Electronic effects in the oxidative addition of iodomethane with mixed-aryl β-diketiminate chromium complexes

Article abstract of DOI:10.1021/om100961f

The Cr^II complexes CpCr[DppNC(Me)CHC(Me)NC₆H ₄Y] (1; Dpp = 2,6-(Me₂CH)₂C₆H ₃) were used to prepare the corresponding Cr^III iodo (2) and methyl (3) mixed-aryl β-diketim

Full text of DOI:10.1021/om100961f

Organometallics 2011, 30, 603–610 603
DOI: 10.1021/om100961f
Electronic Effects in the Oxidative Addition of Iodomethane with
Mixed-Aryl β-Diketiminate Chromium Complexes
Wen Zhou,† Liming Tang,† Brian O. Patrick,‡ and Kevin M. Smith*,†
†Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna,
British Columbia, Canada V1V 1V7, and ‡Department of Chemistry, University of British Columbia,
Vancouver, British Columbia, Canada V6T 1Z1
Received October 4, 2010
The CrII complexes CpCr[DppNC(Me)CHC(Me)NC6H4Y] (1; Dpp = 2,6-(Me2CH)2C6H3) were
used to prepare the corresponding CrIII iodo (2) and methyl (3) mixed-aryl β-diketiminate com-
pounds with Y = OMe (a), CH3 (b), H (c), CF3 (d). Oxidation of the CrII precursors with iodine gave
CpCr[DppNC(Me)CHC(Me)NC6H4Y](I) (2a-d). The CrIII methyl compounds 3a-d were prepared
by reaction of the CrII complexes with iodomethane followed by MeMgI. The CrIII chloride
complexes 4a-d were synthesized by salt metathesis of isolated CpCrCl2(THF) with the appropriate
deprotonated mixed-aryl β-diketiminate. The structures of the paramagnetic complexes 1b,c, 2a,b,
3a-d, and 4a-d were determined by single-crystal X-ray diffraction. The rate constants for the
single-electron oxidative addition of iodomethane with 1a-d were determined by monitoring the
reaction by UV-vis spectroscopy. The rate constants correlated well with the electronic parameters
of the Y substituents (F = -0.36). The rate constants for 1a-d were an order of magnitude greater
than those previously obtained for the same reaction with CpCr[(DppNCMe)2CH] and related
ortho-disubstituted symmetric β-diketiminate complexes.
Introduction
radical polymerization.2 With well-defined complexes in the
absence of halide sources as the starting point, the reversible
Cr-R homolysis forms the basis of organometallic mediated
radical polymerization (OMRP).2,6 Alternatively, the con-
centration of R• can also be minimized via an atom-transfer
radical polymerization (ATRP) mechanism if halides are
available.2,6b,7 For the current system, we have demonstrated
that a well-defined CrIII neopentyl complex can serve to both
initiate and control polymerization of vinyl acetate by OMRP.8
However, the barrier between C and B was determined to be
prohibitively high in this system: both experimental and
computational studies indicated that the ATRP mechanism
was not competitive with OMRP.9
The ease with which the steric and electronic properties of
the β-diketiminate ligand can be varied has contributed to its
widespread use over the past decade.10 Certain aspects of the
reaction coordinate shown in Figure 1 appear to be amenable
to control through ancillary ligand steric effects. Increasing
the size of the NAr substituents raises the relative energy of
the CpCr[(ArNCMe)2CH](R) complex through increased
The reaction of CrII with organic halides is a critical
feature of chromium-catalyzed carbon-carbon bond-form-
ing reactions for organic synthesis.1 Single-electron oxida-
tive addition is also intimately connected mechanistically to
transition-metal-mediated radical polymerization.2 We have
been interested in the single-electron oxidative addition
reaction of organic halides (R-X) with well-defined CpCr-
[(ArNCMe)2CH] complexes, as shown in Figure 1.3 The
initial slow step is the halogen atom abstraction of R-X
with CrII to generate the CrIII-X compound and an alkyl
radical.4 The energy difference between A and C is deter-
mined by the strength of the R-X bond that is broken
compared to the weaker CrIII-X bond that is formed. The
subsequent fast step relies on the remarkable ability of CrII to
efficiently trap carbon-based radicals.5 The strength of the
Cr-R bond formed dictates the energy difference between C
and E.
The reaction coordinate shown in Figure 1 also illustrates
the energetics for the use of these complexes for controlled
(6) (a) Smith, K. M.; McNeil, W. S.; Abd-El-Aziz, A. S. Macromol.
Chem. Phys. 2010, 211, 10–16. (b) di Lena, F.; Matyjasezwski, K. Prog.
Polym. Sci. 2010, 35, 959–1021.
(7) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009, 109,
4963–5050.
*To whom correspondence should be addressed. E-mail: kevin.m.
smith@ubc.ca.
€
(1) (a) Furstner, A. Chem. Rev. 1999, 99, 991–1045. (b) Wessjohann,
L. A.; Scheid, G. Synthesis 1999, 1–36.
(2) Poli, R. Angew. Chem., Int. Ed. 2006, 45, 5058–5070.
(3) (a) Doherty, J. C.; Ballem, K. H. D.; Patrick, B. O.; Smith, K. M.
Organometallics 2004, 23, 1487–1489. (b) Smith, K. M. Organometallics
2005, 24, 778–784.
(4) (a) Kochi, J. K.; Powers, J. W. J. Am. Chem. Soc. 1970, 92, 137–
146. (b) Smith, K. M. Coord. Chem. Rev. 2006, 250, 1023–1031.
(5) Espenson, J. H. Prog. Inorg. Chem. 1983, 30, 189–212.
(8) Champouret, Y.; MacLeod, K. C.; Baisch, U.; Patrick, B. O.;
Smith, K. M.; Poli, R. P. Organometallics 2010, 29, 167–176.
(9) Champouret, Y.; MacLeod, K. C.; Smith, K. M.; Patrick, B. O.;
Poli, R. P. Organometallics 2010, 29, 3125–3132.
(10) (a) Bourget-Merle, L.; Lappert, M. F.; Severn, J. S. Chem. Rev.
