Synthesis and Structural Studies of Platinum Complexes Containing Monodentate, Bridging, and Chelating Formamidine Ligands

Article abstract of DOI:10.1021/ic950741e

The reactions of K₂PtCl₄ with N,N'-diphenylformamidine (HDPhF) and N.N'-di-p-tolylformamidine (HDTolF) produce the trans square-planar compounds PtCl₂(HDPhF)₂, 1a, and PtCl₂(HDTolF)₂, 1b. Compound la crystallizes as yellow parallelepipeds in the space group P2₁/c with two independent molecules in the asymmetric unit and unit cell dimensions a = 23.427(7) ?, b = 16.677(6) ?, c = 12.980(4) ?, and β= 96.10(2)°. These compounds are soluble in common organic solvents and have been used as starting materials for the preparation of diplatinum compounds. Treatment of 1a and 1b with NaOMe and the halide abstraction reagent TIPF₆ produces the compounds Pt₂(μ-DArF)₂(η²-DArF)₂, Ar = Ph (2a) and Tol (2b), respectively. Compound 2a crystallizes as yellow rods in the space group P1? with unit cell dimensions a = 12.296(3) ?, b = 12.310(4) ?, c = 15.374(4) ?, α = 90.75(2)°, β = 91.02(2)°, and γ = 110.20(2)°. Compound 2b crystallizes with a molecule of THF, as yellow rods in the space group P2₁/c with a = 17.883(3) ?, b = 14.517(3) ?, c = 22.581(3) ?, and β = 98.17(1)°. These compounds contain two cis bridging formamidinato ligands and two formamidinato ligands that are chelated to separate Pt centers. Upon heating, they further react to give the tetrabridged compounds Pt₂(μ-DArF)₄ Ar = Ph (3a), Tol (3b). Compound 3a crystallizes as orange cubes in the cubic space group I432 with a = 19.671(1) ?. On going from the bis-bridged, bis-chelate structure in 2a to the tetrabridged structure in 3a, the metal-metal separation decreases from 2.910(1) to 2.649(1) ?. Both 2b and 3b have been oxidized to give the Pt^II-Pt^III compound Pt₂(μ-DTolF)₄(PF₆), 4. Compound 4 crystallizes as cubes in the tetragonal space group P4/ncc with a = 14.392(1) ? and c = 14.436(1) ?. The Pt-Pt distance in 4 is 2.5304(6) ?.

Full text of DOI:10.1021/ic950741e

498
Inorg. Chem. 1996, 35, 498-503
Synthesis and Structural Studies of Platinum Complexes Containing Monodentate,
Bridging, and Chelating Formamidine Ligands
F. Albert Cotton,*,† John H. Matonic,† and Carlos A. Murillo*,†,‡
Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University,
College Station, Texas 77843-3255, and Department of Chemistry, University of Costa Rica,
Ciudad Universitaria, Costa Rica
ReceiVed June 15, 1995X
The reactions of K2PtCl4 with N,N′-diphenylformamidine (HDPhF) and N,N′-di-p-tolylformamidine (HDTolF)
produce the trans square-planar compounds PtCl2(HDPhF)2, 1a, and PtCl2(HDTolF)2, 1b. Compound 1a crystallizes
as yellow parallelepipeds in the space group P21/c with two independent molecules in the asymmetric unit and
unit cell dimensions a ) 23.427(7) Å, b ) 16.677(6) Å, c ) 12.980(4) Å, and â ) 96.10(2)°. These compounds
are soluble in common organic solvents and have been used as starting materials for the preparation of diplatinum
compounds. Treatment of 1a and 1b with NaOMe and the halide abstraction reagent TlPF6 produces the compounds
Pt2(µ-DArF)2(η2-DArF)2, Ar ) Ph (2a) and Tol (2b), respectively. Compound 2a crystallizes as yellow rods in
the space group P1h with unit cell dimensions a ) 12.296(3) Å, b ) 12.310(4) Å, c ) 15.374(4) Å, R ) 90.75(2)°,
â ) 91.02(2)°, and γ ) 110.20(2)°. Compound 2b crystallizes with a molecule of THF, as yellow rods in the
space group P21/c with a ) 17.883(3) Å, b ) 14.517(3) Å, c ) 22.581(3) Å, and â ) 98.17(1)°. These compounds
contain two cis bridging formamidinato ligands and two formamidinato ligands that are chelated to separate Pt
centers. Upon heating, they further react to give the tetrabridged compounds Pt2(µ-DArF)4, Ar ) Ph (3a), Tol
(3b). Compound 3a crystallizes as orange cubes in the cubic space group I432 with a ) 19.671(1) Å. On going
from the bis-bridged, bis-chelate structure in 2a to the tetrabridged structure in 3a, the metal-metal separation
decreases from 2.910(1) to 2.649(1) Å. Both 2b and 3b have been oxidized to give the PtII-PtIII compound
Pt2(µ-DTolF)4(PF6), 4. Compound 4 crystallizes as cubes in the tetragonal space group P4/ncc with a ) 14.392(1)
Å and c ) 14.436(1) Å. The Pt-Pt distance in 4 is 2.5304(6) Å.
