Caged phosphite complexes of copper(I) halides

Article abstract of DOI:10.1023/A:1024217727932

The coordination chemistry of the caged phosphites 4-methyl-2,6,7-trioxa-1- phosphabicyclo [2.2.2]octane (MeCage) and 4-nitro-2,6,7-trioxa-1- phosphabicyclo[2.2.2]octane (NO₂Cage) with copper(I) halides is reported. Reactions of CuX = CuCl, CuBr, and CuI with the phosphite ligands afford complexes of the type [CuX(MeCage)]₄, [CuX(NO ₂Cage)]₄, [CuX(MeCage)₂]₂, [CuX(NO₂Cage)₂]₂, and [CuX(MeCage) ₃]. Recrystallization of [CuBr(NO₂Cage)₂] ₂ in MeCN produced [CuBr(NO₂Cage)(NCMe)]₂. Three X-ray crystal structures are reported: [CuCl(MeCage)]₄ (I - 4, a = 13.4292(5) A, b = 13.4292(5) A, c = 9.4641(5) A, V = 1706.79(13) A³, Z = 2), [CuBr(MeCage)₂]₂ (I41/a, a = 19.5751(17) A, b= 19.5751(17) A, c = 16.4513(15) A, V = 6303.9(10) A³, Z = 8), and [CuBr(NO₂Cage)(NCMe)] ₂ (P2₁/c, a = 9.0506(4) A, b = 15.2428(7) A, c = 8.8673(4) A, β = 109.7640(10)°, V = 1151.24(9) A ³, Z = 2).

Full text of DOI:10.1023/A:1024217727932

Journal of Chemical Crystallography, Vol. 33, Nos. 5/6, June 2003 (�
2003)
C
Caged phosphite complexes of copper(I) halides
(
1)
(1)
(1)
Jenine R. Cole, Megan E. Dellinger, T. Jason Johnson,
(1)
(1)�
(2)
Bryn A. Reinecke, Robert D. Pike, William T. Pennington,
Mariusz Krawiec, and Arnold L. Rheingold
(2)
(3)
Received August 2, 2002
The coordination chemistry of the caged phosphites 4-methyl-2,6,7-trioxa-1-phosphabicyclo
2.2.2]octane(MeCage)and4-nitro-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (NO2Cage)
[
with copper(I) halides is reported. Reactions of CuX = CuCl, CuBr, and CuI with the
phosphite ligands afford complexes of the type [CuX(MeCage)]4, [CuX(NO2Cage)]4,
[
[
CuX(MeCage)2]2, [CuX(NO2Cage)2]2, and [CuX(MeCage)3]. Recrystallization of
CuBr(NO2Cage)2]2 in MeCN produced [CuBr(NO2Cage)(NCMe)]2. Three X-ray crystal
˚
˚
structures are reported: [CuCl(MeCage)]4 (I � 4, a = 13.4292(5) A, b = 13.4292(5) A,
˚
˚ 3
c = 9.4641(5) A, V = 1706.79(13) A , Z = 2), [CuBr(MeCage)2]2 (I41/a, a =
˚
˚
˚
˚ 3
1
9.5751(17) A, b = 19.5751(17) A, c = 16.4513(15) A, V = 6303.9(10) A , Z = 8),
˚
˚
and [CuBr(NO2Cage)(NCMe)]2 (P21/c, a = 9.0506(4) A, b = 15.2428(7) A,
˚
�
˚ 3
c = 8.8673(4) A, � = 109.7640(10) , V = 1151.24(9) A , Z = 2).
KEY WORDS: Copper(I); halide; phosphite; cubane; tetramer; dimer.
4
Introduction
to date, should prove to be an even stronger
�
-acceptor than MeCage due to inductive
The caged phosphite 4-methyl-2,6,7-trioxa-
-phosphabicyclo[2.2.2]octane (MeCage) has
electron-withdrawal by the nitro group.
1
1
been known for over 40 years. However, it has
found surprisingly limited use as a ligand in coor-
2
dination chemistry. The ligand is of interest as a
result of its combination of strong �-acceptor na-
ture (exceeding that of P(OPh) ) and its very small
3
�
3
cone angle of 101 . An analogous ligand, 4-
nitro-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane
(
NO Cage), which has been virtually unstudied
2
The foregoing combination of electronic and
steric characteristics should make the caged phos-
phites excellent ligands for the soft copper(I)
ion. A previous study has shown that excess
MeCage reacts with CuY (Y = NO , ClO ) in
(
(
(
1)
Department of Chemistry, College of William and Mary,
Williamsburg, Virginia 23187.
