J. Am. Chem. SOC.1982, 104, 2041-2042
204 1
of the structure of karachine (2) indicates the degree of complexity attainable in nature starting from a relatively simple berbinoid skeleton.6
Acknowledgment. This project was supported by Grant CA11450 from the National Cancer Institute, NIH, DHHS. G.B. is the recipient of an IREX fellowship. We are grateful to Theresa S. Dunne for technical assistance. A.R. and A.A.A. express their gratitude to Hakim Mohammad Said, Chairman, Hamdard National Foundation, Karachi, for providing the plant material.
QC"
c25
Registry No. 2, 80908-02-7; 3, 80908-03-8. (6) For a recent review on purely synthetic adducts of berberine with acetone, see: Govindachari, T. R.; Pai, B. R.; Rajeswari, S.; Natarajan, S . ; Chandrasekaran, S.; Premila, M. S.; Charubala, R.; Venkatesan, K.; Bhadbhade, M. M.; Nagarajan, K.; Richter, W. J.; Heterocycles 1981, 15, 1463. Such adducts bear minimal structural resemblance to karachine.
Figure 1. ORTEPview of M O ~ ( O ~ C M ~ ) ~ ( O S ~ M ~Important ~)~(PM~~)~. bond lengths and angles: Mo-Mo = 2.114 (1) A, Mo-O(siloxide) =
A, Mo-O(acetate) = 2.11 (1) A, Mo-Mo-P = 94.02 ( 3 ) O , Mo-Mo-O(si1oxide) = 116.9(5)O, P-Mo-O(si1oxide) = 149.0(5)'. 2.019 (9)
Quadruply Bonded Dimolybdenum Compounds of the Type M O ~ ( O ~ C R ) ~ X ~ ( P Evidence R ~ ) ~ : for Kinetic and Structural Trans Effects Gregory S. Girolami, Vera V. Mainz, and Richard A. Andersen* Department of Chemistry, University of California Berkeley, California 94720 Received October 19, 1981
It has been shown recently that the PEt, ligands in Mo2Me4(PEt&, of stereochemistry I, undergo stepwise exchange with
X
L
X
L
0
A.0
1
0-0
Y ' R
Table I. Magnitudes of 'Jpp in the Compounds Mo, (O,CCMe,),X,(PMe,Et)(PMe,) X CH,CMe, CH,SiMe, Me I
,Jpp,
X
Hz
4
c1
6 6
Br N( SiMe, H), OSiMe,
20
'JPP, Hz
25 29
33 39
asymmetric unit. Each molecule contains a crystallographic inversion center, and each molecules possesses nearly Ca symmetry, though the siloxide SiMe, groups are rotated slightly out of the Mo2O2PZplane, giving Mo-Mo-O-Si torsion angles of 166.0 and 145.6' for the two independent molecules in the unit cell. The overall geometry is similar to that found in the electronically and sterically equivalent alkyl derivative M O ~ ( O ~ C M ~ ) ~ (CI12SiMe3)2(PMe3)2.7 The most notable difference between the two structures occurs in the Mo-P distances: 2.547 (1) A in the alkyl and 2.487 (1) A in the siloxide. This difference is most reasonably ascribed to the low position of siloxide ligands relative to alkyls on the trans-influence series, which agrees with the order deduced in square-planar Pt(I1) chemistry.* In our initial attempts to study the phosphine substitution kinetics, we found that the exchange rates in the series Mo2(02CCMe3)2X2(PEt3)2 where X is alkyl, halide, amide, or siloxide were too rapid to be followed conveniently by ,'P('H) N M R ~pectroscopy.~Since kinetics studies in the Mo2Me4(PRJ4system showed that smaller phosphines exhibit slower rates,' we prepared the series M O ~ ( O , C C M ~ ~ ) , X ~ ( P M These ~ ~ Ecompounds ~)~.~~