Introduction
formamidinato ion as a ligand,6 because they form the neutral
M2(RNC(H)NR)4 compounds. These can be oxidized by
removal of one electron, but studies have shown this to be a
ligand-based oxidation in the case of the Pd compound.
The possibility of altering the electronic properties of the
ligand by changing the substituents on the nitrogen and methine
carbon atoms is another advantage of the amidine ligand. By
using N,N′-diphenylbenzamidine (DPhBz) in the palladium
Formamidinato ligands in a paddlewheel arrangement about
two metal centers provide a unique opportunity to study many
metal-metal interactions within the same structural motif. This
type of compound is known for many of the transition metals,
including V, Cr, Fe, Co, Ni, Cu, Mo, Tc, Ru, Rh, Pd, W, Re,
Os, and Ir.1 The flexibility of the formamidinato ligand,
exemplified by its ability to bridge two metals that are separated
at distances as short as 1.930(2) Å2 and as long as 3.049(1) Å,3
and its highly basic nature are probably the main reasons that
it stabilizes such a variety of dimetal systems.
Within group 10 (Ni, Pd, and Pt) the possibility of a metal-
metal bond of order 1 arises if the metal centers have a d7
electronic configuration, that is, are in a formal oxidation state
of +3. The bonding, formally described as σ2π4δ2δ*2π*4, is
frequently found in Rh(II),1a but it is not as common for Ir(II)4
and Pt(III).1a,5 For the d8 metals, Ni(II) and Pd(II), the formation
of a metal-metal bond does not occur, at least with the
system, Bear and co-workers found that the neutral Pd (DPhBz)4
+27
could be oxidized to the d7-d8 species Pd2(DPhBz)4
.
Chang-
ing the substituents on the amidine ligand can also influence
the mode of binding. For Pd(II), DPhBz is known to form
complexes that include a bis-chelate mononuclear species,8 a
bis-chelate, bis-bridged dimetal compound, and a tetrabridged
compound.7 In contrast, for the N,N′-di-p-tolylformamidine
(DTolF) ligand, only the dinuclear tetrabridged species has been
observed. For platinum, the DPhBz ligand produces a mono-
nuclear bis-chelate species, with no evidence of other com-
pounds being formed in solution.9
We wish to present the syntheses and X-ray structures of
several complexes of Pt with the formamidine ligand. PtCl2-
(HDArF)2, Ar ) Ph (1a), Tol (1b), were prepared as a
convenient method for solubilizing Pt(II) chloride. Pt2(µ-
DArF)2(η2-DArF)2, Ar ) Ph (2a), Tol (2b‚THF), were prepared
from 1a or 1b by deprotonating the formamidine ligand and
† Texas A&M University.
‡ University of Costa Rica.
X Abstract published in AdVance ACS Abstracts, December 1, 1995.
(1) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms,
2nd ed.; Oxford University Press: New York, 1993; see also references
therein. (b) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219.
(2) See for example: Cotton, F. A.; Ren, T. J. Am. Chem. Soc. 1992,
114, 2237.
(3) See for example: Bailey, J. A; Miskowski, V. M.; Gray, H. B. Acta
Crystallogr. 1993, C49, 793.
(6) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988,
110, 1144.
(4) Cotton, F. A.; Poli, R. Polyhedron 1987, 6, 1625 and references therein.
(5) (a) Appleton, T. G.; Byriel, K. A.; Hall, J. R.; Kennard, C. H. L.;
Mathieson, M. T. J. Am. Chem. Soc. 1992, 114, 7305. (b) Baxter, L.
A. M.; Heath, G. A.; Raptis, R. G.; Willis, A. C. J. Am. Chem. Soc.
1992, 114, 6944. (c) Cini, R.; Fanizzi, F. P.; Intini, F. P.; Natile, G.
J. Am. Chem. Soc. 1991, 113, 7805.
(7) Yao, C.-L.; He, L.-P.; Korp, J. D.; Bear, J. L. Inorg. Chem. 1988, 27,
4389.
(8) Barker, J.; Cameron, N.; Kilner, M.; Mahoud, M. M.; Wallwork, S.
C. J. Chem. Soc., Dalton Trans. 1986, 1359.
(9) Barker, J.; Kilner, M.; Gould, R. O. J. Chem Soc., Dalton Trans. 1987,
2687.