Department of Chemistry, Clemson University, Clemson, South
Carolina.
Department of Chemistry and Biochemistry, University of
Delaware, Newark, Delaware.
To whom correspondence should be addressed. E-mail:
rdpike@wm.edu
2)
2
3
4
3)
acetonitrile solution to reduce the copper, pro-
5
�
ducing [Cu(MeCage) ]Y. Solutions of the latter
4
species proved to be conductive; however, no
341
1
074-1542/03/0600-0341/0 �C 2003 Plenum Publishing Corporation

3
42
Cole et al.
crystallographic studies were conducted. Herein,
we report a study of the coordination chem-
25 mL of CH CN. The solution was filtered
3
through Celite to remove traces of Cu(II) species,
�
istry of the MeCage and NO Cage ligands with
and stirred under nitrogen at 25 C. A solution
2
CuX (X = Cl, Br, I), including the crystal struc-
of caged phosphite ligand (1–12 mmol) in about
tures of [CuCl(MeCage)] , [CuBr(MeCage) ] ,
5 mL of CH CN was added to the CuX solution
4
2 2
3
and [CuBr(NO Cage)(NCMe)] .
via syringe. The pale yellow or green CuX solu-
tion became colorless over about 0.5–2 h. In most
cases the mixtures remained as solutions. How-
ever, spontaneous precipitation of [CuI(Cage)2]
product occurred for 1:1, 1:2, 1:3, and 1:4.5
CuI:Cage ratios. The solution was concentrated to
about 15 mL and diethyl ether added until slight
clouding was evident. Slow precipitation occurred
2
2
Experimental
General methods
All syntheses were carried out under nitrogen
atmosphere. Triols were purchased from Aldrich
and used as received. Phosphorus trichloride was
distilled before use. Copper(I) chloride and bro-
mide were freshly recrystallized from aqueous
HCl and HBr. Copper(I) iodide was used as re-
ceived from Aldrich. Copper analysis was carried
�
upon cooling the mixture to � 5 C. All products
were white solids and were collected via filtration;
yields and analyses are reported in Table 1.
X-ray data collection, structure solutions,
and refinements. The complexes were crystallized
6
out by atomic absorption. Microanalyses for C,
by layering CH CN solutions of the complexes
3
H, and N were carried out by Atlantic Microlab,
Inc., Norcross, GA.
Synthesis of caged phosphites. 1,1,1-
Tris(hydroxymethyl)ethane (7.21 g, 60.0 mmol)
and triethylamine (14.0 g, 138 mmol) were
with diethyl ether in 5-mm tubes. Crystals slowly
formed over a period of 2 weeks. Crystals were
mounted on glass fibers. For [CuCl(MeCage)]4
and [CuBr(NO Cage)(NCMe)] , all measure-
2
2
�
mentsweremadeat� 100 � 1 ConaBrukerAXS
dissolved in about 70 mL of CHCl . A solution
3
of PCl (8.20 g, 59.7 mmol) in CHCl was
3
3
�
Table 1. Synthetic and Analytical Results
added dropwise at 0 C. The resulting orange
solution was refluxed overnight. The mixture
was extracted with water and then dried over
anhydrous MgSO4 and filtered. The solvent
was then removed under vacuum yielding the
%
% Cu,
% Cu,
CuX:Cage ratio
complex
Yield theory experimental
1
1
1
:1
:1
:0.5
[CuCl(MeCage)]4a
[CuBr(MeCage)]4
[CuI(MeCage)]4
[CuCl(MeCage)2]2
[CuBr(MeCage)2]2
[CuI(MeCage)2]2
[CuCl(MeCage)3]
[CuBr(MeCage)3]c
[CuI(MeCage)3]
94 25.72
68 21.79
77 18.77
83 16.08
78 14.45
93 13.06
77 11.70
79 10.81
75 10.01
73 22.85
91 19.70
83 17.20
24.74
22.72
18.30
15.91
13.90
13.33
11.56
10.58
9.86
23.07
20.23
17.22
13.42
12.86
11.31
MeCage as a white solid (5.74 g, 38.7 mmol,
1
b
6
3
(
5%). H NMR, ppm (CDCl ) 0.69 (s, CH ),
1:2
1:2.5
1
1
3
1
3
3
.91 (d, J = 1.6 Hz, CH ). C NMR, ppm
2
:2
:4.5
CDCl ) 16.9 (d, J = 5.4 Hz, CH ), 32.2 (d,
3
3
J = 22.3 Hz, CH ), 71.9 (s, C---CH ). Anal.,
2
3
1:4.5
1
1
1
1
1:3
1:3
:6
:1
:1
:1
%
C 40.61 (40.55 theory), %H 6.30 (6.13 theory).
