Synthesis, structure, and chemical vapor deposition studies of molecular precursors to NbP films

Article abstract of DOI:10.1021/ic990198s

Treatment of niobium pentachloride with cyclohexylphosphine and phenylphosphine in dichloromethane afforded octachlorotetrakis(cyclohexylphosphine)diniobium(IV) (85%) and octachlorotetrakis(phenylphosphine)diniobium-(IV) (88%), respectively, as brown-red and dark red crystalline solids. In addition, the phosphonium compound [P(C₆H₁₁)H₃]₂[NbCl₆] was isolated in 68% yield from the same preparation of octachlorotetrakis(cyclohexylphosphine)diniobium(IV). Studies are described which demonstrate that the primary phosphine serves as the hydrogen atom source for the formation of the phosphonium salts. Octachlorotetrakis(cyclohexylphosphine)-diniobium(IV) can be prepared in 77% yield by addition of cyclohexylphosphine to NbCl₄(THF)₂ in toluene. The X-ray crystal structure determinations for two dimeric niobium(IV) complexes_ are described. Octachlorotetrakis-(cyclohexylphosphine)diniobium(IV) crystallizes in the triclinic space group Pī with a = 9.904(2) ?,b = 10.243-(2) ?, c = 10.642(1) ?, α = 76.33(2)°, β= 67.231(8)°, γ = 73.76(2)°, V = 945.7(3) ?³, and Z = 1. Octachlorotetrakis(phenylphosphine)diniobium(IV) crystallizes in the orthorhombic space group Pbca with a = 8.873(2) ?, b = 17.394(6) ?, c = 21.778(3) ?, V = 3361.1(9) ?³, and Z = 4. Octachlorotetrakis-(cyclohexylphosphine)diniobium(IV) and octachlorotetrakis(phenylphosphine)diniobium(IV) were evaluated as molecular precursors to niobium(III) phosphide (NbP) films in a chemical vapor deposition reactor. Octachlorotetrakis(phenylphosphine)diniobium(IV) was not volatile and was therefore not a useful precursor. However, octachlorotetrakis(cyclohexylphosphine)diniobium(IV) afforded specular, silver-colored films of NbP with substrate temperatures between 400 and 600 °C. The analysis of the films is presented.

Full text of DOI:10.1021/ic990198s

4354
Inorg. Chem. 1999, 38, 4354-4360
Synthesis, Structure, and Chemical Vapor Deposition Studies of Molecular Precursors to
NbP Films
Joseph T. Scheper,1a Kumudini C. Jayaratne,1a Louise M. Liable-Sands,1b Glenn P. A. Yap,1b
Arnold L. Rheingold,1b and Charles H. Winter*,1a
Departments of Chemistry, Wayne State University, Detroit, Michigan 48202, and
University of Delaware, Newark, Delaware 19716
ReceiVed February 18, 1999
Treatment of niobium pentachloride with cyclohexylphosphine and phenylphosphine in dichloromethane afforded
octachlorotetrakis(cyclohexylphosphine)diniobium(IV) (85%) and octachlorotetrakis(phenylphosphine)diniobium-
(IV) (88%), respectively, as brown-red and dark red crystalline solids. In addition, the phosphonium compound
[P(C6H11)H3]2[NbCl6] was isolated in 68% yield from the same preparation of octachlorotetrakis(cyclohexyl-
phosphine)diniobium(IV). Studies are described which demonstrate that the primary phosphine serves as the
hydrogen atom source for the formation of the phosphonium salts. Octachlorotetrakis(cyclohexylphosphine)-
diniobium(IV) can be prepared in 77% yield by addition of cyclohexylphosphine to NbCl4(THF)2 in toluene. The
X-ray crystal structure determinations for two dimeric niobium(IV) complexes are described. Octachlorotetrakis-
(cyclohexylphosphine)diniobium(IV) crystallizes in the triclinic space group P1h with a ) 9.904(2) Å, b ) 10.243-
(2) Å, c ) 10.642(1) Å, R ) 76.33(2)°, â ) 67.231(8)°, γ ) 73.76(2)°, V ) 945.7(3) Å3, and Z ) 1.
Octachlorotetrakis(phenylphosphine)diniobium(IV) crystallizes in the orthorhombic space group Pbca with a )
8.873(2) Å, b ) 17.394(6) Å, c ) 21.778(3) Å, V ) 3361.1(9) Å3, and Z ) 4. Octachlorotetrakis-
(cyclohexylphosphine)diniobium(IV) and octachlorotetrakis(phenylphosphine)diniobium(IV) were evaluated as
molecular precursors to niobium(III) phosphide (NbP) films in a chemical vapor deposition reactor. Octachlo-
rotetrakis(phenylphosphine)diniobium(IV) was not volatile and was therefore not a useful precursor. However,
octachlorotetrakis(cyclohexylphosphine)diniobium(IV) afforded specular, silver-colored films of NbP with substrate
temperatures between 400 and 600 °C. The analysis of the films is presented.
Introduction
calcium phosphide7 or sodium phosphide,8 or through treatment
of niobium(V) oxide with aluminum metal in the presence of
red phosphorus.9 Whiskers and pillars of crystalline NbP have
been grown on quartz substrates coated with gold, platinum,
palladium, or nickel layers.13 These crystals were produced by
chemical vapor deposition (CVD) using a three-precursor system
consisting of niobium pentachloride, phosphorus trichloride, and
hydrogen. The same precursors were later reported to yield thin
films of NbP at a substrate temperature of 1050 °C.10
We previously reported that adducts of early transition metal
halides with primary amines serve as single-source precursors
for the fabrication of metal nitride phases using CVD
techniques.14-19 Additionally, we reported that a primary
phosphine adduct of titanium tetrachloride affords TiP films
when used as a single-source precursor in a CVD process.20,21
In attempting to extend our studies to the phosphides of niobium,
The fabrication of niobium(III) phosphide (NbP) films has
received little attention, despite the useful properties of this
material.2-12 It is a metallic conductor, is as hard as high-carbon
steel, and is resistant to attack by oxygen, sulfur dioxide, and
hydrogen sulfide at temperatures as high as 600 °C.2-13 The
excellent chemical resistance of NbP has led to its application
as a matrix electrolyte in phosphoric acid fuel cells.11 An
isostructural phosphide, tantalum(III) phosphide (TaP), has been
used as a diffusion barrier layer in semiconductor devices.12
Bulk NbP can be prepared from the elements at high
temperature,2-5 by the reaction of niobium pentachloride with
(1) (a) Wayne State University. (b) University of Delaware.
(2) Xu, J.; Greenblatt, M.; Emge, T.; Ho¨hn, P.; Hughbanks, T.; Tian, Y.
