Direct hydrosilylation by a zirconacycle with β-hydrogen

Article abstract of DOI:10.1039/c4dt00658e

Azasilazirconacycle Cp₂Zr{κ²-N(SiHMe ₂)SiHMeCH₂} (1) and formaldehyde react through an uncatalyzed addition reaction (hydrosilylation) to form an exocyclic methoxysilyl-substituted zirconacycle. Although 1 contains 2-center-2-electron SiH groups, this transformation parallels the reactions of non-classical [Cp₂ZrN(SiHMe₂)₂]⁺ ([2]⁺) with carbonyls. Reactions of 1 with a series of nucleophilic and electrophilic agents were explored, as well as reactions of related β-SiH-containing silazidozirconium compounds, to develop a rationale for the unexpected hydrosilylation. For example, carbon monoxide and 1 react at the Zr-C bond to form Cp₂Zr{κ²-OC(CH₂)SiHMeN(SiHMe ₂)} (7). The Lewis acid B(C₆F₅)₃ also reacts at the Zr-C bond to give Cp₂Zr{N(SiHMe ₂)SiHMeCH₂B(C₆F₅)₃} (8). OPEt₃ and N,N-dimethylaminopyridine (DMAP) do not appear to interact with 1. In contrast, OPEt₃ and DMAP react with non-classical compounds [2]⁺ and zwitterionic 8.

Full text of DOI:10.1039/c4dt00658e

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Direct hydrosilylation by a zirconacycle with
β-hydrogen†
Cite this: Dalton Trans., 2014, 43,
8644
KaKing Yan, Aradhana Pindwal, Arkady Ellern and Aaron D. Sadow*
Azasilazirconacycle Cp2Zr{κ2-N(SiHMe2)SiHMeCH2} (1) and formaldehyde react through an uncatalyzed
addition reaction (hydrosilylation) to form an exocyclic methoxysilyl-substituted zirconacycle. Although 1
contains 2-center-2-electron SiH groups, this transformation parallels the reactions of non-classical
[Cp2ZrN(SiHMe2)2]+ ([2]+) with carbonyls. Reactions of 1 with a series of nucleophilic and electrophilic
agents were explored, as well as reactions of related β-SiH-containing silazidozirconium compounds, to
develop a rationale for the unexpected hydrosilylation. For example, carbon monoxide and 1 react at the
Zr–C bond to form Cp2Zr{κ2-OC(vCH2)SiHMeN(SiHMe2)} (7). The Lewis acid B(C6F5)3 also reacts at the
Zr–C bond to give Cp2Zr{N(SiHMe2)SiHMeCH2B(C6F5)3} (8). OPEt3 and N,N-dimethylaminopyridine
(DMAP) do not appear to interact with 1. In contrast, OPEt3 and DMAP react with non-classical
compounds [2]+ and zwitterionic 8.
Received 5th March 2014,
Accepted 10th April 2014
DOI: 10.1039/c4dt00658e
www.rsc.org/dalton
interactions, but not always. Often non-classical structures are
fluxional, and therefore it is difficult to experimentally assess
Introduction
The β position in organometallic compounds is well-known as if the intermolecular reactions of the β-H containing ligand
a reactive site involved in intramolecular abstraction and elimi- occur from non-classical structures or molecular configur-
nation processes.1,2 In metal alkyl systems lacking the appro- ations in which the β-E–H is a classical 2-center-2-electron
priate vacant metal orbitals, β-H elimination processes can be bond.
slow. As a result, alternative reaction pathways may become
Therefore, we sought to compare the reactivity of classical
accessible. Similarly, d0 metal amido compounds containing and non-classical β-hydrogen-containing compounds. The
β-H are reluctant to undergo β-elimination. This inertness is recently reported azasilazirconacyclobutane compound Cp2Zr-
proposed to be related to the geometric and electronic struc- {κ2-N(SiHMe2)SiHMeCH2} (1) with a pendent β-SiHMe2 group
ture of β-agostic amides, which contain features that are gene- and an endocyclic β-SiH provides such an opportunity.10 The
rally dissimilar to those of the expected transition state of rigid cyclic structure of the molecule results in classical
β-elimination.3 Notably, the structural distortion and spec- bonding for both Si–H groups. In contrast, the cationic com-
troscopy of β-agostic amides are distinguished from those of pound [Cp2ZrN(SiHMe2)2][HB(C6F5)3] ([2]+) is characterized by
alkyl ligands (that readily β-eliminate) by larger M–N–C angles unusual spectroscopic and geometric features supporting the
and downfield 1H NMR chemical shifts for the β-H.
presence of two non-classical Zr↼H–Si structures.7 Related
In the absence of intramolecular elimination processes, diagostic structures were previously reported for rare earth
alkyl and amide ligands containing β hydrogen are still reac- tetramethyldisilazido compounds,11–14 but before that only
tive at that position. For example, rare earth and main group molybdenum, niobium, tantalum and zirconium tetramethyl-
compounds of the tris(dimethylsilyl)methide or bis(dimethyl- disilazido compounds were reported.15–17
silyl)amido ligands react with Lewis acids via β-H abstraction.4–6
Moreover, studies of these two zirconium compounds
Cp2Zr{N(SiHMe2)2}Et reacts with B(C6F5)3 via β-H abstraction reveal unusual reactivity. The Zr–C bond of 1 reacts with ethyl-
from the amide or ethyl ligands.7 Other main group alkyls, ene through a sequence involving C–H bond activation
such as ZnEt2 and [Al]CH2CHMe2, also react with B(C6F5)3 via (Scheme 1A), whereas a β-hydrogen-free analog was unchanged
β-H abstraction.8,9 In some of these cases, the compounds after treatment with C2H4 under forcing conditions.10
display features associated with non-classical or agostic β-H
Interestingly, the non-classical compound [2]+ and carbo-
nyls react by addition of the Si–H bonds to the CvO
(Scheme 1B). Compound [2]+ also reacts with N,N-dimethyl-
aminopyridine (DMAP) via attack at a silicon center to give Si–N
bond formation and a new zirconium hydride.7 In order to
understand the effect of classical and non-classical β-hydrogen
Department of Chemistry, 1605 Gilman Hall, Iowa State University, Ames, IA 50011,
USA. E-mail: sadow@iastate.edu
†CCDC 988789–988792. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00658e
8644 | Dalton Trans., 2014, 43, 8644–8653
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purchased from Aldrich and were used as received. Acetone,
acetophenone, and cyclohexanecarbaldehyde were degassed
and stored over
4 Å molecular sieves before use. The
compounds Cp2Zr{κ2-N(SiHMe2)SiHMeCH2} (1),10 Cp2Zr-
{N(SiHMe2)2}Et,7 Cp2Zr{N(SiHMe2)2}H,18 Cp2Zr{κ2-NSiMe2-
19
(SiHMe2)}PMe3 (6·PMe3),10 and B(C6F5)3 were prepared fol-
lowing literature procedures.
1H, 13C{1H}, 11B and 29Si{1H} NMR spectra were collected
on a Bruker DRX-400 spectrometer, a Bruker Avance III-600
spectrometer, or an Agilent MR 400 spectrometer. 11B NMR
spectra were referenced to an external sample of BF3·Et2O.
