Trend-Analysis of Solid-State Structures: Low-Energy Conformational 'Reactions' Involving Directed and Coupled Movements in Half-Sandwich Compounds [CpFe(CO){C(=O)R}PPh3].

Trends in solid-state structures were used to identify preferred intramolecular movements in half-sandwich compounds [CpFe(CO){C(=O)R}PPh3]. Three weak interactions were analyzed: 1) the CH/π donor-acceptor interaction of phenyl rings in the PPh3 ligand, 2) the PhPPh3 face-on Cp stabilization, and 3) the hydrogen bond between the oxygen atom of the acyl group and an ortho-C-H bond of one of the PPh3 phenyl rings. Clockwise and counter-clockwise rotations established directed and coupled movements of the PPh3 ligand, the acyl group, and the phenyl rings within the PPh3 ligand.

Normally, the arrangement of sample points within an energy minimum is statistical (Figure 1, left side). A concentration, indicated by an inclined best‐fit line, contains additional information (Figure 1, right side). For half‐sandwich compounds [CpFe(CO){C(=O)R}PPh3], we show that such trends in solid‐state structures can be used to identify direction and coupling of movements inside the molecules. Such movements confirm and specify weak intramolecular interactions. This approach connects structure and movement by the relation: trends in solid‐state structures−preferred movements.

Figure 1

Statistical arrangement of sample points (left side) and concentration of sample points along an inclined best‐fit line (right side).

Disregarding the conformational flexibility of the acyl substituent R at the outside of the molecules, there are five parameters in [CpFe(CO){C(=O)R}PPh3] compounds that change the shape of the molecule: the three propeller angles τ of the phenyl rings of the PPh3 ligand and the rotation angles ρ of the PPh3 and acyl substituents. The propeller angles τ are defined as Co−Ci−P−Fe <90° (i, o=ipso, ortho), the rotation angle ρ PPh3 as Ci−P−Fe−Cpcent, and the rotation angle ρ acyl as O=C−Fe−Cpcent (Figure 2). The rotation of the Cp ring around the axis Cpcent−Fe is not regarded to be a substantial change of the molecular shape.

Figure 2

Labelling of phenyl rings, propeller angles τ, rotation angles ρ, and rotation axes in CALWAN [CpFe(CO){C(=O)sec‐Bu}PPh3]. The arrows indicate the CH/π interaction Phface→Phedge within the PPh3 ligand. The bold line and the dashed line show the PPh3 interaction with the fragment CpFe(CO)‐ {C(=O)R} by PhPPh3 face‐on Cp interaction and CH⋅⋅⋅O hydrogen bond, respectively.

The example CALWAN, [CpFe(CO){C(=O)sec‐Bu}PPh3], is shown in Figure 2, including the designations of the phenyl rings, as well as the propeller and rotation axes. In addition, the arrows indicate the CH/π interactions of the C−H bond of the donor Phface to the acceptor Phedge in the internal stabilization within the PPh3 ligand. The interactions of the PPh3 phenyl rings with the substituents of the fragment CpFe(CO)‐ {C(=O)R} are shown by a bold line for the Cp/Phface interaction and a dashed line for the hydrogen bond CH⋅⋅⋅O from Phface to the acyl oxygen atom.

A CSD search1 of the Cambridge Crystallographic Data Centre provided 47 [CpFe(CO){C(=O)R}PPh3] compounds, the propeller and rotation angles of which are given in Table 1.2 Four compounds have two independent molecules in the unit cell. Thus, 51 different structures are available for analysis. In 23 cases, the CSD cif files were inverted to allow for a consistent stereochemistry in all the compounds.

Table 1

CSD symbols, rotation angles ρ, propeller angles τ, and torsion angle acyl and (Co)H−Ci−P−Fe.

