β-Diketiminato Organolead Complexes - American Chemical Society

Mar 31, 2015 - Several lead(II) β-diketiminate complexes, [(BDI)PbX] (BDI = [{N(2,6-iPr2C6H3)C(Me)}2CH]. −. ), have been reported over the past dec...
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β‑Diketiminato Organolead Complexes: Structures, Hammett Correlations

207

Pb NMR, and

Morgan J. Taylor, Emma J. Coakley, Martyn P. Coles,† Hazel Cox,* and J. Robin Fulton*,† Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9RH, U.K. S Supporting Information *

ABSTRACT: The synthesis, structure, and spectroscopic details of a series of βdiketiminato lead(II) alkyl complexes are described. The Hammett correlation between the 207Pb NMR chemical shifts and σpara Hammett constant was examined computationally and found to be due to the paramagnetic shielding contribution, whereas the diamagnetic and spin−orbit coupling contributions remained fairly constant across this series of compounds.

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INTRODUCTION Several lead(II) β-diketiminate complexes, [(BDI)PbX] (BDI = [{N(2,6-iPr2C6H3)C(Me)}2CH]−), have been reported over the past decade, including lead halide, alkoxide, aryloxide, amide, and alkyl complexes1−6 as well as more novel compounds such as the lead phosphanide complexes [(BDI)PbPR2] (R = Ph, Cy, SiMe3) and the lead silylidenephosphanide [(BDI)Pb−PSi(R)Si(tBu)3] (R = 2,4,6-iPr3C6H2).7,8 The geometry of the ligands around the three-coordinate metal center is always trigonal pyramidal, with the lone pair having some directionality due to it possessing approximately 10% p character.1,9−11 The metal center for these neutral molecules lies outside the plane defined by the BDI backbone (NCCCN plane), which is similar to four-coordinate metal complexes bearing a BDI ligand12 and thus consistent with the presence of a stereochemically active lone pair.13 The acute bond angles observed at lead has also been attributed to a high 6p orbital contribution to the lead−ligand bonds.14 Two different types of isomers have been observed in these three-coordinate lead complexes (Figure 1): one in which the metal center and X ligand are below the NCCCN plane and the Pb−X bond is approximately perpendicular to the NCCCN plane (“endo” conformation) and the other in which the lead center lies above the NCCCN plane and the X ligand points away from the “(BDI)Pb” core (“exo” conformation).15 βDiketiminato lead complexes with sterically unconstrained

ligands such as methyl or ligands that are able to achieve a planar geometry at the center bound to lead (e.g., N(SiMe3)2, Ph) adopt an endo conformation, and complexes with sterically encumbering ligands (e.g., OtBu, PCy2) possess an exo conformation. Although the observation of the different isomers is based purely on solid-state data, solution-phase NMR studies on isostructural tin complexes, notably [(BDI)SnCl] and [(BDI)SnPCy2] (R = Cy), have revealed that these isomers are maintained in solution.8 However, these solutionphase studies relied on through-space coupling, JSnC and JSnH, between Sn and the isopropyl groups on the BDI aryl groups. Unfortunately, through-space coupling between the lead and any other nuclei has not been observed. The 207Pb NMR chemical shift of β-diketiminato lead complexes ranges from 800 to 3981 ppm, with one high-field anomaly, [(BDI)Pb{P(SiMe3)2}], at −1737 ppm. Although this is a small range inside the 207Pb window (∼17000 ppm), it is relatively unpredictable. That is, the electron-withdrawing and donating capacity of the “X” ligand does not govern the chemical shift. The general explanation for the differing chemical shifts is that the relative contribution to the isotropic shielding (σ) of the 207Pb nuclei from the diamagnetic (σD), paramagnetic (σP) and spin−orbit (σSO) shieldings (eq 1) varies dramatically with small changes in the ligand. σ = σ D + σ P + σ SO

(1)

We were interested in examining the effect of the ligand on the 207Pb chemical shift by focusing on β-diketiminato organolead complexes, enabling us to ascertain differences in Special Issue: Mike Lappert Memorial Issue Received: November 30, 2014

Figure 1. Two different conformations of [(BDI)PbX]. © XXXX American Chemical Society

