Inorg. Chem. 1986, 25, 957-964 The 31P(1HJ N M R spectra of the nitrile complexes appear as sharp singlets at 129.6-138.8 ppm, indicating the presence of four equivalent phosphorus nuclei, as expected for the trans arrangement of the carbonyl and nitrile ligands in the complexes. Acetonitrile reacts with the diazene complexes to give yellowwhite products whose elemental analyses and conductivity data agree with those of the [Fe(CO)(CH3CN)L4]*+derivative. Although the infrared spectra of both solid and solution samples do not show any absorption in the C N stretching region$l the 'H N M R spectra in hexadeuterioacetone or CD2CI2, besides the phosphite and BPh4 signals, show a broad resonance near 6 2.20 attributable to the methyl protons of the acetonitrile ligand. Furthermore, in CD3CN solvent ligand exchange is observed with the appearance, after 10-20 min, of the singlet at S 1.96 of the free acetonitrile ligand, thus confirming the proposed formulation for the compound. Adiponitrile also reacts with the starting Fe(I1) complexes but only gives the monosubstituted dication [Fe(CO)(NC(CH2)4CN)L4]2+at all 1igand:complex ratios. The infrared spectrum shows two v(CN) bands at 2296 and 2248 cm-I, attributable to the coordinated and free C N group, respectively. An increase of 48 cm-l as compared to the free ligand value is observed, like that found for the aryl cyanide complexes, and suggests similar bonding properties. Finally, the 31P('H)N M R spectra also show that the alkyl cyanide derivatives have the carbonyl and RCN ligands in the trans position. (41) The low intensity or even absence of v(CN) in acetonitrile complexes is a feature that has already been observed: Rouschias, G.; Wilkinson, G. J . Chem. SOC.A 1967, 993.
957
Acknowledgment. The financial support of the MPI and CNR, Rome, is gratefully acknowledged. We thank Daniela Baldan for technical assistance. Registry No. trans-[FeH(CO)(P(OMe)3}4]BPh4, 1001 12-66-1; trans-[FeH(CO){P(OEt),l4] BPh,, 95738-79-7; trans-[FeH(CO){PhP(OEt),},] BPh,, 100112-68-3; trans-[FeH(CO){P(OEt),],] Br, 1001 12(BPh,),, 95738-8 1- 1; 69-4; [Fe(4-CH,C6H,N=NH)(CO){P(OEt),)4] [Fe(4-C1C6H,N=NH)(CO){P(OEt)3)4] (BPh,),, 1001 12-71-8; [Fe( C O ) ((CH3)2CO)(P(OMe)3)4] (BPh,),, 1001 12-73-0; [Fe(CO)I(CH3)2CO)(P(OEt)3141(BPh4)2, 95738-89-9; [Fe(CO){(CHd(C6Hs)CO)(P(OEt),J,] (BPh,),, 100112-75-2; [ Fe(CO)((CD,),CO}{P(OEt),},](BPh,),, 1001 12-77-4; [Fe(CO)((CH,)(C2H,)CO](P(OEt)3]4](BPh4)2, 100112-79-6; [Fe(CO)((C5H,o)CO)(P(OEt)~)4](BPh4)2, 100112-81-0; [Fe(H20)(CO){P(OEt),),](BPh,),, 100112-83-2; [FeCI(CO)(P(OMe)Ja]BPh4, 100112-85-4; [FeCI(CO)(P(OEt),),]BPh,, 1001 12-87-6; [FeCI(CO){PhP(OEt)2]BPh,, 1001 12-89-8; cis- [FeBr(CO)(P(OEt),],]Br, 100227-13-2; 100112-90-1; [Fe(CO)(4-CH3C6H4NC)(P(OEt)3]4]2+, [Fe(CO)(C,H5NC){P(OEt)3)4]2+, 100112-91-2; [ F e ( C 0 ) ( 4 C H 3 0 C 6 H 4 N C ) ( P ( O E t ) , ) 4 ] 2 + , 1001 12-92-3; ' [ F e ( C 0 ) ( 4 N 0 2 C 6 H 4 N C ) ( P ( o E t ) , ) 4 ] 2 + , 1001 12-93-4; [ F e ( C 0 ) ( 2 , 6 (CH,)2C6H,NC){P(oEt),)4]2+, 1001 12-94-5; [ F e ( C 0 ) ( 4 CH3C6H4CN)(P(OMe)3)4]2+, 100112-95-6; [Fe(C0)(4-CH,C6H4CN)100227- 14-3; [Fe(CO)(C6HsCN){P(OEt)3}4]2+, 100112(P(OEt),),] 100112-97-8; [Fe(CO)96-7; [Fe(CO)(2-CH,C6H4CN)(P(OEt),)4]2*, (CH,CN)(P(OEt),)4]2+, 1001 12-98-9; [Fe(CO)(NC(CH,),CN)(P(OEt),]4]2+, IO01 12-99-0; [Fe(CO)2(P(OEt)3]4]2+,100227-15-4; [Fe(C0)(4-CH,C6H4NC)(P(OMe),)4]2+, 1001 13-00-6; 4-CH,C6H4N2+BFC, 459-44-9; 4-C1C6H4N2+BF;, 673-41-6; Zn, 7440-66-6.
