Mechanism-based design of labile precursors for chromium(i) chemistry

Article abstract of DOI:10.1039/c5cc05993c

Dinitrogen complexes of the type Tp^R,RCr-N₂-CrTp^R,R are not the most labile precursors for Cr(i) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex-Tp^tBu,MeCr(η²-C₂(SiMe₃)₂)-proved to be much more reactive.

Full text of DOI:10.1039/c5cc05993c

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Mechanism-based design of labile precursors for
chromium(I) chemistry†
Cite this: Chem. Commun., 2015,
51, 15402
Eser S. Akturk, Glenn P. A. Yap and Klaus H. Theopold*
Received 18th July 2015,
Accepted 27th August 2015
DOI: 10.1039/c5cc05993c
www.rsc.org/chemcomm
Dinitrogen complexes of the type TpR,RCr–N2–CrTpR,R are not the
most labile precursors for Cr(I) chemistry, as they are sterically
protected from obligatory associative ligand substitution. A mono-
nuclear alkyne complex – TptBu,MeCr(g2-C2(SiMe3)2) – proved to be
much more reactive.
Half a century after the discovery of the first dinitrogen complex,
by Allen and Senoff,1 coordination compounds of the rather
inert N2 molecule are still much sought after, due in large part to
their substitutional lability and concomitant role as precursors
for a wide variety of transition metal complexes.2 For example,
our interest in the activation of O2 and other small molecules
has benefited greatly from the availability of TptBu,MeCo(N2) and
[(i-Pr2Ph)2nacnacCr]2(m-Z2:Z2-N2), respectively.3,4 While these
two molecules differ in the mode of coordination of the desig-
Fig. 1 The molecular structure of [TptBu,MeCr]2(m-Z1:Z1-N2) (1, 30% probability
nated leaving group, both undergo facile ligand substitution to level). Selected interatomic distances (Å) and angles (1): N7–N7A, 1.213(5);
yield a plethora of compounds incorporating the TptBu,MeCo and
(i-Pr2Ph)2nacnacCr fragments.5,6 We were interested in the inter-
section of these two chemistries, and accordingly we now report
Cr–N7, 1.838(3), Cr–N1, 2.205(3); Cr–N3, 2.200(3); Cr–N5, 2. 190(3); NTp
–
Cr–NTp,avg, 87.3; NTp–Cr–N7avg, 127.2.
the preparation of dinitrogen complexes of various TpCr frag- distance (2.198 Å). Both measures are consistent with strong
ments, which exhibited some notable differences in reactivity. p-backbonding from the low-valent chromium to the dinitrogen
KC8 reduction under nitrogen of blue TptBu,MeCr(THF)Cl in ligand. In accord with the crystallographically imposed inversion
Et2O/THF (4 : 1) at room temperature yielded green needles of symmetry of 1, its IR spectrum (KBr) did not show a discernable
[TptBu,MeCr]2(m-Z1:Z1-N2) (1) in 42% yield (see ESI† for experi- N–N stretching vibration. 1 is a paramagnetic substance with
mental detail and characterization of all compounds). The isotropically shifted and broadened 1H NMR resonances. At room
molecular structure of 1, as determined by X-ray diffraction, temperature, it has an effective magnetic moment of meff = 3.9(1)
is shown in Fig. 1. The dinuclear complex contains a single N2 mB, a possible interpretation of which is that the bridging N2 ligand
ligand bridging the two staggered TpCrI fragments, featuring mediates antiferromagnetic coupling between the two CrI (high-
end-on coordination of the dinitrogen to chromium. The N–N spin d5, S = 5/2) ions.
bond distance of 1.211(4) Å is substantially elongated over that
With 1 in hand, we embarked on an exploration of its reactivity
of the free ligand (1.098 Å),7 and the Cr–N7 bond – at 1. 838(3) Å – with a variety of small molecules. As expected, the low-valent
is very short, certainly by comparison to the average Cr–NTp dinitrogen complex reacted rapidly with molecules that yielded
products in which the chromium was oxidized. Examples include
O2, S8, N2O, and RN3. While the chalcogenide chemistry will be
Department of Chemistry and Biochemistry, University of Delaware, Newark,
detailed elsewhere, we offer the product of the reaction of 1 with
DE 19716, USA. E-mail: theopold@udel.educ
adamantyl azide, i.e. purple TptBu,MeCrQNAd (2) as a representative
† Electronic supplementary information (ESI) available: Experimental details and
example. 2 is the sole terminal imido complex of trivalent chromium.8
molecular structures of 3–5 and 7–10. CCDC 1058333–1058342. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc05993c Its molecular structure is depicted in Fig. 2. The pseudo-tetrahedral
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Table0
1
Selected structural parameters of dinitrogen complexes
[TpR,R Cr]2(m-N2)
Compound
1 (TptBu,Me
)
3 (TptBu,iPr
)
4 (TpiPr,iPr
)
N–N [Å]
Cr–N [Å]
Cr–NTp [Å]
1.213(5)
1.838(3)
2. 198
1.209(3)
1.8395(16)
2.191
1.214(4)
1.773(2)
2.094
and (ii) the steric blocking of associative ligand substitution
pathways. A dissociation of 1 in the absence of N2 must yield
either one or two equivalents of TptBu,MeCr or a solvate thereof
(TptBu,MeCr(S), S = Et2O, THF). Alternatively, in the presence of
gaseous N2, an associative reaction with the latter may produce
two equivalents of mononuclear intermediate TptBu,MeCr(N2).
