Organometallics 1996, 14, 2718-2724
2718
p2-q2:ql-Benzylidene(dipheny1phosphino)maleic
Anhydride Mediated PMe3 Addition to the Tricobalt I
Cluster Cos(CO)a[lr2-q2:q1-C(Ph)C=C(PPh2)C( 0)OC(O)]@-PPh2). Structure and Redox Properties of the Arachno Cluster Cos(C0)4(PMe3)2@2-q2:q l-C(Ph)C=C(PPh2)C(0)OC( O ) ](/zz-PPhs) Kaiyuan Yang, Simon G . Bott,* a n d Michael G. Richmond* Center for Organometallic Research and Education, Department of Chemistry, University of North Texas, Denton, Texas 76203 Received January 18, 1995@
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The reaction of PMe3 (2.5 equiv) with the tricobalt arachno cluster Co3(C0)&2-q2:q1-
C(Ph)C=C(PPh2)C(O)OC(O)](,D2-PPh2) (1)at room temperature proceeds rapidly to give the disubstituted arachno cluster Co~(C0)~(PMe~)~~~-q2:q1-C(Ph)C=C(PPh~)C~O~OC~O~l(,D~-P I (4)in a stepwise process via the known clusters Co3(C0)5(,D2-CO)(PMe3)CU2-q2:q1-C(Ph)C=CI (PPh2)C(O)OC(O)](,D2-PPh2)(2) and Co3(C0)5(PMe3)1U2-q2:q1-C(Ph)C=C(PPh2)C(O)OC(O)l(,D2PPh2)(3).Independent experiments using cluster 2 show that CO loss occurs in preference to PMe3 substitution, giving cluster 3 first, and it is cluster 3 that reacts rapidly with added
PMe3 to afford the disubstituted cluster Co3(C0)4(PMe3)2~2-q2:q1-C(Ph)C=C(PPh2)C(0)OCI
(0)](p2-PPh2). The kinetics for the reaction between Co3(C0)5(PMe3)CU2-q2:q1-C(Ph)C=CI
(PP~~)C(O)OC(O)I(,D~-PP~Z) and PMe3 have been measured in CH2C12 by UV-vis spectros-
copy. On the basis of the second-order rate constants and the activation parameters (@ = 11.9 f 0.6 kcal mol-l and AS'* = -20 f 2 eu), a n associative reaction that involves PMe3 addition to cluster 3 is supported. Cluster 4 has been isolated and characterized in solution by IR and NMR (31Pand 13C)spectroscopy and in the solid state by X-ray diffraction analysis.
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Co~(C0)~(PMe~)~~~-q2:q1-C(Ph)C-C(PPh~)C~O)OC(O)l(,D~-PPh~) crystallizes in the monoclinic
A,
A,
A,
space grou P21/n: a = 11.0047(8) b = 19.558(2) c = 17.683(1) p = 94.947(6)", V = 4664.4(6) 3, Z = 4, dcalc= 1.419 g ~ m - R ~ ;= 0.0453, R, = 0.0489 for 3795 observed reflections. The solid-state structure of 4 indicates that the second PMe3 ligand adds t o the cobalt center that is substituted by the maleic anhydride n bond. The electrochemical properties of cluster 4 were examined by cyclic voltammetry in CHzClz solvent. Two diffisioncontrolled, one-electron redox responses at E112= 0.30 V and E112 = -1.06 V were observed in addition to a n irreversible reduction a t EPC= -1.87 V, with the first two processes assignable to the 0/1+ and 0/1- redox couples, respectively. The importance of the p2-q2:q1 -benzylidene(diphenylphosphino)maleicanhydride ligand in cluster 3 in directing the site of PMe3 ligand addition is discussed in terms of a n associative PMe3 process that is coupled with a dissociation of the maleic anhydride moiety from the cobalt center in 3.
1
Introduction The ligand substitution chemistry of polynuclear metal clusters remains an intensely studied field, in part because of the myriad pathways that are available for the replacement of the cluster-bound ligand, which is usually a CO group, by an incoming 1igand.l The vast majority of the cluster substitution reactions proceed by a pathwayb) involving a dissociative, associative, or @Abstractpublished in Advance ACS Abstracts, May 1, 1995. ( l ) ( a ) Darensbourg, D. J. In The Chemistry of Metal Cluster Complexes; Shriver, D. F., Kaesz, H.D., Adams, R. D., Eds.; VCH Publishers: New York, 1990;Chapter 4.(b) Muetterties, E. L.; Burch, R. R.; Stolzenberg, A. M. Annu. Rev. Phys. Chem. 1982,33,89.
0276-7333/95/2314-2718$09.00/0
cluster fragmentation sequence.2 In 1979, Huttner and co-workers provided the first experimental data dealing with a ligand addition to a metal cluster that was accompained by metal-metal bond ~leavage.~ Since that (2) (a) Richmond, M. G.; Kochi, J. K. Inorg. Chem. 1986,25, 1334 and references therein. (b) Don, M.-J.; Richmond, M. G.; Watson, W. H.; Nagl, A. J . Organomet. Chem. 1989,372,417.(c) Poe, A. J.;Farrar, A. J.; Zheng, Y. J. Am. Chem. SOC.1992,114,5146.(d) Shen, J.-K.; Basolo, F. Organometallics 1993,12,2942.(e) Dahlinger, K.; Falcone, (0 Atwood, J. D.; Wovkulich, F.; Poe, A. J. Inorg. Chem. 1986,25,2654. M. J.; Sonnenberger, D. C . Acc. Chem. Res. 1983,16,350and references therein. (g) Shojaie, R.; Atwood, J . D. Inorg. Chem. 1987,26,2199; 1988,27,2558.(h) Taube, D. J.; Ford, P. C. Organometallics 1986,5, 99.(i) Darensbourg, D. J.; Incorvia, M. J. Inorg. Chem. 1980,19,2585. (j) Darensbourg, D.J.; Peterson, B. S.; Schmidt, R. E., Jr. Organome-
tallics 1982,1, 306.
