J . Phys. Chem. 1992, 96, 8765-8777 (22) For simplicity, we choose harmonic excited-statepotentials in all of the following examples, although the theoretical method is not restricted by the functional form of the potentials. The potentials are given by V,(Q) = (1!2)k,(Q - AQ,)2 + E, with k! = 4r2.M(ho,)' being the force constant, AQ, being the position of the potential minimum along Q, and E, being the energy of the potential minimum for state i. These uncoupled potentials are shown as dashed lines in Figure 1 (diabatic potentials). The coupling between the diabatic potentials for states 1 and 2 is chosen to be coordinate independent; Le., V,,= V2, = constant. Again, the computational method allows us to use coordinate-dependent coupling. The most important coupling mechanism in transition-metal spectra is spin-orbit coupling, which does not stronglydepend on nuclear coordinates. For simplicity, we assume a harmonic ground-state potential in all the examples presented here. The wave functions 4, at t = 0 are therefore Gaussians. Also for simplicity. the transition moments pi were chosen to be coordinate-independent,i.e., constants, in all of the following examples.
8765
(23) Hitchman,M. A.; Stratemeier, H.; Hoppe, R. Inorg. Chem. 1988,27, 2506. (24) Nowitzki, B.; Hoppe, R. Croat. Chem. Acta 1984, 57, 537. (25) Chang, T. H.; Zink, J. I. J . Am. Chem. SOC.1984, 106, 287. (26) Sugano, S.; Tanabe, Y.; Kamimura, H. Multiplets of Transition Metal Ions in Crystals; Academic Press: New York, 1970; Chapter 5.2. (27) Reber. C.; Zink, J. I. Comments Inorg. Chem. 1992, 13, 177. (28) Jacobsen, S. M.; Gudel, H. U.; Dad, C. A. J. Am. Chem. Soc. 1988, 110, 7610. (29) Greenough, P.; Paulusz, A. G. J . Chem. Phys. 1979, 70, 1967. (30) Wood,D. L.; Ferguson, J.; Knox, K.; Dillon, J. F. J. Chem. Phys. 1963, 39, 890. (31) Ferguson, J.; Guggenheim, H. J.; Wood, D. L. J . Chem. Phys. 1971, 54, 504. (32) Chodos, S. L.; Black, A. M.; Flint, C. D. J. Chem. Phys. 1976,65, 4816.
Stereodynamics of Sterically Crowded Metal-Phosphine Complexes: trans-[ (f-Bu),P(i-Pr)],MCi2 [M = Pt( I I ) and Pd( I I)]. One-Dimensional Dynamic and Two-Dlmenslonal Chemical Exchange NMR Studies, X-ray Crystallographic Studies, Molecular Conformation Trapping, and Molecular Mechanics Calculations Christine M. DiMeglio,*Vt Kazi J. Ahmed; Linda A. Luck,t Eugen E. Weltin,t Arnold L. Rheingold,t and C. Hackett BushweUer*st Departments of Chemistry, University of Vermont, Burlington, Vermont 05405, and University of Delaware, Newark, Delaware 1971 6 (Received: April 15, 1992; In Final Form: July 20, 1992)
By using complementary techniques including dynamic NMR (DNMR) spectroscopy, X-ray crystallography, molecular conformation trapping and molecular mechanics calculations, incisive pictures of the stereodynamics of two sterically crowded Pd(I1) and Pt(I1) complexes have been elucidated. X-ray crystallography shows that the molecular geometry in the only crystalline modification (PZ,/a space group) of tr~ns-[(t-Bu)~P(i-Pr)]~PdCl~ (1) has C, symmetry. The isopropyl groups are mutually anti; the dihedral angle between the two P-CH bonds is 180'. For the isopropyl group on each phosphine ligand, one methyl group is anti and the other gauche to the palladium atom. There are significant torsions about Pd-P, P-(i-Pr) and P-(t-Bu) bonds including a dihedral angle between proximate P-CH and Pd-CI bonds of 21.6'. On the basis of the hypotheses that (a) when placed in solution, the conformation of each phosphine ligand of 1 will adopt an essentially exclusive preference for that in the crystal and (b) the two phosphines act essentially independentlyof each other, four stable diastereomericequilibrium conformations are predicted. In two diasteromericconformations, the isopropyl groups are mutually syn with dihedral angles between the P-CH bonds of 44' (C2symmetry) and ' 0 (C, symmetry). In the other two diastereomeric forms, the isopropyl groups are mutually anti with dihedral angles between the P-CH bonds of 136' (C2symmetry) and 1 80' (C, symmetry). Two-dimensional 31P('H)NMR chemical exchange spectroscopy in conjunction with theoretical simulations of the one-dimensional 31P{'HJand 13C{IH]DNMR spectra do reveal the presence of four diastereomeric equilibrium conformations and allow elucidation of the preferred conformational interconversion pathways. Dissolution of a sample of crystalline 1 at 150 K where conformational exchange is very slow on the laboratory time scale resulted in a solution of the pure C,-symmetric molecular conformation. The 31P(lH)NMR spectrum of this solution at 150 K and spectra recorded at higher temperatures, where equilibrationto the other three stable conformations