A

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Organometallics chemical shift that are due to both a wide range of steric environments and a full range of hybridization of the carbon directly bound to the lead, thereby controlling the relative “s” and “p” contributions to the Pb−X bond as the hybridization changes from sp3 (alkyl) to sp2 (aryl) to sp (allynyl). Three divalent organolead complexes, [(BDI)PbMe] (1), [(BDI)PbPh] (2), and [(BDI)PbCCPh] (3), were reported elsewhere,6 all possessing an endo conformation. The 207Pb chemical shifts for 1−3 are 3009 ppm (1), 2424 ppm (2), and 1463 ppm (3). Although this is a broad range of chemical shifts, this series lacks organolead complexes possessing an exo conformation. We have expanded the range of known βdiketiminato organolead complexes by synthesizing β-diketiminato lead complexes bearing bulkier alkyl groups, including tBu and neopentyl (Np) (Figure 2). This has not only provided

Figure 2. List of compounds used in this study and arranged by their solid-state conformations. Complexes 1 − 3 have been previously reported;6 complexes 4−8 are reported for the first time. Figure 3. ORTEP diagrams of lead isopropyl complex 4 (a), tert-butyl complex 5 (b), sec-butyl complex 6 (c), neopentyl complex 7 (d), and benzyl complex 8 (e). H atoms are omitted for all diagrams, and BDI C atoms are minimized in parts (b)−(e) for clarity. Ellipsoids are shown at the 30% probability level.

examples of complexes with an exo conformation but also increases the range of electron donors, allowing for a more detailed study of the 207Pb chemical shift.

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RESULTS AND DISCUSSION Synthesis of Lead Alkyl Complexes. Five new βdiketiminato lead complexes were generated by treatment of lead chloride, [(BDI)PbCl], with the appropriate Grignard (in the case of isopropyl, sec-butyl, and benzyl lead complexes 4, 6, and 8, respectively) or alkyllithium reagent (in the case of tertbutyl and neopentyl lead complexes 5 and 7, respectively). The 1 H NMR spectra of these complexes are similar, with two multiplets observed between δ 3 and 4 ppm for the isopropyl groups on the N-aryl substituent, consistent with a trigonalpyramidal geometry at lead. Coupling between 1H and 207Pb was only observed in the benzyl lead complex 8 (JHPb = 86 Hz). The 207Pb NMR chemical shift ranged from δ 2872 ppm for benzyl complex 8 to δ 3684 ppm for tert-butyl complex 5. The solid-state structure was determined for compounds 4− 8 (Figure 3). See the Supporting Information for the data collection parameters of complexes 4−8. The bond lengths and bond angles for compounds 4 and 5 are given in Table 1, and the bond lengths and bond angles for compounds 6−8 are given in Table 2. As expected, consistent with other βdiketiminato lead(II) complexes, a pyramidal geometry is observed at lead, and the degree of pyramidalization (DOP)16 is consistent with that of other β-diketiminato group 14 complexes.3,17−20 However, in contrast to the previously reported β-diketiminato organolead complexes 1−3, an exo conformation is observed at the metal center of complexes 4− 8, with the lead atom significantly displaced from the NCCCN plane (range 1.179−1.436 Å). This displacement is also reflected in the torsion angle between the NCCCN plane and a plane defined by N−Pb−N (NPbN plane). The exo complexes 4−8 have a more acute torsion angle (range 131.3−

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Isopropyl Complex 4 and tert-Butyl Complex 5 (L = BDI) Pb−C(30) Pb−N(1) Pb−N(2) N(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−N(2) N(1)−Pb−C(30) N(2)−Pb−C(30) N(1)−Pb−N(2) Pb−N(1)−C(1) Pb−N(2)−C(2) sum of angles DOPa Pb−NCCCN plane torsion angleb

[LPbiPr] (4)

[LPbtBu] (5)

2.305(4) 2.327(3) 2.330(3) 1.323(4) 1.404(5) 1.404(5) 1.332(5) 98.51(14) 97.67(14) 79.90(10) 120.6(2) 120.2(2) 276.08 93.2 1.436 141.68

2.333(6) 2.364(4) 2.362(4) 1.326(7) 1.402(8) 1.409(8) 1.315(7) 102.32(19) 104.22(18) 79.14(15) 114.8(3) 114.0(3) 285.68 82.6 1.359 131.27

a

Degree of pyramidalization.16 bAngle between the plane defined by N(1)−C(1)−C(2)−C(3)−N(2) and the plane defined by N(1)−Pb− N(2).