Inorg. Chem. 1996, 35, 845.
(3) Willerstrom, J.-O. J. Less-Common Met. 1984, 99, 273.
(4) Furuseth, S.; Kjekshus, A. Acta Crystallogr. 1964, 17, 1077.
(5) Boller, H.; Parthe, H. Acta Crystallogr. 1963, 16, 1095.
(6) Scho¨nberg, N. Acta Chem. Scand. 1954, 8, 226.
(7) Ripley, R. L. J. Less-Common Met. 1962, 4, 496.
(8) Fitzmaurice, J. C.; Hector, A.; Parkin, I. P.; Keynes, M. J. Mater.
Sci. Lett. 1994, 13, 1.
(14) Winter, C. H.; McKarns, P. J.; Scheper, J. T. Mater. Res. Soc. Symp.
Proc. 1998, 495, 95.
(15) Jayaratne, K. C.; Yap, G. P. A.; Haggerty, B. S.; Rheingold, A. L.;
Winter, C. H. Inorg. Chem. 1996, 35, 4910.
(16) Lewkebandara, T. S.; Sheridan, P. H.; Heeg, M. J.; Rheingold, A. L.;
Winter, C. H. Inorg. Chem. 1994, 33, 5879.
(17) Winter, C. H.; Lewkebandara, T. S.; Proscia, J. W.; Rheingold, A. L.
Inorg. Chem. 1994, 33, 1227.
(9) Martin, J.; Gruehn, R. Solid State Ionics 1990, 43, 19.
(10) Motojima, S.; Wakamatsu, T.; Sugiyama, K. J. Less-Common Met.
1981, 82, 379.
(11) Takahashi, K.; Shimamoto, K.; Shunichi, M.; Watai, H.; Nakajo, H.;
Ido, T. Japan Kokai Tokkyo Koho JP 63 29 457, 1986; Chem. Abstr.
1988, 109, 76602n.
(12) Komaki, K. Japan Kokai Tokkyo Koho JP 02 150 067, 1990; Chem.
Abstr. 1990, 113, 182994j.
(13) Motojima, S.; Izushi, T.; Sugiyama, K.; Takahashi, Y. Bull. Chem.
Soc. Jpn. 1976, 49, 2122.
(18) Winter, C. H.; Jayaratne, K. C.; Proscia, J. W. Mater. Res. Soc. Symp.
Proc. 1994, 327, 103.
(19) Winter, C. H.; Viejo, V. C.; Proscia, J. W. Mater. Res. Soc. Symp.
Proc. 1994, 327, 109.
(20) Lewkebandara, T. S.; Proscia, J. W.; Winter, C. H. Chem. Mater. 1995,
7, 1053.
(21) Lewkebandara, T. S.; Winter, C. H. AdV. Mater. CVD 1996, 2, 75.
10.1021/ic990198s CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/27/1999

Molecular Precursors to NbP Films
Inorganic Chemistry, Vol. 38, No. 19, 1999 4355
we envisioned that adducts of niobium(V) halides with phos-
phine ligands might be sufficiently volatile to serve as molecular
precursors to thin films under CVD conditions. The coordination
chemistry of niobium halides with tertiary phosphines has been
extensively studied;22-28 however, it is likely that the use of
tertiary phosphines would lead to high levels of carbon in the
resulting films.29 Herein, we report a study of the reaction of
niobium pentachloride with primary phosphines and subsequent
evaluation of the resultant complexes as precursors for niobium
phosphide phases. Treatment of niobium pentachloride with
cyclohexylphosphine or phenylphosphine affords niobium(IV)
adducts of the formula Nb2Cl8(H2PR)4, along with other products
resulting from phosphine oxidation. The crystal structures of
Nb2Cl8(H2PC6H11)4 and Nb2Cl8(H2PC6H5)4 are reported. The
cyclohexylphosphine adduct Nb2Cl8(H2PC6H11)4 functions as
a precursor to NbP films. This is the first report of a molecular
precursor for NbP films and is the first low-temperature
deposition of NbP by CVD techniques.
in the compound. A C, H, Cl microanalysis of the precipitate
resulting from the formation of 1 was consistent with a
formulation of [C6H11PH3]2[NbCl6] (3, 68%). The presence of
cyclohexylphosphonium ions in 3 was supported by the infrared
spectrum, which showed P-H stretches at 2443 and 2404 cm-1
.
For comparison, neat cyclohexylphosphine has a P-H stretch
at 2285 cm-1. Furthermore, the infrared spectrum of 3 was
virtually identical to that of [C6H11PH3]2[Cl5TaOTaCl5], which
we have characterized by X-ray crystallography and which
contains cyclohexylphosphonium ions.30 The presumed phen-
ylphosphonium salt 4 was not submitted for elemental analysis,
since the infrared spectrum showed a mixture of 4 and 2. The
insolubility of 2 and 4 precluded further separation.
A significant question is the origin of the hydrogen atom in
3. Poli and co-workers have reported a study of the reaction of
CpNbCl4 with tertiary phosphines to afford CpNbCl3(PR3)2 and
phosphonium salts.31 It was proposed in Poli’s study that the
hydrogen atom in the phosphonium ion product originated from
the toluene solvent. To test for a solvent-derived hydrogen atom
in 3, we repeated the reaction of niobium pentachloride with
cyclohexylphosphine (2 equiv) in dichloromethane-d2. Workup
afforded 1 (28%) and 3 (40%). Inspection of the infrared spectra
of both 1 and 3 did not show any new stretches that could be
assigned to phosphorus-deuterium bonds (expected νP-D ca.
1700 cm-1). Accordingly, the hydrogen atoms in 3 are not
derived from dichloromethane solvent. The fact that the yields
of 1 and 3 were substantially lower with a 1:2 molar ratio of
niobium pentachloride to cyclohexylphosphine than with excess
cyclohexylphosphine supported the idea that the hydrogen atom
in 3 originated from cyclohexylphosphine. To test for this
possibility, the reaction of niobium pentachloride with cyclo-
hexylphosphine in dichloromethane-d2 was monitored by 31P-
{1H} NMR spectroscopy at ambient temperature. After filtration
to remove insoluble 3, the 31P{1H} NMR spectrum revealed a
resonance due to 1 at -2.6 ppm, as well as new broad, roughly
equal-intensity resonances at 23.5, -69.0, and -87.8 ppm. The
resonance at -87.8 ppm is close to the chemical shifts reported
for meso- and D,L-(C6H11)2P2H2 (-86.3 and -89.7 ppm in
THF32). The broad resonance at 23.5 ppm is similar to the
chemical shifts found for oligophosphanes derived from phen-
ylphospine (ca. 20 ppm), which were obtained by a catalytic
coupling reaction of phenylphosphine.33 We were unable to
assign the resonance at -69.0 ppm, but it may correspond to a
low oligomer derived from cyclohexylphosphine, since the
chemical shifts of low molecular weight oligomers of phen-
ylphosphine were intermediate between those of phenylphos-
phine and the high oligomers derived therefrom.33 Hence, the
hydrogen atom in 3 appears to originate from the cyclohex-
ylphosphine. Such a hydrogen source is not surprising, in view
of the relatively low phosphorus-hydrogen bond strength (76.4
kcal/mol in PH334). Clearly, more than 2 equiv amounts of
primary phosphine are required to achieve maximum yields of
1 and 3. The stoichiometry in eq 1 is suggested to reflect the
hydrogen abstraction step.
