1053
Organometallics 1996, 14, 1053-1060
Experimental and Theoretical Study of #-Effects in P-Coordinated (Dipheny1phosphino)alkynes Elmostafa Louattani, Agusti Lled6s,* and Joan Suades" Departament de Quimica, Edifici C, Universitat Autbnoma de Barcelona, 08193 Bellaterra, Spain
Angel Alvarez-Larena and Joan F. Piniella Departament de Geologia, Edifici C, Universitat Autdnoma de Barcelona, 08193 Bellaterra, Spain Received July 1, 1994@
In order to study the nature of n-effects on phosphinoalkynes, we have carried out a combined experimental and theoretical study on [(X)AzPC=CR]+systems where X = H, CH3, Fp (Fp = CpFe(C0)d and A = H, Ph. The P-coordinated (dipheny1phosphino)alkynemetal complexes [(Fp)PhzPC=CRI[BF41 have been prepared and characterized by microanalysis and IR, lH, 13C, and 31PNMR spectroscopy. The crystal structure of [(Fp)PhzPC=CPhI+ has been determined by X-ray diffraction. This compound crystallizes in the triclinic space group P-1 with unit cell parameters a = 9.7770) A, b = 10.351(1) c = 12.857(5) a = 92.38(2)",p = 103.07(2)",y = 100.02(1)",D,= 1.469 g ~ m -2~=, 2. Least-squares refinement using all 4068 independent reflections led to a final R value of 0.049 (all data). Ab initio calculations with geometry optimization have been performed in related model systems. A natural population and natural bond orbital analysis of the wave functions has been performed. The experimental difference between the 13CNMR chemical shifts of acetylenic carbons and the calculated difference between NPA atomic charges is linearly correlated. A n-electron transfer from the filled n ( C W ) orbitals to the empty phosphorus d orbitals has not been observed. When X = H, CH3 a strong polarization of the n(C=C) bond is detected, when X = Fp the polarization is reduced and n-back-donation from metal d orbitals to the empty d p - A orbitals is found.
A,
Introduction Trivalent phosphorus donor ligands P& (A = H, OR, F, C1, alkyl, aryl) play a major role in coordination and organometallic chemistry. The n-bonding properties of this kind of ligand is a controversialsubject. The extent of the n-bonding depends strongly on the nature of the groups attached to the phosphorus atom, particularly on the electronegativity of these groups. If A is relatively electronegative, such as OR, C1 or F, it is generally accepted that the n-acceptor behavior may be important.1 This is especially true for PF3, which forms many compounds comparable to those of C0.2 At the other extreme, tertiary phosphines such as P(CH3)3 exhibit no such tendency. However, it has recently been shown that even a trimethylphosphineligand has nonnegligible n-acceptor properties.3 The nature of the x-acceptor orbitals entails controversy. In the classical picture, the x-acceptor orbitals are the empty P3d. However, it has been proposed that the n-acceptor orbitals are the n*pA3, which have essentially a phosphorus 3p character with a small 3d ~omponent.~ These n-accepting orbitals have local u*
symmetry with respect to the P-A bond axis, and are also called d p - ~ . Recent studies have clearly shown the n-acceptor role of the $P-A orbitak5+j Orpen et al.5a2b have studied the geometry variations in the M-P& units of 24 pairs of transition metal complexes in two different oxidation states, whose crystal structures were known. Metal-phosphorus bond lengths increase on oxidizing the metal, consistent with the presence of an important element of M-P n-back-bonding. Reduction of M-P n-bonding causes a decrease in the average P-A bond lengths, in agreement with the P& n-acceptor orbitals having d character. Reed and Schleyer,6ain a thorough study of the nature of chemical bonding in species of the X3EY type, including E = P, have proven that the ny U*E-X interaction, the socalled negative hyperconjugation, is the primary contribution to ZE-Y bonding. The same conclusion has been reached in an ab initio study of zerovalent metal phosphine complexes.* Two recent articles addressed the study of n-effects of phosphine ligands in transition metal complexe~.~ Phosphinoalkynes are very interesting ligands for the study of n-effects. The presence of the alkyne n-system
-
~
Abstract published in Advance ACS Abstracts, December 15,1994. (1)Cotton, F.A.; Wilkinson, G.Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988;pp 64-68. (2)Nixon, J. F.Adv. Inorg. Chem. Radiochem. 1985,29,42. (3) Wang, S.P.; Richmond, M. G.; Schwartz, M. J. Am. Chem. SOC. 1992,114,1595. (4)(a)Xiao, S.X.; Trogler, W. C.; Ellis, D. E.; Berkovitch-Vellin, Z. J. J.Am. Chem. SOC.1963,105,7033. (b)Marynick, D.S. J.Am. Chem. (c) Braga, M.Inorg. Chem. 1986,24,2702. SOC.1984,106,4064. @
A,
~~~
(5)(a) Orpen, A. G.; Connelly, N. G. J. Chem. SOC.,Chem. Commun. 1985,1310.(b) Orpen, A. G.; Connelly, N. G. Organometallics 1990, 9,1206. (6)(a) Reed, A.E.; Schleyer, P. v. R. J.Am. Chem. Soc. 1990,112, (c) 1434.(b) Pacchioni, G.; Bagus, P. S. Inorg. Chem. 1992,31,4391. Wang, P.;Zhang, Y.;Glaser, R.; Reed, A. E.; Schleyer, P. v. R.; Streitwieser, A. J.Am. Chem. SOC.1991,113, 55. (7)(a) Lichtenberger, D.L.; Jatcko, M. E. Inorg. Chem. 1992,31, 451. (b) Kraatz, H.B.; Jacobsen, H.; Ziegler, T.; Boorman, P. M. Organometallics 1993,12,76.