0020-1669/96/1335-0498$12.00/0 © 1996 American Chemical Society

Platinum Complexes Containing Formamidine Ligands
Inorganic Chemistry, Vol. 35, No. 2, 1996 499
Table 1. Crystallographic Data
1a
2a
2b‚THF
3a
4
formula
fw
space group
a, Å
b, Å
c, Å
C52H48Cl4N8Pt2
1316.96
P21/c
23.427(7)
16.677(6)
12.980(4)
C52H44N8Pt2
1171.13
P1h
C64H60N8OPt2
1347.38
P21/c
17.883(3)
14.517(3)
22.581(3)
C52H44N8Pt2
1171.13
I432
C60H60F6N8PPt2
1428.31
P4/nnc
12.296(3)
12.310(4)
15.374(4)
90.75(2)
91.02(2)
110.20(2)
2183(1)
2
1.782
6.448
0.710 73
293(2)
19.671(1)
14.392(1)
14.436(1)
R, deg
â, deg
γ, deg
V, Å3
Z
96.10(2)
98.17(1)
5043(3)
4
1.735
5.798
0.710 73
293(2)
5803(2)
4
1.542
4.864
0.710 73
293(2)
7611.7(7)
6
1.533
10.475
1.541 84
293(2)
2989.9(4)
2
1.587
4.763
0.710 73
293(2)
d
calc, g cm-3
µ, mm-1
λ, Å
temp, K
transm factors
R1a, wR2b
R1a, wR2b (all data)
0.9981-0.6590
0.033, 0.074
0.034, 0.077
0.9990-0.7108
0.021, 0.053
0.029, 0.054
0.9991-0.8955
0.030, 0.065
0.071, 0.087
1.000-0.7913
0.050, 0.138
0.069, 0.194
0.9987-0.6318
0.019, 0.049
0.023, 0.052
2
2
2
2
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. b wR2 ) [∑w[(Fo - Fc2)2]/∑[w(Fo )2]]1/2; w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) [max(Fo or 0) + 2(Fc2)]/3.
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
abstracting the chloride. Similar to the Pd(II)- benzamidinato
Displacement Parameters (Å2 × 103) for PtCl2(HDPhF)2, 1a
compounds, these compounds contain two bridging and two
x
y
z
U(eq)a
chelating formamidinato ligands and can be converted into the
tetrabridged compounds Pt2(µ-DArF)4, Ar ) Ph (3a), Tol (3b).
The mixed-valence compound [Pt2(µ-DTolF)4](PF6), 4, synthe-
sized from the one-electron oxidation of 2b, is also re-
ported.
Pt(1)
Cl(1)
Cl(2)
N(1)
N(2)
N(3)
N(4)
C(1)
C(2)
Pt(2)
Cl(3)
Cl(4)
N(5)
N(6)
N(7)
N(8)
C(3)
C(4)
924(1)
1031(2)
760(1)
1054(3)
118(3)
923(4)
-58(4)
651(5)
586(1)
1587(2)
-405(2)
1421(5)
1131(1)
2358(2)
-119(2)
26(6)
36(1)
67(1)
46(1)
36(2)
49(2)
49(2)
53(2)
38(3)
48(3)
36(1)
46(1)
72(1)
35(2)
35(2)
41(2)
44(2)
36(3)
42(3)
1888(5)
-110(7)
2263(7)
2390(7)
-362(8)
2683(8)
4130(1)
5680(2)
2592(2)
4909(6)
4818(6)
3340(6)
2774(7)
5094(7)
2826(8)
-250(6)
-330(6)
1911(6)
Experimental Section
478(5)
-519(6)
-5143(1)
-4499(2)
-5811(2)
-6147(5)
-6815(5)
-4096(5)
-4300(5)
-6778(6)
-3892(6)
General Procedures. All reactions were carried out under an argon
or dinitrogen atmosphere using standard Schlenk techniques. Solvents
were purified by conventional methods and distilled under dinitrogen
immediately prior to use. N,N′-diphenylformamidine (HDPhF) was
purchased from Aldrich Chemical Co. and used as received; N,N′-di-
p-tolylformamidine (HDTolF) was prepared by a slightly modified
published procedure.10 The product was washed extensively with
hexanes and recrystallized from toluene/hexanes. 1H NMR studies were
performed on a JEOL JNM GX-400 or -200 spectrometer; IR spectra
were recorded on a Perkin-Elmer 16PC FT-IR spectrophotometer as
Nujol mulls between KBr plates or as KBr pellets. UV-vis spectra
were recorded on a Cary 17D spectrometer.
5893(1)
5836(1)
5907(2)
6146(3)
5273(3)
5749(4)
4791(4)
5831(5)
5276(5)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
Preparation of trans-PtCl2(HDPhF)2, 1a. Method 1. K2PtCl4 (0.5
g, 1.20 mmol) was placed in a round-bottom flask containing HDPhF
(0.470 g, 2.41 mmol). Toluene (35 mL) was added, and the mixture
was refluxed and stirred for 24 h, or until it was observed that the
insoluble pink K2PtCl4 had disappeared. While hot, the mixture was
filtered to remove KCl, giving a yellow filtrate. The toluene was
evaporated under vacuum to give a crude product that was contaminated
with a small amount of free ligand. Washing with hot hexanes,
followed by recrystallization from toluene/hexanes, gave pure 1a.
