Elongation of the quadruple Cr(II)-Cr(II) bond induced by two PtMe2 moieties in the linearly aligned tetrametal system, CtMe2***Cr- Cr***PtMe2

Full text of DOI:10.1021/ja964052t

J. Am. Chem. Soc. 1997, 119, 4307-4308
4307
Elongation of the Quadruple CrII-CrII Bond
Induced by Two PtMe2 Moieties in the Linearly
Aligned Tetrametal System,
PtMe2‚‚‚Cr-Cr‚‚‚PtMe2
Kazushi Mashima,* Mitsuhiro Tanaka, and Kazuhide Tani
Department of Chemistry, Faculty of
Engineering Science, Osaka UniVersity
Toyonaka, Osaka 560, Japan
Akira Nakamura
Department of Macromolecular Science
Faculty of Science, Osaka UniVersity
Toyonaka, Osaka 560, Japan
Sadamu Takeda, Wasuke Mori, and Kizashi Yamaguchi
Department of Chemistry, Faculty of Science
Osaka UniVersity, Toyonaka, Osaka 560, Japan
Figure 1. A drawing of 1 with the labeling scheme. Phenyl groups
bound to the phosphorus atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg): Cr1-Cr2, 2.015(5); Cr1-O1, 1.93-
(2); Cr1-O3, 1.94(1); Cr2-O2, 2.00(2); Cr2-O4, 1.94(2); Cr1-N2,
2.12(2); Cr1-N4, 2.09(2); Cr2-N1, 2.04(2); Cr2-N3, 2.07(2); O1-
Cr1-O3, 167.8(7); N2-Cr1-N4, 179.0(8); O2-Cr2-O4, 167.4(7);
N1-Cr2-N3, 177.1(8).
ReceiVed NoVember 25, 1996
Dinuclear compounds bearing the multiple metal-metal bond,
to date, have been prepared and characterized crystallographi-
cally, and the nature of their multiple bond has been theoretically
and spectroscopically investigated.1,2 We have recently reported
the new reaction of the dinuclear dimolybdenum(II) complex
with two palladium(I) species at both of the axial positions of
the Mo2 core to form metal-metal-bonded tetranuclear com-
plexes such as Mo2Pd2Cl2(pyphos)4 (pyphos ) 6-(diphenylphos-
phino)-2-pyridonate).3,4 This reaction involves the coordination
of two Pd(II) metals at the axial positions of Mo2 and is followed
by the reduction of Pd(II) to Pd(I) to result in the formation of
the Mo-Pd σ-bonds. For the extension of our continuous
interest in this area, we have focused on the axial interaction of
group 10 metal in +2 oxidation state with the quadruple metal-
metal bond of a different metal. Here, we report the axial
interaction of the quadruple Cr-Cr bond with two platinum-
(II) atoms surrounded by pyphos ligands, i.e., d-d dative bond,
whereas the quadruple bonded Cr-Cr core is significantly
elongated by the axial interaction with organic π-systems, (i.e.,
d-p dative bond).5 The strength of this interaction delicately
depends on the nature of the ligands on the Pt(II) atoms.
The starting dinuclear complex Cr2(pyphos)4 (1) was prepared
in 67% yield by the reaction of sodium salt of pyphos with
Cr2(OAc)4 in ethanol.6 The 31P{1H} NMR spectrum of 1
exhibited a broad singlet at δ -4.3 (fwhm ) 660 Hz), indicating
that all four phosphorus atoms are magnetically equivalent and
are free from the coordination of transition metals. Figure 17
shows that 1 is a dinuclear complex comprised of a quadruple
Cr-Cr bond (2.015(5) Å), a value which is longer than those
of Cr2(chp)4 (chp ) 6-chloro-2-pyridonate) (1.955(2) Å)8 and
Cr2(mhp)4 (mhp ) 6-methyl-2-pyridonate) (1.889(1) Å),9 while
it is shorter than that (2.288(2) Å) of Cr2(OAc)4.10 At both
sides of the Cr2 core, 1 has two trans-arranged phosphine atoms
capable of coordinating to transition metals.