0276-733319512314-1053$09.00/0 0 1995 American Chemical Society
Louattani et al.
1054 Organometallics, Vol. 14, No. 2, 1995
offers the possibility to weight up these effects. It has been proven that in phosphinoalkynes, the electronic distribution of the acetylenic group is very sensitive to changes around the P atom that can be related to the retrodonation toward the phosphorus.8 Moreover, these changes can be measured by means of different experimental techniques. The difference between the 13C chemical shifts of acetylenic carbons in a series of phosphonium salts (Ph3PR+, R = unsaturated group) has been used as a measure of the carbon-carbon triple bond polarization. The deshielding extension of the ,8 carbon was interpreted t o be a consequence of Ph-C,, bonding.8b The magnitude of the increase in the v(C=C) between the free PhzPCWPh ligand and its transitionmetal complexes has been taken as a measure of the retrodative bonding from the transition-metal atom Figure 1. Cationic fragment of compound 4. toward the phosphorus atom.8c In order to understand the nature of the n-effects on Scheme 1 phosphinoalkynes, we carried out a combined experimental and theoretical study. A series of new Pcoordinated transition-metal cationic complexes [(FplPh2PC=CR)I+(Fp = (C&t5)Fe(CO)2) have been ~ r e p a r e d . ~ All of them have been characterized by means of infrared and 13C NMR spectroscopic techniques. The structure of the [(Fp)PhzPC=CPhI+ complex has been determined by X-ray diffraction analysis. Ab initio calculations have been performed in the H2PCWR (R (a) (b) = H, CH3, COOCH3) and [(X)HzPC=CRI+(X= H, CH3, Fp; R = H, CH3, COOCH3) systems. The ensemble of Table 1. Selected X-ray Bond Distances (A) and Angles (deg) with Esd's in Parentheses for [(Fp)PhzPC=CPh]+ experimental and theoretical results can shed more light Cation (4) on the controversy about trivalent phosphorus ligands Fe-C3 1.785(3) P-c 1 1.746(3) n-effects.
Experimental Results Synthesis and Characterization. A series of stable cationic P-coordinated (dipheny1phosphino)alkynecomplexes [(Fp)Ph2PCWR]+(R = H (l),CH3 (2), tBu (31, Ph (4), To1 (51, COOMe (6))were prepared in high yield by oxidation of [Fez(CO)&pzl with ferricinium cation in the presence of (diphenylphosphino)alkyne,according t o the reaction 1/2 [Fp,]
+ [FeCp,]' + Ph,PC=CR
-
[(Fp)Ph,PC=CR]+
+ FeCp,
In the infrared spectrum all prepared complexes exhibit two bands (1): 2061,2020 cm-l) assigned t o the v(C0) of the fragment [CpFe(CO)zl+.Moreover, in the lH and l3C NMR spectra the signals corresponding to the presence of an y5-cyclopentadienylligand are observed. In this context, two different ligand coordination possibilities can be proposed according to the bifunctional character of (dipheny1phosphino)alkynes:(a) y2-alkyne coordination, (b) P-coordination (Scheme 1). Both structures have been described in the literature, (a) in y2alkyne iron complexesll [CpFeCO(L)(y2-alkyne)1+ and (8) (a) Nucciarone, D.;MacLaughlin, S. A.; Taylor, N. J.; Carty, A. Organometallics 1988, 7, 106 and references therein. (b) Albright,T. A.; Freeman, W. J.; Schweizer, E. E. J. Am. Chem. SOC.1976,97,2946. (c) King, R. B.;Efraty, A. Inorg. Chim. Acta 1970, 4 , 319. (9) Iron cationic complexes of PhZPCWPPhZ had already been preparedlo by unsymmetrical cleavage of [{ CpzFez(CO)&(PhZPC= CPPhz)]. (10)Carty, A. J.; Efraty, A.; Ng, T. W.; Birchall, T. Inorg. Chem. 1970, 9, 1263. (11)Reger, D. L.; Klaeren, S. A,; Lebioda, L. Organometallics 1988, 7, 189.