2002, 102, 3031–3065. (b) Mindiola, D. J. Angew. Chem., Int. Ed. 2009, 48,
6198–6200.
r
2011 American Chemical Society
Published on Web 01/05/2011
pubs.acs.org/Organometallics

604 Organometallics, Vol. 30, No. 3, 2011
Zhou et al.
Figure 2. Thermal ellipsoid diagram (50%) of CrII complex 1b.
(diethylphenyl, 2,6-(CH3CH2)2C6H3), Xyl (xylyl, 2,6-Me2-
C6H3), Mes (mesityl, 2,4,6-Me3C6H2).14
We now report the use of mixed-aryl β-diketiminate
ligands15 to explore electronic effects in the single-electron
transfer reactivity of CrII complexes.3b In these ligands, one
bulky Dpp subsituent is retained to afford steric protection
tothe paramagnetic CrII and CrIII complexes in Scheme 1. The
absence of ortho substituents is intended to allow the second
aryl moiety to more readily align with the Cr[(NCMe)2CH]
plane, enhancing electronic effects transmitted to the chro-
mium center from the para group (Y = OMe (a), CH3 (b), H
(c), CF3 (d)). We previously reported the synthesis of the
three CrII complexes CpCr[DppNC(Me)CHC(Me)NC6H4Y]
(1a,c,d), the X-ray crystal structures of two of these com-
pounds (1a,d), and their use in the controlled radical polym-
erization of vinyl acetate initiated by V-70.16 In this contri-
bution, we report the independent synthesis of the corresponding
CrIII iodo, methyl, and chloro complexs (2a-d, 3a-d, and
4a-d in Scheme 1, respectively). Kinetic measurements of
the oxidative addition of iodomethane with 1a-d demonstrate
a clear electronic influence of the para group. The significant
increase in the rate constants measured for all four mixed-
aryl CrII complexes in comparison to the previously reported
symmetric β-diketiminate analogues appears to be an unanti-
cipated steric consequence of removing the ortho substituents
to enhance electronic effects.
Figure 1. Schematic energy diagram for single-electron oxida-
tive addition of R-X by CpCr[(ArNCMe)2CH].
Scheme 1. Mixed-Aryl β-Diketiminate CrII and CrIII Complexes
(Y = OMe (a), CH3 (b), H (c), CF3 (d))
steric hindrance. The resulting decrease in energy between E
and D is evident in an increased rate measured for the CrIII-R
homolysis.11 The same decrease in CrIII-R homolytic bond
dissociation energy can be achieved by increasing the steric
requirements of the R group,11 as previously observed for
aqueous CrIII alkyl complexes.5
Results and Discussion
Synthesis of Chromium(II) Complexes. In the initial 2004
communication,3a the symmetric CpCr[(DppNCMe)2CH]
was prepared by treatment of Gibson’s {Cr[(DppNCMe)2-
CH]}2(μ-Cl)2 dimer17 with NaCp. Subsequent syntheses of
CpCr[ArNCMe)2CH] complexes relied on the sequential
addition of NaCp and Li[(ArNCMe)2CH] to CrCl2 in THF.8
The latter route was employed for the previously reported
CrII mixed-aryl complexes 1a,c,d,16 as well as for the new
p-methyl-substituted variant 1b (eq 1). The X-ray crystal
Currently, Cr-mediated coupling reactions of R-X and
aldehydes use nickel cocatalysts to activate aryl or alkenyl
halides,1,12 and alkyl halides require cobalt or iron cocata-
lysts.1,13 Lowering the barrier from A to B through ancillary
ligand modification would extend the range of R-X sub-
strates that can react directly with Cr, simplifying catalyst
design. Unlike the Cr-R BDE, however, the rate-determin-
ing step for R-X activation did not display any significant
sensitivity to steric modification of the β-diketiminate ligand.
We previously reported that the rate of oxidative addition of
iodomethane with CpCr[(ArNCMe)2CH] is relatively constant
for Ar = Dpp (diisopropylphenyl, 2,6-(Me2CH)2C6H3), Dep
(14) MacLeod, K. C.; Conway, J. L.; Tang, L.; Smith, J. J.; Corcoran,
L. D.; Ballem, K. H. D.; Patrick, B. O.; Smith, K. M. Organometallics
2009, 28, 6798–6806.
(15) (a) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.;
Williams, D. J. Dalton Trans. 2004, 570–578. (b) Gong, S.; Ma, H. Dalton
Trans. 2008, 3345–3357. (c) Gong, S.; Ma, H.; Huang, J. J. Organomet.
Chem. 2008, 693, 3509–3518.
(16) Champouret, Y.; Baisch, U.; Poli, R.; Tang, L.; Conway, J. L.;
Smith, K. M. Angew. Chem., Int. Ed. 2008, 47, 6069–6072.
(17) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White,
A. J. P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895–1903.
(11) MacLeod, K. C.; Conway, J. L.; Patrick, B. O.; Smith, K. M.
J. Am. Chem. Soc. 2010, 132, 17325–17334.
(12) Guo, H.; Dong, C.-G.; Kim, D.-S.; Urabe, D.; Wang, J.; Kim,
J. T.; Liu, X.; Sasaki, T.; Kishi, Y. J. Am. Chem. Soc. 2009, 131, 15387–
15393.
(13) Wessjohann, L. A.; Schmidt, G.; Schrekker, H. S. Tetrahedron
2008, 64, 2134–2142.

Article
Organometallics, Vol. 30, No. 3, 2011 605
structure of 1b is shown in Figure 2, and the newly determined
structure of 1c is included in the Supporting Information.