'+,
Supplementary Material Available: Tables of structure factors and thermal parameters and a full list of bond lengths, bond angles, and torsion angles (20 pages). Ordering information is given on any current masthead page.
Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
Synthesis, Structure, and 195PtNMR Solution Studies of a Reactive Binuclear NO2)(NO3)](N03)2*0.5H20 Platinum(111) Complex: [Pt2(en) 2( C5H4N0)2( Thomas V. O'Halloran, Michael M. Roberts, and Stephen J. Lippard* Received September 25, 1985 Oxidation of the a-pyridonate- (C,H,NO-) bridged diplatinum(I1) complex [Pt2(en)2(C5H4N0)2]2+ (2), where en = ethylenediamine, in nitric acid leads to the formation of a metal-metal-bonded diplatinum(II1) complex, [Pt2(en)2(C5H4NO)2(N02)(NO,)],' (1). In the structure of 1, two (ethylenediamine)diplatinum(III) units are linked by a metal-metal single bond and two a-pyridonate ligands bridging in a head-to-head fashion. The 2.6382 (6) A Pt-Pt bond axis is capped by nitrite, Pt-N = 2.109 ( I ) A, and nitrate, Pt-0 = 2.307 (9) A, ligands. These Pt-axial ligand distances nicely illustrate a structural trans influence mediated by the metal-metal bond. The coordinated nitrite group is disordered over two positions about the Pt-N axis and is bonded to the platinum atom having two N - and two 0-donor ligands. As shown by lssPt N M R spectroscopy, 1 has a binuclear, metal-metal-bonded structure in freshly prepared aqueous or dimethylformamide (DMF) solutions. In D M F the platinum resonances occur at +541 (PtN,O,) and -1 141 (PtN,O) ppm from [PtCI6l2-, with J(195Pt-195Pt)= 6850 Hz. Decomposition of 1 to form 2 occurs slowly in solution, as revealed by 195PtN M R spectroscopy. Reductive elimination of nitrite (NO,-) to form the nitronium (NO,') ion is postulated to explain this decomposition, since nitrobenzene or p-nitrophenol is generated in aqueous solutions of 1 containing tetraphenylborate or phenol, respectively. The nitrate salt of compound 1, [Pt(en),(C5H,NO),(N02)(N03)](N03)2.0.5H20, crystallizes in the monoclinic space group P 2 , / c with a = 17.934 (1) A,b = 11.491 ( I ) A, c = 13.507 (1) A, (3 = 108.84 (1)O, V = 2634 A3,2 = 4, pcalcd= 2.369 g c m 3 , and pobd = 2.379 (5) g cmW3.The structure was refined to R = 0.035 using 2686 diffractometer data with F, > 4a(F,).
Introduction Until recently, there were few examples of complexes containing platinum in the +3 oxidation state. Binuclear species comprise the vast majority of structurally characterized platinum( 111) complexes and therefore provide the best opportunity to probe the structural properties and chemical reactivity of this oxidation state.' Platinum-platinum single-bond formation is a natural consequence of the d7-d7electronic configuration in these binuclear complexes, all of which contain two or more bridging ligands. (1) For a review see: OHalloran, T. V.;Lippard, S. J. Isr. J . Chem. 1985, 25, 130-137.