Yield: 0.60 g (82%). Anal. Calcd for C26H24Cl2N4Pt: 47.42% C,
3.67% H, 8.51% N. Found: 47.09% C, 3.19% H, 8.22% N. IR:
1880(m), 1650(s), 1630(s), 1600(s), 1580(s), 1500(s), 1330(s), 1215(s),
1100(w), 1020(m), 990(m), 960(s), 840(m), 720(w), 700(m), 510(s)
cm-1. UV-vis (CH3CN): λmax 285 nm. 1H NMR (CDCl3): δ 7.7,
7.8 (s, C-H), 7.1-7.5 (m, arom), 9.4 (s, br, N-H).
Method 2. K2PtCl4 (0.5 g, 1.20 mmol) was placed in a round-bottom
flask containing HDPhF (0.470 g, 2.41 mmol). 1,2-Dichlorobenzene
(20 mL) was then added, and the mixture was refluxed. The reaction
was essentially complete in 3 h. KCl was removed by filtration.
Addition of hexanes to the filtrate precipitated the product 1a. The
solid was then filtered off and recrystallized from hot toluene/hexanes.
Yield: 0.52 g (71%).
1490(m), 1320(s), 1200(s), 1020(w), 975(w), 800(s), 515(m) cm-1
.
UV-vis (CH3CN): λmax 275 nm. 1H NMR (CDCl3): δ 7.6, 7.7 (s,
C-H), 7.1-7.5 (m, arom), 9.2-9.5 (s, br, N-H), 2.0, 2.1 (m, p-CH3).
Preparation of Pt2(µ-DPhF)2(η2-DPhF)2, 2a. To a THF (10 mL)
solution of PtCl2(HDPhF)2 (0.200 g, 0.30 mmol) were added TlPF6
(0.213 g, 0.61 mmol) and an excess of NaOMe (0.05 g, 0.9 mmol).
The reaction mixture was stirred at room temperature for 2 h, during
which time the solution color changed from yellow to purple. Filtration
was then accomplished through Celite, and a mixture of hexanes was
layered on top of the filtrate. Diffusion of the hexanes into the THF
solution produced large yellow crystals of 2a. Yield: 0.015 g (8%)
IR: 1630(m), 1610(m), 1585(s), 1500(s), 1370(m), 1280(s), 1220(m),
975(m), 830(w), 825(m), 520(w) cm-1
.
1H NMR (CDCl3): δ 9.50 (s,
C-H), 7.70, 7.55, 7.15 (m, arom), 7.8 (s, C-H), 6.95, 6.80, 6.20 (m,
arom).
Preparation of Pt2(µ-DTolF)2(η2-DTolF)2‚THF, 2b. This com-
pound was prepared in a manner similiar to that used for 2a, with PtCl2-
(HDTolF)2 as the starting material. X-ray-quality crystals were prepared
by slow diffusion of hexanes into a solution of 2b in THF. Yields
were only 10-15% due to a small amount of reduction to Pt metal
and incomplete reaction. IR: 1645(m), 1615(m), 1580(s), 1540(s),
1500(s), 1345(m, br), 1260(s), 1220(m), 960(m), 840(w), 815(m),
Preparation of trans-PtCl2(HDTolF)2, 1b. This compound was
prepared in a manner similar to that above, using the HDTolF ligand.
Yields were 70-75%. IR: 1640(s), 1600(s), 1580(m), 1515(s),
500(w) cm-1 1H NMR (CDCl3): δ 9.80 (s, C-H), 7.80, 7.85, 7.20
.
(m, arom), 7.6 (s, C-H), 6.85, 6,90, 6.50 (m, arom), 2.0, 2.1 (m, p-CH3).
Preparation of Pt2(µ-DPhF)4, 3a, and Pt2(µ-DTolF)4, 3b. A 15
mL MeOH solution of 2a (0.10 g, 0.085 mmol) was refluxed for 1 h.
(10) Roberts, R. M. J. Org. Chem. 1949, 14, 277.

500 Inorganic Chemistry, Vol. 35, No. 2, 1996
Cotton et al.