The success of the apparent cyclopentyldienide displace-
ment reaction from Cp2Cr had previously been attributed to
the ability of the bulky NAr substituents to preclude the
formation of CrII bis(β-diketiminate) complexes.14 Indeed,
by using an even more sterically demanding ligand, Mindiola
and co-workers have prepared three-coordinate Cr[(Dpp-
NCtBu)2CH](X) complexes.18 However, it was recently de-
monstratedthatCr[(RNCMe)2CH]2 complexes can be prepared
and structurally characterized not only for R = C6H5,
CH2C6H5 but also for the bulkier R = 2-(Me2CH)C6H4,
2-(Me3C)C6H4, C10H7 derivatives.19-21 Similarly, CpCr(C6H4-
CH2NMe2),22 CpCr[(Me3SiN)2CPh],23 and CpCr(Mes)(L)
(L = DBU, N-heterocyclic carbene)24 compounds have all been
prepared, despite the stability of the known Cr(C6H4CH2-
NMe2)2,25 Cr[(Me3SiN)2CPh]2,26 and Cr(Mes)2(L)2 complexes.27
The factors that favor the synthesis of CpCr(LX) compounds
instead of the corresponding Cr(LX)2 species remain to be
explored.28
Figure 3. Thermal ellipsoid diagram (50%) of CrIII iodo com-
plex 2a.
of 2a is shown in Figure 3; the structure of 2b is included in
the Supporting Information.
Synthesis of Chromium(III) Iodo Complexes. Single-elec-
tron oxidation of well-defined CrII precursors with iodine is a
common route to CrIII iodo complexes.24,29 Reaction of
1a-d with I2 did give the expected iodo complexes 2a-d
(eq 2), although the mixed-aryl β-diketiminate compounds
did not crystallize as readily as the more symmetric CpCr-
[(ArNCMe)2CH](I) analogues.14 Complex 2c (Y = H), in
particular, displayed a tendency to precipitate as a powder
rather than form crystals. The lower yields for compounds
2a-d are presumed to be due to the difficulties in recrystal-
lizing these chiral-at-metal complexes, rather than an inher-
ent problem with the synthetic route in eq 2 or a lack of
stability of the CrIII iodo species. The X-ray crystal structure
Synthesis of Chromium(III) Methyl Complexes. The opti-
mal choice of RMgX and CrIII precursor to prepare CpCr-
[(ArNCMe)2CH](R) complexes by salt metathesis depends
on the steric demands of the NAr and R groups.3a,14,8,11 Both
the CrIII iodide and CrIII chloride mixed-aryl β-diketiminate
complexes reacted readily with MeMgI. However, synthesis
of CrIII methyl compounds 3a-d directly from the corre-
sponding CrII precursors, as shown in eq 3, capitalized on the
ease of synthesis of 1a-d. Addition of iodomethane to CpCr-
[(DppNC(Me)CHC(Me)NC6H4Y] produced a mixture of
CrIII iodo and CrIII methyl species, which were converted
to 3a-d by subsequent addition of MeMgI. Complexes 3b,d
were also prepared by reaction of the appropriate CrII complex
with MeI and SmI2.30 As was observed for CpCr[(DppNCMe)2-
CH](CH3),3a the high solubility of the CrIII methyl complexes
limited the yield of products 3b-d as crystalline solids. The
comparatively high yield of 3a may be due to a slight reduc-
tion in lipophilicity imparted by the polar methoxy substituent.
(18) Fan, H.; Adhikari, D.; Saleh, A. A.; Clark, R. L.; Zuno-Cruz,
F. J.; Sanchez Cabrera, G.; Huffman, J. C.; Pink, M.; Mindiola, D. J.;
Baik, M.-H. J. Am. Chem. Soc. 2008, 130, 17351–17361.
(19) MacAdams, L. A.; Kim, W.-K.; Liable-Sands, L. M.; Guzei,
I. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 2002, 21, 952–
960.
(20) Latreche, S.; Schaper, F. Organometallics 2010, 29, 2180–2185.
(21) Monillas, W.; Bazzoli, T. C.; Yap, G. P. A.; Theopold, K. H.
J. Chem. Crystallogr. 2010, 40, 67–71.
(22) Jonas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295–311.
(23) Gallant, A. J.; Smith, K. M.; Patrick, B. O. Chem. Commun.
2002, 2914–2915.
(24) Zhou, W.; Therrien, J. A.; Wence, D. L. K.; Yallits, E. N.;
Conway, J. L.; Patrick, B. O.; Smith, K. M. Dalton Trans. 2011, 40, 337–
339.
(25) Edema, J. J. H.; Gambarotta, S.; Meetma, A.; Spek, A. L.
Organometallics 1992, 11, 2452–2457.
(26) (a) Buijink, J.-K.; Noltemeyer, M.; Edelmann, F. T. Z. Natur-
forsch., B 1991, 46, 1328–1332. (b) Hao, S.; Gambarotta, S.; Bensimon, C.;
Edema, J. J. H. Inorg. Chim. Acta 1993, 213, 65–74.
(27) (a) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organometallics
1988, 7, 2372–2379. (b) Edema, J. J. H.; Gambarotta, S.; van Bolhuis, F.;
Smeets, W. J. J.; Spek, A. L.; Chiang, M. Y. J. Organomet. Chem. 1990, 389,
47–59. (c) Danopoulos, A. A.; Hankin, D. M.; Wilkinson, G.; Cafferkey,
S. M.; Sweet, T. K. N.; Hursthouse, M. B. Polyhedron 1997, 16, 3879–3892.
(28) Fontaine, P. P.; Yonke, B. L.; Zavalij, P. Y.; Sita, L. R. J. Am.
Chem. Soc. 2010, 132, 12273–12285.
Synthesis of Chromium(III) Chloro Complexes. In light of
the problems encountered in recrystallization of the CrIII
iodo complexes 2a-d, routes were investigated to prepare
CrIII chloro compounds as potential precursors for the
(30) (a) Curran, D. P.; Totleben, M. J. J. Am. Chem. Soc. 1992, 114,
6050–6058. (b) Ogoshi, S.; Stryker, J. M. J. Am. Chem. Soc. 1998, 120,
3514–3515. (c) Nicolaou, K. C.; Ellery, S. P.; Chen, J. S. Angew. Chem., Int.
Ed. 2009, 48, 7140–7165.
(29) Monillas, W.; Yap, G. P. A.; Theopold, K. H. J. Chem. Crystal-
logr. 2009, 39, 842–845.

