2988
J. Am. Chem. SOC.1981, 103, 2988-2994
and lock compounds. Field desorption mass spectra were run on a Kratos AEI MS 30 double-focusing mass spectrometer equipped with hightemperature-activated wire emitters. Analytical and preparative highpressure liquid chromatographies were performed on either a Waters 440 (detector UV, X = 254 nm) or a Prep LC 500 (detector, differential refractometer) apparatus, respectively. Materials. [2.2.2]Paracyclophane (1) was prepared by the modified Wurtz condensation of paraxylylene chloride in the presence of a catalytic amount of tetraphenylethylene as described by Tabushi and c o - ~ o r k e r s . ~ From the crude solid, 1 and 2 were obtained as a mixture by column chromatography (silica, hexane-benzene) and then separated by repeated recrystallization from ethanol. A more rapid and convenient separation of 1 and 2 was performed by preparative HPLC (Prep PAK-500/C18 column; mobile phase methanol). [2.2.2]Paracyclophane (1): mp 168 OC; 'H N M R (CDCIJ see Table I; ' H N M R (CD,OD) 6 2.47 (CH,), 6.23 (Ar); 13C N M R (CDC13) featured resonances at 6 33.56 (CH,), 128.35, and 136.56 (Ar). p-bis(2-ptolylethy1)benzene ( 2 ) : mp 142-143 OC; 'H N M R (CDC1,) see Table I. 1:l Silver Triflate Complex of 1. Into a solution of 25.7 mg (0.1 mmol) of silver triflate in 8 mL of freshly distilled T H F was added 31.2 mg (0.1 mmol) of 1. The mixture was magnetically stirred at room temperature for ten minutes and the solvent evaporated under reduced pressure to give a stoichiometric amount of the 1:l silver triflate complex as a whitish crystalline powder. The crude complex was dissolved in hexane4ichloromethane and allowed to evaporate very slowly in the darkness at room temperature to give tiny colorless needles: mp 192 OC; ' H N M R (CDClJ see Table I; ' H N M R (CD,OD) 6 2.57 (CH,), 6.42 (Ar); I3C N M R featured resonances at 6 33.01 (CH,), 125.95, and 138.19 (Ar);field desorption mass spectrum m / e 312 (M+), 419 (Io7Ag M)', 420 ("Ag + M)+. Anal. Calcd for C2SH24F3S03Ag: C, 52.72; H, 4.21. Found: C, 52.43; H, 4.28. Use of up to 4:l molar ratio of silver triflate to cyclophane did not affect the character of product or yield to any extent; only the monocomplex of 1 could be detected. Determination of the Stability Constant. Considering the equilibrium A + D a AD, where A and D represent the acceptor and donor mole-
+
Table 11. Values of Complexation Constant K , from ' H NMR Chemical Shifts with Measurement in CD,OD at 24 "Ca acceptor4onor system CF3S0,Ag-[2.2.2]PCP
proton conditionsb measured
Ao,
Hz
K,, L mol-'
H aliph Harom
20.2 41.7
197 t 8 189 i. 5
Q~
>>do
a For each concentration, three spectra were averaged to obtain the chemical shift of the individual protons. Q,, acceptor concentration; do, donor concentration.
cules and AD represents the ?r-molecular complex, we can use the following equation, derived from the Benesi-Hildebrand e q ~ a t i o n : ~ -1= - - 1- 1 1 + 1 A Kc 4 a, A, where A = observed shift of the donor protons for the system in equilibrium relative to the shift for the pure donor in solution, 4 = shift for the pure complex relative to the shift for the pure donor in solution, K, = equilibrium constant, and a. = acceptor concentration. K, was evaluated graphically, plotting l / A vs. l/a, (see Table 11). In this study, we kept the donor concentration fixed while the concentrations of the acceptor were varied and we measured the shifts of the donor protons in the complexing media. The physical method chosen here for studying weak complexes requires a large excess of the acceptor (AgCF,SO,) compared with that of the donor ([2.2.2]PCP). Deuterated methanol (CD,OD) was used; this solvent dissolves a large amount of silver salt as well as smaller amounts of cyclophane. a standard solution of cyclophane in CD,OD was prepared with a concentration just sufficient to observe a measurable N M R spectrum (0.01 M). Molar ratios of silver triflate to cyclophane were varied from 2 to 10 in the preparation of a series of N M R samples (200 pL in volume). The N M R instrument was internally locked on a CDpOD peak and chemical shifts were measured at 24 OC with MePSi as an external reference and an estimated accuracy of *0.3 Hz.
