Carbodiimides as catalysts for the reduction of a cadmium hydride complex

D. J. Webb a, C. M. Fitchett b, M. Lein a and J. R. Fulton *a
aSchool of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand. E-mail:
bDepartment of Chemistry, University of Canterbury, P.B. 4800, Christchurch 8041, New Zealand

Received 1st November 2017 , Accepted 4th December 2017

First published on 4th December 2017

A rare terminal cadmium hydride complex [(BDI)CdH] (BDI = [{N(2,6-iPr2C6H3)C(Me)}2CH]) has been synthesised from [(BDI)CdCl] and LiEt3BH. The hydride can be reduced to the cadmium(I) dimer, [(BDI)CdCd(BDI)] upon treatment with a catalytic amount of diisopropyl- or dicyclohexylcarbodiimide.

Group 12 metal hydrides were first synthesised in the mid twentieth century.1,2 Due to their relative stability, the subsequent reactivity studies focused primarily on zinc hydrides, the subject of a recent review.3 Although most studies have focused on their stoichiometric nucleophilic and basic properties, zinc hydride complexes have also been shown to be active catalysts in the hydrosilylation of aldehydes, ketones and nitriles,4,5 hydrogenation of imines,6 and dehydrocoupling of silanes and alcohols.7 In contrast, both cadmium and mercury hydride complexes have received relatively little attention, partly because of their unsuitability as a general industrial catalyst due to their toxicity, but also because of their limited stability relative to zinc.1 Although cadmium hydrides have been implicated as reactive intermediates in the hydrosilylation and hydroboration of carbon–oxygen double bonds,8,9 to date, only three cadmium hydride complexes have been reported: Power's terminal hydride [Ar*CdH] (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2),10 a loosely-associated bridging hydride, [Ar′Cd(μ-H)2CdAr′] (Ar′ = C6H3-2,6-(C6H2-2,6-iPr2)2)11 and Reger's terminal hydride [HB(3-tBuPz)3CdH],12 the molecular structure of which was not determined.

Outside of their implied catalytic activity, little is known about the chemistry of cadmium hydride complexes. The dimeric hydride [Ar′Cd(μ-H)2CdAr′] loses dihydrogen to form cadmium–cadmium bond complex [Ar′CdCdAr′] upon standing in solution at room temperature.11 The mechanism for hydrogen loss is postulated to proceed via an associative pathway involving a three-coordinate cadmium centre. In contrast, [Ar*CdH] does not transform into the corresponding dimeric species, presumably due to kinetic stabilisation from the larger Ar* ligand. We set out to synthesise a three-coordinate β-diketiminato cadmium hydride, [(BDI)CdH] (BDI = [{N(2,6-iPr2C6H3)C(Me)}2CH]). This complex was predicted to be unable to undergo an associative mechanism for the elimination of dihydrogen, thus allowing for an investigation into the chemistry of the terminal cadmium–hydrogen bond.

The synthesis of monomeric cadmium hydride 1 was achieved upon addition of lithium triethylborohydride to [(BDI)CdCl] (Scheme 1).13,14 Treatment of [(BDI)CdCl] with other hydride sources such as NaH, CaH2, NaBH4, LiAlH4 or Et3SiH did not yield either the cadmium hydride 1 or the reduced dinuclear cadmium complex [(BDI)CdCd(BDI)] (2). The 1H NMR spectrum of 1 shows a broad hydride resonance at δ 5.6 ppm, upfield to that of terminal hydrides [Ar*CdH] (δ 6.79 ppm) and [HB(3-tBuPz)3CdH] (δ 6.3 ppm) and bridging hydride [Ar′Cd(μ-H)2CdAr′] (δ 6.84 ppm).10–12 The Cd–H stretch for hydride 1 is found at 1734 cm−1, very similar to that of [Ar*CdH] (1735 cm−1).10 A 113Cd NMR signal is observed at δ −650 ppm.

