Article pubs.acs.org/Organometallics
Synthesis, Thermochemistry, Bonding, and 13C NMR Chemical Shift Analysis of a Phosphorano-Stabilized Carbene of Thorium Danil E. Smiles,† Guang Wu,† Peter Hrobárik,*,‡,§ and Trevor W. Hayton*,† †
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany § Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, SK-84215 Bratislava, Slovakia ‡
S Supporting Information *
ABSTRACT: Reaction of the Th(IV) metallacycle [Th(CH2SiMe2NSiMe3)(NR2)2] (1; R = SiMe3), with Ph3P CH2 affords the Th(IV) carbene [Th(CHPPh3)(NR2)3] (2) in good yield. In solution, complex 2 exists in equilibrium with complex 1 and free ylide, Ph3PCH2. The thermodynamic parameters of this equilibrium were probed using variabletemperature NMR spectroscopy, and these results are compared to those collected for the isostructural U(IV) complex [U(CHPPh3)(NR2)3]. X-ray diffraction studies, together with NMR spectroscopic data and DFT calculations, provide clear evidence for actinide−carbon multiple bonding in the title complex 2, which features the shortest Th−C distance measured thus far. This interaction is best characterized as a strongly polarized σ(Th−C) bond augmented by a three-center two-electron π(Th−C−P)-type interaction. In addition, 13C NMR chemical shifts of carbon atoms bonded to the thorium center were identified as quantitative measures of the An−C bond covalency for a series of structurally related Th carbenes.
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INTRODUCTION
complexes, such as [Th(C(Ph2PNR)2)(NR2)(NCPh2)] (C).1,15,44 Importantly, each of these complexes employs a pincer motif, in addition to two phosphorus substituents, to stabilize the ThC interaction. Indeed, the need for a heteroatom, such as phosphorus, to generate an isolable carbene moiety is a necessity that is exclusive to the f elements, as examples of transition-metal carbenes without this feature are wellknown.45−49 Herein, we report the synthesis of a phosphorano-stabilized carbene of thorium, [Th(CHPPh3)(NR2)3], along with an analysis of its thermochemistry, bonding, and 13C NMR spectroscopic parameters. Notably, this complex represents the first thorium carbene that is stabilized by only one phosphorano substitutent.
Once rare, uranium carbene complexes are now becoming common.1−34 The first example of an actinide carbene, [Cp3U(CHPMe2Ph)], was reported by Gilje and co-workers in 1981,9−11 and since then slow but steady progress has been made toward the synthesis of new uranium carbenes.35−39 In contrast, carbene complexes of other actinides, including thorium, remain relatively rare.13−15 In 2005, Andrews and co-workers reported the formation of [HTh(X)CH2] (X = F, Cl, Br) in an inert-gas matrix.40 They have since isolated several other Th carbenes under similar conditions.29,41−43 In 2011, Cavell and co-workers reported the first structurally characterized thorium methanediide complexes, including [Cp2Th(C(Ph2PNR)2)] (R = SiMe3) (A) (Chart 1).13 Also in 2011, Zi and co-workers reported the synthesis of bis- and trismethanediide complexes of thorium, including [Th(C(Ph2P S)2)2(DME)] (B).14 More recently, Liddle and co-workers reported the synthesis of a series of thorium methanediide
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RESULTS AND DISCUSSION
We recently reported the synthesis of the uranium(IV) carbene complex [U(CHPPh3)(NR2)3] (R = SiMe3).12 In solution this complex exists in equilibrium with the U(IV) metallacycle [U(CH2SiMe2NSiMe3)(NR2)2] and Ph3PCH2.12 Thus, we hypothesized that reaction of the thorium metallacycle [Th(CH2SiMe2NSiMe3)(NR2)2] (1) with Ph3PCH2 would generate the analogous thorium carbene. Gratifyingly, addition of 1 equiv of Ph3PCH2 to a cold (−25 °C) solution of 1 in
Chart 1. Previously Reported Thorium−Methanediide Complexes