606 Organometallics, Vol. 30, No. 3, 2011
Zhou et al.
Figure 4. Thermal ellipsoid diagrams (50%) of CrIII methyl complexes 3a-d.
independent synthesis of 3a-d by salt metathesis. Although
the symmetric CpCr[(DppNCMe)2CH](Cl) could be prepared
by oxidation with PbCl2, it was not readily accessible by salt
metathesis reactions from CpCrCl2(THF).3a This observation
was confirmed by Jin and co-workers, although this route
served well for their preparation of CpCr[(PhNCMe)2CH](Cl)
and related β-keto iminate complexes.31 ForCpCr[(XylNCMe)2-
CH](Cl), sequential reaction of CrCl3 with deprotonated
β-diketiminate followed by NaCp had proved optimal.8
The preparation of CpCr[(DppNC(Me)CHC(Me)NC6-
H4Y](Cl) (4a-d) was achieved using isolated CpCrCl2(THF),
as shown in eq 4, rather than reacting the CrCl3 with the
mixed-aryl Li β-diketiminate. Schaper and co-workers demon-
strated that attempts to prepare bis(β-diketiminate) CrIII
complexes by reaction of Li[(XylNCMe)2CH] with CrCl3-
(THF) yielded only the monosubstituted compound.20 Although
inadvertent formation of Cr(LX)2Cl complexes by salt meta-
thesis is not an issue even for the smallest symmetric 2,6-
disubsituted β-diketiminate, it was a recurring problem for
the synthesis of Cr[(PhNCMe)2CH]X2 derivatives.19,32 Adding
themixed-aryl β-diketiminatetoa well-definedCrIII precursor
that already has a cyclopentadienyl ligand presumably helps
avoid such difficulties in the synthesis of 4a-d.
methyl and CrIII chloro species. The thermal ellipsoid dia-
grams for compounds 3a-d and 4a-d are shown in Figures 4
and 5, respectively, and the Cr-X (X = Cl, CH3) bond
lengths are collected in Table 1. In the symmetric CpCr-
[(ArNCMe)2CH](R) alkyl complexes, the Cr-CH3 species
3a,14
˚
had shorter bonds (between 2.0645(17) and 2.076(2) A)
than were found for substituted primary alkyl Cr-CH2R
complexes (typically between 2.10 and 2.13 A).11 The mixed-
˚
aryl CrIII methyl complexes 3a-d follow the same trend, with
two compounds (3a,d) having Cr-CH3 bond lengths less than
2.06 A. However, no clear connection was evident between
˚
the Cr-CH3 or Cr-Cl bond lengths in Table 1 and the elec-
tronic properties of the NC6H4Y substituents for 3a-d or
4a-d.
The differing steric demands of the N-aryl groups are
displayed in the Cr-N bond lengths for 3a-d and 4a-d
(Table 2). With the exception of the anomalously long
˚
Cr-N(Dpp) bond of 2.134(3) A in 4b, the Cr-N(Dpp) bond
lengths are similar to those observed for the corresponding
symmetric CpCr[(ArNCMe)2CH]X compounds.3a,8,14 In all
cases, the Cr-N(C6H4Y) bond is significantly shorter, pre-
sumably reflecting the reduced steric requirements of the smaller
aryl group. Once again, however, no correlation between the
electronic properties of the Y group and the Cr-N(C6H4Y)
bond length could be discerned.
Kinetic Measurements of Iodomethane Activation. The reac-
tion of the mixed-aryl CrII complexes 1a-dwith at least 10 equiv
of MeI in pentane was monitored by UV-vis spectroscopy at
530 nm (eq 5).14 The second-order rate constants were obtained
by running the pseudo-first-order reactions with different
concentrations of excess iodomethane and then plotting kobs
vs [MeI]. The rate constants were calculated to be k = (9.80 (
0.3) ꢀ 10-1 M-1 s-1 for 1a (Y = OMe), k = (8.94 ( 0.4) ꢀ
10-1 M-1 s-1 for 1b (Y=Me),k=(7.48(0.2) ꢀ 10-1 M-1 s-1
(31) Huang, Y.-B.; Jin, G.-X. Dalton Trans. 2009, 767–769.
(32) For related challenges in the selective synthesis of Cr(LX)Cl2-
(THF)2 complexes, see: (a) Manzer, L. E. Inorg. Chem. 1978, 17, 1552–
1558. (b) Gibson, V. C.; Mastroianni, S.; Newton, C.; Redshaw, C.; Solan,
G. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2000, 1969–1971.
(c) Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 2005, 127, 11037–11046. (d) Xu, T.; An,
H.; Gao, W.; Mu, Y. Eur. J. Inorg. Chem. 2010, 3360–3364.
X-ray Crystal Structures of CrIII Methyl and Chloro Com-
plexes. X-ray crystal structures were obtained for all four
mixed-aryl β-diketiminate complexes for both the CrIII

Article
Organometallics, Vol. 30, No. 3, 2011 607
Figure 5. Thermal ellipsoid diagrams (50%) of CrIII chloro complexes 4a-d.
˚
Table 1. Cr-X Bond Lengths (A) in CpCr[DppNC(Me)CHC(Me)NC6H4Y](X) Complexes (X = CH3 (3a-d), Cl (4a-d))
Cr-X
Y = OMe
Y = CH3
Y = H
Y = CF3
Cr-CH3
Cr-Cl
2.0563(15) (3a)
2.3046(4) (4a)
2.0608(17) (3b)
2.3173(10) (4b)
2.0799(18) (3c)
2.2984(5) (4c)
2.052(2) (3d)
2.2947(2) (4d)
˚
Table 2. Cr-NAr Bond Lengths (A) in CpCr[DppNC(Me)CHC(Me)NC6H4Y](X) Complexes (X = CH3 (3a-d), Cl (4a-d))
Cr-NAr
Y = OMe
Y = CH3
Y = H
Y = CF3
Dpp
2.0262(12) (3a)
2.0128(12) (3a)
2.0274(12) (4a)
2.0067(12) (4a)
2.0254(12) (3b)
2.0175(14) (3b)
2.134(3) (4b)
2.011(3) (4b)
2.0391(13) (3c)
2.0109(13) (3c)
2.0285(11) (4c)
1.9993(12) (4c)
2.0274(19) (3d)
2.0177(19) (3d)
2.024(2) (4d)
2.000(2) (4d)
C6H4Y
Dpp
C6H4Y
for 1c (Y = H), and k = (4.96 ( 0.3) ꢀ 10-1 M-1 s-1 for 1d
(Y = CF3).