Table 3. Selected Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for Pt2(µ-DPhF)2(η2-DPhF)2, 2a,
and Pt2(µ-DTolF)2(η2-DTolF)2, 2b‚THF
2a
2b‚THF
x
y
z
U(eq)a
x
y
z
U(eq)a
Pt(1)
Pt(2)
N(1)
N(2)
N(3)
N(4)
N(5)
N(6)
N(7)
N(8)
C(1)
C(2)
C(3)
C(4)
2412(1)
3533(1)
2509(3)
933(3)
3833(4)
2609(4)
3662(3)
1895(3)
5114(3)
3692(4)
3050(4)
3589(5)
965(4)
1467(1)
2589(1)
3117(3)
1342(3)
1309(4)
-124(4)
4067(3)
2495(3)
2405(4)
1169(4)
4037(4)
224(5)
1690(1)
3310(1)
1461(3)
2331(3)
1110(3)
1766(3)
2673(3)
3534(3)
3237(3)
3890(3)
1951(3)
1357(4)
3051(3)
3643(3)
29(1)
29(1)
31(1)
31(1)
39(1)
39(1)
32(1)
30(1)
38(1)
38(1)
30(1)
44(1)
31(1)
43(1)
4374(1)
4293(1)
3829(3)
5348(3)
3553(3)
4723(3)
4218(3)
5417(3)
3220(3)
4149(3)
3943(4)
5705(4)
4026(4)
3437(4)
324(1)
2266(1)
222(4)
403(4)
292(4)
2300(1)
1931(1)
1446(2)
1938(2)
2839(2)
3212(2)
1117(2)
2003(2)
1979(2)
2696(2)
1025(3)
1923(3)
3335(3)
2512(3)
45(1)
41(1)
46(1)
47(1)
49(1)
51(1)
43(1)
42(1)
46(1)
46(1)
49(2)
46(2)
55(2)
49(2)
310(4)
1649(4)
2028(4)
2737(4)
2951(4)
815(5)
1198(5)
313(5)
1953(4)
1413(5)
4774(4)
3129(5)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
Table 4. Selected Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for Pt2(µ-DPhF)4, 3a, and
[Pt2(µ-DTolF)4](PF6), 4
3a
4
x
y
z
U(eq)a
x
y
z
U(eq)a
Pt(1)
N(1)
C(1)
0
4327(1)
4400(8)
5000
3882(7)
4024(8)
3507(12)
2849(10)
2707(7)
3223(8)
0
40(1)
46(7)
34(5)
32(4)
48(9)
102(20)
88(11)
67(8)
2500
2358(13)
2500
2500
1621(1)
1694(3)
2500
927(5)
56(4)
-700(6)
-617(7)
244(7)
1002(6)
-1475(6)
-2500
-1398(5)
-2500
40(1)
41(3)
48(2)
46(2)
54(2)
68(3)
101(3)
133(5)
93(3)
189(1)
70(1)
92(3)
104(2)
686(17)
934(9)
772(19)
934(9)
755(8)
785(11)
927(14)
1038(13)
1007(11)
865(9)
3907(4)
4321(5)
4433(5)
4266(4)
4757(6)
5449(7)
5600(9)
5109(6)
5950(1)
2500
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
P(1)
1304(7)
1996(8)
2457(6)
2226(9)
1534(10)
1074(7)
2082(4)
2460(1)
2157(5)
1505(8)
1139(9)
1402(6)
1170(1)
2500
47(6)
F(1)
F(2)
2500
3277(3)
2500
1723(3)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij tensor.
A small amount of platinum metal that formed during heating was
removed by filtration, and the MeOH was evaporated under vacuum.
The residue was redissolved in toluene (5 mL), and a layer of hexanes
(10 mL) was placed on the toluene solution. Slow mixing of the
hexanes at -20 °C produced small cubes of 3a. Yield: 0.030 g (30%).
IR: 1610(m), 1585(s), 1540(m), 1510(s), 1355(m), 1230(m), 1000(s),
were corrected for Lorentz and polarization effects and an absorption
correction was applied based on a series of ψ scans. The positions of
the platinum atoms for all of the structures were found in direct-methods
E maps using the structure solution program in SHELXS-86.12 The
positions of the remaining non-hydrogen atoms were found using
subsequent sets of least-squares refinement cycles followed by differ-
ence-Fourier syntheses using the program SHELXL-93.12 In each
model, except 3a, hydrogen atoms were introduced in idealized positions
for the calculation of structure factors.
970(m), 820(m), 540(w) cm-1
.
1H NMR (CDCl3): δ 8.35 (s, C-H),
7.20, 7.23 (m, arom). A similar route was used to prepare 3b. Both
isomerizations can also be done in CH3CN or THF.
Preparation of [Pt2(µ-DTolF)4](PF6), 4. A 10 mL CH2Cl2 solution
of 2b (0.220 g, 0.19 mmol) was treated with AgPF6 (0.05 g, 0.20 mmol).
The solution color immediately changed from yellow-orange to brown-
red. The reaction mixture was stirred for 8 h. The mixture was filtered
through Celite, and Et2O was layered onto the filtrate. Once the solvents
had mixed, the product that formed was collected by filtration (0.109
g, 42%). X-ray-quality crystals were grown after several weeks in the
freezer at -20 °C from a THF/hexanes solution. IR: 1585(s), 1560(s),
1480(m), 1415(m), 1325(s), 1295(m), 1270(s), 1200(s), 1165(s, br),
1000(m), 950(m), 850(m), 710(w), 540(m) cm-1. UV-vis (CH2Cl2):
λmax 442 nm (sh), 685 nm (vbr).