141.7°) than then endo complexes 1−3 (149.7° for acetylide complex 3 to 164.0° for phenyl complex 2). The Pb−C bond lengths range from 2.293(4) Å for neopentyl complex 7 to 2.333(6) for tert-butyl complex 5, a range similar to that of the endo complexes (2.300 Å for methyl complex 1 and phenyl complex 2, 2.276(3) Å for acetylide complex 3). There is no B

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the Hammett σmeta constant (R2 = 0.84).22 The σpara correlation increases when the chemical shift of the β-diketimato lead chloride is included (R2 = 0.95); however, this correlation does not hold with all β-diketimato lead complexes (see the Supporting Information). If lead amido complexes [(BDI)PbNHPh] and [(BDI)PbNMe2] are included, the correlation between σpara and the 207Pb chemical shift vanishes (R2 = 0.0082). Unfortunately, reliable σpara values could not be found for all of the reported β-diketiminato lead complexes. A similar correlation between 103Rh NMR chemical shift and σpara was observed by both the Leitner and Elsevier groups on a series of rhodium complexes bearing bidentate aryl phosphine ligands.23,24 The more electron rich the aryl phosphine, the higher the chemical shift and less shielding of the rhodium nuclei. In contrast, ring-substituted (η5-cyclopentadienyl)dicarbonylrhodium complexes, [(η5-C5H4R)Rh(CO)2], show the opposite trend; electron-withdrawing groups decrease the shielding of the rhodium nuclei. These contrasting results reveal the sensitivity of nuclei to various perturbations, presumably affecting the relative contributions from diamagnetic, paramagnetic, and spin−orbit shieldings. A Hammett correlation was also observed in the 183W NMR chemical shift of a series of para-substituted pyridine tungsten complexes, cis[W(CO)4(PPh3)(4-RC5H4N)].25 Similar to the case for the [(η5-C5H4R)Rh(CO)2] system, decreasing the electron density on the pyridine resulted in greater deshielding of the nuclei. These results were attributed to differences in the polarizability of the pyridine ligand affecting the paramagnetic shielding contribution to the chemical shift. A correlation was investigated for a series of tetrakis(phenyl)germanium complexes;26 however, in this case, only a small correlation was found, highlighting the limits of utilizing Hammett constants in the investigation of trends in chemical shift. However, in all of these examples, a substituent was changed on an aryl group that was either directly bound to the metal or bound through a donor ligand such as phosphine. In our system, we are investigating the effect of the ligand directly bound to the metal center. Computational Studies: 207Pb NMR Chemical Shifts. The 207Pb NMR chemical shifts were calculated using density functional theory (DFT) to determine the factors contributing to the excellent correlation between the experimental chemical shifts and the Hammett σpara parameter. A handful of computational studies have previously been performed on 207 Pb NMR chemical shifts, including calculations on solid-state 207 Pb NMR spectra27−30 and a solution-phase study investigating the importance of spin−orbit coupling in the chemical shift calculations on a series of lead(IV) complexes.31 In this work, it was found necessary to include relativistic effects at the spin− orbit level, as neither scalar relativistic effects nor a relativistic core potential were sufficient to model these systems (see the Experimental Section for method details). These relativistic calculations were performed on both the X-ray (solid-state) and optimized (gas-phase) structures; we note that neither is the perfect model for the solution-phase system in which the 207Pb NMR was recorded. The 207Pb NMR data calculated using the X-ray structures gave a more consistent set of errors and thus will be the data discussed herein. Initially, the absolute isotropic shielding of complexes 1−8 was calculated and converted into chemical shifts by using the shielding of tetramethyllead, [Pb(CH3)4] (TML), which was calculated to have an isotropic shielding value of 3530 ppm. The NMR calculations were performed on a geometry-optimized Td structure of TML at the

Table 2. Selected Bond Lengths (Å) and Angles (deg) for sec-Butyl Complex 6, Neopentyl Complex 7, and Benzyl Complex 8 (L = BDI) Pb−C(16)* Pb−N N−C(1) N−Pb−C(16)a N′−Pb−C(16)a N−Pb−N′ Pb−N−C(1) sum of angles DOPb Pb−NCCCN plane torsion anglec

[LPbsBu] (6)

[LPbNp] (7)

[LPbBn] (8)

2.306(5)* 2.3287(19) 1.325(3) 97.5(6) 98.2(7)* 79.56(10) 118.73(15) 275.26 94.2 1.215 137.89

2.293(4) 2.3273(16) 1.324(2) 91.04(14) 97.54(15) 79.64(8) 119.41(12) 268.22 101.9 1.179 139.32

2.314(5) 2.321(2) 1.329(4) 93.38(11) 93.38(11) 80.22(12) 117.42(19) 266.98 103.4 1.251 135.95

a

Atom bonded to lead is C(17) in structure 6. bDegree of pyramidalization.16 cAngle between the plane defined by N(1)− C(1)−C(2)−C(3)−N(2) and the plane defined by N(1)−Pb−N(2).