OCCUR, allow assignments of NMR resonanca to specific conformationsand the calculation of relative conformational free energies at 200 K C, anti form (0.00 kcal/mol), C, syn (0.28), C, syn (0.29), C2anti (0.49). tr~ns-[(t-Bu)~P(i-Pr)]~PtCI, (2) exists in two crystalline modifications that have space groups P 2 , / a (C, anti molecular geometry) and P2,/c (C2anti molecular geometry). DNMR studies of 2 in solution reveal stereodynamics that are virtually identical to 1 with a similar distribution of relative conformational free energies at 210 K: C, anti (0.00 kcal/mol), C2syn (0.19), C, syn (OSl), C2anti (0.45). Molecular mechanics calculations performed for 1 by using a locally modified version of Allinger's 1985 MM2 force field agree qualitatively with the experimental results and provide important insight into the transition states for conformational exchange. For 1, the crystals belong to space group P2,/a with Z = 4, a = 15.057 (6) A, b = 12.318 (4) A, c = 16.242 (6) A, /3 = 112.88 (3)', V = 2775 (2) A3,R(F) = 4.1776, and R,(F) = 4.82%. For the P2,/a crystalline modification of 2, Z = 4, a = 15.096 (5) A, b = 12.307 (6) A, c = 16.255 (7) A, fi = 112.63 (3)', Y = 2787 (2) A', R(F) = 3.2476, and R,(F) = 3.91%. For the PZ,/ccrystalline modification of 2, Z = 4, a = 8.536 (2) A, b = 27.670 (6) A, c = 12.174 (3) A, fi = 107.11 (2)', V = 2748 (1) A3,R(F) = 3.75%, and R,(F) = 3.93%.
Introduction Tetracoordinate complexes of R(II), pd(II), lr(1), and Rh(I) that show square-planar coordination are ubiquitous in the ,-hemicai literature.12 These complexes constitute an important To whom correspondence should be addressed. + Universitv - ... . .. .., of - - Vermont. ---------
*University of Delaware.
0022-3654/92/2096-8765$03.00/0
class of chemical compounds. They have applications in catalysis and synthesis including asymmetric ind~ction.~They are inherently interesting from structural and conformational perspectives in that a plethora of ligands is available for complexation and, once formed, the complexes can exist as cis and trans geometrical isomers each of which may adopt a variety of equilibrium conformations. While myriad X-ray crystallographic studies have determined the preferred molecular conformation(s) in the 0 1992 American Chemical Society
8766 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
crystalline state, there remains a dearth of knowledge regarding preferred equilibrium conformationsin solution and the internal motions that effect conformational exchange in solution.'--' The internal molecular mechanics that ultimately determine the stereodynamics of these systems in solution are generally not well understood. In the bisphosphinehalocarbonyl complexes of Rh(1) and Ir(1) or the bisphosphinedihalocomplexes of Pt(I1) and Pd(II), bulky phosphine ligands stabilize the trans geometrical isomers over the cis, for steric reasons. This paper concerns a comprehensive study of the stereodynamics of two sterically crowded tram-bisphosphinedihalo complexes of Pd(I1) and Pt( 11). The phosphine ligand of interest in this report is the bulky di-tert-butylisopropylphosphine. Dynamic NMR (DNMR) studies and molecular mechanics calculations for the free phosphine reveal a strong preference for that conformation having one isopropyl methyl group oriented anti (A) and the other gauche (G) to the phosphorus lone pair (see structure AG).4 In the AG form,
P
?I
GG optimization of 1$-dimethyl repulsions leads to twisting about both the (i-Pr)-P and (r-Bu)-P bonds. Molecular mechanics calculations predict that the conformer in which both isopropyl methyl groups are gauche to the lone pair (GG) is 3.5 kcal/mol higher in energy than the AG forma4At room temperature, the phosphine exists 99.7% of the time as the AG conformation. In light of the long phosphorus-metal bonds (2.3-2.4 A) in a metal-phosphine complex, it is reasonable to expect that the overwhelming AG conformational preference in the free phosphine will be retained by the phosphine ligand in a complex. The results of '-'C{'H) and -"P(IH} DNMR studies of tr~ns-[(t-Bu)~P(iPr)],M(CO)CI [M = Rh(1) and Ir(I)] are amistent with a strong AG preference in each phosphine liganda5By invoking staggering of the ( ~ - B U )moiety ~P of each phosphine ligand about the C1M-CO axis and an AG orientation for each phosphine ligand, six stable, diastereomericequilibrium conformations are predicted. In fact, the 31P('H}DNMR spectra of the rhodium and iridium complexes each decoalesce into six different subspectra consistent with the prediction^.^ I-'C('H}DNMR spectra suggest that each isopropyl group prefers the AG orientation. While these studies allowed a circumstantial case to be made for conformational assignments, disorder in the crystals precluded obtaining crystal and molecular structures by using X-ray crystallography. Important information is lacking regarding molecular geometry that could be used to provide critical insight into the solution stereodynamics. In the bisphosphinedihalocomplexes of Pd(I1) and Pt(II), bulky phosphine ligands stabilize the trans geometrical isomer over the cis. In an attempt to shed some light on the stereodynamics of such sterically encumbered complexes, we examined the two systems below (1 and 2). In such complexes, there is evidence