Fe-C4 Fe-P C3-03 C4-04
1.787(4) 2.207(1) 1.138(4) 1.130(4)
P-c21 P-C3 1 Cl-C2 c2-c11
C4-Fe -P C3-Fe-P C3-Fe-C4 Fe-P-C31 Fe-P-C2 1
92.1(0.1) 92.5(0.1) 95.2(0.1) 116.3(0.1) 114.7(0.1)
Fe-P-C1 C21-P-C31 C1 -P-C31 Cl-P-C21 P-Cl-C2
1.825(3) 1.815(3) 1.202(5) 1.432(4) 112.3(0.1) 104.1(0.1) 105.9(0.1) 102.1(0.1) 174.7(0.3)
(b) in triphenylphosphino12 metal complexes [(FplPPh31+. The 31PNMR spectra of 1-6 show a signal at 38-47 ppm which is assigned to the phosphorus atom coordinated to an iron atom. On the other hand, the IR band near 2200 cm-l together with the C1 and CZ 13C NMR chemical shifts indicate the presence of an uncoordinated triple bond. All these data are consistent with structure (b). Crystal Structure of [(Fp)PhzPC=CPhl[BF41 (4). The structure is composed of discrete cations and [BFJ anions without significant interactions between them. The structure of the cation [(Fp)PhzPC=CPh]+with the adopted atom numbering system is shown in Figure 1; selected interatomic distances and angles are given in Table 1. The structure of the cation consists of a PhzPC=CPh ligand bonded to a [FpI+fragment through a P-Fe bond. The existence of the uncoordinated alkyne function is confirmed by the long values of the distances between the iron atom and the acetylenic carbons (Fe-Cl = 3.294(3) 8;Fe-C, = 4.275(3) A) and by the bond distance c1-C~. A search in the Cambridge Structural ~~
(12) Sim, G. A,; Woodhouse, D. I.; Knox, G. R. J.Chem. SOC.,Dalton Trans. 1979, 629.
z-Effectsin P-Coordinated Phosphinoalkynes
Organometallics, Vol. 14, No. 2, 1995 1055
Table 2. Bond Lengths (A) in X-ray Structure Determination of Compounds with the Fragmenv
x\
/ TC1 =c2Ph
Ph
compound
X
[(Fp)PhzPCICPh]+ [Fe3(CO)6013-~~)013-Se)(Ph~PC=CPh)31 [F~~(CO)S~~Z-CO)~~~-S)(P~~PCEC~)]
P-X
Ci-Cz
P-Ci
Fe 2.207(1) 1.202(5) 1.746(3) Fe 2.220(6)b 1.181(3)b 1.753(2)b Fe 2.238(2) 1.171(9) 1.744(6) [(F~(CO)~)Z@-P~ZPC=CC=CPP~Z)]~ Fe 2.218(2)b 1.197(7)b 1.767(6)b [COZ~-~~-[(OC)~W(PP~~)~C"C[C'C(PP~~)W(CO)~~}(CO)~~~ W 2.496(6)b 1.21(4)b 1.74(3)b [{Mn(CO)z(Cp)}2(PhzPCECPPhz)I Mn 2.196(2)b 1.199(5) 1.774(4)b [Ru~(CO)~I(P~ZPC=CP~)] Ru 2.342c 1.182' 1.75ge [ R U ~ ( C ~ ) ~ ~ - C ~ C ' B U ) ~ - ~ ~ - C I C ' B U ) ~ - P P ~ ~ ) ~RU ( P ~2.314' ~ P C ' C ~ 1.199' U)] 1.47ge [ R U ~ ( C O ) S ~ - P P ~ ~ ) Z ~ - ~ ~ - C I C ' B U ) Z ~ ~ - ~ * - C ~ ~RU B U )2.383' ( P ~ Z P C1.19ge ~ C ' B U ) 1.750' ] [FeRuzP-C1)Z(CO)S(P~ZPC=C~BU)Z] Ru 2.380(5)b 1.19(4)b 1.75(2)b [Ru3(CO)s@3-S)z(PhzPCWtB~) Ru 2.284(1) 1.192(6) 1.744(5) [(oS3(co)i dz(PWC'CPPh2)I Os 2.33(1)b 1.13(7) 1.80(5)b [C~N~OS~@-H)S(CO)S(P~~PC~CFY)]~ Os 2.334(9)b 1.18(5)b 1.74(3)b [Pt(SCN)2(Ph2PC=C1Bu)2] Pt 2.256(7)b 1.220(4)b 1.696(3)b [MePhzPCICBPhs] C 1.790(5) 1.216(5) 1.699(4) [MePhzPC=CC(PPhzMe)2l2+ C 1.79(1) 1.19(1) 1.70(1) [Ph,PC=CMnBr(CO)s] C 1.78(1) 1.21(1) 1.68(1) [(Ph3P)C12Pd(CECPPh3)] C 1.79(1) 1.23(2) 1.71(1) [Ph2PC=CPPh2] 1.207(5) 1.765(4) [PhzPCECB(mes)z] 1.217(4) 1.754(3) [Fe(CO)z(Cp)(PPh3)1+ Fe 2.240(1)