	Phedge		Phtrans		Phface		Acyl	(Co)H−Ci−P−Fe
CSD symbol[a]	Ci−P−Fe−Cpcent	Co−Ci−P−Fe	Ci−P−Fe−Cpcent	Co−Ci−P−Fe	Ci−P−Fe−Cpcent	Co−Ci−P−Fe	O=C−Fe−Cpcent
	ρ [°]	τ [°]	ρ [°]	τ [°]	ρ [°]	τ [°]	ρ [°]	[°]
FIHTUL[b]	79.04	9.03	−165.76	−65.69	−40.67	−68.12	78.23	−70.03
LEZVAN	80.73	8.63	−160.05	−33.28	−37.40	−78.08	82.61	−81.74
DOKXIK	80.84	−1.38	−161.32	−57.71	−36.48	−70.33	91.28	−73.64
VOWTUW[1]	82.73	−3.05	−159.53	−49.34	−36.77	−65.70	83.61	−66.63
PIRDOJ	84.87	−3.48	−157.05	−56.56	−34.55	−63.23	61.39	−65.21
KEWSEK[b]	81.09	−3.58	−155.51	−26.69	−33.71	−75.84	84.61	−79.54
CUXBIG10	87.98	−3.79	−154.09	−57.89	−32.19	−60.24	68.97	–[c]
LADFEB[b]	83.71	−5.32	−157.92	−59.19	−33.20	−66.97	75.22	−69.48
WAJYOV	79.90	−6.34	−163.87	−74.24	−39.26	−59.15	99.04	–[c]
KEWSIO	79.40	−6.92	−161.40	−36.31	−41.99	−62.98	77.99	−68.11
JUDNEB	82.71	−8.91	−158.34	−50.62	−35.75	−67.62	70.74	−71.09
JUDNEB01	84.58	−9.76	−155.90	−50.64	−34.31	−68.43	75.80	−71.89
XIKFEC[b]	79.03	−9.86	−164.88	−71.46	−41.71	−56.33	99.61	−55.21
FAMNAI	80.50	−10.07	−162.25	−65.74	−39.69	−57.87	52.24	−67.29
NOCQEB	83.19	−10.22	−157.57	−46.40	−35.81	−65.18	66.63	−66.36
YOTBEO[b]	85.15	−12.11	−156.06	−52.35	−34.69	−54.24	50.66	−56.32
VOWTUW[2][b]	81.07	−12.12	−160.50	−73.23	−38.80	−55.01	68.45	−56.33
YOTBIS	88.91	−13.65	−153.56	−53.45	−30.69	−57.43	61.20	−59.81
KITVAK	86.95	−13.96	−154.79	−48.33	−32.60	−60.97	80.54	−61.53
FIHTEV[b]	84.07	−14.00	−158.05	−63.76	−37.80	−54.57	51.05	−55.84
GOZYAX[1]	83.87	−14.00	−159.27	−73.97	−35.84	−57.89	69.54	−58.48
PIRDID[b]	85.91	−14.39	−157.08	−69.32	−34.47	−58.30	75.02	−58.37
FECPCB10[b]	83.11	−15.17	−160.52	−60.74	−37.79	−51.15	66.12	−51.55
FIHTOF[1][b]	82.64	−15.36	−159.82	−69.80	−37.20	−57.72	68.29	−57.82
GADWEN01[b]	81.61	−15.48	−160.99	−76.90	−38.58	−57.92	75.60	–[c]
VOWVAE	83.65	−16.78	−159.42	−74.77	−35.29	−58.73	78.35	−58.63
RARXAJ	84.10	−16.96	−159.45	−66.78	−38.52	−52.90	66.20	–[c]
FEHTUH[b]	86.95	−17.26	−156.11	−65.67	−34.41	−54.11	72.82	−53.99
WAJYUB	83.83	−17.71	−157.91	−64.71	−35.84	−57.07	71.91	–[c]
HAPSIA	75.30	−17.97	−167.06	−74.26	−45.37	−41.89	56.07	−42.97
ROXQEC	90.58	−18.15	−149.85	−41.19	−29.09	−62.14	75.61	−60.91
FIHTOF[2][b]	83.45	−18.20	−159.93	−70.01	−37.13	−53.62	69.62	−55.12
FIHTIZ[1][b]	84.55	−19.70	−157.11	−54.44	−35.32	−56.60	89.13	−58.59
CALWAN	86.80	−20.11	−156.13	−65.70	−34.67	−53.30	60.05	−52.86
GOZYAX[2][b]	84.80	−20.40	−157.31	−70.70	−35.26	−55.45	80.73	−55.20
GOZXUQ[b]	88.66	−21.14	−153.25	−59.78	−32.22	−55.98	66.80	−57.68
FIHTIZ[2][b]	88.01	−21.88	−154.55	−63.46	−32.70	−56.31	84.08	−58.19
DUHXOT	85.82	−22.10	−156.97	−65.58	−35.34	−52.67	65.57	−52.20
MCXCFE	89.53	−22.39	−150.81	−44.57	−28.87	−61.05	74.95	−63.77
RAZCEA	87.89	−23.44	−154.20	−57.69	−33.24	−53.26	77.40	−53.65
GIBTUG[b]	91.65	−23.56	−150.68	−64.58	−28.00	−56.04	67.41	–[c]
ZIQGIP[b]	82.49	−24.04	−158.64	−65.88	−36.02	−58.25	83.15	−58.18
GADWEN02[b]	81.41	−24.10	−160.15	−62.12	−39.98	−49.50	64.65	−46.94
FELFOR	87.60	−24.88	−155.07	−61.64	−33.20	−54.64	77.38	−54.37
SOGXOB	87.66	−25.34	−154.51	−59.99	−33.53	−53.11	73.84	−54.83
VIVTEZ	83.47	−28.54	−158.53	−72.92	−37.65	−47.14	59.90	−46.30
GAKJEH[b]	80.42	−29.01	−160.51	−57.99	−39.44	−52.64	57.58	−52.87
RARXEN	88.18	−29.06	−152.51	−51.88	−31.96	−54.11	80.17	–[c]
JIDLUD[b]	80.19	−30.71	−159.51	−58.15	−38.60	−49.95	58.15	−51.79
DAWDUA	80.37	−31.39	−161.51	−72.20	−39.24	−55.36	51.92	−50.23
NOCQIF	86.50	−43.18	−153.19	−46.37	−31.60	−56.66	80.43	−57.20