rrans-[(r-B~)~P(i-Pr)]~MCl~ 1: M = Pd(I1) 2: M = Pt(I1)
DiMeglio et al. that trialkylphosphines are primarily a-bonded to the metal: The torsional constants for rotation about the metal-phosphorus bonds are minuscule and, in the absence of steric retardation, there is free rotation about the metal-phosphorus bonds. Conformational exchange in 1 and 2 will occur via rotation about the metalphosphorus, phosphorus-carbon, and carbon-carbon u bonds. The relative stabilities of diastereomeric equilibrium conformations and energy barriers for conformational exchange will be determind primarily by nonbonded interactions and less by molecular orbital stabilization of specific conformations. X-ray crystallographic and NMR studies reported herein show that 1 exists in one crystalline modification (P2,/a space group) in which the molecules are conformationally homogeneous and that 2 exists in two different crystalline modifications (P21/aand R l / c space groups) that differ in molecular conformation. Each of these two different crystalline modifications of 2 is conformationally homogeneous. The DNMR spectra of both complexes decoalesce due to slowing rotation about the M-P, (i-Pr)-P, and (t-Bu)-P bonds. At slow exchange, the spectra of 1 and 2 reveal the presence of four diastereomeric equilibrium conformations. Dissolution of the conformationally homogeneous crystals of 1 at low temperatures where conformationalexchange is very slow allowed unequivocal identification of two of the conformations and circumstantial assignments of the other two. All of the data show that the stereodynamicsof 1 and 2 are remarkably similar. Molecular mechanics calculations for 1 using a modification of Allinger's 1985 MM2 force field agree qualitatively with the experimental findings and provide important insight into the conformational exchange pathways. X-ray CrystaLlognphic and DNMR Studies trans-[(t - B ~ ) ~ P ( i - P r ) l ~ P dComplex Cl~ 1 was recrystallized several times from 1:1 CH30H:CH2C12to yield yellow, prismshaped crystals of the monoclinic space group P2'/a. In the crystallographic unit cell, there are two C, symmetric molecular geometries arising from two crystallographically independent half-molecules situated on inversion centers. The two molecules differ only in very subtle ways. A complete set of crystallographic data for both geometries is provided in the Experimental Section and in the supplementary material (see paragraph at end of paper). Because the two independent molecules are so similar, it is sufficient to focus discussion of salient geometrical features on just one of the molecules. Structure 3 is an ORTEP drawing of that Bc11221
c11211
WJ
c11311
Lllldl
Clllll
c11121
3 molecule. In 3, the metal atom resides on a crystallographic inversion center and the geometry at the metal template is square-planar. The C1( 1)-Pd-Cl( la) and P( 1)-Pd-P( la) bond angles are both 180.0' (l), and the C1( 1)-Pd-P( 1) bond angle is 90S0 (1). Selected bond angles, bond lengths and dihedral angles for 3 are listed in Table I. The preferred AG conformation in the free phosphine is retained in 3. In each phosphine ligand, one isopropyl methyl group is anti (A) and the other is gauche (G) to the palladium atom. The two isopropyl groups are anti to each otner with the G methyl groups oriented on opposite sides of the coordination plane. Significant torsions occur about the Pd-P and (t-Bu)-P bonds (Table I). For example, the C1( 1a)-Pd-P( l)-C( 13) dihedral angle is -21.6'. The (t-Bu),P moieties asymmetrically stagger the CI-Pd-CI axis. Apparently, the torsion about the Pd-P bonds results from the
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8767
Sterically Crowded Metal-Phosphine Complexes
TABLE I: Selected Bond Lengths (A), Bond Angles Mbedral Angles (deg) from X-ray Crystallography
(dq), and
One of Two Crystallographically Independent CrSymmetric Molecular Conformations in Crystalline tram-[(~-BU)~P(~-P~)J~P~CI~ (See ORTEP Drawing 3) Pd-CI(1) 2.307 (2) Pd-P(l) 2.410 (2) 1.880 (8) 1.885 (5) 180.0 (1) 90.5(1) 109.0 (2) 117.7(2) 107.2 (3) CI(la)-Pd-P( 1)-C(13) -21.6 Pd-P( I)