Results
Reaction of Niobium Pentachloride with Primary Phos-
phines. Treatment of niobium pentachloride with excess cy-
clohexylphosphine or phenylphosphine in dichloromethane
resulted in the formation of a brown-red or dark red solution
with concomitant production of a purple precipitate. Workup
as described in the Experimental Section afforded octachlo-
rotetrakis(cyclohexylphosphine)diniobium(IV) (1, 85%) and
octachlorotetrakis(phenylphosphine)diniobium(IV) (2, 88%),
respectively, as brown-red and dark red crystalline solids (eq
1). Complex 1 was fully characterized by spectroscopic and
analytical techniques. X-ray crystal structures of 1 and 2 were
determined as described below and showed them to possess
dimeric, chloride-bridged structures. NMR spectra of 2 could
not be obtained, since the complex was completely insoluble
in unreactive solvents after filtration and removal of the
dichloromethane solvent.
During the preparation of 1 and 2, purple precipitates formed.
Since the formation of 1 and 2 involves one-electron reduction
of niobium(V) to niobium(IV), it was likely that these precipi-
tates contained phosphonium ions resulting from the oxidation
of the respective phosphines. The precipitates were air sensitive
and intensely purple, suggesting the presence of niobium(IV)
(30) Scheper, J. T.; Liable-Sands, L. M.; Rheingold, A. L.; Heeg, M. J.;
Winter, C. H. Manuscript in preparation.
(22) Manzer, L. E. Inorg. Chem. 1977, 16, 525.
(31) Fettinger, J. C.; Keogh, D. W.; Poli, R. Inorg. Chem. 1995, 34, 2343.
(32) Albrand, J. P.; Taieb, C. In Phosphorus Chemistry: Proceedings of
the 1981 International Conference; Quin, L. D., Verkade, J. D., Eds.;
ACS Symposium Series 171; American Chemical Society: Washing-
ton, DC, 1981; pp 577-580.
(23) Samuel, E.; Labauze, G.; Livage, J. NouV. J. Chim. 1977, 1, 93.
(24) Labauze, G.; Samuel, E.; Livage, J. Inorg. Chem. 1980, 19, 1384.
(25) Boyd, P. D.; Nielson, A. J.; Rickard, C. E. F. J. Chem. Soc., Dalton
Trans. 1987, 307.
(26) Cotton, F. A.; Roth, W. J. Inorg. Chem. 1984, 23, 945.
(27) Cotton, F. A.; Duraj, S. A.; Roth, W. J. Inorg. Chem. 1984, 23, 3592.
(28) Cotton, F. A.; Diebold, M. P.; Roth, W. J. Polyhedron 1985, 4, 1103.
(29) Williams, K. B.; Stewart, O.; Reck, G. P.; Proscia, J. W. Mater. Res.
Soc. Symp. Proc. 1994, 327, 121.
(33) Xin, S.; Woo, H. G.; Harrod, J. F.; Samuel, E.; Lebuis, A.-M. J. Am.
Chem. Soc. 1997, 119, 5307.
(34) Toy, A. D. F. In ComprehensiVe Inorganic Chemistry; Bailar, J. C.,
Jr., Emele´us, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.;
Pergamon Press: Oxford, U.K., 1973; Vol. 2, p 415.

4356 Inorganic Chemistry, Vol. 38, No. 19, 1999
Scheper et al.
Table 1. Crystallographic Data for 1 and 2
Poli isolated adducts of the formula CpNbCl4(PR3) upon
treatment of CpNbCl4 with 1 equiv of PR3 and found that
reduction to niobium(IV) occurred only upon addition of excess
PR3.31 To test for a similar situation in our reactions, niobium
pentachloride was treated with 1 equiv of cyclohexylphosphine
at -78 °C in dichloromethane. Workup afforded NbCl5-
(P(C6H11)H2) (5, 88%) as a light orange solid (eq 2). Complex
1
2
empirical formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C24H52Cl8Nb2P4
934.00
P1h
C24H28Cl8Nb2P4
909.76
Pbca
8.873(2)
17.394(6)
21.778(3)
9.904(2)
10.243(2)
10.642(1)
76.33(2)
67.231(8)
73.76(2)
945.7(3)
1
3361.1(9)
4
T (K)
226(2)
0.710 73
1.640
298(2)
0.710 73
1.798
15.25
2.84b
λ
Mo KR (Å)
F
calcd (g cm-3
)
µ (cm-1
)
13.60
R(F)a (%)
2.57a
Rw(F)b (%)
3.74a
6.02b,c
a Quantity minimized: ∑∆2; R ) ∑∆/∑(Fo); Rw ) ∑∆w1/2/∑(Fow1/2),
∆ ) |(Fo - Fc)|. b Quantity minimized: Rw(F2) ) {∑[w(Fo2 - Fc2)2]/
5 was characterized by spectroscopic and analytical techniques.
While it was not possible to grow crystals of 5 of sufficient
quality for X-ray crystallography, we have determined the crystal
structure of the analogous tantalum complex and found it to be
an octahedral monomer.30 It is very likely that 5 has a similar
structure. While 5 showed no evidence for decomposition to 1
and 3 in dichloromethane at ambient temperature over 24 h,
addition of excess cyclohexylphosphine to 5 under the conditions
described in eq 1 resulted in the formation of 1 (66%) and 3
(52%). These experiments are consistent with 5 being an
intermediate in the formation of 1 and 3.
∑[(wFo )2]}1/2. R ) ∑∆/∑∆(Fo), ∆ ) |(Fo - Fc)|. c Rw(F2), %.