[a] Brackets [] indicate independent molecules. [b] Inverted into the mirror image orientation. [c] No hydrogen atoms.

The architecture of the PPh3 propeller in half‐sandwich compounds [CpFe(CO){C(=O)R}PPh3] is determined by CH/π interactions.3, 4 Contrary to the T‐shaped benzene dimer, these CH/π interactions are intramolecular and entropically almost neutral.5 In the PPh3 ligand, there are six Co−H bonds: three inside the propeller (inCoH) and three outside (outCoH). It is the interaction between the inCo−H bonds and Ci, inCo, and outCo atoms of neighboring phenyl rings that add up to an appreciable stabilization.3, 4 Each of the three phenyl rings plays a specific role in the donor–acceptor interactions, as indicated by the arrows in Figure 2 for Phface→Phedge.

In the histogram of Figure 3, a correlation of the propeller angles of the acceptor Phedge with the donor Phface is shown. The Phedge propeller angles span the range of τ=9.0° to −43.2°, and the Phface propeller angles from τ=−41.9° to −78.1°. The best‐fit line shows a good correlation with quality factors R 2=0.5005 and p<6.5×10−9.

Figure 3

Correlation of the propeller angles of acceptor Phedge and donor Phface (R 2=0.5005, p<6.5×10−9) (top). The ‘reaction’ JIDLUD→FIHTUL (clw=clockwise, c‐clw=counter‐clockwise) (bottom).