P-CphcnyIa
R
1.820(3) 1.832' 1.812(5) 1.807(4) 1.82(1) 1.830(5) 1.828' 1.828' 1.827' 1.83(2) 1.822(5) 1.86(4) 1.81(2) 1.801(3) 1.788(4) 1.80(1) 1.78(1) 1.79(1) 1.832(3) 1.830(3) 1.817(5)
0.049 0.088 0.033 0.042 0.055 0.036 0.035 0.039 0.037 0.083 0.029 0.085 0.055 0.057 0.066 0.047 0.063 0.097 0.061 0.040 0.055
reference (REFCODE")
this work 14b (LAGHIK) 14a (GETMEN) 14c (SUBNOS) 14c (SUBNIM) 17a (FMLlR) 17b (CARTEU) 1 7 (BAYCOS) ~ 17d (BOBTUH) 17e (FEBARU10) 14a (GEJMIR) 17f (TAGNUK10) 17g (DIFBAV) 17h (CBYNPT) 16a (SEDRAU) 16b(FITDAN) 16c (MNPACY10) 16d (VUPDOZ) 15a (DPHPAC) 15b (PELVAD) 12
Search on Cambridge Structural Data Base; X = carbon or transition metal; the structures where the fragment is placed in a part of a cycle are omitted. Mean value. The unit cell contains two independent molecules. Nonacetylenic compound for data comparison. e Esd not reported.
Data Base13 (v. 5.07,April 1994)shows that only a few complete X-ray structures of complexes having the [FePPhzC=C-I fragment with uncoordinated alkynes are reported.14 In Table 2 structural data for (diphe(diphenylphosphonium)alkynes,16 nylpho~phino)alkynes,1~ and P-coordinated metal complexes of (diphenylphosphino)alkynes1*J7 are presented. On comparing the Fe-P bond distance of 2.207(1)A observed in complex 4 with the values of the related previously reported complexes, a slight contraction is observed. The CpFe(C0)2 fragment has very similar geometry to that found in [(Fp)APh3]+(A = P, As, Sb, Bi) complexes.ls Spectroscopic Analysis of the Acetylenic Fragment. IR Spectra. The v(C=C) frequencies of free (dipheny1phosphino)alkynes appear at lower values than the normal range for disubstituted alkynes (2190(13)Allen, F.H.; Davies, J. E.; Galloy, J. J.; Jhonson, 0.;Kennnard, 0.; Mitchell, G. F.; Smith, J. M.; Watson, D. G. J. Chem. Inf. Comp. Sci. 1991,31,187. (14)(a)Hogarth, G.; Taylor, N. J.; Carty, A. J.; Meyer, A. J. Chem. SOC..Chem. Commun. 1988.834.(b) Imhof. W.Eber. B.: Huttner. G.: Emmerich, C. J . Organomet. Chem. 1993,'447,21.(c) Adams, C'. J.: Bruce, M. I.; Horn, E.; Tieknik, E. R. T. J . Chem. SOC.,Dalton Trans. 1992,1157. (15)(a) Bart, J. C. J. Acta Crystullogr., Sect. B 1969,25,489.(b) Yuan, Z.;Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams, I. D.; Cheng, L. J. Organomet. Chem. 1993,449, 27. (16)(a) Bestmann, H. J.; Behl, H.; Bremer, M. Angew. Chem., Int. Ed. Engl. 1989,28, 1219. (b) Schmidbaur, H.; Pollok, T.; Reber, G.; Muller, G. Chem. Ber. 1987,120,1403.(c) Goldberg, S.Z.; Duesler, E. N.; Raymond, K. N. Inorg. Chem. 1972,11, 1397. (d) Sunkel, K. J . Organomet. Chem. 1992,436,101. (17)(a) Orama, 0.J . Organomet. Chem. 1986,314,273.(b) Carty, A. J.; MacLaughlin, S. A.; Taylor, N. J. J . Organomet. Chem. 1981, 204,C27.(c) Carty, A. J.; Taylor, N. J.; Smith, W. F. J . Chem. SOC., Chem. Commun. 