15N NMR chemical shifts were determined by 1H–15N HMBC
experiments either on a Bruker Avance II 700 spectrometer
with a Bruker Z-gradient inverse TXI 1H/13C/15N 5 mm cryo-
probe or by 1H–15N CIGARAD experiments on an Agilent MR400
spectrometer; 15N chemical shifts were originally referenced to
liquid NH3 and recalculated to the CH3NO2 chemical shift
scale by adding −381.9 ppm. Assignments of NMR resonances
Scheme 1 (A) C–H bond activation by a classical β-SiH containing
zirconacycle 1. (B) Hydrosilylation of formaldehyde by non-classical SiH
in [2]+.
on reactivity, we decided to investigate the reactions of 1 with
two-electron donors and carbonyls to compare with the reactiv-
ity of [2]+. The results of those studies are described in the
present disclosure. Unexpectedly, 1 and formaldehyde react to
give a methoxysilyl, and we describe a number of combi-
nations of related SiH-containing zirconium silazidos and car-
bonyls that provide some insight into the unusual addition
reaction. We also compare the reactivity of 1, [2]+, and a
zwitterionic analog of [2]+ prepared by the reaction of 1 and
B(C6F5)3.
1
are supported by H–1H and heteronuclear correlation experi-
ments. Infrared spectra were measured on a Bruker IFS66v
FTIR. Elemental analyses were performed using a Perkin-
Elmer 2400 Series II CHN/S. X-ray diffraction data was col-
lected on a Bruker APEX II diffractometer (Table 1).
Cp2Zr{κ2-N(SiMe2OMe)SiHMeCH2} (3)
Metallacycle 1 (0.115 g, 0.326 mmol) and paraformaldehyde
(0.010 g, 0.33 mmol) were suspended in benzene (2 mL) and
heated to 80 °C for 5.5 h. The reaction mixture was filtered,
and the volatile components of the soluble portion were evap-
Experimental
All manipulations were performed under a dry argon atmos- orated under reduced pressure to give a yellow oil. Extraction
phere using standard Schlenk techniques or under a nitrogen with pentane and evaporation of the pentane provided 3 as a
atmosphere in a glovebox unless otherwise indicated. Water viscous yellow oil (0.119 g, 0.31 mmol, 95%). 1H NMR
and oxygen were removed from benzene and pentane solvents (benzene-d6, 600 MHz, 25 °C): δ 5.91 (s, 5 H, C5H5), 5.87 (s,
using an IT PureSolv system. Benzene-d6 was heated to reflux 5 H, C5H5), 4.50 (m, 1JSiH = 202 Hz, 1 H, NSiHMe), 3.29 (s, 3 H,
2
3
over Na/K alloy and vacuum-transferred. Methylene chloride-d2 SiMe2OMe), 1.90 (dd, JHH = 12.4 Hz, JHH = 4.0 Hz, 1 H, CH2),
2
3
was vacuum transferred from CaH2 and stored under N2 in 1.55 (dd, JHH = 12.4 Hz, JHH = 2.2 Hz, 1 H, CH2), 0.32 (d,
the glovebox. PMe3, triethylphosphine oxide, DMAP, benzo- 3JHH = 2.4 Hz, 3 H, CH2SiHMe), 0.12 (s, 3 H, SiMe2OMe), 0.10
phenone, paraformaldehyde and paraformaldehyde-d2 were (s, 3 H, SiMe2OMe). 13C{1H} NMR (benzene-d6, 150 MHz,
Table 1 Crystallographic data for compounds 4, 5, 6·PMe3, and 9
4
5
6·PMe3
9
Chemical formula
Formula weight
Crystal system
C17H29NOSiZr
382.72
C17H29NOSi2Zr
410.82
C17H32NPSi2Zr
428.81
C38H35BF15NO2Si2Zr
980.88
Orthorhombic
a = 8.586(1) Å
b = 13.622(2) Å
c = 32.542(5) Å
α = β = γ = 90°
3805.8(10) Å3
P212121
Monoclinic
a = 17.1498(14) Å
b = 8.7333(7) Å
c = 15.0209(13) Å
α = γ = 90°; β = 114.845(1)°
2041.5(3) Å3
P21/c
Monoclinic
a = 9.947(3) Å
b = 8.477(2) Å
c = 12.816(3) Å
α = γ = 90°; β = 97.646(4)°
1071.0(5) Å3
P21
Orthorhombic
a = 17.6574(16) Å
b = 19.6319(18) Å
c = 22.653(2) Å
α = β = γ = 90°
7852.6(12) Å3
Pbca
Unit-cell dimensions
Volume
Space group
Z
8
4
2
8
Reflections collected
Independent reflections
Rint
36 156
8690
0.0888
17 916
4179
0.0436
9483
4374
0.0408
53 924
10 942
0.0882
R I > 2σ(I)
R1 = 0.1220
wR2 = 0.2888
R1 = 0.1324
wR2 = 0.2940
R1 = 0.0491
wR2 = 0.1260
R1 = 0.0621
wR2 = 0.1345
R1 = 0.0369
wR2 = 0.0999
R1 = 0.0377
wR2 = 0.1009
R1 = 0.0443
wR2 = 0.0911
R1 = 0.0954
wR2 = 0.1114
Rall
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25 °C): δ 112.32 (C5H5), 112.29 (C5H5), 50.02 (SiMe2OMe), 61%). Red X-ray quality needle crystals were grown from a con-
35.56 (ZrCH2), 2.14 (CH2SiHMe), 1.26 (SiMe2OMe), 0.80 centrated pentane solution at −30 °C. 1H NMR (benzene-d6,
(SiMe2OMe). 15N{1H} NMR (benzene-d6, 61 MHz, 25 °C): δ 600 MHz, 25 °C): δ 5.40 (s, 10 H, C5H5), 5.18 (br, 1 H, SiH),
2
3
−246.8. 29Si{1H} NMR (benzene-d6, 119.3 MHz, 25 °C): δ −6.4 0.93 (d, JPH = 9 Hz, 9 H, PMe), 0.42 (d, JHH = 4.8 Hz, 6 H,
(SiMe2OMe), −65.9 (SiHMe). IR (KBr, cm−1): 2952 m, 2827 w, SiHMe2), 0.40 (s, 6 H, SiMe2). 13C{1H} NMR (benzene-d6,
1
2072 m br (νSiH), 1597 w, 1443 w, 1249 s, 1087 s, 1016 s, 900 s, 150 MHz, 25 °C): δ 104.72 (C5H5), 20.98 (d, JPC = 21.8 Hz,
796 s br, 720 s. Anal. Calcd for C15H25Si2NOZr: C, 47.07; H, PMe), 3.62 (SiMe2 and SiHMe overlapped). 15N{1H} NMR
6.58; N, 3.66. Found: C, 47.37; H, 6.59; N, 3.44.
(benzene-d6, 61 MHz, 25 °C): δ −331.1. 31P NMR (benzene-d6,
243 MHz, 25 °C):
δ
18.67. 29Si{1H} NMR (benzene-d6,
Cp2Zr{N(SiHMe2)t-Bu}OMe (4)
119.3 MHz, 25 °C): δ −12.9 (1JSiH = 177.0 Hz, SiHMe2), −26.9
Paraformaldehyde (0.008 g, 0.255 mmol) and Cp2Zr{N(SiHMe2)- (2JPSi = 74.0 Hz, SiMe2).
t-Bu}H (0.090 g, 0.255 mmol) were suspended in benzene
Cp2Zr{κ2-OC(vCH2)SiHMeN(SiHMe2)} (7)
(5 mL) at room temperature, and the mixture was heated at
60 °C for 7 h. The resulting yellow solution was filtered, and Compound 1 (0.116 g, 0.329 mmol) was dissolved in benzene
the benzene supernatant was evaporated to dryness under (15 mL), the solution was placed in a flask with a resealable
reduced pressure to afford compound 4 as an analytically and teflon valve, and the reaction mixture was degassed with
spectroscopically pure yellow solid (0.075 g, 0.195 mmol, 87%). freeze–pump–thaw cycles (3×). The vessel was charged with CO
1H NMR (benzene-d6, 600 MHz, 25 °C): δ 6.00 (s, 10 H, C5H5), (1 atm), and the reaction mixture was heated to 85 °C for 9 h.