JIDLUD and FIHTUL are close to the best‐fit line in Figure 3. The descent from JIDLUD to FIHTUL along the best‐fit line implies not only a coupled movement of the propeller angles of Phedge and Phface, but also preferred directions of this movement. The change of the propeller angle of Phedge of JIDLUD from −30.7° to 9.0° of FIHTUL corresponds to a counter‐clockwise (c‐clw) rotation of Phedge by 39.7° around the Ci−P bond, whereas the simultaneous change of the Phface angle from −50.0° to −68.1° is a clockwise (clw) rotation of 18.1°. A conformational change, such as the descent from JIDLUD to FIHTUL, will be called a ‘reaction’. In such a ‘reaction’, a fragment changes, which two different molecules have in common. Here, the two molecules differ in the R substituents of their acyl ligands. In the ‘reaction’ JIDLUD→FIHTUL, there is a concomitant change of the rotation angle of the PPh3 ligand around the P−Fe axis from ρ=−38.6° to −40.7°.The other way round, the ascent from FIHTUL to JIDLUD is associated with a clockwise rotation of Phedge and a counter‐clockwise rotation of Phface. Thus, in a conformational change of the type JIDLUD→FIHTUL, Phedge and Phface do not move independently, but concertedly. The directions clockwise and counter‐clockwise by no means are equal. There is a preferred direction for the synchronized movement of the two phenyl rings.

In the interaction Phface/Phedge of JIDLUD, the inCo−H bond of Phface is the donor to Ci and inCo of Phedge (arrows in Figure 3). For a CH/π interaction, in JIDLUD the distance inCoH−Ci (2.57 Å) is very short, whereas the distance inCoH−inCo (2.85 Å) is relatively long. In the ‘reaction’ JIDLUD→FIHTUL, Phface and Phedge of JIDLUD move to their positions in FIHTUL. In FIHTUL, the distances inCoH−Ci (2.63 Å) and inCoH−inCo (2.65 Å) are in the middle range. Given the counter‐clockwise movement of Phedge in JIDLUD→FIHTUL, a counter‐clockwise instead of the observed clockwise movement of Phface would elongate both distances and weaken the CH/π interaction. Had the CH/π donor–acceptor interaction between Phedge and Phface not been established yet,3, 4 the ‘reaction’ JIDLUD→FIHTUL would indicate a weak bonding interaction between Phedge and Phface.

In (π‐Ar)MPPh3 complexes, a weak bonding stabilization is ascribed to the PhPPh3 face‐on π‐Ar interaction.6 For the complexes [CpFe(CO){C(=O)R}PPh3], this means a weak attraction of Cp and Phface. In these molecules, a rotation around the P−Fe axis moves the PPh3 ligand with respect to the fragment CpFe(CO){C(=O)R}. Figure 4 shows that the sample points of the three phenyl rings crowd within narrow ranges of their rotation angles, constraining the rotation around the P−Fe axis to small degree intervals (Table 1). A change of the face/edge/trans‐character of the phenyl rings would afford passage over higher transition states.

Figure 4

Correlation of the propeller angles τ of Phtrans, Phface, and Phedge and the rotation angle ρ of the PPh3 ligand (R 2=0.0399, p<0.160 for Phface) (top). The ‘reaction’ VIVTEZ→KEWSEK (clw=clockwise, c‐clw=counter‐clockwise) (bottom).

We had previously described the compression of the rotation angles of the face/edge/trans‐phenyls of the PPh3 ligand in half‐sandwich complexes to narrow degree intervals, without recognizing the directionality, popping up in the best‐fit lines of Figure 4.7 For Phface, the ‘reaction’ VIVTEZ→KEWSEK is shown at the bottom of Figure 4. Going down from VIVTEZ to KEWSEK, Phface performs a clockwise rotation of τ=28.7°. Simultaneously, the PPh3 ligand rotates in the counter‐clockwise direction by ρ=4.0°. This is not much; however, it has to be kept in mind that PPh3 rotation is strictly limited to narrow intervals and for the ‘reaction’ HAPSIA→GIBTUG the PPh3 rotation amounts to ρ=17.4°.

For the complexes [CpFe(CO){C(=O)R}PPh3], the PhPPh3 face‐on Cp stabilization requires that Phface is as close and as much face‐on to the Cp ligand as possible. The decreasing rotation angle in the ‘reaction’ VIVTEZ→KEWSEK brings Phface closer to Cp and the simultaneous clockwise rotation of Phface increases its face‐on character. The coupled movements work hand in hand to strengthen the PhPPh3 face‐on Cp stabilization. Eclipsing of Phedge and Phtrans with the carbonyl and acyl substituents prevents a further decrease of the rotation angle of Phface.6 In the back ‘reaction’ KEWSEK→VIVTEZ, the increase of the rotation angle ρ weakens the PhPPh3 face‐on Cp interaction. However, the counter‐clockwise rotation of Phface increases its face‐on character, making sure that as much stabilization as possible is maintained.