X-ray Crystallographic Procedures. Geometric and intensity data
were collected on the following diffractometers: Syntex P3 (1, 2a),
Enraf-Nonius CAD-4 (2b‚THF, 4), and Rigaku AFC5R (3). Detailed
procedures have previously been described.11
Each crystal was attached to the tip of a glass fiber. Reflections for
cell indexing were found by an automated search routine (CAD-4 and
Rigaku) or a 360° φ-rotation photograph (P3). In each case the cell
dimensions and Laue group were verified by axial photography. Data
For compound 3a, the structure was initially solved in the centro-
symmetric cubic space group Im3hm. The solution showed the structure
to be disordered in two ways: the molecule resided on 4mm site
symmetry, in which mirror planes contained the ligands. Because the
orientation of the phenyl groups in the ligand does not allow for this,
they were disordered on either side of the mirror plane. Additionally,
peaks corresponding to a second ligand orientation came up at 45°
relative to the first ligand set. It was found that moving to a
noncentrosymmetric space group (I432) with the same systematic
absences improved the refinement and removed the first disorder
because the molecule resided on a 422-symmetry site. This however
did not remove the second ligand orientation, and subsequently each
orientation was refined at 50% occupancy. To keep the number of
parameters low, only the Pt, N, and methine carbon atoms were refined
anisotropically, and the phenyl groups were constrained to be perfect
hexagons (C-C 1.395 Å). This gave a final agreement factor of R1
) 0.05. Other information pertinent to data collection and structure
solution is given in Table 1. Selected bond distances and angles are
(12) (a) Sheldrick, G. M. SHELXS-86. Institut fu¨r Anorganische Chemie,
Universita¨t Go¨ttingen, Germany, 1986. (b) Sheldrick, G. M. SHELXL-
93: A program for crystal structure refinement. Institut fu¨r Anorgan-
ische Chemie, Universita¨t Go¨ttingen, Germany, 1993.
(11) (a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18,
3558. (b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. J.
Organomet. Chem. 1973, 50, 227.

Platinum Complexes Containing Formamidine Ligands
Inorganic Chemistry, Vol. 35, No. 2, 1996 501
Table 5. Selected Bond Lengths (Å) and Angles (deg) for 1a
Table 8. Torsion Angles for Selected Ni, Pd, and Pt Amidine
Compounds
Pt(1)-Cl(1)
Pt(1)-Cl(2)
Pt(1)-N(1)
Pt(1)-N(3)
2.302(3)
2.319(3)
2.045(8)
2.026(9)
Pt(2)-Cl(3)
Pt(2)-Cl(4)
Pt(2)-N(5)
Pt(2)-N(7)
2.297(3)
2.289(3)
2.014(8)
2.033(8)
compound
Ni2(DTolF)4
Ni2(DTolF)4(BF4)
angle, deg
ref
16.8(3)
27.4(4)
15.1(6)
17(1)
19
6
6
6
6
7
7
Pd2(DTolF)4‚H2O
N(1)-Pt(1)-N(3)
N(3)-Pt(1)-Cl(1)
N(1)-Pt(1)-Cl(1)
N(3)-Pt(1)-Cl(2)
N(1)-Pt(1)-Cl(2)
Cl(1)-Pt(1)-Cl(2)
N(1)-C(1)-N(2)
N(3)-C(2)-N(4)
171.5(3)
90.4(3)
88.7(2)
90.2(3)
91.2(2)
176.8(1)
123.0(9)
125(1)
N(5)-Pt(2)-Cl(4)
N(7)-Pt(2)-Cl(4)
N(5)-Pt(2)-Cl(3)
N(7)-Pt(2)-Cl(3)
Cl(4)-Pt(2)-Cl(3)
N(6)-C(3)-N(5)
N(7)-C(4)-N(8)
N(5)-Pt(2)-N(7)
90.1(2)
89.7(3)
89.4(2)
91.0(2)
177.2(1)
122.9(9)
126(1)
Pd2(DTolF)4(PF6)
Pd2(µ-DPhBz)2(η2-DPhBz)2
Pd2(µ-DPhBz)4
14
Pt2(µ-DTolF)2(η2-DTolF)2‚THF
Pt2(µ-DPhF)2(η2-DPhF)2
Pt2(µ-DPhF)4
23.9(2)
9.3(2)
6.8(9)
11.6(9)
this work
this work
this work
this work
Pt2(µ-DTolF)4(PF6)
172.5(3)
Table 6. Selected Bond Lengths (Å) and Angles (deg) for 2a and
2b‚THF
2a
2b‚THF
Pt(1)-Pt(2)
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-N(3)
Pt(1)-N(4)
Pt(2)-N(5)
Pt(2)-N(6)
Pt(2)-N(7)
Pt(2)-N(8)
2.9128(9)
2.028(4)
2.043(4)
2.043(4)
2.060(4)
2.037(4)
2.012(4)
2.038(4)
2.039(4)
2.9376(7)