1
4.10 (m, JSiH = 164 Hz, 1 H, SiHMe2), 3.69 (s, 3 H, OMe), 1.27 Evaporation of the volatile components, extraction of the
3
(s, 9 H, CMe3), 0.33 (d, JHH = 3.6 Hz, 6 H, SiHMe2). 13C{1H} resulting yellow residue with pentane, and evaporation of the
NMR (benzene-d6, 150 MHz, 25 °C): δ 112.37 (C5H5), 61.89 pentane provided 7 as yellow microcrystalline solid (0.045 g,
(OMe), 57.66 (CMe3), 35.10 (CMe3), 3.07 (SiHMe2). 15N{1H} 0.12 mmol, 36%). 1H NMR (benzene-d6, 400 MHz, 25 °C):
1
NMR (benzene-d6, 61 MHz, 25 °C): δ −263. 29Si{1H} NMR δ 6.09 (s, 5 H, C5H5), 6.06 (s, 5 H, C5H5), 5.43 (m, JSiH = 207
(benzene-d6, 119.3 MHz, 25 °C): δ −34.9. IR (KBr, cm−1): Hz, 1 H, NSiHMe), 4.91 (s br, 1 H, CvCH2), 4.48 (s br, 1 H,
1
2950 w, 2908 s, 2801 s, 2085 s (νSiH), 1600 w, 1456 m, 1382 s, CvCH2), 4.27 (m, JSiH = 190 Hz, 1 H, NSiHMe2), 0.41 (d,
3
1354 s, 1244 s, 1189 s, 1144 s br, 1015 s, br, 921 s br, 794 s br, 3JHH = 3.2 Hz, 3 H, OCNSiHMe) 0.24 (d, JHH = 3.1 Hz, 3 H,
3
687 s. Anal. Calcd for C15H27Si2NOZr: C, 53.35; H, 7.64; N, ZrNSiHMe2), 0.18 (d, JHH = 3.2 Hz, 3 H, ZrNSiHMe2). 13C{1H}
3.66. Found: C, 53.05; H, 7.60; N, 3.47. mp 82–84 °C.
NMR (benzene-d6, 125 MHz, 25 °C): δ 173.42 (OCCH2), 115.31
(C5H5), 115.27 (C5H5), 98.41 (OCCH2), 3.35 (OCSiHMe), 2.27
(SiMe2), 0.73 (SiMe2). 29Si{1H} NMR (benzene-d6, 119.3 MHz,
Cp2Zr{κ2-O,N-OCMe2SiMe2N(SiHMe2)} (5)
A benzene solution (2 mL) of Cp2Zr{N(SiHMe2)2}Et (0.063 g, 25 °C): δ −10.5 (SiHMe), −24.6 (SiHMe2). IR (KBr, cm−1): 3083
0.165 mmol) and acetone (14.5 μL 0.197 mmol) was heated at w, 2953 s, 2075 s br (νSiH), 1609 m, 1576 s (νCvC), 1442 s,
60 °C for 4 h. The volatiles were evaporated under reduced 1361 m, 1249 s, 1176 s, 1015 s, 916 s br, 796 s br. Anal. Calcd
pressure, and the product was extracted with pentane for C15H23Si2NOZr: C, 47.32; H, 6.09; N, 3.68. Found: C, 47.73;
(2 × 3 mL) to give 5 (0.059 g, 0.144 mmol, 87%) as a yellow H, 5.85; N, 3.28. mp 120 °C (decomp).
solid. X-ray quality crystals were grown from a concentrated
Cp2Zr{N(SiHMe2)SiHMeCH2B(C6F5)3} (8)
pentane solution at −30 °C. 1H NMR (benzene-d6, 600 MHz,
1
25 °C): δ 6.10 (s, 10 H, C5H5), 4.30 (m, JSiH = 188.4 Hz, 1 H, Compound 1 (0.082 g, 0.232 mmol) and B(C6F5)3 (0.125 g,
SiH), 1.25 (s, 6 H, ZrOCMe2), 0.25 (d, JHH = 3.2 Hz, 6 H, 0.244 mmol) were allowed to react in benzene (5 mL). A light-
3
SiHMe2), 0.22 (s, 6 H, CSiMe2). 13C{1H} NMR (benzene-d6, yellow oil precipitated from the benzene solution. The benzene
150 MHz, 25 °C): δ 114.27 (C5H5), 83.88 (OCMe2), 29.01 was separated from the oil with a pipet. The oil was washed
(ZrOCMe2), 2.90 (CSiMe2), 2.25 (SiHMe2). 29Si{1H} NMR with benzene (2 × 5 mL), extracted with methylene chloride
(benzene-d6, 119.3 MHz, 25 °C): δ 24.6 (CSiMe2), −23.6 (1 × 5 mL), and evaporated to dryness to give a pale yellow
(SiHMe2). IR (KBr, cm−1): 3104 w, 3070 w, 2961 s, 2897 m, solid. This solid was washed with pentane (2 × 5 mL) and
2851 m, 2085 m br (νSiH), 1443 m, 1369 m, 1352 m, 1246 s, dried under vacuum to give 8 as a white solid (0.159 g,
1163 m, 1148 m, 1113 s, 1015 s, 993 s, 946 s, 916 s br, 795 s br, 0.184 mmol, 79%). 1H NMR (bromobenzene-d5, 600 MHz,
764 s. Anal. Calcd for C17H29NOSi2Zr: C, 49.70; H, 7.12; N, 25 °C): δ 5.54 (s, 5 H, C5H5), 5.46 (s, 5 H, C5H5), 1.64 (br d,
2
3.40. Found: C, 50.01; H, 6.81; N, 2.84. mp 82–90 °C.
2JHH = 15 Hz, 1 H, CH2), 0.61 (br d, JHH = 13.0 Hz, 1 H, CH2),
1
0.02 (br overlapping signal, JSiH = 81 Hz, 4 H, SiHMe2 and
SiHMeCH2B(C6F5)3), −0.04 (br s, 3 H, SiHMe2), −0.07 (br s,
Cp2Zr{κ2-NSiMe2(SiHMe2)}PMe3 (6·PMe3)
1
The synthesis and NMR characterization data of 6·PMe3 was 3 H, SiHMeCH2B(C6F5)3), −0.54 (br s, JSiH = 83 Hz, 1 H,
previously reported in ref. 10. A benzene solution of Cp2Zr- SiHMe2). 13C{1H} NMR (bromobenzene-d5, 150 MHz, 25 °C):
{N(SiHMe2)2}Et (0.028 g, 0.073 mmol) and 1.1 equiv. of PMe3 δ 148.23 (C6F5), 146.65 (C6F5), 138.08 (C6F5), 136.49 (C6F5),
were heated at 65 °C for 2.5 h to give a deep red solution. The 134.90 (C6F5), 108.17 (C5H5), 107.74 (C5H5), 8.87 (CH2B-
volatiles were removed under reduced pressure and the (C6F5)3), −2.05 (SiMe2), −2.80 (CH2SiHMe), −2.88 (SiMe2).
product was extracted with pentane (0.019 g, 0.044 mmol, 11B NMR (bromobenzene-d5, 119.3 MHz, 25 °C): δ −14.4.