0020-1669/86/1325-0957$01.50/0
Binuclear cis-diammineplatinum(II1) complexes, bridged by two 1-methyluracilate2or two a-pyridonate3 ligands in either a head-to-head or head-to-tail manner, have been previously described. They can be prepared by chemical or electrochemical oxidation of the corresponding platinum(I1) dimers3 or tetranuclear (2) Lippert, B.; Schollhorn, H.; Thewalt, U. Z . Naturforsch., 8: Anorg. Chem. Urg. Chem. 1983, 388, 1441-1445. (3) Hollis, L. S.; Lippard, S. J. J. Am. Chem. SOC.1983, 105, 3494-3503. (4) (a) For a review see: Lippard, S.J. Science (Washington, D.C.) 1982, 218, 1075-1082. (b) Barton, J. K.; Szalda, D. J.; Rabinowitz, H. N.; Waszczak, J. W.; Lippard, S . J. J. Am. Chem. SOC. 1979, 101, 1434-1441.
0 1986 American Chemical Society
958 Inorganic Chemistry, Vol. 25, No. 7, 1986 0
a-
pyridone
C5H4NOH
0
l
CH3
1 - methyluracil 1 - MeUH
platinum blue (Pt2.25)cations, and undergo quasi-reversible two-electron redox reaction^.^ The Pt-Pt bond distances in the head-to-tail complexes vary as a function of the axial ligand in a manner consistent with the established structural trans influence series in mononuclear platinum complexes.6 Electronic and steric effects associated with the in-plane, or equatorial, ligands also influence the Pt-Pt distance in these [Pt"'], complexes, but the extent of these interactions has not been investigated.6b In this paper we examine the effects of steric interactions involving the equatorial ligands on the structure, reactivity, and metal-metal bonding for the a-pyridonate-bridged diplatinum( 111) cations [Pt2(en)2(CsH4NO)2X,]2+(en = ethylenediamine, X = anion). This system was chosen for study because it already has provided a clear example of the influence of such steric interactions on structure and reactivity for the related binuclear platinum(I1) complex [Pt2(en)2(CSH4N0)2]2(N03)4.7~* The X-ray crystal structure of this binuclear (ethylenediamine)platinum(II) cation revealed pronounced torsional strain and an increased metal-metal distance relative to the cis-diammineplatinum( 11) analogue. These distortions alter the lability of the platinum(I1)-a-pyridonate bonds, resulting in a dramatic enhancement in the rate of linkage isomerization of the bridging a-pyridonate ligand.* The distortions observed in the platinum(I1) ethylenediamine dimers initially led us to question whether they could be oxidized to metal-metalbonded [Pt2,25]4and [Ptl"], analogues of the cis-diammineplatinum(II)/a-pyridone system, since such clusters require much shorter metal-metal distances. A preliminary study suggested that harsh conditions were required to oxidize [Pt2(en),(C5H4NO),12+compared to the [Pt2(NH3)4(CSH4N0)2]2+ cation, and that the blue [Pt2,25]4 species was observed only as a transient in this r e a ~ t i o n .Subsequently, ~ however, we have succeeded in synthesizing and crystallizing the binuclear platinum(II1) ethylenediamine complex and report here the structure and chemical reactivity of this interesting species. We have also been able to react this new Pt(II1) complex with the known Pt(I1) dimer under mild conditions to prepare and characterize the (ethylenediamine)platinum a-pyridone blue, the structure and properties of which are described e l ~ e w h e r e . ~ Experimental Section Preparation of Compounds. The starting material, [Pt,(en),(C5H4NO)2]z(N03)4, was obtained as described previously' and recrystallized from water. Dimethylformamide (DMF) was distilled over BaO and stored under N,. All other reagents were used as obtained from commercial sources. [Pt2(en),(CsH4NO)2(N02)(N03)](N03)2.0.5H20 (l), Head-to-Head Isomer. This compound was obtained by oxidizing [Pt,(en),(CSH4N0)2](N03)2 ( 2 ) with either nitric acid (low yield) or with 1 equiv of NaNO, in nitric acid (high yield). Method A: Nitric Acid Oxidation. A slurry of 100 mg (0.12 mmol) of finely ground 2 in 2 mL of 4 M H N 0 3 was stirred without heating for 30 min. Four additional 20-mg portions of 2 were added over a 2-h interval. The solution gradually changed color from pale yellow to olive (a) Hollis, L. S.; Lippard, S . J. J . Am. Chem. SOC.1981, 103, 6761-6763. (b) Hollis, L. S . ; Lippard, S. J. Inorg. Chem. 1983, 22, 2605-2614. (a) Hollis, L. S . ; Lippard, S. J. Inorg. Chem. 1982, 21, 2116-21 17. (b) Hollis, L. S.; Roberts, M. M.; Lippard, S . J. Inorg. Chem. 1983, 22, 3637-3644. (c) See also: Alexander, K. A.; Bryan, S . A.; Fronczek, F. R.; Fultz, W. C.; Rheingold, A. L.; Roundhill, D. M.; Stein, P.; Watkins, S. F. Inorg. Chem. 1985, 24, 2803-2808. Hollis, 1. S.; Lippard, S . J. Inorg. Chem. 1983, 22, 2600-2604. OHalloran, T. V.; Lippard, S . J. J . Am. Chem. SOC.1983, 105, 3341-3342. (a) O'Halloran, T. V.; Roberts, M. M.; Lippard, S. J. J . Am. Chem. SOC.1984, 106, 6427-6428; and manuscript in preparation.