[CuCl(NO2Cage)]4
[CuBr(NO2Cage)]4
[CuI(NO2Cage)]4
The NO Cage was prepared similarly from
2
tris(hydroxymethyl)nitromethane (58% yield).
1
H NMR, ppm (CDCl ) 4.44 (d, J = 2.3 Hz,
[CuCl(NO2Cage)2]2 80 13.90
3
1
3
[CuBr(NO2Cage)2]2 91 12.67
CH ). C NMR, ppm (CDCl ) 67.7 (s, CH ),
2
3
2
d
1
:2
[CuI(NO2Cage)2]2
75 11.58
C---NO not observed. Anal., %C 26.83 (26.83
2
a
theory), %H 3.41 (3.38 theory), %N 7.82
%C 24.38 (24.30 theory), %H 3.69 (3.67 theory).
C 29.65 (30.39 theory), %H 4.54 (4.59 theory).
C 29.26 (30.65 theory), %H 4.58 (4.63 theory).
C 18.40 (17.52 theory), %H 2.32 (2.20 theory), %N 5.23 (5.11
b
c
d
%
%
%
(
7.82 theory).
Synthesis of copper halide complexes. Cop-
per(I) halide (about 2 mmol) was dissolved in
theory).

Caged phosphite complexes
343
Table 2. Crystal and Structure Refinement Dataa
Complex
[CuCl(MeCage)]4
[CuBr(MeCage)2]2
[CuBr(NO2Cage)(NCMe)]2
Color and habit
Size, mm
Formula
CCDC Deposit No.
Formula weight
Space group
Colorless needle
0.4 � 0.1 � 0.1
C20H36Cl4Cu4O12P4
201729
Colorless block
0.4 � 0.4 � 0.4
C20H36Br2Cu2O12P4
201730
Colorless block
0.5 � 0.3 � 0.2
C12H12Br2Cu2N4O10P2
201731
988.33
879.27
721.20
I � 4 (#82)
13.4292(5)
13.4292(5)
9.4641(5)
90
1706.79(13)
2
1.923
I41/a (#88)
19.5751(17)
19.5751(17)
16.4513(15)
90
6303.9(10)
8
1.853
P21/c (#14)
9.0506(4)
15.2428(7)
8.8673(4)
109.7640(10)
1151.24(9)
2
2.080
700
5.512
˚
a, A
˚
b, A
˚
c, A
, deg
Volume, A
�
˚
3
Z
�
calc
F000
, g cm 3
�
992
3.013
3520
4.143
(Mo K�), mm 1
�
�
˚
˚
˚
radiation
Mo K� (� = 0.71073 A) Mo K� (� = 0.71073 A) Mo K� (� = 0.71073 A)
�
Temperature, C
Residuals:a R; Rw
Goodness of fit
� 100
20
� 100
0.017; 0.056
0.509
0.052; 0.148
1.015
0.031; 0.091
0.729
P
P
P
P
a
kFo| � |Fck/ |Fo| for observed data only. Rw = wR2 = { [w(F o2 � F ) ]/ [w(F ) ]}
2 2
2 2 1/2
R = R1 =
c
o
for all data.