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19F NMR (bromobenzene-d5, 564 MHz, 25 °C): δ −131.9 (d, s, 1 H, ZrH), 3.17 (s, 6 H, NMe2), 1.17 (br d, 2JHH = 8.2 Hz, 2 H,
3
3JFF = 21.3 Hz, 6 F, o-F), −162.2 (t, JFF = 21.5 Hz, 3 F, p-F), BCH2), 0.53 (s, 3 H, SiMe2), 0.48 (s, 3 H, SiMe2), 0.28 (over-
3
−165.7 (t, JFF = 19.9 Hz, 6 F, m-F). 29Si{1H} NMR (bromo- lapped with SiHMe2 resonance, assigned by 1H–1H COSY
benzene-d5, 119.3 MHz, 25 °C): δ −36.2 (SiMe), −47.3 (SiMe2). experiment, SiH), 0.25 (br s, 6 H, SiHMe2). 13C{1H} NMR
IR (KBr, cm−1): 3125 w, 2963 w, 2905 w, 1737 m (νSiH), 1641 s, (methylene chloride-d2, 125 MHz, −53 °C): δ 155.28 (ipso-
1579 m br, 1512 s, 1456 vs br, 1257 s, 1169 s, 1101 s, 1081 s, NC5H4NMe2), 142.31 (α-NC5H4NMe2), 106.92 (β-NC5H4NMe2),
1017 s, 972 s, 925 s, 860 s, 811 s, 778 s, 750 m. Anal. Calcd for 105.39 (C5H5), 105.01 (C5H5), 40.13 (NC5H4NMe2), 8.30 (br,
BC32F15H23Si2NZr: C, 44.45; H, 2.68; N, 1.62. Found: C, 44.01; BCH2), 1.05 (SiMe2), 1.03 (SiMe2), −1.96 (SiHMe2). 11B NMR
H, 2.51; N, 1.51. mp 113–120 °C.
(methylene chloride-d2, 119.3 MHz, −53 °C): δ −14.7. 15N{1H}
NMR (methylene chloride-d2, 61 MHz, −53 °C): δ −193.8
(NC5H4NMe2), −295.8 (NC5H4NMe2), −317.7 (ZrN). 19F NMR
Cp2ZrN{SiMe2(OCHMe2)}{SiMe(OCHMe2)CH2B(C6F5)3} (9)
Acetone (32.5 μL, 0.444 mmol) was added to a methylene (methylene chloride-d2, 564 MHz, −53 °C): δ −133.3 (br, 6 F,
3
3
chloride solution (10 mL) of 8 (0.128 g, 0.148 mmol), and the o-F), −164.4 (t, JFF = 20.2 Hz, 3 F, p-F), −167.7 (d, JFF = 21.1
mixture was stirred for 20 min. All volatile substances were Hz,
6
F, m-F). 29Si{1H} NMR (methylene chloride-d2,
1
removed under reduced pressure. The residue was washed 119.3 MHz, −53 °C): δ −4.6 (SiMe2DMAP), −63.9 (d, JSiH
=
with pentane (2 × 2 mL). The solid was dried in vacuo to give 9 112.3 Hz, SiHMe2). 10b: 1H NMR (methylene chloride-d2,
as a yellow solid (0.141 g, 0.144 mmol, 97%). X-ray quality crys- 600 MHz, −53 °C): δ 7.48 (d, 3JHH = 6.8 Hz, 2 H, α-NC5H4NMe2),
3
tals were grown from a concentrated methylene chloride solu- 6.53 (d, JHH = 6.5 Hz, 2 H, β-NC5H4NMe2), 5.86 (s, 5 H, C5H5),
tion at −30 °C. 1H NMR (methylene chloride-d2, 600 MHz, 5.47 (s, 5 H, C5H5), 3.99 (br s, 1 H, ZrH), 3.17 (s, 6 H, NMe2),
2
25 °C): δ 6.47 (s, 5 H, C5H5), 6.32 (s, 5 H, C5H5), 4.13 (m, 1.33 (br d, JHH = 14.1 Hz, 2 H, BCH2), 1.29 (br, 1 H, SiH), 0.39
3
1 H, Me2SiOCH), 3.95 (m, 1 H, BCH2SiOCH), 1.53 (d, JHH
=
(br s, 6 H, SiMe and SiHMe2), 0.27 (br s, 3 H, SiHMe2). 13C{1H}
3
6.3 Hz, 3 H, BCH2SiOCHMe2), 1.52 (d, JHH = 6.2 Hz, 3 H, NMR (methylene chloride-d2, 125 MHz, −53 °C): δ 156.08 (ipso-
Me2SiOCHMe2), 1.49 (d, JHH = 6.5 Hz, 3 H, Me2SiOCHMe2), NC5H4NMe2), 142.47 (α-NC5H4NMe2), 105.81 (C5H5), 105.73
3
3
1.29 (d, JHH = 6.5 Hz, 3 H, BCH2SiOCHMe2), 0.31 (s br, 3 H, (β-NC5H4NMe2), 105.21 (C5H5), 40.20 (NC5H4NMe2), 10.50 (br,
SiMe2), 0.13 (s br, 3 H, SiMe2), –0.21 (s br, 3 H, SiMe). 13C{1H} BCH2), −0.47 (SiMe2), −1.20 (SiMe2), −1.96 (SiHMe2). 11B NMR
NMR (methylene chloride-d2, 150 MHz, 25 °C): δ 149.54 (methylene chloride-d2, 119.3 MHz, −53 °C): δ −14.7. 15N{1H}
(br, C6F5), 147.95 (br, C6F5), 139.36 (br, C6F5), 137.90 (br, NMR (methylene chloride-d2, 61 MHz, 25 °C): δ −192.3
C6F5), 136.39 (br, C6F5), 116.26 (C5H5), 115.29 (C5H5), 75.07 (NC5H4NMe2), −295.8 (NC5H4NMe2), −311.8 (ZrN). 19F NMR
3
(Me2SiOCH), 74.91 (BCH2SiOCH), 25.44 (Me2SiOCHMe2), 25.15 (methylene chloride-d2, 564 MHz, −53 °C): δ −135.3 (d, JFF
=
3
(BCH2SiOCHMe2), 24.77 (Me2SiOCHMe2), 24.29 (BCH2SiOCHMe2), 22.3 Hz, 6 F, o-F), −164.7 (t, JFF = 20.2 Hz, 3 F, p-F), −167.5
4.29 (SiMe2), 4.20 (SiMeCH), 4.15 (SiMe2). 29Si{1H} NMR (methyl- (br,
6
F, m-F). 29Si{1H} NMR (methylene chloride-d2,
1
ene chloride-d2, 119.3 MHz, 25 °C): δ 13.8 (SiMe2), 5.7 (SiMeCH2). 119.3 MHz, −53 °C): δ 11.4 (SiMe2DMAP), −61.1 (d, JSiH
=
19F NMR (methylene chloride-d2, 376 MHz, 25 °C): δ −133.1 (d, 115.0 Hz, SiHMe2). IR (KBr, cm−1): 2959 m br, 1845 m vbr
3JFF = 22.1 Hz, 6 F, o-F), −165.2 (t, JFF = 20.4 Hz, 3 F, p-F), (νSiH), 1641 m (νC6F5), 1601 m, 1565 s, 1511 s (νC6F5), 1458 s br
3
−168.3 (t, 3JFF = 20.5 Hz, 6 F, m-F). 11B NMR (methylene chloride- (νC6F5), 1403 m, 1309 m, 1255 m, 1233 m, 1102 s br, 1069 s br,
d2, 79.5 MHz, 25 °C): δ −15.1. Anal. Calcd for BC38F15H35Si2- 1030 m, 972 s, 846 m, 804 s br, 681 m. Anal. Calcd for
NO2Zr: C, 46.53; H, 3.60; N, 1.43. Found: C, 46.34; H, 4.00; BC39F15H33Si2N3Zr: C, 47.46; H, 3.37; N, 4.26. Found: C, 47.06;
N, 1.14. mp 77–85 °C.