correlation between the Pb−C bond length and torsion angle. However, the bond angles around lead are more acute for the endo conformers (average 87.5°) than for the exo conformers (91.5°). The differences between the endo and exo conformers are presumably due to the differences in steric bulk at the alkyl ligand; that is, in the absence of steric bulk, there is a preference for the endo conformation, as the ligand is small enough to fit between the bulky N-aryl substituents on the β-diketiminate ligand. Preliminary density functional theory (DFT) investigations on the relative stabilities of these complexes revealed that complexes 2, 5, and 7 all optimize to the observed conformation, whereas for complexes 1, 4, 6, and 8 both endo and exo structures are obtained as minima with the relative energy difference between the conformers being small.21 207 Pb NMR Spectroscopy. The electronic effect of the alkyl group on the 207Pb NMR chemical shift was examined by plotting the Hammett σpara constant versus the 207Pb chemical shift.22 A good correlation (R2 = 0.94) was observed between the chemical shift and σpara (Figure 4). Thus, the 207Pb NMR chemical shifts of complexes 1−8 are directly related to the electron donor capacity of the alkyl group; the more electron donating the alkyl group, the higher the chemical shift. It is notable that a Hammett correlation exists with complexes 1−8, even though the data from both the endo and exo conformers have been combined. A weaker correlation was observed with

Figure 4. Hammett σpara constant versus complexes 1−8.

207

Pb chemical shifts of C

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Table 3. Experimental and Calculated 207Pb NMR Chemical Shifts (ppm) Relative to TML and Linear Regression Reference, Hammett Constants (σpara), Absolute Isotropic Shielding (σcalcd), Spin−Orbit Shielding (σSO), Diamagnetic Shielding (σD), and Paramagnetic Shielding (σP) for Complexes 1−8 using Eq 2 calcd exptl entry

R

1 2 3 4 5 6 7 8

Me Ph CCPh iPr tBu sBu Np Bn

207 Pb chemical shift δexptl

3002 2419 1463 3145 3684 3262 3506 2872

Pb chemical shift

isotropic shielding σcalcd = σD + σP+ σSO

Hammett constant σpara

absolute isotropic shielding σcalcd

TML δTML

linear regression δcalcd

σSO

σD

σP

−0.17 −0.01 0.16 −0.15 −0.20 −0.12 −0.17 −0.09

1731 2318 3142 1550 971 1568 1303 1938

1799 1212 388 1980 2559 1962 2227 1592

3008 2376 1509 3185 3794 3166 3445 2776

−2140 −2130 −2034 −2200 −2354 −2198 −2247 −2225

9955 9956 9956 9955 9955 9955 9954 9954

−6084 −5508 −4780 −6205 −6630 −6189 −6404 −5791

same level of theory as that used for the lead complexes. This resulted in significant error between the experimental and calculated chemical shifts (Table 3); however, as the variation in the error is relatively small, 225 ppm (1075−1300 ppm), these errors can be considered systematic and a reflection of the unsuitability of TML as a reference standard. This unsuitability is not unusual and has been examined in detail in the calculation of 13C NMR chemical shifts. When tetramethylsilane (TMS) is used as a reference, errors can arise due to the differences in the isotropic shielding values for carbon atoms attached to other carbon atoms versus silicon atoms.32 To reduce error, one technique is to choose a new reference molecule that has properties similar to that of the molecules of interest. Due to the limited number of known three-coordinate divalent lead complexes, this is not possible. Another method used in 13 C NMR chemical shift calculations32 and used by Bühl for calculating transitionmetal NMR chemical shifts33 is linear regression analysis, in which the calculated isotropic shielding is plotted against the experimental chemical shift, δexptl. The resulting linear correlation allows for the determination of a new reference, taken as the intercept at δexptl = 0. This can then be used to convert absolute shielding (σcalcd) to calculated chemical shift (δcalcd) by eq 2, thus avoiding the use of an unsuitable reference point. δcalcd = (intercept − σcalcd)/slope

207

Figure 5. Correlation plot for calculated absolute isotropic shielding, σcalcd, and experimental chemical shift, δexptl, for the 207Pb NMR of compounds 1−8.

spin−orbit (σSO) shielding contributions (Table 3).31 The diamagnetic contribution, σD, remained relatively constant between all complexes (2 ppm difference). Diamagnetic shielding is a result of electrons moving in orbitals determined by the ground-state wave function and is thereby dependent upon the electron density at the nucleus. Thus, for a series of structurally similar compounds, this term is not anticipated to vary significantly. The spin−orbit contribution, σSO, showed a variation of 320 ppm among complexes 1−8, which is relatively small in comparison to the variation in calculated chemical shift (over 2000 ppm). In previous studies, it was found that spin−orbit coupling is very sensitive to the atomic number of the atoms directly coordinated to the Pb center.31 Thus, only minor variations would be expected between these complexes. In contrast, the paramagnetic term, σP, increases significantly (by up to 1850 ppm) as the chemical shift decreases. This term is due to the circulation of electrons within the ground- and excited-state orbitals. Its magnitude depends on the asymmetric electron distribution close to the nucleus; therefore, for s orbitals it is 0 but can be quite large for an asymmetric distribution of p and d electrons when these electrons have lowlying excited states.34 A strong correlation was found between the Hammett constant and paramagnetic shielding (Figure 6). That is, the paramagnetic shielding alone correlates with the Hammett constant and thus the electron-withdrawing and