0 1995 American Chemical Society
Anhydride-Mediated PMe3 Addition to a Cog Cluster Scheme 1 Ph I
0
Th
+b
0
%o
Organometallics, Vol. 14,No. 6,1995 2719
-
coupled with re-formation of the Co-Co bond, to ultimately afford the simple substitution product Co3(C0)5-
(PMe3)[CL2-~2:q1-C(Ph)C=C(PPh~)C(O)OC(O)l(u2-PPh2) (3) at room temperature. Equation 1 depicts this transformation.
1
2 2
time, other reports of ligand substitution reactions involving polyhedral cluster expansion have been publ i ~ h e dwith , ~ the polyhedral opening being successfully rationalized, in many cases, through the use of electrontopoiogy ruies.5 We have recently published our results concerning the PMe3 ligand substitution in the tricobalt cluster Cog-
-
3
(C0)~[CL2-~2:~1-C(Ph)C=C(PPh2)C(O)OC(O)l(uz-PPh2) (1;
where the , ~ 2 , 7 7 ~ , 1descriptors 7~ refer to the coordination Accordingly, we wished to examine the reaction mode adopted by the benzylidene, maleic anhydride, and phosphine groups, respectively, of the six-electron benbetween cluster 3 and additional PMe3 to see if Co-Co zylidene(dipheny1phosphino)maleicanhydride l i g a ~ ~ d ) . ~ ,bond ~ heterolysis could again be achieved. However, Of interest in this reaction is the first step that leads what we observed was the rapid substitution of CO by to the unexpected regioselective addition of the PMe3 PMe3 a t the maleic anhydride-substituted cobalt center. ligand to the phosphine-substituted cobalt center in 1, Herein, we report our results on the substitution as shown in Scheme 1. The first intermediate zwittechemistry of cluster 3,which gives the new cluster COSrionic cluster in Scheme 1illustrates the importance of (C~)~(PM~~)~~UU~-I~~~:~~-C(P~)C=C(PP~~ polar metal-metal scission in cluster substitution reacPPh2) (4). The solution and solid-state structures of tions that possess a strongly electron-withdrawing functionality, a concept first put forward by Johnson for C~~(C~)~(PM~~)Z~Z-~~:~~-C(P~)C=C(PP~ carbonyl ligand substitution in M3(CO)12 (where M = bz-PPh2) have been determined by spectroscopic methFe, Ru, Os).* In the case of cluster 1, the maleic ods (IR and NMR) and X-ray crystallography. The anhydride moiety assists in directing the site of PMe3 electrochemical properties of cluster 4 have been exattack by helping to stabilize the developing anionic plored by cyclic voltammetry a t a platinum electrode charge on the adjacent cobalt center. The observed in CHzClz and are discussed relative to the redox polyhedral changes are consistent within the context of properties of the related diphosphine-substituted cluspolyhedral skeletal electron pair (PSEP) theory, which ters PhCCo3(C0)7P2. A plausible substitution mechasupports the arachno hypho structural expansion. nism outlining the role of a coordinatively flexible maleic The monosubstituted cluster 2 is stable at low temanhydride ligand in controlling the site of ligand attack perature but readily loses CO at elevated temperatures, in cluster 3 and supported by kinetic measurements is presented.
-
-
(3)Huttner, G.; Schneider, J.;Muller, H.-D.; Mohr, G.; von Seyerl, J.; Wohlfahrt, L. Angew. Chem., Int. Ed. Engl. 1979,18,76. (4)(a) Vahrenkamp, H. Adu. Organomet. Chem. 1983,22,169.(b) Adams, R.D.; Yang, L.-W. J.Am. Chem. SOC.1983,105,235. (c) Knoll, K.;Huttner, G.; Zsolnai, L.; Jibril, I.; Wasiucionek, M. J. Organomet. Chem. 1985,294, 91. (d) Schneider, J.; Minelli, M.; Huttner, G. J. Organomet. Chem. 1985,294,75.(e) Planalp, R.P.; Vahrenkamp, H. Organometallics 1987,6,492.(0 Huttner, G.Angew. Chem., Int. Ed. Engl. 1987,26,743.(g) Curtis, M. D.; Curnow, 0. J. Organometallics 1994, 13, 2489. (h) Richmond, M. G.; Kochi, J. K. Inorg. Chem. 1987, 26,541. (5)(a) Wade, K.In Transition Metal Clusters; Johnson, B. F. G., Ed.; Wiley: New York, 1980,Chapter 3.(b)Wade, R Adu. Inorg. Chem. Radiochem. 1976,18, 1. (c) Mingos, D. M. P. Acc. Chem. Res. 1964, 17, 311. (d) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice-Hall: New York, 1990. (6)Yang, K.;Bott, S. G.; Richmond, M. G. Organometallics 1995, 14,919. (7) Yang, K.;Smith, J. M.; Bott, S. G.; Richmond, M. G. Organometallics 1993,12,4779. (8)Johnson, B.F. G. Inorg. Chim. Acta 1986,115,L39.