H, 3.84; N, 4.40.
Cp2ZrN{(SiMe2DMAP)SiHMeCH2B(C6F5)3}H (10a) and Cp2ZrN- Cp2Zr{N(SiHMe2)SiHMeCH2B(C6F5)3}OPEt3 (11)
{(SiHMe2)SiMe(DMAP)CH2B(C6F5)3}H (10b)
Compound 8 (0.053 g, 0.061 mmol) and OPEt3 (0.008 g,
Compound 8 (0.049 g, 0.056 mmol) and DMAP (0.125 g, 0.061 mmol) react in methylene chloride (2 mL) to form a
0.057 mmol) were allowed to react in methylene chloride yellow solution. The solution was stirred for 5 min., the volatile
(2 mL) to form a yellow solution. The solution was stirred for materials were removed under reduced pressure, and the solid
5 min., and then the solvent was evaporated. The resulting was washed with pentane (2 × 5 mL). The resulting material
material was washed with pentane (2 × 5 mL) and then dried was then dried under vacuum to give 11 (0.057 g, 0.057 mmol,
to give a mixture of Cp2ZrN{(SiMe2DMAP)SiHMeCH2B(C6F5)3}H 93%) as a yellow crystalline solid. 1H NMR (methylene chlor-
(10a) and Cp2ZrN{(SiHMe2)SiMe(DMAP)CH2B(C6F5)3}H (10b) ide-d2, 600 MHz, 25 °C): δ 6.37 (s, 5 H, C5H5), 6.24 (s, 5 H,
1
1
(0.055 g, 0.056 mmol, 99%) as a yellow crystalline solid in a C5H5), 4.09 (m, JSiH = 169.4 Hz, 1 H, SiHMe2), 3.82 (m, JSiH
=
1.65 : 1 ratio as determined by integration of the 1H NMR 177 Hz, 1 H, SiHMe), 1.98 (m, 6 H, PCH2), 1.19 (m, 9 H,
spectrum. 19F NMR resonances were assigned based on the PCH2CH3), 0.47 (d, JHH = 2.8 Hz, 3 H, SiHMe2), 0.37 (br, 2 H,
3
3
3
ratio of 10a : 10b, but 13C{1H} NMR signals associated with the CH2), 0.27 (d, JHH = 3.0 Hz, 3 H, SiHMe2), −0.45 (d, JHH
=
C6F5 groups were broad and not assigned to 10a or 10b. 10a: 2.3 Hz, 3 H, SiHMe). 13C{1H} NMR (methylene chloride-d2,
1H NMR (methylene chloride-d2, 600 MHz, −53 °C): δ 7.87 (d, 150 MHz, 25 °C): δ 149.65 (C6F5), 148.05 (C6F5), 139.17 (C6F5),
3JHH = 7.3 Hz, 2 H, α-NC5H4NMe2), 6.72 (d, JHH = 7.2 Hz, 2 H, 137.82 (C6F5), 137.48 (C6F5), 136.26 (C6F5), 115.13 (C5H5),
3
1
β-NC5H4NMe2), 5.61 (s, 5 H, C5H5), 5.60 (s, 5 H, C5H5), 3.93 (br 114.96 (C5H5), 18.79 (d, JPC = 65.5 Hz, PCH2CH3), 11.57 (br,
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2
BCH2), 5.94 (d, JPC = 4.7 Hz, PCH2CH3), 2.79 (SiHMe), 2.06
Apparently, the pendent β-SiH in 1 is essential for its reac-
(SiHMe2), 1.34 (SiHMe2). 11B NMR (methylene chloride-d2, tion with formaldehyde because 2 equiv. of paraformaldehyde
119.3 MHz, 25 °C): δ −14.7. 19F NMR (methylene chloride-d2, and Cp2Zr{κ2-N,C-N(t-Bu)SiHMeCH2} (which contains only an
3
564 MHz, 25 °C): δ −133.5 (br, 6 F, o-F), −165.9 (t, JFF
=
endocyclic SiH)18 are unchanged after heating in benzene-d6 at
19.9 Hz, 3 F, p-F), −168.8 (br, 6 F, m-F). 31P NMR (methylene 120 °C for 12 h. Note that Cp2Zr{κ2-N,C-N(t-Bu)SiHMeCH2} is
chloride-d2, 243 MHz, 25 °C): δ 101.9. 29Si{1H} NMR (methyl- available only as a mixture with {Cp2ZrN(SiHMe2)}2. These
ene chloride-d2, 119.3 MHz, 25 °C): δ −7.7 (SiHMe), −26.3 experiments strongly suggest that the reaction of compound 1
(SiHMe2). IR (KBr, cm−1): 2956 m, 2914 w, 2890 w, 2135 m br and formaldehyde is unlikely to involve insertion into the Zr–C
(νSiH), 2099 m br (νSiH), 1641 m (νC6F5), 1511 s (νC6F5), 1457 s br bond followed by a rearrangement. Such a pathway was con-
(νC6F5), 1271 m, 1250 m, 1166 m, 1082 s br, 1016 m, 973 s, sidered because Cp2Zr{κ2-CH2SiMe2CH2} and formaldehyde
930 s, 861 s, 804 s, 681 m. Anal. Calcd for BC38F15H38Si2- react through insertion into the Zr–C bond to give Cp2Zr{κ2-
NOPZr: C, 45.69; H, 3.83; N, 1.40. Found: C, 45.81; H, 3.81; N, OCH2CH2SiMe2CH2}.20 In addition, it is worth noting that the
1.36. mp 95–98 °C.
β-SiH in 1 are classically bonded (2-center-2-electron bonding)
based on its signals in the infrared spectrum (2074 cm−1) and
1
1H NMR spectrum (4.84 ppm, JSiH = 193 Hz).10 Thus, there is
no evidence for a side-on Zr↼H–Si interaction that might
enhance reactivity toward formaldehyde. In addition, the rigid
cyclic structure of 1 likely inhibits non-classical bonding. Still,
the cationic compound [2]+ that contains two non-classical
Zr↼H–Si interaction also reacts with formaldehyde to give
[Cp2ZrN(SiMe2OMe)2]+.7
Reactions of related zirconium hydrosilazido compounds
with carbonyl compounds were explored based on this
unusual insertion reaction; however, in many cases the reac-
tions provide the expected chemistry at Zr–R site (X = H, Me,
Et) rather than at the β-SiH. For instance, the acyclic hydrido
disilazido zirconium compound Cp2Zr{N(SiHMe2)2}H reacts
with formaldehyde or acetone at the Zr–H bond rather
than the β-SiH to afford the zirconium methoxide Cp2Zr-
{N(SiHMe2)2}OMe or isopropoxide Cp2Zr{N(SiHMe2)2}Oi-
Results and discussion
The metallacycle Cp2Zr{κ2-N(SiHMe2)SiHMeCH2} (1) and para-
formaldehyde (H2CO)n react in benzene at 80 °C over 5.5 h to
give Cp2Zr{κ2-N(SiMe2OMe)SiHMeCH2} (3) in 95% yield (eqn
(1)). In this transformation, a β-methoxy group is formed from
the addition of formaldehyde to the exocyclic β-SiH. Neither
the Zr–C bond nor the β-SiH on the endocyclic silicon center
are affected by formaldehyde.