1979,750.(d) Carty, A.J.; MacLaughlin, S. A.; Van Wagner, J.; Taylor, N. J. Organometallics 1982,I , 1013.(e) Jones, D. F.; Dixneuf, P. H.; Southern, T. G.; Le Marouille, J . Y.; Grandjean, D.; Guenot, P. Inorg. Chem. 1981,20,3247.(0Amoroso, A. J.; Johnson, B. F. G.; Lewis, J.; Massey, A. D.; Raithby, P. R.; Wong, W. T. J . Organomet. Chem. 1992,440, 219.(g) Sappa, E.;Predieri, G.; Tipipicchio, A.; Tiripicchio Camellini, M. J . Organomet. Chem. 1985,297, 103.(h) Wong, S.;Jacobson, S.; Chieh, P. C.; Carty, A. J . Inorg. Chem. 1974,13,284. (18)Schumann, H.; Eguren, L. J. Organomet. Chem. 1991,403,183.
2260 cm-1).8c In [(Fp)Ph2PC=CRl+ complexes 2 to 6, the v(C=C) frequencies are observed in the range 22002150 cm-l as a band of medium intensity, with the exception of 1 where no v(C=C) stretching is 0b~erved.l~ In complex 3,two bands a t 2209 and 2169 cm-l are observed, one is assigned to the v(C=C) and the other one is assigned to a Fermi resonance observed in substituted tert-butylalkynes.20The v(C=C) frequencies of complexes 2, 4, 5, and 6 are shifted 15-42 cm-' to higher frequencies in comparison with those of the free (diphenylphosphino)alkynes,and a similar value (Av = 45 cm-l) was found in the propyinyltriphenylphosphonium cation.21 We wish to emphasize that the intensity of the v(C=C) band increases in complexes 2 , 4 , 5 , and 6 in comparison with those of the free (diphenylphosphino)alkynes. This fact may be related t o an increase in the triple bond dipole moment upon coordination. lSC NMR Spectra. The 13CNMR spectra of (dipheny1phosphino)alkynes ligands and their metal complexes 1-6 show two characteristic signals in the region 70-130 ppm assigned to acetylenic carbons (Table 3). The spectra of complexes 1-6 show an important increase in IIJc-pl with respect to the free ligands (85115 Hz compared to 0-24 Hz). IlJc-pl high values were reported for methyltriphenylphosphonium salt [PhsPC=CMe]+ (191.7 H Z ) where ~ ~ the P atom is also quaternized. The 13C resonances of acetylenic carbon atoms C1 and C2 in complexes 1-6 are all shifted in the same direction, upfield and downfield respectively, with regard to that of the corresponding free (dipheny1phosphino)alkyne (Table 3). The resonances of C1 (19)The presence of the CECH group in this complex is revealed by the observation of the Characteristic signal of v(C-H)st at 3225 cm-' and by NMR results. (20)Grindley, T. B.; Johnson, K. F.; Katritzky, A. R.; Keogh, H. J.; Thirkettle, C.; Topsom, R. D. J . Chem. SOC.,Perkin Trans. II 1974, 282. (21)Schweizer, E. E.;Goff, S. D.; Murray, W. P. J . Org. Chem. 1977, 42,200.
1056 Organometallics, Vol. 14, No. 2, 1995
Louattani et al.
Chemical Shifts (8, ppm) for the Acetylenic Atoms in Cationic Compounds [(X)PhflC1=CZR]+(in Parentheses, Values for the Correponding Free Phosphinoalkynes [Ph$C1=CZR])
Table 3.