As demonstrated by the Hammett plot in Figure 6, the
observed rate constants for the single-electron oxidative
addition of MeI fit well with the electronic parameters (σ)
for the NC6H4Y substituents.33 The observed electronic
effect of the Y group is relatively modest (F = -0.36),
corresponding to about a 2-fold decrease in rate constant
from the most electron-donating (1a, Y = OMe, σ = -0.27)
to the most electron-withdrawing substituent (1d, Y = CF3,
σ = þ0.54). The ability of electron-donating groups to acce-
lerate the oxidative addition of iodomethane can be under-
stood in terms of the potential energy surface discussed
Figure 6. Hammett plot showing the correlation between the Y
group and the rate constant for MeI oxidative addition for
1a-d.
above. Increasing the electron-donating ability of the β-
diketiminate ligand stabilizes the CrIII iodo compound with
respect to the CrII species, thereby lowering the barrier between
A and C (Figure 1). The correlation is especially remarkable
given the absence of obvious structural differences in the
X-ray structures attributable to any electronic influence of
the Y group. In all of the mixed-aryl X-ray structures, the
NC6H4Y groups are aligned roughly perpendicular to the
Cr[(NCMe)2CH] plane. However, in solution the absence of
(33) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195.

608 Organometallics, Vol. 30, No. 3, 2011
Zhou et al.
enhancement for 1a-d compared to CpCr[(DppNCMe)2-
CH] and related symmetric ortho-disubstituted β-diketimi-
nate complexes was attributed to differences in steric con-
gestion between the CrII and CrIII complexes in the two systems.
It remains to be determined if continued steric reduction will
lead to yet further enhancement of single-electron oxidative
addition reactivity for CpCr(LX) complexes.
Experimental Section
General Considerations. All reactions were carried out under
nitrogen using standard Schlenk and glovebox techniques. Hexanes,
pentane, Et2O, CH2Cl2, and THF were purified by passage
through activated alumina and deoxygenizer columns from
Glass Contour Co. (Laguna Beach, CA, USA). Celite (Aldrich)
was dried overnight at 120 °C before being evacuated and then
stored under nitrogen. Iodine was purified by sublimation
before use. n-BuLi (1.6 M in hexanes), NaCp (2.0 M in THF),
CrCl2, CrCl3 (anhydrous), iodomethane (2.0 M in MTBE), 1,4-
dioxane (anhydrous), SmI2 (0.1 M in THF), and methylmagne-
sium iodide (3.0 M in Et2O) were purchased from Aldrich and
used as received. The β-diketiminate ligands were prepared accord-
ing to the literature procedure.15a,16 CpCrCl2(THF) was prepared
by treatment of Cp2Cr with excess anhydrous HCl (1.0 M in
Et2O) in THF at 0 °C.35 The CrII β-diketiminate compounds 1a,
c,d were prepared as described in the literature.16
UV/vis spectroscopic data were collected on a Varian Cary
100 Bio UV-visible spectrophotometer in pentane or hexanes
solution in a specially constructed cell for air-sensitive samples:
a Kontes Hi-Vac Valve with PTFE plug was attached by a
professional glassblower to a Hellma 10 mm path length quartz
absorption cell with a quartz-to-glass graded seal. Elemental
analyses were performed by Guelph Chemical Laboratories,
Guelph, ON, Canada.
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4Me] (1b). A
solution of NaCp (1.9 mL, 2.0 M in THF, 3.8 mmol) was added
dropwise to a suspension of CrCl2 (418.2 mg, 3.4 mmol) in
25 mL of THF, and the resulting mixture was stirred at room
temperature for 2 h. In a separate reaction vessel, a solution of
n-BuLi (2.4 mL, 1.6 M in hexanes, 3.8 mmol) was added dropwise
to a -30 °C solution of DppNHC(Me)CHC(Me)NC6H4Me
(1.18 g, 3.38 mmol) in 15 mL of THF, and the yellow solution
was warmed to room temperature and stirred for 1 h. The ligand
solution was then added to the CrII reaction mixture. After this
mixture was stirred overnight, the solvent was removed in
vacuo, the residue was extracted with hexanes, and the extract
was filtered through Celite and cooled to -30 °C. 1b (1.279 g,
81%) was isolated in three crops over several days. Anal. Calcd
for C29H36N2Cr: C, 74.97; H, 7.81; N, 6.01. Found: C, 71.59; H,
6.15; N, 8.71. UV/vis (hexanes; λmax, nm (ε, M-1cm-1)): 306
(10 300), 428 (5800), 557 (1080).
Figure 7. Relative steric repulsion in CpCr(LX) and CpCr-
(LX)(X) complexes with symmetric and mixed-aryl β-diketimi-
nate ligands.
ortho substituents may allow the NC6H4Y group to access a
planar conformation where the electronic influence of the Y
group may more readily be transmitted to the chromium
center.34
The observed sensitivity to electronic effects is in marked
contrast to the absence of pronounced steric effects on the
rate of MeI oxidative addition by symmetric CpCr[(ArNC-
Me)2CH] CrII complexes.14 Surprisingly, even the least reactive
mixed-aryl CrII complex, 1d, has a rate constant that is an
order of magnitude larger than all of those previously deter-
mined for the symmetric complexes, which ranged from
1.9 ꢀ 10-2 to 2.8 ꢀ 10-2 M-1 s-1 14
.