ð1Þ
A singlet resonance at 3.29 ppm in the H NMR spectrum C3H7.7 The reactions with excess carbonyl molecules under
of 3 was assigned to the methoxy group, and only one SiH these conditions do not result in further conversion of the
signal was detected at 4.50 ppm. That signal was assigned by N(SiHMe2)2 group.7 Similarly, hydridozirconium silazide con-
1H–29Si HMQC and COSY experiments; the latter showed taining one β-SiH group Cp2Zr{N(SiHMe2)t-Bu}H also under-
scalar coupling to the ZrCH2 proving that the SiH is part of the goes insertion with paraformaldehyde exclusively into the
cyclic structure. As in 1, the (C5H5)2, CH2, and SiMe2 groups Zr–H bond to give Cp2Zr{N(SiHMe2)t-Bu}OMe (4). This com-
are diastereotopic. The transformation of the exocyclic β-SiH is pound is isolated and fully characterized by spectroscopic
supported by observation of two SiMe resonances (3 H each) methods, and its connectivity is confirmed by a single crystal
assigned to the SiMe2OMe group that appeared as singlets X-ray diffraction study (Fig. 1).
rather than doublets as in 1. The product and approximate
1
rate of conversion are not affected when the reaction is per-
formed in acetonitrile or in the presence of donor ligands
such as PMe3, OPEt3 or DMAP.
In addition, only starting materials are observed upon
treatment of 1 with excess benzophenone, acetophenone, or
cyclohexanecarbaldehyde even for 12 h at 80 °C. Reaction with
acetone (2 equiv.) at 80 °C overnight in benzene provides
multiple unidentified products. Benzaldehyde gives a mixture
of at least three products, one of which is tentatively assigned
as a Zr–C insertion product based on the observation of a new
resonance for the benzylic CH in the 1H NMR spectrum.
Upon thermolysis of 1 with excess benzaldehyde at 60 °C, a
small amount of toluene is generated (<5%) among un-
identified zirconium-containing products. Compound 3 is
inert toward further formaldehyde insertions under the con-
ditions of eqn (1).
Fig. 1 Thermal ellipsoid plot of one of two crystallographically inde-
pendent molecules of Cp2Zr{N(SiHMe2)t-Bu}OMe (4).
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Scheme 2 Proposed pathway for the formation of 5.
In contrast to the reactivity of hydridozirconium silazides,
the alkyl compound Cp2Zr{N(SiHMe2)2}Me and formaldehyde
react to give a complex mixture after 9 h at 85 °C in benzene-
d6. The combination of Cp2Zr{N(SiHMe2)2}Et and acetone
affords a zirconacyclopentane product Cp2Zr{κ2-O,N-OCMe2Si-
Me2N(SiHMe2)} (5) and ethane (Scheme 2). The latter bypro-
duct was detected in 1H NMR spectra of micromolar scale
experiments performed in Teflon-sealed NMR tubes. Com-
pound 5 was structurally characterized by a single crystal X-ray
diffraction study (Fig. 2) that proves Zr–O bond formation. In
Fig. 3 Thermal ellipsoid plot of Cp2Zr{κ2-NSiMe2(SiHMe2)}PMe3
(6·PMe3). Selected interatomic distances (Å): Zr1–N1, 2.195(2); Zr1–Si2,
2.660(1); Zr1–P2, 2.683(1); Zr1–Si1, 3.605(2); Zr1–H99, 3.7(2); Si1–N1,
1.680(3); Si1–N1, 1.693(3). Selected interatomic angles (°): Si1–N1–Si2,
138.1(2); Zr1–N1–Si1, 136.6(2); Zr1–N1–Si2, 85.3(1).
(6·PMe3). Recently, we obtained crystals of 6·PMe3, and a
single crystal X-ray diffraction study reported here provides
unambiguous evidence for formation of the zirconium silan-
imine complex (Fig. 3).
1
addition, the H NMR spectrum clearly showed the two C5H5,
the SiMe2, and CMe2 were equivalent, contrasting the
diastereotopic groups present in 1 and 3. A signal at 4.30 ppm
(1JSiH = 188 Hz) was assigned to a single, terminal SiH moiety.
Presumably, compound 5 is formed through a two-step
β-SiH abstraction and carbonyl insertion sequence. Abstraction
gives a zirconium silanimine Cp2Zr{κ2-N(SiHMe2)SiMe2} inter-
mediate. Subsequent insertion of acetone into the Zr–Si bond
provides 5. In support of this pathway, thermolysis of Cp2Zr-
{N(SiHMe2)2}Et in the absence of acetone provides the pro-
posed silanimine intermediate Cp2Zr{κ2-N(SiHMe2)SiMe2}
(6).10 Notably, the proposed silanimine species contains a non-
In this case, the non-classical β-SiH in 6 is not the site of
reactivity with acetone, which instead inserts into the Zr–Si
bond to form a Zr–O bond. Again, this contrasts the reactivity
of non-classical β-SiH-containing [2]+, which reacts with
acetone to give β-SiOCHMe2 groups.
Thus, the interaction of formaldehyde and Cp2Zr-
{N(SiHMe2)2}Et apparently does not involve insertion into a
Zr–C bond. This insertion pathway also seems unlikely for the
reaction of 1 and formaldehyde. For this reason, we considered
an alternative mechanism based on the reaction of compound
1 and ethylene. That reaction was proposed to involve vinylic
C–H bond activation by the Zr–C moiety to give a [Zr]CHvCH2
intermediate.10 Our next hypothesis, then, was that C–H bond
activation of formaldehyde by 1 could give 3 via a Zr formyl
intermediate followed by an isomerization (Pathway A,
Scheme 3). Zirconium formyl compounds are exceedingly
rare,21 and have been postulated as intermediates in zirconium
1
classical Zr↼H–Si feature (δSiH = −2.23 ppm, JSiH = 89 Hz);
however, we have not yet obtained X-ray quality crystals of 6.
Further evidence that Cp2Zr{N(SiHMe2)2}Et reacts to give a
zirconium silanimine is provided by a trapping experiment with
PMe3. Previously, we reported the solution-phase characteriz-
ation of the trapped product, Cp2Zr{κ2-NSiMe2(SiHMe2)}PMe3
Fig. 2 Rendered thermal ellipsoid plot of Cp2Zr{κ2-O,N-OCMe2SiMe2N-
(SiHMe2)} (5).
Scheme 3 A deuterium labeling experiment rules out CH bond acti-
vation as a first step in the conversion of 1 to 3.