Results and Discussion
I. Synthesis and Spectroscopic Characteriza-
I
tion of COS(CO)~(PM~S)~[CC~-~~:~~.C(P~)C=C(
-
1
(O)OC(O)1(1(2-PPh2)(4). The reaction of the monosubstituted arachno cluster Co3(C0)5(PMe3)1U2-r2:11-
C(P~)C-C(PP~~)C(O)OC(O)I(M~-PP~~) with 1.2 mol equiv
of PMe3 in CHzClz proceeds rapidly at room temperature to give the new cluster C O ~ ( C O ) ~ ( P M ~ ~ ) ~ ~ Z - ~ ~ : ~ ~ -
C=C(PP~Z)C(O)OC(O)](~~-PP~~) as the sole observable product by IR and TLC analyses. Alternatively, cluster 4 may also be synthesized by starting with either the
Yang et al.
2720 Organometallics, Vol. 14,No.6,1995
Table 1. Observed Rate Constants for the Disappearance of Co3(CO)s(PMe3)[lc~-t~:1~I
I
C(Ph)C=C(PPhz)C(0)OC(O)l(lr~-PPh~) (3)in the Presence of PMe3"
entry no.
temp, "C
104[PMe31,M
104k,b,d, S-'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
15.1 15.1 15.1 15.1 23.3 23.3 23.3 23.3 23.3 28.3 28.3 28.3 28.3 33.3 33.3 33.3 33.3
15.6 31.3 46.9 62.5 15.6 31.3 31.3 46.9 62.5 15.6 31.3 46.9 62.5 15.6 31.3 46.9 62.5
3.5 f 0.01 9.4 f 0.2 12.5 f 0.3 20.7 f 0.9 10.5 f 0.04 18.8 f 0.2 16.6 f 0.9* 30.4 f 0.9 39.1 f 1.0 14.5 f 0.2 33.8 f 0.7 44.7 f 1.1 56.4 f 2.1 19.4 i 0.3 40.5 f 4.9 60.4 f 13.2 73.2 f 1.9
Table 2. Second-Order Rate Constants for the Reaction between Cos(C0)5(PMe3)[lr~-1~:1 lI
I
C(P~)C=C(PP~Z)C(O)OC(O)I(.~~-PP~Z) (3)and PMe3 temp, "C
102kz,M-' s-l
temp, "C
10% M-' s-'
15.1 23.3
35.0 f 4.3 62.3 f 2.9
28.3 33.3
87.4 f 8.6 116.0 f 8.7
Scheme 2
3
I
a
From 8.10 x 10-5 M C O ~ ( C O ) ~ ( P M ~ ~ ) C ~ ~ - ~ ~ ~ : ~ ~ - C ( P ~ ) C = C ( P P ~ ~ ) -
1
C(O)OC(O)]@z-PPhz)in CH2Clz by following the decrease absorbance of the W-vis band at 374 nm. b In the presence of 1 atm of co. l
l
'
l
~
l
'
l
'
1.1700
4
first-order rate constants ( k o b s d ) as a function of PMe3 concentration (not shown) were found to be linear and gave intercepts of zero within experimental error; the slopes of these plots afforded the second-order rate constants given in Table 2. The activation parameters of AfF = 11.9 f 0.6 kcal mo1-l and AS* = -20 k 2 eu fully support an associative mechanism that obeys the rate lawlaq9
1.1250 1.0800 1.0350
0.9900 0.9450
0
500
1000
1500 2000
2500
Seconds Figure 1. Plot of A, - At for the reaction of CO~(CO)~-
rate =
,
C=C(PPh2)C(O)OC(O)I@~-PPh2)1[PMe~1
I We have also examined the effect of CO on this ( P M ~ ~ ) ~ Z - ~ ~ : ~ ~ - C ( P ~ ) C = C ( P P ~ ~ ) C ( O )with O C ( O ) I reaction. ~ Z - P P ~ ZEntry ) 7, which represents the reaction conPMe3 at 23.3 "C (entry 6, Table 1) showing the raw data 1 atm of CO, is, for all purposes, identical ducted under (0) and the experimentally fitted (-1 curve. with the reaction run under argon (entry 6). Taken collectively, this leads us to propose a mechanism parent cluster C O ~ ( C O ) G C U ~ - ~ ~ : ~ ~ - C ( P ~ ) C = C ( P P ~whereby ~ ) C ( O the ) - incoming PMe3 ligand displaces the alkene1 coordinated bond in 3,affording the putative cluster c03OC(O)l(p2-PPh2)or the monosubstituted hypho cluster
I (C0)5(PMe3)2~2-)72:+C(Ph)C=C(PPh2)C(0)OC(O)l@2CO~(CO)~@~-CO)(PM~~)~~-~~:~,W(P~)C=C(PP~~)C(O)PPhz), whose maleic anhydride moiety then rapidly
1
OC(O)I@2-PPh2)and a slight excess of PMe3, as these reactions eventually give cluster 3, which is the direct precursor to 4. The reaction leading to 4 is sensitive to the temperature at which the reaction is conducted. Whereas the parent cluster 1 reacts with excess PMe3 below 0 "C to yield the hypho cluster 2, clusters 2 and 3 are substitutionally inert under these conditions. The rates of the reaction for 3 to 4 in CH2Cl2 in the presence of a measured excess of PMe3 were monitored by W-vis spectroscopyby following the decrease in the absorption of the 374 nm band belonging to 3. The reaction followed first-order kinetics for at least 5 halflives over the temperature range 15-33 "C,as evidenced by the linear plots of In (A, - A,) vs time, from which the rate constants quoted in Table 1 were obtained. Figure 1shows a sample plot of the raw absorbance data a t 374 nm as a function of time. Plots of the pseudo-
ejects CO by an associative coordination of the alkene moiety. Scheme 2 illustrates the course of this reaction, starting with cluster 3. Cluster 4 was isolated routinely in yields of 50-65% as a green-black solid by chromatography over silica gel using CH2C12 as the eluant. The FT-IR spectrum of 4 in CHzClz exhibits two terminal carbonyl bands at 1992 (sh) and 1978 (vs) along with an absorption at 1865 (m) cm-l, which we ascribe to the semibridging CO ligand that spans the benzylidene-bridged Co-Co bond in the X-ray structure of 4 (vide infra). A strong v(C0)band at 1871 cm-l in the solid-state IR spectrum (KBr) of 4 further strengthens this assignment as that of a semibridging ligand.1° The maleic anhydride moiety is (9)Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Brooks/Cole Publishing: Monterey, CA, 1985. (10)Cotton, F. A. Prog. Inorg. Chem. 1976,21, 1.