I I1 either PMe, or PMe2Ph in toluene solution by a dissociative mechanism.' In this class of molecules, the phosphine ligands L are cis to the anionic groups X. An analogous study of binuclear compounds in which the phosphines are trans to the anionic group was of interest. The bis(carboxy1ato) complexes of stereochemistry I1 are well suited for such a s t ~ d y . ~We - ~ now present kinetic, crystallographic, and spectroscopic data as a function of the anionic (6) Single crystals of Mo,(O~CM~)~(OS~M~~)~(PM~~), are triclinic, space group, X, which provide the first evidence for both kinetic and group PI, with a = 11.465 (2) A b = 11.544 (1) A c = 13.695 (2) A, a = 70.54 ( 1 ) O , B = 64.12 (1)O, y = 77.54 (I)', V = 1532.4 (3) A'; Z = 2. X-ray structural trans effects operating in quadruply bonded dimers. diffraction data were collected for 4015 independent reflections having 28 < The red siloxide derivative MO~(O~CCM~~)~(OS~M~~)~(PM~~)? 45O on an Enraf-Nonius CAD-4 diffractometer using graphite-monocan be prepared from Mo2(02CCMe3),, LiOSiMe,, and PMe, chromated Mo Ka radiation and 0-2 8 scans. The structure was solved by in diethyl ether, followed by crystallization from pentane at -10 the heavy-atom method. The final residuals for 254 variables refined against > 3@) were R, = 2.29'76, RwF= 4.22'76, and the 3564 data for which O C . The NMR parameters indicate a structure of type 11, and GOF = 2.23. In the last cycle, hydrogen atom positions were predicted and this was confirmed by an X-ray crystallographic study (see Figure included in the structure factor calculations, but not refined. 1).6 The structure has two independent half-molecules in the (7) Hursthouse, M. B.; Malik, K. M. A. Acta Crystallogr, Sect. B 1979, (1) Girolami, G. S.; Mainz, V. V.; Andersen, R. A,; Vollmer, S. H.; Day, V. W. J. Am. Chem. SOC.1981, 103, 3953-3955. (2) Andersen, R. A.; Jones, R. A.; Wilkinson, G. J. Chem. SOC.,Dalton Trans. 1978, 446-453. (3) Mainz, V. V.; Andersen, R. A. Znorg. Chem. 1980, 19, 2165-2168. (4) Arenivar, J. D.; Mainz, V. V.; Ruben, H.; Andersen, R. A.; Zalkin, A. Znorg. Chem., in press. (5) Anal. Calcd: C, 30.0; H, 6.61. Found: C, 30.3; H, 6.66. Mp 157-158 OC; 'H NMR (PhH-d6,25 "C) 6 2.63 (02CMe, s), 1.28 (PMe,, AA'X9Xf9, 'JM + 'JAPX= 9 HZ), 0.20 (OSiMe3,s); "C ('HI NMR (PhH-d6, 20 "C) 6 183.4 (02CMe, s), 23.2 (02CMe,s), 11.9 (PMe3,ABX, 'JAX+ 4JBx= 26 Hz), 4.1 (OSiMe3, s); 31P('H]NMR (PhH-d6, 25 "C) 6 5.0, s.
0002-7863/82/1504-2041$01.25/0
83.5, 2709-2712. (8) cis-Pt(OSiMe3)2(PMe3)2has IJpt-p= 3400 H Z , * ~ while cis-Pt(CH2SiMe3),(PMe2Ph)2has IJpt+ = 1999 H z , * showing ~ that CH2SiMe3> OSiMe, on a trans series. (a) Schmidbaur, H.; Adlkofer, J. Chem. Ber. 1974, 107, 3680-3683. (b) Cardin, C. J.; Cardin, D. J.; Lappert, M. F.; Muir, K. W. J. Chem. SOC.,Dalton Trans. 1978, 46-50. For general references on trans-influence series, see: Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Reu. 1973, 10, 335-422. Manojlovic-Muir, L.; Muir, K.W. Znorg. Chim. Acta 1914, 10, 47-49. (9) The pivalate derivatives were used since they are more soluble in toluene at low temperature than their corresponding acetate analogues. Rate studies have shown that phosphine-exchange rates for the neopentyl derivatives MO~(O,CR)~(CH~CM~~)~(PM~E~)~ are independent of the carboxylate group (R = Me, CF,, or CMe,).