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models of Fisher–Tropsch chemistry.22–24 A rare isolated zirco-
nium formyl is stabilized by interaction with a phosphine,25
and such an intermediate might rearrange by an unanticipated
pathway to give 3.
ð3Þ
A deuterium labeling experiment tested this hypothesis:
1 and paraformaldehyde-d2 react under the conditions of eqn
(1) to give Cp2Zr{κ2-N(SiMe2OCHD2)SiHMeCH2} (3-d2).
A singlet resonance at −14.4 ppm in the 11B NMR spectrum
(acquired in bromobenzene-d5) provided support for B–C bond
formation, and a small signal at −25 ppm, assigned to
2
1H NMR and H NMR spectroscopy of this product indicated
−
that deuterium was incorporated solely into the methoxy
group, whereas a C–H bond activation would partly label the
SiMe groups. Apparently, the reaction of 1 with paraformalde-
hyde occurs directly at the β-SiH without cleavage of the Zr–C
bond.
HB(C6F5)3 was also detected. The product precipitated as an
oil from benzene, the oil was isolated and washed with more
benzene to produce a solid, and methylene chloride extraction
removes the trace borate signal to produce an off-white pure
sample. As in compound 1, the stereogenic silicon center
resulted in diastereotopic C5H5, CH2, and SiHMe2 methyl
Therefore, reactions of 1 and coordinating molecules, such
as OPEt3, PMe3, pyridine, and DMAP, were tested, but the
1H NMR spectrum of 1 was unchanged in the presence of
these molecules. As noted above, these molecules apparently
do not inhibit the reaction of 1 and formaldehyde. These
experiments do not provide evidence for coordination of for-
maldehyde to the zirconium center in 1, but that possibility is
not unambiguously ruled out either. Therefore, other small-
molecule reactivity was investigated to probe sites for co-
ordination and reactivity. Reaction of 1 and CO at 85 °C for 3 h
gives the five-membered cyclic insertion product containing a
Zr–O bond and an exocyclic methylene group (7, eqn (2)).
Carbon monoxide and azametallasilacyclobutanes (for
group 4, Th and U centers) were previously suggested to
involve 1,1-insertion of CO to give an acyl that rearranges to
the product through 1,2-silyl migration of an oxycarbene
intermediate.26–30
1
groups, which each gave two signals in the H NMR spectrum.
In addition, two upfield 1H NMR resonances at 0.02 and
−0.54 ppm were assigned as silicon hydrides on the basis of
strong coupling to the two silicon centers (1JSiH = 81 and
83 Hz). The far upfield chemical shifts and low silicon-hydro-
gen coupling constants are consistent with a rare multicenter
non-classical interactions involving two side-on Zr↼H–Si
structures. Further support for this feature is provided by a low
energy SiH stretch (νSiH = 1737 cm−1) in IR spectrum. For com-
parison, the νSiH band in 1 appear at 2074 cm−1 10
.
Neutral diagostic d0 amido compounds were previously
reported for trivalent rare earth disilazido compounds, and
these contain upfield SiH chemical shifts, low 1JSiH values, and
low energy IR bands.11–14,35,36 In addition, the reaction of
B(C6F5)3 and Cp2Zr{N(SiHMe2)2}H provides the cationic Zr
amido complex [Cp2ZrN(SiHMe2)2]+, which contains two side-
on Zr↼H–Si interactions identified through a single crystal
X-ray diffraction study and distinct NMR and IR spectro-
scopic features.7 In that structure, the closest cation–anion
contact is a long distance (3.27 Å) between one of the Si
centers in [Cp2ZrN(SiHMe2)2]+ and a meta-F in the [HB(C6F5)3]
anion. Thus, compound 8 represents the first zwitterionic
complex of this type. Unfortunately, the material obtained
proved recalcitrant toward forming X-ray quality crystals.
Further support for the proposed connectivity and zwitterio-
nic nature of compound 8 was obtained through derivatization
reactions. Following the reactivity of [Cp2ZrN(SiHMe2)2]+
([2]+),7 zwitterionic 8 reacts with two equiv. of acetone in
methylene chloride to give the product Cp2ZrN-
{SiMe2(OCHMe2)}{SiMe(OCHMe2)CH2B(C6F5)3} (9; eqn (4)) in
quantitative yield.
ð2Þ
Resonances in the 1H NMR spectrum of 7 at 4.91 and
4.48 ppm were assigned to the exocyclic methylene. These
signals showed correlations in 1H–13C HMBC experiments to
13C NMR signals at 173.4 and 98.4 ppm, which were assigned
to the terminal and disubstituted unsaturated carbon centers.
Because hydrosilylation of CO would have been unexpected,
we next attempted reactions of 1 and tris(perfluorophenyl)-
borane. This strong Lewis acid is known to activate organo-
silanes to give [R3Si–H–B(C6F5)3]-type species,31–33 while orga-
nometallic alkyl and hydride compounds LnMR react to form
[LnM-R-B(C6F5)3].34 Recently, we have found that β-SiH-contain-
ing organometallic compounds such as M[C(SiHMe2)3]2(THF)2
(M = Ca, Yb) react with B(C6F5)3 by β-hydrogen abstraction.5
Interestingly, the reaction of B(C6F5)3 and Cp2Zr{N(SiHMe2)2}R
(R = Me, Et, CHvCHSiMe3) involves competitive β-H abstrac-
tion and alkyl group abstraction.7 Thus, the interaction of 1
and B(C6F5)3 should identify the most nucleophilic site in 1.
Alkyl group abstraction is observed to give the zwitterionic
ð4Þ
The 1H NMR spectrum of 9 contained four doublets at 1.53,
Cp2Zr{N(SiHMe2)SiHMeCH2B(C6F5)3} (8; eqn (3)) as the major 1.52, 1.49, and 1.29 ppm assigned to diastereotopic inequiv-
product.
alent isopropyl groups. Three singlets at 0.31, 0.13, and
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zwitterionic 8 and its reactivity with acetone. Addition of DMAP
to 8 results in Si–N bond formation and β-H migration to Zr to
give the zwitterionic zirconium hydride Cp2Zr{N(SiMe2DMAP)-
SiHMeCH2B(C6F5)3}H (10a, 62%) and Cp2Zr{N(SiHMe2)SiMe-
(DMAP)CH2B(C6F5)3}H (10b, 38%) as a mixture of isomers
(eqn (5)). As noted above, a similar reaction of [2]+ and DMAP
was previously described.7
Fig. 4 ORTEP diagram of Cp2ZrN{SiMe2(OCHMe2)}SiMe(OCHMe2)-
CH2B(C6F5)3 (9). Ellipsoids are drawn at 35% probability. All hydrogen
atoms are not included for clarity. Significant interatomic distances (Å):
Zr1–N1, 2.154(2); Zr1–O1, 2.319(2); Zr1–O2, 2.321(2); N1–Si1, 1.678(2);
N1–Si2, 1.688(2); Si1–O2, 1.712(2); Si2–O1, 1.729(2); B1–C5, 1.657(4).
Selected interatomic angles (°): Zr1–N1–Si1, 105.1(1); Zr1–N1–Si2,
105.0(1); Si1–N1–Si2, 146.2(1); O1–Zr1–O2, 131.13(6); Zr1–N1–Si1–O2,
1.3(1); Zr1–N1–Si2–O1, −9.8(1); O2–Si1–Si2–O1, −9.0(1).
ð5Þ
The formation of two isomers is due to the addition of
DMAP to one of the two β-Si groups (SiHMe vs. SiHMe2).