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
Nb-Cl(1)
Nb-Cl(3)
Nb-Cl(3A)
Nb-P(1)
2.486(1)
2.545(1)
2.529(1)
2.613(1)
2.809(1)
Nb-Cl(2)
Nb-Cl(4)
Nb-Cl(4A)
Nb-P(2)
2.511(1)
2.541(1)
2.543(1)
2.617(1)
Nb-Nb(A)
Cl(1)-Nb-Cl(2)
Cl(2)-Nb-Cl(3)
Cl(2)-Nb-Cl(4)
Cl(1)-Nb-P(1)
106.1(1) Cl(1)-Nb-Cl(3)
144.0(1) Cl(1)-Nb-Cl(4)
143.3(1) Cl(3)-Nb-Cl(4)
71.4(1) Cl(2)-Nb-P(2)
143.3(1) Cl(4)-Nb-P(2)
113.0(1) Cl(1)-Nb-Nb(A)
127.3(1) Cl(3)-Nb-Nb(A)
56.5(1) P(1)-Nb-Nb(A)
123.8(1) Cl(1)-Nb-Cl(3A)
81.9(1) Cl(3)-Nb-Cl(3A)
72.6(1) P(1)-Nb-Cl(3A)
79.1(1) Nb(A)-Nb-Cl(3A)
143.2(1) Cl(2)-Nb-Cl(4A)
72.3(1) Cl(4)-Nb-Cl(4A)
78.4(1) P(2)-Nb-Cl(4A)
56.4(1) Cl(3A)-Nb-Cl(4A)
67.2(1) Nb-Cl(4)-Nb(A)
81.7(1)
81.8(1)
71.8(1)
69.6(1)
79.9(1)
126.6(1)
56.1(1)
123.2(1)
143.8(1)
112.8(1)
141.4(1)
56.7(1)
82.5(1)
112.9(1)
142.2(1)
72.1(1)
67.1(1)
The formation of 1 and 2 described above wasted some of
the starting reagents through formation of 3 and 4. We
envisioned that it might be possible to prepare 1 and 2 from a
niobium(IV) chloride precursor and thereby avoid use of extra
Cl(3)-Nb-P(2)
P(1)-Nb-P(2)
Cl(2)-Nb-Nb(A)
Cl(4)-Nb-Nb(A)
P(2)-Nb-Nb(A)
Cl(2)-Nb-Cl(3A)
Cl(4)-Nb-Cl(3A)
P(2)-Nb-Cl(3A)
Cl(1)-Nb-Cl(4A)
Cl(3)-Nb-Cl(4A)
P(1)-Nb-Cl(4A)
Nb(A)-Nb-Cl(4A)
Nb-Cl(3)-Nb(A)
22
phosphine ligands. Accordingly, treatment of NbCl4(THF)2
with cyclohexylphosphine (2 equiv) in toluene at ambient
temperature led to displacement of the THF ligands and
formation of 1 in 77% yield (eq 3). The spectroscopic properties
of 1 so produced were identical to those of 1 prepared as
described above.
distance in 1 is 2.809(1) Å, and in 2 it is 2.7949(10) Å. Cotton
has suggested that the structurally similar dimeric molecule Nb2-
Cl8(PMe2Ph)4 has a niobium-niobium bond, on the basis of a
metal-metal distance of 2.838(1) Å.27 The similar bond lengths
in 1 and 2 imply that they also contain metal-metal bonds.
The angles about each niobium center reflect the expected values
for square antiprismatic geometry. For example, the cis chlorine-
niobium-chlorine angles for 1 range between 71.8 and 106.1°,
while cis chlorine-niobium-phosphorus angles encompass
69.6-80.1°. For 1, the trans chlorine-niobium-chlorine angles
are 144.0(1) and 143.3(1)°, while the trans chlorine-niobium-
phosphorus angles are 143.3(1) and 143.7(1)°. The phosphorus-
niobium-phosphorus angle in 1 is 113.0(1)°. The small values
of the niobium-chlorine-niobium angles in 1 associated with
the bridging chloride ligands (67.1(1), 67.2(1)°) reflect the
distortions required for the formation of the niobium-niobium
bond. The related angles in 2 are very similar.
X-ray Crystal Structures of 1 and 2. The X-ray crystal
structures of 1 and 2 were determined to establish their
nuclearities and molecular geometries. Experimental crystal-
lographic data are summarized in Table 1, selected bond lengths
and angles are presented in Tables 2 and 3, and perspective
views are provided in Figures 1 and 2. Further data are available
in the Supporting Information.
Complexes 1 and 2 crystallize as dimers that are held together
by four bridging chloride ions. Additionally, each niobium atom
bears two terminal chloride ions and two phosphine ligands,
and there are short niobium-niobium distances suggesting
metal-metal bonds. The geometry about each niobium atom
in 1 and 2 is square antiprismatic, with one face consisting of
mutually trans phosphine and chloride ligands and the other
face consisting of four bridging chloride ligands. The terminal
niobium-chlorine bond lengths in 1 and 2 range between 2.49
and 2.51 Å, while the bridging niobium-chlorine bond lengths
range between 2.52 and 2.57 Å. The niobium-phosphorus bond
lengths range between 2.61 and 2.63 Å. The niobium-niobium
Deposition of NbP Films from 1. The major goal of this
work was to develop molecular precursors to NbP films.
Attempts to deposit films from 2 were not successful, since it
did not sublime. However, 1 sublimes slowly at 140 °C (0.1
mmHg) with partial decomposition (ca. 50% sublimed yields
of 1 were obtained in sublimations carried out on a 1.0 g scale).

Molecular Precursors to NbP Films
Inorganic Chemistry, Vol. 38, No. 19, 1999 4357
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 2
Nb-Cl(1)
Nb-Cl(3)
Nb-Cl(3)#1
Cl(3)-Nb#1
Nb-P(1)
2.4906(13)
2.5723(13)
2.5530(13)
2.5529(13)
2.633(2)
Nb-Cl(2)
Nb-Cl(4)
Nb-Cl(4)#1
Cl(4)-Nb#1
Nb-P(2)
2.4956(13)
2.5223(13)
2.5227(13)
2.5226(13)
2.634(2)
Nb-Nb#1
2.7949(10)
Cl(1)-Nb-Cl(2)
Cl(2)-Nb-Cl(4)#1
Cl(2)-Nb-Cl(4)
Cl(1)-Nb-Cl(3)#1
Cl(4)#1-Nb-Cl(3)#1
Cl(1)-Nb-Cl(3)
Cl(4)#1-Nb-Cl(3)
Cl(3)#1-Nb-Cl(3)
Cl(2)-Nb-P(1)
104.85(5) Cl(1)-Nb-Cl(4)#1 143.35(4)
82.18(4) Cl(1)-Nb-Cl(4) 82.56(5)
144.52(4) Cl(4)#1-Nb-Cl(4) 112.72(3)
143.67(4) Cl(2)-Nb-Cl(3)#1
72.32(4) Cl(4)-Nb-Cl(3)#1
81.98(4) Cl(2)-Nb-Cl(3)
72.51(4) Cl(4)-Nb-Cl(3)
113.91(3) Cl(1)-Nb-P(1)
70.62(4) Cl(4)#1-Nb-P(1)
82.38(4)
72.84(4)
142.81(4)
72.00(4)
69.66(5)
79.52(4)
Cl(4)-Nb-P(1)
141.63(4) Cl(3)#1-Nb-P(1) 143.24(4)
78.19(4) Cl(1)-Nb-P(2) 73.31(4)
69.58(4) Cl(4)#1-Nb-P(2) 140.07(5)
Cl(3)-Nb-P(1)
Cl(2)-Nb-P(2)
Figure 2. Perspective view of 2 with thermal ellipsoids at the 30%
probability level.