0 1982 American Chemical Society
J . Am. Chem. SOC.1982, 104, 2042-2044
2042
undergo phosphine exchange with PMe3 at rates that can be Characterization of Several Novel Iron Nitrosyl followed satisfactorily by 3'P(1H)N M R spectroscopy. Porphyrins Reaction with excess PMe3 at low temperature in toluene forms the mixed-phosphine intermediates M O ~ ( O ~ C C M ~ ~ )L.~ W. X ~Olson,t D. Schaeper, D. Lanpon, and K. M. Kadish* (PMe2Et)(PMe3)before the second exchange occurs to give the Department of Chemistry, University of Houston fully substituted bis-PMe, product. The ''P{lH) spectra of the Houston, Texas 77004 mixed-phosphine intermediates are characteristic of AB spin systems, with each phosphine resonance being split into a doublet Received November 16, 1981 of separation 3Jpp. The magnitude of the coupling constant reflects Considerable attention has been given to the chemistry of the trans-influence ability of the X group as shown in Table I. As X becomes a poorer trans-influence ligand, the molybdenumsynthetic transition-metal metalloporphyrins complexed by diaphosphorus bond becomes stronger (as shown by the Mo-P bond tomic molecules.'v2 Because of their relevance to biological lengths discussed above), and the phosphorus-phosphorus coupling systems, the reactions of Fe(I1) porphyrin complexes with diatomic constants increase. Thus, the trans-influence series alkyl > halide molecules such as 02,CO, CS, and N O have been of special > amide > siloxide may be deduced for binuclear complexes of interest. stereochemistry 11. It has been reported' that the five-coordinate complexes of Having established that the exchange process is stepwise, as PorFeNO (where Por = TPP2- (tetraphenylporphyrin) or OEP" was found in molecules of stereochemistry I,' we studied the (octaethylporphyrin)) can be reversibly oxidized by cyclic volkinetics of substitution for the first phosphine ligand as a function tammetry at a Pt electrode to yield [PorFeNO]+. In this comof the X group. We can minimize complications due to steric munication we report the isolation and characterization of a novel effects by choosing the two anionic groups at the extremes of Table bis(nitrosy1) complex of Fe(III), [PorFe(NO),]+. We also report I (CH2CMe3and OSiMe,), as they are very similar in size. Thus, the reversible electrochemical reduction of PorFeNO to yield the alkyl derivative M O ~ ( O ~ C C M ~ ~ ) ~ ( C H ~ C M ~ ~ ) ~( PM~~E~)~ [Por FeNOI-. reacts with an excess of PMe3 at -30 O C in toluene following (TPP)FeNO and (0EP)FeNO are low-spin, five-coordinate first-order kinetics with koW = (1.00 f 0.05) X 10-4 s-I. No rate complexes of Fe(II).24 Cyclic voltammetry of these complexes dependence on [PMe3] was observed over a range of 10-20 molar at a Pt electrode in CH2CI2, 0.1 M TBAP (tetra-n-butylequiv/binuclear unit. An Arrhenius plot over three temperatures ammonium perchlorate), reveals that each neutral species ungave AH* = 22.2 kcal mol-' and AS* = 15 eu. The positive dergoes a reversible one-electron oxidation and a reversible oneentropy of activation implies a dissociative mechanism for molelectron reduction during the time scale of the experiment. This ecules of structure 11, as observed previously for M O ~ M ~ ~ ( P R ~ is ) ~shown ' in Figure 1, and the potentials of the reactions are listed ( P MI. ~ ~ E ~listed ) ~ in this table are potentials for several The siloxide derivative M O ~ ( O ~ C C M ~ ~ ) ~ ( O S ~ M ~ ~ ) in~Table Also reacts in a similar manner with PMe,, except that a higher temrepresentative five- and six-coordinate complexes of (TPP)Fe and perature is necessary to achieve a similar rate. At 0 OC, kow = (0EP)Fe in both CH2C12and pyridine. As seen from this table, (1 -97 f 0.05) X lo4 s-', and Arrhenius parameters of AH*= the potentials for oxidation of PorFeNO are extremely positive 24.1 kcal mol-' and AS' = 15 eu can be obtained. and are, in fact, the most positive ever reported for the reaction Extrapolating the observed rate data to similar temperatures, Fe(I1) F? Fe(II1). This substantial stability of the ferrous form we find that the phosphine dissociation rates are lo2 times faster is reflected in the relative inertness of PorFeNO to air oxidation in the alkyl derivative than in the siloxide. This rate difference and to displacement by other ligand^.