DMAP coordination to the sterically less hindered Si position
is favored (10a, 62%). The 1H NMR spectrum acquired at room
temperature in methylene chloride-d2 was slightly broad, but
the complete disappearance of 8 and the formation of two new
−0.21 ppm indicated that all three SiMe groups were inequiv- species can be identified.
alent, as were the C5H5 ligands. X-ray quality crystals of 9 were
The low temperature NMR signals corresponding to 10 were
grown from methylene chloride solution at room temperature. sharper and better resolved. At 220 K, four 29Si resonances
A single crystal X-ray diffraction study (Fig. 4) confirms the were observed at 11.4, −4.6, −61.1 and −63.9 ppm. The signals
Si–CH2–B(C6F5)3 connectivity of 9 and supports the related at −61.1 ppm (1JSiH = 115 Hz) and −63.9 ppm (1JSiH = 112 Hz)
assignment in 8. The B1–C5 distance is 1.657(4) Å, and this were the only ones observed in a 29Si INEPT experiment opti-
distance is similar to the B–C distance in methyl-abstracted mized for JSiH = 120 Hz (∼1-bond coupling), and the signals at
tris(pentafluorophenyl)borane
compounds
such
as 11.4 ppm and −4.6 ppm were only detected in a 29Si INEPT
(C5Me2H3)2ZrMe(μ-Me)B(C6F5)3 (1.663(5) Å).34
experiment optimized for JSiH = 7 Hz (long range proton coup-
In addition, the X-ray diffraction study confirms the hydro- ling). In a H–29Si HMBC experiment optimized for long range
1
silylation of acetone by the β-SiH. The resulting isopropoxy 1H–29Si coupling (JSiH
= 7
Hz), the 29Si resonance at
groups bridge between the silicon and zirconium centers. The −63.9 ppm correlated with the 1H NMR signal at 0.25 ppm
Zr1–O1 and Zr1–O2 distances of 2.319(2) and 2.321(2) Å are assigned to a SiHMe; the 29Si resonance at −61.1 ppm corre-
1
similar to the distances in cationic [Cp2ZrN(SiMe2OCHMe2)2]- lated with the H NMR signals at 0.39 ppm and 0.27 ppm are
[HB(C6F5)3]. The oxygen in that cationic compound and in assigned to a SiHMe2. In that experiment, crosspeaks from the
zwitterionic 9 are best described as silyl ethers that coordinate 29Si signal of the major product at −4.6 ppm correlated to
to the zirconium center.7 That is, the positive charge in the SiMe2 singlet resonances (0.53 ppm and 0.48 ppm), the signal
zwitterionic is located on zirconium rather than on the silicon at 3.93 ppm (1 H) assigned to the ZrH, and the aromatic reson-
1
centers. The methoxysilyl group on the silicon with the CH2B- ances assigned to DMAP (7.87 ppm) in the H NMR spectrum.
(C6F5)3 group is slightly twisted. Thus, the torsion angle Zr1– Similarly, crosspeaks from the 29Si signal of the minor product
N1–Si2–O2 of −9.8(1)° is not planar, whereas the torsion angle at 11.4 ppm were correlated to a second SiMe resonance
associated with the SiMe2OMe, Zr1–N1–Si1–O1 (1.3(1)°) is (0.39 ppm), the signal at 3.99 ppm (1 H) assigned to the ZrH,
nearly planar.
and the aromatic resonances assigned to DMAP (7.48 ppm).
The reaction of formaldehyde and 8 provides a mixture of These long-range correlations between the silicon atom and
compounds. The 1H NMR spectrum of the crude reaction the aromatic signals provide convincing evidence that DMAP
mixture contains signals that may be assigned to SiOMe is coordinated to silicon rather than zirconium.
groups, but the product was not isolated.
The 1H NMR signal at 3.93 ppm and 3.99 ppm were
We were further interested in the chemistry of 8 with 2-elec- assigned as zirconium hydrides rather than silicon hydrides
tron donors given the similar reactivity of 1 and [2]+ with car- on the basis of their lack of 1-bond coupling to the silicon
bonyls, but the contrasting reactivity of the two compounds centers. In addition, a COSY experiment showed correlations
with 2-electron donors, as well as the unusual structure of between those signals and the non-classical SiH resonances at
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0.28 and 1.29 ppm assigned to a Zr↼H–Si. A broad band at ligands appears to rule out a pathway involving precoordina-
1845 cm−1 was observed in the crude mixture of 10a and 10b tion to the silicon center.
that suggested the non-classical nature of β-SiH groups in 10.
The reaction of 1 with B(C6F5)3 or CO appears to involve the
The interaction of 8 and OPEt3 affords Cp2Zr{N(SiHMe2)- zirconium carbon bond. Coordination of CO prior to 1,1-inser-
SiHMeCH2B(C6F5)3}OPEt3 (11, eqn (6)). Although the coordi- tion would occur on the distal side with respect to the SiHMe2
nation of OPEt3 to Zr distinguishes this reaction from that of 8 group, and this reaction does not appear to provide evidence
and DMAP, this result follows the reactions of [2]+ with OPEt3 for coordination of formaldehyde to zirconium prior to
and DMAP.
hydrosilylation.
Notably, the metallacyclic structure of 1 appears to dis-
tinguish its reactivity from that of acyclic β-SiH containing sila-
zidozirconium compounds. It may be that the strain
associated with the 4-membered ring results in enhanced reac-
tivity of the exocyclic β-hydrogen. Alternatively, the rigidity of
the zirconacycle may limit even transient non-classical inter-
actions that otherwise stabilize the reactive β-hydrogen.
ð6Þ
The structure of 11 is dissimilar to that of the DMAP
adducts 10a/b, as demonstrated by NMR and IR spectrosco-
pies. In the H NMR spectrum of 11, two doublet resonances
1
at 0.27 ppm (3JHH = 3.0 Hz) and −0.45 ppm (3JHH = 2.3 Hz)
were assigned to SiHMe2 and SiHMe groups based on inte-
gration. Furthermore, two signals at 4.09 ppm (1JSiH = 169 Hz)
and 3.82 ppm (1JSiH = 177 Hz) were assigned as SiHMe2 and
SiHMe groups, respectively, based on their correlation to the
Acknowledgements
The authors gratefully thank the National Science Foundation
(CHE-0955635) for financial support.
1
SiMe2 and SiMe signals in a COSY experiment. The JSiH value
Notes and references
suggests that 11 contains terminal silicon hydride groups. The
observation of NMR signals assigned to SiHMe2 and SiHMe
groups provide evidence that OPEt3 is coordinated to Zr center
instead of either β-Si positions. In addition, the νSiH at
2099 cm−1 in the infrared spectrum is consistent with classi-
cally bonded silicon hydrides.
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Conclusion
The two β-SiH groups in azasilazirconacycle 1 show NMR
and IR spectroscopic features of classical 2-center-2-electron
bonds. Neither spectral signatures of non-classical interactions
nor intramolecular hydrogen elimination and abstraction
pathways are observed. Nonetheless, the reactions of 1 with
ethylene or formaldehyde diverge from the chemistry of β-H
free analogs. The reaction of 1 and formaldehyde at the exo-
cyclic SiH gives a methoxysilyl moiety. A related reaction
occurs with the highly non-classical β-SiH in cationic [2]+ and
zwitterionic 8. In the latter reactions, nucleophilic attack of a
carbonyl on silicon or zirconium centers is readily rationalized
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