Treatment of 1 with 2 equiv of PtMe2(cod) in THF afforded
2 in quantitative yield.11 During the reaction course, the
geometry of the four pyphos ligands changed from trans-fashion
to cis-one, which is attributed to the trans-effect of the methyl
group on the platinum. Complex 2 is also prepared by the
treatment of PtMe2(pyphos-H)2 (pyphos-H ) 6-(diphenylphos-
phino)-2-pyridone) with Cr2(OAc)4 in the presence of NaOMe
in THF. In sharp contrast to 1, compound 2 exhibited no signal
in the 31P{1H} NMR spectrum due to the formation of the
paramagnetic species, which was produced by the partial
disruption of the quadruple Cr-Cr bond by the interaction with
the two PtMe2 cores. The similar partial contribution of the
open-shell (σ)2(π)4(δ)1(δ*)1 configuration on the Cr-Cr bond
was reported for the Cr2 species coordinated by organic ligands
at both of the axial positions.5 The temperature-dependence of
the paramagnetism has been used to estimate the separation
(1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
John Wiley & Sons: New York, 1988.
(2) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: New York, 1993.
(3) Mashima, K.; Nakano, H.; Nakamura, A. J. Am. Chem. Soc. 1993,
115, 11632.
(4) Mashima, K.; Nakano, H.; Nakamura, A. J. Am. Chem. Soc. 1996,
118, 9083.
(5) Cotton, F. A.; Chen, H.; Daniels, L. M.; Feng, X. J. Am. Chem. Soc.
1992, 114, 8980.
(6) For 1: mp 159-160 °C; 1H NMR (CDCl3) δ 5.94 (1H, d, J ) 8
Hz), 6.23 (1H, d, J ) 7Hz), 7.03 (1H, t, J ) 7 Hz), 7.24-7.33 (10H, m);
31P NMR (CDCl3) δ -4.3 (bs, fwhm ) 660 Hz); UV-vis (CH2Cl2) λmax
) 275 (ꢀ, 3.7 × 104), 330 (ꢀ, 2.1 × 104), 455 (ꢀ, 4.7 × 102); FAB MS m/z
) 1217 (MH+). Anal. Calcd. for C60H52Cr2N4O4P4‚CH2Cl2: C, 63.65; H,
4.18; N, 4.30. Found: C, 63.87; H, 4.12; N, 4.36.
(8) Cotton, F. A.; Ilsley, W. H.; Kaim, W. Inorg. Chem. 1980, 19, 1453.
(9) Cotton, F. A.; Fanwick, P. E.; Niswander, R. H.; Sekutowski, J. C.
J. Am. Chem. Soc. 1978, 100, 4725.
(10) Cotton, F. A.; Rice, C. E.; Rice, G. W. J. Am. Chem. Soc. 1977,
99, 4704.
(7) Crystal data for 1: fw ) 1302.00, monoclinic space group P21/a, a
) 18.645(5) Å, b ) 17.896(3) Å, c ) 19.338(3) Å, â ) 104.12(2)°, V )
6257(2) Å3, Z ) 4, dcalcd ) 1.382, µ ) 5.88 cm-1, no. of unique data )
9983, no. of observations with I > 3σ(I) ) 2251, goodness of fit ) 1.85,
R ) 0.072, Rw ) 0.082.
(11) For 2: mp > 300 °C; UV-vis (CH2Cl2) λmax ) 303 (ꢀ, 1.7 × 104),
386 nm (ꢀ, 8.9 × 103); ESI MS m/z ) 1668.1 (MH+). Anal. Calcd. for
C72H64Cr2N4O4P4Pt2‚2(C4H10O): C, 52.92; H, 4.66; N, 3.09. Found: C,
53.17; H, 4.72; N, 3.05.
S0002-7863(96)04052-8 CCC: $14.00 © 1997 American Chemical Society

4308 J. Am. Chem. Soc., Vol. 119, No. 18, 1997
Communications to the Editor
between the singlet ground state and the lowest triplet state (the
open-shell (σ)2(π)4(δ)1(δ*)1 configuration on the Cr-Cr
bond).5,12,13 We measured the solid state 31P{1H}-MAS (magic
angle spinning) NMR spectra for 1 and 2 between 138 and 350
K at the resonance frequency of 121.5 MHz to obtain the J
values for 1 (-340 cm-1) and 2 (∼-10 cm-1).14 By measuring
the temperature-dependence of the magnetic susceptibilities of
1 and 2 by the SQUID magnetometer, we obtain similar J values
of -340 and -29 cm-1, respectively.14 The singlet-triplet
separation of 1 is thus comparable to those found for Cr2(O2-
CR)4L2,5 while that of 2 is small and is consistent with the highly
paramagnetic character of 2 observed on its 31P{1H} NMR
spectrum. Consequently, the PtMe2 core interacted significantly
with the Cr2 core to induce the elongation of the Cr-Cr bond,
which was revealed by the crystal structure of 2 (vide infra).