Special Issue: Organometallic Actinide and Lanthanide Chemistry Received: March 15, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00202 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
ppm, while the 13C{1H} spectrum features a characteristic doublet (JPC = 22.5 Hz) at 116.54 ppm, which is assignable to the carbene carbon (Figures S6 and S9 in the Supporting Information). Note that this 13C resonance is significantly shifted downfield from that of the free ylide, Ph3PCH2, which appears at −4.18 ppm (Figure S11 in the Supporting Information) and also from that measured for the metal-bound carbon in metallacycle 1 (δ(13C) +68.4 ppm) and the methanediide carbon in pincer complex C (δ(13C) +78 ppm).15 Interestingly, the room-temperature 1H NMR spectrum of 2 in C6D6 also features signals assignable to free ylide and complex 1 (Figure S2 in the Supporting Information), suggesting that 2 exists in equilibrium with 1 and Ph3P CH2 in solution. In fact, this equilibrium is both temperature and solvent dependent. For example, nonpolar solvents, such as pentane, and lower temperatures favor the formation of 2 and allow for its isolation. However, polar solvents shift this equilibrium dramatically in the other direction. In particular, complex 2 is almost completely converted into 1 and Ph3P CH2 when it is dissolved in THF-d8 (Figure S13 in the Supporting Information), regardless of the temperature. A similar solvent dependence was observed for [U(CHPPh3)(NR2)3].12 This equilibrium behavior was probed using variabletemperature NMR spectroscopy in toluene-d8 (Figure S15 in the Supporting Information), and the thermodynamic parameters were determined from the equilibrium concentrations of 1, 2, and Ph3PCH2. The van’t Hoff plot is linear (Figure 2)
diethyl ether affords a light yellow solution, from which [Th(CHPPh3)(NR2)3] (2) can be isolated as a yellow crystalline solid, in 70% yield, upon crystallization from pentane (Scheme 1). Scheme 1. Preparation of 2 from Metallacycle 1 and Ph3P CH2
Complex 2 is isostructural with its uranium(IV) analogue (Figure 1) and features the shortest Th−C distance (2.362(2) Å) known.
Figure 1. Solid-state molecular structure of [Th(CHPPh3)(NR2)3] (2) with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Th1−C19 = 2.362(2), C19−P1 = 1.680(2), Th1−C19−P1 = 148.2(1).
This distance is markedly shorter than those reported for other structurally characterized thorium methanediide complexes (average 2.49 Å, range 2.436−2.552 Å).1,13−15 Furthermore, the Th−C distance in 2 is intermediate between the sums of single-bond (2.50 Å) and double-bond (2.10 Å) covalent radii for Th and C,50 suggestive of some multiple-bond character (see electronic structure analysis below). However, the Th−C distance in 2 is longer than the U−C distance in its uranium analogue (2.278(8) Å), which is consistent with the larger ionic radius of Th4+ vs U4+.51 Finally, the P−C distance (1.680(2) Å) is identical with that observed for the uranium analogue (1.679(8) Å) and the parent ylide (1.661(8) Å),52 suggesting that no activation of the P−C bond has occurred. The 1H NMR spectrum of 2, in benzene-d6 at room temperature, features a sharp singlet at 0.49 ppm, assignable to the methyl groups of the silylamide ligands (Figure S2 in the Supporting Information). The spectrum also features a doublet at 1.69 ppm (JPH = 20.5 Hz), which is assignable to the methine proton of the carbene ligand. This signal collapses to a singlet in the 1H{31P} NMR spectrum (Figure S5 in the Supporting Information). The 31P{1H} spectrum exhibits a singlet at 17.55
Figure 2. van’t Hoff plot for conversion of [M(CHPPh3)(NR2)3] (M = Th, U; R = SiMe3) into [M(CH2SiMe2NSiMe3)(NR2)2] and Ph3P CH2, in toluene-d8. Concentrations: Th, 27.9 mM; U, 25.3 mM. Data for [U(CHPPh3)(NR2)3] were taken from ref 12.