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Organometallics, Vol. 14,No.6, 1995 2721
Anhydride-Mediated PMe3 Addition to a Cog Cluster
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Table 3. X-ray Crystallographic Data and Processing Parameters for Co3(C0)4(PMes)2[lr2-q2:q1-
C(Ph)C=C(PPh2)C(O)OC(O)I(p2-PPh2) (4) space group a,A b, A c, A
A deg
v, A3
mol formula fw formula units per cell (2) e, g c m 3 abs coeff b),cm-1 XMo Ka), A collecn range, deg temp, "C max scan time, s scan speed range, deg min-' total no. of data collcd no. of indep data, I > 3dI)
R RW
GOF weights
C(Ph)C=C(PPhz)C(0)OC(011(Ir2-PPhz) (4) with Estimated Standard Deviations in Parentheses" ~
P21/n 11.0047(8) 19.558(2) 17.683(1) 94.947(6) 4664.4(6) C46H43C0307P4 996.54 4 1.419 12.33 0.710 73 2.0 5 28 5 44.0 24 120 0.67-8.0 6253 3795 0.0453 0.0489 0.94 [ O . O 4 F + (ufl21-1
ascertained by the presence of v(C0) bands at 1791 (m) and 1728 (m) cm-l, whose frequency and intensity are consistent with related compounds of this genre.637J1 Cluster 4 exhibits four 31PNMR resonances, as a result of inequivalent phosphine groups. The resonance observed a t 6 172.9 in the 31P{1H} NMR spectrum of 4, which was recorded in THF a t -97 "C, may be confidently assigned t o the p2-phosphido group, on the basis of the chemical shift data reported for clusters 1-3 and related s y s t e m ~ . ~ The , ~ J ~remaining three 31P resonances at 6 7.2, 0.9 (Jp-p = 100.6 Hz), and -10.5 (Jp-p = 100.6 Hz) are tentatively assigned to the PhnP(ma1eic anhydride) and PMe3 groups, respectively. The assignment of these last two phosphorus resonances is strengthened by the X-ray structure of 4, which displays trans PMe3 groups that are situated on adjacent cobalt centers. The 13C{lH} NMR spectrum of 13CO-enriched 4 exhibited three carbonyl resonances at 6 224.0 (Jc-p = 25.5 Hz), 207.4, and 206.7 in an integral ratio of 1:l: 2, respectively. While no attempt has been made to assign these last two carbonyl resonances to specific groups, the first resonance is highly suggestive of a semibridging CO group, as was observed in the solidstate structure of 4. 11. X-ray Crystallographic Results. The molecular structure of 4 was determined by X-ray diffraction analysis. Cluster 4 exists as discrete molecules in the unit cell with no unusually short inter- or intramolecular contacts. The X-ray data collection and processing parameters for 4 are given in Table 3, and the final fractional coordinates are listed in Table 4. The ORTEP diagram in Figure 2 shows the molecular structure of cluster 4 and establishes the disposition of the ancillary PMe3 ligands about the cluster polyhedron and the arachno nature (7 SEP) of this cluster. Table 5 contains selected bond distances and angles for 4. The Co-Co bonds are disparate in length, ranging from 2.422(1) to 2.832(1) A, with the longest Co-Co bond (11)Yang, K.; Bott, S. G.; Richmond, M. G.Organometallics 1994, 13,3767, 3788. (12) Don, M.-J.; Richmond, M. G.Inorg. Chem. 1991,30,1703 and
references therein.