Dynamic FTNMR Studies of Hindered Metal-Cage Rotation in Twelve-Vertex closo-Phosphinometallacarborane Complexes+ Todd B. Marder,'" R. Thomas Baker,lbJudith A. Long,lCJames A. Doi, and M. Frederick Hawthorne* Contribution from the Department of Chemistry, University of California, Los Angeles, California 90024. Received August 28, 1980
Abstract: Dynamic 'H and 31P('H)F T N M R spectra of a series of 12-vertex closo-phosphinometallacarboranesare presented which suggest that the metal vertex undergoes hindered rotation with respect to the five-membered face of the carborane cage. Studies of the Rh(II1) and Ir(II1) complexes, [L2HM(carb)], where L = PPh3, PEt3, or PMezPh and carb = 1,2-, 1,7-, or 1,12-C2B9Hl0R(R = H, Me, Ph, or n-Bu), the Ru(1V) complex, [2,2-(PPh3)z-2,2-H2-2,1,7-RuC2B9Hll], the Ru(I1) complex, [2,2-(PPh3)2-2-CO-2,1 ,7-RuC2B9Hl'1, and the Ru(I1) and Rh(1) complexes, [3,3-(PPh3),-3-(H),-4-CSH5N-3,1 ,2-MC2B?H!o] (n = 0, M = Rh; n = 1, M = Ru), constitute the first direct determination of rotational barriers in solution for [ML,] moieties with respect t o planar $-bonded ligands in which the K system is continuous. Free energies of activation (AG') vary from C8.4 to >17.5 kcal/mol.
The potential barrier to internal rotation about t h e n-fold axis in organometallic *-complexes containing a cyclic +C,,R, ligand ( n = 3-8) is so low2s3that it evidently cannot be measured directly by d y n a m i c FTNMR i n solution. Although free energies of activation for complexes with noncontinuous *-systems (i.e., polyenes$ polyenyls? nido-5,6-C2B8HI1,6 and nido-Bl,,Hl?-') have been reported, no barriers have thus f a r appeared for complexes containing continuous b u t nonuniform A systems such a s those + A preliminary account of this work has been presented. T.B. Marder, J. A. Doi, R. T.Baker, and M. F. Hawthorne, Pacific Conference on Chemistry and Spectroscopy, Pasadena, CA, October 1979.
0002-7863/81/1503-2988$01.25/0
containing C-substituents8 or one or more heteroatoms in t h e A-bonding n e t ~ o r k . ~Extended Hiickel molecular orbital cal(1) (a) University of California Regents' Intern Fellow, 1976-1980. (b) University of California Regents' Fellow, 1978-1979. (c) University of California Chancellor's Intern Fellow, 1977-1981. (2) For reviews and theoretical papers see: (a) H. H. Jaffe, J . Chem. Phys., 21, 156 (1953); (b) W. Moffitt, J . Am. Chem. SOC.,76,3386 (1954); (c) L. A. Federov, Russ. Chem. Reu. (Engl. Transl.), 42, 678 (1973); (d) D. M. P. Mingos, J . Chem. SOC.,Dalton Trans., 602 (1977); (e) T. A. Albright, P. Hofmann, and R. Hoffmann, J . Am. Chem. Soc., 99, 7546 (1977); (f) G. Wilkinson, M. Rosenblum, M. C. Whiting, and R. B. Woodward, ibid., 74, 2125 (1952); (9) T.A. Albright, R. Hoffmann, Y . Tse, and T. D. D'Ottavio, ibid., 101, 3812 (1979).