X = Fp, R = H X = Fp, R = CH3 X = Fp, R = 'Bu X = Fp, R = Ph X = Fp, R = To1 X = Fp, R = COOMe X = Ph, R = CH3
76.1 (92.0) 70.9 (75.4) 70.9 (75.2) 80.3 (86.5) 80.0 (85.3) 77.2 (87.0) 60.4" (75.4)
107.0 (98.8) 117.9 (107.2) 127.9 (119.5) 115.7 (109.4) 116.5 (108.9) 102.2 (98.9) 121.8" (107.2)
183.1 (190.8) 188.8 (182.6) 198.8 (194.7) 196.0 (195.9) 196.5 (194.2) 179.4 (185.9) 182.2 (182.6)
30.9 (6.8) 47.0 (31.8) 57.0 (44.3) 35.4 (22.9) 36.5 (23.6) 25 (11.9) 61.4 (31.8)
Reference 8b.
Table 4. Most Relevant Optimized GeometricalPParameters for the L and [X-L]+ Systems (X = H, CH3, Fp; L = PH3, HflC=CH, HSC=CCHJ, HzPC=CCOOCHJ)with the 6-31G' Basis Set X
X-P
P-Ci
P-H
1.403 1.380 1.380
95.4 109.5 106.8
Ci-Cz
XF'Ci
HPH
XPH HPCi
L = PH3 H 1.380 CH3 1.822
109.5 106.8
1.785 H 1.379 1.717 CH3 1.820 1.726 Fpb 2.207c 1.756
L = HzPCZCH 1.401 1.191 95.5 98.1 1.379 1.189 110.9 108.0 108.0 110.9 1.380 1.189 113.1 106.0 110.5 108.3 1.393 1.189 121.4 98.7 116.0 100.5
1.780 H 1.380 1.705 CH3 1.821 1.714 Fpb 2.207c 1.749
L = H2PCECCH3 1.402 1.192 95.4 98.6 1.380 1.195 111.6 107.3 107.3 111.6 1.380 1.194 113.6 105.4 109.7 109.0 1.394 1.192 121.7 98.3 115.4 101.3 L = HzPCWCOOCH3
H 1.380 CH3 1.820 Fpb 2.207' a
1.791 1.717 1.726 1.758
1.400 1.380 1.380 1.393
1.190 95.4 97.2 1.190 111.1 108.0 108.0 110.7 1.190 113.0 105.2 110.1 109.0 1.188 120.9 98.8 116.5 100.2
Distances in angstroms, angles in degrees. Valence-double f basis
set for iron, minimal basis set for the CO and Cp ligands. Fe-P distance fixed at their x-ray-determined value.
atoms exhibit a high-field chemical shift of 4-16 ppm, whereas the CZatoms show a down-field chemical shift of 3-11 ppm. The chemical shift differences (dC2 - 6C1) of acetylenic carbons for different compounds have been related to the triple bond polarization, and the sum (6C1+ 6C2) has been connected with the charge changes.22 According to this hypothesis, the data presented in Table 3 indicate that complexation of (diphenylphosphin0)alkynes to [Fpl+ has an important effect in the polarization of the triple bond whereas the influence in charge transfer is small. Furthermore, the C=C triple bond polarization increases following the sequence Ph,PC=CR
[(Fp)PhzPC=CRlf
[Ph,PC=CR]+
Theoretical Results Optimized Geometries. The most relevant optimized geometrical parameters for the free ligands L = HzPCWR (R = H, CH3, COOCH3) and the P-coordinated phosphinoalkynes [(X)H2PC=CRl+(X = H, CH3, Fp) are shown in Table 4. For comparative purposes, the results with L = PH3 have also been included. The optimized values for ethynylphosphine (HzPCrCH) agree well with the structural parameters determined for this molecule.23 The geometries of the P ligands in [(Fp)HzPCzCRl+systems are in good agreement with (22) Rosenberg, D.; Drenth, W. Tetrahedron 1971,27, 3893.