image file: c7cc08393a-s1.tif
Scheme 1

The molecular structure of hydride 1 is shown in Fig. 1. The hydrogen atom was located in the difference map and modelled as isotropic. The geometry at cadmium is planar, with a Cd–H bond length of 1.65(3) Å, similar to that reported for [Ar*CdH] (1.79(4) Å). The nitrogen–cadmium distances of 2.160(1) and 2.163(1) Å are similar to that of [(BDI)Cd(C6F5)] (2.168(4) Å) and similar to the shorter bond length reported for [(BDI)CdMe] (2.149(4), 2.152(3)), but shorter than the longer bond length reported for that structure (2.234(4), 2.240(4)).15,16 Hydride 1 is stable in solution, only slowly degrading to a black precipitate and small amounts of dinuclear cadmium complex [(BDI)Cd]2 (2) over a month at room temperature (vide infra). This is in contrast to [Ar′Cd(μ-H)2CdAr′] and implies that the β-diketiminate ligand is able to prevent a thermal pathway to 2. Addition of THF to a C6D6 solution of cadmium hydride 1 does not alter the 1H NMR chemical shifts, indicating that hydride 1 does not form strong complexes with this Lewis base. This is in contrast to cadmium anilido complex, [(BDI)Cd{NH(2,6-iPr2C6H3)}], which readily coordinates THF to form a four-coordinate cadmium complex.13 Note that exposure of 1 to NaH, KH or LiEt3BH resulted in decomposition to a black precipitate.

image file: c7cc08393a-f1.tif
Fig. 1 ORTEP diagram of [(BDI)CdH] (1) with H atoms omitted and BDI aryl groups minimised for clarity; ellipsoid probability shown at 30%. Selected bond distances (Å) and bond angles (°): Cd–H 1.65(3); Cd–N(1) 2.1599(12), Cd–N(2) 2.1625(12); N(1)–Cd–N(2) 88.44(5); N(1)–Cd–H 134.6 (1); N(2)–Cd–H 136.8 (1). Cd(1)–{N(1)C(1)C(2)C(3)N(2)-plane} 0.031 Å.

Although the reactivity of the isostructural zinc complex, [(BDI)ZnH], towards nucleophiles was not reported,17 the related [(BDIMe2)ZnH] (BDIMe2 = [{N(2,6-Me2C6H3)C(Me)}2CH]) has shown nucleophilic behaviour by inserting into heterocumulenes such as di-tert-butylcarbodiimide (DBC) and tert-butyl isothiocyanate to form formamidinato and thioformamido complexes, respectively.18 Thus, the nucleophilic behaviour of hydride 1 was probed. Addition of benzophenone, phenylacetylene, PhNCO, TEMPO or DBC to 1 results in the formation of multiple unisolable products. In contrast, when one equivalent of the less sterically encumbered carbodiimide, freshly purified dicyclohexylcarbodiimide (DCC), was added to hydride 1, dinuclear cadmium(I) complex 2 is formed with >90% conversion after 16 hours at room temperature (Scheme 2).

image file: c7cc08393a-s2.tif
Scheme 2

The expected product, amidinate [(BDI)Cd(HDCC)] (3, HDIC = [C(H)(NCy)2]) was not observed. Interestingly, complex 2 also forms when only 0.1 equivalents of DCC was added to 1, although a longer reaction time (3 days) was required. Shorter reaction times were observed when 2 equivalents of DCC was added. In both cases, dihydrogen gas is observed by 1H NMR spectroscopy when the reaction is performed in a sealable NMR tube. The only non-solvent volatile detected in the GC-MS analysis of the product mixture is DCC.

Addition of di-iso-propylcarbodiimide (DIC) to 1 also yields cadmium dimer 2 with approximately 50% conversion. Although other products were observed in the 1H NMR spectrum of the reaction mixture, only dimer 2 could be isolated. As with DCC, when only 0.1 equivalents of DIC is added to 1, dimer 2 is formed in greater conversion, albeit at longer reaction times. Attempts at independently generating the expected amidinate complexes (3) [(BDI)Cd(HDCC)] or [(BDI)Cd(HDIC)] (HDIC = [C(H)(NiPr)2]) were unsuccessful, either from salt metathesis between [(BDI)CdCl] and Li(HDIC) or Li(HDCC) or from protonolysis of [(BDI)Cd{N(SiMe3)2}] with H(HDIC) or H(HDCC).13