diffractometer, equipped with a SMART APEX
CCD detector. Data for [CuBr(MeCage) ] were
Results and discussion
2
2
�
collected at 27 � 2 C using a Mercury CCD de-
tector coupled to a Rigaku AFC-8S diffractome-
ter. Initial space group determination was based
on a matrix of 60 frames. Data were collected
in 10-, 20-, or 30-s frames. The data were re-
Synthesis and characterization of products
The solid products resulting from the reac-
tions of copper(I) halides with caged phosphites
in ratios varying between 1:0.5 and 1:6 were
prepared (see Table 1). The MeCage phosphite
7
duced using SAINT+, and empirical absorp-
8
tion correction applied using SADABS. Data for
4
[
CuBr(MeCage) ] were collected and processed
Table 3. Atomic coordinates (10 ) and Equivalent Isotropic
2
2
9
3 ˚ 2
Displacement Parameters (10 A ) for [CuCl(MeCage)]4
using Crystal Clear.
Structures were solved using direct methods.
x
y
z
Ueq
Least-squares refinement for all structures was
2
carried out on F . The non-hydrogen atoms were
Cu(1)
Cl(1)
P(1)
8946(1)
10600(1)
7935(1)
7813(1)
8232(1)
6798(1)
7126(2)
7611(1)
6171(1)
6728(1)
6047(1)
549(1)
1244(1)
884(1)
� 18(1)
1796(1)
1145(1)
154(1)
1928(1)
1386(1)
1216(1)
1398(1)
1219(1)
1252(1)
2909(1)
3988(1)
3912(1)
2526(1)
5160(2)
5169(2)
3740(2)
5123(2)
6384(2)
22(1)
18(1)
16(1)
24(1)
24(1)
22(1)
25(1)
22(1)
21(1)
17(1)
27(1)
refined anisotropically. Hydrogen atoms were
placed in theoretical positions. Structure solution,
refinement, and the calculation of derived results
were performed using the SHELXTL package of
O(1)
O(2)
O(3)
C(1)
C(2)
C(3)
C(4)
C(5)
1
0
computer programs. Details of the X-ray exper-
iments and crystal data are summarized in Table 2.
Non-hydrogen atomic coordinates for the three
structures are given in Tables 3, 4, and 5. Selected
bond lengths and bond angles for all structures are
given in Table 6.
Note: Ueq is defined as one third of the trace of the orthogonalized
i j
U
tensor.

3
44
Cole et al.
4
4
Table 4. Atomic Coordinates (10 ) and Equivalent Isotropic
˚ 2
Table 5. Atomic Coordinates (10 ) and Equivalent Isotropic
3
3 ˚ 2
Displacement Parameters (10 A ) for [CuBr(MeCage)2]2
Displacement Parameters (10 A ) for [CuBr(NO2Cage)(NCMe)]2
x
y
z
Ueq
x
y
z
Ueq
Cu(1)
Br(1)
P(1)
4263(1)
4559(1)
3711(1)
3717(1)
2960(2)
3596(4)
4000(2)
3209(2)
3222(2)
4170(2)
2590(3)
3217(7)
3620(4)
2921(3)
3809(3)
2821(3)
3009(2)
2605(3)
3042(2)
2674(3)
2129(1)
3375(1)
1814(1)
1914(1)
1557(3)
2332(2)
1188(3)
2496(2)
1266(2)
1802(2)
1341(6)
2108(4)
968(4)
1114(3)
1732(3)
2345(3)
1394(2)
1153(3)
1683(2)
1554(3)
6291(1)
6299(1)
7410(1)
5130(1)
7272(3)
8098(3)
7880(3)
4838(2)
5108(2)
4346(2)
7991(4)
8823(5)
8597(5)
4323(4)
3578(3)
4105(3)
8732(2)
9468(3)
3725(3)
2930(4)
50(1)
55(1)
44(1)
43(1)
127(2)
155(3)
123(2)
66(1)
82(1)
80(1)
134(4)
176(6)
131(4)
87(2)
69(1)
74(2)
47(1)
67(1)
48(1)
79(2)
Cu(1)
Br(1)
P(1)
9136(1)
8600(1)
7302(1)
10120(2)
4243(2)
5529(2)
7015(3)
7700(2)
3188(2)
4563(2)
10524(3)
11033(3)
4407(3)
5980(3)
6603(3)
5301(2)
9200(1)
9963(1)
9007(1)
8032(2)
8361(1)
8785(2)
9744(1)
8165(1)
7844(1)
8651(1)
7325(2)
6416(2)
8621(2)
9531(2)
7926(2)
8622(1)
5291(1)
2749(1)
6322(1)
5171(3)
8766(2)
5232(2)
7470(3)
7473(3)
8146(2)
10128(2)
5369(3)
5659(3)
6050(3)
8373(3)
8259(4)
7850(3)
17(1)
18(1)
16(1)
23(1)
18(1)
32(1)
39(1)
38(1)
26(1)
28(1)
19(1)
27(1)
21(1)
26(1)
28(1)
16(1)
P(2)
N(1)
N(2)
O(1)
O(2)
O(3)
O(4)
O(5)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
Note: Ueq is defined as one third of the trace of the orthogonalized
i j
U
tensor.