O'Halloran et al. green, and a small fraction of 2 remained undissolved. Upon addition of the final 20" portion, the solution changed from green to orange-red and, with continued stirring, most of the starting material dissolved. The solution was filtered and allowed to evaporate in air overnight. A 10-mg crop (5% yield) of well-formed red crystals was collected. Anal. Calcd for Pt2Cl,H2sNioOi,,,(1): Pt, 41.5; C, 17.90; H , 2.68; N , 14.91; 0, 22.99. Found: Pt, 40.3; C , 17.98; H , 2.73; N , 14.56. Method B: Sodium Nitrite/Nitric Acid Oxidation. The nitric acid concentration of a freshly prepared aqueous solution of 2 (0.36 g/15 mL) was raised to 3 M by the addition of 3 mL of concentrated nitric acid. Solid N a N 0 , (30 mg) was immediately added, and the solution rapidly changed from pale yellow to olive green to orange-red. The mixture was filtered, and crystals began forming in the filtrate after 2 h at room temperature. The solution was allowed to evaporate in air overnight, and 0.27 g (66%) of brick red crystals were collected and washed with 5 mL of H 2 0 . Typical yields range from 50 to 70%. Anal. Calcd: See above. Found:Io C, 17.82 (17.89); H , 2.54 (2.71); N , 14.91 (14.82); 0, 23.30 (23.40). Infrared Absorptions. The I R spectra of 1 in Nujol mulls or KBr pellets revealed the following peaks, which were not observed in the spectrum of 2: 1490 (sh), 1445 (w), 1410 (m), 1290 (s), 1260 (s), 1145 (m), 1010 (m), 805 (m), 635 (w) cm-I. Nitration Reactions. Nitrobenzene, released when sodium tetraphenylborate was added to an aqueous or nitric acid solution of 1, was initially identified by its characteristic odor and was subsequently verified by gas chromatography. A 30-pL aliquot of an aqueous solution containing 15 @molof sodium tetraphenylborate was added to 0.4 mL of a solution containing 3.2 pmol of 1, and this mixture was extracted with an equal volume of ether. The extract was injected into a Perkin-Elmer Sigma 3 gas chromatograph equipped with a 0.25 in. X 10 ft 4.1% Carbowax/Chromabsorb G column maintained at 180 OC. Three peaks were observed with retention times of 3.2, 3.5, and 6.6 min, respectively. The 3.2-min peak is, a t present, unidentified. Co-injection with an authentic sample of nitrobenzene established the 3.5-min peak to be nitrobenzene. Injection of authentic samples of phenol in ether revealed the 6.6-min peak to be phenol. The phenol apparently arises from hydrolysis of tetraphenylborate in a reaction independent of the decomposition of 1, since ether extracts of 0.5 M HNO, or aqueous solutions of tetraphenylborate also revealed the 6.6-min peak characteristic of phenol in their gas chromatograms. The odor of nitrobenzene was also detected when sodium tetraphenylborate was added to a D M F solution of 1. Phenol was added to either a neutral or an acidic solution of 1, under a layer of ether. After 10 min the ether layer was separated and extracted with an aliquot of 1 M NaOH solution, which exhibited a strong absorbance maximum a t 400 nm, characteristic of deprotonated 4nitrophenol." Control reactions using nitrite, nitrate, or complex 2 alone in neutral solutions showed no formation of 4-nitrophenol, judging by the lack of a 400-nm absorption band. Physical Measurements. Magnetic susceptibility measurements were made on an S.H.E. Corp. SQUID type variable-field susceptometer operating at 10 kG. For typical procedures used in our laboratory see ref 12. I9'Pt N M R spectra were acquired on a Bruker WM-250 spectrometer operating at 