The ancillary ligand effects in the reaction of CpCr(LX)
with alkyl halides may be attributable to the difference in
steric demands between crowded CrIII-X and open CrII
complexes. Figure 7a,b illustrates the CrII and CrIII-X
species, respectively, for the symmetric ortho-disubstituted
β-diketiminate system. The steric demands of the N(2,6-
R2C6H3) groups are more acutely felt in CpCr(LX)(X) than
in CpCr(LX), resulting in the relative destabilization of the
CrIII product. For the mixed-aryl β-diketiminate system
(Figure 7c,d), the relative destabilization of CrIII is lessened
due to reduced steric repulsion. Increasing the relative stabi-
lity of CpCr(LX)(X) results in a diminished barrier between
A and C in Figure 1, corresponding to the observed enhance-
ment in the rate of oxidative addition of MeI for 1a-d.
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4(OMe)](I) (2a).
To a solution of 1a (100 mg, 0.208 mmol) in 5 mL of Et2O was
added a solution of I2 (27.9 mg, 0.110 mmol) in 3 mL of Et2O.
After the mixture was stirred for 20 h, the solvent was removed
in vacuo, the residue was extracted with 3 mL of Et2O, and the
extract was filtered through Celite and cooled to -30 °C. 2a (50
mg, 40%) was isolated after 2 days. Anal. Calcd for C29H36N2O-
CrI: C, 57.34; H, 5.97; N, 4.61. Found: C, 57.62; H, 6.04; N, 4.82.
UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 432 (5500).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4Me](I) (2b).
To a solution of 1b (200 mg, 0.431 mmol) in 10 mL of Et2O was
added a solution of I2 (60.1 mg, 0.237 mmol) in 4 mL of Et2O.
After the mixture was stirred for 20 h, the solvent was removed
in vacuo, the residue was extracted with 5 mL of Et2O, and the
extract was filtered through Celite and cooled to-30 °C. 2b (103mg,
40%) was isolated after 2 days. Anal. Calcd for C29H36N2CrI: C,
Conclusions
Paramagnetic CrIII CpCr[DppNC(Me)CHC(Me)NC6-
H4Y](X) complexes with mixed-aryl β-diketiminate com-
plexes were prepared from CpCrCl2(THF) (X = Cl) or by
single-electron oxidation of the corresponding CrII species
(X = I, CH3). Despite the absence of structural differences in
the CrIII complexes as Y was varied, a distinct electronic
effect on the rate of oxidative addition of iodobenzene was
observed. The rate constants ranged from k = (9.80 ( 0.3) ꢀ
10-1 M-1 s-1 for 1a (Y = OMe) to k = (4.96 ( 0.3) ꢀ 10-1
M-1 s-1 for 1d (Y = CF3) and correlated well with the σp
substituent constants for the Y groups. The substantial rate
(34) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2002, 124, 1378–1398.
€
(35) Furstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 12349–12357.

Article
Organometallics, Vol. 30, No. 3, 2011 609
58.89; H, 6.13; N, 4.74. Found: C, 58.55; H, 6.37; N, 5.04. UV/
vis (hexanes; λmax, nm (ε, M-1 cm-1)): 433 (6300), 579 (700).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H5](I) (2c). To
a solution of 1c (100 mg, 0.222 mmol) in 3 mL of Et2O was added
a solution of I2 (24.0 mg, 0.0946 mmol) in 4 mL of Et2O. After
the mixture was stirred for 20 h, the solvent was removed in
vacuo, the residue was extracted with 10 mL of Et2O, and the
extract was filtered through Celite and cooled to -30 °C. 2c
(10 mg, 8%) was isolated after several days. Anal. Calcd for
C28H34N2CrI: C, 58.24; H, 5.93; N, 4.85. Found: C, 57.85; H,
6.20; N, 4.82. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 431
(4200), 577 (620).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4CF3](I) (2d).
To a solution of 1d (115 mg, 0.222 mmol) in 6 mL of Et2O was
added a solution of I2 (45 mg, 0.177 mmol) in 4 mL of Et2O.
After the mixture was stirred for 20 h, the solvent was removed
in vacuo, the residue was extracted with 3 mL of Et2O, and the
extract was filtered through Celite and cooled to -30 °C. 2d
(48.3 mg, 34%) was isolated after several days. Anal. Calcd for
C29H33N2F3CrI: C, 53.96; H, 5.15; N, 4.34. Found: C, 54.04; H,
5.44; N, 4.21. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 431
(5600).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4(OMe)]-
(CH3) (3a). To a solution of 1a (200 mg, 0.416 mmol) in Et2O
was added a solution of MeI (0.12 mL, 2.0 M in MTBE, 0.24
mmol). After the reaction mixture was stirred for 1 h, MeMgI
(0.08 mL, 3.0 M in Et2O, 0.24 mmol) was added. After this
mixture was stirred overnight, excess 1,4-dioxane (0.3 mL, 3.3
mmol) was added and the reaction mixture was stirred for an
additional 2 h. The solvent was removed in vacuo, the residue
was extracted with 18 mL of hexanes, the extract was filtered
through Celite, and the solvent was again removed in vacuo. The
purple solid was extracted with 15 mL of hexanes, and the
extract was filtered through Celite and cooled to -30 °C. 3a
(170 mg, 82%) was isolated after 2 days. Anal. Calcd for C30H39-
N2OCr: C, 72.70; H, 7.32; N, 5.65. Found: C, 72.50; H, 7.66; N,
5.52. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 417 (4600), 542
(1300).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4Me](CH3)
(3b). Method A. To a solution of 1b (250 mg, 0.538 mmol) in
15 mL of Et2O was added a solution of MeI (0.15 mL, 2.0 M in
MTBE, 0.30 mmol). After the reaction mixture was stirred for
1 h, MeMgI (0.10 mL, 3.0 M in Et2O, 0.30 mmol) was added.
After this mixture was stirred overnight, excess 1,4-dioxane
(0.3 mL, 3.3 mmol) was added and the reaction mixture was
stirred for an additional 2 h. The solvent was removed in vacuo,
the residue was extracted with 20 mL of hexanes, the extract was
filtered through Celite, and the solvent was again removed in
vacuo. The purple solid was extracted with 10 mL of hexanes,
and the extract was filtered through Celite and cooled to -30 °C.
