
Treatment of cis-[M(CO)4(NHCsH]o)2] (M = Mo, W) with an equimolar quantity
of the unsymmetrical diphosphazane in dichloromethane yields the chelate complexes
cis-[M (CO),, {X2 PN (Pr i) PYY' }] (scheme 1). The molybdenum complexes are readily
formed within 10 min at room temperature, whereas the formation of the tungsten
complexes requires the heating of the reaction mixture under reflux for three hours.
The presence of an excess of the diphosphazane ligand also favours the formation of
cis-chelate complexes.
The infra-red spectra of these cOmplexes exhibit three strong Vco absorptions in
the range 2030 to 1865cm-] (table 1), characteristic of an M(CO)4 moiety bonded
to strong 7t-acceptor ligands such'as MeN(PF:) 2 (King and Lee 1982; Cotton and
Kraihanzel 1962). These values are in the range observed for similar type of diphosphazane complexes (Balakrishna et al 1990).
The ~H NMR spectra for the complexes 2a and 2b display a doublet for the methyl
protons of the isopropyl group (table 1). These protons are shielded by ,--0"5ppm
Ci.~s-[M(CO)t,(NHC'~HIo)2]+ X2PN(Pri)PYY'
la - l h
M=Mo,
2cs- 2._hh
M=w,
Lf.
la, 2a :
X=Ph'YYI = ~J)O~
lb, 2b :
X=Ph, yyI =
v
1~, 2_c :
X =Ph, Y=Ph~ yl= _OCGH~Mr
ld ,2d :
X =Ph,Y=Phs YI=-OCGH4Br-4
1r 2_er
X= Ph, Y=Ph, yl= _OC6H3Mr
lf, 2f, 3f " X= Ph, Y = Phi yI = - - O " ~
Me
lg,2g, 3g : X = Ph, Y=Ph, yl=
Me
Sckem~ 1.
5
42
R P Kamalesh Babu and S S Krishnaraurthy
in comparison with the observed chemical shifts for the free iigand. The spectra of
2c-2h show two different resonance for the two methyl groups owing to the presence
of an adjacent chiral phosphorus centre; one of these resonances is strongly shielded
(--- 1 ppm) and appears in the region 0"3-0.0 ppm suggesting that these protons lie in
the shielding zone of one of the phenyi groups on the phosphorus as observed in
[Mo(CO)3(MeCN) ~Ph 2 PN (Pr i) PPh (N: C3 HMe2-3_, 5) }] (Babu et al 1993).
The z3C { t H } N M R spectra displays four different chemical shifts for the carbonyl
carbons (table 2). The carbonyls which are trans- to phosphorus atoms resonate as
a doublet of doublets, whereas the cis-carbonyls display triplet resonances. The 2 j ~
values are in the range 10-30 Hz and are higher for the trans carbonyls (25-30 Hz),
with the exception of 2e.
The 31p {IH} N MR spectra of the complexes exhibit a doublet of doublets owing
to the non-equivalence of the phosphorus nuclei (table 3). The resonances are considerably deshielded. The extent of deshielding of the PPh2 phosphorus remains more or
less the same whereas the coordination shift (Balakrishna et al 1990) (A6 = 6comp,c, 6Up,d) (table 3) for the PYY' resonance depends upon the electronegativity of the
substituents on the phosphorus which in turn determines its n-acceptor capabilities.
There is no regular trend in 2jpp values. Diphosphazane ligands can exist as different
conformers in solution (Keat et al 1981), whereas in the tetracarbonyl complexes the
conformational mobility is severely restricted. Furthermore, in the complexes the
P - P coupling will be an algebraic sum of the coupling through the nitrogen as well
I
120
115
I
I
110
105
//
I
I
65
6o
G (ppm)
Figure I. The 31p NM R spectrum (81"02MHz) of ~V(CO)4 { Phz PN(Pr ) PPh(OC sH4 N -
2_)}3, ~f).
Organometallic c h e m i s t r y o f diphosphazanes
43
as t h r o u g h the metal a n d it is difficult to assess each of these two c o n t r i b u t i o n s
separately.
The 31p N M R spectra of the t u n g s t e n complexes 3f a n d 3g display l saW satellites.
The spectrum of 3f is s h o w n in figure 1. The t u n g s t e n - p h o s p h o r u s c o u p l i n g c o n s t a n t s
lie in the range 2 0 8 - 2 5 6 Hz, the m o r e electronegative p h o s p h o r u s being associated
with a higher c o u p l i n g constant. In spite of the presence of a n a d d i t i o n a l d o n o r a t o m
in these d i p h o s p h a z a n e s , the P P chelate f o r m a t i o n is clearly favoured. This result
further s u b s t a n t i a t e s the earlier report that d i p h o s p h a z a n e s with b u l k y s u b s t i t u e n t s
at the p h o s p h o r u s a t o m show a p r o n o u n c e d tendency to form four-membered m o n o metallic chelate complexes ( B a l a k r i s h n a et al 1991; B r o w n i n g et al 1992).
Acknowledgement
The a u t h o r s t h a n k the D e p a r t m e n t of Science a n d T e c h n o l o g y , New Delhi, for
support.
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