53.6 MHz. The 90° pulse in a fixed-frequency, 10-mm probe was 25 ps. Infrared measurements were made with a Beckman Acculab 10 instrument. Collection and Reduction of X-ray Data for [Pt(en),(C,H,NO),(N0,)(N03)](N0,),~0.5H,0 (1). The orange crystal used in the X-ray diffraction study was a parallelepiped of approximate dimensions 0.05 mm X 0.08 mm X 0.12 mm and was bounded by the faces (001) and (1 101. The quality of the crystal, examined by taking open-counter o scans of several low-angle reflections, was found to be acceptable ( A G J ] , ~ = 0.15O). Study on the diffractometer showed the crystal to belong to the monoclinic system, space group PZ,/r (C,, No. 14).13 Data collection and reduction proceeded by methods standard in our l a b o r a t ~ r y , ' ~ the details of which are presented in Table I. Structure Solution and Refinement. The positions of the platinum atoms were found by using standard Patterson and direct methods.15 All The first number represents analytical results for a ground sample dried in air for 2 days. The numbers in parentheses report results for a portion of the same sample after it was dried in a vacuum desiccator for 24 h. The agreement between these numbers suggests the lattice water was not removed by vacuum desiccation at room temperature. Roberts, J. D.; Caserio, M. C. "Basic Principles of Organic Chemistry"; Benjamin: Menlo Park, CA, 1977; p 1402. Armstrong, W. H.; Spool, A,; Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S . J. J . Am. Chem. SOC.1984, 106, 3653-3667. 'International Tables for X-ray Crystallography", 3rd ed.; Kynoch Press: Birmingham, England, 1973; Vol. I, p 99. Silverman, L. D.; Dewan, J. C.; Giandomenico, C. M.; Lippard, S. J. Inorg. Chem. 1980, 19, 3379-3383.
Inorganic Chemistry, Vol. 25, No. 7, 1986 959
A Reactive Binuclear Pt(II1) Complex
(A) Crystal Parameters' at 25 OC space group = P 2 , / c A z = 4 b = 11.491 ( I ) 8, c = 13.507 (1) A PEalcd = 2.369 g p = 108.84 (1)' Pobsdb = 2.379 (5) g V = 2634 A3 M , = 939.6 a = 17.934 (1)
(B) Measurement of Intensity DataC Enraf-Nonius CAD-4F K-geometry diffractometer Mo K a (Ae, = 0.709 30 A,X, radiation 0.710 73 A) graphite monochromatized stds," measd every (313), (313), (302) 2.5 h of X-ray exposure time no. of reflcns collcd 5041 [3O d 20 d 50' (+h,+k,fl)] (excluding syst abs) instrument
(C) Treatment of Intensity DataC 107.9 transmissn factor rangee 0.421-0.605 averaging,' R,, 0.0 16 no. of reflcns after averaging 4613 no. of obsd. unique data [F,> 4a(F,,)] 2686 k , cm-'
From a least-squares fit of the setting angles of 25 reflections. By suspension in a mixture of CHBr, and CHCI, (four determinations). 70u) longer in 1 compared to the average bond length of 3 and 5-10. Opening of the twist angle, w , further minimizes the nonbonded repulsion between the in-plane ligands. The tilt angle, 7,increases with the Pt-Pt distance in a straightforward manner as described in detail elsewhere.' The large intradimer Pt-Pt distance of 2.992 (1) A in 2 corresponds to a tilt angle of 39.6O; both of these values are greater than the analogous parameters in 1, 3, and 5-10. The bond lengths and angles in both ethylenediamine ligands are within the range found in other ethylenediamine/Pt(II) complexes (C-C-N angles 109-1 19O and C-C distances 1.29-1.54 A).21 In contrast to the ethylenediamine [Pt"I2 precursor 2,7 which also has adjacent ethylenediamine rings, there are no disordered ethylene carbon atoms, and both of the torsion angles around these carbon atoms (N( l)-C(2)