Either way, the reversible dissociation into mononuclear frag-
ments should lead to scrambling of mixtures of suitably labeled
dinuclear N2 complexes. In order to test this prediction we have
prepared [TptBu,iPrCr]2(m-N2) (3), a close analog of 1. 3 has been fully
characterized, and selected structural parameters are listed in
Table 1. In a control experiment, the reduction of an equimolar
mixture of TptBu,MeCr(THF)Cl and TptBu,iPrCr(tBu,iPrpzH)Cl yielded a
1:2:1 mixture of 1, [TptBu,MeCr](m-N2)[CrTptBu,iPr], and 3; the
proportions of the products were measured by LIFDI-MS,12 which
Fig. 2 The molecular structure of TptBu,MeCrQNAd (2, 30% probability
level). Selected interatomic distances (Å) and angles (1): Cr–N7, 1.687(2);
N7–C25, 1.455(3); Cr–N1, 2.132(2); Cr–N3, 2.151(2); Cr–N5, 2.160(2); Cr1–
N7–C25, 178.8(2)1; NTp–Cr–NTp,avg, 88.0; NTp–Cr–N7avg, 126.7.
complex features a linear imido ligand with a Cr–N distance of exhibited strong molecular ion (M+) peaks for these compounds.
1.687(2) Å; the latter is close the computationally predicted The ratio of the products did not change upon heating the mixture
1.708 Å for TptBu,MeCrQNtBu.9 Consistent with the intermediate to reflux in THF. However, when a mixture of 1 and 3 in THF under
formal oxidation state of chromium it is also on the very long vacuum was heated to 70 1C for two days, subsequent analysis of
side of such distances.10 The effective magnetic moment of 2 the mixture by LIFDI-MS showed no evidence for the formation
measured meff = 3.7(1) mB, which is consistent with a quartet spin of the mixed ligand complex ([TptBu,MeCr](m-N2)[CrTptBu,iPr]).
ground state (d3, S = 3/2).
Similarly, when the same experiment was repeated under a N2
To our surprise, reactions of 1 with good p-acceptors did either atmosphere, no signal for the mixed compound was detected in
not proceed at all, or yielded decomposition products only after the mass spectrum. These results prove that 1 (and 3) do not
prolonged exposure. Thus, 1 did not react with alkenes (e.g., detectably dissociate in THF solution, even when heated for
ethylene) or alkynes (e.g. 2-butyne), and lengthy exposure to an prolonged periods. A dissociative mechanism (Id or D) for the
excess of CO (1 atm, 18 h) yielded only the ligand fragmentation ligand substitution of 1 is thereby ruled out.13
product (tBu,MepzH)2Cr(CO)4, possibly due to traces of adventitious
An alternative associative mechanism should be facilitated
impurities (H2O?). We have reason to believe (vide infra) that all of by lesser steric hindrance of the Tp ligands. To explore this
these attempted reactions are thermodynamically favorable and possibility, we have prepared [TpiPr,iPrCr]2(m-N2) (4). It is inter-
would yield stable p-complexes. However, they apparently face esting to note that the N–N bond distance of 4 (see Table 1)
insurmountable kinetic barriers, distinguishing 1 as a peculiarly does not significantly differ from those of 1 or 3; the extent of
substitution inert dinitrogen complex. To rationalize this disparity in p-backbonding is apparently similar in all three compounds.
reactivities, which has some precedent in titanium chemistry,11 we However, the Cr–N distances in 4 are appreciably shorter (by
hypothesized that the reactions with oxidants may proceed via initial 0.066(2) Å), suggesting that lesser steric interactions between
outer sphere electron transfer, thereby activating the Cr–N2 bond the opposing ligands allow for a closer approach of the two
with respect to dissociation. Non-oxidizing ligands, on the other TpCr fragments. Space filling models of 1 and 4 (see Fig. S3,
hand, may be forced to undergo an associative ligand substitution, ESI†) also suggest greater accessibility of the chromium centers in
because the Cr–N2 bond of 1 is too strong to permit a dissociative 4. In stark contrast to 1, exposure of 4 to 1 atm of CO(g) resulted in
reaction path. The 13-electron configuration of the individual Cr an immediate color change from violet to yellow and precipitation
atoms may make a ligand dissociation – yielding a bare, trigonal of octahedral TpiPr,iPrCr(CO)3 (5, see Fig. S4, ESI†). It appears that
pyramidal 11-electron TptBu,MeCr fragment – energetically unfeasible. the diminished steric protection of Cr by the TpiPr,iPr-ligand causes
In this scenario, the effective steric shielding of the metal atoms by a dramatic increase in the rate of ligand substitution; this
interleaving tert-butyl substituents of the opposing TptBu,Me ligands observation argues strongly in favor of an associative substitution
may prove impossible to penetrate, rendering the Cr–N2–Cr core of 1 mechanism (Ia or A).
impervious to ligand attack.