Table 4. Positional Parameters for the Non-Hydrogen Atoms in C0~(C0)4(PMe~)~[lr~-q~:q~-
atom Co(l)
X
Y
z
0.98841(8) 1.00280(9) 1.19085(8) 1.1779(2) 0.9439(2) 0.8319(2) 1.3509(2) 1.1373(5) 0.8520(7) 1.2018(5) 1.3432(5) 0.9313(5) 0.8201(4) 0.7611(5) 1.0838(7) 0.9082(8) 1.1380(8) 1.2829(7) 1.0146(6) 0.9262(7) 0.8396(6) 0.9669(6) 1.0455(6) 1.0410(6) 0.9869(7) 0,9867(8) 1.0419(8) 1.0908(7) 1.0906(7) 0.6778(8) 0.8117(8) 0.8556(9) 1.3501(8) 1.4839(8) 1.4069(9) 1.2245(6) 1.1608(8) 1.1989(9) 1.297(1) 1.365(1) 1.3277(9) 1.2370(7) 1.3502(7) 1.4002(8) 1.3333(9) 1.2219(8) 1.1719(7) 1.0309(7) 1.0800(8) 1.151(1) 1.171(1) 1.123(1) 1.0519(9) 0.7863(7) 0.7499(8) 0.6257(9) 0.5470(9) 0.576(1) 0.7003(8)
0.18816(5) 0.07004(5) 0.09797(5) 0.1505(1) 0.1620(1) 0.2616(1) 0.0289( 1) 0.3088(3) -0.0425(4) 0.0409(3) 0.1877(3) 0.1970(3) 0.1260(3) 0.0454(3) 0.2596(4) 0.0040(5) 0.0572(4) 0.1544(4) 0.1508(4) 0.1640(4) 0.0818(4) 0.0927(4) 0.0407(4) -0.0313(4) -0.0514(4) -0.1190(4) -0.1678(4) -0.1501(4) -0.0827(4) 0.2325(5) 0.3252(4) 0.3183(5) -0.0349(5) 0.0776(5) -0.0182(5) 0.1098(4) 0.0553(5) 0.0183(5) 0.0386(6) 0.0922(6) 0.1296(5) 0.2373(4) 0.2535(4) 0.3188(5) 0.3650(5) 0.3511(5) 0.2861(4) 0.1985(4) 0.2631(5) 0.2859(6) 0.2447(6) 0.1814(6) 0.1576(5) 0.1679(4) 0.2148(5) 0.2177(5) 0.1760(5) 0.1291(6) 0.1261(5)
0.25954(4) 0.20679(5) 0.26958(4) 0.35979(9) 0.15959(9) 0.2682(1) 0.2947(1) 0.2483(3) 0.1590(4) 0.1359(3) 0.2032(3) 0.4389(2) 0.3741(2) 0.3036(3) 0.2529(3) 0.1796(4) 0.1726(4) 0.2305(4) 0.3487(3) 0.3922(3) 0.3254(4) 0.3106(3) 0.2893(3) 0.3090(3) 0.3608(4) 0.3797(4) 0.3472(4) 0.2942(4) 0.2758(3) 0.2728(5) 0.2074(4) 0.3349(4) 0.3552(4) 0.3240(5) 0.2327(5) 0.4344(3) 0.4536(4) 0.5067(4) 0.5411(5) 0.5250(6) 0.4706(5) 0.3738(3) 0.3572(4) 0.3687(4) 0.3962(5) 0.4122(4) 0.4020(4) 0.1005(3) 0.1029(4) 0.0570(5) 0.0110(5) 0.0062(5)
B, A2 2.95(2)
0.0518(5)
0.1238(4) 0.0773(4) 0.0531(5) 0.0738(5) 0.1190(5) 0.1450(4)
a Starred values denote atoms refined isotropically. Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter, defined as 4/3[u2B(l,l) b2B(2,2)+ c2B(3,3) + ab(cos y)B(1,2) + ac(cos /3)B(1,3) + bc(cos a)B(2,3)1.
+
belonging to the PMes-substituted co(l)-c0(3)vector. This elongated co(l)-Co(3) bond is presumed t o result from unfavorable steric interactions and/or a trans influence that originates from the two trans PMe3 groups. The introduction of the first PMe3 ligand in cluster 3 leads t o a 0.083 A increase in the co(l)-co(3) bond relative to the parent cluster 1. Coordination of
Yang et al.
2722 Organometallics, Vol. 14,No. 6, 1995
c4
c4
ClZO
-
Figure 2. ORTEP diagram of the non-hydrogen atoms of C ~ ~ ( C ~ ) ~ ( P M ~ ~ ) Z ~ Z - ~ ~ : ~ ~ - C ( P ~ ) C = C ( P P ~ Z ) C ~ O showing the thermal ellipsoids at the 50% probability level. a second PMe3 ligand as one goes from 3 to 4 lengthens this same bond by 0.053 A. Similar ligand-inducedbond length alterations have been observed by us in other structurally characterized cobalt ~1usters.l~ The two p2-benzylidene-Co bond distances of 1.900(7) and 2.035(7) A and the terminal Co-CO distances that range from 1.725(9) t o 1.775(8) A are in close agreement with the ps-benzylidyne-Co and Co-CO distances, respectively, reported for the nonacarbonyl cluster P ~ C C O ~ ( C OThe ) ~ .semibridging ~~ interaction between co(3) and the C(3)0(3)grou bound to Co(2) is confirmed by the observed 2.278(8) bond length and the distinctly nonlinear bond angles of 128.3(6) and 159.1(7)"exhibited by the Co(3)-C(3)-0(3) and Co(2)C(3)-0(3) groups.l0J5 The bond distances and angles exhibited by the Co-PMes and the p2-v2:~l-benzylidene(dipheny1phosphino)maleicanhydride ligands are unexceptional and require no comment. Unequal Co-phosphido bond lengths are observed in 4, and interestingly enough, when these Co-phosphido bonds in 4 are viewed by using a conventional electroncounting formalism, the shorter C0(2)-P(2) bond of 2.139(2)A corresponds to the donor-acceptor (2e)bond while the longer covalent (le) Co(l)-P(l) bond displays a length of 2.241(2) A. This trend appears to serve as a reliable indicator as t o the coordination mode exhib-
f
(13) (a) Schulman, C. L.; Richmond, M. G . ;Watson, W. H.; Nagl, A. J . Organomet. Chem. 1989,368, 367. (b) Richmond, M. G.; Kochi, J. K. Organometallics 1987,6, 254. (14)(a) Colbran, S. B.; Robinson, B. H.; Simpson, J. Acta Crystallogr. Sect. C 1986,42,972. (b) Ahlgren, M.; Pakkanen, T. T.; Tahvanainen, I. J . Organomet. Chem. 1987,323, 91. (15) Horwitz, C. P.; Shriver, D. F. Adu. Organomet. Chem. 1984, 23, 219.