Note: Ueq is defined as one third of the trace of the orthogonalized
and CuBr or >1:6 for CuI produced either the
:3 product or what appeared to be a mixture
i j
U
tensor.
1
of 1:2 and 1:3 products. The difficulty in crys-
tallizing a 1:3 CuI complex is probably related
to the relative insolubility of the 1:2 complex,
afforded CuX:ligand product stoichiometries of
:1, 1:2, and 1:3. Formation of the 1:3 product
1
necessitated the use of about 1:4 CuX:ligand ra-
tio for CuCl and CuBr and 1:6 ratio for CuI.
Copper halide:Cage ratios of >1:4 for CuCl
[CuI(MeCage) ]. In fact, CuI:MeCage mixing ra-
tios of 1:1, 1:2, 1:3, and 1:4.5 each produced the
insoluble 1:2 product immediately upon mixing.
2
˚
�
Table 6. Selected Bond Distances (A) and Angles ( )
[
CuCl(MeCage)]4
[CuBr(MeCage)2]2
2.5070(7), 2.5094(7)
[CuBr(NO2Cage)(NCMe)]2
Bond distance
Cu--- X
2.4096(4), 2.4838(4),
2.4358(4), 2.5377(4)
2
.3527(4)
Cu--- P
Cu--- N
2.1468(4)
—
2.2224(12), 2.2297(11) 2.1683(6)
—
2.009(2)
Cu ���Cu
3.191, 3.226
3.230
3.0350(5)
Bond angle
X--- Cu--- X
95.841(13), 93.879(13),
99.83(2)
104.817(12)
9
8.597(14)
119.483(17), 110.433(16) 112.22(4), 107.39(4),
09.46(4), 111.77(4)
X--- Cu--- P
120.89(2), 109.58(2)
75.183(12)
1
Cu--- X --- Cu
85.290(14), 83.642(13),
1.384(14)
80.16(2)
8
P--- Cu--- P
X--- Cu--- N
P--- Cu--- N
—
—
—
115.13(5)
—
—
—
109.11(6), 102.24(6)
108.60(6)

Caged phosphite complexes
345
To obtain the 1:1 product, a 1:0.5 mixture of CuI
and MeCage was required.
The NO Cage phosphite proved to be a
2
somewhat weaker ligand than MeCage for Cu(I).
Thus, only 1:1 and 1:2 NO Cage products were
2
obtained for the three CuX salts, despite the
use of CuX:Cage ratios of up to 1:6. Further
evidence for the lability of NO Cage was un-
2
covered during recrystallization attempts. The
recrystallization of [CuBr(NO Cage) ] for X-
2
2
ray analysis resulted in partial substitution of
the caged ligand by the acetonitrile solvent,
yielding [CuBr(NO Cage)(NCMe)] (see below).
2
2
The weakness of the NO Cage ligand presum-
2
ably is the result of the electron-withdrawing
nitro group. However, preparation and study
of the [(MeCp)Mn(CO) (Cage)] complexes re-
2
Fig. 1. Molecular structure of [CuCl(MeCage)]
4
. Thermal
vealed only a slight increase in �C O for the
ellipsoids shown at 50%.
11
NO Cage complex over that of the MeCage.
2
This result suggests that the inductive effect of
the nitro group at the phosphorus atom is rather
weak.
the large thermal parameters for C(1), C(2), C(3),
and for O(1), O(2), O(3) (Table 4). Since the dimer
rhomboid core (Cu X ) can be regarded as half of
2
2
X-ray crystallography
a cubane core (Cu X ), the metal–halide angles
4 4
may be directly compared to those in the cubane.