and reveals that conversion of the carbene into the metallacycle and free ylide is endothermic (ΔHr = 9.4 ± 0.3 kcal/mol) but entropically favored (ΔSr = 16.5 ± 1.2 eu), with an overall reaction free energy, ΔGr, of +4.5 kcal/mol. As expected, the enthalpic component in this example is similar to that of the analogous uranium system (ΔHr = 10.38 ± 0.03 kcal/mol), consistent with the fact that similar bonds are being formed and broken in both cases. There is, however, a rather large difference in the entropic component. The entropy of formation of the U(IV) metallacycle and ylide (ΔSr = 30.5 ± 0.1 eu) is nearly twice that of the analogous thorium case, resulting in ΔGr = +1.8 kcal/mol for the uranium example. The origin of this entropic difference is not known. However, the B
DOI: 10.1021/acs.organomet.7b00202 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Table 1. Experimental and Calculated M−C Bond Lengths (M = Th, U), Selected QTAIM Topological Indicators and Compositions of the M−C Bonding NLMOsa d(M−C) (Å)
QTAIM (M−C)b
NLMO (M−C)
X-ray
calcd
ρb
ε
DI
type
%M
M(s)
M(d)
M(f)
%C
C(s)
C(p)
[Th(CH2SiMe2NR)(NR2)2] (1) [Th(CH2SiMe2NR)2(NR2)]− (1′) [Cp2Th{C(Ph2PNR)2}] (A)
2.562c 2.436d
2.462 2.547 2.437
0.085 0.073 0.083
0.06 0.07 0.28
0.659 0.586 0.647
[Th{C(Ph2PS)2}2(DME)] (B)
2.492e
2.459
0.080
0.27
0.625
[Th{C(Ph2PNR)2}(NR2)(NCPh2)] (C)
2.475f
2.487
0.076
0.30
0.615
[Th(CHPPh3)(NR2)3] (2)
2.362g
2.351
0.095
0.28
0.808
[Th(CHPPh3)Cl3]
2.293
0.111
0.38
0.964
[ThCH2(NR2)3]−
2.201
0.128
0.56
1.208
[HTh(F)=CH2]
2.136
0.151
0.58
1.581
2.401 2.289
0.093 0.107
0.13 0.25
0.734 0.902
σ σ σ π σ π σ π σ π σ π σ π σ π σ σ π
15.3 14.8 10.6 9.8 10.7 10.1 10.6 8.2 13.8 10.8 16.1 12.7 19.7 24.9 24.9 29.8 20.3 15.9 14.4
16 19 10 0 9 0 4 2 5 0 13 0 6 0 9 5 12 8 0
72 70 68 59 62 58 64 64 79 61 73 68 78 73 74 77 57 78 50
12 11 21 41 29 42 32 33 14 39 14 32 15 26 17 18 31 12 50
81.4 81.6 78.8 74.9 79.6 74.9 77.6 75.5 82.4 75.9 80.7 75.2 79.3 73.1 75.1 70.2 76.5 79.9 73.3
24 20 19 0 24 0 20 2 41 0 38 0 46 0 36 2 23 38 0
76 80 81 100 76 100 80 98 59 100 62 100 54 100 64 98 77 62 100
complex (R = SiMe3)
[U(CH2SiMe2NR)(NR2)2] [U(CHPPh3)(NR2)3]
2.278h
a
Electronic structure analysis done at the PBE0/def2-TZVP/ECP level for B3LYP-D3(BJ)/def2-TZVP/ECP optimized structures (see Computational Details in the Supporting Information). bρb and ε stand for electron density and ellipticity at the bond critical point, respectively, DI stands for delocalization index as a measure of the M−C bond covalency. cSee ref 55. dSee ref 13. eSee ref 14. fSee ref 15. gThis work. hSee ref 12.