On the other hand, we observed the complete opposite effect
of the chloro ligand on the platinum atom. The 31P{1H} NMR
spectrum of the chloro derivative 3,15 which was synthesized
by the reaction of 1 with 2 equiv of PtCl2(cod) (cod ) 1,5-
cyclooctadiene), displayed a sharp singlet at δ 12.6 (JPPt ) 3590
Hz), indicating that 3 is the typical diamagnetic complex. The
quadruple Cr-Cr bond is highly sensitive to the electron-
donating property of ligand on the Pt(II) atom.
The bonding nature of 2 was verified by the X-ray crystal-
lographic study.16 Figure 2 shows the ORTEP diagram of 2, in
which the Cr-Cr distance (2.389(9) Å) is longer by 0.374 Å
than that of 1 and is comparable to those (2.329(2)-2.396(2)
Å) of Cr2(O2CCH3)4L2 where L ) H2O,17 Py,18 MeOH,5 and
MeCN.5 This is the first example where transition metals act
as axial donors to the quadruple Cr-Cr bond resulting in
elongation of the bond. The donor properties of the metal atoms
in square planar complexes have been recognized.19-24 The
interatomic distances (2.806(9) and 2.811(3) Å) between the
platinum atom and the chromium atom indicate that they are
the dative PtfCr bonds, though they are longer than the sum
of Cr and Pt atomic radii (2.6 Å).25 Similar dative bonding
between platinum and palladium (PtfPd bond) has been
reported so far.22-24 It is noteworthy that the two square planes
comprised of the two methyl groups and the two phosphorus
atoms in a cis-arrangement are parallel to each other and
perpendicular to the vector of the Cr-Cr bond. The platinum
atom shifted close to the chromium atom by 0.03 Å from the
normal position in the square planar geometry. The angles of
Cr-Pt-P [83.2(3), 86.5(2), 85.38(6), and 83.3(1)°] and Cr-
Pt-Me [92(1), 97(2), 97(2), and 96.08(8)°] are acutely and
obtusely deviated form 90°, respectively.
Figure 2. Drawings of 2 with the labeling scheme: a perspective
perpendicular to the Cr-Cr vector (top) and a view down the metal-
metal bond (bottom). Phenyl groups bound to the phosphorus atoms
are omitted for clarity (top). Selected bond distances (Å) and angles
(deg): Cr1-Cr2, 2.389(9); Cr1-Pt1, 2.806(9); Cr2-Pt2, 2.811(3);
Pt1-C81, 2.12(5); Pt2-C82, 2.088(9); Pt1-P3, 2.282(7); Pt1-P4,
2.304(4); Pt2-P1, 2.313(3); Pt2-P2, 2.261(5) Cr1-O1, 1.96(2); Cr1-
O2, 1.95(3); Cr2-O3, 1.99(3); Cr2-O4, 1.98(3); Cr1-N3, 2.14(5);
Cr1-N4, 2.145(2); Cr2-N1, 2.16(3); Cr2-N2, 2.12(2); Pt1-Cr1-
Cr2, 178.87(3); Cr1-Cr2-Pt2, 178.1(2); Cr1-Pt1-C81, 92(1); Cr1-
Pt1-C82, 97(2); Cr1-Pt1-P3, 83.2(3); Cr1-Pt1-P4, 86.5(2); Cr2-
Pt2-C91, 97(1); Cr2-Pt2-C92, 96.08(8); Cr2-Pt2-P1, 85.38(6);
Cr2-Pt2-P2, 83.3(1); O1-Cr1-N3, 176(1); O2-Cr1-N4, 176.1(9);
O3-Cr2-N1, 175(15); O4-Cr2-N2, 176(1).