the X-ray-determined structure of [(Fp)PhzPC=CPhI+ (see Table 1). Only minor differences between the P bond angles are found. They can be attributed to the steric effects due to the more bulky phenyl groups. When comparing the values reported in Table 4 we should bear in mind the different charge of the neutral free ligand and the cationic P-coordinated phosphinoalkynes. This cationic character could be responsible for the contraction of some distances. In the free ligands the optimized C=C distance is slightly lengthened with respect to the calculated distances for nonconjugated alkynes (1.19A in front of 1.18 A). The P-C distances (1.78-1.79 A) are significantly shorter than P-C,,3 (1.85 A) and P-CNl (1.83 The free phosphinoalkynes and the PH3 ligand show similar P-H distances and angles. Now we will consider the effect of coordination on the ligand geometry. The structural changes observed are very similar for the three different R groups studied. The CIC bond distance is practically unaffected by coordination. Conversely other parameters are affected by coordination, showing different behavior depending on the nature of the group X. When X = H or CH3, i.e. pure a-donors, the P-C distance is strongly shortened and the P-H distances and P-bond angles are close to the values found in [PH41+. When X = Fp the P-C1 and P-H distances become longer than the values calculated for X = H, CH3. For X = Fp the P-H distances are close to its value in the free PH3 and the phosphorus bond angles show a marked deviation from the tetrahedral geometry. The H-P-H and H-P-C angles are reduced to ca. loo",and the Fe-P-C and Fe-P-H are raised to ca. 120" in such a way that the P ligands are pyramidalized away from the metal. Atomic Charges. The electronic distribution of an alkyne group bonded to a phosphorus atom has been shown to be very sensitive to changes in the atoms coordinated to phosphorus.8 This sensitivity should be reflected on the atomic charges on the acetylenic carbons. So, an accurate calculation of these charges seems to be necessary. To assess the validity of the calculated atomic charges, we have chosen the two simplest systems HzPCsCH and [H3PCWHl+. In both cases we have determined the charges by means of two different methods: The Mulliken population analysis (MPA)25and the natural population analysis (NPAInZ6 (23)Cohen, E. A.; McRae, G. A.; Goldwhite, H.; Di Stefano, S.; Beaudet, R. A. Inorg. Chem. 1987,26, 4000. (24) Allen, F. H.; Kennard, Watson, D.; Brammer, L.; Orpen, A. G., Taylor R. J. Chem. Soc., Perkin Trans. II 1987, 51. (25) Mulliken, R. S. J.Chem. Phys. I D S , 23,1833,1841,2338,2343. (26) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J . Chem. Phys. 1985, 83, 735. (b) Reed, A. E.; Weinhold, F. J. Am. Chem. Soc. 1986, 108, 3586.
n-Effects in P-Coordinated Phosphinoalkynes
Organometallics, Vol. 14,No. 2, 1995 1057 Table 6. Percentage of Coefficients in C1 and C2 of the Three NBOs Corresponding to the C-C Triple Bond for the H~PCGCCH~ and [(X)H$CSCCH3]+ Systems
Table 5. NPA Atomic Charges for the HzPCSCR and [(X)H$C=CRl+ Systems (R =' H, CH3, COOCH3; X = H, CH?. FD)
d'cc
UCC
0.00
f0.24 f0.28 f0.28 f0.26
L = HzPCECCH3 +0.44 -0.43 +0.03 f1.04 -0.64 f0.33 +1.27 -0.63 +0.30 f0.24 -0.51 +0.16
-0.04 +0.05 f0.03 -0.01
f0.05 f0.13 fO.ll f0.09
L = H2PCWCOOCH3 -0.31 -0.12 f1.02 -0.55 f0.20 -0.53 +0.17 +1.26 f0.23 -0.41 f0.02
-0.03 +0.06 f0.04 0.00
f0.05 +0.15 +0.14 fO.10
f0.44
H CH3 FP
H CH3 FP
L = HzPC'CH -0.42 -0.60 -0.59 -0.49
+0.06 -0.11 f 1.06
+1.02 f1.26 f0.23
f0.05 -0.12 f1.05
-0.19 +0.12 f0.09 -0.06
f0.44
H CH3 FP
+0.06 -0.11 1.07
+
-0.04 +0.06
+OM
As has been pointed O U ~ ,the~ MPA ~ , charges ~ ~ ~ are ~ very sensitive to the basis set, particularly as the basis set is enlarged to higher accuracy. The charges of the acetylenic carbons are specially affected by this fact. The much smaller basis set sensitivity of the NPA charges has been tested. The NPA charges converge as the basis set expands. The largest changes occur when passing from the 3-21Gto the 3-21G"basis set (i.e. a set of d atomic orbitals have been added to the P atom). Basis set expansion beyond the 6-31G*does not produce any significant change of the atomic charges. Therefore we have chosen the NPA method and the 6-31G*basis set to calculate the atomic charges. It is noteworthy that the 3-21G charges (without inclusion of d orbitals on the P atom) already give the C-C bond polarized in the same way. NPA atomic charges for the H2PCWR and [(X)H2PC=CRl+systems are presented in Table 5. In the [(X)H2PCWRl+systems the phosphorus atom charges and the X group charges ( q x ) are very different when X = H, CH3 or X = Fp. As expected, in the first case the charge +l is placed on the P atom; on the contrary, in the Fp complex the charge is found on the metallic fragment, mainly located on the Fe atom. The most appealing result from Table 5 is the CzC bond polarization. In all the systems considered, electronic density is concentrated on the C1 atom which becomes more negative than CZ. This polarization is also present in the free ligands, but it is strongly enhanced in the coordinated systems. The polarization reaches a maximum when X is H or CH3, and for X = Fp intermediate values between the phosphoniumalkynes (X= H, CH3) and the free ligands are obtained. In spite of the important variations in each atomic carbon charge, the variations in the total charge (q(C1) q(C2))are considerably less important. For instance, in case of L = H2PC=CCH3 the total charge goes from 0.40 electrons (e) in the free ligand to 0.36 e in [(Fp)H2PC=CCH31f and 0.33 e in [(CH~)H~PCSCCH~]+, and the charge difference between the two carbon atoms goes from 0.46 e to 0.68e and 0.93 e, respectively. It is interesting to point out the opposite C s C acetylenic bond polarization of the alkyl-substituted alkynes: in propyne the 8, carbon bears a charge of -0.26 and the a carbon -0.03. "BO Analysis. The NPA charges from ab initio calculations have clearly shown the polarization of the C-C triple bond. The C-C framework is constituted
+
(27) Baker, J. Theor. Chim. Acta 1986,68,621.