(2)

A plot of absolute shielding versus experimental 207Pb chemical shifts gives a very good correlation (R2 = 0.99) with a slope of −0.95, indicative of a well-performing method, and an intercept of 4576 ppm (Figure 5).32 Using this linear regression reference point for calculating the chemical shifts reduced the errors between the experimental and calculated chemical shifts by 1 order of magnitude or more. The errors ranged from −110 to 95 ppm (Table 3), which are less than 10% of the chemical shift span reported in this study. There is a very good correlation (R2 = 0.93) between the calculated chemical shift (δcalcd) and the Hammett parameter, enabling the theory to elucidate the reasons for it. The calculated chemical shifts did not correlate directly to the Pb−C bond length. This indicates that electronic rather than structural factors are primarily responsible for the correlation. It was noted earlier that spin−orbit coupling was very important; therefore, the contributions to the absolute shielding were evaluated. The absolute shielding can be deconstructed into the relative diamagnetic (σD), paramagnetic (σP), and D

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CH), 3.59 (sept, J = 6.9 Hz, 2H, CHMe2), 3.48 (sept, J = 6.8 Hz, 2H, CHMe2), 1.75 (s, 6H, NCMe), 1.26 (d, J = 6.9 Hz, 12H, CHMe2), 1.24 (d, J = 6.8 Hz, 12H, CHMe2), 0.59 (s, JMe‑Pb = 89 Hz, 3H, PbMe). UV−vis: λmax 396.0 nm, λsecondary 353.0 nm. [CH{(CH3)CN-2,6-iPr2C6H3}2PbPh] (2): Modified Procedure.6 [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. PhMgBr (3.0 M, 150 μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the LPbCl solution. The mixture was left to react for 2 h, after which the toluene was removed in vacuo, pentane was added, and the yellow solution was filtered through Celite. The solution was concentrated and the product crystallized at −30 °C, giving cubic yellow crystals. Yield: 80 mg (25.4%). 1H NMR (500 MHz, C6D6, 30 °C): δ 8.52 (d, J = 6.9 Hz, 2H, o-HPh), 7.58 (t, J = 7.6 Hz, 2H, m-HPh), 7.12 (m, 6H, Haryl), 7.05 (m, 1H, p-HPh), 4.80 (s, 1H, middle CH), 3.58 (sept, J = 6.9 Hz, 2H, CHMe2), 3.22 (sept, J = 6.8 Hz, 2H, CHMe2), 1.77 (s, 6H, NCMe), 1.38 (d, J = 6.9 Hz, 6H, CHMe2), 1.25 (d, J = 6.9 Hz, 6H, CHMe2), 1.07 (d, J = 6.9 Hz, 6H, CHMe2), 0.52 (d, J = 6.7 Hz, 6H, CHMe2). UV−vis: λmax 397.0 nm, λsecondary 360.0 nm. CH{(CH3)CN-2,6-iPr2C6H3}2PbiPr] (4). [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. iPrMgCl (2.0 M, 230 μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the cold LPbCl solution. The mixture was stirred for 2 h, after which the toluene was removed in vacuo. Pentane was then added and the orange solution filtered through Celite. The solution was concentrated and placed at −30 °C for crystallization. Orange crystals suitable for X-ray diffraction were grown from a heptane solution, also held at −30 °C. Yield: 161 mg (53.1%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.07 (m, 6H, Haryl), 4.62 (s, 1H, middle CH), 3.79 (sept, J = 6.9 Hz, 2H, CHMe2), 3.32 (sept, J = 6.9 Hz, 2H, CHMe2), 3.31 (sept, J = 6.9 Hz, 1H, PbCHMe2) 1.72 (s, 6H, NCMe), 1.54 (d, J = 4.0 Hz, 6H, PbCHMe2), 1.42 (d, J = 6.9 Hz, 6H, CHMe2), 1.25 (d, J = 6.9 Hz, 6H, CHMe2), 1.23 (d, J = 6.9 Hz, 6H, CHMe2), 1.19 (d, J = 6.8 Hz, 6H, CHMe2). 13C NMR (400 MHz, C6D6, 30 °C): δ 165.9 (NCMe), 143.7, 143.3, 142.5, 126.6, 125.4, 124.0, 123.6, 123.2 (Caryl), 101.7 (Pb-CHMe2), 97.5 (middle CH), 28.2 (CHMe2), 27.6 (CHMe2), 24.7 (CHMe2), 24.3 (CHMe2), 24.2 (CHMe2), 24.1 (CHMe2), 23.0 (NCMe), 17.2 (PbCMe2). 207Pb NMR (600 MHz, C6D6, 30 °C): 3145 ppm. Anal. Calcd for C32H48N2Pb: C, 57.55; H, 7.19; N, 4.20. Found: C, 57.49; H, 7.27; N, 4.15. UV−vis: λmax 354.0 nm, λsecondary 448.9 nm. [CH{(CH3)CN-2,6-iPr2C6H3}2PbtBu] (5). [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. tBuLi (1.89 M, 240 μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the cold LPbCl solution. The mixture was stirred at −78 °C overnight, after which the toluene was removed in vacuo and replaced with pentane and the bright orange solution was filtered through Celite. Large red crystals, suitable for X-ray analysis, were obtained from a concentrated pentane solution at −30 °C. Yield: 174 mg (56.0%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.08 (m, 6H, Haryl), 4.67 (s, 1H, middle CH), 3.85 (sept, J = 6.9 Hz, 2H, CHMe2), 3.22 (sept, J = 6.8 Hz, 2H, CHMe2), 2.20 (s, 9H, CMe3), 1.75 (s, 6H, NCMe), 1.44 (d, J = 6.9 Hz, 6H, CHMe2), 1.26 (d, J = 6.9 Hz, 6H, CHMe2), 1.21 (d, J = 6.8 Hz, 6H, CHMe2), 1.18 (d, J = 6.8 Hz, 6H, CHMe2). 13C NMR (400 MHz, C6D6, 30 °C): δ 167.9 (NCMe), 144.6, 143.3, 142.3, 125.3, 123.8, 123.6 (Caryl), 123.2 (PbCMe3), 97.7 (middle CH), 28.6 (CHMe2), 28.0 (CHMe2), 26.9 (CHMe2), 26.6 (CHMe2), 25.0 (CHMe2), 24.6 (CHMe2), 24.1 (PbCMe3), 23.5 (NCMe), 23.0 (PbCMe3). 207Pb NMR (600 MHz, C6D6, 30 °C): 3684 ppm. Anal. Calcd for C33H50N2Pb: C, 58.13; H, 7.34; N, 4.11. Found: C, 58.06; H, 7.28; N, 4.03. UV−vis: λmax 375.0 nm, λsecondary 451.1 nm. [CH{(CH3)CN-2,6-iPr2C6H3}2PbsBu] (6). [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. sBuMgCl (1.9 M, 260 μL, 0.5 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the cold LPbCl solution. The mixture was stirred at −78 °C for 2 h, after which the toluene was evaporated and replaced with

Figure 6. Hammett constant σpara versus calculated paramagnetic shielding, σP, of complexes 1−8.

-donating properties of the ligand. This confirms that the paramagnetic shielding alone is trend setting and that the more electron donating the substituent, R, the more negative the paramagnetic shielding contribution, resulting in a smaller isotropic shielding and a more downfield (larger) chemical shift.

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CONCLUSIONS The 207Pb NMR chemical shift of β-diketiminato lead complexes, [(BDI)PbX], is largely unpredictable on comparison of different classes of terminal X ligands. This is presumably due to differences in the contributions to the nuclear shielding from the diamagnetic, paramagnetic, and spin−orbit terms. However, across a class of terminal X ligands, notably the organolead derivatives, the relative contributions of the diamagnetic and spin−orbit terms to the overall shielding are constant, allowing for trends in 207Pb NMR chemical shifts to appear. The observed correlation with the Hammet σpara constant highlights influence of subtle electronic effects well beyond organic molecules.