Cl(4)-Nb-P(2)
79.99(4) Cl(3)#1-Nb-P(2)
144.69(5) P(1)-Nb-P(2)
127.40(4) Cl(2)-Nb-Nb#1
56.36(3) Cl(4)-Nb-Nb#1
57.29(3) Cl(3)-Nb-Nb#1
122.92(4) P(2)-Nb-Nb#1
66.09(3) Nb#1-Cl(4)-Nb
76.34(4)
114.69(4)
127.74(4)
56.36(3)
56.62(3)
122.37(4)
67.28(3)
Cl(3)-Nb-P(2)
Cl(1)-Nb-Nb#1
Cl(4)#1-Nb-Nb#1
Cl(3)#1-Nb-Nb#1
P(1)-Nb-Nb#1
Nb#1-Cl(3)-Nb
Figure 3. X-ray diffraction spectrum of a film deposited on Corning
7059 glass at 600 °C using 1 as the precursor. The broad reflection
centered at 2θ ) 24° is from the substrate.
deposited NbP film was 2.18 Å, which is identical to the value
given in the JCPDS reference file.36 The films deposited at 400
and 500 °C were specular and silver-colored but were too thin
to give observable reflections in the X-ray diffraction spectra.
X-ray photoelectron spectroscopy was used to determine the
surface composition of a film deposited at 600 °C. Such analysis
showed the presence of two types of niobium environments (Nb
3d5/2 ) 207.9, 203.3 eV; 70:30 ratio), oxygen (O 1s ) 532.6
eV), carbon (C 1s ) 285.6 eV), and two kinds of phosphorus
ionizations (P 2p ) 133.8, 128.9 eV; 70:30 ratio). The first Nb
3d5/2 binding energy is similar to that reported for Nb2O5 (Nb
3d5/2 ) 207.5 eV).37 The first P 2p ionization, 133.8 eV, is
consistent with the binding energy of the phosphate ion.37 The
second P 2p peak, 128.9 eV, is consistent with values for
reported metal phosphides.37 After sputtering with argon ions
for 45 min, the main niobium ionization was centered at 203.0
eV; the ionization at 207.9 eV was significantly reduced in
intensity. The ratio between these two ionizations was 70:30.
Presumably, the Nb 3d5/2 peak at 203.0 eV is due to NbP. The
phosphorus region still contained two peaks after sputtering (P
2p ) 133.8, 129.0 eV), but the ratio had changed to 20:80. The
ionization at 129.0 eV probably corresponds to the phosphido
ion of NbP. The oxygen (O 1s ) 531.4 eV) and carbon (C 1s
) 285.0 eV) peaks, although still present after sputtering, were
significantly reduced in intensity. The X-ray photoelectron
spectroscopic data suggest a surface that is terminated by
niobium phosphate.
Figure 1. Perspective view of 1 with thermal ellipsoids at the 30%
probability level.
Accordingly, 1 was deemed sufficiently volatile to allow film
fabrication trials. Depositions were conducted on Corning 7059
glass and crystalline silicon at 400, 500, and 600 °C using 1 as
the precursor in a horizontal hot-walled reactor. Details of the
reactor configuration have been previously described.35 The
films were specular or matte in appearance and were silver-
colored. The precursor did not sublime quantitatively. Some of
the precursor decomposed in the precursor vial to give silver-
colored granules with a metallic sheen. The powder X-ray
diffraction spectrum of these granules was consistent with that
of NbP.36 This experiment indicates that 1 decomposes in the
solid state to bulk NbP at about the same temperature at which
it sublimes.
A film deposited at 600 °C was analyzed by X-ray diffraction
(Figure 3). The film exhibited preferred orientation along the
reflections with 2θ ) 27, 32, and 41.5°. The d spacing for the
(35) Winter, C. H.; Lewkebandara, T. S.; Sheridan, P. H.; Proscia, J. W.
Mater. Res. Soc. Symp. Proc. 1993, 282, 293.
(36) Inorganic Phases; JCPDS-International Centre for Diffraction Data:
Swarthmore, PA, 1988; Card No. 17-882.
(37) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, G. E. Handbook
of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp., Physical
Electronics Division: Eden Prairie, MN, 1979.

4358 Inorganic Chemistry, Vol. 38, No. 19, 1999
Scheme 1. Proposed Reaction Process
Scheper et al.
by the isolation and characterization of 5. In the presence of
additional phosphine, NbCl5(PRH2) reacts to form the dimeric
niobium(IV) complex Nb2Cl8(PRH2)4, isolated examples of
which include 1 and 2. To account for the reduction to niobium-
(IV), we propose the formation of a seven-coordinate transition
state or intermediate, which can eliminate a chlorine atom with
concomitant reduction of the niobium(V) center. A similar
process was proposed by Poli in the reaction of CpNbCl4 with
excess trimethylphosphine.31 Poli suggested that the chlorine
atom abstracts a hydrogen atom from toluene solvent to afford
hydrogen chloride, which then reacts with trimethylphosphine
to give trimethylphosphonium chloride. Our results suggest that
the hydrogen source for 3 is not dichloromethane solvent but
rather additional cyclohexylphosphine. This proposal is sup-
ported by the lack of deuterium incorporation when the synthesis
of 3 is conducted in dichloromethane-d2 and by the spectroscopic
detection of oligomeric phosphines derived from cyclohexyl-
phosphine. We note that Poli’s system utilized trimethylphos-
phine, which has no phosphorus-hydrogen bonds to be
abstracted by chlorine atoms, leaving toluene solvent as the most
likely hydrogen atom source. Once formed, the hydrogen
chloride reacts with the primary phosphine to form a primary
phosphonium chloride. The phosphonium ions are isolated as
highly insoluble hexachloroniobate(IV) salts, which presumably
form by capture of two chloride ions by a niobium(IV) chloride
species. While mechanistically interesting, the formation of 3
and 4 can be avoided through use of NbCl4(THF)2 in place of
niobium pentachloride.