The angles within the core typically show increas-
ingdistortionfrom90 withincreasinghalidesize:
X-ray crystallographic determination of the
:1 complex revealed a cubane tetrameric struc-
�
1
ture, [CuCl(MeCage)] , as shown in Fig. 1.
4
This type of structure is well-known for amine,
phosphine, and phosphite complexes of cop-
1
2
per(I) halides. Despite the approximate tetra-
hedral symmetry imparted by the cubane struc-
ture, the molecule lacks all symmetry elements
save for a fourfold inversion axis. The bond an-
gles of the Cu Cl core reveal the usual small
4
4
distortions from those of a perfect cube. The
�
�
Cl---Cu---Cl bond angles average 96.1(19) and
the Cu---Cl---Cu angles average 83.4(16) . The
copper atoms, which form a tetrahedron, are too
distant from one another to be regarded as bonding
˚
(
3.191, 3.226 A).
The 1:2 complex [CuBr(MeCage) ] proved
2
to be a halide-bridged dimer, as shown in Fig. 2.
This molecule lies at a center of crystallographic
symmetry. Conformational disorder was present
in one of the MeCage ligands. This is reflected in
2 2
Fig. 2. Molecular structure of [CuBr(MeCage) ] . Thermal
ellipsoids shown at 50%.

3
46
Cole et al.
�
I---Cu---I > Br---Cu---Br > Cl---Cu---Cl > 90
and Cu---I---Cu < Cu---Br---Cu < Cu---Cl---Cu
Conclusions
�
12
<
90 . In the current case the Br---Cu---Br and
Cu---Br---Cu angles are 99.8 and 80.2 , respec-
tively. The Cu---Cu distance in the dimer is similar
The ligands MeCage and NO2Cage readily
coordinate to copper(I) halides to form complexes
of the type [CuX(Cage)]4, [CuX(Cage)2]2, and
[CuX(Cage)3]. The latter class of complexes is
not formed with NO2Cage.
�
˚
to that in the cubane structure above (3.230 A).
The Cu---P bonds are slightly elongated in the
dimeric complex, possibly reflecting the competi-
tion of the two strong �-acceptor ligands coordi-
nated to a single copper center.
Supplementary material
Finally, the acetonitrile-substituted dimer
Crystallographic data in CIF format for
[
CuBr(NO Cage)(NCMe)] produced by recrys-
2 2
[
(
CuCl(MeCage)] , [CuBr(MeCage) ] , and [CuBr
4
2 2
tallization of the 1:2 complex was determined by
X-ray crystallography (see Fig. 3). This rhomboid
dimer lies on a center of crystallographic symme-
try. The core angles Br---Cu---Br and Cu---Br---Cu
NO Cage)(NCMe)] .
2
2
Acknowledgments
are more significantly distorted than those in
�
the MeCage complexes: 104.8 and 75.2 , respec-
We gratefully acknowledge support from the
Henry Dreyfus Teacher-Scholar Awards Program of
the Camille and Henry Dreyfus Foundation (Grant No.
TH-99-010). This research was also supported in part
by a Howard Hughes Medical Institute grant through
the Undergraduate Biological Sciences Education Pro-
gram to the College of William and Mary. We also
acknowledge the College of William & Mary for a fac-
ulty semester research assignment. Financial support
of the National Science Foundation for purchase of the
CCD-based X-ray system at Clemson (CHE-9808165)
is gratefully acknowledged.
tively. As a result of the acute Cu---Br---Cu angles,
˚
the copper atoms are separated by only 3.035 A,
possibly indicating a very weak Cu ���Cu
interaction.
X-ray quality crystals of 1:3 complexes were
not obtained, so no structure could be deter-
mined. However, it is likely that the structure
would consist of a monomeric tetrahedron hav-
ing one terminal halide ligand and three phosphite
1
3
ligands.
References
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1
Fig. 3. Molecular structure of [CuBr(NO
2 2
Cage)(NCMe)] .
. CRYSTAL CLEAR: Rigaku/Molecular Corporation, The
Thermal ellipsoids shown at 50%.
Woodlands, Texas, 1999.

Caged phosphite complexes
347
1
0. Sheldrick, G.M. SHELXTL: Crystallographic Computing Sys-
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1