The results described above suggest that the preparation
We then resolved to test the two essential pillars of this of coordination compounds of the TptBu,MeCrI fragment will
mechanistic hypothesis, namely (i) the lack of dissociation of 1, require a precursor that is subject to facile associative ligand
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Fig. 3 The molecular structure of TptBu,MeCr(Z2-C2(SiMe3)2) (6, 30% probability
level). Selected interatomic distances (Å) and angles (1): Cr–C25, 2.0480(19);
Cr–C26, 2.0835(18); C25–C26, 1.288(3); Cr–N1, 2.1015(15); Cr–N3, 2.1614(16);
Cr–N5, 2. 1504(16); NTp–Cr–NTp,avg, 87.7; N1–Cr–C25/C26centroid, 172.5;
a (angle of deviation of alkyne centroid from B–Cr axis) = 49.31.
Scheme 1 Ligand substitution reactions of 6.
metals,16 finds precedent in the analogous [(i-Pr2Ph)2nacnacCr]2-
(m-Z2:Z2-C2H4).4 Like the latter, it did not react further with
ethylene, exhibiting no activity for catalytic oligomerization or
polymerization of ethylene.6a The irreversible reactions of 6 with
less hindered alkynes were expected, being of interest mostly for
the formation of pseudotetrahedral alkyne complexes 9 and 10,
substitution; in all likelihood this will require a mononuclear
structure to disrupt the molecular sheath protecting the Cr–N2–Cr
core of 1. Based on related nacnacCr chemistry, and inspired by
Rosenthal et al.,14 we selected TptBu,MeCr(Z2-C2(SiMe3)2) (6) as a
likely candidate.15 KC8 reduction of TptBu,MeCr(THF)Cl in Et2O/
THF under vacuum in the presence of bis(trimethylsilyl)acetylene
yielded brown crystals of 6 in 75% yield. The molecular structure
of 6 (depicted in Fig. 3) features a severely distorted coordination
environment, in which the centroid of the alkyne’s triple bond is
displaced from the B–Cr axis of the threefold symmetric TpCr
fragment by 491. This cis-divacant octahedral structure creates two
symmetry equivalent openings for attack by external ligands.
The relatively long Cr–Calkyne distances (2.048(2) and 2.084(2) Å)
and the comparatively modest structural reorganization of the
coordinated alkyne – by comparison with other complexes of the
type TptBu,MeCr(Z2-C2R2) (R = Me, Ph; see ESI†) – herald a rather
tenuous hold of Cr upon this sterically encumbered alkyne. In
accord with this notion, ‘spring-loaded’ 6 proved much more
reactive toward ligand substitution than 1!
1
as evidenced by H NMR. More surprising was the observation
that 6 reacted with N2 (1 atm), forming 1 and free alkyne
quantitatively! The spontaneous substitution of an alkyne ligand
by N2 is rather unusual. It is a measure of the instability and
lability of 6 and – if additional proof was needed – suggests that
it is an excellent precursor for TptBu,MeCrI chemistry.
We are now exploring the small molecule activation chemistry
of TpCr(I) fragments, judiciously using the synthons described
above. The results of these studies will be reported in due course.
This research was supported by DOE (DE-FG02-92ER14273).
Shared instrumentation for NMR, LIFDI-MS, and X-ray diffraction
was supported by grants from NIGMS (1 P30 GM110758-01), NSF
(CHE-1229234), and NSF (CRIF 1048367), respectively.
The reactions of 6 with various p-acceptors are summarized
in Scheme 1; the molecular structures of the products – as
determined by X-ray diffraction – are included in the ESI.†
When carried out in ethereal solvents (THF, Et2O), these reac-
tions were facile and proceeded in good yield. The carbonyl-
ation of 6 is notable in that it stopped short of the formation
of TptBu,MeCr(CO)3 (i.e., the analog of 5). The actual product,
k2-TptBu,MeCr(CO)2(m-Z1:Z1-CO)(Et2O)CrTptBu,Me (7) is best
rationalized as the product of a disproportionation, resulting
in a mixed-valent (Cr0CrII) isocarbonyl complex. The divalent
chromium – formally a cation – has apparently lost its affinity for
additional p-acids. The dinuclear ethylene complex, [k2-TptBu,MeCr]2-
(m-Z2:Z2-C2H4) (8), while a rare case of ethylene p-bonded to two
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