Rutherford backscattering spectrometry was used to determine
the stoichiometry of an NbP film deposited at 600 °C on a
silicon substrate. The backscattering spectrum revealed a
phosphorus-rich stoichiometry of Nb1.00P1.20. In addition, traces
of carbon, oxygen, and chlorine were detected, but the back-
scattering peaks for these elements were barely above the noise
level and were correspondingly difficult to quantify. We estimate
that the total concentration of these elements was less than 10%
relative to niobium.
Initial deposition studies demonstrate that 1 functions as a
molecular precursor to slightly phosphorus-rich NbP films using
a CVD reactor. It was found that a deposition temperature of
600 °C was required to obtain films that were sufficiently thick
for analysis. This deposition temperature can be contrasted with
the 1050 °C required to obtain NbP films from niobium
pentachloride, phosphorus trichloride, and hydrogen.10 The low
volatility of 1 is a distinct limitation in its use as a precursor.
In fact, 1 partially decomposes to bulk NbP at temperatures
required for its vapor transport. Future source compound
development should focus on monomeric complexes to achieve
higher volatility. Interestingly, monomeric 5 decomposes before
it sublimes and is therefore not a viable source compound. RBS
analysis of the films deposited at 600 °C demonstrates that the
bulk of the film is NbP containing e10% of carbon, chlorine,
and oxygen. However, XPS studies indicate that the surface of
the film is terminated by phosphate ions rather than phosphido
ions. The formation of surface phosphate undoubtedly occurs
rapidly upon exposure to ambient atmosphere. The formation
of niobium(V) phosphate on the film surface is probably the
key to the air stability and remarkable chemical resistance of
NbP films.10 The formation of phosphate phases in the presence
of oxygen donors seems to be a general feature in the attempted
deposition of early transition metal phosphide phases. Watson,
Connor, and Whyman reported that film depositions using the
precursors M(CO)5(PH3) (M ) Cr, Mo, W) yielded metal
phosphate films rather than metal phosphide films.38 Apparently,
the carbon monoxide ligands serve as the oxygen source. Of
relevance to this study, Watson, Connor, and Whyman found
that the metal phosphate films were unaffected by immersion
in aqueous hydrochloric acid for several hours. The stability of
these metal phosphate phases toward chemical attack supports
our proposal that it is the phosphate-terminated surface of the
NbP films that leads to their stability in air.
Discussion
We have documented an interesting reaction course that
occurs upon treatment of niobium pentachloride with primary
phosphines. The niobium(IV) products 1 and 2 are obtained in
good yields and possess dimeric structures that are very similar
to tertiary phosphine adducts of niobium(IV) chloride that have
been documented by the groups of Nielson and Cotton.25-28 It
is clear that the dimeric framework adopted by 1 and 2 is very
stable, since this structure type seems to be generally adopted
by complexes of the empirical formula NbCl4(PR3)2 (R ) H,
alkyl, aryl). Despite the fact that 1 has a niobium(IV) oxidation
state, 1H, 13C{1H}, and 31P{1H} NMR spectra exhibited reason-
ably sharp resonances. Such behavior implies that the d1 metal
centers couple in an antiferromagnetic fashion to afford a
complex that has substantial diamagnetic character, at least at
ambient temperature. The diamagnetic character of 1 also
suggests that the dimeric structure observed in the solid state is
retained in solution. By contrast, Cotton and co-workers reported
that the tertiary phosphine adduct Nb2Cl8(PMe2Ph)4, which
possesses a structure similar to that of 1 and 2, dissolves in
dichloromethane with a drastic change in color and exhibits an
ESR spectrum in this medium.26,27 These observations suggest
dissociation of the dimer to NbCl4(PMe2Ph)2 in dichlo-
romethane. Retention of the dimeric structure of 1 in solution
probably reflects the smaller steric profile of a primary phos-
phine compared to a tertiary phosphine and argues that Nb2-
Cl8(PMe2Ph)4 dissociates in dichloromethane to relieve steric
crowding.
The origin of the hydrogen atom that is incorporated in 3
and 4 has been probed. A proposed mechanistic sequence that
is consistent with the experimental data is outlined in Scheme
1. Treatment of niobium pentachloride with 1 equiv of a primary
phosphine affords the adduct NbCl5(PRH2), which is stable in
the absence of additional phosphine. This initial step is supported
(38) Watson, I. M.; Connor, J. A.; Whyman, R. Thin Solid Films 1991,
201, 337.

Molecular Precursors to NbP Films
Inorganic Chemistry, Vol. 38, No. 19, 1999 4359
Spectroscopic and analytical data for bis(cyclohexylphospho-
nium) hexachloroniobate(IV) (3): mp 128-132 °C dec; IR (Nujol,
cm-1) 2446 (νP-H, s), 2405 (νP-H, s), 1352 (m), 1328 (m), 1301 (m),
1275 (m), 1217 (m), 1183 (w), 1127 (w), 1078 (w), 1053 (m), 1037
(m), 1009 (m), 980 (s), 918 (m), 892 (m), 857 (m), 773 (w), 736 (w).
NMR data could not be obtained because 3 was insoluble in all common
solvents. Anal. Calcd for C12H28Cl6NbP2: C, 26.69; H, 5.23; Cl, 39.40.
Found: C, 26.97; H, 5.29; Cl, 39.15.
Experimental Section
General Considerations. All synthetic manipulations were carried
out under argon using either drybox or Schlenk-line techniques.
Dichloromethane was distilled from calcium hydride. Diethyl ether was
distilled from sodium/benzophenone ketyl. Chloroform-d and dichlo-
romethane-d2 were dried over 4 Å molecular sieves. Niobium pen-
tachloride, cyclohexylphosphine, and phenylphosphine were obtained
from commercial vendors and were used as received. NbCl4(THF)2 was
prepared according to a literature method.22
1H, 13C{1H}, and 31P{1H} NMR spectra were obtained at 400, 100,
and 161 MHz, respectively, in the indicated solvents. Infrared spectra
were obtained using Nujol mulls. Elemental analyses were performed
by Midwest Microlab, Indianapolis, IN. X-ray diffraction spectra were
obtained on a Rigaku Rotaflex powder diffractometer equipped with a
rotating-anode source. X-ray photoelectron spectroscopy was carried
out on an SSX-100 ESCA spectrometer using Al KR radiation.
Rutherford backscattering spectrometry was performed by the Michigan
Ion Beam Laboratory, Ann Arbor, MI.