Complex 3 has two PtCl2 moieties, and thereby the orbital
2
interaction between the filled dz -orbital on PtCl2 and σ-orbital
of Cr2 core induces the closed-shell repulsion, although the
vacant pz-orbital interacts attractively with the filled σ orbital
on Cr2 to stabilize the bonding.26,27 In partial support of this
repulsive effect, the X-ray analysis of the molybdenum anologue
of 3, Mo2Pt2Cl4(pyphos)4 (4),3,4 showed that the two platinum
atoms at both of the axial positions of the Mo2 core slightly
deviated outside from the normal position in the square planar
geometry. Such 2-fold ligand effect of the axial metal center
is the first case observed for influencing the metal-metal bond.
In summary, we demonstrated that the two Pt atoms can
interact with the Cr2 moiety attractively or repulsively, depend-
ing upon the nature of the ligands on the platinum atom, i.e.,
methyl or chloro.
(12) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. J. Am. Chem.
Soc. 1992, 114, 4915.
(13) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. Inorg. Chem.
1993, 32, 2104.
(14) Experimental details are given in the Supporting Information.
(15) For 3: mp > 300 °C; 1H NMR (CDCl3) δ 6.50 (1H, br), 6.80 (1H,
br), 7.27-7.57 (11H, m); 31P NMR (CDCl3) δ 12.6 (JPPt ) 3590 Hz); UV-
vis (CH2Cl2) λmax ) 275 (ꢀ, 2.5 × 104), 310 (ꢀ, 2.3 × 104); FAB MS m/z
) 1607 (M+ - 4Cl). Anal. Calcd. for C68H52Cl4Cr2N4O4P4Pt2‚2(CH2Cl2):
C, 43.82; H, 2.94; N, 2.92. Found: C, 43.62; H, 3.80; N, 2.86.
(16) Crystal data for 2: fw ) 1571.30, monoclinic space group P21/n,
a ) 15.58(1) Å, b ) 17.534(4) Å, c ) 28.455(7) Å, â ) 99.74(3)°, V )
7663(4) Å3, Z ) 4, dcalcd ) 1.362, µ ) 40.25 cm-1, no. of unique data )
8499, no. of observations with I > 3σ(I) ) 8097, goodness of fit ) 2.75,
R ) 0.064, Rw ) 0.064.
(17) Kranz, M.; Witkowska, A. Inorg. Synth. 1960, 6, 144.
(18) Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1980, 19, 328.
(19) Balch, A. L.; Nagle, J. K.; Olmstead, M. M.; Reedy, P. E., Jr. J.
Am. Chem. Soc. 1987, 109, 4123.
(20) Balch, A. L.; Olmstead, M. M.; Oram, D. E.; Reedy, P. E., Jr.;
Reimer, S. H. J. Am. Chem. Soc. 1989, 111, 4021.
Supporting Information Available: Experimental details for the
preparation of 1-3, crystallographic data, final positional parameters
and final thermal parameters for 1 and 2, and figures giving additional
atom labeling for 1 and 2 (25 pages). See any current masthead page
for ordering and Internet access instructions.
(21) Balch, A. L.; Catalano, V. J.; Chatfield, M. A.; Nagle, J. K.;
Olmstead, M. M.; Reedy, P. E., Jr. J. Am. Chem. Soc. 1991, 113, 1252.
(22) Krumm, M.; Lippert, B.; Randaccio, L.; Zangrando, E. J. Am. Chem.
Soc. 1991, 113, 5129.
(23) Krumm, M.; Zangrando, E.; Randaccio, L.; Menzer, S.; Lippert, B.
Inorg. Chem. 1993, 32, 700.
JA964052T
(26) Aullo´n, G.; Alvarez, S. Inorg. Chem. 1996, 35, 3137. Aullo´n, G.;
Alemany, P.; Alvarez, S. Inorg. Chem. 1996, 35, 5061.
(24) Mealli, C.; Pichierri, F.; Randaccio, L.; Zangrando, E.; Krumm, M.;
Holtenrich, D.; Lippert, B. Inorg. Chem. 1995, 34, 3418.
(25) Emsley, J. The Elements; Oxford University Press: New York, 1989.
(27) Smith, D. C.; Miskowski, V. M.; Mason, W. R.; Gray, H. B. J.
Am. Chem. Soc. 1990, 112, 3759. Connick, W. B.; Henling, L. M.; Marsh,
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