ited by the Co-phosphido bond in parent cluster 1 and its relatives that have been structurally characterized to date, as is summarized in Table 6 for clusters 1-4. 111. Cyclic Voltammetry Data. The cyclic voltammetry studies were conducted at a platinum electrode in CH2C12 containing 0.2 M TBAP as the supporting electrolyte. Figure 3 shows the room-temperature cyclic voltammogram of 4 at a scan rate of 0.1 V s-l. Two well-defined, diffusion-controlled redox responses are observed at E112= 0.30 V and El12= -1.06 V, along with = -1.87 V. an irreversible reduction (not shown) a t EPC The first two redox responses are readily assignable to the O h + and 011- redox couples. These two redox couples represent fully reversible, one-electron processes, on the basis of the measured peak current ratios of unity and plots of the current function (Ip)vs the square root of the scan rate, which were found to be linear over the scan range of 0.1-1.0 V s-l.16 Calibration of the peak currents against ferrocene recorded under analogous conditions and application of Walden's rule confirmed the one-electron nature of each couple.16b The reduction potential recorded for cluster 4 is in good agreement with the electrochemical data reported for the disubstituted clusters PhCCo3(C0)7P2(where P2 = two monodentate ligands or one bidentate ligand).17 (16) (a)Rieger, P. H. Electrochemistry; Chapman & Hall: New York, 1994. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (17)(a) Watson, W. H.; Nagl, A.; Hwang, S.; Richmond, M. G. J . Organomet. Chem. 1993,445,163. (b)Yang, K.; Bott, S. G . ;Richmond, M. G . J . Organomet. Chem. 1993, 454, 273. (c) Downard, A. J.; Robinson, B. H.; Simpson, J. Organometallics 1986,5, 1122, 1132, 1140. (d) Hinkelmann, K.; Heinze, J.; Schacht, H.-T.; Field, J. S.; Vahrenkamp, H. J . Am. Chem. SOC.1989,111,5078.
Anhydride-Mediated PMe3 Addition to a Cog Cluster
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Organometallics, Vol. 14, No. 6, 1995 2723
Table 5. Selected Bond Distances (di) and Angles (deg) in Co3(C0)4(PMe3)2[C12-t12:t11-
C(Ph)C=C(PPh3C(O)OC(O)l~2-PPhz)(4)" Co(l)-Co(2) c0(2)-c0(3) c O ( 1)-P(3) Co(l)-C(11) Co(2)-P(2) c0(2)-c(3) c0(3)-P(1) c0(3)-c(3) C0(3)-C(16) P(l)-C(111) P(2)-C(211) P(3)-C(31) P(3)-C(33) P(4)-C(42) 0(1)-C(l) 0(3)-C(3) 0(12)-C(12) 0(13)-C(14) C(1l)-C( 12) C(14)-C(15)
Bond Distances 2.592(1) cO(l)-C0(3) 2.422(1) Co(l)-P(2) 2.265(2) CO(l)-C(l) 2.068(7) c0(l)-C(15) 2.139(2) C0(2)-C(2) 1.750(9) C0(2)-C(16) 2.235(2) c0(3)-P(4) 2.278(8) c0(3)-c(4) 2.035(7) P(l)-C(ll) 1.836(7) P(l)-C(117) 1.827(8) P(2)-C(217) l.802(9) P(3)-C(32) 1.83(1) P(4)-C(41) 1.806(9) P(4)-C(43) 1.138(9) 0(2)-C(2) 1.16(1) 0(4)-C(4) 1.201(9) 0(13)-C(12) 1.402(9) 0(14)-C(14) 1.45(1) C(ll)-C(15) 1.48(1) C(15)-C(16)
2.832(1) 2.241(2) 1.762(7) 2.198(7) 1.725(9) 1.900(7) 2.242(2) 1.775(8) 1.788(7) 1.832(7) 1.830(8) 1.813(9) 1.815(9) 1.80(1) 1.16(1) 1.14(1) 1.405(9) 1.180(9) 1.47(1) 1.44(1)
Bond Angles C O ( ~ ) - C O ( ~ ) - C O ( ~52.81(3) ) C O ( ~ ) - C O ( ~ ) - C O ( ~68.68(4) ) C O ( ~ ) - C O ( ~ ) - C O ( ~58.51(4) ) C O ( ~ ) - C O ( ~ ) - C ( ~82.5(2) ~) Co(l)-C0(3)-P(4) 170.33(8) C O ( ~ ) - C O ( ~ ) - C ( ~74.3(2) ~) C O ( ~ ) - C O ( ~ ) - P ( ~127.28(7) ) P(l)-c0(3)-P(4) 100.48(8) p(4)-C0(3)-C(3) 97.7(2) P(4)-c0(3)-C(4) 91.1(2) P(4)-C0(3)-C(16) 103.4(2) C(4)-C0(3)-C(16) 160.8(3) C0(3)-P(l)-C(ll) 92.2(2) Co(l)-P(2)-C0(2) 72.53(7) Cdl)-C(l)-O(l) 174.6(7) C0(2)-C(2)-0(2) 174.6(7) c0(2)-C(3)-C0(3) 72.6(3) C0(2)-C(3)-0(3) 159.1(7) C0(3)-C(3)-0(3) 128.3(6) C0(3)-C(4)-0(4) 176.2(7) Co(l)-C(ll)-P(l) 99.7(3) C0(l)-C(ll)-C(l2) 120.9(5) C0(l)-C(ll)-C(l5) 74.6(4) P(l)-C(ll)-C(l2) 130.2(5) P(l)-C(ll)-C(l5) 111.6(5) C(l2)-C(ll)-C(l5) 106.4(6) 0(12)-C(12)-0(13) 119.1(7) 0(12)-C(l2)-C(ll) 132.3(7) 0(13)-C(12)-C(ll) 108.5(6) 0(13)-C(14)-0(14) 120.8(7) 0(13)-C(14)-C(15) 107.3(6) 0(14)-C(14)-C(15) 131.8(7) C0(l)-C(l5)-C(ll) 65.1(4) C O ( ~ ) - C ( ~ ~ ) - C O75.9(3) (~) C(15)-C(16)-C(17) 122.8(6) a Numbers in parentheses are estimated standard deviations in the least significant digits.