2.040(5)
2.032(5)
2.035(5)
2.066(5)
2.032(5)
2.023(5)
2.054(5)
2.042(5)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-N(3)
N(2)-Pt(1)-N(3)
N(1)-Pt(1)-N(4)
N(2)-Pt(1)-N(4)
N(3)-Pt(1)-N(4)
N(6)-Pt(2)-N(5)
N(6)-Pt(2)-N(8)
N(5)-Pt(2)-N(8)
N(6)-Pt(2)-N(7)
N(5)-Pt(2)-N(7)
N(8)-Pt(2)-N(7)
N(1)-C(1)-N(5)
N(2)-C(3)-N(6)
N(3)-C(2)-N(4)
N(7)-C(4)-N(8)
85.0(2)
86.8(2)
104.8(2)
170.3(2)
168.2(2)
106.8(2)
63.5(2)
106.0(2)
167.1(2)
168.2(2)
104.3(2)
63.0(2)
85.0(2)
86.3(2)
104.9(2)
170.2(2)
168.6(2)
106.3(2)
63.8(2)
126.5(4)
125.9(4)
110.1(5)
109.5(5)
105.0(2)
168.6(2)
167.7(2)
104.7(2)
64.0(2)
125.5(6)
126.1(6)
109.7(6)
110.2(6)
Figure 1. ORTEP diagram for one the two molecules found in the
asymmetric unit in the structure of PtCl2(HDPhF)2.
formamidine ligands bound to the Pt. These compounds are
soluble in a number of organic solvents (toluene, dichloro-
methane, benzene, THF, and CH3CN), and because they already
contain the correct ratio of formamidine ligands to metal atoms,
we considered them good starting materials for the formation
of dinuclear compounds of the type M2(DArF)4. The neutral
compounds 2a and 2b‚THF were prepared by the reactions
described in eq 1.
Table 7. Selected Bond Lengths (Å) and Angles (deg) for 3a and
4a
3a
4
Pt(1)-Pt(1)i
Pt(1)-N(1)
2.649(2)
2.04(2)
2.5304(6)
2.039(6)
N(1)ii-Pt(1)-N(1)
N(1)iii-Pt(1)-N(1)
N(1)-C(1)-N(1)iv
89.72(7)
171.9(9)
129(2)
89.81(6)
174.1(2)
126.4(8)
PtCl2(HDArF)2 + 2NaOMe + 2TlPF6 f
Pt2(µ-DArF)2(η2-DArF)2
+ 2TlCl +
a Symmetry transformations used to generate equivalent atoms are
Ar ) Ph (2a), Tol (2b)
2MeOH + 2NaPF6 (1)
as follows. 3a: (i) -z, -y + 1, -x; (ii) -z, y, x; (iii) -x, y, -z; (iv)
1
1
1
z, -y + 1, x. 4: (i) x, -y + /2, -z + /2; (ii) -y + /2, x, z; (iii) -x
1
1
1
1
+ /2, -y + /2, z; (iv) -x + /2, y, -z + /2.
The deprotonation of the coordinated formamidine ligand was
best accomplished with NaOMe. n-Butyllithium or other strong
bases played noninnocent roles, with reduction to platinum metal
usually being the end result. TlPF6 was used as a chloride
abstraction reagent; formation of the very insoluble TlCl served
as a driving force for the reaction. The structure of 2a is shown
in Figure 2. It consists of two bridging DPhF ligands cis to
each other and two DPhF ligands that are chelating different Pt
atoms. The structure of 2b contains a molecule of THF, but
the main features are very similar to those in 2a. The interligand
repulsion between the two chelating DPhF ligands is presumably
responsible for the long Pt-Pt separations of 2.913(1) and
2.938(1) Å in 2a and 2b‚THF, respectively. An indication of
the repulsion between the two chelating formamidines is given
by the dihedral angle of the Pt(N)4 planes. These angles are
23.3(2) and 30.1(1)° for 2a and 2b, respectively. They are
given in Table 6 for compound 1a, Table 7 for compounds 2a and
2b‚THF, and Table 8 for compounds 3a and 4.
Results and Discussion
In a previous paper,13 we reported the syntheses of several
M(II) chloride complexes with formamidine. Because of their
solubility properties, they have been used as starting materials
in the preparation of metal-metal-bonded compounds in
nonaqueous solution. We now report that the reaction of
K2PtCl4 with 2 equiv of the formamidine ligand produces the
mononuclear square-planar compounds 1a and 1b. The struc-
ture of 1a, shown in Figure 1, features two trans monodentate
(13) Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Matonic, J. H.; Murillo,
C. A. Polyhedron 1994, 13, 815.

502 Inorganic Chemistry, Vol. 35, No. 2, 1996
Cotton et al.