^ The differences between is most reasonably ascribed to a kinetic trans effect, where the OEP and TPP complexes reflect the greater basicity of the CH2SiMe3> OSiMe, in a trans series. Hence the rate data, in OEP2- ring and are consistent with differences observed for other conjunction with the structural and spectroscopic results, strongly complexes of the two porphyrins.s indicate that both a trans influence and a trans effect are operating Oxidation of PorFeNO greatly increases the lability of the in these binuclear, quadruply bonded compounds. nitrosyl ligand. Attempts to isolate [PorFeNO]+ by controlledpotential coulometry in CH2C12at 1 .O V vs. SCE, 0.1 M TBAP, Acknowledgment. We thank the N S F for departmental grants produced only PorFeC104 as determined by electronic spectra.6 used to purchase the N M R spectrometers and X-ray diffractomHowever, electrolysis under an atmosphere of nitric oxide rather than nitrogen produced a new compound whose electronic speceter used in this work and Chevron (G.S.G.)and the UCB equal opportunity fund (V.V.M.) for fellowships. We also thank Dr. trum is shown in Figure 2. Two Soret bands of significantly lowered intensity are observed for both OEP and TPP derivatives. F. J. Hollander, staff crystallographer at the UCB X-ray crysPurging the solution with an inert gas results in appearance of tallographic facility (CHEXRAY), for performing the X-ray study. the perchlorate spectrum. Conversely, exposure of solutions of PorFeC104 to nitric oxide regenerates the spectrum in Figure 2. Registry No. MO~(O~CCM~,)~(OS~M~~)~(PM~~)~, 80925-85-5; MolCrystalline solids may be obtained from CH2Cl2/hexane so(02CCMe3)2(CH2CMe3)2(PMe2Et)(PMe3), 80925-86-6; Mozlutions, but decomposition occurs unless they are stored under (02CCMe3)2(CH2SiMe3)(PMe2Et)(PMe3), 80925-87-7; Mo2nitric oxide. Two N - 0 stretching frequencies in the infrared (02CCMe3)2(Me)2(PMe2Et)(PMe3), 80925-88-8; M O ~ ( O ~ C C M ~ , ) ~ I ~spectrum at 1940 and 1860 an-'(TPP derivative) provide evidence (PMe2Et)(PMe,), 80925-89-9; MO,(O~CCM~~)~C~~(PM~~E~)(PM~~), that this new species is the bis(nitrosy1) complex [PorFe80925-90-2; MO~(O~CCM~~)~B~~(PM~~E~)(PM~,), 80925-91-3; Mo2(NO)z]+C104-. The perchlorate region is typical of ionic rather (02CCMe3)2(N(SiMe2H)2)2(PMe2Et)(PMe3), 80925-92-4; Mo2than coordinated C 1 0 ~ Conductivity .~ studies in CH2C12under (02CCMe3)2(0SiMe3)2(PMe2Et)(PMe3), 80925-93-5; Mo2(02CCMe3)2(CH2CMe3)2(PMe2Et)z, 80939-27-1; M O ~ ( O ~ C C M ~ ~ ) N ~ -O show a molar conductivity of 40.8 Q-' cm2 mol-', consistent with a 1:l electrolyte such as TBAP and much larger than a (OSiMe3)2(PMqEt)2,80925-94-6; M%(O2CCMe3),, 55946-68-4; PMe,, 594-09-2.
Supplementary Material Available: A listing of positional and thermal parameters and their estimated standard deviations ( 1 page). Ordering information is given on any current masthead page. ~~
~
~
(IO) The alkyl derivatives were prepared as in ref 2, the amide as in ref 3, and the halides as in ref 4. All compounds gave satisfactory elemental analysis, and NMR spectral properties (lH,13C(1HJ, and ,IP(lH)) show them to be of the structural type 11.
0002-7863/82/1504-2042$01.25/0
'On leave of absence from Grand Canyon College, Phoenix, A 2 85017. (1) "The Porphyrins";D. Dolphin, Ed.;Academic Press: New York, 1979; Vol. I-VII. (2) Buchler, J. W.; Kokisch, W.; Smith, P. D. In "Structureand Bonding"; Bunitz, J. D., Ed.; Springer-Verlag: New York, 1978; Vol. 34. (3) Buchler, J. W.; Kokisch, W.; Smith, P.; Tonn, B. 2.Nururforsch, B 1978, 336, 1371. (4) Wayland, B. B.; Olson, L. W. J. Am. Chem. SOC.1974, 96, 6037. (5) Fuhrhop, J. H. In "Porphyrins and Metalloporphyrins";Smith, K. M., Ed.; Elsevier: New York, 1975; Chapter 14. (6) Reed, C. A,; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, W. R.; Spartalion, K.; Lang, G. J . Am. Chem. SOC.1979, 101, 2948.
0 1982 American Chemical Society