0 1981 American Chemical Society
J . Am. Chem. SOC.,Vol. 103, No. 11, 1981 2989
closo-PhosphinometallacarboraneComplexes
Table I. The 200.133-MHz H NMR Spectral Data for Twelve-Vertex closo-Phosphinometallacarborane Complexesa JRh-H’,
JRh-H,
complex I ac
IbC Ib IC Id 1ec If Ig IIa IIb IIC IIdC IIec IIf I Ig IIhCsd IIIC IVd
6 (M-H)
8 (M-H’)
-8.40 -9.25 -9.73 (br) - 10.19 - 10.10 -11.03 -12.76 -13.25 -10.74 -12.14 -13.79 -14.16 -12.22 -13.00 -16.33 -8.94 -8.74 -10.04
a Spectra recorded in CD’CI,.
‘JP,-H
Hz
‘JP-H,
Hz
HZ
4 > IJI for all Ss and thus the simple expression given above should be an excellent approximation. Since the chemical shift differences are fairly large, errors due to natural line widths are negligible. In the case of complexes which show unequal populations at low temperatures the value of the rotational (16) E. H. S. Wong and M. F. Hawthorne, J. Chem. Soc., Chem. Commum, 257 (1976); Inorg. Chem., 17, 2863 (1978). (1 7) R. T. Baker, M. S. Delaney, R. E. King 111, J. A. Long, T. B. Marder, T. E. Paxson, R. G . Teller, and M. F. Hawthorne, J . Am. Chem. Soc., to be
__
submitted . - ._._. - -. (18) J. A. Doi, Ph.D. Thesis, UCLA, 1980,
(19) B. E. Mann, J . Chem. SOC.,Perkin Tmns. 2, 30 (1972).
If I1
"
m -
I
Io
Figure 3. Graphic representation of the low-temperature 8 1.02-MHz jlP('H) N M R spectra of the closo complexes. barrier given is that for the conversion from the minor to the major rotamer (i.e., complexes Ib,c,g, IIc,g, and IVb). In these cases, the values of AG* are most probably within *0.5 kcal/mol, whereas for the remaining complexes, the maximum error in AG' is f 0 . 3 kcal/mol. Due to the nature of the observed dynamic N M R spectra, computer simulation was not undertaken and all calculations were performed by using a program written for a Texas Instruments TI-59 magnetic card programmable calculator by Mr. T. B. Marder.
closo- Phosphinometallacarborane Complexes
J . Am. Chem. SOC.,Vol. 103, No. 11, 1981 2991
U
Ph3P&PPh3
+
11 F
0 BH 0 CH
ph3p&pph3
+
L
-533-80
-90
-100
-110
Figure 6. Variable-temperature 'H NMR spectra of complex IC. Figure 4. Variable-temperature31P{lH) NMR spectra of complexes IIa,f (asterisk = trace impurity of Ia). U
Figure 5. Variable-temperature 3'P{'H)NMR spectra of complex IC.