X-ray structural analysis of compound 2 reveals an approximate D2d symmetry in the solid state, with an 83.4° angle between the two β-diketiminate (NCCCN) planes, similar to the isostructural magnesium,19 manganese20 and zinc21 complexes (Fig. 2). Complex 2 is a rare example of a low-coordinate dinuclear cadmium complex,22–25 with a cadmium–cadmium bond length of 2.56952(4) Å, shorter those previously reported. In contrast to 1, both cadmium atoms are significantly displaced from the NCCCN planes (0.657 and 0.526 Å).

image file: c7cc08393a-f2.tif
Fig. 2 ORTEP diagram of [(BDI)CdCd(BDI)] (2) with H atoms omitted and BDI aryl groups minimised for clarity; ellipsoid probability shown at 30%. Selected bond distances (Å) and bond angles (°): Cd(1)–Cd(2) 2.5952(3); Cd(1)–N(1) 2.200(2); Cd(1)–N(2) 2.212(2); Cd(2)–N(3) 2.197(2); Cd(2)–N(4) 2.205(2); N(1)–Cd–N(2) 87.14(8); N(1)–Cd(1)–Cd(2) 141.66(6); N(2)–Cd(1)–Cd(2) 130.96(6); N(3)–Cd–N(4) 87.45(8); N(3)–Cd(2)–Cd(1) 142.14(6); N(4)–Cd(2)–Cd(1) 130.00(5). Cd(1)–{N(1)C(1)C(2)C(3)N(2)-plane} 0.657 Å; Cd(2)–{N(3)C(30)C(31)C(32)N(4)} 0.526 Å.

DFT studies were performed on hydride 1, dimer 2 and the isostructural zinc analogues (PBE0-D3(BJ)/def2-TZVP, see ESI for further details). In the gas phase, the D2 conformer of 2 is 4.2 and 12.9 kcal mol−1 more stable than the D2d and D2h conformers, respectively. In contrast, the D2 conformer does not exist for the isostructural zinc dimer, and the D2d conformer is 18.0 kcal mol−1 more stable than the D2h conformer. The trend is similar to that reported for Robinson's model system, RZn–ZnR (R = [(HNCH)2CH]);21 however, the magnitude of the difference is significantly greater and highlights the importance of sterics in determining the relative stability of these conformers. The smaller energy differences between the D2d and D2h conformers of the cadmium dimer compared to the zinc dimer can be attributed to the increase of ionic radii of the metal atom. The Cd–Cd σ-bond was located at HOMO−2, and is primarily s in character (96.0%). Similarly, the Cd–H bond is comprised of 95.3% s-orbital from the metal, with the cadmium atom contributing to 27.6% to the Cd–H bond with the hydrogen atom contributing 72.4% to the bond. The Natural bond orbitals (NBOs) of 1 and 2 are shown in Fig. 3.

image file: c7cc08393a-f3.tif
Fig. 3 NBO of Cd–H bond of 1 (left) and Cd–Cd bond of 2 (right).

The calculated ΔH for conversion of cadmium hydride 1 to cadmium dimer 2 and dihydrogen gas is −142.3 kJ mol−1. The zinc reduction is more thermally neutral (ΔH = −14.5 kJ mol−1). The difference in reaction enthalpies between the cadmium and zinc systems is a reflection of the concurrent weakening of the M–H bond and strengthening of the M–M bond as the group is descended.26,27

In summary, we have synthesised a remarkably stable terminal cadmium hydride complex that only undergoes reduction to the cadmium(I) dimer upon treatment with small carbodiimides. The observed reactivity is unprecedented, not only for β-diketiminato metal hydrides, but also other metal hydrides complexes as well, all of which insert carbodiimides into the metal–hydrogen bond to form corresponding amidinate complexes.18,28–32 Although we cannot entirely rule out that a catalytic impurity is responsible for the observed reactivity, our inability to generate β-diketiminato cadmium amidinate complexes leads us to speculate about the inherent stability of said complexes. We are currently undergoing further investigations to understand the mechanism for this transformation.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available: Experimental detail for 1 and 2, data collection parameters and thermal ellipsoid plots of 1 and 2, computational details, including Cartesian coordinates, for 1 and 2. CCDC 1582730–1582731. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc08393a

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