Table 6. Bond-Length Comparisons of the Asymmetric Cobalt-Phosphido Moiety in Clusters 1-4 cluster 1 2
3 4
donor-acceptor (2e) bond, A
covalent (le) bond, A
ref
2.129(3) 2.129(7) 2.135(3) 2.139(2)
2.264(3) 2.188(6) 2.253(3) 2.241(2)
7 6 6 this work
This suggests that the LUMO in 4 is best described as an antibonding Co-Co orbital, with little or no contribution from the p2-q2:q1-benzylidene(diphenylphosphinolmaleic anhydride ligand. This stands in contrast to compounds that are substituted with an intact 2,3-bis(dipheny1phosphino)maleic anhydride (bma) ligand, where it has been shown that the initial reduction occurs over a potential range of -0.5 to -0.8 V and at the x* orbital that is localized on the bma ligand.llJs To verify this premise, we have carried out preliminary extended Huckel calculations on the model compound
-
I
0: 4
7
-0.4
-1.2
Potential (Volts)
Figure 3. Cathodic scan cyclic voltammogram of CO~(CO)~-
(PM~~)Z~~-~~:~~-C(P~)C=C(PP~~)C(O)OC(O)I(~Z (ca. M) in CHzClz containing 0.2 M TBAP at 0.1 V s-l. order to explore the nature of the LUMO in this genre of cluster. Our MO results indicate that the LUMO corresponds to an antibonding a2 orbital that involves the three cobalt atoms, the nature of which is consistent with the MO data published by Hoffmann on ligandcapped M3L9 ~1usters.l~ Conclusions I
The reaction of Co3(CO)s(PMe3)l&-q2:q1-C(Ph)C=CI
( P P ~ Z ) C ( O ) O C ( O ) ] + ~ - Pwith P ~ ~added ) PMe3 proceeds to give the bis(phosphine1 cluster C O ~ ( C O ) ~ ( P M ~ ~ ) ~ C U ~ -
q2:q1-C(Ph)C=C(PPhz)C(0)OC(O)l+2-PPh2), as a result of a regioselective addition of PMe3 to the maleic anhydride substituted cobalt center in 3. The observed second-order kinetics and activation parameters support this contention. The X-ray structure of 4 confirms the site of PMe3 addition in 3. Cyclic voltammetry data indicate the existence of accessible oxidation (O/l+) and reduction ( O h - ) couples, with the nature of the latter redox couple being best described as an antibonding metal-based a2 orbital. Experimental Section General Considerations. The PhCCo3(C0)gZ0and the bma ligandz1 that were used in the synthesis of Co~(C0)5I
(PMe~)[u~-y2:~1-C(Ph)C=C(PPh~)C(O)OC(O)l~~-PPh~)6 were prepared according to literature procedures, while PMe3 was synthesized from P(OPh)3 and MeMgI.22 The solvents THF, BuzO, pentane, and benzene used in these studies were distilled from sodiumhenzophenone ketyl under argon. CHzClz was distilled similarly from CaHz. All distilled solvents were handled under argon using Schlenk techniques, and when not in use they were stored in Schlenk vessels equipped with Teflon stopcocks.23 The 13C0 (99%)used in the synthesis of 13CO-enriched 4 was purchased from Isotec, Inc. The tetran-butylammonium perchlorate (TBAP; Caution! strong oxidant) was obtained from Johnson Matthey Electronics and recrystallized from ethyl acetate/petroleum ether and dried for (19)Schilling,B. E. R.; Hoffmann, R. J . Am. Chem. SOC.1979,101, 3456.
CO~(CO)&-~~:T,J-C(P~)C=C(PH~)C(O)OC(O)I+~-PH~) in (20) Nestle, M. 0.;Hallgren, J. E.; Seyferth, D. Inorg. Synth. 1980, (18)(a)Yang, K; Bott, S. G.; Richmond,M. G. Organometallics 1995, 14,2387.(b)Tyler, D.R. Acc. Chem. Res. 1991,24,325and references therein. (c) Mao, F.; Tyler, D. R.; Bruce, M. R. M.; Bruce, A. E.; Rieger, A. L.; Rieger, P. H. J . Am. Chem. SOC.1992,114,6418. (d) Fenske, D.; Becher, H. J. Chem. Ber. 1974,107,117;1975,108, 2115.(e) Becher, H. J.; Bensmann, W.; Fenske, D. Chem. Ber. 1977,110,315.
20,226. (21)Mao, F.; Philbin, C. E.; Weakley, T. J. R.; Tyler, D. R. Organometallics 1990,9, 1510. (22)Luetkens, M. L.,Jr.; Sattelberger, A. P.; Murray, H. H.; Basil, J. D.;Fackler, J. P., Jr. Inorg. Chem. 1989,26,7. (23)Shriver, D. F. The Manipulation ofAir-Sensitiue Compounds;
McGraw-Hill: New York, 1969.
Yang et al.