large change in entropy is likely responsible for the different speciation observed for the thorium and uranium systems. For example, in the uranium case, the metallacycle is favored by 4.5:1 over the carbene, in toluene-d8 at room temperature, while in the thorium system, the reverse speciation is observed. Specifically, the carbene (2) is favored by ∼20:1 over the metallacycle (1) under similar conditions. The somewhat larger thermodynamic stability of the Th carbene over that of its U(IV) congener and decreasing stability of the carbene complexes upon increasing solvent polarity are also supported by DFT calculations of the reaction free energies (Table S3 in the Supporting Information), even though decomposition of both Th and U carbenes is predicted to be slightly endoenergetic in highly polar solvents (with ΔG°r values of 9.8 and 6.5 kcal/mol for Th and U, respectively; B3LYPD3(BJ)/ECP/6-311++G** data using a SMD solvation model, considering THF as the solvent). DFT calculations of 1, 2, the structurally related carbenes A− C, and [U(CHPPh3)(NR2)3] provide excellent agreement between optimized and X-ray determined structural parameters, where available, with differences in An−C bond lengths of less than 0.03 Å (Table 1). This justifies the use of DFT methods for in-depth electronic structure analysis. Unlike the metallacycle 1 and bis(metallacycle) anion 1′, which have well-defined single Th−C bonds (Table 1), the thorium−carbon interaction in 2 is best described as a double bond with two-center twoelectron σ(Th−C) and three-center two-electron (3c2e) π(Th−C−P)-type components (see Figure 3 for corresponding natural localized molecular orbitals, NLMOs, and Figure S17 in the Supporting Information for relevant frontier MOs and Mulliken populations). Interestingly, a somewhat similar 3c2e interaction is observed in the recently prepared scandium phosphinoalkylidene, [LSc{C(SiMe3)PPh2}(THF)] (L = CH{C(Me)N(2,6-iPr2C6H3)}2).53
Figure 3. Two-center two-electron σ(Th−C) and three-center twoelectron π(Th−C−P) bonding NLMOs in [Th(CHPPh3)(NR2)3] (2) (isosurface plots ±0.03 au; hydrogen atoms omitted for clarity).
From electronic structure analysis (Table 1) it is evident that both σ- and π-type Th−C bonds in 2, and also in structurally related carbene complexes A−C, are strongly polarized toward the carbene carbon, with about 8−15% of Th participation in bonding (see also Figure 4 for an electron localization function (ELF) plot). In addition, a sum of metal (%M) and carbon (% C) atomic contributions to the ThC bond deviates notably from 100%, hinting at the electron delocalization (multicenter character) within both σ(Th−C) and, in particular, π(Th−C)type components (and a necessity of resonance structures to describe the delocalized electrons within the Lewis structure notation properly). Note that while compositions of the π-type bond are virtually identical for all carbene complexes, notable reduction of σ(Th−C) covalency is observed for pincer complexes A−C (Table 1). This feature can be attributed to the presence of the second phosphorus atom (with adjacent electronegative S or N coordinating atoms) attached to the methanediide carbon. This structural motif facilitates the C
DOI: 10.1021/acs.organomet.7b00202 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
negative) π donors, such as Cl−; however, this modification has a somewhat smaller (but still notable) effect (Table 1). Notably, the 5f-orbital involvement in Th−C bonding of 2 is rather minor but, interestingly, the overall Th(5f) contribution in π bonding is more than twice as large as that in σ(Th−C) bond (4.2% vs 1.9%). In keeping with previous findings, the analogous U(IV) carbene [U(CHPPh3)(NR2)3] possesses both larger An−C bond covalency (by ∼11%) and larger 5f-orbital involvement, with overall U(5f) contributions of 1.9% and 7.2% to σ and π bonds, respectively. On the basis of the electronic structure analysis discussed above, we can ascribe the large downfield shift of the methine carbon in 2 to the notably larger σ(Th−C) bond covalency, in comparison to previously characterized Th carbenes (see QTAIM delocalization indices, DI, and composition of Th−C bonding NLMOs in Table 1), and also to the spin−orbit (SO) effects, which are expected to be particularly important for ligand atoms (with an appreciable L(s)-orbital contribution in An−L bonding) attached directly to an actinide f0 center.56−59 In analogy with our previous findings for 77Se/125Te chemical shifts in the thorium chalcogenide series [Th(E)n(NR2)3]− (E = Se, Te; n = 1, 2),60 we hypothesized that the Th−C delocalization index would correlate with 13C NMR shifts, which would allow for examination of the An−C bond covalency/ionicity with a simple spectroscopic measurement.61 Excellent agreement between theoretical and experimental 13C NMR shifts with deviations of