3b (67 mg, 26%) was isolated after several days.
hexanes, the extract was filtered through Celite, and the solvent
was again removed in vacuo. The purple solid was extracted
with a minimum amount of hexanes, and the extract was filtered
through Celite and cooled to -30 °C. 3c (50 mg, 24%) was
isolated after several days. Anal. Calcd for C29H37N2Cr: C,
74.81; H, 8.01; N, 6.02. Found: C, 75.20; H, 7.63; N, 6.21. UV/
vis (hexanes; λmax, nm (ε, M-1 cm-1)): 417 (5100), 541 (1500).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4CF3](CH3)
(3d). Method A. To a solution of 1d (200 mg, 0.386 mmol) in
15 mL of Et2O was added a solution of MeI (0.11 mL, 2.0 M in
MTBE, 0.22 mmol). After the reaction mixture was stirred for
1 h, MeMgI (0.07 mL, 3.0 M in Et2O, 0.21 mmol) was added.
After this mixture was stirred overnight, excess 1,4-dioxane
(0.3 mL, 3.3 mmol) was added and the reaction mixture was
stirred for an additional 2 h. The solvent was removed in vacuo,
the residue was extracted with hexanes, the extract was filtered
through Celite, and the solvent was again removed in vacuo. The
purple solid was extracted with a minimum amount of hexanes,
and the extract was filtered through Celite and cooled to -30 °C.
3d (63 mg, 31%) was isolated after several days.
Method B. To a solution of 1d (70 mg, 0.135 mmol) in 10 mL
of THF was added a solution of MeI (0.075 mL, 2.0 M in
MTBE, 0.15 mmol), followed by a solution of SmI2 (1.5 mL, 0.1
M in THF, 0.15 mmol). After the reaction mixture was stirred
for 3 h, the solvent was removed in vacuo, the residue was
extracted with 5 mL of hexanes, and the extract was filtered
through Celite and cooled to -30 °C. 3d (37 mg, 52%) was
isolated after several days.
Anal. Calcd for C30H36N2F3Cr: C, 67.53; H, 6.80; N, 5.25.
Found: C, 67.37; H, 7.06; N, 5.51. UV/vis (hexanes; λmax, nm
(ε, M-1 cm-1)): 417 (5600), 542 (1600).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4(OMe)](Cl)
(4a). A solution of n-BuLi (0.4 mL, 1.6 M in hexanes, 0.64 mmol)
was added dropwise to a -78 °C solution of DppNHC-
(Me)CHC(Me)NC6H4(OMe) (0.223 g, 0.612 mmol) in 15 mL
of THF, and the yellow solution was warmed to room tempera-
ture and stirred for 1 h. A solution of CpCrCl2(THF) (150 mg,
0.576 mmol) in 15 mL of THF was added, and the reaction
mixture was stirred at room temperature overnight. The solvent
was removed in vacuo, the residue was extracted with a total
of 10 mL of hexanes and 10 mL of CH2Cl2, and the extract
was filtered through Celite and cooled to -30 °C. 4a (158 mg,
53%) was isolated after several days. Anal. Calcd for C29H36-
N2OCrCl: C, 67.50; H, 7.03; N, 5.43. Found: C, 67.23; H, 7.33;
N, 5.25. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 421 (8300),
582 (810).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4Me](Cl) (4b).
A solution of n-BuLi (2.06 mL, 1.6 M in hexanes, 3.30 mmol)
was added dropwise to a -78 °C solution of DppNHC(Me)-
CHC(Me)NC6H4Me (1.147 g, 3.43 mmol) in THF, and the
yellow solution was warmed to room temperature and stirred
for 1 h. A solution of CpCrCl2(THF) (781 mg, 3.00 mmol) in
THF was added, and the reaction mixture was stirred at room
temperature overnight. The solvent was removed in vacuo, the
residue was extracted with a total of 10 mL of hexanes and 5 mL
of CH2Cl2, and the extract was filtered through Celite and
cooled to -30 °C. 4b (555 mg, 37%) was isolated after several
days. Anal. Calcd for C29H36N2CrCl: C, 69.66; H, 7.26; N, 5.60.
Found: C, 69.30; H, 6.95; N, 5.33. UV/vis (hexanes; λmax, nm
(ε, M-1 cm-1)): 421 (5100), 585 (480).
Method B. To a solution of 1b (86 mg, 0.185 mmol) in 10 mL
of THF was added a solution of MeI (0.10 mL, 2.0 M in MTBE,
0.20 mmol), followed by a solution of SmI2 (2.24 mL, 0.1 M in
THF, 0.224 mmol). After the reaction mixture was stirred for
3 h, the solvent was removed in vacuo, the residue was extracted
with 5 mL of hexanes, and the extract was filtered through Celite
and cooled to -30 °C. 3b (39 mg, 44%) was isolated after several
days.
Anal. Calcd for C30H39N2Cr: C, 75.13; H, 8.20; N, 5.84.
Found: C, 74.98; H, 8.44; N, 5.49. UV/vis (hexanes; λmax, nm
(ε, M-1 cm-1)): 417 (5700), 541 (1700).
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H5](Cl) (4c). A
solution of n-BuLi (0.40 mL, 1.6 M in hexanes, 0.634 mmol) was
added dropwise to a -78 °C solution of DppNHC(Me)CHC-
(Me)NC6H5 (0.194 g, 0.581 mmol) in 15 mL of THF, and the
yellow solution was warmed to room temperature and stirred
for 1 h. A solution of CpCrCl2(THF) (150 mg, 0.576 mmol) in
15 mL of THF was added, and the reaction mixture was stirred
at room temperature overnight. The solvent was removed in
vacuo, the residue was extracted with a total of 10 mL of hexanes
and 10 mL of CH2Cl2, and the extract was filtered through Celite
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H5](CH3) (3c).