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EXPERIMENTAL SECTION

All manipulations were carried out under an atmosphere of dry nitrogen or argon using standard Schlenk techniques or in an inertatmosphere glovebox. Solvents were dried from the appropriate drying agent, distilled, degassed, and stored over 4 Å sieves. [(BDI)PbCl] (1) and [(BDI)PbCCPh] were prepared according to the literature.1,6 The 1 H and 13C NMR spectra were recorded on a Varian 400, 500, or 600 MHz spectrometer. The 1H and 13C NMR spectroscopic chemical shifts are given relative to residual solvent peaks, and the 207Pb shifts were externally referenced to PbMe4. Reagents were purchased from Acros Organics or Sigma-Aldrich. Neopentyllithium was donated by Prof. Geoff Cloke’s research group. UV−vis spectra were recorded on a Varian Cary 50 instrument. The data for the X-ray structure were collected at 173 K on a Nonius Kappa CCD diffractometer (λ(Mo Kα) = 0.71073 Å) and refined using the SHELXL-97 software package.35 [CH{(CH3)CN-2,6-iPr2C6H3}2PbMe] (1): Modified Procedure.6 [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. MeMgBr (1.6 M, 330 μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the LPbCl solution. The mixture was stirred for 1 h, after which the toluene was removed in vacuo, pentane was added, and the lemon yellow solution was filtered through Celite. The solution was concentrated and the product left to crystallize at −30 °C, giving bright yellow crystals. Yield: 115 mg (39.6%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.06 (m, 6H, Haryl), 4.81 (s, 1H, middle E

DOI: 10.1021/om501223a Organometallics XXXX, XXX, XXX−XXX

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Organometallics pentane and the orange solution was filtered through Celite. The solution was placed at −30 °C, and small, dark orange crystals were observed after a few days. Yield: 186 mg (60.7%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.06 (m, 6H, Haryl), 4.63 (s, 1H, middle CH), 3.80 (sept, J = 6.9 Hz, 2H, CHMe2), 3.32 (sept, J = 6.6 Hz, 2H, CHMe2), 2.46 (m, 1H, PbCH(Me)CH2Me), 1.73 (s, 6H, NCMe), 1.61 (d, J = 7.3 Hz, 3H, PbCH(Me)CH2Me), 1.45 (d, J = 6.7 Hz, 6H, CHMe2), 1.25 (m, 2H, PbCH(Me)CH2Me), 1.18 (d, J = 6.7 Hz, 6H, CHMe2), 0.80 (t, J = 7.3 Hz, 3H, PbCH(Me)CH2Me). 13C NMR (400 MHz, C6D6, 30 °C): δ 169.5, 166.1 (NCMe), 143.9, 143.3, 142.5, 131.8, 125.4, 124.0, 123.6 (Caryl), 108.7 (PbCH(Me)CH2Me), 97.5 (middle CH), 28.3, 28.2 (CHMe2), 27.7 (PbCH(Me)CH2Me), 26.9, 26.8 (CHMe2), 25.0, 24.7, 24.3, 24.2, 24.1 (CHMe2), 23.1, 23.0 (NCMe), 15.6 (PbCH(Me)CH2Me), 13.0 (PbCH(Me)CH2Me). 