X
c1
cz
H CH3
49.1 50.2 50.1 49.8
50.9 49.8 49.9 50.2
FP
CI 51.0 60.2 59.5 54.9
nlcc c2
49.0 39.8 40.5 45.1
CI 54.2 60.2 59.4 57.0
c 2
45.8 39.8 40.6 43.0
Table 7. NBOs Occupancies for the H2PC=CCH3 and [(X)H2PC=CCH31f Systems X ncca n*CCb u*PHC U*PC dpd H CH3
FP
3.955 3.916 3.920 3.946
0.092 0.078 0.078 0.082
0.024 0.050 0.056 0.108
0.014 0.025 0.024 0.050
0.04 0.06 0.06 0.06
Total occupancy of the two ncc NBOs. Total occupancy of the two n'cc NBOS. Total OCCUP~IICY of two U*PH NBOs. Total O C C U P ~ ~ Cof Y ~
3d orbitals on P atom, by NPA.
by one Q and two n bonds. In order t o get a deeper insight into the charge redistribution in the triple bond we have carried out an NBO analysis28of the calculated wave functions. This analysis allows us to separate 0and n-effects. Natural bond orbitals (NBOs) are computed in the natural atomic orbital basis and are the localized one- or two-center orbitals that form an orthogonal set. In this way, three NBOs are found in the acetylenic compounds, with only coefficients on C1 and C2. One NBO, formed by combination of two sp hybrids, one for each carbon, corresponds to the QC-c bond, and two NBOs, formed each one by combination of two pc orbitals, correspond to the two nc-c bonds (n" and d). Polarization will be reflected on the values of the coefficients of the NBO for each carbon atom. The percentage of coefficients for C1 and C2 of the three NBOs are presented in Table 6 for the HzPCsCCH3 and [(X)H2PCWCH31+(X = H, CH3, Fp) systems. Similar values are found for the ligands H2PC=CH and H2PCECCOOCH~(supplementary material). From values in Table 6 it is clear that the polarization of the C-C bond is a n-effect. The a - c bond is not polarized in all the studied systems (% coefficient for C1 % coefficient for (22). In the free ligands the n" NBO in the PCC plane (PCC angle deviates slightly from BO0)is not polarized and only a slight polarization of the nLNBO perpendicular to the PCC plane is found. It is interesting to point out that in the H~PCECCOOCH~ ligand the slight polarization of the nL NBO occurs in the opposite way (49% coefficient in C1 and 51% coefficient in CZ). This fact can be related to the COOCH3 group n-acceptor character. In the cationic systems with X = H, CH3, a strong polarization of the two n-bonds is found (approximately 60% C1,40% C2). This polarization is remarkably reduced when X = Fp. To go further into the n-effects, we have also looked at the occupancies of different NBOs in the calculated systems. These occupancies are collected in Table 7 for the systems H2PCWCH3 and [(X)H2PC=CCH31+ (X = H, CH3, Fp) and will be commented on in the next section. This kind of analysis has proved to be very useful in the study of hypervalence and hypercon(28) (a) Foster, J. P.; Weinhold, F. J . Am. Chem. SOC.1980,102, 7211. (b)Reed, A. E.; Weinhold, F. J . Chem. Phys. 1983,78,4066.(c) Reed, A. E.; Weinhold, F. J . Chem. Phys. 1986,83,1736.(d) Reed, A. E.; Curtiss, J. A,; Weinhold, F. Chem. Rev. 1988,88,899.
Louattani et al.
1058 Organometallics, Vol. 14,No. 2, 1995 Aq=0.0264+0.015A6
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