Figure 4. ORTEP diagram of Pt2(DTolF)4(PF6). The tolyl carbon atoms
are shown with spheres of arbitrary size.
Figure 2. ORTEP drawing of Pt2(µ-DPhF)2(η2-DPhF)2. Ellipsoids are
drawn at the 40% probability level.
Figure 5. Packing of Pt2(DTolF)4(PF6) along the c axis.
which is 2.622(3) Å. Compared to other PtII-PtII distances,
that in 3a is the shortest yet reported. A similar distance was
reported for Pt2(pyt)4, namely 2.680(2) Å;15 the distance in 3a
is 0.26 Å shorter than that reported in K4[Pt2(H2P2O5)4]‚2H2O
(2.925(1) Å)16 and 0.10 Å shorter than the Pt-Pt distance in
Pt2(CH3CS2)4 (2.767(1)Å).17
A one-electron oxidation of either 2b or 3b produces the
mixed-valence compound Pt2(DTolF)4(PF6), 4, as shown in eq
2.
Figure 3. ORTEP drawing of Pt2(µ-DPhF)4, showing one orientation
of the ligand. Ellipsoids are drawn at the 40% level.
Pt2(µ-DTolF)2(η2-DTolF)2 or
Pt2(µ-DTolF)4 + AgPF6 f Pt2(µ-DTolF)4(PF6) + Ag0V (2)
comparable to that in the Pd2(µ-DPhBz)2(η2-DPhBz)2 compound
(35°), which also has a similar metal-metal distance of 2.900-
(3) Å.6 An interesting difference between the two compounds,
2a and 2b, namely, the torsion angles, is shown in Table 4; the
twist is considerably less in 2a, being 9° as compared to 23° in
2b.
Compounds 2a and 2b are reaction intermediates; heating
them in weakly coordinating solvents forms the more stable
tetrabridged compounds 3a and 3b. The structure of 3a is
shown in Figure 3. This isomerization occurs to relieve strain
in the four-membered chelate ring. These chelating ligands have
N-Pt-N angles of 63.5(2) and 63.8(2)° for 2a and 63.0(2) and
64.0(2)° for 2b. Even though they are comparable to those in
the very stable tris-chelates,14 the effect of the nearby metal
atom makes the isomerization to a bridging ligand more
favorable. On going to the tetrabridged isomer, the metal-
metal separation decreases from 2.910(1) Å in 2a to 2.649(1)
Å in 3a. The distance is very similar to that in Pd2(DTolF)4,
The structure of 4 is similar to those of the analogous Ni
and Pd di-p-tolylformamidinato compounds. The Pt atoms,
along with the P atom, lie on a 4-fold axis, forcing the alternation
of anions and cations, as shown in Figure 5. The metal-metal
distance decreases from 2.649(1) Å in the neutral complex 3a
to 2.5034(6) Å in 4. This may be explained by the formation
of a partial metal-metal bond on going from a σ2π4δ2δ*2π*4σ*2
(BO ) 0) electronic configuration in 3a to σ2π4δ2δ*2π*4σ*1
(BO ) 0.5) in 4. In the palladium system, no such decrease is
seen, and in fact the metal-metal distance increases by 0.015(6)
Å to 2.637(6) Å. This is consistent with a ligand-based
(15) Umakoshi, K.; Kinoshita, I.; Ichimura, A.; Ooi, S. Inorg. Chem. 1987,
26, 3551.
(16) Filomena Dos Remedios Pinto, M. A.; Sadler, P. J.; Neidle, S.;
Sanderson, M. R.; Subbiah, A.; Kuroda, R. J. Chem Soc., Chem.
Commun. 1980, 13.
(14) Cotton F. A.; Daniels, L. M.; Maloney, D. J. Inorg. Chim. Acta, in
(17) Bellitto, C.; Flamini, A.; Piovesana, O.; Zanazzi, P. F. Inorg. Chem.
1980, 19, 3632.
press.

Platinum Complexes Containing Formamidine Ligands
Inorganic Chemistry, Vol. 35, No. 2, 1996 503
Supporting Information Available: Tables of crystallographic
data, atomic positional and displacement parameters, complete bond
distances and angles, anisotropic displacement parameters, and hydrogen
positional and thermal parameters for 1a, 2a, 2b‚THF, 3a, and 4 and
labeled ORTEP diagrams for 1a and 2b‚THF (50 pages). Ordering
information is given on any current masthead page.
oxidation. In the nickel system, the metal-metal distance does
in fact decrease on going from the neutral compound to the
oxidized compound, but this may be partly due to a large
increase in the torsion angle on going to the oxidized species.
Acknowledgment. The support of the Robert A. Welch
Foundation (Grant No. A-494) and the Department of Chem-
istry, University of Costa Rica, is gratefully acknowleged.
IC950741E