207Pb NMR (600 MHz, C6D6, 30 °C): 3262 ppm. Anal. Calcd for C33H50N2Pb: C, 58.13; H, 7.34; N, 4.11. Found: C, 57.03; H, 7.29; N, 4.02. UV−vis: λmax 360.1 nm, λsecondary 452.0 nm. [CH{(CH3)CN-2,6-iPr2C6H3}2PbNp] (7). [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to 0 °C. NpLi (35 mg, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the cold LPbCl solution. The mixture was stirred for 2 h, after which the toluene was removed in vacuo. Pentane was then added, and the orange solution was filtered through Celite. The solution was concentrated and placed at −30 °C. After 1 week, orange crystals suitable for X-ray analysis were observed. Yield: 209 mg (66.8%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.27 (m, 6H, Haryl), 4.73 (s, 1H, middle CH), 3.89 (sept, J = 6.9 Hz, 2H, CHMe2), 3.41 (sept, J = 6.8 Hz, 2H, CHMe2), 1.84 (s, 6H, NCMe), 1.62 (d, J = 6.9 Hz, 6H, CHMe2), 1.36 (d, J = 6.9 Hz, 6H, CHMe2), 1.34 (d, J = 6.9 Hz, 6H, CHMe2), 1.29 (d, J = 6.8 Hz, 6H, CHMe2), 1.01 (s, 2H, PbCH2tBu), 0.79 (s, 9H, PbCH2tBu). 13C NMR (400 MHz, C6D6, 30 °C): δ 166.2 (NCMe), 143.4, 143.3, 142.8, 125.4, 123.9 (C aryl ), 114.0 (PbCH2CMe3), 97.3 (middle CH), 36.6 (PbCH2CMe3), 28.3 (CHMe2), 27.7 (CHMe2), 26.6 (CHMe2), 24.8 (CHMe2), 24.6 (CHMe2), 24.4 (CHMe2), 22.9 (NCMe). 207Pb NMR (600 MHz, C6D6, 30 °C): 3506 ppm. Anal. Calcd for C34H52N2Pb: C, 58.69; H, 7.48; N, 4.03. Found: C, 58.64; H, 7.56; N, 3.98. UV−vis: λmax 344.0 nm, λsecondary 451.0 nm. [CH{(CH3)CN-2,6-iPr2C6H3}2PbBn] (8). [(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and cooled to −78 °C. BnMgCl (20 wt %, 343 mg, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk tube and was added dropwise to the LPbCl solution. The mixture was stirred for 2 h, after which the toluene was removed in vacuo, pentane was added, and the orange solution was filtered through Celite. The solution was concentrated and the product left to crystallize at −30 °C, giving pale orange crystals. Yield: 126 mg (39.2%). 1H NMR (400 MHz, C6D6, 30 °C): δ 7.13 (m, 6H, Haryl), 6.98 (t, J = 7.6 Hz, 2H, m-HPh), 6.57 (t, J = 7.4 Hz, 1H, p-HPh), 5.94 (d, J = 7.4 Hz, 2H, o-HPh), 4.64 (s, 1H, middle CH), 3.74 (sept, J = 6.9 Hz, 2H, CHMe2), 3.19 (sept, J = 6.8 Hz, 2H, CHMe2), 1.75 (s, JCH2‑Pb = 87 Hz, 2H, PbCH2), 1.68 (s, 6H, NCMe), 1.44 (d, J = 6.9 Hz, 6H, CHMe2), 1.24 (d, J = 6.8 Hz, 6H, CHMe2), 1.15 (d, J = 6.8 Hz, 6H, CHMe2), 1.05 (d, J = 6.9 Hz, 6H, CHMe2). 13C NMR (400 MHz, C6D6, 30 °C): δ 165.5 (NCMe), 143.4, 143.2, 142.7, 141.9, 138.4, 134.9, 127.2, 125.7, 124.2, 124.0, 122.4 (Caryl), 98.2 (middle CH), 94.6 (Pb-CH2), 28.3 (CHMe2), 27.6 (CHMe2), 26.5 (CHMe2), 24.7 (CHMe2), 24.6 (CHMe2), 24.2 (CHMe2), 23.0 (NCMe). 207Pb NMR (400 MHz, C6D6, 30 °C): δ 2871. Anal. Calcd for C36H46N2Pb: C, 60.40; H, 6.71; N, 3.91. Found: C, 60.45; H, 6.79; N, 3.87. UV−vis: λmax 345.0 nm, λsecondary 446.0 nm. Computational Details. DFT NMR calculations were performed on the lead complexes 1−8 and tetramethyllead (TML) using the Amsterdam Density Functional (ADF) package,36−38 which allows for a detailed treatment of relativistic effects. Relativistic effects were accounted for using the zeroth-order regular approximation (ZORA).39 All calculations were performed using the BP86 functional,40−42 with a Slater-type orbital all-electron relativistic quadrupleζ basis set, with four sets of polarization functions (ZORA-QZ4P) on the Pb center, and a triple-ζ basis set with polarization (ZORA-TZP)