To a solution of 1c (200 mg, 0.444 mmol) in Et2O was added a
solution of MeI (0.13 mL, 2.0 M in MTBE, 0.26 mmol). After
the reaction mixture was stirred for 1 h, MeMgI (0.09 mL, 3.0 M
in Et2O, 0.27 mmol) was added. After this mixture was stirred
overnight, excess 1,4-dioxane (0.3 mL, 3.3 mmol) was added and
the reaction mixture was stirred for an additional 2 h. The
solvent was removed in vacuo, the residue was extracted with

610 Organometallics, Vol. 30, No. 3, 2011
Zhou et al.
and cooled to -30 °C. 4c (69 mg, 25%) was isolated after several
days. Anal. Calcd for C28H34N2CrCl: C, 69.20; H, 7.05; N, 5.76.
Found: C, 69.53; H, 7.39; N, 5.81. UV/vis (hexanes; λmax, nm
(ε, M-1 cm-1)): 420 (6100), 585 (570).
Bruker X8 APEX diffractometer with graphite-monochro-
mated Mo KR radiation. The data were collected at a tempera-
ture of -100.0 ( 0.1 °C. Crystal data and refinement parameters
are given in the Supporting Information: Table S1 for 1b,c and
2a,b, Table S2 for 3a-d, and Table S3 for 4a-d.
Data Reduction. Data for all complexes were collected and
integrated using the Bruker SAINT software package.36 Data
were corrected for absorption effects using the multiscan tech-
nique (SADABS).37 The data were corrected for Lorentz and
polarization effects.
Structure Solution and Refinement. The structures were solved
by direct methods.38 All non-hydrogen atoms were refined
anisotropically (unless otherwise mentioned below). All hydro-
gen atoms were included in calculated positions but not refined
(unless otherwise mentioned below). All refinements were per-
formed using SHELXTL.39 In complex 2b, the N(1)-C(17)
fragment is disordered and was modeled in two orientations in
roughly equal proportions. In complex 3d, the fluorine atoms of
the CF3 group are disordered and were modeled in two orienta-
tions. Complex 4a crystallizes with disordered solvent in the
lattice. No reasonable model of the solvent could be obtained;
therefore, the PLATON/SQUEEZE program40 was used to
generate a second, solvent-free data set. In complex 4d, the Cp
ring was disordered and subsequently modeled in two orienta-
tions. All non-hydrogen atoms were refined anisotropically
except for C25, C26, C27, C28, and C29.
Synthesis of CpCr[DppNC(Me)CHC(Me)NC6H4CF3](Cl)
(4d). A solution of n-BuLi (0.40 mL, 1.6 M in hexanes, 0.634
mmol) was added dropwise to a -78 °C solution of DppNHC-
(Me)CHC(Me)NC6H4CF3 (0.237 g, 0.588 mmol) in 10 mL of
THF, and the yellow solution was warmed to room temperature
and stirred for 1 h. A solution of CpCrCl2(THF) (150 mg,
0.576 mmol) in 10 mL of THF was added, and the reaction
mixture was stirred at room temperature overnight. The solvent
was removed in vacuo, the residue was extracted with a total of
30 mL of Et2O, and the extract was filtered through Celite and
cooled to -30 °C. 4d (59 mg, 19%) was isolated after several
days. Anal. Calcd for C29H33N2F3CrCl: C, 62.87; H, 6.00; N,
5.06. Found: C, 62.50; H, 6.29; N, 5.33. UV/vis (hexanes; λmax
,
nm (ε, M-1 cm-1)): 420 (3700), 584 (430).
Kinetics Measurements. All kinetics measurements were per-
formed according to the same experimental protocol. A typical
experiment is described as a representative example with 1a. In a
glovebox, 17.1 mg (0.0356 mmol) of 1a was dissolved in pentane
and diluted to the mark in a 10 mL volumetric flask. An aliquot
of 1 mL of this 3.56 ꢀ 10-3 M solution was transferred using a
pipet to a 25 mL volumetric flask. Pentane was added, and then
80 μL of a 2.0 M solution of MeI in MTBE (0.16 mmol) was
added by microliter syringe. Pentane was added up to the mark,
and the solution was thoroughly mixed. A portion of the 1.42 ꢀ
10-4 M solution was loaded into the UV-visible cell for air-
sensitive samples (described above) and transferred from the
glovebox to the spectrophotometer, where the absorption at
530 nm was recorded every 0.1 min for 20 min. The resulting
kinetics trace was fit to a first-order decay curve to give the rate
constant kobs = 6.24 ꢀ 10-3 s-1. The pseudo-first-order experi-
ment was repeated three more times with varying excess con-
centrations of MeI. The second-order rate constant, k = (4.96 (
0.3) ꢀ 10-1 M-1 s-1, for the reaction was extracted from the
slope of the straight-line plot of kobs vs MeI concentration (R2 =
0.9973). The reactions with 1b-d were performed in the same
manner, with R2 = 0.9912, R2 = 0.9985, and R2 = 0.9905,
respectively. The Hammett plot in Figure 6 was generated by
graphing log(kY/kH) (where kH is the rate constant for 1c, Y = H)
vs the substituent constants taken from ref 33 (σp = -0.27, -0.27,
0, and 0.54 for Y = OMe, CH3, H, CF3, respectively) to give
F = -0.36 with R2 = 0.9954.
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada (NSERC),
the Canadian Foundation for Innovation, and the
University of British Columbia for financial support.
Supporting Information Available: Tables and CIF files giving
complete crystallographic data for complexes 1b,c, 2a,b, 3a-d,
and 4a-d. This material is available free of charge via the
Internet at http://pubs.acs.org.
(36) SAINT, Version 7.46A; Bruker AXS Inc., Madison, WI, 1997-
2007.
(37) SADABS: Bruker Nonius area detector scaling and absorption
correction, V2.10; Bruker AXS Inc., Madison, WI, 2003.
(38) SIR97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano,
G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
(39) SHELXTL, Version 5.1; Bruker AXS Inc., Madison, WI, 1997.
(40) SQUEEZE: Sluis, P. v. d.; Spek, A. L. Acta Crystallogr., Sect. A
1990, 46, 194–201.
X-ray Crystallography. Data Collection. All crystals were
mounted on a glass fiber, and measurements were made on a