2724 Organometallics, Vol. 14, No. 6, 1995 up to 48 h prior to use. The microanalysis was performed by Atlantic Microlab, Norcross, GA. All infrared spectra were recorded on a Nicolet 20SXB FTIR spectrometer. Room-temperature spectra were recorded in 0.1 mm NaCl cells, while low-temperature IR spectra were recorded by using a Specac Model P/N 21.000 variabletemperature cell equipped with inner and outer CaFz windows. Dry ice/acetone was employed as the coolant, and the cell temperature was measured with the aid of a copper-constan(75 MHz) and the 31P(121 MHz) tan thermocouple. The NMR spectra were recorded on a Varian 300-VXR spectrometer. All 31Pchemical shifts are reported relative to external H3P04 (85%),taken to have 6 0. Positive chemical shifts represent resonances t h a t are to low field of the external standard.
maps. With the exception of the phenyl groups associated with the phosphorus atoms, all non-hydrogen atoms were refined anisotropically. Refinement converged at R = 0.0453 and R, = 0.0489 for 3795 unique reflections with Z > 3dZ). Electrochemical Studies. Cyclic voltammograms were obtained with a PAR Model 273 potentiostatJgalvanostat, with all CV experiments carried out by using positive feedback circuitry to compensate for iR drop. The airtight cyclic voltammetry cell was of homemade design and employed a three-electrode configuration. All electrochemical experiments employed platinum disks as the working and auxiliary electrodes. The reference electrode in all experiments consisted of a silver-wire quasi-reference electrode, with all potential data reported relative to the formal potential of the Cp*zFe/ Cp*zFe+(internally added) redox couple, taken to have Eli2 = -0.23 V.24 The Cp*zFe/Cp*zFe+potential used as a n offset Synthesisof Cos(C0)4(PMes)z[C1~.11~:11 W(Ph)C=C(PPhz)value in these studies was obtained from a CV experiment I using ferrocene as a reference. C(O)OC(O)](pz-PPh2).To a Schlenk tube containing 0.1 g I Kinetic Studies. All kinetic studies were carried out under (0.11m o l ) of Co3(CO)s(PMe3)Cuz-rlz:r11-C(Ph)C=C(PPh~)C(O~C- pseudo-first-order conditions with concentrations of PMe3 (O)](pz-PPhz) in 20 mL of CHzClz a t room temperature was being at least 20 times greater than that of cluster 3. The added 1.5 equiv of PMe3 (0.34 mL of a 0.48 M solution of PMe3 reactions were monitored spectrophotometrically by using a in CHzC12). The reaction mixture was stirred for 1.0 h, Hewlett-Packard 8452A W - v i s spectrometer equipped with followed by IR and TLC examination, which revealed the a variable-temperature cell. The extent of the reaction was complete conversion to the desired bis(phosphine) cluster 4. determined by following the W - v i s changes in the 374 nm The product was subsequently isolated by chromatography band of 3 for at least 5 half-lives. A VWR refrigerated constant over silica gel using CHzClz as the eluant. The analytical temperature circulator was used to maintain a constant sample was recrystallized from a benzene solution containing temperature, t o within *0.2 "C. Plots of ln(A, - At) vs time 4 that had been layered with pentane. Yield: 0.058 g (53%). were linear and gave the pseudo-first-order rate constants IR (CHZC12): v(C0) 1992 (sh), 1978 (vs), 1865 (m), 1791 (m, (kobad) shown in Table 1. The second-order rate constants asym bma C-0),1728 (m, sym bma C=O) cm-l. 31P(1H)NMR displayed in Table 2 were obtained from plots of k&sd vs [PMe31, (THF, 176 K): 6 172.9 (pz-PPhz), 7.2 [PhzP(maleic anhydride)], while the activation parameters were calculated by using the 0.9 (PMe3, J p - p = 100.6 Hz), -10.5 (PMe3, J P - p = 100.6 Hz). Eyring equation.25 13C{1H)NMR (THF, 176 K): 6 224.0 (semibridging CO, J c - p = 25.5 Hz), 207.4 (lCO), 206.7 (2CO). Anal. Calcd (found) Acknowledgment. We are grateful for financial for C45H43Co30,P4"/2CsHs: C, 55.67 (55.29); H, 4.48 (4.76).
X-ray Diffraction Structure of Cos(C0)4(PMes)~[C1z-11~:
, ~ ] W ( P ~ ) C = C ( P P ~ ~ ) C ( O ) O C ( O ) ] ( ~ ~Single - P P ~crystals Z). for X-ray diffraction analysis were grown from a CHzClz solution of 4 t h a t had been layered with pentane. A suitable black crystal, of dimensions 0.25 x 0.27 x 0.30 mm3, was chosen and sealed inside a Lindemann capillary, followed by mounting on the goniometer of a n Enraf-Nonius 0 - 4 diffractometer that employed Mo K a radiation. Cell constants were obtained from a least-squares refinement of 25 reflections with 28 > 30". Intensity data in the range 2.0 5 28 5 44.0" were collected at room temperature using the w-scan technique in the variable-scan speed mode and were corrected for Lorentz, polarization, and absorption (DIFABS). Three reflections (600; 0,10,0; 0,0,10) were measured after every 3600 s of exposure time in order to monitor crystal decay (-=1%). The structure was solved by SHELX-86, which enabled all of the non-hydrogen atoms to be located in the difference Fourier
support from the Robert A. Welch Foundation (Grant B-1202-SGB and B-1039-MGR) and the UNT Faculty Research Program. Supplementary Material Available: Textual presentations of the crystallographic experimental details and listings of crystallographic data, bond distances, bond angles, and hydrogen positional and thermal parameters for Co3(C0)4-
-
( P M ~ ~ ) ~ [ ~ Z - ~ ~ : ~ ~ - C ( P ~ ) C = C ( P P ~ ~ ) C ( O )(15 OC(O)I(~Z-P pages). Ordering information is given on any current masthead page.
OM9500376 (24) Ryan, M. F.; Richardson, D. E.; Lichtenberger, D. L.; Gruhn, N. E. Organometallics 1994, 13, 1190. (25) Carpenter, B. K. Determination of Organic Reaction Mechanisms; Wiley-Interscience: New York, 1984.