on all other atoms. Frequency analysis was used to confirm minima obtained via geometry optimization; however, only the single-point energies calculated using the X-ray crystal structure are discussed in the main text (coordinates are provided in the CIF files of the Supporting Information). Magnetic shielding tensors were calculated using the gauge-including atomic orbitals (GIAO) method.43,44 NMR calculations were performed on both the X-ray crystal structure and optimized geometry for each complex 1−8. The maximum chemical shift errors using the geometry-optimized structures were larger (∼475 ppm) than those using the X-ray structure (∼250 ppm), and thus the geometry-optimized results are not presented. Furthermore, it was found that the spin−orbit coupling was extremely important in order to provide quantitative agreement with experimental chemical shifts; therefore, only the calculations which include spin−orbit relativistic effects are presented in the main text.

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ASSOCIATED CONTENT

* Supporting Information S

Figures, a table, and CIF files giving crystallographic data and complete ORTEP diagrams for complexes 4−8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.C.: [email protected]. *E-mail for J.R.F.: [email protected]. Present Address †

School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.R.F. and M.J.T. are grateful for financial support from the EPSRC (Grant No. EP/E-32575/1). H.C. and E.J.C. acknowledge the use of the EPSRC UK National Service for Computational Chemistry Software (NSCCS) at Imperial College London in carrying out this work.

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DEDICATION We dedicate this article to Professor Mike Lappert, an inspiration and a true hero of Group 14. REFERENCES

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DOI: 10.1021/om501223a Organometallics XXXX, XXX, XXX−XXX