Preparation of 1 from Tetrachlorobis(tetrahydrofuran)niobium-
(IV). A 100-mL Schlenk flask was charged with NbCl4(THF)2 (0.55
g, 1.44 mmol) and toluene (25 mL). Using a syringe, cyclohexylphos-
phine (0.35 g, 3.01 mmol) was added to this suspension. The reaction
mixture was stirred for 18 h. During this time, the reaction solution
became brown and a brown-red precipitate formed. The brown-red
precipitate was collected on a medium-porosity glass frit in a drybox.
The solid was washed with toluene (15 mL) and dried under vacuum
to afford the first portion of 1 (0.39 g, 58%). The volume of the filtrate
obtained from filtration of the brown-red solid was reduced to about
20 mL under reduced pressure. The flask was cooled to -20 °C. Brown-
red crystals of 1 formed over 24 h. The crystals were isolated and dried
under vacuum to afford a second portion of 1 (0.13 g, 19%). The
Preparation of Octachlorotetrakis(cyclohexylphosphine)diniobium-
(IV) (1). A 100-mL Schlenk flask was charged with niobium pen-
tachloride (0.53 g, 2.0 mmol) and dichloromethane (25 mL). Using a
syringe, cyclohexylphosphine (0.57 g, 4.9 mmol) was added to this
suspension. The resultant brown-red mixture was stirred for 18 h at
ambient temperature. Over this period, a purple precipitate formed and
the dichloromethane layer took on a brown-red color. The purple
precipitate was collected on a medium-porosity glass frit in the glovebox
and was washed with dichloromethane (15 mL) and vacuum-dried to
afford 3 (0.36 g, 68% based on niobium pentachloride). Analytical and
spectroscopic data for 3 are given below. The clear brown-red filtrate
was layered with diethyl ether (20 mL). The crystallization system was
allowed to equilibrate for 24 h. The resultant crystals were isolated by
cannulating away the solvent and vacuum-drying to afford 1 as brown-
red crystals (0.39 g, 85% based on niobium pentachloride): mp 155-
160 °C (dec); IR (Nujol, cm-1) 2413 (νP-H, w), 2396 (νP-H, w), 2368
(νP-H, m), 2363 (νP-H, m), 1449 (s), 1354 (w), 1344 (w), 1295(w),
1270 (w), 1214 (w), 1180 (w), 1126 (m), 1059 (m), 1027 (w), 1003
1
combined yield of 1 was 77%. The H, 13C{1H}, and 31P{1H} NMR
and infrared spectra of the crystals and precipitate were identical with
those of 1 prepared as described above.
Preparation of Pentachloro(cyclohexylphosphine)niobium(V) (5).
A 100-mL Schlenk flask was charged with niobium pentachloride (1.08
g, 4.00 mmol) and dichloromethane (25 mL). The flask and its contents
were cooled to -78 °C. Cyclohexylphosphine (0.35 g, 3.0 mmol) was
added by syringe to this stirred suspension. After the addition, the
resultant orange mixture was stirred at -78 °C for 1 h. The mixture
was then warmed to room temperature and was stirred for an additional
13 h. Filtration of the reaction solution through a 1-cm pad of Celite
on a coarse glass frit, followed by removal of the volatile components
under reduced pressure, afforded 5 as a bright orange powder (1.02 g,
88% based on cyclohexylphosphine): mp 78-83 °C dec; IR (Nujol,
cm-1) 2387 (νP-H, w), 2367 (νP-H, w), 2347 (νP-H, w), 1443 (s), 1346
(w), 1327 (w), 1297 (m), 1274 (w), 1258 (w), 1209 (w), 1179 (m),
1125 (m), 1086 (w), 1078 (w), 1053(s), 1041 (s), 1003 (m), 921 (s),
1
(m), 916 (w), 893 (s), 869 (s), 841 (s), 819 (m), 733 (w); H NMR
1
893 (s), 864 (m), 831 (s), 818 (s), 785 (w); H NMR (CDCl3, δ) 4.60
(CDCl3, δ) 4.69 (br d, JHP ) 355.6 Hz, PH2), 2.39 (br s, CHPH2),
2.14, 1.77, 1.36 (br s, PCH(CH2)2(CH2)2CH2); 13C{1H} NMR (CDCl3,
ppm) 33.88 (s, PCH(CH2)2(CH2)2CH2), 32.23 (s, PCH(CH2)2(CH2)2-
CH2), 26.84 (br s, PCH(CH2)2(CH2)2CH2), 25.62 (s, PCH(CH2)2-
(CH2)2CH2); 31P{1H} NMR (CDCl3, ppm) -2.62 (br s). Anal. Calcd
for C24H52Cl8Nb2P4: C, 30.86; H, 5.61. Found: C, 28.88; H, 5.31.
(dd, JHP ) 347 Hz, JHH ) 6.0 Hz, PH2), 2.38 (br m, PCH(CH2)2(CH2)2-
CH2), 2.21, 1.85, 1.43 (br m, PCH(CH2)2(CH2)2CH2); 13C{1H} NMR
(CDCl3, ppm) 32.63 (d, JCP ) 5.2 Hz, PCH(CH2)2(CH2)2CH2), 30.57
(d, JCP ) 17.8 Hz, PCH(CH2)2(CH2)2CH2), 26.57 (d, JCP ) 9.6 Hz,
PCH(CH2)2(CH2)2CH2), 25.33 (s, PCH(CH2)2(CH2)2CH2); 31P{1H}
NMR (CDCl3, ppm) -40.07 (s, PH2). Anal. Calcd for C6H13Cl5NbP:
C, 18.65; H, 3.39. Found: C, 15.73; H, 3.02.
Reaction of 5 with Cyclohexylphosphine. A 100 mL Schlenk flask
was charged with 5 (0.61 g, 1.5 mmol) and dichloromethane (25 mL).
Cyclohexylphosphine (0.37 g, 3.1 mmol) was added by syringe to this
solution. The reaction mixture turned brown-red immediately and was
stirred for 18 h. A purple precipitate was collected on a medium-porosity
glass frit and vacuum-dried to afford 3 (0.22 g, 52%). The filtrate was
layered with diethyl ether (25 mL). The crystallization flask was allowed
to equilibrate for 24 h. The resultant brown-red crystals were isolated
and dried under vacuum to afford 1 (0.34 g, 66%). Compounds 1 and
3 were identified by comparison of their spectral properties with those
of materials prepared as described above.