Results The closo-phosphinometallacarborane complexes employed in this study are shown in Figure 1. The IH and 'lP('H] FTNMR spectral data are presented in Tables I and 11, respectively, and graphic representations of the low-temperature spectra are shown in Figures 2 and 3. Coupling constants (JRh-H and 'Jp-") were deduced by comparison of the rhodium complexes with the indium analogues and by single frequency, and/or broad-band 31Pdecoupling of the hydride resonance. The spectra of the analogous unsubstituted complexes Ia and 111, IIh, Ie, and IIj consisted of apparent A2MX (M = lo3Rh, 100% abundance, Z = ' / 2 ) , A2X2, A2X, and A2 spin systems, respectively, and were invariant from -80 to +40 'C. The unsubstituted complexes IIa and IId-f, however, exhibited broad resonances in the 31P(1H)FTNMR spectra at room temperature, as shown in Figure 4 for complexes IIa and f. At ca. -80 O C the AA' portion of an AA'MX (AA'X for IIf) spectrum was observed, implying the absence of a mirror plane bisecting both the carborane ligand and the P-Rh-P angle. The 'H FTNMR spectra of IIa and f were invariant over the temperature range -88 to +27 'C and the metal hydride retained coupling to the phosphorus (and rhodium for IIa) throughout, demonstrating that phosphine dissociation cannot account for the observed dynamic process. The 31P(IH)FTNMR spectra of IIi and boron-substituted IVa exhibited similar behavior to that of IIa,f. The dynamic FTNMR spectra of the carbon-substituted closephosphinometallacarborane complexes were more complex. The
Figure 7. Variable-temperature 3'P('H) NMR spectra of complex Ib.
'lP('H) FTNMR spectrum of IC at -73 'C, shown in Figure 5, exhibited four different phosphorus resonances and was consistent with the AA' portions of two separate AA'MX spin systems of approximate ratio 1O:l. If PI and PI' are arbitrarily designated to be those phosphorus resonances possessing the smaller chemical shift difference (Av),then the coalescence of PI and PI' is observed at ca. 15 OC,while that of P2 and P2' occurs at 40 'C. The 'H FTNMR spectrum of ICat -88 'C, shown in Figure 6,consisted of two rhodium hydride resonances, with an approximate intensity ratio of lO:l, which coalesced at ca. 20 'C; however, the hightemperature limiting spectrum was not obtained for either nucleus. The FTNMR spectra of the analogous iridacarborane complex, Ig, displayed similar behavior, although the intensity ratio of the two species at the low-temperature limit was ca.5:l. The dynamic FTNMR spectra of Ib were similar to that of IC; however, the lower activation energy for this complex permitted the observation of the high-temperature limiting spectrum but precluded that of the low-temperature limiting spectrum, as shown in Figures 7 and 8. The spectra of Id and f were also similar to those of IC, except that the ratio of the two species observed at low temperature was approximately unity. Complexes IIb,c,g and IVb all possessed
2992 J . A m . Chem. Soc.. Vol. 103, No. 11, 1981
Marder et al.
Table 11. The 81.02-MHz 3 1 P{'H} NMR Spectral Data for Twelvevertex closc-Phosphinometallacarborane Complexes' ~ ( P z(JRh-p,) )
6 ( p i ) (JRh-p,)
Ia
39.5 (125) 40.0 (131) 40.3 (134) 37.5 (134) 38.5 (134) 11.0 7.8 7.2 41.7 (125) 49.7 (139) 26.6 (116) 26.7 (112) 30.5 (125) 41.4 (129) 10.9 (127) 19.0 (134) 10.8 13.5 7.2 61.8 43.6 52.2 32.8 34.8 (112) 46.1 (198) 52.8 (221) 57.7
Ibbid Ib IC
Id Ie
If Ig IIad IIa
IIbC IIC IIdbtC IIdC IIebvC
IIeC 119
IIf IIg IIhbSC
IIib*C IIic IIjb I I1
IVab IVa
IVbC
6(P1')(JRh-p,')
6(P,') (JRh-p,')
'Jp,-p,
*Jp,-p,'
T , "C -83
37.4 28.5 24.1 26.1
(131) (108) (110) (110)
8.7 3.8
41.2 (122) 40.5 (127) 39.2 (122)
48.6 (15 3) 48.5 (158) 46.0 (159)
18 22 20 22
14.3 13.8
16 15
5 5
47.5 (145) 47.1 (157)
21 13 15
12 10
8.3 8.7
33.2 (112) 43.7 (143) 39.0 (147)
30.5 (122) 33.3 (127)
22.7 (115)
22
7.2 (110)
24
9.2 8.5
15 17
12 10
12.8
8.8
brf
36.7