In the realm of Phedge, the ‘reaction’ FIHTUL→NOCQIF results in a clockwise rotation of Phedge by τ=52.2° and a counter‐clockwise rotation of PPh3 by ρ=7.5°. For Phtrans, a ‘reaction’ with points close to the best‐fit line is KEWSEK→GADWEN01, involving a clockwise rotation of Phedge by τ=52.2° and a counter‐clockwise rotation of PPh3 by ρ=7.5°.

In the compounds [CpFe(CO){C(=O)R}PPh3], the outCo−H bond of Phface, abbreviated (Co)H, forms a weak hydrogen bond to the oxygen atom of the acyl group (Figure 2).4 Figure 5 shows the correlation of the torsion angle O=C−Fe−Cpcent and the torsion angle (Co)H−Ci−P−Fe (Table 1). Although there is no bond between (Co)H and Ci in Phface, the torsion angle (Co)H−Ci−P−Fe perfectly positions (Co)H, because it contains the propeller axis Ci−P of Phface and also the rotation axis P−Fe of PPh3. Both axes contribute to the orientation of (Co)H within the molecule.

Figure 5

Correlation of the torsion angle (Co)H−Ci−P−Fe of Phface and the torsion angle O=C−Fe−Cpcent (R 2=0.1807, p<0.004026) (top). The ‘reaction’ JIDLUD→FIHTUL (clw=clockwise, c‐clw=counter‐clockwise) (bottom).

In Figure 5, JIDLUD and FIHTUL are close to the best‐fit line. The descent from JIDLUD to FIHTUL changes the torsion angle (Co)H−Ci−P−Fe of Phface of JIDLUD from 70.0° to 51.8° of FIHTUL, corresponding to a clockwise rotation of Phface of 18.2°, whereas the concomitant change of the acyl rotation angle from 58.2° to 78.2° is a counter‐clockwise rotation of 20.0°. The change of the torsion angle (Co)H−Ci−P−Fe of Phface in the ‘reaction’ JIDLUD→FIHTUL implies two components: a clockwise rotation of Phface by 18.1° around the Ci−P axis and a clockwise rotation of the PPh3 ligand around the P−Fe axis by 2.1°. Vice versa, the ascent from FIHTUL to JIDLUD involves a counter‐clockwise rotation of Phface, a counter‐clockwise rotation of the PPh3 ligand, and a clockwise rotation of the acyl group.

In all of the 51 structures of Table 1, a weak hydrogen bond from outCoH of Phface to the oxygen atom of the acyl group is present. In the ‘reaction’ JIDLUD→FIHTUL, there is a clockwise rotation of (Co)H of Phface and a counter‐clockwise rotation of the acyl group (Figure 5). These concerted movements keep the hydrogen bond outCoH⋅⋅⋅O intact, as shown in the distances Co−O of 3.22 Å in JIDLUD and 3.15 Å in FIHTUL. A rotation of (Co)H and acyl in the same direction, both clockwise or counter‐clockwise, would disrupt the hydrogen bond.

Using the correct absolute configuration, the trend analysis gives the correct chiral movements within the molecules—not their mirror images. In molecules with a symmetry plane, the movements on the two sides are image and mirror image to each other.

The molecular movements described here are below full rotations around the Ci−P and P−Fe axes and even below the transition states, which interconvert the face/edge/trans‐phenyl rings of the PPh3 ligand.6 By trend analysis of solid‐state structures, they are easily recognized. Directed and coupled movements may play a general role, for example, in biochemical processes such as protein folding.8, 9

Experimental Section

The Cambridge Structural Database1 was used for a search of the complexes discussed in this paper. The programs OLEX 2,10 Mercury CSD ver. 3.9,11 and ConQuest ver. 1.192 were used.

Conflict of interest

The authors declare no conflict of interest.