Preparation of 1 and 3 in Dichloromethane-d2. In a glovebox, a
10-mL glass vial was charged with niobium pentachloride (0.54 g, 2.0
mmol) and a stir bar. Dichloromethane-d2 (5.0 mL) was added, followed
by cyclohexylphosphine (0.44 g, 3.8 mmol). The resultant brown-red
mixture was stirred for 18 h at ambient temperature. A purple precipitate
was collected by filtration through a medium-porosity glass frit. The
purple solid was dried under vacuum and subsequently identified as 3
(0.22 g, 40%) by comparison of its infrared spectrum with that of
material prepared as described above. The filtrate was cooled to -20
°C for 24 h. A brown-red crystalline solid was isolated by removal of
the solvent and identified as 1 (0.13 g, 28%) by comparison of its NMR
spectra with those of material prepared as described above. The infrared
spectra of 1 and 3 prepared herein were identical to material that was
prepared in protiodichloromethane.
Preparation of Octachlorotetrakis(phenylphosphine)diniobium-
(IV) (2). A 100-mL Schlenk flask was charged with niobium pen-
tachloride (2.02 g, 7.50 mmol) and dichloromethane (15 mL). Using a
syringe, a solution of phenylphosphine (1.70 g, 14.6 mmol) in
dichloromethane (15 mL) was added to this mixture. After the addition,
the resultant dark red mixture was stirred at ambient temperature for
18 h. During this time, a purple precipitate formed. The precipitate
(containing 4; vide infra) was removed by filtration through a medium-
porosity glass frit in a drybox. The resultant dark red filtrate was layered
with diethyl ether (15 mL). The crystallization system was allowed to
equilibrate for 24 h. The resultant crystals were isolated by cannulating
away the solvent and vacuum-drying to afford 2 as dark red crystals
(1.503 g, 88% based on niobium pentachloride): mp 170-175 °C dec;
IR (Nujol, cm-1) 2409 (νP-H, w), 2393 (νP-H, w), 2386 (νP-H, w), 1484
(w), 1437 (m), 1333 (w), 1306 (w), 1106 (w), 1069 (w), 1048 (w),
1038 (w), 1023 (w), 1001 (w), 917 (w), 872 (s), 847 (m), 737 (m),
732 (m), 703 (w), 687 (m), 668 (w). NMR data could not be obtained
because 2 was insoluble in all common solvents after isolation as a
solid. Anal. Calcd for C24H28Cl8Nb2P4: C, 31.68; H, 3.10. Found: C,
28.01, 28.11; H, 2.96, 2.96.
The purple precipitate was probably 4, in analogy with 3. Due to
the insolubility of compound 2, however, 4 could not be isolated as a
single compound. The infrared spectrum of the purple precipitate
showed absorptions for 2, as well as absorptions consistent with a
phosphonium ion (νPH 2408, 2392, 2385 cm-1). No further attempts
were made to separate 2 from 4.

4360 Inorganic Chemistry, Vol. 38, No. 19, 1999
Scheper et al.
For the NMR experiment, a mixture of 1 and 3 was prepared as
above by starting from niobium pentachloride (0.16 g, 0.6 mmol) and
cyclohexylphosphine (0.18 g, 1.5 mmol) in dichloromethane-d2 (2.0
mL). Insoluble 3 was removed by filtration through a plug of Celite
on a coarse glass frit to afford a solution containing 1 and products of
cyclohexylphosphine oxidation: 31P{1H} NMR (CD2Cl2, ppm) 23.50
(br s), -3.53 (br m, 1), -69.04 (br s), -87.80 (br s). For discussion,
see text.
NbP Film Deposition from 1. Film depositions were carried out in
a horizontal hot-wall reactor, the details of which have been previously
described.35 The precursor (ca. 1 g) was finely ground and placed in a
vial. The vial was placed in the sealed end of a 1 in. diameter quartz
tube located in a tube furnace. The substrates were situated directly
ahead of the precursor vial in the reactor. Corning 7059 low-sodium
glass substrates were wiped with a Kimwipe until visually clean just
prior to use, while silicon substrates were used as received. The open
end of the tube was fitted with a vacuum adapter. Depositions were
conducted at low pressure (ca. 0.01 mmHg). The substrates were
equilibrated at the deposition temperatures for ca. 1 h prior to deposition,
with the precursor region of the tube outside the furnace. Once the
substrates were equilibrated at the deposition temperature, the precursor
vial was placed just inside the furnace (temperature 150-200 °C),
resulting in sublimation of the precursor over the heated substrates.
Depositions were carried out for 15.0 min with substrate temperatures
of 400, 500, and 600 °C. Details of the film analysis are described in
the text.
absences in the diffraction data for 2 are uniquely consistent with the
space group Pbca. E statistics suggested the centrosymmetric space
group option P1h for 1, which yielded chemically reasonable and
computationally stable results of refinement. The structures were solved
using direct methods, completed by subsequent difference Fourier
syntheses, and refined by full-matrix least-squares procedures. Absorp-
tion corrections were applied to the data of 2 but were ignored for 1
because there was <10% variation in the integrated ψ-scan intensity
data. The molecules of 1 and 2 each lie on an inversion center. All
non-hydrogen atoms were refined with anisotropic displacement
coefficients. The hydrogen atoms on phosphorus were located from
the difference map, and their distances in 1 were fixed to an average
phosphorus-hydrogen distance. All other hydrogen atoms were treated
as idealized contributions.
All software and sources of the scattering factors are contained in
either the SHELXTL (5.03) or SHELXTL PLUS (4.2) program libraries
(G. Sheldrick, Siemens XRD, Madison, WI).
Acknowledgment. We are grateful to the National Science
Foundation for support of this research and the Institute for
Manufacturing Research of Wayne State University for fellow-
ships to K.C.J. and J.T.S. The Rutherford backscattering
spectrometry was performed at the Michigan Ion Beam Labora-
tory by Dr. Victor Rotberg.
Crystallographic Structure Determinations for 1 and 2. Crystal,
data collection, and refinement parameters are given in Table 1. Suitable
crystals were selected and mounted in thin-walled glass capillaries under
nitrogen, and the capillaries were then flame-sealed. Diffraction data
were collected on a Siemens P4 diffractometer. The unit cell parameters
were obtained by the least-squares refinement of the angular settings
of 24 reflections (20 ° e 2θ e 25°).
Supporting Information Available: X-ray photoelectron spectra,
an RBS spectrum, and an X-ray diffraction spectrum for NbP films,
an ORTEP diagram and listings of crystallographic data, atomic
coordinates, thermal parameters, bond lengths, and bond angles for 1,
and an X-ray crystallographic file, in CIF format, for the structure
determination of 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
No evidence of symmetry higher than triclinic was observed in either
the photographic or the diffraction data for 1, and the systematic
IC990198S